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1 IMPACT OF VIRAL MEDIATED INSULIN LIKE GROWTH FACTOR I ON SKELETAL MUSCLE FOLLOWING CAST IMMOBILIZATION By FAN YE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010
2 2010 Fan Ye
3 To my parents, wife and son
4 ACKNOWLEDGMENTS I would never have been able to finish my dissertation without the guidance of my committee members, help from friends and support from my family. I would like to take this opportunity to thank all the people involved in my journey towards obtaining a doctorate degree in this foreign land. First and foremost, I would like to express my deepest gratitude t o my advisor, Dr. Krista Vandenborne, for her excellent guidance, care and patience in providing me with an incomparable environment for pursuing research. With her academic knowledge in the field of muscle rehabilitation, Dr.Vandenborne has encouraged an d led me through this fascinating area of science. As a mentor, she also taught me a lot of communication and social skills, required for success in America. As an international student, my gratefulness for her invaluable contribution to my academic development rises from the bottom of my heart. I would also like to thank Dr. Glenn Walter. His wisdom and passion for science has provided the most amazing journey in science for me. I would like thank Dr. Floyd Thompson for his broad knowledge and patience and his help in developing my background in physiology. I thank Dr. David Fuller for the positive encouragement he gave me each time I came to his office. He always made me feel welcome and put me at ease. I thank Dr. Orit Shechtman for her consistent encouragement and incredible scientific advice. I would also like to express my appreciation to Dr. Sally Johnson, Dr. David Criswell and Dr. Steve Borst for sharing their research experience, constructive critiques and invaluable contributions to my research. Special thanks are also extended to Dr. H. Lee Sweeney and Dr. Elisabeth Barton at the University of Pennsylvania for providing support for my research project.
5 It has been a great opportunity to work in Dr. Vandenbornes laboratory. My deepest thanks go to Dr. Min L i u for his generosity and thoughts, which kept me in good cheer during my research. This project would not have been completed without his support and devotion. Special thanks to Wendy Han for teaching me how to do w estern blot and to Dr. Jennifer Stevens for showing me the cast immobilization model. I would also like thank Dr. Donovan Lott for being my mentor, while he was a post doc in the lab. I owe great thanks to Dr. Sunita Mathur, who as a good friend was always willing to help me and give her best suggestions. Many thanks to Dr. Sean Forbes for assisting me in writing my dissertation. Special thanks go to my fellow lab members, Dr. Claudia Senesac, Dr. Chris Gregory Dr. Arun Jayaraman, Dr. Prithvi Shah, Sean Germain, Dr. Celine Baligand, Nat han Br yant, Ravneet Vohra, Jasjit Deol Ishu Arpan, Emily Senesac, Wootaek Lim Soren De Vos and Christa Richmond. I am grateful to all for their help and friendship. Last but not least, I would like to express my deepest appreciation to my family. To my mother, I thank you for coming to America to take care of my son during the most critical year for me To my father, I thank you for encouraging me and always trusting me. I also thank my lovely son, Andrew, who made my graduate study wonderful. And words cannot adequately express how grateful I am to my wife and love of my life, Li. She has always been there to cheer me up and stand by me through the good and bad times. Finally, I would thank all my friends being supportive of me. Thank you all!
6 TABLE OF CONTENTS ACKNOWLEDGMENTS .................................................................................................. 4 page LIST OF TABLES ............................................................................................................ 9 LIST OF FIGURES ........................................................................................................ 10 ABSTRACT ................................................................................................................... 11 CHAPTER 1 LIMB DISUSE ANIMAL MODELS ........................................................................... 13 1.1 Introduction ....................................................................................................... 13 1.2 Cast Immobilization........................................................................................... 14 1.3 Summary .......................................................................................................... 16 2 SKELETAL MUSCLE PLASTICITY TO DECREASED LOADING CONDITIONS ... 17 2.1 Introduction ....................................................................................................... 17 2.2 Structural Properties ......................................................................................... 17 2.2.1 Muscle Weight ......................................................................................... 17 2.2.2 Muscle Fiber Size and Susceptibility of Different Fiber Types to Atrophy .......................................................................................................... 18 2.2.3 Fiber Ty pe Transition ............................................................................... 19 2.2.4 Myonuclear Number and Domain Size .................................................... 19 2.3 Muscle Metabolism ........................................................................................... 20 2.3.1 Energy Metabolism .................................................................................. 20 2.3.2 Protein Turnover ...................................................................................... 21 2.4 Contractile Properties ....................................................................................... 22 2.5 Summary .......................................................................................................... 25 3 RECOVERY OF ATROPHIED SKELETAL MUSCLE FOLLOWING DISUSE ........ 26 3.1 Introduct ion ....................................................................................................... 26 3.2 Muscle Damage During Reloading ................................................................... 27 3.2.1 Early Structural Changes During Reloading ............................................ 27 3.2.2 Muscle Inflammation ................................................................................ 28 3.2.3 Transcriptional Programming During Early Reloading ............................. 29 3.2.4 S low Twitch Muscles Are More Susceptible to Muscle Damage During Reloading ...................................................................................................... 29 3.3 Muscle Regeneration Following Disuse and Reloading .................................... 30 3.3.1 Protein Changes ...................................................................................... 30 3.3.2 Satellite Cells as Adult Muscle Precursor Cells Contribute to Muscle Regeneration ................................................................................................. 31
7 3.4 S ummary .......................................................................................................... 33 4 EFFECTS OF IGFI ON SKELETAL MUSCLE ....................................................... 34 4.1 The IGF I System .............................................................................................. 34 4.1.1 IGF I ........................................................................................................ 34 4.1.2 IGF I Receptor ......................................................................................... 35 4.1.3 IGF Binding Proteins ............................................................................... 36 4.2 IGF I Actions on Skeletal Muscle ...................................................................... 37 4.2.1 Introduction .............................................................................................. 37 4.2.2 Anabolic Actions of IGF I ......................................................................... 37 184.108.40.206 IGF I and protein synthesis ............................................................ 38 220.127.116.11 IGF I and protein degradation ........................................................ 40 4.2.3 IGF I and Satellite Cells ........................................................................... 41 4.2.4 IGF I and Apoptosis in Skeletal Muscle ................................................... 42 4.3 IGF I Isoforms in Skeletal Muscle ..................................................................... 43 4.4 AdenoAssociated Virus Based Gene Therapy in Skeletal Muscle ................... 44 4.5 Summary .......................................................................................................... 45 5 OUTLINE OF EXPERIMENTS ................................................................................ 47 5.1 Overall Objective............................................................................................... 47 5.2 Experiment One: Assess the Effect of Viral Mediated IGF I Gene Transfer on the Anterior Compartment Fast Twitch Muscles Following Cast Immobilization ...................................................................................................... 47 5.2.1 Experimental Design ............................................................................... 47 5 .2.2 Specific Aims and Hypotheses ................................................................ 48 5.3 Experiment Two: To Assess the Effect of Viral Mediated IGF I Gene Transfer on the Posterior Compartment Muscles Following Cast Immobilization ...................................................................................................... 49 5.3.1 Experimental Design ............................................................................... 49 5.3.2 Specific Aims and Hypotheses ................................................................ 50 6 E XPERIMENT ONE: ASSESS THE EFFECT OF VIRAL MEDIATED IGF I GENE TRANSFER ON THE ANTERIOR COMPARTMENT FAST TWITCH MUSCLES FOLLOWING CAST IMMOBILIZATION ............................................... 52 6.1 Abstract ............................................................................................................. 52 6.2 Introduction ....................................................................................................... 53 6.3 Material and Methods ....................................................................................... 55 6.3.1 Animals and Viral Injection ...................................................................... 55 6.3.2 Cast Immobilization ................................................................................. 56 6.3.3 Force Measurements ............................................................................... 57 6.3.4 Musc le Morphological and Regeneration Assessment ............................ 57 6.3.5 RNA Isolation and Assessment of mRNA Abundance for Real Time Quantitative PCR .......................................................................................... 58 6.3.6 Determination of IGF I Protein Concentration ......................................... 59
8 6.3.7 Data Analysis .......................................................................................... 60 6.4 Results .............................................................................................................. 60 6.5 Discussion ........................................................................................................ 64 7 EXPERIMENT TWO: TO ASSESS THE EFFECT OF VIRAL MEDIATED IGF I GENE TRANSFER ON THE POSTERIOR COMPARTMENT SLOW TWITCH MUSCLES FOLLOWING CA ST IMMOBILIZATION ............................................... 83 7.1 Abstract ............................................................................................................. 83 7.2 Introduction ....................................................................................................... 84 7.3 Mat erials and Methods ...................................................................................... 87 7.3.1 Animal Care and Experimental Design .................................................... 87 7.3.2 Cast Immobilization and Reambulation ................................................... 88 7.3.3 Magnetic Resonance Imaging ................................................................. 88 7.3.4 Muscle Mechanical Measurements ......................................................... 89 7.3 .5 Muscle Morphology and Regeneration Assessment ............................... 90 7.3.6 RNA Extraction, Reverse Transcription and TaqMan Quantitative PCR Analysis ......................................................................................................... 91 7.3.7 Data Analysis .......................................................................................... 91 7.4 Results .............................................................................................................. 92 7.5 Discussion ........................................................................................................ 95 8 CONCLUSION ...................................................................................................... 117 LIST OF REFERENCES ............................................................................................. 119 BIOGRAPHICAL SKETCH .......................................................................................... 155
9 LIST OF TABLES Table page 6 1 Changes in IGF I, receptor, and binding proteins mRNA expression in the TA muscles .............................................................................................................. 82
10 LIST OF FIGURES Figure page 4 1 IGF I signaling in skeletal muscl e ...................................................................... 46 6 1 Schematic diagram of the rAAV IGF I gene construct ........................................ 72 6 2 Lac Z expression ................................................................................................ 72 6 3 EDL wet weights tetanic force and specific force ............................................... 73 6 4 EDL fiber cross sectional area ............................................................................ 74 6 5 TA IGF I protein levels ........................................................................................ 77 6 6 Presence of central nuclei (%) in IGF I and PBS injected muscle. ................... 78 6 7 Immunofluorescent staining of muscle crosssections for Pax7.. ....................... 79 6 8 Satellite cell activity in muscle fibers for IGF I and PBSinjected muscle. ........ 80 6 9 Presence of embryonic myosin in IGF I and PBS injected muscle. .................... 81 7 1 Representative transaxial proton MRIs of the mouse lower hindlimb. .............. 103 7 2 Maximal cross sectional area (CSAmax) of the triceps surae. Soleus wet weight, g astrocnemius wet wei ght, s ol eus tetanic force and specific force. .... 104 7 3 Representative T2 image and corresponding Hematoxylin and eosin stained crosssections of the soleus muscle ................................................................ 107 7 4 Muscle T2 and percentage of pixels wit h elevated T2 in the lower hindlimb muscles of mice at different loading time points. .............................................. 108 7 5 H&E stained cros ssect ions of the soleus and area fractions of abnormal muscle in the IGF I and PBS injectio n group. ................................................... 109 7 6 Percentage of central nuclei at each time point ................................................ 110 7 7 A Presence of Pax 7 and embryonic myosin in the soleus muscle) .................. 112 7 8 mRNAs expression of muscle MyoD at different time points ............................ 116
11 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy IMPACT OF VIRAL MEDIATED INSULIN LIKE GROWTH FACTOR I ON SKEL ETAL MUSCLE FOLLOWING CAST IMMOBILIZATION By Fan Ye M ay 2010 Chair: Krista Vandenborne Major: Rehabilitation Science Muscle weakness is a common clinical phenomena observed following bed rest, surgery, cast immobilization or chronic disease. The conseq uences of muscle dysfunction ar e far reaching and can include functional impairments, decreased motor control, reduced fitness and long term disability. As such, the maintenance and recovery of muscle function presents an important challenge to the field o f rehabilitation sciences. The overall objective of this dissertation was to investigate the potential of virus mediated gene transfer of insulinlike growth factor I (IGF I) to maintain skeletal muscle size and function during cast immobilization and ream bulation. A combination of magnetic resonance imaging (MRI), in vitro force measurements, immunohistochemical assays and real timePCR techniques were performed in an animal model of cast immobilization to explore this objective. We investigated both the i mpact of IGF I overexpression in the anterior compartment and posterior compartment hindlimb muscles. Our findings demonstrate that local overexpression of IGF I results in significant increases in muscle mass, muscle fiber size and concomitant increases i n muscle force production under normal loading conditions in both slow twitch and fast twitch muscles
12 In addition, the relative gains in muscle size and force production are maintained during cast immobilization and reambulation. Thirdly, mus cles overexpr essing IGF I show protection from muscle damage during early reambulat ion and evidence of enhanced muscle regeneration during later reambulation. Additionally, IGF I receptor and binding proteins demonstrate concomitant molecular responses with IGF I prote in and mRNAs overexpression during cast immobilization and reambulation. Collectively, findings from this series of studies show that viral delivery of IGF I can protect skeletal muscle from the deleterious effects of immobilization/disuse and accelerate t he subsequent restoration of muscle function. These findings may also set the stage for possible clinical applica tions of gene therapy to ameliorate muscle weakness and atrophy and expedite muscle recovery following disuse or in chronic diseases associated with muscle wasting (e.g. cancer, AIDS).
13 CHAPTER 1 LIMB DISUSE ANIMAL MODELS 1.1 Introduction The loss of skeletal muscle mass secondary to inactivity or disuse is a common clinical phenomenon following surgery, immobilization, long term bed rest, et c. In particular, the susceptibility of skeletal muscle to disuse induced atrophy became the focus of attention after the Skylab mission of the mid1970s, because morphological and physiological deleterious adaptation of skeletal muscle to inactivity were recognized to endanger the daily life of humans (396, 397) Given the ethical concerns associated with inducing muscle atrophy in humans, as well as limitations in performing invasive measurements in human subjects (18) it is not surpri sing that m ost mechanical disuse studies have been performed in animal models which allow for more comprehensive collection of data from which mechanisms can be elucidated, as well as more controlled experiments. Because of the complexities and difficult ies involved in investigating the mechanisms responsible for disuse muscle atrophy in humans, researchers have developed many experimental animal models to study muscle adaptations to disuse. The most common models include cast immobilization (19, 55) hindlimb suspension (30, 266, 363) denervation (26) and spinal cord injury (220, 310) These models have advantages and disadvantages in their ability to mimic the changes in muscle morphology and physiology seen with limb disuse in humans, and there are limitations in making comparisons among the findings obtained from different animal models. For example, hindlimb s uspension unloads the muscles but allows for activity and is often used to mimic prolonged bed rest and spaceflight in humans. C asting immobilization of
14 a limb on the other hand, permits minimal muscle activity when fixed in their shortened position (18, 19) Lieber commented some models are very clean in terms of creating a reproducible response, but are not very relevant to real clinical problems, others are very relevant but so complex that they are difficult to decipher (219) As a result, there are conflicting results in the literature on the effects of limb disuse on skeletal muscle adaptations. 1.2 Cast Immobilization Cast immobilization has been used extensively in the clinical setting to help protect fractured bones from loading injury and allow for proper healing of the fracture site. It is well known that the most common complication of immobil ization is the loss of muscle mass and strength (179) In a study on patients submitted to 8 weeks of cast immobilization following ankle surgery, Shaffer found a 4563% decrease in ankle plantar flexor strength (331) and a 1632% decrease in maximal muscle cross se ctional area (111) Similarly, the amount of strength loss in adductor pollicis muscle was ~22% in both you and old men after immobilization for 2 weeks (381) On the other hand, cast immobilization in animals is relatively easy to implement, noninvasiv e and can be targeted to affect specific muscles of interest. I mmobilization in animals (rats) was first applied by Solandt et al (1942) (105) and later modified by Fischbach and Robbins (1969) (122) These early investigators immobilized the calf muscle of the rat, using a needle inserted percutaneously through the calcaneus into the distal part of the tibia, and another needle inserted through the distal femur into the proximal tibia, so as to fix each joint at a right angle. Recently, Frimel et al (139) established cast immobilization model for mice, which successfully induced muscle atrophy in either the anterior or posterior compartment of the hindlimb s by varying the angle of the ankle joint.
15 Therefore, this model of muscle atrophy is highly relevant for disuse studies in bot h slow and fast twitch muscle s. In addition, t his technique produces similar conditions that observed in patients subjected to cast immobilization. From the point of view of studying muscle plasticity, the hi ndlimb cast immobilization and hindlimb suspension models more closely mimic decreased use in which the upper and lower motor neurons are intact and muscle tension is extremely low. Compared to hindlimb suspension, cast immobilization may provide a cleaner model of muscle disuse (219) EMG studies of cast immobilized muscles (fixed in their shortened position) show s a n 85~90% reduction in muscle activity compared to control s (122, 171) In contrast, hindlimb suspension only induces a transient decrease in EMG which retur ns toward control values after 7 days of suspension (10) This is likely due to the fact that the hindlimb suspended animals can still perform dynamic muscle contractions of their hindlimbs. Secondly, although both models produce a large degree of atrophy, only the limb immobilization model has been shown to cause a significant amount of muscle atrophy in the fast twitch EDL muscles, by fixing the muscle at resting or shorter than resting lengths (56) In contrast, in the hindlimb suspension model, the minimal atrophy is observed in the fast twitch EDL muscle (368) Finally, hindlimb suspension produces transient stress in the rats as evidenced by a small increase in adrenal gland mass and elevated plasma corticosterone levels, which is absent in the hindlimb immobilization model (256) In summary, while both suspension and immobilization models unload hindlimb skeletal muscles, it appears that the two mod els induce distinct physiological responses and may serve different research purposes.
16 1.3 Summary Models of disuse can be placed along a spectrum from a simple reduction in normal activity to complete lack of electrical activity in the muscles. When we draw a spectrum of disuse we can easily find the places of these models. Simply reducing normal activity levels represents the first element in a spectrum of disuse. More dramatic changes occur with prolonged bed rest or spaceflight, and in animal hindlimb suspension and immobilization models. The most severe model of muscle atrophy spinal cord injury model combines unloading and disruptions of the neutral input to the muscles. In this study, we used mice cast immobilization model as a disuse animal m odel to study the effect of insulinlike grow factor I (IGF I) on both fast twitch and slow twitch muscle s during cast immobilization/reambulation.
17 CHAPTER 2 SKELETAL MUSCLE PLASTICITY TO DECREASED LOADING CONDITIONS 2.1 Introduction Skeletal muscle is cap able of remarkable adaptations in response to altered loading and activity. A djustments to mechanical demands initiate modified transcriptional programs which elicit a gain (hypertrophy) or loss (atrophy) in muscle mass. Muscle atrophy is characterized by a wasting or decline in overall muscle mass and is associated with a number of deleterious adaptations (152) In contrast, muscle hypertrophy is defined as an increase in muscle mass (313) which in adult animal comes as a result of an increase in the size, as opposed to the number, of preexisting skeletal muscle fibers (152) 2.2 Structural Properties 2.2 .1 Muscle W eight The reduction in muscle weight with disuse h as been well described in animal studies, and has also been extrapolated from decreases in limb girth measurements in humans (126, 193) T he greatest decrease in muscle mass is observed during the early phase of disus e, with a prolonged period of inactivity resulting in littl e additional decline in mass For example, Max et al (238) (1971) found a muscle weight loss of 30% in rat gastrocnemius muscle, only 3 days after immobilization; at the end of the experimental p eriod (15 days), the weight loss amounted to 50%. Similar findings have been reported by Herbison et al (165) In that study a muscle weight reduction in the gastrocnemius of 58% after six weeks of immobilization was reported. It should be noted that immobilizi ng muscles in a stretched position has been shown to delay the development of disuse atrophy or even produce a transient
18 hypertrophy in animal models. For example, Ferguson (1957) (120) showed that when the lower extremity of rabbits was immobilized with the ankle joint in a plantarflexed position (i.e., stretching the anterior c ompartme nt), the anterior tibialis muscle gained weight but gastrocnemius showed atrophy. Similarly, Summers and Hines (1951) (358) found increasing atrophy in the shortened position compared to stretch or neutral position when comparing the effects of immobilization in the cat soleus muscle at d ifferent lengths. Collectively, previous studies show that disuse causes a significant decrease in muscle mass, with the greatest loss occurring during the early unloading period. However, this atrophy may be altered by certain stimuli, such as the muscle being positioned in a lengthened position. 2.2.2 Muscle Fiber Size and Susceptibility of Different Fiber Types to A trophy Following a period of inactivity, reductions in muscle mass and strength is associated with a decrease in muscle f iber cross sectional area (CSA). In animals, muscles predominantly composed of type I (slow, oxidative) fibers are more susceptible to the effects of limb disuse. Tischler et al (375) showed that following s paceflight, the weights of muscle s composed of larger proportions o f slow twitch fibers showed the greatest reductions in muscle mass. Specifically, the gastrocnemius, plantaris and soleus demonstrated reductions of 16%, 24%, and 38% respectively, but ther e was no change in the weight of fast twitch muscles, the tibialis anterior and extensor digitorum longus. Booth (55) reported that after four weeks of immobilization there was a decrease in the size o f slow twitch fibers in the soleus, but the cross sectional area of fast twitch fibers in the soleus muscles was not significantly changed. Interestingly it wa s shown that the slow twitch type I fibers atrophied more than the fast type II fibers,
19 while in human studies, the fast type II fibers were at least susceptible to space induced atrophy as the slow fiber type (107, 125, 398) 2.2.3 Fiber Type T ransition In addition to muscle atrophy from inactivity, rodent st udies have shown a transition from slow twitch fibers to fast twitch fibers. Martin et al (234) observed a 3950% decrease in the number of slow twitch fibers in soleus muscles, whereas the number of fast twitch muscle fibers in the plantaris and superficial region of medial gastrocnemius was increased after seven days of spaceflight in rats. This f inding i s also con sistent with other studies which suggest that the increase in the number of fast twitch fiber s is at the expens e of a decrease in the slow twitch fibers after limb disuse (6, 29, 30, 67, 351) Histochemical studies also support the fact the there are significant increases in faster (type II) fibers and a decrease in slower (type I) fibers following disuse atrophy (28, 106, 232, 362) This slow to fast fiber transition after in activity is also observed in human studies although the evidence in humans is limited (107, 363) Fiber type transitions during inactivity may represent a compensatory effect to minimize the devastating results of atrophy. As fast twitch fibers can generate much more power at a given contraction speed than slow twitch fibers, increasing the number of fast twitch fibers may be an attempt to compensate, but not entirely offset, the loss of power generating capacity during unloading caused by muscle atrophy and weakness (125, 243) 2.2.4 Myonuclear Number and Domain S ize Studies have shown a reduction in the number of myonuclei in spinal cord injury and hindlimb immobilization mo dels (103, 341) and apoptosis (programmed cell death) has been indicated as a mechanism contributing to remodeling of skeletal muscle in
20 response to hindlimb unweighting (13) Apoptosis has been proposed to be controlled by three pathways: sarcoplasmic reticulum mediated, receptor mediated, and mitochondrial mediated (291) Roy et al (309) introduced the concept nuclear domain in skeletal muscle plasticity which refers to the specific area of the myofiber that is controlled by a myonucleus. A decrease in the number of myonuclei during atrophy is coincident with the decrease of muscle fiber CSA (230) and it is also proportional to the myo nuclear domain size (277, 341) The nuclear domain of fast twitch muscles is twice the size of the domain found in slow twitch muscles. In other words, the density of nuclear domain in slow twitch muscles is higher than in fast twitch muscles (376) Due to the difference in the muscle fiber type ratios, the question regarding myonuclear loss is whether specific fiber types are more or less sensitive to myonuclear shifts in comparison with the others (13) Several microscopic studies in adult rats found greater myonuclei loss in slow twitch muscle fibers than fast twitch fibers (12, 13, 49, 167) Similar findings were also reported in human studies (12, 13, 167, 254) However, this does not appear t o be the case in neonatal muscle fibers of rats, in which the same amount of myonuclear loss was found in both fiber types (276) 2.3 Muscle Metabolism 2.3.1 Energy M etabolism Fiber type transitions during inactivit y also result in a shift in energy metabolism from lipid as the primary fuel source to glucose. This is consistent with the fiber type transition from slow to fast fibers, since fast twitch muscles have a greater dependence on glycolytic activity and have less capacity for lipid metabolism (126, 257, 303, 349) However, this greater reliance on glycolysis also appears to be the case for slow twitch
21 fibers. Several studies have shown a decreased capacity for lipid uti lization and greater reliance on glucose for energy in both slow and fast muscles (126, 164, 211, 356, 375) Glycolysis is very effective for high intensity, short duration exercise which might also be part of the c ompensation for loss of slow twitch muscles. I n terestingly, even though rats have larger proportions of IIb/x fibers in the vastus lateralis muscle compared to humans, d isus e decreases succinate dehydrogenase activity more in the rats (151) 2.3 .2 Protein Turnover Protein maintenance is physiologically important to maintain lean body mass. All body proteins including muscle proteins are in a constant state of turnover, with concurrent synthesis and degradation. It has been reported that disuse leads to a negative nitrogen balance (89) decreased protein concentration (92) loss of contractile proteins (24) and decreased cytochrome c mRNA (24, 258) Loss of muscle protein with disuse occurs due to both a decrease in protein synthesis and an incr ease in protein degradation. A reduction in protein synthesis is thought to be the initial cause of protein turnover that results in the loss of protein as evident by a ~20% after two days of immobilization and slowly decrease to ~50% of original level s by seven days in rats (379) Molecular studies indicate that changes in translation occur very rapidly followed by transcriptional changes (20, 91, 168, 240, 357, 360, 367) The large majority of the protein loss after the first few days of immobilization has also been attributed to the increased protein deg radation rate (367) During unloading, there is approximately a two day lag in the increase in myofibrillar protein degradatio n. The degradation increases up to ~200% above normal at day 15 and slowly returns to normal by day 24 of unloading in rats (367) In rodents, the greater susceptibility of anti gravity, slow muscle fibers may
22 also depend on their more rapid rates of turnover and thus a higher rate constant for protein degradation compared to humans (367) The mechanism causing musclespecific atrophy is characterized by increases in protein degradation processes, particularly the ATP dependent proteolytic ubiquitinproteasome pathway (183) Three E3 ubiquitin ligases: Muscle Ring Finger 1 (MuRF1) (54) Muscle Atro phy F box (MAFbx) also called Atrogin1 (150) II (208) have been identified as s II mRNAs have been shown to be increased during cancer cachexia (208) however little is known about its mechanism. MuRF1 and atrogin1 encode ubiquitin ligases, which function to conjugate ubiquitin to protein substrates (54, 150) Numerous distinct in vitro and in vivo models of skeletal muscle atrophy have shown to result in an upregulation of MAFbx/Atrogin and MuRF1 (54, 88, 91, 150, 212) Studies in genet ically deficient mice further demonstrate that MuRF1 or atrogin1 knockouts lose less muscle under atrophy conditions (54) 2.4 Contractile Properties The structural changes summarized in the preceding sections lea d to substantial modifications of muscle contractile function. The models of hindlimb immobilization and hindlimb suspension both lead to muscle atrophy wit h a preferential effect on slow twitch muscles in rodents, such as the soleus, changes towar ds the f ast twitch muscle type. For example, i n the atrophied soleus muscle the following changes have been noted: (i) the time to peak tension and the half relaxation time become shorter, (ii) the maximal shortening velocity becomes faster, (iii) tetanic tension and specific tension (i.e. tetanic force per cross sectional area) are usually reduced, and (iv) force and power generated are decreased (125, 143, 283, 288, 334, 377, 399) Contractile properties of most of the fas t twitch muscles, such as tibialis anter ior muscles, are generally
23 unaffected by limb disuse. Interestingly, data from human Skylab studies suggest that type II fibers are more susceptible to microact ivity induced strength loss compared to slow type I fib ers in the vastus lateralis and soleus muscles (107, 123, 398) Similar to muscle mass loss, time dependent changes in protein turnover result in rapid strength loss during the initial stages of disuse, and the rate of loss slows down in the later stages of disuse (243) Compared with spaceflight and bed rest, immobilization appears to result in a more rapid decline in muscle strength (2). It is often observed that strength loss during inactivity is larger than the loss of muscle mass in the whole muscle (animal studies) and single fibers ( humans ), which ma y contribute to the decrease in specific force/tension during disuse (81, 165, 365, 370, 398) The decrease in specific force may be due to a reduction in the ratio of surface electromyographical activity to mechani cal response of the muscle (98, 202, 251) Interestingly, the fall in specific tension i s less than the increases in shortening velocity, which is found to counteract the fall in maximal power, at least at the muscl e fiber level (398) Increased fatigability is a consequence of muscle atrophy, which greatly impacts muscle performance (249) There are a number of definitions of muscle fatigue that have been used including: 1) reduced maximal voluntary contraction (MV C) or power output, 2) a failure to maintain the required or expected power output, and 3) an increased effort to maintain force (11, 108, 224) Although researchers are familiar with the phenomenon of muscle fatigue, it is often difficult to determine the precise causes of fatigue. In general, the major sources of fatigue can arise from either central (afferent feedback or psychological motivation) or peripheral (neural transmission or muscular
24 activation or transmi ssion) factors (50, 342) Muscle fatigue can refer to the capacity of the central nervous system to generate motivation, it can be the ability of the conducting pathways to deliver motor impulses to the muscle fibers, and it can be the intrinsic contractile properties of the muscle fibers themselves (52, 114) Early studies by Merton (249) suggested that increasing electrical stimulation immediately after maximal voluntary contraction (MVC) could not restore tension which indicated the fatigue could be entirely peripheral. However, later s tudies found that electrical stimulation of fatigued muscle could increase tension in about half of subjects examined dur ing voluntary contractions (51) Researchers that study muscle fatigue seem to generally agree that the cause of fatigue is very specific to different activities task dependency of muscle fatigue (21, 51, 114) There are several examples to support this principle. First, it was shown that old men were more fatigable than young adults when performing maximal shortening or lengthening contraction (42) Second, women were observed to be more tolerable to fatigue under lower intensity contractions than men (72, 166, 175) ; but there was no difference between at maximal contractions (42, 72, 176) In addition, the duration of muscle contraction in different muscle fiber types also responded differently to electrical stimulation compared to voluntarily contraction (61, 63, 109) Collectively, these studies suggest that muscle fatigue could be attributed to multiple factors that depend on the intensity, duration and type of contractions, gender, and fiber types. In atrophied muscle, the significant effect on fatigability of muscle atrophy could be attributed to a greater number of motor units recruited to accomplish the same amount of work load because of slow twitch muscle fibers transitioning to fast muscle types
25 (233) It is well known that motor units which are recruited at a higher threshold demonstrate increased fatigue. Thus, in an atrophied muscle, more motor units are recruited to perform a given task, and consequently, fatigability is increased. In addition to motor unit recruitment, there might be several other causes of increased fatigability in atrophied muscle: i) fuel shif t from lipid to glucose; ii) loss of blood supply to the atrophied muscles; and iii) oxidative stress and reactive oxygen species (ROS) (125, 171, 291) 2.5 Summary Muscle disuse atrophy is induced by muscle inactiv ity or decreased activity for an extended period (203) Many animal models such as hindlimb suspension, immobilization, and denervation are available to study the effects of disuse on skeletal muscle. Regardless of the inciting event, skeletal muscle atrophy is characterized by the loss of muscle mass, fiber size, force production and changes in fi ber phenotype and metabolism. One might question whether skeletal muscle atrophy is simply the converse of skeletal muscle hypertrophy. However, during skeletal muscle atrophy, an entirely distinct process is stimulated, leading to a dramatic increase in protein degradation and turnover.
26 CHAPTER 3 RECOVERY OF ATROPHIED SKELETAL MUSCLE F OLLOWING DISUSE 3.1 Introduction Morphological and functional properties of postural or antigravity muscles change significantly depending on weight bearing status A s descri bed above, a reduction in weight bearing or physical activity leads to a reduction in muscle mass, fiber cross sectional area, total myonuclear number and a shift towards a fast fiber phenotype (160, 173, 290). These changes are accompanied by an upregulat ion in gene expression involved in protein degradation, glycolysis, and growth arrest, as well as an attenuation of cell proliferation and differentiation, and fat metabolism (81, 96, 190, 229, 262, 268). When the factors causing atrophy are removed, musc le begins to recover. This is a more complicated process than activation of typical muscle growth programs for several reasons. First, disuse atrophy causes a molecular program leading to protein degradation and myonuclear loss; therefore it is tempting to speculate that this program must be turned off before muscle can regrow. Second, de novo formation of new myofibers can occur in some cases of hypertrophy, whereas muscle recovery from disuse normally starts from repairing existing muscle fibers. Third, although increased mechanical load is known to increase skeletal muscle mass, it also causes muscle injury in the initial reloading phase, which is later followed by muscle fiber regeneration (46) Finally, it has been demonstrated that atrophy negatively affects muscle precursor cells (satellite cells). Despite the fact that satellite cells are mobilized after reloading, the t ransient nature of the compensation in the satellite cell population is insufficient for extended periods of reloading (264, 328) while under hypertrophic conditions, the
27 morphology and functions of satellite cells are well preserved and easily respond to stimuli (254) 3.2 Muscle Damage During R eloading S tudies have found that when humans return to terrestrial gravity after being in microgravity environment in space for an extended period of time, muscle weakness, fatigue, impaired coordination and delayed onset muscl e soreness were observed (301) Morphological changes such as segmental tearing of muscl e fibers were observed five hours post flight, and appeared similar to lesions seen after eccentric contraction (300, 301) Resuming normal activity also induced an increase in the serum creatine kinase (CK) (371) Similarly, in rodent studies, subsequent reloading of the hindlimbs by resuming normal cage activity initiates muscle fiber damage. Muscle damage during reloading has been linked to the inability of the muscle fibers to bear eccentric contractions and the consequent inflammatory proce ss (49, 126, 193, 300) 3.2.1 Early Structural C hanges During Reloading During the early phase of muscle reloading following disuse, mechanical stress and consequent damage of some fibers is apparent. Numerous studi es have demonstrated that reloading following short term unloading induces damage to myofibers (e.g., Z line streaming, sarcomere lesions), increased interstitial edema, disruptions in excitationcontraction (E C) coupling, infiltration of inflammatory cel ls, and force decrement. These changes parallel those seen following a bout of eccentric contractions in normally loaded skeletal muscle (180, 193, 204, 205, 300, 301, 394) Activated capillary en dothelial cells and high proportion of synthetically active fibroblasts with ribosomal endoplasmic reticulum was also detected, which indicates the secretion of connective tissue components and subsequently alters cell adhesion (136)
28 Increased sarcolemmal permeability and calcium influx into the damaged myofibers activates calcium dependent, nonlysosomal muscle proteases such as calpain leading to further muscle fiber degradation (44, 343) 3.2.2 Muscle I nflammation Muscle inflammation is one of the processes contributing to muscle injury/repair and begins at the onset of reloading. The possibility that inflammatory cells contribute to muscle inj ury is evident by the observation that significant elevations in muscle neutrophils and macrophages occur at the time point when the ultrastructural signs of injury are exacerbated (138) The number of neutrophils is elevated within just two hours of reloading and this is followed by morphological changes of muscle fibers after five hours of reloading (138, 289, 372) The concentration of neutrophils in the muscle peaks within 12 hours of perturbation (372) whereas the number of macrophages is signif icantly elevated at 24 hours reloading in a rat hindlimb reloading model (101) Dumont et al reported that the level of neutrophils and macrophages did not return to normal until 14 days after reloading in a mouse model of hindlimb unloading (100) Current evidence indicates that it is not clear whether inflammation has a net negative or positive impact on muscles that experience modified loading, or whether it is possible t o remove the negative consequences of inflammation while still maintaining its beneficial effects, by manipulating the muscle inflammatory response (371) Neutrophils are capable of initiating muscle injury and releasing reactive oxygen intermediates, such as peroxides and superoxide (100, 371) Yet, the depletion of circulating neutrophil populations before muscle injury does not appear to affect involuntary force production (221) Macrophages can promote muscle injury in vitro but typical increase at a later
29 stage of reloading which is more related to muscle growth, regeneration and repair, such as satellite cell activation and proliferation (348, 373) Overall, activation of the inflammatory response plays an important role in proteolys is and removal of debris; it also leads to release of high concentrations of cytolytic and cytotoxic molecules (374) Because of the dual role of neutrophils and macrophages in both the inflammatory and healing processes, the temporal coordination of the processes occurring during the initial phase of reloading after disuse is critical for the su ccessful recovery of muscle mass (65) 3.2.3 Tran scriptional Programming D uring Early R eloading Molecular studies indicate early, dynamic muscle adaptations within three days of reloading. These adaptations include: 1) an early transcriptional increase in cell cycle regulatory factors (CDK4, cyclin D1) and myogenic regulatory factors (MyoD and Myf6) expression; 2) enhanced gene expression of some mechanosensitivity sarcomere alignment proteins (integrin 1, desmin, titin); 3) alterations in metabolic enzyme expression (carbohydrate uptake, glycolysis, fat metabolism) and muscle p henotype (MyHC I, IIa, and IIx); and 4) normalization of ubiquitin proteasomeproteolys is and downregulation of mitochondrial associated apoptotic pathways (68, 137, 155, 184, 352, 353, 360, 401) 3.2.4 Slow Twitch M uscles A re More Susceptible to Muscle D amage D uring R eloading Reloading following disuse results in the production of sarcomere lesions primarily in the antigravity muscles (e.g. adductor longus, soleus) (299, 387) There may be several reasons for this finding. First, the sequence of motor unit recruitment selectively activates slow fiber motor units before fast twitch during voluntary contractions (115,
30 149) In human stu dies, slow fibers are activated earlier than fast fibers during normal recruitment which is evidenced by recruiting fast fibers only after slow fibers are depleted of glycogen (125, 213) Second, it is reported that muscle precursor cells are only needed in the soleus muscle during regrowth from atrophy, but not in fast twitch muscles (104, 254) Finally, it has been clearly shown that atrophic muscle is more prone to injury (19, 369) The fact that antigravity muscle s such as slow twitch soleus muscle is the most susceptible to disuse may make it more affected by the increased mechanical stress during reloading than fast twitch muscles. 3 .3 Muscle Regeneration F ollowing Disuse and Reloading Tissue repair occurs in four interdependent phases: degeneration, inflammation, regeneration, and fibrosis. Muscle recovery after reloading can be considered a continuous sequence of inflammation and repair/regeneration. In young healthy muscles, sarcomere reorganization is visible after two days of reloading characterized by very wide Z bands, which presumably prevents further damage and serves as a scaffold for the regeneration of contractile filamen ts (194, 205, 267) 3.3.1 Protein C hanges During reloading, upregulation of ubiquitin is progressively suppressed and protein synthesis is stimulated (359) Based on extensiv e data from the Booth lab, studies have shown that protein synthesis in the gastrocnemius muscle is significantly increased during the first six hours of immobilization and returned to control levels after four days of reambulation (57, 258, 259) Protein degradation remains elevated during the initial stage of recovery from disuse atrophy, which might be caused by muscle damage. L arge numbe rs of lysosomal like bodies are observed during the reloading phase (201, 410) After seven
31 days of recovery, proteolytic proteins and gene expression return to control values in a rat hindlimb suspension model (359) In human studies of cast immobil ization, the expression of ubiquitin dependent 20S proteasome and musclespecific proteolytic genes MuRF1 and atro gin 1 return to baseline within 24 hours following cast removal (189) Therefore, stimulation of both protein synthesis and degradation results in elimination of abnormal proteins and allow for the reloaded muscle to return to previous levels following disuse atrophy. 3.3.2 Satellite Cells as Adult Muscle Precursor Cells Contribute to Muscle R egeneration Ever since the concept of nuclear domain was introduced, more and more researchers have demonstrated that extended periods of disuse lead to the loss of myonuclei, while when muscles regenerate from atrophy, new myonuclei need to be added to maintain a certain volume of cytoplasm. During recovery of the soleus muscle following disuse atrophy the 35% decrease in myonuclear number after 2 wk of hindlimb unloading was restored to control values (254) .The primary source for the generation of new myonuclei in vivo in skeletal muscle are muscle stem cells, called satellite cells (261, 262, 335) Sat ellite cells are mononucleated cells that normally lie between the basal lamina and the plasma membrane of muscle fibers in a quiescent state (237) Young animal muscle contains 30% more satellite cells than old animals (146) Recent data suggest that satellite cells al one are sufficient to mediate extensive regeneration of damaged adult skeletal muscle in vivo (78) and contribute to muscle growth after atrophy (254) Upon muscle injury resulting from mechanical stress, the satellite cells are activated, at which point they are considered myoblasts, and depart from quiescence
32 and enter the G1 phase of the cell cycle. After several rounds of proliferation, the majority of the satellite cells differentiate and fuse to form new myofibers or repair damaged myofibers (329) Satellite cell mitotic activity is required for hypertrophy (308) and it is higher in the reloading muscle compared to normal control rats (264) The number of satellite cells gradually increases in response to reloading (195, 236) The importance of satellite cells is because they can fuse together, or with existing myofibers, and they have self renewing capability (78) (15) Satellite cell self renewal is a necessary process. Without this process recurrent muscle regeneration would rapidly lead to the depletion of the satellite cell pool. Upon activation, a large proportion of satellite cells proliferate and differentiate to provide new myonuclei for growing muscle. The remaining satellite cells return to the quiescent state, thereby replenishing the satellite cell pool (329) Satellite cell self renewal may result from an asymmetric division generating two distinguishable daughter cells (79) The process of satellite cell activation and differentiation during muscle regeneration is modified by myogenic regulatory factors, growth factors, macrophages and other factors. Myogenic regulatory factors are muscle specific helix loophelix transcription factors that regulate muscle specific genes (82, 227) MyoD and Myogenin are the primary factors that are expressed in act ivated satellite cells, and they are necessary for satellite cell proliferation and differentiation (82, 405) At the molecular level, activation and proliferation of satellite cells is characterized by the rapid upregulation of MyoD (329) The expression of myogenin is also upregulated in cells beginning their terminal differentiation program. A variety of alterations in the surrounding environment of the satellite cell, including platelet derived growth factor,
33 basic fibroblast growth factor, leukemia inhibitory factor, and insulinlike growth factor (IGF I), are all known to stimulate muscleprecursor cell mitotic activity (58, 161) 3 4 Summary This chapter provided a synopsis of how structural and functional properties of skeletal muscle adapt with different loading s tates imposed on the muscle. Muscle disuse diminishes skeletal muscle mass and strength and causes alterations in metabol ism and fiber phenotypes When physical activity or normal loading conditions are resumed, skeletal muscle recovers from atrophy. In th e early reloading phase, muscle injury dominates the recovery process; in the later reloading phase, regeneration of damaged fibers occurs, as well as increases in the number of myonuclei and transformation of muscle fiber types, are part of the normal cou rse of reestablishment of normal characteristics of atrophied muscles (137)
34 CHAPTER 4 EFFECTS OF IGFI ON SKELETAL MUSCLE 4 .1 The IGFI System The InsulinLike Growth Factor I (IGF I) was first identified in 1957 (255) It was named on the basis of both its insulinlike activity a nd growth promoting properties (302) IGF I has three unique characteristics: its mediation of the skeletal growthpromoting actions of growth hormone (GH), its mitogenic properties, and its mimicry of the actions of insulin. The IGF I family includes the ligand IGF I, the IGF I receptor, and IGF binding proteins (IGFBPs), which are involved in all phases of mammalian growth (160) 4 .1.1 IGF I Mature IGF I is a 70 amino acid single chained polypeptide with structural homology with proinsulin. The majority of circulating IGF I is produced by the liver (endocrine action) under the control of pituitary growth hormone (197, 260) There are several important differences between insul in and IGF I. IGF I is a more primitive hormone than insulin, produced by multiple tissues and it has the capability of acting in an autocrine/paracrine fashion (197) T he affinity of IGF I to insulin receptor is only 1/100th of the potency of insulin (250) In addition, only a small amount of liver derived IGF I circulates as a free hormone, and over 99% of plasma IGF I is bound to special carrier proteins, called IGF I binding proteins (IGFBPs); the most important of which seems to be IGFBP 3 (188) The biological significance of IGF I in skeletal muscle was most demonstrated when depression of the circulating and tissue levels of the somatic growth factor environment through surgical hypophysectomy, resulted in a robust increase in the expression of IGF I mRNA and peptide in overloaded muscles (4, 93)
35 4 .1.2 IGF I Receptor The various biological actions of IGF I are mediated mainly by interacting with its cell surface receptor type I (IGF IR). IGF IR is a heterotetrameric tyrosine protein kinase consisting of two subunits responsible for ligand binding and two subunits possessing tyrosine kinase activity (407) Despite the striking similarity in overall structure between insulin receptor and IGF IR (~60% homology at the amino acid sequence level with the insulin receptor), IGF IR and insulin receptor developed their own identities and spectrum of activities. Insulin primarily functions as a regulator of metabolism (388) In contrast, IGF I appears to be one of the primary regulators of the growth of organisms (248, 297) Thus, one might predict that IGF I receptor might be more potent than the insulin receptor in mediating differentiation and survival by stimulating DNA synthesis and mitogenic events (53, 354) Mice l acking insulin receptors are born with modest growth retardation (~10%) (226) IGF I receptor deficient mice, on the other hand, show severely impaired growth (reaching only ~45% of normal size) and have multiple abnormalities, including lung, neurologic, skin and bone defects. These mice also die within minutes after birth of respiratory failure (206, 222, 278) Baudry et al showed that IGF IR can represent an alternative to the insulin receptor for metabolic signaling (41) Transgenic MCK KR hIGF IR (MKR) mice overexpress a dominant negative IGF IR specifically in skeletal muscle, and exhibit significantly lower levels of muscle mass and hypoplasia than wild type mice (121)
36 Upon ligand binding, the intrinsic tyrosine kinase of the IGF IR is activated, and and triggers the appropriate downstream pa thways. 4 .1.3 IGF Binding Proteins The functions of IGF I are not only complicated by its receptor, but also by a family of at least six binding proteins (IGFBPs 1 6) which are responsible for modulating the actions of IGF I (74) IGFBPs are found in the circulation and widely expresse d in adult tissues at different levels. IGFBPs are multifunctional proteins that transport IGF I in the circulation to their location and alter IGF I binding ability to its receptors. Approximately 95% of circulating IGF I is carried by IGFBP 3 and acidla bile subunit (ALS), which forms a ternary complex and is essentially confined to the vascular compartment (177) IGFBPs were first thought to be primarily responsible for extending the half life of IGF I and inhibiting IGF I binding to its receptors (354) Thus, tissue IGF bioactivity was determined by the circulating IGFs, and also the local expression of IGFs, IGFBPs and proteases. Proteases cleave IGFBPs into smaller fragments thereby releasing IGF I and increasing the availability of circulating free IGF (25) Now it is realized that IGFBPs can localize in the extracellular matrix, modulate IGF Is effectiveness and exert IGF I independent effects on various tissues, as well as act as a localized storage depot for IGF I (345, 354) Binding IGFBPs to IGFs may have a dual role: on the one hand, IGFBP blocks the interaction between IGFs and IGF IR; on the other hand, it can protect IGFs from proteolytic degradation and, thus enhanc e the action of IGFs by increasing their bioavailability in local tissues (77, 141, 298) Among the six IGFBPs, skeletal muscle produces IGFBP 3, 4, 5, and 6, with IGFBP 3 and 5 being the most abundant (338) While IGFBP 3 is primarily invo lved in
37 IGF I transport as mentioned above, IGFBP 4 is believed to inhibit the actions of IGF I whereas IGFBP 5 can either inhibit or stimulate proliferation and differentiation depending on culture conditions in vitro (117, 131, 279) 4 .2 IGF I Actions on Skeletal Muscle 4 .2.1 Introduction Knock out models of IGF I, the receptor or IGFBPs have demonstrated that every component of the IGF I system is very important in muscle growth and development (403) Mouse embryos lacking IGF I have impaired development and the pups die immediately after birth, because they cannot breathe (290 ) and demonstrate generalized organ hypoplasia including the muscles. (222) The importance of IGF I has also been studied by the opposite approach. Using transgenic mice with systemic IGF I overexpression, Mathews et al found that muscle and bone growth were increased approximat ely 30% when circulating levels of IGF I were 50% above control values (235) Furthermore, Adams showed that the muscles of rats with surgical hypophysectomy responded to increased loading, despite a drastic depression in growth hormone (4). It has become increasingly clear that IGF I may be acting as an autocrine and paracrine signal, thereby playing a major role in the development, growth, differentiation, and maintenance of skeletal muscles, both in culture and in intact animals (86, 93, 225, 282) Unlike other growth factors, IGF I is reported to have a variety of anabolic effects on skeletal muscle and it stimulates both myoblast proliferation and differenti ation (113, 128, 129, 131) 4 .2.2 Anabolic Actions of IGFI The fact that IGF I mediates many anabolic effects has been reported through in vivo and in vitro experiments by many investigators (14, 54, 96, 99, 162, 186, 244, 252,
38 306, 384) and even on human cells (182) In these experiments, IGF I acts both by stimulating protein synthesis and inhibiting protein degradation pathways. Thi s well established ability of IGF I to mediate anabolism could clearly contribute to the hypertrophy process in skeletal muscle. 4 .2.2.1 IGF I and protein s ynthesis Hypertrophy of skeletal muscle is a multidimensional process and IGF I is involved in this process. In human exercise studies, IGF I mRNA and protein expression in muscles has been found to increase after either single bouts of aerobic exercise (33, 158, 198) or periods of strength training (207, 286) ; and even in older men and women after 10 weeks of strength training (339) Numerous studies employing in vivo animal models known to in duce muscle hypertrophy also demonstrate that muscle expression of IGF I increases very early in the hypertrophy process. These studies also show that increased IGF I is localized to the myofibers of the affected muscles (47, 85, 110) The importance of local muscle produced IGF I in response to hypertrophy is emphasized by the normal growth seen in mice that lack the circulating liver form of IGF I (340) and increased IGF I mRNA levels in the overloaded muscles of surgical hypophysectomy rats (4, 93) The central role of IGF I in muscle growth and hypertrophy has led to suggestions that IGF I administration might prevent ageassociated myofiber loss, necrosis of dystrophic myofibers (36) and myofiber atrophy resulting from spaceflight or disuse (383) The various studies cited in the previous secti ons clearly suggest a cause and effect relationship between the hypertrophy process and increased, GH independent, autocrine/paracrine IGF I expression in skeletal muscle. Therefore, understanding the
39 downstream signaling mechanism induced by IGF I that ar e required for hypertrophy in skeletal muscle is of great importance (Figure 4 1) a) The IGFI/PI3K/Akt/mTOR pathway : Genetic disruption of PI3K and/or the mammalian target of rapamycine (mTOR) gene result in decreases in cell size (411, 412) Furthermore, myotube hypertrophy induced by IGF I can be inhibited by a pharmacological inhibitor of PI3K (307) The requirement for mTOR mediated signaling in hypertrophy has been dem onstrated pharmacologically (54, 307) mTOR can be activated downstream of PI3K/Akt; it can also be activated by IGF I directly (64, 159) Once activated, mTOR can increase protein synthesis in skeletal muscle by modulating at least 2 signaling pathways: the p70S6 kinase (p70S6K) pathway and PHAS 1 (also known as 4E BP) pathway (181) mTOR activates p70S6K, a positive regulator of protein translation (389) ; on the other hand, mTOR inhibits the activity of PHAS 1, a negative regulator of the protein initiation factors eIF 4E (312) It is also interesting to point out that stimulation of p70S6K might be secondary to the blockade of PHAS 1 when mTOR is activated (275) Therefore, the PI3K/A kt/mTOR pathway can induce hypertrophy, by enhancing the translation of mRNAs encoding ribosomal proteins and elongation factors; integral components of the protein synthesis machinery (366) b) The IGF: is another substrate of IGF I/PI3K/Akt downstream target. Phosphorylation of Akt results (84) Expression of a do minant has been shown to induce myotube hypertrophy, which is a way to mimic Akt activity (306)
40 (147) inhibition of this kinase produces the desired effect. There is a concern whether domi nant sectional area and completely prevents the atrophic effects of glucocorticoids in rats (321) c) Other pathways: New evidence suggests that IGF I induced hypertrophy is independent of PI3K signaling pathways (270) Myogenic differentiation is dependent upon nuclear export of a histone deacetylase (HDAC) (245) IGF I and the calcium/calmod ulin dependent protein kinase (CaMK) can stimulate the phosphorylation of HDAC, thereby exporting HDAC out of the nuclei and facilitating the hypertrophic action of IGF I (228) Calcineurin, a serine/threonine phos phatase activated by calcium/calmodulin, is also established as a link with IGF I (270, 330) Overexpression of IGF in C2C12 mouse (330) or L6E9 rat (270) myoblasts has shown to result in ca lcineurin mediated hypertrophy. 4 .2.2.2 IGF I and protein degradation It has been shown that IGF I reduces atrogin1 and MuRF1 expression and attenuates muscle wasting in diabetic rats (253) and in C2C12 myotubes, even when atrophy i s induced by serum deprivation or a synthetic glucocorticoid, dexamethasone (355) The mechanism by which Akt inhibits atrogin1 and MuRF1 upregulation was demonstrated to involve the FOXO family of transcription factors, a subgroup of the Forkhead box O (FOXO) family of transcription factors (215, 318, 355) IGF I activates the PI3K/AKT pathway, which in turn results in phosphorylation of the FOXO proteins, Phosphorylation of the FOXO transcript ion factors results in excluding ubiquitin ligases
41 genes from the nucleus (60) which prevents transcription of the atrogin1 and MuRF1 genes. This finding demonstrated a novel role for PI3K/Akt, not only in stimulating muscle hypertrophy, but also inhibiting the induction of atrophy signaling. This is important because it shows that during atrophy, deactivated PI3K/Akt may not only lead to decreased protein synthesis, but also cause increases in protein degradation rate, actin cleavage, and expression of muscle ubiquitin ligases. 4 .2.3 IGF I and Satellite Cells It is obvious that IGF I acts in an autocrine/paracrine factor mediating skeletal muscle hypertrophy (284) However, the importance of IGF I induced act ions on muscle satellite cells may be less evident. After birth, skeletal muscle fibers become terminally differentiated and cannot undergo mitotic division or directly increase their myonuclear number (i.e., myonuclear division) (70) As discussed earlier, muscle hypertrophy requires addition of new myonuclei to maintain a certain ratio between myonuclei and the size of myofibers (i.e. nuclear domain) (363) In this case, satellite cells, which are small mononucleated skeletal muscle stem cells, appear to be mobilized to enter activation, proliferation and differentiation processes. In order to distinguish the effects of IGF I on anabolism versus proliferation in muscle Barton Davis et al used radiation to eliminate satellite cells in mice (36) Overexpression of IGF I through adenoviral delivery enhanced muscle mass by only about 7% in the radiated muscles compared to 15% enhancement in nonirradiated controls. This suggested that IGF I acts to promote muscle recovery by stimulating anabolism of muscle fibers as well as by activating satellite cell proliferation and differentiation. Among the well characterized growth factors, IGF I is the only one that
42 has been consistently reported to facilitate each of these processes in satellite primary cell culture and muscle cell lines (294) The effects of IGF I on proliferation and differenti ation are temporally separated. The proliferative response is mediated by the Ras Raf MEK ERK pathway (80) which stimulates the expression of proliferation proteinMyoD; whereas the pathway leading to differentiati on involves the activation of PI3K /Akt pathway (380) which stimulates prodifferentiation proteinmyogenin (8). In satellite cell culture, the muscle regenerative markers my oD and myogenin are reported to be present in activated but not in quiescent satellite cells. It was found that myoD mRNA increased prior to proliferation, while the appearance of myogenin mRNA was found to coincide with differentiation (294) IGF I has be en shown to stimulate gene expression of both myoD and myogenin in various conditions (293, 294) These seemingly contradictory results emphasize the role of IGF I in cooperating with other mitogenic factors to stim ulate either proliferation or differentiation, or both. Under most in vitro conditions, by assessing cell morphology and other markers of myogenesis the stimulation of muscle cell growth by IGF I exhibits a strikingly biphasic concentration dependency curv e; high IGF I concentrations favor proliferation, whereas lower IGF I concentrations seem to promote differentiation (80, 130, 132, 294) Besides that, it is clear that there is a temporal link between the two effec ts: 1) a proliferative (mitogenic) response to IGF I that lasts 24 to 36 hours and 2) this is followed by myogenic differentiation (132) 4 .2.4 IGF I and Apoptosis in Skeletal Muscle IGF I has been reported to act as a survival factor that prevents apoptosis in some tissues (97) It has been s hown that actin is a substrate of the apoptotic protease and
43 caspase3 mediated fragmentation of actin occurring during atrophy (97) In addition, Lee et al (2004) showed that caspase 2 activates the proapoptotic protein Bax (215) Activation of caspase 3 and cleavage of actin were prevented by supplement ing with IGF I (54) In addition, activation of PI3K/Akt inhibits proapoptotic proteins of the Bcl 2 family (Bax, Bad) and also induces anti apoptotic proteins of the Bcl 2 family (Bcl X) (54) Therefore, the signaling pathways of IGF I are highly complex, and its effects on skeletal muscle atrophy/ hypertrophy acts not only via regulations of protein synthesis and degradation, but also on apoptosis, myoblast proliferation and differentiation (Figure 4 1) (263) 4 .3 IGF I Isoforms in Skeletal M uscle The fact that IGF I can act either as a circulating hormone or as a local factor has driven scientists to further study the structure of different isoforms of IGF I. The IGF I gene is comprised of six exons, and depending on alternative splicing, there are at least four different IGF I isoforms in rodents (400) The different isoforms vary in structure and function and are referred to as local (class 1) and circulating (class 2) IGF I. Cla ss 1 and 2 denote the use of exon 1 or 2, respectively, and class A and B denote the absence or presence of exon 5, respectively (39, 332, 400) The rodent IGF IA and IB is equivalent to human IGF IEa and I GF IEc, r espectively. IGF IEc i s sensitive to mechanical loading induced by stretch or damage and this has prompted this isoform to be termed mechano growth factor or MGF (406) MGF has been proposed to be a potent form of IGF I for promoting skeletal muscle hypertrophy. Barton used viral mediated gene carried either IGF IA or IGF IB to study the effects of these two isoforms on skel etal muscle hypertrophy in young and old mice (39) It turned out that both IGF IA and IGF IB were equally effective in increasing muscle mass in young mice, whereas
44 only IGF IA overexpression produced significant increases in muscle size of old mice. Further studies are needed to determine the production, distribution and stability of IGF IA and IGF IB in skeletal musc le under different conditions. 4.4 AdenoAssociated Virus Based Gene Therapy in Skeletal Muscle Gene therapy has become a rapid emerging field for a treatment for a variety of genetic and acquired diseases. A spectrum of viral and nonviral based vectors have been used on skeletal muscle because of its accessibility and capability to maintain and express the target genes (34, 229, 265, 402) N onviral based vectors include plasmid DNA alone, liposome complexed DNA, etc (402) Although nonvrial based vector delivery can reach the desired expression quickly, it is associated with transient transgene expression in vivo and needs repeated delivery ; a nd it only affect s dividing cells in vitro and in vivo (217) Adenoassociated virus (AAV) is a relatively new viral gene delivery system One of th e attractive advantages of this system is its broad range of infectivity, including both dividing and nondividing cells (9, 133135, 191, 192) More importantly, recombinant AAV (rAAV) vectors have all the viral seq uences removed, they are nontoxic and elicit a minimal immune response (45) In addition, AAV viral particles are very stable, resistant to many physical and chemical factors and readily purified and concentrat ed (271) All these properties make it convenient for gene therapy. AAV consists of two open reading frames (OPFs) flanked by inverted terminal repeats (ITR) (317, 347) AAV packages and delivers a singlestranded (ss) DNA genome that is transcriptionally inactive until it is converted int o a double strands (ds) (391) Conventional rAAV vector replication in vivo is a ratelimiting step because it requires the second (complementary) strand synthesis (239) By using half size of the
45 wild type genome, the rAAV is packaged into a self complementary doublestranded molecule (239) This recent technology is more efficient to induce earlier transgene expression by bypassing the requirement for host dependent leading strand synthesis (391, 404) Many serotypes of AAV have been isolated and have a broad host range. All rAAV vectors used currently are based on AAV type 2 (314) It is shown th at AAV1 i s considerably more efficient than AAV2 vectors in gene delivery to adult rat brains (62, 280, 390) AAV serotype 8 vector is of particular interest because of its exceptionally high gene transfer efficiency and has very high tropism for skeletal muscle (142, 272, 392) Previously, Barton has successfully used rAAV mediated IGF I gene transfer to study aging related loss of skeletal muscle function (35, 40) Therefore, muscle can be efficiently transfected by AAV to reach stable transgene expression. This technique shows great promise as a therapeutic intervention for a variety of genetic and clinic al medical conditions. 4 .5 Sum mary The studies cited in this section emphasize the importance of the role of IGF I in mediating skeletal muscle adaptations and suggest the following conclusions: 1) IGF I operates in both autocrine and paracrine modes in skeletal muscle; 2) IGF I stimul ates myofiber anabolic processes and satellite cell proliferation and differentiation; and 3) IGF I protects myofibers from apoptosis and contributes to myofiber survival .
46 Fi gure 41. IGF I signaling in skeletal muscle modified from Mourkioti F (263)
47 CHAPTER 5 OUTLINE OF EXPERIMENTS 5 .1 Overall Objective 1. To investigate the potential of IGF I enhancement to guard skeletal muscle from the deleterious impact of immobilization/disuse. 2. To study the effect of IGF I gene transfer on muscle damage and recovery of muscle size and function following cast immobilization. 3. Further elucidate the role of IGF I in the regulation of muscle size under varying loading/activity conditions 5 .2 Experiment One: Assess the Effect of Viral Mediated IGF I Gene Transfer on the Anterior Compartment Fast Twitch Muscles Following Cast Immobilization 5 2.1 Experimental Design R ecombinant adenoassociated vir us vector serotype 1 (rAAV 2/cap1: AAV 2 genomes pseudopackaged into AAV 1 capsids) for IGF I (rAAV IGF I MLC1/3 promoter ) was injected in the anterior compartment of one of the hindlimbs of young C57BL6 female mice (3 weeks of age; n = 24). At 20 weeks o f age, these mice were randomly allocated into 4 groups (n = 6/group): 1) normal weight bearing; 2) 2 wk cast immobilization; 3) 1 wk reambulation; and 4) 3 wk reambulation. For the cast immobilization procedures both hindlimbs were immobilized with the k nees extended and the ankles in 60of dorsiflexion to maximize atrophy of the tibialis anterior (TA) and extensor longus digitorum (EDL) muscles. TA and EDL muscles were removed from both legs after 2 w eeks of cast immobilization and after 1 and 3 weeks of free cage reambulation. In vitro force mechanics was implemented to measure EDL muscle strength. Immediately following force mechanics, muscle wet weights were measured and immunohistochemistry was performed to quantify EDL muscle fiber cross sectional
48 ar eas (CSAs). Quantitative real time PCR was used to quantify mRNA expression levels of IGF I, IGF I receptor (IGF IR), IGF binding protein 3 ( IGFBP 3 ), IGF binding protein four ( IGFBP 4 ), and IGF binding protein 5 ( IGFBP 5 ). ). Markers of muscle fiber regeneration were determined using hematoxylin & eosin staining for central nuclei and immunofluorescence techniques using monoclonal antibodies to detect Paired box protein Pax 7 and embryonic myosin. 5 .2 .2 Specific Aims and Hypothese s Specific Aim 1: Hypothesis a: IGF I gene transfer induces an increase in EDL muscle mass muscle strength and muscle fiber CSAs under normal loading conditions. To char acterize the effect of IGF I gene transfer on the morphological and functional properties of a fast twitch muscle (extensor digitorum longus muscle ) under altered loading conditions. Hypothesis b: EDL muscles overexpressing IGF I will show greater muscle mass and strength during as we well as post cast immobilization, compared to PBS injected EDL muscles. Specific Aim 2: Hyp othesis a: Four months after injection, IGF I mRNA expression levels are significantly elevated in the IGF I gene transfer TA muscles. To determine the impact of IGF I gene t ransfer on mRNA expression of IGF I and its related receptor and binding proteins during cast immobilization and after 1 week and 3 weeks of reambulation in a fast twitch muscle (tibialis anterior muscle). Hypothesis b: IGF I gene transfer will not only alter IGF I mRNA expression but also induce concomitant alterations in I GF IR, IGFBP 3, IGFBP 4, and IGFBP 5 mRNA expression during different loading conditions.
49 Specific Aim 3 : Hypothesis a : IGF I gene transfer increases the number of EDL muscle fibers with central nuclei and muscle fibers expressing Pax 7 compared to PBS injected EDL muscles under normal loading conditions. To assess the impact of IGF I gene transfer on markers of muscle regeneration in the fast twitch muscle (EDL muscle) during 1 week and 3 weeks of ream bulation. Hypothesis b: Reambulation will induce a gr eater regenerative response in IGF I overexpressing muscles, as evidenced by more central nuclei, and increased Pax 7 and embryonic myosin expression. 5 .3 Experiment Two: To Assess the Effect of Viral Mediated IGF I Gene Transfer on the Posterior Compartme nt Muscles Following Cast Immobilization 5 .3 .1 Experimental Design R ecombinant adenoassociated virus vector for IGF I (rAAV IGF I CBA promoter ) was injected in the posterior compartment of one of the hindlimbs of 48 young C57BL6 female mice (3 weeks of age). At 20 weeks of age, these mice were randomly divided into 6 groups (n = 8 /group): 1) normal weight bearing; 2) 2 wk cast immobilization; 3) 2 d reambulation; 4) 5 d reambulation ; 5) 10 d reambulation; 6) 21 d reambulation In the cast immobilization group, both hindlimbs were immobilized with the knees extended and the ankles in 1 60 to maximize atrophy of the soleus muscle. The soleus ( SOL ) and gastrocnemius ( GAST ) muscles were removed from both legs after 2 wks of cast immobilization or after cage reambulation. Magnetic resonance imag ing was conducted in vivo at 4, 8, 12, 16 weeks after IGF I gene transfer and 2 weeks of cast immobilization as well as at 2 5 10 or 21 days during reambulation. Changes in the maximal muscle CSAs of the triceps surae (GAS T and SOL) were assessed using MRI
50 M uscle transverse relaxation time (muscle T2), which is indicative of muscle damage and/or edema, was measured using in vivo T2 weighted MRI In vitro force mechanics was used to measure soleus muscle strength. Immediately following force mechanics, muscle wet weights were measured. Immunohistochemistry, hematoxylin & eosin staining, and real time PCR were used to quantify markers of muscle regeneration (central nuclei, embryonic myosin and Pax 7) and expression of myogenic regulatory factors (MyoD mRNA) in the soleus at different loading time points 5 .3.2 Specific Aim s and Hypothes e s Specific Aim 1: Hypothesis a: IGF I gene transfer leads to an increase of the cross sectional area of the triceps surae (GAST and SOL) muscles under normal loading conditions. The increase in muscle wet weight, tetanic force and muscle CSAs observed in IGF I overexpressed muscles under normal loading conditions is maintained during cast immobilization and reambulation. T o study the effect of IGF I gene transfer on the size and strength of a slow twitch muscle (soleus) during cast immobili zation and early reambulation. Hypothesis b : I GF I gene transfer induces a faster recovery of contractile properties in the soleus muscle during reambulati on following cast immobilization. Specific Aim 2 : Hypothesis a : IGF I gene transfer will not alter the muscle T2 relaxation pr operties of the soleus muscle d uring normal loading conditions or cast immobilization. To non invasively monitor muscle damage in the postural soleus muscle of AAV1 IGF I and contralateral PBS injected hindlimbs during early reloading/reambulatio n following cast immobilization
51 Hypothesis b : Early reambulation will cause an increase in muscle T2 relaxation time of the soleus muscle following 2 weeks of cast immobilization, esp ecially in the PBS injected soleus muscle. Specific Aim 3 : Hypothesis a: The expression of markers of muscle regeneration will be greatly enhanced in the soleus muscles during the reloading phases after cast immobilization, especially in IGF I transfected muscles. T o assess the impact of IGF I gene transfer on muscle regeneration and myogenic regulatory factors in the soleus muscle during early reambulation foll owing cast immobilization. Hypothesis b : IGF I tr ansfected soleus muscles will demonstrate a greater response in mRNA expression of MyoD during early reambulation compared to the PBS injected muscles.
52 CHAPTER 6 EXPERIMENT ONE : ASSESS THE EFFECT OF VIRAL MEDIATED IGF I GENE TRANSFER ON THE ANTE RIOR COMP ARTMENT FAST TWITCH MUSCLES FOLLOWING CAST IMMOB ILIZATION 6 .1 Abstract Insulin like growth factor I (IGF I) is a potent anabolic and myogenic factor that plays a critical role in muscle hypertrophy and muscle regeneration The purpose of the cu rrent study was to examine the effect of IGF I overexpression on muscle size and strength under altered loading conditions. In addition, we investigated concomitant molecular responses in IGF I receptor and binding proteins (BPs). Thirdly, we determine d the impact of IGF I overexpression on markers of musc le regeneration following cast immobilization. rAAVIGF I or PBS (control solution) was directly injected into the anterior compartment of one of the hindlimbs of each mouse at 3 weeks of age. Four months after injec tion, a greater increase in muscle mass, tetanic force and fiber size was observed in the EDL muscle with IGF I overexpression, compared to the PBS injected limb After 2 weeks of cast immobilization and 1 and 3 weeks of free cage reambulation, the relativ e gains in muscle mass, force production and fiber size in the AAV1 IGF I injected muscles were maintained. Changes in IGFBP 5 mRNA expression during cast immobilization and reambulation paralleled those of IGF I, while IGFBP 3 expression changed inversely to IGFBP 5 IGF I gene transfer resulted in significant increases in central nuclei and Pax 7 expression, indicative of muscle regeneration, during normal weight bearing and cast immobilization com pared to the contralateral, PBS injected E DL muscles. Enha nced muscle regeneration was observed during early reloading (one week of reambulation). Therefore, local overexpression of IGF I in fast twitch muscle s results in muscle hypertrophy and the relative gains in muscle mass and
53 strength are better maintained during cast immobilization and reambulation compared to contralateral limbs. IGFBP 5 may play an important role in mediating the effect of IGF I during varied loading conditions 6 .2 Introduction Skeletal muscle fibers are characterized by high plasticity that involve s constantly adapt ing to external stimuli (e.g., mechanical overload), by changing structural functional and physiological properties (3). A consequence of an extended period of reduced load on skeletal muscle is atr ophy The results of these negative muscular changes are far reaching and may include development of functional limitations, decreased motor control loss of fitness, and long term disability. Although skeletal muscle has an inherent capacity to recover from these maladaptations, the maintenance and recovery of muscle function after disuse can be slow, and in many cases, inefficient and incomplete (273, 350) Therefore, understanding the mechanisms invo lved in muscle atrophy and r egrowth are important for the development of therapeutic approaches to reduce the loss of muscle function and promote muscle recovery following disuse When skeletal muscle is challenged with an increased mechanical load, there are rapid increases in autocrine/paracrine growth factors that activate signaling mechanisms that promote skeletal muscle hypertrophy (5, 315) One such growth factor is insulin like growth factor I (IGF I), which plays an important role in the formation and growth of skeletal muscle (121) Overloading of skeletal muscle produces hypertrophy and is associated with increases in IGF I mRNA and peptide level (4). It has been shown that adenoassociated virus (AAV) can effectively deliver the IGF I gene to skeletal muscle and initiate a hypertrophic process, similar to that observed from
54 exercise strength training (39, 216) Experimental manipulations of muscle IGF I levels have been shown to induce muscle hypertrophy in vivo For example, expression of IGF I has been correlated with muscle hypertrophy in various transgenic mouse lines (35, 76, 269) Furthermore, i n models of muscle atrophy such as glucocorticoid treatment or denervation, IGF I overexpression has been found to have positive effects on muscle size (320) Moreover, viral mediated gene transfer of IGF I has been reported to block or counter muscle loss in aging (37) and muscular dystrophy mice models (38) These studies suggest that IGF I has the potential to attenuate muscle atrophy from disuse or accelerate the recovery of skeletal muscle following a period of inactivity. IGF I activity is modified by its receptor (IGF I receptor, IGF IR) and a family of binding proteins (IGFBPs). Mice lacking the IGF IR exhibit marked muscle hypoplasia and die soon after birth due to respiratory failure (222) Also, mice with a dominant negative IGF I r eceptor have smaller skeletal muscles than control mice (121) IGF binding proteins (IGFBPs) are multifunctional proteins that transport IGFs in circulation, localize IGFs in specific cell types, and alter binding c haracteristics of IGFs to receptors (177) Six different IGFBP proteins have been cloned and sequenced (354) Initially, IGFBPs were thought to function as proteins that extend the half life of IGFs in the circulation and also inhibit the binding of IGF to specific receptors such as insulin receptors (354) Since IGF I concentrations in the serum are 1,000 times higher than those of insulin they can interfere with normal functioning of the cell (327) It is now known that IGFBPs can localize in the ext racellular matrix, modulate IGFs interaction with a receptor, and/or act as a localized storage depot for IGFs (354) Thus IGFBPs
55 likely modulate IGF Is effectiveness and exert IGF I independent effects on various tissues. Although the interactions of IGFBPs and IGF I have been previously investigated, relatively little is know n about the changes in IGFBP s gene expression in skeletal muscle with IGF I overexpression during altered loading patterns Thus, the purpose s of this study w ere: 1) to determine the impact of viral mediated IGF I gene transfer on EDL muscle mass and stre ngth after 2 weeks of cast immobilization and following one and three weeks of cage reambulation; 2) to determine transcriptional changes of IGF I, IGF IR, IGFBP 3 4, and 5 in response to overexpression of IGF I during cast immobilization and reambulati on; 3) to compar e the markers of muscle regeneration (Pax 7, embryonic myosin, central nuclei) in the IGF I injected limbs to the contralateral P BS injected limb s during the reambulation phase following 2 weeks of cast immobilization in the fast twitch m us cles 6 .3 Material and M ethods 6 .3.1 Animals and Viral Injection This study was conducted with approval from the Institutional Animal Care and Use Committee of the University of Florida. Twenty four female C57BL6 mice (3 weeks of age) were studied. A reco mbinant AAV plasmid (pSUB201) was constructed using the entire rat IGF I c DNA which encodes for IGF I, a m yos in light c hain (MLC) 1/3 promoter and enhancer, and SV40 polyadenylation sequence as previously described ( 35, 36) A recombinant AAV serotype 1 (rAAV 2/cap1: AAV 2 genomes pseudopackaged into AAV 1 capsids) was prepared by the University of Pennsylvania Vector Core, following published procedures. Reverse transcription PCR analyses has shown persistent, local expression of the myosin light chaindriven IGF I mRNA up to 9 months post injection (35) T he effects of IGF I production have also been shown to be
56 local (35) Figure 6 1 demonstrates expression of the MLC driven IGF I mRNA in the tibialis anterior muscle 3 months following direct muscle injection of the recombinant AAV. Under isoflurane anesthesia, the anterior compartment of one hindlimb was injected with 5 x 1010 viral particles in 85l of phosphatebuffered saline (PBS). This method of delivery targeted the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles, allowing the entire muscles surface to be bathed in virus solution. The contralateral limb was injec ted with the same volume of sterile PBS Previous studies have shown that PBS injection did not cause any significant changes in muscle mass and body weight (35, 40) Once these mice were recovered from anesthesi a they were sent back to animal facilities for future studies. Four months after injection, viral injected mi ce were randomly assigned into four (n=6 each condition): 1) noncasted; 2 ) two weeks of cast immobilization ( 2 wk Immob); 3) 1 week of free cage r eambulation ( 1 wk Reamb); and 4) three weeks of free cage reambulation ( 3 wk Reamb) as described below. The animals were housed in an accredited animal facility room controlled for temperature (22 + 1C), humidity (50 + 10%), and light (12h light/dark cycle) Mice in the noncasted group remained in their cages and experienced only normal incage activity. 6 .3.2 Cast Immobilization Both hindlimbs were performed under isoflurane anesthesia with hip and knee joints fixed at ~160 and ~180, respectively, as previously described (139) In order to unload the TA and EDL muscles, the ankle was fixed at 60. T he plaster of Paris cast encompassed both hindlimbs and the caudal fourth of the body (superior to the wings of the ilium). The animals were free to move using their forelimbs and they ate and drank
57 ad libitum. The mice were monitored on a daily basis for chewed plaster, abrasions, venous occlusion, and problems with ambulation. In addition, the mice were checked daily for fecal clearance. Following 2 weeks of cast immobilization, six mice were sacrificed. The remaining twelve mice were allowed to freely reambulate in their cages for either 1 week or 3 weeks following cast immobilization. 6 .3.3 Force Measurements In vitro force mechanics were performed on both hindlimb muscles as previously described (35) Briefly, E DL muscles were immersed horizontally in Ringers solution, at 25C, equilibrated with 95% O2/5% CO2, pH = 7.4. The distal tendon was tied to a nonmovable post and the proximal tendon attached to a servomotor (Aurora Scientific, Inc., Aurora, Ontario, Canada). Muscle length was adjusted to the optimal length (Lo) at which maximal twitch force was reached. EDL maximal tetanic forces were determined using 120 Hz 500 ms supramaximal electrical pulses. Stimulation was delivered via two parallel platinum electrodes that were positioned along the length of the muscle. Immediately following force mechanics, m uscle wet weights were measured. M uscles for histology measurements were pinned at resting lengths and cooled in liquid n itrogen melting with isopentane; muscl es for other measurements were snap frozen in liquid nitrogen. Muscles were stored at 80 0C. 6 .3.4 Muscle Morphological and Regeneration A ssess ment Frozen cross sections (10m) from the midbelly of the EDL muscle were subjected to hematoxylin & eosin (H &E) staining to assess the proportion of central nuclei in the muscle fibers. Immunohistochemistry was used to determine the muscle fiber size and assess the distribution of embryonic myosin and paired box transcription factor seven (Pax7). Sections were i ncubated with rabbit anti laminin (Neomarker,
58 Labvision, Fremont, CA) and mouse anti embryonic myosin MHC antibodies (DSHB, Iowa City, IA) (4C overnight), followed by incubation with rhodamineconjugated anti rabbit IgG (Nordic Immunological Laboratories) and Fitc conjugated anti mouse IgG (Nordic Immunological Laboratories). A mouse blocking kit (BMK 2202, Mouse on Mouse, Vector Lab) was used in staining of Pax7. Pax7 staining has previously been shown to correspond well to satellite cell content (385) Primary antibody mouse anti Pax7(R&D) and secondary antibody Alexa 488 (InvitrogenMolecular Probes) were used. These slides were mounted with Vectashield mounting medium with DAPI (Vector lab). Image acquisition was performed on a Leitz DMR microscope with a digital camera (Leica Microsystems, Solms, Germany ). The NIH image J program (version 1.62) was used to analyze the data. The pixels setting used for conversion of pixels to micrometer were 1.50 pixels 1 m2 for a 10X objective. The average positive number per 100 muscle fibers was quantified from a sampl e of 150 250 fibers on randomly selected slides. 6 .3.5 RNA Isolation and Assessment of mRNA Abundance for Real Time Quantitative PCR Total RNA from frozen TA muscle samples was isolated with TRIzol Reagent (Invitrogen Life Technologies, Carlsbad, CA). Br iefly, approximately 30 mg of muscle was cut in 1.0 ml of TRIzol Reagent using an RNase free razor blade homogenizer. Each muscle sample was centrifuged, incubated and precipitated. After isolation RNA was dissolved in DEPC treated water and stored at 80 C. The resulting RNA was quantitated by optical density at 260 nm. One microgram of total RNA was reverse transcribed to cDNA with the SuperScript III First Strand Synthesis for RT PCR kit (Invitrogen Life Technologies,
59 Carlsbad, CA) and a mix of oligo(dT) (100 ng/reaction) and random primers (200 ng/reaction) in a 20 l total reaction volume according to the manufacturers instructions. Following reserves transcription (RT), samples were stored at 80C for PCR reactions. Real time PCR was performed usi ng the protocols and detection systems of ABI PrismTM 7900 Sequence Detection System (Applied Biosystems). The reaction components for a single 20ul reaction contained 10ul TaqMan Universal PCR Master Mix, 1 ul primers mix and 9 ul RNase free water. Primer sequences were selected from NCBI database and purchased from Applied Biosystems (IGF I: Rn00710306_m1; IGF IR: Mm00802831_m1; IGF BP3: Mm00515156_m1; IGF BP4: Mm00494922 _m1; IGF BP5: Mm00516037_m1; and 18S ribosomal RNA (18S rRNA):18S 4319413E). Each as say contains primer and specific intronspanning FAM or VIC labeled (18S ribosomal RNA)TaqMan MGB probes. The standard curve was calculated automatically via software by plotting the Ct values against each standard of known concentration and calculation of the linear regression line of this curve. The deltadelta Ct method (364) was used to calculate the relative expression ratio [2CT] based on the change in threshold values (Ct). The normalization of the target genes with an endogenous standard was done via the reference gene (18S ribosomal RNA ) expression and 18S expression was not different at any time points. All samples and standards were run in duplicate or quadruplicate. 6 .3.6 Determination of IGF I Protein Concentration Frozen TA muscles were rinsed with PBS to remove excess blood, homoge nized in 1 mL of 1X PBS using FastPrep Homogenizer and Isolation System (Thermo Fisher
60 Scientific, Franklin, MA) and stored overnight at 200C. The homogenates and serum were then centrifuged for 5 minutes at 5000 x g. The supernatants and serum were utilized for measurements of total IGF I in a commercially available ELISA kit specific for rodent IGF I (R&D Systems, Minneapolis, MN). IGF I concentration was calculated based on a standard curve generated from recombinant mouse IGF I. This kit detects total rodent IGF I, and the measurements are not affected by the presence of IGF I binding proteins or IGF II (40) This kit has been validated for the determination of rat IGF I at 30 3000 pg/ml with an intraassay precision of ~4.3% and an inter assay pr ecision of ~6.0% (40) All samples were measured on a microplate reader at 450nm in duplicate. 6 .3.7 Data A nalysis Data are presented as meansSEM. Using SPSS software (version 16.0) a twoway ANOVA was used to analyze the main effects of injection, loading, or an interaction effect of the two. Paired t tests were used for comparisons between rAAV 1 injected and contralateral, control limbs. A oneway analysis of variance was used to compare all variables within the same injection groups. Post hoc te sting for ANOVAs w as performed using a Bonferroni test. A significance level of p<0.05 was used for all comparisons. 6 .4 Results 1) Validation of IGF I gene t ransfer: Prior to using the r AAV virus coded with IGF I cDNA, we injected the same virus carrying the LacZ gene into the anterior compartment of the mice hindlimbs. LacZ is a common reporter gene which encodes the protein galactosidase This enzyme causes bacteria expressing the gene to appear blue when grown on a medium that contains the substrate analog X gal (209) In
61 our experiments, close to 100% of EDL muscle fibers showed positive staining for LacZ, whi ch indicated that ge ne transfer was efficient (Fig 6 2). 2) Effects of IGF I o verexpression under normal loading conditions : IGF I overexpression caused significant increases in EDL muscle wet weight ( ~20%, Figure 6 3A p < 0.05), tetanic force (~ 18%, Fig ure 6 3 B, p < 0.05) and fiber CSA ( ~20%, Figure 6 4 A p < 0.05) during normal weight bearing conditions compared with the contralateral EDL muscles, respectively. Changes in muscle size were matched by changes in muscle force, such that there were no changes in specific force with IGF I overexpression. Similar increases in a second anterior compartment muscle, tibialis anterior (TA), muscle weight were also seen (data not shown). To estimate the extent to which IGF I, IGF IR and its binding proteins were affected by IGF I gene transfer under normal weight bearing, we analyzed the mRNAs expression levels for IGF I, IGF IR and IGFBP 3 4 and 5 in the TA muscle. Dramatic increases in IGF I mRNA (185 fold) and protein (26 fold) levels were observed following t he IGF I gene transfer (Table 6 1, Figure 6 5) IGFBP 5 mRNA expression was significantly higher (~70%) in the TA muscle overexpressing IGF I compared with the contralateral control limb (p <0.05). However, no significant changes in IGF IR, IGFBP 3 or IGFB P 4 mRNA were detected in the transfected TA muscles (Table 6 1). Under normal loading conditions, there was a significant higher baseline l evel of central nuclei (Figure 6 6 ) and satellite cells (Pax 7 positive fibers; Figure 6 8 ) in the IGF I injected ED L muscles compared to the PBS injected contralateral limb. Serum IGF I protein levels did not show any significant difference between rAAV IGF I (500 20 ng/ml) and PBS (510 14 ng/ml) injected groups.
62 3) Effects of IGF I overexpression during cast immobili zation : Two weeks of cast immobilization without IGF I overexpression resulted in significant decreases in EDL muscle wet weight (17%), muscle fiber CSA (23%), tetanic force (40%) and specific force (34 %) compared to control musc les (normal loading) (Fi gures 6 3 p < 0.05). Immobilization with IGF I overexpression caused similar decreases in EDL weight, tetanic force and specific force. However, the relative gains in muscle weight, muscle fiber CSA and tetanic force that were present before immobilizati on were preserved with IGF I overexpression (Figures 6 3 p < 0.05). Cast immobilization increased IGF IR and IGFBP 3 mRNA levels in the TA muscle by, on average, ~60% and ~80% respectively; in contrast, IGFBP 5 mRNA decreased by ~80% (p < 0.05) in the PBS injected TA muscles. Similar changes in IGF I, IGFBP 3 mRNA levels, and IGFBP 5 mRNA were observed in IGF I injected TA muscles. Cast immobilization did not change IGF I mRNA and IGF I protein levels significantly compar ed to baseline in either group unde r normal weight bearing conditions (Figure 6 5, Table 6 1). We also did not detect significant changes in IGFBP 4 mRNA expression af ter cast immobilization (Table 6 1). Following cast immobilization, there were greater central nuclei and Pax 7 positive fib ers in IGF I injected EDL muscles compared to PBS injected EDL muscle (Figure 6 6, 6 8 ). There was no expression of embryonic myosin positive fibers during normal loading and cast immobili zation in the EDL from either group (Figure 6 9 ). 4) Effects of IGF I overexpression during reambulation : After one week of reambulation (1w k Reamb), specific force returned to its baseline levels, while EDL wet
63 weights and tetanic force remained significantly below baseline after one week (p<0.05) but returned to baselin e levels within three weeks of reambulation (3w k R eamb) in both IGF I injected and PB S injected muscles (Figure 6 3). However, the absolute tetanic force in the IGF I injected EDL muscle at 1week of reambulation was almost the same as that of the noncasted PBSinjected muscles. Especially, after 3 weeks of reambulation, the difference in tetanic force between IGF I and PBS injected EDLs was evident. Of particular interest, after three weeks of reambulation there was a shift in EDL fiber CSAs towards larger muscle fibers (CSA > 1400 m) in the IGF I injected muscles, as shown in Figure 6 4 E IGF I mRNA increased significantly after the first week of reambulation following immobilization and returned to control levels after 3 weeks of reambulation in PBS injec ted TA muscles. In both IGF I and PBS injected TA muscles, IGFBP 3 mRNA and IGF IR mRNA (which had both increased with cast immobilization) returned to baseline after 1 week and 3 weeks of reambulation respectively. Also in both groups, IGFBP 5 mRNA expre ssion (which had decreased during immobilization), showed a significant increase from baseline after 1 week of reambulation. After 3 weeks of reambulation, IGFBP 5 mRNA returned to baseline in PBS injected muscle, but it remained elevated in the IGF I over expressing muscle. No significant differences were detected in IGFBP 4 mRNAs expression in the TA muscles of both groups after 1 week or 3 weeks of reambulation (Table 6 1) IGF I protein levels were elevated at 1 week of reambulation in both IGF I overexpr essed and PBS injected TA muscles (p < 0.05) and returned to preimmobilization levels by 3 weeks reambulation. There was also a significantly larger
64 increase in IGF I protein levels (Figure 6 5 ) at 1week reambulation in the AAV IGF I injected muscles (sig nificant interaction effect, loading x injection, p < 0.05). Importantly, we observed a greater increase in the markers of muscle regeneration in IGF I injected EDL muscle after one week of reambulation compared to the PBS injected muscles. In the control muscles, < 0.5% and < 0.3% of the fibers contained central nuclei and embryonic myosin, respectively. In EDL overexpressing IGF I, the proportion of centrally nucleated and embryonic myosin fibers increased to 2% and 1.2%, respectively (p<0.05, Figures 6 6 and 6 9 ). Pax 7 positive myonuclei were increased up to a 12 fold after 1 week reambulation in the IGF I injected muscles following cast immobilization (p<0.05, Figure 6 8 ). 6 .5 Discussion In this study, local delivery of IGF I by AAV virus mediated gene transfer to the EDL muscle of mice resulted in an 1820% increase in EDL muscle mass (wet weights), fiber CSA and tetani c force. These gains were maintained in proportion to baseline levels throughout immobilization and reambulation, suggesting that IGF I overexpression does protect skeletal muscle from the deleterious effects of immobilization. Evidence of enhanced muscle regeneration was observed in IGF I overexpressed muscles at 1 week of reambulation with an increased prevalence of central nuclei, embr yonic myosin and Pax7 positive fibers. In addition, after 3 weeks of reambulation, a greater proportion of muscle fibers with larger CSAs were observed in IGF I inje cted EDL muscles, suggesting that hypertrophy contributed to the increase in muscle mass. T o our knowledge, this is the first study to detect the effect of viral mediated IGF I gene transfer on the IGF I, IGF I receptor and its binding proteins, IGFBP 3 IGFBP 4 and IGFBP 5 mRNA expression in the TA muscle during cast
65 immobilization and following reambulation. Our data showed that gene expression of IGF IR and IGFBP 3 and IGFBP 5 is acutely regulated during adaptive changes induced in skeletal muscle by unloading and reloading. We also verified that the regulation of IGF I transcriptional and translational expressions exists under these loading conditions. Our cast immobilization model was successful in causing muscle atrophy in the EDL as previously shown (139) and immobilization produced changes in muscle fiber CSA and force that paralleled the changes in muscle mass. This model induced 20% atrophy of the EDL which is greater than reported previously in the EDL using hindlimb suspension models (83, 157) In addition, 3 weeks of reambulation was sufficient to restore normal muscle size, confirming that the disuse model and time course for evaluation were appropriate. To our knowledge, this is the first investigation to quantify the decline of tetanic muscle force after 2 weeks of cast immobilization and its recovery with reambulation. IGF I is a potent factor for growth in a number of tissues. Limiting its expression in the local skeletal muscle without causing circulating changes by a fast muscle specific promoter is the primary goal of IGF I gene transfer (269) In our model, the increased muscle mass and muscle force measured before cast immobilization are c onsistent with that reported by Barton et al. using a rAAVs vector delivery system for IGF overexpression (35, 39) In addition, our results are also consistent with increases in muscle mass and force seen in transg enic mice overexpressing IGF I (269) These results, in conjunction with an extensive body of literature regarding the hypertrophic
66 and anabolic effects of IGF I in vitro lead us to suggest that this growth factor acts in an autocrine and/or paracrine mode to mediate the hypertrophy process in skeletal muscle. In the present study, unloading reduced IGF I mRNA expression i n the PBS injected muscle. Some studies have also reported the same findings regarding IGF I expression in atrophic muscle in various disuse models. For example, Awede et al. reported that unloading by hindlim b suspension resulted in loss of soleus muscle mass (20%) associated with a 30% decrease of IGF I mRNA (23) However, other studies showed that neither 5 weeks of unilateral limb suspension in human (154) nor 4 days of spinal cord isolation in rat (156) resulted in a decrease in IGF I mRNA. Furthermore, some studies even indicate an increase in IGF I mRNA in human muscle during chronic disuse (296) and rat muscle following spinal cord isolation (199) Overall, the effect of unloading on IGF I levels seems to be variable among the previous models utilized. Unlike the ambiguous results of studies examining the role of IGF I in muscle atrophy, numerous studies have consistently shown that IGF I plays a critical role in mediating muscle mass after a variety of exercis es and training programs. A number of studies have shown that increased load on muscles can significantly increase expression of IGF I and the loading sensitive IGF I isoform, mechano growth factor (MGF) (22, 39, 148) In addition, a number of studies demonstrate that IGF I mRNA and peptide levels increased during compensatory hypertrophy in both animal models and humans (3, 4, 30, 47) In the present study, reloading induced regrowth of the EDL and TA muscle s was associated with increased IGF I mRNA and protein levels. Additionally, t he preservation of skeletal muscle mass and muscle fiber CSA with local overexpression of IGF I has been reported in models of skeletal muscle atrophy
67 involving glucocorticoids and denervation (295) Schakman et al. found that local overexpression of IGF I protein by gene transfer attenuated skeletal muscle atrophy induced by glucocorticoids (320) Similarly, Rabinovsky et al. used a transgenic IGF I mouse model to demonstr ate that IGF I intensifies muscle regeneration after denervation by accelerating the myogenic differentiation pathway (295) In contrast, Criswell et al. reported that local overexpression of IGF I did not prevent atrophy of the hindlimb muscles during hindlimb suspension (83) However, in that study the absolute muscle mass of the TA and EDL muscle in IGF I transgenic animals after 2 weeks suspension was equivalent to that of weight bearing muscles that had not undergone suspension in wildtype mice. Similarly, in our study we found that while IGF I did not imp act the rate of atrophy, IGF I overexpressed muscles were larger and produced more force compared to PBS injected muscles, before as well as after 2 weeks cast immobilization. We also found that AAV IGF I injected muscles demonstrated an increased prevalence of large muscle fibers (>1400m) at 3 weeks of reambulation. Although muscle mass and tetanic force was greater throughout reambulation in the IGF I injected muscles compared to contralateral PBS injected muscle, we found that IGF I did not impact the rate of atrophy or the recovery during reambulation. The recovery rate in muscle wet weight and tetanic force following cast immobilization was not significantly different in r AAVIGF I inje cted muscles compared to contralateral limbs One explanation for these findings not showing a greater effect on the rate of recovery following IGF I administration is that significant mechanical loading and activity is a necessary factor to optimize the effects of IGF I (7). Since the EDL muscle is not a
68 primary antigravity muscle in rodents (292) (112) we anticipate that the (re)loading experienced in this muscle from normal cage activity is relatively low. Therefore, the effect of IGF I administration may be more pronounced in the antigravity muscles, such as the soleus. Further examination of this muscle is warranted. It has been shown that the number of myonuclei decreases in a variety of atrophy models, such as denervation (386) spinal cord transection (103) spaceflight (13, 167) and hindlimb suspension (12, 87) Thus, in order to maintain a certain nuclei domai n, the transcriptional and translational demands placed on myonuclei are attenuated. On the other hand, functional overload requires a satellite cellinduced increase in myonuclear number (378) Schiaffino et al demonstrated that satellite cell proliferative activity is increased during the early stages (3 days) of the compensatory hypertrophy of the rat soleus or EDL that accompanied synergist ablation (323, 324) In the present study, increases in mech anical loading following immobilization significantly enhanced the expression of markers of muscle regeneration in both IGF I and PBSinjected groups. These observations suggest that recovery of the loss of myonuclei numbers due to disuse is required for the regrowth of skeletal muscle from inactivity induced atrophy. Satellite cells are critical for the regeneration of muscle tissue after injury (337) Jennische et al showed that IGF I immunoreactivity was detected in the cytoplasm of myoblasts and myotubes and in satellite cells during muscle regeneration (185) Pax 7 has been identified as a satellite cell specific marker and its function in satellite cell specification, survival, proliferation and self renewal is consistent with its expression in all satellite cells. In this study, there were significantly more centrally nucleated fibers
69 and satellite cell activity (Pax7) in IGF injected compared to PBS injected limbs during early reloading, su ggesting that IGF I over expression successfully augmented muscle regenerative capabilities. In addition, we found shifts in fiber sizes towards larger fibers with IGF I overexpression after 3 weeks of reambulatio n. Interestingly, there was a trend towards increased muscle mass and muscle force with IGF I overexpression fr om 1 to 3 weeks of reambulation. Collectively, those results provide further support that IGF I enhances muscle recovery. Although no studies have specifically evaluated the regenerative ca pabilities of muscle with reambulation after a period of disuse, Lee et al examined the roles of IGF I overexpression with resistance training in rats and found that the combination of resistance training plus IGF I overexpression resulted in significantly elevated muscle mass and force than either treatment alone (216) Since the gains in the aforementioned study were noted after 8 weeks of resistance training, it is possible that 3 weeks was not sufficient time to fully reveal the gains in muscle mass and force that may have occurred following IGF I overexpression. The role of the IGF IR in mediating changes in muscle function with overexpression of IGF I is still not clear. Studies have demonstrated that IGF IR is essential during early normal development (121) and other investigations have suggested that the in vivo effects of growth hormone (GH) on muscle mass and strength are primarily m ediated by activation of the IGF IR (198) More recently, Spangenbur g et al. found that increased mechanical load can induce muscle hypertrophy independent of the function of IGF IR (346) The present study found that IGF IR mRNA level was elevated after cast immobilization and dropped back to baseline levels with r eambulation. If IGF IR plays a role in muscle atrophy or hypertrophy, it is possible that
70 an elevation of IGF IR during atrophy may offer a compensatory mechanism to attenuate muscle atrophy by increasing the availability for IGF I binding. It is less cle ar how the cellular effects of IGF I may be influenced by its binding proteins. IGF I binding proteins modulate the effectiveness of IGF I (344) (Spangenburg 2003). Among the six known IGF binding proteins, IGFBP 4 and IGFBP 5 h ave been shown to play an important role in skeletal muscle adaptations, while IGFBP 3 appears to impact the availability of free circulating IGF I (23, 117, 131, 153) In the present study, IGFBP 5 mRNA levels were significantly reduced following 2 weeks of cast immobilization and increased above baseline in both AAV IGF I and PBSinjected muscles after one week of reambulation. In contrast, IGFBP 4 mRNA expression was not significantly altered at any of the time points. Hypertrophic conditions such as electromyostimulation (48) and overload (1, 4, 23) have been reported to be associated with increases in IGFBP 4 mRNA, whereas findings in regard to IGFBP 5 have been le ss consistent (23, 31, 345, 353) IGFBP 5 has been shown to both stimulate and suppress cell survival, proliferation, and differentiation (16, 17, 43, 408, 409) Interestingl y, in our study, IGFBP 5 mRNA returned to baseline in control muscles, but remained elevated in AAV IGFI injected muscles following 3 weeks reambulation. In addition, changes in IGFBP 5 mRNA during cast immobilization/reambulation paralleled those of IGF I, while IGFBP 3 expression changed inversely to IGFBP 5 and IGF I. Few studies have investigated changes in IGFBP 3 with unloading, yet IGFBP 3 is thought to play a critical role towards modulating the availability of IGF I in the bloodstream (23) Since IGFBP 3 is reported to suppress the proliferation of the cultured porcine myogenic cells (163) unloading induced increases in IGFBP 3 may inhibit the
71 proliferation of muscle satellite cells or IGF I binding to its receptors. The lack of clarity regarding the regulation of IGF binding proteins across studies may also be a result of transient increases in mRNA that are extremely sensitive to the timing of the measurements or because of transcription in binding protein levels through mechanisms of regulatio n which are yet unknown. In conclusion, local delivery of recombinant AAV1IGF I into fast twitch, lower li mb muscle s resulted in muscle hypertrophy and a concomitant increase in muscle strength during normal activity Importantly, r elative gains in muscle size and force production observed under normal loading conditions were maintained during cast immobilization and reambulation. In addition, the results of this study show ed that overexpress ion of IGF I contributed to muscle regeneration during reambulati on as evident by increased percentage of central nuclei satellite cell activity and expression of embryonic myosin. These results that local overexpression of IGF I was effective in maintaining muscle morphology and function following cast immobilization are consistent with the notion that IGF I may be an effective therapeutic intervention in certain clinical setting. The findings of the current study also demonstrated that gene expression of IGF I, IGF IR and its binding proteins, IGFBP 3 and IGFBP 5 in predominately fast twitch muscles were regulated during unloading and regrowth fro m limb immobilization. T he absence of an increase in IGFBP 4 expression was unexpected. Alterations in IGFBP 5 mRNA most close ly parallel ed the changes in IGF I during cast i mmobilization and reambulation, while IGFBP 3 mRNA expression changed inversely to IGFBP 5 mRNA. Further experiments are necessary to determine how IGF I receptor, IGFBP 3 and IGFBP 5 modulate IGF I function in skeletal muscle.
72 F igure 6 1. Schemati c diagram of the rAAV IGF I gene construct modified from BartonDavis ER (35) Figure 6 2. Lac Z expression ( blue stained muscle fibers) in the injected EDL muscles at different magnific ations Bar= 100 m
73 F igure 6 3 A) EDL wet weights. B) EDL tetanic force. C) E DL specific force. Values are means SEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading time point was significantly different from noncasted group. (p<0.05) A B
74 Figure 6 3 Continued Figure 6 4. EDL fiber cross sectional area ( m2) (n=6/group at each time point) (A). Fiber type size distributions as a percentage of total fibers before casting (B), after casting (C), after 1 wk reamb (D), and after 3 wk s reamb (E). Note the fiber size shift in muscle injected with IGF I by 3 wks reamb. Values are means SEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading time point was significantly different from noncasted group. (p<0.05) C A
75 Figure 6 4. Continued B C
76 Figure 6 4. Continued D E
77 Figure 6 5 TA IGF I protein lev els. Noncasted ( n=6 ); 2 wk Immob (n=6); 1 wk Reamb (n=6); 3 wk Reamb (n=6) ; Values are means SEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading ti me point was significantly different from noncasted group. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significant higher increase in IGF I protein (not indicated on figure). (p<0.05)
78 Figure 6 6 Presence of central nuclei (%) in IGF I and PBS injected muscle A) Cross sections of EDL muscle stained with hematoxylin and eosin. Central nuclei (arrows) were most apparent after 1 wk reambulation in IGF I injected muscle. B) Percent age of central nuclei at each time point. Values are meansSEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading time point was significantly different from noncasted group. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significant higher increase in central nuclei and embryonic myosin positive f ibers (not indicated on figure), p<0.05. Bar = 5 0 m A B
79 Fig ure 67 Immunofluorescent staining of muscle cross sections for Pax7. A) C ross sections of EDL muscle stained with laminin B) C ross sections of EDL muscle stained with Pax7. C) C ross sections of EDL muscle st ained with laminin + DAPI + P ax7. .
80 Fig ure 6 8 Satellite cell activity in muscle fibers for IGF I and PBSinjected muscle A) C ross sections of EDL muscle stained with laminin + DAPI + Pax7. Pax7 positive fibers (arrows) are most apparent after 1 wk reambulation in IGF I inje cted muscle. B) Percentage of Pax7 positive fibers at each time point. Values are means SEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading time point was significantly different from noncasted group. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significant higher increase in Pax7 positive fibers (not indicated on figure), p<0.05. B ar= 25 m. A B
81 Figure 6 9 A) Cross sections of EDL muscle stained with monoclonal antibody against embryonic myosin isoform (green). Muscle fibers expressing embryonic myosin were most notable after 1 wk reambulati on in IGF I injected muscle. B ) Number of fibers expressing embryonic myosin per 100 fibers Values are meansSEM. indicates significant difference within each loading time point between IGF I injected and PBS injected muscles. indicates that muscles at this loading time point was significan tly different from noncasted group. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significant higher increase in central nuclei and embryonic myosin positive f ibers (not indicated on figur e), p<0.05. Bar=50 m. A B
82 Table 6 1. Changes in IGF I, receptor, and binding proteins mRNA expression in TA muscles (expressed as fold difference compared to the PBS injected/noncasted TA muscle) Gene Noncasted 2 wk Immob 1 wk Reamb 3 wk Reamb PBS IGF I PBS IGF I PBS IGF I PBS IGF I IGF I 1.00.1 185.421.6 0.40.1 176.99.9 2.80.9 17148.0 1.20.2 98.324.4 IGF IR 1.00.1 1.10.1 1.60.2 1.80.03 0.50.08 0.50.1 0.80.07 0.80.1 IGF B P3 1.00.1 1.30.2 1.80.4 2.20.3 0.90.1 0.80.1 1.10.1 1.20.2 IGF BP4 1.00.1 1.40.2 1.00.1 1.30.1 1.30.4 1.60.4 1.00.1 1.20.3 IGF BP5 1.00.1 1.70.2 0.20.01 0.30.03 2.50.4 3.30.08 1.00.1 2.20.08 Values are meansSEM. indicates significant difference within each loading time point between IGF I injec ted and PBS injected muscles. indicates that muscles at this loading time point was significantly different from noncasted group. (p<0.05)
83 CHAPTER 7 EXPERIMENT TWO: TO ASS ESS THE EFFECT OF VI RAL MEDIATED IGF I GENE TRANSFER ON THE POST ERIOR COMPARTMENT SLOW TWITCH MUSCLES FOLLOWING CAST IMMOB ILIZATION 7 .1 Abstract In the previous experiment we observed that overexpression of IGF I enhanced muscle mass, strength, and the regenerative capacity of muscle during normal activity and reloading following cast immobilization in muscles composed of primarily fast twitch fibers (TA and EDL). However, fast twitch anterior compartment hindlimb muscles experience minimal loading during ambulation, compared to postural muscles, and have a smaller proportion of satellite cells. Thus, the purpose of this study was to i nvestigate the effects of AAVIGF I treatment during cast immobilization/ reambulation in posterior compartment hindlimb mus cles, which have a higher proportion of slow twitch fibers and experience high levels of load during walking. METHODS: The posterior compartment of the hindlimb was injected with rAAV IGF I in 48 mice at 3 weeks of age and the contralateral limb served as a control (injection of PBS solution). Magnetic resonance imaging (MRI) was used to monitor longitudinal changes in the cross sectional area of the triceps surae (gastrocnemius and soleus) during growth. Four months after injection, 40 mice were immobilized by casting the posterior compartment muscles into a shortened position to induce maximal atrophy in the soleus and gastrocnemius muscles. We measured muscle strength, muscle fiber morphology and the MR proton transverse relaxation time (T2), an in vivo m arker of muscle damage, in the mice hindlimb muscles under the following conditions: normal weight bearing, after 2 weeks of cast immobilization and during reloading at 2, 5, 10, and 21 days. RESULTS: Starting 8 weeks post injection, IGF I significantly au gmented triceps surae size as assessed by
84 MRI. Relative gains in muscle size, wet weight and force production observed under normal loading conditions in the IGF I injected group were maintained during cast immobilization and reambulation. Muscle T2 values of the soleus were significantly higher after 2 and 5 days of reambulation, with a greater increase in the PBS injected muscles compared to the IGF I injected group. Muscle strength returned to baseline levels in the IGF I injected soleus after 10 days of reambulation, while it took 21 days for the PBS injected limb to recover. Moreover, evidence of enhanced muscle regeneration with IGF I overexpression was supported by a higher percentage of central nuclei, embryonic myosin, Pax7 positive muscle fibers and increased expression of the myogenic regulatory factor, MyoD. In conclusion, viral mediated IGF I gene transfer protects the soleus muscle from reloading damage during the early phase of reambulation and accelerates muscle recovery during the later phase of reambulation following a period of cast immobilization. 7 .2 Introduction Primary functions of skeletal muscle are to provide postural support and movement. The functional characteristics of muscles are generally reflected by the muscle fiber composition and in part determined by the activity pattern of the motor neurons innervating the muscle (316) One of the earliest classification schemes for muscle was based on color, and thus muscles were classified as red or white (287) However, as experimental methods have become more sophisticated, classification of muscle fiber types based on the m etabolic profile (oxidative vs. glycolytic), contractile characteristics (slow twitch or fast twitch) or myosin heavy chain composition (type I or type II) have become more accepted. Slow twitch muscles typically have a slower rate of muscle contraction, are more resistant to fatigue and have a higher percentage
85 of capillaries, mitochondria and enzymes for aerobic metabolism, making them better suited to long term activity such as postural maintenance compared to fast twitch muscles. Among mammalian skeletal muscles, the soleus muscle of the lower limb and the vastus intermedius of the thigh are excellent examples of highly oxidative slow twitch muscles with an important anti gravity function. In contrast, fast twitch muscles of small mammals such as the plantaris, extensor digitorum longus, and tibialis anterior are better suited for higher velocity contractions (30, 90, 124, 304, 325, 363) During an extended period of inactivity, disuse not only results in skeletal muscle atrophy, but also increases the susceptibility of slow twitch antigravity muscles to muscle damage during reloading (140) Muscle damage following disuse presents a challenging problem for rehabilitation specialists, as injured muscles heal slowly and often incompletely (218, 247) Muscle d amage that occurs as a result of reloading is likely due to the inability of muscle fibers to maintain their integrity during eccentric contrac tions resulting in structural and physiological alterations in atrophied muscle, such as the disruption of sarcolemmal integrity (187) sar comere lesions (299) and disruptions in excitationcontraction (E C) coupling (180) Increased interstitial edema occurs rapidly after injury and subsequent inflammatory cells are infiltrated into the damage area (136 ) The activated inflammatory cells simultaneously secrete growth factors and several cytokines, which lead to a wide array of functions in the later phases of muscle regeneration. The changes observed with muscle reloading are parallel to those seen foll owing a bout of eccentric contractions in normally loaded skeletal muscle (32, 180, 394)
86 Several methods have been implemented to investigate muscle damage including histological, biochemical, and plasma markers s uch as creatine kinase (CK) (393) Among them magnetic resonance imaging (MRI) is unique because it provides a noninvasive and in vivo method of monitoring skeletal muscle damage. This method has the advantage of studying structure and function of intact skeletal muscle. The MR transverse relaxation time (T2) of muscle has been shown to be sensitive to changes in muscle damage after eccentric exercise (241, 336) and as a consequence of local injections of myotoxin (140) More specifically, increased muscle T2 correlates well with plasma CK and histological markers of muscle damage (214, 305) Muscle healing after injury normally includes four interrelated and timedependent phases: necrosis/degeneration, inflammation, regeneration, and fibrosis (281) The subsequent secretion of growth factors and cytokines following muscle damage contributes to the removal of necrotic material and further stimulates satellite cell activation (174) Among the various growth factors that have the ability to stimulate and accelerate healing and growth in muscle tissue, insulinlike growth factor I (IGF I) is the most important (174) Local upregulation of IGF I has been observed in a variety of models of muscle injury, including myotoxin injection and eccentric exercise (169, 170, 196, 246) Previously, Musaro et al. reported that transgenic expression of a muscle restricted IGF I (mIGF I) could modulate the inflammatory response, thus accelerating muscle regeneration after myotoxic injury (281) It has also been suggested that IGF I can reduce early myofiber necros is (332) protect muscle from contractioninduced damage (144) and promote muscle regeneration (38) in dystrophic mice ( mdx mice). In
87 addition, mIGF I enhances muscle regeneration by recruiting bone marrow derived stem cells to the injury site and augments muscle repair (268) Previously, we have observed that IGF I overexpression induces muscle hypertrophy and signs of enhanced muscle regeneration in mice fast twitch anterior compartment muscles (extensor digitorum longus). However, we did not observe an accelerated rate of recovery in the EDL muscle or amelioration of the rate of muscle atrophy. Considering that the impact of mechanical loading on skeletal muscle is more pronounced in slow twitch, antigravity muscles than in fast twitch muscles (27, 66, 242, 368) the aim of the current study was to monitor the time course of muscle damage following immobilization and reloading in the primary antigravity slow twitch soleus muscle overexpressing IGF I and further explore the role of IGF I in muscle atrophy and recovery. 7 .3 Materials and Methods 7 .3.1 Animal Care and Experimental Design All procedures were approved by the University of Florida Institutional Animal Care and Use Committee. A total of 48 female C57BL6 mice (age 3 weeks; Charles River, Inc., Wilmington, Massachusetts) were studied at multiple time points prior to and following cast immobilization. Animals were housed in an accredited animal facility at the University of Florida under a 12 h light 12 h dark cycle, with drinking water and standard chow provided ad libitum. At 3 weeks of age, all mice were injected with a recombinant AAV harboring rat IGF I cDNA into the posterior compartment of one of their lower hindlimbs, targeting the soleus muscle. The contralat eral limb received an injection of the same volume of sterile PBS solution. The AAV vector used in this study, AAV2/8sc. CBA rat IGF IA (sc, self -
88 actin) was prepared by the University of Pennsylvania Vector Core. A preliminary study from our lab showed that the elevated protein expression of this virus was localized to the posterior compartment of hindlimb for up to 6 months and no significant increase of serum IGF I was detected (data not shown here). After injection, mice were returned to their cages and maintained normal activity for the next four months. Four months after injection, the mice were randomly assigned into six groups (n=8 for each condition): 1) noncasted; 2) two weeks of cast immobilization (2wk Immob); 3) two day s of reambulation (2d Reamb); 4) five days of reambulation (5d Reamb); 5) ten days of reambulation (10d Reamb); and 6) twenty one days of reambulation (21 d Reamb). 7 .3.2 Cast Immobilization and Reambulation Mice were anesthetized with gaseous isoflurane w hile the bilateral hindlimb cast was applied. The cast immobilized the mouse bilaterally with the hip and knee joints extended and the ankle plantarflexed (~160) to maximize atrophy in the posterior compartment muscles. The plaster of Paris cast encompass ed both hindlimbs and the caudal fourth of the body T he mice were able to move freely in the cage using their forelimbs and reach their food and water I n addition, the mice were checked daily for abrasions, venous occlusion and fecal clearance. F ollowing two weeks of cast immobilization, the casts were removed from the mice in the reloading groups and mice were allowed to reambulat e in their cages for 2, 5, 10 or 21 days 7 .3.3 M agnetic R esonance I maging MRI was performed using a horizontal, 4.7 Tesla mag net with Paravision 3.0.2 software (Bruker Medical, Ettlingen, Germany). The spectrometer was equipped with
89 the Bruker S116 actively shielded gradient coil and a custom built 5turn, 1 cm inner diameter, single tuned, 1H solenoid coil (200 MHz). Mice were placed on a cradle with both hindlimbs secured in the center of the coil, covering the region from the thigh to the ankle. Mice were initially anesthetized with 4% gaseous isoflourane in oxygen (1 L/min flow rate) and maintained with 12% isoflourane in ox ygen for the duration of the MR experiment. Heart rate, oxygen saturation, respiration rate and body temperature were monitored to ensure constant values. 3D gradient echo imaging was performed at repeated time points after IGF I gene transfer ( 4, 8, 12 an d 16 weeks ), 2 weeks of cast immobilization as well as during reambulation ( 2 5 10, and 21 days ). The data were acquired with an encoding matrix of 384 x 192 x 64, field of view of 1.65 x 1.2 x 2.4 cm, pulse repetition time (TR) of 100 ms, and an echo ti me (TE) of 7.5 ms. The total region imaged extended from the midthigh to the calcaneus. Cross sectional area (CSA) of the triceps surae muscle was manually outlined using O sirix software. The average of the three highest consecutive slices was recorded as the maximal CSA (CSAmax). T o evaluate muscle damage, T2weighted, multiple slice, single spin echo images were acquired with the following parameters: TR = 2000ms, TE = 14 and 40ms, FOV = 1020mm, slice thickness 0.5 1mm, acquisition matrix = 128 x 256 and two signal averages. Mean T2 values and the percentage of damaged muscle (or percentage of pixels with elevated T2 values defined as 2SD above the mean of controls) in the soleus muscles were quantified using inhouse software with Interactive Data Language (IDL). 7 .3.4 M uscle M echanical M easurements I solated in vi tr o force mechanics was performed on the soleus muscles as previously described (40) Optimal muscle length was defined as the length that
90 resulted in the maximum twitch tension. M aximal tetanic forces were determined using 80Hz 1500ms supramaximal electrica l pulses at the optimal length. S timulation was delivered via two parallel platinum electrodes that were positioned along the length of the muscle. At the end of the mechanical measurements, soleus muscles were weighed, blotted at optimal length and rapidl y frozen in melting isopentane, precooled in liquid nitrogen and stored at 80C 7 .3.5 Musc l e M orphology and Regeneration Assessment Transverse muscle sections (10 m thick) were cut from the midbelly of the soleus and mounted serially on gelatincoated glass slides. Muscle s ections were stained with hematoxylin and eosin (H&E) for examination of central nuclei I mmunohistochemistry was used on serial sections to as sess the distribution of embryonic myosin and Pax 7. Sections were incubated with primary antibodies ( rabbit anti laminin ( Neomarker Labvision), mouse anti embryonic myosin MHC (DSHB, 1:20), mouse anti Pax7 (R&D, 1:50) ) at 4 C overnight, followed by incubation with rhodamineconjugated anti rabbit IgG FITC conjugated anti mouse IgG (Nordic Immunological Laboratories) and Alexa Fluor 488 anti mouse (InvitrogenMolecular Probes) A mouse blocking kit (BMK 2202, Mouse on Mouse, Vector Lab) was used in staining of Pax7. Sections were mounted with Vectashield mounting medium with DAPI (Vector lab) Image acquisition was performed on a Leitz DMR microscope with a digital camera (Leica). The NIH image program (version 1.62) was used to analyze the data. The pixels setting used for conversion of pixels to micrometer were 1.50 pixels 1 m2 for a 10X objective. The average positive number per 100 muscle fibers was quantified from a sample of 150250 fibers on randomly selected slides.
91 7 .3.6 RNA Extraction, Reverse Tr anscription and TaqMan Quantitative PCR Analysis RNA was isolated from homogenized soleus muscles using the TRIzol Reagent (Invitrogen Life Technologies, Carlsbad, CA) according to the manufacturers protocol. RNA was quantified using a Nanodrop ND 1000 Spectrophotometer (Nanodrop Technologies, ND Software version 3.3.0). One microgram of total RNA was reverse transcribed using a commercially available kit (Invitrogen Life Technologies SuperScript III First Strand Synthesis for RT PCR, Carlsbad, CA). Realt ime PCR was performed for the following using TaqMan PCR master mix and gene expression primers that are inventory items of Applied Biosystems: MyoD (assay ID: Mm00440387_m1) and 18s (4352339E). Each sample was run in duplicate. The relative quantification method ( CT, where CT is threshold cycle) was used to document results, and data were normalized to 18s contr ols. 7 .3.7 Data Analysis Data are presented as meansSEM. Using SPSS software (version 16.0) twoway analysis of variance (ANOVA) was used to an alyze the main effects of injection and loading and the interaction of the main effects. Paired t tests were used for comparisons between IGF I injected and contralateral, PBS injected limbs at each time point. A oneway ANOVA was used to compare all variables within the same injection group. Post hoc testing for ANOVAs measures was performed using a Bonferroni test. A significance level of p<0.05 was used for all comparisons.
92 7 .4 Results 1) Muscle size, mass and contractile properties: Comparison of MR images acquired at baseline and before cast immobilization showed enhanced muscle growth in the IGF I injected hindlimbs of young mice. Starting at 8 weeks post injection, the CSAmax of the triceps surae (TS) muscle group (gastrocnemius and soleus) in the IGF I injected limb was significantly larger than the PBS injected limb (Figure 7 1, 7 2). On average, the TS CSAmax in the IGF I injected limb was ~15% higher than that of the contralateral limb. This difference between IGF I and PBS injected TS CSAmax was also prevalent immediately following cast immobilization and at each of the reambulation time points (Figure 72). Similar results were observed in muscle wet weights. Viral mediated IGF I expression promoted a significant augmentation in muscle mass in both the soleus (18%) and gastrocnemius muscles (17%) under normal loading conditions. The difference in muscle mass between IGF I and PBS injected groups was even more noticeable after 2 weeks of unloading and at 2 days reloading, especially in the soleus muscle. At 2 days reloading the wet weight of the AAV injected soleus was 22% higher than that of the PBS injected muscle. In both the treated (IGF I) and untreated (PBS) soleus muscle, a rapid recovery to baseline or preimmobilization values was noted. In contrast the recovery in gastrocnemius muscle mass mirrored the changes observed in the CSAmax of the TS using MRI (Figure 7 2). Both cast immobilization and reloading significantly affected muscle contractility. Maximum isometric tetanic force and specific force of the soleus muscles dropped by ~35% and ~39%, respectively, during cast immobilization in both AAV IGF I and PBS injected soleus m uscles. An additional 18 and 14% decline in tetanic force was measured in the PBS and IGF I injected muscles at 2 days of reloading. The largest
93 contractile difference between IGF I and PBS injected soleus muscles was observed at 2, 5 and 10 days reambulation, with a 32 37 % higher isometric force in IGF I overexpressing muscles. In contrast, a 16 and 22% difference was noted between IGF I and PBS injected muscles at baseline and following 2 weeks of cast immobilization. Importantly, tetanic force in the IGF I injected soleus muscle returned to its baseline levels (no statistical difference) after 10 days of reambulation, whereas the contralateral (PBS injected) soleus still remained significantly lower (25% less than its baseline value) at 10d reambulation. There was also a significant augmentation in specific force with IGF I overexpression at 10 days reambulation, which resulted in a significant difference in specific force (20%) between IGF I and PBS injected soleus muscles (Figure 7 2). 2) Muscle T2 values: Muscle T2 and the percent of pixels with elevated T2 in the soleus muscle were compared between IGF I an d the control (PBS injected) limbs before and at different time points of reloading. Average muscle T2 of the soleus in the IGF I and PBS injected limbs with baseline cage activity was 28.58 0.39ms and 28.22 0.34ms, respectively. There was also no significant difference in muscle T2 in either limb following 2 weeks of cast immobilization compared with baseline. As expected T2 was greatly elevated during reloading especially at 2 days and 5 days reloading, in both the IGF I and PBS injected soleus muscl es. Interestingly, T2 of the soleus in the PBS injected limb was significantly higher at 2 and 5 days of reloading compared with the AAVIGF I injected limb. IGF I overexpression significantly reduced the percent of pixels with elevated T2 (i.e. % muscle damage) compared to the PBS injected soleus (27.4% 1.4% vs. 45.5% 2.4% at 2 days reloading; 15.6% 1.8% vs. 28.4% 2.8% at 5
94 days in the IGF I and PBS injected limbs, respectively). After 10 days of reambulation, the percent of pixels with elevated muscle T2 was no longer different between the PBS and IGF I injected limbs (IGF I: 4.4 4% 1.08%, PBS: 6.06% 1.12%) (Figure 7 3 7 4 ). 3) Histological evaluations: Muscle crosssections from the soleus acquired at different reloading time points were stained with hematoxylin and eosin (H&E). During the early reloading phases (2 and 5 days), both IGF I and PBS injected groups showed larger muscle fibers with rounded shape, infiltration of leukocytes, widened interstitial space and necrotic fi bers. These changes were more severe in PBS than the IGF 1 injected muscles (Figure 7 3B). Although we did not detect any significant changes in muscle T2 or the percent of pixels with elevated T2 during 2 weeks of cast immobilization, we found the morphology of muscle fibers was better preserved in IGF I injected soleus muscles. We assessed the area fraction of abnormal muscle based on a 63point counting technique as described in principle by Weibel et al (395) A significantly higher percentage of abnormal muscle (inflammatory cells in the interstitial tissue, a few scattered necrotic fibers, and swollen fibers) were observed in the PBS injected soleus compared with the IGF I injected soleus (Figure 7 5). 4) Markers of muscle regeneration : The percentage of centrally nucleated fibers identified in H&E transvers e sections of the soleus was significantly increased from baseline after 5 and 10 days of reambulation in both the IGF I injected limb (21.2% 2.0% and 16.5% 1.8%, respectively) and PBS injected limb (8.3% 1.4% and 11.9% 1.6%, respectively). The num ber of fibers with central nuclei in the IGF I injected group was significantly higher than that of the PBS injected group at all of the reloading time points. Similarly, Pax7 positive muscle fibers identified using
95 immunohistochemistry were significantly elevated at 5 and 10 days reloading in both groups, with the changes significantly higher in the IGF I treated gr oup than the PBS group (Figure 76, 7 7B & D). The percentage of muscle fibers positive for embryonic myosin in the soleus was increased after reloading in both the AAV IGF I injected and PBS injected group at 2 days reambulation (6.3% 0.9% and 3.2% 0.5%, respectively). At 5 days of reambulation, a significantly higher number of EM positive fibers were observed in the IGF I injected group (11.3% 1.1% in IGF I vs. 8.9% 1.5% in PBS, p < 0.05). The greatest difference in EM positive fibers between these two groups was observed at 10 days of reambulation (36.8% 5.1% vs. 21.7% 2.4%, respectively, p < 0.05) (Figure 7 7A & C). 5) MyoD expression : MyoD, the basic helix loophelix transcription factor, plays an essential role in determining the myogenic fate of satel lite cells. As shown in Figure 7 8, 2 days after reloading, the IGF I treated soleus muscle exhibited a sevenfold increase in MyoD expression from baseline levels before gradually declining, whereas a fourfold increase was observed in the PBS injected group (p<0.05). MyoD expression in the PBS injected group remained significantly lower than that of IGF I treated muscle at each of the reloading time points, and returned to baseline levels at 21 days of reambulation. Interestingly, there was a significant increase in MyoD with cast immobilization prior to reloading in the IGF I treated soleus muscles, which was not observed in the PBS i njected muscle. 7 .5 Discussion This study examined the effects of IGF I overexpression on a predominantly slow twitch postural muscle (soleus) during cast immobilization and reambulation in mice.
96 The main findings were that viral mediated IGF I gene transf er: 1) increased muscle size in the targ eted muscles at all time points ; 2) faster recovery in muscle strength following immobilization; 3) protected muscle from early reloading damage as indicated by the attenuated increase in muscle T2 and the percent of pixels with elevated T2; 4) enhanced muscle regeneration as shown by an increased prevalence in central nuclei, embryonic myosin, Pax7 positive fibers, and elevated MyoD mRNA; and 5) adaptations during cast immobilization including increase in MyoD mRNA and less abnormality in muscle morphology. Collectively, to our knowledge, this is the first study to show that IGF I overexpression in a slow twitch muscle not only protects muscle from reloading damage and enhances muscle regeneration following cast immobilization, but also has a protective role during disuse. In this study, we found a marked increase in muscle T2 of the soleus at 2 and 5 days of reloading after cast immobilization, which demonstrates selective muscle damage in this muscle during reloading Previous studies have reported an increase in muscle T2 following eccentric exercise, and this has been suggested to be due to an increase in muscle free water concentration due to edema, inflammation and vascular changes (116, 127, 223, 274) Also, previous studies examining the impact of acute reloading or acute compensatory overload have found increases in interstitial space between muscle cells indicative of muscle edema (116, 140) Thus, increases in muscle T2 after reloading may be attributable to edemalike changes during early reloading which are considered precursors of muscle fiber necrosis and inflammation (200) In addition, a loss of specific and tetanic force and histological features of injury in our study are consistent with muscle damage during early reloading. For example, specific
97 force is affected by the integrity of sarcolemma and by factors that influence muscle fiber size. Kandarian et al. found that muscle edema contributed to ~50% loss of tetanic force during acute compensatory overload (190) Also, the decrease in muscle strength has been previously noted to occur at the time point when the ultrastructural signs of injury were exacerbated (138, 289) T herefore, our results are consistent with muscle damage induced by reloading contributing to a reduction in strength following immobilization. One of the primary results of this study is that muscle T2 was significantly lower in the AAV IGF I injected soleus muscles compared to PBS injected limbs during early reloading, indicating that IGF I overexpression reduced muscle damage. In addition, the percent of pixels with elevated T2 was lower in the IGF I injected muscle, especially after 2 days of reloading. The attenuation in muscle T2 response during reloading in the IGF I overexpressed muscles may be caused by a number of factors. First, muscle injury and regeneration are intimately related to the inflammatory process. IGF I might accelerate muscle healing by rapidly modulating the inflammatory response. It has been reported that a variety of inflammatory mediators decrease the prevailing circulating concentration of IGF I (118, 119, 210) and inhibition of IGF I sign aling is induced through increased proinflammatory cytokine levels (1, 94, 95, 361) On the other hand, Musaro et al. demonstrated that IGF I did not block the activation of the inflammatory response after cytotoxic muscle injury but had a role in selectively downregulating proinflammatory cytokines at 5 days of post injury and accelerated the inflammatory process (281) Considering the shorter timeframe of injury caused by reloading compared to myotoxin injection, we would expect that the resolution of inflammation by
98 IGF I could occur at an earlier time point. Second, overexpression of IGF I might help to ameliorate muscle degeneration during the acute onset of muscle damage or reduce the susceptibility of atrophic muscle to contractioninduced damage (322) Previous studies have shown that IGF I can protect myofibers from breakdown/necrosis in young mdx mice under normal loading conditions (38, 332, 333) or improve excitat ion contraction coupling in adult mdx mice (144) In addition, IGF I protects the injured muscle by inhibiting hypoactive mutant superoxide dismutase (75) and enhances the oxidative status in skeletal muscle of mdx mice (322) Finally, the persistence of the inflammatory response is often associated with muscle fibrosis. IGF I can increase the production of matrix components and decrease the matrix degrading enzymes expression, which will ameliorate fibrosis associated with the persistence of inflammation res ponse (172, 188, 319) In the current study, we observed that the area fraction of abnormal muscle on histology was significantly higher in the PBS injected soleus muscle compared to the IGF I injected soleus after 2 weeks of cast immobilization. Histological abnormalities mainly consisted of changes in fiber size and shape as well as enlarged and roundshaped nuclei in the interstitial tissue, indicative of edema and muscle degeneration. These histological data support the conjecture that IGF I overexpression provides the slow twitch soleus muscle protection during cast immobilization and makes the muscle less prone to subsequent damage when resuming normal activity (332) To our knowledge, this is the first study to show that IGF I overexpression in the soleus leads to an accelerated response in muscle regeneration after reloading in atrophied muscle in a mouse cast immobilization/reambulation model. Notably, IGF I
99 overexpression facilitated the recovery of the sol eus muscle following cast immobilization as is evident by a faster return of strength to baseline, and larger differences in isometric tetanic force between the PBS and AAV IGF I injected muscles at 2 10 days reloading. Reloading of skeletal muscle after a period of disuse will initiate muscle regrowth that may involve elements of both muscle hypertrophy and regeneration pathways (231) We have previously shown that fast twitch muscles overexpressing IGF I maintain the relative gains in muscle force production observed under normal loading conditions during cast immobilization/reambul ation, but we did not find attenuation of muscle atrophy or an accelerated rate of muscle recovery. In the present study, IGF I overexpression increased force production and accelerated the rate of recovery in the soleus. This result supports the hypothesi s that myogenic and mitogenic effects of IGF I are dependent on mechanical stimulation of muscle tissue (1) In vitro studies show that the additio n of the growth factor alone results in a modest upregulation of protein synthesis in the myoblast cell culture, and when combined with mechanical stimulation the rates of protein synthesis double (73) Lee et al. also showed that administration of IGF I has an additive effect on resistance training resulting in greater hypertrophy than either treatment alone (216) Given the fact that the soleus muscle is a primary postural antigravity muscle, the impact of mechanical loading is more pronounced in the soleus than the EDL (27, 66, 242, 368) Therefore, the beneficial effects of IGF I on the r ecovery of muscle strength with reloading is more obvious in the soleus muscle. It is well known that skeletal muscle is a post mitotic tissue, so a progenitor cell group is necessary for regeneration to occur. The main cells involved in the
100 regeneration o f damaged skeletal muscle are muscle stem cells (i.e. satellite cells). During muscle regeneration after damage, permanently differentiated mammalian myofibers can be repaired by satellite cells by fusing with existing myofibers or becoming new myofibers (15) Jennische et al. showed that IGF I was produced by satellite cells in regenerating muscles (110, 185) One of the primary roles of IGF I is its involvement in the activation, proliferation and differentiation of satellite cells (263) In addition, IGF I has been shown to restore skeletal muscle regenerative capacity by stimulat ing proliferative potential of satellite cells in immobilized, old rats (69) In this study, enhanced muscle regeneration with IGF I overexpression was supported by the finding that the increased number of Pax7 positive fibers and central nuclei in the IGF I injected soleus muscles were almost three times higher at 5 days reambulation, and embryonic myosin positive muscle fibers was 150% higher at 10 days reambulation compared to PBS injected soleus muscles. Importantly, the peak expression of central nuclei and Pax7 in the IGF I treated muscle occurred at 5 days of reloading, whereas it occurred later in the PBS injected limb (10 days of reambulation). Also, it should be noted that the number of Pax7 and central nuclei observed in the IGF I injected soleus at different time points was two to five fold higher than obs erved in the EDL, which is consistent with a higher number of satellite cells being found in slow twitch than fast twitch muscles (71, 145, 326) This difference in satellite cell availability may also contribute to the ability of IGF I overexpression to accelerate soleus muscle recovery. Muscle repair and regeneration is dependent on the ability of satellite cells to activate, proliferate and differentiate (15) and this pathway is modified by a f amily of factors known as myogenic regulatory factors (MRFs) (173, 311) Among them, MyoD is
101 thought to be essential for muscle regeneration and can be used as a marker of satellite cell activation in response to muscle damage (5, 59, 178, 285) In vitro MyoD expression is increased by the addition of IGF I (76) However, the time course for MyoD expression from in vivo studies is uncl ear. In the current study, we found that following cast immobilization, mRNA expression of MyoD was significantly upregulated in the soleus muscle after 2 days of reloading and returned to baseline levels after 5 days of reloading. Our findings were consis tent with Schols et al., who used a hindlimb suspension model in mice (382) Peterson et al. showed that acute (30 minutes) and chronic (5 days) pedaling exercise did not change MyoD and myogenin mRNA expression in spinal cord transected rats (102) The lack of clarity regarding MyoD expression in vivo is likely due to the use of different models and the complex interaction between the muscle regeneration processes with the se cretion of cytokines and growth factors following muscle damage. The present study showed that mRNA expression of MyoD was threefold greater in the IGF I injected soleus muscle compared to control. These data indicate that overexpression IGF I might posit ively increase satellite cell activation by intrinsically regulating MyoD expression. Without reloading, a low but significant increased expression of MyoD suggests that IGF I overexpression may create a pool of resting satellite cells already committed to the myogenic program. In early reloading, IGF I overexpression induces a more dramatic increase in MyoD compared to PBS injected muscle, which is consistent with the role of IGF I in inducting myogenic factors in vitro and in vivo (295) Combined with the increased markers of muscle regeneration, these results suggest that IGF I acts early in
102 the regeneration process by activating satellite cells thus enhancing muscle healing from injury. In summary, rAAV mediated I GF I gene transfer induces a significant increase in muscle size in the targeted posterior compartment hindlimb muscles during normal activity, immobilization and reloading. Importantly, our findings indicate that increased IGF I levels protects the soleus muscle from reloading induced muscle damage, enhances muscle regeneration and is associated with an accelerated recovery in muscle function following cast immobilization. These findings suggest that IGF I may ultimately have therapeutic applications durin g rehabilitation after prolonged inactivity. Further studies are needed to clarify the precise mechanism for the protective effects of IGF I after muscle injury and explore more details about the beneficial effects of IGF I in muscle recovery.
103 Fig ure 7 1. A) Representative transaxial proton MRIs of the mouse lower hindlimb with IGF I injection in one lower hindlimb and PBS injection on the contralateral side before casting immobilization. B) Muscle maximal cross sectional area (CSAmax) was determined by tracing the triceps surae muscles. IGF I injected PBS injected B A
104 Figure 7 2. A) Maximal cross sectional area (CSAmax) of the triceps surae. B) Soleus wet weight. C) Gastrocnemius wet weight. D) Soleus tetanic force and E) soleus specific force. Values are expressed as means SEM *d enotes significant difference (p<0.05) between IGF I injected and PBS injected group. indicates significant difference (p<0.05) from the noncasted group in the PBS injected muscles indicates significant difference (p<0.05) from noncasted group in the IGF I injected muscles. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significant ly higher increase in tetanic force (not indicated on figure) (p<0.05) A
105 Figure 7 2. Continued B C
106 Figure 7 2. Continued D E
107 Figure 7 3 A) Representative T2 images (TE=40ms) from the mice lower hindlimb muscles acquired at different time points. B) Representative T2 image and corresponding Hematoxylin and eosinstained crosssections of the soleus at 2 days of reloading following cast immobilization in IGF I and PBS injected groups. A B
108 Figure 7 4. A) Muscle T2 of the lower hindlimb muscles of mice at different loading time points. B) Percentage of pixels with elevated T2 (% damaged muscl e) in the lower hindlimb muscles of mice at different loading time points. Values are expressed as means SEM *d enotes significant difference (p<0.05) between IGF I injected and PBS injected group indicates significant difference (p<0.05) from the noncasted group in the PBS injected muscles indicates significant difference (p<0.05) from noncasted group in the IGF I injected muscles. A B
109 Figure 7 5 A) Light micrographs of H&E stained cross sections of the soleus from both IGF I (Right) and PB S (Left) injected groups following 2 weeks of cast immobilization B) Area fractions of abnormal muscle in the IGF I and PBS injection group. Values are expressed as means SEM. *Denotes significant difference (p<0.05) between IGF I injected and PBS inje cted group. Bar=50 m A A B
110 Figure 7 6 A) Light micrographs of H&E stained cross sections of the soleus from both IGF I (Right) and PBS(Left) injected groups at 5 days and 10 days of reloading following 2 weeks of cast immobilization. B) Percentage of cen tral nuclei at each time point. Values are expressed as means SEM *d enotes significant difference (p<0.05) between IGF I injected and PBS injected group. indicates significant difference (p<0.05) from the noncasted group in the PBS injected muscles indicates significant difference (p<0.05) from noncasted group in the IGF I injected muscles. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significantly greater increase in central nuclei positive f ibers (not indicated on figure), p<0.05. Bar=50 m A
111 Figure 7 6 Continued B
112 Figure 7 7. A) Cross sections of the soleus muscle stained with monoclonal antibody against embryonic myosin isoform (green) at 10 days of reloading. B) Cross sections of the soleus muscle stained with laminin + DAPI + Pax7. Pax7 positive fibers (arrows) at 5 days of reloading. C) Number of fibers expressing embryonic myosin per 100 fibers. D) Percentage of Pax7 positive fibers at each time point. Values are expressed as means SEM *d enotes significant difference (p<0.05) between IGF I injected and PBS injected group. indicates significant difference (p<0.05) from the noncasted group in the PBS injected muscles. indicates significant difference (p<0.05) from noncas ted group in the IGF I injected muscles. An interaction of group (IGF I injected vs PBS injected) and time was detected where IGF I injected muscles had a significantly higher increase in Pax7 positive f ibers (not indicated on figure), p<0.05 Bar=12.5 m
113 A B
114 Figure 7 7 Continued C
115 Figure 7 7 Continued D
116 Figure 7 8 mRNAs expression of muscle MyoD during normal activity, immobilization, and reambulation. Signal intensity of the mRNA bands was normalized to 18s. Values are expressed as means SEM *d enotes significant difference (p<0.05) between IGF I injected and PBS injected group indicates significant difference (p<0.05) from the noncasted group in the PBS injected muscles indicates significant difference (p<0.05) from noncasted group in t he IGF I injected muscles.
117 CHAPTER 8 CONCLUSION Gene therapy presents new opportunities for the development of novel rehabilitation strategies to improve muscle function in patients following bed rest, cast immobilization, surgery, injury and chronic disease s. This dissertation presented a series of experiments using a mouse model of cast immobilization and reambulation to examine the effects of viral mediated insulinlike growth factor I (IGF I) overexpression on skeletal muscle atrophy and recovery. The f indings from Experiment one demonstrated that local overexpression of IGF I in lower hindlimb, fast twitch muscles (tibialis anterior and extensor digitorum longus) induces significant hypertrophy and a concomitant increase in muscle strength. Relative gai ns in muscle size and force production observed under normal loading conditions were maintained during cast immobilization and reambulation. Muscles overexpressing IGF I also demonstrated evidence of enhanced muscle regeneration, with an increase number of embryonic myosin positive fibers, central nuclei, and muscle satellite cells. In addition, IGF I receptor and binding proteins demonstrated concomitant molecular responses with IGF I overexpression during cast immobilization and reambulation. Results fro m Experiment two demonstrated that local overexpression of IGF I induces significant muscle hypertrophy and increased strength in a predominantly slow twitch, lower hindlimb muscle (soleus) under normal loading conditions. Most importantly, the results indicated that elevated levels of IGF I could protect the soleus muscle from damag e induced by reloading, enhance muscle regeneration and muscle recovery following injury. The largest impact of IGF I overexpression was observed between 2 and 10 days of reloading. Interestingly, IGF I over expression also induced
118 adaptations in the soleus muscle during cast immobilization, with increased expression of M yoD and an attenuated degenerative response in the muscle fiber morphology. Collectively, the findings from this dissertation show that IGF I can protect skeletal muscle by minimizing the deleterious effects of disuse as well as reloading and accelerate the subsequent restoration of muscle function. These findings enhance our understanding of the mechanisms underl ying muscle atrophy and regeneration in immobilized muscle. The results of this work may also have applicability to clinical conditions associated with muscle wasting and may help in the development of pharmacological or gene therapy strategies that can be used in conjunction with rehabilitation interventions to counteract the deleterious effects of inactivity.
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155 BIOGRAPHICAL SKETCH Fan Ye was born in Nanjing, China in February 1979. He received his M.D. Degree from Southeast University, China in 2002 and was ranked in the top 1% of medical students. He then finished his threeyear orthopedics residency training in the ZhongDa Hospital affiliated to Southeast University and received his Masters of Science degree in orthopedic surgery in 2005. He joined the rehabilitation science doctoral program in the Department of Physical Therapy at the University of Florida in 2005.