The Role and Mechanism of Apoptosis as an Innate Immune Response against Viral Infection in Insects


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The Role and Mechanism of Apoptosis as an Innate Immune Response against Viral Infection in Insects
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1 online resource (128 p.)
Liu, Bo
University of Florida
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Gainesville, Fla.
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Doctorate ( Ph.D.)
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University of Florida
Degree Disciplines:
Medical Sciences, Genetics (IDP)
Committee Chair:
Zhou, Lei
Committee Members:
Bloom, David C
Snyder, Richard O
Jin, Shouguang
Becnel, James J


Subjects / Keywords:
apoptosis -- drosophila -- immunity -- mosquito -- virus
Genetics (IDP) -- Dissertations, Academic -- UF
Medical Sciences thesis, Ph.D.
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theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
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Arthropod-borne pathogens account for millions of death each year. Understanding the genetic basis controlling vector susceptibility to pathogens is pivotal to novel disease control strategies. The fact that many vertebrate and insect viruses possess anti-apoptotic genes that are required for their infectivity has long led to the hypothesis that induction of apoptosis could be a fundamental innate immune response.  However, the cellular mechanisms mediating the induction of apoptosis following viral infection remained enigmatic, which prevented experimental verification of the functional significance of apoptosis in limiting/preventing viral infection in insects. In addition, studies with cultured insect cells showed that either there is a lack of apoptosis, or the pro-apoptotic response happens relatively late, thus casting doubt on the functional significance of apoptosis as an innate immunity.  To prove the hypothesis that apoptosis can act as an innate immunity against viral infection in insects, I developed two kinds of in vivo infection systems. The first system is a native infection system established in mosquitoes. In this system, the mosquitoes were orally infected with virus which mimics the natural infection route. Using this system, I found that there is a rapid induction of reaper-like pro-apoptotic genes and apoptosis within a few hours following exposure to DNA or RNA viruses specifically in refractory mosquitoes rather than susceptible species. The correlation between apoptosis and mosquito susceptibility strongly suggests that apoptosis can serve as a defense against viral infection.  Given the limited genetic tools in mosquito, to elucidate the mechanisms of virus-induced apoptosis, I established the second in vivo infection system in the powerful model organism Drosophila. Similar to the results in mosquitoes, I found that pro-apoptotic genes (e.g. reaper and hid) and apoptosis were also rapidly induced in Drosophila by viral infection. Moreover, I found that this rapid induction of apoptosis requires the function of transcription factor p53and is mediated by a stress –responsive region upstream of reaper. More importantly, I showed that the rapid induction of apoptosis is responsible for denying the expression of viral genes and blocking/limiting the infection which proved the role of apoptosis as an anti-viral defense.
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by Bo Liu.
Thesis (Ph.D.)--University of Florida, 2012.
Adviser: Zhou, Lei.
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2012 Bo Liu 2


To Yanchao, my dear wife and to our family 3


ACKNOWLEDGMENTS First of all, Id like to thank my ment or and close friend, Dr. Lei Zhou who has given me excellent guidance and selfless support in the past four years, in both scientific research and life. It is Dr. Zhou w ho makes me truly feel the charm of science. I will never forget the key fa ctors that Dr. Zhou emphas izes repeatedly to become a successful scientist: honesty, interest, logica l thinking and perseverance. These words will accompany and benefit me throughout my whole career. Second, I would like to thank all of my committee members, Dr James Becnel, Dr. David Bloom, Dr. Shouguang Jin and Dr. Richar d Snyder for their inspirational instructions. Their comments and suggestions hel p to keep my research going along the right way. I would also like to thank IDP program and Department of Molecular Genetics & Microbiology for offering me such a w onderful training environment: competitive, passionate and meanwhile, full of fun. Third, I would like to thank all of the former and current colleagues of Zhou lab, Yanping Zhang, Nianwei Lin, Can Zhang, Guangyao Li, Kiet La, John Pang, Michael Novo, Brandon McDonald, Hannah Wang, Michel e Chung, Denis Tito v, Jordan Reuter, Erika Choi, Alex Gomez et al. Their contributions make the whole lab operate smoothly and their kindness makes the env ironment warm like a home. In addition, I would also like to pass my t hanks to several wonderful collaborators: Dr. James Becnel at the US De partment of Agriculture, Dr Rollie Clem at Kansas State University, Dr. Anette Schneemann at Sc ripps Research Institute and Dr. David Severson at the University of Notre Dame Without their generous sharing of reagents and ideas, my project could not have gone so smoothly. 4


Finally, I would like to thank my parents fo r their selfless love ever since I came into this world. They taught me how to become a wonderful person, which is the basis for everything. I also want to thank my beloved wife Yanchao Zhang, who makes every day in my life amazingly different. 5


TABLE OF CONTENTS page ACKNOWLEDG MENTS .................................................................................................. 4LIST OF TABLES ............................................................................................................ 9LIST OF FI GURES ........................................................................................................ 10LIST OF ABBR EVIATION S ........................................................................................... 12ABSTRACT ................................................................................................................... 131 BACKGROUND AND INTRODUCTION ................................................................. 15Apoptosis and General Regulatory Pathways ......................................................... 15Apoptosis Regulation in Drosophila and Mosqui to .................................................. 19Drosophila ........................................................................................................ 19Mosquito ........................................................................................................... 20Apoptosis and Vira l Infect ion .................................................................................. 21Viral Inhibition of Apopto sis .............................................................................. 22Adenovirus ................................................................................................. 22Baculoviruses ............................................................................................. 22Herpesviru ses ............................................................................................ 23Poxviruses ................................................................................................. 24Other viruses .............................................................................................. 25Viral Induction of Apopto sis .............................................................................. 25Adenovirus ................................................................................................. 25Human immunodeficiency vi rus type 1 (H IV-1) .......................................... 25Other viruses .............................................................................................. 262 INDUCTION OF REAPER ORTHOLOG MX IN MOSQUITO MIDGUT CELLS FOLLOWING BACULOVI RUS INFECT ION ........................................................... 30Summary ................................................................................................................ 30Introducti on ............................................................................................................. 30Materials and Methods ............................................................................................ 34Bioinformatics, Gene Cloning, and Cell Deat h Assa y ....................................... 34Mosquito Rearing and CuniNPV Infection ........................................................ 35RNA Extraction and Quantitative Real-Time PCR (Q-PCR) ............................. 35PI Staini ng ........................................................................................................ 36TUNEL As say ................................................................................................... 36Fluorescent in Situ Hy bridization (FISH)........................................................... 37Blocking Apoptosis with Z-VAD and Q-VD -OPH .............................................. 37Results .................................................................................................................... 38Molecular Cloning and Characterization of Mx in C. quinquefasciatus ............. 38The Mx Gene Is Induced in Midgut Cells Infected by CuniNPV ........................ 40 6


Necrosis, Not Apoptosis, Is Associ ated with CuniNPV Infection in the Susceptible Mosquito C. quinquefasciatus .................................................... 41Rapid Induction of Is Followed by Apoptosis in the Refractory Mosquito A. aegypti ....................................................................................... 42Inhibition of Apoptosis Leads to Expre ssion of Viral Genes in the Refractory Mosquito A. aegypti ....................................................................................... 43Discussio n .............................................................................................................. 45A Race to A poptosis ......................................................................................... 47Apoptosis and MosquitoH ost Compatibility .................................................... 483 IDENTIFICATION OF THE ANTI -APOPTOTIC GENE(S) IN CUNINPV GENOME ................................................................................................................ 59Summary ................................................................................................................ 59Introducti on ............................................................................................................. 59Materials and Methods ............................................................................................ 62Cell Culture and Transfect ion ........................................................................... 62Extraction of CuniNP V Genomic DNA .............................................................. 63Construction of CuniNPV Genomic Li brary ...................................................... 63Quantitative Real-Tim e PCR (Q-P CR) ............................................................. 64Cloning Full Length Mx_Culex .......................................................................... 64In Vitro Cell Death Assay .................................................................................. 65Results .................................................................................................................... 65Extraction of High Quality CuniNPV Genom ic DNA ......................................... 65Construction of CuniNPV Genomic Li brary ...................................................... 65The Gene Contained in the Plasmid Can Be Successfully Expressed ............. 66Discussio n .............................................................................................................. 674 P53-MEDIATED RAPID INDUCTION OF APOPTOSIS CONVEYS RESISTANCE TO VIRAL IN FECTION IN INSECTS .............................................. 72Summary ................................................................................................................ 72Introducti on ............................................................................................................. 73Materials and Methods ............................................................................................ 76Drosophila Strains ............................................................................................ 76Cell Culture, Viral Production and Drosophila Infection .................................... 76RNA Extraction and Q-PC R ............................................................................. 77FHV Genome Es timation .................................................................................. 78Fluorescent in Situ Hy bridization (FISH)........................................................... 78Antibody and Immunostaining .......................................................................... 79Results .................................................................................................................... 80Rapid Induction of Mx Following DEN-2 Infection in a Refractory, but Not in a Susceptible, Strain of Aedes aegypti .......................................................... 80Rapid Induction of RHG Genes Following Viral Infection in Live Fruit Flies but Not in Cult ured Cells ............................................................................... 80Rapid Induction of Reaper and Hid Requires P53 and the Regulatory Region IR ER ................................................................................................. 83 7


Rapid Induction of Apoptosis Blocks/ limits Viral Gene Expression and Proliferat ion ................................................................................................... 85Animals Lacking the Rapid Induction of Apoptosis Are Hyper-susceptible to FHV Infect ion ................................................................................................ 86Discussio n .............................................................................................................. 87Animal Models vs. Cultured Cells for Arbovirus Infection ................................. 88Rapid Induction of Apoptosis is a Key Innate Immunity against Viral Infection......................................................................................................... 89What is the Signal Transduction Pathway that Activates P53 .......................... 915 DISCUSSION AND PERSPECTI VES................................................................... 103The Role of Apoptosis as an Innate Imm une Response against Viral Infection .... 103Innate Immune Responses in Drosophila and Mosqui to ....................................... 105Drosophila ...................................................................................................... 105Mosquito ......................................................................................................... 106Significance of Studying t he Defensive Mechanisms in Drosophila and Mosquitoes ........................................................................................................ 107LIST OF REFE RENCES ............................................................................................. 111BIOGRAPHICAL SK ETCH .......................................................................................... 128 8


LIST OF TABLES Table page 1-1 Comparison of the numbers of id entified cell death regulatory genes in Anopheles mosquito and Drosophila .................................................................. 29 9


LIST OF FIGURES Figure page 1-1 Three major cell death regulatory pathways in differ ent organisms .................... 271-2 Simplified schematic presentation of the integration of cell death regulatory control in the Gas and Brake model. ................................................................ 282-1 Gene cloning and characteri zation of mx _Cu.qu. ............................................... 502-2 Induction of mx_Cu.qu and following CuniNPV infection. .................. 512-3 Necrosis in CuniNPV-infected C. quinquefasciatus larvae. ................................ 522-4 Rapid apoptosis following the induction in A. aegypti larvae exposed to CuniNPV. ......................................................................................... 532-5 induced cell death could be partially rescued by caspase inhibitor z-VAD-fmk ......................................................................................................... 542-6 Suppressing apoptosis in A. aegypti leads to expressions of viral genes. .......... 552-7 Suppressing apoptosis with caspase inhibitor Q-VD and combination of zVAD and Q-VD allowed expression of viral genes in the refractory Ae.aegy larvae. ................................................................................................................. 562-8 Viral gene expression was confined to the nuclei of a few cells at early stage of CuniNPV infection in Culex quinquefasciatus ................................................ 572-9 The race to apoptosis through the mosquito reaper .......................................... 583-1 Gel picture of extrac ted CuniNPV genom ic DNA ............................................... 693-2 Currently sequenced CuniNPV fr agments .......................................................... 703-3 Amplification plots of several tar get genes ........................................................ 714-1 Specific induction of mx in the refractory strain (MOYO-R), but not in the susceptible strain (MOYO-S), fo llowing exposure to DEN-2. .............................. 934-2 Rapid induction of reaper and hid following viral infection of Drosophila larvae or adults ............................................................................................................. 944-3 Viral gene expression is required for t he induction of pro-apoptotic response. .. 954-4 Virus -induced reaper / hid expression and apoptosis requires P53 and IRER. ... 96 10


4-5 Rapid induction of apopt osis functions to block/limit viral gene expression and proliferat ion. ................................................................................................. 974-6 Rapid induction of apoptos is functions to block/limit viral prolif eration. .............. 984-7 Standard curve for estima ting FHV genome /titter ............................................. 1004-8 Diagram summarizes the role of rapi d induction of apoptosis as an innate immune response against viral infect ion ........................................................... 1014-9 Monitoring the status of the JAK/STAT pathway. ............................................. 1025-1 The schematic representation of Toll and IMD immune pathways in Drosophila ........................................................................................................ 110 11


LIST OF ABBREVIATIONS ACMNPV Autographa californica nucleopolyhedrovirus BIR Baculovirus IAP repeats BMNPV Bombyx mori NPV CUNINPV Culex nigripalpus nucleopolyhedrovirus DIAP1 Drosophila inhibitor of apoptosis 1 EBV Epstein-Barr virus FADD Fas associated protein with death domain FBS Fetal bovine serum FHV Flock house virus FISH Fluorescent in situ hybridization HBV Hepatitis B virus IAP Inhibitor of apoptosis IBM IAP binding motif PI Propidium Iodide PRRSV Porcine reproductive and respiratory syndrome virus Q-PCR Quantitative real-time PCR RING Really interesting new gene SlNPV Spodoptera litura NPV TNF Tumor necrosis factor TNFR Tumor necrosis factor receptor TnMNPV Trichoplusia ni MNPV TRADD TNF receptor associated protein with death domain TUNEL Terminal deoxynucleotidyl transferase dUTP nick end labeling XIAP X-linked inhi bitor of apoptosis 12


Abstract of Dissertation Pr esented to the Graduate School of the University of Florida in Partial Fulf illment of the Requirements for t he Degree of Doctor of Philosophy THE ROLE AND MECHANISM OF APOPTOS IS AS AN INNATE IMMUNE RESPONSE AGAINST VIRAL INFECT ION IN INSECTS By Bo Liu December 2012 Chair: Lei Zhou Major: Medical Sciences-Genetics Arthropod-borne pathogens account for millions of deaths each year. Understanding the genetic basis controlling ve ctor susceptibility to pathogens is pivotal to developing novel disease control strategies The fact that many vertebrate and insect viruses possess anti-apoptotic genes that are required for their infectivity has long led to the hypothesis that induction of apoptosis could be a fundamental innate immune response. However, the cellular mechani sms mediating the induction of apoptosis following viral infection remained enigmatic which prevented experimental verification of the functional significance of apoptosis in limiting/preventing viral infection in insects. In addition, studies with cultured insect cells showed that either there is a lack of apoptosis, or the pro-apoptotic response happens relatively late, thus casting doubt on the functional significance of apopt osis as an innate immunity. To prove the hypothesis that apoptosis can act as an innate immunity against viral infection in insects, I developed two kinds of in vivo infection systems. The first is a native infection system established in mosqui toes. In this system, the mosquitoes were orally infected with virus mimicing the natural infection route. Using this system, I found that there is a rapid induction of reaper -like pro-apoptotic genes and apoptosis within a 13


14 few hours following exposure to DNA or RNA viruses specifically in refractory mosquitoes rather than susceptible spec ies. The correlation between apoptosis and mosquito susceptibility strongly suggests that apoptosis can serve as a defense against viral infection. To elucidate the mechanisms of virusinduced apoptosis, I established two in vivo infection systems in the genetic model organism Drosophila Similar to the results in mosquitoes, I found that pro-apoptotic genes (e.g. reaper and hid ) and apoptosis were also rapidly induced in Drosophila following viral infection. Moreover, I found that this rapid induction of apoptosis requires the function of transcription factor p53 and is mediated by a stress res ponsive region upstream of reaper More importantly, I showed that the rapid induction of apoptosis is responsible for denying the expression of viral genes and blocking/limiting the infect ion which proved the role of apoptosis as an anti-viral defense.


CHAPTER 1 BACKGROUND AND INTRODUCTION Apoptosis and Regulatory Pathways Apoptosis is a genetically controlled cell su icide process in multicellular organisms to remove obsolete, damaged and potential dangerous cells. A poptosis is characterized by a series of morphological features and biol ogical processes including cell shrinkage, membrane blebbing, chromatin condensatio n, DNA fragmentation and breaking down into apoptotic bodies. Eventually the apopt otic bodies are engulfed and digested by cells with phagocytotic activity such as microphage without causing inflammation. (Bergmann et al., 1998b; Vaux a nd Strasser, 1996; White, 1996). Apoptosis plays important roles in development. For ex ample, during the development of the brain, half of the neurons that are originally generated die later on when the adult brain forms (Hutchins and Barger, 1998). The formation of the individual digits is also due to massive cell death in interdigital mesenchymal tissues(Zuzarte-Luis and Hurle, 2002). Apoptosis is also critical for maintaining homeostasis in adult tissues. Cell proliferation in adult organs must be balanced by cell death to keep cell numbers constant. Moreover, apoptosis also has a prot ective role by removing the potentially dangerous cells such as those carrying muta tions in the genome or cells infected by intracellular pathogens. The dysre gulation of apoptosis manifests itself in a variety of human diseases. Insufficient apoptosis can cause cancer, autoimmune disease or the spreading of the viral infection; in cont rast, excessive apoptosis can result in neurodegenerative disease (Fade el et al., 1999). Most of the current kno wledge about apoptosis was deri ved from the work on the nematode C.elegans The fate of every somatic cell during C.elegans development is 15


traceable (Kimble and Hirsh, 1979; Sulston and Horvitz, 1977). Out of the 1090 somatic cells that are originally generated during development, 131 always die by apoptosis (Ellis et al., 1991; Horvitz et al., 1994). This strongly suggests that apoptosis is a strictly regulated instead of randomly occurring process. Later on, the genetic analysis in C.elegans identified the pathway controlling apoptosis which is called CED-9/CED4/CED-3 pathway (Figure 1-1, top green letters). Animals carrying mutation for ced-3 lost all cell death that would have occurr ed in normal development (Yuan et al., 1993; Yuan and Horvitz, 1990). Subsequently, the r egulatory pathways have been found to be highly conserved across all metazoans. Figure 1-1 listed three major cell death regulatory pathways in nematode, fly and mammal (Zhou et al., 2003). In all three pathways, the executors of apoptosis are a family of proteases called caspases ( c ysteine as partic acid-specific prote ase ). In normal situations, the caspases are inactive zymogens and ubiquitously expres sed in both dying and living cells. Upon apoptotic stimuli such as ioni zing radiation, a series of cascading events are triggered which can eventually activate the caspases The activated caspases further cleave multiple cellular components such as the cytoskeleton, enzyme inhibitors and so on which will lead to collapse of the cells (B udihardjo et al., 1999). Thus far, eleven caspases have been described in humans, ten in mice, seven in Drosophila and four in C.elegans (Lamkanfi et al., 2002). The mammalian orthologs of CED-9 are the BCL-2 family proteins which can positively or negatively regulate mitochondria membrane integrity (Adams and Cory, 2001; Chao and Korsmeyer, 1998). The member s that possess pro-apoptotic activities can cause mitochondria permeabilization and result in the release of cytochrome c 16


(CYT c) into the cytosol, which further activates downstream APAF-1, the mammalian ortholog of CED-4 (Zou et al., 1997). CYT c, together with activated APAF-1 and ATP can form a complex namely the apoptosome, which, once formed, can recruit and activate caspases (Hu et al., 1998; Pan et al., 1998). The middle pathway presented with blue letters in figure 1-1 was termed IAPpathway which was originally characterized in Drosophila The details concerning this pathway will be discussed in next section. Generally speaking, the core regulator in this pathway is a protein called DIAP1 ( D rosophila I nhibitor of Ap optosis). DIAP1 can inhibit the enzymatic activity of caspases thr ough direct interaction (Salvesen and Duckett, 2002). The IAPs may be considered as a universal brake to caspase activation (Figure 1-2). Upon apoptotic stimuli su ch as steroid hormones, radi ation etc., a family of upstream genes including reaper hid grim and sickle are transcriptionally upregulated (Brodsky et al., 2000; Chen et al., 1996; Christ ich et al., 2002; Jiang et al., 2000; Lohmann et al., 2002; Nordstrom et al., 1996; Robinow et al., 1997). The protein products of these genes can compete with ca spases for DIAP1 binding and thus these genes are called IAP-antagonists (Chai et al., 2003; Yan et al., 2004). In Drosophila unlike DIAP1 and caspases which are ubiqu itously expressed in cells, the IAPantagonists are selectively expressed in cells doomed to die. To date, at least two mammalian proteins, SMAC/DIABLO (Chai et al., 2000; Du et al., 2000) and OMI1/HTRA2 (Martins et al., 2002; Suzuki et al., 2001), have been found to have IBM ( I AP B inding M otif) and act as IAP-ant agonists. However, unlike IAP-antagonists in Drosophila these mammalian IAP-antagonists are ubiquitously expressed and reside in the mitochondria, from which they are re leased into the cytosol when apoptosis 17


happens. Moreover, mice deficient for SMAC /DIABLO or OMI1/HTRA2 did not show increased resistance to apoptosis (Jones et al., 2003; Okada et al., 2002). Thus, the physiological role of these proteins in regulating IAPs is still unclear. So far eight mammalian IAPs have been identified (Salvesen and Duckett, 2002) and the best studied is XIAP ( X -linked I nhibitor of Ap optosis). It is regarded as the most potent caspase inhibitor in vitro (Eckelman and Salv esen, 2006). However, XIAP-deficient mice present the normal phenotype, so the physiologic al role of XIAP in vivo remains to be studied (Harlin et al., 2001). In addition to those pathways discussed above, the bottom pathway presented in black letters in figure 1-1 is another conser ved cell death regulatory pathway, namely death receptor pathway. Death receptors such as TNFR ( T umor N ecrosis F actor R eceptor), FasR ( Fas R eceptor) etc. are a subset of transmembrane receptors with intracellular death domain (D aniel and Krammer, 1994; It oh and Nagata, 1993; Nagata, 1999; Schmitz et al., 2000). After bindi ng with corresponding ligands through the extracellular sequence, the deat h receptors become oligom erized and recruit caspase 8 through the adaptor proteins such as FADD ( F as A ssociated Protein with D eath D omain) or TRADD ( T NF R eceptor A ssociated Protein with D eath D omain) (Chinnaiyan et al., 1995; Hsu et al., 1995), which further ac tivate caspase 8 and induces apoptosis. Although I discussed the cell death regulatory pathways independently, there is complicated crosstalk among different pathways. For example, caspase 8 is activated by the death receptor pathway. Meanw hile, its target, pro-apoptotic gene bid is also involved in regulating mitochondrial membrane int egrity (Li et al., 1998). Deciphering the 18


interactions between different pathways can help to establish the whole network of apoptosis regulation. Apoptosis Regulation in Drosophila and Mosquito Drosophila As a powerful model organism, Drosophila provides us with an excellent tool to study apoptosis regulation. In terestingly, when searching Drosophila genome for the motifs presented in major cell death regulatory proteins, it seems that the orthologs of the proteins involving in all three major cell death regulatory pathways listed in figure 1 exist in Drosophila which suggests that all three cell death regulatory pathways might be conserved in Drosophila However, the IAP pathway that was discussed in above section has been found to be the most important cell death regul atory pathways in flies. The IAP ( I nhibitor of Ap optosis) which was initially identifi ed from insect baculoviruses is a potent inhibitor of apoptosis (Crook et al ., 1993). Subsequently, ce llular orthologs of IAP were identified in mu ltiple organisms such as Drosophila and mammals (Hay, 2000). The core factor in the IAP pathway is DIAP1 which is the Drosophila ortholog of IAP. DIAP1 can inhibit the enzymatic activi ty of caspases through direct interaction which is mediated by its N-terminal BIR domain ( B aculovirus I AP R epeats). Moreover, DIAP1 also possesses ubiquitin E3 ligase ac tivity through the C-terminal RING ( R eally I nteresting N ew G ene) domain which can lead to t he ubiquitination and degradation of caspases (Vaux and Silke, 2005). Loss-of-function of diap1 mutants are embryonic lethal due to the widespread apop tosis (Wang et al., 1999). Upon apoptotic stimuli, the inhibitory function of DIAP1 can be counteracted by a family of upstream proteins including REAP ER, HID, GRIM and SICKLE, all of which are termed IAP-antagonist. I AP-antagonists were initiall y identified through genetic 19


studies in flies. Interestingly, the four I AP-antagonists genes are loca ted in proximity at the 75C1-2 chromosome region and share regul atory enhancer regions (Zhang et al., 2008). These IAP-antagonists function by bindi ng to the BIR domain of DIAP1, the selfsame domain normally bound to caspases As IAP-antagonists displace caspases from DIAP1, the caspases become free to i nduce apoptosis (Chai et al., 2003; Yan et al., 2004). The N-terminal IBMs ( I AP B inding M otif) in IAP-antagoni sts are responsible for the DIAP1 binding. The sequences of those IAP-antagonists are highly divergent except for the short IBM. Af ter genetically removing thes e IAP-antagonists, almost all developmental apoptosis was blocked (White et al., 1994). On the other hand, overexpression of these genes can result in potent apoptosis induction (Chen et al., 1996; Grether et al., 1995). This indi cates that releasing the brak e is a major mechanism of cell death regulation in Drosophila (figure 1-2). Indeed, in Drosophila while DIAP1 and caspases are ubiquitously expressed in cells, the IAP-antagonist s are selectively expressed in cells doomed to die. Those IAP-antagonists act in a partially redundant manner because removing an individual gene only causes a mild apoptosis phenotype (Grether et al., 1995; Peterson et al., 2002; Zhou et al., 1997). Mosquito The mosquito is a common insect in the order of Diptera, the same as Drosophila They are very important vectors for a va riety of pathogens including malaria, dengue fever, yellow fever etc. Although mosquitoes are in the same orde r as the fruit fly and the apoptosis regulation in Drosophila has been extensively studied, the apoptosis regulation in mosquitoes has been a mystery until the completion of the genome project of Anophele gambiae in 2002 (Christophides et al., 2002). It seems that at the sequence level, the general organization of cell death regulatory machinery in mosquitoes is highly 20


comparable to that in Drosophila (Table 1-1). Twelve caspases and seven IAPs were identified in the genome sequence of Anophele gambiae representing a significant increase compared to Drosophila which has seven and four respectively (Christophides et al., 2002). The significant increase of caspases and IAPs may reflect the functional requirement of fine-tuning cell death in response to parasites and viruses commonly encountered as a consequence of blood feeding. The specific expansion of IAPs also implies that, as in Drosophila the IAP pathway also plays important roles in cell death regulation in Anopheles mosquitoes. However, the genome project did not identify any orthologs of IAP-antagonists (i.e. reaper hid grim sickle ) due to the fast divergence of these genes (Table 1-1). The orthologs of IAP ant agonist were later identified by using a bioinformatics method and functionally characte rized (Zhou et al., 2005). More orthologs were then identified by BLAST (Bryant et al., 2008; Wang and Clem, 2011). The orthologs of core factors in IAP pat hway were subsequently identified from other mosquito species such as the yellow fever mosquito Aedes aegypti (Bryant et al., 2008; Nene et al., 2007). Functional tests in mo squito cell lines verified the role of those genes in regulating apoptosis e.g. silencing the mosquito IAP caused spontaneous apoptosis; silencing the mosquito caspases completely or partially inhibited apoptosis induced by different apoptotic stimuli (Liu and Clem, 2011). These findings further suggest the potential roles of IAP pathway in apoptosis regulation in mosquitoes. Apoptosis and Viral Infection It has been well established that apoptosis of host cells are usually associated with viral infection. Many viral gene products c an manipulate host cell apoptosis. On the one hand, it appears that many viruses carry anti -apoptotic genes in their genome to prevent premature death of host cells to maximize viral yield. On the other hand, a variety of 21


viral gene products have been shown to ac tively promote apoptosis, which was proposed to facilitate viral spreading while evading host infl ammatory response. Therefore, the exact role of apoptosis in vira l infection is still to be determined. Viral Inhibition of Apoptosis Adenovirus Human adenovirus is the first virus that has been shown to manipulate host cell apoptosis (Ezoe et al., 1981). The E1B 19K protein of adenovirus was found to be the homolog of anti-apoptotic protei n BCL-2 (Chiou et al., 1994; Pilder et al., 1984). Loss-offunction viral mutants can caus e a strong apoptosis phenotype in infected cells (Ezoe et al., 1981). Moreover, E1B 19K is also able to suppress TNF (White et al., 1992) or Fas ligand (Huang et al., 1997) induced apoptosis. Another adenovirus protein, E4orf6, has been shown to block P53-mediated apoptosis by preventing P53 accumulation in infected cells (Querido et al., 1997). Recently, E4orf6 was found to promote de novo heterochromatin formation at P53-targeted promoters and led to the epigenetic silencing of the P53-targeted genes, many of which can induce apoptos is (Soria et al., 2010). In addition to human adenovirus, anti-apoptotic genes were also identified from other adenoviruses. For example, GAM1 which en codes a 30kDa protein was discovered from chicken adenovirus type 1. Although GAM1 did not show any homology to so-far sequenced proteins at the sequence level, it has been shown to function like BCL-2 and E1B 19K (Chiocca et al., 1997). Baculoviruses Baculoviruses are a family of lar ge rod-shaped viruses which have a very restricted host range. They can only in fect certain insect species and are nonpathogenic to humans (Okano et al., 2006). To date, baculov irus might be the one in 22


which manipulation of host cell apoptosis has been best studied. Two well-known antiapoptotic proteins, P35 and IAP, were both init ially identified from baculovirus (Clem et al., 1991; Crook et al., 1993). P35 can act as bait for a variety of activated effecter caspases such as human caspases 1, 3, 6, 7, 8, 10 (Z hou et al., 1998) and Drosophila DrICE (Fraser et al., 1997), but not for initiator caspases such as human caspase 9 (Vier et al., 2000) and Drosophila DRONC (Meier et al., 2000). Mutant viruses lacking functional P35 can strongly in duce apoptosis in infected cells (Clem et al., 1991). Ever since the initial identif ication from AcMNPV ( Autographa californica nucleopolyhedrovirus), p35 has been subsequently identified from many other baculoviruses including BmNPV ( Bombyx mori NPV), SlNPV ( Spodoptera litura NPV), TnMNPV ( Trichoplusia ni MNPV). However, no cellular p35 has been identified so far. Unlike P35 which can only act on effecter caspases, IAP can directly interact with both initiator and effecter caspases and inhibit their activity. Cellular IAP was also later identified from multiple organi sms. For example, the DIAP1 in the IAP pathway was the Drosophila homolog of IAP. I AP proteins have also been descr ibed in other viruses that can infect arthropods such as entomopoxvirus (Li et al., 2005), iridovirus (Crook et al., 1993), Hz-1 virus (Cheng et al., 2002) and Afric an swine fever virus (Chacon et al., 1995). Interestingly, to date, no IAP homol ogs have been described from viruses that can infect vertebrates. Given the notion that viruses usually target the most important proteins or pathways, this suggests that IAP proteins ma y be more important for cell death regulation in arthr opods than vertebrates. Herpesviruses Like many other viruses, herpesviruses are also able to inhibit host cell apoptosis by expressing BCL-2 homologs or sequeste ring P53 activity. Human cytomegalovirus 23


(CMV) IE2 has been shown to bind to P53 and r epress its transcriptional activity (Speir et al., 1994). Epstein-Barr virus (EBV) lat ent membrane protein 1 (LMP1) can upregulate BCL-2 expression (Hen derson et al., 1991) or inhi bit P53-mediated apoptosis to prevent B cells from killing themselves (Okan et al., 1995). Moreover, two other EBV proteins, EBNA-5 (Szekely et al., 1993) and BZLF1 (Zhang et al., 1994), have been found to bind with P53, which may play a ro le in blocking apoptosis. An EBV protein involved in the lytic cycle, BHFR1, has hom ology with BCL-2. Functional verification indicated that BHFR1 can suppress apoptosis triggered by mu ltiple stimuli such as serum depletion (Henderson et al., 1993), DN A damage or infection by adenovirus mutants lacking E1B 19K (Tarodi et al., 1994). Moreover, BHFR1 has been shown to inhibit TNF or Fas induced apoptosis in intesti nal epithelial cells (Kawanishi, 1997). The product of ORF16 of Herpes simian B vi rus is a BCL-2 homolog, which can block apoptosis induced by viral infection (Nava et al., 1997). Kaposi sarcoma-associated virus contains a homolog of BCL-2, KSBCL-2, that can inhibit viral infection-induced apoptosis (Cheng et al., 1997). The LAT protei n of Herpes Simplex Virus (HSV) was also shown to inhibit cell death by blocking caspse 8 and caspase 9 pathways (Bloom, 2004). Poxviruses The CRMA ( c ytokine r esponse m odifier) in cowpox is an inhibitor of caspases which can block apoptosis induced by TNF or Fas signaling (Miura et al., 1995; Talley et al., 1995). Likewise, vaccinia virus also inhibits apoptosis through a gene namely SPI-2 which shows high homology to CrmA (Dobbelstein and Shenk, 1996; Kettle et al., 1997). T2 protein contained in myxomavirus is a homolog of TNF receptor which is used by the virus to antagonize TNF -induced apoptosis (Macen et al., 1996). Molluscum 24


contagiosum, another poxvirus encodes a vFLIP ( v iral FL ICEl ike inhibitory p roteins) to inhibit FADD-mediated caspase activa tion and apoptosis (Thome et al., 1997). Other viruses Hepatitis B virus (HBV) encodes a pX prot ein which can bind to P53 and inhibit the DNA binding property of P53 thus blocking P53-mediated transcriptional activation and suppress P53-dependent apoptosis (Wang et al., 1995). Hepatitis C virus core protein was shown to repress P53 expression to inhi bit apoptosis (Ray et al., 1997). In addition, the large T antigen of SV40 has been shown to inactivate P53 to block P53-dependent apoptosis (McCarthy et al., 1994). Viral Induction of Apoptosis As discussed above, many viral proteins can trigger apoptosis instead of blocking it. This active induction of apoptosis might re present a strategy used by many viruses, particularly non-enveloped, non-ly tic viruses, to spread wit hout initiating host immune response. Compared to the abundant knowl edge about how viruses inhibit apoptosis, much less information is available about how viruses actively induce apoptosis. Adenovirus The adenovirus E1A protein has been shown to induce apoptosis in cultured cells when expressed (Rao et al., 1992). E1A has been found to stabilize P53, which further mediates apoptosis. Moreover, the intera ction between E1A and the product of E4 transcription complex has been implicated in apoptosis when cells lack functional P53 (Marcellus et al., 1996). Human immunodeficiency virus type 1 (HIV-1) HIV-1 TAT protein has been found to up-regulate Fas ligand which further activates Fas receptor and apoptosis (Westendorp et al., 1995). When expressed in 25


haematopoietic cell lines, TAT was shown to down-regulate anti-apopt otic protein BCL2 and up-regulate pro-apoptotic pr otein BAX (Sastry et al., 1996). Other viruses The alphavirus, Sindbis virus, has been descr ibed to cause cell death in a variety of mammalian cells (Levine et al., 1993). Howe ver, the mechanisms responsible for this effect have not been determined. The B 19 human parvovirus has been found to promote apoptosis through the nonstructural protein (NSP) (Morey et al., 1993; Ozawa et al., 1988). The PRRSV (Porcine reproducti ve and respiratory syndrome virus) can induce apoptosis through the P25 protein, encoded by ORF5 (Suarez et al., 1996). 26


Figure 1-1. Three major cell death regulatory pathways in different organisms. When activated, all pathways lead to the acti vation of a family of protease termed caspases (encircled by red dashed s quare). Different colors represent different organisms (Green:C.elegan; bl ue: Drosophila and black: mammals). Stimulatory and inhibitory interactions are indicated by arrows and T-bar respectively. (Zhou et al., 2003) 27


Figure 1-2. Simplified schematic presentation of the integration of cell death regulatory control in the Gas and Brake model. Apaf-1-like molecules function as Gas for caspase activation, which is checked by the Brake IAPs. Drosophila gene names are separated from their mamm alian counterpart wit h /. Due to space limitation only one nam e is chosen for each gene. 28


29 Table 1-1. Comparison of the numbers of identified cell death regulatory genes in Anopheles mosquito and Drosophila Data on Anopheles genes are mainly based on sequence similarity analysis (Christophides et al., 2002; Holt et al., 2002) ? indicate s that the ortholog was not identified by the genome projec t, i.e. cannot be identified based on routine similarity search such as BLAST analysis. Genes Anopheles gambiae Drosophila melanogaster Caspases 12 7 IAP 7 4 IAP-antagonists 0? 4 Apaf-1/Hac-1 1 1 Bcl-2 family 2 2 TNFR family 1 1 TNF 0? 1


CHAPTER 2 INDUCTION OF REAPER ORTHOLOG MX IN MOSQUITO MIDGUT CELLS FOLLOWING BACULOVIRUS INFECTION Summary Many vertebrate and insect viruses posse ss anti-apoptotic genes that are required for their infectivity. This led to th e hypothesis that apoptosis is an innate immunoresponse important for limiting virus in fections. The role of apoptosis may be especially important in insect antiviral defense because of the lack of adaptive immunity. However, the cellular mechanism that elicits apoptosis in response to viral infection in insects has not been determined. Using an in vivo infection system with the mosquito baculovirus CuniNPV ( Culex nigripalpus nucleopolyhedrovirus), I demonstrated that michelob_x (mx ), the mosquito ortholog of Drosophila pro-apoptotic gene reaper is specifically induced in larval midgut cells following viral infection. Interestingly, the dynamics of mx induction corresponds with the outco me of the infection. In the permissive mosquito Culex quinquefasciatus ( C. quinquefasciatus), a slow induction of mx failed to induce prompt apoptosis, and t he infected cells eventually undergo necrosis with heavy loads of encapsulated viruses. In contrast, in the refractory mosquito Aedes aegypti ( A.aegypti ), a rapid induction of mx within 30 min p.i. is followed by apoptosis within 2 h p.i., suggesting a possible role for apoptosis in limiting viral infection. When the execution of apoptosis was delayed by caspase inhibitors, viral gene expression became detectable in the Aedes aegypti larvae. Introduction The theory that apoptosis has a very important role in virus infections was mainly supported by evidence that many viruses po ssess one or multiple genes that interfere with cellular apoptosis during the infection process (Benedict et al., 2002). Studies on 30


viruses have uncovered a multitude of vira l arsenals that can manipulate cellular apoptotic response in essentially all aspects and levels of the process. For instance, both the intrinsic and extrinsic cell death regul atory pathways are targeted by proteins and/or small RNAs encoded by mammalian viru ses (Galluzzi et al., 2008; Thomson, 2001). The extensiveness of viral interfer ence of apoptosis ranges from manipulating upstream sensing and regulatory mechanisms to blocking the enzymatic activity of downstream effectors such as caspases. Several very important cell death regulators we re initially identified in viruses. For instance, IAP (inhibitor of apoptosis) was originally identified in lepidopteran baculoviruses (Clem et al., 1991; Crook et al., 1993). Viruses mutated for iap induce rapid cell death in infected cells (Li et al., 2005; Means et al., 2003). Viral IAPs not only can block cell death associated with viral in fection but also apoptosis induced by other cytotoxic stimuli. Independently, genetic study in Drosophila melanogaster identified reaper -like genes as the pivotal regulators of programmed cell death (White et al., 1994). Subsequent genetic and biochem ical analysis revealed that reaper -like proapoptotic genes function as IAP ant agonists. One of the cellular iap genes, diap1 is ubiquitously expressed and requir ed for the survival of cells. Essentially all cells in the developing embryo undergo apoptosis when functi onal DIAP1 is absent (Goyal et al., 2000; Hawkins et al., 1999). Se lective cell death during Drosophila development is mainly achieved by specific expression of the IAP antagonists reaper, hid, grim and sickle. With the exception of HI D, whose pro-apoptotic activity is subject to posttranslational modification (Bergmann et al., 1998a), IAP antagonists such as reaper are mainly regulated at the transcriptional level. In addition to mediating developmental cell 31


death, IAP antagonists are also responsible for mediating cell death in response to environmental stimuli. For ex ample, the expression of reaper in Drosophila can be activated/induced by X ray, UV irradiation, or hormonal surges (Jiang et al., 2000; White et al., 1994; Zhou and Steller, 2003). As insects lack adaptive immunity, it has been postulated that apoptosis would have an even more important role in antiv iral response. Indeed, apoptosis has been observed during pathogen infection of mosqui toes and has been associated with host susceptibility to viral infection. It has been documented that ingestion of blood containing West Nile virus induces apoptos is in the midgut of a refractory Culex pipiens strain (Vaidyanathan and Scott, 2006). In co ntrast, necrosis has been associated with Western Equine Encephalomyelitis vi rus infection in susceptible Culex. tarsalis strains (Weaver et al., 1992). Although these eviden ces strongly suggest that pro-apoptotic response may have a very important role in determining vector comp atibility, detailed mechanistic study has been hindered by the lack of knowledge about the underlying genetic mechanisms mediating pro-apoptotic response against viral infection. The genome projects of Anopheles gambiae and Aedes aegypti revealed that, compared with Drosophila these arbovirus vectors have expanded families of IAPs and caspases (Christophides et al., 2002). This was speculated as an adaptation to repeated exposure to pathogens a ssociated with blood ingesti on, although expansion of caspases has also been found in other Drosophila species (Bryant et al., 2010). The A. gambiae genome project did not initially annot ate any IAP antagonists because of the fast divergence of their sequences. The missing IAP antagonist was uncovered using an advanced bioinformatics approach, which identified michelob_x (mx ) as the reaper 32


like IAP antagonist in both Anopheles and Aedes mosquitoes (Zhou et al., 2005). Another IAP antagonist that is related to mx was subsequently characterized in A. aegypti (Bryant et al., 2008). Despite the low s equence similarity of the entire gene, the functional domain, that is, the IAP-binding motif (IBM), was very well conserved, and the functional mechanism of MX appears to be very similar to that of REAPER. The identification of IAP antagonists in mo squitoes allowed us to ask whether reaper -like genes are involved in pro-apoptotic response to viral infection. We took advantage of the mosquito baculovirus CuniNPV ( Culex nigripalpus nucleopolyhedrovirus) because of the accessib ility of this system and the established insect pathology associated with CuniNPV infection (Andr eadis et al., 2003). CuniNPV is originally isolated from the mosquito Culex.nigripalpus (Becnel et al., 2001). It is related to lepidopteran and hymenopteran baculoviruses, but genomic sequence comparison indicated that there is a lar ge evolutionary distance between CuniNPV and lepidopteran baculoviruses (Moser et al., 2001) CuniNPV infects only epithelial cells of the larval midgut, has a restricted host range, and mainly infects Culex (Andreadis et al., 2003). None of the examined Aedes mosquitoes, including A. aegypti is susceptible to CuniNPV infection (Andreadis et al., 2003). Cuni NPV can exist either as the occluded form or the budded form. The virus exists out side the mosquito in the occluded form, which allows the virus to survive under harsh environmental conditions. Ingested occluded virus initiates the infection in the presence of the divale nt cation magnesium. Not all larval midgut cells are receptive to CuniNPV infection, which is limited to a particular group of resorbing/secreting cells in the gastric caeca and the posterior 33


midgut (Moser et al., 2001). Once inside the midgut, the virus can spread from infected cells to neighboring cells via the budded form. In this study, I showed that mx is induced in larval midgut cells following exposure to a mosquito baculovirus CuniNPV. More im portantly, the dynamics of this induction is different in the susceptible C. quinquefasciatus versus the refractory A. aegypti The relatively timid and delayed response of mx in C. quinquefasciatus (mx_Cu.qu ) is associated with necrosis, whereas t he robust and immediate induction of mx in A. aegypti ( ) is followed by apoptosis within 6 h of viral exposure, suggesting that apoptosis may contribute to lim iting viral infection in Aedes Materials and Methods Bioinformatics, Gene Cloni ng, and Cell Death Assay Data-mining strategy, plasmid construc tion, and in vitro cell death assay were performed as described previously (Zhou et al., 2005). Briefly, a mo tif search program implemented in C was customized to search for the IBM motifs in the genomic and EST sequences from mosquito genomes. An intronless cDNA for mx_Cu.qu was then obtained by reverse transcriptase PCR usin g RNA extracted from CuniNPV-infected mosquito C. quinquefasciatus that serves as the template for PCR reaction. The primers used here are 5-ACCGGCCGGCGCT GGTTACGTGATTCT-3 and 3CGGGATCCATACACTTTGCGGAGCAGA-5. P CR product was then subcloned into the pIE-3 vector for transfection assay, and cloned into pBS for synthesizing cRNA probe. The other plasmids used in this st udy, including Pie-LacZ,, PieP35, and Pie-DIAP1, are described previously (Zhou et al., 2005). 34


Mosquito Rearing a nd CuniNPV Infection C. quinquefasciatus (Gainesville, FL strain, maintained since 1995) and A. aegypti (Orlando, FL strain, maintained since 1952) were reared in the insect ary of the Mosquito and Fly Research Unit at the Center for Medical, Agricultural, and Veterinary Entomology, USDA-ARS, Gainesville, FL, U SA. CuniNPV viral OBs were purified from infected C. quinquefasciatus larvae as described previously (Moser et al., 2001). The standard assay involves groups of 100 threeto four-day-old C. quinquefasciatus and A. aegypti larvae obtained from the laboratory co lony. Larvae are exposed at a dose of 1.29 OBs per ml in 100 ml of deionized water with 15mM MgCl2 plus 50 mg alfalfa and potbelly pig chow mixture (2 : 1). Groups without the addition of the virus serve as controls. RNA Extraction and Quantitative Real-Time PCR (Q-PCR) Total RNA was extracted from frozen larvae with RNe asy Mini Kit (QIAGEN, Valencia, CA, USA) according to the prot ocol provided by the manufacturer. RNA samples were treated with DNase I to re move genomic DNA. cDNA was prepared by reverse transcription of total RNA with a High-Capacity cDNA Archive Kit (Applied Biosystems, Foster City, CA, USA). Q-PCR was performed with an ABI 7500 Fast thermocycler (Applied Biosystems) following protocols provided by the manufacturer. Triplicates were measured for each gene/sample combination. To control for variation of mx expression levels during normal develop ment, each pool of larvae was hatched on the same day and randomly separated into control and viral treatment groups. A sample of 10 larvae was taken from each group at a given time point and frozen at -80 oC. The cellular expression levels of mx were measured as relative to the glyceraldehyde 335


phosphate dehydrogenase (GAPDH ) or actin levels before the calculation of the ratio between CuniNPV-treated and control samples. PI Staining Live mosquito larvae were removed from the culture media, washed briefly with water, and transferred to 6-we ll plates with 5ml of fresh culture media containing 1 mg/ml PI (Sigma, St. Louis, MO, USA). After 10 min of in cubation, the staining was stopped by transferring the larvae to fresh media without PI. The larvae were than dissected quickly, and the midgut and a ttached tissues were fixed in 4% paraformaldehyde in PBS. The fixed samp les were washed three times in PBS. Samples were mounted with the Vectorsh ield mounting medium (Vector Lab, Burlingame, CA, USA) containing DAPI. The pictures were taken with a Leica upright fluorescent microscope (Leica, Bannock burn, IL, USA) using OpenLab software (Improvision, Coventry, UK). TUNEL Assay Apoptosis was detected usi ng the FragEL Kit (Calbiochem, Gibbstown, NJ, USA) with a modified protocol. Midguts were di ssected and fixed with 4% paraformaldehyde, 100mM PIPES buffer, pH 7.4, 2mM MgSO4, and 1mM EGTA for 20 min at room temperature. They were washed with TBS (20mM Tris-HCl, pH 8.0, and 140mM NaCl), followed by 5 min incubations in a TBS-methanol step gradient from 0, 25, 50, 75 to 100% methanol and back to TBS buffer follo wing a reversed methanol series of concentrations. The tissue was then incubated for 5 min at room temperature with 20 mg/ml protease K in TBS, and the reac tion was stopped by washing with 4% paraformaldehyde in TBS. This was follo wed by another 10 min fixation with 4% paraformaldehyde in TBS. The sample was equilibrated for 20 min with equilibration 36


buffer at room temperature and incubated with TdT labeling reaction mixture for 1 h at 37 oC. The samples were then mounted with Ve ctorshield mounting medium (Vector Lab) containing DAPI. The pictures were taken with a Leica upright fluorescent microscope using OpenLab software. Fluorescent in Situ Hy bridization (FISH) Probes were synthesized using DIGor Fluorescein-RNA Labeling Mix (Roche, Madison, WI, USA). Midguts were dissected, fixed, and processed as described above. After prefixing with 4% paraformaldehyde in PBT_DEPC (0.3% Triton in PBS made with DEPC pretreated double-distilled water) for 30 min, the tissue was incubated for 7 min with 50 mg/ml protease K in PBT_DEPC, and reaction was stopped by washing with 4% paraformaldehyde. Samples were incubated wit h probes diluted in hybridization buffer (50% formamide, 25% 2SSC, 20 mg/ml yeast tRNA, 100 mg/ml ssRNA, 50 mg/ml heparin, and 0.1% Tween-20). Hybridizat ion was performed overnight at 60 oC. If necessary, HRP-conjugated anti-DIG or antiFITC (Roche) antibody (depends on what marker the probes carry) was applied after hybridization, fo llowed by signal amplification using the Tyramid Signal Amplificati on Kit (PerkinElmer, Waltham, MA, USA). Blocking Apoptosis with Z-VAD and Q-VD-OPH Immediately following the administration of virus and MgCl2, exposed A. aegypti larvae were removed from t he culture pan together with the virus containing media to 24-well or 12-well culture dishes. z-VAD-f mk (R&D Bioscience, Minneapolis, MN, USA) and Q-VD-OPH (BioVision, Mountain View, CA, USA) were added to individual set of the wells for final concentration of 100 and 50 mM respectively. The combination of 50 mM z-VAD-fmk and 20 mM Q-VD-OPH was also applied. Meanwhile, equal amounts of DMSO (solvent for z-VAD and Q-VD-OPH) were added to a parallel set of wells as 37


control. Larvae were collected at discrete time points following the infection and processed for Q-PCR or FISH as described above. To assess the inhibitory efficiency of above caspase inhibitors, cultur ed C6/36 cells were transf ected with 0.2 mg lacZ and 0.2 mg Mx in the presence or absence of corresponding caspase inhibitors. The number of viable (lacZ positive) cells were counted at 24 h after transfection. Results Molecular Cloning and Charac terization of MX in C. quinquefasciatus mx was originally identified in Anopheles and Aedes mosquito genomes as the ortholog of Drosophila reaper using an integrated bioinformatics strategy and verified via functional assays (Zhou et al., 2005). A similar bioinformatics approach was applied to identify potential IAP antagonists in the C. pipiens genome. Using the sequence information, we were able to clone the mx ortholog ( mx_Cu.qu ) gene from a cDNA made from C. quinquefasciatus larvae. MX_Cu.qu is ~80% identical to its orthologs in A. aegypti ( or A. albopictus ( The three mx orthologs in the Culicinae tribe share considerable similarity beyond the IAP-binding motif (Figur e 2-1A). In contrast they share little similarity with the mx ortholog in A .gambiae except the IBM. Given the evolution history of these groups, we would expect a signifi cant difference between the subfamilies Anophelinae and Culicinae, which may also be partially reflected in the MX protein distance tree (Figure 2-1B). Like the mx orthologs in Aedes and Anopheles MX_Cu.qu induces rapid cell death when expressed in C3/36 cells (Figure 2-1C ). This pro-apoptotic activity is largely, if not totally, dependent on the N-terminal IBM. Removing the first three amino acids of this motif (2; AIA) abolis hes most of the killing ability of MX_Cu.qu (Figure 2-1C). 38


The pro-apoptotic activity of MX_Cu. qu and appear to be very similar when assayed in C6/36 cells. Cell killing induced by either MX_Cu.qu or is significantly suppressed by co-transfection of DIAP1, and to a less degree, by cotransfection of the viral inhibitor P35 (Fi gure 2-1D). DIAP1 is the major anti-apoptotic cellular protein in Drosophila and is highly conserved from insects to mammals. P35 is the viral caspase inhibitor that was original ly identified from l epidopteran baculovirus (Clem et al., 1991). The fact t hat MX_Cu.qu-induced cell death can be blocked by either DIAP1 or P35 indicates that the functiona l mechanism of MX_Cu.qu is the same as previously characterized for MX orthologs in Anopheles and Aedes and REAPER in Drosophila Cell death induced by expressing MX_Cu. qu or in C6/36 has typical apoptotic hallmarks, that is, nuclear c ondensation, fragment ation, and so on. Interestingly, C6/36 cells killed by expres sion of MX_Cu.qu or appear to be quickly phagocytosed by neighboring cells (Figure 2-1E). Since pIE-LacZ was cotransfected with pIE-MX_Cu.qu, cells ex pressing MX_Cu.qu or are also galactosidase ( -gal) positive. At 20 h after transfect ion, most of the remaining blue gal-positive cells in MX_Cu.qu-transfected samp les appear to be in the later stages of apoptosis. These -gal-positive cells are often rounded up and/or fr agmented. Small phagosomes with -gal staining can be clearly seen in cells surrounding the fragmented -gal-positive cell, indicati ng that dying or dead cells were quickly phagocytosed before the breakdown of the LacZ pr otein. A similar phenomenon was observed for REAPER and HID-induced cell death in Drosophila embryos where most dying cells were quickly phagocytosed by hemocyte/macrophages (Abrams et al., 1993; White et al., 1994; Zhou 39


et al., 1995). However, not all Drosophila cell lines have endocytotic capability (Abrams et al., 1992). The fact that C6 /36 cells are actively attracted to apoptotic cells and aggressively phagocytose fragmen ted apoptotic bodies suggests that this cell line, similar to the Drosophila S2 cell line, may have a mesoderm origin and display hemocyte/macrophage functionality. The Mx Gene Is Induced in Midgut Cells Infected by CuniNPV Using quantitative real-time PCR (Q-P CR) I first monitored the expression of mx following CuniNPV infection in pools of whole larvae. I found that in C. quinquefasciatus there was a lack of significant increase of mx_Cu.qu expression during the initial phase of infection, that is, before 8 h p.i. However, 48 h after the init iation of virus infection, the level of mx_Cu.qu reached significantly higher levels (Figure 2-2A). At this time, the level of mx_Cu.qu in infected larvae was about six times higher than the uninfected controls. In sharp contrast, Q-PCR analysis indicat ed that there was a quick response of induction in the first 2 h following ex posure to CuniNPV in the refractory A. aegypti larvae (Figure 2-2A). Significant induction of was found within 1 h of exposure to the virus (Data not shown). The elevated level of in virus challenged larvae was transient and retreated to background or previous infection level at about 8 h p.i. The dynamics of mx induction in both C. quinquifasciatus and A. aegypti were verified via fluorescent in situ hybridization (FISH) with cRNA probes to mx_Cu.qu and respectively. Consistent with the Q-PCR data, only timid induction of mx_Cu.qu could be detected in a few cells in t he gastric caeca at early stage of virus infection (2 h) in CuniNPV infected C. quinquifasciatus larvae (Figure 2b). However, the 40


mRNA level of mx_Cu.qu is very high at later stages of infection. At 48 h p.i., it appears that most cells in the gastric caeca and t he posterior midgut have very high levels of mx_Cu.qu (Figure 2-2B). These cells also disp lay necrotic nuclei morphology (described below). In contrast, there is a rapid induction of in the gastric caeca of A.aegypti larvae exposed to the same treatment of CuniNPV. The number of cells expressing as detected by FISH, reached a peak at about 2 h p.i. At 4 h p.i., the number of -positive cells had decreased signific antly. By 8 h p.i., there was no difference in FISH signal between virus-treated and control A. aegypti larvae (Figure 2-2C). Necrosis, Not Apoptosis, Is Associated wi th CuniNPV Infection in the Susceptible Mosquito C. quinquefasciatus Previous histological analysis has shown that many cells at the gastric caeca and posterior midgut of infected C. nigripalpus or C. quinquefasciatus larvae were heavily infected with CuniNPV occlusion bodies (OBs) and display signs of necrosis at the final stages of infection (Becnel et al., 2003; Mo ser et al., 2001). When TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) was applied to monitor possible apoptotic response to viral infection in C. quinquefasciatus larvae, there was no significant level of apoptosis at either early stage (2 h p.i.) or la te stages (24 and 48 h p.i.) of the infection (data not shown). FISH analysis with cRNA probes for CuniNPV genes allowed us to pinpoint virusinfected cells even before the final stages of infection. Counter stain with DAPI indicated that the nuclei of infected cells, that is, cells with CuniNPV gene expression, were greatly hypertrophied at 48 h p.i. (Figure 2-3A). These nucle i were swollen to 2 times 41


the diameter compared with corresponding cells in uninfe cted larvae. FISH analysis confirmed that before 8 h p. i., only a small group of cells at the gastric caeca and posterior midgut were infected (i.e., have vira l gene expression). However, by 48 h p.i., most cells at the posterior midgut and gastric caeca were positive for viral gene expression and had swollen nucle i (Moser et al., 2001). This indicates that the virus initially infects only a small gr oup of cells but later spreads horizontally to infect other cells, presumably via budded virus. Interestingly, while most of the cells in the posterior midgut were positive for viral gene expression, none of the cells at the anterior midgut was positive for viral gene expression. To confirm that virus-infected cells in C. quinquefasciatus larvae undergo necrotic instead of apoptotic cell death, I examine the membrane permeability of these infected cells. When live infected C. quinquefasciatus larvae were incubated for 10 min in normal culture media containing 1 mg/ml propidium i odide (PI), all of these swollen cells at posterior midgut were labeled with PI, w hereas none of the uninfected cells in the anterior midgut were labeled (Figure 2-3B). The swollen morphology and the compromised membrane integrity of these cells are characteristics of necrosis. Indeed, tissues of the infected specimens at 48 h p.i. were very fragile and easily disintegrated when touched or pulled duri ng dissection. Eventually, at about 60 h p.i., most mosquitoes died and tissues disint egrated into the culture media. Rapid Induction of Is Followed by Apoptosis in the Refractory Mosquito A. aegypti The dynamics of induction of mx was very different between the susceptible C.quinquefasciatus larvae and the refractory A. aegypti larvae. Cells expressing high level of could be detected within 1 h of infection in the gastric caeca and 42


reach a peak at 2 h p.i. TUNEL analysis indicated that this rapid induction of was followed by a wave of apoptosis in the mi dgut epithelial cells. TUNEL-positive cells could be observed in the gastric caeca at 2 h p. i. (Figure 2-4). This wave of apoptosis in CuniNPV-infected A. aegypti larvae appears to be relatively synchronized. The number of TUNEL-positive cells is the highest at about 2 h p.i. (Figure 2-4A). By 8 h p.i., few cells in the gastric caeca or posterior midgut can be found to be TUNEL positive. The quick apoptotic process following induction helps to explain the sharp decline of positive cells after 2 h p.i. Co localization analysis confirmed that TUNEL positive cells, especially those at early apoptotic stages before the onset of significant nuclei condensati on, are also positive for (Figure 2-4B). The timeline of the events indicates that cells undergo apoptosis shortly after the expression of This is not surprising given that in Drosophila embryogenesis, the expression of reaper in cells destined to die is followed immediately by apoptosis within approximately 1 h (White et al., 1994). Inhibition of Apoptosis Leads to Express ion of Viral Genes in the Refractory Mosquito A. aegypti The rapid induction of mx and apoptosis following CuniN PV infection in the larval midgut suggests that cells invaded by t he virus underwent apoptosis and were quickly eliminated from the midgut epithelium. Under normal conditions, I were not able to detect viral gene expression in A. aegypti larvae exposed to CuniNPV at any time point by either FISH or Q-PCR. This could potentially be attributed to at least three possibilities. First, maybe the virus could not enter the cell because of incompatibility of cell surface receptors. Second, virus that entered the cell c ould not replicate because of incompatibility of the cellular transcription system. Thir d, it could be that the quick 43


apoptotic response prevented viral gene expression from reaching a detectable level. To discern these possibilities, I tested w hether blocking or delay ing apoptosis following CuniNPV infection in A. aegypti larvae could have an impact on the infection process. The refractory A. aegypti larvae were infected with CuniNPV and split into two populations after the addition of virus and MgCl2. To one series of samples, pan caspase inhibitors z-VAD-FMK, or Q-VD-O PH, or a combination of both was added to the medium. As a control, the same amount of DMSO (the solvent for both z-VAD and Q-VD) was added to the parallel samples. This treatment of both z-VAD and/or Q-VD was able to suppress the cell deat h induced by transfection of mx in a tissue culturebased assay (Figure 2-5). From both the caspase inhibitor-treated and control populations, larvae were collected at the indicated time points and processed for Q-PCR and FISH analyses. Immediate early (ie) gene cun103 is one of the first genes expressed following CuniNPV infection of C. quinquefasciatus. Although it is not detected in A. aegypti larvae treated with DMSO, it was reliably detectable in ca spase inhibitor-treated samples (Figure 26A). The expression of cun103 was detectable via Q-PCR at 8, 24, and up to 30 h p.i. In most of the independent tr ials, the expression of cun103 diminished after 48 h p.i. Only in one out of five trials, I detected persistent cun103 expression in a 72 h p.i. sample. Correspondingly, structural (nucleocapsid) genes cun024 and cun035 were detectable in that sample, indicating that at least in one larva, late-stage viral expression was achieved by inhibiting/delaying apoptosis (Figure 2-7). FISH analysis confirmed that about 10% of larvae treated with caspase inhibitors have cells with detectable viral gene expression at 24 h p.i. (Figure 2-6B). The 44


viral gene signals were confined to the nuclei, similar to what we observed with earlystage infection of CuniNPV in the permissive C. quinquefasciatus (Figure 2-8). However, repeated trials with different caspas e inhibitors or combinations of caspase inhibitors indicated that the efficacy of ca spase inhibitor in transforming the resistant A. aegypti larvae was limited. I were never able to im prove the infection rate to more than 20%, nor could I reliably detect the expression of virus structural genes in every trial. Discussion A significant contribution to our knowledge about the role of apoptosis in virus host interaction came from studying l epidopteran baculoviruses such as AcMNPV ( Autographa californica multicapsid nucleopolyhedrovirus ) (Clem, 2007). Several antiapoptotic genes have been characterized in lepidopteran baculoviruses, including iap p35 and p49 (Clem and Miller, 1994; Zoog et al., 2002) While injecting wild-type virus to moth larvae usually leads to necrosis, injecting viruses with mutated p35 induced massive apoptosis (Clarke and Clem, 2003). Conseq uently, the infectivity of the mutant virus was much lower than that of the wild-ty pe virus. Most of t hese previous studies relied on directly injecting budded viruses into the hemocoel. However, in natural conditions, baculovirus infection of insects is initiated in the midgut epithelia by occluded viruses. Mine is the first evidence showing that infection by the occluded baculovirus induces pro-apoptotic response. In additi on, the correlation between the cellular outcome of CuniNPV-infected midgut epithelia l cells and the organism susceptibility to viral infection strongly suggests that apoptosis has a very important role in limiting viral infection. Apoptosis in the mosquito midgut epithel ia has long been observed accompanying the infection of a variety of pathogens (Han et al., 2000; Vaidyanathan and Scott, 2006). 45


However, we know little as to which regul atory pathway is responsible for mediating apoptosis against pathogen infection (Abr aham and Jacobs-Lorena, 2004). My study here indicated t hat the mosquito ortholog of reaper mx is activated following CuniNPV infection. This, to our knowledge, is the first evidence suggesting that reaper -like genes, known for their pivotal role in regulati ng programmed cell death during development, are also involved in the innate immune response against viral infection. In Drosophila reaper -like genes ( hid, grim, and sickle ) are required for most developmental cell death (Steller, 2008). The reaper grim and sickle are exclusively expressed in cells destined to die during devel opment or following cytot oxic stimuli such as irradiation. Transcr iptional regulation of mx also appears to be tightly regulated in the mosquito larval midgut. In the absence of viral infection, only a few cells have detectable levels of mx expression and presumably reflec ts the routine turnover of midgut epithelial cells. The induction of mx in CuniNPV infected C. quinquefasciatus larval midgut is limited to virus infected ti ssues, that is, gastric caeca and posterior midgut. As we have reported previously, like reaper mx is also transcriptionally responsive to UV irradiation (Zhang et al ., 2008). Thus it appears that although the protein sequence of MX has diverged signific antly from that of REAPER, their regulation at the transcriptional leve l share much similarity. My study strongly indicate s that the induction of mx is very likely responsible for the pro-apoptotic response following CuniNPV infection. Howe ver, direct assessment of this hypothesis was hindered by the lack of te chnical means to specifically block the surge of expression following viral infecti on. There has not been any reported success with RNAi in Aedes larvae. My attempts with d ouble-stranded RNA, using a 46


strategy that worked in A. gambiae larvae (Zhang et al., 2010), failed to produce any damping effect on the induction of following CuniNPV infection. Besides potential problems associated with the efficacy of RNAi in Aedes larvae, it is also questionable whether the RNAi mechanism, which can bring down the level of constitutively expressed genes, can be effect ive in suppressing the quick induction of associated with viral infect ion. It is possible that because the induction is so robust and followed quickly by apoptosis, RNAi mechanism could not function fast enough to have a significant, if any, im pact on the rapid increase of It might be necessary to first identify the enhanc er(s) mediating viral infection-induced and applying this to genetically en gineered mosquito to truly test the hypothesis that the induction of is responsible for the apoptosis of midgut cells following CuniNPV infection. A Race to Apoptosis My study highlights the importance of t he timing of pro-apoptot ic response on the cellular and organismal outcome In the refractory host A. aegypti the induction of and apoptosis happened so fast that viral gene expression was never detectable in the process. In contrast, in the susceptible host C. quinquefasciatus, the delayed and timid induction of mx_Cu.qu did not reach to detectable level until viral genes were detectable (Figure 2-9). The data presented here do not exclusively suggest that the quick induction of apoptosis is responsible for the refractory phenotype, as there are many other differences between the two species. However, the fact that viral gene can be detected in A. aegypti when apoptosis is delayed/suppressed suggests that apoptosis is responsible for preventing viral gene expression. 47


The extremely high level of mx_Cu.qu in CuniNPV-infected midgut cells in C. quinquefasciatus indicates that the virus has a very efficient way to block mx -induced apoptosis. In the tissue culture system, mx -induced cell death can be effectively blocked by co-expression of iap However, there was no cl ear ortholog of either iap or p49 in CuniNPV. A predicted viral pr otein, CUN75, has marginal si milarity to AcMNPV P35, but lacks a conserved reactive site loop that is required for caspase-inhibition activity (dela Cruz et al., 2001). We did not find any anti-apoptotic effect of CUN75 when cotransfected with MX_Cu.qu or As the only Dipteran baculovirus with genomic information, CuniNPV is distantly related to lepidopteran baculoviruses and lacks identifiable orthologs to many important genes such as the ie genes (Afonso et al., 2001). It is very likely that CuniNPV utilizes a yet unknown, but powerful, mechanism to block the apoptotic pathway downstream of mx activation. Apoptosis and MosquitoHost Compatibility Correlation between apoptosis and resistant phenotype has been found with refractory Culex mosquito exposed to the West Nile virus (Vaidyanathan and Scott, 2006). The comparison of the CuniNPV infection in the A. aegypti and C. quinquefasciatus also suggests that apoptosis has an important role in eliminating CuniNPV infected midgut cells and conv ey resistance to viral infection. Partially blocking apoptosis by feeding the refractory A. aegypti larvae with caspase inhibitors allowed reliable detection of several immediate early viral genes. This indicates that the virus was able to enter into the nuclei of the midgut cells and successfully initiate gene expression when a poptosis was blocked or delayed. The fact that the application of caspase inhibitor alone was not reliable in allowing expression of structural genes may be because of several po ssibilities. First, there may be additional 48


antiviral responses that can block the developm ental program of the vi rus. However, it is also possible that while caspase inhibitor parti ally blocks the activity of MX-activated caspases, it did not block the other antiv irus mechanisms elicited by MX. It has been found that REAPER can nonspecifi cally inhibit protein synthesis (Holley et al., 2002), an activity that is independent of caspase activa tion. It remains to be seen as to whether MX also has this activity and can inhibit vira l protein synthesis. It is notable that in the tissue culture system, while co-expressing I AP was very effective in blocking MXinduced cell death (Figure 2-1), application of very high level of caspase inhibitor only partially blocked/delayed cell death (Figure 2-5). In summary, my study found that mx the mosquito ortholog of the pro-apoptotic gene reaper is transcriptionally activated in ce lls infected by CuniNPV and that the cellular outcome correlates with organism resist ance/susceptibility. This should pave the way for more mechanistic studies addressing the role of apoptosis in determining host susceptibility or vector compatibility for human pathogens. For instance, as controlled expression of has been successfully used as an effecter for sterile insect approaches (Fu et al., 2010), it will be very interesting to see whether adding viral responsiveness to such a system could also make the vector more resistant to viral infection. 49


Figure 2-1. Gene cloning and characterization of mx_Cu.qu (A) Alignment of protein sequences of MX orthologs in mosquitoes and REAPER from D. melanogaster The blue triangles denote the relative position of the intron in MX sequences (there is no intron in R eaper). (B) Distance tree of these orthologs. (C) Expression of MX_Cu. qu kills C6/36 cells. However, when amino acids 2 are removed ( IBM), the protein has little, if any, proapoptotic activity. (D) MX_Cu.qu-induc ed cell death is almost completely blocked by co-transfection of DIAP1 and significantly inhibited by cotransfection of P35. Numbers in parent heses denote the ratio of respective genes. (E) C6/36 cells killed by expre ssion of MX_Cu.qu or show typical apoptotic morphology incl uding cell fragmentat ion (arrow) and phagocytosis by neighboring cells (arrow head). Cells co-transfected with pIELacZ and pIE-MX_Cu.qu were fixed 20 h later and processed for X-Gal staining. 50


Figure 2-2. Induction of mx_Cu.qu and following CuniNPV infection. (A). The level of mx_Cu.qu and in pooled larvae following CuniNPV infection. Expression level was firs t normalized with house-keeping genes before calculating the fold induction. Da ta are presented as meanS.D. of at least three independent experiments. (*P-value< 0.05 and ***P-value<0.001.) (B) and (C). Expression of mx_Cu.qu and in the midguts of CuniNPV-infected or contro l (Ctrl) larvae was monitored with FISH. Photos are the representatives of three independent experiments. 51


Figure 2-3. Necrosis in CuniN PV-infected C. quinquefasciatus larvae. (A) Cells infected with CuniNPV at 48 h p.i. were identif ied with a pool of fluorescein-labeled cRNA probes against CuniNPV genes, cun24, cun75, and cun85. Virusinfected cells at this time demons trated hypertrophied nuclei (arrow) compared with uninfected cells (arrow head). Note that there is clear separation between cells that were positive for viral gene expression versus those that were negative (dashed line). More than 100 midguts were examined in several independent experim ents; all cells positive for viral gene expression at this time point have hypertrophied nuclei. (B) Cells with hypertrophied nuclei have compromised cell membrane integrity. Live larvae at 48 h p.i. were exposed to 1 mg/ml PI in culture media for 10 min before the midguts were dissected out, fixed with paraformaldehyde, and counterstained using DAPI. All the cells with hypertrophied nuclei (arrow) are also permeable to PI, indicating necrotic cell death. In contrast, cells in the anterior midgut (arrow head) have normal nuclei and intact membrane. More than 40 midguts were observed in three independent experiments, and essentially all of them showed the described associati on between nuclei morphology and permeability to PI. 52


Figure 2-4. Rapid apoptosis following the induction in A. aegypti larvae exposed to CuniNPV. (A) TUNEL assay in A. aegypti midgut. At 2 h after CuniNPV infection (2 h p.i.), signific ant increase of TUNEL-positive cells can be detected in over 80% of the examined midguts. In contrast, few TUNEL-positive cells were detectable in the control animal (C trl) or the midgut of infected animal at late time points (24 h p.i.). (B) Co localization analysis indicated that TUNEL-positive cells are the ones that express Early-stage TUNEL-positive cells (open a rrow), indicated by relatively normal nuclei morphology and intact cytoplasm, have high levels of FISH signal for The signal is not visible in later stage TUNEL-positive cells with significantly condensed nuclei (solid arrow), which is likely because of the shrinkage of cytoplasm and/or degradation of macromolecules at this stage. Some mx -expressing cells are not TUNE L positive, which is likely because of the lag between mx expression and the activation of caspasedependent DNase. 53


Figure 2-5. induced cell death could be partially rescued by caspase inhibitor z-VAD-fmk. Cultured C6/36 cell was transfe cted with Vector or Mx in the absence or presence of z-VAD. 100uM z-VAD partially suppressed MXinduced cell death. 54


Figure 2-6. Suppressing apoptosis in A. aegypti leads to expressions of viral genes. (A) Suppressing apoptosis allowed the expressi on of ie viral genes. None of the three ie CuniNPV genes, cun16, cun86, and cun103 could be detectable by Q-PCR in A. aegypti larvae exposed to CuniNPV and treated with DMSO only. In contrast, all the three genes could be detected in A. aegypti larvae treated with capsase inhibitors z-VAD -FMK (z-VAD). Bottom panels are gel pictures of the Q-PCR pr oducts. (B) FISH was performed with a pool of cRNA probes against CuniNPV genes, cun16, cun86 and cun103 Top panel, no viral gene could be detected in the midgut of any of the DMSO-treated larvae (over 100 examined) at 24 h after CuniNP V infection. In contrast, viral gene expression was detectable in ~20% of the midguts dissected from caspase inhibitor-treated larvae. 55


Figure 2-7. Suppressing apoptosis with cas pase inhibitor Q-VD and combination of zVAD and Q-VD allowed expression of viral genes in the refractory A.aegypti larvae. (A) Detection of cun103 in virus exposed A. aegypti larvae treated with 50uM Q-VD but not in the larvae samp les treated only with DMSO. (B-D) At one of the trials, a combined treatment of Q-VD and z-VAD led to the detection of capsid genes cun24 cun35 and occluded body gene cun85 at 48 hr p.i. The specificity of QPCR analysis (B) was verified by gel electrophoresis (C) and sequencing of the DNA fr agments. Interestingly, in this trial, the level of at 48hr p.i. is significantly hi gher than that of the DMSO-treated larvae (D). This could be either due to more cells were infected through secondary infection at this late time poi nt, or, similar to what we observed in C. quinquefasciatus, due to the accumulation of message in infected cells that failed to undergo apoptosis. 56


Figure 2-8. Viral gene expression was confined to the nuclei of a few cells at early stage of CuniNPV infection in Culex quinquefasciatus FISH was performed with a pool of cRNA probes against viral genes cun16, cun86 and cun103 in the midgut of the susceptible C. quinquefasciatus larvae at 2-4 hr post infection. Bottom panel, higher magnification of insets in top panels. 57


58 Figure 2-9. The race to apoptosis through the mosquito reaper This is a schematic presentation of our findings On CuniNPV infection, there is a competing race between the host cell to expr ess cellular pro-apoptotic gene(s) to eliminate the infected cell and the viru s to express early genes to block apoptosis and initiate proliferation. A prompt induction of mx and apoptosis leads to elimination of the infected ce ll before the viral genes are detectable in the refractory mosquito A. aegypti A delay in this process, either due to insufficient pro-apoptotic response or inte rference of caspase inhibitors, could subjugate the cell under viral control and render the organism susceptible to the virus.


CHAPTER 3 IDENTIFICATION OF THE ANTI-APOPTOTIC GENE(S) IN CUNINPV GENOME Summary CuniNPV ( Culex nigripalpus nucleopolyhedrovirus) is a kind of baculovirus which can specifically infect Culex mosquitoes. My previous studi es suggested that similar to other baculoviruses, CuniNPV is also capable of inhibiting host cell apoptosis. However, sequence analysis did not find any ortholog s of known anti-apoptotic genes from the CuniNPV genome, suggesting that CuniNPV carries novel anti-apoptotic gene(s). Identification of the antiapoptotic gene(s) in the C uniNPV genome can enrich our knowledge about apoptosis regul ation. To identify the anti-apoptotic gene(s) in CuniNPV, I extracted intact CuniNPV genom e and partially digested it with restriction enzymes to get genomic fragments. A Cuni NPV genomic DNA library was subsequently constructed by cloning those genomic fragments into a plasmid vector. Once the library is established, functional tests will be done in cultured cells to identify which genomic fragment possesses the ant i-apoptotic activity. After the initial round of screening, the genomic range will be further narrowed down until the ant i-apoptotic gene(s) is(are) localized. So far, I have finished ~90% of the library construction work. Introduction Baculoviruses are a family of ro d-shaped viruses with double stranded DNA genome (dsDNA). They have a very restri cted host range, i.e., they only infect arthropods and are non-pathogenic to humans (Clem, 2007). T herefore, they have been widely used in biological control of pest insects. Moreover, the baculovirus expression system is powerful for foreign gene expr ession and is being widely used nowadays to produce foreign gene products. Given the fact that baculoviruses can enter almost any 59


kind of cell but only replicate in certain inse ct cells, they have been studied to serve as vectors for human gene ther apy (Clem, 2001). In addition to those benefits mentioned above, another important discovery derived from baculovirus study is their ab ility to suppress host cell apoptosis. Several well-known anti-apoptotic genes have been or iginally identified from various baculoviruses (relative information can al so be found in Chapter 1). For example, p35 which was identified from baculovirus AcMNPV, can serve as an inhibitor of activated effecter caspases (Bump et al., 1995; Clem et al., 1991; X ue and Horvitz, 1995). It is the only known gene that can inhibit apoptosis in all three major model organisms, the nematode, the fly and the mouse. Later on, the crystal struct ure of P35 revealed a loop which contains a caspase cleavage site. Af ter being recognized by activated caspase, P35 and caspase can form a stable complex to sequester the caspase activity (Fisher et al., 1999). Below the loop is an -helix followed by a -sheet. By disrupting the -helix structure, the anti-apoptotic activity of P35 can be abolished (Zoog et al., 1999). So far, no P35 homologs have been repor ted from cells. However, beside AcMNPV, other baculoviruses have been found to carry p35 -homologous genes such as BmNPV ( Bombyx mori NPV) and SlNPV ( Spodoptora littoralis NPV). BmNPV P35 is ~90% homologous to AcMNPV P35 and can suppress BmNPV-induced apoptosis in Bombyx mori cells (Kamita et al., 1993). SlNPV P35 which is called P49 is more divergent. The sequence of P49 is ~49% si milar to AcMNPV P35 (Du et al., 1999). In addition to the p35 another anti-apoptotic gene, iap was originally identified from baculovirus CpGV ( Cydia pomonella granulovirus) using a genetic screen (Crook et al., 1993). To date, iap -homologous genes have been ident ified from more than 10 60


different baculoviruses. Other than this, many cellular iap -homologous genes have been found in multiple organisms. The functi onal principle of IAP has been discussed previously in Chapter 1. So far, most of the baculovirus st udies that have been done are focused on lepidopteron baculovirus. CuniNPV, which wa s identified in 2001, is the only sequenced Dipteran baculovirus (Afonso et al., 2001; Bec nel et al., 2001). CuniNPV replicates in the nuclei of midgut epithelial cells in the gastric caeca and posterior midgut. Similarly to most of the baculoviruses, the CuniNPV lifecycle also involves two forms of virus: ODV (occlusion-derived virion) and BV (budded virion) ODV is encapsulated in the occlusion bodies that are composed of the proteins polyhedrin or granulin. Upon ingestion by the insects, the occlusion bodies dissolve under the alkaline condition s of the midgut. The ODV is released starting the init ial infection. After replication in infected cells, a large number of viruses escape by budding, thereby initiating the secondary infection. The CuniNPV genome is a circular, doubl e-stranded DNA molecule of 108252 bp containing 252 ORFs, of which 109 are likel y to encode proteins. Only 36 of the 109 putative genes show homology to genes from other baculoviruses (Afonso et al., 2001). The 109 putative genes encode proteins that have multiple functions such as DNA polymerase, RNA polymerase, tran scriptional factors, structural proteins etc. Moreover, CuniNPV genome contains a gene, cun075 which shows homology to anti-apoptotic gene p35 except that CUN075 lacks the 110-amino-acid C-terminal region of AcMNPV P35. Moreover, no homologs of iap has been found in CuniNPV genome. My previous study indicated that in CuniNPV-infected Culex mosquito, at the late stage of infection, the cells become necro sis (Figure 2-3) rather than apoptosis, 61


although at this point the level of pro-apoptotic gene, mx was very high (Figure 2-2). It has been shown that if the apoptotic machi nery has been initiated by certain stimuli such as TNF (Tumor Necrosis Factor) tr eatment but get inhibi ted from downstream using caspase inhibitor, t he cells eventually undergo necr osis (Nicotera and Melino, 2004). This strongly suggests that CuniN PV is capable of inhibiting apoptosis downstream of mx induction. As mentioned, cun075 shows certain level of homology to p35 however, functional test by us did not find any anti-apoptotic activity of cun075, which suggests that CuniNPV inhibits apoptosis using a novel gene(s). In this study, I constructed the genomic library of CuniNPV through partial digestion by restriction enzymes and molecular cloning. The following functional test will be performed to identify the novel anti-apoptotic gene(s) carri ed in CuniNPV genome. Identification of the novel anti-apoptotic gene( s) will increase our underst anding of the processes involved in apoptosis regulation. Materials and Methods Cell Culture and Transfection The mosquito Aedes albopictus cell line C6/36 was maintained with MEM culture medium (Cellgro, Manassas, VA, USA) supplied with 10% FBS ( F etal B ovine S erum) (Sigma, St. Louis, MO, USA), 1% penicillin /streptomycin (Sigma) and 1% nonessential amino acid (Cellgro) at 28oC with 5% CO2. The Drosophila cell line S2 was maintained with Schneiders medium (S igma) with 10% FBS, 1% peni cillin/streptomycin and 1% nonessential amino acid at 25oC without CO2. Transfection of C6/36 and S2 was done with Insect GeneJuice trans fection reagent (EMD Millpore, Billerica, MA, USA) and Cellfectin (Invitrogen) respectively following the manual provided by the manufacturer. 62


Extraction of CuniNPV Genomic DNA CuniNPV genomic DNA was extr acted from occlusion bodi es following a protocol adapted from a previously reported method (Bruce et al., 1991; Wilson, 2001). Generally speaking, CuniN PV occlusion bodies (~2.512) were dissolved in 60ul of Buffer 1 (0.1M NaOH+10% SDS v/v=5:1) by in cubating at room temp erature for 8 min. Then the alkaline condition was neutralized with 50ul of 1M Tris (PH 8.0). After treatment with 6ul of 5mg/ml protease K (fi nal conc. 100ug/ml) at 37oC for 1 hr, 16ul of Sarkosyl was added (final conc. 2.5mg/ml) and kept incubating at 37oC for 1.5 hr. 45.6ul of 5M NaCl was then added to a final concentration of 0.5M and followed by addition of 0.15 volumes of CTAB (Hexadecyl trimethylammonium bromide)/NaCl (~53.6ul). After incubating at 65oC for 1 hr, the suspension wa s extracted once with 412ul of chloroform/isoamyl-alcohol (24:1) and once with an equal volume of phenol/chloroform/isoamyl-alc ohol (25:24:1). The DNA was precipitated by adding 0.6 volumes of isopropanol and centrifugation at 15000g for 10 min. The pellet was washed with 70% ethanol and resuspended in a small volume of TE buffer (0.01M Tris, 0.001M EDTA, PH 8.0). Construction of CuniNPV Genomic Library To construct the CuniNPV genomic lib rary, the purified CuniNPV genomic DNA was digested with restriction enzymes. To obtain large genomic fragments, partial enzyme digestion was performed instead of co mplete digestion. To ensure a certain level of overlapping among different fr agments, the extracted genomic DNA was partially digested with several different restriction enzymes. Specifically, CuniNPV genomic DNA was digested with 0.5u BamH1 or Kpn1 for 10 min at 37oC. The digestion products were separated by electrophoresis on 0.7% agarose gel. Another option of 63


partial digestion is using high unit of re striction enzyme combined with short digestion time (e.g. 10u restricti on enzymes for 20 sec at 37oC). The fragments larger than 5 kb were recovered from the gel with QIAEXII Ge l Extraction Kit (QIAGEN, Valencia, CA, USA) following the manual provided by t he manufacturer. The recovered fragments were then ligated into the pBlueScript pl asmid vector and transformed into DH5 competent E.Coli cells (Invitrogen, Grand Island, NY, USA). The bacterial colonies were grown on the LB (Lysogeny Broth) agar plate containing Ampicillin (100ug/ml) and x-gal (5-bromo-4-chloro-indolyl-D-galactopyranoside) in a 37oC incubator. White colonies were picked and the contained insertions we re verified by sequencing. The fragments were aligned to the Cu niNPV genome with BLAST ( B asic L ocal A lignment S earch T ool) provided by NCBI ( N ational C enter for B iotechnology I nformation). Quantitative Real-Time PCR (Q-PCR) Plasmids were transfected in to the S2 cells and 24 hr later, the cells were lysed and total RNA was extracted with RNeasy Mi ni Kit (QIAGEN, Valencia, CA, USA) according to the protocol provided by the manufacturer. RNA samples were treated with DNase I to remove genomic DNA. cDNA was prepared by reverse transcription of total RNA with a High-Capacity cDNA Archive Ki t (Applied Biosystems, Foster City, CA, USA). Q-PCR was performed with an ABI 7500 Fast thermocycler (Applied Biosystems) following protocols provided by the manufactu rer. Triplicates were measured for each gene/sample combination. Cloning Full Length Mx_Culex Full length mx_Culex was obtained by PCR using Culex quinquefasciatus genomic DNA as the template. The primers used for amplifying mx are 5GGCAAGCAGTCGTGTTTGTA-3 and 5-GAG CAGGTACTGGCTGGTTC-3. PCR 64


product was sub-cloned into pM T vector which contains t he copper-inducible promoter to achieve controllable expression of mx In Vitro Cell Death Assay Cell death assay will be performed essentia lly as previously described (Jones et al., 2000). For each test, a total of 1.0 ug of DNA mixed with 8ug of liposome Cellfectin in serum free media was distributed into 2 wells in a 24-well plate. This included 0.1 ug pIE-lacZ and 0.9 ug of the test DNA sample or a comb ination of samples in the pIE vector. Intact pIE vector was used as the co ntrol. The transfection lasted for 4 hours and was stopped by replacing t he transfection mix with cult ure media supplemented with 10% FBS. At 20 hours post-transfection, ce lls were fixed and stained with x-Gal/IPTG. Blue cells were counted to calculate the percentage of cell survival. Results Extraction of High Qua lity CuniNPV Genomic DNA To construct the genomic library of CuniNP V, I first extracted intact genomic DNA from occlusion bodies. On the basis of so me previously published methods (Bruce et al., 1991; Wilson, 2001), I devel oped a protocol to extract CuniNPV genome. Figure 3-1 is the agarose gel picture of purified C uniNPV genomic DNA which indicated a clear band of the CuniNPV genome. Th is demonstrates that our newly developed protocol worked well to extract CuniNPV genomic DNA. Construction of CuniNPV Genomic Library By using restriction enzyme partial di gestion strategy, I have been trying to construct the CuniNPV genomic library which can cover the ent ire CuniNPV genome. The CuniNPV genomic fragments were cloned into the pBlueScript plasmid vector which allows me to do bluewhite screening for the colonies containing insertions. 65


Moreover, the T3/T7 sites at pBlueScript c an facilitate the sequencing of the insertions. The sequenced genomic fragments were mapped to the CuniNPV genome using BLAST. Figure 3-2 lists all current ava ilable fragments. The red backbone represents the linearized genomic map of CuniNPV and the black lines above indicate the cloned and sequenced fragments. So far, I have co vered ~90% of the entire CuniNPV genome and the nearby fragments have certai n level of overlapping. The Gene Contained in the Plasmid Can Be Successfully Expressed Since pBlueScript vector has no promoter in it, the genes inserted were presumably driven by their ow n promoters. To ensure t hat the inserted genes can be expressed successfully, I randomly picked several plasmids and transfected into Drosophila S2 cells. RNA was then extracted and gene expression was examined by QPCR. The plasmids that I transfected we re #5 and #10 (Figure 3-2). The corresponding genes that I examined were cun026 for #5 and cun068 cun071 for #10. Q-PCR results indicated that in #5-tran sfected cells, viral gene cun026 was easily detected. Under threshold of 1, the Ct va lue is ~30. In contrast, cun026 was undetectable in #10transfected cells, which prov ed the specificity of the cun026 signal (Figure 3-3A). Likewise, cun068 and cun071 were easily detected in #10-transfected cells but not #5transfected cells (Figure 3-3B & C). Those signals cannot be due to the contamination of the transfected plasmid DNA because I cannot detect any signal for any gene from the RNA of #5 or #10-transfected S2 cells (d ata not shown). Taken together, these data indicates that the genes in the plasmid can be successfully expressed when transfected into the cultured cells. 66


Discussion Baculoviruses have made great contribut ions to humans. There are several benefits that came from baculoviruses studi es. First, baculoviruses have been widely used to express foreign proteins in eukaryotic cells. Second, baculoviruses are useful to control certain insect pests. Last but not l east, baculoviruses have been used to study virus-host interactions and to aid in the identification of several anti-apoptotic genes. This has largely increased our understanding of the pathways regulating apoptosis. CuniNPV is a the first baculovirus isolated from mos quitoes. It is the only sequenced dipteran baculovirus so far. My pr evious studies suggest that similar to lepidopteran baculoviruses, CuniNPV is also capable of inhibiting host cell apoptosis. However, only cun075 shows a certain level of homology to AcMNPV p35 ; functional tests found that cun075 had no anti-apoptotic activity at all. This suggests that CuniNPV carries novel anti-apoptotic genes to inhibit host cell apoptosis. The aim of this chapter is to identify and functionally characteri ze the anti-apoptotic gene(s) in CuniNPV genome. The strategy is to construct t he CuniNPV genomic library and screen for the fragments that possess ant i-apoptotic activity. To date, I have covered ~90% of the ent ire CuniNPV genome (Figure 3-2). Once the library is established, we will screen for the fragments posse ssing anti-apoptotic activity by in vitro cell death assay. To include the possibility that CuniNPV uses microRNA to inhibit mx -induced apoptosis, I will use full length mx which contains 5UTR (Untranslated Region) and 3-UTR besides the open reading frame in the in vitro cell death assay because 3-UTR is usua lly the target region of microRNA. The plasmid vector that I used to c onstruct the CuniNPV genomic library is pBlueScript. The benefit of th is vector is it allows doing blue-white screening which 67


saves much of the effort in isolating colo nies that contain insertions. However, one potential drawback of using this vector is that pBlueScript is not an expressing vector, i.e. there is no promoter in the vector so the expression of the inserted genes cannot be ensured. Therefore, the successful expressi ons of inserted viral genes need to rely on their own promoters. Q-PCR result indicated that indeed, the inserted genes can be expressed after transfection of the plasmids into the cells (F igure 3-3). So far, I have only test two genes. After finishing cons truction the genomic library, it might be necessary to examine the expressions of all 109 putative genes using Q-PCR to ensure their expression. If certain genes failed to be expressed, I will clone those fragments to insect cell expressing vectors such as pIE. 68


Figure 3-1. Gel picture of extracted CuniNPV genomic DNA. The CuniNPV genomic DNA band is indicated by the white arrow. 69


Figure 3-2. Currently sequenced CuniNP V fragments. The red backbone is the linearized genomic map of CuniNPV and the red boxes are the 109 of putative genes. The black lines above indicate the fragments that have been cloned into the plasmid vector and sequenc ed. The identifier of each plasmid is labeled in front of t he fragments (picture adapted fr om (Afonso et al., 2001)) #4 #3 #1 #5 #2 #9 #8 #7 #6 #11 #10 #13 #14 #15 #12 70


71 Figure 3-3. Amplification pl ots of several target genes, cun026 (A), cun068 (B), cun071 (C) and Drosophila housekeeping gene rp49 (D). The red line in each panel represents the thre shold of 1.


CHAPTER 4 P53-MEDIATED RAPID INDUCTION OF APO PTOSIS CONVEYS RESISTANCE TO VIRAL INFECTION IN INSECTS Summary Arthropod-borne pathogens account fo r millions of death each year. Understanding the genetic basis controllin g vector susceptibility to pathogens has profound implication for developing novel stra tegies for controlling insect transmitted infectious diseases. The fact that many viral genes have an ti-apoptotic activity has long led to the hypothesis that induction of apopt osis could be a fundamental innate immune response. However, the cellular mechani sms mediating the induction of apoptosis following viral infection remained enigmatic which prevented experimental verification of the functional significance of apoptosis in limiting/preventing viral infection in insects. In addition, studies with cultured insect cells showed that either there is a lack of apoptosis, or the pro-apoptotic response happens relatively late, thus casting doubt on the functional significance of apoptos is as an innate immunity. Using in vivo mosquito models mimicking the native route of infection, I found that there is a rapid induction of reaper -like pro-apoptotic genes within a few hours following exposure to DNA/RNA viruses. Recapitulating similar response in Drosophila I found that this rapid induction of apoptosis requires the function of P 53 and is mediated by a stress responsive regulatory region upstream of reaper More importantly, I showed that the rapid induction of apoptosis is responsible fo r denying the expression of viral genes and blocking/limiting the infection. Genetic c hanges influencing this rapid induction of reaper -like pro-apoptotic genes lead to significant differences in susceptibility to viral infection. 72


Introduction As a genetically regulated mechanism of cell elimination, apoptosis plays an important role in maintaining tissue homeostasis through the removal of obsolete or potentially dangerous cells. The c ontrolled collapse of intracellular infrastructures and encapsulation of cell bodies associated wit h apoptotic cell death has long led to the speculation that apoptosis could function as an efficient innate immune mechanism against intracellular pathogens such as vi rus (Clouston and Ke rr, 1985; Everett and McFadden, 1999; Hardwick, 1998). The majority of evidences supporting the role of apoptosis as an important antiviral immune response came from the study of viruses. Many viruses encode genes that can interfere with the regulation of apoptosis at various levels (Benedict et al., 2002). For example, the pivo tal upstream regulator p53 is a frequent target of viral inhibition. It can be sequestered by the SV (Simian virus) 40 T antigen or degraded by proteins encoded by Adenovirus or human papillomavirus es. In addition, it was found recently that adenovirus E4orf3 can block P53-i nduced gene expression by promoting de novo heterochromatin formation at P53-target ed promoters (Soria et al., 2010). Besides blocking the sensors / upstream r egulators, viral proteins can also directly interfere with the apoptotic machinery. For instance, many viruses (including adenovirus, Epstein-Barr virus, Kaposis sarcoma-associated -herpesvirus, and mouse -herpesvirus, etc.) encode functional homologs of the anti-apoptoti c regulator BCL-2, which can directly inhibit the intrinsic apoptotic pathway. Si milarly, key components of the extrinsic pathway are targeted by viruses such as Shope fibroma virus, myxoma virus, and smallpox virus, etc. (Best, 2008). Last but not least, some vi ruses, particularly insect baculoviruses, encode caspase inhibitors. P 35 and IAP (Inhibitor of Apoptosis) were 73


initially identified in lepidopteran baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) and Cydia pomonella granulosis virus (CpGV), respectively (Clem et al., 1991; Crook et al., 1993). It has been very well demonstrated that these two genes are required for the infect ivity of baculoviruses (Clem, 2005). All of these evidences strongly suggest that evading/delaying apoptosis is an important mechanism for viruses to succeed in es tablishing prolifer ative infection. Despite of these evidences from virology studies, the functional role of apoptosis in mediating insect immunity has been under debate. Since insects do not have adaptive immunity, induction of apoptosis could conceivably play an even more prominent role in antiviral defense than in mammalian or vertebrate hosts. Indeed, it has been documented that ingestion of blood containi ng West Nile virus induces apoptosis in the midgut of a refractory Culex pipiens strain (Vaidyanathan and Scott, 2006). In contrast, necrosis has been associated wit h Western Equine Encephalomyelitis virus infection in susceptible Culex tarsalis strains (Weaver et al., 1992). Although these observations suggested that apoptosis might play an important role in insect antiviral defense, we know little about what mechanism is involved in the induction of apoptosis following viral infection. Consequently, there is a conspicuous lack of direct evidence supporting the role of apoptosis in insect inna te immunity. In the m eantime, a series of studies conducted in cultured insect cells reported that apoptos is was either not observed (Blitvich et al., 2002; Borucki et al ., 2002), or as is the case for AcMNPV or flock house virus (FHV), only observed relatively late in the infection cycle (i.e. at or after 24 hrs p.i.) (Settles and Fries en, 2008; Vandergaast et al., 2011). More importantly, blocking apoptosis in these infection systems seem s to have little effect on 74


the infection and proliferation of the viruse s. These observations raised the question as whether apoptosis is an innate immune response that can prevent/limit the infection, or it is simply one of the cellular outcomes a ssociated with late stage viral infection. Genetic studies in Drosophila revealed that the four IAP-antagonist genes, reaper hid grim and sickle (also referred to as the RHG genes) together play a pivotal role in mediating developmental cell death (Steller, 2008). With t he exception of HID, whose pro-apopotic activity can be suppressed by the MAP Kinase pathway (Bergmann et al., 1998a), RHG genes are mostly regulated at t he transcription level and are selectively expressed in cells destined to die during ani mal development. Transcriptional activation of the RHG genes is also responsible for me diating the induction of apoptosis following cytotoxic stimuli such as i rradiation. Interestingly, t he sequences of the RHG genes diverged very fast during evolution. Cons equently, no RHG ortholog was identified by the annotation of the genome of Anopheles gambiae. The first RHG gene in mosquitoes, michelob_x (mx ), was identified with an adv anced bioinformatics approach and verified as a bona fide IAP-antagonist (Zhou et al., 2005). Although the sequence of mx has diverged so much from that of reaper to the extent that it is almost beyond recognition, its transcriptional regulation was surprisingly similar in that it is induced rapidly following UV irradiation. Using a baculovirus-mosquito system mimi cking the native rout e of infection, I found that mx is rapidly induced in virus-infected larval midgut cells in a refractory species (Liu et al., 2011). This rapid induction of mx in virus infected cells was followed by quick apoptotic cell death and e limination of the in fected cells. In contrast, this rapid induction of apoptosis is absent in a species that is susceptible to this virus (Liu et al., 75


2011) (refer to Chapter 2 for details). Interestingly, the rapid induction of mx following blood meal containing Dengue virus seroty pe 2 (DEN-2) was also observed in a refractory Aedes mosquito strain but not in a susceptible strain. While these observations strongly suggested that the r apid induction of pro-apoptotic response could be responsible for the resistance phen otype, the lack of genetic tools prevented mechanistic test of this hypothesis. In this study, by using two in vivo virus infection systems in Drosophila melanogester I demonstrated that upon viral injection, several RHG genes are quickly induced at 1-2hr post infection. The inducti on of the RHG genes requires the function of dP53 and is mediated by a highly conserved r egulatory region in the vicinity of the reaper gene. More importantly, I showed that, in live animals, the rapid induction of apoptosis is an important innate immune res ponse that is capable of blocking/limiting viral gene expression and infection. Materials and Methods Drosophila Strains Drosophila white 1118 (w1118) strain was used as a standard wild-type strain. p53 deficient line p53[5A-1-4] which has a 3.3k deletion in p53 gene (Rong et al., 2002) was obtained from the Bloomington Stock Center (Indiana University, Bloomington, IN, USA). The IRER deficient strain B11 was generated by us as previously reported (Zhang et al., 2008). All strains were maintained on a standard cornmeal medium at room temperature. Cell Culture, Viral Production and Drosophila Infection Baculovirus AcMNPV was produced as previ ously described (Huang et al., 2011). Generally speaking, Spodoptera frugiperda cell line sf9 was cultured with sf900 medium 76


at 28oC incubator. Infectious Autographa californica nucleopolyhedrovirus (AcMNPV) was obtained by transfection of sf9 cells with bacmid DNA containing AcMNPV genome. Virus was tittered in sf9 cells by standard end point dilution assay. For AcMNPV infection, Drosophila 3rd instar larvae were injected with budded AcMNPV at a dosage of 3x104 PFU/larvae from the dorsal-posterior par t of larval body. Injected larvae were kept in sf900 medium with or without virus for indicated times bef ore subject to RNA extraction and Q-PCR analysis. Flock house vi rus particles were purified following established protocols (Schneemann and Mars hall, 1998). For flock house virus (FHV) infection, adult flies 4-6 day of age were used. FHV stock was prepared with sf900 medium. Infection was achieved by injection of viral suspension into the thoraces of adult flies. The injected flies were t hen cultured with standard fly food at room temperature. RNA Extraction and Q-PCR Larval total RNA was extracted with RN easy Mini Kit (QIAGEN, Valencia, CA, USA) according to the protocol provided by the manufacturer. Adult RNA was extracted with TRIZol Reagent (Invitrogen, Grand Island, NY, USA) following manufacturers manual and purified with RNeasy Mini Spin Column (QIAGEN). RNA samples were treated with DNase I to remove genomic DNA. cDNA was prepared by reverse transcription of total RNA with a High-Capacity cDNA Archive Kit (Applied Biosystems, Foster City, CA, USA). Q-PCR was perfo rmed with an ABI 7500 Fast thermocycler (Applied Biosystems) fo llowing protocols provided by the manufacturer. Triplicates were measured for each gene/sample combination. The oligo sequences of the main target genes are as follows: reaper : 5-ACGGGGAAAACCAATAGTCC-3 and 5TGGCTCTGTGTCCTTGACTG-3; hid : 5-CTAAAACGCTTGGCGAACTT-3 and 577


CCCAAAAATCGCATTGATCT-3; rp49 : 5-GCTAAGCTGTCGCACAAATG-3 and 5GTTCGATCCGTAACCGATGT-3; AcMNPV ie0 : 5CGAGACGCGTTGAAGCTAAT-3 and 5CGCAACATTCTTTTGGCTTT-3; AcMNPV ie1 : 5GGCAGCTTCAAACTTTTTGG-3 and 5TTCACACCAGCAGAATGCTC-3; FHV RNA1: 5CCAGATCACCCGAACTGAAT-3 and 5-AGGCTGTCAAGCGGATAGAA-3; FHV RNA2: 5-CGTCACAAC AACCCAAACAG-3 and 5GGTCGGTGTTGAAGTCAGGT-3. FHV Genome Estimation The amount of FHV genome was estimated according to the pre-generated standard curve and regression equation. To get the standard curve of the viral dosage/Ct value, a serial dilution of know n dosage of FHV was mixed with wild type adult male (one fly for one dilution). RNA extraction and Q-PCR were performed as described above to get the Ct value of viral RNA1 or RNA2. Standard curve and regression equation were generated using Mi crosoft Office Excel (version 2007). Fluorescent in Situ Hy bridization (FISH) Probes were synthesized using digoxin (DIG)-RNA Labeling Mix (Roche, Madison, WI, USA). Drosophila 3rd instar larval skins were parti ally removed to expose inside tissue in 4% paraformaldehyde. After prefixing with 4% paraformaldehyde in PBT_DEPC (0.3% Triton in PBS made with DEPC pretreated double-distilled water) for 30 min, the tissue was incubated for 7 mi n with 50 mg/ml protease K in PBT_DEPC, and reaction was stopped by washing with 4% paraformaldehyde. Samples were incubated with probes diluted in hybridizat ion buffer (50% formamide, 25% 2xSSC, 20 mg/ml yeast tRNA, 100 mg/ml ssRNA, 50 mg/ml heparin, and 0.1% Tween-20). Hybridization was perfo rmed overnight at 60 oC. Larvae were incubated with 78


horseradish peroxidase (HRP)-conjugat ed anti-DIG (Roche) antibody after hybridization, followed by si gnal amplification using the Ty ramid Signal Amplification Kit (PerkinElmer, Waltham, MA, USA). Antibody and Immunostaining Rabbit monoclonal antibody to cleav ed caspase-3 was purchased from Cell Signaling (Danvers, MA, USA). The antibody was used at a dilution of 1:200. AlexaFluor 488 labeled goat anti rabbit antibody was purchased from Molecular Probes and was used at a dilution of 1:1000. Propidium iodi de was purchased from Sigma (St. Louis, MO, USA). To detect cell necrosis, 100nL of PI (1mg/mL) was injected into the thoraces of adult flies. Injected flies were kept culturing with standard fly food at room temperature for 20 min before subjected to immunostaining. Standard procedure was used for immunostaining. Fly fat bodies were dissected in PBS containing 4% paraformaldehyde and fixed for 20min at ro om temperature. After being washed with PBS containing 0.1% Triton X-100 (PBST), the samples were blocked with PBST containing 5% normal goat serum for 30min. Samples were then incubated overnight with anti-cleaved casp ase-3 antibody (1:200 dilution) at 4oC. Labeling with secondary antibody was done at 25oC for 2hr. Slides were mounted with Vectorshield Mounting Medium (Vector Laboratories, Burlingame, CA USA). Pictures were taken with a Leica upright fluorescent microscope (Leica, Bann ockburn, IL, USA) using OpenLab software (Improvision, Coventry, UK). 79


Results Rapid Induction of Mx Following DEN-2 Infection in a Refractory, but Not in a Susceptible, Strain of Aedes aegypti My previous work found that following ex posure to the mosquito baculovirus CuniNPV, the mosquito reaper ortholog mx was rapidly induced (within 2 hour p.i.) in infected midgut cells of the Aedes aegypti larvae. This induction of mx is followed by apoptotic death of the infected midgut cells at about 4-6 hour p.i.. However, this rapid induction of mx and apoptosis was absent in a mosquito species that is susceptible to CuniNPV infection (Liu et al., 2011). Interestingly, similar corre lation between the induction of mx and the sensitivity to virus infection was also observed in two Aedes aegypti strains following exposure to DEN-2. Blood meals contai ning DEN-2 (dengue virus serotype 2) was introduced to adult mosquitoes of the refrac tory (MOYO-R) or the susc eptible (MOYO-S) strains. When the expression level of mx was monitored via Q-PCR, we found that it is significantly induced in the MOYO-R stra in following DEN-2 ex posure when compared with the control-fed mosquitoes (Figure 4-1). This induction of mx in the refractory strain was rapid, since at 3 hrs post bl ood meal (p.b.m.) the level of mx was about 2.5 fold higher in the virus-fed compared to the control-fed. The level of mx receded to lower levels at 18 hr p.b.m.. In contrast, there was no difference between control-fed or DEN-2 fed mosquitoes of the MOYO-S strain. This indicates that the susceptible MOYO-S strain lacks the rapid induction of mx Rapid Induction of RHG Genes Following Vira l Infection in Live Fruit Flies but Not in Cultured Cells To test whether similar rapid induction of RHG genes and apoptosis can be observed following viral infection in the fruit fly, I tried to infect Drosophila larvae and 80


adults with AcMNPV and FHV, respectively. AcMNPV, a lepidopteran baculovirus with a dsDNA genome of about 134 kb, infects the la rvae of susceptible hosts (Vail et al., 1971) but do not replicate in Drosophila cells. Budded AcMNPV virus was propagated in SF9 cells and introduced into the abdominal hemocoel of 3rd instar Drosophila larvae through micro-injection. Q-PCR analysis in dicates that following AcMNPV injection (3x104 PFU per animal), two RHG genes, hid and reaper were quickly induced as early as 1hr post injection (p.i.). By 2hr p.i., the level of pro-apoptotic genes has gone back to normal (Figure 4-2A). This rapid induction of RHG genes likely requires immediate early gene expression from the bacul ovirus, since UV-inactivated virus failed to induce hid expression (Figure 4-3). In parallel, I also introduced AcMNPV to cultured Drosophila DL-1 cells. Even at MOI of 20, AcMNPV fa iled to induce significant induction of reaper or hid at early stage of the infection. Moderate level of induction was observed at 6 hr p.i. and significant induction was obser ved at 24 hr p.i. (Figure 4-2B). Since the RNA was extracted from homogenized whole larvae, the level of gene induction revealed by Q-PCR cannot fully reflect the magnitude of change of gene expression in specific cells To monitor the level of hid in individual cells, FISH was performed with DIG-label ed RNA probes against hid and reaper Several tissues, including fat body, midgut, hindgut, malpighian tube, ovary/test is, etc. were examined. I found that the induction of hid following AcMNPV infection was mainly observed in scattered fat body cells (Figure 4-2C). The number of hid -positive cells in the fat body following AcMNPV injection was about 10-20% of the total fat body cells. A few midgut cells, less than 1% of all cells in that tissue, were also found to be hid -positive in the infected larvae (Figure 4-2D). 81


The flock house virus (FHV) is a positiv e-sense single strand RNA virus of the nodavirus family (reviewed in (van Rij et al., 2006)). Originally isolated from grass grubs, it has been shown to replicate in plants, y east, and a variety of insects (Galiana-Arnoux et al., 2006; Lanman et al., 2008). I injected FHV into the thoraces of the adult flies at dosages ranging from 22 to 2 106PFU/adult. Q-PCR result indicated that similar to AcMNPV, at 1-2hr post viral injection, the expression of reaper was significantly induced in FHV-injected adults. The level of reaper in FHV infected adults was about 1.6 fold of that in the control-injected sa mple (Figure 4-2E). This result demonstrated that besides DNA virus, infection by RNA virus can also induce rapid induction of RHG genes. This rapid induction of RHG genes was not observ ed when parallel infection was performed in Drosophila DL-1 cells. Addition of FHV, at 40 MOI, to cultured DL -1 cells did not induce reaper or hid expression until at about 36 hr p.i. (Figure 4-2F). This corresponds well with a previous report which showed that caspase activation and apoptosis happened after 36 hr p.i. when a similar dosage of FHV was applied to the DL-1 cells (Settles and Friesen, 2008). It has been shown t hat fat body is the major target of FHV in Drosophila (Dostert et al., 2005), to prove that the reaper induction is indeed due to viral infection, I performed FISH for reaper mRNA and immunostaining for FHV capsid protein in the fly strain that has DIAP1 ov erexpressed in the fat body cells to inhibit apoptosis. Indeed, in the cells that had FHV capsid signal, reaper signal is positive. In contrast, in the cells lacking FHV, there is no reaper expression (Figure 4-2G). This result indicates that reaper induction is indeed due to viral infection. Taken together, these observations indicated that the rapid induction of RHG genes following viral exposure is a general phenomenon that can be observed in both 82


mosquitoes and Drosophila In addition, this response is not limited to DNA viruses. Infection by RNA virus such as the FHV can elicit a rapid induction of pro-apoptotic genes as well. The significant difference in the timing of pro-apoptotic response following AcMNPV and FHV infection in ani mal models vs. that in cultured cells indicated that the dynamics of pro-apoptotic response, and possibly the mechanism, differs significantly. To elucidate the ro le of rapid induction of apoptosis in limiting/blocking viral infection in animals, I focused the following analysis on in vivo models. Rapid Induction of Reaper and Hid Requires P53 and the Regulatory Region IRER The rapid induction of both reaper and hid following viral infection was reminiscent of what was observed following ionizing irradiation, in whic h case the function of the transcriptional factor P53 is required for t he rapid induction of the RHG genes (Brodsky et al., 2004). To investigate the mechanism responsible for mediating virus -induced pro-apoptotic gene expression, I introduced AcMN PV infection to larvae of different genotypes and monitored the induction of hid following the injection. The P53 loss-offunction allele 5A-1-4 is a deletion gener ated via homologous recombination (Rong et al., 2002). Animals homozygous to this mutant allele have reduced level of stressinduced apoptosis, but are otherwise viable and have no obvious phenotype. When the level of RHG genes were monitored following in jection of AcMNPV infection, I found that the induction of hid and reaper was completely blocked in this p53 null mutant (Figure 44A). Somewhat to my surprise, FHV -induced ex pression of RHG genes in the adult is also p53 -dependent. Q-PCR results indi cated that while in wild type flies, FHV infection can result in about 1.6 fold induction of reaper at 1-2hr post infection, FHV injection 83


failed to induce reaper expression in animals lacking p53 function (Figure 4-4B). Taken together, these results indicated that P53 plays a pivotal role in mediating the rapid induction of RHG genes follo wing viral infection. The induction of RHG genes following ionizing irradiat ion requires a regulatory region upstream of reaper/grim/hid which was referred to as the IRER (Irradiation responsive enhancer region) (Zhang et al., 2 008). In animals deficient for IRER, the induction of RHG genes reaper and hid following irradiation is either completely blocked or significantly suppressed depending on th e tissue and development stage examined. The proximal breaking point of the del etion is about 3 kb upstream of the reaper promoter. Thus this regulator y mutant specifically blo cks stress-induced expression of the RHG genes without deleting any transcri bed region. More importantly, other DNA damage induced responses, such as the inducti on of DNA repair proteins KU70/KU80, remains intact in this mutant (Zhang et al., 2008). My data indi cated that similar to what was observed in the P53 mutant st rain, AcMNPV and FHV -induced RHG gene expression is blocked in Df(IRER) animals, indicating this regulatory region is required for induction of reaper/hid following virus infection (Figure 4-4 A & B). My previous work showed that midgut cells infected by CuniNP V following a native route of infection become TUNEL-positive at 4-6 hr p.i. (Liu et al., 2011). In wild type animals infected by AcMNPV or FHV, the rapid induction of RHG genes in fat body cells is followed by apoptosis at about 2.5 hrs p.i. (Fi gure 4-4 C & D). Apoptotic cells were recognized with an antibody developed against activated (cleaved) capase-3, which labels cells with activated upstream caspase DRONC (Fan and Bergmann, 2008). There is little if any apoptotic cells in la rvae or adult fat bodies injected with control 84


media or suspension buffer, respectively. Ho wever, significant increase of apoptotic cells was observed at 2.5 hours followin g either AcMNPV or FHV injection. Corresponding with the absence of the i nduction of the RHG genes following viral infection, the rapid induction of apoptosis in both larval and adult fat body was blocked in the P53 null mutant. The induction of apoptosis was also blocked in homozygous Df(IRER) animals. These evidences indicate that P53 induced expression of the RHG genes following viral infection is responsib le for the rapid induction of apoptosis. Significant induction of RHG genes was not observed in P53-/animals even at later time point of the infection. At 4-7 days p.i., essentially all cells in the fat body of the P53-/animals are filled with FHV (containing capsid protei n immune-reactivity) (Figure 4-3E). These cells, unlike those in wild type animals, have lost the integrity of cell membrane and become permeable to propidium iodide (PI). Similar loss of membrane integrity, a typical feature of necrotic cells, was also observed in CuniNPV infected susceptible mosquitoes at 48-72 hr p.i. (Liu et al., 2011). Rapid Induction of Apopt osis Blocks/limits Viral Gene Expression and Proliferation To investigate the functional significance of the rapid induction of apoptosis as an innate immunity against viral infection, I first examined the expression of AcMNPV immediate early genes, ie0 and ie1 (Stewart et al., 2005) at 6hr post infection in wild type, p53 deficient, and IRER deficient strains. For ie0 it was not detectable in wild type larvae following virus injection. However, it was reliably detectable in either P53 null or Df(IRER) animals at 6 hr post injection (Figure 4-5 A). For ie1 its level of expression was very low, but nonetheless detectable, in wild type animals at 6 hr post infection. Its level of expression was dramatically higher in p53 mutant or Df(IRER) animals that 85


lacks the rapid induction of apoptosis. This indicates that the rapid induction of apoptosis mediated by P53 and IRER plays an impor tant role in blocking/ inhibiting viral gene expression. To test whether the rapid induction of apoptosis is responsible for blocking / limiting viral proliferat ion, I monitored the genome levels of FHV at 24 hr post injection of 200 PFU per animals. In this assay, a group of 5 animals for each genotype was homogenized and relative abundance of the FHV RNA genome was assayed with QPCR primers targeting both RNA1 and RNA2 of the FHV (Fi gure 4-5B). I found that comparing to wild type animals, the relative levels of FHV genome were significantly higher in P53-/and Df(IRER) mutant than that in wild type animals. To verified whether the difference in FHV proliferation is indeed due to the lack (or delay) of cell death, I next injected 200 PF U per animal to flies that carrying LSPGal4/UAS-DroncRNAi. DRONC is an upstream ca spase that play a pivotal role in mediating cell death induced by RHG genes. I found that knocking down dronc specifically in the fat body (where LSP-Gal4 is expressed), allowed the proliferation of FHV viral genome to a level comparable to t hat observed in Df(IRER) animals at 24 hr p.i. (Figure 4-5C). These evidences indi cate that rapid induction of apoptosis is responsible for limiting vi ral proliferation at early stage of the infection. Animals Lacking the Rapid I nduction of Apoptosis Are Hyper-susceptible to FHV Infection To test whether the lack of rapid inducti on of apoptosis could lead to established virus proliferation and infection, I monitor ed the proliferation of viral genomes following FHV injection in individual wild type or muta nt animals. At 4 days following FHV injection of 20 PFU per animal, the levels of the vi ral genome in most wild type animals did not 86


increase at all (Figure 4-6 A), indicating that there is no successful proliferation of the virus. In contrast, in both P53 mutant and Df(IRER) animals, the level of FHV genome have increased dramatically, indicating successful proliferation of the virus. When 200 PFU per animal of FHV was injected, the level of viral genomes was unchanged in most wild type animals at 4 days post injection. In c ontrast, the levels of viral RNA in P53-/and Df(IRER) animals indicated that signifi cant proliferation had occurred in those mutants that lack rapid inducti on of apoptosis (Figure 4-6B). The successful proliferation of FHV in animals lacking the rapid induction of apoptosis was also verified by visualizi ng packaged viruses using an antiserum raised against the FHV coat protein (Figure 4-6C ). At 4 days followin g 200 PFU per animal injection, no cells can be detected positive for FHV in wild type flies. In contrast, all fat body cells in the P53 mutant (or Df(IRER)) animals are pos itive for FHV. Furthermore, as early as 4 days p.i., cells containing FH V capsid protein can be identified in the salivary gland of P53-/animals (Figure 46D), indicating systematic infection has been established. These evidences indicated that the rapid induction of apoptosis, observed in wild type animals, is capable of blocking th e infection when the viral titter is not too high. Conversely, lack of rapid induction of apoptosis, as was observed for the P53-/and Df(IRER), leads to significant ly increased susceptibility. Discussion Although apoptosis was long ago postulated to be a major mechanism against viral infection in insect vectors, the mec hanistic detail remained elusive. The lack of mechanistic understanding, and the conflicti ng results obtained from cultured cells, casted serious doubt on the functional significance of apoptosis as an innate immunity against viral infection. My study revealed that that the RHG gene s, known for their 87


pivotal role in regulating devel opmental cell death, is also responsible for mediating the rapid induction of apoptosis following viral in fection. The induction of the RHG genes following viral infection requires the function of P53 as well as the regulatory region IRER. Furthermore, I showed that the rapid induction of apoptosis is capable of blocking/limiting viral gene expression and infecti on. Genetic variations in this response likely play an important role in determining the susceptibility of insect host to viral infection. Animal Models vs. Cultured Cells for Arbovirus Infection So far, the rapid inducti on of RHG genes and apoptosis following viral infection have only been observed in live animals. Dengue viral infection of the mosquito cells line C6/36 did not induce apoptosis and there appears to be no significant induction of pro-apoptotic genes (Lin et al., 2007; Sh ih et al., 2010). When AcMNPV or FHV was applied to a Drosophila cell line, apoptosis was only observed 24 or 36 hr post infection, respectively (Settles and Friesen, 2008; Vandergaast et al., 2011). Correspondingly, I observed that there was no significant i nduction of RHG genes before 24 hr p.i. for either virus (Figure 4-2). It is unclear as to why cell lines lack t he rapid pro-apoptotic response observed in both live mosquitoes and fruit flies. One possibility is that only certain types of cells can launch the rapid pro-apoptotic response, and such cell types are not represented in cultured cell lines. Another possibility is t hat cultured cell lines were selected to have reduced sensitivity to stress-induced cell death. The regulatory region required for mediating viral infection i nduced pro-apoptotic genes, i.e. IRER, serves as a locus control region mediating the induction of RHG genes in response to a variety of stresses, such as x-ray, UV, oncogenic stresse s, etc. In addition, the accessibility of 88


IRER is controlled by epigenetic regulation. When IRER is epige netically blocked, i.e. in heterochromatin-like conforma tion, the RHG genes are no longer responsive to stresses such as DNA damage (Zhang et al., 2008). Our analysis of several Drosophila cell lines (S2, Kc167, etc.) indicated that the IRER region in these cell lines is enriched for heterochromatic modifications and resistant to DNase I treatment (Lin et al., 2011)(and unpublished observations). It is possible that cells with r educed sensitivity to stressinduced cell death, either through genetic muta tion or epigenetic silencing of IRER, are inadvertently selected during in vitro cell cult ure processes. As a result, the ability to launch the rapid induction of RHG genes followi ng viral infection could have been lost in long term cultured cells. Rapid Induction of Apoptosis is a Key Innate Immunity against Viral Infection My data indicated that the rapid inducti on of apoptosis is capable of blocking the infection at its initiation stage when the anima ls are exposed to relatively small amount of viruses. This response very likely contri bute to the midgut infe ction barrier that has long been observed for arbovirus transmission through insect vectors (Bosio et al., 2000). Apoptosis of midgut cells followi ng viral exposure has been observed before, when a refractor strain of Culex pipiens was fed with blood meal containing West Nile virus (Vaidyanathan and Scott, 2006). Similarly, rapid induction of mx was observed in refractory Aedes aegypti (MOYO-R) fed with blood meal c ontaining DEN-2 (Figure 4-1). There is a conspicuous la ck of rapid induction of mx in the susceptible strain (MOYO-S). My data with FHV-infected P53-/and Df(IRER) animals indicat ed that the lack of rapid induction of apoptosis following viral infect ion could lead to dramatically increased susceptibility to established systematic infection. 89


The rapid induction of apoptosis effectively denies the opportunity of viral gene expression and accumulation (Figure 4-8). Th is has have been previously demonstrated for CuniNPV infection through a native route of infection (Liu et al., 2011), where I showed that viral gene expression was only detected when apoptosis was delayed/suppressed with caspase inhibitors. In this study, I showed the lack of rapid induction of apoptosis in P53 and Df(IRER) animals allowed viral gene expression and proliferation. In addition, signi ficant viral proliferation can be achieved when the level of DRONC in fat body cells was knocked down by tissue-specific RNAi. All of these evidenced indicate that rapid e limination of the infected cell is responsible for blocking the infection at the initiation stage, before significant expression of viral genes could take control of the cellular system. Rapid induction of apoptosis as an innate i mmunity against viral infection is not restricted to insects. For instance, rapid induction of apoptosis was observed at 8 hr following infection of human embryonic stem cells (hESCs) with recombinant AAV (Hirsch et al., 2011). The hESC s are extreme sensitivity to various stresses. And the rapid induction of apoptosis in hESC cells fo llowing rAAV infection also requires P53. Rapid induction of apoptosis was also obs erved following the infection of primary dendritic cells by t he intracellular pathogen legionella pneumophila which induces apoptosis within the first hour of infection (N ogueira et al., 2009). Similarly, influenza A viruses (IAV) mutated for NS1 can also induces rapid apopt osis in primary macrophages (Stasakova et al., 2005). Similar to what I discovered with Drosophila P53-/mice are hypersensitive to IAV infe ction (Munoz-Fontela et al., 2011). However, the anti-viral effect of P53 in mice may include induction of pro-inflammatory genes in 90


addition to its pro-apoptosis function. Study of innate immunity in C. elegans also revealed that P53 has an ancient role as an immune/stress sensor (Fuhrman et al., 2009). In this study, I found that although FHV vi rus can proliferate in Df(IRER) that lacks rapid induction of apoptosis, the titer of FHV was consistently lower than that observed for P53 null mutant animals. It is very likel y that the observed difference between P53-/and Df(IRER) animals is due to anti-viral activi ty of P53 besides its role in the rapid induction of apoptosis. Indeed, a genomic survey of dengue virus-induced changes of gene expression revealed that many P53 targeted genes were activated in the refractory (MOYO-R) strain but not in the susceptible stra in (Behura et al., 2011). However, my analysis with Df(IRER) animals and those with DRONC KD indicated that a major mechanism of P53 mediated anti-viral activity in insects is through the rapid induction of RHG genes and apoptosis. What is the Signal Transduction Pathway that Activates P53 It is not clear as to how is P53 ac tivated in virus -infected cells. In Drosophila the well characterized immune pathways includi ng the Toll pathway, the IMD pathway, and the Jak-STAT pathway (reviewed in (Che rry and Silverman, 2006)). Toll pathway is mainly responsive to fungi and Gram-positive bacteria while IMD pathway is activated by Gram-negative bacteria (Hoffmann and Reic hhart, 2002). Recent studies indicated that except for anti-fungal and anti-bacterial functions, the Toll pathway is also involved in antiviral response (Nakamoto et al., 2012; Zambon et al., 2005). The JaK-STAT pathway has been shown to be activated by Drosophila C virus. Loss of function of JaK led to increased viral load and decreased survival ra te after viral infecti on (Dostert et al., 2005). However, I found that vir-1 a target gene of JaK-STAT, was not induced by 91


AcMNPV (or FHV) infection when reaper / hid were significantly induced, which suggested that JaK-STAT pathway is unlikely in volved in activating P53 (Figure 4-9). The fact that both AcMNPV and FHV induc ed rapid transcriptional activation of RHG genes requires P53 and IRER suggests that a common mechanism may be responsible. The two viruses are quite differ ent, i.e. dsDNA virus vs. ssRNA virus. AcMNPV cannot replicate in Drosophila whereas FHV can replicate in a variety of insects including Drosophila The fact that these two vi ruses, and very likely other viruses such as DEN-2 and CuniNPV, induce rapid induction through the same transcription factor and regulatory region strongly suggests that a more general mechanism is involved. Revealing this mechanism should shed great light on our understanding of arbovirusvector interaction. 92


Figure 4-1. Specific induction of mx in the refractory strain (MOYO-R), but not in the susceptible strain (MOYO-S), following exposure to DEN-2. Expression level of mx was measured with Q-PCR and norma lized against housekeeping gene GAPDH before calculating the fold of induction, i.e. the ratio of mx in virus-fed vs. the control-fed samples. At 3hr post feeding, the level of mx in mosquitoes exposed to DEN-2 was more than 2 fo ld higher than those fed with control blood meal. This level of mx expression goes down at 18hr, possibly due to death of mx -expressing cells as we observed for CuniNPV infected Aedes aegypti larvae. Data represented as Mean + STD. 93


Figure 4-2. Rapid induction of reaper and hid following viral infection of Drosophila larvae or adults. (A). At 1hr post AcMNPV injection (p.i.), hid mRNA level in injected larvae was induced ~2 fold. By 2hr p.i., hid level has gone back to normal. (B). AcMNPV infection in cu ltured DL-1 cell line (MOI=20) did not induce reaper / hid until relative late stage (24hr p.i.). Data are shown as Mean STD of two to th ree independent experiments. (C) and (D). Following AcMNPV injection, hid is mainly induced in fat body cells and some gut cells as revealed by FISH using digoxin-l abeled cRNA probe. (E). At 1hr post FHV injection, reaper mRNA level in infected adults was induced ~1.6 fold. (F). FHV infection in cultured DL-1 cell (MOI=40) did not induce reaper / hid expression until 36hr p.i.. (G). Induced reaper is due to FHV infection. Immunostaining for FHV capsid protein (red) and reaper FISH (green) were performed in the fly strain that has DIAP1 overexpression in the fat body. Note the colocalization of red and green si gnal (white arrows). In contrast, the FHV negative cells have no reaper signal either (white arrow heads). Photos are representatives of at least 2 independent experiments. 94


A Figure 4-3. Viral gene expression is required for the inducti on of pro-apoptotic response. (A). UV irradiation can dramatically decrease the early gene transcription of AcMNPV. sf9 cells were infected with wild type and UV-irradiated AcMNPV. Two immediate early genes ie0 and ie1 mRNA level were examined with QPCR to indicate the UV effect. (B). UV-inactivated AcMNPV has decreased ability to induce hid expression. Drosophila larvae were injected with wild type and UV-inactivated AcMNPV. mRNA level was detected with Q-PCR. B 95


Figure 4-4. Virus -induced reaper / hid expression and apoptosis requires P53 and IRER. (A). The induction of hid following AcMNPV injection is absent in animals homozygous to a P53 null mutation (p53-/-) or deficiency removing the regulatory region IRER (Df(IRER). (B). Likewise, the induction of reaper following FHV infection also requires p53 and IRER. Data are shown as Mean STD of two independent Q-PCR ex periments. (C) and (D). Activated caspase 3 can be detected in the fat body cells of wild type fly larvae or adults infected with AcMNPV or FHV, respective ly. In contrast, cells with activated caspase 3 were not observed in p53-/or IRER deficient flies. (E) Fat body cells in P53-/became necrotic at 4-7 days p.i.. PI was injected to either control-injected or FHV (200 PFU /per animal) injected anim als at 7 days p.i. and the animals were sacrificed and fixed 20 minutes later. The presence of FHV was visualized with an antibody against the capsid protein. The presence of PI indicating the integrit y of the cell membrane was compromised in fat body cells infected by FHV. Photos are representatives of two independent experiments. 96


Figure 4-5. Rapid induction of apoptosis functions to block/limit viral gene expression and proliferation. (A). T he mRNA levels of AcMNPV immediate early gene ie0 and ie1 were examined at 6hr post virus injection by Q-PCR in Drosophila larvae. Data indicates that in both p53 -/and IRER deficient strains that lacks rapid induction of apoptosis, ie0 and ie1 expression levels were dramatically higher than that in the wild type strain. Data are shown as Mean STD. (B) FHV gene RNA1 and RNA2 levels were examined at 24hr post injection in wild type, p53-/and Df(IRER) adults by Q-PCR. Note the significantly higher copy levels of RNA1 and RNA2 in p53-/and Df(IRER) flies than that in wild type flies. p-value<0.05, ** p-value<0.01, t-test. (C) RNA1 and RNA2 levels were also higher when apoptosis wa s suppressed by knocking down the upstream caspases dronc (Dronc KD) in the fat body cells (genotype LSP2Gal4;UAS-dronc_RNAi). Data are shown as Mean STD. p-value<0.05, ttest. 97


Figure 4-6. Rapid induction of apoptosis functions to block/lim it viral proliferation. (A) When a low dosage FHV (20 PFU/per animal) was injected, no viral proliferation was observed at 4 days p.i.. In contrast, si gnificant proliferation of FHV was observed in p53-/or Df(IRER) flies. The dotted line indicated the amount of virus injected to each animal. Total RNA extraction and Q-PCR were performed in individual fly to estimate the copy numbers of RNA1 and RNA2 (Material and Methods and Figure 4-7). ** p-value<0. 01, t-test. (B) When 200 PFU/per animal was inject ed. Most wild type flies has no significant increase of FHV copy number s, although the virus proliferated significantly in P53-/and Df(IRER) animals. ** p-value< 0.01, t-test. (C) Immunostaining was performed with ant iserum against FHV capsid protein and AlexaFluor 488 labeled goat anti r abbit secondary antibody. Cells in the fat body of p53-/animal ar e filled with FHV capsid protein at 3 days post 20 PFU injection. No FHV positive cells can be detected in wild type (wt) animals inject 20PFU FHV. (D) At 4 days p.i., cells in the salivary gland of P53-/flies are positive for FHV capsid protein immunity, while no spreading of virus can be detected in wild type ani mals. Photos are representatives of two independent experiments. 98


Figure 4-6. Continued. 99

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y = 3.1966x 18.136 R = 0.9983 20 15 10 5 0 5 0246 C Viral dosage (lg)RNA1 standard curve8 Figure 4-7. Standard curve for estimating FHV genome/titter. FHV of various titter was mixed with 1 adult fly and processed immediate for RNA extraction. Q-PCR were performed for RNA1 and RNA2 and the relative CT to GAPDH was calculated plotted against the viral dosage. This information was used to estimate viral genome number / ti tter in Figure 4-6 A & B. y = 3.3944x 18.938 R = 0.995 20 15 10 5 0 5 02468 C Viral dosage (lg)RNA2 standard curve 100

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Fig 4-8. Diagram summarizes the role of rapid induction of apoptosis as an innate immune response against viral infection. By eliminating the infected cells before accumulation of viral gene producti on, the infection can be blocked at the initiation stage. 101

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102 Figure 4-9. Monitoring the st atus of the JAK/STAT pat hway. QPCR measurements of hid and vir-1 mRNA were first normalized ag ainst GAPDH before calculating the ratio (AcMNPV injected / Control media injected).

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CHAPTER 5 DISCUSSION AND PERSPECTIVES The Role of Apoptosis as an Innate I mmune Response agains t Viral Infection The role of apoptosis as a defense against viral infection has long been under debate. On the one hand, a variety of viral genes have been found to inhibit host cell apoptosis (refer to Chapter 1 for details), mor eover, it has been found that the viral antiapoptotic genes are essential for the infectivity of the viruses (Clem, 2001). On the other hand, many viral genes have been found to acti vely trigger host cell apoptosis (also discussed in Chapter 1). In addition, results derived from cultured insect cells indicated that inhibition of virus-induced apoptosis had no effect on viral yield (Settles and Friesen, 2008; Vandergaast et al., 2011). Due to the lack of efficient experimental systems, the exact role of apoptosis in viral in fection has always been a mystery. In this study, I tried to demonstrate the role of apoptosis in viral infection by establishing two virus-host interaction systems: the mosquito system and the Drosophila system. The mosquito system is a native infection system, i.e. animals were orally infected by viruses which mimicked the native infecti on route. In this system, mosquito larvae and adults were used respectively. The mosquito larvae were infected by directly adding baculoviruses to the culture medium of the larvae. The mo squito adults were fed with blood meal containing the hum an pathogen, Dengue virus. In both larvae and adults, I found that upon viral infecti on, there was rapid induct ion of pro-apoptotic gene, mx in the refractory Aedes aegypti strain but not susceptible strain. In larvae, I further found that the apoptosis correlated with mosquito competence, i.e. rapid apoptosis can be detected in refractory species but not suscept ible species. After inhibiting apoptosis in 103

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refractory species, viral genes became det ectable. The correlation between apoptosis and mosquito susceptibility strongly sugges ts that apoptosis serves as a defense against viral infection. Mor eover, the correlation between mx induction and apoptosis suggests that mx might be the mediator of virusinduced apoptosis. To further prove this, I need to specifically knock down mx from refractory species and examine if apoptosis still happens upon viral infection. However, to date, no mature RNAi technique has been established in mosquito larvae. Moreover, given the limited genetic tools in mosquito, it is hard to inhibit apopt osis through genetic methods. Therefore, the major drawback of the mosquito system is t hat it is hard to study the mechanisms of virus-induced apoptosis. To study the mechanisms of virus-induced a poptosis and further prove the role of apoptosis as an anti-viral response, I established two infection systems in Drosophila As a powerful model system, there are a large number of genetic resources in Drosophila In this system, viruses were injected into Drosophila larvae or adults because very few viruses have been reported to infect Drosophila through a native route. By using the DNA virus AcMNPV and RNA virus FHV, I found that, similar to the mosquito, pro-apoptotic genes are quickly induced in Drosophila upon viral infection. More importantly, I found that virus induced pro-apoptotic gene expression is P53dependent and mediated by IRER. In P53-defici ent or IRER-deficient animals, there was no rapid apoptosis following FHV infectio n and the animals were hyper susceptible to FHV infection, which proved the role of apoptosis in defending against viral infection. 104

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Innate Immune Responses in Drosophila and Mosquito Drosophila The well characterized immune pathways in Drosophila include the Toll pathway and the IMD pathway (Figure 5-1) (Hoffm ann, 2003). The Toll pathway is mainly activated by gram-positive bacteria and fungi while the IMD pathway is mainly activated by gram-negative bacteria infection. Consequently, the Toll mutants are highly susceptible to fungal infection and the IM D mutants die after Gram-negative bacteria infection (Lemaitre et al., 1995; Lemaitre et al., 1996). In both Toll and IMD pathways, upon pathogen infection, the transcription factor NFB-like protein ( Dorsal/Dif in the Toll pathway and Relish in the IMD pathway) is activated. The activated NFB then translocates into the nuclei where it binds to the promoters of the target genes and triggers their expression. The known target genes include antimicrobial peptides such as Diptericin and Cecropins To date, seven antimicrobi al peptides have been identified (Hoffmann, 2003). The secretion of these antimicrobial peptides into the haemolymph (insect blood) is an important aspect of insect humoral immunity. Following the identification of the role s of Toll and IMD pathways in immune response, a third pathway, JaK-STAT, was later reported to be ac tivated by viral infection and involved in anti-viral response (Dostert et al., 2005). One gene, vir-1 whose promoter contains two STAT binding sites, has been found to be the target of JaK-STAT pathway and can be strongly induced by DCV ( Drosophila C virus) infection (Dostert et al., 2005). Using the Drosophila infection system, I found that virus-induced apoptosis is P53dependent and mediated by an enhancer regi on IRER. The responsible signaling transduction pathways remain to be identified. The preliminary results indicated that vir1 cannot be induced by viral infection, whic h suggested that the JaK-STAT pathway is 105

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unlikely involved. To elucidate the in volved signaling transduction pathways, a nonbiased RNAi screening could be performed. I will take advantage of the available RNAi strains generated by TRiP (Transgenic RNA i Project) to specifically knockdown the key regulators in major signal trans duction pathways such as the DNA damage pathways, Toll pathway, IMD pathways, et c. to identify the pathways involved. Mosquito The sequencing and annotation of majo r mosquito genomes have largely prompted the understanding of the mosquito immunity. Research on mosquito immunity has been strongly influenced by work on Drosophila Homologs of the major factors in Drosophila Toll and IMD pathways have also been found in mosquitoes. However, unlike Drosophila mosquitoes lack Dif but use Dorsal ortholog Rel1 and Relish ortholog Rel2 to induce the expression of antimic robial peptides (Fragkoudis et al., 2009). The heat-shock protein cognate 70B has been found to be upregulated by virus infection, and the silencing of this gene decreased the lifespan of ONNV-infected A. gambiae (Kang et al., 2008; Sim et al., 2007). In DEN-2-infected A. aegypti strong upregulation of the Toll and JaK-STAT pathwa ys can be observed. It was further proved that the Toll pathway played a role in c ontrolling DEN-2 (Xi et al., 2008). In addition to the classical immune pathways described above, the siRNAmediated RNAi has been demonstrated to be an important antiviral mechanism in mosquitoes (Keene et al., 2004; Li et al., 2004). Virus-derived long dsRNAs can be cleaved by the RNase DCR-2 to generate s hort siRNAs (~21-25 bp in length), often called viRNAs. DCR-2 and dsRNA binding prot ein R2D2 then integrate one strand of viRNA into the RNA-induced silencing comple x RISC, which further activates RISC. The activated RISC mediates the sequence specif ic cleavage of viral RNA through its AGO106

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2 component. The orthologs of DCR-2, R2D2 and AGO-2 have all been found in major mosquito genomes, e.g. A. gambiae, C. pipiens and A. aegypti. Moreover, AGO-2 has been found to evolve quickly, which suggests it s role in antiviral response (Campbell et al., 2008a). In Drosophila all three genes DCR-2, R2D2 and AGO-2 were found to evolve quickly under positively selection (O bbard et al., 2006). To date, genetic mutants for these siRNA proteins have not been availabl e in mosquitoes, however, the artificial system by injection of long dsRNA for DCR-2, R2D2 or AGO-2 has proved that these genes are important in contro lling flaviviruses and alphaviruses infection or spreading (Campbell et al., 2008b; Keene et al., 2004; Sanchez-Vargas et al., 200 9). Significance of Studying the Defensive Mechanisms in Drosophila and Mosquitoes A striking finding through studying Drosophila innate immunity is that most of the genes involved are very similar to genes im plicated in mammalian innate immunity. For example, the Toll-like receptor (TLR) in mammals is similar to Drosophila Toll and TLR has been found to be able to activate NFB (Medzhitov et al., 1997). Likewise, the apoptosis regulatory mechanisms are also conserved between Drosophila and mammals. Studying the role of apoptos is as an innate immune response in Drosophila may enrich the understanding of innate immunity in mammals. More importantly, studying the antiviral defensive mechanisms in mosquitoes might be useful to develop novel strategies to control mosquito-borne viruses. The transmission of mosquito-borne viruses r equires the established infection in the mosquito vector. When the viruses enter the mosquito body, they infect certain cells and replicate inside those cells. After the initial replicat ion, the viruses spread to the salivary gland (SG) of the mosquito and further r eplicate in SG. The viruses can then be 107

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transmitted to other hosts upon mosquito biti ng. Therefore, identifying the antiviral mechanisms in mosquito and genetically ma nipulating the defensive mechanism can help to break the viral transmission chain. Nowadays, many labs are focusing on genetic manipulation of vector mosquitoes to make them refractory to viruses and other parasites such as malaria. Then, the genetically manipulated mosquit oes can be released into the wild to replace the wild species. This strategy is termed popul ation replacement. Unlike the traditional population reduction strategy, po pulation replacement can avoid the potential ecological effect elicited by population reduction (Tereni us et al., 2008). Indeed, this strategy has successfully generated virus-refractory mosquitoes (Mathur et al., 2010) or malariaresistant mosquitoes (Ito et al., 2002; Moreira et al., 2002). However, the trangenes usually lead to fitness cost which make s the transgenic mosquit oes less competitive than the wild species (Catteruccia et al., 2003; Irvin et al., 2004; Moreira et al., 2004). Therefore, Mendelian i nheritance alone will not be suffici ent to drive the spreading of the refractory gene. A maternal-effect selfish genetic element, medea described in Drosophila might be useful to spread the tran sgenes in the wild (Chen et al., 2007). The Sterile Insect Technique (SIT) is a method to reduce and eliminate a certain population of the pest insect. After mass rearing and steriliz ing the males by exposing them to low doses of radiation, the sterile males are released into the wild to compete with wild male for female mating. Eventua lly, the population size will be reduced or totally eliminated. SIT has been succe ssfully used against the screwworm fly cocbliomyia bominivorax in the USA. SIT in mosquitoes were also conducted during 1970s and 80s on several mosquito species such as Ae. aegypti Ae. Albopictus C. 108

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pipiens C. quinquefasciatus A. gambiae and A. albimanus (Benedict and Robinson, 2003). However, although many of these tr ials showed a reduction in mosquito population, very few can eliminate the popul ation in the released area (Benedict and Robinson, 2003). One major problem with SIT is that it is difficult to separate the males from the females. This step is crucial as the female s transmit the disease through bite. The proapoptotic gene, mx initially identified by our lab, has been successfully used by others to specifically express in the flight muscl e of female and cause t he flightless phenotype of the female (Fu et al., 2010). This is an efficient way to remove females. Another drawback of traditional SIT is t he sterilization. It is very ha rd to find an ideal method to sterilize males while mitigating fitness cost. To overcome these drawbacks of traditional SIT, an improved technique, RIDL (Release of Insects carrying a Dominant Lethal) was developed. Unlike SIT, which releases the males sterilized with irradiation, RIDL releases males that are homozygous for a dominant lethal gene. Mating with wild populations produces offspring that are het erozygous for the dominant lethal gene, resulting in the deat h of all progeny. All in all, the identification of more and more factors involved in the mosquito defensive mechanisms will greatly benefit the development of novel disease-controlling strategies, and the elimination of mosquito-borne pathogens will no longer be a dream. 109

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110 Figure 5-1. The schematic representat ion of Toll and IMD immune pathways in Drosophila (cited from (Hoffmann, 2003))

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128 BIOGRAPHICAL SKETCH Bo Liu was born in 1981, in Wendeng, a ci ty in Shandong Province, China. He received his B.S. degree from t he Department of Biology at Xiamen University, China in 2004 and continued pursuing his M.S. degree at Department of Biomedical Science, Xiamen University under the guidance of Dr. Qi ao Wu. In 2008, he completed his thesis and received his M.S. degree. In the same year, he traveled from Ch ina to the United State of America and enrolled as a graduate st udent in the Interdisci plinary Program in Biomedical Sciences (IDP) at the University of Florida. In 2009, he joined the laboratory of Dr. Lei Zhou in the Department of Mole cular Genetics and Microbiology. He passed his qualifying exam and became an official Ph.D. candidate in October 2010. He received his Ph.D. degree in December 2012.