Abortive replication of Bombyx mori nucleopolyhedrovirus in Sf9 and High Five cells: defective nuclear transport of the virions

The DEAD/H-box helicase DHX9 contributes to suppression of Bombyx mori nucleopolyhedrovirus propagation in B. mori cells

Antiviral immune responses are mainly triggered through the recognition of virus-derived nucleic acids by host-specific pattern recognition receptors (PRRs). Here, we identified and characterized homologs of human PRRs for virus-derived DNA in Bombyx mori upon infection with a nucleopolyhedrovirus (NPV), a member of the family Baculoviridae. We found that progeny virus production of B. mori NPV was promoted in B. mori cells silenced with B. mori homolog of DEAD/H box polypeptide 9 gene (Bm-DHX9), but not in cells silenced with the other examined genes. Silencing of Bm-DHX9 expression has no effect on apoptosis induction, one of the major antiviral responses in B. mori cells. We also showed that Bm-DHX9 has the ability to bind DNA containing unmethylated C-phosphate-G-motif, which are characteristic of microbial pathogens and contained in the NPV genome with high frequency. Our findings suggest that Bm-DHX9 has the potential for sensing NPV-derived DNA to induce antiviral immune responses.

Label-free proteomics analysis on the envelope of budded viruses of Bombyx mori nucleopolyhedrovirus harboring differential localized GP64

Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 is the key membrane fusion protein that mediates budded virus (BV) infection. We recently reported that BmNPV GP64's n-region of signal peptide (SP) blocked the SP-cleavage and mediated GP64 localization on the plasma membrane (PM); n-region (SP^∆nGP64) absence caused GP64 intracellular localization, however, SP^∆nGP64 was still incorporated into virion to generate BVs with lower infectivity. To better understand the biogenesis of the envelope of BmNPV BV, we conducted a label-free ESI mass spectrometry analysis of the envelope of purified BVs harboring PM localized GP64 or intracellular localized SP^∆nGP64. The results indicated that 31 viral proteins were identified on the envelope, among which 15 were reported in other viruses. The other 16 proteins were first reported in BmNPV BV, including the BmNPV-specific protein BRO-A and proteins associated with vesicle transportation. Six proteins with significant intensity differences were detected in virions with differential localized GP64, and five specific proteins were identified in virions with GP64. Meanwhile, we identified 81 host proteins on the envelope, and seven lipoproteins were first identified in baculovirus virion; other 74 proteins are involved in the cytoskeleton, DNA-binding, vesicle transport, etc. In the meantime, eight and five specific host proteins were, respectively, identified in GP64 and SP^∆nGP64's virions. The two virions shared 68 common host proteins, and 8 proteins were identified on their envelopes with a significant difference. This study provides new insight into the protein composition of BmNPV BV and a clue for further investigation of the budding mechanism of BmNPV.

Bombyx mori Nucleopolyhedrovirus Hijacks Multivesicular Body as an Alternative Envelopment Platform for Budded Virus Egress

Baculovirus budded virus (BV) acquires its envelope and viral membrane fusion proteins from the plasma membrane (PM) of the host cell during the budding process. However, this classical BV egress pathway has been questioned because an intracellularly localized membrane fusion protein, SPΔnGP64 (glycoprotein 64 [GP64] lacking the signal peptide [SP] n region), was assembled into the envelope to generate infective BVs in our recent studies. Here, we identify an additional pathway for Bombyx mori nucleopolyhedrovirus (BmNPV) BV assembly and release that differs, in part, from the currently accepted model for the egress pathway of baculovirus. Electron microscopy showed that during infection, BmNPV-infected cells contained many newly formed multivesicular body (MVB)-like compartments that included mature virions at 30 h postinfection (p.i.). Immunoelectron microscopy demonstrated that the MVBs contained CD63, an MVB endosome marker, and GP64, a BmNPV fusion glycoprotein. MVB fusion with the PM and the release of mature virions, together with naked nucleocapsids, were observed at the cell surface. Furthermore, MVB egress mediated the translocation of SPΔnGP64 to the PM, which induced cell-cell fusion until 36 h p.i. This BV egress pathway can be partially inhibited by U18666A incubation and RNA interference targeting MVB biogenesis genes. Our findings indicate that BmNPV BVs are enveloped and released through MVBs via the cellular exosomal pathway, which is a subordinate BV egress pathway that produces virions with relatively inferior infectivity. This scenario has significant implications for the elucidation of the BmNPV BV envelopment pathway. IMPORTANCE BmNPV is a severe pathogen that infects mainly Bombyx mori, a domesticated insect of economic importance, and accounts for approximately 15% of economic losses in sericulture. BV production plays a key role in systemic BmNPV infection of larvae. Despite the progress made in the functional gene studies of BmNPV, BmNPV BV egress is ill-understood. This study reports a previously unreported MVB envelopment pathway in BmNPV BV egress. To our knowledge, this is the first report of a baculovirus using dual BV egress pathways. This specific BV egress mechanism explains the cause of the non-PM-localized SPΔnGP64-rescued gp64-null bacmid infectivity, elucidating the reason underlying the retention of SP by BmNPV GP64. The data obtained elucidate an alternate molecular mechanism of baculovirus BV egress.

Expression and localization of Bombyx mori nucleopolyhedrovirus GP37

Mitochondria play an essential role in intracellular energy metabolism. This study described the involvement of Bombyx mori nucleopolyhedrovirus (BmNPV) GP37 (BmGP37) in host mitochondria. Herein, the proteins associated with host mitochondria isolated from BmNPV-infected or mock-infected cells by two-dimensional gel electrophoresis were compared. One mitochondria-associated protein in virus-infected cells was identified as BmGP37 by liquid chromatography-mass spectrometry analysis. Furthermore, the BmGP37 antibodies were generated, which could react specifically with BmGP37 in the BmNPV-infected BmN cells. Western blot experiments showed that BmGP37 was expressed at 18 h post-infection and was verified as a mitochondria-associated protein. Immunofluorescence analysis demonstrated that BmGP37 localized to the host mitochondria during BmNPV infection. Furthermore, western blot analysis revealed that BmGP37 is a novel component protein of the occlusion-derived virus (ODV) of BmNPV. The present results indicated that BmGP37 is one of the ODV-associated proteins and may have important roles in host mitochondria during BmNPV infection.

Identification of the Key Functional Domains of Bombyx mori Nucleopolyhedrovirus IE1 Protein

The immediate early protein 1 (IE1) acts as a transcriptional activator and is essential for viral gene transcription and viral DNA replication. However, the key regulatory domains of IE1 remain poorly understood. Here, we analyzed the sequence characteristics of Bombyx mori nucleopolyhedrovirus (BmNPV) IE1 and identified the key functional domains of BmNPV IE1 by stepwise truncation. Our results showed that BmNPV IE1 was highly similar to Autographa californica nucleopolyhedrovirus (AcMNPV) IE1, but was less conserved with IE1 of other baculoviruses, the C-terminus of IE1 was more conserved than the N-terminus, and BmNPV IE1 was also necessary for BmNPV proliferation. Moreover, we found that IE1_158–208 was a major nuclear localization element, and IE1_1–157 and IE1_539–559 were minor nuclear localization elements, but the combination of these two minor elements was equally sufficient to fully mediate the nuclear entry of IE1. Meanwhile, IE1_1–258, IE1_560–584, and the association of amino acids 258 and 259 were indispensable for the transactivation activity of BmNPV IE1. These results systematically resolve the functional domains of BmNPV IE1, which contribute to the understanding of the mechanism of baculovirus infection and provide a possibility to synthesize a small molecule IE1-truncated mutant as an agonist or antagonist.

The Bombyx mori Nucleopolyhedrovirus GP64 Retains the Transmembrane Helix of Signal Peptide to Contribute to Secretion across the Cytomembrane

Bombyx mori nucleopolyhedrovirus (BmNPV) is the primary pathogen of silkworms that causes severe economic losses in sericulture. GP64 is the key membrane fusion protein that mediates budded virus (BV) fusion with the host cell membrane. Previously, we found that the n-region of the GP64 signal peptide (SP) is required for protein secretion and viral pathogenicity; however, our understanding of BmNPV GP64 remains limited. Here, we first reported that BmNPV GP64 retained its SP in the mature protein and virion in only host cells but did not retain in nonhost cells. Uncleaved SP mediates protein targeting to the cytomembrane or secretion in Bombyx mori cells. The exitance of the n-region extended the transmembrane helix length, which resulted in the cleavage site to be located in the helix structure and thus blocked cleavage from signal peptidase (SPase). Without the n-region, the protein fails to be transported to the cytomembrane, but this failure can be rescued by the cleavage site mutation of SP. Helix-breaking mutations in SP abolished protein targeting to the cytomembrane and secretion. Our results revealed a previously unrecognized mechanism by which SP of membrane fusion not only determines protein localization but also determines viral pathogenicity, which highlights the escape mechanism of SP from the cleavage by SPase. IMPORTANCE BmNPV is the primary pathogen of silkworms, which causes severe economic losses in sericulture. BmNPV and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) are closely related group I alphabaculoviruses, but they exhibit nonoverlapping host specificity. Recent studies suppose that GP64 is a determinant of host range, while knowledge remains limited. In this study, we revealed that BmNPV GP64 retained its SP in host cells but not in nonhost cells, and the SP retention is required for GP64 secretion across the cytomembrane. This is the first report that a type I membrane fusion protein retained its SP in mature proteins and virions. Our results unveil the mechanism by which SP GP64 escapes cleavage and the role of SP in protein targeting. This study will help elucidate an important mechanistic understanding of BmNPV infection and host range specificity.

Identification and Characterization of Genes Related to Resistance of Autographa californica Nucleopolyhedrovirus Infection in Bombyx mori

Simple Summary

Autographa californica nucleopolyhedrovirus (AcMNPV) is a kind of baculovirus that was initially found and named for its host, but the previous study reveals several silkworm strains are preferentially susceptible to AcMNPV through intrahemocelical injection method. In the following study, genetics analysis showed that a set of potential genes which controlled resistance of AcMNPV was located on chromosome 3. In the present research, we performed Genome-Wide Association Studies to identify the gene that controls the resistance of AcMNPV, results show that the Niemann-Pick C1 (NPC-1) gene is strongly associated with this resistance. Then we found that there are several amino acid mutations in the protein sequence of BmNPC1 between two different resistance strains of Bombyx mori. RNAi results showed that BmNPC1 successfully suppressed virus infection ability and changed the expression pattern of viral genes.

Abstract

In Bombyx mori, as an important economic insect, it was first found that some strains were completely refractory to infection with Autographa californica nucleopolyhedrovirus (AcMNPV) through intrahemocelical injection; whereas almost all natural strains had difficulty resisting Bombyx mori nucleopolyhedrovirus (BmNPV), which is also a member of the family Baculoviridae. Previous genetics analysis research found that this trait was controlled by a potentially corresponding locus on chromosome 3, but the specific gene and mechanism was still unknown. With the help of the massive silkworm strain re-sequencing dataset, we performed the Genome-Wide Association Studies (GWAS) to identify the gene related to the resistance of AcMNPV in this study. The GWAS results showed that the Niemann-Pick type C1 (NPC-1) gene was the most associated with the trait. The knockdown experiments in BmN cells showed that BmNPC1 has a successful virus suppression infection ability. We found a small number of amino acid mutations among different resistant silkworms, which indicates that these mutations contributed to the resistance of AcMNPV. Furthermore, inhibition of the BmNPC1 gene also changed the viral gene expression of the AcMNPV, which is similar to the expression profile in the transcriptome data of p50 and C108 strains.

Carbohydrate metabolism is a determinant for the host specificity of baculovirus infections

Baculoviruses Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV) have highly similar genome sequences but exhibit no overlap in their host range. After baculovirus infects nonpermissive larvae (e.g., AcMNPV infecting B. mori or BmNPV infecting Spodoptera litura), we found that stored carbohydrates, including hemolymph trehalose and fat body glycogen, are rapidly transformed into glucose; enzymes involved in glycolysis and the TCA cycle are upregulated and produce more ATP; adenosine signaling that regulates glycolytic activity is also increased. Subsequently, phagocytosis in cellular immunity and the expression of genes involved in humoral immunity increase significantly. Moreover, inhibiting glycolysis and the expression of gloverins in nonpermissive hosts increased baculovirus infectivity, indicating that the stimulated energy production is designed to support the immune response against infection. Our study highlights that alteration of the host's carbohydrate metabolism is an important factor determining the host specificity of baculoviruses, in addition to viral factors.

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Nonpermissive infections by AcMNPV and BmNPV alter host carbohydrate metabolism

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Increased carbohydrate metabolism produces energy to launch immune responses

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Immune responses including antimicrobial peptide production inhibit virus infection

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Host metabolic alterations affect the determination of virus's host specificity

NISES-AnPe-428 cell line derived from the Chinese oak silkworm Antheraea pernyi is permissive for multiple nucleopolyhedrovirus species from insects of four different families

The cell line NISES-AnPe-428 (AnPe), derived from the Chinese oak silkworm Antheraea pernyi, was characterized for its permissiveness and productivity for six different nucleopolyhedrovirus (NPV) species. These NPVs included homologous Antheraea pernyi NPV (AnpeNPV) and heterologous Autographa californica multiple NPV (AcMNPV), Bombyx mori NPV (BmNPV), Hyphantria cunea MNPV (HycuMNPV), Spodoptera exigua MNPV (SeMNPV), and Lymantria dispar MNPV (LdMNPV), representing viruses that had been isolated from insect species belonging to five different families (Saturniidae, Noctuidae, Bombycidae, Arctiidae, and Lymantriidae). We found that AnPe cells supported productive replication of AnpeNPV, AcMNPV, BmNPV, HycuMNPV, and SeMNPV to varying degrees. Upon infection with SeMNPV, a subset of AnPe cell population in the culture underwent apoptosis, while remaining cells produced limited amounts of progeny virions and polyhedra. AnPe cells were refractory to LdMNPV infection and failed to support replication of viral DNA, indicating that viral replication was restricted at or prior to the step of viral DNA replication. These results indicated that AnPe cells have the potential to provide excellent systems for studying the molecular mechanisms underlying cellular permissiveness for NPV replication and host-range determination of NPVs.

18 Additional Amino Acids of the Signal Peptide of the Bombyx mori Nucleopolyhedrovirus GP64 Activates Immunoglobulin Binding Protein (BiP) Expression by RNA-seq Analysis

GP64 is the key membrane fusion protein of Group I baculovirus, and while the Bombyx mori nucleopolyhedrovirus (BmNPV) GP64 contains a longer n-region (18 amino acid) of the signal peptide than does the Autographa californica multiple nucleopolyhedrovirus (AcMNPV), the function of the n-region has not been determined. In this study, we first showed that n-region is required for membrane protein localization in BmN cells, then the transcriptome sequencing was conducted on proteins guided by different signal peptide regions, and the results were analyzed and validated by quantitative PCR and luciferase assays. The results indicated that 1049 differentially expressed genes (DEGs) were identified among the different region of signal peptides and the control. With the n-region, the protein export pathway was upregulated significantly, the Wnt-1 signaling pathway was downregulated, and BiP was significantly activated by the GP64 full-length signal peptide. Furthermore, RNA interference on BiP efficiently increased luciferase secretion. These results indicate that the GP64 n-region plays a key role in protein expression and regulation.

Methyl-Beta-Cyclodextrin-Induced Macropinocytosis Results in Increased Infection of Sf21 Cells by Bombyx Mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is closely related to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with over ~93% amino acid sequence identity. However, their host ranges are essentially nonoverlapping. The mechanism of BmNPV entry into host cells is completely different from that of AcMNPV, and whether the entry mechanism difference relates to the host range remains unclear. BmNPV produces an abortive infection in nonhost cells due to virion nuclear transportation failure. Here, we performed a detailed study by increasing BmNPV infection in Sf21 cells with the aid of methyl-beta-cyclodextrin (MβCD). We found that low-concentration MβCD incubation efficiently activates membrane ruffling in Sf21 cells, which mediates the increase in BmNPV infection. Interestingly, MβCD incubation after virion internalization also increases the infection, which suggests that macropinocytosis is involved in BmNPV infection in Sf21 cells after virion internalization. Further study revealed that clathrin-mediated endocytosis (CME) is employed by BmNPV to facilitate entry into Sf21 cells, and chlorpromazine application abolishes BmNPV infection in cells incubated both with and without MβCD. Based on these studies, we show that BmNPV enters Sf21 cells via CME and that parallel induction of macropinocytosis facilitates BmNPV infection in Sf21 cells. This study reveals the mechanism of BmNPV entry into Sf21 cells and provides clues for improving BmNPV infections in nonpermissive cells.

Transport Via Macropinocytic Vesicles is Crucial for Productive Infection with Bombyx Mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is a serious viral pathogen in the sericulture industry and enters host cells via macropinocytic endocytosis; however, the current understanding of the BmNPV entry mechanism remains limited. To confirm whether direct membrane fusion (DMF) results in productive BmNPV infection, DMF infectivity induced by low pH during BmNPV infection was investigated, and the infectious viral particle was traced using an eGFP-labeled virion. We found that BmNPV infection efficiently induced fluid uptake, which allowed BmNPV to bypass the cell membrane barrier via macropinocytosis. However, DMF induced by a low pH abolished the infection. While low pH is an essential condition for membrane fusion triggering, it is not sufficient for productive BmNPV infection, and DMF results in failure to transport the nucleocapsid into the nucleus. These results indicate that transport via macropinocytic vesicles facilitates BmNPV entry into the nucleus and contribute to our understanding of the BmNPV entry mechanism.

Antiviral immune responses of Bombyx mori cells during abortive infection with Autographa californica multiple nucleopolyhedrovirus

Lepidopteran cells rely on multiple antiviral responses to defend against baculovirus infections, including apoptosis, global protein synthesis shutdown, and rRNA degradation. Here, we characterized apoptosis and rRNA degradation in Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Bombyx mori cells, a system resulting in abortive infection, in relation to viral DNA replication and viral late gene expression. RNAi-mediated silencing of viral DNA replication-related genes prevented apoptosis, but not rRNA degradation, in B. mori cells infected with p35-deficient AcMNPV. Additionally, AcMNPV, but not B. mori nucleopolyhedrovirus (BmNPV), drastically reduced B. mori cellular iap1 transcript levels and p35-deficient AcMNPV induced more prominent apoptosis than did p35-deficient BmNPV. These results, together with previous results that global protein synthesis shutdown follows viral DNA replication, demonstrate that rRNA degradation is the primary antiviral response that abolishes productive AcMNPV infection of B. mori cells. Our results also demonstrate that B. mori cells induce apoptosis to a different extent depending on NPV species.

HCF-1 encoded by baculovirus AcMNPV is required for productive nucleopolyhedrovirus infection of non-permissive Tn368 cells

Baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) replicates in both Spodoptera frugiperda Sf21 and Trichoplusia ni Tn368 cells, whereas AcMNPV defective in hcf-1 (host cell-factor 1) gene productively infects only Sf21 cells, indicating that HCF-1 is indispensable for the AcMNPV productive infection of Tn368 cells. Here, we demonstrated that HCF-1 protein transiently expressed in Tn368 cells promotes the DNA synthesis of Hyphantria cunea MNPV (HycuMNPV), Orygia pseudotsugata MNPV and Bombyx mori NPV, which are normally unable to replicate in Tn368 cells. We also demonstrated that a recombinant HycuMNPV harboring the hcf-1 gene successfully replicates in Tn368 cells, generating substantial yields of progeny viruses and polyhedra. These results indicate that HCF-1 encoded by AcMNPV is an essential viral factor for productive NPV infection of Tn368 cells. Taken together with the previous findings on HRF-1 (host range factor 1), the present results provide strong evidence that viral genes acquired through horizontal gene transfer play an important role in baculovirus evolution, serving to expand the host range of baculoviruses.

Midgut-based resistance to oral infection by a nucleopolyhedrovirus in the laboratory-selected strain of the smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae)

A strain of Adoxophyes honmai resistant to Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) was established from a field-collected colony by repeated selection. Fifth-instar larvae of this resistant strain (R-strain) had over 66 666-fold greater resistance in terms of 50 % lethal concentration values to oral infection of AdhoNPV than non-selected strain larvae (susceptible for AdhoNPV; S2-strain). In this study, the mechanism of resistance to AdhoNPV was determined in R-strain larvae. An assessment of viral genome replication in AdhoNPV-infected S2- and R-strain larvae by quantitative PCR showed no viral genome replication occurring in R-strain larvae. Transcription of AdhoNPV ie-1, vp39 and polyhedrin genes was also not detected in R-strain midgut cells. Besides, a fluorescent brightener had no effect on AdhoNPV infection in either S2- or R-strain. However, binding and fusion of occlusion-derived virus with R-strain were significantly lower than those of S2-strain. These findings suggest that R-strain Adoxophyeshonmai larvae possess a midgut-based resistance to oral infection by AdhoNPV in which midgut epithelial cells are infected less efficiently.

Entry of Bombyx mori nucleopolyhedrovirus into BmN cells by cholesterol-dependent macropinocytic endocytosis

Bombyx mori nucleopolyhedrovirus (BmNPV) is a serious viral pathogen of silkworm, and no drug or specific protection against BmNPV infection is available at present time. Although functions of most BmNPV genes were depicted in recent years, knowledge on the mechanism of BmNPV entry into insect cells is still limited. Here BmNPV cell entry mechanism is investigated by different endocytic inhibitor application and subcellular analysis. Results indicated that BmNPV enters BmN cells by clathrin-independent macropinocytic endocytosis, which is mediated by cholesterol in a dose-dependent manner, and cholesterol replenishment rescued the BmNPV infection partially.

Differential susceptibilities to BmNPV infection of two cell lines derived from the same silkworm ovarian tissues

We previously established and characterized two insect cell lines (BmN-SWU1 and BmN-SWU2) from Bombyx mori ovaries. Here, we examined their differential susceptibilities to Bombyx mori nucleopolyhedrovirus (BmNPV) despite having originated from the same tissue source. BmN-SWU1 cells were susceptible and supported high titers of BmNPV replication, while BmN-SWU2 cells were resistant to BmNPV infection. Subcellular localization analysis demonstrated that very few BmNPV particles could be imported into BmN-SWU2 cells. However, initiation of BmNPV DNA replication but not amplification was detected in BmN-SWU2 cells after transfection with vA4^prm-VP39-EGFP bacmid DNA. BmNPV transcription assays showed that late and very late but not early viral genes apparently were blocked in BmNSWU2 cells by unknown mechanisms. Further syncytium formation assays demonstrated that the BmNPV envelope fusion protein GP64 could not mediate BmN-SWU2 host cell-cell membrane fusion. Taken together, these results indicate that these two cell lines represent optimal tools for investigating host-virus interactions and insect antiviral mechanisms.

Degradation of rRNA in BM-N cells from the silkworm Bombyx mori during abortive infection with heterologous nucleopolyhedroviruses

Cell lines derived from the silkworm, Bombyx mori, are only permissive for B. mori nucleopolyhedrovirus (NPV), with other NPVs generally resulting in abortive infection. Here, we demonstrate that rRNA of B. mori BM-N cells undergoes rapid degradation through site-specific cleavage upon infection with NPVs from Autographa californica (AcMNPV), Hyphantria cunea (HycuMNPV), Spodoptera exigua (SeMNPV) and Spodoptera litura (SpltMNPV). No significant decreases in cellular RNA were observed in Ld652Y, Se301, Sf9, SpIm and S2 cells infected with AcMNPV or HycuMNPV, indicating the response is unique to BM-N cells. A transient expression assay using a cosmid library of the HycuMNPV genome demonstrated that HycuMNPV P143 is responsible for rRNA degradation, which was also detected in BM-N cells transfected with plasmids expressing the P143 proteins from AcMNPV, SeMNPV and SpltMNPV. These results indicate that B. mori evolved to acquire a unique antiviral immune mechanism that is activated by P143 proteins from heterologous NPVs.

Mutualistic polydnaviruses share essential replication gene functions with pathogenic ancestors

Viruses are usually thought to form parasitic associations with hosts, but all members of the family Polydnaviridae are obligate mutualists of insects called parasitoid wasps. Phylogenetic data founded on sequence comparisons of viral genes indicate that polydnaviruses in the genus Bracovirus (BV) are closely related to pathogenic nudiviruses and baculoviruses. However, pronounced differences in the biology of BVs and baculoviruses together with high divergence of many shared genes make it unclear whether BV homologs still retain baculovirus-like functions. Here we report that virions from Microplitis demolitor bracovirus (MdBV) contain multiple baculovirus-like and nudivirus-like conserved gene products. We further show that RNA interference effectively and specifically knocks down MdBV gene expression. Coupling RNAi knockdown methods with functional assays, we examined the activity of six genes in the MdBV conserved gene set that are known to have essential roles in transcription (lef-4, lef-9), capsid assembly (vp39, vlf-1), and envelope formation (p74, pif-1) during baculovirus replication. Our results indicated that MdBV produces a baculovirus-like RNA polymerase that transcribes virus structural genes. Our results also supported a conserved role for vp39, vlf-1, p74, and pif-1 as structural components of MdBV virions. Additional experiments suggested that vlf-1 together with the nudivirus-like gene int-1 also have novel functions in regulating excision of MdBV proviral DNAs for packaging into virions. Overall, these data provide the first experimental insights into the function of BV genes in virion formation.

Microorganisms form symbiotic associations with animals and plants that range from parasitic (pathogens) to beneficial (mutualists). Although numerous examples of obligate, mutualistic bacteria, fungi, and protozoans exist, viruses are almost always considered to be pathogens. An exception is the family Polydnaviridae, which consists of large DNA viruses that are obligate mutualists of insects called parasitoid wasps. Prior studies show that polydnaviruses in the genus Bracovirus evolved approximately 100 million years ago from a group of viruses called nudiviruses, which are themselves closely related to a large family of insect pathogens called baculoviruses. Polydnaviruses are thus of fundamental interest for understanding the processes by which viruses can evolve into mutualists. In this study we characterized the composition of virus particles from Microplitis demolitor bracovirus (MdBV) and conducted functional experiments to assess whether BV genes share similar functions with related essential baculovirus replication genes. Our results indicate that several genes in MdBV retain ancestral functions, but select other genes have novel functions unknown from baculoviruses. Our results also provide the first experimental data on the function of polydnavirus replication genes and enhance understanding of the similarities between these viruses and their pathogenic ancestors.

AcMNPV enhances infection by ThorNPV in Sf21 cells and SeMNPV in Hi5 cells

An expression cassette containing the DsRed2 gene, which encodes the red fluorescent protein (RFP), was inserted into the wide-host-range Autographa californica M nucleopolyhedrovirus (AcMNPV) at the polyhedrin locus (vAcDsRed2). An expression cassette containing the enhanced green fluorescent protein (EGFP) gene was inserted at the gp37 locus of the narrow-host-range Thysanoplusia orichalcea MNPV (ThorMNPV) and the p10 locus of Spodoptera exigua MNPV (SeMNPV) to produce vThGFP and vSeGFP, respectively. vThGFP and vSeGFP are poor at infecting Sf21 and Hi5 cells, respectively, whereas vAcDsRed2 is highly infectious to both cell lines. During co-infection, vAcDsRed2 enhanced vThGFP infection in Sf21 cells by approximately 20-fold, and it enhanced vSeGFP infection in Hi5 cells by more than 300-fold, as detected by fluorescence measurements. In contrast, vThGFP reduced vAcDsRed2 infection by 5.4-fold in Sf21 cells, while vSeGFP reduced vAcDsRed2 by 3.2-fold in Hi5 cells. Plaque assay data did not suggest viral recombination, but vThGFP plaques surrounded by vAcDsRed2 plaques were observed. A viral DNA replication assay performed by real-time quantitative PCR suggested that the detected fluorescence correlated with virus replication. Sf21 cells infected with vAcDsRed2 were resistant to superinfection by viruses of the same type expressing EGFP (vAcGFP). These results demonstrated that AcMNPV could enhance replication of ThorMNPV and SeMNPV in non-permissive cells without recombination.

A baculovirus isolated from wild silkworm encompasses the host ranges of Bombyx mori nucleopolyhedrosis virus and Autographa californica multiple nucleopolyhedrovirus in cultured cells

A baculovirus, named BomaNPV S2, was isolated from a diseased larva of the wild silkworm, Bombyx mandarina. Notably, BomaNPV S2 exhibited a distinguishing feature in that its host range covered that of both Bombyx mori nucleopolyhedrosis virus (BmNPV) and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in cultured cells. It could replicate in cells of B. mori (Bm5 and BmN), Spodoptera frugiperda (Sf9) and Trichoplusia ni (Tn-5B1-4). However, occlusion-derived virions of BomaNPV S2 in B. mori cells contained only a single nucleocapsid, whereas they contained multiple nucleocapsids in Tn-5B1-4 cells. The complete genome sequence of BomaNPV S2, including predicted ORFs, was determined and compared with the genome sequence of its close relatives. The comparison results showed that most of the BomaNPV S2 genome sequence was shared with BmNPV (BmNPV T3) or BomaNPV S1, but several regions seemed more similar to regions of AcMNPV. This observation might explain why BomaNPV S2 covers the host ranges of BmNPV and AcMNPV. Further recombinant virus infection experiments demonstrated that GP64 plays an important role in BomaNPV S2 host-range determination.

Cloning and characterization of a dronc homologue in the silkworm, Bombyx mori

We cloned and characterized a novel Bombyx mori homologue (bm-dronc) of Drosophila melanogaster dronc (dm-dronc), which could encode a polypeptide of 438 amino acid residues. Bm-Dronc shares relatively low amino acid sequence identities of 25% and 26% with Dm-Dronc and Aedes aegypti Dronc (Aa-Dronc), respectively. Bm-Dronc has the sequence QACRG surrounding the catalytic site (C), which is consistent with the QAC(R/Q/G)(G/E) consensus sequence in most caspases but distinct from the sequences PFCRG and SICRG of Dm-Dronc and Aa-Dronc, respectively. Bm-Dronc possesses a long N-terminal prodomain containing a caspase recruitment domain (CARD), a p20 domain and a p10 domain, exhibiting cleavage activities on synthetic substrates Ac-VDVAD-AMC, Ac-IETD-AMC and Ac-LEHD-AMC, which are preferred by human initiator caspases-2, -8 and -9, respectively. Bm-Dronc transiently expressed in insect cells and Escherichia coli cells underwent spontaneous cleavage and caused apoptosis and stimulation of caspase-3-like protease activity in various lepidopteran cell lines, but not in the dipteran cell line D. melanogaster S2. The apoptosis and the stimulation of caspase-3-like protease activity induced by Bm-Dronc overexpression were abrogated upon transfection with either a double-stranded RNA against bm-dronc or a plasmid expressing functional anti-apoptotic protein Hycu-IAP3 encoded by the baculovirus Hyphantria cunea multiple nucleopolyhedrovirus (MNPV). Apoptosis induction in BM-N cells by infection with a p35-defective Autographa californica MNPV or exposure to actinomycin D and UV promoted the cleavage of Bm-Dronc. These results indicate that Bm-Dronc serves as the initiator caspase responsible for the induction of caspase-dependent apoptosis.

Baculovirus IAP1 induces caspase-dependent apoptosis in insect cells

Baculoviruses encode inhibitors of apoptosis (IAPs), which are classified into five groups, IAP1-5, based on their sequence homology. Most of the baculovirus IAPs with anti-apoptotic functions belong to the IAP3 group, with certain exceptions. The functional roles of IAPs from other groups during virus infection have not been well established. We have previously shown that Hyphantria cunea multiple nucleopolyhedrovirus (HycuMNPV) encodes three iap genes, hycu-iap1, hycu-iap2 and hycu-iap3, and that only Hycu-IAP3 has anti-apoptotic activity against actinomycin D-induced apoptosis of Spodoptera frugiperda Sf9 cells. In the present study, we demonstrate that transient expression of Hycu-IAP1 is capable of inducing apoptosis and/or stimulating caspase-3-like protease activity in various lepidopteran and dipteran cell lines. Transient-expression assay analysis also demonstrates that not only Hycu-IAP1 but also IAP1s from Autographa californica MNPV, Bombyx mori NPV and Orgyia pseudotsugata MNPV (OpMNPV) are capable of inducing apoptosis, and that apoptosis induced by Hycu-IAP1 is precluded by the functional anti-apoptotic baculovirus protein Hycu-IAP3. In HycuMNPV-infected Spilosoma imparilis (SpIm) cells and OpMNPV-infected Ld652Y cells, caspase-3-like protease activity is markedly stimulated during the late stages of infection, and the caspase-3-like protease activity stimulated in HycuMNPV-infected SpIm cells is repressed by RNA interference-mediated silencing of hycu-iap1. In addition, initiator caspase Bm-Dronc, the B. mori homologue of Dronc, is cleaved upon transfection of BM-N cells with a plasmid expressing Hycu-IAP1. These results provide the first evidence that baculovirus IAP1s act to induce caspase-dependent apoptosis, possibly by replacing the cellular IAP1 that prevents Dronc activation.

A silencing suppressor protein (NSs) of a tospovirus enhances baculovirus replication in permissive and semipermissive insect cell lines

The nonstructural protein (NSs) of the Tomato spotted wilt virus (TSWV) has been identified as an RNAi suppressor in plant cells. A recombinant Autographa californica multiple nucleopolyhedrovirus (AcMNPV) designated vAcNSs, containing the NSs gene under the control of the viral polyhedrin (polh) gene promoter, was constructed and the effects of NSs in permissive, semipermissive and nonpermissive insect cells to vAcNSs infection were evaluated. vAcNSs produced more budded virus when compared to wild type in semipermissive cells. Co-infection of vAcNSs with wild type baculoviruses clearly enhanced polyhedra production in all host cells. Confocal microscopy analysis showed that NSs accumulated in abundance in the cytoplasm of permissive and semipermissive cells. In contrast, high amounts of NSs were detected in the nuclei of nonpermissive cells. Co-infection of vAcNSs with a recombinant AcMNPV containing the enhanced green fluorescent protein (egfp) gene, significantly increased EGFP expression in semipermissive cells and in Anticarsia gemmatalis-hemocytes. Absence of small RNA molecules of egfp transcripts in this cell line and in a permissive cell line indicates the suppression of gene silencing activity. On the other hand, vAcNSs was not able to suppress RNAi in a nonpermissive cell line. Our data showed that NSs protein of TSWV facilitates baculovirus replication in different lepidopteran cell lines, and these results indicate that NSs could play a similar role during TSWV-infection in its thrips vector.

Comparative transient expression assay analysis of hycu-hr6- and IE1-dependent regulation of baculovirus gp64 early promoters in three insect cell lines

We previously demonstrated that the Hyphantria cunea multicapsid nucleopolyhedrovirus (HycuMNPV) gp64 gene (hycu-gp64) is uniquely localized on the viral genome with a large homologous region of 1582bp, hycu-hr6, immediately upstream of the hycu-gp64 gene. In the present study, we compared the regulation of gp64 early promoters from HycuMNPV, Autographa californica multicapsid NPV (AcMNPV) and Bombyx mori NPV (BmNPV) by cis-acting hycu-hr6 and trans-acting IE1s in three cell lines (Spodoptera frugiperda Sf9, Bombyx mori BM-N and Spilosoma imparilis SpIm). A transient expression assay with plasmids harboring a reporter luciferase gene demonstrated that the gp64 early promoters are positively regulated by hycu-hr6, independent of virus and cell types. In contrast, gp64 early promoters were regulated positively or negatively by trans-acting IE1s, in a cell- and virus-type dependent manner, indicating that cellular factors, as well as viral factors, are responsible for IE1-dependent regulation of gp64 early promoters. However, hycu-gp64 early promoter activity was consistently suppressed by HycuMNPV IE1 (Hycu-IE1), irrespective of the cell lines used. Analysis of the hycu-gp64 early promoter region revealed two novel sequence elements that were involved in Hycu-IE1-dependent negative regulation of the hycu-gp64 early promoter. These two novel regulatory sequence elements could compensate for each other but could not be substituted with AcMNPV IE1 binding motif (Ac-IBM). These results suggest that IE1 regulates gp64 early promoters to produce the proper amount of GP64 protein, depending upon NPV-insect cell systems.

A single amino acid substitution modulates low-pH-triggered membrane fusion of GP64 protein in Autographa californica and Bombyx mori nucleopolyhedroviruses

We have previously shown that budded viruses of Bombyx mori nucleopolyhedrovirus (BmNPV) enter the cell cytoplasm but do not migrate into the nuclei of non-permissive Sf9 cells that support a high titer of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) multiplication. Here we show, using the syncytium formation assay, that low-pH-triggered membrane fusion of BmNPV GP64 protein (Bm-GP64) is significantly lower than that of AcMNPV GP64 protein (Ac-GP64). Mutational analyses of GP64 proteins revealed that a single amino acid substitution between Ac-GP64 H155 and Bm-GP64 Y153 can have significant positive or negative effects on membrane fusion activity. Studies using bacmid-based GP64 recombinant AcMNPV harboring point-mutated ac-gp64 and bm-gp64 genes showed that Ac-GP64 H155Y and Bm-GP64 Y153H substitutions decreased and increased, respectively, the multiplication and cell-to-cell spread of progeny viruses. These results indicate that Ac-GP64 H155 facilitates the low-pH-triggered membrane fusion reaction between virus envelopes and endosomal membranes.

Comparative studies of lepidopteran baculovirus-specific protein FP25K: development of a novel Bombyx mori nucleopolyhedrovirus-based vector with a modified fp25K gene

Lepidopteran baculovirus-specific protein FP25K performs many roles during the infection cycle, including functions in the production of occlusion bodies (OBs) and budded viruses (BVs), oral infection, and postmortem host degradation. To explore the common and specific functions of FP25K proteins among lepidopteran baculoviruses, we performed comparative analyses of FP25K proteins from group I and group II nucleopolyhedroviruses (NPVs) and granulovirus (GV). Using recombinant Bombyx mori NPVs (BmNPVs), we showed that the FP25Ks from NPVs were able to eliminate all the phenotypic defects observed in an infection with a BmNPV mutant lacking functional fp25K but that FP25K from GV did not show abilities to recover oral infectivity and postmortem host degradation. We also observed that introduction of Autographa californica multiple NPV (AcMNPV) fp25K into the BmNPV genome enhanced OB and BV production. According to these results, we generated a novel BmNPV-based expression vector with AcMNPV fp25K and examined its potential in BmN cells and B. mori larvae. Our results showed that the introduction of AcMNPV fp25K significantly increases the expression of foreign gene products in cultured cells and shortens the time for obtaining the secreted recombinant proteins from larval hemolymph.

Establishment and characterization of the Bombyx mandarina cell line

A new cell line, designated as NIAS-Boma-529b, was established from the larval fat bodies of Bombyx mandarina (B. mandarina), which is believed to be an ancestor of Bombyx mori (B. mori). This cell line has been cultured for approximately 150 passages during 2years in an IPL-41 medium supplemented with 10% fetal bovine serum at a constant temperature of 26 degrees C. The morphology of this line includes adhesive round and spindle-shaped cells. Random-amplified polymorphic DNA analysis (RAPD) using 7 primers and a statistical analysis based on Nei's genetic distance revealed that this cell line was closely related to B. mori-derived cell lines. An infection study also revealed that this cell line was susceptible to B. mori nucleopolyhedrovirus (BmNPV); however, it had no apparent susceptibility to Autographa californica NPV (AcNPV), which is closely related to BmNPV. Nevertheless, cells infected with AcNPV showed an extensive cytopathic effect (CPE), including a rough cell surface, rounding, nuclear expansion, and cell blebbing. These results suggest that this cell line can be useful to clarify the mechanism of host range determination of BmNPV and AcNPV.

Slow cell infection, inefficient primary infection and inability to replicate in the fat body determine the host range of Thysanoplusia orichalcea nucleopolyhedrovirus

Thysanoplusia orichaceamulticapsid nucleopolyhedrovirus (ThorMNPV) carrying an enhanced green fluorescent protein (EGFP) gene expression cassette (vThGFP) was used to study host-range mechanisms. Infection kinetics showed that vThGFP replication in Sf21 cells was too slow to suppress cell growth. Wide-host-rangeAutographa californicaMNPV (AcMNPV) could speed up vThGFP infection and enhance the vThGFP infection rate in Sf21 cells. The enhancement was not due to recombination, as no recombinant virus was isolated from co-infection by plaque assay. No improvement of vThGFP infection in Sf21 was found by AcMNPV cosmid transactivation assay. However, culture medium from Sf21 cells infected with AcMNPV did enhance vThGFP replication in Sf21. Third-instar larvae ofSpodoptera frugiperda,S. exiguaandHelicoverpa zeawere not killed by feeding with vThGFP polyhedra but were killed by intrahaemocoelic injection using budded viruses (BVs). This suggested that insufficient BVs were generated during the primary infection in the midgut. vThGFP infected haemocytes, tracheae and Malpighian tubules but not fat bodies of larvae ofS. frugiperda,S. exiguaandH. zea. Third-instarS. frugiperdalarvae co-infected by injection with vThGFP and vAcDsRed2, an AcMNPV expressing a red fluorescent protein gene, showed EGFP expression in the fat body. This result suggests that vAcDsRed2 could help vThGFP to replicate in the fat body ortrans-activate EGFP expression in the fat body. All these results suggested that slow cell infection, insufficient primary infection and inability to replicate in the fat body control the host range of ThorMNPV.