Stimulation of cell motility by a viral fibroblast growth factor homolog: proposal for a role in viral pathogenesis

Successful Rescue of Synthetic AcMNPV with a ~17 kb Deletion in the C1 Region of the Genome

Baculoviruses have been widely used as expression vectors. However, numerous genes in the baculoviral genome are non-essential for cellular infection and protein expression, making the optimisation of baculovirus expression vectors possible. We used a synthetic biological method to reduce the number of genes in a partial region of the autograph californica multiple nucleopolyhedrovirus (AcMNPV), the most widely used baculovirus expression vector. The C1 region of the AcMNPV is 46.4 kb and is subdivided into B1, B2, and B3 fragments. We first designed modified B1, B2, and B3 fragments by deleting the non-essential genes, and then synthesised complete viral genomes containing either individual modified B fragments or joint modified B fragments through transformation-related recombination in yeast. The synthetic genomes were then transfected into Sf9 cells to rescue the progeny viruses and test their infectivity. The design-build-test cycle was repeated until the ultimately rescued virus could produce progeny viruses efficiently. Finally, AcMNPV-Syn-mC1-1.1 by deleting approximately 17.2 kb, including 20 ORFs, in the C1 region, was obtained. This is essential to the synthesis of a minimal AcMNPV genome that can generate infectious progeny viruses and can be further used to optimise the foundation of baculovirus expression vectors.

Alteration in Cellular Signaling and Metabolic Reprogramming during Viral Infection

Cellular activities are finely regulated by numerous signaling pathways to support specific functions of complex life processes. Viruses are obligate intracellular parasites. Each step of viral replication is ultimately governed by the interaction of a virus with its host cells. Because of the demands of viral replication, the nutritional needs of virus-infected cells differ from those of uninfected cells. To improve their chances of survival and replication, viruses have evolved to commandeer cellular processes, including cell metabolism, augmenting these processes to support their needs. This article summarizes recent findings regarding virus-induced alterations to major cellular metabolic pathways focusing on how viruses modulate various signaling cascades to induce these changes. We begin with a general introduction describing the role played by signaling pathways in cellular metabolism. We then discuss how different viruses target these signaling pathways to reprogram host metabolism to favor the viral needs. We highlight the gaps in understanding metabolism-related virus-host interactions and discuss how studying these changes will enhance our understanding of fundamental processes involved in metabolic regulation. Finally, we discuss the potential to harness these processes to combat viral diseases, as well as other diseases, including metabolic disorders and cancers.

Systematic Analysis of 42 Autographa Californica Multiple Nucleopolyhedrovirus Genes Identifies An Additional Six Genes Involved in the Production of Infectious Budded Virus

Baculoviruses have been widely used as a vector for expressing foreign genes. Among numerous baculoviruses, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most frequently used and it encodes 155 open reading frames (ORFs). Here, we systematically investigated the impact of 42 genes of AcMNPV on the production of infectious budded viruses (BVs) by constructing gene-knockout bacmids and subsequently conducting transfection and infection assays. The results showed that among the 39 functionally unverified genes and 3 recently reported genes, 36 are dispensable for infectious BV production, as the one-step growth curves of the gene-knockout viruses were not significantly different from those of the parental virus. Three genes (ac62, ac82 and ac106/107) are essential for infectious BV production, as deletions thereof resulted in complete loss of infectivity while the repaired viruses showed no significant difference in comparison to the parental virus. In addition, three genes (ac13, ac51 and ac120) are important but not essential for infectious BV production, as gene-knockout viruses produced significantly lower BV levels than that of the parental virus or repaired viruses. We then grouped the 155 AcMNPV genes into three categories (Dispensable, Essential, or Important for infectious BV production). Based on our results and previous publications, we constructed a schematic diagram of a potential mini-genome of AcMNPV, which contains only essential and important genes. The results shed light on our understanding of functional genomics of baculoviruses and provide fundamental information for future engineering of baculovirus expression system.

Baculovirus actin-rearrangement-inducing factor ARIF-1 induces the formation of dynamic invadosome clusters

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), a pathogen of lepidopteran insects, has a striking dependence on the host cell actin cytoskeleton. During the delayed-early stage of infection, AcMNPV was shown to induce the accumulation of actin at the cortex of infected cells. However, the dynamics and molecular mechanism of cortical actin assembly remained unknown. Here, we show that AcMNPV induces dynamic cortical clusters of dot-like actin structures that mediate degradation of the underlying extracellular matrix and therefore function similarly to clusters of invadosomes in mammalian cells. Furthermore, we find that the AcMNPV protein actin-rearrangement-inducing factor-1 (ARIF-1), which was previously shown to be necessary and sufficient for cortical actin assembly and efficient viral infection in insect hosts, is both necessary and sufficient for invadosome formation. We mapped the sequences within the C-terminal cytoplasmic region of ARIF-1 that are required for invadosome formation and identified individual tyrosine and proline residues that are required for organizing these structures. Additionally, we found that ARIF-1 and the invadosome-associated proteins cortactin and the Arp2/3 complex localize to invadosomes and Arp2/3 complex is required for their formation. These ARIF-1-induced invadosomes may be important for the function of ARIF-1 in systemic virus spread.

Effects of Manipulating Fibroblast Growth Factor Expression on Sindbis Virus Replication In Vitro and in Aedes aegypti Mosquitoes

Fibroblast growth factors (FGFs) are conserved among vertebrate and invertebrate animals and function in cell proliferation, cell differentiation, tissue repair, and embryonic development. A viral fibroblast growth factor (vFGF) homolog encoded by baculoviruses, a group of insect viruses, is involved in escape of baculoviruses from the insect midgut by stimulating basal lamina remodeling. This led us to investigate whether cellular FGF is involved in the escape of an arbovirus from mosquito midgut. In this study, the effects of manipulating FGF expression on Sindbis virus (SINV) replication and escape from the midgut of the mosquito vector Aedes aegypti were examined. RNAi-mediated silencing of either Ae. aegypti FGF (AeFGF) or FGF receptor (AeFGFR) expression reduced SINV replication following oral infection of Ae. aegypti mosquitoes. However, overexpression of baculovirus vFGF using recombinant SINV constructs had no effect on replication of these viruses in cultured mosquito or vertebrate cells, or in orally infected Ae. aegypti mosquitoes. We conclude that reducing FGF signaling decreases the ability of SINV to replicate in mosquitoes, but that overexpression of vFGF has no effect, possibly because endogenous FGF levels are already sufficient for optimal virus replication. These results support the hypothesis that FGF signaling, possibly by inducing remodeling of midgut basal lamina, is involved in arbovirus midgut escape following virus acquisition from a blood meal.

The Autographa californica Multiple Nucleopolyhedrovirus ac51 Gene Is Required for Efficient Nuclear Egress of Nucleocapsids and Is Essential for In Vivo Virulence

Alphabaculoviruses are lepidopteran-specific nucleopolyhedroviruses that replicate within the nucleus; however, the anterograde transport of the nucleocapsids of these viruses, which is an obligatory step for progeny virion production, is not well understood. In the present study, a unique Alphabaculovirus gene with unknown function, namely, the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac51 gene, was found to be required for efficient nuclear egress of AcMNPV nucleocapsids. Our results indicate that ac51 is a late gene, and Ac51 protein was detectable from 24 to 72 h postinfection using an antibody raised against Ac51. Ac51 is distributed in both the cytoplasm and nuclei of infected cells. Upon ac51 deletion, budded virion (BV) production by 96 h posttransfection was reduced by approximately 1,000-fold compared with that of wild-type AcMNPV. Neither viral DNA synthesis nor viral gene expression was affected. Ac51 was demonstrated to be a nucleocapsid protein of BVs, and ac51 deletion did not interrupt nucleocapsid assembly and occlusion-derived virion (ODV) formation. However, BV production in the supernatants of transfected cells during a viral life cycle was substantially decreased when ac51 was deleted. Further analysis showed that, compared with wild-type AcMNPV, ac51 deletion decreased nucleocapsid egress, while the numbers of nucleocapsids in the nuclei were comparable. Deletion of ac51 also eliminated the virulence of AcMNPV in vivo Taken together, our results support the conclusion that ac51 plays an important role in the nuclear egress of nucleocapsids during BV formation and is essential for the in vivo virulence of AcMNPV.

Global Analysis of Baculovirus Autographa californica Multiple Nucleopolyhedrovirus Gene Expression in the Midgut of the Lepidopteran Host Trichoplusia ni

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a large double-stranded DNA (dsDNA) virus that encodes approximately 156 genes and is highly pathogenic to a variety of larval lepidopteran insects in nature. Oral infection of larval midgut cells is initiated by the occlusion-derived virus (ODV), while secondary infection of other tissues is mediated by the budded virus (BV). Global viral gene expression has been studied in detail in BV-infected cell cultures, but studies of ODV infection in the larval midgut are limited. In this study, we examined expression of the ∼156 AcMNPV genes in Trichoplusia ni midgut tissue using a transcriptomic approach. We analyzed expression profiles of viral genes in the midgut and compared them with profiles from a T. ni cell line (Tnms42). Several viral genes (p6.9, orf76, orf75, pp31, Ac-bro, odv-e25, and odv-ec27) had high expression levels in the midgut throughout the infection. Also, the expression of genes associated with occlusion bodies (polh and p10) appeared to be delayed in the midgut in comparison with the cell line. Comparisons of viral gene expression profiles revealed remarkable similarities between the midgut and cell line for most genes, although substantial differences were observed for some viral genes. These included genes associated with high level BV production (fp-25k), acceleration of systemic infection (v-fgf), and enhancement of viral movement (arif-1/orf20). These differential expression patterns appear to represent specific adaptations for virus infection and transmission through the polarized cells of the lepidopteran midgut.IMPORTANCE Baculoviruses such as AcMNPV are pathogens that are natural regulators of certain insect populations. Baculovirus infections are biphasic, with a primary phase initiated by oral infection of midgut epithelial cells by occlusion-derived virus (ODV) virions and a secondary phase in which other tissues are infected by budded-virus (BV) virions. While AcMNPV infections in cultured cells have been studied extensively, comparatively little is known regarding primary infection in the midgut. In these studies, we identified gene expression patterns associated with ODV-mediated infection of the midgut in Trichoplusia ni and compared those results with prior results from BV-infected cultured cells, which simulate secondary infection. These studies provide a detailed analysis of viral gene expression patterns in the midgut, which likely represent specific viral strategies to (i) overcome or avoid host defenses in the gut and (ii) rapidly move infection from the midgut, into the hemocoel to facilitate systemic infection.

Genome-wide diversity in temporal and regional populations of the betabaculovirus Erinnyis ello granulovirus (ErelGV)

Background

Erinnyis ello granulovirus (ErelGV) is a betabaculovirus infecting caterpillars of the sphingid moth E. ello ello (cassava hornworm), an important pest of cassava crops (Manihot esculenta). In this study, the genome of seven field isolates of the virus ErelGV were deep sequenced and their inter- and intrapopulational sequence diversity were analyzed.

Results

No events of gene gain/loss or translocations were observed, and indels were mainly found within highly repetitive regions (direct repeats, drs). A naturally occurring isolate from Northern Brazil (Acre State, an Amazonian region) has shown to be the most diverse population, with a unique pattern of polymorphisms. Overall, non-synonymous substitutions were found all over the seven genomes, with no specific gathering of mutations on hotspot regions. Independently of their sizes, some ORFs have shown higher levels of non-synonymous changes than others. Non-core genes of known functions and structural genes were among the most diverse ones; and as expected, core genes were the least variable genes. We observed remarkable differences on diversity of paralogous genes, as in multiple copies of p10, fgf, and pep. Another important contrast on sequence diversity was found on genes encoding complex subunits and/or involved in the same biological processes, as late expression factors (lefs) and per os infectivity factors (pifs). Interestingly, several polymorphisms in coding regions lie on sequences encoding specific protein domains.

Conclusions

By comparing and integrating information about inter- and intrapopulational diversity of viral isolates, we provide a detailed description on how evolution operates on field isolates of a betabaculovirus. Our results revealed that 35–41% of the SNPs of ErelGV lead to amino acid changes (non-synonymous substitutions). Some genes, especially non-core genes of unknown functions, tend to accumulate more mutations, while core genes evolve slowly and are more conserved. Additional studies would be necessary to understand the actual effects of such gene variations on viral infection and fitness.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5070-6) contains supplementary material, which is available to authorized users.

Exploitation of receptor tyrosine kinases by viral-encoded growth factors

Receptor tyrosine kinases (RTKs) are essential components of cell communication pathways utilized from the embryonic to adult stages of life. These transmembrane receptors bind polypeptide ligands, such as growth factors, inducing signalling cascades that control cellular processes such as proliferation, survival, differentiation, motility and inflammation. Many viruses have acquired homologs of growth factors encoded by the hosts that they infect. Production of growth factors during infection allows viruses to exploit RTKs for entry and replication in cells, as well as for host and environmental dissemination. This review describes the genetic diversity amongst virus-derived growth factors and the mechanisms by which RTK exploitation enhances virus survival, then highlights how viral ligands can be used to further understanding of RTK signalling and function during embryogenesis, homeostasis and disease scenarios.

Comprehensive analysis of single molecule sequencing-derived complete genome and whole transcriptome of Hyposidra talaca nuclear polyhedrosis virus

We sequenced the Hyposidra talaca NPV (HytaNPV) double stranded circular DNA genome using PacBio single molecule sequencing technology. We found that the HytaNPV genome is 139,089 bp long with a GC content of 39.6%. It encodes 141 open reading frames (ORFs) including the 37 baculovirus core genes, 25 genes conserved among lepidopteran baculoviruses, 72 genes known in baculovirus, and 7 genes unique to the HytaNPV genome. It is a group II alphabaculovirus that codes for the F protein and lacks the gp64 gene found in group I alphabaculovirus viruses. Using RNA-seq, we confirmed the expression of the ORFs identified in the HytaNPV genome. Phylogenetic analysis showed HytaNPV to be closest to BusuNPV, SujuNPV and EcobNPV that infect other tea pests, Buzura suppressaria, Sucra jujuba, and Ectropis oblique, respectively. We identified repeat elements and a conserved non-coding baculovirus element in the genome. Analysis of the putative promoter sequences identified motif consistent with the temporal expression of the genes observed in the RNA-seq data.

Genome Sequencing and Analysis of Catopsilia pomona nucleopolyhedrovirus: A Distinct Species in Group I Alphabaculovirus

The genome sequence of Catopsilia pomona nucleopolyhedrovirus (CapoNPV) was determined by the Roche 454 sequencing system. The genome consisted of 128,058 bp and had an overall G+C content of 40%. There were 130 hypothetical open reading frames (ORFs) potentially encoding proteins of more than 50 amino acids and covering 92% of the genome. Among all the hypothetical ORFs, 37 baculovirus core genes, 23 lepidopteran baculovirus conserved genes and 10 genes conserved in Group I alphabaculoviruses were identified. In addition, the genome included regions of 8 typical baculoviral homologous repeat sequences (hrs). Phylogenic analysis showed that CapoNPV was in a distinct branch of clade “a” in Group I alphabaculoviruses. Gene parity plot analysis and overall similarity of ORFs indicated that CapoNPV is more closely related to the Group I alphabaculoviruses than to other baculoviruses. Interesting, CapoNPV lacks the genes encoding the fibroblast growth factor (fgf) and ac30, which are conserved in most lepidopteran and Group I baculoviruses, respectively. Sequence analysis of the F-like protein of CapoNPV showed that some amino acids were inserted into the fusion peptide region and the pre-transmembrane region of the protein. All these unique features imply that CapoNPV represents a member of a new baculovirus species.

Characterization of the viral fibroblast growth factor homolog of Helicoverpa armigera single nucleopolyhedrovirus

Fibroblast growth factor (FGF) is found throughout multicellular organisms; however, fgf homologs (vfgf) have only been identified among viruses in lepidopteran baculoviruses. The function of vFGFs from Group I alphabaculoviruses, including Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV), involves accelerated killing of infected larvae by both viruses. The vFGF of Group II alphabaculovirus is structurally different from that of Group I alphabaculovirus, with a larger C-terminal region and additional N-linked glycosylation sites. In this study, we characterized the Group II alphabaculovirus vFGF of Helicoverpa armigera single nucleopolyhedrovirus (HearNPV). The transcription and expression of vfgf was detected at 3 h and 16 h post-infection in HearNPV-infected cells. To further study vFGF function, we constructed vfgf-knockout and -repaired HearNPV bacmids and investigated their affect in both cultured cells and insects. Deletion of vfgf had no effect on budded-virus production or viral DNA replication in cultured HzAM1 cells. However, bioassays showed that HearNPV vfgf deletion significantly increased the median lethal dose and delayed the median lethal time by ∼12 h in the host insect when the virus was delivered orally. These results suggested that vFGF is an important virulent factor for HearNPV infection and propagation in vivo.

Genome scale transcriptomics of baculovirus-insect interactions

Baculovirus-insect cell technologies are applied in the production of complex proteins, veterinary and human vaccines, gene delivery vectors' and biopesticides. Better understanding of how baculoviruses and insect cells interact would facilitate baculovirus-based production. While complete genomic sequences are available for over 58 baculovirus species, little insect genomic information is known. The release of the Bombyx mori and Plutella xylostella genomes, the accumulation of EST sequences for several Lepidopteran species, and especially the availability of two genome-scale analysis tools, namely oligonucleotide microarrays and next generation sequencing (NGS), have facilitated expression studies to generate a rich picture of insect gene responses to baculovirus infections. This review presents current knowledge on the interaction dynamics of the baculovirus-insect system' which is relatively well studied in relation to nucleocapsid transportation, apoptosis, and heat shock responses, but is still poorly understood regarding responses involved in pro-survival pathways, DNA damage pathways, protein degradation, translation, signaling pathways, RNAi pathways, and importantly metabolic pathways for energy, nucleotide and amino acid production. We discuss how the two genome-scale transcriptomic tools can be applied for studying such pathways and suggest that proteomics and metabolomics can produce complementary findings to transcriptomic studies.

Reduced expression of Autographa californica nucleopolyhedrovirus ORF34, an essential gene, enhances heterologous gene expression

Autographa californica multiple nucleopolyhedrovirus ORF34 is part of a transcriptional unit that includes ORF32, encoding a viral fibroblast growth factor (FGF) and ORF33. We identified ORF34 as a candidate for deletion to improve protein expression in the baculovirus expression system based on enhanced reporter gene expression in an RNAi screen of virus genes. However, ORF34 was shown to be an essential gene. To explore ORF34 function, deletion (KO34) and rescue bacmids were constructed and characterized. Infection did not spread from primary KO34 transfected cells and supernatants from KO34 transfected cells could not infect fresh Sf21 cells whereas the supernatant from the rescue bacmids transfection could recover the infection. In addition, budded viruses were not observed in KO34 transfected cells by electron microscopy, nor were viral proteins detected from the transfection supernatants by western blots. These demonstrate that ORF34 is an essential gene with a possible role in infectious virus production.

The genome sequence of the emerging common midwife toad virus identifies an evolutionary intermediate within ranaviruses

Worldwide amphibian population declines have been ascribed to global warming, increasing pollution levels, and other factors directly related to human activities. These factors may additionally be favoring the emergence of novel pathogens. In this report, we have determined the complete genome sequence of the emerging common midwife toad ranavirus (CMTV), which has caused fatal disease in several amphibian species across Europe. Phylogenetic and gene content analyses of the first complete genomic sequence from a ranavirus isolated in Europe show that CMTV is an amphibian-like ranavirus (ALRV). However, the CMTV genome structure is novel and represents an intermediate evolutionary stage between the two previously described ALRV groups. We find that CMTV clusters with several other ranaviruses isolated from different hosts and locations which might also be included in this novel ranavirus group. This work sheds light on the phylogenetic relationships within this complex group of emerging, disease-causing viruses.

Barriers to success: how baculoviruses establish efficient systemic infections

The mechanisms used by baculoviruses to exit the midgut and cause systemic infection of their insect hosts have been debated for decades. After being ingested, baculoviruses reach the midgut, where several host barriers need to be overcome in order to establish successful infection. One of these barriers is the basal lamina, a presumably virus-impermeable extracellular layer secreted by the epithelial cells lining the midgut and trachea. This review discusses new evidence that demonstrates how these viruses breach the basal lamina and establish efficient systemic infections. The biochemical mechanisms involved in dismantling basal lamina during baculovirus infection may also provide new insights into the process of basal lamina remodeling in invertebrate and vertebrate animals.

Autographa californica multiple nucleopolyhedrovirus Ac92 (ORF92, P33) is required for budded virus production and multiply enveloped occlusion-derived virus formation

The Autographa californica multiple nucleopolyhedrovirus orf92 (p33), ac92, is one of 31 genes carried in all sequenced baculovirus genomes, thus suggesting an essential function. Ac92 has homology to the family of flavin adenine dinucleotide-linked sulfhydryl oxidases and is related to the ERV/ALR family of sulfhydryl oxidases. The role of ac92 during virus replication is unknown. Ac92 was associated with the envelope of both budded and occlusion-derived virus (ODV). To investigate the role of Ac92 during virus replication, an ac92-knockout bacmid was generated through homologous recombination in Escherichia coli. Titration and plaque assays showed no virus spread in ac92-knockout bacmid DNA-transfected insect cells. Deletion of ac92 did not affect viral DNA replication. However, ac92-knockout bacmid DNA-transfected cells lacked multiply enveloped occlusion-derived nucleocapsids; instead, singly enveloped nucleocapsids were detected. To gain insight into the requirement for sulfhydryl oxidation during virus replication, a virus was constructed in which the Ac92 C(155)XXC(158) amino acids, important for sulfhydryl oxidase activity, were mutated to A(155)XXA(158). The mutant virus exhibited a phenotype similar to that of the knockout virus, suggesting that the C-X-X-C motif was essential for sulfhydryl oxidase activity and responsible for the altered ODV phenotype.

Virion-associated viral fibroblast growth factor stimulates cell motility

The Autographa californica M nucleopolyhedrovirus (AcMNPV) viral fibroblast growth factor (vFGF) has functional parallels to cellular FGFs. Deletion of the AcMNPV vfgf has no obvious phenotype in cell culture but delays the time of insect death. Here, we determined vFGF production during virus infection. vFGF was detected at 24 hours post infection and through the remainder of the infection cycle. Since vFGF is thought to be a secreted membrane-binding protein and virions acquire an envelope derived from the cell membrane, we examined virions for the presence of vFGF using microscopy, flow cytometry, and affinity chromatography. We found that vFGF associated with virions. Furthermore, budded virus carrying vFGF had more affinity to heparin than vFGF-deficient budded virus, consistent with the affinity of FGFs for heparan sulfate proteoglycans. Although the function of virion-associated vFGF is not clear, we found that virion-associated vFGF stimulated cell motility and affected virus attachment.

Baculovirus genes affecting host function

Baculoviruses are insect-specific viruses. These large DNA viruses encode many genes in addition to those required to replicate and build new virions. These auxiliary genes provide selective advantages to the virus for invading and infecting host insects. Eight of these genes, which help the virus overcome insect defenses against invasion, are discussed. These include genes whose products help the virus traverse physical or physiological barriers and those that overcome host immune defenses.

The fibroblast growth factor homolog of Bombyx mori nucleopolyhedrovirus enhances systemic virus propagation in B. mori larvae

All lepidopteran baculovirus genomes sequenced to date encode a viral fibroblast growth factor homolog (vFGF). Recently, we generated a Bombyx mori nucleopolyhedrovirus (BmNPV) mutant lacking functional vfgf and found that BmNPV vfgf contributes to virus virulence in B. mori larvae. However, the steps at which BmNPV vFGF works during in vivo virus infection were unclear. To uncover the role of vFGF during systemic infection of silkworm larvae, we generated a BmNPV mutant, BmIEGFP, possessing an ie-1 promoter-driven green fluorescent protein gene, and its derivative BmIEGFP/FGFD, in which vfgf was partially deleted from the genome of BmIEGFP. Intrahemocoelic and oral infection experiments using these viruses revealed that the loss of functional vFGF reduces viral infectivity in B. mori hemocytes. Our results suggest that BmNPV vFGF is required for efficient systemic infection, presumably by a chemotactic effect that allows budded virus to infect hemocytes efficiently.

Eicosanoids mediate insect hemocyte migration

Hemocyte migration toward infection and wound sites is an essential component of insect defense reactions, although the biochemical signal mechanisms responsible for mediating migration in insect cells are not well understood. Here we report on the outcomes of experiments designed to test the hypotheses that (1) insect hemocytes are able to detect and migrate toward a source of N-formyl-Met-Leu-Phe (fMLP), the major chemotactic peptide from Escherichia coli and (2) that pharmaceutical modulation of eicosanoid biosynthesis inhibits hemocyte migration. We used primary hemocyte cultures prepared from fifth-instar tobacco hornworms, Manduca sexta in Boyden chambers to assess hemocyte migration toward buffer (negative control) and toward buffer amended with fMLP (positive control). Approximately 42% of negative control hemocytes migrated toward buffer and about 64% of positive control hemocytes migrated toward fMLP. Hemocyte migration was inhibited (by >40%) by treating hornworms with pharmaceutical modulators of cycloxygenase (COX), lipoxygenase and phospholipase A2 (PLA2) before preparing primary hemocyte cultures. The influence of the COX inhibitor, indomethacin, and the glucocorticoid, dexamethasone, which leads to inhibition of PLA2, was expressed in a dose-dependent way. The influence of dexamethasone was reversed by injecting arachidonic acid (precursor to eicosanoid biosynthesis) into hornworms before preparing primary hemocyte cultures. The saturated fatty acid, palmitic acid, did not reverse the inhibitor effect. These findings support both our hypotheses, first that insect hemocytes can detect and respond to fMLP, and second, that insect hemocyte migration is mediated by eicosanoids.

ERK- and JNK-dependent signaling pathways contribute to Bombyx mori nucleopolyhedrovirus infection

Mitogen-activated protein kinases (MAPKs) often play important roles in virus infection. To explore intracellular signaling pathways induced by baculovirus infection, we examined the involvement of MAPKs in Bombyx mori nucleopolyhedrovirus (BmNPV) infection of BmN cells. We found that specific inhibitors of extracellular signal-regulated kinase (ERK) kinase and c-Jun NH(2)-terminal kinase (JNK) significantly reduced occlusion body (OB) formation and budded virus (BV) production. Next, we quantified OB and BV production after applying the inhibitors at different times postinfection (p.i.). The inhibitors significantly reduced OB and BV production to various extents when applied at 12 h p.i., indicating that the reduction of BmNPV infectivity by these inhibitors occurs at the late stage of infection. Also, we observed that these inhibitors markedly repressed or deregulated the expression of delayed early, late, and very late gene products. Western blot analysis using phospho-MAPK-specific antibodies showed that ERK and JNK were activated at the late stage of BmNPV infection. In addition, the magnitude and pattern of MAPK activation were dependent on the multiplicity of infection. To verify the effects of the inhibitors on BmNPV infection, we also attempted to knock down the B. mori genes BmErk and BmJnk, which encode ERK and JNK, respectively. Knockdown of BmErk and BmJnk resulted in the reduced production of OBs and BVs, confirming that BmERK and BmJNK are involved in the BmNPV infection process. Taken together, these results indicate that the activation of MAPK signaling pathways is required for efficient infection by BmNPV.

The AcMNPV pp31 gene is not essential for productive AcMNPV replication or late gene transcription but appears to increase levels of most viral transcripts

The pp31 gene of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes a phosphorylated DNA binding protein that associates with virogenic stroma in the nuclei of infected cells. Prior studies of pp31 by transient late expression assays suggested that pp31 may play an important role in transcription of AcMNPV late genes [Todd, J. W., Passarelli, A. L., and Miller, L. K. (1995). Eighteen baculovirus genes, including lef-11, p35, 39K, and p47, support late gene expression. J. Virol. 69, 968-974] although genetic studies of the closely related BmNPV pp31 gene suggested that pp31 may be dispensable [Gomi, S., Zhou, C. E., Yih, W., Majima, K., and Maeda, S. (1997). Deletion analysis of four of eighteen late gene expression factor gene homologues of the baculovirus, BmNPV. Virology 230 (1), 35-47]. In the current study, we examined the role of the pp31 gene in the context of the AcMNPV genome during infection. We used a BACmid-based system to generate a pp31 knockout in the AcMNPV genome. The pp31 knockout was subsequently rescued by reinserting the pp31 gene into the polyhedrin locus of the same virus genome. We found that pp31 was not essential for viral replication although the absence of pp31 resulted in a lower viral titer. Analysis of viral DNA replication in the absence of pp31 showed that the kinetics of viral DNA replication were unaffected. An AcMNPV oligonucleotide microarray was used to compare gene expression from all AcMNPV genes in the presence or absence of pp31. In the absence of pp31, a modest reduction in transcripts was detected for many viral genes (99 genes) while no substantial increase or decrease was observed for 43 genes. Transcripts from 6 genes (p6.9, ORF 97, ORF 60, ORF 98, ORF 102 and chitinase) were reduced by 66% or more compared to the levels detected from the control virus. Microarray results were further examined by qPCR analysis of selected genes. In combination, these data show that deletion of the pp31 gene was not lethal and did not appear to affect viral DNA replication but resulted in an apparent modest down-regulation of a subset of AcMNPV genes that included both early and late genes.

Lepidopteran ortholog of Drosophila breathless is a receptor for the baculovirus fibroblast growth factor

The Bombyx mori nucleopolyhedrovirus (BmNPV) encodes a gene homologous to the mammalian fibroblast growth factor (FGF) family. We report the cloning of B. mori and Spodoptera frugiperda orthologous genes (Bmbtl and Sfbtl, respectively) of Drosophila melanogaster breathless (btl) encoding a receptor for Branchless/FGF and show that these genes encode the receptor for a baculovirus-encoded FGF (vFGF). Sequence analysis showed that BmBtl is composed of 856 amino acid residues, which potentially encodes a 97.3-kDa polypeptide and shares structural features and sequence similarities with the FGF receptor family. Reverse transcription-PCR experiments showed that Bmbtl was abundantly expressed in the trachea and midgut in B. mori larvae, with moderate expression observed in the hemocytes and the B. mori cultured cell line BmN. We generated Sf-9 cells that stably expressed His-tagged BmBtl. Western blot analysis revealed that BmBtl was an approximately 110-kDa protein. Immunoprecipitation experiments showed that BmNPV vFGF markedly phosphorylated BmBtl in Sf-9 cells. In addition, we found that BmBtl overexpression enhanced the migration activity for BmNPV vFGF. Furthermore, we generated Sf-9 cells in which Sfbtl was knocked down by transfection with double-strand RNA-expressing plasmids. In these cells, cell motility triggered by vFGF was markedly reduced. These results strongly suggest that the Btl orthologs, BmBtl and SfBtl, are the receptors for vFGF, which mediate vFGF-induced host cell chemotaxis.

The Autographa californica M nucleopolyhedrovirus fibroblast growth factor accelerates host mortality

The fibroblast growth factor (vfgf) gene encoded by Autographa californica M nucleopolyhedrovirus (AcMNPV) has been shown to share functional properties with cellular fgfs; it is a secreted protein, binds heparin, and stimulates motility of insect cells. We previously reported that viruses containing or lacking vfgf produced similar yields of budded virus and had similar kinetics of viral DNA and protein syntheses in cultured cells. In this study, we characterized these viruses in two permissive hosts, Spodoptera frugiperda and Trichoplusia ni, using two insect developmental stages and two infection routes, by feeding and intrahemocoelic injection. In addition, we constructed an AcMNPV bacmid overexpressing vfgf under polyhedrin promoter control and characterized it in both cell culture and insects. Deletion of vfgf had no effect on the infectivity of AcMNPV. However, lack of vfgf delayed the time of death in two host species when the virus was delivered by feeding but not by intrahemocoelic injection. The virus overexpressing vfgf produced less budded virus than the control virus in cultured cells. In insect bioassays, the infectivity of this virus was greater than that of the parental virus in both insect species and significantly accelerated time of death of both hosts tested. Our results suggest that the AcMNPV vfgf may play a role in dissemination of virus infection from the midgut in the insect species tested.

PTP-H2 and PTP-H3 from Microplitis demolitor Bracovirus localize to focal adhesions and are antiphagocytic in insect immune cells

Viruses in the family Polydnaviridae are symbiotically associated with parasitoid wasps. Wasps inject polydnaviruses (PDVs) when laying an egg into their insect host, and expression of viral gene products causes several physiological alterations, including immunosuppression, that allow the wasp's progeny to develop. As with other PDVs, most Microplitis demolitor bracovirus (MdBV) genes are related variants that form gene families. The largest MdBV gene family includes 13 members that encode predicted proteins related to protein tyrosine phosphatases (PTPs). Sequence analysis during the present study indicated that five PTP family members (PTP-H2, -H3, -N1, and -N2) have fully conserved catalytic domains, whereas other family members exhibited replacements, deletions, or rearrangements of amino acids considered essential for tyrosine phosphatase activity. Expression studies indicated that most MdBV PTP genes are expressed in virus-infected host insects, with transcript abundance usually being highest in hemocytes. MdBV-infected hemocytes also exhibited higher levels of tyrosine phosphatase activity than noninfected hemocytes. We produced expression constructs for four of the most abundantly expressed PTP family members and conducted functional studies with hemocyte-like Drosophila S2 cells. These experiments suggested that recombinant PTP-H2 and PTP-H3 are functional tyrosine phosphatases whereas PTP-H1 and PTP-J1 are not. PTP-H2 and -H3 localized to focal adhesions in S2 cells, and coexpression with another MdBV gene product, Glc1.8, resulted in complete inhibition of phagocytosis.

N-linked glycans of Bombyx mori nucleopolyhedrovirus fibroblast growth factor are crucial for its secretion

Bombyx mori nucleopolyhedrovirus (BmNPV) fibroblast growth factor (BmFGF) is a glycosylated protein that is efficiently secreted into the medium. Here, we constructed mutant NPVs expressing His-tagged wild-type (wt) or mutant BmFGFs and showed that the two residues, asparagine 44 and 171, are the glycosylation sites of BmFGF. Also, removal of N-linked glycans from BmFGF resulted in almost complete inhibition of the secretion into the medium, suggesting that N-linked glycans of BmFGF are required for its secretion. These residues are not conserved in closely related Autographa californica NPV (AcMNPV)-encoded vFGF (AcFGF). Western blot analysis suggested that AcFGF is not glycosylated and is poorly secreted. A mutant AcFGF possessing two N-linked glycosylation sites was secreted into the medium more abundantly than that which occurred for wt AcFGF. This is the first direct evidence showing the role of N-linked glycans in the secretion process of a baculovirus protein.

Virus-derived genes for insect-resistant transgenic plants

Insect viruses have evolved to counter physiological barriers to infection presented by the host insect. For the Lepidoptera (butterflies and moths), these barriers include (1) the peritrophic membrane (PM) lining the gut, which presents a physical barrier to virus infection of the midgut epithelial cells, (2) the basement membrane (BM) that overlies the gut thereby restricting secondary infection of other tissues, and (3) the immune system of the host insect. Hence, insect viruses provide a resource for genes that disrupt host physiology in a specific manner, and these genes in turn serve as a resource both for the study of physiological processes, and for disruption of these processes for pest management purposes. There are several examples of the application of genes used by an insect virus to overcome the PM barrier for production of insect-resistant transgenic plants. There are other examples of intrahemocoelic effectors, such as BM-degrading proteases that can only be used with an appropriate system for delivery of the agent from the gut into the hemocoel (body cavity) of the insect pest. In this chapter, we describe (1) baculovirus- and entomopoxvirus-derived genes that alter the physiology of the host insect, (2) use of these and homologous genes for production of insect-resistant transgenic plants, (3) other viral genes that have potential for use in development of insect-resistant transgenic plants, and (4) the use of plant lectins for delivery of intrahemocoelic toxins from transgenic plants. Plant expression of polydnavirus-derived genes is described by Gill et al. (this volume, pp. 393-426).

Analysis of the temporal expression of Trichoplusia ni single nucleopolyhedrovirus genes following transfection of BT1-Tn-5B1-4 cells

Trichoplusia ni single nucleopolyhedrovirus (TnSNPV), a 134,394-bp double-stranded DNA group II Nucleopolyhedrovirus, is pathogenic to the lepidopteran T. ni. TnSNPV transcription is temporally regulated and divided into three promoter sequence-dependent classes (early, late and very late genes). A viral oligonucleotide DNA microarray containing all potential (144) viral genes of TnSNPV was designed to investigate global viral gene expression during cell infection. Total BT1-Tn-5B1-4 cellular mRNAs extracted between 0 and 72 h posttransfection with TnSNPV genomic DNA were hybridized to the microarray. Initial average expression of early genes was detected between 12 and 24 h posttransfection while late genes were mainly detected between 24 and 72 h posttransfection. The microarray expression profiling data verified many computer predicted promoter assignments. K-means clustering was used to sort the 144 genes based on their temporal expression pattern similarities. This clustering resulted in the confirmation and temporal class assignment of previously unidentified genes and promoters.

In vivo and in vitro analyses of a Bombyx mori nucleopolyhedrovirus mutant lacking functional vfgf

All lepidopteran baculovirus genomes sequenced to date encode a viral fibroblast growth factor homolog (vfgf), suggesting that vfgf may play an important role in the infection cycle of lepidopteran baculoviruses. Here, we describe the characterization of a Bombyx mori nucleopolyhedrovirus (BmNPV) mutant lacking functional vfgf. We constructed a vfgf deletion mutant (BmFGFD) and characterized it in BmN cells and B. mori larvae. We observed that budded virus (BV) production was reduced in BmFGFD-infected BmN cells and B. mori larvae. The larval bioassays also revealed that deletion of vfgf did not reduce the infectivity; however, the mutant virus did take 20 h longer to kill B. mori larvae than wild-type BmNPV, when tested either by BV injection or by polyhedrin-inclusion body ingestion. These results suggest that BmNPV vfgf is involved in efficient virus production in BmN cells and B. mori larvae.

Comparative studies of Bombyx mori nucleopolyhedrovirus chitinase and its host ortholog, BmChi-h

Baculovirus-encoded chitinases (V-CHIAs) were first proposed to be acquired from a bacterium via horizontal gene transfer. However, we have recently reported that lepidopteran hosts also encode v-chiA orthologs. Here we describe comparative studies of Bombyx mori nucleopolyhedrovirus (BmNPV) chitinase and its host ortholog, BmChi-h. We constructed recombinant BmNPVs in which native and modified forms of BmChi-h were driven under the polyhedrin promoter and the authentic v-chiA was deleted. Western blot analysis indicated that BmCHI-h was rapidly secreted from virus-infected BmN cells whereas BmNPV CHIA was localized within the virus-infected cells; probably because of the presence of a C-terminal endoplasmic reticulum retention motif on BmNPV CHIA. Enzymological studies showed that BmNPV CHIA was able to retain much higher chitinolytic activity under alkaline conditions. For B. mori larvae infected with v-chiA-deleted BmNPV, the terminal liquefaction of dead larvae and the activation of baculovirus-encoded cysteine protease were not observed, and the introduction of BmChi-h did not rescue these defects. Our findings show that BmNPV chiA possesses unique features that are not shared by host orthologs, which may reflect functional specialization of baculovirus chitinases.

Analysis of a baculovirus lacking a functional viral fibroblast growth factor homolog

Baculoviruses encode fibroblast growth factor (vfgf) homologs whose function during virus infection is unknown. We constructed a recombinant bacmid of Autographa californica M nucleopolyhedrovirus (AcMNPV) lacking a functional vfgf and characterized it in two insect cell lines. The kinetics of budded virus production were similar in the parental and vfgf-deficient viruses in both cell lines at both high and low multiplicities of infection. In addition, no obvious differences were observed between the mutant and parental viruses in protein or DNA synthesis. Finally, coinfection of vfgf-containing and -deficient viruses and passage for several generations did not reveal a consistent growth advantage for either virus.