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

The tracheal immune system of insects - A blueprint for understanding epithelial immunity

The unique design of respiratory organs in multicellular organisms makes them prone to infection by pathogens. To cope with this vulnerability, highly effective local immune systems evolved that are also operative in the tracheal system of insects. Many pathogens and parasites (including viruses, bacteria, fungi, and metazoan parasites) colonize the trachea or invade the host via this route. Currently, only two modules of the tracheal immune system have been characterized in depth: 1) Immune deficiency pathway-mediated activation of antimicrobial peptide gene expression and 2) local melanization processes that protect the structure from wounding. There is an urgent need to increase our understanding of the architecture of tracheal immune systems, especially regarding those mechanisms that enable the maintenance of immune homeostasis. This need for new studies is particularly exigent for species other than Drosophila.

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.

Autophagy induced by Vip3Aa has a pro-survival role in Spodoptera frugiperda Sf9 cells

Vip3Aa is an insecticidal protein that can effectively control certain lepidopteran pests and has been used widely in biological control. However, the mechanism of action of Vip3Aa is unclear. In the present study, we showed that Vip3Aa could cause autophagy in Sf9 cells, which was confirmed by the increased numbers of GFP-Atg8 puncta, the appearance of autophagic vacuoles, and an elevated Atg8-II protein level. Moreover, we found that the AMPK-mTOR-ULK1 pathway is involved in Vip3Aa-induced autophagy, which might be associated with the destruction of ATP homeostasis in Vip3Aa-treated cells. Both the elevated p62 level and the increased numbers of GFP-RFP-Atg8 yellow fluorescent spots demonstrated that autophagy in Sf9 cells was inhibited at 24 h after Vip3Aa treatment. With the prolongation of Vip3Aa treatment time, this inhibition became more serious and led to autophagosome accumulation. Genetic knockdown of ATG5 or the use of the autophagy inhibitor 3-MA further increased the sensitivity of Sf9 cells to Vip3Aa. Overexpression of ATG5 reduced the cell mortality of Vip3Aa-treated cells. In summary, the results revealed that autophagy induced by Vip3Aa has a pro-survival role, which might be related to the development of insect resistance.

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.

Cross-talking between baculoviruses and host insects towards a successful infection

Baculoviridae is a family of large DNA viruses that infect insects. They have been extensively used as safe and efficient biological agents for the control of insect pests. As a result of coevolution with their hosts, baculoviruses developed unique life cycles characterized by the production of two distinctive virion phenotypes, occlusion-derived virus and budded virus, which are responsible for mediating primary infection in insect midgut epithelia and spreading systemic infection within infected insects, respectively. In this article, advances associated with virus-host interactions during the baculovirus life cycle are reviewed. We mainly focus on how baculoviruses exploit versatile strategies to overcome diverse host barriers and establish successful infections. For example, in the midgut, baculoviruses encode enzymes to degrade peritrophic membranes and use a series of per os infectivity factors to initiate primary infection. A viral fibroblast growth factor is expressed to attract tracheoblasts that spread the virus for systemic infection. Baculoviruses use different strategies to suppress host defence systems, including apoptosis, melanization and RNA interference. Additionally, baculoviruses can manipulate host physiology and induce 'tree-top disease' for optimal virus replication and dispersal. These advances in our understanding of baculoviruses will greatly inform the development of more effective baculoviral pesticides. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

CYPs in different families are involved in the divergent regio-specific epoxidation of alkenyl sex pheromone precursors in moths

In moth species that utilize alkenyl sex pheromones, the epoxidation of alkenes confers further diversity on the chemical structures of pheromone components. Hc_epo1 (CYP341B14), the first pheromone gland (PG)-specific epoxidase identified from the fall webworm Hyphantria cunea (Erebidae), specifically epoxidizes the Z9 double bond in the triene precursor, (3Z,6Z,9Z)-3,6,9-henicosatriene (Z3,Z6,Z9-21:H). In the present study, we identified a novel PG-specific epoxidase, As_epo1, from the Japanese giant looper Ascotis selenaria (Geometridae), which secretes cis-3,4-epoxy-(6Z,9Z)-6,9-nonadecadiene (epo3,Z6,Z9-19:H) as the main sex pheromone component. A functional assay using the Sf9 insect cell line-baculovirus expression system showed that As_epo1 specifically epoxidizes the Z3 double bond in the pheromone precursor triene, (3Z,6Z,9Z)-3,6,9-nonadecatriene (Z3,Z6,Z9-19:H). As_epo1 also Z3-specifically epoxidized a triene with a longer carbon chain, Z3,Z6,Z9-21:H, which does not occur in this species. A phylogenetic analysis indicated that As_epo1 belonged to the CYP340 family, not the CYP341 family to which Hc_epo1 belongs. These results suggest that moth PG-specific epoxidases with divergent regio-specificities have evolved independently.

Functional characterization of the epoxidase gene, Li_epo1 (CYP341B14), involved in generation of epoxyalkene pheromones in the mulberry tiger moth Lemyra imparilis

Epoxidation of alkenes derived from essential fatty acids is a key step in the biosynthesis of sex pheromones in moth species that utilize alkenyl sex pheromones. The position of the epoxy ring in the pheromone molecule differs depending on the species, thereby conferring diversities on sex pheromones. To date, only one pheromone gland (PG)-specific epoxidase, Hc_epo1 (CYP341B14), has been reported. Hc_epo1, which was identified from an arctiid moth Hyphantria cunea, catalyzes the epoxidation of a double bond at position 9 of the triene, Z3,Z6,Z9-21:H. In the present study, we investigated the PG-specific epoxidase from another arctiid, the mulberry tiger moth Lemyra imparilis, in order to verify whether cytochrome P450 in the CYP341B subfamily, to which Hc_epo1 belongs to, is responsible for the epoxidation of pheromone precursors at position 9 in moths other than H. cunea. A fragment of the Hc_epo1 homolog was amplified from cDNA prepared from the PG of L. imparilis by PCR with degenerate primers. The deduced amino acid sequence of the subsequently cloned homolog, Li_epo1, showed 88.5% identity to Hc_epo1. A functional assay using the Sf9 insect cell line-baculovirus expression system showed that Li_epo1 exhibited epoxidase activity with high selectivity to the double bond at position 9 of two trienes, Z3,Z6,Z9-21:H and Z3,Z6,Z9-23:H, precursors of epoxy diene sex pheromone components in L. imparilis.

Fibroblast Growth Factor Receptor, a Novel Receptor for Vegetative Insecticidal Protein Vip3Aa

Vegetative insecticidal proteins (Vips), which are secreted by some Bacillus thuringiensis strains during vegetative growth, exhibit high virulence to many pests. Vip3A proteins have been used commercially both in some bio-insecticides and in transgenic crops; however, compared with insecticidal crystal proteins, the mechanism of action of Vip3A is still unclear. In this work, we indicated that the fibroblast growth factor receptor-like protein (Sf-FGFR) from the membrane of Sf9 cells could bind to Vip3Aa. The interaction between Vip3Aa and Sf-FGFR was confirmed by pull-down assays and dot blotting experiment in vitro. The binding affinity between Vip3Aa and extracellular regions of Sf-FGFR (GST-FGFR-N) was determined by microscale thermophoresis assay (MST). Moreover, Vip3Aa-Flag could be co-immunoprecipitated with Sf-FGFR-V5 ex vivo. Furthermore, knockdown of Sf-FGFR gene in Sf9 cells resulted in reducing the mortality of those cells to Vip3Aa. In summary, our data indicated that Sf-FGFR is a novel receptor for Vip3Aa.

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.

Scavenger receptor-C acts as a receptor for Bacillus thuringiensis vegetative insecticidal protein Vip3Aa and mediates the internalization of Vip3Aa via endocytosis

The vegetative insecticidal proteins (Vip), secreted by many Bacillus thuringiensis strains during their vegetative growth stage, are genetically distinct from known insecticidal crystal proteins (ICPs) and represent the second-generation insecticidal toxins. Compared with ICPs, the insecticidal mechanisms of Vip toxins are poorly understood. In particular, there has been no report of a definite receptor of Vip toxins to date. In the present study, we identified the scavenger receptor class C like protein (Sf-SR-C) from the Spodoptera frugiperda (Sf9) cells membrane proteins that bind to the biotin labeled Vip3Aa, via the affinity magnetic bead method coupled with HPLC-MS/MS. We then certified Vip3Aa protoxin could interact with Sf-SR-C in vitro and ex vivo. In addition, downregulation of SR-C expression in Sf9 cells and Spodoptera exigua larvae midgut reduced the toxicity of Vip3Aa to them. Coincidently, heterologous expression of Sf-SR-C in transgenic Drosophila midgut significantly enhanced the virulence of Vip3Aa to the Drosophila larvae. Moreover, the complement control protein domain and MAM domain of Sf-SR-C are involved in the interaction with Vip3Aa protoxin. Furthermore, endocytosis of Vip3Aa mediated by Sf-SR-C correlates with its insecticidal activity. Our results confirmed for the first time that Sf-SR-C acts as a receptor for Vip3Aa protoxin and provides an insight into the mode of action of Vip3Aa that will significantly facilitate the study of its insecticidal mechanism and application.

Bacillus thuringiensis Vip3A has potential in control of Lepidopteran pest and has been used in transgenic plants. However, studies of the insecticidal mechanisms of Vip3A are rare, and none of their definite receptors have been reported so far, which seriously restricts the study of its insecticidal mechanism and application. This work identified and confirmed the scavenger receptor class C like protein (Sf-SR-C) acts as the receptor of Vip3Aa protoxin, demonstrated that Sf-SR-C mediates the toxicity of Vip3Aa to Sf9 cells in an internalized manner. These results extend our understanding of SR-C proteins in insects and explain the specificity of Vip3Aa insecticidal activity, which strongly support it as a safe biopesticide. More importantly, it suggests the insecticidal mechanism of Vip3Aa different from the well-known “pore formation” model, “signal transduction” model, as well as newly found “necrosis” model of Cry toxins, which will significantly promote the relevant study of Vip3Aa. Last but not least, because scavenger receptors play a crucial role in innate immunity, our results provide relevant insights into host-pathogen interactions.

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.

A single amino acid substitution in the Bombyx-specific mucin-like membrane protein causes resistance to Bombyx mori densovirus

Bombyx mori densovirus type 1 (BmDV) is a pathogen that causes flacherie disease in the silkworm. The absolute nonsusceptibility to BmDV among certain silkworm strains is determined independently by two genes, nsd-1 and Nid-1. However, neither of these genes has been molecularly identified to date. Here, we isolated the nsd-1 gene by positional cloning and characterized the properties of its product, NSD-1. Sequence and biochemical analyses revealed that this gene encodes a Bombyx-specific mucin-like glycoprotein with a single transmembrane domain. The NSD-1 protein was specifically expressed in the larval midgut epithelium, the known infection site of BmDV. Sequence analysis of the nsd-1 gene from 13 resistant and 12 susceptible strains suggested that a specific arginine residue in the extracellular tail of the NSD-1 protein was common among susceptible strains. Germline transformation of the susceptible-type nsd-1 (with a single nucleotide substitution) conferred partial susceptibility to resistant larvae, indicating that the +  ^nsd-1 gene is required for the susceptibility of B. mori larvae to BmDV and the susceptibility is solely a result of the substitution of a single amino acid with arginine. Taken together, our results provide striking evidence that a novel membrane-bound mucin-like protein functions as a cell-surface receptor for a densovirus.

A Conserved Glycine Residue Is Required for Proper Functioning of a Baculovirus VP39 Protein

The baculovirus VP39 protein is a major nucleocapsid protein essential for viral propagation. However, the critical domains or residues of the VP39 protein have not yet been identified. Here, we performed mutagenesis experiments with Bombyx mori nucleopolyhedrovirus (BmNPV) using 5-bromo-2'-deoxyuridine and isolated a BmNPV mutant that produced fewer occlusion bodies than the wild-type virus. This mutant also produced fewer infectious budded viruses (BVs) than the wild-type virus in both cultured cells and B. mori larvae. Marker rescue experiments using genomic libraries identified a single nucleotide mutation in the vp39 gene. This mutation resulted in an amino acid substitution at glycine 276 (Gly-276) to serine, which was required for all the defective phenotypes observed in the mutant. Sequence comparison revealed that this residue is completely conserved among the VP39 proteins of the sequenced alphabaculoviruses, betabaculoviruses, and gammabaculoviruses. Although early viral gene expression was not significantly affected, the level of expression of a late gene, vcath, was reduced. In addition, two of the very late genes were markedly downregulated in cells infected with this mutant. Western blot and quantitative PCR analyses revealed that the BVs produced from cells infected with this mutant contained smaller amounts of the VP39 protein and viral genomic DNA than those produced from wild-type virus-infected cells. Combined with the results of transmission electron microscopy, VP39 Gly-276 can be concluded to be essential for correct nucleocapsid assembly, viral DNA packaging, and viral gene expression, especially of very late genes.IMPORTANCE The major nucleocapsid protein gene vp39 is one of the most well-known baculovirus genes. Although several viral and host proteins that interact with the VP39 protein have been identified, the functionally important domains or residues of this protein remain unknown. The present study revealed that the glycine residue at residue 276, which is completely conserved among sequenced alphabaculoviruses, betabaculoviruses, and gammabaculoviruses, is important for the VP39 function, i.e., structural assembly of nucleocapsids and viral DNA packaging. Moreover, our results provide evidence for the link between nucleocapsid formation and the transcription of viral very late genes.

Bombyx mori nucleopolyhedrovirus BM5 protein regulates progeny virus production and viral gene expression

Bombyx mori nucleopolyhedrovirus (BmNPV) orf5 (Bm5) is a core gene of lepidopteran baculoviruses and encodes the protein with the conserved amino acid residues (DUF3627) in its C-terminus. Here, we found that Bm5 disruption resulted in lower titers of budded viruses and fewer numbers of occlusion bodies (OBs) in B. mori cultured cells and larvae, although viral genome replication was not affected. Bm5 disruption also caused aberrant expression of various viral genes at the very late stage of infection. Immunocytochemical analysis revealed that BM5 localized to the nuclear membrane. We also found that DUF3627 is important for OB production, transcriptional regulation of viral genes, and subcellular localization of BM5. Compared with wild-type BmNPV infection, larval death was delayed when B. mori larvae were infected with Bm5 mutants. These results suggest that BM5 is involved in progeny virus production and regulation of viral gene expression at the very late stage of infection.

Drosophila and the hallmarks of cancer

: Cancer was the disease of the twentieth century. Today it is still a leading cause of death worldwide despite being intensively investigated. Abundant knowledge exists regarding the pathological and molecular mechanisms that drive healthy cells to become malignant and form metastatic tumors. The relation of oncogenes and tumor suppressors to the genetic trigger of carcinogenesis is unquestionable. However, the development of the disease requires many characteristics that due to their proven role in cancer are collectively described as the "hallmarks of cancer." We highlight here the historic discoveries made using the model organism Drosophila melanogaster and its contributions to biomedical and cancer research. Flies are utilized as a model organism for the investigation of each and every aspect of cancer hallmarks. Due to the significant conservation between flies and mammals at the signaling and tissue physiology level it is possible to explore the genes and mechanisms responsible for cancer pathogenesis in flies. Recent Drosophila studies suggest novel aspects of therapeutic intervention and are expected to guide cancer research in the twenty-first century.

Phosphatase activity of Bombyx mori nucleopolyhedrovirus PTP is dispensable for enhanced locomotory activity in B. mori larvae

Baculovirus-induced enhanced locomotory activity (ELA) is not induced in caterpillars infected with a mutant Bombyx mori nucleopolyhedrovirus (BmNPV) or Autographa californica multiple nucleopolyhedrovirus (AcMNPV) lacking a functional protein tyrosine phosphatase gene (ptp). Previous studies suggest that the PTP proteins from BmNPV and AcMNPV act in different ways to induce ELA, i.e., BmNPV PTP is utilized as a virion structural component, whereas AcMNPV PTP requires its phosphatase activity. Here, I generated and characterized two new BmNPV mutants expressing enzymatically inactive PTP proteins and confirmed that the phosphatase activity of PTP is not required for ELA induction in BmNPV-infected B. mori larvae.

Characterization of Cryptopygus antarcticus endo-β-1,4-glucanase from Bombyx mori expression systems

Endo-β-1,4-glucanase (CaCel) from Antarctic springtail, Cryptopygus antarcticus, a cellulase with high activity at low temperature, shows potential industrial use. To obtain sufficient active cellulase for characterization, CaCel gene was expressed in Bombyx mori-baculovirus expression systems. Recombinant CaCel (rCaCel) has been expressed in Escherichia coli (Ec-CaCel) at temperatures below 10°C, but the expression yield was low. Here, rCaCel with a silkworm secretion signal (Bm-CaCel) was successfully expressed and secreted into pupal hemolymph and purified to near 90% purity by Ni-affinity chromatography. The yield and specific activity of rCaCel purified from B. mori were estimated at 31 mg/l and 43.2 U/mg, respectively, which is significantly higher than the CaCel yield obtained from E. coli (0.46 mg/l and 35.8 U/mg). The optimal pH and temperature for the rCaCels purified from E. coli and B. mori were 3.5 and 50°C. Both rCaCels were active at a broad range of pH values and temperatures, and retained more than 30% of their maximal activity at 0°C. Oligosaccharide structural analysis revealed that Bm-CaCel contains elaborated N- and O-linked glycans, whereas Ec-CaCel contains putative O-linked glycans. Thermostability of Bm-CaCel from B. mori at 60°C was higher than that from E. coli, probably due to glycosylation.

Characterization and recombinant protein expression of ferritin light chain homologue in the silkworm, Bombyx mori

The silkworm genome encodes three iron storage proteins or ferritins, Fer1HCH, Fer2LCH, and Fer3HCH. Probing our EST library constructed from 1-day-old silkworm eggs revealed only Fer2LCH mRNA, which encoded for a protein with a predicted putative N-glycosylation site. Developmental and tissue expression analyses during embryogenesis revealed that Fer2LCH mRNA was abundant from 6 h to 6 days after oviposition. Transcriptional expression of Fer2LCH during the postembryonic stage is also high in the larval fat body and mid-gut, and then is upregulated in all pupal tissues tested. We found that Fer2LCH mRNA contains an iron-responsive element, suggesting this ferritin subunit is subject to translational control. Although ferritin expression has been shown to increase following immune challenge in other insects, the levels of Fer2LCH mRNA were not significantly induced following viral or bacterial infection of Bombyx mori. Using a baculovirus expression system we expressed recombinant BmFer2LCH protein, which was detectable in the cytoplasmic fraction, likely in a compartment of the secretory pathway, and was shown to undergo posttranslational modifications including N-glycosylation. In particular, rBmFer2LCH carbohydrate chains were composed of mannose and GlcNAc. We suggest that Fer2LCH is important for iron homeostasis and maintaining normal organ function in silkworms.

Ectopic expression of the male BmDSX affects formation of the chitin plate in female Bombyx mori

Mating structures are involved in successful copulation, intromission, and/or insemination. These structures enable tight coupling between external genitalia of two sexes. During Bombyx mori copulation, the double harpagones in the external genitalia of males clasp the female chitin plate, which is derived from the larval eighth abdominal segment; abnormal development of the female chitin plate affects copulation. We report that ERK phosphorylation (p-ERK) and expression of Abdominal-B (Abd-B) in the posterior abdomen of the female adult is lower than in the male. Ectopic expression of the male-specific spliced form of B. mori doublesex (Bmdsx(M)) in females, however, up-regulates Abd-B and spitz (spi) expression, increasing EGFR signaling activity, and thus forming an abnormal chitin plate and reduced female copulation. These findings indicate that Bmdsx affects the development of the eighth abdominal segment by regulating the activity of EGFR signaling and the expression of Abd-B, resulting in an extra eighth abdominal segment (A8) in males versus the loss of this segment in adult females.

Discovery of a disused desaturase gene from the pheromone gland of the moth Ascotis selenaria, which secretes an epoxyalkenyl sex pheromone

Female Ascotis selenaria (Geometridae) moths use 3,4-epoxy-(Z,Z)-6,9-nonadecadiene, which is synthesized from linolenic acid, as the main component of their sex pheromone. While the use of dietary linolenic or linoleic fatty acid derivatives as sex pheromone components has been observed in moth species belonging to a few families including Geometridae, the majority of moths use derivatives of a common saturated fatty acid, palmitic acid, as their sex pheromone components. We attempted to gain insight into the differentiation of pheromone biosynthetic pathways in geometrids by analyzing the desaturase genes expressed in the pheromone gland of A. selenaria. We demonstrated that a Δ11-desaturase-like gene (Asdesat1) was specifically expressed in the pheromone gland of A. selenaria in spite of the absence of a desaturation step in the pheromone biosynthetic pathway in this species. Further analysis revealed that the presumed transmembrane domains were degenerated in Asdesat1. Phylogenetic analysis demonstrated that Asdesat1 anciently diverged from the lineage of Δ11-desaturases, which are currently widely used in the biosynthesis of sex pheromones by moths. These results suggest that an ancestral Δ11-desaturase became dysfunctional in A. selenaria after a shift in pheromone biosynthetic pathways.

Activation of glial FGFRs is essential in glial migration, proliferation, and survival and in glia-neuron signaling during olfactory system development

Development of the adult olfactory system of the moth Manduca sexta depends on reciprocal interactions between olfactory receptor neuron (ORN) axons growing in from the periphery and centrally-derived glial cells. Early-arriving ORN axons induce a subset of glial cells to proliferate and migrate to form an axon-sorting zone, in which later-arriving ORN axons will change their axonal neighbors and change their direction of outgrowth in order to travel with like axons to their target areas in the olfactory (antennal) lobe. These newly fasciculated axon bundles will terminate in protoglomeruli, the formation of which induces other glial cells to migrate to surround them. Glial cells do not migrate unless ORN axons are present, axons fail to fasciculate and target correctly without sufficient glial cells, and protoglomeruli are not maintained without a glial surround. We have shown previously that Epidermal Growth Factor receptors and the IgCAMs Neuroglian and Fasciclin II play a role in the ORN responses to glial cells. In the present work, we present evidence for the importance of glial Fibroblast Growth Factor receptors in glial migration, proliferation, and survival in this developing pathway. We also report changes in growth patterns of ORN axons and of the dendrites of olfactory (antennal lobe) neurons following blockade of glial FGFR activation that suggest that glial FGFR activation is important in reciprocal communication between neurons and glial cells.

The baculovirus uses a captured host phosphatase to induce enhanced locomotory activity in host caterpillars

The baculovirus is a classic example of a parasite that alters the behavior or physiology of its host so that progeny transmission is maximized. Baculoviruses do this by inducing enhanced locomotory activity (ELA) that causes the host caterpillars to climb to the upper foliage of plants. We previously reported that this behavior is not induced in silkworms that are infected with a mutant baculovirus lacking its protein tyrosine phosphatase (ptp) gene, a gene likely captured from an ancestral host. Here we show that the product of the ptp gene, PTP, associates with baculovirus ORF1629 as a virion structural protein, but surprisingly phosphatase activity associated with PTP was not required for the induction of ELA. Interestingly, the ptp knockout baculovirus showed significantly reduced infectivity of larval brain tissues. Collectively, we show that the modern baculovirus uses the host-derived phosphatase to establish adequate infection for ELA as a virion-associated structural protein rather than as an enzyme.

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.

Drosophila melanogaster as a model for human intestinal infection and pathology

Recent findings concerning Drosophila melanogaster intestinal pathology suggest that this model is well suited for the study of intestinal stem cell physiology during aging, stress and infection. Despite the physiological divergence between vertebrates and insects, the modeling of human intestinal diseases is possible in Drosophila because of the high degree of conservation between Drosophila and mammals with respect to the signaling pathways that control intestinal development, regeneration and disease. Furthermore, the genetic amenability of Drosophila makes it an advantageous model species. The well-studied intestinal stem cell lineage, as well as the tools available for its manipulation in vivo, provide a promising framework that can be used to elucidate many aspects of human intestinal pathology. In this Perspective, we discuss recent advances in the study of Drosophila intestinal infection and pathology, and briefly review the parallels and differences between human and Drosophila intestinal regeneration and disease.

Sex pheromone desaturase functioning in a primitive Ostrinia moth is cryptically conserved in congeners' genomes

(E)-11- and (Z)-11-tetradecenyl acetate are the most common female sex pheromone components in Ostrinia moths. The Δ11-desaturase expressed in the pheromone gland (PG) of female moths is a key enzyme that introduces a double bond into pheromone molecules. A single Δ11-desaturase of Ostrinia nubilalis, OnubZ/E11, has been shown to produce an ∼7:3 mixture of (E)-11- and (Z)-11-tetradecenoate from the substrate tetradecanoate. In contrast, the sex pheromone of Ostrinia latipennis, a primitive species of Ostrinia, is (E)-11-tetradecenol. This pheromone is unique in that it is not acetylated, and includes no Z isomer. In the present study, through the cloning and functional analysis of a PG-specific Δ11-desaturase in O. latipennis, we showed that the absence of the Z isomer in the pheromone is attributable to the strict product specificity of the Δ11-desaturase in this species, LATPG1. Phylogenetic analysis revealed that LATPG1 was not closely related to OnubZ/E11. Rather, it was closely related to retroposon-linked cryptic Δ11-desaturases (ezi-Δ11) found in the genomes of O. nubilalis and Ostrinia furnacalis. Taken together, the results showed that an unusual Δ11-desaturase is functionally expressed in O. latipennis, although the genes encoding this enzyme appear to be cryptic in congeners.

Viral fibroblast growth factor, matrix metalloproteases, and caspases are associated with enhancing systemic infection by baculoviruses

Most arthropod-borne and invertebrate viruses are orally ingested and commence infection in cells of the invertebrate intestine. Infection of secondary sites and eventual transmission to other hosts is hindered by basal lamina, a tightly interwoven and virus-impenetrable noncellular layer, lining the intestine and other organ cell layers. The mechanisms for viral escape across basal laminae are unknown. We describe an elegant mechanism mediated by a baculovirus-encoded fibroblast growth factor (vFGF) that signals a previously undescribed stepwise cascade of protease activation wherein matrix metalloproteases activate effector caspases, leading to remodeling of basal lamina lining tracheal cells associated with the intestine and culminating in the establishment of efficient systemic infections. Because FGFs coordinate diverse functions during development, metabolic processes, and tissue repair, it is plausible that the vFGF-mediated pathway described here is widely used during developmental and pathogenic processes that involve basal lamina remodeling.

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.

Molecular and functional characterization of an acetyl-CoA acetyltransferase from the adzuki bean borer moth Ostrinia scapulalis (Lepidoptera: Crambidae)

Two types of thiolases are involved in the synthesis and catabolism of fatty acids; acetyl-CoA acetyltransferase (AT) which catalyzes the formation of acetoacetyl-CoA from acetyl-CoA by transferring an acetyl group from one acetyl-CoA molecule to another, and 3-ketoacyl-CoA thiolase which catalyzes a reversible thiolytic cleavage of 3-ketoacyl-CoA into acetyl-CoA and acyl-CoA. Although many mammalian thiolases have been characterized in detail, no thiolases from insects have been functionally characterized to date. Here we report first characterization of an insect AT gene, Osat1, from the pheromone gland of the adzuki bean borer moth Ostrinia scapulalis (Lepidoptera; Crambidae). Osat1 encodes a 41.2 kDa protein comprising 396 amino acid residues (OsAT1), which possesses structural features of the thiolase family. An Osat1 homologue of Bombyx mori (Bmat1) was cloned through exploration of an EST library of the silkworm. Subsequent survey of the genome database revealed that B. mori has at least six Osat1 homologues, among which Bmat1 was most closely related to Osat1. We expressed recombinant OsAT1 using a baculovirus expression system, and verified that OsAT1 catalyzes the formation of acetoacetyl-CoA from acetyl-CoA. Osat1 was expressed in all adult tissues examined. These results indicate that OsAT1 is a functional AT ubiquitously expressed in O. scapulalis tissues.

Identification of protein-protein interactions of the occlusion-derived virus-associated proteins of Helicoverpa armigera nucleopolyhedrovirus

The purpose of this study was to identify protein-protein interactions among the components of the occlusion-derived virus (ODV) of Helicoverpa armigera nucleopolyhedrovirus (HearNPV), a group II alphabaculovirus in the family Baculoviridae. To achieve this, 39 selected genes of potential ODV structural proteins were cloned and expressed in the Gal4 yeast two-hybrid (Y2H) system. The direct-cross Y2H assays identified 22 interactions comprising 13 binary interactions [HA9-ODV-EC43, ODV-E56-38K, ODV-E56-PIF3, LEF3-helicase, LEF3-alkaline nuclease (AN), GP41-38K, GP41-HA90, 38K-PIF3, 38K-PIF2, VP80-HA100, ODV-E66-PIF3, ODV-E66-PIF2 and PIF3-PIF2] and nine self-associations (IE1, HA44, LEF3, HA66, GP41, CG30, 38K, PIF3 and P24). Five of these interactions - LEF3-helicase and LEF3-AN, and the self-associations of IE1, LEF3 and 38K - have been reported previously in Autographa californica multiple nucleopolyhedrovirus. As HA44 and HA100 were two newly identified ODV proteins of group II viruses, their interactions were further confirmed. The self-association of HA44 was verified with a His pull-down assay and the interaction of VP80-HA100 was confirmed by a co-immunoprecipitation assay. A summary of the protein-protein interactions of baculoviruses reported so far, comprising 68 interactions with 45 viral proteins and five host proteins, is presented, which will facilitate our understanding of the molecular mechanisms of baculovirus infection.

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.

The Bombyx ovary-derived cell line endogenously expresses PIWI/PIWI-interacting RNA complexes

Genetic studies and large-scale sequencing experiments have revealed that the PIWI subfamily proteins and PIWI-interacting RNAs (piRNAs) play an important role in germ line development and transposon control. Biochemical studies in vitro have greatly contributed to the understanding of small interfering RNA (siRNA) and microRNA (miRNA) pathways. However, in vitro analyses of the piRNA pathway have been thus far quite challenging, because their expression is largely restricted to the germ line. Here we report that Bombyx mori ovary-derived cultured cell line, BmN4, endogenously expresses two PIWI subfamily proteins, silkworm Piwi (Siwi) and Ago3 (BmAgo3), and piRNAs associated with them. Siwi-bound piRNAs have a strong bias for uridine at their 5' end and BmAgo3-bound piRNAs are enriched for adenine at position 10. In addition, Siwi preferentially binds antisense piRNAs, whereas BmAgo3 binds sense piRNAs. Moreover, we identified many pairs in which Siwi-bound antisense and BmAgo3-bound sense piRNAs are overlapped by precisely 10 nt at their 5' ends. These signatures are known to be important for secondary piRNA biogenesis in other organisms. Taken together, BmN4 is a unique cell line in which both primary and secondary steps of piRNA biogenesis pathways are active. This cell line would provide useful tools for analysis of piRNA biogenesis and function.

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.

N-linked glycans located in the pro-region of Bombyx mori nucleopolyhedrovirus V-CATH are essential for the proper folding of V-CATH and V-CHIA

Post-mortem host degradation by infection of Bombyx mori nucleopolyhedrovirus (BmNPV) requires the synergistic activation of two virus-encoded genes, cathepsin (v-cath) and chitinase (v-chiA). Previous studies have suggested that V-CHIA is essential for the proper folding of the nascent V-CATH polypeptide in the endoplasmic reticulum, and that the putative V-CHIA-V-CATH interaction might be mediated by N-linked glycans of V-CATH. Sequence analysis shows that BmNPV V-CATH includes three consensus N-linked glycosylation sites (asparagine 38, 65 and 158). To clarify the role of N-linked glycans of V-CATH in its biological activity, we generated three recombinant BmNPVs expressing mutant V-CATHs, and found that the two residues, asparagine 38 and 65, which are localized in the pro-region of V-CATH, are the glycosylation sites of BmNPV V-CATH. Western blot analysis also showed that removal of N-linked glycans from BmNPV V-CATH resulted in production of the insoluble forms of V-CATH and V-CHIA. These results demonstrate that N-linked glycans located in the pro-region of BmNPV V-CATH are essential for the proper folding of V-CATH and V-CHIA.

Two short-chain dehydrogenase/reductases, NON-YELLOW COLORING 1 and NYC1-LIKE, are required for chlorophyll b and light-harvesting complex II degradation during senescence in rice

Yellowing, which is related to the degradation of chlorophyll and chlorophyll-protein complexes, is a notable phenomenon during leaf senescence. NON-YELLOW COLORING 1 (NYC1) in rice encodes a membrane-localized short-chain dehydrogenase/reductase (SDR) that is thought to represent a chlorophyll b reductase necessary for catalyzing the first step of chlorophyll b degradation. Analysis of the nyc1 mutant, which shows the stay-green phenotype, revealed that chlorophyll b degradation is required for the degradation of light-harvesting complex II and thylakoid grana in leaf senescence. Phylogenetic analysis further revealed the existence of NYC1-LIKE (NOL) as the most closely related protein to NYC1. In the present paper, the nol mutant in rice was also found to show a stay-green phenotype very similar to that of the nyc1 mutant, i.e. the degradation of chlorophyll b was severely inhibited and light-harvesting complex II was selectively retained during senescence, resulting in the retention of thylakoid grana even at a late stage of senescence. The nyc1 nol double mutant did not show prominent enhancement of inhibition of chlorophyll degradation. NOL was localized on the stromal side of the thylakoid membrane despite the lack of a transmembrane domain. Immunoprecipitation analysis revealed that NOL and NYC1 interact physically in vitro. These observations suggest that NOL and NYC1 are co-localized in the thylakoid membrane and act in the form of a complex as a chlorophyll b reductase in rice.

Identification and characterization of globin genes from two lepidopteran insects, Bombyx mori and Samia cynthia ricini

We describe the characterization of hemoglobin-like genes from two lepidopteran insects, Bombyx mori (Bmglobin) and Samia cynthia ricini (Scglobin). Bmglobin and Scglobin are predicted to be intracellular proteins and contain amino acids required for heme and oxygen binding. Expression profiles of two lepidopteran globins, especially Bmglobin, were different from that of other insect globins. Although other insect globins are mainly associated with the tracheal system, Bmglobin was expressed almost exclusively in the Malpighian tubules, and the strongest signal for Scglobin was detected in the fat body. Furthermore, biochemical fractionation analysis revealed that both Bmglobin and Scglobin were localized in the cytoplasm. These results suggest that each lepidopteran globin has a distinct role in the tissues in which it is expressed and that the functions of insect globins are more divergent than previously thought.

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 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.

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.

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.