AcMNPV P74 is cleaved at R325 and R334 by proteinases of both OB and BBMV to expose a potential fusion peptide for oral infection

Baculoviruses enter insect midgut epithelial cells via a set of occlusion-derived virion (ODV) envelope proteins called per os infectivity factors (PIFs). P74 of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), which was the first identified PIF, is cleaved by an endogenous proteinase embedded within the occlusion body during per os infection, but the target site(s) and function of the cleavage have not yet been ascertained. Here, based on bioinformatics analyses, we report that cleavage was predicted at an arginine and lysine-rich region in the middle of P74. A series of recombinant viruses with site-directed mutants in this region of P74 were generated. R325 or R334 was identified as primary cleavage site. In addition, we showed that P74 is also cleaved by brush border membrane vesicles (BBMV) of the host insect at R325 or R334, instead of R195, R196, and R199, as previously reported. Simultaneous mutations in R195, R196, and R199 lead to instability of P74 during ODV release. Bioassays showed that mutations at both R325 and R334 significantly affected oral infectivity. Taken together, our data show that both R325 and R334 of AcMNPV P74 are the primary cleavage site for both occlusion body endogenous proteinase and BBMV proteinase during ODV release and are critical for oral infection.

IMPORTANCE

Cleavage of viral envelope proteins is usually an important trigger for viral entry into host cells. Baculoviruses are insect-specific viruses that infect host insects via the oral route. P74, a per os infectivity factor of baculoviruses, is cleaved during viral entry. However, the function and precise cleavage sites of P74 remain unknown. In this study, we found that R325 or R334 between the N- and C-conserved domains of P74 was the primary cleavage site by proteinase either from the occlusion body or host midgut. The biological significance of cleavage seems to be the release of the potential fusion peptide at the N-terminus of the cleaved C-terminal P74. Our results shed light on the cleavage model of P74 and imply its role in membrane fusion in baculovirus per os infection.

Midgut membrane protein BmSUH facilitates Bombyx mori nucleopolyhedrovirus oral infection

Baculoviruses are virulent pathogens that infect a wide range of insects. They initiate infections via specific interactions between the structural proteins on the envelopes of occlusion-derived virions (ODVs) and the midgut cell surface receptors in hosts. However, host factors that are hijacked by baculoviruses for efficient infection remain largely unknown. In this study, we identified a membrane-associated protein sucrose hydrolase (BmSUH) as an ODV binding factor during Bombyx mori nucleopolyhedrovirus (BmNPV) primary infection. BmSUH was specifically expressed in the midgut microvilli where the ODV-midgut fusion happened. Knockout of BmSUH by CRISPR/Cas9 resulted in a significantly higher survival rate after BmNPV orally infection. Liquid chromatography-tandem mass spectrometry analysis and co-immunoprecipitation analysis demonstrated that PIF protein complex required for ODV binding could interact with BmSUH. Furthermore, fluorescence dequenching assay showed that the amount of ODV binding and fusion to the midgut decreased in BmSUH mutants compared to wild-type silkworm, suggesting the role of BmSUH as an ODV binding factor that mediates the ODV entry process. Based on a multilevel survey, the data showed that BmSUH acted as a host factor that facilitates BmNPV oral infection. More generally, this study indicated that disrupting essential protein-protein interactions required for baculovirus efficient entry may be broadly applicable to against viral infection.

Baculoviridae is a large family of pathogens that infect insects and frequently cause fatal diseases. Bombyx mori nucleopolyhedrovirus (BmNPV) is a major threat to the sericulture industry. Although we have learned a lot about baculoviruses over the past several decades, the detailed interaction patterns between host proteins and viral proteins that lead to infection remain underexplored. Here, we determined that BmSUH, a midgut microvilli protein, was required for the efficient oral infection of BmNPV. Our research suggests that BmSUH mediates the entry of occlusion-derived virions into the midgut epithelia by interacting with per os infectivity factors. According to the findings, inhibition of viral binding to host cells is an attractive strategy to prevent infection. This study provides an approach for preventing BmNPV infection through developing genetic resistance to viruses by using CRISPR/Cas9 system to abolish the host factors that are essential for viral entry.

Coocclusion of Helicoverpa armigera Single Nucleopolyhedrovirus (HearSNPV) and Helicoverpa armigera Multiple Nucleopolyhedrovirus (HearMNPV): Pathogenicity and Stability in Homologous and Heterologous Hosts

Helicoverpa armigera single nucleopolyhedrovirus (HearSNPV) is a virulent pathogen of lepidopterans in the genera Heliothis and Helicoverpa, whereas Helicoverpa armigera multiple nucleopolyhedrovirus (HearSNPV) is a different virus species with a broader host range. This study aimed to examine the consequences of coocclusion of HearSNPV and HearMNPV on the pathogenicity, stability and host range of mixed-virus occlusion bodies (OBs). HearSNPV OBs were approximately 6-fold more pathogenic than HearMNPV OBs, showed faster killing by approximately 13 h, and were approximately 45% more productive in terms of OB production per larva. For coocclusion, H. armigera larvae were first inoculated with HearMNPV OBs and subsequently inoculated with HearSNPV OBs at intervals of 0–72 h after the initial inoculation. When the interval between inoculations was 12–24 h, OBs collected from virus-killed insects were found to comprise 41–57% of HearSNPV genomes, but the prevalence of HearSNPV genomes was greatly reduced (3–4%) at later time points. Quantitative PCR (qPCR) analysis revealed the presence of HearSNPV genomes in a small fraction of multinucleocapsid ODVs representing 0.47–0.88% of the genomes quantified in ODV samples, indicating that both viruses had replicated in coinfected host cells. End-point dilution assays on ODVs from cooccluded mixed-virus OBs confirmed the presence of both viruses in 41.9–55.6% of wells that were predicted to have been infected by a single ODV. A control experiment indicated that this result was unlikely to be due to the adhesion of HearSNPV ODVs to HearMNPV ODVs or accidental contamination during ODV band extraction. Therefore, the disparity between the qPCR and end-point dilution estimates of the prevalence of mixed-virus ODVs likely reflected virus-specific differences in replication efficiency in cell culture and the higher infectivity of pseudotyped ODVs that were produced in coinfected parental cells. Bioassays on H. armigera, Spodoptera frugiperda and Mamestra brassicae larvae revealed that mixed-virus OBs were capable of infecting heterologous hosts, but relative potency values largely reflected the proportion of HearMNPV present in each mixed-virus preparation. The cooccluded mixtures were unstable in serial passage; HearSNPV rapidly dominated during passage in H. armigera whereas HearMNPV rapidly dominated during passage in the heterologous hosts. We conclude that mixed-virus coocclusion technology may be useful for producing precise mixtures of viruses with host range properties suitable for the control of complexes of lepidopteran pests in particular crops, although this requires validation by field testing.

The Membrane-Anchoring Region of the AcMNPV P74 Protein Is Expendable or Interchangeable with Homologs from Other Species

Baculoviruses are insect pathogens that are characterized by assembling the viral dsDNA into two different enveloped virions during an infective cycle: occluded virions (ODVs; immersed in a protein matrix known as occlusion body) and budded virions (BVs). ODVs are responsible for the primary infection in midgut cells of susceptible larvae thanks to the per os infectivity factor (PIF) complex, composed of at least nine essential viral proteins. Among them, P74 is a crucial factor whose activity has been identified as virus-specific. In this work, the p74 gene from AcMNPV was pseudogenized using CRISPR/Cas9 technology and then complemented with wild-type alleles from SeMNPV and HearSNPV species, as well as chimeras combining the P74 amino and carboxyl domains. The results on Spodoptera exigua and Rachiplusia nu larvae showed that an amino terminal sector of P74 (lacking two potential transmembrane regions but possessing a putative nuclear export signal) is sufficient to restore the virus infectivity whether alone or fused to the P74 transmembrane regions of the other evaluated viral species. These results provide novel information about the functional role of P74 and delimit the region on which mutagenesis could be applied to enhance viral activity and, thus, produce better biopesticides.

Cross-Resistance of the Codling Moth against Different Isolates of Cydia pomonella Granulovirus Is Caused by Two Different but Genetically Linked Resistance Mechanisms

Cydia pomonella granulovirus (CpGV) is a widely used biological control agent of the codling moth. Recently, however, the codling moth has developed different types of field resistance against CpGV isolates. Whereas type I resistance is Z chromosomal inherited and targeted at the viral gene pe38 of isolate CpGV-M, type II resistance is autosomal inherited and targeted against isolates CpGV-M and CpGV-S. Here, we report that mixtures of CpGV-M and CpGV-S fail to break type II resistance and is expressed at all larval stages. Budded virus (BV) injection experiments circumventing initial midgut infection provided evidence that resistance against CpGV-S is midgut-related, though fluorescence dequenching assay using rhodamine-18 labeled occlusion derived viruses (ODV) could not fully elucidate whether the receptor binding or an intracellular midgut factor is involved. From our peroral and intra-hemocoel infection experiments, we conclude that two different (but genetically linked) resistance mechanisms are responsible for type II resistance in the codling moth: resistance against CpGV-M is systemic whereas a second and/or additional resistance mechanism against CpGV-S is located in the midgut of CpR5M larvae.

Functional analysis of the baculovirus per os infectivity factors 3 and 9 by imaging the interaction between fluorescently labelled virions and isolated midgut cells

Baculovirus occlusion-derived viruses (ODVs) contain ten known per os infectivity factors (PIFs). These PIFs are crucial for midgut infection of insect larvae and form, with the exception of PIF5, an ODV entry complex. Previously, R18-dequenching assays have shown that PIF3 is dispensable for binding and fusion with midgut epithelial cells. Oral infection nevertheless fails in the absence of PIF3. PIF9 has not been analysed in much depth yet. Here, the biological role of these two PIFs in midgut infection was examined by monitoring the fate of fluorescently labelled ODVs when incubated with isolated midgut cells from Spodoptera exigua larvae. Confocal microscopy showed that in the absence of either PIF3 or PIF9, the ODVs bound to the brush borders, but the nucleocapsids failed to enter the cells. Finally, we discuss how the results obtained for PIF3 with dequenching assays and confocal microscopy can be explained by a two-phase fusion process.

An Advanced View on Baculovirus per Os Infectivity Factors

Baculoviruses are arthropod-specific large DNA viruses that orally infect the larvae of lepidopteran, hymenopteran and dipteran insect species. These larvae become infected when they eat a food source that is contaminated with viral occlusion bodies (OBs). These OBs contain occlusion-derived viruses (ODVs), which are released upon ingestion of the OBs and infect the endothelial midgut cells. At least nine different ODV envelope proteins are essential for this oral infectivity and these are denoted per os infectivity factors (PIFs). Seven of these PIFs form a complex, consisting of PIF1, 2, 3 and 4 that form a stable core complex and PIF0 (P74), PIF6 and PIF8 (P95) that associate with this complex with lower affinity than the core components. The existence of a PIF complex and the fact that the pif genes are conserved in baculovirus genomes suggests that PIF-proteins cooperatively mediate oral infectivity rather than as individual functional entities. This review therefore discusses the knowledge obtained for individual PIFs in light of their relationship with other members of the PIF complex.

Genome Sequencing of the Behavior Manipulating Virus LbFV Reveals a Possible New Virus Family

Parasites are sometimes able to manipulate the behavior of their hosts. However, the molecular cues underlying this phenomenon are poorly documented. We previously reported that the parasitoid wasp Leptopilina boulardi which develops from Drosophila larvae is often infected by an inherited DNA virus. In addition to being maternally transmitted, the virus benefits from horizontal transmission in superparasitized larvae (Drosophila that have been parasitized several times). Interestingly, the virus forces infected females to lay eggs in already parasitized larvae, thus increasing the chance of being horizontally transmitted. In a first step towards the identification of virus genes responsible for the behavioral manipulation, we present here the genome sequence of the virus, called LbFV. The sequencing revealed that its genome contains an homologous repeat sequence (hrs) found in eight regions in the genome. The presence of this hrs may explain the genomic plasticity that we observed for this genome. The genome of LbFV encodes 108 ORFs, most of them having no homologs in public databases. The virus is however related to Hytrosaviridae, although distantly. LbFV may thus represent a member of a new virus family. Several genes of LbFV were captured from eukaryotes, including two anti-apoptotic genes. More surprisingly, we found that LbFV captured from an ancestral wasp a protein with a Jumonji domain. This gene was afterwards duplicated in the virus genome. We hypothesized that this gene may be involved in manipulating the expression of wasp genes, and possibly in manipulating its behavior.

Baculovirus-assisted Reovirus Infection in Monolayer and Spheroid Cultures of Glioma cells

The mammalian orthoreovirus Type 3 Dearing has great potential as oncolytic agent in cancer therapy. One of the bottlenecks that hampers its antitumour efficacy in vivo is the limited tumour-cell infection and intratumoural distribution. This necessitates strategies to improve tumour penetration. In this study we employ the baculovirus Autographa californica multiple nucleopolyhedrovirus as a tool to expand the reovirus' tropism and to improve its spread in three-dimensional tumour-cell spheroids. We generated a recombinant baculovirus expressing the cellular receptor for reovirus, the Junction Adhesion Molecule-A, on its envelope. Combining these Junction Adhesion Molecule-A-expressing baculoviruses with reovirus particles leads to the formation of biviral complexes. Exposure of the reovirus-resistant glioblastoma cell line U-118 MG to the baculovirus-reovirus complexes results in efficient reovirus infection, high reovirus yields, and significant reovirus-induced cytopathic effects. As compared to the reovirus-only incubations, the biviral complexes demonstrated improved penetration and increased cell killing of three-dimensional U-118 MG tumour spheroids. Our data demonstrate that reovirus can be delivered with increased efficiency into two- and three-dimensional tumour-cell cultures via coupling the reovirus particles to baculovirus. The identification of baculovirus' capacity to penetrate into tumour tissue opens novel opportunities to improve cancer therapy by improved delivery of oncolytic viruses into tumours.

Per os infectivity factors: a complicated and evolutionarily conserved entry machinery of baculovirus

Baculoviruses are a family of arthropod-specific large DNA viruses that infect insect species belonging to the orders Lepidoptera, Hymenoptera and Diptera. In nature, occlusion-derived viruses (ODVs) initiate baculovirus primary infection in the midgut epithelium of insect hosts, and this process is largely dependent on a number of ODV envelope proteins designated as per os infectivity factors (PIFs). Interestingly, PIF homologs are also present in other invertebrate large DNA viruses, which is indicative that per os infection is an ancient and phylogenetically conserved entry mechanism shared by these viruses. Here, we review the advances in the knowledge of the functions of individual PIFs and recent discoveries about the PIF complex, and discuss the evolutionary implications of PIF homologs in invertebrate DNA viruses. Furthermore, future research highlights on the per os infection mechanism are also prospected.

Protein-protein interactions of the baculovirus per os infectivity factors (PIFs) in the PIF complex

After ingestion of occlusion bodies, the occlusion-derived viruses (ODVs) of the baculoviruses establish the first round of infection within the larval host midgut cells. Several ODV envelope proteins, called per os infectivity factors (PIFs), have been shown to be essential for oral infection. Eight PIFs have been identified to date, including P74, PIFs 1-6 and Ac110. At least six PIFs, P74, PIFs 1-4 and PIF6, together with three other ODV-specific proteins, Ac5, P95 (Ac83) and Ac108, have been reported to form a complex on the ODV surface. In this study, in order to understand the interactions of these PIFs, the direct protein-protein interactions of the nine components of the Autographa californica multiple nucleopolyhedrovirus PIF complex were investigated using yeast two-hybrid (Y2H) screening combined with bimolecular fluorescence complementation (BiFC) assay. Six direct interactions, comprising PIF1-PIF2, PIF1-PIF3, PIF1-PIF4, PIF1-P95, PIF2-PIF3 and PIF3-PIF4, were identified in the Y2H analysis, and these results were further verified by BiFC. For P74, PIF6, Ac5 and Ac108, no direct interaction was identified. P95 (Ac83) was identified to interact with PIF1, and further Y2H analysis of the truncation and deletion mutants showed that the predicted P95 chitin-binding domain and amino acids 100-200 of PIF1 were responsible for P95 interaction with PIF1. Furthermore, a summary of the protein-protein interactions of PIFs reported so far, comprising 10 reciprocal interactions and two self-interactions, is presented, which will facilitate our understanding of the characteristics of the PIF complex.

Unbiased RNA Shotgun Metagenomics in Social and Solitary Wild Bees Detects Associations with Eukaryote Parasites and New Viruses

The diversity of eukaryote organisms and viruses associated with wild bees remains poorly characterized in contrast to the well-documented pathosphere of the western honey bee, Apis mellifera. Using a deliberate RNA shotgun metagenomic sequencing strategy in combination with a dedicated bioinformatics workflow, we identified the (micro-)organisms and viruses associated with two bumble bee hosts, Bombus terrestris and Bombus pascuorum, and two solitary bee hosts, Osmia cornuta and Andrena vaga. Ion Torrent semiconductor sequencing generated approximately 3.8 million high quality reads. The most significant eukaryote associations were two protozoan, Apicystis bombi and Crithidia bombi, and one nematode parasite Sphaerularia bombi in bumble bees. The trypanosome protozoan C. bombi was also found in the solitary bee O. cornuta. Next to the identification of three honey bee viruses Black queen cell virus, Sacbrood virus and Varroa destructor virus-1 and four plant viruses, we describe two novel RNA viruses Scaldis River bee virus (SRBV) and Ganda bee virus (GABV) based on their partial genomic sequences. The novel viruses belong to the class of negative-sense RNA viruses, SRBV is related to the order Mononegavirales whereas GABV is related to the family Bunyaviridae. The potential biological role of both viruses in bees is discussed in the context of recent advances in the field of arthropod viruses. Further, fragmentary sequence evidence for other undescribed viruses is presented, among which a nudivirus in O. cornuta and an unclassified virus related to Chronic bee paralysis virus in B. terrestris. Our findings extend the current knowledge of wild bee parasites in general and addsto the growing evidence of unexplored arthropod viruses in valuable insects.

Trichoplusia ni Kinesin-1 Associates with Autographa californica Multiple Nucleopolyhedrovirus Nucleocapsid Proteins and Is Required for Production of Budded Virus

The mechanism by which nucleocapsids of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) egress from the nucleus to the plasma membrane, leading to the formation of budded virus (BV), is not known. AC141 is a nucleocapsid-associated protein required for BV egress and has previously been shown to be associated with β-tubulin. In addition, AC141 and VP39 were previously shown by fluorescence resonance energy transfer by fluorescence lifetime imaging to interact directly with the Drosophila melanogaster kinesin-1 light chain (KLC) tetratricopeptide repeat (TPR) domain. These results suggested that microtubule transport systems may be involved in baculovirus nucleocapsid egress and BV formation. In this study, we investigated the role of lepidopteran microtubule transport using coimmunoprecipitation, colocalization, yeast two-hybrid, and small interfering RNA (siRNA) analyses. We show that nucleocapsid AC141 associates with the lepidopteran Trichoplusia ni KLC and kinesin-1 heavy chain (KHC) by coimmunoprecipitation and colocalization. Kinesin-1, AC141, and microtubules colocalized predominantly at the plasma membrane. In addition, the nucleocapsid proteins VP39, FP25, and BV/ODV-C42 were also coimmunoprecipitated with T. ni KLC. Direct analysis of the role of T. ni kinesin-1 by downregulation of KLC by siRNA resulted in a significant decrease in BV production. Nucleocapsids labeled with VP39 fused with three copies of the mCherry fluorescent protein also colocalized with microtubules. Yeast two-hybrid analysis showed no evidence of a direct interaction between kinesin-1 and AC141 or VP39, suggesting that either other nucleocapsid proteins or adaptor proteins may be required. These results further support the conclusion that microtubule transport is required for AcMNPV BV formation.

IMPORTANCE

In two key processes of the replication cycle of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), nucleocapsids are transported through the cell. These include (i) entry of budded virus (BV) into the host cell and (ii) egress and budding of nucleocapsids newly produced from the plasma membrane. Prior studies have shown that the entry of nucleocapsids involves the polymerization of actin to propel nucleocapsids to nuclear pores and entry into the nucleus. For the spread of infection, progeny viruses must rapidly exit the infected cells, but the mechanism by which AcMNPV nucleocapsids traverse the cytoplasm is unknown. In this study, we examined whether nucleocapsids interact with lepidopteran kinesin-1 motor molecules and are potentially carried as cargo on microtubules to the plasma membrane in AcMNPV-infected cells. This study indicates that microtubule transport is utilized for the production of budded virus.