Baculovirus Per Os Infectivity Factor Complex: Components and Assembly

The nucleocapsid architecture and structural atlas of the prototype baculovirus define the hallmarks of a new viral realm

Baculovirus is the most studied insect virus owing to a broad ecological distribution and ease of engineering for biotechnological applications. However, its structure and evolutionary place in the virosphere remain enigmatic. Using cryo-electron microscopy, we show that the nucleocapsid forms a covalently cross-linked helical tube protecting a highly compacted 134-kilobase pair DNA genome. The ends of the tube are sealed by the base and cap substructures, which share a 126-subunit hub but differ in components that promote actin tail-mediated propulsion and nuclear entry of the nucleocapsid, respectively. Unexpectedly, sensitive searches for hidden evolutionary links show that the morphogenetic machinery and conserved oral infectivity factors originated within the lineage of baculo-like viruses (class Naldaviricetes). The unique viral architecture and structural atlas of hallmark proteins firmly place these viruses into a separate new realm, the highest taxonomy rank, and provide a structural framework to expand their use as sustainable bioinsecticides and biomedical tools.

Autographa californica multiple nucleopolyhedrovirus ac106 is required for the nuclear egress of nucleocapsids and intranuclear microvesicle formation

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf106 (ac106) is highly conserved in baculoviruses. Previous studies have shown that ac106 is required for the production of infectious budded virions (BVs). However, the functional role of ac106 in virion morphogenesis remains unknown. In this report, an ac106 knockout virus and an ac106 repair virus were constructed. The effect of ac106 deletion on virion morphogenesis was investigated, and the expression and subcellular localization of the Ac106 protein were characterized. Our data indicated that ac106 is required for the nuclear egress of nucleocapsids and intranuclear microvesicle formation, as well as subsequent BV and occlusion-derived virion (ODV) production and the embedding of ODVs into polyhedra. Ac106 is a baculovirus late protein that is concentrated in discrete foci of virus-induced membrane structures in the intranuclear ring zone of virus-infected cells. Further studies on the relationship between Ac106 and four other proteins that are also required for intranuclear microvesicle formation, Ac75, Ac76, Ac93, and P48 (Ac103), revealed that Ac106 is associated with Ac75, Ac76, Ac93, P48, and itself. Ac106 is required for Ac75, Ac93, and P48 accumulation in foci of virus-induced intranuclear membrane structures and the intranuclear transport of Ac76. Analysis of the subcellular localization of ODV integral envelope proteins upon deletion of the genes required for intranuclear microvesicle formation indicated that intranuclear microvesicle formation may be essential for ODV integral envelope protein transport into the nucleus, supporting the hypothesis that intranuclear microvesicles originate from the nuclear membrane.IMPORTANCEBaculovirus occlusion-derived virions (ODVs) are known to acquire their envelopes from virus-induced intranuclear microvesicles within the nucleoplasm, and this strategy of intranuclear envelopment of nucleocapsids to form virions is unique among viruses. However, the mechanism of ODV morphogenesis, particularly intranuclear microvesicle formation, remains unclear. In this study, we identified ac106 as the fifth gene, in addition to ac75, ac76, ac93, and p48 (ac103), which are required for intranuclear microvesicle formation. Further studies on the relationship between ac106 and the other four genes, as well as the effect of ac106 or ac75 deletion on the localization of ODV integral envelope proteins, indicated that intranuclear microvesicle formation may be essential for the transport of ODV integral envelope proteins into the nucleus, which strongly supports the hypothesis that intranuclear microvesicles originate from the nuclear membrane. These findings greatly enhance our understanding of the molecular mechanism of baculovirus ODV morphogenesis.

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.

A qPCR Assay for the Quantification of Selected Genotypic Variants of Spodoptera frugiperda Multiple Nucleopolyhedrovirus (Baculoviridae)

Alphabaculoviruses are lethal dsDNA viruses of Lepidoptera that have high genetic diversity and are transmitted in aggregates within proteinaceous occlusion bodies. This mode of transmission has implications for their efficacy as biological insecticides. A Nicaraguan isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV-NIC) comprising nine genotypic variants has been the subject of considerable study due to the influence of variant interactions on the insecticidal properties of mixed-variant occlusion bodies. As part of a systematic study on the replication and transmission of variant mixtures, a tool for the accurate quantification of a selection of genotypic variants was developed based on the quantitative PCR technique (qPCR). First, primer pairs were designed around a region of high variability in four variants named SfNic-A, SfNic-B, SfNic-C and SfNic-E to produce amplicons of 103-150 bp. Then, using cloned purified amplicons as standards, amplification was demonstrated over a dynamic range of 108-101 copies of each target. The assay was efficient (mean ± SD: 98.5 ± 0.8%), reproducible, as shown by low inter- and intra-assay coefficients of variation (<5%), and specific to the target variants (99.7-100% specificity across variants). The quantification method was validated on mixtures of genotype-specific amplicons and demonstrated accurate quantification. Finally, mixtures of the four variants were quantified based on mixtures of budded virions and mixtures of DNA extracted from occlusion-derived virions. In both cases, mixed-variant preparations compared favorably to total viral genome numbers by quantification of the polyhedrin (polh) gene that is present in all variants. This technique should prove invaluable in elucidating the influence of variant diversity on the transmission and insecticidal characteristics of this pathogen.

Discovery and characterization of novel DNA viruses in Apis mellifera: expanding the honey bee virome through metagenomic analysis

To date, many viruses have been discovered to infect honey bees. In this study, we used high-throughput sequencing to expand the known virome of the honey bee, Apis mellifera, by identifying several novel DNA viruses. While the majority of previously identified bee viruses are RNA, our study reveals nine new genomes from the Parvoviridae family, tentatively named Bee densoviruses 1 to 9. In addition, we characterized a large DNA virus, Apis mellifera filamentous-like virus (AmFLV), which shares limited protein identities with the known Apis mellifera filamentous virus. The complete sequence of AmFLV, obtained by a combination of laboratory techniques and bioinformatics, spans 152,678 bp. Linear dsDNA genome encodes for 112 proteins, of which 49 are annotated. Another large virus we discovered is Apis mellifera nudivirus, which belongs to a group of Alphanudivirus. The virus has a length of 129,467 bp and a circular dsDNA genome, and has 106 protein encoding genes. The virus contains most of the core genes of the family Nudiviridae. This research demonstrates the effectiveness of viral binning in identifying viruses in honey bee virology, showcasing its initial application in this field.IMPORTANCEHoney bees contribute significantly to food security by providing pollination services. Understanding the virome of honey bees is crucial for the health and conservation of bee populations and also for the stability of the ecosystems and economies for which they are indispensable. This study unveils previously unknown DNA viruses in the honey bee virome, expanding our knowledge of potential threats to bee health. The use of the viral binning approach we employed in this study offers a promising method to uncovering and understanding the vast viral diversity in these essential pollinators.

Genome characteristics and the ODV proteome of a second distinct alphabaculovirus from Spodoptera litura

BACKGROUND

Spodoptera litura is a harmful pest that feeds on more than 80 species of plants, and can be infected and killed by Spodoptera litura nucleopolyhedrovirus (SpltNPV). SpltNPV-C3 is a type C SpltNPV clone, that was observed and collected in Japan. Compared with type A or type B SpltNPVs, SpltNPV-C3 can cause the rapid mortality of S. litura larvae.

METHODS

In this study, occlusion bodies (OBs) and occlusion-derived viruses (ODVs) of SpltNPV-C3 were purified, and OBs were observed by scanning electron microscopy (SEM). ODVs were observed under a transmission electron microscope (TEM).

RESULTS

Both OBs and ODVs exhibit morphological characteristics typical of nucleopolyhedroviruses (NPVs).The genome of SpltNPV-C3 was sequenced and analyzed; the total length was 148,634 bp (GenBank accession 780,426,which was submitted as SpltNPV-II), with a G + C content of 45%. A total of 149 predicted ORFs were found. A phylogenetic tree of 90 baculoviruses was constructed based on core baculovirus genes. LC‒MS/MS was used to analyze the proteins of SpltNPV-C3; 34 proteins were found in the purified ODVs, 15 of which were core proteins. The structure of the complexes formed by per os infectivity factors 1, 2, 3 and 4 (PIF-1, PIF-2, PIF-3 and PIF-4) was predicted with the help of the AlphaFold multimer tool and predicted conserved sequences in PIF-3. SpltNPV-C3 is a valuable species because of its virulence, and the analysis of its genome and proteins in this research will be beneficial for pest control efforts.

Genome-wide nonessential gene identification of Autographa californica multiple nucleopolyhedrovirus

The Baculovirus Expression Vector System (BEVS) is an insect cell-based heterologous protein expression system that possesses powerful potential in the development of protein drugs and vaccines. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most widely-used vector in BEVS with 151 open reading frames (ORFs) containing essential and nonessential genes. Deletion of nonessential genes has many advantages including increased foreign gene insertion. In this study, the λ red recombination system was used to knock out genes in a modified AcMNPV that carried an enhanced yellow fluorescent protein (eYFP) at the Ac126-Ac127 locus. Eighty genes were almost completely deleted respectively and 69 gene knockout AcMNPVs (KOVs) were obtained to evaluate their infection efficiency. After infecting Spodoptera frugiperda 9 (Sf9) cells, 51 KOVs including 62 genes showed similar infectivity as wide type (WT) and hence were defined as nonessential genes. However, 18 KOVs produced fewer infectious virions, indicating that these genes were influential in the production of progeny viruses. Combining our research with previous studies, a desired minimal AcMNPV genome containing 86 ORFs and all of the homologous regions (hrs) was brought up, facilitating genetic modification of baculovirus vectors and improvement of recombinant protein expression in the future.

Identification and characterization of BmNPV Bm5 protein required for the formation of nuclear vesicle structures

BmNPV infection induces nuclear vesicle-like structures and its Bm5 protein mediates the intranuclear lipid accumulation, which is thought to participate in the formation of nuclear vesicles. However, the relationship between viral-induced nuclear vesicles and Bm5 protein is still unclear. Here, our results indicated that BmNPV Bm5 protein participated in the baculovirus infection-induced nuclear vesicle-like structures' invagination thereby influencing the production of occlusion-derived virion (ODV) and occlusion body (OB). The process of nuclear vesicle-like structures' formation was dispensable for the transport or recruitment of ODV major envelope proteins, such as P74 and Bm14. Furthermore, baculovirus-induced nuclear F-actin might provide a direct mechanical force to mediate the scission of large vesicle-like structures from the nuclear membrane. Collectively, these findings illustrated a BmNPV Bm5 protein-induced nuclear membrane invagination pathway and revealed the function of nuclear vesicle-like structures in ODV production.

Protein-Gene Orthology in Baculoviridae: An Exhaustive Analysis to Redefine the Ancestrally Common Coding Sequences

Baculoviruses are entomopathogens that carry large, double-stranded circular DNA genomes and infect insect larvae of Lepidoptera, Hymenoptera and Diptera, with applications in the biological control of agricultural pests, in the production of recombinant proteins and as viral vectors for various purposes in mammals. These viruses have a variable genetic composition that differs between species, with some sequences shared by all known members, and others that are lineage-specific or unique to isolates. Based on the analysis of nearly 300 sequenced genomes, a thorough bioinformatic investigation was conducted on all the baculoviral protein coding sequences, characterizing their orthology and phylogeny. This analysis confirmed the 38 protein coding sequences currently considered as core genes, while also identifying novel coding sequences as candidates to join this set. Accordingly, homology was found among all the major occlusion body proteins, thus proposing that the polyhedrin, granulin and CUN085 genes be considered as the 39th core gene of Baculoviridae.

Functional Peroral Infectivity Complex of White Spot Syndrome Virus of Shrimp

White spot syndrome virus (WSSV) is a major cause of disease in shrimp cultures worldwide. The infection process of this large circular double-stranded DNA virus has been well studied, but its entry mechanism remains controversial. The major virion envelope protein VP28 has been implicated in oral and systemic viral infection in shrimp. However, genetic analysis of viral DNA has shown the presence of a few genes related to proteins of per os infectivity factor (PIF) complex in baculoviruses. This complex is essential for the entry of baculoviruses, large terrestrial circular DNA viruses, into the midgut epithelial cells of insect larvae. In this study, we aimed to determine whether a PIF complex exists in WSSV, the components of this complex, whether it functions as an oral infectivity complex in shrimp, and the biochemical properties that contribute to its function in a marine environment. The results revealed a WSSV PIF complex (~720 kDa) comprising at least eight proteins, four of which were not identified as PIF homologs: WSV134, VP124 (WSV216), WSSV021, and WSV136. WSV134 is suggested to be a PIF4 homolog due to predicted structural similarity and amino acid sequence identity. The WSSV PIF complex is resistant to alkali, proteolysis, and high salt, properties that are important for maintaining infectivity in aquatic environments. Oral infection can be neutralized by PIF-specific antibodies but not by VP28-specific antibodies. These results indicate that the WSSV PIF complex is critical for WSSV entry into shrimp; the complex's evolutionary significance is also discussed. IMPORTANCE White spot disease, caused by the white spot syndrome virus (WSSV), is a major scourge in cultured shrimp production facilities worldwide. This disease is only effectively controlled by sanitation. Intervention strategies are urgently needed but are limited by a lack of appropriate targets. Our identification of a per os infectivity factor (PIF) complex, which is pivotal for the entry of WSSV into shrimp, could provide new targets for antibody- or dsRNA-based intervention strategies. In addition, the presence of a PIF complex with at least eight components in WSSV, which is ancestrally related to the PIF complex of invertebrate baculoviruses, suggests that this complex is structurally and functionally conserved in disparate virus taxa.

AC81 Is a Putative Disulfide Isomerase Involved in Baculoviral Disulfide Bond Formation

The correct formation of native disulfide bonds is critical for the proper structure and function of many proteins. Cellular disulfide bond formation pathways commonly consist of two parts: sulfhydryl oxidase-mediated oxidation and disulfide isomerase-mediated isomerization. Some large DNA viruses, such as baculoviruses, encode sulfhydryl oxidases, but viral disulfide isomerases have not yet been identified, although G4L in poxvirus has been suggested to serve such a function. Here, we report that the baculovirus core gene ac81 encodes a putative disulfide isomerase. ac81 is conserved in baculoviruses, nudiviruses, and hytrosaviruses. We found that AC81 homologs contain a typical thioredoxin fold conserved in disulfide isomerases. To determine the role of AC81, a series of Autographa californica nucleopolyhedrovirus (AcMNPV) bacmids containing ac81 knockout or point mutations was generated, and the results showed that AC81 is essential for budded virus production, multinucleocapsid occlusion-derived virus (ODV) formation, and ODV embedding in occlusion bodies. Nonreducing Western blot analysis indicated that disulfide bond formation in per os infectivity factor 5 (PIF5), a substrate of the baculoviral sulfhydryl oxidase P33, was abnormal when ac81 was knocked out or mutated. Pulldown assays showed that AC81 interacted with PIF5 and P33 in infected cells. In addition, two critical regions that harbor key amino acids for function were identified in AC81. Taken together, our results suggest that AC81 is a key component involved in the baculovirus disulfide bond formation pathway and likely functions as a disulfide isomerase. IMPORTANCE Many large DNA viruses, such as poxvirus, asfarvirus, and baculovirus, encode their own sulfhydryl oxidase to facilitate the disulfide bond formation of viral proteins. Here, we show that AC81 functions as a putative disulfide isomerase and is involved in multiple functions of the baculovirus life cycle. Interestingly, AC81 and P33 (sulfhydryl oxidase) are conserved in baculoviruses, nudiviruses, and hytrosaviruses, which are all insect-specific large DNA viruses replicating in the nucleus, suggesting that viral disulfide bond formation is an ancient mechanism shared by these viruses.

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.

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.

Structural Characterization of Per Os Infectivity Factor 5 (PIF5) Reveals the Essential Role of Intramolecular Interactions in Baculoviral Oral Infectivity

Baculoviruses initiate oral infection in the highly alkaline midgut of insects via a group of envelope proteins called per os infectivity factors (PIFs). To date, no high-resolution structural information has been reported for any PIF. Here, we present the crystal structure of the PIF5 ectodomain (PIF5e) from Autographa californica multiple nucleopolyhedrovirus (AcMNPV) at a 2.2-Å resolution. It revealed an open cavity between the N-terminal E1 domain and the C-terminal E2 domain and a cysteine-rich region with three pairs of disulfide bonds in the E2 domain. Multiple conserved intramolecular interactions within PIF5 are essential for maintaining its tertiary structure. Two conserved arginines (Arg8 and Arg74) play critical roles in E1-E2 interactions, and mutagenesis analysis supported their crucial role in oral infection. Importantly, the reduction in the oral infectivity of the Arg8, Arg74, or cysteine mutant viruses was related to the proteolytic cleavage of PIF5 by the endogenous protease embedded in occlusion bodies during alkaline treatment. This suggested that the structural stability of PIF5 under physiological conditions in the insect midgut is critical for baculoviral oral infectivity. IMPORTANCEPer os infection mediated by PIFs is the highly complex mechanism by which baculoviruses initiate infection in insects. Previous studies revealed that multiple PIF proteins form a large PIF complex on the envelope of virions, while PIF5 functions independently of the PIF complex. Here, we report the crystal structure of AcMNPV PIF5e, which, to our knowledge, is the first atomic structure reported for a PIF protein. The structure revealed the precise locations of three previously proposed disulfide bonds and other conserved intramolecular interactions, which are important for the structural stability of PIF5 and are also essential for oral infectivity. These findings advance our understanding of the molecular mechanism of baculovirus oral infection under alkaline conditions.

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.

Nucleopolyhedrovirus Coocclusion Technology: A New Concept in the Development of Biological Insecticides

Nucleopolyhedroviruses (NPV, Baculoviridae) that infect lepidopteran pests have an established record as safe and effective biological insecticides. Here, we describe a new approach for the development of NPV-based insecticides. This technology takes advantage of the unique way in which these viruses are transmitted as collective infectious units, and the genotypic diversity present in natural virus populations. A ten-step procedure is described involving genotypic variant selection, mixing, coinfection and intraspecific coocclusion of variants within viral occlusion bodies. Using two examples, we demonstrate how this approach can be used to produce highly pathogenic virus preparations for pest control. As restricted host range limits the uptake of NPV-based insecticides, this technology has recently been adapted to produce custom-designed interspecific mixtures of viruses that can be applied to control complexes of lepidopteran pests on particular crops, as long as a shared host species is available for virus production. This approach to the development of NPV-based insecticides has the potential to be applied across a broad range of NPV-pest pathosystems.

Genome Analysis of an Alphabaculovirus Isolated from the Larch Looper, Erannis ankeraria

The larch looper, Erannis ankeraria Staudinger (Lepidoptera: Geometridae), is one of the major insect pests of larch forests, widely distributed from southeastern Europe to East Asia. A naturally occurring baculovirus, Erannis ankeraria nucleopolyhedrovirus (EranNPV), was isolated from E. ankeraria larvae. This virus was characterized by electron microscopy and by sequencing the whole viral genome. The occlusion bodies (OBs) of EranNPV exhibited irregular polyhedral shapes containing multiple enveloped rod-shaped virions with a single nucleocapsid per virion. The EranNPV genome was 125,247 bp in length with a nucleotide distribution of 34.9% G+C. A total of 131 hypothetical open reading frames (ORFs) were identified, including the 38 baculovirus core genes and five multi-copy genes. Five homologous regions (hrs) were found in the EranNPV genome. Phylogeny and pairwise kimura 2-parameter analysis indicated that EranNPV was a novel group II alphabaculovirus and was most closely related to Apocheima cinerarium NPV (ApciNPV). Field trials showed that EranNPV was effective in controlling E. ankeraria in larch forests. The above results will be relevant to the functional research on EranNPV and promote the use of this virus as a biocontrol agent.

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.

Solutions against emerging infectious and noninfectious human diseases through the application of baculovirus technologies

Abstract

Baculoviruses are insect pathogens widely used as biotechnological tools in different fields of life sciences and technologies. The particular biology of these entities (biosafety viruses 1; large circular double-stranded DNA genomes, infective per se; generally of narrow host range on insect larvae; many of the latter being pests in agriculture) and the availability of molecular-biology procedures (e.g., genetic engineering to edit their genomes) and cellular resources (availability of cell lines that grow under in vitro culture conditions) have enabled the application of baculoviruses as active ingredients in pest control, as systems for the expression of recombinant proteins (Baculovirus Expression Vector Systems—BEVS) and as viral vectors for gene delivery in mammals or to display antigenic proteins (Baculoviruses applied on mammals—BacMam). Accordingly, BEVS and BacMam technologies have been introduced in academia because of their availability as commercial systems and ease of use and have also reached the human pharmaceutical industry, as incomparable tools in the development of biological products such as diagnostic kits, vaccines, protein therapies, and—though still in the conceptual stage involving animal models—gene therapies. Among all the baculovirus species, the Autographa californica multiple nucleopolyhedrovirus has been the most highly exploited in the above utilities for the human-biotechnology field. This review highlights the main achievements (in their different stages of development) of the use of BEVS and BacMam technologies for the generation of products for infectious and noninfectious human diseases.

Key points

• Baculoviruses can assist as biotechnological tools in human health problems.

• Vaccines and diagnosis reagents produced in the baculovirus platform are described.

• The use of recombinant baculovirus for gene therapy–based treatment is reviewed.

Construction and Characterization of a Novel Bacmid AcBac-Syn Based on a Synthesized Baculovirus Genome

Baculoviruses are large DNA viruses which have been widely used as expression vectors and biological insecticides. Homologous recombination and Bac-to-Bac system have been the main methods for manipulating the baculovirus genome. Recently, we generated a synthetic baculovirus AcMNPV-WIV-Syn1 which fully resembled its parental virus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Here, we report the modification of AcMNPV-WIV-Syn1 into a novel bacmid, AcBac-Syn, which can be used as a backbone for Bac-to-Bac system. To achieve this, a vector contained a LacZ:attTn7 and egfp cassette was constructed, and recombined with a linearized AcMNPV-WIV-Syn1 genome by transformation-associated recombination in yeast to generate bacmid AcBac-Syn. The bacmid was then transfected to insect cells and the rescued virus showed similar biological characteristics to the wild-type virus in terms of the kinetics of budded virus production, the morphology of occlusion bodies, and the oral infectivity in insect larvae. For demonstration, a red fluorescent protein gene Dsred was transposed into the attTn7 site by conventional Bac-to-Bac method, and the transfection and infection assays showed that AcBac-Syn can be readily used for foreign gene insertion and expression. AcBac-Syn has several advantages over the conventional AcMNPV bacmids, such as it contains an egfp reporter gene which facilitates visualization of virus propagation and titration; its DNA copy numbers could be induced to a higher level in E. coli; and the retaining of the native polyhedrin gene in the genome making it an attractive system for studying the functions of gene related to occlusion body assembly and oral infection.

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.

Depletion of Gut Microbiota Impairs Gut Barrier Function and Antiviral Immune Defense in the Liver

Commensal gut microbiota protects the immune defense of extra-intestinal organs. Gut microbiota depletion by antibiotics can impair host antiviral immune responses and alter hepatitis B virus (HBV) infection outcomes. However, how gut microbiota modulates antiviral immune response in the liver remains unclear. Here, mice were treated with broad-spectrum antibiotics to deplete gut microbiota. Gut integrity was evaluated, and translocation of live commensal gut bacteria and their components into the liver was investigated. An HBV infection model was established to evaluate impairment of antiviral immune response in the liver after gut microbiota depletion. We found that gut microbiota depletion was associated with impairment of colon epithelial integrity, and live commensal gut microbiota could translocate to the liver. Further, T cell antiviral function in the liver was impaired, partially relying on enhanced PD-1 expression, and HBV immune clearance was hampered. In conclusion, gut microbiota depletion by antibiotics can impair gut barrier function and suppress T cell antiviral immune response in the liver.

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.

Advances in the Arms Race Between Silkworm and Baculovirus

Insects are the largest group of animals. Nearly all organisms, including insects, have viral pathogens. An important domesticated economic insect is the silkworm moth Bombyx mori. B. mori nucleopolyhedrovirus (BmNPV) is a typical baculovirus and a primary silkworm pathogen. It causes major economic losses in sericulture. Baculoviruses are used in biological pest control and as a bioreactor. Silkworm and baculovirus comprise a well-established model of insect–virus interactions. Several recent studies have focused on this model and provided novel insights into viral infections and host defense. Here, we focus on baculovirus invasion, silkworm immune response, baculovirus evasion of host immunity, and enhancement of antiviral efficacy. We also discuss major issues remaining and future directions of research on silkworm antiviral immunity. Elucidation of the interaction between silkworm and baculovirus furnishes a theoretical basis for targeted pest control, enhanced pathogen resistance in economically important insects, and bioreactor improvement.

Comparative genomic analysis of three geographical isolates from China reveals high genetic stability of Plutella xylostella granulovirus

In this study, the genomes of three Plutella xylostella granulovirus (PlxyGV) isolates, PlxyGV-W and PlxyGV-Wn from near Wuhan and PlxyGV-B from near Beijing, China were completely sequenced and comparatively analyzed to investigate genetic stability and diversity of PlxyGV. PlxyGV-W, PlxyGV-B and PlxyGV-Wn consist of 100,941bp, 100,972bp and 100,999bp in length with G + C compositions of 40.71–40.73%, respectively, and share nucleotide sequence identities of 99.5–99.8%. The three individual isolates contain 118 putative protein-encoding ORFs in common. PlxyGV-W, PlxyGV-B and PlxyGV-Wn have ten, nineteen and six nonsynonymous intra isolate nucleotide polymorphisms (NPs) in six, fourteen and five ORFs, respectively, including homologs of five DNA replication/late expression factors and two per os infectivity factors. There are seventeen nonsynonymous inter isolate NPs in seven ORFs between PlxyGV-W and PlxyGV-B, seventy three nonsynonymous NPs in forty seven ORFs between PlxyGV-W and PlxyGV-Wn, seventy seven nonsynonymous NPs in forty six ORFs between PlxyGV-B and PlxyGV-Wn. Alignment of the genome sequences of nine PlxyGV isolates sequenced up to date shows that the sequence homogeneity between the genomes are over 99.4%, with the exception of the genome of PlxyGV-SA from South Africa, which shares a sequence identity of 98.6–98.7% with the other ones. No events of gene gain/loss or translocations were observed. These results suggest that PlxyGV genome is fairly stable in nature. In addition, the transcription start sites and polyadenylation sites of thirteen PlxyGV-specific ORFs, conserved in all PlxyGV isolates, were identified by RACE analysis using mRNAs purified from larvae infected by PlxyGV-Wn, proving the PlxyGV-specific ORFs are all genuine genes.

Per Os Infectivity Factor 5 Identified as a Substrate of P33 in the Baculoviral Disulfide Bond Formation Pathway

Disulfide bonds are critical for the structure and function of many proteins. Some large DNA viruses encode their own sulfhydryl oxidase for disulfide bond formation. Previous studies have demonstrated that the baculovirus-encoded sulfhydryl oxidase P33 is necessary for progeny virus production, and its enzymatic activity is important for morphogenesis and oral infectivity of baculoviruses. However, the downstream substrates of P33 in the putative redox pathway of baculoviruses are unknown. In this study, we showed that PIF5, one of the per os infectivity factors (PIFs), contained intramolecular disulfide bonds and that the disulfide bond formation was interrupted in the absence of P33. In vivo pulldown and colocalization analyses revealed that PIF5 and P33 interacted with each other during virus infection. Further, in vitro assays validated that the reduced PIF5 proteins could be oxidized by P33. To understand the contribution of disulfide bonds to the function of PIF5, several cysteine-to-serine mutants were constructed, which all interfered with the disulfide bond formation of PIF5 to different extents. All the mutants lost their oral infectivity but had no impact on infectious budding virus (BV) production or virus morphogenesis. Taken together, our results indicated PIF5 as the first identified substrate of P33. Further, the disulfide bonds in PIF5 play an essential role in its function in oral infection.IMPORTANCE Similar to some large DNA viruses that encode their own disulfide bond pathway, baculovirus encodes a viral sulfhydryl oxidase, P33. Enzyme activity of P33 is related to infectious BV production, occlusion-derived virus (ODV) envelopment, occlusion body morphogenesis, and oral infectivity, suggesting that P33 is involved in disulfide bond formation of multiple proteins. A complete disulfide bond formation pathway normally contains a sulfhydryl oxidase, a disulfide-donating enzyme, and one or more substrates. In baculovirus, apart from P33, other components of the putative pathway remain unknown. In this study, we identified PIF5 as the first substrate of P33, which is fundamental for revealing the complete disulfide bond formation pathway in baculovirus. PIF5 is essential for oral infection and is absent from the PIF complex. Our study demonstrated that native disulfide bonds in PIF5 are required for oral infection, which will help us to reveal its mode of action.

Downregulation of PIF1, a potential new target of MYCN, induces apoptosis and inhibits cell migration in neuroblastoma cells

Neuroblastoma (NB) is one of the most common malignant tumors in children. Chemotherapy resistance is one of the significant challenges in the treatment of high-risk NB patients, and it is necessary to search for new valid targets for NB treatment. This study aims to explore the possible role of PIF1 in NB by using bioinformatic analysis and downregulation of PIF1 with specific siRNA. Kyoto genome encyclopedia and R language based gene ontology was used to analyze the differentially expressed genes (DEGs) (including PIF1) when MYCN expression was silenced in NB cells. Analysis based on the R2 database showed a lower expression of PIF1 correlated with good prognosis in NB patients. Downregulation of MYCN expression by transfecting MYCN siRNA (#1, #2) into NB cells decreased the PIF1 expression at both mRNA and protein levels, while upregulation of MYCN expression by transfecting MYCN overexpressed plasmid increased the PIF1 expression. We further found that downregulation of PIF1 expression by transfecting PIF1 siRNA (#1, #2) into NB cells, increased the number of apoptotic cells, inhibited the cell survival, decreased the ability of cell migration and induced a cell cycle arrest at G1 phase. These data indicated that PIF1, as a potential new target of MYCN, maybe a novel target for NB treatment.