The p95 gene of Bombyx mori nuclear polyhedrosis virus: temporal expression and functional properties

The cysteine-rich region of a baculovirus VP91 protein contributes to the morphogenesis of occlusion bodies

The baculovirus core gene vp91 has been reported to be essential for nucleocapsid assembly and oral infection. Here, we studied the function of vp91 by analyzing its homologue, ha76, in Helicoverpa armigera nucleopolyhedrovirus (HearNPV). HA76 was expressed at the late stage of HearNPV infection; deletion of ha76 showed that the gene is required for budded virus production. A series of recombinants with truncated ha76 was constructed and analyzed in vitro and in vivo. The results showed that the region encoding the C-terminus of HA76 was essential for nucleocapsid assembly, whereas the N-terminal cysteine-rich region was responsible for oral infection. Electron microscope analyses further showed that the cysteine-rich region contributed to morphogenesis of occlusion bodies (OBs), with amino acids 136-223 of HA76 being critical for this function. The results revealed a novel function of VP91 and suggested that the impact on OB morphogenesis is partially related to oral infectivity.

Autographa californica Multiple Nucleopolyhedrovirus AC83 is a Per Os Infectivity Factor (PIF) Protein Required for Occlusion-Derived Virus (ODV) and Budded Virus Nucleocapsid Assembly as well as Assembly of the PIF Complex in ODV Envelopes

Baculovirus occlusion-derived virus (ODV) initiates infection of lepidopteran larval hosts by binding to the midgut epithelia, which is mediated by per os infectivity factors (PIFs). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes seven PIF proteins, of which PIF1 to PIF4 form a core complex in ODV envelopes to which PIF0 and PIF6 loosely associate. Deletion of any pif gene results in ODV being unable to bind or enter midgut cells. AC83 also associates with the PIF complex, and this study further analyzed its role in oral infectivity to determine if it is a PIF protein. It had been proposed that AC83 possesses a chitin binding domain that enables transit through the peritrophic matrix; however, no chitin binding activity has ever been demonstrated. AC83 has been reported to be found only in the ODV envelopes, but in contrast, the Orgyia pseudotsugata MNPV AC83 homolog is associated with both ODV nucleocapsids and envelopes. In addition, unlike known pif genes, deletion of ac83 eliminates nucleocapsid formation. We propose a new model for AC83 function and show AC83 is associated with both ODV nucleocapsids and envelopes. We also further define the domain required for nucleocapsid assembly. The cysteine-rich region of AC83 is also shown not to be a chitin binding domain but a zinc finger domain required for the recruitment or assembly of the PIF complex to ODV envelopes. As such, AC83 has all the properties of a PIF protein and should be considered PIF8. In addition, pif7 (ac110) is reported as the 38th baculovirus core gene.IMPORTANCE ODV is essential for the per os infectivity of the baculovirus AcMNPV. To initiate infection, ODV binds to microvilli of lepidopteran midgut cells, a process which requires a group of seven virion envelope proteins called PIFs. In this study, we reexamined the function of AC83, a protein that copurifies with the ODV PIFs, to determine its role in the oral infection process. A zinc finger domain was identified and a new model for AC83 function was proposed. In contrast to previous studies, AC83 was found to be physically located in both the envelope and nucleocapsid of ODV. By deletion analysis, the AC83 domain required for nucleocapsid assembly was more finely delineated. We show that AC83 is required for PIF complex formation and conclude that it is a true per os infectivity factor and should be called PIF8.

Transcriptome analysis of interactions between silkworm and cytoplasmic polyhedrosis virus

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) specifically infects silkworm midgut (MG) and multiplication occurs mainly in posterior midgut (PM). In this study, MG and fat body (FB) were extracted at 0, 3, 24, and 72 h after BmCPV infection. The total sequence reads of each sample were more than 1510000, and the mapping ratio exceeded 95.3%. Upregulated transcripts increased in MG during the infection process. Gene ontology (GO) categories showed that antioxidants were all upregulated in FB but not in MG. BGI001299, BGI014434, BGI012068, and BGI009201 were MG-specific genes with transmembrane transport function, the expression of which were induced by BmCPV. BGI001299, BGI014434, and BGI012068 expressed in entire MG and may be involved in BmCPV invasion. BGI009201 expressed only in PM and may be necessary for BmCPV proliferation. BmPGRP-S2 and BGI012452 (a putative serine protease) were induced by BmCPV and may be involved in immune defense against BmCPV. The expression level of BmCPV S1, S2, S3, S6, and S7 was high and there was no expression of S9 in MG 72 h, implying that the expression time of structural protein coding genes is earlier. These results provide insights into the mechanism of BmCPV infection and host defense.

Bombyx mori nucleopolyhedrovirus lef8 gene: effects of deletion and implications for gene transduction applications

In this study, we have deleted the lef8 gene of the baculovirus BmNPV, which encodes one of the viral RNA polymerase subunits, in order to create a knockout bacmid, Δlef8, directing cytopathology-free single-cell infections for gene transduction and recombinant protein production. However, while removal of the complete lef8 ORF produced the expected phenotype, it also affected the function of the closely linked essential gene orf40, thus hampering the mutant bacmid rescue in cultured Bombyx cells expressing recombinant LEF8. Subsequently, we determined that several diverse sequences can substitute for the orf40 5'-upstream sequences that were removed by the deletion of the lef8 gene and also showed that neither a physical linkage nor expression of the two relevant genes under native promoter control is a prerequisite for a fully functional virus. Based on these findings, we generated a rescue-competent lef8-null vector, which contained a heterologous promoter-driven orf40. This lef8-deficient vector, which produces productive infections and progeny virus lacking lef8 in deficiency-complementing cells expressing LEF8, could be used as the basis for an alternative to current silkmoth transduction systems.

The baculovirus core gene ac83 is required for nucleocapsid assembly and per os infectivity of Autographa californica nucleopolyhedrovirus

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac83 is a baculovirus core gene whose function in the AcMNPV life cycle is unknown. In the present study, an ac83-knockout AcMNPV (vAc83KO) was constructed to investigate the function of ac83 through homologous recombination in Escherichia coli. No budded virions were produced in vAc83KO-transfected Sf9 cells, although viral DNA replication was unaffected. Electron microscopy revealed that nucleocapsid assembly was aborted due to the ac83 deletion. Domain-mapping studies revealed that the expression of Ac83 amino acid residues 451 to 600 partially rescued the ability of AcMNPV to produce infectious budded virions. Bioassays indicated that deletion of the chitin-binding domain of Ac83 resulted in the failure of oral infection of Trichoplusia ni larvae by AcMNPV, but AcMNPV remained infectious following intrahemocoelic injection, suggesting that the domain is involved in the binding of occlusion-derived virions to the peritrophic membrane and/or to other chitin-containing insect tissues. It has been demonstrated that Ac83 is the only component with a chitin-binding domain in the per os infectivity factor complex on the occlusion-derived virion envelope. Interestingly, a functional inner nuclear membrane sorting motif, which may facilitate the localization of Ac83 to the envelopes of occlusion-derived virions, was identified by immunofluorescence analysis. Taken together, these results demonstrate that Ac83 plays an important role in nucleocapsid assembly and the establishment of oral infection.

Bombyx mori nucleopolyhedrovirus BmP95 plays an essential role in budded virus production and nucleocapsid assembly

Bombyx mori nucleopolyhedrovirus (BmNPV) BmP95 is a highly conserved gene that is found in all of the baculovirus genomes sequenced to date and is also found in nudiviruses. To investigate the role of BmP95 in virus infection in vitro, a BmP95 deletion virus (vBmP95-De) was generated by homologous recombination in Escherichia coli. Fluorescence and light microscopy and titration analysis indicated that the BmP95 deletion bacmid led to a defect in production of infectious budded virus (BV). However, deletion of BmP95 did not affect viral DNA replication. Electron microscopy showed that masses of aberrant tubular structures were present in cells transfected with the BmP95 deletion bacmid, indicating that deletion of BmP95 affected assembly of the nucleocapsid. This defect could be rescued by insertion of full-length BmP95 into the polyhedrin locus of the BmP95-knockout bacmid but not the N-terminal domain of BmP95. Together, these results showed that full-length BmP95 is essential for BV production and is required for nucleocapsid assembly.

Characterization of novel components of the baculovirus per os infectivity factor complex

Baculovirus occlusion-derived virus (ODV) infects insect midgut cells under alkaline conditions, a process mediated by highly conserved per os infectivity factors (PIFs), P74 (PIF0), PIF1, PIF2, PIF3, PIF4, and PIF5 (ODV-E56). Previously, a multimolecular complex composed of PIF1, PIF2, PIF3, and P74 was identified which was proposed to play an essential role during ODV entry. Recently, more proteins have been identified that play important roles in ODV oral infectivity, including PIF4, PIF5, and SF58, which might work in concert with previously known PIFs to facilitate ODV infection. In order to understand the ODV entry mechanism, the identification of all components of the PIF complex is crucial. Hence, the aim of this study was to identify additional components of the PIF complex. Coimmunoprecipitation (CoIP) combined with proteomic analysis was used to identify the components of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) PIF complex. PIF4 and P95 (AC83) were identified as components of the PIF complex while PIF5 was not, and this was confirmed with blue native PAGE and a second CoIP. Deletion of the pif4 gene impaired complex formation, but deletion of pif5 did not. Differentially denaturing SDS-PAGE further revealed that PIF4 forms a stable complex with PIF1, PIF2, and PIF3. P95 and P74 are more loosely associated with this complex. Three other proteins, AC5, AC68, and AC108 (homologue of SF58), were also found by the proteomic analysis to be associated with the PIF complex. Finally the functional significance of the PIF protein interactions is discussed.

Differential structure of the intronic promoter of the Bombyx mori A3 actin gene correlated with silkworm sensitivity/resistance to nucleopolyhedrovirus

Previous reports demonstrated that actin is necessary for nucleocapsid transport and viral gene expression during nucleopolyhedrovirus infection of Bombyx mori. The first intron of B. mori A3 actin contains a cryptic promoter that drives expression of a rare isoform. We detected differences in the size and nucleotide composition of the first intron of the A3 actin gene from B. mori strain C24A, which is more resistant to nucleopolyhedrovirus than the M11A strain (22 and 95% lethality, respectively). We sought to determine if resistance to BmMNPV infection and the A3 actin promoter structure are correlated. Intrinsically bent DNA sites in these sequences, which determine curved structures, were analyzed by electrophoretic mobility assays and the helical parameters ENDS ratio, roll and twist. We found both fragments to have non-centralized bent DNA sites with distinct ENDS ratio values, nucleotide positions and two-dimensional structures. Additionally, a conformational-sensitive gel electrophoresis assay identified an allelic variation found in strain M11A that is absent in strain C24A. These data suggest that A3 actin intronic sequence variations impair virus propagation and are markers of BmMNPV-resistant populations.

Soluble forms of the cell adhesion molecule L1 produced by insect and baculovirus-transduced mammalian cells enhance Schwann cell motility

For biotechnological applications, insect cell lines are primarily known as hosts for the baculovirus expression system that is capable to direct synthesis of high levels of recombinant proteins through use of powerful viral promoters. Here, we demonstrate the implementation of two alternative approaches based on the baculovirus system for production of a mammalian recombinant glycoprotein, comprising the extracellular part of the cell adhesion molecule L1, with potential important therapeutic applications in nervous system repair. In the first approach, the extracellular part of L1 bearing a myc tag is produced in permanently transformed insect cell lines and purified by affinity chromatography. In the second approach, recombinant baculoviruses that express L1-Fc chimeric protein, derived from fusion of the extracellular part of L1 with the Fc part of human IgG1, under the control of a mammalian promoter are used to infect mammalian HEK293 and primary Schwann cells. Both the extracellular part of L1 bearing a myc tag accumulating in the supernatants of insect cultures as well as L1-Fc secreted by transduced HEK293 or Schwann cells are capable of increasing the motility of Schwann cells with similar efficiency in a gap bridging bioassay. In addition, baculovirus-transduced Schwann cells show enhanced motility when grafted on organotypic cultures of neonatal brain slices while they retain their ability to myelinate CNS axons. This proof-of-concept that the migratory properties of myelin-forming cells can be modulated by recombinant protein produced in insect culture as well as by means of baculovirus-mediated adhesion molecule expression in mammalian cells may have beneficial applications in the field of CNS therapies.

Characterization of an early gene orf122 from Bombyx mori nucleopolyhedrovirus

The open reading frame 122 of Bombyx mori nucleopolyhedrovirus (BmNPV) (Bm122) has been observed to be a conserved gene in the lepidopteran baculoviruses that have been completely sequenced so far. Its transcript was detected at 3 h post infection (h p.i.) and remained detectable at up to 96 h p.i. Temporal transcription analysis indicated that Bm122 is transcribed by host RNA polymerase. The size of the translational product of the Bm122 gene in Tn5B-1-4 cells was approximately 23 kDa, which is in agreement with the predicted value of 22.9 kDa, suggesting that no major posttranslational modification occurred in the primary protein product. The subcellular localization of Bm122 was studied using EGFP-Bm122, which revealed that Bm122 protein was accumulated within the nuclear region of virus-infected BmN cells. All these results suggest that Bm122 is an early gene encoding a protein that functions in the nucleus.

Baculovirus-mediated gene delivery into Mammalian cells does not alter their transcriptional and differentiating potential but is accompanied by early viral gene expression

Gene delivery to neural cells is central to the development of transplantation therapies for neurological diseases. In this study, we used a baculovirus derived from the domesticated silk moth, Bombyx mori, as vector for transducing a human cell line (HEK293) and primary cultures of rat Schwann cells. Under optimal conditions of infection with a recombinant baculovirus containing the reporter green fluorescent protein gene under mammalian promoter control, the infected cells express the transgene with high efficiency. Toxicity assays and transcriptome analyses suggest that baculovirus infection is not cytotoxic and does not induce differential transcriptional responses in HEK293 cells. Infected Schwann cells retain their characteristic morphological and molecular phenotype as determined by immunocytochemistry for the marker proteins S-100, glial fibrillary acidic protein, and p75 nerve growth factor receptor. Moreover, baculovirus-infected Schwann cells are capable of differentiating in vitro and express the P0 myelination marker. However, transcripts for several immediate-early viral genes also accumulate in readily detectable levels in the transduced cells. This transcriptional activity raises concerns regarding the long-term safety of baculovirus vectors for gene therapy applications. Potential approaches for overcoming the identified problem are discussed.

Transformed lepidopteran cells expressing a protein of the silkmoth fat body display enhanced susceptibility to baculovirus infection and produce high titers of budded virus in serum-free media

Baculovirus vectors constitute important tools for therapeutic protein production and mammalian cell transduction for gene therapy applications. A prerequisite for such applications is that the cell lines in which baculoviruses are propagated be maintained in serum-free media that are devoid of potential human pathogens. However, in serum-free media, the performance of baculovirus-based systems can be significantly reduced. In this report, we show that silkmoth-derived host cell lines for the Bombyx mori-nuclear polyhedrosis virus (BmNPV) that are transformed with the gene for the promoting protein (PP), a silkmoth-derived secreted factor containing a lipid-binding domain, display enhanced susceptibility to BmNPV infection and enhanced budded virus productivity in serum-free media. For transformed silkmoth cells maintained in serum-free media, the rate of BmNPV entry is enhanced by two orders of magnitude relative to the untransformed cells, while the rate of budded virus production is increased five-fold. The infectivity-enhancing effect can be also conferred to normal cells grown in serum-free media by addition of conditioned media from the transformed cells, which contain the secreted recombinant PP. Thus, PP substitutes for serum factors whose presence facilitates baculovirus entry into the cells. However, the effects of silkmoth-derived PP may be specific to the BmNPV-silkmoth system since little or no changes in viral infectivity are obtained by PP expression in Trichoplusia ni-derived High-Fivetrade mark cells grown in serum-free media and infected with a different baculovirus (AcNPV).

Sequence analysis of the Xestia c-nigrum granulovirus genome

The nucleotide sequence of the Xestia c-nigrum granulovirus (XcGV) genome was determined and found to comprise 178,733 bases with a G+C content of 40.7%. It contained 181 putative genes of 150 nucleotides or greater that showed minimal overlap. Eighty-four of these putative genes, which collectively accounted for 43% of the genome, are homologs of genes previously identified in the Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) genome. These homologs showed on average 33% amino acid sequence identity to those from AcMNPV. Several genes reported to have major roles in AcMNPV biology including ie-2, gp64, and egt were not found in the XcGV genome. However, open reading frames with homology to DNA ligase, two DNA helicases (one similar to a yeast mitochondrial helicase and the other to a putative AcMNPV helicase), and four enhancins (virus enhancing factors) were found. In addition, several ORFs are repeated; there are 7 genes related to AcMNPV orf2, 4 genes related to AcMNPV orf145/150, and a number of repeated genes unique to XcGV. Eight major repeated sequences (XcGV hrs) that are similar to sequences found in the Trichoplusia ni GV genome (TnGV) were found.