Cell entry of BmCPV can be promoted by tyrosine-protein kinase Src64B-like protein

Multiple conformations of trimeric spikes visualized on a non-enveloped virus

Many viruses utilize trimeric spikes to gain entry into host cells. However, without in situ structures of these trimeric spikes, a full understanding of this dynamic and essential process of viral infections is not possible. Here we present four in situ and one isolated cryoEM structures of the trimeric spike of the cytoplasmic polyhedrosis virus, a member of the non-enveloped Reoviridae family and a virus historically used as a model in the discoveries of RNA transcription and capping. These structures adopt two drastically different conformations, closed spike and opened spike, which respectively represent the penetration-inactive and penetration-active states. Each spike monomer has four domains: N-terminal, body, claw, and C-terminal. From closed to opened state, the RGD motif-containing C-terminal domain is freed to bind integrins, and the claw domain rotates to expose and project its membrane insertion loops into the cellular membrane. Comparison between turret vertices before and after detachment of the trimeric spike shows that the trimeric spike anchors its N-terminal domain in the iris of the pentameric RNA-capping turret. Sensing of cytosolic S-adenosylmethionine (SAM) and adenosine triphosphate (ATP) by the turret triggers a cascade of events: opening of the iris, detachment of the spike, and initiation of endogenous transcription.

Zhang and Cui et al. present in situ cryoEM structures of the trimeric spike of cytoplasmic polyhedrosis virus in both open and close conformations, and demonstrate that spike detachment from the capsid is triggered by the presence of SAM and ATP.

Bombyx mori cypovirus (BmCPV) induces PINK1-Parkin mediated mitophagy via interaction of VP4 with host Tom40

The mechanism by which infection by Bombyx mori cytoplasmic nucleopolyhedrosis virus (BmCPV) causes autophagy has not been studied in detail. Herein we have observed by electron microscopy that infection with BmCPV causes autophagosome and mitochondrial structure damage in Bombyx mori midgut. In BmN cells infected with BmCPV and expressing eGFP-LC3, fluorescence spots and LC3-II levels increased, suggesting that BmCPV infection causes autophagy. Autophagy inducer rapamycin (Rap) and autophagy inhibitor 3-methyladenine (3-MA) were used to monitor the effects of mitophagy on BmCPV proliferation. It was found BmCPV proliferation to be promoted by mitophagy. Transient transfection experiments in cultured BmN cells showed that mitophagy can be triggered by expression of BmCPV structural protein VP4. Moreover, VP4 caused upregulation of p-Drp1, PINK1 and Parkin proteins in the mitophagy pathway and downregulation of mitochondrial membrane protein Tom20. Furthermore, interaction between VP4 with Tom40 was confirmed by Co-IP, western blot and colocalization experiment, and overexpression of Tom40 reduce the level of mitochondrial autophagy induced by VP4. These results suggested that VP4 induced PINK1-Parkin-mediated mitophagy interacting with Tom40. These findings deepen our understanding of the interaction between BmCPV and silkworm and also provide a molecular target for screening anti-BmCPV drugs.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Characterization of the lipidomic profile of BmN cells in response to Bombyx mori cytoplasmic polyhedrosis virus infection

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV)that belongs to the genus Cypovirus in the family of Reoviridae is one of the problematic pathogens in sericulture. In our previous study, we have found that lipid-related constituents in the host cellular membrane are associated with the BmCPV life cycle. It is important to note that the lipids not only affect the cellular biological processes, they also impact the virus life cycle. However, the intracellular lipid homeostasis in BmN cells after BmCPV infection remains unclear. Here, the lipid metabolism in BmCPV-infected BmN cells was studied by lipidomics analysis. Our results revealed that the intracellular lipid homeostasis was disturbed in BmN cells upon BmCPV infection. Major lipids constituents in cellular membrane were found to be significantly induced upon BmCPV infection, which included triglycerides, phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, phospholipids, glucoside ceramide, monoetherphosphatidylcholin, ceramide, ceramide phosphoethanolamine and cardiolipin. Further analysis of the pathways related to these altered lipids (such as PE and PC) showed that glycerophospholipid metabolism was one of the most enriched pathways. These results suggested that BmCPV may manipulate the lipid metabolism of cells for their own interest. The findings may facilitate a better understanding of the roles of lipid metabolic changes during virus infection in future studies.

Construction and Application of PLP Self-sufficient Biocatalysis System for Threonine Aldolase

A number of organic synthesis involve threonine aldolase (TA), a pyridoxal phosphate (PLP)-dependent enzyme. Although the addition of exogenous PLP is necessary for the reactions, it increases the cost and complicates the purification of the product. This work constructed a PLP self-sufficient biocatalysis system for TA, which included an improvement of the intracellular PLP level and co-immobilization of TA with PLP. Engineered strain BL-ST was constructed by introducing PLP synthase PdxS/T to Escherichia coli BL21(ED3). The intracellular PLP concentration of the strain increased approximately fivefold to 48.5 μmol/g_DCW. l-TA, from Bacillus nealsonii (BnLTA), was co-expressed in the strain BL-ST with PdxS/T, resulting in the engineered strain BL-BnLTA-ST. Compared with the control strain BL-BnLTA (254.1 U/L), the enzyme activity of the strain BL-BnLTA-ST reached 1518.4 U/L without the addition of exogenous PLP. An efficient co-immobilization system was then designed. The epoxy resin LX-1000HFA wrapped by polyethyleneimine (PEI) acted as a carrier to immobilize the crude enzyme solution of the strain BL-BnLTA-ST mixed with an extra 100 μM of exogenous PLP, resulting in the catalyst HFA^PEI-BnLTA-STPLP 100. HFA^PEI-BnLTA-STPLP 100 exhibited a half-life of approximately 450 h, and the application of the catalyst in the continuous biosynthesis of 3-[4-(methylsulfonyl) phenyl] serine had more than 180 batch reactions (>60%_conv) without the extra addition of exogenous PLP. The excellent compatibility and stability of the system were further confirmed by other TAs. This work introduced a PLP self-sufficient biocatalysis system that can reduce the cost of PLP and contribute to the industrial application of TA. In addition, the system may also be applied in other PLP-dependent enzymes.

Effect of Titanium Dioxide Nanoparticles on the Resistance of Silkworm to Cytoplasmic Polyhedrosis Virus in Bombyx mori

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a serious disease harmful to silk industry, which is one of the major sources of financial support for farmers in many developing countries. So far, there is still no good way to prevent or treat this disease. In this study, titanium dioxide nanoparticles (TiO₂ NPs) were used to pretreat silkworm larvae, and good results were achieved in improving silkworm immunity and alleviating the damage of cytoplasmic polyhedrosis virus. The results showed that nano-titanium dioxide pretreatment could inhibit the proliferation of BmCPV in the midgut of silkworm, activate JAK/STAT and PI3K-AKT immune signaling pathways, and upregulate the expression of key immune genes, so as to improve the immunity of silkworm and enhance the resistance of silkworm to BmCPV.

Antiviral defense against Cypovirus 1 (Reoviridae) infection in the silkworm, Bombyx mori

Recent years have shown a large increase in studies of infection of the silkworm (Bombyx mori) with Cypovirus 1 (previously designated as B. mori cytoplasmic polyhedrosis virus), that causes serious damage in sericulture. Cypovirus 1 has a single-layered capsid that encapsulates a segmented double-strand RNA (dsRNA) genome which are attractive features for the establishment of a biotechnological platform for the production of specialized gene silencing agents, either as recombinant viruses or as viral-like particles with nonreplicative dsRNA cargo. For both combatting viral disease and application of Cypovirus-based pest control, however, a better understanding is needed of the innate immune response caused by Cypovirus infection of the midgut of lepidopteran larvae. Studies of deep sequencing of viral small RNAs have indicated the importance of the RNA interference pathway in the control of Cypovirus infection although many functional aspects still need to be elucidated and conclusive evidence is lacking. A considerable number of transcriptome studies were carried out that revealed a complex response that hitherto remains uncharacterized because of a dearth in functional studies. Also, the uptake mechanism of Cypovirus by the midgut cells remains unclarified because of contrasting mechanisms revealed by electron microscopy and functional studies. The field will benefit from an increase in functional studies that will depend on transgenic silkworm technology and reverse genetics systems for Cypovirus 1.

Litopenaeus vannamei Src64B restricts white spot syndrome virus replication by modulating apoptosis

The Src family kinases (SFK) are involved in signaling transductions that regulate numerous biological activities including host-virus interaction. These features of SFK have been well explored in vertebrates, however, in shrimp, the invertebrate SFK family member Src64B, has not been characterized and therefore its role in shrimp-virus interaction remains unknown. In this study, two Litopenaeus vannamei Src64B isoforms (designated LvSrc64B1 and LvSrc64B2) were first cloned and their role in white spot syndrome virus (WSSV) infection was explored. Bioinformatics analysis revealed that LvSrc64B1 and LvSrc64B2 were similar to other Src64B family members, with high homology in primary and tertiary structures, and contained the conserved SFK functional domains, as well as the putative myristylation and phosphorylation sites. Tissue distribution analysis showed that both LvSrc64B isoforms were ubiquitously expressed, albeit distinctively in the tested tissues. In addition, transcript levels of LvSrc64B1 and LvSrc64B2 were significantly induced following WSSV challenge and had similar expression patterns. Furthermore, siRNA-mediated knockdown of LvSrc64B1 and LvSrc64B2 followed by WSSV infection resulted in increased expression of viral genes, enhanced viral DNA replication, and elevation of hemocytes apoptosis. Depletion of LvSrc64B1 and LvSrc64B2 also reduced shrimp survival upon WSSV infection. In conclusion, the current data strongly suggest that Src64B is a host factor that inhibits WSSV replication by modulating apoptosis in shrimp.

Bombyx mori cypovirus encoded small peptide inhibits viral multiplication

Bombyx mori cypovirus (BmCPV) is one of the most infectious pathogen in sericulture and a member of the family Reoviridae. It specifically infects the midgut of silkworm. The BmCPV genome consists of 10 dsRNAs segments (S1-S10), which have generally been assumed to be monocistronic. In this study, a small open reading frame encoding the peptide S5-sORF, containing 27 amino acid residues, was predicted in a region of the negative (-) strand of BmCPV segment S5. An immunofluorescence assay detected S5-sORF in the cytoplasm and nuclei of BmCPV-infected cells, and it was also detected in the virion with western blotting, suggesting that S5-sORF may be assembled into the BmCPV virion. Viral gene expression was inhibited by overexpressed S5-sORF, and viral multiplication was dose-dependently suppressed by the S5-sORF peptide. A viable recombinant virus, BmCPV-S5-sORF^mut, in which the start codon (ATG) of S5-sORF was mutated to a stop codon (TGA), was generated with reverse genetics. The proliferation of BmCPV was increased by the abolition of S5-sORF expression. Furthermore, the RNA transcript of S5-sORF and small peptide of S5-sORF were involved in BmCPV replication. The expression of genes related to the innate immune pathways and apoptosis in the silkworm were not significantly affected by S5-sORF overexpression. Our results suggest that a viral nucleotide sequence is utilized by the host to generate an antiviral peptide, which may be a novel strategy protecting the host from viral infection.