Introgression of OsAP47 by marker-assisted selection enhanced resistance against southern rice black-streaked dwarf virus disease

Southern rice black-streaked dwarf virus disease (SRBSDVD) is the most destructive viral disease in rice. In order to breeding resistant cultivars, Insertion-Deletion (InDel) markers were developed linked to OsAP47, the first isolated major resistance gene against SRBSDVD. Marker-assisted selection (MAS) was conducted to introduce this gene into the commercial variety. A rice line carrying homozygous resistance allele of OsAP47 was selected and named Kanghei No. 201 (KH201). Evaluated by artificial inoculation, KH201 showed significantly higher resistance than the recurrent parent Suxiu No.867 (SX867). And no significant differences were detected for KH201 in the yield-related components, including spikelets per panicle (SPP), ripened grains per panicle (RGPP), 1000-grain weight (TGW) and panicles per square meter (PPSM), leading to stable theoretical yield. The results indicated that introgression of OsAP47 improved rice resistance and can avoid yield losses produced by SRBSDVD. KH201 was demonstrated as a resistance material that could be used in rice breeding.

Investigation and characterization of rice dwarfing epidemic caused by southern rice black-streaked dwarf virus in Jiangsu in 2023

During the field surveys in Jiangsu Province, China, contiguous patches of rice plants with varying degrees of dwarfing, wax-white or dark brown enations at the base of stems, and abnormal heading symptoms were observed in the fields located in Jiangning District in Nanjing City, Jurong County in Zhenjiang City, and Zhangjiagang County in Suzhou City. Through molecular analyses, the presence of southern rice black-streaked dwarf virus was confirmed in symptomatic rice plants. The infections of other rice viruses that cause dwarfing were also ruled out. Additionally, Koch's postulates were fulfilled, further validating SRBSDV as the causal agent for the observed dwarfing disease epidemic. Furthermore, the phylogenetic analyses revealed that the SRBSDV prevalent in Jiangsu in 2023 may originate from multiple regions in Vietnam. Our study has documented the emergence of an SRBSDV epidemic in Jiangsu in 2023, marking the first incidence of southern rice black-streaked dwarf disease in this region.

The miR-184-3p promotes rice black-streaked dwarf virus infection by suppressing Ken in Laodelphax striatellus (Fallén)

BACKGROUND

MicroRNAs (miRNAs) play a key role in various biological processes by influencing the translation of target messenger RNAs (mRNAs) through post-transcriptional regulation. The miR-184-3p has been identified as an abundant conserved miRNA in insects. However, less is known about its functions in insect-plant virus interactions.

RESULTS

The function of miR-184-3p in regulation of plant viral infection in insects was investigated using a rice black-streaked dwarf virus (RBSDV) and Laodelphax striatellus (Fallén) interaction system. We found that the expression of miR-184-3p increased in L. striatellus after RBSDV infection. Injection of miR-184-3p mimics increased RBSDV accumulation, while treatment with miR-184-3p antagomirs inhibits the viral accumulation in L. striatellus. Ken, a zinc finger protein, was identified as a target of miR-184-3p. Knockdown of Ken increased the virus accumulation and promoted RBSDV transmission by L. striatellus.

CONCLUSION

This study demonstrates that RBSDV infection induces the expression of miR-184-3p in its insect vector L. striatellus. The miR-184-3p targets Ken to promote RBSDV accumulation and transmission. These findings provide a new insight into the function of the miRNAs in regulating plant viral infection in its insect vector. © 2023 Society of Chemical Industry.

Structure of orthoreovirus RNA chaperone σNS, a component of viral replication factories

The mammalian orthoreovirus (reovirus) σNS protein is required for formation of replication compartments that support viral genome replication and capsid assembly. Despite its functional importance, a mechanistic understanding of σNS is lacking. We conducted structural and biochemical analyses of a σNS mutant that forms dimers instead of the higher-order oligomers formed by wildtype (WT) σNS. The crystal structure shows that dimers interact with each other using N-terminal arms to form a helical assembly resembling WT σNS filaments in complex with RNA observed using cryo-EM. The interior of the helical assembly is of appropriate diameter to bind RNA. The helical assembly is disrupted by bile acids, which bind to the same site as the N-terminal arm. This finding suggests that the N-terminal arm functions in conferring context-dependent oligomeric states of σNS, which is supported by the structure of σNS lacking an N-terminal arm. We further observed that σNS has RNA chaperone activity likely essential for presenting mRNA to the viral polymerase for genome replication. This activity is reduced by bile acids and abolished by N-terminal arm deletion, suggesting that the activity requires formation of σNS oligomers. Our studies provide structural and mechanistic insights into the function of σNS in reovirus replication.

Development and application of a multiplex PCR method for the simultaneous detection of goose parvovirus, waterfowl reovirus, and goose astrovirus in Muscovy ducks

A multiplex polymerase chain reaction (PCR) method was developed to detect and distinguish goose parvovirus (GPV), waterfowl reovirus (WRV), and goose astrovirus (GAstV). Three pairs of primers were designed based on conserved regions in the genomic sequences of these enteric viruses and were used to specifically amplify targeted fragments of 493 bp from the viral protein 3 (VP3) gene of GPV, 300 bp from the sigma A-encoding gene of WRV, and 156 bp from the capsid protein-encoding gene of GAstV. The results showed that the primers can specifically amplify target fragments, without any cross-amplification with other viruses, indicating that the method had good specificity. A sensitivity test showed that the detection limit of the multiplex PCR method was 1 × 103 viral copies. A total of 102 field samples from Muscovy ducks with clinically suspected diseases were evaluated using the newly developed multiplex PCR method. The ratio of positive samples to total samples for GPV, WRV, and GAstV was 73.53% (75/102) for multiplex PCR and was 73.53% (75/102) for routine PCR. Seventy-five positive samples were detected by both methods, for a coincidence ratio of 100%. This multiplex PCR method can simultaneously detect GPV, WRV, and GAstV, which are associated with viral enteritis, thereby providing a specific, sensitive, efficient, and accurate new tool for clinical diagnosis and laboratory epidemiological investigations.

Mammalian orthoreovirus can exit cells in extracellular vesicles

Several egress pathways have been defined for many viruses. Among these pathways, extracellular vesicles (EVs) have been shown to function as vehicles of non-lytic viral egress. EVs are heterogenous populations of membrane-bound structures released from cells as a form of intercellular communication. EV-mediated viral egress may enable immune evasion and collective viral transport. Strains of nonenveloped mammalian orthoreovirus (reovirus) differ in cell lysis phenotypes, with T3D disrupting cell membranes more efficiently than T1L. However, mechanisms of reovirus egress and the influence of transport strategy on infection are only partially understood. To elucidate reovirus egress mechanisms, we infected murine fibroblasts (L cells) and non-polarized human colon epithelial (Caco-2) cells with T1L or T3D reovirus and enriched cell culture supernatants for large EVs, medium EVs, small EVs, and free reovirus. We found that both reovirus strains exit cells in association with large and medium EVs and as free virus particles, and that EV-enriched fractions are infectious. While reovirus visually associates with large and medium EVs, only medium EVs offer protection from antibody-mediated neutralization. EV-mediated protection from neutralization is virus strain- and cell type-specific, as medium EVs enriched from L cell supernatants protect T1L and T3D, while medium EVs enriched from Caco-2 cell supernatants largely fail to protect T3D and only protect T1L efficiently. Using genetically barcoded reovirus, we provide evidence that large and medium EVs can convey multiple particles to recipient cells. Finally, T1L or T3D infection increases the release of all EV sizes from L cells. Together, these findings suggest that in addition to exiting cells as free particles, reovirus promotes egress from distinct cell types in association with large and medium EVs during lytic or non-lytic infection, a mode of exit that can mediate multiparticle infection and, in some cases, protection from antibody neutralization.

Dynamic gene expression of southern rice black-streaked dwarf virus during its infection in rice plants and vector insects

In this century, a disease caused by southern rice black-streaked dwarf virus (SRBSDV) has resulted in significant loss in rice production in Asia. Aside from infecting rice plants, SRBSDV is transmitted by white-backed planthopper (WBPH) in a persistent propagative manner. Recent studies showed that SRBSDV can dynamically modulate the host cells throughout the infection progress. However, the expression dynamics of the SRBSDV genes during infection remain unclear. Here we established an absolute real-time quantitative PCR method to assess the dynamic of the SRBSDV genes expression in rice plants and planthoppers. Apart from displaying the expression levels of viral genes, we discovered that the expression level of viral genes in insects significantly surpasses that in plant cells. In addition, we identified two nonstructural proteins with unknown functions that exhibit the highest expression levels in plant and insect cells, respectively, which provide possible targets for restraining the disease outbreaks.

A novel chimeric RNA originating from BmCPV S4 and Bombyx mori HDAC11 transcripts regulates virus proliferation

Polymerases encoded by segmented negative-strand RNA viruses cleave 5'-m7G-capped host transcripts to prime viral mRNA synthesis ("cap-snatching") to generate chimeric RNA, and trans-splicing occurs between viral and cellular transcripts. Bombyx mori cytoplasmic polyhedrosis virus (BmCPV), an RNA virus belonging to Reoviridae, is a major pathogen of silkworm (B. mori). The genome of BmCPV consists of 10 segmented double-stranded RNAs (S1-S10) from which viral RNAs encoding a protein are transcribed. In this study, chimeric silkworm-BmCPV RNAs, in which the sequence derived from the silkworm transcript could fuse with both the 5' end and the 3' end of viral RNA, were identified in the midgut of BmCPV-infected silkworms by RNA_seq and further confirmed by RT-PCR and Sanger sequencing. A novel chimeric RNA, HDAC11-S4 RNA 4, derived from silkworm histone deacetylase 11 (HDAC11) and the BmCPV S4 transcript encoding viral structural protein 4 (VP4), was selected for validation by in situ hybridization and Northern blotting. Interestingly, our results indicated that HDAC11-S4 RNA 4 was generated in a BmCPV RNA-dependent RNA polymerase (RdRp)-independent manner and could be translated into a truncated BmCPV VP4 with a silkworm HDAC11-derived N-terminal extension. Moreover, it was confirmed that HDAC11-S4 RNA 4 inhibited BmCPV proliferation, decreased the level of H3K9me3 and increased the level of H3K9ac. These results indicated that during infection with BmCPV, a novel mechanism, different from that described in previous reports, allows the genesis of chimeric silkworm-BmCPV RNAs with biological functions.

Isolation and genomic characterization of a cypovirus from Clanis bilineata

Clanis bilineata Walker, soybean hawkmoth, belongs to the subfamily Ambulicinae (Sphingidae, Lepidoptera) and is an edible insect that usually grows on soybean leaves. In this study, we isolated a new cypovirus from naturally diseased Clanis bilineata larvae (named CbCPV), scanned its structure, sequenced its genome, and studied its phylogenetic relationship to other cypoviruses. Microscopy showed that CbCPV polyhedral occlusion bodies were about 1.878 μm on average and contained many virions in the ultrathin sections. The complete genome sequence of CbCPV is 22,812 bp comprising 10 segmented double-stranded RNAs. Apart from segment 1 containing one open reading frame (ORF) and one sub-ORF, the other nine segments all contain one open reading frame and encoded one putative protein. The non-coding regions contained conserved sequences at 5' termini (AGUCAAA) and 3' termini (AGC), except segment 4 containing a different 5' termini (AUGUUUA). The whole sequence of the polyhedrin gene in CbCPV contained 892 nucleotides, encoding a protein of 246 amino acids. Based on amino acid sequences of polyhedrin or RNA dependent RNA polymerase (RdRp), the phylogenetic analysis indicated that CbCPV was closely related to DnCPV-23. The putative function of all segments differed from each other, but the most closely related species of segments were DnCPV-23 with 98.2-99.8% nucleotide identity. Overall, the evidence of morphology, protein analysis and nucleic acids (genomic pattern) showed that CbCPV is a new isolate in the cypovirus-23 type and can be termed Clanis bilineata cypovirus type 23 (CbCPV-23).

Reovirus genomic diversity confers plasticity for protease utility during adaptation to intracellular uncoating

IMPORTANCE

Reoviruses infect many mammals and are widely studied as a model system for enteric viruses. However, most of our reovirus knowledge comes from laboratory strains maintained on immortalized L929 cells. Herein, we asked whether naturally circulating reoviruses possess the same genetic and phenotypic characteristics as laboratory strains. Naturally circulating reoviruses obtained from sewage were extremely diverse genetically. Moreover, sewage reoviruses exhibited poor fitness on L929 cells and relied heavily on gut proteases for viral uncoating and productive infection compared to laboratory strains. We then examined how naturally circulating reoviruses might adapt to cell culture conditions. Within three passages, virus isolates from the parental sewage population were selected, displaying improved fitness and intracellular uncoating in L929 cells. Remarkably, selected progeny clones were present at 0.01% of the parental population. Altogether, using reovirus as a model, our study demonstrates how the high genetic diversity of naturally circulating viruses results in rapid adaptation to new environments.

Comparative transcriptional analysis between virulent isolate HN1307 and avirulent isolate GD1108 of grass carp reovirus genotype II

As a widespread epidemic virus, genotype II of the grass carp reovirus poses a significant threat to the grass carp farming industry in China. Different genotype II isolates cause different degrees of virulence, although the underlying pathogenic mechanisms remain largely unknown. In this work, infections of grass carp with the virulent isolate grass carp reovirus (GCRV)-HN1307 and the avirulent isolate GCRV-GD1108 were performed to reveal a possible mutual transcriptional discrepancy. More differentially expressed genes (DEGs) were identified in the HN1307-infected group, which defined a grossly similar gene ontology (GO) pattern and different pathway landscape as the GD1108-infected group. Gene set enrichment analysis revealed that pathways related to innate immunity and metabolism were reciprocally activated and suppressed, respectively, following infection withHN1307, compared with GD1108. The trend analysis further indicated that immune-related pathways were involved in one of the four statistically significant profiles. Network analysis of transcription factor-gene interactions and protein-protein interactions on the immune-related profile suggested that among the core transcriptional factors (TFs) (UBTF, HCFC1, MAZ, MAX, and NRF1) and the hub proteins (Tlr3, Tlr7, Tlr9, Irf3, and Irf7), the latter were highly enriched in the toll-like receptor signaling pathway. Real-time quantitative PCR performed on the selected mRNAs validated the relative expression. This work will provide insights into the distinct transcriptional signatures from avirulent and virulent isolates of GCRV, which may contribute to the development of products for prevention.

A systematic review of southern rice black-streaked dwarf virus in the age of omics

Southern rice black-streaked dwarf virus (SRBSDV) is one of the most damaging rice viruses. The virus decreases rice quality and yield, and poses a serious threat to food security. From this perspective, this review performed a survey of published studies in recent years to understand the current status of SRBSDV and white-backed planthopper (WBPH, Sogatella furcifera) transmission processes in rice. Recent studies have shown that the interactions between viral virulence proteins and rice susceptibility factors shape the transmission of SRBSDV. Moreover, the transmission of SRBSDV is influenced by the interactions between viral virulence proteins and S. furcifera susceptibility factors. This review focused on the molecular mechanisms of key genes or proteins associated with SRBSDV infection in rice via the S. furcifera vector, and the host defense response mechanisms against viral infection. A sustainable control strategy using RNAi was summarized to address this pest. Finally, we also present a model for screening anti-SRBSDV inhibitors using viral proteins as targets. © 2023 Society of Chemical Industry.

Characterization of the complete genome of oat sterile dwarf virus: a fijivirus occurring in the temperate climate zone of Europe

Oat sterile dwarf virus (OSDV) is a fijivirus whose genome segments 7 to 10 were sequenced earlier. In the current study, the complete genome was sequenced. To confirm the genome ends, rapid amplification and sequencing of cDNA ends were performed. The complete OSDV genome consists of 10 double-stranded RNA (dsRNA) segments with a total size of 28,686 bp. The sense strand sequence of all segments has the terminal consensus sequence motif 5'-AACGA(5-7)… U(6-8)(A/U)GUC-3', in which the length of the stretches of A and U varies, being slightly shorter for segments 1-4 and longer for segments 5-10. The 3' end of segment 3 is …UGUC, not AGUC as in the other segments. Segments 5, 7, and 10 contain two small ORFs, while each of the other segments contains one long ORF. ORF7-2 and ORF9 are slightly longer than annotated before. Phylogenetic analysis based on amino acid sequences of the RNA-directed RNA polymerase (RdRP) placed OSDV between the plant fijiviruses and Nilaparvata lugens reovirus (NLRV), an insect fijivirus that does not replicate in plants. OSDV RdRP shares 48-49% sequence identity with other plant-infecting fijivirus RdRPs and 30% identity with that of NLRV. OSDV has earlier been reported in several Northern and Central European countries. The sequencing of the complete genome serves as a reference for identifying all segments in future high-throughput sequencing datasets, enabling the investigation of the molecular epidemiology and evolution of OSDV.

Nonstructural protein NS17 of grass carp reovirus Honghu strain promotes virus infection by mediating cell-cell fusion and apoptosis

Fusion-associated small transmembrane (FAST) proteins can promote cell fusion, alter membrane permeability and trigger apoptosis to promote virus proliferation in orthoreoviruses. However, it is unknown whether FAST proteins perform these functions in aquareoviruses (AqRVs). Non-structural protein 17 (NS17) carried by grass carp reovirus Honghu strain (GCRV-HH196) belongs to the FAST protein family, and we preliminarily explored its relevance to virus infection. NS17 has similar domains to FAST protein NS16 of GCRV-873, comprising a transmembrane domain, a polybasic cluster, a hydrophobic patch and a polyproline motif. It was observed in the cytoplasm and the cell membrane. Overexpression of NS17 enhanced the efficiency of cell-cell fusion induced by GCRV-HH196 and promoted virus replication. Overexpression of NS17 also led to DNA fragmentation and reactive oxygen species (ROS) accumulation, and it triggered apoptosis. The findings illuminate the functions of NS17 in GCRV infection, and provide a reference for the development of novel antiviral strategies.

Piscine Vitamin D Receptors Vdra/Vdrb in the Absence of Vitamin D Are Utilized by Grass Carp Reovirus for Promoting Viral Replication

The vitamin D receptor (VDR) plays a pivotal role in the biological actions of vitamin D (VitD). However, little is known about the functions of VDR in the production of viral inclusion bodies (VIBs). Using a representative strain of grass carp reovirus (GCRV) genotype I, GCRV-873, we show that GCRV-873 recruits grass carp Vdrs for promoting the production of VIBs in the absence of VitD. Inhibition of cholesterol synthesis by lovastatin impairs the production of VIBs and blocks the effects of grass carp Vdrs in promoting the production of VIBs in the absence of VitD. Furthermore, grass carp Vdrs are found to form the Vdra-Vdrb heterodimer, which is vital for 3-hydroxy-3-methylglutaryl-coenzyme A reductase (hmgcr)-dependent cholesterol synthesis and GCRV replication. Intriguingly in the presence of VitD, grass carp Vdra but not Vdrb forms the heterodimer with the retinoid X receptor beta b (Rxrbb), which induces the transcription of those genes involved in the RIG-I-like receptor (RLR) antiviral signaling pathway for inhibiting GCRV infection. Furthermore, the VitD-activated Vdra-Vdrb heterodimer attenuates the transcription of the RLR antiviral signaling pathway induced by VitD. In the presence of VitD, a balance between the Vdra-Rxrbb heterodimers as coactivators and Vdra-Vdrb heterodimers as corepressors in affecting the transcriptional regulation of the RLR antiviral signaling pathway may eventually determine the outcome of GCRV infection. Transfection with VitD can abolish the effect of grass carp Vdrs in promoting GCRV replication in a dose-dependent manner. Taken together, these findings demonstrate that GCRV utilizes host Vdrs to increase hmgcr-dependent cholesterol synthesis for promoting its replication, which can be prevented by VitD treatment. IMPORTANCE Grass carp reovirus (GCRV) is the causative agent of grass carp hemorrhagic disease, which seriously harms freshwater fish. Although many positive or negative regulators of GCRV infection have been identified in teleosts, little is known about the molecular mechanisms by which GCRV utilizes host factors to generate its infectious compartments beneficial for viral replication and infection. Here, we show that in the absence of VitD, the GCRV-873 strain utilizes host vitamin D receptors Vdra/Vdrb to increase hmgcr-dependent cholesterol synthesis for promoting the production of VIBs, which are important functional sites for aquareovirus replication and assembly. The negative regulation of Vdrs during viral infection can be prevented by VitD treatment. Thus, this present work broadens understanding of the pivotal roles of Vdrs in the interaction between the host and GCRV in the absence or presence of VitD, which might provide a rational basis for developing novel anti-GCRV strategies.

An Aquareovirus Exploits Membrane-Anchored HSP70 To Promote Viral Entry

Temperature dependency of viral diseases in ectotherms has been an important scientific issue for decades, while the molecular mechanism behind this phenomenon remains largely mysterious. In this study, deploying infection with grass carp reovirus (GCRV), a double-stranded RNA aquareovirus, as a model system, we demonstrated that the cross talk between HSP70 and outer capsid protein VP7 of GCRV determines temperature-dependent viral entry. Multitranscriptomic analysis identified HSP70 as a key player in the temperature-dependent pathogenesis of GCRV infection. Further biochemical, small interfering RNA (siRNA) knockdown, pharmacological inhibition, and microscopic approaches revealed that the primary plasma membrane-anchored HSP70 interacts with VP7 to facilitate viral entry during the early phase of GCRV infection. Moreover, VP7 functions as a key coordinator protein to interact with multiple housekeeping proteins and regulate receptor gene expression, concomitantly facilitating viral entry. This work illuminates a previously unidentified immune evasion mechanism by which an aquatic virus hijacks heat shock response-related proteins to enhance viral entry, pinpointing targeted preventives and therapeutics for aquatic viral diseases. IMPORTANCE The seasonality of viral diseases in ectotherms is a prevailing phenomenon in the aquatic environment, which causes huge economic losses every year worldwide and hinders sustainable development of the aquaculture industry. Nevertheless, our understanding of the molecular mechanism of how temperature determines the pathogenesis of aquatic viruses remains largely unexplored. In this study, by deploying grass carp reovirus (GCRV) infection as a model system, we demonstrated that temperature-dependent, primarily membrane-localized HSP70 interacts with major outer capsid protein VP7 of GCRV to bridge the virus-host interaction, reshape the host's behaviors, and concomitantly facilitate viral entry. Our work unveils a central role of HSP70 in the temperature-dependent pathogenesis of aquatic viruses and provides a theoretical basis for the formulation of prevention and control strategies for aquatic viral diseases.

Peptidoglycan Recognition Protein S5 of Bombyx mori Facilitates the Proliferation of Bombyx mori Cypovirus 1

Bombyx mori cypovirus 1 (BmCPV1), a primary pathogen of the silkworm, is a typical dsRNA virus belonging to the Reoviridae family. In this study, a total of 2520 differentially expressed genes (DEGs) were identified by RNA-seq analysis of the silkworm midgut after BmCPV1 infection and Gene Ontology (GO) functional annotation showed that the DEGs predominantly functioned in binding (molecular function), cell (cellular component), and cellular processes (biological process). Additionally, the Kyoto Encyclopedia of Genes and Genomes (KEGG) functional annotation revealed that the DEGs were mainly distributed in global and overview metabolism maps, translation, and signal transduction. Among the identified DEGs, BmPGRP-S5 belongs to the peptidoglycan recognition protein (PGRP) family. Previous studies have revealed that PGRPs were involved in the interactions between silkworm and BmCPV1. Here, we explored the effect of BmPGRP-S5 on BmCPV1 replication and demonstrated that BmPGRP-S5 promotes the proliferation of BmCPV1 in BmN cells through overexpression or knockdown experiments. Knocking down of BmPGRP-S5 in silkworm larvae similarly promoted the proliferation of BmCPV1. Through experimental validation, we therefore determined that BmPGRP-S5 acts as a proviral host factor for BmCPV1 infection. This study clarifies the proliferation mechanism of BmCPV1 and provides new insights into the functional role of BmPGRP-S5.

GAPDH mediates plant reovirus-induced incomplete autophagy for persistent viral infection in leafhopper vector

Macroautophagy/autophagy is a conserved mechanism launched by host organisms to fight against virus infection. Double-membraned autophagosomes in arthropod vectors can be remodeled by arboviruses to accommodate virions and facilitate persistent viral propagation, but the underlying mechanism is unknown. Rice gall dwarf virus (RGDV), a plant nonenveloped double-stranded RNA virus, induces the formation of virus-containing double-membraned autophagosomes to benefit persistent viral propagation in leafhopper vectors. In this study, it was found that the capsid protein P2 of RGDV alone induced autophagy. P2 specifically interacted with GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and ATG4B both in vitro and in vivo. Furthermore, the GAPDH-ATG4B complex could be recruited to virus-induced autophagosomes. Silencing of GAPDH or ATG4B expression suppressed ATG8 lipidation, autophagosome formation, and efficient viral propagation. Thus, P2 could directly recruit the GAPDH-ATG4B complex to induce the formation of initial autophagosomes. Furthermore, such autophagosomes were modified to evade fusion with lysosomes for degradation, and thus could be persistently exploited by viruses to facilitate efficient propagation. GAPDH bound to ATG14 and inhibited the interaction of ATG14 with SNAP29, thereby preventing ATG14-SNARE proteins from mediating autophagosome-lysosome fusion. Taken together, these results highlight how RGDV activates GAPDH to initiate autophagosome formation and block autophagosome degradation, finally facilitating persistent viral propagation in insect vectors. The findings reveal a positive regulation of immune response in insect vectors during viral infection.

Cell Killing by Reovirus: Mechanisms and Consequences

Infection of host cells by mammalian reovirus in culture or in tissues of infected animals results in cell death. Cell death of infected neurons and myocytes contributes to the pathogenesis of reovirus-induced encephalitis and myocarditis in a newborn mouse model. Thus, reovirus-induced cell death has been used to investigate the basis of viral disease. Depending on the cell type, infection of host cells by reovirus results in one of two forms of cell death-apoptosis and necroptosis. In addition to the obvious differences in how these two forms of cell death are executed, the mechanisms by which reovirus infection initiates and transduces signals that lead to each of these types of cell death are distinct. In this review, we discuss how apoptosis and necroptosis are triggered by events at different stages of infection. We also describe how innate immune recognition of reovirus genomic material and type I interferon signaling pathways connect with the core components of the apoptosis and necroptosis machinery. The impact of different cell death mediators on viral pathogenesis and the potential of reovirus as an oncolytic vector are also outlined.

Distinct Whole Transcriptomic Profiles of the Bursa of Fabricius in Muscovy Ducklings Infected by Novel Duck Reovirus with Different Virulence

Novel duck reovirus (NDRV) is a newly identified reovirus that brings about more severe damage on multiple organs and mortality in various species of waterfowl. We previously characterized the transcriptomic profiles responding to NDRV in the bursa of Fabricius of Muscovy ducklings, which is a major immunological organ against virus infection. However, the molecular mechanisms of variant cell responses in the bursa of Fabricius to NDRV with different virulence is unclear. Here, we conducted a whole transcriptomic analysis to study the effects of two strains, HN10 (virulent NDRV) and JDm10 (artificially attenuated NDRV), on the bursa of Fabricius of Muscovy ducklings. We harvested a large number of differentially expressed genes (DEGs) of the bursa of Fabricius specially induced by HN10 and JDm10, and we found that HN10 induced DEGs enriched in differentiation and development in multiple organs beyond JDm10. Moreover, the ceRNA regulatory network also indicated the different connections among mRNA, lncRNA and miRNA. Interestingly, we further noticed that a population of differential expressed miRNA could particularly target to transcripts of HN10 and JDm10. We took miR-24 as an example and observed that miR-24 could reduce the transcription of GLI family zinc finger 3 (Gli3) and membrane-associated guanylate kinase, WW and PDZ domain containing 1 (Magi1) via recognition 3' UTR of these two genes by a dual luciferase reporter gene assay in vitro. However, this effect could be compromised by HN10 infection or the ectopic over-expression of the putative miR-24 targeting regions in L1 and L3 fragments of HN10. Taken together, we examined and proposed a novel regulatory competitive mechanism between transcripts of NDRV and Muscovy ducklings for miRNA. These findings may advance the understanding of the molecular pathogenesis of NDRV in Muscovy ducklings, and help provide the potential targets for vaccine and drug development against NDRV.

Golden 2-like transcription factor contributes to the major QTL against rice black-streaked dwarf virus disease

A major resistance QTL was identified on chromosome 6 in rice variety Wuke; both overexpression and knockdown experiments confirmed that OsGLK1 is the candidate gene for association with Rice black-streaked dwarf virus disease. Rice black-streaked dwarf virus disease is one of the most destructive rice viral diseases in China and East Asia. Progress has been limited in RBSDVD resistance breeding due to inadequate knowledge on the underlying functional genes. In this study, a major QTL for RBSDV (rice black-streaked dwarf virus) independent of SBPH (small brown planthopper) resistance was mapped in a 1.8 Mb interval on chromosome 6 by using an F_2:3 population originated from resistant rice variety Wuke. Representative transcripts within this region were analysed and three genes showing amino acid sequence variation in functional domains were selected for transformation. Overexpression experiments showed that one gene exhibited significant enhanced resistance compared to control lines, encoding protein involving Myb domain and probable transcription factor Golden 2-like1 (GLK1). Furthermore, OsGLK1 knockdown rice lines were investigated and the resistance ability was significantly declined without this gene compared to the wild type. Taken together, both overexpression and knockdown experiments strongly suggested that OsGLK1 plays an important role for RBSDV resistance and contributes to the major QTL. The study paves the way for elucidating the molecular mechanism underlying RBSDVD resistance and the molecular markers associated with OsGLK1 may be used for marker-assisted selection.

The μ2 and λ1 Proteins of Mammalian Reovirus Modulate Early Events Leading to Induction of the Interferon Signaling Network

It has been previously shown that amino acid polymorphisms in reovirus proteins μ2 and λ1 are associated with differing levels of interferon induction. In the present study, viruses carrying these polymorphisms in either or both proteins, were further studied. The two viral determinants exert a synergistic effect on the control of β-interferon induction at the protein and mRNA level, with a concomitant increase in RIG-I. In contrast, levels of phospho-Stat1 and interferon-stimulated genes are increased in singly substituted viruses but with no further increase when both substitutions were present. This suggests that the viral determinants are acting during initial events of viral recognition. Accordingly, difference between viruses was reduced when infection was performed with partially uncoated virions (ISVPs) and transfection of RNA recovered from early-infected cells recapitulates the differences between viruses harboring the different polymorphisms. Altogether, the data are consistent with a redundant or complementary role of μ2 and λ1, affecting either early disassembly or the nature of the viral RNA in the incoming viral particle. Proteins involved in viral RNA synthesis are thus involved in this likely critical aspect of the ability of different reovirus variants to infect various cell types, and to discriminate between parental and transformed/cancer cells.

Characterization of replication and variations in genome segments of a bat reovirus, BatMRV/B19-02, by RNA-seq in infected Vero-E6 cells

Mammalian orthoreoviruses (MEVs) that can cause enteric, respiratory, and encephalitic infections have been identified in a wide variety of mammalian species. Here, we report a novel MRV type 1 strain detected in Miniopterus schreibersii that may have resulted from reassortment events. Using next-generation RNA sequencing (RNA-seq), we found that the ratios of the RNA levels of the 10 reovirus segments in infected cells were constant during the late stages of infection. We also discovered that the relative abundance of each segment differed. Notably, the relative abundance of M2 (encoding the µ1 protein) and S4 (encoding the σ3 protein) RNAs was higher than that of the others throughout the infection. Additionally, massive junctions were identified. These results support the hypothesis that defective genome segments are generated and that cross-family recombination occurs. These data may further the study of gene function, viral replication, and virus evolution.

Silencing suppressors of rice black-streaked dwarf virus and rice stripe virus hijack the 26S proteasome of Laodelphax striatellus to facilitate virus accumulation and transmission

BACKGROUND

Rice black-streaked dwarf virus (RBSDV) is transmitted by small brown planthopper (Laodelphax striatellus [L. striatellus]) and causes devastating disease in rice. P9-1 has silencing suppression activity and is the key protein for viroplasm formation in RBSDV-infected plants and insects; however, its exact function is poorly understood.

RESULTS

In this study, the P9-1 of RBSDV interacted with L. striatellus 26S proteasome subunit RPN8. RBSDV accumulation in L. striatellus increased after the 26S proteasome was disrupted by silencing the RPN8 expression. This finding indicated that L. striatellus 26S proteasome played a defense role against RBSDV infection by regulating RBSDV accumulation. Further investigations revealed that P9-1 could competitively bind to RPN8 with RPN7, thereby disrupting the assembly of 26S proteasome in L. striatellus and promoting the infection of RBSDV in insect vectors, and further affecting the transmission of the virus to rice by insect vectors. Similar to P9-1, rice stripe virus (RSV) NS2, a weak silencing suppressor, regulated virus accumulation and transmission by hijacking RPN8 to interfere with the function of 26S proteasome in L. striatellus.

CONCLUSION

These results suggest that viruses promote their own infection via interfering with ubiquitination pathway of insect vectors, and this mechanism might be of universal importance. These findings provide a new insight into the mechanism of virus transmission in insect vectors. © 2022 Society of Chemical Industry.

Characterization of virus-like particles assembled by co-expression of BmCPV capsid shell protein and large protrusion protein

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a typical single-layer capsid dsRNA virus belonging to the Reoviridae family of the Cypovirus genus. Previous studies have shown that the BmCPV major capsid shell protein (CSP) has the ability to self-assemble into virus-like particles (VLPs), and cryo-electron microscopy of the BmCPV virions has revealed a tight mutual binding region between CSP and another capsid protein known as the Large Protrusion Protein (LPP), which further stabilizes the capsid shell. In this study, the multi-gene baculovirus expression system, Ac-MultiBac, was used to produce both solely CSP-based and CSP-LPP co-assembled VLPs. Transmission electron microscopy (TEM) results showed that addition of LPP did not affect the assembly of VLPs resulting in almost identical structure in both cases. However, ex vivo administration of VLPs to silkworm midgut tissue showed that CSP-based VLPs did not induce a significant transcriptional response in the innate immunity and RNAi gene cascades, compared to the co-assembled CSP-LPP based VLPs and the natural BmCPV virions isolated from polyhedra. The experimental results indicate that CSP and LPP attach tightly ("Plug and Display" model with CSP acting as "catcher" and LPP as "tag") to form VLPs that have a structure similar to that of the native CPV virions. Moreover, our results showed that the formation of VLPs with the two BmCPV capsid proteins is feasible, which can form the basis for the production of BmCPV-based VLPs as a new type of biological material to display exogenous proteins.

RNA Packaging in the Cystovirus Bacteriophages: Dynamic Interactions during Capsid Maturation

The bacteriophage family Cystoviridae consists of a single genus, Cystovirus, that is lipid-containing with three double-stranded RNA (ds-RNA) genome segments. With regard to the segmented dsRNA genome, they resemble the family Reoviridae. Therefore, the Cystoviruses have long served as a simple model for reovirus assembly. This review focuses on important developments in the study of the RNA packaging and replication mechanisms, emphasizing the structural conformations and dynamic changes during maturation of the five proteins required for viral RNA synthesis, P1, P2, P4, P7, and P8. Together these proteins constitute the procapsid/polymerase complex (PC) and nucleocapsid (NC) of the Cystoviruses. During viral assembly and RNA packaging, the five proteins must function in a coordinated fashion as the PC and NC undergo expansion with significant position translation. The review emphasizes this facet of the viral assembly process and speculates on areas suggestive of additional research efforts.

Viral Capsid and Polymerase in Reoviridae

The members of the family Reoviridae (reoviruses) consist of 9-12 discrete double-stranded RNA (dsRNA) segments enclosed by single, double, or triple capsid layers. The outer capsid proteins of reoviruses exhibit the highest diversity in both sequence and structural organization. By contrast, the conserved RNA-dependent RNA polymerase (RdRp) structure in the conserved innermost shell in all reoviruses suggests that they share common transcriptional regulatory mechanisms. After reoviruses are delivered into the cytoplasm of a host cell, their inner capsid particles (ICPs) remain intact and serve as a stable nanoscale machine for RNA transcription and capping performed using enzymes in ICPs. Advances in cryo-electron microscopy have enabled the reconstruction at near-atomic resolution of not only the icosahedral capsid, including capping enzymes, but also the nonicosahedrally distributed complexes of RdRps within the capsid at different transcriptional stages. These near-atomic resolution structures allow us to visualize highly coordinated structural changes in the related enzymes, genomic RNA, and capsid protein during reovirus transcription. In addition, reoviruses encode their own enzymes for nascent RNA capping before RNA releasing from their ICPs.

Transcriptional responses of Daphnis nerii larval midgut to oral infection by Daphnis nerii cypovirus-23

BACKGROUND

Daphnis nerii cypovirus-23 (DnCPV-23) is a new type of cypovirus and has a lethal effect on the oleander hawk moth, Daphnis nerii which feeds on leave of Oleander and Catharanthus et al. After DnCPV-23 infection, the change of Daphnis nerii responses has not been reported.

METHODS

To better understand the pathogenic mechanism of DnCPV-23 infection, 3rd-instar Daphnis nerii larvae were orally infected with DnCPV-23 occlusion bodies and the transcriptional responses of the Daphnis nerii midgut were analyzed 72 h post-infection using RNA-seq.

RESULTS

The results showed that 1979 differentially expressed Daphnis nerii transcripts in the infected midgut had been identified. KEGG analysis showed that protein digestion and absorption, Toll and Imd signaling pathway were down-regulated. Based on the result, we speculated that food digestion and absorption in insect midgut might be impaired after virus infection. In addition, the down-regulation of the immune response may make D. nerii more susceptible to bacterial infections. Glycerophospholipid metabolism and xenobiotics metabolism were up-regulated. These two types of pathways may affect the viral replication and xenobiotic detoxification of insect, respectively.

CONCLUSION

These results may facilitate a better understanding of the changes in Daphnis nerii metabolism during cypovirus infection and serve as a basis for future research on the molecular mechanism of DnCPV-23 invasion.

Functional analysis of a putative Bombyx mori cypovirus miRNA BmCPV-miR-10 and its effect on virus replication

Bombyx mori cypovirus (BmCPV) is an important pathogen of silkworm (B. mori), the economically beneficial insect. The mechanism of its interaction with host immune defence system in the process of infection is still not yet completely clear. Researches have demonstrated that virus-encoded microRNAs (miRNA) play a crucial role in regulating host-pathogen interaction, but few reports are available so far on miRNAs encoded by insect viruses, especially the RNA viruses. In this study, a putative miRNA encoded by the 10th segment of BmCPV genomic RNA, BmCPV-miR-10, was identified and functionally analysed. The expression of the putative BmCPV-miR-10 could be detected via stem-loop RT-PCR (reverse transcription-Polymerase Chain Reaction) in the midgut of silkworm larvae infected with BmCPV. BmCSDE1 (B. mori cold shock domain E1 protein) gene was predicted to be a candidate target gene for BmCPV-miR-10 with the miRNA binding site located in 3' untranslated region of its mRNA. The regulation effect of the putative BmCPV-miR-10 on BmCSDE1 was verified in HEK293 cells by lentiviral expression system, in BmN cells by transfecting BmCPV-miR-10 mimics. The qRT-PCR (quantitative real-time PCR) results showed that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1. By injection of BmCPV-miR-10 mimics into the silkworm larvae infected with BmCPV, it was further proved that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1 in vivo, then inhibit the expression of BmApaf-1 (B. mori apoptotic protease activating factor 1), while enhance the replication of BmCPV genomic RNAs to a certain extent. These results implied that the putative BmCPV-miR-10 could down-regulate the expression of BmCSDE1, then suppress the expression of BmApaf-1, thereby created a favourable intracellular environment for virus replication and proliferation.

Resistance Evaluation of Dominant Varieties against Southern Rice Black-Streaked Dwarf Virus in Southern China

Southern rice black-streaked dwarf virus (SRBSDV), a Fijivirus in the Reoviridae family, is transmitted by the white-backed planthopper (Sogatella furcifera, WBPH), a long-distance migratory insect, and presents a serious threat to rice production in Asia. It was first discovered in China’s Guangdong Province in 2001 and has been endemic in the south of China and north of Vietnam for two decades, with serious outbreaks in 2009, 2010, and 2017. In this study, we evaluated the resistance of 10 dominant rice varieties from southern China, where the virus overwinters and accumulates as a source of early spring reinfection, against this virus by artificial inoculation. The results showed that in all tested varieties there was no immune resistance, but there were differences in the infection rate, with incidence rates from 21% to 90.7%, and in symptom severity, with plant weight loss from 66.71% to 91.20% and height loss from 34.1% to 65.06%. Additionally, and valuably, the virus titer and the insect vector virus acquisition potency from diseased plants were significantly different among the varieties: an over sixfold difference was determined between resistant and susceptible varieties, and there was a positive correlation between virus accumulation and insect vector virus acquisition. The results can provide a basis for the selection of rice varieties in southern China to reduce the damage of SRBSDV in this area and to minimize the reinfection source and epidemics of the virus in other rice-growing areas.

MicroRNA-315-5p promotes rice black-streaked dwarf virus infection by targeting a melatonin receptor in the small brown planthopper

BACKGROUND

MicroRNAs (miRNAs), a class of small non-coding endogenous RNAs, play key roles in various biological processes. Most plant viruses are transmitted by insect vectors. However, little is known about the function of miRNAs on plant virus-insect host interaction.

RESULTS

We investigated the role of miR-315-5p in regulation of plant viral infection in insects using a rice black-streaked dwarf virus (RBSDV) and small brown planthopper (SBPH) interaction system. Our results showed that miR-315-5p had the highest expression level in 2nd-instar nymph, and was highly expressed in the salivary gland and midgut in SBPH. miR-315-5p was in response to and regulated RBSDV infection in SBPH. Injection of miR-315-5p mimic, agomir-315, significantly increased the RBSDV accumulation, whereas injection of miR-315-5p inhibitor, antagomir-315, reduced virus accumulation in SBPH. Furthermore, a melatonin receptor was identified as a target gene of miR-315-5p by the dual luciferase reporter assay. Knockdown of the melatonin receptor significantly increased the expression of RBSDV coat protein gene S10 and replication related genes, S5-1, S6, and S9-1. Furthermore, treatment with melatonin receptor antagonist luzindole and activator agomelatine significantly increased and reduced RBSDV accumulation in SBPH, respectively. Compared to the control, miR-315-5p did not affect the efficiency of RBSDV acquisition in SBPH. However, the efficiency of RBSDV transmission was significantly reduced after injecting antagomir-315.

CONCLUSION

Taken together, our data reveal that miR-315-5p is beneficial for RBSDV infection in its insect vector by directly targeting a melatonin receptor. These findings provide a new insight to the function of miRNAs in virus-insect vector interaction. © 2021 Society of Chemical Industry.

Reovirus Low-Density Particles Package Cellular RNA

Packaging of segmented, double-stranded RNA viral genomes requires coordination of viral proteins and RNA segments. For mammalian orthoreovirus (reovirus), evidence suggests either all ten or zero viral RNA segments are simultaneously packaged in a highly coordinated process hypothesized to exclude host RNA. Accordingly, reovirus generates genome-containing virions and “genomeless” top component particles. Whether reovirus virions or top component particles package host RNA is unknown. To gain insight into reovirus packaging potential and mechanisms, we employed next-generation RNA-sequencing to define the RNA content of enriched reovirus particles. Reovirus virions exclusively packaged viral double-stranded RNA. In contrast, reovirus top component particles contained similar proportions but reduced amounts of viral double-stranded RNA and were selectively enriched for numerous host RNA species, especially short, non-polyadenylated transcripts. Host RNA selection was not dependent on RNA abundance in the cell, and specifically enriched host RNAs varied for two reovirus strains and were not selected solely by the viral RNA polymerase. Collectively, these findings indicate that genome packaging into reovirus virions is exquisitely selective, while incorporation of host RNAs into top component particles is differentially selective and may contribute to or result from inefficient viral RNA packaging.

A Novel Rubi-Like Virus in the Pacific Electric Ray (Tetronarce californica) Reveals the Complex Evolutionary History of the Matonaviridae

Rubella virus (RuV) is the causative agent of rubella ("German measles") and remains a global health concern. Until recently, RuV was the only known member of the genus Rubivirus and the only virus species classified within the Matonaviridae family of positive-sense RNA viruses. Recently, two new rubella-like matonaviruses, Rustrela virus and Ruhugu virus, have been identified in several mammalian species, along with more divergent viruses in fish and reptiles. To screen for the presence of additional novel rubella-like viruses, we mined published transcriptome data using genome sequences from Rubella, Rustrela, and Ruhugu viruses as baits. From this, we identified a novel rubella-like virus in a transcriptome of Tetronarce californica-order Torpediniformes (Pacific electric ray)-that is more closely related to mammalian Rustrela virus than to the divergent fish matonavirus and indicative of a complex pattern of cross-species virus transmission. Analysis of host reads confirmed that the sample analysed was indeed from a Pacific electric ray, and two other viruses identified in this animal, from the Arenaviridae and Reoviridae, grouped with other fish viruses. These findings indicate that the evolutionary history of the Matonaviridae is more complex than previously thought and highlights the vast number of viruses that remain undiscovered.

Genome-Wide Association Study on Resistance to Rice Black-Streaked Dwarf Disease Caused by Rice black-streaked dwarf virus

Rice black-streaked dwarf disease caused by Rice black-streaked dwarf virus (RBSDV) is one of the most destructive viral diseases of rice. Thus, it is imperative that resistant rice germplasms are screened for novel RBSDV-resistant genes. RBSDV resistance of a diverse global collection comprising 1,953 rice accessions was evaluated under natural conditions across 3 years. The average disease incidences of the Xian/indica (XI) subgroup were significantly lower than those of the Geng/japonica (GJ) subgroup. Interestingly, most XI-1A accessions in the Xian subgroup were significantly more susceptible than XI-1B accessions, even though XI-1A and XI-1B have a close phylogenetic relationship. Four Xian accessions stably and highly resistant to RBSDV were consistently identified in 2 years. Ten genomic regions (GRs) with 147 single nucleotide polymorphisms associated with RBSDV resistance were detected by a single-locus genome-wide association study (GWAS), of which five were repeatedly identified in a multilocus GWAS. Two previously reported GRs, grRBSDV-6.1 and grRBSDV-6.3, which were repeatedly detected as stably and highly associated with RBSDV resistance, contained 17 and seven genes, respectively, with significant differences of resistance among haplotypes. Haplotype analyses of the candidate genes LOC_Os06g03150 in grRBSDV-6.1 and LOC_Os06g31190 in grRBSDV-6.3 suggested that the former gene is mainly associated with the differentiation of resistance within the Xian subgroup and the latter gene mainly explains the difference in the resistance between Xian and Geng. Another three novel resistance GRs (grRBSDV-1.1, grRBSDV-7.1, and grRBSDV-9.1) were identified. Our findings may enhance the application of disease-resistant rice germplasms for breeding RBSDV-resistant varieties.

Captivating Perplexities of Spinareovirinae 5' RNA Caps

RNAs with methylated cap structures are present throughout multiple domains of life. Given that cap structures play a myriad of important roles beyond translation, such as stability and immune recognition, it is not surprising that viruses have adopted RNA capping processes for their own benefit throughout co-evolution with their hosts. In fact, that RNAs are capped was first discovered in a member of the Spinareovirinae family, Cypovirus, before these findings were translated to other domains of life. This review revisits long-past knowledge and recent studies on RNA capping among members of Spinareovirinae to help elucidate the perplex processes of RNA capping and functions of RNA cap structures during Spinareovirinae infection. The review brings to light the many uncertainties that remain about the precise capping status, enzymes that facilitate specific steps of capping, and the functions of RNA caps during Spinareovirinae replication.

Noncanonical Cell Death Induction by Reassortant Reovirus

Triple-negative breast cancer (TNBC) constitutes 10 to 15% of all breast cancer and is associated with worse prognosis than other subtypes of breast cancer. Current therapies are limited to cytotoxic chemotherapy, radiation, and surgery, leaving a need for targeted therapeutics to improve outcomes for TNBC patients. Mammalian orthoreovirus (reovirus) is a nonenveloped, segmented, double-stranded RNA virus in the Reoviridae family. Reovirus preferentially kills transformed cells and is in clinical trials to assess its efficacy against several types of cancer. We previously engineered a reassortant reovirus, r2Reovirus, that infects TNBC cells more efficiently and induces cell death with faster kinetics than parental reoviruses. In this study, we sought to understand the mechanisms by which r2Reovirus induces cell death in TNBC cells. We show that r2Reovirus infection of TNBC cells of a mesenchymal stem-like (MSL) lineage downregulates the mitogen-activated protein kinase/extracellular signal-related kinase pathway and induces nonconventional cell death that is caspase-dependent but caspase 3-independent. Infection of different MSL lineage TNBC cells with r2Reovirus results in caspase 3-dependent cell death. We map the enhanced oncolytic properties of r2Reovirus in TNBC to epistatic interactions between the type 3 Dearing M2 gene segment and type 1 Lang genes. These findings suggest that the genetic composition of the host cell impacts the mechanism of reovirus-induced cell death in TNBC. Together, our data show that understanding host and virus determinants of cell death can identify novel properties and interactions between host and viral gene products that can be exploited for the development of improved viral oncolytics.IMPORTANCE TNBC is unresponsive to hormone therapies, leaving patients afflicted with this disease with limited treatment options. We previously engineered an oncolytic reovirus (r2Reovirus) with enhanced infective and cytotoxic properties in TNBC cells. However, how r2Reovirus promotes TNBC cell death is not known. In this study, we show that reassortant r2Reovirus can promote nonconventional caspase-dependent but caspase 3-independent cell death and that the mechanism of cell death depends on the genetic composition of the host cell. We also map the enhanced oncolytic properties of r2Reovirus in TNBC to interactions between a type 3 M2 gene segment and type 1 genes. Our data show that understanding the interplay between the host cell environment and the genetic composition of oncolytic viruses is crucial for the development of efficacious viral oncolytics.

A novel cypovirus found in a betabaculovirus co-infection context contains a poxvirus immune nuclease (poxin)-related gene

The cassava hornworm Erinnyis ello ello (Lepidoptera: Sphingidae) is an important pest in Brazil. This insect feeds on host plants of several species, especially Manihot esculenta (cassava) and Hevia brasiliensis (rubber tree). Cassava hornworm outbreaks are quite common in Brazil and can cause great impact over crop production. Granulare and polyhedral-shaped occlusion bodies (OBs) were observed in extracts of dead E. ello larvae from rubber-tree plantations by light and scanning electron microscopy (SEM), suggesting a mixed infection. The polyhedral-shaped OB surface revealed indentations that resemble those found in cypovirus polyhedra. After OB nucleic acid extraction followed by cDNA production and Illumina deep-sequencing analysis, the results confirmed for the presence of a putative novel cypovirus that carries ten segments and also a betabaculovirus (Erinnyis ello granulovirus, ErelGV). Phylogenetic analysis of the predicted segment 1-enconded RdRP showed that the new cypovirus isolate is closely related to a member of species Cypovirus 2, which was isolated from Inachis io (Lepidoptera: Nymphalidae). Therefore, we named this new isolate Erinnyis ello cypovirus 2 (ErelCPV-2). Genome in silico analyses showed that ErelCPV-2 segment 8 (S8) has a predicted amino acid identity of 35.82 % to a hypothetical protein of betabaculoviruses. This putative protein has a cGAMP-specific nuclease domain related to the poxvirus immune nucleases (poxins) from the 2',3'-cGAMP-degrading enzyme family.

Aquareovirus NS31 protein serves as a specific inducer for host heat shock 70-kDa protein

Elevation of heat-shock protein expression, known as cellular heat-shock responses, occurs during infection of many viruses, which is involved in viral replication through various mechanisms. Herein, transcriptome analysis revealed that over-expression of non-structural protein NS31 of grass carp reovirus (GCRV) in grass carp Ctenopharyngodon idellus kidney (CIK) cells specifically induced expression of heat-shock response (HSR) genes HSP30 and HSP70. We further found that, among the HSR genes, only HSP70 protein were shown to be efficiently induced in fish cells following NS31 over-expression or GCRV infection. Employing a luciferase assay, we were able to show that the promoter of the HSP70 gene can be specifically activated by NS31. In addition, over-expressing HSP70 in grass carp CIK cells resulted in enhanced replication efficiency of GCRV, and an inhibitor for HSP70 resulted in the inhibition of GCRV replication, indicating that HSP70 should serve as a pro-viral factor. We also found that NS31 could activate HSP70 expression in cells of other vertebrate animals. Similar inducing effect on HSP70 expression was demonstrated for NS31-homologue proteins of other aquareoviruses including American grass carp reovirus (AGCRV) and GRCV (green river chinook virus). All these results indicated NS31 proteins in the Aquareovirus genus should play a key role for up-regulating specific HSP70 gene during viral replication.

Studies on the clinical symptoms, virus distribution, and mRNA expression of several antiviral immunity-related genes in grass carp after infection with genotype II grass carp reovirus

The viral hemorrhage disease caused by grass carp reovirus (GCRV) is a serious contagious disease of grass carp that mainly infects fingerlings and yearlings. Epidemiological studies have shown that GCRV genotype II is currently the prominent genotype. However, little is known about the histopathological characteristics, virus distribution, and expression of immunity-related genes in grass carp infected by GCRV genotype II. In this study, we found that grass carp infected by GCRV genotype II lost appetite, swam alone, and rolled, and their fins, eyes, operculum, oral cavity, abdomen, intestine, and muscles showed pronounced punctate hemorrhage. Congestion, swelling, deformation, thinning of membranes, dilatation and darkened color of nucleoli, cathepsis, erythrocyte infiltration, and vacuole formation were observed in some infected tissues. A qRT-PCR test showed that the 11 genome segments of GCRV had similar expression patterns in different tissues. The S8 segment, with unknown function and no homologous sequences, had the highest expression level, while the most conserved segment, L2, had the lowest expression level. GCRV particles were distributed in different tissues, especially in the intestine. In the infected intestine, the expression of various receptors and adaptor molecules was modulated at different levels. Pro-inflammatory cytokine interleukin-1β (IL-1β) expression was 2160.9 times higher than that in the control group. The upregulation of immunity-related genes activated the antiviral immunity pathways. Therefore, the intestine might play a dual role in mediating GCRV infection and the antiviral immune response. This study provides detailed information about the pathogenicity of GCRV and expression of immunity-related genes, laying the foundation for further research on virus control and treatment.

Recognition of Reovirus RNAs by the Innate Immune System

Mammalian orthoreovirus (reovirus) is a dsRNA virus, which has long been used as a model system to study host–virus interactions. One of the earliest interactions during virus infection is the detection of the viral genomic material, and the consequent induction of an interferon (IFN) based antiviral response. Similar to the replication of related dsRNA viruses, the genomic material of reovirus is thought to remain protected by viral structural proteins throughout infection. Thus, how innate immune sensor proteins gain access to the viral genomic material, is incompletely understood. This review summarizes currently known information about the innate immune recognition of the reovirus genomic material. Using this information, we propose hypotheses about host detection of reovirus.

Functional analysis of a miRNA-like small RNA derived from Bombyx mori cytoplasmic polyhedrosis virus

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a major pathogen of the economic insect silkworm, Bombyx mori. Virus-encoded microRNAs (miRNAs) have been proven to play important roles in host-pathogen interactions. In this study we identified a BmCPV-derived miRNA-like 21 nt small RNA, BmCPV-miR-1, from the small RNA deep sequencing of BmCPV-infected silkworm larvae by stem-loop quantitative real-time PCR (qPCR) and investigated its functions with qPCR and lentiviral expression systems. Bombyx mori inhibitor of apoptosis protein (BmIAP) gene was predicted by both target prediction software miRanda and Targetscan to be one of its target genes with a binding site for BmCPV-miR-1 at the 5' untranslated region. It was found that the expression of BmCPV-miR-1 and its target gene BmIAP were both up-regulated in BmCPV-infected larvae. At the same time, it was confirmed that BmCPV-miR-1 could up-regulate the expression of BmIAP gene in HEK293T cells with lentiviral expression systems and in BmN cells by transfecting mimics. Furthermore, BmCPV-miR-1 mimics could up-regulate the expression level of BmIAP gene in midgut and fat body in the silkworm. In the midgut of BmCPV-infected larvae, BmCPV-miR-1 mimics could be further up-regulated and inhibitors could lower the virus-mediated expression of BmIAP gene. With the viral genomic RNA segments S1 and S10 as indicators, BmCPV-miR-1 mimics could up-regulate and inhibitors down-regulate their replication in the infected silkworm. These results implied that BmCPV-miR-1 could inhibit cell apoptosis in the infected silkworm through up-regulating BmIAP expression, providing the virus with a better cell circumstance for its replication.

Grass Carp Reovirus Major Outer Capsid Protein VP4 Interacts with RNA Sensor RIG-I to Suppress Interferon Response

Diseases caused by viruses threaten the production industry and food safety of aquaculture which is a great animal protein source. Grass carp reovirus (GCRV) has caused tremendous loss, and the molecular function of viral proteins during infection needs further research, as for most aquatic viruses. In this study, interaction between GCRV major outer capsid protein VP4 and RIG-I, a critical viral RNA sensor, was screened out by GST pull-down, endogenous immunoprecipitation and subsequent LC-MS/MS, and then verified by co-IP and an advanced far-red fluorescence complementation system. VP4 was proved to bind to the CARD and RD domains of RIG-I and promoted K48-linked ubiquitination of RIG-I to degrade RIG-I. VP4 reduced mRNA and promoter activities of key genes of RLR pathway and sequential IFN production. As a consequence, antiviral effectors were suppressed and GCRV replication increased, resulting in intensified cytopathic effect. Furthermore, results of transcriptome sequencing of VP4 stably expressed CIK (C. idella kidney) cells indicated that VP4 activated the MyD88-dependent TLR pathway. Knockdown of VP4 obtained opposite effects. These results collectively revealed that VP4 interacts with RIG-I to restrain interferon response and assist GCRV invasion. This study lays the foundation for anti-dsRNA virus molecular function research in teleost and provides a novel insight into the strategy of immune evasion for aquatic virus.

Reovirus uses macropinocytosis-mediated entry and fast axonal transport to infect neurons

Several barriers protect the central nervous system (CNS) from pathogen invasion. Yet viral infections of the CNS are common and often debilitating. Understanding how neurotropic viruses co-opt host machinery to overcome challenges to neuronal entry and transmission is important to combat these infections. Neurotropic reovirus disseminates through neural routes and invades the CNS to cause lethal encephalitis in newborn animals. To define mechanisms of reovirus neuronal entry and directional transport, we used primary neuron cultures, which reproduce in vivo infection patterns displayed by different reovirus serotypes. Treatment of neurons with small-molecule inhibitors of different endocytic uptake pathways allowed us to discover that the cellular machinery mediating macropinocytosis is required for reovirus neuronal entry. This mechanism of reovirus entry differs from clathrin-mediated endocytosis, which is used by reovirus to invade non-neuronal cells. Analysis of reovirus transport and release from isolated soma or axonal termini of neurons cultivated in microfluidic devices indicates that reovirus is capable of retrograde but only limited anterograde neuronal transmission. The dynamics of retrograde reovirus movement are consistent with fast axonal transport coordinated by dynein along microtubules. Further analysis of viral transport revealed that multiple virions are transported together in axons within non-acidified vesicles. Reovirus-containing vesicles acidify after reaching the soma, where disassembly of virions and release of the viral core into the cytoplasm initiates replication. These results define mechanisms of reovirus neuronal entry and transport and establish a foundation to identify common host factors used by neuroinvasive viruses. Furthermore, our findings emphasize consideration of cell type-specific entry mechanisms in the tailored design of neurotropic viruses as tracers, oncolytic agents, and delivery vectors.

Viral infections of the central nervous system (CNS) cause a significant health burden globally and compel a better mechanistic understanding of neural invasion by viruses to develop effective interventions. Neurotropic reovirus disseminates through neural routes to infect the CNS and serves as a tractable model to study neural invasion by viruses. Despite knowledge of reovirus neurotropism for decades, mechanisms mediating reovirus neuronal infection remain undefined. We used primary neurons cultured in microfluidic devices to study entry and directional transport of reovirus. We discovered that reovirus uses macropinocytosis for neuronal entry as opposed to the use of a clathrin-mediated pathway in non-neuronal cells. We are unaware of another virus using macropinocytosis to enter neurons. Following internalization, reovirus spreads in the retrograde direction using dynein-mediated fast axonal transport but exhibits limited anterograde spread. We further demonstrate that reovirus disassembly and replication occur in the neuronal soma subsequent to axonal transport. Remarkably, these entry and transport mechanisms mirror those used by misfolded proteins implicated in neurodegenerative diseases. Our findings establish the mechanics of reovirus neuronal uptake and spread and provide clues about therapeutic targets to limit neuropathology inflicted by pathogens and misfolded proteins.

Delivery of Oncolytic Reovirus by Cell Carriers

Oncolytic virus therapy is a rapidly expanding branch of cancer immunotherapy and represents a genuine opportunity to improve currently available treatment options. However, as single agents oncolytic viruses have shown only moderate clinical benefit and many challenges remain before their full potential is realized. Central to this is the efficient delivery of the virus to the tumor site and potentiation of the antitumor immune response. This chapter describes the loading of oncolytic reovirus onto monocytes which act as carriers for delivery of the virus to the tumor site and, as antigen presenting cells, may also thereby potentiate the development of an adaptive antitumor immune response.

A real-time reverse-transcription isothermal recombinase polymerase amplification assay for the rapid detection of genotype III grass carp (Ctenopharyngodon idella) reovirus

Grass carp (Ctenopharyngodon idella) hemorrhagic disease, which is characterized by external and internal hemorrhage, is a serious infectious disease affecting grass carp production. Strains of the causative agent, grass carp reovirus (GCRV), are divided into genotypes I, II and III, which are represented by the isolates GCRV-873, GCRV-HZ08 and GCRV-104, respectively. In this study, a real-time reverse-transcription recombinase polymerase amplification (real-time RT-RPA) assay was developed to detect the genotype III grass carp reovirus GCRV-104. The assay was based on the detection of the vp55 gene which encodes the outer fiber protein of the virus. A portable ESE-Quant Tube scanner, with a dimension of 17.4 × 18.8 cm, weighing about 1 kg, and equipped with temperature settings to amplify the DNA isothermally and spectral devices to detect the amplified products using fluorescence, was used to complete the assay. Under the optimal conditions, the assay took approximately 10 min to complete at 37 °C and showed no cross-reactions with other aquatic viruses. Consequently, this rapid real-time RT-RPA assay is a useful method for the simple, rapid and reliable detection of genotype III GCRV strains in resource-limited diagnostic laboratories.

Emergence of Southern Rice Black-Streaked Dwarf Virus in the Centuries-Old Chinese Yuanyang Agrosystem of Rice Landraces

Southern rice black-streaked dwarf virus (SRBSDV), which causes severe disease symptoms in rice (Oriza sativa L.) has been emerging in the last decade throughout northern Vietnam, southern Japan and southern, central and eastern China. Here we attempt to quantify the prevalence of SRBSDV in the Honghe Hani rice terraces system (HHRTS)-a Chinese 1300-year-old traditional rice production system. We first confirm that genetically diverse rice varieties are still being cultivated in the HHRTS and categorize these varieties into three main genetic clusters, including the modern hybrid varieties group (MH), the Hongyang improved modern variety group (HY) and the traditional indica landraces group (TIL). We also show over a 2-year period that SRBSDV remains prevalent in the HHRTS (20.1% prevalence) and that both the TIL (17.9% prevalence) and the MH varieties (5.1% prevalence) were less affected by SRBSDV than were the HY varieties (30.2% prevalence). Collectively we suggest that SRBSDV isolates are freely moving within the HHRTS and that TIL, HY and MH rice genetic clusters are not being preferentially infected by particular SRBSDV lineages. Given that SRBSDV can cause 30-50% rice yield losses, our study emphasizes both the need to better monitor the disease in the HHRTS, and the need to start considering ways to reduce its burden on rice production.

Dufulin Intervenes the Viroplasmic Proteins as the Mechanism of Action against Southern Rice Black-Streaked Dwarf Virus

Southern rice black-streaked dwarf virus (SRBSDV) causes disease in crops, which reduces the quality and yield. Several commercial antiviral agents are available to control the SRBSDV induced disease. However, the mechanism of antiviral agents controlling SRBSDV is largely unknown. Identifying targets in SRBSDV is a key step of antiviral agent discovery. Here, we investigated the potential protein target of the antiviral agent dufulin. We cloned and expressed a soluble viroplasmic P6 protein in the prokaryote Escherichia coli and the eukaryote Spodoptera frugiperda 9. The dissociation constants of dufulin with the purified P6 protein from E. coli and S. frugiperda 9 expression systems were 4.49 and 4.95 μM, respectively, indicating a strong binding affinity between dufulin and P6 protein. In vivo, dufulin significantly inhibited the expression of both P6 protein and P6 gene in the SRBSDV-infected rice leaves. This inhibition on P6 protein expression was also observed in transformed Nicotiana benthamiana where the P6 was overexpressed. Our data also showed that dufulin inhibited the duplication of SRBSDV in a dose-dependent manner in infected rice leaves with a half maximum effective concentration of 3.32 mM. It is therefore concluded that dufulin targets the viroplasmic protein P6 to inhibit the virulence of SRBSDV.

Sustained Neurotrophin Release from Protein Nanoparticles Mediated by Matrix Metalloproteinases Induces the Alignment and Differentiation of Nerve Cells

The spatial and temporal availability of cytokines, and the microenvironments this creates, is critical to tissue development and homeostasis. Creating concentration gradients in vitro using soluble proteins is challenging as they do not provide a self-sustainable source. To mimic the sustained cytokine secretion seen in vivo from the extracellular matrix (ECM), we encapsulated a cargo protein into insect virus-derived proteins to form nanoparticle co-crystals and studied the release of this cargo protein mediated by matrix metalloproteinase-2 (MMP-2) and MMP-8. Specifically, when nerve growth factor (NGF), a neurotrophin, was encapsulated into nanoparticles, its release was promoted by MMPs secreted by a PC12 neuronal cell line. When these NGF nanoparticles were spotted onto a cover slip to create a uniform circular field, movement and alignment of PC12 cells via their extended axons along the periphery of the NGF nanoparticle field was observed. Neural cell differentiation was confirmed by the expression of specific markers of tau, neurofilament, and GAP-43. Connections between the extended axons and the growth cones were also observed, and expression of connexin 43 was consistent with the formation of gap junctions. Extensions and connection of very fine filopodia occurred between growth cones. Our studies indicate that crystalline protein nanoparticles can be utilized to generate a highly stable cytokine gradient microenvironment that regulates the alignment and differentiation of nerve cells. This technique greatly simplifies the creation of protein concentration gradients and may lead to therapies for neuronal injuries and disease.

Rice Dwarf Virus Small RNA Profiles in Rice and Leafhopper Reveal Distinct Patterns in Cross-Kingdom Hosts

RNA silencing has evolved as a widespread antiviral strategy in many eukaryotic organisms. Antiviral RNA silencing is mediated by virus-derived small RNAs (vsiRNAs), created by the cleavage of double-stranded viral RNA substrates by Dicer (Dcr) in animals or Dicer-like (DCL) proteins in plants. However, little is known about how the RNA silencing mechanisms of different hosts respond to the same virus infection. We performed high-throughput small RNA sequencing in Nephotettix cincticeps and Oryza sativa infected with Rice dwarf phytoreovirus and analyzed the distinct accumulation of vsiRNAs in these two hosts. The results suggested a potential branch in the evolution of antiviral RNA silencing of insect and plant hosts. The rice vsiRNAs were predominantly 21 and 22 nucleotides (nt) long, suggesting that OsDCL4 and OsDCL2 are involved in their production, whereas 21-nt vsiRNAs dominated in leafhopper, suggesting the involvement of a Dcr-2 homolog. Furthermore, we identified ~50-fold more vsiRNAs in rice than in leafhoppers, which might be partially attributable to the activity of RNA-dependent RNA polymerase 6 (RDR6) in rice and the lack of RDR genes in leafhoppers. Our data established a basis for further comparative studies on the evolution of RNA silencing-based interactions between a virus and its hosts, across kingdoms.

Endangered wild salmon infected by newly discovered viruses

The collapse of iconic, keystone populations of sockeye (Oncorhynchus nerka) and Chinook (Oncorhynchus tshawytscha) salmon in the Northeast Pacific is of great concern. It is thought that infectious disease may contribute to declines, but little is known about viruses endemic to Pacific salmon. Metatranscriptomic sequencing and surveillance of dead and moribund cultured Chinook salmon revealed a novel arenavirus, reovirus and nidovirus. Sequencing revealed two different arenavirus variants which each infect wild Chinook and sockeye salmon. In situ hybridisation localised arenavirus mostly to blood cells. Population surveys of >6000 wild juvenile Chinook and sockeye salmon showed divergent distributions of viruses, implying different epidemiological processes. The discovery in dead and dying farmed salmon of previously unrecognised viruses that are also widely distributed in wild salmon, emphasizes the potential role that viral disease may play in the population dynamics of wild fish stocks, and the threat that these viruses may pose to aquaculture.