Functional characterization of Malabar grouper (Epinephelus malabaricus) interferon regulatory factor 9 involved in antiviral response.

IRF9 is a crucial component in the JAK-STAT pathway. IRF9 interacts with STAT1 and STAT2 to form IFN-I-stimulated gene factor 3 (ISGF3) in response to type I IFN stimulation, which promotes ISG transcription. However, the mechanism by which IFN signaling regulates Malabar grouper (Epinephelus malabaricus) IRF9 is still elusive. Here, we explored the nd tissue-specific mRNA distribution of the MgIRF9 gene, as well as its antiviral function in E. malabaricus. MgIRF9 encodes a protein of 438 amino acids with an open reading frame of 1317 base pairs. MgIRF9 mRNA was detected in all tissues of a healthy M. grouper, with the highest concentrations in the muscle, gills, and brain. It was significantly up-regulated by nervous necrosis virus infection and poly (I:C) stimulation. The gel mobility shift test demonstrated a high-affinity association between MgIRF9 and the promoter of zfIFN in vitro. In GK cells, grouper recombinant IFN-treated samples showed a significant response in ISGs and exhibited antiviral function. Subsequently, overexpression of MgIRF9 resulted in a considerable increase in IFN and ISGs mRNA expression (ADAR1, ADAR1-Like, and ADAR2). Co-immunoprecipitation studies demonstrated that MgIRF9 and STAT2 can interact in vivo. According to the findings, M. grouper IRF9 may play a role in how IFN signaling induces ISG gene expression in grouper species.

Bipartite viral RNA genome heterodimerization influences genome packaging and virion thermostability.

Multi-segmented viruses often multimerize their genomic segments to ensure efficient and stoichiometric packaging of the correct genetic cargo. In the bipartite Nodaviridae family, genome heterodimerization is also observed and conserved among different species. However, the nucleotide composition and biological function for this heterodimer remain unclear. Using Flock House virus as a model system, we developed a next-generation sequencing approach ("XL-ClickSeq") to probe heterodimer site sequences. We identified an intermolecular base-pairing site which contributed to heterodimerization in both wild-type and defective virus particles. Mutagenic disruption of this heterodimer site exhibited significant deficiencies in genome packaging and encapsidation specificity to viral genomic RNAs. Furthermore, the disruption of this intermolecular interaction directly impacts the thermostability of the mature virions. These results demonstrate that the intermolecular RNA-RNA interactions within the encapsidated genome of an RNA virus have an important role on virus particle integrity and thus may impact its transmission to a new host.IMPORTANCEFlock House virus is a member of Nodaviridae family of viruses, which provides a well-studied model virus for non-enveloped RNA virus assembly, cell entry, and replication. The Flock House virus genome consists of two separate RNA molecules, which can form a heterodimer upon heating of virus particles. Although similar RNA dimerization is utilized by other viruses (such as retroviruses) as a packaging mechanism and is conserved among Nodaviruses, the role of heterodimerization in the Nodavirus replication cycle is unclear. In this research, we identified the RNA sequences contributing to Flock House virus genome heterodimerization and discovered that such RNA-RNA interaction plays an essential role in virus packaging efficiency and particle integrity. This provides significant insight into how the interaction of packaged viral RNA may have a broader impact on the structural and functional properties of virus particles.

Slc43a2+ T cell metastasis from spleen to brain in RGNNV infected teleost.

The origin of T cells in the teleost's brain is unclear. While viewing the central nervous system (CNS) as immune privileged has been widely accepted, previous studies suggest that T cells residing in the thymus but not in the spleen of the teleost play an essential role in communicating with the peripheral organs. Here, we identified nine T cell subpopulations in the thymus and spleen of orange-spotted grouper (Epinephelus coioices) through single-cell RNA-sequencing analysis. After viral CNS infection with red-spotted grouper nervous necrosis virus (RGNNV), the number of slc43a2+ T cells synchronously increased in the spleen and brain. During the infection tests in asplenic zebrafish (tlx1▲ zebrafish model), no increase in the number of slc43a2+ T cells was observed in the brain. Single-cell transcriptomic analysis indicated that slc43a2+ T cells mature and functionally differentiate within the spleen and then migrate into the brain to trigger an immune response. This study suggests a novel route for T cell migration from the spleen to the brain during viral infection in fish.

Sea perch (Lateolabrax japonicus) UBC9 augments RGNNV infection by hindering RLRs-interferon response.

Small ubiquitin-like modifier (SUMO) is a reversible post-translational modification that regulates various biological processes in eukaryotes. Ubiquitin-conjugating enzyme 9 (UBC9) is the sole E2-conjugating enzyme responsible for SUMOylation and plays an important role in essential cellular functions. Here, we cloned the UBC9 gene from sea perch (Lateolabrax japonicus) (LjUBC9) and investigated its role in regulating the IFN response during red-spotted grouper nervous necrosis virus (RGNNV) infection. The LjUBC9 gene consisted of 477 base pairs and encoded a polypeptide of 158 amino acids with an active site cysteine residue and a UBCc domain. Phylogenetic analysis showed that LjUBC9 shared the closest evolutionary relationship with UBC9 from Paralichthys olivaceus. Tissue expression profile analysis demonstrated that LjUBC9 was significantly increased in multiple tissues of sea perch following RGNNV infection. Further experiments showed that overexpression of LjUBC9 significantly increased the mRNA and protein levels of RGNNV capsid protein in LJB cells infected with RGNNV, nevertheless knockdown of LjUBC9 had the opposite effect, suggesting that LjUBC9 exerted a pro-viral effect during RGNNV infection. More importantly, we found that the 93rd cysteine is crucial for its pro-viral effect. Additionally, dual luciferase assays revealed that LjUBC9 prominently attenuated the promoter activities of sea perch type Ⅰ interferon (IFN) in RGNNV-infected cells, and overexpression of LjUBC9 markedly suppressed the transcription of key genes associated with RLRs-IFN pathway. In summary, these findings elucidate that LjUBC9 impairs the RLRs-IFN response, resulting in enhanced RGNNV infection.

Characterization and functional analysis of SOCS9 from orange-spotted grouper (Epinephelus coioides) during virus infection.

The suppressor of cytokine signaling (SOCS) proteins family have twelve members including eight known mammalian SOCS members (CISH, SOCS1-7) and four new discovery members (SOCS3b, SOCS5b, SOCS8 and SOCS9) that is regarded as a classic feedback inhibitor of cytokine signaling. Although the function of the mammalian SOCS proteins have been well studied, little is known about the roles of SOCS in fish during viral infection. In this study, the molecular characteristics of SOCS9 from orange-spotted grouper (Epinephelus coioides, EcSOCS9) is investigated. The EcSOCS9 protein encoded 543 amino acids with typical SH2 (389-475aa) and SOCS_box (491-527aa), sharing high identities with reported fish SOCS9. EcSOCS9 was expressed in all detected tissues and highly expressed in kidney. After red-spotted grouper nervous necrosis virus (RGNNV) infection, the expression of EcSOCS9 was significantly induced in vitro. Furthermore, EcSOCS9 overexpression enhanced RGNNV replication, promoted virus-induced mitophagy that evidenced by the increased level of LC3-Ⅱ, BCL2, PGAM5 and decreased level of BNIP3 and FUNDC1. Besides, EcSOCS9 overexpression suppressed the expression levels of ATP6, CYB, ND4, ATP level and induced ROS level. The expression levels of interferon (IFN) related factors (IRF1, IRF3, IRF7, P53), inflammatory factors (IL1-ß, IL8, TLR2, TNF-α) and IFN-3, ISRE, NF-κB, AP1 activities were also reduced by overexpressing EcSOCS9. These date suggests that EcSOCS9 impacts RGNNV infection through modulating mitophagy, regulating the expression levels of IFN- related and inflammatory factors, which will expand our understanding of fish immune responses during viral infection.

Detection of different Betanodavirus genotypes in wild fish from Spanish Atlantic coastal waters (Galicia, northwestern Spain).

OBJECTIVE: The nervous necrosis virus (NNV; genus Betanodavirus) is an aquatic pathogen that is responsible for a neurological disease affecting marine fish. Despite its almost worldwide distribution, global warming could favor the spread of NNV to new areas, highlighting the importance of conducting epidemiological surveys on both wild and farmed marine fish species. In this study, we assessed NNV prevalence in wild fish caught along the Galician Atlantic coast. METHODS: In total, 1277 fish were analyzed by reverse transcription real-time polymerase chain reaction. RESULT: Twenty two (1.72%) of those fish tested positive for NNV, including two species in which the pathogen had not yet been reported. CONCLUSION: The reassortant RGNNV/SJNNV (red-spotted grouper NNV/striped jack NNV) was detected in 55% of NNV-positive individuals, while the remaining 45% harbored the SJNNV-type genome. Moreover, from European Pilchard Sardina pilchardus and Atlantic Mackerel Scomber scombrus, we isolated four reassortant strains that carried amino acid mutations at key sites related to NNV-host interaction.

Selection and characterization of ssDNA aptamer targeting Macrobrachium rosenbergii nodavirus capsid protein: A potential capture agent in gold-nanoparticle-based aptasensor for viral protein detection.

The giant freshwater prawn holds a significant position as a valuable crustacean species cultivated in the aquaculture industry, particularly well-known and demanded among the Southeast Asian countries. Aquaculture production of this species has been impacted by Macrobrachium rosenbergii nodavirus (MrNV) infection, which particularly affects the larvae and post-larvae stages of the prawn. The infection has been recorded to cause mortality rates of up to 100% among the affected prawns. A simple, fast, and easy to deploy on-site detection or diagnostic method is crucial for early detection of MrNV to control the disease outbreak. In the present study, novel single-stranded DNA aptamers targeting the MrNV capsid protein were identified using the systematic evolution of ligands by exponential enrichment (SELEX) approach. The aptamer was then conjugated with the citrate-capped gold nanoparticles (AuNPs), and the sensitivity of this AuNP-based aptasensor for the detection of MrNV capsid protein was evaluated. Findings revealed that the aptamer candidate, APT-MrNV-CP-1 was enriched throughout the SELEX cycle 4, 9, and 12 with the sequence percentage of 1.76%, 9.09%, and 12.42%, respectively. The conjugation of APT-MrNV-CP-1 with citrate-capped AuNPs exhibited the highest sensitivity in detecting the MrNV capsid protein, where the presence of 62.5 nM of the viral capsid protein led to a significant agglomeration of the AuNPs. This study demonstrated the practicality of an AuNP-based aptasensor for disease diagnosis, particularly for detecting MrNV infection in giant freshwater prawns.

Cytotoxic activity and gene expression during in vitro adaptive cell-mediated cytotoxicity of head-kidney cells from betanodavirus-infected European sea bass.

Cell-mediated cytotoxicity (CMC) is essential in eradicating virus-infected cells, involving CD8+ T lymphocytes (CTLs) and natural killer (NK) cells, through the activation of different pathways. This immune response is well-studied in mammals but scarcely in teleost fish. Our aim was to investigate the adaptive CMC using head-kidney (HK) cells from European sea bass infected at different times with nodavirus (NNV), as effector cells, and the European sea bass brain cell line (DLB-1) infected with different NNV genotypes, as target cells. Results showed low and unaltered innate cytotoxic activity through the infection time. However, adaptive CMC against RGNNV and SJNNV/RGNNV-infected target cells increased from 7 to 30 days post-infection, peaking at 15 days, demonstrating the specificity of the cytotoxic activity and suggesting the involvement of CTLs. At transcriptomic level, we observed up-regulation of genes related to T cell activation, perforin/granzyme and Fas/FasL effector pathways as well as apoptotic cell death. Further studies are necessary to understand the adaptive role of European sea bass CTLs in the elimination of NNV-infected cells.

Identification of candidate SNPs and genes associated with resistance to nervous necrosis virus in leopard coral grouper (Plectropomus leopardus) using GWAS.

The leopard coral grouper (Plectropomus leopardus), which has become increasingly popular in consumption due to its bright body color and great nutritional, holds a high economic and breeding potential. However, in recent years, the P.leopardus aquaculture industry has been impeded by the nervous necrosis virus (NNV) outbreak, leading to widespread mortality among fry and juvenile grouper. However, the genetic basis of resistance to NNV in P. leopardus remains to be investigated. In the present study, we conducted a genome-wide association analysis (GWAS) on 100 resistant and 100 susceptible samples to discover variants and potential genes linked with NNV resistance. For this study, 157,926 high-quality single nucleotide polymorphisms (SNPs) based on whole genome resequencing were discovered, and eighteen SNPs loci linked to disease resistance were discovered. We annotated six relevant candidate genes, including sik2, herc2, pip5k1c, npr1, mybpc3, and arhgap9, which showed important roles in lipid metabolism, oxidative stress, and neuronal survival. In the brain tissues of resistant and susceptible groups, candidate genes against NNV infection showed significant differential expression. The results indicate that regulating neuronal survival or pathways involved in lipid metabolism may result in increased resistance to NNV. Understanding the molecular mechanisms that lead to NNV resistance will be beneficial for the growth of the P. leopardus breeding sector. Additionally, the identified SNPs could be employed as biomarkers of disease resistance in P. leopardus, which will facilitate the selective breeding of grouper.

Orange-spotted grouper IFNh response to NNV or MSRV and its potential antiviral activities.

Type I interferon (IFN) plays a crucial role in the antiviral immune response. Nervous necrosis virus (NNV) and Micropterus salmoides rhabdovirus (MSRV) are the most important viruses in cultured larvae and juveniles, causing great economic losses to fish farming. To better understand the antiviral activities and immunoregulatory role of IFN from orange-spotted grouper (Epinephelus coioides), EcIFNh was cloned from NNV infected sample. EcIFNh has an open reading frame (ORF) of 552 bp and encodes a polypeptide of 183 amino acids. Phylogenetic tree analysis showed that EcIFNh was clustered into the IFNh branch. The tissue distribution analysis revealed that EcIFNh was highly expressed in the liver and brain of healthy orange-spotted grouper. The mRNA levels of EcIFNh were significantly upregulated after poly (I:C) stimulation and NNV or MSRV infection. Furthermore, the promoter of EcIFNh was characterized and significantly activated by EcMDA5, EcMAVS, EcSTING, EcIRF3, and EcIRF7 in the luciferase activity assays. We found that EcIFNh overexpression resisted the replication of NNV and MSRV, while EcIFNh silencing facilitated NNV replication in GB cells. In addition, EcIFNh recombinant protein (rEcIFNh) enhanced the immune response by inducing the expression of ISGs in vivo and in vitro, suggesting the potential application of rEcIFNh for anti-NNV and anti-MSRV. Taken together, our research may offer the foundation for virus-IFN system interaction in orange-spotted grouper.

Nervous Necrosis Virus Modulation of European Sea Bass (Dicentrarchus labrax, L.) Immune Genes and Transcriptome towards Establishment of Virus Carrier State.

Viral infections of teleost fish have great environmental and economic implications in aquaculture. Nervous necrosis virus (NNV) is a pathogen affecting more than 120 different species, causing high mortality and morbidity. Herein, we studied the course of NNV experimental infection of D. labrax, focusing on survivors which indicated viral carrier state. To determine the carrier state of D. labrax head kidney, we performed a gene expression analysis of selected immune-related genes and we profiled its transcriptome 14 days post infection (dpi). All tested genes showed clear differentiations in expression levels while most of them were up-regulated 14 dpi suggesting that their role is not limited in early antiviral responses, but they are also implicated in disease persistence. To gain a better understanding of the fish that survived the acute infection but still maintained a high viral load, we studied the differential expression of 124 up-regulated and 48 down-regulated genes in D. labrax head kidney, at 14 dpi. Concluding, the NNV virus persistent profile was assessed in D. labrax, where immune-related gene modification was intense (14 dpi) and the head kidney transcriptome profile at this time point offered a glimpse into host attempts to control the infection in asymptomatic carriers.

Grouper Rab1 inhibits nodovirus infection by affecting virus entry and host immune response.

Rab1, a GTPase, is present in all eukaryotes, and is mainly involved in vesicle trafficking between the endoplasmic reticulum and Golgi, thereby regulating many cellular activities and pathogenic infections. However, little is known of how Rab1 functions in fish during virus infection. Groupers (Epinephelus spp.) are high in economic value and widely cultivated in China and Southeast Asia, although they often suffer from diseases. Red-spotted grouper nervous necrosis virus (RGNNV), a highly pathogenic RNA virus, is a major pathogen in cultured groupers, and causes huge economic losses. A series of host cellular proteins involved in RGNNV infection was identified. However, the impact of Rab1 on RGNNV infection has not yet been reported. In this study, a novel Rab1 homolog (EcRab1) from Epinephelus coioides was cloned, and its roles during virus infection and host immune responses were investigated. EcRab1 encoded a 202 amino acid polypeptide, showing 98% and 78% identity to Epinephelus lanceolatus and Homo sapiens, respectively. After challenge with RGNNV or poly(I:C), the transcription of EcRab1 was altered both in vitro and in vivo, implying that EcRab1 was involved in virus infection. Subcellular localization showed that EcRab1 was displayed as punctate structures in the cytoplasm, which was affected by EcRab1 mutants. The dominant negative (DN) EcRab1, enabling EcRab1 to remain in the GDP-binding state, caused EcRab1 to be diffusely distributed in the cytoplasm. Constitutively active (CA) EcRab1, enabling EcRab1 to remain in the GTP-binding state, induced larger cluster structures of EcRab1. During the late stage of RGNNV infection, some EcRab1 co-localized with RGNNV, and the size of EcRab1 clusters was enlarged. Importantly, overexpression of EcRab1 significantly inhibited RGNNV infection, and knockdown of EcRab1 promoted RGNNV infection. Furthermore, EcRab1 inhibited the entry of RGNNV to host cells. Compared with EcRab1, overexpression of DN EcRab1 or CA EcRab1 also promoted RGNNV infection, suggesting that EcRab1 regulated RGNNV infection, depending on the cycles of GTP- and GDP-binding states. In addition, EcRab1 positively regulated interferon (IFN) immune and inflammatory responses. Taken together, these results suggest that EcRab1 affects RGNNV infection, possibly by regulating host immunity. Our study furthers the understanding of Rab1 function during virus infection, thus helping to design new antiviral strategies.

Genomic characterization and transcription analysis of European sea bass (Dicentrarchus labrax) rtp3 genes.

Fish RTP3, belonging to the receptor-transporting protein family, display several functions, including a putative antiviral role as virus-responsive gene. In this work, we have identified and characterized two different European sea bass rtp3 genes. In addition, an in vivo transcription analysis in response to LPS, poly I:C and betanodavirus infection (RGNNV genotype) has been performed. The sequence analysis showed that European sea bass displays two rtp3 genes, X1 and X2, composed of two exons and a single intron (1007-bp and 888-bp long, respectively), located within the ORF sequence. The full-length cDNA is 1969 bp for rtp3 X1, and 1491 bp for rtp3 X2. Several ATTTA motifs have been found in the intron sequence of both genes, whereas rtp3 X1 also contains this motif in both untranslated regions. The transcription analyses revealed significant level of rtp3 X2 mRNA in brain and head kidney after LPS and poly I:C inoculation; however, the induction elicited by RGNNV infection was much higher, suggesting an essential role for this protein in controlling NNV infections.

The Viromes of Mosquitoes from the Natural Landscapes of Western Siberia.

The metagenomic analysis of mosquitoes allows for the genetic characterization of mosquito-associated viruses in different regions of the world. This study applied a metagenomic approach to identify novel viral sequences in seven species of mosquitoes collected from the Novosibirsk region of western Siberia. Using NGS sequencing, we identified 15 coding-complete viral polyproteins (genomes) and 15 viral-like partial sequences in mosquitoes. The complete sequences for novel viruses or the partial sequences of capsid proteins, hypothetical viral proteins, and RdRps were used to identify their taxonomy. The novel viral sequences were classified within the orders Tymovirales and Picornavirales and the families Partitiviridae, Totiviridae, Tombusviridae, Iflaviridae, Nodaviridae, Permutotetraviridae, and Solemoviridae, with several attributed to four unclassified RNA viruses. Interestingly, the novel putative viruses and viral sequences were mainly associated with the mosquito Coquillettidia richardii. This study aimed to increase our understanding of the viral diversity in mosquitoes found in the natural habitats of Siberia, which is characterized by very long, snowy, and cold winters.

Methyltransferase-like 3 suppresses red spotted grouper nervous necrosis virus and viral hemorrhagic septicemia virus infection by enhancing type I interferon responses in sea perch.

Methylation at the N6 position of adenosine (m6A) is the most abundant internal mRNA modification in eukaryotes, tightly associating with regulation of viral life circles and immune responses. Here, a methyltransferase-like 3 homolog gene from sea perch (Lateolabrax japonicus), designated LjMETTL3, was cloned and characterized, and its negative role in fish virus pathogenesis was uncovered. The cDNA of LjMETTL3 encoded a 601-amino acid protein with a MT-A70 domain, which shared the closest genetic relationship with Echeneis naucrates METTL3. Spatial expression analysis revealed that LjMETTL3 was more abundant in the immune tissues of sea perch post red spotted grouper nervous necrosis virus (RGNNV) or viral hemorrhagic septicemia virus (VHSV) infection. LjMETTL3 expression was significantly upregulated at 12 and 24 h post RGNNV and VHSV infection in vitro. In addition, ectopic expression of LjMETTL3 inhibited RGNNV and VHSV infection in LJB cells at 12 and 24 h post infection, whereas knockdown of LjMETTL3 led to opposite effects. Furthermore, we found that LjMETTL3 may participate in boosting the type I interferon responses by interacting with TANK-binding kinase. Taken together, these results disclosed the antiviral role of fish METTL3 against RGNNV and VHSV and provided evidence for understanding the potential mechanisms of fish METTL3 in antiviral innate immunity.

In vitro infection efficiency of nervous necrosis virus alters depending on amount of viral particles adsorbed onto cells.

Nervous necrosis virus (NNV) in the family Nodaviridae is one of the simplest spherical RNA viruses and is pathogenic to many fish species. We investigated the effect of purified NNV on striped snakehead cells (SSN-1) in terms of adsorption ratio and infection efficiency using the 96-well titration system. The proportion of cytopathic effect (CPE)-positive wells among total number of wells inoculated with the virus (CPE appearance ratio) reduced by 17% each time the NNV infectivity dose was halved (y = 55.7x + 50.6). Thus, subtle differences in NNV infectivity could be accurately detected using this system. Experiments performed to observe alteration of CPE appearance ratio with changing viral doses and adsorption times showed that NNV particles introduced into microplate wells as suspensions in ≤ 100 µl inoculum were adsorbed almost completely onto cells seeded on the wells within 4 days of incubation. Density profile analysis of NNV coat proteins revealed that the NNV suspension at 1 50% tissue culture infectious dose (TCID50) contained 60 particles. Infection efficiency/NNV peaked at 20 particles (1.20%/particle) and then declined gradually with increasing NNV doses. Therefore, in vitro infection efficiency of NNV may alter depending on the quantity of viral particles adsorbed onto cells.

Novel insights into virus-host interactions using the model organism C. elegans.

Viruses continue to pose a public health threat raising the need for effective management strategies. Currently existing antiviral therapeutics are often specific to only a single viral species, and resistance to the therapeutic can often arise, and therefore new therapeutics are needed. The C. elegans-Orsay virus system offers a powerful platform for studying RNA virus-host interactions that could ultimately lead to novel targets for antiviral therapy. The relative simplicity of C. elegans, the well-established experimental tools, and its extensive evolutionary conservation of genes and pathways with mammals are key features of this model. Orsay virus, a bisegmented positive sense RNA virus, is a natural pathogen of C. elegans. Orsay virus infection can be studied in a multicellular organismal context, overcoming some of the limitations inherent to tissue culture-based systems. Moreover, compared to mice, the rapid generation time of C. elegans enables robust and facile forward genetics. This review aims to summarize studies that have laid the foundation for the C. elegans-Orsay virus experimental system, experimental tools, and key examples of C. elegans host factors that impact Orsay virus infection that have evolutionarily conserved function in mammalian virus infection.

Ring Finger Protein 34 Facilitates Nervous Necrosis Virus Evasion of Antiviral Innate Immunity by Targeting TBK1 and IRF3 for Ubiquitination and Degradation in Teleost Fish.

Ubiquitination, as one of the most prevalent posttranslational modifications of proteins, enables a tight control of host immune responses. Many viruses hijack the host ubiquitin system to regulate host antiviral responses for their survival. Here, we found that the fish pathogen nervous necrosis virus (NNV) recruited Lateolabrax japonicus E3 ubiquitin ligase ring finger protein 34 (LjRNF34) to inhibit the RIG-I-like receptor (RLR)-mediated interferon (IFN) response via ubiquitinating Lateolabrax japonicus TANK-binding kinase 1 (LjTBK1) and interferon regulatory factor 3 (LjIRF3). Ectopic expression of LjRNF34 greatly enhanced NNV replication and prevented IFN production, while deficiency of LjRNF34 led to the opposite effect. Furthermore, LjRNF34 targeted LjTBK1 and LjIRF3 via its RING domain. Of note, the interactions between LjRNF34 and LjTBK1 or LjIRF3 were conserved in different cellular models derived from fish. Mechanically, LjRNF34 promoted K27- and K48-linked ubiquitination and degradation of LjTBK1 and LjIRF3, which in turn diminished LjTBK1-induced translocation of LjIRF3 from the cytoplasm to the nucleus. Ultimately, NNV capsid protein (CP) was found to bind with LjRNF34, CP induced LjTBK1 and LjIRF3 degradation, and IFN suppression depended on LjRNF34. Our finding demonstrates a novel mechanism by which NNV CP evaded host innate immunity via LjRNF34 and provides a potential drug target for the control of NNV infection. IMPORTANCE Ubiquitination plays an essential role in the regulation of innate immune responses to pathogens. NNV, a type of RNA virus, is the causal agent of a highly destructive disease in a variety of marine and freshwater fish. A previous study reported NNV could hijack the ubiquitin system to manipulate the host's immune responses; however, how NNV utilizes ubiquitination to facilitate its own replication is not well understood. Here, we identified a novel distinct role of E3 ubiquitin ligase LjRNF34 as an IFN antagonist to promote NNV infection. NNV capsid protein utilized LjRNF34 to target LjTBK1 and LjIRF3 for K27- and K48-linked ubiquitination and degradation. Importantly, the interactions between LjRNF34 and CP, LjTBK1, or LjIRF3 are conserved in different cellular models derived from fish, suggesting it is a general immune evasion strategy exploited by NNV to target the IFN response via RNF34.

Cells and Fugu Response to Capsid of BFNNV Genotype.

The nervous necrosis virus (NNV) of the BFNNV genotype is the causative agent of viral encephalopathy and retinopathy (VER) in cold water fishes. Similar to the RGNNV genotype, BFNNV is also considered a highly destructive virus. In the present study, the RNA2 of the BFNNV genotype was modified and expressed in the EPC cell line. The subcellular localization results showed that the capsid and N-terminal (1-414) were located in the nucleus, while the C-terminal (415-1014) of the capsid was located in the cytoplasm. Meanwhile, cell mortality obviously increased after expression of the capsid in EPC. EPC cells were transfected with pEGFP-CP and sampled at 12 h, 24 h and 48 h for transcriptome sequencing. There are 254, 2997 and 229 up-regulated genes and 387, 1611, and 649 down-regulated genes post-transfection, respectively. The ubiquitin-activating enzyme and ubiquitin-conjugating enzyme were up-regulated in the DEGs, indicating that cell death evoked by capsid transfection may be related to ubiquitination. The qPCR results showed that heat stock protein 70 (HSP70) is extremely up-regulated after expression of BFNNV capsid in EPC, and N-terminal is the key region to evoke the high expression. For further study, the immunoregulation of the capsid in fish pcDNA-3.1-CP was constructed and injected into the Takifugu rubripes muscle. pcDNA-3.1-CP can be detected in gills, muscle and head kidney, and lasted for more than 70 d post-injection. The transcripts of IgM and interferon inducible gene Mx were up-regulated after being immunized in different tissues, and immune factors, such as IFN-γ and C3, were also up-regulated in serum, while C4 was down-regulated one week after injection. It was suggested that pcDNA-3.1-CP can be a potential DNA vaccine in stimulating the immune system of T. rubripes; however, NNV challenge needs to be conducted in the following experiments.

Grouper annexin A2 affects RGNNV by regulating the host immune response.

Annexin A2 (AnxA2) is ubiquitous in vertebrates and has been identified as a multifunctional protein participating in a series of biological processes, such as endocytosis, exocytosis, signal transduction, transcription regulation, and immune responses. However, the function of AnxA2 in fish during virus infection still remains unknown. In this study, we identified and characterized AnxA2 (EcAnxA2) in Epinephelus coioides. EcAnxA2 encoded a 338 amino acids protein with four identical annexin superfamily conserved domains, which shared high identity with other AnxA2 of different species. EcAnxA2 was widely expressed in different tissues of healthy groupers, and its expression was significantly increased in grouper spleen cells infected with red-spotted grouper nervous necrosis virus (RGNNV). Subcellular locatio n analyses showed that EcAnxA2 diffusely distributed in the cytoplasm. After RGNNV infection, the spatial distribution of EcAnxA2 was unaltered, and a few EcAnxA2 co-localized with RGNNV during the late stage of infection. Furthermore, overexpression of EcAnxA2 significantly increased RGNNV infection, and knockdown of EcAnxA2 reduced RGNNV infection. In addition, overexpressed EcAnxA2 reduced the transcription of interferon (IFN)-related and inflammatory factors, including IFN regulatory factor 7 (IRF7), IFN stimulating gene 15 (ISG15), melanoma differentiation related gene 5 (MDA5), MAX interactor 1 (Mxi1) laboratory of genetics and physiology 2 (LGP2), IFN induced 35 kDa protein (IFP35), tumor necrosis factor receptor-associated factor 6 (TRAF6) and interleukin 6 (IL-6). The transcription of these genes was up-regulated when EcAnxA2 was inhibited by siRNA. Taken together, our results showed that EcAnxA2 affected RGNNV infection by down-regulating the host immune response in groupers, which provided new insights into the roles of AnxA2 in fish during virus infection.