MAVS splicing variants contribute to the induction of interferon and interferon-stimulated genes mediated by RIG-I-like receptors

MAVS disruption impairs downstream signaling and results in higher virus replication levels of salmonid alphavirus subtype 3 but not infectious pancreatic necrosis virus in vitro

The mitochondrial anti-viral signaling (MAVS) protein is an intermediary adaptor protein of retinoic acid-inducible gene-1 (RIG-I) like receptor (RLR) signaling, which activates the transcription factor interferon (IFN) regulatory factor 3 (IRF3) and NF-kB to produce type I IFNs. MAVS expression has been reported in different fish species, but few studies have shown its functional role in anti-viral responses to fish viruses. In this study, we used the transcription activator-like effector nuclease (TALEN) as a gene editing tool to disrupt the function of MAVS in Chinook salmon (Oncorhynchus tshawytscha) embryonic cells (CHSE) to understand its role in induction of interferon I responses to infections with the (+) RNA virus salmonid alphavirus subtype 3 (SAV-3), and the dsRNA virus infectious pancreatic necrosis virus (IPNV) infection. A MAVS-disrupted CHSE clone with a 7-aa polypeptide (GVFVSRV) deletion mutation at the N-terminal of the CARD domain infected with SAV-3 resulted in significantly lower expression of IRF3, IFNa, and ISGs and increased viral titer (1.5 log10) compared to wild-type. In contrast, the IPNV titer in MAVS-disrupted cells was not different from the wild-type. Furthermore, overexpression of salmon MAVS in MAVS-disrupted CHSE cells rescued the impaired type I IFN-mediated anti-viral effect against SAV-3.

Role of alternative splicing in fish immunity

Alternative splicing serves as a pivotal source of complexity in the transcriptome and proteome, selectively connecting various coding elements to generate a diverse array of mRNAs. This process encodes multiple proteins with either similar or distinct functions, contributing significantly to the intricacies of cellular processes. The role of alternative splicing in mammalian immunity has been well studied. Remarkably, the immune system of fish shares substantial similarities with that of humans, and alternative splicing also emerges as a key player in the immune processes of fish. In this review, we offer an overview of alternative splicing and its associated functions in the immune processes of fish, and summarize the research progress on alternative splicing in the fish immunity. Furthermore, we review the impact of alternative splicing on the fish immune system's response to external stimuli. Finally, we present our perspectives on future directions in this field. Our aim is to provide valuable insights for the future investigations into the role of alternative splicing in immunity.

Zebrafish spop promotes ubiquitination and degradation of mavs to suppress antiviral response via the lysosomal pathway

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) signaling pathways are required to be tightly controlled to initiate host innate immune responses. Fish mitochondrial antiviral signaling (mavs) is a key determinant in the RLR pathway, and its ubiquitination is associated with mavs activation. Here, we identified the zebrafish E3 ubiquitin ligase Speckle-type BTB-POZ protein (spop) negatively regulates mavs-mediated the type I interferon (IFN) responses. Consistently, overexpression of zebrafish spop repressed the activity of IFN promoter and reduced host ifn transcription, whereas knockdown spop by small interfering RNA (siRNA) transfection had the opposite effects. Accordingly, overexpression of spop dampened the cellular antiviral responses triggered by spring viremia of carp virus (SVCV). A functional domain assay revealed that the N-terminal substrate-binding MATH domain regions of spop were necessary for IFN suppression. Further assays indicated that spop interacts with mavs through the C-terminal transmembrane (TM) domain of mavs. Moreover, zebrafish spop selectively promotes K48-linked polyubiquitination and degradation of mavs through the lysosomal pathway to suppress IFN expression. Our findings unearth a post-translational mechanism by which mavs is regulated and reveal a role for spop in inhibiting antiviral innate responses.

Liver X Receptor LXRα Promotes Grass Carp Reovirus Infection by Attenuating IRF3-CBP Interaction and Inhibiting RLR Antiviral Signaling

Liver X receptors (LXRs) are nuclear receptors involved in metabolism and the immune response. Different from mammalian LXRs, which include two isoforms, LXRα and LXRβ, only a single LXRα gene exists in the piscine genomes. Although a study has suggested that piscine LXR inhibits intracellular bacterial survival, the functions of piscine LXRα in viral infection are unknown. In this study, we show that overexpression of LXRα from grass carp (Ctenopharyngodon idellus), which is named as gcLXRα, increases host susceptibility to grass carp reovirus (GCRV) infection, whereas gcLXRα knockdown in CIK (C. idellus kidney) cells inhibits GCRV infection. Consistent with these functional studies, gcLXRα knockdown promotes the transcription of antiviral genes involved in the RIG-I-like receptor (RLR) antiviral signaling pathway, including IFN regulatory factor (IRF3) and the type I IFN IFN1. Further results show that gcLXRα knockdown induces the expression of CREB-binding protein (CBP), a transcriptional coactivator. In the knockdown of CBP, the inhibitory effect of gcLXRα knockdown in limiting GCRV infection is completely abolished. gcLXRα also interacts with IRF3 and CBP, which impairs the formation of the IRF3/CBP transcription complex. Moreover, gcLXRα heterodimerizes with RXRg, which cooperatively impair the transcription of the RLR antiviral signaling pathway and promote GCRV infection. Taken together, to our knowledge, our findings provide new insight into the functional correlation between nuclear receptor LXRα and the RLR antiviral signaling pathway, and they demonstrate that gcLXRα can impair the RLR antiviral signaling pathway and the production of type I IFN via forming gcLXRα/RXRg complexes and attenuating IRF3/CBP complexes.

Transcriptomic profiling of clobetasol propionate-induced immunosuppression in challenged zebrafish embryos

In the ecotoxicological hazard assessment of chemicals, the detection of immunotoxicity is currently neglected. This is mainly due to the complexity of the immune system and the consequent lack of standardized procedures and markers for the comprehensive assessment of immunotoxic modes of action. In this study, we present a new approach applying transcriptome profiling to an immune challenge with a mixture of pathogen-associated molecular patterns (PAMPs) in zebrafish embryos, analyzing differential gene expression during acute infection with and without prior exposure to the immunosuppressive drug clobetasol propionate (CP). While PAMP injection itself triggered biological processes associated with immune activation, some of these genes were more differentially expressed upon prior exposure to CP than by immune induction alone, whereas others showed weaker or no differential regulation in response to the PAMP stimulus. All of these genes responding differently to PAMP after prior CP exposure showed additivity of PAMP- and CP-induced effects, indicating independent regulatory mechanisms. The transcriptomic profiles suggest that CP impaired innate immune induction by attenuating the response of genes involved in antigen processing, TLR signaling, NF-КB signaling, and complement activation. We propose this approach as a powerful method for detecting gene biomarkers for immunosuppressive modes of action, as it was able to identify alternatively regulated processes and pathways in a sublethal, acute infection zebrafish embryo model. This allowed to define biomarker candidates for immune-mediated effects and to comprehensively characterize immunosuppression. Ultimately, this work contributes to the development of molecular biomarker-based environmental hazard assessment of chemicals in the future.

Histone H2A Nuclear/Cytoplasmic Trafficking Is Essential for Negative Regulation of Antiviral Immune Response and Lysosomal Degradation of TBK1 and IRF3

Histone H2A is a nuclear molecule tightly associated in the form of the nucleosome. Our previous studies have demonstrated the antibacterial property of piscine H2A variants against gram-negative bacteria Edwardsiella piscicida and Gram-positive bacteria Streptococcus agalactiae. In this study, we show the function and mechanism of piscine H2A in the negative regulation of RLR signaling pathway and host innate immune response against spring viremia of carp virus (SVCV) infection. SVCV infection significantly inhibits the expression of histone H2A during an early stage of infection, but induces the expression of histone H2A during the late stage of infection such as at 48 and 72 hpi. Under normal physiological conditions, histone H2A is nuclear-localized. However, SVCV infection promotes the migration of histone H2A from the nucleus to the cytoplasm. The in vivo studies revealed that histone H2A overexpression led to the increased expression of SVCV gene and decreased survival rate. The overexpression of histone H2A also significantly impaired the expression levels of those genes involved in RLR antiviral signaling pathway. Furthermore, histone H2A targeted TBK1 and IRF3 to promote their protein degradation via the lysosomal pathway and impair the formation of TBK1-IRF3 functional complex. Importantly, histone H2A completely abolished TBK1-mediated antiviral activity and enormously impaired the protein expression of IRF3, especially nuclear IRF3. Further analysis demonstrated that the inhibition of histone H2A nuclear/cytoplasmic trafficking could relieve the protein degradation of TBK1 and IRF3, and blocked the negative regulation of histone H2A on the SVCV infection. Collectively, our results suggest that histone H2A nuclear/cytoplasmic trafficking is essential for negative regulation of RLR signaling pathway and antiviral immune response in response to SVCV infection.

MAVS splicing variants associated with TRAF3 and TRAF6 in NF-κB and IRF3 signaling pathway in large yellow croaker Larimichthys crocea

Mitochondrial antiviral signaling protein (MAVS) acts as an essential adaptor in host RIG-I-like receptors (RLRs) mediated antiviral signaling pathway. In the present study, two MAVS transcript variants, the typical form and a splicing variant, namely Lc-MAVS_tv1 and Lc-MAVS_tv2 were characterized in large yellow croaker (Larimichthys crocea). The putative Lc-MAVS_tv1 protein contains 512 aa, with an N-terminal CARD domain, a central proline-rich region, and a C-terminal transmembrane (TM) domain, whereas Lc-MAVS_tv2 contains 302 aa and lacks the C-terminal TM domain due to a premature stop in the 102 bp intron fragment insertion. Lc-MAVS_tv1 was identified as a mitochondrion localized protein whereas Lc-MAVS_tv2 exhibited an entire cytosolic distribution. Quantitative real-time PCR revealed that Lc-MAVS_tv1 mRNA was broadly expressed in examined organs/tissues and showed extremely higher level than that of Lc-MAVS_tv2, and both of them could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Interestingly, overexpression of Lc-MAVS_tv2 could induce the activation of NF-κB but not IRF3, and Lc-MAVS_tv2 co-transfected with Lc-MAVS_tv1 induced a significantly higher level of NF-κB and IRF3 promoter activity. In addition, Lc-MAVS_tv2 overexpression could enhance TRAF3 and TRAF6 mediated NF-κB activation, but suppress TRAF3 and TRAF6 mediated IRF3 activation, implying that the splicing variant Lc-MAVS_tv2 may function as an important regulator in MAVS mediated signaling pathway.

Genomic analysis and functional characterization of immune genes from the RIG-I- and MAVS-mediated antiviral signaling pathway in lamprey

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are well-known viral RNA sensors in the cytoplasm. RIG-I-mediated antiviral signals are activated by interacting with the adapter protein mitochondrial antiviral signaling (MAVS), which triggers interferon (IFN) responses via a signaling cascade. Although the complete RIG-I receptor signaling pathway has been traced back to teleosts, definitive evidence of its presence in lampreys is lacking. Here, we identified 13 pivotal molecules in the RIG-I signaling pathway in lamprey, and demonstrated that the original RIG-I/MAVS signaling pathway was activated and mediated the expression of unique immunity factors such as RRP4, to inhibit viral proliferation after viral infection in vivo and in vitro. This study confirmed the conservation of the RIG-I pathway, and the uniqueness of the RRP4 effector molecule in lamprey, and further clarified the evolutionary process of the RIG-I antiviral signaling pathway, providing evidence on the origins of innate antiviral immunity in vertebrates.

Grass carp cGASL negatively regulates interferon activation through autophagic degradation of MAVS

In mammals, cyclic GMP-AMP synthase (cGAS) is a crucial cytosolic DNA sensor responsible for activating the interferon (IFN) response. A cGAS-like (cGASL) gene was previously identified from grass carp Ctenopharyngodon idellus, which is evolutionarily closest to cGAS but not a true ortholog of cGAS. Here, we found that grass carp cGASL targets mitochondrial antiviral signaling protein (MAVS) for autophagic degradation to negatively regulate fish IFN response. Firstly, the transcriptional level of cellular cgasl was upregulated by poly I:C stimulation, and overexpression of cGASL significantly decreased poly I:C- and MAVS-induced promoter activities and transcriptional levels of IFN and IFN-stimulated genes (ISGs). In addition, cGASL associated with MAVS and prompted autophagic degradation of MAVS in a dose-dependent manner. Finally, overexpression of cGASL attenuated MAVS-mediated cellular antiviral response. These results collectively indicate that cGASL negatively regulates fish IFN response by triggering autophagic degradation of MAVS.

Cytosolic Sensors for Pathogenic Viral and Bacterial Nucleic Acids in Fish

Recognition of the non-self signature of invading pathogens is a crucial step for the initiation of the innate immune mechanisms of the host. The host response to viral and bacterial infection involves sets of pattern recognition receptors (PRRs), which bind evolutionarily conserved pathogen structures, known as pathogen-associated molecular patterns (PAMPs). Recent advances in the identification of different types of PRRs in teleost fish revealed a number of cytosolic sensors for recognition of viral and bacterial nucleic acids. These are DExD/H-box RNA helicases including a group of well-characterized retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) and non-RLR DExD/H-box RNA helicases (e.g., DDX1, DDX3, DHX9, DDX21, DHX36 and DDX41) both involved in recognition of viral RNAs. Another group of PRRs includes cytosolic DNA sensors (CDSs), such as cGAS and LSm14A involved in recognition of viral and intracellular bacterial dsDNAs. Moreover, dsRNA-sensing protein kinase R (PKR), which has a role in antiviral immune responses in higher vertebrates, has been identified in fish. Additionally, fish possess a novel PKR-like protein kinase containing Z-DNA binding domain, known as PKZ. Here, we review the current knowledge concerning cytosolic sensors for recognition of viral and bacterial nucleic acids in teleosts.

A Novel Transcript Isoform of TBK1 Negatively Regulates Type I IFN Production by Promoting Proteasomal Degradation of TBK1 and Lysosomal Degradation of IRF3

TANK-binding kinase 1 (TBK1), an IKK-related serine/threonine kinase, is pivotal for the induction of antiviral type I interferon (IFN) by TLR and RLR signaling pathways. In a previous study, we demonstrated that TBK1 spliced isoforms (TBK1_tv1 and TBK1_tv2) from zebrafish were dominant negative regulators in the RLR antiviral pathway by targeting the functional TBK1–IRF3 complex formation. In this study, we show that the third TBK1 isoform (namely TBK1_tv3) inhibits zebrafish type I IFN production by promoting TBK1 and IRF3 degradation. First, ectopic expression of TBK1_tv3 suppresses poly(I:C)- and Spring viremia of carp virus-induced type I IFN response, and also inhibits the up-regulation of IFN promoter activities stimulated by RIG-I, MDA5, MAVS, TBK1, and IRF3. Second, TBK1_tv3 targets TBK1 and IRF3 to impair the formation of TBK1 dimer, TBK1–IRF3 complex, and IRF3 dimer. Notably, TBK1_tv3 promotes the degradation of TBK1 through the ubiquitin–proteasome pathway and the degradation of IRF3 through the lysosomal pathway. Further analysis demonstrates that TBK1_tv3 promotes the degradation of TBK1 for K48-linked ubiquitination by targeting the K251, K256, and K271 sites of TBK1. Collectively, our results suggest a novel TBK1 isoform-mediated negative regulation mechanism, which serves to balance the production of type I IFN and ISGs.

Zebrafish RBM47 Promotes Lysosome-Dependent Degradation of MAVS to Inhibit IFN Induction

IFN is essential for hosts to defend against viral invasion, whereas it must be tightly regulated to prevent hyperimmune responses. Fish mitochondrial antiviral signaling protein (MAVS) is a vital factor for IFN production, but until now, there have been few studies on the regulation mechanisms of fish MAVS enabling IFN to be properly controlled. In this study, we show that zebrafish RNA-binding motif protein 47 (RBM47) promotes MAVS degradation in a lysosome-dependent manner to suppress IFN production. First, the transcription of IFN activated by polyinosinic/polycytidylic acid (poly I:C), spring viremia of carp virus, or retinoic acid-inducible gene I (RIG-I)-like receptor pathway components were significantly suppressed by RBM47. Second, RBM47 interacted with MAVS and promoted lysosome-dependent degradation of MAVS, changing the cellular location of MAVS from the cytoplasm to the lysosome region. Finally, RBM47 inhibited downstream MITA and IRF3/7 activation, impairing the host antiviral response. Collectively, these data suggest that zebrafish RBM47 negatively regulates IFN production by promoting lysosome-dependent degradation of MAVS, providing insights into the role of RBM47 in the innate antiviral immune response in fish.

NOD1 Promotes Antiviral Signaling by Binding Viral RNA and Regulating the Interaction of MDA5 and MAVS

Nucleotide oligomerization domain-like receptors (NLRs) and RIG-I-like receptors (RLRs) detect diverse pathogen-associated molecular patterns to activate the innate immune response. The role of mammalian NLR NOD1 in sensing bacteria is well established. Although several studies suggest NOD1 also plays a role in sensing viruses, the mechanisms behind this are still largely unknown. In this study, we report on the synergism and antagonism between NOD1 and MDA5 isoforms in teleost. In zebrafish, the overexpression of NOD1 enhances the antiviral response and mRNA abundances of key antiviral genes involved in RLR-mediated signaling, whereas the loss of NOD1 has the opposite effect. Notably, spring viremia of carp virus-infected NOD1^-/- zebrafish exhibit reduced survival compared with wild-type counterparts. Mechanistically, NOD1 targets MDA5 isoforms and TRAF3 to modulate the formation of MDA5-MAVS and TRAF3-MAVS complexes. The cumulative effects of NOD1 and MDA5a (MDA5 normal form) were observed for the binding with poly(I:C) and the formation of the MDA5a-MAVS complex, which led to increased transcription of type I IFNs and ISGs. However, the antagonism between NOD1 and MDA5b (MDA5 truncated form) was clearly observed during proteasomal degradation of NOD1 by MDA5b. In humans, the interactions between NOD1-MDA5 and NOD1-TRAF3 were confirmed. Furthermore, the roles that NOD1 plays in enhancing the binding of MDA5 to MAVS and poly(I:C) are also evolutionarily conserved across species. Taken together, our findings suggest that mutual regulation between NOD1 and MDA5 isoforms may play a crucial role in the innate immune response and that NOD1 acts as a positive regulator of MDA5/MAVS normal form-mediated immune signaling in vertebrates.

Genome-wide identification, characterization of RLR genes in Yesso scallop (Patinopecten yessoensis) and functional regulations in responses to ocean acidification

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), are crucial sensors with a conserved structure in cytoplasm, inducing the production of cytokines, chemokines and host restriction factors which mediate a variety of intracellular activities to interfere with distinct PAMPs (pathogen-associated molecular patterns) for eliminating pathogens in innate immune system. Although RLR genes have been investigated in most vertebrates and some invertebrates, the systematic identification and characterization of RLR genes have not been reported in scallops. In this study, four RLR genes (PY-10413.4, PY-10413.5, PY-443.7 and PY-443.8, designated PyRLRs) were identified in Yesso scallop (Patinopecten yessoensis) through whole-genome scanning through in silico analysis, including two pairs of tandem duplicate genes located on the same scaffold (PY-10413.4 and PY-10413.5, PY-443.7 and PY-443.8, respectively). Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of these genes. The expression profiles of PyRLRs were determined in all developmental stages, in healthy adult tissues, and in mantles that simulated ocean acidification (OA) exposure (pH = 6.5 and 7.5) at different time points (3, 6, 12 and 24 h). Spatiotemporal expression patterns suggested the functional roles of PyRLRs in all stages of development and growth of the scallop. Regulation expressions revealed PY-10413.4 and PY-10413.5 with one or two CARD(s) (caspase activation and recruitment domain) were up-regulated expressed at most time points, whereas PY-443.8 and PY-10413.4 without CARD were significantly down-regulated at each time points, suggesting functional differentiations in the two pairs of PyRLRs based on the structural differences in response to OA. Collectively, this study demonstrated gene duplication of RLR family genes and provide primary analysis for versatile roles in the response of the bivalve innate immune system to OA challenge.

The negative regulation of piscine CD44c in viral and bacterial infection

CD44 gene is a cell surface receptor which undergoes complex alternative splicing and extensive post-translational modifications. Although many studies have showed that CD44 is involved in the process of host defense, the function of piscine CD44 in antibacterial or antiviral defense response remains unclear. In the present study, we report the functional characterization of zebrafish CD44c, which is more similar to CD44b antigen isoforms rather than CD44a based on amino acid composition and phylogenetic analysis. The expression of zebrafish CD44c was inducible in response to bacterial and viral infections. During SVCV infection, the in vivo studies revealed that CD44c overexpression led to the increased virus loads and decreased survival rate. The attenuated response by zebrafish CD44c in response to SVCV infection were characterized by the impaired production of inflammatory cytokines and the impaired expressions of IFNs, IFN-stimulated genes, MHC class I and II genes. During Edwardsiella piscicida infection, the overexpression of zebrafish CD44c facilitated bacterial growth and dissemination, but did not impact on larvae survival. The detrimental role of CD44c in host defense against E. piscicida infection was supported by a decreased production of several antibacterial molecules including defbl2, defbl3, NK-lysin and RNase3. All together, these results firstly demonstrate the negative regulation of piscine CD44c in viral and bacterial infection.

MAVS of triploid hybrid of red crucian carp and allotetraploid possesses the improved antiviral activity compared with the counterparts of its parents

Triploid hybrid (3n = 150) of red crucian carp (♀, 2n = 100) and allotetraploid (♂, 4n = 200) presents the obviously stronger disease resistance than its parents. To elucidate the innate immunity of triploid hybrid, the MAVS homologues of triploid hybrid (3nMAVS), red crucian carp (2nMAVS) and allotetraploid (4nMAVS) have been identified and characterized separately in this study. 2nMAVS and 4nMAVS were evolutionarily conserved; however, 3nMAVS showed lower amino acid similarity and differently predicted structure to 2nMAVS or 4nMAVS. 3nMAVS transcription increase rate in host cells were obviously higher than 2nMAVS or 4nMAVS in response to different stimuli, which included spring viraemia of carp virus (SVCV), grass carp reovirus (GCRV) and poly (I:C). The reporter assay in EPC cells showed that 3nMAVS owned much stronger ability to induce the production of DrIFNφ1 and eIFN than either 2nMAVS or 4nMAVS. Accordingly, EPC cells transfected with 3nMAVS presented obviously stronger antiviral activity against both GCRV and SVCV than the cells expressing 2nMAVS or 4nMAVS. All the data support the conclusion that 3nMAVS-mediated antiviral signaling during innate immune activation was stronger than those of 2nMAVS and 4nMAVS, which provided us the new insight on the innate immune system of triploid hybrid.

Comparative study on pattern recognition receptors in non-teleost ray-finned fishes and their evolutionary significance in primitive vertebrates

Pattern recognition receptors (PRRs) play important roles in innate immunity system and trigger the specific pathogen recognition by detecting the pathogen-associated molecular patterns. The main four PRRs components including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), NOD-like receptors (NLRs) and C-type lectin receptors (CLRs) were surveyed in the five genomes of non-teleost ray-finned fishes (NTR) including bichir (Polypterus senegalus), American paddlefish (Polyodon spathula), alligator gar (Atractosteus spatula), spotted gar (Lepisosteus oculatus) and bowfin (Amia calva), representing all the four major basal groups of ray-finned fishes. The result indicates that all the four PRRs components have been well established in these NTR fishes. In the RLR-MAVS signal pathway, which detects intracellular RNA ligands to induce production of type I interferons (IFNs), the MAVS was lost in bichir particularly. Also, the essential genes of recognition of Lipopolysaccharide (LPS) commonly in mammals like MD2, LY96 and LBP could not be identified in NTR fishes. It is speculated that TLR4 in NTR fishes may act as a cooperator with other PRRs and has a different pathway of recognizing LPS compared with that in mammals. In addition, we provide a survey of NLR and CLR in NTR fishes. The CLRs results suggest that Group V receptors are absent in fishes and Group II and VI receptors are well established in the early vertebrate evolution. Our comprehensive research of PRRs involving NTR fishes provides a new insight into PRR evolution in primitive vertebrate.

Transcriptomic profiles of striped snakehead cells (SSN-1) infected with snakehead vesiculovirus (SHVV) identifying IFI35 as a positive factor for SHVV replication

Snakehead vesiculovirus (SHVV) has caused great economic loss in snakehead fish culture in China. However, there is no effective strategy to prevent the epidemic of the virus. Understanding the host factors in response to virus infection is the basis for the prevention of viral disease. In this study, the transcriptomic profiles of SHVV-infected and mock-infected SSN-1 cells (derived from striped snakehead, Channa striatus) at 3 and 24 h (h) post of infection (poi) were obtained using high-throughput sequencing technique. A total of 93,372 unigenes were obtained. The differently expressed genes (DEGs) of SSN-1 cells upon SHVV infection were thereby identified, including 3668 and 3536 DEGs at 3 and 24 h poi, respectively. These DEGs were involved in many pathways of viral pathogenesis, including retinoic acid-inducible gene I (RIG-I) like receptors pathway, Toll-like receptor signaling pathway, NF-kappa B signaling pathway, PI3K-Akt signaling pathway and MAPK signaling pathway. Therefore, several immune-related DEGs were randomly selected and confirmed by quantitative real-time PCR (qRT-PCR). In addition, the effects of the interferon inducible protein 35 (IFI35) on SHVV replication were further investigated. Over-expression or inhibition of IFI35 significantly promoted or reduced SHVV replication at the level of viral gene expression, which indicated that IFI35 might be a positive factor for SHVV replication in SSN-1 cells. Our findings presented some valuable information, which will benefit for future study on SHVV-host interactions.

The antiviral mechanism of viperin and its splice variant in spring viremia of carp virus infected fathead minnow cells

Viperin is known to play an important role in innate immune and its antiviral mechanisms are well demonstrated in mammals. Fish Viperin mediates antiviral activity against several viruses. However, little has been done to the underlying mechanism. Here, we discovered a novel Viperin splice variant named Viperin_sv1 from viral-infected FHM cells. Spring varimia of carp virus (SVCV) was able to increase the mRNA levels of both Viperin and Viperin_sv1, while poly(I:C) only has effect on Viperin. Viperin functions as an antiviral protein at 24 h post-SVCV infection, but the antiviral activity dramatically declined at late infection stages. However, Viperin_sv1 inhibited SVCV replication significantly at all the tested time. Viperin_sv1, but not Viperin can facilitate the production of type I IFN and IFN stimulate genes (ISGs) through activation of RIG-1, IRF3 and IRF7 signaling cascades. On the other hand, SVCV down-regulated Viperin_sv1 at the protein level through the proteasome pathway to keep itself away from the immune system monitoring. Taken together, these findings provide new insights into the regulation of Viperin from the posttranscriptional modification perspective and the role of splicing variant Viperin_sv1 in virus-host interaction.

Molecular characterization and function analysis of three RIG-I-like receptor signaling pathway genes (MDA5, LGP2 and MAVS) in Oreochromis niloticus

The recognition of microbial pathogens, which is mediated by pattern recognition receptors (PRRs), is critical to the initiation of innate immune responses. In the present study, we isolated the full-length cDNA and genomic DNA sequences of the MDA5, LGP2 and MAVS genes in Nile tilapia, termed OnMDA5, OnLGP2 and OnMAVS. The OnMDA5 gene encodes 974 amino acids and contains two caspase-associated recruitment domains (CARDs), a DExDc domain (DExD/H box-containing domain), a HELICc (helicase superfamily C-terminal) domain and a C-terminal regulatory domain (RD). The OnLGP2 gene encodes 679 amino acids and contains a DExDc, a HELICc and an RD. The OnMAVS gene encodes 556 amino acids and contains a CARD, a proline-rich domain, a transmembrane helix domain and a putative TRAF2-binding motif (269PVQDT273). Phylogenetic analyses showed that all three genes from Nile tilapia were clustered together with their counterparts from other teleost fishes. Real-time PCR analyses showed that all three genes were constitutively expressed in all examined tissues in Nile tilapia. OnMDA5 presented the highest expression level in the blood and the lowest expression level in the liver, while OnMAVS presented the highest expression level in the kidney. The highest expression level of OnLGP2 was detected in the liver. An examination of the expression patterns of these RIG-I-like receptors (RLRs) during embryonic development showed that the highest expression levels of OnMDA5 occurred at 2 days postfertilization (dpf), and the expression significantly decreased from 3 to 8 dpf. The expression levels of OnLGP2 significantly increased from 4 to 8 dpf. The expression levels of OnMAVS mRNA were stable from 2 to 8 dpf. Upon stimulation by intraperitoneal injection of Streptococcus agalactiae, the expression levels of OnMDA5 were first downregulated and then upregulated in the blood, gill and spleen. In the intestine and kidney, the expression of OnMDA5 was first upregulated, then downregulated, and then upregulated again. The expression of OnLGP2 was upregulated in the kidney and intestine, and the expression of OnMAVS was upregulated in the spleen. Overexpression of OnMAVS increased NF-κB activation in 293 T cells (p < 0.05), and after cotransfection with OnMDA5, the OnMAVS-dependent NF-κB activation was slightly increased (p > 0.05), after cotransfection with OnLGP2, the OnMAVS-dependent NF-κB activation was significantly decreased (p < 0.05). These findings suggest that, although the deduced protein structure of OnMDA5 is evolutionarily conserved with the structures of other RLR members, its signal transduction function is markedly different. The results also suggest that OnLGP2 has a negative regulatory effect on the OnMAVS gene. OnMDA5 and OnMAVS were uniformly distributed throughout the cytoplasm in 293 T cells, whereas OnLGP2 was distributed throughout the cytoplasm and nucleus. These results are helpful for clarifying the innate immune response against bacterial infection in Nile tilapia.

Nucleotide-binding and oligomerization domain (NOD)-like receptors in teleost fish: Current knowledge and future perspectives

Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are a group of intracellular pathogen recognition receptors (PRRs) that play key roles in pathogen recognition and subsequent activation of innate immune signalling pathways. Expressions of several NLR subfamily members, including NOD1, NOD2, NLR-C3, NLR-C5 and NLR-X1 have been reported in many different teleost fish species. These receptors are activated by a variety of ligands, including lipopolysaccharides (LPS), peptidoglycans (PGN) and polyinosinic-polycytidylic acid [Poly(I:C)]. Synthetic dsRNA and bacterial or viral infections are known to stimulate these receptors both in vitro and in vivo. In this review, we focus on the identification, expression and function of teleost NLRs in response to bacterial or viral pathogens. Additionally, NLR ligand specificity and signalling pathways involved in the recognition of bacterial or viral stimuli are also summarized. This review focuses on current knowledge in this area and provides future perspectives regarding topics in need of additional investigation. Understanding the response of innate immune system to bacterial or viral infections in diverse species could inform the development of more effective therapies and vaccines.

TANK-Binding Kinase 1 (TBK1) Isoforms Negatively Regulate Type I Interferon Induction by Inhibiting TBK1-IRF3 Interaction and IRF3 Phosphorylation

TANK-binding kinase 1 (TBK1) is an important serine/threonine-protein kinase that mediates phosphorylation and nuclear translocation of IRF3, which contributes to induction of type I interferons (IFNs) in the innate antiviral response. In mammals, TBK1 spliced isoform negatively regulates the virus-triggered IFN-β signaling pathway by disrupting the interaction between retinoic acid-inducible gene I (RIG-I) and mitochondria antiviral-signaling protein (MAVS). However, it is still unclear whether alternative splicing patterns and the function of TBK1 isoform(s) exist in teleost fish. In this study, we identify two alternatively spliced isoforms of TBK1 from zebrafish, termed TBK1_tv1 and TBK1_tv2. Both TBK1_tv1 and TBK1_tv2 contain an incomplete STKc_TBK1 domain. Moreover, the UBL_TBK1_like domain is also missing for TBK1_tv2. TBK1_tv1 and TBK1_tv2 are expressed in zebrafish larvae. Overexpression of TBK1_tv1 and TBK1_tv2 inhibits RIG-I-, MAVS-, TBK1-, and IRF3-mediated activation of IFN promoters in response to spring viremia of carp virus infection. Also, TBK1_tv1 and TBK1_tv2 inhibit expression of IFNs and IFN-stimulated genes induced by MAVS and TBK1. Mechanistically, TBK1_tv1 and TBK1_tv2 competitively associate with TBK1 and IRF3 to disrupt the formation of a functional TBK1-IRF3 complex, impeding the phosphorylation of IRF3 mediated by TBK1. Collectively, these results demonstrate that TBK1 spliced isoforms are dominant negative regulators in the RIG-I/MAVS/TBK1/IRF3 antiviral pathway by targeting the functional TBK1-IRF3 complex formation. Identification and functional characterization of piscine TBK1 spliced isoforms may contribute to understanding the role of TBK1 expression in innate antiviral response.

Comparative transcriptome analysis of zebrafish (Danio rerio) brain and spleen infected with spring viremia of carp virus (SVCV)

Spring viremia of carp virus (SVCV) is the pathogen of spring viremia of carp (SVC) and often causes acute hemorrhagic symptoms in various kinds of cyprinids and induces serious environmental and economic losses. However, the molecular mechanisms of infection remain poorly understood, especially at the individual level. In this study, zebrafish was employed as the infection model to explore the pathogenesis of SVCV. 4 groups of zebrafish tissues were set and RNA sequencing (RNA-Seq) technology was employed to analyze the differentially expressed genes (DEGs) after SVCV-infection. A total of 360,971,498 clean reads were obtained from 12 samples, 382 DEGs in the brain and 926 DEGs in the spleen were identified. These DEGs were annotated into three ontologies after gene ontology (GO) enrichment analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that these DEGs were primarily related to Influenza A pathway and Herpes simplex infection pathway in brain and Tuberculosis and Toxoplasmosis pathways in spleen, and all of these pathways may be involved in response to pathogen invasion. At the same time, 3' and 5' alternative splicing (AS) events were significantly up-regulated in the spleen. The transcriptome analysis results demonstrated changes and tissue-specific influences caused by SVCV in vivo, which provided us with more information to understand the complex relationships between SVCV and its host.

Alternative Pre-mRNA Splicing in Mammals and Teleost Fish: A Effective Strategy for the Regulation of Immune Responses Against Pathogen Infection

Pre-mRNA splicing is the process by which introns are removed and the protein coding elements assembled into mature mRNAs. Alternative pre-mRNA splicing provides an important source of transcriptome and proteome complexity through selectively joining different coding elements to form mRNAs, which encode proteins with similar or distinct functions. In mammals, previous studies have shown the role of alternative splicing in regulating the function of the immune system, especially in the regulation of T-cell activation and function. As lower vertebrates, teleost fish mainly rely on a large family of pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) from various invading pathogens. In this review, we summarize recent advances in our understanding of alternative splicing of piscine PRRs including peptidoglycan recognition proteins (PGRPs), nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) and their downstream signaling molecules, compared to splicing in mammals. We also discuss what is known and unknown about the function of splicing isoforms in the innate immune responses against pathogens infection in mammals and teleost fish. Finally, we highlight the consequences of alternative splicing in the innate immune system and give our view of important directions for future studies.

Mutual Regulation of NOD2 and RIG-I in Zebrafish Provides Insights into the Coordination between Innate Antibacterial and Antiviral Signaling Pathways

Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) and retinoic acid-inducible gene I (RIG-I) are two important cytosolic pattern recognition receptors (PRRs) in the recognition of pathogen-associated molecular patterns (PAMPs), initiating innate antibacterial and antiviral signaling pathways. However, the relationship between these PRRs, especially in teleost fish models, is rarely reported. In this article, we describe the mutual regulation of zebrafish NOD2 (DrNOD2) and RIG-I (DrRIG-I) in innate immune responses. Luciferase assays were conducted to determine the activation of NF-κB and interferon signaling. Morpholino-mediated knockdown and mRNA-mediated rescue were performed to further confirm the regulatory roles between DrNOD2 and DrRIG-I. Results showed that DrNOD2 and DrRIG-I shared conserved structural hallmarks with their mammalian counterparts, and activated DrRIG-I signaling can induce DrNOD2 production. Surprisingly, DrNOD2-initiated signaling can also induce DrRIG-I expression, indicating that a mutual regulatory mechanism may exist between them. Studies conducted using HEK293T cells and zebrafish embryos showed that DrRIG-I could negatively regulate DrNOD2-activated NF-κB signaling, and DrNOD2 could inhibit DrRIG-I-induced IFN signaling. Moreover, knocking down DrRIG-I expression by morpholino could enhance DrNOD2-initiated NF-κB activation, and vice versa, which could be rescued by their corresponding mRNAs. Results revealed a mutual feedback regulatory mechanism underlying NOD2 and RIG-I signaling pathways in teleosts. This mechanism reflects the coordination between cytosolic antibacterial and antiviral PRRs in the complex network of innate immunity.

CARD and TM of MAVS of black carp play the key role in its self-association and antiviral ability

Mitochondrial antiviral signaling protein (MAVS) is an adaptor protein of the innate immune system of higher vertebrate. In this paper, the transcription profile of black carp MAVS (bcMAVS) in host cells in response to spring viremia of carp virus (SVCV) and grass carp reovirus (GCRV) infection was identified. EPC cells expressing bcMAVS possessed obviously enhanced antiviral activity against both SVCV and GCRV. Immunofluorescence (IF) staining data demonstrated that bcMAVS molecules were redistributed and formed aggregates on the mitochondria of EPC cells after virus infection. Co-immunoprecipitation (co-IP) assay in HEK293T cells demonstrated that bcMAVS proteins bound to each other, which suggested that this fish protein owned self-association in vivo. IF assay identified that the transmembrane (TM) domain of bcMAVS was crucial for its mitochondrial localization. Co-IP assays among bcMAVS mutants demonstrated that both N-terminal caspase recruitment domain (CARD) and TM domain were indispensible for dimerization of bcMAVS. It was interesting that Truncated-bcMAVS possessed much enhanced interferon-inducing activity and antiviral ability than wild type bcMAVS, which only contains CARD and TM. All the data generated in this study support the idea that oligomerization of bcMAVS on mitochondrion is crucial for the antiviral ability of bcMAVS, which is depend on both CARD and TM domain of this fish MAVS orthologue.

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) in fish: current knowledge and future perspectives

Retinoic acid-inducible gene I (RIG-I) -like receptors (RLRs) are found conservatively present in teleost fish. All three members, RIG-I, MDA5 and LGP2, together with the downstream molecules such as MITA, TRAF3 and TBK1, have been identified in a range of fish species. However, it is unexpected that RIG-I has not been reported in fish of Acanthopterygii, and it would be important to clarify the presence and role of the RIG-I gene in a broad range of taxa in Teleostei. RLRs in fish can be induced in vivo and in vitro by viral pathogens as well as synthetic dsRNA, poly(I:C), leading to the production of type I interferons (IFNs) and the expression of IFN-stimulated genes (ISGs). Bacterial pathogens, such as Edwardsiella tarda, and their components, such as lipopolysaccharide are also found to induce the expression of RLRs, and whether such induction was mediated through the direct recognition by RLRs or through crosstalk with other pattern recognition receptors recognizing directly bacterial pathogen-associated molecular patterns awaits to be investigated. On the other hand, RLR-activated type I IFN production can be negatively regulated in fish by molecules, such as TBK-1-like protein and IRF10, which are found to negatively regulate RIG-I and MAVS-activated type I IFN production, and to block MITA or bind ISRE motifs, respectively. It is considered that the evolutionary occurrence of RLRs in fish, and their recognized ligands, especially those from their fish pathogens, as well as the mechanisms involved in the RLR signalling pathways, are of significant interest for further investigation.

Molecular cloning and functional characterization of feline MAVS

The mitochondrial anti-viral signaling protein (MAVS) plays an important role in the type I IFN response. In this study, two feline MAVS transcripts were cloned. Both transcripts have the same open reading frame encoding 523 amino acids. The putative protein shares 76.6 % similarity with canine and exhibits similarity to human, mouse, rat, bovine and porcine, ranging from 46.1 to 65.8 %. Deletion mutant analysis indicated that the transmembrane (TM) domain is necessary for localization in the mitochondrial membrane, and both the caspase activation and recruitment domain and TM domain are indispensible for activating the IFN-β response. Additionally, Sendai virus-induced IFN-β promoter activation was significantly inhibited by siRNA targeting MAVS. Finally, miniMAVS, a second protein encoded by MAVS mRNA, was identified, which interfered with the IFN-β response via the inhibition of NF-κB activation. The identification of MAVS will promote the understanding and control of feline infectious diseases.

Zebrafish foxo3b Negatively Regulates Antiviral Response through Suppressing the Transactivity of irf3 and irf7

Forkhead box O (FOXO)3, a member of the FOXO family of transcription factors, plays key roles in various cellular processes, including development, longevity, reproduction, and metabolism. Recently, FOXO3 has also been shown to be involved in modulating the immune response. However, how FOXO3 regulates immunity and the underlying mechanisms are still largely unknown. In this study, we show that zebrafish (Danio rerio) foxo3b, an ortholog of mammalian FOXO3, is induced by polyinosinic-polycytidylic acid stimulation and spring viremia of carp virus (SVCV) infection. We found that foxo3b interacted with irf3 and irf7 to inhibit ifr3/irf7 transcriptional activity, thus resulting in suppression of SVCV or polyinosinic-polycytidylic acid-induced IFN activation. By suppressing expression of key antiviral genes, foxo3b negatively regulated the cellular antiviral response. Furthermore, upon SVCV infection, the expression of the key antiviral genes was significantly enhanced in foxo3b-null zebrafish larvae compared with wild-type larvae. Additionally, the replication of SVCV was inhibited in foxo3b-null zebrafish larvae, leading to a higher survival rate. Our findings suggest that by suppressing irf3/irf7 activity, zebrafish foxo3b negatively regulates the antiviral response, implicating the vital role of the FOXO gene family in innate immunity.

NOD2 in zebrafish functions in antibacterial and also antiviral responses via NF-κB, and also MDA5, RIG-I and MAVS

NOD2/RIPK2 signalling plays essential role in the modulation of innate and adaptive immunity in mammals. In this study, NOD2 was functionally characterized in zebrafish (Danio rerio), and its interaction with a receptor-interaction protein, RIPK2, and RLRs such as MDA5 and RIG-I, as well as the adaptor, MAVS was revealed in fish innate immunity. The expression of NOD2 and RIPK2 in ZF4 cells has been constitutive and can be induced by the infection of Edwardsiella tarda and SVCV. The NOD2 can sense MDP in PGN from Gram-negative and -positive bacteria. It is further revealed that the NOD2 and RIPK2 can activate NF-κB and IFN promoters, inducing significantly antiviral defense against SVCV infection. As observed in the reduced bacterial burden in RIPK2 overexpressed cells, RIPK2 also has a role in inhibiting the bacterial replication. The overexpression of NOD2 in zebrafish embryos resulted in the increase of immune gene expression, especially those encoding PRRs and cytokines involved in antiviral response such as MDA5, RIG-I, and type I IFNs, etc. Luciferase reporter assays and co-immunoprecipitation assays demonstrated that zebrafish NOD2 is associated with MDA5 and RIG-I in signalling pathway. In addition, it is further demonstrated that RIPK2 and MAVS in combination with NOD2 have an enhanced role in NOD2-mediated NF-κB and type I IFN activation. It is concluded that teleost fish NOD2 can not only sense MDP for activating innate immunity as reported in mammals, but can also interact with other PRRs to form a network in antiviral innate response.

Functional characterizations of IPS-1 in CIK cells: Potential roles in regulating IFN-I response dependent on IRF7 but not IRF3

IPS-1, as the sole adaptor of RIG-I and MDA5, plays a central role in innate antiviral immunity. In this study, we investigated potential roles of IPS-1 in innate immunity and the domain-requirement of IPS-1 for its signaling in grass carp (Ctenopharyngodon idella). Overexpression experiment showed that CiIPS-1 mediated IFN-I signal possibly dependent on CiIRF7 but not CiIRF3. Post GCRV challenge, CiIPS-1 could enhance antiviral immune responses. CARD and TM domains were crucial for antiviral function of CiIPS-1, and TRAF motif played an assistant role. PRO domain seemed as a negative regulator but was pivotal for the initiation of CiIFN-I and CiMx1. Post viral/bacterial PAMPs stimulation, CiIPS-1-mediated signaling was tightly controlled. CARD domain of CiIPS-1 could significantly elicit poly I:C/LPS/PGN-mediated signaling. PRO domain negatively regulated CiIRF7 and CiIFN-I but was indispensable for inductions of CiMx1 and CiIL-1β. TRAF motif and TM domain regulated the signaling presumably in a cooperative fashion. Post poly I:C stimulation, TRAF motif negatively regulated CiIRF7, CiIFN-I and CiIL-1β at a relative early time while TM domain functioned at a relative late time. TRAF motif was indispensable for the production of CiMx1, while TM domain slightly negatively regulated the expression. Post LPS and PGN stimulation, TRAF motif excited an assistant and persistent negative role on CiIFN-I, CiIRF7 and CiIL-1β induction, but was crucial for induction of CiMx1. TM domain slightly negatively regulated LPS- and PGN-triggered signaling. Taken together, CiIPS-1 not only exerted important functions in antiviral immune response but also participated in viral/bacterial PAMPs-triggered immune response which was tightly controlled to prevent harmful effects resulting from excessive activation. This study provided novel insights into the pivotal role of IPS-1 in innate immunity.

Identification and characterization of IKKε gene from grass carp Ctenopharyngodon idella

IKKε is an IKK-related kinase implicated in antiviral immune response in higher vertebrates. To elucidate the function of IKKε in teleost fish, grass carp IKKε (gcIKKε) has been cloned and characterized in this paper. The full-length cDNA of gcIKKε is composed of 2529 nucleotides and encodes a polypeptide of 723 amino acids. The mRNA transcription of gcIKKε was constitutively detected in all the selected tissues and the gcIKKε mRNA level increased at 36 h after GCRV infection. Western blot data of both HEK293T cells and EPC cells demonstrated that gcIKKε was around 80 KDa; and immunofluorescence staining data of both NIH3T3 cells and EPC cells determined gcIKKε was a cytosolic protein. The mRNA level of gcIKKε in CIK cells was increased more than 150 times right after poly(I:C) treatment and PMA treatment triggered gcIKKε mRNA transcription in CIK cells more than 100 times. Over-expression of gcIKKε in EPC cells activated the promoter activity of both zebrafish IFN and fathead minnow IFN. gcIKKε mRNA transcription level in CIK cells was increased from 48 h post GCRV infection with different MOIs. All the data support the idea that gcIKKε is a novel teleost IκB kinase recruited in the IFN-mediated antiviral immunity of grass carp.

Functions of the two zebrafish MAVS variants are opposite in the induction of IFN1 by targeting IRF7

IFNs create the first line of host cells to defense viral infection, however, unrestricted expression of IFN can be hazardous to the host. IRF7 is the master regulator of type I IFN expression. To our knowledge, non research about the inhibition of IFN expression by targeting IRF7 has been reported in fish. In this study, we reported that the splicing variant of wildtype MAVS (MAVS_tv1), MAVS_tv2, negatively regulated IRF7-mediated IFN production. Firstly, in vivo, the transcriptional levels of MAVS_tv2 in trunk kidney and spleen from the zebrafish infected with SVCV were monitored. Then, in vitro, the protein expression pattern of MAVS_tv2 in zebrafish cell lines was detected using anti-MAVS_tv2 antibody. Furthermore, overexpression of MAVS_tv2 decreased the activation of IFN1 promoter that induced by IRF7 in a dose-dependent manner, whereas it had little effect on IRF3, a close relative of IRF7. In addition, such inhibition was also observed in IRF7-mediated epcIFN promoter and ISRE activities, but not in the activation of the promoters of type II IFNs and NF-ĸB, due to IRF7 not regulating their expression. Lastly, overexpression of MAVS_tv2 decreased the transcriptional levels of several IFN-stimulated genes activated by IRF7. These findings suggest that MAVS_tv2 is a negative regulator of IFN1 by targeting IRF7.

Sensors of Infection: Viral Nucleic Acid PRRs in Fish

Viruses produce nucleic acids during their replication, either during genomic replication or transcription. These nucleic acids are present in the cytoplasm or endosome of an infected cell, or in the extracellular space to be sensed by neighboring cells during lytic infections. Cells have mechanisms of sensing virus-generated nucleic acids; these nucleic acids act as flags to the cell, indicating an infection requiring defense mechanisms. The viral nucleic acids are called pathogen-associated molecular patterns (PAMPs) and the sensors that bind them are called pattern recognition receptors (PRRs). This review article focuses on the most recent findings regarding nucleic acids PRRs in fish, including: Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), cytoplasmic DNA sensors (CDSs) and class A scavenger receptors (SR-As). It also discusses what is currently known of the downstream signaling molecules for each PRR family and the resulting antiviral response, either type I interferons (IFNs) or pro-inflammatory cytokine production. The review highlights what is known but also defines what still requires elucidation in this economically important animal. Understanding innate immune systems to virus infections will aid in the development of better antiviral therapies and vaccines for the future.

Abortive infection of snakehead fish vesiculovirus in ZF4 cells was associated with the RLRs pathway activation by viral replicative intermediates

Snakehead fish vesiculovirus (SHVV) is a negative strand RNA virus which can cause great economic losses in fish culture. To facilitate the study of SHVV-host interactions, the susceptibility of zebrafish embryonic fibroblast cell line (ZF4) to the SHVV was investigated in this report. The results showed that high amount of viral mRNAs and cRNAs were detected at the 3 h post-infection. However, the expressions of the viral mRNAs and cRNA were decreased dramatically after 6 h post-infection. In addition, the expressions of interferon (IFN) and interferon-induced GTP-binding protein Mx were all up regulated significantly at the late stage of the infection. Meanwhile, the expressions of Retinoic acid-inducible gene I (RIG-I) and Melanoma differentiation-associated gene 5 (MDA5) were also all up-regulated significantly during the infection. Two isoforms of DrLGP2 from zebrafish were also cloned and analyzed. Interestingly, the expression of DrLGP2a but not DrLGP2b was significantly up-regulated at both mRNA and protein levels, indicating that the two DrLGP2 isoforms might play different roles during the SHVV infection. Transfection experiment showed that viral replicative intermediates were required for the activation of IFN-α expression. Taken together, the abortive infection of SHVV in ZF4 cells was associated with the activation of RLRs pathway, which was activated by viral replicative intermediates.

Insights into the antiviral immunity against grass carp (Ctenopharyngodon idella) reovirus (GCRV) in grass carp

Global fish production from aquaculture has rapidly grown over the past decades, and grass carp shares the largest portion. However, hemorrhagic disease caused by grass carp reovirus (GCRV) results in tremendous loss of grass carp (Ctenopharyngodon idella) industry. During the past years, development of molecular biology and cellular biology technologies has promoted significant advances in the understanding of the pathogen and the immune system. Immunoprophylaxis based on stimulation of the immune system of fish has also got some achievements. In this review, authors summarize the recent progresses in basic researches on GCRV; viral nucleic acid sensors, high-mobility group box proteins (HMGBs); pattern recognition receptors (PRRs), Toll-like receptors (TLRs) and retinoic acid inducible gene I- (RIG-I-) like receptors (RLRs); antiviral immune responses induced by PRRs-mediated signaling cascades of type I interferon (IFN-I) and IFN-stimulated genes (ISGs) activation. The present review also notices the potential applications of molecule genetic markers. Additionally, authors discuss the current preventive and therapeutic strategies (vaccines, RNAi, and prevention medicine) and highlight the importance of innate immunity in long term control for grass carp hemorrhagic disease.