Involvement of fish signal transducer and activator of transcription 3 (STAT3) in nodavirus infection induced cell death

IL-6/STAT3 axis is hijacked by GCRV to facilitate viral replication via suppressing type Ⅰ IFN signaling

Grass carp reovirus (GCRV) infections and hemorrhagic disease (GCHD) outbreaks are typically seasonally periodic and temperature-dependent, yet the molecular mechanism remains unclear. Herein, we depicted that temperature-dependent IL-6/STAT3 axis was exploited by GCRV to facilitate viral replication via suppressing type Ⅰ IFN signaling. Combined multi-omics analysis and qPCR identified IL-6, STAT3, and IRF3 as potential effector molecules mediating GCRV infection. Deploying GCRV challenge at 18 °C and 28 °C as models of resistant and permissive infections and switched to the corresponding temperatures as temperature stress models, we illustrated that IL-6 and STAT3 expression, genome level of GCRV, and phosphorylation of STAT3 were temperature dependent and regulated by temperature stress. Further research revealed that activating IL-6/STAT3 axis enhanced GCRV replication and suppressed the expression of IFNs, whereas blocking the axis impaired viral replication. Mechanistically, grass carp STAT3 inhibited IRF3 nuclear translocation via interacting with it, thus down-regulating IFNs expression, restraining transcriptional activation of the IFN promoter, and facilitating GCRV replication. Overall, our work sheds light on an immune evasion mechanism whereby GCRV facilitates viral replication by hijacking IL-6/STAT3 axis to down-regulate IFNs expression, thus providing a valuable reference for targeted prevention and therapy of GCRV.

Identifying transcriptomic indicators of tertiary treated municipal effluent in longnose dace (Rhinichthys cataractae) caged under semi-controlled conditions in experimental raceways

To evaluate effects of tertiary treated wastewater treatment plant effluent (MWWE) on transcriptomic responses in longnose dace (Rhinichthys cataractae; LND) we conducted a semi-controlled study in experimental raceways (Advancing Canadian Water Assets facility) imbedded in the Pine Creek treatment plant (Calgary, AB). LND collected from a reference site in the Bow River (REF) were caged in raceways containing either 5 % Pine Creek effluent (PC) or Bow River water (BR; control) over 28 d. Liver transcriptomes were analyzed in males and females sampled on days 7, 14 and 28 from BR and PC, and compared to REF fish on day 0. Concurrent with the caging, selected environmental substances of concern were analyzed in the BR and PC. Significantly different unigenes (SDUs) in females (vs males) within both BR and PC raceways increased over time and compared to REF fish. Moreover, SDUs in females and males within the same treatment (i.e., BR, PC) showed a temporal increase as well as compared to REF fish. Time was the dominant factor affecting SDUs, whereas sex and treatment had less of an impact on the transcriptome profiling. Gene Set Enrichment Analysis of BR vs PC over time revealed effects on genes involved in growth, metabolism of carbohydrates and lipids, and immune system on day 7; however, by day 28, 80-100 % of the transcripts localized to enriched biomarkers were associated with tissue immune responses in both sexes. Exposure to 5 % effluent had significant effects on female liver somatic index but no effects were observed on other phenotypic health indices in either sex. BR was used as the source of reference water, but analyses showed trace amounts of ESOCs. Analyses did not point towards definitive response patterns that could be used in field-based ecotoxicogenomic studies on the impacts of well-treated MWWE but suggested compromised adaptive immune responses.

Toxicity of tributyltin chloride on haarder (Liza haematocheila) after its acute exposure: Bioaccumulation, antioxidant defense, histological, and transcriptional analyses

Liza haematocheila is exposed to various chemical contaminants from anthropogenic sources, including tributyltin chloride (TBTC). Yet the toxicity mechanism of TBTC on haarder remains unclear. The haarder was exposed to different doses (0, 10%, 20%, and 50% of LC₅₀-96 h) of TBTC. In this study, the results revealed its high bioaccumulation in the livers and significant alteration for development. The activities of antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase decreased after 96-h exposure to TBTC, this accompanied by an increased malondialdehyde level. TBTC exposure caused the intense production of reactive oxygen species, a reduction in total blood cell count in serum, and apoptosis-related alterations in livers, indicating that enhanced oxidative stress occurred in the process of TBTC exposure. Histological results revealed angiorrhexis and infiltration of inflammatory cells, vacuolar degeneration of hepatocytes in the livers, and swelling, fusion, and disintegration of gill organs. Interestingly, the obtained transcriptional profiles indicated that high doses of TBTC caused energy disorder, apoptosis, and adipogenesis restriction mediated by cytokines and adipokines in Jak-STAT and adipocytokine signaling pathways. In summary, acute exposure to high doses of TBTC could impair the antioxidant system and pathways related to energy, apoptosis and adipogenesis, eventually posing a serious challenge to the fitness of haarder individuals and its fish populations as marine resources.

Identification of BAG5 from orange-spotted grouper (Epinephelus coioides) involved in viral infection

Bcl-2-associated athanogene 5 (BAG5) is a kind of molecular chaperone that can bind to the Bcl-2 and modulate cell survival. However, little is known about the functions of fish BAG5. In this study, we characterized a BAG5 homolog from orange-spotted grouper (Epinephelus coioides) gene (Ec-BAG5) and investigated its roles during viral infection. The Ec-BAG5 protein encoded 468 amino acids with four BAG domains, which shared high identities with reported BAG5. The highest transcriptional level of Ec-BAG5 was found in the peripheral blood lymphocyte (PBL). And the Ec-BAG5 expression were significantly up-regulated after red-spotted grouper nervous necrosis virus (RGNNV) or Lipopolysaccharide (LPS) stimulation in vitro. Furthermore, Ec-BAG5 overexpression could inhibited viral replication and the expression of viral genes (coat protein (CP) and RNA-dependent RNA polymerase (RdRp)). Also, overexpression of Ec-BAG5 significantly increased the expression of interferon pathway-related factors including interferon regulatory factor 3 (IRF3), interferon-stimulated gene 15 (ISG15), interferon-induced protein 35 (IFP35), myxovirus resistance gene 1 (Mx1) and inflammatory-related factors including tumor necrosis factor receptor-associated factor 6 (TRAF6), tumor necrosis factor-α (TNF-α), interleukin-1 beta (IL-1β), as well as the activities of NF-κB, ISRE and IFN-1. These data indicate that Ec-BAG5 can affect viral infection through regulating the expression of IFN- and inflammation-related factors, which provide useful information to better understand the immune response against viral infection.

Grouper interferon-induced transmembrane protein 3 (IFITM3) inhibits the infectivity of iridovirus and nodavirus by restricting viral entry

Interferon-induced transmembrane proteins (IFITMs) have been identified as important host restriction factors in mammals for the control of infection by multiple viruses. However, the antiviral functions of IFITMs against fish viruses remain largely uncertain. In this study, the IFITM3 homolog from orange spotted grouper (EcIFITM3) was cloned and its roles in grouper virus infection were investigated. The full-length cDNA of EcIFITM3 was 737 bp, which was composed of a 16 bp 5'-UTR, a 274 bp 3'-UTR, and a 447 bp ORF. EcIFITM3 encodes a 148-amino-acid polypeptide, which contains five domains, i.e., the N-terminal domain (aa 1-65), TM1 (aa 66-90), the cytoplasmic domain (aa 91-110), TM2 (aa 111-140), and the C-terminal domain (aa 141-148), and shares 78% and 47% identity with IFITM3 of gilthead seabream (Sparus aurata) and human (Homo sapiens), respectively. EcIFITM3 mRNA was detected in 12 tissues of healthy groupers, with the highest expression levels in the head kidney. Additionally, the in vitro mRNA levels of EcIFITM3 were significantly upregulated by infection with Singapore grouper iridovirus (SGIV) or red spotted grouper nervous necrosis virus (RGNNV), or treatment with polyinosinic-polycytidylic acid (poly I:C) or lipopolysaccharide (LPS). Subcellular localization analysis showed that EcIFITM3 was mainly distributed in the cell membrane of grouper cells. In vitro, the ectopic expression of EcIFITM3 inhibited SGIV and RGNNV infection, as demonstrated by the reduced severity of the cytopathic effect, decreased virus production, and low levels of viral mRNA and proteins. Consistently, knockdown of EcIFITM3 by small interfering RNAs (siRNAs) enhanced SGIV and RGNNV replication. EcIFITM3 overexpression and knockdown experiments both suggested that EcIFITM3 inhibits the infection of SGIV and RGNNV by restricting virus entry.

Metabolic profiles of fish nodavirus infection in vitro: RGNNV induced and exploited cellular fatty acid synthesis for virus infection

Red-spotted grouper nervous necrosis virus (RGNNV), the causative agent of viral nervous necrosis disease, has caused high mortality and heavy economic losses in marine aquaculture worldwide. However, changes in host cell metabolism during RGNNV infection remain largely unknown. Here, the global metabolic profiling during RGNNV infection and the roles of cellular fatty acid synthesis in RGNNV infection were investigated. As the infection progressed, 71 intracellular metabolites were significantly altered in RGNNV-infected cells compared with mock-infected cells. The levels of metabolites involved in amino acid biosynthesis and metabolism were significantly decreased, whereas those that correlated with fatty acid synthesis were significantly up-regulated during RGNNV infection. Among them, tryptophan and oleic acid were assessed as the most crucial biomarkers for RGNNV infection. In addition, RGNNV infection induced the formation of lipid droplets and re-localization of fatty acid synthase (FASN), indicating that RGNNV induced and required lipogenesis for viral infection. The exogenous addition of palmitic acid (PA) enhanced RGNNV infection, and the inhibition of FASN and acetyl-CoA carboxylase (ACC) significantly decreased RGNNV replication. Additionally, not only inhibition of palmitoylation and phospholipid synthesis, but also destruction of fatty acid β-oxidation significantly decreased viral replication. These data suggest that cellular fatty acid synthesis and mitochondrial β-oxidation are essential for RGNNV to complete the viral life cycle. Thus, it has been demonstrated for the first time that RGNNV infection in vitro overtook host cell metabolism and, in that process, cellular fatty acid synthesis was an essential component for RGNNV replication.

Autophagy Participates in Lysosomal Vacuolation-Mediated Cell Death in RGNNV-Infected Cells

Nervous necrosis virus (NNV) is the etiological agent of viral nervous necrosis (VNN), also known as viral encephalopathy and retinopathy (VER), which results in heavy economic losses to the aquaculture industry worldwide. Dramatic cytoplasmic vacuoles were observed during NNV infection both in vitro and in vivo; however, the origin and mechanism of cytoplasmic vacuolization remains unknown. In this report, we found that the cytoplasmic vacuole morphology became fused and enlarged during infection with red spotted grouper nervous necrosis virus (RGNNV), which was accompanied by increased cell death. Notably, Lyso-Tracker, but not Mito-Tracker or ER-Tracker, was accumulated in the vacuoles, and abnormal lysosome swelling was observed in RGNNV-infected cells, suggesting that the cytoplasmic vacuoles originated from lysosomal organelles. Cytoplasmic vacuolization and cell death in RGNNV-infected cells was completely blocked by the vacuolar H⁺-ATPase inhibitor (bafilomycin A1), and was significantly weakened by chloroquine (CQ), a lysosomotropic agent that induces the acidification of the lysosomes. This suggests that lysosome acidification was essential for vacuole formation. Significant inhibitory effects on vacuolization and cell death were also observed in the RGNNV-infected cells following treatment with nigericin and monensin (ionophores that uncouple the proton gradient present in lysosomes). This indicated that lysosome function was tightly associated with RGNNV infection-induced cell death. In addition, vacuoles were found to be partially co-localized with GFP-LC3II punctate dots during RGNNV infection. Moreover, the severity of vacuolization and cell death were both significantly decreased after treatment with the autophagy inhibitor, 3-MA, suggesting that autophagy was involved in lysosomal vacuolization and cell death evoked by RGNNV infection. Thus, our results demonstrate that autophagy participates in lysosomal vacuolation-mediated cell death during RGNNV infection, and provides new insight into our understanding of the potential mechanisms underlying nodavirus pathogenesis in vitro.

Fine-Tuning of Type I Interferon Response by STAT3

Type I interferon (IFN-I) is induced during innate immune response and is required for initiating antiviral activity, growth inhibition, and immunomodulation. STAT1, STAT2, and STAT3 are activated in response to IFN-I stimulation. STAT1, STAT2, and IRF9 form ISGF3 complex which transactivates downstream IFN-stimulated genes and mediates antiviral response. However, the role of STAT3 remains to be characterized. Here, we review the multiple actions of STAT3 on suppressing IFN-I responses, including blocking IFN-I signaling, downregulating the expression of ISGF3 components, and antagonizing the transcriptional activity of ISGF3. Finally, we discuss the evolution of the suppressive activity of STAT3 and the therapeutic potential of STAT3 inhibitors in host defense against viral infections and IFN-I-associated diseases.

ELP2 negatively regulates osteoblastic differentiation impaired by tumor necrosis factor α in MC3T3-E1 cells through STAT3 activation

Tumor necrosis factor‐α (TNF‐α) is a pluripotent signaling molecule. The biological effect of TNF‐α includes slowing down osteogenic differentiation, which can lead to bone dysplasia in long‐term inflammatory microenvironments. Signal transducer and activator of transcription 3 (STAT3)‐interacting protein 1 (StIP1, also known as elongator complex protein 2, ELP2) play a role in inhibiting TNF‐α‐induced osteoblast differentiation. In the present study, we investigated whether and how ELP2 activation mediates the effects of TNF‐α on osteoblastic differentiation. Using in vitro cell cultures of preosteoblastic MC3T3‐E1 cells, we found that TNF‐α inhibited osteoblastic differentiation accompanied by an increase in ELP2 expression and STAT3 activation. Forced ELP2 expression inhibited osteogenic differentiation of MC3T3‐E1 cells, with a decrease in the expression of osteoblast marker genes, alkaline phosphatase activity, and matrix mineralization capacity. In contrast, ELP2 silencing ameliorated osteogenic differentiation in MC3T3‐E1 cells, even after TNF‐α stimulation. The TNF‐α‐induced inhibitory effect on osteoblastic differentiation was therefore mediated by ELP2, which was associated with Janus kinase 2 (JAK2)/STAT3 activation. These results suggest that ELP2 is upregulated at the differentiation of MC3T3‐E1 cells into osteoblasts and inhibits osteogenic differentiation in response to TNF‐α through STAT3 activation.

Grouper viperin acts as a crucial antiviral molecule against iridovirus

Virus inhibitory protein, endoplasmic reticulum-associated, IFN-inducible (viperin), is an antiviral protein, induced by interferon (IFN), poly(I:C) and viral infection to exert antiviral function. To investigate the roles of viperin during fish virus infection, a viperin homolog from orange spotted grouper (Epinephelus coioides) (Ecviperin) was cloned and characterized in this study. Ecviperin encoded a 361-aa protein which shared 87% and 69% identity with Siniperca undulata and Homo sapiens, respectively. Amino acid alignment analysis showed that Ecviperin contained a conserved radical-SAM domain (aa73-281). Phylogenetic analysis indicated that Ecviperin showed the nearest relationship with S. undulata. In healthy grouper, Ecviperin was distributed in all tissues, and the expression of Ecviperin was the highest in kidney and spleen. In vitro, the mRNA expression of Ecviperin was significantly up-regulated in response to Singaporean grouper iridovirus (SGIV) infection. Subcellular localization analysis showed that Ecviperin was distributed in the cytoplasm and co-localized with endoplasmic reticulum (ER). The ectopic expression of Ecviperin significantly inhibited the replication of SGIV. Furthermore, overexpression of Ecviperin positively regulated the interferon related molecules, including interferon regulatory factor 3 (IRF3), IRF7, interferon stimulated gene 15 (ISG15), myxovirus resistance gene I (MXI), interferon-induced 35-kDa protein (IFP35), and TNF receptor-associated factor 6 (TRAF6). In addition, the expression of pro-inflammation cytokines was differently regulated by Ecviperin overexpression. Furthermore, reporter gene analysis showed that the overexpression of Ecviperin enhanced the activity of nuclear factor of kappa B (NF-κB), IFN-1 and interferon-stimulated response element (ISRE) promoter, suggesting that Ecviperin might restrict SGIV replication by the positive regulation of interferon and inflammatory response. Taken together, our results demonstrated that Ecviperin encoded an ER-localized protein, and exerted antiviral function against fish DNA virus by up-regulating interferon and pro-inflammatory response.

Immuno-related gene transcription and antibody response in nodavirus (RGNNV and SJNNV)-infected European sea bass (Dicentrarchus labrax L.)

The immune response of European sea bass to RGNNV and SJNNV infections has been evaluated by quantifying the transcription of some genes involved in the IFN I system, as well as in the inflammatory and adaptive immune mechanisms. The transcription of IFN-I, ISG-12, ISG-15 and MxA genes has been analyzed in brain and head kidney, showing that RGNNV genotype induces a more intense response of the IFN I system than SJNNV in both organs. In addition, the results obtained indicate the importance of the inflammatory response in nodavirus pathogenesis, with the transcription of IL-8 and TNF-α significantly higher in brain than in head kidney, being RGNNV the strongest inductor. An important difference between the immune response induced by both genotypes refers to the IgM titre in sera, which was higher in SJNNV-inoculated fish. The acquired response is also important locally, since TR-γ transcription is higher in brain than in head kidney (especially in the RGNNV-inoculated group). To our knowledge, this is the first study addressing the sea bass anti-SJNNV immune response.

Identification of two Stat3 variants lacking a transactivation domain in grass carp: New insights into alternative splicing in the modification of teleost Stat3 signaling

Signal transducer and activator of transcription 3 (STAT3) is a member of the STAT family in response to cytokines and growth factors. In mammals, alternative splicing of STAT3 generates STAT3α and STAT3β, which have distinct and overlapping functions. In the previous study, we have identified two spliceforms of Stat3α (Stat3α1 and Stat3α2) possessing all functional domains of Stat3 in grass carp (Ctenopharyngodon idella). In the present study, two Stat3β variants (Stat3β1 and Stat3β2) without C-terminal transactivation domain were isolated from this species, and their transcripts were ubiquitously expressed in all examined tissues with the highest levels in liver. Further studies showed that Stat3β1/2 had the ability to translocate into the nucleus upon activation, indicating their roles in transcriptional regulation. In support of this notion, grass carp Stat3β1 and Stat3β2 displayed the abilities to inhibit Interleukin-10 (Il-10) signaling and competitively impaired the transcriptional activities of Stat3α1/2. In particular, similar to their mammalian counterparts, grass carp Stat3β1 and Stat3β2 could enhance Stat3α1/2 phosphorylation upon cytokine stimulation. Interestingly, stat3β1 and stat3β2 transcripts were also found in zebrafish (Danio rerio) and goldfish (Carassius auratus), and each variant in these teleosts is generated through similar alternative splicing events, including exon skipping and intron retention. This highlights a conserved splicing event of stat3 gene during vertebrate evolution and indicates a potential physiological significance of generating unique Stat3 variants in fish. These results, along with the findings regarding Stat3α1/2, demonstrate the existence of Stat3 isoforms with functional diversity and redundancy in teleosts. It leads to the hypothesis that teleost-specific spliceforms of Stat3 gene may contribute to the complexity of Stat3 signaling in fishes, thereby benefiting them to adapt to evolution and environmental changes.

Micropterus salmoides rhabdovirus (MSRV) infection induced apoptosis and activated interferon signaling pathway in largemouth bass skin cells

Largemouth bass (Micropterus salmoides) rhabdovirus (MSRV) was isolated from infected juveniles of largemouth bass, and the infected fish exhibited corkscrew, irregular swimming, and crooked body. To our knowledge, the potential molecular mechanisms underlying the pathogenesis of MSRV infection remain largely unknown. In the current study, we found that MSRV infection in largemouth bass skin (LBS) cells induced typical apoptosis, evidenced by the presence of apoptotic bodies and caspase-3 activation. To further analyze the host factors involved in MSRV infection in LBS cells, the transcriptomic profiles during MSRV infection were uncovered using deep RNA sequencing technique, and several differentially expressed genes (DEGs) were validated by quantitative PCR. Our results showed that a total of 124483 unigenes were assembled. Among them, 34465 and 27273 had significant hits to those in the NR and SwissProt databases. After MSRV infection, a total of 2432 and 2480 genes which involved in multiples pathways including TNF signaling, NF-κB signaling, Toll-like receptor signaling and RIG-I signaling pathway were differentially expressed in MSRV infected LBS cells compared to mock-infected cells at 12 h, respectively. Furthermore, quantitative PCR showed that the expression levels of 9 differentially expressed genes (DEGs) related to apoptosis and interferon signaling pathway was consistent with that from transcriptomic profiles. Together, our results not only demonstrated that interferon signaling pathway and apoptosis pathway might exerted crucial roles during MSRV infection, but also provided a useful resource for subsequent investigation of other immune-related genes related to virus infection.

Grouper MAVS functions as a crucial antiviral molecule against nervous necrosis virus infection

Mitochondrial antiviral signaling protein (MAVS), also known as IPS-1, VISA, and Cardif, has been well studied for its crucial roles in the mammalian interferon immune response. To better understand the actions of MAVS in fish immune response, a MAVS homolog from orange spotted grouper (Epinephelus coioides) (EcMAVS) was cloned and characterized in this study. EcMAVS encoded a 563-amino acid peptide which showed 64% and 20% identity to rock bream (Oplegnathus fasciatus) and human (Homo sapiens), respectively. Sequence alignment analysis showed that EcMAVS shared a conserved CARD domain at N terminal, a central proline-rich region and a TM domain at C terminal. Phylogenetic analysis indicated that EcMAVS showed the nearest relationship to rock bream, followed by other fishes, birds and mammals. In healthy grouper, the transcript of EcMAVS was predominantly detected in gill, intestine and skin. In vitro, the expression level of EcMAVS was significantly increased during red-spotted grouper nervous necrosis virus (RGNNV) infection, but only slightly increased at the late stage of Singapore grouper iridovirus (SGIV) infection, suggested the EcMAVS might exert various roles in response to different viruses. Subcellular localization analysis showed that the fluorescence in EcMAVS transfected cells primarily co-localized with mitochondria. Overexpression of EcMAVS in grouper cells significantly inhibited the replication of RGNNV, demonstrated by the delay of CPE progression and the decrease of viral gene transcription. Differently, the replication of SGIV was almost not affected by the ectopic expression of EcMAVS. Furthermore, our results also showed that EcMAVS overexpression significantly increased the expression of interferon related cytokines, and activated both IRF3- and IRF7-mediated interferon promoter activities. Taken together, our results demonstrated that grouper MAVS exerted antiviral function against nodavirus infection via up-regulating the interferon immune response.

Identification and functional evaluation of two STAT3 variants in grass carp: Implication for the existence of specific alternative splicing of STAT3 gene in teleost

A STAT family member, STAT3, becomes activated as a DNA binding protein in response to cytokines and growth factors. In teleost, STAT3 cDNA has been cloned and identified in a few species, but only a single STAT3 transcript is revealed in these studies. In the present study, two variants of STAT3 gene generated by alternative splicing were isolated from grass carp and nominated as STAT3α1 and STAT3α2 based on the homology with their mammalian orthologs. In particular, the homologs of STAT3α1/2 were also found in various fish species, including zebrafish, takifugu, tilapia, medaka and goldfish. Intriguingly, sequence alignment and genomic structure analysis revealed that fish STAT3α1/2 are generated through similar alternative splicing events, implying the potential physiological significance of generating STAT3 variants in fish. Grass carp STAT3α1/2 (gcSTAT3α1/2) were ubiquitously expressed although the transcript levels of STAT3α2 were markedly higher than STAT3α1 in all examined tissues. In vivo and in vitro studies showed that the expression patterns of these two variants were similar under the stimulation of immune stimuli. To reveal the role of gcSTAT3α1/2 in fish immunity, their phosphorylation and involvement in IL-17A/F1 mRNA expression were demonstrated in grass carp peripheral blood lymphocytes upon LPS or PHA challenge, providing evidence for the functional conservation of STAT3 signaling in fish. These findings also raise a question of whether both gcSTAT3α1/2 participate in transcriptional regulation in fish. Actually, our results showed that both of them had the ability to translocate into the nucleus upon activation, and to amplify IL-10 signaling, indicating the existence of STAT3 isoforms with functional redundancy in teleost.

Grouper STAT1a is involved in antiviral immune response against iridovirus and nodavirus infection

Signal Transducer and Activator of Transcription 1 (STAT1) has been demonstrated to function as a critical mediator in multiple cell processes, such as cell proliferation, cell death, and innate immune response. Interestingly, two orthologues of human STAT1, including STAT1a and STAT1b genes have been identified in different fish. However, the detailed roles of fish STAT1a in virus replication still remained largely uncertain. Here, we cloned a STAT1a from orange-spotted grouper Epinephelus coioides (EcSTAT1a) and characterized its roles during fish virus infection. EcSTAT1a encoded a 751-aa peptide which shared 97% and 93% identity to STAT1 from mandarin fish (Siniperca chuatsi) and Malabar grouper (Epinephelus malabaricus), respectively. Amino acid alignment analysis showed that EcSTAT1a contained a STAT-int domain, a STAT-alpha domain, a STAT-bind domain (DNA binding domain), a SH2 domain and a STAT1-TAZ2 bind domain. In examined tissues from healthy grouper, the expression of EcSTAT1a was predominant in intestine, gill and liver. In grouper cells, the relative expression levels of EcSTAT1a was significantly increased during red-spotted grouper nervous necrosis virus (RGNNV) or Singapore grouper iridovirus (SGIV) infection. Under fluorescence microscopy, we found that EcSTAT1a mainly localized in the cytoplasm. The ectopic expression of EcSTAT1a in vitro significantly delayed the cytopathic effect (CPE) progression evoked by RGNNV and SGIV. Further studies showed that the expression levels of viral genes, including SGIV major capsid protein (MCP), VP19, ICP-18, LITAF and RGNNV coat protein (CP), RNA-dependent RNA polymerase (RdRp) were all significantly reduced in EcSTAT1a overexpressing cells compared to the control vector transfected cells, suggested that EcSTAT1a exerted antiviral activity against iridovirus and nodavirus. Furthermore, overexpression of EcSTAT1a significantly increased the expression of interferon related cytokines or effectors and pro-inflammatory factors. Together, our results elucidated that EcSTAT1a might function as a critical antiviral factor by regulating the host interferon immune and inflammation response.

Fish TRIM35 negatively regulates the interferon signaling pathway in response to grouper nodavirus infection

Tripartite motif-containing protein 35 (TRIM35) has been demonstrated to exert critical roles in cancer, cell death and other multiple cell processes. However, the precisely roles of TRIM35 during virus infection still remained largely unknown. In the current study, we cloned a TRIM35 gene from orange spotted grouper (EcTRIM35) and uncovered its roles in response to nodavirus infection. EcTRIM35 encoded a 456-aa protein which showed 65% and 32% identity to large yellow croaker (Larimichthys crocea) and human (Homo sapiens), respectively. Structure prediction and amino acid alignment analysis indicated that EcTRIM35 contained three conserved domains, including RING domain, B-BOX and SPRY domain. In healthy grouper, the high expression level of EcTRIM35 could be detected in liver, spleen and intestine. After infection with red-spotted grouper nervous necrosis (RGNNV) and Singapore grouper iridovirus (SGIV) in GS cells, the transcript of EcTRIM35 was significantly up-regulated with the infection time increased. Under fluorescence microscopy, the bright fluorescence aggregates were observed in EcTRIM35 transfected cells, but the fluorescence distribution was obviously altered in the EcTRIM35-ΔRING transfected cells. After incubation with RGNNV, the overexpression of EcTRIM35 in vitro significantly enhanced the viral replication, evidenced by the enhancement of cytopathic effect (CPE) severity and the up-regulation of the viral gene transcription. Moreover, the ectopic expression of EcTRIM35 significantly decreased the expression of interferon signaling molecules or effectors. Further studies elucidated that EcTRIM35 overexpression significantly weakened the MAVS-, MITA- or TBK1-induced interferon immune response, but showed no effects on MDA5-induced immune response. Thus, our results will shed new lights on the roles of fish TRIM35 in innate immune response against grouper virus infection.

Zika virus induces massive cytoplasmic vacuolization and paraptosis-like death in infected cells

The cytopathic effects of Zika virus (ZIKV) are poorly characterized. Innate immunity controls ZIKV infection and disease in most infected patients through mechanisms that remain to be understood. Here, we studied the morphological cellular changes induced by ZIKV and addressed the role of interferon-induced transmembrane proteins (IFITM), a family of broad-spectrum antiviral factors, during viral replication. We report that ZIKV induces massive vacuolization followed by "implosive" cell death in human epithelial cells, primary skin fibroblasts and astrocytes, a phenomenon which is exacerbated when IFITM3 levels are low. It is reminiscent of paraptosis, a caspase-independent, non-apoptotic form of cell death associated with the formation of large cytoplasmic vacuoles. We further show that ZIKV-induced vacuoles are derived from the endoplasmic reticulum (ER) and dependent on the PI3K/Akt signaling axis. Inhibiting the Sec61 ER translocon in ZIKV-infected cells blocked vacuole formation and viral production. Our results provide mechanistic insight behind the ZIKV-induced cytopathic effect and indicate that IFITM3, by acting as a gatekeeper for incoming virus, restricts virus takeover of the ER and subsequent cell death.

CXCR3.1 and CXCR3.2 Differentially Contribute to Macrophage Polarization in Teleost Fish

The study of multiple copies of chemokine receptor genes in various teleosts has long appealed to investigators seeking to understand the evolution of the immune system. The CXCR CXCR3 gene has two isoforms, CXCR3.1 and CXCR3.2, which are both expressed in macrophages. The distinct roles of teleost CXCR3s have not been identified previously. In this article, we found that CXCR3.1 and CXCR3.2 differentially contributed to macrophage polarization in the teleosts: ayu (Plecoglossus altivelis), grass carp (Ctenopharyngodon idella), and spotted green pufferfish (Tetraodon nigroviridis). In ayu macrophages, the P. altivelis CXCR3.1 (PaCXCR3.1) gene was constitutively expressed, whereas the P. altivelis CXCR3.2 (PaCXCR3.2) gene was induced postinfection with Escherichia coli Upon E. coli infection, PaCXCR3.1⁺ and PaCXCR3.2⁺ macrophages showed an M1 and an M2 phenotype, respectively. CXCL9-11-like proteins mediated M1 and M2 polarization by interacting with the PaCXCR3.1 and PaCXCR3.2 proteins on macrophages, respectively. The transcription factors P. altivelis STAT1 and P. altivelis STAT3 were activated in PaCXCR3.1⁺ and PaCXCR3.2⁺ macrophages, respectively. Furthermore, the prognosis of septic ayu adoptively transferred with PaCXCR3.2⁺ macrophages was improved. Our data reveal a previously unknown mechanism for macrophage polarization, suggesting that redundant genes may regulate crucial functions in the teleost immune system.

A homolog of teleostean signal transducer and activator of transcription 3 (STAT3) from rock bream, Oplegnathus fasciatus: Structural insights, transcriptional modulation, and subcellular localization

Signal transducer and activator of transcription 3 (STAT3) is one of the crucial transcription factors in the Janus kinase (JAK)/STAT signaling pathway, and it was previously considered as acute phase response factor. A number of interleukins (ILs) such as IL-5, IL-6, IL-9, IL-10, IL-12, and IL-22 are known to be involved in activation of STAT3. In addition, various growth factors and pathogenic or oxidative stresses mediate the activation of a wide range of functions via STAT3. In this study, a STAT3 homolog was identified and functionally characterized from rock bream (RbSTAT3), Oplegnathus fasciatus. In silico characterization revealed that the RbSTAT3 amino acid sequence shares highly conserved common domain architectural features including N-terminal domain, coiled coil domain, DNA binding domain, linker domain, and Src homology 2 (SH2) domains. In addition, a fairly conserved transcriptional activation domain (TAD) was located at the C-terminus. Comparison of RbSTAT3 with other counterparts revealed higher identities (>90%) with fish orthologs. The genomic sequence of RbSTAT3 was obtained from a bacterial artificial chromosome (BAC) library, and was identified as a multi-exonic gene (24 exons), as found in other vertebrates. Genomic structural comparison and phylogenetic studies have showed that the evolutionary routes of teleostean and non-teleostean vertebrates were distinct. Quantitative real time PCR (qPCR) analysis revealed that the spatial distribution of RbSTAT3 mRNA expression was ubiquitous and highly detectable in blood, heart, and liver tissues. Transcriptional modulation of RbSTAT3 was examined in blood and liver tissues after challenges with bacteria (Edwardsiella tarda and Streptococcus iniae), rock bream irido virus (RBIV), and immune stimulants (LPS and poly (I:C)). Significant changes in RbSTAT3 transcription were also observed in response to tissue injury. In addition, the transcriptional up-regulation of RbSTAT3 was detected in rock bream heart cells upon recombinant rock bream IL-10 (rRbIL-10) treatment. Subcellular localization and nuclear translocation of rock bream STAT3 following poly (I:C) treatment were also demonstrated. Taken together, the results of the current study provide important evidence for potential roles of rock bream STAT3 in the immune system and wound healing processes.

Negative regulation of the antiviral response by grouper LGP2 against fish viruses

Laboratory of genetics and physiology 2 (LGP2), a member of RIG-I like receptor (RLR) family, plays crucial roles in modulating cellular antiviral response during viral infection. However, the detailed roles of LGP2 in different virus infection were controversial up to now. Here, we cloned a LGP2 gene from orange-spotted grouper (EcLGP2) and investigated its roles in response to grouper virus infection. EcLGP2 encoded a 678-aa protein which shared 83% identity to sea perch (Lateolabrax japonicas). Amino acid alignment showed that EcLGP2 contained three conserved domains, including a DEAD/DEAH box helicase domain, a helicase superfamily C-terminal domain and a C-terminal domain of RIG-I. In healthy grouper, the transcript of EcLGP2 could be predominantly detected in kidney, gill, fin, spleen and skin. Subcellular localization analysis showed that EcLGP2 distributed throughout the cytoplasm in grouper cells. Notably, the intracellular distribution of EcLGP2 was altered at the late stage of Singapore grouper iridovirus (SGIV) infection, but remained unchanged during red-spotted grouper nervous necrosis virus (RGNNV) infection. Moreover, overexpression of EcLGP2 in vitro significantly enhanced the viral replication of SGIV and RGNNV, evidenced by the acceleration of CPE occurrence and the up-regulation of the viral gene transcription or protein synthesis. Further studies indicated that overexpression of EcLGP2 decreased the expression level of interferon related molecules or effectors, including IRF3, IRF7, ISG15, IFP35, MXI, MXII, and MDA5, suggesting that the negative feedback of interferon immune response by EcLGP2 might contribute to the enhancement of RGNNV infection. Moreover, the expression levels of pro-inflammation cytokines, including IL-8 and TNFα were significantly decreased, but that of IL-6 was increased by the ectopic expression of EcLGP2. Thus, our results will contribute greatly to understanding the roles of fish LGP2 in innate immune response during iridovirus and nodavirus infection.

RING domain is essential for the antiviral activity of TRIM25 from orange spotted grouper

Tripartite motif-containing 25 (TRIM25) has been demonstrated to exert crucial roles in the regulation of innate immune signaling. However, the roles of fish TRIM25 in antiviral immune response still remained uncertain. Here, a novel fish TRIM25 gene from orange spotted grouper (EcTRIM25) was cloned and its roles in grouper virus infection were elucidated. EcTRIM25 encoded a 734-aa protein which shared 68% identity to large yellow croaker (Larimichthys crocea). Amino acid alignment showed that EcTRIM25 contained three conserved domains, including a RING-finger domain, a B box/coiled-coil domain and a SPRY domain. In healthy grouper, the transcript of EcTRIM25 was predominantly detected in skin, spleen and intestine. After stimulation with Singapore grouper iridovirus (SGIV) or poly I:C, the relative expression of EcTRIM25 in grouper spleen was significantly increased at the early stage of injection. Subcellular localization analysis showed that EcTRIM25 distributed throughout the cytoplasm in grouper cells. Notably, the deletion RING domain affected its accurate localization and displayed microtubule like structures or bright aggregates in GS cells. After incubation with SGIV or red spotted grouper nervous necrosis virus (RGNNV), overexpression of full length of EcTRIM25 in vitro significantly decreased the viral gene transcription of SGIV and RGNNV. Consistently, the deletion of RING domain obviously affected the inhibitory effect of EcTRIM25. Furthermore, overexpression of EcTRIM25 significantly increased the expression level of interferon related signaling molecules, including interferon regulatory factor (IRF) 3, interferon-induced 35-kDa protein (IFP35), MXI, IRF7 and myeloid differentiation factor 88 (MyD88), suggesting that the positive regulation of interferon immune response by EcTRIM25 might affected RGNNV replication directly. Meanwhile, the expression levels of pro-inflammation cytokines were differently regulated by the ectopic expression of EcTRIM25. We proposed that the regulation of IRF7, MyD88 and pro-inflammation cytokines might contribute more important roles in SGIV infection. In addition, the RING domain of EcTRIM25 also played critical roles in the regulation of interferon immune and inflammation response. Together, our results will provide new evidences that the RING domain was essential for the antiviral action of fish TRIM25 against iridovirus and nodavirus infection.

Fish TRIM8 exerts antiviral roles through regulation of the proinflammatory factors and interferon signaling

The tripartite motif (TRIM)-containing proteins usually exert important regulatory roles during multiple biological processes. TRIM8 has been demonstrated to be a RING domain-containing E3 ubiquitin ligase which plays critical roles in inflammation and cancer. In this study, a TRIM8 homolog from grouper, Epinephelus coioides (EcTRIM8) was cloned, and its effects on fish virus replication were investigated. The full-length EcTRIM8 cDNA encoded a polypeptide of 568 amino acids with 92% identity to TRIM8 homolog from large yellow croaker (Larimichthys crocea). Sequence alignment analysis indicated that EcTRIM8 contained conserved RING finger, B-box and coiled-coil domain. Expression patterns analysis showed that EcTRIM8 was predominant in kidney, gill, fin, liver, spleen and brain. After challenging with Singapore grouper iridovirus (SGIV) or polyinosin-polycytidylic acid (poly I:C), the EcTRIM8 transcript was significantly increased at the early stage of injection. Under fluorescence microscopy, we observed different distribution patterns of EcTRIM8 in grouper spleen (GS) cells, including punctate fluorescence evenly situated throughout the cytoplasm and bright aggregates. The ectopic expression of EcTRIM8 in vitro significantly inhibited the replication of SGIV and red spotted grouper nervous necrosis virus (RGNNV), evidenced by the obvious reduction in the severity of cytopathic effect (CPE) and the significant decrease in viral gene transcription and protein synthesis. Moreover, the transcription of the proinflammatory factors and interferon related immune factors were differently regulated by EcTRIM8 during SGIV or RGNNV infection. In addition, overexpression of EcTRIM8 significantly increased the transcription of interferon regulator factor 3 (IRF3) and IRF7, and enhanced IRF3 or IRF7 induced interferon-stimulated response element (ISRE) promoter activity. Together, our results firstly demonstrated that fish TRIM8 could exert antiviral function through the regulation of the expression of proinflammatory cytokines and interferon related transcription factors in response to fish viruses.

Antiviral function of grouper MDA5 against iridovirus and nodavirus

Melanoma differentiation-associated gene 5 (MDA5) is a critical member of retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) family which can recognize viral RNA and enhances antiviral response in host cells. In this study, a MDA5 homolog from orange spotted grouper (Epinephelus coioides) (EcMDA5) was cloned, and its roles on grouper virus infection were characterized. The full-length EcMDA5 cDNA encoded a polypeptide of 982 amino acids with 74% identity with MDA5 homolog from rock bream (Oplegnathus fasciatus). Amino acid alignment analysis indicated that EcMDA5 contained three functional domains: two caspase activation and recruitment domain (CARDs), a DEAD box helicase-like (DExDc) domain, a helicase superfamily C-terminal domain (HELICc), and a C-terminal regulatory domain (RD). Upon challenge with Singapore grouper iridovirus (SGIV) or polyinosin-polycytidylic acid (poly I:C), the transcript of EcMDA5 was significantly up-regulated especially at the early stage post-injection. Under fluorescence microscopy, we observed that EcMDA5 mostly localized in the cytoplasm of grouper spleen (GS) cells. Interestingly, during virus infection, the distribution pattern of EcMDA5 was significantly altered in SGIV infected cells, but not in red spotted grouper nervous necrosis virus (RGNNV) infected cells, suggested that EcMDA5 might interact with viral proteins during SGIV infection. The ectopic expression of EcMDA5 in vitro obviously delayed virus infection induced cytopathic effect (CPE) progression and significantly inhibited viral gene transcription of RGNNV and SGIV. Moreover, overexpression of EcMDA5 not only significantly increased interferon (IFN) and IFN-stimulated response element (ISRE) promoter activities in a dose dependent manner, but also enhanced the expression of IRF3, IRF7 and TRAF6. In addition, the transcription level of the proinflammatory factors, including TNF-α, IL-6 and IL-8 were differently altered by EcMDA5 overexpression during SGIV or RGNNV infection, suggesting that the regulation on proinflammatory cytokines by EcMDA5 were also important for RGNNV infection. Together, our results demonstrated for the first time that the inhibitory effect of fish MDA5 on iridovirus replication might be mainly through the regulation of proinflammatory cytokines.

Understanding the interaction between Betanodavirus and its host for the development of prophylactic measures for viral encephalopathy and retinopathy

Over the last three decades, the causative agent of viral encephalopathy and retinopathy (VER) disease has become a serious problem of marine finfish aquaculture, and more recently the disease has also been associated with farmed freshwater fish. The virus has been classified as a Betanodavirus within the family Nodaviridae, and the fact that Betanodaviruses are known to affect more than 120 different farmed and wild fish and invertebrate species, highlights the risk that Betanodaviruses pose to global aquaculture production. Betanodaviruses have been clustered into four genotypes, based on the RNA sequence of the T4 variable region of their capsid protein, and are named after the fish species from which they were first derived i.e. Striped Jack nervous necrosis virus (SJNNV), Tiger puffer nervous necrosis virus (TPNNV), Barfin flounder nervous necrosis virus (BFNNV) and Red-spotted grouper nervous necrosis virus (RGNNV), while an additional genotype turbot betanodavirus strain (TNV) has also been proposed. However, these genotypes tend to be associated with a particular water temperature range rather than being species-specific. Larvae and juvenile fish are especially susceptible to VER, with up to 100% mortality resulting in these age groups during disease episodes, with vertical transmission of the virus increasing the disease problem in smaller fish. A number of vaccine preparations have been tested in the laboratory and in the field e.g. inactivated virus, recombinant proteins, virus-like particles and DNA based vaccines, and their efficacy, based on relative percentage survival, has ranged from medium to high levels of protection to little or no protection. Ultimately a combination of effective prophylactic measures, including vaccination, is needed to control VER, and should also target larvae and broodstock stages of production to help the industry deal with the problem of vertical transmission. As yet there are no commercial vaccines for VER and the aquaculture industry eagerly awaits such a product. In this review we provide an overview on the current state of knowledge of the disease, the pathogen, and interactions between betanodavirus and its host, to provide a greater understanding of the multiple factors involved in the disease process. Such knowledge is needed to develop effective methods for controlling VER in the field, to protect the various aquaculture species farmed globally from the different Betanodavirus genotypes to which they are susceptible.

Fish TRIM39 regulates cell cycle progression and exerts its antiviral function against iridovirus and nodavirus

The tripartite motif (TRIM)-containing proteins exert important immune regulatory roles through regulating different signaling pathways in response to different stimuli. TRIM39, a member of the TRIM family, is a RING domain-containing E3 ubiquitin ligase which could regulate cell cycle progression and apoptosis. However, the antiviral activity of TRIM39 is not explored. Here, a TRIM39 homolog from grouper, Epinephelus coioides (EcTRIM39) was cloned, and its effects on cell cycle progression and fish virus replication were investigated. The full-length EcTRIM39 cDNA was composed of 2535 bp and encoded a polypeptide of 543 amino acids with 70% identity with TRIM39 homologs from bicolor damselfish. Amino acid alignment analysis indicated that EcTRIM39 contained a RING finger, B-box and SPRY domain. Expression profile analysis revealed that EcTRIM39 was abundant in intestine, spleen and skin. Upon different stimuli in vivo, the EcTRIM39 transcript was obviously up-regulated after challenging with Singapore grouper iridovirus (SGIV), and polyinosinic-polycytidylic acid (poly I:C). Using fluorescence microscopy, we found that EcTRIM39 localized in the cytoplasm and formed aggregates in grouper spleen (GS) cells. The ectopic expression of EcTRIM39 in vitro affected the cell cycle progression via mediating G1/S transition. Moreover, the RING domain was essential for its accurate localization and effect on cell cycle. In addition, overexpression of EcTRIM39 significantly inhibited viral gene transcription of SGIV and red-spotted grouper nervous necrosis virus (RGNNV) in vitro, and the mutant of RING exerted the opposite effect. Together, our results demonstrated that fish TRIM39 not only regulated the cell cycle progression, but also acted as an important regulator of fish innate immune response against viruses.

Antiviral role of grouper STING against iridovirus infection

Stimulator of interferon genes (STING, also known as MITA, ERIS, MPYS or TMEM173) has been identified as a central component in the innate immune response to cytosolic DNA and RNA derived from different pathogens. However, the detailed role of STING during fish iridovirus infection still remained largely unknown. Here, the STING homolog from grouper Epinephelus coioides (EcSTING) was cloned and its effects on IFN response and antiviral activity were investigated. The full-length EcSTING cDNA was composed of 1590 bp and encoded a polypeptide of 409 amino acids with 80% identity to STING homolog from large yellow croaker. Amino acid alignment analysis indicated that EcSTING contained 4 predicated transmembrane motifs (TMs) in the N terminal, and a C-terminal domain (CTD) which consisted of a dimerization domain (DD), c-di-GMP-binding domain (CBD) and a C-terminal tail (CTT). Expression profile analysis revealed that EcSTING was abundant in gill, spleen, brain, skin, and liver. Upon different stimuli in vivo, the EcSTING transcript was dramatically up-regulated after challenging with Singapore grouper iridovirus (SGIV), lipopolysaccharide (LPS) and polyinosin-polycytidylic acid (poly I:C). Reporter gene assay showed that EcSTING activated ISRE, zebrafish type I IFN and type III IFN promoter in vitro. Mutant analysis showed that IFN promoter activity was mostly mediated by the phosphorylation sites at serine residue S379 and S387. Moreover, EcSTING induced type I and III IFN promoter activity could be impaired by overexpression of EcIRF3-DN or EcIRF7-DN, suggesting that EcSTING mediated IFN response in IRF3/IRF7 dependent manner. In addition, the cytopathic effect (CPE) progression of SGIV infection and viral protein synthesis was significantly inhibited by overexpression of EcSTING, and the inhibitory effect was abolished in serine residue S379 and S387 mutant transfected cells. Together, our results demonstrated that EcSTING might be an important regulator of grouper innate immune response against iridovirus infection.