Antiviral immunity of grouper MAP kinase phosphatase 1 to Singapore grouper iridovirus infection

Singapore grouper iridovirus (SGIV), with various mechanisms for evading and modulating host, has inflicted heavy economic losses in the grouper aquaculture. MAP kinase phosphatase 1 (MKP-1) regulates mitogen-activated protein kinases (MAPKs) to mediate the innate immune response. Here, we cloned EcMKP-1, an MKP-1 homolog from the orange-spotted grouper Epinephelus coioides, and investigated its role in the infection of SGIV. In juvenile grouper, EcMKP-1 was highly upregulated and peaked at different times after injection with lipopolysaccharide, polyriboinosinic polyribocytidylic acid and SGIV. EcMKP-1 expression in heterologous fathead minnow cells was able to suppress SGIV infection and replication. Furthermore, EcMKP-1 was a negative regulator of c-Jun N-terminal kinase (JNK) phosphorylation early in SGIV infection. EcMKP-1 decreased the apoptotic percentage and caspase-3 activity during the late stage of SGIV replication. Our results demonstrate critical functions of EcMKP-1 in antiviral immunity, JNK dephosphorylation and anti-apoptosis during SGIV infection.

Modulatory effects of curcumin on Singapore grouper iridovirus infection-associated apoptosis and autophagy in vitro

Singapore grouper iridovirus (SGIV) with high pathogenicity can cause great economic losses to aquaculture industry. Thus, it is of urgency to find effective antiviral strategies to combat SGIV. Curcumin has been demonstrated effective antiviral activity on SGIV infection. However, the molecular mechanism behind this action needs to be further explanations. In view of the fact that apoptosis (type I programmed cell death) and autophagy (type II programmed cell death) were key regulators during SGIV infection, we aimed to investigate the relevance between antiviral activity of curcumin and SGIV-associated programmed and clarify the role of potential signaling pathways. Our results showed that curcumin suppressed SGIV-induced apoptosis. At the same time, the activities of caspase-3/8/9 and activating protein-1 (AP-1), P53, nuclear factor-κB (NF-ΚB) promoters were inhibited. Besides, the activation of extracellular regulated protein kinases (ERK), c-Jun N-terminal kinase (JNK) and p38 mitogen activate protein kinase (p38 MAPK) signal pathways were suppressed in curcumin-treated cells. On the other hand, curcumin down-regulated protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway to promote autophagy representing by increased LC3 II and Beclin1 expression. Curcumin also hindered the transition of cells from G1 to S phase, as well as down-regulating the expression of CyclinD1. Our findings revealed the resistance curcumin induced to the effects of DNA virus on cell apoptosis and autophagy and the insights gained from this study may be of assistance to understand the molecular mechanism of curcumin against DNA virus infection.

Grouper ATF1 plays an antiviral role in response to iridovirus and nodavirus infection

Transcription factor ATF1 is a member of the ATF/CREB family of the CREB subfamily and is involved in physiological processes such as tumorigenesis, organ development, reproduction, cell survival, and apoptosis in mammals. However, studies on ATF1 in fish have been relatively poorly reported, especially on its role in antiviral immunity in fish. In this study, ATF1 from orange-spotted grouper (named EcATF1) were cloned and characterized. Molecular characterization analysis showed that EcATF1 encodes a 307-amino-acid protein, containing PKID and bZIP_CREB1 domains. Homology analysis showed that had the highest homology with E. lanceolatus(88.93%). Tissue expression pattern showed that EcATF1 was extensively distributed in twelve selected tissues, with higher expression in the skin, gill, liver and spleen. Subcellular localization analysis showed that EcATF1 was distributed in the nucleus of GS cells. EcATF1 overexpression inhibits Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) replication, as evidenced by a diminished degree of CPE induced by SGIV and RGNNV and a reduction in the level of viral gene transcription and viral capsid protein expression. Furthermore, EcATF1 overexpression upregulated interferon pathway-related genes and proinflammatory factors, and increased the promoter activities of IFN, IFN stimulated response element (ISRE), and nuclear factor κB(NFκB). Meanwhile, EcATF1 overexpression positive regulate the MHC-I signaling pathway, and upregulated the promoter activity of MHC-I. Collectively, these data demonstrate that EcATF1 plays an important role during the host antiviral immune response. This study provides insights into the function of ATF1 in the immune system of lower vertebrates.

Functional analysis of the Cystatin F gene response to SGIV infection in orange-spotted grouper, Epinephelus coioides

Cystatin F (CyF), an inhibitor of cysteine protease, was widely studied in immune defense and cancer therapy. However, the function of CyF and its latent molecular mechanism during virus infection in fish remain vacant. In our research, we cloned the open reading frame (ORF) of CyF homology from orange-spotted grouper (Ec-CyF) consisting of 342 nucleotides and encoding a 114-amino acid protein. Ec-CyF included two cystatins family sequences containing one KXVXG sequence without the signal peptide, and a hairpin ring containing proline and tryptophan (PW). Tissue distribution analysis indicated that Ec-CyF was highly expressed in spleen and head kidney. Besides, further analysis showed that the expression of Ec-CyF increased during SGIV infection in grouper spleen (GS) cells. Subcellular localization assay demonstrated that Ec-CyF was mainly distributed in cytoplasm in GS cells. Overexpressed Ec-CyF demoted the mRNA level of viral genes MCP, VP19 and LITAF. Meanwhile, SGIV-induced apoptosis in fat head minnow (FHM) cells was impeded, as well as the restraint of caspase 3/7 and caspase 8. In addition, Ec-CyF overexpression up-regulated the expression of IFN related molecules including ISG15, IFN, IFP35, IRF3, IRF7, MYD88 and down-regulated proinflammatory factors such as IL-1β, IL-8 and TNF-α. At the same time, Ec-CyF-overexpressing increased the activity of IFN3 and ISRE promoter, but impeded NF-κB promoter activity by luciferase reporter gene assay. In summary, our findings suggested that Ec-CyF was involved in innate immunity response and played a key role in DNA virus infection.

Functional Analysis of the Cathepsin D Gene Response to SGIV Infection in the Orange-Spotted Grouper, Epinephelus coioides

(1) Background: Lysosomal aspartic protease Cathepsin D (CD) is a key regulator and signaling molecule in various biological processes including activation and degradation of intracellular proteins, the antigen process and programmed cell death. However, the function of fish CD in virus infection remains largely unknown. (2) Methods: The functions of the CD gene response to SGIV infection was determined with light microscopy, reverse transcription quantitative PCR, Western blot and flow cytometry. (3) Results: In this study, Ec-Cathepsin D (Ec-CD) was cloned and identified from the orange-spotted grouper, Epinephelus coioides. The open reading frame (ORF) of Ec-CD consisted of 1191 nucleotides encoding a 396 amino acid protein with a predicted molecular mass of 43.17 kDa. Ec-CD possessed typical CD structural features including an N-terminal signal peptide, a propeptide region and a mature domain including two glycosylation sites and two active sites, which were conserved in other CD sequences. Ec-CD was predominantly expressed in the spleen and kidneys of healthy groupers. A subcellular localization assay indicated that Ec-CD was mainly distributed in the cytoplasm. Ec-CD expression was suppressed by SGIV stimulation and Ec-CD-overexpressing inhibited SGIV replication, SGIV-induced apoptosis, caspase 3/8/9 activity and the activation of reporter gene p53 and activating protein-1 (AP-1) in vitro. Simultaneously, Ec-CD overexpression obviously restrained the activated mitogen-activated protein kinase (MAPK) pathways, including extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). In addition, Ec-CD overexpression negatively regulated the transcription level of pro-inflammatory cytokines and activation of the NF-κB promotor. (4) Conclusions: Our findings revealed that the Ec-CD possibly served a function during SGIV infection.

Identification of virion-associated transcriptional transactivator (VATT) of SGIV ICP46 promoter and their binding site on promoter

BACKGROUND

Iridoviruses are large DNA viruses that cause diseases in fish, amphibians and insects. Singapore grouper iridovirus (SGIV) is isolated from cultured grouper and characterized as a ranavirus. ICP46 is defined to be a core gene of the family Iridoviridae and SGIV ICP46 was demonstrated to be an immediate-early (IE) gene associated with cell growth control and could contribute to virus replication in previous research.

METHODS

The transcription start site (TSS) and 5'-untranslated region (5'-UTR) of SGIV ICP46 were determined using 5' RACE. The core promoter elements of ICP46s were analyzed by bioinformatics analysis. The core promoter region and the regulation model of SGIV ICP46 promoter were revealed by the construction of serially deleted promoter plasmids, transfections, drug treat and luciferase reporter assays. The identification of virion-associated transcriptional transactivator (VATT) that interact with SGIV ICP46 promoter and their binding site on promoter were performed by electrophoretic mobility shift assays (EMSA), DNA pull-down assays and mass spectrometry (MS).

RESULTS

SGIV ICP46 was found to have short 5'-UTR and a presumptive downstream promoter element (DPE), AGACA, which locates at + 36 to + 39 nt downstream of the TSS. The core promoter region of SGIV ICP46 located from - 22 to + 42 nt relative to the TSS. VATTs were involved in the promoter activation of SGIV ICP46 and further identified to be VP12, VP39, VP57 and MCP. A 10-base DNA sequence "ATGGCTTTCG" between the TSS and presumptive DPE was determined to be the binding site of the VATTs.

CONCLUSION

Our study showed that four VAATs (VP12, VP39, VP57 and MCP) might bind with the SGIV ICP46 promoter and be involved in the promoter activation. Further, the binding site of the VATTs on promoter was a 10-base DNA sequence between the TSS and presumptive DPE.

Functional characterization of Cystatin C in orange-spotted grouper, Epinephelus coioides

Cystatin C is an endogenous inhibitor of cysteine proteases and widely exist in organisms. Several studies in mammals have showed that Cystatin C plays critical role in the immune defense against microorganisms. It is also well known that some fish Cystatin C have important immune regulation functions in inflammatory responses. However, the function of fish Cystatin C in virus infection as well as its underlying molecular mechanisms remain to be elucidated. In the present study, a Cystatin C gene termed Ec-CysC was identified from orange-spotted grouper, Epinephelus coioides. The full-length of Ec-CysC cDNA was 817 bp with a 387 bp open reading frame (ORF) that encoded a 129-amino acid (aa) protein, including 18-aa signal peptide and 111-aa mature polypeptide. The deduced amino acid of Ec-CysC shared three conserved domains containing Glycine at the N-terminus region, QVVAG motif in the middle and PW motif near the C-terminus region. Transcription analysis of the Ec-CysC gene showed its expression in all twelve examined tissues including liver, spleen, kidney, brain, intestine, heart, skin, muscle, fin, stomach, gill and head kidney. Its expression following stimulation with Singapore grouper iridovirus (SGIV) was further tested in spleen, the relative expression of Ec-CysC was significantly up-regulated at 12 h post-infection. The subcellular localization experiment revealed that Ec-CysC was mainly distributed in the cytoplasm in Grouper Spleen (GS) cells. In vitro, Overexpression of Ec-CysC in GS cells significantly reduced the expression of viral genes, namely, ORF162, ORF049 and ORF072. Meanwhile, we found that overexpression of Ec-CysC resulted in upward trend of expression of inflammatory cytokines TNF-a, IL-1β and IL8 during SGIV infection. Further, SGIV-inducible apoptosis and Caspase-3 activity were also weakened by overexpression Ec-CysC in fathead minnow (FHM) cells. These results indicated that Ec-CysC might have a deeper involvement in fish immune defense, and played important roles in inflammation and apoptosis induced by SGIV.

Characterization of cathepsin C from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

The lysosomal cysteine protease cathepsin C plays a pivotal role in regulation of inflammatory and immune responses. However, the function of fish cathepsin C in virus replication remains largely unknown. In this study, cathepsin C gene (Ec-CC) was cloned and characterized from orange-spotted grouper, Epinephelus coioides. The full-length Ec-CC cDNA was composed of 2077 bp. It contained an open reading frame (ORF) of 1374 bp and encoded a 458-amino acid protein which shared 89% identity to cathepsin C from bicolor damselfish (Stegastes partitus). Amino acid alignment analysis showed that Ec-CC contained an N-terminal signal peptide, the propeptide region and the mature peptide. RT-PCR analysis showed that Ec-CC transcript was expressed in all the examined tissues which abundant in spleen and head kidney. After challenged with Singapore grouper iridovirus (SGIV) stimulation, the relative expression of EC-CC was significantly increased at 24 h post-infection. Subcellular localization analysis revealed that Ec-CC was distributed mainly in the cytoplasm. Further studies showed that overexpression of Ec-CC in vitro significantly delayed the cytopathic effect (CPE) progression evoked by SGIV and inhibited the viral genes transcription. Moreover, overexpression of Ec-CC significantly increased the expression of proinflammatory cytokines during SGIV infection. Taken together, our results demonstrated that Ec-CC might play a functional role in SGIV infection by regulating the inflammation response.

Transcriptomics analysis reveals candidate genes and pathways for susceptibility or resistance to Singapore grouper iridovirus in orange-spotted grouper (Epinephelus coioides)

In this study, the transcriptional response of grouper to Singapore grouper iridovirus (SGIV) stimulation was characterized using RNA sequencing. Transcriptome sequencing of three test groups in the grouper was performed using the Illumina MiSeq platform. The three test groups were a control group, which was injected with PBS buffer; a high-susceptible (HS) group, which died shortly after the SGIV injection; and a high-resistance (HR) group, which survived the SGIV injection. In total, 38,253 unigenes were generated. When the HS group was compared with the control group, 885 unigenes were upregulated and 487 unigenes were downregulated. When the HR and control groups were compared, 1114 unigenes were upregulated and 420 were downregulated, and when the HR and HS groups were compared, 1010 unigenes were upregulated and 375 were downregulated. In the KEGG analysis, two immune-related pathways, the p53 and peroxisome proliferator-activated receptor pathways, were detected with highly significant enrichment. In addition, 7465 microsatellites and 22,1569 candidate single nucleotide polymorphisms were identified from our transcriptome data. The results suggested several pathways that are associated with traits of disease susceptibility or disease resistance, and provided extensive information about novel gene sequences, gene expression profiles, and genetic markers. This may contribute to vaccine research and a breeding program against SGIV infection in grouper.

Identification of a Bcl-xL homolog from orange-spotted grouper (Epinephelus coioides) involved in SGIV-induced nonapoptotic cell death

Bcl-2 family proteins play essential roles in modulating immune response and controlling cells' fate. Bcl-xL is one of anti-apoptotic protein in this family. In this study, a new Bcl-xL homolog (EcBcl-xL) was identified and characterized from orange-spotted grouper, Epinephelus coioides. EcBcl-xL encoded a 221 amino acid peptides that shared 86% identity to Larimichthys crocea Bcl-xL protein, contained four conserved BH domains and one transmembrane region. The predicted three-dimensional structure of EcBcl-xL was similar with Homo sapiens Bcl-xL. EcBcl-xL widely expressed in all tested tissues with highest expression in head kidney. Its expression level was significantly up-regulated after SGIV infection in vivo. Furthermore, overexpression of EcBcl-xL could inhibit SGIV-induced nonapoptotic cell death and suppressed viral genes transcriptions in GS cells. Our findings suggested that EcBcl-xL might play a role during virus infection through modulating SGIV-induced nonapoptotic cell death.

BNIP3, a cell pro-apoptotic protein, involved in response to viral infection in orange spotted grouper, Epinephelus coioides

BNIP3 is a kind of BH3-only protein that induces both cell death and autophagy. Here, a BNIP3 gene (EcBNIP3) was identified from orange spotted grouper, Epinephelus coioides. EcBNIP3 possessed 236 amino acids residues, contained a conservative BNIP3 domain and a transmembrane region. Besides, EcBNIP3 expressed at a relative high level in heart and spleen. EcBNIP3 transcript was up-regulated after SGIV infection in vitro. Subcellular localization analysis revealed that EcBNIP3 was predominantly localized in the cytoplasm and co-localized with mitochondria. In addition, overexpression EcBNIP3 accelerated SGIV infection induced cell death but inhibited viral genes transcription. Taken together, these results provided new evidence that fish BNIP3 might involved in response to virus infection.

c-Jun N-terminal kinases 3 (JNK3) from orange-spotted grouper, Epinephelus coioides, inhibiting the replication of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis

C-Jun N-terminal kinases (JNKs), a subgroup of serine-threonine protein kinases that activated by phosphorylation, are involve in physiological and pathophysiological processes. JNK3 is one of JNK proteins involved in JNK3 signaling transduction. In the present study, two JNK3 isoforms, Ec-JNK3 X1 and Ec-JNK3 X2, were cloned from orange-spotted grouper, Epinephelus coioides. Both Ec-JNK3 X1 and Ec-JNK3 X2 were mainly expressed in liver, gill, skin, brain and muscle of juvenile grouper. The relative expression of Ec-JNK3 X2 mRNA was much higher in muscle and gill than that of Ec-JNK3 X1. Isoform-specific immune response to challenges was revealed by the expression profiles in vivo. Immunofluorescence staining indicated that JNK3 was localized in the cytoplasm of grouper spleen (GS) cells and shown immune response to SGIV infection in vitro. Over-expressing Ec-JNK3 X1 and/or Ec-JNK3 X2 inhibited the SGIV infection and replication and the SGIV-induced apoptosis. To achieve the antiviral and anti-apoptosis activities, JNK3 promoted the activation of genes ISRE and type I IFN in the antiviral IFN signaling pathway, and inhibited the activation of transcription factors NF-κB and p53 relating to apoptosis, respectively. Ec-JNK3 X2 showed stronger activities in antivirus and anti-apoptosis than that of Ec-JNK3 X1. Our results not only define the characterization of JNK3 but also reveal new immune functions and the molecular mechanisms of JNK3 on iridoviruses infection and the virus-induced apoptosis.

MKK7 confers different activities to viral infection of Singapore grouper iridovirus (SGIV) and nervous necrosis virus (NNV) in grouper

Mitogen-activated protein kinase 7 (MKK7) is one of the major stress-activated protein kinase (SAPK)-activating kinases in response to environmental or physiological stimuli. Here a MKK7 named as Ec-MKK7 was identified from orange-spotted grouper, Epinephelus coioides. The full-length cDNA of Ec-MKK7 was 1853 bp, with an open reading frame (ORF) of 1272 bp encoding a putative protein of 423 amino acids. A characteristic S-K-A-K-T motif was contained in the domain of dual-specificity protein kinase, mitogen-activated protein kinase kinase 7 (PKc_MKK7). Intracellular localization showed that Ec-MKK7 was localized in both the cytoplasm and the nucleus of grouper spleen (GS) and/or grouper brain (EAGB) cells. Moreover, Ec-MKK7 was universally expressed in all examined tissues and showed expression modulation to challenges of lipopolysacchride (LPS), Singapore grouper iridovirus (SGIV) and polyriboinosinic polyribocytidylic acid (poly I:C) in vivo. A gene targeting strategy over-expressing Ec-MKK7 was performed to examine the activities of MKK7 to viral infection in vitro. Our data showed that Ec-MKK7 was involved in the evasion and replication of SGIV but played an antiviral role to the infection of nervous necrosis virus (NNV). All results demonstrated that Ec-MKK7 could play important roles in grouper innate immunity and show distinct functions on virus infection.

Identification of the Bcl-2 family protein gene BOK from orange-spotted grouper (Epinephelus coioides) involved in SGIV infection

Apoptosis plays vital roles in many physiological process and immune response. BOK is one of the central regulators in apoptosis. In this study, a new BOK homolog (Ec-BOK) was cloned and characterized from Orange-spotted grouper, Epinephelus coioides. Ec-BOK encoded a 210 amino acid peptides which shared 97% identity to Stegastes partitus BOK protein, contained four BH domains and one transmembrane region. Ec-BOK widely expressed in all analyzed tissues with the higher expressions in kidney and spleen. Its expression level was up-regulated after SGIV infection in vitro. Further analysis revealed that overexpression of Ec-BOK inhibited viral genes transcriptions and virus replication in fish cell. Our findings suggested that Ec-BOK might play a role in the immune response against virus.

JNK1 Derived from Orange-Spotted Grouper, Epinephelus coioides, Involving in the Evasion and Infection of Singapore Grouper Iridovirus (SGIV)

c-Jun N-terminal kinase (JNK) regulates cellular responses to various extracellular stimuli, environmental stresses, pathogen infections, and apoptotic agents. Here, a JNK1, Ec-JNK1, was identified from orange-spotted grouper, Epinephelus coioides. Ec-JNK1 has been found involving in the immune response to pathogen challenges in vivo, and the infection of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis in vitro. SGIV infection activated Ec-JNK1, of which phosphorylation of motif TPY is crucial for its activity. Over-expressing Ec-JNK1 phosphorylated transcription factors c-Jun and promoted the infection and replication of SGIV, while partial inhibition of the phosphorylation of Ec-JNK1 showed the opposite effects by over-expressing the dominant-negative EcJNK1-Δ183-185 mutant. Interestingly, SGIV enhanced the viral infectivity by activating Ec-JNK1 which in turn drastically inhibited the antiviral responses of type 1 IFN, indicating that Ec-JNK1 could be involved in blocking IFN signaling during SGIV infection. In addition, Ec-JNK1 enhanced the activation of AP-1, p53, and NF-κB, and resulted in increasing the levels of SGIV-induced cell death. The caspase 3-dependent activation correlated with the phosphorylation of Ec-JNK1 and contributed to SGIV-induced apoptosis. Taken together, SGIV modulated the phosphorylation of Ec-JNK1 to inactivate the antiviral signaling, enhance the SGIV-induced apoptosis and activate transcription factors for efficient infection and replication. The “positive cooperativity” molecular mechanism mediated by Ec-JNK1 contributes to the successful evasion and infection of iridovirus pathogenesis.

Molecular clone and characterization of c-Jun N-terminal kinases 2 from orange-spotted grouper, Epinephelus coioides

c-Jun N-terminal kinase 2 (JNK2) is a multifunctional mitogen-activated protein kinases involving in cell differentiation and proliferation, apoptosis, immune response and inflammatory conditions. In this study, we reported a new JNK2 (Ec-JNK2) derived from orange-spotted grouper, Epinephelus coioides. The full-length cDNA of Ec-JNK2 was 1920 bp in size, containing a 174 bp 5'-untranslated region (UTR), 483 bp 3'-UTR, and a 1263 bp open reading frame (ORF), which encoded a putative protein of 420 amino acids. The deduced protein sequence of Ec-JNK2 contained a conserved Thr-Pro-Tyr (TPY) motif in the domain of serine/threonine protein kinase (S-TKc). Ec-JNK2 has been found to involve in the immune response to pathogen challenges in vivo, and the infection of Singapore grouper iridovirus (SGIV) in vitro. Immunofluorescence staining showed that Ec-JNK2 was localized in the cytoplasm of grouper spleen (GS) cells, and moved to the nucleus after infecting with SGIV. Ec-JNK2 distributed in all immune-related tissues examined. After challenging with lipopolysaccharide (LPS), SGIV and polyriboinosinic polyribocytidylic acid (poly I:C), the mRNA expression of Ec-JNK2 was significantly (P < 0.01) up-regulated in juvenile orange-spotted grouper. Over-expressing Ec-JNK2 in fathead minnow (FHM) cells increased the SGIV infection and replication, while over-expressing the dominant-negative Ec-JNK2Δ181-183 mutant decreased it. These results indicated that Ec-JNK2 could be an important molecule in the successful infection and evasion of SGIV.

Characterization of c-Jun from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

The nuclear phosphoprotein c-Jun is a member of the AP1 family of transcription activating complex, can be induced by various extracellular stimuli such as virus infection. In this study, the c-Jun gene (Ec-c-Jun) was cloned from orange-spotted grouper, Epinephelus coioides. The full-length Ec-c-Jun cDNA is composed of 2046 bp and encodes a polypeptide of 328 amino acids with 81% identity of zebrafish. Amino acid alignment analysis indicated that Ec-c-Jun contained three conserved domains including a transactivation domain (TAD), a DNA-binding domain (DBD) and leucine zipper domain (LZD). RT-PCR results showed that Ec-c-Jun transcript was most abundant in spleen, kidney, heart and gill. The expression of Ec-c-Jun was up-regulated after challenged with Singapore grouper iridovirus (SGIV). To investigate the roles of Ec-c-Jun during SGIV infection, we constructed its dominant-negative mutant (DN-Ec-c-Jun) by deleting the major TAD that lacks amino acids 3-122. Fluorescence microscopy observation revealed that Ec-c-Jun and DN-Ec-c-Jun were expressed predominantly in the nucleus in transfected cells. Interestingly, the green fluorescence of Ec-c-Jun was congregated and co-localized with virus assembly sites at the late stage of SGIV infection. However, in DN-Ec-c-Jun transfected cells, no virus assembly sites were observed, and the distribution of fluorescence remained unchanged. Moreover, overexpression of DN-Ec-c-Jun in vitro delayed the occurrence of CPE induced by SGIV infection and inhibited the virus gene transcription. In addition, ectopic expression of DN-Ec-c-Jun was able to inhibit SGIV induced c-Jun/AP1 promoter activity in GS cells. Thus, we proposed that c-Jun transcription factor was essential for SGIV replication in vitro. Our results will contribute to understanding the crucial roles of JNK signaling pathway in fish virus infection.

Identification and characterization of Rab7 from orange-spotted grouper, Epinephelus coioides

Rab7 is a small GTPase that regulates vesicular traffic from early to late endosomal stages of the endocytic pathway. During the virus-host co-evolution, host Rab7 was also exploited by virus to complete their life cycle. To date, however, the roles of fish Rab7 in virus infection remained largely unknown. Here, we cloned and characterized a Rab7 gene from grouper, Epinephelus coioides (Ec-Rab7). The full-length Ec-Rab7 cDNA was composed of 1182 bp and encoded a polypeptide of 207 amino acids which shared 99% identity with that from Anoplopoma fimbria or Oreochromis niloticus. Ec-Rab7 contained five conserved domains of Rab GTPase family including GTP-binding or GTPase regions as well as an effector site. RT-PCR analysis revealed that Ec-Rab7 ubiquitously expressed in all detected tissues and its transcript in spleen was up-regulated after challenge with Singapore grouper iridovirus (SGIV). Subcellular localization analysis revealed that Ec-Rab7 was distributed in the cytoplasm as spots and mostly colocalized with lysosomes. Notably, the ectopic expressed Ec-Rab7 partly aggregated into the viral factories in cells infected by SGIV. Furthermore, overexpression of Ec-Rab7 accelerated the occurrence of cytopathic effect (CPE) induced by SGIV infection and promoted viral gene transcription. In addition, far western blotting assay revealed that Ec-Rab7 might interact with viral proteins, including SGIV VP69 and VP101. Taken together, our data suggested that Ec-Rab7 might be potentially involved in SGIV replication.

Characterization of cathepsin B gene from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

The lysosomal cysteine protease cathepsin B of papain family is a key regulator and signaling molecule that involves in various biological processes, such as the regulation of apoptosis and activation of virus. In the present study, cathepsin B gene (Ec-CB) was cloned and characterized from orange-spotted grouper, Epinephelus coioides. The full-length Ec-CB cDNA was composed of 1918 bp and encoded a polypeptide of 330 amino acids with higher identities to cathepsin B of teleosts and mammalians. Ec-CB possessed typical cathepsin B structural features including an N-terminal signal peptide, the propeptide region and the cysteine protease domain which were conserved in other cathepsin B sequences. Phylogenetic analysis revealed that Ec-CB was most closely related to Lutjanus argentimaculatus. RT-PCR analysis showed that Ec-CB transcript was expressed in all the examined tissues which abundant in spleen, kidney and gill. After challenged with Singapore grouper iridovirus (SGIV) stimulation, the mRNA expression of cathepsin B in E. coioides was up-regulated at 24 h post-infection. Subcellular localization analysis revealed that Ec-CB was distributed predominantly in the cytoplasm. When the fish cells (GS or FHM) were treated with the cathepsin B specific inhibitor CA-074Me, the occurrence of CPE induced by SGIV was delayed, and the viral gene transcription was significantly inhibited. Additionally, SGIV-induced typical apoptosis was also inhibited by CA-074Me in FHM cells. Taken together, our results demonstrated that the Ec-CB might play a functional role in SGIV infection.

Characterization of LPS-induced TNFα factor (LITAF) from orange-spotted grouper, Epinephelus coioides

Lipopolysaccharide-induced TNFα factor (LITAF) is an important transcription factor that mediates cell apoptosis and inflammatory response. In the present study, we cloned and characterized a LITAF gene from orange-spotted grouper (Epinephelus coioides) (Ec-LITAF). Ec-LITAF encoded a predicted 142 amino acid protein which shared 74% identity to sablefish (Anoplopoma fimbria) LITAF homolog. Multiple amino acid alignment showed that Ec-LITAF contained a typical LITAF domain with two CXXC motifs. Phylogenetic analysis indicated that Ec-LITAF was closely related to that of sablefish. Ec-LITAF mRNA was widely expressed in different tissues and its expression level in spleen was up-regulated after Singapore grouper iridovirus (SGIV) infection. Subcellular localization analysis revealed that the distribution of Ec-LITAF showed diffuse and aggregated patterns in cytoplasm. Interestingly, the distribution of Ec-LITAF overlayed with a viral LITAF homolog (vLITAF) encoded by SGIV. Overexpression of Ec-LITAF in vitro up-regulated the expression of tumor necrosis factors (TNF1 and TNF2) and TNF receptors (TNFR1 and TNFR2), and the expression of itself initiated apoptosis in fish cells. In addition, overexpression of Ec-LITAF not only accelerated SGIV infection induced CPE and cell death, but also increased viral gene transcription. Taken together, our data suggested that Ec-LITAF might play crucial roles during SGIV replication.

Identification and characterization of TRP14, a thioredoxin-related protein of 14 kDa from orange-spotted grouper, Epinephelus coioides

Thioredoxin (abbreviated as Trx) is an important ubiquitous disulfide reductase, which can protect organisms against various oxidative stresses. In the present study, a thioredoxin-related protein of 14 kDa (named as Ec-TRP14) was identified from the marine fish grouper, Epinephelus coioides by RACE PCR. The full-length cDNA of Ec-TRP14 was comprised of 1066 bp with a 372 bp open reading frame that encodes a putative protein of 123 amino acids. Similar to most TRP14s, Ec-TRP14 contained the conserved motif C-P-D-C. Ec-TRP14 mRNA is predominately expressed in liver, brain and muscle. The expression of Ec-TRP14 was up-regulated in the liver of grouper challenged with SGIV. Ec-TRP14 was recombined and expressed in Escherichia coli BL21 (DE3), and the rEc-Ec-TRP14 fusion protein was demonstrated to possess the antioxidant activity. The grouper spleen (GS) cells were treated with a high concentration of rEc-TRP14 (8.3 μg/ml), which significantly enhanced cells viability under damage caused by viral infection. These results together indicated that Ec-TRP14 could function as an important antioxidant in a physiological context, and might be involved in the responses to viral challenge.