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

Structural and Functional Characterization of a Fish Type I Subgroup d IFN Reveals Its Binding to Receptors

Teleost fish type I IFNs and the associated receptors from the cytokine receptor family B (CRFB) are characterized by remarkable diversity and complexity. How the fish type I IFNs bind to their receptors is still not fully understood. In this study, we demonstrate that CRFB1 and CRFB5 constitute the receptor pair through which type I subgroup d IFN (IFNd) from large yellow croaker, Larimichthys crocea, activates the conserved JAK-STAT signaling pathway as a part of the antiviral response. Our data suggest that L. crocea IFNd (LcIFNd) has a higher binding affinity with L. crocea CRFB5 (LcCRFB5) than with LcCRFB1. Furthermore, we report the crystal structure of LcIFNd at a 1.49-Å resolution and construct structural models of LcIFNd in binary complexes with predicted structures of extracellular regions of LcCRFB1 and LcCRFB5, respectively. Despite striking similarities in overall architectures of LcIFNd and its ortholog human IFN-ω, the receptor binding patterns between LcIFNd and its receptors show that teleost and mammalian type I IFNs may have differentially selected helices that bind to their homologous receptors. Correspondingly, key residues mediating binding of LcIFNd to LcCRFB1 and LcCRFB5 are largely distinct from the receptor-interacting residues in other fish and mammalian type I IFNs. Our findings reveal a ligand/receptor complex binding mechanism of IFNd in teleost fish, thus providing new insights into the function and evolution of type I IFNs.

Singapore grouper iridovirus infection counteracts poly I:C induced antiviral immune response in vitro

Groupers are important mariculture fish in South China and Southeast Asian countries. However, the increasing frequency of infectious disease outbreaks has caused great economic losses in the grouper industry. Among these pathogens, Singapore grouper iridovirus (SGIV) infection causes high mortality in larval and juvenile stages of grouper. However, the mechanism underlying the action of viral manipulation on cellular immune response still remained largely uncertain. Here, using RNA-seq technology, we investigated the regulatory roles of SGIV infection on synthetic RNA duplex poly I:C induced immune response in vitro. Using reporter gene assays, we found that SGIV infection decreased poly I:C induced interferon promoter activation. Transcriptomic analysis showed that the mRNA expression levels of 2238 genes were up-regulated, while 1247 genes were down-regulated in poly I:C transfected grouper spleen (GS) cells. Interestingly, SGIV infection decreased the expression of 1479 up-regulated genes and increased the expression of 297 down-regulated genes in poly I:C transfected cells. The differentially expressed genes (DEGs) down-regulated by SGIV were directly related to immune, inflammation and viral infection, and JUN, STAT1, NFKB1, MAPK14A, TGFB1 and MX were the 6 top hub genes in the down-regulated DEGs' protein-protein interaction (PPI) network. Furthermore, quantitative real-time PCR (qPCR) analysis confirmed that the interferon signaling and inflammatory-related genes, including cGAS, STING, TBK1, MAVS, TNF, IRAK4 and NOD2 were up-regulated by poly I:C stimulation, but all significantly down-regulated after SGIV infection. Thus, we speculated that SGIV infection counteracted poly I:C induced antiviral immune response and this ability helped itself to escape host immune surveillance. Together, our data will contribute greatly to understanding the potential immune evasion mechanism of iridovirus infection in vitro.

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.

Comparative transcriptomic analysis reveals different host cell responses to Singapore grouper iridovirus and red-spotted grouper nervous necrosis virus

Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) are important pathogens that cause high mortality and heavy economic losses in grouper aquaculture. Interestingly, SGIV infection in grouper cells induces paraptosis-like cell death, while RGNNV infection induces autophagy and necrosis characterized morphologically by vacuolation of lysosome. Here, a comparative transcriptomic analysis was carried out to identify the different molecular events during SGIV and RGNNV infection in grouper spleen (EAGS) cells. The functional enrichment analysis of DEGs suggested that several signaling pathways were involved in CPE progression and host immune response against SGIV or RGNNV. Most of DEGs featured in the KEGG "lysosome pathway" were up-regulated in RGNNV-infected cells, indicating that RGNNV induced lysosomal vacuolization and autophagy might be due to the disturbance of lysosomal function. More than 100 DEGs in cytoskeleton pathway and mitogen-activated protein kinase (MAPK) signal pathway were identified during SGIV infection, providing additional evidence for the roles of cytoskeleton remodeling in cell rounding during CPE progression and MAPK signaling in SGIV induced cell death. Of note, consistent with changes at the transcriptional levels, the post-translational modifications of MAPK signaling-related proteins were also detected during RGNNV infection, and the inhibitors of extracellular signal-regulated kinase (ERK) and p38 MAPK significantly suppressed viral replication and virus induced vacuoles formation. Moreover, the majority of DEGs in interferon and inflammation signaling were obviously up-regulated during RGNNV infection, but down-regulated during SGIV infection, suggesting that SGIV and RGNNV differently manipulated host immune response in vitro. In addition, purine and pyrimidine metabolism pathways were also differently regulated in SGIV and RGNNV-infection cells. Taken together, our data will provide new insights into understanding the potential mechanisms underlying different host cell responses against fish DNA and RNA virus.

Characterization of a novel activating protein-1 (AP-1) gene and the association of its single nucleotide polymorphisms with vibrio resistance in Tegillarca granosa

The blood clam Tegillarca granosa is a commercial marine bivalve of economic value, accounting for approximately 50% of clam production in China. In recent years, the yield of blood clams has been threatened by bacterial infections caused by marine Vibrio species that thrive under a rising sea temperature. The transcription factor activating protein-1 (AP-1) is emerging as an important player in the innate immunity of marine bivalves against viral or bacterial infections. In this study, the full-length cDNA of a novel T. granosa AP-1 (TgAP-1) was cloned for the first time. The 1591-bp cDNA encoded a protein of 292 amino acid residues with a calculated molecular weight of 32.8 kDa. The TgAP-1 protein contained an N-terminal Jun domain and a C-terminal basic region leucine zipper domain typically found in Jun proteins (a subfamily of AP-1 proteins). TgAP-1 was ubiquitously expressed in T. granosa, with the highest expression detected in the gill and foot, followed by the mantle, hemolymph, and hepatopancreas. Exposure to Vibrio harveyi induced TgAP-1 expression in gill tissues and the expression levels of TgAP-1 of resistant blood clams were always lower than that of control population whether Vibro infection or not. A total of 18 single nucleotide polymorphisms (SNPs) of TgAP-1 were detected in T. granosa. SNP-typing and haplotyping of resistant and susceptible populations revealed that six SNPs (AG type of TgSNP-1, GA type of TgSNP-2, TG type of TgSNP-4, CT type of TgSNP-7, AG type of TgSNP-11, and GA type of TgSNP-12) and four haplotypes (fHap2, fHap3, fHap6, and fHap7) were significantly associated with V. harveyi resistance. Risk assessment showed that fHap2 (CG) and fHap7 (GA) were associated with an increased resistance, while fHap3 (CT) and fHap6 (AG) were associated with an increased susceptibility. The results from this study supported a potential role of TgAp-1 in the anti-Vibro immunity of T. granosa. The discovery of the genetic molecular markers and haplotypes related to Vibrio resistance can provide guidance for selective breeding of T. granosa in the future.

Retroposition of the Long Transcript from Multiexon IFN-β Homologs in Ancestry Vertebrate Gave Rise to the Proximal Transcription Elements of Intronless IFN-β Promoter in Humans

IFN-β is a unique member of type I IFN in humans and contains four positive regulatory domains (PRDs), I-II-III-IV, in its promoter, which are docking sites for transcription factors IFN regulatory factor (IRF) 3/7, NF-κB, IRF3/7, and activating transcription factor 2/Jun proto-oncogene, respectively. In chicken IFN-β and zebrafish IFNφ1 promoters, a conserved PRD or PRD-like sequences have been reported. In this study, a type I IFN gene, named as xl-IFN1 in the amphibian model Xenopus laevis, was found to contain similar PRD-like sites, IV-III/I-II, in its promoter, and these PRD-like sites were proved to be functionally responsive to activating transcription factor 2/Jun proto-oncogene, IRF3/IRF7, and p65, respectively. The xl-IFN1, as IFNφ1 in zebrafish, was transcribed into a long and a short transcript, with the long transcript containing all of the transcriptional elements, including PRD-like sites and TATA box in its proximal promoter. A retroposition model was then proposed to explain the transcriptional conservation of IFNφ1, xl-IFN1, and IFN-β in chicken and humans.

Identification of IκBα in Japanese eel Anguilla japonica that impairs the IKKα-dependent activation of NF-κB, AP1, and type I IFN signaling pathways

As a member of inhibitory κB family (IκB) family, IκBα is best-characterized and plays a central negative feedback regulator of NF-κB pathway in mammals, but the information about IκBα in the regulation of immune responses is still limited in teleost fishes. In the present study, the full-length cDNA of an IκBα homologue, AjIκBα, was cloned by 5' and 3' SMART RACE from Japanese eel, and its characteristics of expression in response to various PAMPs and A. hydrophila infection were investigated both in vivo and in vitro using quantitative real-time polymerase chain reaction (qRT-PCR). In addition, the subcellular localization of AjIκBα GFP fusion protein and the induction of AjIκBα alone or co-expression with Japanese eel IKKα (AjIKKα) in the activation of NF-κB, type I IFN and AP1 performed using Dual-Glo luciferase assay system were also detected. Sequence comparison analysis revealed that AjIκBα has typical conserved domains, including the N-terminal conserved degradation motif, the ankyrin repeats, and the C-terminal PEST domain. The predicted three-dimensional structure of AjIκBα is similar to that of human IκBα. qRT-PCR analysis revealed a broad expression for AjIκBα in a wide range of tissues, with the highest expression in the spleen, followed by intestine, liver, gills, skin, kidney, and with a lower expression in the heart and muscle. The AjIκBα expressions in the kidney, spleen, and especially in liver were significantly induced following injection with Gram-negative bacterial component LPS, the viral mimic poly I:C and Aeromonas hydrophila infection. In vitro, the AjIκBα transcripts of Japanese eel liver cells were significantly enhanced by the treatment of LPS, poly I:C, or the stimulation of different concentration of Aeromonas hydrophil. Luciferase assays demonstrated that not only could the AjIκBα expression significantly decrease the activation of NF-κB, AP1, and IFNβ-responsive promoters in HEK293 cells and EPC cells, but also robustly inhibited the activity of these three promoters in HEK293 cells or NF-κB and AP1-responsive promoters in EPC cells induced by AjIKKα. Additionally, subcellular localization studies showed that AjIκBα was evenly distributed in the cytoplasm and nucleus both in HEK293 cells and EPC cells under natural state. AjIκBα was found to aggregate into spots in the cytoplasm and nucleus stimulated by LPS or mostly aggregate into nucleus with the treatment of poly I:C in HEK293 cells, whereas the elevated expression of AjIκBα was observed in the cytoplasm of EPC cells upon the stimulation of poly I:C. These results collectively indicated that AjIκBα function as an important negative regulation in innate immunity of host against antibacterial and antiviral infection likely via the inhibition of the activation of NF-κB, AP1, and type I IFN signaling pathways.

Fish IKKα from Japanese eel (Anguilla japonica) can activate NF-κB, AP1, and type I IFN signaling pathways

Inhibitor of nuclear factor kappa-B kinase subunit alpha (IKKα) plays a pivotal role in the activation of nuclear factor kappa-B (NF-κB) pathway in response to pathogens infections in mammals, but the information about IKKα in the regulation of immune responses is still limited in teleost fishes. In the present study, the full-length cDNA of an IKKα homologue, AjIKKα, was cloned by 5' and 3' SMART RACE from Japanese eel, and its characteristics of expression in response to various PAMPs and A. hydrophila infection were investigated both in vivo and in vitro using quantitative real-time polymerase chain reaction (qRT-PCR). In addition, the subcellular localization of AjIKKα GFP fusion protein and the induction of AjIKKα in the activation of NF-κB, type I IFN and AP1 performed using Dual-Glo luciferase assay system were also detected. Sequence comparison analysis revealed that AjIKKα has typical conserved domains, including an N-terminal kinase domain, an ubiquitin-like domain, a scaffold dimerization domain, and a C-terminal NEMO-binding domain. The predicted three-dimensional structure of AjIKKα is similar to that of human IKKα. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed a broad expression for AjIKKα in a wide range of tissues, with the highest expression in the liver, followed by the intestine, gills, and spleen, and with a lower expression in the muscle and heart. The AjIKKα expressions in the liver and kidney were significantly induced following injection with the viral mimic poly I:C and Aeromonas hydrophila infection, whereas the bacterial mimic LPS down-regulated the expression of AjIKKα in the spleen. In vitro, the AjIKKα transcripts of Japanese eel liver cells were significantly enhanced by the treatment of LPS, poly I:C, CpG-DNA, and PGN or the stimulation of different concentration of Aeromonas hydrophila (1 × 10⁶ cfu/mL, 1 × 10⁷ cfu/mL, and 1 × 10⁸ cfu/mL). Luciferase assays demonstrated that AjIKKα expression could significantly induce NF-κB, AP-1 and type I IFN promoter activation in a dose-dependent manner. Additionally, subcellular localization studies showed that AjIKKα was evenly distributed in the cytoplasm in the natural state, but AjIKKα was found to aggregate into spots in the cytoplasm after the stimulation of LPS and poly I:C. These results collectively indicated that AjIKKα plays an important role in innate immunity of host against antibacterial and antiviral infection likely via the activation of NF-κB, AP1and type I IFN signaling pathway.

Conservation of eATP perception throughout multicellular animal evolution: Identification and functional characterization of coral and amphioxus P2X7-like receptors and flounder P2X7 receptor

Perception of extracellular ATP (eATP), a common endogenous damage-associated molecular pattern, is through its receptor P2X7R. If eATP/P2X7R signaling is conserved throughout animal evolution is unknown. Moreover, little information is currently available regarding P2X7R in invertebrates. Here we demonstrated that the coral P2X7-like receptor, AdP2X7RL, the amphioxus P2X7-like receptor, BjP2X7RL and the flounder P2X7 receptor, PoP2X7R, shared common features characteristic of mammalian P2X7R, and their 3D structures displayed high resemblance to that of human P2X7R. Expression of Adp2x7rl, Bjp2x7rl and Pop2x7r was all subjected to the regulation by LPS and ATP. We also showed that AdP2X7RL, BjP2X7RL and PoP2X7R were distributed on the plasma membrane in AdP2X7RL-, BjP2X7RL- and PoP2X7R-expressing HEK cells, and had strong affinity to eATP. Importantly, the binding of AdP2X7RL, BjP2X7RL and PoP2X7R to eATP all induced similar downstream responses, including induction of cytokines (IL-1β, IL-6, IL-8 and CCL-2), enhancement of phagocytosis and activation of AKT/ERK-associated signaling pathway observed for mammalian P2X7R. Collectively, our results indicate for the first time that both coral and amphioxus P2X7RL as well as flounder P2X7R can interact with eATP, and induce events that trigger mammalian mechanisms, suggesting the high conservation of eATP perception throughout multicellular animal evolution.

Molecular cloning

Transcription factor c-Jun is a member of AP-1 transcription complex that can be induced by various pathogens and plays a broad regulatory role in vertebrate immune response. In this study, the complete c-Jun cDNA of large yellow croaker Larimichthys crocea (Lcc-Jun) was cloned, whose open reading frame (ORF) is 984 bp long and encodes a protein of 327 amino acids (aa). The deduced Lcc-Jun protein contains three highly conserved domains, a transactivation domain (TAD, Met¹-His¹¹⁸), a DNA binding domain (DBD, Lys²¹⁸-Arg²⁴³), and a Leucine zipper domain (LZD, Leu²⁷¹-Leu²⁹⁹), as found in other specie c-Jun. Lcc-Jun was constitutively expressed in all examined tissues, with the higher levels in blood, heart, and head kidney. Its transcripts were not only induced in spleen and head kidney by poly (I: C) or LPS, but also up-regulated in primary head kidney leukocytes (PKL), macrophages (PKM), and granulocytes (PKG), suggesting that Lcc-Jun may be involved in immune responses induced by poly (I: C), a viral mimic, and LPS, a Gram-negative bacterial component. Overexpression of Lcc-Jun in PKL increased the expression of cytokines and transcription factors involved in T helper 1 (Th1: TNF-α, IFN-γ, and T-bet) and Th2 (IL-4/13 A/B, IL-6, and GATA3) cell development and differentiation, suggesting that Lcc-Jun may play a role in regulation of Th1/Th2 cell response. These results therefore led us to suggest that the c-Jun-mediated signaling pathways may have an important immune-modulatory function in teleost fish.

Insight into the molecular function and transcriptional regulation of activator protein 1 (AP-1) components c-Jun/c-Fos ortholog in red lip mullet (Liza haematocheila)

The transcription factor, activator protein-1 (AP-1), is a dimeric protein and a downstream member of the mitogen-activated protein kinase (MAPK) signaling pathway. It regulates a wide array of functions including, cell proliferation, survival, differentiation, response to UV-irradiation, immune responses, and inflammatory conditions. AP-1 belongs to the basic leucine zipper (bZIP) protein family, which consists of members from Jun, Fos, Maf, and ATF subfamilies. In the present study, c-Jun and c-Fos homologs were identified from a transcriptome database of Liza haematocheila and designated as Lhc-Jun and Lhc-Fos. In both sequences, the signature bZIP domain was identified and also the DNA binding sites, dimerization sites, as well as the phosphorylation sites, were found to be highly conserved through evolution. Tissue distribution analysis revealed that both Lhc-Jun and Lhc-Fos transcripts were ubiquitously expressed in all examined tissues of healthy mullets. In order to determine the transcriptional modulations of Lhc-Jun and Lhc-Fos, challenge experiments were carried out using LPS, poly I:C, and L. garvieae. The qRT-PCR analysis revealed significant upregulation of Lhc-Jun and Lhc-Fos in blood, gill, liver, and spleen. This is the first study that explores the correlation between UV-irradiation and AP-1 ortholog expression in teleosts. Also, this is the first time that the functional characterization of the teleost c-Fos ortholog has been carried out. Sub-cellular localization of Lhc-Jun and Lhc-Fos was observed in the nucleus. AP-1-Luc reporter assays revealed significant higher luciferase activities in both Lhc-Jun and Lhc-Fos proteins compared to mock controls. These results strongly suggest that Lhc-Jun and Lhc-Fos might play a significant role in Liza haematocheila immunity by regulating AP-1 promoter sequences in immune and stress-related genes.

Characterization and function of a group II type I interferon in the perciform fish, large yellow croaker (Larimichthys crocea)

Teleost fish possess two groups of type I interferons (IFNs) with two (group I IFNs) or four (group II IFNs) conserved cysteines, which are further classified into seven subgroups. In our previous study, two group I type I IFNs, LcIFNd and LcIFNh (a new subgroup member), were identified in the perciform fish, large yellow croaker (Larimichthys crocea). Here, we identified a group II type I IFN, LcIFNc, in this species. The deduced LcIFNc contained six cysteines, four of which are highly conserved (C1: C²⁸, C2:C⁵³, C3: C¹³⁰, and C4:C¹⁵⁹) in the fish group II type I IFNs, and a typical type I IFN signature motif was also found in it. Phylogenetic analysis indicated that LcIFNc belongs to the IFNc subgroup of fish group II type I IFNs. LcIFNc was constitutively expressed in all examined tissues, and was rapidly up-regulated in spleen and head kidney by poly(I:C) and Aeromonas hydrophila. Recombinant LcIFNc protein (rLcIFNc) could increase the expression of antiviral genes, Mx1, PKR and ISG15, in large yellow croaker peripheral blood leukocytes (PBLs). The rLcIFNc also exhibited obvious antiviral activity based on less cytopathic effect (CPE) and decreased expression levels of several viral genes in the rLcIFNc-treated grouper spleen (GS) cells following Singapore grouper iridovirus (SGIV) infection. Additionally, rLcIFNc was able to induce the expression of LcIFNc, as well as LcIFNd and LcIFNh in the PBLs and primary head kidney cells (HKCs) from large yellow croaker. These results therefore indicated that LcIFNc not only had antiviral activity, but also mediated the regulation of type I IFN response.

Molecular Characterization of Two Mitogen-Activated Protein Kinases: p38 MAP Kinase and Ribosomal S6 Kinase From Bombyx mori (Lepidoptera: Bombycidae), and Insight Into Their Roles in Response to BmNPV Infection

Proteins p38 map kinase and ribosomal S6 kinase (S6K) as members of mitogen-activated protein kinases (MAPKs) play important roles against pathogens. In this study, Bmp38 and BmS6K were identified as differentially expressed proteins from iTRAQ database. Bmp38 and BmS6K were expressed, and recombinant proteins were purified. The bioinformatics analysis showed that both proteins have serine/threonine-protein kinases, catalytic domain (S_TKc) with 360 and 753 amino acids, respectively. The real-time quantitative polymerase chain reaction (RT-qPCR) results suggest that Bmp38 and BmS6K had high expression in the midgut and hemolymph. The comparative expression level of Bmp38 and BmS6K in BC9 was upregulated than in P50 in the midgut after Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Western bolt results showed a positive correlation between RT-qPCR and iTRAQ data for Bmp38, but BmS6K data showed partial correlation with iTRAQ. Injection of anti-Bmp38 and anti-BmS6K serum suggested that Bmp38 may be involved against BmNPV infection, whereas BmS6K may require phosphorylation modification to inhibit BmNPV infection. Taken together, our results suggest that Bmp38 and BmS6k might play an important role in innate immunity of silkworm against BmNPV.

Identification of Two Subgroups of Type I IFNs in Perciforme Fish Large Yellow Croaker Larimichthys crocea Provides Novel Insights into Function and Regulation of Fish Type I IFNs

Like mammals, fish possess an interferon regulatory factor (IRF) 3/IRF7-dependent type I IFN responses, but the exact mechanism by which IRF3/IRF7 regulate the type I IFNs remains largely unknown. In this study, we identified two type I IFNs in the Perciforme fish large yellow croaker Larimichthys crocea, one of which belongs to the fish IFNd subgroup and the other is assigned to a novel subgroup of group I IFNs in fish, tentatively termed IFNh. The two IFN genes are constitutively expressed in all examined tissues, but with varied expression levels. Both IFN genes can be rapidly induced in head kidney and spleen tissues by polyinosinic–polycytidylic acid. The recombinant IFNh was shown to be more potent to trigger a rapid induction of the antiviral genes MxA and protein kinase R than the IFNd, suggesting that they may play distinct roles in regulating early antiviral immunity. Strikingly, IFNd, but not IFNh, could induce the gene expression of itself and IFNh through a positive feedback loop mediated by the IFNd-dependent activation of IRF3 and IRF7. Furthermore, our data demonstrate that the induction of IFNd can be enhanced by the dimeric formation of IRF3 and IRF7, while the IFNh expression mainly involves IRF3. Taken together, our findings demonstrate that the IFN responses are diverse in fish and are likely to be regulated by distinct mechanisms.

Developmental transcriptome analysis and identification of genes involved in formation of intestinal air-breathing function of Dojo loach, Misgurnus anguillicaudatus

Dojo loach, Misgurnus anguillicaudatus is a freshwater fish species of the loach family Cobitidae, using its posterior intestine as an accessory air-breathing organ. Little is known about the molecular regulatory mechanisms in the formation of intestinal air-breathing function of M. anguillicaudatus. Here high-throughput sequencing of mRNAs was performed from six developmental stages of posterior intestine of M. anguillicaudatus: 4-Dph (days post hatch) group, 8-Dph group, 12-Dph group, 20-Dph group, 40-Dph group and Oyd (one-year-old) group. These six libraries were assembled into 81300 unigenes. Totally 40757 unigenes were annotated. Subsequently, 35291 differentially expressed genes (DEGs) were scanned among different developmental stages and clustered into 20 gene expression profiles. Finally, 15 key pathways and 25 key genes were mined, providing potential targets for candidate gene selection involved in formation of intestinal air-breathing function in M. anguillicaudatus. This is the first report of developmental transcriptome of posterior intestine in M. anguillicaudatus, offering a substantial contribution to the sequence resources for this species and providing a deep insight into the formation mechanism of its intestinal air-breathing function. This report demonstrates that M. anguillicaudatus is a good model for studies to identify and characterize the molecular basis of accessory air-breathing organ development in fish.

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.

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.

Evolutionary conservation of molecular structure and antiviral function of a viral receptor, LGP2, in amphioxus Branchiostoma japonicum

RIG-I-like (where RIG-I is retinoic acid inducible gene I) receptor LGP2 (where LGP2 is laboratory of genetics and physiology) is an important intracellular receptor that recognizes viral RNAs in innate immunity, but its origin and evolution remains unknown. Here we clearly demonstrate the presence of a RIG-I-like receptor, BjLGP2, in the basal chordate amphioxus. It is predominantly expressed in the hepatic caecum and hindgut, and is upregulated following challenge with poly(I:C). BjLGP2 is distributed in the cytoplasm of both grouper spleen and flounder gill (FG) cells, and the recombinant BjLGP2 interacts with poly(I:C). BjLGP2 can enhance the expression of IFN and IFN-inducible genes in FG cells upon poly(I:C) challenge. It also significantly induces the expression of the antiviral genes ifn-i and Mx as well as the signal transduction relevant genes MAVS, NF-κB, and IRF-3 in FG cells upon lymphocystis disease virus challenge. Moreover, BjLGP2 inhibits the replication of lymphocystis disease virus in FG cells and the gene transcription of Singapore grouper iridovirus in grouper spleen cells. This is the first report showing that a LGP2 protein in invertebrate species (amphioxus) is structurally conserved and plays an antiviral role similar to that of vertebrate LGP2 proteins.