Molecular cloning and immune responsive expression of MDA5 gene, a pivotal member of the RLR gene family from grass carp Ctenopharyngodon idella

Expression profiles of four Nile Tilapia innate immune genes during early stages of Aeromonas veronii infection

OBJECTIVE

During Egypt's hot summer season, Aeromonas veronii infection causes catastrophic mortality on Nile Tilapia Oreochromis niloticus farms. Egypt is ranked first in aquaculture production in Africa, sixth in aquaculture production worldwide, and third in global tilapia production. This study aimed to investigate, at the molecular level, the early innate immune responses of Nile Tilapia to experimental A. veronii infection.

METHODS

The relative gene expression, co-expression clustering, and correlation of four selected immune genes were studied by quantitative real-time polymerase chain reaction in four organs (spleen, liver, gills, and intestine) for up to 72 h after a waterborne A. veronii challenge. The four genes studied were nucleotide-binding oligomerization domain 1 (NOD1), lipopolysaccharide-binding protein (LBP), natural killer-lysin (NKL), and interleukin-1 beta (IL-1β).

RESULT

The four genes showed significant transcriptional upregulation in response to infection. At 72 h postchallenge, the highest NOD1 and IL-1β expression levels were recorded in the spleen, whereas the highest LBP and NKL expression levels were found in the gills. Pairwise distances of the data points and the hierarchical relationship showed that NOD1 clustered with IL-1β, whereas LBP clustered with NKL; both genes within each cluster showed a significant positive expression correlation. Tissue clustering indicated that the responses of only the gill and intestine exhibited a significant positive correlation.

CONCLUSION

The results suggest that NOD1, LBP, NKL, and IL-1β genes play pivotal roles in the early innate immune response of Nile Tilapia to A. veronii infection, and the postinfection expression profile trends of these genes imply tissue-/organ-specific responses and synchronized co-regulation.

Comparative Transcriptomics Analysis Reveals Unique Immune Response to Grass Carp Reovirus Infection in Barbel Chub (Squaliobarbus curriculus)

Grass carp (Ctenopharyngodon idella) and barbel chub (Squaliobarbus curriculus)-both Leuciscinae subfamily species-demonstrate differences in grass carp reovirus (GCRV) infection resistance. We infected barbel chubs with type II GCRV and subjected their liver, spleen, head kidney, and trunk kidney samples to investigate anti-GCRV immune mechanisms via RNA sequencing and quantitative real-time polymerase chain reaction (qRT-PCR). We identified 139, 970, 867, and 2374 differentially expressed genes (DEGs) in the liver, spleen, head kidney, and trunk kidney, respectively. Across all four tissues, gene ontology analysis revealed significant immune response-related DEG enrichment, and the Kyoto Encyclopedia of Genes and Genomes analysis revealed pattern recognition receptor (PRR) and cytokine-related pathway enrichment. We noted autophagy pathway enrichment in the spleen, head kidney, and trunk kidney; apoptosis pathway enrichment in the spleen and trunk kidney; and complement- and coagulation-cascade pathway enrichment in only the spleen. Comparative transcriptome analysis between GCRV-infected barbel chubs and uninfected barbel chubs comprehensively revealed that PRR, cytokine-related, complement- and coagulation-cascade, apoptosis, and autophagy pathways are potential key factors influencing barbel chub resistance to GCRV infection. qRT-PCR validation of 11 immune-related DEGs confirmed our RNA-seq data's accuracy. These findings provide a theoretical foundation and empirical evidence for the understanding of GCRV infection resistance in barbel chub and hybrid grass carp-barbel chub breeding.

Proteomic and Phosphoproteomic Analysis Reveals Differential Immune Response to Hirame Novirhabdovirus (HIRRV) Infection in the Flounder (Paralichthys olivaceus) under Different Temperature

Hirame novirhabdovirus (HIRRV) is one of most serious viral pathogens causing significant economic losses to the flounder (Paralichthys olivaceus)-farming industry. Previous studies have shown that the outbreak of HIRRV is highly temperature-dependent, and revealed the viral replication was significantly affected by the antiviral response of flounders under different temperatures. In the present study, the proteome and phosphoproteome was used to analyze the different antiviral responses in the HIRRV-infected flounder under 10 °C and 20 °C. Post viral infection, 472 differentially expressed proteins (DEPs) were identified in the spleen of flounder under 10 °C, which related to NOD-like receptor signaling pathway, RIG-I-like receptor signaling pathway, RNA transport and so on. Under 20 °C, 652 DEPs were identified and involved in focal adhesion, regulation of actin cytoskeleton, phagosome, NOD-like receptor signaling pathway and RIG-I-like receptor signaling pathway. Phosphoproteome analysis showed that 675 differentially expressed phosphoproteins (DEPPs) were identified in the viral infected spleen under 10 °C and significantly enriched in Spliceosome, signaling pathway, necroptosis and RNA transport. Under 20 °C, 1304 DEPPs were identified and significantly enriched to Proteasome, VEGF signaling pathway, apoptosis, Spliceosome, mTOR signaling pathway, mRNA surveillance pathway, and RNA transport. To be noted, the proteins and phosphoproteins involved in interferon production and signaling showed significant upregulations in the viral infected flounder under 20 °C compared with that under 10 °C. Furthermore, the temporal expression profiles of eight selected antiviral-related mRNA including IRF3, IRF7, IKKβ, TBK1, IFIT1, IFI44, MX1 and ISG15 were detected by qRT-PCR, which showed a significantly stronger response at early infection under 20 °C. These results provided fundamental resources for subsequent in-depth research on the HIRRV infection mechanism and the antiviral immunity of flounder, and also gives evidences for the high mortality of HIRRV-infected flounder under low temperature.

Integrative analysis of miRNA and mRNA expression associated with the immune response in the intestine of rainbow trout (Oncorhynchus mykiss) infected with infectious hematopoietic necrosis virus

Rainbow trout (Oncorhynchus mykiss), an economically important cold-water fish cultured worldwide, suffers from infectious hematopoietic necrosis virus (IHNV) infection, resulting in huge financial losses. In order to understand the immune response of rainbow trout during virus infection, we explored trout intestine transcriptome profiles following IHNV challenge, and identified 3355 differentially expressed genes (DEGs) and 80 differentially expressed miRNAs (DEMs). Transcriptome analysis revealed numerous DEGs involved in immune responses, such as toll-like receptor 3 (TLR3), toll-like receptor 7/8 (TLR7/8), tripartite motif-containing 25 (TRIM25), DExH-Box helicase 58 (DHX58), interferon-induced with helicase C domain 1 (IFIH1), interferon regulatory factor 3 (IRF3/7), signal transducer and activator of transcription 1 (STAT1) and heat shock protein 90-alpha 1 (HSP90A1). Integrated analysis identified five key miRNAs (miR-19-y, miR-181-z, miR-203-y, miR-143-z and miR-206-y) targeting at least two important immune genes (TRIM25, DHX58, STAT1, TLR7/8 and HSP90A1). Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses showed that DEGs and target genes were significantly enriched in various immune-related terms including immune system process, binding, cell part and pathways of Toll-like receptor signalling, RIG-I-like receptor signalling, NOD-like receptor signalling, JAK-STAT signalling, PI3K-Akt signalling, NF-kappa B signalling, IL-17 signalling and AGE-RAGE signalling. In addition, protein-protein interaction networks (PPI) was used to display highly interactive DEG networks involving eight immune-related pathways. The expression trends of 12 DEGs and 10 DEMs were further verified by quantitative real-time PCR, which confirmed the reliability of the transcriptome sequencing results. This study expands our understanding of the immune response of rainbow trout infected with IHNV, and provides valuable resources for future studies on the immune molecular mechanism and disease resistance breeding.

Transcriptome analysis identifies LGP2 as an MDA5-mediated signaling activator following spring viremia of carp virus infection in common carp (Cyprinus carpio L.)

The common carp (Cyprinus carpio L.) is an important farmed species worldwide. Mucosal-associated lymphoid tissues play an essential role in the fight against pathogen infection. Spring viremia of carp virus (SVCV) poses a serious threat to the common carp aquaculture industry. Understanding the molecular mechanisms driving mucosal immune responses to SVCV infection is critical. In this study, the mucosal tissues (gills, foregut and hindgut) were collected from normal and infected fishes for transcriptome analysis. A total of 932,378,600 clean reads were obtained, of which approximately 80% were successfully mapped to the common carp genome. 577, 1,054 and 1,014 differential expressed genes (DEGs) were identified in the gills, foregut and hindgut, respectively. A quantitative polymerase chain reaction assay indicated that the DEGs expression in the foregut following SVCV infection was consistent with the transcriptome results. Among them, two key genes of the retinoic acid-inducible gene I (RIG-I)-like receptor family, melanoma-differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) (i.e., CcMDA5 and CcLGP2), underwent further analysis. Overexpression of CcMDA5 or CcLGP2 increased phosphorylation of TANK-binding kinase 1 and interferon regulatory factor 3 and the expression of interferon-1 (ifn-1), myxovirus resistance (mx), viperin and interferon-stimulated gene 15 (isg15), and inhibited SVCV replication in epithelioma papulosum cyprini cells. Furthermore, CcLGP2 significantly upregulated the CcMDA5-induced ifn-1 mRNA expression and the activation of the ifn-1 promoter. Finally, confocal microscopy and coimmunoprecipitation experiments revealed that CcLGP2 colocalizes and interacts with CcMDA5 via the C-terminal regulatory domain. This study provides essential gene resources for understanding the fish immune response to SVCV infection and sheds light on the potential role of fish LGP2 in the MDA5 regulation.

Identification and comparative study of melanoma differentiation-associated gene 5 homologues of triploid hybrid fish and its parents

Sterile triploid fish (3n = 150), derived from the hybridization between red crucian carp (Carassius auratus red var., ♀, 2n = 100) and allotetraploid (♂, 4n = 200), exhibits the improved disease resistance compared with its parents, but the current knowledge of the immunity of triploid fish is limited. Here, we report the identification and characterization of melanoma differentiation-associated gene 5 (MDA5) homologues from red crucian carp, triploid fish and allotetraploid. In this study, one red crucian carp MDA5 transcript (2nMDA5), two triploid fish MDA5 transcripts (3nMDA5-a and 3nMDA5-b) and two allotetraploid fish MDA5 transcripts (4nMDA5-a and 4nMDA5-b) have been cloned and identified separately. Immunofluorescence staining assay displayed that these MDA5 proteins were cytoplasmic proteins. RT-qPCR assay showed that, in response to spring viremia of carp virus (SVCV) and poly (I:C) stimuli, the increase of 3nMDA5 mRNA level was obviously higher than those of 2nMDA5 and 4nMDA5. Interestingly, the reporter assay and plaque assay revealed collectively that 3nMDA5-b, a shorter splicing form of MDA5, exhibited the strongest IFN promoter-inducing ability and antiviral activity. Additionally, when co-expressed with 3nMAVS, 3nMDA5-b induced a considerably higher level of IFN promoter activation than 3nMDA5-a; and the interactions between 3nMAVS/3nMDA5-a and 3nMAVS/3nMDA5-b were verified by co-IP assay. Taken together, our findings support the conclusion that in triploid fish, 3nMDA5-b mediates a robust antiviral signaling in host innate immune response.

Presence of two RIG-I-like receptors, MDA5 and LGP2, and their dsRNA binding capacity in a perciform fish, the snakehead Channa argus

Fish retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are critical RNA sensors in cytoplasm and are involved in antiviral innate immunity. However, some species of fish lack RIG-I gene, and the function of RLR members in RIG-I-absent fish is poorly understood. In the present study, MDA5, LGP2 and MAVS genes were identified in commercially important snakehead Channa argus. But, RIG-I gene was not found in this fish, and a systematic analysis of RLRs in available genome database of fish indicated the absence of RIG-I in the Acanthomorphata, Clupeiformes and Polypteriformes, suggesting that loss events of RIG-I gene may have occurred independently three times in the evolutionary history of fish. The MDA5, LGP2 and MAVS in snakehead have conserved protein domains and genomic location based on sequence, phylogenetic and syntenic analyses. These genes are constitutively expressed in healthy fish and can be induced by polyinosinic and polycytidylic acid (poly(I:C)) stimulation in vitro. It is further revealed that the snakehead MDA5 and LGP2 have binding capacity with dsRNA, such as poly(I:C), and MDA5 can interact with MAVS, implying the antiviral function of MDA5 in the RIG-I-absent fish.

Grass Carp Reovirus VP35 Degrades MAVS Through the Autophagy Pathway to Inhibit Fish Interferon Production

Fish interferon (IFN) is a crucial cytokine for a host to resist external pathogens, conferring cells with antiviral capacity. Meanwhile, grass carp reovirus (GCRV) is a strong pathogen that causes high mortality in grass carp. Therefore, it is necessary to study the strategy used by GCRV to evade the cellular IFN response. In this study, we found that GCRV 35-kDa protein (VP35) inhibited the host IFN production by degrading mitochondrial antiviral signaling (MAVS) protein through the autophagy pathway. First, the overexpression of VP35 inhibited the IFN activation induced by polyinosinic-polycytidylic acid (poly I:C) and MAVS, and the expression of downstream IFN-stimulated genes (ISGs) was also decreased by using VP35 under the stimulation. Second, VP35 interacted with MAVS; the experiments of truncated mutants of MAVS demonstrated that the caspase recruitment domain (CARD) and proline-rich (PRO) domains of MAVS were not necessary for this binding. Then, MAVS was degraded by using VP35 in a dose-dependent manner, and 3-MA (the autophagy pathway inhibitor) significantly blocked the degradation, meaning that MAVS was degraded by using VP35 in the autophagy pathway. The result of MAVS degradation suggested that the antiviral capacity of MAVS was remarkably depressed when interrupted by VP35. Finally, in the host cells, VP35 reduced ifn transcription and made the cells vulnerable to virus infection. In conclusion, our results reveal that GCRV VP35 impairs the host IFN response by degrading MAVS through the autophagy pathway, supplying evidence of a fish virus immune evasion strategy.

Cytosolic Sensors for Pathogenic Viral and Bacterial Nucleic Acids in Fish

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

Structural features and antiviral function of the MDA5 gene in ducks (Anas platyrhynchos)

Melanoma differentiation-associated gene 5 (MDA5) is an important cytoplasmic RNA sensor that detects viral double-stranded RNA in innate immunity. The objective of this study was to characterize the structure and function of the MDA5 gene in the duck. In this study, full-length duck MDA5 (duMDA5) complementary DNA (cDNA) was obtained using the reverse transcription-polymerase chain reaction and rapid amplification of the cDNA ends. The cDNA consisted of a 123 nucleotide 5′ untranslated region (UTR), a 735 nucleotide 3′ UTR, and a 3012 nucleotide open-reading frame, encoding 1003 amino acids. Multiple sequence alignments showed that duMDA5 had 91.18% and 83.11% amino acid sequence similarity with geese and chicken MDA5, respectively, as well as 59.76%–61.26% sequence identity with mammalian homologs. Phylogenetic analysis demonstrated that MDA5 has been highly conserved throughout vertebrate evolution. Quantitative real-time polymerase chain reaction analysis indicated that the duMDA5 mRNA is scarcely detected in healthy tissues and the highest relative transcript level of duMDA5 was induced during poly(I:C) stimulation. Furthermore, knockdown duMDA5 significantly inhibited the transcription of poly(I:C)-induced beta interferons, nuclear factor kappa-B, interferon regulatory factor 7, translocated intimin receptor domain-containing adaptor protein inducing beta interferons, interferon-induced GTP-binding protein, signal transducer and activator of transcription 1 and 2 mRNA. Taken together, these results suggest that duMDA5 is an important receptor for inducing antiviral activity in the duck’s innate immune response.

Black carp NAP1 positively regulates MDA5-mediated antiviral signaling during the innate immune activation

NAK-associated protein 1 (NAP1) is involved in NF-κB activation and interferon (IFN) induction in human and mammal; however, the role of teleost NAP1 in innate immunity remains unknown. In this paper, NAP1 homologue of black carp (Mylopharyngodon piceus) has been cloned and characterized. Black carp NAP1 (bcNAP1) migrated around 47 kDa in immunoblot assay and was identified as a cytosolic protein by immunofluorescent staining. bcNAP1 showed little IFN promoter-inducing ability in the reporter assay and bcNAP1 presented no antiviral activity against either grass carp reovirus (GCRV) or spring viremia of carp virus (SVCV) in the plaque assay. However, when co-expressed with black carp MDA5 (bcMDA5), bcNAP1 enhanced bcMDA5-mediated IFN promoter induction in the reporter assay. Accordingly, the plaque assay data demonstrated that the antiviral activity of bcMDA5 against both GCRV and SVCV was upregulated by bcNAP1. Additionally, the association between bcNAP1 and bcMDA5 has been identified through immunofluorescent staining and co-immunoprecipitation (co-IP) assay. Thus, the data generated in this study support the conclusion that bcNAP1 interacts with bcMDA5 and up-regulates bcMDA5-mediated antiviral signaling during host innate immune activation.

Dietary Astragalus polysaccharides ameliorates the growth performance, antioxidant capacity and immune responses in turbot (Scophthalmus maximus L.)

Supplying immunostimulants to aquatic feed has been an effective way to enhance the health of aquatic animals and substitute for antibiotics. In the present study, the potential effects of Astragalus polysaccharides (APS) were evaluated in turbot, Scophthalmus maximus. Two levels of APS (50 and 150 mg/kg) were added to the basal diet (CON) and a 63-day growth trial (initial weight 10.13 ± 0.04 g) was conducted. As the results showed, significant improvement on growth performance in the APS groups were observed. In addition, dietary 150 mg/kg APS significantly increased the total antioxidant capacity (T-AOC), glutathione peroxidase (GSH-PX) and lysozyme activities in liver. Meanwhile, APS diets induced the mRNA expression of toll-like receptors (TLRs) such as tlr5α, tlr5β, tlr8 and tlr21, while reduced the expression of tlr3 and tlr22. The expression of inflammatory genes myeloid differentiation factor 88 and nuclear factor kappa b p65 and pro-inflammatory cytokines tumor necrosis factor-α and interleukin-1β were up-regulated in APS groups while the expression of anti-inflammatory cytokine transforming growth factor beta was inhibited. Taken together, the present study indicated that Astragalus polysaccharides could remarkably enhance the growth performance, antioxidant activity and maintain an active immune response in turbot.

Characterization and immune function of the interferon-β promoter stimulator-1 in the barbel chub, Squaliobarbus curriculus

To elucidate the immunity-protecting role of the interferon-β promoter stimulator-1 (ScIPS-1) in barbel chub Squaliobarbus curriculus, the full-length cDNA of ScIPS-1 was cloned and expression levels in response to stimulation were investigated. In addition, the function of ScIPS-1 and its domains were analyzed. The full-length cDNA of ScIPS-1 is 2524 bp and encodes 601 aa. The N-terminal caspase activation and recruitment domain, central proline-rich domain, C-terminal transmembrane domain, C₂HC-zinc finger, and Cwf21 domains were identified. The mRNA level of ScIPS-1 was the highest in the kidney, whereas the highest protein level was observed in the liver. The ScIPS-1 expressions were significantly up-regulated after lipopolysaccharide and poly I:C treatment. The ScIPS-1 protein level was up-regulated at 12 h in the head kidney and was up-regulated at 12 h and then down-regulated from 12 to 48 h in the liver after grass carp reovirus (GCRV) infection. The CiIFN and CiMx transcription levels were significantly enhanced in pEGFP-C1-IPS-1 and pcDNA3.1-ΔCwf21 overexpressing cells after GCRV infection. The results indicate that ScIPS-1 may function in the immune response against pathogens and provide a basis for achieving resistance to diseases in fish breeding.

Raptor/mTORC1 Acts as a Modulatory Center to Regulate Anti-bacterial Immune Response in Rockfish

The mammalian target of rapamycin (mTOR) is an evolutionarily highly conserved atypical serine/threonine protein kinase, which regulates cell growth, proliferation, apoptosis, autophagy, and metabolism. As a regulatory protein, Raptor is awfully important for the stability and function of mTOR complex 1 (mTORC1). However, the studies about how Raptor/mTORC1 participates in and regulates immune response in lower vertebrates are still limited. In this study, we investigated the regulation of immune response by the Raptor/mTORC1 signaling pathway in rockfish Sebastes schlegelii. Sebastes schlegelii Raptor (Ss-Raptor) is a highly conserved protein during the evolution, in both primary and tertiary structure. Ss-Raptor mRNA was widely distributed in various tissues of rockfish and has a relative higher expression in spleen and blood. After infected by Micrococcus luteus or Listonella anguillarum, mRNA expression of Ss-Raptor rapidly increased within 48 h. Once Raptor/mTORC1 signaling was blocked by rapamycin, expression of the pro-inflammatory cytokines IL-1β and IL-8 was severely impaired, suggesting potential regulatory role of Raptor/mTORC1 signaling in the innate immune response of rockfish. In addition, Raptor/mTORC1 pathway participated in lymphocyte activation of rockfish through promoting 4EBP1 and S6 phosphorylation. Inhibition of Raptor/mTORC1 signaling crippled the lymphocyte expansion during primary adaptive immune response, manifesting by the decrease of lymphoid organ weight and lymphocyte numbers. More importantly, inhibition of Raptor/mTORC1 signaling impaired the lymphocyte mediated cytotoxic response, and made the fish more vulnerable to the bacterial infection. Together, our results suggested that Raptor and its tightly regulated mTORC1 signaling acts as modulatory center to regulate both innate and lymphocyte-mediated adaptive immune response during bacterial infection. This research has shed new light on regulatory mechanism of teleost immune response, and provide helpful evidences to understand the evolution of immune system.

Sequence and expression analysis of the cytoplasmic pattern recognition receptor melanoma differentiation-associated gene 5 from the barbel chub Squaliobarbus curriculus

MDA5 is a cytoplasmic viral double-stranded RNA recognition receptor that plays a pivotal role in the aquatic animal innate immune system. To decipher the role of MDA5 of Squaliobarbus curriculus (ScMDA5) in the immune response, full-length cDNA of ScMDA5 was cloned using the RACE technology, mRNA and protein expression levels of ScMDA5 signalling pathway members in response to stimulation were detected and effects of overexpression of ScMDA5 on the immune response were investigated. ScMDA5 comprises 3597 bp and is composed of an open reading frame (2958 nucleotides long) that translates into a putative peptide of 985 amino acid residues. ScMDA5 possesses two N-terminal caspase-recruiting domains, DEAD-like helicases superfamily, helicase superfamily C-terminal and RIG-I_C-RD domains, and differences in these domains among species were mainly observed with respect to their length and location. ScMDA5 was closely clustered with those of Carassius auratus, Ctenopharyngodon idellus and Mylopharyngodon piceus. ScMDA5 transcripts were most abundant in the spleen and the lowest in the liver. Expression levels of ScMDA5 in healthy tissues were significantly correlated with those of ScIRF3, ScIRF7 and ScIFN. Besides, mRNA expression levels of ScIRF3 were significantly correlated with those of ScIRF7 (0.956, P < 0.01). Expression level changes, including downregulation, upregulation and initial upregulation followed by downregulation, were found in ScMDA5 signalling pathway molecules in tissues after grass carp reovirus infection. Protein levels of ScMDA5 were the highest in the liver and the lowest in the spleen in detected healthy tissues. Overexpression of ScMDA5 led to significantly enhanced CiIRF7 and CiMx transcription in grass carp ovary cells (P < 0.05). The results of this study helped to clarify the role of ScMDA5 in the immune reaction against grass carp reovirus and provided fundamental information for fish breeding to achieve strong resistance to infection.

Pattern Recognition by Melanoma Differentiation-Associated Gene 5 (Mda5) in Teleost Fish: A Review

Teleost fish, as with other vertebrates, rely on their innate immune system as a first line of defense against invading pathogens. A very important characteristic of the innate immune response is its ability to recognize conserved molecular structures, such as viral dsRNA and ssRNA. Mda5 is one of the three pattern recognition receptors (PRRs) that recognize cytoplasmic viral ligands. Teleost Mda5 is widely conserved among several fish species and possesses the same structural domains as those seen in their mammalian counterparts. Fish Mda5 has been shown to be capable of initiating an inflammatory response both in vitro (in different fish cell lines) and in vivo using synthetic viral analogs or virus. The interferon (IFN) pathway is triggered as a result of Mda5 activation, leading to the expression of type I IFNs, IFN- stimulated genes and pro-inflammatory cytokines. Although it is known that Mda5 acts as a receptor for virally-produced ligands, it has been shown more recently that it can also initiate an immune response against bacterial challenges. This review discusses recent advances in the characterization of teleost Mda5 and its potential role in antiviral and antibacterial immunity in teleost fish.

Cloning and characterization of the LEF/TCF gene family in grass carp (Ctenopharyngodon idella) and their expression profiles in response to grass carp reovirus infection

T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) proteins from the High Mobility Group (HMG) box family act as the main downstream effectors of the Wnt signaling pathway. HMGB proteins play multifaceted roles in the immune system of mammals. To clarify the immunological characteristics of LEF/TCF genes in grass carp (Ctenopharyngodon idella), five LEF/TCF genes (TCF7, LEF1, TCF7L1A, TCF7L1B, and TCF7L2) were identified and characterized. All five LEF/TCF proteins contained two characteristic domains: a HMG-BOX domain and a CTNNB1_binding region. Phylogenetic tree analysis revealed that the LEF/TCF proteins were represented different lineages. These results of subcellular localization showed that four of the LEF/TCF genes were localized exclusively within the nucleus, while TCF7L2 was localized in the cytoplasm and nucleus. The mRNA expression profiles of these LEF/TCF family genes differed across different tissues. The mRNA expression levels of TCF7, TCF7L1A, and TCF7L2 changed significantly in liver after grass carp reovirus (GCRV) challenge; TCF7 and TCF7L1A responded early while TCF7L2 responded late. This suggests that these genes may participate in GCRV-related immune responses. Moreover, TCF7 promoted Bcl6 transcription in response to the GCRV challenge. These findings further our understanding of the function of LEF/TCF genes in teleosts.

MDA5 and LGP2 acts as a key regulator though activating NF-κB and IRF3 in RLRs signaling of mandarinfish

RIG-I-like receptors (RLRs), as key cytoplasmic sensors of viral pathogen-associated molecular patterns, can recognise viral RNA and enhance the antiviral response. Some investigations have focused on the roles of RLRs in the innate immune response in grass carp, large yellow croaker, and rainbow trout. However, little is known about the function of RLRs in mandarinfish (Siniperca chuatsi), an important economic fish in Perciformes. Here, we functionally characterized the RLRs involved in the immune responses of mandarinfish (Siniperca chuatsi), by evaluating three RLRs, namely, RIG-I, MDA5, and LGP2. The results revealed that MDA5 and LGP2 were present in mandarinfish, whereas RIG-I was absent. The MDA5 and LGP2 cDNA sequences contained 2976 and 2046 bp and encoded 991 and 681 amino acids, respectively. Multiple sequence alignments showed that MDA5 and LGP2 of mandarinfish were clustered together with their homologs from other teleost fishes and shared high similarities with those from other vertebrates, and RIG-I of mandarinfish was absent. Moreover, quantitative real-time PCR (qPCR) analysis suggested that MDA5 and LGP2 were constitutively expressed in all tissues tested, and MDA5 mRNA expression was relatively high in the gill, and spleen, whereas LGP2 mRNA expression was high in the liver, gill, and head kidney. After stimulation with lipopolysaccharide or poly I:C, the expression of MDA5 and LGP2 was upregulated in spleen, gill and head kidney, but the pattern was not exactly the same, MDA5 transcripts generally increased and then declined with the prolonged infection, while LGP2 transcripts went up continuously, which showed that mandarinfish MDA5 and LGP2 may play independent roles in antiviral response. Besides, it is further revealed that the MDA5 could activate NF-κB and IRF3 to inducing the production of IFN-β by constructing tet-on stable strain of 293T cell, however over-expression of LGP2 resulted in decreased NF-κB, IRF3 and IFN-β production in cells challenged with LPS and polyI:C Taken together, our results demonstrated that MDA5 and LGP2, as a positive and negative regulator, respectively, played an important role in modulating antibacterial andantiviral immune responses though activating NF-κB and IRF3 in RLRs signaling of mandarinfish.

Identification of four type I IFNs from Japanese eel with differential expression properties and Mx promoter inducibility

Type I IFNs are a family of cytokines with antiviral, anti-proliferative and immune-modulatory functions. In this study, four type I IFNs (termed AjIFN1-4) have been cloned from the Japanese eel, Anguilla japonica. The open reading frames of AjIFN1-4 are 552, 534, 546 and 561 bp in length, encoding 183, 177, 181, and 186 amino acids (aa), respectively. Sequence comparison and phylogenetic analysis results revealed that AjIFN1 and AjIFN2 belong to group one (2C-containing) IFNs, while AjIFN3 and AjIFN4 belong to group two (4C-containing) IFNs. Syntenic comparison showed that chromosome block duplication and rearrangement events might have occurred at IFN loci in different teleost lineages. Expression analysis revealed the rapid induction of AjIFNl and AjIFN2 in response to poly I:C stimulation, while AjIFN3 and AjIFN4 were predominantly expressed at later time points. Two Mx promoter reporter assays were conducted to assess the Mx-inducing capability of AjIFN1-4. It is shown that the overexpression of AjIFN1-4 all promoted the luciferase activity of MxB reporter, but the activity of MxC reporter increased only in cells transfected with AjIFN1. Collectively, it is suggested that teleost IFNs were evolved independently in different lineages of fish and may function differently in teleost antiviral immunity.

Cloning, expression and bioinformatics analysis of a putative pigeon melanoma differentiation-associated gene 5

1. Melanoma differentiation-associated gene 5 (MDA5) is a critical member of cytosolic pattern recognition receptors (PRRs) that recognise viral RNA and mediate type I interferon secretion in host cells. 2. The objective of the present study was to identify and characterise the structure and expression of pigeon MDA5. 3. The full-length MDA5 cDNA was cloned from pigeon spleen using RT-PCR and RACE. The distribution and expression level of pigeon MDA5 in different tissues were determined by QRT-PCR. 4. The results showed that the full-length pigeon MDA5 cDNA had 3858 nucleotides (containing a 210-bp 5'-UTR, a 3030-bp open reading frame and a 618-bp 3'-UTR) encoding a polypeptide of 1009 amino acids. The deduced amino acid sequence contained six conserved structural domains typical of RIG-I-like receptor (RLR), including two tandem arranged N-terminal caspase activation and recruitment domains (CARDs), a DEAH/DEAD box helicase domain (DExDc), a helicase superfamily c-terminal domain (HELICc), a type III restriction enzyme (ResIII) and a C-terminal regulatory domain (RD). 5. The pigeon MDA5 showed 84.8%, 87.3%, 87.9% and 87.2% amino acid sequence identities with previously described homologues from chicken, duck, goose and Muscovy ducks, respectively, and phylogenetic analysis revealed a close relationship among these MDA5. 6. Pigeon MDA5 transcript was ubiquitously expressed in all seven tissues tested in healthy pigeons and showed a high level in the thymus gland and kidney. 7. These findings lay the foundation for further research on the function and mechanism of MDA5 in innate immune responses related to vaccinations and infectious diseases in the pigeon.

Oral delivery of Bacillus subtilis spores expressing grass carp reovirus VP4 protein produces protection against grass carp reovirus infection

Grass carp (Ctenopharyngodon idellus) hemorrhagic disease (GCHD), caused by grass carp reovirus (GCRV), has given rise to an enormous loss in grass carp industry during the past years. Up to date, vaccination remained to be the most effective way to protect grass carp from GCHD. Oral vaccination is of major interest due to its advantages of noninvasive, time-saving, and easily-operated. The introduction of oral vaccination has profound impact on aquaculture industry because of its feasibility of extensive application for fish in various size and age. However, the main challenge in developing oral vaccine is that antigens are easily degraded and are easy to induce tolerance. Bacillus subtilis (B. subtilis) spores would be an ideal oral vaccine delivery system for their robust specialty, gene operability, safety and adjuvant property. VP4 protein is the major outer capsid protein encoded by GCRV segment 6 (S6), which plays an important role in viral invasion and replication. In this study, we used B. subtilis spores as the oral delivery system and successfully constructed the B. subtilis CotC-VP4 recombinant spores (CotC-VP4 spores) to evaluate its protective efficacy in grass carp. Grass carp orally immunized with CotC-VP4 spores showed a survival rate of 57% and the relative percent survival (RPS) of 47% after the viral challenge. Further, the specific IgM levels in serum and the specific IgZ levels in intestinal mucus were significantly higher in the CotC-VP4 group than those in the Naive group. The immune-related genes including three innate immune-related genes (IL-4/13A, IL-4/13B, CSF1R), four adaptive immune-related genes (BAFF, CD4L, MHC-II, CD8), three inflammation-related genes (IL-1β, TNF-α, TGF-β) and interferon type I (IFN-I) related signaling pathway genes were significantly up-regulated in the CotC-VP4 group. The study demonstrated that the CotC-VP4 spores produced protection in grass carp against GCRV infection, and triggered both innate and adaptive immunity post oral immunization. This work highlighted that Bacillus subtilis spores were powerful platforms for oral vaccine delivery, and the combination of Bacillus subtilis spores with GCRV VP4 protein was a promising oral vaccine.

Cyprinid viral diseases and vaccine development

In the past decades, global freshwater fish production has been rapidly growing, while cyprinid takes the largest portion. Along with the rapid rise of novel forms of intensive aquaculture, increased global aquatic animal movement and various anthropogenic stress to aquatic ecosystems during the past century, freshwater fish farming industry encounter the emergence and breakout of many diseases, especially viral diseases. Because of the ability to safely and effectively prevent aquaculture diseases, vaccines have become the mainstream technology for prevention and control of aquatic diseases in the world. In this review, authors summarized six major cyprinid viral diseases, including koi herpesvirus disease (KHVD), spring viraemia of carp (SVC), grass carp hemorrhagic disease (GCHD), koi sleepy disease (KSD), carp pox disease (CPD) and herpesviral haematopoietic necrosis (HPHN). The present review described the characteristics of these diseases from epidemiology, pathology, etiology and diagnostics. Furthermore, the development of specific vaccines respective to these diseases is stated according to preparation methods and immunization approaches. It is hoped that the review could contribute to aquaculture in prevention and controlling of cyprinid viral diseases, and serve the healthy and sustainable development of aquaculture industry.

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

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

Molecular characterization and immune responsive expression of feline MDA5 gene

The retinoic acid-inducible gene-I-like receptor (RLR) family is a group of cytosolic RNA helicase proteins that play an important role in sensing viral RNAs. Melanoma differentiation-associated gene 5 (MDA5), an RLR protein, recognizes viral double-stranded RNA and 5’-triphosphate single-stranded RNA in the cytoplasm for the expression of type I interferon (IFN). The expression of MDA5 is also induced by type I IFN. In the present study, we determined the complete coding sequence of the feline MDA5 gene, and analyzed its structure. In addition, we examined tissue expression patterns, inducibilities of the feline MDA5 by polyinosinic-polycytidylic acid and type I IFN, and a functional role of feline MDA5 on type I IFN expression.

Mechanisms of Fish Macrophage Antimicrobial Immunity

Overcrowding conditions and temperatures shifts regularly manifest in large-scale infections of farmed fish, resulting in economic losses for the global aquaculture industries. Increased understanding of the functional mechanisms of fish antimicrobial host defenses is an important step forward in prevention of pathogen-induced morbidity and mortality in aquaculture setting. Like other vertebrates, macrophage-lineage cells are integral to fish immune responses and for this reason, much of the recent fish immunology research has focused on fish macrophage biology. These studies have revealed notable similarities as well as striking differences in the molecular strategies by which fish and higher vertebrates control their respective macrophage polarization and functionality. In this review, we address the current understanding of the biological mechanisms of teleost macrophage functional heterogeneity and immunity, focusing on the key cytokine regulators that control fish macrophage development and their antimicrobial armamentarium.

Molecular characterization, ontogeny and expression profiling of mitochondrial antiviral signaling adapter, MAVS from Asian seabass Lates calcarifer, Bloch (1790)

Mitochondrial antiviral signaling protein (MAVS), an innate immune signaling adapter coordinates the signals received from two independent cytosolic pathogen recognition receptors (RIG-1 and MDA5) to induce antiviral genes. In the present study the MAVS gene of Lates calcarifer (LcMAVS) was cloned and characterized. The complete cDNA sequence of LcMAVS was 3160 bp and encodes a poly peptide of 577 amino acids. Structural analysis of LcMAVS revealed an N-terminal CARD-like domain, central proline-rich domain and a C-terminal transmembrane domain. Phylogenetic analysis indicated that LcMAVS exhibited the closest relationship to P. olivaceous MAVS. LcMAVS was ubiquitously expressed in all tested tissues of healthy fish viz., brain, gill, heart, liver, spleen, kidney and intestine, with highest transcript level in spleen. The mRNA transcript level of LcMAVS in different developmental stages showed constitutive expression in all the stages tested suggesting the maternal transfer of the gene. Significant up regulation in MAVS expression was observed post nervous necrosis virus (NNV) challenge in vivo in all the selected tissues. Further, time course analysis showed that LcMAVS transcripts significantly increased in the brain and spleen tissues after NNV infection. These findings provide useful information for further elucidating the function of LcMAVS in antiviral innate immune response against NNV in Asian seabass.

Molecular characterisation, ontogeny and expression analysis of melanoma differentiation-associated factor 5 (MDA5) from Asian seabass, Lates calcarifer

MDA5 is the pivotal member of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) and is reported to play a crucial role in type I IFN-mediated responses against pathogen-associated molecular patterns (PAMPs), especially nucleic acids. In this study, we have identified and cloned the full-length cDNA sequence of MDA5, which comprises 3398 nucleotides and encodes for a putative protein of 978 AA length, in Asian seabass, Lates calcarifer. From the putative amino acid sequence of AsMDA5, four different conserved domains could be predicted: two N-terminal CARD domains, a DExDc domain, a HELICc domain and a C-terminal RIG-1_C-RD domain. The mRNA transcript of AsMDA5 could be detected in all the 11 tissues tested in healthy animals with the highest expression in heart followed by gill and skin. The ontogenetic expression profile showed constitutive expression in developmental stages starting from unfertilized eggs, which implies the possibility of maternally acquired immunity of RLRs in offspring. The viral analogue poly I:C could modulate the AsMDA5 expression both in vivo and in vitro. In all the tissues, AsMDA5 expression was found to be highly regulated following injection with poly I:C with the highest expression observed in kidney. The expression level of AsMDA5 was found to be modulated at different time-points following challenge with Gram-negative bacterium, Vibrio alginolyticus, and Gram-positive bacterium, Staphylococcus aureus. Similarly, noticeable change in AsMDA5 expression was detected in SISK cell line induced with either LPS or PGN. The observations made in this study suggest that in euryhaline marine teleosts like Asian seabass, MDA5 gene serves as one of the pivotal receptor for the detection of viral and bacterial PAMP, and might play an important antimicrobial role during early embryonic development.

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

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

Mannan-binding lectin-associated serine protease-1 (MASP-1) mediates immune responses against Aeromonas hydrophila in vitro and in vivo in grass carp

The mannan-binding lectin-associated serine protease-1 (MASP-1) gene is a crucial component of the lectin pathway in the complement and coagulation cascade. Although MASP-1 has been found in the immune system of teleosts, its immune functions in response to bacterial infection are unclear. In this study, we identified a MASP-1 homolog (gcMASP-1) in the grass carp (Ctenopharyngodon idella). The full-length 3308-bp gcMASP-1 cDNA includes a 2160-bp open reading frame encoding a protein composed of 719 amino acids with epidermal growth factor-like, complement control protein, and trypsin-like domains. gcMASP-1 shares a high similarity with MASP-1 counterparts in other species, and it is most closely related to Cyprinus carpio MASP-1 and Sinocyclocheilus anshuiensis MASP-1. Transcription of gcMASP-1 was widely distributed in different tissues and induced by Aeromonas hydrophila in vivo and in vitro. Expression of gcMASP-1 was also affected by lipopolysaccharide and flagellin stimulation in vitro. In cells over-expressing gcMASP-1, transcript levels of almost all components, except gcMBL and gcC5, were significantly enhanced, and gcIL1β, gcTNF-α, gcIFN, gcCD59, gcC5aR1, and gcITGβ-2 were significantly upregulated after exposure to A. hydrophila; gcMASP-1 interference downregulated the transcript levels after A. hydrophila challenge. In addition, gcMASP-1 activated NF-κB signaling. These findings indicate the vital role of gcMASP-1 in innate immunity in C. idella.

The antiviral signaling mediated by black carp MDA5 is positively regulated by LGP2

Melanoma differentiation-associated gene 5 (MDA5) belongs to RIG-I like receptor (RLR) family, which detects cytosolic viral RNA component in immune response. In this study, MDA5 orthologue of black carp (Mylopharyngodon piceus) has been cloned and characterized. The full-length cDNA of black carp MDA5 (bcMDA5) comprises 3244 nucleotides and the predicted bcMDA5 protein contains 984 amino acids. The constitutive transcription of bcMDA5 was extremely low in all the tested tissues, which included gill, skin, muscle, intestine, kidney, spleen, liver and heart. However, bcMDA5 mRNA level was much enhanced in most selected tissues in response to GCRV or SVCV infection. bcMDA5 migrated around 120 KDa in immunoblot and was identified as a cytosolic protein by immunofluorescent staining in both EPC and HeLa cells. Expressing bcMDA5 in EPC cells resulted in the induction of promoter activity of zebrafish IFN3 or fathead minnow IFN. The EPC cells expressing bcMDA5 obtained improved antiviral ability against both SVCV and GCRV. When EPC cells were co-transfected with plasmids expressing bcMDA5 and bcLGP2, the induced IFN expression by bcMDA5 was obviously enhanced. EPC cells expressing both bcMDA5 and bcLGP2 owned much improved antiviral ability than those cells expressing only bcMDA5 or bcLGP2. In general, our data support the conclusion that bcMDA5 plays an important role in the antiviral innate immune response of black carp and bcLGP2 acts as a positive regulator in bcMDA5 mediated signaling.

Grass Carp Laboratory of Genetics and Physiology 2 Serves As a Negative Regulator in Retinoic Acid-Inducible Gene I- and Melanoma Differentiation-Associated Gene 5-Mediated Antiviral Signaling in Resting State and Early Stage of Grass Carp Reovirus Infection

Laboratory of genetics and physiology 2 (LGP2) is a key component of RIG-I-like receptors (RLRs). However, the lack of the caspase recruitment domains (CARDs) results in its controversial functional performance as a negative or positive regulator in antiviral responses. Especially, no sufficient evidence uncovers the functional mechanisms of LGP2 in RLR signaling pathways in teleost. Here, negative regulation mechanism of LGP2 in certain situations in retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5)-mediated antiviral responses was identified in Ctenopharyngodon idella kidney cells. LGP2 overexpression inhibits synthesis and phosphorylation of interferon regulatory factor 3/7 (IRF3/7), and mRNA levels and promoter activities of IFNs and NF-κBs in resting state and early phase of grass carp reovirus (GCRV) infection. Knockdown of LGP2 obtains opposite effects. Luciferase report assay indicates that LGP2 works at the upstream of RIG-I and MDA5. LGP2 binds to RIG-I and MDA5 with diverse domain preference and which is independent of GCRV infection. Furthermore, LGP2 restrains K63-linked ubiquitination of RIG-I and MDA5 in various degrees. These differences result in disparate repressive mechanisms of LGP2 to RIG-I- and MDA5-mediated signal activations of IFN-β promoter stimulator 1 and mediator of IRF3 activation. Interestingly, LGP2 also inhibits K48-linked RIG-I and MDA5 ubiquitination to suppress proteins degradation, which guarantees the basal protein levels for subsequently rapid signal activation. All these results reveal a mechanism that LGP2 functions as a suppressor in RLR signaling pathways to maintain cellular homeostasis in resting state and early phase during GCRV infection.

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

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

MDA5 Induces a Stronger Interferon Response than RIG-I to GCRV Infection through a Mechanism Involving the Phosphorylation and Dimerization of IRF3 and IRF7 in CIK Cells

Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5) are critical cytosolic sensors that trigger the production of interferons (IFNs). Though their recognition functions are well identified, their unique roles in the downstream signal transduction remain to be elucidated. Herein, we report the differential effect between grass carp (Ctenopharyngodon idella) MDA5 (CiMDA5) and CiRIG-I on the production of various IFNs upon grass carp reovirus (GCRV) infection in C. idella kidney (CIK) cell line. In CIK cells, grass carp IFN1 (CiIFN1) and CiIFN3 are relatively highly expressed while CiIFN2 and CiIFN4 are relatively slightly expressed. Following GCRV infection, CiMDA5 induces a more extensive type I IFN response than CiRIG-I. Further investigation reveals that both CiMDA5 and CiRIG-I facilitate the expression and total phosphorylation levels of grass carp IFN regulatory factor (IRF) 3 (CiIRF3) and CiIRF7 upon GCRV infection or poly(I:C) stimulation. However, the difference is that CiRIG-I decreases the threonine phosphorylation level of CiIRF7. As a consequence, CiMDA5 enhances the heterodimerization of CiIRF3 and CiIRF7 and homodimerization of CiIRF7, whereas CiRIG-I facilitates the heterodimerization but attenuates homodimerization of CiIRF7. Moreover, the present study suggests that CiIRF3 and CiIRF7 heterodimers and CiIRF7 homodimers are able to induce more extensive IFN-I responses than CiIRF3 homodimers under GCRV infection. Additionally, CiMDA5 induces a stronger type II IFN (IFN-II) response against GCRV infection than CiRIG-I. Collectively, these results demonstrate that CiMDA5 plays a more potent role than CiRIG-I in IFN response to GCRV infection through differentially regulating the phosphorylation and dimerization of CiIRF3 and CiIRF7.

Immune transcriptome reveals the mincle C-type lectin receptor acts as a partial replacement for TLR4 in lipopolysaccharide-mediated inflammatory response in barramundi (Lates calcarifer)

Fish represent the most diverse and abundant extant vertebrate infraclass. They are also one of the earliest divergent phyla with adaptive immunity based on antigen recognition by MHC and immunoglobulin. The aquaculture industry, which currently provides more than half of the fish for human consumption globally, has successfully exploited the adaptive immune system of fish through mass vaccination programs. However, vaccination against highly diverse antigens, mostly carbohydrates, such as capsular polysaccharides and lipopolysaccharide (LPS) is challenging. Fish have a subdued innate response to LPS, but adaptive response is generally high and type-specific. To better understand the link between initial innate response and early onset of adaptive immunity to carbohydrate antigens in the perciform barramundi (Lates calcarifer), an immune transcriptome was prepared from pronephros and spleen following vaccination with LPS and peptidoglycan. From 163,661 transcripts derived by Illumina mRNA-Seq, most grouped in neuronal, endocrine or immune system categories, suggesting a close relationship between the three systems. Moreover, digestive enzyme transcripts in spleen appeared to be highly inducible in barramundi. Most of the known TLRs were transcribed in the barramundi spleen and HK transcriptome, with the notable exception of TLR4, which is primarily responsible for LPS recognition in mammals. Several C-type lectin receptors were also identified, including CD209, CD205, and CLEC4E (Mincle). As Mincle has been shown to bind LPS and is abundant on dendritic cells, its role in response to LPS in barramundi was further investigated. A high dose of LPS induced TNF-alpha expression via Mincle. However, IL-6 regulation, whilst still regulated in response to LPS, did not depend upon the Mincle pathway, suggesting other routes of activation. This study thus suggests that Mincle acts as a partial substitute for TLR4 in barramundi in the processing of LPS.

The evolution and functional characterization of miiuy croaker cytosolic gene LGP2 involved in immune response

The laboratory of genetics and physiology 2 (LGP2) is a member of retinoic acid-inducible gene I (RIG-I)-like receptors (RLR receptors), which may participate in the immune regulation process. The role of LGP2 on modulating signaling was ambiguous, some researchers suggested that the regulation mechanism of LGP2 to melanoma differentiation-associated gene 5 (MDA5) and retinoic acid inducible gene-I (RIG-I) were different. In this study, the bioinformatics and functions of LGP2 from miiuy croaker (mmLGP2) were characterized. Comparative genomic analysis showed that the evolution of LGP2 in mammals was more conserved than it in fish. LGP2 contains three structural domains: ResIII, HelicaseC and RD, and ResIII structural domain of LGP2 was extremely conservative. The mmLGP2 was ubiquitously expressed in the tested miiuy croaker tissues and the expressions were significantly upregulated after stimulation with poly(I:C), indicating that LGP2 might participate in the immune response, especially antiviral immunity. Furthermore, immunofluorescence of miiuy croaker LGP2 presents in the cytoplasm in Hela cells. The overexpression of mmLGP2 can activate ISRE, but cannot activate NF-κB luciferase reporter, implying that mmLGP2 might act as a positive regulator in immune responses through activating ISRE to induce the expression of IFN. The research of mmLGP2 will enrich the information of fish LGP2, and the functional experiments will be helpful for the future research about fish immune systems.

Identification and characterization of the melanoma differentiation - associated gene 5 in sea perch, Lateolabrax japonicus

The RIG-I-like receptors family is a group of cytosolic RNA helicase proteins that can recognize viral RNA via binding to pathogen associated molecular pattern motifs within RNA ligands. A novel vertebrate RLR counterpart named LjMDA5 was firstly identified from the marine fish sea perch Lateolabrax japonicus in this study. The full-length cDNA of LjMDA5 is 3750 bp and encodes a polypeptide of 988 amino acids, containing two N-terminal tandem caspase activation and recruitment domains, a DExH (Asp-Glu-X-His) box domain, an HELICc domain, and a C-terminal domain RIG-I. Phylogenetic analysis showed that LjMDA5 shared the closest genetic relationship with the MDA5 of Larimichthys crocea. Quantitative RT-PCR analysis showed that LjMDA5 was ubiquitously expressed and up-regulated significantly in all selected tissues in vivo post NNV infection. Time course analysis showed that LjMDA5 transcripts significantly increased in spleen and kidney. We found LjMDA5 could be regulated in the sea perch LJB and LJF cell lines after lipopolysaccharide, polyinosinic-polycytidylic acid treatment and NNV challenge. RNA interference experiment indicated that silencing of LjMDA5 significantly increased RGNNV replication and virus production in NNV infected LJF cells. Our results revealed that MDA5 was essential for host defense against NNV, which provided new insights into the function of RLR signaling pathway during NNV infection in fish.

Molecular characterization and expression analyses of three RIG-I-like receptor signaling pathway genes (MDA5, LGP2 and MAVS) in Larimichthys crocea

In this study, we sequenced and characterized melanoma differentiation-associated antigen 5 (LcMDA5), laboratory of genetics and physiology 2 (LcLGP2) and mitochondrial antiviral signaling protein (LcMAVS) from large yellow croaker (Larimichthys crocea). The LcMDA5 encodes 969 amino acids and contains two caspase-associated and recruitment domains (CARDs), a DExDc (DExD/H box-containing domain), a HELICc (helicase superfamily C-terminal domain) and a C-terminal regulatory domain (RD). The LcLGP2 encodes 679 amino acids and contains a DExDc, a HELICc and a RD. The LcMAVS encodes 512 amino acids and contains a CARD, a proline-rich domain, a transmembrane helix domain and a putative TRAF2-binding motif ((269)PVQDT(273)). Phylogenetic analyses showed that all the three genes of large yellow croaker are clustered together with their counterparts from other teleost fishes. The Real-time PCR analyses showed that all the three genes were found to be constitutively expressed in all examined tissues in large yellow croaker, but all with relatively low expression levels. Expression analyses showed that the three genes were all rapidly and significantly upregulated in vivo after poly (I:C) challenge in peripheral blood, liver, spleen and head kidney tissues. The results indicate that the LcMDA5, LcLGP2 and LcMAVS might play important roles in antiviral immune responses.

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.

Promoter structures and differential responses to viral and non-viral inducers of chicken melanoma differentiation-associated gene 5

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A 2.5 kb chicken MDA5 promoter fragment was cloned and characterized (Accession number KT335979).

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The activity of the promoter is extremely low under basal conditions.

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Viral (IBDV) and non-viral (IFN-β or poly (I:C) inducers could activate the promoter.

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Gene expression driven by exogenous MDA5 promoter is in accordance with endogenous MDA5.

Melanoma differentiation-associated gene 5 (MDA5) is a member of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) family and plays a pivotal role in the anti-viral innate immune response. As RIG-I is absent in chickens, MDA5 is hypothesized to be important in detecting viral nucleic acids in the cytoplasm. However, the molecular mechanism of the regulation of chicken MDA5 (chMDA5) expression has yet to be fully elucidated. With this in mind, a ∼2.5 kb chMDA5 gene promoter region was examined and PCR amplified to assess its role in immune response. A chMDA5 promoter reporter plasmid (piggybac-MDA5-DsRed) was constructed and transfected into DF-1 cells to establish a Piggybac-MDA5-DsRed cell line. The MDA5 promoter activity was extremely low under basal condition, but was dramatically increased when cells were stimulated with polyinosinic: polycytidylic acid (poly I:C), interferon beta (IFN-β) or Infectious Bursal Disease Virus (IBDV). The DsRed mRNA level represented the promoter activity and was remarkably increased, which matched the expression of endogenous MDA5. However, Infectious Bronchitis Virus (IBV) and Newcastle disease virus (NDV) failed to increase the MDA5 promoter activity and the expression of endogenous MDA5. The results indicated that the promoter and the Piggybac-MDA5-DsRed cell line could be utilized to determine whether a ligand regulates MDA5 expression. For the first time, this study provides a tool for testing chMDA5 expression and regulation.

Characterization and immune response expression of the Rig-I-like receptor mda5 in common carp Cyprinus carpio

In this study, the full-length complementary (c)DNA of common carp Cyprinus carpio melanoma differentiation-associated gene 5 (mda5) was cloned. The complete open reading frame of C. carpio mda5 contained 2982 bp and encodes 993 amino acids. The deduced amino acids contained six functional domains: two caspase activation and recruitment domains (CARD), a conserved restriction domain of bacterial type III restriction enzyme (ResIII), a DExD/H box-containing domain (DEXDc), a helicase super family C-terminal domain (HELICc) and a C-terminal regulatory domain (RD). The mda5 gene was expressed in all tested tissues, with high levels in the gills and spleen, while lower expressed in gonad and blood. The copy numbers of mda5 were increased in the liver, spleen, head kidney and the mucosal-associated immune tissues such as the foregut, hindgut, gills and skin after stimulation with polyinosinic polycytidylic [poly(I:C)] and Aeromonas hydrophila. The myxovirus resistance gene (mx) messenger (m)RNA levels in the spleen, head kidney, foregut and gills were significantly up-regulated after poly(I:C) injection. When injected with poly(I:C), mda5 and mx transcripts were also significantly induced in vitro. These results implied that mda5 might be involved in both antiviral and antibacterial innate immune processes in C. carpio. © 2016 The Authors. Journal of Fish Biology © 2016 The Fisheries Society of the British Isles.

Transcriptomic analysis of the host response to an iridovirus infection in Chinese giant salamander, Andrias davidianus

The emergence of an infectious viral disease caused by the Chinese giant salamander iridovirus (GSIV) has led to substantial economic losses. However, no more molecular information is available for the understanding of the mechanisms associated with virus–host interaction. In this study, de novo sequencing was used to obtain abundant high-quality ESTs and investigate differentially-expressed genes in the spleen of Chinese giant salamanders that were either infected or mock infected with GSIV. Comparative expression analysis indicated that 293 genes were down-regulated and 220 genes were up-regulated. Further enrichment analysis showed that the most enriched pathway is “complement and coagulation cascades”, and significantly enriched diseases include “inherited thrombophilia”, “immune system diseases”, “primary immunodeficiency”, “complement regulatory protein defects”, and “disorders of nucleotide excision repair”. Additionally, 30 678 simple sequence repeats (SSRs) from all spleen samples, 26 355 single nucleotide polymorphisms (SNPs) from the spleens of uninfected animals and 36 070 SNPs from the spleens of infected animals were detected. The large amount of variation was specific for the Chinese giant salamanders that were infected with GSIV. The results reported herein provided significant and new EST information that could contribute greatly in investigations into the molecular functions of immune genes in the Chinese giant salamander.

Transcriptome comparative analysis revealed poly(I:C) activated RIG-I/MDA5-mediated signaling pathway in miiuy croaker

Miiuy croker (Miichthys miiuy) as an important economical aquaculture species has challenged many more diseases caused by various pathogens recently. To better explore the immune response to virus, we have analyzed the transcriptome profiling of miiuy croaker challenged with poly(I:C) synthetic analog of virus dsRNA. We have obtained differentially expressed genes (DEGs) with up/down-relevant from comparison of the Ctrl and Poly transcriptome libraries. Through GO and KEGG enrichment analysis, immune-relevant DEGs whose expression are significantly rise or fall after challenged have been identified and classified. In order to detailedly analysis host immune response patterns for dsRNA virus, we have performed a map based on RIG-I/MDA5-mediated and TLR3-mediated signaling pathway which both induced type I IFNs response. In this pathway, both MDA5 and LGP2 are important RLRs in host surveillance against infection of dsRNA viruses and induce type I IFNs response which subsequently form a transcription factor complex ISGF3 that promote downstream genes referred to as ISGs to inhibits virus replication.

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.

Functions of MDA5 and its domains in response to GCRV or bacterial PAMPs

Melanoma differentiation-associated gene 5 (MDA5) is a member of retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) family which can initiate type I IFN expression in response to RNA virus infection. In this study, we constructed six mutants of Ctenopharyngodon idella MDA5 (CiMAD5) overexpression plasmids and generated stable transfected C. idella kidney (CIK) cell lines to study the function of different domains of CiMAD5. After ploy(I:C) stimulation, the downstream genes of CiMDA5 in transfected cells was repressed. Overexpression of CiMDA5 or its variant repressed the replication of grass carp reovirus (GCRV) in CIK cells and decreased the viral titer of GCRV more or less compared to that in control cells. After GCRV or bacterial pathogen-associated molecular patterns (PAMPs) stimulation, overexpression of CiMDA5 or CARD domain significantly induced the expression of CiIFN-I, CiIL-1β and CiMx1. The deletion of Helicase or RD domain reduced the inductive effect of CiMDA5 on CiIFN-I, CiIL-1β and CiMx1 expression. RD overexpression resulted in an enhanced expression of CiIFN-I, CiIL-1β and CiMx1. These observations collectively demonstrate that, in CIK cells, after GCRV or bacterial PAMPs stimulation, CARD domain alone can mediate signaling; Helicase or RD domain alone negatively regulates CARD function by intramolecular interaction with CARD. However, RD domain acts as an enhancer by intermolecular interaction. These results enlarge the response spectrum of MDA5 and contribute to a further understanding of the functions of MDA5 and its domains in evolution.

A DN-mda5 transgenic zebrafish model demonstrates that Mda5 plays an important role in snakehead rhabdovirus resistance

Melanoma Differentiation-Associated protein 5 (MDA5) is a member of the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) family, which is a cytosolic pattern recognition receptor that detects viral nucleic acids. Here we show an Mda5-dependent response to rhabdovirus infection in vivo using a dominant-negative mda5 transgenic zebrafish. Dominant-negative mda5 zebrafish embryos displayed an impaired antiviral immune response compared to wild-type counterparts that can be rescued by recombinant full-length Mda5. To our knowledge, we have generated the first dominant-negative mda5 transgenic zebrafish and demonstrated a critical role for Mda5 in the antiviral response to rhabdovirus.

Sensors of Infection: Viral Nucleic Acid PRRs in Fish

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

Characterization of the IFN pathway in the teleost fish gonad against vertically transmitted viral nervous necrosis virus

One of the most powerful innate immune responses against viruses is mediated by type I IFN. In teleost fish, it is known that virus infection triggers the expression of ifn and many IFN-stimulated genes, but the viral RNA sensors and mediators leading to IFN production are scarcely known. Thus, we have searched for the presence of these genes in gilt-head sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax), and evaluated their expression after infection with viral nervous necrosis virus (VNNV) in the brain, the main viral target tissue, and the gonad, used to transmit the virus vertically. In sea bream, a fish species resistant to the VNNV strain used, we found an upregulation of the genes encoding MDA5 (melanoma differentiation-associated gene 5), TBK1 (TANK-binding kinase 1), IRF3 (IFN regulatory factor 3), IFN, Mx [myxovirus (influenza) resistance protein] and PKR (dsRNA-dependent protein kinase receptor) proteins in the brain, which were unaltered in the gonad and could favour the dissemination by gonad fluids or gametes. Strikingly, in European sea bass, a very susceptible species, we also identified, transcripts coding for LGP2 (Laboratory of Genetics and Physiology 2), MAVS (mitochondrial antiviral signalling), TRAF3 (TNF receptor-associated factor 3), TANK (TRAF family member-associated NFκB activator) and IRF7 (IFN regulatory factor 7), and found that all the genes analysed were upregulated in the gonad, but only mda5, lgp2, irf3, mx and pkr were upregulated in the brain. These findings supported the notion that the European sea bass brain innate immune response is unable to clear the virus and pointed to the importance of gonad immunity to control the dissemination of VNNV to the progeny--an aspect that is worth investigating in aquatic animals.

Proteomic identification and characterization of Ctenopharyngodon idella tumor necrosis factor receptor-associated protein 1 (CiTrap1): an anti-apoptosis factor upregulated by grass carp reovirus infection

Human tumor necrosis factor receptor-associated protein 1 (Trap1) is a mitochondrial protein identical to heat shock protein 75 (HSP75) that plays an important role in protecting cells from oxidative stress and apoptosis. In this study, grass carp (Ctenopharyngodon idella) tumor necrosis factor receptor-associated protein 1 (designated as CiTrap1) was identified through two-dimensional electrophoresis (2-DE) analysis and its pattern of expression was investigated in grass carp kidney (CIK) cells infected with grass carp reovirus (GCRV). The full length cDNA of CiTrap1 contained an opening reading frame of 2157 bp that encoded a peptide of 718 amino acids. Phylogenetic analyses indicated that the CiTrap1 shared 87% identity with its homologue from zebrafish (Danio rerio). The transcriptional level of CiTrap1 in CIK cells was upregulated post virus infection as well as poly (I: C) stimulation. Following virus infection, grass carp PTEN-induced putative kinase 1 (PINK1) and Sorcin, whose coding proteins interact with Trap1 in human, were simultaneously upregulated with CiTrap1. Typical characteristics of apoptosis were observed in CIK cells infected with GCRV by DAPI staining, DNA ladder electrophoresis, TUNEL assay and Annexin Ⅴ labeling. RNAi-mediated silencing of CiTrap1 in CIK cells resulted in the increased rate of virus-induced apoptotic cells. The results of this study suggest that CiTrap1 is involved in the host's innate immune response to viral infection possibly through protecting infected cells from apoptosis.

LGP2 plays extensive roles in modulating innate immune responses in Ctenopharyngodon idella kidney (CIK) cells

LGP2 (laboratory of genetics and physiology 2), RIG-I (retinoic acid inducible gene-I) and MDA5 (melanoma differentiation associated gene 5) constitute the RLR (RIG-I-like receptor) family. LGP2 plays a pivotal role in modulating signaling of RIG-I and MDA5 in innate immune responses. In this study, three representative overexpression vectors were constructed and transfected into C. idella kidney (CIK) cell line to research functional characterizations of CiLGP2 (C. idella LGP2). CiLGP2 overexpression led to the induction of CiRIG-I transcripts. After GCRV challenge, CiLGP2 enhanced CiMDA5 and CiIPS-1 to reinforce the immune response, however, impaired the expression of CiRIG-I. Meanwhile, antiviral activity assays showed that overexpression of CiLGP2 or its domains could inhibit GCRV replication and protect cells from death. Besides, CiLGP2 lingeringly induced CiRIG-I mRNA expression and inhibited CiMDA5 transcripts post poly(I:C) simulation. As a result, CiLGP2 suppressed the RLR-mediated signaling pathway against poly(I:C). Furthermore, CiLGP2 played active roles in RLR signaling response to bacterial PAMPs (LPS and PGN) stimulation. CiLGP2 altered the expression pattern of CiIPS-1 after LPS treatment, while it significantly enhanced the RLR signaling pathway against PGN stimulation. These results collectively suggested that CiLGP2 played a strikingly broad regulation in RLR mediated innate immune responses in C. idella, responding to not only the dsRNA virus or synthetic dsRNA but also bacterial PAMPs, which contribute to the understanding of C. idella LGP2 and RLR signaling pathways. In addition, these results lay a foundation for the further functional mechanism research of LGP2 in fishes.