Expression and functional analysis of receptor-interacting serine/threonine kinase 2 (RIP2) in Japanese flounder (Paralichthys olivaceus)

Antiviral effect of miR-206 in rainbow trout (Oncorhynchus mykiss) against infectious hematopoietic necrosis virus infection

Infectious hematopoietic necrosis (IHN), caused by IHN virus, is a highly contagious and lethal disease that seriously hampers the development of rainbow trout (Oncorhynchus mykiss) aquaculture. However, the immune response mechanism of rainbow trout underlying IHNV infection remains largely unknown. MicroRNAs act as post-transcriptional regulators of gene expression and perform a crucial role in fish immune response. Herein, the regulatory mechanism and function of miR-206 in rainbow trout resistance to IHNV were investigated by overexpression and silencing. The expression analysis showed that miR-206 and its potential target receptor-interacting serine/threonine-protein kinase 2 (RIP2) exhibited significant time-dependent changes in headkidney, spleen and rainbow trout primary liver cells infected with IHNV and their expression displayed a negative correlation. In vitro, the interaction between miR-206 and RIP2 was verified by luciferase reporter assay, and miR-206 silencing in rainbow trout primary liver cells markedly increased RIP2 and interferon (IFN) expression but significantly decreased IHNV copies, and opposite results were obtained after miR-206 overexpression or RIP2 knockdown. In vivo, overexpressed miR-206 with agomiR resulted in a decrease in the expression of RIP2 and IFN in liver, headkidney and spleen. This study revealed the key role of miR-206 in anti-IHNV, which provided potential for anti-viral drug screening in rainbow trout.

Functional characterization of RIP2 in large yellow croaker (Larimichthys crocea), a protein involved in the host antiviral responses via NF-κΒ, IRF3/7 related signaling

As an adaptor protein functions essentially in the activation of NF-κΒ and MAPK signaling pathways mediated by NOD1 and NOD2, RIP2 plays important roles in the host innate immune responses. In the present study, the RIP2 ortholog termed Lc-RIP2 was identified and characterized in large yellow croaker (Larimichthys crocea). It was revealed that Lc-RIP2 is consisted of an open reading frame (ORF) of 1695 bp, encoding a protein of 564 aa, with an N-terminal kinase domain and a C-terminal caspase activation and recruitment domain (CARD). Subcellular localization assays demonstrated that Lc-RIP2 was a cytosolic protein, which was broadly distributed in the examined tissues/organs, and could be induced in response to poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulations in vivo according to qRT-PCR analysis. Notably, Lc-RIP2 overexpression in vitro was sufficient to abolish SVCV proliferation in EPC cells, and could significantly induce the activation of NF-κB, IRF3, IRF7, and IFN1 promoters. In addition, luciferase assays found that Lc-RIP2 could cooperate with Lc-MAVS, Lc-TRAF3, Lc-TRAF6, Lc-IRF3, and Lc-IRF7 in NF-κB activation, associate with Lc-TRIF, Lc-MAVS, Lc-TRAF3, Lc-IRF3, and Lc-IRF7 in IRF3 activation, enhance Lc-TRIF, Lc-MAVS, Lc-TRAF3, and Lc-TRAF6 mediated IRF7 activation, and Lc-IRF3 mediated IFN1 activation, whereas suppress NF-κB activation when co-expressed with Lc-TRIF. Co-immunoprecipitation (Co-IP) assays also demonstrated that Lc-RIP2 interacts separately with Lc-TRIF, Lc-MAVS, Lc-TRAF3, Lc-TRAF6, Lc-IRF3, and Lc-IRF7. It is thus collectively indicated that Lc-RIP2 function dominantly in the regulation of the host innate immune signaling.

Integrative mRNA-miRNA interaction analysis associated with the immune response in the head kidney of rainbow trout (Oncorhynchus mykiss) after infectious hematopoietic necrosis virus infection

Rainbow trout (Oncorhynchus mykiss) is an important cold-water fish widely cultivated in China. The frequent occurrence of viral diseases caused by infectious hematopoietic necrosis virus (IHNV) seriously restricted the healthy development of the rainbow trout farming industry. However, the immune defense mechanism induced by IHNV in rainbow trout has not been fully elucidated. In the present study, we detected mRNA and miRNA expression profiles in rainbow trout head kidney after IHNV infection using RNA-seq and identified key immune-related genes and miRNAs. The results showed that a total of 7486 genes and 277 miRNAs were differentially expressed, and numerous differentially expressed genes (DEGs) enriched in the immune-related pathways such as Toll-like receptor signaling pathway, RIG-I-like receptor signaling pathway, NOD-like receptor signaling pathway, cytokine-cytokine receptor interaction, and JAK-STAT signaling pathway were significantly up-regulated, including LGP2, MDA5, TRIM25, IRF3, IRF7, TLR3, TLR7, TLR8, MYD88, and IFN1. Integration analysis identified six miRNAs (miR-141-y, miR-200-y, miR-144-y, miR-2188-y, miR-725-y, and miR-203-y) that target at least six key immune-related genes (TRIM25, LGP2, TLR3, TLR7, IRF3, and IRF7). Further, we verified selected immune-related mRNAs and miRNAs through qRT-PCR and confirmed the reliability of the RNA-seq results. These findings improve our understanding of the immune mechanism of rainbow trout infected with IHNV and provide basic data for future breeding for disease resistance in rainbow trout.

Identification and expression profiling of receptor-interacting serine/threonine-protein kinase 2 in Siberian sturgeon (Acipenser baerii)

Receptor-interacting serine/threonine-protein kinase 2 (RIPK2) is an adaptor protein of the pattern recognition receptors NOD1 and NOD2 involved in regulating inflammatory response and resisting pathogenic microbial infection. In this study, Acipenser baerii RIPK2 (AbRIPK2) was identified. The open reading frame of AbRIPK2 was 1815 bp encoding 604 amino acids. AbRIPK2 possessed the typical N-terminal kinase domain (KD) and C-terminal caspase recruitment domain (CARD). The phylogenetic tree analysis revealed that AbRIPK2 shared a relatively high identity with bony fish. Real-time fluorescence quantitative PCR (qRT-PCR) results indicated that AbRIPK2 was highly expressed in the gill, followed by muscle, liver and heart. AbRIPK2 was significantly induced in the spleen and valvular intestine after Streptococcus iniae and Aeromonas hydrophila infection. AbRIPK2 was significantly upregulated after peptidoglycan (PGN) treatment in the splenic leukocytes. This study indicated that AbRIPK2 played a potential role in resisting the pathogenic infection of Siberian sturgeon by responding to bacteria.

Cytokines Induced by Edwardsiella tarda: Profile and Role in Antibacterial Immunity

Edwardsiella tarda is a Gram-negative bacterial pathogen with a broad range of hosts, including fish and mammals. In the present study, we used an advanced antibody array technology to identify the expression pattern of cytokines induced by E. tarda in a mouse infection model. In total, 31 and 24 differentially expressed cytokines (DECs) were identified in the plasma at 6 h and 24 h post-infection (hpi), respectively. The DECs were markedly enriched in the Gene Ontology (GO) terms associated with cell migration and response to chemokine and in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with immunity, diseases, and infection. Ten key DECs, including IL6 and TNF-α, were found to form extensive protein-protein interaction networks. IL6 was demonstrated to inhibit E. tarda infection and be required for E. tarda-induced inflammatory response. TNF-α also exerted an inhibitory effect on E. tarda infection, and knockdown of fish (Japanese flounder) TNF-α promoted E. tarda invasion in host cells. Together, the results of this study revealed a comprehensive profile of cytokines induced by E. tarda, thus adding new insights into the role of cytokine-associated immunity against bacterial infection and also providing the potential plasma biomarkers of E. tarda infection for future studies.

Cloning, Expression, Characterization, and Tissue Distribution of Cystatin C from Silver Carp (Hypophthalmichthys molitrix)

Cystatins are proteins, which inhibit cysteine proteases, such as papain. In this study, the 336-bp cystatin C gene (family II, HmCysC) of silver carp (Hypophthalmichthys molitrix) was cloned and expressed in Escherichia coli BL21 (DE3). HmCysC encodes the mature peptide of cystatin C (HmCystatin C), with 111 amino acids. A typical QXXXG motif was found in HmCystatin C and it formed a cluster with Cyprinus carpio and Danio rerio cystatin C in the phylogenetic tree. Quantitative real-time polymerase chain reaction analysis indicated that HmCysC was transcribed at different levels in five tested tissues of silver carp. Following purification with Ni²⁺- nitrilotriacetic acid agarose affinity chromatography, HmCystatin C displayed a molecular weight of 20 kDa in sodium dodecyl sulfate polyacrylamide gel electrophoresis. Purified HmCystatin C had strong inhibitory effects toward the proteolytic activity of papain. Immunochemical staining with anti-HmCystatin C antibody showed that HmCystatin C was widely distributed in silver carp tissues. These results collectively demonstrated the properties of HmCystatin C, providing information for further studies of cystatins from fish organisms.

Grass carp (Ctenopharyngodon idella) stefin A: Systematic research on its cloning, expression, characterization and tissue distribution

To fully understand the properties of piscine stefins (family I cystatins), the 294-bp stefinA gene from grass carp (Ctenopharyngodon idella, Ci) was cloned and expressed in E. coli BL21 (DE3). After purification by Ni²⁺-NTA agarose affinity chromatography, the CiStefin A protein was tested to have a molecular weight of 11.48 kDa and an isoelectric point of 8.7. The typical motif of the cystatins superfamily was characterized from CiStefin A (QVVQG). CiStefin A specifically inhibited the activity of papain and cathepsin B/L. The K_i value of CiStefin A against papain was 6.5 × 10^-11 M. CiStefin A showed excellent heat and acid-base tolerance. StefinA gene transcription occurred in all tested tissues of grass carp, with the highest level in the hepatopancreas. Immunolocalization staining with an anti-CiStefinA antibody revealed the CiStefinA protein distribution in all tested tissues at various levels. Overall, these results clarified the physical and biochemical properties of grass carp stefin A.

Expression patterns of innate immunity-related genes in response to polyinosinic:polycytidylic acid (poly[I:C]) stimulation in DF-1 chicken fibroblast cells

Polyinosinic:polycytidylic acid (poly[I:C]) can stimulate Toll-like receptor 3 (TLR3) signaling pathways. In this study, DF-1 cells were treated with poly(I:C) at various concentrations and time points to examine the comparative expression patterns of innate immune response genes. The viability of DF-1 cells decreased from 77.41% to 38.68% when cells were treated different dose of poly(I:C) from 0.1 µg/mL to 100 µg/mL for 24 h respectively. The expressions of TLR3, TLR4, TLR7, TLR15, TLR21, IL1B, and IL10 were increased in dose- and time-dependent manners by poly(I:C) treatment. On the contrary, the expression patterns of interferon regulatory factors 7 (IRF7), Jun proto-oncogene, AP-1 transcription factor subunit (JUN), Nuclear Factor Kappa B Subunit 1 (NF-κB1), and IL8L2 were varied; IRF7 and IL8L2 were increasingly expressed whereas the expressions of JUN and NF-κB1 were decreased in a dose-dependent manner after they were early induced. In time-dependent analysis, IRF7 expression was significantly upregulated from 3 h to 24 h, whereas JUN and NF-κB1 expressions settled down from 6 h to 24 h after poly(I:C) treatment although they were induced at early time from 1 h to 3 h. Poly(I:C) treatment rapidly increased the expression of IL8L2 from 3 h to 6 h with a plateau at 6 h and then the expression of IL8L2 was dramatically decreased until 24 h after poly(I:C) treatment although the expression level was still higher than the non-treated control. These results may provide the basis for understanding host response to viral infection and its mimicry system in chickens.

Edwardsiella tarda-induced miR-7a functions as a suppressor in PI3K/AKT/GSK3β signaling pathway by targeting insulin receptor substrate-2 (IRS2a and IRS2b) in Paralichthys olivaceus

To study the effect of Edwardsiella tarda infection on miRNAs expression profile in Japanese flounder, fish were injected intraperitoneally with E. tarda. The miRNAs involved in regulating immune responses were analyzed by high-throughput sequencing. A total of 164 mature miRNAs were identified, of which 17 miRNAs were differentially expressed (DE miRNAs) after E. tarda infection, indicating that they were immune-related miRNAs. To further examine the relationship between the miRNAs and their predicted target mRNAs, a total of 22 predicted target mRNAs, mainly related to endocytic signaling pathway, NF-κB signaling pathway, and p53 signaling pathway, were detected with miRNA mimics in HEK-293T cells by dual-luciferase reporter experiments. Finally, we confirmed that insulin receptor substrate-2 (IRS2a and IRS2b) were regulated by miR-7a. And the target sites of the 3' untranslated region (UTR) of IRS2a and IRS2b were verified by dual-luciferase reporter experiments. Furthermore, we found that the E. tarda and LPS significantly increased host miR-7a expression. In vivo and in vitro studies revealed that IRS2-mediated PI3K/AKT/GSK3β signaling pathway was suppressed. Taken together, these results implied that miR-7a might be a key regulator of PI3K/AKT/GSK3β signaling pathway via suppressing the IRS2a and IRS2b genes.

Molecular characterization, expression and immune functions of cystatin B in Japanese flounder (Paralichthys olivaceus)

Cystatin B is an intracellular inhibitor that regulates the activities of cysteine proteases. In this study, cystatin B in Japanese flounder (Paralichthys olivaceus) was characterized and its immune function was analyzed. This gene had a high similarity with the sequence of cystatin B in other fish species, and the derived peptide shared typical features of cystatin proteins including the QXVXG motif. The results of quantitative real-time PCR showed that cystatin B mRNA was constitutively expressed in all examined tissues, with the highest level in gill. The stimulations of lipopolysaccharide, peptidoglycan and polyinosinic-polycytidylic acid effectively increased the expression level of cystatin B mRNA. Functional analysis implied that the recombinant P. olivaceus cystatin B purified from Escherichia coli had cysteine protease inhibitory activity and could inhibit bacterial growth by binding to bacteria. Furthermore, we found that P. olivaceus cystatin B had no effects on the expression of inflammatory factors cytokines tumor necrosis factor α, interleukin 10, interleukin 1β and interferon γ. These results indicate that cystatin B of P. olivaceus is potentially involved in immune responses against invading microbial pathogens, and provide a better understanding of the immune mechanisms of cystatins in teleosts.