Identification and functional characterization of a fish-specific tlr19 in common carp (Cyprinus carpio L.) that recruits TRIF as an adaptor and induces ifn expression during the immune response

Effects on growth performance and immunity of Monopterus albus after high temperature stress

To investigate the impact of the effect of high temperature stimulation on Monopterus albus larvae after a certain period of time, five experimental groups were established at different temperatures. Then, the M. albus under high temperature stress was fed at 30°C for 70 days. After that, the growth index of the M. albus was counted and analyzed. In terms of growth index, high temperature stress had significant effects on FCR, FBW, WGR, and SGR of M. albus (p < 0.05). The SR increased after being stimulated by temperature (p < 0.1). The study revealed that liver cells of M. albus were harmed by elevated temperatures of 36°C and 38°C. In the experimental group, the activities of digestive enzymes changed in the same trend, reaching the highest point in the 32°C group and then decreasing, and the AMS activity in the 38°C group was significantly different from that in the 30°C group (p < 0.05). The activities of antioxidase in liver reached the highest at 34°C, which was significantly different from those at 30°C (p < 0.05). In addition, the expression levels of TLR1, C3, TNF-α, and other genes increased in the experimental group, reaching the highest point at 34°C, and the expression level of the IL-1β gene reached the highest point at 32°C, which was significantly different from that at 30°C (p < 0.05). However, the expression level of the IRAK3 gene decreased in the experimental group and reached its lowest point at 34°C (p < 0.05). The expression level of the HSP90α gene increased with the highest temperature stimulus and reached its highest point at 38°C (p < 0.05). In the α diversity index of intestinal microorganisms in the experimental group, the observed species, Shannon, and Chao1 indexes in the 34°C group were the highest (p < 0.05), and β diversity analysis revealed that the intestinal microbial community in the experimental group was separated after high temperature stimulation. At the phylum level, the three dominant flora are Proteus, Firmicutes, and Bacteroides. Bacteroides and Macrococcus abundance increased at the genus level, but Vibrio and Aeromonas abundance decreased. To sum up, appropriate high-temperature stress can enhance the immunity and adaptability of M. albus. These results show that the high temperature stimulation of 32°C-34°C is beneficial to the industrial culture of M. albus.

Characterization of STING from common carp (Cyprinus carpio L.) involved in spring viremia of carp virus infection

Stimulator of interferon genes (STING) serve as an endoplasmic reticulum (ER) protein and modulates innate immune responses to viral contagion. Most investigations involving teleost STING antiviral immunity have examined DNA viruses. Therefore, fish STING signaling events against RNA viruses require additional exploration. Here, common carp STING (named CcSTING) was cloned and characterized. The bioinformatics analyses of CcSTING showed evolutionary conservations and were most closely related to other cyprinid STINGs. Immunofluorescence staining discovered that the CcSTING was chiefly placed in the cytoplasm, specifically within the ER. CcSTING was ubiquitously generated in all analyzed organs, with especially strong expression in the gills and head kidney. Spring viremia of carp virus (SVCV) stimulation and poly(I:C) infection induced the generation of CcSTING in immune-associated organs, as well as in peripheral blood leukocytes. Additional investigations revealed that CcSTING overexpression strongly suppressed SVCV replication in EPC cells. Mechanistically, CcSTING enhanced IFN-1 and ISGs expression following SVCV infection. CcSTING also substantially increased both IFN and NF-κB promoter luciferase activity via a dosage-dependent fashion. Lastly, CcSTING significantly up-regulated both TBK1 and p65 phosphorylation. Collectively, these findings demonstrated the critical role and underlying mechanism of fish STING in response to RNA virus.

Stimulator of interferon genes from Asian swamp eel (MaSTING) is involved in host defense against bacterial infection

Stimulator of interferon genes (STING) is an endoplasmic reticulum (ER)-associated protein that plays critical roles in innate immunity and pathogenesis of various diseases. To date, teleost STING against viral stimulation has been identified, whereas STING signaling events in fish against bacteria are not well understood. In the present study, the open reading frame (ORF) of STING from Asian swamp eel (Monopterus albus) was cloned (named MaSTING) and its roles in bacterial infection were investigated. Amino acid sequence alignment and phylogenetic analysis revealed that MaSTING had conserved structures with mammalian STING and shared the closest relationship with mandarin fish STING. Subcellular localization analysis showed that MaSTING distributed in the whole cytoplasm and mainly co-localized with ER. Expression pattern analysis found that MaSTING was constitutively expressed in all the examined tissues with the highest expression in the liver and spleen. Post stimulation with bacteria and various PAMPs, the expression of MaSTING was induced at indicated time points in the immune-related organs and isolated peripheral blood leucocytes. Furthermore, the mechanism underlying MaSTING against bacterial infection was further studied. The qPCR analysis showed that MaSTING overexpression promoted 2'3'-cGAMP induced the expression of IFN-1, ISG15, Viperin, Mx, IL-1β and TNF-α. Western blotting assay suggested that MaSTING significantly enhanced the phosphorylation of TANK-binding kinase 1 (TBK1) and p65. MaSTING also significantly increased the luciferase activity of IFN-1 and NF-κB promoters. Taken together, MaSTING is involved in host defense against bacterial infection by inducing the inflammatory response.

Genome-wide identification and characterization of toll-like receptor 5 (TLR5) in fishes

Toll-like receptors 5 (TLR5), a member of the toll-like receptors (TLRs) family, is a class of pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). It responds to vertebrate recognition of bacterial flagellin and participates in innate immune responses. However, genome-wide identification and characterization of TLR5 in fishes have not been investigated. Here, three TLR5M isotypes (TLR5Ma, TLR5Mb1, and TLR5Mb2) and a TLR5S are all extracted from fish genomes on the basis of phylogenetic and synteny analyses. We confirmed that the non-teleost fishes have one TLR5M gene, as well as additional TLR5 genes (TLR5M and TLR5S) in teleost fishes. In addition, some special teleost fishes possess two to three TLR5 genes, which have undergone the fourth whole-genome duplication (WGD). According to our results, we inferred that the diversity of TLR5 genes in fishes seems to be the result of combinations of WGD and gene loss. Furthermore, TLR5 isoforms displayed differences at the flagellin interaction sites and viral binding sites, and showed lineage-specific, which indicated that TLR5 duplicates may generate functional divergence. Bacterial experiments also supported the idea that CiTLR5Ma and CiTLR5Mb are subfunctionalized to sense bacterial flagellin. In summary, our present comparative genomic survey will benefit for further functional investigations of TLR5 genes in fish.

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.

TRIM25 inhibits spring viraemia of carp virus replication by positively regulating RIG-I signaling pathway in common carp (Cyprinus carpio L.)

Common carp (Cyprinus carpio L.) is one of the most widely cultivated fish in China. Spring viraemia of carp virus (SVCV) is a highly pathogenic virus and has often caused excessive losses in carp pond fisheries. Innate immune play important roles against virus infection. To better understand the immune response of common carp against SVCV infection, transcriptome analysis was performed using the Illumina Novaseq 6000 platform. It was showed that a total of 3953 differentially expressed unigenes were identified, and the RLR signaling pathway were significantly enriched after SVCV infection. Subsequently, the role of RLR signaling pathway in SVCV infection was studied. The results showed that common carp RIG-I (CcRIG-I) and TRIM25 (CcTRIM25) significantly decreased the replication of SVCV by inducing the phosphorylation of TBK1, IRF3 and p65 and the expression of ifn-1, viperin, isg15 and mx. Further studies illustrated that CcTRIM25 could positive regulate CcRIG-I mediated downstream signaling pathway. Finally, the mechanism of CcTRIM25 promoting CcRIG-I-mediated signaling was investigated. CcTRIM25 could interact with the caspase activation and recruitment domain (CARD) of CcRIG-I and promoted K63-linked polyubiquitination of CcRIG-I. Altogether, the study revealed a mechanism of CcTRIM25 regulating CcRIG-I mediated immune response in SVCV infection.

Fish-specific Toll-like receptor 14 (TLR14) from Asian swamp eel (Monopterus albus) is involved in immune response to bacterial infection

Toll-like receptors (TLRs) are a class of pattern recognition receptors (PRRs) that play a critical role in innate immune responses against pathogens. In the present study, a fish-specific TLR14 was identified and characterized from Monopterus albus (named MaTLR14), which consisted of a 2658 bp open reading frame encoding a protein of 885 amino acids. Phylogenetic analysis revealed that MaTLR14 belong to the TLR1 subfamily and shared the highest similarity to Paralichthys olivaceus TLR14. Immunohistochemistry assay showed that MaTLR14 mainly located in intestinal epithelial cells of hindgut. Immunofluorescence revealed that MaTLR14 largely localized to the intracellular region and partially co-localized with cell membrane of HeLa cells. The expression levels of MaTLR14 were upregulated in the liver, spleen, foregut and hindgut post infection with Aeromonas hydrophila. When stimulated with LPS and Flagellin, the MaTLR14 expression was elevated in isolated peripheral blood leukocytes. Further studies showed that recombinant MaTLR14-LRR could bind to both the gram-negative and gram-positive bacteria and cause agglutination. Subsequently, the signaling pathway of MaTLR14 was investigated. Confocal microscopy and co-immunoprecipitation assay demonstrated that MaTLR14 recruited MyD88 as adaptor. When overexpressed, MaTLR14 augmented the expression of TRAF6 and phosphorylation of ERK and p65, activated NF-κB and AP-1 and elicited the expression of il-6 and tnf-α. Collectively, MaTLR14 plays an important role in the microorganism recognition and signaling transduction.

TLR1 in Nile tilapia: The conserved receptor cannot interact with MyD88 and TIRAP but can activate NF-κB in vitro

Toll-like receptors (TLRs) play a critical role in the innate immune response of fish. In this study, we isolated the cDNA sequence of Nile tilapia TLR1 (OnTLR1). The deduced OnTLR1 protein contains a signal peptide, 7 leucine-rich repeats (LRRs), a C-terminal LRR (LRR-CT), a transmembrane region and a highly conserved TIR domain. In healthy Nile tilapia, the OnTLR1 transcript was broadly expressed in all examined tissues, with the highest expression levels in the spleen. After infection with Streptococcus agalactiae, the OnTLR1 transcripts were upregulated in the gill and kidney. After stimulation with polyinosinic-polycytidylic acid (poly(I:C)), the expression levels of OnTLR1 were significantly downregulated in the intestine, whereas OnTLR1 transcripts were significantly upregulated in the kidney. After challenge with lipopolysaccharide (LPS), the expression levels of OnTLR1 were significantly upregulated in the spleen and kidney. The subcellular localization showed that OnTLR1 was expressed in the cytoplasm. TLR1 significantly increased MyD88-dependent NF-κB activity. However, the results of a pull-down assay showed that OnTLR1 did not interact with MyD88 or TIRAP. Binding assays revealed the specificity of OnTLR1 for pathogen-associated molecular patterns (PAMPs) and bacteria that included S. agalactiae, Aeromonas hydrophila and poly(I:C) and LPS. Taken together, these findings suggest that OnTLR1, as a pattern recognition receptor (PRR), might play an important role in the immune response to pathogen invasion.