Identification and functional characterization of IRAK-4 in grass carp (Ctenopharyngodon idellus)

The role of HMGB2 in the immune response of Nile tilapia (Oreochromis niloticus) to streptococcal infection

High mobility group protein B2 (HMGB2) is an abundant chromatin-associated protein with pivotal roles in transcription, cell proliferation, differentiation, inflammation, and tumorigenesis. However, its immune function in Nile tilapia (Oreochromis niloticus) remains unclear. In this study, we identified a homologue of HMGB2 from Nile tilapia (On-HMGB2) and investigated its functions in the immune response against streptococcus infection. The open reading frame (ORF) of On-HMGB2 spans 642 bp, encoding 213 amino acids, and contains two conserved HMG domains. On-HMGB2 shares over 80 % homology with other fish species and 74%-76 % homology with mammals. On-HMGB2 was widely distributed in various tissues, with its highest transcript levels in the liver and the lowest in the intestine. Knockdown of On-HMGB2 promoted the inflammatory response in Nile tilapia, increased the bacterial load in the tissues, and led to elevated mortality in Nile tilapia following Streptococcus agalactiae infection. Taken together, On-HMGB2 significantly influences the immune system of Nile tilapia in response to streptococcus infection.

Both IRAK3 and IRAK1 Activate the MyD88-TRAF6 Pathway in Zebrafish

IL-1R-associated kinases (IRAKs) are signal transducers of the TLR/IL-1R-MyD88-TRAF6 pathways. Vertebrates possess two IRAK lineages, IRAK1/2/3 and IRAK4. In mammals, IRAK4/IRAK1 and IRAK4/IRAK2 are pathway enhancers, whereas IRAK3 is a repressor. However, in bony fish, IRAK2 is absent, and it remains elusive how fish IRAK1/3/4 functionally differ from their mammalian counterparts. In this study, we explored this using the zebrafish model. First, we showed that in human 293T cells, zebrafish IRAK1 and IRAK4 were components of the Myddosome (MyD88-IRAK4-IRAK1) complex, with IRAK1 serving as a potent pathway enhancer. Then, we discovered two zebrafish IRAK3 variants: one (IRAK3a) contains an N-terminal Death domain, a middle pseudokinase domain, and a C-terminal TRAF6-binding domain, whereas the other (IRAK3b) lost both the kinase and TRAF6-binding domains. This truncation of IRAK3 variants could be a conserved phenomenon in fish, because it is also observed in trout and grass carp. We proceeded to show that zebrafish IRAK3a acts as a pathway enhancer by binding with MyD88 and TRAF6, but its activity is milder than IRAK1, possibly because it has no kinase activity. Zebrafish IRAK3b, however, plays a sheer negative role, apparently because of its lack of kinase and TRAF6-binding domains. Moreover, zebrafish IRAK3a/3b inhibit the activity of IRAK1/4, not by interacting with IRAK1/4 but possibly by competing for MyD88 and TRAF6. Finally, we have verified the essential activities of zebrafish IRAK1/3a/3b/4 in zebrafish cells and embryos. In summary, to our knowledge, our findings provide new insights into the molecular functions of fish IRAKs and the evolution of the IRAK functional modes in vertebrates.

Molecular characterization and expression analyses of five genes involved in the MyD88-dependent pathway of yellow catfish (Pelteobagrus fulvidraco) responding to challenge of Aeromonas hydrophila

MyD88-dependent pathway mediated by Toll-like receptor is one of the vital ways activating immune responses. In order to identify the role of MyD88-dependent signaling pathway in yellow catfish, the Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 (p105) (Pf: abbreviation of Pelteobagrus fulvidraco) were cloned and characterized respectively. The Pf_MyD88, Pf_IRAK4, Pf_IRAK1 and Pf_TRAF6 were all highly conserved among species and showed the highest homology to that of Pangasianodon hypophthalmus. Pf_NFκB1 showed the highest homology to that of Ictalurus punetaus. All of the five genes showed similar expression patterns in various tissues, with the highest expression level in the liver. These genes also showed similar expression levels in different embryonic development stages, except Pf_IRAK4. The higher expression level was detected from fertilized eggs to 1 day post hatching (dph), lower expression from 3 dph to 30 dph. After stimulation of inactivated Aeromonas hydrophila, the mRNA expressions of Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 were significantly increased at 24 h in the liver, spleen, head kidney and trunk kidney, suggesting that all the five genes were involved in the innate immune response of yellow catfish. These results showed that MyD88-dependent signaling pathway plays important roles for disease defensing in the innate immune response. Meanwhile, inactivated A. hydrophila can cause strong innate immune response, which provides theoretical bases for the application of inactivated vaccines in defense against bacterial diseases of teleost.

Non-coding RNAs targeting NF-κB pathways in aquatic animals: A review

Nuclear factor-kappa B (NF-κB) pathways have a close relationship with many diseases, especially in terms of the regulation of inflammation and the immune response. Non-coding RNAs (ncRNAs) are a heterogeneous subset of endogenous RNAs that directly affect cellular function in the absence of proteins or peptide products; these include microRNAs (miRNAs), long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), etc. Studies on the roles of ncRNAs in targeting the NF-κB pathways in aquatic animals are scarce. A few research studies have confirmed detailed regulatory mechanisms among ncRNAs and the NF-κB pathways in aquatic animals. This comprehensive review is presented concerning ncRNAs targeting the NF-κB pathway in aquatic animals and provides new insights into NF-κB pathways regulatory mechanisms of aquatic animals. The review discusses new possibilities for developing non-coding-RNA-based antiviral applications in fisheries.

A novel interleukin-1 receptor-associated kinase 4 from blunt snout bream (Megalobrama amblycephala) is involved in inflammatory response via MyD88-mediated NF-κB signal pathway

Interleukin-1 receptor-associated kinase 4 (IRAK4) plays a crucial role in the Toll-like receptor/IL-1R signal pathway, which mediates the downstream signal transduction involved in innate and adaptive immunity. In the present study, an IRAK4 homologue (named as MaIRAK4) from blunt snout bream (Megalobrama amblycephala) was cloned and characterized. The open reading frame (ORF) of MaIRAK4 contains 1422 nucleotides, encoding a putative protein of 473 amino acids. Protein structural analysis revealed that MaIRAK4 has an N-terminal death domain (DD) and a central kinase domain (S_TKc), similar to those of mammals and other fishes. Multiple sequence alignment demonstrated that MaIRAK4 is highly homologous with that of grass carp (97.67%). The qRT-PCR analysis showed that MaIRAK4 expressed widely in all examined tissues, including heart, liver, spleen, kidney, head-kidney, gill, intestine and muscle, with the highest expression in the liver and spleen. After stimulation with LPS, MaIRAK4 expression upregulated significantly and reached a peak at 6 h and 12 h post LPS stimulation in the spleen and head-kidney, respectively. After challenge with Aeromonas hydrophila, MaIRAK4 expression peaked at 48 h and 72 h in spleen/head-kidney and liver, respectively. These results implied that MaIRAK4 is involved in the host defense against bacterial infection. Subcellular localization analysis indicated that MaIRAK4 distributed in the cytoplasm. Co-immunoprecipitation and subcellular co-localization assay revealed that MaIRAK4 can combine with MaMyD88 through DD domain. MaIRAK4 overexpression can induce slightly the NF-κB promoter activity in HEK 293 cells. However, the activity of NF-κB promoter was dramatically enhanced after co-transfection with MaIRAK4 and MaMyD88 plasmids. The results showed that MaIRAK4 was involved in NF-κB signal pathway mediated by maMyD88. The expression level of pro-inflammatory cytokines (IL-1β, IL-6, IL-8 and TNF-α) decreased significantly after the siRNA-mediated knockdown of MaIRAK4. Together, these results suggest that MaIRAK4 plays an important function in the innate immunity of M. amblycephala by inducing cytokines expression.

Cloning and functional analysis of SNX8 from grass carp (Ctenopharyngodon idellus)

Sorting nexin 8 (SNX8), a member of sorting nexin protein family, plays important roles in endocytosis, endosomal sorting, and innate immune response. To date, a few homologs of SNX8 have been found in fish except in mammals. In this study, a teleost SNX8 cDNA was identified from grass carp (Ctenopharyngodon idellus). CiSNX8 was up-regulated significantly after infection with poly I:C or GCRV. We found that SNX8 was mainly distributed in the endoplasmic reticulum (ER) in CIK cells. Further analysis indicated that CiSNX8 might negatively regulate RLR signaling pathway that is quite distinct from mammalian SNX8. In addition, CiSNX8 could interact with MAVS, STING, TBK1, IRF3 and IRF7. Either wild type CiSNX8 or mutants of N-terminal PX domain (aa 1-245) and C-terminal BAR domain (aa 256-519) could associate with STING. These results suggested that fish SNX8 participated in innate immune response through different molecular mechanisms.

HMG20A from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

High-mobility group 20 A (HMG20A) has important biological functions, such as inhibiting the differentiation of red blood cells and nerve cells, promoting the proliferation and metastasis of cancer cells, and regulating inflammatory reaction. However, the role of HMG20A in the response to bacterial infection in the economic fish Nile tilapia (Oreochromis niloticus) remains unclear. In this study, a HMG20A homolog was successfully identified and characterized from Nile tilapia (On-HMG20A), and its expression model and biological effects on bacterial infection were analyzed. The open reading frame (ORF) of On-HMG20A was 876 bp in length, which encoded 291 amino acids and possessed a HMG domain (High mobility group domains) and coiled coil region. Results of the expression model showed that On-HMG20A was widely distributed in immune-related tissues of healthy tilapia and upregulated in a time-dependent manner after being challenged by Streptococcus agalactiae. Meanwhile, knocking down the expression of On-HMG20A can reduce the inflammatory response of tilapia and the degree of tissue damage caused by S. agalactiae. Moreover, knocking down the expression of On-HMG20A can reduce the bacterial load of tilapia tissues after being challenged by S. agalactiae and improve the survival rate. Collectively, these results showed that On-HMG20A may be related to the immune response of Nile tilapia against bacterial infection.

Ctenopharyngodon idella Tollip regulates MyD88-induced NF-κB activation

Toll-interacting protein (Tollip) and MyD88 are key components of the TLR/IL-1R signaling pathway in mammals. MyD88 is known as a universal adaptor protein involving in TLR/IL-1R-induced NF-κB activation. Tollip is a crucial negative regulator of TLR-mediated innate immune responses. Previous studies have demonstrated that teleost Tollip served as a negative regulator of MyD88-dependent TLR signaling pathway. However, the mechanism is still unclear. In particular, the effect of TBD, C2, and CUE domains of Tollip on MyD88-NF-κB signaling pathway remains to be elucidated. In this study, we found that the response of grass carp Tollip (CiTollip) to LPS stimulation was faster and stronger than that of poly I:C treatment, and CiTollip diminished the expression of tnf-α induced by LPS. Further assays indicated that except for the truncated mutant of △CUE2 (1-173 aa), wild type CiTollip and other truncated mutants (△N-(52-276 aa), △C2-(173-276 aa) and △CUE1-(1-231 aa)) could associate with MyD88 and negatively regulate MyD88-induced NF-κB activation. It suggested that the C-terminal (173-276 aa), in particular the connection section between C2 and CUE domains (173-231 aa), played a pivotal role in suppressing MyD88-induced activation of NF-κB.

Molecular mechanisms of intermuscular bone development in fish: a review

Intermuscular bones (IBs) are slender linear bones embedded in muscle, which ossify from tendons through a process of intramembranous ossification, and only exist in basal teleosts. IBs are essential for fish swimming, but they present a choking risk during human consumption, especially in children, which can lead to commercial risks that have a negative impact on the aquaculture of these fish. In this review, we discuss the morphogenesis and functions of IBs, including their underlying molecular mechanisms, as well as the advantages and disadvantages of different methods for IB studies and techniques for breeding and generating IB-free fish lines. This review reveals that the many key genes involved in tendon development, osteoblast differentiation, and bone formation, e.g., scxa, msxC, sost, twist, bmps, and osterix, also play roles in IB development. Thus, this paper provides useful information for the breeding of new fish strains without IBs via genome editing and artificial selection.

Grouper (Epinephelus coioides) IRAK-4 regulates activation of NF-κB and expression of inflammatory cytokines in grouper spleen cells

IRAK-4 is a serine/threonine kinase that can bind to interleukin-1 receptor induced by interleukin-1. It plays a key role in the Toll-like receptor signaling pathway and is involved in innate and adaptive immune responses. In this study, piscine IRAK-4 significantly activated nuclear factor (NF)-κB signaling in grouper spleen cells. Grouper (Epinephelus coioides) IRAK-4 (EcIRAK-4) co-localized with EcMyD88 and did not impair EcMyD88-dependent NF-κB activation. Different doses of EcIRAK-4 caused different degrees of nuclear translocation of the transcription factor NF-κB p65 subunit, and it induced transcription of multiple pro-inflammatory cytokines. Using expression vectors of deletion domains or mutations at important sites of EcIRAK-4, we found that the EcIRAK-4 kinase domain is necessary for its signal transduction function. The conserved amino acid sites performed functions similar to those in mammals, and grouper-specific amino acids such as E339 also played important roles. These findings provide information about the functional characteristics of IRAK-4 in lower vertebrates.

Grass carp (Ctenopharyngodon idella) IRAK1 and STAT3 up-regulate synergistically the transcription of IL-10

In vertebrates, IL-10 is an anti-inflammatory factor that serves as a key inhibitory role in a wide range of immune responses. IRAK1 (IL-1 receptor-associated kinase 1), a key molecule in the inflammatory signal of IL-1R/TLR, plays an important pivotal role in regulating the autoimmunity of body. STAT3 (Signal transducer and activator of transcription 3) activated by IRAK1 participates in inflammation, tumorigenesis, metabolic disorders and immune response. Under the stimulation of LPS, IRAK1 enters the nucleus to form a dimer with STAT3 and regulates the expression of IL-10. However, the relationship between fish IRAK1 and STAT3 has not been reported. To explain the anti-inflammation in fish, we amplified and identified the complete open reading frame of grass carp IRAK1 (CiIRAK1) and STAT3 (CiSTAT3) based on the existing sequences. The expression of CiIRAK1 and CiSTAT3 were up-regulated significantly under the stimulation of LPS. This result suggests that both CiIRAK1 and CiSTAT3 may be involved in LPS-induced TLR4 pathway. The subcellular localization experiment revealed that CiIRAK1 is distributed in cytoplasm and enters nucleus after LPS stimulation. CiSTAT3 is distributed in both cytoplasm and nucleus with or without LPS stimulation. Immunoprecipitation assay revealed that CiIRAK1 interacted with CiSTAT3 under LPS stimulation. However in absence of LPS stimulation, CiIRAK1 and CiSTAT3 cannot interact with each other. Subsequently, immunofluorescence colocalization experiment further proved the interaction of CiIRAK1 and CiSTAT3 in nucleus under LPS stimulation. The dual luciferase reporter assays indicated that the binding of CiIRAK1 and CiSTAT3 synergistically enhanced the activity of CiIL-10 promoter.

Molecular features of interleukin-1 receptor-associated kinase-b and a in Mytilus coruscus, regulating their function by infection of aquatic pathogens and the expression of their serine/threonine protein kinase functional domains

Interleukin-1 receptor-associated kinases (IRAKs) play important roles in the innate immune system of TLR (Toll-like receptor) signaling pathway. In this paper, interleukin-1 receptor-associated kinase-b (designated as McIRAK-b) and interleukin-1 receptor-associated kinase-a (named as McIRAK-a) were obtained based on the transcriptome data, the full length of McIRAK-b was 1815 bp and McIRAK-a was 3168bp, encoding 532 and 978 amino acids, respectively. BLASTp analysis and phylogenetic relationship strongly suggested that the deduced amino acid sequence of McIRAK-b had high homology with IRAK-4 and McIRAK-a was similar to IRAK-1 of other mollusks, especially at their function domains. The expressions of McIRAK-b and McIRAK-a were detected in six tissues including adductor muscle, hemocyte, gills, gonad and hepatopancreas, and the highest expressions appeared both in gills. The expressions of McIRAK-b and McIRAK-a in gills were observed with time-dependent manners after bacterial infections. After being challenged with Vibrio alginolyticus, McIRAK-b expressed significantly and got the peak at 8 h (9.47 times compared with the control group), but the peak appeared at 4 h by being infected with Vibrio parahaemolyticus (12.02 times compared with the control group). The highest point of McIRAK-a mRNA appeared at 12 h (5.16 times) after being challenged with V.alginolyticus and 8 h (4.21 times) for V.parahaemolyticus challenge. The results suggested that IRAK-b and IRAK-a might be important in immune signaling pathway of mussels. The kinase functional domain sequences (S_TKc) of McIRAK-b and McIRAK-a expressed in BL21(DE3) and purified by Ni-NAT Superflow resin conforming to the expected molecular weight with many active sites for their conferring protein-protein interaction functions. This study may provide some further understandings of the regulatory mechanisms in the bivalve innate immune system for IRAKs family.

Grass carp (Ctenopharyngodon idellus) TRAF6 up-regulates IFN1 expression by activating IRF5

In mammals, interferon regulatory factor 5 (IRF5) can be activated by tumor necrosis factor receptor-associated factor 6 (TRAF6). Upon activation, IRF5 translocates into the nucleus, where it binds to IFN promoter and up-regulates IFN expression. However, there are few reports on the molecular mechanism by which TRAF6 up-regulates IFN expression in fish. In this study, we explored how Grass carp (Ctenopharyngodon idellus) TRAF6 initiated innate immunity by activating IRF5. We found that CiTRAF6, CiIRF5 and CiIFN1 were all significantly up-regulated in LPS-stimulated CIK cells and the expression of CiTRAF6 was earlier than the expressions of CiIRF5 and CiIFN1. These findings suggested that CiIFN1 expression might be induced by CiTRAF6 in fish. CiIFN1 expression, CiIFN1 promoter activity and CO cells viability were all significantly up-regulated in the overexpression experiments, but they were significantly down-regulated in the gene silencing experiments. This indicated that CiTRAF6, along with CiIRF5, regulated CiIFN1 expression. The localization analysis found that both CiTRAF6 and CiIRF5 located in the cytoplasm. Following LPS stimulation, CiIRF5 was observed to translocate to the nucleus. GST-pull down and co-IP experiments revealed that CiTRAF6 interacted with CiIRF5. The colocalization analysis also showed that CiTRAF6 bound with CiIRF5 in the cytoplasm. Overexpression of CiTRAF6 increased the endogenous CiIRF5, promoted its ubiquitination and nuclear translocation. In conclusion, CiTRAF6 bound to CiIRF5 in the cytoplasm, and then activated CiIRF5, resulting in up-regulating the expression of CiIFN1.