Characterization of a novel toll-like receptor and activation NF-κB signal pathway in triangle sail mussel Hyriopsis cumingii

Genome-wide identification of toll-like receptors in Octopus sinensis and expression analysis in response to different PAMPs stimulation

Toll-like receptors (TLRs) are one of the extensively studied pattern recognition receptors (PRRs) and play crucial roles in the immune responses of vertebrates and invertebrates. In this study, 14 TLR genes were identified from the genome-wide data of Octopus sinensis. Protein structural domain analysis showed that most TLR proteins had three main structural domains: extracellular leucine-rich repeats (LRR), transmembrane structural domains, and intracellular Toll/IL-1 receptor domain (TIR). The results of subcellular localization prediction showed that the TLRs of O. sinensis were mainly located on the plasma membrane. The results of quantitative real-time PCR (qPCR) showed that the detected TLR genes were differentially expressed in the hemolymph, white bodies, hepatopancreas, gills, gill heart, intestine, kidney, and salivary gland of O. sinensis. Furthermore, the present study investigated the expression changes of O. sinensis TLR genes in hemolymph, white bodies, gills, and hepatopancreas in different phases (6 h, 12 h, 24 h, 48 h) after stimulation with PGN, poly(I: C) and Vibrio parahaemolyticus. The expression of most of the TLR genes was upregulated at different time points after infection with pathogens or stimulation with PAMPs, a few genes were unchanged or even down-regulated, and many of the TLR genes were much higher after V. parahaemolyticus infection than after PGN and poly(I:C) stimulation. The results of this study contribute to a better understanding of the molecular immune mechanisms of O. sinensis TLRs genes in resistance to pathogen stimulation.

Signaling events induced by lipopolysaccharide-activated Toll in response to bacterial infection in shrimp

Toll-like receptors (TLR) play a crucial role in the detection of microbial infections in vertebrates and invertebrates. Mammalian TLRs directly recognize a variety of structurally conserved microbial components. However, invertebrates such as Drosophila indirectly recognize microbial products by binding to the cytokine-like ligand Spätzle, which activates signaling cascades that are not completely understood. In this study, we investigated the signaling events triggered by Toll in response to lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria, and Vibrio parahaemolyticus infection in the arthropod shrimp Litopenaeus vannamei. We found that five of the nine Tolls from L. vannamei bound to LPS and the RNAi of LvToll1, LvToll2, LvToll3, LvToll5, and LvToll9 weakened LvDorsal-L phosphorylation induced by V. parahaemolyticus. All nine Tolls combined with MyD88 via the TIR domain, thereby conferring signals to the tumor necrosis factor receptor-associated factor 6 (TRAF6)-transforming growth factor-β activated kinase 1 binding protein 2 (TAB2)-transforming growth factor-β activated kinase 1 (TAK1) complex. Further examination revealed that the LvTRAF6-LvTAB2-LvTAK1 complex contributes to Dorsal-L phosphorylation and nuclear translocation during V. parahaemolyticus infection. Overall, shrimp Toll1/2/3/5/9-TRAF6/TAB2/TAK1-Dorsal cascades protect the host from V. parahaemolyticus infection, which provides a better understanding of how the innate immune system recognizes and responds to bacterial infections in invertebrates.

β-arrestin interacts with TRAF6 to negatively regulate the NF-κB pathway in triangle sail mussel Hyriopsis cumingii

As members of arrestins family, β-arrestins are widely expressed in monocytes, macrophages, neutrophils and other immune cells. They can regulate the immune response of bodies through various ways. In the present study, a β-arrestin homolog named Hcβ-arrestin was cloned and identified from Hyriopsis cumingii. Predicted Hcβ-arrestin protein contained a conserved arrestin domain, which could be further divided into arrestin-N (39-192aa) and arrestin-C (211-365aa). Amino acid sequence alignment showed that it had the highest identity with Mytilus galloprovincialis and Mytilus edulis counterpart, which was 89.02% and 87.68%, respectively. Furthermore, real-time quantitative PCR analysis showed that the Hcβ-arrestin gene was widely expressed in the detected tissues and with the highest expression in hepatopancreas. The transcription of Hcβ-arrestin in hepatopancreas and gill of mussels was significantly up-regulated after stimulation with peptidoglycan, lipopolysaccharide (LPS) and polyinosinic polycytidylic acid. Knockdown of Hcβ-arrestin gene significantly increased the expression of some antibacterial effector genes, such as lysozyme, LPS-binding protein/bactericidal permeability increasing protein and theromacin in hepatopancreas and gills of LPS stimulated mussels, but only had little effect on TLR pathway genes. In addition, GST pull-down assay confirmed that Hcβ-arrestin can bind to HcTRAF6 protein in vitro. Dual luciferase reporter assay showed that the co-expression of HcTRAF6 and Hcβ-arrestin inhibited the activation of NF-κB reporter by HcTRAF6. These findings indicated that Hcβ-arrestins could interact with HcTRAF6 to negatively regulate the NF-κB pathway in H. cumingii.

A novel molluscan TLR molecule engaged in inflammatory response through MyD88 adapter recruitment

Toll-like receptors (TLRs) mediated signaling plays a vital role in activating innate and adaptive immunity. Although TLR mediated signaling has been comprehensively investigated in mammalian species, the mechanisms underlying TLR signaling in molluscs remain obscure. In the present study, a novel TLR isoform namely McTLR-like1 was identified in the thick shell mussel Mytilus coruscus. McTLR-like1 was highly expressed in molluscan immune-related tissues, and its transcriptional levels in hemocytes were significantly increased when challenged by V. alginolyticus. McTLR-like1 activated nuclear factor κB (NF-κB) and strengthened the transcription and phosphorylation of NF-κB subunit P65 in mammalian cells. Upon the silencing of McTLR-like1, the mRNA expression levels of pro-inflammatory cytokines were down-regulated, and the animals exhibited higher levels of resistance when challenged with V. alginolyticus. McMyD88a mRNA expression was also downregulated alongside McTLR-like1. Furthermore, GST-pull down assays revealed a visible affinity between McTLR-like1 and McMyD88a. Collectively, these results demonstrated that the newly identified gene affiliated to the molluscan TLR family and plays a role in the TLR-mediated activation of inflammatory response via its affinity with MyD88. The present study enhances our knowledge of TLR signaling mechanisms in molluscs and provides new insights into the evolution of TLRs.