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  • br Conclusion br Acknowledgements This work is supported


    Acknowledgements This work is supported by the National Natural Science Foundation of China (41806151, 41706144), the National Natural Science Foundation of China (31330082). We would like to thanks the support of young science and technology talents training fund of South China Agricultural University.
    Introduction Chemokines are members of chemotactic cytokines that play an important role in cell movement and activation under inflammatory conditions [[1], [2], [3]]. Furthermore, chemokine superfamilies are an important bridge between innate and adaptive immunity [4]. Most chemokines generally contain four types of cysteine; according to differences in the first two cysteines, chemokines can be classified into five subtypes: CXC, XC, CX, CC and CX3C [5]. Chemokine receptors are G protein-coupled receptors containing seven transmembrane structures [6]. These receptors contain 7 helical transmembrane domains, three intracellular and three extracellular ras inhibitor rings, a short acidic N-terminal and an intracellular C-terminal, with serine and threonine residues that act as phosphorylation sites in the receptor regulation process [7]. The N-terminal of a chemokine receptor is the key position that specifically binds to a ligand, whereas G proteins couple to the C-terminal end, which is an important signal pathway after ligand binds to the receptor [7]. Chemokine receptors are also divided into four subfamilies, CXC, CC, CX3C and XC, based on the spacing of cysteine residues near the N-terminal of the receptor [6,8]. Most chemokines can bind to multiple receptors, and one receptor can also bind to multiple chemokines [9]. The ligand receptor complex then initiates a series of signaling pathways that regulate various physiological and pathological processes [10,11]. Although chemokine and chemokine receptors are widely studied in mammals, particularly humans, the involvement of the chemokine-receptor system in different diseases [7] and the chemokine and chemokine receptor biology of teleost fish and its involvement in fish immunity are ill-defined. CXC chemokine receptor 1 (CXCR1) was the first defined chemokine receptor subtype, and this is the only known receptor for ELR + CXC chemokines in mammals. CXCR1 is often present on the surface of neutrophils as a receptor for il-8, and IL-8 carries out its function by binding and activating the receptor. Il-8 binds to CXCR1 to induce chemotaxis and cytotoxic reactions, such as the exocytosis of lysosomal enzymes and the production of superoxide anions [[12], [13], [14]]. Hence, CXCR1 has fundamental regulatory effects on natural immunity, inflammation, white blood cell transport, neutrophil recruitment, immune system signal transduction pathways and host anti-infection [15]. To date, CXCR1 has been identified and characterized only from a few fish species [[16], [17], [18], [19], [20], [21], [22], [23]]. In our previous study, we found two CXCR1 (CXCR1a and CXCR1b) genes in orange-spotted grouper, in an SGIV-challenged experiment, CXCR1a was highly expressed in resistant fish [24]. However, the function of CXCR1a in viral infection has not been studied. Orange-spotted grouper (Epinephelus coioides) is an economically valuable fish in China and Southeast Asian countries. However, in recent years, outbreaks of various viral diseases have affected the development of grouper aquaculture [25]. Specifically, Singapore grouper iridovirus (SGIV) infection causes spleen and liver hemorrhage and enlargement, resulting in more than 90% mortality in fish farms and challenge experiments [26]. As chemokine receptors play an important role in the natural immunity and anti-viral, elucidating the molecular relationship between chemokine receptors and SGIV will help to provide ideas for the treatment of SGIV infection, like increase disease resistance through enhancing CXCR1a expression.
    Materials and methods
    Discussion The role of chemokine receptors in the vertebrate immune response has attracted much attention in recent years [35,36], and understanding the role of chemokine receptors in pathogen defense is critically important. In this study, we obtained the full length of the CXCR1a gene from orange-spotted grouper. Typical features of CXCRs, such as seven hydrophobic membrane-spanning domains, the most highly conserved DRY motif, were found in CXCR1a. This motif is important for the ability of CXCR to signal upon ligand binding and G-protein interaction [37,38]. A phylogenetic tree was constructed based on the amino acid sequence of CXCR1a and CXCR1 from other known species. The results showed that the orange-spotted group CXCX1a clustered only with other fish CXCR1 and formed distinct clades. The overall topology of the tree was consistent with traditional taxonomy and phylogenetic transition.