BLNK is involved in host defense to Streptococcus agalactiae infection and BCR signaling pathway in Nile tilapia (Oreochromis niloticus)

B cell linker protein (BLNK), an adapter of SLP-76 family, plays an extremely important role in B cell receptor (BCR) signaling pathway that regulates proliferation, differentiation, and apoptosis of B cell. BLNK participates in the signal transduction of BCR activation through its typical structure and interaction network with SYK, BTK etc. In this study, Nile tilapia BLNK (OnBLNK) open reading frame is 1470 bp in length for 489 amino acids. OnBLNK composes of a SAM domain at the N-terminal, a central proline-rich region and a highly conserved SH2 domain containing four conserved YXXP tyrosine phosphorylation sites at the C-terminal. Alignment of BLNK sequence and phylogenetic analysis indicated that OnBLNK was highly homologous to that of other teleost fish and mammals. Tissue distribution analysis revealed that OnBLNK showed a wide distribution in all examined tissues and with the highest expression in head kidney . After challenge with Streptococcus agalactiae ( S. agalactiae ) in vivo , the OnBLNK expression was significantly up-regulated in spleen and head kidney. In addition, the significant up-regulation of OnBLNK mRNA expression was also observed in head kidney leukocytes (HKLs) stimulated with S. agalactiae and lipopolysaccharide (LPS) in vitro . Further, it was also demonstrated that expression of OnBLNK mRNA was obviously up-regulated when HKLs were stimulated by mouse anti-tilapia IgM monoclonal antibody in vitro , and declined to normal after treatment with LYN, SYK or BTK inhibitor, respectively. Overall, the results suggest that OnBLNK might take part in host defense during bacterial infection, and get involved in the BCR signaling pathway. This study might aid to better understand the regulatory mechanism of BCR signaling pathway in teleost fish.