The role of SYK phosphorylation in LPS-induced immunoglobulin responses of B cells in large yellow croaker (Larimichthys crocea).

Spleen tyrosine kinase (SYK), a non-receptor protein tyrosine kinase, is a key component of B cell receptor signaling and can regulate multiple physiological functions of B cells in mammals. In this study, a SYK gene was cloned and characterized from large yellow croaker (Larimichthys crocea) (LcSYK), whose open reading frame consists of 1851 base pairs and encodes 616 amino acid residues. The predicted LcSYK protein contains two N-terminal tandem Src homology 2 domains and a C-terminal tyrosine kinase catalytic domain, and shares a high amino acid sequence identity with SYK sequences in other vertebrate species. LcSYK was mainly expressed in immune tissues, such as head kidney, trunk kidney, spleen, and gill. The mRNA expression of LcSYK in primary head kidney leukocytes was not changed at 12, 24, and 48 h after lipopolysaccharide (LPS) stimulation. LPS stimulation upregulated the mRNA expression and protein production of IgM in IgM+ B cells, accompanied by an increase in the phosphorylation level, but not the total protein level, of LcSYK. Moreover, when we used PRT062607 HCl to inhibit the phosphorylation of LcSYK, both mRNA expression and protein production of IgM in IgM+ B cells were significantly suppressed. These results suggest that SYK phosphorylation may play a role in LPS-induced IgM production by IgM+ B cells, improving our understanding of the role of SYK in immunoglobulin responses of B cells in fish.