CDK8 mediates coelomocyte proliferation via promoting β-catenin nuclear translocation in Vibrio splendidus-challenged Apostichopus japonicus

Cyclin-dependent kinase 8 (CDK8) play a vital role in the regulation of essential biological processes such as cell proliferation and differentiation in various organisms. However, the immune functional of CDK8 in economic marine animals remains unclear. We aimed to explore the functional roles of CDK8 and its underlying mechanism in the pathogen Vibrio splendidus-challenged sea cucumber, Apostichopus japonicus. In this study, we cloned the full-length cDNA sequence of the CDK8 gene of A. japonicus (AjCDK8) and performed its bioinformatics analysis. AjCDK8 possessed the classical function domain of S_TKc and was highly conserved in various species. qRT-PCR analysis showed that AjCDK8 was significantly up-regulated in coelomocytes and intestines after being challenged with V. splendidus infection and/or LPS exposure. Importantly, the coelomocyte proliferation levels were significantly increased by the 105 and 106 CFU/mL V. splendidus infection, whereas it was inhibited by 108 CFU/mL V. splendidus in A. japonicus. Moreover, the underlying mechanism of coelomocytes proliferation mediated by AjCDK8 was investigated under 105 CFU/mL V. splendidus treatment. Silencing of AjCDK8 (siAjCDK8) or A. japonicus β-catenin (siAjβ-catenin) could both suppress the coelomocytes proliferation in 105 CFU/mL V. splendidus-challenged sea cucumbers. Furthermore, AjCDK8 knockdown significantly inhibited the expression of Ajβ-catenin in vivo and in vitro, and Ajβ-catenin nuclear translocation was promoted by recombinant AjCDK8 (rAjCDK8) treatment followed by V. splendidus challenge. Additionally, there were no visible changes in coelomocytes proliferation levels in sea cucumbers post-treated with rAjCDK8 + V. splendidus and recombinant histidine-tag protein + V. splendidus in Ajβ-catenin knockdown coelomocytes. Collectively, these results confirmed that AjCDK8 played a key role in regulating coelomocyte proliferation via promoting Ajβ-catenin expression in V. splendidus-challenged A. japonicus.