p38 mitogen-activated protein kinases (MAPKs) are involved in intestinal immune response to bacterial muramyl dipeptide challenge in Ctenopharyngodon idella.

The p38 mitogen-activated protein kinases (MAPKs) are essential cytoplasmic signal molecules of innate immune pathways that play a vital role in host immune defense responses to pathogenic challenges. In this study, two fish p38 genes (Cip38α and Cip38β) were characterized for the first time from the grass carp Ctenopharyngodon idella. Similar to other reported p38MAPKs, both Cip38α and Cip38β contained a conserved phosphorylation motif (Thr-Gly-Tyr, TGY) and a substrate binding site (Ala-Thr-Arg-Trp, ATRW) in the serine/threonine protein kinase (S_TKc) domain. Expression profile analysis showed that Cip38α and Cip38β mRNAs were broadly expressed in all of the examined tissues and developmental stages of C. idella. In addition, in vivo injection experiments directly revealed that Cip38α and Cip38β showed strong responsiveness to Aeromonas hydrophila and muramyl dipeptide (MDP) challenges, and their expression levels were significantly upregulated in the intestine of grass carp. Additionally, the MDP-induced expression levels of intestinal inflammatory cytokines (TNF-α and IL-15) and an antimicrobial peptide (β-defensin) were significantly inhibited by the p38MAPK-specific inhibitor SB203580. Moreover, the nutritional dipeptide carnosine and Ala-Gln were found to significantly suppress the bacterial MDP-induced expression of p38MAPK pathway genes and inflammatory cytokines in the intestine of grass carp. Finally, overexpression analysis demonstrated that Cip38α and Cip38β could act as efficient activators in the regulation of AP-1 signaling pathways through interaction with CiMKK6. Altogether, this study provided experimental evidence of the presence of a functional p38 pathway in grass carp, which revealed its involvement in the intestinal immune response to bacterial challenges in bony fish.