p38 MAPK is involved in the immune response to pathogenic Vibrio in the clam Meretrix petechialis

p38 mitogen-activated protein kinases (MAPKs) are involved in the response to various extracellular stimuli via regulating gene expression. In the present study, a p38 MAPK gene (MpP38) was identified from the clam Meretrix petechialis. The full-length cDNA of MpP38 measures 1,720 bp, consisting of a 134-bp 5'-UTR, a 1,095-bp ORF and a 491-bp 3'-UTR. Both the mRNA and protein expression levels of MpP38 increased after Vibrio challenge, implying that MpP38 is involved in clam immunity. Based on our previous study, a transcription factor activated by p38 MAPK, i.e. microphthalmia-associated transcription factor (MITF), participated in clam immunity by regulating the expression of phenoloxidase (PO). Coupled with other related reports, the mechanism underlying the involvement of MpP38 in clam immunity is most likely that pathogen stimuli induce the phosphorylation of p38 MAPK and thus activate MITF to regulate the expression of the immune-related gene PO. The results obtained in this study support this mechanism. First, we found that the MpP38 phosphorylation level increased in response to Vibrio challenge. Second, as revealed by a yeast two-hybrid assay, there was a direct interaction between MpP38 and MITF. Meanwhile, inhibiting the phosphorylation of MpP38 decreased the phosphorylation level of MpMITF, implying that MpP38 phosphorylation is required for MpMITF activation. Additionally, our results showed that there was a regulatory relationship between MpP38 phosphorylation level and PO expression level. With increased MpP38 phosphorylation level, the PO expression level was also increased after Vibrio challenge; when MpP38 phosphorylation was inhibited, the PO expression level was significantly decreased. This study describes the immune function of p38 MAPK in the clam for the first time and analyses its potential underlying mechanism, which will help to elucidate the immune mechanism in the clam M. petechialis.