Molecular characterization of a glutathione peroxidase gene and its expression in the selected Vibrio-resistant population of the clam Meretrix meretrix

Glutathione peroxidase (GPx) is an important member of cellular enzymatic antioxidant system, which may be involved in pathogen defense of host. In the present study, a selenium-dependent glutathione peroxidase (MmeGPx) gene from clam Meretrix meretrix was cloned and analyzed. The MmeGPx gene was composed of two introns of 723 bp and 238 bp and an open reading frame (ORF) of 711 bp. The ORF encodes a protein of 237 amino acids with a putative selenocysteine residue encoded by an unusual stop codon. MmeGPx shares a higher level of similarity with human GPx 3 than with other human GPx isozymes. The level of MmeGPx mRNA roughly paralleled GPx enzyme activity in different tissues except in gills, with the highest mRNA expression and enzyme activity occurring in hepatopancreas. MmeGPx mRNA expressions were detected in different larval stages and the results showed that MmeGPx mRNA increased significantly in pediveliger stage, which may be a response to oxidative stress. After challenge of clam with a Vibrio parahaemolyticus-related bacterium (MM21), the expression of MmeGPx was significantly up-regulated at 6 h and 12 h in hepatopancreas, which suggested that MmeGPx may be involved in the immune response to MM21 infection. To better understand its role in the immunity of clam, the expression of MmeGPx in hepatopancreas was compared between a selected Vibrio-resistant population and a control population after immersion challenge with MM21. Early up-regulation of MmeGPx was observed in the resistant population. These results suggested that MmeGPx might be involved in maintaining the redox state of immune system, and the early immune response to pathogen infection may help the clam against pathogen infection.