cDNA structure of an insulin-related peptide in the Pacific oyster and seasonal changes in the gene expression

Insulin-related peptide cDNA was characterized in the Pacific oyster Crassostrea gigas. It was determined that three transcripts with differing lengths of 3'-untranslated region (3'-UTR) were expressed in the visceral ganglia. The insulin-related peptide cDNA contained a number of AUUUA motifs that were typical of adenylate/uridylate-rich elements in the 3'-UTR. The deduced preprohormone was a polypeptide of 161 residues and showed a conformation typical of preprohormones of the insulin superfamily, which included conserved amino acids necessary to adopt the globular insulin structure. The expression of the three different transcripts was variable throughout the year, with the highest expression observed in March and lower expression in November and July. The expression of the shortest mRNA in March was about tenfold higher than in July, while the expression of the longest transcript varied approximately twofold during the year. The accumulation of glycogen in the soft body rapidly increased in October and November, and robust body growth and gametogenetic development occurred in March to May. The period of the highest expression of the oyster insulin-related peptide gene corresponded to the onset of body growth and gametogenetic development, but did not overlap with the period of glycogen accumulation. This is the first report that fully details the structure and expression of the insulin-related peptide gene in bivalves.