Expression of glycogen synthase (GYS) and glycogen synthase kinase 3β (GSK3β) of the Fujian oyster, Crassostrea angulata, in relation to glycogen content in gonad development

To investigate the regulation of glycogen metabolism at the mRNA level in Crassostrea angulata, we cloned and characterized glycogen synthase and glycogen synthase kinase 3β cDNAs (Ca-GYS and Ca-GSK3β, respectively), which encode the primary enzymes involved in glycogen storage. We examined their expression profiles in different tissues and during different reproductive stages. The full-length cDNA of GYS was 4771 bp in length with a 2023 bp open reading frame (ORF), predicted to encode a protein of 674 aa. The full-length GSK3β cDNA was 2333 bp long, with an ORF of 1242 bp. High expression levels of both genes were observed in the gonad and the adductor muscle, as compared to the mantle, gill, or visceral mass, which correlates well with the ability to store glucose. The regulation of both genes was correlated with glycogen content via qPCR and in situ hybridization and was dependent upon the stage of the reproductive cycle (initiation stage, maturation stage, ripeness stage). Thus, it appears that the expression of Ca-GYS and Ca-GSK3β is driven by the reproductive cycle of the oyster, reflecting the central role played by glycogen in energy storage and gametogenic development in C. angulata. We suggest that Ca-GYS and Ca-GSK3β can be used as useful molecular markers for identifying the stages of glycogen metabolism and reproduction in C. angulata.