Glycogen content relative to expression of glycogen phosphorylase (GPH) and hexokinase (HK) during the reproductive cycle in the Fujian Oyster, Crassostrea angulata

Glycogen, a polymer of glucose, is an important means of storing energy. It is degraded by glycogen phosphorylase (GPH) and hexokinase (HK), glycogen phosphorylase, and hexokinase cDNAs (Ca-GPH and Ca-HK, respectively), which encode the primary enzymes involved in glycogen use, cloned and characterized and used to investigate the regulation of glycogen metabolism at the mRNA level in Crassostrea angulata. Their expression profiles were examined in different tissues and during different reproductive stages. Full-length cDNA of GPH was 3 078 bp in length with a 2 607 bp open reading frame (ORF) predicted to encode a protein of 868 amino acids (aa). The full-length HK cDNA was 3 088 bp long, with an ORF of 1 433 bp, predicted to encode a protein of 505 aa. Expression levels of both genes were found to be significantly higher in the gonads and adductor muscle than in the mantle, gill, and visceral mass. They were especially high in the adductor muscle, which suggested that these oysters can use glycogen to produce a readily available supply of glucose to support adductor muscle activity. The regulation of both genes was also found to be correlated with glycogen content via qRT-PCR and in situ hybridization and was dependent upon the stage of the reproductive cycle (initiation, maturation, ripeness). In this way, it appears that the expression of Ca-GPH and Ca-HK is driven by the reproductive cycle of the oyster, reflecting the central role played by glycogen in energy use and gametogenic development in C. angulata. It is here suggested that Ca-GPH and Ca-HK can be used as useful molecular markers for identifying the stages of glycogen metabolism and reproduction in C. angulata.