TSENG, Yung-Che; HWANG, Pung-Pung; Institute of Fisheries Science, National Taiwan University; Institute of Zoology, Academia Sinica: Role of Glycogen Phosphorylase in Energy Supplying for Osmoregulation in Tilapia (Oreochromis mossambicus )

Upon external salinity challenges, euryhaline teleosts perform rapid modulations of functions of several ion transporters or channels in gills to maintain the internal homeostasis. Ion- and osmo-regulation are highly energy consuming, therefore there must be an efficient metabolism system, which supplies immediate and sufficient energy sources for the active ion transport in gills. Glycogen is the main energy reserve in highly energy depletion tissues, such as brain, liver and muscle, and has been proposed to be an emergency fuel store during physiological stresses. In the processes of glycogen metabolism, glycogen phosphorylase (GP) catalyzes the initial glycogen degradation, i.e., the breakdown of glycogen to glucose-1-phosphate, which enters glycolysis or glycogenolysis to fulfill the energetic requirements of different cell types. The homodimeric enzyme exists as 3 isoforms named according to the tissues in which they predominate; LL (liver), MM (skeletal muscle) and BB (brain) forms in most mammalians. Using subtraction PCR, a clone of tGP was found to express differentially in the gills between freshwater (FW) – and seawater (SW) -acclimated tilapia. In present study, we have cloned and sequenced the full length cDNA of the tGP from tilapia gill. Alignment analysis showed that the tGP may be a homologue of the liver form of the mammalian GP. RT-PCR and real-time PCR analysis indicated that the tGP gene expression in SW tilapia gills is about 1.5 times that in FW ones. In situ hybridization result indicated that the tGP mRNA is localized in the epithelial cells of tilapia gills. Taken together, the tGP gene expression is stimulated in tilapia gills upon salinity challenge, suggesting that the breakdown of glycogen may be directly providing energy for the osmoregulation mechanisms in gill cells.