Glycogen metabolism in tilapia (Oreochromis mossambicus) during acclimation to environmental salinity changes
Date Issued
2005
Date
2005
Author(s)
Chang, Joshua Chia-Hsi
DOI
en-US
Abstract
Glycogen, a high-molecular-weight polysaccharide, is a major energy supply for routine and emergency needs. Functions of glycogenesis and glycogenolysis are highly regulated by the relative activities of glycogen synthase (GS; EC 2.4.1.11) and glycogen phosphorylase (GP; EC 2.4.1.1).
Gill is the most important extrarenal organ responsible for ion/osmoregulation in teleosts. Furthermore, mitochondrial-rich (MR) cells, which abundantly located in branchial epithelium, have been identified as the major sites for active transport of ions and acid/base regulation. Acclimation to seawater (SW) in euryhaline teleosts is achieved by activation of ion secretion pathway in the gill. The activation of these ion transport processes demands timely and sufficiently extra energy. However, nothing was known about the cellular and molecular basis of the energy metabolism for the ion/osmoregulation in teleosts during salinity challenges.
The purpose of the present study was to examine the role of GS and GP in the energy metabolism for the ion/osmoregulation in teleosts. We have successfully cloned and sequenced the full-length cDNA of GS form tilapia gill epithelial cells. The results of deduced amino acid sequence alignment and phylogenetic tree analysis showed that the tilapia GS (tGS) cloned from tilapia gill is a homologue of the mammalian GS1 (muscle form). The results of immunohistochemical experiments demonstrated that GS, GP, and glycogen were co-localized in the un-identified cells of gill. These cells were just adjacent to the Na+-K+-ATPase-expressing mitochondrial-rich cells (MR cells), the major ionocytes in fish gill. The gene expression levels of GS in gill measured by quantitative real-time PCR and semi-quantitative RT-PCR showed no significant change in tilapia after a long-term acclimation from fresh water (FW) to 35-ppt seawater (SW). However, the Western blotting analysis indicated that the GS protein expression in gill decreased immediately after transferring to 25-ppt SW. This finding was opposite to the protein expression profile of GP, which increased right after transfer to SW and eventually recovered. Similar phenomena of up-regulation of GP and down-regulation of GS protein expressions were also found in liver and brain right after SW transfer. However, the differential expressions of GP and GS in brain and liver occurred at 6 and 12 hours post-transfer, respectively.
According to these results, we proposed that GS and GP are involved in energy re-storing and degradation to maintain the internal glucose homeostasis in gill and brain of fish during acclimation to environmental salinity. Gill and brain initialize the metabolism of the local glycogen right after salinity challenge, and subsequently liver follows to responsible for the energy supply for longer acclimation to salinity change.
Subjects
肝糖
肝糖生合成酶
肝糖磷酸化酶
滲透壓
吳郭魚
glycogen
glycogen synthase
glycogen phosphorylase
salinity
osmoregulation
tilapia
Type
other
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