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Title: Molecular Biology of Glutamate Dehydrogenase and Glutamine Synthetase in two air breathing teleosts
Keywords: glutamate, glutamine, Monopterus albus, Misgurnus anguillicaudatus, air-breathing, mRNA expression
Issue Date: 17-Mar-2011
Citation: TOK CHIA YEE (2011-03-17). Molecular Biology of Glutamate Dehydrogenase and Glutamine Synthetase in two air breathing teleosts. ScholarBank@NUS Repository.
Abstract: Air-breathing fishes such as the weatherloach Misgurnus anguillicaudatus and the swamp eel Monopterus albus often encounter the problem of endogenous ammonia buildup leading to ammonia toxicity during emersion or exposure to increased environmental ammonia. Occasionally, M. albus also faces hyperosmotic stress when it inhabits swamps. Both M. anuguillicaudatus and M. albus are capable of coping with the various adverse conditions by synthesizing glutamine, which is a product of ammonia detoxification. Moreover, glutamine may also act as an organic osmolyte in M. albus. As glutamine synthesis involves glutamate dehydrogenase (Gdh) and glutamine synthetase (Gs), this study was undertaken to examine the molecular biology of Gdh and Gs in M. anguillicaudatus and M. albus, so as to better understand the mechanisms affecting and regulating their function in these two air-breathing fishes. Results obtained from this study reveal that M. anguillicaudatus and M. albus each express one form of gdh in the liver, which may be influenced by different transcriptional and translational controls. Early phases of terrestrial exposure induced increased hepatic gdh mRNA expression in both M. anguillicaudatus and M. albus. On the other hand, increased environmental ammonia led to an initial increase in hepatic gdh mRNA expression in M. albus but not in M. anguillicaudatus. Additionally, intestinal gdh mRNA expression was down-regulated in M. anguillicaudatus exposed to terrestrial conditions, but up-regulated in M. albus exposed to increased ambient salinity. As such, it appears that unlike M. albus, the intestine of M. anguillicaudatus was unlikely to be involved in increased glutamate synthesis to facilitate increased glutamine synthesis This study also reveals for the first time that a single form of gs is expressed in the liver of M. anguillicaudatus, but three isoforms of gs are expressed in the liver, intestine and brain of M. albus. Terrestrial exposure resulted in a significant down-regulation of gs mRNA expression in the liver and intestine of M. anguillicaudatus. Furthermore, even though ammonia loading conditions led to an initial up-regulation of hepatic and intestinal gs mRNA expression in M. anguillicaudatus, gs mRNA expressions in both organs were subsequently down-regulated. In contrast, M. albus exposed to terrestrial conditions up-regulated hepatic gs1 mRNA expression and intestinal and hepatic gs2 mRNA expression. Additionally, exposure to elevated environmental ammonia also induced a significant up-regulation of hepatic gs1 mRNA expression. This differential regulation of gs between M. anguillicaudatus and M. albus is indicative of the latter utilizing mainly the strategy of glutamine synthesis while the former relying on more than one strategy to deal with increased endogenous ammonia during terrestrial exposure and ammonia loading.
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