Metabolic trafficking between astrocytes and neurons under hyperammonemia and manganism: Nitrogen- and Carbon metabolism

Abstract

Hyperammonemia is defined as an elevated ammonia concentration, which is caused by an impairment of liver function. The increased ammonia concentrations in the brain exert toxic effects on neural cells.Despite of intensive investigations on the role and importance of ammonia in cerebral metabolism, the exact molecular mechanism could not be yet clarified. In the line of this thesis, the effects of extracellular ammonia and glutamine on cerebral metabolism were investigated. For this purpose, apart form multinuclear NMR spectroscopy, molecular biological methods, e.g. Western Blotting, and RT-PCR were used. The data of the present study clearly show that ammonia causes an increased glucose metabolism by stimulation of glycolytic- and TCA cycle-activity in both astrocytes and neurons. Furthermore, it could be shown that glutamine was utilized as an anaplerotic substrate. In addition, the nitrogen from glutamate was transferred to oxaloacetate and pyruvate by aminotransferase. This indicates to alternative ammonia detoxification processes in neurons, compared to glutamine synthesis in astrocytes. The results of the present study confirm the higher susceptibility of the energy-status in astrocytes to ammonia. To study the effect of extracellular glutamine on neural metabolism, primary astrocytes and neurons were incubated for 24 hours in a glutamine-containing medium. The data of the present study show that deprivation of extracellular glutamine leads to a disturbance in the neuronal TCA cycle, demonstrating a crucial role of glutamine on the normal, physiological metabolism in neurons Furthermore, the results prove that the glutamine-glutamate cycle is not stochiometric. Using Western blot analysis, it was shown that both ammonia and extracellular glutamine cause a remarkably increase of glutamine synthetase expression in astrocytes. This finding indicates to an important role of GS in the elimination of ammonia and the regulation of the glutamine-glutamate cycle.