Ammonia inhibits energy metabolism in astrocytes in a rapid and GDH2-dependent manner

In hepatic encephalopathy (HE) astrocyte dysfunction is a primary factor impairing neuronal activity under hyperammonemia. We show that mitochondria in cellular HE models undergo rapid fragmentation under hyperammonemia in a reversible manner. Mitochondrial respiration and glycolysis were instantaneously hampered in a pH-independent manner. A metabolomics approach revealed a subsequent accumulation of numerous amino acids, including branched chain amino acids, and glucose. N 15 labeling of ammonia shows rapid incorporation of ammonia-derived nitrogen into glutamate and glutamate-derived amino acids. Downregulating human GLUD2 , encoding mitochondrial glutamate dehydrogenase 2 (GDH2), inhibiting GDH2 activity by SIRT4 overexpression, and supplementing cells with glutamate or glutamine alleviated ammonia-induced inhibition of mitochondrial respiration. Thus, under hyperammonemic conditions, GDH2 catalyzes the removal of ammonia by reductive amination of α-ketoglutarate but at the same time inhibits the TCA-cycle by depleting α-ketoglutarate. Overall, we propose a mitochondria-dependent mechanism contributing to the early steps in the pathogenesis of HE where the interplay between energy metabolism and ammonia removal plays a pivotal role.