Ammonia induces MK‐801‐sensitive nitration and phosphorylation of protein tyrosine residues in rat astrocytes

Abstract

Astrocytes play a key role in the pathogenesis of ammonia-induced neurotoxicity and hepatic encephalopathy. As shown here, ammonia induces protein tyrosine nitration in cultured rat astrocytes, which is sensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801. A similar pattern of nitrated proteins is produced by NMDA. Ammonia-induced tyrosine nitration depends on a rise in [Ca2+]i, IkB degradation, and NO synthase (iNOS) induction, which are prevented by MK-801 and the intracellular Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). Moreover, the increase in tyrosine nitration is blunted by L-NMMA, 1400W, uric acid, Cu, Zn-superoxide dismutase/catalase treatment, and methionine-sulfoximine, which indicate the involvement of reactive nitrogen intermediates and intracellular glutamine accumulation. Such reactive nitrogen intermediates additionally mediate ammonia-induced phosphorylation of the MAP-kinases Erk-1/Erk-2 and p38MAPK. Among the proteins, which are tyrosine -nitrated by ammonia, glyceraldehyde-3-phosphate dehydrogenase, the peripheral-type benzodiazepine receptor, Erk-1, and glutamine synthetase are identified. Ammonia-induced nitration of glutamine synthetase is associated with a loss of enzymatic activity. Astroglial protein tyrosine nitration is found in brains from rats after acute ammonia-intoxication or after portacaval anastomosis, indicating the in vivo relevance of the present findings. The production of reactive nitrogen intermediates and protein tyrosine nitration may alter astrocyte function and contribute to ammonia neurotoxicity.

Direct molecular and spectroscopic evidence for increased ammonia removal capacity of skeletal muscle in acute liver failure

“…A possible explanation for these findings has recently emerged. Exposure of cultured rat astrocytes (the cells that express GS in the brain) to pathophysiologically relevant (low millimolar) concentrations of ammonia results in significant nitration of GS on the tyrosine residues [18] leading to a significant reduction of GS activity. Whether, or not similar mechanisms are responsible for the loss of brain GS expression in ALF awaits the results of ongoing studies.…”

Ammonium-Induced Impairment of Axonal Growth Is Prevented through Glial Creatine

“…The glial mechanism of axonal growth protection might involve protein phosphorylation, which is directly linked to cell content in high-energy phosphates and the Cr/PCr/CK system and altered in glial cells under ammonium exposure (Neary et al, 1987;Schliess et al, 2002) and was proposed as a main signaling pathway in axon-glia inter-relationships (Witt and Brady, 2000). Crdependent modification of glial protein phosphorylation could support axonal growth, which per se is regulated through phosphorylation of axonal cytoskeleton proteins (e.g., NFs) under direct influence of oligodendrocytes (de Waegh et al, 1992).…”

Hyperammonemia in gene-targeted mice lacking functional hepatic glutamine synthetase

“…Astrocytes in close proximity to the blood-brain barrier exhibit strong PTN, possibly affecting its permeability (11). Data derived from HE animal models suggest a relationship between impaired functions of brain regions involved in cognition, learning, memory formation, and motor control, as well as elevated markers for oxidative stress in the hippocampus (12,13), cerebral cortex (14), and cerebellum (15,16).…”