Dysregulated cellular redox status during hyperammonemia causes mitochondrial dysfunction and senescence by inhibiting sirtuin-mediated deacetylation

Perturbed metabolism of ammonia, an endogenous cytotoxin, causes mitochondrial dysfunction, reduced NAD(+)/NADH (redox) ratio, and postmitotic senescence. Sirtuins are NAD(+)-dependent deacetylases that delay senescence. In multiomics analyses, NAD metabolism and sirtuin pathways are enriched during hyperammonemia. Consistently, NAD(+)-dependent Sirtuin3 (Sirt3) expression and deacetylase activity were decreased, and protein acetylation was increased in human and murine skeletal muscle/myotubes. Global acetylomics and subcellular fractions from myotubes showed hyperammonemia-induced hyperacetylation of cellular signaling and mitochondrial proteins. We dissected the mechanisms and consequences of hyperammonemia-induced NAD metabolism by complementary genetic and chemical approaches. Hyperammonemia inhibited electron transport chain components, specifically complex I that oxidizes NADH to NAD(+), that resulted in lower redox ratio. Ammonia also caused mitochondrial oxidative dysfunction, lower mitochondrial NAD(+)-sensor Sirt3, protein hyperacetylation, and postmitotic senescence. Mitochondrial-targeted Lactobacillus brevis NADH oxidase (MitoLbNOX), but not NAD+ precursor nicotinamide riboside, reversed ammonia-induced oxidative dysfunction, electron transport chain supercomplex disassembly, lower ATP and NAD(+) content, protein hyperacetylation, Sirt3 dysfunction and postmitotic senescence in myotubes. Even though Sirt3 overexpression reversed ammonia-induced hyperacetylation, lower redox status or mitochondrial oxidative dysfunction were not reversed. These data show that acetylation is a consequence of, but is not the mechanism of, lower redox status or oxidative dysfunction during hyperammonemia. Targeting NADH oxidation is a potential approach to reverse and potentially prevent ammonia-induced postmitotic senescence in skeletal muscle. Since dysregulated ammonia metabolism occurs with aging, and NAD(+) biosynthesis is reduced in sarcopenia, our studies provide a biochemical basis for cellular senescence and have relevance in multiple tissues.

Automated summary

Perturbed metabolism of ammonia causes mitochondrial dysfunction and senescence. Sirtuins delay senescence and their pathways are enriched during hyperammonemia. Hyperammonemia leads to decreased Sirt3 expression, increased protein acetylation, and lower redox status.