The mitochondrial calcium uniporter (MCU) facilitates mitochondrial calcium influx, and several enzymes that catalyze key steps of the TCA cycle require calcium for their activation. As such, it is generally accepted that rates of mitochondrial oxidation correlate with mitochondrial calcium activity. Decreased rates of hepatic mitochondrial oxidation are associated with metabolic dysfunction and ectopic lipid accumulation as seen with type 2 diabetes and nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis. Despite this, a role for MCU and mitochondrial calcium in regulating hepatic mitochondrial oxidation is still unclear. To address this question, we generated a liver-specific MCU knockout (MCU KO) mouse model which displayed reduced mitochondrial calcium ([Ca2+]mt; p<0.05) and increased cytosolic calcium ([Ca2+]cyt; p<0.01). Despite decreased [Ca2+]mt, we found that deletion of hepatic MCU increased rates of isocitrate dehydrogenase flux (VIDH; p<0.01), α-ketoglutarate dehydrogenase flux (VOGDH; p<0.01), and succinate dehydrogenase flux (VSDH; p<0.05) in vivo, using a novel [13C5]glutamine tracer infusion technique (Q-Flux). Further, we show that rates of [14C16]palmitate oxidation (p<0.01) and intrahepatic lipolysis (p<0.05) were significantly increased in liver slices collected from MCU KO mice compared to WT mice, and were associated with reductions in hepatic triacylglycerol content in vivo (p<0.01). Hepatocellular redox state was also altered with MCU KO, as indicated by a decreased cytosolic redox ratio ([lactate]:[pyruvate]; p<0.05) and increased mitochondrial redox ratio ([β-hydroxybutyrate]:[acetoacetate]; p<0.01). Conclusion: Here we show that increases in [Ca2+]cyt potently increase hepatic mitochondrial fat oxidation and alters hepatic redox balance to reduce hepatic lipid content independently of [Ca2+]mt in vivo and targeting [Ca2+]cyt may represent a novel therapeutic target to treat NAFLD. Disclosure T.E.Lamoia: None. B.T.Hubbard: None. M.Guerra: None. R.Goodman: None. M.H.Nathanson: None. G.I.Shulman: Consultant; Novo Nordisk, DiCerna Pharmaceuticals, Inc., Bayer Consumer Care AG, Kriya Therapeutics, Arrowhead Pharmaceuticals, Inc., ESPERION Therapeutics, Inc., Other Relationship; AstraZeneca, Merck & Co., Inc., Janssen Research & Development, LLC, iMetabolic Biopharma Corporation, Maze Therapeutics, 89bio, Inc., Equator Therapeutics, Inc., Generian Pharmaceuticals, Fortress Biotech, Inc., OrsoBio, Aro Biotherapeutics Company, Stock/Shareholder; Levels Health, Inc. Funding National Institutes of Health (F31DK126362, T32GM007324)

1558-P: the Mitochondrial Calcium Uniporter Regulates Hepatic Mitochondrial Oxidation and Intracellular Redox in Vivo