The Mitochondrial Calcium Uniporter Regulates Hepatic Lipid Accumulation and Mitochondrial Oxidation

The mitochondrial calcium uniporter (MCU) facilitates mitochondrial calcium influx, and several enzymes that catalyze key steps of the tricarboxylic acid (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. 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 significantly reduced mitochondrial calcium ([Ca2+]mt, p<0.05) and increased cytosolic calcium ([Ca2+]cyt, p<0.01) in isolated hepatocytes. Despite decreased [Ca2+]mt, we found that deletion of hepatic MCU resulted in 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) , using a novel in vivo [13C5]glutamine tracer technique. Further, we report that rates of [14C16]palmitate oxidation (p<0.01) and intrahepatic lipolysis (p<0.05) were significantly increased in precision-cut liver slices collected from MCU KO mice compared to WT, consistent with [Ca2+]cyt activation of CAMKII (p<0.01) and ATGL (p<0.001) in MCU KO mice, and were associated with reductions in hepatic triacylglycerol content in vivo (p<0.01) . Taken together, these data demonstrate that hepatic mitochondrial activity can be dissociated from mitochondrial calcium concentrations and reveal a key role for cytosolic calcium and mitochondrial anaplerosis in the regulation of hepatic mitochondrial fat oxidation. Disclosure T. E. Lamoia: n/a. B. T. Hubbard: None. M. Guerra: None. R. Goodman: None. M. H. Nathanson: None. V. Mootha: Advisory Panel; 5AM Ventures, LLC, Janssen Research & Development, LLC. G. I. Shulman: Advisory Panel; 89bio, Inc., AstraZeneca, Equator Therapeutics, Inc., Janssen Research & Development, LLC, Merck & Co., Inc., Consultant; DiCerna Pharmaceuticals, Inc. , Novo Nordisk, Other Relationship; Generian Pharmaceuticals, iMetabolic Biopharma Corporation, Maze Therapeutics, The Liver Company, Stock/Shareholder; Levels Health, Inc. . Funding F31 DK126362/DK/NIDDK NIH HHST32 GM007324/GM/NIGMS NIH HHS