Cytosolic, but not matrix, calcium is essential for adjustment of mitochondrial pyruvate supply

Mitochondrial oxidative phosphorylation (OXPHOS) and cellular workload are tightly balanced by the key cellular regulator, calcium (Ca(2+)). Current models assume that cytosolic Ca(2+) regulates workload and that mitochondrial Ca(2+) uptake precedes activation of matrix dehydrogenases, thereby matching OXPHOS substrate supply to ATP demand. Surprisingly, knock-out (KO) of the mitochondrial Ca(2+) uniporter (MCU) in mice results in only minimal phenotypic changes and does not alter OXPHOS. This implies that adaptive activation of mitochondrial dehydrogenases by intramitochondrial Ca(2+) cannot be the exclusive mechanism for OXPHOS control. We hypothesized that cytosolic Ca(2+), but not mitochondrial matrix Ca(2+), may adapt OXPHOS to workload by adjusting the rate of pyruvate supply from the cytosol to the mitochondria. Here, we studied the role of malate aspartate shuttle (MAS)-dependent substrate supply on OXPHOS responses to changing Ca(2+) concentrations in isolated brain and heart mitochondria, synaptosomes, fibroblasts, and thymocytes from wild-type (WT) and MCU KO mice, and the isolated working rat heart. Our results indicate that extramitochondrial Ca(2+) controls up to 85% of maximal pyruvate-driven OXPHOS rates, mediated by the activity of the complete MAS, and that intramitochondrial Ca(2+) accounts for the remaining 15%. Of note, the complete MAS as applied here, included besides its classical NADH oxidation reaction the generation of cytosolic pyruvate. Part of this largely neglected mechanism has previously been described as the "mitochondrial gas pedal". Its implementation into OXPHOS control models integrates seemingly contradictory results and warrants a critical reappraisal of metabolic control mechanisms in health and disease.