Mitochondrial oxidative phosphorylation (OXPHOS) and cellular workload are tightly balanced by the key cellular regulator, calcium (Ca2+). Current models assume that cytosolic Ca2+ regulates workload and that mitochondrial Ca2+ uptake precedes activation of matrix dehydrogenases, thereby matching OXPHOS substrate supply to ATP demand. Surprisingly, knockout (KO) of the mitochondrial Ca2+ 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 Ca2+ cannot be the exclusive mechanism for OXPHOS control. We hypothesized that cytosolic Ca2+, but not mitochondrial matrix Ca2+, 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 in OXPHOS responses to changing Ca2+ concentrations in isolated brain and heart mitochondria, synaptosomes, fibroblasts, and thymocytes from WT and MCU KO mice and the isolated working rat heart. Our results indicate that extramitochondrial Ca2+ controls up to 85% of maximal pyruvate-driven OXPHOS rates, mediated by the activity of the complete MAS, and that intramitochondrial Ca2+ 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.

中文翻译：

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钙是钙的，但不是基质的钙，对于调节线粒体丙酮酸的供应至关重要。

线粒体氧化磷酸化（OXPHOS）和细胞工作量由关键的细胞调节剂钙（Ca2 +）紧密平衡。当前模型假设胞质Ca2 +调节工作量，线粒体Ca2 +吸收先于基质脱氢酶激活，从而使OXPHOS底物供应与ATP需求匹配。令人惊讶的是，小鼠中线粒体Ca2 +单向转运蛋白（MCU）的敲除（KO）仅导致最小的表型变化，并且不会改变OXPHOS。这意味着线粒体内Ca2 +激活线粒体脱氢酶不能成为OXPHOS控制的唯一机制。我们假设胞质Ca2 +而非线粒体基质Ca2 +可能通过调节从细胞质到线粒体的丙酮酸供应速率来使OXPHOS适应工作量。这里，我们研究了苹果酸-天冬氨酸穿梭（MAS）依赖性底物供应在OXPHOS响应中的作用，该OXPHOS响应来自WT和MCU KO小鼠以及分离的工作大鼠心脏的分离的脑和心脏线粒体，突触小体，成纤维细胞和胸腺细胞中Ca2 +浓度变化。我们的结果表明，线粒体外Ca2 +控制了丙酮酸驱动的最大OXPHOS速率的85％，这是由完整的MAS的活性介导的，线粒体内Ca2 +占了剩余的15％。值得注意的是，此处所用的完整MAS除了其经典的NADH氧化反应外，还包含胞溶丙酮酸的产生。这种被广泛忽略的机制的一部分先前已被描述为“线粒体油门踏板”。