Decreased agonist-stimulated mitochondrial ATP production caused by a pathological reduction in endoplasmic reticulum calcium content in human complex I deficiency

Although a large number of mutations causing malfunction of complex I (NADH:ubiquinone oxidoreductase) of the OXPHOS system is now known, their cell biological consequences remain obscure. We previously showed that the bradykinin (Bk)-induced increase in mitochondrial [ATP] ([ATP]_M) is significantly reduced in primary skin fibroblasts from a patient with an isolated complex I deficiency. The present work addresses the mechanism(s) underlying this impaired response. Luminometry of fibroblasts from 6 healthy subjects and 14 genetically characterized patients expressing mitochondria targeted luciferase revealed that the Bk-induced increase in [ATP]_M was significantly, but to a variable degree, decreased in 10 patients. The same variation was observed for the increases in mitochondrial [Ca²⁺] ([Ca²⁺]_M), measured with mitochondria targeted aequorin, and cytosolic [Ca²⁺] ([Ca²⁺]_C), measured with fura-2, and for the Ca²⁺ content of the endoplasmic reticulum (ER), calculated from the increase in [Ca²⁺]_C evoked by thapsigargin, an inhibitor of the ER Ca²⁺ ATPase. Regression analysis revealed that the increase in [ATP]_M was directly proportional to the increases in [Ca²⁺]_C and [Ca²⁺]_M and to the ER Ca²⁺ content. Our findings provide evidence that a pathological reduction in ER Ca²⁺ content is the direct cause of the impaired Bk-induced increase in [ATP]_M in human complex I deficiency.