Mitochondrial ATP Synthase in Cardiac Biology and Disease

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The mitochondrial ATP synthase is a multi-subunit complex that catalyzes the synthesis of >90% of ATP in mammalian cells. The ATP synthase is also hypothesized to function as the mitochondrial permeability transition pore (mPTP), a major trigger for necrotic cell death. Except for short-term drug inhibitor experiments, the functions of the ATP synthase have never been assessed in the heart in vivo. We have created the first mouse models deficient in the entire ATP synthase complex in cardiomyocytes. To accomplish this, we individually deleted at 5 weeks of age ATP5L and ATP5J, ATP synthase subunits required for complex assembly. Thus far, we have analyzed the ATP5L KO mice. Because the half-lives of most mitochondrial ATP synthase subunits exceed 35 days in cardiomyocytes, the abundance of the complex decreased gradually with 15% remaining at 12 weeks post-deletion. KO mice uniformly developed heart failure (HF) with reduced systolic function and died between 12-16 weeks post-deletion. Analysis of cardiac mitochondria confirmed reduced ATP synthesis rates as expected. Unexpectedly, however, ATP concentrations in whole heart lysates, as well as in cytoplasmic and mitochondrial fractions, were elevated in KO, compared with control, mice. Parallel investigations into the role of the ATP synthase as the mPTP revealed that, rather than inhibiting Ca2+-induced mPTP opening, deficiency of the ATP synthase sensitized this event. Moreover, mice with cardiomyocyte-specific deficiency of the ATP synthase exhibited larger – not smaller – infarcts following myocardial ischemia/reperfusion in vivo. Finally, we observed that ATP synthase levels and activity in mitochondria decrease during pressure overload-induced HF in wild type mice. These results suggest: (a) Loss of the mitochondrial ATP synthase activates marked metabolic/energetic responses and unleashes previously unrecognized mechanisms that promote lethal HF. Regarding the latter, our preliminary studies implicate Complex II to I reverse electron transport (RET) promoting ROS-induced cardiomyocyte apoptosis. (b) Our studies cast doubt that the ATP synthase also functions as the mPTP and rather suggest that it is a negative regulator. (c) Deficient ATP synthase function may contribute to acquired forms of HF. We propose studies to understand the mechanistic basis of our observations and to assess the role deficient mitochondrial ATP synthase function in human HF. Aim 1. To define metabolic/energetic pathways that are activated and mechanisms that contribute to HF in mice with cardiomyocyte-specific deficiency of the mitochondrial ATP synthase. Aim 2. To test definitively whether the mitochondrial ATP synthase is the mPTP. Aim 3. To assess the role of deficient mitochondrial ATP synthase abundance/function in pressure overload-induced HF in mice and in human HF. These studies break new ground in investigating functions of the mitochondrial ATP synthase in cardiomyocytes in vivo. Deliverables include the assessment of RET as a novel HF mechanism, a definitive determination of the role of the ATP synthase as the mPTP, and a delineation of the role deficient ATP synthase function in human HF.