Project Summary Heart disease results in over 650,000 deaths per year in the United States, and nearly half of all adults are currently living with some form of cardiovascular disease. Currently there is no curative treatment option except for a heart transplant. Thus, there is significant need for better therapeutic interventions. Heart failure can take years to develop, but is ultimately characterized by a reduction in cardiac output. There is a considerable body of research demonstrating maladaptive metabolic reprogramming of cardiomyocytes at this stage, which contributes to heart dysfunction and adverse remodeling of the left ventricle. The hypothesis detailed in this application proposes that nicotinamide adenine dinucleotide (NAD+) deficiency in the mitochondria is sufficient to cause heart failure. Recent literature demonstrates tissue-level NAD+ decline during ischemic injury. NAD+ precursors have been found to be therapeutic for heart failure in multiple pre-clinical models. Preliminary data generated in our lab suggests that the NAD+ precursor nicotinamide riboside (NR) prevented heart dysfunction, protected mitochondrial function, and improved survival in a model of Friedreich’s Ataxia (FA), in which mice typically develop progressive hypertrophic cardiomyopathy. This result suggests that NAD+ deficiency may be involved in the pathogenesis of heart failure in FA. Alternatively, it provides preclinical evidence that NR is therapeutic in FA even without NAD+ deficiency, suggesting that boosting NAD+ levels in the heart tissue can be protective of heart function under mitochondrial stress. To address these points, this proposal first aims to determine whether low mitochondrial NAD+ levels are observed with loss of frataxin, and whether increasing mitochondrial NAD+ synthesis with NR ex vivo can directly support mitochondrial respiration (Aim 1). This proposal also aims to define the metabolic consequences of mitochondrial NAD+ deficiency in cardiomyocytes, to determine whether mitochondrial NAD+ loss demonstrates characteristic metabolism of the hypertrophic heart in vivo (Aim 2). Together, this set of experiments will demonstrate whether NAD+ decline is a metabolic consequence of frataxin loss and mitochondrial damage, and define the importance of mitochondrial NAD+ maintenance in supporting the normal metabolism of cardiomyocytes. Further, it will provide mechanistic understanding of the specific action of NAD+ precursors to protect against mitochondrial- dysfunction mediated heart failure, which will inform future therapeutic interventions in progressive heart disease.