More than 1 in 5000 children are born with inherited mitochondrial diseases from which 30-40% are complex I (CI) defects that lead to cardiomyopathy, heart failure and death. A large proportion of chronic conditions (diabetes, coronary artery disease, heart failure, aging) develop mitochondrial CI defects that are key factors promoting disease progression and cardiac complications. Pathogenic mechanisms that link CI defect with heart disease include energy deficit, oxidative stress, and an increased [NADH]. Either inherited or acquired, mitochondrial CI defects have no cure. We propose a novel therapeutic target that preserves cardiac oxidative metabolism and function in mitochondrial cardiomyopathies. The objective of this project is to determine if the mitochondrial enzyme, nicotinamide nucleotide transhydrogenase (NNT), is necessary to sustain cardiac fatty acid (FA) β-oxidation for ATP production under conditions of reductive stress (increased NADH) caused by a mitochondrial CI defect. We hypothesize that NADH accumulation induced by a CI defect decreases cardiac oxidative metabolism thus decreasing FA oxidation and ATP generation, and inducing oxidative stress. An intact NNT provides an alternate route to consume the excessive NADH and form FADH2 to maintain FA oxidation and prevent oxidative stress and ATP depletion while preserving cardiac function. First specific aim is to determine if NNT is necessary to protect the heart function and structure in mitochondrial CI defect. The second specific aim is to delineate the role of NNT in sustaining complete FA β-oxidation and ATP production in CI defective mitochondria. The third specific aim is to decipher the contribution of NNT to normalize redox status and decrease oxidative stress in CI defective mitochondria. Our specific objective aligns with the goal of our research to cure mitochondrial cardiomyopathies. We will compare hearts, cardiomyocytes and mitochondria from mice with normal and absent (systemic and cardiac specific) CRISPR NNT knockout on the C57BL6N background. Rotenone-induced and human-like NDUFS4-deficient cardiac specific CI defects will be used. Expected outcomes will determine if NNT is a potential therapeutic target to preserve cardiac oxidative metabolism and function in mitochondrial cardiomyopathy.