Abstract Body: Introduction: During fetal development, the heart primarily relies on glycolysis due to the hypoxic environment. After birth, increased oxygen availability prompts a metabolic switch to mitochondrial oxidative phosphorylation (OXPHOS), and usage of fatty acids as substrates to meet the high energy demands of the maturing myocardium. This adaptation is essential for sustaining the increased workload and contractile function of the maturing heart. Nicotinamide adenine dinucleotide (NAD+) plays a pivotal role in this transition, by acting as a key cofactor for fatty acid oxidation, supplying acetyl-CoA to the tricarboxylic acid (TCA) cycle. Moreover, NAD+ is essential for TCA cycle enzymes, enabling the production of NADH for OXPHOS to sustain ATP production. Notably, the mitochondrial NAD+ (mtNAD+) pool constitutes up to 70% of the total cellular NAD+ in cardiomyocytes. Recently, SLC25A51 was identified as a primary mammalian mtNAD+ transporter. However, the specific role of mtNAD+ transport by SLC25A51 in the regulation of heart function remains unknown.
Hypothesis: SLC25A51 is critical for the postnatal changes in cardiac metabolism and function.
Methods and Results: We generated cardiac-specific SLC25A51 knockout (cKO) mice. cKO mice experienced retarded growth and premature death by 8 weeks of age. We observed pronounced eccentric cardiac hypertrophy with significantly slower heart rates in cKO mice compared to control. 6-weeks old cKO mice have depleted mtNAD+ levels by ~85% compared to controls. Interestingly, total cellular NAD+ levels were also diminished in cKO hearts. cKO hearts showed disrupted TCA cycle with no changes in glycolysis intermediates. Furthermore, NADH-dependent Complex I-driven respiration was impaired in cKO cardiac mitochondria. Transcriptomics analysis revealed upregulation of genes involved in Hif1a signaling, which promotes anaerobic metabolism, as well as upregulation of the fetal forms of contractile proteins (e.g. Myh7, Tnni1) and downregulation of adult forms (e.g. Myh6, Tnni3).
Conclusion: SLC25A51 is indispensable for the maintenance of mtNAD⁺ pool in the heart and is essential for mitochondrial and cardiac adaptations during postnatal maturation.