Background: Cardiac substrate oxidation switches from predominantly glucose oxidation during embryogenesis toward fatty acid oxidation in the postnatal heart, which is mediated by mitophagic replacement of fetal by adult mitochondria. We sought to test the hypothesis that mitochondrial motility mediated by Ras Homolog Family Member T (RHOT) 1 and RHOT2 is required for metabolic transformation and transcriptional reprogramming in the context of increased workload in the postnatal heart.
Methods: We investigated mice with embryonic cardiomyocyte-selective deletion of RHOT1 and RHOT2 (cRHOT1/2-KO), which exhibit impaired mitochondrial motility. Differential gene expression analysis was performed by RNA sequencing using left ventricular tissue at 1-day and 1-week of age, which represent an early phase of postnatal cardiac adaptation. Contractile function was determined by transthoracic echocardiography. Abundance of proteins involved in autophagy and mitophagy was determined by immunoblotting. Mitochondrial morphology was assessed by electron micrographs; mitochondrial oxygen consumption was determined in isolated mitochondria.
Results: Deletion of RHOT isoforms had a relatively mild effect on expression of genes involved in oxidative phosphorylation and in cardiac substrate oxidation as evidenced by the comparisons of cRhot1/2-KO at 1 d and 1 week of age relative to age-matched controls. Principal component analysis and hierarchical clustering revealed that gene expression was similar between genotypes at the 1-day time point. Differences between genotypes became more apparent at 1-week of age. Contractile function was similar between genotypes at 1-week of age. Abundance of proteins involved in autophagy and mitophagy was unchanged in cRhot1/2-KO relative to controls. Mitochondrial volume density, size, and ADP-stimulated state III oxygen consumption with palmitoyl-carnitine and pyruvate each combined with malate as substrats were not different in cRhot1/2-KO relative to controls.
Conclusion: Mitochondrial motility mediated by RHOT proteins is dispensable for transcriptional reprogramming of metabolic and energetic genes and mitophagic replacement of fetal mitochondria in the early postnatal period.
