Heart-Specific and Conditional Deletion of the Immt Gene Reveals Its Role in Regulating Mitochondrial Structure and Total Heart Metabolism

Mitochondria comprise ~1/3rd of the volume of an adult ventricular cardiomyocyte. The gene <i>Immt</i> encodes the Mic60/Mitofilin protein that is hypothesized to organize the mitochondrial contact site and cristae organization system (MICOS) complex that generates mitochondrial cristae junctions between the inner and outer membranes. To investigate the function of the <i>Immt</i> gene in the mouse heart, we generated and characterized mice in which this gene was specifically deleted in the mouse heart using a loxP-targeted allele (<i>Immt^fl</i>^/<i>fl</i>) and either the constitutive heart-specific Myh6-Cre transgene or the conditional Myh6-MerCreMer transgene, each of which showed lethality in several weeks. Hearts from these mice showed progressive hypertrophic cardiomyopathy and failure with lost contractility and lung edema. At the ultrastructural level, hearts from these mice showed extreme abnormalities in mitochondrial architecture characterized by lost cristae junctions, stacking of the inner mitochondrial membranes, mitophagy and areas with complete absence of mitochondria. Analysis of mitochondria showed loss of the MICOS complex of proteins as well as loss of mitochondrial membrane potential (Δψ) and increased expression of mitophagy proteins and mitochondrial biogenesis transcription factors. Hearts from these mice also showed widespread cardiomyocyte necrosis and induction of the universal mitochondrial stress response at the mRNA level, as well as major alterations in cardiac metabolites, suggesting greater use of glucose, ketones and amino acids. We conclude that the <i>Immt</i> gene is required for cardiac mitochondrial structure and function, although the ensuing mitochondrial stress response provides molecular clues as to how the heart can compensate metabolically and maintain viability for weeks after mitochondria are absent or unfunctional.