Mitochondrial functional resilience after TFAM ablation in adult cardiomyocytes

The adult heart is a terminally differentiated tissue that depends on mitochondria for its energy supply. Respiratory chain energy supply deficits due to alterations in the mitochondrial genome (mtDNA) or in nuclear genome (nDNA)-encoded mtDNA regulators are associated with cardiac pathologies ranging from primary mitochondrial cardiomyopathies to heart failure. Mitochondrial transcription factor A (TFAM) is an nDNA-encoded regulator of mtDNA transcription, replication, and maintenance. Insufficiency of this protein in embryonic and postnatal cardiomyocytes causes cardiomyopathy and/or lethality, establishing TFAM as indispensable to the developing heart; its role in adult tissue has been inferred from these findings. Here, we provide evidence that challenges this long-standing paradigm using Tfam ablation in the adult heart. Unexpectedly, loss of Tfam in adult cardiomyocytes resulted in a prolonged period of functional resilience characterized by preserved mtDNA content, mitochondrial function, and cardiac function despite mitochondrial structural alterations and decreased transcript abundance. Remarkably, TFAM protein levels did not directly dictate mtDNA content in the adult heart, and mitochondrial translation was preserved with acute TFAM inactivation, suggesting a mechanism whereby respiratory chain assembly and function can be sustained, which we term functional resilience. Finally, long-term Tfam inactivation induced a coordinated downregulation of the core mtDNA transcription and replication machinery that ultimately resulted in mitochondrial dysfunction and cardiomyopathy. Taken together, adult-onset cardiomyocyte-specific Tfam inactivation reveals a striking resilience of the adult heart to acute insults to mtDNA regulatory mechanisms and provides insight into critical differences between the developing versus differentiated heart.