OPA1 processing by YEM1L and OMA1 is dispensable for mitochondrial fusion and instead drives mitochondrial fragmentation, which is crucial for mitochondrial integrity and quality control.

Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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Mitochondrial morphology is shaped by fusion and division of their membranes. Here, we found that adult myocardial function depends on balanced mitochondrial fusion and fission, maintained by processing of the dynamin-like guanosine triphosphatase OPA1 by the mitochondrial peptidases YME1L and OMA1. Cardiac-specific ablation of Yme1l in mice activated OMA1 and accelerated OPA1 proteolysis, which triggered mitochondrial fragmentation and altered cardiac metabolism. This caused dilated cardiomyopathy and heart failure. Cardiac function and mitochondrial morphology were rescued by Oma1 deletion, which prevented OPA1 cleavage. Feeding mice a high-fat diet or ablating Yme1l in skeletal muscle restored cardiac metabolism and preserved heart function without suppressing mitochondrial fragmentation. Thus, unprocessed OPA1 is sufficient to maintain heart function, OMA1 is a critical regulator of cardiomyocyte survival, and mitochondrial morphology and cardiac metabolism are intimately linked.

The dynamic network of mitochondria fragments under stress allowing the segregation of damaged mitochondria and, in case of persistent damage, their selective removal by mitophagy. Mitochondrial fragmentation upon depolarisation of mitochondria is brought about by the degradation of central components of the mitochondrial fusion machinery. The OMA1 peptidase mediates the degradation of long isoforms of the dynamin-like GTPase OPA1 in the inner membrane. Here, we demonstrate that OMA1-mediated degradation of OPA1 is a general cellular stress response. OMA1 is constitutively active but displays strongly enhanced activity in response to various stress insults. We identify an amino terminal stress-sensor domain of OMA1, which is only present in homologues of higher eukaryotes and which modulates OMA1 proteolysis and activation. OMA1 activation is associated with its autocatalyic degradation, which initiates from both termini of OMA1 and results in complete OMA1 turnover. Autocatalytic proteolysis of OMA1 ensures the reversibility of the response and allows OPA1-mediated mitochondrial fusion to resume upon alleviation of stress. This differentiated stress response maintains the functional integrity of mitochondria and contributes to cell survival.

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A decline in mitochondrial activity has been associated with aging and is a hallmark of many neurological diseases. Surveillance mechanisms acting at the molecular, organellar, and cellular level monitor mitochondrial integrity and ensure the maintenance of mitochondrial proteostasis. Here we will review the central role of mitochondrial chaperones and proteases, the cytosolic ubiquitin-proteasome system, and the mitochondrial unfolded response in this interconnected quality control network, highlighting the dual function of some proteases in protein quality control within the organelle and for the regulation of mitochondrial fusion and mitophagy.