MNRR1 (CHCHD2) is a bi-organellar regulator of mitochondrial function that directly activates cytochrome c oxidase in the mitochondria and functions in the nucleus as a transcriptional activator for hundreds of genes. Since MNRR1 depletion contains features of a mitochondrial disease phenotype, we evaluated the effects of forced expression of MNRR1 on the mitochondrial disease MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) syndrome. MELAS is a multisystem encephalomyopathy disorder that can result from a heteroplasmic mutation in the mitochondrial DNA (mtDNA; m.3243A > G) at heteroplasmy levels of ∼50 to 90%. Since cybrid cell lines with 73% m.3243A > G heteroplasmy (DW7) display a significant reduction in MNRR1 levels compared to the wild type (0% heteroplasmy) (CL9), we evaluated the effects of MNRR1 levels on mitochondrial functioning. Overexpression of MNRR1 in DW7 cells induces the mitochondrial unfolded protein response (UPRmt), autophagy, and mitochondrial biogenesis, thereby rescuing the mitochondrial phenotype. It does so primarily as a transcription activator, revealing this function to be a potential therapeutic target. The role of MNRR1 in stimulating UPRmt, which is blunted in MELAS cells, was surprising and further investigation uncovered that under conditions of stress the import of MNRR1 into the mitochondria was blocked, allowing the protein to accumulate in the nucleus to enhance its transcription function. In the mammalian system, ATF5, has been identified as a mediator of UPRmt MNRR1 knockout cells display an ∼40% reduction in the protein levels of ATF5, suggesting that MNRR1 plays an important role upstream of this known mediator of UPRmt.
Keywords: CHCHD2; cytochrome c oxidase; mitochondria; transcription; unfolded protein response.