The conserved MICOS complex functions as a primary determinant of mitochondrial inner membrane structure. We address the organization and functional roles of MICOS and identify two independent MICOS subcomplexes: Mic27/Mic10/Mic12, whose assembly is dependent on respiratory complexes and the mitochondrial lipid cardiolipin, and Mic60/Mic19, which assembles independent of these factors. Our data suggest that MICOS subcomplexes independently localize to cristae junctions and are connected via Mic19, which functions to regulate subcomplex distribution, and thus, potentially also cristae junction copy number. MICOS subunits have non-redundant functions as the absence of both MICOS subcomplexes results in more severe morphological and respiratory growth defects than deletion of single MICOS subunits or subcomplexes. Mitochondrial defects resulting from MICOS loss are caused by misdistribution of respiratory complexes in the inner membrane. Together, our data are consistent with a model where MICOS, mitochondrial lipids and respiratory complexes coordinately build a functional and correctly shaped mitochondrial inner membrane.

Mitochondria form a dynamic network within the cell as a result of balanced fusion and fission. Despite the established role of mitofusins (MFN1 and MFN2) in mitochondrial fusion, only MFN2 has been associated with metabolic and neurodegenerative diseases, which suggests that MFN2 is needed to maintain mitochondrial energy metabolism. The molecular basis for the mitochondrial dysfunction encountered in the absence of MFN2 is not understood. Here we show that loss of MFN2 leads to impaired mitochondrial respiration and reduced ATP production, and that this defective oxidative phosphorylation process unexpectedly originates from a depletion of the mitochondrial coenzyme Q pool. Our study unravels an unexpected and novel role for MFN2 in maintenance of the terpenoid biosynthesis pathway, which is necessary for mitochondrial coenzyme Q biosynthesis. The reduced respiratory chain function in cells lacking MFN2 can be partially rescued by coenzyme Q10 supplementation, which suggests a possible therapeutic strategy for patients with diseases caused by mutations in the Mfn2 gene.

Mutations in mitochondrial DNA (mtDNA) accumulate at a higher rate than mutations in nuclear DNA, and although somatic mtDNA mutations are known to be involved in mammalian ageing, the role of germline mutations in this process is unclear: here germline-transmitted mtDNA mutations are shown to be associated with ageing and brain malformations, and maternally transmitted mtDNA mutations may thus influence both development and ageing. Mutations in mitochondrial DNA (mtDNA) accumulate at a higher rate than mutations in nuclear DNA. But whereas somatic mtDNA mutations are firmly associated with mammalian ageing, the extent to which such mutations contribute to ageing when inherited via the mother is not clear. Here it is shown that germline-transmitted mtDNA mutations can induce ageing phenotypes in offspring, aggravate ageing induced by somatic mitochondrial mutations and cause brain malformations in the presence of somatic mtDNA mutations. These findings suggest that inherited human mtDNA sequence variants may have a key influence on the rate of human ageing. Ageing is due to an accumulation of various types of damage1,2, and mitochondrial dysfunction has long been considered to be important in this process3,4,5,6,7,8. There is substantial sequence variation in mammalian mitochondrial DNA (mtDNA)9, and the high mutation rate is counteracted by different mechanisms that decrease maternal transmission of mutated mtDNA10,11,12,13. Despite these protective mechanisms14, it is becoming increasingly clear that low-level mtDNA heteroplasmy is quite common and often inherited in humans15,16. We designed a series of mouse mutants to investigate the extent to which inherited mtDNA mutations can contribute to ageing. Here we report that maternally transmitted mtDNA mutations can induce mild ageing phenotypes in mice with a wild-type nuclear genome. Furthermore, maternally transmitted mtDNA mutations lead to anticipation of reduced fertility in mice that are heterozygous for the mtDNA mutator allele (PolgAwt/mut) and aggravate premature ageing phenotypes in mtDNA mutator mice (PolgAmut/mut). Unexpectedly, a combination of maternally transmitted and somatic mtDNA mutations also leads to stochastic brain malformations. Our findings show that a pre-existing mutation load will not only allow somatic mutagenesis to create a critically high total mtDNA mutation load sooner but will also increase clonal expansion of mtDNA mutations17 to enhance the normally occurring mosaic respiratory chain deficiency in ageing tissues18,19. Our findings suggest that maternally transmitted mtDNA mutations may have a similar role in aggravating aspects of normal human ageing.

Dysfunction of the oxidative phosphorylation (OXPHOS) system is a major cause of human disease and the cellular consequences are highly complex. Here, we present comparative analyses of mitochondrial proteomes, cellular transcriptomes and targeted metabolomics of five knockout mouse strains deficient in essential factors required for mitochondrial DNA gene expression, leading to OXPHOS dysfunction. Moreover, we describe sequential protein changes during post-natal development and progressive OXPHOS dysfunction in time course analyses in control mice and a middle lifespan knockout, respectively. Very unexpectedly, we identify a new response pathway to OXPHOS dysfunction in which the intra-mitochondrial synthesis of coenzyme Q (ubiquinone, Q) and Q levels are profoundly decreased, pointing towards novel possibilities for therapy. Our extensive omics analyses provide a high-quality resource of altered gene expression patterns under severe OXPHOS deficiency comparing several mouse models, that will deepen our understanding, open avenues for research and provide an important reference for diagnosis and treatment.