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Mitochondrial fragmentation drives selective removal of deleterious mtDNA in the germline.

ABSTRACT: Mitochondria contain their own genomes that, unlike nuclear genomes, are inherited only in the maternal line. Owing to a high mutation rate and low levels of recombination of mitrochondrial DNA (mtDNA), special selection mechanisms exist in the female germline to prevent the accumulation of deleterious mutations1-5. However, the molecular mechanisms that underpin selection are poorly understood6. Here we visualize germline selection in Drosophila using an allele-specific fluorescent in situ-hybridization approach to distinguish wild-type from mutant mtDNA. Selection first manifests in the early stages of Drosophila oogenesis, triggered by reduction of the pro-fusion protein Mitofusin. This leads to the physical separation of mitochondrial genomes into different mitochondrial fragments, which prevents the mixing of genomes and their products and thereby reduces complementation. Once fragmented, mitochondria that contain mutant genomes are less able to produce ATP, which marks them for selection through a process that requires the mitophagy proteins Atg1 and BNIP3. A reduction in Atg1 or BNIP3 decreases the amount of wild-type mtDNA, which suggests a link between mitochondrial turnover and mtDNA replication. Fragmentation is not only necessary for selection in germline tissues, but is also sufficient to induce selection in somatic tissues in which selection is normally absent. We postulate that there is a generalizable mechanism for selection against deleterious mtDNA mutations, which may enable the development of strategies for the treatment of mtDNA disorders.

SUBMITTER: Lieber T

PROVIDER: S-EPMC6614061 | biostudies-literature | 2019 Jun

REPOSITORIES: biostudies-literature

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Project description:Integrity of mitochondrial DNA (mtDNA), encoding several subunits of the respiratory chain, is essential to maintain mitochondrial fitness. Mitochondria, as a central hub for metabolism, are affected in a wide variety of human diseases but also during normal ageing, where mtDNA integrity is compromised. Mitochondrial quality control mechanisms work at different levels, and mitophagy and its variants are critical to remove dysfunctional mitochondria and mtDNA to maintain cellular homeostasis. Understanding the mechanisms governing a selective turnover of mutation-bearing mtDNA without affecting the entire mitochondrial pool is fundamental to design therapeutic strategies against mtDNA diseases and ageing. Here, we show that mtDNA damage after expressing a dominant negative version of the mitochondrial helicase Twinkle, or by chemical means, leads to an exacerbated mtDNA turnover. mtDNA removal depends on lysosomal function and requires the autophagy protein Atg5 but is independent of canonical mitophagy or autophagy. Using proximity labelling, we demonstrated that the area of influence of mitochondrial nucleoids differs upon mtDNA damage, which induces mitochondrial membrane remodelling and endosomal recruitment in close proximity to mitochondrial nucleoid sub compartments. Targeting of nucleoids is controlled by the mitochondrial transmembrane proteins ATAD3 and SAMM50, which together with the endosomal trafficking protein VPS35, orchestrate endosomal nucleoid engulfment. SAMM50 acts as a gatekeeper to avoid mtDNA release to the cytoplasm and facilitating mtDNA transfer to VPS35. Lastly, we show that stimulation of lysosomal activity by rapamycin selectively removes mtDNA deletions in vivo, without affecting mtDNA copy number. With these results, we unveil the molecular players of a new complex mechanism specifically targeting and removing mutant mtDNA which occurs outside the mitochondrial network, a process with multiple potential benefits to understand human mtDNA related diseases, either inherited, acquired or due to normal ageing.

Project description:BackgroundMitochondrial DNA (mtDNA) is present at high copy numbers in animal cells, and though characterized by a single haplotype in each individual due to maternal germline inheritance, deleterious mutations and intact mtDNA molecules frequently co-exist (heteroplasmy). A number of factors, such as replicative segregation, mitochondrial bottlenecks, and selection, may modulate the exitance of heteroplasmic mutations. Since such mutations may have pathological consequences, they likely survive and are inherited due to functional complementation via the intracellular mitochondrial network. Here, we hypothesized that compromised mitochondrial fusion would hamper such complementation, thereby affecting heteroplasmy inheritance.ResultsWe assessed heteroplasmy levels in three Caenorhabditis elegans strains carrying different heteroplasmic mtDNA deletions (ΔmtDNA) in the background of mutant mitofusin (fzo-1). Animals displayed severe embryonic lethality and developmental delay. Strikingly, observed phenotypes were relieved during subsequent generations in association with complete loss of ΔmtDNA molecules. Moreover, deletion loss rates were negatively correlated with the size of mtDNA deletions, suggesting that mitochondrial fusion is essential and sensitive to the nature of the heteroplasmic mtDNA mutations. Introducing the ΔmtDNA into a fzo-1;pdr-1;+/ΔmtDNA (PARKIN ortholog) double mutant resulted in a skewed Mendelian progeny distribution, in contrast to the normal distribution in the fzo-1;+/ΔmtDNA mutant, and severely reduced brood size. Notably, the ΔmtDNA was lost across generations in association with improved phenotypes.ConclusionsTaken together, our findings show that when mitochondrial fusion is compromised, deleterious heteroplasmic mutations cannot evade natural selection while inherited through generations. Moreover, our findings underline the importance of cross-talk between mitochondrial fusion and mitophagy in modulating the inheritance of mtDNA heteroplasmy.

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Mitochondrial fragmentation drives selective removal of deleterious mtDNA in the germline.

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