RESUMO
Mammalian mitochondrial DNA (mtDNA) is replicated and transcribed by phage-like DNA and RNA polymerases, and our understanding of these processes has progressed substantially over the last several decades. Molecular mechanisms have been elucidated by biochemistry and structural biology and essential in vivo roles established by cell biology and mouse genetics. Single molecules of mtDNA are packaged by mitochondrial transcription factor A into mitochondrial nucleoids, and their level of compaction influences the initiation of both replication and transcription. Mutations affecting the molecular machineries replicating and transcribing mtDNA are important causes of human mitochondrial disease, reflecting the critical role of the genome in oxidative phosphorylation system biogenesis. Mechanisms controlling mtDNA replication and transcription still need to be clarified, and future research in this area is likely to open novel therapeutic possibilities for treating mitochondrial dysfunction.
Assuntos
Replicação do DNA , DNA Mitocondrial , Transcrição Gênica , Humanos , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Animais , Mitocôndrias/metabolismo , Mitocôndrias/genética , Doenças Mitocondriais/genética , Doenças Mitocondriais/metabolismo , Doenças Mitocondriais/patologia , Fatores de Transcrição/metabolismo , Fatores de Transcrição/genética , Mutação , Proteínas Mitocondriais/metabolismo , Proteínas Mitocondriais/genéticaRESUMO
Mitochondrial DNA is compacted into nucleoprotein complexes denoted mitochondrial nucleoids, the focus of this SnapShot.
Assuntos
Proteínas de Ligação a DNA/metabolismo , Mitocôndrias/metabolismo , Proteínas Mitocondriais/metabolismo , Animais , Proteínas de Ligação a DNA/genética , Humanos , Mitocôndrias/genética , Proteínas Mitocondriais/genéticaRESUMO
Mammalian mitochondrial DNA (mtDNA) encodes 13 proteins that are essential for the function of the oxidative phosphorylation system, which is composed of four respiratory-chain complexes and adenosine triphosphate (ATP) synthase. Remarkably, the maintenance and expression of mtDNA depend on the mitochondrial import of hundreds of nuclear-encoded proteins that control genome maintenance, replication, transcription, RNA maturation, and mitochondrial translation. The importance of this complex regulatory system is underscored by the identification of numerous mutations of nuclear genes that impair mtDNA maintenance and expression at different levels, causing human mitochondrial diseases with pleiotropic clinical manifestations. The basic scientific understanding of the mechanisms controlling mtDNA function has progressed considerably during the past few years, thanks to advances in biochemistry, genetics, and structural biology. The challenges for the future will be to understand how mtDNA maintenance and expression are regulated and to what extent direct intramitochondrial cross talk between different processes, such as transcription and translation, is important.
Assuntos
DNA Mitocondrial/genética , Mitocôndrias/genética , Proteínas Mitocondriais/genética , Biossíntese de Proteínas , Transcrição Gênica , Animais , Evolução Biológica , Núcleo Celular/genética , Núcleo Celular/metabolismo , Replicação do DNA , DNA Mitocondrial/metabolismo , Transporte de Elétrons/genética , Regulação da Expressão Gênica , Mamíferos , Mitocôndrias/metabolismo , Mitocôndrias/ultraestrutura , Proteínas Mitocondriais/metabolismo , Ribossomos Mitocondriais/química , Ribossomos Mitocondriais/metabolismo , Modelos Moleculares , Fosforilação Oxidativa , Transporte Proteico , Transdução de SinaisRESUMO
Although the mammalian mtDNA transcription machinery is simple and resembles bacteriophage systems, there are many reports that nuclear transcription regulators, as exemplified by MEF2D, MOF, PGC-1α, and hormone receptors, are imported into mammalian mitochondria and directly interact with the mtDNA transcription machinery. However, the supporting experimental evidence for this concept is open to alternate interpretations, and a main issue is the difficulty in distinguishing indirect regulation of mtDNA transcription, caused by altered nuclear gene expression, from direct intramitochondrial effects. We provide a critical discussion and experimental guidelines to stringently assess roles of intramitochondrial factors implicated in direct regulation of mammalian mtDNA transcription.
Assuntos
DNA Mitocondrial , Mitocôndrias , Animais , Mitocôndrias/genética , Mitocôndrias/metabolismo , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Núcleo Celular/genética , Núcleo Celular/metabolismo , Mamíferos/genética , Mamíferos/metabolismo , Fator 1 Nuclear Respiratório/metabolismoRESUMO
Oligoribonucleases are conserved enzymes that degrade short RNA molecules of up to 5 nt in length and are assumed to constitute the final stage of RNA turnover. Here we demonstrate that REXO2 is a specialized dinucleotide-degrading enzyme that shows no preference between RNA and DNA dinucleotide substrates. A heart- and skeletal-muscle-specific knockout mouse displays elevated dinucleotide levels and alterations in gene expression patterns indicative of aberrant dinucleotide-primed transcription initiation. We find that dinucleotides act as potent stimulators of mitochondrial transcription initiation in vitro. Our data demonstrate that increased levels of dinucleotides can be used to initiate transcription, leading to an increase in transcription levels from both mitochondrial promoters and other, nonspecific sequence elements in mitochondrial DNA. Efficient RNA turnover by REXO2 is thus required to maintain promoter specificity and proper regulation of transcription in mammalian mitochondria.
Assuntos
Proteínas 14-3-3/metabolismo , Biomarcadores Tumorais/metabolismo , Exorribonucleases/metabolismo , Mitocôndrias/enzimologia , Oligonucleotídeos/metabolismo , Regiões Promotoras Genéticas , Estabilidade de RNA , RNA Mitocondrial/metabolismo , Proteínas 14-3-3/deficiência , Proteínas 14-3-3/genética , Animais , Biomarcadores Tumorais/genética , Exorribonucleases/genética , Regulação da Expressão Gênica no Desenvolvimento , Regulação Enzimológica da Expressão Gênica , Humanos , Camundongos Endogâmicos C57BL , Camundongos Knockout , RNA Mitocondrial/genética , Células Sf9 , SpodopteraRESUMO
Altered expression of mitochondrial DNA (mtDNA) occurs in ageing and a range of human pathologies (for example, inborn errors of metabolism, neurodegeneration and cancer). Here we describe first-in-class specific inhibitors of mitochondrial transcription (IMTs) that target the human mitochondrial RNA polymerase (POLRMT), which is essential for biogenesis of the oxidative phosphorylation (OXPHOS) system1-6. The IMTs efficiently impair mtDNA transcription in a reconstituted recombinant system and cause a dose-dependent inhibition of mtDNA expression and OXPHOS in cell lines. To verify the cellular target, we performed exome sequencing of mutagenized cells and identified a cluster of amino acid substitutions in POLRMT that cause resistance to IMTs. We obtained a cryo-electron microscopy (cryo-EM) structure of POLRMT bound to an IMT, which further defined the allosteric binding site near the active centre cleft of POLRMT. The growth of cancer cells and the persistence of therapy-resistant cancer stem cells has previously been reported to depend on OXPHOS7-17, and we therefore investigated whether IMTs have anti-tumour effects. Four weeks of oral treatment with an IMT is well-tolerated in mice and does not cause OXPHOS dysfunction or toxicity in normal tissues, despite inducing a strong anti-tumour response in xenografts of human cancer cells. In summary, IMTs provide a potent and specific chemical biology tool to study the role of mtDNA expression in physiology and disease.
Assuntos
Mitocôndrias/efeitos dos fármacos , Mitocôndrias/metabolismo , Bibliotecas de Moléculas Pequenas/química , Bibliotecas de Moléculas Pequenas/farmacologia , Transcrição Gênica/efeitos dos fármacos , Animais , Proliferação de Células/efeitos dos fármacos , Microscopia Crioeletrônica , DNA Mitocondrial/efeitos dos fármacos , DNA Mitocondrial/genética , RNA Polimerases Dirigidas por DNA/metabolismo , Regulação para Baixo/efeitos dos fármacos , Estabilidade Enzimática/efeitos dos fármacos , Feminino , Regulação da Expressão Gênica/efeitos dos fármacos , Genes Mitocondriais/efeitos dos fármacos , Humanos , Masculino , Camundongos , Neoplasias/tratamento farmacológico , Neoplasias/patologia , Especificidade por Substrato/efeitos dos fármacos , Ensaios Antitumorais Modelo de XenoenxertoRESUMO
How mtDNA replication is terminated and the newly formed genomes are separated remain unknown. We here demonstrate that the mitochondrial isoform of topoisomerase 3α (Top3α) fulfills this function, acting independently of its nuclear role as a component of the Holliday junction-resolving BLM-Top3α-RMI1-RMI2 (BTR) complex. Our data indicate that mtDNA replication termination occurs via a hemicatenane formed at the origin of H-strand replication and that Top3α is essential for resolving this structure. Decatenation is a prerequisite for separation of the segregating unit of mtDNA, the nucleoid, within the mitochondrial network. The importance of this process is highlighted in a patient with mitochondrial disease caused by biallelic pathogenic variants in TOP3A, characterized by muscle-restricted mtDNA deletions and chronic progressive external ophthalmoplegia (CPEO) plus syndrome. Our work establishes Top3α as an essential component of the mtDNA replication machinery and as the first component of the mtDNA separation machinery.
Assuntos
Segregação de Cromossomos/genética , Replicação do DNA/genética , DNA Topoisomerases Tipo I/metabolismo , DNA Mitocondrial/biossíntese , Dinâmica Mitocondrial/genética , Linhagem Celular Tumoral , DNA Mitocondrial/genética , Células HeLa , Humanos , Mitocôndrias/genética , Doenças Mitocondriais/genética , Oftalmoplegia Externa Progressiva Crônica/genéticaRESUMO
In mammals, the leucine-rich pentatricopeptide repeat protein (LRPPRC) and the stem-loop interacting RNA-binding protein (SLIRP) form a complex in the mitochondrial matrix that is required throughout the life cycle of most mitochondrial mRNAs. Although pathogenic mutations in the LRPPRC and SLIRP genes cause devastating human mitochondrial diseases, the in vivo function of the corresponding proteins is incompletely understood. We show here that loss of SLIRP in mice causes a decrease of complex I levels whereas other OXPHOS complexes are unaffected. We generated knock-in mice to study the in vivo interdependency of SLIRP and LRPPRC by mutating specific amino acids necessary for protein complex formation. When protein complex formation is disrupted, LRPPRC is partially degraded and SLIRP disappears. Livers from Lrpprc knock-in mice had impaired mitochondrial translation except for a marked increase in the synthesis of ATP8. Furthermore, the introduction of a heteroplasmic pathogenic mtDNA mutation (m.C5024T of the tRNAAla gene) into Slirp knockout mice causes an additive effect on mitochondrial translation leading to embryonic lethality and reduced growth of mouse embryonic fibroblasts. To summarize, we report that the LRPPRC/SLIRP protein complex is critical for maintaining normal complex I levels and that it also coordinates mitochondrial translation in a tissue-specific manner.
RESUMO
Mammalian mitochondrial DNA (mtDNA) is inherited uniparentally through the female germline without undergoing recombination. This poses a major problem as deleterious mtDNA mutations must be eliminated to avoid a mutational meltdown over generations. At least two mechanisms that can decrease the mutation load during maternal transmission are operational: a stochastic bottleneck for mtDNA transmission from mother to child, and a directed purifying selection against transmission of deleterious mtDNA mutations. However, the molecular mechanisms controlling these processes remain unknown. In this study, we systematically tested whether decreased autophagy contributes to purifying selection by crossing the C5024T mouse model harbouring a single pathogenic heteroplasmic mutation in the tRNAAla gene of the mtDNA with different autophagy-deficient mouse models, including knockouts of Parkin, Bcl2l13, Ulk1, and Ulk2. Our study reveals a statistically robust effect of knockout of Bcl2l13 on the selection process, and weaker evidence for the effect of Ulk1 and potentially Ulk2, while no statistically significant impact is seen for knockout of Parkin. This points at distinctive roles of these players in germline purifying selection. Overall, our approach provides a framework for investigating the roles of other important factors involved in the enigmatic process of purifying selection and guides further investigations for the role of BCL2L13 in the elimination of non-synonymous mutations in protein-coding genes.
Assuntos
DNA Mitocondrial , Transmissão Vertical de Doenças Infecciosas , Animais , Camundongos , Feminino , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Mitocôndrias/genética , Células Germinativas/metabolismo , Mutação , Autofagia/genética , Mamíferos/genética , Proteínas Proto-Oncogênicas c-bcl-2/genética , Proteínas Proto-Oncogênicas c-bcl-2/metabolismoRESUMO
Mitochondrial dysfunction is heavily implicated in the multifactorial aging process. Aging humans have increased levels of somatic mtDNA mutations that tend to undergo clonal expansion to cause mosaic respiratory chain deficiency in various tissues, such as heart, brain, skeletal muscle, and gut. Genetic mouse models have shown that somatic mtDNA mutations and cell type-specific respiratory chain dysfunction can cause a variety of phenotypes associated with aging and age-related disease. There is thus strong observational and experimental evidence to implicate somatic mtDNA mutations and mosaic respiratory chain dysfunction in the mammalian aging process. The hypothesis that somatic mtDNA mutations are generated by oxidative damage has not been conclusively proven. Emerging data instead suggest that the inherent error rate of mitochondrial DNA (mtDNA) polymerase gamma (Pol gamma) may be responsible for the majority of somatic mtDNA mutations. The roles for mtDNA damage and replication errors in aging need to be further experimentally addressed.
Assuntos
Envelhecimento/fisiologia , DNA Mitocondrial/genética , Mitocôndrias/genética , Animais , Transporte de Elétrons , Humanos , Mitocôndrias/fisiologia , MutaçãoRESUMO
Mitochondrial DNA (mtDNA) maintenance disorders are caused by mutations in ubiquitously expressed nuclear genes and lead to syndromes with variable disease severity and tissue-specific phenotypes. Loss of function mutations in the gene encoding the mitochondrial genome and maintenance exonuclease 1 (MGME1) result in deletions and depletion of mtDNA leading to adult-onset multisystem mitochondrial disease in humans. To better understand the in vivo function of MGME1 and the associated disease pathophysiology, we characterized a Mgme1 mouse knockout model by extensive phenotyping of ageing knockout animals. We show that loss of MGME1 leads to de novo formation of linear deleted mtDNA fragments that are constantly made and degraded. These findings contradict previous proposal that MGME1 is essential for degradation of linear mtDNA fragments and instead support a model where MGME1 has a critical role in completion of mtDNA replication. We report that Mgme1 knockout mice develop a dramatic phenotype as they age and display progressive weight loss, cataract and retinopathy. Surprisingly, aged animals also develop kidney inflammation, glomerular changes and severe chronic progressive nephropathy, consistent with nephrotic syndrome. These findings link the faulty mtDNA synthesis to severe inflammatory disease and thus show that defective mtDNA replication can trigger an immune response that causes age-associated progressive pathology in the kidney.
Assuntos
Nefropatias , Doenças Mitocondriais , Animais , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Rim/metabolismo , Nefropatias/genética , Camundongos , Camundongos Knockout , Mitocôndrias/metabolismo , Doenças Mitocondriais/metabolismo , MutaçãoRESUMO
Cancer cells depend on mitochondria to sustain their increased metabolic need and mitochondria therefore constitute possible targets for cancer treatment. We recently developed small-molecule inhibitors of mitochondrial transcription (IMTs) that selectively impair mitochondrial gene expression. IMTs have potent antitumor properties in vitro and in vivo, without affecting normal tissues. Because therapy-induced resistance is a major constraint to successful cancer therapy, we investigated mechanisms conferring resistance to IMTs. We employed a CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats)-(CRISP-associated protein 9) whole-genome screen to determine pathways conferring resistance to acute IMT1 treatment. Loss of genes belonging to von Hippel-Lindau (VHL) and mammalian target of rapamycin complex 1 (mTORC1) pathways caused resistance to acute IMT1 treatment and the relevance of these pathways was confirmed by chemical modulation. We also generated cells resistant to chronic IMT treatment to understand responses to persistent mitochondrial gene expression impairment. We report that IMT1-acquired resistance occurs through a compensatory increase of mitochondrial DNA (mtDNA) expression and cellular metabolites. We found that mitochondrial transcription factor A (TFAM) downregulation and inhibition of mitochondrial translation impaired survival of resistant cells. The identified susceptibility and resistance mechanisms to IMTs may be relevant for different types of mitochondria-targeted therapies.
Assuntos
Sistemas CRISPR-Cas , DNA Mitocondrial , DNA Mitocondrial/genética , Regulação para Baixo , Edição de Genes , Mitocôndrias/genética , Mitocôndrias/metabolismo , Transcrição GênicaRESUMO
In this issue, Lee et al. (2009) present a crystal structure of the human mitochondrial DNA polymerase (POLgamma). The structure of this heterotrimeric enzyme lays a foundation for understanding how POLgamma mutations cause human mitochondrial disease and why some antiviral nucleoside analogs cause cellular toxicity.
Assuntos
Replicação do DNA , DNA Mitocondrial/metabolismo , DNA Polimerase Dirigida por DNA/química , DNA Polimerase gama , Humanos , Mitocôndrias/genética , Mitocôndrias/metabolismoRESUMO
The in vivo role for RNase H1 in mammalian mitochondria has been much debated. Loss of RNase H1 is embryonic lethal and to further study its role in mtDNA expression we characterized a conditional knockout of Rnaseh1 in mouse heart. We report that RNase H1 is essential for processing of RNA primers to allow site-specific initiation of mtDNA replication. Without RNase H1, the RNA:DNA hybrids at the replication origins are not processed and mtDNA replication is initiated at non-canonical sites and becomes impaired. Importantly, RNase H1 is also needed for replication completion and in its absence linear deleted mtDNA molecules extending between the two origins of mtDNA replication are formed accompanied by mtDNA depletion. The steady-state levels of mitochondrial transcripts follow the levels of mtDNA, and RNA processing is not altered in the absence of RNase H1. Finally, we report the first patient with a homozygous pathogenic mutation in the hybrid-binding domain of RNase H1 causing impaired mtDNA replication. In contrast to catalytically inactive variants of RNase H1, this mutant version has enhanced enzyme activity but shows impaired primer formation. This finding shows that the RNase H1 activity must be strictly controlled to allow proper regulation of mtDNA replication.
Assuntos
DNA Mitocondrial , Ribonuclease H , Camundongos , Animais , DNA Mitocondrial/química , Ribonuclease H/genética , Ribonuclease H/metabolismo , RNA/química , Replicação do DNA/genética , Mitocôndrias/genética , Mamíferos/genéticaRESUMO
Dopamine (DA) neurons of the midbrain are at risk to become affected by mitochondrial damage over time and mitochondrial defects have been frequently reported in Parkinson's disease (PD) patients. However, the causal contribution of adult-onset mitochondrial dysfunction to PD remains uncertain. Here, we developed a mouse model lacking Mitofusin 2 (MFN2), a key regulator of mitochondrial network homeostasis, in adult midbrain DA neurons. The knockout mice develop severe and progressive DA neuron-specific mitochondrial dysfunction resulting in neurodegeneration and parkinsonism. To gain further insights into pathophysiological events, we performed transcriptomic analyses of isolated DA neurons and found that mitochondrial dysfunction triggers an early onset immune response, which precedes mitochondrial swelling, mtDNA depletion, respiratory chain deficiency and cell death. Our experiments show that the immune response is an early pathological event when mitochondrial dysfunction is induced in adult midbrain DA neurons and that neuronal death may be promoted non-cell autonomously by the cross-talk and activation of surrounding glial cells.
Assuntos
Neurônios Dopaminérgicos/metabolismo , Imunidade , Mesencéfalo/metabolismo , Mitocôndrias/metabolismo , Animais , DNA Mitocondrial/genética , Modelos Animais de Doenças , Homeostase , Camundongos , Transtornos Parkinsonianos/genéticaRESUMO
Deletions and duplications in mitochondrial DNA (mtDNA) cause mitochondrial disease and accumulate in conditions such as cancer and age-related disorders, but validated high-throughput methodology that can readily detect and discriminate between these two types of events is lacking. Here we establish a computational method, MitoSAlt, for accurate identification, quantification and visualization of mtDNA deletions and duplications from genomic sequencing data. Our method was tested on simulated sequencing reads and human patient samples with single deletions and duplications to verify its accuracy. Application to mouse models of mtDNA maintenance disease demonstrated the ability to detect deletions and duplications even at low levels of heteroplasmy.
Assuntos
DNA Mitocondrial/genética , Deleção de Genes , Duplicação Gênica , Sequenciamento de Nucleotídeos em Larga Escala/métodos , Análise de Sequência de DNA/métodos , Animais , DNA Mitocondrial/química , Sequenciamento de Nucleotídeos em Larga Escala/normas , Camundongos , Reprodutibilidade dos Testes , Análise de Sequência de DNA/normasRESUMO
Mitochondrial dynamics is an essential physiological process controlling mitochondrial content mixing and mobility to ensure proper function and localization of mitochondria at intracellular sites of high-energy demand. Intriguingly, for yet unknown reasons, severe impairment of mitochondrial fusion drastically affects mtDNA copy number. To decipher the link between mitochondrial dynamics and mtDNA maintenance, we studied mouse embryonic fibroblasts (MEFs) and mouse cardiomyocytes with disruption of mitochondrial fusion. Super-resolution microscopy revealed that loss of outer mitochondrial membrane (OMM) fusion, but not inner mitochondrial membrane (IMM) fusion, leads to nucleoid clustering. Remarkably, fluorescence in situ hybridization (FISH), bromouridine labeling in MEFs and assessment of mitochondrial transcription in tissue homogenates revealed that abolished OMM fusion does not affect transcription. Furthermore, the profound mtDNA depletion in mouse hearts lacking OMM fusion is not caused by defective integrity or increased mutagenesis of mtDNA, but instead we show that mitochondrial fusion is necessary to maintain the stoichiometry of the protein components of the mtDNA replisome. OMM fusion is necessary for proliferating MEFs to recover from mtDNA depletion and for the marked increase of mtDNA copy number during postnatal heart development. Our findings thus link OMM fusion to replication and distribution of mtDNA.
Assuntos
DNA Mitocondrial/genética , Mitocôndrias Cardíacas/genética , Dinâmica Mitocondrial/genética , Proteínas Mitocondriais/genética , Animais , Variações do Número de Cópias de DNA/genética , Replicação do DNA/genética , Fibroblastos , Humanos , Hibridização in Situ Fluorescente , Fusão de Membrana/genética , Camundongos , Mitocôndrias Cardíacas/metabolismo , Membranas Mitocondriais/metabolismo , Mutagênese , Miócitos Cardíacos/metabolismo , Transcrição GênicaRESUMO
Regulation of replication and expression of mitochondrial DNA (mtDNA) is essential for cellular energy conversion via oxidative phosphorylation. The mitochondrial transcription elongation factor (TEFM) has been proposed to regulate the switch between transcription termination for replication primer formation and processive, near genome-length transcription for mtDNA gene expression. Here, we report that Tefm is essential for mouse embryogenesis and that levels of promoter-distal mitochondrial transcripts are drastically reduced in conditional Tefm-knockout hearts. In contrast, the promoter-proximal transcripts are much increased in Tefm knockout mice, but they mostly terminate before the region where the switch from transcription to replication occurs, and consequently, de novo mtDNA replication is profoundly reduced. Unexpectedly, deep sequencing of RNA from Tefm knockouts revealed accumulation of unprocessed transcripts in addition to defective transcription elongation. Furthermore, a proximity-labeling (BioID) assay showed that TEFM interacts with multiple RNA processing factors. Our data demonstrate that TEFM acts as a general transcription elongation factor, necessary for both gene transcription and replication primer formation, and loss of TEFM affects RNA processing in mammalian mitochondria.
Assuntos
Mitocôndrias/genética , Mitocôndrias/metabolismo , Proteínas Mitocondriais/metabolismo , Processamento Pós-Transcricional do RNA , Elongação da Transcrição Genética , Fatores de Transcrição/metabolismo , Animais , DNA Mitocondrial , Desenvolvimento Embrionário/genética , Deleção de Genes , Regulação da Expressão Gênica , Loci Gênicos , Heterozigoto , Camundongos , Camundongos Knockout , Mitocôndrias/ultraestrutura , Fenótipo , Regiões Promotoras GenéticasRESUMO
Mammals develop age-associated clonal expansion of somatic mtDNA mutations resulting in severe respiratory chain deficiency in a subset of cells in a variety of tissues. Both mathematical modeling based on descriptive data from humans and experimental data from mtDNA mutator mice suggest that the somatic mutations are formed early in life and then undergo mitotic segregation during adult life to reach very high levels in certain cells. To address whether mtDNA mutations have a universal effect on aging metazoans, we investigated their role in physiology and aging of fruit flies. To this end, we utilized genetically engineered flies expressing mutant versions of the catalytic subunit of mitochondrial DNA polymerase (DmPOLγA) as a means to introduce mtDNA mutations. We report here that lifespan and health in fruit flies are remarkably tolerant to mtDNA mutations. Our results show that the short lifespan and wide genetic bottleneck of fruit flies are limiting the extent of clonal expansion of mtDNA mutations both in individuals and between generations. However, an increase of mtDNA mutations to very high levels caused sensitivity to mechanical and starvation stress, intestinal stem cell dysfunction, and reduced lifespan under standard conditions. In addition, the effects of dietary restriction, widely considered beneficial for organismal health, were attenuated in flies with very high levels of mtDNA mutations.
Assuntos
DNA Mitocondrial , Longevidade/genética , Mutação , Animais , DNA Mitocondrial/genética , DNA Mitocondrial/metabolismo , Drosophila melanogasterRESUMO
Mitochondrial DNA (mtDNA) mutations become more prevalent with age and are postulated to contribute to the ageing process. Point mutations of mtDNA have been suggested to originate from two main sources, i.e. replicative errors and oxidative damage, but the contribution of each of these processes is much discussed. To elucidate the origin of mtDNA mutations, we measured point mutation load in mice with deficient mitochondrial base-excision repair (BER) caused by knockout alleles preventing mitochondrial import of the DNA repair glycosylases OGG1 and MUTYH (Ogg1 dMTS, Mutyh dMTS). Surprisingly, we detected no increase in the mtDNA mutation load in old Ogg1 dMTS mice. As DNA repair is especially important in the germ line, we bred the BER deficient mice for five consecutive generations but found no increase in the mtDNA mutation load in these maternal lineages. To increase reactive oxygen species (ROS) levels and oxidative damage, we bred the Ogg1 dMTS mice with tissue specific Sod2 knockout mice. Although increased superoxide levels caused a plethora of changes in mitochondrial function, we did not detect any changes in the mutation load of mtDNA or mtRNA. Our results show that the importance of oxidative damage as a contributor of mtDNA mutations should be re-evaluated.