Mitochondrial Respiratory Dysfunction Is Not Correlated With Mitochondrial Genotype in Premature Aging Mice

mtDNA mutator mice (Polg^mut/mut mice) have reinforced the mitochondrial theory of aging. These mice accumulate multiple mutations in mtDNA with age due to a homozygous proofreading-deficient mutation in mtDNA polymerase gamma (Polg), resulting in mitochondrial respiratory dysfunction and premature aging phenotypes. However, whether the accumulation of multiple mutations in Polg^mut/mut mice induces mitochondrial respiratory dysfunction remains unclear. Here, we determined the accurate mtDNA genotype, including the frequency of total mutations and the number of non-synonymous substitutions and pathogenic mutations, using next-generation sequencing in the progeny of all three genotypes obtained from the mating of heterozygous mtDNA mutator mice (Polg^+/mut mice) and examined their correlation with mitochondrial respiratory activity. Although Polg^+/mut mice showed equivalent mtDNA genotype to Polg^+/+ (wild-type) mice, the mitochondrial respiratory activity in the Polg^+/mut mice was mildly reduced. To further investigate the causal relationship between mtDNA genotype and mitochondrial respiratory activity, we experimentally varied the mtDNA genotype in Polg mice. However, mitochondrial respiratory activity was mildly reduced in Polg^+/mut mice and severely reduced in Polg^mut/mut mice, regardless of the mtDNA genotype. Moreover, by varying the mtDNA genotype, some Polg^+/+ mice showed mtDNA genotype equivalent to those of Polg^mut/mut mice, but mitochondrial respiratory activity in Polg^+/+ mice was normal. These results indicate that the mitochondrial respiratory dysfunction observed in mice with proofreading-deficient mutation in Polg is correlated with the nuclear genotype of Polg rather than the mtDNA genotype. Thus, the mitochondrial theory of aging in Polg^mut/mut mice needs further re-examination.