Estimation of the mtDNA mutation rate in aging mice by proteome analysis and mathematical modeling

The accumulation of mitochondria containing mutated genomes was proposed to be an important factor involved in aging. Although the level of mutated mtDNA has shown to increase over time, it is currently not possible to directly measure the mtDNA mutation rate within living cells. The combination of mathematical modeling and controlled experiments is an alternative approach to obtain an estimate for the mutation rate in a well-defined system. In order to judge the relevance of mitochondrial mutations for the aging process, we used a mouse model to study age-related alterations of the mitochondrial proteins. Based on these experimental data we constructed a mathematical model of the mitochondrial population dynamics to estimate mtDNA mutation rates. Mitochondria were isolated from mouse brain and liver at six different ages (newborn to 24-months). A large-gel 2D-electrophoresis-based proteomics approach was used to analyze the mitochondrial proteins. The expression of two respiratory chain complex I subunits and one complex IV subunit decreased significantly with age. One subunit of complex III and one subunit of complex V increased in expression during aging. Together, these data indicate that complex I and IV deficiency in aged tissues might be accompanied by feedback regulation of other protein complexes in the respiratory chain. When we fitted our experimental data to the mathematical model, mtDNA mutation rate was estimated to be 2.7 x 10-8 per mtDNA per day for brain and 3.2 x 10-9 per mtDNA per day for liver. According to our model and in agreement with the mitochondrial theory of aging, mtDNA mutations could cause the detrimental changes seen in mitochondrial populations during the normal lifespan of mice, while at the same time ensure that the mitochondrial population remains functional during the developmental and reproductive period of mice.