Mitochondrial DNA Mutations Induce Mitochondrial Dysfunction, Apoptosis and Sarcopenia in Skeletal Muscle of Mitochondrial DNA Mutator Mice

TLDR

Aging causes progressive loss of skeletal muscle (sarcopenia), and accumulating mtDNA mutations in muscle correlate with this loss, though a causal link has not been proven. The study sought to determine whether mtDNA mutations directly cause sarcopenia. Using a Pol γ proofreading‑deficient (D257A) mouse that generates high mtDNA mutation rates, the authors examined gene expression and biochemical changes in skeletal muscle. D257A mice exhibited profound downregulation of mitochondrial genes, marked loss of ETC complexes I, III, and IV, impaired respiration, reduced ATP and membrane potential, yet no rise in ROS, culminating in muscle apoptosis and sarcopenia, thereby establishing mtDNA mutations as a causal factor.

Abstract

Background Aging results in a progressive loss of skeletal muscle, a condition known as sarcopenia. Mitochondrial DNA (mtDNA) mutations accumulate with aging in skeletal muscle and correlate with muscle loss, although no causal relationship has been established. Methodology/Principal Findings We investigated the relationship between mtDNA mutations and sarcopenia at the gene expression and biochemical levels using a mouse model that expresses a proofreading-deficient version (D257A) of the mitochondrial DNA Polymerase γ, resulting in increased spontaneous mtDNA mutation rates. Gene expression profiling of D257A mice followed by Parametric Analysis of Gene Set Enrichment (PAGE) indicates that the D257A mutation is associated with a profound downregulation of gene sets associated with mitochondrial function. At the biochemical level, sarcopenia in D257A mice is associated with a marked reduction (35–50%) in the content of electron transport chain (ETC) complexes I, III and IV, all of which are partly encoded by mtDNA. D257A mice display impaired mitochondrial bioenergetics associated with compromised state-3 respiration, lower ATP content and a resulting decrease in mitochondrial membrane potential (Δψm). Surprisingly, mitochondrial dysfunction was not accompanied by an increase in mitochondrial reactive oxygen species (ROS) production or oxidative damage. Conclusions/Significance These findings demonstrate that mutations in mtDNA can be causal in sarcopenia by affecting the assembly of functional ETC complexes, the lack of which provokes a decrease in oxidative phosphorylation, without an increase in oxidative stress, and ultimately, skeletal muscle apoptosis and sarcopenia.