Mutations and deletions in mitochondrial DNA (mtDNA) lead to a number of human diseases characterized by neuromuscular degeneration. Accumulation of truncated mtDNA molecules (Δ‐mtDNA) lacking a specific 4977‐bp fragment, the common deletion, leads to three related mtDNA diseases : Pearson's syndrome; Kearns‐Sayre syndrome; and chronic progressive external ophthalmoplegia (CPEO). In addition, the proportion of Δ‐mtDNA present increases with age in a range of tissues. Consequently, there is considerable interest in the effects of the accumulation of Δ‐mtDNA on cell function. The 4977‐bp deletion affects genes encoding 7 polypeptide components of the mitochondrial respiratory chain, and 5 of the 22 tRNAs necessary for mitochondrial protein synthesis. To determine how the accumulation of Δ‐mtDNA affects oxidative phosphorylation we constructed a series of cybrids by fusing a human osteosarcoma cell line depleted of mtDNA (@gR⁰) with enucleated skin fibroblasts from a CPEO patient. The ensuing cybrids contained 0−86 %Δ‐mtDNA and all had volumes, protein contents, plasma‐membrane potentials and mitochondrial contents similar to those of the parental cell line. The bioenergetic consequences of accumulating Δ‐mtDNA were assessed by measuring the mitochondrial membrane potential, rate of ATP synthesis and ATP/ADP ratio. In cybrids containing less than 50−55 %Δ‐mtDNA, these bioenergetic functions were equivalent to those of cybrids with intact mtDNA. However, once the proportion of Δ‐mtDNA exceeded this threshold, the mitochondrial membrane potential, rate of ATP synthesis, and cellular ATP/ADP ratio decreased. These bioenergetic deficits will contribute to the cellular pathology associated with the accumulation of Δ‐mtDNA in the target tissues of patients with mtDNA diseases.

There is much interest in the capacity of resistance exercise to prevent the age-related loss of skeletal muscle mass and function, known as sarcopenia. This study investigates the molecular basis underlying the benefits of resistance exercise in aging C57BL/6J mice of both sexes. This study is the first to demonstrate that long-term (34 weeks) voluntary resistance wheel exercise (RWE) initiated at middle age, from 15 months, prevents sarcopenia in selected hindlimb muscles and causes hypertrophy in soleus, by 23 months of age in both male and female C57BL/6J mice. Compared with 23-month-old sedentary (SED) controls, RWE (0–6 g of resistance) increased intramuscular mitochondrial density and oxidative capacity (measured by citrate synthase and NADH-TR) and increased LC3II/I ratios (a marker of autophagy) in exercised mice of both sexes. RWE also reduced mRNA expression of Gadd45α (males only) and Runx1 (females only) but had no effect on other markers of denervation including Chrng, Chrnd, Musk, and Myog. RWE increased heart mass in all mice, with a more pronounced increase in females. Significant sex differences were also noted among SED mice, with Murf1 mRNA levels increasing in male, but decreasing in old female mice between 15 and 23 months. Overall, long-term RWE initiated from 15 month of age significantly improved some markers of the mitochondrial and autophagosomal pathways and prevented age-related muscle wasting.