4.1. Oxidative Stress, mtDNA Mutagenesis, Apoptosis, and Respiration

2 −• and H 2 O 2 production increases with advancing age and is inversely correlated to lifespan in multiple mammalian species and flies [46,47, m ), and induction of cell-death pathways in post-mitotic tissues of both prematurely (progeroid) and physiologically aged animal models. For example, mtDNA polymerase gamma-deficient mice (PolG; ↑ mtDNA mutagenesis) exhibit an accelerated aging phenotype (shorter lifespan, muscle atrophy, cardiomyopathy, anemia, thin dermis, gray fur, and kyphosis), deficits in OXPHOS function and ATP synthesis, and increased ROS-induced damage to mitochondrial proteins and nucleic acids [ m in PolG mice is associated with the release of pro-apoptotic factors and induction of apoptosis, which likely contributes to organ dysfunction and muscle wasting in this model [51,2+ dysregulation, and/or loss of ΔΨ m may mediate mitochondrial outer membrane permeabilization and activation of intrinsic apoptotic pathways by opening of the mitochondrial permeability transition pore (mPTP) and the Bax/Bcl2-controlled mitochondrial apoptosis channel. ROS also contribute to telomere shortening and nuclear DNA instability (mainly in stem cells [54,58, Although direct evidence from human trials is lacking, mitochondrial Oand Hproduction increases with advancing age and is inversely correlated to lifespan in multiple mammalian species and flies [ 45 48 ]. Excessive mitochondrial ROS production (and/or reduced antioxidant capacity) is associated with oxidative damage, MRC dysfunction, loss of mitochondrial membrane potential (ΔΨ), and induction of cell-death pathways in post-mitotic tissues of both prematurely (progeroid) and physiologically aged animal models. For example, mtDNA polymerase gamma-deficient mice (PolG; ↑ mtDNA mutagenesis) exhibit an accelerated aging phenotype (shorter lifespan, muscle atrophy, cardiomyopathy, anemia, thin dermis, gray fur, and kyphosis), deficits in OXPHOS function and ATP synthesis, and increased ROS-induced damage to mitochondrial proteins and nucleic acids [ 49 ]. Consistent with observations made in old Fisher 344 Brown Norway rats [ 48 ], reduced ΔΨin PolG mice is associated with the release of pro-apoptotic factors and induction of apoptosis, which likely contributes to organ dysfunction and muscle wasting in this model [ 50 52 ]. As cogently summarized by others [ 53 54 ], ROS imbalance, Cadysregulation, and/or loss of ΔΨmay mediate mitochondrial outer membrane permeabilization and activation of intrinsic apoptotic pathways by opening of the mitochondrial permeability transition pore (mPTP) and the Bax/Bcl2-controlled mitochondrial apoptosis channel. ROS also contribute to telomere shortening and nuclear DNA instability (mainly in stem cells [ 55 ]), and genotoxic damage is a known activator of p53-mediated mPTP opening and apoptosis [ 56 ], which is the basis of the telomere-p53-mitochondrion model of aging [ 57 ]. In other words, several intrinsic (mitochondrial, ER, and lysosomal) and extrinsic (death receptor-induction by TNF-α and FasL) pathways may cooperate in myonuclear and satellite cell apoptosis, while mitochondria-driven cell death is believed to play the most important role in sarcopenia of aging [ 53 59 ].

60,61,62,70,76,77,78,79, c oxidase-negative (COX−) areas within skeletal muscle that contained atrophied and broken myofibers with high apoptotic susceptibility [76,83,84,85,86, vastus lateralis (VL) muscle fibers displayed a ‘ragged-blue phenotype’ (e.g., succinate dehydrogenase-hyperactive (SDH++) and COX−) in older humans (>90 years), and that >80% of the total mtDNA pool was mutated in affected fibers [ Biological aging in humans is characterized by a progressive accumulation of oxidative damage and mutations to the mitochondrial genome from the third decade of life onward in several post-mitotic tissues (for example, muscle, heart, and brain) [ 31 63 ]. Concurrent with (or as a result of) increased ROS-induced damage and/or mtDNA mutagenesis, aging mitochondria display morphological abnormalities [ 30 64 ], lower MRC and OXPHOS activities [ 65 66 ], and impaired ATP synthesis [ 67 68 ]. Age-associated mitochondrial dysfunction, as assessed in vivo or at the whole tissue level [ 69 ], is attributable to intrinsic mitochondrial deficiency and a reduction in organellar number [ 68 71 ]. Due to the close proximity of mtDNA to the source of ROS, lack of protection by histones, and limited capacity for DNA repair [ 72 73 ], mtDNA is more susceptible to oxidative damage than nuclear DNA (nDNA), resulting in a nearly 20-fold higher mutation rate [ 74 ], including deletions [ 75 80 ], tandem duplications [ 81 ], and single base modifications [ 82 ]. In a series of landmark publications by the groups of Aiken and Turnbull, it was shown that clonal expansion of mtDNA mutations were linked to energy-deficient, cytochromeoxidase-negative (COX) areas within skeletal muscle that contained atrophied and broken myofibers with high apoptotic susceptibility [ 75 87 ]. In one elegant study, Bua et al. found that a significant number of(VL) muscle fibers displayed a ‘ragged-blue phenotype’ (e.g., succinate dehydrogenase-hyperactive (SDH) and COX) in older humans (>90 years), and that >80% of the total mtDNA pool was mutated in affected fibers [ 76 ]. Other findings suggest that random deletions may be present in up to 70% of mtDNA molecules in VL muscle of ‘the oldest old’, primarily affecting MRC complexes that contain mtDNA-encoded subunits [ 88 ].