and inflammation [54]. Ultimately, carbons that stem from the BCAA catabolism enter the tricarboxylic acid (TCA) cycle as either acetyl‐CoA or succinyl‐CoA, getting completely oxidized. In summary, impaired BCAA entry in skeletal myofibers appears to be associated with reduced muscle quality, impaired mitochondrial biogenesis, and decreased mitochondrial substrate delivery.
Accumulation of somatic mutations in mitochondrial and nuclear DNA
It has been speculated that the accumulation of somatic mutations in mtDNA over time contributes to mitochondrial dysfunction observed with aging in skeletal muscle [55]. Human mtDNA is a small circular double‐stranded DNA molecule of approximately 16.5 thousand base pairs that encodes 37 genes, including two ribosomal RNAs, 22 transfer RNAs, and 13 proteins that are subunits of respiratory chain complexes. It has been suggested that mtDNA is particularly susceptible to oxidative stress damage because of its proximity to the locus of ROS generation, but most recent theories suggest that these mutations are caused by errors in replication fidelity of the mtDNA polymerase, POLGγ [56]. Thus, damage progressively accumulates through the aging process but, as there are hundreds or thousands of mitochondria in a single myofiber, and multiple mtDNA in each mitochondrion, a low rate of random accumulations across the genome is probably compatible with normal functioning [55]. In contrast to this theory, it has been suggested that even small percentages of mutated mtDNA molecules (micro‐heteroplasmy) may have functional consequences [57]. The role of mtDNA somatic mutations in the genesis of age‐related decline of muscle strength remains controversial [58, 59]. The strongest evidence on the matter comes from a mouse model that contains a mutated proofreading‐deficient form of POLGγ, which leads to a severe and rapid accumulation of mtDNA mutations and early development of severe sarcopenia [60].
Both in the nucleus and in mitochondria, the integrity of DNA is continuously challenged by ROS and other damaging agents, such as radiation and chemical mutagens. Endogenous DNA damages occur frequently, estimated as more than 10 000 oxidative damages per day per nucleus, are much higher in mtDNA than in nuclear DNA, especially in high‐energy‐demanding tissues such as skeletal muscle. Different types of DNA damage are continuously repaired by a very complex and sophisticated repair machinery. However, if the mutational rate is higher than the repair capacity, damage accumulates over time and may determine genomic instability, with severe consequences for the cell’s fate. If the unrepaired damage in the coding and control regions of the DNA becomes pervasive, the chance of impairment of critical cell functions may become substantial [61]. Indeed, there is evidence that somatic mutations accumulate with aging and may contribute to important pathologies [62].
Accelerated DNA repair draws energy from the available pool and directly reduces the availability of ATP used by myocytes to produce mechanical force or for maintenance/repair of macromolecules and organelles. In addition, and perhaps more importantly, DNA repair and mitochondrial energetic metabolism compete for the use of nicotinamide adenine dinucleotide (NAD+) as an essential metabolite [63]. In the mitochondrion, NAD+ is a critical electron acceptor during the OXPHOS. Thus, a decrease of cellular NAD+ levels or NAD+/NADH ratio leads to a decrease in the rate of ATP production. In addition, NAD+ is also an essential co‐factor of Poly [ADP‐ribose] polymerase 1 (PARP‐1), a protein that plays a key role in the early phase of the cellular response to different forms of DNA damage. DNA repair consumes large quantities of NAD+, consumption that may overcome the rate of NAD+ synthesis, leading to NAD+ depletion and impaired OXPHOS. In addition, since NAD+ is a co‐factor for Sirtuin1, a protein that by de‐acetylating the PGC1‐alpha/ERR‐alpha complex modulates energetic metabolism, mitochondrial biogenesis, and mitophagy, scarcity of NAD+ may compromise these functions leading to further mitochondrial impairment. This hypothesis is consistent with studies that found a downregulation of mitochondrial biogenesis with aging in human cardiac muscle [64]. Finally, the activation of the NAD+ salvage synthesis pathway in response to the decline in intracellular NAD+ consumes large quantities of ATP, thus contributes to the energetic crisis. However, there is evidence showing that the efficiency of this pathway may be improved by physical activity [65]. Although the precise nature of these complex interactions has not been fully established, there is consensus that NAD+ deficiency is a potential cause of mitochondrial dysfunction and sarcopenia during aging. As the administration of NAD+ and/or NAD+ precursors or PARP inhibitors can increase NAD+ levels, these seem promising therapeutic approaches to prevent sarcopenia.
Fission, fusion, and mitochondrial recycling
The many different types of damage accumulated in mitochondria over time probably underlie the decline of mitochondrial mass and function observed with aging in skeletal muscle. Noteworthy, the rate of damage accumulation is blunted by several resilience mechanisms, but unfortunately the efficiency of this resilience declines with aging.
A primary mechanism aimed at maintaining mitochondrial integrity is the alternation of constant cycles of fission and fusion. Fission is the mechanism by which mitochondria divide and duplicate, while through fusion two or more smaller mitochondria form a unique larger structure. The conceptualization of fission and fusion is usually referred to mitochondria as single organelles, but how mitochondrial fission and fusion occur in skeletal muscle mitochondria that form a highly connected network is not understood. Thus, our knowledge of these mechanisms in humans is scant, although it is evident that the functionality of fission and fusion is essential to the preservation of mitochondrial health and energetic metabolism [66]. Mitochondria exert a tight quality control on protein integrity, they can repair misfolded proteins or eliminate them through chaperon‐mediated autophagy and proteasome proteolysis, without the need of activating other mechanisms [67]. However, when the severity of damage is overt, damaged mitochondria may fuse with other mitochondria in the same myofiber, share undamaged components and, as long as the mtDNA mutation load remains below a certain threshold, maintain a good level of function [68]. If mitochondrial functionality cannot be restored through these mechanisms, damaged and dysfunctional mitochondria can be eliminated though mitophagy (mitochondrial autophagy, see later). In particular, distressed mitochondria undergo fission and segregate most of the damaged components into small mitochondrial vesicles with partially depolarized membranes. These smaller vesicles can either be targeted for autophagy or fuse with other healthy mitochondria. However, if the level of damage is overt and above a certain threshold, both fission and fusion are inhibited to avoid the transfer of damaged material to healthy mitochondria. Under these conditions, the whole mitochondrial membrane would become depolarized and ATP production declines below a critical threshold, activating autophagy (mitophagy), a process that consists in the sequestration of a mitochondrion in an autophagic vacuole (autophagosome) and the hydrolytic degradation of its cargo by fusion with lysosomes (autolysosome) [69].
Profound alterations in mitochondrial fusion and fission proteins have been associated with many age‐related conditions, including obesity, neurodegenerative diseases, cardiovascular diseases, type 2 diabetes, and age‐related sarcopenia. Although most of these studies were conducted in animal models, they suggest that a dysregulation of mitochondrial dynamics may play an important role in age‐related sarcopenia and other age‐related chronic conditions in humans [70]. Supporting evidence for this theory comes from the fact that “in vivo;” genetic manipulations of proteins involved in fission accelerated muscle mass decline with aging in mice, while the inhibition of fission prevented sarcopenia. Moreover, overexpression of dynamin‐like 120‐kDa protein (OPA1), an essential protein for mitochondrial fusion, is protective against denervation‐induced muscle atrophy [71]. The expression of mitochondrial fusion and fission proteins is reduced during aging in human skeletal muscle [72, 73]. Reduced levels of Mitofusin‐2 (MFN‐2), an outer membrane mitochondrial protein essential for mitochondrial fusion, have been found in sarcopenic older individuals, as compared to age‐matched controls [74]. In addition, several lines of research suggest that autophagy becomes defective with aging, including some evidence regarding human skeletal muscle.
The persistence of severely damaged mitochondria jeopardizes attempts to replace them with new, healthy mitochondria. Stressed mitochondria also release non‐methylated
