Figure 3.2 Hypothesized mechanisms leading to mitochondrial dysfunction, decline in muscle performance, and mobility impairment.
CAUSES OF THE DECLINE OF MITOCHONDRIAL MASS AND OXIDATIVE CAPACITY IN AGING SKELETAL MUSCLE
In spite of some inconsistencies in the literature, there is enough evidence to conclude that mitochondrial volume and function in skeletal muscle decline with aging. However, there is substantial disagreement on possible causal mechanisms that lead to such decline. Here, we review briefly the main theories and mention some of the supportive evidence.
It is important to recognize that most of the available evidence is based on studies conducted on animal models or human studies with a cross‐sectional design, and substantial work still needs to be conducted to better identify the molecular changes that are associated with the decline of muscle mitochondrial health with aging.
Decline of physical activity with aging
It is generally acknowledged that a substantial portion of the decline of mitochondrial function with aging is attributable to the decline of physical activity, observed even in very healthy aging individuals [32]. The level of physical activity is by far the strongest predictor of muscle function with aging and has been linked with the preservation of mitochondrial function. Indeed, the discovery proteomic studies conducted on muscle biopsies from healthy individuals have found a strong association between the levels of self‐reported physical activity and increased representation of all mitochondrial proteins, both structural and functional [33]. A reduced physical activity, however, does not exhaustively explain the decline of mitochondrial function, since skeletal muscle oxidative capacity declines even after the effect of age is estimated after adjusting for physical activity [23, 26]. Although observational studies assessing changes of mitochondrial function with aging in individuals who maintain a high level of physical activity are lacking, randomized clinical trials have shown that regular physical activity and resistance exercise prevent age‐related sarcopenia [34–36]. The beneficial effect of physical activity on mitochondrial function is mostly mediated by mitochondrial biogenesis, driven by the upregulation of the peroxisome proliferator‐activated receptor γ coactivator‐1 α (PGC‐1α), [37, 38]. PGC‐1α modulates the biological activity of several transcription factors including the nuclear respiratory factors (NRF1 and NRF2) and mitochondrial transcription factors (TFAM and TF2B) [39], and its concentration in skeletal muscle has been associated with the degree of oxidative capacity and shown to decline with aging [29]. PGC‐1α also inhibits the Forkhead box O3a (FoxO3a) and nuclear factor κB (NF‐κB), both of which enhance muscle catabolism [40, 41]. In mice, the overexpression of PGC‐1α levels in skeletal muscle prevents age‐dependent sarcopenia [42]. Thus, increasing mitochondrial biogenesis or enhanced quality control processes by exercise or exercise mimetics is considered one of the possible strategies for sarcopenia prevention.
Oxidative stress
The generation of ATP from ADP in mitochondria requires a perfectly tuned and coordinated activity of metabolic mechanisms that process different substrates to ultimately produce highly charged spared electrons, which are transported through a respiratory chain composed of five proteins complexes. The energy released by these electrons drives the accumulation of hydrogen ions (H+) against a concentration gradient into the intermembrane space. H+ returns to the mitochondrial matrix through the fifth complex (ATP‐synthase), and releases the energy accumulated to produce ATP and, if oxygen is available, water. Even in ideal conditions, the electron transport system (ETS) is imperfect and some leakage of highly charged electrons occurs at complex I and III and through a partial reduction of oxygen to form superoxide (•O−2) that is quickly transformed into hydrogen peroxide (H2O2) by two dismutases, superoxide dismutase 2 (SOD2) in the mitochondrial matrix and superoxide dismutase 1 (SOD1). Small amounts of reactive oxygen species (ROS) are now considered essential signaling molecules and beneficial to mitochondrial health, probably through the modulation of mitophagy (explained later in the text) [43]. However, conditions that impair and slow down the normal flux of electrons through the ETC facilitate the production of ROS above the physiologic quantity and this excess of ROS causes substantial damage to cell membranes, organelles, DNA, and proteins, leading to mitochondrial damage, in an hypothetical vicious cycle where mitochondrial damage facilitates the production of ROS and increased ROS production further damages mitochondria integrity. There is substantial evidence that ROS production increases with aging and it is not adequately buffered by increased level of scavenging enzymes, such as manganese superoxide dismutase (MnSOD or mitochondria‐specific SOD2), catalase (CAT), and glutathione peroxidase (GPx). Consistently, aging is associated with increased oxidative damage to macromolecules, especially DNA and proteins [44]. However, a direct link between ROS production, oxidative damage to macromolecules, and mitochondrial dysfunction has not been definitively established.
An important emergent target for ROS‐induced mitochondrial damage are cardiolipins. Cardiolipins are glycerophospholipids specific to the mitochondria with a unique dimeric structure of two phosphatidic acid moieties connected to a glycerol backbone. Although the variety of phosphatidic acids found in cardiolipin is extremely large, cardiolipin containing 18‐carbon fatty alkyl chains with two unsaturated bonds (18:2), the most frequent form, has probably the highest affinity to the inner membrane proteins of mammalian mitochondria, and is important to the structural integrity of the inner mitochondrial membrane and the preservation of the proper shape of the mitochondrial cristae. This is consistent with findings that show how low circulating levels of lysophosphatidylcholine 18:2 are strongly associated with the risk of losing mobility [45–47]. In eukaryotes, cardiolipins are only synthesized in the mitochondrion, where they remain throughout the mitochondrion lifespan. Functionally, cardiolipin binds to the complexes of the ETC to stabilize their reciprocal positioning and regulate their activation. In particular, cardiolipins interact with NADH‐dependent Coenzyme Q oxidoreductase (complex I), succinate dehydrogenase (complex II), ubiquinol cytochrome oxidoreductase (complex III), cytochrome c oxidase (complex IV), and complex V [48]. Cardiolipins are also responsible for anchoring to the mitochondrial membranes two important enzymes: the creatine kinase, which produces PCr from Cr, and the nucleoside diphosphate kinase, which catalyzes the exchange of terminal phosphate between different nucleoside diphosphates (NDPs) and triphosphates (NTPs), such as ATP, in a reversible manner. Cardiolipins participate in the induction of apoptosis by interacting with cytochrome c [48], and are essential for mitochondrial fission and fusion [49]. Excessive, unopposed oxidative stress may determine peroxidation of the cardiolipin, whose structure and proximity to the ETC makes it particularly vulnerable to ROS, leading to structural and functional changes in mitochondria that strongly affect oxidative phosphorylation (OXPHOS) capacity. Of note, to recycle oxidized cardiolipins and newly resynthesize them may be energetically challenging in an environment where energy production is already scarce. The interaction between oxidized cardiolipin and cytochrome c could be responsible for the substantial upregulation of the apoptotic pathways that is often detected in sarcopenic muscle. Indeed, while the cardiolipin content of mitochondria has been shown to decrease with aging, changes in different species of oxidized cardiolipins have not been studied in depth because of the lack of quantitative, reliable, and sensitive measurement methods. Because of its potential therapeutic implications, the study of cardiolipins is an active area of research.
It is important to remember that oxidative stress may also affect structures outside the mitochondria. In particular, the less reactive hydrogen peroxide may cross the mitochondrial membrane and attack molecules in the cytoplasm, nucleus, and other organelles, particularly in the presence of heavy metals, such as iron or copper, which react with hydrogen peroxide to generate highly reactive hydroxyl radicals (Fenton reaction). In muscles, this may involve damage to the contractile apparatus, ion channels, and signaling receptors that may impair muscle function.
Anabolic