Aging muscle exhibits a progressive decline in mass and strength, known as sarcopenia, and a decrease in the adaptive response to contractile activity. The molecular mechanisms mediating this reduced plasticity have yet to be elucidated. The purposes of this study were 1) to determine whether denervation-induced muscle disuse would increase the expression of autophagy genes and 2) to examine whether selective autophagy pathways (mitophagy) are altered in aged animals. Denervation reduced muscle mass in young and aged animals by 24 and 16%, respectively. Moreover, young animals showed a 50% decrease in mitochondrial content following denervation, an adaptation that was not matched by aged animals. Basal autophagy protein expression was higher in aged animals, whereas young animals exhibited a greater induction of autophagy proteins following denervation. Localization of LC3II, Parkin, and p62 was significantly increased in the mitochondrial fraction of young and aged animals following denervation. Moreover, the unfolded protein response marker CHOP and the mitochondrial dynamics protein Fis1 were increased by 17-and 2.5-fold, respectively, in aged animals. Lipofuscin granules within lysosomes were evident with aging and denervation. Thus reductions in the adaptive plasticity of aged muscle are associated with decreases in disuse-induced autophagy. These data indicate that the expression of autophagy proteins and their localization to mitochondria are not decreased in aged muscle; however, the induction of autophagy in response to disuse, along with downstream events such as lysosome function, is impaired. This may contribute to an accumulation of dysfunctional mitochondria in aged muscle. reactive oxygen species; muscle atrophy; mitochondria; mitophagy; apoptosis SKELETAL MUSCLE IS A REMARKABLY plastic tissue that undergoes a striking transformation in response to decreases in contractile activity. This distinctive response is attributable to the multinucleated composition of muscle fibers and the coordinated activation of several catabolic signaling pathways. Although muscle retains its adaptability throughout the life of an organism, tissue malleability is reduced with advancing age (4, 22). Aged muscle is further affected by an age-associated loss of skeletal muscle mass and strength, a condition known as sarcopenia (8,22,42). Although the precise cellular mechanisms responsible for mediating sarcopenia have yet to be fully elucidated, several studies have implicated decreases in mitochondrial function and a corresponding increase in mitochondrially mediated cell death (apoptosis) as factors contributing to this age-induced decline (4, 8). Indeed, mitochondrially mediated apoptosis can be activated by increases in reactive oxygen species (ROS), which have also been associated with several other deleterious effects, including the oxidation of mitochondrial DNA, lipids, and proteins (10, 39). Our laboratory has shown that mitochondria from aged muscle generate more ROS, possess a lower mitochondrial membrane pot...

Chronic muscle use increases the ratio of fusion:fission proteins, leading to reticular mitochondria, whereas muscle disuse and aging result in a decrease in this ratio, culminating in fragmented organelles.

Cardiolipin (CL) is a phospholipid that maintains the integrity of mitochondrial membranes. We previously demonstrated that CL content increases with chronic muscle use, and decreases with denervation-induced disuse. To investigate the underlying mechanisms, we measured the mRNA expression of 1) CL synthesis enzymes cardiolipin synthase (CLS) and CTP:PA-cytidylyltransferase-1 (CDS-1); 2) remodeling enzymes tafazzin and acyl-CoA:lysocardiolipin acyltransferase-1 (ALCAT1); and 3) outer membrane CL enzymes, mitochondrial phospholipase D and phospholipid scramblase 3 (Plscr3), during chronic contractile activity (CCA)-induced mitochondrial biogenesis and denervation. With CCA, CDS-1 expression increased by 128%, parelleling CL levels. Surprisingly, denervation also led to large increases in CDS-1 and CLS, despite a decrease in mitochondria, possibly due to a compensatory mechanism to restore lost CL. ALCAT1 decreased by 32% with CCA, but increased by 290% following denervation, indicating that both CCA and denervation alter CL remodeling. CCA and denervation also elicited 60-90% increases in Plscr3, likely to facilitate CL movement to the outer membrane. The expression of these genes was not affected by aging, but changes due to CCA and denervation were attenuated compared with young animals. The absence of PPARγ coactivator-1α in knockout animals led to a decrease in CDS-1 and an increase in ALCAT1 mRNA levels, implicating PGC-1α in regulating both CL synthesis and remodeling. These data suggest that chronic muscle use and disuse modify the expression of mRNAs encoding CL metabolism enzymes. Our data also illustrate, for the first time, that PPARγ coactivator-1α regulates the CL metabolism pathway in muscle.

Regular exercise can maintain skeletal muscle mass as we age. However, the molecular mechanisms that contribute to muscle health with exercise are incompletely defined. Recently, a group of three p53‐regulated proteins called Sestrins (Sesn1–3) have been identified. Whether these proteins are altered with age, or are induced by acute exercise in muscle is currently unknown. Thus, the purposes of our investigation were to determine: 1) if Sesns are expressed in the presence or absence of p53 after acute exercise, and 2) whether the expression of Sesns is altered in aging muscle. p53 wild‐type (WT) and knock‐out (KO) mice were run on a treadmill for 90mins and either sacrificed immediately or allowed to recover for 3hrs. Basally, muscle from p53 KO animals had 27% lower mRNA expression of Sesn2, while Sesn1 mRNA was increased by 2.3‐fold compared to WT animals. Acute exercise had no effect on Sesn2 expression, but resulted in a 1.9‐fold increase in Sesn1 which was evident in the recovery phase, but only in WT animals. To examine the effect of age, muscle from 6‐ and 36‐month old rats was analyzed. Aged animals displayed 71% and 26% reductions in Sesn1 and Sesn2 mRNA, respectively, compared to younger animals. Similar trends were observed at the protein level. Our results suggest that Sesn expression is reduced with age, and that Sesn1 can be induced by acute exercise in a p53‐dependent manner. Supported by NSERC.