Parkin is required for exercise-induced mitophagy in muscle: impact of aging.

The maintenance of muscle health with advancing age is dependent on mitochondrial homeostasis. While reductions in mitochondrial biogenesis have been observed with age, less is known regarding organelle degradation. Parkin is an E3 ubiquitin ligase implicated in mitophagy, but few studies have examined Parkin's contribution to mitochondrial turnover in muscle. Wild-type (WT) and Parkin knockout (KO) mice were used to delineate a role for Parkin-mediated mitochondrial degradation in aged muscle, in concurrence with exercise. Aged animals exhibited declines in muscle mass and mitochondrial content, paralleled by a nuclear environment endorsing the transcriptional repression of mitochondrial biogenesis. Mitophagic signaling was enhanced following acute endurance exercise in young WT mice but was abolished in the absence of Parkin. Basal mitophagy flux of the autophagosomal protein lipidated microtubule-associated protein 1A/1B-light chain 3 was augmented in aged animals but did not increase additionally with exercise when compared with young animals. In the absence of Parkin, exercise increased the nuclear localization of Parkin-interacting substrate, corresponding to a decrease in nuclear peroxisome proliferator gamma coactivator-1α. Remarkably, exercise enhanced mitochondrial ubiquitination in both young WT and KO animals. This suggested compensation of alternative ubiquitin ligases that were, however, unable to restore the diminished exercise-induced mitophagy in KO mice. Under basal conditions, we demonstrated that Parkin was required for mitochondrial mitofusin-2 ubiquitination. We also observed an abrogation of exercise-induced mitophagy in aged muscle. Our results demonstrate that acute exercise-induced mitophagy is dependent on Parkin and attenuated with age, which likely contributes to changes in mitochondrial content and quality in aging muscle.

The role of Nrf2 in skeletal muscle contractile and mitochondrial function.

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that confers cellular protection by upregulating antioxidant enzymes in response to oxidative stress. However, Nrf2 function within skeletal muscle remains to be further elucidated. We examined the role of Nrf2 in determining muscle phenotype using young (3 mo) and older (12 mo) Nrf2 wild-type (WT) and knockout (KO) mice. Basally, the absence of Nrf2 did not impact mitochondrial content. In intermyofibrillar mitochondria, lack of Nrf2 resulted in a 40% reduction in state 4 respiration, which coincided with a 68% increase in reactive oxygen species (ROS) emission. Nrf2 abrogation impaired in situ muscle performance, characterized by a 48% greater rate of fatigue and a 35% decrease in force within the first 5 min of stimulation. Acute treadmill exercise resulted in a 1.5-fold increase in Nrf2 activation via enhanced DNA binding in WT animals. In response to training, cytochrome-c oxidase activity increased by 20% in the WT animals; however, this response was attenuated in KO mice. Nrf2 protein was reduced 30% by training. Despite this, exercise training normalized respiration, ROS production, and muscle performance in KO mice. Our results suggest that Nrf2 transcriptional activity is increased by exercise and that Nrf2 is required for the maintenance of basal mitochondrial function as well as for the normal increase in specific mitochondrial proteins in response to training. Nonetheless, the decrements in mitochondrial function in Nrf2 KO muscle can be rescued by exercise training, suggesting that this restorative function operates via a pathway independent of Nrf2.

Function of specialized regulatory proteins and signaling pathways in exercise-induced muscle mitochondrial biogenesis.

Skeletal muscle mitochondrial content and function are regulated by a number of specialized molecular pathways that remain to be fully defined. Although a number of proteins have been identified to be important for the maintenance of mitochondria in quiescent muscle, the requirement for these appears to decrease with the activation of multiple overlapping signaling events that are triggered by exercise. This makes exercise a valuable therapeutic tool for the treatment of mitochondrially based metabolic disorders. In this review, we summarize some of the traditional and more recently appreciated pathways that are involved in mitochondrial biogenesis in muscle, particularly during exercise.

Exercise and the Regulation of Mitochondrial Turnover.

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Effect of denervation on the regulation of mitochondrial transcription factor A expression in skeletal muscle.

The purpose of this study was to determine how the expression of mitochondrial transcription factor A (Tfam), a protein that governs mitochondrial DNA (mtDNA) transcription and replication, is regulated during a state of reduced organelle content imposed by muscle disuse. We measured Tfam expression at 8 h, 16 h, 24 h, 3 days, or 7 days following denervation and hypothesized that decreases in Tfam expression would precede mitochondrial loss. Muscle mass was lowered by 13% and 38% at 3 and 7 days postdenervation, while cytochrome c oxidase activity fell by 33% and 39% at the same time points. Tfam promoter activation in vivo was reduced by 30-65% between 8 h and 3 days of denervation, while Tfam transcript half-life was increased following 8-24 h of denervation. Protein expression of RNA-binding proteins that promote mRNA degradation (CUG repeat-binding protein and K homology splicing regulator protein) was elevated at 3 and 7 days of denervation. Tfam localization within subsarcolemmal mitochondria was reduced after 3 and 7 days of denervation and was associated with suppression of the cytochrome c oxidase type I transcript at 3 days, indicating that denervation impairs both mitochondrial Tfam import and mtDNA transcription during an early period following denervation. These data suggest that putative signals downregulate Tfam transcription during the earliest stages following denervation but are counteracted by increases in Tfam mRNA stability. Import of Tfam into the mitochondrion seems to be the most critical point of regulation of this protein during the early onset of denervation, an impairment of which is coincident with the loss of mitochondria during muscle disuse.

Recent advances in mitochondrial turnover during chronic muscle disuse.

Chronic muscle disuse, such as that resulting from immobilization, denervation, or prolonged physical inactivity, produces atrophy and a loss of mitochondria, yet the molecular relationship between these events is not fully understood. In this review we attempt to identify the key regulatory steps mediating the loss of muscle mass and the decline in mitochondrial content and function. An understanding of common intracellular signaling pathways may provide much-needed insight into the possible therapeutic targets for treatments that will maintain aerobic energy metabolism and preserve muscle mass during disuse conditions.