Mitochondrial health is critical for skeletal muscle function and is improved by exercise training through both mitochondrial biogenesis and removal of damaged/dysfunctional mitochondria via mitophagy. The mechanisms underlying exercise-induced mitophagy have not been fully elucidated. Here, we show that acute treadmill running in mice causes mitochondrial oxidative stress at 3–12 h and mitophagy at 6 h post-exercise in skeletal muscle. These changes were monitored using a novel fluorescent reporter gene, pMitoTimer, that allows assessment of mitochondrial oxidative stress and mitophagy in vivo, and were preceded by increased phosphorylation of AMP activated protein kinase (Ampk) at tyrosine 172 and of unc-51 like autophagy activating kinase 1 (Ulk1) at serine 555. Using mice expressing dominant negative and constitutively active Ampk in skeletal muscle, we demonstrate that Ulk1 activation is dependent on Ampk. Furthermore, exercise-induced metabolic adaptation requires Ulk1. These findings provide direct evidence of exercise-induced mitophagy and demonstrate the importance of Ampk-Ulk1 signaling in skeletal muscle.

Our understanding of the molecular mechanisms underlying adaptations to resistance exercise remains elusive despite the significant biological and clinical relevance. We developed a novel voluntary mouse weightlifting model, which elicits squat‐like activities against adjustable load during feeding, to investigate the resistance exercise‐induced contractile and metabolic adaptations. RNAseq analysis revealed that a single bout of weightlifting induced significant transcriptome responses of genes that function in posttranslational modification, metabolism, and muscle differentiation in recruited skeletal muscles, which were confirmed by increased expression of fibroblast growth factor‐inducible 14 (Fn14), Down syndrome critical region 1 (Dscr1) and Nuclear receptor subfamily 4, group A, member 3 (Nr4a3) genes. Long‐term (8 weeks) voluntary weightlifting training resulted in significantly increases of muscle mass, protein synthesis (puromycin incorporation in SUnSET assay) and mTOR pathway protein expression (raptor, 4e‐bp‐1, and p70S6K proteins) along with enhanced muscle power (specific torque and contraction speed), but not endurance capacity, mitochondrial biogenesis, and fiber type transformation. Importantly, weightlifting training profound improved whole‐body glucose clearance and skeletal muscle insulin sensitivity along with enhanced autophagy (increased LC3 and LC3‐II/I ratio, and decreased p62/Sqstm1). These data suggest that resistance training in mice promotes muscle adaptation and insulin sensitivity with simultaneous enhancement of autophagy and mTOR pathway.

Deterioration of neuromuscular junction (NMJ) integrity and function is causal to muscle atrophy and frailty, ultimately hindering quality of life and increasing the risk of death. In particular, NMJ is vulnerable to ischemia reperfusion (IR) injury when blood flow is restricted followed by restoration. However, little is known about the underlying mechanism(s) and hence the lack of effective interventions. New evidence suggests that mitochondrial oxidative stress plays a causal role in IR injury, which can be precluded by enhancing mitochondrial protein S-nitrosation (SNO). To elucidate the role of IR and mitochondrial protein SNO in skeletal muscle, we utilized a clinically relevant model and showed that IR resulted in significant muscle and motor nerve injuries with evidence of elevated muscle creatine kinase in the serum, denervation at NMJ, myofiber degeneration and regeneration, as well as muscle atrophy. Interestingly, we observed that neuromuscular transmission improved prior to muscle recovery, suggesting the importance of the motor nerve in muscle functional recovery. Injection of a mitochondria-targeted S-nitrosation enhancing agent, MitoSNO, into ischemic muscle prior to reperfusion reduced mitochondrial oxidative stress in the motor nerve and NMJ, attenuated denervation at NMJ, and resulted in accelerated functional recovery of the muscle. These findings demonstrate that enhancing mitochondrial protein SNO protects against IR-induced denervation at NMJ in skeletal muscle and accelerates functional regeneration. This could be an efficacious intervention for protecting neuromuscular injury under the condition of IR and other related pathological conditions.

the common clinical symptoms of Friedreich's ataxia (FRDA) include ataxia, muscle weakness, type 2 diabetes and heart failure, which are caused by impaired mitochondrial function due to the loss of frataxin (FXN) expression. Endurance exercise is the most powerful intervention for promoting mitochondrial function; however, its impact on FRDA has not been studied. Here we found that mice with genetic knockout and knock-in of the Fxn gene (KIKO mice) developed exercise intolerance, glucose intolerance and moderate cardiac dysfunction at 6 months of age. These abnormalities were associated with impaired mitochondrial respiratory function concurrent with reduced iron regulatory protein 1 (Irp1) expression as well as increased oxidative stress, which were not due to loss of mitochondrial content and antioxidant enzyme expression. Importantly, long-term (4 months) voluntary running in KIKO mice starting at a young age (2 months) completely prevented the functional abnormalities along with restored Irp1 expression, improved mitochondrial function and reduced oxidative stress in skeletal muscle without restoring Fxn expression. We conclude that endurance exercise training prevents symptomatic onset of FRDA in mice associated with improved mitochondrial function and reduced oxidative stress. These preclinical findings may pave the way for clinical studies of the impact of endurance exercise in FRDA patients.Friedreich's ataxia (FRDA) is the most common autosomal recessive ataxia in the Caucasian population 1-4 with detrimental clinical symptoms, including ataxia, muscle weakness, type 2 diabetes and heart failure 5,6 . These symptoms usually first appear in childhood or adolescence and worsen over time. A hypertrophic cardiomyopathy is an important clinical trait, which contributes significantly to disability and early death 7,8 . A high percentage of FRDA patients have glucose intolerance or diabetes mellitus 4 , and exercise capacity is usually severely diminished 9 , leading to wheelchair binding within 10 to 20 years after the disease onset 10 .FRDA is caused by GAA repeat expansions in both alleles of the frataxin (FXN) gene within intron 1 located in chromosome 9 11,12 from normally around 5-30 to >90 6,12 . The GAA expansion mutations lead to reduced expression of frataxin, a mitochondrial protein, which is involved in assembly of iron-sulfur clusters (ISC) and/or function as an iron chaperone or an iron storage protein 13 . Although the precise function of frataxin is not entirely clear yet, substantial evidence shows that frataxin deficiency leads to inefficient use of iron during ISC synthesis, inhibition of ISC formation and/or increased production of reactive oxygen species (ROS) via the Fenton reaction, resulting in iron deposition, mitochondrial dysfunction and oxidative damage 14,15 . It is these abnormal functions in the metabolically active tissues/organs that eventually lead to the onset of clinical symptoms. Unfortunately, no pharmacologic treatment up to date has been proven to impede FRDA progression ...

Resistance exercise promotes skeletal muscle hypertrophy, prevents muscle atrophy and improves muscle function. However, despite the significant biological and clinical relevance we still have a poor understanding of the molecular and cellular mechanisms. A critical barrier is the lack of an appropriate animal model of resistance training. We have developed a novel weightlifting cage for mice to perform squat like movements against weight and subjected adult wild type mice (C57BL/6) to an acute bout of weightlifting (WL; n = 4) using sedentary mice as control (CON; n = 3). Following 3 days of acclimatization in the cage, mice in the WL group were allowed to perform weightlifting against 150% of their body weight overnight during the dark cycle (12 hrs). Immediately following the dark cycle, all the mice were sacrificed, and skeletal muscle, liver and heart tissues were harvested for RNA and protein analyses. Mice in the WL group pushed 419±83 times overnight. Semi‐quantitative RT‐PCR showed 3‐fold increase of tumor necrosis factor receptor superfamily, member 12 (Tnfrsf12a or Fn14) mRNA and 2‐fold increase of down syndrome critical region gene 1 (Dscr1) mRNA with no change in TNF‐related weak inducer of apoptosis (Tweak) mRNA in gastrocnemius muscle. Western blot analysis no significant changes in 5′ AMP‐activated protein kinase (AMPK), phospho‐AMPK (p‐AMPKThr172), protein kinase B (Akt), p‐AKTSer473 and cleaved Notch1 proteins. Therefore, we have developed a novel model of resistance exercise and confirmed that weightlifting results in increased Fn14 and Dscr1 mRNA in recruited skeletal muscle. This novel weightlifting cage may be used as a model of resistant exercise for future research.Support or Funding InformationChina Scholarship Council Fund (201506140108)