Autophagy and protein turnover signaling in slow-twitch muscle during exercise

Obesity-induced skeletal muscle remodeling: A comparative analysis of exercise training and ACE-inhibitory drug in male mice

Obesity over-activates the classical arm of the renin-angiotensin system (RAS), impairing skeletal muscle remodeling. We aimed to compare the effect of exercise training and enalapril, an angiotensin-converting enzyme inhibitor, on RAS modulation in the skeletal muscle of obese animals. Thus, we divided C57BL/6 mice into two groups: standard chow (SC) and high-fat (HF) diet for 16 weeks. At the eighth week, the HF-fed animals were divided into four subgroups-sedentary (HF), treated with enalapril (HF-E), exercise training protocol (HF-T), and combined interventions (HF-ET). After 8 weeks of treatment, we evaluated body mass and index (BMI), body composition, exercise capacity, muscle morphology, and skeletal muscle molecular markers. All interventions resulted in lower BMI and attenuation of overactivation in the classical arm, while favoring the B2R in the bradykinin receptors profile. This was associated with reduced apoptosis markers in obese skeletal muscles. The HF-T group showed an increase in muscle mass and expression of biosynthesis markers and a reduction in expression of degradation markers and muscle fiber atrophy due to obesity. These findings suggest that the combination intervention did not have a synergistic effect against obesity-induced muscle remodeling. Additionally, the use of enalapril impaired muscle's physiological adaptations to exercise training.

Comparison between low, moderate, and high intensity aerobic training with equalized loads on biomarkers and performance in rats

This study investigated the physiological and molecular responses of Wistar Hannover rats, submitted to three 5-week chronic training models, with similar training loads. Twenty-four Wistar Hanover rats were randomly divided into four groups: control (n = 6), low-intensity training (Z1; n = 6), moderate-intensity training (Z2; n = 6) and high-intensity training (Z3; n = 6). The three exercise groups performed a 5-week running training three times a week, with the same prescribed workload but the intensity and the volume were different between groups. An increase in maximal speed was observed after four weeks of training for the three groups that trained, with no difference between groups. Higher rest glycogen was also observed in the soleus muscle after training for the exercise groups compared to the control group. We also found that the Z2 group had a higher protein content of total and phosphorylated GSK3-β compared to the control group after five weeks of training. In conclusion, the present study shows that five weeks of treadmill training based on intensity zones 1, 2, and 3 improved performance and increased resting glycogen in the soleus muscle, therefore intensity modulation does not change the training program adaptation since the different program loads are equalized.

The autophagic response to exercise in peripheral blood mononuclear cells from young men is intensity-dependent and is altered by exposure to environmental heat

Autophagy is essential to maintaining cellular homeostasis in all eukaryotic cells and to tolerance of acute stressors such as starvation, heat, and recovery following exercise. Limited information exists regarding the exercise intensity-dependent autophagic response in humans, and it is unknown how environmental heat stress may modulate this response. Therefore, we evaluated autophagy and accompanying pathways of cellular stress (the heat shock response [HSR], apoptosis, and acute inflammation) in peripheral blood mononuclear cells (PBMCs) from 10 young men (mean [SD]; 22 [2] years) before, immediately after and up to 6h post-exercise recovery from 30 minutes of low-, moderate-, and high-intensity semi-recumbent cycling (40, 55 and 70% of maximal oxygen consumption (VO_2max), respectively)in a temperate environment (25°C) and at 70% of VO_2max in a hot environment (40°C). Changes in protein content were analyzed via Western blot. Each increase in exercise intensity was associated with elevations in mean body temperature. LC3-II increased following moderate-intensity exercise, with further increases following high-intensity exercise (p < 0.05). However, an increase in beclin-2 and ULK1, with a decrease in p62 was only observed after high-intensity exercise, which was paralleled by elevated TNF-α and cleaved-caspase-3, with the HSR peaking at 6h after exercise (p < 0.05). When exercise was performed in the heat, greater LC3-II and cleaved-caspase-3 accumulation was observed, however beclin-2 declined in recovery (p < 0.05). Therefore, our findings indicate that autophagy in PBMCs during exercise may be associated with greater heat strain exhibited during increasing exercise intensities, which is modulated by exposure to heat.

Targeting parvalbumin promotes M2 macrophage polarization and energy expenditure in mice

Exercise benefits M2 macrophage polarization, energy homeostasis and protects against obesity partially through exercise-induced circulating factors. Here, by unbiased quantitative proteomics on serum samples from sedentary and exercised mice, we identify parvalbumin as a circulating factor suppressed by exercise. Parvalbumin functions as a non-competitive CSF1R antagonist to inhibit M2 macrophage activation and energy expenditure in adipose tissue. More importantly, serum concentrations of parvalbumin positively correlate with obesity in mouse and human, while treating mice with a recombinant parvalbumin blocker prevents its interaction with CSF1R and promotes M2 macrophage polarization and ameliorates diet-induced obesity. Thus, although further studies are required to assess the significance of parvalbumin in mediating the effects of exercise, our results implicate parvalbumin as a potential therapeutic strategy against obesity in mice.

Mitochondrial E3 ubiquitin ligase 1 (MUL1) as a novel therapeutic target for diseases associated with mitochondrial dysfunction

Mitochondrial E3 ubiquitin ligase 1 (MUL1) is a mitochondrial outer membrane-anchored protein-containing transmembrane domain in its N- and C-terminal regions, where both are exposed to the cytosol. Interestingly the C-terminal region has a RING finger domain responsible for its E3 ligase activity, as ubiquitin or in SUMOylation, interacting with proteins related to mitochondrial fusion and fission, cell survival, and tumor suppressor process, such as Akt. Therefore, MUL1 is involved in various cellular processes, such as mitochondrial dynamics, inter-organelle communication, proliferation, mitophagy, immune response, inflammation and cell apoptosis. MUL1 is expressed at a higher basal level in the heart, immune system organs, and blood. Here, we discuss the role of MUL1 in mitochondrial dynamics and its function in various pathological models, both in vitro and in vivo. In this context, we describe the role of MUL1 in: (1) the inflammatory response, by regulating NF-κB activity; (2) cancer, by promoting cell death and regulating exonuclear function of proteins, such as p53; (3) neurological diseases, by maintaining communication with other organelles and interacting with proteins to eliminate damaged organelles and; (4) cardiovascular diseases, by maintaining mitochondrial fusion/fission homeostasis. In this review, we summarize the latest advances in the physiological and pathological functions of MUL1. We also describe the different substrates of MUL1, acting as a positive or negative regulator in various pathologies associated with mitochondrial dysfunction. In conclusion, MUL1 could be a potential key target for the development of therapies that focus on ensuring the functionality of the mitochondrial network and, furthermore, the quality control of intracellular components by synchronously modulating the activity of different cellular mechanisms involved in the aforementioned pathologies. This, in turn, will guide the development of targeted therapies.

Ergothioneine Improves Aerobic Performance Without Any Negative Effect on Early Muscle Recovery Signaling in Response to Acute Exercise

Physical activity is now recognized as an essential element of healthy lifestyles. However, intensive and repeated exercise practice produces a high level of stress that must be managed, particularly oxidative damage and inflammation. Many studies investigated the effect of antioxidants, but reported only few positive effects, or even muscle recovery impairment. Secondary antioxidants are frequently highlighted as a way to optimize these interactions. Ergothioneine is a potential nutritional supplement and a secondary antioxidant that activates the cellular NRF2 pathway, leading to antioxidant response gene activation. Here, we hypothesized that ergothioneine could improve performance during aerobic exercise up to exhaustion and reduce exercise-related stress without impairing early muscle recovery signaling. To test this hypothesis, 5-month-old C56B6J female mice were divided in two groups matched for maximal aerobic speed (MAS): control group (Ctrl; n = 9) and group supplemented with 70 mg ergothioneine/kg/day (ET; n = 9). After 1 week of supplementation (or not), mice performed a maximum time-to-exhaustion test by running on a treadmill at 70% of their MAS, and gastrocnemius and soleus muscles were collected 2 h after exercise. Time to exhaustion was longer in the ET than Ctrl group (+41.22%, p < 0.01). Two hours after exercise, the ET group showed higher activation of protein synthesis and satellite cells, despite their longer effort. Conversely, expression in muscles of metabolic stress and inflammation markers was decreased, as well as oxidative damage markers in the ET group. Moreover, ergothioneine did not seem to impair mitochondrial recovery. These results suggest an important effect of ergothioneine on time-to-exhaustion performance and improved muscle recovery after exercise.

Exercise and Training Regulation of Autophagy Markers in Human and Rat Skeletal Muscle

Autophagy is a key intracellular mechanism by which cells degrade old or dysfunctional proteins and organelles. In skeletal muscle, evidence suggests that exercise increases autophagosome content and autophagy flux. However, the exercise-induced response seems to differ between rodents and humans, and little is known about how different exercise prescription parameters may affect these results. The present study utilised skeletal muscle samples obtained from four different experimental studies using rats and humans. Here, we show that, following exercise, in the soleus muscle of Wistar rats, there is an increase in LC3B-I protein levels immediately after exercise (+109%), and a subsequent increase in LC3B-II protein levels 3 h into the recovery (+97%), despite no change in Map1lc3b mRNA levels. Conversely, in human skeletal muscle, there is an immediate exercise-induced decrease in LC3B-II protein levels (−24%), independent of whether exercise is performed below or above the maximal lactate steady state, which returns to baseline 3.5 h following recovery, while no change in LC3B-I protein levels or MAP1LC3B mRNA levels is observed. SQSTM1/p62 protein and mRNA levels did not change in either rats or humans following exercise. By employing an ex vivo autophagy flux assay previously used in rodents we demonstrate that the exercise-induced decrease in LC3B-II protein levels in humans does not reflect a decreased autophagy flux. Instead, effect size analyses suggest a modest-to-large increase in autophagy flux following exercise that lasts up to 24 h. Our findings suggest that exercise-induced changes in autophagosome content markers differ between rodents and humans, and that exercise-induced decreases in LC3B-II protein levels do not reflect autophagy flux level.

Mitochondrial Quality Control in Sarcopenia: Updated Overview of Mechanisms and Interventions

Sarcopenia is a common geriatric disorder characterized by decreased muscle strength, low muscle mass and poor physical performance. This aging-related skeletal muscle deterioration leads to adverse outcomes and severely impairs the quality of life of patients. The accumulation of dysfunctional mitochondria with aging is an important factor in the occurrence and progression of sarcopenia. Mitochondrial quality control (MQC) fundamentally ensures the normal mitochondrial functions and is comprised of four main parts: proteostasis, biogenesis, dynamics and autophagy. Therefore, any pathophysiologic factors compromising the quality control of homeostasis in the skeletal muscle may lead to sarcopenia. However, the specific theoretical aspects of these processes have not been fully elucidated. Current therapeutic interventions using nutritional and pharmaceutical treatments show a modest therapeutic efficacy; however, only physical exercise is recommended as the first-line therapy for sarcopenia, which can ameliorate skeletal muscle deficiency by maintaining the homeostatic MQC. In this review, we summarized the known mechanisms that contribute to the pathogenesis of sarcopenia by impairing normal mitochondrial functions and described potential interventions that mitigate sarcopenia through improving MQC.

EFFECTS OF EXHAUSTIVE EXERCISE AND CONTUSION ON AUTOPHAGY-RELATED FACTORS IN SKELETAL MUSCLE OF RATS

ABSTRACT Objective: To study the effects of exhaustive exercise and contusion on autophagy-related factors Beclin1, LC3 and PINK1 expression in the skeletal muscle of rats. Methods: Forty-two male SD rats were randomly divided into 7 groups, 6 rats in each group: C, D0, D24, D48, E0, E24, and E48. Each group of rats was killed and dissected at the different respective time points specified above. The whole quadriceps femoris of the left hind limbs were removed and divided into two parts, one for mRNAs of Beclin1, LC3 and PINK1 by real-time fluorescent quantitative PCR, and the other for LC3 protein by Western blotting. Results: Compared with group C, the contents of Beclin1 mRNA, PINK1 mRNA, and LC3 mRNA in the immediate exhaustive exercise group (E0) were significantly reduced p<0.01. However, the levels of PINK1 mRNA, LC3 mRNA, and LC3 protein in skeletal muscle cells increased significantly in the 48 hours after exhaustion (E48) p<0.05, suggesting that cell autophagy had an increasing trend during the recovery period. Meanwhile, compared with the C group, the contents of Beclin1 mRNA, PINK1 mRNA, and LC3 mRNA in the immediate blunt contusion group (D0) increased significantly p<0.01 and were followed by a downward trend. Conclusion: Generally, there were differences between the blunt contusion and exhausted exercise models at each recovery phase. The gene expression of the autophagy-related factors was not high in the early exhaustive exercise recovery phase and subsequently followed an upward trend. But the above factors increased significantly in the immediate and early recovery phases after blunt contusion. Injury from blunt contusion may be more severe than exhaustive exercise-induced-injury, so the autophagy starts earlier according to the changes in autophagy-related factors. Level of evidence III; Therapeutic studies investigating the results of treatment.

Mechanisms of TLR4-Mediated Autophagy and Nitroxidative Stress

Pathogenic infections have badly affected public health and the development of the breeding industry. Billions of dollars are spent every year fighting against these pathogens. The immune cells of a host produce reactive oxygen species and reactive nitrogen species which promote the clearance of these microbes. In addition, autophagy, which is considered an effective method to promote the destruction of pathogens, is involved in pathological processes. As research continues, the interplay between autophagy and nitroxidative stress has become apparent. Autophagy is always intertwined with nitroxidative stress. Autophagy regulates nitroxidative stress to maintain homeostasis within an appropriate range. Intracellular oxidation, in turn, is a strong inducer of autophagy. Toll-like receptor 4 (TLR4) is a pattern recognition receptor mainly involved in the regulation of inflammation during infectious diseases. Several studies have suggested that TLR4 is also a key regulator of autophagy and nitroxidative stress. In this review, we describe the role of TLR4 in autophagy and oxidation, and focus on its function in influencing autophagy-nitroxidative stress interactions.

Electrical Stimulation of Cultured Myotubes in vitro as a Model of Skeletal Muscle Activity: Current State and Future Prospects

Skeletal muscles comprise more than a third of human body mass and critically contribute to regulation of body metabolism. Chronic inactivity reduces metabolic activity and functional capacity of muscles, leading to metabolic and other disorders, reduced life quality and duration. Cellular models based on progenitor cells isolated from human muscle biopsies and then differentiated into mature fibers in vitro can be used to solve a wide range of experimental tasks. The review discusses the aspects of myogenesis dynamics and regulation, which might be important in the development of an adequate cell model. The main function of skeletal muscle is contraction; therefore, electrical stimulation is important for both successful completion of myogenesis and in vitro modeling of major processes induced in the skeletal muscle by acute or regular physical exercise. The review analyzes the drawbacks of such cellular model and possibilities for its optimization, as well as the prospects for its further application to address fundamental aspects of muscle physiology and biochemistry and explore cellular and molecular mechanisms of metabolic diseases.

Autophagy and ALS: mechanistic insights and therapeutic implications

Mechanisms of protein homeostasis are crucial for overseeing the clearance of misfolded and toxic proteins over the lifetime of an organism, thereby ensuring the health of neurons and other cells of the central nervous system. The highly conserved pathway of autophagy is particularly necessary for preventing and counteracting pathogenic insults that may lead to neurodegeneration. In line with this, mutations in genes that encode essential autophagy factors result in impaired autophagy and lead to neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS). However, the mechanistic details underlying the neuroprotective role of autophagy, neuronal resistance to autophagy induction, and the neuron-specific effects of autophagy-impairing mutations remain incompletely defined. Further, the manner and extent to which non-cell autonomous effects of autophagy dysfunction contribute to ALS pathogenesis are not fully understood. Here, we review the current understanding of the interplay between autophagy and ALS pathogenesis by providing an overview of critical steps in the autophagy pathway, with special focus on pivotal factors impaired by ALS-causing mutations, their physiologic effects on autophagy in disease models, and the cell type-specific mechanisms regulating autophagy in non-neuronal cells which, when impaired, can contribute to neurodegeneration. This review thereby provides a framework not only to guide further investigations of neuronal autophagy but also to refine therapeutic strategies for ALS and related neurodegenerative diseases.Abbreviations: ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CHMP2B: charged multivesicular body protein 2B; DPR: dipeptide repeat; FTD: frontotemporal dementia; iPSC: induced pluripotent stem cell; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PINK1: PTEN induced kinase 1; RNP: ribonuclear protein; sALS: sporadic ALS; SPHK1: sphingosine kinase 1; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK-binding kinase 1; TFEB: transcription factor EB; ULK: unc-51 like autophagy activating kinase; UPR: unfolded protein response; UPS: ubiquitin-proteasome system; VCP: valosin containing protein.

Understanding the molecular mechanisms and role of autophagy in obesity

Vital for growth, proliferation, subsistence, and thermogenesis, autophagy is the biological cascade, which confers defence against aging and various pathologies. Current research has demonstrated de novo activity of autophagy in stimulation of biological events. There exists a significant association between autophagy activation and obesity, encompassing expansion of adipocytes which facilitates β cell activity. The main objective of the manuscript is to enumerate intrinsic role of autophagy in obesity and associated complications. The peer review articles published till date were searched using medical databases like PubMed and MEDLINE for research, primarily in English language. Obesity is characterized by adipocytic hypertrophy and hyperplasia, which leads to imbalance of lipid absorption, free fatty acid release, and mitochondrial activity. Detailed evaluation of obesity progression is necessary for its treatment and related comorbidities. Data collected in regard to etiological sustaining of obesity, has revealed hypothesized energy misbalance and neuro-humoral dysfunction, which is stimulated by autophagy. Autophagy regulates chief salvaging events for protein clustering, excessive triglycerides, and impaired mitochondria which is accompanied by oxidative and genotoxic stress in mammals. Autophagy is a homeostatic event, which regulates biological process by eliminating lethal cells and reprocessing physiological constituents, comprising of proteins and fat. Unquestionably, autophagy impairment is involved in metabolic syndromes, like obesity. According to an individual's metabolic outline, autophagy activation is essential for metabolism and activity of the adipose tissue and to retard metabolic syndrome i.e. obesity. The manuscript summarizes the perception of current knowledge on autophagy stimulation and its effect on the obesity.

Molecular Regulation of Skeletal Muscle Growth and Organelle Biosynthesis: Practical Recommendations for Exercise Training

The regulation of skeletal muscle mass and organelle homeostasis is dependent on the capacity of cells to produce proteins and to recycle cytosolic portions. In this investigation, the mechanisms involved in skeletal muscle mass regulation-especially those associated with proteosynthesis and with the production of new organelles-are presented. Thus, the critical roles of mammalian/mechanistic target of rapamycin complex 1 (mTORC1) pathway and its regulators are reviewed. In addition, the importance of ribosome biogenesis, satellite cells involvement, myonuclear accretion, and some major epigenetic modifications related to protein synthesis are discussed. Furthermore, several studies conducted on the topic of exercise training have recognized the central role of both endurance and resistance exercise to reorganize sarcomeric proteins and to improve the capacity of cells to build efficient organelles. The molecular mechanisms underlying these adaptations to exercise training are presented throughout this review and practical recommendations for exercise prescription are provided. A better understanding of the aforementioned cellular pathways is essential for both healthy and sick people to avoid inefficient prescriptions and to improve muscle function with emergent strategies (e.g., hypoxic training). Finally, current limitations in the literature and further perspectives, notably on epigenetic mechanisms, are provided to encourage additional investigations on this topic.

Self-Eating for Muscle Fitness: Autophagy in the Control of Energy Metabolism

Cellular processes that sense and transmit metabolic changes are crucial for adaptation to external signals. In this regard, autophagy provides energy upon nutrient deprivation and represents a quality control mechanism that eliminates damaged organelles or proteins. Here, we review recent findings on the metabolic pathways controlling autophagy in skeletal muscle, a plastic tissue that undergoes major changes in energy demands. We also analyze the implications of autophagy in the regulation of energy metabolism in muscle and how alterations in this process affect energy homeostasis at the whole-body level and the development of metabolic diseases and aging.

Analysis of Activity-Dependent Energy Metabolism in Mice Reveals Regulation of Mitochondrial Fission and Fusion mRNA by Voluntary Physical Exercise in Subcutaneous Fat from Male Marathon Mice (DUhTP)

Physical inactivity is considered as one of the main causes of obesity in modern civilizations, and it has been demonstrated that resistance training programs can be used to reduce fat mass. The effects of voluntary exercise on energy metabolism are less clear in adipose tissue. Therefore, the effects of three different voluntary exercise programs on the control of energy metabolism in subcutaneous fat were tested in two different mouse lines. In a cross-over study design, male mice were kept for three or six weeks in the presence or absence of running wheels. For the experiment, mice with increased running capacity (DUhTP) were used and compared to controls (DUC). Body and organ weight, feed intake, and voluntary running wheel activity were recorded. In subcutaneous fat, gene expression of browning markers and mitochondrial energy metabolism were analyzed. Exercise increased heart weight in control mice (p < 0.05) but significantly decreased subcutaneous, epididymal, perinephric, and brown fat mass in both genetic groups (p < 0.05). Gene expression analysis revealed higher expression of browning markers and individual complex subunits present in the electron transport chain in subcutaneous fat of DUhTP mice compared to controls (DUC; p < 0.01), independent of physical activity. While in control mice, voluntary exercise had no effect on markers of mitochondrial fission or fusion, in DUhTP mice, reduced mitochondrial DNA, transcription factor Nrf1, fission- (Dnm1), and fusion-relevant transcripts (Mfn1 and 2) were observed in response to voluntary physical activity (p < 0.05). Our findings indicate that the superior running abilities in DUhTP mice, on one hand, are connected to elevated expression of genetic markers for browning and oxidative phosphorylation in subcutaneous fat. In subcutaneous fat from DUhTP but not in unselected control mice, we further demonstrate reduced expression of genes for mitochondrial fission and fusion in response to voluntary physical activity.

Precision remodeling: how exercise improves mitochondrial quality in myofibers

The profound energetic demand of prolonged exercise imposed upon skeletal muscle and the heart is met by oxidation of substrate within mitochondria. As such, several coordinated events are initiated in order to maintain mitochondria, collectively known as mitochondrial quality control. In this review, we discuss how mitochondrial quality control functions to maintain the integrity of the reticulum and energy production in response to prolonged exercise, as well as the relevant signaling events that dictate these responses. Based upon the prevailing data in the field, we propose a model where exercise-mediated quality control may be chiefly regulated through local mechanisms, thus allowing for the remarkable precision in mitochondrial quality control events.

Regulatory role of exercise-induced autophagy for sarcopenia

Sarcopenia is an aging-related disease, described as the progressive reduction in mass and strength of skeletal muscle. Sarcopenia is typically characterized as the accumulation of damaged products due to an imbalance between protein synthesis and protein degradation. This imbalance between protein synthesis and degradation is attributed to impaired autophagic signal pathways. Sarcopenia can predispose elderly patients to several complications that may significantly impact patient quality of life. Recent evidence indicates that autophagy is required for the control of skeletal muscle mass under catabolic conditions and plays a crucial role in maintaining the homeostasis and integrity of skeletal muscle, specifically at appropriate level of autophagy. Exercise may be considered as a stress stimulus that can substantially modulate cellular signaling to promote metabolic adaptations. Appropriate exercise can induce autophagy or regulate the functional status of autophagy. Additionally, exercise-induced autophagy is the most effective treatment available in slowing down sarcopenia, improving mitochondrial quality, and the number of quiescent satellite cells, as a process that depends on basal autophagy. The molecular mechanisms underpinning the development of sarcopenia, however, remained largely unknown. In this narrative review, the current molecular mechanisms of sarcopenia are discussed from the perspective of exercise-induced autophagy and the effect of different exercise modalities on this response. This narrative review will aim to provide the references for developing scientific and optimal intervention strategies including exercise intervention for the prevention and treatment of sarcopenia through regulating autophagic signal pathways.

Effects of Exercise Training on the Autophagy-Related Muscular Proteins Expression in Ovariectomized Rats

Ovariectomy disrupts estrogen production and homeostasis. However, whether exercise training (ET) could counteract the ovariectomy-induced effect on muscular autophagy has remained elusive. This study examined muscular autophagy in ovariectomized (OVX) rats following 8 weeks of swimming ET. Here, 40 6-month-old female Sprague-Dawley rats were randomly divided into five groups: sham-operated control (Sham), OVX control (OVX), OVX with 60-min ET (OVX-60ET), 90-min ET (OVX-90ET), and 120-min ET (OVX-120ET) for 6 days/week. According to the results of Western blotting, the expression levels of autophagy-related proteins in the OVX gastrocnemius muscle, including mammalian target of rapamycin, uncoordinated 51-like kinase 1, Beclin-1, autophagy-related gene (Atg-7), and microtubule-associated protein light chains 3 were significantly decreased (all P < 0.05), while there was an elevation on the p62 level. ET appreciably mitigated the OVX-induced negative effects on muscle quality and the autophagy pathway, which seemed to be dependent on ET volume. The most optimal outcomes were observed in the OVX-90ET group. The OVX-120 group had an adversely augmented catabolic process associated with gastrocnemius muscle atrophy. In conclusion, the expression levels of autophagy proteins are decreased in OVX rats, which can be appreciably mitigated following 8 weeks of swimming ET.

Targeting Autophagy in Obesity-Associated Heart Disease

Over the past three decades, the increasing rates of obesity have led to an alarming obesity epidemic worldwide. Obesity is associated with an increased risk of cardiovascular diseases; thus, it is essential to define the molecular mechanisms by which obesity affects heart function. Individuals with obesity and overweight have shown changes in cardiac structure and function, leading to cardiomyopathy, hypertrophy, atrial fibrillation, and arrhythmia. Autophagy is a highly conserved recycling mechanism that delivers proteins and damaged organelles to lysosomes for degradation. In the hearts of patients and mouse models with obesity, this process is impaired. Furthermore, it has been shown that autophagy flux restoration in obesity models improves cardiac function. Therefore, autophagy may play an important role in mitigating the adverse effects of obesity on the heart. Throughout this review, we will discuss the benefits of autophagy on the heart in obesity and how regulating autophagy might be a therapeutic tool to reduce the risk of obesity-associated cardiovascular diseases.

Effects of exercise training with short-duration intermittent hypoxia on endurance performance and muscle metabolism in well-trained mice

The author previously reported that short-duration intermittent hypoxia had additive effects on improvements in endurance capacity by enhancing fatty acid metabolism. The present study was designed to investigate the effects of short-duration intermittent hypoxia on endurance capacity, metabolic enzyme activity, and protein levels associated with mitochondrial biogenesis in well-trained mice. Mice in the training group were housed in a cage with a running wheel for 7 weeks from 5 weeks old. Voluntary running markedly increased maximal work values by 5.0-fold. Trained mice were then subjected to either endurance treadmill training (ET) for 60 min or hybrid training (HT, ET for 30 min followed by sprint interval exercise (5-sec run-10-sec rest) for 30 min) with (H-ET or H-HT) or without (ET or HT) short-duration intermittent hypoxia (4 cycles of 12-13% O2 for 15 min and 20.9% O2 for 10 min) for 4 weeks. Maximal endurance capacity was markedly greater in the H-ET and H-HT than ET and HT groups, respectively. H-ET and H-HT increased activity levels of 3-hydroxyacyl-CoA-dehydrogenase in oxidative muscle portion and pyruvate dehydrogenase complex in glycolytic muscle portion. These activity levels were significantly correlated with maximal endurance capacity. Protein levels of dynamin-related protein-1 were increased more by H-ET and H-HT than by ET and HT, but were not significantly correlated with maximal work. These results suggest that intermittent hypoxic exposure has beneficial effects on endurance and hybrid training to improve the endurance capacity via improving fatty acid and pyruvate oxidation in highly trained mice.

Nutrition and exercise in Pompe disease

The current standard of care for Pompe disease (PD) is the administration of enzyme replacement therapy (ERT). Exercise and nutrition are often considered as complementary strategies rather than "treatments" per se. Nutritional assessment is important in patients with locomotor disability because the relative hypodynamia limits energy expenditure and thus the total amount of energy must be reduced to avoid obesity. A lower total energy intake often leads to lower protein and micronutrient intake. Consequently, ensuring that Pompe patients are tested for and replaced for deficiencies (protein, vitamin D, vitamin B12, etc.) is an important aspect of care. Furthermore, given the role of autophagy in the pathophysiology of PD and the fact that fasting induces autophagy, it is important that strategies such as nutritional timing and amino acid intake (L-arginine, L-leucine) be evaluated as therapies. Exercise interventions have been shown to improve six-minute walk testing distance by more than what was seen in the seminal ERT study in late-onset PD. Exercise therapy can also activate autophagy, and this is likely another component of its efficacy. The current review will evaluate the theoretical and practical aspects of nutrition and exercise as therapies for patients with PD.

Recent Data on Cellular Component Turnover: Focus on Adaptations to Physical Exercise

Significant progress has expanded our knowledge of the signaling pathways coordinating muscle protein turnover during various conditions including exercise. In this manuscript, the multiple mechanisms that govern the turnover of cellular components are reviewed, and their overall roles in adaptations to exercise training are discussed. Recent studies have highlighted the central role of the energy sensor (AMP)-activated protein kinase (AMPK), forkhead box class O subfamily protein (FOXO) transcription factors and the kinase mechanistic (or mammalian) target of rapamycin complex (MTOR) in the regulation of autophagy for organelle maintenance during exercise. A new cellular trafficking involving the lysosome was also revealed for full activation of MTOR and protein synthesis during recovery. Other emerging candidates have been found to be relevant in organelle turnover, especially Parkin and the mitochondrial E3 ubiquitin protein ligase (Mul1) pathways for mitochondrial turnover, and the glycerolipids diacylglycerol (DAG) for protein translation and FOXO regulation. Recent experiments with autophagy and mitophagy flux assessment have also provided important insights concerning mitochondrial turnover during ageing and chronic exercise. However, data in humans are often controversial and further investigations are needed to clarify the involvement of autophagy in exercise performed with additional stresses, such as hypoxia, and to understand the influence of exercise modality. Improving our knowledge of these pathways should help develop therapeutic ways to counteract muscle disorders in pathological conditions.

Mitochondria and Aging-The Role of Exercise as a Countermeasure

Mitochondria orchestrate the life and death of most eukaryotic cells by virtue of their ability to supply adenosine triphosphate from aerobic respiration for growth, development, and maintenance of the ‘physiologic reserve’. Although their double-membrane structure and primary role as ‘powerhouses of the cell’ have essentially remained the same for ~2 billion years, they have evolved to regulate other cell functions that contribute to the aging process, such as reactive oxygen species generation, inflammation, senescence, and apoptosis. Biological aging is characterized by buildup of intracellular debris (e.g., oxidative damage, protein aggregates, and lipofuscin), which fuels a ‘vicious cycle’ of cell/DNA danger response activation (CDR and DDR, respectively), chronic inflammation (‘inflammaging’), and progressive cell deterioration. Therapeutic options that coordinately mitigate age-related declines in mitochondria and organelles involved in quality control, repair, and recycling are therefore highly desirable. Rejuvenation by exercise is a non-pharmacological approach that targets all the major hallmarks of aging and extends both health- and lifespan in modern humans.

Exercise and exercise training-induced increase in autophagy markers in human skeletal muscle

Moderately trained male subjects (mean age 25 years; range 19–33 years) completed an 8‐week exercise training intervention consisting of continuous moderate cycling at 157 ± 20 W for 60 min (MOD; n = 6) or continuous moderate cycling (157 ± 20 W) interspersed by 30‐sec sprints (473 ± 79 W) every 10 min (SPRINT; n = 6) 3 days per week. Sprints were followed by 3:24 min at 102 ± 17 W to match the total work between protocols. A muscle biopsy was obtained before, immediately and 2 h after the first training session as well as at rest after the training session. In both MOD and SPRINT, skeletal muscle AMPK^Thr172 and ULK^Ser317 phosphorylation was elevated immediately after exercise, whereas mTOR^Ser2448 and ULK^Ser757 phosphorylation was unchanged. Two hours after exercise LC3I, LC3II and BNIP3 protein content was overall higher than before exercise with no change in p62 protein. In MOD, Beclin1 protein content was higher immediately and 2 h after exercise than before exercise, while there were no differences within SPRINT. Oxphos complex I, LC3I, BNIP3 and Parkin protein content was higher after the training intervention than before in both groups, while there was no difference in LC3II and p62 protein. Beclin1 protein content was higher after the exercise training intervention only in MOD. Together this suggests that exercise increases markers of autophagy in human skeletal muscle within the first 2 h of recovery and 8 weeks of exercise training increases the capacity for autophagy and mitophagy regulation. Hence, the present findings provide evidence that exercise and exercise training regulate autophagy in human skeletal muscle and that this in general was unaffected by interspersed sprint bouts.

Effects of intermittent hyperbaric exposure on endurance and interval exercise performance in well-trained mice

NEW FINDINGS

What is the central question of this study? Intermittent hyperbaric exposure (1.3 atmospheres absolute with 20.9% O₂ ) enhances endurance capacity by facilitating oxidative and glycolytic capacities in skeletal muscle. It remains unclear whether this strategy enhances endurance performance in well-trained individuals. What is the main finding and its importance? Hyperbaric exposure with endurance training enhanced oxidative and glycolytic capacities and protein levels of mitochondrial transcription factor A, dynamin-related protein-1 and heat shock protein 70. Hyperbaric exposure with sprint interval training increased the proportion of type I muscle fibres and promoted capillary growth and muscle fibre hypertrophy. These results may lead to a new strategy for enhancing exercise capacity in well-trained mice.

ABSTRACT

The study was designed to clarify the mechanisms by which hyperbaric exposure (1.3 atmospheres absolute with 20.9% O₂ ) improves endurance and interval exercise capacities in highly trained mice. Male mice in the training group were housed in a cage with a wheel activity device for 7 weeks from 5 weeks old. Voluntary running markedly increased maximal endurance capacity by 6.4-fold. Trained mice were then subjected to either endurance treadmill training (20-32.5 m min^-1 ) or sprint interval training (5 s run-10 s rest, 30-42.5 m min^-1 ) with (HypET or HypSIT, respectively) and without (ET or SIT, respectively) 1 h hyperbaric exposure for 4 weeks. Maximal endurance capacity was significantly increased by HypET and HypSIT, and maximal interval capacity was significantly enhanced by HypSIT. Peroxisome proliferator-activated receptor gamma coactivator 1-alpha expression levels were markedly increased after HypET and HypSIT. Activity levels of 3-hydroxyacyl-CoA-dehydrogenase, citrate synthase and phosphofructokinase in the red gastrocnemius muscle were increased more by HypET than by ET. Protein levels of mitochondrial transcription factor A, dynamin-related protein-1 and heat shock protein 70 were increased more by HypET than by ET. The proportion of type I fibres in the soleus muscle was remarkably increased by HypSIT. Capillary-to-fibre ratio values in the white gastrocnemius were increased more by HypSIT than by SIT. These results suggest that hyperbaric exposure has beneficial effects for endurance and interval training to improve exercise capacity in highly trained mice.

Effect of aerobic training on baseline expression of signaling and respiratory proteins in human skeletal muscle

Most studies examining the molecular mechanisms underlying adaptation of human skeletal muscles to aerobic exercise focused on the response to acute exercise. Here, we examined the effect of a 2-month aerobic training program on baseline parameters in human muscle. Ten untrained males performed a one-legged knee extension exercise for 1 h with the same relative intensity before and after a 2-month aerobic training program. Biopsy samples were taken from vastus lateralis muscle at rest before and after the 2 month training program (baseline samples). Additionally, biopsy samples were taken from the exercised leg 1 and 4 h after the one-legged continuous knee extension exercise. Aerobic training decreases baseline phosphorylation of FOXO1Ser256 , increases that of CaMKIIThr286 , CREB1Ser133 , increases baseline expression of mitochondrial proteins in respiratory complexes I-V, and some regulators of mitochondrial biogenesis (TFAM, NR4A3, and CRTC2). An increase in the baseline content of these proteins was not associated with a change in baseline expression of their genes. The increase in the baseline content of regulators of mitochondrial biogenesis (TFAM and NR4A3) was associated with a transient increase in transcription after acute exercise. Contrariwise, the increase in the baseline content of respiratory proteins does not seem to be regulated at the transcriptional level; rather, it is associated with other mechanisms. Adaptation of human skeletal muscle to regular aerobic exercise is associated not only with transient molecular responses to exercise, but also with changes in baseline phosphorylation and expression of regulatory proteins.

Autophagy is induced by resistance exercise in young men, but unfolded protein response is induced regardless of age

AIM

Autophagy and unfolded protein response (UPR) appear to be important for skeletal muscle homoeostasis and may be altered by exercise. Our aim was to investigate the effects of resistance exercise and training on indicators of UPR and autophagy in healthy untrained young men (n = 12, 27 ± 4 years) and older men (n = 8, 61 ± 6 years) as well as in resistance-trained individuals (n = 15, 25 ± 5 years).

METHODS

Indicators of autophagy and UPR were investigated from the muscle biopsies after a single resistance exercise bout and after 21 weeks of resistance training.

RESULTS

Lipidated LC3II as an indicator of autophagosome content increased at 48 hours post-resistance exercise (P < .05) and after a resistance training period (P < .01) in untrained young men but not in older men. Several UPR_ER markers, typically induced by protein misfolding in endoplasmic reticulum, were increased at 48 hours post-resistance exercise in untrained young and older men (P < .05) but were unaltered after the 21-week resistance training period regardless of age. UPR was unchanged within the first few hours after the resistance exercise bout regardless of the training status. Changes in autophagy and UPR_ER indicators did not correlate with a resistance training-induced increase in muscle strength and size.

CONCLUSION

Autophagosome content is increased by resistance training in young previously untrained men, but this response may be blunted by ageing. However, unfolded protein response is induced by an unaccustomed resistance exercise bout in a delayed manner regardless of age.

Modelling performance and skeletal muscle adaptations with exponential growth functions during resistance training

System theory is classically applied to describe and to predict the effects of training load on performance. The classic models are structured by impulse-type transfer functions, nevertheless, most biological adaptations display exponential growth kinetics. The aim of this study was to propose an extension of the model structure taking into account the exponential nature of skeletal muscle adaptations by using a genetic algorithm. Thus, the conventional impulse-type model was applied in 15 resistance trained rodents and compared with exponential growth-type models. Even if we obtained a significant correlation between actual and modelled performances for all the models, our data indicated that an exponential model is associated with more suitable parameters values, especially the time constants that correspond to the positive response to training. Moreover, positive adaptations predicted with an exponential component showed a strong correlation with the main structural adaptations examined in skeletal muscles, i.e. hypertrophy (R² = 0.87, 0.96 and 0.99, for type 1, 2A and 2X cross-sectional area fibers, respectively) and changes in fiber-type composition (R² = 0.81 and 0.79, for type 1 and 2A fibers, respectively). Thus, an exponential model succeeds to describe both performance variations with relevant time constants and physiological adaptations that take place during resistance training.

Adaptations of Skeletal Muscle Mitochondria to Obesity, Exercise, and Polyunsaturated Fatty Acids

Mitochondria intricately modulate their energy production through the control of mitochondrial adaptation (mitochondrial biogenesis, fusion, and/or fission) to meet energy demands. Nutrient overload may result in dysregulated mitochondrial biogenesis, morphology toward mitochondrial fragmentation, and oxidative stress in the skeletal muscle. In addition, physical activity and diet components influence mitochondrial function. Exercise may stimulate mitochondrial biogenesis and promote mitochondrial fusion/fission in the skeletal muscle. Moreover, some dietary fatty acids, such as n-3 polyunsaturated fatty acids and conjugated linoleic acid, have been identified to positively regulate mitochondrial adaptation in the skeletal muscle. This review discusses the association of mitochondrial impairments and obesity, and presents an overview of various mechanisms of which exercise training and mitochondrial nutrients promote mitochondrial function in the skeletal muscle.

Branched-chain amino acids administration suppresses endurance exercise-related activation of ubiquitin proteasome signaling in trained human skeletal muscle

We tested whether post exercise ingestion of branched-chain amino acids (BCAA < 10 g) is sufficient to activate signaling associated with muscle protein synthesis and suppress exercise-induced activation of mechanisms associated with proteolysis in endurance-trained human skeletal muscle. Nine endurance-trained athletes performed a cycling bout with and without BCAA ingestion (0.1 g/kg). Post exercise ACCSer79/222 phosphorylation (endogenous marker of AMPK activity) was increased (~3-fold, P < 0.05) in both sessions. No changes were observed in IGF1 mRNA isoform expression or phosphorylation of the key anabolic markers - p70S6K1Thr389 and eEF2Thr56 - between the sessions. BCAA administration suppressed exercise-induced expression of mTORC1 inhibitor DDIT4 mRNA, eliminated activation of the ubiquitin proteasome system, detected in the control session as decreased FOXO1Ser256 phosphorylation (0.83-fold change, P < 0.05) and increased TRIM63 (MURF1) expression (2.4-fold, P < 0.05). Therefore, in endurance-trained human skeletal muscle, post exercise BCAA ingestion partially suppresses exercise-induced expression of PGC-1a mRNA, activation of ubiquitin proteasome signaling, and suppresses DDIT4 mRNA expression.

mTOR Signaling Pathway and Protein Synthesis: From Training to Aging and Muscle Autophagy

In muscle tissue there is a balance between the processes muscle synthesis and degradation. The mammalian target of rapamycin (mTOR) signaling pathway plays a critical role in regulating protein synthesis in order to maintain muscular protein turnover and trophism. Studies have shown that both down- and upregulation mechanisms are involved in this process in a manner dependent on stimulus and cellular conditions. Additionally, mTOR signaling has recently been implicated in several physiological conditions related to cell survival, such as self-digestion (autophagy), energy production, and the preservation of cellular metabolic balance over the lifespan. Here we briefly describe the mTOR structure and its regulatory protein synthesis pathway. Furthermore, the role of mTOR protein in autophagy, aging, and mitochondrial function in muscle tissue is presented.

Exercise-Induced Autophagy in Fatty Liver Disease

Hepatic steatosis prevails each year. Autophagy is integral in mitochondrial quality control and lipid homeostasis in the liver. No pharmacological strategies are currently available to reduce hepatic steatosis, but exercise has been known to improve clinical outcomes of chronic liver disease, particularly nonalcoholic fatty liver disease (NAFLD). Recent studies suggest that exercise may improve NAFLD through enhancing autophagy.

Mitophagy in maintaining skeletal muscle mitochondrial proteostasis and metabolic health with ageing

Skeletal muscle is important for overall functionality and health. Ageing is associated with an accumulation of damage to mitochondrial DNA and proteins. In particular, damage to mitochondrial proteins in skeletal muscle, which is a loss of mitochondrial proteostasis, contributes to tissue dysfunction and negatively impacts systemic health. Therefore, understanding the mechanisms underlying the regulation of mitochondrial proteostasis and how those mechanisms change with age is important for the development of interventions to promote healthy ageing. Herein, we examine how impairment in the selective degradation of damaged/dysfunctional mitochondria through mitophagy may play a central role in the loss of mitochondrial proteostasis in skeletal muscle ageing, as well as its broader implications for systemic health. Further, we explore how stimulating mitophagy through exercise may promote healthy ageing.

Diabetes-Induced Dysfunction of Mitochondria and Stem Cells in Skeletal Muscle and the Nervous System

Diabetes mellitus is one of the most common metabolic diseases spread all over the world, which results in hyperglycemia caused by the breakdown of insulin secretion or insulin action or both. Diabetes has been reported to disrupt the functions and dynamics of mitochondria, which play a fundamental role in regulating metabolic pathways and are crucial to maintain appropriate energy balance. Similar to mitochondria, the functions and the abilities of stem cells are attenuated under diabetic condition in several tissues. In recent years, several studies have suggested that the regulation of mitochondria functions and dynamics is critical for the precise differentiation of stem cells. Importantly, physical exercise is very useful for preventing the diabetic alteration by improving the functions of both mitochondria and stem cells. In the present review, we provide an overview of the diabetic alterations of mitochondria and stem cells and the preventive effects of physical exercise on diabetes, focused on skeletal muscle and the nervous system. We propose physical exercise as a countermeasure for the dysfunction of mitochondria and stem cells in several target tissues under diabetes complication and to improve the physiological function of patients with diabetes, resulting in their quality of life being maintained.

Ampk phosphorylation of Ulk1 is required for targeting of mitochondria to lysosomes in exercise-induced mitophagy

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.Exercise is associated with biogenesis and removal of dysfunctional mitochondria. Here the authors use a mitochondrial reporter gene to demonstrate the occurrence of mitophagy following exercise in mice, and show this is dependent on AMPK and ULK1 signaling.

Exercise-mediated modulation of autophagy in skeletal muscle

Although exercise exerts multiple beneficial health effects, it may also damage cellular structures. Damaged elements are continuously degraded and its constituents recycled to produce renovated structures through a process called autophagy, which is essential for the adaptation to training. Autophagy is particularly active in skeletal muscle, where it can be evaluated using specific molecular markers of activation (unc-51-like kinase 1 [ULK1] phosphorylation) and specific proteins indicating increased autophagosome content (increased total LC3, LC3-II, LC3-II/LC3-I ratio). Studies in humans are technically limited but have provided evidence suggesting the activation of autophagy in skeletal muscle through AMP-activated protein kinase (AMPK) and its downstream target ULK1. Autophagy activation is more likely when the intensity is elevated and the exercise performed in the fasted state. The autophagy-gene program and autophagosome content are upregulated after ultraendurance running competitions. However, autophagosome content is reduced after endurance exercise at moderate intensities (50% and 70% of VO₂ max) for 60-120 minutes. Autophagosome content is decreased within the first few hours after resistance training. The effects of regular endurance and strength training on basal autophagy remain to be established in humans. One study has reported that acute severe hypoxia increases autophagosome content in human skeletal muscle, which is reverted by 20 minutes of low-intensity exercise. Experiments with transgenic mice have shown that autophagy is necessary for skeletal muscle adaptation to training. Little is known on how genetic factors, environment, nutrition, drugs and diseases may interact with exercise to modulate autophagy at rest and during exercise in humans.

Autophagy-Dependent Beneficial Effects of Exercise

Exercise has long been recognized as a powerful physiological stimulus for a wide variety of metabolic adaptations with implications for health and performance. The metabolic effects of exercise occur during and after each exercise bout and manifest as cumulative adaptive responses to successive exercise bouts. Studies on the beneficial effects of exercise have traditionally focused on the biosynthesis of metabolic proteins and organelles. However, the recycling of cellular components by autophagy has recently emerged as an important process involved in the adaptive responses to exercise. This review covers the regulation of autophagy by exercise, with emphasis on the potential autophagy-dependent beneficial effects of exercise.

Regulation of Exercise-Induced Autophagy in Skeletal Muscle

PURPOSE OF REVIEW

Physical exercise is a highly effective method to prevent several pathogenic conditions, such as obesity, type 2 diabetes and cardiovascular diseases, largely due to metabolic adaptations induced by exercise in skeletal muscle. Yet how exercise induces the beneficial effects in muscle remains to be fully elucidated. Autophagy is a lysosomal degradation pathway that regulates nutrient recycling, energy production and organelle quality control. The autophagy pathway is upregulated in response to stress during exercise and muscle contraction, and may be an important mechanism mediating exercise-induced health benefits.

RECENT FINDINGS

A number of studies have indicated that physical exercise induces non-selective autophagy and selective mitophagy in skeletal muscle in animal models and humans. The AMPK-ULK1 and the FoxO3 signaling pathways play an essential role in the activation of the upstream autophagy machinery in skeletal muscle during exercise. The autophagy activity is required for health benefits of exercise, as in different autophagy-deficient mouse lines exercise-induced effects are abolished.

SUMMARY

This review aims to summarize and highlight the most recent findings on the role of autophagy in muscle maintenance, the molecular pathways that upregulate autophagy during exercise, and the potential functions of exercise-induced autophagy and mitophagy in skeletal muscle.

Role of the mitochondrial DNA replication machinery in mitochondrial DNA mutagenesis, aging and age-related diseases

As regulators of bioenergetics in the cell and the primary source of endogenous reactive oxygen species (ROS), dysfunctional mitochondria have been implicated for decades in the process of aging and age-related diseases. Mitochondrial DNA (mtDNA) is replicated and repaired by nuclear-encoded mtDNA polymerase γ (Pol γ) and several other associated proteins, which compose the mtDNA replication machinery. Here, we review evidence that errors caused by this replication machinery and failure to repair these mtDNA errors results in mtDNA mutations. Clonal expansion of mtDNA mutations results in mitochondrial dysfunction, such as decreased electron transport chain (ETC) enzyme activity and impaired cellular respiration. We address the literature that mitochondrial dysfunction, in conjunction with altered mitochondrial dynamics, is a major driving force behind aging and age-related diseases. Additionally, interventions to improve mitochondrial function and attenuate the symptoms of aging are examined.

Intact initiation of autophagy and mitochondrial fission by acute exercise in skeletal muscle of patients with Type 2 diabetes

The present study finds important steps in the initiation of autophagy and mitochondrial fission are intact in skeletal muscle of patients with Type 2 diabetes (T2D) compared with non-diabetic, weight-matched controls.

Effects of skeletal muscle energy availability on protein turnover responses to exercise

Skeletal muscle adaptation to exercise training is a consequence of repeated contraction-induced increases in gene expression that lead to the accumulation of functional proteins whose role is to blunt the homeostatic perturbations generated by escalations in energetic demand and substrate turnover. The development of a specific 'exercise phenotype' is the result of new, augmented steady-state mRNA and protein levels that stem from the training stimulus (i.e. endurance or resistance based). Maintaining appropriate skeletal muscle integrity to meet the demands of training (i.e. increases in myofibrillar and/or mitochondrial protein) is regulated by cyclic phases of synthesis and breakdown, the rate and turnover largely determined by the protein's half-life. Cross-talk among several intracellular systems regulating protein synthesis, breakdown and folding is required to ensure protein equilibrium is maintained. These pathways include both proteasomal and lysosomal degradation systems (ubiquitin-mediated and autophagy, respectively) and the protein translational and folding machinery. The activities of these cellular pathways are bioenergetically expensive and are modified by intracellular energy availability (i.e. macronutrient intake) and the 'training impulse' (i.e. summation of the volume, intensity and frequency). As such, exercise-nutrient interactions can modulate signal transduction cascades that converge on these protein regulatory systems, especially in the early post-exercise recovery period. This review focuses on the regulation of muscle protein synthetic response-adaptation processes to divergent exercise stimuli and how intracellular energy availability interacts with contractile activity to impact on muscle remodelling.

Exercise-induced skeletal muscle signaling pathways and human athletic performance

Skeletal muscle is a highly malleable tissue capable of altering its phenotype in response to external stimuli including exercise. This response is determined by the mode, (endurance- versus resistance-based), volume, intensity and frequency of exercise performed with the magnitude of this response-adaptation the basis for enhanced physical work capacity. However, training-induced adaptations in skeletal muscle are variable and unpredictable between individuals. With the recent application of molecular techniques to exercise biology, there has been a greater understanding of the multiplicity and complexity of cellular networks involved in exercise responses. This review summarizes the molecular and cellular events mediating adaptation processes in skeletal muscle in response to exercise. We discuss established and novel cell signaling proteins mediating key physiological responses associated with enhanced exercise performance and the capacity for reactive oxygen and nitrogen species to modulate training adaptation responses. We also examine the molecular bases underpinning heterogeneous responses to resistance and endurance exercise and the dissociation between molecular 'markers' of training adaptation and subsequent exercise performance.

Autophagy plays a role in skeletal muscle mitochondrial biogenesis in an endurance exercise-trained condition

Mitochondrial homeostasis is tightly regulated by two major processes: mitochondrial biogenesis and mitochondrial degradation by autophagy (mitophagy). Research in mitochondrial biogenesis in skeletal muscle in response to endurance exercise training has been well established, while the mechanisms regulating mitophagy and the interplay between mitochondrial biogenesis and degradation following endurance exercise training are not yet well defined. The purpose of this study was to examine the effects of a short-term inhibition of autophagy in response to acute endurance exercise on skeletal muscle mitochondrial biogenesis and dynamics in an exercise-trained condition. Male wild-type C57BL/6 mice performed five daily bouts of 1-h swimming per week for 8 weeks. In order to measure autophagy flux in mouse skeletal muscle, mice were treated with or without 2 days of 0.4 mg/kg/day intraperitoneal colchicine (blocking the degradation of autophagosomes) following swimming exercise training. The autophagic flux assay demonstrated that swimming training resulted in an increase in the autophagic flux (~100 % increase in LC3-II) in mouse skeletal muscle. Mitochondrial fusion proteins, Opa1 and MFN2, were significantly elevated, and mitochondrial fission protein, Drp1, was also increased in trained mouse skeletal muscle, suggesting that endurance exercise training promotes both mitochondrial fusion and fission processes. A mitochondrial receptor, Bnip3, was further increased in exercised muscle when treated with colchicine while Pink/Parkin protein levels were unchanged. The endurance exercise training induced increases in mitochondrial biogenesis marker proteins, SDH, COX IV, and a mitochondrial biogenesis promoting factor, PGC-1α but this effect was abolished in colchicine-treated mouse skeletal muscle. This suggests that autophagy plays an important role in mitochondrial biogenesis and this coordination between these opposing processes is involved in the cellular adaptation to endurance exercise training.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Effects of treadmill exercise on skeletal muscle mTOR signaling pathway in high-fat diet-induced obese mice

[Purpose] The aim of this study was to investigate the effects of regular treadmill exercise on skeletal muscle Rictor-Akt and mTOR-Raptor-S6K1 signaling pathway in high-fat diet-induced obese mice. [Subjects and Methods] Four- week-old C57BL/6 mice were adopted and classified into normal diet group (ND, n = 10), normal diet and training group (NDT, n = 10), high-fat diet group (HF, n = 10), and high-fat diet and training group (HFT, n = 10). The exercise program consisted of a treadmill exercise provided at low intensity for 1–4 weeks, and moderate intensity for 5–8 weeks. [Results] The Western blot method was used to measure the expression of mTOR, Raptor, S6K1, Rictor, and Akt proteins in the soleus muscle. mTOR levels were significantly higher in the HF group than in the ND and NDT groups. Raptor/mTORC1 and S6K1 levels were significantly higher in the HF group than in all the other groups. Akt levels were significantly lower in the HF group than in the NDT group. The risk of obesity may be associated with the overactivation of the mTOR-Raptor-S6K1 signaling pathway and a decrease in Akt levels. [Conclusion] This study also indicates that performing aerobic exercise may be associated with the downregulation of the mTOR-Raptor-S6K1 pathway.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Effects of aerobic training on markers of autophagy in the elderly

Autophagy is a molecular process essential for the maintenance of cellular homeostasis, which appears to (i) decline with age and (ii) respond to physical exercise. In addition, recent evidence suggests a crosstalk between autophagy and toll-like receptor (TLR)-associated inflammatory responses. This study assessed the effects of aerobic exercise training on autophagy and TLR signaling in older subjects. Twenty-nine healthy women and men (age, 69.7 ± 1.0 year) were randomized to a training (TG) or a control (CG) group. TG performed an 8-week aerobic training program, while CG followed their daily routines. Peripheral blood mononuclear cells were isolated from blood samples obtained before and after the intervention, and protein levels of protein 1 light chain 3 (LC3), sequestosome 1 (p62/SQSTM1), beclin-1, phosphorylated unc-51-like kinase (ULK-1), ubiquitin-like autophagy-related (Atg)12, Atg16, and lysosome-associated membrane protein (LAMP)-2 were measured. TLR2 and TLR4 signaling pathways were also analyzed. Peak oxygen uptake increased in TG after the intervention. Protein expression of beclin-1, Atg12, Atg16, and the LC3II/I ratio increased following the training program (p < 0.05), while expression of p62/SQSTM1 and phosphorylation of ULK-1 at Ser(757) were lower (p < 0.05). Protein content of TLR2, TLR4, myeloid differentiation primary response gen 88 (MyD88), and TIR domain-containing adaptor-inducing interferon (TRIF) were not significantly modified by exercise. The current data indicate that aerobic exercise training induces alterations in multiple markers of autophagy, which seem to be unrelated to changes in TLR2 and TLR4 signaling pathways. These results expand knowledge on exercise-induced autophagy adaptations in humans and suggest that the exercise type employed may be a key factor explaining the potential relationship between autophagy and TLR pathways.