Short-term endurance training results in a muscle-specific decrease of myostatin mRNA content in the rat

An open-source, lockable mouse wheel for the accessible implementation of time- and distance-limited elective exercise

Current methods of small animal exercise involve either voluntary (wheel running) or forced (treadmill running) protocols. Although commonly used, each have several drawbacks which cause hesitancy to adopt these methods. While mice will instinctively run on a wheel, the distance and time spent running can vary widely. Forced exercise, while controllable, puts animals in stressful environments in which they are confined and often shocked for "encouragement." Additionally, both methods require expensive equipment and software, which limit these experiments to well-funded laboratories. To counter these issues, we developed a non-invasive mouse running device aimed to reduce handler-induced stress, provide time- and distance-based stopping conditions, and enable investigators with limited resources to easily produce and use the device. The Lockable Open-Source Training-Wheel (LOST-Wheel) was designed to be 3D printed on any standard entry-level printer and assembled using a few common tools for around 20 USD. It features an on-board screen and is capable of tracking distances, running time, and velocities of mice. The LOST-Wheel overcomes the largest drawback to voluntary exercise, which is the inability to control when and how long mice run, using a servo driven mechanism that locks and unlocks the running surface according to the protocol of the investigator. While the LOST-Wheel can be used without a computer connection, we designed an accompanying application to provide scientists with additional analyses. The LOST-Wheel Logger, an R-based application, displays milestones and plots on a user-friendly dashboard. Using the LOST-Wheel, we implemented a timed running experiment that showed distance-dependent decreases in serum myostatin as well as IL-6 gene upregulation in muscle. To make this device accessible, we are releasing the designs, application, and manual in an open-source format. The implementation of the LOST-Wheel and future iterations will improve upon existing murine exercise equipment and research.

Exercise-induced myokines and their effect on prostate cancer

Exercise is recognized by clinicians in the field of clinical oncology for its potential role in reducing the risk of certain cancers and in reducing the risk of disease recurrence and progression; yet, the underlying mechanisms behind this reduction in risk are not fully understood. Studies applying post-exercise blood serum directly to various types of cancer cell lines provide insight that exercise might have a role in inhibiting cancer growth via altered soluble and cell-free blood contents. Myokines, which are cytokines produced by muscle and secreted into the bloodstream, might offer multiple benefits to cellular metabolism (such as a reduction in insulin resistance, improved glucose uptake and reduced adiposity), and blood myokine levels can be altered with exercise. Alterations in the levels of myokines such as IL-6, IL-15, IL-10, irisin, secreted protein acidic risk in cysteine (SPARC), myostatin, oncostatin M and decorin might exert a direct inhibitory effect on cancer growth via inhibiting proliferation, promoting apoptosis, inducing cell-cycle arrest and inhibiting the epithermal transition to mesenchymal cells. The association of insulin resistance, hyperinsulinaemia and hyperlipidaemia with obesity can create a tumour-favourable environment; exercise-induced myokines can manipulate this environment by regulating adipose tissue and adipocytes. Exercise-induced myokines also have a critical role in increasing cytotoxicity and the infiltration of immune cells into the tumour.

[Myokines and adipomyokines: inflammatory mediators or unique molecules of targeted therapy for obesity?]

Skeletal muscles make up about 25% of the total mass in children and more than 40% in adults. Studies of the last twenty years have shown that along with the main functions, muscle tissue has hormonal activity. It was found that myocytes are able to release signaling molecules-myokines. They act auto-and paracrine within the muscle, and at a high level-through the systemic circulation, carrying out interactions between skeletal muscles and various organs and tissues, such as the liver, bone and adipose tissue, the brain. It is proved that the key factor in the expression of myokines is physical activity, and their level largely depends on physical fitness, the amount of skeletal muscle mass and its composition (the ratio of fast and slow fibers), on the intensity and duration of physical activity. Myokines have a wide range of physiological effects: myostatin suppresses the growth and differentiation of muscle tissue, and decorin, acting as its antagonist, promotes muscle hypertrophy. Interleukin 6 provides an energy substrate for contracting muscle fibers, fibroblast growth factor 21 activates the mechanisms of energy production during fasting and improves tissue sensitivity to insulin; irisin stimulates thermogenesis, glucose uptake by myocytes, and also contributes to an increase in bone mineral density. The study of myokines is one of the key links in understanding the mechanisms underlying obesity and metabolic complications, the consequences of a sedentary lifestyle, as well as the implementation of the action of physical activity. Taking into account the physiological effects of myokines in the body, in the future they can become therapeutic targets for the treatment of these conditions.

Effects of Dietary Fat Restriction on Endurance Training-induced Metabolic Adaptations in Rat Skeletal Muscle

Endurance exercise training enhances muscle fat oxidation while concomitantly reducing carbohydrate (glycogen) utilization during exercise, thereby delaying the onset of fatigue. This study examined the effects of dietary fat restriction on endurance training-induced metabolic adaptations in rat skeletal muscle. Male Sprague-Dawley rats were placed on either a control diet (CON: 19.2% protein, 21.6% fat, and 59.2% carbohydrate as a percentage of total energy) or a fat-restricted diet (FR: 21.5% protein, 2.4% fat, and 76.1% carbohydrate as a percentage of total energy) for 4 wks. Half the rats in each dietary group performed daily 6-h swimming exercise (two 3-h sessions separated by 45 min of rest) on 5 days each wk. Endurance training significantly increased the expression of β-hydroxyacyl CoA dehydrogenase (βHAD), a key enzyme of fat oxidation, and pyruvate dehydrogenase kinase 4 (PDK4), an inhibitory regulator of glycolytic flux, in the skeletal muscle of rats fed the CON diet. However, such endurance training-induced increases in muscle βHAD and PDK4 were partially suppressed by the FR diet, suggesting that a FR diet may diminish the endurance training-induced enhancement of fat oxidation and reduction in glycogen utilization during exercise. We then assessed the muscle glycogen utilization rate during an acute bout of swimming exercise in the trained rats fed either the CON or the FR diet and consequently found that rats fed the FR diet had a significantly higher muscle glycogen utilization rate during exercise compared with rats fed the CON diet. In conclusion, dietary fat restriction may attenuate the endurance training-induced metabolic adaptations in skeletal muscle.

Exercise-Induced Myokines can Explain the Importance of Physical Activity in the Elderly: An Overview

Physical activity has been found to aid the maintenance of health in the elderly. Exercise-induced skeletal muscle contractions lead to the production and secretion of many small proteins and proteoglycan peptides called myokines. Thus, studies on myokines are necessary for ensuring the maintenance of skeletal muscle health in the elderly. This review summarizes 13 myokines regulated by physical activity that are affected by aging and aims to understand their potential roles in metabolic diseases. We categorized myokines into two groups based on regulation by aerobic and anaerobic exercise. With aging, the secretion of apelin, β-aminoisobutyric acid (BAIBA), bone morphogenetic protein 7 (BMP-7), decorin, insulin-like growth factor 1 (IGF-1), interleukin-15 (IL-15), irisin, stromal cell-derived factor 1 (SDF-1), sestrin, secreted protein acidic rich in cysteine (SPARC), and vascular endothelial growth factor A (VEGF-A) decreased, while that of IL-6 and myostatin increased. Aerobic exercise upregulates apelin, BAIBA, IL-15, IL-6, irisin, SDF-1, sestrin, SPARC, and VEGF-A expression, while anaerobic exercise upregulates BMP-7, decorin, IGF-1, IL-15, IL-6, irisin, and VEGF-A expression. Myostatin is downregulated by both aerobic and anaerobic exercise. This review provides a rationale for developing exercise programs or interventions that maintain a balance between aerobic and anaerobic exercise in the elderly.

Modulation of exercise training related adaptation of body composition and regulatory pathways by anabolic steroids

Anabolic steroids have a long history of abuse in amateur and professional athletics. However, their interaction with training and the resulting effects on body composition and tissue adaptation, relying on a concert of factors and pathways, remain under investigation. This study aims at investigating the changes of body composition and the expression of selected genes and pathways essential for this adaptation process. Therefore, male wistar rats were treated with the anabolic steroid metandienone in two groups (n = 16; metandienone, metandienone + exercise) alongside with control groups (n = 16; control, exercise). Following a 6-week steep-angle treadmill training protocol, weight of organs, visceral fat and muscles was determined. M. gastrocnemius was histologically assessed by ATPase staining, mRNA and protein levels of factors of regeneration, hypertrophy and myogenesis and selected master regulators and markers were determined. Results show additive effects of anabolic steroids and exercise on body, tibia and reproductive organs weight. Mm. gastrocnemius and soleus weight was increased by training but not anabolic steroids. Muscle fiber diameter and composition remained unchanged. Visceral fat mass and fat cell size was affected by training and anabolic steroids but no additive effects could be observed. Exercise and anabolic steroids result in a complex regulation of the expression of genes in M. Gastrocnemius involved in skeletal muscle metabolism, hypertrophy, inflammation and regeneration. In summary, our data suggests distinct molecular mechanisms involved in the adaptation of the skeletal muscle to anabolic androgenic steroids and exercise. Metandienone treatment neither results in skeletal muscle hypertrophy nor liver-toxic effects but in an induction of skeletal muscle regeneration and an activation of endocrine negative feedback. Moreover our study demonstrates that visceral fat and bone responds with higher sensitivity to ASS and exercise than the skeletal muscle. This apparent plasticity of adipose and bone tissue rather than skeletal muscle could indicate a potentially superior future role of fat rather than muscle related parameters to detect and AAS abuse in a biologic passport strategy in professional athletes.

The Role of Systemic Inflammation in Cancer-Associated Muscle Wasting and Rationale for Exercise as a Therapeutic Intervention

Progressive skeletal muscle wasting in cancer cachexia involves a process of dysregulated protein synthesis and breakdown.  This catabolism may be the result of mal-nutrition, and an upregulation of both pro-inflammatory cytokines and the ubiquitin proteasome pathway (UPP), which can subsequently increase myostatin and activin A release.  The skeletal muscle wasting associated with cancer cachexia is clinically significant, it can contribute to treatment toxicity or the premature discontinuation of treatments resulting in increases in morbidity and mortality.  Thus, there is a need for further investigation into the pathophysiology of muscle wasting in cancer cachexia to develop effective prophylactic and therapeutic interventions.  Several studies have identified a central role for chronic-systemic inflammation in initiating and perpetuating muscle wasting in patients with cancer.  Interestingly, while exercise has shown efficacy in improving muscle quality, only recently have investigators begun to assess the impact that exercise has on chronic-systemic inflammation.  To put this new information into context with established paradigms, here we review several biological pathways (e.g. dysfunctional inflammatory response, hypothalamus pituitary adrenal axis, and increased myostatin/activin A activity) that may be responsible for the muscle wasting in patients with cancer.  Additionally, we discuss the potential impact that exercise has on these pathways in the treatment of cancer cachexia.  Exercise is an attractive intervention for muscle wasting in this population, partially because it disrupts chronic-systemic inflammation mediated catabolism.  Most importantly, exercise is a potent stimulator of muscle synthesis, and therefore this therapy may reverse muscle damage caused by cancer cachexia. 

Irisin and myostatin responses to acute high-intensity interval exercise in humans

Background The purpose of this study was to investigate irisin and myostatin responses to acute high-intensity interval exercise. Materials and methods Ten male professional kick-boxers aged between 18 and 24 years and 10 sedentary males with similar age and body weight participated in the present study. Participants performed 4 × 30-s Wingate test separated with 4 min of rest. Blood samples were taken immediately before and after exercise, and 3 and 6 h of recovery. Results and conclusion At rest, irisin levels were higher in the kick-boxers (p < 0.05). Immediately after the exercise, irisin levels were decreased in both groups (p < 0.05). A trend toward a return to baseline appeared after 3 h of recovery in the kick-boxers (p < 0.05). At rest, myostatin concentrations were not different between the groups (p > 0.05). Immediately after the exercise, myostatin levels were increased in both groups (p < 0.05). A trend toward a return to baseline appeared after 3 h of recovery in the kick-boxers (p < 0.05). Acute high-intensity interval exercise decreased irisin levels and increased myostatin levels.

Myokine Expression in Muscle and Myotubes in Response to Exercise Stimulation

PURPOSE

Myokines have been shown to affect muscle physiology and exert systemic effects. We endeavored to investigate a panel of myokine mRNA expression after a single exercise bout (studies 1 and 2) to measure myokine mRNA in primary human myotubes in an in vitro exercise model (study 2).

METHODS

Vastus lateralis muscle biopsies were obtained from 20 healthy males (age, 24.0 ± 4.5 yr; BMI, 23.6 ± 1.8 kg·m)(-2) before and after a single exercise bout (650 kcal at 50% V˙O2max). Primary myotubes from active and sedentary male donors were treated with a pharmacological cocktail (palmitate, forskolin, and ionomycin (PFI)) to mimic exercise-stimulated contractions in vitro.

RESULTS

Interleukin 6 and 8 (IL-6 and IL-8), leukocyte-inducing factor, and connective tissue growth factor (CTGF) mRNA levels increased approximately 10-fold after a single exercise bout (all P < 0.001), whereas myostatin levels decreased (P < 0.05). Key correlations between myokine expression and parameters of muscle and whole-body physiology were found: myostatin versus skeletal muscle citrate synthase activity (r = -0.69, P < 0.001), V˙O2max (r = -0.64, P = 0.002) and the percentage of Type I fibers (r = -0.55, P = 0.01); IL-6 versus the RER (r = 0.45, P = 0.04), homeostatic model assessment of insulin resistance (r = 0.44, P = 0.05), and serum lactate (r = 0.50, P = 0.02). Myokine expressions in myotubes from sedentary donors for CTGF and myostatin decreased, whereas IL-6 and IL-8 increased after PFI treatment. In myotubes from active donors, myokine expression increased for IL-6, CTGF, and myostatin but decreased for IL-8 after PFI treatment.

CONCLUSION

These data offer insight into the differences in regulation of myokine expression and their possible physiologic relationships.

microRNAs: Modulators of the underlying pathophysiology of sarcopenia?

Skeletal muscle homeostasis depends on an intricate balance between muscle hypertrophy, atrophy and regeneration. As we age, maintenance of muscle homeostasis is perturbed, resulting in a loss of muscle mass and function, termed sarcopenia. Individuals with sarcopenia exhibit impaired balance, increased falls (leading to subsequent injury) and an overall decline in quality of life. The mechanisms mediating sarcopenia are still not fully understood but clarity in our understanding of the precise pathophysiological changes occurring during skeletal muscle ageing has improved dramatically. Advances in transcriptomics has highlighted significant deregulation in skeletal muscle gene expression with ageing, suggesting epigenetic alterations may play a crucial and potentially causative role in the skeletal muscle ageing process. microRNAs (miRNAs, miRs), novel regulators of gene expression, can modulate many processes in skeletal muscle, including myogenesis, tissue regeneration and cellular programming. Expression of numerous evolutionary conserved miRNAs is disrupted in skeletal muscle with age. Given that a single miRNA can simultaneously affect the functionality of multiple signaling pathways, miRNAs are potent modulators of pathophysiological changes. miRNA-based interventions provide a promising new therapeutic strategy against alterations in muscle homeostasis. The aim of this review is two-fold; firstly to outline the latest understanding of the pathophysiological alterations impacting the deregulation of skeletal muscle mass and function with ageing, and secondly, to highlight the mounting evidence for a role of miRNAs in modulating muscle mass, and the need to explore their specific role in sarcopenia.

Skeletal muscle as an endocrine organ: PGC-1α, myokines and exercise

An active lifestyle is crucial to maintain health into old age; inversely, sedentariness has been linked to an elevated risk for many chronic diseases. The discovery of myokines, hormones produced by skeletal muscle tissue, suggests the possibility that these might be molecular mediators of the whole body effects of exercise originating from contracting muscle fibers. Even though less is known about the sedentary state, the lack of contraction-induced myokines or the production of a distinct set of hormones in the inactive muscle could likewise contribute to pathological consequences in this context. In this review, we try to summarize the most recent developments in the study of muscle as an endocrine organ and speculate about the potential impact on our understanding of exercise and sedentary physiology, respectively. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Mixed lactate and caffeine compound increases satellite cell activity and anabolic signals for muscle hypertrophy

We examined whether a mixed lactate and caffeine compound (LC) could effectively elicit proliferation and differentiation of satellite cells or activate anabolic signals in skeletal muscles. We cultured C2C12 cells with either lactate or LC for 6 h. We found that lactate significantly increased myogenin and follistatin protein levels and phosphorylation of P70S6K while decreasing the levels of myostatin relative to the control. LC significantly increased protein levels of Pax7, MyoD, and Ki67 in addition to myogenin, relative to control. LC also significantly increased follistatin expression relative to control and stimulated phosphorylation of mTOR and P70S6K. In an in vivo study, male F344/DuCrlCrlj rats were assigned to control (Sed, n = 10), exercise (Ex, n = 12), and LC supplementation (LCEx, n = 13) groups. LC was orally administered daily. The LCEx and Ex groups were exercised on a treadmill, running for 30 min at low intensity every other day for 4 wk. The LCEx group experienced a significant increase in the mass of the gastrocnemius (GA) and tibialis anterior (TA) relative to both the Sed and Ex groups. Furthermore, the LCEx group showed a significant increase in the total DNA content of TA compared with the Sed group. The LCEx group experienced a significant increase in myogenin and follistatin expression of GA relative to the Ex group. These results suggest that administration of LC can effectively increase muscle mass concomitant with elevated numbers of myonuclei, even with low-intensity exercise training, via activated satellite cells and anabolic signals.

Alleviating exercise-induced muscular stress using neat and processed bee pollen: oxidative markers, mitochondrial enzymes, and myostatin expression in rats

BACKGROUND

The current study was designed to investigate the influence of monofloral Indian mustard bee pollen (MIMBP) and processed monofloral Indian mustard bee pollen (PMIMBP) supplementation on chronic swimming exercise-induced oxidative stress implications in the gastrocnemius muscle of Wistar rats.

METHODS

MIMBP was processed with an edible lipid-surfactant mixture (Captex 355:Tween 80) to increase the extraction of polyphenols and flavonoid aglycones as analyzed by UV spectroscopy and high performance liquid chromatography-photo diode array. Wistar rats in different groups were fed with MIMBP or PMIMBP supplements at a dose of 100 mg/kg, 200 mg/kg and 300 mg/kg individually, while being subjected to chronic swimming exercise for 4 weeks (5 d/wk). Various biochemical [superoxide dismutase (SOD), glutathione (GSH), malonaldehyde (MDA), nitric oxide (NO), and total protein content], mitochondrial (Complex I, II, III, and IV enzyme activity), and molecular (myostatin mRNA expression) parameters were monitored in the gastrocnemius muscle of each group.

RESULTS

Administration of both MIMBP (300 mg/kg) and PMIMBP (100 mg/kg, 200 mg/kg, and 300 mg/kg) wielded an antioxidant effect by significantly improving SOD, GSH, MDA, NO, and total protein levels. Further MIMBP (300 mg/kg) and PMIMBP (200 mg/kg and 300 mg/kg) significantly improved impaired mitochondrial Complex I, II, III, and IV enzyme activity. Significant down-regulation of myostatin mRNA expression by MIMBP (300 mg/kg) and PMIMBP (200 mg/kg and 300 mg/kg) indicates a muscle protectant role in oxidative stress conditions.

CONCLUSION

The study establishes the antioxidant, mitochondrial upregulatory, and myostatin inhibitory effects of both MIMBP and PMIMBP in exercise-induced oxidative stress conditions, suggesting their usefulness in effective management of exercise-induced muscular stress. Further, processing of MIMBP with an edible lipid-surfactant mixture was found to improve the therapeutic efficiency of pollen.

Phenotypic flexibility of skeletal muscle and heart masses and expression of myostatin and tolloid-like proteinases in migrating passerine birds

Migrant birds require large flight muscles and hearts to enhance aerobic capacity and support sustained flight. A potential mechanism for increasing muscle and heart masses during migration in birds is the muscle growth inhibitor myostatin and its metalloproteinase activators, tolloid-like proteinases (TLL-1 and TLL-2). We hypothesized that myostatin, TLL-1 and TLL-2 are downregulated during migration in pectoralis and hearts of migratory passerines to promote hypertrophy. We measured seasonal variation of tissue masses, mRNA expression of myostatin, TLL-1, and TLL-2, and myostatin protein levels in pectoralis muscle and heart for yellow warblers (Setophaga petechia), warbling vireos (Vireo gilvus), and yellow-rumped warblers (Setophaga coronata). Pectoralis mass was greatest in spring for warbling vireos and yellow warblers, but was stable between spring and fall for yellow-rumped warblers. Heart mass was higher in spring than in fall for yellow-rumped warblers, lowest in fall for warbling vireos, and seasonally stable for yellow warblers. Pectoralis and heart mRNA expression of myostatin and the TLLs did not differ significantly for any of the three species, offering little support for our hypothesis for a prominent role for myostatin in regulating migration-induced variation in pectoralis and heart masses. In contrast, pectoralis myostatin protein levels were lowest in spring for all three species, consistent with our hypothesis. Myostatin protein levels in heart, however, were seasonally stable for warbling vireos and yellow warblers, and increased in spring relative to fall for yellow-rumped warblers. These data offer mixed support for our hypothesis for the pectoralis, but suggest that myostatin is not a prominent regulator of migration-induced heart hypertrophy. Moreover, the different seasonal patterns for pectoralis mRNA and protein expression suggest that post-transcriptional modification of myostatin may contribute to pectoralis mass regulation during migration.

Aerobic exercise training prevents heart failure-induced skeletal muscle atrophy by anti-catabolic, but not anabolic actions

Background

Heart failure (HF) is associated with cachexia and consequent exercise intolerance. Given the beneficial effects of aerobic exercise training (ET) in HF, the aim of this study was to determine if the ET performed during the transition from cardiac dysfunction to HF would alter the expression of anabolic and catabolic factors, thus preventing skeletal muscle wasting.

Methods and Results

We employed ascending aortic stenosis (AS) inducing HF in Wistar male rats. Controls were sham-operated animals. At 18 weeks after surgery, rats with cardiac dysfunction were randomized to 10 weeks of aerobic ET (AS-ET) or to an untrained group (AS-UN). At 28 weeks, the AS-UN group presented HF signs in conjunction with high TNF-α serum levels; soleus and plantaris muscle atrophy; and an increase in the expression of TNF-α, NFκB (p65), MAFbx, MuRF1, FoxO1, and myostatin catabolic factors. However, in the AS-ET group, the deterioration of cardiac function was prevented, as well as muscle wasting, and the atrophy promoters were decreased. Interestingly, changes in anabolic factor expression (IGF-I, AKT, and mTOR) were not observed. Nevertheless, in the plantaris muscle, ET maintained high PGC1α levels.

Conclusions

Thus, the ET capability to attenuate cardiac function during the transition from cardiac dysfunction to HF was accompanied by a prevention of skeletal muscle atrophy that did not occur via an increase in anabolic factors, but through anti-catabolic activity, presumably caused by PGC1α action. These findings indicate the therapeutic potential of aerobic ET to block HF-induced muscle atrophy by counteracting the increased catabolic state.

Single muscle fiber gene expression with run taper

This study evaluated gene expression changes in gastrocnemius slow-twitch myosin heavy chain I (MHC I) and fast-twitch (MHC IIa) muscle fibers of collegiate cross-country runners (n = 6, 20±1 y, VO_2max = 70±1 ml•kg⁻¹•min⁻¹) during two distinct training phases. In a controlled environment, runners performed identical 8 kilometer runs (30∶18±0∶30 min:s, 89±1% HR_max) while in heavy training (∼72 km/wk) and following a 3 wk taper. Training volume during the taper leading into peak competition was reduced ∼50% which resulted in improved race times and greater cross-section and improved function of MHC IIa fibers. Single muscle fibers were isolated from pre and 4 hour post run biopsies in heavily trained and tapered states to examine the dynamic acute exercise response of the growth-related genes Fibroblast growth factor-inducible 14 (FN14), Myostatin (MSTN), Heat shock protein 72 (HSP72), Muscle ring-finger protein-1 (MURF1), Myogenic factor 6 (MRF4), and Insulin-like growth factor 1 (IGF1) via qPCR. FN14 increased 4.3-fold in MHC IIa fibers with exercise in the tapered state (P<0.05). MSTN was suppressed with exercise in both fiber types and training states (P<0.05) while MURF1 and HSP72 responded to running in MHC IIa and I fibers, respectively, regardless of training state (P<0.05). Robust induction of FN14 (previously shown to strongly correlate with hypertrophy) and greater overall transcriptional flexibility with exercise in the tapered state provides an initial molecular basis for fast-twitch muscle fiber performance gains previously observed after taper in competitive endurance athletes.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Combinatory effects of siRNA-induced myostatin inhibition and exercise on skeletal muscle homeostasis and body composition

Abstract Inhibition of myostatin (Mstn) stimulates skeletal muscle growth, reduces body fat, and induces a number of metabolic changes. However, it remains unexplored how exercise training modulates the response to Mstn inhibition. The aim of this study was to investigate how siRNA-mediated Mstn inhibition alone but also in combination with physical activity affects body composition and skeletal muscle homeostasis. Adult mice were treated with Mstn-targeting siRNA and subjected to a treadmill-based exercise protocol for 4 weeks. Effects on skeletal muscle and fat tissue, expression of genes, and serum concentration of proteins involved in myostatin signaling, skeletal muscle homeostasis, and lipid metabolism were investigated and compared with Mstn(-/-) mice. The combination of siRNA-mediated Mstn knockdown and exercise induced skeletal muscle hypertrophy, which was associated with an upregulation of markers for satellite cell activity. SiRNA-mediated Mstn knockdown decreased visceral fat and modulated lipid metabolism similar to effects observed in Mstn(-/-) mice. Myostatin did not regulate its own expression via an autoregulatory loop, however, Mstn knockdown resulted in a decrease in the serum concentrations of myostatin propeptide, leptin, and follistatin. The ratio of these three parameters was distinct between Mstn knockdown, exercise, and their combination. Taken together, siRNA-mediated Mstn knockdown in combination with exercise stimulated skeletal muscle hypertrophy. Each intervention or their combination induced a specific set of adaptive responses in the skeletal muscle and fat metabolism which could be identified by marker proteins in serum.

Exercise is the real polypill

The concept of a "polypill" is receiving growing attention to prevent cardiovascular disease. Yet similar if not overall higher benefits are achievable with regular exercise, a drug-free intervention for which our genome has been haped over evolution. Compared with drugs, exercise is available at low cost and relatively free of adverse effects. We summarize epidemiological evidence on the preventive/therapeutic benefits of exercise and on the main biological mediators involved.

Differential gene expression in high- and low-active inbred mice

Numerous candidate genes have been suggested in the recent literature with proposed roles in regulation of voluntary physical activity, with little evidence of these genes' functional roles. This study compared the haplotype structure and expression profile in skeletal muscle and brain of inherently high- (C57L/J) and low- (C3H/HeJ) active mice. Expression of nine candidate genes [Actn2, Actn3, Casq1, Drd2, Lepr, Mc4r, Mstn, Papss2, and Glut4 (a.k.a. Slc2a4)] was evaluated via RT-qPCR. SNPs were observed in regions of Actn2, Casq1, Drd2, Lepr, and Papss2; however, no SNPs were located in coding sequences or associated with any known regulatory sequences. In mice exposed to a running wheel, Casq1 (P = 0.0003) and Mstn (P = 0.002) transcript levels in the soleus were higher in the low-active mice. However, when these genes were evaluated in naïve animals, differential expression was not observed, demonstrating a training effect. Among naïve mice, no genes in either tissue exhibited differential expression between strains. Considering that no obvious SNP mechanisms were determined or differential expression was observed, our results indicate that genomic structural variation or gene expression data alone is not adequate to establish any of these genes' candidacy or causality in relation to regulation of physical activity.

AMPK activation by AICAR inhibits myogenic differentiation and myostatin expression in cattle

AMP-activated protein kinase (AMPK) regulates metabolism in skeletal muscle, and myostatin (MSTN) negatively regulates skeletal muscle development and growth. In the present study, AMPK activation and the relationship between AMPK and MSTN during myogenic differentiation were investigated in cultures derived from bovine skeletal muscle. Myoblasts capable of forming myotubes were obtained from bovine skeletal muscle and treated with AICAR to activate AMPK, resulting in suppressed myotube formation. AICAR treatment significantly reduced the expression of MSTN mRNA during myogenic differentiation. Combined treatment with AICAR and MSTN suppressed myotube formation to a greater extent than AICAR alone. SB431542, an inhibitor of MSTN signaling, promoted myotube formation during myogenic differentiation. However, simultaneous treatment with AICAR blocked this effect of SB431542. Therefore, AMPK activation inhibits myogenic differentiation but may suppress MSTN expression to balance muscle development.

Expression and function of myostatin in obesity, diabetes, and exercise adaptation

Myostatin is a member of the transforming growth factor-β/bone morphogenetic protein (TGF-β/BMP) superfamily of secreted factors that functions as a potent inhibitor of skeletal muscle growth. Moreover, considerable evidence has accumulated that myostatin also regulates metabolism and that its inhibition can significantly attenuate the progression of obesity and diabetes. Although at least part of these effects on metabolism can be attributable to myostatin's influence over skeletal muscle growth and therefore on the total volume of metabolically active lean body mass, there is mounting evidence that myostatin affects the growth and metabolic state of other tissues, including the adipose and the liver. In addition, recent work has explored the role of myostatin in substrate mobilization, uptake, and/or utilization of muscle independent of its effects on body composition. Finally, the effects of both endurance and resistance exercise on myostatin expression, as well as the potential role of myostatin in the beneficial metabolic adaptations occurring in response to exercise, have also begun to be delineated in greater detail. The purpose of this review was to summarize the work to date on the expression and function of myostatin in obesity, diabetes, and exercise adaptation.

Endocrine characterization of the designer steroid methyl-1-testosterone: investigations on tissue-specific anabolic-androgenic potency, side effects, and metabolism

Various products containing rarely characterized anabolic steroids are nowadays marketed as dietary supplements. Herein, the designer steroid methyl-1-testosterone (M1T) (17β-hydroxy-17α-methyl-5α-androst-1-en-3-one) was identified, and its biological activity, potential adverse effects, and metabolism were investigated. The affinity of M1T toward the androgen receptor (AR) was tested in vitro using a yeast AR transactivation assay. Its tissue-specific androgenic and anabolic potency and potential adverse effects were studied in a Hershberger assay (sc or oral), and tissue weights and selected molecular markers were investigated. Determination of M1T and its metabolites was performed by gas chromatography mass spectrometry. In the yeast AR transactivation assay, M1T was characterized as potent androgen. In rats, M1T dose-dependently stimulated prostate and levator ani muscle weight after sc administration. Oral administration had no effect but stimulated proliferation in the prostate and modulated IGF-I and AR expression in the gastrocnemius muscle in a dose-dependent manner. Analysis of tyrosine aminotransferase expression provided evidence for a strong activity of M1T in the liver (much higher after oral administration). In rat urine, 17α-methyl-5α-androstane-3α,17β-diol, M1T, and a hydroxylated metabolite were identified. In humans, M1T was confirmed in urine in addition to its main metabolites 17α-methyl-5α-androst-1-ene-3α,17β-diol and 17α-methyl-5α-androstane-3α,17β-diol. Additionally, the corresponding 17-epimers as well as 17β-hydroxymethyl-17α-methyl-18-nor-5α-androsta-1,13-dien-3-one and its 17-epimer were detected, and their elimination kinetics was monitored. It was demonstrated that M1T is a potent androgenic and anabolic steroid after oral and sc administration. Obviously, this substance shows no selective AR modulator characteristics and might exhibit liver toxicity, especially after oral administration.

Migration- and exercise-induced changes to flight muscle size in migratory birds and association with IGF1 and myostatin mRNA expression

Seasonal adjustments to muscle size in migratory birds may result from preparatory physiological changes or responses to changed workloads. The mechanisms controlling these changes in size are poorly understood. We investigated some potential mediators of flight muscle size (myostatin and insulin-like growth factor, IGF1) in pectoralis muscles of wild wintering or migrating white-throated sparrows (Zonotrichia albicollis), captive white-throated sparrows that were photoperiod manipulated to be in a `wintering' or `migratory' (Zugunruhe) state, and captive European starlings (Sturnus vulgaris) that were either exercised for 2 weeks in a wind tunnel or untrained. Flight muscle size increased in photo-stimulated `migrants' and in exercised starlings. Acute exercise but not long-term training caused increased expression of IGF1, but neither caused a change in expression of myostatin or its metalloprotease activator TLL1. Photo-stimulated `migrant' sparrows demonstrated increased expression of both myostatin and IGF1, but wild sparrows exhibited no significant seasonal changes in expression of either myostatin or IGF1. Additionally, in both study species we describe several splice variants of myostatin that are shared with distantly related bird species. We demonstrate that their expression patterns are not different from those of the typical myostatin, suggesting that they have no functional importance and may be mistakes of the splicing machinery. We conclude that IGF1 is likely to be an important mediator of muscle phenotypic flexibility during acute exercise and during endogenous, seasonal preparation for migration. The role of myostatin is less clear, but its paradoxical increase in photo-stimulated `migrants' may indicate a role in seasonal adjustments of protein turnover.

Post-exercise changes in myostatin and actRIIB expression in obese insulin-resistant rats

We evaluated the expression of MSTN and ActRIIB mRNA in muscle and adipose tissue in diet-induced obesity and insulin resistance in rats subjected to exercise. There was no difference in the expression of MSTN between exercised and sedentary high-fat fed rats in muscle after swimming training. The expression of ActRIIB mRNA in muscle was not significantly different among the groups. In BAT, MSTN mRNA expression was higher in exercised high-fat fed group (EHF) compared with sedentary high-fat fed group (SHF). ActRIIB mRNA expression in BAT was higher in EHF compared with SHF. In mesenteric fat, MSTN mRNA was lower in EHF compared with SHF and ActRIIB mRNA was lower in EHF compared with SHF. In conclusion, the results demonstrate that the expression of MSTN and ActRIIB mRNA changes in both adipose tissue and skeletal muscle in diet-induced obese and exercised rats and suggest the participation of MSTN in energy homeostasis.

Myostatin decreases with aerobic exercise and associates with insulin resistance

PURPOSE

There is mounting evidence that skeletal muscle produces and secretes biologically active proteins or "myokines" that facilitate metabolic cross talk between organ systems. The increased expression of myostatin, a secreted anabolic inhibitor of muscle growth and development, has been associated with obesity and insulin resistance. Despite these intriguing findings, there have been few studies linking myostatin and insulin resistance.

METHODS

To explore this relationship in more detail, we quantified myostatin protein in muscle and plasma from 10 insulin-resistant, middle-aged (53.1 ± 5.5 yr) men before and after 6 months of moderate aerobic exercise training (1200 kcal·wk−¹ at 40%-55% VO2peak). To establish a cause-effect relationship, we also injected C57/Bl6 male mice with high physiological levels of recombinant myostatin protein.

RESULTS

Myostatin protein levels were shown to decrease in muscle (37%, P = 0.042, n = 10) and matching plasma samples (from 28.7 ng·mL−¹ pretraining to 22.8 ng·mL−¹ posttraining, P = 0.003, n = 9) with aerobic exercise. Furthermore, the strong correlation between plasma myostatin levels and insulin sensitivity (R² = 0.82, P < 0.001, n = 9) suggested a cause-effect relationship that was subsequently confirmed by inducing insulin resistance in myostatin-injected mice. A modest increase (44%) in plasma myostatin levels was also associated with significant reductions in the insulin-stimulated phosphorylation of Akt (Thr308) in both muscle and liver of myostatin-treated animals.

CONCLUSIONS

These findings indicate that both muscle and plasma myostatin protein levels are regulated by aerobic exercise and, furthermore, that myostatin is in the causal pathway of acquired insulin resistance with physical inactivity.

Increased mass of slow-type skeletal muscles and depressed myostatin gene expression in cold-tolerant chicks

Temperature is maintained in birds by skeletal muscle shivering as well as by non-shivering thermogenesis in a cold environment because they lack brown adipose tissue, which is a mammalian thermogenic organ. Chicks acquire cold tolerance after their skeletal muscles mature. Here, we found that muscle fibers transformed to the slow-twitch type with increasing gene expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), and that the mass increased with decreasing myostatin gene expression, in the leg muscles of 7-day-old and younger chicks within 24 h of cold exposure. Muscle fibers did not transform and the mass did not increase within 24 h of cold exposure in muscles from chicks older than 8 days of age. Myostatin mRNA expression remained depressed in cold-tolerant muscles for 24 h, whereas cold-enhanced growth of the muscle continued for 48 h. Myostatin expression was depressed and muscle mass was increased only in chick leg muscles that comprised both fast- and slow-twitch fibers. These results suggest that the acute regulation of PGC-1alpha and myostatin gene expression in leg muscles is required for chicks to acquire cold tolerance up to 7 days of age.

The Impact of Pre-Dialytic Endurance Training on Nutritional Status and Quality of Life in Stable Hemodialysis Patients (Sawada Study)

BACKGROUND

Protein-energy malnutrition and decreased quality of life (QOL) are common in hemodialysis (HD) patients. Although several studies have proved that regular exercise has beneficial effects, few control studies have shown the effects of exercise training on the nutritional status and QOL in HD patients.

METHODS

Fifty-five HD patients were recruited, and 22 of them were trained to exercise on an ergometer prior to dialysis three times a week for one year. Serum albumin levels, creatinine generation rate (CGR), and the Short Form 36 were assessed as outcome measures.

RESULTS

The serum albumin levels and CGR increased in the training group compared with baseline. The QOL scores also increased in half of the physical health and mental health dimensions in the training group.

CONCLUSION

These observations suggest that low-dose, long-term pre-dialytic endurance training might reverse the poor clinical outcome by improving the nutritional status and QOL in HD patients.

Possible roles of myostatin and PGC-1alpha in the increase of skeletal muscle and transformation of fiber type in cold-exposed chicks: expression of myostatin and PGC-1alpha in chicks exposed to cold

This study examined the hypothesis that myostatin and PGC-1alpha are involved in the increase in skeletal muscle mass and transformation of fiber type in cold-exposed chicks. One-week-old chicks were exposed to acute (24h) or long-term (8d) cold at 4 degrees C or kept warm at 30 degrees C. Acute cold exposure induced a significant increase in the skeletal muscle weight and the ratio of slow- to fast-fiber specific troponin I expression (sTnI/fTnI), accompanied by a significant decrease in lactate dehydrogenase activity. Expression of myostatin mRNA in the muscle was significantly lower in cold-exposed chicks than in the controls, whereas PGC-1alpha mRNA expression was significantly enhanced. These changes in the gene expression rapidly returned to the levels of the control chicks after the end of cold exposure, whereas the changes in fiber type and enzymatic activity were not resumed within 24h after removal of cold exposure. On the other hand, long-term exposure to cold resulted in a remarkable increase in skeletal muscle weight, accompanied by a significant increase in the ratio of sTnI/fTnI and the enzymatic activities of cytochrome oxidase and lactate dehydrogenase. However, the expression level of myostatin mRNA in cold-exposed chicks was not different from that in their age-matched control chicks and that of PGC-1alpha mRNA was significantly lower than in the controls. These results indicate that myostatin and PGC-1alpha expression in the skeletal muscle rapidly change in response to acute cold, suggesting the possibility that these two genes could be involved in the increase in muscle mass and transformation of fiber type, respectively, at the initial stage of adaptation in cold-exposed chicks.

Histomorphometric analysis of the response of rat skeletal muscle to swimming, immobilization and rehabilitation

The objective of the present study was to determine to what extent, if any, swimming training applied before immobilization in a cast interferes with the rehabilitation process in rat muscles. Female Wistar rats, mean weight 260.52 +/- 16.26 g, were divided into 4 groups of 6 rats each: control, 6 weeks under baseline conditions; trained, swimming training for 6 weeks; trained-immobilized, swimming training for 6 weeks and then immobilized for 1 week; trained-immobilized-rehabilitated, swimming training for 6 weeks, immobilized for 1 week and then remobilized with swimming for 2 weeks. The animals were then sacrificed and the soleus and tibialis anterior muscles were dissected, frozen in liquid nitrogen and processed histochemically (H&E and mATPase). Data were analyzed statistically by the mixed effects linear model (P < 0.05). Cytoarchitectural changes such as degenerative characteristics in the immobilized group and regenerative characteristics such as centralized nucleus, fiber size variation and cell fragmentation in the groups submitted to swimming were more significant in the soleus muscle. The diameters of the lesser soleus type 1 and type 2A fibers were significantly reduced in the trained-immobilized group compared to the trained group (P < 0.001). In the tibialis anterior, there was an increase in the number of type 2B fibers and a reduction in type 2A fibers when trained-immobilized rats were compared to trained rats (P < 0.001). In trained-immobilized-rehabilitated rats, there was a reduction in type 2B fibers and an increase in type 2A fibers compared to trained-immobilized rats (P < 0.009). We concluded that swimming training did not minimize the deleterious effects of immobilization on the muscles studied and that remobilization did not favor tissue re-adaptation.

Impact of exercise training on myostatin expression in the myocardium and skeletal muscle in a chronic heart failure model

AIMS

In late-stage chronic heart failure (CHF), elevated cytokines and cachexia are often observed. Several studies have shown that exercise training exerts beneficial effects on skeletal muscle in this setting. Furthermore, it has been shown that the expression of myostatin, a key regulator of skeletal muscle mass, is increased in a variety of cachectic states. This study aimed to investigate the expression of myostatin in CHF, the influence of exercise training on myostatin levels, and regulation of myostatin by tumour necrosis factor-alpha (TNF-alpha).

METHODS AND RESULTS

In an animal model of CHF (LAD-ligation model), protein expression of myostatin was elevated 2.4-fold in the skeletal muscle and more than four-times in the myocardium, compared with control (Co). Exercise training on a treadmill over 4 weeks led to a significant reduction in myostatin protein expression in the skeletal muscle and the myocardium of CHF animals, with values returning to baseline levels. In differentiated C2C12 cells, TNF-alpha induced the expression of myostatin through a p38MAPK-dependent pathway involving nuclear factor kappa-B (NF-kappaB). The increased TNF-alpha mRNA levels in the skeletal muscle of CHF animals correlated significantly with myostatin expression.

CONCLUSION

These alterations in myostatin expression in the skeletal and heart muscle following exercise training could help to explain the beneficial anti-catabolic effects of exercise training in CHF.

Presence, actions, and regulation of myostatin in rat uterus and myometrial cells

Myostatin, a member of the TGF-beta superfamily of proteins, is known to suppress skeletal muscle mass and myocyte proliferation. The muscular component of the uterus is the myometrium, a tissue that regulates its mass in response to different physiological conditions under the influence of sex steroids. Recently, our laboratory reported effects of activin-A, another TGF-beta family member, on signalling and proliferation of rat uterine explants and human myometrial cell lines in culture. Here, we explore the expression, actions, and regulation of myostatin in uterine smooth muscle. Myostatin mRNA was demonstrated to be expressed in a myometrial cell line, pregnant human myometrial 1 cell line (PHM1). Functional assays showed that myostatin induced phosphorylation of Smad-2 and reduced proliferation of PHM1 number in a time and dose-dependent manner. Furthermore, myostatin activated smad-2 specific signalling pathways in rat uterine explants. To expand on our in vitro findings, we found that myostatin is expressed in rat uterus and determined that myostatin mRNA expression varies as a function of the phase of the estrous cycle. Uterine levels of myostatin peaked during late estrus and were the lowest at proestrus. Ovariectomy increased myostatin expression; estrogen treatment strongly decreased myostatin levels, whereas progesterone weakly decreased myostatin expression. In conclusion, myometrial cells are myostatin sensitive, myostatin mRNA levels are modulated in vivo in rats during the estrous cycle, and in response to steroid deprivation and replacement.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Time course of proteolytic, cytokine, and myostatin gene expression after acute exercise in human skeletal muscle

The aim of this study was to examine the time course induction of select proteolytic [muscle ring finger-1 (MuRF-1), atrogin-1, forkhead box 3A (FOXO3A), calpain-1, calpain-2], myostatin, and cytokine (IL -6, -8, -15, and TNF-α) mRNA after an acute bout of resistance (RE) or run (RUN) exercise. Six experienced RE (25 ± 4 yr, 74 ± 14 kg, 1.71 ± 0.11 m) and RUN (25 ± 4 yr, 72 ± 5 kg, 1.81 ± 0.07 m) subjects had muscle biopsies from the vastus lateralis (RE) or gastrocnemius (RUN) before, immediately after, and 1, 2, 4, 8, 12, and 24 h postexercise. RE increased ( P < 0.05) mRNA expression of MuRF-1 early (3.5-fold, 1–4 h), followed by a decrease in atrogin-1 (3.3-fold) and FOXO3A (1.7-fold) 8–12 h postexercise. Myostatin mRNA decreased (6.3-fold; P < 0.05) from 1 to 24 h postexercise, whereas IL-6, IL-8, and TNF-α mRNA were elevated 2–12 h. RUN increased ( P < 0.05) MuRF-1 (3.6-fold), atrogin-1 (1.6-fold), and FOXO3A (1.9-fold) 1–4 h postexercise. Myostatin was suppressed (3.6-fold; P < 0.05) 8–12 h post-RUN. The cytokines exhibited a biphasic response, with immediate elevation ( P < 0.05) of IL-6, IL-8, and TNF-α, followed by a second elevation ( P < 0.05) 2–24 h postexercise. In general, the timing of the gene induction indicated early elevation of proteolytic genes, followed by prolonged elevation of cytokines and suppression of myostatin. These data provide basic information for the timing of human muscle biopsy samples for gene expression studies involving exercise. Furthermore, this information suggests a greater induction of proteolytic genes following RUN compared with RE.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Load-mediated downregulation of myostatin mRNA is not sufficient to promote myofiber hypertrophy in humans: a cluster analysis

Myostatin is a potent inhibitor of myogenesis; thus differential expression might be expected across individuals varying in responsiveness to myogenic stimuli. We hypothesized that myostatin would be differentially regulated across humans with markedly different hypertrophic responses to resistance training (RT; 16 wk). Targets were assessed in muscle biopsies at baseline (T1) and 24 h after the first (T2) and last (T3) loading bouts in previously untrained subjects statistically clustered based on mean myofiber hypertrophy as extreme (Xtr; n = 17, 2,475 microm(2)), modest (n = 32, 1,111 microm(2)), and nonresponders (n = 17, -16 microm(2)). We assessed protein levels of latent full-length myostatin protein complex and its propeptide; mRNA levels of myostatin, cyclin D1, p21(cip1), p27(kip1), and activin receptor IIB; and serum myostatin protein concentration. Total RNA concentration increased by T3 in nonresponders (37%) and modest responders (40%), while it increased acutely (T2) only in Xtr (26%), remaining elevated at T3 (40%). Myostatin mRNA decreased at T2 (-44%) and remained suppressed at T3 (-52%), but not differentially across clusters. Cyclin D1 mRNA increased robustly by T2 (38%) and T3 (74%). The increase at T2 was driven by Xtr (62%, P < 0.005), and Xtr had the largest elevation at T3 (82%, P < 0.001). No effects were found for other target transcripts. Myostatin protein complex increased 44% by T3 (P < 0.001), but not differentially by cluster. Myostatin protein complex propeptide and circulating myostatin were not influenced by RT or cluster. Overall, we found no compelling evidence that myostatin is differentially regulated in humans demonstrating robust RT-mediated myofiber hypertrophy vs. those more resistant to growth.

Short-term strength training and the expression of myostatin and IGF-I isoforms in rat muscle and tendon: differential effects of specific contraction types

In skeletal muscle, an increased expression of insulin like growth factor-I isoforms IGF-IEa and mechano-growth factor (MGF) combined with downregulation of myostatin is thought to be essential for training-induced hypertrophy. However, the specific effects of different contraction types on regulation of these factors in muscle are still unclear, and in tendon the functions of myostatin, IGF-IEa, and MGF in relation to training are unknown. Female Sprague-Dawley rats were subjected to 4 days of concentric, eccentric, or isometric training ( n = 7–9 per group) of the medial gastrocnemius, by stimulation of the sciatic nerve during general anesthesia. mRNA levels for myostatin, IGF-IEa, and MGF in muscle and Achilles' tendon were measured by real-time RT-PCR. Muscle myostatin mRNA decreased in response to all types of training (2- to 8-fold) ( P < 0.05), but the effect of eccentric training was greater than concentric and isometric training ( P < 0.05). In tendon, myostatin mRNA was detected, but no changes were seen after exercise. IGF-IEa and MGF increased in muscle (up to 15-fold) and tendon (up to 4-fold) in response to training ( P < 0.01). In tendon no difference was seen between training types, but in muscle the effect of eccentric training was greater than concentric training for both IGF-IEa and MGF ( P < 0.05), and for IGF-IEa isometric training had greater effect than concentric ( P < 0.05). The results indicate a possible role for IGF-IEa and MGF in adaptation of tendon to training, and the combined changes in myostatin and IGF-IEa/MGF expression could explain the important effect of eccentric actions for muscle hypertrophy.

Effect of swimming on myostatin expression in white and red gastrocnemius muscle and in cardiac muscle of rats

The aim of this study was to test the hypothesis that swimming training might impact differentially myostatin expression in skeletal muscles, depending on fibre type composition, and in cardiac muscle of rats. Myostatin expression was analysed by real time reverse transcriptase-polymerase chain reaction, Western blot and immunohistochemistry of the red deep portion (mainly composed of slow and type II A fibres) and in the superficial, white portion (composed of fast type II X and II B fibres) of the gastrocnemius muscle in adult male Wistar rats: (i) subjected to two consecutive swimming bouts for 3 h; (ii) subjected to intensive swimming training for 4 weeks; and (iii) sedentary control rats. Myostatin mRNA content was in all cases higher in white than in red muscles. Two bouts of swimming did not alter myostatin expression, whereas swimming training for 4 weeks resulted in a significant reduction of myostatin mRNA contents, significant both in white and red muscles but more pronounced in white muscles. Western blot did not detect any change in the amount of myostatin protein. Immunohistochemistry showed that, in control rats, myostatin was localized in presumptive satellite cells of a few muscle fibres. After training, the number of myostatin-positive spots decreased significantly. Myostatin mRNA content in cardiac muscle was lower than in skeletal muscle and was significantly increased by swimming training. In conclusion, the results obtained showed that intense training caused a decreased expression of myostatin mRNA in white and red skeletal muscles but an increase in cardiac muscle.

Resistance training, and IGF involvement in the maintenance of muscle mass during the aging process

Sarcopenia is the decline of muscle mass and strength with age. Sarcopenia leads to significant impairment in the ability to carry out normal daily function and thus there is a great need for interventions that will lead to muscle regeneration and repair in the aging population. Age-related sarcopenia in humans, characterized by loss of type I and type II muscle fibers and a decrease in fiber cross-sectional area primarily in type II fibers, can be attenuated by mechanical load on the muscle, which increases cross-sectional area of the remaining fibers, but does not restore fiber numbers characteristic of young muscle. Considerable evidence also implicates age-related declines in muscle insulin-like growth factor action in sarcopenia. IGF-I promotes myoblast proliferation, differentiation, and protein accretion in muscle through multiple signaling mechanisms, including the PI3-kinase, MAP kinase and calcineurin pathways. Exercise and injury induce increases in IGF-I, IGF-I receptors and IGF-I-activated signaling pathways. Although there is evidence that aging muscle retains the ability to synthesize IGF-I, there is also evidence that aging may be associated with attenuation of the ability of exercise to induce an isoform of IGF-I that promotes satellite cell proliferation. Moreover, aging muscle may be resistant to IGF-I, an effect that is reversed by exercise. However, it is clear that over-expression of IGF-I in muscle can protect against age-related sarcopenia.

Work-induced changes in skeletal muscle IGF-1 and myostatin gene expression in uremia

Resistance to growth hormone (GH)-induced insulin-like growth factor-1 (IGF-1) gene expression contributes to uremic muscle wasting. Since exercise stimulates muscle IGF-1 expression independent of GH, we tested whether work overload (WO) could increase skeletal muscle IGF-1 expression in uremia and thus bypass the defective GH action. Furthermore, to provide insight into the mechanism of uremic wasting and the response to exercise we examined myostatin expression. Unilateral plantaris muscle WO was initiated in uremic and pairfed (PF) normal rats by ablation of a gastrocnemius tendon and adjoining part of this muscle with the contralateral plantaris as a control. Some rats were GH treated for 7 days. WO led to similar gains in plantaris weight in both groups and corrected the uremic muscle atrophy. GH increased plantaris IGF-1 mRNA >twofold in PF rats but the response in uremia was severely attenuated. WO increased the IGF-1 mRNA levels significantly in both uremic and PF groups, albeit less brisk in uremia; however, after 7 days IGF-1 mRNA levels were elevated similarly, >2-fold, in both groups. In the atrophied uremic plantaris muscle basal myostatin mRNA levels were increased significantly and normalized after an increase in WO suggesting a myostatin role in the wasting process. In the hypertrophied uremic left ventricle the basal myostatin mRNA levels were reduced and likely favor the cardiac hypertrophy. Together the findings provide insight into the mechanisms of skeletal muscle wasting in uremia and the hypertrophic response to exercise, and suggest that alterations in the balance between IGF-1 and myostatin play an important role in these processes.

Contractile properties of EDL and soleus muscles of myostatin-deficient mice

Myostatin is a negative regulator of muscle mass. The impact of myostatin deficiency on the contractile properties of healthy muscles has not been determined. We hypothesized that myostatin deficiency would increase the maximum tetanic force (P(o)), but decrease the specific P(o) (sP(o)) of muscles and increase the susceptibility to contraction-induced injury. The in vitro contractile properties of extensor digitorum longus (EDL) and soleus muscles from wild-type (MSTN(+/+)), heterozygous-null (MSTN(+/-)), and homozygous-null (MSTN(-/-)) adult male mice were determined. For EDL muscles, the P(o) of both MSTN(+/-) and MSTN(-/-) mice were greater than the P(o) of MSTN(+/+) mice. For soleus muscles, the P(o) of MSTN(-/-) mice was greater than that of MSTN(+/+) mice. The sP(o) of EDL muscles of MSTN(-/-) mice was less than that of MSTN(+/+) mice. For soleus muscles, however, no difference in sP(o) was observed. Following two lengthening contractions, EDL muscles from MSTN(-/-) mice had a greater force deficit than that of MSTN(+/+) or MSTN(+/-) mice, whereas no differences were observed for the force deficits of soleus muscles. Myostatin-deficient EDL muscles had less hydroxyproline, and myostatin directly increased type I collagen mRNA expression and protein content. The difference in the response of EDL and soleus muscles to myostatin may arise from differences in the levels of a myostatin receptor, activin type IIB. Compared with the soleus, the amount of activin type IIB receptor was approximately twofold greater in EDL muscles. The results support a significant role for myostatin not only in the mass of muscles but also in the contractility and the composition of the extracellular matrix of muscles.

Interaction of contractile activity and training history on mRNA abundance in skeletal muscle from trained athletes

Skeletal muscle displays enormous plasticity to respond to contractile activity with muscle from strength- (ST) and endurance-trained (ET) athletes representing diverse states of the adaptation continuum. Training adaptation can be viewed as the accumulation of specific proteins. Hence, the altered gene expression that allows for changes in protein concentration is of major importance for any training adaptation. Accordingly, the aim of the present study was to quantify acute subcellular responses in muscle to habitual and unfamiliar exercise. After 24-h diet/exercise control, 13 male subjects (7 ST and 6 ET) performed a random order of either resistance (8 x 5 maximal leg extensions) or endurance exercise (1 h of cycling at 70% peak O2 uptake). Muscle biopsies were taken from vastus lateralis at rest and 3 h after exercise. Gene expression was analyzed using real-time PCR with changes normalized relative to preexercise values. After cycling exercise, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (ET approximately 8.5-fold, ST approximately 10-fold, P < 0.001), pyruvate dehydrogenase kinase-4 (PDK-4; ET approximately 26-fold, ST approximately 39-fold), vascular endothelial growth factor (VEGF; ET approximately 4.5-fold, ST approximately 4-fold), and muscle atrophy F-box protein (MAFbx) (ET approximately 2-fold, ST approximately 0.4-fold) mRNA increased in both groups, whereas MyoD (approximately 3-fold), myogenin (approximately 0.9-fold), and myostatin (approximately 2-fold) mRNA increased in ET but not in ST (P < 0.05). After resistance exercise PDK-4 (approximately 7-fold, P < 0.01) and MyoD (approximately 0.7-fold) increased, whereas MAFbx (approximately 0.7-fold) and myostatin (approximately 0.6-fold) decreased in ET but not in ST. We conclude that prior training history can modify the acute gene responses in skeletal muscle to subsequent exercise.

The expression of IGF-I and myostatin mRNAs in skeletal muscle of hypophysectomized and underfed rats during postnatal growth

AIM

To determine the roles of myostatin and insulin-like growth factor-I (IGF-I) during postnatal growth, we examined IGF-I and myostatin mRNA expression in the skeletal muscles of hypophysectomized and underfed rats during postnatal growth.

METHODS

Five-week-old rats were divided into four groups: freely fed control, moderately underfed, severely underfed and hypophysectomized. Four weeks later, blood and muscle samples were gathered to determine serum IGF-I, myosin heavy chain (MHC) isoforms, IGF-I Ea, IGF-I Eb and myostatin mRNA.

RESULTS

The weights of soleus, plantaris and masseter muscles were decreased in underfed and hypophysectomized rats. Hypophysectomy resulted in significant increases of type I MHC at the expense of type IIx in plantaris muscle and of neonatal MHC at the expense of types IIx and IIb in masseter muscle. Serum IGF-I was decreased by underfeeding and hypophysectomy. Plantaris muscle IGF-I Ea mRNA in underfed and hypophysectomized rats was significantly lower than in normal controls. Plantaris muscle IGF-I Eb mRNA in underfed rats was significantly lower than in normal controls. Masseter muscle IGF-I Eb mRNA in severely underfed rats was significantly lower than in normal control and hypophysectomized rats. Soleus muscle myostatin mRNA in hypophysectomized rats was significantly higher than in normal and significantly underfed rats. No significant differences in plantaris and masseter muscle myostatin mRNA were observed between groups.

CONCLUSION

Suppressed muscle growth caused by hypophysectomy and underfeeding may be attributed mainly to reduced circulating IGF-I and partially to reduced IGF-I mRNA, rather than to a change in myostatin.