Cellular and molecular events controlling skeletal muscle mass in response to altered use

Rheb, an activator of target of rapamycin, in the blackback land crab, Gecarcinus lateralis: cloning and effects of molting and unweighting on expression in skeletal muscle

Molt-induced claw muscle atrophy in decapod crustaceans facilitates exuviation and is coordinated by ecdysteroid hormones. There is a 4-fold reduction in mass accompanied by remodeling of the contractile apparatus, which is associated with an 11-fold increase in myofibrillar protein synthesis by the end of the premolt period. Loss of a walking limb or claw causes a loss of mass in the associated thoracic musculature; this unweighting atrophy occurs in intermolt and is ecdysteroid independent. Myostatin (Mstn) is a negative regulator of muscle growth in mammals; it suppresses protein synthesis, in part, by inhibiting the insulin/metazoan target of rapamycin (mTOR) signaling pathway. Signaling via mTOR activates translation by phosphorylating ribosomal S6 kinase (s6k) and 4E-binding protein 1. Rheb (Ras homolog enriched in brain), a GTP-binding protein, is a key activator of mTOR and is inhibited by Rheb-GTPase-activating protein (GAP). Akt protein kinase inactivates Rheb-GAP, thus slowing Rheb-GTPase activity and maintaining mTOR in the active state. We hypothesized that the large increase in global protein synthesis in claw muscle was due to regulation of mTOR activity by ecdysteroids, caused either directly or indirectly via Mstn. In the blackback land crab, Gecarcinus lateralis, a Mstn-like gene (Gl-Mstn) is downregulated as much as 17-fold in claw muscle during premolt and upregulated 3-fold in unweighted thoracic muscle during intermolt. Gl-Mstn expression in claw muscle is negatively correlated with hemolymph ecdysteroid level. Full-length cDNAs encoding Rheb orthologs from three crustacean species (G. lateralis, Carcinus maenas and Homarus americanus), as well as partial cDNAs encoding Akt (Gl-Akt), mTOR (Gl-mTOR) and s6k (Gl-s6k) from G. lateralis, were cloned. The effects of molting on insulin/mTOR signaling components were quantified in claw closer, weighted thoracic and unweighted thoracic muscles using quantitative polymerase chain reaction. Gl-Rheb mRNA levels increased 3.4-fold and 3.9-fold during premolt in claw muscles from animals induced to molt by eyestalk ablation (ESA) and multiple leg autotomy (MLA), respectively, and mRNA levels were positively correlated with hemolymph ecdysteroids. There was little or no effect of molting on Gl-Rheb expression in weighted thoracic muscle and no correlation of Gl-Rheb mRNA with ecdysteroid titer. There were significant changes in Gl-Akt, Gl-mTOR and Gl-s6k expression with molt stage. These changes were transient and were not correlated with hemolymph ecdysteroids. The two muscles differed in terms of the relationship between Gl-Rheb and Gl-Mstn expression. In thoracic muscle, Gl-Rheb mRNA was positively correlated with Gl-Mstn mRNA in both ESA and MLA animals. By contrast, Gl-Rheb mRNA in claw muscle was negatively correlated with Gl-Mstn mRNA in ESA animals, and no correlation was observed in MLA animals. Unweighting increased Gl-Rheb expression in thoracic muscle at all molt stages; the greatest difference (2.2-fold) was observed in intermolt animals. There was also a 1.3-fold increase in Gl-s6k mRNA level in unweighted thoracic muscle. These data indicate that the mTOR pathway is upregulated in atrophic muscles. Gl-Rheb, in particular, appears to play a role in the molt-induced increase in protein synthesis in the claw muscle.

Electrical stimulation influences satellite cell proliferation and apoptosis in unloading-induced muscle atrophy in mice

Muscle atrophy caused by disuse is accompanied by adverse physiological and functional consequences. Satellite cells are the primary source of skeletal muscle regeneration. Satellite cell dysfunction, as a result of impaired proliferative potential and/or increased apoptosis, is thought to be one of the causes contributing to the decreased muscle regeneration capacity in atrophy. We have previously shown that electrical stimulation improved satellite cell dysfunction. Here we test whether electrical stimulation can also enhance satellite cell proliferative potential as well as suppress apoptotic cell death in disuse-induced muscle atrophy. Eight-week-old male BALB/c mice were subjected to a 14-day hindlimb unloading procedure. During that period, one limb (HU-ES) received electrical stimulation (frequency: 20 Hz; duration: 3 h, twice daily) while the contralateral limb served as control (HU). Immunohistochemistry and western blotting techniques were used to characterize specific proteins in cell proliferation and apoptosis. The HU-ES soleus muscles showed significant improvement in muscle mass, cross-sectional area, and peak tetanic force relative to the HU limb (p<0.05). The satellite cell proliferative activity as detected within the BrdU+/Pax7+ population was significantly higher (p<0.05). The apoptotic myonuclei (detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) and the apoptotic satellite cells (detected by cleaved Poly [ADP-ribose] polymerase co-labeled with Pax7) were reduced (p<0.05) in the HU-ES limb. Furthermore the apoptosis-inducing factor and cleaved caspase-3 were down-regulated while the anti-apoptotic Bcl-2 protein was up-regulated (p<0.05), in the HU-ES limb. These findings suggest that the electrical stimulation paradigm provides an effective stimulus to rescue the loss of myonuclei and satellite cells in disuse muscle atrophy, thus maintaining a viable satellite cell pool for subsequent muscle regeneration. Optimization of stimulation parameters may enhance the outcome of the intervention.

Low-level laser (light) therapy (LLLT) on muscle tissue: performance, fatigue and repair benefited by the power of light

The use of low level laser (light) therapy (LLLT) has recently expanded to cover areas of medicine that were not previously thought of as the usual applications such as wound healing and inflammatory orthopedic conditions. One of these novel application areas is LLLT for muscle fatigue and muscle injury. Since it is becoming agreed that mitochondria are the principal photoacceptors present inside cells, and it is known that muscle cells are exceptionally rich in mitochondria, this suggests that LLLT should be highly beneficial in muscle injuries. The ability of LLLT to stimulate stem cells and progenitor cells means that muscle satellite cells may respond well to LLLT and help muscle repair. Furthermore the ability of LLLT to reduce inflammation and lessen oxidative stress is also beneficial in cases of muscle fatigue and injury. This review covers the literature relating to LLLT and muscles in both preclinical animal experiments and human clinical studies. Athletes, people with injured muscles, and patients with Duchenne muscular dystrophy may all benefit.

Muscle wasting and the temporal gene expression pattern in a novel rat intensive care unit model

BACKGROUND

Acute quadriplegic myopathy (AQM) or critical illness myopathy (CIM) is frequently observed in intensive care unit (ICU) patients. To elucidate duration-dependent effects of the ICU intervention on molecular and functional networks that control the muscle wasting and weakness associated with AQM, a gene expression profile was analyzed at time points varying from 6 hours to 14 days in a unique experimental rat model mimicking ICU conditions, i.e., post-synaptically paralyzed, mechanically ventilated and extensively monitored animals.

RESULTS

During the observation period, 1583 genes were significantly up- or down-regulated by factors of two or greater. A significant temporal gene expression pattern was constructed at short (6 h-4 days), intermediate (5-8 days) and long (9-14 days) durations. A striking early and maintained up-regulation (6 h-14d) of muscle atrogenes (muscle ring-finger 1/tripartite motif-containing 63 and F-box protein 32/atrogin-1) was observed, followed by an up-regulation of the proteolytic systems at intermediate and long durations (5-14d). Oxidative stress response genes and genes that take part in amino acid catabolism, cell cycle arrest, apoptosis, muscle development, and protein synthesis together with myogenic factors were significantly up-regulated from 5 to 14 days. At 9-14 d, genes involved in immune response and the caspase cascade were up-regulated. At 5-14d, genes related to contractile (myosin heavy chain and myosin binding protein C), regulatory (troponin, tropomyosin), developmental, caveolin-3, extracellular matrix, glycolysis/gluconeogenesis, cytoskeleton/sarcomere regulation and mitochondrial proteins were down-regulated. An activation of genes related to muscle growth and new muscle fiber formation (increase of myogenic factors and JunB and down-regulation of myostatin) and up-regulation of genes that code protein synthesis and translation factors were found from 5 to 14 days.

CONCLUSIONS

Novel temporal patterns of gene expression have been uncovered, suggesting a unique, coordinated and highly complex mechanism underlying the muscle wasting associated with AQM in ICU patients and providing new target genes and avenues for intervention studies.

Inhibition of FoxO transcriptional activity prevents muscle fiber atrophy during cachexia and induces hypertrophy

Cachexia is characterized by inexorable muscle wasting that significantly affects patient prognosis and increases mortality. Therefore, understanding the molecular basis of this muscle wasting is of significant importance. Recent work showed that components of the forkhead box O (FoxO) pathway are increased in skeletal muscle during cachexia. In the current study, we tested the physiological significance of FoxO activation in the progression of muscle atrophy associated with cachexia. FoxO-DNA binding dependent transcription was blocked in the muscles of mice through injection of a dominant negative (DN) FoxO expression plasmid prior to inoculation with Lewis lung carcinoma cells or the induction of sepsis. Expression of DN FoxO inhibited the increased mRNA levels of atrogin-1, MuRF1, cathepsin L, and/or Bnip3 and inhibited muscle fiber atrophy during cancer cachexia and sepsis. Interestingly, during control conditions, expression of DN FoxO decreased myostatin expression, increased MyoD expression and satellite cell proliferation, and induced fiber hypertrophy, which required de novo protein synthesis. Collectively, these data show that FoxO-DNA binding-dependent transcription is necessary for normal muscle fiber atrophy during cancer cachexia and sepsis, and further suggest that basal levels of FoxO play an important role during normal conditions to depress satellite cell activation and limit muscle growth.

[Reduction of muscle mass mediated by myostatin in an experimental model of pulmonary emphysema]

INTRODUCTION

Among the extrapulmonary manifestations of COPD, dysfunction and loss of muscle mass/weight are those that have the greatest impact on the quality of life of patients. Our objective was to evaluate the molecular mechanisms that are potentially implicated in the limited development of muscle mass in the diaphragm and gastrocnemius of mice with experimentally-induced emphysema.

METHODS

An experimental model in mice, in which emphysema was induced by means of the local instillation of elastase (n=6), while saline was administered to the controls (n=7). We determined the levels of oxidative stress, proteolytic systems, signaling pathways, growth factors and cell differentiation (western-blot) in the diaphragm and gastrocnemius of all the mice after 34 weeks.

RESULTS

Upon comparing the mice with emphysema with the controls, the following findings were observed: (1) lower total body weight and lower weight of the diaphragm and gastrocnemius; (2) in the diaphragm, the levels of protein oxidation were increased, the mitochondrial antioxidant systems reduced, the levels of myostatin and of the ERK1/2 and FoxO1 signaling pathways were higher, and the myosin content was lower (67%); and (3) in the gastrocnemius of the emphysematous mice, the cytosolic antioxidants were decreased and the levels of myostatin and of the JNK and NF-kB signaling pathways were increased.

CONCLUSIONS

The reduction of the myosin content observed in the diaphragm of mice with emphysema could explain their smaller size. Oxidative stress, myostatin and FoxO could be implicated in the loss of this structural protein.

Enhanced survival of skeletal muscle myoblasts in response to overexpression of cold shock protein RBM3

Cold-inducible RNA-binding protein (RBM3) is suggested to be involved in the regulation of skeletal muscle mass. Cell death pathways are implicated in the loss of muscle mass and therefore the role of RBM3 in muscle apoptosis in C(2)C(12) myoblasts was investigated in this study. RBM3 overexpression was induced by either cold shock (32°C exposure for 6 h) or transient transfection with a myc-tagged RBM3 expression vector. Cell death was induced by H(2)O(2) (1,000 μM) or staurosporine (StSp, 5 μM), and it was shown that cold shock and RBM3 transfection were associated with attenuation of morphological changes and an increase in cell viability compared with normal temperature or empty vector, respectively. No changes in proliferation were observed with either cold shock or RBM3 transfection. DNA fragmentation was not increased in response to H(2)O(2), and a cell permeability assay indicated that cell death in response to H(2)O(2) is more similar to necrosis than apoptosis. RBM3 overexpression reduced apoptosis and the collapse of the membrane potential in response to StSp. Moreover, the increase in caspase-3, -8, and -9 activities in response to StSp was returned to control levels with RBM3 overexpression. These results indicate that increased RBM3 expression decreases muscle cell necrosis as well as apoptosis and therefore RBM3 could potentially serve as an intervention for the loss of muscle cell viability during muscle atrophy and muscle diseases.

Striated muscle activator of Rho signalling (STARS) is a PGC-1α/oestrogen-related receptor-α target gene and is upregulated in human skeletal muscle after endurance exercise

The striated muscle activator of Rho signalling (STARS) is an actin-binding protein specifically expressed in cardiac, skeletal and smooth muscle. STARS has been suggested to provide an important link between the transduction of external stress signals to intracellular signalling pathways controlling genes involved in the maintenance of muscle function. The aims of this study were firstly, to establish if STARS, as well as members of its downstream signalling pathway, are upregulated following acute endurance cycling exercise; and secondly, to determine if STARS is a transcriptional target of peroxisome proliferator-activated receptor gamma co-activator 1-α (PGC-1α) and oestrogen-related receptor-α (ERRα). When measured 3 h post-exercise, STARS mRNA and protein levels as well as MRTF-A and serum response factor (SRF) nuclear protein content, were significantly increased by 140, 40, 40 and 40%, respectively. Known SRF target genes, carnitine palmitoyltransferase-1β (CPT-1β) and jun B proto-oncogene (JUNB), as well as the exercise-responsive genes PGC-1α mRNA and ERRα were increased by 2.3-, 1.8-, 4.5- and 2.7-fold, 3 h post-exercise. Infection of C2C12 myotubes with an adenovirus-expressing human PGC-1α resulted in a 3-fold increase in Stars mRNA, a response that was abolished following the suppression of endogenous ERRα. Over-expression of PGC-1α also increased Cpt-1β, Cox4 and Vegf mRNA by 6.2-, 2.0- and 2.0-fold, respectively. Suppression of endogenous STARS reduced basal Cpt-1β levels by 8.2-fold and inhibited the PGC-1α-induced increase in Cpt-1β mRNA. Our results show for the first time that the STARS signalling pathway is upregulated in response to acute endurance exercise. Additionally, we show in C2C12 myotubes that the STARS gene is a PGC-1α/ERRα transcriptional target. Furthermore, our results suggest a novel role of STARS in the co-ordination of PGC-1α-induced upregulation of the fat oxidative gene, CPT-1β.

Upregulation of the CaV 1.1-ryanodine receptor complex in a rat model of critical illness myopathy

The processes that trigger severe muscle atrophy and loss of myosin in critical illness myopathy (CIM) are poorly understood. It has been reported that muscle disuse alters Ca(2+) handling by the sarcoplasmic reticulum. Since inactivity is an important contributor to CIM, this finding raises the possibility that elevated levels of the proteins involved in Ca(2+) handling might contribute to development of CIM. CIM was induced in 3- to 5-mo-old rats by sciatic nerve lesion and infusion of dexamethasone for 1 wk. Western blot analysis revealed increased levels of ryanodine receptor (RYR) isoforms-1 and -2 as well as the dihydropyridine receptor/voltage-gated calcium channel type 1.1 (DHPR/Ca(V) 1.1). Immunostaining revealed a subset of fibers with elevation of RYR1 and Ca(V) 1.1 that had severe atrophy and disorganization of sarcomeres. These findings suggest increased Ca(2+) release from the sarcoplasmic reticulum may be an important contributor to development of CIM. To assess the endogenous functional effects of increased intracellular Ca(2+) in CIM, proteolysis of α-fodrin, a well-known target substrate of Ca(2+)-activated proteases, was measured and found to be 50% greater in CIM. There was also selective degradation of myosin heavy chain relative to actin in CIM muscle. Taken together, our findings suggest that increased Ca(2+) release from the sarcoplasmic reticulum may contribute to pathology in CIM.

Increased cellular proliferation in rat skeletal muscle and tendon in response to exercise: use of FLT and PET/CT

PURPOSE

The purpose of this study is to investigate exercise-induced cellular proliferation in rat skeletal muscle/tendon with the use of 3'-[F-18]fluoro-3'deoxythymidine (FLT) and to quantitatively study concomitant changes in the proliferation-associated factor, Ki67.

PROCEDURES

Wistar rats (n = 13) performed 3 days of treadmill running. Cellular proliferation was investigated 3 days before and 48 h after the running exercise with the use of FLT and positron emission tomography/computed tomography (PET/CT). Results were compared to a sedentary control group (n = 10). Image-derived standardized uptake values were calculated for Achilles tendons and calf muscles and compared to gene expression and immunohistochemical evaluations of Ki67.

RESULTS

Treadmill running induced increased uptake of FLT uptake in calf muscles (30%; p < 0.001) and in Achilles tendon (21%, p < 0.001). The image-derived results were supported by a correlation in calf muscle to Ki67 (protein and mRNA level), while this coherence was not found in tendon.

CONCLUSION

FLT-PET seems to be a promising tool for imaging of exercise-induced cellular proliferation in musculo-tendinous tissue.

Differential apoptosis-related protein expression, mitochondrial properties, proteolytic enzyme activity, and DNA fragmentation between skeletal muscles

Increased skeletal muscle apoptosis has been associated with a number of conditions including aging, disuse, and cardiovascular disease. Skeletal muscle is a complex tissue comprised of several fiber types with unique properties. To date, no report has specifically examined apoptotic differences across muscles or fiber types. Therefore, we measured several apoptotic indices in healthy rat red (RG) and white gastrocnemius (WG) muscle, as well as examined the expression of several key proteins across fiber types in a mixed muscle (mixed gastrocnemius). The protein content of apoptosis-inducing factor (AIF), apoptosis repressor with caspase recruitment domain (ARC), Bax, Bcl-2, cytochrome c, heat shock protein 70 (Hsp70), and second mitochondria-derived activator of caspases (Smac) were significantly (P < 0.05) higher in RG vs. WG muscle. Cytosolic AIF, cytochrome c, and Smac as well as nuclear AIF were also significantly (P < 0.05) higher in RG compared with WG muscle. In addition, ARC protein expression was related to muscle fiber type and found to be highest (P < 0.001) in type I fibers. Similarly, AIF protein expression was differentially expressed across fibers; however, AIF was correlated to oxidative potential (P < 0.001). Caspase-3, -8, and -9 activity, calpain activity, and DNA fragmentation (a hallmark of apoptosis) were also significantly higher (P < 0.05) in RG compared with WG muscle. Furthermore, total muscle reactive oxygen species generation, as well as Ca(2+)-induced permeability transition pore opening and loss of membrane potential in isolated mitochondria were greater in RG muscle. Collectively, these data suggest that a number of apoptosis-related indices differ between muscles and fiber types. Given these findings, muscle and fiber-type differences in apoptotic protein expression, signaling, and susceptibility should be considered when studying cell death processes in skeletal muscle.

A preliminary study on the changes in some potential markers of muscle-cell degradation in sub-maximally exercised horses supplemented with a protein and amino acid mixture

In this preliminary study, time-dependent changes in plasma CK and AST activity, tyrosine (Tyr), 3-methyl-histidine (3mHis), glucose and lactate concentrations were analysed in nine horses under two different conditions. Furthermore, intramuscular concentrations of Tyr, 3mHis and activities of cathepsin B, acid phosphatase (ACP), glucose-6-phosphate dehydrogenase (G6PDH) and mRNA expression of ubiquitin were determined at the same time. After studying the effects of exercise alone, the effects of exercise and feeding of an experimental protein/amino acid (AA) supplement were analysed. Horses were submitted to a total of four standardised exercise tests (SETs) of high intensity. Potential markers of muscle break down were determined prior to, immediately after, 4 and 18 h after exercise. The experiment was subdivided into two consecutive periods of 3 weeks. In each period, two SETs were performed. In the second period, horses were fed with the protein/AA supplement within 1 h after exercise. Significant changes in plasma, intramuscular Tyr levels and mRNA expression of ubiquitin were caused both by time in relation to exercise and by treatment with the protein/AA supplement. The experimental supplement significantly decreased the 4-h post-exercise expression of ubiquitin mRNA in muscle. Only a borderline increase of markers of lysosomal involvement was seen and CK and AST activity generally showed their normal post-exercise patterns. A clear post-exercise reduction of this CK activity, however, was not observed after supplementation with the protein/AA mixture. The current findings indicate that horses might benefit from protein and AA supplementation directly after training by decreasing post-exercise proteolysis. The results support that further studies should be performed to characterize changes in equine protein metabolism caused by exercise including underlying molecular mechanisms.

Cellular responses in skeletal muscle to a season of ice hockey

We hypothesized that a season of ice hockey would result in extensive remodeling of muscle. Tissue sampled from the vastus lateralis of 15 players (age = 20.6 ± 0.4 years; mean ± SE) prior to (PRE) and following (POST) a season was used to characterize specific adaptations. Measurement of representative metabolic pathway enzymes indicated higher maximal activities in POST than in PRE (p < 0.05) for succinic dehydrogenase (3.26 ± 0.31 vs. 3.91 ± 0.11 mol mg protein(-1) min(-1)), citrate synthase (7.26 ± 0.70 vs. 8.70 ± 0.55 mol mg protein(-1) min(-1)), and phosphofructokinase (12.8 ± 1.3 vs. 14.4 ± 0.96 mol mg protein(-1) min(-1)) only. The season resulted in an increase in Na+-K+-ATPase concentration (253 ± 6.3 vs. 265 ± 6.0 pmol g(-1) wet weight), a decrease (p < 0.05) in maximal activity of the sarcoplasmic reticulum Ca2+-ATPase (107 ± 4.2 micromol g protein(-1) min(-1) vs. 92.0 ± 4.6 micromol g protein(-1) min(-1)), and no change in the distribution (%) of fibre types. A smaller (p < 0.05) cross-sectional area (CSA) for both type I (-11.7%) and type IIA (-18.2%) fibres and a higher (p < 0.05) capillary count/CSA for type I (+17.9%) and type IIA (+17.2%) were also found over the season. No changes were found in peak oxygen consumption (51.4 ± 1.2 mL kg(-1) min(-1) vs. 52.3 ± 1.3 mL kg(-1) min(-1)). The results suggest, based on the alterations in oxidative and perfusion potentials and muscle mass, that the dominant adaptations are in support of oxidative metabolism, which occurs at the expense of fibre CSA and possibly force-generating potential.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Physiological loading of joints prevents cartilage degradation through CITED2

Both overuse and disuse of joints up-regulate matrix metalloproteinases (MMPs) in articular cartilage and cause tissue degradation; however, moderate (physiological) loading maintains cartilage integrity. Here, we test whether CBP/p300-interacting transactivator with ED-rich tail 2 (CITED2), a mechanosensitive transcriptional coregulator, mediates this chondroprotective effect of moderate mechanical loading. In vivo, hind-limb immobilization of Sprague-Dawley rats up-regulates MMP-1 and causes rapid, histologically detectable articular cartilage degradation. One hour of daily passive joint motion prevents these changes and up-regulates articular cartilage CITED2. In vitro, moderate (2.5 MPa, 1 Hz) intermittent hydrostatic pressure (IHP) treatment suppresses basal MMP-1 expression and up-regulates CITED2 in human chondrocytes, whereas high IHP (10 MPa) down-regulates CITED2 and increases MMP-1. Competitive binding and transcription assays demonstrate that CITED2 suppresses MMP-1 expression by competing with MMP transactivator, Ets-1 for its coactivator p300. Furthermore, CITED2 up-regulation in vitro requires the p38δ isoform, which is specifically phosphorylated by moderate IHP. Together, these studies identify a novel regulatory pathway involving CITED2 and p38δ, which may be critical for the maintenance of articular cartilage integrity under normal physical activity levels.

Aging with muscular dystrophy: pathophysiology and clinical management

Major advances in the fields of medical science and physiology, molecular genetics, biomedical engineering, and computer science have provided individuals with muscular dystrophy (MD) with more functional equipment, allowing better strategies for improvement of quality of life. These advances have also allowed a significant number of these patients to live much longer. As progress continues to change management, it also changes patients' expectations. A comprehensive medical and rehabilitative approach to management of aging MD patients can often fulfill expectations and help them enjoy an enhanced quality of life.

The time course of myonuclear accretion during hypertrophy in young adult and older rat plantaris muscle

To investigate whether accretion of myonuclei precedes or follows the increase in fibre cross-sectional area and whether this time course is affected by age, left plantaris muscle of 5- and 25-month-old male Wistar rats was overloaded by denervation of its synergists for 1, 2 or 4 weeks. Contralateral plantaris muscle served as control. Myonuclei were counted in haematoxylin-stained cross-sections. While hypertrophy (33% in young adult) became significant after 2 weeks overload (p<0.01), the myonuclear number was increased only at 4 weeks of overload (p<0.001). The time course and magnitude of hypertrophy were similar in young adult and older rats. Older muscles contained 26% more myonuclei per mm fibre length (p=0.001) and had a 10-fold larger proportion of central myonuclei (p<0.001) than young adult muscles. In conclusion, our data indicate that muscle fibre hypertrophy precedes the acquisition of new myonuclei and that the ability to develop hypertrophy is not attenuated or delayed in 25-month-old rat muscle.

Alcohol, athletic performance and recovery

Alcohol consumption within elite sport has been continually reported both anecdotally within the media and quantitatively in the literature. The detrimental effects of alcohol on human physiology have been well documented, adversely influencing neural function, metabolism, cardiovascular physiology, thermoregulation and skeletal muscle myopathy. Remarkably, the downstream effects of alcohol consumption on exercise performance and recovery, has received less attention and as such is not well understood. The focus of this review is to identify the acute effects of alcohol on exercise performance and give a brief insight into explanatory factors.

Leucine attenuates skeletal muscle wasting via inhibition of ubiquitin ligases

The aim of this study was to assess the effect of leucine supplementation on elements of the ubiquitin-proteasome system (UPS) in rat skeletal muscle during immobilization. This effect was evaluated by submitting the animals to a leucine supplementation protocol during hindlimb immobilization, after which different parameters were determined, including: muscle mass; cross-sectional area (CSA); gene expression of E3 ligases/deubiquitinating enzymes; content of ubiquitinated proteins; and rate of protein synthesis. Our results show that leucine supplementation attenuates soleus muscle mass loss driven by immobilization. In addition, the marked decrease in the CSA in soleus muscle type I fibers, but not type II fibers, induced by immobilization was minimized by leucine feeding. Interestingly, leucine supplementation severely minimized the early transient increase in E3 ligase [muscle ring finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/atrogin-1] gene expression observed during immobilization. The reduced peak of E3 ligase gene expression was paralleled by a decreased content of ubiquitinated proteins during leucine feeding. The protein synthesis rate decreased by immobilization and was not affected by leucine supplementation. Our results strongly suggest that leucine supplementation attenuates muscle wasting induced by immobilization via minimizing gene expression of E3 ligases, which consequently could downregulate UPS-driven protein degradation. It is notable that leucine supplementation does not restore decreased protein synthesis driven by immobilization.

How is the balance between protein synthesis and degradation achieved?

Unlike most substances that cells manufacture, proteins are not produced and broken down by a common series of chemical reactions, but by completely different (independent and disconnected) mechanisms that possess no intrinsic means of making the rates of the two processes equal and attaining steady state concentrations. Balance between them is achieved extrinsically and is often imagined today to be the result of the actions of chemical feedback agents. But however instantiated, chemical feedback or any similar mechanism can only rectify induced imbalances in a system previously balanced by other means. Those "other means" necessarily involve reversible mass action or equilibrium-based interactions between native and altered forms of protein molecules somewhere in time and space between their synthesis and degradation.

Eccentric exercise activates novel transcriptional regulation of hypertrophic signaling pathways not affected by hormone changes

Unaccustomed eccentric exercise damages skeletal muscle tissue, activating mechanisms of recovery and remodeling that may be influenced by the female sex hormone 17β-estradiol (E2). Using high density oligonucleotide based microarrays, we screened for differences in mRNA expression caused by E2 and eccentric exercise. After random assignment to 8 days of either placebo (CON) or E2 (EXP), eighteen men performed 150 single-leg eccentric contractions. Muscle biopsies were collected at baseline (BL), following supplementation (PS), +3 hours (3H) and +48 hours (48H) after exercise. Serum E2 concentrations increased significantly with supplementation (P<0.001) but did not affect microarray results. Exercise led to early transcriptional changes in striated muscle activator of Rho signaling (STARS), Rho family GTPase 3 (RND3), mitogen activated protein kinase (MAPK) regulation and the downstream transcription factor FOS. Targeted RT-PCR analysis identified concurrent induction of negative regulators of calcineurin signaling RCAN (P<0.001) and HMOX1 (P = 0.009). Protein contents were elevated for RND3 at 3H (P = 0.02) and FOS at 48H (P<0.05). These findings indicate that early RhoA and NFAT signaling and regulation are altered following exercise for muscle remodeling and repair, but are not affected by E2.

Notch and Wnt signaling, physiological stimuli and postnatal myogenesis

Adult skeletal muscle stem cells, termed satellite cells are imperative to muscle regeneration. Much work has been performed on satellite cell identification and the subsequent activation of the myogenic response but the regulation of satellite cells including its activation is not well elucidated. The purpose of this review article is to synthesize what the literature reveals in regards to the current understanding of satellite cells including their contribution to muscle repair and growth following physiological stimuli. In addition, this review article will describe the recent findings on the roles of the classic developmental signaling pathways, Notch and Wnt, to the myogenic response in various muscle injury models. This purpose of this summary is to bring awareness of the impact that muscle contraction models have on the local and systemic environment of adult muscle stem cells which will be beneficial for comprehending and treatment development for muscle -associated ailments and other organ diseases.

Research in the exercise sciences: where we are and where do we go from here--Part II

This decadal perspective summarizes novel, insightful observations achieved in exercise science. The topics span genomics and gene function, epigenetics, cell signaling, epidemiological phenomena, and other important areas. A future strategy is presented along two parallel, integrated paths involving the following: 1) a continuance of genomic discovery and gene function, and 2) classical biochemical/physiological approaches toward solving biological- and health/disease-related phenomena.

The cardiomyocyte circadian clock: emerging roles in health and disease

Circadian misalignment has been implicated in the development of obesity, diabetes mellitus, and cardiovascular disease. Time-of-day-dependent synchronization of organisms with their environment is mediated by circadian clocks. This cell autonomous mechanism has been identified within all cardiovascular-relevant cell types, including cardiomyocytes. Recent molecular- and genetic-based studies suggest that the cardiomyocyte circadian clock influences multiple myocardial processes, including transcription, signaling, growth, metabolism, and contractile function. Following an appreciation of its physiological roles, the cardiomyocyte circadian clock has recently been linked to the pathogenesis of heart disease in response to adverse stresses, such as ischemia/reperfusion, in animal models. The purpose of this review is therefore to highlight recent advances regarding the roles of the cardiomyocyte circadian clock in both myocardial physiology and pathophysiology (ie, health and disease).

Downregulation of Akt/mammalian target of rapamycin pathway in skeletal muscle is associated with increased REDD1 expression in response to chronic hypoxia

Although it is well established that chronic hypoxia leads to an inexorable loss of skeletal muscle mass in healthy subjects, the underlying molecular mechanisms involved in this process are currently unknown. Skeletal muscle atrophy is also an important systemic consequence of chronic obstructive pulmonary disease (COPD), but the role of hypoxemia in this regulation is still debated. Our general aim was to determine the molecular mechanisms involved in the regulation of skeletal muscle mass after exposure to chronic hypoxia and to test the biological relevance of our findings into the clinical context of COPD. Expression of positive and negative regulators of skeletal muscle mass were explored 1) in the soleus muscle of rats exposed to severe hypoxia (6,300 m) for 3 wk and 2) in vastus lateralis muscle of nonhypoxemic and hypoxemic COPD patients. In rodents, we observed a marked inhibition of the mammalian target of rapamycin (mTOR) pathway together with a strong increase in regulated in development and DNA damage response 1 (REDD1) expression and in its association with 14-3-3, a mechanism known to downregulate the mTOR pathway. Importantly, REDD1 overexpression in vivo was sufficient to cause skeletal muscle fiber atrophy in normoxia. Finally, the comparative analysis of skeletal muscle in hypoxemic vs. nonhypoxemic COPD patients confirms that hypoxia causes an inhibition of the mTOR signaling pathway. We thus identify REDD1 as a negative regulator of skeletal muscle mass during chronic hypoxia. Translation of this fundamental knowledge into the clinical investigation of COPD shows the interest to develop therapeutic strategies aimed at inhibiting REDD1.

Mitogen-activated protein kinase-activated protein kinase 2 (MK2) in skeletal muscle atrophy and hypertrophy

Skeletal muscle is a highly plastic tissue. Overall muscle growth (hypertrophy) or muscle wasting (atrophy) results from alterations in intracellular signaling pathways with important regulatory steps occurring in the nucleus as well as in the cytoplasm. Previous studies have identified components of the Akt/mTor pathway as well as the p38 MAPK pathway as important for skeletal muscle hypertrophy and/or atrophy. The present study tests the hypothesis that MK2, a substrate of p38 which following phosphorylation, can be exported from the nucleus in a complex with p38, may be important for skeletal muscle growth. The expression of MK2 was examined in denervated mouse hind-limb (atrophic) and hemidiaphragm (transiently hypertrophic) muscles. MK2 mRNA expression decreased after denervation in both atrophic (48% of innervated controls, P < 0.001) and hypertrophic muscle (34% of innervated controls, P < 0.01) but MK2 protein expression decreased only in atrophic muscle (32% of innervated controls, P < 0.01). The level of T205 phosphorylated MK2 increased after denervation in both atrophic (fourfold increase, P < 0.01) and hypertrophic muscles (almost sevenfold increase, P < 0.001) whereas the level of T317 phosphorylated MK2 (necessary for nuclear export) increased after denervation in hypertrophic muscle (nearly threefold increase, P < 0.001) but not in atrophic muscle. Logarithmically transformed relative changes in MK2 phosphorylated at T317 correlated well (r(2) = 0.7737) with relative changes in muscle weight. The results suggest a role for MK2 in the regulation of muscle mass, a role which, at least in part, may be related to determining the subcellular localization of p38 in muscle fibers.

Molt cycle regulation of protein synthesis in skeletal muscle of the blackback land crab, Gecarcinus lateralis, and the differential expression of a myostatin-like factor during atrophy induced by molting or unweighting

In decapod crustaceans, claw muscle undergoes atrophy in response to elevated ecdysteroids while thoracic muscle undergoes atrophy in response to unweighting. The signaling pathways that regulate muscle atrophy in crustaceans are largely unknown. Myostatin is a negative regulator of muscle growth in mammals, and a myostatin-like cDNA is preferentially expressed in muscle of the land crab, Gecarcinus lateralis (Gl-Mstn). Contrary to prediction, levels of Gl-Mstn mRNA decreased dramatically in both the claw closer and weighted thoracic muscles when molting was induced by either eyestalk ablation (ESA) or multiple limb autotomy (MLA). However, the effect of molt induction was greater in the claw muscle. By late premolt, Gl-Mstn mRNA in the claw muscle decreased 81% and 94% in ESA and MLA animals, respectively, and was negatively correlated with ecdysteroids. Gl-Mstn mRNA in thoracic muscle decreased 68% and 82% in ESA and MLA animals, respectively, but was only weakly correlated with ecdysteroid. Claw and thoracic muscles also differed to varying extents in the expression of ecdysteroid receptor (Gl-EcR and Gl-RXR), elongation factor-2 (Gl-EF-2), and calpain T (Gl-CalpT) in response to molt induction, but levels of the four transcripts were not correlated with ecdysteroid. The downregulation of Gl-Mstn expression in premolt claw muscle coincided with 11- and 13-fold increases in protein synthesis in the myofibrillar and soluble protein fractions, respectively. Furthermore, the rate of the increase in the synthesis of soluble proteins was greater than that of myofibrillar proteins during early premolt (1.4:1, soluble:myofibrillar), but the two were equivalent during late premolt. By contrast, Gl-Mstn mRNA increased 3-fold and Gl-CalpT mRNA decreased 40% in unweighted thoracic muscle; there was little or no effect on Gl-EF-2, Gl-EcR, and Gl-RXR mRNA levels. These data indicate that Gl-Mstn expression is negatively regulated by both ecdysteroids and load-bearing contractile activity. The downregulation of Gl-Mstn in claw muscle may induce the elevated protein turnover associated with remodeling of the contractile apparatus during molt-induced atrophy. The upregulation of Gl-Mstn in unweighted thoracic muscle suggests that this factor is also involved in disuse atrophy when hemolymph ecdysteroid levels are low.

A subpopulation of rat muscle fibers maintains an assessable excitation-contraction coupling mechanism after long-standing denervation despite lost contractility

To define the time course and potential effects of electrical stimulation on permanently denervated muscle, we evaluated excitation-contraction coupling (ECC) of rat leg muscles during progression to long-term denervation by ultrastructural analysis, specific binding to dihydropyridine receptors, ryanodine receptor 1 (RYR-1), Ca channels and extrusion Ca pumps, gene transcription and translation of Ca-handling proteins, and in vitro mechanical properties and electrophysiological analyses of sarcolemmal passive properties and L-type Ca current (ICa) parameters. We found that in response to long-term denervation: 1) isolated muscle that is unable to twitch in vitro by electrical stimulation has very small myofibers but may show a slow caffeine contracture; 2) only roughly half of the muscle fibers with "voltage-dependent Ca channel activity" are able to contract; 3) the ECC mechanisms are still present and, in part, functional; 4)ECC-related gene expression is upregulated; and 5) at any time point, there are muscle fibers that are more resistant than others to denervation atrophy and disorganization of the ECC apparatus. These results support the hypothesis that prolonged "resting" [Ca] may drive progression of muscle atrophy to degeneration and that electrical stimulation-induced [Ca] modulation may mimic the lost nerve influence, playing a key role in modifying the gene expression of denervated muscle. Hence, these data provide a potential molecular explanation for the muscle recovery that occurs in response to rehabilitation strategies developed based on empirical clinical observations.

Myofiber apoptosis occurs in the inflammation and regeneration phase following eccentric contractions in rats

Eccentric contractions (ECC) induce myofibrillar collapse, edema, and inflammation in muscle cells. Although apoptosis of myonuclei following ECC is activated during the inflammatory phase, the apoptosis response of the regenerative phase remains to be elucidated. The aim of the present study was to determine the inflammatory and regenerative phase of the apoptosis responses induced by ECC. In anesthetized rats, the tibialis anterior muscles were subjected to ECC repeated 40 times, evoked by surface electric stimulation (100 Hz, 10 V) with mechanical muscle stretch. Apoptosis was examined in the control group and in groups 1, 3, 7, and 14 days after ECC (each group, n = 4-6). Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL)-positive myonuclei were assessed by further labeling with dystrophin staining and DAPI. The expression of proteins related to apoptosis (Bcl-2 and Bax) was examined by Western blot assay. At 1 and 3 days, focal edema and necrotic myofibers invaded by mononuclear phagocytes were present, whereas regenerated myofibers with central nuclei were detected at 7 and 14 days. The occurrence of TUNEL-positive myonuclei increased significantly at 7 (7.0 +/- 1.5%) and 14 days (5.6 +/- 0.6%) compared with control (0.9 +/- 0.5%). Further we found that myonuclear apoptosis was restricted to the subsarcolemmal space at 7 and 14 days and markedly absent from the central nucleus. The Bax/Bcl-2 ratio was significantly higher at 3 (4.5 +/- 0.9) and 7 days (3.4 +/- 0.5) after ECC. In conclusion, myofiber apoptotic responses following ECC are present not only in the inflammatory phase but also persist during the regenerative phase.

Chronic high fat feeding attenuates load-induced hypertrophy in mice

The incidence of obesity and obesity-related conditions, such as metabolic syndrome and insulin resistance, is on the increase. The effect of obesity on skeletal muscle function, especially the regulation of muscle mass, is poorly understood. In this study we investigated the effect of diet-induced obesity on the ability of skeletal muscle to respond to an imposed growth stimulus, such as increased load. Male C57BL/6 mice were randomized into two diet groups: a low fat, high carbohydrate diet (LFD) and a high fat, low carbohydrate diet (HFD) fed ad libitum for 14 weeks. Mice from each diet group were divided into two treatment groups: sedentary control or bilateral functional overload (FO) of the plantaris muscle. Mice were evaluated at 3, 7, 14 or 30 days following FO. By 14 days of FO, there was a 10% reduction (P < 0.05) in absolute growth of the plantaris in response to overload in HFD mice vs. LFD mice. By 30 days the attenuation in growth increased to 16% in HFD mice compared to LFD mice. Following FO, there was a reduction in the formation of polysomes in the HFD mice relative to the LFD mice, suggesting a decrease in protein translation. Further, activation of Akt and S6K1, in response to increased mechanical loading, was significantly attenuated in the HFD mice relative to the LFD mice. In conclusion, chronic high fat feeding impairs the ability of skeletal muscle to hypertrophy in response to increased mechanical load. This failure coincided with a failure to activate key members of the Akt/mTOR signalling pathway and increase protein translation.

Effects of endotoxaemia on protein metabolism in rat fast-twitch skeletal muscle and myocardium

BACKGROUND

It is unclear if the rat myocardium undergoes the same rapid reductions in protein content that are classically observed in fast-twitch skeletal muscle during endotoxaemia.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate this further, and to determine if there is any divergence in the response of skeletal muscle and myocardium in the mechanisms that are thought to be largely responsible for eliciting changes in protein content, Sprague Dawley rats were implanted with vascular catheters and administered lipopolysaccharide (LPS; 150 microg kg(-1) h(-1)) intravenously for 2 h, 6 h or 24 h (saline administered control animals were also included), after which the extensor digitorum longus (EDL) and myocardium were removed under terminal anaesthesia. The protein-to-DNA ratio, a marker of protein content, was significantly reduced in the EDL following 24 h LPS administration (23%; P<0.05), but was no different from controls in the myocardium. At the same time point, a significant increase in MAFbx/atrogin-1 and MuRF1 mRNA (3.7+/-0.7- and 19.5+/-1.9-fold increase vs. controls, respectively; P<0.05), in addition to protein levels of alpha1-3, 5-7 subunits of the 20S proteasome, were observed in EDL but not myocardium. In contrast, elevations in phosphorylation of p70 S6K residues Thr(421)/Ser(424), and 4E-BP1 residues Thr(37)/Thr(46) (P<0.05), consistent with an elevation in translation initiation, were seen exclusively in the myocardium of LPS-treated animals.

CONCLUSIONS/SIGNIFICANCE

In summary, these findings suggest that the myocardium does not undergo the same catabolic response as skeletal muscle during early endotoxaemia, partly due to the absence of transcriptional and signalling events in the myocardium typically associated with increased muscle proteolysis and the suppression of protein synthesis.

Rehabilitation management of neuromuscular disease: the role of exercise training

This paper summarizes the current state of knowledge regarding exercise and neuromuscular diseases/disorders (NMDs) and reviews salient studies in the literature. Unfortunately, there is inadequate evidence in much of the NMDs to make specific recommendations regarding exercise prescriptions. This review focuses on the role of exercise in a few of the specific NMDs where most research has taken place and recommends future research directions.

[Exercise therapy and myopathies]

Since the first consensus papers published early in the 2000s, a growing number of recent publications has shown that adapted physical activity is not only safe in the context of myopathy but also potentially effective as a therapeutic tool. After a short recall of the different exercise modalities, the mechanical strain they induce and the expected muscular benefits, the present paper reviews the different studies related to exercise therapy in myopathic patients and provides a critical analysis of the topic. Myopathies are rare diseases with many different etiologies and a large number of training modalities which could be useful for the different muscular challenges have been proposed. We have chosen to focus on several specific training modalities and to discuss the results from the most recent papers. The purpose of this review is to, firstly, update physical training guidelines for patients with myopathy and, secondly, highlight some common pitfalls associated with this strategy. This is particularly important for medical and allied professionals involved in prescribing and managing exercise therapy protocols.

Exercise-training in young Drosophila melanogaster reduces age-related decline in mobility and cardiac performance

Declining mobility is a major concern, as well as a major source of health care costs, among the elderly population. Lack of mobility is a primary cause of entry into managed care facilities, and a contributing factor to the frequency of damaging falls. Exercise-based therapies have shown great promise in sustaining mobility in elderly patients, as well as in rodent models. However, the genetic basis of the changing physiological responses to exercise during aging is not well understood. Here, we describe the first exercise-training paradigm in an invertebrate genetic model system. Flies are exercised by a mechanized platform, known as the Power Tower, that rapidly, repeatedly, induces their innate instinct for negative geotaxis. When young flies are subjected to a carefully controlled, ramped paradigm of exercise-training, they display significant reduction in age-related decline in mobility and cardiac performance. Fly lines with improved mitochondrial efficiency display some of the phenotypes observed in wild-type exercised flies. The exercise response in flies is influenced by the amount of protein and lipid, but not carbohydrate, in the diet. The development of an exercise-training model in Drosophila melanogaster opens the way to direct testing of single-gene based genetic therapies for improved mobility in aged animals, as well as unbiased genetic screens for loci involved in the changing response to exercise during aging.

Prolonged exercise training induces long-term enhancement of HSP70 expression in rat plantaris muscle

Skeletal muscle may develop adaptive molecular chaperone enhancements as a potential defense system through repeated daily exercise stimulation. The present study investigated whether prolonged exercise training alters the expression of molecular chaperone proteins for the long term in skeletal muscle. Mature male Wistar rats were subjected for 8 wk to either a single bout of acute intermittent treadmill running (30 m/min, 5 min x 4, 5 degrees grade) or prolonged treadmill running training (15-40 m/min, 5 min x 4, 5-7 degrees grade). Levels of five molecular chaperone proteins [heat shock protein (HSP)25, HSP60, glucose-regulated protein (GRP)78, HSP70, and heat shock cognate (HSC)70] were measured in response to acute exercise and prolonged training. HSP70 levels were increased 6 and 24 h after acute exercise, but expression returned to control level within 2 days. In contrast, prolonged training had a long-term effect on HSP70 expression. Levels of HSP70 were notably increased by 4.5-fold over control 2 days after prolonged training; the enhancement was maintained for at least 14 days after training ended. However, other molecular chaperone proteins did not show adaptive changes in response to prolonged training. In addition, HSP70 enhancement by prolonged exercise training was not accompanied by transcription of HSP70 mRNA. These findings demonstrate that prolonged training can induce long-term enhancement of HSP70 expression without change at the mRNA level in skeletal muscle.

Down-regulation of Akt/mammalian target of rapamycin signaling pathway in response to myostatin overexpression in skeletal muscle

Myostatin, a member of the TGF-beta family, has been identified as a master regulator of embryonic myogenesis and early postnatal skeletal muscle growth. However, cumulative evidence also suggests that alterations in skeletal muscle mass are associated with dysregulation in myostatin expression and that myostatin may contribute to muscle mass loss in adulthood. Two major branches of the Akt pathway are relevant for the regulation of skeletal muscle mass, the Akt/mammalian target of rapamycin (mTOR) pathway, which controls protein synthesis, and the Akt/forkhead box O (FOXO) pathway, which controls protein degradation. Here, we provide further insights into the mechanisms by which myostatin regulates skeletal muscle mass by showing that myostatin negatively regulates Akt/mTOR signaling pathway. Electrotransfer of a myostatin expression vector into the tibialis anterior muscle of Sprague Dawley male rats increased myostatin protein level and decreased skeletal muscle mass 7 d after gene electrotransfer. Using RT-PCR and immunoblot analyses, we showed that myostatin overexpression was ineffective to alter the ubiquitin-proteasome pathway. By contrast, myostatin acted as a negative regulator of Akt/mTOR pathway. This was supported by data showing that the phosphorylation of Akt on Thr308, tuberous sclerosis complex 2 on Thr1462, ribosomal protein S6 on Ser235/236, and 4E-BP1 on Thr37/46 was attenuated 7 d after myostatin gene electrotransfer. The data support the conclusion that Akt/mTOR signaling is a key target that accounts for myostatin function during muscle atrophy, uncovering a novel role for myostatin in protein metabolism and more specifically in the regulation of translation in skeletal muscle.

Premature aging in skeletal muscle lacking serum response factor

Aging is associated with a progressive loss of muscle mass, increased adiposity and fibrosis that leads to sarcopenia. At the molecular level, muscle aging is known to alter the expression of a variety of genes but very little is known about the molecular effectors involved. SRF (Serum Response Factor) is a crucial transcription factor for muscle-specific gene expression and for post-natal skeletal muscle growth. To assess its role in adult skeletal muscle physiology, we developed a post-mitotic myofiber-specific and tamoxifen-inducible SRF knockout model. Five months after SRF loss, no obvious muscle phenotype was observed suggesting that SRF is not crucial for myofiber maintenance. However, mutant mice progressively developed IIB myofiber-specific atrophy accompanied by a metabolic switch towards a more oxidative phenotype, muscular lipid accumulation, sarcomere disorganization and fibrosis. After injury, mutant muscles exhibited an altered regeneration process, showing smaller regenerated fibers and persistent fibrosis. All of these features are strongly reminiscent of abnormalities encountered in aging skeletal muscle. Interestingly, we also observed an important age associated decrease in SRF expression in mice and human muscles. Altogether, these results suggest that a naturally occurring SRF down-regulation precedes and contributes to the muscle aging process. Indeed, triggering SRF loss in the muscles of mutant mice results in an accelerated aging process.

Hypertrophy of mature Xenopus muscle fibres in culture induced by synergy of albumin and insulin

The aim of this study was to investigate effects of albumin and insulin separately as well as in combination on mature muscle fibres during long-term culture. Single muscle fibres were dissected from m. iliofibularis of Xenopus laevis and attached to a force transducer in a culture chamber. Fibres were cultured in a serum-free medium at slack length (mean sarcomere length 2.3 mum) for 8 to 22 days. The medium was supplemented with (final concentrations): (1) bovine insulin (6 nmol/L or 200-600 nmol/L), (2) 0.2% bovine albumin or (3) 0.2% bovine albumin in combination with insulin (120 nmol/L). In culture medium with insulin, 50% of the muscle fibres became in-excitable within 7-12 days, whereas the other 50% were stable. Caffeine contractures of in-excitable muscle fibres produced 80.4 +/- 2.4% of initial peak tetanic force, indicating impaired excitation-contraction (E-C) coupling in in-excitable fibres. In the presence of albumin, all cultured muscle fibres were stable for at least 10 days. Muscle fibres cultured in medium with insulin or albumin exclusively did not hypertrophy or change the number of sarcomeres in series. In contrast, muscle fibres cultured with both albumin and insulin showed an increase in tetanic force and fibre cross-sectional area of 19.6 +/- 2.8% and 32.5 +/- 4.9%, respectively, (means +/- SEM.; P = 0.007) after 16.3 +/- 1.7 days, whereas the number of sarcomeres in series remained unchanged. We conclude that albumin prevents muscle fibre damage and preserves E-C coupling in culture. Furthermore, albumin is important in regulating muscle fibre adaptation by a synergistic action with growth factors like insulin.

Slow- and fast-twitch rat hind limb skeletal muscle phenotypes 8 months after spinal cord transection and olfactory ensheathing glia transplantation

Paralysed skeletal muscle of rats with spinal cord injury (SCI) undergoes atrophy and a switch in gene expression pattern which leads to faster, more fatigable phenotypes. Olfactory ensheathing glia (OEG) transplants have been reported to promote axonal regeneration and to restore sensory-motor function in animals with SCI. We hypothesized that OEG transplants could attenuate skeletal muscle phenotypic deterioration and that this effect could underlie the functional recovery observed in behavioural tests. A variety of morphological, metabolic and molecular markers were assessed in soleus (SOL) and extensor digitorum longus (EDL) muscles of spinal cord transected (SCT), OEG-transplanted rats 8 months after the intervention and compared with non-transplanted SCT rats and sham-operated (without SCT) controls (C). A multivariate analysis encompassing all the parameters indicated that OEG-transplanted rats displayed skeletal muscle phenotypes intermediate between non-transplanted and sham-operated controls, but different from both. A high correlation was observed between behaviourally tested sensory-motor functional capacity and expression level of slow- and fast-twitch hind limb skeletal muscle phenotypic markers, particularly the histochemical glycerol-3-phosphate dehydrogenase activity (-0.843, P < 0.0001) and the fraction of variant 2s of the slow regulatory myosin light chain isoform (0.848, P < 0.0001) in SOL. Despite the mean overall effect of OEG transplants in patterning skeletal muscle protein expression towards normal, in 6 out of 9 animals they appeared insufficient to overcome fibre type switching and to support a consistent and generalized long-term maintenance of normal skeletal muscle characteristics. The interplay of OEG and exercise-mediated neurotrophic actions is a plausible mechanism underlying OEG transplantation effects on paralysed skeletal muscle.