Molecular basis of skeletal muscle plasticity--from gene to form and function

Adipose tissue and skeletal muscle plasticity modulates metabolic health

Obesity, accumulation of adipose tissue, develops when energy intake exceeds energy expenditure. Adipose tissue is essential for buffering the differences between energy intake and expenditure by accumulating lipids while skeletal muscle is the energy burning machine. Here we adopted the concept that (i) adipose tissue ability to regulate the storage capacity for lipids as well as (ii) dynamic regulation of muscle and adipose tissue secretory and metabolic activity is important for maintaining the metabolic health. This might be at least in part related to tissue plasticity, a phenomenon enabling dynamic modulation of the tissue phenotype in different physiological and pathophysiological situations. Recent advances in our understanding of the complex endocrine function of adipose tissue in regulating lipid metabolism, adipogenesis, angiogenesis, extracellular matrix remodelling, inflammation and oxidative stress prompted us to review the role of tissue plasticity--dynamic changes in adipose tissue and skeletal muscle metabolic and endocrine phenotype--in determining the difference between metabolic health and disease.

Rapid activation by 3,5,3'-L-triiodothyronine of adenosine 5'-monophosphate-activated protein kinase/acetyl-coenzyme a carboxylase and akt/protein kinase B signaling pathways: relation to changes in fuel metabolism and myosin heavy-chain protein content in rat gastrocnemius muscle in vivo

T3 stimulates metabolic rate in many tissues and induces changes in fuel use. The pathways by which T3 induces metabolic/structural changes related to altered fuel use in skeletal muscle have not been fully clarified. Gastrocnemius muscle (isolated at different time points after a single injection of T3 into hypothyroid rats), displayed rapid inductions of AMP-activated protein kinase (AMPK) phosphorylation (threonine 172; within 6 h) and acetyl-coenzyme A carboxylase phosphorylation (serine 79; within 12 h). As a consequence, increases occurred in mitochondrial fatty acid oxidation and carnitine palmitoyl transferase activity. Concomitantly, T3 stimulated signaling toward increased glycolysis through a rapid increase in Akt/protein kinase B (serine 473) phosphorylation (within 6 h) and a directly related increase in the activity of phosphofructokinase. The kinase specificity of the above effects was verified by treatment with inhibitors of AMPK and Akt activity (compound C and wortmannin, respectively). In contrast, glucose transporter 4 translocation to the membrane (activated by T3 within 6 h) was maintained when either AMPK or Akt activity was inhibited. The metabolic changes were accompanied by a decline in myosin heavy-chain Ib protein [causing a shift toward the fast-twitch (glycolytic) phenotype]. The increases in AMPK and acetyl-coenzyme A carboxylase phosphorylation were transient events, both levels declining from 12 h after the T3 injection, but Akt phosphorylation remained elevated until at least 48h after the injection. These data show that in skeletal muscle, T3 stimulates both fatty acid and glucose metabolism through rapid activations of the associated signaling pathways involving AMPK and Akt/protein kinase B.

Training in hypoxia and its effects on skeletal muscle tissue

It is well established that local muscle tissue hypoxia is an important consequence and possibly a relevant adaptive signal of endurance exercise training in humans. It has been reasoned that it might be advantageous to increase this exercise stimulus by working in hypoxia. However, as long-term exposure to severe hypoxia has been shown to be detrimental to muscle tissue, experimental protocols were developed that expose subjects to hypoxia only for the duration of the exercise session and allow recovery in normoxia (live low-train high or hypoxic training). This overview reports data from 27 controlled studies using some implementation of hypoxic training paradigms. Hypoxia exposure varied between 2300 and 5700 m and training duration ranged from 10 days to 8 weeks. A similar number of studies was carried out on untrained and on trained subjects. Muscle structural, biochemical and molecular findings point to a specific role of hypoxia in endurance training. However, based on the available data on global estimates of performance capacity such as maximal oxygen uptake (VO2max) and maximal power output (Pmax), hypoxia as a supplement to training is not consistently found to be of advantage for performance at sea level. There is some evidence mainly from studies on untrained subjects for an advantage of hypoxic training for performance at altitude. Live low-train high may be considered when altitude acclimatization is not an option.

Activity-dependent regulation of myelin maintenance in the adult rat

Hindlimb unloading (HU) is known to induce changes in the neuromuscular system. However, no data describing the effects of HU on morphological characteristics of peripheral nerve have been reported so far. Therefore, we used soleus and radial nerves obtained from control and rats submitted to 14 days of HU to study the consequences of a decrease (soleus) or an increase (radial) in neural activity on its morphology. The mean number of fibers was not changed after HU. The soleus nerve axon diameter was weakly affected after HU, whereas the myelin thickness was reduced. For the radial nerve, both axon and fiber diameter were increased, and the myelin thickness and internodal distance were higher in HU rats. These results suggest that regulation of myelin maintenance undergoes plastic mechanisms. Neural activity and/or neural pattern might be essential in the maintenance of myelin sheath in adults.

Resistance training of long duration modulates force and unloaded shortening velocity of single muscle fibres of young women

The aim of the present study was to clarify the impact of long term (1 year) resistance training (RT) on structure and function of single muscle fibres of vastus lateralis in young female subjects. Five young women (age: 25.4+/-6.2 year) performed exercise sessions at 60% of single subject own repetition maximum (1 RM) 1h twice a week. Maximum voluntary force was determined pre- and post-RT and was found to significantly increase post-RT ensuring a successful impact of RT on muscle performance in vivo. Needle muscle biopsy samples were obtained both pre- and post-RT and the following determinations were performed: myosin heavy chain isoform (MHC) distribution of the whole muscle samples by SDS-PAGE; cross sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (Vo) of a large population (n=358) of single skinned muscle fibres classified on the basis of MHC isoform composition by SDS-PAGE. The results suggest that the long duration of RT can determine a significant increase in specific force (Po/CSA) and unloaded shortening velocity (Vo) of single muscle fibres in female subjects, whereas no muscle fibre hypertrophy and no shift in MHC isoform content was observed.

The role of exercise and PGC1alpha in inflammation and chronic disease

Inadequate physical activity is linked to many chronic diseases. But the mechanisms that tie muscle activity to health are unclear. The transcriptional coactivator PGC1alpha has recently been shown to regulate several exercise-associated aspects of muscle function. We propose that this protein controls muscle plasticity, suppresses a broad inflammatory response and mediates the beneficial effects of exercise.

Gene expression profiling of skeletal muscle in exercise-trained and sedentary rats with inborn high and low VO2max

The relationship between inborn maximal oxygen uptake (VO(2max)) and skeletal muscle gene expression is unknown. Since low VO(2max) is a strong predictor of cardiovascular mortality, genes related to low VO(2max) might also be involved in cardiovascular disease. To establish the relationship between inborn VO(2max) and gene expression, we performed microarray analysis of the soleus muscle of rats artificially selected for high- and low running capacity (HCR and LCR, respectively). In LCR, a low VO(2max) was accompanied by aggregation of cardiovascular risk factors similar to the metabolic syndrome. Although sedentary HCR were able to maintain a 120% higher running speed at VO(2max) than sedentary LCR, only three transcripts were differentially expressed (FDR <or=0.05) between the groups. Sedentary LCR expressed high levels of a transcript with strong homology to human leucyl-transfer RNA synthetase, of whose overexpression has been associated with a mutation linked to mitochondrial dysfunction. Moreover, we studied exercise-induced alterations in soleus gene expression, since accumulating evidence indicates that long-term endurance training has beneficial effects on the metabolic syndrome. In terms of gene expression, the response to exercise training was more pronounced in HCR than LCR. HCR upregulated several genes associated with lipid metabolism and fatty acid elongation, whereas LCR upregulated only one transcript after exercise training. The results indicate only minor differences in soleus muscle gene expression between sedentary HCR and LCR. However, the inborn level of fitness seems to influence the transcriptional adaption to exercise, as more genes were upregulated after exercise training in HCR than LCR.

AMPK and PPARdelta agonists are exercise mimetics

The benefits of endurance exercise on general health make it desirable to identify orally active agents that would mimic or potentiate the effects of exercise to treat metabolic diseases. Although certain natural compounds, such as reseveratrol, have endurance-enhancing activities, their exact metabolic targets remain elusive. We therefore tested the effect of pathway-specific drugs on endurance capacities of mice in a treadmill running test. We found that PPARbeta/delta agonist and exercise training synergistically increase oxidative myofibers and running endurance in adult mice. Because training activates AMPK and PGC1alpha, we then tested whether the orally active AMPK agonist AICAR might be sufficient to overcome the exercise requirement. Unexpectedly, even in sedentary mice, 4 weeks of AICAR treatment alone induced metabolic genes and enhanced running endurance by 44%. These results demonstrate that AMPK-PPARdelta pathway can be targeted by orally active drugs to enhance training adaptation or even to increase endurance without exercise.

Hypoxia: the third wheel between nerve and muscle

Skeletal muscles not only obey carefully all motor commands received via motor nerves from nervous system, but also are ready to modify their structure and function to be more suited to the tasks assigned by nervous system. Thus, nervous system appears as the major modulator of the muscle structure and function. Other factors, however, may interfere with the nerve-muscle partnership and among them, hypoxia plays a pivotal role because skeletal muscles exhibit a great variability of the oxygen fluxes and because hypoxia per se has a powerful influence on muscle fibers. The adaptation of skeletal muscles to nerve-induced activity is particularly evident with low frequency tonic patterns and examples are given by chronic low frequency stimulation and by endurance training. Adaptation includes fiber type transitions towards a slow-oxidative phenotype, increased mitochondrial density and increased capillary/fiber ratio. Hypoxia can trigger some of such changes and this has suggested that low oxygen tension at fiber level might be a mediator, possibly based on HIF and VEGF, of the muscle adaptation to increased contractile activity. Chronic hypoxia can, however, induce opposite modifications, such as a fiber type transition from slow-oxidative to fast-glycolytic and mitochondrial loss. In such conditions, the increased contractile activity can antagonize hypoxia effects. Thus, hypoxia can play a double role in the nerve-muscle relationship, either reinforcing the nerve influence or antagonizing it. This short review aims to re-examine the ambiguous relationships between nerve-induced contractile activity and hypoxic conditions and to suggest possible interpretations of the double role played by hypoxia.

Heat shock factor activation in human muscles following a demanding intermittent exercise protocol is attenuated with hyperthermia

AIM

The present study investigated whether increased activation of heat shock factors (HSF) following exercise relates primarily to the increased muscle temperature or to exercise in general.

METHODS

Six subjects completed 40 min of intermittent cycling (15s:15s exercise:recovery at 300 +/- 22 W) at an ambient temperature of either 20.0 +/- 1.3 or 40.3 +/- 0.7 degrees C. Muscle biopsies were taken prior to and immediately following the exercise protocol with samples analysed for HSF DNA binding by electrophoretic mobility shift assay.

RESULTS

Exercise at 40 degrees C resulted in significantly increased oesophageal (39.3 +/- 0.2 degrees C) and muscle temperature (40.0 +/- 0.2 degrees C) at the end of the exercise protocol compared with 20 degrees C (oesophageal, 38.1 +/- 0.1 degrees C; muscle, 38.9 +/- 0.2 degrees C). However, an increased DNA binding of HSF was not evident following exercise at 40 degrees C (reduced by 21 +/- 22%) whereas it increased by 29 +/- 51% following exercise at 20 degrees C.

CONCLUSION

It appears that increased temperature is not the major factor responsible for activation of HSF DNA binding.

Opposite pathobiochemical fate of pyruvate kinase and adenylate kinase in aged rat skeletal muscle as revealed by proteomic DIGE analysis

Sarcopenia is the drastic loss of skeletal muscle mass and strength during ageing. In order to better understand the molecular pathogenesis of age-related muscle wasting, we have performed a DIGE analysis of young adult versus old rat skeletal muscle. Proteomic profiling revealed that out of 2493 separated 2-D spots, 69 proteins exhibited a drastically changed expression. Age-dependent alterations in protein abundance indicated dramatic changes in metabolism, contractile activity, myofibrillar remodelling and stress response. In contrast to decreased levels of pyruvate kinase (PK), enolase and phosphofructokinase, the mitochondrial ATP synthase, succinate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase and adenylate kinase (AK) were increased in senescent fibres. Higher expression levels of myoglobin and fatty acid binding-protein indicated a shift to more aerobic-oxidative metabolism in a slower-twitching aged fibre population. The drastic increase in alphaB-crystallin and myotilin demonstrated substantial filament remodelling during ageing. An immunoblotting survey of selected muscle proteins confirmed the pathobiochemical transition process in aged muscle metabolism. The proteomic analysis of aged muscle has identified a large cohort of new biomarkers of sarcopenia including opposite changes in PK and AK, which might be useful for the design of improved diagnostic procedures and/or therapeutic strategies to counteract ageing-induced muscle degeneration.

The effects of Ankrd2 alteration indicate its involvement in cell cycle regulation during muscle differentiation

Ankrd2 is a member of the Muscle Ankyrin Repeat Protein family (MARPs), consisting of sarcomere-associated proteins that can also localize in the nucleus. There are indications that MARPs might function as shuttle proteins between the cytoplasm and nucleus, likely sending information to the nucleus concerning the changes in the structure or function of the contractile machinery. Even though recent findings suggest that the MARP gene family is not essential for the basal functioning of skeletal muscle, its influence on the gene expression program of skeletal muscle cells was highlighted. To investigate this regulatory role we produced and examined both morphological and functional features of myocytes stable overexpressing or silencing the Ankrd2 protein. The transcriptional profiles of the myocytes revealed that the molecular pathways perturbed by changes in Ankrd2 protein level are congruent with the morpho-physiological and biochemical data obtained in Ankrd2-modified myoblasts induced to differentiate. Our results suggest that Ankrd2 gives an important contribution to the coordination of proliferation and apoptosis during myogenic differentiation in vitro, mainly through the p53 network.

Mechanotransduction in striated muscle via focal adhesion kinase

Contractile tissues demonstrate a pronounced capacity to remodel their composition in response to mechanical challenges. Descriptive evidence suggests the upstream involvement of the phosphotransfer enzyme FAK (focal adhesion kinase) in the molecular control of load-dependent muscle plasticity. Thereby FAK evolves as a myocellular transducer of mechanical signals towards downstream transcript expression in myofibres. Recent advances in somatic gene therapy now allow the exploration of the functional involvement of this enzyme in mechanotransduction in intact muscle.

Molecular biomarkers monitoring human skeletal muscle fibres and microvasculature following long-term bed rest with and without countermeasures

The cellular mechanisms of human skeletal muscle adaptation to disuse are largely unknown. The aim of this study was to determine the morphological and biochemical changes of the lower limb soleus and vastus lateralis muscles following 60 days of head-down tilt bed rest in women with and without exercise countermeasure using molecular biomarkers monitoring functional cell compartments. Muscle biopsies were taken before (pre) and after bed rest (post) from a bed rest-only and a bed rest exercise group (n = 8, each). NOS1 and NOS3/PECAM, markers of myofibre 'activity' and capillary density, and MuRF1 (E3 ubiquitin-ligase), a marker of proteolysis, were documented by confocal immunofluorescence and immunoblot analyses. Morphometrical parameters (myofibre cross-sectional area, type I/II distribution) were largely preserved in muscles from the exercise group with a robust trend for type II hypertrophy in vastus lateralis. In the bed rest-only group, the relative NOS1 immunostaining intensity was decreased at type I and II myofibre membranes, while the bed rest plus exercise group compensated for this loss particularly in soleus. In the microvascular network, NOS3 expression and the capillary-to-fibre ratio were both increased in the exercise group. Elevated MuRF1 immunosignals found in subgroups of atrophic myofibres probably reflected accelerated proteolysis. Immunoblots revealed overexpression of the MuRF1 protein in the soleus of the bed rest-only group (> 35% vs. pre). We conclude that exercise countermeasure during bed rest affected both NOS/NO signalling and proteolysis in female skeletal muscle. Maintenance of NO signalling mechanisms and normal protein turnover by exercise countermeasure may be crucial steps to attenuate human skeletal muscle atrophy and to maintain cell function following chronic disuse.

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.

Recovery of skeletal muscle mass after extensive injury: positive effects of increased contractile activity

The present study was designed to test the hypothesis that increasing physical activity by running exercise could favor the recovery of muscle mass after extensive injury and to determine the main molecular mechanisms involved. Left soleus muscles of female Wistar rats were degenerated by notexin injection before animals were assigned to either a sedentary group or an exercised group. Both regenerating and contralateral intact muscles from active and sedentary rats were removed 5, 7, 14, 21, 28 and 42 days after injury (n = 8 rats/group). Increasing contractile activity through running exercise during muscle regeneration ensured the full recovery of muscle mass and muscle cross-sectional area as soon as 21 days after injury, whereas muscle weight remained lower even 42 days postinjury in sedentary rats. Proliferator cell nuclear antigen and MyoD protein expression went on longer in active rats than in sedentary rats. Myogenin protein expression was higher in active animals than in sedentary animals 21 days postinjury. The Akt-mammalian target of rapamycin (mTOR) pathway was activated early during the regeneration process, with further increases of mTOR phosphorylation and its downstream effectors, eukaryotic initiation factor-4E-binding protein-1 and p70(s6k), in active rats compared with sedentary rats (days 7-14). The exercise-induced increase in mTOR phosphorylation, independently of Akt, was associated with decreased levels of phosphorylated AMP-activated protein kinase. Taken together, these results provided evidence that increasing contractile activity during muscle regeneration ensured early and full recovery of muscle mass and suggested that these beneficial effects may be due to a longer proliferative step of myogenic cells and activation of mTOR signaling, independently of Akt, during the maturation step of muscle regeneration.

Contrast between cardiac left ventricle and diaphragm muscle in expression of genes involved in carbohydrate and lipid metabolism

The heart and diaphragm both need appropriate metabolic machinery to ensure long-term energy supplies, as they must contract rhythmically without cessation for the entire lifetime of the organism to ensure homeostasis of oxygen and carbon dioxide exchange. However, their energy requirements differ due to disparities in mechanical loads. Understanding how these two muscles converge and diverge in their approaches to meeting their metabolic demands may suggest novel strategies for improving cardiac and skeletal muscle long-term performance in health and disease. To assess this at a transcriptional level, expression of genes involved in carbohydrate and lipid metabolism was assessed using microarrays in rats. There were 594 genes with >2-fold differential expression between left ventricle of the heart and diaphragm; 307 were expressed heart>diaphragm and 287 diaphragm>heart. Assignment to gene ontology groups revealed over-representation for "carbohydrate metabolism" (P=0.005, n=32 genes or 5.4% of all genes with differential expression) and "lipid metabolism" (P=0.0012, n=48 genes or 8.1% of all genes with differential expression). For carbohydrate there were 14 genes with heart>diaphragm and 18 genes with diaphragm>heart, and for lipid there were 30 genes with heart>diaphragm and 18 genes with diaphragm>heart. The magnitude of differential expression between heart and diaphragm ranged up to 30-fold for carbohydrate and up to 59-fold for lipid. Carbohydrate-related genes were almost all involved in energy metabolism (e.g. Pfkm, Pgm1, Pgam1, Pfkfb1, Pfkfb2), whereas lipid-related genes were involved in energetics as well as other cellular processes; for both groups this included genes involved in rate-limiting metabolic steps. Data thus indicate that diaphragm and heart have both shared and differential transcriptional strategies for ensuring long-term energy supplies, with a relative favoring of lipid metabolism in the heart and carbohydrate metabolism in the diaphragm.

cDNA cloning and characterization of temperature-acclimation-associated light meromyosins from grass carp fast skeletal muscle

The three types of cDNA clones, previously defined as the 10 degrees C, intermediate and 30 degrees C-types [Tao, Y., Kobayashi, M., Liang, C.S., Okamoto, T., Watabe, S., 2004. Temperature-dependent expression patterns of grass carp fast skeletal myosin heavy chain genes. Comp. Biochem. Physiol. B 139, 649-656], were determined for their 5'-regions which encoded at least the C-terminal half of myosin rod, light meromyosin (LMM), in fast skeletal muscles of grass carp Ctenopharyngodon idella. The deduced amino acid sequence identity was 91.1% between the 10 degrees C and 30 degrees C-types and 91.4% between the 10 degrees C and intermediate-types, whereas a high sequence identity of 97.8% was found between the intermediate and 30 degrees C-types. These three grass carp LMMs all had a characteristic seven-residue (heptad) repeat (a, b, c, d, e, f, g)(n), where positions a and d were normally occupied by hydrophobic residues, and positions b, c and f by charged residues. However, the ratios of hydrophobic residues to the total were higher for the intermediate- and 30 degrees C- than 10 degrees C-type LMM, suggesting that the former both types may form more stable coiled-coils of alpha-helices than the latter type. These differences in the primary structures of LMM isoforms might be partially implicated in differences in the thermostabilities and gel-forming profiles of myosins from grass carp in different seasons reported previously [Tao, Y., Kobayashi, M., Fukushima, H., Watabe, S., 2005. Changes in enzymatic and structural properties of grass carp fast skeletal myosin induced by the laboratory-conditioned thermal acclimation and seasonal acclimatization. Fish. Sci. 71, 195-204; Tao, Y., Kobayashi, M., Fukushima, H., Watabe, S., 2007. Changes in rheological properties of grass carp fast skeletal myosin induced by seasonal acclimatization. Fish. Sci. 73, 189-196].

Redox regulation of vitamin C transporter SVCT2 in C2C12 myotubes

We have previously demonstrated that skeletal muscle cells possess efficient systems for vitamin C accumulation; in particular, the SVCT2 transporter for ascorbic acid uptake seems to play a crucial role. In this study, we investigated the regulatory mechanism(s) accounting for SVCT2 activity in C2C12 myotubes. We found that transcription of the SVCT2 gene could be positively or negatively modulated by the presence of oxidant (H(2)O(2)) or antioxidant (lipoate) compounds, respectively. This redox-mediated regulation of SVCT2 expression seemed to be achieved via AP-1 and NF-kappaB signaling. Our findings could be relevant in skeletal muscle, where reactive oxygen species, naturally produced during physical exercise, can induce muscle damage. Thus, the redox-sensitive SVCT2 expression can be placed among the adaptive responses induced by contractile activity.

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.

Plasticity of human skeletal muscle: gene expression toin vivofunction

Human skeletal muscle is a highly heterogeneous tissue, able to adapt to the different challenges that may be placed upon it. When overloaded, a muscle adapts by increasing its size and strength through satellite-cell-mediated mechanisms, whereby protein synthesis is increased and new nuclei are added to maintain the myonuclear domain. This process is regulated by an array of mechanical, hormonal and nutritional signals. Growth factors, such as insulin-like growth factor I (IGF-I) and testosterone, are potent anabolic agents, whilst myostatin acts as a negative regulator of muscle mass. Insulin-like growth factor I is unique in being able to stimulate both the proliferation and the differentiation of satellite cells and works as part of an important local repair and adaptive mechanism. Speed of movement, as characterized by maximal velocity of shortening (V(max)), is regulated primarily by the isoform of myosin heavy chain (MHC) contained within a muscle fibre. Human fibres can express three MHCs: MHC-I, -IIa and -IIx, in order of increasing V(max) and maximal power output. Training studies suggest that there is a subtle interplay between the MHC-IIa and -IIx isoforms, with the latter being downregulated by activity and upregulated by inactivity. However, switching between the two main isoforms appears to require significant challenges to a muscle. Upregulation of fast gene programs is caused by prolonged disuse, whilst upregulation of slow gene programs appears to require significant and prolonged activity. The potential mechanisms by which alterations in muscle composition are mediated are discussed. The implications in terms of contractile function of altering muscle phenotype are discussed from the single fibre to the whole muscle level.

Proteomic profiling of animal models mimicking skeletal muscle disorders

Over the last few decades of biomedical research, animal models of neuromuscular diseases have been widely used for determining pathological mechanisms and for testing new therapeutic strategies. With the emergence of high-throughput proteomics technology, the identification of novel protein factors involved in disease processes has been decisively improved. This review outlines the usefulness of the proteomic profiling of animal disease models for the discovery of new reliable biomarkers, for the optimization of diagnostic procedures and the development of new treatment options for skeletal muscle disorders. Since inbred animal strains show genetically much less interindividual differences as compared to human patients, considerably lower experimental repeats are capable of producing meaningful proteomic data. Thus, animal model proteomics can be conveniently employed for both studying basic mechanisms of molecular pathogenesis and the effects of drugs, genetic modifications or cell-based therapies on disease progression. Based on the results from comparative animal proteomics, a more informed decision on the design of clinical proteomics studies could be reached. Since no one animal model represents a perfect pathobiochemical replica of all of the symptoms seen in complex human disorders, the proteomic screening of novel animal models can also be employed for swift and enhanced protein biochemical phenotyping.

Muscle transcriptome adaptations with mild eccentric ergometer exercise

The muscle has a wide range of possibilities to adapt its phenotype. Repetitive submaximal concentric exercise (i.e., shortening contractions) mainly leads to adaptations of muscle oxidative metabolism and endurance while eccentric exercise (i.e., lengthening contractions) results in muscle growth and gain of muscle strength. Modified gene expression is believed to mediate these exercise-specific muscle adjustments. In the present study, early alterations of the gene expression signature were monitored by a muscle-specific microarray. Transcript profiling was performed on muscle biopsies of vastus lateralis obtained from six male subjects before and in a 24-h time course after a single bout of mild eccentric ergometer exercise. The eccentric exercise consisted of 15 min of eccentric cycling at 50% of the individual maximal concentric power output leading to muscle soreness (5.9 on a 0-10 visual analogue scale) and limited muscle damage (1.7-fold elevated creatine kinase activity). Muscle impairment was highlighted by a transient reduction in jumping height after the eccentric exercise. On the gene expression level, we observed a general early downregulation of detected transcripts, followed by a slow recovery close to the control values within the first 24 h post exercise. Only very few regulatory factors were increased. This expression signature is different from the signature of a previously published metabolic response after an intensive endurance-type concentric exercise as well as after maximal eccentric exercise. This is the first description of the time course of changes in gene expression as a consequence of a mild eccentric stimulus.

Cellular patterns of the atrophic response in murine soleus and gastrocnemius muscles submitted to simulated weightlessness

The purpose of the present study was to investigate the mechanisms of cell death (apoptosis vs. necrosis) during muscle atrophy induced by 1 week of hindlimb suspension. Biochemical and morphological parameters were examined in murine soleus and gastrocnemius muscles. A total of 70 male Charles River CD1 mice were randomly assigned to seven groups (n = 10/group): Cont (loading control conditions) and 6HS, 12HS, 24HS, 48HS, 72HS and 1wkHS with respect to the period of hindlimb suspension (HS). Compared to the Cont, skeletal muscle atrophy was confirmed by a significant decrease of 44 and of 17% in fiber cross-sectional areas in the gastrocnemius and soleus, respectively. A significant increase in caspase-3 activity was noticed in 6HS (196%, P < 0.05) and in 12HS (201%, P < 0.05), as well as the amount of cytosolic mono- and oligonucleosomes at 12HS (142%, P < 0.05) and 24HS (203%, P < 0.05) in the gastrocnemius and soleus, respectively. The profile of necrotic markers showed a peak of myeloperoxidase activity at 24HS (170%, P < 0.05) and at 72HS (114%, P < 0.05) in the gastrocnemius and soleus, respectively. The analysis of N-acetylglucosaminidase activity evidenced more increment in the soleus at 72HS (60%, P < 0.05). The analysis of the basal values of these parameters suggested that apoptosis prevailed in the slow-twitch muscle analyzed, whereas lysosomic activity seemed to be more pronounced in the gastrocnemius. The morphological data supported the biochemical results pointing towards a shift from apoptosis to necrosis, which seems to corroborate the aponecrosis theory.

Fuel economy in food‐deprived skeletal muscle: signaling pathways and regulatory mechanisms

Energy deprivation poses a tremendous challenge to skeletal muscle. Glucose (ATP) depletion causes muscle fibers to undergo rapid adaptive changes toward the use of fatty acids (instead of glucose) as fuel. Physiological situations involving energy deprivation in skeletal muscle include exercise and fasting. A vast body of evidence is available on the signaling pathways that lead to structural/metabolic changes in muscle during exercise and endurance training. In contrast, only recently has a systematic, overall picture been obtained of the signaling processes (and their kinetics and sequential order) that lead to adaptations of the muscle to the fasting state. It has become clear that the reaction of the organism to food restraint or deprivation involves a rapid signaling process causing skeletal muscles, which generally use glucose as their predominant fuel, to switch to the use of fat as fuel. Efficient sensing of glucose depletion in skeletal muscle guarantees maintained activity in those tissues that rely entirely on glucose (such as the brain). To metabolize fatty acids, skeletal muscle needs to activate complex transcription, translation, and phosphorylation pathways. Only recently has it become clear that these pathways are interrelated and tightly regulated in a rapid, transient manner. Food deprivation may trigger these responses with a timing/intensity that differs among animal species and that may depend on their individual ability to induce structural/metabolic changes that serve to safeguard whole-body energy homeostasis in the longer term. The increased cellular AMP/ATP ratio induced by food deprivation, which results in activation of AMP-activated protein kinase (AMPK), initiates a rapid signaling process, resulting in the recruitment of factors mediating the structural/metabolic shift in skeletal muscle toward this change in fuel usage. These factors include peroxisome proliferator-activated receptor (PPAR)gamma coactivator-1alpha (PGC-1alpha), PPARdelta, and their target genes, which are involved in the formation of oxidative muscle fibers, mitochondrial biogenesis, oxidative phosphorylation, and fatty acid oxidation. Fatty acids, besides being the fuel for mitochondrial oxidation, have been identified as important signaling molecules regulating the transcription and/or activity of the genes or gene products involved in fatty acid metabolism during food deprivation. It is thus becoming increasingly clear that fatty acids determine the economy of their own usage. We discuss the order of events from the onset of food deprivation and their importance.

Deterioration of contractile properties of muscle fibres in elderly subjects is modulated by the level of physical activity

The impact of ageing on force and velocity of human skeletal muscle fibres has been extensively studied. As discrepancies have been reported, it is still unclear whether or not a deterioration of the capacity of muscle fibres to develop force and shortening is involved in determining weakness and decrease in shortening velocity of skeletal muscle of elderly people. We compared myosin heavy chain (MHC) isoform distribution of vastus lateralis muscle, and specific force (Po/CSA) and maximum shortening velocity (Vo) of skeletal muscle fibres among one population of young controls (CTRL) and three populations of elderly (EL) subjects with very variable levels of physical activity: sedentary (EL-SED, n = 3); controls (EL-CTRL, n = 4); endurance trained (EL-END, n = 3). Muscle phenotype was progressively faster in the order EL-END --> CTRL --> EL-CTRL --> EL-SED. Po/CSA and Vo also varied among the different populations of elderly subjects generally showing a decreasing deterioration with increasing activity levels. The results suggest that discrepancies observed so far in age-induced deterioration of contractile properties of muscle fibres could depend on the different activity levels of the populations of elderly subjects enrolled in the studies.

Skeletal muscle fiber type comparison of pyruvate dehydrogenase phosphatase activity and isoform expression in fed and food-deprived rats

Fiber type specificity of pyruvate dehydrogenase (PDH) phosphatase (PDP) was determined in fed (CON) and 48-h food-deprived (FD) rats. PDP activity and isoform protein content were determined in soleus (slow-twitch oxidative), red gastrocnemius (RG; fast-twitch oxidative glycolytic), and white gastrocnemius (WG; fast-twitch glycolytic) muscles. When normalized for mitochondrial volume, there was no difference in PDP activity between muscle types or CON and FD. When expressed per gram wet tissue weight, PDP activity was higher in RG compared with soleus and WG in both CON and FD rats. PDP activities from CON muscles were 1.48 +/- 0.19, 2.68 +/- 0.65, and 1.20 +/- 0.33 nmol x min(-1) x g wet tissue wt(-1) in soleus, RG, and WG, respectively, and decreased in FD muscles (1.22 +/- 0.22, 2.00 +/- 0.57, and 0.84 +/- 0.18 nmol x min(-1) x g wet tissue wt(-1)). This correlated with increased PDP2 protein, however, only in RG, as PDP2 was not detectable in soleus or WG. PDP1 protein was not responsive to food deprivation in all fiber types. In conclusion, PDP activity and protein content were higher in fast-twitch oxidative glycolytic muscles from CON and FD rats, identifying a unique inter- and intramuscular distribution. FD induced a small but significant decrease in PDP activity that was partially due to decreases in PDP2 protein. As a result, coordinate changes to PDP activity opposite to those of the other regulatory enzyme, PDH kinase, during food deprivation would maximize the inactivation of skeletal muscle PDH and enhance carbohydrate conservation during periods of limited carbohydrate supply.

Effect of hindlimb suspension on activation and MHC content of triceps brachii and on the representation of forepaw on the sensorimotor cortex

The aims of this work were to analyze the effects of a chronic (14 days) increase in the functional demand imposed on the triceps brachii and to evaluate the changes of the cortical representation of forelimb to this increased activity. The activation of triceps brachii was obtained by the hindlimb unloading (HU) model. Electromyographic activity changed from a phasic to a tonic pattern. Response amplitude increased during the first days of hyperactivity and then stabilized at an intermediate level. A transient decrease (-13% to -36% on day 2) in the mean frequency of motor units was observed. Content in myosin heavy chain of muscle fibers showed a reduction in IIb+IIx fibers in HU rats, whereas IIa+IIx fibers were more numerous. Thus, fibers tend to be more resistant to fatigue. Taken together, these observations reveal a dual plastic process. First, the nervous system reacts immediately to an environmental change, and second it reorganizes its motor command to impose a pattern of activity that is more adapted to a postural function. The extent of the cortical forelimb representation was delimited by oxidase histochemistry. No differences were detectable between control and HU animals for the period corresponding to enlarged receptive fields in the HU condition. Our observation lends support to our hypothesis that activation patterns contribute to the maintenance of neuronal properties in the somatosensory cortex. Moreover, the new tonic pattern resulting from the long contact of the paw with the floor may contribute to the adaptation of the central control of motoneuronal activity.

Musclin gene expression is strongly related to fast-glycolytic phenotype

Musclin has been described as a muscle-derived secretory peptide, responsive to insulin in vivo, and inducing insulin resistance in vitro. Because muscle fibers display very different metabolic properties and insulin sensitivity, we tested the hypothesis that musclin expression could depend on myofiber type. Musclin mRNA was detected at high level in fast gastrocnemius and plantaris muscles, but only as traces in soleus, a slow-twitch muscle. A single fiber analysis showed that musclin was produced by muscle fibers themselves, almost exclusively type IIb fibers. Slow to fast transition of soleus phenotype after hindlimb suspension increased musclin mRNA levels, whereas fast to slow transition of plantaris phenotype after functional overload decreased musclin mRNA levels. This clearly suggests that musclin transcription is strongly related to fast-glycolytic phenotype. We conclude that musclin is produced by myocytes in a highly fiber-type specific manner and that physiological changes in type IIb MHC lead to coordinated musclin expression.

Acute and long-term effects of botulinum neurotoxin on the function and structure of developing extraocular muscles

Strabismus is a misalignment of the visual axes, due to an imbalance in extraocular muscle (EOM) function. Botulinum neurotoxin (BoNT) treatment can correct the misalignment with permanent therapeutic effects in infants, possibly because the toxin causes structural alterations in developing EOM. To determine whether BoNT indeed permanently weakens developing EOMs, we examined the chicken oculomotor system. Following injections of BoNT in hatchling chicks, we quantified physiological parameters (contractile force measurements) and morphological parameters (myofiber morphometry, innervation, quantitative transmission electron microscopy of mitochondria/fiber types). Treatment of developing EOM with BoNT caused acute reductions of muscle strength and mitochondrial densities, but minimal changes in muscle fiber diameter and neuromuscular junction structures. Contrary to expectations, contractile force was fully recovered by 3-4 months after treatment. Thus, permanent therapeutic effects of BoNT most likely do not cause permanent changes at the level of the peripheral effector organ, but rather involve central (CNS) adaptive responses.

Exercise training in normobaric hypoxia in endurance runners. III. Muscular adjustments of selected gene transcripts

We hypothesized that specific muscular transcript level adaptations participate in the improvement of endurance performances following intermittent hypoxia training in endurance-trained subjects. Fifteen male high-level, long-distance runners integrated a modified living low-training high program comprising two weekly controlled training sessions performed at the second ventilatory threshold for 6 wk into their normal training schedule. The athletes were randomly assigned to either a normoxic (Nor) (inspired O2 fraction = 20.9%, n = 6) or a hypoxic group exercising under normobaric hypoxia (Hyp) (inspired O2 fraction = 14.5%, n = 9). Oxygen uptake and speed at second ventilatory threshold, maximal oxygen uptake (VO2 max), and time to exhaustion (Tlim) at constant load at VO2 max velocity in normoxia and muscular levels of selected mRNAs in biopsies were determined before and after training. VO2 max (+5%) and Tlim (+35%) increased specifically in the Hyp group. At the molecular level, mRNA concentrations of the hypoxia-inducible factor 1alpha (+104%), glucose transporter-4 (+32%), phosphofructokinase (+32%), peroxisome proliferator-activated receptor gamma coactivator 1alpha (+60%), citrate synthase (+28%), cytochrome oxidase 1 (+74%) and 4 (+36%), carbonic anhydrase-3 (+74%), and manganese superoxide dismutase (+44%) were significantly augmented in muscle after exercise training in Hyp only. Significant correlations were noted between muscular mRNA levels of monocarboxylate transporter-1, carbonic anhydrase-3, glucose transporter-4, and Tlim only in the group of athletes who trained in hypoxia (P < 0.05). Accordingly, the addition of short hypoxic stress to the regular endurance training protocol induces transcriptional adaptations in skeletal muscle of athletic subjects. Expressional adaptations involving redox regulation and glucose uptake are being recognized as a potential molecular pathway, resulting in improved endurance performance in hypoxia-trained subjects.

Exertional heat illness and human gene expression

Microarray analysis of gene expression at the level of RNA has generated new insights into the relationship between cellular responses to acute heat shock in vitro, exercise, and exertional heat illness. Here we discuss the systemic physiology of exertional hyperthermia and exertional heat illness, and compare the results of several recent microarray studies performed in vitro on human cells subjected to heat shock and in vivo on samples obtained from subjects performing exercise or suffering from exertional heat injury. From these comparisons, a concept of overlapping component responses emerges. Namely, some of the gene expression changes observed in peripheral blood mononuclear cells during exertional heat injury can be accounted for by normal cellular responses to heat, exercise, or both; others appear to be specific to the disease state itself. If confirmed in future studies, these component responses might provide a better understanding of adaptive and pathological responses to exercise and exercise-induced hyperthermia, help find new ways of identifying individuals at risk for exertional heat illness, and perhaps even help find rational molecular targets for therapeutic intervention.

The role of HIF-1 in hypoxic response in the skeletal muscle

During endurance training, exercising skeletal muscle experiences severe and repetitive oxygen stress, and the muscle's ability to cope with and improve its function through that stress is central to its role in the body. The primary transcriptional response factor for hypoxic adaptation is hypoxia inducible factor-1alpha (HIF-1alpha), which upregulates glycolysis and angiogenesis in response to low levels of tissue oxygenation. To examine the role of HIF-1alpha in endurance training, we have created mice specifically lacking skeletal muscle HIF-1alpha and subjected them to an endurance training protocol. We found that only wild type mice improve their oxidative capacity, as measured by the respiratory exchange ratio; surprisingly, we found that HIF-1alpha null mice have already upregulated this parameter without training. Furthermore, untrained HIF-1alpha null mice have an increased capillary to fiber ratio, and elevated oxidative enzyme activities. These changes correlate with constitutively activated AMP-activated protein kinase in the HIF-1alpha null muscles. Additionally, HIF-1alpha null muscles have decreased expression of pyruvate dehydrogenase kinase I, a HIF-1alpha target that inhibits oxidative metabolism. This data demonstrates that removal of HIF-1alpha causes an adaptive response in skeletal muscle akin to endurance training, and provides evidence for the suppression of mitochondrial biogenesis by HIF-1alpha in normal tissue.

Suppression of testosterone does not blunt mRNA expression of myoD, myogenin, IGF, myostatin or androgen receptor post strength training in humans

We hypothesized that suppression of endogenous testosterone blunts mRNA expression post strength training (ST). Twenty-two young men were randomized for treatment with the GnRH analogue goserelin (3.6 mg every 4 weeks) or placebo for a period of 12 weeks. The ST period of 8 weeks started at week 4. Strength test, blood sampling, muscle biopsies, and whole-body dual-energy X-ray absorptiometry (DXA) scan were performed at weeks 4 and 12. Muscle biopsies were taken during the final ST session (pre, post 4 h, and post 24 h). Resting serum testosterone decreased significantly (P < 0.01) in the goserelin group from 22.6 +/- 1.6 (mean +/- s.e.m.) to 2.0 +/- 0.1 nmol l(-1) (week 4), whereas it remained unchanged in the placebo group. An acute increase of serum testosterone was observed during the final ST session in the placebo group (P < 0.05), whereas a decreased response was observed in the goserelin group (P < 0.05). mRNA expression of IGF-IE(bc) and myogenin increased, while expression of myostatin decreased (P < 0.01); however, no differences were observed between the groups. Muscle strength and muscle mass showed a tendency to increase more in the placebo group than in the goserelin group (P = 0.05). In conclusion, despite blocked acute responses of testosterone and 10- to 20-fold lower resting levels in the goserelin group, ST resulted in a similar mRNA expression of myoD, myogenin, IGF-IE(abc), myostatin and androgen receptor as observed in the placebo group. Therefore, in the present study, the molecular events were the same, despite divergent muscle hypertrophy and strength gains.

PGC1alpha expression is controlled in skeletal muscles by PPARbeta, whose ablation results in fiber-type switching, obesity, and type 2 diabetes

Mice in which peroxisome proliferator-activated receptor beta (PPARbeta) is selectively ablated in skeletal muscle myocytes were generated to elucidate the role played by PPARbeta signaling in these myocytes. These somatic mutant mice exhibited a muscle fiber-type switching toward lower oxidative capacity that preceded the development of obesity and diabetes, thus demonstrating that PPARbeta is instrumental in myocytes to the maintenance of oxidative fibers and that fiber-type switching is likely to be the cause and not the consequence of these metabolic disorders. We also show that PPARbeta stimulates in myocytes the expression of PGC1alpha, a coactivator of various transcription factors, known to play an important role in slow muscle fiber formation. Moreover, as the PGC1alpha promoter contains a PPAR response element, the effect of PPARbeta on the formation and/or maintenance of slow muscle fibers can be ascribed, at least in part, to a stimulation of PGC1alpha expression at the transcriptional level.

Effect of Exercise on mRNA Levels for Growth Factors in Skeletal Muscle of Hemodialysis Patients

OBJECTIVES

Muscle mass and muscle mRNA levels for certain growth factors are reduced in maintenance hemodialysis (MHD) patients. This study tested the hypothesis that in MHD patients endurance exercise training (EET) increases mRNA levels for insulin-like growth factors and reduces myostatin mRNA.

DESIGN

Biopsies of the right vastus lateralis muscle were performed before and at the end of 8.9 +/- 0.9 (SEM) weeks of EET in MHD patients. Muscle tissue was analyzed histologically by electron microscopy and for fiber cross-sectional area, and, in 8 pairs of biopsies, muscle was examined for mRNA levels for the following proteins: myostatin, insulin-like growth factor-I (IGF-I), IGF-I receptor (IGF-IR), IGF binding proteins (IGFBPs)-1, -2, -3, -4, and -5, and IGF-binding protein-related protein-1 (IGFBP-rP1).

SETTING

Outpatient MHD centers.

PATIENTS

This was a pilot study conducted in sedentary clinically stable MHD patients undergoing EET with no control group.

INTERVENTION

EET that was carefully supervised by exercise trainers.

MAIN OUTCOME MEASURE

Skeletal muscle mRNA levels, especially myostatin mRNA.

RESULTS

With EET, skeletal muscle myostatin mRNA decreased by 51%, mRNA levels increased significantly for IGF-IR (by 41%), IGFBP-2, -4, and -5, and IGFBP-rP1. IGF-I mRNA increased by 35%; this change was not significant. IGFBP-3 mRNA did not change, and IGFBP-1 mRNA was undetectable. There were mild to moderate alterations in skeletal muscle ultrastructure that did not change significantly with EET. Muscle fiber size, measured in 5 patients, did not change.

CONCLUSION

In MHD patients who undergo approximately 9 weeks of EET, skeletal muscle mRNA for myostatin decreases and mRNA for IGF-IR, IGFBPs -2, -4, and -5 and IGFBP-rP1 increases. These changes may indicate mechanisms by which EET improves muscle exercise capacity in MHD patients.

Development of a diagnostic method for detecting increased muscle protein degradation in patients with catabolic conditions

Muscle atrophy in catabolic illnesses is due largely to accelerated protein degradation. Unfortunately, methods for detecting accelerated muscle proteolysis are cumbersome. The goal of this study was to develop a method for detecting muscle protein breakdown and assess the effectiveness of anticatabolic therapy. In rodent models of catabolic conditions, it was found that accelerated muscle protein degradation is triggered by activation of caspase-3. Caspase-3 cleaves actomyosin/myofibrils to form substrates for the ubiquitin-proteasome system and leaves a characteristic 14-kD actin fragment in the insoluble fraction of a muscle lysate. Muscle biopsies were obtained from normal adults and three groups of patients: 14 who were undergoing hip arthroplasty, 28 hemodialysis patients who were participating in exercise programs, and seven severely burned patients. In muscle of patients who were undergoing hip arthroplasty, the 14-kD actin fragment level was correlated (r = 0.787, P < 0.01) with the fractional rate of protein degradation. In muscle of hemodialysis patients who were undergoing endurance exercise training, the 14-kD actin fragment decreased to values similar to levels in normal adults; strength training did not significantly decrease the actin fragment. Severely burned patients had increased muscle protein degradation and actin fragment levels, but the two measures were not significantly correlated. The experimental results suggest that the 14-kD actin fragment in muscle biopsies is increased in catabolic states and could be used in conjunction with other methods to detect and monitor changes in muscle proteolysis that occur in patients with mild or sustained increases in muscle proteolysis.

Effects of intermittent hypoxic training on amino and fatty acid oxidative combustion in human permeabilized muscle fibers

The effects of concurrent hypoxic/endurance training on mitochondrial respiration in permeabilized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1-W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired Po(2) (Pi(O(2)))] or hypoxic conditions ( approximately 100 Torr Pi(O(2)), approximately 3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) increased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O(2) uptake (Vo(2 max)) remained unchanged: 58.1 +/- 0.8 vs. 61.0 +/- 1.2 ml.kg(-1).min(-1) and 58.5 +/- 0.7 vs. 58.3 +/- 0.6 ml.kg(-1).min(-1) for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 +/- 0.37 vs. 8.51 +/- 0.33 mumol O(2).min(-1).g dry weight(-1)) and significantly decreased for palmitate + malate (3.88 +/- 0.23 vs. 2.77 +/- 0.08 mumol O(2).min(-1).g dry weight(-1)) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in Vo(2 max), and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.

Temperature- and exercise-induced gene expression and metabolic enzyme changes in skeletal muscle of adult zebrafish (Danio rerio)

Both exercise training and cold acclimatization induce muscle remodelling in vertebrates, producing a more aerobic phenotype. In ectothermic species exercise training and cold-acclimatization represent distinct stimuli. It is currently unclear if these stimuli act through a common mechanism or if different mechanisms lead to a common phenotype. The goal of this study was to survey responses that represent potential mechanisms responsible for contraction- and temperature-induced muscle remodelling, using an ectothermic vertebrate. Separate groups of adult zebrafish (Danio rerio) were either swim trained or cold acclimatized for 4 weeks. We found that the mitochondrial marker enzyme citrate synthase (CS) was increased by 1.5x in cold and by 1.3x with exercise (P<0.05). Cytochrome c oxidase (COx) was increased by 1.2x following exercise training (P<0.05) and 1.2x (P=0.07) with cold acclimatization. However, only cold acclimatization increased beta-hydroxyacyl-CoA dehydrogenase (HOAD) compared to exercise-trained (by 1.3x) and pyruvate kinase (PK) relative to control zebrafish. We assessed the whole-animal performance outcomes of these treatments. Maximum absolute sustained swimming speed (Ucrit) was increased in the exercise trained group but not in the cold acclimatized group. Real-time PCR analysis indicated that increases in CS are primarily transcriptionally regulated with exercise but not with cold treatments. Both treatments showed increases in nuclear respiratory factor (NRF)-1 mRNA which was increased by 2.3x in cold-acclimatized and 4x in exercise-trained zebrafish above controls. In contrast, peroxisome proliferator-activated receptor (PPAR)-alpha mRNA levels were decreased in both experimental groups while PPAR-beta1 declined in exercise training only. Moreover, PPAR-gamma coactivator (PGC)-1alpha mRNA was not changed by either treatment. In zebrafish, both temperature and exercise produce a more aerobic phenotype, but there are stimulus-dependent responses (i.e. HOAD and PK activities). While similar changes in NRF-1 mRNA suggest that common responses might underlie aerobic muscle remodelling there are distinct changes (i.e. CS and PPAR-beta1 mRNA) that contribute to specific temperature- and exercise-induced phenotypes.

Acetyl-l-carnitine feeding to unloaded rats triggers in soleus muscle the coordinated expression of genes involved in mitochondrial biogenesis

The expressional profile of mitochondrial transcripts and of genes involved in the mitochondrial biogenesis pathway induced by ALCAR daily supplementation in soleus muscle of control and unloaded 3-month-old rats has been analyzed. It has been found that ALCAR treatment is able to upregulate the expression level of mitochondrial transcripts (COX I, ATP6, ND6, 16 S rRNA) in both control and unloaded animals. Interestingly, ALCAR feeding to unloaded rats resulted in the increase of transcript level for master factors involved in mitochondrial biogenesis (PGC-1alpha, NRF-1, TFAM). It also prevented the unloading-induced downregulation of mRNA levels for kinases able to transduce metabolic (AMPK) and neuronal stimuli (CaMKIIbeta) into mitochondrial biogenesis. No significant effect on the expressional level of such genes was found in control ALCAR-treated rats. In addition, ALCAR feeding was able to prevent the loss of mitochondrial protein content due to unloading condition. Correlation analysis revealed a strong coordination in the expression of genes involved in mitochondrial biogenesis only in ALCAR-treated suspended animals, supporting a differentiated effect of ALCAR treatment in relation to the loading state of the soleus muscle. In conclusions, we demonstrated the ability of ALCAR supplementation to promote only in soleus muscle of hindlimb suspended rats an orchestrated expression of genes involved in mitochondrial biogenesis, which might counteract the unloading-induced metabolic changes, preventing the loss of mitochondrial proteins.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.