Application of Animal Models: Chronic Electrical Stimulation-Induced Contractile Activity

Indirect Structural Muscle Injuries of Lower Limb: Rehabilitation and Therapeutic Exercise

Muscle injuries are the most common trauma in team and individual sports. The muscles most frequently affected are those of the lower limb, and in particular hamstrings, adductors, rectus femoris and calf muscles. Although several scientific studies have tried to propose different rehabilitation protocols, still too often the real rehabilitation process is not based on scientific knowledge, especially in non-elite athletes. Moreover, the growing use of physical and instrumental therapies has made it increasingly difficult to understand what can be truly effective. Therefore, the aim of the present paper is to review proposed therapeutic algorithms for muscle injuries, proposing a concise and practical summary. Following a three-phase rehabilitation protocol, this review aims to describe the conservative treatment of indirect structural muscle injuries, which are the more routinely found and more challenging type. For each phase, until return to training and return to sport are completed, the functional goal, the most appropriate practitioner, and the best possible treatment according to current evidence are expressed. Finally, the last section is focused on the specific exercise rehabilitation for the four main muscle groups with a structured explanatory timetable.

Chronic electrical stimulation reduces reliance on anaerobic metabolism in locust jumping muscle

Chronic electrical stimulation (CES) is a well-documented method for changing mammalian muscle from more fast-twitch to slow-twitch metabolic and contractile profiles. Although both mammalian and insect muscles have many similar anatomical and physiological properties, it is unknown if CES produces similar muscle plasticity changes in insects. To test this idea, we separated Schistocerca americana grasshoppers into two groups (n = 37 to 47): one that was subjected to CES for 180 min each day for five consecutive days and one group that was not. Each group was then electrically stimulated for a single time period (0, 5, 30, 60, or 180 min) before measuring jumping muscle lactate, a characteristic of fast-twitch type fibers. At each time point, CES led to a significantly reduced jumping muscle lactate concentration. Based on similar short-term CES mammalian studies, the reduction in lactate production was most likely due to a reduced reliance on anaerobic metabolism. Thus, longer stimulation periods should result in greater aerobic enzymatic activities, altered myosin ATPase, and shift fiber types. This is the first study to use electrical stimulation to explore insect muscle plasticity and our results show that grasshopper jumping muscle responds similarly to mammalian muscle.

Establishment of an acute extraocular muscle injury model in cats

AIM

To describe an acute extraocular muscle injury model in cats.

METHODS

Seventy-two cats were randomly divided into 6 groups (12 cats per group). Cats' left lateral recti were clamped using a surgical needle holder with a clamping strength of 2 (Groups A and D), 4 (Groups B and E) and 6 kg (Groups C and F). The right lateral recti were treated as controls. On the 4th and 7th days, hematoxylin eosin (HE) staining, immunohistochemical staining for proliferating cell nuclear antigen (PCNA), muscle force measurements and ocular alignment changes were performed to evaluate the extent of injuries.

RESULTS

The morphological changes were graded as mild, moderate or severe by HE staining in all experiment groups. PCNA immunohistochemical staining indicated repairment of muscle fibers in the damaged area. On the 4th and 7th days after clamping, the injured lateral muscle exhibited an elevated threshold for electric stimulation. The muscle forces among groups 2, 4 and 6 kg injury at 4d (Groups A, B and C) were statistically significant (P<0.05), but no significant differences were noted among groups 2, 4 and 6 kg injury at 7d (Groups D, E and F) (P>0.05), respectively. In addition, medial deviation in ocular alignment was also present to various degrees in all groups.

CONCLUSION

A cat model of acute extraocular muscle injury can be established by rectus clamping. Different clamping strengths can make different degrees of muscle injury. This model may help the future study in the acute extraocular muscle injury.

Italian consensus conference on guidelines for conservative treatment on lower limb muscle injuries in athlete

Provide the state of the art concerning (1) biology and aetiology, (2) classification, (3) clinical assessment and (4) conservative treatment of lower limb muscle injuries (MI) in athletes. Seventy international experts with different medical backgrounds participated in the consensus conference. They discussed and approved a consensus composed of four sections which are presented in these documents. This paper represents a synthesis of the consensus conference, the following four sections are discussed: (i) The biology and aetiology of MIs. A definition of MI was formulated and some key points concerning physiology and pathogenesis of MIs were discussed. (ii) The MI classification. A classification of MIs was proposed. (iii) The MI clinical assessment, in which were discussed anamnesis, inspection and clinical examination and are provided the relative guidelines. (iv) The MI conservative treatment, in which are provided the guidelines for conservative treatment based on the severity of the lesion. Furthermore, instrumental therapy and pharmacological treatment were discussed. Knowledge of the aetiology and biology of MIs is an essential prerequisite in order to plan and conduct a rehabilitation plan. Another important aspect is the use of a rational MI classification on prognostic values. We propose a classification based on radiological investigations performed by ultrasonography and MRI strongly linked to prognostic factors. Furthermore, the consensus conference results will able to provide fundamental guidelines for diagnostic and rehabilitation practice, also considering instrumental therapy and pharmacological treatment of MI. Expert opinion, level IV.

Application of Chronic Stimulation to Study Contractile Activity-induced Rat Skeletal Muscle Phenotypic Adaptations

Skeletal muscle is a highly adaptable tissue, as its biochemical and physiological properties are greatly altered in response to chronic exercise. To investigate the underlying mechanisms that bring about various muscle adaptations, a number of exercise protocols such as treadmill, wheel running, and swimming exercise have been used in the animal studies. However, these exercise models require a long period of time to achieve muscle adaptations, which may be also regulated by humoral or neurological factors, thus limiting their applications in studying the muscle-specific contraction-induced adaptations. Indirect low frequency stimulation (10 Hz) to induce chronic contractile activity (CCA) has been used as an alternative model for exercise training, as it can successfully lead to muscle mitochondrial adaptations within 7 days, independent of systemic factors. This paper details the surgical techniques required to apply the treatment of CCA to the skeletal muscle of rats, for widespread application in future studies.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

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

Chronology of UPR activation in skeletal muscle adaptations to chronic contractile activity

The mitochondrial and endoplasmic reticulum unfolded protein responses (UPR(mt) and UPR(ER)) are important for cellular homeostasis during stimulus-induced increases in protein synthesis. Exercise triggers the synthesis of mitochondrial proteins, regulated in part by peroxisome proliferator activator receptor-γ coactivator 1α (PGC-1α). To investigate the role of the UPR in exercise-induced adaptations, we subjected rats to 3 h of chronic contractile activity (CCA) for 1, 2, 3, 5, or 7 days followed by 3 h of recovery. Mitochondrial biogenesis signaling, through PGC-1α mRNA, increased 14-fold after 1 day of CCA. This resulted in 10-32% increases in cytochrome c oxidase activity, indicative of mitochondrial content, between days 3 and 7, as well as increases in the autophagic degradation of p62 and microtubule-associated proteins 1A/1B light chain 3A (LC3)-II protein. Before these adaptations, the UPR(ER) transcripts activating transcription factor-4, spliced X-box-binding protein 1, and binding immunoglobulin protein were elevated (1.3- to 3.8-fold) at days 1-3, while CCAAT/enhancer-binding protein homologous protein (CHOP) and chaperones binding immunoglobulin protein and heat shock protein (HSP) 70 were elevated at mRNA and protein levels (1.5- to 3.9-fold) at days 1-7 of CCA. The mitochondrial chaperones 10-kDa chaperonin, HSP60, and 75-kDa mitochondrial HSP, the protease ATP-dependent Clp protease proteolytic subunit, and the regulatory protein sirtuin-3 of the UPR(mt) were concurrently induced 10-80% between days 1 and 7 To test the role of the UPR in CCA-induced remodeling, we treated animals with the endoplasmic reticulum stress suppressor tauroursodeoxycholic acid and subjected them to 2 or 7 days of CCA. Tauroursodeoxycholic acid attenuated CHOP and HSP70 protein induction; however, this failed to impact mitochondrial remodeling. Our data indicate that signaling to the UPR is rapidly activated following acute contractile activity, that this is attenuated with repeated bouts, and that the UPR is involved in chronic adaptations to CCA; however, this appears to be independent of CHOP signaling.

Mitochondria, muscle health, and exercise with advancing age

Skeletal muscle health is dependent on the optimal function of its mitochondria. With advancing age, decrements in numerous mitochondrial variables are evident in muscle. Part of this decline is due to reduced physical activity, whereas the remainder appears to be attributed to age-related alterations in mitochondrial synthesis and degradation. Exercise is an important strategy to stimulate mitochondrial adaptations in older individuals to foster improvements in muscle function and quality of life.

Muscle-Specific Myosin Heavy Chain Shifts in Response to a Long-Term High Fat/High Sugar Diet and Resveratrol Treatment in Nonhuman Primates

Shifts in myosin heavy chain (MHC) expression within skeletal muscle can be induced by a host of stimuli including, but not limited to, physical activity, alterations in neural activity, aging, and diet or obesity. Here, we hypothesized that both age and a long-term (2 year) high fat/high sugar diet (HFS) would induce a slow to fast MHC shift within the plantaris, soleus, and extensor digitorum longus (EDL) muscles from rhesus monkeys. Furthermore, we tested whether supplementation with resveratrol, a naturally occurring compound that has been attributed with augmenting aerobic potential through mitochondrial proliferation, would counteract any diet-induced MHC changes by promoting a fast to slow isoform switch. In general, we found that MHC isoforms were not altered by aging during mid-life. The HFS diet had the largest impact within the soleus muscle where the greatest slow to fast isoform shifts were observed in both mRNA and protein indicators. As expected, long-term resveratrol treatment counteracted, or blunted, these diet-induced shifts within the soleus muscle. The plantaris muscle also demonstrated a fast-to-slow phenotypic response to resveratrol treatment. In conclusion, diet or resveratrol treatment impacts skeletal muscle phenotype in a muscle-specific manner and resveratrol supplementation may be one approach for promoting the fatigue-resistant MHC (type I) isoform especially if its expression is blunted as a result of a long-term high fat/sugar diet.

Exercise and the Regulation of Mitochondrial Turnover

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Humanized animal exercise model for clinical implication

Exercise and physical activity function as a patho-physiological process that can prevent, manage, and regulate numerous chronic conditions, including metabolic syndrome and age-related sarcopenia. Because of research ethics and technical difficulties in humans, exercise models using animals are requisite for the future development of exercise mimetics to treat such abnormalities. Moreover, the beneficial or adverse outcomes of a new regime or exercise intervention in the treatment of a specific condition should be tested prior to implementation in a clinical setting. In rodents, treadmill running (or swimming) and ladder climbing are widely used as aerobic and anaerobic exercise models, respectively. However, exercise models are not limited to these types. Indeed, there are no golden standard exercise modes or protocols for managing or improving health status since the types (aerobic vs. anaerobic), time (morning vs. evening), and duration (continuous vs. acute bouts) of exercise are the critical determinants for achieving expected beneficial effects. To provide insight into the understanding of exercise and exercise physiology, we have summarized current animal exercise models largely based on aerobic and anaerobic criteria. Additionally, specialized exercise models that have been developed for testing the effect of exercise on specific physiological conditions are presented. Finally, we provide suggestions and/or considerations for developing a new regime for an exercise model.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

Exercise training combined with electromyostimulation in the rehabilitation of patients with chronic heart failure: A randomized trial

AIM

Both aerobic training (AT) and electromyostimulation (EMS) of leg muscles improve exercise tolerance in patients suffering from chronic heart failure (CHF). It was speculated that combination of both methods might have an additive effect. This study was performed to evaluate the effects of a combination of AT and EMS in rehabilitation (RHB) of CHF patients.

PATIENTS AND METHODS

Patients (n=71; age 59 ± 10.2 yrs, NYHA II/III, EF 32 ± 7.1%) were randomized into 3 groups: a) group AT, b) group EMS, and c) group AT+EMS. AT protocol included standard activity on bicycle 3x a week at the level of individual anaerobic threshold. EMS (10 Hz, mode 20s "on"/20s "off") was applied to leg extensors for 2 h/day. Total time of given type of RHB was 12 weeks.

RESULTS

Data analysis revealed statistically significant improvements of patients in all experimental groups (averaged difference after 12 weeks of exercise as related to initial value: ∆VO2peak: +12.9%, ∆VO2AT: +9.3%, ∆Wpeak: +22.7%). No statistically significant difference among experimental groups was found. Quality of life (Minnesota Living with Heart Failure - MLHF) global score was significantly improved in all 3 groups: AT (∆MLHF: -27.9%; P=0.001), AT+EMS (∆MLHF: -29.1%; P=0.002), and EMS (∆MLHF: -16.6%; P=0.008). MLHF score in EMS group showed the smallest time-related improvement compared to AT and AT+EMS groups, and this difference in improvement between the groups was statistically significant (P=0.021).

CONCLUSION

No significant difference was found between the two types of exercise training.and nor did, their combination have any significant additional improvement.

Regeneration of skeletal muscle

Skeletal muscle has a robust capacity for regeneration following injury. However, few if any effective therapeutic options for volumetric muscle loss are available. Autologous muscle grafts or muscle transposition represent possible salvage procedures for the restoration of mass and function but these approaches have limited success and are plagued by associated donor site morbidity. Cell-based therapies are in their infancy and, to date, have largely focused on hereditary disorders such as Duchenne muscular dystrophy. An unequivocal need exists for regenerative medicine strategies that can enhance or induce de novo formation of functional skeletal muscle as a treatment for congenital absence or traumatic loss of tissue. In this review, the three stages of skeletal muscle regeneration and the potential pitfalls in the development of regenerative medicine strategies for the restoration of functional skeletal muscle in situ are discussed.

Identification of secondary effects of hyperexcitability by proteomic profiling of myotonic mouse muscle

Myotonia is a symptom of various genetic and acquired skeletal muscular disorders and is characterized by hyperexcitability of the sarcolemma. Here, we have performed a comparative proteomic study of the genetic mouse models ADR, MTO and MTO*5J of human congenital myotonia in order to determine myotonia-specific changes in the global protein complement of gastrocnemius muscle. Proteomic analyses of myotonia in the mouse, which is caused by mutations in the gene encoding the muscular chloride channel Clc1, revealed a generally perturbed protein expression pattern in severely affected ADR and MTO muscle, but less pronounced alterations in mildly diseased MTO*5J mice. Alterations were found in major metabolic pathways, the contractile machinery, ion handling elements, the cellular stress response and cell signaling mechanisms, clearly confirming a glycolytic-to-oxidative transformation process in myotonic fast muscle. In the long-term, a detailed biomarker signature of myotonia will improve our understanding of the pathobiochemical processes underlying this disorder and be helpful in determining how a single mutation in a tissue-specific gene can trigger severe downstream effects on the expression levels of a very large number of genes in contractile tissues.

Proteomics of skeletal muscle differentiation, neuromuscular disorders and fiber aging

Skeletal muscle fibers are the most abundant cellular structure in the human body. Altered neuromuscular activity, traumatic injury or genetic abnormalities have profound effects on muscle integrity, tissue mass, fiber type distribution, metabolic integration and contractile function. The recent application of mass spectrometry-based proteomics has decisively advanced our molecular understanding of numerous physiological adaptations in healthy muscle and pathophysiological mechanisms associated with major muscle diseases. Skeletal muscle proteomics promises to play a major role in the establishment of a disease-specific biomarker signature for the major classes of neuromuscular disorders. New muscle markers will be crucial for the development of improved diagnostics, the monitoring of disease progression, evaluation of drug action and the identification of novel therapeutic targets.

Effect of chronic contractile activity on mRNA stability in skeletal muscle

Repeated bouts of exercise promote the biogenesis of mitochondria by multiple steps in the gene expression patterning. The role of mRNA stability in controlling the expression of mitochondrial proteins is relatively unexplored. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 or 6 h/day) rat muscle for 7 days. Chronic contractile activity (CCA) increased the protein expression of PGC-1alpha, c-myc, and mitochondrial transcription factor A (Tfam) by 1.6-, 1.7- and 2.0-fold, respectively. To determine mRNA stability, we incubated total RNA with cytosolic extracts using an in vitro cell-free system. We found that the intrinsic mRNA half-lives (t(1/2)) were variable within control muscle. Peroxisome proliferator-activated receptor-gamma, coactivator-1alpha (PGC-1alpha) and Tfam mRNAs decayed more rapidly (t(1/2) = 22.7 and 31.4 min) than c-myc mRNA (t(1/2) = 99.7 min). Furthermore, CCA resulted in a differential response in degradation kinetics. After CCA, PGC-1alpha and Tfam mRNA half-lives decreased by 48% and 44%, respectively, whereas c-myc mRNA half-life was unchanged. CCA induced an elevation of both the cytosolic RNA-stabilizing human antigen R (HuR) and destabilizing AUF1 (total) by 2.4- and 1.8-fold, respectively. Increases in the p37(AUF1), p40(AUF1), and p45(AUF1) isoforms were most evident. Thus these data indicate that CCA results in accelerated turnover rates of mRNAs encoding important mitochondrial biogenesis regulators in skeletal muscle. This adaptation is likely beneficial in permitting more rapid phenotypic plasticity in response to subsequent contractile activity.

Relationship between Sirt1 expression and mitochondrial proteins during conditions of chronic muscle use and disuse

Sirt1 is a NAD(+)-dependent histone deacetylase that interacts with the regulatory protein of mitochondrial biogenesis PGC-1alpha and is sensitive to metabolic alterations. We assessed whether a strict relationship between the expression of Sirt1, mitochondrial proteins, and PGC-1alpha existed across tissues possessing a wide range of oxidative capabilities, as well as in skeletal muscle subject to chronic use (voluntary wheel running or electrical stimulation for 7 days, 10 Hz; 3 h/day) or disuse (denervation for up to 21 days) in which organelle biogenesis is altered. PGC-1alpha levels were not closely associated with the expression of Sirt1, measured using immunoblotting or via enzymatic deacetylase activity. The mitochondrial protein cytochrome c increased by 70-90% in soleus and plantaris muscles of running animals, whereas Sirt1 activity remained unchanged. In chronically stimulated muscle, cytochrome c was increased by 30% compared with nonstimulated muscle, whereas Sirt1 activity was increased modestly by 20-25%. In contrast, in denervated muscle, these markers of mitochondrial content were decreased by 30-50% compared with the control muscle, whereas Sirt1 activity was increased by 75-80%. Our data suggest that Sirt1 and PGC-1alpha expression are independently regulated and that, although Sirt1 activity may be involved in mitochondrial biogenesis, its expression is not closely correlated to changes in mitochondrial proteins during conditions of chronic muscle use and disuse.

Molecular basis for an attenuated mitochondrial adaptive plasticity in aged skeletal muscle

Our intent was to investigate the mechanisms driving the adaptive potential of subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria in young (6 mo) and senescent (36 mo) animals in response to a potent stimulus for organelle biogenesis. We employed chronic electrical stimulation (10 Hz, 3 h/day, 7 days) to induce contractile activity of skeletal muscle in 6 and 36 mo F344XBN rats. Subsequent to chronic activity, acute stimulation (1 Hz, 5 min) in situ revealed greater fatigue resistance in both age groups. However, the improvement in endurance was significantly greater in the young, compared to the old animals. Chronic muscle use also augmented SS and IMF mitochondrial volume to a greater extent in young muscle. The molecular basis for the diminished organelle expansion in aged muscle was due, in part, to the collective attenuation of the chronic stimulation-evoked increase in regulatory proteins involved in mediating mitochondrial protein import and biogenesis. Furthermore, adaptations in mitochondrial function were also blunted in old animals. However, chronic contractile activity evoked greater reductions in mitochondrially-mediated proapoptotic signaling in aged muscle. Thus, mitochondrial plasticity is retained in aged animals, however the magnitude of the changes are less compared to young animals due to attenuated molecular processes regulating organelle biogenesis.

Specific attenuation of protein kinase phosphorylation in muscle with a high mitochondrial content

Acute contractile activity increases the activation of protein kinases involved in signal transduction. We hypothesized that the contractile activity-induced kinase phosphorylation would occur to a lesser degree in muscle with elevated mitochondrial content. We compared red and white sections of tibialis anterior (TA) muscle with two- to threefold differences in mitochondrial volume, and we increased the mitochondrial content in the TA muscle by 40% with unilateral chronic stimulation-induced contractile activity (10 Hz, 7 days, 3 h/day). Both the chronically stimulated and the contralateral control muscles were then acutely stimulated in situ for 15 min (10 Hz). We investigated 1) the total protein content and 2) the phosphorylation of kinases important for mitochondrial biogenesis in skeletal muscle, including AMPKalpha and p44, p42, and p38 MAPKs, as well as Akt by immunoblotting. In response to chronic stimulation, a selective upregulation of kinase protein content was observed, suggesting unique transcriptional/translational processing for these enzymes. Inverse relationships were observed between mitochondrial volume and 1) kinase protein content and 2) basal levels of kinase phosphorylation. In general, the kinase phosphorylation response to acute exercise depended, in part, on the oxidative capacity of the fiber type, evidenced by a greater absolute level of acute contractile activity-induced kinase signaling in muscle with a lower mitochondrial volume. The attenuation of contraction-evoked kinase phosphorylation in muscle with high mitochondrial content suggests that these proteins may become less sensitive to upstream signaling and require greater stimulation for activation to propagate these adaptive cues downstream toward transcription or translation events.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

Diminished contraction-induced intracellular signaling towards mitochondrial biogenesis in aged skeletal muscle

The intent of this study was to determine whether aging affects signaling pathways involved in mitochondrial biogenesis in response to a single bout of contractile activity. Acute stimulation (1 Hz, 5 min) of the tibialis anterior (TA) resulted in a greater rate of fatigue in old (36 month), compared to young (6 month) F344XBN rats, which was associated with reduced ATP synthesis and a lower mitochondrial volume. To investigate fiber type-specific signaling, the TA was sectioned into red (RTA) and white (WTA) portions, possessing two- to 2.5-fold differences in mitochondrial content. The expression and contraction-mediated phosphorylation of p38, MKK3/6, CaMKII and AMPKalpha were assessed. Kinase protein expression tended to be higher in fiber sections with lower mitochondrial content, such as the WTA, relative to the RTA muscle, and this was exaggerated in tissues from senescent, compared to young animals. At rest, kinase activation was generally similar between young and old animals, despite the age-related variations in mitochondrial volume. In response to contractile activity, age did not influence the signaling of these kinases in the high-oxidative RTA muscle. However, in the low-oxidative WTA muscle, contraction-induced kinase activation was attenuated in old animals, despite the greater metabolic stress imposed by contractile activity in this muscle. Thus, the reduction of contraction-evoked kinase phosphorylation in muscle from old animals is fiber type-specific, and depends on factors which are, in part, independent of the metabolic milieu within the contracting fibers. These findings imply that the downstream consequences of kinase signaling are reduced in aging muscle.

Stem cells for the treatment of skeletal muscle injury

Skeletal muscle injuries are extremely common, accounting for up to 35%-55% of all sports injuries and quite possibly affecting all musculoskeletal traumas. These injuries result in the formation of fibrosis, which may lead to the development of painful contractures, increases patients' risk for repeat injuries, and limits their ability to return to a baseline or pre-injury level of function. The development of successful therapies for these injuries must consider the pathophysiology of these musculoskeletal conditions. We discuss the direct use of muscle-derived stem cells and some key cell population dynamics as well as the use of clinically applicable modalities that may enhance the local supply of stem cells to the zone of injury by promoting angiogenesis.

Kinase-specific responsiveness to incremental contractile activity in skeletal muscle with low and high mitochondrial content

Muscle contractions activate protein kinases, leading to signal transduction. We hypothesized that kinase activation would be influenced by mitochondrial content, as well as by contractile activity-induced increases in muscle O(2) consumption (Vo(2)). Kinase phosphorylation in high-oxidative red and low-oxidative white tibialis anterior (TA) muscle (RTA and WTA, respectively) with 2.5-fold differences in mitochondrial content were compared. Stimulation of the TA muscle elicited large increases in Vo(2) (3- to 6-fold and 4- to 60-fold above resting levels in WTA and RTA, respectively). At rest, AMP-activated protein kinase (AMPK), p38, p42, and p44 activation were nearly twofold greater in WTA than in RTA, suggesting an inverse relationship between mitochondrial content and kinase activation in resting muscle. During contractions, similar degrees of phosphorylation in RTA and WTA were evident as a function of Vo(2) for p38 and p42. During increases in Vo(2) up to sixfold above rest, greater responses were observed in RTA than in WTA for AMPK and p44, whereas Akt activation was greater in WTA. In RTA, elevations in Vo(2) elicited increases in AMPK and p44 activation, whereas Akt, p38, and p42 were less sensitive to increments in Vo(2). Reactive oxygen species (ROS) production was greater in mitochondria from white muscle, but when it was calculated in the context of the whole muscle, ROS production was twofold greater in red than in white myofibers. Thus mitochondrial content influences ROS production and is inversely related to kinase activation in resting muscle. During contractions, kinases are differentially sensitive to contraction-induced increments in Vo(2), suggesting that muscle mitochondrial content is important, but it is not the sole determinant of kinase activation during exercise of different intensities.

Proteomic profiling of chronic low-frequency stimulated fast muscle

Skeletal muscle fibre transitions occur in many biological processes, in response to alterations in neuromuscular activity, in muscular disorders, during age-induced muscle wasting and in myogenesis. It was therefore of interest to perform a comprehensive proteomic profiling of muscle transformation. Chronic low-frequency stimulation of the rabbit tibialis anterior muscle represents an established model system for studying the response of fast fibres to enhanced neuromuscular activity under conditions of maximum activation. We have conducted a DIGE analysis of unstimulated control specimens versus 14- and 60-day conditioned muscles. A differential expression pattern was observed for 41 protein species with 29 increased and 12 decreased muscle proteins. Identified classes of proteins that are changed during the fast-to-slow transition process belong to the contractile machinery, ion homeostasis, excitation-contraction coupling, capillarization, metabolism and stress response. Results from immunoblotting agreed with the conversion of the metabolic, regulatory and contractile molecular apparatus to support muscle fibres with slower twitch characteristics. Besides confirming established muscle elements as reliable transition markers, this proteomics-based study has established the actin-binding protein cofilin-2 and the endothelial marker transgelin as novel biomarkers for evaluating muscle transformation.

Effect of chronic contractile activity on SS and IMF mitochondrial apoptotic susceptibility in skeletal muscle

Chronic contractile activity of skeletal muscle induces an increase in mitochondria located in proximity to the sarcolemma [subsarcolemmal (SS)] and in mitochondria interspersed between the myofibrils [intermyofibrillar (IMF)]. These are energetically favorable metabolic adaptations, but because mitochondria are also involved in apoptosis, we investigated the effect of chronic contractile activity on mitochondrially mediated apoptotic signaling in muscle. We hypothesized that chronic contractile activity would provide protection against mitochondrially mediated apoptosis despite an elevation in the expression of proapoptotic proteins. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 h/day) rat muscle for 7 days. Chronic contractile activity did not alter the Bax/Bcl-2 ratio, an index of apoptotic susceptibility, and did not affect manganese superoxide dismutase levels. However, contractile activity increased antiapoptotic 70-kDa heat shock protein and apoptosis repressor with a caspase recruitment domain by 1.3- and 1.4-fold (P<0.05), respectively. Contractile activity elevated SS mitochondrial reactive oxygen species (ROS) production 1.4- and 1.9-fold (P<0.05) during states IV and III respiration, respectively, whereas IMF mitochondrial state IV ROS production was suppressed by 28% (P<0.05) and was unaffected during state III respiration. Following stimulation, exogenous ROS treatment produced less cytochrome c release (25-40%) from SS and IMF mitochondria, and also reduced apoptosis-inducing factor release (approximately 30%) from IMF mitochondria, despite higher inherent cytochrome c and apoptosis-inducing factor expression. Chronic contractile activity did not alter mitochondrial permeability transition pore (mtPTP) components in either subfraction. However, SS mitochondria exhibited a significant increase in the time to Vmax of mtPTP opening. Thus, chronic contractile activity induces predominantly antiapoptotic adaptations in both mitochondrial subfractions. Our data suggest the possibility that chronic contractile activity can exert a protective effect on mitochondrially mediated apoptosis in muscle.

Neuromuscular electrical stimulation in neurorehabilitation

This review provides a comprehensive overview of the clinical uses of neuromuscular electrical stimulation (NMES) for functional and therapeutic applications in subjects with spinal cord injury or stroke. Functional applications refer to the use of NMES to activate paralyzed muscles in precise sequence and magnitude to directly accomplish functional tasks. In therapeutic applications, NMES may lead to a specific effect that enhances function, but does not directly provide function. The specific neuroprosthetic or "functional" applications reviewed in this article include upper- and lower-limb motor movement for self-care tasks and mobility, respectively, bladder function, and respiratory control. Specific therapeutic applications include motor relearning, reduction of hemiplegic shoulder pain, muscle strengthening, prevention of muscle atrophy, prophylaxis of deep venous thrombosis, improvement of tissue oxygenation and peripheral hemodynamic functioning, and cardiopulmonary conditioning. Perspectives on future developments and clinical applications of NMES are presented.

Effect of satellite cell ablation on low-frequency-stimulated fast-to-slow fibre-type transitions in rat skeletal muscle

The purpose of this study was to determine whether satellite cell ablation within rat fast-twitch muscles exposed to chronic low-frequency stimulation (CLFS) would limit fast-to-slow fibre-type transitions. Twenty-nine male Wistar rats were randomly assigned to one of three groups. Satellite cells of the left tibialis anterior were ablated by weekly exposure to a 25 Gy dose of gamma-irradiation during 21 days of CLFS (IRR-Stim), whilst a second group received only 21 days of CLFS (Stim). A third group received weekly doses of gamma-irradiation (IRR). Non-irradiated right legs served as internal controls. Continuous infusion of 5-bromo-2'-deoxyuridine (BrdU) revealed that CLFS induced an 8.0-fold increase in satellite cell proliferation over control (mean +/-s.e.m.: 23.9 +/- 1.7 versus 3.0 +/- 0.5 mm(-2), P < 0.0001) that was abolished by gamma-irradiation. M-cadherin and myogenin staining were also elevated 7.7- and 3.8-fold (P < 0.0001), respectively, in Stim compared with control, indicating increases in quiescent and terminally differentiating satellite cells; these increases were abolished by gamma-irradiation. Myonuclear content was elevated 3.3-fold (P < 0.0001) in Stim, but remained unchanged in IRR-Stim. Immunohistochemical analyses revealed attenuation of fast-to-slow fibre-type transitions in IRR-Stim compared with Stim. Comparable changes were observed at the protein level by SDS-PAGE. It is concluded that although considerable adaptive potential exists within myonuclei, satellite cells play a role in facilitating fast-to-slow fibre-type transitions.