Relationship between muscle fiber types and sizes and muscle architectural properties in the mouse hindlimb

Acute antibody-directed myostatin inhibition attenuates disuse muscle atrophy and weakness in mice

Counteracting the atrophy of skeletal muscle associated with disuse has significant implications for minimizing the wasting and weakness in plaster casting, joint immobilization, and other forms of limb unloading, with relevance to orthopedics, sports medicine, and plastic and reconstructive surgery. We tested the hypothesis that antibody-directed myostatin inhibition would attenuate the loss of muscle mass and functional capacity in mice during 14 or 21 days of unilateral hindlimb casting. Twelve-week-old C57BL/10 mice were subjected to unilateral hindlimb plaster casting or served as controls. Mice received subcutaneous injections of saline or a mouse chimera of anti-human myostatin antibody (PF-354, 10 mg/kg; n = 6-9) on days 0 and 7 and were tested for muscle function on day 14, or were treated on days 0, 7, and 14 and tested for muscle function on day 21. Hindlimb casting reduced muscle mass, fiber size, and function of isolated soleus and extensor digitorum longus (EDL) muscles (P < 0.05). PF-354 attenuated the loss of muscle mass, fiber size, and function with greater effects after 14 days than after 21 days of casting, when wasting and weakness had plateaued (P < 0.05). Antibody-directed myostatin inhibition therefore attenuated the atrophy and loss of functional capacity in muscles from mice subjected to unilateral hindlimb casting with reductions in muscle size and strength being most apparent during the first 14 days of disuse. These findings highlight the therapeutic potential of antibody-directed myostatin inhibition for disuse atrophy especially within the first 2 wk of disuse.

Lateral transmission of force is impaired in skeletal muscles of dystrophic mice and very old rats

The dystrophin–glycoprotein complex (DGC) provides an essential link from the muscle fibre cytoskeleton to the extracellular matrix. In dystrophic humans and mdx mice, mutations in the dystrophin gene disrupt the structure of the DGC causing severe damage to muscle fibres. In frog muscles, transmission of force laterally from an activated fibre to the muscle surface occurs without attenuation, but lateral transmission of force has not been demonstrated in mammalian muscles. A unique ‘yoke' apparatus was developed that attached to the epimysium of muscles midway between the tendons and enabled the measurement of lateral force. We now report that in muscles of young wild-type (WT) mice and rats, compared over a wide range of longitudinal forces, forces transmitted laterally showed little or no decrement. In contrast, for muscles of mdx mice and very old rats, forces transmitted laterally were impaired severely. Muscles of both mdx mice and very old rats showed major reductions in the expression of dystrophin. We conclude that during contractions, forces developed by skeletal muscles of young WT mice and rats are transmitted laterally from fibre to fibre through the DGC without decrement. In contrast, in muscles of dystrophic or very old animals, disruptions in DGC structure and function impair lateral transmission of force causing instability and increased susceptibility of fibres to contraction-induced injury.

Physiology, structure, and susceptibility to injury of skeletal muscle in mice lacking keratin 19-based and desmin-based intermediate filaments

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. Our previous results show that the tibialis anterior (TA) muscles of mice lacking keratin 19 (K19) lose costameres, accumulate mitochondria under the sarcolemma, and generate lower specific tension than controls. Here we compare the physiology and morphology of TA muscles of mice lacking K19 with muscles lacking desmin or both proteins [double knockout (DKO)]. K19-/- mice and DKO mice showed a threefold increase in the levels of creatine kinase (CK) in the serum. The absence of desmin caused a larger change in specific tension (-40%) than the absence of K19 (-19%) and played the predominant role in contractile function (-40%) and decreased tolerance to exercise in the DKO muscle. By contrast, the absence of both proteins was required to obtain a significantly greater loss of contractile torque after injury (-48%) compared with wild type (-39%), as well as near-complete disruption of costameres. The DKO muscle also showed a significantly greater misalignment of myofibrils than either mutant alone. In contrast, large subsarcolemmal gaps and extensive accumulation of mitochondria were only seen in K19-null TA muscles, and the absence of both K19 and desmin yielded milder phenotypes. Our results suggest that keratin filaments containing K19- and desmin-based intermediate filaments can play independent, complementary, or antagonistic roles in the physiology and morphology of fast-twitch skeletal muscle.

Monitoring murine skeletal muscle function for muscle gene therapy

The primary function of skeletal muscle is to generate force. Muscle force production is compromised in various forms of acquired and/or inherited muscle diseases. An important goal of muscle gene therapy is to recover muscle strength. Genetically engineered mice and spontaneous mouse mutants are readily available for preclinical muscle gene therapy studies. In this chapter, we outlined the methods commonly used for measuring murine skeletal muscle function. These include ex vivo and in situ analysis of the contractile profile of a single intact limb muscle (the extensor digitorium longus for ex vivo assay and the tibialis anterior muscle for in situ assay), grip force analysis, and downhill treadmill exercise. Force measurement in a single muscle is extremely useful for pilot testing of new gene therapy protocols by local gene transfer. Grip force and treadmill assessments offer body-wide evaluation following systemic muscle gene therapy.

Mitochondrial biogenesis and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) deacetylation by physical activity: intact adipocytokine signaling is required

OBJECTIVE Transcriptional peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) plays a key role in mitochondrial biogenesis and energy metabolism and is suggested to be involved in the exercise-induced increase in mitochondrial content. PGC-1α activity is regulated by posttranslational modifications, among them acetylation or phosphorylation. Accordingly, the deacetylase SIRT1 and the kinase AMPK increase PGC-1α activity. RESEARCH DESIGN AND METHODS We tested whether chronic treadmill exercise or a single exercise session modifies PGC-1α activation and mitochondrial biogenesis differentially in obese ob/ob mice with dysregulated adiponectin/leptin-mediated AMPK activation compared with C57BL/6J wild-type mice. RESULTS Exercise training (12 weeks) induced adiponectin and lowered plasma insulin and glucose, suggesting improved insulin sensitivity in wild-type mice. It enhanced mitochondrial biogenesis in red gastrocnemius muscle, as indicated by increased mRNA expression of transcriptional regulators and primary mitochondrial transcripts, increased mtDNA content, and citrate synthase activity. Parallel to this, we observed AMPK activation, PGC-1α deacetylation, and SIRT1 induction in trained wild-type mice. Although none of these exercise-induced changes were detected in ob/ob mice, comparable effects on mitochondrial respiration were observed. A single exercise session resulted in comparable changes in wild-type mice. These changes remained detectable 6 h after the exercise session but had disappeared after 24 h. Treatment of C2C12 myoblasts with leptin or adiponectin resulted in increased AMPK phosphorylation and PGC-1α deacetylation. CONCLUSIONS Chronic exercise induces mitochondrial biogenesis in wild-type mice, which may require intact AMPK activation by adipocytokines and involve SIRT1-dependent PGC-1α deacetylation. Trained ob/ob mice appear to have partially adapted to reduced mitochondrial biogenesis by AMPK/SIRT1/PGC-1α-independent mechanisms without mtDNA replication.

In vivo assessment of muscle fascicle length by extended field-of-view ultrasonography

The present study examined the reliability and validity of in vivo vastus lateralis (VL) fascicle length ( Lf) assessment by extended field-of-view ultrasonography (EFOV US). Intraexperimenter and intersession reliability of EFOV US were tested. Further, Lf measured from EFOV US images were compared to Lf measured from static US images (6-cm FOV) where out-of-field fascicle portions were trigonometrically estimated (linear extrapolation). Finally, spatial accuracy of the EFOV technique was assessed by comparing Lf measured on swine VL by EFOV US to actual measurements from digital photographs. The difference between repeated VL Lf measurements by the same experimenter was 2.1 ± 1.7% with an intraclass correlation (ICC) of 0.99 [95% confidence interval (CI) = 0.95–1.00]. In terms of intersession reliability, no difference ( P = 0.48) was observed between Lf measured on two different occasions, with ICC = 0.95 (CI = 0.80–0.99). The average absolute difference between Lf measured by EFOV US and using linear extrapolation was 12.6 ± 8.1% [ICC = 0.76 (CI = −0.20–0.94)]; EFOV Lf was always longer than extrapolated Lf. The relative error of measurement between Lf measured by EFOV US and by dissective assessment (digital photographs) in isolated swine VL was 0.84% ± 2.6% with an ICC of 0.99 (CI = 0.94–1.00). These results show that EFOV US is a reliable and valid method for the measurement of long muscle fascicle in vivo. Thus EFOV US analysis was proven more accurate for the assessment of skeletal muscle fascicle length than conventional extrapolation methods.

Nitrosative stress elicited by nNOSµ delocalization inhibits muscle force in dystrophin-null mice

The mechanism of force reduction is not completely understood in Duchenne muscular dystrophy (DMD), a dystrophin-deficient lethal disease. Nitric oxide regulates muscle force. Interestingly, neuronal nitric oxide synthase µ (nNOSµ), a major source of muscle nitric oxide, is lost from the sarcolemma in DMD muscle. We hypothesize that nNOSµ delocalization contributes to force reduction in DMD. To test this hypothesis, we generated dystrophin/nNOSµ double knockout mice. Genetic elimination of nNOSµ significantly enhanced force in dystrophin-null mice. Pharmacological inhibition of nNOS yielded similar results. To further test our hypothesis, we studied δ-sarcoglycan-null mice, a model of limb-girdle muscular dystrophy. These mice had minimal sarcolemmal nNOSµ delocalization and muscle force was less compromised. Annihilation of nNOSµ did not improve their force either. To determine whether nNOSµ delocalization itself inhibited force, we corrected muscle disease in dystrophin-null mice with micro-dystrophins that either restored or did not restore sarcolemmal nNOSµ. Similar muscle force was obtained irrespective of nNOSµ localization. Additional studies suggest that nNOSµ delocalization selectively inhibits muscle force in dystrophin-null mice via nitrosative stress. In summary, we have demonstrated for the first time that nitrosative stress elicited by nNOSµ delocalization is an important mechanism underlying force loss in DMD.

Lifting the Nebula: Novel Insights into Skeletal Muscle Contractility

Nebulin is a giant protein and a constituent of the skeletal muscle sarcomere. The name of this protein refers to its unknown (i.e., nebulous) function. However, recent rapid advances reveal that nebulin plays important roles in the regulation of muscle contraction. When these functions of nebulin are compromised, muscle weakness ensues, as is the case in patients with nemaline myoptahy.

Orderly recruitment of motor units under optical control in vivo

A drawback of electrical stimulation for muscle control is that large, fatigable motor units are preferentially recruited before smaller motor units by the lowest-intensity electrical cuff stimulation. This phenomenon limits therapeutic applications because it is precisely the opposite of the normal physiological (orderly) recruitment pattern; therefore, a mechanism to achieve orderly recruitment has been a long-sought goal in physiology, medicine and engineering. Here we demonstrate a technology for reliable orderly recruitment in vivo. We find that under optical control with microbial opsins, recruitment of motor units proceeds in the physiological recruitment sequence, as indicated by multiple independent measures of motor unit recruitment including conduction latency, contraction and relaxation times, stimulation threshold and fatigue. As a result, we observed enhanced performance and reduced fatigue in vivo. These findings point to an unanticipated new modality of neural control with broad implications for nervous system and neuromuscular physiology, disease research and therapeutic innovation.

Early response of heat shock proteins to functional overload of the soleus and plantaris in rats and mice

Heat shock proteins (HSPs) are important factors in the response of skeletal muscles to chronic increases or decreases in activation and loading. The purpose of this study was to compare species-, time- and muscle-dependent changes in protein expression of Hsp20, Hsp25, αB-crystallin, Hsp72 and Hsp90 in response to functional overload (FO) in rats and mice. We compared protein levels of Hsp20, Hsp25, αB-crystallin, Hsp72 and Hsp90 in soleus and plantaris in baseline conditions and following 0.5, 1, 2, 3 and 7 days (rats) or 3 and 7 days (mice) of FO. Baseline levels of all HSPs were higher in rat soleus than plantaris, whereas only baseline expression of Hsp20 was higher in mouse soleus than plantaris. Levels of Hsp72 and Hsp90 were higher in plantaris and soleus of FO than control mice and rats after 3 and 7 days of FO. Protein levels and phosphorylation of Hsp25 in mouse plantaris and soleus were higher than control levels after 3 and 7 days of FO, except for soleus at 3 days. αB-crystallin levels were higher in plantaris of FO than control mice after 3 and 7 days of FO and in FO than control rats after 7 days of FO. Heat shock protein 20 was the least responsive, increasing only in 7 day FO rat plantaris compared with control rats. Overall, the results demonstrate that levels of both large and small HSPs increase with FO, suggesting a contributory role during the compensatory hypertrophy response.

Micro-computed tomography with iodine staining resolves the arrangement of muscle fibres

We illustrate here microCT images in which contrast between muscle and connective tissue has been achieved by means of staining with iodine. Enhancement is shown to be dependent on the concentration of iodine solution (I(2)KI), time in solution and specimen size. Histological examination confirms that the arrangement of individual muscle fibres can be visualised on the enhanced microCT images, and that the iodine accumulates in the muscle fibres in preference to the surrounding connective tissues. We explore the application of this technique to describe the fibrous structure of skeletal muscle, and conclude that it has the potential to become a non-destructive and cost-effective method for investigating muscle fascicle architecture, particularly in comparative morphological studies.

The effects of selective breeding on the architectural properties of the pelvic limb in broiler chickens: a comparative study across modern and ancestral populations

Intensive artificial selection has led to the production of the modern broiler chicken, which over the last few decades has undergone a dramatic increase in growth rate and noticeable changes in body conformation. Unfortunately, this has been associated with musculoskeletal abnormalities which have altered the walking ability of these birds, raising obvious welfare concerns, as well as causing economic losses. Here we present a comparative study of ancestral and derived muscle anatomy in chickens to begin to tease apart how evolutionary alterations of muscle form in chickens have influenced their locomotor function and perhaps contributed to lameness. We measured the muscle architectural properties of the right pelvic limb in 50 birds, including the Giant Junglefowl, a commercial strain broiler and four pureline commercial broiler breeder lines (from which the broiler populations are derived) to identify which features of the broiler's architectural design have diverged the most from the ancestral condition. We report a decline in pelvic limb muscle mass in the commercial line birds that may compromise their locomotor abilities because they carry a larger body mass. This greater demand on the pelvic limb muscles has mostly led to changes in support at the hip joint, revealing significantly larger abductors and additionally much larger medial rotators in the broiler population. Differences were seen within the commercial line bird populations, which are likely attributed to different selection pressures and may reflect differences in the walking ability of these birds. In addition, Junglefowl seem to have both greater force-generating capabilities and longer, presumably faster contracting muscles, indicative of superior musculoskeletal/locomotor function. We have provided baseline data for generating hypotheses to investigate in greater depth the specific biomechanical constraints that compromise the modern broiler's walking ability and propose that these factors should be considered in the selection for musculoskeletal health in the chickens of the future. Our new anatomical data for a wide range of domestic and wild-type chickens is useful in a comparative context and for deeper functional analysis including computer modelling/simulation of limb mechanics.

Skeletal muscle weakness in osteogenesis imperfecta mice

Exercise intolerance, muscle fatigue and weakness are often-reported, little-investigated concerns of patients with osteogenesis imperfecta (OI). OI is a heritable connective tissue disorder hallmarked by bone fragility resulting primarily from dominant mutations in the proα1(I) or proα2(I) collagen genes and the recently discovered recessive mutations in post-translational modifying proteins of type I collagen. In this study we examined the soleus (S), plantaris (P), gastrocnemius (G), tibialis anterior (TA) and quadriceps (Q) muscles of mice expressing mild (+/oim) and moderately severe (oim/oim) OI for evidence of inherent muscle pathology. In particular, muscle weight, fiber cross-sectional area (CSA), fiber type, fiber histomorphology, fibrillar collagen content, absolute, relative and specific peak tetanic force (P(o), P(o)/mg and P(o)/CSA respectively) of individual muscles were evaluated. Oim/oim mouse muscles were generally smaller, contained less fibrillar collagen, had decreased P(o) and an inability to sustain P(o) for the 300-ms testing duration for specific muscles; +/oim mice had a similar but milder skeletal muscle phenotype. +/oim mice had mild weakness of specific muscles but were less affected than their oim/oim counterparts which demonstrated readily apparent skeletal muscle pathology. Therefore muscle weakness in oim mice reflects inherent skeletal muscle pathology.

Tropomodulin isoforms regulate thin filament pointed-end capping and skeletal muscle physiology

In skeletal muscle fibers, tropomodulin 1 (Tmod1) can be compensated for, structurally but not functionally, by Tmod3 and -4.

During myofibril assembly, thin filament lengths are precisely specified to optimize skeletal muscle function. Tropomodulins (Tmods) are capping proteins that specify thin filament lengths by controlling actin dynamics at pointed ends. In this study, we use a genetic targeting approach to explore the effects of deleting Tmod1 from skeletal muscle. Myofibril assembly, skeletal muscle structure, and thin filament lengths are normal in the absence of Tmod1. Tmod4 localizes to thin filament pointed ends in Tmod1-null embryonic muscle, whereas both Tmod3 and -4 localize to pointed ends in Tmod1-null adult muscle. Substitution by Tmod3 and -4 occurs despite their weaker interactions with striated muscle tropomyosins. However, the absence of Tmod1 results in depressed isometric stress production during muscle contraction, systemic locomotor deficits, and a shift to a faster fiber type distribution. Thus, Tmod3 and -4 compensate for the absence of Tmod1 structurally but not functionally. We conclude that Tmod1 is a novel regulator of skeletal muscle physiology.

Peroxisome proliferator-activated receptor gamma coactivator 1alpha or 1beta overexpression inhibits muscle protein degradation, induction of ubiquitin ligases, and disuse atrophy

Overexpression of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha), like exercise, increases mitochondrial content and inhibits muscle atrophy. To understand these actions, we tested whether PGC-1alpha or its close homolog, PGC-1beta, influences muscle protein turnover. In myotubes, overexpression of either coactivator increased protein content by decreasing overall protein degradation without altering protein synthesis rates. Elevated PGC-1alpha or PGC-1beta also prevented the acceleration of proteolysis induced by starvation or FoxO transcription factors and prevented the induction of autophagy and atrophy-specific ubiquitin ligases by a constitutively active FoxO3. In mouse muscles, overexpression of PGC-1beta (like PGC-1alpha) inhibited denervation atrophy, ubiquitin ligase induction, and transcription by NFkappaB. However, increasing muscle PGC-1alpha levels pharmacologically by treatment of mice with 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside failed to block loss of muscle mass or induction of ubiquitin ligases upon denervation atrophy, although it prevented loss of mitochondria. This capacity of PGC-1alpha and PGC-1beta to inhibit FoxO3 and NFkappaB actions and proteolysis helps explain how exercise prevents muscle atrophy.

New insights into the structural roles of nebulin in skeletal muscle

One important feature of muscle structure and function that has remained relatively obscure is the mechanism that regulates thin filament length. Filament length is an important aspect of muscle function as force production is proportional to the amount of overlap between thick and thin filaments. Recent advances, due in part to the generation of nebulin KO models, reveal that nebulin plays an important role in the regulation of thin filament length. Another structural feature of skeletal muscle that is not well understood is the mechanism involved in maintaining the regular lateral alignment of adjacent sarcomeres, that is, myofibrillar connectivity. Recent studies indicate that nebulin is part of a protein complex that mechanically links adjacent myofibrils. Thus, novel structural roles of nebulin in skeletal muscle involve the regulation of thin filament length and maintaining myofibrillar connectivity. When these functions of nebulin are absent, muscle weakness ensues, as is the case in patients with nemaline myopathy with mutations in nebulin. Here we review these new insights in the role of nebulin in skeletal muscle structure.

Increased superoxide in vivo accelerates age-associated muscle atrophy through mitochondrial dysfunction and neuromuscular junction degeneration

Oxidative stress has been implicated in the etiology of age-related muscle loss (sarcopenia). However, the underlying mechanisms by which oxidative stress contributes to sarcopenia have not been thoroughly investigated. To directly examine the role of chronic oxidative stress in vivo, we used a mouse model that lacks the antioxidant enzyme CuZnSOD (Sod1). Sod1(-/-) mice are characterized by high levels of oxidative damage and an acceleration of sarcopenia. In the present study, we demonstrate that muscle atrophy in Sod1(-/-) mice is accompanied by a progressive decline in mitochondrial bioenergetic function and an elevation of mitochondrial generation of reactive oxygen species. In addition, Sod1(-/-) muscle exhibits a more rapid induction of mitochondrial-mediated apoptosis and loss of myonuclei. Furthermore, aged Sod1(-/-) mice show a striking increase in muscle mitochondrial content near the neuromuscular junctions (NMJs). Despite the increase in content, the function of mitochondria is significantly impaired, with increased denervated NMJs and fragmentation of acetylcholine receptors. As a consequence, contractile force in aged Sod1(-/-) muscles is greatly diminished. Collectively, we show that Sod1(-/-) mice display characteristics of normal aging muscle in an accelerated manner and propose that the superoxide-induced NMJ degeneration and mitochondrial dysfunction are potential mechanisms of sarcopenia.

Diversification of Intrinsic Motoneuron Electrical Properties During Normal Development and Botulinum Toxin–Induced Muscle Paralysis in Early Postnatal Mice

During early postnatal development, between birth and postnatal days 8–11, mice start to achieve weight-bearing locomotion. In association with the progression of weight-bearing locomotion there are presumed developmental changes in the intrinsic electrical properties of spinal α-motoneurons. However, these developmental changes in the properties of α-motoneuron properties have not been systematically explored in mice. Here, data are presented documenting the developmental changes of selected intrinsic motoneuron electrical properties, including statistically significant changes in action potential half-width, intrinsic excitability and diversity (quantified as coefficient of variation) of rheobase current, afterhyperpolarization half-decay time, and input resistance. In various adult mammalian preparations, the maintenance of intrinsic motoneuron electrical properties is dependent on activity and/or transmission-sensitive motoneuron–muscle interactions. In this study, we show that botulinum toxin–induced muscle paralysis led to statistically significant changes in the normal development of intrinsic motoneuron electrical properties in the postnatal mouse. This suggests that muscle activity during early neonatal life contributes to the development of normal motoneuron electrical properties.

Aerobic exercise alters analgesia and neurotrophin-3 synthesis in an animal model of chronic widespread pain

BACKGROUND

Present literature and clinical practice provide strong support for the use of aerobic exercise in reducing pain and improving function for individuals with chronic musculoskeletal pain syndromes. However, the molecular basis for the positive actions of exercise remains poorly understood. Recent studies suggest that neurotrophin-3 (NT-3) may act in an analgesic fashion in various pain states.

OBJECTIVE

The purpose of the present study was to examine the effects of moderate-intensity aerobic exercise on pain-like behavior and NT-3 in an animal model of widespread pain.

DESIGN

This was a repeated-measures, observational cross-sectional study.

METHODS

Forty female mice were injected with either normal (pH 7.2; n=20) or acidic (pH 4.0; n=20) saline in the gastrocnemius muscle to induce widespread hyperalgesia and exercised for 3 weeks. Cutaneous (von Frey monofilament) and muscular (forceps compression) mechanical sensitivity were assessed. Neurotrophin-3 was quantified in 2 hind-limb skeletal muscles for both messenger RNA (mRNA) and protein levels after exercise training. Data were analyzed with 2-factor analysis of variance for repeated measures (group x time).

RESULTS

Moderate-intensity aerobic exercise reduced cutaneous and deep tissue hyperalgesia induced by acidic saline and stimulated NT-3 synthesis in skeletal muscle. The increase in NT-3 was more pronounced at the protein level compared with mRNA expression. In addition, the increase in NT-3 protein was significant in the gastrocnemius muscle but not in the soleus muscle, suggesting that exercise can preferentially target NT-3 synthesis in specific muscle types.

LIMITATIONS

Results are limited to animal models and cannot be generalized to chronic pain syndromes in humans.

CONCLUSIONS

This is the first study demonstrating the effect of exercise on deep tissue mechanical hyperalgesia in a rodent model of pain and providing a possible molecular basis for exercise training in reducing muscular pain.

Repeatability of DTI-based skeletal muscle fiber tracking

Diffusion tensor imaging (DTI)-based muscle fiber tracking enables the measurement of muscle architectural parameters, such as pennation angle (theta) and fiber tract length (L(ft)), throughout the entire muscle. Little is known, however, about the repeatability of either the muscle architectural measures or the underlying diffusion measures. Therefore, the goal of this study was to investigate the repeatability of DTI fiber tracking-based measurements and theta and L(ft). Four DTI acquisitions were performed on two days that allowed for between acquisition, within day, and between day analyses. The eigenvalues and fractional anisotropy were calculated at the maximum cross-sectional area of, and fiber tracking was performed in, the tibialis anterior muscle of nine healthy subjects. The between acquisitions condition had the highest repeatability for the DTI indices and the architectural parameters. The overall inter class correlation coefficients (ICC's) were greater than 0.6 for both theta and L(ft) and the repeatability coefficients were theta < 10.2 degrees and L(ft) < 50 mm. In conclusion, under the experimental and data analysis conditions used, the repeatability of the diffusion measures is very good and repeatability of the architectural measurements is acceptable. Therefore, this study demonstrates the feasibility for longitudinal studies of alterations in muscle architecture using DTI-based fiber tracking, under similar noise conditions and with similar diffusion characteristics.

Nerve terminal degeneration is independent of muscle fiber genotype in SOD1 mice

Background

Motor neuron degeneration in SOD1^G93A transgenic mice begins at the nerve terminal. Here we examine whether this degeneration depends on expression of mutant SOD1 in muscle fibers.

Methodology/Principal Findings

Hindlimb muscles were transplanted between wild-type and SOD1^G93A transgenic mice and the innervation status of neuromuscular junctions (NMJs) was examined after 2 months. The results showed that muscles from SOD1^G93A mice did not induce motor terminal degeneration in wildtype mice and that muscles from wildtype mice did not prevent degeneration in SOD1^G93A transgenic mice. Control studies demonstrated that muscles transplanted from SOD1^G93A mice continued to express mutant SOD1 protein. Experiments on wildtype mice established that the host supplied terminal Schwann cells (TSCs) at the NMJs of transplanted muscles.

Conclusions/Significance

These results indicate that expression of the mutant protein in muscle is not needed to cause motor terminal degeneration in SOD1^G93A transgenic mice and that a combination of motor terminals, motor axons and Schwann cells, all of which express mutant protein may be sufficient.

The architectural design of the gluteal muscle group: implications for movement and rehabilitation

The organization of fibers within a muscle (architecture) defines the performance capacity of that muscle. In the current commentary, basic architectural terms are reviewed in the context of the major hip muscles and then specific illustrative examples relevant to lower extremity rehabilitation are presented. These data demonstrate the architectural and functional specialization of the hip muscles, and highlight the importance of muscle physiology and joint mechanics when evaluating and treating musculoskeletal disorders.

Conditional knockout of Mn-SOD targeted to type IIB skeletal muscle fibers increases oxidative stress and is sufficient to alter aerobic exercise capacity

In vitro studies of isolated skeletal muscle have shown that oxidative stress is limiting with respect to contractile function. Mitochondria are a potential source of muscle function-limiting oxidants. To test the hypothesis that skeletal muscle-specific mitochondrial oxidative stress is sufficient to limit muscle function, we bred mice expressing Cre recombinase driven by the promoter for the inhibitory subunit of troponin (TnIFast-iCre) with mice containing a floxed Sod2 (Sod2(fl/fl)) allele. Mn-SOD activity was reduced by 82% in glycolytic (mainly type II) muscle fiber homogenates from young TnIFastCreSod2(fl/fl) mice. Furthermore, Mn-SOD content was reduced by 70% only in type IIB muscle fibers. Aconitase activity was decreased by 56%, which suggests an increase in mitochondrial matrix superoxide. Mitochondrial superoxide release was elevated more than twofold by mitochondria isolated from glycolytic skeletal muscle in TnIFastCreSod2(fl/fl) mice. In contrast, the rate of mitochondrial H(2)O(2) production was reduced by 33%, and only during respiration with complex II substrate. F(2)-isoprostanes were increased by 36% in tibialis anterior muscles isolated from TnIFastCreSod2(fl/fl) mice. Elevated glycolytic muscle-specific mitochondrial oxidative stress and damage in TnIFastCreSod2(fl/fl) mice were associated with a decreased ability of the extensor digitorum longus and gastrocnemius muscles to produce contractile force as a function of time, whereas force production by the soleus muscle was unaffected. TnIFastCreSod2(fl/fl) mice ran 55% less distance on a treadmill than wild-type mice. Collectively, these data suggest that elevated mitochondrial oxidative stress and damage in glycolytic muscle fibers are sufficient to reduce contractile muscle function and aerobic exercise capacity.

Built for speed: musculoskeletal structure and sprinting ability

The musculoskeletal structure of the foot and ankle has the potential to influence human sprinting performance in complex ways. A large Achilles'tendon moment arm improves the mechanical advantage of the triceps surae but also produces larger shortening velocity during rapid plantarflexion, which detracts from the force-generating capacity of the plantarflexors. The lever arm of the ground reaction force that resists the muscular plantarflexor moment during propulsive push-off is constrained in part by the skeletal structure of the foot. In this study, we measured the plantarflexion moment arms of the Achilles' tendon, lateral gastrocnemius fascicle lengths and pennation angles, and anthropometric characteristics of the foot and lower leg in collegiate sprinters and height-matched non-sprinters. The Achilles' tendon moment arms of the sprinters were 25% smaller on average in sprinters than in non-sprinters (P&lt;0.001) whereas the sprinters' fascicles were 11%longer on average (P=0.024). The ratio of fascicle length to moment arm was 50% larger in sprinters (P&lt;0.001). Sprinters were found to have longer toes (P=0.032) and shorter lower legs (P=0.026)than non sprinters. A simple computer simulation of the sprint push-off demonstrated that shorter plantarflexor moment arms and longer toes, like those measured in sprinters, permit greater generation of forward impulse. Simulated propulsion was enhanced in both cases by increasing the `gear ratio'of the foot, thus maintaining plantarflexor fibre length and reducing peak fibre shortening velocity. Longer toes especially prolonged the time of contact, giving greater time for forward acceleration by propulsive ground reaction force.

Aging and the force-velocity relationship of muscles

Aging in humans is associated with a loss in neuromuscular function and performance. This is related, in part, to the reduction in muscular strength and power caused by a loss of skeletal muscle mass (sarcopenia) and changes in muscle architecture. Due to these changes, the force-velocity (f-v) relationship of human muscles alters with age. This change has functional implications such as slower walking speeds. Different methods to reverse these changes have been investigated, including traditional resistance training, power training and eccentric (or eccentrically-biased) resistance training. This review will summarise the changes of the f-v relationship with age, the functional implications of these changes and the various methods to reverse or at least partly ameliorate these changes.

Nesprin 1 is critical for nuclear positioning and anchorage

Nesprin 1 is an outer nuclear membrane protein that is thought to link the nucleus to the actin cytoskeleton. Recent data suggest that mutations in Nesprin 1 may also be involved in the pathogenesis of Emery-Dreifuss muscular dystrophy. To investigate the function of Nesprin 1 in vivo, we generated a mouse model in which all isoforms of Nesprin 1 containing the C-terminal spectrin-repeat region with or without KASH domain were ablated. Nesprin 1 knockout mice are marked by decreased survival rates, growth retardation and increased variability in body weight. Additionally, nuclear positioning and anchorage are dysfunctional in skeletal muscle from knockout mice. Physiological testing demonstrated no significant reduction in stress production in Nesprin 1-deficient skeletal muscle in either neonatal or adult mice, but a significantly lower exercise capacity in knockout mice. Nuclear deformation testing revealed ineffective strain transmission to nuclei in muscle fibers lacking Nesprin 1. Overall, our data show that Nesprin 1 is essential for normal positioning and anchorage of nuclei in skeletal muscle.

Muscle-specific adaptations, impaired oxidative capacity and maintenance of contractile function characterize diet-induced obese mouse skeletal muscle

Background

The effects of diet-induced obesity on skeletal muscle function are largely unknown, particularly as it relates to changes in oxidative metabolism and morphology.

Principal Findings

Compared to control fed mice, mice fed a high fat diet (HFD; 60% kcal: fat) for 8 weeks displayed increased body mass and insulin resistance without overt fasting hyperglycemia (i.e. pre-diabetic). Histological analysis revealed a greater oxidative potential in the HFD gastrocnemius/plantaris (increased IIA, reduced IIB fiber-type percentages) and soleus (increased I, IIA cross-sectional areas) muscles, but no change in fiber type percentages in tibialis anterior muscles compared to controls. Intramyocellular lipid levels were significantly increased relative to control in HFD gastrocnemius/plantaris, but were similar to control values in the HFD soleus. Using a novel, single muscle fiber approach, impairments in complete palmitate and glucose oxidation (72.8±6.6% and 61.8±9.1% of control, respectively; p<0.05) with HFD were detected. These reductions were consistent with measures made using intact extensor digitorum longus and soleus muscles. Compared to controls, no difference in succinate dehydrogenase or citrate synthase enzyme activities were observed between groups in any muscle studied, however, short-chain fatty acyl CoA dehydrogenase (SCHAD) activity was elevated in the HFD soleus, but not tibialis anterior muscles. Despite these morphological and metabolic alterations, no significant difference in peak tetanic force or low-frequency fatigue rates were observed between groups.

Conclusions

These findings indicate that HFD induces early adaptive responses that occur in a muscle-specific pattern, but are insufficient to prevent impairments in oxidative metabolism with continued high-fat feeding. Moreover, the morphological and metabolic changes which occur with 8 weeks of HFD do not significantly impact muscle contractile properties.

Mechanical constraints on the functional morphology of the gibbon hind limb

Gibbons utilize a number of locomotor modes in the wild, including bipedalism, leaping and, most of all, brachiation. Each locomotor mode puts specific constraints on the morphology of the animal; in some cases these may be complementary, whereas in others they may conflict. Despite several studies of the locomotor biomechanics of gibbons, very little is known about the musculoskeletal architecture of the limbs. In this study, we present quantitative anatomical data of the hind limb for four species of gibbon (Hylobates lar, H. moloch, H. pileatus and Symphalangus syndactylus). Muscle mass and fascicle lengths were obtained from all of the major hind limb muscles and the physiological cross-sectional area was calculated and scaled to remove the effect of body size. The results clearly indicate that, for all of the species studied, the major hip, knee and ankle extensors are short-fascicled and pennate. The major hip and knee flexors, however, are long-fascicled, parallel muscles with relatively small physiological cross-sectional areas. We hypothesize that the short-fascicled muscles could be coupled with a power-amplifying mechanism and are predominantly useful in leaping. The long-fascicled knee and hip flexors are adapted for a wide range of joint postures and can play a role in flexing the legs during brachiation.

Changes in contraction-induced phosphorylation of AMP-activated protein kinase and mitogen-activated protein kinases in skeletal muscle after ovariectomy

Recent evidence suggests that ovarian hormones contribute to altered function of skeletal muscle, however the signaling processes thought to regulate muscle function remain undefined in females. Thus, the purpose of this investigation is to determine if ovarian hormone status is critical for contraction-induced activation of AMPK or MAPK in skeletal muscle. Female mice were divided into two groups, ovariectomy (OVX) and SHAM, which were then subjected to in situ isometric contractile protocols. AMPK, ERK 1/2, p38, and JNK phosphorylation were measured in the control and contracting limb. In the in situ protocol, OVX muscles were significantly more resistant to fatigue compared to the SHAM animals. In addition, the muscles from OVX mice demonstrated significantly lower levels of normalized AMPK phosphorylation at rest. AMPK phosphorylation was not increased in the muscles from SHAM mice after the in situ contractile protocol, while the OVX demonstrated significant increases in AMPK phosphorylation. After contraction, normalized ERK2 phosphorylation was significantly higher in the OVX group compared to the SHAM group. Both p38 and JNK phosphorylation increased in response to contraction; but no group differences were detected. A second set of SHAM and OVX animals were subjected to fatigue stimulated under in vitro conditions. Significant increases in AMPK and ERK2 phosphorylation were detected, but no differences were found between groups. In conclusion, removal of the ovaries results in different responses to contraction-induced changes in phosphorylation of AMPK and ERK2 in female mice and suggests hormones secreted from the ovaries significantly impacts cellular signaling in skeletal muscle.

Hormone therapy and skeletal muscle strength: a meta-analysis

BACKGROUND

Our objective was to perform a systematic review and meta-analysis of the research literature that compared muscle strength in postmenopausal women who were and were not on estrogen-based hormone therapy (HT).

METHODS

Twenty-three relevant studies were found. Effect sizes (ESs) were calculated as the standardized mean difference, and meta-analyses were completed using a random effects model.

RESULTS

HT was found to result in a small beneficial effect on muscle strength in postmenopausal women (overall ES = 0.23; p = .003) that equated to an approximately 5% greater strength for women on HT. Among the 23 studies, various muscle groups were assessed for strength, and those that benefitted the most were the thumb adductors (ES = 1.14; p < .001). Ten studies that compared muscle strength in rodents that were and were not estradiol deficient were also analyzed. The ES for absolute strength was moderate but not statistically significant (ES = 0.44; p = .12), whereas estradiol had a large effect on strength normalized to muscle size (ES = 0.66; p = .03).

CONCLUSION

Overall, estrogen-based treatments were found to beneficially affect strength.

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end-organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle-spindle capsules; however, information on the other types of end-organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle-specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver-impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle-spindle afferents in hind-limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind-foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike-initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first-order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind-foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo-distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle-specific variability. Finally, we present some comparative data on muscle-spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.

Thin filament length dysregulation contributes to muscle weakness in nemaline myopathy patients with nebulin deficiency

Nemaline myopathy (NM) is the most common non-dystrophic congenital myopathy. Clinically the most important feature of NM is muscle weakness; however, the mechanisms underlying this weakness are poorly understood. Here, we studied the muscular phenotype of NM patients with a well-defined nebulin mutation (NM-NEB), using a multidisciplinary approach to study thin filament length regulation and muscle contractile performance. SDS-PAGE and western blotting revealed greatly reduced nebulin levels in skeletal muscle of NM-NEB patients, with the most prominent reduction at nebulin's N-terminal end. Muscle mechanical studies indicated approximately 60% reduced force generating capacity of NM-NEB muscle and a leftward-shift of the force-sarcomere length relation in NM-NEB muscle fibers. This indicates that the mechanism for the force reduction is likely to include shorter and non-uniform thin filament lengths in NM-NEB muscle compared with control muscle. Immunofluorescence confocal microscopy and electron microscopy studies indicated that average thin filament length is reduced from approximately 1.3 microm in control muscle to approximately 0.75 microm in NM-NEB muscle. Thus, the present study is the first to show a distinct genotype-functional phenotype correlation in patients with NM due to a nebulin mutation, and provides evidence for the notion that dysregulated thin filament length contributes to muscle weakness in NM patients with nebulin mutations. Furthermore, a striking similarity between the contractile and structural phenotypes of nebulin-deficient mouse muscle and human NM-NEB muscle was observed, indicating that the nebulin knockout model is well suited for elucidating the functional basis of muscle weakness in NM and for the development of treatment strategies.

Are current measurements of lower extremity muscle architecture accurate?

Skeletal muscle architecture is defined as the arrangement of fibers in a muscle and functionally defines performance capacity. Architectural values are used to model muscle-joint behavior and to make surgical decisions. The two most extensively used human lower extremity data sets consist of five total specimens of unknown size, gender, and age. Therefore, it is critically important to generate a high-fidelity human lower extremity muscle architecture data set. We disassembled 27 muscles from 21 human lower extremities to characterize muscle fiber length and physiologic cross-sectional area, which define the excursion and force-generating capacities of a muscle. Based on their architectural features, the soleus, gluteus medius, and vastus lateralis are the strongest muscles, whereas the sartorius, gracilis, and semitendinosus have the largest excursion. The plantarflexors, knee extensors, and hip adductors are the strongest muscle groups acting at each joint, whereas the hip adductors and hip extensors have the largest excursion. Contrary to previous assertions, two-joint muscles do not necessarily have longer fibers than single-joint muscles as seen by the similarity of knee flexor and extensor fiber lengths. These high-resolution data will facilitate the development of more accurate musculoskeletal models and challenge existing theories of muscle design; we believe they will aid in surgical decision making.

Reduced thin filament length in nebulin-knockout skeletal muscle alters isometric contractile properties

Nebulin (NEB) is a large, rod-like protein believed to dictate actin thin filament length in skeletal muscle. NEB gene defects are associated with congenital nemaline myopathy. The functional role of NEB was investigated in gastrocnemius muscles from neonatal wild-type (WT) and NEB knockout (NEB-KO) mice, whose thin filaments have uniformly shorter lengths compared with WT mice. Isometric stress production in NEB-KO skeletal muscle was reduced by 27% compared with WT skeletal muscle on postnatal day 1 and by 92% on postnatal day 7, consistent with functionally severe myopathy. NEB-KO muscle was also more susceptible to a decline in stress production during a bout of 10 cyclic isometric tetani. Length-tension properties in NEB-KO muscle were altered in a manner consistent with reduced thin filament length, with length-tension curves from NEB-KO muscle demonstrating a 7.4% narrower functional range and an optimal length reduced by 0.13 muscle lengths. Expression patterns of myosin heavy chain isoforms and total myosin content did not account for the functional differences between WT and NEB-KO muscle. These data indicate that NEB is essential for active stress production, maintenance of functional integrity during cyclic activation, and length-tension properties consistent with a role in specifying normal thin filament length. Continued analysis of NEB's functional properties will strengthen the understanding of force transmission and thin filament length regulation in skeletal muscle and may provide insights into the molecular processes that give rise to nemaline myopathy.

Architecture and fiber type of the pyramidalis muscle

The paired pyramidalis muscles are small triangular-shaped muscles that lie between the anterior surface of the rectus abdominus and the posterior surface of the rectus sheath. The precise function of pyramidalis muscles is unclear, but together the muscles are thought to tense the linea alba. The muscles are not always present, or are often unilateral, and vary greatly in size. Their wider inferior margins attach to the pubic symphyses and pubic crests, whereas their narrow superior margins attach to the linea alba. The gross anatomy and innervation of the pyramidalis muscles has been described by others, but their architecture and fiber type have not been determined in previous publications. The purpose of the present paper was therefore to investigate these parameters and place the findings into context for the literature available on this muscle. An example of bilateral pyramidalis muscles was recently encountered in a male cadaver that provided ample tissue for an analysis of its architecture and fiber type. The muscle mass, muscle length, fiber length, and pennation angle of muscle fibers were measured to ascertain physiological cross-sectional area and thereby estimate force production. Fiber type composition was also examined using immunofluorescent labeling. The results show that this is a muscle of mixed fiber type composition, similar to the rectus abdominus, and that the estimated forces generated by this muscle are relatively small.

The cardiac glycoside binding site on the Na,K-ATPase alpha2 isoform plays a role in the dynamic regulation of active transport in skeletal muscle

The physiological significance of the cardiac glycoside-binding site on the Na,K-ATPase remains incompletely understood. This study used a gene-targeted mouse (alpha2(R/R)) which expresses a ouabain-insensitive alpha2 isoform of the Na,K-ATPase to investigate whether the cardiac glycoside-binding site plays any physiological role in active Na(+)/K(+) transport in skeletal muscles or in exercise performance. Skeletal muscles express the Na,K-ATPase alpha2 isoform at high abundance and regulate its transport over a wide dynamic range under control of muscle activity. Na,K-ATPase active transport in the isolated extensor digitorum longus (EDL) muscle of alpha2(R/R) mice was lower at rest and significantly enhanced after muscle contraction, compared with WT. During the first 60 s after a 30-s contraction, the EDL of alpha2(R/R) mice transported 70.0 nmol/g.min more (86)Rb than WT. Acute sequestration of endogenous ligand(s) in WT mice infused with Digibind to sequester endogenous cardiac glycoside(s) produced similar effects on both resting and contraction-induced (86)Rb transport. Additionally, the alpha2(R/R) mice exhibit an enhanced ability to perform physical exercise, showing a 2.1- to 2.8-fold lower failure rate than WT within minutes of the onset of moderate-intensity treadmill running. Their enhanced exercise performance is consistent with their enhanced contraction-induced Na,K-ATPase transport in the skeletal muscles. These results demonstrate that the Na,K-ATPase alpha2 isozyme in skeletal muscle is regulated dynamically by a mechanism that utilizes the cardiac glycoside-binding site and an endogenous ligand(s) and that its cardiac glycoside-binding site can play a physiological role in the dynamic adaptations to exercise.

AMPK regulates basal skeletal muscle capillarization and VEGF expression, but is not necessary for the angiogenic response to exercise

5'-AMP-activated protein kinase (AMPK) is a metabolic fuel sensor that monitors cellular energy charge, while the vasculature is important for maintaining cellular energy homeostasis. Mice with muscle-specific inactive AMPK (AMPK DN) were used to investigate if AMPK regulates skeletal muscle capillarization and the angiogenic responses to exercise. Two hours of the AMP analogue AICAR (1.0 g kg(-1)) or systemic hypoxia (6% O(2)) increased vascular endothelial growth factor (VEGF) mRNA in wild-type (WT), but not in AMPK DN mice. In contrast, the increase in VEGF mRNA with acute exercise (1 h at 20 m min(-1), 10% gradient) was greater in AMPK DN compared to WT mice. Nuclear run-on assay demonstrated that exercise increased VEGF transcription, while hypoxia decreased VEGF transcription. There was no difference in VEGF transcription between WT and AMPK DN. There was a strong correlation between VEGF transcription and VEGF mRNA at rest and with exercise. Resting capillarization was lower in AMPK DN compared to WT. Wheel running (28 days) increased capillarization and this response was AMPK independent. Significant correlations between VEGF protein and muscle capillarization are consistent with VEGF being an important determinant of skeletal muscle capillarization. These data are to our knowledge the first to demonstrate in skeletal muscle in vivo that: (1) AMPK is necessary for hypoxia-induced VEGF mRNA stabilization, (2) acute exercise increases VEGF transcription, (3) inhibition of AMPK augments the VEGF mRNA response to acute exercise, and (4) AMPK regulates basal VEGF expression and capillarization, but is not necessary for exercise-induced angiogenesis.

Molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a truncated dystrophin

Myotendinous strain injury is the most common injury of human skeletal muscles because the majority of muscle forces are transmitted through this region. Although the immediate response to strain injury is well characterized, the chronic response to myotendinous strain injury is less clear. Here we examined the molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a microdystrophin transgene (microdystrophin(DeltaR4-R23)). We found that muscles with myotendinous strain injury had an increased expression of utrophin and alpha7-integrin together with the dramatic restructuring of peripheral myofibrils into concentric rings. The sarcolemma of the microdystrophin(DeltaR4-R23)/mdx gastrocnemius muscles was highly protected from experimental lengthening contractions, better than wild-type muscles. We also found a positive correlation between myotendinous strain injury and ringed fibers in the HSA(LR) (human skeletal actin, long repeat) mouse model of myotonic dystrophy. We suggest that changes in protein expression and the formation of rings are adaptations to myotendinous strain injury that help to prevent muscle necrosis and retain the function of necessary muscles during injury, ageing and disease.

Suppression of autophagy in skeletal muscle uncovers the accumulation of ubiquitinated proteins and their potential role in muscle damage in Pompe disease

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, the genetic deficiency of the glycogen-degrading lysosomal enzyme acid-alpha glucosidase. Accumulation of lysosomal glycogen, presumably transported from the cytoplasm by the autophagic pathway, occurs in multiple tissues, but pathology is most severe in skeletal and cardiac muscle. Skeletal muscle pathology also involves massive autophagic buildup in the core of myofibers. To determine if glycogen reaches the lysosome via autophagy and to ascertain whether autophagic buildup in Pompe disease is a consequence of induction of autophagy and/or reduced turnover due to defective fusion with lysosomes, we generated muscle-specific autophagy-deficient Pompe mice. We have demonstrated that autophagy is not required for glycogen transport to lysosomes in skeletal muscle. We have also found that Pompe disease involves induction of autophagy but manifests as a functional deficiency of autophagy because of impaired autophagosomal-lysosomal fusion. As a result, autophagic substrates, including potentially toxic aggregate-prone ubiquitinated proteins, accumulate in Pompe myofibers and may cause profound muscle damage.

Scaling of muscle architecture and fiber types in the rat hindlimb

The functional capacity of a muscle is determined by its architecture and metabolic properties. Although extensive analyses of muscle architecture and fiber type have been completed in a large number of muscles in numerous species, there have been few studies that have looked at the interrelationship of these functional parameters among muscles of a single species. Nor have the architectural properties of individual muscles been compared across species to understand scaling. This study examined muscle architecture and fiber type in the rat (Rattus norvegicus) hindlimb to examine each muscle's functional specialization. Discriminant analysis demonstrated that architectural properties are a greater predictor of muscle function (as defined by primary joint action and anti-gravity or non anti-gravity role)than fiber type. Architectural properties were not strictly aligned with fiber type, but when muscles were grouped according to anti-gravity versusnon-anti-gravity function there was evidence of functional specialization. Specifically, anti-gravity muscles had a larger percentage of slow fiber type and increased muscle physiological cross-sectional area. Incongruities between a muscle's architecture and fiber type may reflect the variability of functional requirements on single muscles, especially those that cross multiple joints. Additionally, discriminant analysis and scaling of architectural variables in the hindlimb across several mammalian species was used to explore whether any functional patterns could be elucidated within single muscles or across muscle groups. Several muscles deviated from previously described muscle architecture scaling rules and there was large variability within functional groups in how muscles should be scaled with body size. This implies that functional demands placed on muscles across species should be examined on the single muscle level.

PPAR alpha, more than PPAR delta, mediates the hepatic and skeletal muscle alterations induced by the PPAR agonist GW0742

Therapeutic use of certain peroxisome proliferator-activated receptor (PPAR) alpha agonists (fibrates) for the treatment of dyslipidemia has infrequently been associated with the untoward side effect of myopathy. With interest in PPAR-delta as a therapeutic target, this study assessed whether a PPAR-delta agonist induced similar hepatic and skeletal muscle alterations as noted with some fibrates. PPAR-alpha null (KO) and corresponding wild-type (WT) mice were administered toxicological dosages of a potent PPAR-delta agonist tool ligand (GW0742; which also has weak PPAR-alpha agonist activity) or a potent PPAR-alpha agonist (WY-14,643) for 10 days. Increases in liver weights and clinical chemistry indicators of skeletal muscle damage and/or liver injury were more pronounced in WT mice compared with KO mice administered the PPAR-delta agonist. Likewise, the incidence and severity of skeletal myopathy were greater in WT mice given GW0742 compared with KO mice. Ultrastructural and immunohistochemical analyses revealed significant peroxisome proliferation in muscle and liver of WT mice treated with each agonist; however, KO animals showed little or no evidence of hepatic and muscle peroxisome proliferation. PMP-70 protein expression in liver was consistent with these results. The hepatomegaly, hepatic and skeletal muscle peroxisome proliferation, and skeletal myopathy induced by this PPAR-delta ligand was predominantly mediated by its cross-activation of PPAR-alpha, though PPAR-delta agonism contributed slightly to these effects.

Poloxamer 188 reduces the contraction-induced force decline in lumbrical muscles from mdx mice

Duchenne Muscular Dystrophy is a genetic disease caused by the lack of the protein dystrophin. Dystrophic muscles are highly susceptible to contraction-induced injury, and following contractile activity, have disrupted plasma membranes that allow leakage of calcium ions into muscle fibers. Because of the direct relationship between increased intracellular calcium concentration and muscle dysfunction, therapeutic outcomes may be achieved through the identification and restriction of calcium influx pathways. Our purpose was to determine the contribution of sarcolemmal lesions to the force deficits caused by contraction-induced injury in dystrophic skeletal muscles. Using isolated lumbrical muscles from dystrophic (mdx) mice, we demonstrate for the first time that poloxamer 188 (P188), a membrane-sealing poloxamer, is effective in reducing the force deficit in a whole mdx skeletal muscle. A reduction in force deficit was also observed in mdx muscles that were exposed to a calcium-free environment. These results, coupled with previous observations of calcium entry into mdx muscle fibers during a similar contraction protocol, support the interpretation that extracellular calcium enters through sarcolemmal lesions and contributes to the force deficit observed in mdx muscles. The results provide a basis for potential therapeutic strategies directed at membrane stabilization of dystrophin-deficient skeletal muscle fibers.

Magnetic resonance imaging of mouse skeletal muscle to measure denervation atrophy

We assessed the potential of different MRI measures to detect and quantify skeletal muscle changes with denervation in two mouse models of denervation/neurogenic atrophy. Acute complete denervation and chronic partial denervation were examined in calf muscles after sciatic nerve axotomy and in transgenic SOD1(G93A) mice, respectively. Serial T(2), diffusion tensor, and high resolution anatomical images were acquired, and compared to behavioral, histological, and electrophysiological data. Increase in muscle T(2) signal was first detected after sciatic nerve axotomy. Progressive muscle atrophy could be monitored with MRI-based volume measurements, which correlated strongly with postmortem muscle mass measurements. Significant increase in muscle fractional anisotropy and decreases in secondary and tertiary eigenvalues obtained from diffusion tensor imaging (DTI) were observed after denervation. In SOD1(G93A) animals, muscle denervation was detected by elevated muscle T(2) and atrophy in the medial gastrocnemius at 10 weeks. Changes in T(2) and muscle volume were first observed in medial gastrocnemius and later in other calf muscles. Alterations in secondary and tertiary eigenvalues obtained from DTI were first observed in tibialis anterior and medial gastrocnemius muscles at age 12 weeks. We propose that MRI of skeletal muscle is a sensitive surrogate outcome measure of denervation atrophy in animal models of neuromuscular disorders, with potential applicability in preclinical therapeutic screening studies in rodents.

Quantitative analysis of neonatal skeletal muscle functional improvement in the mouse

Postnatal skeletal muscle growth is classically attributed to fiber hypertrophy and myogenic differentiation, but these processes do not account for the size-independent increase of muscle mechanical performance that occurs during postnatal growth. There is also little knowledge about the precise time-course of contractile function or the underlying factors that affect it. The present study investigated morphological factors (muscle fiber size and myofibrillar packing), biochemical factors (myosin heavy chain isoform and desmin intermediate filament protein expression), and muscle architecture during postnatal development in mice. Physiological testing of the mouse tibialis anterior revealed that maximum isometric stress increased from 27±3 kPa at postnatal day 1 to 169±10 kPa by postnatal day 28,roughly a sixfold increase. Morphological measurements revealed a robust increase in the size-independent packing of myofibrillar matrix material occurring with the functional improvement, with just 48.1±5.5% of the cross-sectional area filled with myofibrils at postnatal day 1 whereas 92.5±0.9% was filled by day 28. Expression of four myosin heavy chain isoforms (embryonic, neonatal, IIX and IIB), as well as desmin, correlated significantly with muscle mechanical function. Stepwise multiple regression showed that, of the variables measured, percentage content of neonatal myosin heavy chain was the best predictor of mechanical function during the postnatal time-course. These data provide the first specific structural basis for increases in muscle tension development during growth. Therefore, models of muscle growth must be modified to include an intrinsic quality enhancement component.