Effect of voluntary wheel-running exercise on muscles of the mdx mouse

Voluntary wheel running complements microdystrophin gene therapy to improve muscle function in mdx mice

We tested the hypothesis that voluntary wheel running would complement microdystrophin gene therapy to improve muscle function in young mdx mice, a model of Duchenne muscular dystrophy. mdx mice injected with a single dose of AAV9-CK8-microdystrophin or vehicle at age 7 weeks were assigned to three groups: mdxRGT (run, gene therapy), mdxGT (no run, gene therapy), or mdx (no run, no gene therapy). Wild-type (WT) mice were assigned to WTR (run) and WT (no run) groups. WTR and mdxRGT performed voluntary wheel running for 21 weeks; remaining groups were cage active. Robust expression of microdystrophin occurred in heart and limb muscles of treated mice. mdxRGT versus mdxGT mice showed increased microdystrophin in quadriceps but decreased levels in diaphragm. mdx final treadmill fatigue time was depressed compared to all groups, improved in mdxGT, and highest in mdxRGT. Both weekly running distance (km) and final treadmill fatigue time for mdxRGT and WTR were similar. Remarkably, mdxRGT diaphragm power was only rescued to 60% of WT, suggesting a negative impact of running. However, potential changes in fiber type distribution in mdxRGT diaphragms could indicate an adaptation to trade power for endurance. Post-treatment in vivo maximal plantar flexor torque relative to baseline values was greater for mdxGT and mdxRGT versus all other groups. Mitochondrial respiration rates from red quadriceps fibers were significantly improved in mdxGT animals, but the greatest bioenergetic benefit was observed in the mdxRGT group. Additional assessments revealed partial to full functional restoration in mdxGT and mdxRGT muscles relative to WT. These data demonstrate that voluntary wheel running combined with microdystrophin gene therapy in young mdx mice improved whole-body performance, affected muscle function differentially, mitigated energetic deficits, but also revealed some detrimental effects of exercise. With microdystrophin gene therapy currently in clinical trials, these data may help us understand the potential impact of exercise in treated patients.

Effects of PDE5 inhibition on dystrophic muscle following an acute bout of downhill running and endurance training

Lack of sarcolemma-localized neuronal nitric oxide synthase mu (nNOSμ) contributes to muscle damage and fatigue in dystrophic muscle. In this study, we examined the effects of compensating for lack of nNOSμ with a phosphodiesterase type 5 (PDE5) inhibitor in mdx mice following downhill running and endurance training. Dystrophic mice (mdx) were treated with sildenafil citrate and compared with untreated mdx and wild-type mice after an acute bout of downhill running and during a progressive low-intensity treadmill running program (5 days/wk, 4 wk). Magnetic resonance imaging (MRI) and spectroscopy (MRS) transverse relaxation time constant (T₂) of hindlimb and forelimb muscles were measured as a marker of muscle damage after downhill running and throughout training. The MRI blood oxygenation level dependence (BOLD) response and ³¹phosphorus MRS (³¹P-MRS) data were acquired after stimulated muscle contractions. After downhill running, the increase in T₂ was attenuated (P < 0.05) in treated mdx and wild-type mice compared with untreated mdx. During training, resting T₂ values did not change in wild-type and mdx mice from baseline values; however, the running distance completed during training was greater (P < 0.05) in treated mdx (>90% of target distance) and wild-type (100%) than untreated mdx (60%). The post-contractile BOLD response was greater (P < 0.05) in treated mdx that trained than untreated mdx, with no differences in muscle oxidative capacity, as measured by ³¹P-MRS. Our findings indicate that PDE5 inhibition reduces muscle damage after a single bout of downhill running and improves performance during endurance training in dystrophic mice, possibly because of enhanced microvascular function. NEW & NOTEWORTHY This study examined the combined effects of PDE5 inhibition and exercise in dystrophic muscle using high-resolution magnetic resonance imaging and spectroscopy. Our findings demonstrated that sildenafil citrate reduces muscle damage after a single bout of downhill running, improves endurance-training performance, and enhances microvascular function in dystrophic muscle. Collectively, the results support the combination of exercise and PDE5 inhibition as a therapeutic approach in muscular dystrophies lacking nNOSμ.

Dystrophinopathy-associated dysfunction of Krebs cycle metabolism

Duchenne muscular dystrophy is a deadly muscle-wasting disorder caused by loss of dystrophin protein. Studies suggest that metabolic alterations are important to disease pathogenesis. Because muscle accounts for ~40% of body mass, we hypothesized that dystrophy-mediated metabolic changes would be measurable in biofluids and that a metabolomic analysis of urine would provide insight into the metabolic status of dystrophic muscle. Using the mdx mouse model, we performed a large-scale metabolomic screen at 1 and 3 months. While 10% of metabolites were altered at age 1 month, 40% were changed at 3 months. Principal component analysis distinguished wild-type from mdx animals, with the greatest separation at 3 months. A critical distinguishing pathway was Krebs cycle metabolite depletion in mdx urine. Five of seven detected Krebs cycle metabolites were depleted in mdx urine, with succinate being the most robustly affected metabolite. Using selected reaction monitoring mass spectrometry, we demonstrated that muscle-specific dystrophin expression corrects mdx succinate depletion. When subjected to downhill treadmill running, wild-type and mdx mice expressing recombinant dystrophin in skeletal muscle displayed significant increases in urinary succinate levels. However, mdx succinate levels were unchanged, suggesting urinary succinate depletion may reflect an inability to upregulate the Krebs cycle following exercise. Finally, we show that supplementing the Krebs cycle in an ex vivo fatigue/recovery assay significantly impacts mdx muscle performance but has no effect on wild-type muscle. Our results suggest that global metabolic impairment is associated with mdx disease progression and that Krebs cycle deficiencies are a downstream consequence of dystrophin loss.

Voluntary wheel running: patterns and physiological effects in mice

Exercise can prevent and improve the pathophysiology of diseases and promote healthy aging. Thus, understanding the mechanisms that regulate the beneficial effects of exercise may lead to the development of new strategies to enhance quality of life and to counteract chronic diseases. Voluntary wheel running is an interesting model to study the effects of exercise in mice. Compared to forced treadmill exercise, voluntary wheel running presents several advantages such as: 1) running pattern is similar to natural running behavior of mice; 2) it is performed under non-stressed conditions, according to the rhythmicity of the animal; 3) it does not require direct interference from the researcher, and can be easily applied in long-term studies. Mice run spontaneously when given access to running wheels, for a total distance of ∼4 to 20 km per day and a total activity time of ∼3 to 7 hours a day. Hence, voluntary wheel running can result in robust endurance-like adaptation in skeletal and cardiac muscles and protect from sarcopenia. However, due to the lack of control over exercise parameters in voluntary exercise models, it is important for the researcher to understand the patterns and variability of wheel running in mice, as well as the factors that can affect voluntary running activity. Overall, voluntary wheel running in mice is a very interesting approach to study the chronic adaptation to exercise, analyze the effects of exercise, and test exercise capacity in different experimental models.

The effects of 8 weeks voluntary wheel running on the contractile performance of isolated locomotory (soleus) and respiratory (diaphragm) skeletal muscle during early ageing

Decreased skeletal muscle performance with increasing age is strongly associated with reduced mobility and quality of life. Increased physical activity is a widely prescribed method of reducing the detrimental effects of ageing on skeletal muscle contractility. The present study uses isometric and work loop testing protocols to uniquely investigate the effects of 8 weeks of voluntary wheel running on the contractile performance of isolated dynapenic soleus and diaphragm muscles of 38 week old CD1 mice. When compared to untrained controls, voluntary wheel running induced significant improvements in maximal isometric stress and work loop power, a reduced resistance to fatigue, but greater cumulative work during fatiguing work loop contractions in isolated muscle. These differences occurred without appreciable changes in LDH, CS, SERCA or MHC expression synonymous with this form of training in younger rodent models. Despite the given improvement in contractile performance, the average running distance significantly declined over the course of the training period, indicating that this form of training may not be sufficient to fully counteract the longer term ageing induced decline in skeletal muscle contractile performance. Although these results indicate that regular low intensity physical activity may be beneficial in offsetting the age-related decline in skeletal muscle contractility, the present findings infer that future work focusing on the maintenance of a healthy body mass with increasing age and its effects on myosin-actin cross bridge kinetics and Ca2+ handling, is needed to clarify the mechanisms causing the improved contractile performance in trained dynapenic skeletal muscle.

FVB/NJ Mice Are a Useful Model for Examining Cardiac Adaptations to Treadmill Exercise

Mice are commonly used to examine the mechanisms by which exercise improves cardiometabolic health; however, exercise compliance and adaptations are often strain-dependent or are variable due to inconsistency in exercise training protocols. In this study, we examined nocturnal/diurnal behavior, treadmill exercise compliance, and systemic as well as cardiac-specific exercise adaptations in two commonly used mouse strains, C57BL/6J, and FVB/NJ mice. Metabolic cage analysis indicated a strong nocturnal nature of C57BL/6J mice, whereas FVB/NJ mice showed no circadian element to activity, food or water intake, VO₂, or VCO₂. Initial exercise capacity tests revealed that, compared with C57BL/6J mice, FVB/NJ mice are capable of achieving nearly 2-fold higher workloads prior to exhaustion. FVB/NJ mice tested during the day were capable of achieving significantly more work compared with their night-tested counterparts. Following 4 weeks of training, FVB/NJ mice showed significant increases in exercise capacity as well as physiologic cardiac growth characterized by enlarged myocytes and higher mitochondrial DNA content. C57BL/6J mice showed no increases in exercise capacity or cardiac growth regardless of whether they exercised during the day or the night. This lack of adaptation in C57BL/6J mice was attributable, at least in part, to their progressive loss of compliance to the treadmill training protocol. We conclude that the FVB/NJ strain is a useful and robust mouse model for examining cardiac adaptations to treadmill exercise and that treadmill training during daytime hours does not negatively affect exercise compliance or capacity.

Oral quercetin administration transiently protects respiratory function in dystrophin-deficient mice

KEY POINT

PGC-1α pathway activation has been shown to decrease disease severity and can be driven by quercetin. Oral quercetin supplementation protected respiratory function for 4-6 months during a 12 month dosing regimen. This transient protection was probably due to a failure to sustain elevated SIRT1 activity and downstream PGC-1α signalling. Quercetin supplementation may be a beneficial treatment as part of a cocktail provided continued SIRT1 activity elevation is achieved.

ABSTRACT

Duchenne muscular dystrophy (DMD) impacts 1 : 3500 boys and leads to muscle dysfunction culminating in death due to respiratory or cardiac failure. There is an urgent need for effective therapies with the potential for immediate application for this patient population. Quercetin, a flavonoid with an outstanding safety profile, may provide therapeutic relief to DMD patients as the wait for additional therapies continues. This study evaluated the capacity of orally administered quercetin (0.2%) in 2 month old mdx mice to improve respiratory function and end-point functional and histological outcomes in the diaphragm following 12 months of treatment. Respiratory function was protected for the first 4-6 months of treatment but appeared to become insensitive to quercetin thereafter. Consistent with this, end-point functional measures were decreased and histopathological measures were more severe in dystrophic muscle compared to C57 and similar between control-fed and quercetin-fed mdx mice. To better understand the transient nature of improved respiratory function, we measured PGC-1α pathway activity, which is suggested to be up-regulated by quercetin supplementation. This pathway was largely suppressed in dystrophic muscle compared to healthy muscle, and at the 14 month time point dietary quercetin enrichment did not increase expression of downstream effectors. These data support the efficacy of quercetin as an intervention for DMD in skeletal muscle, and also indicate the development of age-dependent quercetin insensitivity when continued supplementation fails to drive the PGC-1α pathway. Continued study is needed to determine if this is related to disease severity, age or other factors.

Voluntary Aerobic Exercise Reverses Frailty in Old Mice

Frailty is a major cause of disability and loss of independence in the elderly. Using clinically relevant criteria from our previously established mouse frailty index, we investigated the effects of aerobic exercise on frailty in male C57BL/6 mice. In order to measure the effect of treatment on the individual animals, we constructed a composite score, the Frailty Intervention Assessment Value. We hypothesized voluntary aerobic exercise would improve individual criteria and reverse or prevent frailty in the old mice. Five adult and 11 old mice (6 and 28+ months, respectively) were housed individually in cages with running wheels for 4 weeks. Controls (adult, n = 5 and old, n = 17) were housed without wheels. Inverted cling grip and rotarod tests were performed pre- and postintervention. Hind limb muscles were used for biochemical analysis and contractility experiments. We conclude that the exercise stimulus reversed frailty and was sufficient to maintain or improve functional performance in old mice, as well as to produce measurable morphological changes. In addition, the Frailty Intervention Assessment Value proved to be a valuable tool with increased power to detect treatment effects and to examine the intervention efficacy at the level of the individual mouse.

Exercise increases utrophin protein expression in the mdx mouse model of Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a lethal genetic disease caused by mutations in the dystrophin gene resulting in chronic muscle damage, muscle wasting, and premature death. Utrophin is a dystrophin protein homologue that increases dystrophic muscle function and reduces pathology. Currently, no treatments that increase utrophin protein expression exist. However, exercise increases utrophin mRNA expression in healthy humans. Therefore, the purpose was to determine whether exercise increases utrophin protein expression in dystrophic muscle.

METHODS

Utrophin protein was measured in the quadriceps and soleus muscles of mdx mice after 12 weeks of voluntary wheel running exercise or sedentary controls. Muscle pathology was measured in the quadriceps.

RESULTS

Exercise increased utrophin protein expression 334 ± 63% in the quadriceps relative to sedentary controls. Exercise increased central nuclei 4 ± 1% but not other measures of pathology.

CONCLUSIONS

Exercise may be an intervention that increases utrophin expression in patients with DMD.

Longitudinal features of STIR bright signal in FSHD

INTRODUCTION

Magnetic resonance imaging of muscle shows short tau-inversion recovery (STIR) brightness in autosomal dominant facioscapulohumeral muscular dystrophy (FSHD1) suggestive of active inflammation/injury. We measured the longitudinal stability/progression of this potential disease biomarker.

METHODS

Nine subjects underwent calf MRI imaging over 2 years. Two radiologists evaluated qualitative muscle changes.

RESULTS

In 3/9 subjects, calf muscles demonstrated moderate/severe STIR hyperintensity at Time 1 that had progressed to fatty replacement 2 years later (Time 2). In the remaining subjects, moderate/severe muscle STIR abnormalities, when present, were consistent between exams. Mild STIR+ elevations had roughly similar patterns between exams.

CONCLUSIONS

Moderate/severe STIR hyperintensities often foreshadow fatty replacement over a 2-year interval. Whether longer time courses are required to observe muscle degeneration and fatty replacement in some subjects remains to be explored.

Enhancement of neuromuscular dynamics and strength behavior using extremely low magnitude mechanical signals in mice

Exercise in general, and mechanical signals in particular, help ameliorate the neuromuscular symptoms of aging and possibly other neurodegenerative disorders by enhancing muscle function. To better understand the salutary mechanisms of such physical stimuli, we evaluated the potential for low intensity mechanical signals to promote enhanced muscle dynamics. The effects of daily brief periods of low intensity vibration (LIV) on neuromuscular functions and behavioral correlates were assessed in mice. Physiological analysis revealed that LIV increased isometric force production in semitendinosus skeletal muscle. This effect was evident in both young and old mice. Isometric force recordings also showed that LIV reduced the fatiguing effects of intensive synaptic muscle stimulation. Furthermore, LIV increased evoked neurotransmitter release at neuromuscular synapses but had no effect on spontaneous end plate potential amplitude or frequency. In behavioral studies, LIV increased mouse grip strength and potentiated initial motor activity in a novel environment. These results provide evidence for the efficacy of LIV in producing changes in the neuromuscular system that translate into performance gains at a behavioral scale.

Long-term wheel running compromises diaphragm function but improves cardiac and plantarflexor function in the mdx mouse

Dystrophin-deficient muscles suffer from free radical injury, mitochondrial dysfunction, apoptosis, and inflammation, among other pathologies that contribute to muscle fiber injury and loss, leading to wheelchair confinement and death in the patient. For some time, it has been appreciated that endurance training has the potential to counter many of these contributing factors. Correspondingly, numerous investigations have shown improvements in limb muscle function following endurance training in mdx mice. However, the effect of long-term volitional wheel running on diaphragm and cardiac function is largely unknown. Our purpose was to determine the extent to which long-term endurance exercise affected dystrophic limb, diaphragm, and cardiac function. Diaphragm specific tension was reduced by 60% (P < 0.05) in mice that performed 1 yr of volitional wheel running compared with sedentary mdx mice. Dorsiflexor mass (extensor digitorum longus and tibialis anterior) and function (extensor digitorum longus) were not altered by endurance training. In mice that performed 1 yr of volitional wheel running, plantarflexor mass (soleus and gastrocnemius) was increased and soleus tetanic force was increased 36%, while specific tension was similar in wheel-running and sedentary groups. Cardiac mass was increased 15%, left ventricle chamber size was increased 20% (diastole) and 18% (systole), and stroke volume was increased twofold in wheel-running compared with sedentary mdx mice. These data suggest that the dystrophic heart may undergo positive exercise-induced remodeling and that limb muscle function is largely unaffected. Most importantly, however, as the diaphragm most closely recapitulates the human disease, these data raise the possibility of exercise-mediated injury in dystrophic skeletal muscle.

Exercise training improves plantar flexor muscle function in mdx mice

PURPOSE

We tested the hypothesis that low-intensity exercise in mdx mice improves plantar flexor muscle contractile function, resistance to fatigue, and mitochondrial adaptations without exacerbating muscular dystrophy.

METHODS

We subjected mdx mice to 12 wk of voluntary low-resistance wheel running (Run, n = 17) or normal cage activities (sedentary (Sed), n = 16) followed by in vivo analyses for plantar flexor torque generation and fatigue resistance or running capacity on a treadmill. Gastrocnemius muscles were further evaluated for exercise-induced mitochondrial adaptations and fiber type distribution and central nuclei. t-tests were used to determine differences between the Sed and Run groups.

RESULTS

Plantar flexor submaximal isometric torques and maximal isometric torque at multiple ankle joint angles and resistance to fatigue were greater in Run compared with Sed mdx mice (P G 0.05). Citrate synthase and A-hydroxyacyl-CoA dehydrogenase enzyme activities and cytochrome c oxidase IV protein expression in gastrocnemius muscles were greater in Run than in Sed mdx mice(P e 0.04), along with a trend of fiber type transformation from Type IIb to Type IIx fibers. Exercise training in mdx mice did not elevate serum creatine kinase levels but led to a significant reduction of centrally nucleated myofibers.

CONCLUSIONS

Voluntary low-resistance wheel running in mdx mice can result in skeletal muscle adaptation, leading to improved contractile function and reduced fatigability,with no indication that exercise was detrimental. This study supports the need for further investigation of low-intensity exercise as an early therapeutic intervention in ambulatory boys with Duchenne muscular dystrophy.

Creatine kinase response to high-intensity aerobic exercise in adult-onset muscular dystrophy

INTRODUCTION

We investigated the effect of high-intensity exercise on plasma creatine kinase (CK) in patients with muscular dystrophies.

METHODS

Fourteen patients with Becker (BMD), facioscapulohumeral (FSHD), or limb-girdle type 2 (LGMD2) muscular dystrophy, and 8 healthy subjects performed 5 cycling tests: an incremental max test, and tests at 65%, 75%, 85%, and 95% of maximal oxygen uptake (VO2max ). Heart rate and oxygen consumption were measured during the tests, and plasma CK was measured before, immediately after, and 24 hours after exercise.

RESULTS

All subjects were able to perform high-intensity exercise at the different levels. In patients with LGMD2 and FSHD, CK normalized 24 hours after exercise compared with the pre-exercise value, whereas those with BMD and healthy controls had elevated CK values 24 hours after exercise.

CONCLUSIONS

The findings suggest that high-intensity exercise is generally well tolerated in patients with LGMD2 and FSHD, whereas those with BMD may be more prone to exercise-induced damage.

Voluntary physical activity protects from susceptibility to skeletal muscle contraction-induced injury but worsens heart function in mdx mice

It is well known that inactivity/activity influences skeletal muscle physiological characteristics. However, the effects of inactivity/activity on muscle weakness and increased susceptibility to muscle contraction-induced injury have not been extensively studied in mdx mice, a murine model of Duchenne muscular dystrophy with dystrophin deficiency. In the present study, we demonstrate that inactivity (ie, leg immobilization) worsened the muscle weakness and the susceptibility to contraction-induced injury in mdx mice. Inactivity also mimicked these two dystrophic features in wild-type mice. In contrast, we demonstrate that these parameters can be improved by activity (ie, voluntary wheel running) in mdx mice. Biochemical analyses indicate that the changes induced by inactivity/activity were not related to fiber-type transition but were associated with altered expression of different genes involved in fiber growth (GDF8), structure (Actg1), and calcium homeostasis (Stim1 and Jph1). However, activity reduced left ventricular function (ie, ejection and shortening fractions) in mdx, but not C57, mice. Altogether, our study suggests that muscle weakness and susceptibility to contraction-induced injury in dystrophic muscle could be attributable, at least in part, to inactivity. It also suggests that activity exerts a beneficial effect on dystrophic skeletal muscle but not on the heart.

Resistance training in patients with limb-girdle and becker muscular dystrophies

INTRODUCTION

In this study we investigated the effect of strength training in patients with limb-girdle muscular dystrophy (LGMD) and Becker muscular dystrophy (BMD).

METHODS

In 2 studies we compared the effect of low-intensity training (LOIT; n = 8) and high-intensity training (HIT; n = 4) in muscles of the upper and lower extremities. Patients were tested for maximal strength and endurance before and after the training program.

RESULTS

LOIT training over 6 months resulted in increased biceps strength and endurance. HIT training increased endurance and strength in wrist flexion and extension and in elbow flexion. One patient discontinued HIT training due to muscle soreness and mildly increased plasma CK levels without strength deterioration.

CONCLUSIONS

Both LOIT and HIT increased muscle strength and endurance in some of the muscles tested and were well tolerated in most patients. Our findings suggest that supervised resistance training may be considered in the management of patients with LGMD2 and BMD.

Combined effect of AMPK/PPAR agonists and exercise training in mdx mice functional performance

The present investigation was undertaken to test whether exercise training (ET) associated with AMPK/PPAR agonists (EM) would improve skeletal muscle function in mdx mice. These drugs have the potential to improve oxidative metabolism. This is of particular interest because oxidative muscle fibers are less affected in the course of the disease than glycolitic counterparts. Therefore, a cohort of 34 male congenic C57Bl/10J mdx mice included in this study was randomly assigned into four groups: vehicle solution (V), EM [AICAR (AMPK agonist, 50 mg/Kg-1.day-1, ip) and GW 1516 (PPARδ agonist, 2.5 mg/Kg-1.day-1, gavage)], ET (voluntary running on activity wheel) and EM+ET. Functional performance (grip meter and rotarod), aerobic capacity (running test), muscle histopathology, serum creatine kinase (CK), levels of ubiquitined proteins, oxidative metabolism protein expression (AMPK, PPAR, myoglobin and SCD) and intracellular calcium handling (DHPR, SERCA and NCX) protein expression were analyzed. Treatments started when the animals were two months old and were maintained for one month. A significant functional improvement (p<0.05) was observed in animals submitted to the combination of ET and EM. CK levels were decreased and the expression of proteins related to oxidative metabolism was increased in this group. There were no differences among the groups in the intracellular calcium handling protein expression. To our knowledge, this is the first study that tested the association of ET with EM in an experimental model of muscular dystrophy. Our results suggest that the association of ET and EM should be further tested as a potential therapeutic approach in muscular dystrophies.

Voluntary wheel running in dystrophin-deficient (mdx) mice: Relationships between exercise parameters and exacerbation of the dystrophic phenotype

Voluntary wheel running can potentially be used to exacerbate the disease phenotype in dystrophin-deficient mdx mice. While it has been established that voluntary wheel running is highly variable between individuals, the key parameters of wheel running that impact the most on muscle pathology have not been examined in detail. We conducted a 2-week test of voluntary wheel running by mdx mice and the impact of wheel running on disease pathology. There was significant individual variation in the average daily distance (ranging from 0.003 ± 0.005 km to 4.48 ± 0.96 km), culminating in a wide range (0.040 km to 67.24 km) of total cumulative distances run by individuals. There was also variation in the number and length of run/rest cycles per night, and the average running rate. Correlation analyses demonstrated that in the quadriceps muscle, a low number of high distance run/rest cycles was the most consistent indicator for increased tissue damage. The amount of rest time between running bouts was a key factor associated with gastrocnemius damage. These data emphasize the need for detailed analysis of individual running performance, consideration of the length of wheel exposure time, and the selection of appropriate muscle groups for analysis, when applying the use of voluntary wheel running to disease exacerbation and/or pre-clinical testing of the efficacy of therapeutic agents in the mdx mouse.

Endpoint measures in the mdx mouse relevant for muscular dystrophy pre-clinical studies

Loss of mobility influences the quality of life for patients with neuromuscular diseases. Common measures of mobility and chronic muscle damage are the six-minute walk test and serum creatine kinase. Despite extensive pre-clinical studies of therapeutic approaches, characterization of these measures is incomplete. To address this, a six-minute ambulation assay, serum creatine kinase, and myoglobinuria were investigated for the mdx mouse, a dystrophinopathy mouse model commonly used in pre-clinical studies. mdx mice ambulated shorter distances than normal controls, a disparity accentuated after mild exercise. An asymmetric pathophysiology in mdx mice was unmasked with exercise, and peak measurements of serum creatine kinase and myoglobinuria were identified. Our data highlights the necessity to consider asymmetric pathology and timing of biomarkers when testing potential therapies for muscular dystrophy.

nNOS regulation of skeletal muscle fatigue and exercise performance

Neuronal nitric oxide synthases (nNOS) are Ca²⁺/calmodulin-activated enzymes that synthesize the gaseous messenger nitric oxide (NO). nNOSμ and the recently described nNOSβ, both spliced nNOS isoforms, are important enzymatic sources of NO in skeletal muscle, a tissue long considered to be a paradigmatic system for studying NO-dependent redox signaling. nNOS is indispensable for skeletal muscle integrity and contractile performance, and deregulation of nNOSμ signaling is a common pathogenic feature of many neuromuscular diseases. Recent evidence suggests that both nNOSμ and nNOSβ regulate skeletal muscle size, strength, and fatigue resistance, making them important players in exercise performance. nNOSμ acts as an activity sensor and appears to assist skeletal muscle adaptation to new functional demands, particularly those of endurance exercise. Prolonged inactivity leads to nNOS-mediated muscle atrophy through a FoxO-dependent pathway. nNOS also plays a role in modulating exercise performance in neuromuscular disease. In the mdx mouse model of Duchenne muscular dystrophy, defective nNOS signaling is thought to restrict contractile capacity of working muscle in two ways: loss of sarcolemmal nNOSμ causes excessive ischemic damage while residual cytosolic nNOSμ contributes to hypernitrosylation of the ryanodine receptor, causing pathogenic Ca²⁺ leak. This defect in Ca²⁺ handling promotes muscle damage, weakness, and fatigue. This review addresses these recent advances in the understanding of nNOS-dependent redox regulation of skeletal muscle function and exercise performance under physiological and neuromuscular disease conditions.

Impaired Wheel Running Exercise in CLC-1 Chloride Channel-Deficient Myotonic Mice

Background: Genetic deficiency of the muscle CLC-1 chloride channel leads to myotonia, which is manifested most prominently by slowing of muscle relaxation. Humans experience this as muscle stiffness upon initiation of contraction, although this can be overcome with repeated efforts (the “warm-up” phenomenon). The extent to which CLC-1 deficiency impairs exercise activity is controversial. We hypothesized that skeletal muscle CLC-1 chloride channel deficiency leads to severe reductions in spontaneous exercise. Methodology/Principal Findings: To examine this quantitatively, myotonic CLC-1 deficient mice were provided access to running wheels, and their spontaneous running activity was quantified subsequently. Differences between myotonic and normal mice in running were not present soon after introduction to the running wheels, but were fully established during week 2. During the eighth week, myotonic mice were running significantly less than normal mice (322 ± 177 vs 5058 ± 1253 m/day, P = 0.025). Furthermore, there were considerable reductions in consecutive running times (18.8 ± 1.5 vs 59.0 ± 3.7 min, P < 0.001) and in the distance per consecutive running period (58 ± 38 vs 601 ± 174 m, P = 0.048) in myotonic compared with normal animals. Conclusion/Significance: These findings indicate that CLC-1 chloride deficient myotonia in mice markedly impairs spontaneous exercise activity, with reductions in both total distance and consecutive running times.

Use of imaging biomarkers to assess perfusion and glucose metabolism in the skeletal muscle of dystrophic mice

Background

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease that affects 1 in 3500 boys. The disease is characterized by progressive muscle degeneration that results from mutations in or loss of the cytoskeletal protein, dystrophin, from the glycoprotein membrane complex, thus increasing the susceptibility of contractile muscle to injury. To date, disease progression is typically assessed using invasive techniques such as muscle biopsies, and while there are recent reports of the use of magnetic resonance, ultrasound and optical imaging technologies to address the issue of disease progression and monitoring therapeutic intervention in dystrophic mice, our study aims to validate the use of imaging biomarkers (muscle perfusion and metabolism) in a longitudinal assessment of skeletal muscle degeneration/regeneration in two murine models of muscular dystrophy.

Methods

Wild-type (w.t.) and dystrophic mice (weakly-affected mdx mice that are characterized by a point mutation in dystrophin; severely-affected mdx:utrn-/- (udx) mice that lack functional dystrophin and are null for utrophin) were exercised three times a week for 30 minutes. To follow the progression of DMD, accumulation of ¹⁸ F-FDG, a measure of glucose metabolism, in both wild-type and affected mice was measured with a small animal PET scanner (GE eXplore Vista). To assess changes in blood flow and blood volume in the hind limb skeletal muscle, mice were injected intravenously with a CT contrast agent, and imaged with a small animal CT scanner (GE eXplore Ultra).

Results

In hind limb skeletal muscle of both weakly-affected mdx mice and in severely-affected udx mice, we demonstrate an early, transient increase in both ¹⁸F-FDG uptake, and in blood flow and blood volume. Histological analysis of H&E-stained tissue collected from parallel littermates demonstrates the presence of both inflammatory infiltrate and centrally-located nuclei, a classic hallmark of myofibrillar regeneration. In both groups of affected mice, the early transient response was succeeded by a progressive decline in muscle perfusion and metabolism; this was also evidenced histologically.

Conclusions

The present study demonstrates the utility of non-invasive imaging biomarkers in characterizing muscle degeneration/regeneration in murine models of DMD. These techniques may now provide a promising alternative for assessing both disease progression and the efficacy of new therapeutic treatments in patients.

Exercise and Duchenne muscular dystrophy: toward evidence-based exercise prescription

To develop a rational framework for answering questions about the role of exercise in Duchenne muscular dystrophy (DMD), we focused on five pathophysiological mechanisms and offer brief hypotheses regarding how exercise may beneficially modulate pertinent cellular and molecular pathways. We aimed to provide an integrative overview of mechanisms of DMD pathology that may improve or worsen as a result of exercise. We also sought to stimulate discussion of what outcomes/dependent variables most appropriately measure these mechanisms, with the purpose of defining criteria for well-designed, controlled studies of exercise in DMD. The five mechanisms include pathways that are both intrinsic and extrinsic to the diseased muscle cells.

Voluntary exercise induces structural remodeling in the hearts of dystrophin-deficient mice

In this exploratory study, we test the hypothesis that voluntary exercise affects the progression of dystrophic changes in the left ventricle of the heart. Wild-type (C57BL/10ScSn) and dystrophin-deficient (mdx) mice, aged 7 weeks, were divided into sedentary and exercise-treated groups and tested for differences in cardiac histomorphometry. Exercised mdx mice were found to exhibit significantly enlarged ventricles and thinner lateral ventricular walls than sedentary mdx mice (P < 0.05). Trichrome staining indicated the presence of fibrotic lesions in the left ventricular myocardium in 20% of the exercised mdx group. Fibrotic lesions were not found in control or sedentary mdx mice. No histomorphometric differences were found between treatment groups in wild-type mice. Our findings suggest voluntary exercise may accelerate the progression of ventricular dilation and fibrosis in young mdx mice. The effects of exercise on cardiac remodeling should be considered during the treatment of cardiac disease in dystrophin-deficient patients.

Progressive resistance voluntary wheel running in the mdx mouse

Exercise training has been minimally explored as a therapy to mitigate the loss of muscle strength for individuals with Duchenne muscular dystrophy (DMD). Voluntary wheel running is known to elicit beneficial adaptations in the mdx mouse model for DMD. The aim of this study was to examine progressive resistance wheel running in mdx mice by comprehensively testing muscle function before, during, and after a 12-week training period. Male mdx mice at ~4 weeks age were randomized into three groups: Sedentary, Free Wheel, and Resist Wheel. Muscle strength was assessed via in vivo dorsiflexion torque, grip strength, and whole body tension intermittently throughout the training period. Contractility of isolated soleus muscles was analyzed at the study's conclusion. Both Free and Resist Wheel mice had greater grip strength (~22%) and soleus muscle specific tetanic force (26%) compared with Sedentary mice. This study demonstrates that two modalities of voluntary exercise are beneficial to dystrophic muscle and may help establish parameters for an exercise prescription for DMD.

A human-specific deletion in mouse Cmah increases disease severity in the mdx model of Duchenne muscular dystrophy

During the evolution of humans, an inactivating deletion was introduced in the CMAH (cytidine monophosphate-sialic acid hydroxylase) gene, which eliminated biosynthesis of the common mammalian sialic acid N-glycolylneuraminic acid from all human cells. We found that this human-specific change in sialylation capacity contributes to the marked discrepancy in phenotype between the mdx mouse model for Duchenne muscular dystrophy (DMD) and the human disease. When compared to human patients with DMD, mdx mice show reduced severity or slower development of clinically relevant disease phenotypes, despite lacking dystrophin protein in almost all muscle cells. This is especially true for the loss of ambulation, cardiac and respiratory muscle weakness, and decreased life span, all of which are major phenotypes contributing to DMD morbidity and mortality. These phenotypes occur at an earlier age or to a greater degree in mdx mice that also carry a human-like mutation in the mouse Cmah gene, possibly as a result of reduced strength and expression of the dystrophin-associated glycoprotein complex and increased activation of complement. Cmah-deficient mdx mice are a small-animal model for DMD that better approximates the human glycome and its contributions to muscular dystrophy.

The chondrogenic response to exercise in the proximal femur of normal and mdx mice

Background

Submaximal exercise is used in the management of muscular dystrophy. The effects of mechanical stimulation on skeletal development are well understood, although its effects on cartilage growth have yet to be investigated in the dystrophic condition. The objective of this study was to investigate the chondrogenic response to voluntary exercise in dystrophin-deficient mice.

Methods

Control and dystrophin-deficient (mdx) mice were divided into sedentary and exercise-treated groups and tested for chondral histomorphometric differences at the proximal femur.

Results

Control mice ran 7 km/week further than mdx mice on average, but this difference was not statistically significant (P > 0.05). However, exercised control mice exhibited significantly enlarged femur head diameter, articular cartilage thickness, articular cartilage tissue area, and area of calcified cartilage relative to sedentary controls and exercised mdx mice (P < 0.05). No differences were found between other treatment groups.

Conclusions

Mdx mice exhibit a reduced chondrogenic response to increased mechanical stimulation relative to controls. However, no significant reduction in articular dimensions was found, indicating loss of chondral tissue may not be a clinical concern with dystrophinopathy.

Aging with muscular dystrophy: pathophysiology and clinical management

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

Muscle weakness and atrophy are associated with decreased regenerative capacity and changes in mTOR signaling in skeletal muscles of venerable (18-24-month-old) dystrophic mdx mice

The muscles of mdx mice progressively deteriorate with age. We wanted to know whether this is associated with a decrease in regenerative capacity and/or changes in the mammalian target of rapamycin complex (mTOR) signaling pathway. Muscles of mdx mice aged 5 weeks, 5, 12, and 18-24 months were studied. Maximal force and muscle weight of the older mice were decreased as compared to younger adult mice. Activation of the mTOR signaling pathway, i.e., phosphorylation of Akt (also known as protein kinase B) and ribosomal protein S6 was also reduced in the older mice. Moreover, 14 days after cardiotoxin injury the degree of recovery of maximal force and muscle weight were less in the older mice. In contrast to younger mice, there was also activation of the mTOR pathway during regeneration in the older mice. Progressive muscle weakness and atrophy in mdx mouse muscle is associated with a decline in regenerative potential and changes in activation of the mTOR signaling pathway.

Assessment and management of fatigue in neuromuscular disease

Fatigue is a common and potentially debilitating symptom of neuromuscular disease (NMD). Studies show that patients with NMD subjectively report increased levels of fatigue. Laboratory testing has demonstrated that patients with NMD show objective physiological signs of increased fatigue, with both central and peripheral components. To date, no treatment has been proven to be truly effective through evidence-based medicine. Thus, the clinician must use a multimodality approach to treating fatigue in patients with NMD. Management interventions are generally based on a sequential approach including treatment of comorbid factors, with the goal of maximizing physical and psychological functioning. This might include low-intensity exercise training, cognitive therapy, treatment of associated depression, correction of risk factors such as obesity, poor nutrition, and inactivity (deconditioning). Optimizing cardiopulmonary function is also critical and measures such as noninvasive, positive pressure ventilation may reduce fatigue in patients with NMD. Novel medications such as modafinil, a nonamphetamine stimulant, may be a helpful pharmacological treatment. Nutraceutical agents, such as creatine monohydrate, coenzyme Q10 (CoQ10), and alpha-lipoic acid, may also improve neuromuscular function and reduce fatigue.

Genetic background affects properties of satellite cells and mdx phenotypes

Duchenne muscular dystrophy (DMD) is the most common lethal genetic disorder of children. The mdx (C57BL/10 background, C57BL/10-mdx) mouse is a widely used model of DMD, but the histopathological hallmarks of DMD, such as the smaller number of myofibers, accumulation of fat and fibrosis, and insufficient regeneration of myofibers, are not observed in adult C57BL/10-mdx except for in the diaphragm. In this study, we showed that DBA/2 mice exhibited decreased muscle weight, as well as lower myofiber numbers after repeated degeneration-regeneration cycles. Furthermore, the self-renewal efficiency of satellite cells of DBA/2 is lower than that of C57BL/6. Therefore, we produced a DBA/2-mdx strain by crossing DBA/2 and C57BL/10-mdx. The hind limb muscles of DBA/2-mdx mice exhibited lower muscle weight, fewer myofibers, and increased fat and fibrosis, in comparison with C57BL/10-mdx. Moreover, remarkable muscle weakness was observed in DBA/2-mdx. These results indicate that the DBA/2-mdx mouse is a more suitable model for DMD studies, and the efficient satellite cell self-renewal ability of C57BL/10-mdx might explain the difference in pathologies between humans and mice.

Use of pifithrin to inhibit p53-mediated signalling of TNF in dystrophic muscles of mdx mice

Tumour Necrosis Factor (TNF) plays a major role in exacerbating necrosis of dystrophic muscle; however, the precise molecular mechanism underlying this effect of TNF is unknown. This study investigates the role that p53 plays in TNF-mediated necrosis of dystrophic myofibres by inhibiting p53 using pifithrin-alpha and three pifithrin-beta analogues. Tissue culture studies using C2C12 myoblasts established that pifithrin-alpha was toxic to differentiating myoblasts at concentrations greater than 10 muM. While non-toxic concentrations of pifithrin-alpha did not prevent the TNF-mediated inhibition of myoblast differentiation, Western blots indicated that nuclear levels of p53 were higher in TNF-treated myoblasts indicating that TNF does elevate p53. In contrast, in vivo studies in adult mdx mice showed that pifithrin-alpha significantly reduced myofibre necrosis that resulted from voluntary wheel running over 48 h. These results support the hypothesis that p53 plays some role in TNF-mediated necrosis of dystrophic muscle and present a potential new target for therapeutic interventions.

Adaptive and nonadaptive responses to voluntary wheel running by mdx mice

The purpose of this study was to determine the extent to which hindlimb muscles of mdx mice adapt to a voluntary endurance type of exercise. mdx and C57BL mice engaged in 8 weeks of wheel running or maintained normal cage activities. Beneficial adaptations that occurred in mdx mice included changes in muscle mass, fiber size, and fiber types based on myosin heavy chain (MHC) isoform expression. These adaptations occurred without increases in fiber central nuclei and embryonic MHC expression. An undesirable outcome, however, was that muscle mitochondrial enzyme activities did not improve with exercise in mdx mice as they did in C57BL mice. Cellular remodeling of dystrophic muscle following exercise has not been studied adequately. In this study we found that some, but not all, of the expected adaptations occurred in mdx mouse muscle. We must better understand these (non)adaptations in order to inform individuals with DMD about the benefits of exercise.

Endurance capacity in maturing mdx mice is markedly enhanced by combined voluntary wheel running and green tea extract

Duchenne muscular dystrophy is characterized by the absence of dystrophin from muscle cells. Dystrophic muscle cells are susceptible to oxidative stress. We tested the hypothesis that 3 wk of endurance exercise starting at age 21 days in young male mdx mice would blunt oxidative stress and improve dystrophic skeletal muscle function, and these effects would be enhanced by the antioxidant green tea extract (GTE). In mice fed normal diet, average daily running distance increased 300% from week 1 to week 3, and total distance over 3 wk was improved by 128% in mice fed GTE. Running, independent of diet, increased serum antioxidant capacity, extensor digitorum longus tetanic stress, and total contractile protein content, heart citrate synthase, and heart and quadriceps beta-hydroxyacyl-CoA dehydrogenase activities. GTE, independent of running, decreased serum creatine kinase and heart and gastrocnemius lipid peroxidation and increased gastrocnemius citrate synthase activity. These data suggest that both endurance exercise and GTE may be beneficial as therapeutic strategies to improve muscle function in mdx mice.

Duchenne muscle activity evaluation and muscle function preservation: is it possible a prophylactic strategy?

Duchenne muscular dystrophy yields pervasive and progressive muscle mass loss. In the current measures relating to the monitoring of disease progression the following are relevant (i): the type of scale used, (ii) the clinical significance of the attribute being measured and (iii) the mathematical properties of the data provided. The high prevalence of obesity at an early stage of this pathology could result not only from reduced physical activity, but also from low resting energy expenditure, abnormal nutrient utilization or overfeeding. This muscle weakness may be attenuated by regular low-intensity exercise. However, there is a critical lack of data to support appropriate exercise prescription. Because inappropriate activity may exacerbate the dystrophic process, a systematic analysis of muscle function to determine potential exercise load thresholds to avoid injury in dystrophic mice and dogs, and then in humans is recommended.

Low intensity training decreases markers of oxidative stress in skeletal muscle of mdx mice

Reactive oxygen species may contribute to the pathogenesis of muscular dystrophy. High intensity exercise clearly induces muscle damage in mdx mice; however, the effects of low intensity exercise training (LIT) on mdx muscle are less clear. We examined the effect of LIT on markers of oxidative stress (malondialdehyde and protein carbonyls), antioxidant (superoxide dismutase, catalase, and glutathione peroxidase), and mitochondrial (2-oxoglutarate dehydrogenase and cytochrome oxidase) enzymes in skeletal muscle of mdx and wild-type mice. Mdx and wild-type mice were allocated to LIT and sedentary groups. Malondialdehyde levels were higher in white muscle from sedentary mdx as compared to both sedentary and LIT wild-type mice (P<0.001). Protein carbonyl content was higher in white and red muscle of mdx versus wild-type mice (P<0.05). LIT was associated with lower levels of malondialdehyde and protein carbonyls in white muscle of mdx mice (decreased 38 and 44%, P<0.001 and P<0.01, respectively). Antioxidant and mitochondrial enzyme activities were higher in white muscle of mdx than in wild-type mice (P<0.05). LIT in mdx mice induced physiological adaptation resulting in lower levels of markers of oxidative stress that were not different than those from wild type. These results are of relevance for therapeutic exercise in patients with dystrophinopathy where exercise prescription remains controversial.

Upregulation of the creatine synthetic pathway in skeletal muscles of mature mdx mice

Duchenne muscular dystrophy (DMD) is a fatal neuromuscular human disease caused by dystrophin deficiency. The mdx mouse lacks dystrophin protein, yet does not exhibit the debilitating DMD phenotype. Investigating compensatory mechanisms in the mdx mouse may shed new insights into modifying DMD pathogenesis. This study targets two metabolic genes, guanidinoacetate methyltransferase (GAMT) and arginine:glycine amidinotransferase (AGAT) which are required for creatine synthesis. We show that GAMT and AGAT mRNA are up-regulated 5.4- and 1.9-fold respectively in adult mdx muscle compared to C57. In addition, GAMT protein expression is up-regulated at least 2.5-fold in five different muscles of mdx vs. control. Furthermore, we find GAMT immunoreactivity in up to 80% of mature mdx muscle fibers in addition to small regenerating fibers and rare revertants; while GAMT immunoreactivity is equal to background levels in all muscle fibers of mature C57 mice. The up-regulation of the creatine synthetic pathway may help maintain muscle creatine levels and limit cellular energy failure in leaky mdx skeletal muscles. These results may help better understand the mild phenotype of the mdx mouse and may offer new treatment horizons for DMD.

Recommendations to Define Exercise Prescription for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy yields pervasive and progressive muscle weakness. This weakness may be attenuated by regular, low-intensity exercise. However, there is a critical lack of data to support appropriate exercise prescription. Because inappropriate activity may exacerbate the dystrophic process, a systematic analysis of muscle function to determine potential exercise load thresholds to avoid injury in dystrophic mice and dogs, and then in humans, is recommended.

RAG2 gene knockout in mice causes fatigue

The effect of a disrupted immune system on the neuromuscular system is poorly characterized. We compared the strength and fatigue of RAG2(-/-) mice, which lack T-cells and B-cells, with immune intact controls. RAG2(-/-) mice demonstrated fatigue with shorter inverted hang-times (HT) and voluntary wheel-running (VWR) distance and total run times; they increased body weight more slowly but had proportionally normal forelimb grip strength (FGS) and VWR speed. Medial rectus femoris histopathology showed no change in fiber type proportions, no variation in type 2b fiber diameter, and no change in the percentage of central nuclei. There was no change in serum creatine kinase (CK) levels. Thus, in RAG2(-/-) mice body weight and fatigue were directly affected by a hypoactive immune system. Whether these effects were centrally or peripherally mediated is unknown. This model may help to explain fatigue in human conditions in which the immune system is suppressed or absent.

Cromolyn administration (to block mast cell degranulation) reduces necrosis of dystrophic muscle in mdx mice

Duchenne muscular dystrophy is a lethal muscle wasting disorder, resulting from mutations in the gene encoding for the skeletal muscle protein dystrophin. The absence of functional dystrophin leaves the muscle membrane vulnerable to damage during contraction. Damage initially occurs as 'tears' in the membrane, this damage can be exacerbated by the inflammatory response leading to myofibre necrosis rather than repair. Mast cells resident within skeletal muscle represent an immediate source of pro-inflammatory cytokines. We hypothesise that blockade of mast cell degranulation would reduce the extent of myofibre necrosis in the mdx mouse. Daily cromolyn injections were performed on young and exercised adult mdx mice and histological analysis confirmed that mast cell degranulation contributes to myofibre necrosis. This research identified high biological variation between individual mdx mice in the severity of the dystrophic pathology, and supported a relationship between extent of muscle damage in adult mdx mice and their individual enthusiasm for voluntary wheel running.

Electromyographic studies in mdx and wild-type C57 mice

The electromyographic (EMG) characteristics of human Duchenne muscular dystrophy (DMD) have been well-described. However, to our knowledge, no prior needle electromyographic (EMG) studies of motor unit morphology have been undertaken in muscles from the mdx mouse, an animal that is genetically homologous to DMD. There are significant phenotypic differences between the human and murine dystrophic conditions, bringing into question whether the mdx mouse is an appropriate animal model for DMD. This study was done in order to characterize the EMG findings in mdx mice, compared to normal wild-type mice, and to assess for similarities to DMD. The tibialis anterior and gastrocnemius/soleus muscles from 34 mice (16 C57 wild-type and 18 mdx), divided into four age groups (3, 12, 18, and 24 months), were examined. Wild-type muscles showed normal insertional activity and no abnormal activity at rest. Motor unit action potential (MUAP) parameters were characterized. In contrast to wild-type muscles, mdx muscles showed increased insertional activity, abnormal spontaneous potentials, and the presence of complex repetitive discharges (CRDs). MUAPs showed increased numbers of phases (4.0 +/- 0.6, P < 0.001) and duration (7.1 +/- 1.2 ms, P < 0.02), as well as late components (15%). These EMG data indicate that mdx muscles display EMG characteristics similar to those found in muscles from boys with DMD, lending credence to the mdx mouse as an animal model for this disease. The data obtained in this study indicate a potential role for EMG as an in vivo, objective measurement tool that could be used longitudinally to monitor the effects of therapeutic interventions in mdx mice. This is important as there are few objective measures of muscle function in murine models that do not require killing the animal.