The capacity of mdx mouse diaphragm muscle to do oscillatory work

The extracellular matrix of dystrophic mouse diaphragm accounts for the majority of its passive stiffness and is resistant to collagenase digestion

The healthy skeletal muscle extracellular matrix (ECM) has several functions including providing structural integrity to myofibers, enabling lateral force transmission, and contributing to overall passive mechanical properties. In diseases such as Duchenne Muscular dystrophy, there is accumulation of ECM materials, primarily collagen, which results in fibrosis. Previous studies have shown that fibrotic muscle is often stiffer than healthy muscle, in part due to the increased number and altered architecture of collagen fibers within the ECM. This would imply that the fibrotic matrix is stiffer than the healthy matrix. However, while previous studies have attempted to quantify the extracellular contribution to passive stiffness in muscle, the outcomes are dependent on the type of method used. Thus, the goals of this study were to compare the stiffness of healthy and fibrotic muscle ECM and to demonstrate the efficacy of two methods for quantifying extracellular-based stiffness in muscle, namely decellularization and collagenase digestion. These methods have been demonstrated to remove the muscle fibers or ablate collagen fiber integrity, respectively, while maintaining the contents of the extracellular matrix. Using these methods in conjunction with mechanical testing on wildtype and D2.mdx mice, we found that a majority of passive stiffness in the diaphragm is dependent on the ECM, and the D2.mdx diaphragm ECM is resistant to digestion by bacterial collagenase. We propose that this resistance is due to the increased collagen cross-links and collagen packing density in the ECM of the D2.mdx diaphragm. Taken altogether, while we did not find increased stiffness of the fibrotic ECM, we did observe that the D2.mdx diaphragm conveyed resistance against collagenase digestion. These findings demonstrate how different methods for measuring ECM-based stiffness each have their own limitations and can produce different results.

Breathing in Duchenne muscular dystrophy: Translation to therapy

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease caused by a deficiency in dystrophin - a structural protein which stabilizes muscle during contraction. Dystrophin deficiency adversely affects the respiratory system leading to sleep-disordered breathing, hypoventilation, and weakness of the expiratory and inspiratory musculature, which culminate in severe respiratory dysfunction. Muscle degeneration associated respiratory impairment in neuromuscular disease is a result of disruptions at multiple sites of the respiratory control network, including sensory and motor pathways. As a result of this pathology, respiratory failure is a leading cause of premature death in DMD patients. Currently available treatments for DMD respiratory insufficiency attenuate respiratory symptoms without completely reversing the underlying pathophysiology. This underscores the need to develop curative therapies to improve quality of life and longevity of DMD patients. This review summarises research findings on the pathophysiology of respiratory insufficiencies in DMD disease in humans and animal models, the clinical interventions available to ameliorate symptoms, and gene-based therapeutic strategies uncovered by preclinical animal studies. Abstract figure legend: Summary of the therapeutic strategies for respiratory insufficiency in DMD (Duchenne muscular dystrophy). Treatment options currently in clinical use only attenuate respiratory symptoms without reversing the underlying pathology of DMD-associated respiratory insufficiencies. Ongoing preclinical and clinical research is aimed at developing curative therapies that both improve quality of life and longevity of DMD patients. AAV - adeno-associated virus, PPMO - Peptide-conjugated phosphorodiamidate morpholino oligomer This article is protected by copyright. All rights reserved.

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged mdx dystrophic mice by reducing pathological fibre branching

The common null polymorphism (R577X) in the ACTN3 gene is present in over 1.5 billion people worldwide and results in the absence of the protein α-actinin-3 from the Z-discs of fast-twitch skeletal muscle fibres. We have previously reported that this polymorphism is a modifier of dystrophin-deficient Duchenne Muscular Dystrophy. To investigate the mechanism underlying this, we use a double knockout (dk)Actn3KO/mdx (dKO) mouse model, which lacks both dystrophin and sarcomere α-actinin-3. We used dKO mice and mdx dystrophic mice at 12 months (aged) to investigate the correlation between morphological changes to the fast-twitch dKO EDL and the reduction in force deficit produced by an in vitro eccentric contraction protocol. In the aged dKO mouse, we found a marked reduction in fibre branching complexity that correlated with protection from eccentric contraction induced force deficit. Complex branches in the aged dKO EDL fibres (28%) were substantially reduced compared to aged mdx EDL fibres (68%), and this correlates with a graded force loss over three eccentric contractions for dKO muscles (~36% after first contraction, ~66% overall) compared to an abrupt drop in mdx upon the first eccentric contraction (~75% after first contraction, ~89% after three contractions). In dKO, protection from eccentric contraction damage was linked with a doubling of SERCA1 pump density the EDL. We propose that the increased oxidative metabolism of fast-twitch glycolytic fibres characteristic of the null polymorphism (R577X) and increase in SR Ca2+ pump proteins reduces muscle fibre branching and decreases susceptibility to eccentric injury in the dystrophinopathies.

Lifespan Analysis of Dystrophic mdx Fast-Twitch Muscle Morphology and Its Impact on Contractile Function

Duchenne muscular dystrophy is caused by the absence of the protein dystrophin from skeletal muscle and is characterized by progressive cycles of necrosis/regeneration. Using the dystrophin deficient mdx mouse model, we studied the morphological and contractile chronology of dystrophic skeletal muscle pathology in fast-twitch Extensor Digitorum Longus muscles from animals 4–22 months of age containing 100% regenerated muscle fibers. Catastrophically, the older age groups lost ∼80% of their maximum force after one eccentric contraction (EC) of 20% strain with the greatest loss of ∼92% recorded in senescent 22-month-old mdx mice. In old age groups, there was minimal force recovery ∼24% after 120 min, correlated with a dramatic increase in the number and complexity of branched fibers. This data supports our two-phase model where a “tipping point” is reached when branched fibers rupture irrevocably on EC. These findings have important implications for pre-clinical drug studies and genetic rescue strategies.

Extensor carpi ulnaris muscle shows unexpected slow-to-fast fiber type switch in Duchenne muscular dystrophy dogs

Aged dystrophin-null canines are excellent models for studying experimental therapies for Duchenne muscular dystrophy, a lethal muscle disease caused by dystrophin deficiency. To establish the baseline, we studied the extensor carpi ulnaris (ECU) muscle in 15 terminal age (3-year-old) male affected dogs and 15 age/sex-matched normal dogs. Affected dogs showed histological and anatomical hallmarks of dystrophy, including muscle inflammation and fibrosis, myofiber size variation and centralized myonuclei, as well as a significant reduction of muscle weight, muscle-to-body weight ratio and muscle cross-sectional area. To rigorously characterize the contractile properties of the ECU muscle, we developed a novel in situ assay. Twitch and tetanic force, contraction and relaxation rate, and resistance to eccentric contraction-induced force loss were significantly decreased in affected dogs. Intriguingly, the time-to-peak tension and half-relaxation time were significantly shortened in affected dogs. Contractile kinetics predicted an unforeseen slow-to-fast myofiber-type switch, which we confirmed at the protein and transcript level. Our study establishes a foundation for studying long-term and late-stage therapeutic interventions in dystrophic canines. The unexpected myofiber-type switch highlights the complexity of muscle remodeling in dystrophic large mammals.

This article has an associated First Person interview with the first author of the paper.

Summary: A slow-to-fast fiber-type switch in dystrophic canine ECU muscle is revealed by contraction kinetics and myosin protein and transcript expression. This highlights the complexity of muscle remodeling in Duchenne muscular dystrophy.

A novel ex vivo protocol to mimic human walking gait: implications for Duchenne muscular dystrophy

We developed a novel ex vivo mouse protocol to mimic in vivo human soleus muscle function predicted by musculoskeletal simulations to better understand eccentric contractions during gait and ultimately to better understand their effects in Duchenne muscular dystrophy (DMD) muscles. DMD muscles are susceptible to eccentric injury because the protein dystrophin is absent. The mdx mouse, a DMD model that also lacks dystrophin, is often subjected to ex vivo acute but nonphysiological eccentric injury protocols. It is possible these acute protocols either over- or underestimate eccentric stresses and strains compared with those from humans during gait. To explore this possibility, healthy human soleus excitation, force, and length change profiles during a single walking stride (gait cycle) were simulated using OpenSim and then scaled to an ex vivo mouse soleus preparation based on muscle architectural measurements. Aurora Scientific, Inc., software and a 701C electrical stimulator were modified to discretely modulate muscle stimulation voltage at constant frequency and finely control muscle length changes to produce a force pattern that correctly mimicked the gait cycle from simulations. In a proof-of-principle study, wild-type and mdx mice soleus muscles were subjected to 25 gait cycles. Modest fatigue was evident in the muscles at the 25th versus first gait cycle for both genotypes, but both rapidly recovered isometric force within 1 min of the last cycle. These data indicate that the ex vivo gait protocol was well tolerated. More important, this protocol provides a novel assessment tool to determine the effects of physiological eccentric contractions on dystrophic muscle.NEW & NOTEWORTHY A novel ex vivo mouse soleus protocol that mimics scaled length change and excitation profiles predicted by a mathematical model of human soleus during gait is presented. A custom stimulator was developed that enabled an innovative muscle stimulation technique to modulate voltage to closely match the excitation pattern of human soleus during gait. This ex vivo protocol provides assessment of simulated human movement in mouse muscle, including components of eccentric contractions.

Breathing with neuromuscular disease: Does compensatory plasticity in the motor drive to breathe offer a potential therapeutic target in muscular dystrophy?

Duchenne muscular dystrophy is a fatal neuromuscular disease associated with respiratory-related morbidity and mortality. Herein, we review recent work by our group exploring deficits and compensation in the respiratory control network governing respiratory homeostasis in a pre-clinical model of DMD, the mdx mouse. Deficits at multiple sites of the network provide considerable challenges to respiratory control. However, our work has also revealed evidence of compensatory neuroplasticity in the motor drive to breathe enhancing diaphragm muscle activity during increased chemical drive. The finding may explain the preserved capacity for mdx mice to increase ventilation in response to chemoactivation. Given the profound dysfunction in the primary pump muscle of breathing, we argue that activation of accessory muscles of breathing may be especially important in mdx (and perhaps DMD). Notwithstanding the limitations resulting from respiratory muscle dysfunction, it may be possible to further leverage intrinsic physiological mechanisms serving to compensate for weak muscles in attempts to preserve or restore ventilatory capacity. We discuss current knowledge gaps and the need to better appreciate fundamental aspects of respiratory control in pre-clinical models so as to better inform intervention strategies in human DMD.

Combined XIL-6R and urocortin-2 treatment restores MDX diaphragm muscle force

INTRODUCTION

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle degeneration leading to immobility, respiratory failure, and premature death. As chronic inflammation and stress are implicated in DMD pathology, the efficacy of an anti-inflammatory and anti-stress intervention strategy in ameliorating diaphragm dysfunction was investigated.

METHODS

Diaphragm muscle contractile function was compared in wild-type and dystrophin-deficient mdx mice treated with saline, anti-interleukin-6 receptor antibodies (xIL-6R), the corticotrophin-releasing factor receptor 2 (CRFR2) agonist, urocortin 2, or both xIL-6R and urocortin 2.

RESULTS

Combined treatment with xIL-6R and urocortin 2 rescued impaired force in mdx diaphragms. Mechanical work production and muscle shortening was also improved by combined drug treatment.

DISCUSSION

Treatment which neutralizes peripheral IL-6 signaling and stimulates CRFR2 recovers force-generating capacity and the ability to perform mechanical work in mdx diaphragm muscle. These findings may be important in the search for therapeutic targets in DMD. Muscle Nerve 56: E134-E140, 2017.

Reduction of Oxidative Damage and Inflammatory Response in the Diaphragm Muscle of mdx Mice Using Iron Chelator Deferoxamine

Oxidative stress and inflammatory processes strongly contribute to pathogenesis in Duchenne muscular dystrophy (DMD). Based on evidence that excess iron may increase oxidative stress and contribute to the inflammatory response, we investigated whether deferoxamine (DFX), a potent iron chelating agent, reduces oxidative stress and inflammation in the diaphragm (DIA) muscle of mdx mice (an experimental model of DMD). Fourteen-day-old mdx mice received daily intraperitoneal injections of DFX at a dose of 150 mg/kg body weight, diluted in saline, for 14 days. C57BL/10 and control mdx mice received daily intraperitoneal injections of saline only, for 14 days. Grip strength was evaluated as a functional measure, and blood samples were collected for biochemical assessment of muscle fiber degeneration. In addition, the DIA muscle was removed and processed for histopathology and Western blotting analysis. In mdx mice, DFX reduced muscle damage and loss of muscle strength. DFX treatment also resulted in a significant reduction of dystrophic inflammatory processes, as indicated by decreases in the inflammatory area and in NF-κB levels. DFX significantly decreased oxidative damage, as shown by lower levels of 4-hydroxynonenal and a reduction in dihydroethidium staining in the DIA muscle of mdx mice. The results of the present study suggest that DFX may be useful in therapeutic strategies to ameliorate dystrophic muscle pathology, possibly via mechanisms involving oxidative and inflammatory pathways.

Skeletal muscle dysfunction is associated with derangements in mitochondrial bioenergetics (but not UCP3) in a rodent model of sepsis

Muscle dysfunction is a common feature of severe sepsis and multiorgan failure. Recent evidence implicates bioenergetic dysfunction and oxidative damage as important underlying pathophysiological mechanisms. Increased abundance of uncoupling protein-3 (UCP3) in sepsis suggests increased mitochondrial proton leak, which may reduce mitochondrial coupling efficiency but limit reactive oxygen species (ROS) production. Using a murine model, we examined metabolic, cardiovascular, and skeletal muscle contractile changes following induction of peritoneal sepsis in wild-type and Ucp3(-/-) mice. Mitochondrial membrane potential (Δψm) was measured using two-photon microscopy in living diaphragm, and contractile function was measured in diaphragm muscle strips. The kinetic relationship between membrane potential and oxygen consumption was determined using a modular kinetic approach in isolated mitochondria. Sepsis was associated with significant whole body metabolic suppression, hypothermia, and cardiovascular dysfunction. Maximal force generation was reduced and fatigue accelerated in ex vivo diaphragm muscle strips from septic mice. Δψm was lower in the isolated diaphragm from septic mice despite normal substrate oxidation kinetics and proton leak in skeletal muscle mitochondria. Even though wild-type mice exhibited an absolute 26 ± 6% higher UCP3 protein abundance at 24 h, no differences were seen in whole animal or diaphragm physiology, nor in survival rates, between wild-type and Ucp3(-/-) mice. In conclusion, this murine sepsis model shows a hypometabolic phenotype with evidence of significant cardiovascular and muscle dysfunction. This was associated with lower Δψm and alterations in mitochondrial ATP turnover and the phosphorylation pathway. However, UCP3 does not play an important functional role, despite its upregulation.

What has the mdx mouse model of Duchenne muscular dystrophy contributed to our understanding of this disease?

Duchenne muscular dystrophy (DMD) is a fatal X-chromosome linked recessive disorder caused by the truncation or deletion of the dystrophin gene. The most widely used animal model of this disease is the dystrophin-deficient mdx mouse which was first discovered 30 years ago. Despite its extensive use in DMD research, no effective treatment has yet been developed for this devastating disease. This review explores what we have learned from this mouse model regarding the pathophysiology of DMD and asks if it has a future in providing a better more thorough understanding of this disease or if it will bring us any closer to improving the outlook for DMD patients.

A novel protocol for assessing exercise performance and dystropathophysiology in the mdx mouse

INTRODUCTION

Dystrophinopathy in the young mdx mouse model of Duchenne muscular dystrophy is comparatively mild, requires induction, and is rarely assessed with tests of systemic muscle function in whole animals.

METHODS

A modified TREAT-NMD induction protocol was used to evaluate respiratory and exercise performance, starting and ending with maximum oxygen consumption (VO2max ) tests.

RESULTS

The initial and/or final VO2max , time to exhaustion, speed at exhaustion, and total expended calories were significantly lower in mdx mice. Episodic VO2 and VCO2 fluctuations occurred during training and resulted in dissociated patterns of VO2 and respiratory exchange ratio (RER). These fluctuations further resulted in significantly greater VO2 coefficient of variation and RER values and lower minimal VO2 values.

CONCLUSIONS

Quantifying respiratory performance during exercise is a potentially useful means for studying pathophysiology in mdx mice, as it assesses intact animals over time, is more sensitive than some histological markers, and assesses systemic muscle function.

Concurrent muscle and bone deterioration in a murine model of cancer cachexia

Cachexia is defined as an excessive, involuntary loss of fat and lean tissue. We tested the validity of the Lewis lung carcinoma (LLC) as a model of cancer cachexia and examined its effect on the two major lean tissue components, skeletal muscle and bone. LLC cells (0.75 × 10⁶) were injected into the left thigh of C57BL/6 mice. Control mice received an equal volume injection of growth media. Tumors were observed in all LLC-injected animals 21 and 25 days post inoculation. LLC-injected animals showed significant reductions in fat and lean mass despite having the same average daily caloric intake as media-treated mice. Global bone mineral density (BMD) had fallen by 5% and 6% in the LLC animals at 21 and 25 days, respectively, compared to a BMD increase of 5% in the 25-day media-treated animals. Extensor digitorum longus (EDL) muscles (isolated from the noninjected hindlimb) showed earlier and quantitatively greater losses in mass, physiological cross-sectional area (pCSA), and tetanic force compared to soleus muscles from the same hindlimb. By the 25th day post-LLC inoculation, EDL force/pCSA was reduced by 19% versus media treatment. This loss in specific force was not trivial as it accounted for about one-third of the reduction in EDL absolute force at this time point. Muscle strips dissected from the diaphragm of LLC mice also exhibited significant reductions in force/pCSA at day 25. We conclude that LLC is a valid model of cachexia that induces rapid losses in global BMD and in limb and respiratory muscle function.

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.

The force-temperature relationship in healthy and dystrophic mouse diaphragm; implications for translational study design

In the field of muscular dystrophy, striated muscle function is often assessed in vitro in dystrophin-deficient mdx mice in order to test the impact of a potential treatment strategy. Although many past studies have assessed diaphragm contractile function at or near room temperature, the diaphragm performs in vivo at 37°C. To improve translation of bench-top results to possible clinical application, we studied temperature-dependence of contractile performance in wild-type (C57BL/10) and mdx muscle strips at temperatures from 25°C to 37°C. Maximal tetanic force in wild-type muscles was higher at 37°C (198 ± 11 vs. 155 ± 9 mN/mm² at 25°C), while the difference between wild-type and mdx was extremely similar: wild-type muscles produced 45.9% and 45.1% more force at 25°C and 37°C respectively. At 37°C twitch contraction kinetics and 50% rise time to tetanic plateau were slower in mdx diaphragm. A fatigue/injury protocol indicated 2-fold fatigue/contraction-induced force deficit in mdx muscles. We conclude that assessment of diaphragm muscle strips can be reliably and reproducibly performed at 37°C.

Inflammation in muscular dystrophy and the beneficial effects of non-steroidal anti-inflammatory drugs

INTRODUCTION

Glucocorticoids are the only drugs available for the treatment of Duchenne muscular dystrophy (DMD), but it is unclear whether their efficacy is dependent on their anti-inflammatory activity.

METHODS

To address this issue, mdx mice were treated daily with methylprednisolone and non-steroidal anti-inflammatory drugs (NSAIDs: aspirin, ibuprofen, parecoxib).

RESULTS

NSAID treatment was effective in ameliorating muscle morphology and reducing macrophage infiltration and necrosis. The percentage of regenerating myofibers was not modified by the treatments. The drugs were effective in reducing COX-2 expression and inflammatory cytokines, but they did not affect utrophin levels. The effects of the treatments on contractile performance were analyzed. Isometric tension did not differ in treated and untreated muscle, but the resistance to fatigue was decreased by treatment with methylprednisolone and aspirin.

CONCLUSIONS

NSAIDs have a beneficial effect on mdx muscle morphology, pointing to a crucial role of inflammation in the progression of DMD.

An octaguanidine-morpholino oligo conjugate improves muscle function of mdx mice

INTRODUCTION

Skeletal muscles of mdx mice lack functional levels of dystrophin due to a mutation in Dmd exon 23. Morpholino antisense oligomers can induce expression of a truncated dystrophin by redirecting splicing to skip processing of exon 23.

METHODS

We tested whether systemic administration of Vivo-Morpholino, an octaguanidine delivery moiety-Morpholino conjugate that targets exon 23 (VMO23), restored function to muscles of mdx mice.

RESULTS

Extensor digitorum longus (EDL) muscles of mdx mice were weaker, less powerful, and showed greater functional deficits after eccentric contractions than normal. VMO23 treatment normalized EDL force and power of mdx mice and eliminated their exaggerated sensitivity to eccentric contractions. Diaphragm muscle strips from mdx mice also produced lower-than-normal force and power, and these variables were restored to normal, or near-normal, levels by VMO23 treatment.

CONCLUSION

These results provide a functional basis for continuing development of VMO23 as a treatment for Duchenne muscular dystrophy.

Exacerbation of pathology by oxidative stress in respiratory and locomotor muscles with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most devastating type of muscular dystrophy, leading to progressive weakness of respiratory (e.g. diaphragm) and locomotor muscles (e.g. gastrocnemius). DMD is caused by X-linked defects in the gene that encodes for dystrophin, a key scaffolding protein of the dystroglycan complex (DCG) within the sarcolemmal cytoskeleton. As a result of a compromised dystroglycan complex, mechanical integrity is impaired and important signalling proteins (e.g. nNOS, caveolin-3) and pathways are disrupted. Disruption of the dystroglycan complex leads to high susceptibility to injury with repeated, eccentric contractions as well as inflammation, resulting in significant damage and necrosis. Chronic damage and repair cycling leads to fibrosis and weakness. While the link between inflammation with damage and weakness in the DMD diaphragm is unresolved, elevated oxidative stress may contribute to damage, weakness and possibly fibrosis. While utilization of non-specific antioxidant interventions has yielded inconsistent results, recent data suggest that NAD(P)H oxidase could play a pivotal role in elevating oxidative stress via integrated changes in caveolin-3 and stretch-activated channels (SACs). Oxidative stress may act as an amplifier, exacerbating disruption of the dystroglycan complex, upregulation of the inflammatory transcription factor NF-B, and thus functional impairment of force-generating capacity.

Effects of glutathione depletion and age on skeletal muscle performance and morphology following chronic stretch-shortening contraction exposure

The involvement of glutathione in the response of skeletal muscle following repetitive, high-intensity mechanical loading is not known. We examined the influence of a glutathione antagonist [L: -Buthionine Sulfoximine (BSO)] had on the adaptability of skeletal muscle during chronic mechanical loading via stretch-shortening contractions (SSCs) in young and old rats. Left dorsiflexor muscles of young (12 weeks, N = 16) and old (30 months, N = 16), vehicle- and BSO-treated rats were exposed three times per week for 4.5-weeks to a protocol of 80 maximal SSCs per exposure in vivo. Skeletal muscle response to the SSC exposure was characterized by muscle performance, as well as muscle wet-weight and quantitative morphological analyses following the exposure period. Results reveal that generally, muscle performance increased in the young rats only following chronic SSC exposure. BSO treatment had no effect on muscle performance or morphology following the chronic SSC exposure in old rats. Muscle wet-weight was increased following exposure compared with the contra-lateral control limb, irrespective of age (p < 0.05). Muscle cross-sectional area increased approximately 20% with SSC loading in the young, vehicle rats, while increasing approximately 10% with SSC loading in old, vehicle rats compared with control rat muscle. No degenerative myofibers were noted in either age group, but edema were increased as a result of aging (p < 0.05). We conclude that our results indicate that glutathione depletion does not adversely affect muscle performance or morphology in old rats. Nevertheless, we continue to show that aging negatively influences performance and morphology following chronic SSC exposure.

Dynamic properties of a locomotory muscle of the tobacco hornworm Manduca sexta during strain cycling and simulated natural crawling

Caterpillars are soft-bodied terrestrial climbers that perform a wide variety of complex movements with several hundred muscles and a relatively small number of neurons. Control of movements is therefore expected to place unusual demands on the mechanical properties of the muscles. The muscles develop force slowly (1-6 s to peak) yet over a strain range extending from under 60% to more than 160% of resting length, with a length-tension relationship resembling that of supercontracting or cross-striated muscle. In passive and active sinusoidal strain cycling, muscles displayed viscoelastic qualities, with very low and stretch-velocity dependent resilience; there was a positive linear relationship between stretch velocity and the fraction of work dissipation attributable to passive muscle properties (20-80%). In linear stretches of unstimulated muscles at velocities bracketing those encountered in natural crawling, the rise in tension showed a distinct transition to a lower rate of increase, with transition tension dependent upon stretch velocity; peak force was exponentially related to stretch velocity. When stretching ceased, force decayed exponentially, with slower decay associated with lower stretch velocities; the decay time constant was exponentially related to stretch velocity. From the kinematics of caterpillars crawling horizontally we determined that the ventral interior lateral muscle (VIL) of the third abdominal segment (A3) is at or near resting length for most of the crawl cycle, with a fairly linear shortening by 25-30% and re-lengthening occupying about 45% of cycle duration. Synchronized kinematic and EMG recordings showed that during horizontal crawling A3 VIL is stimulated as the muscle shortens from about 95% to 75% of its resting length. We subjected in vitro VIL preparations to strain cycling and stimulus phase and duration similar to that of natural crawling. The resulting work loops were figure-eight shaped, with the muscle performing work during the shortest 45-65% of the strain cycle but dissipating work during the rest of the cycle. The muscle remained in the ascending limb of its length-tension relationship throughout the crawl cycle. Peak force occurred at the end of re-lengthening, nearly a full second after stimulation ceased, underscoring the importance of understanding passive muscle properties to explain caterpillar locomotion. Whether A3 VIL functions as an actuator at all during simulated natural strain cycling is highly sensitive to stimulus timing but far less so to stimulus duration. The muscle's elastomer-like properties appear to play a major role in its function.

Physiological properties of human diaphragm muscle fibres and the effect of chronic obstructive pulmonary disease

The contractile and actomyosin ATPase properties of single fibres were examined in human diaphragm muscle obtained from patients with and without chronic obstructive pulmonary disease (COPD). Costal diaphragm biopsies were taken from five patients without evidence of COPD and from 11 age-matched individuals with varying degrees of the disease. Our aim was to establish whether changes in contractile properties of COPD diaphragm could be fully explained by the previously documented shift towards a greater proportion of type I myosin heavy chain isoform in COPD. The relative proportion of type I diaphragm fibres from non-COPD and COPD patients was measured by gel electrophoresis, and was negatively correlated with FEV(1) over the full range of values investigated. There was also significant atrophy of the type I fibre population in COPD diaphragms. Isometric tension was similar among the fibre types and between the COPD and non-COPD patients. The intrinsic energetic properties of diaphragm fibres were examined by monitoring the time-resolved actomyosin ATPase activity in COPD and non-COPD fibres that produced similar isometric forces. The isometric ATPase rate in COPD fibres was reduced to 50% of the rate in non-COPD fibres; hence, the cost of isometric contraction in type I and type IIA COPD fibres was reduced to between one-third and one-half of the tension cost calculated for non-COPD fibres. The rate of force development in type I COPD fibres was reduced to 50% of the rate seen in non-COPD type-I fibres. No difference in the rate of ATP consumption between COPD and non-COPD fibres was evident during isovelocity shortening. These data extend previous findings showing that aspects of breathing mechanics during progressive COPD are associated with remodelling of the diaphragm fibre-type distribution; on top of the increase in type I fibres there are fibre-specific reductions in force development rate (type I fibres) and ATPase rate that are consistent with the impairment of cross-bridge cycling kinetics.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Tendons of myostatin-deficient mice are small, brittle, and hypocellular

Tendons play a significant role in the modulation of forces transmitted between bones and skeletal muscles and consequently protect muscle fibers from contraction-induced, or high-strain, injuries. Myostatin (GDF-8) is a negative regulator of muscle mass. Inhibition of myostatin not only increases the mass and maximum isometric force of muscles, but also increases the susceptibility of muscle fibers to contraction-induced injury. We hypothesized that myostatin would regulate the morphology and mechanical properties of tendons. The expression of myostatin and the myostatin receptors ACVR2B and ACVRB was detectable in tendons. Surprisingly, compared with wild type (MSTN(+/+)) mice, the tendons of myostatin-null mice (MSTN(-/-)) were smaller and had a decrease in fibroblast density and a decrease in the expression of type I collagen. Tendons of MSTN(-/-) mice also had a decrease in the expression of two genes that promote tendon fibroblast proliferation: scleraxis and tenomodulin. Treatment of tendon fibroblasts with myostatin activated the p38 MAPK and Smad2/3 signaling cascades, increased cell proliferation, and increased the expression of type I collagen, scleraxis, and tenomodulin. Compared with the tendons of MSTN(+/+) mice, the mechanical properties of tibialis anterior tendons from MSTN(-/-) mice had a greater peak stress, a lower peak strain, and increased stiffness. We conclude that, in addition to the regulation of muscle mass and force, myostatin regulates the structure and function of tendon tissues.

Chronic exposure to stretch-shortening contractions results in skeletal muscle adaptation in young rats and maladaptation in old rats

The objective of this research was to investigate skeletal muscle response to a chronic administration of stretch-shortening cycles (SSCs) in young and old rats. Dorsiflexor muscles of old (30 months, n = 5) and young (12 weeks, n = 6) rats were exposed 3 times/week for 4.5 weeks to a protocol of 80 maximal SSCs per exposure in vivo. Skeletal muscle response was characterized by isometric and dynamic performance, as well as by muscle wet mass and quantitative morphological analyses following the exposure period. The performance of the young and old groups was not statistically different at the start of the exposure. By the end of the exposure, however, a statistical difference was noted, as performance increased significantly in the young animals and decreased significantly in the old animals. Muscle wet mass of the left tibialis anterior (TA) in the treated limb was significantly greater in the youngthan in the old animals (p < 0.001), whereas there was no difference in the contra-lateral TA. No degenerative myofibers or changes in non-cellular interstitium were noted in either age group, but a significant increase was observed in the volume of the cellular interstitium in the exposed limb of the old animals (p = 0.01), which is indicative of an inflammatory response. Thus, a chronic exposure of SSCs results in significant performance increase and muscle hypertrophy in young animals, and a significant performance decrease and an increased cellular interstitial response in old animals. These findings suggest that age may impair the ability of skeletal muscle to adapt to repetitive mechanical loading, even in the absence of degeneration.

The influence of velocity of stretch–shortening contractions on muscle performance during chronic exposure: age effects

Aging increases injury susceptibility and impairs the ability to adapt to repetitive exposures of mechanical loading. The objective of this research was to investigate if movement velocity affects muscle response to a chronic administration of stretch–shortening cycles (SSCs) differently in young vs. old rats. Dorsiflexor muscles of old (30 months, n = 5) and young rats (12 weeks, n = 6) were exposed 3 times/week for 4.5 weeks to a protocol of 80 maximal SSCs per exposure in vivo. Skeletal muscle response was characterized by high- (500°/s) and low- (60°/s) velocity dynamic performance, which was evaluated using peak eccentric force, isometric pre-stretch force, eccentric force enhancement above the isometric pre-stretch force, negative work, and positive work. The performance of the young and old groups was not statistically different at the start of the exposure. By the end of the exposure, however, a statistical difference was noted—performance increased significantly in the young animals and decreased significantly in the old animals. The SSC velocity had a profound effect on muscle response. The young animals’ high- and low-velocity performances increased during the chronic exposure period, whereas the old animals’ performances declined. High-velocity performance increased more than low-velocity performance in young animals. In contrast, old animals suffered the most loss in high-velocity performance over the chronic exposure period. A chronic exposure of SSCs results in a significant performance increase in young animals, and a significant performance decrease in old animals. These differences are more profound during high-velocity movements. These findings suggest that age may impair the ability of skeletal muscle to adapt to repetitive mechanical loading, particularly during high-velocity movements.

Impact of muscle length during stretch-shortening contractions on real-time and temporal muscle performance measures in rats in vivo

The objective of the present study was to investigate the impact of muscle length during stretch-shortening cycles on static and dynamic muscle performance. Animals were randomly assigned to an isometric (control, Con, n = 12), a short-muscle-length (S-Inj, 1.22-2.09 rad, n = 12), or a long-muscle-length (L-Inj, 1.57-2.44 rad, n = 12) group. The dorsiflexor muscles were exposed in vivo to 7 sets of 10 stretch-shortening contractions (conducted at 8.72 rad/s) or 7 sets of isometric contractions of the same stimulation duration by using a custom-designed dynamometer. Performance was characterized by multipositional isometric exertions and positive, negative, and net work before exposure, 6 h after exposure, and 48 h after exposure to contractions. Real-time muscle performance during the stretch-shortening cycles was characterized by stretch-shortening parameters and negative, positive, and net work. The S-Inj group recovery (force difference) was similar to the Con group force difference at 48 h, whereas the L-Inj group force difference was statistically greater at 1.39, 1.57, and 1.74 rad than the Con group force difference ( P < 0.05). Negative work ( P < 0.05) and net work ( P < 0.05) were statistically lower in the S-Inj and L-Inj groups than in the Con group 48 h after exposure to contractions. Of the real-time parameters, there was a difference in cyclic force with treatment during the stretch-shortening cycles ( P < 0.0001), with the L-Inj group being the most affected. Thus longer ranges of motion result in a more profound isometric force decrement 48 h after exposure to contractions and in real-time changes in eccentric forces.

Functional characteristics of dystrophic skeletal muscle: insights from animal models

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders that show myofiber degeneration and regeneration. Identification of animal models of muscular dystrophy has been instrumental in research on the pathogenesis, pathophysiology, and treatment of these disorders. We review our understanding of the functional status of dystrophic skeletal muscle from selected animal models with a focus on 1) the mdx mouse model of Duchenne muscular dystrophy, 2) the Bio 14.6 delta-sarcoglycan-deficient hamster model of limb-girdle muscular dystrophy, and 3) transgenic null mutant murine lines of sarcoglycan (alpha, beta, delta, and gamma) deficiencies. Although biochemical data from these models suggest that the dystrophin-sarcoglycan-dystroglycan-laminin network is critical for structural integrity of the myofiber plasma membrane, emerging studies of muscle physiology suggest a more complex picture, with specific functional deficits varying considerably from muscle to muscle and model to model. It is likely that changes in muscle structure and function, downstream of the specific, primary biochemical deficiency, may alter muscle contractile properties.

Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that connects the cytoskeleton of a muscle fiber to its surrounding extracellular matrix. Mutations in the DGC disrupt the complex and lead to muscular dystrophy. There are a few naturally occurring animal models of DGC-associated muscular dystrophy (e.g. the dystrophin-deficient mdx mouse, dystrophic golden retriever dog, HFMD cat and the delta-sarcoglycan-deficient BIO 14.6 cardiomyopathic hamster) that share common genetic protein abnormalities similar to those of the human disease. However, the naturally occurring animal models only partially resemble human disease. In addition, no naturally occurring mouse models associated with loss of other DGC components are available. This has encouraged the generation of genetically engineered mouse models for DGC-linked muscular dystrophy. Not only have analyses of these mice led to a significant improvement in our understanding of the pathogenetic mechanisms for the development of muscular dystrophy, but they will also be immensely valuable tools for the development of novel therapeutic approaches for these incapacitating diseases.