Animal models of muscular dystrophy--what can they teach us?

A Novel Method for Detecting Duchenne Muscular Dystrophy in Blood Serum of mdx Mice

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy, typically affecting males in infancy. The disease causes progressive weakness and atrophy of skeletal muscles, with approximately 20,000 new cases diagnosed yearly. Currently, methods for diagnosing DMD are invasive, laborious, and unable to make accurate early detections. While there is no cure for DMD, there are limited treatments available for managing symptoms. As such, there is a crucial unmet need to develop a simple and non-invasive method for accurately detecting DMD as early as possible. Raman spectroscopy with chemometric analysis is shown to have the potential to fill this diagnostic need.

Diagnosis of a model of Duchenne muscular dystrophy in blood serum of mdx mice using Raman hyperspectroscopy

Duchenne muscular dystrophy (DMD) is the most common and severe form of muscular dystrophy and affects boys in infancy or early childhood. Current methods for diagnosing DMD are often laborious, expensive, invasive, and typically diagnose the disease late in its progression. In an effort to improve the accuracy and ease of diagnosis, this study focused on developing a novel method for diagnosing DMD which combines Raman hyperspectroscopic analysis of blood serum with advanced statistical analysis. Partial least squares discriminant analysis was applied to the spectral dataset acquired from blood serum of a mouse model of Duchenne muscular dystrophy (mdx) and control mice. Cross-validation showed 95.2% sensitivity and 94.6% specificity for identifying diseased spectra. These results were verified via external validation, which achieved 100% successful classification accuracy at the donor level. This proof-of-concept study presents Raman hyperspectroscopic analysis of blood serum as an easy, fast, non-expensive, and minimally invasive detection method for distinguishing control and mdx model mice, with a strong potential for clinical diagnosis of DMD.

Circulating serum CK level vs. muscle impairment for in situ monitoring burden of disease in Mdx-mice

BACKGROUND

Duchenne muscular dystrophy (DMD) consists of a lack in the expression of the subsarcolemmal protein dystrophin causing progressive muscle dysfunction. Among the widely applied animal models in DMD research is the C57BL/1010ScSn-Dmdmdx mouse, commonly referred to as the "mdx mouse". The potential benefit of novel interventions in this model is often assessed by variables such as functional improvement, histological changes, and creatine kinase (CK) serum levels as an indicator for the extent of in situ muscle damage.

OBJECTIVE

Our objective was to determine to what extent the serum CK-level serves a surrogate for muscle dysfunction.

METHODS

In this trial mdx mice were subjected to a four-limb wire-hanging test (WHT) to assess the physical performance as a reference for muscle function. As CK is a component of the muscle fiber cytosol, its serum activity is supposed to positively correlate with progressing muscle damage. Hence serum CK levels were measured to detect the degree of muscle impairment. The functional tests and the serum CK levels were analyzed for their specific correlation.

RESULTS

Although physical performance decreased during the course of the experiment, latency to fall times in the WHT did not correlate with the CK level in mdx mice.

CONCLUSION

Our data suggests that the serum CK activity might be a critical parameter to monitor the progression of muscle impairment in mdx mice. Further this study emphasizes the complexity of the DMD phenotype in the mdx mouse, and the care with which isolated parameters in this model should be interpreted.

Spatial and age-related changes in the microstructure of dystrophic and healthy diaphragms

Duchenne muscular dystrophy (DMD) is a progressive degenerative disease that results in fibrosis and atrophy of muscles. The main cause of death associated with DMD is failure of the diaphragm. The diaphragm is a dome-shaped muscle with a fiber microstructure that differs across regions of the muscle. However, no studies to our knowledge have examined spatial variations of muscle fibers in dystrophic diaphragm or how aging affects those variations in DMD. In this study, diaphragms were obtained from mdx and healthy mice at ages three, seven, and ten months in the dorsal, midcostal, and ventral regions. Through immunostaining and confocal imaging, we quantified sarcomere length, interstitial space between fibers, fiber branching, fiber cross sectional area (CSA), and fiber regeneration measured by centrally located nuclei. Because DMD is associated with chronic inflammation, we also investigated the number of macrophages in diaphragm muscle cross-sections. We saw regional differences in the number of regenerating fibers and macrophages during the progression of DMD in the mdx diaphragm. Additionally, the number of regenerating fibers increased with age, while CSA and the number of branching fibers decreased. Dystrophic diaphragms had shorter sarcomere lengths than age-matched controls. Our results suggest that the dystrophic diaphragm in the mdx mouse is structurally heterogeneous and remodels non-uniformly over time. Understanding regional changes in dystrophic diaphragms over time will facilitate the development of targeted therapies to prevent or minimize respiratory failure in DMD patients.

The muscular dystrophic chicken is hypernatremic

1. The E3 ubiquitin protein ligase 1 (WWP1) gene, the mutation of which causes muscular dystrophy in chickens, is expressed not only in the pectoral muscle, but also in a number of tissues such as the kidney. Therefore, this study examined some parameters related to kidney function in muscular dystrophic (MD) chickens. 2. Plasma osmolality, Na⁺ and K⁺ concentrations, aldosterone levels, and the expression of aquaporin (AQP) 2, AQP3, and α subunits of the amiloride-sensitive epithelial sodium channel (αENaC) were analysed in the kidneys of 5-week-old MD chickens and White Leghorn (WL) chickens under physiological conditions or after one day of water deprivation. 3. Plasma osmolality, Na⁺ concentrations, and plasma aldosterone levels were significantly higher in MD chickens than in WL chickens. αENaC mRNA expression levels were lower in MD chickens than in WL chickens. AQP2 and AQP3 mRNA expression levels were similar in the two strains of chickens. 4. Plasma osmolality correlated with aldosterone levels and AQP2 and αENaC mRNA levels in WL chickens. In MD chickens, plasma osmolality correlated with AQP2 mRNA levels, but not with plasma aldosterone or αENaC mRNA levels. 5. These results suggest that neither water reabsorption nor the expression of AQP2 and AQP3 is impaired in MD chickens and that a WWP1 gene mutation may or may not directly induce an abnormality in Na⁺-reabsorption in the kidneys of MD chickens, potentially through αENaC.

Nutraceuticals and Their Potential to Treat Duchenne Muscular Dystrophy: Separating the Credible from the Conjecture

In recent years, complementary and alternative medicine has become increasingly popular. This trend has not escaped the Duchenne Muscular Dystrophy community with one study showing that 80% of caregivers have provided their Duchenne patients with complementary and alternative medicine in conjunction with their traditional treatments. These statistics are concerning given that many supplements are taken based on purely "anecdotal" evidence. Many nutraceuticals are thought to have anti-inflammatory or anti-oxidant effects. Given that dystrophic pathology is exacerbated by inflammation and oxidative stress these nutraceuticals could have some therapeutic benefit for Duchenne Muscular Dystrophy (DMD). This review gathers and evaluates the peer-reviewed scientific studies that have used nutraceuticals in clinical or pre-clinical trials for DMD and thus separates the credible from the conjecture.

Molecular hydrogen alleviates motor deficits and muscle degeneration in mdx mice

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a devastating muscle disease caused by a mutation in DMD encoding dystrophin. Oxidative stress accounts for dystrophic muscle pathologies in DMD. We examined the effects of molecular hydrogen in mdx mice, a model animal for DMD.

METHODS

The pregnant mother started to take supersaturated hydrogen water (>5 ppm) ad libitum from E15.5 up to weaning of the offspring. The mdx mice took supersaturated hydrogen water from weaning until age 10 or 24 weeks when they were sacrificed.

RESULTS

Hydrogen water prevented abnormal body mass gain that is commonly observed in mdx mice. Hydrogen improved the spontaneous running distance that was estimated by a counter-equipped running-wheel, and extended the duration on the rota-rod. Plasma creatine kinase activities were decreased by hydrogen at ages 10 and 24 weeks. Hydrogen also decreased the number of central nuclei of muscle fibers at ages 10 and 24 weeks, and immunostaining for nitrotyrosine in gastrocnemius muscle at age 24 weeks. Additionally, hydrogen tended to increase protein expressions of antioxidant glutathione peroxidase 1, as well as anti-apoptotic Bcl-2, in skeletal muscle at age 10 weeks.

DISCUSSION

Although molecular mechanisms of the diverse effects of hydrogen remain to be elucidated, hydrogen potentially improves muscular dystrophy in DMD patients.

Muscular dystrophy in the mdx mouse is a severe myopathy compounded by hypotrophy, hypertrophy and hyperplasia

BACKGROUND

Preclinical testing of potential therapies for Duchenne muscular dystrophy (DMD) is conducted predominantly of the mdx mouse. But lack of a detailed quantitative description of the pathology of this animal limits our ability to evaluate the effectiveness of putative therapies or their relevance to DMD.

METHODS

Accordingly, we have measured the main cellular components of muscle growth and regeneration over the period of postnatal growth and early pathology in mdx and wild-type (WT) mice; phalloidin binding is used as a measure of fibre size, myonuclear counts and BrdU labelling as records of myogenic activity.

RESULTS

We confirm a two-phase postnatal growth pattern in WT muscle: first, increase in myonuclear number over weeks 1 to 3, then expansion of myonuclear domain. Mdx muscle growth lags behind that of WT prior to overt signs of pathology. Fibres are smaller, with fewer myonuclei and smaller myonuclear domains. Moreover, satellite cells are more readily detached from mdx than WT muscle fibres. At 3 weeks, mdx muscles enter a phase of florid myonecrosis, accompanied by concurrent regeneration of an intensity that results in complete replacement of pre-existing muscle over the succeeding 3 to 4 weeks. Both WT and mdx muscles attain maximum size by 12 to 14 weeks, mdx muscle fibres being up to 50% larger than those of WT as they become increasingly branched. Mdx muscle fibres also become hypernucleated, containing twice as many myonuclei per sarcoplasmic volume, as those of WT, the excess corresponding to the number of centrally placed myonuclei.

CONCLUSIONS

The best-known consequence of lack of dystrophin that is common to DMD and the mdx mouse is the conspicuous necrosis and regeneration of muscle fibres. We present protocols for measuring this in terms both of loss of muscle nuclei previously labelled with BrdU and of the intensity of myonuclear labelling with BrdU administered during the regeneration period. Both measurements can be used to assess the efficacy of putative antinecrotic agents. We also show that lack of dystrophin is associated with a number of previously unsuspected abnormalities of muscle fibre structure and function that do not appear to be directly associated with myonecrosis.

The effect of the muscle environment on the regenerative capacity of human skeletal muscle stem cells

Background

Muscle stem cell transplantation is a possible treatment for muscular dystrophy. In addition to the intrinsic properties of the stem cells, the local and systemic environment plays an important role in determining the fate of the grafted cells. We therefore investigated the effect of modulating the host muscle environment in different ways (irradiation or cryoinjury or a combination of irradiation and cryoinjury) in two immunodeficient mouse strains (mdx nude and recombinase-activating gene (Rag)2-/γ chain-/C5-) on the regenerative capacity of two types of human skeletal muscle-derived stem cell (pericytes and CD133+ cells).

Methods

Human skeletal muscle-derived pericytes or CD133+ cells were transplanted into muscles of either mdx nude or recombinase-activating gene (Rag)2-/γ chain-/C5- host mice. Host muscles were modulated prior to donor cell transplantation by either irradiation, or cryoinjury, or a combination of irradiation and cryoinjury. Muscles were analysed four weeks after transplantation, by staining transverse cryostat sections of grafted muscles with antibodies to human lamin A/C, human spectrin, laminin and Pax 7. The number of nuclei and muscle fibres of donor origin and the number of satellite cells of both host and donor origin were quantified.

Results

Within both host strains transplanted intra-muscularly with both donor cell types, there were significantly more nuclei and muscle fibres of donor origin in host muscles that had been modulated by cryoinjury, or irradiation+cryoinjury, than by irradiation alone. Irradiation has no additive effects in further enhancing the transplantation efficiency than cryodamage. Donor pericytes did not give rise to satellite cells. However, using CD133+ cells as donor cells, there were significantly more nuclei, muscle fibres, as well as satellite cells of donor origin in Rag2-/γ chain-/C5- mice than mdx nude mice, when the muscles were injured by either cryodamage or irradiation+cryodamage.

Conclusions

Rag2-/γ chain-/C5- mice are a better recipient mouse strain than mdx nude mice for human muscle stem cell transplantation. Cryodamage of host muscle is the most effective method to enhance the transplantation efficiency of human skeletal muscle stem cells. This study highlights the importance of modulating the muscle environment in preclinical studies to optimise the efficacy of transplanted stem cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0036-8) contains supplementary material, which is available to authorized users.

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.

Resveratrol improves muscle function but not oxidative capacity in young mdx mice

Patients with Duchenne muscular dystrophy (DMD) have reduced muscle function due to chronic muscle damage, inflammation, oxidative stress, and reduced oxidative capacity. Resveratrol reduces inflammation and oxidative stress, and increases oxidative capacity in other disease models. The purpose of this study was to determine the effects of resveratrol on muscle function, muscle pathology, and oxidative capacity in young mdx mice. For this, 4- to 5-week-old male mdx mice were randomized into control or resveratrol-treated groups and given resveratrol (100 mg/kg body mass) or an equal volume of water by gavage every other day for 8 weeks. Muscle function was assessed pre- and post-treatment. Central nucleation, total immune cell infiltrate, oxidative stress, and oxidative capacity were measured post-treatment. Resveratrol mediated substantial improvements in rotarod performance and in-situ peak tension by 53% and 17%, respectively, and slight improvements in central nucleation and oxidative stress. Resveratrol did not affect total immune cell infiltrate at 12 weeks of age, and had no effect on oxidative capacity. Resveratrol improves muscle function in mdx mice despite small changes in muscle pathology. The likely mechanism is a resveratrol-mediated reduction in immune cell infiltrate at the early stages of this disease, as previously reported by our laboratory.

Delayed bone regeneration is linked to chronic inflammation in murine muscular dystrophy

Duchenne muscular dystrophy (DMD) patients exhibit skeletal muscle weakness with continuous cycles of muscle fiber degeneration/regeneration, chronic inflammation, low bone mineral density, and increased risks of fracture. Fragility fractures and associated complications are considered as a consequence of the osteoporotic condition in these patients. Here, we aimed to establish the relationship between muscular dystrophy and fracture healing by assessing bone regeneration in mdx mice, a model of DMD with absence of osteoporosis. Our results illustrate that muscle defects in mdx mice impact the process of bone regeneration at various levels. In mdx fracture calluses, both cartilage and bone deposition were delayed followed by a delay in cartilage and bone remodeling. Vascularization of mdx fracture calluses was also decreased during the early stages of repair. Dystrophic muscles are known to contain elevated numbers of macrophages contributing to muscle degeneration. Accordingly, we observed increased macrophage recruitment in the mdx fracture calluses and abnormal macrophage accumulation throughout the process of bone regeneration. These changes in the inflammatory environment subsequently had an impact on the recruitment of osteoclasts and the remodeling phase of repair. Further damage to the mdx muscles, using a novel model of muscle trauma, amplified both the chronic inflammatory response and the delay in bone regeneration. In addition, PLX3397 treatment of mdx mice, a cFMS (colony stimulating factor receptor 1) inhibitor in monocytes, partially rescued the bone repair defect through increasing cartilage deposition and decreasing the number of macrophages. In conclusion, chronic inflammation in mdx mice contributes to the fracture healing delay and is associated with a decrease in angiogenesis and a transient delay in osteoclast recruitment. By revealing the role of dystrophic muscle in regulating the inflammatory response during bone repair, our results emphasize the implication of muscle in the normal bone repair process and may lead to improved treatment of fragility fractures in DMD patients.

Rhabdomyosarcomas: an overview on the experimental animal models

Rhabdomyosarcomas (RMS) are aggressive childhood soft-tissue malignancies deriving from mesenchymal progenitors that are committed to muscle-specific lineages. Despite the histopathological signatures associated with three main histological variants, termed embryonal, alveolar and pleomorphic, a plethora of genetic and molecular changes are recognized in RMS. Over the years, exposure to carcinogens or ionizing radiations and gene-targeting approaches in vivo have greatly contributed to disclose some of the mechanisms underlying RMS onset. In this review, we describe the principal distinct features associated with RMS variants and focus on the current available experimental animal models to point out the molecular determinants cooperating with RMS development and progression.

Resveratrol decreases inflammation and increases utrophin gene expression in the mdx mouse model of Duchenne muscular dystrophy

BACKGROUND & AIMS

Duchenne muscular dystrophy (DMD) is a lethal genetic disease with no cure. Reducing inflammation or increasing utrophin expression can alleviate DMD pathology. Resveratrol can reduce inflammation and activate the utrophin promoter. The aims of this study were to identify an active dose of resveratrol in mdx mice and examine if this dose decreased inflammation and increased utrophin expression.

METHODS

5-week old mdx mice were given 0, 10, 100, or 500 mg/kg of resveratrol everyday for 10 days. Sirt1 was measured by qRT-PCR and used to determine the most active dose. Muscle inflammation was measured by H&E staining, CD45 and F4/80 immunohistochemistry. IL-6, TNFα, PGC-1α, and utrophin gene expression were measured by qRT-PCR. Utrophin, Sirt1, and PGC-1α protein were quantified by western blot.

RESULTS

The 100 mg/kg dose of resveratrol, the most active dose, increased Sirt1 mRNA 60 ± 10% (p < 0.01), reduced immune cell infiltration 21 ± 6% (H&E) and 42 ± 8% (CD45 immunohistochemistry (p < 0.05)), reduced macrophage infiltration 48 ± 10% (F4/80 immunohistochemistry (p < 0.05)), and increased IL-6, PGC-1α, and utrophin mRNA 247 ± 77%, 27 ± 17%, and 43 ± 23% respectively (p ≤ 0.05). Utrophin, Sirt1, and PGC-1α protein expression did not change.

CONCLUSIONS

Resveratrol may be a therapy for DMD by reducing inflammation.

Connective tissue growth factor is overexpressed in muscles of human muscular dystrophy

The detailed process of how dystrophic muscles are replaced by fibrotic tissues is unknown. In the present study, the immunolocalization and mRNA expression of connective tissue growth factor (CTGF) in muscles from normal and dystrophic human muscles were examined with the goal of elucidating the pathophysiological function of CTGF in muscular dystrophy. Biopsies of frozen muscle from patients with Duchenne muscular dystrophy (DMD), Becker muscular dystrophy, congenital muscular dystrophy, spinal muscular atrophy, congenital myopathy were analyzed using anti-CTGF polyclonal antibody. Reverse transcription-polymerase chain reaction (RT-PCR) was also performed to evaluate the expression of CTGF mRNA in dystrophic muscles. In normal muscle, neuromuscular junctions and vessels were CTGF-immunopositive, which suggests a physiological role for CTGF in these sites. In dystrophic muscle, CTGF immunoreactivity was localized to muscle fiber basal lamina, regenerating fibers, and the interstitium. Triple immunolabeling revealed that activated fibroblasts were immunopositive for CTGF and transforming growth factor-beta1 (TGF-beta1). RT-PCR analysis revealed increased levels of CTGF mRNA in the muscles of DMD patients. Co-localization of TGF-beta1 and CTGF in activated fibroblasts suggests that CTGF expression is regulated by TGF-beta1 through a paracrine/autocrine mechanism. In conclusion, TGF-beta1-CTGF pathway may play a role in the fibrosis that is commonly observed in muscular dystrophy.

Hematopoietic cell transplantation directly into dystrophic muscle fails to reconstitute satellite cells and myofibers

We sought to determine whether wild-type hematopoietic cell transplantation directly into muscle could restore dystrophin expression in a relevant preclinical canine model of Duchenne muscular dystrophy. In recipients rendered tolerant to their dog leukocyte antigen-matched unaffected littermates through hematopoietic stem cell transplantation, intramuscular injection of donor marrow cells produced no evidence of dystrophin expression, and clonal analysis of satellite cells failed to reveal any donor contribution.

Dystrophin-deficient mdx mice display a reduced life span and are susceptible to spontaneous rhabdomyosarcoma

Duchenne muscular dystrophy (DMD) is the most common, lethal genetic disorder of children. A number of animal models of muscular dystrophy exist, but the most effective model for characterizing the structural and functional properties of dystrophin and therapeutic interventions has been the mdx mouse. Despite the approximately 20 years of investigations of the mdx mouse, the impact of the disease on the life span of mdx mice and the cause of death remain unresolved. Consequently, a life span study of the mdx mouse was designed that included cohorts of male and female mdx and wild-type C57BL/10 mice housed under specific pathogen-free conditions with deaths restricted to natural causes and with examination of the carcasses for pathology. Compared with wild-type mice, both mdx male and female mice had reduced life spans and displayed a progressively dystrophic muscle histopathology. Surprisingly, old mdx mice were prone to develop muscle tumors that resembled the human form of alveolar rhabdomyosarcoma, a cancer associated with poor prognosis. Rhabdomyosarcomas have not been observed previously in nontransgenic mice. The results substantiate the mdx mouse as an important model system for studies of the pathogenesis of and potential remedies for DMD.

Fine mapping of the muscular dystrophy (AM) gene on chicken chromosome 2q

Our previous studies revealed that the genetic locus for chicken muscular dystrophy of abnormal muscle (AM) mapped to chromosome 2q, and that the region showed conserved synteny with human chromosome 8q11-24.3. In the current study, we mapped the chicken orthologues of genes from human chromosome 8q11-24 in order to identify the responsible gene. Polymorphisms in the chicken orthologues were identified in the parents of the resource family. Twenty-three genes and expressed sequence tags (ESTs) were mapped to chicken chromosome 2 by linkage analysis. The detailed comparative map shows a high conservation of synteny between chicken chromosome 2q and human chromosome 8q. The AM locus was mapped between [inositol(myo)-1(or4)-monophosphatase 1] (IMPA1) gene and [core-binding factor, runt domain, alpha-subunit 2; translocated to 1; cyclin D-related] (CBFA2T1) gene. The genes located between IMPA1 and CBFA2T1 are the most likely candidates for chicken muscular dystrophy.

Electrodiagnostic evaluation in feline hypertrophic muscular dystrophy

Standard needle electromyography (EMG) of 56 muscles and nerve conduction velocities (NCV) of the ulnar and common peroneal nerves were investigated in each of six cats affected with hypertrophic feline muscular dystrophy, 10 related heterozygote carriers and 10 normal cats. The EMG findings were considered normal in carrier and control cats, and consisted of 33% normal readings, 22% myotonic discharges, 18% fibrillation potentials, 11% prolonged insertional potentials, 10% complex repetitive discharges and 6% positive sharp waves in affected cats. Muscles of the proximal limbs were most frequently affected. No differences in NCV were found between the three cat groups. It was concluded that dystrophin-deficient dystrophic cats have widespread and frequent EMG changes, predominantly myotonic discharges and fibrillation potentials, which are most pronounced in the proximal appendicular muscles.

Regenerative capacity of the dystrophic (mdx) diaphragm after induced injury

Duchenne muscular dystrophy is characterized by myofiber necrosis, muscle replacement by connective tissue, and crippling weakness. Although the mdx mouse also lacks dystrophin, most muscles show little myofiber loss or functional impairment. An exception is the mdx diaphragm, which is phenotypically similar to the human disease. Here we tested the hypothesis that the mdx diaphragm has a defective regenerative response to necrotic injury, which could account for its severe phenotype. Massive necrosis was induced in mdx and wild-type (C57BL10) mouse diaphragms in vivo by topical application of notexin, which destroys mature myofibers while leaving myogenic precursor satellite cells intact. At 4 h after acute exposure to notexin, >90% of diaphragm myofibers in both wild-type and mdx mice demonstrated pathological sarcolemmal leakiness, and there was a complete loss of isometric force-generating capacity. Both groups of mice showed strong expression of embryonic myosin within the diaphragm at 5 days, which was largely extinguished by 20 days after injury. At 60 days postinjury, wild-type diaphragms exhibited a persistent loss ( approximately 25%) of isometric force-generating capacity, associated with a trend toward increased connective tissue infiltration. In contrast, mdx diaphragms achieved complete functional recovery of force generation to noninjured values, and there was no increase in muscle connective tissue over baseline. These data argue against any loss of intrinsic regenerative capacity within the mdx diaphragm, despite characteristic features of major dystrophic pathology being present. Our findings support the concept that significant latent regenerative capacity resides within dystrophic muscles, which could potentially be exploited for therapeutic purposes.

Temporal gene expression profiling of dystrophin-deficient (mdx) mouse diaphragm identifies conserved and muscle group-specific mechanisms in the pathogenesis of muscular dystrophy

Mutations in dystrophin are the proximate cause of Duchenne muscular dystrophy (DMD), but pathogenic mechanisms linking the absence of dystrophin from the sarcolemma to myofiber necrosis are not fully known. The muscular dystrophies also have properties not accounted for by current disease models, including the temporal delay to disease onset, broad species differences in severity, and diversity of skeletal muscle responses. To address the mechanisms underlying the differential targeting of muscular dystrophy, we characterized temporal expression profiles of the diaphragm in dystrophin-deficient (mdx) mice between postnatal days 7 and 112 using oligonucleotide microarrays and contrasted these data with published hindlimb muscle data. Although the diaphragm and hindlimb muscle groups differ in severity of response to dystrophin deficiency, and exhibited substantial divergence in some transcript categories including inflammation and muscle-specific genes, our data show that the general mechanisms operative in muscular dystrophy are highly conserved. The two muscle groups principally differed in expression levels of differentially regulated genes, as opposed to the non-conserved induced/repressed transcripts defining fundamentally distinct mechanisms. We also identified a postnatal divergence of the two wild-type muscle group expression profiles that temporally correlated with the onset and progression of the dystrophic process. These findings support the hypothesis that conserved disease mechanisms interacting with baseline differences in muscle group-specific transcriptomes underlie their differential responses to DMD. We further suggest that muscle group-specific transcriptional profiles contribute toward the muscle targeting and sparing patterns observed for a variety of metabolic and neuromuscular diseases.

The Feline Genome Project

The compilation of a dense gene map and eventually a whole genome sequence (WGS) of the domestic cat holds considerable value for human genome annotation, for veterinary medicine, and for insight into the evolution of genome organization among mammals. Human association and veterinary studies of the cat, its domestic breeds, and its charismatic wild relatives of the family Felidae have rendered the species a powerful model for human hereditary diseases, for infectious disease agents, for adaptive evolutionary divergence, for conservation genetics, and for forensic applications. Here we review the advantages, rationale, and present strategy of a feline genome project, and we describe the disease models, comparative genomics, and biological applications posed by the full resolution of the cat's genome.

Constitutive properties, not molecular adaptations, mediate extraocular muscle sparing in dystrophic mdx mice

Extraocular muscle (EOM) is spared in Duchenne muscular dystrophy. Here, we tested putative EOM sparing mechanisms predicted from existing dystrophinopathy models. Data show that mdx mouse EOM contains dystrophin-glycoprotein complex (DGC)-competent and DGC-deficient myofibers distributed in a fiber type-specific pattern. Up-regulation of a dystrophin homologue, utrophin, mediates selective DGC retention. Counter to the DGC mechanical hypothesis, an intact DGC is not a precondition for EOM sarcolemmal integrity, and active adaptation at the level of calcium homeostasis is not mechanistic in protection. A partial, fiber type-specific retention of antiischemic nitric oxide to vascular smooth muscle signaling is not a factor in EOM sparing, because mice deficient in dystrophin and alpha-syntrophin, which localizes neuronal nitric oxide synthase to the sarcolemma, have normal EOMs. Moreover, an alternative transmembrane protein, alpha7beta1 integrin, does not appear to substitute for the DGC in EOM. Finally, genomewide expression profiling showed that EOM does not actively adapt to dystrophinopathy but identified candidate genes for the constitutive protection of mdx EOM. Taken together, data emphasize the conditional nature of dystrophinopathy and the potential importance of nonmechanical DGC roles and support the hypothesis that broad, constitutive structural cell signaling, and/or biochemical differences between EOM and other skeletal muscles are determinants of differential disease responsiveness.

Persistent over-expression of specific CC class chemokines correlates with macrophage and T-cell recruitment in mdx skeletal muscle

Prior studies and the efficacy of immunotherapies provide evidence that inflammation is mechanistic in pathogenesis of Duchenne muscular dystrophy. To identify putative pro-inflammatory mechanisms, we evaluated chemokine gene/protein expression patterns in skeletal muscle of mdx mice. By DNA microarray, reverse transcription-polymerase chain reaction, quantitative polymerase chain reaction, and immunoblotting, convergent evidence established the induction of six distinct CC class chemokine ligands in adult MDX: CCL2/MCP-1, CCL5/RANTES, CCL6/mu C10, CCL7/MCP-3, CCL8/MCP-2, and CCL9/MIP-1gamma. CCL receptors, CCR2, CCR1, and CCR5, also showed increased expression in mdx muscle. CCL2 and CCL6 were localized to both monocular cells and muscle fibers, suggesting that dystrophic muscle may contribute toward chemotaxis. Temporal patterns of CCL2 and CCL6 showed early induction and maintained expression in mdx limb muscle. These data raise the possibility that chemokine signaling pathways coordinate a spatially and temporally discrete immune response that may contribute toward muscular dystrophy. The chemokine pro-inflammatory pathways described here in mdx may represent new targets for treatment of Duchenne muscular dystrophy.

Analyses of Beta-1 Syntrophin, Syndecan 2 and Gem GTPase as Candidates for Chicken Muscular Dystrophy

Despite intensive studies of muscular dystrophy of chicken, the responsible gene has not yet been identified. Our recent studies mapped the genetic locus for abnormal muscle (AM) of chicken with muscular dystrophy to chromosome 2q using the Kobe University (KU) resource family, and revealed the chromosome region where the AM gene is located has conserved synteny to human chromosome 8q11-24.3, where the beta-1 syntrophin (SNTB1), syndecan 2 (SDC2) and Gem GTPase (GEM) genes are located. It is reasonable to assume those genes might be candidates for the AM gene. In this study, we cloned and sequenced the chicken SNTB1, SDC2 and GEM genes, and identified sequence polymorphisms between parents of the resource family. The polymorphisms were genotyped to place these genes on the chicken linkage map. The AM gene of chromosome 2q was mapped 130 cM from the distal end, and closely linked to calbindin 1 (CALB1). SNTB1 and SDC2 genes were mapped 88.5 cM distal and 27.6 cM distal from the AM gene, while the GEM gene was mapped 18.5 cM distal from the AM gene and 9.1 cM proximal from SDC2. Orthologues of SNTB1, SDC2 and GEM were syntenic to human chromosome 8q. SNTB1, SDC2 and GEM did not correspond to the AM gene locus, suggesting it is unlikely they are related to chicken muscular dystrophy. However, this result also suggests that the genes located in the proximal region of the CALB1 gene on human chromosome 8q are possible candidates for this disease.

Alpha1-syntrophin-deficient skeletal muscle exhibits hypertrophy and aberrant formation of neuromuscular junctions during regeneration

Alpha1-syntrophin is a member of the family of dystrophin-associated proteins; it has been shown to recruit neuronal nitric oxide synthase and the water channel aquaporin-4 to the sarcolemma by its PSD-95/SAP-90, Discs-large, ZO-1 homologous domain. To examine the role of alpha1-syntrophin in muscle regeneration, we injected cardiotoxin into the tibialis anterior muscles of alpha1-syntrophin-null (alpha1syn-/-) mice. After the treatment, alpha1syn-/- muscles displayed remarkable hypertrophy and extensive fiber splitting compared with wild-type regenerating muscles, although the untreated muscles of the mutant mice showed no gross histological change. In the hypertrophied muscles of the mutant mice, the level of insulin-like growth factor-1 transcripts was highly elevated. Interestingly, in an early stage of the regeneration process, alpha1syn-/- mice showed remarkably deranged neuromuscular junctions (NMJs), accompanied by impaired ability to exercise. The contractile forces were reduced in alpha1syn-/- regenerating muscles. Our results suggest that the lack of alpha1-syntrophin might be responsible in part for the muscle hypertrophy, abnormal synapse formation at NMJs, and reduced force generation during regeneration of dystrophin-deficient muscle, all of which are typically observed in the early stages of Duchenne muscular dystrophy patients.

Extraocular muscle is spared despite the absence of an intact sarcoglycan complex in gamma- or delta-sarcoglycan-deficient mice

Models of the dystrophin-glycoprotein complex do not reconcile the novel sparing of extraocular muscle in muscular dystrophy. Extraocular muscle sparing in Duchenne muscular dystrophy implies the existence of adaptive properties in these muscles that may extend protection to other neuromuscular diseases. We studied the extraocular muscle morphology and dystrophin-glycoprotein complex organization in murine targeted deletion of the gamma-sarcoglycan (gsg(-/-)) and delta-sarcoglycan (dsg(-/-)) genes, two models of autosomal recessive limb girdle muscular dystrophy. In contrast to limb and diaphragm, the principal extraocular muscles were intact in gsg(-/-) and dsg(-/-) mice. However, central nucleated, presumptive regenerative, fibers were seen in the accessory extraocular muscles (retractor bulbi, levator palpebrae superioris) of both strains. Skeletal muscles of gsg(-/-) mice exhibited in vivo Evans Blue dye permeability, while the principal extraocular muscles did not. Disruption of gamma-sarcoglycan produced secondary displacement of alpha- and beta-sarcoglycans in the extraocular muscles. The intensity of immunofluorescence for dystrophin and alpha- and beta-dystroglycan also appeared to be slightly reduced. Utrophin localization was unchanged. The finding that sarcoglycan disruption was insufficient to elicit alterations in extraocular muscle suggests that loss of mechanical stability and increased sarcolemmal permeability are not inevitable consequences of mutations that disrupt the dystrophin-glycoprotein complex organization and must be accounted for in models of muscular dystrophy.

Identification of the increased expression of monocyte chemoattractant protein-1, cathepsin S, UPIX-1, and other genes in dystrophin-deficient mouse muscles by suppression subtractive hybridization

The lack of dystrophin results in muscular dystrophy characterized by degeneration, inflammation, and partial regeneration of skeletal muscles. The fate of these muscles may be determined by the extent of adaptation to the defect and the efficiency of regeneration that is affected by inflammatory cells. We have used suppression subtractive hybridization and quantitative Northern blot analysis to identify differentially expressed genes. Increased expression of murine monocyte chemoattractant protein-1 (JE/MCP-1), cathepsin S, UPIX-1, nmb, cathepsin B, and lysozyme M mRNAs were identified in 2-month-old mdx mouse leg muscles. UPIX-1 is a novel gene. Although it was not expressed in control muscles, it was expressed in control brain, heart, and spleen. JE/MCP-1 and cathepsin S proteins in mdx muscles, as well as JE/MCP-1 protein in the serum of mdx mice were also detected. JE/MCP-1 may be responsible for attraction of inflammatory cells, and cathepsin S, a potent elastolytic protease, may contribute to the remodeling of the extracellular matrix that is required for the migration of these cells to the injured muscles.

Why do cultured transplanted myoblasts die in vivo? DNA quantification shows enhanced survival of donor male myoblasts in host mice depleted of CD4+ and CD8+ cells or Nk1.1+ cells

Overcoming the massive and rapid death of injected donor myoblasts is the primary hurdle for successful myoblast transfer therapy (MTT), designed as a treatment for the lethal childhood myopathy Duchenne muscular dystrophy. The injection of male myoblasts into female host mice and quantification of surviving male DNA using the Y-chromosome-specific (Y1) probe allows the speed and extent of death of donor myoblasts to be determined. Cultured normal C57BL/10Sn male donor myoblasts were injected into untreated normal C57BL/10Sn and dystrophic mdx female host mice and analyzed by slot blots using a 32P-labeled Y1 probe. The amount of male DNA from donor myoblasts showed a remarkable decrease within minutes and by 1 h represented only about 10-18% of the 2.5 x 10(5) cells originally injected (designated 100%). This declined further over 1 week to approximately 1-4%. The host environment (normal or dystrophic) as well as the extent of passaging in tissue culture (early "P3" or late "P15-20" passage) made no difference to this result. Modulation of the host response by CD4+/CD8+ -depleting antibodies administered prior to injection of the cultured myoblasts dramatically enhanced donor myoblast survival in dystrophic mdx hosts (15-fold relative to untreated hosts after 1 week). NK1.1 depletion also dramatically enhanced donor myoblast survival in dystrophic mdx hosts (21-fold after 1 week) compared to untreated hosts. These results provide a strategic approach to enhance donor myoblast survival in clinical trials of MTT.

Mechanisms of resistance to pathogenesis in muscular dystrophies

A mechanistic definition of the dystrophic process is proposed, and the effects of growth factors vs. down-regulation of growth are critically analyzed. A conceptual scheme is presented to illustrate the steps leading to pathology, and various compensatory systems which ameliorate the pathology are examined, particularly in regards to the mdv mouse which is resistant to the deficiency of dystrophin, the main protein product of the Duchenne and Becker muscular dystrophy (DMD/BMD) gene. These compensatory systems are analyzed in terms of the differential resistance of fiber types to pathogenesis. The generation of a stable population of maturationally arrested centronucleated fibers which express the mature adult myosin isoforms is proposed to be the main strategy of mdx muscle to minimize apoptosis. Physiological properties of these fibers, such as utrophin expression, and high mitochondrial and endoplasmic reticulum content, together with probable increased glycerophosphorylcholine concentrations and facile access to the vascular system, are hypothesized to be instrumental in their resistance to pathogenesis. It is proposed that the major element that determines the susceptibility of most human muscles to the dystrophic process is their inability to arrest the maturation of regenerated fibers at the centronucleated stage with a concomitant expression of the adult myosins.

Myonuclear apoptosis in dystrophic mdx muscle occurs by perforin-mediated cytotoxicity

Myonuclear apoptosis is an early event in the pathology of dystrophin-deficient muscular dystrophy in the mdx mouse. However, events that initiate apoptosis in muscular dystrophy are unknown, and whether elimination of apoptosis can ameliorate subsequent muscle wasting remains a major question. We have tested the hypothesis that cytotoxic T-lymphocytes initiate myonuclear apoptosis in dystrophic muscle, and examined whether perforin-mediated cytotoxicity plays a role in the pathophysiology of muscular dystrophy. Mdx mice showed muscle invasion by cytotoxic T cells and helper T cells at the onset of histologically detectable muscle fiber pathology. At this time, perforin-expressing cells were also present at elevated concentration. Mdx mice depleted of CD8(+) cells showed a significant reduction of apoptotic myonuclei concentration and a reduction in necrosis, judged by macrophage invasion of muscle fibers. Double-mutant mice, deficient in dystrophin and perforin, showed nearly complete absence of myonuclear apoptosis, and a significant reduction in the concentration of macrophages in the connective tissue surrounding muscle fibers. However, muscle fiber invasion by macrophages was not reduced significantly in double mutant mice. Thus, cytotoxic T-lymphocytes contribute significantly to apoptosis and necrosis in mdx dystrophy, and perforin-mediated killing is primarily responsible for myonuclear apoptosis.

Congenital myopathy in Braunvieh x Brown Swiss calves

A hitherto unknown skeletal muscle disorder is described in six Braunvieh x Brown Swiss calves. The animals showed rapidly progressing muscular weakness and became recumbent within 2 weeks of birth. Histological examination of skeletal muscle revealed a marked variation in muscle fibre size, internally placed nuclei, segmental loss of cross-striation with disorganization of myofibrils, and accumulation of nemaline rods. The most distinctive histological finding was intracytoplasmic, homogeneous, mostly crescent-shaped areas at the periphery of numerous muscle fibres. Electron microscopically, accumulations of tightly packed, parallel filamentous structures, about 20 nm in diameter, were detected in these areas. Enzyme histochemistry showed that all muscle fibre types were affected. Vimentin and dystrophin immunohistochemistry revealed normal antigen distribution within connective tissue components and at the periphery of each muscle fibre, respectively. The lesions could be readily distinguished from other neurological and neuromuscular disorders previously described in Braunvieh x Brown Swiss or American Brown Swiss Cattle. The disease appears to be a novel congenital myopathy in this breed, and a hereditary aetiology is suspected.

Spatial learning and hippocampal long-term potentiation are not impaired in mdx mice

Moderate non-progressive cognitive impairment is a consistent feature of Duchenne muscular dystrophy (DMD), although few central nervous system abnormalities have yet been identified. A model for DMD is provided by the mdx mouse which fails to produce full length dystrophin in muscle and brain. In this study we have compared performances in a hippocampal-dependent spatial learning task, the Morris water maze, in mdx mice and in age-matched normal (C57BL/10) mice. There was no difference in acquisition rates or in retention between the two groups. We also found no difference in the magnitude of long-term potentiation (LTP) between the two groups, either in the dentate gyrus or in area CA. These experiments demonstrate that neither spatial learning nor hippocampal synaptic plasticity are significantly affected by the lack of full-length dystrophin.

A new adenoviral vector: Replacement of all viral coding sequences with 28 kb of DNA independently expressing both full-length dystrophin and beta-galactosidase

Adenoviral vector-mediated gene transfer offers significant potential for gene therapy of many human diseases. However, progress has been slowed by several limitations. First, the insert capacity of currently available adenoviral vectors is limited to 8 kb of foreign DNA. Second, the expression of viral proteins in infected cells is believed to trigger a cellular immune response that results in inflammation and in only transient expression of the transferred gene. We report the development of a new adenoviral vector that has all viral coding sequences removed. Thus, large inserts are accommodated and expression of all viral proteins is eliminated. The first application of this vector system carries a dual expression cassette comprising 28.2 kb of nonviral DNA that includes the full-length murine dystrophin cDNA under control of a large muscle-specific promoter and a lacZ reporter construct. Using this vector, we demonstrate independent expression of both genes in primary mdx (dystrophin-deficient) muscle cells.

Dystrophin-positive muscle fibers following C2 myoblast transplantation into mdx nude mice

To determine when and how the dystrophin-positive muscle fibers are formed after myoblast transplantation into dystrophin-negative muscles, the tibialis anterior (TA) muscle from mdx nude mouse was chronologically examined after C2 myoblast transplantation by immunohistochemical and glucose 6-phosphate isomerase (GPI) isoenzyme analyses. The host TA muscle transplanted with C2 myoblasts became necrotic with accumulation of basic fibroblast growth factor in the necrotic areas. This may stimulate concomitant proliferation of the host satellite cells and C2 myoblasts. Small dystrophin-positive muscle fibers appeared in the necrotic areas 3 days after transplantation. This TA muscle contained two different kinds of homodimer GPI isoenzymes but did not contain the heterodimer, suggesting rare fusion of host and donor cells. The dystrophin-positive muscle fibers in the necrotic areas rapidly increased in number and in size by 7 days, but they were smaller than the original host muscle fibers. They had central nuclei, indicating that they were regenerating fibers. The presence of heterodimer GPI isoenzyme in these muscles indicated that the regenerating fibers were mosaic host/donor muscle fibers. The dystrophin-positive muscle fibers are probably formed first by fusion of donor cells with each other and then later by the fusion of host satellite and donor cells.

Recombinant truncated dystrophin minigenes: construction, expression, and adenoviral delivery

Duchenne muscular dystrophy (DMD) is a lethal genetic disorder for which there is currently no effective treatment. Although clinical application of adenoviral vector-mediated gene transfer has not been fully developed, it shows promise for the treatment of DMD. One significant problem posed by adenoviral vector-mediated gene transfer for DMD is that currently available adenoviral vectors cannot accommodate the entire 14-kb dystrophin cDNA. To address this problem, we selectively deleted regions of the murine dystrophin cDNA to produce truncated constructs. We created three constructs, each with an in-frame deletion of a segment (3.0, 4.4, and 5.7 kb) of the spectrin-like repeat region of dystrophin. As an additional modification, we removed the majority of the 3' untranslated region of the cDNA in expression vectors encoding some of these truncated constructs. Comparative quantitative expression studies after transfection into COS and C2C12 mouse muscle cells demonstrate variations in the level of expression with different deletions in the spectrin-like repeat region. Furthermore, deletion of the 3' untranslated region was tested for one recombinant construct and resulted in a reduction in the level of expression in both cell culture systems. Toward the ultimate goal of gene transfer therapy for DMD, we created an adenoviral vector from one of our truncated constructs. Using this vector, we demonstrated truncated dystrophin expression in vitro in primary mdx (dystrophin-deficient) muscle cells and in vivo in mdx mouse muscle. In vivo, recombinant dystrophin was properly localized to the muscle membrane.

Age-related differences in regeneration of dystrophic (mdx) and normal muscle in the mouse

The mdx mouse, a genetic homologue of human Duchenne muscular dystrophy (DMD), has been attributed with a greater regenerative capacity of its skeletal muscles. Here, we have tested the hypothesis that muscles of mdx mice regenerate better than those of nondystrophic animals. We studied muscle regeneration resulting from a denervation-devascularization injury (DD) of extensor digitorum longus muscle (EDL) at 3 weeks and 2 months in mdx and wild-type (C57BL/10) mice. Histological and morphometrical studies of muscle regeneration were made from 3 to 180 days later. When DD was performed in 3-week-old C57BL/10 mice, the percentages of nonperipheral nuclei in regenerated fibers decreased progressively over 3 months. This decrease did not occur in animals where DDs were performed at 2 months, suggesting that two different populations of muscle precursor cells are mobilized in muscle regeneration in mice at these two ages. Moreover, mdx EDL muscle regenerated similarly to the controls for up to 60 postoperative days, as shown by distribution of mean diameters and percentage of nonperipheral nuclei of muscle fibers. After 60 postoperative days, necrosis/regeneration characteristics of mdx muscles recurred, suggesting that mdx-regenerated muscle fibers remain susceptible to degeneration.

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

The purpose of this study is to determine whether dystrophin-deficient mdx mice are more susceptible to muscle injury and functional impairment than normal C57 mice when allowed to exercise voluntarily on mouse wheels. The mdx mice were significantly impaired when compared to controls as shown by functional, contractile and morphometric responses. The distance young mdx mice ran was 67-78% of young C57 mice, while adult mdx mice ran 31-48% of adult controls. After exercise the slow, oxidative soleus of young and adult mdx mice exhibited hypertrophy with no changes in strength or fatiguability, while the young C57 mice increased strength and the adults became less fatiguable. In the adult mdx mice the fast EDL, which is primarily glycolytic, exhibits slight hypertrophy with a loss of strength, while the young exhibit no changes. These results indicate that the mdx mouse adapts differently than the C57 mouse to even moderate exercise.

mdx mice show progressive weakness and muscle deterioration with age

The time-course of degeneration/regeneration was investigated in leg muscles throughout the life of the mdx mutant mouse, which is a biochemical homologue of Duchenne muscular dystrophy (DMD). In young and adult mice (up to 52 weeks old), muscle fibre necrosis was compensated by a vigorous regeneration, but in old mdx mice (65-104 weeks) this regeneration slightly declined, while the necrotic process persisted. Body and muscles weights declined strikingly after 52 weeks. Life span of mdx mutants was reduced in comparison with the control C57BL/10 animals. Immunostaining of old mdx muscles showed clusters of dystrophin-positive fibres. Muscle fibres in old mdx mice showed great variation in size, many being atrophied or split. Endomysial fibrosis became increasingly conspicuous, and there was some accumulation of adipose tissue. These progressive degenerative changes of old mdx mice resemble those found in DMD and imply that basic pathological similarities between the murine and human diseases previously observed in diaphragm of mdx mice may be extended to other skeletal muscles.

Connective tissue proliferation and growth factors in animal models of Duchenne muscular dystrophy

The difference in the lifespan of dy and mdx mice could be due to different muscle regeneration capabilities. In mdx an involvement of bFGF in stimulating regeneration has been postulated. The aim of our work was to detect the presence, and to study the distribution, of muscular and connective tissue growth factors in mdx and dy mice at different stages of muscle pathology. From 7 to 10 weeks of age the difference between the two dystrophic mice becomes evident. At 13 weeks the dy mouse presents a predominance of fibrosis and degenerative muscular phenomena while the main pathological feature in mdx mouse is the muscle regeneration. In both animal models fibrosis proliferation is correlated to the presence of EGF and its receptor and TGF beta 1. bFGF was localized to regenerating and degenerating fibers in both dy and mdx mice. The bFGF presented a normal pattern in mdx mice at 20 weeks when regenerative and degenerative phenomena were no longer present. Our data suggest that growth factors could influence the outcome of muscular regenerative and degenerative processes.