Differential expression of muscular dystrophy in diaphragm versus hindlimb muscles of mdx mice

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Prolonged voluntary wheel running reveals unique adaptations in mdx mice treated with microdystrophin constructs ± the nNOS-binding site

We tested the effects of prolonged voluntary wheel running on the muscle function of mdx mice treated with one of two different microdystrophin constructs. At 7 weeks of age mdx mice were injected with a single dose of AAV9-CK8-microdystrophin with (gene therapy 1, GT1) or without (gene therapy 2, GT2) the nNOS-binding domain and were assigned to one of four gene therapy treated groups: mdxRGT1 (run, GT1), mdxGT1 (no run, GT1), or mdxRGT2 (run,GT2), mdxGT2 (no run, GT2). There were two mdx untreated groups injected with excipient: mdxR (run, no gene therapy) and mdx (no run, no gene therapy). A third no treatment group, Wildtype (WT) received no injection and did not run. mdxRGT1, mdxRGT2 and mdxR performed voluntary wheel running for 52 weeks; WT and remaining mdx groups were cage active. Robust expression of microdystrophin occurred in diaphragm, quadriceps, and heart muscles of all treated mice. Dystrophic muscle pathology was high in diaphragms of non-treated mdx and mdxR mice and improved in all treated groups. Endurance capacity was rescued by both voluntary wheel running and gene therapy alone, but their combination was most beneficial. All treated groups increased in vivo plantarflexor torque over both mdx and mdxR mice. mdx and mdxR mice displayed ∼3-fold lower diaphragm force and power compared to WT values. Treated groups demonstrated partial improvements in diaphragm force and power, with mdxRGT2 mice experiencing the greatest improvement at ∼60% of WT values. Evaluation of oxidative red quadriceps fibers revealed the greatest improvements in mitochondrial respiration in mdxRGT1 mice, reaching WT levels. Interestingly, mdxGT2 mice displayed diaphragm mitochondrial respiration values similar to WT but mdxRGT2 animals showed relative decreases compared to the no run group. Collectively, these data demonstrate that either microdystrophin construct combined with voluntary wheel running increased in vivo maximal muscle strength, power, and endurance. However, these data also highlighted important differences between the two microdystrophin constructs. GT1, with the nNOS-binding site, improved more markers of exercise-driven adaptations in metabolic enzyme activity of limb muscles, while GT2, without the nNOS-binding site, demonstrated greater protection of diaphragm strength after chronic voluntary endurance exercise but decreased mitochondrial respiration in the context of running.

Diverse effector and regulatory functions of fibro/adipogenic progenitors during skeletal muscle fibrosis in muscular dystrophy

Fibrosis is a prominent pathological feature of skeletal muscle in Duchenne muscular dystrophy (DMD). The commonly used disease mouse model, mdx ^5cv , displays progressive fibrosis in the diaphragm but not limb muscles. We use single-cell RNA sequencing to determine the cellular expression of the genes involved in extracellular matrix (ECM) production and degradation in the mdx ^5cv diaphragm and quadriceps. We find that fibro/adipogenic progenitors (FAPs) are not only the primary source of ECM but also the predominant cells that express important ECM regulatory genes, including Ccn2, Ltbp4, Mmp2, Mmp14, Timp1, Timp2, and Loxs. The effector and regulatory functions are exerted by diverse FAP clusters which are different between diaphragm and quadriceps, indicating their activation by different tissue microenvironments. FAPs are more abundant in diaphragm than in quadriceps. Our findings suggest that the development of anti-fibrotic therapy for DMD should target not only the ECM production but also the pro-fibrogenic regulatory functions of FAPs.

The Many Roles of Macrophages in Skeletal Muscle Injury and Repair

Skeletal muscle is essential to physical activity and energy metabolism. Maintaining intact functions of skeletal muscle is crucial to health and wellbeing. Evolutionarily, skeletal muscle has developed a remarkable capacity to maintain homeostasis and to regenerate after injury, which indispensably relies on the resident muscle stem cells, satellite cells. Satellite cells are largely quiescent in the homeostatic steady state. They are activated in response to muscle injury. Activated satellite cells proliferate and differentiate into myoblasts. Myoblasts fuse to form myotubes which further grow and differentiate into mature myofibers. This process is tightly regulated by muscle microenvironment that consists of multiple cellular and molecular components, including macrophages. Present in both homeostatic and injured muscles, macrophages contain heterogeneous functional subtypes that play diverse roles in maintaining homeostasis and promoting injury repair. The spatial-temporal presence of different functional subtypes of macrophages and their interactions with myogenic cells are vital to the proper regeneration of skeletal muscle after injury. However, this well-coordinated process is often disrupted in a chronic muscle disease, such as muscular dystrophy, leading to asynchronous activation and differentiation of satellite cells and aberrant muscle regeneration. Understanding the precise cellular and molecular processes regulating interactions between macrophages and myogenic cells is critical to the development of therapeutic manipulation of macrophages to promote injury repair. Here, we review the current knowledge of the many roles played by macrophages in the regulation of myogenic cells in homeostatic, regenerating, and dystrophic skeletal muscles.

Reduced Sarcolemmal Membrane Repair Exacerbates Striated Muscle Pathology in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a common X-linked degenerative muscle disorder that involves mutations in the DMD gene that frequently reduce the expression of the dystrophin protein, compromising the structural integrity of the sarcolemmal membrane and leaving it vulnerable to injury during cycles of muscle contraction and relaxation. This results in an increased frequency of sarcolemma disruptions that can compromise the barrier function of the membrane and lead to death of the myocyte. Sarcolemmal membrane repair processes can potentially compensate for increased membrane disruptions in DMD myocytes. Previous studies demonstrated that TRIM72, a muscle-enriched tripartite motif (TRIM) family protein also known as mitsugumin 53 (MG53), is a component of the cell membrane repair machinery in striated muscle. To test the importance of membrane repair in striated muscle in compensating for the membrane fragility in DMD, we crossed TRIM72/MG53 knockout mice into the mdx mouse model of DMD. These double knockout (DKO) mice showed compromised sarcolemmal membrane integrity compared to mdx mice, as measured by immunoglobulin G staining and ex vivo muscle laser microscopy wounding assays. We also found a significant decrease in muscle ex vivo contractile function as compared to mdx mice at both 6 weeks and 1.5 years of age. As the DKO mice aged, they developed more extensive fibrosis in skeletal muscles compared to mdx. Our findings indicate that TRIM72/MG53-mediated membrane repair can partially compensate for the sarcolemmal fragility associated with DMD and that the loss of membrane repair results in increased pathology in the DKO mice.

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

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

Expression of the organic cation/carnitine transporter family (Octn1,-2 and-3) in mdx muscle and heart: Implications for early carnitine therapy in Duchenne muscular dystrophy to improve cellular carnitine homeostasis

INTRODUCTION

Carnitine is essential for long-chain fatty acid oxidation in muscle and heart. Tissue stores are regulated by organic cation/Cn transporter plasmalemmal Octn2. We previously demonstrated low carnitine in quadriceps/gluteus and heart of adult mdx mice.

METHODS

We studied protein and mRNA expression of Octn2, mitochondrial Octn1 and peroxisomal Octn3 in adult male C57BL/10ScSn-DMD mdx/J quadriceps, heart, and diaphragm compared to C57BL/10SnJ mice.

RESULTS

We demonstrated reduction in mOctn2 expression on Western blot and similar expression of mOctn1 and mOctn3 in mdx quadriceps, heart and diaphragm. There was a significant upregulation of mOctn1 and mOctn2 mRNA by qRT-PCR in mdx quadriceps and of mOctn2 and mOctn3 mRNA in mdx heart. We showed upregulation of mdx mOctn1 and mOctn3 mRNA but no increase in protein expression.

DISCUSSION

Dystrophin deficiency likely disrupts Octn2 expression decreasing muscle carnitine uptake thus contributing to membranotoxic long-chain acyl-CoAs with sarcolemmal and organellar membrane oxidative injury providing a treatment rationale for early L-carnitine in DMD.

α-smooth muscle actin is not a marker of fibrogenic cell activity in skeletal muscle fibrosis

α-Smooth muscle actin (α-SMA) is used as a marker for a subset of activated fibrogenic cells, myofibroblasts, which are regarded as important effector cells of tissue fibrogenesis. We address whether α-SMA-expressing myofibroblasts are detectable in fibrotic muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy (DMD), and whether the α-SMA expression correlates with the fibrogenic function of intramuscular fibrogenic cells. α-SMA immunostaining signal was not detected in collagen I (GFP)-expressing cells in fibrotic muscles of ColI-GFP/mdx^5cv mice, but it was readily detected in smooth muscle cells lining intramuscular blood vessel walls. α-SMA expression was detected by quantitative RT-PCR and Western blot in fibrogenic cells sorted from diaphragm and quadriceps muscles of the ColI-GFP/mdx^5cv mice. Consistent with the more severe fibrosis in the ColI-GFP/mdx^5cv diaphragm, the fibrogenic cells in the diaphragm exerted a stronger fibrogenic function than the fibrogenic cells in the quadriceps as gauged by their extracellular matrix gene expression. However, both gene and protein expression of α-SMA was lower in the diaphragm fibrogenic cells than in the quadriceps fibrogenic cells in the ColI-GFP/mdx^5cv mice. We conclude that myofibroblasts are present in fibrotic skeletal muscles, but their expression of α-SMA is not detectable by immunostaining. The level of α-SMA expression by intramuscular fibrogenic cells does not correlate positively with the level of collagen gene expression or the severity of skeletal muscle fibrosis in the mdx^5cv mice. α-SMA is not a functional marker of fibrogenic cells in skeletal muscle fibrosis associated with muscular dystrophy.

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.

Skeletal muscle contractile properties in a novel murine model for limb girdle muscular dystrophy 2i

Limb-girdle muscular dystrophy (LGMD) 2i results from mutations in fukutin-related protein and aberrant α-dystroglycan glycosylation. Although this significantly compromises muscle function and ambulation, the comprehensive characteristics of contractile dysfunction are unknown. Therefore, we quantified the in situ contractile properties of the medial gastrocnemius in young adult P448L mice, an affected muscle of a novel model of LGMD2i. Normalized maximal twitch force, tetanic force, and power were significantly smaller in P448L mice, compared with sex-matched, wild-type mice. These differences were consistent with the replacement of contractile fibers by passive tissue. The shape of the active force-length relationships were similar in both groups, regardless of sex, consistent with an intact sarcomeric structure in P448L mice. Passive force-length curves normalized to maximal isometric force were steeper in P448L mice, and passive elements contribute disproportionately more to total contractile force in P448L mice. Sex differences were mostly noted in the force-velocity curves, as normalized values for maximal and optimal velocities were significantly slower in P448L males, compared with wild-type, but not in P448L females. This suggests that the dystrophic phenotype, which may include possible changes in cross-bridge kinetics and fiber-type proportions, progresses more quickly in P448L males. These results together indicate that active force and power generation are compromised in both sexes of P448L mice, while passive forces increase. More importantly, the results identified several functional markers of disease pathophysiology that could aid in developing and assessment of novel therapeutics for LGMD2i and possibly other dystroglycanopathies as well. NEW & NOTEWORTHY Comprehensive assessments of muscle contractile function have, until now, never been performed in an animal model for any dystroglycanopathy. This study suggests that skeletal muscle contractile properties are significantly compromised in a recently developed model for limb-girdle muscular dystrophy 2i, the P448L mouse. It further identifies novel pathological markers of muscle function that are suitable for developing therapeutics and for better understanding of disease pathogenesis.

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.

Anatomical and mesoscopic characterization of the dystrophic diaphragm: An in vivo nuclear magnetic resonance imaging study in the Golden retriever muscular dystrophy dog

Because respiratory failure remains a major issue in Duchenne Muscular Dystrophy patients, respiratory muscles are a key target of systemic therapies. In the Golden Retriever Muscular Dystrophy (GRMD) dogs, the disease shows strong clinical and histological similarities with the human pathology, making it a valuable model for preclinical therapeutic trials. We report here the first nuclear magnetic resonance (NMR) imaging anatomical study of the diaphragm in GRMD dogs and healthy controls. Both T1- and T2-weighted images of the diaphragm of seven healthy and thirteen GRMD dogs, from 3 to 36 months of age, were acquired on a 3 tesla NMR scanner. Abnormalities of texture and shape were revealed and consisted of increases in signal intensity on T2-weighted images and in signal heterogeneity on both T1- and T2-weighted images of the dystrophic diaphragm. These abnormalities were associated with a significant thickening of the muscle and we identified a clear 8-mm-threshold distinguishing clinically preserved GRMD dogs from those more severely affected. In this study, we demonstrated the feasibility of NMR imaging of the diaphragm and depicted several anatomical and mesoscopic anomalies in the dystrophic diaphragm. NMR imaging of the diaphragm shows a promise as an outcome measure in preclinical trials using GRMD dogs.

Lifelong quercetin enrichment and cardioprotection in Mdx/Utrn+/− mice

Duchenne Muscular Dystrophy (DMD) is associated with progressive cardiac pathology; however, the SIRT1/PGC1-α activator quercetin may cardioprotect dystrophic hearts. We tested the extent to which long-term 0.2% dietary quercetin enrichment attenuates dystrophic cardiopathology in Mdx/Utrn+/− mice. At 2 mo, Mdx/Utrn+/− mice were fed quercetin-enriched (Mdx/Utrn+/−-Q) or control diet (Mdx/Utrn+/−) for 8 mo. Control C57BL/10 (C57) animals were fed a control diet for 10 mo. Cardiac function was quantified by MRI at 2 and 10 mo. Spontaneous physical activity was quantified during the last week of treatment. At 10 mo hearts were excised for histological and biochemical analysis. Quercetin feeding improved various physiological indexes of cardiac function in diseased animals. Mdx/Utrn+/−-Q also engaged in more high-intensity physical activity than controls. Histological analyses of heart tissues revealed higher expression and colocalization of utrophin and α-sarcoglycan. Lower abundance of fibronectin, cardiac damage (Hematoxylin Eosin-Y), and MMP9 were observed in quercetin-fed vs. control Mdx/Utrn+/− mice. Quercetin evoked higher protein abundance of PGC-1α, cytochrome c, ETC complexes I–V, citrate synthase, SOD2, and GPX compared with control-fed Mdx/Utrn+/−. Quercetin decreased abundance of inflammatory markers including NFκB, TGF-β1, and F4/80 compared with Mdx/Utrn+/−; however, P-NFκB, P-IKBα, IKBα, CD64, and COX2 were similar between groups. Dietary quercetin enrichment improves cardiac function in aged Mdx/Utrn+/− mice and increases mitochondrial protein content and dystrophin glycoprotein complex formation. Histological analyses indicate a marked attenuation in pathological cardiac remodeling and indicate that long-term quercetin consumption benefits the dystrophic heart.

NEW & NOTEWORTHY The current investigation provides first-time evidence that quercetin provides physiological cardioprotection against dystrophic pathology and is associated with improved spontaneous physical activity. Secondary findings suggest that quercetin-dependent outcomes are in part due to PGC-1α pathway activation.

Identification and Function of Fibrocytes in Skeletal Muscle Injury Repair and Muscular Dystrophy

We identified and characterized the function of CD45⁺/collagen I⁺ fibrocytes in acutely injured skeletal muscle of wild-type (WT) and Ccr2^-/- mice, and in quadriceps and diaphragm muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy. Fibrocytes were not detected in peripheral blood of WT mice after acute muscle injury or mdx^5cv mice. Fibrocytes were detected in acutely injured muscles and in mdx^5cv quadriceps and diaphragm muscles. These cells expressed F4/80 and CCR2, and they were mostly Ly6C^lo They expressed a low level of collagens but a high level of profibrotic growth factors as compared with i.m. fibroblasts. Fibrocyte expression of collagens and profibrotic growth factors was not increased in Ccr2^-/- mice as compared with WT controls. Fibrocyte expression of both proinflammatory and profibrotic cytokines was significantly higher in mdx^5cv diaphragm than in mdx^5cv quadriceps. In cocultures, fibrocytes from the mdx^5cv diaphragm stimulated a higher level of fibroblast expression of extracellular matrix genes than did those from the mdx^5cv quadriceps. Our findings suggest that i.m. fibrocytes most likely originate from infiltrating monocytes/macrophages and differentiate within injured muscles. They likely contribute to the normal muscle injury repair by producing growth factors. They do not appear to contribute to the persistent muscle fibrosis associated with poor injury repair in Ccr2^-/- mice. However, they likely contribute to the persistent inflammation and progressive fibrosis in the mdx^5cv diaphragm.

CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx5cv mice

Genetic ablation or pharmacologic inhibition of CC chemokine receptor type 2 (CCR2) reduced macrophage (MP) infiltration and improved muscle pathology and function in mdx diaphragm muscle at early stages. We addressed whether CCR2 deficiency resulted in sustained improvement of mdx^5cv-Ccr2^-/- diaphragm. Compared to mdx^5cv controls, CCR2 deficiency in mdx^5cv-Ccr2^-/- mice markedly reduced intramuscular Ly6C^hi MPs at all stages, but it reduced Ly6C^low MPs only at early stages (4 and 9 wk). CCR2 deficiency reduced quadriceps and diaphragm muscle damage and fibrosis at 14 wk but not at 6 mo, and it improved diaphragm muscle regeneration and respiratory function at 14 wk but not at 6 mo. Intramuscular MPs in mdx^5cv-Ccr2^-/- diaphragm expressed a low level of IL-1β, IL-6, and IFN-γ genes, a similar level of TNF-α, TGF-β1, and platelet-derived growth factor α genes, and a high level of IGF-1 and osteopontin genes compared to mdx^5cv controls. Diaphragm fibroblasts at 14 wk showed a similar cell number with a similar level of collagen and profibrogenic growth factor gene expression in mdx^5cv-Ccr2^-/- and mdx^5cv mice. Diaphragm MPs from both mdx^5cv-Ccr2^-/- and mdx^5cv mice stimulated collagen gene expression by cocultured fibroblasts. The findings suggest that CCR2 deficiency does not provide a sustained benefit and that Ly6C^low MPs may contribute to the progressive fibrosis and dysfunction of mdx^5cv diaphragm.-Zhao, W., Wang, X., Ransohoff, R. M., Zhou, L. CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx^5cv mice.

Second-generation compound for the modulation of utrophin in the therapy of DMD

Duchenne muscular dystrophy (DMD) is a lethal, X-linked muscle-wasting disease caused by lack of the cytoskeletal protein dystrophin. There is currently no cure for DMD although various promising approaches are progressing through human clinical trials. By pharmacologically modulating the expression of the dystrophin-related protein utrophin, we have previously demonstrated in dystrophin-deficient mdx studies, daily SMT C1100 treatment significantly reduced muscle degeneration leading to improved muscle function. This manuscript describes the significant disease modifying benefits associated with daily dosing of SMT022357, a second-generation compound in this drug series with improved physicochemical properties and a more robust metabolism profile. These studies in the mdx mouse demonstrate that oral administration of SMT022357 leads to increased utrophin expression in skeletal, respiratory and cardiac muscles. Significantly, utrophin expression is localized along the length of the muscle fibre, not just at the synapse, and is fibre-type independent, suggesting that drug treatment is modulating utrophin transcription in extra-synaptic myonuclei. This results in improved sarcolemmal stability and prevents dystrophic pathology through a significant reduction of regeneration, necrosis and fibrosis. All these improvements combine to protect the mdx muscle from contraction induced damage and enhance physiological function. This detailed evaluation of the SMT C1100 drug series strongly endorses the therapeutic potential of utrophin modulation as a disease modifying therapeutic strategy for all DMD patients irrespective of their dystrophin mutation.

Disease course in mdx:utrophin+/- mice: comparison of three mouse models of Duchenne muscular dystrophy

The mdx mouse model of Duchenne muscular dystrophy (DMD) is used to study disease mechanisms and potential treatments, but its pathology is less severe than DMD patients. Other mouse models were developed to more closely mimic the human disease based on knowledge that upregulation of utrophin has a protective effect in mdx muscle. An mdx:utrophin^−/− (dko) mouse was created, which had a severe disease phenotype and a shortened life span. An mdx:utrophin^+/− mouse was also created, which had an intermediate disease phenotype compared to the mdx and dko mice. To determine the usefulness of mdx:utrophin^+/− mice for long-term DMD studies, limb muscle pathology and function were assessed across the life span of wild-type, mdx, mdx:utrophin^+/−, and dko mice. Muscle function assessment, specifically grip duration and rotarod performance, demonstrated that mdx:utrophin^+/− mice were weaker for a longer time than mdx mice. Mean myofiber area was smaller in mdx:utrophin^+/− mice compared to mdx mice at 12 months. Mdx:utrophin^+/− mice had a higher percentage of centrally nucleated myofibers compared to mdx mice at 6 and 12 months. Collagen I and IV density was significantly higher in mdx:utrophin^+/− muscle compared to mdx at most ages examined. Generally, mdx:utrophin^+/− mice showed an intermediate disease phenotype over a longer time course compared to the mdx and dko mice. While they do not genetically mirror human DMD, mdx:utrophin^+/− mice may be a more useful animal model than mdx or dko mice for investigating long-term efficacy of potential treatments when fibrosis or muscle function is the focus.

Dystropathology increases energy expenditure and protein turnover in the mdx mouse model of duchenne muscular dystrophy

The skeletal muscles in Duchenne muscular dystrophy and the mdx mouse model lack functional dystrophin and undergo repeated bouts of necrosis, regeneration, and growth. These processes have a high metabolic cost. However, the consequences for whole body energy and protein metabolism, and on the dietary requirements for these macronutrients at different stages of the disease, are not well-understood. This study used juvenile (4- to 5- wk-old) and adult (12- to 14-wk-old) male dystrophic C57BL/10ScSn-mdx/J and age-matched C57BL/10ScSn/J control male mice to measure total and resting energy expenditure, food intake, spontaneous activity, body composition, whole body protein turnover, and muscle protein synthesis rates. In juvenile mdx mice that have extensive muscle damage, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were higher than in age-matched controls. Adaptations in food intake and decreased activity were insufficient to meet the increased energy and protein needs of juvenile mdx mice and resulted in stunted growth. In (non-growing) adult mdx mice with less severe dystropathology, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were also higher than in age-matched controls. Food intake was sufficient to meet their protein and energy needs, but insufficient to result in fat deposition. These data show that dystropathology impacts the protein and energy needs of mdx mice and that tailored dietary interventions are necessary to redress this imbalance. If not met, the resultant imbalance blunts growth, and may limit the benefits of therapies designed to protect and repair dystrophic muscles.

Comparative proteomic profiling of soleus, extensor digitorum longus, flexor digitorum brevis and interosseus muscles from the mdx mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy is due to genetic abnormalities in the dystrophin gene and represents one of the most frequent genetic childhood diseases. In the X-linked muscular dystrophy (mdx) mouse model of dystrophinopathy, different subtypes of skeletal muscles are affected to a varying degree albeit the same single base substitution within exon 23 of the dystrophin gene. Thus, to determine potential muscle subtype-specific differences in secondary alterations due to a deficiency in dystrophin, in this study, we carried out a comparative histological and proteomic survey of mdx muscles. We intentionally included the skeletal muscles that are often used for studying the pathomechanism of muscular dystrophy. Histological examinations revealed a significantly higher degree of central nucleation in the soleus and extensor digitorum longus muscles compared with the flexor digitorum brevis and interosseus muscles. Muscular hypertrophy of 20–25% was likewise only observed in the soleus and extensor digitorum longus muscles from mdx mice, but not in the flexor digitorum brevis and interosseus muscles. For proteomic analysis, muscle protein extracts were separated by fluorescence two-dimensional (2D) gel electrophoresis. Proteins with a significant change in their expression were identified by mass spectrometry. Proteomic profiling established an altered abundance of 24, 17, 19 and 5 protein species in the dystrophin-deficient soleus, extensor digitorum longus, flexor digitorum brevis and interosseus muscle, respectively. The key proteomic findings were verified by immunoblot analysis. The identified proteins are involved in the contraction-relaxation cycle, metabolite transport, muscle metabolism and the cellular stress response. Thus, histological and proteomic profiling of muscle subtypes from mdx mice indicated that distinct skeletal muscles are differentially affected by the loss of the membrane cytoskeletal protein, dystrophin. Varying degrees of perturbed protein expression patterns in the muscle subtypes from mdx mice may be due to dissimilar downstream events, including differences in muscle structure or compensatory mechanisms that counteract pathophysiological processes. The interosseus muscle from mdx mice possibly represents a naturally protected phenotype.

Diaphragm muscle sarcopenia in aging mice

Sarcopenia, defined as muscle weakness and fiber atrophy, of respiratory muscles such as the diaphragm (DIAm) has not been well characterized. The DIAm is the main inspiratory muscle and knowledge of DIAm sarcopenia is important for establishing the effects of aging on respiratory function. We hypothesized that aging is associated with a loss of DIAm force and reduced fiber cross-sectional area (CSA), and that these changes vary across fiber types. DIAm sarcopenia was assessed in young (5 month; n = 11) and old (23 month; n = 12) wild-type mice reflecting ~100 and 75% survival, respectively. In addition, DIAm sarcopenia was evaluated in BubR1(H/H) mice (n = 4) that display accelerated aging (~60% survival at 5 months) as a result of expression of a hypomorphic allele (H) of the mitotic checkpoint protein BubR1. Maximum specific force (normalized for CSA) of the DIAm was 34% less in old mice and 57% lower in BubR1(H/H) mice compared to young mice. Mean CSA of type IIx and/or IIb DIAm fibers was 27% smaller in old wild-type mice and 47% smaller in BubR1(H/H) mice compared to young mice. Mean CSA of type I or IIa fibers was not different between groups. Collectively these results demonstrate sarcopenia of the DIAm in aging wild-type mice and in BubR1(H/H) mice displaying accelerated aging. Sarcopenia may limit the ability of the DIAm to accomplish expulsive, non-ventilatory behaviors essential for airway clearance. As a result, these changes in the DIAm may contribute to respiratory complications with aging.

In vivo tracking of murine adipose tissue-derived multipotent adult stem cells and ex vivo cross-validation

Stem cells are characterized by the ability to renew themselves and to differentiate into specialized cell types, while stem cell therapy is believed to treat a number of different human diseases through either cell regeneration or paracrine effects. Herein, an in vivo and ex vivo near infrared time domain (NIR TD) optical imaging study was undertaken to evaluate the migratory ability of murine adipose tissue-derived multipotent adult stem cells [mAT-MASC] after intramuscular injection in mice. In vivo NIR TD optical imaging data analysis showed a migration of DiD-labelled mAT-MASC in the leg opposite the injection site, which was confirmed by a fibered confocal microendoscopy system. Ex vivo NIR TD optical imaging results showed a systemic distribution of labelled cells. Considering a potential microenvironmental contamination, a cross-validation study by multimodality approaches was followed: mAT-MASC were isolated from male mice expressing constitutively eGFP, which was detectable using techniques of immunofluorescence and qPCR. Y-chromosome positive cells, injected into wild-type female recipients, were detected by FISH. Cross-validation confirmed the data obtained by in vivo/ex vivo TD optical imaging analysis. In summary, our data demonstrates the usefulness of NIR TD optical imaging in tracking delivered cells, giving insights into the migratory properties of the injected cells.

Loss of cIAP1 attenuates soleus muscle pathology and improves diaphragm function in mdx mice

The cellular inhibitor of apoptosis 1 (cIAP1) protein is an essential regulator of canonical and noncanonical nuclear factor κB (NF-κB) signaling pathways. NF-κB signaling is known to play important roles in myogenesis and degenerative muscle disorders such as Duchenne muscular dystrophy (DMD), but the involvement of cIAP1 in muscle disease has not been studied directly. Here, we asked whether the loss of cIAP1 would influence the pathology of skeletal muscle in the mdx mouse model of DMD. Double-mutant cIAP1(-/-);mdx mice exhibited reduced muscle damage and decreased fiber centronucleation in the soleus, compared with single-mutant cIAP1(+/+);mdx mice. This improvement in pathology was associated with a reduction in muscle infiltration by macrophages and diminished expression of inflammatory cytokines such as IL-6 and tumor necrosis factor-α. Furthermore, the cIAP1(-/-);mdx mice exhibited reduced serum creatine kinase, and improved exercise endurance associated with improved exercise resilience by the diaphragm. Mechanistically, the loss of cIAP1 was sufficient to drive constitutive activation of the noncanonical NF-κB pathway, which led to increased myoblast fusion in vitro and in vivo. Collectively, these results show that the loss of cIAP1 protects skeletal muscle from the degenerative pathology resulting from systemic loss of dystrophin.

Ca2+-dependent proteolysis of junctophilin-1 and junctophilin-2 in skeletal and cardiac muscle

Excessive increases in intracellular [Ca(2+)] in skeletal muscle fibres cause failure of excitation-contraction coupling by disrupting communication between the dihydropyridine receptors in the transverse tubular system and the Ca(2+) release channels (RyRs) in the sarcoplasmic reticulum (SR), but the exact mechanism is unknown. Previous work suggested a possible role of Ca(2+)-dependent proteolysis in this uncoupling process but found no proteolysis of the dihydropyridine receptors, RyRs or triadin. Junctophilin-1 (JP1; ∼90 kDa) stabilizes close apposition of the transverse tubular system and SR membranes in adult skeletal muscle; its C-terminal end is embedded in the SR and its N-terminal associates with the transverse tubular system membrane. Exposure of skeletal muscle homogenates to precisely set [Ca(2+)] revealed that JP1 undergoes Ca(2+)-dependent proteolysis over the physiological [Ca(2+)] range in tandem with autolytic activation of endogenous μ-calpain. Cleavage of JP1 occurs close to the C-terminal, yielding a ∼75 kDa diffusible fragment and a fixed ∼15 kDa fragment. Depolarization-induced force responses in rat skinned fibres were abolished following 1 min exposure to 40 μm Ca(2+), with accompanying loss of full-length JP1. Supraphysiological stimulation of rat skeletal muscle in vitro by repeated tetanic stimulation in 30 mm caffeine also produced marked proteolysis of JP1 (and not RyR1). In dystrophic mdx mice, JP1 proteolysis is seen in limb muscles at 4 and not at 10 weeks of age. Junctophilin-2 in cardiac and skeletal muscle also undergoes Ca(2+)-dependent proteolysis, and junctophilin-2 levels are reduced following cardiac ischaemia-reperfusion. Junctophilin proteolysis may contribute to skeletal muscle weakness and cardiac dysfunction in a range of circumstances.

The mdx mouse as a model for carnitine deficiency in the pathogenesis of Duchenne muscular dystrophy

INTRODUCTION

Muscle and cardiac metabolism are dependent on the oxidation of fats and glucose for adenosine triphosphate production, for which L-carnitine is an essential cofactor.

METHODS

We measured muscle carnitine concentrations in skeletal muscles, diaphragm, and ventricles of C57BL/10ScSn-DMDmdx/J mice (n = 10) and compared them with wild-type C57BL/6J (n = 3), C57BL/10 (n = 10), and C3H (n = 12) mice. Citrate synthase (CS) activity was measured in quadriceps/gluteals and ventricles of mdx and wild-type mice.

RESULTS

We found significantly lower tissue carnitine in quadriceps/gluteus (P < 0.05) and ventricle (P < 0.05), but not diaphragm of mdx mice, when compared with controls. CS activity was increased in mdx quadriceps/gluteus (P < 0.03) and ventricle (P < 0.02). This suggests compensatory mitochondrial biogenesis.

CONCLUSIONS

Decreased tissue carnitine has implications for reduced fatty acid and glucose oxidation in mdx quadriceps/gluteus and ventricle. The mdx mouse may be a useful model for studying the role of muscle carnitine deficiency in DMD bioenergetic insufficiency and providing a targeted and timed rationale for L-carnitine therapy.

The paradox of muscle hypertrophy in muscular dystrophy

Mutations in the dystrophin gene cause Duchenne and Becker muscular dystrophy in humans and syndromes in mice, dogs, and cats. Affected humans and dogs have progressive disease that leads primarily to muscle atrophy. Mdx mice progress through an initial phase of muscle hypertrophy followed by atrophy. Cats have persistent muscle hypertrophy. Hypertrophy in humans has been attributed to deposition of fat and connective tissue (pseudohypertrophy). Increased muscle mass (true hypertrophy) has been documented in animal models. Muscle hypertrophy can exaggerate postural instability and joint contractures. Deleterious consequences of muscle hypertrophy should be considered when developing treatments for muscular dystrophy.

Bmi1 is expressed in postnatal myogenic satellite cells, controls their maintenance and plays an essential role in repeated muscle regeneration

Satellite cells are the resident stem cell population of the adult mammalian skeletal muscle and they play a crucial role in its homeostasis and in its regenerative capacity after injury. We show here that the Polycomb group (PcG) gene Bmi1 is expressed in both the Pax7 positive (+)/Myf5 negative (−) stem cell population as well as the Pax7+/Myf5+ committed myogenic progenitor population. Depletion of Pax7+/Myf5− satellite cells with reciprocal increase in Pax7+/Myf5+ as well as MyoD positive (+) cells is seen in Bmi1−/− mice leading to reduced postnatal muscle fiber size and impaired regeneration upon injury. Bmi1−/− satellite cells have a reduced proliferative capacity and fail to re-enter the cell cycle when stimulated by high serum conditions in vitro, in keeping with a cell intrinsic defect. Thus, both the in vivo and in vitro results suggest that Bmi1 plays a crucial role in the maintenance of the stem cell pool in postnatal skeletal muscle and is essential for efficient muscle regeneration after injury especially after repeated muscle injury.

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.

Dystrophin is required for the normal function of the cardio-protective K(ATP) channel in cardiomyocytes

Duchenne and Becker muscular dystrophy patients often develop a cardiomyopathy for which the pathogenesis is still unknown. We have employed the murine animal model of Duchenne muscular dystrophy (mdx), which develops a cardiomyopathy that includes some characteristics of the human disease, to study the molecular basis of this pathology. Here we show that the mdx mouse heart has defects consistent with alteration in compounds that regulate energy homeostasis including a marked decrease in creatine-phosphate (PC). In addition, the mdx heart is more susceptible to anoxia than controls. Since the cardio-protective ATP sensitive potassium channel (K_ATP) complex and PC have been shown to interact we investigated whether deficits in PC levels correlate with other molecular events including K_ATP ion channel complex presence, its functionality and interaction with dystrophin. We found that this channel complex is present in the dystrophic cardiac cell membrane but its ability to sense a drop in the intracellular ATP concentration and consequently open is compromised by the absence of dystrophin. We further demonstrate that the creatine kinase muscle isoform (CKm) is displaced from the plasma membrane of the mdx cardiac cells. Considering that CKm is a determinant of K_ATP channel complex function we hypothesize that dystrophin acts as a scaffolding protein organizing the K_ATP channel complex and the enzymes necessary for its correct functioning. Therefore, the lack of proper functioning of the cardio-protective K_ATP system in the mdx cardiomyocytes may be part of the mechanism contributing to development of cardiac disease in dystrophic patients.

Impaired respiratory function in mdx and mdx/utrn(+/-) mice

Muscle fibrosis is a prominent pathological feature that directly causes muscle dysfunction in Duchenne muscular dystrophy (DMD). The DMD mouse models, mdx mice and mdx mice with haploinsufficiency of the utrophin gene (mdx/utrn(+/-) ), display progressive diaphragm fibrosis. We performed unrestrained whole-body plethysmography (WBP) in mdx and mdx/utrn(+/-) mice, and compared them with wild-type controls. Respiratory function gauged by respiratory frequency, tidal volume, minute volume, peak inspiratory flow, and peak expiratory flow was significantly impaired in the mdx mice. Consistent with more severe diaphragm fibrosis in the mdx/utrn(+/-) mice, respiratory impairment was worse than in mdx mice at 6 months. WBP is useful for monitoring in vivo respiratory function of mdx and mdx/utrn(+/-) mice, and it may serve as an outcome measurement for therapies that target diaphragm fibrosis. The mdx/utrn(+/-) mouse model may be better than the mdx model for testing antifibrotic therapies, especially at the severe stage.

Muscle-derived collagen XIII regulates maturation of the skeletal neuromuscular junction

Formation, maturation, stabilization, and functional efficacy of the neuromuscular junction (NMJ) are orchestrated by transsynaptic and autocrine signals embedded within the synaptic cleft. Here, we demonstrate that collagen XIII, a nonfibrillar transmembrane collagen, is another such signal. We show that collagen XIII is expressed by muscle and its ectodomain can be proteolytically shed into the extracellular matrix. The collagen XIII protein was found present in the postsynaptic membrane and synaptic basement membrane. To identify a role for collagen XIII at the NMJ, mice were generated lacking this collagen. Morphological and ultrastructural analysis of the NMJ revealed incomplete adhesion of presynaptic and postsynaptic specializations in collagen XIII-deficient mice of both genders. Strikingly, Schwann cells erroneously enwrapped nerve terminals and invaginated into the synaptic cleft, resulting in a decreased contact surface for neurotransmission. Consistent with morphological findings, electrophysiological studies indicated both postsynaptic and presynaptic defects in Col13a1(-/-) mice, such as decreased amplitude of postsynaptic potentials, diminished probabilities of spontaneous release and reduced readily releasable neurotransmitter pool. To identify the role of collagen XIII at the NMJ, shed ectodomain of collagen XIII was applied to cultured myotubes, and it was found to advance acetylcholine receptor (AChR) cluster maturation. Together with the delay in AChR cluster development observed in collagen XIII-deficient mutants in vivo, these results suggest that collagen XIII plays an autocrine role in postsynaptic maturation of the NMJ. Altogether, the results presented here reveal that collagen XIII is a novel muscle-derived cue necessary for the maturation and function of the vertebrate NMJ.

A muscle resident cell population promotes fibrosis in hindlimb skeletal muscles of mdx mice through the Wnt canonical pathway

Previous work has pointed to a role for the Wnt canonical pathway in fibrosis formation in aged skeletal muscles. In the present study, we studied the dystrophic mdx mouse, which displays skeletal muscle fibrosis. Our results indicated that the muscle resident stromal cell (mrSC) population in the muscles of dystrophic mice is higher than in the muscles of age-matched wild-type mice. Wnt3a promoted the proliferation of and collagen expression by cultured mrSCs but arrested the growth of and collagen expression by cultured myoblasts. Injections of Wnt3A in the tibialis anterior muscles of adult wild-type mice significantly enhanced the mrSC population and collagen deposition compared with the contralateral muscles. Conversely, an injection of the Wnt antagonist Dickkof protein (DKK1) into the skeletal muscles of mdx mice significantly reduced collagen deposition. These results suggested that the Wnt canonical pathway expands the population of mrSCs and stimulates their production of collagen as observed during aging and in various myopathies.

Targeting fibrosis in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common genetic muscle disease affecting 1 in 3,500 live male births. It is an X-linked recessive disease caused by a defective dystrophin gene. The disease is characterized by progressive limb weakness, respiratory and cardiac failure, and premature death. Fibrosis is a prominent pathological feature of muscle biopsies from patients with DMD. It directly causes muscle dysfunction and contributes to the lethal DMD phenotype. Although gene therapy and cell therapy may ultimately provide a cure for DMD, currently the disease is devastating, with no effective therapies. Recent studies have demonstrated that ameliorating muscle fibrosis may represent a viable therapeutic approach for DMD. By reducing scar formation, antifibrotic therapies may not only improve muscle function but also enhance muscle regeneration and promote gene and stem cell engraftment. Antifibrotic therapy may serve as a necessary addition to gene and cell therapies to treat DMD in the future. Therefore, understanding cellular and molecular mechanisms underlying muscle fibrogenesis associated with dystrophin deficiency is key to the development of effective antifibrotic therapies for DMD.

Muscle genome-wide expression profiling during disease evolution in mdx mice

Mdx mice show a milder phenotype than Duchenne patients despite bearing an analogous genetic defect. Our aim was to sort out genes, differentially expressed during the evolution of skeletal muscle mdx mouse disease, to elucidate the mechanisms by which these animals overcome the lack of dystrophin. Genome-wide microarray-based gene expression analysis was carried out at 3 wk and 1.5 and 3 mo of life. Candidate genes were selected by comparing: 1) mdx vs. controls at each point in time, and 2) mdx mice and 3) control mice among the three points in time. The first analysis showed a strong upregulation (96%) of inflammation-related genes and in >75% of genes related to cell adhesion, muscle structure/regeneration, and extracellular matrix remodeling during mdx disease evolution. Lgals3, Postn, Ctss, and Sln genes showed the strongest variations. The analysis performed among points in time demonstrated significant changes in Ecm1, Spon1, Thbs1, Csrp3, Myo10, Pde4b, and Adamts-5 exclusively during mdx mice lifespan. RT-PCR analysis of Postn, Sln, Ctss, Thbs1, Ecm1, and Adamts-5 expression from 3 wk to 9 mo, confirmed microarray data and demonstrated variations beyond 3 mo of age. A high-confidence functional network analysis demonstrated a strong relationship between them and showed two main subnetworks, having Dmd- Utrn- Myo10 and Adamts5- Thbs1- Spon1-Postn as principal nodes, which are functionally linked to Abca1, Actn4, Crebbp, Csrp3, Lama1, Lama3, Mical2, Mical3, Myf6, Pxn, and Sparc genes. Candidate genes may participate in the decline of muscle necrosis in mdx mice and could be considered potential therapeutic targets for Duchenne patients.

Imatinib attenuates skeletal muscle dystrophy in mdx mice

Duchenne-Meryon muscular dystrophy (DMD) is the most common and lethal genetic muscle disease. Ameliorating muscle necrosis, inflammation, and fibrosis represents an important therapeutic approach for DMD. Imatinib, an antineoplastic agent, demonstrated antiinflammatory and antifibrotic effects in liver, kidney, lung, and skin of various animal models. This study tested antiinflammatory and antifibrotic effects of imatinib in mdx mice, a DMD mouse model. We treated mdx mice with intraperitoneal injections of imatinib at the peak of limb muscle inflammation and the onset of diaphragm fibrosis. Controls received PBS vehicle or were left untreated. Muscle necrosis, inflammation, fibrosis, and function were evaluated by measuring serum CK activity, endomysial CD45 immunoreactive inflammation area, endomysial collagen III deposition, and hind limb grip strength. Phosphorylation of the tyrosine kinase targets of imatinib was assessed by Western blot in diaphragm tissue and in primary cultures of peritoneal macrophages and skeletal muscle fibroblasts. Imatinib markedly reduced muscle necrosis, inflammation, and fibrosis, and significantly improved hind limb grip strength in mdx mice. Reduced clinical disease was accompanied by inhibition of c-abl and PDGFR phosphorylation and suppression of TNF-alpha and IL-1beta expression. Imatinib therapy for DMD may hold promise for ameliorating muscle necrosis, inflammation, and fibrosis by inhibiting c-abl and PDGFR signaling pathways and downstream inflammatory cytokine and fibrotic gene expression.

Functional overloading of dystrophic mice enhances muscle-derived stem cell contribution to muscle contractile capacity

OBJECTIVES

To evaluate the effect of functional overloading on the transplantation of muscle derived stem cells (MDSCs) into dystrophic muscle and the ability of transplanted cells to increase dystrophic muscle's ability to resist overloading-induced weakness.

DESIGN

Cross-sectional.

SETTING

Laboratory.

ANIMALS

Male mice (N=10) with a dystrophin gene mutation.

INTERVENTIONS

MDSCs were intramuscularly transplanted into the extensor digitorum longus muscle (EDL). Functional overloading of the EDL was performed by surgical ablation of the EDL's synergist.

MAIN OUTCOME MEASURES

The total number of dystrophin-positive fibers/cross-section (as a measure of stem cell engraftment), the average number of CD31+ cells (as a measure of capillarity), and in vitro EDL contractile strength. Independent t tests were used to investigate the effect of overloading on engraftment, capillarity, and strength. Paired t tests were used to investigate the effect of MDSC engraftment on strength and capillarity.

RESULTS

MDSC transplantation protects dystrophic muscles against overloading-induced weakness (specific twitch force: control 4.5N/cm2+/-2.3; MDSC treated 7.9N/cm2+/-1.4) (P=.02). This improved force production following overloading is concomitant with an increased regeneration by transplanted MDSCs (MDSC: 26.6+/-20.2 dystrophin-positive fibers/cross-section; overloading + MDSC: 170.6+/-130.9 dystrophin-positive fibers/cross-section [P=.03]). Overloading-induced increases in skeletal muscle capillarity is significantly correlated with increased MDSC engraftment (R2=.80, P=.01).

CONCLUSIONS

These findings suggest that the functional contribution of transplanted MDSCs may rely on activity-dependent mechanisms, possibly mediated by skeletal muscle vascularity. Rehabilitation modalities may play an important role in the development of stem cell transplantation strategies for the treatment of muscular dystrophy.

Treatment with inhibitors of the NF-kappaB pathway improves whole body tension development in the mdx mouse

The whole body tension (WBT) method was used to evaluate the hypothesis that long term treatment with NF-kappaB inhibitors improves the total forward pulling tension exerted by the limb musculature of the mdx mouse. Mdx mice exhibited significantly reduced WBT values and more profound weakening during the course of generating multiple forward pulling movements than age-matched nondystrophic mice. Long term treatment with the NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC) did not significantly reduce nuclear p65 activation in the costal diaphragm, but increased WBT by 12% in mature (12 month) mice. Daily treatment (30 days) of 1 month old mdx mice with the inhibitor ursodeoxycholic acid (UDCA) reduced costal diaphragm nuclear p65 activation by 40% and increased WBT by 21%. These results indicate that treatment with NF-kappaB inhibitors improves WBT in the mdx mouse and further establishes the utility of the WBT procedure in assessing therapeutic efficacy.

Systemic microdystrophin gene delivery improves skeletal muscle structure and function in old dystrophic mdx mice

Restoring dystrophin expression in the muscles of patients with Duchenne muscular dystrophy (DMD) may halt or reverse the degenerative wasting and weakness that causes premature death. However, the therapeutic efficacy of an intervention may be limited by the extent of disease progression prior to treatment. In this study, we considered the potential for ameliorating the pathology in a mouse model of advanced-stage muscular dystrophy by systemic administration of recombinant adeno-associated viral (rAAV6) vectors encoding a microdystrophin expression construct. The treatment of 20-month-old mdx mice restored body-wide expression of a dystrophin-based protein in striated musculature. In aged mice that received treatment, the resultant dystrophin expression was associated with improved hindlimb and respiratory muscle morphology and function, concomitant with reduced muscle fiber degeneration. The findings demonstrate that an established dystrophic state remains amenable to improvement with appropriate intervention and, by some measures, may even achieve benefits similar to those observed with intervention early in disease progression. The capacity to ameliorate the pathology in an animal model of advanced-stage muscular dystrophy suggests that interventions ultimately proven to exert a therapeutic effect in young patients may offer benefits to older patients or those with advanced conditions of progressive muscular dystrophy.

Diaphragm muscle strip preparation for evaluation of gene therapies in mdx mice

1. Duchenne muscular dystrophy (DMD), a severe muscle wasting disease of young boys with an incidence of one in every 3000, results from a mutation in the gene that encodes dystrophin. The absence of dystrophin expression in skeletal muscles and heart results in the degeneration of muscle fibres and, consequently, severe muscle weakness and wasting. The mdx mouse discovered in 1984, with some adjustments for differences, has proven to be an invaluable model for scientific investigations of dystrophy. 2. The development of the diaphagm strip preparation provided an ideal experimental model for investigations of skeletal muscle impairments in structure and function induced by interactions of disease- and age-related factors. Unlike the limb muscles of the mdx mouse, which show adaptive changes in structure and function, the diaphragm strip preparation reflects accurately the deterioration in muscle structure and function observed in boys with DMD. 3. The advent of sophisticated servo motors and force transducers interfaced with state-of-the-art software packages to drive complex experimental designs during the 1990s greatly enhanced the capability of the mdx mouse and the diaphragm strip preparation to evaluate more accurately the impact of the disease on the structure-function relationships throughout the life span of the mouse. 4. Finally, during the 1990s and through the early years of the 21st century, many promising, sophisticated genetic techniques have been designed to ameliorate the devastating impact of muscular dystrophy on the structure and function of skeletal muscles. During this period of rapid development of promising genetic therapies, the combination of the mdx mouse and the diaphragm strip preparation has provided an ideal model for the evaluation of the success, or failure, of these genetic techniques to improve dystrophic muscle structure, function or both. With the 2 year life span of the mdx mouse, the impact of age-related effects can be studied in this model.

Stimulation of calcineurin Aalpha activity attenuates muscle pathophysiology in mdx dystrophic mice

Calcineurin activation ameliorates the dystrophic pathology of hindlimb muscles in mdx mice and decreases their susceptibility to contraction damage. In mdx mice, the diaphragm is more severely affected than hindlimb muscles and more representative of Duchenne muscular dystrophy. The constitutively active calcineurin Aalpha transgene (CnAalpha) was overexpressed in skeletal muscles of mdx (mdx CnAalpha*) mice to test whether muscle morphology and function would be improved. Contractile function of diaphragm strips and extensor digitorum longus and soleus muscles from adult mdx CnAalpha* and mdx mice was examined in vitro. Hindlimb muscles from mdx CnAalpha* mice had a prolonged twitch time course and were more resistant to fatigue. Because of a slower phenotype and a decrease in fiber cross-sectional area, normalized force was lower in fast- and slow-twitch muscles of mdx CnAalpha* than mdx mice. In the diaphragm, despite a slower phenotype and a approximately 35% reduction in fiber size, normalized force was preserved. This was likely mediated by the reduction in the area of the diaphragm undergoing degeneration (i.e., mononuclear cell and connective and adipose tissue infiltration). The proportion of centrally nucleated fibers was reduced in mdx CnAalpha* compared with mdx mice, indicative of improved myofiber viability. In hindlimb muscles of mdx mice, calcineurin activation increased expression of markers of regeneration, particularly developmental myosin heavy chain isoform and myocyte enhancer factor 2A. Thus activation of the calcineurin signal transduction pathway has potential to ameliorate the mdx pathophysiology, especially in the diaphragm, through its effects on muscle degeneration and regeneration and endurance capacity.

Episodic hypoxia exacerbates respiratory muscle dysfunction in DMD(mdx) mice

Many patients with Duchenne muscular dystrophy (DMD) are eventually diagnosed with sleep-disordered breathing (SDB). SDB is associated with reduced ventilation, decreased arterial oxygen tension, and increased respiratory muscle recruitment during sleep, factors that could be especially detrimental to respiratory muscles in DMD. To assess whether SDB impacts dystrophin-deficient respiratory muscle function and fibrosis, diaphragm strength, and collagen content were evaluated in dystrophic mice (Dmd(mdx)) exposed to experimental SDB. Diurnal exposure to episodic hypoxia resulted in a 30% reduction in diaphragm strength without affecting collagen content. Episodic hypoxia secondary to SDB can exacerbate respiratory muscle dysfunction in DMD.

Haploinsufficiency of utrophin gene worsens skeletal muscle inflammation and fibrosis in mdx mice

To address whether mdx mice with haploinsufficiency of utrophin (mdx/utrn+/-) develop more severe skeletal muscle inflammation and fibrosis than mdx mice, to represent a better model for Duchenne muscular dystrophy (DMD), we performed qualitative and quantitative analysis of skeletal muscle inflammation and fibrosis in mdx and mdx/utrn+/- littermates. Inflammation was significantly worse in mdx/utrn+/- quadriceps at age 3 and 6 months and in mdx/utrn+/- diaphragm at age 3 but not 6 months. Fibrosis was more severe in mdx/utrn+/- diaphragm at 6 months, and at this age, mild fibrosis was noted in quadriceps of mdx/utrn+/- but not mdx mice. The findings indicate that utrophin compensates, although insufficiently, for the effects of dystrophin loss with regard to inflammation and fibrosis of both quadriceps and diaphragm muscles in mdx mice. With more severe muscle dystrophy than mdx mice and a longer life span than utrophin-dystrophin-deficient (dko) mice, mdx/utrn+/- mice provide a better mouse model for testing potential therapies for muscle inflammation and fibrosis associated with DMD.

Corticortophin releasing factor 2 receptor agonist treatment significantly slows disease progression in mdx mice

BACKGROUND

Duchenne muscular dystrophy results from mutation of the dystrophin gene, causing skeletal and cardiac muscle loss of function. The mdx mouse model of Duchenne muscular dystrophy is widely utilized to evaluate the potential of therapeutic regimens to modulate the loss of skeletal muscle function associated with dystrophin mutation. Importantly, progressive loss of diaphragm function is the most consistent striated muscle effect observed in the mdx mouse model, which is the same as in patients suffering from Duchenne muscular dystrophy.

METHODS

Using the mdx mouse model, we have evaluated the effect that corticotrophin releasing factor 2 receptor (CRF2R) agonist treatment has on diaphragm function, morphology and gene expression.

RESULTS

We have observed that treatment with the potent CRF2R-selective agonist PG-873637 prevents the progressive loss of diaphragm specific force observed during aging of mdx mice. In addition, the combination of PG-873637 with glucocorticoids not only prevents the loss of diaphragm specific force over time, but also results in recovery of specific force. Pathological analysis of CRF2R agonist-treated diaphragm muscle demonstrates that treatment reduces fibrosis, immune cell infiltration, and muscle architectural disruption. Gene expression analysis of CRF2R-treated diaphragm muscle showed multiple gene expression changes including globally decreased immune cell-related gene expression, decreased extracellular matrix gene expression, increased metabolism-related gene expression, and, surprisingly, modulation of circadian rhythm gene expression.

CONCLUSION

Together, these data demonstrate that CRF2R activation can prevent the progressive degeneration of diaphragm muscle associated with dystrophin gene mutation.

MicroRNA-206 is overexpressed in the diaphragm but not the hindlimb muscle of mdx mouse

MicroRNAs are highly conserved, noncoding RNAs involved in posttranscriptional gene silencing. MicroRNAs have been shown to be involved in a range of biological processes, including myogenesis and muscle regeneration. The objective of this study was to test the hypothesis that microRNA expression is altered in dystrophic muscle, with the greatest change occurring, of the muscles examined, in the diaphragm. The expression of the muscle-enriched microRNAs was determined in the soleus, plantaris, and diaphragm muscles of control and dystrophin-deficient ( mdx) mice by semiquantitative PCR. In the soleus and plantaris, expression of the mature microRNA 133a (miR-133a) and miR-206, respectively, was decreased by ∼25%, whereas in the diaphragm, miR-206 expression increased by 4.5-fold relative to control. The increased expression of miR-206 in the mdx diaphragm was paralleled by a 4.4-fold increase in primary miRNA-206 (pri-miRNA-206) transcript level. Expression of Myod1 was elevated 2.7-fold only in the mdx diaphragm, consistent with an earlier finding demonstrating Myod1 can activate pri-miRNA-206 transcription. Transcript levels of Drosha and Dicer, major components of microRNA biogenesis pathway, were unchanged in mdx muscle, suggesting the pathway is not altered under dystrophic conditions. Previous in vitro analysis found miR-206 was capable of repressing utrophin expression; however, under dystrophic conditions, both utrophin transcript and protein levels were significantly increased by 69% and 3.9-fold, respectively, a finding inconsistent with microRNA regulation. These results are the first to report alterations in expression of muscle-enriched microRNAs in skeletal muscle of the mdx mouse, suggesting microRNAs may have a role in the pathophysiology of muscular dystrophy.

Ventilation during air breathing and in response to hypercapnia in 5 and 16 month-old mdx and C57 mice

Previous studies have shown a blunted ventilatory response to hypercapnia in mdx mice older than 7 months. We test the hypothesis that in the mdx mice ventilatory response changes with age, concomitantly with the increased functional impairment of the respiratory muscles. We thus studied the ventilatory response to CO(2) in 5 and 16 month-old mdx and C57BL10 mice (n = 8 for each group). Respiratory rate (RR), tidal volume (VT), and minute ventilation (VE) were measured, using whole-body plethysmography, during air breathing and in response to hypercapnia (3, 5 and 8% CO(2)). The ventilatory protocol was completed by histological analysis of the diaphragm and intercostals muscles. During air breathing, the 16 month-old mdx mice showed higher RR and, during hypercapnia (at 8% CO(2) breathing), significantly lower RR (226 +/- 26 vs. 270 +/- 21 breaths/min) and VE (1.81 +/- 0.35 vs. 3.96 +/- 0.59 ml min(-1) g(-1)) (P < 0.001) in comparison to C57BL10 controls. On the other hand, 5 month-old C57BL10 and mdx mice did not present any difference in their ventilatory response to air breathing and to hypercapnia. In conclusion, this study shows similar ventilation during air breathing and in response to hypercapnia in the 5 month-old mdx and control mice, in spite of significant pathological structural changes in the respiratory muscles of the mdx mice. However in the 16 month-old mdx mice we observed altered ventilation under air and blunted ventilation response to hypercapnia compared to age-matched control mice. Ventilatory response to hypercapnia thus changes with age in mdx mice, in line with the increased histological damage of their respiratory muscles.

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.

Weekly oral prednisolone improves survival and strength in male mdx mice

Although corticosteroids alleviate weakness in mdx mice, no long-term treatment has determined whether this benefit is maintained. We studied mdx mice forelimb grip strength and fatigue from 3 through 84 weeks and followed survival through 104 weeks. The mdx mice were given twice weekly oral prednisolone (5 mg/kg) beginning at 3 or 4 weeks. Treated mdx mice survived longer than untreated mice. Between 3 and 10 weeks, treated and untreated mdx mice had similar strength. Between 10 and 24 weeks, strength and strength per gram body weight declined more slowly in treated than untreated mdx mice. Between 24 and 84 weeks, treated and untreated mdx mice declined in strength at the same rate, although treated mice remained stronger. Forelimb grip fatigue was present in untreated mdx mice at all time-points compared to wild-type and was not changed significantly by treatment. We have demonstrated long-term benefit of oral prednisolone in the mdx mouse model of Duchenne muscular dystrophy (DMD). As corticosteroids remain the most validated long-term treatment of DMD, this work may allow for better prediction of synergistic treatments likely to translate to effective improvement for boys with this progressive muscular dystrophy.