Reduced muscle necrosis and long-term benefits in dystrophic mdx mice after cV1q (blockade of TNF) treatment

Age-progressive stratification of Becker muscular dystrophy patients: a focus on muscle biopsy fibrosis, inflammation and capillary network

Skeletal muscle dystrophies comprise a group of inherited disorders characterized by progressive muscle weakness, with Duchenne and Becker muscular dystrophies (DMD/BMD) being among the most severe. These dystrophies are caused by mutations in the dystrophin gene, resulting in muscle cell instability, chronic inflammation, fibrosis, and impaired muscle regeneration. Although skeletal muscle has intrinsic regenerative potential via satellite cells, the ongoing muscle damage in DMD/BMD depletes these cells and promotes fibrosis. Inflammation also plays a pivotal role, with immune cell infiltration correlating with disease severity. This study investigates fibrosis, inflammation, and capillarization in BMD patients across different age groups to clarify how disease progression varies over time. Morphological analyses of muscle biopsies revealed an increase in connective tissue, particularly in adult patients. Pediatric patients showed reduced capillarization, whereas adult patients displayed vascular adaptations, including elevated capillary-to-fibre ratios and capillary contacts, indicative of compensatory mechanisms in response to chronic muscle degeneration. Inflammatory profiles also varied with age: younger adult patients exhibited a predominance of CD68-positive macrophages, while older adults demonstrated increased CD4/CD8 T-cell activity. Our findings highlight pronounced age-dependent differences in muscle pathology, encompassing structural adaptations, fibrosis, and inflammation, which may be crucial for developing age-tailored therapeutic approaches.

Identifying hub genes and dysregulated pathways in Duchenne muscular dystrophy

PURPOSE

The aim of this study was to identify the hub genes and dysregulated pathways in the progression of duchenne muscular dystrophy (DMD) and to unveil detailedly the cellular and molecular mechanisms associated with DMD for developing efficacious treatments in the future.

MATERIAL AND METHODS

Three mRNA microarray datasets (GSE13608, GSE38417 and GSE109178) were downloaded from Gene Expression Omnibus (GEO). The differentially expressed genes (DEGs) between DMD and normal tissues were obtained via R package. Function enrichment analyses were implemented respectively using DAVID online database. The network analysis of protein-protein interaction network (PPI) was conducted using String. Cytoscape and String were used to analyse modules and screen hub genes. The expression of the identified hub genes was confirmed in mdx mice through using qRT-PCR.

RESULTS

In total, 519 DEGs were identified, consisting of 393 upregulated genes and 126 downregulated genes. The enriched functions and pathways of the DEGs mainly involve extracellular matrix organization, collagen fibril organization, interferon-gamma-mediated signaling pathway, muscle contraction, endoplasmic reticulum lumen, MHC class II receptor activity, phagosome, graft-versus-host disease, cardiomyocytes, calcium signaling pathway. Twelve hub genes were discovered and biological process analysis proved that these genes were mainly enriched cell cycle, cell division. The result of qRT-PCR suggested that increase in expression of CD44, ECT2, TYMS, MAGEL2, HLA-DMA, SERPINH1, TNNT2 was confirmed in mdx mice and the downregulation of ASB2 and LEPREL1 was also observed.

CONCLUSION

In conclusion, DEGs and hub genes identified in the current research help us probe the molecular mechanisms underlying the pathogenesis and progression of DMD, and provide candidate targets for diagnosis and treatment of DMD.

In dystrophic mdx hindlimb muscles where fibrosis is limited, versican haploinsufficiency transiently improves contractile function without reducing inflammation

Versican is increased with inflammation and fibrosis, and is upregulated in Duchenne muscular dystrophy. In fibrotic diaphragm muscles from dystrophic mdx mice, genetic reduction of versican attenuated macrophage infiltration and improved contractile function. Versican is also implicated in myogenesis. Here, we investigated whether versican modulated mdx hindlimb muscle pathology, where inflammation and regeneration are increased but fibrosis is minimal. Immunohistochemistry and qRT-PCR were used to assess how fiber type and glucocorticoids (α-methylprednisolone) modify versican expression. To genetically reduce versican, female mdx and male versican haploinsufficient (hdf) mice were bred resulting in male mdx-hdf and mdx (control) pups. Versican expression, contractile function, and pathology were evaluated in hindlimb muscles. Versican immunoreactivity was greater in slow versus fast hindlimb muscles. Versican mRNA transcripts were reduced by α-methylprednisolone in soleus, but not in fast extensor digitorum longus, muscles. In juvenile (6-wk-old) mdx-hdf mice, versican expression was most robustly decreased in soleus muscles leading to improved force output and a modest reduction in fatiguability. These functional benefits were not accompanied by decreased inflammation. Muscle architecture, regeneration markers, and fiber type also did not differ between mdx-hdf mice and mdx littermates. Improvements in soleus contractile function were not retained in adult (20-wk-old) mdx-hdf mice. In conclusion, soleus muscles from juvenile mdx mice were most responsive to pharmacological or genetic approaches targeting versican; however, the benefits of versican reduction were limited due to low fibrosis. Preclinical matrix research in dystrophy should account for muscle phenotype (including age) and the interdependence between inflammation and fibrosis. NEW & NOTEWORTHY The proteoglycan versican is upregulated in muscular dystrophy. In fibrotic diaphragm muscles from mdx mice, versican reduction attenuated macrophage infiltration and improved performance. Here, in hindlimb muscles from 6- and 20-wk-old mdx mice, where pathology is mild, versican reduction did not decrease inflammation and contractile function improvements were limited to juvenile mice. In dystrophic mdx muscles, the association between versican and inflammation is mediated by fibrosis, demonstrating interdependence between the immune system and extracellular matrix.

Macrophages in the Context of Muscle Regeneration and Duchenne Muscular Dystrophy

Macrophages are essential to muscle regeneration, as they regulate inflammation, carry out phagocytosis, and facilitate tissue repair. These cells exhibit phenotypic switching from pro-inflammatory (M1) to anti-inflammatory (M2) states during muscle repair, influencing myoblast proliferation, differentiation, and myofiber formation. In Duchenne Muscular Dystrophy (DMD), asynchronous muscle injuries disrupt the normal temporal stages of regeneration, leading to fibrosis and failed regeneration. Altered macrophage activity is associated with DMD progression and physiopathology. Gaining insight into the intricate relationship between macrophages and muscle cells is crucial for creating effective therapies aimed at treating this muscle disorder. This review explores the dynamic functions of macrophages in muscle regeneration and their implications in DMD.

The Bi-directional Relationship Between Sleep and Inflammation in Muscular Dystrophies: A Narrative Review

Muscular dystrophies vary in presentation and severity, but are associated with profound disability in many people. Although characterised by muscle weakness and wasting, there is also a very high prevalence of sleep problems and disorders which have significant impacts on quality of life in these individuals. There are no curative therapies for muscular dystrophies, with the only options for patients being supportive therapies to aid with symptoms. Therefore, there is an urgent need for new therapeutic targets and a greater understanding of pathogenesis. Inflammation and altered immunity are factors which have prominent roles in some muscular dystrophies and emerging roles in others such as type 1 myotonic dystrophy, signifying a link to pathogenesis. Interestingly, there is also a strong link between inflammation/immunity and sleep. In this review, we will explore this link in the context of muscular dystrophies and how it may influence potential therapeutic targets and interventions.

Muscle Pathology in Dystrophic Rats and Zebrafish Is Unresponsive to Taurine Treatment, Compared to the mdx Mouse Model for Duchenne Muscular Dystrophy

Inflammation and oxidative stress are strongly implicated in the pathology of Duchenne muscular dystrophy (DMD), and the sulphur-containing amino acid taurine ameliorates both and decreases dystropathology in the mdx mouse model for DMD. We therefore further tested taurine as a therapy using dystrophic DMD^mdx rats and dmd zebrafish models for DMD that have a more severe dystropathology. However, taurine treatment had little effect on the indices of dystropathology in both these models. While we and others have previously observed a deficiency in taurine in mdx mice, in the current study we show that the rat and zebrafish models had increased taurine content compared with wild-type, and taurine treatment did not increase muscle taurine levels. We therefore hypothesised that endogenous levels of taurine are a key determinate in potential taurine treatment efficacy. Because of this, we felt it important to measure taurine levels in DMD patient plasma samples and showed that in non-ambulant patients (but not in younger patients) there was a deficiency of taurine. These data suggest that taurine homeostasis varies greatly between species and may be influenced by age and disease progression. The potential for taurine to be an effective therapy may depend on such variables.

Inhibition of TNF-α Restores Muscle Force, Inhibits Inflammation, and Reduces Apoptosis of Traumatized Skeletal Muscles

BACKGROUND

Muscle injuries are common in humans and are often associated with irrecoverable damage and disability. Upon muscle injury, TNF-α signaling pathways modulate the healing process and are predominantly associated with tissue degradation. In this study we assumed that TNF-α inhibition could reduce the TNF-α-associated tissue degradation after muscle injury.

MATERIALS AND METHODS

Therefore, the left soleus muscle of 42 male Wistar rats was injured using a standardized open muscle injury model. All rats were treated immediately after injury either with infliximab (single i.p. injection; 10 mg/kg b.w.) or saline solution i.p. Final measurements were conducted at day one, four, and 14 post injury. The muscle force, the muscle cell proliferation, the muscle cell coverage as well as the myofiber diameter served as read out parameters of our experiment.

RESULTS

Systemic application of infliximab could significantly reduce the TNF-α levels in the injured muscle at day four upon trauma compared to saline treated animals. The ratio of muscle weight to body weight was increased and the twitch muscle force showed a significant rise 14 days after trauma and TNF-α inhibition. Quantification of myofiber diameter in the penumbra zone showed a significant difference between both groups at day one and four after injury, indicated by muscle hypertrophy in the infliximab group. Planimetric analysis of the injured muscle at day 14 revealed increased muscle tissue fraction in the infliximab group compared to the control animals. Muscle cell proliferation did not differ between both groups.

CONCLUSIONS

These data provide evidence that the TNF-α blockade positively regulates the restauration of skeletal muscles upon injury.

Description of Osmolyte Pathways in Maturing Mdx Mice Reveals Altered Levels of Taurine and Sodium/Myo-Inositol Co-Transporters

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration. Osmotic stress participates to DMD pathology and altered levels of osmolyte pathway members have been reported. The goal of this study was to gain insight in osmoregulatory changes in the mdx mouse model by examining the expression of osmolyte pathway members, including taurine transporter (TauT), sodium myo-inositol co-transporter (SMIT), betaine GABA transporter (BGT), and aldose reductase (AR) in the skeletal muscles and diaphragm of mdx mice aged 4, 8, 12, and 26 weeks. Necrosis was most prominent in 12 week-old mdx mice, whereas the amount of regenerated fibers increased until week 26 in the tibialis anterior. TauT protein levels were downregulated in the tibialis anterior and gastrocnemius of 4 to 12 week-old mdx mice, but not in 26 week-old mice, whereas TauT levels in the diaphragm remained significantly lower in 26 week-old mdx mice. In contrast, SMIT protein levels were significantly higher in the muscles of mdx mice when compared to controls. Our study revealed differential regulation of osmolyte pathway members in mdx muscle, which points to their complex involvement in DMD pathogenesis going beyond general osmotic stress responses. These results highlight the potential of osmolyte pathway members as a research interest and future therapeutic target in dystrophinopathy.

Macrophages in Skeletal Muscle Dystrophies, An Entangled Partner

While skeletal muscle remodeling happens throughout life, diseases that result in its dysfunction are accountable for many deaths. Indeed, skeletal muscle is exceptionally capable to respond to stimuli modifying its homeostasis, such as in atrophy, hypertrophy, regeneration and repair. In particular conditions such as genetic diseases (muscular dystrophies), skeletal muscle’s capacity to remodel is strongly affected and undergoes continuous cycles of chronic damage. This induces scarring, fatty infiltration, as well as loss of contractibility and of the ability to generate force. In this context, inflammation, primarily mediated by macrophages, plays a central pathogenic role. Macrophages contribute as the primary regulators of inflammation during skeletal muscle regeneration, affecting tissue-resident cells such as myogenic cells and endothelial cells, but also fibro-adipogenic progenitors, which are the main source of the fibro fatty scar. During skeletal muscle regeneration their function is tightly orchestrated, while in dystrophies their fate is strongly disturbed, resulting in chronic inflammation. In this review, we will discuss the latest findings on the role of macrophages in skeletal muscle diseases, and how they are regulated.

A Blood Biomarker for Duchenne Muscular Dystrophy Shows That Oxidation State of Albumin Correlates with Protein Oxidation and Damage in Mdx Muscle

Duchenne muscular dystrophy (DMD) is a severe X-linked muscle wasting disease with no cure. While the precise mechanisms of progressive dystropathology remain unclear, oxidative stress caused by excessive generation of oxidants is strongly implicated. Blood biomarkers that could track oxidant levels in tissues would be valuable to measure the effectiveness of clinical treatments for DMD; our research has focused on developing such biomarkers. One target of oxidants that has the potential to be harnessed as a clinical biomarker is the thiol side chain of cysteine 34 (Cys34) of the blood protein albumin. This study using the mdx mouse model of DMD shows that in plasma, albumin Cys34 undergoes thiol oxidation and these changes correlate with levels of protein thiol oxidation and damage of the dystrophic muscles. A comparison with the commonly used biomarker protein carbonylation, confirmed that albumin thiol oxidation is the more sensitive plasma biomarker of oxidative stress occurring in muscle tissue. We show that plasma albumin oxidation reflects muscle dystropathology, as increased after exercise and decreased after taurine treatment of mdx mice. These data support the use of albumin thiol oxidation as a blood biomarker of dystropathology to assist with advancing clinical development of therapies for DMD.

Dystrophin Deficiency Causes Progressive Depletion of Cardiovascular Progenitor Cells in the Heart

Duchenne muscular dystrophy (DMD) is a devastating condition shortening the lifespan of young men. DMD patients suffer from age-related dilated cardiomyopathy (DCM) that leads to heart failure. Several molecular mechanisms leading to cardiomyocyte death in DMD have been described. However, the pathological progression of DMD-associated DCM remains unclear. In skeletal muscle, a dramatic decrease in stem cells, so-called satellite cells, has been shown in DMD patients. Whether similar dysfunction occurs with cardiac muscle cardiovascular progenitor cells (CVPCs) in DMD remains to be explored. We hypothesized that the number of CVPCs decreases in the dystrophin-deficient heart with age and disease state, contributing to DCM progression. We used the dystrophin-deficient mouse model (mdx) to investigate age-dependent CVPC properties. Using quantitative PCR, flow cytometry, speckle tracking echocardiography, and immunofluorescence, we revealed that young mdx mice exhibit elevated CVPCs. We observed a rapid age-related CVPC depletion, coinciding with the progressive onset of cardiac dysfunction. Moreover, mdx CVPCs displayed increased DNA damage, suggesting impaired cardiac muscle homeostasis. Overall, our results identify the early recruitment of CVPCs in dystrophic hearts and their fast depletion with ageing. This latter depletion may participate in the fibrosis development and the acceleration onset of the cardiomyopathy.

Long-term human IgG treatment improves heart and muscle function in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease caused by mutations in the dystrophin gene, which leads to structural instability of the dystrophin-glycoprotein-complex with subsequent muscle degeneration. In addition, muscle inflammation has been implicated in disease progression and therapeutically addressed with glucocorticosteroids. These have numerous adverse effects. Treatment with human immunoglobulin G (IgG) improved clinical and para-clinical parameters in the early disease phase in the well-established mdx mouse model. The aim of the present study was to confirm the efficacy of IgG in a long-term pre-clinical study in mdx mice.

METHODS

IgG (2 g/kg body weight) or NaCl solution as control was administered monthly over 18 months by intraperitoneal injection in mdx mice beginning at 3 weeks of age. Several clinical outcome measures including endurance, muscle strength, and echocardiography were assessed. After 18 months, the animals were sacrificed, blood was collected for analysis, and muscle samples were obtained for ex vivo muscle contraction tests, quantitative PCR, and histology.

RESULTS

IgG significantly improved the daily voluntary running performance (1.9 m more total daily running distance, P < 0.0001) and slowed the decrease in grip strength by 0.1 mN, (P = 0.018). IgG reduced fatigability of the diaphragm (improved ratio to maximum force by 0.09 ± 0.04, P = 0.044), but specific tetanic force remained unchanged in the ex vivo muscle contraction test. Cardiac function was significantly better after IgG, especially fractional area shortening (P = 0.012). These results were accompanied by a reduction in cardiac fibrosis and the infiltration of T cells (P = 0.0002) and macrophages (P = 0.0027). In addition, treatment with IgG resulted in a significant reduction of the infiltration of T cells (P ≤ 0.036) in the diaphragm, gastrocnemius, quadriceps, and a similar trend in tibialis anterior and macrophages (P ≤ 0.045) in gastrocnemius, quadriceps, tibialis anterior, and a similar trend in the diaphragm, as well as a decrease in myopathic changes as reflected by a reduced central nuclear index in the diaphragm, tibialis anterior, and quadriceps (P ≤ 0.002 in all).

CONCLUSIONS

The present study underscores the importance of an inflammatory contribution to the disease progression of DMD. The data demonstrate the long-term efficacy of IgG in the mdx mouse. IgG is well tolerated by humans and could preferentially complement gene therapy in DMD. The data call for a clinical trial with IgG in DMD.

The SINE Compound KPT-350 Blocks Dystrophic Pathologies in DMD Zebrafish and Mice

Duchenne muscular dystrophy (DMD) is an X-linked muscle wasting disease that is caused by the loss of functional dystrophin protein in cardiac and skeletal muscles. DMD patient muscles become weakened, leading to eventual myofiber breakdown and replacement with fibrotic and adipose tissues. Inflammation drives the pathogenic processes through releasing inflammatory cytokines and other factors that promote skeletal muscle degeneration and contributing to the loss of motor function. Selective inhibitors of nuclear export (SINEs) are a class of compounds that function by inhibiting the nuclear export protein exportin 1 (XPO1). The XPO1 protein is an important regulator of key inflammatory and neurological factors that drive inflammation and neurotoxicity in various neurological and neuromuscular diseases. Here, we demonstrate that SINE compound KPT-350 can ameliorate dystrophic-associated pathologies in the muscles of DMD models of zebrafish and mice. Thus, SINE compounds are a promising novel strategy for blocking dystrophic symptoms and could be used in combinatorial treatments for DMD.

The mammalian LINC complex component SUN1 regulates muscle regeneration by modulating drosha activity

Here we show that a major muscle specific isoform of the murine LINC complex protein SUN1 is required for efficient muscle regeneration. The nucleoplasmic domain of the isoform specifically binds to and inhibits Drosha, a key component of the microprocessor complex required for miRNA synthesis. Comparison of the miRNA profiles between wildtype and SUN1 null myotubes identified a cluster of miRNAs encoded by a non-translated retrotransposon-like one antisense (Rtl1as) transcript that are decreased in the WT myoblasts due to SUN1 inhibition of Drosha. One of these miRNAs miR-127 inhibits the translation of the Rtl1 sense transcript, that encodes the retrotransposon-like one protein (RTL1), which is also required for muscle regeneration and is expressed in regenerating/dystrophic muscle. The LINC complex may therefore regulate gene expression during muscle regeneration by controlling miRNA processing. This provides new insights into the molecular pathology underlying muscular dystrophies and how the LINC complex may regulate mechanosignaling.

Glycine administration attenuates progression of dystrophic pathology in prednisolone-treated dystrophin/utrophin null mice

Duchenne muscular dystrophy (DMD) is an X-linked genetic disease characterized by progressive muscle wasting and weakness and premature death. Glucocorticoids (e.g. prednisolone) remain the only drugs with a favorable impact on DMD patients, but not without side effects. We have demonstrated that glycine preserves muscle in various wasting models. Since glycine effectively suppresses the activity of pro-inflammatory macrophages, we investigated the potential of glycine treatment to ameliorate the dystrophic pathology. Dystrophic mdx and dystrophin-utrophin null (dko) mice were treated with glycine or L-alanine (amino acid control) for up to 15 weeks and voluntary running distance (a quality of life marker and strong correlate of lifespan in dko mice) and muscle morphology were assessed. Glycine increased voluntary running distance in mdx mice by 90% (P < 0.05) after 2 weeks and by 60% (P < 0.01) in dko mice co-treated with prednisolone over an 8 week treatment period. Glycine treatment attenuated fibrotic deposition in the diaphragm by 28% (P < 0.05) after 10 weeks in mdx mice and by 22% (P < 0.02) after 14 weeks in dko mice. Glycine treatment augmented the prednisolone-induced reduction in fibrosis in diaphragm muscles of dko mice (23%, P < 0.05) after 8 weeks. Our findings provide strong evidence that glycine supplementation may be a safe, simple and effective adjuvant for improving the efficacy of prednisolone treatment and improving the quality of life for DMD patients.

Fibro-adipogenic progenitors of dystrophic mice are insensitive to NOTCH regulation of adipogenesis

Fibro-adipogenic progenitors (FAPs) promote satellite cell differentiation in adult skeletal muscle regeneration. However, in pathological conditions, FAPs are responsible for fibrosis and fatty infiltrations. Here we show that the NOTCH pathway negatively modulates FAP differentiation both in vitro and in vivo. However, FAPs isolated from young dystrophin-deficient mdx mice are insensitive to this control mechanism. An unbiased mass spectrometry-based proteomic analysis of FAPs from muscles of wild-type and mdx mice suggested that the synergistic cooperation between NOTCH and inflammatory signals controls FAP differentiation. Remarkably, we demonstrated that factors released by hematopoietic cells restore the sensitivity to NOTCH adipogenic inhibition in mdx FAPs. These results offer a basis for rationalizing pathological ectopic fat infiltrations in skeletal muscle and may suggest new therapeutic strategies to mitigate the detrimental effects of fat depositions in muscles of dystrophic patients.

Immunobiology of Inherited Muscular Dystrophies

The immune response to acute muscle damage is important for normal repair. However, in chronic diseases such as many muscular dystrophies, the immune response can amplify pathology and play a major role in determining disease severity. Muscular dystrophies are inheritable diseases that vary tremendously in severity, but share the progressive loss of muscle mass and function that can be debilitating and lethal. Mutations in diverse genes cause muscular dystrophy, including genes that encode proteins that maintain membrane strength, participate in membrane repair, or are components of the extracellular matrix or the nuclear envelope. In this article, we explore the hypothesis that an important feature of many muscular dystrophies is an immune response adapted to acute, infrequent muscle damage that is misapplied in the context of chronic injury. We discuss the involvement of the immune system in the most common muscular dystrophy, Duchenne muscular dystrophy, and show that the immune system influences muscle death and fibrosis as disease progresses. We then present information on immune cell function in other muscular dystrophies and show that for many muscular dystrophies, release of cytosolic proteins into the extracellular space may provide an initial signal, leading to an immune response that is typically dominated by macrophages, neutrophils, helper T-lymphocytes, and cytotoxic T-lymphocytes. Although those features are similar in many muscular dystrophies, each muscular dystrophy shows distinguishing features in the magnitude and type of inflammatory response. These differences indicate that there are disease-specific immunomodulatory molecules that determine response to muscle cell damage caused by diverse genetic mutations. © 2018 American Physiological Society. Compr Physiol 8:1313-1356, 2018.

The dual CCR2/CCR5 chemokine receptor antagonist Cenicriviroc reduces macrophage infiltration and disease severity in Duchenne muscular dystrophy (Dmdmdx-4Cv) mice

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle weakness which is ultimately fatal, most often due to involvement of the diaphragm. Macrophage infiltration of dystrophic muscles has been strongly linked to muscle damage and fibrosis in DMD. We hypothesized that cenicriviroc (CVC), a dual chemokine receptor (CCR2/CCR5) antagonist currently under clinical evaluation for other diseases, could prevent macrophage accumulation and blunt disease progression in the diaphragms of mdx mice (genetic homologue of DMD). Treatment with CVC (20 mg/kg/day intraperitoneally) or vehicle was initiated in mdx mice at 2 weeks of age (prior to the onset of muscle necrosis) and continued for 4 weeks. Flow cytometry to assess inflammatory cell subsets as well as histological and force generation parameters were determined in mdx diaphragms at the conclusion of the treatment. CVC therapy induced a major (3.9-fold) reduction in total infiltrating macrophages, whereas total numbers of neutrophils and T lymphocytes (CD4+ and CD8+) were unaffected. No changes in macrophage polarization status (inflammatory versus anti-inflammatory skewing based on iNOS and CD206 expression) were observed. Muscle fiber size and fibrosis were not altered by CVC, whereas a significant reduction in centrally nucleated fibers was found suggesting a decrease in prior necrosis-regeneration cycles. In addition, maximal isometric force production by the diaphragm was increased by CVC therapy. These results suggest that CVC or other chemokine receptor antagonists which reduce pathological macrophage infiltration may have the potential to slow disease progression in DMD.

Matrix Metalloproteinases and Tissue Inhibitor of Metalloproteinases in Inflammation and Fibrosis of Skeletal Muscles

In skeletal muscles, levels and activity of Matrix MetalloProteinases (MMPs) and Tissue Inhibitors of MetalloProteinases (TIMPs) have been involved in myoblast migration, fusion and various physiological and pathological remodeling situations including neuromuscular diseases. This has opened perspectives for the use of MMPs' overexpression to improve the efficiency of cell therapy in muscular dystrophies and resolve fibrosis. Alternatively, inhibition of individual MMPs in animal models of muscular dystrophies has provided evidence of beneficial, dual or adverse effects on muscle morphology or function. We review here the role played by MMPs/TIMPs in skeletal muscle inflammation and fibrosis, two major hurdles that limit the success of cell and gene therapy. We report and analyze the consequences of genetic or pharmacological modulation of MMP levels on the inflammation of skeletal muscles and their repair in light of experimental findings. We further discuss how the interplay between MMPs/TIMPs levels, cytokines/chemokines, growth factors and permanent low-grade inflammation favor cellular and molecular modifications resulting in fibrosis.

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.

Bilateral muscle fiber and nerve influences by TNF-alpha in response to unilateral muscle overuse - studies on TNF receptor expressions

BACKGROUND

TNF-alpha is suggested to be involved in muscle damage and muscle inflammation (myositis). In order to evaluate whether TNF-alpha is involved in the myositis that occurs in response to muscle overuse, the aim was to examine the expression patterns of TNF receptors in this condition.

METHODS

A rabbit muscle overuse model leading to myositis in the soleus muscle was used. The expression patterns of the two TNF receptors Tumor Necrosis Factor Receptor type 1 (TNFR1) and Tumor Necrosis Factor Receptor type 2 (TNFR2) were investigated. In situ hybridization and immunofluorescence were utilized. Immunostainings for desmin, NK-1R and CD31 were made in parallel.

RESULTS

Immunoreactions (IR) for TNF receptors were clearly observed in white blood cells, fibroblasts and vessel walls, and most interestingly also in muscle fibers and nerve fascicles in the myositis muscles. There were very restricted reactions for these in the muscles of controls. The upregulation of TNF receptors was for all types of structures seen for both the experimental side and the contralateral nonexperimental side. TNF receptor expressing muscle fibers were present in myositis muscles. They can be related to attempts for reparation/regeneration, as evidenced from results of parallel stainings. Necrotic muscle fibers displayed TNFR1 mRNA and TNFR2 immunoreaction (IR) in the invading white blood cells. In myositis muscles, TNFR1 IR was observed in both axons and Schwann cells while TNFR2 IR was observed in Schwann cells. Such observations were very rarely made for control animals.

CONCLUSIONS

The findings suggest that there is a pronounced involvement of TNF-alpha in the developing myositis process. Attempts for reparation of the muscle tissue seem to occur via both TNFR1 and TNFR2. As the myositis process also occurs in the nonexperimental side and as TNF receptors are confined to nerve fascicles bilaterally it can be asked whether TNF-alpha is involved in the spreading of the myositis process to the contralateral side via the nervous system. Taken together, the study shows that TNF-alpha is not only associated with the inflammation process but that both the muscular and nervous systems are affected and that this occurs both on experimental and nonexperimental sides.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

Dystrophic phenotype improvement in the diaphragm muscle of mdx mice by diacerhein

Chronic inflammation and oxidative stress are striking features of Duchenne muscular dystrophy disease. Diacerhein is an anthraquinone, which exhibits anti-inflammatory and antioxidant properties. Based on their actions, the present study evaluated the effects of diacerhein against myonecrosis, oxidative stress and inflammatory response in the diaphragm muscle of mdx mice and compared these results to current treatment widely used in DMD patients, with a main focus on the impact of prednisone. The results demonstrated that diacerhein treatment prevented muscle damage indicated by a decrease in the IgG uptake by muscle fibers, lower CK levels in serum, reduction of fibers with central nuclei with a concomitant increase in fibers with peripheral nuclei. It also had an effect on the inflammatory process, decreasing the inflammatory area, macrophage staining and TNF-α and IL-1β content. Regarding oxidative stress, diacerhein treatment was effective in reducing the ROS and lipid peroxidation in the diaphragm muscle from mdx mice. Compared to prednisone treatment, our findings demonstrated that diacerhein treatment improved the dystrophic phenotype in the diaphragm muscle of mdx mice similar to that of glucocorticoid therapy. In this respect, this work suggests that diacerhein has a potential use as an alternative drug in dystrophinopathy treatment and recommends that its anti-inflammatory and antioxidants properties in the dystrophic muscle should be better understood.

Dysferlinopathy Promotes an Intramuscle Expansion of Macrophages with a Cyto-Destructive Phenotype

Dysferlinopathies are a group of muscular dystrophies resulting from a genetic deficiency in Dysf. Macrophages, highly plastic cells that mediate tissue repair and destruction, are prominent within dystrophic skeletal muscles of dysferlinopathy patients. We hypothesized that Dysf-deficient muscle promotes recruitment, proliferation, and skewing of macrophages toward a cyto-destructive phenotype in dysferlinopathy. To track macrophage dynamics in dysferlinopathy, we adoptively transferred enhanced green fluorescent protein-labeled monocytes into Dysf-deficient BLA/J mice with age-related (2 to 10 months) muscle disease and Dysf-intact (C57BL/6 [B6]) mice. We detected an age- and disease-related increase in monocyte recruitment into Dysf-deficient muscles. Moreover, macrophages recruited into muscle proliferated locally and were skewed toward a cyto-destructive phenotype. By comparing Dysf-deficient and -intact monocytes, our data showed that Dysf in muscle, but not in macrophages, mediate intramuscle macrophage recruitment and proliferation. To further elucidate macrophage mechanisms related to dysferlinopathy, we investigated in vitro macrophage-myogenic cell interactions and found that Dysf-deficient muscle i) promotes macrophage proliferation, ii) skews macrophages toward a cyto-destructive phenotype, and iii) is more vulnerable to macrophage-mediated apoptosis. Taken together, our data suggest that the loss of Dysf expression in muscle, not macrophages, promotes the intramuscle expansion of cyto-destructive macrophages likely to contribute to dysferlinopathy. Identifying pathways within the Dysf-deficient muscle milieu that regulate cyto-destructive macrophages will potentially uncover therapeutic strategies for dysferlinopathies.

In vivo neutralization of IL-6 receptors ameliorates gastrointestinal dysfunction in dystrophin-deficient mdx mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a fatal disease characterized by progressive deterioration and degeneration of striated muscle. A mutation resulting in the loss of dystrophin, a structural protein which protects cells from contraction-induced damage, underlies DMD pathophysiology. Damage to muscle fibers results in chronic inflammation and elevated levels of proinflammatory cytokines such as interleukin-6 (IL-6). However, loss of cellular dystrophin also affects neurons and smooth muscle in the gastrointestinal (GI) tract with complaints such as hypomotility, pseudo-obstruction, and constipation reported in DMD patients.

METHODS

Using dystrophin-deficient mdx mice, studies were carried out to examine colonic morphology and function compared with wild-type mice. Treatment with neutralizing IL-6 receptor antibodies (xIL-6R) and/or the corticotropin-releasing factor (CRF) 2 receptor agonist, urocortin 2 (uro2) was tested to determine if they ameliorated GI dysfunction in mdx mice.

KEY RESULTS

Mdx mice exhibited thickening of colonic smooth muscle layers and delayed stress-induced defecation. In organ bath studies, neurally mediated IL-6-evoked contractions were larger in mdx colons. In vivo treatment of mdx mice with xIL-6R normalized defecation rates and colon lengths. Uro2 treatment did not affect motility or morphology. The potentiated colonic contractile response to IL-6 was attenuated by treatment with xIL-6R.

CONCLUSIONS & INFERENCES

These findings confirm the importance of dystrophin in normal GI function and implicate IL-6 as an important regulator of GI motility in the mdx mouse. Inhibition of IL-6 signaling may offer a potential new therapeutic strategy for treating DMD-associated GI symptoms.

Effect of the IL-1 Receptor Antagonist Kineret® on Disease Phenotype in mdx Mice

Duchenne muscular dystrophy (DMD) is an X-linked muscle disease caused by mutations in the dystrophin gene. The pathology of DMD manifests in patients with progressive muscle weakness, loss of ambulation and ultimately death. One of the characteristics of DMD is muscle inflammation, and dystrophin-deficient skeletal muscles produce higher levels of the pro-inflammatory cytokine interleukin 1β (IL-1β) in response to toll like receptor (TLR) stimulation compared to controls; therefore, blocking the IL-1β pathway could improve the disease phenotype in mdx mice, a mouse model of DMD. Kineret^® or IL-1Ra is a recombinant IL-1 receptor antagonist approved by the FDA for treating rheumatoid arthritis. To determine the efficacy of IL-1Ra in a DMD model, we administered subcutaneous injections of saline control or IL-1Ra (25 mg/kg/day) to mdx mice daily for 45 days beginning at 5 weeks of age. Functional and histological parameters were measured at the conclusion of the study. IL-1Ra only partially inhibited this signaling pathway in this study; however, there were still interesting observations to be noted. For example, although not significantly changed, splenocytes from the IL-1Ra-treated group secreted less IL-1β after LPS stimulation compared to control mice indicating a blunted response and incomplete inhibition of the pathway (37% decrease). In addition, normalized forelimb grip strength was significantly increased in IL-1Ra-treated mice. There were no changes in EDL muscle-specific force measurements, histological parameters, or motor coordination assessments in the dystrophic mice after IL-1Ra treatment. There was a significant 27% decrease in the movement time and total distance traveled by the IL-1Ra treated mice, correlating with previous studies examining effects of IL-1 on behavior. Our studies indicate partial blocking of IL-1β with IL-1Ra significantly altered only a few behavioral and strength related disease parameters; however, treatment with inhibitors that completely block IL-1β, pathways upstream of IL-1β production or combining various inhibitors may produce more favorable outcomes.

Invited Review: Polycomb group genes in the regeneration of the healthy and pathological skeletal muscle

The polycomb group (PcG) proteins are epigenetic repressors required during key developmental processes, such as maintenance of cell identity and stem cell differentiation. To exert their repressive function, PcG proteins assemble on chromatin into multiprotein complexes, known as polycomb repressive complex 1 and 2. In this review, we will focus on the role and mode of function of PcG proteins in the development and regeneration of the skeletal muscle, both in normal and pathological conditions and we will discuss the emerging concept of modulation of their expression to enhance the muscle-specific regenerative process for patient benefit.

Inflammatory monocytes promote progression of Duchenne muscular dystrophy and can be therapeutically targeted via CCR2

Myofiber necrosis and fibrosis are hallmarks of Duchenne muscular dystrophy (DMD), leading to lethal weakness of the diaphragm. Macrophages (MPs) are required for successful muscle regeneration, but the role of inflammatory monocyte (MO)-derived MPs in either promoting or mitigating DMD is unclear. We show that DMD (mdx) mouse diaphragms exhibit greatly increased expression of CCR2 and its chemokine ligands, along with inflammatory (Ly6C(high)) MO recruitment and accumulation of CD11b(high) MO-derived MPs. Loss-of-function of CCR2 preferentially reduced this CD11b(high) MP population by impeding the release of Ly6C(high) MOs from the bone marrow but not the splenic reservoir. CCR2 deficiency also helped restore the MP polarization balance by preventing excessive skewing of MPs toward a proinflammatory phenotype. These effects were linked to amelioration of histopathological features and increased muscle strength in the diaphragm. Chronic inhibition of CCR2 signaling by mutated CCL2 secreted from implanted mesenchymal stem cells resulted in similar improvements. These data uncover a previously unrecognized role of inflammatory MOs in DMD pathogenesis and indicate that CCR2 inhibition could offer a novel strategy for DMD management.

Chromatin signaling in muscle stem cells: interpreting the regenerative microenvironment

Muscle regeneration in the adult occurs in response to damage at expenses of a population of adult stem cells, the satellite cells. Upon injury, either physical or genetic, signals released within the satellite cell niche lead to the commitment, expansion and differentiation of the pool of muscle progenitors to repair damaged muscle. To achieve this goal satellite cells undergo a dramatic transcriptional reprogramming to coordinately activate and repress specific subset of genes. Although the epigenetics of muscle regeneration has been extensively discussed, less emphasis has been put on how extra-cellular cues are translated into the specific chromatin reorganization necessary for progression through the myogenic program. In this review we will focus on how satellite cells sense the regenerative microenvironment in physiological and pathological circumstances, paying particular attention to the mechanism through which the external stimuli are transduced to the nucleus to modulate chromatin structure and gene expression. We will discuss the pathways involved and how alterations in this chromatin signaling may contribute to satellite cells dysfunction during aging and disease.

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

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

Functional and Morphological Improvement of Dystrophic Muscle by Interleukin 6 Receptor Blockade

The anti-inflammatory agents glucocorticoids (GC) are the only available treatment for Duchenne muscular dystrophy (DMD). However, long-term GC treatment causes muscle atrophy and wasting. Thus, targeting specific mediator of inflammatory response may be more specific, more efficacious, and with fewer side effects. The pro-inflammatory cytokine interleukin (IL) 6 is overproduced in patients with DMD and in the muscle of mdx, the animal model for human DMD. We tested the ability of inhibition of IL6 activity, using an interleukin-6 receptor (Il6r) neutralizing antibody, to ameliorate the dystrophic phenotype. Blockade of endogenous Il6r conferred on dystrophic muscle resistance to degeneration and alleviated both morphological and functional consequences of the primary genetic defect. Pharmacological inhibition of IL6 activity leaded to changes in the dystrophic muscle environment, favoring anti-inflammatory responses and improvement in muscle repair. This resulted in a functional homeostatic maintenance of dystrophic muscle.

These data provide an alternative pharmacological strategy for treatment of DMD and circumvent the major problems associated with conventional therapy.

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Inhibition of IL6 activity leads to changes in the dystrophic muscle environment.

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IL6R neutralizing antibody ameliorates the dystrophic phenotype.

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IL6 blockade counters muscle decline in mdx mice.

The immune system in Duchenne muscular dystrophy: Friend or foe

Duchenne muscular dystrophy (DMD) is a genetic disease caused by mutations in the X-linked dystrophin gene, resulting in reduced or absent protein production, subsequently leading to the structural instability of the dystroglycan complex (DGC), muscle degeneration, and early death in males. Thus, current treatments have been targeting the genetic defect either by bypassing the mutation through exon skipping or replacing the defective gene through gene therapy and stem cell approaches. However, what has been an underappreciated mediator of muscle pathology and, ultimately, of muscle degeneration and fibrotic replacement, is the prominent inflammatory response. Of potentially critical importance, however, is the fact that the elements mediating the inflammatory response also play an essential role in tissue repair. In this opinion piece, we highlight the detrimental and supportive immune parameters that occur as a consequence of the genetic disorder and discuss how changes to immunity can potentially ameliorate the disease intensity and be employed in conjunction with efforts to correct the genetic disorder.

Toll-like receptor 4 ablation in mdx mice reveals innate immunity as a therapeutic target in Duchenne muscular dystrophy

Toll-like receptor 4 (TLR4) recognizes specific structural motifs associated with microbial pathogens and also responds to certain endogenous host molecules associated with tissue damage. In Duchenne muscular dystrophy (DMD), inflammation plays an important role in determining the ultimate fate of dystrophic muscle fibers. In this study, we used TLR4-deficient dystrophic mdx mice to assess the role of TLR4 in the pathogenesis of DMD. TLR4 expression was increased and showed enhanced activation following agonist stimulation in mdx diaphragm muscle. Genetic ablation of TLR4 led to significantly increased muscle force generation in dystrophic diaphragm muscle, which was associated with improved histopathology including decreased fibrosis, as well as reduced pro-inflammatory gene expression and macrophage infiltration. TLR4 ablation in mdx mice also altered the phenotype of muscle macrophages by inducing a shift toward a more anti-inflammatory (iNOS(neg) CD206(pos)) profile. In vitro experiments confirmed that lack of TLR4 is sufficient to influence macrophage activation status in response to classical polarizing stimuli such as IFN-gamma and IL-4. Finally, treatment of dystrophic mice with glycyrrhizin, an inhibitor of the endogenous TLR4 ligand, high mobility group box (HMGB1), also pointed to involvement of the HMGB1-TLR4 axis in promoting dystrophic diaphragm pathology. Taken together, our findings reveal TLR4 and the innate immune system as important players in the pathophysiology of DMD. Accordingly, targeting either TLR4 or its endogenous ligands may provide a new therapeutic strategy to slow disease progression.

Milk-derived ribonuclease 5 preparations induce myogenic differentiation in vitro and muscle growth in vivo

Ribonuclease 5, also known as angiogenin, is a stable and abundant ribonuclease in milk whey protein, which is able to regulate several cellular functions, including capillary formation, neuron survival, and epithelial cell growth. Ribonuclease 5 is important for protein synthesis directly stimulating rRNA synthesis in the nucleolus. Here, we show that biologically active RNase5 can be purified from bovine milk. Furthermore, we show that milk-derived RNase5 directly stimulates muscle cell differentiation in vitro, inducing C2C12 cell differentiation and myogenesis. When supplemented into the diet of healthy adult mice, milk-derived RNase5 preparations promoted muscle weight gain and grip strength. Collectively, these data indicate that milk-derived RNase5 preparations exhibit a novel role in skeletal muscle cell function.

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.

Long-term administration of the TNF blocking drug Remicade (cV1q) to mdx mice reduces skeletal and cardiac muscle fibrosis, but negatively impacts cardiac function

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease caused by mutations in the gene encoding dystrophin (DYS). Tumor necrosis factor (TNF) has been implicated in the pathogenesis since short-term treatment of mdx mice with TNF blocking drugs proved beneficial; however, it is not clear whether long-term treatment will also improve long-term outcomes of fibrosis and cardiac health. In this investigation, short and long-term dosing studies were carried out using the TNF blocking drug Remicade and a variety of outcome measures were assessed. Here we show no demonstrable benefit to muscle strength or morphology with 10mg/kg or 20mg/kg Remicade; however, 3mg/kg produced positive strength benefits. Remicade treatment correlated with reductions in myostatin mRNA in the heart, and concomitant reductions in cardiac and skeletal fibrosis. Surprisingly, although Remicade treated mdx hearts were less fibrotic, reductions in LV mass and ejection fraction were also observed, and these changes coincided with reductions in AKT phosphorylation on threonine 308. Thus, TNF blockade benefits mdx skeletal muscle strength and fibrosis, but negatively impacts AKT activation, leading to deleterious changes to dystrophic heart function. These studies uncover a previously unknown relationship between TNF blockade and alteration of muscle growth signaling pathways.

Andrographolide attenuates skeletal muscle dystrophy in mdx mice and increases efficiency of cell therapy by reducing fibrosis

Background

Duchenne muscular dystrophy (DMD) is characterized by the absence of the cytoskeletal protein dystrophin, muscle wasting, increased transforming growth factor type beta (TGF-β) signaling, and fibrosis. At the present time, the only clinically validated treatments for DMD are glucocorticoids. These drugs prolong muscle strength and ambulation of patients for a short term only and have severe adverse effects. Andrographolide, a bicyclic diterpenoid lactone, has traditionally been used for the treatment of colds, fever, laryngitis, and other infections with no or minimal side effects. We determined whether andrographolide treatment of mdx mice, an animal model for DMD, affects muscle damage, physiology, fibrosis, and efficiency of cell therapy.

Methods

mdx mice were treated with andrographolide for three months and skeletal muscle histology, creatine kinase activity, and permeability of muscle fibers were evaluated. Fibrosis and TGF-β signaling were evaluated by indirect immunofluorescence and Western blot analyses. Muscle strength was determined in isolated skeletal muscles and by a running test. Efficiency of cell therapy was determined by grafting isolated skeletal muscle satellite cells onto the tibialis anterior of mdx mice.

Results

mdx mice treated with andrographolide exhibited less severe muscular dystrophy than untreated dystrophic mice. They performed better in an exercise endurance test and had improved muscle strength in isolated muscles, reduced skeletal muscle impairment, diminished fibrosis and a significant reduction in TGF-β signaling. Moreover, andrographolide treatment of mdx mice improved grafting efficiency upon intramuscular injection of dystrophin-positive satellite cells.

Conclusions

These results suggest that andrographolide could be used to improve quality of life in individuals with DMD.

Identification of FHL1 as a therapeutic target for Duchenne muscular dystrophy

Utrophin is a potential therapeutic target for the fatal muscle disease, Duchenne muscular dystrophy (DMD). In adult skeletal muscle, utrophin is restricted to the neuromuscular and myotendinous junctions and can compensate for dystrophin loss in mdx mice, a mouse model of DMD, but requires sarcolemmal localization. NFATc1-mediated transcription regulates utrophin expression and the LIM protein, FHL1 which promotes muscle hypertrophy, is a transcriptional activator of NFATc1. By generating mdx/FHL1-transgenic mice, we demonstrate that FHL1 potentiates NFATc1 activation of utrophin to ameliorate the dystrophic pathology. Transgenic FHL1 expression increased sarcolemmal membrane stability, reduced muscle degeneration, decreased inflammation and conferred protection from contraction-induced injury in mdx mice. Significantly, FHL1 expression also reduced progressive muscle degeneration and fibrosis in the diaphragm of aged mdx mice. FHL1 enhanced NFATc1 activation of the utrophin promoter and increased sarcolemmal expression of utrophin in muscles of mdx mice, directing the assembly of a substitute utrophin-glycoprotein complex, and revealing a novel FHL1-NFATc1-utrophin signaling axis that can functionally compensate for dystrophin.

Transcriptional response to GAA deficiency (Pompe disease) in infantile-onset patients

Pompe disease is a genetic disorder resulting from a deficiency of lysosomal acid alpha-glucosidase (GAA) that manifests as a clinical spectrum with regard to symptom severity and rate of progression. In this study, we used microarrays to examine gene expression from the muscle of two cohorts of infantile-onset Pompe patients to identify transcriptional differences that may contribute to the disease phenotype. We found strong similarities among the gene expression profiles generated from biceps and quadriceps, and identified a number of signaling pathways altered in both cohorts. We also found that infantile-onset Pompe patient muscle had a gene expression pattern characteristic of immature or regenerating muscle, and exhibited many transcriptional markers of inflammation, despite having few overt signs of inflammatory infiltrate. Further, we identified genes exhibiting correlation between expression at baseline and response to therapy. This combined dataset can serve as a foundation for biological discovery and biomarker development to improve the treatment of Pompe disease.

The effect of physiological stimuli on sarcopenia; impact of Notch and Wnt signaling on impaired aged skeletal muscle repair

The age-related loss of skeletal muscle mass and function that is associated with sarcopenia can result in ultimate consequences such as decreased quality of life. The causes of sarcopenia are multifactorial and include environmental and biological factors. The purpose of this review is to synthesize what the literature reveals in regards to the cellular regulation of sarcopenia, including impaired muscle regenerative capacity in the aged, and to discuss if physiological stimuli have the potential to slow the loss of myogenic potential that is associated with sarcopenia. In addition, this review article will discuss the effect of aging on Notch and Wnt signaling, and whether physiological stimuli have the ability to restore Notch and Wnt signaling resulting in rejuvenated aged muscle repair. The intention of this summary is to bring awareness to the benefits of consistent physiological stimulus (exercise) to combating sarcopenia as well as proclaiming the usefulness of contraction-induced injury models to studying the effects of local and systemic influences on aged myogenic capability.

TNF-Alpha in the Locomotor System beyond Joints: High Degree of Involvement in Myositis in a Rabbit Model

The importance of TNF-alpha in arthritis is well documented. It may be that TNF-alpha is also markedly involved in muscle inflammation (myositis). An animal model where this can be investigated is needed. A newly developed rabbit myositis model involving pronounced muscle overuse and local injections of substances having proinflammatory effects was therefore used in the present study. The aim was to investigate the patterns of TNF-alpha expression in the developing myositis and to evaluate the usefulness of this myositis model for further TNF-alpha research. Human rheumatoid arthritis (RA) synovial tissue was examined as a reference. TNF-alpha immunoexpression and TNF-alpha mRNA, visualized via in situ hybridization, were detected in cells in the inflammatory infiltrates of the affected muscle (soleus muscle). Coexistence of TNF-alpha and CD68 immunoreactions was noted, suggesting that the TNF-alpha reactive cells are macrophages. Expression of TNF-alpha mRNA was also noted in muscle fibers and blood vessel walls in areas with inflammation. These findings demonstrate that TNF-alpha is highly involved in the myositis process. The model can be used in further studies evaluating the importance of TNF-alpha in developing myositis.

Amelioration of Duchenne muscular dystrophy in mdx mice by elimination of matrix-associated fibrin-driven inflammation coupled to the αMβ2 leukocyte integrin receptor

In Duchenne muscular dystrophy (DMD), a persistently altered and reorganizing extracellular matrix (ECM) within inflamed muscle promotes damage and dysfunction. However, the molecular determinants of the ECM that mediate inflammatory changes and faulty tissue reorganization remain poorly defined. Here, we show that fibrin deposition is a conspicuous consequence of muscle-vascular damage in dystrophic muscles of DMD patients and mdx mice and that elimination of fibrin(ogen) attenuated dystrophy progression in mdx mice. These benefits appear to be tied to: (i) a decrease in leukocyte integrin α(M)β(2)-mediated proinflammatory programs, thereby attenuating counterproductive inflammation and muscle degeneration; and (ii) a release of satellite cells from persistent inhibitory signals, thereby promoting regeneration. Remarkably, Fib-gamma(390-396A) (Fibγ(390-396A)) mice expressing a mutant form of fibrinogen with normal clotting function, but lacking the α(M)β(2) binding motif, ameliorated dystrophic pathology. Delivery of a fibrinogen/α(M)β(2) blocking peptide was similarly beneficial. Conversely, intramuscular fibrinogen delivery sufficed to induce inflammation and degeneration in fibrinogen-null mice. Thus, local fibrin(ogen) deposition drives dystrophic muscle inflammation and dysfunction, and disruption of fibrin(ogen)-α(M)β(2) interactions may provide a novel strategy for DMD treatment.

Gender differences in contractile and passive properties of mdx extensor digitorum longus muscle

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a severe, muscle-wasting disease caused by mutations in the dystrophin gene. The mdx mouse is the first and perhaps the most commonly used animal model for study of DMD. Both male and female mdx mice are used. However, it is not completely clear whether gender influences contraction and the passive mechanical properties of mdx skeletal muscle.

METHODS

We compared isometric tetanic forces and passive forces of the extensor digitorum longus muscle between male and female mdx mice.

RESULTS

At age 6 months, female mdx mice showed better-preserved specific tetanic force. Interestingly, at 20 months of age, female mdx muscle appeared stiffer.

CONCLUSIONS

Our results suggest that gender may profoundly influence physiological measurement outcomes in mdx mice.

Motor performance of young dystrophic mdx mice treated with long-circulating prednisolone liposomes

For Duchenne muscular dystrophy (DMD), a common myopathy that leads to severe disability, no causal therapy is available. Glucocorticosteroids improve patients' muscle strength, but their long-term use is limited by negative side effects. Thus, pharmacological modifications of glucocorticosteroids are required to increase the efficacy by drug targeting. Liposomal encapsulation augments systemic half-life and local tissue concentrations of glucocorticosteroids and, at the same time, reduces systemic side effects. In this study, the efficacy of novel, long-circulating, polyethylene-glycol-coated liposomes encapsulating prednisolone was compared with free prednisolone in the treatment of mdx mice, a well-established animal model for DMD. Using an objective and sensitive computerized 24-hr detection system of voluntary wheel-running in single cages, we demonstrate a significant impairment of the running performance in mdx compared with black/10 control mice aged 3-6 weeks. Treatment with liposomal or free prednisolone did not improve running performance compared with saline control or empty liposomes. Histopathological parameters, including the rate of internalized nuclei and fiber size variation, and mRNA and protein expression levels of transforming growth factor (TGF)-β and monocytes chemotactic protein (MCP)-1 also remained unchanged. Bioactivity in skeletal muscle of liposomal and free prednisolone was demonstrated by elevated mRNA expression of muscle ring finger protein 1 (MuRF1), a mediator of muscle atrophy, and its forkhead box transcription factors (Foxo1/3). Our data support the assessment of voluntary running to be a robust and reproducible outcome measure of skeletal muscle performance during the early disease course of mdx mice and suggest that liposomal encapsulation is not superior in treatment efficacy compared with conventional prednisolone. Our study helps to improve the future design of experimental treatment in animal models of neuromuscular diseases.

The different impact of a high fat diet on dystrophic mdx and control C57Bl/10 mice

The absence of functional dystrophin protein in patients with Duchenne muscular dystrophy (DMD) and dystrophic mdx mice leads to fragile myofibre membranes and cycles of myofibre necrosis and regeneration. It is proposed that both DMD patients and mdx mice have an altered metabolism and impaired energy status and that nutritional supplementation may reduce the severity of dystropathology. This research compares the in vivo responses of dystrophic mdx and normal control C57Bl/10 mice to a high protein (50%) or a high fat (16%) diet. Consumption of a high protein diet had minimal effects on the body composition or muscle morphology in both strains of mice. In contrast, differences between the strains were seen in response to the high fat diet; this response also varied between mdx mice aged <24 weeks, and mdx mice aged 24 - 40 weeks. C57Bl/10 mice demonstrated many negative side effects after consuming the high fat diet, including weight gain, increased body fat, and elevated inflammatory cytokines. In contrast, after consuming the high fat diet for 16 weeks the mdx mice (< 24 weeks) remained lean with minimal fat deposition and were resistant to changes in body composition. These results support the proposal that energy metabolism in dystrophic mdx mice is altered compared to normal C57Bl/10 mice and this enables the mdx mice to better metabolise the high fat diet and avoid fat deposition. However, older mdx mice (24 - 40-week-old), with increased energy intake, exhibited some mild adverse effects of a high fat diet but to a far lesser extent than age-matched C57Bl/10 mice. Benefits of the high fat diet on dystrophic muscles of young mice were demonstrated by the significantly increased running ability (km) of voluntarily exercised mdx mice and significantly reduced myofibre necrosis in 24-week-old sedentary mdx mice. These novel data clearly identify an 'altered' response to a high fat diet in dystrophic mdx compared to normal C57Bl/10 mice. Our data indicate that the high fat diet may better meet the energy needs of mdx mice to reduce muscle damage and improve muscle function.

A single 30 min treadmill exercise session is suitable for 'proof-of concept studies' in adult mdx mice: a comparison of the early consequences of two different treadmill protocols

The extent of muscle pathology in sedentary adult mdx mice is very low and treadmill exercise is often used to increase myofibre necrosis; however, the early events in dystrophic muscle and blood in response to treadmill exercise (leading to myofibre necrosis) are unknown. This study describes in detail two standardised protocols for the treadmill exercise of mdx mice and profiles changes in molecular and cellular events after a single 30 min treadmill session (Protocol A) or after 4 weeks of (twice weekly) treadmill exercise (Protocol B). Both treadmill protocols increased multiple markers of muscle damage. We conclude that a single 30 min treadmill exercise session is a sufficient and conveniently fast screening test and could be used in 'proof-of-concept' studies to evaluate the benefits of pre-clinical drugs in vivo. Myofibre necrosis, blood serum CK and oxidative stress (specifically the ratio of oxidised to reduced protein thiols) are reliable markers of muscle damage after exercise; many parameters demonstrated high biological variation including changes in mRNA levels for key inflammatory cytokines in muscle. The sampling (sacrifice and tissue collection) time after exercise for these parameters is critical. A more precise understanding of the changes in dystrophic muscle after exercise aims to identify biomarkers and new potential therapeutic drug targets for Duchenne Muscular Dystrophy.

Cellular and molecular mechanisms regulating fibrosis in skeletal muscle repair and disease

The repair of an injured tissue is a complex biological process involving the coordinated activities of tissue-resident and infiltrating cells in response to local and systemic signals. Following acute tissue injury, inflammatory cell infiltration and activation/proliferation of resident stem cells is the first line of defense to restore tissue homeostasis. However, in the setting of chronic tissue damage, such as in Duchenne Muscular Dystrophy, inflammatory infiltrates persist, the ability of stem cells (satellite cells) is blocked and fibrogenic cells are continuously activated, eventually leading to the conversion of muscle into nonfunctional fibrotic tissue. This review explores our current understanding of the cellular and molecular mechanisms underlying efficient muscle repair that are dysregulated in muscular dystrophy-associated fibrosis and in aging-related muscle dysfunction.

Identification of muscle necrosis in the mdx mouse model of Duchenne muscular dystrophy using three-dimensional optical coherence tomography

Three-dimensional optical coherence tomography (3D-OCT) was used to image the structure and pathology of skeletal muscle tissue from the treadmill-exercised mdx mouse model of human Duchenne muscular dystrophy. Optical coherence tomography (OCT) images of excised muscle samples were compared with co-registered hematoxylin and eosin-stained and Evans blue dye fluorescence histology. We show, for the first time, structural 3D-OCT images of skeletal muscle dystropathology well correlated with co-located histology. OCT could identify morphological features of interest and necrotic lesions within the muscle tissue samples based on intrinsic optical contrast. These findings demonstrate the utility of 3D-OCT for the evaluation of small-animal skeletal muscle morphology and pathology, particularly for studies of mouse models of muscular dystrophy.

Aberrant repair and fibrosis development in skeletal muscle

The repair process of damaged tissue involves the coordinated activities of several cell types in response to local and systemic signals. Following acute tissue injury, infiltrating inflammatory cells and resident stem cells orchestrate their activities to restore tissue homeostasis. However, during chronic tissue damage, such as in muscular dystrophies, the inflammatory-cell infiltration and fibroblast activation persists, while the reparative capacity of stem cells (satellite cells) is attenuated. Abnormal dystrophic muscle repair and its end stage, fibrosis, represent the final common pathway of virtually all chronic neurodegenerative muscular diseases. As our understanding of the pathogenesis of muscle fibrosis has progressed, it has become evident that the muscle provides a useful model for the regulation of tissue repair by the local microenvironment, showing interplay among muscle-specific stem cells, inflammatory cells, fibroblasts and extracellular matrix components of the mammalian wound-healing response. This article reviews the emerging findings of the mechanisms that underlie normal versus aberrant muscle-tissue repair.