Impending therapies for Duchenne muscular dystrophy

Cellular pathogenesis of Duchenne muscular dystrophy: progressive myofibre degeneration, chronic inflammation, reactive myofibrosis and satellite cell dysfunction

Duchenne muscular dystrophy is a highly progressive muscle wasting disease of early childhood and characterized by complex pathophysiological and histopathological changes in the voluntary contractile system, including myonecrosis, chronic inflammation, fat substitution and reactive myofibrosis. The continued loss of functional myofibres and replacement with non-contractile cells, as well as extensive tissue scarring and decline in tissue elasticity, leads to severe skeletal muscle weakness. In addition, dystrophic muscles exhibit a greatly diminished regenerative capacity to counteract the ongoing process of fibre degeneration. In normal muscle tissues, an abundant stem cell pool consisting of satellite cells that are localized between the sarcolemma and basal lamina, provides a rich source for the production of activated myogenic progenitor cells that are involved in efficient myofibre repair and tissue regeneration. Interestingly, the self-renewal of satellite cells for maintaining an essential pool of stem cells in matured skeletal muscles is increased in dystrophin-deficient fibres. However, satellite cell hyperplasia does not result in efficient recovery of dystrophic muscles due to impaired asymmetric cell divisions. The lack of expression of the full-length dystrophin isoform Dp427-M, which is due to primary defects in the DMD gene,  appears to affect key regulators of satellite cell polarity causing a reduced differentiation of myogenic progenitors, which are essential for myofibre regeneration. This review outlines the complexity of dystrophinopathy and describes the importance of the pathophysiological role of satellite cell dysfunction. A brief discussion of the bioanalytical usefulness of single cell proteomics for future studies of satellite cell biology is provided.

The skeletal muscle phenotype of the DE50-MD dog model of Duchenne muscular dystrophy

Background: Animal models of Duchenne muscular dystrophy (DMD) are essential to study disease progression and assess efficacy of therapeutic intervention, however dystrophic mice fail to display a clinically relevant phenotype, limiting translational utility. Dystrophin-deficient dogs exhibit disease similar to humans, making them increasingly important for late-stage preclinical evaluation of candidate therapeutics. The DE50-MD canine model of DMD carries a mutation within a human 'hotspot' region of the dystrophin gene, amenable to exon-skipping and gene editing strategies. As part of a large natural history study of disease progression, we have characterised the DE50-MD skeletal muscle phenotype to identify parameters that could serve as efficacy biomarkers in future preclinical trials. Methods: Vastus lateralis muscles were biopsied from a large cohort of DE50-MD dogs and healthy male littermates at 3-monthly intervals (3-18 months) for longitudinal analysis, with multiple muscles collected post-mortem to evaluate body-wide changes. Pathology was characterised quantitatively using histology and measurement of gene expression to determine statistical power and sample sizes appropriate for future work. Results: DE50-MD skeletal muscle exhibits widespread degeneration/regeneration, fibrosis, atrophy and inflammation. Degenerative/inflammatory changes peak during the first year of life, while fibrotic remodelling appears more gradual. Pathology is similar in most skeletal muscles, but in the diaphragm, fibrosis is more prominent, associated with fibre splitting and pathological hypertrophy. Picrosirius red and acid phosphatase staining represent useful quantitative histological biomarkers for fibrosis and inflammation respectively, while qPCR can be used to measure regeneration ( MYH3, MYH8), fibrosis ( COL1A1), inflammation ( SPP1), and stability of DE50-MD dp427 transcripts. Conclusion: The DE50-MD dog is a valuable model of DMD, with pathological features similar to young, ambulant human patients. Sample size and power calculations show that our panel of muscle biomarkers are of strong pre-clinical value, able to detect therapeutic improvements of even 25%, using trials with only six animals per group.

Muscle histological changes in a large cohort of patients affected with Becker muscular dystrophy

Becker muscular dystrophy (BMD) is a severe X-linked muscle disease. Age of onset, clinical variability, speed of progression and affected tissues display wide variability, making a clinical trial design for drug development very complex. The histopathological changes in skeletal muscle tissue are central to the pathogenesis, but they have not been thoroughly elucidated yet. Here we analysed muscle biopsies from a large cohort of BMD patients, focusing our attention on the histopathological muscle parameters, as fibrosis, fatty replacement, fibre cross sectional area, necrosis, regenerating fibres, splitting fibres, internalized nuclei and dystrophy evaluation. We correlated histological parameters with both demographic features and clinical functional evaluations. The most interesting results of our study are the accurate quantification of fibroadipose tissue replacement and the identification of some histopathological aspects that well correlate with clinical performances. Through correlation analysis, we divided our patients into three clusters with well-defined histological and clinical features. In conclusion, this is the first study that analyses in detail the histological characteristics of muscle biopsies in a large cohort of BMD patients, correlating them to a functional impairment. The collection of these data help to better understand the histopathological progression of the disease and can be useful to validate any pharmacological trial in which the modification of muscle biopsy is utilized as outcome measure.

The effect of hypoxia on myogenic differentiation and multipotency of the skeletal muscle-derived stem cells in mice

Background

Skeletal muscle-derived stem cells (SC) have become a promising approach for investigating myogenic differentiation and optimizing tissue regeneration. Muscle regeneration is performed by SC, a self-renewal cell population underlying the basal lamina of muscle fibers. Here, we examined the impact of hypoxia condition on the regenerative capacity of SC either in their native microenvironment or via isolation in a monolayer culture using ectopic differentiation inductions. Furthermore, the effect of low oxygen tension on myogenic differentiation protocols of the myoblasts cell line C2C12 was examined.

Methods

Hind limb muscles of wild type mice were processed for both SC/fiber isolation and myoblast extraction using magnetic beads. SC were induced for myogenic, adipogenic and osteogenic commitments under normoxic (21% O2) and hypoxic (3% O2) conditions. SC proliferation and differentiation were evaluated using histological staining, immunohistochemistry, morphometric analysis and RT-qPCR. The data were statistically analyzed using ANOVA.

Results

The data revealed enhanced SC proliferation and motility following differentiation induction after 48 h under hypoxia. Following myogenic induction, the number of undifferentiated cells positive for Pax7 were increased at 72 h under hypoxia. Hypoxia upregulated MyoD and downregulated Myogenin expression at day-7 post-myogenic induction. Hypoxia promoted both SC adipogenesis and osteogenesis under respective induction as shown by using Oil Red O and Alizarin Red S staining. The expression of adipogenic markers; peroxisome proliferator activated receptor gamma (PPARγ) and fatty acid-binding protein 4 (FABP4) were upregulated under hypoxia up to day 14 compared to normoxic condition. Enhanced osteogenic differentiation was detected under hypoxic condition via upregulation of osteocalcin and osteopontin expression up to day 14 as well as, increased calcium deposition at day 21. Hypoxia exposure increases the number of adipocytes and the size of fat vacuoles per adipocyte compared to normoxic culture. Combining the differentiation medium with dexamethasone under hypoxia improves the efficiency of the myogenic differentiation protocol of C2C12 by increasing the length of the myotubes.

Conclusions

Hypoxia exposure increases cell resources for clinical applications and promotes SC multipotency and thus beneficial for tissue regeneration.

The Dystrophin Node as Integrator of Cytoskeletal Organization, Lateral Force Transmission, Fiber Stability and Cellular Signaling in Skeletal Muscle

The systematic bioanalytical characterization of the protein product of the DMD gene, which is defective in the pediatric disorder Duchenne muscular dystrophy, led to the discovery of the membrane cytoskeletal protein dystrophin. Its full-length muscle isoform Dp427-M is tightly linked to a sarcolemma-associated complex consisting of dystroglycans, sarcoglyans, sarcospan, dystrobrevins and syntrophins. Besides these core members of the dystrophin-glycoprotein complex, the wider dystrophin-associated network includes key proteins belonging to the intracellular cytoskeleton and microtubular assembly, the basal lamina and extracellular matrix, various plasma membrane proteins and cytosolic components. Here, we review the central role of the dystrophin complex as a master node in muscle fibers that integrates cytoskeletal organization and cellular signaling at the muscle periphery, as well as providing sarcolemmal stabilization and contractile force transmission to the extracellular region. The combination of optimized tissue extraction, subcellular fractionation, advanced protein co-purification strategies, immunoprecipitation, liquid chromatography and two-dimensional gel electrophoresis with modern mass spectrometry-based proteomics has confirmed the composition of the core dystrophin complex at the sarcolemma membrane. Importantly, these biochemical and mass spectrometric surveys have identified additional members of the wider dystrophin network including biglycan, cavin, synemin, desmoglein, tubulin, plakoglobin, cytokeratin and a variety of signaling proteins and ion channels.

Tissue Engineering in Musculoskeletal Tissue: A Review of the Literature

Tissue engineering refers to the attempt to create functional human tissue from cells in a laboratory. This is a field that uses living cells, biocompatible materials, suitable biochemical and physical factors, and their combinations to create tissue-like structures. To date, no tissue engineered skeletal muscle implants have been developed for clinical use, but they may represent a valid alternative for the treatment of volumetric muscle loss in the near future. Herein, we reviewed the literature and showed different techniques to produce synthetic tissues with the same architectural, structural and functional properties as native tissues.

Biomarkers for Duchenne muscular dystrophy: myonecrosis, inflammation and oxidative stress

Duchenne muscular dystrophy (DMD) is a lethal, X-linked disease that causes severe loss of muscle mass and function in young children. Promising therapies for DMD are being developed, but the long lead times required when using clinical outcome measures are hindering progress. This progress would be facilitated by robust molecular biomarkers in biofluids, such as blood and urine, which could be used to monitor disease progression and severity, as well as to determine optimal drug dosing before a full clinical trial. Many candidate DMD biomarkers have been identified, but there have been few follow-up studies to validate them. This Review describes the promising biomarkers for dystrophic muscle that have been identified in muscle, mainly using animal models. We strongly focus on myonecrosis and the associated inflammation and oxidative stress in DMD muscle, as the lack of dystrophin causes repeated bouts of myonecrosis, which are the key events that initiate the resultant severe dystropathology. We discuss the early events of intrinsic myonecrosis, along with early regeneration in the context of histological and other measures that are used to quantify its incidence. Molecular biomarkers linked to the closely associated events of inflammation and oxidative damage are discussed, with a focus on research related to protein thiol oxidation and to neutrophils. We summarise data linked to myonecrosis in muscle, blood and urine of dystrophic animal species, and discuss the challenge of translating such biomarkers to the clinic for DMD patients, especially to enhance the success of clinical trials.

Summary: This Review discusses biomarkers in blood and urine linked to myonecrosis, inflammation and oxidative stress, to enhance development of therapies for DMD, and the challenges to be overcome for clinical translation.

Monitoring disease activity noninvasively in the mdx model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a rare, muscle degenerative disease resulting from the absence of the dystrophin protein. DMD is characterized by progressive loss of muscle fibers, muscle weakness, and eventually loss of ambulation and premature death. Currently, there is no cure for DMD and improved methods of disease monitoring are crucial for the development of novel treatments. In this study, we describe a new method of assessing disease progression noninvasively in the mdx model of DMD. The reporter mice, which we term the dystrophic Degeneration Reporter strains, contain an inducible CRE-responsive luciferase reporter active in mature myofibers. In these mice, muscle degeneration is reflected in changes in the level of luciferase expression, which can be monitored using noninvasive, bioluminescence imaging. We monitored the natural history and disease progression in these dystrophic report mice and found that decreases in luciferase signals directly correlated with muscle degeneration. We further demonstrated that this reporter strain, as well as a previously reported Regeneration Reporter strain, successfully reveals the effectiveness of a gene therapy treatment following systemic administration of a recombinant adeno-associated virus-6 (rAAV-6) encoding a microdystrophin construct. Our data demonstrate the value of these noninvasive imaging modalities for monitoring disease progression and response to therapy in mouse models of muscular dystrophy.

Tackling muscle fibrosis: From molecular mechanisms to next generation engineered models to predict drug delivery

Muscle fibrosis represents the end stage consequence of different diseases, among which muscular dystrophies, leading to severe impairment of muscle functions. Muscle fibrosis involves the production of several growth factors, cytokines and proteolytic enzymes and is strictly associated to inflammatory processes. Moreover, fibrosis causes profound changes in tissue properties, including increased stiffness and density, lower pH and oxygenation. Up to now, there is no therapeutic approach able to counteract the fibrotic process and treatments directed against muscle pathologies are severely impaired by the harsh conditions of the fibrotic environment. The design of new therapeutics thus need innovative tools mimicking the obstacles posed by the fibrotic environment to their delivery. This review will critically discuss the role of in vivo and 3D in vitro models in this context and the characteristics that an ideal model should possess to help the translation from bench to bedside of new candidate anti-fibrotic agents.

Emerging new tools to study and treat muscle pathologies: genetics and molecular mechanisms underlying skeletal muscle development, regeneration, and disease

Skeletal muscle is the most abundant tissue in our body, is responsible for generating the force required for movement, and is also an important thermogenic organ. Skeletal muscle is an enigmatic tissue because while on the one hand, skeletal muscle regeneration after injury is arguably one of the best-studied stem cell-dependent regenerative processes, on the other hand, skeletal muscle is still subject to many degenerative disorders with few therapeutic options in the clinic. It is important to develop new regenerative medicine-based therapies for skeletal muscle. Future therapeutic strategies should take advantage of rapidly developing technologies enabling the differentiation of skeletal muscle from human pluripotent stem cells, along with precise genome editing, which will go hand in hand with a steady and focused approach to understanding underlying mechanisms of skeletal muscle development, regeneration, and disease. In this review, I focus on highlighting the recent advances that particularly have relied on developmental and molecular biology approaches to understanding muscle development and stem cell function. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

An artificial niche preserves the quiescence of muscle stem cells and enhances their therapeutic efficacy

A promising therapeutic strategy for diverse genetic disorders involves transplantation of autologous stem cells that have been genetically corrected ex vivo. A major challenge in such approaches is a loss of stem cell potency during culture. Here we describe an artificial niche for maintaining muscle stem cells (MuSCs) in vitro in a potent, quiescent state. Using a machine learning method, we identified a molecular signature of quiescence and used it to screen for factors that could maintain mouse MuSC quiescence, thus defining a quiescence medium (QM). We also engineered muscle fibers that mimic the native myofiber of the MuSC niche. Mouse MuSCs maintained in QM on engineered fibers showed enhanced potential for engraftment, tissue regeneration and self-renewal after transplantation in mice. An artificial niche adapted to human cells similarly extended the quiescence of human MuSCs in vitro and enhanced their potency in vivo. Our approach for maintaining quiescence may be applicable to stem cells isolated from other tissues.

Pathoproteomic profiling of the skeletal muscle matrisome in dystrophinopathy associated myofibrosis

The gradual accumulation of collagen and associated proteins of the extracellular matrix is a crucial myopathological parameter of many neuromuscular disorders. Progressive tissue damage and fibrosis play a key pathobiochemical role in the dysregulation of contractile functions and often correlates with poor motor outcome in muscular dystrophies. Following a brief introduction into the role of the extracellular matrix in skeletal muscles, we review here the proteomic profiling of myofibrosis and its intrinsic role in X-linked muscular dystrophy. Although Duchenne muscular dystrophy is primarily a disease of the membrane cytoskeleton, one of its most striking histopathological features is a hyperactive connective tissue and tissue scarring. We outline the identification of novel factors involved in the modulation of the extracellular matrix in muscular dystrophy, such as matricellular proteins. The establishment of novel proteomic markers will be helpful in improving the diagnosis, prognosis, and therapy monitoring in relation to fibrotic substitution of contractile tissue. In the future, the prevention of fibrosis will be crucial for providing optimum conditions to apply novel pharmacological treatments, as well as establish cell-based approaches or gene therapeutic interventions. The elimination of secondary abnormalities in the matrisome promises to reduce tissue scarring and the loss of skeletal muscle elasticity.

Mesoangioblast delivery of miniagrin ameliorates murine model of merosin-deficient congenital muscular dystrophy type 1A

Background

Merosin-deficient congenital muscular dystrophy type-1A (MDC1A) is characterized by progressive muscular dystrophy and dysmyelinating neuropathy caused by mutations of the α2 chain of laminin-211, the predominant laminin isoform of muscles and nerves. MDC1A has no available treatment so far, although preclinical studies showed amelioration of the disease by the overexpression of miniagrin (MAG). MAG reconnects orphan laminin-211 receptors to other laminin isoforms available in the extracellular matrix of MDC1A mice.

Methods

Mesoangioblasts (MABs) are vessel-associated progenitors that can form the skeletal muscle and have been shown to restore defective protein levels and motor skills in animal models of muscular dystrophies. As gene therapy in humans still presents challenging technical issues and limitations, we engineered MABs to overexpress MAG to treat MDC1A mouse models, thus combining cell to gene therapy.

Results

MABs synthesize and secrete only negligible amount of laminin-211 either in vitro or in vivo. MABs engineered to deliver MAG and injected in muscles of MDC1A mice showed amelioration of muscle histology, increased expression of laminin receptors in muscle, and attenuated deterioration of motor performances. MABs did not enter the peripheral nerves, thus did not affect the associated peripheral neuropathy.

Conclusions

Our study demonstrates the potential efficacy of combining cell with gene therapy to treat MDC1A.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0055-5) contains supplementary material, which is available to authorized users.

Framing optional genetic testing in the context of mandatory newborn screening tests

Background

Parents are increasingly faced with decisions about optional newborn bloodspot screening (NBS) despite no consistent policy for communicating information about such testing. We examined whether framing optional NBS alongside mandatory NBS influenced intention to participate in optional NBS.

Methods

For this Internet-administered study, 2,991 adults read a hypothetical vignette in which optional NBS for Duchenne muscular dystrophy (DMD) was either presented by itself (in isolation), alongside a description including the total number of mandatory NBS tests (“bundled” mandatory context), or alongside a listing of each mandatory NBS test (“unbundled” mandatory context). We assessed associations with participants’ intended participation using ordered logistic regression models, and associations with attitudes towards optional DMD NBS and subjective norms using Analysis of Variance.

Results

Participants were more likely to choose optional DMD NBS if they also read information about mandatory NBS (either bundled or unbundled) versus when DMD NBS was presented in isolation. Participants who read about optional DMD NBS in isolation also reported such testing to be less important and that they would worry more about the results than those who also saw mandatory NBS information.

Conclusions

Future NBS programs should pay attention to the framing of optional testing communication, as it influences parental behavior. Predictors of NBS uptake will become increasingly important as NBS programs continue expanding.

Electronic supplementary material

The online version of this article (doi:10.1186/s12911-015-0173-3) contains supplementary material, which is available to authorized users.

Comparative Label-Free Mass Spectrometric Analysis of Mildly versus Severely Affected mdx Mouse Skeletal Muscles Identifies Annexin, Lamin, and Vimentin as Universal Dystrophic Markers

The primary deficiency in the membrane cytoskeletal protein dystrophin results in complex changes in dystrophic muscles. In order to compare the degree of secondary alterations in differently affected subtypes of skeletal muscles, we have conducted a global analysis of proteome-wide changes in various dystrophin-deficient muscles. In contrast to the highly degenerative mdx diaphragm muscle, which showed considerable alterations in 35 distinct proteins, the spectrum of mildly to moderately dystrophic skeletal muscles, including interosseus, flexor digitorum brevis, soleus, and extensor digitorum longus muscle, exhibited a smaller number of changed proteins. Compensatory mechanisms and/or cellular variances may be responsible for differing secondary changes in individual mdx muscles. Label-free mass spectrometry established altered expression levels for diaphragm proteins associated with contraction, energy metabolism, the cytoskeleton, the extracellular matrix and the cellular stress response. Comparative immunoblotting verified the differences in the degree of secondary changes in dystrophin-deficient muscles and showed that the up-regulation of molecular chaperones, the compensatory increase in proteins of the intermediate filaments, the fibrosis-related increase in collagen levels and the pathophysiological decrease in calcium binding proteins is more pronounced in mdx diaphragm as compared to the less severely affected mdx leg muscles. Annexin, lamin, and vimentin were identified as universal dystrophic markers.

Label-free mass spectrometric analysis of the mdx-4cv diaphragm identifies the matricellular protein periostin as a potential factor involved in dystrophinopathy-related fibrosis

Proteomic profiling plays a decisive role in the identification of novel biomarkers of muscular dystrophy and the elucidation of new pathobiochemical mechanisms that underlie progressive muscle wasting. Building on the findings of recent comparative analyses of tissue samples and body fluids from dystrophic animals and patients afflicted with Duchenne muscular dystrophy, we have used here label-free MS to study the severely dystrophic diaphragm from the not extensively characterized mdx-4cv mouse. This animal model of progressive muscle wasting exhibits less dystrophin-positive revertant fibers than the conventional mdx mouse, making it ideal for the future monitoring of experimental therapies. The pathoproteomic signature of the mdx-4cv diaphragm included a significant increase in the fibrosis marker collagen and related extracellular matrix proteins (asporin, decorin, dermatopontin, prolargin) and cytoskeletal proteins (desmin, filamin, obscurin, plectin, spectrin, tubulin, vimentin, vinculin), as well as decreases in proteins of ion homeostasis (parvalbumin) and the contractile apparatus (myosin-binding protein). Importantly, one of the most substantially increased proteins was identified as periostin, a matricellular component and apparent marker of fibrosis and tissue damage. Immunoblotting confirmed a considerable increase of periostin in the dystrophin-deficient diaphragm from both mdx and mdx-4cv mice, suggesting an involvement of this matricellular protein in dystrophinopathy-related fibrosis.

In silico screening based on predictive algorithms as a design tool for exon skipping oligonucleotides in Duchenne muscular dystrophy

The use of antisense 'splice-switching' oligonucleotides to induce exon skipping represents a potential therapeutic approach to various human genetic diseases. It has achieved greatest maturity in exon skipping of the dystrophin transcript in Duchenne muscular dystrophy (DMD), for which several clinical trials are completed or ongoing, and a large body of data exists describing tested oligonucleotides and their efficacy. The rational design of an exon skipping oligonucleotide involves the choice of an antisense sequence, usually between 15 and 32 nucleotides, targeting the exon that is to be skipped. Although parameters describing the target site can be computationally estimated and several have been identified to correlate with efficacy, methods to predict efficacy are limited. Here, an in silico pre-screening approach is proposed, based on predictive statistical modelling. Previous DMD data were compiled together and, for each oligonucleotide, some 60 descriptors were considered. Statistical modelling approaches were applied to derive algorithms that predict exon skipping for a given target site. We confirmed (1) the binding energetics of the oligonucleotide to the RNA, and (2) the distance in bases of the target site from the splice acceptor site, as the two most predictive parameters, and we included these and several other parameters (while discounting many) into an in silico screening process, based on their capacity to predict high or low efficacy in either phosphorodiamidate morpholino oligomers (89% correctly predicted) and/or 2'O Methyl RNA oligonucleotides (76% correctly predicted). Predictions correlated strongly with in vitro testing for sixteen de novo PMO sequences targeting various positions on DMD exons 44 (R² 0.89) and 53 (R² 0.89), one of which represents a potential novel candidate for clinical trials. We provide these algorithms together with a computational tool that facilitates screening to predict exon skipping efficacy at each position of a target exon.

Discovery of serum protein biomarkers in the mdx mouse model and cross-species comparison to Duchenne muscular dystrophy patients

It is expected that serum protein biomarkers in Duchenne muscular dystrophy (DMD) will reflect disease pathogenesis, progression and aid future therapy developments. Here, we describe use of quantitative in vivo stable isotope labeling in mammals to accurately compare serum proteomes of wild-type and dystrophin-deficient mdx mice. Biomarkers identified in serum from two independent dystrophin-deficient mouse models (mdx-Δ52 and mdx-23) were concordant with those identified in sera samples of DMD patients. Of the 355 mouse sera proteins, 23 were significantly elevated and 4 significantly lower in mdx relative to wild-type mice (P-value < 0.001). Elevated proteins were mostly of muscle origin: including myofibrillar proteins (titin, myosin light chain 1/3, myomesin 3 and filamin-C), glycolytic enzymes (aldolase, phosphoglycerate mutase 2, beta enolase and glycogen phosphorylase), transport proteins (fatty acid-binding protein, myoglobin and somatic cytochrome-C) and others (creatine kinase M, malate dehydrogenase cytosolic, fibrinogen and parvalbumin). Decreased proteins, mostly of extracellular origin, included adiponectin, lumican, plasminogen and leukemia inhibitory factor receptor. Analysis of sera from 1 week to 7 months old mdx mice revealed age-dependent changes in the level of these biomarkers with most biomarkers acutely elevated at 3 weeks of age. Serum analysis of DMD patients, with ages ranging from 4 to 15 years old, confirmed elevation of 20 of the murine biomarkers in DMD, with similar age-related changes. This study provides a panel of biomarkers that reflect muscle activity and pathogenesis and should prove valuable tool to complement natural history studies and to monitor treatment efficacy in future clinical trials.

Pharmacological prospects in the treatment of Duchenne muscular dystrophy

PURPOSE OF REVIEW

The most encouraging recent advances regarding pharmacological agents for treating Duchenne muscular dystrophy (DMD) are summarized. Emphasis is given to compounds acting downstream of dystrophin, the protein lacking in DMD, on cellular pathways leading to pathological consequences. The author highlights the progress that may have the greatest potential for clinical use in DMD.

RECENT FINDINGS

Modifying the transcripts of the mutated gene by exon skipping has led to expression of shortened dystrophins in DMD patients. Currently, the most promising potential drugs are the exon-skipping agents eteplirsen and drisapersen. Biglycan and SMTC1100 upregulate utrophin. The steroid receptor modulating compounds VBP15 and tamoxifen, and specific antioxidants appear promising agents for symptomatic therapy.

SUMMARY

The past 18 months have seen a strong increase in the number of exciting reports on novel therapeutic agents for DMD. Exon-skipping agents have been fine-tuned to improve tissue delivery and stability. Impressive discoveries regarding pathogenic events in cellular signalling have revealed targets that were unknown in the context of DMD, thus enabling approaches that limit inflammation, fibrosis and necrosis. The targets are nuclear hormone receptors, NADPH-oxidases and Ca channels. Inhibition of NF-KB, transforming growth factor-alpha (TGF-α) and transforming growth factor-beta (TGF-β)/myostatin production or action are also promising routes in counteracting the complex pathogenesis of DMD.

Comparative Genomics of X-linked Muscular Dystrophies: The Golden Retriever Model

Duchenne muscular dystrophy (DMD) is a devastating disease that dramatically decreases the lifespan and abilities of affected young people. The primary molecular cause of the disease is the absence of functional dystrophin protein, which is critical to proper muscle function. Those with DMD vary in disease presentation and dystrophin mutation; the same causal mutation may be associated with drastically different levels of disease severity. Also contributing to this variation are the influences of additional modifying genes and/or changes in functional elements governing such modifiers. This genetic heterogeneity complicates the efficacy of treatment methods and to date medical interventions are limited to treating symptoms. Animal models of DMD have been instrumental in teasing out the intricacies of DMD disease and hold great promise for advancing knowledge of its variable presentation and treatment. This review addresses the utility of comparative genomics in elucidating the complex background behind phenotypic variation in a canine model of DMD, Golden Retriever muscular dystrophy (GRMD). This knowledge can be exploited in the development of improved, more personalized treatments for DMD patients, such as therapies that can be tailor-matched to the disease course and genomic background of individual patients.

Targeting endothelial junctional adhesion molecule-A/ EPAC/ Rap-1 axis as a novel strategy to increase stem cell engraftment in dystrophic muscles

Muscular dystrophies are severe genetic diseases for which no efficacious therapies exist. Experimental clinical treatments include intra-arterial administration of vessel-associated stem cells, called mesoangioblasts (MABs). However, one of the limitations of this approach is the relatively low number of cells that engraft the diseased tissue, due, at least in part, to the sub-optimal efficiency of extravasation, whose mechanisms for MAB are unknown. Leukocytes emigrate into the inflamed tissues by crossing endothelial cell-to-cell junctions and junctional proteins direct and control leukocyte diapedesis. Here, we identify the endothelial junctional protein JAM-A as a key regulator of MAB extravasation. We show that JAM-A gene inactivation and JAM-A blocking antibodies strongly enhance MAB engraftment in dystrophic muscle. In the absence of JAM-A, the exchange factors EPAC-1 and 2 are down-regulated, which prevents the activation of the small GTPase Rap-1. As a consequence, junction tightening is reduced, allowing MAB diapedesis. Notably, pharmacological inhibition of Rap-1 increases MAB engraftment in dystrophic muscle, which results into a significant improvement of muscle function offering a novel strategy for stem cell-based therapies.

Ongoing therapeutic trials and outcome measures for Duchenne muscular dystrophy

Muscular dystrophy is a heterogeneous group of genetic disorders characterised by progressive muscle tissue degeneration. No effective treatment has been discovered for these diseases. Preclinical and clinical studies aimed at the development of new therapeutic approaches have been carried out, primarily in subjects affected with dystrophinopathies (Duchenne and Becker muscular dystrophy). In this review, we outline the current therapeutic approaches and past and ongoing clinical trials, highlighting both the advantages and limits of each one. The experimental designs of these trials were based on different rationales, including immunomodulation, readthrough strategies, exon skipping, gene therapy, and cell therapy. We also provide an overview of available outcome measures, focusing on their reliability in estimating meaningful clinical improvement in order to aid in the design of future trials. This perspective is extremely relevant to the field considering the recent development of novel therapeutic approaches that will result in an increasing number of clinical studies over the next few years.

Chemical shift-based MRI to measure fat fractions in dystrophic skeletal muscle

PURPOSE

The relationship between fat fractions (FFs) determined based on multiple TE, unipolar gradient echo images and (1) H magnetic resonance spectroscopy (MRS) was evaluated using different models for fat-water decomposition, signal-to-noise ratios, and excitation flip angles.

METHODS

A combination of single-voxel proton spectroscopy ((1) H-MRS) and gradient echo imaging was used to determine muscle FFs in both normal and dystrophic muscles. In order to cover a large range of FFs, the soleus and vastus lateralis muscles of 22 unaffected control subjects, 16 subjects with collagen VI deficiency (COL6), and 71 subjects with Duchenne muscular dystrophy (DMD) were studied. (1) H-MRS-based FF were corrected for the increased muscle (1) H2 O T1 and T2 values observed in dystrophic muscles.

RESULTS

Excellent agreement was found between coregistered FFs derived from gradient echo images fit to a multipeak model with noise bias correction and the relaxation-corrected (1) H-MRS FFs (y = 0.93x + 0.003; R(2)  = 0.96) across the full range of FFs. Relaxation-corrected (1) H-MRS FFs and imaging-based FFs were significantly elevated (P < 0.01) in the muscles of COL6 and DMD subjects.

CONCLUSION

FFs, T2 , and T1 were all sensitive to muscle involvement in dystrophic muscle. MRI offered an additional advantage over single-voxel spectroscopy in that the tissue heterogeneity in FFs could be readily determined.

Proteomic identification of biomarkers of skeletal muscle disorders

Disease-specific biomarkers play a central diagnostic and therapeutic role in muscle pathology. Serum levels of a variety of muscle-derived enzymes are routinely used for the detection of muscle damage in diagnostic procedures, as well as for the monitoring of physical training status in sports medicine. Over the last few years, the systematic application of mass spectrometry-based proteomics for studying skeletal muscle degeneration has greatly expanded the range of muscle biomarkers, including new fiber-associated proteins involved in muscle transformation, muscular atrophy, muscular dystrophy, motor neuron disease, inclusion body myositis, myotonia, hypoxia, diabetes, obesity and sarcopenia of old age. These mass spectrometric studies have clearly established skeletal muscle proteomics as a reliable method for the identification of novel indicators of neuromuscular diseases.

Proteomic profiling of cardiomyopathic tissue from the aged mdx model of Duchenne muscular dystrophy reveals a drastic decrease in laminin, nidogen and annexin

The majority of patients afflicted with Duchenne muscular dystrophy develop cardiomyopathic complications, warranting large-scale proteomic studies of global cardiac changes for the identification of new protein markers of dystrophinopathy. The aged heart from the X-linked dystrophic mdx mouse has been shown to exhibit distinct pathological aspects of cardiomyopathy. In order to establish age-related alterations in the proteome of dystrophin-deficient hearts, cardiomyopathic tissue from young versus aged mdx mice was examined by label-free LC-MS/MS. Significant age-dependent alterations were established for 67 proteins, of which 28 proteins were shown to exhibit a lower abundance and 39 proteins were found to be increased in their expression levels. Drastic changes were demonstrated for 17 proteins, including increases in Ig chains and transferrin, and drastic decreases in laminin, nidogen and annexin. An immunblotting survey of young and old wild-type versus mdx hearts confirmed these proteomic findings and illustrated the effects of natural aging versus dystrophin deficiency. These proteome-wide alterations suggest a disintegration of the basal lamina structure and cytoskeletal network in dystrophin-deficient cardiac fibres, increased levels of antibodies in a potential autoimmune reaction of the degenerating heart, compensatory binding of excess iron and a general perturbation of metabolic pathways in dystrophy-associated cardiomyopathy.

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.

Ibuprofen plus isosorbide dinitrate treatment in the mdx mice ameliorates dystrophic heart structure

BACKGROUND

Co-administration of ibuprofen (IBU) and isosorbide dinitrate (ISDN) provides synergistic beneficial effects on dystrophic skeletal muscle. Whether this treatment has also cardioprotective effects in this disease was still unknown.

AIMS

To evaluate the effects of co-administration of IBU and ISDN (a) on left ventricular (LV) structure and function, and (b) on cardiac inflammatory response and fibrosis in mdx mice.

METHODS

Three groups of mice were studied: mdx mice treated with IBU (50 mg kg⁻¹)+ISDN (30 mg kg⁻¹) administered daily in the diet, mdx mice that received standard diet without drugs and wild type aged-matched mice. Animals were analysed after 10-11 months of treatment. Structural and functional parameters were evaluated by echocardiography while histological analyses were performed to evaluate inflammatory response, collagen deposition, cardiomyocyte number and area.

RESULTS

Treatment for 10-11 months with IBU+ISDN preserved LV wall thickness and LV mass. Drug treatment also preserved the total number of cardiomyocytes in the LV and attenuated the increase in cardiomyocyte size, when compared to untreated mdx mice. Moreover, a trend towards a decreased number of inflammatory cells, a reduced LV myocardial interstitial fibrosis and an enhanced global LV function response to stress was observed in treated mdx mice.

CONCLUSIONS

Treatment for 10-11 months with IBU+ISDN is effective in preventing the alterations in LV morphology of mdx mice while not reaching statistical significance on LV function and cardiac inflammation.

Activation of p38 signaling increases utrophin A expression in skeletal muscle via the RNA-binding protein KSRP and inhibition of AU-rich element-mediated mRNA decay: implications for novel DMD therapeutics

Several therapeutic approaches are currently being developed for Duchenne muscular dystrophy (DMD) including upregulating the levels of endogenous utrophin A in dystrophic fibers. Here, we examined the role of post-transcriptional mechanisms in controlling utrophin A expression in skeletal muscle. We show that activation of p38 leads to an increase in utrophin A independently of a transcriptional induction. Rather, p38 controls the levels of utrophin A mRNA by extending the half-life of transcripts via AU-rich elements (AREs). This mechanism critically depends on a decrease in the functional availability of KSRP, an RNA-binding protein known to promote decay of ARE-containing transcripts. In vitro and in vivo binding studies revealed that KSRP interacts with specific AREs located within the utrophin A 3' UTR. Electroporation experiments to knockdown KSRP led to an increase in utrophin A in wild-type and mdx mouse muscles. In pre-clinical studies, treatment of mdx mice with heparin, an activator of p38, causes a pronounced increase in utrophin A in diaphragm muscle fibers. Together, these studies identify a pathway that culminates in the post-transcriptional regulation of utrophin A through increases in mRNA stability. Furthermore, our results constitute proof-of-principle showing that pharmacological activation of p38 may prove beneficial as a novel therapeutic approach for DMD.

Duchenne muscular dystrophy drug discovery - the application of utrophin promoter activation screening

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a devastating genetic muscle wasting disease caused by mutations in the DMD gene that in turn lead to an absence of dystrophin. Currently, there is no definitive therapy for DMD. Gene- and cell-based therapies designed to replace dystrophin have met some degree of success, as have strategies that seek to improve the dystrophic pathology independent of dystrophin.

AREAS COVERED

In this review the authors focus on utrophin promoter activation-based strategies and their implications on potential therapeutics for DMD. These strategies in common are designed to identify drugs/small molecules that can activate the utrophin promoter and would allow the functional substitution of dystrophin by upregulating utrophin expression in dystrophic muscle. The authors provide an overview of utrophin biology with a focus on regulation of the utrophin promoter and discuss current attempts in identifying utrophin promoter-activating molecules using high-throughput screening (HTS).

EXPERT OPINION

The characterisation of utrophin promoter regulatory mechanisms coupled with advances in HTS have allowed researchers to undertake screens and identify a number of promising lead compounds that may prove useful for DMD. In principle, these pharmacological compounds offer significant advantages from a translational viewpoint for developing DMD therapeutics.

Reawakening atlas: chemical approaches to repair or replace dysfunctional musculature

Muscle diseases are major health concerns. For example, ischemic heart disease is the third most common cause of death. Cell therapy is an attractive approach for treating muscle diseases, although this is hampered by the need to generate large numbers of functional muscle cells. Small molecules have become established as attractive tools for modulating cell behavior and, in this review, we discuss the recent, rapid research advances made in the development of small molecule methods to facilitate the production of functional cardiac, skeletal, and smooth muscle cells. We also describe how new developments in small molecule strategies for muscle disease aim to induce repair and remodelling of the damaged tissues in situ. Recent progress has been made in developing small molecule cocktails that induce skeletal muscle regeneration, and these are discussed in a broader context, because a similar phenomenon occurs in the early stages of salamander appendage regeneration. Although formidable technical hurdles still remain, these new advances in small molecule-based methodologies should provide hope that cell therapies for patients suffering from muscle disease can be developed in the near future.

Skeletal muscle degenerative diseases and strategies for therapeutic muscle repair

Skeletal muscle is a highly specialized, postmitotic tissue that must withstand chronic mechanical and physiological stress throughout life to maintain proper contractile function. Muscle damage or disease leads to progressive weakness and disability, and manifests in more than 100 different human disorders. Current therapies to treat muscle degenerative diseases are limited mostly to the amelioration of symptoms, although promising new therapeutic directions are emerging. In this review, we discuss the pathological basis for the most common muscle degenerative diseases and highlight new and encouraging experimental and clinical opportunities to prevent or reverse these afflictions.

Recombinant MG53 protein modulates therapeutic cell membrane repair in treatment of muscular dystrophy

Mitsugumin 53 (MG53), a muscle-specific TRIM family protein, is an essential component of the cell membrane repair machinery. Here, we examined the translational value of targeting MG53 function in tissue repair and regenerative medicine. Although native MG53 protein is principally restricted to skeletal and cardiac muscle tissues, beneficial effects that protect against cellular injuries are present in nonmuscle cells with overexpression of MG53. In addition to the intracellular action of MG53, injury to the cell membrane exposes a signal that can be detected by MG53, allowing recombinant MG53 protein to repair membrane damage when provided in the extracellular space. Recombinant human MG53 (rhMG53) protein purified from Escherichia coli fermentation provided dose-dependent protection against chemical, mechanical, or ultraviolet-induced damage to both muscle and nonmuscle cells. Injection of rhMG53 through multiple routes decreased muscle pathology in the mdx dystrophic mouse model. Our data support the concept of targeted cell membrane repair in regenerative medicine, and present MG53 protein as an attractive biological reagent for restoration of membrane repair defects in human diseases.

Profiling of age-related changes in the tibialis anterior muscle proteome of the mdx mouse model of dystrophinopathy

X-linked muscular dystrophy is a highly progressive disease of childhood and characterized by primary genetic abnormalities in the dystrophin gene. Senescent mdx specimens were used for a large-scale survey of potential age-related alterations in the dystrophic phenotype, because the established mdx animal model of dystrophinopathy exhibits progressive deterioration of muscle tissue with age. Since the mdx tibialis anterior muscle is a frequently used model system in muscular dystrophy research, we employed this particular muscle to determine global changes in the dystrophic skeletal muscle proteome. The comparison of mdx mice aged 8 weeks versus 22 months by mass-spectrometry-based proteomics revealed altered expression levels in 8 distinct protein species. Increased levels were shown for carbonic anhydrase, aldolase, and electron transferring flavoprotein, while the expressions of pyruvate kinase, myosin, tropomyosin, and the small heat shock protein Hsp27 were found to be reduced in aged muscle. Immunoblotting confirmed age-dependent changes in the density of key muscle proteins in mdx muscle. Thus, segmental necrosis in mdx tibialis anterior muscle appears to trigger age-related protein perturbations due to dystrophin deficiency. The identification of novel indicators of progressive muscular dystrophy might be useful for the establishment of a muscle subtype-specific biomarker signature of dystrophinopathy.

Stem cell therapies for muscle disorders

PURPOSE OF REVIEW

This review focuses on stem cell-based therapies to treat skeletal muscle disorders, with a special emphasis on muscular dystrophies.

RECENT FINDINGS

We briefly review previous attempts at cell therapy by the use of donor myoblasts, explaining the likely reasons for the poor clinical results; we then describe the use of the same cells in current promising trials for localized treatments of different diseases of skeletal muscle. Moreover, we discuss important novel findings on muscle stem/progenitor cell biology and their promise for future clinical translation. Preclinical and clinical applications of novel myogenic stem/progenitor cells are also described.

SUMMARY

We summarize several ongoing clinical trials for different muscle disorders and the advances in the understanding of the biology of the myogenic progenitors used in such trials. On the basis of the currently available information, a prediction of developments in the field is proposed.

Extensive and prolonged restoration of dystrophin expression with vivo-morpholino-mediated multiple exon skipping in dystrophic dogs

Duchenne muscular dystrophy (DMD) is a severe and the most prevalent form of muscular dystrophy, characterized by rapid progression of muscle degeneration. Antisense-mediated exon skipping is currently one of the most promising therapeutic options for DMD. However, unmodified antisense oligos such as morpholinos require frequent (weekly or bi-weekly) injections. Recently, new generation morpholinos such as vivo-morpholinos are reported to lead to extensive and prolonged dystrophin expression in the dystrophic mdx mouse, an animal model of DMD. The vivo-morpholino contains a cell-penetrating moiety, octa-guanidine dendrimer. Here, we sought to test the efficacy of multiple exon skipping of exons 6-8 with vivo-morpholinos in the canine X-linked muscular dystrophy, which harbors a splice site mutation at the boundary of intron 6 and exon 7. We designed and optimized novel antisense cocktail sequences and combinations for exon 8 skipping and demonstrated effective exon skipping in dystrophic dogs in vivo. Intramuscular injections with newly designed cocktail oligos led to high levels of dystrophin expression, with some samples similar to wild-type levels. This is the first report of successful rescue of dystrophin expression with morpholino conjugates in dystrophic dogs. Our results show the potential of phosphorodiamidate morpholino oligomer conjugates as therapeutic agents for DMD.

Proteomics reveals drastic increase of extracellular matrix proteins collagen and dermatopontin in the aged mdx diaphragm model of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a lethal genetic disease of childhood caused by primary abnormalities in the gene coding for the membrane cytoskeletal protein dystrophin. The mdx mouse is an established animal model of various aspects of X-linked muscular dystrophy and is widely used for studying fundamental mechanisms of dystrophinopathy and testing novel therapeutic approaches to treat one of the most frequent gender-specific diseases in humans. In order to determine global changes in the muscle proteome with the progressive deterioration of mdx tissue with age, we have characterized diaphragm muscle from mdx mice at three ages (8-weeks, 12-months and 22-months) using mass spectrometry-based proteomics. Altered expression levels in diaphragm of 8-week vs. 22-month mice were shown to occur in 11 muscle-associated proteins. Aging in the mdx diaphragm seems to be associated with a drastic increase in the extracellular matrix proteins, collagen and dermatopontin, the molecular chaperone αB-crystallin, and the intermediate filament protein vimentin, suggesting increased accumulation of connective tissue, an enhanced cellular stress response and compensatory stabilization of the weakened membrane cytoskeleton. These proteomic findings establish the aged mdx diaphragm as an excellent model system for studying secondary effects of dystrophin deficiency in skeletal muscle tissue.