Dystrophin and dystrophin-related protein (utrophin) distribution in normal and dystrophin-deficient skeletal muscles

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Endogenous bioluminescent reporters reveal a sustained increase in utrophin gene expression upon EZH2 and ERK1/2 inhibition

Duchenne muscular dystrophy (DMD) is an X-linked disorder caused by loss of function mutations in the dystrophin gene (Dmd), resulting in progressive muscle weakening. Here we modelled the longitudinal expression of endogenous Dmd, and its paralogue Utrn, in mice and in myoblasts by generating bespoke bioluminescent gene reporters. As utrophin can partially compensate for Dmd-deficiency, these reporters were used as tools to ask whether chromatin-modifying drugs can enhance Utrn expression in developing muscle. Myoblasts treated with different PRC2 inhibitors showed significant increases in Utrn transcripts and bioluminescent signals, and these responses were independently verified by conditional Ezh2 deletion. Inhibition of ERK1/2 signalling provoked an additional increase in Utrn expression that was also seen in Dmd-mutant cells, and maintained as myoblasts differentiate. These data reveal PRC2 and ERK1/2 to be negative regulators of Utrn expression and provide specialised molecular imaging tools to monitor utrophin expression as a therapeutic strategy for DMD.

Muscle spindle function in healthy and diseased muscle

Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. Many neuromuscular diseases affect muscle spindle function contributing, among others, to an unstable gait, frequent falls and ataxic behavior in the affected patients. Nevertheless, muscle spindles are usually ignored during examination and analysis of muscle function and when designing therapeutic strategies for neuromuscular diseases. This review summarizes the development and function of muscle spindles and the changes observed under pathological conditions, in particular in the various forms of muscular dystrophies.

Biochemical and biomechanical characteristics of dystrophin-deficient mdx3cv mouse lens

The molecular and cellular basis for cataract development in mice lacking dystrophin, a scaffolding protein that links the cytoskeleton to the extracellular matrix, is poorly understood. In this study, we characterized lenses derived from the dystrophin-deficient mdx3cv mouse model. Expression of Dp71, a predominant isoform of dystrophin in the lens, was induced during lens fiber cell differentiation. Dp71 was found to co-distribute with dystroglycan, connexin-50 and 46, aquaporin-0, and NrCAM as a large cluster at the center of long arms of the hexagonal fibers. Although mdx3cv mouse lenses exhibited dramatically reduced levels of Dp71, only older lenses revealed punctate nuclear opacities compared to littermate wild type (WT) lenses. The levels of dystroglycan, syntrophin, and dystrobrevin which comprise the dystrophin-associated protein complex (DAPC), and NrCAM, connexin-50, and aquaporin-0, were significantly lower in the lens membrane fraction of adult mdx3cv mice compared to WT mice. Additionally, decreases were observed in myosin light chain phosphorylation and lens stiffness together with a significant elevation in the levels of utrophin, a functional homolog of dystrophin in mdx3cv mouse lenses compared to WT lenses. The levels of perlecan and laminin (ligands of α-dystroglycan) remained normal in dystrophin-deficient lens fibers. Taken together, although mdx3cv mouse lenses exhibit only minor defects in lens clarity possibly due to a compensatory increase in utrophin, the noted disruptions of DAPC, stability, and organization of membrane integral proteins of fibers, and stiffness of mdx3cv lenses reveal the importance of dystrophin and DAPC in maintaining lens clarity and function.

Microutrophin expression in dystrophic mice displays myofiber type differences in therapeutic effects

Gene therapy approaches for DMD using recombinant adeno-associated viral (rAAV) vectors to deliver miniaturized (or micro) dystrophin genes to striated muscles have shown significant progress. However, concerns remain about the potential for immune responses against dystrophin in some patients. Utrophin, a developmental paralogue of dystrophin, may provide a viable treatment option. Here we examine the functional capacity of an rAAV-mediated microutrophin (μUtrn) therapy in the mdx4cv mouse model of DMD. We found that rAAV-μUtrn led to improvement in dystrophic histopathology & mostly restored the architecture of the neuromuscular and myotendinous junctions. Physiological studies of tibialis anterior muscles indicated peak force maintenance, with partial improvement of specific force. A fundamental question for μUtrn therapeutics is not only can it replace critical functions of dystrophin, but whether full-length utrophin impacts the therapeutic efficacy of the smaller, highly expressed μUtrn. As such, we found that μUtrn significantly reduced the spacing of the costameric lattice relative to full-length utrophin. Further, immunostaining suggested the improvement in dystrophic pathophysiology was largely influenced by favored correction of fast 2b fibers. However, unlike μUtrn, μdystrophin (μDys) expression did not show this fiber type preference. Interestingly, μUtrn was better able to protect 2a and 2d fibers in mdx:utrn-/- mice than in mdx4cv mice where the endogenous full-length utrophin was most prevalent. Altogether, these data are consistent with the role of steric hindrance between full-length utrophin & μUtrn within the sarcolemma. Understanding the stoichiometry of this effect may be important for predicting clinical efficacy.

Impaired muscle spindle function in murine models of muscular dystrophy

KEY POINTS

Muscular dystrophy patients suffer from progressive degeneration of skeletal muscle fibres, sudden spontaneous falls, balance problems, as well as gait and posture abnormalities. Dystrophin- and dysferlin-deficient mice, models for different types of muscular dystrophy with different aetiology and molecular basis, were characterized to investigate if muscle spindle structure and function are impaired. The number and morphology of muscle spindles were unaltered in both dystrophic mouse lines but muscle spindle resting discharge and their responses to stretch were altered. In dystrophin-deficient muscle spindles, the expression of the paralogue utrophin was substantially upregulated, potentially compensating for the dystrophin deficiency. The results suggest that muscle spindles might contribute to the motor problems observed in patients with muscular dystrophy.

ABSTRACT

Muscular dystrophies comprise a heterogeneous group of hereditary diseases characterized by progressive degeneration of extrafusal muscle fibres as well as unstable gait and frequent falls. To investigate if muscle spindle function is impaired, we analysed their number, morphology and function in wildtype mice and in murine model systems for two distinct types of muscular dystrophy with very different disease aetiology, i.e. dystrophin- and dysferlin-deficient mice. The total number and the overall structure of muscle spindles in soleus muscles of both dystrophic mouse mutants appeared unchanged. Immunohistochemical analyses of wildtype muscle spindles revealed a concentration of dystrophin and β-dystroglycan in intrafusal fibres outside the region of contact with the sensory neuron. While utrophin was absent from the central part of intrafusal fibres of wildtype mice, it was substantially upregulated in dystrophin-deficient mice. Single-unit extracellular recordings of sensory afferents from muscle spindles of the extensor digitorum longus muscle revealed that muscle spindles from both dystrophic mouse strains have an increased resting discharge and a higher action potential firing rate during sinusoidal vibrations, particularly at low frequencies. The response to ramp-and-hold stretches appeared unaltered compared to the respective wildtype mice. We observed no exacerbated functional changes in dystrophin and dysferlin double mutant mice compared to the single mutant animals. These results show alterations in muscle spindle afferent responses in both dystrophic mouse lines, which might cause an increased muscle tone, and might contribute to the unstable gait and frequent falls observed in patients with muscular dystrophy.

Screening identifies small molecules that enhance the maturation of human pluripotent stem cell-derived myotubes

Targeted differentiation of pluripotent stem (PS) cells into myotubes enables in vitro disease modeling of skeletal muscle diseases. Although various protocols achieve myogenic differentiation in vitro, resulting myotubes typically display an embryonic identity. This is a major hurdle for accurately recapitulating disease phenotypes in vitro, as disease commonly manifests at later stages of development. To address this problem, we identified four factors from a small molecule screen whose combinatorial treatment resulted in myotubes with enhanced maturation, as shown by the expression profile of myosin heavy chain isoforms, as well as the upregulation of genes related with muscle contractile function. These molecular changes were confirmed by global chromatin accessibility and transcriptome studies. Importantly, we also observed this maturation in three-dimensional muscle constructs, which displayed improved in vitro contractile force generation in response to electrical stimulus. Thus, we established a model for in vitro muscle maturation from PS cells.

Alternative utrophin mRNAs contribute to phenotypic differences between dystrophin-deficient mice and Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disorder caused by absence of functional dystrophin protein. Compensation in dystrophin‐deficient (mdx) mice may be achieved by overexpression of its fetal paralogue, utrophin. Strategies to increase utrophin levels by stimulating promoter activity using small compounds are therefore a promising pharmacological approach. Here, we characterise similarities and differences existing within the mouse and human utrophin locus to assist in high‐throughput screening for potential utrophin modulator drugs. We identified five novel 5′‐utrophin isoforms (A′,B′,C,D and F) in adult and embryonic tissue. As the more efficient utrophin‐based response in mdx skeletal muscle appears to involve independent transcriptional activation of conserved, myogenic isoforms (A′ and F), elevating their paralogues in DMD patients is an encouraging therapeutic strategy.

ECM-Related Myopathies and Muscular Dystrophies: Pros and Cons of Protein Therapies

Extracellular matrix (ECM) myopathies and muscular dystrophies are a group of genetic diseases caused by mutations in genes encoding proteins that provide critical links between muscle cells and the extracellular matrix. These include structural proteins of the ECM, muscle cell receptors, enzymes, and intracellular proteins. Loss of adhesion within the myomatrix results in progressive muscle weakness. For many ECM muscular dystrophies, symptoms can occur any time after birth and often result in reduced life expectancy. There are no cures for the ECM-related muscular dystrophies and treatment options are limited to palliative care. Several therapeutic approaches have been explored to treat muscular dystrophies including gene therapy, gene editing, exon skipping, embryonic, and adult stem cell therapy, targeting genetic modifiers, modulating inflammatory responses, or preventing muscle degeneration. Recently, protein therapies that replace components of the defective myomatrix or enhance muscle and/or extracellular matrix integrity and function have been explored. Preclinical studies for many of these biologics have been promising in animal models of these muscle diseases. This review aims to summarize the ECM muscular dystrophies for which protein therapies are being developed and discuss the exciting potential and possible limitations of this approach for treating this family of devastating genetic muscle diseases. © 2017 American Physiological Society. Compr Physiol 7:1519-1536, 2017.

Targeting muscle stem cell intrinsic defects to treat Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a genetic disease characterised by skeletal muscle degeneration and progressive muscle wasting, which is caused by loss-of-function mutations in the DMD gene that encodes for the protein dystrophin. Dystrophin has critical roles in myofiber stability and integrity by connecting the actin cytoskeleton to the extracellular matrix. Absence of dystrophin leads to myofiber fragility and contributes to skeletal muscle degeneration in DMD patients, however, accumulating evidence also indicate that muscle stem cells (also known as satellite cells) are defective in dystrophic muscles, which leads to impaired muscle regeneration. Our recent work demonstrated that dystrophin is expressed in activated satellite cells, where it regulates the establishment of satellite cell polarity and asymmetric cell division. These findings indicate that dystrophin-deficient satellite cells have intrinsic dysfunctions that contribute to muscle wasting and progression of the disease. This discovery suggests that satellite cells could be targeted to treat DMD. Here we discuss how these new findings affect regenerative therapies for muscular dystrophies. Therapies targeting satellite cells hold great potential and could have long-term efficiency owing to the high self-renewal ability of these cells.

Studying the role of dystrophin-associated proteins in influencing Becker muscular dystrophy disease severity

Becker muscular dystrophy is characterized by a variable disease course. Many factors have been implicated to contribute to this diversity, among which the expression of several components of the dystrophin associated glycoprotein complex. Together with dystrophin, most of these proteins anchor the muscle fiber cytoskeleton to the extracellular matrix, thus protecting the muscle from contraction induced injury, while nNOS is primarily involved in inducing vasodilation during muscle contraction, enabling adequate muscle oxygenation. In the current study, we investigated the role of three components of the dystrophin associated glycoprotein complex (beta-dystroglycan, gamma-sarcoglycan and nNOS) and the dystrophin homologue utrophin on disease severity in Becker patients. Strength measurements, data about disease course and fresh muscle biopsies of the anterior tibial muscle were obtained from 24 Becker patients aged 19 to 66. The designation of Becker muscular dystrophy in this study was based on the mutation and not on the clinical severity. Contrary to previous studies, we were unable to find a relationship between expression of nNOS, beta-dystroglycan and gamma-sarcoglycan at the sarcolemma and disease severity, as measured by muscle strength in five muscle groups and age at reaching several disease milestones. Unexpectedly, we found an inverse correlation between utrophin expression at the sarcolemma and age at reaching disease milestones.

Muscle structure influences utrophin expression in mdx mice

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by mutations in the dystrophin gene. To examine the influence of muscle structure on the pathogenesis of DMD we generated mdx^4cv:desmin double knockout (dko) mice. The dko male mice died of apparent cardiorespiratory failure at a median age of 76 days compared to 609 days for the desmin^−/− mice. An ∼2.5 fold increase in utrophin expression in the dko skeletal muscles prevented necrosis in ∼91% of 1a, 2a and 2d/x fiber-types. In contrast, utrophin expression was reduced in the extrasynaptic sarcolemma of the dko fast 2b fibers leading to increased membrane fragility and dystrophic pathology. Despite lacking extrasynaptic utrophin, the dko fast 2b fibers were less dystrophic than the mdx^4cv fast 2b fibers suggesting utrophin-independent mechanisms were also contributing to the reduced dystrophic pathology. We found no overt change in the regenerative capacity of muscle stem cells when comparing the wild-type, desmin^−/−, mdx^4cv and dko gastrocnemius muscles injured with notexin. Utrophin could form costameric striations with α-sarcomeric actin in the dko to maintain the integrity of the membrane, but the lack of restoration of the NODS (nNOS, α-dystrobrevin 1 and 2, α1-syntrophin) complex and desmin coincided with profound changes to the sarcomere alignment in the diaphragm, deposition of collagen between the myofibers, and impaired diaphragm function. We conclude that the dko mice may provide new insights into the structural mechanisms that influence endogenous utrophin expression that are pertinent for developing a therapy for DMD.

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by mutations in the dystrophin gene. Utrophin is structurally similar to dystrophin and improving its expression can prevent skeletal muscle necrosis in the mdx mouse model of DMD. Consequently, improving utrophin expression is a primary therapeutic target for treating DMD. While the downstream mechanisms that influence utrophin expression and stability are well described, the upstream mechanisms are less clear. Here, we found that perturbing the highly ordered structure of striated muscle by genetically deleting desmin from mdx mice increased utrophin expression to levels that prevented skeletal muscle necrosis. Thus, the mdx:desmin double knockout mice may prove valuable in determining the upstream mechanisms that influence utrophin expression to develop a therapy for DMD.

Progress in therapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy is a devastating muscular dystrophy of childhood. Mutations in the dystrophin gene destroy the link between the internal muscle filaments and the extracellular matrix, resulting in severe muscle weakness and progressive muscle wasting. There is currently no cure and, whilst palliative treatment has improved, affected boys are normally confined to a wheelchair by 12 years of age and die from respiratory or cardiac complications in their twenties or thirties. Therapies currently being developed include mutation-specific treatments, DNA- and cell-based therapies, and drugs which aim to modulate cellular pathways or gene expression. This review aims to provide an overview of the different therapeutic approaches aimed at reconstructing the dystrophin-associated protein complex, including restoration of dystrophin expression and upregulation of the functional homologue, utrophin.

Biglycan recruits utrophin to the sarcolemma and counters dystrophic pathology in mdx mice

Duchenne muscular dystrophy (DMD) is caused by mutations in dystrophin and the subsequent disruption of the dystrophin-associated protein complex (DAPC). Utrophin is a dystrophin homolog expressed at high levels in developing muscle that is an attractive target for DMD therapy. Here we show that the extracellular matrix protein biglycan regulates utrophin expression in immature muscle and that recombinant human biglycan (rhBGN) increases utrophin expression in cultured myotubes. Systemically delivered rhBGN up-regulates utrophin at the sarcolemma and reduces muscle pathology in the mdx mouse model of DMD. RhBGN treatment also improves muscle function as judged by reduced susceptibility to eccentric contraction-induced injury. Utrophin is required for the rhBGN therapeutic effect. Several lines of evidence indicate that biglycan acts by recruiting utrophin protein to the muscle membrane. RhBGN is well tolerated in animals dosed for as long as 3 months. We propose that rhBGN could be a therapy for DMD.

Myotendinous junction defects and reduced force transmission in mice that lack alpha7 integrin and utrophin

The alpha7beta1 integrin, dystrophin, and utrophin glycoprotein complexes are the major laminin receptors in skeletal muscle. Loss of dystrophin causes Duchenne muscular dystrophy, a lethal muscle wasting disease. Duchenne muscular dystrophy-affected muscle exhibits increased expression of alpha7beta1 integrin and utrophin, which suggests that these laminin binding complexes may act as surrogates in the absence of dystrophin. Indeed, mice that lack dystrophin and alpha7 integrin (mdx/alpha7(-/-)), or dystrophin and utrophin (mdx/utr(-/-)), exhibit severe muscle pathology and die prematurely. To explore the contribution of the alpha7beta1 integrin and utrophin to muscle integrity and function, we generated mice lacking both alpha7 integrin and utrophin. Surprisingly, mice that lack both alpha7 integrin and utrophin (alpha7/utr(-/-)) were viable and fertile. However, these mice had partial embryonic lethality and mild muscle pathology, similar to alpha7 integrin-deficient mice. Dystrophin levels were increased 1.4-fold in alpha7/utr(-/-) skeletal muscle and were enriched at neuromuscular junctions. Ultrastructural analysis revealed abnormal myotendinous junctions, and functional tests showed a ninefold reduction in endurance and 1.6-fold decrease in muscle strength in these mice. The alpha7/utr(-/-) mouse, therefore, demonstrates the critical roles of alpha7 integrin and utrophin in maintaining myotendinous junction structure and enabling force transmission during muscle contraction. Together, these results indicate that the alpha7beta1 integrin, dystrophin, and utrophin complexes act in a concerted manner to maintain the structural and functional integrity of skeletal muscle.

Laminin-111 protein therapy prevents muscle disease in the mdx mouse model for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease caused by mutations in the gene encoding dystrophin. Loss of dystrophin results in reduced sarcolemmal integrity and increased susceptibility to muscle damage. The alpha(7)beta(1)-integrin is a laminin-binding protein up-regulated in the skeletal muscle of DMD patients and in the mdx mouse model. Transgenic overexpression of the alpha(7)-integrin alleviates muscle disease in dystrophic mice, making this gene a target for pharmacological intervention. Studies suggest laminin may regulate alpha(7)-integrin expression. To test this hypothesis, mouse and human myoblasts were treated with laminin and assayed for alpha(7)-integrin expression. We show that laminin-111 (alpha(1), beta(1), gamma(1)), which is expressed during embryonic development but absent in normal or dystrophic skeletal muscle, increased alpha(7)-integrin expression in mouse and DMD patient myoblasts. Injection of laminin-111 protein into the mdx mouse model of DMD increased expression of alpha(7)-integrin, stabilized the sarcolemma, restored serum creatine kinase to wild-type levels, and protected muscle from exercised-induced damage. These findings demonstrate that laminin-111 is a highly potent therapeutic agent for the mdx mouse model of DMD and represents a paradigm for the systemic delivery of extracellular matrix proteins as therapies for genetic diseases.

NF-kappaB-dependent expression of the antiapoptotic factor c-FLIP is regulated by calpain 3, the protein involved in limb-girdle muscular dystrophy type 2A

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a recessive genetic disorder caused by mutations in the cysteine protease calpain 3 (CAPN3) that leads to selective muscle wasting. We previously showed that CAPN3 deficiency is associated with a profound perturbation of the NF-kappaB/IkappaB alpha survival pathway. In this study, the consequences of altered NF-kappaB/IkappaB alpha pathway were investigated using biological materials from LGMD2A patients. We first show that the antiapoptotic factor cellular-FLICE inhibitory protein (c-FLIP), which is dependent on the NF-kappaB pathway in normal muscle cells, is down-regulated in LGMD2A biopsies. In muscle cells isolated from LGMD2A patients, NF-kappaB is readily activated on cytokine induction as shown by an increase in its DNA binding activity. However, we observed discrepant transcriptional responses depending on the NF-kappaB target genes. IkappaB alpha is expressed following NF-kappaB activation independent of the CAPN3 status, whereas expression of c-FLIP is obtained only when CAPN3 is present. These data lead us to postulate that CAPN3 intervenes in the regulation of the expression of NF-kappaB-dependent survival genes to prevent apoptosis in skeletal muscle. Deregulations in the NF-kappaB pathway could be part of the mechanism responsible for the muscle wasting resulting from CAPN3 deficiency.

Increasing alpha 7 beta 1-integrin promotes muscle cell proliferation, adhesion, and resistance to apoptosis without changing gene expression

The dystrophin-glycoprotein complex maintains the integrity of skeletal muscle by associating laminin in the extracellular matrix with the actin cytoskeleton. Several human muscular dystrophies arise from defects in the components of this complex. The alpha(7)beta(1)-integrin also binds laminin and links the extracellular matrix with the cytoskeleton. Enhancement of alpha(7)-integrin levels alleviates pathology in mdx/utrn(-/-) mice, a model of Duchenne muscular dystrophy, and thus the integrin may functionally compensate for the absence of dystrophin. To test whether increasing alpha(7)-integrin levels affects transcription and cellular functions, we generated alpha(7)-integrin-inducible C2C12 cells and transgenic mice that overexpress the integrin in skeletal muscle. C2C12 myoblasts with elevated levels of integrin exhibited increased adhesion to laminin, faster proliferation when serum was limited, resistance to staurosporine-induced apoptosis, and normal differentiation. Transgenic expression of eightfold more integrin in skeletal muscle did not result in notable toxic effects in vivo. Moreover, high levels of alpha(7)-integrin in both myoblasts and in skeletal muscle did not disrupt global gene expression profiles. Thus increasing integrin levels can compensate for defects in the extracellular matrix and cytoskeleton linkage caused by compromises in the dystrophin-glycoprotein complex without triggering apparent overt negative side effects. These results support the use of integrin enhancement as a therapy for muscular dystrophy.

Pathological pattern of Mdx mice diaphragm correlates with gradual expression of the short utrophin isoform Up71

Utrophin gene is transcribed in a large mRNA of 13 kb that codes for a protein of 395 kDa. It shows amino acid identity with dystrophin of up to 73% and is widely expressed in muscle and non-muscle tissues. Up71 is a short utrophin product of the utrophin gene with the same cysteine-rich and C-terminal domains as full-length utrophin (Up395). Using RT-PCR, Western blots analysis, we demonstrated that Up71 is overexpressed in the mdx diaphragm, the most pathological muscle in dystrophin-deficient mdx mice, compared to wild-type C57BL/10 or other mdx skeletal muscles. Subsequently, we demonstrated that this isoform displayed an increased expression level up to 12 months, whereas full-length utrophin (Up395) decreased. In addition, beta-dystroglycan, the transmembrane glycoprotein that anchors the cytoplasmic C-terminal domain of utrophin, showed similar increase expression in mdx diaphragm, as opposed to other components of the dystrophin-associated protein complex (DAPC) such as alpha-dystrobrevin1 and alpha-sarcoglycan. We demonstrated that Up71 and beta-dystroglycan were progressively accumulated along the extrasynaptic region of regenerating clusters in mdx diaphragm. Our data provide novel functional insights into the pathological role of the Up71 isoform in dystrophinopathies.

A 1.3 kb promoter fragment confers spatial and temporal expression of utrophin A mRNA in mouse skeletal muscle fibers

Upregulation of utrophin in muscle is currently being examined as a potential therapy for Duchenne muscular dystrophy patients. In this context, we generated transgenic mice harboring a 1.3 kb human utrophin A promoter fragment driving expression of the lacZ gene. Characterization of reporter expression during postnatal muscle development revealed that the levels and localization of beta-galactosidase parallel expression of utrophin A transcripts. Moreover, we noted that the utrophin A promoter is more active in slow soleus muscles. Additionally, expression of the reporter gene was regulated during muscle regeneration in a manner similar to utrophin A transcripts. Together, these results show that the utrophin A promoter-lacZ construct mirrors expression of utrophin A mRNAs indicating that this utrophin A promoter fragment confers temporal and spatial patterns of expression in skeletal muscle. This transgenic mouse will be valuable as an in vivo model for developing and testing molecules aimed at increasing utrophin A expression.

Pharmacological strategies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal, genetic disorder whose relentless progression underscores the urgency for developing a cure. Although Duchenne initiated clinical trials roughly 150 years ago, therapies for DMD remain supportive rather than curative. A paradigm shift towards developing rational therapeutic strategies occurred with identification of the DMD gene. Gene- and cell-based therapies designed to replace the missing gene and/or dystrophin protein have achieved varying degrees of success. However, pharmacological strategies not designed to replace dystrophin per se appear promising, and can circumvent many hurdles hampering gene- and cell-based therapy. Here, we will review present pharmacological strategies, in particular those dealing with functional substitution of dystrophin by utrophin and enhancing muscle progenitor commitment by myostatin blockade, with a view toward facilitating drug discovery for DMD.

The utrophin gene is transcriptionally up-regulated in regenerating muscle

The utrophin gene codes for a large cytoskeletal protein closely related to dystrophin, the gene mutated in Duchenne's muscular dystrophy. Although utrophin could functionally substitute for dystrophin, in Duchenne's muscular dystrophy patients it did not compensate for the absence of dystrophin because in adult muscle utrophin was poorly expressed and limited to subsynaptic nuclei. However, increased levels of utrophin have been observed in regenerated muscles fibers suggesting that utrophin up-regulation in muscle is feasible. We observed that utrophin mRNA was transiently up-regulated at early time points after muscle injury with a peak already 24 h after muscle damage and utrophin induction in activated satellite cells before fusion into young regenerated fibers. Injection of utrophin lacZ constructs into regenerating muscle demonstrated that the utrophin upstream promoter under the control of its intronic enhancer activated the transcription that leads to the expression of the reporter gene in the newly formed fibers, which was not limited to neuromuscular junctions. Utrophin enhancer activity was dependent on an AP-1 site, and in satellite cells of regenerating muscle the AP-1 factors Fra1, Fra2, and JunD were strongly induced. These results establish that utrophin was induced in adult muscle independently from neuromuscular junctions and suggest that growth factors and cytokines that mediate the muscle repair up-regulate utrophin transcription.

Identification of altered gene expression in skeletal muscles from Duchenne muscular dystrophy patients

Mutations in the dystrophin gene lead to dystrophin deficiency, which is the cause of Duchenne muscular dystrophy (DMD). This important discovery more than 10 years ago opened a new field for very productive investigations. However, the exact functions of dystrophin are still not fully understood and the complex process leading to subsequent muscle fiber necrosis has not been clearly described; hence there has not yet been any marked improvement in patient treatment. To decipher the molecular mechanisms induced by a lack of dystrophin, we started identifying genes whose expression is altered in DMD skeletal muscles. The approach was based on differential screening of a human muscle cDNA array. Nine genes were found to be up- or downregulated. Our results indicate expression alterations in mitochondrial genes, titin, a muscle transcription factor and three novel genes. First characterizations of these novel genes indicated that two of them have striated muscle tissue specificity.

The artificial zinc finger coding gene 'Jazz' binds the utrophin promoter and activates transcription

Up-regulation of utrophin gene expression is recognized as a plausible therapeutic approach in the treatment of Duchenne muscular dystrophy (DMD). We have designed and engineered new zinc finger-based transcription factors capable of binding and activating transcription from the promoter of the dystrophin-related gene, utrophin. Using the recognition 'code' that proposes specific rules between zinc finger primary structure and potential DNA binding sites, we engineered a new gene named 'Jazz' that encodes for a three-zinc finger peptide. Jazz belongs to the Cys2-His2 zinc finger type and was engineered to target the nine base pair DNA sequence: 5'-GCT-GCT-GCG-3', present in the promoter region of both the human and mouse utrophin gene. The entire zinc finger alpha-helix region, containing the amino acid positions that are crucial for DNA binding, was specifically chosen on the basis of the contacts more frequently represented in the available list of the 'code'. Here we demonstrate that Jazz protein binds specifically to the double-stranded DNA target, with a dissociation constant of about 32 nM. Band shift and super-shift experiments confirmed the high affinity and specificity of Jazz protein for its DNA target. Moreover, we show that chimeric proteins, named Gal4-Jazz and Sp1-Jazz, are able to drive the transcription of a test gene from the human utrophin promoter.

Altered membrane proteins and permeability correlate with cardiac dysfunction in cardiomyopathic hamsters

A mutation in the delta-sarcoglycan (SG) gene with absence of delta-SG protein in the heart has been identified in the BIO14.6 cardiomyopathic (CM) hamster, but how the defective gene leads to myocardial degeneration and dysfunction is unknown. We correlated left ventricular (LV) function with increased sarcolemmal membrane permeability and investigated the LV distribution of the dystrophin-dystroglycan complex in BIO14.6 CM hamsters. On echocardiography at 5 wk of age, the CM hamsters showed a mildly enlarged diastolic dimension (LVDD) with decreased LV percent fractional shortening (%FS), and at 9 wk further enlargement of LVDD with reduction of %FS was observed. The percent area of myocardium exhibiting increased membrane permeability or membrane rupture, assessed by Evans blue dye (EBD) staining and wheat germ agglutinin, was greater at 9 than at 5 wk. In areas not stained by EBD, immunostaining of dystrophin was detected in CM hamsters at sarcolemma and T tubules, as expected, but it was also abnormally expressed at the intercalated discs; in addition, the expression of beta-dystroglycan was significantly reduced compared with control hearts. As previously described, alpha-SG was completely deficient in CM hearts compared with control hearts. In myocardial areas showing increased sarcolemmal permeability, neither dystrophin nor beta-dystroglycan could be identified by immunolabeling. Thus, together with the known loss of delta-SG and other SGs, abnormal distribution of dystrophin and reduction of beta-dystroglycan are associated with increased sarcolemmal permeability followed by cell rupture, which correlates with early progressive cardiac dysfunction in the BIO14.6 CM hamster.

Utrophin may be a precursor of dystrophin during skeletal muscle development

Expression patterns of utrophin were investigated and compared to those of dystrophin and associated proteins in skeletal muscle of rat embryos from E12 to E21 by immunohistochemistry. Utrophin was readily detected from E12 on, earlier than full-length dystrophin on E14. A shorter dystrophin isoform was observed from E12 to E16. The level of utrophin reached a maximum on E16-17 and then declined while that of dystrophin increased after E17. A complementary distribution of these two molecules was observed on E18. Beta-dystroglycan appeared as early as utrophin. Sarcoglycans, appearing from E14 on, were anchored first by utrophin and then by dystrophin. These results elucidate the chronological order of expression of the dytrophin/utrophin protein complex and indicate that this protein complex is originally stabilized by utrophin. This study supports our hypothesis that utrophin might be a developmental precursor of dystrophin.

Quantitative analysis of relative protein contents by Western blotting: comparison of three members of the dystrophin-glycoprotein complex in slow and fast rat skeletal muscle

We have developed a method for accurate quantitative analysis and statistical comparison of the relative contents of the dystrophin-glycoprotein complex (DGC) in skeletal muscle. This method was applied to compare DGC contents in slow (soleus) and in fast (extensor digitorum longus, EDL) rat skeletal muscles. The quantitative analysis combines a modified bicinchoninic acid (BCA) assay with Western blotting and enhanced chemiluminescence (ECL). This combination allows the use of high levels of detergents and reducing reagents essential for extracting DGC. In addition, the evaluation of the total amount of proteins in each sample makes it possible to have a reference and to accurately compare relative protein levels without using a specific standard. With a large gradient gel, we could concomitantly compare two groups (n = 9) and quantify all protein contents differing highly in their molecular masses (from 35 kDa to 427 kDa). Each experiment was triplicated and normalized; the two muscles were compared using the Mann-Whitney test (P<0.001) to establish their protein content. The DGC relative levels for the slow muscle soleus and the fast muscle EDL differed significantly: dystrophin, beta-dystroglycan, and gamma-sarcoglycan levels were 130%, 110% and 120% higher in the soleus, respectively. The differences observed in the expression level of cytoskeletal associated protein (dystrophin) and transmembranous anchorage components may correspond to a physiological response of the muscle fibers to duration, magnitude, and frequency of the imposed mechanical loading.

Dystrophin and utrophin complexed with different associated proteins in cardiac Purkinje fibres

Abnormal dystrophin expression is directly responsible for Duchenne and Becker muscular dystrophies. In skeletal muscle, dystrophin provides a link between the actin network and the extracellular matrix via the dystrophin-associated protein complex. In mature skeletal muscle, utrophin is a dystrophin-related protein localized mainly at the neuromuscular junction, with the same properties as dystrophin in terms of linking the protein complex. Utrophin could potentially overcome the absence of dystrophin in dystrophic skeletal muscles. In cardiac muscle, dystrophin and utrophin were both found to be present with a distinct subcellular distribution in Purkinje fibres, i.e. utrophin was limited to the cytoplasm, while dystrophin was located in the cytoplasmic membrane. In this study, we used this particular characteristic of cardiac Purkinje fibres and demonstrated that associated proteins of dystrophin and utrophin are different in this structure. We conclude, contrary to skeletal muscle, dystrophin-associated proteins do not form a complex in Purkinje fibres. In addition, we have indirect evidence of the presence of two different 400 kDa dystrophins in Purkinje fibres.

Utrophin and dystrophin-associated glycoproteins in normal and dystrophin deficient cardiac muscle

In this study, various members of the dystrophin family (dystrophin, the short dystrophin product Dp 71, utrophin and DRP2), and different members of the dystrophin-associated glycoprotein (DAG) complex (beta-dystroglycan, alpha-, beta-, gamma- and delta-sarcoglycans) were localized in bovine cardiac muscle using a battery of specific antibodies. We have established that dystrophin is exclusively associated with beta-dystroglycan and both alpha- and delta-sarcoglycans in cardiac muscle cell membranes. In contrast, utrophin is a specific component of intercalated disks together with beta- and gamma-sarcoglycans, while beta-dystroglycan, alpha- and delta-sarcoglycans are not present. Dp 71 is mainly localized at the T tubule transverse area. In dystrophin deficient cardiac muscle, utrophin and beta-sarcoglycan were observed in intercalated disks and at the sarcolemma of each cardiocyte. Our results revealed that complexes of associated glycoproteins differ in cardiac muscle when associated with dystrophin or utrophin. Despite the described sequence homologies between dystrophin and utrophin, the present results indicate that these proteins have different roles in some specific cardiac cell areas.

Exclusion of muscle specific actinin-associated LIM protein (ALP) gene from 4q35 facioscapulohumeral muscular dystrophy (FSHD) candidate genes

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder for which no candidate gene has yet been identified. The gene corresponding to one of the novel human cDNAs that we cloned on the basis of a muscle restricted expression pattern [Piétu G, Alibert O, Guichard B, et al. Genome Res 1996;6:492-503] was mapped in the region of the FSHD1A genetic locus, i.e. one of the loci involved in this muscular dystrophy. The corresponding encoded protein contains a PDZ and a LIM domain, two protein-protein interaction domains, and was very recently shown to bind alpha-actinin-2 and was named ALP (actinin-associated LIM protein) [Xia H, Winokur S, Kuo W, Altherr M, Bredt D. J Cell Biol 1997;139:507-515]. We raised a specific polyclonal anti-ALP serum against an ALP recombinant polypeptide to evaluate the size, level of expression and subcellular localization of ALP in three patients, clearly diagnosed with FSHD disease. Quantitative or qualitative alterations of ALP expression have not been detected in any of them, thus prompting us to exclude ALP as a FSHD gene candidate.

Characterization of monoclonal antibodies to calpain 3 and protein expression in muscle from patients with limb-girdle muscular dystrophy type 2A

Monoclonal antibodies were raised to two regions of calpain 3 (muscle-specific calcium-activated neutral protease), which is the product of the gene that is defective in limb-girdle muscular dystrophy type 2A. The antibodies produced characteristic patterns of bands on Western blots: normal calpain 3 protein was represented by bands at 94 kd, plus additional fragments at approximately 60 or 30 kd, according to the antibody used. Specificity was confirmed by the loss of all bands in patients with null gene mutations. The "normal" profile of bands was observed in muscle from 33 control subjects and 70 disease-control patients. Calpain 3 protein was found to be extremely stable in fresh human muscle, with full-size protein being detected 8 hours after the muscle had been removed. Blots of muscle from nine limb-girdle muscular dystrophy type 2A patients with defined mutations showed variation in protein expression, with seven showing a clear reduction in the abundance of protein detected. No simple relationship was found between the abundance and clinical severity. Two patients showed normal expression of the full-size 94 kd band accompanied by a clear reduction in the smaller fragments. This pattern was also observed in one patient with an undefined form of limb-girdle dystrophy. These results indicate that immunodiagnosis is feasible, but caution will need to be exercised with the interpretation of near-normal protein profiles.

Mosaic expression of two dystrophins in a boy with progressive muscular dystrophy

A boy with a Becker muscular dystrophy (BMD) phenotype presented unique muscular dystrophin expression. Western blot analysis showed the presence of two dystrophins of different sizes, i.e., a 400-kDa dystrophin and a 500-kDa form. An immunofluorescent study revealed mosaic expression of these dystrophins in the sarcolemma, with matching alpha-sarcoglycan and beta-dystroglycan staining patterns. DNA and RNA analysis did not reveal any mutation in the dystrophin gene, and the karyotype was normal.

Different utrophin and dystrophin properties related to their vascular smooth muscle distributions

Monoclonal antibodies used to distinguish between dystrophin and utrophin were systematically applied to skeletal muscles containing arteries and veins. Small arteries were found to contain long forms of both utrophin and dystrophin, while small veins contained only long forms of utrophin. In addition, all sizes of vascular smooth muscles were demonstrated to contain another related Mr 80 kDa protein (possibly a short utrophin transcript). Regardless of their tissue distributions, we assumed that each of these molecules had distinct properties, i.e. dystrophin with a mechanical function and utrophin with an architectural function. This difference in the roles of dystrophin and utrophin could reduce the efficiency of protection against muscle membrane degeneration when utrophin overexpression is programmed.