Dystrophin and a dystrophin-related protein in intrafusal muscle fibers, and neuromuscular and myotendinous junctions

Sarcospan increases laminin-binding capacity of α-dystroglycan to ameliorate DMD independent of Galgt2

In Duchenne muscular dystrophy (DMD), mutations in dystrophin result in a loss of the dystrophin-glycoprotein complex (DGC) at the myofiber membrane, which functions to connect the extracellular matrix with the intracellular actin cytoskeleton. The dystroglycan subcomplex interacts with dystrophin and spans the sarcolemma where its extensive carbohydrates (matriglycan and CT2 glycan) directly interact with the extracellular matrix. In the current manuscript, we show that sarcospan overexpression enhances the laminin-binding capacity of dystroglycan in DMD muscle by increasing matriglycan glycosylation of α-dystroglycan. Furthermore, we find that this modification is not affected by loss of Galgt2, a glycotransferase, which catalyzes the CT2 glycan. Our findings reveal that the matriglycan carbohydrates, and not the CT2 glycan, are necessary for sarcospan-mediated amelioration of DMD. Overexpression of Galgt2 in the DMD mdx murine model prevents muscle pathology by increasing CT2 modified α-dystroglycan. Galgt2 also increases expression of utrophin, which compensates for the loss of dystrophin in DMD muscle. We found that combined loss of Galgt2 and dystrophin reduced utrophin expression; however, it did not interfere with sarcospan rescue of disease. These data reveal a partial dependence of sarcospan on Galgt2 for utrophin upregulation. In addition, sarcospan alters the cross-talk between the adhesion complexes by decreasing the association of integrin β1D with dystroglycan complexes. In conclusion, sarcospan functions to re-wire the cell to matrix connections by strengthening the cellular adhesion and signaling, which, in turn, increases the resilience of the myofiber membrane.

In the human, true myocutaneous junctions of skeletal muscle fibers are limited to the face

Within the panniculus carnosus‐associated skeletal muscles in the human, the palmaris brevis and the platysma showed myotendinous/myofascial junctions with clear distance to the corium and the specific connection collagen type XXII. The orbicularis oris muscle, in contrast, contained bundles of striated muscle fibers reaching the corium at two distinct levels: the predominant inner ending was connected to the elastic network of the inner corium and the outer ending was within the more superficial collagen network. At both locations, the striated muscle fibers showed brush‐like cytoplasmic protrusions connecting a network which was not oriented toward the muscle fibers. Collagen type XXII was not present.

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.

In vivo dynamics of skeletal muscle Dystrophin in zebrafish embryos revealed by improved FRAP analysis

Dystrophin forms an essential link between sarcolemma and cytoskeleton, perturbation of which causes muscular dystrophy. We analysed Dystrophin binding dynamics in vivo for the first time. Within maturing fibres of host zebrafish embryos, our analysis reveals a pool of diffusible Dystrophin and complexes bound at the fibre membrane. Combining modelling, an improved FRAP methodology and direct semi-quantitative analysis of bleaching suggests the existence of two membrane-bound Dystrophin populations with widely differing bound lifetimes: a stable, tightly bound pool, and a dynamic bound pool with high turnover rate that exchanges with the cytoplasmic pool. The three populations were found consistently in human and zebrafish Dystrophins overexpressed in wild-type or dmd(ta222a/ta222a) zebrafish embryos, which lack Dystrophin, and in Gt(dmd-Citrine)(ct90a) that express endogenously-driven tagged zebrafish Dystrophin. These results lead to a new model for Dystrophin membrane association in developing muscle, and highlight our methodology as a valuable strategy for in vivo analysis of complex protein dynamics.

Hanging on for the ride: adhesion to the extracellular matrix mediates cellular responses in skeletal muscle morphogenesis and disease

Skeletal muscle specification and morphogenesis during early development are critical for normal physiology. In addition to mediating locomotion, skeletal muscle is a secretory organ that contributes to metabolic homeostasis. Muscle is a highly adaptable tissue, as evidenced by the ability to increase muscle cell size and/or number in response to weight bearing exercise. Conversely, muscle wasting can occur during aging (sarcopenia), cancer (cancer cachexia), extended hospital stays (disuse atrophy), and in many genetic diseases collectively known as the muscular dystrophies and myopathies. It is therefore of great interest to understand the cellular and molecular mechanisms that mediate skeletal muscle development and adaptation. Muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction. This process requires carefully orchestrated interactions between cells and their extracellular matrix microenvironment. These interactions are dynamic, allowing muscle cells to sense biophysical, structural, organizational, and/or signaling changes within their microenvironment and respond appropriately. In many musculoskeletal diseases, these cell adhesion interactions are disrupted to such a degree that normal cellular adaptive responses are not sufficient to compensate for accumulating damage. Thus, one major focus of current research is to identify the cell adhesion mechanisms that drive muscle morphogenesis, with the hope that understanding how muscle cell adhesion promotes the intrinsic adaptability of muscle tissue during development may provide insight into potential therapeutic approaches for muscle diseases. Our objectives in this review are to highlight recent studies suggesting conserved roles for cell-extracellular matrix adhesion in vertebrate muscle morphogenesis and cellular adaptive responses in animal models of muscle diseases.

Biochemical and morphological alterations in the Achilles tendon of mdx mice

Dystrophin-deficient muscles have repeated cycles of necrosis and regeneration, being susceptible to injury induced by muscle contractions. Some studies have demonstrated that tendons are also affected in mdx mice, based especially on the changes in biomechanical properties arising from the respective linked muscles. However, most studies have focused only on alterations in the myotendinous junction. Thus, the purpose of this work was to study biochemical and morphological alterations in the Achilles tendons of 60-day-old mdx mice. Hydroxyproline quantification, showed higher collagen concentration in the mdx mice as compared with the control. No difference between the tendons of both groups was found in the noncollagenous proteins dosage, and in the amount of collagen type III detected in the western blotting analysis. The zymography for gelatinases detection showed higher amounts of metaloproteinase-2 (active isoform) and of metalloproteinase-9 (latent isoform) in the mdx mice. Measurements of birefringence, using polarization microscopy, showed higher molecular organization of the collagen fibers in the tendons of mdx mice in comparison to the control group, with presence of larger areas of crimp. Ponceau SS-stained tendon sections showed stronger staining of the extracellular matrix in the mdx groups. Toluidine blue-stained sections showed more intense basophilia in tendons of the control group. In morphometry, a higher number of inflammatory cells was detected in the epitendon of mdx group. In conclusion, the Achilles tendon of 60-day-old mdx mice presents higher collagen concentration and organization of the collagen fibers, enhanced metalloproteinase-2 activity, as well as prominent presence of inflammatory cells and lesser proteoglycans.

The potential of sarcospan in adhesion complex replacement therapeutics for the treatment of muscular dystrophy

Three adhesion complexes span the sarcolemma and facilitate critical connections between the extracellular matrix and the actin cytoskeleton: the dystrophin- and utrophin-glycoprotein complexes and α7β1 integrin. Loss of individual protein components results in a loss of the entire protein complex and muscular dystrophy. Muscular dystrophy is a progressive, lethal wasting disease characterized by repetitive cycles of myofiber degeneration and regeneration. Protein-replacement therapy offers a promising approach for the treatment of muscular dystrophy. Recently, we demonstrated that sarcospan facilitates protein-protein interactions amongst the adhesion complexes and is an important potential therapeutic target. Here, we review current protein-replacement strategies, discuss the potential benefits of sarcospan expression, and identify important experiments that must be addressed for sarcospan to move to the clinic.

Sarcospan: a small protein with large potential for Duchenne muscular dystrophy

Purification of the proteins associated with dystrophin, the gene product responsible for Duchenne muscular dystrophy, led to the discovery of the dystrophin-glycoprotein complex. Sarcospan, a 25-kDa transmembrane protein, was the last component to be identified and its function in skeletal muscle has been elusive. This review will focus on progress over the last decade revealing that sarcospan is an important regulator of muscle cell adhesion, strength, and regeneration. Investigations using several transgenic mouse models demonstrate that overexpression of sarcospan in the mouse model for Duchenne muscular dystrophy ameliorates pathology and restores muscle cell binding to laminin. Sarcospan improves cell surface expression of the dystrophin- and utrophin-glycoprotein complexes as well as α7β1 integrin, which are the three major laminin-binding complexes in muscle. Utrophin and α7β1 integrin compensate for the loss of dystrophin and the finding that sarcospan increases their abundance at the extra-synaptic sarcolemma supports the use of sarcospan as a therapeutic target. Newly discovered phenotypes in sarcospan-deficient mice, including a reduction in specific force output and increased drop in force in the diaphragm muscle, result from decreased utrophin and dystrophin expression and further reveal sarcospan’s role in determining abundance of these complexes. Dystrophin protein levels and the specific force output of the diaphragm muscle are further reduced upon genetic removal of α7 integrin (Itga7) in SSPN-deficient mice, demonstrating that interactions between integrin and sarcospan are critical for maintenance of the dystrophin-glycoprotein complex and force production of the diaphragm muscle. Sarcospan is a major regulator of Akt signaling pathways and sarcospan-deficiency significantly impairs muscle regeneration, a process that is dependent on Akt activation. Intriguingly, sarcospan regulates glycosylation of a specific subpopulation of α-dystroglycan, the laminin-binding receptor associated with dystrophin and utrophin, localized to the neuromuscular junction. Understanding the basic mechanisms responsible for assembly and trafficking of the dystrophin- and utrophin-glycoprotein complexes to the cell surface is lacking and recent studies suggest that sarcospan plays a role in these essential processes.

Sarcospan-dependent Akt activation is required for utrophin expression and muscle regeneration

Utrophin is normally confined to the neuromuscular junction (NMJ) in adult muscle and partially compensates for the loss of dystrophin in mdx mice. We show that Akt signaling and utrophin levels were diminished in sarcospan (SSPN)-deficient muscle. By creating several transgenic and knockout mice, we demonstrate that SSPN regulates Akt signaling to control utrophin expression. SSPN determined α-dystroglycan (α-DG) glycosylation by affecting levels of the NMJ-specific glycosyltransferase Galgt2. After cardiotoxin (CTX) injury, regenerating myofibers express utrophin and Galgt2-modified α-DG around the sarcolemma. SSPN-null mice displayed delayed differentiation after CTX injury caused by loss of utrophin and Akt signaling. Treatment of SSPN-null mice with viral Akt increased utrophin and restored muscle repair after injury, revealing an important role for the SSPN-Akt-utrophin signaling axis in regeneration. SSPN improved cell surface expression of utrophin by increasing transportation of utrophin and DG from endoplasmic reticulum/Golgi membranes. Our experiments reveal functions of utrophin in regeneration and new pathways that regulate utrophin expression at the cell surface.

The value of mammalian models for duchenne muscular dystrophy in developing therapeutic strategies

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy. There is no effective treatment and patients typically die in approximately the third decade. DMD is an X-linked recessive disease caused by mutations in the dystrophin gene. There are three mammalian models of DMD that have been used to understand better the pathogenesis of disease and develop therapeutic strategies. The mdx mouse is the most widely used model of DMD that displays some features of muscle degeneration, but the pathogenesis of disease is comparatively mild. The severity of disease in mice lacking both dystrophin and utrophin is similar to DMD, but one has to account for the discrete functions of utrophin. Canine X-linked muscular dystrophy (cxmd) is the best representation of DMD, but the phenotype of the most widely used golden retriever (GRMD) model is variable, making functional endpoints difficult to ascertain. Although each mammalian model has its limitations, together they have been essential for the development of several treatment strategies for DMD that target dystrophin replacement, disease progression, and muscle regeneration.

Multiple regulatory events controlling the expression and localization of utrophin in skeletal muscle fibers: insights into a therapeutic strategy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most prevalent inherited muscle disease and results from mutations/deletions in the X-linked dystrophin gene. Although several approaches have been envisaged to counteract the effects of this progressive disease, there is currently no cure available. One strategy consists in utilizing a protein normally expressed in DMD muscle which, once expressed at appropriate levels and at the correct subcellular location, could compensate for the lack of dystrophin. A candidate for such a role is the dystrophin-related protein now referred to as utrophin. In contrast to dystrophin, which is expressed along the length of healthy muscle fibers, utrophin accumulates at the neuromuscular junction in both normal and DMD fibers. Several years ago, we began a series of experiments to determine the mechanisms responsible for the expression of utrophin at the neuromuscular synapse. Initially, we showed that utrophin transcripts accumulate preferentially within the postsynaptic sarcoplasm. To determine whether selective transcription of the utrophin gene accounts for this synaptic accumulation of utrophin mRNAs, we injected several utrophin promoter-reporter constructs directly into mouse muscle and demonstrated the preferential synaptic expression of the reporter gene. These results suggested that local transcriptional activation of the utrophin gene is responsible for the accumulation of utrophin mRNAs at the neuromuscular junction. In these studies, we also demonstrated that an N-box motif contained within the utrophin promoter plays a critical role in directing the synapse-specific expression of the utrophin gene. Additionally, our studies have shown that the ets-factors GABP alpha and beta are part of a protein complex that can bind to the N-box motif to transactivate the gene in muscle cells in culture and in vivo. In these experiments, we also noted that the nerve-derived trophic factors agrin and ARIA/heregulin regulate expression of utrophin via the activation of GABP alpha and beta which in turn, transactivate the utrophin gene via the N-box motif. Although these studies demonstrate that transcriptional activation can regulate utrophin mRNA levels, it is possible that additional mechanisms are also involved. In particular, the association of mRNAs with cytoskeletal elements and RNA-binding proteins may contribute to the accumulation of utrophin transcripts within the postsynaptic sarcoplasm. In recent studies, we have begun to examine this and we have now identified specific regions within the 3' untranslated region that are necessary for targeting and stabilizing utrophin mRNAs in skeletal muscle cells. A series of in vivo studies have also led us to conclude that post-transcriptional mechanisms are indeed important in regulating the abundance of utrophin transcripts in muscle. Together, these studies should lead to the identification of cis- and trans-acting elements regulating transcription of the utrophin gene as well as the stability and targeting of its mRNA in muscle cells. The results should therefore, identify specific targets that may become important in designing specific pharmacological interventions directed at increasing the expression of utrophin into extrasynaptic regions of DMD muscle fibers. In addition, these findings will contribute to our basic understanding of the cellular and molecular events involved in the formation, maintenance and plasticity of the neuromuscular synapse.

Application of passive stretch and its implications for muscle fibers

To increase range of motion, physical therapists frequently use passive stretch as a means of gaining increased excursion around a joint. In addition to clinical studies showing effectiveness, thereby supporting evidence-based practice, the basic sciences can provide an explanation how a technique might work once a technique is known to be effective. The goal of this article is to review the potential cellular events that may occur when muscle fibers are stretched passively. A biomechanical example of passive stretch applied to the ankle is used to provide a means to discuss passive stretch at the cellular and molecular levels. The implications of passive stretch on muscle fibers and the related connective tissue are discussed with respect to tissue biomechanics. Emphasis is placed on structures that are potentially involved in the sensing and signal transduction of stretch, and the mechanisms that may result in myofibrillogenesis are explored.

Regulation and functional significance of utrophin expression at the mammalian neuromuscular synapse

Duchenne muscular dystrophy (DMD) is caused by the absence of full-length dystrophin molecules in skeletal muscle fibers. In normal muscle, dystrophin is found along the length of the sarcolemma where it links the intracellular actin cytoskeleton to the extracellular matrix, via the dystrophin-associated protein (DAP) complex. Several years ago, an autosomal homologue to dystrophin, termed utrophin, was identified and shown to be expressed in a variety of tissues, including skeletal muscle. However, in contrast to the localization of dystrophin in extrajunctional regions of muscle fibers, utrophin preferentially accumulates at the postsynaptic membrane of the neuromuscular junction in both normal and DMD adult muscle fibers. Since it has recently been suggested that the upregulation of utrophin might functionally compensate for the lack of dystrophin in DMD, considerable interest is now directed toward the elucidation of the various regulatory mechanisms presiding over expression of utrophin in normal and dystrophic skeletal muscle fibers. In this review, we discuss some of the most recent data relevant to our understanding of the impact of myogenic differentiation and innervation on the expression and localization of utrophin in skeletal muscle fibers.

Molecular mechanisms and putative signalling events controlling utrophin expression in mammalian skeletal muscle fibres

The absence of full-length dystrophin molecules in skeletal muscle fibres results in the most severe form of muscular dystrophy, the Duchenne form (DMD). Several years ago, an autosomal homologue to dystrophin, termed utrophin, was identified. Although utrophin is expressed along the sarcolemma in developing, regenerating and DMD muscles, it nonetheless accumulates at the postsynaptic membrane of the neuromuscular junction in both normal and DMD adult muscle fibres. Due to the high degree of sequence identity between dystrophin and utrophin, it has been previously suggested that utrophin could in fact functionally compensate for the lack of dystrophin. Recent studies using transgenic mouse model systems have directly tested this hypothesis and revealed that upregulation of utrophin throughout dystrophic muscle fibres represents indeed, a viable approach for the treatment of DMD. Current studies are therefore focusing on the elucidation of the various regulatory mechanisms presiding over expression of utrophin in muscle fibres in attempts to ultimately identify small molecules which could systematically increase utrophin levels in extrasynaptic compartments of dystrophic muscle fibres. This review presents some of the recent data relevant for our understanding of the transcriptional regulatory mechanisms involved in maintaining expression of utrophin at the neuromuscular junction. In addition, the contribution of specific cues originating from motoneurons and the putative involvement of signalling events are also discussed.

Extracellular matrix and transmembrane linkages at the termination of intrafusal fibers and the outer capsule in chicken muscle spindles

Attachments of intrafusal fibers and of the outer spindle capsule at the far polar region were examined by immunohistochemistry in serially sectioned chicken leg muscles. Patterns of distribution of connective tissues and intracellular filaments suggest that, in this segment of the muscle spindle, intrafusal fibers bind laterally with the capsule. Contrary to extrafusal fibers at myotendinous junctions, folded plasmalemmas at the ends of intrafusal fibers were rare. Thus, there was little end-to-end interlocking between intrafusal fibers and the extracellular matrix. The tapered contours of terminating intrafusal fibers resembled those of extrafusal fibers which end in fascicles without tendinous connections. At points where the distal portions of intrafusal fibers closely adjoined and overlapped extrafusal fibers, alpha-actinin, vinculin, filamin, talin, beta 1 integrin, spectrin, and dystrophin occurred with moderate to great frequency. It is generally accepted that these compounds are links in molecular chains that extend from the intracellular space across cell membranes to the extracellular matrix. Their location along substantial lengths of extrafusal fibers, distal capsule, and terminating intrafusal fibers suggests the presence of numerous transverse connections between elements of the terminal portion of the spindle and nonspindle tissues. Hence, it is likely that forces monitored by chicken spindles in muscles undergoing length changes are transferred from extrafusal fibers and extracellular matrix to the receptors in large part via lateral shear instead of by longitudinal tension.

Expression of utrophin and its mRNA in denervated mdx mouse muscle

Utrophin is a large cytoskeletal protein which shows high homology to dystrophin. In contrast to the sarcolemmal distribution of dystrophin, utrophin accumulates at the postsynaptic membrane of the neuromuscular junction. Because of its localization within this compartment of muscle fibers, expression of utrophin may be significantly influenced by the presence of the motor nerve. We tested this hypothesis by denervating muscles of mdx mouse and monitoring levels of utrophin and its mRNA by immunofluorescence, immunoblotting and RT-PCR. A significant increase in the number of utrophin positive fibers was observed by immunofluorescence 3 to 21 days after sectioning of the sciatic nerve. Quantitative analyses of utrophin and its transcripts in hindlimb muscles denervated for two weeks showed only a moderate increase in the levels of both utrophin (approximately 2-fold) and its transcript (approximately 60 to 90%). The present data suggest that although utrophin is a component of the postsynaptic membrane, its neural regulation is distinct from that of the acetylcholine receptor.

Immunohistochemical study of calpain and its endogenous inhibitor in the skeletal muscle of muscular dystrophy

A calcium-dependent proteinase (calpain) has been suggested to play an important role in muscle degradation in Duchenne muscular dystrophy (DMD). In immunohistochemical studies, calpain and its endogenous inhibitor (calpastatin) were located exclusively in the cytoplasm in normal human muscles. The intensity of the staining was stronger in type 1 than in type 2 fibers. Quantitative immunohistochemical study showed an increase of calpain in biopsied muscles from the patients with DMD and Becker muscular dystrophy. Abnormal increases in calpain and calpastatin were demonstrated mainly in atrophic fibers, whereas necrotic fibers showed moderate or weak immunoreactions for the enzymes. Opaque fibers and hypertrophic fibers were negative. Not all dystrophin-deficient muscle fibers necessarily showed a strong reaction for calpain. We suggest that calpain may play an important role in muscle fiber degradation, especially in the early stage of muscle degradation in muscular dystrophy.

Interaction of chromosome-6-encoded dystrophin related protein with the extracellular matrix

Dystrophin-related protein/utrophin is a large, cytoskeletal protein that shares significant sequence similarity with dystrophin. Dystrophin-related protein is known to be enriched where cell-extracellular matrix contacts are well defined; however, the mechanism of dystrophin-related protein enrichment and its functional role(s) at these sites are yet to be defined. Here, we demonstrate that dystrophin-related protein is concentrated in patches of astrocyte membrane in apposition with the extracellular matrix and that the distribution of dystrophin-related protein is temporally modulated by the extracellular matrix constituent laminin. Furthermore, we demonstrate the existence of a specific biochemical association between dystrophin-related protein and laminin in astrocytes. In these astrocytes, the depletion of dystrophin-related protein by the use of antisense dystrophin-related protein oligonucleotides causes marked reduction in the formation of functional substratum-membrane attachments. Taken together, these data suggest that dystrophin-related protein may function in the generation and maintenance of regional substratum-associated membrane specializations, such as those found at the blood-brain barrier.

The structural and functional diversity of dystrophin

Duchenne and Becker muscular dystrophies are caused by defects of the dystrophin gene. Expression of this large X-linked gene is under elaborate transcriptional and splicing control. At least five independent promoters specify the transcription of their respective alternative first exons in a cell-specific and developmentally controlled manner. Three promoters express full-length dystrophin, while two promoters near the C terminus express the last domains in a mutually exclusive manner. Six exons of the C terminus are alternatively spliced, giving rise to several alternative forms. Genetic, biochemical and anatomical studies of dystrophin suggest that a number of distinct functions are subserved by its great structural diversity. Extensive studies of dystrophin may lead to an understanding of the cause and perhaps a rational treatment for muscular dystrophy.