Molecular and genetic characterization of sarcospan: insights into sarcoglycan-sarcospan interactions

Targeted next‐generation sequencing determined a novel SGCG variant that is associated with limb‐girdle muscular dystrophy type 2C: A case report

Limb‐girdle muscular dystrophy‐type 2C (LGMD2C) is caused by mutations in the SGCG gene. Here, we report a case of a 26‐year‐old male who had inactive walking due to proximal muscle weakness. Targeted next‐generation sequencing found a novel variant c.412C > T (Q138*) in the SGCG gene.

Multi-omics analysis of sarcospan overexpression in mdx skeletal muscle reveals compensatory remodeling of cytoskeleton-matrix interactions that promote mechanotransduction pathways

BACKGROUND

The dystrophin-glycoprotein complex (DGC) is a critical adhesion complex of the muscle cell membrane, providing a mechanical link between the extracellular matrix (ECM) and the cortical cytoskeleton that stabilizes the sarcolemma during repeated muscle contractions. One integral component of the DGC is the transmembrane protein, sarcospan (SSPN). Overexpression of SSPN in the skeletal muscle of mdx mice (murine model of DMD) restores muscle fiber attachment to the ECM in part through an associated increase in utrophin and integrin adhesion complexes at the cell membrane, protecting the muscle from contraction-induced injury. In this study, we utilized transcriptomic and ECM protein-optimized proteomics data sets from wild-type, mdx, and mdx transgenic (mdx^TG) skeletal muscle tissues to identify pathways and proteins driving the compensatory action of SSPN overexpression.

METHODS

The tibialis anterior and quadriceps muscles were isolated from wild-type, mdx, and mdx^TG mice and subjected to bulk RNA-Seq and global proteomics analysis using methods to enhance capture of ECM proteins. Data sets were further analyzed through the ingenuity pathway analysis (QIAGEN) and integrative gene set enrichment to identify candidate networks, signaling pathways, and upstream regulators.

RESULTS

Through our multi-omics approach, we identified 3 classes of differentially expressed genes and proteins in mdx^TG muscle, including those that were (1) unrestored (significantly different from wild type, but not from mdx), (2) restored (significantly different from mdx, but not from wild type), and (3) compensatory (significantly different from both wild type and mdx). We identified signaling pathways that may contribute to the rescue phenotype, most notably cytoskeleton and ECM organization pathways. ECM-optimized proteomics revealed an increased abundance of collagens II, V, and XI, along with β-spectrin in mdx^TG samples. Using ingenuity pathway analysis, we identified upstream regulators that are computationally predicted to drive compensatory changes, revealing a possible mechanism of SSPN rescue through a rewiring of cell-ECM bidirectional communication. We found that SSPN overexpression results in upregulation of key signaling molecules associated with regulation of cytoskeleton organization and mechanotransduction, including Yap1, Sox9, Rho, RAC, and Wnt.

CONCLUSIONS

Our findings indicate that SSPN overexpression rescues dystrophin deficiency partially through mechanotransduction signaling cascades mediated through components of the ECM and the cortical cytoskeleton.

High-Throughput Screening to Identify Modulators of Sarcospan

High-throughput screening enables the discovery of disease-modifying small molecules. Here, we describe the development of a scalable, cell-based assay to screen for small molecules that modulate sarcospan for the treatment of Duchenne muscular dystrophy. We detail the hit validation pipeline, which includes secondary screening, gene/protein quantification, and an in vitro membrane stability assay.

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.

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.

Loss of sarcospan exacerbates pathology in mdx mice, but does not affect utrophin amelioration of disease

The dystrophin-glycoprotein complex (DGC) is a membrane adhesion complex that provides structural stability at the sarcolemma by linking the myocyte's internal cytoskeleton and external extracellular matrix. In Duchenne muscular dystrophy (DMD), the absence of dystrophin leads to the loss of the DGC at the sarcolemma, resulting in sarcolemmal instability and progressive muscle damage. Utrophin (UTRN), an autosomal homolog of dystrophin, is upregulated in dystrophic muscle and partially compensates for the loss of dystrophin in muscle from patients with DMD. Here, we examine the interaction between Utr and sarcospan (SSPN), a small transmembrane protein that is a core component of both UTRN-glycoprotein complex (UGC) and DGC. We show that additional loss of SSPN causes an earlier onset of disease in dystrophin-deficient mdx mice by reducing the expression of the UGC at the sarcolemma. In order to further evaluate the role of SSPN in maintaining therapeutic levels of Utr at the sarcolemma, we tested the effect of Utr transgenic overexpression in mdx mice lacking SSPN (mdx:SSPN -/-:Utr-Tg). We found that overexpression of Utr restored SSPN to the sarcolemma in mdx muscle but that the ablation of SSPN in mdx muscle reduced Utr at the membrane. Nevertheless, Utr overexpression reduced central nucleation and improved grip strength in both lines. These findings demonstrate that high levels of Utr transgenic overexpression ameliorate the mdx phenotype independently of SSPN expression but that loss of SSPN may impair Utr-based mechanisms that rely on lower levels of Utr protein.

Stabilization of the cardiac sarcolemma by sarcospan rescues DMD-associated cardiomyopathy

In the current preclinical study, we demonstrate the therapeutic potential of sarcospan (SSPN) overexpression to alleviate cardiomyopathy associated with Duchenne muscular dystrophy (DMD) utilizing dystrophin-deficient mdx mice with utrophin haploinsufficiency that more accurately represent the severe disease course of human DMD. SSPN interacts with dystrophin, the DMD disease gene product, and its autosomal paralog utrophin, which is upregulated in DMD as a partial compensatory mechanism. SSPN transgenic mice have enhanced abundance of fully glycosylated α-dystroglycan, which may further protect dystrophin-deficient cardiac membranes. Baseline echocardiography reveals SSPN improves systolic function and hypertrophic indices in mdx and mdx:utr-heterozygous mice. Assessment of SSPN transgenic mdx mice by hemodynamic pressure-volume methods highlights enhanced systolic performance compared to mdx controls. SSPN restores cardiac sarcolemma stability, the primary defect in DMD disease, reduces fibrotic response and improves contractile function. We demonstrate that SSPN ameliorates more advanced cardiac disease in the context of diminished sarcolemma expression of utrophin and β1D integrin that mitigate disease severity and partially restores responsiveness to β-adrenergic stimulation. Overall, our current and previous findings suggest SSPN overexpression in DMD mouse models positively impacts skeletal, pulmonary and cardiac performance by addressing the stability of proteins at the sarcolemma that protect the heart from injury, supporting SSPN and membrane stabilization as a therapeutic target for DMD.

Humanizing the mdx mouse model of DMD: the long and the short of it

Duchenne muscular dystrophy (DMD) is a common fatal heritable myopathy, with cardiorespiratory failure occurring by the third decade of life. There is no specific treatment for DMD cardiomyopathy, in large part due to a lack of understanding of the mechanisms underlying the cardiac failure. Mdx mice, which have the same dystrophin mutation as human patients, are of limited use, as they do not develop early dilated cardiomyopathy as seen in patients. Here we summarize the usefulness of the various commonly used DMD mouse models, highlight a model with shortened telomeres like humans, and identify directions that warrant further investigation.

Next-Generation Sequencing to Diagnose Muscular Dystrophy, Rhabdomyolysis, and HyperCKemia

Background:Neuromuscular disorders are a phenotypically and genotypically diverse group of diseases that can be difficult to diagnose accurately because of overlapping clinical features and nonspecific muscle pathology. Next-generation sequencing (NGS) is a high-throughput technology that can be used as a more time- and cost-effective tool for identifying molecular diagnoses for complex genetic conditions, such as neuromuscular disorders.Methods:One hundred and sixty-nine patients referred to a Canadian neuromuscular clinic for evaluation of possible muscle disease were screened with an NGS panel of muscular dystrophy–associated genes. Patients were categorized by the reason of referral (1) muscle weakness (n=135), (2) recurrent episodes of rhabdomyolysis (n=18), or (3) idiopathic hyperCKemia (n=16).Results:Pathogenic and likely pathogenic variants were identified in 36.09% of patients (61/169). The detection rate was 37.04% (50/135) in patients with muscle weakness, 33.33% (6/18) with rhabdomyolysis, and 31.25% (5/16) in those with idiopathic hyperCKemia.Conclusions:This study shows that NGS can be a useful tool in the molecular workup of patients seen in a neuromuscular clinic. Evaluating the utility of large panels of a muscle disease-specific NGS panel to investigate the genetic susceptibilities of rhabdomyolysis and/or idiopathic hyperCKemia is a relatively new field. Twenty-eight of the pathogenic and likely pathogenic variants reported here are novel and have not previously been associated with disease.

Transcriptome profiling of claw muscle of the mud crab (Scylla paramamosain) at different fattening stages

In crustaceans, muscle growth and development is complicated, and to date substantial knowledge gaps exist. In this study, the claw muscle, hepatopancreas and nervous tissue of the mud crab (Scylla paramamosain) were collected at three fattening stages for sequence by the Illumina sequencing. A total of 127.87 Gb clean data with no less than 3.94 Gb generated for each sample and the cycleQ30 percentages were more than 86.13% for all samples. De Bruijn assembly of these clean data produced 94,853 unigenes, thereinto, 50,059 unigenes were found in claw muscle. A total of 121 differentially expressed genes (DEGs) were revealed in claw muscle from the three fattening stages with a Padj value < 0.01, including 63 genes with annotation. Functional annotation and enrichment analysis showed that the DEGs clusters represented the predominant gene catalog with roles in biochemical processes (glycolysis, phosphorylation and regulation of transcription), molecular function (ATP binding, 6-phosphofructokinase activity, and sequence-specific DNA binding) and cellular component (6-phosphofructokinase complex, plasma membrane, and integral component of membrane). qRT-PCR was employed to further validate certain DEGs. Single nucleotide polymorphism (SNP) analysis obtained 159,322, 125,963 and 166,279 potential SNPs from the muscle transcriptome at stage B, stage C and stage D, respectively. In addition, there were sixteen neuropeptide transcripts being predicted in the claw muscle. The present study provides a comprehensive transcriptome of claw muscle of S. paramamosain during fattening, providing a basis for screening the functional genes that may affect muscle growth of S. paramamosain.

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.

Nanospan, an alternatively spliced isoform of sarcospan, localizes to the sarcoplasmic reticulum in skeletal muscle and is absent in limb girdle muscular dystrophy 2F

Background

Sarcospan (SSPN) is a transmembrane protein that interacts with the sarcoglycans (SGs) to form a tight subcomplex within the dystrophin-glycoprotein complex that spans the sarcolemma and interacts with laminin in the extracellular matrix. Overexpression of SSPN ameliorates Duchenne muscular dystrophy in murine models.

Methods

Standard cloning approaches were used to identify nanospan, and nanospan-specific polyclonal antibodies were generated and validated. Biochemical isolation of skeletal muscle membranes and two-photon laser scanning microscopy were used to analyze nanospan localization in muscle from multiple murine models. Duchenne muscular dystrophy biopsies were analyzed by immunoblot analysis of protein lysates as well as indirect immunofluorescence analysis of muscle cryosections.

Results

Nanospan is an alternatively spliced isoform of sarcospan. While SSPN has four transmembrane domains and is a core component of the sarcolemmal dystrophin-glycoprotein complex, nanospan is a type II transmembrane protein that does not associate with the dystrophin-glycoprotein complex. We demonstrate that nanospan is enriched in the sarcoplasmic reticulum (SR) fractions and is not present in the T-tubules. SR fractions contain membranes from three distinct structural regions: a region flanking the T-tubules (triadic SR), a SR region across the Z-line (ZSR), and a longitudinal SR region across the M-line (LSR). Analysis of isolated murine muscles reveals that nanospan is mostly associated with the ZSR and triadic SR, and only minimally with the LSR. Furthermore, nanospan is absent from the SR of δ-SG-null (Sgcd^−/−) skeletal muscle, a murine model for limb girdle muscular dystrophy 2F. Analysis of skeletal muscle biopsies from Duchenne muscular dystrophy patients reveals that nanospan is preferentially expressed in type I (slow) fibers in both control and Duchenne samples. Furthermore, nanospan is significantly reduced in Duchenne biopsies.

Conclusions

Alternative splicing of proteins from the SG-SSPN complex produces δ-SG3, microspan, and nanospan that localize to the ZSR and the triadic SR, where they may play a role in regulating resting calcium levels as supported by previous studies (Estrada et al., Biochem Biophys Res Commun 340:865–71, 2006). Thus, alternative splicing of SSPN mRNA generates three protein isoforms (SSPN, microspan, and nanospan) that differ in the number of transmembrane domains affecting subcellular membrane association into distinct protein complexes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-017-0127-9) contains supplementary material, which is available to authorized users.

Dilated cardiomyopathy mutations in δ-sarcoglycan exert a dominant-negative effect on cardiac myocyte mechanical stability

Delta-sarcoglycan is a component of the sarcoglycan subcomplex within the dystrophin-glycoprotein complex located at the plasma membrane of muscle cells. While recessive mutations in δ-sarcoglycan cause limb girdle muscular dystrophy 2F, dominant mutations in δ-sarcoglycan have been linked to inherited dilated cardiomyopathy (DCM). The purpose of this study was to investigate functional cellular defects present in adult cardiac myocytes expressing mutant δ-sarcoglycans harboring the dominant inherited DCM mutations R71T or R97Q. This study demonstrates that DCM mutant δ-sarcoglycans can be stably expressed in adult rat cardiac myocytes and traffic similarly to wild-type δ-sarcoglycan to the plasma membrane, without perturbing assembly of the dystrophin-glycoprotein complex. However, expression of DCM mutant δ-sarcoglycan in adult rat cardiac myocytes is sufficient to alter cardiac myocyte plasma membrane stability in the presence of mechanical strain. Upon cyclical cell stretching, cardiac myocytes expressing mutant δ-sarcoglycan R97Q or R71T have increased cell-impermeant dye uptake and undergo contractures at greater frequencies than myocytes expressing normal δ-sarcoglycan. Additionally, the R71T mutation creates an ectopic N-linked glycosylation site that results in aberrant glycosylation of the extracellular domain of δ-sarcoglycan. Therefore, appropriate glycosylation of δ-sarcoglycan may also be necessary for proper δ-sarcoglycan function and overall dystrophin-glycoprotein complex function. These studies demonstrate that DCM mutations in δ-sarcoglycan can exert a dominant negative effect on dystrophin-glycoprotein complex function leading to myocardial mechanical instability that may underlie the pathogenesis of δ-sarcoglycan-associated DCM.

The Dystrophin Complex: Structure, Function, and Implications for Therapy

The dystrophin complex stabilizes the plasma membrane of striated muscle cells. Loss of function mutations in the genes encoding dystrophin, or the associated proteins, trigger instability of the plasma membrane, and myofiber loss. Mutations in dystrophin have been extensively cataloged, providing remarkable structure-function correlation between predicted protein structure and clinical outcomes. These data have highlighted dystrophin regions necessary for in vivo function and fueled the design of viral vectors and now, exon skipping approaches for use in dystrophin restoration therapies. However, dystrophin restoration is likely more complex, owing to the role of the dystrophin complex as a broad cytoskeletal integrator. This review will focus on dystrophin restoration, with emphasis on the regions of dystrophin essential for interacting with its associated proteins and discuss the structural implications of these approaches.

Sarcospan Regulates Cardiac Isoproterenol Response and Prevents Duchenne Muscular Dystrophy-Associated Cardiomyopathy

Background

Duchenne muscular dystrophy is a fatal cardiac and skeletal muscle disease resulting from mutations in the dystrophin gene. We have previously demonstrated that a dystrophin‐associated protein, sarcospan (SSPN), ameliorated Duchenne muscular dystrophy skeletal muscle degeneration by activating compensatory pathways that regulate muscle cell adhesion (laminin‐binding) to the extracellular matrix. Conversely, loss of SSPN destabilized skeletal muscle adhesion, hampered muscle regeneration, and reduced force properties. Given the importance of SSPN to skeletal muscle, we investigated the consequences of SSPN ablation in cardiac muscle and determined whether overexpression of SSPN into mdx mice ameliorates cardiac disease symptoms associated with Duchenne muscular dystrophy cardiomyopathy.

Methods and Results

SSPN‐null mice exhibited cardiac enlargement, exacerbated cardiomyocyte hypertrophy, and increased fibrosis in response to β‐adrenergic challenge (isoproterenol; 0.8 mg/day per 2 weeks). Biochemical analysis of SSPN‐null cardiac muscle revealed reduced sarcolemma localization of many proteins with a known role in cardiomyopathy pathogenesis: dystrophin, the sarcoglycans (α‐, δ‐, and γ‐subunits), and β1D integrin. Transgenic overexpression of SSPN in Duchenne muscular dystrophy mice (mdx^TG) improved cardiomyofiber cell adhesion, sarcolemma integrity, cardiac functional parameters, as well as increased expression of compensatory transmembrane proteins that mediate attachment to the extracellular matrix.

Conclusions

SSPN regulates sarcolemmal expression of laminin‐binding complexes that are critical to cardiac muscle function and protects against transient and chronic injury, including inherited cardiomyopathy.

Costamere proteins and their involvement in myopathic processes

Muscle fibres are very specialised cells with a complex structure that requires a high level of organisation of the constituent proteins. For muscle contraction to function properly, there is a need for not only sarcomeres, the contractile structures of the muscle fibre, but also costameres. These are supramolecular structures associated with the sarcolemma that allow muscle adhesion to the extracellular matrix. They are composed of protein complexes that interact and whose functions include maintaining cell structure and signal transduction mediated by their constituent proteins. It is important to improve our understanding of these structures, as mutations in various genes that code for costamere proteins cause many types of muscular dystrophy. In this review, we provide a description of costameres detailing each of their constituent proteins, such as dystrophin, dystrobrevin, syntrophin, sarcoglycans, dystroglycans, vinculin, talin, integrins, desmin, plectin, etc. We describe as well the diseases associated with deficiency thereof, providing a general overview of their importance.

The limb-girdle muscular dystrophies

A collection of more than 30 genetic muscle diseases that share certain key features, limb-girdle muscular dystrophies are characterized by progressive weakness and muscle atrophy of the hips, shoulders, and proximal extremity muscles with postnatal onset. This article discusses clinical, laboratory, and histologic features of the 6 most prevalent limb-girdle dystrophies. In this large group of disorders, certain distinctive features often can guide clinicians to a correct diagnosis.

Clinical aspects of patients with sarcoglycanopathies under steroids therapy

Patients with sarcoglycanopathies, which comprise four subtypes of autosomal recessive limb-girdle muscular dystrophies, usually present with progressive weakness leading to early loss of ambulation and premature death, and no effective treatment is currently available.

OBJECTIVE

To present clinical aspects and outcomes of six children with sarcoglycanopathies treated with steroids for at least one year.

METHOD

Patient files were retrospectively analyzed for steroid use.

RESULTS

Stabilization of muscle strength was noted in one patient, a slight improvement in two, and a slight worsening in three. In addition, variable responses of forced vital capacity and cardiac function were observed.

CONCLUSIONS

No overt clinical improvement was observed in patients with sarcoglycanopathies under steroid therapy. Prospective controlled studies including a larger number of patients are necessary to determine the effects of steroids for sarcoglycanopathies.

Sarcospan integration into laminin-binding adhesion complexes that ameliorate muscular dystrophy requires utrophin and α7 integrin

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene that result in loss of the dystrophin-glycoprotein complex, a laminin receptor that connects the myofiber to its surrounding extracellular matrix. Utrophin, a dystrophin ortholog that is normally localized to the neuromuscular junction, is naturally upregulated in DMD muscle, which partially compensates for the loss of dystrophin. Transgenic overexpression of utrophin causes broad sarcolemma localization of utrophin, restoration of laminin binding and amelioration of disease in the mdx mouse model of DMD. We previously demonstrated that overexpression of sarcospan, a dystrophin- and utrophin-binding protein, ameliorates mdx muscular dystrophy. Sarcospan boosts levels of utrophin to therapeutic levels at the sarcolemma, where attachment to laminin is restored. However, understanding the compensatory mechanism is complicated by concomitant upregulation of α7β1 integrin, which also binds laminin. Similar to the effects of utrophin, transgenic overexpression of α7 integrin prevents DMD disease in mice and is accompanied by increased abundance of utrophin around the extra-synaptic sarcolemma. In order to investigate the mechanisms underlying sarcospan 'rescue' of muscular dystrophy, we created double-knockout mice to test the contributions of utrophin or α7 integrin. We show that sarcospan-mediated amelioration of muscular dystrophy in DMD mice is dependent on the presence of both utrophin and α7β1 integrin, even when they are individually expressed at therapeutic levels. Furthermore, we found that association of sarcospan into laminin-binding complexes is dependent on utrophin and α7β1 integrin.

Finding the sweet spot: assembly and glycosylation of the dystrophin-associated glycoprotein complex

The dystrophin-associated glycoprotein complex (DGC) is a collection of glycoproteins that are essential for the normal function of striated muscle and many other tissues. Recent genetic studies have implicated the components of this complex in over a dozen forms of muscular dystrophy. Furthermore, disruption of the DGC has been implicated in many forms of acquired disease. This review aims to summarize the current state of knowledge regarding the processing and assembly of dystrophin-associated proteins with a focus primarily on the dystroglycan heterodimer and the sarcoglycan complex. These proteins form the transmembrane portion of the DGC and undergo a complex multi-step processing with proteolytic cleavage, differential assembly, and both N- and O-glycosylation. The enzymes responsible for this processing and a model describing the sequence and subcellular localization of these events are discussed.

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.

Fine mapping of a QTL on chromosome 13 for submaximal exercise capacity training response: the HERITAGE Family Study

Although regular exercise improves submaximal aerobic capacity, there is large variability in its response to exercise training. While this variation is thought to be partly due to genetic differences, relatively little is known about the causal genes. Submaximal aerobic capacity traits in the current report include the responses of oxygen consumption (ΔVO(2)60), power output (ΔWORK60), and cardiac output (ΔQ60) at 60% of VO2max to a standardized 20-week endurance exercise training program. Genome-wide linkage analysis in 475 HERITAGE Family Study Caucasians identified a locus on chromosome 13q for ΔVO(2)60 (LOD = 3.11). Follow-up fine mapping involved a dense marker panel of over 1,800 single-nucleotide polymorphisms (SNPs) in a 7.9-Mb region (21.1-29.1 Mb from p-terminus). Single-SNP analyses found 14 SNPs moderately associated with both ΔVO(2)60 at P ≤ 0.005 and the correlated traits of ΔWORK60 and ΔQ60 at P < 0.05. Haplotype analyses provided several strong signals (P < 1.0 × 10(-5)) for ΔVO(2)60. Overall, association analyses narrowed the target region and included potential biological candidate genes (MIPEP and SGCG). Consistent with maximal heritability estimates of 23%, up to 20% of the phenotypic variance in ΔVO(2)60 was accounted for by these SNPs. These results implicate candidate genes on chromosome 13q12 for the ability to improve submaximal exercise capacity in response to regular exercise. Submaximal exercise at 60% of maximal capacity is an exercise intensity that falls well within the range recommended in the Physical Activity Guidelines for Americans and thus has potential public health relevance.

Founder mutation for α-sarcoglycan-LGMD2D in a Magdalen Islands Acadian cluster

BACKGROUND

We have recruited a group of four living and reviewed the records of six deceased distantly related French-Canadians of Acadian descent affected by a childhood-onset form of recessive limb-girdle muscular dystrophy (LGMD). All cases originate from the small archipelago of the Magdalen Islands (population: 13,000) isolated in the Gulf of St-Lawrence.

METHODS

Based on the likely sharing of the same founder mutation we completed a 319K SNPs genome-wide scan to identify the disease locus and then screen candidate genes in this region.

RESULTS

All patients had normal initial motor milestones. They presented with limb girdle weakness at the average age of seven years (5-11). Progressive weakness led to loss of ambulation at a wide range of ages (10-39). Patients also developed macroglossia, large calves and mild to moderate contractures, hyperlordosis and decreased pulmonary function. Creatine kinase levels were elevated (1,800-10,000 U/L) in the first decades, but decreased with progression of disease. Homozygosity mapping uncovered a shared chromosomal region of 6.33Mb. The alpha sarcoglycan (SGCA) gene, mutated in LGMD2D, lay in this candidate interval. Sequencing of all SGCA exons uncovered a shared homozygous missense mutation (c. 229C>T, p.R77C), the most common SGCA mutation internationally reported. Using demographic data, we estimated a high carrier rate of 1/22.

CONCLUSION

The p.R77C mutation has also been observed in many populations, including in France and Spain (Basques). This corresponds to the first reported recessive founder disease for the Magdalen Islands, an archipelago settled in the XIXth century, largely by Acadian immigrants.

Sarcoglycanopathies

The so-called sarcoglycanopathies form a subgroup of four genetically closely related autosomal recessive limb-girdle muscular dystrophies (LGMD2C-F) caused by mutations of the α-, β-, γ-, and δ-sarcoglycan genes. All four sarcoglycans are glycosylated transmembrane proteins and form a tetrameric complex that is part of dystrophin-associated proteins. The clinical phenotype associated with sarcoglycanopathies is characterized by a slowly progressive proximal muscle weakness with onset during childhood in most cases. The disease course is often similar but more variable than X-linked Duchenne muscular dystrophy. Diagnosis is usually based on muscle biopsy findings that confirm dystrophic changes and deficiency of one or more sarcoglycan proteins. Genetic testing is used to confirm the diagnosis. A number of different animal models have been developed to study the function of sarcoglycans and to develop specific therapeutic strategies such as gene transfer, but so far none of these techniques has entered clinical practice. Therefore, treatment is symptomatic and aims at amelioration of locomotor, respiratory, and cardiac manifestations of the disease.

The roles of the dystrophin-associated glycoprotein complex at the synapse

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene and is characterized by progressive muscle wasting. A number of Duchenne patients also present with mental retardation. The dystrophin protein is part of the highly conserved dystrophin-associated glycoprotein complex (DGC) which accumulates at the neuromuscular junction (NMJ) and at a variety of synapses in the peripheral and central nervous systems. Many years of research into the roles of the DGC in muscle have revealed its structural function in stabilizing the sarcolemma. In addition, the DGC also acts as a scaffold for various signaling pathways. Here, we discuss recent advances in understanding DGC roles in the nervous system, gained from studies in both vertebrate and invertebrate model systems. From these studies, it has become clear that the DGC is important for the maturation of neurotransmitter receptor complexes and for the regulation of neurotransmitter release at the NMJ and central synapses. Furthermore, roles for the DGC have been established in consolidation of long-term spatial and recognition memory. The challenges ahead include the integration of the behavioral and mechanistic studies and the use of this information to identify therapeutic targets.

Sarcoglycanopathies: molecular pathogenesis and therapeutic prospects

Sarcoglycanopathies are a group of autosomal recessive muscle-wasting disorders caused by genetic defects in one of four cell membrane glycoproteins, alpha-, beta-, gamma- or delta-sarcoglycan. These four sarcoglycans form a subcomplex that is closely linked to the major dystrophin-associated protein complex, which is essential for membrane integrity during muscle contraction and provides a scaffold for important signalling molecules. Proper assembly, trafficking and targeting of the sarcoglycan complex is of vital importance, and mutations that severely perturb tetramer formation and localisation result in sarcoglycanopathy. Gene defects in one sarcoglycan cause the absence or reduced concentration of the other subunits. Most genetic defects generate mutated proteins that are degraded through the cell's quality control system; however, in many cases, conformational modifications do not affect the function of the protein, yet it is recognised as misfolded and prematurely degraded. Recent evidence shows that misfolded sarcoglycans could be rescued to the cell membrane by assisting their maturation along the ER secretory pathway. This review summarises the etiopathogenesis of sarcoglycanopathies and highlights the quality control machinery as a potential pharmacological target for therapy of these genetic disorders.

Overexpression of Galgt2 reduces dystrophic pathology in the skeletal muscles of alpha sarcoglycan-deficient mice

Recent studies have shown that a number of genes that are not mutated in various forms of muscular dystrophy may serve as surrogates to protect skeletal myofibers from injury. One such gene is Galgt2, which is also called cytotoxic T cell GalNAc transferase in mice. In this study, we show that Galgt2 overexpression reduces the development of dystrophic pathology in the skeletal muscles of mice lacking alpha sarcoglycan (Sgca), a mouse model for limb girdle muscular dystrophy 2D. Galgt2 transgenic Sgca(-/-) mice showed reduced levels of myofiber damage, as evidenced by i) normal levels of serum creatine kinase activity, ii) a lack of Evans blue dye uptake into myofibers, iii) normal levels of mouse locomotor activity, and iv) near normal percentages of myofibers with centrally located nuclei. In addition, the overexpression of Galgt2 in the early postnatal period using an adeno-associated virus gene therapy vector protected Sgca(-/-) myofibers from damage, as observed using histopathology measurements. Galgt2 transgenic Sgca(-/-) mice also had increased levels of glycosylation of alpha dystroglycan with the CT carbohydrate, but showed no up-regulation of beta, gamma, delta, or epsilon sarcoglycan. These data, coupled with results from our previous studies, show that Galgt2 has therapeutic effects in three distinct forms of muscular dystrophy and may, therefore, have a broad spectrum of therapeutic potential for the treatment of various myopathies.

Sarcoglycan complex: implications for metabolic defects in muscular dystrophies

The sarcoglycans are known as an integral subcomplex of the dystrophin glycoprotein complex, the function of which is best characterized in skeletal muscle in relation to muscular dystrophies. Here we demonstrate that the white adipocytes, which share a common precursor with the myocytes, express a cell-specific sarcoglycan complex containing β-, δ-, and ϵ-sarcoglycan. In addition, the adipose sarcoglycan complex associates with sarcospan and laminin binding dystroglycan. Using multiple sarcoglycan null mouse models, we show that loss of α-sarcoglycan has no consequence on the expression of the adipocyte sarcoglycan complex. However, loss of β- or δ-sarcoglycan leads to a concomitant loss of the sarcoglycan complex as well as sarcospan and a dramatic reduction in dystroglycan in adipocytes. We further demonstrate that β-sarcoglycan null mice, which lack the sarcoglycan complex in adipose tissue and skeletal muscle, are glucose-intolerant and exhibit whole body insulin resistance specifically due to impaired insulin-stimulated glucose uptake in skeletal muscles. Thus, our data demonstrate a novel function of the sarcoglycan complex in whole body glucose homeostasis and skeletal muscle metabolism, suggesting that the impairment of the skeletal muscle metabolism influences the pathogenesis of muscular dystrophy.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Sarcoglycanopathies: can muscle immunoanalysis predict the genotype?

Muscle immunoanalysis of the sarcoglycan complex is an important part of the diagnostic evaluation of muscle biopsies in patients with autosomal recessive limb-girdle muscular dystrophy. Reduced or absent sarcolemmal expression of one or all of the four sarcoglycans (alpha-, beta-, gamma-, delta-sarcoglycan) can be found in patients with limb-girdle muscular dystrophy 2C-F (LGMD2C-F) and also in patients with Duchenne and Becker muscular dystrophy (DMD/BMD). It has previously been suggested that different patterns of sarcoglycan expression could predict the primary genetic defect, and that genetic analysis could be directed by these patterns. In this first UK study we studied 24 genetically characterized patients with sarcoglycan deficient LGMD, in 22 of whom muscle immunoanalysis data were available. Thirteen patients showed alpha-sarcoglycan deficient LGMD2D, 7 patients beta-sarcoglycan deficient LGMD2E, 3 patients gamma-sarcoglycan deficient LGDM2C, and one patient delta-sarcoglycan deficient LGMD2F. Muscle biopsies were analysed in one centre without knowledge of the established genetic diagnosis. Our results demonstrated that residual sarcoglycan expression is highly variable and does not enable an accurate prediction of the genotype. Considering previous reports of sarcoglycanopathy patients with an isolated loss of one sarcoglycan we recommend to use antibodies against all four sarcoglycans for immunoanalysis of skeletal muscle sections. A concomitant reduction of dystrophin and beta-dystroglycan was observed more frequently than previously reported and illustrates the important differential diagnosis of DMD and BMD for sarcoglycan deficient LGMD.

Limb-girdle muscular dystrophies

PURPOSE OF REVIEW

The aim of this review is to provide an up-to-date analysis of current knowledge about limb-girdle muscular dystrophies (LGMDs).

RECENT FINDINGS

Over the last few years, new and interesting studies have been published on LGMD. New LGMD genes have been discovered and the clinical and genetic heterogeneity in this group of muscular dystrophies has been further enlarged by the description of new forms of LGMD. Several studies have demonstrated involvement of genes causing posttranslational modifications of alpha-dystroglycan in the pathogenesis of autosomal recessive LGMD. This has highlighted an important overlap in pathogenesis between LGMD and congenital muscular dystrophies, prompting further research. Moreover, new pathogenic mechanisms and pathways are emerging for LGMD, in particular calpainopathies, dysferlinopathies and titinopathies. Such new findings may suggest novel therapeutic approaches and future clinical trials.

SUMMARY

The increased understanding of the genes and pathogenic mechanism of the LGMDs will improve diagnostic processes and prognostic accuracy, and promote therapeutic strategies. European and global LGMD patient registries will increase current knowledge on natural history and facilitate translational research.

A common disease-associated missense mutation in alpha-sarcoglycan fails to cause muscular dystrophy in mice

Limb-girdle muscular dystrophy type 2D (LGMD2D) is caused by autosomal recessive mutations in the alpha-sarcoglycan gene. An R77C substitution is the most prevalent cause of the disease, leading to disruption of the sarcoglycan-sarcospan complex. To model this common mutation, we generated knock-in mice with an H77C substitution in alpha-sarcoglycan. The floxed neomycin (Neo)-cassette retained at the targeted H77C alpha-sarcoglycan locus caused a loss of alpha-sarcoglycan expression, resulting in muscular dystrophy in homozygotes, whereas Cre-mediated deletion of the floxed Neo-cassette led to recovered H77C alpha-sarcoglycan expression. Contrary to expectations, mice homozygous for the H77C-encoding allele expressed both this mutant alpha-sarcoglycan and the other components of the sarcoglycan-sarcospan complex in striated muscle, and did not develop muscular dystrophy. Accordingly, conditional rescued expression of the H77C protein in striated muscle of the alpha-sarcoglycan-deficient mice prevented the disease. Adding to the case that the behavior of mutant alpha-sarcoglycan is different between humans and mice, mutant human R77C alpha-sarcoglycan restored the expression of the sarcoglycan-sarcospan complex when introduced by adenoviral vector into the skeletal muscle of previously created alpha-sarcoglycan null mice. These findings indicate that the alpha-sarcoglycan with the most frequent missense mutation in LGMD2D is correctly processed, is transported to the sarcolemma, and is fully functional in mouse muscle. Our study presents an unexpected difference in the behavior of a missense-mutated protein in mice versus human patients, and emphasizes the need to understand species-specific protein quality control systems.

The 16 kDa subunit of vacuolar H+-ATPase is a novel sarcoglycan-interacting protein

The sarcoglycan complex in muscle consists of alpha-, beta-, gamma- and delta-sarcoglycan and is part of the larger dystrophin-glycoprotein complex (DGC), which is essential for maintaining muscle membrane integrity. Mutations in any of the four sarcoglycans cause limb-girdle muscular dystrophies (LGMD). In this report, we have identified a novel interaction between delta-sarcoglycan and the 16 kDa subunit c (16K) of vacuolar H(+)-ATPase. Co-expression studies in heterologous cell system revealed that 16K interacts specifically with delta-sarcoglycan and the highly related gamma-sarcoglycan through the transmembrane domains. In cultured C2C12 myotubes, 16K forms a complex with sarcoglycans at the plasma membrane. Loss of sarcoglycans in the sarcoglycan-deficient BIO14.6 hamster destabilizes the DGC and alters the localization of 16K at the sarcolemma. In addition, the steady state level of beta(1)-integrin is increased. Recent studies have shown that 16K also interacts directly with beta(1)-integrin and our data demonstrated that sarcoglycans, 16K and beta(1)-integrin were immunoprecipitated together in C2C12 myotubes. Since sarcoglycans have been proposed to participate in bi-directional signaling with integrins, our findings suggest that 16K might mediate the communication between sarcoglycans and integrins and play an important role in the pathogenesis of muscular dystrophy.

Disrupted mechanical stability of the dystrophin-glycoprotein complex causes severe muscular dystrophy in sarcospan transgenic mice

The dystrophin-glycoprotein complex spans the muscle plasma membrane and provides a mechanical linkage between laminin in the extracellular matrix and actin in the intracellular cytoskeleton. Within the dystrophin-glycoprotein complex, the sarcoglycans and sarcospan constitute a subcomplex of transmembrane proteins that stabilize α-dystroglycan, a receptor for laminin and other components of the extracellular matrix. In order to elucidate the function of sarcospan, we generated transgenic mice that overexpress sarcospan in skeletal muscle. Sarcospan transgenic mice with moderate (tenfold) levels of sarcospan overexpression exhibit a severe phenotype that is similar to mouse models of laminin-deficient congenital muscular dystrophy (MD). Sarcospan transgenic mice display severe kyphosis and die prematurely between 6 and 10 weeks of age. Histological analysis reveals that sarcospan expression causes muscle pathology marked by increased muscle fiber degeneration and/or regeneration. Sarcospan transgenic muscle does not display sarcolemma damage, which is distinct from dystrophin- and sarcoglycan-deficient muscular dystrophies. We show that sarcospan clusters the sarcoglycans into insoluble protein aggregates and causes destabilization of α-dystroglycan. Evidence is provided to demonstrate abnormal extracellular matrix assembly, which represents a probable pathological mechanism for the severe and lethal dystrophic phenotype. Taken together, these data suggest that sarcospan plays an important mechanical role in stabilizing the dystrophin-glycoprotein complex.

A new evidence for the maintenance of the sarcoglycan complex in muscle sarcolemma in spite of the primary absence of δ-SG protein

delta-Sarcoglycan (delta-SG) is one of the first proteins of the sarcoglycan complex (SGC) to be expressed during muscle development, and it has been considered fundamental for the assembling and insertion of the SGC in the sarcolemma. Studies using heterologous cell systems and co-precipitation have demonstrated that SGC assembly was dependent on the simultaneous synthesis of all four sarcoglycan proteins. Mutations in any one of sarcoglycan genes, including the common disease causing mutation c.656delC in the delta-SG gene, block complex formation and its insertion in the plasma membrane. Failure in complex assembly in patients with this mutation would be therefore expected. In this study, we provide evidence for the possibility of preservation of part of the SG complex in the sarcolemma, even in the absence of delta-SG. This is based on the study of one mildly affected patient with limb-girdle muscular dystrophy type 2F (LGMD2F) due to the homozygous c.656delC mutation in the delta-SG gene. Protein analysis in his muscle biopsy presented a significant deficiency of only delta-SG with retention of the other three SG proteins in the sarcolemma. RNA expression analysis showed that zeta-SG, a functionally homologous to delta-SG, is not atypically upregulated in his muscle and would not replace the absent delta-SG, retaining the complex alpha-beta-gamma-zeta. The patient started clinical manifestation at age 25, with frequent falls, but he is currently able to walk unassisted at age 42. His clinical course is significantly milder when compared to several other affected patients carrying the same mutation associated with a total deficiency of the four SG proteins in the muscle studied by our group and confirmed in other patients. Therefore, our results add a new in vivo evidence that alpha-, beta-, and gamma-SG proteins can be maintained in the sarcolemma without delta-SG. Additionally, LGMD2F, with retention of the part of the SGC, might be associated to a milder clinical course, which has important implications for clinical prognosis and genetic counseling of the family.

SGCE missense mutations that cause myoclonus-dystonia syndrome impair ε-sarcoglycan trafficking to the plasma membrane: modulation by ubiquitination and torsinA

Myoclonus-dystonia syndrome (MDS) is a genetically heterogeneous disorder characterized by myoclonic jerks often seen in combination with dystonia and psychiatric co-morbidities and epilepsy. Mutations in the gene encoding epsilon-sarcoglycan (SGCE) have been found in some patients with MDS. SGCE is a maternally imprinted gene with the disease being inherited in an autosomal dominant pattern with reduced penetrance upon maternal transmission. In the central nervous system, epsilon-sarcoglycan is widely expressed in neurons of the cerebral cortex, basal ganglia, hippocampus, cerebellum and the olfactory bulb. epsilon-Sarcoglycan is located at the plasma membrane in neurons, muscle and transfected cells. To determine the effect of MDS-associated mutations on the function of epsilon-sarcoglycan we examined the biosynthesis and trafficking of wild-type and mutant proteins in cultured cells. In contrast to the wild-type protein, disease-associated epsilon-sarcoglycan missense mutations (H36P, H36R and L172R) produce proteins that are undetectable at the cell surface and are retained intracellularly. These mutant proteins become polyubiquitinated and are rapidly degraded by the proteasome. Furthermore, torsinA, that is mutated in DYT1 dystonia, a rare type of primary dystonia, binds to and promotes the degradation of epsilon-sarcoglycan mutants when both proteins are co-expressed. These data demonstrate that some MDS-associated mutations in SGCE impair trafficking of the mutant protein to the plasma membrane and suggest a role for torsinA and the ubiquitin proteasome system in the recognition and processing of misfolded epsilon-sarcoglycan.

Structural and functional analysis of the sarcoglycan-sarcospan subcomplex

Sarcospan is a component of the dystrophin-glycoprotein complex that forms a tight subcomplex with the sarcoglycans. The sarcoglycan-sarcospan subcomplex functions to stabilize alpha-dystroglycan at the plasma membrane and perturbations of this subcomplex are associated with autosomal recessive limb-girdle muscular dystrophy. In order to characterize protein interactions within this subcomplex, we first demonstrate that sarcospan forms homo-oligomers within the membrane. Experiments with a panel of site-directed mutants reveal that proper structure of the large extracellular loop is an important determinant of oligo formation. Furthermore, the intracellular N- and C-termini contribute to stability of sarcospan-mediated webs. Point mutation of each cysteine residue reveals that Cys 162 and Cys 164 within the large extracellular loop form disulfide bridges, which are critical for proper sarcospan structure. The extracellular domain of sarcospan also forms the main binding site for the sarcoglycans. We propose a model whereby sarcospan forms homo-oligomers that cluster the components of the dystrophin-glycoprotein complex within the membrane.

The β-δ-core of sarcoglycan is essential for deposition at the plasma membrane

Mutations of any of the sarcoglycan complex subunits (alpha, beta, delta, and gamma) cause limb-girdle muscular dystrophy. Furthermore, individual mutations lead to a reduction or loss of all other members of the complex. In some cases of limb-girdle muscular dystrophies, however, residual sarcoglycan expression has been documented. Therefore, in this study we tested the hypothesis that formation of specific sarcoglycan subcomplexes is crucial for plasma membrane deposition. Using co-immunoprecipitation assays, we demonstrated that beta- and delta-sarcoglycan interact with alpha-sarcoglycan and these two subunits must be co-expressed for export from the endoplasmic reticulum. Advanced light-microscopic imaging techniques demonstrated that co-expression of beta-sarcoglycan and delta-sarcoglycan is also responsible for delivery to and retention of sarcoglycan subcomplexes at the cell surface. These data suggest that formation of the beta-delta-core may promote the export and deposition of sarcoglycan subcomplexes at the plasma membrane, and therefore identifies a mechanism for sarcoglycan transport.

Processing and Assembly of the Dystrophin Glycoprotein Complex

The assembly, processing and translocation of proteins occur constantly in all cells, and these processes also take place during the genesis, maintenance and repair of skeletal muscle. Skeletal muscle fibers are composed of myofibrils and are surrounded by a muscle plasma membrane, the sarcolemma. The sarcolemma serves as a docking location for many proteins. These proteins are important for establishing the physical connection between the extracellular matrix and the cytoskeleton and play a role in transmitting force related to muscle contraction. This physical connection is maintained through a myriad of proteins including the dystrophin glycoprotein complex (DGC). Normal sarcolemmal function requires proper DGC synthesis and positioning, and perturbation of the DGC leads to muscle membrane instability and disease.

New p63 targets in keratinocytes identified by a genome-wide approach

p63 is a developmentally regulated transcription factor related to p53. It is involved in the development of ectodermal tissues, including limb, skin and in general, multilayered epithelia. The DeltaNp63alpha isoform is thought to play a 'master' role in the asymmetric division of epithelial cells. It is also involved in the pathogenesis of several human diseases, phenotypically characterized by ectodermal dysplasia. Our understanding of transcriptional networks controlled by p63 is limited, owing to the low number of bona fide targets. To screen for new targets, we employed chromatin immunoprecipitation from keratinocytes (KCs) coupled to the microarray technology, using both CpG islands and promoter arrays. The former revealed 96 loci, the latter yielded 85 additional genes. We tested 40 of these targets in several functional assays, including: (i) in vivo binding by p63 in primary KCs; (ii) expression analysis in differentiating HaCaT cells and in cells overexpressing DeltaNp63alpha; (iii) promoter transactivation and (iv) immunostaining in normal tissues, confirming their regulation by p63. We discovered several new specific targets whose functional categorization links p63 to cell growth and differentiation.

Over-expression of Microspan, a novel component of the sarcoplasmic reticulum, causes severe muscle pathology with triad abnormalities

Sarcospan (SSPN) is a core component of the dystrophin-glycoprotein complex (DGC). Multiple SSPN transcripts are ubiquitously expressed and SSPN splicing is disrupted in many lung tumors, suggesting the importance of SSPN-related mRNAs. We describe the isolation of an alternatively spliced isoform of SSPN, which we designate 'microspan' based on its small size relative to SSPN. Microspan has two transmembrane domains and a novel C-terminus. We demonstrate that microspan is not an integral component of the DGC and is not perturbed by the loss of dystrophin. Microspan protein is detected at the sarcoplasmic reticulum (SR) using indirect immunofluorescence and immunoelectron microscopy. Furthermore, microspan purifies with skeletal muscle SR membranes and not transverse tubules. Mice engineered to over-express microspan display severe kyphosis and die at approximately 8 weeks of age. Levels of ryanodine receptor, dihydropyridine receptor, and SERCA-1 are greatly reduced in microspan transgenic muscle. Furthermore, electron microscopy reveals that microspan over-expression causes a dramatic perturbation in triad structure. Our findings suggest that microspan is an important component of the SR and may contribute to excitation-contraction coupling.

The childhood limb-girdle muscular dystrophies

The heterogeneous childhood limb-girdle muscular dystrophies have originally been defined as a group of autosomal recessive and dominant diseases with progressive weakness and wasting of shoulder and pelvic-girdle muscles. Over the last 12 years, the underlying genetic defects for many of the diseases have been identified and insight into pathomechanisms of disease has been gained. At the same time, improved diagnostic techniques have allowed to extend the phenotypic spectrum for many of these devastating conditions, which showed that clinical symptoms and pathological findings are not restricted to skeletal muscles. Childhood limb-girdle muscular dystrophies are systemic diseases that often affect the musculoskeletal, respiratory, and cardiovascular system and that can go along with central nervous system involvement and gastrointestinal symptoms. The systemic nature of the diseases requires adequate management strategies that improve symptoms, longevity, and quality of life of the patients. As we are entering an era of translational research the need for precise molecular diagnoses, a thorough understanding of the natural history of the diseases and guidelines for standardized assessments of the patients become even more relevant. In this review, the best characterized childhood limb-girdle muscular dystrophies are discussed and their management aspects highlighted.

Common pathological mechanisms in mouse models for muscular dystrophies

Duchenne/Becker and limb-girdle muscular dystrophies share clinical symptoms like muscle weakness and wasting but differ in clinical presentation and severity. To get a closer view on the differentiating molecular events responsible for the muscular dystrophies, we have carried out a comparative gene expression profiling of hindlimb muscles of the following mouse models: dystrophin-deficient (mdx, mdx(3cv)), sarcoglycan-deficient (Sgca null, Sgcb null, Sgcg null, Sgcd null), dysferlin-deficient (Dysf null, SJL(Dysf)), sarcospan-deficient (Sspn null), and wild-type (C57Bl/6, C57Bl/10) mice. The expression profiles clearly discriminated between severely affected (dystrophinopathies and sarcoglycanopathies) and mildly or nonaffected models (dysferlinopathies, sarcospan-deficiency, wild-type). Dystrophin-deficient and sarcoglycan-deficient profiles were remarkably similar, sharing inflammatory and structural remodeling processes. These processes were also ongoing in dysferlin-deficient animals, albeit at lower levels, in agreement with the later age of onset of this muscular dystrophy. The inflammatory proteins Spp1 and S100a9 were up-regulated in all models, including sarcospan-deficient mice, which points, for the first time, at a subtle phenotype for Sspn null mice. In conclusion, we identified biomarker genes for which expression correlates with the severity of the disease, which can be used for monitoring disease progression. This comparative study is an integrating step toward the development of an expression profiling-based diagnostic approach for muscular dystrophies in humans.