The sarcoglycan complex in limb-girdle muscular dystrophy

Nintedanib Reduces Muscle Fibrosis and Improves Muscle Function of the Alpha-Sarcoglycan-Deficient Mice

Sarcoglycanopathies are a group of recessive limb-girdle muscular dystrophies, characterized by progressive muscle weakness. Sarcoglycan deficiency produces instability of the sarcolemma during muscle contraction, leading to continuous muscle fiber injury eventually producing fiber loss and replacement by fibro-adipose tissue. Therapeutic strategies aiming to reduce fibro-adipose expansion could be effective in muscular dystrophies. We report the positive effect of nintedanib in a murine model of alpha-sarcoglycanopathy. We treated 14 Sgca-/- mice, six weeks old, with nintedanib 50 mg/kg every 12 h for 10 weeks and compared muscle function and histology with 14 Sgca-/- mice treated with vehicle and six wild-type littermate mice. Muscle function was assessed using a treadmill and grip strength. A cardiac evaluation was performed by echocardiography and histological study. Structural analysis of the muscles, including a detailed study of the fibrotic and inflammatory processes, was performed using conventional staining and immunofluorescence. In addition, proteomics and transcriptomics studies were carried out. Nintedanib was well tolerated by the animals treated, although we observed weight loss. Sgca-/- mice treated with nintedanib covered a longer distance on the treadmill, compared with non-treated Sgca-/- mice, and showed higher strength in the grip test. Moreover, nintedanib improved the muscle architecture of treated mice, reducing the degenerative area and the fibrotic reaction that was associated with a reversion of the cytokine expression profile. Nintedanib improved muscle function and muscle architecture by reducing muscle fibrosis and degeneration and reverting the chronic inflammatory environment suggesting that it could be a useful therapy for patients with alpha-sarcoglycanopathy.

Muscle Diversity, Heterogeneity, and Gradients: Learning from Sarcoglycanopathies

Skeletal muscle, the most abundant tissue in the body, is heterogeneous. This heterogeneity forms the basis of muscle diversity, which is reflected in the specialized functions of muscles in different parts of the body. However, these different parts are not always clearly delimitated, and this often gives rise to gradients within the same muscle and even across the body. During the last decade, several studies on muscular disorders both in mice and in humans have observed particular distribution patterns of muscle weakness during disease, indicating that the same mutation can affect muscles differently. Moreover, these phenotypical differences reveal gradients of severity, existing alongside other architectural gradients. These two factors are especially prominent in sarcoglycanopathies. Nevertheless, very little is known about the mechanism(s) driving the phenotypic diversity of the muscles affected by these diseases. Here, we will review the available literature on sarcoglycanopathies, focusing on phenotypic differences among affected muscles and gradients, characterization techniques, molecular signatures, and cell population heterogeneity, highlighting the possibilities opened up by new technologies. This review aims to revive research interest in the diverse disease phenotype affecting different muscles, in order to pave the way for new therapeutic interventions.

New genotype-phenotype correlations in a large European cohort of patients with sarcoglycanopathy

Sarcoglycanopathies comprise four subtypes of autosomal recessive limb-girdle muscular dystrophies (LGMDR3, LGMDR4, LGMDR5 and LGMDR6) that are caused, respectively, by mutations in the SGCA, SGCB, SGCG and SGCD genes. In 2016, several clinicians involved in the diagnosis, management and care of patients with LGMDR3-6 created a European Sarcoglycanopathy Consortium. The aim of the present study was to determine the clinical and genetic spectrum of a large cohort of patients with sarcoglycanopathy in Europe. This was an observational retrospective study. A total of 33 neuromuscular centres from 13 different European countries collected data of the genetically confirmed patients with sarcoglycanopathy followed-up at their centres. Demographic, genetic and clinical data were collected for this study. Data from 439 patients from 13 different countries were collected. Forty-three patients were not included in the analysis because of insufficient clinical information available. A total of 159 patients had a confirmed diagnosis of LGMDR3, 73 of LGMDR4, 157 of LGMDR5 and seven of LGMDR6. Patients with LGMDR3 had a later onset and slower progression of the disease. Cardiac involvement was most frequent in LGMDR4. Sixty per cent of LGMDR3 patients carried one of the following mutations, either in a homozygous or heterozygous state: c.229C>T, c.739G>A or c.850C>T. Similarly, the most common mutations in LMGDR5 patients were c.525delT or c.848G>A. In LGMDR4 patients the most frequent mutation was c.341C>T. We identified onset of symptoms before 10 years of age and residual protein expression lower than 30% as independent risk factors for losing ambulation before 18 years of age, in LGMDR3, LGMDR4 and LGMDR5 patients. This study reports clinical, genetic and protein data of a large European cohort of patients with sarcoglycanopathy. Improving our knowledge about these extremely rare autosomal recessive forms of LGMD was helped by a collaborative effort of neuromuscular centres across Europe. Our study provides important data on the genotype-phenotype correlation that is relevant for the design of natural history studies and upcoming interventional trials in sarcoglycanopathies.

Sam68 splicing regulation contributes to motor unit establishment in the postnatal skeletal muscle

Sam68 ensures the establishment of neuromuscular junctions (NMJs) and motor unit integrity by orchestrating a neuronal splicing program.

RNA-binding proteins orchestrate the composite life of RNA molecules and impact most physiological processes, thus underlying complex phenotypes. The RNA-binding protein Sam68 regulates differentiation processes by modulating splicing, polyadenylation, and stability of select transcripts. Herein, we found that Sam68^−/− mice display altered regulation of alternative splicing in the spinal cord of key target genes involved in synaptic functions. Analysis of the motor units revealed that Sam68 ablation impairs the establishment of neuromuscular junctions and causes progressive loss of motor neurons in the spinal cord. Importantly, alterations of neuromuscular junction morphology and properties in Sam68^−/− mice correlate with defects in muscle and motor unit integrity. Sam68^−/− muscles display defects in postnatal development, with manifest signs of atrophy. Furthermore, fast-twitch muscles in Sam68^−/− mice show structural features typical of slow-twitch muscles, suggesting alterations in the metabolic and functional properties of myofibers. Collectively, our data identify a key role for Sam68 in muscle development and suggest that proper establishment of motor units requires timely expression of synaptic splice variants.

Acute Exercise Increases Plasma Levels of Muscle-Derived Microvesicles Carrying Fatty Acid Transport Proteins

CONTEXT

Microvesicles (MVs) are a class of membrane particles shed by any cell in the body in physiological and pathological conditions. They are considered to be key players in intercellular communication, and with a molecular content reflecting the composition of the cell of origin, they have recently emerged as a promising source of biomarkers in a number of diseases.

OBJECTIVE

The effects of acute exercise on the plasma concentration of skeletal muscle-derived MVs (SkMVs) carrying metabolically important membrane proteins were examined.

PARTICIPANTS

Thirteen men with obesity and type 2 diabetes mellitus (T2DM) and 14 healthy male controls with obesity exercised on a cycle ergometer for 60 minutes.

INTERVENTIONS

Muscle biopsies and blood samples-obtained before exercise, immediately after exercise, and 3 hours into recovery-were collected for the analysis of long-chain fatty acid (LCFA) transport proteins CD36 (a scavenger receptor class B protein) and fatty acid transport protein 4 (FATP4) mRNA content in muscle and for flow cytometric studies on circulating SkMVs carrying either LCFA transport protein.

RESULTS

Besides establishing a flow cytometric approach for the detection of circulating SkMVs and subpopulations carrying either CD36 or FATP4 and thereby adding proof to their existence, we demonstrated an overall exercise-induced change of SkMVs carrying these LCFA transport proteins. A positive correlation between exercise-induced changes in skeletal muscle CD36 mRNA expression and concentrations of SkMVs carrying CD36 was found in T2DM only.

CONCLUSIONS

This approach could add important real-time information about the abundance of LCFA transport proteins present on activated muscle cells in subjects with impaired glucose metabolism.

Deficiency in Kelch protein Klhl31 causes congenital myopathy in mice

Maintenance of muscle structure and function depends on the precise organization of contractile proteins into sarcomeres and coupling of the contractile apparatus to the sarcoplasmic reticulum (SR), which serves as the reservoir for calcium required for contraction. Several members of the Kelch superfamily of proteins, which modulate protein stability as substrate-specific adaptors for ubiquitination, have been implicated in sarcomere formation. The Kelch protein Klhl31 is expressed in a muscle-specific manner under control of the transcription factor MEF2. To explore its functions in vivo, we created a mouse model of Klhl31 loss of function using the CRISPR-Cas9 system. Mice lacking Klhl31 exhibited stunted postnatal skeletal muscle growth, centronuclear myopathy, central cores, Z-disc streaming, and SR dilation. We used proteomics to identify several candidate Klhl31 substrates, including Filamin-C (FlnC). In the Klhl31-knockout mice, FlnC protein levels were highly upregulated with no change in transcription, and we further demonstrated that Klhl31 targets FlnC for ubiquitination and degradation. These findings highlight a role for Klhl31 in the maintenance of skeletal muscle structure and provide insight into the mechanisms underlying congenital myopathies.

Expansion of divergent SEA domains in cell surface proteins and nucleoporin 54

SEA (sea urchin sperm protein, enterokinase, agrin) domains, many of which possess autoproteolysis activity, have been found in a number of cell surface and secreted proteins. Despite high sequence divergence, SEA domains were also proposed to be present in dystroglycan based on a conserved autoproteolysis motif and receptor-type protein phosphatase IA-2 based on structural similarity. The presence of a SEA domain adjacent to the transmembrane segment appears to be a recurring theme in quite a number of type I transmembrane proteins on the cell surface, such as MUC1, dystroglycan, IA-2, and Notch receptors. By comparative sequence and structural analyses, we identified dystroglycan-like proteins with SEA domains in Capsaspora owczarzaki of the Filasterea group, one of the closest single-cell relatives of metazoans. We also detected novel and divergent SEA domains in a variety of cell surface proteins such as EpCAM, α/ε-sarcoglycan, PTPRR, collectrin/Tmem27, amnionless, CD34, KIAA0319, fibrocystin-like protein, and a number of cadherins. While these proteins are mostly from metazoans or their single cell relatives such as choanoflagellates and Filasterea, fibrocystin-like proteins with SEA domains were found in several other eukaryotic lineages including green algae, Alveolata, Euglenozoa, and Haptophyta, suggesting an ancient evolutionary origin. In addition, the intracellular protein Nucleoporin 54 (Nup54) acquired a divergent SEA domain in choanoflagellates and metazoans.

Muscular dystrophy modeling in zebrafish

Skeletal muscle performs an essential function in human physiology with defects in genes encoding a variety of cellular components resulting in various types of inherited muscle disorders. Muscular dystrophies (MDs) are a severe and heterogeneous type of human muscle disease, manifested by progressive muscle wasting and degeneration. The disease pathogenesis and therapeutic options for MDs have been investigated for decades using rodent models, and considerable knowledge has been accumulated on the cause and pathogenetic mechanisms of this group of human disorders. However, due to some differences between disease severity and progression, what is learned in mammalian models does not always transfer to humans, prompting the desire for additional and alternative models. More recently, zebrafish have emerged as a novel and robust animal model for the study of human muscle disease. Zebrafish MD models possess a number of distinct advantages for modeling human muscle disorders, including the availability and ease of generating mutations in homologous disease-causing genes, the ability to image living muscle tissue in an intact animal, and the suitability of zebrafish larvae for large-scale chemical screens. In this chapter, we review the current understanding of molecular and cellular mechanisms involved in MDs, the process of myogenesis in zebrafish, and the structural and functional characteristics of zebrafish larval muscles. We further discuss the insights gained from the key zebrafish MD models that have been so far generated, and we summarize the attempts that have been made to screen for small molecules inhibitors of the dystrophic phenotypes using these models. Overall, these studies demonstrate that zebrafish is a useful in vivo system for modeling aspects of human skeletal muscle disorders. Studies using these models have contributed both to the understanding of the pathogenesis of muscle wasting disorders and demonstrated their utility as highly relevant models to implement therapeutic screening regimens.

Myocardial Contractile Dysfunction Is Present without Histopathology in a Mouse Model of Limb-Girdle Muscular Dystrophy-2F and Is Prevented after Claudin-5 Virotherapy

Mutations in several members of the dystrophin glycoprotein complex lead to skeletal and cardiomyopathies. Cardiac care for these muscular dystrophies consists of management of symptoms with standard heart medications after detection of reduced whole heart function. Recent evidence from both Duchenne muscular dystrophy patients and animal models suggests that myocardial dysfunction is present before myocardial damage or deficiencies in whole heart function, and that treatment prior to heart failure symptoms may be beneficial. To determine whether this same early myocardial dysfunction is present in other muscular dystrophy cardiomyopathies, we conducted a physiological assessment of cardiac function at the tissue level in the δ-sarcoglycan null mouse model (Sgcd^−/−) of Limb-girdle muscular dystrophy type 2F. Baseline cardiac contractile force measurements using ex vivo intact linear muscle preparations, were severely depressed in these mice without the presence of histopathology. Virotherapy withclaudin-5 prevents the onset of cardiomyopathy in another muscular dystrophy model. After virotherapy with claudin-5, the cardiac contractile force deficits in Sgcd^−/− mice are no longer significant. These studies suggest that screening Limb-girdle muscular dystrophy patients using methods that detect earlier functional changes may provide a longer therapeutic window for cardiac care.

Enzymatic Activity of the Scaffold Protein Rapsyn for Synapse Formation

Neurotransmission is ensured by a high concentration of neurotransmitter receptors at the postsynaptic membrane. This is mediated by scaffold proteins that bridge the receptors with cytoskeleton. One such protein is rapsyn (receptor-associated protein at synapse), which is essential for acetylcholine receptor (AChR) clustering and NMJ (neuromuscular junction) formation. We show that the RING domain of rapsyn contains E3 ligase activity. Mutation of the RING domain that abolishes the enzyme activity inhibits rapsyn- as well as agrin-induced AChR clustering in heterologous and muscle cells. Further biological and genetic studies support a working model where rapsyn, a classic scaffold protein, serves as an E3 ligase to induce AChR clustering and NMJ formation, possibly by regulation of AChR neddylation. This study identifies a previously unappreciated enzymatic function of rapsyn and a role of neddylation in synapse formation, and reveals a potential target of therapeutic intervention for relevant neurological disorders.

A novel method for identifying disease associated protein complexes based on functional similarity protein complex networks

Background

Protein complexes formed by non-covalent interaction among proteins play important roles in cellular functions. Computational and purification methods have been used to identify many protein complexes and their cellular functions. However, their roles in terms of causing disease have not been well discovered yet. There exist only a few studies for the identification of disease-associated protein complexes. However, they mostly utilize complicated heterogeneous networks which are constructed based on an out-of-date database of phenotype similarity network collected from literature. In addition, they only apply for diseases for which tissue-specific data exist.

Methods

In this study, we propose a method to identify novel disease-protein complex associations. First, we introduce a framework to construct functional similarity protein complex networks where two protein complexes are functionally connected by either shared protein elements, shared annotating GO terms or based on protein interactions between elements in each protein complex. Second, we propose a simple but effective neighborhood-based algorithm, which yields a local similarity measure, to rank disease candidate protein complexes.

Results

Comparing the predictive performance of our proposed algorithm with that of two state-of-the-art network propagation algorithms including one we used in our previous study, we found that it performed statistically significantly better than that of these two algorithms for all the constructed functional similarity protein complex networks. In addition, it ran about 32 times faster than these two algorithms. Moreover, our proposed method always achieved high performance in terms of AUC values irrespective of the ways to construct the functional similarity protein complex networks and the used algorithms. The performance of our method was also higher than that reported in some existing methods which were based on complicated heterogeneous networks. Finally, we also tested our method with prostate cancer and selected the top 100 highly ranked candidate protein complexes. Interestingly, 69 of them were evidenced since at least one of their protein elements are known to be associated with prostate cancer.

Conclusions

Our proposed method, including the framework to construct functional similarity protein complex networks and the neighborhood-based algorithm on these networks, could be used for identification of novel disease-protein complex associations.

Electronic supplementary material

The online version of this article (doi:10.1186/s13015-015-0044-6) contains supplementary material, which is available to authorized users.

Progressive muscular dystrophies

Infancy- or childhood-onset muscular dystrophies may be associated with profound loss of muscle function, affecting ambulation, posture, cardiac and respiratory functions, while those of late onset may be mild and associated with slight weakness or fatigability induced by effort. In addition to the distribution of muscle weakness, symptoms, and course of the disease, the diagnosis of muscular dystrophy is usually ascertained by histological findings. There is connective tissue proliferation in the perimysium and endomysium, variation in muscle fiber size, cytoarchitectural alterations of myofibers such as internal nuclei, myofibrillar whorls, and fiber splitting and lobulation, but, most of all, degeneration and regeneration of myofibers. Causes of muscular dystrophies characterized by muscle weakness and wasting are heterogeneous and include dysfunction of diverse genetic pathways and genes encoding proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. Duchenne and Becker muscular dystrophies are prototypes illustrating advances in the field of myology. Limb-girdle muscular dystrophies (LGMDs) are clinically and genetically heterogeneous, some with autosomal dominant (LGMD1) and others with autosomal recessive (LGMD2) inheritance. Neither clinical and genetic grounds nor biopsy patterns are specific enough to distinguish them, but two common denominators are: (1) weakness and wasting predominating in pelvic and shoulder girdle muscles, with occasional involvement of the myocardium; and (2) necrosis and regeneration of myofibers. While identification of genetic causes and molecular diagnosis are increasingly improved, especially with the advent of new generation sequencing technologies, optimized care, information for the family, and prevention, including genetic counseling and prenatal diagnosis, require multidisciplinary follow-up with genetic, pediatric, and psychological involvement.

Leaky ryanodine receptors in β-sarcoglycan deficient mice: a potential common defect in muscular dystrophy

BACKGROUND

Disruption of the sarcolemma-associated dystrophin-glycoprotein complex underlies multiple forms of muscular dystrophy, including Duchenne muscular dystrophy and sarcoglycanopathies. A hallmark of these disorders is muscle weakness. In a murine model of Duchenne muscular dystrophy, mdx mice, cysteine-nitrosylation of the calcium release channel/ryanodine receptor type 1 (RyR1) on the skeletal muscle sarcoplasmic reticulum causes depletion of the stabilizing subunit calstabin1 (FKBP12) from the RyR1 macromolecular complex. This results in a sarcoplasmic reticular calcium leak via defective RyR1 channels. This pathological intracellular calcium leak contributes to reduced calcium release and decreased muscle force production. It is unknown whether RyR1 dysfunction occurs also in other muscular dystrophies.

METHODS

To test this we used a murine model of Limb-Girdle muscular dystrophy, deficient in β-sarcoglycan (Sgcb-/-).

RESULTS

Skeletal muscle RyR1 from Sgcb-/- deficient mice were oxidized, nitrosylated, and depleted of the stabilizing subunit calstabin1, which was associated with increased open probability of the RyR1 channels. Sgcb-/- deficient mice exhibited decreased muscle specific force and calcium transients, and displayed reduced exercise capacity. Treating Sgcb-/- mice with the RyR stabilizing compound S107 improved muscle specific force, calcium transients, and exercise capacity. We have previously reported similar findings in mdx mice, a murine model of Duchenne muscular dystrophy.

CONCLUSIONS

Our data suggest that leaky RyR1 channels may underlie multiple forms of muscular dystrophy linked to mutations in genes encoding components of the dystrophin-glycoprotein complex. A common underlying abnormality in calcium handling indicates that pharmacological targeting of dysfunctional RyR1 could be a novel therapeutic approach to improve muscle function in Limb-Girdle and Duchenne muscular dystrophies.

Milestones in dystonia

The last 25 years have seen remarkable advances in our understanding of the genetic etiologies of dystonia, new approaches into dissecting underlying pathophysiology, and independent progress in identifying effective treatments. In this review we highlight some of these advances, especially the genetic findings that have taken us from phenomenological to molecular-based diagnoses. Twenty DYT loci have been designated and 10 genes identified, all based on linkage analyses in families. Hand in hand with these genetic findings, neurophysiological and imaging techniques have been employed that have helped illuminate the similarities and differences among the various etiological dystonia subtypes. This knowledge is just beginning to yield new approaches to treatment including those based on DYT1 animal models. Despite the lag in identifying genetically based therapies, effective treatments, including impressive benefits from deep brain stimulation and botulinum toxin chemodenervation, have marked the last 25 years. The challenge ahead includes continued advancement into understanding dystonia's many underlying causes and associated pathology and using this knowledge to advance treatment including preventing genetic disease expression.

New dystrophin/dystroglycan interactors control neuron behavior in Drosophila eye

Background

The Dystrophin Glycoprotein Complex (DGC) is a large multi-component complex that is well known for its function in muscle tissue. When the main components of the DGC, Dystrophin (Dys) and Dystroglycan (Dg) are affected cognitive impairment and mental retardation in addition to muscle degeneration can occur. Previously we performed an array of genetic screens using a Drosophila model for muscular dystrophy in order to find novel DGC interactors aiming to elucidate the signaling role(s) in which the complex is involved. Since the function of the DGC in the brain and nervous system has not been fully defined, we have here continued to analyze the DGC modifiers' function in the developing Drosophila brain and eye.

Results

Given that disruption of Dys and Dg leads to improper photoreceptor axon projections into the lamina and eye neuron elongation defects during development, we have determined the function of previously screened components and their genetic interaction with the DGC in this tissue. Our study first found that mutations in chif, CG34400, Nrk, Lis1, capt and Cam cause improper axon path-finding and loss of SP2353, Grh, Nrk, capt, CG34400, vimar, Lis1 and Cam cause shortened rhabdomere lengths. We determined that Nrk, mbl, capt and Cam genetically interact with Dys and/or Dg in these processes. It is notable that most of the neuronal DGC interacting components encountered are involved in regulation of actin dynamics.

Conclusions

Our data indicate possible DGC involvement in the process of cytoskeletal remodeling in neurons. The identification of new components that interact with the DGC not only helps to dissect the mechanism of axon guidance and eye neuron differentiation but also provides a great opportunity for understanding the signaling mechanisms by which the cell surface receptor Dg communicates via Dys with the actin cytoskeleton.

Hyperthermic seizures and aberrant cellular homeostasis in Drosophila dystrophic muscles

In humans, mutations in the Dystrophin Glycoprotein Complex (DGC) cause muscular dystrophies (MDs) that are associated with muscle loss, seizures and brain abnormalities leading to early death. Using Drosophila as a model to study MD we have found that loss of Dystrophin (Dys) during development leads to heat-sensitive abnormal muscle contractions that are repressed by mutations in Dys's binding partner, Dystroglycan (Dg). Hyperthermic seizures are independent from dystrophic muscle degeneration and rely on neurotransmission, which suggests involvement of the DGC in muscle-neuron communication. Additionally, reduction of the Ca(2+) regulator, Calmodulin or Ca(2+) channel blockage rescues the seizing phenotype, pointing to Ca(2+) mis-regulation in dystrophic muscles. Also, Dys and Dg mutants have antagonistically abnormal cellular levels of ROS, suggesting that the DGC has a function in regulation of muscle cell homeostasis. These data show that muscles deficient for Dys are predisposed to hypercontraction that may result from abnormal neuromuscular junction signaling.

Upregulation of paxillin and focal adhesion signaling follows Dystroglycan Complex deletions and promotes a hypertensive state of differentiation

Anchorage to matrix is mediated for many cells not only by integrin-based focal adhesions but also by a parallel assembly of integral and peripheral membrane proteins known as the Dystroglycan Complex. Deficiencies in either dystrophin (mdx mice) or γ-sarcoglycan (γSG(-/-) mice) components of the Dystroglycan Complex lead to upregulation of numerous focal adhesion proteins, and the phosphoprotein paxillin proves to be among the most prominent. In mdx muscle, paxillin-Y31 and Y118 are both hyper-phosphorylated as are key sites in focal adhesion kinase (FAK) and the stretch-stimulatable pro-survival MAPK pathway, whereas γSG(-/-) muscle exhibits more erratic hyper-phosphorylation. In cultured myotubes, cell tension generated by myosin-II appears required for localization of paxillin to adhesions while vinculin appears more stably integrated. Overexpression of wild-type (WT) paxillin has no obvious effect on focal adhesion density or the physical strength of adhesion, but WT and a Y118F mutant promote contractile sarcomere formation whereas a Y31F mutant shows no effect, implicating Y31 in striation. Self-peeling of cells as well as Atomic Force Microscopy (AFM) probing of cells with or without myosin-II inhibition indicate an increase in cell tension within paxillin-overexpressing cells. However, prednisolone, a first-line glucocorticoid for muscular dystrophies, decreases cell tension without affecting paxillin at adhesions, suggesting a non-linear relationship between paxillin and cell tension. Hypertension that results from upregulation of integrin adhesions is thus a natural and treatable outcome of Dystroglycan Complex down-regulation.

RNA interference improves myopathic phenotypes in mice over-expressing FSHD region gene 1 (FRG1)

Muscular dystrophies, and other diseases of muscle, arise from recessive and dominant gene mutations. Gene replacement strategies may be beneficial for the former, while gene silencing approaches may provide treatment for the latter. In the last two decades, muscle-directed gene therapies were primarily focused on treating recessive disorders. This disparity at least partly arose because feasible mechanisms to silence dominant disease genes lagged behind gene replacement strategies. With the discovery of RNA interference (RNAi) and its subsequent development as a promising new gene silencing tool, the landscape has changed. In this study, our objective was to demonstrate proof-of-principle for RNAi therapy of a dominant myopathy in vivo. We tested the potential of adeno-associated viral (AAV)-delivered therapeutic microRNAs, targeting the human Facioscapulohumeral muscular dystrophy (FSHD) region gene 1 (FRG1), to correct myopathic features in mice expressing toxic levels of human FRG1 (FRG1(-high) mice). We found that FRG1 gene silencing improved muscle mass, strength, and histopathological abnormalities associated with muscular dystrophy in FRG1(-high) mice, thereby demonstrating therapeutic promise for treatment of dominantly inherited myopathies using RNAi. This approach potentially applies to as many as 29 different gene mutations responsible for myopathies inherited as dominant disorders.

From proteins to genes: immunoanalysis in the diagnosis of muscular dystrophies

Muscular dystrophies are a large heterogeneous group of inherited diseases that cause progressive muscle weakness and permanent muscle damage. Very few muscular dystrophies show sufficient specific clinical features to allow a definite diagnosis. Because of the currently limited capacity to screen for numerous genes simultaneously, muscle biopsy is a time and cost-effective test for many of these disorders. Protein analysis interpreted in correlation with the clinical phenotype is a useful way of directing genetic testing in many types of muscular dystrophies. Immunohistochemistry and western blot are complementary techniques used to gather quantitative and qualitative information on the expression of proteins involved in this group of diseases. Immunoanalysis has a major diagnostic application mostly in recessive conditions where the absence of labelling for a particular protein is likely to indicate a defect in that gene. However, abnormalities in protein expression can vary from absence to very subtle reduction. It is good practice to test muscle biopsies with antibodies for several proteins simultaneously and to interpret the results in context. Indeed, there is a degree of direct or functional association between many of these proteins that is reflected by the presence of specific secondary abnormalities that are of value, especially when the diagnosis is not straightforward.

Gait disturbances in dystrophic hamsters

The delta-sarcoglycan-deficient hamster is an excellent model to study muscular dystrophy. Gait disturbances, important clinically, have not been described in this animal model. We applied ventral plane videography (DigiGait) to analyze gait in BIO TO-2 dystrophic and BIO F1B control hamsters walking on a transparent treadmill belt. Stride length was ∼13% shorter (P < .05) in TO-2 hamsters at 9 months of age compared to F1B hamsters. Hindlimb propulsion duration, an indicator of muscle strength, was shorter in 9-month-old TO-2 (247 ± 8 ms) compared to F1B hamsters (272 ± 11 ms; P < .05). Braking duration, reflecting generation of ground reaction forces, was delayed in 9-month-old TO-2 (147 ± 6 ms) compared to F1B hamsters (126 ± 8 ms; P < .05). Hindpaw eversion, evidence of muscle weakness, was greater in 9-month-old TO-2 than in F1B hamsters (17.7 ± 1.2° versus 8.7 ± 1.6°; P < .05). Incline and decline walking aggravated gait disturbances in TO-2 hamsters at 3 months of age. Several gait deficits were apparent in TO-2 hamsters at 1 month of age. Quantitative gait analysis demonstrates that dystrophic TO-2 hamsters recapitulate functional aspects of human muscular dystrophy. Early detection of gait abnormalities in a convenient animal model may accelerate the development of therapies for muscular dystrophy.

Magnetic resonance imaging assessment of cardiac dysfunction in δ-sarcoglycan null mice

Delta-sarcoglycan (δ-sarcoglycan) null, Scgd(-/-), mice develop cardiac and skeletal muscle histopathological alterations similar to those in humans with limb-girdle muscular dystrophy. The objective of this study was to assess the feasibility of using MRI to investigate cardiac dysfunction in Scgd(-/-) mice. Cardiac MRI of 8 month old Scgd(-/-) and wild type (WT) mice was performed. Compared to WT, Scgd(-/-) mice had significantly lower LV ejection fraction (44±5% vs. 66±4%, p=0.014), lower RV ejection fraction (25±2% vs. 51±3%, p<0.001) lower myocardial circumferential strain, (15.0±0.3% vs. 16.9±0.3%, p=0.007) and RV dilatation (54±3 μL vs. 40±3 μL, p=0.007). The regional circumferential strain also demonstrated significant temporal dyssynchrony between opposing regions of the Scgd(-/-) LV. Our results demonstrate severe cardiac dysfunction in Scgd(-/-) mice at 8 months. The study identifies a set of non-invasive markers that could be used to study efficacy of novel therapeutic agents in dystrophic mice.

Intracellular Ca2+ transients in delta-sarcoglycan knockout mouse skeletal muscle

BACKGROUND

delta-Sarcoglycan (delta-SG) knockout (KO) mice develop skeletal muscle histopathological alterations similar to those in humans with limb muscular dystrophy. Membrane fragility and increased Ca(2+) permeability have been linked to muscle degeneration. However, little is known about the mechanisms by which genetic defects lead to disease.

METHODS

Isolated skeletal muscle fibers of wild-type and delta-SG KO mice were used to investigate whether the absence of delta-SG alters the increase in intracellular Ca(2+) during single twitches and tetani or during repeated stimulation. Immunolabeling, electrical field stimulation and Ca(2+) transient recording techniques with fluorescent indicators were used.

RESULTS

Ca(2+) transients during single twitches and tetani generated by muscle fibers of delta-SG KO mice are similar to those of wild-type mice, but their amplitude is greatly decreased during protracted stimulation in KO compared to wild-type fibers. This impairment is independent of extracellular Ca(2+) and is mimicked in wild-type fibers by blocking store-operated calcium channels with 2-aminoethoxydiphenyl borate (2-APB). Also, immunolabeling indicates the localization of a delta-SG isoform in the sarcoplasmic reticulum of the isolated skeletal muscle fibers of wild-type animals, which may be related to the functional differences between wild-type and KO muscles.

CONCLUSIONS

delta-SG has a role in calcium homeostasis in skeletal muscle fibers.

GENERAL SIGNIFICANCE

These results support a possible role of delta-SG on calcium homeostasis. The alterations caused by the absence of delta-SG may be related to the pathogenesis of muscular dystrophy.

Sox9 represses alpha-sarcoglycan gene expression in early myogenic differentiation

Alpha sarcoglycan (alpha-SG) is highly expressed in differentiated striated muscle, and its disruption causes limb-girdle muscular dystrophy. Accordingly, the myogenic master regulator MyoD finely modulates its expression. However, the mechanisms preventing alpha-SG gene expression at early stages of myogenic differentiation remain unknown. In this study, we uncovered Sox9, which was not previously known to directly bind muscle gene promoters, as a negative regulator of alpha-SG gene expression. Reporter gene and chromatin immunoprecipitation assays revealed three functional Sox-binding sites that mediate alpha-SG promoter activity repression during early myogenic differentiation. In addition, we show that Sox9-mediated inhibition of alpha-SG gene expression is independent of MyoD. Moreover, we provide evidence suggesting that Smad3 enhances the repressive activity of Sox9 over alpha-SG gene expression in a transforming growth factor-beta-dependent manner. On the basis of these results, we propose that Sox9 and Smad3 are responsible for preventing precocious activation of alpha-SG gene expression during myogenic differentiation.

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.

Sarcolemmal neuronal nitric oxide synthase defect in limb-girdle muscular dystrophy: an adverse modulating factor in the disease course?

Reduction of neuronal nitric oxide synthase (nNOS) has been associated with the pathogenesis and clinical expression of inherited myopathies. To determine whether a defect in nNOS might be an adverse modulating factor in the course of limb-girdle muscular dystrophy, we investigated cytosolic and sarcolemmal nNOS expression in muscle biopsies from 32 patients with 7 forms of limb-girdle muscular dystrophy. Primary calpainopathy, dysferlinopathy, and caveolinopathy biopsies showed normal levels of cytosolic nNOS and preserved sarcolemmal nNOS immunoreactivity. By contrast, the cytosolic nNOS levels in sarcoglycanopathy muscles were variably reduced. Sarcolemmal nNOS immunoreactivity varied from absent to reduced, depending on the integrity of the sarcoglycan complex. In muscles with loss of the entire sarcoglycan complex, sarcolemmal nNOS was absent; it otherwise depended on the specific sarcoglycan gene and type of mutation. The integrity of the entire sarcoglycan complex is, therefore, essential for the stabilization of nNOS to the sarcolemma. Absence of sarcolemmal nNOS in sarcoglycanopathy muscle was always associated with severe muscular dystrophy and sometimes with dilated cardiomyopathy, supporting the hypothesis that nNOS defect might contribute to skeletal and cardiac muscle disease progression. These results emphasize the value of nNOS immunohistochemical analysis in limb-girdle muscular dystrophy and provide additional insights for future therapeutic interventions in these disorders.

Using protein complexes to predict phenotypic effects of gene mutation

The best predictor of a protein's knockout phenotype is shown to be the knockout phenotype of other proteins that are present in a protein complex with it.

Background

Predicting the phenotypic effects of mutations is a central goal of genetics research; it has important applications in elucidating how genotype determines phenotype and in identifying human disease genes.

Results

Using a wide range of functional genomic data from the yeast Saccharomyces cerevisiae, we show that the best predictor of a protein's knockout phenotype is the knockout phenotype of other proteins that are present in a protein complex with it. Even the addition of multiple datasets does not improve upon the predictions made from protein complex membership. Similarly, we find that a proxy for protein complexes is a powerful predictor of disease phenotypes in humans.

Conclusion

We propose that identifying human protein complexes containing known disease genes will be an efficient method for large-scale disease gene discovery, and that yeast may prove to be an informative model system for investigating, and even predicting, the genetic basis of both Mendelian and complex disease phenotypes.

Revised spectrum of mutations in sarcoglycanopathies

To define the spectrum of mutations in alpha-, beta-, gamma-, and delta-sarcoglycan (SG) genes, we analyzed these genes in 69 probands with clinical and biological criteria compatible with the diagnosis of autosomal recessive limb-girdle muscular dystrophy. For 48 patients, muscle biopsies were available and multiplex western blot analysis of muscle proteins showed significant abnormalities of alpha- and gamma-SG. Our diagnostic strategy includes multiplex western blot, sequencing of SG genes, multiplex quantitative-fluorescent PCR and RT-PCR analyses. Mutations were detected in 57 patients and homozygous or compound heterozygous mutations were identified in 75% (36/48) of the patients with abnormal western blot, and in 52% (11/21) of the patients without muscle biopsy. Involvement of alpha-SG was demonstrated in 55.3% of cases (26/47), whereas gamma- and beta-SG were implicated in 25.5% (12/47) and in 17% (8/47) of cases, respectively. Interestingly, we identified 25 novel mutations, and a significant proportion of these mutations correspond to deletions (identified in 14 patients) of complete exon(s) of alpha- or gamma-SG genes, and partial duplications (identified in 5 patients) of exon 1 of beta-SG gene. This study highlights the high frequency of exonic deletions of alpha- and gamma-SG genes, as well as the presence of a hotspot of duplications affecting exon 1 of the beta-SG gene. In addition, protein analysis by multiplex western blot in combination with mutation screening and genotyping results allowed to propose a comprehensive and efficient diagnostic strategy and strongly suggested the implication of additional genes, yet to be identified, in sarcoglycanopathy-like disorders.

Role of the sarcomeric Z-disc in the pathogenesis of cardiomyopathy

The Z-disc has traditionally been viewed as a structure required to maintain sarcomeric function and integrity. More recently, the sarcomeric Z-disc has also emerged as a nodal point in cardiomyocyte signaling and mechanotransduction. This notion is not only supported by several transgenic animal models, but also by the identification of mutations in various Z-disc proteins, resulting in dilated or hypertrophic cardiomyopathy in patients. This review will thus focus on the role of the sarcomeric Z-disc and its associated proteins in the pathogenesis of cardiomyopathy.

AAV-mediated delivery of a mutated myostatin propeptide ameliorates calpain 3 but not α-sarcoglycan deficiency

Myostatin is a negative regulator of muscle mass whose inhibition has been proposed as a therapeutic strategy for muscle-wasting conditions. Indeed, blocking myostatin action through different strategies has proved beneficial for the pathophysiology of the dystrophin-deficient mdx mouse. In this report, we tested the inhibition of myostatin by AAV-mediated expression of a mutated propeptide in animal models of two limb-girdle muscular dystrophies: LGMD2A caused by mutations in the calpain 3 (CAPN3) gene and LGMD2D caused by mutations in the alpha-sarcoglycan gene (SGCA). In the highly regenerative Sgca-null mice, survival of the alpha-sarcoglycan-deficient muscle fibers did not improve after transfer of the myostatin propeptide. In calpain 3-deficient mice, a boost in muscle mass and an increase in absolute force were obtained, suggesting that myostatin inhibition could constitute a therapeutic strategy in this predominantly atrophic disorder.

Culture of human skeletal muscle myoblasts: timing appearance and localization of dystrophin-glycoprotein complex and vinculin-talin-integrin complex

The dystrophin-glycoprotein complex together with the vinculin-talin-integrin complex plays an important role in muscle function; in fact the mutations of their elements lead to diverse forms of muscular dystrophies. The relationship between the elements of dystrophin-glycoprotein complex and vinculin-talin-integrin and the time course of their formation are still not known in detail. In order to better understand this relationship we studied their expression during development in normal human skeletal muscle culture. Using a standardized muscle cell culture procedure, this study was performed to analyze the timing, appearance and the localization of some proteins of the dystrophin-glycoprotein complex and vinculin-talin-integrin complex during cellular proliferation (myoblast) and differentiation (4, 7, 15 and 21 days). The indirect immunofluorescence technique was used and cells were examined using a Meta Zeiss LSM510 confocal laser scanning inverted microscope. We examined the progressive appearance of the following proteins: alpha, beta, gamma, delta-sarcoglycans, beta-dystroglycan, dystrophin, talin, vinculin and integrin isoform alpha7/beta1. Immunofluorescence of these proteins, in satellite cells entering myogenic differentiation, revealed different patterns of localization depending on the time of culture. We showed that nondifferentiated cultures of human myoblasts expressed a perinuclear distribution of all proteins tested. During myoblast differentiation into myotubes (4 days) immunofluorescence gradually increased and was located in the whole cytoplasm. Subsequently, at day 7, a strong and homogeneous cytoplasmic labelling of all proteins was seen. At 15 days the distribution of the proteins was on the membrane. At this time some myotubes displayed a significant degree of precostameric banding pattern. As fusion proceeded at 21 days, the cytodistribution progressively changed and appeared along fibrillar longitudinal structures, and myotubes showed a clear periodic distribution (costameres). In conclusion, in normal human muscle cultures DGC and vinculin-talin-integrin proteins are first localized in the perinuclear region, then they diffuse in the cytoplasm and finally form at the plasma membrane into typical rib-like structures that are sarcolemma-associated.

The limb-girdle muscular dystrophies--diagnostic strategies

The limb-girdle muscular dystrophies are a group of disorders where our understanding of their underlying molecular basis has made huge strides over the past years, revealing great heterogeneity at the clinical and molecular level. The availability of direct protein and/ or gene based approaches to diagnosis means that these disorders can now be precisely defined, and such definition of a precise diagnosis is increasingly allowing directed management for these diseases by the ability to predict specific complications such as those of the cardiac or respiratory systems. An algorithm combining clinical, biochemical and molecular testing is described which will aid precision of diagnosis and direct specific testing towards the cases most likely to benefit. This brings advantages for the patients of today in recognising the specific risks of their disorders, and in the future will be the starting point for specific gene and protein based therapies.

Hypertrophic response of Duchenne and limb-girdle muscular dystrophies is associated with activation of Akt pathway

Dystrophic muscle undergoes repeated cycles of degeneration/regeneration, characterized by the presence of hypertrophic fibers. In order to elucidate the signaling pathways that govern these events, we investigated Akt activation in normal and dystrophic muscle. Akt is activated in neonatal muscle and in actively dividing myoblasts, supporting a developmental role for Akt signaling. Akt activation was detected at very early, prenecrotic stages of disease pathogenesis, and maximal activation was observed during peak stages of muscle hypertrophy. Duchenne muscular dystrophy patients exhibit a similar pattern of Akt activation. Mice with sarcoglycan-deficient muscular dystrophy possess more severe muscle pathology and display elevated Akt signaling. However, the highest levels of Akt activation were found in dystrophin-utrophin-deficient muscle with very advanced dystrophy. We propose that Akt may serve as an early biomarker of disease and that Akt activation mediates hypertrophy in muscular dystrophy. Current investigations are focused on introducing constitutively active and dominant-negative Akt into prenecrotic mdx mice to determine how early modification of Akt activity influences disease pathogenesis.

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.

Movement disorders and alcohol misuse

Many movement disorders, including tics, chorea, tremor, myoclonus and parkinsonism, may result from substance abuse. However, alcohol in particular is associated in a more complex manner with two specific movement disorders, essential tremor (ET) and myoclonus-dystonia (M-D). In this review we discuss the comorbidity of alcohol abuse in both ET and M-D, the ameliorative effects of alcohol in both diseases, and review the data evaluating alcohol abuse secondary to self-medication. We also discuss shared pathophysiologic mechanisms in the understanding of both of these disorders, as the elucidation of the mechanisms by which alcohol exerts its effects may lead to novel therapeutic approaches.

The sarcomeric Z-disc: a nodal point in signalling and disease

The perception of the Z-disc in striated muscle has undergone significant changes in the past decade. Traditionally, the Z-disc has been viewed as a passive constituent of the sarcomere, which is important only for the cross-linking of thin filaments and transmission of force generated by the myofilaments. The recent discovery of multiple novel molecular components, however, has shed light on an emerging role for the Z-disc in signal transduction in both cardiac and skeletal muscles. Strikingly, mutations in several Z-disc proteins have been shown to cause cardiomyopathies and/or muscular dystrophies. In addition, the elusive cardiac stretch receptor appears to localize to the Z-disc. Various signalling molecules have been shown to interact with Z-disc proteins, several of which shuttle between the Z-disc and other cellular compartments such as the nucleus, underlining the dynamic nature of Z-disc-dependent signalling. In this review, we provide a systematic view on the currently known Z-disc components and the functional significance of the Z-disc as an interface between biomechanical sensing and signalling in cardiac and skeletal muscle functions and diseases.

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.

Sustained Whole-Body Functional Rescue in Congestive Heart Failure and Muscular Dystrophy Hamsters by Systemic Gene Transfer

BACKGROUND

The success of muscular dystrophy gene therapy requires widespread and stable gene delivery with minimal invasiveness. Here, we investigated the therapeutic effect of systemic delivery of adeno-associated virus (AAV) vectors carrying human delta-sarcoglycan (delta-SG) gene in TO-2 hamsters, a congestive heart failure and muscular dystrophy model with a delta-SG gene mutation.

METHODS AND RESULTS

A single injection of double-stranded AAV serotype 8 vector carrying human delta-SG gene without the need of any physical or pharmaceutical interventions achieved nearly complete gene transfer and tissue-specific expression in the heart and skeletal muscles of the diseased hamsters. Broad and sustained (>12 months) restoration of the missing delta-SG gene in the TO-2 hamsters corrected muscle cell membrane leakiness throughout the body and normalized serum creatine kinase levels (a 50- to 100-fold drop). Histological examination revealed minimal or the absence of central nucleation, fibrosis, and calcification in the skeletal muscle and heart. Whole-body functional analysis such as treadmill running showed dramatic improvement, similar to the wild-type F1B hamsters. Furthermore, cardiac functional studies with echocardiography revealed significantly increased percent fractional shortening and decreased left ventricular end-diastolic and end-systolic dimensions in the treated TO-2 hamsters. The survival time of the animals was also dramatically extended.

CONCLUSIONS

Systemic gene transfer of delta-SG by the AAV serotype 8 vector could effectively ameliorate cardiac and skeletal muscle pathology, profoundly improve cardiac and whole-body functions, and significantly prolong the lifespan of the treated TO-2 hamsters.

Deficiency of alpha-sarcoglycan differently affects fast- and slow-twitch skeletal muscles

Alpha-sarcoglycan (Sgca) is a transmembrane glycoprotein of the dystrophin complex located at skeletal and cardiac muscle sarcolemma. Defects in the alpha-sarcoglycan gene (Sgca) cause the severe human-type 2D limb girdle muscular dystrophy. Because Sgca-null mice develop progressive muscular dystrophy similar to human disorder they are a valuable animal model for investigating the physiopathology of the disorder. In this study, biochemical and functional properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of the Sgca-null mice were analyzed. EDL muscle of Sgca-null mice showed twitch and tetanic kinetics comparable with those of wild-type controls. In contrast, soleus muscle showed reduction of twitch half-relaxation time, prolongation of tetanic half-relaxation time, and increase of maximal rate of rise of tetanus. EDL muscle of Sgca-null mice demonstrated a marked reduction of specific twitch and tetanic tensions and a higher resistance to fatigue compared with controls, changes that were not evident in dystrophic soleus. Contrary to EDL fibers, soleus muscle fibers of Sgca-null mice distinctively showed right shift of the pCa-tension (pCa is the negative log of Ca2+ concentration) relationships and reduced sensitivity to caffeine of sarcoplasmic reticulum. Both EDL and soleus muscles showed striking changes in myosin heavy-chain (MHC) isoform composition, whereas EDL showed a larger number of hybrid fibers than soleus. In contrast to the EDL, soleus muscle of Sgca-null mice contained a higher number of regenerating fibers and thus higher levels of embryonic MHC. In conclusion, this study revealed profound distinctive biochemical and physiological modifications in fast- and slow-twitch muscles resulting from alpha-sarcoglycan deficiency.

The syntrophin-dystrobrevin subcomplex in human neuromuscular disorders

The syntrophins and alpha-dystrobrevin form a subcomplex with dystrophin at the skeletal muscle membrane, and are also highly concentrated at the neuromuscular synapse. Here we demonstrate that the different syntrophins and alpha-dystrobrevin isoforms have distinct expression patterns during human skeletal muscle development, and are differentially affected by loss of dystrophin anchorage and denervation in human neuromuscular disease. During normal fetal development, and in Duchenne muscular dystrophy and denervation disorders, alpha1-syntrophin and alpha-dystrobrevin are absent or markedly reduced at the sarcolemmal membrane. beta1-Syntrophin is the predominant syntrophin isoform expressed at the muscle membrane during development, and it undergoes upregulation in response to loss of alpha1-syntrophin in Duchenne muscular dystrophy and in denervation. Upregulation of beta1-syntrophin in neuromuscular disorders is associated with re-expression of the fetal nicotinic acetylcholine receptor gamma-subunit, cardiac actin, and neonatal myosin, suggesting reversion of muscle fibers to an immature phenotype. We show that denervation specifically affects expression of the syntrophin-dystrobrevin subcomplex and does not affect levels or localization of other members of the dystrophin-associated protein complex. Our results confirm that dystrophin is required for anchorage of the syntrophin-dystrobrevin subcomplex and suggest that expression of the syntrophin-dystrobrevin complex may be independently regulated through neuromuscular transmission.

Epsilon-sarcoglycan immunoreactivity and mRNA expression in mouse brain

Myoclonus dystonia (M-D) is a hereditary movement disorder caused by a maternally imprinted gene that is often associated with psychiatric symptoms. Most cases of M-D are believed to result from mutations of the epsilon-sarcoglycan protein. The neuroanatomical distribution of epsilon-sarcoglycan-like immunoreactivity in mouse was investigated by using an antiserum against the epsilon-sarcoglycan protein. The expression of epsilon-sarcoglycan mRNA was studied by a sensitive fluorescence in situ hybridization (FISH) method. Immunohistochemistry and FISH revealed a wide distribution of epsilon-sarcoglycan protein and mRNA throughout the mouse brain. High expression levels of epsilon-sarcoglycan mRNA and immunoreactivity were found in the mitral cell layer of the olfactory bulb, the Purkinje cell layer in cerebellum, and the monoaminergic neurons in the mouse midbrain. Immunohistochemistry revealed a similar distribution of epsilon-sarcoglycan protein. Double-labeling FISH showed colocalization of tyrosine hydroxylase and epsilon-sarcoglycan mRNAs within all the midbrain dopaminergic (DAergic) cell groups. By combining FISH with fluorescence immunohistochemistry, coexpression of epsilon-sarcoglycan mRNA and tryptophan hydroxylase immunoreactivity was found in the serotonergic (5-HTergic) neurons within the dorsal raphe nucleus. The distribution of epsilon-sarcoglycan in the mouse brain suggests that the symptom complex of M-D may be related to the effects of decreased epsilon-sarcoglycan activity on the development or function of monoaminergic neurons.

Characterization of the ATP-hydrolysing activity of alpha-sarcoglycan

Alpha-Sarcoglycan is a glycoprotein associated with the dystrophin complex at sarcolemma of skeletal and cardiac muscles. Gene defects in alpha-sarcoglycan lead to a severe muscular dystrophy whose molecular mechanisms are not yet clear. A first insight into the function of alpha-sarcoglycan was obtained by finding that it is an ATP-binding protein and that it probably confers ability to hydrolyse ATP to the purified dystrophin complex [Betto, Senter, Ceoldo, Tarricone, Biral and Salviati (1999) J. Biol. Chem. 274, 7907-7912]. In the present study, we present definitive evidence showing that alpha-sarcoglycan is an ATP-hydrolysing enzyme. The appearance of alpha-sarcoglycan protein expression was correlated with the increase in ecto-nucleotidase activity during differentiation of C2C12 cells. Approx. 25% of ecto-nucleotidase activity displayed by the C2C12 myotubes was inhibited by preincubating cells with an antibody specific for the ATP-binding motif of alpha-sarcoglycan. This demonstrates that alpha-sarcoglycan substantially contributes to total ecto-nucleotidase activity of C2C12 myotubes. To characterize further this activity, human embryonic kidney 293 cells were transfected with expression plasmids containing alpha-sarcoglycan cDNA. Transfected cells exhibited a significant increase in the ATP-hydrolysing activity that was abolished by the anti-alpha-sarcoglycan antibody. The enzyme had a substrate specificity for ATP and ADP, did not hydrolyse other triphosphonucleosides, and the affinity for ATP was in the low mM range. The ATPase activity strictly required the presence of both Mg2+ and Ca2+ and was completely inhibited by suramin and reactive blue-2. These results show that alpha-sarcoglycan is a Ca2+, Mg2+-ecto-ATPDase. The possible consequences of the absence of alpha-sarcoglycan activity in the pathogenesis of muscular dystrophy are discussed.

Cell stiffness and receptors: evidence for cytoskeletal subnetworks

Viscoelastic models of cells often treat cells as homogeneous objects. However, studies have demonstrated that cellular properties are local and can change dramatically on the basis of the location probed. Because membrane receptors are linked in various ways to the intracellular space, with some receptors linking to the cytoskeleton and others diffusing freely without apparent linkages, the cellular physical response to mechanical stresses is expected to depend on the receptor engaged. In this study, we tested the hypothesis that cellular mechanical stiffness as measured via cytoskeletally linked receptors is greater than stiffness measured via receptors that are not cytoskeletally linked. We used a magnetic micromanipulator to apply linear stresses to magnetic beads attached to living cells via selected receptors. One of the receptor classes probed, the dystroglycan receptors, is linked to the cytoskeleton, while the other, the transferrin receptors, is not. Fibronectin-coated beads were used to test cellular mechanical properties of the cytoskeleton without membrane dependence by allowing the beads to endocytose. For epithelial cells, transferrin-dependent stiffness and endocytosed bead-dependent stiffness were similar, while dystroglycan-dependent stiffness was significantly lower. For smooth muscle cells, dystroglycan-dependent stiffness was similar to the endocytosed bead-dependent stiffness, while the transferrin-dependent stiffness was lower. The conclusion of this study is that the measured cellular stiffness is critically influenced by specific receptor linkage and by cell type and raises the intriguing possibility of the existence of separate cytoskeletal networks with distinct mechanical properties that link different classes of receptors.

Efficient and long-term intracardiac gene transfer in delta-sarcoglycan-deficiency hamster by adeno-associated virus-2 vectors

Intracardiac gene transfer and gene therapy have been investigated with different vector systems. Here we used adeno-associated virus (AAV) vectors to deliver either a reporter gene or a therapeutic gene into the heart of golden Syrian hamsters. The method of gene delivery was direct infusion of the AAV2 vectors into the coronary artery ex vivo in a heterotopically transplanted heart. When an AAV2 vector carrying the Lac-Z gene driven by CMV promoter was delivered into the heart of healthy hamsters, effective gene transfer was achieved in up to 90% of the cardiomyocytes. Lac-Z gene expression persisted for more than 1 year without immune rejection or promoter shutoff. Furthermore, when an AAV2 vector carrying human delta-sarcoglycan gene was similarly delivered into the heart of Bio14.6 Syrian hamster, a congestive heart failure and limb girdle muscular dystrophy animal model, widespread therapeutic gene transfer was achieved in a majority of the cardiomyocytes. Efficient expression of the human delta-sarcoglycan gene in the dystrophic hamster hearts restored the entire sarcoglycan complex that was missing due to the primary deficiency of delta-sarcoglycan. Transgene expression persisted for 4 months (the duration of the study) without immune rejection or promoter shutoff. These results indicate that AAV is a promising vector system for cardiac gene therapy.

Inhibition of dystroglycan cleavage causes muscular dystrophy in transgenic mice

Dystroglycan (DG) is an essential component of the dystrophin-glycoprotein complex, a molecular scaffold that links the extracellular matrix to the actin cytoskeleton. Dystroglycan protein is post-translationally cleaved into alpha dystroglycan, a highly glycosylated peripheral membrane protein, and beta dystroglycan, a transmembrane protein. Despite clear evidence of the importance of dystroglycan and its associated proteins in muscular dystrophy, the purpose of dystroglycan proteolysis is unclear. By introducing a point mutation at the normal site of proteolysis (serine 654 to alanine, DGS654A), we have created a dystroglycan protein that is severely inhibited in its cleavage. Transgenic expression of DGS654A in mouse skeletal muscles inhibited the expression of endogenously cleaved dystroglycan, while overexpression of wild type dystroglycan by similar amounts did not. DGS654A animals had increased serum creatine kinase activity and most muscles had increased numbers of central nuclei. Overexpression of wild type dystroglycan, by contrast, caused no dystrophy by these measures. Dystrophy in DGS654A muscles correlated with reduced binding of antibodies that recognize glycosylated forms of alpha dystroglycan. Lastly, neuromuscular junctions in DGS654A muscles were aberrant in structure. These data show that aberrant processing of the dystroglycan polypeptide causes muscular dystrophy and suggest that dystroglycan processing is important for the proper glycosylation of alpha dystroglycan.

Assessment of left ventricular systolic and diastolic functions in children with merosin-positive congenital muscular dystrophy

Cardiopathy is an expected finding in X-linked Duchenne and Becker muscular dystrophies. This holds true for some other forms such as autosomal recessive limb-girdle dystrophies. However, data on early-onset and usually severe congenital muscular dystrophies are limited. The purpose of this study was to investigate the presence of cardiac involvement in children with merosin-positive congenital muscular dystrophy. A total of 42 patients and 22 healthy subjects were evaluated by M-mode, 2D, and Doppler echocardiography. Cardiac anatomy, left ventricular dimensions, wall thickness and systolic and diastolic functions were investigated in patients and compared with those of healthy control subjects. Mean left ventricular ejection fraction and shortening fraction were significantly lower in the patient group (P<0.05 and P<0.001, respectively) and in three patients ejection fraction was below 55%. Although some impairments in left ventricular inflow indexes which were suggestive of left ventricular diastolic dysfunction were detected in patients with merosin-positive congenital muscular dystrophy they were not statistically significant. Our results suggest that left ventricular systolic abnormalities may occur in children with merosin-positive congenital muscular dystrophy.