Overexpression of gamma-sarcoglycan induces severe muscular dystrophy. Implications for the regulation of Sarcoglycan assembly

Systemic γ-sarcoglycan AAV gene transfer results in dose-dependent correction of muscle deficits in the LGMD 2C/R5 mouse model

Limb-girdle muscular dystrophy (LGMD) type 2C/R5 results from mutations in the γ-sarcoglycan (SGCG) gene and is characterized by muscle weakness and progressive wasting. Loss of functional γ-sarcoglycan protein in the dystrophin-associated protein complex destabilizes the sarcolemma, leading to eventual myofiber death. The SGCG knockout mouse (SGCG ^-/-) has clinical-pathological features that replicate the human disease, making it an ideal model for translational studies. We designed a self-complementary rAAVrh74 vector containing a codon-optimized human SGCG transgene driven by the muscle-specific MHCK7 promoter (SRP-9005) to investigate adeno-associated virus (AAV)-mediated SGCG gene transfer in SGCG ^-/- mice as proof of principle for LGMD 2C/R5. Gene transfer therapy resulted in widespread transgene expression in skeletal muscle and heart, improvements in muscle histopathology characterized by decreased central nuclei and fibrosis, and normalized fiber size. Histopathologic improvements were accompanied by functional improvements, including increased ambulation and force production and resistance to injury of the tibialis anterior and diaphragm muscles. This study demonstrates successful systemic delivery of the hSGCG transgene in SGCG ^-/- mice, with functional protein expression, reconstitution of the sarcoglycan complex, and corresponding physiological and functional improvements, which will help establish a minimal effective dose for translation of SRP-9005 gene transfer therapy in patients with LGMD 2C/R5.

Absolute expressions of hypoxia-inducible factor-1 alpha (HIF1A) transcript and the associated genes in chicken skeletal muscle with white striping and wooden breast myopathies

Development of white striping (WS) and wooden breast (WB) in broiler breast meat have been linked to hypoxia, but their etiologies are not fully understood. This study aimed at investigating absolute expression of hypoxia-inducible factor-1 alpha subunit (HIF1A) and genes involved in stress responses and muscle repair using a droplet digital polymerase chain reaction. Total RNA was isolated from pectoralis major collected from male 6-week-old medium (carcass weight ≤ 2.5 kg) and heavy (carcass weight > 2.5 kg) broilers. Samples were classified as “non-defective” (n = 4), “medium-WS” (n = 6), “heavy-WS” (n = 7) and “heavy-WS+WB” (n = 3) based on abnormality scores. The HIF1A transcript was up-regulated in all of the abnormal groups. Transcript abundances of genes encoding 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 4 (PFKFB4), lactate dehydrogenase-A (LDHA), and phosphorylase kinase beta subunit (PHKB) were increased in heavy-WS but decreased in heavy-WS+WB. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was up-regulated in non-defective samples. The muscle-specific mu-2 isoform of glutathione S-transferases (GSTM2) was up-regulated in the abnormal samples, particularly in the heavy groups. The genes encoding myogenic differentiation (MYOD1) and myosin light chain kinase (MYLK) exhibited similar expression pattern, of which medium-WS and heavy-WS significantly increased compared to non-defective whereas expression in heavy-WS+WB was not different from either non-defective or WS-affected group. The greatest and the lowest levels of calpain-3 (CAPN3) and delta-sarcoglycan (SCGD) were observed in heavy-WS and heavy-WS+WB, respectively. Based on micrographs, the abnormal muscles primarily comprised fibers with cross-sectional areas ranging from 2,000 to 3,000 μm². Despite induced glycolysis at the transcriptional level, lower stored glycogen in the abnormal muscles corresponded with the reduced lactate and higher pH within their meats. The findings support hypoxia within the abnormal breasts, potentially associated with oversized muscle fibers. Between WS and WB, divergent glucose metabolism, cellular detoxification and myoregeneration at the transcriptional level could be anticipated.

Bi-allelic mutations in MYL1 cause a severe congenital myopathy

Congenital myopathies are typically characterised by early onset hypotonia, weakness and hallmark features on biopsy. Despite the rapid pace of gene discovery, ∼50% of patients with a congenital myopathy remain without a genetic diagnosis following screening of known disease genes. We performed exome sequencing on two consanguineous probands diagnosed with a congenital myopathy and muscle biopsy showing selective atrophy/hypotrophy or absence of type II myofibres. We identified variants in the gene (MYL1) encoding the skeletal muscle fast-twitch specific myosin essential light chain (ELC) in both probands. A homozygous essential splice acceptor variant (c.479-2A > G, predicted to result in skipping of exon 5 was identified in Proband 1, and a homozygous missense substitution (c.488T>G, p.(Met163Arg)) was identified in Proband 2. Protein modelling of the p.(Met163Arg) substitution predicted it might impede intermolecular interactions that facilitate binding to the IQ domain of myosin heavy chain, thus likely impacting on the structure and functioning of the myosin motor. MYL1 was markedly reduced in skeletal muscle from both probands, suggesting that the missense substitution likely results in an unstable protein. Knock down of myl1 in zebrafish resulted in abnormal morphology, disrupted muscle structure and impaired touch-evoked escape responses, thus confirming that skeletal muscle fast-twitch specific myosin ELC is critical for myofibre development and function. Our data implicate MYL1 as a crucial protein for adequate skeletal muscle function and that MYL1 deficiency is associated with severe congenital myopathy.

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.

Gene therapy in monogenic congenital myopathies

Current treatment options for patients with monogenetic congenital myopathies (MCM) ameliorate the symptoms of the disorder without resolving the underlying cause. However, gene therapies are being developed where the mutated or deficient gene target is replaced. Preclinical findings in animal models appear promising, as illustrated by gene replacement for X-linked myotubular myopathy (XLMTM) in canine and murine models. Prospective applications and approaches to gene replacement therapy, using these disorders as examples, are discussed in this review.

Reengineering a transmembrane protein to treat muscular dystrophy using exon skipping

Exon skipping uses antisense oligonucleotides as a treatment for genetic diseases. The antisense oligonucleotides used for exon skipping are designed to bypass premature stop codons in the target RNA and restore reading frame disruption. Exon skipping is currently being tested in humans with dystrophin gene mutations who have Duchenne muscular dystrophy. For Duchenne muscular dystrophy, the rationale for exon skipping derived from observations in patients with naturally occurring dystrophin gene mutations that generated internally deleted but partially functional dystrophin proteins. We have now expanded the potential for exon skipping by testing whether an internal, in-frame truncation of a transmembrane protein γ-sarcoglycan is functional. We generated an internally truncated γ-sarcoglycan protein that we have termed Mini-Gamma by deleting a large portion of the extracellular domain. Mini-Gamma provided functional and pathological benefits to correct the loss of γ-sarcoglycan in a Drosophila model, in heterologous cell expression studies, and in transgenic mice lacking γ-sarcoglycan. We generated a cellular model of human muscle disease and showed that multiple exon skipping could be induced in RNA that encodes a mutant human γ-sarcoglycan. Since Mini-Gamma represents removal of 4 of the 7 coding exons in γ-sarcoglycan, this approach provides a viable strategy to treat the majority of patients with γ-sarcoglycan gene mutations.

The Sarcoglycan complex is expressed in the cerebrovascular system and is specifically regulated by astroglial Cx30 channels

Astrocytes, the most prominent glial cell type in the brain, send specialized processes called endfeet, around blood vessels and express a large molecular repertoire regulating the cerebrovascular system physiology. One of the most striking properties of astrocyte endfeet is their enrichment in gap junction proteins Connexin 43 and 30 (Cx43 and Cx30) allowing in particular for direct intercellular trafficking of ions and small signaling molecules through perivascular astroglial networks. In this study, we addressed the specific role of Cx30 at the gliovascular interface. Using an inactivation mouse model for Cx30 (Cx30^Δ/Δ; Δ means deleted allele) we showed that absence of Cx30 does not affect blood-brain barrier (BBB) organization and permeability. However, it results in the cerebrovascular fraction, in a strong upregulation of Sgcg encoding γ-Sarcoglycan (γ-SG), a member of the Dystrophin-associated protein complex (DAPC) connecting cytoskeleton and the extracellular matrix. The same molecular event occurs in Cx30^T5M/T5M mutated mice, where Cx30 channels are closed, demonstrating that Sgcg regulation relied on Cx30 channel functions. We further characterized the expression of other Sarcoglycan complex (SGC) molecules in the cerebrovascular system and showed the presence of α-, β-, δ-, γ-, ε- and ζ- SG, as well as Sarcospan. Their expression was however not modified in Cx30^Δ/Δ. These results suggest that a full SGC might be present in the cerebrovascular system, and that expression of one of its member, γ-SG, depends on Cx30 channels. As described in skeletal muscles, the SGC may contribute to membrane stabilization and signal transduction in the cerebrovascular system, which may therefore be regulated by Cx30 channel-mediated functions.

What do mouse models of muscular dystrophy tell us about the DAPC and its components?

There are over 30 mouse models with mutations or inactivations in the dystrophin-associated protein complex. This complex is thought to play a crucial role in the functioning of muscle, as both a shock absorber and signalling centre, although its role in the pathogenesis of muscular dystrophy is not fully understood. The first mouse model of muscular dystrophy to be identified with a mutation in a component of the dystrophin-associated complex (dystrophin) was the mdx mouse in 1984. Here, we evaluate the key characteristics of the mdx in comparison with other mouse mutants with inactivations in DAPC components, along with key modifiers of the disease phenotype. By discussing the differences between the individual phenotypes, we show that the functioning of the DAPC and consequently its role in the pathogenesis is more complicated than perhaps currently appreciated.

Expression pattern of mRNA A and mRNA B of alpha sarcoglycan gene during mouse embryonic development and regulation of their expression by myogenic and cardiogenic transcription factors

BACKGROUND

Type 2D limb-girdle muscular dystrophy (LGM2D) is a progressive disorder caused by mutations in the alpha sarcoglycan (α-SG) gene. In mice, the α-SG gene contains two promoters that regulate the expression of two different mRNAs (A and B). However, their gene expression pattern during embryonic development has not been explored and their regulation by myogenic and cardiogenic transcription factors has been only partially studied.

RESULTS

During embryonic development, mRNA A and B of α-SG gene were initially detected in hypaxial muscles, heart, stomach, tongue, and mesenchymal cells, which surround the dorsal region of the somites. Moreover, mRNA B was exclusively expressed in the floor plate and notochord and in the interdigits of limbs. In vitro, MyoD and myogenin positively regulated the transcription of mRNA B during skeletal myogenesis, whereas mRNA A was activated only for MyoD in differentiated skeletal muscle. In addition, Gata-4 together with Mef2c may regulate the expression of mRNA B in heart development, whereas Nkx2.5 and myocardin may activate expression of mRNA A in the differentiated cardiomyocyte.

CONCLUSIONS

The differential expression of α-SG mRNAs during mouse embryonic development may be a consequence of the differential regulation of both promoters by myogenic and cardiogenic factors.

S151A δ-sarcoglycan mutation causes a mild phenotype of cardiomyopathy in mice

So far, the role of mutations in the δ-sarcogylcan (Sgcd) gene in causing autosomal dominant dilated cardiomyopathy (DCM) remains inconclusive. A p.S151A missense mutation in exon 6 of the Sgcd gene was reported to cause severe isolated autosomal dominant DCM without affecting skeletal muscle. This is controversial to our previous findings in a large consanguineous family where this p.S151A mutation showed no relevance for cardiac disease. In this study, the potential of the p.S151A mutation to cause DCM was investigated by using two different approaches: (1) engineering and characterization of heterozygous knock-in (S151A-) mice carrying the p.S151A mutation and (2) evaluation of the potential of adeno-associated virus (AAV) 9-based cardiac-specific transfer of p.S151A-mutated Sgcd cDNA to rescue the cardiac phenotype in Sgcd-deficient (Sgcd-null) mice as it has been demonstrated for intact, wild-type Sgcd cDNA. Heterozygous S151A knock-in mice developed a rather mild phenotype of cardiomyopathy. Increased heart to body weight suggests cardiac enlargement in 1-year-old S151A knock-in mice. However, at this age cardiac function, assessed by echocardiography, is maintained and histopathology completely absent. Myocardial expression of p.S151A cDNA, similar to intact Sgcd cDNA, restores cardiac function, although not being able to prevent myocardial histopathology in Sgcd-null mice completely. Our results suggest that the p.S151A mutation causes a mild, subclinical phenotype of cardiomyopathy, which is prone to be overseen in patients carrying such sequence variants. Furthermore, this study shows the suitability of an AAV-mediated cardiac gene transfer approach to analyze whether a sequence variant is a disease-causing mutation.

Gene transfer to muscle from the isolated regional circulation

Vector transport across the endothelium has long been regarded as one of the central "bottlenecks" in gene therapy research, especially as it pertains to the muscular dystrophies where the target tissue approaches half of the total body mass. Clinical studies of gene therapy for hemophilia B revealed the limitations of the intramuscular route, compelling an aggressive approach to the study of scale-independent circulatory means of vector delivery. The apparent permeability of the microvasculature in small animals suggests that gravitational and/or inertial effects on the circulation require progressive restriction of fluid and solute flow across the capillary wall with increasing body size. To overcome this physiological restriction, we initially used a combined surgical and pharmacological approach to temporarily alter permeability within the isolated pelvic limb. Although this was successful, new information about the cell and molecular biology of histamine-induced changes in microvascular permeability suggested an alternative approach, which substituted pressure-induced transvenular extravasation. Here we outline the details of our surgical approaches in the rat. We also discuss the modifications that are appropriate for the dog.

Dysferlin overexpression in skeletal muscle produces a progressive myopathy

OBJECTIVE

The dose-response effects of dysferlin transgenesis were analyzed to determine if the dysferlin-deficient myopathies are good candidates for gene replacement therapy.

METHODS

We have generated 3 lines of transgenic mice, expressing low, mid, and high levels of full-length human dysferlin from a muscle-specific promoter. Transgenic skeletal muscle was analyzed and scored for morphological and functional deficits.

RESULTS

Overexpression of dysferlin in mice resulted in a striking phenotype of kyphosis, irregular gait, and reduced muscle mass and strength. Moreover, protein dosage correlated with phenotype severity. In contrast to dysferlin-null skeletal muscle, no evidence of sarcolemmal impairment was revealed. Rather, increased levels of Ca(2+)-regulated, dysferlin-binding proteins and endoplasmic reticulum stress chaperone proteins were observed in muscle lysates from transgenic mice as compared with controls.

INTERPRETATION

Expression levels of dysferlin are important for appropriate function without deleterious or cytotoxic effects. As a corollary, we propose that future endeavors in gene replacement for correction of dysferlinopathy should be tailored to take account of this.

Restoration of gamma-sarcoglycan localization and mechanical signal transduction are independent in murine skeletal muscle

Limb girdle muscular dystrophy 2C is caused by mutations in the gamma-sarcoglycan gene (gsg) that results in loss of this protein, and disruption of the sarcoglycan (SG) complex. Signal transduction after mechanical perturbation is mediated, in part, through the SG complex and leads to phosphorylation of tyrosines on the intracellular portions of the sarcoglycans. This study tested if the Tyr(6) in the intracellular region of gamma-sarcoglycan protein (gamma-SG) was necessary for proper localization of the protein in skeletal muscle membranes or for the normal pattern of ERK1/2 phosphorylation after eccentric contractions. Viral mediated gene transfer of wild type gsg (WTgsg) and mutant gsg lacking Tyr(6) (Y6Agsg) was performed into the muscles of gsg(-/-) mice. Muscles were examined for production and stability of the gamma-SG, as well as the level of ERK1/2 phosphorylation before and after eccentric contraction. Sarcolemmal localization of gamma-SG was achieved regardless of which construct was expressed. However, only expression of WTgsg corrected the aberrant ERK1/2 phosphorylation associated with the absence of gamma-SG, whereas Y6Agsg failed to have any effect. This study shows that localization of gamma-SG does not require Tyr(6), but localization alone is insufficient for restoration of normal signal transduction patterns after mechanical perturbation.

Genetic defects in muscular dystrophy

The muscular dystrophies are a group of neuromuscular disorders associated with muscle weakness and wasting, which in many forms can lead to loss of ambulation and premature death. A number of muscular dystrophies are associated with loss of proteins required for the maintenance of muscle membrane integrity, in particular with proteins that comprise the dystrophin-associated glycoprotein (DAG) complex. Proper glycosylation of O-linked mannose chains on alpha-dystroglycan, a DAG member, is required for the binding of the extracellular matrix to dystroglycan and for proper DAG function. A number of congenital disorders of glycosylation have now been described where alpha-dystroglycan glycosylation is altered and where muscular dystrophy is a predominant phenotype. Glycosylation is also increasingly being appreciated as a genetic modifier of disease phenotypes in many forms of muscular dystrophy and as a target for the development of new therapies. Here we will review the mouse models available for the study of this group of diseases and outline the methodologies required to describe disease phenotypes.

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.

Sub-physiological sarcoglycan expression contributes to compensatory muscle protection in mdx mice

Sarcoglycans are a group of single-pass transmembrane glycoproteins. In striated muscle, sarcoglycans interact with dystrophin and other dystrophin-associated proteins (DAPs) to form the dystrophin-associated glycoprotein complex (DGC). The DGC protects the sarcolemma from contraction-induced injury. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations. In the absence of dystrophin, the DGC is disassembled from the sarcolemma. This initiates a chain reaction of muscle degeneration, necrosis, inflammation and fibrosis. In contrast to human patients, dystrophin-null mdx mice are only mildly affected. Enhanced muscle regeneration and the up-regulation of utrophin and integrin are thought to protect mdx muscle. Interestingly, trace amounts of sarcoglycans and other DAPs can be detected at the mdx sarcolemma. It is currently unclear whether sub-physiological sarcoglycan expression also contributes to the mild phenotype in mdx mice. To answer this question, we generated delta-sarcoglycan/dystrophin double knockout mice (delta-Dko) in which residual sarcoglycans were completely eliminated from the sarcolemma. Interestingly, utrophin levels were further increased in these mice. However, enhanced utrophin expression did not mitigate disease. The clinical manifestation of delta-Dko mice was worse than that of mdx mice. They showed characteristic dystrophic signs, body emaciation and more macrophage infiltration. Their lifespan was reduced by 60%. Furthermore, delta-Dko muscle generated significantly less absolute muscle force and became more susceptible to contraction-induced injury. Our results suggest that sub-physiological sarcoglycan expression plays a critical role in ameliorating muscle disease in mdx mice. We speculate that low-level sarcoglycan expression may represent a useful strategy to palliate DMD.

Does delta-sarcoglycan-associated autosomal-dominant cardiomyopathy exist?

In this study we clinically and genetically characterize a consanguineous family with a homozygous novel missense mutation in the delta-sarcoglycan gene and a second delta-sarcoglycan mutation that has previously been reported to cause severe autosomal-dominant dilated cardiomyopathy. We identified a novel missense mutation in exon 6 (p.A131P) of the delta-sarcoglycan gene, which in a homozygous state leads to the clinical picture of a limb girdle muscular dystrophy. In four heterozygous carriers for the mutation, aged 3-64 years, a second sequence variant in exon 6 (p.S151A) of the delta-sarcoglycan gene was detected on the other allele. This second missense change had previously been reported to be responsible for fatal autosomal-dominant dilated cardiomyopathy at young age. Comprehensive clinical and cardiac investigation in all of the compound heterozygous family members revealed no signs of cardiomyopathy or limb girdle muscular dystrophy. Our findings demonstrate that, even in the presence of a second disease-causing mutation, the p.S151A mutation in the delta-sarcoglycan gene does not result in cardiomyopathy. This finding questions the pathological relevance of this sequence variant for causing familial autosomal-dominant dilated cardiomyopathy and thereby the role of the delta-sarcoglycan gene in general as a disease-causing gene for autosomal-dominant dilated cardiomyopathy.

A highly stable and nonintegrated human artificial chromosome (HAC) containing the 2.4 Mb entire human dystrophin gene

Episomal vector with the capacity to deliver a large gene containing all the critical regulatory elements is ideal for gene therapy. Human artificial chromosomes (HACs) have the capacity to deliver an extremely large genetic region to host cells without integration into the host genome, thus preventing possible insertional mutagenesis and genomic instability. Duchenne muscular dystrophy (DMD) is caused by mutation in the extremely large dystrophin gene (2.4 Mb). We herein report the development of a HAC vector containing the entire human dystrophin gene (DYS-HAC) that is stably maintained in mice and human immortalized mesenchymal stem cells (hiMSCs). The DYS-HAC was transferred to mouse embryonic stem (ES) cells, and isoforms of the DYS-HAC-derived human dystrophin in the chimeric mice generated from the ES cells were correctly expressed in tissue-specific manner. Thus, this HAC vector containing the entire dystrophin gene with its native regulatory elements is expected to be extremely useful for future gene and cell therapies of DMD.

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.

Inhibition of proteasome activity promotes the correct localization of disease-causing alpha-sarcoglycan mutants in HEK-293 cells constitutively expressing beta-, gamma-, and delta-sarcoglycan

Sarcoglycanopathies are progressive muscle-wasting disorders caused by genetic defects of four proteins, alpha-, beta-, gamma-, and delta-sarcoglycan, which are elements of a key transmembrane complex of striated muscle. The proper assembly of the sarcoglycan complex represents a critical issue of sarcoglycanopathies, as several mutations severely perturb tetramer formation. Misfolded proteins are generally degraded through the cell's quality-control system; however, this can also lead to the removal of some functional polypeptides. To explore whether it is possible to rescue sarcoglycan mutants by preventing their degradation, we generated a heterologous cell system, based on human embryonic kidney (HEK) 293 cells, constitutively expressing three (beta, gamma, and delta) of the four sarcoglycans. In these betagammadelta-HEK cells, the lack of alpha-sarcoglycan prevented complex formation and cell surface localization, wheras the presence of alpha-sarcoglycan allowed maturation and targeting of the tetramer. As in muscles of sarcoglycanopathy patients, transfection of betagammadelta-HEK cells with disease-causing alpha-sarcoglycan mutants led to dramatic reduction of the mutated proteins and the absence of the complex from the cell surface. Proteasomal inhibition reduced the degradation of mutants and facilitated the assembly and targeting of the sarcoglycan complex to the plasma membrane. These data provide important insights for the potential development of pharmacological therapies for sarcoglycanopathies.

Identification of two E-boxes that negatively modulate the activity of MyoD on the alpha-sarcoglycan core promoter

The alpha-SG promoter is composed of a plethora of cis-regulatory elements, whose individual contribution to alpha-SG gene expression modulation remains unknown. We have identified a negative regulatory element in the alpha-SG distal promoter including two conserved E-boxes (E1 and E2), which interact with MyoD. We found that E1 and E2 negatively modulate the transactivation potential of MyoD on the alpha-SG core promoter. Moreover, such negative effect is mainly mediated by E2, which is surrounded by conserved nucleotides conferring MyoD binding capacity. Our results suggest that modulation of MyoD activity by E1, and particularly E2, contributes to the negative regulation of alpha-SG gene expression during myogenic differentiation.

Ins and outs of therapy in limb girdle muscular dystrophies

Muscular dystrophies are hereditary degenerative muscle diseases that cause life-long disability in patients. They comprise the well-known Duchenne Muscular Dystrophy (DMD) but also the group of Limb Girdle Muscular Dystrophies (LGMD) which account for a third to a fourth of DMD cases. From the clinical point of view, LGMD are characterised by predominant effects on the proximal limb muscles. The LGMD group is still growing today and consists of 19 autosomal dominant and recessive forms (LGMD1A to LGMD1G and LGMD2A to LGMD2M). The proteins involved are very diverse and include sarcomeric, sarcolemmal and enzymatic proteins. With respect to this variability and in line with the intense search for a potent therapeutic approach for DMD, many different strategies have been tested in rodent models. These include replacing the lost function by gene transfer or stem cell transplantation, using a related protein for functional substitution, increasing muscle mass, or blocking the molecular pathological mechanisms by pharmacological means to alleviate the symptoms. The purpose of this review is to summarize current data arising from these preclinical studies and to examine the potential of the tested strategies to lead to clinical applications.

Simultaneous dystrophin and dysferlin deficiencies associated with high-level expression of the coxsackie and adenovirus receptor in transgenic mice

The Coxsackie and adenovirus receptor (CAR), a cell adhesion molecule of the immunoglobulin superfamily, is usually confined to the sarcolemma at the neuromuscular junction in mature skeletal muscle fibers. Previously, we reported that adenovirus-mediated gene transfer is greatly facilitated in hemizygous transgenic mice with extrasynaptic CAR expression driven by a muscle-specific promoter. However, in the present study, when these mice were bred to homozygosity, they developed a severe myopathic phenotype and died prematurely. Large numbers of necrotic and regenerating fibers were present in the skeletal muscle of the homozygous CAR transgenics. The myopathy was further characterized by increased levels of caveolin-3 and beta-dystroglycan and decreased levels of dystrophin, dysferlin, and neuronal nitric-oxide synthase. Even the hemizygotes manifested a subtle phenotype, displaying deficits in isometric force generation and perturbed mitogen-activated protein kinase (MAPK-erk1/2) activation during contraction. There are few naturally occurring or engineered mouse lines showing as severe a skeletal myopathy as observed with ectopic expression of CAR in the homozygotes. Taken together, these findings suggest that substantial overexpression of CAR may lead to physiological dysfunction by disturbing sarcolemmal integrity (through dystrophin deficiency), impairing sarcolemmal repair (through dysferlin deficiency), and interfering with normal signaling (through alterations in caveolin-3 and neuronal nitric-oxide synthase levels).

Partial characterization of the mouse alpha-sarcoglycan promoter and its responsiveness to MyoD

The mouse alpha-sarcoglycan gene is expressed in muscle cells during differentiation, but its transcriptional regulation is not understood. We have characterized the promoter region of the mouse alpha-sarcoglycan gene. This region is composed of positive and negative regulatory elements that respond to the myogenic differentiation environment. Accordingly, MyoD transactivates the alpha-sarcoglycan full-length and the proximal promoter. Chromatin immunoprecipitation assays revealed that MyoD, TFIID, and TFIIB interact with the distal promoter in C2C12 myoblasts, a stage at which the alpha-SG promoter appears to drive basal activity. In myotubes, such factors are located concomitantly at the distal promoter and at a DNA region around the proximal promoter. In agreement with these results, TFIID and TFIIB co-immunoprecipitate with MyoD. We conclude that the alpha-SG promoter is activated by MyoD, which interacts with TFIID and TFIIB in a protein complex differentially located at the distal promoter and around the proximal promoter during myogenic cell differentiation.

Smooth muscle trans-membrane sarcoglycan complex in partial bladder outlet obstruction

The urinary bladder experiences both distension and contraction as a part of the normal filling and emptying cycle. To empty properly, tension generated intracellularly in a smooth muscle cell must be smoothly and efficiently transferred across its sarcolemma to the basement membrane, which mediates its binding to both the extracellular matrix and to other cells. As a consequence of urethral obstruction, the bladder cannot generate appropriate force to contract the organ, thereby leading to inefficient emptying and associated sequelae. In this study, an animal model of urethral obstruction was utilized to study the membrane-associated structures that transfer tension across the sarcolemma of bladder smooth muscle cells. Immunohistochemical localization of key components of the smooth muscle tension transfer apparatus (TTA) was performed utilizing specific antibodies against:(1) the alpha-chains of type IV collagen, a basement membrane component, and (2) beta-sarcoglycan, an integral membrane protein that is a participant in the physical linkage between the cytoskeleton and the basement membrane. We demonstrate, in obstructed animals, that there is a pronounced disruption of the TTA with a physical displacement of these two components that can be demonstrated at the level of the light microscope using scanning confocal microscopy. Electron microscopy further demonstrates significant increases in the size of the junctional plaques between smooth muscle cells.

Canine and feline models of human inherited muscle diseases

Animal models are of immense importance for studying mechanisms of disease and testing new therapies, and rodents have been used extensively in the field of neuromuscular disorders. Mice and rats can be genetically manipulated to over-express or not express genes that are important to muscle function, and these animals can be available in large numbers for analysis. Other species, such as cats and dogs, cannot be manipulated in the same ways or be used in large numbers, but they have spontaneously occurring muscle diseases with clinical presentations more closely resembling those of the human disorders. Therefore, cats and dogs may become valuable as intermediate disease models. This review focuses on canine and feline models of human inherited muscle diseases with comparisons to rodent models and an emphasis on the muscular dystrophies.

Molecular and cell biology of the sarcoglycan complex

The original sarcoglycan (SG) complex has four subunits and comprises a subcomplex of the dystrophin-dystrophin-associated protein complex. Each SG gene has been shown to be responsible for limb-girdle muscular dystrophy, called sarcoglycanopathy (SGP). In this review, we detail the characteristics of the SG subunits, and the mechanism of the formation of the SG complex and various molecules associated with this complex. We discuss the molecular mechanisms of SGP based on studies mostly using SGP animal models. In addition, we describe other SG molecules, epsilon- and zeta-SGs, with special reference to their expression and roles in vascular smooth muscle, which are currently in dispute. We further consider the maternally imprinted nature of the epsilon-SG gene. Finally, we stress that the SG complex cannot work by itself and works in a larger complex system, called the transverse fixation system, which forms an array of molecules responsible for various muscular dystrophies.

Disturbed trafficking of dystrophin and associated proteins in targetoid phenomena after chronic muscle denervation

Dystrophin and associated proteins form a complex with an important role at the sarcolemma. Expression of this protein complex is highly regulated during development and regeneration. In order to better understand assembling patterns of these proteins, we have studied their expression in targetoid-like phenomena found in human muscle after chronic denervation, a situation known to give rise to abnormal protein trafficking. In eight biopsies of patients with chronic denervation, mainly resulting from amyotrophic lateral sclerosis, we found a number of targetoid phenomena. Selective accumulation of a number of sarcolemmal and sarcoplasmatic proteins occurred in targetoid phenomena. The larger majority of them contained gamma-sarcoglycan (gammaSG), but none contained the developmental heavy chain myosin isoform. In a series of 166 targetoid phenomena which could be studied with 17 different antibodies recognizing sarcolemmal and cytoplasmatic proteins, a high level of colocalization of gammaSG with desmin and alpha-actinin was found. Colocalization rate was much lower with other proteins, including other members of the dystrophin-associated protein complex. These data show that selective changes in expression of otherwise closely related proteins occur during disturbed trafficking leading to target formation. Because members of the dystrophin-associated protein complex do not accumulate in a similar fashion within targets, we suggest that a complex molecular control of gene expression and trafficking of this complex is involved after chronic muscle denervation.

NFI-C2 negatively regulates α-sarcoglycan promoter activity in C2C12 myoblasts

alpha-Sarcoglycan striated muscle-specific protein is a member of the sarcoglycan-sarcospan complex. Positive and negative transcriptional regulation of sarcoglycan genes are important in sarcoglycan's intracellular localization and sarcolemmal stability. In the present work we assessed the function of NFI transcription factors in the regulation of alpha-sarcoglycan promoter through the C2C12 cell line differentiation. NFI factors act alternatively as activators and negative modulators of alpha-sarcoglycan promoter activity. In myoblasts NFI-A1.1 and NFI-B2 are activators, whereas NFI-C2 and NFI-X2 are negative regulators. In myotubes, all NFI members are activators, being NFI-C2 the less potent. We identified the alpha-sarcoglycan promoter NFI-C2 response element by testing progressive deletion constructs and point mutations in C2C12 cells over-expressing NFI-C2. Gel-shift and chromatin immunoprecipitation experiments demonstrated that NFI factors are indeed interacting in vitro and in vivo with the binding sequence. These results suggest a NFI role in C2C12 cell differentiation.

Calpain System Regulates Muscle Mass and Glucose Transporter GLUT4 Turnover

The experiments in this study were undertaken to determine whether inhibition of calpain activity in skeletal muscle is associated with alterations in muscle metabolism. Transgenic mice that overexpress human calpastatin, an endogenous calpain inhibitor, in skeletal muscle were produced. Compared with wild type controls, muscle calpastatin mice demonstrated normal glucose tolerance. Levels of the glucose transporter GLUT4 were increased more than 3-fold in the transgenic mice by Western blotting while mRNA levels for GLUT4 and myocyte enhancer factors, MEF 2A and MEF 2D, protein levels were decreased. We found that GLUT4 can be degraded by calpain-2, suggesting that diminished degradation is responsible for the increase in muscle GLUT4 in the calpastatin transgenic mice. Despite the increase in GLUT4, glucose transport into isolated muscles from transgenic mice was not increased in response to insulin. The expression of protein kinase B was decreased by approximately 60% in calpastatin transgenic muscle. This decrease could play a role in accounting for the insulin resistance relative to GLUT4 content of calpastatin transgenic muscle. The muscle weights of transgenic animals were substantially increased compared with controls. These results are consistent with the conclusion that calpain-mediated pathways play an important role in the regulation of GLUT4 degradation in muscle and in the regulation of muscle mass. Inhibition of calpain activity in muscle by overexpression of calpastatin is associated with an increase in GLUT4 protein without a proportional increase in insulin-stimulated glucose transport. These findings provide evidence for a physiological role for calpains in the regulation of muscle glucose metabolism and muscle mass.

Smooth muscle cell-extrinsic vascular spasm arises from cardiomyocyte degeneration in sarcoglycan-deficient cardiomyopathy

Vascular spasm is a poorly understood but critical biomedical process because it can acutely reduce blood supply and tissue oxygenation. Cardiomyopathy in mice lacking gamma-sarcoglycan or delta-sarcoglycan is characterized by focal damage. In the heart, sarcoglycan gene mutations produce regional defects in membrane permeability and focal degeneration, and it was hypothesized that vascular spasm was responsible for this focal necrosis. Supporting this notion, vascular spasm was noted in coronary arteries, and disruption of the sarcoglycan complex was observed in vascular smooth muscle providing a molecular mechanism for spasm. Using a transgene rescue strategy in the background of sarcoglycan-null mice, we replaced cardiomyocyte sarcoglycan expression. Cardiomyocyte-specific sarcoglycan expression was sufficient to correct cardiac focal degeneration. Intriguingly, successful restoration of the cardiomyocyte sarcoglycan complex also eliminated coronary artery vascular spasm, while restoration of smooth muscle sarcoglycan in the background of sarcoglycan-null alleles did not. This mechanism, whereby tissue damage leads to vascular spasm, can be partially corrected by NO synthase inhibitors. Therefore, we propose that cytokine release from damaged cardiomyocytes can feed back to produce vascular spasm. Moreover, vascular spasm feeds forward to produce additional cardiac damage.

Smooth muscle cell-extrinsic vascular spasm arises from cardiomyocyte degeneration in sarcoglycan-deficient cardiomyopathy

Vascular spasm is a poorly understood but critical biomedical process because it can acutely reduce blood supply and tissue oxygenation. Cardiomyopathy in mice lacking gamma-sarcoglycan or delta-sarcoglycan is characterized by focal damage. In the heart, sarcoglycan gene mutations produce regional defects in membrane permeability and focal degeneration, and it was hypothesized that vascular spasm was responsible for this focal necrosis. Supporting this notion, vascular spasm was noted in coronary arteries, and disruption of the sarcoglycan complex was observed in vascular smooth muscle providing a molecular mechanism for spasm. Using a transgene rescue strategy in the background of sarcoglycan-null mice, we replaced cardiomyocyte sarcoglycan expression. Cardiomyocyte-specific sarcoglycan expression was sufficient to correct cardiac focal degeneration. Intriguingly, successful restoration of the cardiomyocyte sarcoglycan complex also eliminated coronary artery vascular spasm, while restoration of smooth muscle sarcoglycan in the background of sarcoglycan-null alleles did not. This mechanism, whereby tissue damage leads to vascular spasm, can be partially corrected by NO synthase inhibitors. Therefore, we propose that cytokine release from damaged cardiomyocytes can feed back to produce vascular spasm. Moreover, vascular spasm feeds forward to produce additional cardiac damage.

Specific assembly pathway of sarcoglycans is dependent on beta- and delta-sarcoglycan

Mutations in sarcoglycans (SG) have been reported to cause autosomal-recessive limb-girdle muscular dystrophy (LGMD) and dilated cardiomyopathy. In skeletal and cardiac muscle, sarcoglycans exist as a complex of four transmembrane proteins (alpha-, beta-, gamma-, and delta-SG). In this study, the assembly of the sarcoglycan complex was examined in a heterologous expression system. Our results demonstrated that the assembly process occurs as a discrete stepwise process. We found that beta-SG appears to play an initiating role and its association with delta-SG is essential for the proper localization of the sarcoglycan complex to the cell membrane. The incorporation of alpha-SG into the sarcoglycan complex occurs at the final stage by interaction with gamma-SG. These findings were supported by chemical cross-linking of endogenous sarcoglycans in cultured myotubes. We have also provided evidence that glycosylation-defective mutations in beta-SG and a common mutation in gamma-SG (C283Y) disrupt sarcoglycan-complex formation. Our proposed model for the assembly and structure of sarcoglycans should generate important insight into their function in muscle as well as their role in muscular dystrophies and cardiomyopathies.

The new frontier in muscular dystrophy research: booster genes

More than 30 different forms of muscular dystrophy (MD) have been molecularly characterized and can be diagnosed, but progress toward treatment has been slow. Gene replacement therapy has met with great difficulty because of the large size of the defective genes and because of difficulties in delivering a gene to all muscle groups. Cell replacement therapy has also been difficult to realize. Will it even be possible to design specific therapy protocols for all MDs? Or is a more realistic goal to treat some of the secondary manifestations that are common to several forms of MD, such as membrane instability, necrosis, and inflammation, and to promote regeneration? As reviewed here, enhanced expression of a range of proteins provides a boost for degenerating dystrophic muscle in mouse models. Expression of a mini-agrin promotes basement membrane formation instead of laminin alpha2; integrin alpha7, GalNac transferase, and ADAM12 promote cell adhesion and muscle stability in the absence of dystrophin; calpastatin prevents muscle necrosis; and nitric oxide synthase prevents inflammation. ADAM12, IGF-I, and myostatin blockade promote regeneration and reduce fibrosis. One can envision numerous other candidate booster genes which encode proteins that promote survival and/or regeneration of the compromised muscle or proteins that affect post-translational modifications of critical proteins. Finally, fibrosis, which is the curse of many human diseases, may also be attacked. Once the mechanisms of the boosters are better understood, drugs may be developed to provide the boost to muscle. Some of the experiences in models of muscular dystrophy may inspire new approaches in other genetic degenerative diseases as well.

Experimental and therapeutic approaches to muscular dystrophies

PURPOSE OF REVIEW

Most patients suffering from muscular dystrophies can now obtain a precise diagnosis of their underlying molecular defect, but no efficient treatment to prevent disability and death. This review summarizes recent progress towards developing efficient treatments for these severe diseases.

RECENT FINDINGS

Different levels of progress have been achieved in three main approaches: gene therapy, cell therapy and pharmacological therapy. Gene therapy has progressed by improving different vectors for gene delivery. Adenoviruses (mainly high capacity versions) and adeno-associated viruses were the most explored viral vectors. Progress was made in understanding the factors needed for an efficient transfection of muscle. An understanding of protein structure and function in muscular dystrophies has allowed elegant examples of protein engineering as a way of gene therapy. Non-viral vectors for gene transfer, targeted gene modification and transcription modulation have also been explored recently. Cell therapy (myogenic-cell transplantation) progressed in understanding myoblast transplantation in primates for human applications, evaluating protocols for the control of graft rejection, understanding the biology of donor myogenic cells, and searching for alternative sources of donor cells. Three clinical trials using pharmacological approaches (anabolic agents and gentamicin) show very poor or negative results. Other pharmacological approaches (upregulation of alternative therapeutic proteins) are still being researched in mice.

SUMMARY

This panoply of experimental approaches covered all the current possibilities of attacking the problem of treating muscular dystrophies. It is expected that one or more will progress to provide efficient tools for the ultimate clinical goal: to prolong function and life in severe muscular dystrophy patients.

Delivery of alpha- and beta-sarcoglycan by recombinant adeno-associated virus: efficient rescue of muscle, but differential toxicity

The sarcoglycanopathies are a group of four autosomal recessive limb girdle muscular dystrophies (LGMD 2D, 2E, 2C, and 2F), caused by mutations of the alpha-, beta-, gamma-, or delta-sarcoglycan genes, respectively. The delta-sarcoglycan-deficient hamster has been the most utilized model for gene delivery to muscle by recombinant adeno-associated virus (AAV) vectors; however, human patients with delta-sarcoglycan deficiency are exceedingly rare, with only two patients described in the United States. Here, we report construction and use of AAV vectors expressing either alpha- or beta-sarcoglycan, the genes responsible for the most common forms of the human sarcoglycanopathies. Both vectors showed successful short-term genetic, biochemical, and histological rescue of both alpha- and beta-sarcoglycan-deficient mouse muscle. However, comparison of persistence of expression in 51 injected mice showed substantial differences between AAV alpha-sarcoglycan (alpha-SG) and beta-sarcoglycan (beta-SG) vectors. AAV-beta-SG showed long-term expression with no decrease in expression for more than 21 months after injection, whereas AAV-alpha-SG showed a dramatic loss of positive fibers between 28 and 41 days post-injection (p = 0.006). Loss of immunopositive myofibers was correlated with significant inflammatory cell infiltrate, primarily macrophages. To determine whether the loss of alpha-sarcoglycan-positive fibers was due to an immune response or cytotoxic effect of alpha-sarcoglycan overexpression, severe combined immunodeficient (SCID) mouse muscle was assayed for cytotoxicity after injection with AAV-alpha-SG, AAV-beta-SG, or phosphate-buffered saline. The results were consistent with overexpression of alpha-sarcoglycan causing significant cytotoxicity. The cytotoxicity of alpha-sarcoglycan, and not beta- or delta-sarcoglycan overexpression, was consistent with biochemical studies of the hierarchical order of assembly of the sarcoglycan complex. Our data suggest that even closely related proteins might require different levels of expression to avoid toxicity and achieve long-term tissue rescue.

Defective assembly of sarcoglycan complex in patients with beta-sarcoglycan gene mutations. Study of aneural and innervated cultured myotubes

Mutations in the sarcoglycan (SG) genes cause autosomal recessive muscular dystrophies. The absence of each SG complex component in muscle impairs the proper assembly of the entire SG complex, resulting in sarcolemmal damage. We investigated the consequences of beta-SG gene mutations in cultured muscle from two beta-SG mutated patients, and analysed each individual SG protein expression by cross-sectional immunocytochemistry and Western blot in aneural and innervated myotubes. Patients' muscle biopsy showed total loss of SG complex; however, a limited amount of beta-SG was detected in aneural and innervated myotubes, where the protein was localized to the plasma membrane. This paradoxical beta-SG expression can be attributable to antibody cross-reaction or to the expression of an unknown SG isoform specific of immature muscle. In our cultured myotubes, the other components of the SG complex were absent, suggesting that beta-SG gene mutations result in a defective assembly of the entire SG complex in early stages of muscle development, and that the role of beta-SG is crucial for the normal structure and/or function of the SG complex in the sarcolemma.

Nonviral approaches satisfying various requirements for effective in vivo gene therapy

Development of an efficient method of gene introduction to target cells is the key issue in treating genetic and acquired diseases by in vivo gene therapy. Although various nonviral approaches have been developed, any method needs to be optimized in terms of the target disease and transgene product. The most important information required is (i) target cell-specificity of gene transfer, (ii) efficiency, (iii) duration of transgene expression, and (iv) the number of transfected cells following in vivo application of a vector. These characteristics are determined by the properties of the vector used, as well as the route of its administration, biodistribution, interaction with biological components and the nature of the target cells. Cell-specific gene transfer can be achieved by controlling the tissue disposition of plasmid DNA (pDNA), although the interaction of the pDNA complex with biological components might limit the specificity. Various approaches have been reported to increase the efficiency of transgene expression, from cationic lipids/polymers to physical stimuli, but some of those are ineffective under in vivo conditions. The duration of transgene expression is a complex function involving variables including the cell type, transfection method, and plasmid construct. Immune response often reduces the level and duration of transgene expression. In addition, the number of transfected cells is important, especially in cases in which the therapeutic protein localizes within the target cells. Successful clinical application of nonviral gene delivery methods rely on the development of such methods optimized for a particular target disease.

Limb-girdle muscular dystrophy type 2H associated with mutation in TRIM32, a putative E3-ubiquitin-ligase gene

Limb-girdle muscular dystrophy type 2H (LGMD2H) is a mild autosomal recessive myopathy that was first described in the Manitoba Hutterite population. Previous studies in our laboratory mapped the causative gene for this disease to a 6.5-Mb region in chromosomal region 9q31-33, flanked by D9S302 and D9S1850. We have now used additional families and a panel of 26 microsatellite markers to construct haplotypes. Twelve recombination events that reduced the size of the candidate region to 560 kb were identified or inferred. This region is flanked by D9S1126 and D9S737 and contains at least four genes. Exons of these genes were sequenced in one affected individual, and four sequence variations were identified. The families included in our study and 100 control individuals were tested for these variations. On the basis of our results, the mutation in the tripartite-motif-containing gene (TRIM32) that replaces aspartate with asparagine at position 487 appears to be the causative mutation of LGMD2H. All affected individuals were found to be homozygous for D487N, and this mutation was not found in any of the controls. This mutation occurs in an NHL (named after the proteins NCL1, HT2A, and LIN-41) domain at a position that is highly conserved. NHL domains are known to be involved in protein-protein interactions. Although the function of TRIM32 is unknown, current knowledge of the domain structure of this protein suggests that it may be an E3-ubiquitin ligase. If proven, this represents a new pathogenic mechanism leading to muscular dystrophy.

The ABC's of limb-girdle muscular dystrophy: alpha-sarcoglycanopathy, Bethlem myopathy, calpainopathy and more

Limb-girdle muscular dystrophy is a class of disorders encompassing many forms of this disease. Variation exists between the inheritance patterns, genes responsible, course of disease and symptoms, with the cohesive factor among these disorders being the predominance of proximal muscle weakness. Here we review each form of limb-girdle muscular dystrophy with attention to molecular genetics, clinical features, inheritance, and diagnostic issues pertaining to each primary genetic cause.