Overexpression of the CT GalNAc transferase in skeletal muscle alters myofiber growth, neuromuscular structure, and laminin expression

Dystrophin- and Utrophin-Based Therapeutic Approaches for Treatment of Duchenne Muscular Dystrophy: A Comparative Review

Duchenne muscular dystrophy is a devastating disease that leads to progressive muscle loss and premature death. While medical management focuses mostly on symptomatic treatment, decades of research have resulted in first therapeutics able to restore the affected reading frame of dystrophin transcripts or induce synthesis of a truncated dystrophin protein from a vector, with other strategies based on gene therapy and cell signaling in preclinical or clinical development. Nevertheless, recent reports show that potentially therapeutic dystrophins can be immunogenic in patients. This raises the question of whether a dystrophin paralog, utrophin, could be a more suitable therapeutic protein. Here, we compare dystrophin and utrophin amino acid sequences and structures, combining published data with our extended in silico analyses. We then discuss these results in the context of therapeutic approaches for Duchenne muscular dystrophy. Specifically, we focus on strategies based on delivery of micro-dystrophin and micro-utrophin genes with recombinant adeno-associated viral vectors, exon skipping of the mutated dystrophin pre-mRNAs, reading through termination codons with small molecules that mask premature stop codons, dystrophin gene repair by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genetic engineering, and increasing utrophin levels. Our analyses highlight the importance of various dystrophin and utrophin domains in Duchenne muscular dystrophy treatment, providing insights into designing novel therapeutic compounds with improved efficacy and decreased immunoreactivity. While the necessary actin and β-dystroglycan binding sites are present in both proteins, important functional distinctions can be identified in these domains and some other parts of truncated dystrophins might need redesigning due to their potentially immunogenic qualities. Alternatively, therapies based on utrophins might provide a safer and more effective approach.

A first-in-human phase I/IIa gene transfer clinical trial for Duchenne muscular dystrophy using rAAVrh74.MCK.GALGT2

In a phase 1/2, open-label dose escalation trial, we delivered rAAVrh74.MCK.GALGT2 (also B4GALNT2) bilaterally to the legs of two boys with Duchenne muscular dystrophy using intravascular limb infusion. Subject 1 (age 8.9 years at dosing) received 2.5 × 1013 vector genome (vg)/kg per leg (5 × 1013 vg/kg total) and subject 2 (age 6.9 years at dosing) received 5 × 1013 vg/kg per leg (1 × 1014 vg/kg total). No serious adverse events were observed. Muscle biopsy evaluated 3 or 4 months post treatment versus baseline showed evidence of GALGT2 gene expression and GALGT2-induced muscle cell glycosylation. Functionally, subject 1 showed a decline in 6-min walk test (6MWT) distance; an increase in time to run 100 m, and a decline in North Star Ambulatory Assessment (NSAA) score until ambulation was lost at 24 months. Subject 2, treated at a younger age and at a higher dose, demonstrated an improvement over 24 months in NSAA score (from 20 to 23 points), an increase in 6MWT distance (from 405 to 478 m), and only a minimal increase in 100 m time (45.6-48.4 s). These data suggest preliminary safety at a dose of 1 × 1014 vg/kg and functional stabilization in one patient.

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

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

Adeno-Associated Virus (AAV)-Mediated Gene Therapy for Duchenne Muscular Dystrophy: The Issue of Transgene Persistence

Duchenne muscular dystrophy (DMD) is an X-linked recessive, infancy-onset neuromuscular disorder characterized by progressive muscle weakness and atrophy, leading to delay of motor milestones, loss of autonomous ambulation, respiratory failure, cardiomyopathy, and premature death. DMD originates from mutations in the DMD gene that result in a complete absence of dystrophin. Dystrophin is a cytoskeletal protein which belongs to the dystrophin-associated protein complex, involved in cellular signaling and myofiber membrane stabilization. To date, the few available therapeutic options are aimed at lessening disease progression, but persistent loss of muscle tissue and function and premature death are unavoidable. In this scenario, one of the most promising therapeutic strategies for DMD is represented by adeno-associated virus (AAV)-mediated gene therapy. DMD gene therapy relies on the administration of exogenous micro-dystrophin, a miniature version of the dystrophin gene lacking unnecessary domains and encoding a truncated, but functional, dystrophin protein. Limited transgene persistence represents one of the most significant issues that jeopardize the translatability of DMD gene replacement strategies from the bench to the bedside. Here, we critically review preclinical and clinical studies of AAV-mediated gene therapy in DMD, focusing on long-term transgene persistence in transduced tissues, which can deeply affect effectiveness and sustainability of gene replacement in DMD. We also discuss the role played by the overactivation of the immune host system in limiting long-term expression of genetic material. In this perspective, further studies aimed at better elucidating the need for immune suppression in AAV-treated subjects are warranted in order to allow for life-long therapy in DMD patients.

Short-term treatment of golden retriever muscular dystrophy (GRMD) dogs with rAAVrh74.MHCK7.GALGT2 induces muscle glycosylation and utrophin expression but has no significant effect on muscle strength

We have examined the effects of intravenous (IV) delivery of rAAVrh74.MHCK7.GALGT2 in the golden retriever muscular dystrophy (GRMD) model of Duchenne Muscular Dystrophy (DMD). After baseline testing, GRMD dogs were treated at 3 months of age and reassessed at 6 months. This 3–6 month age range is a period of rapid disease progression, thus offering a relatively short window to establish treatment efficacy. Measures analyzed included muscle AAV transduction, GALGT2 transgene expression, GALGT2-induced glycosylation, muscle pathology, and muscle function. A total of five dogs were treated, 4 at 2x10¹⁴vg/kg and one at 6x10¹⁴vgkg. The 2x10¹⁴vg/kg dose led to transduction of regions of the heart with 1–3 vector genomes (vg) per nucleus, while most skeletal muscles were transduced with 0.25–0.5vg/nucleus. GALGT2-induced glycosylation paralleled levels of myofiber vg transduction, with about 90% of cardiomyocytes having increased glycosylation versus 20–35% of all myofibers across the skeletal muscles tested. Conclusions from phenotypic testing were limited by the small number of dogs. Treated dogs had less pronounced fibrosis and overall lesion severity when compared to control groups, but surprisingly no significant changes in limb muscle function measures. GALGT2-treated skeletal muscle and heart had elevated levels of utrophin protein expression and GALGT2-induced expression of glycosylated α dystroglycan, providing further evidence of a treatment effect. Serum chemistry, hematology, and cardiac function measures were largely unchanged by treatment. Cumulatively, these data show that short-term intravenous treatment of GRMD dogs with rAAVrh74.MHCK7.GALGT2 at high doses can induce muscle glycosylation and utrophin expression and may be safe over a short 3-month interval, but that such treatments had only modest effects on muscle pathology and did not significantly improve muscle strength.

Fukutin-Related Protein: From Pathology to Treatments

Fukutin-related protein (FKRP) is a glycosyltransferase involved in the functional glycosylation of α-dystroglycan (DG), a key component in the link between the cytoskeleton and the extracellular matrix (ECM). Mutations in FKRP lead to dystroglycanopathies with broad severity, including limb-girdle and congenital muscular dystrophy. Studies over the past 5 years have elucidated the function of FKRP, which has expanded the number of therapeutic opportunities for patients carrying FKRP mutations. These include small molecules, gene delivery, and cell therapy. Here we summarize recent findings on the function of FKRP and describe available models for studying diseases and testing therapeutics. Lastly, we highlight preclinical studies that hold potential for the treatment of FKRP-associated dystroglycanopathies.

rAAVrh74.MCK.GALGT2 Demonstrates Safety and Widespread Muscle Glycosylation after Intravenous Delivery in C57BL/6J Mice

rAAVrh74.MCK.GALGT2 is a surrogate gene therapy that inhibits muscular dystrophy in multiple animal models. Here, we report on a dose-response study of functional muscle GALGT2 expression as well as toxicity and biodistribution studies after systemic intravenous (i.v.) delivery of rAAVrh74.MCK.GALGT2. A dose of 4.3 × 10¹⁴vg/kg (measured with linear DNA standard) resulted in GALGT2-induced glycosylation in the majority of skeletal myofibers throughout the body and in almost all cardiomyocytes, while several lower doses also showed significant muscle glycosylation. No adverse clinical signs or treatment-dependent changes in tissue or organ pathology were noted at 1 or 3 months post-treatment. Blood cell and serum enzyme chemistry measures in treated mice were all within the normal range except for alkaline phosphatase (ALP) activity, which was elevated in serum but not in tissues. Some anti-rAAVrh74 capsid T cell responses were noted at 4 weeks post-treatment, but all such responses were not present at 12 weeks. Using intramuscular delivery, GALGT2-induced muscle glycosylation was increased in Cmah-deficient mice, which have a humanized sialoglycome, relative to wild-type mice, suggesting that use of mice may underestimate GALGT2 activity in human muscle. These data demonstrate safety and high transduction of muscles throughout the body plan with i.v. delivery of rAAVrh74.MCK.GALGT2.

AAV-based gene therapies for the muscular dystrophies

Muscular dystrophy (MD) is a group of progressive genetic diseases affecting the musculature that are characterized by inflammatory infiltrates, necrosis and connective tissue and fat replacement of the affected muscles. Unfortunately, treatments do not exist for the vast majority of MD patients. Adeno-associated viral vector (AAV)-based gene therapy is thus emerging as a potential treatment for many types of MD. Treatments strategies based on AAV are being adapted for replacement of mutant disease-causing genes, knockdown of dominant disease-causing genes using antisense oligonucleotides or inhibitory RNAs, delivery of gene editing tools such as clustered regularly interspaced short palindromic repeats/Cas9 and effecting alterations in pre-mRNA splicing and by manipulating expression levels of modifier genes. Translational and clinical trial work focused on these types of AAV treatments for Duchenne MD, various limb girdle MDs, myotonic dystrophy 1, facioscapulohumeral MD, dysferlinopathies and congenital MDs are discussed here, with a focus on recent studies, pre-clinical large animal work and many promising ongoing and upcoming AAV clinical trials.

Soluble Heparin Binding Epidermal Growth Factor-Like Growth Factor Is a Regulator of GALGT2 Expression and GALGT2-Dependent Muscle and Neuromuscular Phenotypes

GALGT2 (also B4GALNT2) encodes a glycosyltransferase that is normally confined to the neuromuscular and myotendinous junction in adult skeletal muscle. GALGT2 overexpression in muscle can inhibit muscular dystrophy in mouse models of the disease by inducing the overexpression of surrogate muscle proteins, including utrophin, agrin, laminins, and integrins. Despite its well-documented biological properties, little is known about the endogenous regulation of muscle GALGT2 expression. Here, we demonstrate that epidermal growth factor receptor (EGFR) ligands can activate the human GALGT2 promoter. Overexpression of one such ligand, soluble heparin-binding EGF-like growth factor (sHB-EGF), also stimulated mouse muscle Galgt2 gene expression and expression of GALGT2-inducible surrogate muscle genes. Deletion analysis of the GALGT2 promoter identified a 45-bp region containing a TFAP4-binding site that was required for sHB-EGF activation. sHB-EGF increased TFAP4 binding to this site in muscle cells and increased endogenous Tfap4 gene expression. sHB-EGF also increased muscle EGFR protein expression and activated EGFR-Akt signaling. sHB-EGF expression was concentrated at the neuromuscular junction, and Hbegf deletion reduced Galgt2-dependent synaptic glycosylation. Hbegf deletion also mimicked Galgt2-dependent neuromuscular and muscular dystrophy phenotypes. These data demonstrate that sHB-EGF is an endogenous regulator of muscle Galgt2 gene expression and can mimic Galgt2-dependent muscle phenotypes.

rAAVrh74.MCK.GALGT2 Protects against Loss of Hemodynamic Function in the Aging mdx Mouse Heart

Dilated cardiomyopathy is a common cause of death in patients with Duchenne muscular dystrophy (DMD). Gene therapies for DMD must, therefore, have a therapeutic impact in cardiac as well as skeletal muscles. Our previous studies have shown that GALGT2 overexpression in mdx skeletal muscles can prevent muscle damage. Here we have tested whether rAAVrh74.MCK.GALGT2 gene therapy in mdx cardiac muscle can prevent the loss of heart function. Treatment of mdx hearts with rAAVrh74.MCK.GALGT2 1 day after birth did not negatively alter hemodynamic function, tested at 3 months of age, and it prevented early left ventricular remodeling and expression of fibrotic gene markers. Intravenous treatment of mdx mice with rAAVrh74.MCK.GALGT2 at 2 months of age significantly improved stroke volume and cardiac output compared to mock-treated mice analyzed at 17 months, both at rest and after stimulation with dobutamine. rAAVrh74.MCK.GALGT2 treatment of mdx heart correlated with increased glycosylation of α-dystroglycan with the CT glycan and increased utrophin protein expression. These data provide the first demonstration that GALGT2 overexpression can inhibit the loss of cardiac function in the dystrophin-deficient heart and, thus, may benefit both cardiac and skeletal muscles in DMD patients.

An Isolated Limb Infusion Method Allows for Broad Distribution of rAAVrh74.MCK.GALGT2 to Leg Skeletal Muscles in the Rhesus Macaque

Recombinant adeno-associated virus (rAAV)rh74.MCK.GALGT2 is a muscle-specific gene therapy that is being developed to treat forms of muscular dystrophy. Here we report on an isolated limb infusion technique in a non-human primate model, where hindlimb blood flow is transiently isolated using balloon catheters to concentrate vector in targeted leg muscles. A bilateral dose of 2.5 × 1013 vector genomes (vg)/kg/limb was sufficient to induce GALGT2-induced glycosylation in 10%-60% of skeletal myofibers in all leg muscles examined. There was a 19-fold ± 6-fold average limb-wide increase in vector genomes per microgram genomic DNA at a bilateral dose of 2.5 × 1013 vg/kg/limb compared with a bilateral dose of 6 × 1012 vg/kg/limb. A unilateral dose of 6 × 1013 vg/kg/limb showed a 12- ± 3-fold increase in treated limb muscles compared to contralateral untreated limb muscles, which received vector only after release into the systemic circulation from the treated limb. Variability in AAV biodistribution between different segments of the same muscle was 125% ± 18% for any given dose, while variability between the same muscle for any given treatment dose was 45% ± 7%. These experiments demonstrate that treatment of muscles throughout the leg with rAAVrh74.MCK.GALGT2 can be accomplished safely using an isolated limb infusion technique, where balloon catheters transiently isolate the limb vasculature, but that intra- and inter-muscle transduction variability is a significant issue.

Lectin-binding characterizes the healthy human skeletal muscle glycophenotype and identifies disease-specific changes in dystrophic muscle

Our understanding of muscle glycosylation to date has derived from studies in mouse models and a limited number of human lectin histochemistry studies. As various therapeutic approaches aimed at treating patients with muscular dystrophies are being translated from rodent models to human, it is critical to better understand human muscle glycosylation and relevant disease-specific differences between healthy and dystrophic muscle. Here, we report the first quantitative characterization of human muscle glycosylation, and identify differentiation- and disease-specific differences in human muscle glycosylation. Utilizing a panel of 13 lectins with varying glycan specificities, we surveyed lectin binding to primary and immortalized myoblasts and myotubes from healthy and dystrophic sources. Following differentiation of primary and immortalized healthy human muscle cells, we observed increased binding of Narcissus pseudonarcissus agglutinin (NPA), PNA, MAA-II and WFA to myotubes compared to myoblasts. Following differentiation of immortalized healthy and dystrophic human muscle cells, we observed disease-specific differences in binding of NPA, Jac and Tricosanthes japonica agglutinin-I (TJA-I) to differentiated myotubes. We also observed differentiation- and disease-specific differences in binding of NPA, Jac, PNA, TJA-I and WFA to glycoprotein receptors in muscle cells. Additionally, Jac, PNA and WFA precipitated functionally glycosylated α-DG, that bound laminin, while NPA and TJA-I did not. Lectin histochemistry of healthy and dystrophic human muscle sections identified disease-specific differences in binding of O-glycan and sialic acid-specific lectins between healthy and dystrophic muscle. These results indicate that specific and discrete changes in glycosylation occur following differentiation, and identify specific lectins as potential biomarkers sensitive to changes in healthy human muscle glycosylation.

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

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

B4GALNT2 (GALGT2) Gene Therapy Reduces Skeletal Muscle Pathology in the FKRP P448L Mouse Model of Limb Girdle Muscular Dystrophy 2I

Overexpression of B4GALNT2 (previously GALGT2) inhibits the development of muscle pathology in mouse models of Duchenne muscular dystrophy, congenital muscular dystrophy 1A, and limb girdle muscular dystrophy 2D. In these models, muscle GALGT2 overexpression induces the glycosylation of α dystroglycan with the cytotoxic T cell glycan and increases the overexpression of dystrophin and laminin α2 surrogates known to inhibit disease. Here, we show that GALGT2 gene therapy significantly reduces muscle pathology in FKRP P448Lneo(-) mice, a model for limb girdle muscular dystrophy 2I. rAAVrh74.MCK.GALGT2-treated FKRP P448Lneo(-) muscles showed reduced levels of centrally nucleated myofibers, reduced variance, increased size of myofiber diameters, reduced myofiber immunoglobulin G uptake, and reduced muscle wasting at 3 and 6 months after treatment. GALGT2 overexpression in FKRP P448Lneo(-) muscles did not cause substantial glycosylation of α dystroglycan with the cytotoxic T cell glycan or increased expression of dystrophin and laminin α2 surrogates in mature skeletal myofibers, but it increased the number of embryonic myosin-positive regenerating myofibers. These data demonstrate that GALGT2 overexpression can reduce the extent of muscle pathology in FKRP mutant muscles, but that it may do so via a mechanism that differs from its ability to induce surrogate gene expression.

Deletion of Pofut1 in Mouse Skeletal Myofibers Induces Muscle Aging-Related Phenotypes in cis and in trans

Sarcopenia, the loss of muscle mass and strength during normal aging, involves coordinate changes in skeletal myofibers and the cells that contact them, including satellite cells and motor neurons. Here we show that the protein O-fucosyltransferase 1 gene (Pofut1), which encodes a glycosyltransferase required for NotchR-mediated cell-cell signaling, has reduced expression in aging skeletal muscle. Moreover, premature postnatal deletion of Pofut1 in skeletal myofibers can induce aging-related phenotypes in cis within skeletal myofibers and in trans within satellite cells and within motor neurons via the neuromuscular junction. Changed phenotypes include reduced skeletal muscle size and strength, decreased myofiber size, increased slow fiber (type 1) density, increased muscle degeneration and regeneration in aged muscles, decreased satellite cell self-renewal and regenerative potential, and increased neuromuscular fragmentation and occasional denervation. Pofut1 deletion in skeletal myofibers reduced NotchR signaling in young adult muscles, but this effect was lost with age. Increasing muscle NotchR signaling also reduced muscle size. Gene expression studies point to regulation of cell cycle genes, muscle myosins, NotchR and Wnt pathway genes, and connective tissue growth factor by Pofut1 in skeletal muscle, with additional effects on α dystroglycan glycosylation.

Differentiation-related glycan epitopes identify discrete domains of the muscle glycocalyx

The neuromuscular junction (NMJ) is enriched with glycoproteins modified with N-acetylgalactosamine (GalNAc) residues, and four nominally GalNAc-specific plant lectins have historically been used to identify the NMJ and the utrophin-glycoprotein complex. However, little is known about the specific glycan epitopes on skeletal muscle that are bound by these lectins, the glycoproteins that bear these epitopes or how creation of these glycan epitopes is regulated. Here, we profile changes in cell surface glycosylation during muscle cell differentiation and identify distinct differences in the binding preferences of GalNAc-specific lectins, Wisteria floribunda agglutinin (WFA), Vicia villosa agglutinin (VVA), soybean agglutinin (SBA) and Dolichos biflorus agglutinin (DBA). While we find that all four GalNAc binding lectins specifically label the NMJ, each of the four lectins binds distinct sets of muscle glycoproteins; furthermore, none of the major adhesion complexes are required for binding of any of the four GalNAc-specific lectins. Analysis of glycosylation-related transcripts identified target glycosyltransferases and glycosidases that could potentially create GalNAc-containing epitopes; reducing expression of these transcripts by siRNA highlighted differences in lectin binding specificities. In addition, we found that complex N-glycans are required for binding of WFA and SBA to murine C2C12 myotubes and for WFA binding to wild-type skeletal muscle, but not for binding of VVA or DBA. These results demonstrate that muscle cell surface glycosylation is finely regulated during muscle differentiation in a domain- and acceptor-substrate-specific manner, suggesting that temporal- and site-specific glycosylation are important for skeletal muscle cell function.

Deletion of Galgt2 (B4Galnt2) reduces muscle growth in response to acute injury and increases muscle inflammation and pathology in dystrophin-deficient mice

Transgenic overexpression of Galgt2 (official name B4Galnt2) in skeletal muscle stimulates the glycosylation of α dystroglycan (αDG) and the up-regulation of laminin α2 and dystrophin surrogates known to inhibit muscle pathology in mouse models of congenital muscular dystrophy 1A and Duchenne muscular dystrophy. Skeletal muscle Galgt2 gene expression is also normally increased in the mdx mouse model of Duchenne muscular dystrophy compared with the wild-type mice. To assess whether this increased endogenous Galgt2 expression could affect disease, we quantified muscular dystrophy measures in mdx mice deleted for Galgt2 (Galgt2(-/-)mdx). Galgt2(-/-) mdx mice had increased heart and skeletal muscle pathology and inflammation, and also worsened cardiac function, relative to age-matched mdx mice. Deletion of Galgt2 in wild-type mice also slowed skeletal muscle growth in response to acute muscle injury. In each instance where Galgt2 expression was elevated (developing muscle, regenerating muscle, and dystrophic muscle), Galgt2-dependent glycosylation of αDG was also increased. Overexpression of Galgt2 failed to inhibit skeletal muscle pathology in dystroglycan-deficient muscles, in contrast to previous studies in dystrophin-deficient mdx muscles. This study demonstrates that Galgt2 gene expression and glycosylation of αDG are dynamically regulated in muscle and that endogenous Galgt2 gene expression can ameliorate the extent of muscle pathology, inflammation, and dysfunction in mdx mice.

Prenatal muscle development in a mouse model for the secondary dystroglycanopathies

Background

The defective glycosylation of α-dystroglycan is associated with a group of muscular dystrophies that are collectively referred to as the secondary dystroglycanopathies. Mutations in the gene encoding fukutin-related protein (FKRP) are one of the most common causes of secondary dystroglycanopathy in the UK and are associated with a wide spectrum of disease. Whilst central nervous system involvement has a prenatal onset, no studies have addressed prenatal muscle development in any of the mouse models for this group of diseases. In view of the pivotal role of α-dystroglycan in early basement membrane formation, we sought to determine if the muscle formation was altered in a mouse model of FKRP-related dystrophy.

Results

Mice with a knock-down in FKRP (FKRP^KD) showed a marked reduction in α-dystroglycan glycosylation and reduction in laminin binding by embryonic day 15.5 (E15.5), relative to wild type controls. In addition, the total number of Pax7⁺ progenitor cells in the FKRP^KD tibialis anterior at E15.5 was significantly reduced, and myotube cluster/myofibre size showed a significant reduction in size. Moreover, myoblasts isolated from the limb muscle of these mice at E15.5 showed a marked reduction in their ability to form myotubes in vitro.

Conclusions

These data identify an early reduction of laminin α2, reduction of myogenicity and depletion of Pax7⁺ progenitor cells which would be expected to compromise subsequent postnatal muscle growth and its ability to regenerate postnatally. These findings are of significance to the development of future therapies in this group of devastating conditions.

A role for Galgt1 in skeletal muscle regeneration

BACKGROUND

Cell surface glycans are known to play vital roles in muscle membrane stability and muscle disease, but to date, roles for glycans in muscle regeneration have been less well understood. Here, we describe a role for complex gangliosides synthesized by the Galgt1 gene in muscle regeneration.

METHODS

Cardiotoxin-injected wild type (WT) and Galgt1 (-/-) muscles, and mdx and Galgt1 (-/-) mdx muscles, were used to study regeneration in response to acute and chronic injury, respectively. Muscle tissue was analyzed at various time points for morphometric measurements and for gene expression changes in satellite cell and muscle differentiation markers by quantitative real-time polymerase chain reaction (qRT-PCR). Primary cell cultures were used to measure growth rate and myotube formation and to identify Galgt1 expression changes after cardiotoxin by fluorescence-activated cell sorting (FACS). Primary cell culture and tissue sections were also used to quantify satellite cell apoptosis.

RESULTS

A query of a microarray data set of cardiotoxin-induced mouse muscle gene expression changes identified Galgt1 as the most upregulated glycosylation gene immediately after muscle injury. This was validated by qRT-PCR as a 23-fold upregulation in Galgt1 expression 1 day after cardiotoxin administration and a 16-fold upregulation in 6-week-old mdx muscles. These changes correlated with increased expression of Galgt1 protein and GM1 ganglioside in mononuclear muscle cells. In the absence of Galgt1, cardiotoxin-induced injury led to significantly reduced myofiber diameters after 14 and 28 days of regeneration. Myofiber diameters were also significantly reduced in Galgt1-deficient mdx mice compared to age-matched mdx controls, and this was coupled with a significant increase in the loss of muscle tissue. Cardiotoxin-injected Galgt1 (-/-) muscles showed reduced gene expression of the satellite cell marker Pax7 and increased expression of myoblast markers MyoD, Myf5, and Myogenin after injury along with a tenfold increase in apoptosis of Pax7-positive muscle cells. Cultured primary Galgt1 (-/-) muscle cells showed a normal growth rate but demonstrated premature fusion into myofibers, resulting in an overall impairment of myofiber formation coupled with a threefold increase in muscle cell apoptosis.

CONCLUSIONS

These experiments demonstrate a role for Galgt1 in skeletal muscle regeneration and suggest that complex gangliosides made by Galgt1 modulate the survival and differentiation of satellite cells.

A comparative study of N-glycolylneuraminic acid (Neu5Gc) and cytotoxic T cell (CT) carbohydrate expression in normal and dystrophin-deficient dog and human skeletal muscle

The expression of N-glycolylneuraminic acid (Neu5Gc) and the cytotoxic T cell (CT) carbohydrate can impact the severity of muscular dystrophy arising from the loss of dystrophin in mdx mice. Here, we describe the expression of these two glycans in skeletal muscles of dogs and humans with or without dystrophin-deficiency. Neu5Gc expression was highly reduced (>95%) in muscle from normal golden retriever crosses (GR, n = 3) and from golden retriever with muscular dystrophy (GRMD, n = 5) dogs at multiple ages (3, 6 and 13 months) when compared to mouse muscle, however, overall sialic acid expression in GR and GRMD muscles remained high at all ages. Neu5Gc was expressed on only a minority of GRMD satellite cells, CD8⁺ T lymphocytes and macrophages. Human muscle from normal (no evident disease, n = 3), Becker (BMD, n = 3) and Duchenne (DMD, n = 3) muscular dystrophy individuals had absent to very low Neu5Gc staining, but some punctate intracellular muscle staining was present in BMD and DMD muscles. The CT carbohydrate was localized to the neuromuscular junction in GR muscle, while GRMD muscles had increased expression on a subset of myofibers and macrophages. In humans, the CT carbohydrate was ectopically expressed on the sarcolemmal membrane of some BMD muscles, but not normal human or DMD muscles. These data are consistent with the notion that altered Neu5Gc and CT carbohydrate expression may modify disease severity resulting from dystrophin deficiency in dogs and humans.

Dissecting the molecular basis of the role of the O-mannosylation pathway in disease: α-dystroglycan and forms of muscular dystrophy

Dystroglycanopathies form a subgroup of muscular dystrophies that arise from defects in enzymes that are implicated in the recently elucidated O-mannosylation pathway, thereby resulting in underglycosylation of α-dystroglycan. The emerging identification of additional brain proteins modified by O-mannosylation provides a broader context for interpreting the range of neurological consequences associated with dystroglycanopathies. This form of glycosylation is associated with protein mucin-like domains that present numerous serine and threonine residues as possible sites for modification. Furthermore, the O-Man glycans coexist in this region with O-GalNAc glycans (conventionally associated with such protein sequences), thus resulting in a complex glycoconjugate landscape. Sorting out the relationships between the various molecular defects in glycosylation and the modes of disease presentation, as well as the regulatory interplay among the O-Man glycans and the effects on other modes of glycosylation in the same domain, is challenging. Here we provide a perspective on chemical biology approaches employing synthetic and analytical methods to address these questions.

Vascular delivery of rAAVrh74.MCK.GALGT2 to the gastrocnemius muscle of the rhesus macaque stimulates the expression of dystrophin and laminin α2 surrogates

Overexpression of GALGT2 in skeletal muscle can stimulate the glycosylation of α dystroglycan and the upregulation of normally synaptic dystroglycan-binding proteins, some of which are dystrophin and laminin α2 surrogates known to be therapeutic for several forms of muscular dystrophy. This article describes the vascular delivery of GALGT2 gene therapy in a large animal model, the rhesus macaque. Recombinant adeno-associated virus, rhesus serotype 74 (rAAVrh74), was used to deliver GALGT2 via the femoral artery to the gastrocnemius muscle using an isolated focal limb perfusion method. GALGT2 expression averaged 44 ± 4% of myofibers after treatment in macaques with low preexisting anti-rAAVrh74 serum antibodies, and expression was reduced to 9 ± 4% of myofibers in macaques with high preexisting rAAVrh74 immunity (P < 0.001; n = 12 per group). This was the case regardless of the addition of immunosuppressants, including prednisolone, tacrolimus, and mycophenolate mofetil. GALGT2-treated macaque muscles showed increased glycosylation of α dystroglycan and increased expression of dystrophin and laminin α2 surrogate proteins, including utrophin, plectin1, agrin, and laminin α5. These experiments demonstrate successful transduction of rhesus macaque muscle with rAAVrh74.MCK.GALGT2 after vascular delivery and induction of molecular changes thought to be therapeutic in several forms of muscular dystrophy.

The expanding roles of the Sd(a)/Cad carbohydrate antigen and its cognate glycosyltransferase B4GALNT2

BACKGROUND

The histo-blood group antigens are carbohydrate structures present in tissues and body fluids, which contribute to the definition of the individual immunophenotype. One of these, the Sd(a) antigen, is expressed on the surface of erythrocytes and in secretions of the vast majority of the Caucasians and other ethnic groups.

SCOPE OF REVIEW

We describe the multiple and unsuspected aspects of the biology of the Sd(a) antigen and its biosynthetic enzyme β1,4-N-acetylgalactosaminyltransferase 2 (B4GALNT2) in various physiological and pathological settings.

MAJOR CONCLUSIONS

The immunodominant sugar of the Sd(a) antigen is a β1,4-linked N-acetylgalactosamine (GalNAc). Its cognate glycosyltransferase B4GALNT2 displays a restricted pattern of tissue expression, is regulated by unknown mechanisms - including promoter methylation, and encodes at least two different proteins, one of which with an unconventionally long cytoplasmic portion. In different settings, the Sd(a) antigen plays multiple and unsuspected roles. 1) In colon cancer, its dramatic down-regulation plays a potential role in the overexpression of sialyl Lewis antigens, increasing metastasis formation. 2) It is involved in the lytic function of murine cytotoxic T lymphocytes. 3) It prevents the development of muscular dystrophy in various dystrophic murine models, when overexpressed in muscular fibers. 4) It regulates the circulating half-life of the von Willebrand factor (vWf), determining the onset of a bleeding disorder in a murine model.

GENERAL SIGNIFICANCE

The expression of the Sd(a) antigen has a wide impact on the physiology and the pathology of different biological systems.

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

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

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

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

Distinct contributions of Galgt1 and Galgt2 to carbohydrate expression and function at the mouse neuromuscular junction

At the mammalian neuromuscular junction (NMJ), the CT (cytotoxic T cell) carbohydrate antigen [GalNAcβ1,4[Neu5Ac/Gcα2,3]Galβ1,4GlcNAc-] is a unique synaptic cell surface carbohydrate present in both the presynaptic and postsynaptic membranes. Here we show that Galgt1, which synthesizes the β1,4GalNAc linkage of the CT carbohydrate on gangliosides, is required for presynaptic expression of the CT carbohydrate at the NMJ, while Galgt2, which can synthesize the β1,4GalNAc of the CT carbohydrate on glycoproteins, is required for postsynaptic expression. Proper postsynaptic localization of the CT carbohydrate also required muscle expression of dystroglycan, a known muscle substrate for Galgt2. Transgenic overexpression of Galgt2 in skeletal myofibers altered the expression of synaptic muscle proteins and altered neuromuscular topography, which was partially NCAM-dependent, while an increase in postsynaptic AChR-rich domains was observed in both neuron- and skeletal muscle-specific Galgt2 transgenic mice. By contrast, overexpression of Galgt1 in muscle did not allow for increased expression of CT carbohydrate on the sarcolemmal membrane and instead caused muscle pathology. Loss of Galgt2 increased intracellular accumulation of acetylcholine receptors and acetylcholinesterase within skeletal myofibers, suggesting an additional role for Galgt2 in neuromuscular stability. These experiments demonstrate that Galgt1 and Galgt2 contribute in distinct ways to the expression and function of synaptic βGalNAc-containing carbohydrates at the NMJ.

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

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

Role of extracellular matrix proteins and their receptors in the development of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ) remains the best-studied model for understanding the mechanisms involved in synaptogenesis, due to its relatively large size, its simplicity of patterning, and its unparalleled experimental accessibility. During neuromuscular development, each skeletal myofiber secretes and deposits around its extracellular surface an assemblage of extracellular matrix (ECM) proteins that ultimately form a basal lamina. This is also the case at the NMJ, where the motor nerve contributes additional factors. Before most of the current molecular components were known, it was clear that the synaptic ECM of adult skeletal muscles was unique in composition and contained factors sufficient to induce the differentiation of both pre- and postsynaptic membranes. Biochemical, genetic, and microscopy studies have confirmed that agrin, laminin (221, 421, and 521), collagen IV (α3-α6), collagen XIII, perlecan, and the ColQ-bound form of acetylcholinesterase are all synaptic ECM proteins with important roles in neuromuscular development. The roles of their many potential receptors and/or binding proteins have been more difficult to assess at the genetic level due to the complexity of membrane interactions with these large proteins, but roles for MuSK-LRP4 in agrin signaling and for integrins, dystroglycan, and voltage-gated calcium channels in laminin-dependent phenotypes have been identified. Synaptic ECM proteins and their receptors are involved in almost all aspects of synaptic development, including synaptic initiation, topography, ultrastructure, maturation, stability, and transmission.

Comparative proteomic profiling of dystroglycan-associated proteins in wild type, mdx, and Galgt2 transgenic mouse skeletal muscle

Dystroglycan is a major cell surface glycoprotein receptor for the extracellular matrix in skeletal muscle. Defects in dystroglycan glycosylation cause muscular dystrophy and alterations in dystroglycan glycosylation can impact extracellular matrix binding. Here we describe an immunoprecipitation technique that allows isolation of beta dystroglycan with members of the dystrophin-associated protein complex (DAPC) from detergent-solubilized skeletal muscle. Immunoprecipitation, coupled with shotgun proteomics, has allowed us to identify new dystroglycan-associated proteins and define changed associations that occur within the DAPC in dystrophic skeletal muscles. In addition, we describe changes that result from overexpression of Galgt2, a normally synaptic muscle glycosyltransferase that can modify alpha dystroglycan and inhibit the development of muscular dystrophy when it is overexpressed. These studies identify new dystroglycan-associated proteins that may participate in dystroglycan's roles, both positive and negative, in muscular dystrophy.

High throughput screening for compounds that alter muscle cell glycosylation identifies new role for N-glycans in regulating sarcolemmal protein abundance and laminin binding

Duchenne muscular dystrophy is an X-linked disorder characterized by loss of dystrophin, a cytoskeletal protein that connects the actin cytoskeleton in skeletal muscle cells to extracellular matrix. Dystrophin binds to the cytoplasmic domain of the transmembrane glycoprotein β-dystroglycan (β-DG), which associates with cell surface α-dystroglycan (α-DG) that binds laminin in the extracellular matrix. β-DG can also associate with utrophin, and this differential association correlates with specific glycosylation changes on α-DG. Genetic modification of α-DG glycosylation can promote utrophin binding and rescue dystrophic phenotypes in mouse dystrophy models. We used high throughput screening with the plant lectin Wisteria floribunda agglutinin (WFA) to identify compounds that altered muscle cell surface glycosylation, with the goal of finding compounds that increase abundance of α-DG and associated sarcolemmal glycoproteins, increase utrophin usage, and increase laminin binding. We identified one compound, lobeline, from the Prestwick library of Food and Drug Administration-approved compounds that fulfilled these criteria, increasing WFA binding to C2C12 cells and to primary muscle cells from wild type and mdx mice. WFA binding and enhancement by lobeline required complex N-glycans but not O-mannose glycans that bind laminin. However, inhibiting complex N-glycan processing reduced laminin binding to muscle cell glycoproteins, although O-mannosylation was intact. Glycan analysis demonstrated a general increase in N-glycans on lobeline-treated cells rather than specific alterations in cell surface glycosylation, consistent with increased abundance of multiple sarcolemmal glycoproteins. This demonstrates the feasibility of high throughput screening with plant lectins to identify compounds that alter muscle cell glycosylation and identifies a novel role for N-glycans in regulating muscle cell function.

The carbohydrate CT1 is expressed in topographically fixed glomeruli in the mouse olfactory bulb

Cell surface carbohydrates define subpopulations of primary olfactory neurons whose axons terminate in select glomeruli in the olfactory bulb. The combination of carbohydrates present on axon subpopulations has been proposed to confer a unique identity that contributes to the establishment of the olfactory topographic map. We have identified a novel subpopulation of primary olfactory neurons in mice that express blood group carbohydrates with GalNAc-ß1,4[NeuAcα 2,3]Galß1 residues recognised by the CT1 antibody. The CT1 carbohydrate has been shown to modulate adhesion of nerve terminals to the extracellular matrix and to synaptic proteins. The axons of the CT1-positive primary olfactory neurons terminate in a subpopulation of glomeruli in the olfactory bulb. Four lines of evidence support the view that CT1 glomeruli are topographically fixed. First, CT1 glomeruli were restricted predominantly to the dorsomedial olfactory bulb and were absent from large patches of the ventrolateral bulb. Second, similar distributions were observed for CT1 glomeruli on both the left and right olfactory bulbs of each animal, and between animals. Third, CT1 glomeruli were typically present as small clusters of 2-4 glomeruli. Fourth, a single CT1 glomerulus was always apposed to the glomeruli innervated by axons expressing the M72 odorant receptor. We also show that the CT1 carbohydrate is lost in gain-of-function transgenic mice over-expressing the blood group A glycosyltransferase in which there is aberrant targeting of M72 axons. Taken together, these results suggest that the CT1 carbohydrate, together with other carbohydrates, contributes to axon guidance during the establishment of the olfactory topographic map.

A human-specific deletion in mouse Cmah increases disease severity in the mdx model of Duchenne muscular dystrophy

During the evolution of humans, an inactivating deletion was introduced in the CMAH (cytidine monophosphate-sialic acid hydroxylase) gene, which eliminated biosynthesis of the common mammalian sialic acid N-glycolylneuraminic acid from all human cells. We found that this human-specific change in sialylation capacity contributes to the marked discrepancy in phenotype between the mdx mouse model for Duchenne muscular dystrophy (DMD) and the human disease. When compared to human patients with DMD, mdx mice show reduced severity or slower development of clinically relevant disease phenotypes, despite lacking dystrophin protein in almost all muscle cells. This is especially true for the loss of ambulation, cardiac and respiratory muscle weakness, and decreased life span, all of which are major phenotypes contributing to DMD morbidity and mortality. These phenotypes occur at an earlier age or to a greater degree in mdx mice that also carry a human-like mutation in the mouse Cmah gene, possibly as a result of reduced strength and expression of the dystrophin-associated glycoprotein complex and increased activation of complement. Cmah-deficient mdx mice are a small-animal model for DMD that better approximates the human glycome and its contributions to muscular dystrophy.

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.

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.

The synaptic CT carbohydrate modulates binding and expression of extracellular matrix proteins in skeletal muscle: Partial dependence on utrophin

The CT carbohydrate, Neu5Ac/Neu5Gcalpha2,3[GalNAcbeta1,4]Galbeta1,4GlcNAcbeta-, is specifically expressed at the neuromuscular junction in skeletal myofibers of adult vertebrates. When Galgt2, the glycosyltransferase that creates the synaptic beta1,4GalNAc portion of this glycan, is overexpressed in extrasynaptic regions of the myofiber membrane, alpha dystroglycan becomes glycosylated with the CT carbohydrate and this coincides with the ectopic expression of synaptic dystroglycan-binding proteins, including laminin alpha4, laminin alpha5, and utrophin. Here we show that both synaptic and extrasynaptic forms of laminin and agrin have increased binding to the CT carbohydrate compared to sialyl-N-acetyllactosamine, its extrasynaptically expressed precursor. Muscle laminins also show increased binding to CT-glycosylated muscle alpha dystroglycan relative to its non-CT-containing glycoforms. Overexpression of Galgt2 in transgenic mouse skeletal muscle increased the mRNA expression of extracellular matrix (ECM) genes, including agrin and laminin alpha5, as well as utrophin, integrin alpha7, and neuregulin. Increased expression of ECM proteins in Galgt2 transgenic skeletal muscles was partially dependent on utrophin, but utrophin was not required for Galgt2-induced changes in muscle growth or neuromuscular development. These experiments demonstrate that overexpression of a synaptic carbohydrate can increase both ECM binding to alpha dystroglycan and ECM expression in skeletal muscle, and they suggest a mechanism by which Galgt2 overexpression may inhibit muscular dystrophy and affect neuromuscular development.

Aberrant development of neuromuscular junctions in glycosylation-defective Large(myd) mice

Mice deficient in the glycosyltransferase Large are characterized by severe muscle and central nervous system abnormalities. In this study, we show that the formation and maintenance of neuromuscular junctions in Large(myd) mice are greatly compromised. Neuromuscular junctions are not confined to the muscle endplate zone but are widely spread and are frequently accompanied by exuberant nerve sprouting. Nerve terminals are highly fragmented and binding of alpha-bungarotoxin to postsynaptic acetylcholine receptors (AChRs) is greatly reduced. In vitro, Large(myd) myotubes are responsive to agrin but produce aberrant AChR clusters, which are larger in area and less densely packed with AChRs. In addition, AChR expression on the cell surface is diminished suggesting that AChR assembly or transport is defective. These results together with the finding that O-linked glycosylation at neuromuscular junctions of Large(myd) mice is compromised indicate that the action of Large is necessary for proper neuromuscular junction development.

Overexpression of Galgt2 in skeletal muscle prevents injury resulting from eccentric contractions in both mdx and wild-type mice

The cytotoxic T cell (CT) GalNAc transferase, or Galgt2, is a UDP-GalNAc:beta1,4-N-acetylgalactosaminyltransferase that is localized to the neuromuscular synapse in adult skeletal muscle, where it creates the synaptic CT carbohydrate antigen {GalNAcbeta1,4[NeuAc(orGc)alpha2, 3]Galbeta1,4GlcNAcbeta-}. Overexpression of Galgt2 in the skeletal muscles of transgenic mice inhibits the development of muscular dystrophy in mdx mice, a model for Duchenne muscular dystrophy. Here, we provide physiological evidence as to how Galgt2 may inhibit the development of muscle pathology in mdx animals. Both Galgt2 transgenic wild-type and mdx skeletal muscles showed a marked improvement in normalized isometric force during repetitive eccentric contractions relative to nontransgenic littermates, even using a paradigm where nontransgenic muscles had force reductions of 95% or more. Muscles from Galgt2 transgenic mice, however, showed a significant decrement in normalized specific force and in hindlimb and forelimb grip strength at some ages. Overexpression of Galgt2 in muscles of young adult mdx mice, where Galgt2 has no effect on muscle size, also caused a significant decrease in force drop during eccentric contractions and increased normalized specific force. A comparison of Galgt2 and microdystrophin overexpression using a therapeutically relevant intravascular gene delivery protocol showed Galgt2 was as effective as microdystrophin at preventing loss of force during eccentric contractions. These experiments provide a mechanism to explain why Galgt2 overexpression inhibits muscular dystrophy in mdx muscles. That overexpression also prevents loss of force in nondystrophic muscles suggests that Galgt2 is a therapeutic target with broad potential applications.

Muscular dystrophy associated with alpha-dystroglycan deficiency in Sphynx and Devon Rex cats

Recent studies have identified a number of forms of muscular dystrophy, termed dystroglycanopathies, which are associated with loss of natively glycosylated alpha-dystroglycan. Here we identify a new animal model for this class of disorders in Sphynx and Devon Rex cats. Affected cats displayed a slowly progressive myopathy with clinical and histologic hallmarks of muscular dystrophy including skeletal muscle weakness with no involvement of peripheral nerves or CNS. Skeletal muscles had myopathic features and reduced expression of alpha-dystroglycan, while beta-dystroglycan, sarcoglycans, and dystrophin were expressed at normal levels. In the Sphynx cat, analysis of laminin and lectin binding capacity demonstrated no loss in overall glycosylation or ligand binding for the alpha-dystroglycan protein, only a loss of protein expression. A reduction in laminin-alpha2 expression in the basal lamina surrounding skeletal myofibers was also observed. Sequence analysis of translated regions of the feline dystroglycan gene (DAG1) in affected cats did not identify a causative mutation, and levels of DAG1 mRNA determined by real-time QRT-PCR did not differ significantly from normal controls. Reduction in the levels of glycosylated alpha-dystroglycan by immunoblot was also identified in an affected Devon Rex cat. These data suggest that muscular dystrophy in Sphynx and Devon Rex cats results from a deficiency in alpha-dystroglycan protein expression, and as such may represent a new type of dystroglycanopathy where expression, but not glycosylation, is affected.

O-fucosylation of muscle agrin determines its ability to cluster acetylcholine receptors

Protein O-fucosyltransferase 1 (Pofut1) transfers fucose to serine or threonine on proteins, including Notch receptors, that contain EGF repeats with a particular consensus sequence. Here we demonstrate that agrin is O-fucosylated in a Pofut1-dependent manner, and that this glycosylation can regulate agrin function. Fucosylation of recombinant C45 agrin, both active (neural, z8) and inactive (muscle, z0) splice forms, was eliminated when agrin was overexpressed in Pofut1-deficient cells or by mutation of a consensus site for Pofut1 fucosylation (serine 1726 in the EGF4 domain). Loss of O-fucosylation caused a gain of function for muscle agrin such that it stimulated AChR clustering and MuSK phosphorylation in cultured myotubes at levels normally only found with the neural splice form. Deletion of Pofut1 in cultured primary myotubes and in adult skeletal muscle increased AChR aggregation. In addition, Pofut1 gene and protein expression and Pofut1 activity of the EGF4 domain of agrin were modulated during neuromuscular development. These data are consistent with a role for Pofut1 in AChR aggregation during synaptogenesis via the regulation of the synaptogenic activity of muscle agrin.

Overexpression of the cytotoxic T cell (CT) carbohydrate inhibits muscular dystrophy in the dyW mouse model of congenital muscular dystrophy 1A

A number of recent studies have demonstrated therapeutic effects of transgenes on the development of muscle pathology in the mdx mouse model for Duchenne muscular dystrophy, but none have been shown also to be effective in mouse models for laminin alpha2-deficient congenital muscular dystrophy (MDC1A). Here, we show that overexpression of the cytotoxic T cell (CT) GalNAc transferase (Galgt2) is effective in inhibiting the development of muscle pathology in the dy(W) mouse model of MDC1A, much as we had previously shown in mdx animals. Embryonic overexpression of Galgt2 in skeletal muscles using transgenic mice or postnatal overexpression using adeno-associated virus both reduced the extent of muscle pathology in dy(W)/dy(W) skeletal muscle. As with mdx mice, embryonic overexpression of the Galgt2 transgene in dy(W)/dy(W) myofibers inhibited muscle growth, whereas postnatal overexpression did not. Both embryonic and postnatal overexpression of Galgt2 in dy(W)/dy(W) muscle increased the expression of agrin, a protein that, in recombinant form, has been shown to ameliorate disease, whereas laminin alpha1, another disease modifier, was not expressed. Galgt2 over-expression also stimulated the glycosylation of a gly-colipid with the CT carbohydrate, and glycolipids accounted for most of the CT-reactive material in postnatal overexpression experiments. These experiments demonstrate that Galgt2 overexpression is effective in altering disease progression in skeletal muscles of dy(W) mice and should be considered as a therapeutic target in MDC1A.

Altered expression of natively glycosylated alpha dystroglycan in pediatric solid tumors

Altered glycosylation and/or expression of dystroglycan have been reported in forms of congenital muscular dystrophy as well as in cancers of the breast, colon, and oral epithelium. To date, however, there has been no study of the expression of dystroglycan in pediatric solid tumors. Using a combination of immunostaining on tissue microarrays and immunoblotting of snap-frozen unfixed tissues, we demonstrate a significant reduction in native alpha dystroglycan expression in pediatric alveolar rhabdomyosarcoma (RMS), embryonal RMS, neuroblastoma (NBL), and medulloblastoma, whereas expression of beta dystroglycan, which is cotranslated with alpha dystroglycan, is largely unchanged. Loss of native alpha dystroglycan expression was significantly more pronounced in stage 4 NBL than in pooled samples of stage 1 and stage 2 NBL, suggesting that loss of native alpha dystroglycan expression increases with advancing tumor stage. Neuroblastoma and RMS samples with reduced expression of native alpha dystroglycan also showed reduced laminin binding in laminin overlay experiments. Expression of natively glycosylated alpha dystroglycan was not altered in several other pediatric tumor types when compared with appropriate normal tissue controls. These data provide the first evidence that alpha dystroglycan glycosylation and laminin binding to alpha dystroglycan are altered in certain pediatric solid tumors and suggest that aberrant dystroglycan glycosylation may contribute to tumor cell biology in patients with RMS, medulloblastoma, and NBL.

Postnatal overexpression of the CT GalNAc transferase inhibits muscular dystrophy in mdx mice without altering muscle growth or neuromuscular development: evidence for a utrophin-independent mechanism

Overexpression of the cytotoxic T cell (CT) GalNAc transferase (Galgt2) in the skeletal muscles of transgenic mdx mice has been reported to inhibit the development of muscular dystrophy. The profound effect of Galgt2 on muscular dystrophy in transgenic mice, where overexpression is begins from embryonic stages, is complicated by its additional effects on muscle growth and neuromuscular structure. Here, we use adeno-associated virus (AAV) to show that overexpression of Galgt2 in skeletal myofibers in the early postnatal period is equally effective in inhibiting muscular dystrophy, but that it does so without altering muscle growth or neuromuscular structure. Unlike embryonic overexpression, postnatal overexpression of Galgt2 did not reproducibly increase the expression of utrophin, synaptic laminins, or dystrophin-associated glycoproteins along infected myofibers. Moreover, Galgt2 overexpression inhibited muscular dystrophy to the same extent in utrophin-deficient mdx muscles as it did in utrophin-expressing mdx muscles. Thus, Galgt2 is a molecular target for therapy in DMD that can be utilized in a manner that separates its clinical benefit from its effects on development, and its clinical benefit is distinct from that achieved by utrophin.

Genetic manipulation of blood group carbohydrates alters development and pathfinding of primary sensory axons of the olfactory systems

Primary sensory neurons in the vertebrate olfactory systems are characterised by the differential expression of distinct cell surface carbohydrates. We show here that the histo-blood group H carbohydrate is expressed by primary sensory neurons in both the main and accessory olfactory systems while the blood group A carbohydrate is expressed by a subset of vomeronasal neurons in the developing accessory olfactory system. We have used both loss-of-function and gain-of-function approaches to manipulate expression of these carbohydrates in the olfactory system. In null mutant mice lacking the alpha(1,2)fucosyltransferase FUT1, the absence of blood group H carbohydrate resulted in the delayed maturation of the glomerular layer of the main olfactory bulb. In addition, ubiquitous expression of blood group A on olfactory axons in gain-of-function transgenic mice caused mis-routing of axons in the glomerular layer of the main olfactory bulb and led to exuberant growth of vomeronasal axons in the accessory olfactory bulb. These results provide in vivo evidence for a role of specific cell surface carbohydrates during development of the olfactory nerve pathways.

Dystroglycan: from biosynthesis to pathogenesis of human disease

α- and β-dystroglycan constitute a membrane-spanning complex that connects the extracellular matrix to the cytoskeleton. Although a structural role for dystroglycan had been identified, biochemical and genetic discoveries have recently highlighted the significance of posttranslational processing for dystroglycan function. Glycosylation is the crucial modification that modulates the function of dystroglycan as a receptor for extracellular binding partners. It has become clear that perturbation of dystroglycan glycosylation is the central event in the pathogenesis of several complex disorders, and recent advances suggest that glycosylation could be modulated to ameliorate the pathological features. Our increased understanding of the mechanisms of interaction of dystroglycan with its ligands has become an essential tool in deciphering the biological processes related to the human diseases in which the proteins are implicated.

Dystrophin glycoprotein complex dysfunction: a regulatory link between muscular dystrophy and cancer cachexia

Cachexia contributes to nearly a third of all cancer deaths, yet the mechanisms underlying skeletal muscle wasting in this syndrome remain poorly defined. We report that tumor-induced alterations in the muscular dystrophy-associated dystrophin glycoprotein complex (DGC) represent a key early event in cachexia. Muscles from tumor-bearing mice exhibited membrane abnormalities accompanied by reduced levels of dystrophin and increased glycosylation on DGC proteins. Wasting was accentuated in tumor mdx mice lacking a DGC but spared in dystrophin transgenic mice that blocked induction of muscle E3 ubiquitin ligases. Furthermore, DGC deregulation correlated positively with cachexia in patients with gastrointestinal cancers. Based on these results, we propose that, similar to muscular dystrophy, DGC dysfunction plays a critical role in cancer-induced wasting.

Motor neurite outgrowth is selectively inhibited by cell surface MuSK and agrin

During neuromuscular development, "stop signals" present on the target myotube inhibit motor axon growth. Mice lacking either the neuronal form of agrin or the muscle-specific tyrosine kinase (MuSK) lose stop signal activity, suggesting that they are part of such signals or induce them in myotubes. To test whether MuSK complexes form stop signals in the absence of myotube signaling, we cultured ciliary ganglion (CG) neurons with nonmuscle cells expressing cell-surface MuSK. Expression of MuSK had no effect on neuronal adhesion. MuSK expression, however, inhibited neurite outgrowth from CG neurons, but not retinal ganglion cell neurons. The neurite-inhibitory effect could be completely reversed by an antibody to the MuSK extracellular domain, and partially reversed by an antibody to agrin, suggesting that inhibition is mediated by a complex of these proteins. Thus, an agrin/MuSK complex may form part of a motor neuron stop signal involved in "reverse signaling" to the motor neuron.

[Utrophin, a way to cure Duchenne muscle dystrophy]

Duchenne muscle dystrophy results from the absence of dystrophin, a cytoskeletal protein of the muscle fibre. Dystrophin plays an essential role in the integrity of the membrane-associated protein complexes connected to the extracellular matrix. On chromosome 6 is located the gene of a protein presenting 80 % homology with dystrophin : utrophin, which is expressed at the neuromuscular junction. The review examines if utrophin can replace dystrophin and correct the structural and functional characteristics of the myopathy, and how the improvements can be quantitatively expressed. In transgenic mice, deficient in dystrophin, but overexpressing large quantities of utrophin, the latter is found on structures where dystrophin is normally located, histological signs of necrosis disappear and the recovery of functional disorders, specially affecting the mechanical properties of the muscle fibres, can be complete. The review examines also several ways of obtaining overexpression of utrophin in adult mdx mice, such as conditioned expression of the utrophin transgene (using a tetracycline-sensitive transactivator), transfection with viral vectors containing the utrophin cDNA (complete or truncated), actions on factor(s) controlling utrophin expression at the neuromuscular junction (heregulin, 4 N-acetylgalactosamine), and pharmacological ways of inducing expression (NO, arginine). Though partial improvements of the myopathy status have been obtained by these various approaches, they remain limited by their localized action and/or by the moderate level of utrophin expression obtained. Further researchs to overcome these limitations are urgently needed in order to transform the very promising effect of utrophin overexpression into a real treatment of Duchenne myopathy.