Distinct structures and functions of related pre- and postsynaptic carbohydrates at the mammalian neuromuscular junction

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.

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.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

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.

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.

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.

Induction of T-Cell Infiltration and Programmed Death Ligand 2 Expression by Adeno-Associated Virus in Rhesus Macaque Skeletal Muscle and Modulation by Prednisone

Use of adeno-associated virus (AAV) to transduce genes into skeletal muscles can be associated with T-cell responses to viral capsid and/or to transgenic protein. Intramuscular mononuclear cell infiltrates primarily consisting of CD8+ T cells and also containing FOXP3+ regulatory T cells were present in rhesus macaque skeletal muscle treated with rAAVrh74.MCK.GALGT2 by vascular delivery. Administration of oral prednisone prior to AAV gene delivery and throughout the study reduced such infiltrates by 60% at 24 weeks post AAV delivery compared with AAV-treated animals not receiving prednisone, regardless of the presence of pre-existing AAV serum antibodies at the time of treatment. The majority of CD8+ T cells in AAV-treated muscles expressed activated caspase 3 and programmed cell death protein 1 (PD1), suggesting ongoing programmed cell death. AAV-transduced skeletal muscles also had elevated expression of programmed death ligand 2 (PDL2) on skeletal myofibers, and this increase in expression extended to muscles where transgene was not overexpressed. These data demonstrate that prednisone can reduce the extent of intramuscular T-cell infiltrates in AAV-treated muscles, which may aid in achieving long-term transgene expression, as may the induction of PDL2 expression on skeletal myofibers to promote PD1-mediated programmed T-cell death.

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.

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.

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.

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.

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.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

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.

Mechanisms of disease: congenital muscular dystrophies-glycosylation takes center stage

Recent studies have defined a group of muscular dystrophies, now termed the dystroglycanopathies, as novel disorders of glycosylation. These conditions include Walker-Warburg syndrome, muscle-eye-brain disease, Fukuyama-type congenital muscular dystrophy, congenital muscular dystrophy types 1C and 1D, and limb-girdle muscular dystrophy type 2I. Although clinical findings can be highly variable, dystroglycanopathies are all characterized by cortical malformations and ocular defects at the more severe end of the clinical spectrum, in addition to muscular dystrophy. All of these disorders are defined by the underglycosylation of alpha-dystroglycan. Defective glycosylation of dystroglycan severs the link between this important cell adhesion molecule and the extracellular matrix, thereby contributing to cellular pathology. Recent experiments indicate that glycosylation might not only define forms of muscular dystrophy but also provide an avenue to the development of therapies for these disorders.

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.

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.

Versican in the developing brain: lamina-specific expression in interneuronal subsets and role in presynaptic maturation

Chondroitin sulfate proteoglycans (CSPGs) of the extracellular matrix help stabilize synaptic connections in the postnatal brain and impede regeneration after injury. Here, we show that a CSPG of the lectican family, versican, also promotes presynaptic maturation in the developing brain. In the embryonic chick optic tectum, versican is expressed selectively by subsets of interneurons confined to the retinorecipient laminae, in which retinal axons arborize and form synapses. It is a major receptor for the Vicia villosa B4 lectin (VVA), shown previously to inhibit invasion of the retinorecipient lamina by retinal axons (Inoue and Sanes, 1997). In vitro, versican promotes enlargement of presynaptic varicosities in retinal axons. Depletion of versican in ovo, by RNA interference, results in retinal arbors with smaller than normal varicosities. We propose that versican provides a lamina-specific cue for presynaptic maturation and discuss the related but distinct effects of versican depletion and VVA blockade.

Disruption of the mouse Large gene in the enr and myd mutants results in nerve, muscle, and neuromuscular junction defects

The autosomal recessive neuromuscular disorder associated with the enervated (enr) mouse transgene insertion manifests impaired peripheral nerve regeneration due to defects in Schwann cells and resembles the myodystrophy (Large(myd)) phenotype. Here we show that the enr transgene has integrated into Chr 8 approximately 160 kb downstream from the 3' end of the Large gene disrupting its expression as confirmed by the lack of genetic complementation between Large(myd) and enr mice, the very low Large mRNA levels in enr tissues and hypoglycosylation of alpha-dystroglycan, a known substrate of LARGE. Mutant nerve conduction and grip strength were impaired whereas sodium channel clustering at the nodes of Ranvier was unaffected. Interestingly, the mutant neuromuscular junctions displayed abnormal acetylcholine receptor clustering with reduced immunostaining for beta-dystroglycan, laminin, agrin, MuSK, and to a lesser extent acetylcholinesterase and rapsyn. These data implicate LARGE in nerve, muscle, and neuromuscular junction function.

Transgenic overexpression of dystroglycan does not inhibit muscular dystrophy in mdx mice

Recently, there have been a number of studies demonstrating that overexpression of molecules in skeletal muscle can inhibit or ameliorate aspects of muscular dystrophy in the mdx mouse, a model for Duchenne muscular dystrophy. Several such studies involve molecules that increase the expression of dystroglycan, an important component of the dystrophin-glycoprotein complex. To test whether dystroglycan itself inhibits muscular dystrophy in mdx mice, we created dystroglycan transgenic mdx mice (DG/mdx). The alpha and beta chains of dystroglycan were highly overexpressed along the sarcolemmal membrane in most DG/mdx muscles. Increased dystroglycan expression, however, did not correlate with increased expression of utrophin or sarcoglycans, but rather caused their decreased expression. In addition, the percentage of centrally located myofiber nuclei and the level of serum creatine kinase activity were not decreased in DG/mdx mice relative to mdx animals. Therefore, dystroglycan overexpression does not cause the concomitant overexpression of a utrophin-glycoprotein complex in mdx muscles and has no effect on the development of muscle pathology associated with muscular dystrophy.

A metabolic labeling approach toward proteomic analysis of mucin-type O-linked glycosylation

Mucin-type O-linked glycoproteins are involved in a variety of biological interactions in higher eukaryotes. The biosynthesis of these glycoproteins is initiated by a family of polypeptide N-acetyl-alpha-galactosaminyltransferases (ppGalNAcTs) that modify proteins in the secretory pathway. The lack of a defined consensus sequence for the ppGalNAcTs makes the prediction of mucin-type O-linked glycosylation difficult based on primary sequence alone. Herein we present a method for labeling mucin-type O-linked glycoproteins with a unique chemical tag, the azide, which permits their selective covalent modification from complex cell lysates. From a panel of synthetic derivatives, we identified an azido GalNAc analog (N-azidoacetylgalactosamine, GalNAz) that is metabolized by numerous cell types and installed on mucin-type O-linked glycoproteins by the ppGalNAcTs. The azide serves as a bioorthogonal chemical handle for selective modification with biochemical or biophysical probes using the Staudinger ligation. The approach was validated by labeling a recombinant glycoprotein that is known to possess O-linked glycans with GalNAz. In addition, GalNAz efficiently labeled mucin-type O-linked glycoproteins expressed at endogenous levels. The ability to label mucin-type O-linked glycoproteins with chemical tags should facilitate their identification by proteomic strategies.

Core 1 glycans on alpha-dystroglycan mediate laminin-induced acetylcholine receptor clustering but not laminin binding

Although unique O-linked oligosaccharides on alpha-dystroglycan are important for binding to a variety of extracellular ligands, the function(s) of more generic carbohydrate structures on alpha-dystroglycan remain unclear. Recent studies suggest a role for glycoconjugates bearing the core 1 disaccharide Galbeta(1-3)GalNAc in acetylcholine receptor (AChR) clustering on the surface of muscle cells. Here, we report experiments demonstrating that the core 1-specific lectin jacalin almost completely abrogated laminin-induced AChR clustering in C2C12 myotubes and that alpha-dystroglycan was the predominant jacalin-binding protein detected in C2C12 myotube lysates. Although jacalin likely inhibited laminin-induced AChR clustering by directly binding to alpha-dystroglycan, jacalin had no effect on laminin binding to the myotube surface or to alpha-dystroglycan. Like jacalin, peanut agglutinin lectin also binds the core 1 disaccharide but not when it is terminally sialylated as expressed on alpha-dystroglycan. We show that C2C12 alpha-dystroglycan bound to peanut agglutinin only after digestion with neuraminidase. Simultaneous treatment of myotubes with neuraminidase and endo-O-glycosidase diminished alpha-dystroglycan binding to peanut agglutinin and inhibited neuraminidase-induced AChR clustering. We conclude that sialylated core 1 oligosaccharides of alpha-dystroglycan are important for laminin-induced AChR clustering and that their function in this process is distinct from the established role of alpha-dystroglycan oligosaccharides in laminin binding.

Definition of pre- and postsynaptic forms of the CT carbohydrate antigen at the neuromuscular junction: ubiquitous expression of the CT antigens and the CT GalNAc transferase in mouse tissues

At the rodent neuromuscular junction, the synaptic expression of the CT carbohydrate antigens is defined by the binding of two monoclonal antibodies, CT1 and CT2. CT1 preferentially stains the presynaptic membrane, while CT2 preferentially stains the postsynaptic apparatus. Here we show that the differential subsynaptic distribution of these antigens is due to a preference of CT1 for structures containing N-acetyl neuraminic acid (NeuAc) and a preference of CT2 for structures containing N-glycolyl neuraminic acid (NeuGc). This was found to be the case both in binding to cultured myotubes, where NeuAc/NeuGc levels were manipulated by feeding acetylated N-acetyl mannosamine precursors, and in binding to purified GM2 ganglioside containing either NeuAc or NeuGc. At human neuromuscular junctions, where the enzymatic machinery to make NeuGc is absent [Proc. Natl. Acac. Sci. USA 95 (1998) 11751], CT1 and GM2(NeuAc) antibodies stained, while CT2 did not. Thus, the N-glycolyl modification of sialic acid helps to define the differential distribution of the CT antigens at the rodent neuromuscular junction, and this difference is lost in humans. In addition, sulfatase and 9-O-acetylesterase treatment of cells or tissues increased the amount of CT1 and CT2 antibody binding, with sulfatase differentially unmasking CT antigen expression on particular glycoproteins. Despite its uniquely synaptic localization in skeletal muscle, the CT antigens and the CT GalNAc transferase are ubiquitously expressed in other mouse tissues, including brain, spinal cord, and peripheral nerve. One of the proteins that can be co-purified with a CT-reactive glycoprotein is alpha dystroglycan. These data better define the sub-synaptic structures of the CT carbohydrate antigens at the neuromuscular junction and demonstrate their ubiquitous presence in mouse tissues, including the brain.

Glycobiology of the synapse: the role of glycans in the formation, maturation, and modulation of synapses

Synapses, which are the fundamental functional unit of the nervous system, are considered to be highly specialized cell adhesion structures. Studies since the 1960s demonstrated that various carbohydrates and glycoproteins are expressed in synapses in the central and peripheral nervous system. Although the functional roles of these synaptic carbohydrates and glycoproteins remain to be determined, rapidly accumulating data suggest that they may play critical roles in the formation, maturation, and functional modulation of synapses.

Spatiotemporal distribution of heparan sulfate epitopes during myogenesis and synaptogenesis: a study in developing mouse intercostal muscle

Formation of a basal lamina (BL) ensheathing developing skeletal muscle cells is one of the earliest events in mammalian skeletal muscle myogenesis. BL-resident heparan sulfate proteoglycans have been implicated in various processes during myogenesis, including synaptic differentiation. However, attention has focused on the proteoglycan protein core, ignoring the glycosaminoglycan moiety mainly because of a lack of appropriate tools. Recently, we selected a panel of anti-heparan sulfate antibodies applied here to study the spatiotemporal distribution of specific heparan sulfate (HS) epitopes during myogenesis. In mouse intercostal muscle at embryonic day (E14), formation of acetylcholine receptor clusters at synaptic sites coincides with HS deposition. Although some HS epitopes show a general appearance throughout the BL, one epitope preferably clusters at synaptic sites but does so only from E16 onward. During elongation and maturation of primary myotubes, a process preceding secondary myotube development, significant changes in the HS epitope constitution of both synaptic and extrasynaptic BL were observed. As a whole, the data presented here strengthen previous observations on developmental regulation by BL components, and add to the putative roles of specific HS epitopes in myogenesis and synaptogenesis.

Overexpression of the cytotoxic T cell GalNAc transferase in skeletal muscle inhibits muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is a congenital X-linked myopathy caused by lack of dystrophin protein expression. In DMD, the expression of many dystrophin-associated proteins (DAPs) is reduced along the sarcolemmal membrane, but the same proteins remain concentrated at the neuromuscular junction where utrophin, a dystrophin homologue, is expressed [Matsumura, K., Ervasti, J. M., Ohlendieck, K., Kahl, K. D. & Campbell, K. (1992) Nature (London) 360, 588-591]. This outcome has led to the concept that ectopic expression of a "synaptic scaffold" of DAPs and utrophin along myofibers might compensate for the molecular defects in DMD. Here we show that transgenic overexpression of the synaptic CT GalNAc transferase in the skeletal muscles of mdx animals (mdx/CT) increases the expression of utrophin and many DAPs, including dystroglycans, sarcoglycans, and dystrobrevins, along myofibers. Protein expression of utrophin and DAPs was equal to or above that of wild-type mice. In addition, alpha-dystroglycan was glycosylated with the CT carbohydrate antigen in mdx/CT but not in mdx muscles. mdx/CT mice have little or no evidence of muscular dystrophy by several standard measures; Serum creatine kinase levels, percentage of centrally located myofiber nuclei, and variance in myofiber diameter in mdx/CT muscles were dramatically reduced compared with mdx mice. These data suggest that ectopic expression of the CT GalNAc transferase creates a functional dystrophin-related complex along myofibers in the absence of dystrophin and should be considered as a target for therapeutic intervention in DMD.

Modulation of agrin binding and activity by the CT and related carbohydrate antigens

Agrin is a nerve-derived signal that is essential for the proper organization of postsynaptic acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction. It is likely that carbohydrates play a significant role in regulating agrin activity, as agrin binds multiple glycan structures and is itself a highly glycosylated protein. Here we provide support for this contention by showing that agrin can be modified with the CT antigen, a carbohydrate structure expressed at the neuromuscular junction, and by describing the resulting changes in agrin binding to neoglycoconjugates and cultured myotubes, as well as changes in agrin-dependent AChR clustering. Glycosylation of agrin with the CT antigen required the mucin domain and the dystroglycan/heparin-binding domain. The presence of the mucin domain lowered agrin binding to several N-acetyllactosaminyl-containing saccharides and C2 myotubes and lowered agrin activity in AChR clustering. Glycosylation of agrin with the CT antigen, by contrast, increased agrin binding to myotubes and potentiated its AChR clustering activity at subsaturating concentrations. Last, sialylated and nonsialylated variants of N-acetyllactosamine differentially modulated AChR clustering and agrin activity, and these changes correlated with the ability of MuSK, an agrin-stimulated kinase, to bind to these structures. These experiments demonstrate that the glycosylation state of agrin affects its activity and suggest a role for the CT antigen in modulating agrin function.

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

Carbohydrates have been shown to mediate or modulate a number of important events in the development of the nervous system; however, there is little evidence that they participate directly in the development of synapses. One carbohydrate structure that is likely to be important in synaptic development of the neuromuscular junction is the CT carbohydrate antigen [GalNAcbeta1,4[NeuAcalpha2,3]Galbeta1(-3GalNAc or -4GlcNAc)]. The synaptic localization of the CT antigen is due to the presence of the terminal beta1,4 GalNAc linkage, and such linkages are localized to the neuromuscular junction in many species. Here we show that an enzyme that can create the synaptic CT structure, the CT GalNAc transferase, is also confined to the neuromuscular junction in mice. Using transgenic mice, we show that overexpression of the CT GalNAc transferase in extrasynaptic regions in skeletal myofibers caused as much as a 60% reduction in the diameter of adult myofibers and an order of magnitude increase in satellite cells. Neuromuscular junctions of transgenic mice had severely reduced numbers of secondary folds, Schwann cell processes were present in the synaptic cleft, and secondary folds were often misaligned with active zones. In addition, multiple presynaptic specializations occurred on individual myofibers. In addition, some normally synaptic proteins, including laminin alpha4, laminin alpha5, utrophin, and NCAM, were expressed along extrasynaptic regions of myofibers. One of the muscle proteins that displayed increased glycosylation with the CT antigen in the transgenic mice was alpha-dystroglycan. These experiments provide the first in vivo evidence that a synaptic carbohydrate antigen has important roles in the development of the neuromuscular synapse and suggest that the CT antigen is involved in controlling the expression of synaptic molecules.

Developmental expression of a unique carbohydrate antigen, Tn antigen, in mouse central nervous tissues

Using an anti-Tn monoclonal antibody, the Tn antigen was detected immunohistochemically in prenatal and early postnatal central nervous tissues. On embryonic day 9 (E9), the antigen was distributed throughout the single neuroepithelial layer in the neocortex and then became more prominent in the preplate than in the ventricular zone along with formation of the preplate. Following division of the preplate and concomitant formation of the cortical plate, distinct labeling of the neocortex occurred in the marginal, subplate and intermediate zones, whereas in the cortical plate and ventricular zone were virtually not immunostained. It is notable that thalamocortical afferent fibers were also immunostained specifically on E14. After birth, the localization of the antigen became less noticeable and by 3 weeks after birth, the antigen had substantially disappeared. In the developing cerebellum, prominent labeling was also observed in the molecular layer and outskirts of the cerebellar nuclei on early postnatal days. To characterize the glycoprotein bearing the Tn antigen biochemically, immunoblot analysis was performed. The glycoprotein, most of which was extracted with a salt solution, migrated as a broad smeared band corresponding to a molecular weight of about 250 kDa on SDS-PAGE. Among the various tissues examined, this glycoprotein was only detected in the brain and its amount increased until an early postnatal stage with a peak on postnatal day 3 (P3), and then decreased gradually with age. This spatially and developmentally regulated expression of the Tn antigen suggests that this antigen plays a significant role in brain development.

Heparan sulfate heterogeneity in skeletal muscle basal lamina: demonstration by phage display-derived antibodies

The basal lamina (BL) enveloping skeletal muscle fibers contains different glycoproteins, including proteoglycans. To obtain more information on the glycosaminoglycan moiety of proteoglycans, we have selected a panel of anti-heparan sulfate (HS) antibodies from a semisynthetic antibody phage display library by panning against glycosaminoglycan preparations derived from skeletal muscle. Epitope recognition by the antibodies is strongly dependent on O- and N-sulfation of the heparan sulfate. Immunostaining with these antibodies showed a distinct distribution of heparan sulfate epitopes in muscle basal lamina of various species. Clear differences in staining intensity were observed between neural, synaptic, and extrasynaptic basal laminae. Moreover, temporal and regional changes in abundancy of heparan sulfate epitopes were observed during muscle development both in vitro and in vivo. Taken together, these data suggest a role for specific heparan sulfate domains/species in myogenesis and synaptogenesis. Detailed analysis of the functions of heparan sulfate epitopes in muscle morphogenesis has now become feasible with the isolation of antibodies specific for distinct heparan sulfate epitopes.

Alpha-galactosidase stimulates acetylcholine receptor aggregation in skeletal muscle cells via PNA-binding carbohydrates

Aggregation of nicotinic acetylcholine receptors (AChRs) in skeletal muscle is an essential step in the formation of the mammalian neuromuscular junction. While proteins that bind to myotube receptors such as agrin and laminin can stimulate AChR aggregation in cultured myotubes, removal of cell surface sialic acids stimulates aggregation in a ligand-independent manner. Here, we show that removal of cell surface alpha-galactosides also stimulates AChR aggregation in the absence of added laminin or agrin. AChR aggregation stimulated by alpha-galactosidase was blocked by peanut agglutinin (PNA), which binds to lactosamine-containing disaccharides, but not by the GalNAc-binding lectin Vicia villosa agglutinin (VVA-B4). AChR aggregation stimulated by alpha-galactosidase potentiated AChR clustering induced by either neural agrin or laminin-1 and could be inhibited by muscle agrin. These data suggest that capping of cell surface lactosamines or N-acetyllactosamines with alpha-galactose affects AChR aggregation much as capping with sialic acids does.

N-acetyllactosamine and the CT carbohydrate antigen mediate agrin-dependent activation of MuSK and acetylcholine receptor clustering in skeletal muscle

Galbeta1,3GalNAc and Galbeta1,4GIcNAc are the subterminal saccharide structures present on the CT carbohydrate antigen GalNAcbeta1,4[NeuAcalpha2,3]-Galbeta1-(3GalNAc or 4GIcNAc)-R, which is localized at the mammalian neuromuscular junction. Here we show that Galbeta1,3GalNAc, Galbeta1,4GIcNAc, and the CT carbohydrate antigen affect postsynaptic assembly in cultured muscle cells. Treatment of C2C12 myotubes with benzyl-O-alpha-GalNAc or neuraminidase increased peanut agglutinin (PNA) expression and AChR clustering. Induction of AChR clustering was blocked by PNA and by muscle agrin. Addition of Galbeta1,4GIcNAc or Galbeta1,3GalNAc increased AChR clustering in myotubes and muscle-specific kinase (MUSK) autophosphorylation in vitro, while NeuAcalpha2,3Galbeta1,4GIcNAc and Galbeta1,4GIc did not. Neural agrin activated MuSK in vitro if the lactosamine-containing mucin domain was present, and this activation was blocked in large part by Galbeta1,3GalNAc and Galbeta1,4GIcNAc. Agrin fragments and MuSK bound to these disaccharides with differing specificities. Overexpression of the CT carbohydrate antigen also increased AChR clustering and MuSK autophosphorylation in the presence of neural agrin. These data suggest a model in which different portions of the CT carbohydrate structure contribute to agrin-dependent signal transduction.