Defective glycosylation in congenital muscular dystrophies

Structural and functional alterations of muscle fibres in the novel mouse model of facioscapulohumeral muscular dystrophy

We recently generated a mouse model of facioscapulohumeral muscular dystrophy (FSHD) by selectively overexpressing FRG1, a candidate gene for FSHD, in skeletal muscle. The muscles of the FRG-1 mice did not show any plasmamembrane defect suggesting a novel pathogenetic mechanism for FSHD. Here, we study structure and function of muscle fibres from three lines of mice overexpressing FRG1 at different levels: FRG1-low, FRG1-med, FRG1-high. Cross-sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (V(o)) of identified types of muscle fibres from FRG1-low and FRG1-med mice were analysed and found to be lower than in WT mice. Fast fibres and especially type 2B fibres (the fastest type) were preferentially involved in the dystrophic process showing a much larger force deficit than type 1 (slow) fibres. Consistent with the latter observation, the MHC isoform distribution of several muscles of the three FRG1 lines showed a shift towards slower MHC isoforms in comparison to WT muscle. Moreover, fast muscles showed a more evident histological deterioration, a larger atrophy and a higher percentage of centrally nucleated fibres than the soleus, the slowest muscle in mice. Interestingly, loss in CSA, Po/CSA and V(o) of single muscle fibres and MHC isoform shift towards a slower phenotype can be considered early signs of muscular dystrophy (MD). They were, in fact, found also in FRG1-low mice which did not show any impairment of function in vivo and of muscle size in vitro and in soleus muscles, which had a completely preserved morphology. This study provides a detailed characterization of structure and function of muscle fibres in a novel murine model of one of the main human MDs and suggests that fundamental features of the dystrophic process, common to most MDs, such as the intrinsic loss of contractile strength of muscle fibres, the preferential involvement of fast fibres and the shift towards a slow muscle phenotype can occur independently from obvious alterations of the plasma membrane.

Dictyostelium gnt15 encodes a protein with similarity to LARGE and plays an essential role in development

LARGE is a putative glycosyltransferase found to be mutated in mice with myodystrophy or patients with congenital muscular dystrophy. By homology searches, we identified in the Dictyostelium discoideum genome four open reading frames, i.e. gnt12-15, encoding proteins with sequence similarity to LARGE. Semi-quantitative RT-PCR analysis revealed distinct temporal expression patterns of the four gnt genes throughout Dictyostelium development. To explore the gene function, we performed targeted disruptions of gnt14 and gnt15. The gnt14(-) strains showed no obvious phenotypes. However, gnt15(-) cells grew slowly, changed in morphology, and displayed a developmental phenotype arresting at early stages. Compared with the wild type, gnt15(-) cells were more adhesive and exhibited altered levels of some surface adhesion molecules. Moreover, lectin-binding analysis demonstrated that gnt15 disruption affected profiles of membrane glycoproteins. Taken together, our data suggest that Gnt15 is essential for Dictyostelium development and may have a role in modulating cell adhesion and glycosylation.

Dystroglycan regulates structure, proliferation and differentiation of neuroepithelial cells in the developing vertebrate CNS

In the developing CNS alpha- and beta-dystroglycan are highly concentrated in the endfeet of radial neuroepithelial cells at the contact site to the basal lamina. We show that injection of anti-dystroglycan Fab fragments, knockdown of dystroglycan using RNAi, and overexpression of a dominant-negative dystroglycan protein by microelectroporation in neuroepithelial cells of the chick retina and optic tectum in vivo leads to the loss of their radial morphology, to hyperproliferation, to an increased number of postmitotic neurons, and to an altered distribution of several basally concentrated proteins. Moreover, these treatments also altered the oriented growth of axons from retinal ganglion cells and from tectal projection neurons. In contrast, expression of non-cleavable dystroglycan protein in neuroepithelial cells reduced their proliferation and their differentiation to postmitotic neurons. These results demonstrate that dystroglycan plays a key role in maintaining neuroepithelial cell morphology, and that interfering with dystroglycan function influences proliferation and differentiation of neuroepithelial cells. These data also suggest that an impaired dystroglycan function in neuroepithelial cells might be responsible for some of the severe brain abnormalities observed in certain forms of congenital muscular dystrophy.

Expression of the murine Pomt1 gene in both the developing brain and adult muscle tissues and its relationship with clinical aspects of Walker-Warburg syndrome

Walker-Warburg syndrome (WWS) is the most severe of a group of congenital disorders that have in common defects in the O-glycosylation of alpha-dystroglycan. WWS is characterized by congenital muscular dystrophy coupled with severe ocular and brain malformations. Moreover, in at least one-fifth of the reported cases, mutations in the POMT1 gene are responsible for this disease. During embryonic development (E8.5 to E11.5), the mouse Pomt1 gene is expressed in the tissues most severely affected in WWS, the muscle, eye, and brain. In this study, we show that mPomt1 expression is maintained in the muscle and eye in later developmental stages and, notably, that its expression is particularly strong in regions of brain and cerebellum that, when affected, could generate the defects observed in patients with WWS. We show that the Pomt1 protein is localized to the sarcoplasmic reticulum of muscle tissue cells in adult mice, where alpha-dystroglycan is O-glycosylated. Furthermore, the Pomt1 protein is localized to the acrosome of maturing spermatids, where alpha-dystroglycan is not glycosylated, so that Pomt1 might have a different target for O-mannosylation in the testes. This expression pattern in the testes could also be related to the gonadal anomalies observed in some patients with WWS.

A Putative Src Homology 3 Domain Binding Motif but Not the C-terminal Dystrophin WW Domain Binding Motif Is Required for Dystroglycan Function in Cellular Polarity in Drosophila

The conserved dystroglycan-dystrophin (Dg.Dys) complex connects the extracellular matrix to the cytoskeleton. In humans as well as Drosophila, perturbation of this complex results in muscular dystrophies and brain malformations and in some cases cellular polarity defects. However, the regulation of the Dg.Dys complex is poorly understood in any cell type. We now find that in loss-of-function and overexpression studies more than half (34 residues) of the Dg proline-rich conserved C-terminal regions can be truncated without significantly compromising its function in regulating cellular polarity in Drosophila. Notably, the truncation eliminates the WW domain binding motif at the very C terminus of the protein thought to mediate interactions with dystrophin, suggesting that a second, internal WW binding motif can also mediate this interaction. We confirm this hypothesis by using a sensitive fluorescence polarization assay to show that both WW domain binding sites of Dg bind to Dys in humans (K(d) = 7.6 and 81 microM, respectively) and Drosophila (K(d) = 16 and 46 microM, respectively). In contrast to the large deletion mentioned above, a single proline to an alanine point mutation within a predicted Src homology 3 domain (SH3) binding site abolishes Dg function in cellular polarity. This suggests that an SH3-containing protein, which has yet to be identified, functionally interacts with Dg.

Defective peripheral nerve myelination and neuromuscular junction formation in fukutin-deficient chimeric mice

Dystroglycan is a central component of the dystrophin-glycoprotein complex that links the extracellular matrix with cytoskeleton. Recently, mutations of the genes encoding putative glycosyltransferases were identified in several forms of congenital muscular dystrophies accompanied by brain anomalies and eye abnormalities, and aberrant glycosylation of alpha-dystroglycan has been implicated in their pathogeneses. These diseases are now collectively called alpha-dystroglycanopathy. In this study, we demonstrate that peripheral nerve myelination is defective in the fukutin-deficient chimeric mice, a mouse model of Fukuyama-type congenital muscular dystrophy, which is the most common alpha-dystroglycanopathy in Japan. In the peripheral nerve of these mice, the density of myelinated nerve fibers was significantly decreased and clusters of abnormally large non-myelinated axons were ensheathed by a single Schwann cell, indicating a defect of the radial sorting mechanism. The sugar chain moiety and laminin-binding activity of alpha-dystroglycan were severely reduced, while the expression of beta1-integrin was not altered in the peripheral nerve of the chimeric mice. We also show that the clustering of acetylcholine receptor is defective and neuromuscular junctions are fragmented in appearance in these mice. Expression of agrin and laminin as well as the binding activity of alpha-dystroglycan to these ligands was severely reduced at the neuromuscular junction. These results demonstrate that fukutin plays crucial roles in the myelination of peripheral nerve and formation of neuromuscular junction. They also suggest that defective glycosylation of alpha-dystroglycan may play a role in the impairment of these processes in the deficiency of fukutin.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Identification of a novel recessiveRELN mutation using a homozygous balanced reciprocal translocation

Two siblings from a consanguineous Egyptian marriage showed an identical phenotype of cortical lissencephaly with cerebellar hypoplasia, severe epilepsy, and mental retardation. Examination of karyotype revealed 46, t(7;12)(q22;p13)mat (7;12)(q22;p13)pat in both affected children, suggesting a homozygous reciprocal balanced translocation. Each healthy parent was a carrier of the balanced translocation in the heterozygous state, suggesting homozygous disruption of a gene involved in brain development. There were early spontaneous abortions in this family, as would be expected from transmission of an unbalanced chromosome. A disruption of RELN at 7q22.1 with absence of encoded protein was identified. This is the first demonstration that such rare homozygous translocations can be used to identify recessive disease gene mutations.

Cerebral developmental disorders

PURPOSE OF REVIEW

The development of the cerebral cortex progresses through defined stages including neural proliferation, neuroblast migration and neuronal differentiation. Disruptions in each of these developmental stages can lead to characteristic cerebral cortical malformations. This review provides an overview of the known genetic causes of human cerebral developmental disorders and discusses the potential molecular mechanisms that contribute to these malformations.

RECENT FINDINGS

Mutations in genes that are involved in neural proliferation give rise to microcephaly (small brain). Mutations in genes that direct the onset of neuroblast migration give rise to periventricular heterotopia (clusters of neurons along the ventricles of the brain). Mutations in genes that are required for neuroblast migration cause type I lissencephaly (smooth brain) and subcortical band heterotopia (smooth brain with a band of neurons beneath the cortex). Mutations in genes that direct migratory neurons to arrest in the cortex lead to type II lissencephaly (smooth brain with clusters of neurons along the surface of the brain).

SUMMARY

The identification of causative genes involved in the formation of the cerebral cortex now allows for a rational approach with which to interpret the underlying mechanistic basis for many of these disorders.

Fukutin gene mutations in steroid-responsive limb girdle muscular dystrophy

OBJECTIVE

Defects in glycosylation of alpha-dystroglycan are associated with several forms of muscular dystrophy, often characterized by congenital onset and severe structural brain involvement, collectively known as dystroglycanopathies. Six causative genes have been identified in these disorders including fukutin. Mutations in fukutin cause Fukuyama congenital muscular dystrophy. This is the second most common form of muscular dystrophy in Japan and is invariably associated with mental retardation and structural brain defects. The aim of this study was to determine the genetic defect in two white families with a dystroglycanopathy.

METHODS

The six genes responsible for dystroglycanopathies were studied in three children with a severe reduction of alpha-dystroglycan in skeletal muscle.

RESULTS

We identified pathogenic fukutin mutations in these two families. Affected children had normal intelligence and brain structure and shared a limb girdle muscular dystrophy (LGMD) phenotype, had marked elevation of serum creatine kinase, and were all ambulant with remarkable steroid responsiveness.

INTERPRETATION

Our data suggest that fukutin mutations occur outside Japan and can be associated with much milder phenotypes than Fukuyama congenital muscular dystrophy. These findings significantly expand the spectrum of phenotypes associated with fukutin mutations to include this novel form of limb girdle muscular dystrophy that we propose to name LGMD2L.

Protein Glycosylation, Conserved from Yeast to Man: A Model Organism Helps Elucidate Congenital Human Diseases

Proteins can be modified by a large variety of covalently linked saccharides. The present review concentrates on two types, protein N-glycosylation and protein O-mannosylation, which, with only a few exceptions, are evolutionary conserved from yeast to man. They are also distinguished by some special features: The corresponding glycosylation processes start in the endoplasmatic reticulum, are continued in the Golgi apparatus, and require dolichol-activated precursors for the initial biosynthetic steps. With respect to the molecular biology of both types of protein glycosylation, the pathways and the genetic background of the reactions have most successfully been studied with the genetically easy-to-handle baker's yeast, Saccharomyces cerevisae. Many of the severe developmental disturbances in children are related to protein glycosylation, for example, the CDG syndrome (congenital disorders of glycosylation) as well as congenital muscular dystrophies with neuronal-cell-migration defects have been elucidated with the help of yeast.

Loss of glial fibrillary acidic protein (GFAP) impairs Schwann cell proliferation and delays nerve regeneration after damage

Axonal loss causes disabling and permanent deficits in many peripheral neuropathies, and may result from inefficient nerve regeneration due to a defective relationship between Schwann cells, axons and the extracellular matrix. These interactions are mediated by surface receptors and transduced by cytoskeletal molecules. We investigated whether peripheral nerve regeneration is perturbed in mice that lack glial fibrillary acidic protein (GFAP), a Schwann-cell-specific cytoskeleton constituent upregulated after damage. Peripheral nerves develop and function normally in GFAP-null mice. However, axonal regeneration after damage was delayed. Mutant Schwann cells maintained the ability to dedifferentiate but showed defective proliferation, a key event for successful nerve regeneration. We also showed that GFAP and the other Schwann-cell-intermediate filament vimentin physically interact in two distinct signaling pathways involved in proliferation and nerve regeneration. GFAP binds integrin αvβ8, which initiates mitotic signals soon after damage by interacting with fibrin. Consistently, ERK phosphorylation was reduced in crushed GFAP-null nerves. Vimentin instead binds integrin α5β1, which regulates proliferation and differentiation later in regeneration, and may compensate for the absence of GFAP in mutant mice. GFAP might contribute to form macro-complexes to initiate mitogenic and differentiating signaling for efficient nerve regeneration.

Genetic defects in the human glycome

The spectrum of all glycan structures--the glycome--is immense. In humans, its size is orders of magnitude greater than the number of proteins that are encoded by the genome, one percent of which encodes proteins that make, modify, localize or bind sugar chains, which are known as glycans. In the past decade, over 30 genetic diseases have been identified that alter glycan synthesis and structure, and ultimately the function of nearly all organ systems. Many of the causal mutations affect key biosynthetic enzymes, but more recent discoveries point to defects in chaperones and Golgi-trafficking complexes that impair several glycosylation pathways. As more glycosylation disorders and patients with these disorders are identified, the functions of the glycome are starting to be revealed.

Brain alpha-dystroglycan displays unique glycoepitopes and preferential binding to laminin-10/11

alpha-Dystroglycan was quantitatively enriched from mammalian brain based on its uniform reactivity with Vicia villosa agglutinin and resolved into sub-populations possessing or lacking the sulfated glucuronic acid epitope recognized by monoclonal antibody HNK-1. We generated a new monoclonal antibody specific for a glycoepitope on brain alpha-dystroglycan but absent from alpha-dystroglycan expressed in all other tissues examined. Finally, we found that laminin-10/11 preferentially bound to brain alpha-dystroglycan compared to skeletal muscle alpha-dystroglycan. Our results suggest that tissue-specific glycosylation modifies the laminin binding specificity of alpha-dystroglycan.

Cerebellar synaptic defects and abnormal motor behavior in mice lacking alpha- and beta-dystrobrevin

The dystrobrevins (alphaDB and betaDB) bind directly to dystrophin and are components of a transmembrane dystrophin-glycoprotein complex (DGC) that links the cytoskeleton to extracellular proteins in many tissues. We show here that alphaDB, betaDB, and dystrophin are all concentrated at a discrete subset of inhibitory synapses on the somata and dendrites of cerebellar Purkinje cells. Dystrophin is depleted from these synapses in mice lacking both alphaDB and betaDB, and DBs are depleted from these synapses in mice lacking dystrophin. In dystrophin mutants and alphaDB,betaDB double mutants, the size and number of GABA receptor clusters are decreased at cerebellar inhibitory synapses, and sensorimotor behaviors that reflect cerebellar function are perturbed. Synaptic and behavioral abnormalities are minimal in mice lacking either alphaDB or betaDB. Together, our results show that the DGC is required for proper maturation and function of a subset of inhibitory synapses, that DB is a key component of this DGC, and that interference with this DGC leads to behavioral abnormalities. We suggest that motor deficits in muscular dystrophy patients, which are their cardinal symptoms, may reflect not only peripheral derangements but also CNS defects.

Distal myopathies

PURPOSE OF REVIEW

The distal myopathies are a heterogeneous group of disorders that pose a challenge to both the clinician and geneticist. This article summarizes the findings of recent clinical, genetic and molecular studies and the current diagnostic approach to this group of patients.

RECENT FINDINGS

Publications over the past 5 years describe a number of new clinical phenotypes and genetic loci and further emphasize the overlap in clinical phenotype between a number of these disorders and between the distal and limb girdle myopathies and hereditary inclusion body myopathies. Recent studies have led to the identification of the genes and mutations responsible for early onset (Laing) myopathy and tibial (Udd) myopathy, and for distal myopathy with rimmed vacuoles (Nonaka), which has been shown to be allelic with quadriceps sparing hereditary inclusion body myopathy (IBM2), and have elucidated the underlying pathogenetic mechanisms in these conditions. New diagnostic approaches using magnetic resonance imaging, and a blood-based assay for dysferlin deficiency, have also been reported.

SUMMARY

These findings have important implications for future genetic linkage and gene expression studies and for the diagnostic approach to patients with a distal myopathy phenotype. They also hold promise for the eventual development of therapies for this group of disorders.

Carriers and patients with muscle–eye–brain disease can be rapidly diagnosed by enzymatic analysis of fibroblasts and lymphoblasts

We report a new fibroblast and lymphoblast based protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzymatic assay, which allows rapid and accurate diagnosis of carriers and patients with muscle-eye-brain type of congenital muscular dystrophy. Seven patients with genetically confirmed muscle-eye-brain disease were assayed for protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 enzyme activity. In three patients and their heterozygous parents, the assays were done on EBV-transformed lymphoblasts, in another three patients they were done on cultured fibroblasts and in the last patient on both fibroblasts and lymphoblasts. Cultured fibroblasts and lymphoblasts from the muscle-eye-brain patients showed a highly significant decrease in protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity relative to controls. The residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in fibroblasts (average 0.11 nmoles/h per mg) was about 13% of normal controls. The ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity to the activity of a glycosyltransferase control (N-acetylglucosaminyltransferase 1; GnT1) in fibroblasts was on average 0.006 in muscle-eye-brain patients and 0.045 in controls. The average residual protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 level in lymphoblasts was 15% of normal controls. The average ratio of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1/GnT1 activity was 0.007 in muscle-eye-brain patients, 0.026 in heterozygous carriers and 0.046 in normal controls. Assay of protein O-mannosyl beta-1,2-N-acetylglucosaminyltransferase 1 activity in fibroblasts and lymphoblasts from muscle-eye-brain carriers and patients provides a rapid and relatively simple diagnostic test for this disease and could be used as a screening test in carriers and patients with complex congenital muscular dystrophy.

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.

Therapy insight: cardiovascular complications associated with muscular dystrophies

The muscular dystrophies are commonly associated with cardiovascular complications, including cardiomyopathy and cardiac arrhythmias. These complications are caused by intrinsic defects in cardiomyocyte and cardiac conduction system function, and by the presence of severe skeletal muscle disease, which also contributes to cardiac dysfunction. Unlike the skeletal muscle degenerative process, for which treatment options are currently limited, therapy is available for the cardiovascular complications that accompany muscular dystrophy. New therapies for skeletal muscle degeneration are moving into clinical trials and, ultimately, into clinical practice. These therapies are expected to also improve the cardiac function, longevity and wellbeing of muscular dystrophy patients.

Developmental regulation of biglycan expression in muscle and tendon

Biglycan is an extracellular ligand for the dystrophin-associated protein complex (DAPC) that is upregulated in both dystrophic and regenerating muscle. Biglycan also binds to collagen VI, mutations of which cause a congenital muscular dystrophy (Ullrich's; UCMD) that is also characterized by connective tissue abnormalities. The expression of biglycan in early development and postnatal ages has not been well characterized. Here we show that biglycan transcript levels peak at approximately 21 weeks' gestation in human fetal muscle. Immunocytochemical analysis of developing mouse muscle shows that biglycan can be detected in muscle as early as embryonic day (E)16 and is most abundant between postnatal day (P)1 and P7. Biglycan is also highly expressed in developing tendon, with maximal levels observed at E16-18. This robust tendon expression is correlated with a sharp peak in biglycan transcript levels in the hindlimb. Finally, at E18 collagen VI colocalizes with biglycan in tendon. These results suggest that biglycan has a particularly important function during muscle and connective tissue development. Moreover, biglycan may play a role in the pathogenesis of collagen VI-associated congenital muscular dystrophies.

Sub-cellular localisation of fukutin related protein in different cell lines and in the muscle of patients with MDC1C and LGMD2I

MDC1C and LGMD2I are two allelic forms of muscular dystrophies caused by mutations in the gene encoding for fukutin related protein (FKRP). FKRP encodes for a putative glycosyltransferase, the precise function of which is unknown. However, the marked reduction of alpha-dystroglycan glycosylation in the muscle of MDC1C and LGMD2I patients suggests a role for FKRP in dystroglycan processing. Using a polyclonal antibody raised against FKRP we now show that endogenous FKRP locates to the Golgi apparatus of neuronal, oligodendroglial, and the cardiac muscle cell line H9c2. In differentiated C2C12 myotubes and in transverse sections of normal skeletal and cardiac muscle, endogenous FKRP surrounded the myonuclei. This localisation was unaffected in the skeletal muscle of patients with MDC1C and LGMD2I carrying various FKRP mutations. These observations imply a specific role for FKRP during striated muscle, neuronal and glial development and suggest that protein mis-localisation is not a common mechanism of disease in FKRP-related dystrophies.

Alpha-dystroglycan, the usual suspect?

An increasing number of congenital muscular dystrophies might originate from genetic abnormalities of glycosyltransferases genes which are believed to target the alpha subunit of the dystroglycan (DG) adhesion complex as their major enzymatic substrate. alpha-DG is highly glycosylated and peripherally associated with the sarcolemma of skeletal muscle and the plasma membrane in a wide variety of cells. Several lines of evidence indicate that alpha-DG hypoglycosylation might represent the primary molecular event characterizing congenital dystrophies, since it is likely to alter alpha-DG high-affinity binding to laminin and other extracellular molecules, thus negatively influencing the basement-membrane/cytoskeleton axis and eventually leading to sarcolemmal instability, infiltration of myofibers and congenital weakness. For this reason, congenital diseases such as Walker-Warburg Syndrome or Muscle-Eye-Brain disease, have been recently denominated 'secondary dystroglycanopathies'. However, some crucial points need to be fully addressed in order to finally assess the degree of involvement of alpha-DG in congenital muscular diseases, for example: the possibility that mutations hitting the DG gene might lead to primary dystroglycanopathies; the putative functional or pathological role of hypoglycosylated - or even hyperglycosylated - alpha-DG molecules; or also the compensatory role played by the recently identified paralogue glycosyltransferases in alpha-DG sugar decoration.

Enhanced laminin binding by alpha-dystroglycan after enzymatic deglycosylation

Carbohydrate modifications are clearly important to the function of alpha-dystroglycan but their composition and structure remain poorly understood. In the present study, we describe experiments aimed at identifying the alpha-dystroglycan oligosaccharides important for its binding to laminin-1 and carbohydrate-dependent mAbs (monoclonal antibodies) IIH6 and VIA4(1). We digested highly purified skeletal muscle alpha-dystroglycan with an array of linkage-specific endo- and exoglycosidases, which were verified for action on alpha-dystroglycan by loss/gain of reactivity for lectins with defined glyco-epitopes. Notably, digestion with a combination of Arthrobacter ureafaciens sialidase, beta(1-4)galactosidase and beta-N-acetylglucosaminidase substantially degraded SiaAalpha2-3Galbeta1-4GlcNAcbeta1-2Man glycans on highly purified alpha-dystroglycan that nonetheless exhibited enhanced IIH6, VIA4(1) and laminin-1 binding activity. Additional results indicate that alpha-dystroglycan is probably modified with other anionic sugars besides sialic acid and suggest that rare alpha-linked GlcNAc moieties may block its complete deglycosylation with currently available enzymes.

Proteolysis of beta-dystroglycan in muscular diseases

Alpha-dystroglycan is a cell surface peripheral membrane protein which binds to the extracellular matrix (ECM), while beta-dystroglycan is a type I integral membrane protein which anchors alpha-dystroglycan to the cell membrane via the N-terminal extracellular domain. The complex composed of alpha-and beta-dystroglycan is called the dystroglycan complex. We reported previously a matrix metalloproteinase (MMP) activity that disrupts the dystroglycan complex by cleaving the extracellular domain of beta-dystroglycan. This MMP creates a characteristic 30 kDa fragment of beta-dystroglycan that is detected by the monoclonal antibody 43DAG/8D5 directed against the C-terminus of beta-dystroglycan. We also reported that the 30 kDa fragment of beta-dystroglycan was increased in the skeletal and cardiac muscles of cardiomyopathic hamsters, the model animals of sarcoglycanopathy, and that this resulted in the disruption of the link between the ECM and cell membrane via the dystroglycan complex. In this study, we investigated the proteolysis of beta-dystroglycan in the biopsied skeletal muscles of various human muscular diseases, including sarcoglycanopathy, Duchenne muscular dystrophy (DMD), Becker muscular dystrophy, Fukuyama congenital muscular dystrophy, Miyoshi myopathy, LGMD2A, facioscapulohumeral muscular dystrophy, myotonic dystrophy and dermatomyositis/polymyositis. We show that the 30 kDa fragment of beta-dystroglycan is increased significantly in sarcoglycanopathy and DMD, but not in the other diseases. We propose that the proteolysis of beta-dystroglycan may contribute to skeletal muscle degeneration by disrupting the link between the ECM and cell membrane in sarcoglycanopathy and DMD.

Glyc-O-genetics of Walker-Warburg syndrome

Walker-Warburg syndrome (WWS) is the most severe of a group of multiple congenital anomaly disorders known as the cobblestone lissencephalies. These are characterized by congenital muscular dystrophy in conjunction with severe brain malformation and ocular abnormalities. In the last 3 years, important progress has been made towards the elucidation of the genetic causes of these disorders. Mutations in three genes, POMT1, fukutin and FKRP, have been described for WWS, which together account for approximately 20% of patients with Walker-Warburg. It has become evident that some of the underlying genes may cause a broad spectrum of phenotypes, ranging from limb girdle muscular dystrophy type 2I to WWS. In some cases, a genotype-phenotype correlation can be recognized. In line with the known or proposed functions of the resolved genes, all patients with cobblestone lissencephaly show defects in the O-linked glycosylation of the glycoprotein alpha-dystroglycan. Perhaps, the missing genes underlying the remainder of the unexplained WWS patients have also to be sought in the pathways involved in O-linked protein glycosylation.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

POMT2 mutations cause alpha-dystroglycan hypoglycosylation and Walker-Warburg syndrome

BACKGROUND

Walker-Warburg syndrome (WWS) is an autosomal recessive condition characterised by congenital muscular dystrophy, structural brain defects, and eye malformations. Typical brain abnormalities are hydrocephalus, lissencephaly, agenesis of the corpus callosum, fusion of the hemispheres, cerebellar hypoplasia, and neuronal overmigration, which causes a cobblestone cortex. Ocular abnormalities include cataract, microphthalmia, buphthalmos, and Peters anomaly. WWS patients show defective O-glycosylation of alpha-dystroglycan (alpha-DG), which plays a key role in bridging the cytoskeleton of muscle and CNS cells with extracellular matrix proteins, important for muscle integrity and neuronal migration. In 20% of the WWS patients, hypoglycosylation results from mutations in either the protein O-mannosyltransferase 1 (POMT1), fukutin, or fukutin related protein (FKRP) genes. The other genes for this highly heterogeneous disorder remain to be identified.

OBJECTIVE

To look for mutations in POMT2 as a cause of WWS, as both POMT1 and POMT2 are required to achieve protein O-mannosyltransferase activity.

METHODS

A candidate gene approach combined with homozygosity mapping.

RESULTS

Homozygosity was found for the POMT2 locus at 14q24.3 in four of 11 consanguineous WWS families. Homozygous POMT2 mutations were present in two of these families as well as in one patient from another cohort of six WWS families. Immunohistochemistry in muscle showed severely reduced levels of glycosylated alpha-DG, which is consistent with the postulated role for POMT2 in the O-mannosylation pathway.

CONCLUSIONS

A fourth causative gene for WWS was uncovered. These genes account for approximately one third of the WWS cases. Several more genes are anticipated, which are likely to play a role in glycosylation of alpha-DG.

Neuronal migration in cortical development

Cortical formation in the developing brain is a highly complicated process involving neuronal production (through symmetric or asymmetric cell division) interaction of radial glia with neuronal migration, and multiple modes of neuronal migration. It has been convincingly demonstrated by numerous studies that radial glial cells are neural stem cells. However, the processes by which neurons arise from radial glia and migrate to their final destinations in vivo are not yet fully understood. Recent studies using time-lapse imaging of neuronal migration are giving investigators an increasingly more detailed understanding of the mitotic behavior of radial glia and the migrating behavior of their daughter cells. In this review, we describe recent progress in elucidating neuronal migration in brain formation and how neuronal migration is disturbed by mutations in genes that control this process.

Mouse large can modify complex N- and mucin O-glycans on alpha-dystroglycan to induce laminin binding

The human LARGE gene encodes a protein with two putative glycosyltransferase domains and is required for the generation of functional alpha-dystroglycan (alpha-DG). Monoclonal antibodies IIH6 and VIA4-1 recognize the functional glycan epitopes of alpha-DG that are necessary for binding to laminin and other ligands. Overexpression of full-length mouse Large generated functionally glycosylated alpha-DG in Pro(-5) Chinese hamster ovary (CHO) cells, and the amount was increased by co-expression of protein:O-mannosyl N-acetylglucosaminyltransferase 1. However, functional alpha-DG represented only a small fraction of the alpha-DG synthesized by CHO cells or expressed from an alpha-DG construct. To identify features of the glycan epitopes induced by Large, the production of functionally glycosylated alpha-DG was investigated in several CHO glycosylation mutants. Mutants with defective transfer of sialic acid (Lec2), galactose (Lec8), or fucose (Lec13) to glycoconjugates, and the Lec15 mutant that cannot synthesize O-mannose glycans, all produced functionally glycosylated alpha-DG upon overexpression of Large. Laminin binding and the alpha-DG glycan epitopes were enhanced in Lec2 and Lec8 cells. In Lec15 cells, functional alpha-DG was increased by co-expression of core 2 N-acetylglucosaminyltransferase 1 with Large. Treatment with N-glycanase markedly reduced functionally glycosylated alpha-DG in Lec2 and Lec8 cells. The combined data provide evidence that Large does not transfer to Gal, Fuc, or sialic acid on alpha-DG nor induce the transfer of these sugars to alpha-DG. In addition, the data suggest that human LARGE may restore functional alpha-DG to muscle cells from patients with defective synthesis of O-mannose glycans via the modification of N-glycans and/or mucin O-glycans on alpha-DG.

Immunodetection of partially glycosylated isoforms of alpha-dystroglycan by a new monoclonal antibody against its beta-dystroglycan-binding epitope

The alpha/beta dystroglycan (DG) complex links the extracellular matrix to the actin cytoskeleton. The extensive glycosylation of alpha-DG is believed to be crucial for the interaction with its extracellular matrix-binding partners. We characterized a monoclonal antibody, directed against the beta-DG-binding epitope ( approximately positions 550-565), which recognizes preferentially hypoglycosylated alpha-DG. In Western blot, the antibody was able to detect a number of partially glycosylated alpha-DG isoforms from rat brain and chicken skeletal muscle tissue samples. In addition, we demonstrated its inhibitory effect on the interaction between alpha- and beta-DG in vitro and preliminary immunostaining experiments suggest that such hypoglycosylated alpha-DG isoforms could play a role within cells.

α-Dystroglycan does not play a major pathogenic role in autosomal recessive hereditary inclusion-body myopathy

Mutations of the GNE gene are responsible for autosomal recessive hereditary inclusion-body myopathy (HIBM). In this study we searched for the presence of any significant abnormality of alpha-dystroglycan (alpha-DG), a highly glycosylated component of the dystrophin-glycoprotein complex, in 5 HIBM patients which were previously clinically and genetically characterized. Immunocytochemical and immunoblot analysis showed that alpha-DG extracted from muscle biopsies was normally expressed and displayed its typical molecular mass. Immunoblot analysis on the wheat germ lectin-enriched glycoprotein fraction of muscles and primary myotubes showed a reduced amount of alpha-DG in 4 out of 5 HIBM patients, compared to normal and other diseased muscles. However, such altered lectin-binding behaviour, possibly reflecting a partial hyposialylation of alpha-DG, did not affect the laminin binding properties of alpha-DG. Therefore, the subtle changes within the alpha-DG glycosylation pattern, detected in HIBM muscles, likely do not play a key pathogenic role in this disorder.

Dystroglycan: important player in skeletal muscle and beyond

Dystroglycan is a transmembrane protein that connects the extracellular matrix to the cytoskeleton. Given the ubiquitous tissue expression of dystroglycan, different functional roles in various organ systems have been characterized during the past decade. More recently, aberrant glycosylation of dystroglycan has been identified as a novel pathogenetic mechanism in several forms of congenital and late onset muscular dystrophy syndromes. The current review summarizes the recent scientific achievements as they relate to the function of dystroglycan under normal and pathophysiological conditions.

Localization and functional analysis of the LARGE family of glycosyltransferases: significance for muscular dystrophy

The dystroglycanopathies are a novel group of human muscular dystrophies due to mutations in known or putative glycosyltransferase enzymes. They share the common pathological feature of a hypoglycosylated form of alpha-dystroglycan, diminishing its ability to bind extracellular matrix ligands. The LARGE glycosyltransferase is mutated in both the myodystrophy mouse and congenital muscular dystrophy type 1D (MDC1D). We have transfected various cell lines with a variety of LARGE expression constructs in order to characterize their subcellular localization and effect on alpha-dystroglycan glycosylation. Wild-type LARGE co-localized with the Golgi marker GM130 and stimulated the production of highly glycosylated alpha-dystroglycan (hyperglycosylation). MDC1D mutants had no effect on alpha-dystroglycan glycosylation and failed to localize correctly, confirming their pathogenicity. The two predicted catalytic domains of LARGE contain three conserved DxD motifs. Systematically mutating each of these motifs to NNN resulted in the mislocalization of one construct, while all failed to have any effect on alpha-dystroglycan glycosylation. A construct lacking the transmembrane domain also failed to localize at the Golgi apparatus. These results indicate that LARGE needs to both physically interact with alpha-dystroglycan and function as a glycosyltransferase in order to stimulate alpha-dystroglycan hyperglycosylation. We have also cloned and overexpressed a homologue of LARGE, glycosyltransferase-like 1B (GYLTL1B). Like LARGE it localized to the Golgi apparatus and stimulated alpha-dystroglycan hyperglycosylation. These results suggest that GYLTL1B may be a candidate gene for muscular dystrophy and that its overexpression could compensate for the deficiency of both LARGE and other glycosyltransferases.

Heparan sulfates in skeletal muscle development and physiology

Recent years have seen an emerging interest in the composition of the skeletal muscle extracellular matrix (ECM) and in the developmental and physiological roles of its constituents. Many cell surface-associated and ECM-embedded molecules occur in highly organized spatiotemporal patterns, suggesting important roles in the development and functioning of skeletal muscle. Glycans are historically underrepresented in the study of skeletal muscle ECM, even though studies from up to 30 years ago have demonstrated specific carbohydrates and glycoproteins to be concentrated in neuromuscular junctions (NMJs). Changes in glycan profile and distribution during myogenesis and synaptogenesis hint at an active involvement of glycoconjugates in muscle development. A modest amount of literature involves glycoconjugates in muscle ion housekeeping, but a recent surge of evidence indicates that glycosylation defects are causal for many congenital (neuro)muscular disorders, rendering glycosylation essential for skeletal muscle integrity. In this review, we focus on a single class of ECM-resident glycans and their emerging roles in muscle development, physiology, and pathology: heparan sulfate proteoglycans (HSPGs), notably their heparan sulfate (HS) moiety.

Cardiac involvement in muscular dystrophies: Molecular mechanisms

In this review, we draw attention to the multiple mechanisms responsible for the pathogenesis of cardiomyopathies in patients with muscular dystrophies. More than one single mechanism is likely to be involved in the development of skeletal and cardiac muscle pathology even when there is a single protein defect. The best example is dystrophin deficiency, in which increased sarcolemmal permeability following eccentric exercise, reduced force generation, and abnormal signaling are all likely to contribute to the progressive muscle damage observed. In other conditions, such as the sarcoglycanopathies, a protein deficiency both in the striated cardiomyocte and the vascular smooth muscle appears to play a significant role. An entirely different mechanism of disease is likely in defects of nuclear envelope proteins, although the precise pathogenesis of this group of conditions is still not clear. Differences between the organization of skeletal and cardiac muscle protein complex are also only starting to emerge and will very likely be the focus of future research.

The dystroglycan complex: From biology to cancer

Dystroglycan (DG), a non-integrin adhesion molecule, is a pivotal component of the dystrophin-glycoprotein complex, that is expressed in skeletal muscle and in a wide variety of tissues at the interface between the basement membrane (BM) and the cell membrane. DG has been mainly studied for its role in skeletal muscle cell stability and its alterations in muscular diseases, such as dystrophies. However, accumulating evidence have implicated DG in a variety of other biological functions, such as maturation of post-synaptic elements in the central and peripheral nervous system, early morphogenesis, and infective pathogens targeting. Moreover, DG has been reported to play a role in regulating cytoskeletal organization, cell polarization, and cell growth in epithelial cells. Recent studies also indicate that abnormalities in the expression of DG frequently occur in human cancers and may play a role in both the process of tumor progression and in the maintenance of the malignant phenotype. This paper reviews the available information on the biology of DG, the abnormalities found in human cancers, and the implications of these findings with respect to our understanding of cancer pathogenesis and to the development of novel strategies for a better management of cancer patients.

The congenital muscular dystrophies in 2004: a century of exciting progress

The congenital muscular dystrophies are a heterogeneous group of inherited disorders. The clinical features range from severe and often early fatal disorders to relatively mild conditions compatible with survival into adult life. The recent advances in the genetic basis of congenital muscular dystrophies have allowed to significantly improve our understanding of their pathogenesis and clinical diversity. These advances have also allowed to classify these forms according to a combination of clinical features and primary biochemical defects. In this review we present how the congenital muscular dystrophies field has evolved over the last decade from a clinical and genetic point of view.

Fukutin-related protein mutations that cause congenital muscular dystrophy result in ER-retention of the mutant protein in cultured cells

Mutations in the gene encoding fukutin-related protein (FKRP) cause a spectrum of diseases including congenital muscular dystrophy type 1C (MDC1C), limb girdle muscular dystrophy 2I (LGMD2I) and congenital muscular dystrophies (CMDs) with brain malformations and mental retardation. Although these diseases are associated with abnormal dystroglycan processing, the cellular consequences of the idiosyncratic FKRP mutations have not been determined. Here we show, in cultured cells, that FKRP mutants associated with the more severe disease phenotypes (S221R, A455D, P448L) are retained in the endoplasmic reticulum (ER), whereas the wild-type protein and the mutant L276I that causes LGMD2I are found predominantly in the Golgi apparatus. The ER-retained proteins have a shorter half-life than the wild-type FKRP and are preferentially degraded by the proteasome. Furthermore, calnexin binds preferentially to the ER-retained mutants suggesting that it may participate in the quality control pathway for FKRP. These data provide the first evidence that the ER-retention of mutant FKRP may play a role in the pathogenesis of CMD and potentially explain why the allelic disorder LGMD2I is milder, because the mutated protein is able to reach the Golgi apparatus.

Proteolytic enzymes and altered glycosylation modulate dystroglycan function in carcinoma cells

Alterations in the basement membrane receptor dystroglycan (DG) are evident in muscular dystrophies and carcinoma cells and characterized by a selective loss or modification of the extracellular alpha-DG subunit. Defects in posttranslational modifications of DG have been identified in some muscular dystrophies, but the underlying modifications in carcinoma cells have not yet been defined. We reveal here multiple posttranslational modifications that modulate the composition and function of DG in normal epithelial cells and carcinoma cells. We show that alpha-DG is shed from the cell surface of normal and tumorigenic epithelial cells through a proteolytic mechanism that does not require direct cleavage of either alpha- or beta-DG. Shedding is dependent on metalloprotease activity and the proprotein convertase furin. Surprisingly, furin is also found to directly process alpha-DG as a proprotein substrate, changing the existing model of DG composition. We also show that the glycosylation of alpha-DG is altered in invasive carcinoma cells, and this modification causes complete loss of laminin binding properties. Together, these data elucidate several novel events regulating the functional composition of DG and reveal defects that arise during cancer progression, providing direction for efforts to restore this link with the basement membrane in carcinoma cells.

Glycosylation defects in muscular dystrophies

PURPOSE OF REVIEW

Congenital disorders of glycosylation are caused by defects in the synthesis of the glycan moiety of glycoproteins or other glycoconjugates. There has been a great explosion in the number of neuromuscular diseases caused by mutations in genes that affect carbohydrate metabolism or protein glycosylation. A common defect in these disorders is the defective processing of alpha-dystroglycan.

RECENT FINDINGS

Recent advances demonstrating mutations in glycosyltransferases and dysfunction of the alpha-beta dystroglycan axis causing different forms of muscular dystrophy, especially with brain involvement, shows clearly that muscle integrity is dependent on glycosylation. We first review the newly identified muscular dystrophies, with a focus on the hypoglycosylation of alpha-dystroglycan, from a clinical, biochemical and genetic standpoint, and second hereditary inclusion body myopathies caused by mutations in the gene that encodes an enzyme responsible for the protein's posttranslational modification that cause sialidation defects. It is shown very recently that molecular recognition of dystroglycan by LARGE is a key determinant in the biosynthetic pathway to produce mature and functional dystroglycan. Gene transfer of LARGE into the cells of individuals with congenital muscular dystrophies restores alpha-dystroglycan function.

SUMMARY

The clinical spectrum of congenital disorders of glycosylation is becoming increasingly broad. A demonstration of mutations in glycosyltransferases will further help to design diagnostic tools and therapeutic approaches. Recent findings which show that molecular recognition by LARGE is essential for expression of functional dystroglycan and LARGE can functionally bypass alpha-dystroglycan glycosylation defects in distinct congenital muscular dystrophies, indicate a new therapeutic strategy.

Cortical dysplasia resembling human type 2 lissencephaly in mice lacking all three APP family members

The Alzheimer's disease beta-amyloid precursor protein (APP) is a member of a larger gene family that includes the amyloid precursor-like proteins, termed APLP1 and APLP2. We previously documented that APLP2-/-APLP1-/- and APLP2-/-APP-/- mice die postnatally, while APLP1-/-APP-/- mice and single mutants were viable. We now report that mice lacking all three APP/APLP family members survive through embryonic development, and die shortly after birth. In contrast to double-mutant animals with perinatal lethality, 81% of triple mutants showed cranial abnormalities. In 68% of triple mutants, we observed cortical dysplasias characterized by focal ectopic neuroblasts that had migrated through the basal lamina and pial membrane, a phenotype that resembles human type II lissencephaly. Moreover, at E18.5 triple mutants showed a partial loss of cortical Cajal Retzius (CR) cells, suggesting that APP/APLPs play a crucial role in the survival of CR cells and neuronal adhesion. Collectively, our data reveal an essential role for APP family members in normal brain development and early postnatal survival.

Targeted disruption of the Walker-Warburg syndrome gene Pomt1 in mouse results in embryonic lethality

O-mannosylation is an important protein modification in eukaryotes that is initiated by an evolutionarily conserved family of protein O-mannosyltransferases. The first mammalian protein O-mannosyltransferase gene described was the human POMT1. Mutations in the hPOMT1 gene are responsible for Walker-Warburg syndrome (WWS), a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. During embryogenesis, the murine Pomt1 gene is prominently expressed in the neural tube, the developing eye, and the mesenchyme. These sites of expression correlate with those in which the main tissue alterations are observed in WWS patients. We have inactivated a Pomt1 allele by gene targeting in embryonic stem cells and produced chimeras transmitting the defect allele to offspring. Although heterozygous mice were viable and fertile, the total absence of Pomt1(-/-) pups in the progeny of heterozygous intercrosses indicated that this genotype is embryonic lethal. An analysis of the mutant phenotype revealed that homozygous Pomt1(-/-) mice suffer developmental arrest around embryonic day (E) 7.5 and die between E7.5 and E9.5. The Pomt1(-/-) embryos present defects in the formation of Reichert's membrane, the first basement membrane to form in the embryo. The failure of this membrane to form appears to be the result of abnormal glycosylation and maturation of dystroglycan that may impair recruitment of laminin, a structural component required for the formation of Reichert's membrane in rodents. The targeted disruption of mPomt1 represents an example of an engineered deletion of a known glycosyltransferase involved in O-mannosyl glycan synthesis.

Molecular recognition by LARGE is essential for expression of functional dystroglycan

Reduced ligand binding activity of alpha-dystroglycan is associated with muscle and central nervous system pathogenesis in a growing number of muscular dystrophies. Posttranslational processing of alpha-dystroglycan is generally accepted to be critical for the expression of functional dystroglycan. Here we show that both the N-terminal domain and a portion of the mucin-like domain of alpha-dystroglycan are essential for high-affinity laminin-receptor function. Posttranslational modification of alpha-dystroglycan by glycosyltransferase, LARGE, occurs within the mucin-like domain, but the N-terminal domain interacts with LARGE, defining an intracellular enzyme-substrate recognition motif necessary to initiate functional glycosylation. Gene replacement in dystroglycan-deficient muscle demonstrates that the dystroglycan C-terminal domain is sufficient only for dystrophin-glycoprotein complex assembly, but to prevent muscle degeneration the expression of a functional dystroglycan through LARGE recognition and glycosylation is required. Therefore, molecular recognition of dystroglycan by LARGE is a key determinant in the biosynthetic pathway to produce mature and functional dystroglycan.