Differential heparin inhibition of skeletal muscle alpha-dystroglycan binding to laminins

NAD+ enhances ribitol and ribose rescue of α-dystroglycan functional glycosylation in human FKRP-mutant myotubes

Mutations in the fukutin-related protein (FKRP) cause Walker-Warburg syndrome (WWS), a severe form of congenital muscular dystrophy. Here, we established a WWS human induced pluripotent stem cell-derived myogenic model that recapitulates hallmarks of WWS pathology. We used this model to investigate the therapeutic effect of metabolites of the pentose phosphate pathway in human WWS. We show that functional recovery of WWS myotubes is promoted not only by ribitol but also by its precursor ribose. Moreover, we found that the combination of each of these metabolites with NAD+ results in a synergistic effect, as demonstrated by rescue of α-dystroglycan glycosylation and laminin binding capacity. Mechanistically, we found that FKRP residual enzymatic capacity, characteristic of many recessive FKRP mutations, is required for rescue as supported by functional and structural mutational analyses. These findings provide the rationale for testing ribose/ribitol in combination with NAD+ to treat WWS and other diseases associated with FKRP mutations.

Efficient engraftment of pluripotent stem cell-derived myogenic progenitors in a novel immunodeficient mouse model of limb girdle muscular dystrophy 2I

BACKGROUND

Defects in α-dystroglycan (DG) glycosylation characterize a group of muscular dystrophies known as dystroglycanopathies. One of the key effectors in the α-DG glycosylation pathway is the glycosyltransferase fukutin-related protein (FKRP). Mutations in FKRP lead to a large spectrum of muscular dystrophies, including limb girdle muscular dystrophy 2I (LGMD2I). It remains unknown whether stem cell transplantation can promote muscle regeneration and ameliorate the muscle wasting phenotype associated with FKRP mutations.

RESULTS

Here we transplanted murine and human pluripotent stem cell-derived myogenic progenitors into a novel immunodeficient FKRP-mutant mouse model by intra-muscular injection. Upon both mouse and human cell transplantation, we observe the presence of donor-derived myofibers even in absence of pre-injury, and the rescue of α-DG functional glycosylation, as shown by IIH6 immunoreactivity. The presence of donor-derived cells expressing Pax7 under the basal lamina is indicative of satellite cell engraftment, and therefore, long-term repopulation potential. Functional assays performed in the mouse-to-mouse cohort revealed enhanced specific force in transplanted muscles compared to PBS-injected controls.

CONCLUSIONS

Altogether, our data demonstrate for the first time the suitability of a cell-based therapeutic approach to improve the muscle phenotype of dystrophic FKRP-mutant mice.

Laminin G-like domains: dystroglycan-specific lectins

A unique O-mannose-linked glycan on the transmembrane protein dystroglycan binds a number of extracellular matrix proteins containing laminin G-like (LG) domains. The dystroglycan-matrix interaction is essential for muscle function: disrupted biosynthesis of the matrix-binding modification causes several forms of muscular dystrophy. The complete chemical structure of this modification has been deciphered in the past few years. We now know that LG domains bind to a glycosaminoglycan-like polysaccharide of [-3GlcAβ1,3Xylα1-] units, termed matriglycan, that is attached to a highly unusual heptasaccharide linker. X-ray crystallography has revealed the principles of Ca²⁺-dependent matriglycan binding by LG domains. In this review, the new structural insights are applied to the growing number of LG domain-containing proteins that bind dystroglycan. It is proposed that LG domains be recognised as 'D-type' lectins to indicate their conserved function in dystroglycan binding.

Nanotopography-responsive myotube alignment and orientation as a sensitive phenotypic biomarker for Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a fatal genetic disorder currently having no cure. Here we report that culture substrates patterned with nanogrooves and functionalized with Matrigel (or laminin) present an engineered cell microenvironment to allow myotubes derived from non-diseased, less-affected DMD, and severely-affected DMD human induced pluripotent stem cells (hiPSCs) to exhibit prominent differences in alignment and orientation, providing a sensitive phenotypic biomarker to potentially facilitate DMD drug development and early diagnosis. We discovered that myotubes differentiated from myogenic progenitors derived from non-diseased hiPSCs align nearly perpendicular to nanogrooves, a phenomenon not reported previously. We further found that myotubes derived from hiPSCs of a dystrophin-null DMD patient orient randomly, and those from hiPSCs of a patient carrying partially functional dystrophin align approximately 14° off the alignment direction of non-diseased myotubes. Substrates engineered with micron-scale grooves and/or cell adhesion molecules only interacting with integrins all guide parallel myotube alignment to grooves and lose the ability to distinguish different cell types. Disruption of the interaction between the Dystrophin-Associated-Protein-Complex (DAPC) and laminin by heparin or anti-α-dystroglycan antibody IIH6 disenables myotubes to align perpendicular to nanogrooves, suggesting that this phenotype is controlled by the DAPC-mediated cytoskeleton-extracellular matrix linkage.

Matriglycan: a novel polysaccharide that links dystroglycan to the basement membrane

Associations between cells and the basement membrane are critical for a variety of biological events including cell proliferation, cell migration, cell differentiation and the maintenance of tissue integrity. Dystroglycan is a highly glycosylated basement membrane receptor, and is involved in physiological processes that maintain integrity of the skeletal muscle, as well as development and function of the central nervous system. Aberrant O-glycosylation of the α subunit of this protein, and a concomitant loss of dystroglycan's ability to function as a receptor for extracellular matrix (ECM) ligands that bear laminin globular (LG) domains, occurs in several congenital/limb-girdle muscular dystrophies (also referred to as dystroglycanopathies). Recent genetic studies revealed that mutations in DAG1 (which encodes dystroglycan) and at least 17 other genes disrupt the ECM receptor function of dystroglycan and cause disease. Here, we summarize recent advances in our understanding of the enzymatic functions of two of these disease genes: the like-glycosyltransferase (LARGE) and protein O-mannose kinase (POMK, previously referred to as SGK196). In addition, we discuss the structure of the glycan that directly binds the ECM ligands and the mechanisms by which this functional motif is linked to dystroglycan. In light of the fact that dystroglycan functions as a matrix receptor and the polysaccharide synthesized by LARGE is the binding motif for matrix proteins, we propose to name this novel polysaccharide structure matriglycan.

C-terminal region of teneurin-1 co-localizes with the dystroglycan complex in adult mouse testes and regulates testicular size and testosterone production

Testicular size is directly proportional to fertility potential and is dependent on the integration of developmental proteins, trophic factors, and sex steroids. The teneurins are transmembrane glycoproteins that function as signaling and cell adhesion molecules in the establishment and maintenance of the somatic gonad, gametogenesis, and basement membrane. Moreover, teneurins are thought to function redundantly to the extracellular matrix protein, dystroglycan. Encoded on the last exon of the teneurin genes is a family of bioactive peptides termed the teneurin C-terminal-associated peptides (TCAPs). One of these peptides, TCAP-1, functionally interacts with β-dystroglycan to act as a neuromodulatory peptide with trophic characteristics independent from the teneurins. However, little is known about the localization and relationship between the teneurin-TCAP-1 system and the dystroglycans in the gonad. In the adult mouse testis, immunoreactive TCAP-1 was localized to spermatogonia and spermatocytes and co-localized with β-dystroglycan. However, teneurin-1 was localized to the peritubular myoid cell layer of seminiferous tubules and tubules within the epididymis, and co-localized with α-dystroglycan and α-smooth muscle actin. TCAP-1-binding sites were identified in the germ cell layers and adluminal compartment of the seminiferous tubules, and epithelial cells of the epididymis. In vivo, TCAP-1 administration to adult mice for 9 days increased testicular size, seminiferous and epididymal tubule short-diameter and elevated testosterone levels. TCAP-1-treated mice also showed increased TCAP-1 immunoreactivity in the caput and corpa epididymis. Our data provide novel evidence of TCAP-1 localization in the testes that is distinct from teneurin-1, but is integrated through an association with the dystroglycan complex.

LARGE2 generates the same xylose- and glucuronic acid-containing glycan structures as LARGE

LARGE (like-glycosyltransferase) and LARGE2 (glycosyltransferase-like 1B (GYLTL1B)) are homologous Golgi glycosyltransferases possessing two catalytic domains with homology to members of glycosyltransferase families GT8 and GT49. Mutations in human and mouse Large result in muscular dystrophy due to underglycosylation of dystroglycan. The systemic function of LARGE2 is unknown, but at a cellular level the enzyme can substitute for LARGE in glycosylating dystroglycan. Here, we show that LARGE2 catalyzes the same glycosylation reaction as LARGE. It is a bifunctional glycosyltransferase using uridine diphosphate (UDP)-xylose (Xyl) and UDP-glucuronic acid (GlcA) as donor sugars to produce a xyloglucuronan with alternating Xyl and GlcA residues.

Dystroglycan receptor is involved in integrin activation in intestinal epithelia

The dystroglycans (alpha-DG and beta-DG), which play important roles in the formation of basement membranes, have been well studied in skeletal muscle and nerve, but their expression and localization in intestinal epithelial cells has not been previously investigated. Here, we demonstrated that the DG complex, composed of alpha-DG, beta-DG, and utrophin, is specifically expressed in the basolateral membrane of the Caco-2-BBE monolayer. The DG complex coprecipitated with beta(1)-integrin, suggesting a possible interaction among these proteins. In addition, we observed that activation of DG receptors by laminin-1 enhanced the interaction between beta(1)-integrin and laminin-1, whereas activation of DG receptors by laminin-2 reduced the interaction between beta(1)-integrin and laminin-2. Finally, we demonstrated that the intracellular COOH-terminal tail of beta-DG and its binding to the DG binding domain of utrophin are crucial for the interactions between laminin-1/-2 and beta(1)-integrin. Collectively, these novel results indicate that dystroglycans play important roles in the regulation of interactions between intestinal epithelial cells and the extracellular matrix.

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.

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.

Amelioration of laminin-alpha2-deficient congenital muscular dystrophy by somatic gene transfer of miniagrin

Congenital muscular dystrophy (CMD) is characterized by severe muscle wasting, premature death in early childhood, and lack of effective treatment. Most of the CMD cases are caused by genetic mutations of laminin-alpha2, which is essential for the structural integrity of muscle extracellular matrix. Here, we report that somatic gene delivery of a structurally unrelated protein, a miniature version of agrin, functionally compensates for laminin-alpha2 deficiency in the murine models of CMD. Adeno-associated virus-mediated overexpression of miniagrin restored the structural integrity of myofiber basal lamina, inhibited interstitial fibrosis, and ameliorated dystrophic pathology. Furthermore, systemic gene delivery of miniagrin into multiple vital muscles significantly improved whole body growth and motility and quadrupled the lifespan (50% survival) of the dystrophic mice. Thus, our study demonstrated the efficacy of somatic gene therapy in a mouse model of CMD.

Electron microscopy of an alpha-dystroglycan fragment containing receptor sites for lymphocytic choriomeningitis virus and laminin, and use of the receptoid body as a reagent to neutralize virus

We report the electron microscopic structure of an alpha-dystroglycan (alpha-DG) fragment (DGEKFc4) that contains binding sites for lymphocytic choriomeningitis virus (LCMV) and the extracellular matrix (ECM) molecule laminin. In electron microscopic images, DGEKFc4 appears as dumbbell-shaped rods with a length of 7.5 +/- 0.5 nM and width of 3 +/- 0.3 nM. The C-terminal human Fc allows binding of anti-human Fc antibody resulting in formation of immune complexes that preserve alpha-DG binding to virus. Electron microscopy shows the antibody binding to near one end of the dumbbell-shaped rods. Because arenaviruses like LCMV or Lassa fever virus (LFV) generate poor neutralizing antibodies during natural infection or vaccination, we assayed whether the alpha-DG receptoid bodies generated could be used as an efficient antibody mimic. However, the receptor body formed by either alpha-DG fragment alone or complexed to antibody to human Fc failed to efficiently neutralize virus.

Laminin α2 Is Essential for Odontoblast Differentiation Regulating Dentin Sialoprotein Expression

Laminin alpha2 is subunit of laminin-2 (alpha2beta1gamma1), which is a major component of the muscle basement membrane. Although the laminin alpha2 chain is expressed in the early stage of dental mesenchyme development and localized in the tooth germ basement membrane, its expression pattern in the late stage of tooth germ development and molecular roles are not clearly understood. We analyzed the role of laminin alpha2 in tooth development by using targeted mice with a disrupted lama2 gene. Laminin alpha2 is expressed in dental mesenchymal cells, especially in odontoblasts and during the maturation stage of ameloblasts, but not in the pre-secretory or secretory stages of ameloblasts. Lama2 mutant mice have thin dentin and a widely opened dentinal tube, as compared with wild-type and heterozygote mice, which is similar to the phenotype of dentinogenesis imperfecta. During dentin formation, the expression of dentin sialoprotein, a marker of odontoblast differentiation, was found to be decreased in odontoblasts from mutant mice. Furthermore, in primary cultures of dental mesenchymal cells, dentin matrix protein, and dentin sialophosphoprotein, mRNA expression was increased in laminin-2 coated dishes but not in those coated with other matrices, fibronectin, or type I collagen. Our results suggest that laminin alpha2 is essential for odontoblast differentiation and regulates the expression of dentin matrix proteins.

Molecular dissection of the alpha-dystroglycan- and integrin-binding sites within the globular domain of human laminin-10

The adhesive interactions of cells with laminins are mediated by integrins and non-integrin-type receptors such as alpha-dystroglycan and syndecans. Laminins bind to these receptors at the C-terminal globular domain of their alpha chains, but the regions recognized by these receptors have not been mapped precisely. In this study, we sought to locate the binding sites of laminin-10 (alpha5beta1gamma1) for alpha(3)beta(1) and alpha(6)beta(1) integrins and alpha-dystroglycan through the production of a series of recombinant laminin-10 proteins with deletions of the LG (laminin G-like) modules within the globular domain. We found that deletion of the LG4-5 modules did not compromise the binding of laminin-10 to alpha(3)beta(1) and alpha(6)beta(1) integrins but completely abrogated its binding to alpha-dystroglycan. Further deletion up to the LG3 module resulted in loss of its binding to the integrins, underlining the importance of LG3 for integrin binding by laminin-10. When expressed individually as fusion proteins with glutathione S-transferase or the N-terminal 70-kDa region of fibronectin, only LG4 was capable of binding to alpha-dystroglycan, whereas neither LG3 nor any of the other LG modules retained the ability to bind to the integrins. Site-directed mutagenesis of the LG3 and LG4 modules indicated that Asp-3198 in the LG3 module is involved in the integrin binding by laminin-10, whereas multiple basic amino acid residues in the putative loop regions are involved synergistically in the alpha-dystroglycan binding by the LG4 module.

N-terminal α-dystroglycan binds to different extracellular matrix molecules expressed in regenerating peripheral nerves in a protein-mediated manner and promotes neurite extension of PC12 cells

alpha-dystroglycan is a cell surface receptor that is expressed in many tissues including the nervous system. The study shows that a recombinant, non-glycosylated N-terminal fragment of alpha-dystroglycan comprising residues 30 to 315 [alphaDG (30-315)] bound to laminin-2/-4 and laminin-1, fibronectin and fibrinogen, all molecules highly upregulated in the regenerating peripheral nerve. The interaction was concentration dependent and saturable and could not be inhibited by heparin suggesting only minor involvement of sulfated carbohydrate moieties. In contrast to published data, addition of bivalent cations increased the binding affinity by only ten fold.alphaDG (30-315) promotes neurite extension of PC12 cells in a similar amount as described for laminin isoforms and could be inhibited in a concentration dependent manner by alphaDG (30-315) itself, soluble laminin-1, partially by heparin, EDTA, and an RGD-peptide. Furthermore, co-immunoprecipitations between alpha-dystroglycan and beta1-integrin from PC12 cell surfaces suggested complex interactions between neuronal dystroglycan, integrins, and the ECM that induce neurite extension in vitro.

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.

Skeletal muscle signaling pathway through the dystrophin glycoprotein complex and Rac1

The dystrophin glycoprotein complex has been proposed to be involved in signal transduction. Here we have shown that laminin binding causes syntrophin to recruit Rac1 from the rabbit skeletal muscle. Laminin-Sepharose and syntrophin-Sepharose bind a protein complex containing Rac1 from the muscle membranes. The presence of heparin, which inhibits laminin interactions, prevents recruitment of Rac1. The dystrophin glycoprotein complex recruits Rac1 via syntrophin through a Grb2.Sos1 complex. A syntrophin antibody also prevents recruitment of Rac1, suggesting that the signaling complex requires syntrophin. PAK1 is in turn bound by Rac1. c-Jun NH2-terminal kinase-p46 is phosphorylated and activated only when laminin is present, and the p54 isoform is activated when laminin is depleted or binding is inhibited with heparin. In the presence of laminin, c-Jun is activated in both skeletal muscle microsomes and in C2C12 myoblasts, and proliferation increases in C2C12 myoblasts. We postulate that this pathway signals muscle homeostasis and hypertrophy.

Distinct requirements for heparin and alpha-dystroglycan binding revealed by structure-based mutagenesis of the laminin alpha2 LG4-LG5 domain pair

Laminin-2 (alpha2beta1gamma1) is found in basement membranes surrounding muscle and peripheral nerve cells. Several types of cellular receptors bind to the laminin G-like (LG) domains at the C terminus of the alpha2 chain, the interaction with alpha-dystroglycan (alpha-DG) being particularly important in muscle. We have used site-directed mutagenesis and in vitro binding assays to map the binding sites on the laminin alpha2 chain LG4-LG5 domain pair for alpha-DG, heparin and sulfatides. Calcium-dependent alpha-DG recognition requires the calcium ion in LG4, but not the one in LG5, as well as basic residues in both LG domains. Heparin and sulfatides also bind to basic residues in both LG domains, but there is little overlap in the binding sites for alpha-DG and heparin/sulfatides. The results should prove useful for the molecular dissection of laminin-receptor interactions in vivo.

Opposing roles of integrin alpha6Abeta1 and dystroglycan in laminin-mediated extracellular signal-regulated kinase activation

Laminin-integrin interactions can in some settings activate the extracellular signal-regulated kinases (ERKs) but the control mechanisms are poorly understood. Herein, we studied ERK activation in response to two laminins isoforms (-1 and -10/11) in two epithelial cell lines. Both cell lines expressed beta1-containing integrins and dystroglycan but lacked integrin alpha6beta4. Antibody perturbation assays showed that both cell lines bound to laminin-10/11 via the alpha3beta1and alpha6beta1 integrins. Although laminin-10/11 was a stronger adhesion complex than laminin-1 for both cell lines, both laminins activated ERK in only one of the two cell lines. The ERK activation was mediated by integrin alpha6beta1 and not by alpha3beta1 or dystroglycan. Instead, we found that dystroglycan-binding domains of both laminin-1 and -10/11 suppressed integrin alpha6beta1-mediated ERK activation. Moreover, the responding cell line expressed the two integrin alpha6 splice variants, alpha6A and alpha6B, whereas the nonresponding cell line expressed only alpha6B. Furthermore, ERK activation was seen in cells transfected with the integrin alpha6A subunit, but not in alpha6B-transfected cells. We conclude that laminin-1 and -10/11 share the ability to induce ERK activation, that this is regulated by integrin alpha6Abeta1, and suggest a novel role for dystroglycan-binding laminin domains as suppressors of this activation.

Contributions of the LG modules and furin processing to laminin-2 functions

The alpha2-laminin subunit contributes to basement membrane functions in muscle, nerve, and other tissues, and mutations in its gene are causes of congenital muscular dystrophy. The alpha2 G-domain modules, mutated in several of these disorders, are thought to mediate different cellular interactions. To analyze these contributions, we expressed recombinant laminin-2 (alpha(2)beta(1)gamma(1)) with LG4-5, LG1-3, and LG1-5 modular deletions. Wild-type and LG4-5 deleted-laminins were isolated from medium intact and cleaved within LG3 by a furin-like convertase. Myoblasts adhered predominantly through LG1-3 while alpha-dystroglycan bound to both LG1-3 and LG4-5. Recombinant laminin stimulated acetylcholine receptor (AChR) clustering; however, clustering was induced only by the proteolytic processed form, even in the absence of LG4-5. Furthermore, clustering required alpha(6)beta(1) integrin and alpha-dystroglycan binding activities available on LG1-3, acting in concert with laminin polymerization. The ability of the modified laminins to mediate basement membrane assembly was also evaluated in embryoid bodies where it was found that both LG1-3 and LG4-5, but not processing, were required. In conclusion, there is a division of labor among LG-modules in which (i) LG4-5 is required for basement membrane assembly but not for AChR clustering, and (ii) laminin-induced AChR clustering requires furin cleavage of LG3 as well as alpha-dystroglycan and alpha(6)beta(1) integrin binding.

Function and genetics of dystrophin and dystrophin-related proteins in muscle

The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.

Laminin assembles into separate basement membrane and fibrillar matrices in Schwann cells

Laminins are important for Schwann cell basement membrane assembly and axonal function. In this study, we found that exogenous laminin-1, like neuromuscular laminins-2/4, formed two distinct extracellular matrices on Schwann cell surfaces, each facilitated by laminin polymerization. Assembly of one, a densely-distributed reticular matrix, was accompanied by a redistribution of cell-surface dystroglycan and cytoskeletal utrophin into matrix-receptor-cytoskeletal complexes. The other, a fibrillar matrix,accumulated in separate zones associated with pre-existing β1-integrin arrays. The laminin-1 fragment E3 (LG-modules 4-5), which binds dystroglycan and heparin, inhibited reticular-matrix formation. By contrast,β1-integrin blocking antibody (Ha2/5) prevented fibrillar assembly. Ultrastructural analysis revealed that laminin treatment induced the formation of a linear electron-dense extracellular matrix (lamina densa)separated from plasma membrane by a narrow lucent zone (lamina lucida). This structure was considerably reduced with non-polymerizing laminin, fully blocked by E3, and unaffected by Ha2/5. Although it formed in the absence of type IV collagen, it was nonetheless able to incorporate this collagen. Finally, cell competency to bind laminin and form a basement membrane was passage-dependent. We postulate that laminin induces the assembly of a basement membrane on competent cell surfaces probably mediated by anchorage through LG 4-5. Upon binding, laminin interacts with dystroglycan,mobilizes utrophin, and assembles a `nascent' basement membrane, independent of integrin, that is completed by incorporation of type IV collagen. However,the fibrillar β1-integrin dependent matrix is unlikely to be precursor to basement membrane.

Molecular analysis of the interaction of LCMV with its cellular receptor [alpha]-dystroglycan

alpha-Dystroglycan (DG) has been identified as the cellular receptor for lymphocytic choriomeningitis virus (LCMV) and Lassa fever virus (LFV). This subunit of DG is a highly versatile cell surface molecule that provides a molecular link between the extracellular matrix (ECM) and a beta-DG transmembrane component, which interacts with the actin-based cytoskeleton. In addition, DG exhibits a complex pattern of interaction with a wide variety of ECM and cellular proteins. In the present study, we characterized the binding of LCMV to alpha-DG and addressed the role of alpha-DG-associated host-derived proteins in virus infection. We found that the COOH-terminal region of alpha-DG's first globular domain and the NH2-terminal region of the mucin-related structures of alpha-DG together form the binding site for LCMV. The virus-alpha-DG binding unlike ECM alpha-DG interactions was not dependent on divalent cations. Despite such differences in binding, LCMV and laminin-1 use, in part, an overlapping binding site on alpha-DG, and the ability of an LCMV isolate to compete with laminin-1 for receptor binding is determined by its binding affinity to alpha-DG. This competition of the virus with ECM molecules for receptor binding likely explains the recently found correlation between the affinity of LCMV binding to alpha-DG, tissue tropism, and pathological potential. LCMV strains and variants with high binding affinity to alpha-DG but not low affinity binders are able to infect CD11c+ dendritic cells, which express alpha-DG at their surface. Infection followed by dysfunction of these antigen-presenting cells contributes to immunosuppression and persistent viral infection in vivo.

A unique sequence of the laminin alpha 3 G domain binds to heparin and promotes cell adhesion through syndecan-2 and -4

Laminin-5, consisting of the alpha 3, beta 3, and gamma 2 chains, is localized in the skin basement membrane and supports the structural stability of the epidermo-dermal linkage and regulates various cellular functions. The alpha chains of laminins have been shown to have various biological activities. In this study, we identified a sequence of the alpha 3 chain C-terminal globular domain (LG1-LG5 modules) required for both heparin binding and cell adhesion using recombinant proteins and synthetic peptides. We found that the LG3 and LG4 modules have activity for heparin binding and that LG4 has activity for cell adhesion. Studies with synthetic peptides delineated the A3G75aR sequence (NSFMALYLSKGR, residues 1412--1423) within LG4 as a major site for both heparin and cell binding. Substitution mutations in LG4 and A3G75aR identified the Lys and Arg of the A3G75aR sequence as critical for these activities. Cell adhesion to LG4 and A3G75aR was inhibited by heparitinase I treatment of cells, suggesting that cell binding to the A3G75aR site was mediated by cell surface heparan sulfate proteoglycans. We showed by affinity chromatography that syndecan-2 from fibroblasts bound to LG4. Solid-phase assays confirmed that syndecan-2 interacted with the A3G75aR peptide sequence. Stably transfected 293T cells with expression vectors for syndecan-2 and -4, but not glypican-1, specifically adhered to LG4 and A3G75aR. These results indicate that the A3G75aR sequence within the laminin alpha 3 LG4 module is responsible for cell adhesion and suggest that syndecan-2 and -4 mediate this activity.

Differential Distribution of the Members of the Dystrophin Glycoprotein Complex in Mouse Retina: Effect of the mdx3Cv Mutation

Dystrophin glycoprotein complex (DGC) assembly and function require mediation by dystrophin in skeletal muscle. The existence of such complexes and the correlation with DMD phenotypes are not yet established in the central nervous system. Here we have studied the expression of DMD gene mRNAs and proteins in retina from C57BL/6 and mdx(3Cv) mouse strains. Then we have comparatively investigated the localization of dystrophin and dystrophin-associated proteins (DAPs) in both strains to analyze the repercussion of the mdx(3Cv) mutation on the retinal distributions of alpha/beta-dystroglycan, alpha1-syntrophin, alpha-dystrobrevin, and delta/gamma-sarcoglycan. Results showed that DMD gene product deficiency affects the expression of dystroglycan assembly exclusively at the outer plexiform layer without an apparent effect on the other DAPs. We conclude that the localization of members of the DGC could be independent of the presence of the DMD gene products and/or utrophin.

Anomalous dystroglycan in carcinoma cell lines

Dystroglycan is a receptor responsible for crucial interactions between extracellular matrix and cytoplasmic space. We provide the first evidence that dystroglycan is truncated. In HC11 normal murine and the 184B5 non-tumorigenic mammary human cell lines, the expected beta-dystroglycan 43 kDa band was found but human breast T47D, BT549, MCF7, colon HT29, HCT116, SW620, prostate DU145 and cervical HeLa cancer cells expressed an anomalous approximately 31 kDa beta-dystroglycan band. alpha-Dystroglycan was udetectable in most of the cell lines in which beta-dystroglycan was found as a approximately 31 kDa species. An anomalous approximately 31 kDa beta-dystroglycan band was also observed in N-methyl-N-nitrosurea-induced primary rat mammary tumours. Reverse transcriptase polymerase chain reaction experiments confirmed the absence of alternative splicing events and/or expression of eventual dystroglycan isoforms. Using protein extraction procedures at low- and high-ionic strength, we demonstrated that both the 43 kDa and approximately 31 kDa beta-dystroglycan bands harbour their transmembrane segment.

Laminins of the neuromuscular system

The mammalian neuromuscular system expresses seven laminin genes (alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 1), produces seven isoforms of the laminin trimer (laminins 1, 2, 4, 8, 9, 10, and 11), and distributes these trimers to at least seven distinct basal laminae (perineurial, endoneurial, terminal Schwann cell, myotendinous junction, synaptic cleft, synaptic fold, and extrajunctional muscle). The patterns of expression, assembly, and distribution are regulated during development, and primary and secondary changes in laminin expression occur in several neuromuscular genetic disorders. Functional studies using knockout and transgenic mice, and purified laminins and cell types, demonstrate that laminins are required components of basal laminae in the neuromuscular system. Collectively, laminins have both structural and signaling functions; individually, laminin isoforms have unique roles in regulating the behavior of nerve, muscle, and Schwann cell. Among them, laminin-2 (alpha 2 beta 1 gamma 1) plays an important structural role in supporting the muscle plasma membrane, laminin-4 regulates adhesion and differentiation of the myotendinous junction, and laminin-11 regulates nerve terminal differentiation and Schwann cell motility. Together, these observations reveal remarkable diversity in the formation and function of laminins and basal laminae, and suggest avenues for addressing some neuromuscular diseases.

Molecular basis of muscular dystrophies

Muscular dystrophies represent a heterogeneous group of disorders, which have been largely classified by clinical phenotype. In the last 10 years, identification of novel skeletal muscle genes including extracellular matrix, sarcolemmal, cytoskeletal, cytosolic, and nuclear membrane proteins has changed the phenotype-based classification and shed new light on the molecular pathogenesis of these disorders. A large number of genes involved in muscular dystrophy encode components of the dystrophin-glycoprotein complex (DGC) which normally links the intracellular cytoskeleton to the extracellular matrix. Mutations in components of this complex are thought to lead to loss of sarcolemmal integrity and render muscle fibers more susceptible to damage. Recent evidence suggests the involvement of vascular smooth muscle DGC in skeletal and cardiac muscle pathology in some forms of sarcoglycan-deficient limb-girdle muscular dystrophy. Intriguingly, two other forms of limb-girdle muscular dystrophy are possibly caused by perturbation of sarcolemma repair mechanisms. The complete clarification of these various pathways will lead to further insights into the pathogenesis of this heterogeneous group of muscle disorders.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.

Identification of a major heparin and cell binding site in the LG4 module of the laminin alpha 5 chain

The G domain of the laminin alpha chains consists of five homologous G modules (LG1-5) and has been implicated in various biological functions. In this study, we identified an active site for cell and heparin binding within the laminin alpha5 G domain using recombinant proteins and synthetic peptides. Recombinant LG4, LG5, and LG4-5 modules were generated using a mammalian expression system. The LG4 and LG4-5 modules were highly active for cell binding, whereas the LG5 module alone showed only weak binding. Heparin inhibited cell binding to the LG4-5 module, whereas no inhibition was observed with EDTA or antibodies against the integrin beta(1) subunit. These results suggest that the LG4-5 module interacts with a cell surface receptor containing heparan sulfate but not with integrins. Solid-phase assays and surface plasmon resonance measurements demonstrated strong binding of the LG4 and LG4-5 modules to heparin with K(D) values in the nanomolar range, whereas a 16-fold lower value was determined for the LG5 module. Treatment with glycosidases demonstrated that N-linked carbohydrates on the LG5 module are complex-type oligosaccharides. The LG4-5 module, devoid of N-linked carbohydrates, exhibited similar binding kinetics toward heparin. Furthermore, cell binding was unaffected by removal of N-linked glycosylation. To localize active sites on the LG4 module, various synthetic peptides were used to compete with binding of the tandem module to heparin and cells. Peptide F4 (AGQWHRVSVRWG) inhibited binding, whereas a scrambled peptide of F4 failed to compete binding. Alanine replacements demonstrated that one arginine residue within F4 was important for cell and heparin binding. Our results suggest a critical role of the LG4 module for heparan sulfate-containing receptor binding within the laminin alpha5 chain.

alpha-dystroglycan isoforms are differentially distributed in adult rat retina

alpha-Dystroglycan (alpha -DG) is a laminin/agrin receptor expressed in skeletal muscle as well as in nervous system and other tissues. Glycosylation of the core protein of alpha-DG is extensive, variable from tissue to tissue, and functionally relevant. To address differential glycosylation of alpha-DG in the retina, we have investigated the distribution of this protein using two different antibodies: 1B7 directed against the core protein of alpha-dystroglycan, and IIH6 directed against a carbohydrate moiety (Ervasti and Campbell [1993] J Cell Biol 122:809-823). Monoclonal antibody 1B7 recognizes a broader band than IIH6, which seems to recognize only a subset of alpha-DG forms in retina. These data reflect the existence of differentially glycosylated isoforms of alpha-DG. Monoclonal antibody 1B7 shows an extensive staining for alpha-DG in the inner limiting membrane as well as in the ganglion cell and inner plexiform layers labeling Müller cell processes, whereas monoclonal antibody IIH6 staining is restricted to the inner limiting membrane and blood vessels. Our data indicate that there are distinct isoforms of alpha-DG that are localized in apposition to basal lamina in the inner limiting membrane and blood vessels or within the parenchyma of the retina along Müller glia. Both isoforms are expressed in a Müller cell line in culture and coimmunoprecipitate with beta-dystroglycan. These data suggest that DGs may participate in organizing synapses and basement membrane assembly in the retina.

Sedum telephium L. polysaccharide content affects MRC5 cell adhesion to laminin and fibronectin

In traditional medicine the fresh leaves and juice of Sedum telephium L. are used as wound-healing promoters. Cell adhesion represents a primary event in wound repair and in tissue homeostasis, and therefore we have investigated the effect of Sedum juice and its main fractions, polysaccharides and flavonols, on human fibroblast (MRC5) adhesion to fibronectin and laminin. Our findings revealed that total Sedum juice strongly inhibited cell adhesion to laminin and fibronectin (EC50 1.03+/-0.12 mg mL(-1)). This anti-adhesive feature was concentrated mainly in the two polysaccharide fractions (EC50 values comprised between 0.09 and 0.44 mg mL(-1)). The flavonol fractions did not seem to contribute to this effect. A first attempt to elucidate the polysaccharide-related anti-adhesive feature of Sedum juice was also performed. The results confirmed that natural polysaccharides, with chemical structures different from heparin, were able to interfere with integrin-mediated cell behaviour and they contributed to the outstanding effects of Sedum juice and to the role of polysaccharides in cell-matrix interaction.

Neural regulation of alpha-dystroglycan biosynthesis and glycosylation in skeletal muscle

Alpha-dystroglycan (alpha-DG) is part of a complex of cell surface proteins linked to dystrophin or utrophin, which is distributed over the myofiber surface and is concentrated at neuromuscular junctions. In laminin overlays of muscle extracts from developing chick hindlimb muscle, alpha-DG first appears at embryonic day (E) 10 with an apparent molecular mass of 120 kDa. By E15 it is replaced by smaller (approximately 100 kDa) and larger (approximately 150 kDa) isoforms. The larger form increases in amount and in molecular mass (>200 kDa) as the muscle is innervated and the postsynaptic membrane differentiates (E10-E20), and then decreases dramatically in amount after hatching. In myoblasts differentiating in culture the molecular mass of alpha-DG is not significantly increased by their replication, fusion, or differentiation into myotubes. Monoclonal antibody IIH6, which recognizes a carbohydrate epitope on alpha-DG, preferentially binds to the larger forms, suggesting that the core protein is differentially glycosylated beginning at E16. Consistent with prior observations implicating the IIH6 epitope in laminin binding, the smaller forms of alpha-DG bind more weakly to laminin affinity columns than the larger ones. In blots of adult rat skeletal muscle probed with radiolabeled laminin or monoclonal antibody IIH6, alpha-DG appears as a >200-kDa band that decreases in molecular mass but increases in intensity following denervation. Northern blots reveal a single mRNA transcript, indicating that the reduction in molecular mass of alpha-DG after denervation is not obviously a result of alternative splicing but is likely due to posttranslational modification of newly synthesized molecules. The regulation of alpha-DG by the nerve and its increased affinity for laminin suggest that glycosylation of this protein may be important in myofiber-basement membrane interactions during development and after denervation.

Structural and functional analysis of the N-terminal extracellular region of beta-dystroglycan

A protein fragment corresponding to the mouse beta-dystroglycan N-terminal extracellular region from position 654 to 750, beta-DG(654-750) was recombinantly expressed in BL21(DE3) Escherichia coli cells. Secondary structure prediction of the protein fragment reveals about 70% of random coil, as confirmed by circular dichroism analysis. Moreover, fluorescence analysis shows that the tryptophan residue in position 659 lays in a solvent-exposed fashion. These data suggest that the beta-DG(654-750) is likely to have a quite flexible structure and to be only partially folded. Interestingly, the protein still retains its biological function since using solid-phase assays we have detected binding of biotinylated beta-DG(654-750) both to native alpha-dystroglycan and to a recombinant fragment which spans the C-terminal region of alpha-dystroglycan.

O-mannosyl glycans in mammals

Most proteins within living organisms contain glycans. Glycan structures can modulate the biological properties and functions of glycoproteins. The major glycans of glycoproteins can be classified into two groups, N-glycans and O-glycans, according to their glycan-peptide linkage regions. Developments in glycobiology have revealed a new type of glycosidic linkage to the peptide portion, the O-mannosyl linkage, in mammals, while so far it had been thought to be specific to yeast. This review will give an outline of the O-mannosyl glycans of mammalian glycoproteins. Since one of the most well known O-mannosyl-modified mammalian glycoproteins is dystroglycan, the functional aspects of the O-mannosyl glycan of dystroglycan will be described to help understand this new glycobiological field.

The crystal structure of a laminin G-like module reveals the molecular basis of alpha-dystroglycan binding to laminins, perlecan, and agrin

Laminin G-like (LG) modules in the extracellular matrix glycoproteins laminin, perlecan, and agrin mediate the binding to heparin and the cell surface receptor alpha-dystroglycan (alpha-DG). These interactions are crucial to basement membrane assembly, as well as muscle and nerve cell function. The crystal structure of the laminin alpha 2 chain LG5 module reveals a 14-stranded beta sandwich. A calcium ion is bound to one edge of the sandwich by conserved acidic residues and is surrounded by residues implicated in heparin and alpha-DG binding. A calcium-coordinated sulfate ion is suggested to mimic the binding of anionic oligosaccharides. The structure demonstrates a conserved function of the LG module in calcium-dependent lectin-like alpha-DG binding.

Division of labor among the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor to signal morphogenesis and beta-casein expression in mammary epithelial cells

Contact of cultured mammary epithelial cells with the basement membrane protein laminin induces multiple responses, including cell shape changes, growth arrest, and, in the presence of prolactin, transcription of the milk protein beta-casein. We sought to identify the specific laminin receptor(s) mediating the multiple cell responses to laminin. Using assays with clonal mammary epithelial cells, we reveal distinct functions for the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor. Signals from laminin for beta-casein expression were inhibited in the presence of function-blocking antibodies against both the alpha6 and beta1 integrin subunits and by the laminin E3 fragment. The alpha6-blocking antibody perturbed signals mediated by the alpha6beta4 integrin, and the beta1-blocking antibody perturbed signals mediated by another integrin, the alpha subunit(s) of which remains to be determined. Neither alpha6- nor beta1-blocking antibodies perturbed the cell shape changes resulting from cell exposure to laminin. However, the E3 laminin fragment and heparin both inhibited cell shape changes induced by laminin, thereby implicating an E3 laminin receptor in this function. These results elucidate the multiplicity of cell-extracellular matrix interactions required to integrate cell structure and signaling and ultimately permit normal cell function.

Role for alpha-dystrobrevin in the pathogenesis of dystrophin-dependent muscular dystrophies

A dystrophin-containing glycoprotein complex (DGC) links the basal lamina surrounding each muscle fibre to the fibre's cytoskeleton, providing both structural support and a scaffold for signalling molecules. Mutations in genes encoding several DGC components disrupt the complex and lead to muscular dystrophy. Here we show that mice deficient in alpha-dystrobrevin, a cytoplasmic protein of the DGC, exhibit skeletal and cardiac myopathies. Analysis of double and triple mutants indicates that alpha-dystrobrevin acts largely through the DGC. Structural components of the DGC are retained in the absence of alpha-dystrobrevin, but a DGC-associated signalling protein, nitric oxide synthase, is displaced from the membrane and nitric-oxide-mediated signalling is impaired. These results indicate that both signalling and structural functions of the DGC are required for muscle stability, and implicate alpha-dystrobrevin in the former.

Characterization and expression of the laminin gamma3 chain: a novel, non-basement membrane-associated, laminin chain

Laminins are heterotrimeric molecules composed of an alpha, a beta, and a gamma chain; they have broad functional roles in development and in stabilizing epithelial structures. Here, we identified a novel laminin, composed of known alpha and beta chains but containing a novel gamma chain, gamma3. We have cloned gene encoding this chain, LAMC3, which maps to chromosome 9 at q31-34. Protein and cDNA analyses demonstrate that gamma3 contains all the expected domains of a gamma chain, including two consensus glycosylation sites and a putative nidogen-binding site. This suggests that gamma3-containing laminins are likely to exist in a stable matrix. Studies of the tissue distribution of gamma3 chain show that it is broadly expressed in: skin, heart, lung, and the reproductive tracts. In skin, gamma3 protein is seen within the basement membrane of the dermal-epidermal junction at points of nerve penetration. The gamma3 chain is also a prominent element of the apical surface of ciliated epithelial cells of: lung, oviduct, epididymis, ductus deferens, and seminiferous tubules. The distribution of gamma3-containing laminins on the apical surfaces of a variety of epithelial tissues is novel and suggests that they are not found within ultrastructurally defined basement membranes. It seems likely that these apical laminins are important in the morphogenesis and structural stability of the ciliated processes of these cells.

Association of the dystroglycan complex isolated from bovine brain synaptosomes with proteins involved in signal transduction

Dystroglycan is a transmembrane heterodimeric complex of alpha and beta subunits that links the extracellular matrix to the cell cytoskeleton. It was originally identified in skeletal muscle, where it anchors dystrophin to the sarcolemma. Dystroglycan is also highly expressed in nonmuscle tissues, including brain. To investigate the molecular interactions of dystroglycan in the CNS, we fractionated a digitonin-soluble extract from bovine brain synaptosomes by laminin-affinity chromatography and characterized the protein components. The 120-kDa alpha-dystroglycan was the major 125I-laminin-labeled protein detected by overlay assay. This complex, in addition to beta-dystroglycan, was also found to contain Grb2 and focal adhesion kinase p125FAK (FAK). Anti-FAK antibodies co-immunoprecipitated Grb2 with FAK. However, no direct interaction between beta-dystroglycan and FAK was detected by co-precipitation assay. Grb2, an adaptor protein involved in signal transduction and cytoskeleton organization, has been shown to bind beta-dystroglycan. We isolated both FAK and Grb2 from synaptosomal extracts by chromatography on immobilized recombinant beta-dystroglycan. In the CNS, FAK phosphorylation has been linked to membrane depolarization and neurotransmitter receptor activation. At the synapses, the adaptor protein Grb2 may mediate FAK-beta-dystroglycan interaction, and it may play a role in transferring information between the dystroglycan complex and other signaling pathways.

Analysis of heparin, alpha-dystroglycan and sulfatide binding to the G domain of the laminin alpha1 chain by site-directed mutagenesis

The 395-residue proteolytic fragment E3, which comprises the two most C-terminal LG modules of the mouse laminin alpha1 chain, was previously shown to contain major binding sites for heparin, alpha-dystroglycan and sulfatides. The same fragment (alpha1LG4-5) and its individual alpha1LG4 and alpha1LG5 modules have now been obtained by recombinant production in mammalian cells. These fragments were apparently folded into a native form, as shown by circular dichroism, electron microscopy and immunological assays. Fragment alpha1LG4-5 bound about five- to tenfold better to heparin, alpha-dystroglycan and sulfatides than E3. These binding activities could be exclusively localized to the alpha1LG4 module. Side-chain modifications and proteolysis demonstrated that Lys and Arg residues in the C-terminal region of alpha1LG4 are essential for heparin binding. This was confirmed by 14 single to triple point mutations, which identified three non-contiguous basic regions (positions 2766-2770, 2791-2793, 2819-2820) as contributing to both heparin and sulfatide binding. Two of these regions were also recognized by monoclonal antibodies which have previously been shown to inhibit heparin binding. The same three regions and a few additional basic residues also make major contributions to the binding of the cellular receptor alpha-dystroglycan, indicating a larger binding epitope. The data are also consistent with previous findings that heparin competes for alpha-dystroglycan binding.

Basement membranes in development

The aim of this review is to introduce the reader to the main ECM constituents and to some of their roles in development. The main functions of the ECM during embryogenesis are the production, promotion, and regulation of normal tissue structure. Among the ECM components, LMs have been the most extensively studied in relation to embryo-genesis. Skin and skeletal muscle disorders have been shown to be caused by LM alterations. Additional experiments, e.g., with knockout mice, will help enormously to elucidate the functional significance of many ECM constituents and their involvement in development and disease.

Binding of the G domains of laminin alpha1 and alpha2 chains and perlecan to heparin, sulfatides, alpha-dystroglycan and several extracellular matrix proteins

The C-terminal G domain of the mouse laminin alpha2 chain consists of five lamin-type G domain (LG) modules (alpha2LG1 to alpha2LG5) and was obtained as several recombinant fragments, corresponding to either individual modules or the tandem arrays alpha2LG1-3 and alpha2LG4-5. These fragments were compared with similar modules from the laminin alpha1 chain and from the C-terminal region of perlecan (PGV) in several binding studies. Major heparin-binding sites were located on the two tandem fragments and the individual alpha2LG1, alpha2LG3 and alpha2LG5 modules. The binding epitope on alpha2LG5 could be localized to a cluster of lysines by site-directed mutagenesis. In the alpha1 chain, however, strong heparin binding was found on alpha1LG4 and not on alpha1LG5. Binding to sulfatides correlated to heparin binding in most but not all cases. Fragments alpha2LG1-3 and alpha2LG4-5 also bound to fibulin-1, fibulin-2 and nidogen-2 with Kd = 13-150 nM. Both tandem fragments, but not the individual modules, bound strongly to alpha-dystroglycan and this interaction was abolished by EDTA but not by high concentrations of heparin and NaCl. The binding of perlecan fragment PGV to alpha-dystroglycan was even stronger and was also not sensitive to heparin. This demonstrated similar binding repertoires for the LG modules of three basement membrane proteins involved in cell-matrix interactions and supramolecular assembly.

Laminins of the dermo-epidermal junction

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules constituting a family of possibly more than 50 members. Some members such as laminins 5, 6 and 10 are specific of the basal lamina present under stratified epithelia. Although only few intact laminin isoforms have been purified from cultivated cells or tissues, genetic engineering has opened the way for a rapid development of laminin structural biology. Moreover, the phenotypes resulting from gene targeting in mouse or from laminin defects in acquired or inherited human diseases highlight the pivotal role of laminins in morphogenesis, development, and physiology. Indeed, the laminins display a remarkable repertoire of functions, most importantly as structural elements forming a network throughout the basement membrane to which other collagenous or non-collagenous glycoproteins and proteoglycans attach. Furthermore, they are signaling molecules providing adjacent cells with diverse information by interacting with cell surface components.

Dystrophic phenotype induced in vitro by antibody blockade of muscle alpha-dystroglycan-laminin interaction

alpha-dystroglycan is a glycoprotein expressed on the surface of skeletal muscle fibres and other cell types. In muscle, alpha-dystroglycan provides a link between the myofibre cytoskeleton through its indirect binding to dystrophin, and the basal lamina through its binding to laminin-2, a protein of the extracellular matrix. The disruption of this linkage between the myofibre cytoskeleton and the extracellular matrix is a common feature of Duchenne and other muscular dystrophies, though the pathogenic mechanisms leading to muscle wasting remain unknown. By treating primary mouse muscle cultures with a monoclonal antibody which blocks alpha-dystroglycan binding to laminin, we show here the induction of a dystrophic phenotype in vitro. The phenotype is inducible in differentiated cultures only, is characterised by reduced myotube size, myofibril disorganisation, loss of contractile activity, reduced spontaneous clustering of acetylcholine receptors and is reversed by addition of excess exogenous laminin-2. Thus, alpha-dystroglycan may be part of a signalling pathway for the maturation and maintenance of skeletal myofibres. Detailed knowledge of this signalling pathway may provide insights into the molecular pathology of the various inherited muscular dystrophies, and identify valuable pharmacological targets and new therapeutic strategies.