Laminin-binding protein 120 from brain is closely related to the dystrophin-associated glycoprotein, dystroglycan, and binds with high affinity to the major heparin binding domain of laminin

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.

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.

Sarcospan-deficient mice maintain normal muscle function

Sarcospan is an integral membrane component of the dystrophin-glycoprotein complex (DGC) found at the sarcolemma of striated and smooth muscle. The DGC plays important roles in muscle function and viability as evidenced by defects in components of the DGC, which cause muscular dystrophy. Sarcospan is unique among the components of the complex in that it contains four transmembrane domains with intracellular N- and C-terminal domains and is a member of the tetraspan superfamily of proteins. Sarcospan is tightly linked to the sarcoglycans, and together these proteins form a subcomplex within the DGC. Stable expression of sarcospan at the sarcolemma is dependent upon expression of the sarcoglycans. Here we describe the generation and analysis of mice carrying a null mutation in the Sspn gene. Surprisingly, the Sspn-deficient muscle maintains expression of other components of the DGC at the sarcolemma, and no gross histological abnormalities of muscle from the mice are observed. The Sspn-deficient muscle maintains sarcolemmal integrity as determined by serum creatine kinase and Evans blue uptake assays, and the Sspn-deficient muscle maintains normal force and power generation capabilities. These data suggest either that sarcospan is not required for normal DGC function or that the Sspn-deficient muscle is compensating for the absence of sarcospan, perhaps by utilizing another protein to carry out its function.

Biosynthesis of dystroglycan: processing of a precursor propeptide

Dystroglycan is a cytoskeleton-linked extracellular matrix receptor expressed in many cell types. Dystroglycan is composed of alpha- and beta-subunits which are encoded by a single mRNA. Using a heterologous mammalian expression system, we provide the first biochemical evidence of the alpha/beta-dystroglycan precursor propeptide prior to enzymatic cleavage. This 160 kDa dystroglycan propeptide is processed into alpha- and beta-dystroglycan (120 kDa and 43 kDa, respectively). We also demonstrate that the precursor propeptide is glycosylated and that blockade of asparagine-linked (N-linked) glycosylation did not prevent the cleavage of the dystroglycan precursor peptide. However, inhibition of N-linked glycosylation results in aberrant trafficking of the alpha- and beta-dystroglycan subunits to the plasma membrane. Thus, dystroglycan is synthesized as a precursor propeptide that is post-translationally cleaved and differentially glycosylated to yield alpha- and beta-dystroglycan.

Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain

Human sex hormone-binding globulin (SHBG) transports sex steroids in blood and regulates their access to target tissues. In biological fluids, SHBG exists as a homodimer and each monomer comprises two laminin G-like domains (G domains). The crystal structure of the N-terminal G domain of SHBG in complex with 5alpha-dihydrotestosterone at 1.55 A resolution reveals both the architecture of the steroid-binding site and the quaternary structure of the dimer. We also show that G domains have jellyroll topology and are structurally related to pentraxin. In each SHBG monomer, the steroid intercalates into a hydrophobic pocket within the beta-sheet sandwich. The steroid and a 20 A distant calcium ion are not located at the dimer interface. Instead, two separate steroid-binding pockets and calcium-binding sites exist per dimer. The structure displays intriguing disorder for loop segment Pro130-Arg135. In all other jellyroll proteins, this loop is well ordered. If modelled accordingly, it covers the steroid-binding site and could thereby regulate access of ligands to the binding pocket.

Merosin and congenital muscular dystrophy

Merosin (also called as Laminin-2) is an isoform of laminin comprised of the alpha2, beta1 and gamma1 chains. In European populations, half of the patients with classical congenital muscular dystrophy have mutations of the LAMA2 gene (6q22-23) and present reduced or absence of laminin alpha2 chain. This form is generally referred to as merosin-deficient CMD. Merosin-deficient CMD is characterized by involvement of not only skeletal muscle but also central and peripheral nervous systems: Extensive brain white matter abnormalities are found by magnetic resonance imaging (MRI). However, most patients show no mental retardation. Recent case studies reported that some patients have several structural abnormalities such as abnormal cerebral cortical gyration, hypoplasia of cerebellum and pons, and dilation of ventricles. At present, functions of merosin related to muscle degeneration have not been fully elucidated. In addition, the mechanisms responsible for pathogenesis of diffuse brain white matter abnormalities remain to be determined. As mouse models for merosin-deficient CMD, three spontaneous mutants(dy, dy(2J), dy(PAS1)) and two mutants named dy(W) and dy(3K) by targeted gene disruption have been reported. These mice will help to elucidate the pathogenesis of merosin-deficient CMD and serve to develop therapy.

Self-assembly of laminin induced by acidic pH

The supramolecular architecture of the basement membrane is provided by two enmeshed networks of collagen IV and laminin. The laminin network is maintained exclusively by interactions among individual laminin molecules and does not depend on the presence of other extracellular matrix components. Laminin polymers can be obtained in vitro either in solution or in association with the surface of bilayers containing acidic lipids. In this work, we have tested the hypothesis that the negative charges present on acidic lipids establish an acid microenvironment that is directly responsible for inducing laminin aggregation. Using light-scattering measurements, we show that laminin does not aggregate on vesicles of neutral lipids, whereas instantaneous aggregation occurs to progressively greater extents as the proportion of acidic phospholipids in the vesicles is increased. Aggregation of laminin induced by vesicles containing acidic phospholipids occurs very rapidly, so that maximal aggregation for each condition is reached within 1 min after laminin dilution. Aggregation depends on the presence of Ca(2+) ions, is reversed by increasing ionic strength, and can be detected at laminin concentrations as low as 6 nM. In addition, we show that, in the absence of vesicles, acidification of the bulk solution can also induce laminin self-polymerization through a process that exhibits the same properties as lipid-induced polymerization. The fact that there is a correspondence between the processes of self-polymerization of laminin in acidic medium and in neutral medium but in the presence of vesicles containing negatively charged lipids leads us to propose that the microenvironment of an acidic surface may trigger the assembly of laminin networks. In vivo, such an acidic microenvironment would be provided by negatively charged sialic acid and sulfate groups present in the glycocalyx surrounding the cells.

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 laminin alpha2 expressed by dystrophic dy(2J) mice is defective in its ability to form polymers

Mutations in LAMA2 cause severe congenital muscular dystrophy accompanied by nervous system defects [1]. Mice homozygous for the dy(2J) allele of LAMA2 express a laminin alpha2 subunit that has a deletion in the amino-terminal domain VI, providing an animal model for study of the molecular basis of congenital muscular dystrophy [2] [3]. Domain VI is predicted to be involved in laminin polymerization, along with amino-terminal domains from laminin beta and gamma chains [4]. In a solution-polymerization assay, we found that purified dy(2J) laminin assembled poorly and formed little polymer, in contrast to wild-type muscle laminin. Furthermore, dissolution of the collagen IV network caused dy(2J) laminin to be released into solution, indicating that laminin polymers within the skeletal muscle basement membrane were defective. In addition to loss of polymerization, dy(2J) laminin had a reduced affinity for heparin. Finally, recombinant laminin engineered with the dy(2J) deletion was more sensitive to proteolysis and was readily cleaved near the junction of domains V and VI. Thus, the dy(2J) deletion selectively disrupts polymer formation, reduces affinity for heparin, and destabilizes domain VI. These are the first specific functional defects to be identified in a muscular dystrophy laminin, and it is likely that these defects contribute to the abnormalities seen in dy(2J)/dy(2J) muscle and nerve.

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.

Dystroglycan inside and out

Dystroglycan connects the extracellular matrix and cytoskeleton. Key findings in the past year indicate that dystroglycan interacts with a wider repertoire of extracellular ligands than originally appreciated, that dystroglycan plays a critical role in organizing extracellular matrix molecules on the cell surface and in basement membranes, and that at least two human pathogens utilize dystroglycan to gain access to host cells. Together, these advances begin to help elucidate important biological roles for dystroglycan in development and disease.

Mutation of a basic sequence in the laminin alpha2LG3 module leads to a lack of proteolytic processing and has different effects on beta1 integrin-mediated cell adhesion and alpha-dystroglycan binding

A RRKRRQ sequence unique to the LG3 module of the laminin alpha2 chain was previously shown to be sensitive to endogenous proteolysis during the recombinant production of the tandem array alpha2LG1-3. Mutation of RQ surrounding the cleaved peptide bond did not prevent this processing and intracellular degradation. Alanine mutagenesis of three alternate basic residues, however, was shown to prevent the cleavage in alpha2LG1-3, allowing for the alpha2LG3 module to be obtained as a folded, globular fragment. The mutation did not change heparin and sulfatide binding or cell adhesion of alpha2LG1-3 which can be mediated by alpha3beta1 and alpha6beta1 integrins. It did, however, cause a 10-fold reduction in alpha-dystroglycan binding. The data favor the interpretation that binding epitopes for heparin/sulfatides, beta1 integrins and alpha-dystroglycan occupy different parts of the alpha2LG1-3 structure.

Biochemical characterization of the epithelial dystroglycan complex

Dystroglycan is a widely expressed extracellular matrix receptor that plays a critical role in basement membrane formation, epithelial development, and synaptogenesis. Dystroglycan was originally characterized in skeletal muscle as an integral component of the dystrophin glycoprotein complex, which is critical for muscle cell viability. Although the dystroglycan complex has been well characterized in skeletal muscle, there is little information on the structural composition of the dystroglycan complex outside skeletal muscle. Here we have biochemically characterized the dystroglycan complex in lung and kidney. We demonstrate that the presence of sarcoglycans and sarcospan in lung reflects association with dystroglycan in the smooth muscle. The smooth muscle dystroglycan complex in lung, composed of dystroglycan, dystrophin/utrophin, beta-, delta-, epsilon-sarcoglycan, and sarcospan, can be biochemically separated from epithelial dystroglycan, which is not associated with any of the known sarcoglycans or sarcospan. Similarly, dystroglycan in kidney epithelial cells is not associated with any of the sarcoglycans or sarcospan. Thus, our data demonstrate that there are distinct dystroglycan complexes in non-skeletal muscle organs as follows: one from smooth muscle, which is associated with sarcoglycans forming a similar complex as in skeletal muscle, and one from epithelial cells.

WW and EF hand domains of dystrophin-family proteins mediate dystroglycan binding

The membrane-spanning dystrophin glycoprotein complex mediates an indirect linkage between the actin-based cytoskeleton and the extracellular matrix. Although expressed by diverse cell types, genetic lesions of members of this complex result in muscular dystrophy phenotypes emphasizing the importance of these interactions in muscle cells. We have characterized interactions between dystrophin family members and dystroglycan: cytoskeletal and transmembrane components of the complex, respectively. Our results demonstrate that both the WW and EF hand domains of dystrophin and utrophin, an autosomal homologue of dystrophin, directly bind the cytoplasmic domain of dystroglycan. Furthermore, alpha-dystrobrevin, a more distantly related dystrophin family member which lacks a WW domain but contains the EF hand domain, binds dystroglycan. This is the first demonstration of a direct interaction between a dystrobrevin or utrophin and dystroglycan, and has implications for the organization of the dystrophin glycoprotein complex and the use of dystrophin homologues in muscular dystrophy therapy.

alpha-Dystroglycan is a laminin receptor involved in extracellular matrix assembly on myotubes and muscle cell viability

alpha-Dystroglycan (alpha-DG) is a laminin-binding protein and member of a glycoprotein complex associated with dystrophin that has been implicated in the etiology of several muscular dystrophies. To study the function of DG, C2 myoblasts were transfected stably with an antisense DG expression construct. Myotubes from two resulting clones (11F and 11E) had at least a 40-50% and 80-90% reduction, respectively, in alpha-DG but normal or near normal levels of alpha-sarcoglycan, integrin beta1 subunit, acetylcholine receptors (AChRs), and muscle-specific kinase (MuSK) when compared with parental C2 cells or three clones (11A, 9B, and 10C) which went through the same transfection and selection procedures but expressed normal levels of alpha-DG. Antisense DG-expressing myoblasts proliferate at the same rate as parental C2 cells and differentiate into myotubes, however, a gradual loss of cells was observed in these cultures. This loss correlates with increased apoptosis as indicated by greater numbers of nuclei with condensed chromatin and more nuclei labeled by the TUNEL method. Moreover, there was no sign of increased membrane permeability to Trypan blue as would be expected with necrosis. Unlike parental C2 myotubes, 11F and 11E myotubes had very little laminin (LN) on their surfaces; LN instead tended to accumulate on the substratum between myotubes. Exogenous LN bound to C2 myotubes and was redistributed into plaques along with alpha-DG on their surfaces but far fewer LN/alpha-DG plaques were seen after LN addition to 11F or 11E myotubes. These results suggest that alpha-DG is a functional LN receptor in situ which is required for deposition of LN on the cell and, further, implicate alpha-DG in the maintenance of myotube viability.

Laminin polymerization induces a receptor-cytoskeleton network

The transition of laminin from a monomeric to a polymerized state is thought to be a crucial step in the development of basement membranes and in the case of skeletal muscle, mutations in laminin can result in severe muscular dystrophies with basement membrane defects. We have evaluated laminin polymer and receptor interactions to determine the requirements for laminin assembly on a cell surface and investigated what cellular responses might be mediated by this transition. We found that on muscle cell surfaces, laminins preferentially polymerize while bound to receptors that included dystroglycan and alpha7beta1 integrin. These receptor interactions are mediated through laminin COOH-terminal domains that are spatially and functionally distinct from NH2-terminal polymer binding sites. This receptor-facilitated self-assembly drives rearrangement of laminin into a cell-associated polygonal network, a process that also requires actin reorganization and tyrosine phosphorylation. As a result, dystroglycan and integrin redistribute into a reciprocal network as do cortical cytoskeleton components vinculin and dystrophin. Cytoskeletal and receptor reorganization is dependent on laminin polymerization and fails in response to receptor occupancy alone (nonpolymerizing laminin). Preferential polymerization of laminin on cell surfaces, and the resulting induction of cortical architecture, is a cooperative process requiring laminin- receptor ligation, receptor-facilitated self-assembly, actin reorganization, and signaling events.

Conserved amphipathic helices near the N-terminus and C-terminus of the alpha subunit of cranin (dystroglycan)

Cranin (dystroglycan) is a ubiquitously expressed extracellular matrix receptor, synthesized as a single precursor, which is cleaved into an extracellular subunit (alpha) and a transmembrane subunit (beta). The primary sequence of cranin (dystroglycan) is known from cDNA cloning, and the protein has been strongly implicated in morphogenesis, cell adhesion and human disease. Nevertheless, the domain structure of the alpha subunit has not been well studied; although the protein binds to matrix proteins, to the beta subunit, to cell surfaces, and possibly to other membrane proteins such as sarcoglycans, the domains responsible for mediating these interactions remain unknown. Here I report computer analyses that identify two distinctive amphipathic alpha-helical regions near the N-terminus and C-terminus of the alpha subunit, which are conserved in all species for which sequence information is currently available. This finding should stimulate and guide experimental studies designed to understand how the alpha subunit is associated with the cell surface and with its various ligands.

A PDZ-containing scaffold related to the dystrophin complex at the basolateral membrane of epithelial cells

Membrane scaffolding complexes are key features of many cell types, serving as specialized links between the extracellular matrix and the actin cytoskeleton. An important scaffold in skeletal muscle is the dystrophin-associated protein complex. One of the proteins bound directly to dystrophin is syntrophin, a modular protein comprised entirely of interaction motifs, including PDZ (protein domain named for PSD-95, discs large, ZO-1) and pleckstrin homology (PH) domains. In skeletal muscle, the syntrophin PDZ domain recruits sodium channels and signaling molecules, such as neuronal nitric oxide synthase, to the dystrophin complex. In epithelia, we identified a variation of the dystrophin complex, in which syntrophin, and the dystrophin homologues, utrophin and dystrobrevin, are restricted to the basolateral membrane. We used exogenously expressed green fluorescent protein (GFP)-tagged fusion proteins to determine which domains of syntrophin are responsible for its polarized localization. GFP-tagged full-length syntrophin targeted to the basolateral membrane, but individual domains remained in the cytoplasm. In contrast, the second PH domain tandemly linked to a highly conserved, COOH-terminal region was sufficient for basolateral membrane targeting and association with utrophin. The results suggest an interaction between syntrophin and utrophin that leaves the PDZ domain of syntrophin available to recruit additional proteins to the epithelial basolateral membrane. The assembly of multiprotein signaling complexes at sites of membrane specialization may be a widespread function of dystrophin-related protein complexes.

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.

Significant role of laminin-1 in branching morphogenesis of mouse salivary epithelium cultured in basement membrane matrix

Mouse submandibular epithelium shows branching morphogenesis in mesenchyme-free conditions when covered with a basement membrane matrix (Matrigel) in medium supplemented with epidermal growth factor. In the present study, the role of laminin-1 (LN1), a major glycoprotein of Matrigel, in this culture system was defined. When the epithelium was cultured in a LN1-nidogen gel, the epithelium showed much branching, comparable to that observed with Matrigel. By electron microscopy, only a felt-like matrix was formed on the epithelial surface in the LN1-nidogen gel cultures, while an organized basal lamina structure was formed on the epithelial surface in direct or transfilter recombination cultures with mesenchyme. Next, the epithelium covered with Matrigel was cultured in medium containing either biologically active peptides from LN1, IKVAV-including peptide (2097-2108), AG10 (2183-2194), AG32 (2370-2381) or AG73 (2719-2730) from the alpha1 chain, or YIGSR-including peptide (926-933) from the beta1 chain. Only AG73 (RKRLQVQLSIRT from the alpha1 chain carboxyl-terminal globular domain) inhibited the epithelial branching in Matrigel. These results suggest that LN1-nidogen can support the branching morphogenesis of submandibular epithelium even if LN1-nidogen is not assembled into an intact basal lamina, and that the AG73 sequence is an important site on LN1, which interacts with submandibular epithelial cells.

Extensive but coordinated reorganization of the membrane skeleton in myofibers of dystrophic (mdx) mice

We used immunofluorescence techniques and confocal imaging to study the organization of the membrane skeleton of skeletal muscle fibers of mdx mice, which lack dystrophin. beta-Spectrin is normally found at the sarcolemma in costameres, a rectilinear array of longitudinal strands and elements overlying Z and M lines. However, in the skeletal muscle of mdx mice, beta-spectrin tends to be absent from the sarcolemma over M lines and the longitudinal strands may be disrupted or missing. Other proteins of the membrane and associated cytoskeleton, including syntrophin, beta-dystroglycan, vinculin, and Na,K-ATPase are also concentrated in costameres, in control myofibers, and mdx muscle. They also distribute into the same altered sarcolemmal arrays that contain beta-spectrin. Utrophin, which is expressed in mdx muscle, also codistributes with beta-spectrin at the mutant sarcolemma. By contrast, the distribution of structural and intracellular membrane proteins, including alpha-actinin, the Ca-ATPase and dihydropyridine receptors, is not affected, even at sites close to the sarcolemma. Our results suggest that in myofibers of the mdx mouse, the membrane- associated cytoskeleton, but not the nearby myoplasm, undergoes widespread coordinated changes in organization. These changes may contribute to the fragility of the sarcolemma of dystrophic muscle.

Characterization of the transmembrane molecular architecture of the dystroglycan complex in schwann cells

We have demonstrated previously 1) that the dystroglycan complex, but not the sarcoglycan complex, is expressed in peripheral nerve, and 2) that alpha-dystroglycan is an extracellular laminin-2-binding protein anchored to beta-dystroglycan in the Schwann cell membrane. In the present study, we investigated the transmembrane molecular architecture of the dystroglycan complex in Schwann cells. The cytoplasmic domain of beta-dystroglycan was co-localized with Dp116, the Schwann cell-specific isoform of dystrophin, in the abaxonal Schwann cell cytoplasm adjacent to the outer membrane. beta-dystroglycan bound to Dp116 mainly via the 15 C-terminal amino acids of its cytoplasmic domain, but these amino acids were not solely responsible for the interaction of these two proteins. Interestingly, the beta-dystroglycan-precipitating antibody precipitated only a small fraction of alpha-dystroglycan and did not precipitate laminin and Dp116 from the peripheral nerve extracts. Our results indicate 1) that Dp116 is a component of the submembranous cytoskeletal system that anchors the dystroglycan complex in Schwann cells, and 2) that the dystroglycan complex in Schwann cells is fragile compared with that in striated muscle cells. We propose that this fragility may be attributable to the absence of the sarcoglycan complex in Schwann cells.

A role for dystroglycan in basement membrane assembly

Basement membranes are composed of ordered arrays of characteristic extracellular matrix proteins, but little is known about the assembly of these structures in vivo. We have investigated the function of dystroglycan, a cell-surface laminin receptor expressed by cells contacting basement membranes in developing and adult tissues. We find that dystroglycan is required for the formation of a basement membrane in embryoid bodies. Our results further indicate that dystroglycanlaminin interactions are prerequisite for the deposition of other basement membrane proteins. Dystroglycan may exert its influence on basement membrane assembly by binding soluble laminin and organizing it on the cell surface. These data establish a role for dystroglycan in the assembly of basement membranes and suggest fundamental mechanisms underlying this process.

Detection of O-mannosyl glycans in rabbit skeletal muscle alpha-dystroglycan

alpha-Dystroglycan, which is a cell surface component of dystroglycan complex, is known to bind laminin in basal lamina of muscle cells and Schwann cells. We found previously that a novel O-glycan, Siaalpha2-3Galbeta1-4GlcNAcbeta1-2Man, is the major oligosaccharide in bovine peripheral nerve alpha-dystroglycan, and that this structure might mediate the binding of laminin. In order to determine whether this structure is specific for peripheral nerve alpha-dystroglycan or present on different forms of alpha-dystroglycan, we analyzed the structures of the sialylated O-glycans of rabbit skeletal muscle alpha-dystroglycan. Their structures were elucidated to be a mixture of a core 1 O-glycan and the same O-mannosyl glycan that we found in bovine peripheral nerve. These results indicate that alpha-dystroglycan in different species and tissues share a common structure of its major O-linked acidic carbohydrate, suggesting its relevance to the basic functional role of alpha-dystroglycan.

Laminin isoforms and epithelial development

Several different approaches suggest that basement-membrane assembly is important for epithelial development. Basement membranes contain isoforms of collagen IV, proteoglycans, and noncollagenous glycoproteins such as the laminins and nidogens. The expression and role of laminins for epithelial morphogenesis is reviewed. Laminins are large heterotrimeric proteins composed of alpha, beta, and gamma chains. Many major epithelial laminins and their receptors have been identified recently, and the extracellular protein-protein interactions that drive basement-membrane assembly are beginning to be understood. Three laminin alpha-chains are typically made by epithelial, alpha 1, alpha 3, and alpha 5. Three major epithelial heterotrimers can at present be distinguished--laminin-1 (alpha 1 beta 1 gamma 1), laminin-5 (alpha 3 beta 3 gamma 2), and laminin-10 (alpha 5 beta 1 gamma 1)--but other heterotrimers may exist in epithelia. Laminins containing either alpha 1 or alpha 3 chains are largely limited to epithelia, whereas the alpha 5 is also found in endothelial and muscle basement membranes, particularly in the adult. Some epithelial cell types express several laminin alpha-chains, so it is relevant to test how the different laminins affect epithelial cells. Laminins interact with integrin type of receptors on the cell surface, but binding to other proteins has also recently been demonstrated. Two important recent discoveries are the identification of dystroglycan as a major laminin receptor in muscle and epithelia, and nidogen as a high-affinity laminin-binding protein important for basement-membrane assembly. Antibody perturbation experiments suggest that these protein-protein interactions are important for epithelial morphogenesis.

Dystroglycan in development and disease

Our understanding of the structure and function of dystroglycan, a cell surface laminin/agrin receptor, has increased dramatically over the past two years. Structural studies, analysis of its binding partners, and targeted gene disruption have all contributed to the elucidation of the biological role of dystroglycan in development and disease. It is now apparent that dystroglycan plays a critical role in the pathogenesis of several muscular dystrophies and serves as a receptor for a human pathogen as well as being involved in early development, organ morphogenesis, and synaptogenesis.

The sarcoglycan complex in limb-girdle muscular dystrophy

The involvement of the sarcoglycan complex in the pathogenesis of muscular dystrophy is becoming increasingly clear. Sarcoglycan gene mutations lead to four forms of autosomal recessive limb-girdle muscular dystrophy. Recent progress has been made with the identification of novel mutations and their correlations with disease. Through this research, a better understanding the molecular pathogenesis of limb-girdle muscular dystrophy has been gained. Finally, animal models are now being used to study viral-mediated gene transfer for the future treatment of this disease.

Differential heparin sensitivity of alpha-dystroglycan binding to laminins expressed in normal and dy/dy mouse skeletal muscle

The alpha-dystroglycan binding properties of laminins extracted from fully differentiated skeletal muscle were characterized. We observed that the laminins expressed predominantly in normal adult rat or mouse skeletal muscle bound alpha-dystroglycan in a Ca2+-dependent, ionic strength-sensitive, but heparin-insensitive manner as we had observed previously with purified placental merosin (Pall, E. A., Bolton, K. M., and Ervasti, J. M. 1996 J. Biol. Chem. 271, 3817-3821). Rat skeletal muscle laminins partially purified by heparin-agarose affinity chromatography also bound alpha-dystroglycan without sensitivity to heparin. We also confirm previous studies of dystrophic dy/dy mouse skeletal muscle showing that the alpha2 chain of merosin is reduced markedly and that the laminin alpha1 chain is not up-regulated detectably. However, we further observed a quantitative decrease in the expression of laminin beta/gamma chain immunoreactivity in alpha2 chain-deficient dy/dy skeletal muscle and reduced alpha-dystroglycan binding activity in laminin extracts from dy/dy muscle. Most interestingly, the alpha-dystroglycan binding activity of residual laminins expressed in merosin-deficient dy/dy skeletal muscle was inhibited dramatically (69 +/- 19%) by heparin. These results identify a potentially important biochemical difference between the laminins expressed in normal and dy/dy skeletal muscle which may provide a molecular basis for the inability of other laminin variants to compensate fully for the deficiency of merosin in some forms of muscular dystrophy.

The relationship between perlecan and dystroglycan and its implication in the formation of the neuromuscular junction

Perlecan is a major heparan-sulfate proteoglycan (HSPG) within the basement membrane surrounding skeletal muscle fibers. The C-terminus of its core protein contains three globular domain modules which are also found in laminin and agrin, two proteins that bind to dystroglycan (DG, cranin) on the muscle surface with these modules. In this study, we examined whether perlecan can also bind to DG and is involved in signaling the formation of the neuromuscular junction (NMJ). By labeling cultured muscle cells with a polyclonal anti-perlecan antibody, this protein is found both within the extracellular matrix in a fibrillar network and at the cell surface in a punctate pattern. In Xenopus muscle cells, the cell-surface perlecan is precisely colocalized with DG. Both perlecan and DG are clustered at ACh receptor clusters induced by spinal neurons or by beads coated with HB-GAM, a heparin-binding growth factor. Blot overlay assays have shown that perlecan binds alpha-DG in a calcium and heparin-sensitive manner. Furthermore, perlecan is present in muscle lysate immunoprecipitated with an anti-DG antibody. Immunolabeling also showed colocalization between HB-GAM and perlecan and between HB-GAM and DG. These data suggest that perlecan is anchored to muscle surface via DG-dystrophin complex. Since DG is also a site of agrin binding, the neural agrin secreted by motoneurons during NMJ formation may compete with the pre-existing perlecan for cell surface binding. This competition may result in the presentation of perlecan-bound growth factors such as HB-GAM to effect synaptic induction.

Muscle-specific agrin isoforms reduce phosphorylation of AChR gamma and delta subunits in cultured muscle cells

The accumulation of nicotinic acetylcholine receptors (AChRs) at neuromuscular synapses is triggered by agrin, a protein that is synthesized by both nerve and muscle. Nerve-derived agrin, which contains an amino acid insert at a conserved splice site in the carboxy-terminal part of the protein, induces AChR aggregation and causes tyrosine phosphorylation of the AChR beta subunit. In contrast, agrin isoforms synthesized by muscle cells lack such an insert and have no effect on AChR distribution. In order to identify possible functional roles of muscle-derived agrin we have analyzed further the effect of various fragments of recombinant agrin on AChR phosphorylation. A carboxy-terminal fragment of muscle agrin, c95A0B0, reduced AChR gamma and delta subunit phosphorylation when added to C2C12 myotubes in culture. Although c95A0B0 had no effect on AChR beta subunit phosphorylation when added alone, it inhibited AChR beta subunit phosphorylation and AChR aggregation by the nerve-specific agrin isoform c95A4B8. We conclude that muscle-derived agrin can influence, both directly and indirectly, AChR phosphorylation. Such changes may play a role in the formation, maintenance, or function of the neuromuscular junction.

Caveolin-3 is not an integral component of the dystrophin glycoprotein complex

The dystrophin-glycoprotein complex is a multi-subunit protein complex that spans the muscle plasma membrane (sarcolemma) and forms a link between the intracellular cytoskeleton and the extracellular matrix. Caveolin-3, the muscle specific form of caveolin, is also a major structural and regulatory integral membrane protein found at the sarcolemma. Oligomers of caveolin-3 form the structural framework for small membrane pockets known as caveolae. We directly examined whether caveolin-3 is an integral component of the dystrophin-glycoprotein complex by examining four common biochemical and cellular properties of proteins integrally bound to the dystrophin-glycoprotein complex. We found that caveolin-3 de-enriches with partial purification of the dystrophin-glycoprotein complex although a small amount of caveolin-3 is present. Sucrose gradient fractionation and laminin affinity chromatography completely separate this residual caveolin-3 from the core components of the dystrophin-glycoprotein complex. We also show that caveolin-3 expression at the sarcolemma is not reduced in patients with primary mutations in either dystrophin or the sarcoglycans. This data demonstrates that localization of caveolin-3 to the sarcolemma occurs independently of the dystrophin-glycoprotein complex and that caveolin-3 is not an integral component of the dystrophin-glycoprotein complex.

Structural analysis and proteolytic processing of recombinant G domain of mouse laminin alpha2 chain

Four individual LG modules from the C-terminus of the laminin alpha2 chain (LG1, LG2, LG4 and LG5) and combinations of these modules were prepared as recombinant products from transfected mammalian cells. This demonstrated that LG modules represent autonomously folding protein domains. Successful production depended on proper alignment of module borders and required a sequence correction at the C-terminus which added an extra cysteine. The LG modules were glycosylated and shown by electron microscopy to have a globular shape, indicating proper folding. Evidence is provided for the splicing of a 12 bp exon in LG2, although this did not impair folding. Proteolytic cleavage at the C-terminus of a basic sequence was observed close to the N-terminus of LG3. A similar processing also occurs in tissue-derived laminin-2 and -4 which contain the alpha2 chain.

Distribution of dystroglycan in normal adult mouse tissues

Dystroglycan is a cell surface protein which, in muscle, links the extracellular matrix protein laminin-2 to the intracellular cytoskeleton. Dystroglycan also binds laminin-1 and the binding occurs via the E3 fragment of laminin-1. Recently, it was found that dystroglycan is expressed in developing epithelial cells of the kidney. Moreover, antibodies against dystroglycan can perturb epithelial development in kidney organ culture. Therefore, dystroglycan may be an important receptor for cell-matrix interactions in non-muscle tissues. However, information about the tissue distribution of dystroglycan is limited, especially in adult tissues. Here we show that dystroglycan is present in epithelial cells in several non-muscle organs of adult mice. Dystroglycan is enriched towards the basal side of the epithelial cells that are in close contact with basement membranes. We suggest that dystroglycan is involved in linking basement membranes to epithelial and muscle cells. Dystroglycan may be important for the maintenance of tissue integrity.

Beta-dystroglycan localization in the photoreceptor and Müller cells in the rat retina revealed by immunoelectron microscopy

beta-Dystroglycan (beta-DG) is a dystrophin-associated glycoprotein that is expressed in skeletal muscle and other tissues. In the retina, dystrophin is present in the outer plexiform layer (OPL), where it is enriched under the photoreceptor cell membrane. In this study we determined the immunocytochemical localization of beta-DG at both light and electron microscopic levels. beta-DG immunoreactivity was detected at the inner limiting membrane, OPL, and around blood vessels. Immunoelectron microscopy detected beta-DG immunoreactive products under the photoreceptor cell membrane, which are the same regions of dystrophin localization. In addition, beta-DG was detected under the Müller cell membrane that is attached to the paravitreous or perivascular basement membrane. Our results suggest that beta-DG may interact with dystrophin in photoreceptor membranes. However, beta-DG-related interactions between Müller cells and basement membranes appear to be independent of dystrophin and raise the possibility that beta-DG interacts with other molecules. We speculate that beta-DG plays a role in maintaining the structural relationship between photoreceptor and bipolar cells or between Müller cells and basement membranes.

Bidirectional signaling between sarcoglycans and the integrin adhesion system in cultured L6 myocytes

The rat L6 skeletal muscle cell line was used to study expression of the dystrophin-containing glycoprotein complex and its interaction with the integrin system involved in the cell-matrix adhesion reaction. A complex of dystrophin and its associated proteins was fully expressed in L6 myotubes, from which anti-dystrophin or anti-alpha-sarcoglycan co-precipitated integrin alpha 5 beta 1 and other focal adhesion-associated proteins vinculin, talin, paxillin, and focal adhesion kinase. Immunostaining and confocal microscopy revealed that dystrophin, alpha-sarcoglycan, integrin alpha 5 beta 1, and vinculin exhibited overlapping distribution in the sarcolemma, especially at focal adhesion-like, spotty structures. Adhesion of cells to fibronectin- or collagen type I-coated dishes resulted in induction of tyrosine phosphorylation of alpha- and gamma-sarcoglycans but not beta-sarcoglycan. The same proteins were also tyrosine-phosphorylated when L6 cells in suspension were exposed to Arg-Gly-Asp-Ser peptide. All of these tyrosine phosphorylations were inhibited by herbimycin A. On the other hand, treatment of L6 myotubes with alpha- and gamma-sarcoglycan antisense oligodeoxynucleotides resulted in complete disappearance of alpha- and gamma-sarcoglycans and in significant reduction of levels of the associated focal adhesion proteins, which caused about 50% reduction of cell adhesion. These results indicate the existence of bidirectional communication between the dystrophin-containing complex and the integrin adhesion system in cultured L6 myocytes.

Agrin is a high-affinity binding protein of dystroglycan in non-muscle tissue

Agrin is a basement membrane-associated proteoglycan that induces the formation of postsynaptic specializations at the neuromuscular junction. This activity is modulated by alternative splicing and is thought to be mediated by receptors expressed in muscle fibers. An isoform of agrin that does not induce postsynaptic specializations binds with high affinity to dystroglycan, a component of the dystrophin-glycoprotein complex. Transcripts encoding this agrin isoform are expressed in a variety of non-muscle tissues. Here, we analyzed the tissue distribution of agrin and dystroglycan on the protein level and determined their binding affinities. We found that agrin is most abundant in lung, kidney, and brain. Only a little agrin was detected in skeletal muscle, and no agrin was found in liver. Dystroglycan was highly expressed in all tissues examined except in liver. In a solid-phase radioligand binding assay, agrin bound to dystroglycan from lung, kidney, and skeletal muscle with a dissociation constant between 1.8 and 2.2 nM, while the affinity to brain-derived dystroglycan was 4.6 nM. In adult kidney and lung, agrin co-purified and co-immunoprecipitated with dystroglycan, and both molecules were co-localized in embryonic tissue. These data show that the agrin isoform expressed in non-muscle tissue is a high-affinity binding partner of dystroglycan and they suggest that this interaction, like that between laminin and dystroglycan, may be important for the mechanical integrity of the tissue.

Localization of heparin binding activity in recombinant laminin G domain

Basement membrane laminin (laminin-1) is a multidomain glycoprotein that interacts with itself, heparin and cells. The interaction with heparin/heparan sulfate proteglycans is thought to be important for the architectural formation of basement membranes and adhesion to cells. The major heparin binding site has been known to reside in the long arm globular domain (G domain). The G domain is in turn subdivided into five subdomains (G1-G5). In order to localize the heparin binding regions further, recombinant G domains (rG and rG5) were expressed in Sf9 insect cells using baculovirus expression vector. By the limited proteolysis of recombinant G domains followed by either heparin affinity HPLC or overlay with radiolabeled heparin, the relative affinity of each subdomain to heparin was assigned as G1>G2 = G4>G5>G3, such that G1 bound strongly and G3 not at all. Since the activity in G1-G3 is cryptic in intact laminin long arm [Sung, U., O'Rear, J. J. & Yurchenco, P. D. (1993) J. Cell Biol. 123, 1255-1268], the active heparin binding site of G domain appears to be located in G4 and proximal G5.

The laminin alpha2-chain short arm mediates cell adhesion through both the alpha1beta1 and alpha2beta1 integrins

Laminin-2, a heterotrimer composed of alpha2, beta1, and gamma1 subunits, is the primary laminin isoform found in muscle and peripheral nerve and is essential for the development and stability of basement membranes in these tissues. Expression of a domain VI-truncated laminin alpha2-chain results in muscle degeneration and peripheral nerve dysmyelination in the dy2J dystrophic mouse. We have expressed amino-terminal domains VI through IVb of the laminin alpha2-chain, as well as its laminin-1 alpha1-chain counterpart, to identify candidate cell-interactive functions of this critical region. Using integrin-specific antibodies, recognition sites for the alpha1beta1 and alpha2beta1 integrins were identified in the short arms of both laminin alpha1- and alpha2-chain isoforms. Comparisons with a beta-alpha chimeric short arm protein possessing beta1-chain domain VI further localized these activities to alpha-chain domain VI. In addition, we found that the laminin alpha2-chain short arm supported neurite outgrowth independent of other laminin-2 subunits. A heparin/heparan sulfate binding activity was also localized to this region of the laminin alpha2 subunit. These data provide the first evidence that domain VI of the laminin alpha2-chain mediates interactions with cell surface receptors and suggest that these integrin and heparin binding sites, alone or in concert, may play an important role in muscle and peripheral nerve function.

Laminin-induced acetylcholine receptor clustering: an alternative pathway

The induction of acetylcholine receptor (AChR) clustering by neurally released agrin is a critical, early step in the formation of the neuromuscular junction. Laminin, a component of the muscle fiber basal lamina, also induces AChR clustering. We find that induction of AChR clustering in C2 myotubes is specific for laminin-1; neither laminin-2 (merosin) nor laminin-11 (a synapse-specific isoform) are active. Moreover, laminin-1 induces AChR clustering by a pathway that is independent of that used by neural agrin. The effects of laminin-1 and agrin are strictly additive and occur with different time courses. Most importantly, laminin- 1-induced clustering does not require MuSK, a receptor tyrosine kinase that is part of the receptor complex for agrin. Laminin-1 does not cause tyrosine phosphorylation of MuSK in C2 myotubes and induces AChR clustering in myotubes from MuSK-/- mice that do not respond to agrin. In contrast to agrin, laminin-1 also does not induce tyrosine phosphorylation of the AChR, demonstrating that AChR tyrosine phosphorylation is not required for clustering in myotubes. Laminin-1 thus acts by a mechanism that is independent of that used by agrin and may provide a supplemental pathway for AChR clustering during synaptogenesis.

Down-regulation of the amyloid protein precursor of Alzheimer's disease by antisense oligonucleotides reduces neuronal adhesion to specific substrata

The hallmark of Alzheimer's disease is the cerebral deposition of amyloid which is derived from the amyloid precursor protein (APP). The function of APP is unknown but there is increasing evidence for the role of APP in cell-cell and/or cell-matrix interactions. Primary cultures of murine neurons were treated with antisense oligonucleotides to down-regulate APP. This paper presents evidence that APP mediates a substrate-specific interaction between neurons and extracellular matrix components collagen type I, laminin and heparan sulphate proteoglycan but not fibronectin or poly-L-lysine. It remains to be determined whether this effect is the direct result of APP-matrix interactions, or whether an intermediatry pathway is involved.

Tissue-specific heterogeneity in alpha-dystroglycan sialoglycosylation. Skeletal muscle alpha-dystroglycan is a latent receptor for Vicia villosa agglutinin b4 masked by sialic acid modification

Because the polypeptide core of alpha-dystroglycan is encoded by a single gene, the difference in apparent molecular mass between alpha-dystroglycans expressed in various tissues is presumably due to differential glycosylation. However, little is presently known about the tissue-specific differences in alpha-dystroglycan glycosylation and whether these modifications may confer functional variability to alpha-dystroglycan. We recently observed that laminin-1 binding to skeletal muscle alpha-dystroglycan was dramatically inhibited by heparin, whereas the binding of commercial merosin to skeletal muscle alpha-dystroglycan was only marginally inhibited (Pall, E. A., Bolton, K. M., and Ervasti, J. M. (1996) J. Biol. Chem. 3817-3821). In contrast to 156-kDa skeletal muscle alpha-dystroglycan, both laminin-1 and merosin binding to 120-kDa brain alpha-dystroglycan were sensitive to heparin. We have now examined the laminin binding properties of 140-kDa alpha-dystroglycan purified from cardiac muscle and observed that like skeletal muscle alpha-dystroglycan, heparin inhibited cardiac alpha-dystroglycan binding to laminin-1, but not to merosin. On the other hand, cardiac and brain alpha-dystroglycans could be distinguished from skeletal muscle alpha-dystroglycan by their reactivity with the terminal GalNAc-specific lectin Vicia villosa agglutinin. Interestingly, skeletal muscle alpha-dystroglycan became reactive with V. villosa agglutinin upon digestion with sialidase from Clostridium perfringens, Arthrobacter neurofaciens, or Streptococcus, but not Vibrio cholerae or Newcastle disease virus sialidase. While none of the sialidase treatments affected the laminin binding properties of alpha-dystroglycan, the sum of our results suggests that skeletal muscle alpha-dystroglycan contains a novel sialic acid residue linked alpha2-6 to GalNAc. These properties are also consistent with the cellular characteristics of a GalNAc-terminated glycoconjugate recently implicated in neuromuscular synaptogenesis. Thus, variations in alpha-dystroglycan sialoglycosylation may prove as useful markers to further elucidate the role of alpha-dystroglycan glycoforms in different tissues and perhaps within a single cell type.

Laminins of the adult mammalian CNS; laminin-alpha2 (merosin M-) chain immunoreactivity is associated with neuronal processes

Laminins are glycoproteins with three subunits, i.e. a longer alpha chain, a shorter beta chain and a shorter gamma chain. Well-characterized laminins are laminin-1 (EHS laminin; alpha1-beta1-gamma1), laminin-2 (merosin; alpha2-beta1-gamma1), laminin-3 (alpha1-beta2-gamma1) and laminin-4 (alpha2-beta2-gamma1). The present study shows that in the adult mammalian CNS (rat, rabbit, pig and monkey) alpha2 chain immunoreactivity is associated most evidently with neuronal fibers and punctate, potentially synaptic, structures of limbic brain regions. Third ventricle tanycytes and ensheathing cells of the olfactory nerve also express intense alpha2 chain immunoreactivity. Immunostaining for gamma1 chain is present throughout the central nervous system (CNS) in essentially all neuronal cell bodies and their most proximal processes. Immunoreactivity for all chains investigated (alpha1, alpha2, beta1, beta2 and gamma1) were present around blood vessels, especially evident in lightly fixed tissues. The finding that, other than blood vessels, neurons and other structures exhibited immunoreactivity for only one or two (and not three) chains, suggests that variant forms of laminin with yet undiscovered chains or other configurations than the heterotrimeric form are present in the CNS. The association of alpha2-like immunoreactivity with neuronal fibers and synaptic structures is of great interest in light of the known neurite-promoting and cell attachment activities of laminin-2.

Laminin-alpha2 chain-like antigens in CNS dendritic spines

The laminin-alpha2 chain is a component of brain capillary basement membranes and appears also to be present in neurons of rat, rabbit, pig and non-human primate brain as evidenced by immunohistochemistry. In the present study, we have further characterized this very distinct neuronal laminin-alpha2 chain-like immunoreactivity in the hippocampus of various species. Immunoelectron microscopy with poly- and monoclonal antibodies to the laminin-alpha2 chain G-domain localized laminin-alpha2 chain immunoreactivity in adult rat and rabbit hippocampus to dendritic processes, primarily to dendritic spines. In the developing rat hippocampus, spine-associated laminin-alpha2 chain-like immunoreactivity first appeared at a time corresponding to that of active synaptogenesis. After an entorhinal cortex lesion in adult rats, the time course of denervation-induced loss and reactive reappearance of spines in the molecular layer of the dentate gyrus was correlated closely to the loss and reappearance of laminin-alpha2 chain immunoreactivity. Immunoblot analysis of normal adult rat, rabbit and pig brain revealed a protein similar in size to the reported 80-kDa laminin-alpha2 chain fragment of human placenta as well as 140/160-kDa proteins. These results suggest the presence of proteins with antigenic homology to the laminin-alpha2 chain and/or laminin-alpha2 isoforms in dendrites and dendritic spines in selected areas of the brain, predominately in the hippocampus and other limbic structures. Given the adhesion and neurite promoting functions of laminins, it is possible that neuronal laminin-alpha2 chain-like proteins play a role in synaptic function and plasticity in the CNS.

Cloning and expression of a cDNA encoding a nonintegrin laminin-binding protein from Echinococcus granulosus with localization of the laminin-binding domain

We have isolated a cDNA from the hydatid tapeworm, Echinococcus granulosus, encoding a protein that binds laminin. This is the first report of a helminth parasite laminin-binding protein and the first description of a cDNA encoding a laminin-binding protein from a parasite. The cDNA clone (egmo3) was isolated from an E. granulosus protoscolex cDNA expression library, and identified on the basis of sequence homology to the nonintegrin mammalian metastasis-associated 67-kDa laminin receptor (67-LR). The amino acid sequence predicted from the cDNA sequence is 268 residues long with a calculated molecular mass of 29.9 kDa. Southern blot analysis suggested that many copies of the gene may occur in the E. granulosus genome. A Northern blot revealed that the gene is expressed as a single transcript of approximately 1 kb consistent with the size of the cDNA insert. Antibodies raised to the purified protein interacted with a 30 kDa protein in whole E. granulosus protoscoleces. A Western blot of the purified and refolded recombinant protein specifically bound 125I-labelled laminin, as did a synthetic peptide derived from the inferred amino acid sequence of egmo3 which is similar in homology to peptide G, the active ligand-binding site of 67-LR. We also isolated the 3' end of the cDNA encoding the homologous protein from the closely related species, E. multilocularis. The polypeptide encoded by egmo3 also shares substantial identity with the acidic class of ribosomal proteins which are involved in protein synthesis. As such, the egmo3 protein may be multifunctional in E. granulosus, acting as a laminin-binding molecule but also playing a role in cell division and growth.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

Agrin gene expression in mouse somatosensory cortical neurons during development in vivo and in cell culture

Agrin is an extracellular matrix protein involved in the formation of the postsynaptic apparatus of the neuromuscular junction. In addition to spinal motor neurons, agrin is expressed by many other neuronal populations throughout the nervous system. Agrin's role outside of the neuromuscular junction, however, is poorly understood. Here we use the polymerase chain reaction to examine expression and alternative splicing of agrin in mouse somatosensory cortex during early postnatal development in vivo and in dissociated cell culture. Peak levels of agrin gene expression in developing cortex coincide with ingrowth of thalamic afferent fibres and formation of thalamocortical and intracortical synapses. Analysis of alternatively spliced agrin messenger RNA variants shows that greater than 95% of all agrin in developing and adult somatosensory cortex originates in neurons, including isoforms that have little or no activity in acetylcholine receptor aggregation assays. The levels of expression of "active" and "inactive" isoforms, however, are regulated during development. A similar pattern of agrin gene expression is also observed during a period when new synapses are being formed between somatosensory neurons growing in dissociated cell culture. Changes in agrin gene expression, observed both in vivo and in vitro, are consistent with a role for agrin in synapse formation in the central nervous system.

Laminin-alpha2 but not -alpha1-mediated adhesion of human (Duchenne) and murine (mdx) dystrophic myotubes is seriously defective

It has been suggested that alpha-dystroglycan links the dystrophin-associated protein complex and extracellular matrix and that the absence of dystrophin and alpha-dystroglycan in Duchenne muscular dystrophy (DMD) may lead to the breakdown of this linkage. In the present study, myotubes from DMD patients and murine X-linked muscular dystrophic mice (mdx) were used to measure their adhesive force to the physiological laminin-alpha2 substrate, and it was found that the dystrophic myotubes were selectively unable to sustain adhesion. However, normal and dystrophic myotubes attached equally well to the laminin-alpha1 substrate. As far as we know, this is the first experimental evidence that the absence of dystrophin causes the complete loss of a still unknown laminin-alpha2-dependent adhesion force, therefore suggesting that the primary consequence of Duchenne dystrophy consists of the loss of an authentic mechanical linkage at the level of the alpha-dystroglycan/basal lamina interface.