Department of Neuroscience, Glycobiology Research and Training Center, University of California, San Diego, School of Medicine, La Jolla, CA 92093-0691
accepted on April 24, 2003
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Abstract |
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Key words: agrin / dystroglycan / integrin / laminin / muscular dystrophy
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Introduction |
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Muscular dystrophy and the dystrophinglycoprotein complex |
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Dystroglycan is a highly glycosylated matrix-binding protein that is posttranslationally cleaved into two peptides,
and ß dystroglycan (Ervasti and Campbell, 1991
).
Dystroglycan resides on the outer surface of the myofiber membrane and binds tightly but noncovalently to the ß chain, which is a transmembrane protein (Ervasti and Campbell, 1991
). ß Dystroglycan, in turn, binds via its cytoplasmic domain to dystrophin (Chung and Campanelli, 1999
), a large protein that associates with cytoplasmic and transmembrane members of the dystrophinglycoprotein complex and with actin (Ervasti and Campbell, 1993
). A number of other transmembrane proteins and cytoplasmic proteins are associated with the dystrophinglycoprotein complex, including sarcoglycans, sarcospan, dystrobrevins, syncoilin, and dysbindin (Henry and Campbell, 1999
; Benson et al., 2001
; Poon et al., 2002
).
Mutations that lead to loss of one of these proteins result in the loss of other members of the dystrophinglycoprotein complex along the sarcolemmal membrane, and deficits in most of these proteins cause muscular dystrophy. Loss of dystrophin results in Duchenne muscular dystrophy (DMD), loss of sarcoglycans results in forms of Limb-Girdle muscular dystrophy, and loss of laminin results in merosin (laminin-2)-dependent congenital muscular dystrophy (for a review, see Durbeej et al., 1998; Blake et al., 2002
). In addition, loss of dystrobrevins results in muscular dystrophy in mice (Grady et al., 1999
); defects in other proteins, including collagen VI, titin, desmin, and caveolin-3, cause severe myopathies in humans (for a review, see Carlsson and Thornell, 2001
; Nishino and Ozawa, 2002
). In contrast to these genes, null mutations in dystroglycan (Dag1) are lethal at an early embryonic stage in mice (Williamson et al., 1997
). Thus complete loss of dystroglycan protein in humans may not allow viability. Chimeric skeletal muscles made with dystroglycan-deficient embryonic stem cells do have muscular dystrophy. Thus muscular dystrophy can arise from the absence of dystroglycan in skeletal muscle (Cote et al., 1999
). As will be described in detail later in this article, a host of diseases have now been linked to the aberrant posttranslational modification of
dystroglycan, and all of these disorders involve changes in dystroglycan glycosylation. Thus a large number of genetic and biochemical studies have shown that members of the dystrophinglycoprotein complex form an essential scaffold that links the extracellular matrix to the actin cytoskeleton. Moreover, studies suggest that glycans on
dystroglycan are essential for the formation of this complex.
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The neuromuscular junction and the utrophinglycoprotein complex |
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Thus an additional protein scaffold exists at the neuromuscular junction that is very similar in structure to the dystrophinglycoprotein complex but contains novel proteins. This likely is associated with some proteins that have been glycosylated with uniquely synaptic glycans. These include synaptic forms of laminin, which may interact with a uniquely glycosylated form of dystroglycan. ß Dystroglycan, in turn, can interact with utrophin, which binds to actin filaments via protein motifs that are distinct from those used by dystrophin (Rybakova et al., 2002
). The expression of the utrophinglycoprotein complex at the neuromuscular junction is independent of the extrasynaptic dystrophinglycoprotein complex. In the mdx mouse (a model for DMD), dystrophin and many dystrophin-associated glycoproteins are absent or very reduced along the extrasynaptic muscle membrane; however, utrophin and its associated glycoproteins are still expressed at the neuromuscular junction (Matusumura et al., 1992
). Similarly, mice lacking utrophin maintain expression of dystrophin and the dystrophin-associated glycoproteins in the extrasynaptic membrane (Grady et al., 1997
; Deconinck et al., 1997
).
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Glycosylation of ![]() |
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Preliminary characterization of dystroglycan glycosylation was primarily based on its migratory pattern on sodium dodecyl sulfate gels. Early studies by Campbell and colleagues demonstrated that dystroglycan is a heavily glycosylated protein that migrates in a heterogeneous pattern on such gels (Ibraghimov-Beskrovnaya et al., 1992
). The dystroglycan gene encodes about a 70-kD
chain polypeptide, however, this protein migrates at 120 kDa in brain and peripheral nerve, 140 kDa in cardiac muscle, 150 kDa in lung, 156 kDa in skeletal muscle, and 190 kDa in Torpedo electric organ (see Durbeej et al., 1998
; Henry and Campbell, 1999
). ß Dystroglycan, in contrast, migrates as a 43-kDa protein in all of these tissues, though it, too, clearly is a glycoprotein. The molecular weight of
dystroglycan from chick skeletal muscle differs as development progresses, suggesting that glycosylation is also regulated during development within the same tissue (Leschziner et al., 2000
). Likewise, glycosylation of
dystroglycan is affected by muscle denervation (Leschziner et al., 2000
). Therefore glycosylation of
dystroglycan in skeletal muscle is also regulated by neural activity. Because of its migration pattern on gels and because the primary sequence contains two potential sites for glycosaminoglycan addition,
dystroglycan was originally thought to be a proteoglycan. However, a number of studies using both enzymatic (glycosaminoglycan lyase) and chemical methods (nitrous acid) failed to prove the presence of any glycosaminoglycan (GAG) side chains (Smalheiser and Schwartz, 1987
; Smalheiser, 1993
; Ervasti and Campbell, 1993
; Yamada et al., 1996
; Gee et al., 1993
; Bowe et al., 1994
). The definition absence of GAGs, however, was based on a lack of altered migration on one-dimensional sodium dodecyl sulfatepolyacrylamide gel electrophoresis gels. Therefore, it is still possible that short GAG chains that terminate prior to the GAG elongation reactions (see Esko and Selleck, 2002
) decorate these sites, as these would not have greatly altered the molecular weight of the protein.
Dystroglycan contains several N-linked glycans, and enzymatic removal of N-linked glycans alters its molecular weight by about 4 kDa (Ervasti and Campbell, 1991
). The remainder of dystroglycan glycosylation presumably occurs on O-linked sites in the serine-theronine-rich "mucin" domain. This mucin region is located roughly in the middle third of the protein (Figure 3). Electron microscopy studies show that
dystroglycan has a dumbbell-like appearance, with a globular N-terminal domain, a long stalk-like region in the mucin domain, and a globular C-terminal domain (Brancaccio et al., 1995
). Analysis of the primary sequence suggests that the O-linked glycosylation may be particularly complex. There are about 50 serines and threonines that could be glycosylated in the mucin region of roughly 170 amino acids. Wilson et al. (1991)
have compared O-glycosylated sequences in many proteins and have identified that a proline is very often found at the -1 or +3 position in proteins where a single O-linked glycan is present on serine or threonine.
Dystroglycan has 18 P(-1) sites and 8 P(+3) sites. All but four of these potential sites reside in the mucin region (amino acids 317 to 488 based on sequence reported in Ibraghimov-Beskrovnaya et al., 1991). Surprisingly, the extracellular domain of ß dystroglycan also has a hot spot for O-linked glycosylation with two P(-1) sites and two P(+3) sites in the span of 11 amino acids. In proteins where multiple O-linked sites are present, however, there appears to be no propensity for proline spacing as is found at singly glycosylated sites (Wilson et al., 1991
). Indeed, there are also several small repeating sequences of serine and/or threonines that are devoid of prolines in the mucin domain of
dystroglycan. Because all proteins used for comparative sequence analysis were likely glycosylated with O-linked GalNAc, it may be that the unusual proliferation of proline-spaced sites may define sites for O-linked mannose on this protein (Sasaki et al., 1998
; Smalheiser et al., 1998
; Chiba et al., 1997
). Other repeating sequences are also present and may or may not be important, including a V(x/T)(x/T)P motif, which is present five times in the
dystroglycan sequence. There are no apparent consensus sites for O-linked fucosylation or glucosylation.
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In all three glycan sequencing studies, carbohydrate binding lectins were used to purify the material in question. Sasaki et al. (1998) and Chiba et al. (1997)
used wheat germ agglutinin (WGA) chromatography, and Smalheiser et al. (1998)
used Concanavalin A and Jacilin agglutinin chromatography. In all cases, laminin-1 affinity chromatography was also used. Although the demonstration of these sequences is extremely important for understanding
dystroglycan function, it is entirely possible that some glycan chains were not identified because glycan binding was used in the purification process (Figure 3). For example,
dystroglycan from skeletal muscles of mice overexpressing the cytotoxic T cell GalNAc transferase (Galgt2) binds well to ßGalNAc binding lectins, such as Wisteria floribunda agglutinin, but that this material binds poorly, if at all, to WGA (Nguyen et al., 2002
).
Similarly, aside from the presence of Lewis x in the brain, no tissue-specific heterogeneity in dystroglycan glycosylation was identified in these articles. It would be surprising if such differences did not exist. Lectin and antibody blotting suggest that brain and peripheral nerve dystroglycan have terminal GalNAcs and express the HNK-1 epitope (Yamada et al., 1996
; Smalheiser and Kim, 1995
). Dystroglycan isolated from skeletal muscle, in contrast, contains few if any terminal GalNAcs (Xia et al., 2002
), though some are masked by sialic acid (Ervasti et al., 1997
), and does not express HNK-1 (Ervasti and Campbell, 1993)
.
Dystroglycan also has very different affinities for different forms of laminin (Talts et al., 1999
, 2000
). Therefore, use of laminin-1 may have also eliminated glycoforms that do not bind well to this kind of laminin. This is an especially sobering thought, given that laminin-1, though copiously expressed from the Engelbreth-Holm-Swarm (EHS) tumor (and therefore easily purified), has a fairly restricted distribution in the nervous system and skeletal muscle (Lentz et al., 1997
).
The finding of O-linked mannose is the first description of this linkage on a purified mammalian glycoprotein, but it has been known for several decades that O-linked mannose exists in mammals. O-linked mannose was reported to be present in mammalian brain by Finne et al. (1979) long before its potential value could be fully appreciated. Several studies have confirmed and expanded on this result (Yuen et al., 1997
; Chai et al., 1999
; Kogelberg et al., 2001
). O-linked mannose has been reported to be present on HNK-1 structures in rabbit brain (Yuen et al., 1997
), and structures identical to those reported to be on
dystroglycan in sheep brain (Smalheiser et al., 1998
) were shown to be abundant in rabbit brain (Chai et al., 1999
). Here, glycans were released from pronase-digested lipid/GAG-depleted brain proteins and purified by ion-exchange chromatography. Fractions were then analyzed by liquid secondary ion mass spectrometry. After further fractionation of O-linked pools and derivitization, sequences were identified by gas chromatography mass spectrometry. The authors claim that by this method, 30% of the O-linked glycan in brain is O-linked via mannose (Chai et al., 1999
). Interestingly, in addition to identifying NeuAc
2,3Galß1,4GlcNAcß1,2Man
-O and Gal1,4[Fuc
1,3]GlcNAcß1,2Man
-O structures that had been reported to be present on
dystroglycan in sheep brain (Smalheiser et al., 1998
), they also found 1,6-disubstituted Man-
-O structures, including NeuAc
2,3Galß1, 4GlcNAcß1,2[NeuAc
2,3Galß1,4GlcNAcß1,6]Man
-O (and desialylated variants thereof). Although not yet reported, these branched structures might exist on
dystroglycan. If they do not, then such structures must be present on other O-linked glycoproteins. If Chai et al. (1999)
are correct in their estimate that 30% of O-linked glycans in brain are linked via mannose, then other proteins with O-linked mannose must exist because
dystroglycan is not likely to represent a third of all O-linked glycoprotein in the brain.
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Role of ![]() |
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Laminin
Neither the N- nor C-terminal globular domains of dystroglycan that flank the mucin region bind laminin with high affinity (Brancaccio et al., 1997
; Sciandra et al., 2001
; DiStasio et al., 1999
). Therefore, laminin binding requires the mucin region. It is important to note, however, that there is no direct evidence that O-linked glycopeptides derived from
dystroglycan bind directly to any of these ligands. In addition, binding studies often use dystroglycan that has been isolated from different tissue sources or species. Therefore, comparing such studies is problematic.
Studies on native proteins
Many different forms of laminin exist, and many of these have been used to study binding to dystroglycan. Native laminin trimers (laminin-1, ß1,
1,
1; laminin-2, ß1,
2,
1) as well as recombinantly produced laminin G domains (
1,
2,
4, and
5) have all been used. In skeletal muscle,
1 is only transiently expressed during embryonic development, while
2 is the principal extrasynaptic
chain (Sanes et al., 1990
). Laminin
4 and
5 both are both expressed around myofibers in the perinatal period but become confined to the neuromuscular and myotendinous junctions at early postnatal ages (Patton et al., 1997
).
Using laminin-1 overlays to dystroglycan purified from rabbit skeletal muscle, Ervasti and Campbell (1993)
demonstrated that glycosylation was required for laminin binding. Several lines of evidence were provided. First, chemical deglycosylation of
dystroglycan with trifluoromethanesulfonic acid (TFMS) eliminated binding. Second, a monoclonal antibody that requires the presence of glycans on
dystroglycan for binding, IIH6, also blocked laminin binding. IIH6 also blocks laminin-1 binding to muscle
dystroglycan from Torpedo (Sugiyama et al., 1994
) and bovine peripheral nerve (Matsumura et al., 1997
). Interestingly, neuraminidase digestion of
dystroglycan, which removed sialic acids, did not inhibit binding, nor did VIA4-1, a second carbohydrate-dependent monoclonal antibody. Thus there appears to be some specificity to the type of glycan on
dystroglycan that binds laminin-1, and it does not appear to require sialic acid.
In contrast to skeletal muscle, binding of laminin-1 or laminin-2 to dystroglycan isolated from bovine peripheral nerve is eliminated by neuraminidase treatment. Moreover, addition of soluble sialic acid or NeuAc
2,3Galß1,4GlcNAc inhibited binding (Yamada et al., 1996
). Similar results were found when studying the adhesion of RT4 Scwhannoma cells to laminin (Matsumura et al., 1997
). Both neuraminidase and soluble sialic acid blocked these effects, as did IIH6 (but not VI4A-1) (Matsumura et al., 1997
). It is odd, however, that neuraminidase digestion of muscle dystroglycan does not inhibit binding of the blocking antibody IIH6, nor does it inhibit laminin-1 binding (Ervasti and Campbell, 1993
). This would suggest that sialic acid is not important for binding the muscle form, but it is important for binding to the form in peripheral nerve. Of course, these forms could be quite different.
Dystroglycan in skeletal muscle has almost twice as much carbohydrate per unit molecular weight as it does in peripheral nerve.
The degree to which the glycan blocking experiments using dystroglycan from peripheral nerve are relevant is uncertain. For example, both N-glycolyl and N-acetylneuraminic acid (as well as colominic acid) can block laminin-1 binding in these studies, but addition of any of these sialic acid forms requires concentrations above 1 mM (Yamada et al., 1996). Likewise, NeuAc
2,3Galß1,4GlcNAc can block laminin binding to
dystroglycan (again at concentrations above 1 mM), but NeuAc
2,3Galß1,4Glc cannot (Chiba et al., 1997
). Even at concentrations of 10 mM, NeuAc
2, 3Galß1,4GlcNAc only inhibits laminin-1 binding by 35%. One could interpret these data to mean that the subterminal glycans are more important for binding than sialic acid is. Alternately, the valency of the glycans in the mucin region may be very important for high affinity binding. If so, the glycan blocking experiments done in peripheral nerve (Chiba et al., 1997
; Yamada et al., 1996
) may be reflective of higher-affinity multivalent forms that would work at more reasonable concentrations.
Laminin-1 binding was first described to the brain form of dystroglycan, which was originally called cranin (Smalheiser and Schwartz, 1987
). Brain dystroglycan also binds laminin-2 (Tian et al., 1997
). Brain dystroglycan migrates at a lower molecular weight than does dystroglycan isolated from skeletal muscle (Ibrighimov-Beskrovnaya et al., 1992
) but shows many similar properties. Binding of laminin-1 is blocked by heparin and sulfatides but not chondrotin sulfate (but see Gee et al., 1993
) or lactose (Smalheiser and Kim, 1995
). Treatment of dystroglycan with 50 mM periodic acid eliminates binding (Smalheiser, 1993
), whereas nitrous acid treatment does not block binding (Smalheiser and Schwartz, 1987
). These two experiments suggest that laminin binding occurs via glycans but does not occur via GAGs (for which there is no evidence on any dystroglycan form). Digestion with O-sialoprotease altered the migration of brain
dystroglycan and inhibited laminin-1 binding, suggesting that O-linked glycans are important (Smalheiser and Kim, 1995
). The biochemical data on brain dystroglycan is likely to reflect an amalgam of glycoforms, as dystroglycan is expressed in the vasculature as well a variety of neural cells and structures (Gorecki et al., 1994
; Montanaro et al., 1995
; Drenckhahn et al., 1996
; Tian et al., 1997
; Koulen et al., 1998
; Moukles et al., 2000
; Zaccaria et al., 2001
), including synapses (Cavaldesi et al., 1999
; Levi et al., 2002
; Zaccaria et al., 2001
; Smalheiser and Collins, 2000
). Therefore, the degree to which the binding studies on whole-brain protein reflect the function or glycosylation of dystroglycan in various types of neurons or glia is unclear.
Recent work by Michele et al. (2002) has lent further credence to the notion that the binding of laminins and (other ligands) to
dystroglycan is dependent on glycans. Here gel overlays and solid state binding assays were performed using
dystroglycan isolated from the muscles of patients where glycosylation had been altered. These included patients with muscle-eye-brain disease, Fukuyama congenital muscular dystrophy, and the myodystrophy mouse. In muscles from all of these samples, there is no binding of either the IIH6 or VIA4-1 antibody to
dystroglycan. There is little or no binding of laminin-1 as well. This study is consistent with the findings of Ervasti and Campbell (1993)
and suggests that laminin binding is dependent on glycans. Thus although the glycan structure required by IIH6 is unknown, and high-affinity binding of laminin to particular glycans has yet to be shown, there is a wealth of evidence to suggest that laminins bind
dystroglycan via glycan chains.
Studies on recombinant proteins
G domains of laminin 1,
2, and
4 bind to
dystroglycan isolated from chick muscle (Talts et al., 1999
). Binding of recombinant proteins containing all five G domains as well as smaller groupings of two to three domains and individual domains were studied. Both the G13 and the G45 fragments of laminin
2 bound
dystroglycan with 1550 nM affinity. The G45 fragment of laminin
1 bound at
200 nM, whereas the G13 fragment did not bind at all. Binding of all of these laminin fragments did not differ between
dystroglycan isolated from kidney and skeletal muscle. None of the laminin G domains bound when studied as single domains (at concentrations below 500 nM). Thus certain laminin G domains participate in
dystroglycan binding, but all require more than one domain for high-affinity binding. This conclusion is made stronger by comparative studies with the normally synaptic G domains of laminin
4 (Talts et al., 2000
). The G domains of laminin
4 bind 30100 times more poorly than those of laminin
2 and were not saturating even at 1 µM. This occurred despite the fact that laminin
4 bound equally well as laminin
2 to fibulin-1, fibulin-2, heparin, and sulfatides. Thus it is unlikely that the protein produced was simply not folded correctly. Binding of laminin
1 and
2, both as G domains and as native proteins, is also differentially sensitive to heparin inhibition, with laminin
1 being more sensitive than laminin
2 (Pall et al., 1996
; Talts et al., 1999
). Finally, a recombinant form of laminin
5 produced in Escherichia coli has been reported to bind to
dystroglycan using a gel overlay (Shimizu et al., 1999
). Such studies show that glycosylation of the laminin G domain is not important for binding.
Crystal structures
The crystal structure of several laminin G domains has been solved and has shed a great deal of light on how these domains might interact with glycans on dystroglycan (Hohenester et al., 1999
; Tisi et al., 2000
). The structures of the G5 and G45 regions of laminin
2 have been solved. These show that individual G domains are folded into a beta sandwich composed of 14 beta strands in two sheets. The resulting G domains are roughly spherical in shape with a diameter of
3.5 nm, which is consistent with earlier electron microscopy studies. The edge of one beta sandwich binds calcium. Calcium is required for binding of laminins to
dystroglycan, and mutagenesis studies on laminin G domains suggest that most of the amino acids required for
dystroglycan binding reside near the calcium ion in the crystal structure (Timpl et al., 2000
). This same region also contains amino acids involved in binding of laminins to heparin and sulfatides (Timpl et al., 2000
), which may explain why heparin inhibits laminin binding to
dystroglycan. The amino acids required for binding to heparin are not identical to those required for binding to
dystroglycan. Thus heparin and
dystroglycan binding to laminins are distinct to some extent.
In the G45 crystal structure, the G4 and G5 domains associate via an interface of 450 Å, with both calcium ions being at the periphery of the structure. Based on amino acid mutagenesis, laminin residues important for
dystroglycan binding span a 3-nm distance within these individual G domains (Timpl et al., 2000
). Because at least two domains are required for binding, it is possible that the binding site of laminin for
dystroglycan may span as much as 6 nm. Such a large binding site would be well suited to binding multiple glycan chains such as one would find in a mucin domain. Thus G domains of laminins and other dystroglycan binding proteins may be specific to glycoforms of
dystroglycan that contain multiple O-linked sites.
Agrin
Several groups simultaneously reported that agrin can bind dystroglycan (Campanelli et al., 1994
; Gee et al., 1994
; Bowe et al., 1994
). Agrin is an extracellular matrix proteoglycan involved in neuromuscular formation (for a review, see Sanes and Lichtman, 1999
). Mice lacking agrin fail to properly form neuromuscular junctions and die at birth (Gautam et al., 1996
). Agrin is made by both muscle and neurons (Hoch et al., 1993
), however, splice forms specific for the nervous system have a high activity in inducing the aggregation of nicotinic acetylcholine receptors (AChRs) in skeletal muscle (Ferns et al., 1992
, 1993
). Because the concentration of AChRs is an essential step in the development of the neuromuscular synapse, identifying receptors and binding proteins for agrin has been the focus of a number of groups.
The idea that dystroglycan was a functional receptor for agrin-induced AChR clustering was enticing (e.g., see Gee et al., 1994
), however, four results argue that
dystroglycan is not such a receptor. First, mice lacking dystroglycan in their skeletal muscles have AChR aggregates at the neuromuscular junction (Cote et al., 1999
). The structure of the neuromuscular junctions in these animals is perturbed, suggesting a structural role for dystroglycan at the synapse; however, neuromuscular junctions still form. Second, muscles from mice lacking dystroglycan can make AChR aggregates in culture using purified agrin (Grady et al., 2000
). Again, these aggregates are less stable, suggesting a role for dystroglycan in AChR cluster stability. Third, a tyrosine kinase called MuSK is stimulated by agrin (Glass et al., 1996
) and is essential for agrin signaling and AChR clustering (DeChiara et al., 1996
). Fourth, splice forms of agrin that are active in AChR clustering actually bind more poorly to
dystroglycan than do splice forms derived from skeletal muscle (Campanelli et al., 1996
; Gesemann et al., 1996
), which have little or no activity in such assays. The fact that dystroglycan is essential for this important function of agrin does not make its interaction with this ligand unimportant, however, as neuromuscular structure is clearly aberrant in muscles lacking dystroglycan protein (Cote et al., 1999
). In addition, muscles lacking one of several putative laminins, which bind
dystroglycan, also have aberrant neuromuscular structure (Patton et al., 1999, 2001), as do muscles lacking utrophin (Grady et al., 1997
; Deconinck et al., 1997
). Thus dystroglycan is an integral part of the synaptic utrophinglycoprotein complex.
The region of agrin that binds to dystroglycan has been mapped by deletion analysis to the second (G2) of agrin's three G domains, all of which reside in the C-terminal half of the molecule (Gesemann et al., 1996
). Like laminins, a tandem of two G domains (G2 and G3) is far superior for binding when compared to a single G domain, and binding requires mM levels of calcium (Gesemann et al., 1996
; Campanelli et al., 1996
). The third G domain (G3) contains the splice site that defines the bioactive neural-specific agrin and the inactive muscle form (Ferns et al., 1992
; Gesemann et al., 1996
). The G2 domain also can contain a four amino acid splice insert that conveys heparin binding on
dystroglycan (Ferns et al., 1992
, 1993
). As with the laminin G domains, heparin and
dystroglycan binding appear to have significant overlap. The G2G3 region of neural agrin binds more poorly to
dystroglycan and to heparin than does the muscle form (Gesemann et al., 1996
; Campanelli et al., 1996
), which binds in the 25 nM range (Gesemann et al., 1998
).
Binding to chemically deglycosylated dystroglycan has not been reported; however, agrin does bind to multivalent neoglycoconjugates, including NeuAc
2,3Galß1, 4GlcNAcß-BSA, in the 1020 nM range (Xia and Martin, 2002
). Because such structures contain three of the four saccharides present on the mannose-O-linked chains of
dystroglycan (Sasaki et al., 1998
), agrin could bind
dystroglycan in large part via such glycans. Surprisingly, agrin bound equally well to multivalent Galß1,4GlcNAcß- (or Galß1,3GalNAc
-) as it did to NeuAc
2,3Galß1,4GlcNAcß-, suggesting that sialic acid contributed little to binding. This would be consistent with the findings of Ma et al. (1993)
in Torpedo. There they showed that peanut agglutinin, a Galß1,3GalNAc
-O-binding lectin, blocks most agrin binding to muscle membranes. This lectin binds poorly to glycans containing sialic acid, and therefore would not presumably inhibit binding to any NeuAc
2, 3Galß1,3GalNAc that may have been present.
Agrin binding to N-acetyllactosamines (LacNAcs) would also be consistent with the findings that sialidase treatment of muscle cells increases agrin-independent clustering of AChRs (Martin and Sanes, 1995). Neuraminidase treatment mimics agrin signaling, and this activity is also blocked by peanut agglutinin (Parkhomovskiy et al., 2000
). AChR clustering induced by either agrin or laminin-1 is also inhibited by treatment of muscle cells with certain glycosidases, again suggesting that glycans are involved in signaling (Martin and Sanes, 1995
; McDearmon et al., 2001
). Finally, binding of agrin to
dystroglycan is severely diminished in muscle homogenates where
dystroglycan glycosylation is altered (Michele et al., 2002
). All of these studies suggest that agrin, like laminin, can bind
dystroglycan via glycan chains.
Agrin could also effect dystroglycan binding by proxy through its many other binding partners. For example, agrin binds both laminin-1 and laminin-2 (Denzer et al., 1997
, 1998
; Cotman et al., 1999
). The structural interplay between these two matrix proteins could alter access of the G domains to receptors on the muscle membrane. This interaction may be very complex, as lamininagrin interactions are partly via proteinprotein interactions and partly via heparan sulfate GAGs, which are present on agrin (Cotman et al., 1999
). Both laminin and agrin also bind at least half a dozen other proteins each, and
dystroglycan, laminin, and agrin all bind heparin (Talts et al., 1999
; Pall et al., 1996
; Gesemann et al., 1996
; Campanelli et al., 1996
). Thus it is important to keep in mind that most of the binding studies to date on laminin and agrin have been done using soluble (and often recombinant) ligands, and these proteins may not reflect the behavior of these polymeric matrix molecules as they exist in the basal lamina.
Neurexins
Neurexins are transmembrane proteins that are expressed in the nervous system and are one of the major binding proteins for laitrotoxin, the venom of the black widow spider (Ushkaryov et al., 1992
). There are two families of these proteins (
and ß neurexins), and they contain six or one G domains, respectively. Sugita et al. (2001)
have shown that both
and ß neurexins can bind to
dystroglycan with high affinity. Chemical deglycosylation (with TFMS) of
dystroglycan inhibits binding of recombinant G domains from neurexins to
dystroglycan isolated from skeletal muscle, heart, lung, or brain. Interestingly, enzymatic deglycosylation with a cocktail of N-glycanase, O-glycanase, and sialidase has no effect on binding to
dystroglycan from heart or muscle but increases binding to the lung form. Although there was little logic to the use of this combination of reagents, it nevertheless showed that glycosylation of
dystroglycan is heterogeneous in different tissues. Michele et al. (2002)
have also shown greatly reduced binding of recombinant neurexin to
dystroglycan from patients in whom dystroglycan glycosylation has been altered.
Perlecan
Peng et al. (1998) have shown that perlecan, another proteoglycan present in the muscle basal lamina, can bind to
dystroglycan from sheep brain in gel overlays. Perlecan can also be coprecipitated with dystroglycan from Xenopus muscle extracts. Talts et al. (1999)
have shown binding of a recombinant fragment of perlecan to chick muscle
dystroglycan in the 3 nM range. The first two of the three G domains in perlecan are required for this high-affinity binding. Recombinant perlecan binds with stronger affinity than either recombinant laminin
1 or
2. There is no evidence regarding the extent to which perlecan binds via glycans. Based on comparisons with laminin G domains, it is likely that these interactions are similar, but there is some anecdotal evidence that this might not be the case. First, perlecan binding to
dystroglycan is not blocked by heparin, whereas heparin does block binding of some (but not all) laminins (Pall et al., 1996
; Talts et al., 1999
). Second, a 100-fold excess of laminin only blocks perlecan binding by 50% or less, depending on the fragment used (Talts et al., 1999
). Likewise, mice lacking perlecan appear to have relatively normal muscle and neuromuscular structure (Arikawa-Hirasawa et al., 2002
). The main deficit at the neuromuscular junction appears to be a complete lack of acetylcholinesterase expression (Arikawa-Hirosawa et al., 2002
). This is the enzyme that degrades acetylcholine in the synaptic cleft after it has been secreted by the motor nerve terminal. Thus muscles in these animals could be hyperexcitable due to an inability to degrade neurotransmitter.
Biglycan
Recently, Fallon and colleagues have shown that biglycan, a small chondroitin sulfate proteoglycan, binds dystroglycan (Bowe et al., 2001
). Unlike perlecan, laminins, and agrins, biglycan does not have any G domains. There are several aspects of this interaction that delineate it from the binding of these other extracellular matrix ligands. First, biglycan binding to
dystroglycan does not require the glycosylation of
dystroglycan to occur (Bowe et al., 2001
). Biglycan binding does not occur in the mucin region but via the C-terminal domain of the
dystroglycan protein. Second, chondroitin sulfate chains on biglycan are required for binding. Recombinant biglycan produced in bacteria does not bind to
dystroglycan, and chondrotinase treatment inhibits binding of native biglycan. Thus the conventional wisdom for matrix ligandsthat glycans on
dystroglycan are required but glycans on the ligand are nothas been turned on its head in this instance. This has profound implications for dystroglycan function because it suggests that
dystroglycan could not only bind the extracellular matrix via its glycans but also can bind glycans in the matrix as well. Biglycan is reported to be upregulated in mdx muscles, suggesting that it may be involved in compensatory interactions in muscular dystrophy (Bowe et al., 2001
). Expression of
dystroglycan, however, is reduced, not increased, in mdx muscle. Thus this result would appear to argue that biglycan does not bind strongly enough to dystroglycan to control its localization on the muscle membrane.
Viruses and bacteria
Lymphocytic chorionic meningitis virus (LCMV), Lassa fever virus (LSV), and mycobacterium leprae have all been shown to bind to dystroglycan (Cao et al., 1998
; Rambukkana et al., 1998
). Mycobacterium leprae requires laminin
2 as a cofactor for binding. Treatment of
dystroglycan with 0.1 mM periodate, which will remove N-acyl side chains from sialic acids (Diaz and Varki, 1985
), blocks mycobacterium leprae binding but not heparin, suggesting that there is no overlap between the mycobacterium leprae and heparin binding sites, but that glycans are required (Rambukkana et al., 1998
).
Dystroglycan also inhibits binding of mycobacterium lepraelaminin complexes to rat and human Schwann cells. LCMV and LSV bind directly to
dystroglycan. Both viruses require native protein for binding and cannot bind to recombinant forms of the protein, suggesting that glycans are involved. LCMV binding also can be abolished by pretreatment of
dystroglycan with 10 mM sodium periodate (Kunz et al., 2001
).
Neuraminidase treatment or addition of as much as 100 mM sialic acid, in contrast, does not block binding, nor does heparin (Kunz et al., 2001). These data would appear to be inconsistent, though no analysis of the neuraminidase-treated material was done to confirm that the enzyme actually worked. By expressing recombinant fragments of
dystroglycan in embryonic stem cells lacking dystroglycan, Kunz et al. (2001)
showed that a portion of the mucin domain and an adjoining region of the N-terminal domain are required for virus binding. Thus, unlike laminins, some additional polypeptide structure in
dystroglycan may be required for virus binding. Nevertheless, laminin can block most virus binding in competition experiments. Although not entirely consistent, these data for the most part suggest that glycans mediate pathogen binding to
dystroglycan.
![]() |
Other transmembrane scaffolds to the extracellular matrix potentially involving glycans |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Aside from dystroglycan, integrins are the main binding proteins for laminin. Integrins are heterodimeric transmembrane proteins that mediate many important processes involving laminin signaling (Hynes, 2002
). There a large number of integrin
and ß chains, and different
ß integrin heterodimers bind matrix proteins with varying specificities. In skeletal muscle, the principal integrin along the myofiber membrane and at the neuromuscular and myotendinous junction are splice forms of integrin
7ß1 (Martin et al., 1996
). Mutations in integrin
7 cause a form of congenital myopathy (Hayashi et al., 1998
). In addition, antibodies to the integrin
7 block AChR clustering induced by agrin or laminin-1 in skeletal muscle cells (Burkin et al., 1998
, 2000
), as do antibodies to integrin ß1 (Martin and Sanes, 1997
). Recently, antibodies to integrin
6ß1 were also shown to block laminin-2-induced AChR clustering in skeletal muscle cells (Smirnov et al., 2002
). Integrin
6, however, is not significantly expressed in adult human skeletal muscle (Martin et al., 1996
).
Although there is little evidence that laminins bind integrins via glycans, there are studies that suggest that glycans on laminin modulate integrin binding and/or activity. Integrin 6 can utilize glycans in part to bind to laminin (Chammas et al., 1991
). There is no evidence that integrin
7 binds laminin via glycans; however, both laminin-1 (Zhou and Cummings, 1993
) and integrin
7 (Gu et al., 1994
) bind the lactose lectin L14. L14 is a member of the S-type lectin family, some members of which are highly expressed in skeletal muscle (Catt et al., 1987
; Poirer et al., 1992
; Cooper and Barondes, 1990
; Cooper et al., 1991
). L14 binds both laminin-1 and integrin
7, and it can inhibit the binding of these molecules to one another (Gu et al., 1994
). Therefore, integrin
7-laminin interactions could be bridged by lactosamine moeities on laminin via L14 or another lactose-binding lectin. Laminin also binds a number of glycans directly. These include heparin (Talts et al., 1999
), sulfatides (Talts et al., 1999
), and sulfoglucuronyl glycolipids (Mohan et al., 1990
). Thus glycans on receptors other than
dystroglycan may serve as scaffolds to laminin, and glycans on laminin may form scaffolds to membrane receptors.
![]() |
Conclusions |
---|
![]() |
Acknowledgements |
---|
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Abbreviations |
---|
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References |
---|
Arumugham, R.G., Hsieh, T.C.Y., Tanzer, M.L., and Laine, R.A. (1986) Structures of the asparagine-linked sugar chains of laminin. Biochim. Biophys. Acta, 883, 112126.[ISI][Medline]
Benson, MA., Newey, S.E., Martin-Rendon, E., Hawkes, R., and Blake, D.J. (2001) Dysbindin, a novel coiled-coil-containing protein that interacts with the dystrobrevins in muscle and brain. J. Biol. Chem., 276, 2423224241.
Blake, D.J., Weir, A., Newey, S.E., and Davies, K.E. (2002) Function and genetics of dystrophin and dystrophin-related proteins in muscle. Physiol Rev., 82, 291329.
Bowe, M.A., Deyst, K.A., Leszyk, J.D., and Fallon, J. R. (1994) Identification and purification of an agrin receptor from Torpedo synaptic membranes: a heteromeric complex related to the dystroglycans. Neuron, 12, 11731180.[ISI][Medline]
Bowe, M.A., Mendis, D.B., and Fallon, J.R. (2001) The small leucine-rich repeat proteoglycan biglycan binds to -dystroglycan and is upregulated in dystrophic muscle. J. Cell Biol., 148, 801810.[CrossRef][ISI]
Brancaccio, A., Schulthess, T., Gesemann, M., and Engel, J. (1995) Electron microscopic evidence for a mucin-like region in chick muscle alpha dystroglycan. FEBS Lett., 368, 139142.[CrossRef][ISI][Medline]
Brancaccio, A., Schulthess, T., Gesemann, M., and Engel, J. (1997) The N-terminal region of -dystroglycan is an autonomous globular domain. Eur. J. Biochem., 246, 166172.[Abstract]
Burkin, D.J., Gu, M., Hodges, B.L., Campanelli, J.T., and Kaufman, S.J. (1998) A functional role for specific spliced variants of the alpha7beta1 integrin in acetylcholine receptor clustering. J. Cell Biol., 143, 10671075.
Burkin, D.J., Kim, J.E., Gu, M., and Kaufman, S.J. (2000) Laminin and alpha7beta1 integrin regulate agrin-induced clustering of acetylcholine receptors. J. Cell Sci., 113, 28772886.
Campanelli, J.T., Roberds, S.L., Campbell, K.P., and Scheller, R.H. (1994) A role for dystrophin-associated glycoproteins and utrophin in agrin-induced AChR clustering. Cell, 77, 663674.[ISI][Medline]
Campanelli, J.T., Gayer, G.G., and Scheller, R.H. (1996) Alternative RNA splicing that determines agrin activity regulates binding to heparin and dystroglycan. Development, 122, 16631672.
Cao, W., Henry, M.D., Borrow, P., Yamada, H., Elder, J.H., Ravkov, E.V., Nichol, S.T., Compans, R.W., Campbell, K.P., and Oldstone, M.B.A. (1998) Identification of -dystroglycan as a receptor for lymphocytic choriomeningitis virus and lassa fever virus. Science, 282, 20792081.
Carlsson, L. and Thornell, L.W. (2001) Desmin-related myopathies in mice and man. Acta Physiol. Scand., 171, 341348.[CrossRef][ISI][Medline]
Catt, K.W., Harrison, F.L., and Carleton, J.S. (1987) Distribution of an endogenous beta-galactoside specific lectin during fetal and neonatal rabbit development. J. Cell Sci., 87, 623633.[Abstract]
Cavaldesi, M., Macchia, G., Barca, S., Defilippi, P., Tarone, G., and Petrucci, T. (1999) Association of the dystroglycan complex isolated from bovine brain synaptosomes with proteins involved in signal transduction. J. Neurochem., 72, 16481655.[CrossRef][ISI][Medline]
Chai, W., Yuen, C-T., Kogelberg, H., Carruthers, R.A., Margolis, R.U., Feizi, T., and Lawson, A.M. (1999) High prevalence of 2-mono and 2,6-di-substituted manol-terminating sequences among O-glycans released from brain glycopeptides by reductive alkaline hydrolysis. Eur. J. Biochem., 263, 879888.
Chammas, R., Veiga, S.S., Line, S., Potocnbjak, P., and Bretani, R.R. (1991) Asn-linked oligosaccharide-dependent interaction between laminin and gp120/140. J. Biol. Chem., 266, 33493355.
Chiba, A., Matsumura, K., Yamada, H., Inazu, T., Shimizu, T., Kusunoki, S., Kanazawa, I., Kobata, A., and Endo, T. (1997) Structures of sialylated O-linked oligosaccharides of bovine peripheral nerve -dystroglycan. J. Biol. Chem., 272, 21562162.
Chiu, A.Y. and Sanes, J.R. (1984) Development of basal lamina in synaptic and extrasynaptic portions of embryonic rat muscle. Dev. Biol., 103, 456467.[ISI][Medline]
Chung, W. and Campanelli, J.T. (1999) WW and EF hand domains of dystrophin-family proteins mediate dystroglycan binding. Mol. Cell. Biol. Res. Commun., 2, 162171.[CrossRef][Medline]
Cooper, D.N.W. and Barondes, S.H. (1990) Evidence for the export of a muscle lectin from cytosol to extracellular matrix and for a novel secretory mechanism. J. Cell Biol., 110, 16811691.[Abstract]
Cooper, D.N.W., Massa, S.M., and Barondes, S.H. (1991) Endogenous muscle lectin inhibits myoblast adhesion to laminin. J. Cell Biol., 115, 14371448.[Abstract]
Cote, P.D., Moukhles, H., Lindenbaum, M., and Carbonetto, S. (1999) Chimaeric mice deficient in dystroglycans develop muscular dystrophy and have disrupted myoneural synapses. Nature Genet., 23, 338342.[CrossRef][ISI][Medline]
Cotman, S.L., Halfter, W., and Cole, G.J. (1999) Identification of extracellular matrix ligands for the heparan sulfate proteoglycan agrin. Exp. Cell Res., 249, 5464.[CrossRef][ISI][Medline]
DeChiara, T.M., Bowen, D.C., Valenzuela, D.M., Simmons, MV. Poueymirov. W.T., Thomas, S., Kinetz, E., Compton, D.L., Rojas, E., Park, J.S., and others. (1996) The receptor tyrosine kinase MuSK is required for neuromuscular junction formation in vivo. Cell, 85, 501512.[ISI][Medline]
Deconinck, A.E., Potter, A.C., Tinsley, J.M., Wood, S.J., Vater, R., Young, C., Metzinger, L., Vincent, A., Slater, C.R., and Davies, K.E. (1997) Postsynaptic abnormalities at the neuromuscular junctions of utrophin-deficient mice. J. Cell Biol., 136, 883894.
Denzer, A.J., Brandenberger, R., Gesemann, M., Chiquet, M., and Ruegg, M.A. (1997) Agrin binds to the nerve-muscle basal lamina via laminin. J. Cell Biol., 137, 671683.
Denzer, A.J., Schulthess, T., Fauser, C., Schumacher, B., Kammerer, R.A., Engel, J., and Ruegg, M.A. (1998) Electron microscopic structure of agrin and mapping of its binding site in laminin-1. EMBO J., 17, 335343.
Diaz, S. and Varki, A. (1985) Metabolic labeling of sialic acids in tissue culture cell lines: methods to identify substituted and modified radioactive neuraminic acids. Anal. Biochem., 150, 3246.[ISI][Medline]
DiStasio, E., Sciandra, F., Maras, B., Di Tommaso, F., Petrucci, T.C., Giardina, B., and Brancaccio, A. (1999) Structural and functional analysis of the N-terminal extracellular region of ß-dystroglycan. Biochem. Biophys. Res. Commun., 266, 274278.[CrossRef][ISI][Medline]
Drenckhahn, D., Holbach, M., Ness, W., Schmitz, F., and Anderson, L.V.B. (1996) Dystrophin and the dystrophin-associated glycoprotein, ß-dystroglycan, co-localize in photoreceptor synaptic complexes of the human retina. Neuroscience, 73, 605612.[CrossRef][ISI][Medline]
Durbeej, M., Henry, M., and Campbell, K.P. (1998) Dystroglycan in development and disease. Curr. Opin. Cell Biol., 10, 594601.[CrossRef][ISI][Medline]
Engel, J. (1992) Laminins and other strange proteins. Biochemistry, 31, 1064310651.[ISI][Medline]
Engel, A.G. and Franzini-Armstrong, C., eds. (1994) Myology, 2d ed. McGraw-Hill, New York.
Ervasti, J.M. and Campbell, K.P. (1991) Membrane organization of the dystrophin-glycoprotein complex. Cell, 66, 11211131.[ISI][Medline]
Ervasti, J.M. and Campbell, K.P. (1993) A role for the dystrophin-glycoprotein complex as a transmembrane linker between actin and laminin. J. Cell Biol., 122, 809823.[Abstract]
Ervasti, J.M., Burwell, A.L., and Geissler, A.L. (1997) Tissue-specific heterogeneity in alpha dystroglycan sialoglycosylation. Skeletal muscle alpha-dystroglycan is a latent receptor for Vicia villosa aggutinin b4 masked by sialic acid modification. J. Biol. Chem., 272, 2231522321.
Esko, J.D. and Selleck, S.B. (2002) Order out of chaos: assembly of ligand binding sites in heparan sulfate. Annu. Rev. Biochem., 71, 435471.[CrossRef][ISI][Medline]
Ferns, M.J., Hoch, W., Campanelli, J.T., Rupp, F., Hall, Z.W., and Scheller, R.H. (1992) RNA splicing regulates agrin-mediated acetylcholine receptor clustering activity on cultured myotubes. Neuron, 8, 10791086.[ISI][Medline]
Ferns, M.J., Campanelli, J.T., Hoch, W., Scheller, R.H., and Hall, Z. (1993) The ability of agrin to cluster AChRs depends on alternative splicing and on cell surface proteoglycans. Neuron, 11, 491502.[ISI][Medline]
Finne, J., Krusius, T., Margolis, R.K., and Margolis, R.U. (1979) Novel mannitol-containing oligosaccharides obtained by mild alkaline borohydride treatment of a chondroitin sulfate proteoglycan from brain. J. Biol. Chem., 254, 1029510300.[Abstract]
Fujiwara, S., Shinkai, H., Deutzmann, R., Paulsson, M., and Timpl, R. (1988) Structure and distribution of N-linked oligosaccharide chains on various domains of mouse tumor laminin. Biochem. J., 252, 453461.[ISI][Medline]
Gautam, M., Noakes, P.G., Moscoso, L., Rupp, F., Scheller, R.H., Merlie, J.P., and Sanes, J.R. (1996) Defective neuromuscular synaptogenesis in agrin-deficient mutant mice. Cell, 85, 525535.[ISI][Medline]
Gee, S.H., Blacher, R.W., Douville, P.J., Provost, P.R., Yurchenco, P.D., and Carbonetto, S. (1993) 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. J. Biol. Chem., 268, 1497214980.
Gee, S.H., Montanaro, F, Lindenbaum, M.H., and Carbonetto, S. (1994) Dystroglycan-, a dystrophin-associated glycoprotein, is a functional agrin receptor. Cell, 77, 675686.[ISI][Medline]
Gesemann, M., Cavalli, V., Denzer, A.J., Brancaccio, A., Schumacher, B., and Ruegg, M. (1996) Alternative splicing of agrin alters its binding to heparin, dystroglycan, and the putative agrin receptor. Neuron, 16, 755767.[ISI][Medline]
Gesemann, M., Brancaccio, A., Schumacher, B., and Ruegg, M.A. (1998) Agrin is a high-affinity binding protein of dystroglycan in non-muscle tissue. J. Biol. Chem., 273, 600605.
Glass, D.J., Bowen, D.C., Stitt, T.N., Radziejewski, C., Bruno, J., Ryan, T.E., Gies, D.R., Shah, S., Mattsson, K., Burden, S.J., and others. (1996) Agrin acts via a MuSK receptor complex. Cell, 85, 513523.[ISI][Medline]
Gorecki, D.C., Derry, J.M.J., and Barnard, E.A. (1994) Dystroglycan: brain localization and chromosome mapping in the mouse. Hum. Mol. Genet., 3, 15891597.[Abstract]
Grady, R.M., Merlie, J.P., and Sanes, J.R. (1997) Subtle neuromuscular defects in utrophin-deficient mice. J. Cell Biol., 136, 871882.
Grady, R.M., Grange, R.W., Lau, K.S., Maimone, M.M., Nichol, M., C., Stull, J.T., and Sanes, J.R. (1999) Role for -dystrobrevin in the pathogenesis of dystrophin-dependent muscular dystrophies. Nat. Cell Biol., 1, 215220.[CrossRef][ISI][Medline]
Grady, R.M., Zhou, H., Cunningham, J.M., Henry, M.D., Campbell, K.P., and Sanes, J.R. (2000) Maturation and maintenance of the neuromuscular synapse: genetic evidence for roles of the dystrophin-glycoprotein complex. Neuron, 25, 279293.[ISI][Medline]
Gu, M., Wang, W., Song, W.K., Cooper, D.N.W., and Kaufman, S.J. (1994) Selective modulation of the interaction of 7ß1 integrin with fibronectin and laminin by L-14 lectin during skeletal muscle differentiation. J. Cell Sci., 107, 175181.
Hall, Z.W. and Sanes, J.R. (1993) Synaptic structure and development: the neuromuscular junction. Cell, 72 (suppl.), 99121.[ISI][Medline]
Hayashi, Y.K., Chou, F.L., Engvall, E., Ogawa, M., Matsuda, C., Hirabayashi, S., Yokochi, K., Ziober, B.L., Kramer, R.H., Kaufman, S.J., Ozawa, E., Goto, Y., Nonaka, I., Tsukahara, T., Wang, J.Z., Hoffman, E.P., and Arahata, K. (1998) Mutations in the integrin alpha 7 gene cause congenital myopathy. Nature Genet., 19, 9497.[ISI][Medline]
Henry, M.D. and Campbell, K.P. (1999) Dystroglycan inside and out. Curr. Opin. Cell Biol., 11, 602607.[CrossRef][ISI][Medline]
Hoch, W., Ferns, M., Campanelli, J.T., Hall. Z.W., and Scheller, R.H. (1993) Developmental regulation of highly active alternatively spliced agrin. Neuron, 11, 479490.[ISI][Medline]
Hohenester, E., Tisi, D., Talts, J.F., and Timpl, R. (1999) The crystal structure of a laminin-G-like module reveals the molecular basis of -dystroglycan binding to laminins, perlecan, and agrin. Mol. Cell, 4, 783792.[ISI][Medline]
Hynes, R. (2002) Integrins: bifunctional, allosteric signaling machines. Cell, 110, 673687.[ISI][Medline]
Ibraghimov-Beskrovnaya, O., Ervasti, J.M., Leveille, C.J., Slaughter, C.A., Sernett, S.W., and Campbell, K.P. (1992) Primary structure of dystrophin-associated glycoproteins linking dystrophin to the extracellular matrix. Nature, 355, 696702.[CrossRef][ISI][Medline]
Knibbs, R.N., Perini, F., and Goldstein, I.J. (1989) Structure of the major concanavalin A reactive oligosaccharides of the extracellular matrix component laminin. Biochemistry, 28, 63796392.[ISI][Medline]
Kogelberg, H., Chai, W., Feizi, T., and Lawson, T.M. (2001) NMR studies of mannitol-terminating oligosaccharides derived by reductive alkaline hydrolysis from brain glycoproteins. Carbohyd. Res., 331, 393401.[CrossRef][ISI][Medline]
Koulen, P., Blank, M., and Kroger, S. (1998) Differential distribution of beta-dystroglycan in rabbit and rat retina. J. Neurosci. Res., 51, 735747.[CrossRef][ISI][Medline]
Kunz, S., Sevilla, N., McGavern, D.B., Campbell, K.P., and Oldstone, M.B.A. (2001) Molecular analysis of the interaction of LCMV with its cellular receptor dystroglycan. J. Cell Biol., 155, 301310.
Lentz, S.I., Miner, J.H., Sanes, J.R., and Snider, W.D. (1997) Distribution of the ten known laminin chains in the pathways and targets of developing sensory axons. J. Comp. Neurol., 378, 547561.[CrossRef][ISI][Medline]
Leschziner, A., Moukhles, H., Lindenbaum, M., Gee, S.H., Butterworth, J., Campbell, K.P., and Carbonetto, S. (2000) Neural regulation of alpha dystroglycan biosynthesis and glycosylation in skeletal muscle. J. Neurochem., 74, 7080.[CrossRef][ISI][Medline]
Levi, S., Grady, R.M., Henry, M.D., Campbell, K.P., Sanes, J.R., and Craig, A.M. (2002) Dystroglycan is selectively associated with inhibitory GABAergic synapses but is dispensible for their differentiation. J. Neurosci., 22, 42744285.
Ma, J., Nastuk, M.A., McKechnie, B.A., and Fallon, J.R. (1993) The agrin receptor. J. Biol. Chem., 268, 2510825117.
Martin, P.T. and Sanes, J.R. (1995) Role for a synapse-specific carbohydrate in agrin-induced clustering of acetylcholine receptors. Neuron, 14, 743754.[ISI][Medline]
Martin, P.T. and Sanes, J.R. (1997). Integrins mediate adhesion to agrin and modulate agrin signalling. Development, 124, 39093917.
Martin, P.T., Kaufman, S.J., Kramer, R.H., and Sanes, J.R. (1996) Synaptic integrins in developing, adult, and mutant muscle: selective association of 1,
7A, and
7B integrins with the neuromuscular junction. Dev. Biol., 174, 125139.[CrossRef][ISI][Medline]
Martin, P.T., Scott, L.J.C., Porter, B.E., and Sanes, J.R. (1999) Distinct structures and functions of related pre- and postsynaptic carbohydrates at the mammalian neuromuscular junction. Mol. Cell. Neurosci., 13, 105118.[CrossRef][ISI][Medline]
Matsumura, K., Ervasti, J.M., Ohlendieck, K., Kahl, S.D., and Campbell, K.P. (1992) Association of dystrophin-related protein with dystrophin-associated proteins in mdx mouse muscle. Nature, 360, 588591.[CrossRef][ISI][Medline]
Matsumura, K., Chiba, A., Yamada, H., Fukuta-Ohi, H., Fujita, S., Endo, T., Kobata, A., Anderson, L.V.B., Kanazawa, I., Campbell, K.P., and Shimizu, T. (1997) A role of dystroglycan in schwannoma cell adhesion to laminin. J. Biol. Chem., 272, 1390413910.
McDearmon, E.L., Combs, A.C., and Ervasti, J.M. (2001) Differential Vicia villosa agglutinin reactivity identifies three distinct dystroglycan complexes in skeletal muscle. J. Biol. Chem., 276, 3507835086.
Michele, D.E., Barresi, R., Kanagawa, M., Saito, F., Cohn, R.D., Satz, J.S., Dollar, J., Nishino, I., Kelley, R.I., Somer, H., and others. (2002) Post-translational disruption of dystroglycan-ligand interactions in congential muscular dystrophies. Nature, 418, 417422.[CrossRef][ISI][Medline]
Mohan, P.S., Chou, D.K.H., and Jungalwala, F.B. (1990) Sulfoglucuronyl glycolipids bind laminin. J. Neurochem., 54, 20242031.[ISI][Medline]
Montanaro, F., Carbonetto, S., Campbell, K.P., and Lindenbaum, M. (1995) Dystroglycan expression in wild type and mdx mouse neural retina: synaptic colocalization with dystrophin, dystrophin-related protein, but not laminin. J. Neurosci. Res., 42, 528538.[ISI][Medline]
Moukles, H., Roque, R., and Carbonetto, S. (2000) -Dystroglycan isoforms are differentially distributed in adult rat retina. J. Comp. Neurol., 420, 182194.[CrossRef][ISI][Medline]
Nguyen, H. H., Jayasinha, V., Xia, B., Hoyte, K., and Martin, P.T. (2002) Overexpression of the cytotoxic T cell GalNAc transferase in skeletal muscle inhibits muscular dystrophy in mdx mice. Proc. Natl Acad. Sci. USA, 99, 56165621.
Nishino, I. and Ozawa, E. (2002) Muscular dystrophies. Curr. Opin. Neurol., 15, 539544.[CrossRef][ISI][Medline]
Ohlendieck, K., Ervasti, J.M., Matsumura, K., Kahl, S.D., Leveille, C.J., and Campbell, K.P. (1991) Dystrophin-related protein is localized to neuromuscular junctions of adult skeletal muscle. Neuron, 7, 499508.[ISI][Medline]
Pall, E.A., Bolton, K.M., and Ervasti, J.M. (1996) Differential heparin inhibition of skeletal muscle -dystroglycan binding to laminins. J. Biol. Chem., 271, 38173821.
Parkhomovskiy, N., Kammesheidt, A., and Martin, P.T. (2000) N-acetyllactosamine and the CT carbohydrate antigen mediate agrin-dependent activation of MuSK and acetylcholine receptor clustering in skeletal muscle. Mol. Cell. Neurosci., 15, 380397.[CrossRef][ISI][Medline]
Patel, T.J. and Lieber, R.L. (1997) Force transmission in skeletal muscle: from actomyosin to external tendons. Exercise Sports Sci. Rev., 25, 321363.[Medline]
Patton, B.L., Miner, J.H., Chiu, A.Y., and Sanes, J.R. (1997) Distribution and function of laminins in the neuromuscular system of developing, adult, and mutant mice. J. Cell Biol., 139, 15071521.
Patton, B.L., Cunningham, J.M., Thyboll, J., Kortesmaa, J., Westerblad, H., Edstrom, L., Tryggvason, K., and Sanes, J.R. (2001) Properly formed but improperly localized synaptic specializations in the absence of laminin 4. Nature Neurosci., 4, 597604.[CrossRef][ISI][Medline]
Peng, H.B., Ali, A.A., Daggett, D.F., Rauvala, H., Hassell, J.R., and Smalheiser, N.R. (1998) The relationship between perlecan and dystroglycan and its implication for the formation of the neuromusuclar junction. Cell Adhes. Commun., 5, 475489.[ISI][Medline]
Peters, M.F., Adams, M.E., and Froehner, S.C. (1997) Differential association of syntrophin pairs with the dystrophin complex. J. Cell Biol., 138, 8193.
Peters, M.F., Sadoulet-Puccio, H.M., Grady, M.R., Kramarcy, N.R., Kunkel, L.M., Sanes, J.R., Sealock, R., and Froehner, S.C. (1998) Differential membrane localization and intermolecular associations of alpha-dystrobrevin isoforms in skeletal muscle. J. Cell Biol., 142, 12691278.
Poirer, F., Timmons, P.M, Chan, C.-T.J., Guenet, J.-L., and Rigby, P.W.J. (1992) Expression of the L-14 lectin during mouse embryogenesis suggests multiple roles during pre- and post-implantation development. Development, 115, 143155.[Abstract]
Poon, E., Howman, E.V., Newey, S.E., and Davies, K.E. (2002) Association of syncoilin and desmin: linking intermediate filament proteins to the dystrophin-protein complex. J. Biol. Chem., 277, 34333439.
Rambukkana, A., Yamada, H., Zanazzi, G., Mathus, T., Salzer, J., Yurchenco, P.D., Campbell, K.P., and Fischetti, V.A. (1998) Role of alpha dystroglycan as a Schwann cell receptor for mycobacterium leprae. Science, 282, 20762081.
Rybakova, I.N., Patel, J.R., Davies, K.E., Yurchenco, P.D., and Ervasti, J. M. (2002) Utrophin binds laterally along actin filaments and can couple costameric actin with sarcolemma when overexpressed in dystrophin-deficient muscle. Mol. Cell. Biol., 13, 15121521.[CrossRef]
Sanes, J.R. and Cheney, J.M. (1982) Lectin binding reveals a synapse-specific carbohydrate in skeletal muscle. Nature, 300, 646647.[ISI][Medline]
Sanes, J.R. and Lichtman, J.W. (1999) Development of the neuromuscular junction. Annu. Rev. Neurosci., 22, 389442.[CrossRef][ISI][Medline]
Sanes, J.R., Engvall, E., Butkowski, R., and Hunter, D.D. (1990) Molecular heterogeneity of basal laminae: isoforms of laminin and collagen IV at the neuromuscular junction and elsewhere. J. Cell Biol., 111, 16851699.[Abstract]
Sasaki, T., Yamada, H., Matsumura, K., Shimizu, T., Kobata, A., and Endo, T. (1998) Detection of O-mannosyl glycans in rabbit skeletal muscle alpha-dystroglycan. Biochim. Biophys. Acta, 1425, 599606.[ISI][Medline]
Sciandra, F., Schneider, M., Giardina, B., Baumgartner, S., Petrucci, T., and Brancaccio, A. (2001) Identification of the ß-dystroglycan binding epitope within the C-terminal region of -dystroglycan. Eur. J. Biochem., 268, 45904597.
Scott, L.J.C., Bacou, F., and Sanes, J.R. (1988) A synapse-specific carbohydrate at the neuromuscular junction: association with acetylcholinestease and a glycolipid. J. Neurosci., 8, 932944.[Abstract]
Shimizu, H., Hosokawa, H., Ninomiya, H., Miner, J.H., and Masaki, T. (1999) Adhesion of cultured bovine aortic endothelial cells to laminin-1 mediated by dystroglycan. J. Biol. Chem., 274, 1199512000.
Smalheiser, N. and Kim, E. (1995) Purification of cranin, a laminin binding membrane protein. J. Biol. Chem., 270, 1542515433.
Smalheiser, N.R. (1993) Cranin interacts with the sulfatide-binding domain of laminin. J. Neurosci. Res., 36, 528538.[ISI][Medline]
Smalheiser, N.R. and Collins, B.J. (2000) Coordinate enrichment of cranin (dystroglycan) subunits in synaptic membranes of sheep brain. Brain Res., 887, 469471.[CrossRef][ISI][Medline]
Smalheiser, N.R. and Schwartz, N.B. (1987) Cranin: a laminin-binding protein of cell membranes. Proc. Natl Acad. Sci. USA, 84, 64716461.[Abstract]
Smalheiser, N.R., Haslam, S.M., Sutton-Smith, M., Morris, H.R., and Dell, A. (1998) Structural analysis of sequences O-linked to mannose reveals a novel Lewis x structure in cranin (dystroglycan) purified from sheep brain. J. Biol Chem., 273, 2369823703.
Smirnov, S.P., McDearmon, E.L., Li, S., Ervasti, J.M., Tryggvason, K., and Yurchenco, P.D. (2002) Contributions of the LG modules and furin processing to laminin functions. J. Biol. Chem., 277, 1892818937.
Sugita, S., Saito, F., Tang, J., Satz, J., Campbell, K.P., and Sudhof, T.C. (2001) A stoichiometric complex of neurexins and dystroglycan in brain. J. Cell Biol., 154, 435445.
Sugiyama, J., Bowen, D.C., and Hall, Z.W. (1994) Dystroglycan binds nerve and muscle agrin. Neuron, 13, 103115.[ISI][Medline]
Talts, J.F., Andac, Z., Gohring, W., Brancaccio, A., and Timpl, R. (1999) Binding of the G domains of laminin 1 and
2 chains and perlecan to heparin, sulfatides,
-dystroglycan, and several extracellular matrix proteins. EMBO J., 18, 863870.
Talts, J.F., Sasaki, T., Miosge, N., Gohring, W., Mann, K., Mayne, R., and Timpl, R. (2000) Structural and functional analysis of the recombinant G domain of the laminin 4 chain and its proteolytic processing in tissues. J. Biol. Chem., 275, 3519235199.
Tanzer, M.L., Chandrasekaran, S., Dean, J.W., and Giniger, M.S. (1993) Role of laminin carbohydrates on cellular interactions. Kid. Intl., 43, 6672.[ISI][Medline]
Tian, M., Jacobson, C., Gee, S.H., Campbell, K.P., Carbonetto, S., and Jucker, M. (1997) Dystroglycan in the cerebellum is a laminin 2-chain binding protein at the glial-vascular interface and is expressed in Purkinje cells. Eur. J. Neurosci., 8, 27392747.[ISI]
Timpl, R., Tisi, D., Talts, J.F., Andac, Z., Sasaki, T., and Hohenester, E. (2000) Structure and function of laminin LG modules. Matrix Biol., 19, 309317.[CrossRef][ISI][Medline]
Tisi, D., Talts, J.F., Timpl, R., and Hohenester, E. (2000) Structure of the C-terminal laminin G-like domain pair of the laminin 2 chain harbouring binding sites for
-dystroglycan and heparin. EMBO J., 19, 14321440.
Ushkaryov, Y.A., Petrenko, A.G., Geppert, M., and Sudhof, T.C. (1992) Neurexins: synaptic cell surface proteins related to the alpha-laitrotoxin receptor and laminin. Science, 257, 5056.[ISI][Medline]
Williamson, R.A., Henry, M.D., Daniels, K.J., Hrstka, R.F., Lee, J.C,. Sunada, Y., Ibraghimov-Beskrovnaya, O., Campbell, K.P. (1997) Dystroglycan is essential for early embryonic development: disruption of Reichert's membrane in Dag1-null mice. Hum. Mol. Genet., 6, 831841.
Wilson, I.B.H., Gavel, Y., and von Heijne, G. (1991) Amino acid distributions around O-linked glycosylation sites. Biochem. J., 275, 529534.[ISI][Medline]
Xia, B. and Martin, P.T. (2002) Modulation of agrin binding and activity by the CT and related carbohydrate antigens. Mol Cell. Neurosci., 19, 539551.[CrossRef][ISI][Medline]
Xia, B., Hoyte, K., Kammesheidt, A., Deerinck, T., Ellisman, M., and Martin, P.T. (2002) Overexpression of the CT GalNAc transferase in skeletal muscle alters myofiber growth, neuromuscular structure, and laminin expression. Dev. Biol., 242, 5873.[CrossRef][ISI][Medline]
Yamada, H., Chiba, A., Endo, T, Kobata, A., Anderson, L.V.B., Hori, H., Fukuta-Ohi, H., Kanazawa, I., Campbell, K.P., Shimizu, T., and Matsumura, K. (1996) Characterization of dystroglycan-laminin interaction in peripheral nerve. J. Neurochem., 66, 15181524.[ISI][Medline]
Yuen, C-T., Chai, W., Loveless, R.W., Lawson, A.M., Margolis, R.U., and Feizi, T. (1997) Brain contains HNK-1 immunoreactive O-glycans of the sulfoglucuronyl lactosamine series that terminate in 2-linked or 2,6-linked hexose (mannose). J. Biol. Chem., 272, 89248931.
Zaccaria, M.L., Di Tommaso, F., Brancaccio, A., Paggi, P., and Petrucci, T.C. (2001) Dystroglycan distribution in adult mouse brain: a light and electron microscopy study. Neuroscience, 104, 311324.[CrossRef][ISI][Medline]
Zhou, Q. and Cummings, R.D. (1993) L-14 lectin recognition of laminin and its promotion of in vitro cell adhesion. Arch. Biochem. Biophys., 300, 617.[CrossRef][ISI][Medline]