Dystrophin-Glycoprotein Complex: Post-translational Processing and Dystroglycan Function
2003; Elsevier BV; Volume: 278; Issue: 18 Linguagem: Inglês
10.1074/jbc.r200031200
ISSN1083-351X
AutoresDaniel E. Michele, Kevin P. Campbell,
Tópico(s)Adipose Tissue and Metabolism
Resumodystrophin-glycoprotein complex neuronal nitric-oxide synthase Fukuyama congenital muscular dystrophy muscle-eye-brain disease Walker-Warburg Syndrome fukutin-related protein limb-girdle muscular dystrophy The dystrophin-glycoprotein complex (DGC)1 is a multimeric transmembrane protein complex first isolated from skeletal muscle membranes (1Ervasti J.M. Campbell K.P. Cell. 1991; 66: 1121-1131Abstract Full Text PDF PubMed Scopus (1119) Google Scholar). The central protein of the DGC is dystroglycan (Fig.1). In addition to skeletal muscle, dystroglycan is strongly expressed in heart and smooth muscle, as well as many non-muscle tissues including brain and peripheral nerve. In vertebrates, dystroglycan is generated from a single gene (DAG1), which is cleaved into a peripheral α-dystroglycan protein and a transmembrane β-dystroglycan protein (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar). At the sarcolemma in muscle, β-dystroglycan binds intracellularly to dystrophin, which binds the actin cytoskeleton, and extracellularly to α-dystroglycan. α-Dystroglycan completes the link from the cytoskeleton to the basal lamina by calcium-dependent binding with high affinity to extracellular matrix proteins (Fig. 1) containing LamG domains, such as laminin (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar), neurexin (4Sugita S. Saito F. Tang J. Satz J. Campbell K. Sudhof T.C. J. Cell Biol. 2001; 154: 435-445Crossref PubMed Scopus (375) Google Scholar), agrin (5Bowe M.A. Deyst K.A. Leszyk J.D. Fallon J.R. Neuron. 1994; 12: 1173-1180Abstract Full Text PDF PubMed Scopus (278) Google Scholar, 6Campanelli J.T. Roberds S.L. Campbell K.P. Scheller R.H. Cell. 1994; 77: 663-674Abstract Full Text PDF PubMed Scopus (340) Google Scholar, 7Gee S.H. Montanaro F. Lindenbaum M.H. Carbonetto S. Cell. 1994; 77: 675-686Abstract Full Text PDF PubMed Scopus (455) Google Scholar, 8Sugiyama J. Bowen D.C. Hall Z.W. Neuron. 1994; 13: 103-115Abstract Full Text PDF PubMed Scopus (237) Google Scholar), and perlecan (9Talts J.F. Andac Z. Gohring W. Brancaccio A. Timpl R. EMBO J. 1999; 18: 863-870Crossref PubMed Scopus (398) Google Scholar). In addition to dystroglycan and dystrophin, the DGC in muscle cells contains a sarcoglycan complex composed of four sarcoglycan proteins (α, β, γ, δ) and sarcospan (1Ervasti J.M. Campbell K.P. Cell. 1991; 66: 1121-1131Abstract Full Text PDF PubMed Scopus (1119) Google Scholar, 10Durbeej M. Cohn R.D. Hrstka R.F. Moore S.A. Allamand V. Davidson B.L. Williamson R.A. Campbell K.P. Mol. Cell. 2000; 5: 141-151Abstract Full Text Full Text PDF PubMed Scopus (179) Google Scholar). Intracellularly, the sarcolemma DGC, through dystrophin, interacts with a pair of syntrophins (α1 and β1) (11Peters M.F. Adams M.E. Froehner S.C. J. Cell Biol. 1997; 138: 81-93Crossref PubMed Scopus (213) Google Scholar) and α-dystrobrevin (12Sadoulet-Puccio H.M. Rajala M. Kunkel L.M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 12413-12418Crossref PubMed Scopus (167) Google Scholar). α-Syntrophin and α-dystrobrevin can interact with nNOS and localize it to the sarcolemma (13Kameya S. Miyagoe Y. Nonaka I. Ikemoto T. Endo M. Hanaoka K. Nabeshima Y. Takeda S. J. Biol. Chem. 1999; 274: 2193-2200Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 14Grady R.M. Grange R.W. Lau K.S. Maimone M.M. Nichol M.C. Stull J.T. Sanes J.R. Nat. Cell Biol. 1999; 1: 215-220Crossref PubMed Scopus (283) Google Scholar). Syntrophin also can interact with aquaporin 4 through a PDZ domain and can stabilize it in the sarcolemma (15Neely J.D. Amiry-Moghaddam M. Ottersen O.P. Froehner S.C. Agre P. Adams M.E. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 14108-14113Crossref PubMed Scopus (394) Google Scholar). The C-terminal tail of β-dystroglycan also contains a PPXY motif that can interact with dystrophin or caveolin 3 (16Sotgia F. Lee J.K. Das K. Bedford M. Petrucci T.C. Macioce P. Sargiacomo M. Bricarelli F.D. Minetti C. Sudol M. Lisanti M.P. J. Biol. Chem. 2000; 275: 38048-38058Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar). The exact function of the entire DGC is not completely determined but it is thought to contribute to the structural stability of the muscle cell membrane during cycles of contraction and relaxation (17Campbell K.P. Cell. 1995; 80: 675-679Abstract Full Text PDF PubMed Scopus (762) Google Scholar). In humans, mutations in dystrophin cause Duchenne and Becker muscular dystrophy, mutations in sarcoglycans in skeletal muscle cause limb-girdle muscular dystrophy, and mutations in α2 laminin cause congenital muscular dystrophy (18Cohn R.D. Campbell K.P. Muscle Nerve. 2000; 23: 1456-1471Crossref PubMed Scopus (426) Google Scholar). Despite the central role of dystroglycan in the DGC, no primary mutations in dystroglycan have been identified in any human disease. However, mutations in dystrophin do cause a secondary reduction in sarcolemma expression of dystroglycan (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar). Disruption of the DAG1 gene in mice results in embryonic lethality, and dystroglycan appears essential for the formation of the basement membrane (Reichert's membrane) that separates the embryo from the maternal circulation in the mouse (19Williamson R.A. Henry M.D. Daniels K.J. Hrstka R.F. Lee J.C. Sunada Y. Ibraghimov-Beskrovnaya O. Campbell K.P. Hum. Mol. Genet. 1997; 6: 831-841Crossref PubMed Scopus (450) Google Scholar, 20Henry M.D. Campbell K.P. Cell. 1998; 95: 859-870Abstract Full Text Full Text PDF PubMed Scopus (342) Google Scholar). Emerging genetic data have shown that mutations in proteins with homology to glycosyltransferases are linked to murine and human muscular dystrophies. Biochemical analysis of muscle biopsies has revealed a convergent role for these proteins in the glycosylation of α-dystroglycan, a process that is required for its functional activity. The loss of dystroglycan function by incomplete glycosylation can lead to a variety of clinical symptoms including muscular dystrophy and abnormal central nervous system development and function. Here we review what is known about the biosynthetic pathway of dystroglycan required for its normal structure and function and the new insights into dystroglycan function revealed from the study of mouse models and human patients with incomplete glycosylation-induced “dystroglycanopathies.” Because the only detected DGC defect in these “dystroglycanopathies” is the disruption of the dystroglycan ligand binding domain, the recent work supports the proposal that the functions of components of the DGC in the sarcolemma of differentiated skeletal muscle are largely to support the integrity and sarcolemma localization of the central extracellular matrix receptor, dystroglycan. An N-terminal signal peptide directs insertion of dystroglycan into the endoplasmic reticulum membrane with the N terminus in the lumen (Fig. 2 A). Dystroglycan is then cleaved by an unidentified protease at amino acid 653 into the α- and β-dystroglycan subunits (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar, 21Smalheiser N.R. Kim E. J. Biol. Chem. 1995; 270: 15425-15433Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar). The significance of this cleavage is unknown, particularly because the amino acid sequence around the cleavage site in vertebrate dystroglycan is not conserved inCaenorhabditis elegans and Drosophila melanogaster, and Western blotting with a C-terminal antibody suggests that the dystroglycan peptide is not cleaved into two subunits in Drosophila (22Deng W.M. Schneider M. Frock R. Castillejo-Lopez C. Gaman E.A. Baumgartner S. Ruohola-Baker H. Development. 2003; 130: 173-184Crossref PubMed Scopus (148) Google Scholar). N-Linked and fullO-linked glycosylation and plasma membrane trafficking of dystroglycan are not required for cleavage of dystroglycan, suggesting that it may occur in the endoplasmic reticulum (23Holt K.H. Crosbie R.H. Venzke D.P. Campbell K.P. FEBS Lett. 2000; 468: 79-83Crossref PubMed Scopus (153) Google Scholar, 24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). The C-terminal region of α-dystroglycan (residues 550–585) binds to the N terminus of β-dystroglycan (residues 654–750) independently of glycosylation (25Sciandra F. Schneider M. Giardina B. Baumgartner S. Petrucci T.C. Brancaccio A. Eur. J. Biochem. 2001; 268: 4590-4597Crossref PubMed Scopus (38) Google Scholar). The sarcoglycan complex also appears to be required for a strong interaction of α-dystroglycan with the DGC in skeletal muscle. Sarcoglycan-null mutations, resulting in the loss of the sarcoglycan complex, result in dissociation of α-dystroglycan from muscle membranes and the DGC (10Durbeej M. Cohn R.D. Hrstka R.F. Moore S.A. Allamand V. Davidson B.L. Williamson R.A. Campbell K.P. Mol. Cell. 2000; 5: 141-151Abstract Full Text Full Text PDF PubMed Scopus (179) Google Scholar). Dystroglycan undergoes N-linked and extensiveO-linked glycosylation, which causes α-dystroglycan to migrate on SDS-PAGE as a broad band with an approximate molecular mass of 120–180 kDa depending on tissue type (156 kDa in muscle, predicted molecular mass is ∼75 kDa) (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar). α-Dystroglycan contains a large mucin-like domain with a number of Ser or Thr residues, which are potential sites for O-glycosylation (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar). Dystroglycan also contains four potential N-linked glycosylation sites, three in α-dystroglycan and one in β-dystroglycan (2Ibraghimov-Beskrovnaya O. Ervasti J.M. Leveille C.J. Slaughter C.A. Sernett S.W. Campbell K.P. Nature. 1992; 355: 696-702Crossref PubMed Scopus (1198) Google Scholar). Exhaustive treatment of dystroglycan with N-glycanases does not alter its activity as an extracellular matrix receptor suggesting thatN-linked sugars are not required for ligand binding (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar). However, full chemical deglycosylation of dystroglycan results in the complete loss of ligand binding activity (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar). Interestingly, the same chemical deglycosylation also results in the loss of reactivity of two monoclonal antibodies raised against dystroglycan, clones IIH6 and VIA41 (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar). IIH6 antibody inhibits laminin binding, suggesting thatO-linked sugars are required for both ligand binding and the reaction of these monoclonal antibodies with dystroglycan (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar). TheO-linked glycoconjugates contain a fairly unique sugar linkage where mannose is directly coupled to serine or threonine in the dystroglycan peptide (26Chiba A. Matsumura K. Yamada H. Inazu T. Shimizu T. Kusunoki S. Kanazawa I. Kobata A. Endo T. J. Biol. Chem. 1997; 272: 2156-2162Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 27Smalheiser N.R. Haslam S.M. Sutton-Smith M. Morris H.R. Dell A. J. Biol. Chem. 1998; 273: 23698-23703Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar). This O-mannosyl linkage has only been found in a few other mammalian proteins (28Endo T. Biochim. Biophys. Acta. 1999; 1473: 237-246Crossref PubMed Scopus (127) Google Scholar). In peripheral nerve dystroglycan, this Neu5Ac(α2–3)Gal(β1–4)GlcNAc(β1–2)Man-Ser/Thr glycoconjugate has been proposed to be required for laminin binding (26Chiba A. Matsumura K. Yamada H. Inazu T. Shimizu T. Kusunoki S. Kanazawa I. Kobata A. Endo T. J. Biol. Chem. 1997; 272: 2156-2162Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar). However, millimolar concentrations of this glycoconjugate are required to competitively inhibit laminin binding to dystroglycan (26Chiba A. Matsumura K. Yamada H. Inazu T. Shimizu T. Kusunoki S. Kanazawa I. Kobata A. Endo T. J. Biol. Chem. 1997; 272: 2156-2162Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar). Sialic acid has also been reported to competitively inhibit laminin binding to peripheral nerve dystroglycan at lower concentrations than the above glycoconjugate (29Yamada H. Chiba A. Endo T. Kobata A. Anderson L.V. Hori H. Fukuta-Ohi H. Kanazawa I. Campbell K.P. Shimizu T. Matsumura K. J. Neurochem. 1996; 66: 1518-1524Crossref PubMed Scopus (100) Google Scholar). However, enzymatic removal of sialic acid from skeletal muscle dystroglycan has no effect on laminin binding (30Ervasti J.M. Burwell A.L. Geissler A.L. J. Biol. Chem. 1997; 272: 22315-22321Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Furthermore, dystroglycan from brain and cardiac muscle has decreased terminal sialoglycosylation compared with skeletal muscle dystroglycan, but dystroglycan from all three tissues binds tightly to laminin (30Ervasti J.M. Burwell A.L. Geissler A.L. J. Biol. Chem. 1997; 272: 22315-22321Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Therefore, although it is well accepted that glycoconjugates are indeed important for ligand binding, the hypothesis that specific glycoconjugates on dystroglycan are involved directly in ligand binding is still controversial. The extensive glycosylation of dystroglycan has made the elucidation of the structure of the peptide and glycoconjugates and the generation of peptide antibodies technically challenging. Rotary shadowing and electron microscopy of purified chick α-dystroglycan showed dumbbell-shaped particles with two globular domains linked by a flexible segment (31Brancaccio A. Schulthess T. Gesemann M. Engel J. FEBS Lett. 1995; 368: 139-142Crossref PubMed Scopus (137) Google Scholar). Presumably, the flexible linker domain corresponds to the mucin domain extended by the large number of attached glycoconjugates. The crystal structure of the C terminus of dystrophin with a β-dystroglycan peptide has revealed that an EF hand domain in dystrophin stabilizes an interaction of a WW motif (PPXY) in the C-terminal tail of β-dystroglycan with a WW domain in dystrophin (32Huang X. Poy F. Zhang R. Joachimiak A. Sudol M. Eck M.J. Nat. Struct. Biol. 2000; 7: 634-638Crossref PubMed Scopus (225) Google Scholar). The crystal structure of the α2 laminin G-domains reveals that a coordinated calcium ion is surrounded by a basic surface that is thought to provide the site for interactions with acid glycoconjugates (such as sialic acid) on dystroglycan (33Tisi D. Talts J.F. Timpl R. Hohenester E. EMBO J. 2000; 19: 1432-1440Crossref PubMed Google Scholar). How this structural surface on the G-domain confers a specific interaction with dystroglycan and not with other sialic acid-containing glycoproteins is not clear. The complete enzymatic pathway for dystroglycan processing, the exact binding site for ligands on dystroglycan, and the direct or structural role of dystroglycan glycoconjugates in ligand binding activity is still not determined. Despite the fact that mutations in dystroglycan in humans have not been found, recent studies have indicated, rather surprisingly, that the dystroglycan post-translational processing pathway is a convergent target for many human muscular dystrophies (supplemental Table I). Fukuyama congenital muscular dystrophy (FCMD), muscle-eye-brain disease (MEB), and Walker-Warburg Syndrome (WWS) are severe congenital muscular dystrophies with mental retardation, neuronal migration defects including cobblestone lissencephaly, and variable ocular anomalies (MEB and WWS) (34Voit T. Brain Dev. 1998; 20: 65-74Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). The FCMD gene mutation was first identified as a retrotransposon insertion into the fukutin gene (35Kobayashi K. Nakahori Y. Miyake M. Matsumura K. Kondo-Iida E. Nomura Y. Segawa M. Yoshioka M. Saito K. Osawa M. Hamano K. Sakakihara Y. Nonaka I. Nakagome Y. Kanazawa I. Nakamura Y. Tokunaga K. Toda T. Nature. 1998; 394: 388-392Crossref PubMed Scopus (0) Google Scholar). The function of fukutin is unknown, but the protein has homology to the fringe-like family of enzymes that modify glycolipids and glycoproteins (36Aravind L. Koonin E.V. Curr. Biol. 1999; 9: R836-R837Abstract Full Text Full Text PDF PubMed Google Scholar). More recently, the gene responsible for MEB was identified as anO-mannosyl-β1,2-N-acetylglucosaminyltransferase (POMGNT1)(37). A homologue for fukutin was also identified in humans, termed fukutin-related protein (FKRP), and is mutated in a form of congenital muscular dystrophy and a milder dystrophy, limb-girdle muscular dystrophy 2I (LGMD2I) (38Brockington M. Yuva Y. Prandini P. Brown S.C. Torelli S. Benson M.A. Herrmann R. Anderson L.V. Bashir R. Burgunder J.M. Fallet S. Romero N. Fardeau M. Straub V. Storey G. Pollitt C. Richard I. Sewry C.A. Bushby K. Voit T. Blake D.J. Muntoni F. Hum. Mol. Genet. 2001; 10: 2851-2859Crossref PubMed Google Scholar, 39Brockington M. Blake D.J. Prandini P. Brown S.C. Torelli S. Benson M.A. Ponting C.P. Estournet B. Romero N.B. Mercuri E. Voit T. Sewry C.A. Guicheney P. Muntoni F. Am. J. Hum. Genet. 2001; 69: 1198-1209Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar). The identification of dystroglycan as the post-translational target responsible for the phenotype in these enzyme disorders originated largely from the study of human patient muscle biopsies. A few preliminary reports had suggested that α-dystroglycan was missing in the muscle fiber membrane in MEB and FCMD (40Kano H. Kobayashi K. Herrmann R. Tachikawa M. Manya H. Nishino I. Nonaka I. Straub V. Talim B. Voit T. Topaloglu H. Endo T. Yoshikawa H. Toda T. Biochem. Biophys. Res. Commun. 2002; 291: 1283-1286Crossref PubMed Scopus (114) Google Scholar, 41Hayashi Y.K. Ogawa M. Tagawa K. Noguchi S. Ishihara T. Nonaka I. Arahata K. Neurology. 2001; 57: 115-121Crossref PubMed Scopus (218) Google Scholar). However, recent data convincingly show in MEB and FCMD muscle that core α-dystroglycan protein and the entire DGC are present at the sarcolemma, but α-dystroglycan is shifted in molecular mass by ∼60 kDa (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). The apparent altered post-translational modification of α-dystroglycan results in the absence of epitopes for the monoclonal antibodies, IIH6 and V1A41, similar to the chemical deglycosylation experiments performed by Ervasti and Campbell (3Ervasti J.M. Campbell K.P. J. Cell Biol. 1993; 122: 809-823Crossref PubMed Scopus (1185) Google Scholar). Based also on findings of mutations in POMGNT1 in the MEB patients (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar), this shift can be attributed to abnormal O-mannosyl glycosylation of α-dystroglycan. This hypoglycosylation causes α-dystroglycan to be non-functional as a receptor for its known extracellular matrix proteins, including laminin, neurexin, and agrin (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Interestingly, sarcolemma glycoproteins prepared by lectin chromatography (that also enriched both normal and mutant dystroglycan) revealed an almost complete loss of total high affinity laminin binding activity, suggesting that dystroglycan is one of the major glycoprotein laminin receptors in human muscle sarcolemma (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Because mutations in laminin α2 (42Helbling-Leclerc A. Zhang X. Topaloglu H. Cruaud C. Tesson F. Weissenbach J. Tome F.M. Schwartz K. Fardeau M. Tryggvason K. Guicheney P. Nat. Genet. 1995; 11: 216-218Crossref PubMed Scopus (572) Google Scholar) and dystrophin (43Hoffman E.P. Brown Jr., R.H. Kunkel L.M. Cell. 1987; 51: 919-928Abstract Full Text PDF PubMed Scopus (3689) Google Scholar) also cause muscular dystrophy, the muscle phenotype is likely due to a similar loss of the functional link across the sarcolemma by dystroglycan from dystrophin to laminin. However, a major question remained whether dystroglycan was responsible for the neuronal migration phenotype in human patients. A mutation in the spontaneous mutant myodystrophy (myd) mouse was identified by positional cloning in the gene LARGE, which also encodes a putative glycosyltransferase (44Grewal P.K. Holzfeind P.J. Bittner R.E. Hewitt J.E. Nat. Genet. 2001; 28: 151-154Crossref PubMed Scopus (280) Google Scholar). Despite more than 25 years since the identification of the myd mutant mouse, no brain phenotype had been described (45Lane P.W. Beamer T.C. Myers D.D. J. Hered. 1976; 67: 135-138Crossref PubMed Scopus (63) Google Scholar). Careful examination of the brains of the myd mice revealed abnormal glycosylation of α-dystroglycan leading to the functional loss of ligand binding activity similar to myd skeletal muscle, abnormal cerebral cortical layering resembling human cobblestone lissencephaly, and defects in cerebellar granule cell migration (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Because a number of proteins could be targeted by this enzymatic pathway, it was still unclear if the functional defect in α-dystroglycan was sufficient to account for the brain phenotype. Using cre-LoxP gene targeting, the dystroglycan gene was deleted in the mouse brain, and the abnormal neuronal migration closely mimicked the myd mouse and resembled lissencephaly seen in FCMD, MEB, and WWS patients (46Moore S.A. Saito F. Chen J. Michele D.E. Henry M.D. Messing A. Cohn R.D. Ross-Barta S.E. Westra S. Williamson R.A. Hoshi T. Campbell K.P. Nature. 2002; 418: 422-425Crossref PubMed Scopus (486) Google Scholar). The very recent identification of the human WWS gene as a putativeO-mannosyltransferase (POMT1) and the demonstrated loss of glycosylated α-dystroglycan epitopes in WWS muscle biopsies (47Beltran-Valero De Bernabe D. Currier S. Steinbrecher A. Celli J. Van Beusekom E. Van Der Zwaag B. Kayserili H. Merlini L. Chitayat D. Dobyns W.B. Cormand B. Lehesjoki A.E. Cruces J. Voit T. Walsh C.A. Van Bokhoven H. Brunner H.G. Am. J. Hum. Genet. 2002; 71: 1033-1043Abstract Full Text Full Text PDF PubMed Scopus (619) Google Scholar) suggests that MEB, WWS, FCMD, and mydmuscle and brain phenotypes can be explained by a loss of function of α-dystroglycan due to abnormal glycosylation. Likely, not all the genes participating in this α-dystroglycan processing pathway have been identified, and mutations in these genes may be responsible for unexplained forms of muscular dystrophy and diseases of abnormal neuronal migration (Fig. 2 B). In patients with FKRP mutations, no developmental brain phenotype is apparent (38Brockington M. Yuva Y. Prandini P. Brown S.C. Torelli S. Benson M.A. Herrmann R. Anderson L.V. Bashir R. Burgunder J.M. Fallet S. Romero N. Fardeau M. Straub V. Storey G. Pollitt C. Richard I. Sewry C.A. Bushby K. Voit T. Blake D.J. Muntoni F. Hum. Mol. Genet. 2001; 10: 2851-2859Crossref PubMed Google Scholar, 39Brockington M. Blake D.J. Prandini P. Brown S.C. Torelli S. Benson M.A. Ponting C.P. Estournet B. Romero N.B. Mercuri E. Voit T. Sewry C.A. Guicheney P. Muntoni F. Am. J. Hum. Genet. 2001; 69: 1198-1209Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar) suggesting that another enzyme(s) might compensate for FKRP activity in brain. This also raises the intriguing possibility that enzymes involved in glycosylating dystroglycan specifically in brain may be responsible for unexplained forms of human lissencephaly without muscular dystrophy. The variable eye pathology in MEB, WWS, and FCMD (largely absent in the latter syndrome) suggests there may be additional genes or targets for these pathways in the eye. In the recent report on WWS patients, only 5 of 13 patients had mutations in POMT1 suggesting that additional unidentified enzymes may be required for dystroglycan glycosylation in these patients (47Beltran-Valero De Bernabe D. Currier S. Steinbrecher A. Celli J. Van Beusekom E. Van Der Zwaag B. Kayserili H. Merlini L. Chitayat D. Dobyns W.B. Cormand B. Lehesjoki A.E. Cruces J. Voit T. Walsh C.A. Van Bokhoven H. Brunner H.G. Am. J. Hum. Genet. 2002; 71: 1033-1043Abstract Full Text Full Text PDF PubMed Scopus (619) Google Scholar). myd mice also escape the embryonic lethality seen in dystroglycan knock-out mice suggesting a potential developmentally regulated compensatory enzyme for LARGE. ThePOMT1 gene is a homologue of rotated abdomen inDrosophila (48Jurado L.A. Coloma A. Cruces J. Genomics. 1999; 58: 171-180Crossref PubMed Scopus (108) Google Scholar). Rotated abdomen mutants show defects in myogenesis (49Martin-Blanco E. Garcia-Bellido A. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 6048-6052Crossref PubMed Scopus (92) Google Scholar), although it has not been demonstrated that this phenotype is caused by a defect in Drosophiladystroglycan. Finally, the cellular biology and enzymatic activities of fukutin, FKRP, POMT1, and LARGE have not been experimentally determined. Surprisingly, mutations in LARGE andfukutin cause a similar molecular mass shift (∼60 kDa) of α-dystroglycan as observed in MEB patients (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). This suggests that LARGE and fukutin may interact with POMGNT1 or POMT1 by supplying precursor sugars or directly modulating their enzymatic activity (Fig.2 B). The studies of α-dystroglycan post-translational processing in human patients and mouse models have revealed important new insights into the function of dystroglycan and the DGC. Although the genetic role of the DGC in muscular dystrophy is well established, the functional role of dystroglycan in skeletal muscle is still debated. Because dystroglycan is important for assembly of Reichert's membrane in developing embryos (19Williamson R.A. Henry M.D. Daniels K.J. Hrstka R.F. Lee J.C. Sunada Y. Ibraghimov-Beskrovnaya O. Campbell K.P. Hum. Mol. Genet. 1997; 6: 831-841Crossref PubMed Scopus (450) Google Scholar, 20Henry M.D. Campbell K.P. Cell. 1998; 95: 859-870Abstract Full Text Full Text PDF PubMed Scopus (342) Google Scholar), it was hypothesized that skeletal muscle basement membranes would be severely disrupted when dystroglycan was functionally or genetically disrupted. However, despite large disruptions of basement membrane and matrix protein isoform expression in myd mouse brains, the skeletal muscle basement membranes in myd mice are morphologically intact with normal matrix protein isoform expression and localization (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Muscle basement membrane formation is also normal when the dystroglycan gene is specifically targeted in muscle (50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar). This suggests that dystroglycan may not be essential as a basement membrane organizer in muscle, and additional matrix receptors may partially compensate for the functional loss of dystroglycan. Integrins containing β1 isoforms are also required in concert with dystroglycan for complete embryonic basement membrane assembly (51Henry M.D. Satz J.S. Brakebusch C. Costell M. Gustafsson E. Fassler R. Campbell K.P. J. Cell Sci. 2001; 114: 1137-1144Crossref PubMed Google Scholar, 52Li S. Harrison D. Carbonetto S. Fassler R. Smyth N. Edgar D. Yurchenco P.D. J. Cell Biol. 2002; 157: 1279-1290Crossref PubMed Scopus (256) Google Scholar, 53Fassler R. Meyer M. Genes Dev. 1995; 9: 1896-1908Crossref PubMed Scopus (615) Google Scholar) and may in part compensate for the loss of dystroglycan as a basement membrane organizer in muscle (54Colognato H. Winkelmann D.A. Yurchenco P.D. J. Cell Biol. 1999; 145: 619-631Crossref PubMed Scopus (255) Google Scholar, 55Burkin D.J. Wallace G.Q. Nicol K.J. Kaufman D.J. Kaufman S.J. J. Cell Biol. 2001; 152: 1207-1218Crossref PubMed Scopus (234) Google Scholar). The function of dystroglycan in muscle has also been examined by specific genetic targeting of the DAG1 gene in mice. Chimeric mice generated from targeted embryonic stem cells showed that the genetic reduction of dystroglycan expression in muscle could cause muscular dystrophy (56Cote P.D. Moukhles H. Lindenbaum M. Carbonetto S. Nat. Genet. 1999; 23: 338-342Crossref PubMed Scopus (201) Google Scholar). Similar to myd mice, muscle basement membrane formation was morphologically normal in dystroglycan-null chimeric mice (56Cote P.D. Moukhles H. Lindenbaum M. Carbonetto S. Nat. Genet. 1999; 23: 338-342Crossref PubMed Scopus (201) Google Scholar). In addition, although dystroglycan may still play an important role as an agrin receptor in the development of the neuromuscular junction, dystroglycan expression is not required for initial formation of this sarcolemma specialization in response to agrin (57Grady R.M. Zhou H. Cunningham J.M. Henry M.D. Campbell K.P. Sanes J.R. Neuron. 2000; 25: 279-293Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar, 58Jacobson C. Cote P.D. Rossi S.G. Rotundo R.L. Carbonetto S. J. Cell Biol. 2001; 152: 435-450Crossref PubMed Scopus (171) Google Scholar). More recently, the DAG1 gene was targeted by cre-LoxP technology in differentiated skeletal muscle and resulted in normal muscle basement membrane formation but a surprisingly mild dystrophic phenotype (50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar). The creatine kinase promoter used to express cre-recombinase in these studies failed to target dystroglycan expressed in satellite cells, and aged mice displayed a remarkable ability to continue to regenerate muscle compared with other DGC-associated dystrophic mouse models (50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar). This suggests that dystroglycan may play an important role in satellite cell survival or function. Interestingly, muscle biopsies from human patients with a dystroglycan post-translational processing defect and mild limb-girdle muscular dystrophy were also examined (most likely LGMD2I, although not determined in this study). These patients showed a similar expression pattern of residual normally processed dystroglycan from satellite cells after regeneration (50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar). This could be explained by an additional satellite cell-enzyme that compensates for the mutant enzyme in these LGMD patients, but the expression of the enzyme turns off as the myoblasts differentiate during the regeneration process. This phenomenon of residual normal dystroglycan expression from regenerating satellite cells is not demonstrated in the severe muscular dystrophies of myd mice and FCMD or MEB patients (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Therefore, precise understanding of the dystroglycan processing pathway and its developmental regulation may shed important light into how dystroglycan modulates satellite cell function and how dystroglycan processing might be targeted therapeutically to increase functional dystroglycan expression, promote muscle regeneration, and improve the dystrophic phenotype. Despite the loss of laminin binding activity of dystroglycan inmyd, FCMD, and MEB muscle, the remaining DGC proteins can still localize to the sarcolemma (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). Also, dystrophin and dystrophin-associated proteins can still localize to the sarcolemma after the genetic deletion of the entire dystroglycan protein in muscle (50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar, 59Cote P.D. Moukhles H. Carbonetto S. J. Biol. Chem. 2002; 277: 4672-4679Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar). In contrast, the abnormal glycosylation of α-dystroglycan in myd mouse brain causes the failure of targeting of many normally expressed DGC-related proteins, including dystrophin, to neural synapses and glial end feet (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar). For muscle, this suggests the DGC may be targeted to the sarcolemma either by a stronger association with the actin cytoskeleton through the 400-kDa skeletal muscle dystrophin isoform, or perhaps an additional sarcolemma protein that stabilizes dystrophin or the DGC at the sarcolemma. In brain, dystroglycan, specifically by its ability to bind ligands, has a unique scaffold function to recruit proteins associated with the DGC to the localized structures with glia and neurons. It is still unknown which molecules are targeted to the synapse by dystroglycan, which ligands are important, and if those molecules are directly responsible for the functional synaptic defect in brain-specific dystroglycan knock-out mice (46Moore S.A. Saito F. Chen J. Michele D.E. Henry M.D. Messing A. Cohn R.D. Ross-Barta S.E. Westra S. Williamson R.A. Hoshi T. Campbell K.P. Nature. 2002; 418: 422-425Crossref PubMed Scopus (486) Google Scholar). It is tempting to speculate that the failure of recruiting functional molecules to the synapse by dystroglycan and dystrophin may in part underlie the cognitive impairment in FCMD, MEB, WWS, and a subset of dystrophin-associated muscular dystrophies (34Voit T. Brain Dev. 1998; 20: 65-74Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar,60Bardoni A. Felisari G. Sironi M. Comi G. Lai M. Robotti M. Bresolin N. Neuromuscul. Disord. 2000; 10: 194-199Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). The finding that the DGC proteins can still localize to the sarcolemma in dystroglycan glycosylation-deficient muscular dystrophies (24Michele D.E. Barresi R. Kanagawa M. Saito F. Cohn R.D. Satz J.S. Dollar J. Nishino I. Kelley R.I. Somer H. Straub V. Mathews K.D. Moore S.A. Campbell K.P. Nature. 2002; 418: 417-421Crossref PubMed Scopus (694) Google Scholar, 50Cohn R.D. Henry M.D. Michele D.E. Barresi R. Saito F. Moore S.A. Flanagan J.D. Skwarchuk M.W. Robbins M.E. Mendell J.R. Williamson R.A. Campbell K.P. Cell. 2002; 110: 639Abstract Full Text Full Text PDF PubMed Scopus (233) Google Scholar,59Cote P.D. Moukhles H. Carbonetto S. J. Biol. Chem. 2002; 277: 4672-4679Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar) also sheds new light on how the entire DGC may function. Several studies in mouse models of DGC-associated muscular dystrophies have attempted to assign a signaling molecule scaffold function to the DGC or direct roles of functional molecules associated with the DGC (such as aquaporin 4 and nNOS) in the pathogenesis of muscular dystrophy (reviewed in Refs. 61Winder S.J. Trends Biochem. Sci. 2001; 26: 118-124Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar and 62Bredt D.S. Nat. Cell Biol. 1999; 1: E89-E91Crossref PubMed Scopus (30) Google Scholar). However, in the abnormal glycosylation dystroglycanopathies, dystroglycan, the DGC, and associated proteins are correctly targeted to the sarcolemma. Only the extracellular ligand binding domain of α-dystroglycan is disrupted, and muscular dystrophy still develops. Therefore, the functional roles of the DGC proteins, such as the sarcoglycans and to some extent dystrophin, in relationship to dystroglycan, may be more similar to the relationship of structural accessory proteins to the pore-forming subunits of ion channels. Sarcoglycans may function primarily to stabilize the dystroglycan α and β subunit interactions (10Durbeej M. Cohn R.D. Hrstka R.F. Moore S.A. Allamand V. Davidson B.L. Williamson R.A. Campbell K.P. Mol. Cell. 2000; 5: 141-151Abstract Full Text Full Text PDF PubMed Scopus (179) Google Scholar), and dystrophin provides cytoskeleton interactions to stabilize and target dystroglycan to the sarcolemma. Dystroglycan, as the central component, contributes an important function as a ligand receptor and adhesive protein that helps stabilize the sarcolemma relative to the extracellular matrix. The lack of an essential signaling role of the DGC-associated proteins would be consistent with a lack of muscular dystrophy phenotype in aquaporin 4-null, nNOS-null, and syntrophin-null mutations in mice that dissociate or remove nNOS from the sarcolemma (13Kameya S. Miyagoe Y. Nonaka I. Ikemoto T. Endo M. Hanaoka K. Nabeshima Y. Takeda S. J. Biol. Chem. 1999; 274: 2193-2200Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 63Yang B. Verbavatz J.M. Song Y. Vetrivel L. Manley G. Kao W.M. Ma T. Verkman A.S. Am. J. Physiol. 2000; 278: C1108-C1115Crossref PubMed Google Scholar, 64Crosbie R.H. Straub V. Yun H.Y. Lee J.C. Rafael J.A. Chamberlain J.S. Dawson V.L. Dawson T.M. Campbell K.P. Hum. Mol. Genet. 1998; 7: 823-829Crossref PubMed Scopus (102) Google Scholar). The function of α-dystrobrevin as a cytoplasmic protein of the DGC in this hypothesis is less certain, because dystrobrevin-null mice have normal DGC and dystroglycan localization to the sarcolemma but still have a very mild myopathy (14Grady R.M. Grange R.W. Lau K.S. Maimone M.M. Nichol M.C. Stull J.T. Sanes J.R. Nat. Cell Biol. 1999; 1: 215-220Crossref PubMed Scopus (283) Google Scholar). However, because dystrobrevin isoforms can interact with dystrophin (12Sadoulet-Puccio H.M. Rajala M. Kunkel L.M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 12413-12418Crossref PubMed Scopus (167) Google Scholar) and sarcoglycans (65Yoshida M. Hama H. Ishikawa-Sakurai M. Imamura M. Mizuno Y. Araishi K. Wakabayashi-Takai E. Noguchi S. Sasaoka T. Ozawa E. Hum. Mol. Genet. 2000; 9: 1033-1040Crossref PubMed Scopus (119) Google Scholar), it remains to be tested whether or not dystrobrevin may indirectly destabilize the α-dystroglycan association with the DGC resulting in a partial phenotype (i.e. by altering sarcoglycan function) without modulating the sarcolemma DGC localization. In summary, the work over the last 10 years on the biochemistry of dystroglycan and its interaction with ligands and the development of specific antibody reagents allowed for the identification of the mechanism causing several inherited human muscular dystrophies. In turn, the genetic data on human patients and mutant mice are identifying the important players in the basic biology of theO-mannosylation pathway that is required for dystroglycan function. With the combination of appropriate genetic modeling in mice, the full circle is being completed to fully understand the enzymatic processing and function of dystroglycan in muscle and non-muscle tissues. Hopefully, through this work, appropriate therapeutic targets might be revealed to restore normal dystroglycan processing and/or function to prevent the development of dystroglycan-associated diseases. Download .pdf (.01 MB) Help with pdf files
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