602 Dystroglycan inside and out Michael D Henry* and Kevin P Campbell† Dystroglycan connects the extracellular matrix and present in mature dystroglycan — a processing event that cytoskeleton. Key findings in the past year indicate that still remains largely undefined. α-dystroglycan is an extra- dystroglycan interacts with a wider repertoire of extracellular cellular peripheral membrane glycoprotein, bristling with ligands than originally appreciated, that dystroglycan plays a O-linked carbohydrates, which is noncovalently attached critical role in organizing extracellular matrix molecules on the to the membrane-spanning β-dystroglycan [4]. β-dystro- cell surface and in basement membranes, and that at least two glycan, in turn, binds to a variety of cytoplasmic molecules. human pathogens utilize dystroglycan to gain access to host The best understood examples are its interactions with cells. Together, these advances begin to help elucidate dystrophin and related molecules. Within the context of important biological roles for dystroglycan in development and the dystrophin–glycoprotein complex in skeletal muscle, disease. the carboxy-terminal 15 amino-acid residues of β-dystro- glycan bind tightly to a region of dystrophin containing Addresses WW (Trp–Trp) domain and Ca2+-binding motifs [5,6]. Howard Hughes Medical Institute, Department of Physiology and Biophysics and Department of Neurology, University of Iowa College of Medicine, 400 Eckstein Medical Research Building, Iowa City, Figure 1 IA 52242, USA *e-mail: [email protected] †e-mail: [email protected] PER LM-2 Current Opinion in Cell Biology 1999, 11:602–607 AGR 0955-0674/99/$ — see front matter © 1999 Elsevier Science Ltd. LM-1 All rights reserved. Abbreviations Arenavirus AChR acetylcholine receptor CNS central nervous system HSPG heparan sulphate proteoglycan αDG LG domain laminin globular domain MuSK muscle-specific kinase OUT Introduction SG SSPN Dystroglycan was originally isolated from skeletal muscle as an integral membrane component of the Plasma membrane dystrophin–glycoprotein complex [1]. When the gene βDG encoding dystroglycan was cloned in 1992, expression-pat- Integrins tern analysis suggested roles for dystroglycan beyond muscle [2]. Indeed, it is now apparent that dystroglycan IN does have important roles outside of muscle, which cover a broad range of biological inquiry. Generally, dystroglycan DYS/UTR RAP provides a transmembrane linkage between the extracellu- GRB2 lar matrix and the cytoskeleton [3]. In this review, we focus Current Opinion in Cell Biology on advances made in the past year exploring the molecular associations of dystroglycan both inside and outside of the A guide to the dystroglycan intermolecular interactions discussed in cell (see a guide to these in Figure 1); how this information this review. Outside of the cell (OUT) α-dystroglycan (block dumbbell) has helped further elucidate dystroglycan function; and binds directly to the following extracellular matrix molecules: laminin-1 how understanding dystroglycan function might lead to a (LM-1), laminin-2 (LM-2), perlecan (PER), and agrin (AGR). Several arenaviruses also bind directly to α-dystroglycan. Inside of the cell (IN), deeper understanding of several human diseases. the cytoplasmic domain of β-dystroglycan (black rectangle) binds to cytoskeletal molecules including members of the dystrophin family Interaction of b-dystroglycan with molecules (DYS) and perhaps utrophin (UTR), the signaling adapter molecule inside the cell grb2 (GRB2), and rapsyn (RAP), which is involved in the formation of the neuromuscular junction. Other transmembrane molecules might To date, only one highly conserved dystroglycan gene has interact with dystroglycan. Evidence indicates that the sarcoglycan been discovered in mammals and several dystroglycan complex (SG) and perhaps sarcospan (SSPN) help to stabilize the orthologues have emerged from invertebrate databases association of α-dystroglycan with the cell surface. Integrins are not yet ([2]; S Baumgartner, J Kramer, personal communication). known to interact directly with dystroglycan, but they do bind (dotted arrows) to members of the laminin family and can be co-localized with This gene, which encodes a single mRNA species, is post- dystroglycan in certain cell types. translationally cleaved into α and β polypeptide subunits Dystroglycan inside and out Henry and Campbell 603 Outside of muscle and brain, full-length dystrophin is not antisense-inhibited myotubes, though defective in AChR expressed but alternative products from the dystrophin cluster formation, show normal MuSK and AChR phospho- locus containing the dystroglycan interaction domain are; rylation patterns [20]. Further suggestion of a maintenance as is utrophin, which bears a conserved dystroglycan inter- role for dystroglycan in AChR clustering is provided by the action domain. Saito et al. [7•] has provided some insight to observation that following axotomy (severing a nerve), dys- this linkage in Schwann cells. In those cells, Dp116, a trun- troglycan immunoreactivity disappears before that of the cated version of dystrophin, interacts with β-dystroglycan. AChR clusters as the synapses fall apart [21]. Along with Dp71 [5], Dp116 appears to be an important dystrophin-isoform-binding partner for dystroglycan in the A growing list of extracellular ligands for central nervous system (CNS) [8]. Compared with dys- dystroglycan trophin from skeletal muscle, the connection between α-dystroglycan was originally shown to bind laminin-1 [2]. Dp116 and β-dystroglycan is apparently more tenuous. This high affinity interaction occurs through the carboxy- These authors speculate that this could be due to the lack terminal region of the laminin α1 polypeptide — the of a detectable sarcoglycan complex in Schwann cells. It so-called E3 domain [22]. This domain contains two ~190 was previously suggested that the sarcoglycan complex amino acid laminin G (LG) modules (α1LG4 and α1LG5). would serve to stabilize dystroglycan associations by stud- Andac et al. [23••] showed that chicken lung α-dystrogly- ies showing destabilization of dystroglycan interactions in can preferentially interacts with the α1LG4 module. This skeletal muscle of sarcoglycan-deficient animal models same module demonstrated a heparin-binding site consis- [9,10]. How sarcoglycans stabilize dystroglycan binding tent with heparin inhibition of laminin-1 binding [24]. interactions is not known, but recent evidence suggests Point mutations revealed that a series of basic residues that δ-sarcoglycan is in close molecular proximity to mediated dystroglycan binding and the heparin site consti- α–β-dystroglycan [11]. Furthermore, how dystroglycan tuted a subset of these residues. Surprisingly, Talts et al. function might be affected by sarcoglycans or isoforms of [25••] has shown that α-dystroglycan actually exhibits dystrophin (and vice versa) is unknown, but the tissue spe- slightly greater affinity to the LG1–3 modules of the cific expression patterns of these molecules vis-à-vis laminin α2 chain than its LG4–5 modules. These homolo- dystroglycan suggests a possible regulatory role. Clearly, gous modules would be present in the E8 fragment of defining the makeup of dystroglycan complexes in differ- laminin-1, which does not appreciably bind to α-dystrogly- ent tissues is a critical task. can [22]. Moreover, unlike the α1 chain, the α2LG5 domain bound heparin more strongly than the α2LG4 Other molecules that bind to the cytoplasmic tail of domain. Of course, one should be cautious in using the β-dystroglycan include Grb2 and rapsyn. In the case of activities of recombinant expressed fragments and prote- Grb2, this interaction has been known for some time but olytic cleavage products of laminins to draw firm its biological significance has remained elusive [12]. conclusions about the binding sites in the native protein. Cavaldesi et al. [13] has recently shown that a complex Nevertheless, a picture is emerging from this work and ear- isolated from bovine brain synaptosomes by laminin lier studies [19] that not all LG domains are created equal chromatography contained dystroglycan, Grb2 and focal with respect to their interactions with α-dystroglycan, and adhesion kinase. Although it is not yet clear whether focal that co-operative interactions among LG modules might adhesion kinase and dystroglycan are connected through be required for expression of full dystroglycan binding Grb2 — possibly through Src-homology (SH)2 and SH3 activity. This could have important implications for future mediated interactions respectively — this result, neverthe- work aimed at determining whether dystroglycan interacts less, further fuels speculation that dystroglycan could be with other proteins containing LG domains, even other involved in signal transduction pathways. members of the laminin family. Rapsyn plays an important role in clustering acetylcholine An example of an LG-domain-containing protein that receptors (AChRs) during the formation of the neuromus- binds to dystroglycan is perlecan. Perlecan is a heparan- cular synapse [14]. It is now apparent in electrocytes of sulphate proteoglycan (HSPG) found ubiquitously Torpedo californica that rapsyn binds directly to the jux- throughout basement membranes in tissues. It was tamembrane portion of the cytoplasmic domain, a site demonstrated earlier that binding occurred between distinct from the dystrophin binding site [15•]. This inter- α-dystroglycan and agrin, an HSPG involved in