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Syndecan Signaling: When, Where and Why?

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Syndecan Signaling: When, Where and Why? JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2009, 60, Suppl 4, 31-38 www.jpp.krakow.pl H.A.B. MULTHAUPT1, A. YONEDA1, J.R. WHITEFORD1, E.-S. OH2, W. LEE3, J.R. COUCHMAN1 SYNDECAN SIGNALING: WHEN, WHERE AND WHY? 1Department of Biomedical Sciences, BRIC, University of Copenhagen, Denmark; 2Department of Life Sciences, Ewha Womans University, Seoul, Korea; 3Department of Biochemistry, College of Life Science & Biotechnology, Yonsei University, Seoul, Korea Syndecans are the only family of transmembrane heparan sulphate proteoglycans. Invertebrates all appear to have one Syndecan core protein, but in mammals there are four. Examination of the core protein sequences shows that the cytoplasmic domains are the most conserved. This suggests that Syndecans make important interactions and/or signalling contributions. It has been established that all syndecans can interact with proteins of the actin-associated cytoskeleton, but details of signalling have been harder to ascertain. However, it appears that Syndecans can signal, primarily to the cytoskeleton, and the details are reviewed here. Only for vertebrate syndecan-4 is there substantial detail, and it remains a challenge to dissect the functions and signalling of other vertebrate and invertebrate syndecans. Key words: proteoglycan, cell adhesion, heparan sulphate, cytoskeleton, receptor, trafficking BACKGROUND does not answer a key point as to why both transmembrane and GPI-anchored HSPGs are present in nearly all nucleated cells. It Syndecans are cell surface proteoglycans with a long is rare to find a cell type that has only one HSPG. Likely, the cell evolutionary history. No multicellular animal appears to be surface diffusion kinetics, turnover and dynamics of syndecans without at least one, and in mammals, there are four (1-3). These and the GPI-anchored glypicans are distinct, and perhaps they arose, in common with many other cell surface molecules, occupy different cell surface niches (3, 11, 12). There is evidence through two rounds of gene duplication at the invertebrate- that syndecan entry into lipid raft domains is a regulated process. chordate boundary (4). Wherever it has been examined, However, where undertaken, single knock-outs of syndecan or syndecans are always substituted with heparan sulphate chains, glypican genes in mice yield mild phenotypes (1, 3, 14, 15). This though some have additional galactosaminoglycan, either suggests redundancy among the cell surface proteoglycans, at chondroitin or dermatan sulphate (5). Heparan sulphate is a vital least in embryonic development. Glypican-3 might be an carbohydrate; experiments with C. elegans, Drosophila and mice exception, since mutations in man give rise to the rare Simpson- show that deletions of key polymerases involved in its synthesis Golabi-Behmel overgrowth syndrome and a similar phenotype in are lethal (6, 7). Therefore, heparan sulphate is essential for tissue the null mouse (16). function in animals. In a sense this is unsurprising, heparan In recent years it has become clear that all vertebrate sulphate has the property of interacting with an immense array of syndecans can link to the actin cytoskeleton, through direct 'ligands' that may be growth factors, chemokines, cytokines, interactions with actin-associated proteins (3, 17). This extracellular matrix molecules, morphogens, clotting factors, and distinguishes them from the glypicans. Cytoplasmic interactions even some enzymes involved in lipid metabolism (8). A key of invertebrate syndecans are not as well characterised. Syndecan question is how all these possible interactions are regulated at the linkage to the cytoskeleton can be a means of localisation and cell surface. There are only two major families of cell surface stability on the cell surface. Alternately, the syndecans can proteoglycans, the transmembrane syndecans and the glypicans, partake in the organisation of the cytoskeleton if they can with C-terminal glycosylphosphatidylinositol anchors to the undergo regulated signalling. In this review, the structure and outer membrane leaflet (3, 9). There are other heparan sulphate function, as we understand it today, of the syndecan cytoplasmic proteoglycans in the extracellular matrix, and particularly domains is discussed. Evidence suggests that indeed syndecans basement membranes, and these are unrelated to each other in can signal, and the molecular detail is now emerging. terms of protein structure. They include perlecan, agrin and type XVIII collagen (10). On one level, it is proposed that cell surface heparan sulphate SYNDECAN STRUCTURE is a mechanism to concentrate ligands on the cell surface (11), and they are well designed for that. Experiments with Drosophila Syndecans are type I membrane glycoproteins, having three for example, show that disruption of heparan sulphate in embryos major domains, ectodomain, transmembrane and cytoplasmic. can interfere with morphogen gradients and thereby disrupt Originally it was thought that the protein ectodomains of normal differentiation and cell fate decisions (12). However, this syndecans had solely the function of being substituted with 32 Fig. 1. Syndecan-4 core protein structure and potential interactions. Schematic representation of a syndecan molecule showing the different domains and their potential interaction where known (A). Fig. 2. Solution structures of both mammalian and zebrafish Amino acid sequence of the cytoplasmic domain of syndecan-4 syndecan-4 cytoplasmic domain in the absence (A, B) and variable region V flanked by the constant regions C1 and C2 (B). presence (C, D) of PtdIns (4,5)P2 determined by NMR spectro- scopy. Electrostatic potential surface of structured region and the van der Waals surface of the p-4L/PtdIns(4,5)P2 are displayed. The negative electrostatic potential is represented in red, the glycosaminoglycan chains. While there is not much conservation positive in blue and the neutral in white. The potential surface of sequence between syndecans and even between, say, was calculated using Delphi program (Accelrys Inc.). syndecan-4 from different species (3, 14), in fact it is now clear that there is functional activity in the extracellular core protein. Heparan sulphate chains are usually close to the N-terminus, and there appears to be at least three in each syndecan, though the the transmembrane domains that dominate, and are mostly reason is not clear. Between the carbohydrate and the cell surface responsible for the ability of syndecan core proteins to be SDS- is a domain that, when isolated or expressed as a GST fusion resistant and resolve as dimers on SDS-PAGE (23, 24). Their protein, promotes integrin-mediated adhesion (18-20). This has GXXXG motif is the vital site for this property (24). All syndecans been demonstrated for syndecans 1, 2 and 4. We have proposed have very similar transmembrane domain sequences, and it has that the interaction between syndecan-4 and integrin is indirect, been suggested, on the basis of synthetic peptide experiments, that and the molecular basis for the function is under investigation. In heterodimers are possible (24). However, as yet it has not been the case of syndecans 2 and 4, which make up a subfamily based shown that whole syndecans can form heterodimers, and this will on sequence homology, the integrins promoting cell adhesion be an interesting, but difficult area, since it may be hard to belong to the β1 class, while for syndecan-1, often present on distinguish heterodimers from hetero-oligomers. epithelial cells, the integrins are β3 and β5 (19, 20). Questions Cytoplasmic domains have three regions, two of which are still remain about this property. It is not yet understood whether highly conserved across all syndecans, and are a hallmark of the this function is important when syndecans are shed from the cell entire class (Fig. 1). These are the membrane-proximal C1 and surface (as they are by a variety of metalloproteases (21)), or membrane distal C2 domains. Between them is a variable (V) whether the ectodomain adhesion function is constitutive, acting region that is unique to each syndecan, yet conserved within either in trans or in cis on a single cell. However, this is a each specific syndecan member (1, 3, 17). So, avian syndecan-2 mechanism whereby syndecans can only indirectly signal, since has a V region sequence almost identical to that of mammals. it is independent of contributions from the cytoplasmic domain. Even where there are conservative changes in sequence, the Interestingly, it is now also believed that the ectodomains of the structure is conserved. Syndecan-4 cytoplasmic domain is glypicans also have protein-protein interaction functions (22), amenable to NMR spectroscopy, and forms a stable dimer of and that once again there is more to cell surface core proteins than unusual characteristics (Fig. 2). It forms a twisted clamp, and being decorated by heparan sulphate. some of the key residues that stabilise the structure are at either All three major domains of syndecan core proteins have a end of the V region (25). Moreover, although two residues of the tendency to form dimers. Of these, it is a strong self-association of zebrafish syndecan-4 are different to all mammals, the structure 33 is completely conserved (Fig. 2). Both fish and mammal 4 over-expression with knock-out fibroblasts, where cell syndecan-4 V regions bind the membrane lipid, migration is compromised in both cases (3, 15, 40). A regulated phosphatidylinositol 4,5 bisphosphate (PtdIns4,5P2), and on so expression of these molecules is apparently required for optimal doing undergoes a shape change, revealed by NMR cell migration. GIPC/synectin seems to be involved with syndecan spectroscopy (26). Currently it is believed that the inositide- removal from the cell surface, for which its interaction with syndecan complex then is able to bind protein kinase Cα, to myosin VI and the endocytic vesicle may be relevant (38). form a ternary complex. The kinase is then persistently activated Moreover, GIPC/synectin interacts with the Rho GEF syx-1 and a (2, 3, 17, 27). recent report shows that knock-down of this protein in zebrafish compromises vascular branching (41).
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