The syndecans, tuners of transmembrane signaling

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

Synectin, syndecan-4 cytoplasmic domain binding PDZ protein, inhibits cell migration

Syndecan-4, a member of the syndecan gene family of proteoglycans, is an important regulator of bFGF signaling. In particular, bFGF-dependent regulation of cell growth and migration has been linked to syndecan-4 cytoplasmic domain-mediated interactions. Screening of a yeast two-hybrid library with a cytoplasmic domain of rat syndecan-4 identified a novel binding partner, here termed synectin. Synectin is highly homologous to semaphorin F binding protein semcap1, glucose 1 transporter binding protein glut1cbp, and RGS-GAIP/neuropilin-1 binding protein GIPC. Overexpression of synectin in ECV304 cells in culture led to a dose-dependent inhibition of migration while not affecting cell adhesion or growth rate. We conclude that synectin is involved in syndecan-4-dependent interactions and may play a role in the assembly of syndecan-4 signaling complex.

Syndecans and the lymphoid system

Syndecans, transmembrane proteoglycans, play an important role in cell-matrix and cell-cell interactions, as well as modulators in receptor activation. These functions are partly non-specific and related to the heparan sulfate chains attached to the ectodomain, and partly specific due to the transmembrane and cytoplasmic domains of the core protein. In hemopoietic cells syndecan-1 is expressed only in B cells at certain differentiation stages (pre-B and plasma cells). In lymphoproliferative conditions this selective expression is retained in myelomas/plasmacytomas and other lymphoplasmacytic NHL subtypes, and primary effusional lymphomas. It is probably gained in B-CLL, and lost in other NHLs of pre- or post-follicular origin. It is concluded from these empiric results that the expression of syndecan is essential for some NHLs, probably ensuring the required connections to the microenvironment. From a diagnostic point of view, syndecan-1 is a very useful phenotypic marker to indentify cells with plasmacytic differentiation. The importance of syndecan expression in CLL and Hodgkin's lymphoma still requires further studies.

The cysteine-rich domain of human ADAM 12 supports cell adhesion through syndecans and triggers signaling events that lead to beta1 integrin-dependent cell spreading

The ADAMs (a disintegrin and metalloprotease) family of proteins is involved in a variety of cellular interactions, including cell adhesion and ecto- domain shedding. Here we show that ADAM 12 binds to cell surface syndecans. Three forms of recombinant ADAM 12 were used in these experiments: the cys-teine-rich domain made in Escherichia coli (rADAM 12-cys), the disintegrin-like and cysteine-rich domain made in insect cells (rADAM 12-DC), and full-length human ADAM 12-S tagged with green fluorescent protein made in mammalian cells (rADAM 12-GFP). Mesenchymal cells specifically and in a dose-dependent manner attach to ADAM 12 via members of the syndecan family. After binding to syndecans, mesenchymal cells spread and form focal adhesions and actin stress fibers. Integrin beta1 was responsible for cell spreading because function-blocking monoclonal antibodies completely inhibited cell spreading, and chondroblasts lacking beta1 integrin attached but did not spread. These data suggest that mesenchymal cells use syndecans as the initial receptor for the ADAM 12 cysteine-rich domain-mediated cell adhesion, and then the beta1 integrin to induce cell spreading. Interestingly, carcinoma cells attached but did not spread on ADAM 12. However, spreading could be efficiently induced by the addition of either 1 mM Mn(2+) or the beta1 integrin-activating monoclonal antibody 12G10, suggesting that in these carcinoma cells, the ADAM 12-syndecan complex fails to modulate the function of beta1 integrin.

Syndecan-1 and syndecan-4 can mediate the invasion of OpaHSPG-expressing Neisseria gonorrhoeae into epithelial cells

Neisseria gonorrhoeae (Ngo) expressing the outer membrane protein OpaHSPG can adhere to and invade epithelial cells via binding to heparan sulphate proteoglycan (HSPG) receptors. In this study, we have investigated the role of syndecan-1 and syndecan-4, two members of the HSPG family, in the uptake of Ngo by epithelial cells. When overexpressed in HeLa cells, both syndecans co-localize with adherent Ngo on the host cell surface. This overexpression of syndecan-1 and syndecan-4 leads to a three- and sevenfold increase in Ngo invasion respectively. In contrast, transfection with the syndecan-1 and syndecan-4 mutant constructs lacking the intracellular domain results in an abrogation of the invasion process, characteristic of a dominant-negative mode of action. A concomitant loss of the capacity to mediate Ngo uptake was also observed with syndecan-4 mutant constructs carrying lesions in the dimerization motif necessary for the binding of protein kinase C (PKC) and phosphatidylinositol 4,5-bisphosphate (PIP2), and mutants that are deficient in a C-terminal EFYA amino acid motif responsible for binding to syntenin or CASK. We conclude that syndecan-1 and syndecan-4 can both mediate Ngo uptake into epithelial cells, and that their intracellular domains play a crucial role in this process, perhaps by mediating signal transduction or anchorage to the cytoskeleton.

Syndesmos, a protein that interacts with the cytoplasmic domain of syndecan-4, mediates cell spreading and actin cytoskeletal organization

Syndecan-4 is a cell surface heparan sulfate proteoglycan which, in cooperation with integrins, transduces signals for the assembly of focal adhesions and actin stress fibers in cells plated on fibronectin. The regulation of these cellular events is proposed to occur, in part, through the interaction of the cytoplasmic domains of these transmembrane receptors with intracellular proteins. To identify potential intracellular proteins that interact with the cytoplasmic domain of syndecan-4, we carried out a yeast two-hybrid screen in which the cytoplasmic domain of syndecan-4 was used as bait. As a result of this screen, we have identified a novel cellular protein that interacts with the cytoplasmic domain of syndecan-4 but not with those of the other three syndecan family members. The interaction involves both the membrane proximal and variable central regions of the cytoplasmic domain. We have named this cDNA and encoded protein syndesmos. Syndesmos is ubiquitously expressed and can be myristylated. Consistent with its myristylation and syndecan-4 association, syndesmos colocalizes with syndecan-4 in the ventral plasma membranes of cells plated on fibronectin. When overexpressed in NIH 3T3 cells, syndesmos enhances cell spreading, actin stress fiber and focal contact formation in a serum-independent manner.

Syndecan-4 and integrins: combinatorial signaling in cell adhesion

It is now becoming clear that additional transmembrane components can modify integrin-mediated adhesion. Syndecan-4 is a transmembrane heparan sulfate proteoglycan whose external glycosaminoglycan chains can bind extracellular matrix ligands and whose core protein cytoplasmic domain can signal during adhesion. Two papers in this issue of JCS demonstrate, through transfection studies, that syndecan-4 plays roles in the formation of focal adhesions and stress fibers. Overexpression of syndecan-4 increases focal adhesion formation, whereas a partially truncated core protein that lacks the binding site for protein kinase C(α) and phosphatidylinositol 4, 5-bisphosphate acts as a dominant negative inhibitor of focal adhesion formation. Focal adhesion induction does not require interaction between heparan sulfate glycosaminoglycan and ligand but can occur when non-glycanated core protein is overexpressed; this suggests that oligomerization of syndecan-4 plays a major role in signaling from the extracellular matrix in adhesion.