The signaling mechanisms of syndecan heparan sulfate proteoglycans

Syndecans are membrane proteins controlling cell proliferation, differentiation, adhesion, and migration. Their extracellular domains bear versatile heparan sulfate chains that provide structural determinants for syndecans to function as coreceptors or activators for molecules like growth factors and constituents of the matrix. Syndecans also signal via their protein cores and their conserved transmembrane and cytoplasmic domains. The direct interactions and signaling cascades they support are becoming better characterized. These interactions are regulated by phosphorylation, induced clustering and shedding of the syndecan extracellular domain. Moreover evidence is emerging that syndecans concentrate in unconventional lipid domains and might govern novel vesicular trafficking pathways. Here we focus on recent findings that refine our understanding of the complex structure–function relationships of these cellular effectors.

Introduction

Syndecans are membrane proteins that are expressed at up to one million copies per cell in nearly every cell of the body. Mammals express four different syndecans. Each syndecan family member has a distinctive temporal and spatial pattern of expression. Syndecan-1 is primarily expressed in epithelial and plasma cells; syndecan-2 is mainly expressed in fibroblasts, endothelial cells, neurons, and smooth muscle cells; syndecan-3 is the major syndecan of the nervous system but is also important for chondrocyte proliferation; and syndecan-4 is nearly ubiquitous.

In invertebrates like Drosophila melanogaster and C. elegans, there is only a single syndecan gene, which is essential for neuronal development and axon guidance. In mammals, knock-out animals are viable and fertile but show multiple defects following exposure to physiological stresses [1]. These include perturbed wound healing, angiogenesis, and inflammation for syndecan-1^−/− and syndecan-4^−/−. Interestingly, syndecan-1 null mice are protected from tumor development possibly because of impaired pathogenic activation of progenitor/stem cells [2]. Syndecan-3 deficient mice have difficulties of learning and memory, develop muscular dystrophy and have an abnormal feeding behavior after food deprivation [3]. To date, syndecan-2 null mice have not been reported, but studies in Xenopus and zebrafish point to a role in left–right asymmetry and VEGF induced vascular development [4]. In these same models, syndecan-4 is required for modulating migration processes during embryonic development that are regulated by the noncanonical Wnt pathway and for neural induction [5•, 6, 7•]. Xenopus syndecan-1 regulates dorso-ventral patterning of the ectoderm by modulating BMP signaling [8]. The severity of Xenopus/fish versus mice knock-out phenotypes may be because shorter gestational times are prohibitive of compensation mechanisms. The analysis of combined knock-outs should clarify how mammals accommodate the lack of multiple syndecans.

Structurally, all syndecans are composed of an extracellular domain (ED), a characteristic transmembrane region (TM) and a conserved short C-terminal cytoplasmic domain (CD). The ED is substituted with heparan sulfate (HS)-chains synthesized and modified in the Golgi-apparatus and edited postsynthetically by sulfatases and heparanase (Figure 1). The structural features of the HS-chains are responsible for the interaction of syndecans with a number of soluble factors, cell-associated molecules and extracellular matrix components. In some cases HS engagement can promote the active conformation of the ligand. Generally, syndecans do not function as primary receptors but cooperate with these as coreceptors. Although in vitro experiments clearly indicated that specific HS-structures mediate specific high-affinity binding activities [9], loss-of-function studies of the different HS modifying enzymes in mice have established that compensation mechanisms can take place and that the ‘sugar code’ is quite degenerate [10, 11, 12]. Except for the HS attachment sites, the amino acid sequence of the ED varies considerably between the different syndecan family members. Within a syndecan type there is usually over 70% homology between different mammals. A few studies now document that syndecan EDs contain non-HS intrinsic protein-binding structures such as the NXIP motif in syndecan-4 [13] and the ‘synstatin’ part of syndecan-1 that associates with alpha(v)beta(3) and alpha(v)beta(5) integrins and that can block angiogenesis and tumorigenesis [14^••]. The TM and CD domains harbor structural features that are unique to syndecans and support signal transduction across the membrane. Syndecans do not appear to encode any intrinsic catalytic activity, however multiple molecular interactions have been identified with kinases, GTPases, cytoskeletal molecules, and other intracellular components (Table 1). These interactions are regulated by phosphorylation and clustering of the syndecans. Below we review how structural features of the TM and the CD of syndecans can contribute to their signaling, how heparanase and shedding can modulate their activity and how endocytic routes might support their function. Complementary information can be found in other recent reviews [1, 15, 16, 17].