Examples of key cell surface and extracellular HSPGs. Glypicans (Gpcs) are attached to the cell surface via a glycosylphosphatidylinositol (GPI) anchor. They possess a large globular domain stabilized by conserved di-sulfide (S-S) bonds, and HS chains (represented by chains of pink, blue and purple circles) in their C-terminal part. They can be released into the extracellular matrix following cleavage of their GPI anchor by the lipase Notum. A furin-like convertase can also cleave Gpcs at the C-terminal end of their globular domain, leading to the formation of two subunits that remain attached to each other by disulfide bonds. Syndecans (Sdcs) are single-pass transmembrane proteins with HS chains attached to their N-terminal part. Their intracellular region interacts with many different partners through two conserved domains, constant 1 (C1) and constant 2 (C2), that are separated by a more variable region (V). Like Gpcs, Sdcs can be shed into the extracellular environment after cleavage by proteases such as matrix metalloproteases and a disintegrin and metalloproteinase (ADAM) disintegrins. Agrin and perlecan are large multidomain proteoglycans that carry several HS chains and are secreted as different isoforms generated by alternative splicing. The cleavage of the C-terminal region of perlecan by metalloproteases releases endorepellin, an angiogenesis inhibitor. C-ter, C-terminus; N-ter, N-terminus.

The synthesis of heparan sulfate chains. Heparan sulfate (HS) chains are linked by a xylose and a tetrasaccharide linkage to a specific serine residue of a core protein. This linkage region is identical in HS and chondroitin (CS) chains. The addition of the first N-acetylglucosamine is then catalyzed by the enzyme EXTL3. Elongation of the chains is achieved through the alternative addition of glucuronic acid and N-acetylglucosamine by the transferases EXT1 and EXT2. During polymerization, chains undergo several modifications including epimerization of glucoronic acid and sulfations at different positions. These modifications occur in different clusters and generate N-acetylated, N-sulfated and mixed domains differently involved in interactions with partners. Ser, serine; NS, N-sulfate group on glucosamine residues; 2S, 2-O-sulfate group on glucuronic or iduronic acid residues; 3S, 3-O-sulfate group on glucosamine residues; 6S, 6-O-sulfate group on glucosamine residues. Adapted from Esko et al., 2009.

The regulation of cell signaling pathways by HSPGs. HSPGs regulate signaling pathways in many different ways, both at the intracellular level and in the extracellular matrix (ECM). HSPGs have been shown to: (1) mediate signal transduction by acting as receptors or co-receptors; (2) regulate receptor trafficking to and from the plasma membrane; (3) control the secretion of ligands; (4) signal to other cells by acting themselves as cues or presenting ligands to their receptors; and (5) regulate the structure of the ECM and the establishment of signaling gradients.

Functions of HSPGs in early development. (A) Convergent extension allows body axis elongation along the anteroposterior (AP) axis during gastrulation. The collective migration of lateral mesendoderm progenitors (presumptive somites, green) towards the midline (red arrows) and the cell intercalation (white arrows) of notochord progenitors (pink) lead to the extension of the notochord along the AP axis (green arrows). In zebrafish and Xenopus, Gpc4 regulates these convergent extension (CE) movements by modulating the Wnt11 pathway. (B) Normal left-right (LR) patterning in Xenopus requires both non-phosphorylated Sdc2 on the left and PKCγ-dependent phosphorylated Sdc2 on the right side of the embryo. Sdc2 signals non-cell-autonomously from the ectoderm, possibly by activating or mediating Vg1 signaling on the left side, which leads to asymmetric patterning of the mesoderm characterized by the expression of Nodal, Lefty and Pitx2 on the left side.

Functions of HSPGs in nervous system development. (A) During neurogenesis, neuroepithelial cells (NECs) act as neural stem cells (NSCs) that proliferate and differentiate in the ventricular and subventricular zones (VZ and SVZ) of the cortex, giving rise to neuronal progenitors (NPs), neurons and radial glia. Several factors required for neurogenesis are regulated by HSPGs: the Fgf2 pathway is controlled by Gpc1 and Gpc4; Sdc1 modulates the canonical Wnt pathway. Perlecan also acts in the extracellular matrix (ECM) to regulate NEC proliferation. (B) During neuronal migration in the cortex, Sdc3 regulates cell migration along radial glia (blue arrows), from the intermediate zone (IZ) to the marginal zone (MZ), by mediating pleiotrophin and EGFR signaling. Sdc3 also acts as a receptor for GDNF and mediates the tangential migration of inhibitory neurons along a GDNF gradient present from the medial ganglionic eminence (MGE) towards the cortex (pink arrow). CP, cortical plate; LGE, lateral ganglionic eminence; SP, subplate. (C) During axon elongation, guidance cues present at the midline, such as Netrin, Slits or Shh, require HSPGs for signaling through their respective receptors DCC, Robo and Patched, both in vitro and in vivo in Drosophila and C. elegans. In vivo in vertebrates, Gpc1 mediates the response of axons to Shh: in pre-crossing axons, Gpc1 interacts with Shh bound to Patched to promote the transcription of specific genes, including that of the Shh receptor Hhip. In post-crossing axons, Gpc1 binds to Shh, triggering a repulsive response through Hhip signaling. (D) During synaptogenesis, pre-synaptic Gpcs, in particular Gpc4, interact with the pre-synaptic receptor PTPσ and the post-synaptic protein Lrrtm4 to promote excitatory synapse development. Gpc4 and Gpc6 are also released from astrocytes and promote synapse formation by clustering post-synaptic AMPA glutamate receptors (AMPARs). On the post-synaptic membrane, EphB2 phosphorylates Sdc2 and induces its clustering. The interaction of Sdc2 with adaptor proteins, such as Cask or neurofibromin, promotes the formation of dendritic spines. (E) At the neuromuscular junction, agrin is released from the nerve terminal and becomes stabilized in the basal lamina. It binds and activates the receptor Musk, leading to AChR clustering via the cytoplasmic adaptor protein Rapsn.