Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Glypicans (GPCs)

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101637

Synonyms

 Glypican-1: Glypican; Glypican 1; Glypican-1; GPC1

 Glypican-2: Glypican 2; Glypican-2; GPC2; Cerebroglycan

 Glypican-3: Glypican 3; Glypican-3; GPC3; MXR7; OCI-5

 Glypican-4: Glypican 4; Glypican-4; GPC4; K-glypican

 Glypican-5: Glypican 5; Glypican-5; GPC5

 Glypican-6: Glypican 6; Glypican-6; GPC6

Historical Background

Glypican-3 (GPC3) is the first glypican that was cloned (Filmus et al. 1988). Initially named OCI-5, GPC3 was identified in 1988 as a gene whose expression was developmentally regulated in the intestine. Two years later, the cloning of Glypican-1 (GPC1) was reported. This report characterized GPC1 as a glycosylphosphatidylinositol (GPI)-anchored heparan sulfate proteoglycan (HSPG), but it did not indicate that GPC1 displays significantly homology with GPC3 (David et al. 1990). This homology was noticed 4 years later in a study that reported the cloning of Glypican-2 (initially called cerebroglycan) (Stipp et al. 1994). This study established for the first time the existence of a family of GPI-anchored HSPGs, which in the following years became known as the “glypican family.” In 1995, the identification of dally, a Drosophila glypican, was reported (Nakato et al. 1995). This finding indicated that glypicans represent a protein family that is highly conserved during evolution. In fact, studies performed in Drosophila provided the first functional insights on glypicans by showing that they could regulate Hedgehog, Wnt, and BMP signaling (Jackson et al. 1997; Lin and Perrimon 1999; Lum et al. 2003). Notably, 1 year later, GPC3 was identified as the gene mutated in patients with the Simpson-Golabi-Behmel syndrome (SGBS), a genetic disease characterized by embryonic overgrowth and a wide range of developmental abnormalities (Pilia et al. 1996). In addition, this report proposed that GPC3 was a regulator of insulin-like growth factor 2 (IGF2). However, it was later demonstrated that this was not correct and that the effect of GPC3 on embryonic growth is mediated by the Hedgehog signaling pathway (Capurro et al. 2008).

Molecular Structure of Glypicans

The molecular weight of the core proteins of mammalian glypicans is in the range of 60–70 kDa. Even though the sequence homology between the different members of the family could be as low as 20%, the localization of 14 cysteine residues and of the domain where the glycosaminoglycan chains (GAG) are attached is conserved in all glypicans. In addition, most glypicans are cleaved by furin-like convertases into two subunits that remain attached to each other by one or more disulfide bridges. Notably, glypicans do not display domains with obvious homology to other characterized protein domains, suggesting that these proteoglycans have unique functions (Fig. 1, Table 1).
Glypicans (GPCs), Fig. 1

Schematic diagram of the structure of glypicans. GAG glycosaminoglycan, GPI glycosylphosphatidylinositol, S–S disulfide bond

Glypicans (GPCs), Table 1

Mammalian glypicans. List of signaling pathways and diseases associated with each member of the glypican family

Glypican

Signaling pathway

Disease

GPC1

Hedgehog, BMP, FGF

Biliary atresia, prion disease, pancreatic cancer, glioma

GPC2

Unknown

GPC3

Hedgehog, BMP, Wnt

Simpson-Golabi-Behmel, hepatocellular carcinoma, breast cancer

GPC4

BMP, Wnt, FGF

GPC5

Hedgehog, Wnt, FGF

Nephrotic syndrome, spina bifida, hepatitis B, rhabdomyosarcoma, lung adenocarcinoma

GPC6

Unknown

Recessive omodysplasia, autism, primary sclerosing cholangitis

The crystal structure of GPC1 has been recently reported (Svensson et al. 2012). The structure indicates that this is a densely packed protein of elongated cylindrical shape. It also shows that GPC1 consists of 14 α-helices and 3 major loops. The C-terminus domain containing the attachment sites for the GAGs and the GPI anchor seems to be unstructured, but it places the cylindrical core protein parallel to the membrane, with a surface that is evolutionarily conserved in GPC1 orthologs facing the GAG chains. This suggests that GPC1-binding proteins could interact with the core protein, the GAG chains, or with both simultaneously.

Glypicans display several GAG attachment sites (from 2 in GPC3 to 4 in GPC5 and GPC6). The functional implications of the variation in the number of GAG chains are still unknown. Most glypicans have been shown to only carry heparan sulfate chains.

Functions of Glypicans

One of the main functions of glypicans is to regulate growth factor/morphogen interaction at the level of ligand-receptor interaction (Filmus et al. 2008). This regulatory activity is based on the ability of glypicans to stimulate or inhibit ligand-receptor interaction. Experiments initially performed in Drosophila have demonstrated that glypicans can regulate several signaling pathways, including those triggered by Hedgehogs, Wnts, fibroblast growth factors (FGFs), and bone morphogenetic proteins (BMPs). The role of glypicans in these signaling pathways has been also confirmed in various vertebrates. Genetic and biochemical studies published to date have shown that different glypicans can have opposite effects in specific signaling pathways. In addition, these studies demonstrated that a particular glypican can have a stimulatory effect in one signaling pathway and an inhibitory effect in another. In some cases, glypican activity is mostly mediated by the GAG chains, but in other cases both, the core protein and the GAG chains are required. Whereas the different activities of the core proteins are expected due to the limited sequence homology within the glypican family, it has also been suggested that the opposite function of the GAG chains of different glypicans are due to the fact that the biochemical modifications of these chains can vary from glypican to glypican even in the same cell type (Li et al. 2011). It can be proposed, therefore, that the structural features of glypicans combine with the set of growth factors and growth factor receptors present in a given cell type to determine glypican function.

The studies on the role of GPC3 and GPC5 in the Hedgehog signaling pathway represent a good example of the specificity of glypican function (Filmus and Capurro 2014). Hedgehog signaling is triggered at the primary cilium. GPC3 has been shown to act as a Hedgehog inhibitor. Notably, GPC3 is localized outside of the cilium. The protein core of GPC3 binds with high affinity to Hedgehog and it functions as a competitive inhibitor of the binding of this growth to its receptor Patched-1, which is localized in the cilium. GPC5, on the other hand, stimulates Hedgehog signaling. This glypican is localized at the cilium, where it binds to both Hedgehog and Patched-1, and promotes their interaction.

Glypicans also display specific functions in the context of the nervous system. Initially discovered in Drosophila, these functions have been confirmed in higher organisms and include the regulation of axon guidance and roles in the formation of excitatory synapses in the central nervous system (Allen et al. 2012; Song and Kim 2013). For example, GPC4 has been shown to be required for optimal excitatory synaptic transmission. This glypican forms a complex with members of the presynaptic LAR family of receptor protein tyrosine phosphatases and with the postsynaptic leucine-rich repeat transmembrane protein 4 (Ko et al. 2015). GPC1, on the other hand, plays a role in axon guidance by stimulating Shh activity in postcrossing commissural axons (Wilson and Stoeckli 2013).

The Role of Glypicans in Human Disease

The signaling pathways that are regulated by glypicans are known to play crucial roles during embryonic development and in cancer progression. It is therefore not surprising that mutation or abnormal expression of these proteoglycans have been shown to cause embryonic syndromes and to contribute to the progression of several malignancies.

Loss-of-function mutations of GPC3 cause the Simson-Golabi-Behmel syndrome (SGBS). This is a disorder characterized by embryonic overgrowth and a wide range of developmental abnormalities, including cleft palate, polydactyly, syndactyly, supernumerary nipples, cystic and dysplastic kidneys, congenital heart defects, rib and vertebral fusions, and umbilical and inguinal hernias (Pilia et al. 1996). Studies of GPC3-null mice, which display many of the clinical features of SGBS, have demonstrated that GPC3 is a negative regulator of the Hedgehog signaling pathway and that SGBS is caused, at least in part, by excessive Hedgehog activity during embryonic development (Capurro et al. 2008).

Autosomal recessive omodysplasia is a genetic syndrome caused by loss-of-function mutations of GPC6. This syndrome is characterized by short stature, facial dysmorphism, and proximal limb shortening (Campos-Xavier et al. 2009). The phenotype of these patients suggests that GPC6 is involved in endochondral ossification, which is consistent with the finding that GPC6 is expressed by chondrocytes from the growth plate. The signaling pathway that mediates the regulatory role of GPC6 in this omodysplasia remains to be identified.

The clearest example of the involvement of glypicans in cancer progression is that of GPC3 in the context of hepatocellular carcinoma (HCC) (Filmus and Capurro 2013). Multiple studies have demonstrated that this glypican is not produced by normal liver, whereas it is overexpressed in most HCCs (70–80%). Consequently, GPC3 immunostaining of liver biopsies has been recommended as a tool to confirm HCC diagnosis by the American Association for the Study of Liver Diseases (Bruix and Sherman 2011). Notably, in addition to being an HCC diagnostic marker, GPC3 stimulates the progression of this malignancy through its canonical Wnt-stimulatory activity. Consequently, GPC3 is considered a target for HCC immunotherapy. In fact, there is strong evidence showing that this approach can inhibit HCC growth in mice (Gao et al. 2015), and several clinical trials are currently being performed to test this therapy in HCC patients.

Overexpression of GPC1 has been associated with pancreatic cancer, gliomas, and esophageal squamous cell carcinoma. In the case of pancreatic cancer, a recent report has shown that GPC1 is present in exosomes isolated from the blood of 100% of patients tested. Notably, GPC1 was absent from exosomes obtained from normal controls. It has been proposed, therefore, that detection of exosomal GPC1 could be an excellent test for the detection of pancreatic cancer (Melo et al. 2015, 13,415/id).

Another disease that has been associated with abnormal glypican expression is the nephrotic syndrome. This syndrome is characterized by severe proteinuria with concurrent decrease in serum protein, and it is thought to be caused by podocyte damage. GPC5 has been identified as a susceptibility gene for acquired nephrotic syndrome, and it has been suggested that GPC5 stimulates FGF-2-induced damage in the podocytes (Okamoto et al. 2011).

Summary

Glypicans are a family of proteoglycans that are attached to the cell surface by a GPI anchor. Six members of this family have been identified in mammals, and a variable number of orthologs have been found in lower organisms, including Drosophila and C. elegans. Most glypicans carry heparan sulfate chains, which are inserted close to the C-terminus. Glypicans regulate the activity of various signaling pathways at the level of ligand-receptor interaction. The specific function of a particular glypican is determined by its structural features and by the growth factor systems that are expressed by the cellular system. In addition, glypicans have important roles in the nervous system, particularly in axon guidance and synaptic transmission. Mutations of these proteoglycans can cause developmental abnormalities. In addition, abnormal glypican expression can contribute to the progression of various cancer types.

References

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Sunnybrook Research InstituteUniversity of TorontoTorontoCanada