Glypican-3 inhibits Hedgehog signaling during development by competing with patched for Hedgehog binding

The cell-surface proteins Dally-like and Ihog differentially regulate Hedgehog signaling strength and range during development

Hedgehog (Hh) acts as a morphogen in various developmental contexts to specify distinct cell fates in a concentration-dependent manner. Hh signaling is regulated by two conserved cell-surface proteins: Ig/fibronectin superfamily member Interference hedgehog (Ihog) and Dally-like (Dlp), a glypican that comprises a core protein and heparan sulfate glycosaminoglycan (GAG) chains. Here, we show in Drosophila that the Dlp core protein can interact with Hh and is essential for its function in Hh signaling. In wing discs, overexpression of Dlp increases short-range Hh signaling while reducing long-range signaling. By contrast, Ihog has biphasic activity in Hh signaling in cultured cells: low levels of Ihog increase Hh signaling, whereas high levels decrease it. In wing discs, overexpression of Ihog represses high-threshold targets, while extending the range of low-threshold targets, thus showing opposite effects to Dlp. We further show that Ihog and its family member Boi are required to maintain Hh on the cell surface. Finally, Ihog and Dlp have complementary expression patterns in discs. These data led us to propose that Dlp acts as a signaling co-receptor. However, Ihog might not act as a classic co-receptor; rather, it may act as an exchange factor by retaining Hh on the cell surface, but also compete with the receptor for Hh binding.

Hedgehog pathway antagonist 5E1 binds hedgehog at the pseudo-active site

Proper hedgehog (Hh) signaling is crucial for embryogenesis and tissue regeneration. Dysregulation of this pathway is associated with several types of cancer. The monoclonal antibody 5E1 is a Hh pathway inhibitor that has been extensively used to elucidate vertebrate Hh biology due to its ability to block binding of the three mammalian Hh homologs to the receptor, Patched1 (Ptc1). Here, we engineered a murine:human chimeric 5E1 (ch5E1) with similar Hh-binding properties to the original murine antibody. Using biochemical, biophysical, and x-ray crystallographic studies, we show that, like the regulatory receptors Cdon and Hedgehog-interacting protein (Hhip), ch5E1 binding to Sonic hedgehog (Shh) is enhanced by calcium ions. In the presence of calcium and zinc ions, the ch5E1 binding affinity increases 10-20-fold to tighter than 1 nm primarily because of a decrease in the dissociation rate. The co-crystal structure of Shh bound to the Fab fragment of ch5E1 reveals that 5E1 binds at the pseudo-active site groove of Shh with an epitope that largely overlaps with the binding site of its natural receptor antagonist Hhip. Unlike Hhip, the side chains of 5E1 do not directly coordinate the Zn(2+) cation in the pseudo-active site, despite the modest zinc-dependent increase in 5E1 affinity for Shh. Furthermore, to our knowledge, the ch5E1 Fab-Shh complex represents the first structure of an inhibitor antibody bound to a metalloprotease fold.

A novel mechanism of sequestering fibroblast growth factor 2 by glypican in lipid rafts, allowing skeletal muscle differentiation

Heparan sulfate proteoglycans (HSPGs) are critical modulators of growth factor activities. Skeletal muscle differentiation is strongly inhibited by fibroblast growth factor 2 (FGF-2). We have shown that HSPGs present at the plasma membrane are expressed in myoblasts and are downregulated during muscle differentiation. An exception is glypican-1, which is present throughout the myogenic process. Myoblasts that do not express glypican-1 exhibit defective differentiation, with an increase in the receptor binding of FGF-2, concomitant with increased signaling. Glypican-1-deficient myoblasts show decreased expression of myogenin, the master gene that controls myogenesis, myosin, and the myoblast fusion index. Reversion of these defects was induced by expression of rat glypican-1. Glypican-1 is the only HSPG localized in lipid raft domains in myoblasts, resulting in the sequestration of FGF-2 away from FGF-2 receptors (FGFRs) located in nonraft domains. A chimeric glypican-1, containing syndecan-1 transmembrane and cytoplasmic domains, is located in nonraft domains interacting with FGFR-IV- and enhanced FGF-2-dependent signaling. Thus, glypican-1 acts as a positive regulator of muscle differentiation by sequestering FGF-2 in lipid rafts and preventing its binding and dependent signaling.

Dally-like core protein and its mammalian homologues mediate stimulatory and inhibitory effects on Hedgehog signal response

The distribution and activities of morphogenic signaling proteins such as Hedgehog (Hh) and Wingless (Wg) depend on heparan sulfate proteoglycans (HSPGs). HSPGs consist of a core protein with covalently attached heparan sulfate glycosaminoglycan (GAG) chains. We report that the unmodified core protein of Dally-like (Dlp), an HSPG required for cell-autonomous Hh response in Drosophila embryos, alone suffices to rescue embryonic Hh signaling defects. Membrane tethering but not specifically the glycosylphosphatidylinositol linkage characteristic of glypicans is critical for this cell-autonomous activity. Our studies further suggest divergence of the two Drosophila and six mammalian glypicans into two functional families, an activating family that rescues cell-autonomous Dlp function in Hh response and a family that inhibits Hh response. Thus, in addition to the previously established requirement for HSPG GAG chains in Hh movement, these findings demonstrate a positive cell-autonomous role for a core protein in morphogen response in vivo and suggest the conservation of a network of antagonistic glypican activities in the regulation of Hh response.

Heparan sulfate proteoglycans: structure, protein interactions and cell signaling

Heparan sulfate proteoglycans are ubiquitously found at the cell surface and extracellular matrix in all the animal species. This review will focus on the structural characteristics of the heparan sulfate proteoglycans related to protein interactions leading to cell signaling. The heparan sulfate chains due to their vast structural diversity are able to bind and interact with a wide variety of proteins, such as growth factors, chemokines, morphogens, extracellular matrix components, enzymes, among others. There is a specificity directing the interactions of heparan sulfates and target proteins, regarding both the fine structure of the polysaccharide chain as well precise protein motifs. Heparan sulfates play a role in cellular signaling either as receptor or co-receptor for different ligands, and the activation of downstream pathways is related to phosphorylation of different cytosolic proteins either directly or involving cytoskeleton interactions leading to gene regulation. The role of the heparan sulfate proteoglycans in cellular signaling and endocytic uptake pathways is also discussed.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Soluble glypican 3 inhibits the growth of hepatocellular carcinoma in vitro and in vivo

The heterogeneity of the molecular pathology of HCC poses a formidable obstacle to the development of non-cytotoxic therapies. Several pro-tumorigenic signaling pathways can be aberrantly activated in HCC, including those triggered by Wnts. Glypican-3 (GPC3), a membrane-bound heparan sulfate proteoglycan that is overexpressed in most HCCs, promotes the growth of these tumors by stimulating Wnt signaling. Because GPC3 binds with high affinity to Wnts, and its growth-promoting activity requires attachment to the cell membrane, we have hypothesized that a mutated GPC3 lacking the GPI anchoring domain (sGPC3) will block Wnt signaling and inhibit the growth of Wnt-dependent tumors. In addition, because sGPC3 displays heparan sulfate chains, this secreted glypican could also inhibit HCC growth by blocking the activity of other heparin-binding growth factors. To test this hypothesis, HCC cell lines were infected with an sGPC3-expressing lentivirus or virus control, and the effect of sGPC3 on the in vitro and in vivo growth was investigated. In addition, the signaling pathways targeted by sGPC3 were identified. We observed that sGPC3-expressing cells had lower proliferation rate. In addition, sGPC3 significantly inhibited the in vivo growth of the Huh6, HepG2 and Huh7 HCC cell lines. sGPC3 blocked Wnt signaling in Huh6- and Huh7-derived tumors and Erk1/2 and Akt phosphorylation in tumors generated by Huh7 and HepG2 cells, respectively. An anti-angiogenic effect in Huh7 and HepG2-derived tumors was also observed. We conclude that sGPC3 can inhibit HCC tumorigenicity by blocking the activity of several pro-tumorigenic growth factors.

Advances in the relationship between glypican-3 and primary hepatic carcinoma

Glypican-3 (GPC3) is a membrane heparin sulfate proteoglycan that is expressed abundantly in the fetal liver, inactive in the normal adult liver and frequently reactivated in hepatocellular carcinoma (HCC). Serum soluble GPC3 is a new diagnostic and prognostic biomarker for HCC. GPC3 is also a potential target for targeted therapy of HCC. The expression of GPC3 is upregulated at the early stage of HCC, which is associated with sulfatase-2, zinc-fingers and homeoboxes 2 (ZHX2) and alpha-fetoprotein regulator 2 (AFR2). GPC3 overexpression can activate integrin, insulin-like growth factor and Wnt signaling to promote HCC development.

Craniofacial skeletal defects of adult zebrafish Glypican 4 (knypek) mutants

The heparan sulfate proteoglycan Glypican 4 (Gpc4) is part of the Wnt/planar cell polarity pathway, which is required for convergence and extension during zebrafish gastrulation. To observe Glypican 4-deficient phenotypes at later stages, we rescued gpc4(-/-) (knypek) homozygotes and raised them for more than one year. Adult mutants showed diverse cranial malformations of both dermal and endochondral bones, ranging from shortening of the rostral-most skull to loss of the symplectic. Additionally, the adult palatoquadrate cartilage was disorganized, with abnormal chondrocyte orientation. To understand how the palatoquadrate cartilage normally develops, we examined a juvenile series of wild type and mutant specimens. This identified two novel domains of elongated chondrocytes in the larval palatoquadrate, which normally form prior to endochondral ossification. In contrast, gpc4(-/-) larvae never form these domains, suggesting a failure of chondrocyte orientation, though not differentiation. Our findings implicate Gpc4 in the regulation of zebrafish cartilage and bone morphogenesis.

Heparan sulfate is required for embryonic stem cells to exit from self-renewal

Pluripotent embryonic stem cells (ESCs) must select between alternative fates of self-renewal and lineage commitment at each division during continuous proliferation. Heparan sulfate (HS) is a highly sulfated polysaccharide and is present abundantly on the ESC surface. In this study, we investigated the role of HS in ESC self-renewal by examining Ext1(-/-) ESCs that are deficient in HS. We found that Ext1(-/-) ESCs retained their self-renewal potential but failed to transit from self-renewal to differentiation upon removal of leukemia inhibitory factor. Furthermore, we found that the aberrant cell fate commitment is caused by defects in fibroblast growth factor signaling, which directly retained high expression of the pluripotency gene Nanog in Ext1(-/-) ESCs. Therefore, our studies identified and defined HS as a novel factor that controls ESC fate commitment and also delineates that HS facilitates fibroblast growth factor signaling, which, in turn, inhibits Nanog expression and commits ESCs to lineage differentiation.

Activation of the hedgehog-signaling pathway in human cancer and the clinical implications

The hedgehog pathway, initially discovered by two Nobel laureates Drs E Wieschaus and C Nusslein-Volhard in Drosophila, is a major regulator for cell differentiation, tissue polarity and cell proliferation. Studies from many laboratories reveal activation of this pathway in a variety of human cancer, including basal cell carcinomas (BCCs), medulloblastomas, leukemia, gastrointestinal, lung, ovarian, breast and prostate cancers. It is thus believed that targeted inhibition of hedgehog signaling may be effective in treatment and prevention of human cancer. Even more exciting is the discovery and synthesis of specific signaling antagonists for the hedgehog pathway, which have significant clinical implications in novel cancer therapeutics. In this review, we will summarize major advances in the last 2 years in our understanding of hedgehog signaling activation in human cancer, interactions between hedgehog signaling and other pathways in carcinogenesis, potential antagonists for hedgehog signaling inhibition and their clinical implications for human cancer treatment.

The core protein of glypican Dally-like determines its biphasic activity in wingless morphogen signaling

Dally-like (Dlp) is a glypican-type heparan sulfate proteoglycan (HSPG), containing a protein core and attached glycosaminoglycan (GAG) chains. In Drosophila wing discs, Dlp represses short-range Wingless (Wg) signaling, but activates long-range Wg signaling. Here, we show that Dlp core protein has similar biphasic activity as wild-type Dlp. Dlp core protein can interact with Wg; the GAG chains enhance this interaction. Importantly, we find that Dlp exhibits a biphasic response, regardless of whether its glycosylphosphatidylinositol linkage to the membrane can be cleaved. Rather, the transition from signaling activator to repressor is determined by the relative expression levels of Dlp and the Wg receptor, Frizzled (Fz) 2. Based on these data, we propose that the principal function of Dlp is to retain Wg on the cell surface. As such, it can either compete with the receptor or provide ligands to the receptor, depending on the ratios of Wg, Fz2, and Dlp.

Loss of glypican-3 function causes growth factor-dependent defects in cardiac and coronary vascular development

Glypican-3 (Gpc3) is a heparan sulfate proteoglycan (HSPG) expressed widely during vertebrate development. Loss-of-function mutations cause Simpson-Golabi-Behmel syndrome (SGBS), a rare and complex congenital overgrowth syndrome with a number of associated developmental abnormalities including congenital heart disease. We found that Gpc3-deficient mice display a high incidence of congenital cardiac malformations like ventricular septal defects, common atrioventricular canal and double outlet right ventricle. In addition we observed coronary artery fistulas, which have not been previously reported in SGBS. Coronary artery fistulas are noteworthy because little is known about the molecular basis of this abnormality. Formation of the coronary vascular plexus in Gpc3-deficient embryos was delayed compared to wild-type, and consistent with GPC3 functioning as a co-receptor for fibroblast growth factor-9 (FGF9), we found a reduction in Sonic Hedgehog (Shh) mRNA expression and signaling in embryonic mutant hearts. Interestingly, we found an asymmetric reduction in SHH signaling in cardiac myocytes, as compared with perivascular cells, resulting in excessive coronary artery formation in the Gpc3-deficient animals. We hypothesize that the excessive development of coronary arteries over veins enables the formation of coronary artery fistulas. This work has broad significance to understanding the genetic basis of coronary development and potentially to molecular mechanisms relevant to revascularization following ischemic injury to the heart.

Glypican-1 controls brain size through regulation of fibroblast growth factor signaling in early neurogenesis

BACKGROUND

Cell surface heparan sulfate proteoglycans (HSPGs) act as co-receptors for multiple families of growth factors that regulate animal cell proliferation, differentiation and patterning. Elimination of heparan sulfate during brain development is known to produce severe structural abnormalities. Here we investigate the developmental role played by one particular HSPG, glypican-1 (Gpc1), which is especially abundant on neuronal cell membranes, and is the major HSPG of the adult rodent brain.

RESULTS

Mice with a null mutation in Gpc1 were generated and found to be viable and fertile. The major phenotype associated with Gpc1 loss is a highly significant reduction in brain size, with only subtle effects on brain patterning (confined to the anterior cerebellum). The brain size difference emerges very early during neurogenesis (between embryonic days 8.5 and 9.5), and remains roughly constant throughout development and adulthood. By examining markers of different signaling pathways, and the differentiation behaviors of cells in the early embryonic brain, we infer that Gpc1(-/-) phenotypes most likely result from a transient reduction in fibroblast growth factor (FGF) signaling. Through the analysis of compound mutants, we provide strong evidence that Fgf17 is the FGF family member through which Gpc1 controls brain size.

CONCLUSION

These data add to a growing literature that implicates the glypican family of HSPGs in organ size control. They also argue that, among heparan sulfate-dependent signaling molecules, FGFs are disproportionately sensitive to loss of HSPGs. Finally, because heterozygous Gpc1 mutant mice were found to have brain sizes half-way between homozygous and wild type, the data imply that endogenous HSPG levels quantitatively control growth factor signaling, a finding that is both novel and relevant to the general question of how the activities of co-receptors are exploited during development.

Overgrowth of a mouse model of Simpson-Golabi-Behmel syndrome is partly mediated by Indian hedgehog

Loss-of-function mutations of Glypican 3 (Gpc3) cause the Simpson-Golabi-Behmel overgrowth syndrome (SGBS), and developmental overgrowth is observed in Gpc3-null mice, a mouse model for SGBS. We recently reported that GPC3 inhibits Hedgehog (Hh) signalling by inducing its endocytosis and degradation. Here, we show that the developmental overgrowth observed in Gpc3-null mice is, at least in part, a consequence of the hyperactivation of the Hh pathway. We bred Gpc3-null mice with mice that are Hh signalling-deficient owing to the lack of Indian Hh (Ihh), one of the three mammalian Hhs. We found that the Gpc3-null mice showed a 29.9% overgrowth in an Ihh wild-type background, whereas an Ihh-null background partly rescues the overgrowth caused by the lack of Gpc3 as the double mutants were 19.8% bigger than the Ihh-null mice. Consistent with the role of GPC3 in Hh endocytosis and degradation, the Gpc3-null mice show increased levels of Ihh protein and signalling, but similar levels of Ihh messenger RNA.

Proteoglycan interactions with Sonic Hedgehog specify mitogenic responses

Sonic Hedgehog (Shh) has dual roles in vertebrate development, promoting progenitor cell proliferation and inducing tissue patterning. We found that the mitogenic and patterning functions of Shh can be uncoupled from one another. Using a genetic approach to selectively inhibit Shh-proteoglycan interactions in a mouse model, we found that binding of Shh to proteoglycans was required for proliferation of neural stem/precursor cells, but not for tissue patterning. Shh-proteoglycan interactions regulated both spatial and temporal features of Shh signaling. Proteoglycans localized Shh to specialized mitogenic niches and also acted at the single-cell level to regulate the duration of Shh signaling, thereby promoting a gene expression program that is important for cell division. Because activation of the Shh pathway is a feature of diverse human cancers, selective stimulation of proliferation by Shh-proteoglycan interactions may also figure prominently in neoplastic growth.

Syndecan-1 regulates BMP signaling and dorso-ventral patterning of the ectoderm during early Xenopus development

Extracellular regulation of growth factor signaling is a key event for embryonic patterning. Heparan sulfate proteoglycans (HSPG) are among the molecules that regulate this signaling during embryonic development. Here we study the function of syndecan1 (Syn1), a cell-surface HSPG expressed in the non-neural ectoderm during early development of Xenopus embryos. Overexpression of Xenopus Syn1 (xSyn1) mRNA is sufficient to reduce BMP signaling, induce chordin expression and rescue dorso-ventral patterning in ventralized embryos. Experiments using chordin morpholinos established that xSyn1 mRNA can inhibit BMP signaling in the absence of chordin. Knockdown of xSyn1 resulted in a reduction of BMP signaling and expansion of the neural plate with the concomitant reduction of the non-neural ectoderm. Overexpression of xSyn1 mRNA in xSyn1 morphant embryos resulted in a biphasic effect, with BMP being inhibited at high concentrations and activated at low concentrations of xSyn1. Interestingly, the function of xSyn1 on dorso-ventral patterning and BMP signaling is specific for this HSPG. In summary, we report that xSyn1 regulates dorso-ventral patterning of the ectoderm through modulation of BMP signaling.

Trafficking, development and hedgehog

Embryogenesis is mediated by a relatively small number of developmental signaling pathways, and the morphogens, receptors and transcription factors integral to these cascades are considered the master regulators of development. However, superimposed on this is an additional layer of control by complex intracellular trafficking networks. The importance of trafficking in controlling the processes of morphogenesis and development is highlighted by recent data regarding the transport and localisation of the morphogen sonic hedgehog (Shh) and the machinery that leads to its secretion, modification, cellular internalisation and signal transduction. Here we review the regulation of hedgehog signaling by intracellular trafficking, including the role of the primary cilium and lipids in mediating pathway activity.

Hedgehog signaling in development and cancer

The Hedgehog (Hh) family of secreted proteins governs a wide variety of processes during embryonic development and adult tissue homeostasis. Here we review the current understanding of the molecular and cellular basis of Hh morphogen gradient formation and signal transduction, and the multifaceted roles of Hh signaling in development and tumorigenesis. We discuss how the Hh pathway has diverged during evolution and how it integrates with other signaling pathways to control cell growth and patterning.

Opposing roles for glypicans in Hedgehog signalling

Glypican members of the heparan sulphate proteoglycan family regulate several developmental signalling pathways, such as those involving Hedgehog, Wnt, BMP and FGF. Two studies reveal opposite effects of glypicans on Hedgehog signalling and show how their core protein domains and GPI anchor contribute to their versatile biological functions.

Glypican-mediated endocytosis of Hedgehog has opposite effects in flies and mice

Glypicans are glycosylphosphatidylinositol-linked heparan sulfate proteoglycans that play an essential part in the regulation of morphogen signalling. Two new reports using Drosophila and mice have highlighted the importance of glypican endocytosis in the regulation of Hedgehog (Hh) signalling and in Wingless gradient formation. One Drosophila glypican, Dally-like, acts positively in Hh signalling, whereas mouse Glypican-3 is a negative regulator. This difference seems to be dependent on whether glypicans promote the internalization of Hh alone or as a complex with its receptor, Patched.

Hedgehog nanopackages ready for dispatch

Hedgehog proteins are intercellular long-range signaling molecules that spread within tissues and activate gene expression during development. Vyas et al. (2008) propose that Hedgehog forms nanometer-sized oligomers that localize in proteoglycan-rich clusters at the surface of cells expressing Hedgehog. This nanoscale organization and enrichment in clusters ensures that Hedgehog is able to spread and activate signaling over many cell diameters.

Glypicans

The glypican family of cell-surface heparan sulfate proteoglycans modulate the actions of many developmentally important signal proteins.

Glypicans are heparan sulfate proteoglycans that are bound to the outer surface of the plasma membrane by a glycosyl-phosphatidylinositol anchor. Homologs of glypicans are found throughout the Eumetazoa. There are six family members in mammals (GPC1 to GPC6). Glypicans can be released from the cell surface by a lipase called Notum, and most of them are subjected to endoproteolytic cleavage by furin-like convertases. In vivo evidence published so far indicates that the main function of membrane-attached glypicans is to regulate the signaling of Wnts, Hedgehogs, fibroblast growth factors and bone morphogenetic proteins (BMPs). Depending on the context, glypicans may have a stimulatory or inhibitory activity on signaling. In the case of Wnt, it has been proposed that the stimulatory mechanism is based on the ability of glypicans to facilitate and/or stabilize the interaction of Wnts with their signaling receptors, the Frizzled proteins. On the other hand, GPC3 has recently been reported to inhibit Hedgehog protein signaling during development by competing with Patched, the Hedgehog receptor, for Hedgehog binding. Surprisingly, the regulatory activity of glypicans in the Wnt, Hedgehog and BMP signaling pathways is only partially dependent on the heparan sulfate chains.