K-glypican: a novel GPI-anchored heparan sulfate proteoglycan that is highly expressed in developing brain and kidney

Increased GPC4 and clusterin associated with insulin resistance in patients with PCOS

The aim of the study was to investigate the changes in serum glypican 4 (GPC4) and clusterin (CLU) levels in patients with polycystic ovary syndrome (PCOS) as well as their correlation with sex hormones and metabolic parameters. A total of 40 PCOS patients and 40 age-matched healthy women were selected. Serum GPC4 and CLU levels were compared between the PCOS and control groups, and binary logistic regression was used to analyze the relative risk of PCOS at different tertiles of serum GPC4 and CLU concentrations. Stepwise linear regression was used to identify the factors influencing serum GPC4 and CLU levels in PCOS patients. Serum GPC4 (1.82 ± 0.49 vs 1.30 ± 0.61 ng/mL, P < 0.001) and CLU (468.79 ± 92.85 vs 228.59 ± 82.42 µg/mL, P < 0.001) were significantly higher in PCOS patients than in healthy women after adjustment for body mass index (BMI). In the PCOS group, serum GPC4 was positively correlated with follicle-stimulating hormone, fasting plasma glucose (FPG), fasting insulin (FINS), homeostatic model assessment of insulin resistance (HOMA-IR), triglyceride, and CLU (P < 0.05), whereas serum CLU was positively correlated with BMI, FPG, FINS, and HOMA-IR (P < 0.05). Multiple stepwise linear regression analysis showed that HOMA-IR was independently associated with serum GPC4, and BMI and HOMA-IR were independently associated with CLU (P < 0.05). Serum GPC4 and CLU levels were significantly higher in PCOS patients than in healthy women, suggesting that GPC4 and CLU may be markers associated with insulin resistance in women with PCOS.

GPC3-Unc5 receptor complex structure and role in cell migration

Neural migration is a critical step during brain development that requires the interactions of cell-surface guidance receptors. Cancer cells often hijack these mechanisms to disseminate. Here, we reveal crystal structures of Uncoordinated-5 receptor D (Unc5D) in complex with morphogen receptor glypican-3 (GPC3), forming an octameric glycoprotein complex. In the complex, four Unc5D molecules pack into an antiparallel bundle, flanked by four GPC3 molecules. Central glycan-glycan interactions are formed by N-linked glycans emanating from GPC3 (N241 in human) and C-mannosylated tryptophans of the Unc5D thrombospondin-like domains. MD simulations, mass spectrometry and structure-based mutants validate the crystallographic data. Anti-GPC3 nanobodies enhance or weaken Unc5-GPC3 binding and, together with mutant proteins, show that Unc5/GPC3 guide migrating pyramidal neurons in the mouse cortex, and cancer cells in an embryonic xenograft neuroblastoma model. The results demonstrate a conserved structural mechanism of cell guidance, where finely balanced Unc5-GPC3 interactions regulate cell migration.

Evaluation of the association of serum glypican-4 with prevalent and future kidney function

Serum glypican-4 (GPC4) has been identified as an insulin-sensitizing adipokine serving as a marker for body mass index and insulin resistance in humans. The association of circulating GPC4 with kidney function is to date largely unexplored. Therefore, we aimed to evaluate the association between serum GPC4 and prevalent as well future kidney function in a prospective cohort study. The study included 456 Caucasian coronary angiography patients. After a median follow up period of 3.4 years, data on kidney function was reassessed in all patients. Chronic kidney disease (CKD) was defined by decreased estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m² or albuminuria. At baseline, serum GPC4 was significantly associated with decreased eGFR (adjusted odds ratio (OR) per standard deviation = 4.75 [2.66–8.48]; P <  0.001), albuminuria (OR = 1.49 [1.15–1.92]; P = 0.002), and, accordingly, with CKD (OR = 1.75 [1.35–2.26]; P < 0.001). GPC4 levels also significantly and independently predicted the incidence of newly diagnosed decreased eGFR (OR = 2.74 [1.82–4.14]; P < 0.001, albuminuria (OR = 1.58 [1.01–2.46]; P = 0.043, and CKD (OR = 2.16 [1.45–3.23]; P < 0.001). ROC analysis indicated an additional predictive value of GPC4 to a basic prediction model for newly diagnosed CKD and eGFR < 60 mL/min/1.73 m². Our study, therefore, indicates that high serum GPC4 is associated with decreased prevalent and future kidney function.

Galanin and glypican-4 levels depending on metabolic and cardiovascular risk factors in patients with polycystic ovary syndrome

OBJECTIVE

Galanin is a neuropeptide which has effects not only on metabolic syndrome but also on reproduction. Glypican-4 is an adipokine associated with insulin sensitivity by interacting directly with the insulin receptor. This study evaluated serum concentrations of galanin and glypican-4 in relation with the hormonal profile as well as metabolic and cardiovascular risk factors in patients with and without polycystic ovary syndrome (PCOS).

METHODS

A total of 44 women with PCOS and 44 age-matched controls were eligible. Hirsutism scores, hormonal profile, metabolic and cardiovascular risk factors as well as galanin and glypican-4 levels were evaluated in each subject.

RESULTS

Women with PCOS exhibited lower levels of galanin (20.2 pg/mL versus 26.4 pg/mL, p = 0.002) and higher concentrations of glypican-4 (3.1 ng/mL versus 2.6 ng/mL, p < 0.001) than controls. Both adipokines were correlated positively with body mass index (BMI), insulin, triglyceride and Homeostasis Model Assessment (HOMA) index; glypican-4 also showed positive correlations with fasting blood glucose, free testosterone, modified Ferriman-Gallwey scores (p < 0.05). Multiple Linear Regression analyses showed that PCOS and BMI were the best predictors affecting galanin levels with a decreasing and increasing effect respectively; however BMI was the best predictor affecting glypican-4 levels with an increasing effect (p < 0.001).

CONCLUSION

Galanin levels were lower and glypican-4 levels were higher in women with PCOS than controls. Further studies are needed to determine whether these adipokines could be used as additional markers for insulin sensitivity and lipid profile and whether they might play a role in the pathogenesis of PCOS, in which metabolic cardiovascular risks are increased.

Cripto-1 as a Potential Target of Cancer Stem Cells for Immunotherapy

Simple Summary

Cancer immunotherapy is gaining attention as a potential fourth treatment following surgery, chemotherapy, and radiation therapy. Cancer stem cells have recently been recognized and validated as a key target for cancer treatment. Cripto-1, which is a GPI-anchored membrane-bound protein that functions as a co-receptor of Nodal, is a marker of cancer stem cells. Since Nodal is a member of the TGF-β family, which performs an important role in stem cells and cancer stem cells, the inhibition of Cripto-1 could be a strategy by which to block Nodal signaling and thereby suppress cancer stem cells. We propose that Cripto-1 may be a novel target for cancer immunotherapy.

Abstract

The immune system has been found to be suppressed in cancer patients. Cancer cells are extremely resistant to chemotherapeutic drugs, conventional immunotherapy, or cancer antigen vaccine therapy. Cancer immunotherapy, which is mainly based on immune checkpoint inhibitors, such as those for PD-1, PD-L1, and CTLA4, is an effective treatment method. However, no immunotherapeutic target has been found that retains validity in the face of tumor diversity. The transforming growth factor (TGF)-β cytokine family possesses broad biological activity and is involved in the induction and/or transdifferentiation of helper T cells, which are important in immunotherapy. Nodal is a member of the TGF-β family playing important roles in tissue stem cells and cancer stem cells (CSCs), interacting with the co-receptor Cripto-1, as well as with Activin type IB (Alk4) and Activin typeIIreceptors, and maintaining stemness and Notch and Wnt/β-catenin signaling in CSCs. In recent years, it has been reported that Cripto-1 could be a potential therapeutic target in CSCs. Here, we review the accumulated literature on the molecular mechanisms by which Cripto-1 functions in CSCs and discuss the potential of Cripto-1 as an immunotherapeutic target in CSCs.

Glypicans and Heparan Sulfate in Synaptic Development, Neural Plasticity, and Neurological Disorders

Heparan sulfate proteoglycans (HSPGs) are components of the cell surface and extracellular matrix, which bear long polysaccharides called heparan sulfate (HS) attached to the core proteins. HSPGs interact with a variety of ligand proteins through the HS chains, and mutations in HSPG-related genes influence many biological processes and cause various diseases. In particular, recent findings from vertebrate and invertebrate studies have raised the importance of glycosylphosphatidylinositol-anchored HSPGs, glypicans, as central players in the development and functions of synapses. Glypicans are important components of the synapse-organizing protein complexes and serve as ligands for leucine-rich repeat transmembrane neuronal proteins (LRRTMs), leukocyte common antigen-related (LAR) family receptor protein tyrosine phosphatases (RPTPs), and G-protein-coupled receptor 158 (GPR158), regulating synapse formation. Many of these interactions are mediated by the HS chains of glypicans. Neurexins (Nrxs) are also synthesized as HSPGs and bind to some ligands in common with glypicans through HS chains. Therefore, glypicans and Nrxs may act competitively at the synapses. Furthermore, glypicans regulate the postsynaptic expression levels of ionotropic glutamate receptors, controlling the electrophysiological properties and non-canonical BMP signaling of synapses. Dysfunctions of glypicans lead to failures in neuronal network formation, malfunction of synapses, and abnormal behaviors that are characteristic of neurodevelopmental disorders. Recent human genetics revealed that glypicans and HS are associated with autism spectrum disorder, neuroticism, and schizophrenia. In this review, we introduce the studies showing the roles of glypicans and HS in synapse formation, neural plasticity, and neurological disorders, especially focusing on the mouse and Drosophila as potential models for human diseases.

Heparan Sulfate Proteoglycans Biosynthesis and Post Synthesis Mechanisms Combine Few Enzymes and Few Core Proteins to Generate Extensive Structural and Functional Diversity

Glycosylation is a common and widespread post-translational modification that affects a large majority of proteins. Of these, a small minority, about 20, are specifically modified by the addition of heparan sulfate, a linear polysaccharide from the glycosaminoglycan family. The resulting molecules, heparan sulfate proteoglycans, nevertheless play a fundamental role in most biological functions by interacting with a myriad of proteins. This large functional repertoire stems from the ubiquitous presence of these molecules within the tissue and a tremendous structural variety of the heparan sulfate chains, generated through both biosynthesis and post synthesis mechanisms. The present review focusses on how proteoglycans are “gagosylated” and acquire structural complexity through the concerted action of Golgi-localized biosynthesis enzymes and extracellular modifying enzymes. It examines, in particular, the possibility that these enzymes form complexes of different modes of organization, leading to the synthesis of various oligosaccharide sequences.

The Role of Glypicans in Cancer Progression and Therapy

Glypicans are a family of heparan sulfate proteoglycans that are attached to the cell membrane via a glycosylphosphatidylinositol anchor. Glypicans interact with multiple ligands, including morphogens, growth factors, chemokines, ligands, receptors, and components of the extracellular matrix through their heparan sulfate chains and core protein. Therefore, glypicans can function as coreceptors to regulate cell proliferation, cell motility, and morphogenesis. In addition, some glypicans are abnormally expressed in cancers, possibly involved in tumorigenesis, and have the potential to be cancer-specific biomarkers. Here, we provide a brief review focusing on the expression of glypicans in various cancers and their potential to be targets for cancer therapy.

Mice Lacking Glypican 4 Display Juvenile Hyperactivity and Adult Social Interaction Deficits

Glypican 4 (Gpc4) is a heparan sulfate proteoglycan that regulates glutamatergic synapse formation and function in the developing brain. Gpc4 KO mice have been shown to have decreased excitatory synapse number and less synaptic GluA1 AMPA receptors, leading to decreased synaptic transmission. Further, decreased expression of Gpc4 has been linked to autism spectrum disorder (ASD). Gpc4 is expressed by both astrocytes and neurons during postnatal development, with astrocyte expression higher in juvenile stages, and neuronal expression increasing with maturation. We therefore asked if mice lacking Gpc4 display behavioral alterations that are consistent with loss of GluA1 or ASD, and if so if they occur at juvenile ages when astrocyte Gpc4 is high, or at adult ages when both astrocytes and neurons express Gpc4. We found that juvenile (P14) Gpc4 KO mice display hyperactivity in the open field, which is corrected in adult mice (3 month). Adult Gpc4 KO mice show deficient behavior in social novelty, whilst non-social behaviors such as working memory and anxiety are unaffected. Thus, Gpc4 KO mice show age-specific behavioral alterations that are consistent with altered synaptic levels of GluA1 and behaviors associated with ASD.

An Input-Specific Orphan Receptor GPR158-HSPG Interaction Organizes Hippocampal Mossy Fiber-CA3 Synapses

Pyramidal neuron dendrites integrate synaptic input from multiple partners. Different inputs converging on the same dendrite have distinct structural and functional features, but the molecular mechanisms organizing input-specific properties are poorly understood. We identify the orphan receptor GPR158 as a binding partner for the heparan sulfate proteoglycan (HSPG) glypican 4 (GPC4). GPC4 is enriched on hippocampal granule cell axons (mossy fibers), whereas postsynaptic GPR158 is restricted to the proximal segment of CA3 apical dendrites receiving mossy fiber input. GPR158-induced presynaptic differentiation in contacting axons requires cell-surface GPC4 and the co-receptor LAR. Loss of GPR158 increases mossy fiber synapse density but disrupts bouton morphology, impairs ultrastructural organization of active zone and postsynaptic density, and reduces synaptic strength of this connection, while adjacent inputs on the same dendrite are unaffected. Our work identifies an input-specific HSPG-GPR158 interaction that selectively organizes synaptic architecture and function of developing mossy fiber-CA3 synapses in the hippocampus.

Long-term study of the association of adipokines and glucose variability with diabetic complications

BACKGROUND/AIMS

Recent studies have suggested an important role of adipokines in the development of insulin resistance and diabetes mellitus. The clinical relevance of adipokines on long-term outcomes in patients with diabetes and chronic kidney disease is uncertain. The purpose of this study was to identify a predictable factor in patients with long-term diabetic complications.

METHODS

A total of 161 diabetic individuals were followed-up from 2002 to 2013. Circulating plasma levels of adiponectin, glypican-4, irisin, visfatin, and visit-to-visit glucose variability were measured in diabetic patients. Associations among adipokines and variable metabolic parameters and microvascular, and macrovascular complications were evaluated.

RESULTS

Plasma adiponectin and glypican-4 levels were significantly increased in patients with renal insufficiency. These adipokines were negatively associated with estimated glomerular filtration rate and positively associated with urinary albumin excretion. The relative risk of renal progression to dialysis increased independently with increasing level of adiponectin. Glypican-4 and visfatin were not predictive of any microvascular or macrovascular complications. Glucose variability increased the risk of diabetic nephropathy and cerebrovascular complications.

CONCLUSIONS

Adiponectin and glypican-4 were associated with renal function and might be able to predict renal progression. Glucose variability was a predictable factor for diabetic nephropathy and cerebrovascular complications.

Evaluating clinical and prognostic implications of Glypican-3 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most deadly cancers worldwide. In patients with HCC, histopathogical differentiation is an important indicator of prognosis; however, because determination of HCC differentiation is difficult, the recently described immunohistochemical (IHC) marker glypican3 (GPC3) might assist in HCC prognostication.The goal of our study was to investigate GPC3's IHC staining pattern and define the relationship between its expression and patients' clinicopathologic features and overall survival. We retrieved clinical parameters from 101 pathologically diagnosed HCC patients' medical records and classified these patients into 4 clinical score categories (0–3) based on increasing GPC3 staining intensity and the percentage of stained tumor cells in their resection and biopsy specimens. Histopathological samples were well, moderately, and poorly differentiated in 33, 22, and 12 patients, respectively, and the GPC3 expression rate was 63%, 86%, and 92%,respectively. The median overall survival was 49.9 months (confidence interval (CI): 35.3–64.6 months) for clinical scores 0–1 and 30.7 months (CI: 19.4–41.9 months) for clinical scores 2–3. This difference was not statistically significant (P = .06) but showed a strong trend. In conclusion, a greater GPC3 expression is associated with a worse HCC prognosis and may be a promising prognostic marker.

Glypican-2 levels in cerebrospinal fluid predict the status of adult hippocampal neurogenesis

Adult hippocampal neurogenesis is a remarkable form of brain plasticity through which new neurons are generated throughout life. Despite its important roles in cognition and emotion and its modulation in various preclinical disease models, the functional importance of adult hippocampal neurogenesis in human health has not been revealed because of a lack of tools for monitoring adult neurogenesis in vivo. Therefore, we performed an unbiased proteomics screen to identify novel proteins expressed during neuronal differentiation using a human neural stem cell model, and we identified the proteoglycan Glypican-2 (Gpc2) as a putative secreted marker of immature neurons. Exogenous Gpc2 binds to FGF2 and inhibits FGF2-induced neural progenitor cell proliferation. Gpc2 is enriched in neurogenic regions of the adult brain. Its expression is increased by physiological stimuli that increase hippocampal neurogenesis and decreased in transgenic models in which neurogenesis is selectively ablated. Changes in neurogenesis also result in changes in Gpc2 protein level in cerebrospinal fluid (CSF). Gpc2 is detectable in adult human CSF, and first pilot experiments with a longitudinal cohort indicate a decrease over time. Thus, Gpc2 may serve as a potential marker to monitor adult neurogenesis in both animal and human physiology and disease, warranting future studies.

LARGE2-dependent glycosylation confers laminin-binding ability on proteoglycans

Both LARGE1 (formerly LARGE) and its paralog LARGE2 are bifunctional glycosyltransferases with xylosy- and glucuronyltransferase activities, and are capable of synthesizing polymers composed of a repeating disaccharide [-3Xylα1,3GlcAβ1-]. Post-translational modification of the O-mannosyl glycan of α-dystroglycan (α-DG) with the polysaccharide is essential for it to act as a receptor for ligands in the extracellular matrix (ECM), and both LARGE paralogs contribute to the modification in vivo. LARGE1 and LARGE2 have different tissue distribution profiles and enzymatic properties; however, the functional difference of the homologs remains to be determined, and α-DG is the only known substrate for the modification by LARGE1 or LARGE2. Here we show that LARGE2 can modify proteoglycans (PGs) with the laminin-binding glycan. We found that overexpression of LARGE2, but not LARGE1, mediates the functional modification on the surface of DG^-/-, Pomt1^-/- and Fktn^-/- embryonic stem cells. We identified a heparan sulfate-PG glypican-4 as a substrate for the LARGE2-dependent modification by affinity purification and subsequent mass spectrometric analysis. Furthermore, we showed that LARGE2 could modify several additional PGs with the laminin-binding glycan, most likely within the glycosaminoglycan (GAG)-protein linkage region. Our results indicate that LARGE2 can modify PGs with the GAG-like polysaccharide composed of xylose and glucuronic acid to confer laminin binding. Thus, LARGE2 may play a differential role in stabilizing the basement membrane and modifying its functions by augmenting the interactions between laminin globular domain-containing ECM proteins and PGs.

The role of heparan sulfate deficiency in autistic phenotype: potential involvement of Slit/Robo/srGAPs-mediated dendritic spine formation

Autism Spectrum Disorders (ASD) are the second most common developmental cause of disability in the United States. ASDs are accompanied with substantial economic and emotional cost. The brains of ASD patients have marked structural abnormalities, in the form of increased dendritic spines and decreased long distance connections. These structural differences may be due to deficiencies in Heparin Sulfate (HS), a proteoglycan involved in a variety of neurodevelopmental processes. Of particular interest is its role in the Slit/Robo pathway. The Slit/Robo pathway is known to be involved in the regulation of axonal guidance and dendritic spine formation. HS mediates the Slit/Robo interaction; without its presence Slit's repulsive activity is abrogated. Slit/Robo regulates dendritic spine formation through its interaction with srGAPs (slit-robo GTPase Activating Proteins), which leads to downstream signaling, actin cytoskeleton depolymerization and dendritic spine collapse. Through interference with this pathway, HS deficiency can lead to excess spine formation.

Glypican-3 is a biomarker and a therapeutic target of hepatocellular carcinoma

BACKGROUND

The carcinogenesis of hepatocellular carcinoma (HCC) is a multi-factorial, multi-step and complex process. Early diagnosis and effective treatments are of utmost importance. This review summarized the recent studies of oncofetal glypican-3 (GPC-3), a membrane-associated heparan sulfate proteoglycan, in the diagnosis and treatment of HCC.

DATA SOURCES

English-language reports published from June 2001 to September 2014 were searched from MEDLINE. The key words searched included: GPC-3, biomarker, target and HCC. The sensitivity, specificity, positive and negative predictive values were extracted, and the effect of GPC-3 targeted therapy on HCC was also evaluated.

RESULTS

GPC-3 plays a crucial role in HCC cell proliferation and metastasis. It mediates oncogenesis involving signaling pathways during hepatocyte malignant transformation. GPC-3 expression is increased in atypical hyperplasia and cancerous tissues. GPC-3 levels in HCC patients are related to HBV infection, TNM stage, periportal cancerous embolus, and extrahepatic metastasis. The diagnostic accuracy of the combination of serum GPC-3 and alpha-fetoprotein in HCC is up to 94.3%. Down-regulation of GPC-3 with specific siRNA or anti-GPC-3 antibody alters cell migration, metastasis and invasion behaviors. The nude mice xenograft tumor growth is inhibited by silencing GPC-3 gene transcription.

CONCLUSION

Oncofetal GPC-3 is a highly specific biomarker for the diagnosis of HCC and a promising target molecule for HCC gene therapy.

Sugar glues for broken neurons

Proteoglycans (PGs) regulate diverse functions in the central nervous system (CNS) by interacting with a number of growth factors, matrix proteins, and cell surface molecules. Heparan sulfate (HS) and chondroitin sulfate (CS) are two major glycosaminoglycans present in the PGs of the CNS. The functionality of these PGs is to a large extent dictated by the fine sulfation patterns present on their glycosaminoglycan (GAG) chains. In the past 15 years, there has been a significant expansion in our knowledge on the role of HS and CS chains in various neurological processes, such as neuronal growth, regeneration, plasticity, and pathfinding. However, defining the relation between distinct sulfation patterns of the GAGs and their functionality has thus far been difficult. With the emergence of novel tools for the synthesis of defined GAG structures, and techniques for their characterization, we are now in a better position to explore the structure-function relation of GAGs in the context of their sulfation patterns. In this review, we discuss the importance of GAGs on CNS development, injury, and disorders with an emphasis on their sulfation patterns. Finally, we outline several GAG-based therapeutic strategies to exploit GAG chains for ameliorating various CNS disorders.

Proteoglycans in Normal and Healing Skin

Significance: Proteoglycans have a distinct spatial localization in normal skin and are essential for the correct structural development, organization, hydration, and functional properties of this tissue. The extracellular matrix (ECM) is no longer considered to be just an inert supportive material but is a source of directive, spatial and temporal, contextual information to the cells via components such as the proteoglycans. There is a pressing need to improve our understanding of how these important molecules functionally interact with other matrix structures, cells and cellular mediators in normal skin and during wound healing. Recent Advances: New antibodies to glycosaminoglycan side chain components of skin proteoglycans have facilitated the elucidation of detailed localization patterns within skin. Other studies have revealed important proliferative activities of proteinase-generated fragments of proteoglycans and other ECM components (matricryptins). Knockout mice have further established the functional importance of skin proteoglycans in the assembly and homeostasis of the normal skin ECM. Critical Issues: Our comprehension of the molecular and structural complexity of skin as a complex, dynamic, constantly renewing, layered connective tissue is incomplete. The impact of changes in proteoglycans on skin pathology and the wound healing process is recognized as an important area of pathobiology and is an area of intense investigation. Future Directions: Advanced technology is allowing the development of new artificial skins. Recent knowledge on skin proteoglycans can be used to incorporate these molecules into useful adjunct therapies for wound healing and for maintenance of optimal tissue homeostasis in aging skin.

Caveolae-mediated endocytosis of the glucosaminoglycan-interacting adipokine tartrate resistant acid phosphatase 5a in adipocyte progenitor lineage cells

Adipogenesis depends on growth factors controlling proliferation/differentiation of mesenchymal stem cells (MSCs). Membrane binding and endocytosis of growth factors are often coupled to receptor activation and downstream signaling leading to specific cellular responses. The novel adipokine tartrate-resistant acid phosphatase (TRAP) 5a exhibits a growth factor-like effect on MSCs and pre-adipocytes and induces hyperplastic obesity in vivo. However its molecular interaction with pre-adipocytes remains unknown. Therefore, this study aimed to investigate membrane interaction of TRAP and its endocytosis routes in pre-adipocytes. Confocal and/or electron microscopy were used to detect TRAP in untreated or TRAP 5a/b treated pre-adipocytes under conditions that allow or inhibit endocytosis in combination with co-staining of endocytotic vesicles. TRAP interaction with heparin/heparan sulfate was verified by gel filtration. It could be shown that TRAP 5a, but not 5b, binds to the membrane of pre-adipocytes where it co-localizes with heparin-sulfate proteoglycan glypican-4. Also in vitro, TRAP 5a exhibited affinity for both heparin and heparan sulfate with heparin inhibiting its enzyme activity. Upon caveolae-mediated endocytosis of saturating levels of TRAP 5a, TRAP 5a co-localized intracellularly with glypican-4 and late endosomal marker Rab-7 positive vesicles. The protein was also located in multivesicular bodies (MVBs) but did not co-localize with lysosomal marker LAMP-1. TRAP 5a endocytosis was also detectable in pre-osteoblasts, but not fibroblasts, embryonic MSCs or mature adipocytes. These results indicate that TRAP 5a exhibits binding to cell surface, endocytosis and affinity to glucosaminoglycans (GAGs) in pre-adipocyte and pre-osteoblast lineage cells in a manner similar to other heparin-binding growth factors.

Palmitoyl acyltransferase, Zdhhc13, facilitates bone mass acquisition by regulating postnatal epiphyseal development and endochondral ossification: a mouse model

ZDHHC13 is a member of DHHC-containing palmitoyl acyltransferases (PATs) family of enzymes. It functions by post-translationally adding 16-carbon palmitate to proteins through a thioester linkage. We have previously shown that mice carrying a recessive Zdhhc13 nonsense mutation causing a Zdhcc13 deficiency develop alopecia, amyloidosis and osteoporosis. Our goal was to investigate the pathogenic mechanism of osteoporosis in the context of this mutation in mice. Body size, skeletal structure and trabecular bone were similar in Zdhhc13 WT and mutant mice at birth. Growth retardation and delayed secondary ossification center formation were first observed at day 10 and at 4 weeks of age, disorganization in growth plate structure and osteoporosis became evident in mutant mice. Serial microCT from 4-20 week-olds revealed that Zdhhc13 mutant mice had reduced bone mineral density. Through co-immunoprecipitation and acyl-biotin exchange, MT1-MMP was identified as a direct substrate of ZDHHC13. In cells, reduction of MT1-MMP palmitoylation affected its subcellular distribution and was associated with decreased VEGF and osteocalcin expression in chondrocytes and osteoblasts. In Zdhhc13 mutant mice epiphysis where MT1-MMP was under palmitoylated, VEGF in hypertrophic chondrocytes and osteocalcin at the cartilage-bone interface were reduced based on immunohistochemical analyses. Our results suggest that Zdhhc13 is a novel regulator of postnatal skeletal development and bone mass acquisition. To our knowledge, these are the first data to suggest that ZDHHC13-mediated MT1-MMP palmitoylation is a key modulator of bone homeostasis. These data may provide novel insights into the role of palmitoylation in the pathogenesis of human osteoporosis.

Unbiased discovery of glypican as a receptor for LRRTM4 in regulating excitatory synapse development

Leucine-rich repeat (LRR) proteins have recently been identified as important regulators of synapse development and function, but for many LRR proteins the ligand-receptor interactions are not known. Here we identify the heparan sulfate (HS) proteoglycan glypican as a receptor for LRRTM4 using an unbiased proteomics-based approach. Glypican binds LRRTM4, but not LRRTM2, in an HS-dependent manner. Glypican 4 (GPC4) and LRRTM4 localize to the pre- and postsynaptic membranes of excitatory synapses, respectively. Consistent with a trans-synaptic interaction, LRRTM4 triggers GPC4 clustering in contacting axons and GPC4 induces clustering of LRRTM4 in contacting dendrites in an HS-dependent manner. LRRTM4 positively regulates excitatory synapse development in cultured neurons and in vivo, and the synaptogenic activity of LRRTM4 requires the presence of HS on the neuronal surface. Our results identify glypican as an LRRTM4 receptor and indicate that a trans-synaptic glypican-LRRTM4 interaction regulates excitatory synapse development.

Astrocyte glypicans 4 and 6 promote formation of excitatory synapses via GluA1 AMPA receptors

In the developing central nervous system (CNS), the control of synapse number and function is critical to the formation of neural circuits. We previously demonstrated that astrocyte-secreted factors powerfully induce the formation of functional excitatory synapses between CNS neurons¹. Astrocyte-secreted thrombospondins induce structural synapses, however these synapses are post-synaptically silent². Here we use biochemical fractionation of astrocyte conditioned media (ACM) to identify glypican 4 (Gpc4) and 6 (Gpc6) as astrocyte-secreted signals sufficient to induce functional synapses between purified retinal ganglion cell (RGC) neurons, and show that depletion of these molecules from ACM significantly reduces its ability to induce postsynaptic activity. Application of Gpc4 to purified neurons is sufficient to increase the frequency and amplitude of glutamatergic synaptic events. This is achieved by increasing the surface level and clustering, but not overall cellular protein level, of the GluA1 subunit of the AMPA glutamate receptor (AMPAR). Gpc4&6 are expressed by astrocytes in vivo in the developing CNS, with Gpc4 expression enriched in the hippocampus and Gpc6 in the cerebellum. Finally, we demonstrate that Gpc4-deficient mice have defective synapse formation, with decreased amplitude of excitatory synaptic currents in the developing hippocampus and reduced recruitment of AMPARs to synapses. These data identify glypicans as a family of novel astrocyte-derived molecules that are necessary and sufficient to promote glutamate receptor clustering and receptivity and induce the formation of post-synaptically functioning CNS synapses.

Localization of glypican-4 in different membrane microdomains is involved in the regulation of Wnt signaling

Glypicans are members of the heparan sulfate proteoglycans (HSPGs) and are involved in various growth factor signaling mechanisms. Although HSPGs affect the β-catenin-dependent and -independent pathways of Wnt signaling, how they regulate distinct Wnt pathways is not clear. It has been suggested that the β-catenin-dependent pathway is initiated through receptor endocytosis in lipid raft microdomains and the independent pathway is activated through receptor endocytosis in non-lipid raft microdomains. Here, evidence is presented that glypican-4 (GPC4) is localized to both membrane microdomains and that the localization affects its ability to regulate distinct Wnt pathways. GPC4 bound to Wnt3a and Wnt5a, which activate the β-catenin-dependent and -independent pathways, respectively, and colocalized with Wnts on the cell surface. LRP6, one of Wnt3a coreceptors, was present in lipid raft microdomains, whereas Ror2, one of Wnt5a coreceptors, was localized to non-lipid raft microdomains. Expression of GPC4 enhanced the Wnt3a-dependent β-catenin pathway and the Wnt5a-dependent β-catenin-independent pathway, and knockdown of GPC4 suppressed both pathways. A GPC4 mutant that was localized to only non-lipid raft microdomains inhibited the β-catenin-dependent pathway but enhanced the β-catenin-independent pathway. These results suggest that GPC4 concentrates Wnt3a and Wnt5a to the vicinity of their specific receptors in different membrane microdomains, thereby regulating distinct Wnt signaling.

Models for studies of proteoglycans in kidney pathophysiology

Proteoglycans (PGs) impact many aspects of kidney health and disease. Models that permit genetic dissection of PG core protein and glycosaminoglycan (GAG) function have been instrumental to understanding their roles in the kidney. Matrix-associated PGs do not serve critical structural roles in the organ, nor do they contribute significantly to the glomerular barrier under normal conditions, but their abnormal expression influences fibrosis, inflammation, and progression of kidney disease. Most core proteins are dispensable for nephrogenesis (glypican-3 being an exception) and for maintenance of function in adult life, but their loss alters susceptibility to experimental kidney injury. In contrast, kidney development is exquisitely sensitive to GAG expression and fine structure as evidenced by the severe phenotypes of mutants for genes involved in GAG biosynthesis. This article reviews PG expression in normal kidney and the abnormalities caused by their disruption in mice and man.

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.

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.

Tel2 regulates the stability of PI3K-related protein kinases

We report an unexpected role for Tel2 in the expression of all mammalian phosphatidylinositol 3-kinase-related protein kinases (PIKKs). Although Tel2 was identified as a budding yeast gene required for the telomere length maintenance, we found no obvious telomeric function for mammalian Tel2. Targeted gene deletion showed that mouse Tel2 is essential in embryonic development, embryonic stem (ES) cells, and embryonic fibroblasts. Conditional deletion of Tel2 from embryonic fibroblasts compromised their response to IR and UV, diminishing the activation of checkpoint kinases and their downstream effectors. The effects of Tel2 deletion correlated with significantly reduced protein levels for the PI3K-related kinases ataxia telangiectasia mutated (ATM), ATM and Rad3 related (ATR), DNA-dependent protein kinase catalytic subunit ataxia (DNA-PKcs). Tel2 deletion also elicited specific depletion of the mammalian target of rapamycin (mTOR), suppressor with morphological effect on genitalia 1 (SMG1), and transformation/transcription domain-associated protein (TRRAP), and curbed mTOR signaling, indicating that Tel2 affects all six mammalian PIKKs. While Tel2 deletion did not alter PIKK mRNA levels, in vivo pulse labeling experiments showed that Tel2 controls the stability of ATM and mTOR. Each of the PIKK family members associated with Tel2 in vivo and in vitro experiments indicated that Tel2 binds to part of the HEAT repeat segments of ATM and mTOR. These data identify Tel2 as a highly conserved regulator of PIKK stability.

The oncofetal protein glypican-3 is a novel marker of hepatic progenitor/oval cells

Glypican-3 (Gpc3), a cell surface-linked heparan sulfate proteoglycan is highly expressed during embryogenesis and is involved in organogenesis. Its exact biological function remains unknown. We have studied the expression of Gpc3 in fetal and adult liver, in liver injury models of activation of liver progenitor cells: D-galactosamine and 2-acetylaminofluorene (2-AAF) administration followed by partial hepatectomy (PH) (2-AAF/PH); and in the Solt-Farber carcinogenic model: by initiation with a single dose of diethylnitrosamine and promotion with 2-AAF followed by PH treatment. Gpc3 expression was studied using complementary DNA microarrays, reverse transcriptase-polymerase chain reaction, in situ hybridization (ISH); ISH combined with immunohistochemistry (IHC) and immunofluorescent microscopy. We found that Gpc3 is highly expressed in fetal hepatoblasts from embryonic days 13 through 16 and its expression gradually decreases towards birth. Dual ISH with Gpc3 and alpha-fetoprotein (AFP) probes confirmed that only hepatoblasts and no other fetal liver cells express Gpc3. At 3 weeks after birth the expression of Gpc3 mRNA and protein was hardly detected in the liver. Gpc3 expression was highly induced in oval cell of D-gal and 2-AAF/PH treated animals. Dual ISH/IHC with Gpc3 riboprobe and cytokeratin-19 (CK-19) antibody revealed that Gpc3 is expressed in activated liver progenitor cells. ISH for Gpc3 and AFP performed on serial liver sections also showed coexpression of the two-oncofetal proteins. FACS isolated oval cells with anti-rat Thy1 revealed expression of Gpc3. Gpc3 expression persists in atypical duct-like structures and liver lesions of animals subjected to the Solt-Farber model of initiation and promotion of liver cancer expressing CK-19. In this work we report for the first time that the oncofetal protein Gpc3 is a marker of hepatic progenitor cells and of early liver lesions. Our findings show further that hepatic progenitor/oval cells are the target for malignant transformation in the Solt-Farber model of hepatic carcinogenesis.

Expression pattern of glypican-4 suggests multiple roles during mouse development

Glypicans are glycosylphosphatidylinositol-anchored heparan sulphate proteoglycans, which are thought to modulate signalling by growth factors, including fibroblast growth factors and Wnts. Studies in Xenopus have implicated glypicans, in particular glypican-4, in the process of convergent extension, which is essential for neural tube closure in Xenopus and mouse. Expression of glypican-4 has been reported in the mouse brain at embryonic day 10 and later stages, whereas expression during the developmental stages encompassing initiation and progression of neural tube closure has not been reported. Analysis by in situ hybridization reveals a complex pattern of glypican-4 mRNA localization at embryonic days 7-10.5, including sites in the anterior forebrain neuroepithelium, branchial arches, optic and otic vesicles, limb buds and somites. Glypican-4 expression is not detected in the midline of the embryo at the stage of initiation of neural tube closure, suggesting that glypican-4 is unlikely to play an essential role in convergent extension in the mouse.

Processing by convertases is not required for glypican-3-induced stimulation of hepatocellular carcinoma growth

Glypicans are a family of heparan sulfate proteoglycans that are bound to the cell surface by a lipid anchor. Six members of this family have been identified in mammals (GPC1-GPC6). Glypicans act as regulators of the activity of various cytokines, including Wnts, Hedgehogs, and bone morphogenetic proteins. It has been reported that processing by a convertase is required for GPC3 activity during convergent extension in zebrafish embryos, for GPC3-induced regulation of Wnt signaling, and for the binding of GPC3 to Wnt5a. In our laboratory, we have recently demonstrated that GPC3 promotes the growth of hepatocellular carcinomas (HCCs) by stimulating canonical Wnt signaling. Because there is increasing evidence indicating that the structural requirements for GPC3 activity are cell type specific, we decided to investigate whether GPC3 needs to be processed by convertases to stimulate cell proliferation and Wnt signaling in HCC cells. We report here that a mutant GPC3 that cannot be processed by convertases is still able to play its stimulatory role in Wnt activity and HCC growth.

Molecular mechanisms of optic axon guidance

Axon guidance is one of the critical processes during vertebrate central nervous system (CNS) development. The optic nerve, which contains the axons of retinal ganglion cells, has been used as a powerful model to elucidate some of the mechanisms underlying axon guidance because it is easily manipulated experimentally, and its function is well understood. Recent molecular biology studies have revealed that numerous guidance molecules control the development of the visual pathway. This review introduces the molecular mechanisms involved in each critical step during optic axon guidance. Axonal projections to the optic disc are thought to depend on adhesion molecules and inhibitory extracellular matrices such as chondroitin sulfate. The formation of the head of the optic nerve and the optic chiasm require ligand-receptor interactions between netrin-1 and the deleted in colorectal cancer receptor, and Slit proteins and Robo receptors, respectively. The gradient distributions of ephrin ligands and Eph receptors are essential for correct ipsilateral projections at the optic chiasm and the topographic mapping of axons in the superior colliculus/optic tectum. The precise gradient is regulated by transcription factors determining the retinal dorso-ventral and nasal-temporal polarities. Moreover, the axon guidance activities by Slit and semaphorin 5A require the existence of heparan sulfate, which binds to numerous guidance molecules. Recent discoveries about the molecular mechanisms underlying optic nerve guidance will facilitate progress in CNS developmental biology and axon-regeneration therapy.

Regulatory variation at glypican-3 underlies a major growth QTL in mice

The genetic basis of variation in complex traits remains poorly understood, and few genes underlying variation have been identified. Previous work identified a quantitative trait locus (QTL) responsible for much of the response to selection on growth in mice, effecting a change in body mass of approximately 20%. By fine-mapping, we have resolved the location of this QTL to a 660-kb region containing only two genes of known function, Gpc3 and Gpc4, and two other putative genes of unknown function. There are no non-synonymous polymorphisms in any of these genes, indicating that the QTL affects gene regulation. Mice carrying the high-growth QTL allele have approximately 15% lower Gpc3 mRNA expression in kidney and liver, whereas expression differences at Gpc4 are non-significant. Expression profiles of the two other genes within the region are inconsistent with a factor responsible for a general effect on growth. Polymorphisms in the 3′ untranslated region of Gpc3 are strong candidates for the causal sequence variation. Gpc3 loss-of-function mutations in humans and mice cause overgrowth and developmental abnormalities. However, no deleterious side-effects were detected in our mice, indicating that genes involved in Mendelian diseases also contribute to complex trait variation. Furthermore, these findings show that small changes in gene expression can have substantial phenotypic effects.

The genetic basis of variation in complex traits remains poorly understood. Here, a genetic variant is discovered that leads to increased body size in mice

Murine glypican-4 gene structure and expression; Sp1 and Sp3 play a major role in glypican-4 expression in 3T3-F442A cells

In this report we describe the genomic organization of the mouse glypican-4 (Gpc4), an analysis of its promoter and its transcriptional regulation in the 3T3-F442A adipocyte cell line. The Gpc4 gene consists of nine exons separated by eight introns. A series of deletion mutants and 4391 bp of the 5'-flanking region were cloned into pGL3-BASIC upstream of the luciferase reporter gene and transfected into 3T3-F442A adipocytes. Analysis of a 4.3-kb DNA fragment at the 5'-flanking region of this gene revealed that the Gpc4 promoter is a TATA-less promoter with a large cluster of GC boxes. Competitive electrophoretic mobility shift and supershift assays identified a cluster of nine functional GC boxes binding Sp1 and Sp3 in this region. Transactivation experiments in insect cells showed that both Sp1 and Sp3 are major activators of the Gpc4 promoter. Gpc4 is expressed in adipocytes where its expression is highest in confluent 3T3-F442A adipoblasts and decreases dramatically as cells differentiate. Sp protein analyses demonstrated a major decrease in Sp3 protein in differentiated adipocytes as compared to undifferentiated adipoblasts. These experiments show that Gpc4 is developmentally regulated in 3T3-F442A adipocytes and suggest that Sp transcription factors play a significant role in the regulated expression of Gpc4.

Hepatocyte growth factor-mediated renal epithelial branching morphogenesis is regulated by glypican-4 expression

The glypican (Gpc) family of cell surface heparan sulfate proteoglycans are expressed in a tissue-specific and developmentally regulated fashion. To determine if individual Gpcs can modulate heparin-binding growth factor signaling, we examined hepatocyte growth factor (HGF)-stimulated mitogenic, motogenic, and morphogenic responses of renal tubular cells expressing different Gpcs. Adult inner medullary collecting duct (IMCD) cells were found to express primarily Gpc4 and to proliferate, migrate, and form tubules with HGF, correlating with sustained extracellular signal-regulated kinase (ERK) activation. Embryonic IMCD cells expressing predominantly Gpc3 proliferated and migrated in response to HGF but activated ERK only transiently and failed to form tubules. Overexpressing Gpc-4 but not Gpc-3 or Gpc-1 led to sustained HGF-stimulated ERK activation and rescued the tubulogenic response in these cells. These results demonstrate that both signaling and phenotypic responses to HGF can be regulated by specific Gpc expression patterns.

Heparan sulfate proteoglycans in glomerular inflammation

Heparan sulfate proteoglycans (HSPGs) are glycoproteins consisting of a core protein to which linear heparan sulfate side chains are covalently attached. These heparan sulfate side chains can be modified at different positions by several enzymes, which include N-deacetylases, N- and O-sulfotransferases, and an epimerase. These heparan sulfate modifications give rise to an enormous structural diversity, which corresponds to the variety of biologic functions mediated by heparan sulfate, including its role in inflammation. The HSPGs in the glomerular basement membrane (GBM), perlecan, agrin, and collagen XVIII, play an important role in the charge-selective permeability of the glomerular filter. In addition to these HSPGs, various cell types express HSPGs at their cell surface, which include syndecans, glypicans, CD44, and betaglycan. During inflammation, HSPGs, especially heparan sulfate, in the extracellular matrix (ECM) and at the surface of endothelial cells bind chemokines, which establishes a local concentration gradient recruiting leukocytes. Endothelial and leukocyte cell surface HSPGs also play a role in their direct adhesive interactions via other cell surface adhesion molecules, such as selectins and beta2 integrin. Activated leukocytes and endothelial cells exert heparanase activity, resulting in degradation of heparan sulfate moieties in the ECM, which facilitates leukocyte passage into tissues and the release of heparan sulfate-bound factors. In various renal inflammatory diseases the expression of agrin and GBM-associated heparan sulfate is decreased, while the expression of CD44 is increased. Heparan sulfate or heparin preparations affect inflammatory cell behavior and have promising therapeutic, anti-inflammatory properties by preventing leukocyte adhesion/influx and tissue damage.

Effects of Syndecan-1 and Glypican on Muscle Cell Proliferation and Differentiation: Implications for Possible Functions During Myogenesis

The heparan sulfate proteoglycans, syndecan-1 and glypican, are low-affinity receptors for fibroblast growth factor 2 (FGF2). Because FGF2 is a potent stimulator of skeletal muscle cell proliferation and a strong inhibitor of differentiation, it is likely that changes in syndecan-1 and glypican expression will affect myogenesis as both, in part, regulate FGF-dependent signaling. In the current study, expression vector constructs containing either syndecan-1 or glypican were transfected into turkey myogenic satellite cells resulting in the overexpression of these genes. The amount of expression of each of these genes was measured by semiquantitative reverse transcriptase polymerase chain reaction. The satellite cell cultures overexpressing syndecan-1 were unable to fuse to form multinucleated myotubes after differentiation was induced. The syndecan-1-transfected cells maintained a rounded morphology typical of cells during proliferation. In contrast, the satellite cells transfected with glypican formed larger myotubes. These results suggest that both syndecan-1 and glypican play pivotal, but different, roles in both muscle cell proliferation and differentiation.

GPC5 is a possible target for the 13q31-q32 amplification detected in lymphoma cell lines

Comparative genomic hybridization (CGH) analyses have detected gains of copy number on 13q, especially at 13q31-q32, in cell lines and primary cases of various types of lymphoma. Since amplification of chromosomal DNA is one of the mechanisms that can activate tumor-associated genes, and because 13q amplification had been reported in various other types of tumors as well, we attempted to define by fluorescence in situ hybridization (FISH) a common region at 13q31-q32 in which to explore genes that might be targets for the amplification events. Although the commonly amplified region we defined was relatively large (approximately 4 Mb), only one true gene, GPC5, was found there. GPC5 was over-expressed in lymphoma cell lines that had shown amplification, in comparison with those that had not. Our findings suggest that GPC5 is a likely target for amplification, and that over-expression of this gene may contribute to development and/or progression of lymphomas and other tumors.

Glypican 4 modulates FGF signalling and regulates dorsoventral forebrain patterning in Xenopus embryos

Heparan sulphate proteoglycans such as glypicans are essential modulators of intercellular communication during embryogenesis. In Xenopus laevis embryos, the temporal and spatial distribution of Glypican 4 (Gpc4) transcripts during gastrulation and neurulation suggests functions in early development of the central nervous system. We have functionally analysed the role of Xenopus Gpc4 by using antisense morpholino oligonucleotides and show that Gpc4 is part of the signalling network that patterns the forebrain. Depletion of GPC4 protein results in a pleiotropic phenotype affecting both primary axis formation and early patterning of the anterior central nervous system. Molecular analysis shows that posterior axis elongation during gastrulation is affected in GPC4-depleted embryos, whereas head and neural induction are apparently normal. During neurulation, loss of GPC4 disrupts expression of dorsal forebrain genes, such as Emx2, whereas genes marking the ventral forebrain and posterior central nervous system continue to be expressed. This loss of GPC4 activity also causes apoptosis of forebrain progenitors during neural tube closure. Biochemical studies establish that GPC4 binds FGF2 and modulates FGF signal transduction. Inhibition of FGF signal transduction, by adding the chemical SU5402 to embryos from neural plate stages onwards, phenocopies the loss of gene expression and apoptosis in the forebrain. We propose that GPC4 regulates dorsoventral forebrain patterning by positive modulation of FGF signalling.

Expression of NG2 proteoglycan during endochondral and intramembranous ossification

We have used immunohistochemistry to study the distribution of the NG2 proteoglycan during bone development in the mouse. At embryonic day 15.5, NG2 was strongly detected in the immature cartilage of developing limbs. After transient down-regulation in mature chondrocytes, NG2 was up-regulated during primary ossification, colocalizing with alkaline phosphatase and tenascin C. In the epiphyseal growth plates of newborn mouse tibia, NG2 and alkaline phosphatase exhibited overlapping patterns of expression by hypertrophic chondrocytes and by osteoblasts surrounding newly formed bone trabeculae. NG2 was down-regulated after puberty, being only faintly detectable in the tibial growth plates of 3-month-old mice. In cranial sutures, NG2 was strongly labeled in osteogenic bone fronts and in the suture matrix. Our results indicate that NG2 expression is up-regulated during both endochondral and intramembranous ossification, but is down-regulated as ossification is completed.

Localisation of specific heparan sulfate proteoglycans during the proliferative phase of brain development

Early brain development is characterised by the proliferation of neural precursor cells. Several families of signalling molecules such as the fibroblast growth factors (FGFs) and Wnts are known to play important roles in this early phase of brain development. Accumulating evidence demonstrates that signalling of these molecules requires the presence of heparan sulfate chains attached to a proteoglycan core protein (HSPG). However, the specific identity of the HSPG components in the developing brain is unknown. To determine which HSPGs might be involved at this early phase, we analysed the expression of the major cell surface HSPG families in the developing brain at a time of most active proliferation. Syndecan-1 and glypican-4 were the most highly expressed in the developing brain during the time of peak proliferation and localise to ventricular regions of the brain, where the precursor cells are proliferating. Syndecan-4, although less abundant, also localises to cells in the ventricular zone. We have also examined HSPG involvement in brain development using cultures of embryonic neural precursor cells. We find that FGF2 stimulation of proliferation is inhibited in the presence of sodium chlorate, an inhibitor of heparan sulfate synthesis, and is rescued by addition of exogenous heparan sulfate. These data support a requirement for heparan sulfate in FGF signalling for proliferation of brain precursor cells. The expression of these specific HSPGs within the proliferative zone of the brain suggests that they may be involved in regulation of early brain development, such as FGF-stimulated proliferation.

Identification of Hedgehog pathway components by RNAi in Drosophila cultured cells

Classical genetic screens can be limited by the selectivity of mutational targeting, the complexities of anatomically based phenotypic analysis, or difficulties in subsequent gene identification. Focusing on signaling response to the secreted morphogen Hedgehog (Hh), we used RNA interference (RNAi) and a quantitative cultured cell assay to systematically screen functional roles of all kinases and phosphatases, and subsequently 43% of predicted Drosophila genes. Two gene products reported to function in Wingless (Wg) signaling were identified as Hh pathway components: a cell surface protein (Dally-like protein) required for Hh signal reception, and casein kinase 1alpha, a candidate tumor suppressor that regulates basal activities of both Hh and Wg pathways. This type of cultured cell-based functional genomics approach may be useful in the systematic analysis of other biological processes.

Gene expression of EXT1 and EXT2 during mouse brain development

Heparan sulfate (HS) and heparan sulfate proteoglycans (HSPGs) play significant roles in various biological processes. There is a wealth of circumstantial and experimental evidence suggesting the roles of HS in mammalian neural development. HS synthesis is governed by a series of enzymes. Among them, two enzymes, EXT1 and EXT2, catalyze polymerization of glucuronic acid and N-acetylglucosamine, the crucial step of HS synthesis. To obtain insight into the roles of HS in neural development, we examined the spatiotemporal expression patterns of EXT1 and EXT2 during mice brain development. RT-PCR analyses showed that expression of EXT1 and EXT2 peaks during early postnatal period in the cerebrum and around birth in the cerebellum. In situ hybridization revealed that in the embryonic brain, EXT1 and EXT2 were localized primarily in the neuroepithelial cells surrounding the lateral ventricles, the mesencephalic vesicle, and the fourth ventricle. In the early postnatal stage, intense expression of EXT1 and EXT2 was observed in the cerebral cortex and the hippocampus formation. In the postnatal cerebellum, expression of EXT1 and EXT2 was mainly observed in external and internal granular layers. Our results demonstrate that EXT1 and EXT2 are highly expressed in the developing brain, and that their expression is developmentally regulated, suggesting that HS is involved in various neurodevelopmental processes.

Genotype/phenotype correlations of males affected by Simpson-Golabi-Behmel syndrome with GPC3 gene mutations: patient report and review of the literature

Simpson-Golabi-Behmel syndrome (SGBS) is an X-linked overgrowth syndrome with associated visceral and skeletal anomalies. Deletions or point mutations involving the glypican-3 (GPC3) gene at Xq26 are associated with a relatively milder form of this disorder (SGBS1). GPC3 encodes a putative extracellular proteoglycan, glypican-3, that is inferred to play an important role in growth control in embryonic mesodermal tissues in which it is selectively expressed. It appears to form a complex with insulin-like growth factor-II (IGF-II), and might thereby modulate IGF-II action. We reviewed the clinical findings of all published patients with SGBS1 with GPC3 mutations to confirm the clinical specificity for the SGBS1 phenotype. Moreover, we report on a new patient with a GPC3 deletion and IGF-II evaluation.

The role of glypicans in mammalian development

Glypicans are a family of heparan sulfate proteoglycans that are bound to the cell surface by a glycosyl-phosphatidylinositol anchor. Six members of this family have been identified in mammals. In general, glypicans are highly expressed during development, and their expression pattern suggests that they are involved in morphogenesis. One member of this family, glypican-3, is mutated in the Simpson-Golabi-Behmel syndrome. This syndrome is characterized by overgrowth and various developmental abnormalities that indicate that glypican-3 inhibits proliferation and cell survival in the embryo. It has consequently been proposed that glypicans can regulate the activity of several growth factors that play a critical role in morphogenesis.

Mechanical strain regulation of the chicken glypican-4 gene expression in the avian eggshell gland

Comparison of RNA fingerprinting of the avian eggshell gland (ESG) without and with an egg revealed upregulation of a 382-bp cDNA fragment that showed high homology to the mammalian glypican 4 (GPC-4). The gene sequence revealed a conserved glypican signature, a glycosyl phosphatidyl inositol-anchorage site, and cystein residues, most of which were conserved. GPC-4 was expressed in the ESG in a circadian fashion only during the period of eggshell calcification, when maximal mechanical strain was imposed. Removal of the egg just before to its entry into the ESG, with consequent elimination of the mechanical strain, caused reduction in the gene expression. Artificial application of the mechanical strain induced expression of the GPC-4 gene that was related to the level of the strain. GPC-4 expression was strain dependent in other parts of the oviduct. In the ESG, GPC-4 was expressed exclusively by the glandular epithelium and not by the pseudostratified epithelium facing the lumen. In summary, we cloned the avian homologue of GPC-4, established its pattern of expression in the avian ESG, and demonstrated for the first time that this gene is regulated by mechanical strain.

Regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression by follicle-stimulating hormone, cAMP increase and calcium influx during rat Sertoli cell development

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell- to-cell signaling and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate proteoglycans (HSPG). Glypican-1, syndecans-1 and -4 were identified using a molecular approach. Their differential regulation was demonstrated in immature rat Sertoli cells. Follicle-stimulating hormone (FSH) is the main regulator of Sertoli cell function. Signal transduction triggered by FSH involves both an increased intracellular cAMP synthesis and a calcium influx. This study demonstrates that FSH, through its second messengers (increase in intracellular cAMP and intracellular calcium), downregulated the glypican-1 mRNA expression in Sertoli cells from 20-day-old rats. On the other hand, syndecan-1 mRNA expression is not modulated by FSH as it would result from the antagonistic effects of increased intracellular cAMP and intracellular calcium levels. Finally, syndecan-4 mRNA expression is not regulated by this pathway. The present study was extended during Sertoli cell development. Indeed, Sertoli cells undergo extensive changes during the postnatal period both in structure and function. These important transformations are critical for the establishment of spermatogenesis and development of the adult pattern of testicular function. Our data indicated that the regulation of HSPG mRNA expression is HSPG-specific and depends on the Sertoli cell developmental stage.

Protein kinase C regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression during rat Sertoli cell development

Our previous studies indicated that cell surface proteoglycans were mostly heparan sulfate ones (HSPG) in 20 day-old Sertoli cells [Biochim. Biophys. Acta 1510 (2001) 474]. Among these HSPG, glypican-1, syndecans-1 and -4 mRNAs were expressed and differentially regulated. Glypican-1 and syndecan-1 mRNA expression was up-regulated under PKC activation in contrast to syndecan-4 mRNA expression which was not affected [Biochim. Biophys. Acta 1474 (2000) 31]. Rat Sertoli cells undergo extensive changes during the postnatal period both in structure and function, as the hematotesticular barrier establishment occurs at around 20 day-old. The testicular PKCalpha expression in developing Sertoli cells results in (i) a soluble (inactive) form which is maximal at the age of 1 day and declines gradually thereafter and (ii) a particulate (active) form which is low at birth, increases six-fold on days 8-11 of age and declines thereafter. The present study focused on the glypican-1, syndecan-1 and syndecan-4 mRNA expression and regulation under PKC activation by the phorbol myristate acetate (PMA) in 10-30 day-old Sertoli cells. Our data indicated that the regulation of their expression specifically depends on the nature of HSPG and Sertoli cell developmental stage and evidenced a specific PKC regulation of HSPG mRNA expression.