Embryonic Fibroblasts with a Gene Trap Mutation in Ext1 Produce Short Heparan Sulfate Chains

The exostosin glycosyltransferase 1/STAT3 axis is a driver of breast cancer aggressiveness

The epithelial-mesenchymal transition (EMT) program is crucial for transforming carcinoma cells into a partially mesenchymal state, enhancing their chemoresistance, migration, and metastasis. This shift in cell state is tightly regulated by cellular mechanisms that are not yet fully characterized. One intriguing EMT aspect is the rewiring of the proteoglycan landscape, particularly the induction of heparan sulfate proteoglycan (HSPG) biosynthesis. This proteoglycan functions as a co-receptor that accelerates cancer-associated signaling pathways through its negatively-charged residues. However, the precise mechanisms through which EMT governs HSPG biosynthesis and its role in cancer cell plasticity remain elusive. Here, we identified exostosin glycosyltransferase 1 (EXT1), a central enzyme in HSPG biosynthesis, to be selectively upregulated in aggressive tumor subtypes and cancer cell lines, and to function as a key player in breast cancer aggressiveness. Notably, ectopic expression of EXT1 in epithelial cells is sufficient to induce HSPG levels and the expression of known mesenchymal markers, subsequently enhancing EMT features, including cell migration, invasion, and tumor formation. Additionally, EXT1 loss in MDA-MB-231 cells inhibits their aggressiveness-associated traits such as migration, chemoresistance, tumor formation, and metastasis. Our findings reveal that EXT1, through its role in HSPG biosynthesis, governs signal transducer and activator of transcription 3 (STAT3) signaling, a known regulator of cancer cell aggressiveness. Collectively, we present the EXT1/HSPG/STAT3 axis as a central regulator of cancer cell plasticity that directly links proteoglycan synthesis to oncogenic signaling pathways.

Conditional ablation of heparan sulfate expression in stromal fibroblasts promotes tumor growth in vivo

Heparan sulfate (HS) is a glycocalyx component present in the extracellular matrix and cell-surface HS proteoglycans (HSPGs). Although HSPGs are known to play functional roles in multiple aspects of tumor development and progression, the effect of HS expression in the tumor stroma on tumor growth in vivo remains unclear. We conditionally deleted Ext1, which encodes a glycosyltransferase essential for the biosynthesis of HS chains, using S100a4-Cre (S100a4-Cre; Ext1f/f) to investigate the role of HS in cancer-associated fibroblasts, which is the main component of the tumor microenvironment. Subcutaneous transplantation experiments with murine MC38 colon cancer and Pan02 pancreatic cancer cells demonstrated substantially larger subcutaneous tumors in S100a4-Cre; Ext1f/f mice. Additionally, the number of myofibroblasts observed in MC38 and Pan02 subcutaneous tumors of S100a4-Cre; Ext1f/f mice decreased. Furthermore, the number of intratumoral macrophages decreased in MC38 subcutaneous tumors in S100a4-Cre; Ext1f/f mice. Finally, the expression of matrix metalloproteinase-7 (MMP-7) markedly increased in Pan02 subcutaneous tumors in S100a4-Cre; Ext1f/f mice, suggesting that it may contribute to rapid growth. Therefore, our study demonstrates that the tumor microenvironment with HS-reduced fibroblasts provides a favorable environment for tumor growth by affecting the function and properties of cancer-associated fibroblasts, macrophages, and cancer cells.

Proteoglycans in Cancer: Friends or Enemies? A Special Focus on Hepatocellular Carcinoma

Simple Summary

Proteoglycans affect multiple molecular and cellular processes during the progression of solid tumors with a highly desmoplastic stroma, such as HCC. Due to their role in enhancing or limiting the traits of cancer cells underlying their aggressiveness, such as proliferation, angiogenesis, epithelial to mesenchymal transition (EMT), and stemness, these macromolecules could be exploited as molecular targets or therapeutic agents. Proteoglycans, such as biglycan, versican, syndecan-1, glypican-3, and agrin, promote HCC cell proliferation, EMT, and angiogenesis, while endostatin and proteoglycan 4 were shown to impair cancer neovascularization or to enhance the sensitivity of HCC cells to drugs, such as sorafenib and regorafenib. Based on this evidence, interventional strategies involving the use of humanized monoclonal antibodies, T cells engineered with chimeric antigen receptors, or recombinant proteins mimicking potentially curative proteoglycans, are being employed or may be adopted in the near future for the treatment of HCC.

Abstract

Proteoglycans are a class of highly glycosylated proteins expressed in virtually all tissues, which are localized within membranes, but more often in the pericellular space and extracellular matrix (ECM), and are involved in tissue homeostasis and remodeling of the stromal microenvironment during physiological and pathological processes, such as tissue regeneration, angiogenesis, and cancer. In general, proteoglycans can perform signaling activities and influence a range of physical, chemical, and biological tissue properties, including the diffusivity of small electrolytes and nutrients and the bioavailability of growth factors. While the dysregulated expression of some proteoglycans is observed in many cancers, whether they act as supporters or limiters of neoplastic progression is still a matter of controversy, as the tumor promoting or suppressive function of some proteoglycans is context dependent. The participation of multiple proteoglycans in organ regeneration (as demonstrated for the liver in hepatectomy mouse models) and in cancer suggests that these molecules actively influence cell growth and motility, thus contributing to key events that characterize neoplastic progression. In this review, we outline the main roles of proteoglycans in the physiology and pathology of cancers, with a special mention to hepatocellular carcinoma (HCC), highlighting the translational potential of proteoglycans as targets or therapeutic agents for the treatment of this disease.

Biological role of heparan sulfate in osteogenesis: A review

Heparan sulfate (HS) is extensively expressed in cells, for example, cell membrane and extracellular matrix of most mammalian cells and tissues, playing a key role in the growth and development of life by maintaining homeostasis and implicating in the etiology and diseases. Recent studies have revealed that HS is involved in osteogenesis via coordinating multiple signaling pathways. The potential effect of HS on osteogenesis is a complicated and delicate biological process, which involves the participation of osteocytes, chondrocytes, osteoblasts, osteoclasts and a variety of cytokines. In this review, we summarized the structural and functional characteristics of HS and highlighted the molecular mechanism of HS in bone metabolism to provide novel research perspectives for the further medical research.

Chondrocytes respond to an altered heparan sulfate composition with distinct changes of heparan sulfate structure and increased levels of chondroitin sulfate

Heparan sulfate (HS) regulates the activity of many signaling molecules critical for the development of endochondral bones. Even so, mice with a genetically altered HS metabolism display a relatively mild skeletal phenotype compared to the defects observed in other tissues and organs pointing to a reduced HS dependency of growth-factor signaling in chondrocytes. To understand this difference, we have investigated the glycosaminoglycan (GAG) composition in two mouse lines that produce either reduced levels of HS (Ext1^gt/gt mice) or HS lacking 2-O-sulfation (Hs2st1^-/- mice). Analysis by RPIP-HPLC revealed an increased level of sulfated disaccarides not affected by the mutation in both mouse lines indicating that chondrocytes attempt to restore a critical level of sulfation. In addition, in both mutant lines we also detected significantly elevated levels of CS. Size exclusion chromatography further demonstrated that Ext1^gt/gt mutants produce more but shorter CS chains, while the CS chains produced by (Hs2st1^-/- mice) mutants are of similar length to that of wild type littermates indicating that chondrocytes produce more rather than longer CS chains. Expression analysis revealed an upregulation of aggrecan, which likely carries most of the additionally produced CS. Together the results of this study demonstrate for the first time that not only a reduced HS synthesis but also an altered HS structure leads to increased levels of CS in mammalian tissues. Furthermore, as chondrocytes produce 100-fold more CS than HS the increased CS levels point to an active, precursor-independent mechanism that senses the quality of HS in a vast excess of CS. Interestingly, reducing the level of cell surface CS by chondroitinase treatment leads to reduced Bmp2 induced Smad1/5/9 phosphorylation. In addition, Erk phosphorylation is increased independent of Fgf18 treatment indicating that both, HS and CS, affect growth factor signaling in chondrocytes in distinct manners.

Fluoride regulates the expression of extracellular matrix HSPG and related signaling pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification

Skeletal fluorosis causes growth plate impairment and growth retardation during bone development. Longitudinal bone development is accomplished by endochondral ossification in growth plate. However, the mechanism of fluoride impairs growth plate is unclear. To explore the effect of fluoride on various glycosaminoglycans (GAGs) and related signaling pathways in growth plate during endochondral ossification, SD rats and ATDC5 cells were treated with fluoride and carried out a series of experiments. We found that the expression of heparan sulfate (HS), a kind of GAGs in extracellular matrix, was significantly increased in the growth plate of fluoride-treated rats compared with control rats. Furthermore, the expression of HS synthetic enzyme exostosin 1 (EXT1) and glypican 6 (GPC6), a core protein of HS proteoglycan (HSPG), were significantly increased in fluoride-treated ATDC5 cells compared with control cells (P < 0.05). The expression of related molecules including fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription 1 (STAT1) and parathyroid hormone-related protein (PTHrP) were significantly increased in the fluoride-treated groups compared with control groups (P < 0.05), and there was significantly decreased in the expression of Indian hedgehog (Ihh) in fluoride-treated groups compared with control groups (P < 0.05). Our data suggested that fluoride increased the content of HSPG in extracellular matrix by promoting the expression of EXT1 and GPC6. Fluoride also activated FGFR3 signaling pathway, inhibited Ihh/PTHrP feedback loop and inhibited endochondral ossification. Nevertheless, the regulation of fluoride on HSPG and related pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification needs to be further studied.

The exostosin family of glycosyltransferases: mRNA expression profiles and heparan sulphate structure in human breast carcinoma cell lines

Breast cancer remains a leading cause of cancer-related mortality in women. In recent years, regulation of genes involved in heparan sulphate (HS) biosynthesis have received increased interest as regulators of breast cancer cell adhesion and invasion. The exostosin (EXT) proteins are glycosyltransferases involved in elongation of HS, a regulator of intracellular signaling, cell–cell interactions, and tissue morphogenesis. The EXT family contains five members: EXT1, EXT2, and three EXT-like (EXTL) members: EXTL1, EXTL2, and EXTL3. While the expression levels of these enzymes change in tumor cells, little is known how this changes the structure and function of HS. In the present study, we investigated gene expression profiles of the EXT family members, their glycosyltransferase activities and HS structure in the estrogen receptor (ER), and progesterone receptor (PR) positive MCF7 cells, and the ER, PR, and human epidermal growth factor receptor-2 (HER2) negative MDA-MB-231 and HCC38 epithelial breast carcinoma cell lines. The gene expression profiles for MDA-MB-231 and HCC38 cells were very similar. In both cell lines EXTL2 was found to be up-regulated whereas EXT2 was down-regulated. Interestingly, despite having similar expression of HS elongation enzymes the two cell lines synthesized HS chains of significantly different lengths. Furthermore, both MDA-MB-231 and HCC38 exhibited markedly decreased levels of HS 6-O-sulphated disaccharides. Although the gene expression profiles of the elongation enzymes did not correlate with the length of HS chains, our results indicated specific differences in EXT enzyme levels and HS fine structure characteristic of the carcinogenic properties of the breast carcinoma cells.

Metabolic engineering of mammalian cells to produce heparan sulfates

Heparan sulfate (HS) is a glycosaminoglycan produced by all mammalian cells that plays important roles in physiology and various pathologies. Heparin is a highly sulfated form of HS that is used clinically as an anticoagulant. Heparin and HSs may also have therapeutic benefits for a wide variety of other indications. Cultured mammalian cells produce HS and, through genetic modification, have been used to elucidate the biosynthetic pathway. Recently, metabolic engineering has been used to produce HS from cultured mammalian cells for clinical purposes. This review describes the HS biosynthetic pathway and its manipulation through metabolic engineering to produce bioengineered HSs. We also discuss current challenges and opportunities to advance the field of HS metabolic engineering.

Signaling systems affecting the severity of multiple osteochondromas

Multiple osteochondromas (MO) syndrome is a dominant autosomal bone disorder characterized by the formation of cartilage-capped bony outgrowths that develop at the juxtaposition of the growth plate of endochondral bones. MO has been linked to mutations in either EXT1 or EXT2, two glycosyltransferases required for the synthesis of heparan sulfate (HS). The establishment of mouse mutants demonstrated that a clonal, homozygous loss of Ext1 in a wild type background leads to the development of osteochondromas. Here we investigate mechanisms that might contribute to the variation in the severity of the disease observed in human patients. Our results show that residual amounts of HS are sufficient to prevent the development of osteochondromas strongly supporting that loss of heterozygosity is required for osteochondroma formation. Furthermore, we demonstrate that different signaling pathways affect size and frequency of the osteochondromas thereby modulating the severity of the disease. Reduced Fgfr3 signaling, which regulates proliferation and differentiation of chondrocytes, increases osteochondroma number, while activated Fgfr3 signaling reduces osteochondroma size. Both, activation and reduction of Wnt/β-catenin signaling decrease osteochondroma size and frequency by interfering with the chondrogenic fate of the mutant cells. Reduced Ihh signaling does not change the development of the osteochondromas, while elevated Ihh signaling increases the cellularity and inhibits chondrocyte differentiation in a subset of osteochondromas and might thus predispose osteochondromas to the transformation into chondrosarcomas.

Potential role for Ext1-dependent heparan sulfate in regulating P311 gene expression in A549 carcinoma cells

BACKGROUND

Exostosin-1 (EXT1), a member of the EXT protein family, is indispensable for synthesis of heparan sulfate (HS) chains that bind to and modulate the signaling efficiency of numerous growth factor activities. We have previously shown that Ext1 mutated mouse embryonic fibroblasts produce short sulfated HS chains which dramatically influence tumor cell behavior in a 3-dimensional (3D) heterospheroid system composed of tumor cells and fibroblasts.

METHODS

In this study, we have used both 2D co-culture and 3D heterospheroid models, consisting of human A549 carcinoma cells co-cultured with wild-type or Ext1-mutated mouse embryonic fibroblasts.

RESULTS AND CONCLUSIONS

Gene expression profiling of differentially expressed genes in fibroblast/A549 heterospheroids identified P311 as a gene substantially down-regulated in A549 cells co-cultured with Ext1-mutated fibroblasts. In addition, we observed that the Ext1 mutants displayed reduced Tgf-β1 mRNA levels and lower levels of secreted active TGF-β protein. Re-introduction of Ext1 in the Ext1 mutant fibroblasts rescued the levels of Tgf-β1 mRNA, increased the amounts of secreted active TGF-β in these cells, as well as P311 mRNA levels in adjacent A549 cells. Accordingly, small interfering RNAs (siRNAs) against fibroblast Tgf-β1 reduced P311 expression in neighboring A549 tumor cells. Our data raises the possibility that fibroblast Ext1 levels play a role in P311 expression in A549/fibroblast co-culture through TGF-β1.

GENERAL SIGNIFICANCE

This study considers a possible novel mechanism of Ext1-regulated heparan sulfate structure in modifying tumor-stroma interactions through altering stromal tgf-ß1 expression.

Heparan Sulfate: Biosynthesis, Structure, and Function

Heparan sulfate (HS) proteoglycans (PGs) are ubiquitously expressed on cell surfaces and in the extracellular matrix of most animal tissues, having essential functions in development and homeostasis, as well as playing various roles in disease processes. The functions of HSPGs are mainly dependent on interactions between the HS-side chains with a variety of proteins including cytokines, growth factors, and their receptors. In a given HS polysaccharide, negatively charged sulfate and carboxylate groups are arranged in various types of domains, generated through strictly regulated biosynthetic reactions and with enormous potential for structural variability. The mode of HS-protein interactions is assessed through binding experiments using saccharides of defined composition in vitro, signaling assays in cell models where HS structures are manipulated, and targeted disruption of genes for biosynthetic enzymes in animals (mouse, zebrafish, Drosophila, and Caenorhabditis elegans) followed by phenotype analysis. Whereas some protein ligands appear to require strictly defined HS structure, others bind to variable saccharide domains without apparent dependence on distinct saccharide sequence. These findings raise intriguing questions concerning the functional significance of regulation in HS biosynthesis and the potential for development of therapeutics targeting HS-protein interactions.

Old gene, new phenotype: mutations in heparan sulfate synthesis enzyme, EXT2 leads to seizure and developmental disorder, no exostoses

BACKGROUND

Heparan sulfate proteoglycans are vital components of the extracellular matrix and are essential for cellular homeostasis. Many genes are involved in modulating heparan sulfate synthesis, and when these genes are mutated, they can give rise to early-onset developmental disorders affecting multiple body systems. Herein, we describe a consanguineous family of four sibs with a novel disorder, which we designate as seizures-scoliosis-macrocephaly syndrome, characterised by seizures, intellectual disability, hypotonia, scoliosis, macrocephaly, hypertelorism and renal dysfunction.

METHODS

Our application of autozygosity mapping and whole-exome sequencing allowed us to identify mutations in the patients. To confirm the autosomal-recessive mode of inheritance, all available family members were genotyped. We also studied the effect of these mutations on protein expression and function in patient cells and using an in vitro system.

RESULTS

We identified two homozygous mutations p.Met87Arg and p.Arg95 Cys in exostosin 2, EXT2, a ubiquitously expressed gene that encodes a glycosyltransferase required for heparan sulfate synthesis. In patient cells, we observed diminished EXT2 expression and function. We also performed an in vitro assay to determine which mutation has a larger effect on protein expression and observed reduced EXT2 expression in constructs expressing either one of the mutations but a greater reduction when both residues were mutated.

CONCLUSIONS

In short, we have unravelled the genetic basis of a new recessive disorder, seizures-scoliosis-macrocephaly syndrome. Our results have implicated a well-characterised gene in a new developmental disorder and have further illustrated the spectrum of phenotypes that can arise due to errors in glycosylation.

Reduced Expression of EXTL2, a Member of the Exostosin (EXT) Family of Glycosyltransferases, in Human Embryonic Kidney 293 Cells Results in Longer Heparan Sulfate Chains

Heparan sulfate proteoglycans are ubiquitously located on cell surfaces and in the extracellular matrices. The negatively charged heparan sulfate chains interact with a multitude of different proteins, thereby influencing a variety of cellular and developmental processes, for example cell adhesion, migration, tissue morphogenesis, and differentiation. The human exostosin (EXT) family of genes contains five members: the heparan sulfate polymerizing enzymes, EXT1 and EXT2, and three EXT-like genes, EXTL1, EXTL2, and EXTL3. EXTL2 has been ascribed activities related to the initiation and termination of heparan sulfate chains. Here we further investigated the role of EXTL2 in heparan sulfate chain elongation by gene silencing and overexpression strategies. We found that siRNA-mediated knockdown of EXTL2 in human embryonic kidney 293 cells resulted in increased chain length, whereas overexpression of EXTL2 in the same cell line had little or no effect on heparan sulfate chain length. To study in more detail the role of EXTL2 in heparan sulfate chain elongation, we tested the ability of the overexpressed protein to catalyze the in vitro incorporation of N-acetylglucosamine and N-acetylgalactosamine to oligosaccharide acceptors resembling unmodified heparan sulfate and chondroitin sulfate precursor molecules. Analysis of the generated products revealed that recombinant EXTL2 showed weak ability to transfer N-acetylgalactosamine to heparan sulfate precursor molecules but also, that EXTL2 exhibited much stronger in vitro N-acetylglucosamine-transferase activity related to elongation of heparan sulfate chains.

Human genetic disorders and knockout mice deficient in glycosaminoglycan

Glycosaminoglycans (GAGs) are constructed through the stepwise addition of respective monosaccharides by various glycosyltransferases and maturated by epimerases and sulfotransferases. The structural diversity of GAG polysaccharides, including their sulfation patterns and sequential arrangements, is essential for a wide range of biological activities such as cell signaling, cell proliferation, tissue morphogenesis, and interactions with various growth factors. Studies using knockout mice of enzymes responsible for the biosynthesis of the GAG side chains of proteoglycans have revealed their physiological functions. Furthermore, mutations in the human genes encoding glycosyltransferases, sulfotransferases, and related enzymes responsible for the biosynthesis of GAGs cause a number of genetic disorders including chondrodysplasia, spondyloepiphyseal dysplasia, and Ehlers-Danlos syndromes. This review focused on the increasing number of glycobiological studies on knockout mice and genetic diseases caused by disturbances in the biosynthetic enzymes for GAGs.

Reprint of: Heparan sulfate as a regulator of endochondral ossification and osteochondroma development

Most elements of the vertebrate skeleton are formed by endochondral ossification. This process is initiated with mesenchymal cells that condense and differentiate into chondrocytes. These undergo several steps of differentiation from proliferating into hypertrophic chondrocytes, which are subsequently replaced by bone. Chondrocyte proliferation and differentiation are tightly controlled by a complex network of signaling molecules. During recent years, it has become increasingly clear that heparan sulfate (HS) carrying proteoglycans play a critical role in controlling the distribution and activity of these secreted factors. In this review we summarize the current understanding of the role of HS in regulating bone formation. In human, mutations in the HS synthetizing enzymes Ext1 and Ext2 induce the Multiple Osteochondroma syndrome, a skeletal disorder characterized by short stature and the formation of benign cartilage-capped tumors. We review the current insight into the origin of the disease and discuss its possible molecular basis. In addition, we summarize the existing insight into the role of HS as a regulator of signal propagation and signaling strength in the developing skeleton.

Heparan sulfate as a regulator of endochondral ossification and osteochondroma development

Most elements of the vertebrate skeleton are formed by endochondral ossification. This process is initiated with mesenchymal cells that condense and differentiate into chondrocytes. These undergo several steps of differentiation from proliferating into hypertrophic chondrocytes, which are subsequently replaced by bone. Chondrocyte proliferation and differentiation are tightly controlled by a complex network of signaling molecules. During recent years, it has become increasingly clear that heparan sulfate (HS) carrying proteoglycans play a critical role in controlling the distribution and activity of these secreted factors. In this review we summarize the current understanding of the role of HS in regulating bone formation. In human, mutations in the HS synthetizing enzymes Ext1 and Ext2 induce the Multiple Osteochondroma syndrome, a skeletal disorder characterized by short stature and the formation of benign cartilage-capped tumors. We review the current insight into the origin of the disease and discuss its possible molecular basis. In addition, we summarize the existing insight into the role of HS as a regulator of signal propagation and signaling strength in the developing skeleton.

The exostosin family: proteins with many functions

Heparan sulfates are complex sulfated molecules found in abundance at cell surfaces and in the extracellular matrix. They bind to and influence the activity of a variety of molecules like growth factors, proteases and morphogens and are thus involved in various cell-cell and cell-matrix interactions. The mammalian EXT proteins have glycosyltransferase activities relevant for HS chain polymerization, however their exact role in this process is still confusing. In this review, we summarize current knowledge about the biochemical activities and some proposed functions of the members of the EXT protein family and their roles in human disease.

Glycosaminoglycans in the blood of hereditary multiple exostoses patients: Half reduction of heparan sulfate to chondroitin sulfate ratio and the possible diagnostic application

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder with wide variation in clinical phenotype and is caused by heterogeneous germline mutations in two of the Ext genes, EXT-1 and EXT-2, which encode ubiquitously expressed glycosyltransferases involved in the polymerization of heparan sulfate (HS) chains. To examine whether the Ext mutation could affect HS structures and amounts in HME patients being heterozygous for the Ext genes, we collected blood from patients and healthy individuals, separated it into plasma and cellular fractions and then isolated glycosaminoglycans (GAGs) from those fractions. A newly established method consisting of a combination of selective ethanol precipitation of GAGs, digestion of GAGs recovered on the filter-cup by direct addition of heparitinase or chondroitinase reaction solution and subsequent high-performance liquid chromatography of the unsaturated disaccharide products enabled the analysis using the least amount of blood (200 µL). We found that HS structures of HME patients were almost similar to those of controls in both plasma and cellular fractions. However, interestingly, although both the amounts of HS and chondroitin sulfate (CS) varied depending on the different individuals, the amounts of HS in both the plasma and cellular fractions of HME patient samples were decreased and the ratios of HS to CS (HS/CS) of HME patient samples were almost half those of healthy individuals. The results suggest that HME patients' blood exhibited reduced HS amounts and HS/CS ratios, which could be used as a diagnostic biomarker for HME.

Fibroblast EXT1-levels influence tumor cell proliferation and migration in composite spheroids

Background

Stromal fibroblasts are important determinants of tumor cell behavior. They act to condition the tumor microenvironment, influence tumor growth, support tumor angiogenesis and affect tumor metastasis. Heparan sulfate proteoglycans, present both on tumor and stromal cells, interact with a large number of ligands including growth factors, their receptors, and structural components of the extracellular matrix. Being ubiquitously expressed in the tumor microenvironment heparan sulfate proteoglycans are candidates for playing central roles in tumor-stroma interactions. The objective of this work was to investigate the role of heparan sulfate expressed by stromal fibroblasts in modulating the growth of tumor cells and in controlling the interstitial fluid pressure in a 3-D model.

Methodology/Principal Findings

We generated spheroids composed of fibroblasts alone, or composite spheroids, composed of fibroblasts and tumor cells. Here we show that stromal fibroblasts with a mutation in the heparan sulfate elongating enzyme Ext1 and thus a low heparan sulfate content, formed composite fibroblast/tumor cell spheroids with a significant lower interstitial fluid pressure than corresponding wild-type fibroblast/tumor cell composite spheroids. Furthermore, immunohistochemistry of composite spheroids revealed that the cells segregated, so that after 6 days in culture, the wild-type fibroblasts formed an inner core and the tumor cells an outer layer of cells. For composite spheroids containing Ext1-mutated fibroblasts this segregation was less obvious, indicating impaired cell migration. Analysis of tumor cells expressing the firefly luciferase gene revealed that the changes in tumor cell migration in mutant fibroblast/tumor cell composite spheroids coincided with a lower proliferation rate.

Conclusions/Significance

This is the first demonstration that stromal Ext1-levels modulate tumor cell proliferation and affect the interstitial fluid pressure in a 3-D spheroid model. Learning how structural changes in stromal heparan sulfate influence tumor cells is essential for our understanding how non-malignant cells of the tumor microenvironment influence tumor cell progression.

Heparan sulfate proteoglycans as multifunctional cell regulators: cell surface receptors

Proteoglycans are macromolecules expressed on the cell surfaces and in the extracellular matrix of most animal tissues (Annu Rev Biochem 68:729-777, 1999; Int Rev Cell Mol Biol 276:105-159, 2009). Heparan sulfate proteoglycans (HSPGs) are essential for animal development and homeostasis, and are involved in various pathological processes. The functions of HSPGs are largely exerted through interaction of the heparan sulfate (HS) side chains with different types of ligands, including diverse molecules such as cytokines, enzymes, and pathogens. One of the important roles of cell surface HSPGs is to mediate cytokine-induced cell signaling through interaction with growth factors (GFs) and their cognate receptors. A selective dependence of GFs for different structural features of HS has been demonstrated by applying cell models that are mutated variously in HS structure due to deficiency in enzymes involved in the biosynthesis of HS chains.

Genotype-phenotype correlation study in 529 patients with multiple hereditary exostoses: identification of "protective" and "risk" factors

BACKGROUND

Multiple hereditary exostoses is an autosomal dominant skeletal disorder characterized by wide variation in clinical phenotype. The aim of this study was to evaluate whether the severity of the disease is linked with a specific genetic background.

METHODS

Five hundred and twenty-nine patients with multiple hereditary exostoses from two different European referral centers participated in the study. According to a new clinical classification based on the presence or absence of deformities and functional limitations, the phenotype of the patients was assessed as mild (the absence of both aspects), intermediate, or severe (the concurrent presence of both aspects). An identical molecular screening protocol with denaturing high-performance liquid chromatography and multiplex ligation-dependent probe amplification was performed in both institutions.

RESULTS

In our cohort of patients, variables such as female sex (odds ratio = 1.840; 95% confidence interval, 1.223 to 2.766), fewer than five skeletal sites with exostoses (odds ratio = 7.588; 95% confidence interval, 3.479 to 16.553), EXT2 mutations (odds ratio = 2.652; 95% confidence interval, 1.665 to 4.223), and absence of EXT1/2 mutations (odds ratio = 1.975; 95% confidence interval, 1.051 to 3.713) described patients with a mild phenotype; in contrast, a severe phenotype was associated with male sex (odds ratio = 2.431; 95% confidence interval, 1.544 to 3.826), EXT1 mutations (odds ratio = 6.817; 95% confidence interval, 1.003 to 46.348), and more than twenty affected skeletal sites (odds ratio = 2.413; 95% confidence interval, 1.144 to 5.091). Malignant transformation was observed in 5% of patients, and no evidence of association between chondrosarcoma onset and EXT mutation, sex, severity of disease, or number of lesions was detected.

CONCLUSIONS

The identified "protective" and "risk" factors, as well as the proposed classification system, represent helpful tools for clinical management and follow-up of patients with multiple hereditary exostoses; moreover, homogeneous cohorts of patients, useful for studies on the pathogenesis of multiple hereditary exostoses, have been identified.

Specific glycosaminoglycans modulate neural specification of mouse embryonic stem cells

Mouse embryonic stem (mES) cells express a low sulfated form of heparan sulfate (HS). HS chains displayed by ES cells and their progeny become more complex and more sulfated during progression from pluripotency to neuroectodermal precursors. Sulfated epitopes are important for recognition and binding of a variety of ligands including members of the fibroblast growth factor (FGF) family. We demonstrated previously that mES cells lacking HS cannot undergo neural specification but this activity can be recovered by adding soluble heparin, a highly sulfated glycosaminoglycan (GAG). Therefore, we hypothesized that soluble GAGs might be used to support neural differentiation of HS competent cells and that the mechanisms underlying this activity might provide useful information about the signaling pathways critical for loss of pluripotency and early lineage commitment. In this study, we demonstrate that specific HS/heparin polysaccharides support formation of Sox1(+) neural progenitor cells from wild-type ES cells. This effect is dependent on sulfation pattern, concentration, and length of saccharide. Using a selective inhibitor of FGF signal transduction, we show that heparin modulates signaling events regulating exit from pluripotency and commitment to primitive ectoderm and subsequently neuroectoderm. Interestingly, we were also able to demonstrate that multiple receptor tyrosine kinases were influenced by HS in this system. This suggests roles for additional factors, possibly in cell proliferation or protection from apoptosis, during the process of neural specification. Therefore, we conclude that soluble GAGs or synthetic mimics could be considered as suitable low-cost factors for addition to ES cell differentiation regimes.

Compound heterozygous loss of Ext1 and Ext2 is sufficient for formation of multiple exostoses in mouse ribs and long bones

Multiple Hereditary Exostoses (MHE) syndrome is caused by haploinsufficiency in Golgi-associated heparan sulfate polymerases EXT1 or EXT2 and is characterized by formation of exostoses next to growing long bones and other skeletal elements. Recent mouse studies have indicated that formation of stereotypic exostoses requires a complete loss of Ext expression, suggesting that a similar local loss of EXT function may underlie exostosis formation in patients. To further test this possibility and gain greater insights into pathogenic mechanisms, we created heterozygous Ext1(+/-) and compound Ext1(+/-)/Ext2(+/-) mice. Like Ext2(+/-) mice described previously (Stickens et al. Development 132:5055), Ext1(+/-) mice displayed rib-associated exostosis-like outgrowths only. However, compound heterozygous mice had nearly twice as many outgrowths and, more importantly, displayed stereotypic growth plate-like exostoses along their long bones. Ext1(+/-)Ext2(+/-) exostoses contained very low levels of immuno-detectable heparan sulfate, and Ext1(+/-)Ext2(+/-) chondrocytes, endothelial cells and fibroblasts in vitro produced shortened heparan sulfate chains compared to controls and responded less vigorously to exogenous factors such as FGF-18. We also found that rib outgrowths formed in Ext1(f/+)Col2Cre and Ext1(f/+)Dermo1Cre mice, suggesting that ectopic skeletal tissue can be induced by conditional Ext ablation in local chondrogenic and/or perichondrial cells. The study indicates that formation of stereotypic exostoses requires a significant, but not complete, loss of Ext expression and that exostosis incidence and phenotype are intimately sensitive to, and inversely related to, Ext expression. The data also indicate that the nature and organization of ectopic tissue may be influenced by site-specific anatomical cues and mechanisms.

Amino acid substitutions in the E2 glycoprotein of Sindbis-like virus XJ-160 confer the ability to undergo heparan sulfate-dependent infection of mouse embryonic fibroblasts

We have recently demonstrated an essential role of the domain of 145-150 amino acid in the E2 glycoprotein of Sindbis virus in the interaction with cellular heparan sulfate (HS) and in the infection of mouse embryonic fibroblasts (MEF) cells. In this study, we constructed and characterized the mutants of Sindbis-like virus XJ-160 in which Tyr-146 and/or Asn-149 in the E2 glycoprotein had been substituted with His and Arg, respectively. Unlike parental virus XJ-160, mutants with either or both substitutions were able to infect wild-type mouse embryonic fibroblasts (MEF-wt) or MEF-Epi-/- cells which produce mutant HS. Significantly more infectious particles were released from MEF-wt than from MEF-Epi-/- cells. The mutant virus with both substitutions release was inhibited by pre-incubation of virus with heparin or pre-treatment of BHK-21 cells with HS-degrading enzyme. Both XJ-160 and the mutant viruses retained substantial neurovirulence in suckling mice. Our findings provide further support to the importance of positively charged residues in the HS-binding site of E2 in mediating Sindbis virus infection of MEF cells.

No haploinsufficiency but loss of heterozygosity for EXT in multiple osteochondromas

Multiple osteochondromas (MO) is an autosomal dominant disorder caused by germline mutations in EXT1 and/or EXT2. In contrast, solitary osteochondroma (SO) is nonhereditary. Products of the EXT gene are involved in heparan sulfate (HS) biosynthesis. In this study, we investigated whether osteochondromas arise via either loss of heterozygosity (2 hits) or haploinsufficiency. An in vitro three-dimensional chondrogenic pellet model was used to compare heterozygous bone marrow-derived mesenchymal stem cells (MSCs EXT(wt/-)) of MO patients with normal MSCs and the corresponding tumor specimens (presumed EXT(-/-)). We demonstrated a second hit in EXT in five of eight osteochondromas. HS chain length and structure, in vitro chondrogenesis, and EXT expression levels were identical in both EXT(wt/-) and normal MSCs. Immunohistochemistry for HS, HS proteoglycans, and HS-dependent signaling pathways (eg, TGF-β/BMP, Wnt, and PTHLH) also showed no differences. The cartilaginous cap of osteochondroma contained a mixture of HS-positive and HS-negative cells. Because a heterozygous EXT mutation does not affect chondrogenesis, EXT, HS, or downstream signaling pathways in MSCs, our results refute the haploinsufficiency theory. We found a second hit in 63% of analyzed osteochondromas, supporting the hypothesis that osteochondromas arise via loss of heterozygosity. The detection of the second hit may depend on the ratio of HS-positive (normal) versus HS-negative (mutated) cells in the cartilaginous cap of the osteochondroma.

Biosynthesis of heparan sulfate in EXT1-deficient cells

HS (heparan sulfate) is synthesized by HS co-polymerases encoded by the EXT1 and EXT2 genes (exostosin 1 and 2), which are known as causative genes for hereditary multiple exostoses, a dominantly inherited genetic disorder characterized by multiple cartilaginous tumours. It has been thought that the hetero-oligomeric EXT1-EXT2 complex is the biologically relevant form of the polymerase and that targeted deletion of either EXT1 or EXT2 leads to a complete lack of HS synthesis. In the present paper we show, unexpectedly, that two distinct cell lines defective in EXT1 expression indeed produce small but significant amounts of HS chains. The HS chains produced without the aid of EXT1 were shorter than HS chains formed in concert with EXT1 and EXT2. In addition, biosynthesis of HS in EXT1-defective cells was notably blocked by knockdown of either EXT2 or EXTL2 (EXT-like), but not of EXTL3. Then, to examine the roles of EXTL2 in the biosynthesis of HS in EXT1-deficient cells, we focused on the GlcNAc (N-aetylglucosamine) transferase activity of EXTL2, which is involved in the initiation of HS chains by transferring the first GlcNAc to the linkage region. Although EXT2 alone synthesized no heparan polymers on the synthetic linkage region analogue GlcUAbeta1-3Galbeta1-O-C2H4NH-benzyloxycarbonyl, marked polymerization by EXT2 alone was demonstrated on GlcNAcalpha1-4GlcUAbeta1-3Galbeta1-O-C2H4N-benzyloxycarbonyl (where GlcUA is glucuronic acid and Gal is galactose), which was generated by transferring a GlcNAc residue using recombinant EXTL2 on to GlcUAbeta1-3Galbeta1-O-C2H4NH-benzyloxycarbonyl. These findings indicate that the transfer of the first GlcNAc residue to the linkage region by EXTL2 is critically required for the biosynthesis of HS in cells deficient in EXT1.

Interaction of E2 glycoprotein with heparan sulfate is crucial for cellular infection of Sindbis virus

Cell culture-adapted strains of Sindbis virus (SINV) initially attach to cells by the ability to interact with heparan sulfate (HS) through selective mutation for positively charged amino acid (aa) scattered in E2 glycoprotein (W. B. Klimstra, K. D. Ryman, and R. E. Johnston, J. Virol. 72: 7357-7366, 1998). Here we have further confirmed that interaction of E2 protein with HS is crucial for cellular infection of SINV based on the reverse genetic system of XJ-160 virus, a Sindbis-like virus (SINLV). Both SINV YN87448 and SINLV XJ-160 displayed similar infectivity on BHK-21, Vero, or C6/36 cells, but XJ-160 failed to infect mouse embryonic fibroblast (MEF) cells. The molecular mechanisms underlying the selective infectivity of XJ-160 were approached by substituting the E1, E2, or both genes of XJ-160 with that of YN87448, and the chimeric virus was denominated as XJ-160/E1, XJ-160/E2, or XJ-160/E1E2, respectively. In contrast to the parental XJ-160, all chimeric viruses became infectious to wild-type MEF cells (MEF-wt). While MEF-Ext(-/-) cells, producing shortened HS chains, were resistant not only to XJ-160, but also to YN87448 as well as the chimeric viruses, indicating that the inability of XJ-160 to infect MEF-wt cells likely due to its incompetent discrimination of cellular HS. Treatment with heparin or HS-degrading enzyme resulted in a substantial decrease in plaque formation by YN87448, XJ-160/E2, and XJ-160/E1E2, but had marginal effect on XJ-160 and XJ-160/E1, suggesting that E2 glycoprotein from YN87448 plays a more important role than does E1 in mediating cellular HS-related cell infection. In addition, the peptide containing 145-150 aa from E2 gene of YN87448 specifically bound to heparin, while the corresponding peptide from the E2 gene of XJ-160 essentially showed no binding to heparin. As a new dataset, these results clearly confirm an essential role of E2 glycoprotein, especially the domain of 145-150 aa, in SINV cellular infection through the interaction with HS.

Multiple osteochondromas: mutation update and description of the multiple osteochondromas mutation database (MOdb)

Multiple osteochondromas (MO) is an autosomal dominant skeletal disease characterized by the formation of multiple cartilage-capped bone tumors growing outward from the metaphyses of long tubular bones. MO is genetically heterogeneous, and is associated with mutations in Exostosin-1 (EXT1) or Exostosin-2 (EXT2), both tumor-suppressor genes of the EXT gene family. All members of this multigene family encode glycosyltransferases involved in the adhesion and/or polymerization of heparin sulfate (HS) chains at HS proteoglycans (HSPGs). HSPGs have been shown to play a role in the diffusion of Ihh, thereby regulating chondrocyte proliferation and differentiation. EXT1 is located at 8q24.11-q24.13, and comprises 11 exons, whereas the 16 exon EXT2 is located at 11p12-p11. To date, an EXT1 or EXT2 mutation is detected in 70-95% of affected individuals. EXT1 mutations are detected in +/-65% of cases, versus +/-35% EXT2 mutations in MO patient cohorts. Inactivating mutations (nonsense, frame shift, and splice-site mutations) represent the majority of MO causing mutations (75-80%). In this article, the clinical aspects and molecular genetics of EXT1 and EXT2 are reviewed together with 895 variants in MO patients. An overview of the reported variants is provided by the online Multiple Osteochondromas Mutation Database (http://medgen.ua.ac.be/LOVD).

Mutation in the heparan sulfate biosynthesis enzyme EXT1 influences growth factor signaling and fibroblast interactions with the extracellular matrix

Heparan sulfate (HS) chains bind and modulate the signaling efficiency of many ligands, including members of the fibroblast growth factor (FGF) and platelet-derived growth factor families. We previously reported the structure of HS synthesized by embryonic fibroblasts from mice with a gene trap mutation of Ext1 that encodes a glycosyltransferase involved in HS chain elongation. The gene trap mutation results in low expression of Ext1, and, as a consequence, HS chain length is substantially reduced. In the present study, Ext1 mutant and wild-type mouse embryonic fibroblasts were analyzed for the functional consequences of the Ext1 mutation for growth factor signaling and interaction with the extracellular matrix. Here, we show that the phosphorylation of ERK1/2 in response to FGF2 stimulation was markedly decreased in the Ext1 mutant fibroblasts, whereas neither PDGF-BB nor FGF10 signaling was significantly affected. Furthermore, Ext1 mutants displayed reduced ability to attach to collagen I and to contract collagen lattices, even though no differences in the expression of collagen-binding integrins were observed. Reintroduction of Ext1in the Ext1 mutant fibroblasts rescued HS chain length, FGF2 signaling, and the ability of the fibroblasts to contract collagen. These data suggest that the length of the HS chains is a critical determinant of HS-protein interactions and emphasize the essential role of EXT1 in providing specific binding sites for growth factors and extracellular matrix proteins.

Identification of four novel EXT1 and EXT2 mutations in five Chinese pedigrees with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder most frequently caused by the EXT1 and EXT2 gene mutations resulting in reduction or absence of heparan sulfate (HS) in the exostotic cartilage cap. In this study, we investigated the molecular defects in five Chinese pedigrees with HME by direct sequencing analysis. Two novel EXT1 gene mutations and two novel EXT2 gene mutations were identified in two and three pedigrees, respectively. Of the four mutations identified, the c.651-664delinsTTT and c.680delG mutations in the exon 1 of EXT1 gene would cause frameshift (K218fs and R227fs) and introduce premature stop codon at amino acid site 220 and 251, respectively. The two missense mutations of c.398T > G in exon 2 and c.1016G > A in exon 6 of EXT2 gene result in the Leu133Arg and Cys339Tyr substitution, respectively. As HME is caused by defects in HS synthesis that is a complex process and not fully understood, these naturally occurring EXT mutations may provide important clues to future studies elucidating how EXT proteins contribute to HS biosynthesis.

Lack of heparan sulfate expression in B-cell lines: implications for Kaposi's sarcoma-associated herpesvirus and murine gammaherpesvirus 68 infections

Kaposi's sarcoma-associated herpesvirus (KSHV) and its murine homolog, murine gammaherpesvirus 68 (MHV68), are lymphotropic viruses that establish latent infection in their host. Surprisingly, while B cells are the main viral reservoir in vivo, B-cell lines are poorly permissive to infection by either MHV68 or KSHV. Here, we report that most B-cell lines express very little to no cell surface heparan sulfate (HS), a glycosaminoglycan that is essential for infection by these viruses. We found that Ext1, a key enzyme in the biosynthesis of HS, was expressed at a low level in these cells. Transfection of B-cell lines with Ext1 restored high HS expression at the cell surface. Overexpression of Ext1 in murine A20 and M12 B-cell lines increased MHV68 surface binding and enhanced the efficiency of infection. Finally, although it was not sufficient to allow efficient infection, the expression of HS on BJAB cells promoted KSHV binding at the cell surface. Thus, our results indicate that MHV68 and KSHV cycles are blocked in B-cell lines at the binding step due to a lack of surface HS.

Heparan sulfate biosynthesis enzymes EXT1 and EXT2 affect NDST1 expression and heparan sulfate sulfation

Heparan sulfate (HS) proteoglycans influence embryonic development and adult physiology through interactions with protein ligands. The interactions depend on HS structure, which is determined largely during biosynthesis by Golgi enzymes. How biosynthesis is regulated is more or less unknown. During polymerization of the HS chain, carried out by a complex of the exostosin proteins EXT1 and EXT2, the first modification enzyme, glucosaminyl N-deacetylase/N-sulfotransferase (NDST), introduces N-sulfate groups into the growing polymer. Unexpectedly, we found that the level of expression of EXT1 and EXT2 affected the amount of NDST1 present in the cell, which, in turn, greatly influenced HS structure. Whereas overexpression of EXT2 in HEK 293 cells enhanced NDST1 expression, increased NDST1 N-glycosylation, and resulted in elevated HS sulfation, overexpression of EXT1 had opposite effects. Accordingly, heart tissue from transgenic mice overexpressing EXT2 showed increased NDST activity. Immunoprecipitaion experiments suggested an interaction between EXT2 and NDST1. We speculate that NDST1 competes with EXT1 for binding to EXT2. Increased NDST activity in fibroblasts with a gene trap mutation in EXT1 supports this notion. These results support a model in which the enzymes of HS biosynthesis form a complex, or a GAGosome.

Regulation of Cryptococcus neoformans capsule size is mediated at the polymer level

The fungal pathogen Cryptococcus neoformans regulates its polysaccharide capsule depending on environmental stimuli. To investigate whether capsule polymers change under different growth conditions, we analyzed shed capsules at physiological concentrations without physical perturbation. Our results indicate that regulation of capsule size is mediated at the level of individual polysaccharide molecules.

Syndecans promote integrin-mediated adhesion of mesenchymal cells in two distinct pathways

Syndecans are transmembrane proteoglycans that support integrin-mediated adhesion. Well documented is the contribution of syndecan-4 that interacts through its heparan sulphate chains to promote focal adhesion formation in response to fibronectin domains. This process has requirements for integrin and signaling through the cytoplasmic domain of syndecan-4. Here an alternate pathway mediated by the extracellular domains of syndecans-2 and -4 is characterized that is independent of both heparan sulphate and syndecan signaling. This pathway is restricted to mesenchymal cells and was not seen in any epithelial cell line tested, apart from vascular endothelia. The syndecan ectodomains coated as substrates promoted integrin-dependent attachment, spreading and focal adhesion formation. Syndecan-4 null cells were competent, as were fibroblasts compromised in heparan sulphate synthesis that were unable to form focal adhesions in response to fibronectin. Consistent with actin cytoskeleton organization, the process required Rho-GTP and Rho kinase. While syndecan-2 and -4 ectodomains could both promote integrin-mediated adhesion, their pathways were distinct, as shown by competition assays. Evidence for an indirect interaction of beta1 integrin with both syndecan ectodomains was obtained, all of which suggests a distinct mechanism of integrin-mediated adhesion.

Contribution of EXT1, EXT2, and EXTL3 to heparan sulfate chain elongation

The exostosin (EXT) family of genes encodes glycosyltransferases involved in heparan sulfate biosynthesis. Five human members of this family have been cloned to date: EXT1, EXT2, EXTL1, EXTL2, and EXTL3. EXT1 and EXT2 are believed to form a Golgi-located hetero-oligomeric complex that catalyzes the chain elongation step in heparan sulfate biosynthesis, whereas the EXTL proteins exhibit overlapping glycosyl-transferase activities in vitro, so that it is not apparent what reactions they catalyze in vivo. We used gene-silencing strategies to investigate the roles of EXT1, EXT2, and EXTL3 in heparan sulfate chain elongation. Small interfering RNAs (siRNAs) directed against the human EXT1, EXT2, or EXTL3 mRNAs were introduced into human embryonic kidney 293 cells. Compared with cells transfected with control siRNA, those transfected with EXT1 or EXT2 siRNA synthesized shorter heparan sulfate chains, and those transfected with EXTL3 siRNA synthesized longer chains. We also generated human cell lines overexpressing the EXT proteins. Overexpression of EXT1 resulted in increased HS chain length, which was even more pronounced in cells coexpressing EXT2, whereas overexpression of EXT2 alone had no detectable effect on heparan sulfate chain elongation. Mutations in either EXT1 or EXT2 are associated with hereditary multiple exostoses, a human disorder characterized by the formation of cartilage-capped bony outgrowths at the epiphyseal growth plates. To further investigate the role of EXT2, we generated human cell lines overexpressing mutant EXT2. One of the mutations, EXT2-Y419X, resulted in a truncated protein. Interestingly, the capacity of wild type EXT2 to enhance HS chain length together with EXT1 was not shared by the EXT2-Y419X mutant.

Cellular Adhesion Responses to the Heparin-binding (HepII) Domain of Fibronectin Require Heparan Sulfate with Specific Properties

Cell surface heparan sulfate (HS) proteoglycans are required in development and postnatal repair. Important classes of ligands for HS include growth factors and extracellular matrix macromolecules. For example, the focal adhesion component syndecan-4 interacts with the III(12-14) region of fibronectin (HepII domain) through its HS chains. The fine structure of HS is critical to growth factor responses, and whether this extends to matrix ligands is unknown but is suggested from in vitro experiments. Cell attachment to HepII showed that heparin oligosaccharides of >or=14 sugar residues were required for optimal inhibition. The presence of N-sulfated glucosamine in the HS was essential, whereas 2-O-sulfation of uronic acid or 6-O-sulfation of glucosamine had marginal effects. In the more complex response of focal adhesion formation through syndecan-4, N-sulfates were again required and also glucosamine 6-O-sulfate. The significance of polymer N-sulfation and sulfated domains in HS was confirmed by studies with mutant Chinese hamster ovary cells where heparan sulfation was compromised. Finally, focal adhesion formation was absent in fibroblasts synthesizing short HS chains resulting from a gene trap mutation in one of the two major glucosaminoglycan polymerases (EXT1). Several separate, specific properties of cell surface HS are therefore required in cell adhesion responses to the fibronectin HepII domain.

Expression of rib-1, a Caenorhabditis elegans homolog of the human tumor suppressor EXT genes, is indispensable for heparan sulfate synthesis and embryonic morphogenesis

The proteins encoded by all of the five cloned human EXT family genes (EXT1, EXT2, EXTL1, EXTL2, and EXTL3), members of the hereditary multiple exostoses gene family of tumor suppressors, are glycosyltransferases required for the biosynthesis of heparan sulfate. In the Caenorhabditis elegans genome, only two genes, rib-1 and rib-2, homologous to the mammalian EXT genes have been identified. Although rib-2 encodes an N-acetylglucosaminyltransferase involved in initiating the biosynthesis and elongation of heparan sulfate, the involvement of the protein encoded by rib-1 in the biosynthesis of heparan sulfate remains unclear. Here we report that RIB-1 is indispensable for the biosynthesis and for embryonic morphogenesis. Despite little individual glycosyltransferase activity by RIB-1, the polymerization of heparan sulfate chains was demonstrated when RIB-1 was coexpressed with RIB-2 in vitro. In addition, RIB-1 and RIB-2 were demonstrated to interact by pulldown assays. To investigate the functions of RIB-1 in vivo, we depleted the expression of rib-1 by deletion mutagenesis. The null mutant worms showed reduced synthesis of heparan sulfate and embryonic lethality. Notably, the null mutant embryos showed abnormality at the gastrulation cleft formation stage or later and arrested mainly at the 1-fold stage. Nearly 100% of the embryos died before L1 stage, although the differentiation of some of the neurons and muscle cells proceeded normally. Similar phenotypes have been observed in rib-2 null mutant embryos. Thus, RIB-1 in addition to RIB-2 is indispensable for the biosynthesis of heparan sulfate in C. elegans, and the two cooperate to synthesize heparan sulfate in vivo. These findings also show that heparan sulfate is essential for post-gastrulation morphogenic movement of embryonic cells and is indispensable for ensuring the normal spatial organization of differentiated tissues and organs.

The genotype-phenotype correlation of hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant condition with a wide spectrum of clinical presentations. The purpose of this study was to determine the relationship between the genotype and the phenotype in HME. Thirty-two affected individuals from 10 families participated in the study. An extensive description of HME phenotype in terms of the anatomical burden of disease involved clinical and radiographic examinations and evaluation of 76 parameters. Mutations were determined by sequencing the EXT 1 and EXT 2 genes. Mutations were found in eight families (26 individuals), with one mutation previously reported in the literature and seven novel mutations. There were seven subjects with an EXT 1 mutation and 16 with an EXT 2 mutation. Patients with EXT 1 mutation were found to have more exostoses, more limb malalignment with shorter limb segments and height, and more pelvic and flatbone involvement. A genotype-phenotype correlation exists in HME, with patients with EXT 1 mutations having a higher degree of anatomical burden.

Control of capsular polysaccharide chain length by UDP-sugar substrate concentrations in Streptococcus pneumoniae

Regulation of chain length is essential to the proper functioning of prokaryotic and eukaryotic polysaccharides. Modulation of polymer size by substrate concentration is an attractive but unexplored control mechanism that has been suggested for many polysaccharides. The Streptococcus pneumoniae capsular polysaccharide is essential for virulence, and regulation of its size is critical for survival in different host environments. Synthesis of the type 3 capsule [-4)-beta-d-Glc-(1-3)-beta-d-GlcUA-(1-] from UDP-glucose (UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA) is catalysed by the type 3 synthase, a processive beta-glycosyltransferase, and requires a UDP-Glc dehydrogenase for conversion of UDP-Glc to UDP-GlcUA. Strains containing mutant UDP-Glc dehydrogenases exhibited reduced levels of UDP-GlcUA, along with reductions in total capsule amount and polymer chain length. In both the parent and mutant strains, UDP-Glc levels far exceeded UDP-GlcUA levels, which were very low to undetectable in the absence of blocking synthase activity. The in vivo observations were consistent with in vitro conditions that effect chain termination and ejection of the polysaccharide from the synthase when one substrate is limiting. These data are the first to demonstrate modulation of polysaccharide chain length by substrate concentration and to enable a model for the underlying mechanism. Further, they may have implications for the control of chain length in both prokaryotic and eukaryotic polymers synthesized by similar mechanisms.

Heparan Sulfate Polymerization in Drosophila

The formation of heparan sulfate (HS) chains is catalyzed by glycosyltransferases encoded by EXT (hereditary multiple exostosin gene) family members. Genetic screening for mutations affecting morphogen signaling pathways in Drosophila has identified three genes, tout-velu (ttv), sister of tout-velu (sotv), and brother of toutvelu (botv), which encode homologues of human EXT1, EXT2, and EXTL3, respectively. So far, in vitro glycosyltransferase activities have been demonstrated only for BOTV/DEXTL3, which harbors both N-acetylglucosaminyltransferase-I (GlcNAcT-I) and N-acetylglucosaminyltransferase-II (GlcNAcT-II) activities responsible for the chain initiation and elongation of HS, and no glucuronyltransferase-II (GlcAT-II) activity. Here we demonstrated that TTV/DEXT1 and SOTV/DEXT2 had GlcNAcT-II and GlcAT-II activities required for the biosynthesis of repeating disaccharide units of the HS backbone, and the coexpression of TTV with SOTV markedly augmented both glycosyltransferase activities when compared with the expression of TTV or SOTV alone. Moreover, the polymerization of HS was demonstrated on a linkage region analogue as an acceptor substrate by BOTV and an enzyme complex composed of TTV and SOTV (TTV-SOTV). In contrast to human, TTV-SOTV exhibited no GlcNAcT-I activity, indicating that BOTV/DEXT3, which is an EXT-Like gene and possesses GlcNAcT-I activity required for the initiation of HS, is indispensable for the biosynthesis of HS chains in Drosophila. Thus, all three EXT members in Drosophila, TTV, SOTV, and BOTV, are required for the biosynthesis of full-length HS in Drosophila.

A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells

We have grown polarized epithelial Madin-Darby canine kidney II (MDCK II) cells on filters in the presence of [(35)S]sulfate, [(3)H]glucosamine, or [(35)S]cysteine/[(35)S]methionine to study proteoglycan (PG) synthesis, sorting, and secretion to the apical and basolateral media. Whereas most of the [(35)S]sulfate label was recovered in basolateral PGs, the [(3)H]glucosamine label was predominantly incorporated into the glycosaminoglycan chains of apical PGs, indicating that basolateral PGs are more intensely sulfated than their apical counterparts. Expression of the PG serglycin with a green fluorescent protein tag (SG-GFP) in MDCK II cells produced a protein core secreted 85% apically, which was largely modified by chondroitin sulfate chains. Surprisingly, the 15% of secreted SG-GFP molecules recovered basolaterally were more heavily sulfated and displayed a different sulfation pattern than the apical counterpart. More detailed studies of the differential modification of apically and basolaterally secreted SG-GFP indicate that the protein cores have been designated to apical and basolateral transport platforms before pathway-specific, post-translational modifications have been completed.

When sugars guide axons: insights from heparan sulphate proteoglycan mutants

Although there have previously been hints that heparan sulphate proteoglycans (HSPGs) are important for axon guidance, as they are for many other biological processes, there has been little in vivo evidence for interaction with known axon-guidance pathways. Genetic analyses of fly, mouse, nematode and zebrafish mutants now confirm the role of HSPGs in axon guidance and are beginning to show that they might have a key role in modulating the action of axon-guidance ligands and receptors.

Multiple osteochondromas: clinicopathological and genetic spectrum and suggestions for clinical management

Multiple Osteochondromas is an autosomal dominant disorder characterised by the presence of multiple osteochondromas and a variety of orthopaedic deformities. Two genes causative of Multiple Osteochondromas, Exostosin-1 (EXT1) and Exostosin-2 (EXT2), have been identified, which act as tumour suppressor genes. Osteochondroma can progress towards its malignant counterpart, secondary peripheral chondrosarcoma and therefore adequate follow-up of Multiple Osteochondroma patients is important in order to detect malignant transformation early.This review summarizes the considerable recent basic scientific and clinical understanding resulting in a multi-step genetic model for peripheral cartilaginous tumorigenesis. This enabled us to suggest guidelines for clinical management of Multiple Osteochondroma patients. When a patient is suspected to have Multiple Osteochondroma, the radiologic documentation, histology and patient history have to be carefully reviewed, preferably by experts and if indicated for Multiple Osteochondromas, peripheral blood of the patient can be screened for germline mutations in either EXT1 or EXT2. After the Multiple Osteochondroma diagnosis is established and all tumours are identified, a regular follow-up including plain radiographs and base-line bone scan are recommended.