Sulfotransferases in glycosaminoglycan biosynthesis

Biosynthesis of Dermatan Sulfate

We identified the gene encoding chondroitin-glucuronate C5-epimerase (EC 5.1.3.19) that converts D-glucuronic acid to L-iduronic acid residues in dermatan sulfate biosynthesis. The enzyme was solubilized from bovine spleen, and an approximately 43,000-fold purified preparation containing a major 89-kDa candidate component was subjected to mass spectrometry analysis of tryptic peptides. SART2 (squamous cell carcinoma antigen recognized by T cell 2), a protein with unknown function highly expressed in cancer cells and tissues, was identified by 18 peptides covering 26% of the sequence. Transient expression of cDNA resulted in a 22-fold increase in epimerase activity in 293HEK cell lysate. Moreover, overexpressing cells produced dermatan sulfate chains with 20% of iduronic acid-containing disaccharide units, as compared with 5% for mock-transfected cells. The iduronic acid residues were preferentially clustered in blocks, as in naturally occurring dermatan sulfate. Given the discovered identity, we propose to rename SART2 (Nakao, M., Shichijo, S., Imaizumi, T., Inoue, Y., Matsunaga, K., Yamada, A., Kikuchi, M., Tsuda, N., Ohta, K., Takamori, S., Yamana, H., Fujita, H., and Itoh, K. (2000) J. Immunol. 164, 2565-2574) with a functional designation, chondroitin-glucuronate C5-epimerase (or DS epimerase). DS epimerase activity is ubiquitously present in normal tissues, although with marked quantitative differences. It is highly homologous to part of the NCAG1 protein, encoded by the C18orf4 gene, genetically linked to bipolar disorder. NCAG1 also contains a putative chondroitin sulfate sulfotransferase domain and thus may be involved in dermatan sulfate biosynthesis. The functional relation between dermatan sulfate and cancer is unknown but may involve known iduronic acid-dependent interactions with growth factors, selectins, cytokines, or coagulation inhibitors.

A protein canyon in the FGF-FGF receptor dimer selects from an à la carte menu of heparan sulfate motifs

Heparan sulfate (HS) is an essential and dynamic regulator of fibroblast growth factor (FGF) signaling. Two fundamentally different crystallographic models have been proposed to explain, at the molecular level, how HS/heparin enables FGF and FGF receptor (FGFR) to assemble into a functional dimer on the cell surface. In the symmetric 'two-end' model, the heparin-binding sites of FGF and FGFR merge to form a basic canyon that recruits two HS for binding. Within this canyon, the HS molecules primarily act to orchestrate and fortify multivalent and cooperative protein-protein contacts within the dimer that are the foundations of dimerization. In contrast, in the asymmetric model, which mechanistically resembles the previously proposed trans FGF dimer model, a single heparin molecule facilitates dimerization by cross-linking two FGFs into a trans dimer that brings together the two FGFRs. Interestingly, the crystal structure upon which the asymmetric model is based contains a symmetric dimer reminiscent of the symmetric two-end model, suggesting that a different interpretation of the crystal structure has led to the postulation of the asymmetric model. Importantly, the symmetric two-end model provides an intriguing solution to the problem of how HS selectivity is achieved in FGF signaling. The model reveals that, within the canyon, FGF and FGFR no longer adhere to their individual HS binding specificities, but instead act in unison to search for a unique HS motif from a plethora of HS epitopes that are expressed in a tissue-specific and developmentally regulated fashion. Primary sequence differences within the heparin-binding sites of FGFs and FGFRs, together with ligand-induced changes in FGFR conformation, lead to the formation of distinct canyons with unique HS specificity for individual FGF-FGFR complexes.

Enzymatic synthesis of chondroitin 4-sulfate with well-defined structure

Synthesis of chondroitin sulfate (ChS) with well-defined structure was achieved for the first time by hyaluronidase-catalyzed polymerization. N-Acetylchondrosine (GlcAbeta(1-->3)GalNAc) oxazoline derivatives sulfated at C4 (1a), C6 (1b), and both C4 and C6 (1c) in the GalNAc unit were synthesized as transition state analogue substrate monomers for hyaluronidase (HAase) catalysis. Compound 1a was effectively polymerized by the enzyme, giving rise to synthetic ChS sulfated perfectly at the C4 position in all N-acetylgalactosamine units (Ch4S, 2a) in good yields. Molecular weights (Mn) of 2a ranged from 4000 to 18,400, which were controlled by varying reaction conditions. Compounds 1b and 1c were not catalyzed by the enzyme, affording the corresponding disaccharides through the oxazoline ring-opening without formation of polysaccharides.

Evidence of genetic underexpression in chorionic villi samples of euploid fetuses with increased nuchal translucency at 10–11 weeks' gestation

OBJECTIVE

To retrospectively investigate whether the genetic profile from chorionic villous sampling (CVS) found in euploid fetuses with increased NT differs from matched controls.

STUDY DESIGN

We employed cDNA microarray technology to characterize and compare the gene expression profile of chorionic villous tissues (which encompass the trophoblast and inner mesenchymal core) belonging to four singleton male fetuses with increased NT at 10-11 weeks' gestation. A pool of four normal chorionic villous tissues belonging to four respective fetuses, matched for gestational age and gender, was used as controls.

RESULTS

In euploid fetuses, we found several underexpressed genes, possibly involved in mechanisms associated with the abnormal NT thickness. All these genes are likely to belong to the mesenchymal core of the villus that originates from the extraembryonic mesoderm, and thus might be closely representative of the embryonic genetic profile. They include: (1) genes of embryonic development and differentiation such as Endothelin 3 (EDN3) and secreted frizzled-related protein 4 (SFRP4); (2) genes of the extracellular matrix (ECM) metabolism such as tissue inhibitor of metalloproteinase1 (TIMP1), and disintegrin-like and matrix metalloproteinase (MMP) (reprolysin type) with thrombospondin type 1 Motif or ADAMTS2, exostoses (multiple)-like 1 (EXTL1), heparan sulfate (HS) 6-O-sulfotransferase 1 or HS6ST1, fibronectin 1 (FN1) and Integrin Alpha 10 (ITGA10) involved in HS and proteoglycan bio-synthesis, ECM synthesis and cell-matrix adhesion; (3) genes involved in vessel formation and differentiation such as angiogenic factor (VG5Q), and in blood pressure control and muscle contraction, like Endothelin 3 or EDN3 and sarcolemma associated protein (SLMAP). Such lower expressions of the villous tissues might be related to an immature genetic profile of the embryo development as well as abnormal regulation of ECM bio-synthesis and/or improper vessel growth and blood pressure control. Also, the results partially support the theories proposed for NT enlargement such as altered composition of ECM and abnormal/delayed development of the circulatory system.

CONCLUSIONS

Abnormal extraembryonic genetic expression is found at 10-11 weeks' gestation in euploid fetuses with increased NT. If both extra- and intraembryonic mesoderms express the same genetic alterations, then microarray analyses on CVS could be used to screen several mesoderm-derivate anomalies.

Glycosaminoglycans and their proteoglycans: host‐associated molecular patterns for initiation and modulation of inflammation

Glycosaminoglycans, linear carbohydrates such as heparan sulfate and hyaluronan, participate in a variety of biological processes including cell-matrix interactions and activation of chemokines, enzymes and growth factors. This review will discuss progress in immunology and the science of wound repair that has revealed the importance of glycosaminoglycans, and their proteoglycans, in the inflammatory process. Heparan sulfate enables growth factor function and modifies enzyme/inhibitor functions, such as antithrombin III and heparin cofactor II. Heparan sulfate also interacts with cytokines/chemokines and participates in leukocyte selectin binding to promote the recruitment of leukocytes. Chondroitin sulfate/dermatan sulfate regulates growth factor activity and is an alternate modulator of heparin cofactor II. In addition, dermatan sulfate induces ICAM-1 expression on endothelial cells and also recruits leukocytes via selectin interactions. Hyaluronan alternatively participates in leukocyte recruitment via interaction with CD44, while activating various inflammatory cells, such as macrophages, through CD44-dependent signaling. Hyaluronan also signals through Toll-like receptor 4 to induce dendritic cell maturation and promote cytokine release by dendritic cells and endothelial cells. Taken together, the field of glycosaminoglycan biology provides new clues and explanations of the process of inflammation and suggests new therapeutic approaches to human disease.

3-O-sulfated oligosaccharide structures are recognized by anti-heparan sulfate antibody HS4C3

Antibodies against heparan sulfate (HS) are useful tools to study the structural diversity of HS. They demonstrate the large sequential variation within HS and show the distribution of HS oligosaccharide sequences within their natural environment. We analyzed the distribution and the structural characteristics of the oligosaccharide epitope recognized by anti-HS antibody HS4C3. Biosynthetic and synthetic heparin-related oligosaccharide libraries were used in affinity chromatography, immunoprecipitation, and enzyme-linked immunosorbent assay to identify this epitope as a 3-O-sulfated motif with antithrombin binding capacity. The antibody binds weakly to any N-sulfated, 2-O- and 6-O-sulfated hexa- to octasaccharide fragment but strongly to the corresponding oligosaccharide when there is a 3-O-sulfated glucosamine residue present in the sequence. This difference was highlighted by affinity interaction and immunohistochemistry at salt concentrations from 500 mm. At physiological salt conditions the antibody strongly recognized basal lamina of epithelia and endothelia. At 500 mm salt conditions, when 3-O sulfation is required for binding, antibody recognition was more restricted and selective. Antibody HS4C3 bound similar tissue structures as antithrombin in rat kidney. Furthermore, antithrombin and antibody HS4C3 could compete with one another for binding to heparin. Antibody HS4C3 was also able to inhibit the anti-coagulant activities of heparin and Arixtra as demonstrated using the activated partial thromboplastin time clotting and the anti-factor Xa assays. In summary, antibody HS4C3 selectively detects 3-O-sulfated HS structures and interferes with the coagulation activities of heparin by association with the anti-thrombin binding pentasaccharide sequence.

Characterization of the N-deacetylase domain from the heparan sulfate N-deacetylase/N-sulfotransferase 2

Heparin and heparan sulfate are linear sulfated polysaccharides that exert a multitude of biological functions. Heparan sulfate glucosaminyl N-deacetylase/N-sulfotransferase isoform 2 (NDST-2), a key enzyme in the biosynthesis of heparin, contains two distinct activities. This bifunctional enzyme removes the acetyl group from N-acetylated glucosamine (N-deacetylase activity) and transfers a sulfuryl group to the unsubstituted amino position (N-sulfotransferase activity). The N-sulfotransferase activity of NDST has been unambiguously localized to the C-terminal domain of NDST. Here, we report that the N-terminal domain of NDST-2 retains N-deacetylase activity. The N-terminal domain (A66-P604) of human NDST-2, designated as N-deacetylase (NDase), was cloned as a (His)(6)-fusion protein, and protein expression was carried out in Escherichia coli. Heparosan treated with NDase contains N-unsubstituted glucosamine and is highly susceptible to N-sulfation by N-sulfotransferase. Our results conclude that the N-terminal domain of NDST-2 contains functional N-deacetylase activity. This finding helps further elucidate the mechanism of action of heparan sulfate N-deacetylase/N-sulfotransferases and the biosynthesis of heparan sulfate in general.

The specificity of interactions between proteins and sulfated polysaccharides

Sulfated polysaccharides are capable of binding with proteins at several levels of specificity. As highly acidic macromolecules, they can bind non-specifically to any basic patch on a protein surface at low ionic strength, and such interactions are not likely to be physiologically significant. On the other hand, several systems have been identified in which very specific substructures of sulfated polysaccharides confer high affinity for particular proteins; the best-known example of this is the pentasaccharide in heparin with high affinity for antithrombin, but other examples may be taken from the study of marine invertebrates: the importance of the fine structure of dermatan sulfate (DS) to its interaction with heparin cofactor II (HCII), and the involvement of sea urchin egg-jelly fucans in species specific fertilization. A third, intermediate, kind of specific interaction is described for the cell-surface glycosaminoglycan heparan sulfate (HS), in which patterns of sulfate substitution can show differential affinities for cytokines, growth factors, and morphogens at cell surfaces and in the intracellular matrix. This complex interplay of proteins and glycans is capable of influencing the diffusion of such proteins through tissue, as well as modulating cellular responses to them.

Chemical approaches to deciphering the glycosaminoglycan code

Glycosaminoglycans are sulfated biopolymers with rich chemical diversity and complex functions in vivo, contributing to processes ranging from cell growth and neuronal development to viral invasion and neurodegenerative disease. Recent studies suggest that glycosaminoglycans may encode information in the form of a 'sulfation code,' whereby discrete modifications to the polysaccharide backbone may direct the location or activities of proteins.

Development of structural analysis of sulfated N-glycans by multidimensional high performance liquid chromatography mapping methods

Although the biological importance of sulfated oligosaccharides has been widely recognized, there are only a few reports that describe detailed structures of sulfated N-glycans. This is largely due to the lack of a convenient method to identify structures of sulfated glycans found in low incidence. Here we develop multidimensional high performance liquid chromatography (HPLC) mapping methods for rapid and convenient identification of sulfated N-glycans. By using adequate quantities of sulfated N-glycans derived from LS12 cells, which are transfected with sulfotransferase cDNA, 40 different sulfated glycans have been successfully mapped. Furthermore, we have applied the HPLC data to identification of isomeric products resulting from an enzymatic reaction of N-acetylglucosamine 6-O-sulfotransferase-1 in vitro and revealed that this enzyme preferentially catalyzes sulfation of the GlcNAcbeta1-->2Manalpha1-->3Man branch in a biantennary acceptor.

Maintenance of chondroitin sulfation balance by chondroitin-4-sulfotransferase 1 is required for chondrocyte development and growth factor signaling during cartilage morphogenesis

Glycosaminoglycans (GAGs) such as heparan sulfate and chondroitin sulfate are polysaccharide chains that are attached to core proteins to form proteoglycans. The biosynthesis of GAGs is a multistep process that includes the attachment of sulfate groups to specific positions of the polysaccharide chains by sulfotransferases. Heparan-sulfate and heparan sulfate-sulfotransferases play important roles in growth factor signaling and animal development. However, the biological importance of chondroitin sulfation during mammalian development and growth factor signaling is poorly understood. We show that a gene trap mutation in the BMP-induced chondroitin-4-sulfotransferase 1 (C4st1) gene (also called carbohydrate sulfotransferase 11 - Chst11), which encodes an enzyme specific for the transfer of sulfate groups to the 4-O-position in chondroitin, causes severe chondrodysplasia characterized by a disorganized cartilage growth plate as well as specific alterations in the orientation of chondrocyte columns. This phenotype is associated with a chondroitin sulfation imbalance, mislocalization of chondroitin sulfate in the growth plate and an imbalance of apoptotic signals. Analysis of several growth factor signaling pathways that are important in cartilage growth plate development showed that the C4st1(gt/gt) mutation led to strong upregulation of TGFbeta signaling with concomitant downregulation of BMP signaling, while Indian hedgehog (Ihh) signaling was unaffected. These results show that chondroitin 4-O-sulfation by C4st1 is required for proper chondroitin sulfate localization, modulation of distinct signaling pathways and cartilage growth plate morphogenesis. Our study demonstrates an important biological role of differential chondroitin sulfation in mammalian development.

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.

Sulfated glycosaminoglycans in two hematophagous arthropod vectors of Chagas disease, Triatoma brasiliensis and Rhodnius prolixus (Hemiptera: Reduviidae)

The characterization of sulfated glycosaminoglycans (GAGs) in hematophagous arthropod vectors in general has been limited, with the exception of the studies in the triatomine Rhodnius prolixus. Heparan sulfate (HS) and chondroitin sulfate (CS) were previously identified and structurally characterized in extracts of whole bodies of fourth instar larvae of R. prolixus. Recently, we showed the expression of these two sulfated GAGs in specific body tissues of adult males and females and in embryos of R. prolixus. In the present work, we identified and compared the sulfated GAG composition in specific tissues of adult insects and in embryos of another triatomine species, Triatoma brasiliensis. Sulfated GAGs were isolated from the fat body, intestinal tract, and the reproductive tracts of adult insects and from embryos. Only HS and CS were found in the tissues analyzed. The present results extend the initial observations on the sulfated GAG composition in R. prolixus by showing that these molecules are widely distributed among internal organs of triatomines. These observations may be useful for future investigations aiming to evaluate the possible implication of these compounds in physiological events that take place in a specific organ(s) in these insects.

Glycan array screening reveals a candidate ligand for Siglec-8

Sialic acid-binding immunoglobulin-like lectin 8 (Siglec-8) is selectively expressed on human eosinophils, basophils, and mast cells, where it regulates their function and survival. Previous studies demonstrated sialic acid-dependent binding of Siglec-8 but failed to reveal significant substructure specificity or high affinity of that binding. To test a broader range of potential ligands, a Siglec-8-Ig chimeric protein was tested for binding to 172 different glycan structures immobilized as biotinylated glycosides on a 384-well streptavidin-coated plate. Of these, approximately 40 structures were sialylated. Among these, avid binding was detected to a single defined glycan, NeuAcalpha2-3(6-O-sulfo)Galbeta1-4[Fucalpha1-3]GlcNAc, also referred to in the literature as 6'-sulfo-sLex. Notably, neither unsulfated sLex (NeuAcalpha2-3Galbeta1-4[Fucalpha1-3]GlcNAc) nor an isomer with the sulfate on the 6-position of the GlcNAc residue (6-sulfo-sLex, NeuAcalpha2-3Galbeta1-4[Fucalpha1-3](6-O-sulfo)GlcNAc) supported detectable binding. Subsequent secondary screening was performed using surface plasmon resonance. Biotin glycosides immobilized on streptavidin biosensor chips were exposed to Siglec-8-Ig in solution. Whereas surfaces derivatized with sLex and 6-sulfo-sLex failed to support detectable Siglec-8 binding, 6'-sulfo-sLex supported significant binding with a Kd of 2.3 microm. In a separate test of binding specificity, aminopropyl glycosides were covalently immobilized at different concentrations on activated (N-hydroxysuccinimidyl) glass surfaces (Schott-Nexterion Slide H). Subsequent exposure to Siglec-8-Ig precomplexed with fluorescein isothiocyanate anti-human Fc resulted in fluorescent signals at immobilized concentrations of 6'-sulfo-sLex of <5 pmol/spot. In contrast, sLex and 6-sulfo-sLex did not support any Siglec-8 binding at the highest concentration tested (300 pmol/spot). We conclude that Siglec-8 binds preferentially to the sLex structure bearing an additional sulfate ester on the galactose 6-hydroxyl.

Glycomics: a pathway to a class of new and improved therapeutics

Complex glycans that are located at the surface of cells, deposited in the extracellular matrix and attached to soluble signalling molecules have a crucial role in the phenotypic expression of cellular genotypes. However, owing to their structural complexity and some redundancy in terms of structures that elicit a function, the therapeutic potential of complex glycans has not been well exploited, with a few notable exceptions. This review outlines recent advances that promise to increase our ability to use complex glycans as therapeutics. Opportunities for the development of further structure-function relationships for these complex molecules are also discussed.

A novel strategy for defining critical amino acid residues involved in protein/glycosaminoglycan interactions

The binding of proteins to glycosaminoglycans (GAGs) is the prerequisite for a large number of cellular processes and regulatory events and is associated to many pathologies. However, progress in the understanding of these mechanisms has been hampered by the lack of simple and comprehensive analytical tools for the identification of the structural attributes involved in protein/saccharide interaction. Characterization of GAG binding motifs on proteins has so far relied on site-directed mutagenesis studies, protein sequence mapping using synthetic peptides, molecular modeling, or structural analysis. Here, we report the development of a novel approach for identifying protein residues involved in the binding to heparin, the archetypal member of the GAG family. This method, which uses native proteins, is based on the formation of cross-linked complexes of the protein of interest with heparin beads, the proteolytic digestion of these complexes, and the subsequent identification of the heparin binding containing peptides by N terminus sequencing. Analysis of the CC chemokine regulated on activation, normal T-cell expressed, and secreted (RANTES), the envelope glycoprotein gC from pseudorabies virus and the laminin-5 alpha 3LG4/5 domain validated the techniques and provided novel information on the heparin binding motifs present within these proteins. Our results highlighted this method as a fast and valuable alternative to existing approaches. Application of this technique should greatly contribute to facilitate the structural study of protein/GAG interactions and the understanding of their biological functions.

Production of N-sulfated polysaccharides using yeast-expressed N-deacetylase/N-sulfotransferase-1 (NDST-1)

Heparan sulfate/heparin N-deacetylase/N-sulfotransferase-1 (NDST-1) is a critical enzyme involved in heparan sulfate/heparin biosynthesis. This dual-function enzyme modifies the GlcNAc-GlcA disaccharide repeating sugar backbone to make N-sulfated heparosan. N-sulfation is an absolute requirement for the subsequent epimerization and O-sulfation steps in heparan sulfate/heparin biosynthesis. We have expressed rat liver (r) NDST-1 in Saccharomyces cerevisiae as a soluble protein. The yeast-expressed enzyme has both N-deacetylase and N-sulfotransferase activities. N-acetyl heparosan, isolated from Escherichia coli K5 polysaccharide, de-N-sulfated heparin (DNSH) and completely desulfated N-acetylated heparan sulfate (CDSNAcHS) are all good substrates for the rNDST-1. However, N-desulfated, N-acetylated heparin (NDSNAcH) is a poor substrate. The rNDST-1 was partially purified on heparin Sepharose CL-6B. Purified rNDST-1 requires Mn(2+) for its enzymatic activity, can utilize PAPS regenerated in vitro by the PAPS cycle (PAP plus para-nitrophenylsulfate in the presence of arylsulfotransferase IV), and with the addition of exogenous PAPS is capable of producing 60-65% N-sulfated heparosan from E. coli K5 polysaccharide or Pasteurella multocida polysaccharide.

Loss of chondroitin 6-O-sulfotransferase-1 function results in severe human chondrodysplasia with progressive spinal involvement

We studied two large consanguineous families from Oman with a distinct form of spondyloepiphyseal dysplasia (SED Omani type). By using a genome-wide linkage approach, we were able to map the underlying gene to a 4.5-centimorgan interval on chromosome 10q23. We sequenced candidate genes from the region and identified a missense mutation in the chondroitin 6-O-sulfotransferase (C6ST-1) gene (CHST3) changing an arginine into a glutamine (R304Q) in the well conserved 3'-phosphoadenosine 5'-phosphosulfate binding site. C6ST-1 catalyzes the modifying step of chondroitin sulfate (CS) synthesis by transferring sulfate to the C-6 position of the N-acetylgalactosamine of chondroitin. From the crystal structures of other sulfotransferases, it could be inferred that Arg-304 is essential for the structure of the cosubstrate binding site. We used recombinant C6ST-1 to show that the identified missense mutation completely abolishes C6ST-1 activity. Disaccharide composition analysis of CS chains by anion-exchange HPLC shows that both Delta HexA-GalNAc(6S) and Delta HexA(2S)-GalNAc(6S) were significantly reduced in the patient's cells and that Delta HexA-GalNAc(4S,6S), undetectable in controls, was elevated. Analysis of the patient's urine shows marked undersulfation of CS, in particular reduction in 6-O-sulfated disaccharide and an increase in the nonsulfated unit. Our results indicate that the mutation in CHST3 described here causes a specific but generalized defect of CS chain sulfation resulting in chondrodysplasia with major involvement of the spine.

Chondroitin sulfate chains on syndecan-1 and syndecan-4 from normal murine mammary gland epithelial cells are structurally and functionally distinct and cooperate with heparan sulfate chains to bind growth factors. A novel function to control binding of midkine, pleiotrophin, and basic fibroblast growth factor

A comparative analysis was carried out of heparan sulfate (HS) and chondroitin sulfate (CS) chains of the ectodomains of hybrid type transmembrane proteoglycans, syndecan-1 and -4, synthesized simultaneously by normal murine mammary gland epithelial cells. Although the HS chains were structurally indistinguishable, intriguingly the CS chains were structurally and functionally distinct, probably reflecting the differential regulation of sulfotransferases involved in the synthesis of HS and CS. The CS chains of the two syndecans comprised nonsulfated, 4-O-, 6-O-, and 4,6-O-disulfated N-acetylgalactosamine-containing disaccharide units and were significantly different, with a higher degree of sulfation for syndecan-4. Functional analysis using a BIAcore system showed that basic fibroblast growth factor (bFGF) specifically bound only to the HS chains of both syndecans, whereas midkine (MK) and pleiotrophin (PTN) bound not only to the HS but also to the CS chains. Stronger binding of MK and PTN to the CS chains of syndecan-4 than those of syndecan-1 was revealed, supporting the structural and functional differences. Intriguingly, removal of the CS chains decreased the association and dissociation rate constants of MK, PTN, and bFGF for both syndecans, suggesting the simultaneous binding of these growth factors to both types of chains, producing a ternary complex that transfers the growth factors to the corresponding cell surface receptors more efficiently compared with the HS chains alone. The involvement of the core protein was also shown in the binding of MK and PTN to syndecan-1, suggesting the possibility of cooperation with the HS and/or CS chains in the binding of these growth factors and their delivery to the cell surface receptors.

Irreversible glucuronyl C5-epimerization in the biosynthesis of heparan sulfate

Glucuronyl C5-epimerase catalyzes the conversion of d-glucuronic acid to l-iduronic acid units in heparan sulfate biosynthesis. Substrate recognition depends on the N-substituent pattern of the heparan sulfate precursor polysaccharide and requires the adjacent glucosamine residue toward the non-reducing end to be N-sulfated. Epimerization of an appropriately N-sulfated substrate is freely reversible in a soluble system, with equilibrium favoring retention of d-gluco configuration (Hagner-McWhirter, A., Lindahl, U., and Li, J.-P. (2000) Biochem. J. 347, 69-75). We studied the reversibility of the epimerase reaction in a cellular system, by incubating human embryonic kidney 293 cells with d-[5-(3)H]galactose. The label was incorporated with glucuronic acid units into the heparan sulfate precursor polysaccharide and was lost upon subsequent C5-epimerization to iduronic acid. However, analysis of oligosaccharides obtained by deaminative cleavage of the mature heparan sulfate chains indicated that all glucuronic acid units retained their C5-(3)H label, irrespective of whether they had occurred in sequences susceptible or resistant to the epimerase. All (3)H-labels of the final products resisted incubation with epimerase in a soluble system, apparently due to blocking O-sulfate groups. These results indicate that glucuronic acid C5-epimerization is effectively irreversible in vivo and argue for a stringent organization of the biosynthetic machinery.