Undersulfated heparan sulfate in a Chinese hamster ovary cell mutant defective in heparan sulfate N-sulfotransferase

Unusual beta-D-xylosides that prime glycosaminoglycans in animal cells

The biosynthesis of glycosaminoglycans (GAG) takes place while the polysaccharide chains are usually attached to a proteoglycan core protein. Cells also will assemble GAG chains on beta-D-xylosides containing hydrophobic aglycones. In order to evaluate the relationship of the structure of the sugar to priming activity of the glycoside, we synthesized beta-D-xyloside analogs in which the hydroxyls were substituted with hydrogen, fluorine, -O-methyl, amino, -O-isopropyl, and -O-benzyl groups. Epimers at the 2-, 3-, and 4-position of xylose also were made. Their ability to prime GAGs was tested in Chinese hamster ovary cells by measuring 35SO4 incorporation into polysaccharide chains and by assaying the transfer of galactose to the xylosides by galactosyltransferase I (UDP-D-galactose:xylose beta1-4-galactosyltransferase) in vitro. All of the analogs failed to act as primers of GAGs in vivo and as substrates in vitro with the following exceptions. Substitution of 2-OH and 3-OH with -OCH3 were active at high concentration (1 mM), but the deoxygenated derivatives were inactive. Efficient priming also occurred on a derivative with fluorine instead of the 3-OH group, suggesting that the oxygen atoms at C-2 and C-3 were involved as hydrogen bond acceptors. Methylated and deoxy analogs at C-4 were inactive, due to the loss of the acceptor hydroxyl group. Interestingly, benzyl-beta-D-threo-pentopyranos-4-uloside (4-keto derivative) and benzyl-4-methyl-beta-D-xyloside, with a methyl group in place of an axial hydrogen at C-4, primed GAG chains. Priming by these unusual xylosides suggests the possibility of designing inhibitors of GAG synthesis based on xyloside analogs with reactive groups in key positions.

An animal cell mutant defective in heparan sulfate hexuronic acid 2-O-sulfation

The interaction of heparan sulfate with protein ligands depends on unique oligosaccharide sequences containing iduronic acid (IdUA), N-sulfated glucosamine residues, and O-sulfated sugars. To study the role of O-sulfation in greater detail, we isolated a Chinese hamster ovary cell mutant defective in 2-O-sulfation of iduronic acid. The mutant, pgsF-17, was identified by a colony blotting assay in which colonies of mutagen-treated cells were replica plated to two disks of polyester cloth. One disk was blotted with 125I-labeled basic fibroblast growth factor (bFGF) to measure binding to cell surface proteoglycans. The other disk was incubated with 35SO4 to measure proteoglycan biosynthesis. Autoradiography revealed a colony that did not bind 125I-bFGF, but incorporated 35SO4 normally (mutant pgsF-17). Complete deaminative cleavage of heparan sulfate revealed that material from pgsF-17 lacked IdUA(2OSO3)-GlcNSO3 and IdUA(2OSO3)-GlcNSO3(6OSO3), but contained a higher proportion of glucuronic acid GlcUA-GlcNSO3(6OSO3) and IdUA-GlcNSO3(6OSO3). Assay of the 2-O-sulfotransferase that acts on IdUA residues showed that mutant 17 lacked enzyme activity. Interestingly, the alteration resulted in accumulation of GlcNSO3 groups, suggesting that under normal conditions 2-O-sulfation decreases GlcNAc N-deacetylation/N-sulfation, and that the reactions occur simultaneously. The formation of IdUA and 6-O-sulfated glucosaminyl residues appears to be independent of 2-O-sulfation. pgsF-17 also lacks 2-O-sulfated GlcUA residues, suggesting that the same enzyme is responsible for 2-O-sulfation of IdUA and GlcUA residues. Mutant 17 provides a useful tool for studying the regulation of heparan sulfate biosynthesis and the relationship of heparan sulfate fine structure to its biological function.

Cell surface glycosaminoglycans are not obligatory for Plasmodium berghei sporozoite invasion in vitro

The malaria circumsporozoite (CS) protein binds to glycosaminoglycan chains from heparan sulfate proteoglycans present on the basolateral surface of hepatocytes and hepatoma cells in vitro. When injected into mice, CS protein is rapidly cleared from the blood circulation by hepatocytes. The binding region for the HSPGs is the evolutionarily conserved region II-plus of the CS protein. Here we have asked whether the presence of glycosaminoglycans on the plasma membrane of target cells is required for sporozoite invasion in vitro. Two types of target cells were used: HepG2 cells, which are permissive for Plasmodium berghei sporozoite development into mature exoerythrocytic forms, and CHO cells, in which the intracellular development of the parasites is arrested early after penetration. The invasion of mutant CHO cells expressing undersulfated glycosaminoglycans or no glycosaminoglycans was only inhibited 41-49% or 24-32%, respectively, in comparison to invasion of CHO-K1 cells. Previous cleavage of HepG2 surface membrane glycosaminoglycans with heparinase or heparitinase had no significant inhibitory effect on subsequent P. berghei sporozoite invasion and EEF development in these cells, although the glycosaminoglycan lyase treatments removed over 80% of CS binding sites from the cell surface. These results suggest that although the presence of glycosaminoglycans on the target cell surface enhances sporozoite invasion, glycosaminoglycans are not required for sporozoite penetration or the development of exoerythrocytic forms in vitro.

Acetylcholine receptor clustering in C2 muscle cells requires chondroitin sulfate

Proteoglycans have been implicated in the clustering of acetylcholine receptors (AChRs) on cultured myotubes and at the neuromuscular junction. We report that the presence of chondroitin sulfate is associated with the ability of cultured myotubes to form spontaneous clusters of AChRs. Three experimental manipulations of wild type C2 cells in culture were found to affect both glycosaminoglycans (GAGs) and AChR clustering in concert. Chlorate was found to have dose-dependent negative effects both on GAG sulfation and on the frequency of AChR clusters. When extracellular calcium was raised from 1.8 to 6.8 mM in cultures of wild-type C2 myotubes, increases were observed both in the level of cell layer-associated chondroitin sulfate and in the frequency of AChR clusters. Culture of wild-type C2 myotubes in the presence of chondroitinase ABC eliminated cell layer-associated chondroitin sulfate while leaving heparan sulfate intact and simultaneously prevented the formation of AChR clusters. Treatment with either chlorate or chondroitinase inhibited AChR clustering only if begun prior to the spontaneous formation of clusters. We propose that chondroitin sulfate plays an essential role in the initiation of AChR clustering and in the early events of synapse formation on muscle.

Repetitive Ser-Gly sequences enhance heparan sulfate assembly in proteoglycans

We showed previously that the synthesis of heparan sulfate on betaglycan occurs at a Ser-Gly dipeptide flanked by a cluster of acidic residues and an adjacent tryptophan (Zhang, L., and Esko, J.D. (1994) J. Biol. Chem. 269, 19295-19299). A survey of the protein data base revealed that most heparan sulfate proteoglycans contain repetitive (Ser-Gly)n segments (n = 2) and a nearby cluster of acidic residues. To study the role of these amino acid sequences in controlling heparan sulfate synthesis, we have examined the assembly of glycosaminoglycans on Chinese hamster ovary (CHO) cell syndecan-1. The glycosylation sites were mapped by making chimeric proteoglycans containing segments of CHO syndecan-1 cDNA fused to Protein A. Two sites near the transmembrane domain (-EGS205GEQ- and -ETS215GEN-) were used solely for chondroitin sulfate synthesis, whereas three sites near the N terminus (-DGS35GDDSDNFS45GS47GTG-) supported both heparan sulfate and chondroitin sulfate synthesis. The strongest sites for heparan sulfate synthesis consisted of the repeat unit, -S45GS47G-. An unusual coupling phenomenon occurred across the adjacent SG dipeptides, leading to a greater proportion of heparan sulfate than predicted by the behavior of each site acting independently. The clusters of acidic residues adjacent to the heparan sulfate sites play important roles as well. These sequence motifs suggest a set of rules for predicting whether heparan sulfate assembles at glycosylation sites in proteoglycan core proteins.

Endothelial heparan sulfate proteoglycans that bind to L-selectin have glucosamine residues with unsubstituted amino groups

We earlier reported calcium-dependent, heparin-like L-selectin ligands in cultured bovine endothelial cells (Norgard-Sumnicht, K. E., Varki, N. M., and Varki, A. (1993) Science 261,480-483). Here we show that these are heparan sulfate proteoglycans (HSPGs) associated either with the cultured cells or secreted into the medium and extracellular matrix. Activation of the endothelial cells with bacterial lipopolysaccharide (LPS) does not markedly alter the amount or distribution of this material. A major portion of the glycosaminoglycan (GAG) chains released from these HSPGs by alkaline beta-elimination rebinds to L-selectin in the presence of calcium, indicating that these saccharides alone can mediate the high affinity recognition. Heparin lyase digestions indicate that these GAG chains are enriched in heparan sulfate, not heparin sequences. Current understanding of the biosynthesis of heparan sulfate chains indicates that all glucosamine amino groups must be either N-acetylated or N-sulfated. However, nitrous acid deamination at pH 4.0 suggests the presence of some unsubstituted amino groups in these L-selectin-binding GAG chains from endothelial cell HSPGs. This is confirmed by chemical N-reacetylation and by reactivity with sulfo-N-hydroxysuccinimide-biotin. These unsubstituted amino groups are also found on HSPGs from human umbilical vein endothelial cells, but are not detected in those from Chinese hamster ovary cells. In both bovine and human endothelial cells, these novel groups are enriched for in the HS-GAG chains which bind to L-selectin. Despite this, studies with N-reacetylation and nitrous acid deamination do not show conclusive evidence for the direct involvement of the unsubstituted amino groups in L-selectin binding. This may be because the chemical reactions used to modify the amino groups do not go to completion. Alternatively, the unsubstituted amino groups may only be indirectly involved in generating binding, by dictating the biosynthesis of another critical group. Regardless, these studies shown that HSPGs from cultured endothelial cells which can bind to L-selectin are enriched with unsubstituted amino groups on their GAG chains. The possible biochemical mechanisms for generation of these novel groups are discussed.

Hemagglutination and proteoglycan binding by the Lyme disease spirochete, Borrelia burgdorferi

The ability of the Lyme disease spirochete to attach to host components may contribute to its ability to infect diverse tissues. We present evidence that the Lyme disease spirochete expresses a lectin activity that promotes agglutination of erythrocytes and bacterial attachment to glycosaminoglycans. Among a diverse collection of 21 strains of Lyme disease spirochete, hemagglutinating activity was easily detected in all but 3 strains, and these three strains were noninfectious. The ability to agglutinate erythrocytes was associated with the ability of the spirochete to bind to the sulfated polysaccharide dextran sulfate and to mammalian cells. Soluble dextran sulfate was a potent inhibitor of both hemagglutination and attachment to mammalian cells, while dextran had no effect on either activity, suggesting that dextran sulfate may inhibit attachment by mimicking host cell glycosaminoglycans. Consistent with this, the spirochete bound to immobilized heparin, and soluble heparin inhibited bacterial adhesion to mammalian cells. The bacterium did not bind efficiently to Vero cells treated with heparinase or heparitinase or to mutant CHO cell lines that are deficient in proteoglycan synthesis. Sulfation of glycosaminoglycans was critical for efficient bacterial recognition, as Vero cells treated with an inhibitor of sulfation, or a mutant CHO cell line that produces undersulfated heparan sulfate, did not mediate maximal spirochetal binding. Binding of the spirochete to extracellular matrix also appeared to be dependent upon this attachment pathway. These findings suggest that a glycosaminoglycan-binding activity which can be detected by hemagglutination contributes to the attachment of the Lyme disease spirochete to host cells and matrix.

Heparan sulfate primed on beta-D-xylosides restores binding of basic fibroblast growth factor

Heparan sulfate proteoglycans (HSPG) are obligatory for receptor binding and mitogenic activity of basic fibroblast growth factor (bFGF). Mutant Chinese hamster ovary cells (pgsA-745) deficient in xylosyltransferase are unable to initiate glycosaminoglycan synthesis and hence can not bind bFGF to low- and high-affinity cell surface receptors. Exposure of pgsA-745 cells to beta-D-xylopyranosides containing hydrophobic aglycones resulted in restoration of bFGF binding in a manner similar to that induced by soluble heparin or by heparan sulfate (HS) normally associated with cell surfaces. Restoration of bF-GF binding correlated with the ability of the beta-D-xylosides to prime the synthesis of heparan sulfate. Thus, both heparan sulfate synthesis and bFGF receptor binding were induced by low concentrations (10-30 microM) of estradiol-beta-D-xyloside and naphthyl-beta-D-xyloside, but not by cis/trans-decahydro-2-naphthyl-beta-D-xyloside, which at low concentration primes mainly chondroitin sulfate. The obligatory involvement of xyloside-primed heparan sulfate in restoration of bFGF-receptor binding was also demonstrated by its sensitivity to heparinase treatment and by the lack of restoration activity in CHO cell mutants that lack enzymatic activities required to form the repeating disaccharide unit characteristic of heparan sulfate. Xyloside-primed heparan sulfate binds to the cell surface. Restoration of bFGF receptor binding was induced by both soluble and cell bound xyloside-primed heparan sulfate and was abolished in cells that were exposed to 0.5-1.0 M NaCl prior to the bFGF binding reaction. These results indicate that heparan sulfate chains produced on xyloside primers behave like heparan sulfate chains attached to cellular core proteins in terms of affinity for bFGF and ability to function as low-affinity sites in a dual receptor mechanism characteristic of bFGF and other heparin-binding growth promoting factors.

Irradiation of bovine aortic endothelial cells enhances the synthesis and secretion of sulphated glycosaminoglycans

The effect of X-irradiation on the synthesis of heparan sulphate (HS) and chondroitin/dermatan sulphate (CS/DS) by bovine aortic endothelial cells (BAEC), was studied by measuring the incorporation of [35S]sulphate and [3H]glucosamine into newly synthesized glycosaminoglycan (GAG) chains. Medium extracts from irradiated cultures (5Gy) were found to contain approx. 130% more HS and 200% more CS/DS than unirradiated controls. Smaller increases were observed in cellular extracts, irradiated cultures (5Gy) containing approx. 60% more HS and 100% more CS/DS than unirradiated controls. Structural studies showed no significant changes occurred upon irradiation in either the amounts or distribution of N- and O-sulphate groups in the HS molecule. Values for N-sulphation of 41.1% control and 41.5% irradiated (5Gy) were obtained, the corresponding values for O-sulphation being 19.9% control and 20.2% irradiated. Isotope incorporation data indicated that sulphation of CS/DS may decrease after irradiation, however, analysis of chondroitin ABC lyase derived disaccharides showed no changes in the proportion of non-sulphated and O-sulphated disaccharides. The present study indicates that X-irradiation stimulates the synthesis and secretion of HS and CS/DS proteoglycans (PGs) by BAEC. This could be relevant to many features which are found to be indicative of radiation-induced damage.

Herpesvirus-induced cell fusion that is dependent on cell surface heparan sulfate or soluble heparin

The entry of enveloped viruses into animal cells and the cell-to-cell spread of infection via cell fusion require the membrane-fusing activity of viral glycoproteins. This activity can be dependent on variable cell factors or triggered by environmental factors. Here we show that cell fusion induced by herpes simplex virus glycoproteins is dependent on the presence of cell surface glycosaminoglycans, principally heparan sulfate, or on the addition of heparin to the medium. The role of the glycosaminoglycan is probably to alter the conformation of a viral heparin-binding glycoprotein required for the fusion.

Herpes simplex virus infection and propagation in a mouse L cell mutant lacking heparan sulfate proteoglycans

We have isolated a variant line of mouse L cells, termed gro2C, which is partially resistant to infection by herpes simplex virus type 1 (HSV-1). Characterization of the genetic defect in gro2C cells revealed that this cell line harbors a specific defect in the heparan sulfate synthesis pathway. Specifically, anion-exchange high-performance liquid chromatography of metabolically radiolabeled glycosaminoglycans indicated that chondroitin sulfate moieties were synthesized normally in the mutant cells, whereas heparin-like chains were absent. Because of these properties, we have used these cells to investigate the role of heparan sulfate proteoglycans in the HSV-1 life cycle. In this report, we demonstrate that the partial block to HSV-1 infection in gro2C cells occurs in the virus entry pathway. Virus adsorption assays using radiolabeled HSV-1 (KOS) revealed that the gro2C cell surface is a relatively poor target for HSV-1 in that virus attachment was 85% lower in the mutant cells than in the parental L cell controls. A portion of the 15% residual virus adsorption was functional, however, insofar as gro2C cells were susceptible to HSV-1 infection in plaque assays and in single-step growth experiments. Moreover, although the number of HSV-1 plaques that formed in gro2C monolayers was reduced by 85%, the plaque morphology was normal, and the virus released from the mutant cells was infectious. Taken together, these results provide strong genetic evidence that heparan sulfate proteoglycans enhance the efficiency of HSV attachment to the cell surface but are otherwise not essential at any stage of the lytic cycle in culture. Moreover, in the absence of heparan sulfate, other cell surface molecules appear to confer susceptibility to HSV, leading to a productive viral infection.

Selection of COS cell mutants defective in the biosynthesis of heparan sulfate proteoglycan

A simple procedure using human basic fibroblast growth factor (FGF) was utilized for the selection of COS cell mutants with defects in the biosynthesis or expression of heparan sulfate proteoglycan (HSPG). Our approach was based on the strong binding affinity exhibited by COS cells to human basic FGF that had been adsorbed to plastic dishes. Cell binding to basic FGF could be inhibited by heparin and heparan sulfate (HS), but not by chondroitin sulfate, dermatan sulfate, keratan sulfate, or hyaluronic acid, suggesting that the cell binding involved an interaction between basic FGF and cell surface heparin-like molecules. COS cells were treated with ethyl methanesulfonate and four stable mutants were subsequently isolated that did not bind strongly to basic FGF adsorbed to plastic. These mutants cell lines (CM-2, CM-8, CM-9, and CM-15) exhibited significantly reduced 35SO4 incorporation into HS (40-70% depending on the cellular pool analyzed). In one of these cell lines, CM-15, the incorporation of [6-3H]glucosamine into HS was unaltered, suggesting that the extent of oligosaccharide polymerization was equivalent to that observed for the wild-type cells. Structural analysis revealed that N-sulfated glucosamine residues were present much less frequently in HS derived from these cells as compared with that derived from wild-type cells. Furthermore, CM-15 was found to be three-fold deficient in HS N-sulfotransferase activity, but contained wild-type levels of HS O-sulfotransferase activities.(ABSTRACT TRUNCATED AT 250 WORDS)

New approaches for defining sequence specific synthesis of heparan sulfate chains

Mammalian cells synthesize heparan sulfate proteoglycans (HSPG) which consist of core proteins with covalently linked glycosaminoglycans (GAGs) of 50-150 disaccharide units. The GAGs exhibit great structural diversity which arise from differing arrangements of alternate disaccharide units. It has been hypothesized that HSPG may be involved in regulating the most basic aspects of cell biologic systems such as adhesion, proliferation and differentiation. However, considerable doubt exists about the specific nature of the above interactions because of a failure to isolate GAGs of unique monosaccharide sequence with appropriate biologic activities. We have demonstrated that mouse LTA cells synthesize cell surface heparan sulfate proteoglycans with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). However, it remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethylmethane sulfonate and then identified mutants that exhibit decreased antithrombin binding to heparan sulfate chains but possess no gross defects in glycosaminoglycan biosynthesis. After screening 40,000 colonies, we isolated 7 stable mutants which synthesize 8-27% of the wild type HSPGact but produce normal amounts of other HSPG. These mutants are recessive in nature, and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should greatly improve our understanding of how cells synthesize HSPG with regions of defined monosaccharide sequence.

Heparan sulfate glycosaminoglycans as primary cell surface receptors for herpes simplex virus

Our current incomplete picture of the earliest events in HSV infection may be summarized as follows. The initial interaction of virus with cells is the binding of virion gC to heparan sulfate moieties of cell surface proteoglycans. Stable binding of virus to cells may require the interaction of other virion glycoproteins with other cell surface receptors as well (including the interaction of gB with heparan sulfate). Penetration of virus into the cell is mediated by fusion of the virion envelope with the cell plasma membrane. Events leading up to this fusion require the action of at least three viral glycoproteins (gB, gD and gH), one or more of which may interact with specific cell surface components. It seems likely that binding of gB to cell surface heparan sulfate may occur and may be important in the activation of some event required for virus penetration. Heparan sulfate is present not only as a constituent of cell surface proteoglycans but also as a component of the extracellular matrix and basement membranes in organized tissues. In addition, body fluids contain both heparin and heparin-binding proteins, either of which can prevent the binding of HSV to cells (WuDunn and Spear, 1989). As a consequence, the spread of HSV infection is probably influenced, not only by immune responses to the virus, but also by the probability that virus will be entrapped or inhibited from binding to cells by extracellular forms of heparin or heparan sulfate.

Genetic analysis of proteoglycan structure, function and metabolism

Significant progress has been made in understanding the structure, function, and metabolism of proteoglycans. Many of the advances derive from the application of recombinant DNA methodology to their core proteins and from the characterization of animal cell mutants altered in glycosaminoglycan synthesis.

Developmental abnormalities in transgenic mice expressing a sialic acid-specific 9-O-acetylesterase

9-O-acetylation of sialic acids is tissue specific and developmentally regulated. We have selectively destroyed these O-acetyl groups during murine embryogenesis by expressing the 9-O-acetyl-sialic acid-specific esterase of influenza C. DNA constructs driven by the metallothionein promoter arrested development at the 2-cell stage and gave a markedly decreased yield of live mice. A similar construct driven by the phenylethanolamine-N-methyltransferase promoter did not cause this block, but gave transgenic mice with selective expression of esterase in the retina and the adrenal gland. These organs showed variable abnormalities in organization, while all other tissues examined appeared normal. The ganglioside 9-O-acetyl-GD3 was selectively destroyed in target tissues. Thus, 9-O-acetylated sialic acids may play an role in murine development at the 2-cell stage and in certain differentiated tissues.

Cell surface, heparin-like molecules are required for binding of basic fibroblast growth factor to its high affinity receptor

The role of low affinity, heparin-like binding sites for basic fibroblast growth factor (bFGF) was investigated in CHO cells mutant in their metabolism of glycosaminoglycans. Heparan sulfate-deficient mutants transfected to express a cloned mouse FGF receptor cDNA are not able to bind bFGF. It is demonstrated that free heparin and heparan sulfate can reconstitute a low affinity receptor that is, in turn, required for the high affinity binding of bFGF. These studies suggest that the low affinity receptor is an accessory molecule required for binding of bFGF to the high affinity site. Such an obligatory interaction of low and high affinity FGF receptors suggests a physiological role for heparin-like, low affinity receptors and constitutes a novel mechanism for the regulation of growth factor-receptor interactions.

New approaches for defining the molecular basis of anticoagulantly active heparan sulfate production

Mammalian cells synthesize heparan sulfate proteoglycans, which consist of core proteins with covalently linked glycosaminoglycans of 50-150 disaccharide units. The GAGs exhibit great structural diversity, which arises from differing arrangements of alternate disaccharide units. It has been hypothesized that HSPG may be involved in regulating the most basic aspects of cell biologic systems, such as adhesion, proliferation, and differentiation. However, considerable doubt exists about the specific nature of the above interactions because of a failure to isolate GAGs of unique monosaccharide sequence with appropriate biologic activities. We have demonstrated that mouse LTA cells synthesize cell surface heparan sulfate proteoglycans with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). However, it remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethylmethane sulfonate and then identified mutants that exhibit decreased antithrombin binding to heparan sulfate chains but possess no gross defects in glycosaminoglycan biosynthesis. After screening 40,000 colonies, we isolated seven stable mutants that synthesize 8-27% of the wild type HSPGact but produce normal amounts of other HSPG. These mutants are recessive in nature and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should greatly improve our understanding of how cells synthesize HSPG with regions of defined monosaccharide sequence.