Changes in disaccharide composition of heparan sulphate fractions with increasing degrees of sulphation

Interaction of classical swine fever virus with membrane-associated heparan sulfate: role for virus replication in vivo and virulence

Passage of native classical swine fever virus (CSFV) in cultured swine kidney cells (SK6 cells) selects virus variants that attach to the surface of cells by interaction with membrane-associated heparan sulfate (HS). A Ser-to-Arg change in the C terminus of envelope glycoprotein E(rns) (amino acid 476 in the open reading frame of CSFV) is responsible for selection of these HS-binding virus variants (M. M. Hulst, H. G. P. van Gennip, and R. J. M. Moormann, J. Virol. 74:9553-9561, 2000). In this investigation we studied the role of binding of CSFV to HS in vivo. Using reverse genetics, an HS-independent recombinant virus (S-ST virus) with Ser(476) and an HS-dependent recombinant virus (S-RT virus) with Arg(476) were constructed. Animal experiments indicated that this adaptive Ser-to-Arg mutation had no effect on the virulence of CSFV. Analysis of viruses reisolated from pigs infected with these recombinant viruses indicated that replication in vivo introduced no mutations in the genes of the envelope proteins E(rns), E1, and E2. However, the blood of one of the three pigs infected with the S-RT virus contained also a low level of virus particles that, when grown under a methylcellulose overlay, produced relative large plaques, characteristic of an HS-independent virus. Sequence analysis of such a large-plaque phenotype showed that Arg(476) was mutated back to Ser(476). Removal of HS from the cell surface and addition of heparin to the medium inhibited infection of cultured (SK6) and primary swine kidney cells with S-ST virus reisolated from pigs by about 70% whereas infection with the administered S-ST recombinant virus produced in SK6 cells was not affected. Furthermore, E(rns) S-ST protein, produced in insect cells, could bind to immobilized heparin and to HS chains on the surface of SK6 cells. These results indicated that S-ST virus generated in pigs is able to infect cells by an HS-dependent mechanism. Binding of concanavalin A (ConA) to virus particles stimulated the infection of SK6 cells with S-ST virus produced in these cells by 12-fold; in contrast, ConA stimulated infection with S-ST virus generated in pigs no more than 3-fold. This suggests that the surface properties of S-ST virus reisolated from pigs are distinct from those of S-ST virus produced in cell culture. We postulate that due to these surface properties, in vivo-generated CSFV is able to infect cells by an HS-dependent mechanism. Infection studies with the HS-dependent S-RT virus, however, indicated that interaction with HS did not mediate infection of lung macrophages, indicating that alternative receptors are also involved in the attachment of CSFV to cells.

Passage of classical swine fever virus in cultured swine kidney cells selects virus variants that bind to heparan sulfate due to a single amino acid change in envelope protein E(rns)

Infection of cells with Classical swine fever virus (CSFV) is mediated by the interaction of envelope glycoprotein E(rns) and E2 with the cell surface. In this report we studied the role of the cell surface glycoaminoglycans (GAGs), chondroitin sulfates A, B, and C (CS-A, -B, and -C), and heparan sulfate (HS) in the initial binding of CSFV strain Brescia to cells. Removal of HS from the surface of swine kidney cells (SK6) by heparinase I treatment almost completely abolished infection of these cells with virus that was extensively passaged in swine kidney cells before it was cloned (clone C1.1.1). Infection with C1.1.1 was inhibited completely by heparin (a GAG chemically related to HS but sulfated to a higher extent) and by dextran sulfate (an artificial highly sulfated polysaccharide), whereas HS and CS-A, -B, and -C were unable to inhibit infection. Bound C1.1.1 virus particles were released from the cell surface by treatment with heparin. Furthermore, C1.1.1 virus particles and CSFV E(rns) purified from insect cells bound to immobilized heparin, whereas purified CSFV E2 did not. These results indicate that initial binding of this virus clone is accomplished by the interaction of E(rns) with cell surface HS. In contrast, infection of SK6 cells with virus clones isolated from the blood of an infected pig and minimally passaged in SK6 cells was not affected by heparinase I treatment of cells and the addition of heparin to the medium. However, after one additional round of amplification in SK6 cells, infection with these virus clones was affected by heparinase I treatment and heparin. Sequence analysis of the E(rns) genes of these virus clones before and after amplification in SK6 cells showed that passage in SK6 cells resulted in a change of an Ser residue to an Arg residue in the C terminus of E(rns) (amino acid 476 in the polyprotein of CSFV). Replacement of the E(rns) gene of an infectious DNA copy of C1.1.1 with the E(rns) genes of these virus variants proved that acquisition of this Arg was sufficient to alter an HS-independent virus to a virus that uses HS as an E(rns) receptor.

Separation of hydroxyl protected heparin derived disaccharides using reversed-phase high-performance liquid chromatography

A simple and efficient method for the separation of hydrophobic derivatives of glycosaminoglycan-derived disaccharides is described. Hydroxyl-protected derivatives of a trisulfated disaccharide, prepared from heparin using heparin lyase, were separated by reversed-phase high-performance liquid chromatography. These disaccharide derivatives differed by the number, position, and stereochemistry of acetyl and pivaloyl groups. Separation was achieved on a C18 column using a reversed gradient of ammonium sulfate in water. This method has application in the purification of disaccharide derivatives being used as chiral synthons in the preparation of higher oligosaccharides.

Capillary electrophoresis for the analysis of chondroitin sulfate- and dermatan sulfate-derived disaccharides

High-voltage capillary zone electrophoresis (CZE) has been used for the first time in the analysis of non-, mono-, di-, and trisulfated disaccharides derived from chondroitin sulfate, dermatan sulfate, and hyaluronic acid. These glycosaminoglycans are first depolymerized using polysaccharide lyases. The resulting unsaturated disaccharide products can be detected by their ultraviolet absorbance at 232 nm. Different retention times were obtained for each unsaturated disaccharide analyzed by CZE. The application of a constant voltage across a 70-cm fused silica capillary using a single, simple buffer system resolved an eight-component mixture within 40 min. Quantitation of disaccharides derived from chondroitin sulfate using chondroitin ABC lyase (EC 4.2.2.4) and mixtures of unsaturated disaccharide standards was possible requiring only picogram quantities of sample. The disaccharides examined had a net charge of from -1 to -4 and were resolved primarily on the basis of net charge and secondarily on the basis of charge distribution. Two unsulfated disaccharides both containing the same unsaturated uronic acid residue were analyzed. One was from chondroitin having an N-acetylgalactosyl residue and one from hyaluronate having an N-acetylglycosyl residue. Despite the fact that they differed only by the chirality at one center, these disaccharides were resolved by CZE. CZE is a fast and simple method that represents a powerful new tool for analysis and separation of acidic disaccharide components of glycosaminoglycans.

Distribution of iduronate 2-sulphate residues in heparan sulphate. Evidence for an ordered polymeric structure

The structure of human skin fibroblast heparan sulphate has been examined by depolymerization with heparinase, which specifically cleaves highly sulphated disaccharides of structure GlcNSO3 (+/-6S)-alpha 1,4IdoA(2S) [N-sulphated glucosamine (6-sulphate)-alpha 1,4-iduronic acid 2-sulphate]. Heparan sulphate contained only a small proportion (approximately 10%) of linkages susceptible to this enzyme. The major products of depolymerization with heparinase were large oligosaccharides with an average molecular mass of 10 kDa (dp approximately 40, where dp is degree of polymerization; for disaccharides, dp = 2 etc.) as assessed by gel filtration on Sepharose CL-6B, compared with a molecular mass of 45 kDa (dp approximately 200) for the intact chains. The large heparinase-resistant oligosaccharides were highly susceptible to depolymerization with the enzyme heparitinase, which cleaves heparan sulphate in areas of low sulphation, where N-acetylated disaccharides [GlcNAc-alpha 1,4GlcA (N-acetylglucosaminyl-alpha 1,4-glucuronic acid)] are the predominant structural unit. Further analysis of the location of the heparinase cleavage sites indicated that they were predominantly found in a central position in GlcNSO3-alpha 1,4IdoA repeat sequences of average length four to seven disaccharides (dp 8-14). These results indicate that heparinase cleaves heparan sulphate in approximately four or five N-sulphated domains, each domain containing a cluster of two or three susceptible disaccharides; the domains are separated by long N-acetyl-rich sequences that are markedly deficient in sulphate groups. On the basis of these findings a model is proposed which depicts heparan sulphate as an ordered polymeric structure composed of an alternate arrangement of sulphate-rich and sulphate-poor regions. The sulphate-rich regions are likely to be flexible areas of the chain because of their high content of the conformationally versatile IdoA and IdoA(2S) residues. The model has important implications for the biosynthesis and functions of heparan sulphate.

Analysis of glycosaminoglycan-derived oligosaccharides using reversed-phase ion-pairing and ion-exchange chromatography with suppressed conductivity detection

Oligosaccharides prepared from glycosaminoglycans (GAGs) including heparin, heparan sulfate, chondroitin sulfates, dermatan sulfate, and keratan sulfate were analyzed using reverse-phase ion-pairing HPLC and ion-exchange HPLC with suppressed conductivity detection. The results were compared with those obtained by strong anion-exchange HPLC using uv detection. These oligosaccharides were first prepared by enzymatically depolymerizing the GAGs with enzymes including heparin lyase (EC 4.2.2.7), heparan sulfate lyase (EC 4.2.2.8), chondroitin ABC lyase (EC 4.2.2.4), and keratan sulfate hydrolase (EC 3.2.1.103). Analysis was then performed without derivitization under isocratic conditions with a limit of sensitivity in the picomole range. Preliminary studies suggest that this approach may be particularly useful in examining oligosaccharides having no uv chromophore such as those prepared from keratan sulfate.

Analysis of heparan sulfate from the Engelbreth-Holm-Swarm (EHS) tumor

The size of the heparan sulfate chains from the Engelbreth-Holm-Swarm (EHS) tumor heparan sulfate proteoglycan (PG) was measured by several techniques in order to resolve uncertainty about their size and the chains were chemically characterized for comparison with other basement membrane heparan sulfate PGs. Heparan sulfate size was determined by gel filtration (Mr = 5.5 - 6.0 x 10(4], by equilibrium sedimentation centrifugation (Mw = 6.8 x 10(4], and by end group analysis (Mn = 7.1 x 10(4]. A higher molecular weight (HMW) (Mw = 2.13 x 10(5] calculated from scattering measurements may reflect chain-chain interactions. Forty percent of newly synthesized chains eluted on gel filtration as a lower molecular weight (LMW) shoulder and in vivo turned over faster than the larger species. A large heparan sulfate PG was present after 4 hours of in vivo 35SO4 labeling in both a low density form and a high density, slightly smaller form with large heparan sulfate chains (Mr approximately 8.0 x 10(4]. Heparan sulfate PG of intermediate size (Kav = 0.3-0.65, Sepharose CL-4B) and of smaller size (Kav = 0.75, CL-4B) were found predominantly as high density species. These PGs contained chains (Mr = 3.5 x 10(4) and Mr = 1.2 x 10(4), respectively) which were partially sensitive to chondroitinase ABC (CABC) and may include a hybrid heparan sulfate/chondroitin sulfate PG. Heparan sulfate chains, possibly intracellular degradation products, were also found. Heparan sulfate chains were normal in N-sulfation (58% of hexosamine residues) and in iduronate content (approximately 30%). N-sulfation started within two disaccharides of the linkage region. The EHS heparan sulfate was unusually low in O-sulfation (10% of the total sulfation) and no 6-O sulfated, N-acetylated glucosamine residues adjacent to N-sulfated block regions were found.

Molecular distinctions between heparan sulphate and heparin. Analysis of sulphation patterns indicates that heparan sulphate and heparin are separate families of N-sulphated polysaccharides

Heparan sulphate and heparin are chemically related alpha beta-linked glycosaminoglycans composed of alternating sequences of glucosamine and uronic acid. The amino sugars may be N-acetylated or N-sulphated, and the latter substituent is unique to these two polysaccharides. Although there is general agreement that heparan sulphate is usually less sulphated than heparin, reproducible differences in their molecular structure have been difficult to identify. We suggest that this is because most of the analytical data have been obtained with degraded materials that are not necessarily representative of complete polysaccharide chains. In the present study intact heparan sulphates, labelled biosynthetically with [3H]glucosamine and Na2(35)SO4, were isolated from the surface membranes of several types of cells in culture. The polysaccharide structure was analysed by complete HNO2 hydrolysis followed by fractionation of the products by gel filtration and high-voltage electrophoresis. Results showed that in all heparan sulphates there were approximately equal numbers of N-sulpho and N-acetyl substituents, arranged in a similar, predominantly segregated, manner along the polysaccharide chain. O-Sulphate groups were in close proximity to the N-sulphate groups but, unlike the latter, the number of O-sulphate groups could vary considerably in heparan sulphates of different cellular origins ranging from 20 to 75 O-sulphate groups per 100 disaccharide units. Inspection of the published data on heparin showed that the N-sulphate frequency was very high (greater than 80% of the glucosamine residues are N-sulphated) and the concentration of O-sulphate groups exceeded that of the N-sulphate groups. We conclude from these and other observations that heparan sulphate and heparin are separate families of N-sulphated glycosaminoglycans.

Very-high-field n.m.r. studies of bovine lung heparan sulphate tetrasaccharides produced by nitrous acid deaminative cleavage. Determination of saccharide sequence, uronate composition and degrees of sulphation

Tetrasaccharides with the general structure UA-GlcNAc-GlcUA-aManOH (where UA represents uronate, GlcNAc N-acetylglucosamine, GlcUA glucuronate and aManOH anhydromannitol) were prepared from low-sulphated heparan sulphates of bovine lung origin by complete nitrous acid deaminative cleavage followed by reduction and fractionated by gel filtration. Ion-exchange chromatography of the tetrasaccharides yielded three major fractions in approximate yields of 37%, 45% and 14%. These were shown to be non-, mono- and di-sulphated respectively. Complete structural characterization of the tetrasaccharide fractions by quantitative high-field n.m.r. spectroscopy showed that each fraction contained only two discrete species and led to the following observations. (1) All of the uronate residues in the tetrasaccharides (and in larger oligosaccharides) are unsulphated, and hence sulphated iduronate [IdUA(2SO3)] must occur exclusively within -GlcNSO3-IdUA(2SO3)-GlcNSO3- sequences (where GlcNSO3 represents N-sulpho-glucosamine) in the parent polymers. (2) The GlcNAc residues in the tetrasaccharides are more highly C-6-O-sulphated than are the aManOH residues, and furthermore sulphation on the aManOH appears to occur only where the GlcNAc is also sulphated. (3) Where the GlcNAc is C-6-O-sulphated, iduronate is the major non-reducing terminal residue, whereas glucuronate predominates in this position if the GlcNAc is unsulphated. The quantitative data obtained are used to determine the degree of C-6-O-sulphation of glucosamine residues in specific sequences within the parent heparan sulphates.

Hydrazinolysis of heparin and other glycosaminoglycans

Heparin, carboxy-group-reduced heparin, several sulphated monosaccharides and disaccharides formed from heparin, and a tetrasaccharide prepared from chondroitin sulphate were treated at 100 degrees C with hydrazine containing 1% hydrazine sulphate for periods sufficient to cause complete N-deacetylation of the N-acetylhexosamine residues. Under these hydrazinolysis conditions both the N-sulphate and the O-sulphate substituents on these compounds were completely stable. However, the uronic acid residues were converted into their hydrazide derivatives at rates that depended on the uronic acid structures. Unsubstituted L-iduronic acid residues reacted much more slowly than did unsubstituted D-glucuronic acid or 2-O-sulphated L-iduronic acid residues. The chemical modification of the carboxy groups resulted in a low rate of C-5 epimerization of the uronic acid residues. The hydrazinolysis reaction also caused a partial depolymerization of heparin but not of carboxy-group-reduced heparin. Treatment of the hydrazinolysis products with HNO2 at either pH 4 or pH 1.5 or with HIO3 converted the uronic acid hydrazides back into uronic acid residues. The use of the hydrazinolysis reaction in studies of the structures of uronic acid-containing polymers and the implications of the uronic acid hydrazide formation are discussed.