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

Heparan sulfate structure in mice with genetically modified heparan sulfate production

Using a high throughput heparan sulfate (HS) isolation and characterization protocol, we have analyzed HS structure in several tissues from mice/mouse embryos deficient in HS biosynthesis enzymes (N-deacetylase/N-sulfotransferase (NDST)-1, NDST-2, and C5-epimerase, respectively) and in mice lacking syndecan-1. The results have given us new information regarding HS biosynthesis with implications on the role of HS in embryonic development. Our main conclusions are as follows. 1) The HS content, disaccharide composition, and the overall degree of N- and O-sulfation as well as domain organization are characteristic for each individual mouse tissue. 2) Removal of a key biosynthesis enzyme (NDST-1 or C5-epimerase) results in similar structural alterations in all of the tissues analyzed. 3) Essentially no variation in HS tissue structure is detected when individuals of the same genotype are compared. 4) NDST-2, although generally expressed, does not contribute significantly to tissue-specific HS structures. 5) No change in HS structure could be detected in syndecan-1-deficient mice.

Crystal structure and mutational analysis of heparan sulfate 3-O-sulfotransferase isoform 1

Heparan sulfate interacts with antithrombin, a protease inhibitor, to regulate blood coagulation. Heparan sulfate 3-O-sulfotransferase isoform 1 performs the crucial last step modification in the biosynthesis of anticoagulant heparan sulfate. This enzyme transfers the sulfuryl group (SO(3)) from 3'-phosphoadenosine 5'-phosphosulfate to the 3-OH position of a glucosamine residue to form the 3-O-sulfo glucosamine, a structural motif critical for binding of heparan sulfate to antithrombin. In this study, we report the crystal structure of 3-O-sulfotransferase isoform 1 at 2.5-A resolution in a binary complex with 3'-phosphoadenosine 5'-phosphate. This structure reveals residues critical for 3'-phosphoadenosine 5'-phosphosulfate binding and suggests residues required for the binding of heparan sulfate. In addition, site-directed mutagenesis analyses suggest that residues Arg-67, Lys-68, Arg-72, Glu-90, His-92, Asp-95, Lys-123, and Arg-276 are essential for enzymatic activity. Among these essential amino acid residues, we find that residues Arg-67, Arg-72, His-92, and Asp-95 are conserved in heparan sulfate 3-O-sulfotransferases but not in heparan N-deacetylase/N-sulfotransferase, suggesting a role for these residues in conferring substrate specificity. Results from this study provide information essential for understanding the biosynthesis of anticoagulant heparan sulfate and the general mechanism of action of heparan sulfate sulfotransferases.

Alpha C protein of group B Streptococcus binds host cell surface glycosaminoglycan and enters cells by an actin-dependent mechanism

Group B Streptococcus (GBS) colonizes mucosal surfaces of the human gastrointestinal and gynecological tracts and causes disease in a wide range of patients. Invasive illness occurs after organisms traverse an epithelial boundary and enter deeper tissues. Previously we have reported that the alpha C protein (ACP) on the surface of GBS mediates GBS entry into ME180 cervical epithelial cells and GBS translocation across layers of these cells. We now demonstrate that ACP interacts with host cell glycosaminoglycan (GAG); the interaction of ACP with ME180 cells is inhibited if cells are pretreated with sodium chlorate, an inhibitor of sulfate incorporation, or with heparitinases. The interaction is also inhibited in the presence of soluble heparin or heparan sulfate or host cell-derived GAG. In addition, ACP binds soluble heparin specifically in inhibition and dot blot assays. After interaction with host GAG, soluble ACP enters ME180 cells and fractionates to the eukaryotic cell cytosol. These events are inhibited in cells pretreated with cytochalasin D or with Clostridium difficile toxin B. These data indicate that full-length ACP interacts with ME180 cell GAG and enters the eukaryotic cell cytosol by a mechanism that involves Rho GTPase-dependent actin rearrangements. We suggest that these molecular interactions drive ACP-mediated translocation of GBS across epithelial barriers, thereby facilitating invasive GBS infection.

Altered expression of NDST-1 messenger RNA in puromycin aminonucleoside nephrosis

Sulfated portions of glycosaminoglycan (GAG) side chains in heparan sulfate proteoglycan (HSPG) are thought to play an important role in charge-dependent selectivity of glomerular filtration against plasma proteins. Heparan sulfate N-acetylglucosamine N-deacetylase/adenosine 3'-phosphate 5'-phosphosulfate: unsubstituted glucosamine N-sulfotransferase (NDST) is the key enzyme regulating sulfation of GAG chains. In this study we investigated transcriptional expression of NDST-1, 1 of 4 isozymes of NDST, in glomeruli of rats with puromycin aminonucleoside (PAN) nephrosis. Nephrosis was induced in rats with a single intraperitoneal injection of 150 mg/kg PAN. On days 10 and 35, expression of NDST-1 messenger RNA (mRNA) in glomeruli was analyzed with the use of Northern-blot analysis. Immunohistochemical studies were also performed with the use of monoclonal antibodies that react specifically with the N-sulfated portion of the GAG chain of HSPG and agrin, a major core protein of HSPG in glomerular basement membrane (GBM). In addition, we studied the expression of NDST-1 mRNA in cultured glomerular epithelial cells (GECs) and glomerular mesangial cells in the presence of PAN. On day 10, when significant proteinuria developed, the ratios of glomerular expression of NDST-1 mRNA against glyceraldehyde-phosphate dehydrogenase mRNA in PAN-treated rats were decreased to 48% +/- 6% of those in controls (P<.05). Immunohistochemical studies revealed that staining for N-sulfated GAG chains of HSPG on GBM was markedly reduced on day 10 in PAN-treated rats but that staining for agrin was unchanged. In contrast, on day 35, when PAN-treated rats recovered from proteinuria, we noted no differences in glomerular expression of NDST-1 mRNA and staining intensity for N-sulfated GAG chains on GBM between PAN-treated rats and controls. Incubation of GECs for 24 hours in the presence of 50 ng/mL PAN resulted in the reduction of the expression of NDST-1 mRNA (67% +/- 12% of those in controls, P<.05). In summary, we found alteration of the expression of NDST-1 mRNA, accompanying a loss of N-sulfated GAG chains of HSPG on GBM without changes in the core protein agrin, in the course of PAN nephrosis. These data suggest an important role for this enzyme in heparan sulfate assembly in GBM and GEC and in the pathogenesis of proteinuria in PAN nephrosis.

Temperature-sensitive glycosaminoglycan biosynthesis in a Chinese hamster ovary cell mutant containing a point mutation in glucuronyltransferase I

In previous studies, we reported the isolation and characterization of a Chinese hamster ovary cell mutant (pgsG) defective in glucuronyltransferase I (GlcATI). This enzyme adds the terminal GlcA residue in the core protein-linkage tetrasaccharide (GlcAbeta1,3Galbeta1,3Galbeta1, 4Xylbeta-O-) on which glycosaminoglycan assembly occurs (Bai, X. M., Wei, G., Sinha, A., and Esko, J. D. (1999) J. Biol. Chem. 274, 13017-13024; Wei, G., Bai, X. M., Sarkar, A. K., and Esko, J. D. (1999) J. Biol. Chem. 274, 7857-7864). Here we show that incorporation of 35SO4 into glycosaminoglycans in the mutant is temperature-sensitive, with greater synthesis occurring at 33 degrees C compared with 37 degrees C. Wild-type cells show the opposite thermal dependence. Rabbit antiserum to hamster GlcATI failed to detect cross-reactive material in pgsG cells by immunofluorescence and Western blotting. Furthermore, expression of chimeric proteins composed of mutant GlcATI fused to IgG binding domain of protein A or to green fluorescent protein did not yield the proteins at the expected mass. The green fluorescent protein-tagged version appeared as a truncated protein, and immunofluorescence showed large perinuclear bodies at 30 degrees C. At 37 degrees C, the fusion protein was not readily detectable. Sequencing cDNAs from mutant and wild-type cells revealed a single base transition (G331A) in the open reading frame in pgsG cells, which resulted in a Val-111-->Met substitution. These data suggest that pgsG cells contain a labile form of GlcATI that causes conditional expression of glycosaminoglycans dependent on temperature.

Biosynthesis of 3-O-sulfated heparan sulfate: unique substrate specificity of heparan sulfate 3-O-sulfotransferase isoform 5

Heparan sulfate 3-O-sulfotransferase transfers sulfate to the 3-OH position of a glucosamine to generate 3-O-sulfated heparan sulfate (HS), which is a rare component in HS from natural sources. We previously reported that 3-O- sulfotransferase isoform 5 (3-OST-5) generates both an antithrombin-binding site to exhibit anticoagulant activity and a binding site for herpes simplex virus 1 glycoprotein D to serve as an entry receptor for herpes simplex virus. In this study, we characterize the substrate specificity of 3-OST-5 using the purified enzyme. The enzyme was expressed in insect cells using the baculovirus expression approach and was purified by using heparin-Sepharose and 3',5'-ADP- agarose chromatographies. As expected, the purified enzyme generates both an antithrombin binding site and a glycoprotein D binding site. We isolated IdoUA-AnMan3S and IdoUA-AnMan3S6S from nitrous acid-degraded 3-OST-5-modified HS (pH 1.5), suggesting that 3-OST-5 enzyme sulfates the glucosamine residue that is linked to an iduronic acid residue at the nonreducing end. We also isolated a disaccharide with a structure of DeltaUA2S-GlcNS3S and a tetrasaccharide with a structure of DeltaUA2S-GlcNS-IdoUA2S-GlcNH23S6S from heparin lyases-digested 3-OST-5-modified HS. Our results suggest that 3-OST-5 enzyme sulfates both N-sulfated glucosamine and N-unsubstituted glucosamine residues. Taken together, the results indicate that 3-OST-5 has broader substrate specificity than those of 3-OST-1 and 3-OST-3. The unique substrate specificity of 3-OST-5 serves as an additional tool to study the mechanism for the biosynthesis of biologically active HS.

Heparan sulfates and coxsackievirus-adenovirus receptor: each one mediates coxsackievirus B3 PD infection

Amino acid exchanges in the virus capsid protein VP1 allow the coxsackievirus B3 variant PD (CVB3 PD) to replicate in decay accelerating factor (DAF)-negative and coxsackievirus-adenovirus receptor (CAR)-negative cells. This suggests that molecules other than DAF and CAR are involved in attachment of this CVB3 variant to cell surfaces. The observation that productive infection associated with cytopathic effect occurred in Chinese hamster ovary (CHO-K1) cells, whereas heparinase-treated CHO-K1 cells, glucosaminoglycan-negative pgsA-745, heparan sulfate (HS)-negative pgsD-677, and pgsE-606 cells with significantly reduced N-sulfate expression resist CVB3 PD infection, indicates a critical role of highly sulfated HS. 2-O-sulfate-lacking pgsF-17 cells represented the cell line with minimum HS modifications susceptible for CVB3 PD. Inhibition of virus replication in CHO-K1 cells by polycationic compounds, pentosan polysulfate, lung heparin, and several intestinal but not kidney HS supported the hypothesis that CVB3 PD uses specific modified HS for entry. In addition, recombinant human hepatocyte growth factor blocked CVB3 PD infection. However, CAR also mediates CVB3 PD infection, because this CVB3 variant replicates in HS-lacking but CAR-bearing Raji cells, infection could be prevented by pretreatment of cells with CAR antibody, and HS-negative pgsD-677 cells transfected with CAR became susceptible for CVB3 PD. These results demonstrate that the amino acid substitutions in the viral capsid protein VP1 enable CVB3 PD to use specific modified HS as an entry receptor in addition to CAR.

Disturbed Ca2+ kinetics in N-deacetylase/N-sulfotransferase-1 defective myotubes

The biosynthesis of heparan sulfate, present on the cell surface and in the basal lamina surrounding cells, is a multistep process in which each step is mediated by a specific enzyme. The initial modification of the precursor polysaccharide, N-deacetylation followed by N-sulfation of selected N-acetyl-D-glucosamine residues, is catalyzed by the enzyme glucosaminyl N-deacetylase/N-sulfotransferase (NDST). This event is a key step that regulates the overall sulfate content of the polysaccharide. Here, we report on the effects of NDST deficiency on Ca2+ kinetics in myotubes from NDST-1- and NDST-2-deficient mice, indicating a novel role for heparan sulfate in skeletal muscle physiology. Immunostaining for specific heparan sulfate epitopes showed major changes in the heparan sulfate composition in skeletal muscle tissue derived from NDST-1-/- mice and NDST-/- cultured myotubes. Biochemical analysis indicates a relative decrease in both N-sulfation and 2-O-sulfation of skeletal muscle heparan sulfate. The core protein of heparan sulfate proteoglycan perlecan was not affected, as judged by immunohistochemistry. Also, acetylcholine receptor clustering and the occurrence of other ion channels involved in excitation-contraction coupling were not altered. In NDST-2-/- mice and heterozygous mice no changes in heparan sulfate composition were observed. Using high-speed UV confocal laser scanning microscopy, aberrant Ca2+ kinetics were observed in NDST-1-/- myotubes, but not in NDST-2-/- or heterozygous myotubes. Electrically induced Ca2+ spikes had significantly lower amplitudes, and a reduced removal rate of cytosolic Ca2+, indicating the importance of heparan sulfate in muscle Ca2+ kinetics.

The Caenorhabditis elegans genes sqv-2 and sqv-6, which are required for vulval morphogenesis, encode glycosaminoglycan galactosyltransferase II and xylosyltransferase

In mutants defective in any of eight Caenorhabditis elegans sqv (squashed vulva) genes, the vulval extracellular space fails to expand during vulval morphogenesis. Strong sqv mutations result in maternal-effect lethality, caused in part by the failure of the progeny of homozygous mutants to initiate cytokinesis and associated with the failure to form an extracellular space between the egg and the eggshell. Recent studies have implicated glycosaminoglycans in these processes. Here we report the cloning and characterization of sqv-2 and sqv-6. sqv-6 encodes a protein similar to human xylosyltransferases. Transfection of sqv-6 restored xylosyltransferase activity to and rescued the glycosaminoglycan biosynthesis defect of a xylosyltransferase mutant hamster cell line. sqv-2 encodes a protein similar to human galactosyltransferase II. A recombinant SQV-2 fusion protein had galactosyltransferase II activity with substrate specificity similar to that of human galactosyltransferase II. We conclude that C. elegans SQV-6 and SQV-2 likely act in concert with other SQV proteins to catalyze the stepwise formation of the proteoglycan core protein linkage tetrasaccharide GlcAbeta1,3Galbeta1, 3Galbeta1,4Xylbeta-O-(Ser), which is common to the two major types of glycosaminoglycans in vertebrates, chondroitin and heparan sulfate. Our results strongly support a model in which C. elegans vulval morphogenesis and zygotic cytokinesis depend on the expression of glycosaminoglycans.

Heparan sulfate and development: differential roles of the N-acetylglucosamine N-deacetylase/N-sulfotransferase isozymes

Heparan sulfates (HSs) are N- and O-sulfated polysaccharide components of proteoglycans, which are important constituents of the cell surface as well as the extracellular matrix. Heparin, with extensive clinical application as an anticoagulant, is a highly sulfated form of HS present within the granules of connective tissue type mast cells. The diverse functions of HS, which include the modulation of growth factor/cytokine activity, interaction with matrix proteins and binding of enzymes to cell surfaces, depend greatly on the presence of specific, high affinity regions on the chains. N-acetylglucosamine N-deacetylase/N-sulfotransferases, NDSTs, are an important group of enzymes in HS biosynthesis, initiating the sulfation of the polysaccharide chains and thus determining the generation of the high affinity sites. Here, we review the role of the four vertebrate NDSTs in HS biosynthesis as well as their regulated expression. The main emphasis is the phenotypes of mice lacking one or more of the NDSTs.

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

Mutations that affect the tropism of DA and GDVII strains of Theiler's virus in vitro influence sialic acid binding and pathogenicity

Theiler's murine encephalomyelitis virus (TMEV) is a natural pathogen of the mouse. The different strains of TMEV are divided into two subgroups according to the pathology they provoke. The neurovirulent strains GDVII and FA induce an acute fatal encephalitis, while persistent strains, like DA and BeAn, cause a chronic demyelinating disease associated with viral persistence in the central nervous system. Different receptor usage was proposed to account for most of the phenotype difference between neurovirulent and persistent strains. Persistent but not neurovirulent strains were shown to bind sialic acid. We characterized DA and GDVII derivatives adapted to grow on CHO-K1 cells. Expression of glycosaminoglycans did not influence infection of CHO-K1 cells by parental and adapted viruses. Mutations resulting from adaptation of DA and GDVII to CHO-K1 cells notably mapped to the well-characterized VP1 CD and VP2 EF loops of the capsid. Adaptation of the DA virus to CHO-K1 cells correlated with decreased sialic acid usage for entry. In contrast, adaptation of the GDVII virus to CHO-K1 cells correlated with the appearance of a weak sialic acid usage for entry. The sialic acid binding capacity of the GDVII variant resulted from a single amino acid mutation (VP1-51, Asn-->Ser) located out of the sialic acid binding region defined for virus DA. Mutations affecting tropism in vitro and sialic acid binding dramatically affected the persistence and neurovirulence of the viruses.

Biosynthesis of the linkage region of glycosaminoglycans: cloning and activity of galactosyltransferase II, the sixth member of the beta 1,3-galactosyltransferase family (beta 3GalT6)

A family of five beta1,3-galactosyltransferases has been characterized that catalyze the formation of Galbeta1,3GlcNAcbeta and Galbeta1,3GalNAcbeta linkages present in glycoproteins and glycolipids (beta3GalT1, -2, -3, -4, and -5). We now report a new member of the family (beta3GalT6), involved in glycosaminoglycan biosynthesis. The human and mouse genes were located on chromosomes 1p36.3 and 4E2, respectively, and homologs are found in Drosophila melanogaster and Caenorhabditis elegans. Unlike other members of the family, beta3GalT6 showed a broad mRNA expression pattern by Northern blot analysis. Although a high degree of homology across several subdomains exists among other members of the beta3-galactosyltransferase family, recombinant enzyme did not utilize glucosamine- or galactosamine-containing acceptors. Instead, the enzyme transferred galactose from UDP-galactose to acceptors containing a terminal beta-linked galactose residue. This product, Galbeta1,3Galbeta is found in the linkage region of heparan sulfate and chondroitin sulfate (GlcAbeta1,3Galbeta1,3Galbeta1,4Xylbeta-O-Ser), indicating that beta3GalT6 is the so-called galactosyltransferase II involved in glycosaminoglycan biosynthesis. Its identity was confirmed in vivo by siRNA-mediated inhibition of glycosaminoglycan synthesis in HeLa S3 cells. Its localization in the medial Golgi indicates that this is the major site for assembly of the linkage region.

Enhanced 3-O-sulfation of galactose in Asn-linked glycans and Maackia amurensis lectin binding in a new Chinese hamster ovary cell line

We report the characterization of two Chinese hamster ovary cell lines that produce large amounts of sulfated N-linked oligosaccharides. Clones 26 and 489 were derived by stable transfection of the glycosaminoglycan-deficient cell mutant pgsA-745 with a cDNA library prepared from wild-type cells. Peptide:N-glycanase F released nearly all of the sulfate label, indicating that sulfation had occurred selectively on the Asn-linked glycans. Hydrazinolysis followed by nitrous acid treatment at pH 4 and borohydride reduction yielded reduced sulfated disaccharides that comigrated with standard Gal3SO4beta1-4anhydromannitol. The disaccharides were resistant to periodate oxidation but became sensitive after the sulfate group was removed by methanolysis, indicating that the sulfate was located at C3 of the galactose residues. Maackia amurensis lectin bound to the sulfated glycopeptides on the cell surface and in free form, even after sialidase treatment. This finding indicates that the lectin requires only a charged group at C3 of the galactose unit and not an intact sialic acid. Growth of cells with chlorate restored sialidase sensitivity to lectin binding, indicating that sulfation and sialylation occurred largely at the same sites. The enhanced sulfation was due to elevated sulfotransferase activity that catalyzed transfer of sulfate from phosphoadenosine-5'-phosphosulfate to Galbeta1-4(3)GlcNAcbeta-O-naphthalenemethanol.

Highly sulfated forms of heparin sulfate are involved in japanese encephalitis virus infection

Japanese encephalitis virus (JEV) infects a broad range of cell types in vitro, though little is known about the initial events of JEV infection. In the present study, we found that highly sulfated glycosaminoglycans (GAGs) are involved in infection of both neurovirulent (RP-9) and attenuated (RP-2ms) JEV strains. Competition experiments using highly sulfated GAGs, heparin and dextran sulfate, demonstrated an inhibition of JEV's attachment and subsequent infection of BHK-21 cells. Treatment of target cells by a potent sulfation inhibitor, sodium chlorate, greatly reduced viral binding ability as well as infection, suggesting a critical role of GAGs' sulfation status on the cellular surface in JEV infection. This phenomenon was confirmed by the manifestation of a distinct binding efficiency of JEV to the wild-type CHO cell line and its mutants with defects in GAG biosynthesis. We also demonstrated the binding of JEV particles and virus envelope glycoprotein to immobilized heparin beads. Furthermore, the addition of heparin suppressed the cytopathic effects induced by JEV infection in cultured cells. Our results establish that the highly sulfated form of GAGs on cell surfaces plays a determining role in the early stage of in vitro JEV infection.

Analysis of heparan sulfate oligosaccharides by nano-electrospray ionization mass spectrometry

A highly sensitive method to identify and quantify heparan sulfate (HS) oligosaccharides by using nano-electrospray ionization mass spectrometry (nESI-MS) is described. The new approach allows us to detect approximately 50 nM of a chemically synthesized pentasaccharide with a structure of GlcNS6S-GlcA-GlcNS6S-IdoA2S-GlcNS6SOMe (3-OH pentasaccharide). Typically, solutions were infused for a total of 5 min, at an average flow rate of 30 nl/min, and the remaining sample was recovered from the nanovial. The spectra shown were obtained by summing scans for 1--3 min. Hence, our data indicated that as little as 3 x 10(-15) mole of the pentasaccharide was consumed to obtain a reasonable spectrum at the concentration as low as 50 nM. In addition, we found a linear relationship between the relative response of the molecular ion and the concentration of the analyzed 3-OH pentasaccharide, demonstrating that this approach can be used to determine the amount of HS oligosaccharides. To this end, a 3-O-sulfated pentasaccharide was prepared by incubating the 3-OH pentasaccharide with purified HS 3-O-sulfotransferase-1 and 3'-phosphoadenosine-5'-phospho[(35)S]sulfate. The resulting 3-O-sulfated pentasaccharide was purified and analyzed by nESI-MS. Based on the standard curve constructed with the 3-OH pentasaccharide, we calculated the concentration of the 3-O-sulfated pentasaccharide by the relative response. The result indicates that this value is very close to the value measured by [(35)S]sulfate radioactivity. In conclusion, nESI-MS provides both high sensitivity and the capacity to quantify HSs. This approach is likely to become a very important tool for structural analysis and sequencing of HS and heparin oligosaccharides.

Heparanases: endoglycosidases that degrade heparan sulfate proteoglycans

Heparanases are endoglycosidases that cleave the heparan sulfate glycosaminoglycans from proteoglycan core proteins and degrade them to small oligosaccharides. Inside cells, these enzymes are important for the normal catabolism of heparan sulfate proteoglycans (HSPGs), generating glycosaminoglycan fragments that are then transported to lysosomes and completely degraded. When secreted, heparanases are thought to degrade basement membrane HSPGs at sites of injury or inflammation, allowing extravasion of immune cells into nonvascular spaces and releasing factors that regulate cell proliferation and angiogenesis. Heparanases have been described in a wide variety of tissues and cells, but because of difficulties in developing simple assays to follow activity, very little has been known about enzyme diversity until recently. Within the last 10 years, heparanases have been purified from platelets, placenta, and Chinese hamster ovary cells. Characterization of the enzymes suggests there may be a family of heparanase proteins with different substrate specificities and potential functions.

Genetic dissection of proteoglycan function in Drosophila and C. elegans

Genetic analysis of the signaling pathways that govern patterning during development in the fruitfly Drosophila melanogaster and in the nematode C. elegans have provided insight into the in vivo functions of proteoglycans and their associated glycosaminoglycans. These studies have shown that patterning events dictated by Fibroblast Growth Factor Receptors, Wnt, Transforming Growth Factor- beta(TGF- beta), and Hedgehog families of growth factors are regulated by proteoglycans. Recent biochemical and structural analyses have shown that the molecular machinery of glycosaminoglycan biosynthesis is highly conserved between these invertebrate organisms and mammals. Drosophila and C. elegans therefore provide powerful model systems for exploring the varied functions proteoglycans and their glycosaminoglycan modifications.

Multiple isozymes of heparan sulfate/heparin GlcNAc N-deacetylase/GlcN N-sulfotransferase. Structure and activity of the fourth member, NDST4

We report the cloning and partial characterization of the fourth member of the vertebrate heparan sulfate/heparin: GlcNAc N-deacetylase/GlcN N-sulfotransferase family, which we designate NDST4. Full-length cDNA clones containing the entire coding region of 872 amino acids were obtained from human and mouse cDNA libraries. The deduced amino acid sequence of NDST4 showed high sequence identity to NDST1, NDST2, and NDST3 in both species. NDST4 maps to human chromosome 4q25-26, very close to NDST3, located at 4q26-27. These observations, taken together with phylogenetic data, suggest that the four NDSTs evolved from a common ancestral gene, which diverged to give rise to two subtypes, NDST3/4 and NDST1/2. Reverse transcription-polymerase chain reaction analysis of various mouse tissues revealed a restricted pattern of NDST4 mRNA expression when compared with NDST1 and NDST2, which are abundantly and ubiquitously expressed. Comparison of the enzymatic properties of the four murine NDSTs revealed striking differences in N-deacetylation and N-sulfation activities; NDST4 had weak deacetylase activity but high sulfotransferase, whereas NDST3 had the opposite properties. Molecular modeling of the sulfotransferase domains of the murine and human NDSTs showed varying surface charge distributions within the substrate binding cleft, suggesting that the differences in activity may reflect preferences for different substrates. An iterative model of heparan sulfate biosynthesis is suggested in which some NDST isozymes initiate the N-deacetylation and N-sulfation of the chains, whereas others bind to previously modified segments to fill in or extend the section of modified residues.

Glycosaminoglycan sulfation requirements for respiratory syncytial virus infection

Glycosaminoglycans (GAGs) on the surface of cultured cells are important in the first step of efficient respiratory syncytial virus (RSV) infection. We evaluated the importance of sulfation, the major biosynthetic modification of GAGs, using an improved recombinant green fluorescent protein-expressing RSV (rgRSV) to assay infection. Pretreatment of HEp-2 cells with 50 mM sodium chlorate, a selective inhibitor of sulfation, for 48 h prior to inoculation reduced the efficiency of rgRSV infection to 40%. Infection of a CHO mutant cell line deficient in N-sulfation was three times less efficient than infection of the parental CHO cell line, indicating that N-sulfation is important. In contrast, infection of a cell line deficient in 2-O-sulfation was as efficient as infection of the parental cell line, indicating that 2-O-sulfation is not required for RSV infection. Incubating RSV with the purified soluble heparin, the prototype GAG, before inoculation had previously been shown to neutralize its infectivity. Here we tested chemically modified heparin chains that lack their N-, C6-O-, or C2-O-sulfate groups. Only heparin chains lacking the N-sulfate group lost the ability to neutralize infection, confirming that N-sulfation, but not C6-O- or C2-O-sulfation, is important for RSV infection. Analysis of heparin fragments identified the 10-saccharide chain as the minimum size that can neutralize RSV infectivity. Taken together, these results show that, while sulfate modification is important for the ability of GAGs to mediate RSV infection, only certain sulfate groups are required. This specificity indicates that the role of cell surface GAGs in RSV infection is not based on a simple charge interaction between the virus and sulfate groups but instead involves a specific GAG structural configuration that includes N-sulfate and a minimum of 10 saccharide subunits. These elements, in addition to iduronic acid demonstrated previously (L. K. Hallak, P. L. Collins, W. Knudson, and M. E. Peeples, Virology 271:264-275, 2000), partially define cell surface molecules important for RSV infection of cultured cells.

sqv-3, -7, and -8, a set of genes affecting morphogenesis in Caenorhabditis elegans, encode enzymes required for glycosaminoglycan biosynthesis

sqv (squashed vulva) genes comprise a set of eight independent loci in Caenorhabditis elegans required zygotically for the invagination of vulval epithelial cells and maternally for normal oocyte formation and embryogenesis. Sequencing of sqv-3, sqv-7, and sqv-8 suggested a role for the encoded proteins in glycolipid or glycoprotein biosynthesis. Using a combination of in vitro analysis of SQV enzymatic activities, sqv(+)-mediated rescue of vertebrate cell lines, and biochemical characterization of sqv mutants, we show that sqv-3, -7, and -8 all affect the biosynthesis of glycosaminoglycans and therefore compromise the function of one specific class of glycoconjugates, proteoglycans. These findings establish the importance of proteoglycans and their associated glycosaminoglycans in epithelial morphogenesis and patterning during C. elegans development.

Location of the glucuronosyltransferase domain in the heparan sulfate copolymerase EXT1 by analysis of Chinese hamster ovary cell mutants

Heparan sulfate formation occurs by the copolymerization of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc) residues. Recent studies have shown that these reactions are catalyzed by a copolymerase encoded by EXT1 and EXT2, members of the exostosin family of putative tumor suppressors linked to hereditary multiple exostoses. Previously, we identified a collection of Chinese hamster ovary cell mutants (pgsD) that failed to make heparan sulfate (Lidholt, K., Weinke, J. L., Kiser, C. S., Lugemwa, F. N., Bame, K. J., Cheifetz, S., Massagué, J., Lindahl, U., and Esko, J. D. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 2267-2271). Here, we show that pgsD mutants contain mutations that either alter GlcA transferase activity selectively or that affect both GlcNAc and GlcA transferase activities. Expression of EXT1 corrects the deficiencies in the mutants, whereas EXT2 and the related EXT-like cDNAs do not. Analysis of the EXT1 mutant alleles revealed clustered missense mutations in a domain that included a (D/E)X(D/E) motif thought to bind the nucleotide sugar from studies of other transferases. These findings provide insight into the location of the GlcA transferase subdomain of the enzyme and indicate that loss of the GlcA transferase domain may be sufficient to cause hereditary multiple exostoses.

Disruption of gastrulation and heparan sulfate biosynthesis in EXT1-deficient mice

Mutations in the EXT1 gene are responsible for human hereditary multiple exostosis type 1. The Drosophila EXT1 homologue, tout-velu, regulates Hedgehog diffusion and signaling, which play an important role in tissue patterning during both invertebrate and vertebrate development. The EXT1 protein is also required for the biosynthesis of heparan sulfate glycosaminoglycans that bind Hedgehog. In this study, we generated EXT1-deficient mice by gene targeting. EXT1 homozygous mutants fail to gastrulate and generally lack organized mesoderm and extraembryonic tissues, resulting in smaller embryos compared to normal littermates. RT-PCR analysis of markers for visceral endoderm and mesoderm development indicates the delayed and abnormal development of both of these tissues. Immunohistochemical staining revealed a visceral endoderm pattern of Indian hedgehog (Ihh) in wild-type E6.5 embryos. However, in both EXT1-deficient embryos and wild-type embryos treated with heparitinase I, Ihh failed to associate with the cells. The effect of the EXT1 deletion on heparan sulfate formation was tested by HPLC and cellular glycosyltransferase activity assays. Heparan sulfate synthesis was abolished in EXT1 -/- ES cells and decreased to less than 50% in +/- cell lines. These results indicate that EXT1 is essential for both gastrulation and heparan sulfate biosynthesis in early embryonic development.

Adeno-associated virus (AAV)-3-based vectors transduce haematopoietic cells not susceptible to transduction with AAV-2-based vectors

Although adeno-associated virus (AAV)-2 has a broad tissue-host range and can transduce a wide variety of tissue types, some cells, such as erythro-megakaryoblastoid cells, are non-permissive and appear to lack the AAV-2 receptor. However, limited studies have been reported with the related dependovirus AAV-3. We have previously cloned this virus, characterized its genome and produced an infectious clone. In this study, the gene for green fluorescent protein (GFP) was inserted into AAV-2- and AAV-3-based plasmids and recombinant viruses were produced. These viruses were then used to transduce haematopoietic cells and the transduction efficiencies were compared. In contrast to recombinant (r) AAV-2, rAAV-3 successfully transduced erythroid and megakaryoblastoid cells, although rAAV-2 was superior in transduction of lymphocyte-derived cell lines. Recently, it was reported that heparan sulphate can act as a receptor of AAV-2. The infectivity of rAAV-2 and rAAV-3 was tested with mutant cell lines of Chinese hamster ovary cells that were defective for heparin or heparan sulphate expression on the cell surface. There was no correlation between the ability of rAAV-2 or rAAV-3 to infect cells and the cell surface expression of heparan sulphate and, although heparin blocked both rAAV-2 and rAAV-3 transduction, the ID(50) of rAAV-3 was higher than that of rAAV-2. In addition, virus-binding overlay assays indicated that AAV-2 and AAV-3 bound different membrane proteins. These results suggest not only that there are different cellular receptors for AAV-2 and AAV-3, but that rAAV-3 vectors may be preferred for transduction of some haematopoietic cell types.

Structural analysis of glycosaminoglycans in animals bearing mutations in sugarless, sulfateless, and tout-velu. Drosophila homologues of vertebrate genes encoding glycosaminoglycan biosynthetic enzymes

Mutations that disrupt developmental patterning in Drosophila have provided considerable information about growth factor signaling mechanisms. Three genes recently demonstrated to affect signaling by members of the Wnt, transforming growth factor-beta, Hedgehog, and fibroblast growth factor families in Drosophila encode proteins with homology to vertebrate enzymes involved in glycosaminoglycan synthesis. We report here the biochemical characterization of glycosaminoglycans in Drosophila bearing mutations in sugarless, sulfateless, and tout-velu. We find that mutations in sugarless, which encodes a protein with homology to UDP-glucose dehydrogenase, compromise the synthesis of both chondroitin and heparan sulfate, as would be predicted from a defect in UDP-glucuronate production. Defects in sulfateless, a gene encoding a protein with similarity to vertebrate N-deacetylase/N-sulfotransferases, do not affect chondroitin sulfate levels or composition but dramatically alter the composition of heparin lyase-released disaccharides. N-, 6-O-, and 2-O-sulfated disaccharides are absent and replaced entirely with an unsulfated disaccharide. A mutation in tout-velu, a gene related to the vertebrate Exostoses 1 heparan sulfate co-polymerase, likewise does not affect chondroitin sulfate synthesis but reduces all forms of heparan sulfate to below the limit of detection. These findings show that sugarless, sulfateless, and tout-velu affect glycosaminoglycan biosynthesis and demonstrate the utility of Drosophila as a model organism for studying the function and biosynthesis of glycosaminoglycans in vivo.

Exploring the glycan repertoire of genetically modified mice by isolation and profiling of the major glycan classes and nano-NMR analysis of glycan mixtures

The production of mice with genetic alterations in glycosyltransferases has highlighted the need to isolate and study complex mixtures of the major classes of oligosaccharides (glycans) from intact tissues. We have found that nano-NMR spectroscopy of whole mixtures of N- and O-glycans can complement HPLC profiling methods for elucidating structural details. Working toward obtaining such glycan mixtures from mouse tissues, we decided to develop an approach to isolate not only N- and O-glycans, but also to separate out glycosphingolipids, glycosaminoglycans and glycosylphosphatidylinositol anchors. We describe here a comprehensive Glycan Isolation Protocol that is based primarily upon the physicochemical characteristics of the molecules, and requires only commonly available reagents and equipment. Using radiolabeled internal tracers, we show that recovery of each major class of glycans is as good or better than with conventional approaches for isolating individual classes, and that cross-contamination is minimal. The recovered glycans are of sufficient purity to provide a "glycoprofile" of a cell type or tissue. We applied this approach to compare the N- and O-glycans from wild type mouse tissues with those from mice genetically deficient in glycosyltransferases. N- and O-glycan mixtures from organs of mice deficient in ST6Gal-I (CMP-Sia:Galbeta1-4GlcNAc alpha2-6 sialyltransferase) were studied by the nano-NMR spectroscopy approach, showing no detectable alpha2-6-linked sialic acids. Thus, ST6Gal-I is likely responsible for generating most or all of these residues in normal mice. Similar studies indicate that this linkage is very rare in ganglioside glycans, even in wild-type tissues. In mice deficient in GalNAcT-8 (UDP-GalNAc:polypeptide O-Ser/Thr GalNAc transferase 8), HPLC profiling indicates that O-glycans persist in the thymus in large amounts, without a major change in overall profile, suggesting that other enzymes can synthesize the GalNAc-O-Ser/Thr linkage in this tissue. These results demonstrate the applicability of nano-NMR spectroscopy to complex glycan mixtures, as well as the versatility of the Glycan Isolation Protocol, which makes possible the concurrent examination of multiple glycan classes from intact vertebrate tissues.

The spacing of S-domains on HS glycosaminoglycans determines whether the chain is a substrate for intracellular heparanases

Heparanases are mammalian endoglucuronidases that degrade heparan sulfate (HS) glycosaminoglycans to short 5-6 kDa pieces. In the Golgi, HS glycosaminoglycans are modified by a series of interdependent reactions which result in chains that have regions rich in N- and O-sulfate groups and iduronate residues (S-domains), separated by regions that are nearly devoid of sulfate. Structural analysis of the short HS chains produced by Chinese hamster ovary (CHO) cell heparanases indicate that the enzymes recognize differences in sulfate content between S-domains and unmodified sequences, and cleave the chain at junctions between these regions. To look more closely at whether the spacing of S-domains on the gly- cosaminoglycan influences its ability to be cleaved by heparanases, we examined the susceptibility of the HS chains synthesized by the proteoglycan synthesis mutant, pgsE-606. PGS:E-606 cells are deficient in the modification enzyme N-deacetylase/N-sulfotransferase I, and synthesize HS chains that have fewer N- and O-sulfate groups and iduronate residues compared to wild-type (Bame et al., (1991), J. Biol. Chem., 266, 10287). HS glycosaminoglycans were isolated from wild-type and pgsE-606 cells and separated into populations based on sulfate content. Compared to wild-type HS, which has 14 S-domains, pgsE-606 cells synthesize three HS species, 606-1, 606-2, and 606-3, with 1, 4, and 8 S-domains, respectively. The spacing of the S-domains on the pgsE-606 HS chains is similar to the spacing the modified sequences on wild-type HS, indicating that each mutant glycosaminoglycan is composed of wild-type-like sequences and sequences devoid of S-domains. When incubated with partially purified CHO heparanases, only the portion of the mutant HS chains that had S-domains were degraded. Structural analysis of the heparanase-products confirmed that both the number and the arrangement of S-domains on the HS glycosaminoglycan are important for heparanase susceptibility. The structure of the different pgsE-606 HS chains also suggests mechanisms for the placement of S-domains when the gly- cosaminoglycan is synthesized.

The interaction of heparin sulfate and adeno-associated virus 2

Recently heparan sulfate was proposed as the host cell receptor for the dependovirus, adeno-associated virus type 2 (AAV2). We show that although heparan sulfate on the cell surface may contribute to the binding of AAV2 to permissive cells, the amount of heparan sulfate on the cell surface as determined by flow cytometry using four different monoclonal antibodies does not correlate with AAV2 binding to cells or recombinant AAV2 transduction efficiency. Experiments with either mutant CHO cells or cells treated with chlorate to remove sulfate groups showed that sulfation was not absolutely required for infection or binding: in the absence of cell surface sulfation, recombinant AAV2 was still able to be transduced in previously permissive cells. Heparin is commonly used as a substitute in studies of the interaction between heparan sulfate and ligand, and we demonstrate that the binding affinity of AAV2/heparin is low, with a K(d) value of approximately 2.0 nM. A study of the direct interaction between AAV2 and artificial glycosaminoglycans showed that a high degree of sulfation on heparin was critical for the ability to bind AAV2 and compete rAAV2 transduction and that both O- and N-sulfate groups are required. Overall, our data suggest that, as has been shown for other viruses, the presence of a high-affinity AAV2 receptor mediates AAV2 infection in addition to the low-affinity heparan sulfate binding.

Differential ability of heparan sulfate proteoglycans to assemble the fibroblast growth factor receptor complex in situ

Fibroblast growth factors (FGFs) require heparan sulfate proteoglycans (HSPGs) as cofactors for signaling. The heparan sulfate chains (HS) mediate stable high affinity binding of FGFs to their receptor tyrosine kinases (FR) and may specifically regulate FGF activity. A novel in situ binding assay was developed to examine the ability of HSPGs to promote FGF/FR binding using a soluble FR fusion construct (FR1-AP). This fusion protein probe forms a dimer in solution, simulating the dimerization or oligomerization that is thought to occur at the cell surface physiologically. In frozen sections of human skin, FGF-2 binds to keratinocytes and basement membranes of epidermis and dermal blood vessels. In contrast, in skin preincubated with FGF-2, FR1-AP binds avidly to FGF-2 immobilized on keratinocyte cell surfaces, but fails to bind to basement membranes at the dermo-epidermal junction or dermal microvessels despite the fact that these structures bind large amounts of FGF-2. Apparently, basement membrane and cell surface HSPGs differ in their ability to mediate the assembly of a FGF/FR signaling complex presumably due to structural differences of the heparan sulfate chains.

A novel role for 3-O-sulfated heparan sulfate in herpes simplex virus 1 entry

Herpes simplex virus type 1 (HSV-1) binds to cells through interactions of viral glycoproteins gB and gC with heparan sulfate chains on cell surface proteoglycans. This binding is not sufficient for viral entry, which requires fusion between the viral envelope and cell membrane. Here, we show that heparan sulfate modified by a subset of the multiple D-glucosaminyl 3-O-sulfotransferase isoforms provides sites for the binding of a third viral glycoprotein, gD, and for initiation of HSV-1 entry. We conclude that susceptibility of cells to HSV-1 entry depends on (1) presence of heparan sulfate chains to which virus can bind and (2) 3-O-sulfation of specific glucosamine residues in heparan sulfate to generate gD-binding sites or the expression of other previously identified gD-binding receptors.

Efficient infection of mature skeletal muscle with herpes simplex virus vectors by using dextran sulfate as a co-receptor

The use of herpes simplex virus (HSV) vectors for gene delivery to skeletal muscle is hampered by a maturation-dependent loss of muscle fiber infectivity. Previous studies of HSV type 1 (HSV-1) infection in the rodent show that the loss of infectivity may be due, at least in part, to the development of the basal lamina throughout the course of maturation, which may block the initial events in HSV infection. To initiate infection, HSV normally attaches to cell surface heparan sulfate, which stabilizes the virus such that it can interact with secondary protein receptors required for entry into host cells. In this study, we demonstrate that heparan sulfate biosynthesis is downregulated during skeletal muscle maturation. When myofibers were treated with a variety of enzymes, including collagenase type IV or chondroitin ABC lyase, HSV infection was restored, which suggests that virus secondary receptors were present but not readily accessible to the virus in the intact myofiber. Surprisingly, we also found that HSV-1 infectivity could be restored in vitro and in vivo by exposing myofibers to low concentrations of the glycosaminoglycan analog dextran sulfate, which appears to act as a surrogate receptor to stabilize the virus at the myofiber surface such that HSV can engage additional receptors. This demonstration that the basal lamina is not an absolute block to HSV-1 infection is remarkable because it allows for the nondestructive targeting of HSV-1 to mature myofibers and greatly expands the usefulness of HSV as a gene therapy vector for the treatment of inherited and acquired diseases.

Human homolog of Caenorhabditis elegans sqv-3 gene is galactosyltransferase I involved in the biosynthesis of the glycosaminoglycan-protein linkage region of proteoglycans

A cDNA encoding a novel galactosyltransferase was identified based on BLAST analysis of expressed sequence tags, and the cDNA clones were isolated from a human melanoma line library. The new cDNA sequence encoded a type II membrane protein with 327 amino acid sequence and showed 38% homology to the Caenorhabditis elegans sqv-3 gene involved in the vulval invagination and oocyte development. Extracts from L cells transfected with the galactosyltransferase cDNA in an expression vector and a fusion protein with protein A exhibited marked galactosyltransferase activity specific for p-nitrophenyl-beta-D-xylopyranoside. Moreover, transfection with the cloned cDNA restored glycosaminoglycan synthesis of galactosyltransferase I-deficient Chinese hamster ovary mutant pgsB-761 cells. Analysis of the enzyme product by beta-galactosidase digestion, mass spectroscopy, and NMR spectroscopy revealed that the reaction product was formed via beta-1,4 linkage, indicating that the enzyme is galactosyltransferase I (UDP-galactose:O-beta-D-xylosylprotein 4-beta-D-galactosyltransferase, EC 2.4.1.133) involved in the synthesis of the glycosaminoglycan-protein linkage region of proteoglycans.

Chinese hamster ovary cell mutants defective in glycosaminoglycan assembly and glucuronosyltransferase I

The proteoglycans of animal cells typically contain one or more heparan sulfate or chondroitin sulfate chains. These glycosaminoglycans assemble on a tetrasaccharide primer, -GlcAbeta1, 3Galbeta1,3Galbeta1,4Xylbeta-O-, attached to specific serine residues in the core protein. Studies of Chinese hamster ovary cell mutants defective in the first or second enzymes of the pathway (xylosyltransferase and galactosyltransferase I) show that the assembly of the primer occurs by sequential transfer of single monosaccharide residues from the corresponding high energy nucleotide sugar donor to the non-reducing end of the growing chain. In order to study the other reactions involved in linkage tetrasaccharide assembly, we have devised a powerful selection method based on induced resistance to a mitotoxin composed of basic fibroblast growth factor-saporin. One class of mutants does not incorporate 35SO4 and [6-3H]GlcN into glycosaminoglycan chains. Incubation of these cells with naphthol-beta-D-xyloside (Xylbeta-O-Np) resulted in accumulation of linkage region intermediates containing 1 or 2 mol of galactose (Galbeta1, 4Xylbeta-O-Np and Galbeta1, 3Galbeta1, 4Xylbeta-O-Np) and sialic acid (Siaalpha2,3Galbeta1, 3Galbeta1, 4Xylbeta-O-Np) but not any GlcA-containing oligosaccharides. Extracts of the mutants completely lacked UDP-glucuronic acid:Galbeta1,3Gal-R glucuronosyltransferase (GlcAT-I) activity, as measured by the transfer of GlcA from UDP-GlcA to Galbeta1,3Galbeta-O-naphthalenemethanol (<0.2 versus 3.6 pmol/min/mg). The mutation most likely lies in the structural gene encoding GlcAT-I since transfection of the mutant with a cDNA for GlcAT-I completely restored enzyme activity and glycosaminoglycan synthesis. These findings suggest that a single GlcAT effects the biosynthesis of common linkage region of both heparan sulfate and chondroitin sulfate in Chinese hamster ovary cells.

Formation of HNK-1 determinants and the glycosaminoglycan tetrasaccharide linkage region by UDP-GlcUA:Galactose beta1, 3-glucuronosyltransferases

While expression-cloning enzymes involved in heparan sulfate biosynthesis, we isolated a cDNA that encodes a protein 65% identical to the UDP-GlcUA:glycoprotein beta1, 3-glucuronosyltransferase (GlcUAT-P) involved in forming HNK-1 carbohydrate epitopes (3OSO3GlcUAbeta1,3Gal-) on glycoproteins. The cDNA contains an open reading frame coding for a protein of 335 amino acids with a predicted type II transmembrane protein orientation. Cotransfection of the cDNA with HNK-1 3-O-sulfotransferase produced HNK-1 carbohydrate epitopes in Chinese hamster ovary (CHO) cells and COS-7 cells. In vitro, a soluble recombinant form of the enzyme transferred GlcUA in beta-linkage to Galbeta1,3/4GlcNAcbeta-O-naphthalenemethanol, which resembles the core oligosaccharide on which the HNK-1 epitope is assembled. However, the enzyme greatly preferred Galbeta1, 3Galbeta-O-naphthalenemethanol, a disaccharide component found in the linkage region tetrasaccharide in chondroitin sulfate and heparan sulfate. During the course of this study, a human cDNA clone was described that was thought to encode UDP-GlcUA:Galbeta1,3Gal-R glucuronosyltransferase (GlcUAT-I), involved in the formation of the linkage region of glycosaminoglycans (Kitagawa, H., Tone, Y., Tamura, J., Neumann, K. W., Ogawa, T., Oka, S., Kawasaki, T., and Sugahara, K. (1998) J. Biol. Chem. 273, 6615-6618). The deduced amino acid sequences of the CHO and human cDNAs are 95% identical, suggesting that they are in fact homologues of the same gene. Transfection of a CHO cell mutant defective in GlcUAT-I with the hamster cDNA restored glycosaminoglycan assembly in vivo, confirming its identity. Interestingly, transfection of the mutant with GlcUAT-P also restored glycosaminoglycan synthesis. Thus, both GlcUAT-P and GlcUAT-I have overlapping substrate specificities. However, the expression of the two genes was entirely different, with GlcUAT-I expressed in all tissues tested and GlcUAT-P expressed only in brain. These findings suggest that, in neural tissues, GlcUAT-P may participate in both HNK-1 and glycosaminoglycan production.

Molecular cloning and expression of a third member of the heparan sulfate/heparin GlcNAc N-deacetylase/ N-sulfotransferase family

N-Deacetylation and N-sulfation of N-acetylglucosamine residues in heparan sulfate and heparin initiate a series of chemical modifications that ultimately lead to oligosaccharide sequences with specific ligand binding properties. These reactions are catalyzed by GlcNAc N-deacetylase/N-sulfotransferase (NDST), a monomeric enzyme with two catalytic activities. Two genes encoding NDST isozymes have been described, one from rat liver (NDST1) and another from murine mastocytoma (NDST2). Both isozymes are expressed in tissues in varying amounts, but their relative contribution to heparan sulfate formation in any one tissue is unknown. We now report the identification of a third member of the NDST family, designated NDST3. A full-length cDNA clone (3.2 kilobase pairs) encoding a 873-amino acid protein was obtained from a human fetal/infant brain cDNA library. Human NDST3 (hNDST3) has a nucleotide sequence homologous but not identical to hNDST1 and NDST2. The deduced amino acid sequence shows 70% and 65% amino acid identity to that of hNDST1 and NDST2, respectively. A soluble chimera of hNDST3 and protein A exhibited both N-deacetylase and N-sulfotransferase activity, confirming its enzymatic identity. Northern blot analysis of human fetal brain poly(A)+ RNA showed a single transcript of 6.4 kilobase pairs. Reverse transcription polymerase chain reaction analysis revealed more restricted tissue expression of hNDST3 than hNDST1 and NDST2, and high levels in brain, liver, and kidney. Analysis of Chinese hamster ovary cells revealed expression of NDST1 and NDST2, but not NDST3. In a Chinese hamster ovary cell mutant exhibiting reduced N-sulfotransferase activity and reduced sulfation of heparan sulfate (Bame, K. J., and Esko, J. D. (1989) J. Biol. Chem. 264, 8059-8065), expression of NDST1 was greatly reduced, but NDST2 was expressed normally, suggesting that both enzymes are involved in heparan sulfate assembly. The discovery of multiple NDST isozymes suggests that the assembly of heparan sulfate is much complicated than previously appreciated.

A Toxoplasma lectin-like activity specific for sulfated polysaccharides is involved in host cell infection

Toxoplasma gondii is one of the most widespread parasites of humans and animals. The parasite has a remarkable ability to invade a broad range of cells within its mammalian hosts by mechanisms that are poorly understood at the molecular level. This broad host cell specificity suggests that adhesion should involve the recognition of ubiquitous surface-exposed host molecules or, alternatively, the presence of various parasite attachment molecules able to recognize different host cell receptors. We have discovered a sugar-binding activity (lectin) in tachyzoites of T. gondii that plays a role in vitro in erythrocyte agglutination and infection of human fibroblasts and epithelial cells. The ability to agglutinate erythrocytes can be reversed by a variety of soluble glycoconjugates, of which heparin, fucoidan, and dextran sulfate were the most effective. Interestingly, infectivity of tachyzoites for human foreskin fibroblasts, cells that are commonly used to grow T. gondii in vitro, was increased by low concentrations of the sulfated glycoconjugates that inhibited hemagglutination activity (i.e. dextran sulfate and fucoidan) whereas high concentrations inhibited parasite infection. Furthermore, inhibition of glycosaminoglycan biosynthesis and sulfation on the host cells reduced Toxoplasma infectivity. Finally, Toxoplasma tachyzoites showed a reduced ability to infect epithelial cell mutants deficient in the biosynthesis of surface proteoglycans. The probable identity of the hemagglutinin(s) was investigated by 1) direct binding of red blood cells to filter blots of Toxoplasma proteins separated by polyacrylamide gel electrophoresis, and 2) binding of metabolically labeled parasite proteins to fixed mammalian cells. Three parasite bands were thus identified as candidate adhesins. These results suggest that attachment of T. gondii to its target cell is mediated by parasite lectins and that sulfated sugars on the surface of host cells may function as a key parasite receptor.

The retinoic acid and cAMP-dependent up-regulation of 3-O-sulfotransferase-1 leads to a dramatic augmentation of anticoagulantly active heparan sulfate biosynthesis in F9 embryonal carcinoma cells

Retinoic acid (RA) and dibutyryl cAMP plus theophilline (CT) trigger F9 cells to differentiate into parietal endoderm. The differentiation induces a 9-fold increase in total heparan sulfate (HStotal) biosynthesis and a 170-fold increase in anticoagulantly active HS (HSact) biosynthesis. Measurement of 3-O-sulfotransferase-1 mRNA and enzymatic activity demonstrated an increase of over 100-fold whereas determination of N-, 2-O, and 6-O-sulfotransferase enzymatic activities showed elevations of 2-, 3. 5-, and 3.7-fold, respectively. HSact precursor pool measurements reveal that 30% of control F9 HStotal can be converted into HSact while only an additional 10% of RACT F9 HStotal can be transformed into HSact. Disaccharide analysis of metabolic labeled HS indicated that 32% 3-O-sulfate containing disaccharides, i.e. GlcA-anManR3S and GlcA-anManR3S6S, are present in HSact and 68% GlcA-anManR3S and GlcA-anManR3S6S are found in anticoagulantly inactive HS (HSinact). By using adenosine 3'-phosphate 5'-phosphosulfate and purified 3-O-sulfotransferase-1, 30% of 3-O-sulfation occurs in HSact and 70% of 3-O-sulfation occurs in HSinact. The similar ratio of 3-O-sulfate distribution in HSact versus HSinact suggests that HSact production in the F9 system is determined by the abundance of 3-O-sulfotransferase-1 as well as the size of the HSact precursor pool. Extensively 3-O-sulfated HSinact may play an important functional role under in vivo conditions within the murine placenta.

Anosmin-1 underlying the X chromosome-linked Kallmann syndrome is an adhesion molecule that can modulate neurite growth in a cell-type specific manner

Anosmin-1 is an extracellular matrix glycoprotein which underlies the X chromosome-linked form of Kallmann syndrome. This disease is characterized by hypogonadism due to GnRH deficiency, and a defective sense of smell related to the underdevelopment of the olfactory bulbs. This study reports that anosmin-1 is an adhesion molecule for a variety of neuronal and non-neuronal cell types in vitro. We show that cell adhesion to anosmin-1 is dependent on the presence of heparan sulfate and chondroitin sulfate glycosaminoglycans at the cell surface. A major cell adhesion site of anosmin-1 was identified in a 32 amino acid (32R1) sequence located within the first fibronectin-like type III repeat of the protein. The role of anosmin-1 as a substrate for neurite growth was tested on either coated culture dishes or monolayers of anosmin-1-producing CHO cells. In both experimental systems, anosmin-1 was shown to be a permissive substrate for the neurite growth of different types of neurons. Mouse P5 cerebellar neurons cultured on anosmin-1 coated wells developed long neurites; the 32R1 peptide was found to underly part of this neurite growth activity. When the cerebellar neurons were cultured on anosmin-1-producing CHO cells, neurite growth was reduced as compared to wild-type CHO cells; in contrast, no difference was observed for E18 hippocampal and P1 dorsal root ganglion neurons in the same experimental system. These results indicate that anosmin-1 can modulate neurite growth in a cell-type specific manner. Finally, anosmin-1 induced neurite fasciculation of P5 cerebellar neuron aggregates cultured on anosmin-1-producing CHO cells. The pathogenesis of the olfactory defect in the X-linked Kallmann syndrome is discussed in the light of the present results and the recent data reporting the immunohistochemical localisation of anosmin-1 during early embryonic development.

Primers of glycosaminoglycan biosynthesis from Peruvian rain forest plants

We have developed a rapid, high throughput screening assay for compounds that alter the assembly of glycosaminoglycan chains in Chinese hamster ovary cells. The assay uses autoradiography to measure the binding of newly synthesized [35S]proteoglycans and [35S]glycosaminoglycans to a positively charged membrane. Screening over 1000 extracts from a random plant collection obtained from the Amazon rain forest yielded five plants that stimulated glycosaminoglycan assembly in both wild-type cells and a mutant cell line defective in xylosyltransferase (the first committed enzyme involved in glycosaminoglycan biosynthesis). Fractionation of an extract of Maieta guianensis by silica gel and reverse-phase chromatography yielded two pure compounds with stimulatory activity. Spectroscopic analysis by NMR and mass spectrometry revealed that the active principles were xylosides of dimethylated ellagic acid. One of the compounds also contained a galloyl group at C-3 of the xylose moiety. These findings suggest that plants and other natural products may be a source of agents that can potentially alter glycosaminoglycan and proteoglycan formation in animal cells.

The putative tumour suppressor EXT1 alters the expression of cell-surface heparan sulfate

Hereditary multiple exostoses (HME) is an autosomal dominant disorder characterized by the formation of cartilage-capped tumours (exostoses) that develop from the growth plate of endochondral bone. This condition can lead to skeletal abnormalities, short stature and malignant transformation of exostoses to chondrosarcomas or osteosarcomas. Linkage analyses have identified three different genes for HME, EXT1 on 8q24.1, EXT2 on 11p11-13 and EXT3 on 19p (refs 6-9). Most HME cases have been attributed to missense or frameshift mutations in these tumour-supressor genes, whose functions have remained obscure. Here, we show that EXT1 is an ER-resident type II transmembrane glycoprotein whose expression in cells results in the alteration of the synthesis and display of cell surface heparan sulfate glycosaminoglycans (GAGs). Two EXT1 variants containing aetiologic missense mutations failed to alter cell-surface glycosaminoglycans, despite retaining their ER-localization.

Membrane-associated heparan sulfate proteoglycan is a receptor for adeno-associated virus type 2 virions

The human parvovirus adeno-associated virus (AAV) infects a broad range of cell types, including human, nonhuman primate, canine, murine, and avian. Although little is known about the initial events of virus infection, AAV is currently being developed as a vector for human gene therapy. Using defined mutant CHO cell lines and standard biochemical assays, we demonstrate that heparan sulfate proteoglycans mediate both AAV attachment to and infection of target cells. Competition experiments using heparin, a soluble receptor analog, demonstrated dose-dependent inhibition of AAV attachment and infection. Enzymatic removal of heparan but not chondroitin sulfate moieties from the cell surface greatly reduced AAV attachment and infectivity. Finally, mutant cell lines that do not produce heparan sulfate proteoglycans were significantly impaired for both AAV binding and infection. This is the first report that proteoglycan has a role in cellular attachment of a parvovirus. Together, these results demonstrate that membrane-associated heparan sulfate proteoglycan serves as the viral receptor for AAV type 2, and provide an explanation for the broad host range of AAV. Identification of heparan sulfate proteoglycan as a viral receptor should facilitate development of new reagents for virus purification and provide critical information on the use of AAV as a gene therapy vector.

Dengue virus infectivity depends on envelope protein binding to target cell heparan sulfate

Dengue virus is a human pathogen that has reemerged as an increasingly important public health threat. We found that the cellular receptor utilized by dengue envelope protein to bind to target cells is a highly sulfated type of heparan sulfate. Heparin, highly sulfated heparan sulfate, and the polysulfonate pharmaceutical Suramin effectively prevented dengue virus infection of target cells, indicating that the envelope protein-target cell receptor interaction is a critical determinant of infectivity. The dengue envelope protein sequence includes two putative glycosaminoglycan-binding motifs at the carboxy terminus; the first could be structurally modeled and formed an unusual extended binding surface of basic amino acids. Similar motifs were also identified in the envelope proteins of other flaviviridae. Developing pharmaceuticals that inhibit target cell binding may be an effective strategy for treating flavivirus infections.

Cell adhesion to a motif shared by the malaria circumsporozoite protein and thrombospondin is mediated by its glycosaminoglycan-binding region and not by CSVTCG

The malaria circumsporozoite protein (CS), thrombospondin (TSP), and several other proteins including the terminal complement proteins and the neural adhesion molecules F-spondin and Unc-5, share a cell adhesive sequence. In CS this sequence is designated as region II-plus (EWSPCSVTCGNGIQVRIK) and in TSP it is found in the type I repeats. Previous studies aimed at fine mapping the amino acid residues required for cell adhesion have yielded discrepant results. Here we show in three different cell lines that the downstream basic residues are required for cell adhesion whereas the CSVTCG sequence is not. Using mutant Chinese hamster ovary cells selected for deficiencies in proteoglycan synthesis, we show that in wild type cells, heparan sulfate proteoglycans are the binding sites for this motif. This finding is supported by additional experiments with two other cell lines demonstrating that treatment with heparitinase but not chondroitinase abolishes cell adhesion to peptides representing this motif. Using Chinese hamster ovary cell mutants deficient in heparan sulfate proteoglycans but possessing chondroitin sulfate proteoglycans, we show that cell surface chondroitin sulfate proteoglycans can also mediate binding to this motif although higher concentrations of peptides are required for adhesion. Chondroitinase, but not heparitinase, treatment of these cells destroys cell surface-binding sites. Taken together, these results indicate that cell adhesion to this motif involves an interaction between the downstream positively-charged residues and the negatively charged glycosaminoglycan chains of heparan sulfate, or in some cases chondroitin sulfate, proteoglycans on the cell surface.

Abeta(1-40) prevents heparanase-catalyzed degradation of heparan sulfate glycosaminoglycans and proteoglycans in vitro. A role for heparan sulfate proteoglycan turnover in Alzheimer's disease

Alzheimer's disease is characterized by senile plaques composed of polymeric fibrils of beta amyloid (Abeta), a 39-42-amino acid peptide formed after proteolytic processing of the amyloid precursor protein (betaAPP). Heparan sulfate proteoglycans have been shown to colocalize with Abeta in Alzheimer's disease brain, and experimental evidence indicates that the interactions between the proteoglycan and the peptide are important for the promotion, deposition, and/or persistence of the senile plaques. Studies in rat brain indicated that both the core protein and the heparan sulfate glycosaminoglycan chains are required for amyloid fiber formation and deposition in vivo (Snow, A. D., Sekiguchi, R., Nochlin, D., Fraser, P., Kimata, K. , Mizutani, A., Arai, M., Schreier, W. A., and Morgan, D. G. (1994) Neuron 12, 219-234), suggesting that one mechanism to prevent the formation of Abeta-heparan sulfate proteoglycan complexes that lead to deposition of amyloid would be to degrade the proteoglycan. Normally, heparan sulfate proteoglycans are internalized and degraded to short glycosaminoglycans by intracellular heparanases. These reactions occur in the endosomal-lysosomal pathway, which is the same intracellular location where betaAPP is processed to Abeta. Using partially purified heparanase activities from Chinese hamster ovary cells we examined whether Abeta(1-40) affects the catabolism of Chinese hamster ovary heparan sulfate glycosaminoglycans and proteoglycans in vitro. Abeta(1-40) binds to both the long heparan sulfate glycosaminoglycans attached to core proteins and the short, heparanase-derived chains in a concentration-dependent and pH-dependent manner. When Abeta(1-40) is added to heparanase assays, it prevents the partially purified activities from releasing heparan sulfate chains from core proteins and degrading them to short glycosaminoglycans; however, a large molar excess of the peptide to heparan sulfate is required to see the effect. Our results suggest that normally the levels of Abeta in the endosomal pathway are not sufficient to interfere with heparanase activity in vivo. However, once the level of Abeta-peptides are elevated, as they are in Alzheimer's disease, they could interact with heparan sulfate proteoglycans and prevent their catabolism. This could promote the formation and deposition of amyloid, since the binding of Abeta to the proteoglycan species will predominate.

Glycosaminoglycans modulate fibronectin matrix assembly and are essential for matrix incorporation of tenascin-C

We have investigated the role of glycosaminoglycans in fibronectin matrix assembly and the incorporation of tenascin-C into matrix fibrils. Chinese hamster ovary cell mutants with a total block in heparan and chondroitin sulfate production failed to assemble a fibronectin matrix, and incorporated no tenascin-C. Another mutant with reduced heparan sulfate produced a normal fibronectin matrix but failed to incorporate tenascin-C. Excess soluble glycosaminoglycans inhibited the binding of tenascin-C to purified fibronectin in ELISA, and completely blocked incorporation into matrix fibrils. Treating cultured cells with xyloside, which interferes with glycosaminoglycan attachment to proteoglycans, also completely blocked their ability to incorporate tenascin-C into matrix fibrils. We conclude that proteoglycans bound to fibronectin fibrils play a major role in binding tenascin-C to these fibrils. We examined more closely the large heparan sulfate proteoglycan, perlecan, and found that it co-localizes with tenascin-C and fibronectin in the matrix. The perlecan binding site in tenascin-C was mapped to the fibronectin type III domains 3–5, but this binding was strongly enhanced for the small splice variant, which is the major form incorporated into the matrix. Apparently when the alternative splice segment is inserted after domain 5 it inhibits perlecan binding. Thus heparan sulfate glycosaminoglycans, and perlecan in particular, may play a role in incorporation of the small splice variant of tenascin-C into fibronectin matrix fibrils.

Role of the extracellular domain of human herpesvirus 7 glycoprotein B in virus binding to cell surface heparan sulfate proteoglycans

In an attempt to identify the human herpesvirus 7 (HHV-7) envelope protein(s) involved in cell surface binding, the extracellular domain of the HHV-7 glycoprotein B (gB) homolog protein was cloned and expressed as a fusion product with the Fc domain of human immunoglobulin G heavy chain gamma1 (gB-Fc) in an eukaryotic cell system. Indirect immunofluorescence followed by flow cytometric analysis revealed specific binding of gB-Fc to the membrane of SupT1 cells but not to other CD4+ T-lymphoblastoid cell lines, such as Jurkat or PM1, clearly indicating that gB-Fc did not bind to the CD4 molecule. This was also suggested by the ability of gB-Fc to bind to CD4-negative fibroblastoid Chinese hamster ovary (CHO) cells. The binding was abrogated by enzymatic removal of cell surface heparan sulfate proteoglycans by heparinase and heparitinase but not by treatment with condroitinase ABC. In addition, binding of the gB-Fc fusion protein to CHO cells was severely impaired in the presence of soluble heparin, as well as when heparan sulfate-deficient mutant CHO cells were used. Consistent with these findings, soluble heparin was found to block HHV-7 infection and syncytium formation in the SupT1 cell line. Although the CD4 antigen is a critical component of the receptor for the T-lymphotropic HHV-7, these findings suggest that heparin-like molecules also play an important role in HHV-7-cell surface interactions required for infection and that gB represents one of the HHV-7 envelope proteins involved in the adsorption of virus-to-cell surface proteoglycans.

Purification of heparan sulfate D-glucosaminyl 3-O-sulfotransferase

The cellular generation of proteoglycans with anticoagulant heparan sulfate (HSPGact) is determined by microsomal "HSact conversion activity" that functions in concert with the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to convert nonanticoagulant heparan sulfate (HSinact) to anticoagulant heparan sulfate (HSact) (Shworak, N. W., Fritze, L. M. S., Liu, J., Butler, L. D., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27063-27071). Suspension cultures of L-33(+) cells in serum-free medium produce HSPGact and secrete HSact conversion activity. The secreted protein exhibiting HSact conversion activity was isolated by subjecting large volumes of conditioned suspension culture medium to heparin-AF Toyopearl affinity chromatography, Mono Q-FPLC, TSK SW3000-HPLC, and 3',5'-ADP-agarose affinity chromatography. The final product was purified approximately 700,000-fold relative to cellular material with a 5% overall recovery of HSact conversion activity. The isolated protein migrated on SDS-polyacrylamide gel electrophoresis as a broad band of Mr = 46,000 and co-migrated on nondenaturing acidic pH gel electrophoresis with HSact conversion activity. The purified component was identified as heparan sulfate D-glucosaminyl 3-O-sulfotransferase because it transferred sulfate from [35S]PAPS to the 3-O-position of D-glucosamine and D-glucosamine 6-O-sulfate of HSact precursor and HSinact precursor to produce nearly equivalent amounts of labeled HSact and HSinact. The exhaustive modification of wild-type LTA cell [35S]HS with either microsomal HSact conversion activity or purified enzyme increased HSact content from 9 to approximately 36%, which indicates that microsomal HSact conversion activity predominantly reflects the level of a 3-O-sulfotransferase that converts HSact precursor into HSact. The kinetic parameters of purified 3-O-sulfotransferase were determined for modification of HSact precursor and HSinact precursor. The apparent KM* and Vmax* with respect to PAPS concentration for sulfation of HSact precursor and HSinact precursor were 2.4 microM and 23 fmol of sulfate/min/ng of enzyme and 2.1 microM and 38 fmol of sulfate/min/ng of enzyme, respectively. There was substrate inhibition of the sulfation reaction at elevated HS concentration. The apparent KM* and Vmax* with respect to GAG concentration for sulfation of HSact precursor and HSinact precursor were 16 nM and 120 fmol of sulfate/min/ng of enzyme and 17 nM and 240 fmol of sulfate/min/ng of enzyme, respectively. The observation that purified 3-O-sulfotransferase catalyzes sulfation of HSact precursor and HSinact precursor in conjunction with a documented discordant regulation of 3-O-sulfate content in HSinact and HSact suggests that two discrete forms of the enzyme may exist.

Acetylcholine receptor clustering associates with proteoglycan biosynthesis in C2 variant and heterkaryon muscle cells

Several lines of evidence have suggested roles for proteoglycans (PGs) in acetylcholine receptor (AChR) clustering on muscle cells. One line of evidence comes from the correlation between a defect in the biosynthesis of glycosaminoglycans (GAGs), the defining carbohydrates of PGs, and the failure of spontaneous AChR clustering in the S27 cell line, a genetic variant of the C2 muscle cell line. Two approaches were used in the present study to investigate whether GAG and AChR clustering defects are causally linked. First, the formation of AChR clusters was examined in two more variant lines, S11 and S26, also isolated from the C2 muscle cell line on the basis of deficiencies in GAG biosynthesis. S11 and S26, like S27, are also defective in AChR clustering. Ion exchange analysis of the GAGs made by the S11, S26, and S27 lines revealed that the defects in GAG biosynthesis differ between the three lines. Second, heterokaryon myotubes formed between pairs of the GAG defective variants were tested for complementation in both AChR clustering and GAG biosynthesis. AChR clusters were conspicuous on individual heterokaryon myotubes, and GAG biosynthesis was restored to near wild type levels in the heterokaryon cultures. Complementation in GAG biosynthesis corroborates the biochemical data that the relevant mutations in the genetic variants are in different genes and establishes that the defects are not dominant. The consistent correlation between GAG defects and the failure of AChR clustering across three independent genetic variants and the complementary association of GAG biosynthesis with AChR clustering in heterokaryon myotubes argues against a chance association of the two phenotypes and for a causal relationship between PGs and AChR clustering. A prominent chondroitin sulfate peak correlated with AChR clustering in the heterokaryon cultures. This is consistent with earlier results suggesting that chondroitin sulfate in general is required for the spontaneous clustering of AChRs in C2 cultures and further suggests that a particular chondroitin sulfate proteoglycan may be essential for the clustering process.