Cell mutants defective in synthesizing a heparan sulfate proteoglycan with regions of defined monosaccharide sequence

Heparan sulfate proteoglycans (HSPGs) of the ocular lens

Heparan sulfate proteoglycans (HSPGs) reside in most cells; on their surface, in the pericellular milieu and/or extracellular matrix. In the eye, HSPGs can orchestrate the activity of key signalling molecules found in the ocular environment that promote its development and homeostasis. To date, our understanding of the specific roles played by individual HSPG family members, and the heterogeneity of their associated sulfated HS chains, is in its infancy. The crystalline lens is a relatively simple and well characterised ocular tissue that provides an ideal stage to showcase and model the expression and unique roles of individual HSPGs. Individual HSPG core proteins are differentially localised to eye tissues in a temporal and spatial developmental- and cell-type specific manner, and their loss or functional disruption results in unique phenotypic outcomes for the lens, and other ocular tissues. More recent work has found that different HS sulfation enzymes are also presented in a cell- and tissue-specific manner, and that disruption of these different sulfation patterns affects specific HS-protein interactions. Not surprisingly, these sulfated HS chains have also been reported to be required for lens and eye development, with dysregulation of HS chain structure and function leading to pathogenesis and eye-related phenotypes. In the lens, HSPGs undergo significant and specific changes in expression and function that can drive pathology, or in some cases, promote tissue repair. As master signalling regulators, HSPGs may one day serve as valuable biomarkers, and even as putative targets for the development of novel therapeutics, not only for the eye but for many other systemic pathologies.

Characterization of a spontaneous cell line from primary mouse fibroblasts as a model to study Sanfilippo syndrome

To evaluate a new approach to Mucopolysaccharidosis type IIIA (MPS-IIIA), work was initiated on primary fibroblasts from a well-known mouse model in which sulfamidase deficiency correlates with the accumulation of heparan sulfate - the hallmark of this disease. Once the culture of fibroblasts was established, we observed continuous proliferation with a rapid growth rate, loss of contact inhibition and late passage stability, corresponding to a spontaneously immortalized cell line. The presence of the single point D31N mutation was verified and both rapid and abundant intracellular accumulation of low molecular weight HS was observed, confirming both genotype and phenotype. This cell line is a potential in vitro model system for future studies of MPS-IIIA prior to employing animal models.

Control of organization and function of muscle and tendon by thrombospondin-4

Thrombospondins (TSPs) are multifunctional proteins that are deposited in the extracellular matrix where they directly affect the function of vascular and other cell types. TSP-4, one of the 5 TSP family members, is expressed abundantly in tendon and muscle. We have examined the effect of TSP-4 deficiency on tendon collagen and skeletal muscle morphology and function. In Thbs4(-/-) mice, tendon collagen fibrils are significantly larger than in wild-type mice, and there is no compensatory over-expression of TSP-3 and TSP-5, the two TSPs most highly homologous to TSP-4, in the deficient mice. TSP-4 is expressed in skeletal muscle, and higher levels of TSP-4 protein are associated with the microvasculature of red skeletal muscle with high oxidative metabolism. Lack of TSP-4 in medial soleus, red skeletal muscle with predominant oxidative metabolism, is associated with decreased levels of several specific glycosaminoglycan modifications, decreased expression of a TGFβ receptor beta-glycan, decreased activity of lipoprotein lipase, which associates with vascular cell surfaces by binding to glycosaminoglycans, and decreased uptake of VLDL. The soleus muscle is smaller and hind- and fore-limb grip strength is reduced in Thbs4(-/-) mice compared to wild-type mice. These observations suggest that TSP-4 regulates the composition of the ECM at major sites of its deposition, tendon and muscle, and the absence of TSP-4 alters the organization, composition and physiological functions of these tissues.

Glycomics of proteoglycan biosynthesis in murine embryonic stem cell differentiation

Glycosaminoglycans (GAGs) play a critical role in binding and activation of growth factors involved in cell signaling critical for developmental biology. The biosynthetic pathways for GAGs have been elucidated over the past decade and now analytical methodology makes it possible to determine GAG composition in as few as 10 million cells. A glycomics approach was used to examine GAG content, composition, and the level of transcripts encoding for GAG biosynthetic enzymes as murine embryonic stem cells (mESCs) differentiate to embryoid bodies (EBs) and to extraembryonic endodermal cells (ExE) to better understand the role of GAGs in stem cell differentiation. Hyaluronan synthesis was enhanced by 13- and 24-fold, most likely due to increased expression of hyaluronan synthase-2. Chondroitin sulfate (CS)/dermatan sulfate (DS) synthesis was enhanced by 4- and 6-fold, and heparan sulfate (HS) synthesis was enhanced by 5- and 8-fold following the transition from mESC to EB and ExE. Transcripts associated with the synthesis of the early precursors were largely unaltered, suggesting other factors account for enhanced GAG synthesis. The composition of both CS/DS and HS also changed upon differentiation. Interestingly, CS type E and highly sulfated HS both increase as mESCs differentiate to EBs and ExE. Differentiation was also accompanied by enhanced 2-sulfation in both CS/DS and HS families. Transcript levels for core proteins generally showed increases or remained constant upon mESC differentiation. Finally, transcripts encoding selected enzymes and isoforms, including GlcNAc-4,6-O-sulfotransferase, C5-epimerases, and 3-O-sulfotransferases involved in late GAG biosynthesis, were also enriched. These biosynthetic enzymes are particularly important in introducing GAG fine structure, essential for intercellular communication, cell adhesion, and outside-in signaling. Knowing the changes in GAG fine structure should improve our understanding the biological properties of differentiated stem cells.

Synthesis of anticoagulantly active heparan sulfate proteoglycans by glomerular epithelial cells involves multiple 3-O-sulfotransferase isoforms and a limiting precursor pool

Endothelial and other select cell types synthesize a subpopulation of heparan sulfate (HS) proteoglycans (HSPGs), anticoagulant HSPGs (aHSPGs) that bear aHS-HS chains with the cognate 3-O-sulfated pentasaccharide motif that can bind and activate anti-thrombin (AT). Endothelial cells regulate aHSPG production by limiting levels of HS 3-O-sulfotransferase-1 (3-OST-1), which modifies a non-limiting pool of aHS-precursors. By probing kidney cryosections with (125)I-AT and fluorescently tagged AT we found that the glomerular basement membrane contains aHSPGs, with the staining pattern implicating synthesis by glomerular epithelial cells (GECs). Indeed, cultured GECs synthesized aHS with high AT affinity that was comparable with the endothelial product. Disaccharide analyses of human GEC (hGEC) HS in conjunction with transcript analyses revealed that hGECs express predominantly 3-OST-1 and 3-OST-3(A). aHS production has not been previously examined in cells expressing multiple 3-OST isoforms. This unusual situation appears to involve novel mechanisms to regulate aHS production, as HS structural analyses suggest hGECs exhibit excess levels of 3-OST-1 and an extremely limiting pool of aHS-precursor. A limiting aHS-precursor pool may serve to minimize aHS synthesis by non-3-OST-1 isoforms. Indeed, we show that high in vitro levels of 3-OST-3(A) can efficiently generate aHS. Non-3-OST-1 isoforms can generate aHS in vivo, as the probing of kidney sections from 3-OST-1-deficient mice revealed GEC synthesis of aHSPGs. Surprisingly, Hs3st1(-/-) kidney only expresses 3-OST isoforms having a low specificity for aHS synthesis. Thus, our analyses reveal a cell type that expresses multiple 3-OST isoforms and produces minimal amounts of aHS-precursor. In part, this mechanism should prevent aHS overproduction by non-3-OST-1 isoforms. Such a role may be essential, as 3-OST isoforms that have a low specificity for aHS synthesis can generate substantial levels of aHSPGs in vivo.

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.

Mice deficient in heparan sulfate 3-O-sulfotransferase-1: normal hemostasis with unexpected perinatal phenotypes

Heparan sulfate that contains antithrombin binding sites is designated as anticoagulant heparan sulfate (HS(act)) since, in vitro, it dramatically enhances the neutralization of coagulation proteases by antithrombin. Endothelial cell production of HS(act) is controlled by the Hs3st1 gene, which encodes the rate limiting enzyme-heparan sulfate 3-O-sulfotransferase-1 (Hs3st1). It has long been proposed that levels of endothelial HS(act) may tightly regulate hemostatic tone. This potential in vivo role of HS(act) was assessed by generating Hs3st1(-/-) knockout mice. Hs3st1(-/-) and Hs3st1(+/+) mice were evaluated with a variety of methods, capable of detecting altered hemostatic tone. However, both genotypes were indistinguishable. Instead, Hs3st1(-/-) mice exhibited lethality on a specific genetic background and also showed intrauterine growth retardation. Neither phenotypes result from a gross coagulopathy. So although this enzyme produces the majority of tissue HS(act), Hs3st1(-/-) mice do not show an obvious procoagulant phenotype. These results suggest that the bulk of HS(act) is not essential for normal hemostasis and that hemostatic tone is not tightly regulated by total levels of HS(act). Moreover, the unanticipated non-thrombotic phenotypes suggest structure(s) derived from this enzyme might serve additional/alternative biologic roles.

Isolation and characterization of cell lines with reduced urokinase binding

Six cell lines have been generated from the human fibrosarcoma HT-1080 by mutagenesis. They were selected on the basis of reduced urokinase (uPA) binding on replicate polyester filters. Single cell clones were then isolated by limited dilution cloning. All cloned cells showed less uPA binding on filters, and as cell monolayers. These cell lines were able to bind only 10 to 65% as much uPA as the wild-type HT-1080 cells. Surface-bound uPA proteolytic activity and surface activation of plasminogen from these cells were also reduced relative to the wild-type. uPA could activate MAP kinases in the wild-type and two of the cell lines with the least uPA-binding, but the amount of the activated forms of the signalling molecules were reduced. Immunoblotting using two different anti-uPA receptor antibodies showed two cross-reacting protein species of approximately 53 kDa and approximately 38 kDa. The proportion of the lower Mr band to the higher Mr band was found to be reduced in all the cell lines relative to the wild-type. Chemical cross-linking with single-chain urokinase (scuPA) showed only one high-molecular-weight adduct, with Mr approximately 90 kDa, in all the cell lines tested. Similarly, cross-linking with the amino terminal fragment of uPA yielded a single approximately 70 kDa adduct. These would indicate that only the approximately 53 kDa band was responsible for cross-linking reactions. Equilibrium binding experiments showed that only one set of high-affinity binding sites for the wild-type cells. However, the binding of scuPA to two of these cell lines was best fitted to a two-site model, one of which was similar to the high-affinity binding sites of the wild-type, although the number of sites was reduced, while the other was of much lower affinity but was large in number. These results are discussed in relation to changes in the structure of ligand binding machinery in these cells, which affect other cellular functions.

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.

Anticoagulant heparan sulfate precursor structures in F9 embryonal carcinoma cells

To understand the mechanisms that control anticoagulant heparan sulfate (HSact) biosynthesis, we previously showed 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. In this study, HSact precursor structures have been studied by characterizing [6-3H]GlcN metabolically labeled F9 HS tagged with 3-O-sulfates in vitro by 3'-phosphoadenosine 5'-phospho-35S and purified 3-O-sulfotransferase-1. This later in vitro labeling allows the regions of HS destined to become the antithrombin (AT)-binding sites to be tagged for subsequent structural studies. It was shown that six 3-O-sulfation sites exist per HSact precursor chain. At least five out of six 3-O-sulfate-tagged oligosaccharides in HSact precursors bind AT, whereas none of 3-O-sulfate-tagged oligosaccharides from HSinact precursors bind AT. When treated with low pH nitrous or heparitinase, 3-O-sulfate-tagged HSact and HSinact precursors exhibit clearly different structural features. 3-O-Sulfate-tagged HSact hexasaccharides were AT affinity purified and sequenced by chemical and enzymatic degradations. The 3-O-sulfate-tagged HSact hexasaccharides exhibited the following structures, DeltaUA-[6-3H]GlcNAc6S-GlcUA-[6-3H]GlcNS3(35)S+/-6S-++ +IdceA2S-[6-3H]Glc NS6S. The underlined 6- and 3-O-sulfates constitute the most critical groups for AT binding in view of the fact that the precursor hexasaccharides possess all the elements for AT binding except for the 3-O-sulfate moiety. The presence of five potential AT-binding precursor hexasaccharides in all HSact precursor chains demonstrates for the first time the processive assembly of specific sequence in HS. The difference in structures around potential 3-O-sulfate acceptor sites in HSact and HSinact precursors suggests that these precursors might be generated by different concerted assembly mechanisms in the same cell. This study permits us to understand better the nature of the HS biosynthetic pathway that leads to the generation of specific saccharide sequences.

Multiple isoforms of heparan sulfate D-glucosaminyl 3-O-sulfotransferase. Isolation, characterization, and expression of human cdnas and identification of distinct genomic loci

3-O-Sulfated glucosaminyl residues are rare constituents of heparan sulfate and are essential for the activity of anticoagulant heparan sulfate. Cellular production of the critical active structure is controlled by the rate-limiting enzyme, heparan sulfate D-glucosaminyl 3-O-sulfotransferase-1 (3-OST-1) (EC 2.8.2.23). We have probed the expressed sequence tag data base with the carboxyl-terminal sulfotransferase domain of 3-OST-1 to reveal three novel, incomplete human cDNAs. These were utilized in library screens to isolate full-length cDNAs. Clones corresponding to predominant transcripts were obtained for the 367-, 406-, and 390-amino acid enzymes 3-OST-2, 3-OST-3A, and 3-OST-3B, respectively. These type II integral membrane proteins are comprised of a divergent amino-terminal region and a very homologous carboxyl-terminal sulfotransferase domain of approximately 260 residues. Also recovered were partial length clones for 3-OST-4. Expression of the full-length enzymes confirms the 3-O-sulfation of specific glucosaminyl residues within heparan sulfate (Liu, J., Shworak, N. W., Sinaÿ, P., Schwartz, J. J. Zhang, L., Fritze, L. M. S., and Rosenberg, R. D. (1999) J. Biol. Chem. 274, 5185-5192). Southern analyses suggest the human 3OST1, 3OST2, and 3OST4 genes, and the corresponding mouse isologs, are single copy. However, 3OST3A and 3OST3B genes are each duplicated in humans and show at least one copy each in mice. Intriguingly, the entire sulfotransferase domain sequence of the 3-OST-3B cDNA (774 base pairs) was 99.2% identical to the same region of 3-OST-3A. Together, these data argue that the structure of this functionally important region is actively maintained by gene conversion between 3OST3A and 3OST3B loci. Interspecific mouse back-cross analysis identified the loci for mouse 3Ost genes and syntenic assignments of corresponding human isologs were confirmed by the identification of mapped sequence-tagged site markers. Northern blot analyses indicate brain exclusive and brain predominant expression of 3-OST-4 and 3-OST-2 transcripts, respectively; whereas, 3-OST-3A and 3-OST-3B isoforms show widespread expression of multiple transcripts. The reiteration and conservation of the 3-OST sulfotransferase domain suggest that this structure is a self-contained functional unit. Moreover, the extensive number of 3OST genes with diverse expression patterns of multiple transcripts suggests that the novel 3-OST enzymes, like 3-OST-1, regulate important biologic properties of heparan sulfate proteoglycans.

Endothelial-derived nitric oxide preserves anticoagulant heparan sulfate expression in cultured porcine aortic endothelial cells

Nitric oxide (NO) has been shown to inhibit platelet adhesion and aggregation, but there are no reports on its interaction with the coagulation system. We investigated the effect of the L-arginine analogues, N-nitro-L-arginine (LNA), N(G)-nitro-L-arginine methyl ester (L-NAME), and N(G)-monomethyl-L arginine (L-NMMA), competitive inhibitors of NO production, on endothelial-surface heparan sulfate. Addition of LNA to porcine aortic endothelial cells reduced 125I-labeled antithrombin III binding to the cell surface heparan sulfate in a dose- and time-dependent fashion. Significant inhibition was observed with 1 mM LNA, and the maximal suppression (-50% of control) occurred at 10 mM LNA after 12 h. L-NAME (1 mM) and L-NMMA (1 mM) also significantly inhibited the antithrombin III binding. The iron chelator desferrioxamine significantly prevented the reduction of antithrombin III binding to LNA-treated cells. We further investigated the effect of L-NAME on intracellular oxidative stress of endothelial cells using a hydroperoxide-sensitive fluorochrome, carboxy-dichloro-dihydrofluorescein diacetate bisacetoxymethyl ester probe, and revealed that inhibition of NO synthesis by L-NAME led to a marked increase in intracellular oxidative stress. These results demonstrated that the prolonged inhibition of NO synthesis in porcine aortic endothelial cells decreases the expression of anticoagulant heparan sulfate on endothelial cells through the increase in intracellular oxidative stress, perhaps comprising another mechanism by which NO affects the coagulation system in the vasculature.

Molecular cloning and expression of mouse and human cDNAs encoding heparan sulfate D-glucosaminyl 3-O-sulfotransferase

The cellular rate of anticoagulant heparan sulfate proteoglycan (HSPGact) generation is determined by the level of a kinetically limiting microsomal activity, HSact conversion activity, which is predominantly composed of the long sought heparan sulfate D-glucosaminyl 3-O-sulfotransferase (3-OST) (Shworak, N. W., Fritze, L. M. S., Liu, J., Butler, L. D., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27063-27071; Liu, J., Shworak, N. W., Fritze, L. M. S., Edelberg, J. M., and Rosenberg, R. D. (1996) J. Biol. Chem. 271, 27072-27082). Mouse 3-OST cDNAs were isolated by proteolyzing the purified enzyme with Lys-C, sequencing the resultant peptides as well as the existing amino terminus, employing degenerate polymerase chain reaction primers corresponding to the sequences of the peptides as well as the amino terminus to amplify a fragment from LTA cDNA, and utilizing the resultant probe to obtain full-length enzyme cDNAs from a lambda Zap Express LTA cDNA library. Human 3-OST cDNAs were isolated by searching the expressed sequence tag data bank with the mouse sequence, identifying a partial-length human cDNA and utilizing the clone as a probe to isolate a full-length enzyme cDNA from a lambda TriplEx human brain cDNA library. The expression of wild-type mouse 3-OST as well as protein A-tagged mouse enzyme by transient transfection of COS-7 cells and the expression of both wild-type mouse and human 3-OST by in vitro transcription/translation demonstrate that the two cDNAs directly encode both HSact conversion and 3-OST activities. The mouse 3-OST cDNAs exhibit three different size classes because of a 5'-untranslated region of variable length, which results from the insertion of 0-1629 base pairs (bp) between residues 216 and 217; however, all cDNAs contain the same open reading frame of 933 bp. The length of the 3'-untranslated region ranges from 301 to 430 bp. The nucleic acid sequence of mouse and human 3-OST cDNAs are approximately 85% similar, encoding novel 311- and 307-amino acid proteins of 35,876 and 35,750 daltons, respectively, that are 93% similar. The encoded enzymes are predicted to be intraluminal Golgi residents, presumably interacting via their C-terminal regions with an integral membrane protein. Both 3-OST species exhibit five potential N-glycosylation sites, which account for the apparent discrepancy between the molecular masses of the encoded enzyme (approximately 34 kDa) and the previously purified enzyme (approximately 46 kDa). The two 3-OST species also exhibit approximately 50% similarity with all previously identified forms of the heparan biosynthetic enzyme N-deacetylase/N-sulfotransferase, which suggests that heparan biosynthetic enzymes share a common sulfotransferase domain.

Characterization and hormonal modulation of anticoagulant heparan sulfate proteoglycans synthesized by rat ovarian granulosa cells

Anticoagulant heparan sulfate proteoglycans endow the vascular endothelium with antithrombotic properties, but their role outside the vascular bed is unknown. Granulosa cells form an avascular compartment in the ovarian follicle, in which a heparin-like activity has been described. At ovulation extravascular coagulation occurs around ovulatory follicles, and after expulsion of the oocyte, a fibrin clot forms in the antral cavity. Granulosa cells synthesize two major heparan sulfate proteoglycans, whose anticoagulant nature has not been investigated. The purpose of this study was to characterize anticoagulant heparan sulfate proteoglycans synthesized by rat ovarian granulosa cells. Affinity purified 35S-labeled anticoagulant heparan sulfate glycosaminoglycans represent 6.5% of the total heparan sulfate synthesized, and they contain 13% 3-O-sulfated disaccharides that are markers of the antithrombin-binding site of heparin. The biological activity of granulosa cell heparan sulfate proteoglycans was demonstrated by their ability to bind antithrombin and to accelerate the formation of thrombin-antithrombin complexes. The impact of hormonal stimulation on granulosa cell anticoagulant heparan sulfate proteoglycans was studied using 125I-antithrombin binding assays. Follicle-stimulating hormone induced a redistribution of anticoagulant heparan sulfate proteoglycans from the granulosa cell layer to the culture medium, indicating that their distribution could be modulated according to the stage of follicular development. These results suggest that anticoagulant heparan sulfate might be critically located in the follicle to maintain fluidity around the oocyte until its expulsion at ovulation.

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

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

Enzymatic degradation of glycosaminoglycans

Glycosaminoglycans (GAGs) play an intricate role in the extracellular matrix (ECM), not only as soluble components and polyelectrolytes, but also by specific interactions with growth factors and other transient components of the ECM. Modifications of GAG chains, such as isomerization, sulfation, and acetylation, generate the chemical specificity of GAGs. GAGs can be depolymerized enzymatically either by eliminative cleavage with lyases (EC 4.2.2.-) or by hydrolytic cleavage with hydrolases (EC 3.2.1.-). Often, these enzymes are specific for residues in the polysaccharide chain with certain modifications. As such, the enzymes can serve as tools for studying the physiological effect of residue modifications and as models at the molecular level of protein-GAG recognition. This review examines the structure of the substrates, the properties of enzymatic degradation, and the enzyme substrate-interactions at a molecular level. The primary structure of several GAGs is organized macroscopically by segregation into alternating blocks of specific sulfation patterns and microscopically by formation of oligosaccharide sequences with specific binding functions. Among GAGs, considerable dermatan sulfate, heparin and heparan sulfate show conformational flexibility in solution. They elicit sequence-specific interactions with enzymes that degrade them, as well as with other proteins, however, the effect of conformational flexibility on protein-GAG interactions is not clear. Recent findings have established empirical rules of substrate specificity and elucidated molecular mechanisms of enzyme-substrate interactions for enzymes that degrade GAGs. Here we propose that local formation of polysaccharide secondary structure is determined by the immediate sequence environment within the GAG polymer, and that this secondary structure, in turn, governs the binding and catalytic interactions between proteins and GAGs.

Differential partition of anticoagulant heparan sulfate proteoglycans synthesized by endothelial and fibroblastic cell lines

The heparan sulfate proteoglycans that bind and activate antithrombin III (aHSPGs) are synthesized by endothelial cells as well as other nonvascular cells. We determined the amounts of cell surface-associated and soluble aHSPGs generated by the rat fat pad endothelial (RFP) cell line and the fibroblast (LTA) cell line. The RFP cells exhibit higher levels of cell surface-associated aHSPGs as compared to LTA cells, whereas LTA cells release larger amounts of soluble aHSPGs as compared to RFP cells. After confluence RFP cells show an increase in both cell surface-associated and soluble aHSPGs. In contrast, postconfluent LTA cells maintain a constant level of cell surface-associated and soluble aHSPGs. These observations indicate that different cell types can preferentially accumulate aHSPGs as cell surface-associated or soluble forms which could reflect alternate biological functions.

Homocysteine, a thrombogenic agent, suppresses anticoagulant heparan sulfate expression in cultured porcine aortic endothelial cells

Previous studies showed that homocysteine, a thrombo-atherogenic and atherogenic agent, inhibits an endothelial thrombomodulin-protein C anticoagulant pathway. We examined whether homocysteine might affect another endothelial anticoagulant mechanism; i.e., heparin-like glycosaminoglycan-antithrombin III interactions. Incubations of porcine aortic endothelial cell cultures with homocysteine reduced the amount of antithrombin III bound to the cell surface in a dose- and time-dependent fashion. The inhibitory effect was observed at a homocysteine concentration as low as 0.1 mM, and the maximal suppression occurred at 1 mM of homocysteine after 24 h. In contrast with a marked reduction in the maximal antithrombin III binding capacity (approximately 30% of control), the radioactivity of [35S]sulfate incorporated into heparan sulfate on the cell surface was minimally (< 15%) reduced. The cells remained viable after homocysteine treatment. Although neither net negative charge nor proportion in total glycosaminoglycans of cell surface heparan sulfate was altered by homocysteine treatment, a substantial reduction in antithrombin III binding capacity of heparan sulfate isolated from homocysteine-treated endothelial cells was found using both affinity chromatography and dot blot assay techniques. The antithrombin III binding activity of endothelial cells decreased after preincubation with 1 mM homocysteine, cysteine, or 2-mercaptoethanol; no reduction in binding activity was observed after preincubation with the same concentration of methionine, alanine, or valine. This sulfhydryl effect may be caused by generation of hydrogen peroxide, as incubation of catalase, but not superoxide dismutase, with homocysteine-treated endothelial cells prevented this reduction, whereas copper augmented the inhibitory effects of the metabolite. Thus, our data suggest that the inhibited expression of anticoagulant heparan sulfate may contribute to the thrombogenic property resulting from the homocysteine-induced endothelial cell perturbation, mediated by generation of hydrogen peroxide through alteration of the redox potential.