Sequence variation in heparin octasaccharides with high affinity for antithrombin III

Distinct 3-O-sulfated heparan sulfate modification patterns are required for kal-1-dependent neurite branching in a context-dependent manner in Caenorhabditis elegans

Heparan sulfate (HS) is an unbranched glycosaminoglycan exhibiting substantial molecular diversity due to multiple, nonuniformly introduced modifications, including sulfations, epimerization, and acetylation. HS modifications serve specific and instructive roles in neuronal development, leading to the hypothesis of a HS code that regulates nervous system patterning. Although the in vivo roles of many of the HS modifications have been investigated, very little is known about the function of HS 3-O-sulfation in vivo. By examining patterning of the Caenorhabditis elegans nervous system in loss of function mutants of the two 3-O-sulfotransferases, hst-3.1 and hst-3.2, we found HS 3-O-sulfation to be largely dispensable for overall neural development. However, generation of stereotypical neurite branches in hermaphroditic-specific neurons required hst-3.1, hst-3.2, as well as an extracellular cell adhesion molecule encoded by kal-1, the homolog of Kallmann Syndrome associated gene 1/anosmin-1. In contrast, kal-1−dependent neurite branching in AIY neurons required catalytic activity of hst-3.2 but not hst-3.1. The context-dependent requirement for hst-3.2 and hst-3.1 indicates that both enzymes generate distinct types of HS modification patterns in different cell types, which regulate kal-1 to promote neurite branching. We conclude that HS 3-O-sulfation does not play a general role in establishing the HS code in C. elegans but rather plays a specialized role in a context-dependent manner to establish defined aspects of neuronal circuits.

Screening for anticoagulant heparan sulfate octasaccharides and fine structure characterization using tandem mass spectrometry

Heparan sulfate (HS) is a sulfated glycosaminoglycan located on the surface and extracellular matrix of mammalian cells. HS is constituted of highly N-sulfated domains (NS domains) interrupted by lower sulfation domains. The arrangement of these domains dictates the function of HS which is mainly involved in binding proteins and regulating their biological activities. Heparin, a heparan sulfate analogue present in mast cells, resembles the NS domains of HS but lacks the alternating high and low sulfation architecture. Because the NS domains that range up to hexadecasaccharide in size are the main protein binders, heparin has been used as a model for HS in protein binding studies. Heparan sulfate, however, is the more physiologically relevant modulator of growth factor-receptor interactions. In this work, liquid chromatography and mass spectrometry (LC-MS) were used to compare the compositions of affinity-purified heparin and HS octasaccharides with anticoagulant activities versus library octasaccharides. The fine structures of the biologically active HS compositions were then compared against those of library octasaccharides using low-energy collision-induced dissociation tandem mass spectrometry. This approach confirmed isomeric enrichment of these compositions and, most importantly, produces ions diagnostic of their biological activity.

Heparanase expression and activity influences chondrogenic and osteogenic processes during endochondral bone formation

Endochondral bone formation is a highly orchestrated process involving coordination among cell-cell, cell-matrix and growth factor signaling that eventually results in the production of mineralized bone from a cartilage template. Chondrogenic and osteogenic differentiation occur in sequence during this process, and the temporospatial patterning clearly requires the activities of heparin binding growth factors and their receptors. Heparanase (HPSE) plays a role in osteogenesis, but the mechanism by which it does so is incompletely understood. We used a combination of ex vivo and in vitro approaches and a well described HPSE inhibitor, PI-88 to study HPSE in endochondral bone formation. In situ hybridization and immunolocalization with HPSE antibodies revealed that HPSE is expressed in the peri-chondrium, peri-osteum, and at the chondro-osseous junction, all sites of key signaling events and tissue morphogenesis. Transcripts encoding Hpse also were observed in the pre-hypertrophic zone. Addition of PI-88 to metatarsals in organ culture reduced growth and suggested that HPSE activity aids the transition from chondrogenic to osteogenic processes in growth of long bones. To study this, we used high density cultures of ATDC5 pre-chondrogenic cells grown under conditions favoring chondrogenesis or osteogenesis. Under chondrogenic conditions, HPSE/Hpse was expressed at high levels during the mid-culture period, at the onset of terminal chondrogenesis. PI-88 addition reduced chondrogenesis and accelerated osteogenesis, including a dramatic up-regulation of osteocalcin levels. In normal growth medium, addition of PI-88 reduced migration of ATDC-5 cells, suggesting that HPSE facilitates cartilage replacement by bone at the chondro-osseous junction by removing the HS component of proteoglycans, such as perlecan/HSPG2, that otherwise prevent osteogenic cells from remodeling hypertrophic cartilage.

Characterization of heparin oligosaccharides binding specifically to antithrombin III using mass spectrometry

Heparan sulfate (HS) is a sulfated glycosaminoglycan attached to a core protein on the cell surface. Protein binding to cell surface HS is a key regulatory event for many cellular processes such as blood coagulation, cell proliferation, and migration. The concept whereby protein binding to HS is not random but requires a limited number of sulfation patterns is becoming clear. Here we describe a hydrophobic trapping assay for screening a library of heparin hexasaccharides for binders to antithrombin III (ATIII). The hexasaccharide compositions are defined with their building block content in the following format: (DeltaHexA:HexA:GlcN:SO 3:Ac). Of five initial compositions present in the library, (1:2:3:6:1), (1:2:3:7:1), (1:2:3:7:0), (1:2:3:8:0), and (1:2:3:9:0), only two are shown to bind ATIII, namely, (1:2:3:8:0) and (1:2:3:9:0). The use of amide hydrophilic interaction (HILIC) liquid chromatography-mass spectrometry permitted reproducible quantitative analysis of the composition of the initial library as well as that of the binding fraction. The specificity of the hexasaccharides binding ATIII was confirmed by assaying their ability to enhance ATIII-mediated inhibition of Factor Xa in vitro.

The functions of the heparan sulphate proteoglycans

Heparan sulphate (HS)-containing proteoglycans (HS-PGs) are present at the surface of nearly all adherent mammalian cells. The principal mode of attachment is by way of the protein core which is inserted into the plasma membrane. Other forms of HS-PG may be components of pericellular matrices, notably basement membranes. The core proteins of HS-PGs can be small (35K) as in hepatocytes, intermediate (50K) as in many mesenchymal cells, or very large (400K) as in basement membranes. A special case is the HS-PG synthesized by postconfluent fibroblasts. This proteoglycan has a core protein that closely resembles the transferrin receptor glycoprotein. It is possible that this HS-PG is a pro-form of the receptor. Low molecular weight, carbohydrate-rich HS-PG forms are probably derived from larger forms by partial degradation. The HS side-chains can contain 24 different disaccharides in an unknown number of arrangements. The biosynthetic machinery can impose considerable restrictions; for example, the extent of N-sulphation rarely exceeds 40-50%, whereas O-sulphation may range from 20% to 75% of potential sites. Nevertheless, the informational capacity of HS is formidable. By way of the HS chains, HS-PG at the surface of endothelial cells can interact specifically or selectively with a number of plasma proteins. HS-PG at the surface of matrix-producing cells is similarly in a position to interact with matrix proteins, notably collagen, fibronectin and laminin. As the cytoplasmic portion of the HS-PG core protein can bind actin, this proteoglycan can provide a connection between extracellular matrices and the cytoskeleton. A number of studies support a role for HS-PGs in the control of cell growth, and this could be one of their major functions. Whether the HS side-chains or the core protein or both are carrying out such a function remains to be determined.

Finding a needle in a haystack: development of a combinatorial virtual screening approach for identifying high specificity heparin/heparan sulfate sequence(s)

We describe a combinatorial virtual screening approach for predicting high specificity heparin/heparan sulfate sequences using the well-studied antithrombin-heparin interaction as a test case. Heparan sulfate hexasaccharides were simulated in the 'average backbone' conformation, wherein the inter-glycosidic bond angles were held constant at the mean of the known solution values, irrespective of their sequence. Molecular docking utilized GOLD with restrained inter-glycosidic torsions and intra-ring conformations, but flexible substituents at the 2-, 3-, and 6-positions and explicit incorporation of conformational variability of the iduronate residues. The approach reproduces the binding geometry of the sequence-specific heparin pentasaccharide to within 2.5 A. Screening of a combinatorial virtual library of 6,859 heparin hexasaccharides using a dual filter strategy, in which predicted antithrombin affinity was the first filter and self-consistency of docking was the second, resulted in only 10 sequences. Of these, nine were found to bind antithrombin in a manner identical to the natural pentasaccharide, while a novel hexasaccharide bound the inhibitor in a unique but dramatically different geometry and orientation. This work presents the first approach on combinatorial library screening for heparin/heparan sulfate GAGs to determine high specificity sequences and opens up huge opportunities to investigate numerous other physiologically relevant GAG-protein interactions.

Spatiotemporal distribution of heparan sulfate epitopes during murine cartilage growth plate development

Heparan sulfate proteoglycans (HSPGs) are abundant in the pericellular matrix of both developing and mature cartilage. Increasing evidence suggests the action of numerous chondroregulatory molecules depends on HSPGs. In addition to specific functions attributed to their core protein, the complexity of heparan sulfate (HS) synthesis provides extraordinary structural and functional heterogeneity. Understanding the interactions of chondroregulatory molecules with HSPGs and their subsequent outcomes has been limited by the absence of a detailed analysis of HS species in cartilage. In this study, we characterize the distribution and variety of HS species in developing cartilage of normal mice. Cryo-sections of femur and tibia from normal mouse embryos were evaluated using immunostaining techniques. A panel of unique phage display antibodies specific to particular HS species were employed and visualized with secondary antibodies conjugated to Alexa-fluor dyes. Confocal microscopy demonstrates that HS species are dynamic structures within developing growth plate cartilage and the perichondrium. GlcNS6S-IdoUA2S-GlcNS6S species are down regulated and localization of GlcNS6S-IdoUA-GlcNS6S species within the hypertrophic zone of the growth plate is lost during normal development. Regional differences in HS structures are present within developing growth plates, implying that interactions with and responses to HS-binding proteins also may display regional specialization.

Chemoenzymatic synthesis of classical and non-classical anticoagulant heparan sulfate polysaccharides

Heparan sulfate (HS) polysaccharides interact with numerous proteins at the cell surface and orchestrate many different biological functions. Though many functions of HS are well established, only a few specific structures can be attributed to HS functions. The extreme diversity of HS makes chemical synthesis of specific bioactive HS structures a cumbersome and tedious undertaking that requires laborious and careful functional group manipulations. Now that many of the enzymes involved in HS biosynthesis are characterized, we show in this study how one can rapidly and easily assemble bioactive HS structures with a set of cloned enzymes. We have demonstrated the feasibility of this new approach to rapidly assemble antithrombin III-binding classical and non-classical anticoagulant polysaccharide structures for the first time.

New antithrombin-based anticoagulants

Clinically used anticoagulants are inhibitors of enzymes involved in the coagulation pathway, primarily thrombin and factor Xa. These agents can be either direct or indirect inhibitors of clotting enzymes. Heparin-based anticoagulants are indirect inhibitors that enhance the proteinase inhibitory activity of a natural anticoagulant, antithrombin. Despite its phenomenal success, current anticoagulation therapy suffers from the risk of serious bleeding. The need for safer and more effective antithrombotic agents clearly exists. The past decade has seen enormous effort directed toward discovering and/or designing new molecules with anticoagulant activity. These new molecules can be classified into (a). antithrombin and its mutants, (b). natural polysaccharides, (c). synthetic modified heparins and heparin-mimics, (d). synthetic oligosaccharides, and (e). synthetic non-sugar antithrombin activators. This review focuses on these efforts in designing or discovering new molecules that act through the antithrombin pathway of anticoagulation.

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.

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.

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.

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.

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.

Structural analysis of heparin by raman spectroscopy

The Raman spectra of commercially available heparin disaccharide standards exhibit bands associated with the N-sulfate and the 6-O-sulfate groups of the glucosamine and the 2-O-sulfate of the iduronic acid. The N-sulfate has a strong band at 1039 cm-1. The 6-O-sulfate and the 2-O-sulfate exhibit bands at 1055 and 1065 cm-1, respectively. The pattern of these modes, which are assigned to the symmetric SO3 vibrations, was supported by semiempirical quantum mechanical calculations. The above bands were identified in the Raman spectrum of a commercial preparation of porcine mucosal heparin and were used to determine the relative proportion of the N-sulfate, 6-O-sulfate, and 2-O-sulfate groups in the heparin molecule. This information, which is complementary to that obtained by NMR spectroscopy, is of particular importance in relation to biological activity. This study also extends the usefulness of Raman spectroscopy to include structural details required for the quality assurance of pharmaceutical preparations of heparin.

Calcium-dependent heparin-like ligands for L-selectin in nonlymphoid endothelial cells

L-Selectin is a calcium-dependent mammalian lectin that mediates lymphocyte trafficking by recognizing sialylated ligands on high endothelial venules in lymph nodes. Although L-selectin probably mediates neutrophil extravasation into nonlymphoid tissues, no corresponding ligand has been characterized. Staining of cultured endothelial cells with an L-selectin chimera (LS-Rg) showed an internal pool of ligands. Metabolic labeling with sulfur-35-labeled sulfate revealed heparin lyase-sensitive ligands that bound LS-Rg in a calcium-dependent, sialic acid-independent manner. A fraction of commercial heparin bound to LS-Rg and LS-Rg bound to heparin-agarose, both in a calcium-dependent manner. Thus, L-selectin recognizes endothelial heparin-like chains, which could be physiological ligands mediating leucocyte trafficking.

Heparin-like glycosaminoglycans participate in binding of a human trophoblastic cell line (JAR) to a human uterine epithelial cell line (RL95)

In vitro studies in our laboratory have indicated that heparan sulfate proteoglycans (HSPGs) play an important role in murine embryo implantation. In order to investigate the potential function of HSPGs in human implantation, two human cell lines (RL95 and JAR) were used to model uterine epithelium and embryonal trophectoderm, respectively. A heterologous cell-cell adhesion assay was developed to determine if binding of JAR cells to RL95 cells was heparan sulfate-dependent. Labeled, single cell suspensions of JAR cells attached to confluent monolayers of RL95 cells in a dose- and time-dependent manner. Heparin-like glycosaminoglycans and JAR cell proteoglycans competitively inhibited JAR cell adhesion to RL95 cells by 50% or more. A panel of chemically modified heparins were used to demonstrate that O-sulfation and amino group substitution were critical for inhibition of cell-cell adhesion. Treatment with chlorate, an inhibitor of ATP-sulfurylase, resulted in a 56% reduction in cell-cell binding compared to untreated controls. Heparinase and chondroitinase ABC markedly inhibited JAR-RL95 binding, while chondroitinase AC had no significant effect. These observations indicated that HSPGs as well as dermatan sulfate-containing proteoglycans participated in cell-cell binding. Collectively, these results indicate that initial binding interactions between JAR and RL95 cells is mediated by cell surface glycosaminoglycans (GAGs) with heparin-like properties (i.e., heparan sulfate and dermatan sulfate). These observations are consistent with an important role for HS and heparin-like GAGs as well as their corresponding binding sites in early stages of human trophoblast-uterine epithelial cell binding.

The physical biochemistry and molecular genetics of sulfate activation

This article is an overview of current research in the area of sulfate activation. Emphasis is placed on presenting unresolved issues in an appropriate context for critical evaluation by the reader. The energetics of sulfate activation is reevaluated in light of recent findings that demonstrate that the synthesis of activated sulfate is thermodynamically driven by GTP hydrolysis. The structural and functional bases of this GTPase activation are discussed in detail. The bonding and hydrolysis of the high-energy, phosphoric-sulfuric acid anhydride bond of activated sulfate are presented along with an analysis of the importance of the divalent cation and pyrophosphate protonation in the equilibria governing activated sulfate formation. The molecular genetics of sulfate assimilation in prokaryotes is reviewed with an emphasis on the regulation of the pathway. Recent discoveries connecting sulfate activation to plant/microbe symbiogenesis are presented, as are several examples of the importance of activated sulfate in human metabolism and disease.

Thermodynamic analysis of heparin binding to human antithrombin

The binding of heparin to human antithrombin III (ATIII) was investigated by titration calorimetry (TC) and differential scanning calorimetry (DSC). TC measurements of homogeneous high-affinity pentasaccharide and octasaccharide fragments of heparin in 0.02 M phosphate buffer and 0.15 M sodium chloride (pH 7.3) yielded binding constants of (7.1 +/- 1.3) x 10(5) M-1 and (6.7 +/- 1.2) x 10(6) M-1, respectively, and corresponding binding enthalpies of -48.3 +/- 0.7 and -54.4 +/- 5.4 kJ mol-1. The binding enthalpy of heparin in phosphate buffer (0.02 M, 0.15 M NaCl, pH 7.3) was estimated from TC measurements to be -55 +/- 10 kJ mol-1, while the enthalpy in Tris buffer (0.02 M, 0.15 M NaCl, pH 7.3) was -18 +/- 2 kJ mol-1. The heparin-binding affinity was shown by fluorescence measurements not to change under these conditions. The 3-fold lower binding enthalpy in Tris can be attributed to the transfer of a proton from the buffer to the heparin-ATIII complex. DSC measurements of the ATIII unfolding transition exhibited a sharp denaturation peak at 329 +/- 1 K with a van 't Hoff enthalpy of 951 +/- 89 kJ mol-1, based on a two-state transition model and a much broader transition from 333 to 366 K. The transition peak at 329 K accounted for 9-18% of the total ATIII. At sub-saturate heparin concentrations, the lower temperature peak became bimodal with the appearance of a second transition peak at 336 K. At saturate heparin concentration only the 336 K peak was observed. This supports a two domain model of ATIII folding in which the lower stability domain (329 K) binds and is stabilized by heparin.

New approaches for defining sequence specific synthesis of heparan sulfate chains

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

A rapid colorimetric assay for heparinase activity

A rapid, sensitive, assay for enzymes that degrade heparin is described. The procedure is based on the interference of heparin with color development during the interaction of protein with the dye Coomassie brilliant blue. The loss of this property when the glycosaminoglycan is degraded by heparinase can be used to quantify activity of the enzyme in pure form, or in complex biological samples such as tissue homogenates or serum. The assay is also suitable for studying dependence of heparinase activity under conditions such as varying pH and temperature.

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

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

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

We have demonstrated that mouse LTA cells synthesize cell-surface heparan sulfate proteoglycans (HSPGs) with regions of defined monosaccharide sequence that specifically interact with antithrombin (HSPGact). It remains unclear how HSPGact can be generated by a biosynthetic pathway with no simple template for directing the ordered assembly of monosaccharide units. To examine this issue, we treated LTA cells with ethyl methanesulfonate and then isolated seven stable mutants that synthesize only 8-27% of the wild-type HSPGact but produce normal amounts of other HSPGs. These mutants are recessive in nature and fall into at least two different complementation groups. The delineation of the molecular basis of these defects should help to elucidate the manner by which cells synthesize HSPGs with regions of defined monosaccharide sequence.

Inhibition of the early stages of the thrombin generation reaction by various glycosaminoglycans

Inhibition of thrombin generation was studied in strongly diluted human plasma by continuously measuring the splitting of the thrombin-specific chromogenic substrate S2288 as a function of the inhibitor concentration. To avoid the activation of clotting cascade proenzymes other than prothrombin, the thrombin generation reaction was initiated by a mixture of calcium, phospholipids and (bovine) factor Xa. Using this assay we have estimated the relative potency of the following glycosaminoglycans: heparin; a fraction of heparin with high affinity for antithrombin III (high affinity heparin); the low molecular weight heparin Fragmin; the low molecular weight heparinoid Org 10172; a fraction of Org 10172 with high affinity for antithrombin III (high affinity Org 10172) and the O-methyl derivative of the pentasaccharide, representing the minimal structure required for binding to antithrombin III. Based on concentrations expressed in amidolytic anti-Xa units, the descending order of potency observed is: Heparin approximately High Affinity Heparin greater than Fragmin greater than Org 10172 greater than High Affinity Org 10172 greater than O-methyl pentasaccharide. The more potent the glycosaminoglycan the stronger the concentration dependence of its inhibitory effect. These findings could be due to the different, additional anti-thrombin activities of these glycosaminoglycans and/or to their different anti-prothrombinase activities. With the pentasaccharide a striking saturation of the inhibition is observed, due to saturation of the antithrombin III.

Biochemistry of heparin antithrombin interactions, and the physiologic role of this natural anticoagulant mechanism

A small fraction of plasma antithrombin is normally bound to a specific population of heparan sulfate proteoglycans synthesized by macrovascular and microvascular endothelial cells. This permits the protease inhibitor to be selectively activated at blood-surface interfaces where enzymes of the intrinsic coagulation cascade are commonly generated. Thus, antithrombin is critically placed to neutralize these hemostatic enzymes and thereby protect natural surfaces against thrombus formation. Furthermore, the catalytic nature of this specific set of heparan sulfate proteoglycans ensures the continual regeneration of the nonthrombotic properties of the endothelial cell layer. Alterations in the synthesis and/or placement of the anticoagulantly active heparan sulfate proteoglycans on the surface of microvascular and macrovascular endothelial cells could be responsible for arterial and venous thrombotic disease in humans.

Anti-thrombin activities of heparin. Effect of saccharide chain length on thrombin inhibition by heparin cofactor II and by antithrombin

The interactions of two proteinase inhibitors, heparin cofactor II and antithrombin, with thrombin are potentiated by heparin. Using two methods, we have studied the potentiating effects of a series of heparin (poly)saccharides with high affinity for antithrombin and mean Mr ranging from approx. 1700 to 18,800. First, catalytic amounts of heparin (poly)saccharide were added to purified systems containing thrombin and either heparin cofactor II or antithrombin. Residual thrombin activity was determined with a chromogenic substrate. It was found that only the higher-Mr polysaccharides (Mr greater than 8000) efficiently catalysed thrombin inhibition by heparin cofactor II, there being a progressive catalytic effect with increasing Mr of the polysaccharide. Weak accelerating effects were noted with low-Mr saccharides (Mr less than 8000). This contrasted with the well-characterized interaction of heparin with antithrombin and thrombin, where heparin oligosaccharides of Mr less than 5400 had absolutely no ability to accelerate the reaction, while (poly)saccharides of Mr exceeding 5400 showed rapidly increasing catalytic activity with increasing Mr. Secondly, these and other heparin preparations were added in a wide concentration range to plasma with which 125I-labelled thrombin was then incubated for 30 s. Inhibited thrombin was determined from the distribution of labelled thrombin amongst inhibitor-thrombin complexes, predominantly antithrombin-thrombin and heparin cofactor II-thrombin complexes. In this situation, where the inhibitors competed for thrombin and for the (poly)saccharides, it was found that, provided the latter were of high affinity for antithrombin and exceeded a Mr of 5400, thrombin inhibition in plasma was mediated largely through antithrombin. Polysaccharides of Mr exceeding 8000 that were of low affinity for antithrombin accelerated thrombin inhibition in plasma through their interaction with heparin cofactor II. High concentrations of saccharides of Mr 1700-5400 exhibited a size-dependent acceleration of thrombin inhibition, not through their interaction with antithrombin, but through their interaction with heparin cofactor II.

Ex vivo model of an immobilized-enzyme reactor

Immobilized-enzyme reactors are beginning to be studied for a variety of therapeutic applications. To facilitate the design of these devices for different clinical situations and a diverse patient population, mathematical models may be valuable. An immobilized-heparinase (EC 4.2.2.7) reactor was selected as a model system. The device removes heparin from blood that has been anticoagulated to prevent thrombus formation. Heparinase was immobilized to cross-linked agarose particles. A mathematical model was developed to describe the clearance of heparin by the reactor ex vivo and compared to experimental clearances measured in sheep. The model accounted for enzymatic degradation as well as the binding of heparin and its breakdown products to antithrombin. The device was modeled as a steady-state continuously stirred tank reactor. Molar conservation equations within the agarose particles accounted for simultaneous diffusion and chemical reaction. The model had no adjustable parameters and was able to predict the clearance of heparin within 5-25% for three different animals and 12 different perfusions.

Interaction of heparin and antithrombin III. The role of O-sulfate groups

A synthetic pentasaccharide corresponding to the sequence involved in heparin for binding and activation of antithrombin III contains eight sulfate groups. The role of some of them in the interaction with the protein has been demonstrated through the study of fragments obtained from heparin. An approach based on the total chemical synthesis of heparin fragments allows us to provide new information on the O-sulfate groups borne by the iduronic acid and the glucosamine units that constitute the reducing-end disaccharide of the above pentasaccharide sequence. Although not strictly necessary for a weak interaction to take place, these two sulfates co-operate to express maximal activity. This suggests that they belong to a secondary sub-region of interaction with antithrombin III, the primary one being accounted for by other critical parts of the structure and particularly the trisaccharide sequence placed at the non-reducing end of the pentasaccharide.

Contribution of 3-O- and 6-O-sulfated glucosamine residues in the heparin-induced conformational change in antithrombin III

The role of 3-O- and 6-O-sulfated glucosamine residues within the heparin octasaccharide critical for biological activity, iduronic acid----N-acetylglucosamine 6-O-sulfate----glucuronic acid----N-sulfated glucosamine 3,6-di-O-sulfate----iduronic acid 2-O-sulfate----N-sulfated glucosamine 6-O-sulfate----iduronic acid 2-O-sulfate----anhydromannitol 6-O-sulfate, was determined by comparing its ability to bind antithrombin, induce a conformational change in this protease inhibitor as monitored by the enhancement of intrinsic fluorescence, and accelerate (at saturation) the interaction of this protein with human factor Xa. The octasaccharide produced a maximum 48% increase in intrinsic fluorescence at 37 degrees C and a rate of factor Xa inhibition of 6 X 10(5) M-1 s-1 as measured by stopped-flow fluorometry at 25 degrees C. The basal rate of the antithrombin-factor Xa interaction observed in the absence of oligosaccharide was 2 X 10(3) M-1 s-1. The synthetic pentasaccharide, consisting of residues 2-6, produced fluorescence enhancement and rate of inhibition equal to those of the octasaccharide. However, a similar pentasaccharide, identical in all respects except that it lacked the 3-O-sulfate on residue 4, produced less than a 5% fluorescence enhancement and a rate of factor Xa inhibition of 8 X 10(3) M-1 s-1. The tetrasaccharide consisting of residues 2-5 produced a 35% fluorescence enhancement and a rate of factor Xa inhibition of 3 X 10(5) M-1 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)