Introduction to the Molecules Special Edition Entitled 'Heparan Sulfate and Heparin: Challenges and Controversies': Some Outstanding Questions in Heparan Sulfate and Heparin Research.

The scope of this article is to provide a brief general introduction to heparan sulfate (HS) and heparin, and attempt to identify some of the central challenges regarding research into the chemistry and biology of glycosaminoglycans (GAGs), some of which are the subject of contributions to the special issue of Molecules (published in volume 23, 2018) entitled 'Heparan Sulfate and Heparin: Challenges and Controversies' [...].

Cooperative heparin-mediated oligomerization of fibroblast growth factor-1 (FGF1) precedes recruitment of FGFR2 to ternary complexes.

Fibroblast growth factors (FGFs) utilize cell surface heparan sulfate as a coreceptor in the assembly of signaling complexes with FGF-receptors on the plasma membrane. Here we undertake a complete thermodynamic characterization of the assembly of the FGF signaling complex using isothermal titration calorimetry. Heparin fragments of defined length are used as chemical analogs of the sulfated domains of heparan sulfate and examined for their ability to oligomerize FGF1. Binding is modeled using the McGhee-von Hippel formalism for the cooperative binding of ligands to a monodimensional lattice. Oligomerization of FGFs on heparin is shown to be mediated by positive cooperativity (α = 6). Heparin octasaccharide is the shortest length capable of dimerizing FGF1 and on longer heparin chains FGF1 binds with a minimal footprint of 4.2 saccharide units. The thermodynamics and stoichiometry of the ternary complex suggest that in solution FGF1 binds to heparin in a trans-dimeric manner before FGFR recruitment.

Influencing hematopoietic differentiation of mouse embryonic stem cells using soluble heparin and heparan sulfate saccharides.

Heparan sulfate proteoglycans (HSPG) encompass some of the most abundant macromolecules on the surface of almost every cell type. Heparan sulfate (HS) chains provide a key interaction surface for the binding of numerous proteins such as growth factors and morphogens, helping to define the ability of a cell to respond selectively to environmental cues. The specificity of HS-protein interactions are governed predominantly by the order and positioning of sulfate groups, with distinct cell types expressing unique sets of HS epitopes. Embryos deficient in HS-synthesis (Ext1(-/-)) exhibit pre-gastrulation lethality and lack recognizable organized mesoderm and extraembryonic tissues. Here we demonstrate that embryonic stem cells (ESCs) derived from Ext1(-/-) embryos are unable to differentiate into hematopoietic lineages, instead retaining ESC marker expression throughout embryoid body (EB) culture. However hematopoietic differentiation can be restored by the addition of soluble heparin. Consistent with specific size and composition requirements for HS:growth factor signaling, chains measuring at least 12 saccharides were required for partial rescue of hematopoiesis with longer chains (18 saccharides or more) required for complete rescue. Critically N- and 6-O-sulfate groups were essential for rescue. Heparin addition restored the activity of multiple signaling pathways including bone morphogenic protein (BMP) with activation of phospho-SMADs re-established by the addition of heparin. Heparin addition to wild-type cultures also altered the outcome of differentiation, promoting hematopoiesis at low concentrations, yet inhibiting blood formation at high concentrations. Thus altering the levels of HS and HS sulfation within differentiating ESC cultures provides an attractive and accessible mechanism for influencing cell fate.

A developmentally regulated heparan sulfate epitope defines a subpopulation with increased blood potential during mesodermal differentiation.

Heparan sulfate (HS) is a mandatory coreceptor for many growth factors and morphogens involved in embryonic development; its bioactivity is dictated by complex sulfation motifs embedded within the polymer chain. Using a panel of HS-specific antibodies we have identified a unique HS epitope recognized by antibody HS4C3 that is selectively expressed during differentiation of embryonic stem (ES) cells along the mesodermal lineage to the hemangioblast stage. The appearance of this high-affinity HS4C3-binding (HS4C3(high)) epitope is transient; the epitope is specifically expressed within the emerging Brachyury(+) (Bry(+)) population and marks those cells that will become fetal liver kinase 1 (Flk1)(+). Fluorescence-activated cell sorting (FACS) separation and colony forming assays revealed that HS4C3(high)/Flk1(+) cells have a dramatically increased potential to form both blast and endothelial colonies, both of which depend upon the HS-binding growth factor vascular endothelial growth factor. Critically, expression of this HS epitope is tightly regulated, disappearing from the cell surface as the resultant hematopoietic lineages mature, in a similar manner to protein markers Bry and Flk1. In vivo studies showed a remarkable correlation with in vitro findings, with expression of HS4C3-binding epitopes restricted to newly formed mesodermal tissues during gastrulation. We believe this is the first time a defined HS epitope has been implicated in a specific developmental pathway and that this provides, in addition, a novel enrichment technique for the isolation of hemangioblasts from mixed differentiated ES cell cultures.

Altered glycosylation of recombinant NKp30 hampers binding to heparan sulfate: a lesson for the use of recombinant immunoreceptors as an immunological tool.

NKp30 is a natural cytotoxicity receptor expressed by human NK cells and involved in NK lytic activity. We previously published that membranal heparan sulfate serves as a coligand for human NKp30. In the present study, we complement our results by showing direct binding of recombinant NKp30 to immobilized heparin. The heparan sulfate epitope(s) on target tumor cells and the heparin epitope(s) recognized by NKp30 share similar characteristics. Warren and colleagues (Warren HS, Jones AL, Freeman C, Bettadapura J, Parish CR. 2005. Evidence that the cellular ligand for the human NK cell activation receptor NKp30 is not a heparan sulfate glycosaminoglycan. J Immunol. 175:207-212) published that NKp30 does not bind to membranal heparan sulfate on target cells and that heparan sulfate is not involved in NKp30-mediated lysis. In the current study, we examine the binding of six different recombinant NKp30s to membranal heparan sulfate and conclude that NKp30 does interact with membranal heparan sulfate. Yet, two of the six recombinant NKp30s, including the commercially available recombinant NKp30 (employed by Warren et al.) did not show heparan sulfate-dependent binding. We demonstrate that this is due to an altered glycosylation of these two recombinant NKp30s. Upon removal of its N-linked glycans, heparan sulfate-dependent binding to tumor cells and direct binding to heparin were restored. Overall, our results emphasize the importance of proper glycosylation for analysis of NKp30 binding to its ligand and that membranal heparan sulfate could serve as a coligand for NKp30. At the cellular level, soluble heparan sulfate enhanced the secretion of IFNgamma by NK-92 natural killer cells activated with anti-NKp30 monoclonal antibody. We discuss the involvement of heparan sulfate binding to NKp30 in NKp30-mediated activation of NK cells.

Heparan sulfate 6-O-endosulfatases: discrete in vivo activities and functional co-operativity.

HS (heparan sulfate) is essential for normal embryonic development. This requirement is due to the obligatory role for HS in the signalling pathways of many growth factors and morphogens that bind to sulfated domains in the HS polymer chain. The sulfation patterning of HS is determined by a complex interplay of Golgi-located N- and O-sulfotransferases which sulfate the heparan precursor and cell surface endosulfatases that selectively remove 6-O-sulfates from mature HS chains. In the present study we generated single or double knock-out mice for the two murine endosulfatases mSulf1 and mSulf2. Detailed structural analysis of HS from mSulf1-/- fibroblasts showed a striking increase in 6-O-sulfation, which was not seen in mSulf2-/- HS. Intriguingly, the level of 6-O-sulfation in the double mSulf1-/-/2-/- HS was significantly higher than that observed in the mSulf1-/- counterpart. These data imply that mSulf1 and mSulf2 are functionally co-operative. Unlike their avian orthologues, mammalian Sulf activities are not restricted to the highly sulfated S-domains of HS. Mitogenesis assays with FGF2 (fibroblast growth factor 2) revealed that Sulf activity decreases the activating potential of newly-synthesized HS, suggesting an important role for these enzymes in cell growth regulation in embryonic and adult tissues.

Multimers of the fibroblast growth factor (FGF)-FGF receptor-saccharide complex are formed on long oligomers of heparin.

The minimal signalling unit for tyrosine kinase receptors is two protomers dimerized by one or more ligands. However, it is clear that maximal signalling requires the formation of larger complexes of many receptors at discrete foci on the cell surface. The biological interactions that lead to this are likely to be diverse and have system specific components. In the present study, we demonstrate that, in the FGF (fibroblast growth factor)-FGFR (FGF receptor) system, multimers of the minimal complex composed of two FGF1 and two FGFR2 protomers can form on a single chain of the co-receptor heparin. Using size-exclusion chromatography, we show that two complexes can form on heparin chains as small as 16 saccharide units. We also show by MS that discrete complexes containing exactly two copies of the minimal signalling unit are formed. However, the doublet of complexes appears to be less co-operative than the formation of the 2:2:1 FGF1:FGFR2:heparin complex, suggesting that this mechanism is one of a number of weaker interactions that might be involved in the formation of a focal complex on the cell surface.

Regulation of fibroblast growth factor-2 activity by human ovarian cancer tumor endothelium.

Fibroblast growth factor-2 (FGF-2) is a potent angiogenic cytokine that is dependent on heparan sulfate for its biological activity. We have investigated the relationship among heparan sulfate, FGF-2, and the signal-transducing receptors in human, advanced-stage, serous ovarian adenocarcinoma. Using a unique molecular probe, FR1c-Ap, which consisted of a soluble FGF receptor 1 isoform IIIc covalently linked to an alkaline phosphatase moiety, the distribution of heparan sulfate that had the ability to support the formation of a heparan sulfate/FGF-2/FGFR1 isoform IIIc alkaline phosphatase heparan sulfate construct complex was determined. This may be taken as a surrogate marker for the distribution of biologically active heparan sulfate and was distributed predominantly in endothelial cells and stroma but was absent from adenocarcinoma cells. In situ hybridization revealed the expression of FGFR1 mRNA in the endothelium and reverse transcription-PCR confirmed the presence of FGFR1 isoform IIIc but not isoform IIIb. The presence of FGF-2 around tumor endothelium was detected through immunohistochemistry. Double-staining techniques showed that heparan sulfate was found predominantly at the basal aspect of the endothelium and suggested that syndecan-3 might function as one of the proteoglycans involved in FGF-2 signaling in the endothelium. The data suggest that the entire extracellular signaling apparatus, consisting of FGF-2, biologically active heparan sulfate, and FGFRs capable of responding to FGF-2, is present in ovarian cancer endothelium, thereby highlighting the cytokine and its cognate receptor as potential targets for the antiangiogenic treatment of this disease.

Heparin octasaccharides inhibit angiogenesis in vivo.

BACKGROUND: In previous experiments, we showed that heparin oligosaccharides inhibit the angiogenic cytokine fibroblast growth factor-2. Here, we present the first in vivo study of size-fractionated heparin oligosaccharides in four models of angiogenesis that are progressively less dependent on fibroblast growth factor-2. EXPERIMENTAL DESIGN: Heparin oligosaccharides were prepared using size-exclusion gel filtration chromatography and characterized through depolymerization and strong anion exchange high-performance liquid chromatography. Size-defined oligosaccharides (20 mg/kg/d) were given to mice bearing s.c. sponges that were injected with fibroblast growth factor-2 (100 ng/d). After 14 days, octasaccharides and decasaccharides reduced the microvessel density to levels below control. In a second experiment, HEC-FGF2 human endometrial cancer cells that overexpress fibroblast growth factor-2 were implanted in a hollow fiber placed s.c. in vivo. Oligosaccharides were given at 20 mg/kg/d for 2 weeks and the data again showed that octasaccharides significantly reduced microvessel density around the fiber (P = 0.03). In a more complex model, where angiogenesis was induced by a broad spectrum of growth factors, including vascular endothelial growth factor, we implanted H460 lung carcinoma cells in hollow fibers and treated the animals with oligosaccharides at 20 mg/kg/d over 3 weeks. Octasaccharides reduced the microvessel density to that of control. Preliminary investigation of 6-O-desulfated heparins showed that these also had antiangiogenic activity. RESULTS: Finally, we examined the inhibitory potential of hexasaccharides and octasaccharides given at 20 mg/kg/d and these inhibited the growth of H460 lung carcinoma in vivo. At clinically attainable concentrations, significant anticoagulation (activated partial thromboplastin time, anti-factor Xa, and anti-factor IIa) was not observed in vitro unless species containing > or =16 saccharide residues were investigated. CONCLUSIONS: Thus, our preclinical data show that heparin octasaccharides represent novel antiangiogenic compounds that can be given without the anticoagulant effects of low molecular weight heparin.

Prophylactic Use of Nebulized Hypertonic Saline in Mechanically Ventilated Children: A Randomized Blinded Pilot Study.

BACKGROUND: Mucolytic agents, such as nebulized hypertonic saline, may improve airway clearance and shorten the duration of mechanical ventilation, but prospective blinded studies in children undergoing mechanical ventilation are lacking. METHODS: Children <18 y old who had been intubated for <12 h and had an expected duration of mechanical ventilation of >48 additional h were prophylactically given 3 mL of either nebulized hypertonic saline or placebo (0.9% saline) 4 times/d. The primary outcome was duration of mechanical ventilation. Ventilator parameters and the presence of wheezing were recorded before and after study drug administration. RESULTS: The duration of mechanical ventilation was significantly longer in children treated with hypertonic saline (208.1 [interquartile range 136.3-319.8] h) versus those treated with placebo (129.5 [interquartile range 74.4-146.1] h) (P = .03 by Wilcoxon rank-sum test). After adjusting for baseline levels of PEEP, the duration of mechanical ventilation did not differ between groups. Mechanical ventilation parameters, including dead space and dynamic compliance, did not differ between measurements taken before study drug administration versus measurements taken after. New onset wheezing following study drug administration was rare (1.0% with hypertonic saline vs 3.0% with placebo, P = .36 by chi-square test). CONCLUSIONS: Administering prophylactic nebulized hypertonic saline to mechanically ventilated children did not improve clinically relevant outcomes, including duration of mechanical ventilation. Wheezing after hypertonic saline treatment was rare.

Opticin binds to heparan and chondroitin sulfate proteoglycans.

PURPOSE: The extracellular matrix glycoprotein opticin is a small leucine-rich repeat proteoglycan/protein family member that was discovered associated with vitreous humor collagen fibrils. Opticin is present throughout the vitreous, but is particularly concentrated at the internal limiting lamina, where it colocalizes with type XVIII collagen. The present study investigated whether opticin interacts directly with the heparan sulfate (HS) proteoglycan type XVIII collagen. METHODS: Solid-phase opticin binding assays were performed with immobilized type XVIII collagen and heparin albumin. Surface plasmon resonance (SPR) was used to investigate the binding of opticin to heparin and HS. RESULTS: Opticin bound to type XVIII collagen via its HS chains. SPR showed that opticin bound to porcine intestinal mucosa HS and heparin with moderately high affinity (K(D) 73 and 43 nM, respectively). Binding inhibition studies showed that hexasaccharides of heparin had a lower affinity for opticin than larger oligosaccharides; the sulfate groups of heparin contributed variably to opticin binding, with the group at ring position two of iduronate contributing least; and chondroitin sulfate A and B bound to opticin, whereas binding to chondroitin sulfate C and hyaluronan was not observed. CONCLUSIONS: Opticin binds to heparin, HS, chondroitin 4-sulfate, and dermatan sulfate, the binding affinity being dependent on sulfation pattern and oligosaccharide chain length. Opticin may provide a link between cortical vitreous collagen fibrils and the inner limiting lamina by binding HS proteoglycans and stabilize vitreous gel structure by binding chondroitin sulfate proteoglycans.

Distribution and clinical significance of heparan sulfate proteoglycans in ovarian cancer.

PURPOSE: Heparan sulfate proteoglycans have been implicated in cancer cell growth, invasion, metastasis, and angiogenesis. This study was designed to compare their expression in normal ovary and ovarian tumors and then to examine their prognostic significance in ovarian cancer. EXPERIMENTAL DESIGN: The expression of syndecan-1, -2, -3, and -4, glypican-1, and perlecan was assessed by immunohistochemistry in 147 biopsies that included normal ovary and benign, borderline, and malignant ovarian tumors. Clinical data, including tumor stage, performance status, treatment, and survival, were collected. Univariate and multivariate analyses were performed to evaluate prognostic significance. RESULTS: The expression patterns of syndecan-1 and perlecan were altered in ovarian tumors compared with normal ovary. Syndecan-1 was not detected in normal ovary but was present in the epithelial and stromal cells of benign and borderline tumors and in ovarian adenocarcinomas. Perlecan expression was decreased in basement membranes that were disrupted by cancer cells but maintained in the basement membranes of blood vessels. Syndecan-2, -3, and -4, and glypican-1 were expressed in normal ovary and benign and malignant ovarian tumors. Stromal expression of syndecan-1 and glypican-1 were poor prognostic factors for survival in univariate analysis. CONCLUSION: We report for the first time distinct patterns of expression of cell surface and extracellular matrix heparan sulfate proteoglycans in normal ovary compared with ovarian tumors. These data reinforce the role of the tumor stroma in ovarian adenocarcinoma and suggest that stromal induction of syndecan-1 contributes to the pathogenesis of this malignancy.

Binding of endostatin to endothelial heparan sulphate shows a differential requirement for specific sulphates.

Endostatin is a naturally occurring proteolytic fragment of the C-terminal domain of collagen XVIII. It inhibits angiogenesis by a mechanism that appears to involve binding to HS (heparan sulphate). We have examined the molecular interaction between endostatin and HS from micro- and macrovessel endothelial cells. Two discrete panels of oligosaccharides were prepared from metabolically radiolabelled HS, using digestion with either heparinase I or III, and then examined for their endostatin affinity using a sensitive filter-binding assay. Two types of endostatin-binding regions were identified: one comprising sulphated domains of five or more disaccharides in length, enriched in 6-O-sulphate groups, and the other contained long heparinase I-resistant fragments. In the latter case, evidence from the present study suggests that the binding region encompasses a sulphated domain fragment and a transition zone of intermediate sulphation. The contribution to binding of specific O-sulphate groups was determined using selectively desulphated HS species, namely HS from Hs2st-/- mutant cells, and by comparing the compositions of endostatin-binding and non-binding oligosaccharides. The results indicate that 6-O-sulphates play a dominant role in site selectivity and 2-O-sulphates are not strictly essential.

New insights into heparan sulphate biosynthesis from the study of mutant mice.

Heparan sulphate (HS) is an essential co-receptor for a number of growth factors, morphogens and adhesion proteins. The biosynthetic modifications involved in the generation of a mature HS chain may determine the strength and outcome of HS-ligand interactions. These modifications are catalysed by a complex family of enzymes, some of which occur as multiple gene products. Various mutant mice have now been generated, which lack the function of isolated components of the HS biosynthetic pathway. In this discussion, we outline the key findings of these studies, and use them to put into context our own work concerning the structure of the HS generated by the Hs2st -/- mice.

Proteoglycan-specific molecular switch for RPTPσ clustering and neuronal extension.

Heparan and chondroitin sulfate proteoglycans (HSPGs and CSPGs, respectively) regulate numerous cell surface signaling events, with typically opposite effects on cell function. CSPGs inhibit nerve regeneration through receptor protein tyrosine phosphatase sigma (RPTPσ). Here we report that RPTPσ acts bimodally in sensory neuron extension, mediating CSPG inhibition and HSPG growth promotion. Crystallographic analyses of a shared HSPG-CSPG binding site reveal a conformational plasticity that can accommodate diverse glycosaminoglycans with comparable affinities. Heparan sulfate and analogs induced RPTPσ ectodomain oligomerization in solution, which was inhibited by chondroitin sulfate. RPTPσ and HSPGs colocalize in puncta on sensory neurons in culture, whereas CSPGs occupy the extracellular matrix. These results lead to a model where proteoglycans can exert opposing effects on neuronal extension by competing to control the oligomerization of a common receptor.

Antimigratory and antimetastatic effect of heparin-derived 4-18 unit oligosaccharides in a preclinical human melanoma metastasis model.

Heparin and its derivatives have been shown to inhibit angiogenesis and metastasis formation. Accordingly, we investigated the effect of heparin fragments containing 4 to 22 monomers on human melanoma cell proliferation, migration and invasion in vitro as well as on the in vivo metastatic potential in a SCID mouse model. Only oligosaccharide dp18 had significant inhibitory effect on cell proliferation. In contrast, cell migration was inhibited by all oligosaccharides studied except dp8 and dp22. Anti-CD44v3 antibody stimulated cell migration and invasion, and this effect could be attenuated by oligosaccharides dp4 and dp18. These fragments also inhibited the catalytic activity of myosin light chain phosphatase as well. Moreover, oligosaccharides dp4 and dp18 reduced the number of lung colonies formed in SCID mice intravenously injected with human melanoma cells, while dp22 proved to be ineffective in this respect. These studies revealed that fragments of heparin have an antimigratory and antimetastatic potential. These fragments lack the haemostatic effect of heparin, suggesting that they are potential specific antimetastatic agents in anticancer therapy.

The binding properties of minimal oligosaccharides reveal a common heparan sulfate/dermatan sulfate-binding site in hepatocyte growth factor/scatter factor that can accommodate a wide variety of sulfation patterns.

Heparan sulfate (HS)/heparin and dermatan sulfate (DS) both bind with high affinity to hepatocyte growth factor/scatter factor (HGF/SF) and function as necessary co-factors in vitro. How both these two structurally distinct glycosaminoglycans (GAGs) are recognized has remained unclear. We have now reconciled this issue using a panel of minimal tri- and tetrasaccharide sequences of variable but well defined sulfation patterns in combination with further development of the gel mobility shift assay to allow simultaneous comparisons of relative protein affinities/selectivities for different oligosaccharides. From this approach it would seem that a minimum binding sequence is a disulfated trisaccharide comprised of an internal iduronate flanked by monosulfated hexosamine residues and that additional sulfation further enhances affinity. However, the similarity in recognition of HS/heparin and DS seems to arise primarily from a lack of any apparent positional requirement for sulfation. Thus, isomers of HS/heparin tetrasaccharides containing only two sulfates irrespective of whether they are purely N-, 2-O-, or 6-O-sulfates bind with equivalent apparent affinity as a disulfated DS tetrasaccharide. In addition, the NMR chemical shifts induced in NK1 (the truncated variant of HGF/SF comprised of the N-terminal and first Kringle domains) by titration with either heparin or DS oligosaccharides strongly indicate that both bind to essentially the same site. Together, these observations reveal an unexpected degree of flexibility in the GAG-HGF/SF interface, allowing a single binding site in the protein to accommodate iduronate-containing sequences of variable sulfation pattern and/or density from different GAGs.

Evidence that heparin saccharides promote FGF2 mitogenesis through two distinct mechanisms.

Heparin-like saccharides play an essential role in binding to both fibroblast growth factors (FGF) and their receptors at the cell surface. In this study we prepared a series of heparin oligosaccharides according to their size and sulfation level. We then investigated their affinity for FGF2 and their ability to support FGF2 mitogenesis of heparan sulfate-deficient cells expressing FGFR1c. Tetra- and hexasaccharides bound FGF2, but failed to dimerize the growth factor. Nevertheless, these saccharides promoted FGF2-mediated cell growth. Furthermore, whereas enzymatic removal of the non-reducing end 2-O-sulfate group had little effect on the 1:1 interaction with FGF2, it eliminated the mitogenic activity of these saccharides. This evidence supports the symmetric two-end model of ternary complex formation. In contrast, even at very low concentrations, octasaccharide and larger heparin fragments conferred a potent mitogenic activity that was independent of terminal 2-O-sulfation. This correlated with the ability to dimerize FGF2 in an apparently cooperative manner. This data suggests that potent mitogenic signaling results from heparin-mediated trans-dimerization of FGF2, consistent with the asymmetric model of ternary complex formation. We propose that, depending on saccharide structure, there are different architectures and modes of ternary complex assembly that differ in stability and/or efficiency of transmembrane signaling.

Heparin-induced cis- and trans-dimerization modes of the thrombospondin-1 N-terminal domain.

Through its interactions with proteins and proteoglycans, thrombospondin-1 (TSP-1) functions at the interface of the cell membrane and the extracellular matrix to regulate matrix structure and cellular phenotype. We have previously determined the structure of the high affinity heparin-binding domain of TSP-1, designated TSPN-1, in association with the synthetic heparin, Arixtra. To establish that the binding of TSPN-1 to Arixtra is representative of the association with naturally occurring heparins, we have determined the structures of TSPN-1 in complex with heparin oligosaccharides containing eight (dp8) and ten (dp10) subunits, by x-ray crystallography. We have found that dp8 and dp10 bind to TSPN-1 in a manner similar to Arixtra and that dp8 and dp10 induce the formation of trans and cis TSPN-1 dimers, respectively. In silico docking calculations partnered with our crystal structures support the importance of arginine residues in positions 29, 42, and 77 in binding sulfate groups of the dp8 and dp10 forms of heparin. The ability of several TSPN-1 domains to bind to glycosaminoglycans simultaneously probably increases the affinity of binding through multivalent interactions. The formation of cis and trans dimers of the TSPN-1 domain with relatively short segments of heparin further enhances the ability of TSP-1 to participate in high affinity binding to glycosaminoglycans. Dimer formation may also involve TSPN-1 domains from two separate TSP-1 molecules. This association would enable glycosaminoglycans to cluster TSP-1.