Biosynthetic oligosaccharide libraries for identification of protein-binding heparan sulfate motifs. Exploring the structural diversity by screening for fibroblast growth factor (FGF)1 and FGF2 binding

Insights into the role of heparan sulphate in fibroblast growth factor signalling

Signalling from the FGFs (fibroblast growth factors) is crucial for the correct development and homoeostasis of a wide range of cells and tissues. The FGF/FGFR (FGF receptor) signalling system forms an important paradigm for HS (heparan sulphate)-binding proteins, as both the growth factor and receptor bind to HS, and HS or heparin is an absolute requirement for full signalling. The FGF signalling system has been extremely well structurally characterized, and details of each interaction involved in forming a ternary complex of FGF-FGFR-heparin have been elucidated. Recent work has focused on a more thorough understanding of the nature of the FGF-heparin complex in particular, demonstrating that FGFs preferentially bind to similar sites on the co-receptor, and that FGF-FGFR pairs show greater specificity for heparin sulphation patterns than individual FGFs. Further work has suggested that FGF-FGFR-heparin signalling complexes contain one molecule of heparin only, and that when longer fragments of heparin are used to form FGF-FGFR-heparin complexes, multiple complexes form upon the saccharide. These observations form the basis of a model where the range of interactions that FGFs and FGFRs can form with one another and with HS may lead to the formation of complexes with more than two FGFR units. Therefore HS will be crucial to FGF signalling from the initial signalling event to the formation of large receptor clusters.

Heparan sulfate-related oligosaccharides in ternary complex formation with fibroblast growth factors 1 and 2 and their receptors

Biosynthesis of heparan sulfate (HS) is strictly regulated to yield products with cell/tissue-specific composition. Interactions between HS and a variety of proteins, including growth factors and morphogens, are essential for embryonic development and for homeostasis in the adult. Fibroblast growth factors (FGFs) and their various receptors (FRs) form ternary complexes with HS, as required for receptor signaling. Libraries of HS-related, radiolabeled oligosaccharides were generated by chemo-enzymatic modification of heparin and tested for affinity to immobilized FR ectodomains in the presence of FGF1 or FGF2. Experiments were designed to enable assessment of N-sulfated 8- and 10-mers with defined numbers of iduronic acid 2-O-sulfate and glucosamine 6-O-sulfate groups. FGF1 and FGF2 were found to require similar oligosaccharides in complex formation with FR1c-3c, FGF2 affording somewhat more efficient oligosaccharide recruitment than FGF1. FR4, contrary to FR1c-3c, bound oligosaccharides at physiological ionic conditions even in the absence of FGFs, and this interaction was further promoted by FGF1 but not by FGF2. In all systems studied, the stability of FGF-oligosaccharide-FR complexes correlated with the overall level of saccharide O-sulfation rather than on the precise distribution of sulfate groups.

Isolation and characterization of low sulfated heparan sulfate sequences with affinity for lipoprotein lipase

Lipoprotein lipase (LPL), which is an important enzyme in lipid metabolism, binds to heparan sulfate (HS) proteoglycans. This interaction is crucial for several aspects of LPL function, such as intracellular/extracellular transport and high capacity attachment to cell surfaces. Retention of LPL on the capillary walls, and elsewhere, via HS chains is most likely affected by the quality and quantity of HS present. Earlier studies have demonstrated that LPL interacts with highly sulfated HS and heparin oligosaccharides. Since such structures are relatively rare in endothelial HS, we have re-addressed the question of physiological ligand structures for LPL by affinity purification of end-labeled oligosaccharides originating from heparin and HS on immobilized LPL. By a combination of chemical modification and fragmentation of the bound material we identified that the bound fraction contained modestly sulfated oligosaccharides with an average sulfation of one O-sulfate per disaccharide unit and tolerates N-acetylated glucosamine residues. Therefore LPL, containing several clusters of positive charges on each subunit, may constitute an ideal structure for a protein that needs to bind with reasonable affinity to a variety of modestly sulfated sequences of the type that is abundant in HS chains.

Extracellular matrix remodeling in hepatocellular carcinoma: effects of soil on seed?

Extracellular matrix plays two-edged roles, inhibitor and promoter, in the carcinogenesis and progression of hepatocellular carcinoma. On the one hand, extracellular matrix provides the survival signals, and controls the proliferation, differentiation and metastasis of hepatocellular carcinoma. On the other hand, hepatocarcinoma cells create a permissive soil by extracellular matrix remodeling, result in high proliferation, low differentiation, apoptosis block, invasion and metastasis. These malignant phenotypes are related with the change of the capsule around hepatocarcinoma cells that composed by collagens I and IV, the cell-extracellular matrix interaction induced by laminin and its receptor-integrins, and the degradation of ECM which is regulated by proteolytic enzymes and their inhibitor. Thus, normalization of ECM may represent a novel therapeutic strategy for hepatocarcinoma cells.

3-O-sulfated oligosaccharide structures are recognized by anti-heparan sulfate antibody HS4C3

Antibodies against heparan sulfate (HS) are useful tools to study the structural diversity of HS. They demonstrate the large sequential variation within HS and show the distribution of HS oligosaccharide sequences within their natural environment. We analyzed the distribution and the structural characteristics of the oligosaccharide epitope recognized by anti-HS antibody HS4C3. Biosynthetic and synthetic heparin-related oligosaccharide libraries were used in affinity chromatography, immunoprecipitation, and enzyme-linked immunosorbent assay to identify this epitope as a 3-O-sulfated motif with antithrombin binding capacity. The antibody binds weakly to any N-sulfated, 2-O- and 6-O-sulfated hexa- to octasaccharide fragment but strongly to the corresponding oligosaccharide when there is a 3-O-sulfated glucosamine residue present in the sequence. This difference was highlighted by affinity interaction and immunohistochemistry at salt concentrations from 500 mm. At physiological salt conditions the antibody strongly recognized basal lamina of epithelia and endothelia. At 500 mm salt conditions, when 3-O sulfation is required for binding, antibody recognition was more restricted and selective. Antibody HS4C3 bound similar tissue structures as antithrombin in rat kidney. Furthermore, antithrombin and antibody HS4C3 could compete with one another for binding to heparin. Antibody HS4C3 was also able to inhibit the anti-coagulant activities of heparin and Arixtra as demonstrated using the activated partial thromboplastin time clotting and the anti-factor Xa assays. In summary, antibody HS4C3 selectively detects 3-O-sulfated HS structures and interferes with the coagulation activities of heparin by association with the anti-thrombin binding pentasaccharide sequence.

Fibroblast growth factors share binding sites in heparan sulphate

HS (heparan sulphate) proteoglycans bind secreted signalling proteins, including FGFs (fibroblast growth factors) through their HS side chains. Such chains contain a wealth of differentially sulphated saccharide epitopes. Whereas specific HS structures are commonly believed to modulate FGF-binding and activity, selective binding of defined HS epitopes to FGFs has generally not been demonstrated. In the present paper, we have identified a series of sulphated HS octasaccharide epitopes, derived from authentic HS or from biosynthetic libraries that bind with graded affinities to FGF4, FGF7 and FGF8b. These HS species, along with previously identified oligosaccharides that interact with FGF1 and FGF2, constitute the first comprehensive survey of FGF-binding HS epitopes based on carbohydrate sequence analysis. Unexpectedly, our results demonstrate that selective modulation of FGF activity cannot be explained in terms of binding of individual FGFs to specific HS target epitopes. Instead, different FGFs bind to identical HS epitopes with similar relative affinities and low selectivity, such that the strength of these interactions increases with increasing saccharide charge density. We conclude that FGFs show extensive sharing of binding sites in HS. This conclusion challenges the current notion of specificity in HS-FGF interactions, and instead suggests that a set of common HS motifs mediates cellular targeting of different FGFs.

The specificity of interactions between proteins and sulfated polysaccharides

Sulfated polysaccharides are capable of binding with proteins at several levels of specificity. As highly acidic macromolecules, they can bind non-specifically to any basic patch on a protein surface at low ionic strength, and such interactions are not likely to be physiologically significant. On the other hand, several systems have been identified in which very specific substructures of sulfated polysaccharides confer high affinity for particular proteins; the best-known example of this is the pentasaccharide in heparin with high affinity for antithrombin, but other examples may be taken from the study of marine invertebrates: the importance of the fine structure of dermatan sulfate (DS) to its interaction with heparin cofactor II (HCII), and the involvement of sea urchin egg-jelly fucans in species specific fertilization. A third, intermediate, kind of specific interaction is described for the cell-surface glycosaminoglycan heparan sulfate (HS), in which patterns of sulfate substitution can show differential affinities for cytokines, growth factors, and morphogens at cell surfaces and in the intracellular matrix. This complex interplay of proteins and glycans is capable of influencing the diffusion of such proteins through tissue, as well as modulating cellular responses to them.

Xylosyltransferase I acceptor properties of fibroblast growth factor and its fragment bFGF (1-24)

Human basic fibroblast growth factor (bFGF) is a heparin-binding growth factor containing a G-S-G-motif which is a potential recognition sequence of xylosyltransferase I (XT-I). Here, we show that the recombinant human bFGF was xylosylated in vitro by human XT-I and that the fragment bFGF (1-24) is a good XT-I acceptor (K(m) = 20.8 microM for native XT-I and K(m) = 22.3 microM for recombinant XT-I). MALDI and MALDI-PSD time-of-flight mass spectrometric analyses of the xylosylated bFGF protein demonstrate the transfer of xylose to the serine residue of the G-S-G-motif in the amino terminal end of bFGF. The peptide bFGF (1-24) is well suitable as an acceptor substrate for XT-I and can be used in a radiochemical assay to measure the XT-I activity in cell culture supernatant and human body fluids, respectively. Furthermore, we could demonstrate that the XT-I interacts strongly with heparin and that this glycosaminoglycan is a predominantly non-competitive inhibitor of the enzyme using the fragment bFGF (1-24) as xylose acceptor.

Effects of heparin and its 6-O-and 2-O-desulfated derivatives with low anticoagulant activity on proliferation of human neural stem/progenitor cells

Heparin binds various growth factors and activates them to interact with high-affinity cell surface receptors; a specific array of sulfate groups in the heparin backbone structure is very important for this interaction. In the present study, we evaluated the effects of two novel heparin derivatives, 6-O-desulfated heparin (6-DSH) and 2-O-desulfated heparin (2-DSH), on blood coagulation and the proliferation of human neural stem/progenitor cells (NSPCs). 6-DSH showed lower anticoagulant activity than intact heparin or 2-DSH, as measured by the activated partial thromboplastin time and thrombin time. In the presence of FGF-2, 6-DSH and 2-DSH promoted approximately the same rate of proliferation of human NSPCs, without noticeably changing the expression of nestin. The mitotic effects of 6-DSH and 2-DSH on human NSPCs were different from their effects on mouse hematopoietic stem cells and fibroblasts. These findings indicate that 6-DSH and 2-DSH have the same ability to promote the growth of human NSPCs as intact heparin. Our results suggest that these two novel heparin derivates, especially 6-DSH, could be used in clinical applications for ex vivo human NSPC culture, as a lower-risk growth co-adjuvant than intact heparin.

The sweet and sour of cancer: glycans as novel therapeutic targets

A growing body of evidence supports crucial roles for glycans at various pathophysiological steps of tumour progression. Glycans regulate tumour proliferation, invasion, haematogenous metastasis and angiogenesis, and increased understanding of these roles sets the stage for developing pharmaceutical agents that target these molecules. Such novel agents might be used alone or in combination with operative and/or chemoradiation strategies for treating cancer.

Fibrillin-1 interactions with heparin. Implications for microfibril and elastic fiber assembly

Fibrillin-1 assembly into microfibrils and elastic fiber formation involves interactions with glycosaminoglycans. We have used BIAcore technology to investigate fibrillin-1 interactions with heparin and with heparin saccharides that are analogous to S-domains of heparan sulfate. We have identified four high affinity heparin-binding sites on fibrillin-1, localized three of these sites, and defined their binding kinetics. Heparin binding to the fibrillin-1 N terminus has particularly rapid kinetics. Hyaluronan and chondroitin sulfate did not interact significantly with fibrillin-1. Heparin saccharides with more than 12 monosaccharide units bound strongly to all four fibrillin-1 sites. Heparin did not inhibit fibrillin-1 N- and C-terminal interactions or RGD-dependent cell attachment, but heparin and MAGP-1 competed for binding to the fibrillin-1 N terminus, and heparin and tropoelastin competed for binding to a central fibrillin-1 sequence. By regulating these key interactions, heparin can profoundly influence microfibril and elastic fiber assembly.

Sulfation pattern in glycosaminoglycan: does it have a code?

Heparan sulfate chains (HS) are initially synthesized on core proteins as linear polysaccharides composed of glucuronic acid--N-acetylglucosamine repeating units and subjected to marked structural modification by sulfation (N-, 2-O-, 6-O-, 3-O-sulfotransferases) and epimerization (C5-epimerase) at the Golgi lumen and further by desulfation (6-O- endosulfatase) at the cell surface, after which divergent fine structures are generated. The expression patterns and specificity of the modifying enzymes are, at least partly, responsible for the elaboration of these fine structures of heparan sulfate. HS interacts with many proteins including growth factors (GF) and morphogens through specific fine structures. Recent biochemical and genetic studies have presented evidence that HS plays important roles in cell behavior and organogenesis. In knock-down experiments of heparan sulfate 6-O-sulfotransferase, 6-O-sulfated units in HS have been shown to act as a stimulator or suppressor according to individual GF/morphogen signaling systems.

Chemical biology of the sugar code

A high-density coding system is essential to allow cells to communicate efficiently and swiftly through complex surface interactions. All the structural requirements for forming a wide array of signals with a system of minimal size are met by oligomers of carbohydrates. These molecules surpass amino acids and nucleotides by far in information-storing capacity and serve as ligands in biorecognition processes for the transfer of information. The results of work aiming to reveal the intricate ways in which oligosaccharide determinants of cellular glycoconjugates interact with tissue lectins and thereby trigger multifarious cellular responses (e.g. in adhesion or growth regulation) are teaching amazing lessons about the range of finely tuned activities involved. The ability of enzymes to generate an enormous diversity of biochemical signals is matched by receptor proteins (lectins), which are equally elaborate. The multiformity of lectins ensures accurate signal decoding and transmission. The exquisite refinement of both sides of the protein-carbohydrate recognition system turns the structural complexity of glycans--a demanding but essentially mastered problem for analytical chemistry--into a biochemical virtue. The emerging medical importance of protein-carbohydrate recognition, for example in combating infection and the spread of tumors or in targeting drugs, also explains why this interaction system is no longer below industrial radarscopes. Our review sketches the concept of the sugar code, with a solid description of the historical background. We also place emphasis on a distinctive feature of the code, that is, the potential of a carbohydrate ligand to adopt various defined shapes, each with its own particular ligand properties (differential conformer selection). Proper consideration of the structure and shape of the ligand enables us to envision the chemical design of potent binding partners for a target (in lectin-mediated drug delivery) or ways to block lectins of medical importance (in infection, tumor spread, or inflammation).

Sulfatases: Structure, Mechanism, Biological Activity, Inhibition, and Synthetic Utility

Sulfatases, which cleave sulfate esters in biological systems, play a key role in regulating the sulfation states that determine the function of many physiological molecules. Sulfatase substrates range from small cytosolic steroids, such as estrogen sulfate, to complex cell-surface carbohydrates, such as the glycosaminoglycans. The transformation of these molecules has been linked with important cellular functions, including hormone regulation, cellular degradation, and modulation of signaling pathways. Sulfatases have also been implicated in the onset of various pathophysiological conditions, including hormone-dependent cancers, lysosomal storage disorders, developmental abnormalities, and bacterial pathogenesis. These findings have increased interest in sulfatases and in targeting them for therapeutic endeavors. Although numerous sulfatases have been identified, the wide scope of their biological activity is only beginning to emerge. Herein, accounts of the diversity and growing biological relevance of sulfatases are provided along with an overview of the current understanding of sulfatase structure, mechanism, and inhibition.

Amyloidogenesis recapitulated in cell culture: a peptide inhibitor provides direct evidence for the role of heparan sulfate and suggests a new treatment strategy

To date 22 different polypeptides, including Abeta in Alzheimer's disease and PrP(Sc) in prion disorders, are known to re-fold and assemble into highly organized fibrils, which associate with heparan sulfate (HS) proteoglycans to form tissue deposits called amyloid. Mononuclear phagocytes have long been thought to be involved in this process, and we describe a monocytic cell culture system that can transform the acute-phase protein serum amyloid A (SAA1.1) into AA-amyloid and appears to recapitulate all the main features of amyloidogenesis observed in vivo. These features in common include nucleation-dependent kinetics, identical proteolytic processing of SAA1.1, and co-deposition of HS with the fibrils. Heparin and polyvinylsulfonate previously reported to block AA-amyloidogenesis in mice are also effective inhibitors in this cell culture model. Furthermore, a synthetic peptide (27-mer) corresponding to a HS binding site of SAA, blocks amyloid deposition at a concentration that is several-orders-of-magnitude lower than any other peptide-based inhibitor previously reported. The 27-mer's inhibitory activity may target the amyloidogenic pathway specifically as it does not interfere with the binding of SAA to monocytes. These data provide direct evidence that SAA1.1:HS interactions are a critical step in AA-amyloidogenesis and suggest a novel treatment strategy for other amyloidoses.

QSulf1, a heparan sulfate 6-O-endosulfatase, inhibits fibroblast growth factor signaling in mesoderm induction and angiogenesis

The signaling activities of multiple developmental ligands require sulfated heparan sulfate (HS) proteoglycans as coreceptors. QSulf1 and its mammalian orthologs are cell surface HS 6-O-endosulfatases that are expressed in embryonic mesodermal and neural progenitors and promote Wnt signal transduction. In this study, we have investigated the function of QSulf1 in fibroblast growth factor (FGF) signaling, which requires 6-O-sulfated HS for FGF receptor (FGFR) dimerization and tyrosine kinase activation. Here, we report that QSulf1 inhibits FGF2- and FGF4-induced mesoderm formation in the Xenopus embryo and FGF-dependent angiogenesis in the chicken embryo through 6-O-desulfation of cell surface HS. QSulf1 regulates FGF signaling through inhibition of HS-mediated FGFR1 activation by interfering with FGF-HS-FGFR1 ternary complex formation. Furthermore, QSulf1 can produce enzymatically modified soluble heparin that acts as a potent inhibitor of FGF2-induced angiogenesis in the chicken embryo. QSulf1, therefore, has dual regulatory functions as a negative regulator of FGF signaling and a positive regulator of Wnt signaling. Therefore, QSulf1 provides another reagent to produce enzymatically modified heparin compounds, in vivo and in vitro, to modulate cellular signaling in stem cell-based therapies to promote tissue regeneration and in cancer therapies to control cell growth and block angiogenesis.

A new model for the domain structure of heparan sulfate based on the novel specificity of K5 lyase

Elucidation of the molecular structure of heparan sulfate (HS) is the key to understanding its functional versatility as a co-receptor for growth factors and morphogens. We have identified and exploited the novel substrate specificity of the coliphage K5 lyase in studies of the domain organization of HS. We show that K5 lyase cleaves HS principally within non-sulfated sequences of four or more N-acetylated disaccharides. Uniquely, sections comprising alternating N-acetylated and N-sulfated units are resistant to the enzyme, as are the highly sulfated S domains. Spacing of the K5 lyase cleavage sites ( approximately 7-8 kDa) is similar to that of the S domains. On the basis of these findings, we propose a refined model of the structure of HS in which N-acetylated sequences of four to five disaccharide units (GlcNAc-GlcUA)(4-5) are positioned centrally between the S domains. The latter are embedded within N-acetylated and N-sulfated sequences, forming extended regions of hypervariable sulfation distributed at regular intervals along the polymer chain. K5 lyase provides a means of excision of these composite sulfated regions for structural and functional analyses.

Characterization of growth factor-binding structures in heparin/heparan sulfate using an octasaccharide library

Heparan sulfate (HS) chains interact with various growth and differentiation factors and morphogens, and the most interactions occur on the specific regions of the chains with certain monosaccharide sequences and sulfation patterns. Here we generated a library of octasaccharides by semienzymatic methods by using recombinant HS 2-O-sulfotransferase and HS 6-O-sulfotransferase, and we have made a systematic investigation of the specific binding structures for various heparin-binding growth factors. An octasaccharide (Octa-I, DeltaHexA-GlcNSO(3)-(HexA-GlcNSO(3))(3)) was prepared by partial heparitinase digestion from completely desulfated N-resulfated heparin. 2-O- and 6-O-sulfated Octa-I were prepared by enzymatically transferring one to three 2-O-sulfate groups and one to three 6-O-sulfate groups per molecule, respectively, to Octa-I. Another octasaccharide containing 3 units of HexA(2SO(4))-GlcNSO(3)(6SO(4)) was prepared also from heparin. This octasaccharide library was subjected to affinity chromatography for interactions with fibroblast growth factor (FGF)-2, -4, -7, -8, -10, and -18, hepatocyte growth factor, bone morphogenetic protein 6, and vascular endothelial growth factor, respectively. Based upon differences in the affinity to those octasaccharides, the growth factors could be classified roughly into five groups: group 1 needed 2-O-sulfate but not 6-O-sulfate (FGF-2); group 2 needed 6-O-sulfate but not 2-O-sulfate (FGF-10); group 3 had the affinity to both 2-O-sulfate and 6-O-sulfate but preferred 2-O-sulfate (FGF-18, hepatocyte growth factor); group 4 required both 2-O-sulfate and 6-O-sulfate (FGF-4, FGF-7); and group 5 hardly bound to any octasaccharides (FGF-8, bone morphogenetic protein 6, and vascular endothelial growth factor). The approach using the oligosaccharide library may be useful to define specific structures required for binding to various heparin-binding proteins. Octasaccharides with the high affinity to FGF-2 and FGF-10 had the activity to release them, respectively, from their complexes with HS. Thus, the library may provide new reagents to specifically regulate bindings of the growth factors to HS.

Highly Diverse Heparan Sulfate Analogue Libraries: Providing Access to Expanded Areas of Sequence Space for Bioactivity Screening

Structurally diverse heparan sulfate analogue libraries were produced chemicoenzymatically from heparin. They possess vastly more heterogeneity than tissue heparan sulfates, expand the sequence space available for screening, and can help identify minimal structural features associated with activity. Library components are likely to exhibit fewer nonspecific interactions and side-effects than heparin or simple chemically modified heparin. A strategy for their use is illustrated for the fibroblast growth factor-receptor tyrosine kinase signaling system.

Effects of aFGF and genistein on PKC and ERK activity in human colorectal cancer cell line CCL229

AIM

To observe the effects of aFGF and TPK inhibitor genistein on intracellular PKC and ERK activity in CCL229 cell line.

METHODS

The activities of PKC and ERK in cells induced by different concentrations of aFGF (0.15 mg/L, 0.30 mg/L, 0.60 mg/L, 1.20 mg/L) and genistein (6.00 mg/L, 12.00 mg/L, 24.00 mg/L, 48.00 mg/L) were detected by incorporation of [g-³²P]-ATP into exogenous substrates.

RESULTS

The intracellular PKC and ERK activity increased with aFGF in a dose dependent manner (P <0.05). When the concentration of aFGF was 1.20 mg/L , the activity of PKC in cytosol and PKC in membrane and ERK was 2.60, 2.79,1.77 times higher than control group. Genistein suppressed the intracellular PKC and ERK activity also in a dose dependent manner (P <0.05). When the concentration of genistein was 48.00 mg/L, the activity of PKC in cytosol and PKC in membrane and ERK was 0.41,0.36,0.50 times higher than that in control group, The activity of PKC and ERK decreased apparently when the cells were treated with aFGF.

CONCLUSION

aFGF receptor in human colorectal cancer cell line CCL229 possesses TPK activity. Tyrosine-specific protein phosphorylation may initiate a cascade of biochemical events, which may increase the intracellular PKC and ERK activity.

Processing of macromolecular heparin by heparanase

Heparanase is an endo-glucuronidase expressed in a variety of tissues and cells that selectively cleaves extracellular and cell-surface heparan sulfate. Here we propose that this enzyme is involved also in the processing of serglycin heparin proteoglycan in mouse mast cells. In this process, newly synthesized heparin chains (60-100 kDa) are degraded to fragments (10-20 kDa) similar in size to commercially available heparin (Jacobsson, K. G., and Lindahl, U. (1987) Biochem. J. 246, 409-415). A fraction of these fragments contains the specific pentasaccharide sequence required for high affinity binding to antithrombin implicated with anticoagulant activity. Rat skin heparin, which escapes processing in vivo, was used as a substrate in reaction with recombinant human heparanase. An incubation product of commercial heparin size retained the specific pentasaccharide sequence, although oligosaccharides (3-4 kDa) containing this sequence could be degraded by the same enzyme. Commercial heparin was found to be a powerful inhibitor (I50 approximately 20 nM expressed as disaccharide unit, approximately 0.7 nM polysaccharide) of heparanase action toward antithrombin-binding oligosaccharides. Cells derived from a serglycin-processing mouse mastocytoma expressed a protein highly similar to other mammalian heparanases. These findings strongly suggest that the intracellular processing of the heparin proteoglycan polysaccharide chains is catalyzed by heparanase, which primarily cleaves target structures distinct from the antithrombin-binding sequence.

QSulf1 remodels the 6-O sulfation states of cell surface heparan sulfate proteoglycans to promote Wnt signaling

The 6-O sulfation states of cell surface heparan sulfate proteoglycans (HSPGs) are dynamically regulated to control the growth and specification of embryonic progenitor lineages. However, mechanisms for regulation of HSPG sulfation have been unknown. Here, we report on the biochemical and Wnt signaling activities of QSulf1, a novel cell surface sulfatase. Biochemical studies establish that QSulf1 is a heparan sulfate (HS) 6-O endosulfatase with preference, in particular, toward trisulfated IdoA2S-GlcNS6S disaccharide units within HS chains. In cells, QSulf1 can function cell autonomously to remodel the sulfation of cell surface HS and promote Wnt signaling when localized either on the cell surface or in the Golgi apparatus. QSulf1 6-O desulfation reduces XWnt binding to heparin and HS chains of Glypican1, whereas heparin binds with high affinity to XWnt8 and inhibits Wnt signaling. CHO cells mutant for HS biosynthesis are defective in Wnt-dependent Frizzled receptor activation, establishing that HS is required for Frizzled receptor function. Together, these findings suggest a two-state "catch or present" model for QSulf1 regulation of Wnt signaling in which QSulf1 removes 6-O sulfates from HS chains to promote the formation of low affinity HS-Wnt complexes that can functionally interact with Frizzled receptors to initiate Wnt signal transduction.

Oligosaccharide library-based assessment of heparan sulfate 6-O-sulfotransferase substrate specificity

Heparan sulfate mediates numerous complex biological processes. Its action critically depends on the amount and the positions of O-sulfate groups (iduronyl 2-O-sulfates, glucosaminyl 6-O- and 3-O-sulfates) that form binding sites for proteins. The structures and distribution of these protein-binding domains are influenced by the expression and substrate specificity of heparan sulfate biosynthetic enzymes. We describe a general approach to assess substrate specificities of enzymes involved in glycosaminoglycan metabolism, here applied to 6-O-sulfotransferases involved in heparan sulfate biosynthesis. To understand how 2-O-sulfation affects subsequent 6-O-sulfation reactions, the substrate specificity of 6-O-sulfotransferase 3 was probed using substrates from a heparin-based octasaccharide library. Purified 3H-labeled N-sulfated octasaccharides from a library designed to sample 2-O-sulfated motifs were used as sulfate acceptors, 3'-phosphoadenosine 5'-phosphosulfate as sulfate donor, and cell extract from 6-O-sulfotransferase 3-overexpressing 293 cells as enzyme source in the 6-O-sulfotransferase-catalyzed reactions. The first 6-O-sulfate group was preferentially incorporated at the internal glucosamine unit of the octasaccharide substrate. As the reaction proceeded, the octasaccharides acquired three 6-O-sulfate groups. The specificities toward competing octasaccharide substrates, for 6-O-sulfotransferase 2 and 6-O-sulfotransferase 3, were determined using overexpressing 293 cell extracts and purified octasaccharides. Both 6-O-sulfotransferases showed a preference for 2-O-sulfated substrates. The specificity toward substrates with two to three 2-O-sulfate groups was three to five times higher as compared with octasaccharides with no or one 2-O-sulfate group.

Role of heparan sulfate domain organization in endostatin inhibition of endothelial cell function

The anti-angiogenic activity of endostatin (ES) depends on interactions with heparan sulfate (HS). In the present study, intact HS chains of >/=15 kDa bound quantitatively to ES whereas N-sulfated HS decasaccharides, with affinity for several fibroblast growth factor (FGF) species, failed to bind. Instead, ES-binding oligosaccharides composed of mixed N-sulfated and N-acetylated disaccharide units were isolated from pig intestinal HS. A 10/12mer ES-binding epitope was identified, with two N-sulfated regions separated by at least one N-acetylated glucosamine unit (SAS-domain). Cleavage at the N-acetylation site disrupted ES binding. These findings point to interaction between discontinuous sulfated domains in HS and arginine clusters at the ES surface. The inhibitory effect of ES on vascular endothelial growth factor-induced endothelial cell migration was blocked by the ES-binding SAS-domains and by heparin oligosaccharides (12mers) similar in length to the ES-binding SAS-domains, but not by 6mers capable of FGF binding. We propose that SAS-domains modulate the biological activities of ES and other protein ligands with extended HS-binding sites. The results provide a rational explanation for the preferential interaction of ES with certain HS proteoglycan species.

Specific molecular interactions of oversulfated chondroitin sulfate E with various heparin-binding growth factors. Implications as a physiological binding partner in the brain and other tissues

We previously observed that the cortical neuronal cell adhesion mediated by midkine (MK), a heparin (Hep)-binding growth factor, is specifically inhibited by oversulfated chondroitin sulfate-E (CS-E) (Ueoka, C., Kaneda, N., Okazaki, I., Nadanaka, S., Muramatsu, T., and Sugahara, K. (2000) J. Biol. Chem. 275, 37407-37413) and that CS-E exhibits neurite outgrowth promoting activities toward embryonic rat hippocampal neurons. We have also shown oversulfated CS chains in embryonic chick and rat brains and demonstrated that the CS disaccharide composition changes during brain development. In view of these findings, here we tested the possibility of CS-E interacting with Hep-binding growth factors during development, using squid cartilage CS-E. The binding ability of Hep-binding growth factors (MK, pleiotrophin (PTN), fibroblast growth factor-1 (FGF-1), FGF-2, Hep-binding epidermal growth factor-like growth factor (HB-EGF), FGF-10, FGF-16, and FGF-18) toward [(3)H]CS-E was first tested by a filter binding assay, which demonstrated direct binding of all growth factors, except FGF-1, to CS-E. The bindings were characterized further in an Interaction Analysis system, where all of the growth factors, except FGF-1, gave concentration-dependent and specific bindings. The kinetic constants k(a), k(d), and K(d) suggested that MK, PTN, FGF-16, FGF-18, and HB-EGF bound strongly to CS-E, in comparable degrees to the binding to Hep, whereas the intensity of binding of FGF-2 and FGF-10 toward CS-E was lower than that for Hep. These findings suggest the possibility of CS-E being a binding partner, a coreceptor, or a genuine receptor for various Hep-binding growth factors in the brain and possibly also in other tissues.