Requirement for anticoagulant heparan sulfate in the fibroblast growth factor receptor complex

Protein interactors of 3-O sulfated heparan sulfates in human MCI and age-matched control cerebrospinal fluid

Heparan sulfates (HS) proteoglycans are commonly found on the cell surface and mediate many processes. Binding of HS ligands is determined by the sulfation code on the HS chain that can be N-/2-O/6-O- or 3-O-sulfated, generating heterogenous sulfation patterns. 3-O sulfated HS (3S-HS) play a role in several (patho)physiological processes such as blood coagulation, viral pathogenesis and binding and internalization of tau in Alzheimer's disease. However, few 3S-HS-specific interactors are known. Thus, our insight into the role of 3S-HS in health and disease is limited, especially in the central nervous system. Using human CSF, we determined the interactome of synthetic HS with defined sulfation patterns. Our affinity-enrichment mass spectrometry studies expand the repertoire of proteins that may interact with (3S-)HS. Validating our approach, ATIII, a known 3S-HS interactor, was found to require GlcA-GlcNS6S3S for binding, similar to what has been reported. Our dataset holds novel, potential HS and 3S-HS protein ligands, that can be explored in future studies focusing on molecular mechanisms that depend on 3S-HS in (patho)physiological conditions.

HS, an Ancient Molecular Recognition and Information Storage Glycosaminoglycan, Equips HS-Proteoglycans with Diverse Matrix and Cell-Interactive Properties Operative in Tissue Development and Tissue Function in Health and Disease

Heparan sulfate is a ubiquitous, variably sulfated interactive glycosaminoglycan that consists of repeating disaccharides of glucuronic acid and glucosamine that are subject to a number of modifications (acetylation, de-acetylation, epimerization, sulfation). Variable heparan sulfate chain lengths and sequences within the heparan sulfate chains provide structural diversity generating interactive oligosaccharide binding motifs with a diverse range of extracellular ligands and cellular receptors providing instructional cues over cellular behaviour and tissue homeostasis through the regulation of essential physiological processes in development, health, and disease. heparan sulfate and heparan sulfate-PGs are integral components of the specialized glycocalyx surrounding cells. Heparan sulfate is the most heterogeneous glycosaminoglycan, in terms of its sequence and biosynthetic modifications making it a difficult molecule to fully characterize, multiple ligands also make an elucidation of heparan sulfate functional properties complicated. Spatio-temporal presentation of heparan sulfate sulfate groups is an important functional determinant in tissue development and in cellular control of wound healing and extracellular remodelling in pathological tissues. The regulatory properties of heparan sulfate are mediated via interactions with chemokines, chemokine receptors, growth factors and morphogens in cell proliferation, differentiation, development, tissue remodelling, wound healing, immune regulation, inflammation, and tumour development. A greater understanding of these HS interactive processes will improve therapeutic procedures and prognoses. Advances in glycosaminoglycan synthesis and sequencing, computational analytical carbohydrate algorithms and advanced software for the evaluation of molecular docking of heparan sulfate with its molecular partners are now available. These advanced analytic techniques and artificial intelligence offer predictive capability in the elucidation of heparan sulfate conformational effects on heparan sulfate-ligand interactions significantly aiding heparan sulfate therapeutics development.

3-O-Sulfation induces sequence-specific compact topologies in heparan sulfate that encode a dynamic sulfation code

Heparan sulfate (HS) is arguably the most diverse linear biopolymer that is known to modulate hundreds of proteins. Whereas the configurational and conformational diversity of HS is well established in terms of varying sulfation patterns and iduronic acid (IdoA) puckers, a linear helical topology resembling a cylindrical rod is the only topology thought to be occupied by the biopolymer. We reasoned that 3-O-sulfation, a rare modification in natural HS, may induce novel topologies that contribute to selective recognition of proteins. In this work, we studied a library of 24 distinct HS hexasaccharides using molecular dynamics (MD). We discovered novel compact (C) topologies that are populated significantly by a unique group of 3-O-sulfated sequences containing IdoA residues. 3-O-sulfated sequences containing glucuronic acid (GlcA) residue and sequences devoid of 3-O-sulfate groups did not exhibit high levels of the C topology and primarily exhibited only the canonical linear (L) form. The C topology arises under dynamical conditions due to rotation around an IdoA → GlcN glycosidic linkage, especially in psi (Ψ) torsion. At an atomistic level, the L → C transformation is a multi-factorial phenomenon engineered to reduce like-charge repulsion, release one or more HS-bound water molecules, and organize a bi-dentate "IdoA-cation-IdoA" interaction. These forces also drive an L → C transformation in a 3-O-sulfated octasaccharide, which has shown evidence of the unique C topology in the co-crystallized state. The 3-O-sulfate-based generation of unique, sequence-specific, compact topologies indicate that natural HS encodes a dynamic sulfation code that could be exploited for selective recognition of target proteins.

Loss of Hs3st3a1 or Hs3st3b1 enzymes alters heparan sulfate to reduce epithelial morphogenesis and adult salivary gland function

Heparan sulfate 3-O-sulfotransferases generate highly sulfated but rare 3-O-sulfated heparan sulfate (HS) epitopes on cell surfaces and in the extracellular matrix. Previous ex vivo experiments suggested functional redundancy exists among the family of seven enzymes but that Hs3st3a1 and Hs3st3b1 sulfated HS increases epithelial FGFR signaling and morphogenesis. Single-cell RNAseq analysis of control SMGs identifies increased expression of Hs3st3a1 and Hs3st3b1 in endbud and myoepithelial cells, both of which are progenitor cells during development and regeneration. To analyze their in vivo functions, we generated both Hs3st3a1^-/- and Hs3st3b1^-/- single knockout mice, which are viable and fertile. Salivary glands from both mice have impaired fetal epithelial morphogenesis when cultured with FGF10. Hs3st3b1^-/- mice have reduced intact SMG branching morphogenesis and reduced 3-O-sulfated HS in the basement membrane. Analysis of HS biosynthetic enzyme transcription highlighted some compensatory changes in sulfotransferases expression early in development. The overall glycosaminoglycan composition of adult control and KO mice were similar, although HS disaccharide analysis showed increased N- and non-sulfated disaccharides in Hs3st3a1^-/- HS. Analysis of adult KO gland function revealed normal secretory innervation, but without stimulation there was an increase in frequency of drinking behavior in both KO mice, suggesting basal salivary hypofunction, possibly due to myoepithelial dysfunction. Understanding how 3-O-sulfation regulates myoepithelial progenitor function will be important to manipulate HS-binding growth factors to enhance tissue function and regeneration.

Heparan Sulfate as a Therapeutic Target in Tauopathies: Insights From Zebrafish

Microtubule-associated protein tau (MAPT) hyperphosphorylation and aggregation, are two hallmarks of a family of neurodegenerative disorders collectively referred to as tauopathies. In many tauopathies, including Alzheimer's disease (AD), progressive supranuclear palsy (PSP) and Pick's disease, tau aggregates are found associated with highly sulfated polysaccharides known as heparan sulfates (HSs). In AD, amyloid beta (Aβ) peptide aggregates associated with HS are also characteristic of disease. Heparin, an HS analog, promotes misfolding, hyperphosphorylation and aggregation of tau protein in vitro. HS also provides cell surface receptors for attachment and uptake of tau seeds, enabling their propagation. These findings point to HS-tau interactions as potential therapeutic targets in tauopathies. The zebrafish genome contains genes paralogous to MAPT, genes orthologous to HS biosynthetic and chain modifier enzymes, and other genes implicated in AD. The nervous system in the zebrafish bears anatomical and chemical similarities to that in humans. These homologies, together with numerous technical advantages, make zebrafish a valuable model for investigating basic mechanisms in tauopathies and identifying therapeutic targets. Here, we comprehensively review current knowledge on the role of HSs in tau pathology and HS-targeting therapeutic approaches. We also discuss novel insights from zebrafish suggesting a role for HS 3-O-sulfated motifs in tau pathology and establishing HS antagonists as potential preventive agents or therapies for tauopathies.

Sustained delivery of recombinant human bone morphogenetic protein-2 from perlecan domain I - functionalized electrospun poly (ε-caprolactone) scaffolds for bone regeneration

Background

Biomaterial scaffolds that deliver growth factors such as recombinant human bone morphogenetic proteins-2 (rhBMP-2) have improved clinical bone tissue engineering by enhancing bone tissue regeneration. This approach could be further improved if the controlled delivery of bioactive rhBMP-2 were sustained throughout the duration of osteogenesis from fibrous scaffolds that provide control over dose and bioactivity of rhBMP-2. In nature, heparan sulfate attached to core proteoglycans serves as the co-receptor that delivers growth factors to support tissue morphogenesis.

Methods

To mimic this behavior, we conjugated heparan sulfate decorated recombinant domain I of perlecan/HSPG2 onto an electrospun poly(ε-caprolactone) (PCL) scaffold, hypothesizing that the heparan sulfate chains will enhance rhBMP-2 loading onto the scaffold and preserve delivered rhBMP-2 bioactivity.

Results

In this study, we demonstrated that covalently conjugated perlecan domain I increased loading capacity of rhBMP-2 onto PCL scaffolds when compared to control unconjugated scaffolds. Additionally, rhBMP-2 released from the modified scaffolds enhanced alkaline phosphatase activity in W20–17 mouse bone marrow stromal cells, indicating the preservation of rhBMP-2 bioactivity indicative of osteogenesis.

Conclusions

We conclude that this platform provides a sophisticated and efficient approach to deliver bioactive rhBMP-2 for bone tissue regeneration applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s40634-016-0057-1) contains supplementary material, which is available to authorized users.

Expanding the 3-O-Sulfate Proteome--Enhanced Binding of Neuropilin-1 to 3-O-Sulfated Heparan Sulfate Modulates Its Activity

Binding of proteins to heparan sulfate is driven predominantly by electrostatic interactions between positively charged amino acid residues in the protein and negatively charged sulfate groups located at various positions along the polysaccharide chain. Although many heparin/heparan-sulfate-binding proteins have been described, few exhibit preferential binding for heparan sulfates containing relatively rare 3-O-sulfated glucosamine residues. To expand the "3-O-sulfate proteome," affinity matrices were created from Chinese hamster ovary (CHO) cell heparan sulfate engineered in vitro with and without 3-O-sulfate groups. Fractionation of different animal sera yielded several proteins that bound specifically to columns containing 3-O-sulfated heparan sulfate modified by two members of the heparan sulfate 3-O-sulfotransferase superfamily, Hs3st1 and Hs3st2. Neuropilin-1 was analyzed in detail because it has been implicated in angiogenesis and axon guidance. We show that 3-O-sulfation enhanced the binding of neuropilin-1 to heparan sulfate immobilized on plastic plates and to heparan sulfate present on cultured cells. Chemoenzymatically synthesized 3-O-sulfated heparan sulfate dodecamers protected neuropilin-1 from thermal denaturation and inhibited neuropilin-1-dependent, semaphorin-3a-induced growth cone collapse of neurons derived from murine dorsal root ganglia. The effect of 3-O-sulfation was cell autonomous and specific to Hs3st2 based on collapse assays of neurons derived from Hs3st1- and Hs3st2-deficient mice. Finally, 3-O-sulfated heparan sulfate enhanced the inhibition of endothelial cell sprouting by exogenous heparan sulfate. These findings demonstrate a reliable method to identify members of the 3-O-sulfate proteome and that 3-O-sulfation of heparan sulfate can modulate axonal growth cone collapse and endothelial cell sprouting.

Angiopoietin-1 prevents severe bleeding complications induced by heparin-like drugs and fibroblast growth factor-2 in mice

Critically ill children can develop bleeding complications when treated with heparin-like drugs. These events are usually attributed to the anticoagulant activity of these drugs. However, previous studies showed that fibroblast growth factor-2 (FGF-2), a heparin-binding growth factor released in the circulation of these patients, could precipitate intestinal hemorrhages in mice treated with the heparin-like drug pentosan polysulfate (PPS). Yet very little is known about how FGF-2 induces bleeding complications in combination with heparin-like drugs. Here, we examined the mechanisms by which circulating FGF-2 induces intestinal hemorrhages in mice treated with PPS. We used a well-characterized mouse model of intestinal hemorrhages induced by FGF-2 plus PPS. Adult FVB/N mice were infected with adenovirus carrying Lac-Z or a secreted form of recombinant human FGF-2, and injected with PPS, at doses that do not induce bleeding complications per se. Mice treated with FGF-2 in combination with PPS developed an intestinal inflammatory reaction that increased the permeability and disrupted the integrity of submucosal intestinal vessels. These changes, together with the anticoagulant activity of PPS, induced lethal hemorrhages. Moreover, a genetically modified form of the endothelial ligand angiopoietin-1 (Ang-1*), which has powerful antipermeability and anti-inflammatory activity, prevented the lethal bleeding complications without correcting the anticoagulant status of these mice. These findings define new mechanisms through which FGF-2 and Ang-1* modulate the outcome of intestinal bleeding complications induced by PPS in mice and may have wider clinical implications for critically ill children treated with heparin-like drugs.

Discovery of a heparan sulfate 3-O-sulfation specific peeling reaction

Heparan sulfate (HS) 3-O-sulfation determines the binding specificity of HS/heparin for antithrombin III and plays a key role in herpes simplex virus (HSV) infection. However, the low natural abundance of HS 3-O-sulfation poses a serious challenge for functional studies other than the two cases mentioned above. By contrast, multiple distinct isoforms of 3-O-sulfotranserases exist in mammals (up to seven isoenzymes). Here we describe a novel peeling reaction that specifically degrades HS chains with 3-O-sulfated glucosamine at the reducing-end. When HS/heparin is enzymatically depolymerized for compositional analysis, 3-O-sulfated glucosamine at the reducing ends appears to be susceptible to degradation under mildly basic conditions. We propose a 3-O-desulfation initiated peeling reaction mechanism based on the intermediate and side-reaction products observed. Our discovery calls for the re-evaluation of the natural abundance and functions of HS 3-O-sulfation by taking into consideration the negative impact of this novel peeling reaction.

Heparan sulfate 3-O-sulfation: a rare modification in search of a function

Many protein ligands bind to heparan sulfate, which results in their presentation, protection, oligomerization or conformational activation. Binding depends on the pattern of sulfation and arrangement of uronic acid epimers along the chains. Sulfation at the C3 position of glucosamine is a relatively rare, yet biologically significant modification, initially described as a key determinant for binding and activation of antithrombin and later for infection by type I herpes simplex virus. In mammals, a family of seven heparan sulfate 3-O-sulfotransferases installs sulfate groups at this position and constitutes the largest group of sulfotransferases involved in heparan sulfate formation. However, to date very few proteins or biological systems have been described that are influenced by 3-O-sulfation. This review describes our current understanding of the prevalence and structure of 3-O-sulfation sites, expression and substrate specificity of the 3-O-sulfotransferase family and the emerging roles of 3-O-sulfation in biology.

Efficient and inexpensive method for purification of heparin binding proteins

Heparin binding (HB) proteins mediate a wide range of important cellular processes, which makes this class of proteins biopharmaceutically important. Engineering HB proteins may bring many advantages, but it necessitates cost effective and efficient purification methodologies compared to currently available methods. One of the most important classes of HB proteins are fibroblast growth factors (FGFs) and their receptors (FGFRs). In this study, we report an efficient off-column purification of FGF-1 from soluble fractions and purification of the D2 domain of FGFR from insoluble inclusion bodies, using a weak Amberlite cation (IRC) exchanger. FGF-1 and the D2 domain have been expressed in Escherichia coli and purified to homogeneity using IRC resin. This approach is an alternative to conventional affinity column chromatography, which exhibits several disadvantages, including time-consuming experimental procedures for purification and regeneration and results in the expensive production of recombinant proteins. Results of the heparin binding chromatography and steady state fluorescence experiments show that the FGF-1 and the D2 are in a native conformation. The findings of this study will not only aid an in-depth investigation of this class of proteins but will also provide avenues for inexpensive and efficient purification of other important biological macromolecules.

Heparan sulfate proteoglycans

Heparan sulfate proteoglycans are found at the cell surface and in the extracellular matrix, where they interact with a plethora of ligands. Over the last decade, new insights have emerged regarding the mechanism and biological significance of these interactions. Here, we discuss changing views on the specificity of protein-heparan sulfate binding and the activity of HSPGs as receptors and coreceptors. Although few in number, heparan sulfate proteoglycans have profound effects at the cellular, tissue, and organismal level.

Heparan sulfate proteoglycans

Heparan sulfate proteoglycans are found at the cell surface and in the extracellular matrix, where they interact with a plethora of ligands. Over the last decade, new insights have emerged regarding the mechanism and biological significance of these interactions. Here, we discuss changing views on the specificity of protein-heparan sulfate binding and the activity of HSPGs as receptors and coreceptors. Although few in number, heparan sulfate proteoglycans have profound effects at the cellular, tissue, and organismal level.

A peptide found by phage display discriminates a specific structure of a trisaccharide in heparin

A number of recent studies have shown that heparan sulfate can control several important biological events on the cell surface through changes in sulfation pattern. The in vivo modification of sugar chains with sulfates, however, is complicated, and the discrimination of different sulfation patterns is difficult. Heparin, which is primarily produced by mast cells, is closely approximated by the structural analog heparan sulfate. Screening of heparin-associating peptides using phage display and antithrombin-bound affinity chromatography identified a peptide, heparin-associating peptide Y (HappY), that acts as a target of immobilized heparin. The peptide consists of 12 amino acid residues with characteristic three arginines and exclusively binds to heparin and heparan sulfate but does not associate with other glycosaminoglycans. HappY recognizes three consecutive monosaccharide residues in heparin through its three arginine residues. HappY should be a useful probe to detect heparin and heparan sulfate in studies of glycobiology.

Metabolic regulator betaKlotho interacts with fibroblast growth factor receptor 4 (FGFR4) to induce apoptosis and inhibit tumor cell proliferation

In organs involved in metabolic homeostasis, transmembrane α and βklothos direct FGFR signaling to control of metabolic pathways. Coordinate expression of βklotho and FGFR4 is a property of mature hepatocytes. Genetic deletion of FGFR4 or βklotho in mice disrupts hepatic cholesterol/bile acid and lipid metabolism. The deletion of FGFR4 has no effect on the proliferative response of hepatocytes after liver injury. However, its absence results in accelerated progression of dimethynitrosamine-initiated hepatocellular carcinomas, indicating that FGFR4 suppresses hepatoma proliferation. The mechanism underlying the FGFR4-mediated hepatoma suppression has not been addressed. Here we show that βklotho expression is more consistently down-regulated in human and mouse hepatomas than FGFR4. Co-expression and activation by either endocrine FGF19 or cellular FGF1 of the FGFR4 kinase in a complex with βklotho restricts cell population growth through induction of apoptotic cell death in both hepatic and nonhepatic cells. The βklotho-FGFR4 partnership caused a depression of activated AKT and mammalian target of rapamycin while activating ERK1/2 that may underlie the pro-apoptotic effect. Our results show that βklotho not only interacts with heparan sulfate-FGFR4 to form a complex with high affinity for endocrine FGF19 but also impacts the quality of downstream signaling and biological end points activated by either FGF19 or canonical FGF1. Thus the same βklotho-heparan sulfate-FGFR4 partnership that mediates endocrine control of hepatic metabolism plays a role in cellular homeostasis and hepatoma suppression through negative control of cell population growth mediated by pro-apoptotic signaling.

Antiangiogenic forms of antithrombin specifically bind to the anticoagulant heparin sequence

A specific pentasaccharide sequence of heparin binds with high affinity to native antithrombin and induces a conformational change in the inhibitor by a previously described two-step interaction mechanism. In this work, the interactions of heparin with the antiangiogenic latent and cleaved antithrombin forms were studied. Binding of heparin to these antithrombin forms was specific for the same pentasaccharide sequence as native antithrombin. Rapid kinetic studies demonstrated that this pentasaccharide induced a conformational change also in latent and cleaved antithrombin. The binding affinities of these antithrombin forms for the pentasaccharide, as compared to native antithrombin, were approximately 30-fold lower due to two to three fewer ionic interactions, resulting in less stable conformationally altered states. Affinities of latent and cleaved antithrombin for longer heparin chains, containing the pentasaccharide sequence, were 2-fold lower than for the pentasaccharide itself. This contrasts the interaction with native antithrombin and demonstrates that residues flanking the pentasaccharide sequence of heparin are repelled by the latent and cleaved forms. These findings contribute to delineating the mechanism by which heparin or heparan sulfate mediates antiangiogenic activity of antithrombin.

The Importance of Heparan Sulfate in Herpesvirus Infection

Herpes simplex virus type-1 (HSV-1) is one of many pathogens that use the cell surface glycosaminoglycan heparan sulfate as a receptor. Heparan sulfate is highly expressed on the surface and extracellular matrix of virtually all cell types making it an ideal receptor. Heparan sulfate interacts with HSV-1 envelope glycoproteins gB and gC during the initial attachment step during HSV-1 entry. In addition, a modified form of heparan sulfate, known as 3-O-sulfated heparan sulfate, interacts with HSV-1 gD to induce fusion between the viral envelope and host cell membrane. The 3-O-sulfation of heparan sulfate is a rare modification which occurs during the biosynthesis of heparan sulfate that is carried out by a family of enzymes known as 3-O-sulfotransferases. Due to its involvement in multiple steps of the infection process, heparan sulfate has been a prime target for the development of agents to inhibit HSV entry. Understanding how heparan sulfate functions during HSV-1 infection may not only be critical for inhibiting infection by this virus, but it may also be crucial in the fight against many other pathogens as well.

The principal neuronal gD-type 3-O-sulfotransferases and their products in central and peripheral nervous system tissues

Within the nervous system, heparan sulfate (HS) of the cell surface and extracellular matrix influences developmental, physiologic and pathologic processes. HS is a functionally diverse polysaccharide that employs motifs of sulfate groups to selectively bind and modulate various effector proteins. Specific HS activities are modulated by 3-O-sulfated glucosamine residues, which are generated by a family of seven 3-O-sulfotransferases (3-OSTs). Most isoforms we herein designate as gD-type 3-OSTs because they generate HS(gD+), 3-O-sulfated motifs that bind the gD envelope protein of herpes simplex virus 1 (HSV-1) and thereby mediate viral cellular entry. Certain gD-type isoforms are anticipated to modulate neurobiologic events because a Drosophila gD-type 3-OST is essential for a conserved neurogenic signaling pathway regulated by Notch. Information about 3-OST isoforms expressed in the nervous system of mammals is incomplete. Here, we identify the 3-OST isoforms having properties compatible with their participation in neurobiologic events. We show that 3-OST-2 and 3-OST-4 are principal isoforms of brain. We find these are gD-type enzymes, as they produce products similar to a prototypical gD-type isoform, and they can modify HS to generate receptors for HSV-1 entry into cells. Therefore, 3-OST-2 and 3-OST-4 catalyze modifications similar or identical to those made by the Drosophila gD-type 3-OST that has a role in regulating Notch signaling. We also find that 3-OST-2 and 3-OST-4 are the predominant isoforms expressed in neurons of the trigeminal ganglion, and 3-OST-2/4-type 3-O-sulfated residues occur in this ganglion and in select brain regions. Thus, 3-OST-2 and 3-OST-4 are the major neural gD-type 3-OSTs, and so are prime candidates for participating in HS-dependent neurobiologic events.

A novel role of fibroblast growth factor-2 and pentosan polysulfate in the pathogenesis of intestinal bleeding in mice

Pentosan polysulfate (PPS) is a heparin-like polysaccharide that can affect the binding interactions of fibroblast growth factor (FGF-2) with its high-affinity receptors. Patients with angiogenic tumors frequently show high levels of FGF-2 in the circulation. Since FGF-2 is a heparin-binding angiogenic growth factor, PPS has been used successfully to block its activity in patients with angiogenic tumors. However, because of its heparin-like activity, the major toxic effect of PPS is the development of bleeding disorders. The role that circulating FGF-2 plays in the pathogenesis of bleeding disorders in patients treated with PPS is currently unknown. Here we hypothesized that FGF-2 might play a physiological role in the pathogenesis of intestinal bleeding induced by PPS. This hypothesis is supported by previous studies showing that PPS is accumulated in the intestine and that circulating FGF-2 specifically binds to and modulates the angiogenic activity of intestinal submucosal endothelial cells. We used recombinant adenoviral vectors carrying a secreted form of FGF-2 and LacZ control vectors to determine whether high levels of circulating FGF-2 facilitate the development of intestinal bleeding disorders in FVB/N and C57BL/6J mice treated with PPS. We found that PPS, acting together with FGF-2, induced structural changes in intestinal vessels leading to the development of lethal intestinal hemorrhages. These findings might have wider clinical implications for the systemic use of PPS and other heparinoids in the treatment of patients with angiogenic diseases associated with high levels of circulating FGF-2.

Antiangiogenic antithrombin blocks the heparan sulfate-dependent binding of proangiogenic growth factors to their endothelial cell receptors: evidence for differential binding of antiangiogenic and anticoagulant forms of antithrombin to proangiogenic heparan sulfate domains

The anticoagulant serpin antithrombin acquires a potent antiangiogenic activity upon undergoing conformational alterations to cleaved or latent forms. Here we show that antithrombin antiangiogenic activity is mediated at least in part through the ability of the conformationally altered serpin to block the proangiogenic growth factors fibroblast growth factor (FGF)-2 and vascular endothelial growth factor (VEGF) from forming signaling competent ternary complexes with their protein receptors and heparan sulfate co-receptors on endothelial cells. Cleaved and latent but not native forms of antithrombin blocked the formation of FGF-2-FGF receptor-1 ectodomain-heparin ternary complexes, and the dimerization of these complexes in solution and similarly inhibited the formation of FGF-2-heparin binary complexes and their dimerization. Only antiangiogenic forms of antithrombin likewise inhibited (125)I-FGF-2 binding to its low affinity heparan sulfate co-receptor and blocked FGF receptor-1 autophosphorylation and p42/44 MAP kinase phosphorylation in cultured human umbilical vein endothelial cells (HUVECs). Moreover, treatment of HUVECs with heparinase III to specifically eliminate the FGF-2 heparan sulfate co-receptor suppressed the ability of antiangiogenic antithrombin to inhibit growth factor-stimulated proliferation. Antiangiogenic antithrombin inhibited full-length VEGF(165) stimulation of HUVEC proliferation but did not affect the stimulation of cells by the heparin-binding domain-deleted VEGF(121). Taken together, these results demonstrate that antiangiogenic forms of antithrombin block the proangiogenic effects of FGF-2 and VEGF on endothelial cells by competing with the growth factors for binding the heparan sulfate co-receptor, which mediates growth factor-receptor interactions. Moreover, the inability of native antithrombin to bind this co-receptor implies that native and conformationally altered forms of antithrombin differentially bind proangiogenic heparan sulfate domains.

Structural specificity in a FGF7-affinity purified heparin octasaccharide required for formation of a complex with FGF7 and FGFR2IIIb

Variations in sulfation of heparan sulfate (HS) affect interaction with FGF, FGFR, and FGF-HS-FGFR signaling complexes. Whether structurally distinct HS motifs are at play is unclear. Here we used stabilized recombinant FGF7 as a bioaffinity matrix to purify size-defined heparin oligosaccharides. We show that only 0.2%-4% of 6 to 14 unit oligosaccharides, respectively, have high affinity for FGF7 based on resistance to salt above 0.6M NaCl. The high affinity fractions exhibit highest specific activity for interaction with FGFR2IIIb and formation of complexes of FGF7-HS-FGFR2IIIb. The majority fractions with moderate (0.30-0.6M NaCl), low (0.14-0.30M NaCl) or no affinity at 0.14M NaCl for FGF7 supported no complex formation. The high affinity octasaccharide mixture exhibited predominantly 7- and 8-sulfated components (7,8-S-OctaF7) and formed FGF7-HS-FGFR2IIIb complexes with highest specific activity. Deduced disaccharide analysis indicated that 7,8-S-OctaF7 comprised of DeltaHexA2SGlcN6S in a 2:1 ratio to a trisulfated and a variable unsulfated or monosulfated disaccharide. The inactive octasaccharides with moderate affinity for FGF7 were much more heterogenous and highly sulfated with major components containing 11 or 12 sulfates comprised of predominantly trisulfated disaccharides. This suggests that a rare undersulfated motif in which sulfate groups are specifically distributed has highest affinity for FGF7. The same motif also exhibits structural requirements for high affinity binding to dimers of FGFR2IIIb prior to binding FGF7 to form FGF7-HS-FGFR2IIIb complexes. In contrast, the majority of more highly sulfated HS motifs likely play FGFR-independent roles in stability and control of access of FGF7 to FGFR2IIIb in the tissue matrix.

Control of fibroblast growth factor (FGF) 7- and FGF1-induced mitogenesis and downstream signaling by distinct heparin octasaccharide motifs

Variation in length, disaccharide composition, and sulfation of heparan sulfate (HS) affects fibroblast growth factor (FGF) signaling. However, it is unclear whether the specific distribution of groups within oligosaccharides or random variations in charge density underlies the effects. Recently we showed that a mixture of undersulfated octasaccharides exhibiting 7 and 8 sulfates (7,8-S-OctaF7) generated from heparin had the highest affinity for FGF7 monitored by salt resistance (>0.60 M salt) of octasaccharide-FGF7 complexes. 7,8-S-OctaF7 also had the highest specific activity for formation of a complex with dimeric FGFR2IIIb competent to bind FGF7. Here we show that when endogenous HS was inhibited by chlorate treatment, 7,8-S-OctaF7 specifically supported FGF7-stimulated DNA synthesis and downstream signaling in FGFR2IIIb-expressing mouse keratinocytes. It failed to support FGF1 signaling in both HS-deficient mouse keratinocytes and 3T3 fibroblasts. In contrast, abundant, more highly sulfated and heterogenous mixtures of octasaccharides with lower affinity (0.30-0.60 M salt) for FGF7 supported FGF1-induced signaling in both cell types. In contrast to the two-component 7,8-S-OctaF7 mixture from FGF7, the high affinity octasaccharide fraction from FGF1 was a heterogeneous mixture with components ranging from 8 to 12 sulfates with 11-S-octasaccharides the most abundant. The high affinity fraction exhibited similar properties to the lower affinity fractions from both FGF1 and FGF7. Octasaccharide mixtures eluting from FGF1 between 0.30 and 0.60 M and above 0.60 M salt were nearly equal in support of FGF1 signaling in fibroblasts and keratinocytes. Both were deficient in support of FGF7-induced signaling in keratinocytes. The results show that both variations in overall charge density and specific distribution of charged groups within HS motifs exhibit FGF-specific control over formation of FGF-HS-FGFR complexes and downstream signaling.

Symmetric Primary and Tertiary Structure Mutations within a Symmetric Superfold: A Solution, not a Constraint, to Achieve a Foldable Polypeptide

In previous studies designed to increase the primary structure symmetry within the hydrophobic core of human acidic fibroblast growth factor (FGF-1) a combination of five mutations were accommodated, resulting in structure, stability and folding kinetic properties similar to wild-type (despite the symmetric constraint upon the set of core residues). A sixth mutation in the core, involving a highly conserved Met residue at position 67, appeared intolerant to substitution. Structural analysis suggested that the local packing environment of position 67 involved two regions of apparent insertions that distorted the tertiary structure symmetry inherent in the beta-trefoil architecture. It was postulated that a symmetric constraint upon the primary structure within the core could only be achieved after these insertions had been deleted (concomitantly increasing the tertiary structure symmetry). The deletion of these insertions is now shown to permit mutation of position 67, thereby increasing the primary structure symmetry relationship within the core. Furthermore, despite the imposed symmetric constraint upon both the primary and tertiary structure, the resulting mutant form of FGF-1 is substantially more stable. The apparent inserted regions are shown to be associated with heparin-binding functionality; however, despite a marked reduction in heparin-binding affinity the mutant form of FGF-1 is surprisingly approximately 70 times more potent in 3T3 fibroblast mitogenic assays. The results support the hypothesis that primary structure symmetry within a symmetric protein superfold represents a possible solution, rather than a constraint, to achieving a foldable polypeptide.

A conformational change in heparan sulfate 3-O-sulfotransferase-1 is induced by binding to heparan sulfate

The 3-O-sulfation of glucosamine by heparan sulfate 3-O-sulfotransferase-1 (3-OST-1) is a key modification step during the biosynthesis of anticoagulant heparan sulfate (HS). In this paper, we present evidence of a conformational change that occurs in 3-OST-1 upon binding to heparan sulfate. The intrinsic fluorescence of 3-OST-1 was increased in the presence of HS, suggesting a conformational change. This apparent conformational change was further investigated using differential chemical modification of 3-OST-1 to measure the solvent accessibility of the lysine residues. 3-OST-1 was treated with acetic anhydride in either the presence or absence of HS using both acetic anhydride and hexadeuterioacetic anhydride under nondenaturing and denaturing conditions, respectively. The relative reactivity of the lysine residues to acetylation and [2H] acetylation in the presence or absence of HS was analyzed by measuring the ratio of acetylated and deuterioacetylated peptides using matrix-assisted laser desorption ionization mass spectrometry. The solvent accessibilities of the lysine residues were altered differentially depending on their location. In particular, we observed a group of lysine residues in the C-terminus of 3-OST-1 that become more solvent accessible when 3-OST-1 binds to HS. This observation indicates that a conformational change could be occurring during substrate binding. A truncated mutant of 3-OST-1 that lacked this C-terminal region was expressed and found to exhibit a 200-fold reduction in sulfotransferase activity. The results from this study will contribute to our understanding of the interactions between 3-OSTs and HS.

The biosynthesis of anticoagulant heparan sulfate by the heparan sulfate 3-O-sulfotransferase isoform 5

The role of heparan sulfate (HS) in regulating blood coagulation has a wide range of clinical implications. In this study, we investigated the role of 3-O-sulfotransferase isoform 5 (3-OST-5) in generating anticoagulant HS in vivo. A Chinese hamster ovary cell line (3OST5/CHO) stably expressing 3-OST-5 was generated. The expression of 3-OST-5 in 3OST5/CHO cells was confirmed by Northern blot analysis, RT-PCR, and the disaccharide analyses of the HS from the cells. We also determined the effects of the HS from 3OST5/CHO on antithrombin-mediated inhibition of factor Xa. Fluorescently labeled antithrombin bound to the surface of 3OST5/CHO cells, suggesting that the antithrombin-binding HS is indeed present on the cell surface. Our results demonstrate that the 3-OST-5 gene is capable of synthesizing anticoagulant HS in CHO cells and has the potential to contribute to the biosynthesis of anticoagulant HS in humans.

Nucleosomes Bind Fibroblast Growth Factor-2 for Increased Angiogenesis In vitro and In vivo

Solid tumors often display sites of necrosis near regions of angiogenesis in vivo. As tumor cell necrosis would result in the release of nucleosomes into the extracellular environment, we explored the potential role of nucleosomes in the promotion of angiogenesis. Data indicate that nucleosomes acted similar to heparin and bound to several heparin-binding, proangiogenic factors [i.e., fibroblast growth factor (FGF)-1, FGF-2, vascular endothelial growth factor, and transforming growth factor-β1]. Nucleosomes modestly enhanced FGF-2 growth of human umbilical vein endothelial cells when grown in restricted media as well as increased human umbilical vein endothelial cell migration and primitive blood vessel tube formation in vitro. On s.c. injection in mice, nucleosomes aided FGF-2 in promoting angiogenesis. These results suggest that nucleosomes released from dying tumor cells aid in the formation of blood vessels and may provide a novel means by which tumor cells increase angiogenesis.

Mechanistic diversity of cytokine receptor signaling across cell membranes

Circulating cytokines bind to specific receptors on the cell outer surface to evoke responses inside the cell. Binding of cytokines alters the association between receptor molecules that often cross the membrane only once in a single alpha-helical segment. As a consequence, association of protein domains on the inside of the membrane are also altered. Increasing evidence suggests that an initial "off-state" of associated receptors is perturbed, and brought to an activated state that leads to intracellular signaling and eventually effects a change in DNA transcription. The initial detection event that transduces the change in receptor association is sensitive to both proximity and orientation of the receptors, and probably also to the time that the activated state or receptor association is maintained. Ultimately, a cascade of phosphorylation events is triggered. The initial kinases are sometimes part of the intracellular domains of the receptors. The kinases can also be separate proteins that may be pre-associated with intracellular domains of the receptors, or can be recruited after the intracellular association of the activated receptors. We focus here on each of the cases for which structures of the activated cytokine-receptor complexes are known, in a search for underlying mechanisms. The variations in modes of association, stoichiometries of receptors and cytokines, and orientations before and after activation of these receptors are almost as great as the number of complexes themselves. The principles uncovered nevertheless illustrate the basis for high specificity and fidelity in cytokine-mediated signaling.

Ligand-induced coupling versus receptor pre-association: cellular automaton simulations of FGF-2 binding

The binding of basic fibroblast growth factor (FGF-2) to its cell surface receptor (CSR) and subsequent signal transduction is known to be enhanced by heparan sulfate proteoglycans (HSPGs). HSPGs bind FGF-2 with low affinity and likely impact CSR-mediated signaling via stabilization of FGF-2-CSR complexes via association with both the ligand and the receptor. What is unknown is whether HSPG associates with CSR in the absence of FGF-2. In this paper, we determine conditions by which pre-association would impact CSR-FGF-2-HSPG triad formation assuming diffusion-limited surface reactions. Using mean-field rate equations, we show that (i) when [HSPG] is much higher than [CSR], the presence of pre-formed complexes does not affect the steady state of FGF-2 binding, and (ii) when the concentrations are comparable, the presence of pre-formed complexes substantially increases the steady-state concentration of FGF-2 bound to CSR. These findings are supported by explicit cellular automaton simulations, which justify the mean-field treatment. We discuss the advantages of such a two-receptor system compared to a single-receptor model, when the parameters are comparable. Further, we speculate that the observed high concentration of HSPG in intact cells ([HSPG]-100[CSR]) provides a way to ensure that the binding levels of FGF-2 to its signaling receptor remains high, irrespective of the presence of pre-formed CSR-HSPG complexes on the cell surface, while allowing the cell to finely tune the response to FGF-2 via down-regulation of the signaling receptor.

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

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

The involvement of heparan sulfate (HS) in FGF1/HS/FGFR1 signaling complex

Fibroblast growth factor (FGF) signaling begins with the formation of a ternary complex of FGF, FGF receptor (FGFR), and heparan sulfate (HS). Multiple models have been proposed for the ternary complex. However, major discrepancies exist among those models, and none of these models have evaluated the functional importance of the interacting regions on the HS chains. To resolve the discrepancies, we measured the size and molar ratio of HS in the complex and showed that both FGF1 and FGFR1 simultaneously interact with HS; therefore, a model of 2:2:2 FGF1.HS.FGFR1 was shown to fit the data. Using genetic and biochemical methods, we generated HSs that were defective in FGF1 and/or FGFR1 binding but could form the signaling ternary complex. Both genetically and chemically modified HSs were subsequently assessed in a BaF3 cell mitogenic activity assay. The ability of HS to support the ternary complex formation was found to be required for FGF1-stimulated cell proliferation. Our data also proved that specific critical groups and sites on HS support complex formation. Furthermore, the molar ratio of HS, FGF1, and FGFR1 in the ternary complex was found to be independent of the size of HS, which indicates that the selected model can take place on the cell surface proteoglycans. Finally, a mechanism for the FGF.FGFR signaling complex formation on cell membrane was proposed, where FGF and FGFR have their own binding sites on HS and a distinct ternary complex formation site is directly responsible for mitogenic activity.

A heparin binding synthetic peptide from human HIP / RPL29 fails to specifically differentiate between anticoagulantly active and inactive species of heparin

Despite extensive progress in determining structures within heparin and heparan sulfate (Hp/HS) and the discovery of numerous proteinaceous binding partners for Hp/HS so far; the only detailed characterization of a specific protein-glycosaminoglycan interaction is antithrombin III (ATIII) binding to a Hp pentasaccharide containing a unique 3-O-sulfated glucosamine residue. Previously, it was reported from our laboratories that a 16 amino acid synthetic peptide derived from the C-terminus of human HIP/RPL29 (HIP peptide-1) enriched for ATIII-dependent anticoagulant activity, presumably by specifically binding the ATIII pentasaccharide. Herein, we demonstrate that HIP peptide-1 cannot enrich ATIII-dependent anticoagulant activity from a starting pool of porcine intestinal mucosa Hp through a bio-specific interaction. However, a HIP peptide-1 column can be used to enrich for anticoagulantly active Hp from a diverse pool of glycosaminoglycans known as Hp byproducts by a mechanism of nonspecific charge interactions. Thus, HIP peptide-1 cannot recognize Hp via bio-specific interactions but binds glycosaminoglycans by non-specific charge interactions.

Membrane-bound heparin binding proteins from HL-60 cells purified in a two-step affinity chromatography differentially eluted with divalent cations

Solubilized membrane proteins from HL-60 cells were separated by two-step affinity chromatography. Proteins eluted with MgCl2 in the first heparin-gel were applied to the second heparin-gel and eluted with CaCl2. The eluted proteins were analysed and purified by electrophoresis. N-terminal amino acid sequences of eight proteins on the characteristic bands were determined. Homology search for the sequences indicated that three microsomal proteins, two nuclear proteins and a glycolytic enzyme were eluted with divalent cations, whereas a nuclear ribonucleoprotein and a membrane-cytoskelton linker protein were not dissociated with divalent cations, but with 2 M NaCl. Heparin affinity chromatography combined with differential elution with divalent cations can be a useful method for separation of membrane proteins.

Heparan sulfate 3-O-sulfotransferase isoform 5 generates both an antithrombin-binding site and an entry receptor for herpes simplex virus, type 1

Heparan sulfate 3-O-sulfotransferase transfers sulfate to the 3-OH position of a glucosamine residue of heparan sulfate (HS) to form 3-O-sulfated HS. The 3-O-sulfated glucosamine residue contributes to two important biological functions of HS: binding to antithrombin and thereby carrying anticoagulant activity, and binding to herpes simplex viral envelope glycoprotein D to serve as an entry receptor for herpes simplex virus 1. A total of five HS 3-O-sulfotransferase isoforms were reported previously. Here we report the isolation and characterization of a novel HS 3-O-sulfotransferase isoform, designated as HS 3-O-sulfotransferase isoform 5 (3-OST-5). 3-OST-5 cDNA was isolated from a human placenta cDNA library and expressed in COS-7 cells. The disaccharide analysis of 3-OST-5-modified HS revealed that 3-OST-5 generated at least three 3-O-sulfated disaccharides as follows: IdoUA2S-AnMan3S, GlcUA-AnMan3S6S, and IdoUA2S-AnMan3S6S. Transfection of the plasmid expressing 3-OST-5 rendered wild type Chinese hamster ovary cells susceptible to the infection by herpes simplex virus 1, suggesting that 3-OST-5-modified HS serves as an entry receptor for herpes simplex virus 1. In addition, 3-OST-5-modified HS bound to herpes simplex viral envelope protein glycoprotein D. Furthermore, we found that 3-OST-5-modified HS also bound to antithrombin, suggesting that 3-OST-5 also produces anticoagulant HS. In summary, our results indicate that a new member of 3-OST family generates both anticoagulant HS and an entry receptor for herpes simplex virus 1. These results provide a new insight regarding the mechanism for the biosynthesis of biologically active HS.

Heparan sulfate interacting protein (HIP/L29) negatively regulates growth responses to basic fibroblast growth factor in gingival fibroblasts

Basic fibroblast growth factor (bFGF) modulates gingival growth, and its release from heparan sulfate (HS) in the extracellular matrix (ECM) governs local tissue bioavailability. We identified a heparin/HS interacting protein (HIP/L29) that recognizes specific HS sequences. We hypothesize that HIP/L29, by modulating the interactions of bFGF with HS chains on proteoglycans, could regulate bFGF bioavailability. To investigate interactions between bFGF and HIP/L29, we isolated and cultured fibroblasts from normal gingiva and overgrown gingiva from patients on cyclosporine (CSA). bFGF significantly stimulated gingival fibroblast proliferation with or without heparin. Recombinant human HIP/L29 dramatically decreased bFGF-induced proliferation, but did not alter responses to insulin-like growth factor-1 (IGF-1). Analysis of mitogen-activated protein kinase (MAPK) phosphorylation patterns showed that bFGF stimulation of p44 (Erk-1), but not p42 (Erk-2), also was inhibited by HIP/L29 in a dose-dependent manner. Together, these results support our hypothesis that HIP/L29 modulates the bioavailability and action of bFGF.

Heparin-protein interactions

Heparin, a sulfated polysaccharide belonging to the family of glycosaminoglycans, has numerous important biological activities, associated with its interaction with diverse proteins. Heparin is widely used as an anticoagulant drug based on its ability to accelerate the rate at which antithrombin inhibits serine proteases in the blood coagulation cascade. Heparin and the structurally related heparan sulfate are complex linear polymers comprised of a mixture of chains of different length, having variable sequences. Heparan sulfate is ubiquitously distributed on the surfaces of animal cells and in the extracellular matrix. It also mediates various physiologic and pathophysiologic processes. Difficulties in evaluating the role of heparin and heparan sulfate in vivo may be partly ascribed to ignorance of the detailed structure and sequence of these polysaccharides. In addition, the understanding of carbohydrate-protein interactions has lagged behind that of the more thoroughly studied protein-protein and protein-nucleic acid interactions. The recent extensive studies on the structural, kinetic, and thermodynamic aspects of the protein binding of heparin and heparan sulfate have led to an improved understanding of heparin-protein interactions. A high degree of specificity could be identified in many of these interactions. An understanding of these interactions at the molecular level is of fundamental importance in the design of new highly specific therapeutic agents. This review focuses on aspects of heparin structure and conformation, which are important for its interactions with proteins. It also describes the interaction of heparin and heparan sulfate with selected families of heparin-binding proteins.

Cell surface heparan sulfate and its roles in assisting viral infections

Heparan sulfate, a highly sulfated polysaccharide, is present on the surface of mammalian cells and in the extracellular matrix in large quantities. The sulfated monosaccharide sequences within heparan sulfate determine the protein binding specificity and regulate biological functions. Numerous viruses and parasites utilize cell surface heparan sulfate as receptors to infect target cells. Due to the structural complexity of heparan sulfate, it was considered a nonspecific cell surface receptor by interacting with the positive motifs of viral proteins. However, recent studies reveal that heparan sulfate plays multiple roles in assisting viral infection, and the activities in promoting viral infections require unique monosaccharide sequences, suggesting that heparan sulfate could serve as a specific receptor for viral infection. The currently available techniques for the structural analysis of heparan sulfate provide essential information about the specific roles of heparan sulfate in assisting viral infections. The knowledge accumulated in this fast growing field will permit us to have a better understanding of the mechanism of viral infection and will lead to the development of new antiviral agents.

Structural basis for interaction of FGF-1, FGF-2, and FGF-7 with different heparan sulfate motifs

Stromal cell-derived FGF-7 binds and activates only the resident FGFR2IIIb in epithelial cells while FGF-1 and FGF-2 exhibit a broader interaction with multiple isoforms of FGFR. Here we report the structure of FGF-7 that has been solved to 3.1 A resolution by molecular replacement with the structure of a dual function chimera of FGF-7 and FGF-1 (FGF-7/1) which was resolved to 2.3 A. Comparison of the FGF-7 structure to that of FGF-1 and FGF-2 revealed the strongly conserved Calpha backbone among the three FGF polypeptides and the surface hydrophobic patch that forms the primary receptor-binding domain. In contrast, a decrease and dispersion of the positive surface charge density characterized the heparin-binding domain of FGF-7 defined by homology to that of FGF-1 and FGF-2 in complexes with heparin. A simple heparin hexasaccharide that cocrystallized with FGF-1 and FGF-2 and protected both against protease in solution failed to exhibit the same properties with FGF-7. In contrast to FGF-1 and FGF-2, protection of FGF-7 was enhanced by heparin oligosaccharides of increased length with those exhibiting a 3-O-sulfate being the most effective. Protection of FGF-7 required interaction with specifically the fraction of crude heparin retained on antithrombin affinity columns. Conversely, heparin enriched by affinity for immobilized FGF-7 exhibited anti-factor Xa activity similar to that purified on an antithrombin affinity matrix. In contrast, an FGF-1 affinity matrix enriched the fraction of crude heparin with low anti-factor Xa activity. The results provide a structural basis to suggest that the unique FGF-7 heparin-binding (HB) domain underlies a specific restriction in respect to composition and length of the heparan sulfate motif that may impact specificity of localization, stability, and trafficking of FGF-7 in the microenvironment, and formation and activation of the FGFR2IIIb kinase signaling complex in epithelial cells.

Kinase insert domain receptor (KDR) extracellular immunoglobulin-like domains 4-7 contain structural features that block receptor dimerization and vascular endothelial growth factor-induced signaling

The vascular endothelial growth factor (VEGF) receptor tyrosine kinase subtype kinase insert domain receptor (KDR) contains seven extracellular Ig-like domains, of which the three most amino-terminal contain the necessary structural features required for VEGF binding. To clarify the functional role of KDR Ig-like domains 4-7, we compared VEGF-induced signaling in human embryonic kidney and porcine aortic endothelial cells expressing native versus mutant receptor proteins in which Ig-like domains 4-7, 4-6, or 7 had been deleted. Western blotting using an anti-receptor antibody indicated equivalent expression levels for each of the recombinant proteins. As expected, VEGF treatment robustly augmented native receptor autophosphorylation. In contrast, receptor autophosphorylation, as well as downstream signaling events, were VEGF-independent for cells expressing mutant receptors. (125)I-VEGF(165) bound with equal or better affinity to mutant versus native receptor, although the number of radioligand binding sites was significantly reduced because a significant percentage of mutant, but not native, receptors were localized to the cell interior. As was the case for native KDR, (125)I-VEGF(165) binding to the mutant receptors was dependent upon cell surface heparan sulfate proteoglycans, and (125)I-VEGF(121) bound with an affinity equal to that of (125)I-VEGF(165) to the native and mutant receptors. It is concluded that KDR Ig-like domains 4-7 contain structural features that inhibit receptor signaling by a mechanism that is independent of neuropilin-1 and heparan sulfate proteoglycans. We speculate that this provides a cellular mechanism for blocking unwanted signaling events in the absence of VEGF.

Binding of heparin/heparan sulfate to fibroblast growth factor receptor 4

Fibroblast growth factors (FGFs) are heparin-binding polypeptides that affect the growth, differentiation, and migration of many cell types. FGFs signal by binding and activating cell surface FGF receptors (FGFRs) with intracellular tyrosine kinase domains. The signaling involves ligand-induced receptor dimerization and autophosphorylation, followed by downstream transfer of the signal. The sulfated glycosaminoglycans heparin and heparan sulfate bind both FGFs and FGFRs and enhance FGF signaling by mediating complex formation between the growth factor and receptor components. Whereas the heparin/heparan sulfate structures involved in FGF binding have been studied in some detail, little information has been available on saccharide structures mediating binding to FGFRs. We have performed structural characterization of heparin/heparan sulfate oligosaccharides with affinity toward FGFR4. The binding of heparin oligosaccharides to FGFR4 increased with increasing fragment length, the minimal binding domains being contained within eight monosaccharide units. The FGFR4-binding saccharide domains contained both 2-O-sulfated iduronic acid and 6-O-sulfated N-sulfoglucosamine residues, as shown by experiments with selectively desulfated heparin, compositional disaccharide analysis, and a novel exoenzyme-based sequence analysis of heparan sulfate oligosaccharides. Structurally distinct heparan sulfate octasaccharides differed in binding to FGFR4. Sequence analysis suggested that the affinity of the interaction depended on the number of 6-O-sulfate groups but not on their precise location.

Multiple domains contribute to heparin/heparan sulfate binding by human HIP/L29

Human heparin/heparan sulfate interacting protein/L29 (HIP/L29) is thought to be involved in the promotion of cell adhesion, the promotion of cell growth in the cancerous state, and the modulation of blood coagulation. These activities are consistent with the proposed function of HIP/L29 as a heparin/heparan sulfate (Hp/HS) binding growth factor that has a preference for anticoagulantly active Hp/HS. Previous studies showed that a peptide derived from the C terminus of human HIP/L29 (HIP peptide-1) can selectively bind anticoagulant Hp and support cell adhesion. However, a murine ortholog does not have an identical HIP peptide-1 sequence, yet still retains the ability to bind Hp, suggesting that there may be additional Hp/HS binding sites outside of the HIP peptide-1 domain. To test this hypothesis, a systematic study of the domains within human and murine HIP/L29 responsible for Hp/HS binding activity was undertaken. Using deletion mutants, proteolytic fragments, and protease protection of HIP/L29 by Hp, we demonstrate that multiple binding domains contribute to the overall Hp/HS binding activity of HIP/L29 proteins. Furthermore, a conformational change is induced in human HIP/L29 upon Hp binding as detected by circular dichroism spectroscopy. These studies demonstrate the multiplicity of Hp/HS binding sequences within human and murine HIP/L29.

Enzymatic modification of heparan sulfate on a biochip promotes its interaction with antithrombin III

A heparan sulfate glycosaminoglycan chain, biotinylated at its reducing-end, was bound to a streptavidin-coated biochip. Surface plasmon resonance spectroscopy showed a low affinity interaction with antithrombin III (ATIII) when it was flowed over a surface containing heparan sulfate. ATIII bound tightly with high affinity when the same surface was enzymatically modified to using 3-O-sulfotransferase isoform 1 (3-OST-1) in the presence of 3'-phosphoadenosine 5'-phosphosulfate (PAPS). The 3-OST-1 enzyme is involved in heparan sulfate biosynthesis and introduces a critical 3-O-sulfo group into this glycosaminoglycan affording the appropriate pentasaccharide sequence capable of high affinity binding to ATIII. This experiment demonstrates the specific structural modification of a glycosaminoglycan bound to a biochip using a biosynthetic enzyme, suggesting a new approach to rapid screening glycosaminoglycan-protein interactions.