Structural requirements in heparin for binding and activation of FGF-1 and FGF-4 are different from that for FGF-2

Efficient platform for synthesizing comprehensive heparan sulfate oligosaccharide libraries for decoding glycosaminoglycan-protein interactions

Glycosaminoglycans (GAGs) are abundant, ubiquitous carbohydrates in biology, yet their structural complexity has limited an understanding of their biological roles and structure-function relationships. Synthetic access to large collections of well defined, structurally diverse GAG oligosaccharides would provide critical insights into this important class of biomolecules and represent a major advance in glycoscience. Here we report a new platform for synthesizing large heparan sulfate (HS) oligosaccharide libraries displaying comprehensive arrays of sulfation patterns. Library synthesis is made possible by improving the overall synthetic efficiency through universal building blocks derived from natural heparin and a traceless fluorous tagging method for rapid purification with minimal manual manipulation. Using this approach, we generated a complete library of 64 HS oligosaccharides displaying all possible 2-O-, 6-O- and N-sulfation sequences in the tetrasaccharide GlcN-IdoA-GlcN-IdoA. These diverse structures provide an unprecedented view into the sulfation code of GAGs and identify sequences for modulating the activities of important growth factors and chemokines.

Recent Progress on Heparin–Protamine Particles for Biomedical Application

Biomolecules are attractive building blocks with self-assembly ability, structural diversity, and excellent functionality for creating artificial materials. Heparin and protamine, a clinically relevant pair of biomolecules used in cardiac and vascular surgery, have been shown to coassemble into particulate polyelectrolyte complexes in vitro. The resulting heparin–protamine particles exhibit adhesive properties that enable advantageous interactions with proteins, cells, and various other substances and have been employed as functional materials for biomedical applications. In this review article, we summarize recent progress in research on the use of heparin–protamine particles as drug carriers, cell adhesives, and cell labels. Studies have demonstrated that heparin–protamine particles are potentially versatile in biomedical fields from drug delivery and regenerative medicine to plastic surgery.

Heparan Sulfate Proteoglycans: Key Mediators of Stem Cell Function

Heparan sulfate proteoglycans (HSPGs) are an evolutionarily ancient subclass of glycoproteins with exquisite structural complexity. They are ubiquitously expressed across tissues and have been found to exert a multitude of effects on cell behavior and the surrounding microenvironment. Evidence has shown that heterogeneity in HSPG composition is crucial to its functions as an essential scaffolding component in the extracellular matrix as well as a vital cell surface signaling co-receptor. Here, we provide an overview of the significance of HSPGs as essential regulators of stem cell function. We discuss the various roles of HSPGs in distinct stem cell types during key physiological events, from development through to tissue homeostasis and regeneration. The contribution of aberrant HSPG production to altered stem cell properties and dysregulated cellular homeostasis characteristic of cancer is also reviewed. Finally, we consider approaches to better understand and exploit the multifaceted functions of HSPGs in influencing stem cell characteristics for cell therapy and associated culture expansion strategies.

Heparinoid Complex-Based Heparin-Binding Cytokines and Cell Delivery Carriers

Heparinoid is the generic term that is used for heparin, heparan sulfate (HS), and heparin-like molecules of animal or plant origin and synthetic derivatives of sulfated polysaccharides. Various biological activities of heparin/HS are attributed to their specific interaction and regulation with various heparin-binding cytokines, antithrombin (AT), and extracellular matrix (ECM) biomolecules. Specific domains with distinct saccharide sequences in heparin/HS mediate these interactions are mediated and require different highly sulfated saccharide sequences with different combinations of sulfated groups. Multivalent and cluster effects of the specific sulfated sequences in heparinoids are also important factors that control their interactions and biological activities. This review provides an overview of heparinoid-based biomaterials that offer novel means of engineering of various heparin-binding cytokine-delivery systems for biomedical applications and it focuses on our original studies on non-anticoagulant heparin-carrying polystyrene (NAC-HCPS) and polyelectrolyte complex-nano/microparticles (N/MPs), in addition to heparin-coating devices.

Glycosaminoglycan-based biomaterials for growth factor and cytokine delivery: Making the right choices

Controlled, localized drug delivery is a long-standing goal of medical research, realization of which could reduce the harmful side-effects of drugs and allow more effective treatment of wounds, cancers, organ damage and other diseases. This is particularly the case for protein "drugs" and other therapeutic biological cargoes, which can be challenging to deliver effectively by conventional systemic administration. However, developing biocompatible materials that can sequester large quantities of protein and release them in a sustained and controlled manner has proven challenging. Glycosaminoglycans (GAGs) represent a promising class of bio-derived materials that possess these key properties and can additionally potentially enhance the biological effects of the delivered protein. They are a diverse group of linear polysaccharides with varied functionalities and suitabilities for different cargoes. However, most investigations so far have focused on a relatively small subset of GAGs - particularly heparin, a readily available, promiscuously-binding GAG. There is emerging evidence that for many applications other GAGs are in fact more suitable for regulated and sustained delivery. In this review, we aim to illuminate the beneficial properties of various GAGs with reference to specific protein cargoes, and to provide guidelines for informed choice of GAGs for therapeutic applications.

Polyelectrolyte Complexes of Natural Polymers and Their Biomedical Applications

Polyelectrolyte complexes (PECs), composed of natural and biodegradable polymers, (such as positively charged chitosan or protamine and negatively charged glycosaminoglycans (GAGs)) have attracted attention as hydrogels, films, hydrocolloids, and nano-/micro-particles (N/MPs) for biomedical applications. This is due to their biocompatibility and biological activities. These PECs have been used as drug and cell delivery carriers, hemostats, wound dressings, tissue adhesives, and scaffolds for tissue engineering. In addition to their comprehensive review, this review describes our original studies and provides an overview of the characteristics of chitosan-based hydrogel, including photo-cross-linkable chitosan hydrogel and hydrocolloidal PECs, as well as molecular-weight heparin (LH)/positively charged protamine (P) N/MPs. These are generated by electrostatic interactions between negatively charged LH and positively charged P together with their potential biomedical applications.

Sulfonated Copolymers as Heparin-Mimicking Stabilizer of Fibroblast Growth Factor: Size, Architecture, and Monomer Distribution Effects

Fibroblast growth factors (FGF) are involved in a wide range of biological processes such as cell proliferation and differentiation. In living organisms, the binding of FGF to its receptors are mediated through electrostatic interactions between FGF and naturally occurring heparin. Despite its prevalent use in medicine, heparin carries notable limitations; namely, its extraction from natural sources (expensive, low yield and extensive purification), viral contamination, and batch-to-batch heterogeneity. In this work a range of synthetic homopolymers and copolymers of sodium 2-acrylamido-2-methylpropanesulfonate were evaluated as potential FGF stabilizers. This was studied by measuring the proliferation of BaF3-FR1c cells, as a model assay, and the results will be compared with the natural stabilization and activation of FGF by heparin. This study explores the structure-activity relationship of these polysulfonated polymers with a focus on the effect of molecular weight, comonomer type, charge dispersion, and polymer architecture on protein stabilization.

Protective effect of FGF-2 and low-molecular-weight heparin/protamine nanoparticles on radiation-induced healing-impaired wound repair in rats

We examined the effectiveness of localized administration of fibroblast growth factor-2 containing low-molecular-weight heparin/protamine nanoparticles (FGF-2&LMWH/P NPs) on apoptosis in vivo and on healing of radiation-induced skin injury in a rat model. FGF-2 binds onto LMWH/P NPs, which can significantly enhance and stabilize FGF-2 as a local carrier. X-irradiation at a dose of 25 Gy was administered to the lower part of the back (using a lead sheet with two holes) 1 h before the administration of FGF-2&LMWH/P NPs. Cutaneous full-thickness defect wounds were then formed in X-irradiated areas to examine the time-course of wound healing, and the wound tissues were microscopically and histologically compared and examined. Wound healing was significantly delayed by X-irradiation, but FGF-2&LMWH/P NPs administration prior to irradiation led to a significantly shorter delay compared with FGF-2 alone, LMWH/P NPs alone, and controls. Furthermore, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining showed that the proportions of apoptotic dermal fibroblasts in X-irradiated skin were significantly lower in rats administered FGF-2&LMWH/P NPs than in controls. However, 8-hydroxy-2'-deoxyguanosine (8-OHdG) staining showed no differences. Thus, localized administration of FGF-2&LMWH/P NPs prior to irradiation may alleviate X-irradiation-induced healing-impaired wound repair in normal tissue.

The "in and out" of glucosamine 6-O-sulfation: the 6th sense of heparan sulfate

The biological properties of Heparan sulfate (HS) polysaccharides essentially rely on their ability to bind and modulate a multitude of protein ligands. These interactions involve internal oligosaccharide sequences defined by their sulfation patterns. Amongst these, the 6-O-sulfation of HS contributes significantly to the polysaccharide structural diversity and is critically involved in the binding of many proteins. HS 6-O-sulfation is catalyzed by 6-O-sulfotransferases (6OSTs) during biosynthesis, and it is further modified by the post-synthetic action of 6-O-endosulfatases (Sulfs), two enzyme families that remain poorly characterized. The aim of the present review is to summarize the contribution of 6-O-sulfates in HS structure/function relationships and to discuss the present knowledge on the complex mechanisms regulating HS 6-O-sulfation.

In-vitro suppression of IL-6 and IL-8 release from human pulmonary epithelial cells by non-anticoagulant fraction of enoxaparin

BACKGROUND

Enoxaparin, a mixture of anticoagulant and non-anticoagulant fractions, is widely used as an anticoagulant agent. However, it is also reported to possess anti-inflammatory properties. Our study indicated that enoxaparin inhibits the release of IL-6 and IL-8 from A549 pulmonary epithelial cells. Their release causes extensive lung tissue damage. The use of enoxaparin as an anti-inflammatory agent is hampered due to the risk of bleeding associated with its anticoagulant fractions. Therefore, we aimed to identify the fraction responsible for the observed anti-inflammatory effect of enoxaparin and to determine the relationship between its structure and biological activities.

METHODS

A549 pulmonary epithelial cells were pre-treated in the presence of enoxaparin and its fractions. The levels of IL-6 and IL-8 released from the trypsin-stimulated cells were measured by ELISA. The anticoagulant activity of the fraction responsible for the effect of enoxaparin was determined using an anti-factor-Xa assay. The fraction was structurally characterised using nuclear magnetic resonance. The fraction was 2-O, 6-O or N-desulfated to determine the position of sulfate groups required for the inhibition of interleukins. High-performance size-exclusion chromatography was performed to rule out that the observed effect was due to the interaction between the fraction and trypsin or interleukins.

RESULTS

Enoxaparin (60 μg/mL) inhibited the release of IL-6 and IL-8 by >30%. The fraction responsible for this effect of enoxaparin was found to be a disaccharide composed of α-L-iduronic-acid and α-D-glucosamine-6-sulfate. It (15 μg/mL) inhibited the release of interleukins by >70%. The 6-O sulphate groups were responsible for its anti-inflammatory effect. The fraction did not bind to trypsin or interleukins, suggesting the effect was not due to an artefact of the experimental model.

CONCLUSION

The identified disaccharide has no anticoagulant activity and therefore eliminates the risk of bleeding associated with enoxaparin. Future in-vivo studies should be designed to validate findings of the current study.

Distinct patterns of heparan sulphate in pancreatic islets suggest novel roles in paracrine islet regulation

Heparan sulphate proteoglycans (HSPGs) exist in pancreatic beta cells, and HS seems to modulate important interactions in the islet microenvironment. However, the intra-islet structures of HS in health or altered glucose homeostasis are currently unknown. Here we show that distinct spatial distribution of HS motifs is present in islets in the adult, that intra-islet HS motifs are mostly conserved between rodents and humans, and that HS is abundant in glucagon producing islet alpha cells. In beta cells HS is characterised by 2-O, 6-O and N-sulphated moieties, whereas HS in alpha cells is N-acetylated, N-, and 2-O sulphated and low in 6-O groups. Differential expression of three HS modifying genes in alpha and beta cells was observed and may account for the different HS patterns. Furthermore, we found that FGF1 and FGF2 were present in alpha cells, whereas functional FGFRs exist in beta cells, but not in the alpha cell line aTC1-6, or in primary alpha cells in islets. FGF1 induced signalling was dependent on 2-O, and 6-O HS sulphation in beta cells, and HS desulphation reduced beta cell proliferation and potentiated oxidant induced apoptosis. In leptin resistant animals and in islets from streptozotocin treated rats there was a reduction in alpha cell HS expression. These data demonstrate the distinct HS expression patterns in alpha and beta islet cells and propose a novel role for alpha cells as a source of paracrine FGF ligands to neighbouring beta cells with specific cell-associated HS domains mediating the activation and diffusion of paracrine ligands.

Molecular engineering of glycosaminoglycan chemistry for biomolecule delivery

Glycosaminoglycans (GAGs) are linear, negatively charged polysaccharides that interact with a variety of positively charged growth factors. In this review article the effects of engineering GAG chemistry for molecular delivery applications in regenerative medicine are presented. Three major areas of focus at the structure-function-property interface are discussed: (1) macromolecular properties of GAGs; (2) effects of chemical modifications on protein binding; (3) degradation mechanisms of GAGs. GAG-protein interactions can be based on: (1) GAG sulfation pattern; (2) GAG carbohydrate conformation; (3) GAG polyelectrolyte behavior. Chemical modifications of GAGs, which are commonly performed to engineer molecular delivery systems, affect protein binding and are highly dependent on the site of modification on the GAG molecules. The rate and mode of degradation can determine the release of molecules as well as the length of GAG fragments to which the cargo is electrostatically coupled and eventually released from the delivery system. Overall, GAG-based polymers are a versatile biomaterial platform offering novel means to engineer molecular delivery systems with a high degree of control in order to better treat a range of degenerated or injured tissues.

Non-anticoagulant derivatives of heparin for the management of asthma: distant dream or close reality?

INTRODUCTION

Approximately 300 million people worldwide are currently affected by asthma. Improvements in the understanding of the mechanisms involved in such inflammatory airway disorders has led to the recognition of new therapeutic approaches. Heparin, a widely used anticoagulant, has been shown to be beneficial in the management of asthma. It belongs to the family of highly sulphated polysaccharides referred to as glycosaminoglycans, containing a heterogeneous mixture of both anticoagulant and non-anticoagulant polysaccharides. Experimental findings have suggested that heparin has potential anti-asthmatic properties owing to the ability of its non-anticoagulant oligosaccharides to bind and modulate the activity of a wide range of biological molecules involved in the inflammatory process.

AREAS COVERED

This review focuses on the potential mechanisms of action and clinical application of heparin as an anti-inflammatory agent for the management of asthma.

EXPERT OPINION

Heparin may play a significant role in the management of asthma. However, these properties are often hindered by the presence of anticoagulant oligosaccharides, which possess a significant risk of bleeding. Therefore, its therapeutic potential must be explored using well-designed clinical studies that focus on identifying and isolating the anti-inflammatory oligosaccharides of heparin and further elucidating the structure and mechanisms of actions of these non-anticoagulant oligosaccharides.

Convergent stereoselective synthesis of multiple sulfated GlcNα(1,4)GlcAβ(1,4) dodecasaccharides

In this paper, we describe an effective method for the elongation of a GlcNα(1,4)GlcAβ(1,4) sequence using a GlcNTrocα(1,4)GlcA disaccharide unit and the synthesis of the N- and/or O-sulfated GlcNα(1,4)GlcAβ(1,4) oligosaccharides. N-Troc protection of GlcNα(1,4)GlcA units was effective for the synthesis of the GlcNα(1,4)GlcAβ(1,4) oligosaccharides in comparison with the azido substituent. The GlcNα(1,4)GlcAβ(1,4) dodecasaccharide was successfully prepared by the direct β-selective glycosidation of glucuronate in the GlcNα(1,4)GlcAβ(1,4)GlcNα(1,4)GlcAβ(1,4) tetrasaccharide. In addition, the synthesis of the N- and/or O-sulfated GlcNα(1,4)GlcAβ(1,4) oligosaccharides was accomplished by fluorous-assisted deprotection and sulfation. The fluorous-assisted synthetic technology applied to the highly polar sulfated oligosaccharide permits it to be more easily separated from the highly polar reagents, such as SO(3)·NEt(3).

Post treatment with an FGF chimeric growth factor enhances epithelial cell proliferation to improve recovery from radiation-induced intestinal damage

PURPOSE

A fibroblast growth factor (FGF) 1-FGF2 chimera (FGFC) was created previously and showed greater structural stability than FGF1. This chimera was capable of stimulating epithelial cell proliferation much more strongly than FGF1 or FGF2 even without heparin. Therefore FGFC was expected to have greater biologic activity in vivo. This study evaluated and compared the protective activity of FGFC and FGF1 against radiation-induced intestinal injuries.

METHODS AND MATERIALS

We administered FGFC and FGF1 intraperitoneally to BALB/c mice 24 h before or after total-body irradiation (TBI). The numbers of surviving crypts were determined 3.5 days after TBI with gamma rays at doses ranging from 8 to 12 Gy.

RESULTS

The effect of FGFC was equal to or slightly superior to FGF1 with heparin. However, FGFC was significantly more effective in promoting crypt survival than FGF1 (p < 0.01) when 10 μg of each FGF was administered without heparin before irradiation. In addition, FGFC was significantly more effective at promoting crypt survival (p < 0.05) than FGF1 even when administered without heparin at 24 h after TBI at 10, 11, or 12 Gy. We found that FGFC post treatment significantly promoted 5-bromo-2'-deoxyuridine incorporation into crypts and increased crypt depth, resulting in more epithelial differentiation. However, the number of apoptotic cells in FGFC-treated mice decreased to almost the same level as that in FGF1-treated mice.

CONCLUSIONS

These findings suggest that FGFC strongly enhanced radioprotection with the induction of epithelial proliferation without exogenous heparin after irradiation and is useful in clinical applications for both the prevention and post treatment of radiation injuries.

Immobilization, stabilization, and activation of human stem cell factor (SCF) on fragmin/protamine microparticle (F/P MP)-coated plates

Fragmin (low-molecular-weight heparin)/protamine microparticles (F/P MPs) immobilize to culture plates, thereby retaining the binding of heparin-binding cytokines such as human stem cell factor (SCF). The purpose of this study was to evaluate the ability of F/P MP-coating to immobilize, stabilize, and enhance SCF-activity. Cell assays showed that SCF and preimmobilized SCF on F/P MP-coated plates significantly stimulated the proliferation of human erythroleukemia cell line TF-1 in a concentration-dependent manner. Heparin and fragmin enhanced SCF-induced proliferation of chlorate-treated TF-1 cells, in which the biosynthesis of endogenous sulfated polysaccharides was blocked, on noncoated plates at a range of concentrations (2-8 microg/mL). However, heparin and fragmin had no effect on SCF-induced proliferation of chlorate-treated TF-1 cells on F/P MP-coated plates. The interaction of SCF with fragmin and F/P MPs prolonged the half-life of SCF bioactivity, and immobilized and protected SCF from inactivation, such as from heat and proteolysis. These results suggest that F/P MP-coated plates protect SCF and enhance its activity, and F/P MP-coating provides an excellent biomaterial to immobilize and retain heparin-binding cytokines, including SCF, in bioactive form for optimal expansion of hematopoietic cells.

Mice deficient in heparan sulfate 6-O-sulfotransferase-1

Heparan sulfate chains 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 at various places and epimerization of hexuronic acid residues (C5-epimerase) at the Golgi lumen and further by 6-O-desulfation at the cell surface, which generates their characteristic divergent fine structures. This chapter focuses on the biological and physiological functions of 6-O-sulfation in HS and the characterization of the enzymes catalyzing 6-O-sulfation (HS6ST). HS6STs in mammals such as humans and mice comprise of three isoforms (HS6ST-1, -2, and -3) and one alternatively spliced form of HS6ST-2 (HS6ST-2S). Each of these isoforms has distinct substrate preferences, albeit overlapping each other. These HS6ST isoforms are expressed in a spatiotemporally regulated manner in most organs. HS6ST-1-deficient mice are lethal mostly at later embryonic stages and exhibit abnormal angiogenesis in labyrinthine zone of placenta and aberrant lung morphology similar to pulmonary emphysema. These knockout mice also exhibit retinal axon guidance abnormality at the optic chiasm. Other HS6ST-deficient animals reveal various malformations in muscle development and branching morphology of the caudal vein of zebrafish, in tracheal formation of Drosophila, and in axon guidance of ventral nerve cord interneurons of Caenorhabditis elegans. Mouse embryonic fibroblasts prepared from HS6ST-1/HS6ST-2 double knockout mice did produce HS lacking 6-O-sulfation and responded differently to various FGFs dependent signaling.

Compositional analysis of heparin/heparan sulfate interacting with fibroblast growth factor.fibroblast growth factor receptor complexes

Heparan sulfate (HS) proteoglycans (PGs) interact with a number of extracellular signaling proteins, thereby playing an essential role in the regulation of many physiological processes. One major function of HS is to interact with fibroblast growth factors (FGFs) and their receptors (FGFRs) and form FGF.HS.FGFR signaling complexes. Past studies primarily examined the selectivity of HS for FGF or FGFR. In this report, we used a new strategy to study the structural specificity of HS binding to 10 different FGF.FGFR complexes. Oligosaccharide libraries prepared from heparin, 6-desulfated heparin, and HS were used for the interaction studies by solution competition surface plasmon resonance (SPR) and filter trapping assays. Specific oligosaccharides binding to FGF.FGFR complexes were subjected to polyacrylamide gel electrophoresis (PAGE) analysis and disaccharide compositional analysis using liquid chromatography and mass spectrometry. The competition SPR studies using sized oligosaccharide mixtures showed that binding of each of the tested FGFs or FGF.FGFR complexes to heparin immobilized to an SPR chip was size-dependent. The 6-desulfated heparin oligosaccharides exhibited a reduced level of inhibition of FGF and FGF.FGFR complex binding to heparin in the competition experiments. Heparin and the 6-desulfated heparin exhibited higher levels of inhibition of the FGF.FGFR complex binding to heparin than of FGF binding to heparin. In the filter trapping experiments, PAGE analysis showed different affinities between the FGF.FGFR complexes and oligosaccharides. Disaccharide analysis showed that HS disaccharides with a degree of polymerization of 10 (dp10) had high binding selectivity, while dp10 heparin and dp10 6-desulfated heparin showed reduced or no selectivity for the different FGF.FGFR complexes tested.

Specific inhibition of FGF-2 signaling with 2-O-sulfated octasaccharides of heparan sulfate

In fibroblast growth factor (FGF)-2 signaling, the formation of a ternary complex of FGF-2, tyrosine-kinase fibroblast growth factor receptor (FGFR)-1, and cell surface heparan sulfate (HS) proteoglycan is known to be critical for the activation of FGFR-1 and downstream signal transduction. Exogenous heparin polymer and some octasaccharides inhibited FGF-2-induced phosphorylation both of FGFR-1 and of extracellular signal-regulated kinase (ERK1/2) in Chinese hamster ovary (CHO)-K1 cells transfected with FGFR-1, which present HS on their cell surface. The inhibitory effect of octasaccharide was dependent on the number of 2-O-sulfate groups within a molecule but independent of the number of 6-O-sulfate groups. Sulfation at the 2-O-position was a prerequisite not only for the binding of HS to FGF-2 but also for regulation of FGF-2 signaling and competitive inhibition with endogenous HS. Interestingly, FGF-4-induced phosphorylation was impeded only by specific octasaccharides containing both 2-O- and 6-O-sulfated groups, which were necessary for binding FGF-4. In CHO-677 cells deficient in HS biosynthesis, heparin enhanced FGF-2-induced phosphorylation of ERK1/2. On the other hand, an FGF-2-binding octasaccharide inhibited the phosphorylation. Our data suggest that the activity of particular heparin-binding factors can be inhibited by distinctive oligosaccharides that can bind the factors but cannot form functional signaling complexes irrespective of whether cells have a normal complement of HS or lack HS.

6-O-sulfation of heparan sulfate differentially regulates various fibroblast growth factor-dependent signalings in culture

Heparan sulfate (HS) interacts with diverse heparin-binding growth factors and thereby regulates their bioactivities. These interactions depend on the structures characterized by the sulfation pattern and isomer of uronic acid residues. One of the biosynthetic modifications of HS, namely 6-O-sulfation, is catalyzed by three isoforms of HS6-O-sulfotransferase. We generated HS6ST-1- and/or HS6ST-2-deficient mice (6ST1-KO, 6ST2-KO, and double knock-out (dKO)) that exhibited different phenotypes. We examined the effects of HS 6-O-sulfation in heparin-binding growth factor signaling using fibroblasts derived from these mutant mice. Mouse embryonic fibroblasts (MEF) prepared from E14.5 dKO mice produced HS with little 6-O-sulfate, whereas 2-O-sulfation in HS from dKO-MEF (dKO-HS) was increased by 1.9-fold. HS6-O-sulfotransferase activity in the dKO-MEF was hardly detected, and HS2-O-sulfotransferase activity was 1.5-fold higher than that in wild type (WT)-MEFs. The response of dKO-MEFs to fibroblast growth factors (FGFs) was distinct from that of WT-MEFs; in dKO-MEFs, FGF-4- and FGF-2-dependent signalings were reduced to approximately 30 and 60% of WT-MEFs, respectively, and FGF-1-dependent signaling was moderately reduced compared with that of WT-MEFs but only at the lower FGF-1 concentrations. Analysis with a surface plasmon resonance biosensor demonstrated that the apparent affinity of dKO-HS for FGF-4 was markedly reduced and was also reduced for FGF-1. In contrast, the affinity of dKO-HS for FGF-2 was 2.5-fold higher than that of HS from WT-MEFs. Thus, 6-O-sulfate in HS may regulate the signalings of some of HB-GFs, including FGFs, by inducing different interactions between ligands and their receptors.

Effects of growth factors on heparin-carrying polystyrene-coated atelocollagen scaffold for articular cartilage tissue engineering

The specific aim of our investigation is to study the potential use of a collagen/heparin-carrying polystyrene (HCPS) composite extracellular matrix for articular cartilage tissue engineering. Here, we created a high-performance extracellular matrix (HpECM) scaffold to build an optimal extracellular environment using an HCPS we originally developed, and an atelocollagen honeycomb-shaped-scaffold (ACHMS-scaffold) with a membrane seal. This scaffold was coated with HCPS to enable aggregation of heparin-binding growth factors such as FGF-2 and TGF-beta1 within the scaffold. Three-dimensional culture of rabbit articular chondrocytes within the HpECM-scaffold and subsequent preparation of a tissue-engineered cartilage were investigated. The results showed remarkably higher cell proliferative activity within the HpECM-pretreated-FGF-2 scaffold and the sustenance of phenotype within the HpECM-pretreated-TGF-beta1 scaffold. It was thought that both FGF-2 and TGF-beta1 were stably immobilized in the HpEMC-scaffold since HCPS generated an extracellular environment similar to that of heparan sulfate proteoglycan within the scaffold. These results suggest that an ACHMS-scaffold immobilized with HCPS can be a HpECM for cartilage regeneration to retain the heparin-binding growth factors within the scaffolds.

Engineered bio-active polysaccharides from heparin

[Image: see text] Heparin, the well-known anticoagulant polysaccharide, is also active in many other biological systems owing to its structural similarity to HS, but usually lacks selectivity because it is more highly sulfated. A series of straightforward chemical reactions (de-O-sulfation, de-N-sulfation and re-N-acetylation), carried out to partial or complete extent, were combined, resulting in a number of modified heparin polysaccharide derivatives with altered properties. These exhibited a range of abilities to promote cell signalling through the FGF/FGFR tyrosine kinase signalling system, in an in vitro cell assay with combinations of FGF-1, -2, -3 and FGFR 1 and 3. One polysaccharide (N-acetylated, 6-O- and 2-O-sulfated heparin), with only a fraction (<10(-3)) of the anticoagulant activity of heparin (200 U . mg(-1)), promoted FGF-2-mediated angiogenesis (10-fold) and therefore had an improved ratio of pro-angiogenic activity to anticoagulant activity in excess of 10(4) compared to heparin. These results demonstrate that heparin-derived polysaccharides can be engineered for selected activities and have potential in a wide range of medical, biotechnological and tissue-engineering applications. Effect of selected engineered heparin polysaccharides on angiogenesis.

Diversity-oriented chemical modification of heparin: Identification of charge-reduced N-acyl heparin derivatives having increased selectivity for heparin-binding proteins

The diversity-oriented chemical modification of heparin is shown to afford charge-reduced heparin derivatives that possess increased selectivity for binding heparin-binding proteins. Variable N-desulfonation of heparin was employed to afford heparin fractions possessing varied levels of free amine. These N-desulfonated heparin fractions were selectively N-acylated with structurally diverse carboxylic acids using a parallel synthesis protocol to generate a library of 133 heparin-derived structures. Screening library members to compare affinity for heparin-binding proteins revealed unique heparin-derived structures possessing increased affinity and selectivity for individual heparin-binding proteins. Moreover, N-sulfo groups in heparin previously shown to be required for heparin to bind specific proteins have been replaced with structurally diverse non-anionic moieties to afford identification of charge-reduced heparin derivatives that bind these proteins with equivalent or increased affinity compared to unmodified heparin. The methods described here outline a process that we feel will be applicable to the systematic chemical modification of natural polyanionic polysaccharides and the preparation of synthetic oligosaccharides to identify charge-reduced high affinity ligands for heparin-binding proteins.

HSulf-2, an extracellular endoglucosamine-6-sulfatase, selectively mobilizes heparin-bound growth factors and chemokines: effects on VEGF, FGF-1, and SDF-1

BACKGROUND

Heparin/heparan sulfate (HS) proteoglycans are found in the extracellular matrix (ECM) and on the cell surface. A considerable body of evidence has established that heparin and heparan sulfate proteoglycans (HSPGs) interact with numerous protein ligands including fibroblast growth factors, vascular endothelial growth factor (VEGF), cytokines, and chemokines. These interactions are highly dependent upon the pattern of sulfation modifications within the glycosaminoglycan chains. We previously cloned a cDNA encoding a novel human endosulfatase, HSulf-2, which removes 6-O-sulfate groups on glucosamine from subregions of intact heparin. Here, we have employed both recombinant HSulf-2 and the native enzyme from conditioned medium of the MCF-7-breast carcinoma cell line. To determine whether HSulf-2 modulates the interactions between heparin-binding factors and heparin, we developed an ELISA, in which soluble factors were allowed to bind to immobilized heparin.

RESULTS

Our results show that the binding of VEGF, FGF-1, and certain chemokines (SDF-1 and SLC) to immobilized heparin was abolished or greatly diminished by pre-treating the heparin with HSulf-2. Furthermore, HSulf-2 released these soluble proteins from their association with heparin. Native Sulf-2 from MCF-7 cells reproduced all of these activities.

CONCLUSION

Our results validate Sulf-2 as a new tool for deciphering the sulfation requirements in the interaction of protein ligands with heparin/HSPGs and expand the range of potential biological activities of this enzyme.

Controlled release of fibroblast growth factor-2 from an injectable 6-O-desulfated heparin hydrogel and subsequent effect onin vivo vascularization

We prepared a 6-O-desulfated (DS-) heparin (Hep) hydrogel as an excellent carrier for the controlled release of Hep-binding growth factors, such as fibroblast growth factor (FGF)-2. This material, which is partially derived from photoreactive groups, such as cinnamate, is easily crosslinked upon ultraviolet light (UV)-irradiation, resulting in a water-insoluble, viscous, and injectable hydrogel. In the present study, we examined the capacity of 6-O-DS-Hep hydrogel to immobilize FGF-2, as well as the controlled release of FGF-2 molecules from this hydrogel in vitro and in vivo. Only 10% of FGF-2 was gradually released from the FGF-2-containing 6-O-DS-Hep hydrogel (photocrosslinked 6-O-DS-Hep (4%; w/w) hydrogel containing 50 microg/mL FGF-2) into PBS (phosphate-buffered saline) within first 7 days. The 6-O-DS-Hep hydrogel in vitro maintained the original form through 1 weeks incubation in PBS, but it was gradually fragmented and could not maintain the original form by 2-3 week-washing. When the FGF-2-containing 6-O-DS-Hep hydrogel was subcutaneously injected into the back of rats, significant neovascularization and fibrous tissue formation were induced near the injected site from day 3 after the injection. And, the hydrogel had been biodegraded and completely disappeared from the injected sites in vivo within about 15-20 days after the injection. These findings indicate a controlled release of biologically active FGF-2 molecules together with fragmentation and biodegradation of 6-O-DS-Hep hydrogel and the subsequent induction of neovascularization in vivo.

Interactions of multiple heparin binding growth factors with neuropilin-1 and potentiation of the activity of fibroblast growth factor-2

The hypothesis that neuropilin-1 (Npn-1) may interact with heparin-binding proteins other than vascular endothelial growth factor has been tested using an optical biosensor-based binding assay. The results show that fibroblast growth factor (FGF) 1, 2, 4, and 7, FGF receptor 1, hepatocyte growth factor/scatter factor (HGF/SF), FGF-binding protein, normal protease sensitive form of prion protein, antithrombin III, and Npn-1 itself are all able to interact with Npn-1 immobilized on the sensor surface. FGF-2, FGF-4, and HGF/SF are also shown to interact with Npn-1 in a solution assay. Moreover, these protein-protein interactions are dependent on the ionic strength of the medium and are inhibited by heparin, and the kinetics of binding of FGF-2, FGF-4 and HGF/SF to Npn-1 are characterized by fast association rate constants (270,000-1,600,000 m(-1) s(-1)). These results suggest that Npn-1 possesses a "heparin" mimetic site that is able to interact at least in part through ionic bonding with the heparin binding site on many of the proteins studied. Npn-1 was also found to potentiate the growth stimulatory activity of FGF-2 on human umbilical vein endothelial cells, indicating that Npn-1 may not just bind but also regulate the activity of heparin-binding proteins.

Removal of heparan sulfate by heparinase treatment inhibits FGF-2-dependent smooth muscle cell proliferation in injured rat carotid arteries

Smooth muscle cells (SMC) of the rat carotid arterial media proliferate and migrate in response to injury during the formation of a neointima. The interaction of fibroblast growth factor (FGF-2), which is released at the site of injury, with heparan sulfate proteoglycans (HSPGs) is necessary to induce signaling, which elicits an FGF-dependent mitogenic response by arterial smooth muscle cells, and also serves as a mechanism for storage of the growth factor within the extracellular matrix. However, whether these interactions are critical during neointimal formation has not been directly tested. In this study, a model of FGF-2-dependent medial SMC mitogenic response in balloon-injured rat carotid artery was used to test the effect of degradation of vessel wall heparan sulfate on subsequent SMC proliferation. Treatment of balloon-catheterized rat carotid arteries with chondroitin ABC lyase and/or heparin lyases eliminated heparan sulfates in the vessel wall, as determined by immunoperoxidase staining. In contrast, the distribution in the carotid vessel wall of the large core protein of perlecan, a major vessel wall HSPG that binds FGF-2, is not decreased. The effect of glycosaminoglycan digestion in situ on medial SMC proliferation in response to a bolus injection of FGF-2 after injury was determined by measuring the percentage of SMC nuclei that incorporated 5-bromo-2'-deoxyuridine (BrdU) 48 h after injury. Enzymatic removal of heparan sulfate reduced BrdU incorporation into medial SMC by 60-70% (P < 0.001) at 48 h after injury. Moreover, pre-incubation of FGF-2 with heparin prior to injection restored SMC replication to the levels present in injured vessels treated with buffer alone (P < 0.01). These experiments indicate that endogenous HSPGs are essential to promote FGF-2-driven medial SMC proliferation following injury, and that heparinase treatment can abrogate FGF-2-dependent responses in vivo.

Heparin affects signaling pathways stimulated by fibroblast growth factor-1 and -2 in type II cells

Undersulfation of the basement membrane matrix of alveolar type II (AT2) cells compared with that of neighboring type I cells is believed to account for some of the known morphological and functional differences between these pneumocytes. Heparin, a model for sulfated components of basement membrane matrices, is known to inhibit fibroblast growth factor (FGF)-2-stimulated DNA synthesis as well as gene expression of FGF-2 and its receptor in AT2 cells. To determine whether these end points result from specific effects of heparin on FGF-related signaling pathways, isolated rat AT2 cells were treated with 100 ng/ml FGF-1 or FGF-2 in the presence of up to 500 microg/ml heparin. In addition, experiments were done on cells grown in the presence of 20 mM sodium chlorate (sulfation inhibitor). High-dose heparin reduced FGF-1- or FGF-2-stimulated phosphorylation of mitogen-activated protein kinase kinases (MEK1/2), p44/42 mitogen-activated protein kinases (MAPK/ERK1/2), stress-activated protein kinase/c-Jun NH(2)-terminal kinase, Akt/protein kinase B, and p90(RSK). FGF-2-stimulated signaling was more sensitive to heparin's effects than was signaling stimulated by FGF-1. Heparin had an additive effect on the reduced [(3)H]thymidine incorporation in FGF-2-treated AT2 cells caused by inhibition of the MEK/ERK pathway by the MEK inhibitor PD-98059. The data suggest that heparin's known capacity to alter DNA synthesis and, possibly, other biological end points is realized via cross talk between multiple signaling pathways.

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.

Sulfated derivatives of Escherichia coli K5 polysaccharides as modulators of fibroblast growth factor signaling

Heparan sulfate (HS) proteoglycans are intimately involved in the regulation of fibroblast growth factor (FGF) signaling. HS and the related glycosaminoglycan heparin interact with FGFs and FGF receptors (FGFRs), and it is believed that both interactions are required for productive FGF signaling. Attempts to inhibit FGF activity have been made with modified heparin preparations, various heparin-like polysaccharide analogues and other polyanionic molecules, which may all act by interfering with the physiological HS-FGF-FGFR interactions on the cell surface. Here, we have studied the potential of sulfated derivatives of a bacterial polysaccharide (capsular polysaccharide from Escherichia coli K5 (K5PS)) in the modulation of FGF-heparin/HS interactions and FGF signaling. We demonstrate that O-sulfated and N,O-sulfated species of K5PS, with high degrees of sulfation, displaced FGF-1, FGF-2, and FGF-8b from heparin. However, only O-sulfated K5PS efficiently inhibited the FGF-induced proliferation of S115 mammary carcinoma cells and 3T3 fibroblasts, whereas N,O-sulfated K5PS had little or no inhibitory effect. Studies with CHO677 cells lacking endogenous HS, as well as with chlorate-treated S115 cells expressing undersulfated HS, indicated that whereas exogenously administered heparin and N,O-sulfated K5PS restored the cellular response toward FGF stimulation, O-sulfated K5PS was largely devoid of such stimulatory activity. Our data suggest that highly O-sulfated species of K5PS may be efficient inhibitors of FGF signaling.

Heparan sulphate glycosaminoglycans derived from endothelial cells and smooth muscle cells differentially modulate fibroblast growth factor-2 biological activity through fibroblast growth factor receptor-1

Fibroblast growth factor (FGF) signalling is involved in a wide range of important biological activities with differential effects in various cell types. The activity of FGF is modulated by heparin/heparan sulphate-like glycosaminoglycans (HSGAGs), found both in the extracellular matrix and on the cell surface. HSGAGs affect FGF signalling by interacting with both the growth factor and the FGF receptor (FGFR). In this study we sought to investigate whether HSGAGs at the cell surface of bovine aortic endothelial cells (BAEC) and smooth muscle cells (SMC) can differentially modulate FGF signalling in these cell types and modulate their differential response to FGF. We find that SMC and BAEC express the same FGFR isoforms and bind FGF2 with equal affinity at the cell surface, yet FGF has a markedly higher proliferative effect on SMC than on BAEC. Isolated HSGAGs from these two cell types were found to elicit distinct patterns of proliferation in chlorate-treated cells. Furthermore, examination of focal sequences reveals that HSGAGs from SMC, but not those from BAEC, retain the sulphation pattern necessary to induce FGF2 activity. As such, the differences in FGF2-mediated proliferation can be explained by the distinct cell surface HSGAGs of the two cell types. We conclude that the focal sequences of cell surface HSGAGs from SMC and BAEC govern, at least in part, the differential activity of FGF2 on these two cell types.

Controlled release of fibroblast growth factors and heparin from photocrosslinked chitosan hydrogels and subsequent effect on in vivo vascularization

Application of ultraviolet (UV) irradiation to a photocrosslinkable chitosan (Az-CH-LA) aqueous solution resulted within 10 s in an insoluble, flexible hydrogel. A low molecular weight acidic molecule like trypan blue and various high molecular weight molecules such as bovine serum albumin (BSA), heparin and protamine were all retained within the hydrogel, while a low molecular weight basic molecule like toluidine blue was rapidly released from the hydrogel. In the present work, we examined the retaining capability of the chitosan hydrogel for growth factors and controlled release of growth factors from the chitosan hydrogel in vitro and in vivo. Fibroblast growth factor-1 (FGF-1), fibroblast growth factor-2 (FGF-2), vascular endothelial growth factor(165) (VEGF(165)), heparin-binding epidermal growth factor (HB-EGF) in phosphate buffered saline (PBS) were mixed with Az-CH-LA aqueous solution to form growth factor-incorporated chitosan hydrogels. About 10-25% of the growth factor was released from a growth factor-incorporated chitosan hydrogel into PBS within the first day, after which no further substantial release took place. The growth factors interacted with Az-CH-LA molecules poly-ion complexation, and probably were unable to be released after the first day under the in vitro nondegradation conditions of the hydrogel. Although the FGF-1, FGF-2, and VEGF(165)-incorporated chitosan hydrogels on a culture plate significantly stimulated HUVEC growth, the stimulating activity of the growth factor-incorporated chitosan hydrogel was completely cancelled out by washing the hydrogel with PBS solution for 3 days or more. The stimulating activity on the HUVEC growth were however highly recovered by treating the washed growth factor-incorporated chitosan hydrogel during 7 days with chitinase and chitosanase to partly degrade the hydrogel, strongly suggesting that the growth factors within the hydrogel retained their biologically active forms. The chitosan hydrogel (100 microl) when implanted into the back of a mouse was biodegraded in about 10-14 days. When FGF-1- and FGF-2-incorporated chitosan hydrogels were subcutaneously implanted into the back of a mouse, significant neovascularization was induced near the implanted site of the FGF-1- and FGF-2-incorporated chitosan hydrogels. Furthermore, addition of heparin with either FGF-1 or FGF-2 into the hydrogel resulted in a significantly enhanced and prolonged vascularization effect. These results indicate that the controlled release of biologically active FGF-1 and FGF-2 with heparin is caused by biodegradation of the chitosan hydrogel, and subsequent induction of vascularization.

Heparin inhibits DNA synthesis and gene expression in alveolar type II cells

Responses of isolated type II alveolar cells to fibroblast growth factors (FGF) have been shown to be sensitive to the level of sulfation in extracellular matrix (ECM) substrata. These observations may reflect the specific in situ distribution and level of sulfation of ECM within the alveolar basement membranes (ABM) associated with type II cells. The goal of this study was to determine if the model sulfated ECM heparin modified DNA synthesis and gene expression by type II cells in a concentration dependent-manner. Isolated rat type II cells were exposed to different concentrations of heparin (0.005-500 micro g/ml) in serum-free medium for 1-3 d with or without FGF-1 or FGF-2. The effects of heparin were examined by [(3)H]thymidine incorporation into DNA, total cell protein, cell number, and selected gene expression. Results indicated that heparin inhibited [(3)H]thymidine uptake in a concentration-dependent manner. Total protein, cell number, and FGF-2 protein expression and mRNA of FGF-1, -2, and FGF receptor-2 detected by reverse transcriptase-polymerase chain reaction were decreased by heparin. These results demonstrate that sulfated molecules in the ABM may play important regulatory role(s) in selected type II cell activities during normal cell homeostasis, turnover, and repair after lung injury.

Fibroblast growth factor-2 binds to small heparin-derived oligosaccharides and stimulates a sustained phosphorylation of p42/44 mitogen-activated protein kinase and proliferation of rat mammary fibroblasts

We examine the relationship between the chain length of heparin-derived oligosaccharides, fibroblast growth factor (FGF)-2 binding kinetics and the ability of the oligosaccharides to allow FGF-2-induced proliferation of chlorate-treated rat mammary fibroblasts. First, using an optical biosensor, we show that FGF-2 did not bind disaccharides, but definitively bound to tetrasaccharides. As the chain length increased from tetrasaccharide to octasaccharide, there was a substantial increase in k(ass) (564000 M(-1) x s(-1) to 2000000 M(-1) x s(-1), respectively) and affinity (K(d) 77 nM to 11 nM, respectively) for FGF-2. From decasaccharides and longer, the k(ass) and affinity for FGF-2 was reduced substantially (tetradecasaccharide k(ass) 470000 M(-1) x s(-1), K(d) 30 nM). In chlorate-treated, and hence sulphated, glycosaminoglycan-deficient cells, FGF-2 alone or in the presence of disaccharides did not stimulate DNA synthesis and it only elicited an early transient dual phosphorylation of p42/44 mitogen-activated protein kinase (MAPK). In the same cells FGF-2 in the presence of tetrasaccharides and longer oligosaccharides was able to restore DNA synthesis and enable the sustained dual phosphorylation of p42/44(MAPK). However, the oligosaccharides from tetrasaccharides to octasaccharides were less potent in proliferation assays than deca- and longer oligosaccharides. Therefore, there was no correlation between the binding parameters and the potency of the oligosaccharides in DNA synthesis assays. These results demonstrate that tetrasaccharides are able to bind FGF-2 and enable FGF-2 to stimulate cell proliferation, which sets important boundary conditions for models of the FGF-2-heparan sulphate-FGF receptor complex.

Fibroblast growth factor receptors 1 and 2 interact differently with heparin/heparan sulfate. Implications for dynamic assembly of a ternary signaling complex

Heparan sulfate (HS) regulates the kinetics of fibroblast growth factor 2 (FGF2)-stimulated intracellular signaling and differentially activates cell proliferation of cells expressing different FGF receptors (FGFRs). Evidence suggests that HS interacts with both FGFs and FGFRs to form active ternary signaling complexes. Here we compare the interactions of two FGFRs with HS. We show that the ectodomains of FGFR1 IIIc and FGFR2 IIIc exhibit specific interactions with different characteristics for both heparin and porcine mucosal HS. These glycans are both known to activate FGF signaling via these receptors. FGFR2 interacts with a higher apparent affinity than FGFR1 despite both involving 6-O-, 2-O-, and N-sulfates. FGFR1 and FGFR2 bind heparin with mean association rate constants of 1.9 x 10(5) and 2.1 x 10(6) m(-1)s(-1), respectively, and dissociation rate constants of 1.2 x 10(-2) and 2.7 x 10(-2) s(-1), respectively. These produced calculated affinities of 63 and 13 nm, respectively. Hence, FGFR1 and FGFR2 bind to heparin chains with markedly different kinetics and affinities. We propose a mechanistic model where the kinetic parameters of the HS/FGFR interaction are a key element regulating the formation of ternary complexes and the resulting FGF signaling outcomes.

Differential effects of heparin saccharides on the formation of specific fibroblast growth factor (FGF) and FGF receptor complexes

Heparan sulfates (HS) play an important role in the control of cell growth and differentiation by virtue of their ability to modulate the activities of heparin-binding growth factors, an issue that is particularly well studied for fibroblast growth factors (FGFs). HS/heparin co-ordinate the interaction of FGFs with their receptors (FGFRs) and are thought to play a critical role in receptor dimerization. Biochemical and crystallographic studies, conducted mainly with FGF-2 or FGF-1 and FGF receptors 1 and 2, suggests that an octasaccharide is the minimal length required for FGF- and FGFR-induced dimerization and subsequent activation. In addition, 6-O-sulfate groups are thought to be essential for binding of HS to FGFR and for receptor dimerization. We show here that oligosaccharides shorter than 8 sugar units support activation of FGFR2 IIIb by FGF-1 and interaction of FGFR4 with FGF-1. In contrast, only relatively long oligosaccharides supported receptor binding and activation in the FGF-1.FGFR1 or FGF-7.FGFR2 IIIb setting. In addition, both 6-O- and 2-O-desulfated heparin activated FGF-1 signaling via FGFR2 IIIb, whereas neither one stimulated FGF-1 signaling via FGFR1 or FGF-7 via FGFR2 IIIb. These findings indicate that the structure of HS required for activating FGFs is dictated by the specific FGF and FGFR combination. These different requirements may reflect the differences in the mode by which a given FGFR interacts with the various FGFs.

Specific heparan sulfate structures involved in retinal axon targeting

Heparan sulfate (HS), a structurally diverse molecule comprising distinct sequences of sulfated disaccharide units, is abundant in the developing brain and binds to axon guidance molecules. Addition of HS to the developing Xenopus optic pathway causes severe targeting errors yet it is not known how the structural diversity of this molecule relates to its role in axon guidance. We have used an in vivo brain assay to identify the structural characteristics of HS that induce aberrant axon targeting. Inhibiting sulfation of endogenous HS with chlorate causes axons to bypass their target, the tectum, and treatment with chemically modified heparins reveals that 2-O- and 6-O-sulfate groups have potent bypass-inducing activity. Experiments with purified heparin saccharides show that bypass-inducing activity correlates with distinct structures, particularly those containing a combination of 2-O- and 6-O-sulfate groups. Taken together the results indicate that specific sequences, rather than gross structural composition, are critical for activity. In situ hybridisation revealed that HS 6-O-sulfotransferase is regionally expressed along the border of the dorsal optic tract whereas 2-O-sulfotransferase is expressed broadly. Our results demonstrate that specific HS sequences are essential for regulating retinotectal axon targeting and suggest that regionalised biosynthesis of specific HS structures is important for guiding axons into the tectum.

Role of heparan sulfate in fibroblast growth factor signalling: a structural view

Fibroblast growth factors (FGFs) are among the best-studied heparin-binding proteins, and heparan sulfate proteoglycans regulate FGF signalling by direct molecular association with FGF and its tyrosine kinase receptor, FGFR. Two recently determined crystal structures of FGF-FGFR-heparin complexes have provided new structural information on how heparin binds to FGF and FGFR, and lead to different models for receptor dimerisation.

Heparin-carrying polystyrene (HCPS)-bound collagen substratum to immobilize heparin-binding growth factors and to enhance cellular growth

Heparin-carrying polystyrene (HCPS), consisting of low molecular weight heparin chains linked to a synthetic polystyrene core, is able to attach to polymeric surfaces. In this study, HCPS has efficiently bound to collagen-coated micro-plates and collagen membranes thereby retaining the binding of heparin-binding growth factors, such as vascular endothelial growth factor (VEGF)(165) or fibroblast growth factor (FGF)-2. Both human skin fibroblast cells and human umbilical vein endothelial cells have shown a good adherence to both collagen- and HCPS-bound collagen substrata. The growth rate of the fibroblast cells on the HCPS-bound collagen substratum in the presence of low concentrations of FGF-2 is higher than on a collagen surface. The fibroblast cells grow at a significantly higher rate on the HCPS-bound collagen substratum retained with FGF-2. Similarly, the growth rate of the endothelial cells on the HCPS-bound collagen substrata in the presence of low concentrations of either FGF-2 or VEGF(165) is higher than on collagen. The endothelial cells also grow at a significantly higher rate on the HCPS-bound collagen substratum retained with either FGF-2 or VEGF(165). These results indicate that HCPS-bound collagen substrata with various bioactive heparin-binding molecules may provide novel biomaterials controlling cellular activities such as growth and differentiation.

Identification of receptor and heparin binding sites in fibroblast growth factor 4 by structure-based mutagenesis

Fibroblast growth factors (FGFs) comprise a large family of multifunctional, heparin-binding polypeptides that show diverse patterns of interaction with a family of receptors (FGFR1 to -4) that are subject to alternative splicing. FGFR binding specificity is an essential mechanism in the regulation of FGF signaling and is achieved through primary sequence differences among FGFs and FGFRs and through usage of two alternative exons, IIIc and IIIb, for the second half of immunoglobulin-like domain 3 (D3) in FGFRs. While FGF4 binds and activates the IIIc splice forms of FGFR1 to -3 at comparable levels, it shows little activity towards the IIIb splice forms of FGFR1 to -3 as well as towards FGFR4. To begin to explore the structural determinants for this differential affinity, we determined the crystal structure of FGF4 at a 1.8-A resolution. FGF4 adopts a beta-trefoil fold similar to other FGFs. To identify potential receptor and heparin binding sites in FGF4, a ternary FGF4-FGFR1-heparin model was constructed by superimposing the FGF4 structure onto FGF2 in the FGF2-FGFR1-heparin structure. Mutation of several key residues in FGF4, observed to interact with FGFR1 or with heparin in the model, produced ligands with reduced receptor binding and concomitant low mitogenic potential. Based on the modeling and mutational data, we propose that FGF4, like FGF2, but unlike FGF1, engages the betaC'-betaE loop in D3 and thus can differentiate between the IIIc and IIIb splice isoforms of FGFRs for binding. Moreover, we show that FGF4 needs to interact with both the 2-O- and 6-O-sulfates in heparin to exert its optimal biological activity.

Sequence analysis of heparan sulfate epitopes with graded affinities for fibroblast growth factors 1 and 2

Proteins that belong to the fibroblast growth factor (FGF) family regulate proliferation, migration, and differentiation of many cell types. Several FGFs, including the prototype factors FGF-1 and FGF-2, depend on interactions with heparan sulfate (HS) proteoglycans for activity. We have assessed tissue-derived HS fragments for binding to FGF-1 and FGF-2 to identify the authentic saccharide motifs required for interactions. Sequence information on a range of N-sulfated HS octasaccharides spanning from low to high affinity for FGF-1 was obtained. All octasaccharides with high affinity for FGF-1 (> or =0.5 m NaCl required for elution) contained an internal IdoUA(2-OSO(3))-GlcNSO(3)(6-OSO(3))-IdoUA(2-OSO(3))-trisaccharide motif. Octasaccharides with a higher overall degree of sulfation but lacking the specific trisaccharide motif showed lower affinity for FGF-1. FGF-2 was shown to bind to a mono-O-sulfated HS 6-mer carrying a single internal IdoUA(2-OSO(3))-unit. However, a di-O-sulfated -IdoUA(2-OSO(3))-GlcNSO(3)-IdoUA(2-OSO(3))-trisaccharide sequence within a HS 8-mer gave stronger binding. These findings show that not only the number but also the positions of individual sulfate groups determine affinity of HS for FGFs. Our findings support the notion that FGF-dependent processes can be modulated in vivo by regulated expression of distinct HS sequences.

Proteoglycans: pericellular and cell surface multireceptors that integrate external stimuli in the mammary gland

Proteoglycans consist of a core protein and an associated glycosaminoglycan (GAG) chain of heparan sulfate, chondroitin sulfate, dermatan sulfate or keratan sulfate, which are attached to a serine residue. The core proteins of cell surface proteoglycans may be transmembrane, e.g., syndecan, or GPI-anchored, e.g., glypican. Many different cell surface and matrix proteoglycan core proteins are expressed in the mammary gland and in mammary cells in culture. The level of expression of these core proteins, the structure of their GAG chains, and their degradation are regulated by many of the effectors that control the development and function of the mammary gland. Regulatory proteins of the mammary gland that bind GAG include many growth factors and morphogens (fibroblast growth factors, hepatocyte growth factor/scatter factor, members of the midkine family, wnts), matrix proteins (collagen, fibronectin, and laminin), enzymes (lipoprotein lipase) and microbial surface proteins. Structural diversity within GAG chains ensures that each protein-GAG interaction is as specific as necessary and a number of sequences of saccharides that recognize individual proteins have been elucidated. The GAG-protein interactions serve to regulate the signal output of growth factor receptor tyrosine kinase and hence cell fate as well as the storage and diffusion of extracellular protein effectors. In addition, GAGs clearly coordinate stromal and epithelial development, and they are active participants in mediating cell-cell and cell-matrix interactions. Since a single proteoglycan, even if it carries a single GAG chain, can bind multiple proteins, proteoglycans are also likely to act as multireceptors which promote the integration of cellular signals.

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.