An enzyme-linked immunosorbent assay using biotinylated heparan sulfate to evaluate the interactions of heparin-like molecules and basic fibroblast growth factor

Sulfated Alginates as Heparin Analogues: A Review of Chemical and Functional Properties

Heparin is widely recognized for its potent anticoagulating effects, but has an additional wide range of biological properties due to its high negative charge and heterogeneous molecular structure. This heterogeneity has been one of the factors in motivating the exploration of functional analogues with a more predictable modification pattern and monosaccharide sequence, that can aid in elucidating structure-function relationships and further be structurally customized to fine-tune physical and biological properties toward novel therapeutic applications and biomaterials. Alginates have been of great interest in biomedicine due to their inherent biocompatibility, gentle gelling conditions, and structural versatility from chemo-enzymatic engineering, but display limited interactions with cells and biomolecules that are characteristic of heparin and the other glycosaminoglycans (GAGs) of the extracellular environment. Here, we review the chemistry and physical and biological properties of sulfated alginates as structural and functional heparin analogues, and discuss how they may be utilized in applications where the use of heparin and other sulfated GAGs is challenging and limited.

Bioactivity screening of partially desulfated low-molecular-weight heparins: a structure/activity relationship study

A series of size-defined low-molecular-weight heparins were generated by regioselective chemical modifications and profiled for their in vitro and in vivo activities. The compounds displayed reduced anti-coagulant activity, demonstrated varying affinities toward angiogenic growth factors (fibroblast growth factor-2, vascular endothelial growth factor and stromal cell-derived factor-1α), inhibited the P-selectin/P-selectin glycoprotein ligand-1 interaction and, notably, exhibited anti-tumor efficacy in a murine melanoma experimental metastasis model. Our results demonstrate that modulating specific sequences, especially the N-domains (-NS or -NH(2) or -NHCOCH(3)) in these polysaccharide sequences, has a major impact on the participation in a diverse range of biological activities. These results also suggest that the 6-O-sulfates, but not the 2-O-sulfates, critically affect the binding of a desulfated derivative to certain angiogenic proteins as well as its ability to inhibit P-selectin-mediated B16F10 melanoma metastases. Furthermore, N-desulfation followed by N-acetylation regenerates the affinity/inhibition properties to different extents in all the compounds tested in the in vitro assays. This systematic study lays a conceptual foundation for detailed structure function elucidation and will facilitate the rational design of targeted heparan sulfate proteoglycan-based anti-metastatic therapeutic candidates.

Development of an enzyme-linked immunosorbent assay (ELISA)-like fluorescence assay to investigate the interactions of glycosaminoglycans to cells

Sulfated glycosaminoglycans were labeled with biotin to study their interaction with cells in culture. Thus, heparin, heparan sulfate, chondroitin 4-sulfate, chondroitin 6-sulfate and dermatan sulfate were labeled using biotin-hydrazide, under different conditions. The structural characteristics of the biotinylated products were determined by chemical (molar ratios of hexosamine, uronic acid, sulfate and biotin) and enzymatic methods (susceptibility to degradation by chondroitinases and heparitinases). The binding of biotinylated glycosaminoglycans was investigated both in endothelial and smooth muscle cells in culture, using a novel time resolved fluorometric method based on interaction of europium-labeled streptavidin with the biotin covalently linked to the compounds. The interactions of glycosaminoglycans were saturable and number of binding sites could be obtained for each individual compound. The apparent dissociation constant varied among the different glycosaminoglycans and between the two cell lines. The interactions of the biotinylated glycosaminoglycans with the cells were also evaluated using confocal microscopy. We propose a convenient and reliable method for the preparation of biotinylated glycosaminoglycans, as well as a sensitive non-competitive fluorescence-based assay for studies of the interactions and binding of these compounds to cells in culture.

Profiling the Sulfation Specificities of Glycosaminoglycan Interactions with Growth Factors and Chemotactic Proteins Using Microarrays

We report a carbohydrate microarray-based approach for the rapid, facile analysis of glycosaminoglycan-protein interactions. The key structural determinants responsible for protein binding, such as sulfate groups that participate in the interactions, were elucidated. Specificities were also readily compared across protein families or functional classes, and comparisons among glycosaminoglycan subclasses provided a more comprehensive understanding of protein specificity. To validate the approach, we showed that fibroblast growth factor family members have distinct sulfation preferences. We also demonstrated that heparan sulfate and chondroitin sulfate interact in a sulfation-dependent manner with various axon guidance proteins, including slit2, netrin1, ephrinA1, ephrinA5, and semaphorin5B. We anticipate that these microarrays will accelerate the discovery of glycosaminoglycan-binding proteins and provide a deeper understanding of their roles in regulating diverse biological processes.

The Proform of the Eosinophil Major Basic Protein Binds the Cell Surface through a Site Distinct from Its C-type Lectin Ligand-binding Region

The highly basic eosinophil major basic protein (MBP), present in the crystalloid core of eosinophil leukocyte granules, has both cytotoxic and cytostimulatory properties and is directly implicated in a number of diseases. The crystal structure of MBP resembles that of the C-type lectin (CTL) superfamily, and recent data showed that MBP binds heparan sulfate glycosaminoglycan (GAG), with the CTL ligand-binding region as the binding site. MBP is synthesized as a proform (pro-MBP) containing an acidic propiece believed to neutralize the basic MBP domain. Using flow cytometry and site-directed mutagenesis, we demonstrate here that the MBP domain of pro-MBP binds to heparan sulfate GAG on the cell surface and that this is independent of GAG covalently bound to pro-MBP. Eight basic residues located in the CTL ligand-binding region of MBP were hypothesized previously to mediate GAG binding, but we found that surface binding was not compromised by the substitution of these residues with alanine. However, the analysis of a series of mutants with surface-exposed residues substituted with alanine showed that Ser-166, Arg-168, and Arg-171 are involved in surface binding. A binding site formed by these residues is located in the MBP domain between loop 1 and beta-strand 5, outside the CTL ligand-binding region. The binding of a cell-surface heparan sulfate proteoglycan may be important in MBP action, and our findings suggest that two regions shown previously to contain the cytotoxic and cytostimulatory properties of MBP are accessible for ligand interaction in cell surface-bound MBP.

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Viscous drag as the source of active site perturbation during protease translocation: insights into how inhibitory processes are controlled by serpin metastability

The native form of serine protease inhibitors (serpins) is kinetically trapped in a metastable state, which is thought to play a central role in the inhibitory mechanism. The initial binding complex between a serpin and a target protease undergoes a conformational change that forces the protease to translocate toward the opposite pole. Although structural determination of the final stable complex revealed a detailed mechanism of keeping the bound protease in an inactive conformation, it has remained unknown how the serpin exquisitely translocates a target protease with an acyl-linkage unhydrolyzed. We previously suggested that the acyl-linkage hydrolysis is strongly suppressed by active site perturbation during the protease translocation. Here, we address what induces the transient perturbation and how the serpin metastability contributes to the perturbation. Inhibitory activity of alpha1-antitrypsin (alpha1AT) toward elastase showed negative correlations with medium viscosity and Stokes radius of elastase moiety, indicating that viscous drag directly affects the protease translocation. Stopped-flow measurements revealed that the change in the inhibitory activity is primarily caused by the change in the translocation rate. The native stability of alpha1AT cavity mutants showed a negative correlation with the translocation rate but a positive correlation with the acyl-linkage hydrolysis rate, suggesting that the two kinetic steps are not independent but closely related. The degree of active site perturbation was probed by amino acid nucleophiles, supporting the view that the changes in the acyl-linkage hydrolysis rate are due to different perturbation states. These results suggest that the active site perturbation is caused by local imbalance between a pulling force driving protease translocation and a counteracting viscous drag force. The structural architecture of serpin metastability seems to be designed to ensure the active site perturbation by providing a sufficient pulling force, so the undesirable hydrolytic activity of protease is strongly suppressed during the translocation.

Decreased expression of heparanase in glioblastoma multiforme

Object. The authors investigated the presence of endoglycosidase heparanase in human glioblastoma multiforme (GBM) and metastatic brain tumors as well as in healthy brain tissue to explore the relationship between the biological characteristics of GBM and the role of heparanase.

Methods. Heparanase messenger (m)RNA was almost undetectable in GBMs in vivo, whereas it was frequently seen in metastatic brain tumors according to results of reverse transcription—polymerase chain reaction (RT-PCR). Immunohistochemical analysis of paraffin-embedded tissue sections showed that neoplastic cells in metastatic brain tumors, especially in cells that invaded blood vessels, exhibit intense heparanase immunoreactivity. Heparanase was present in two highly invasive glioma cell lines, U87MG and U251MG, in vitro. These cell lines did not have metastatic capability, which was tested in an experimental pulmonary metastases model in mice. The activity of heparanase in these cell lines was almost the same as that in the highly metastatic melanoma cell line B16-F1. After nude mice were inoculated with U87MG cells, however, heparanase was no longer detected in subcutaneous or intracerebral experimental glioma in vivo based on results of immunohistochemical analysis. According to results of real-time quantitative PCR, there was a 10-fold increase in heparanase mRNA in U87MG glioma cells in vitro compared with that in experimental U87MG glioma tissue in vivo in nude mice.

Conclusions. These results indicate that the expression of heparanase was downregulated in GBM in vivo, which rarely metastasizes to distant organs outside the central nervous system. Heparanase is not implicated in the invasiveness of GBM to surrounding healthy brain tissue in vivo.

Solid phase assays in glycoconjugate research: applications to the analysis of proteoglycans, glycosaminoglycans and metalloproteinases

Glycoconjugates are a class of macromolecules consisting of different constituents, one of which is sugar moieties. Glycoconjugates comprise the majority of tissue constituents, both intracellular and extracellular. Extracellular glycoconjugates (glycoproteins and proteoglycans) participate in a wide variety of interactions, through which they maintain tissue integrity. Therefore, their analysis or the study of their possible interactions would give evidence for the state of tissues. Since the amounts of some of the extracellular glycoconjugates are usually low or the amounts of tissue to be examined come from biopsies, specific analytical systems are developed for their study, the most familiar being solid phase assays, which have the advantages of analysis of multiple samples on the same time, cheap instrumentation and high specificity.

Structural specificity of heparin binding in the fibroblast growth factor family of proteins

Heparin and heparan sulfate glycosaminoglycans (HSGAGs) mediate a wide variety of complex biological processes by specifically binding proteins and modulating their biological activity. One of the best studied model systems for protein-HSGAG interactions is the fibroblast growth factor (FGF) family of molecules, and recent observations have demonstrated that the specificity of a given FGF ligand binding to its cognate receptor (FGFR) is mediated by distinct tissue-specific HSGAG sequences. Although it has been known that sulfate and carboxylate groups in the HSGAG chain play a key role by interacting with basic residues on the proteins, there is little understanding of how these ionic interactions provide the necessary specificity for protein binding. In this study, using all of the available crystal structures of different FGFs and FGF-HSGAG complexes, we show that in addition to the ionic interactions, optimal van der Waals contact between the HSGAG oligosaccharide and the protein is also very important in influencing the specificity of FGF-HSGAG interactions. Although the overall helical structure is maintained in the FGF-bound HSGAG compared with unbound HSGAG, we observe distinct changes in the backbone torsion angles of the oligosaccharide chain induced upon protein binding. These changes result in local deviations in the helical axis that provide optimal ionic and van der Waals contact with the protein. A specific conformation and topological arrangement of the HSGAG-binding loops of FGF, on the other hand, impose structural constraints that induce the local deviations in the HSGAG structure, thereby enabling maximum contact between HSGAG and the protein.

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

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

Kinetics of apolipoprotein E isoforms-binding to the major glycosaminoglycans of the extracellular matrix

Apolipoprotein E (apoE), a key lipid transport protein, displays a heparin-binding property that is critical in several apoE functions. The kinetics of the interaction between apoE isoforms and glycosaminoglycans (GAGs) were studied using surface plasmon resonance. The dissociation constant of equilibrium K(D) for apoE3-heparin interaction was estimated to be 12 nM for apoE3 and three common apoE isoforms revealed similar affinities for heparin. ApoE binds to GAGs in the following order: heparin>heparan sulfate>dermatan sulfate>chondroitin sulfate. The affinity parameter of the binding of low molecular weight heparins to apoE is correlated with the chain length. The effective number Z of electrostatic interactions between plasma apoE3 and heparin was assessed to be three. Metal chelators were able to diminish apoE-binding to heparin, suggesting some stabilizing effect of metal ions while reconstitution with lipids did not affect binding affinities for heparin, suggesting that the N-terminal heparin-binding site is responsible for apoE-containing lipoprotein interactions with heparin.

Glycation of apolipoprotein E impairs its binding to heparin: identification of the major glycation site

The increased glycation of plasma apolipoproteins represents a possible major factor for lipid disturbances and accelerated atherogenesis in diabetic patients. The glycation of apolipoprotein E (apoE), a key lipid-transport protein in plasma, was studied both in vivo and in vitro. ApoE was shown to be glycated in plasma very low density lipoproteins of both normal subjects and hyperglycemic, diabetic patients. However, diabetic patients with hyperglycemia showed a 2-3-fold increased level of apoE glycation. ApoE from diabetic plasma showed decreased binding to heparin compared to normal plasma apoE. The rate of Amadori product formation in apoE in vitro was similar to that for albumin and apolipoproteins A-I and A-II. The glycation of apoE in vitro significantly decreased its ability to bind to heparin, a critical process in the sequestration and uptake of apoE-containing lipoproteins by cells. Diethylenetriaminepentaacetic acid, a transition metal chelator, had no effect on the loss of apoE heparin-binding activity, suggesting that glycation rather than glycoxidation is responsible for this effect. In contrast, glycation had no effect on the interaction of apoE with amyloid beta-peptide. ApoE glycation was demonstrated to be isoform-specific. ApoE(2) showed a higher glycation rate and the following order was observed: apoE(2)>apoE(4)>apoE(3). The major glycated site of apoE was found to be Lys-75. These findings suggest that apoE is glycated in an isoform-specific manner and that the glycation, in turn, significantly decreases apoE heparin-binding activity. We propose that apoE glycation impairs lipoprotein-cell interactions, which are mediated via heparan sulfate proteoglycans and may result in the enhancement of lipid abnormalities in hyperglycemic, diabetic patients.

Synthesis and characterization of highly sensitive heparin probes for detection of heparin-binding proteins

Three labeled heparin species were synthesized as probes for heparin-binding protein detection. Heparin conjugated with 5([4,6-dichlorotriazin-2-yl]amino)fluorescein can be iodinated to a high specific activity. This probe specifically detected 40 pg histone on a dot blot without affinity purification. Heparin biotinylated on its naturally occurring primary amino groups also detected known heparin-binding proteins in a specific manner. This probe detected lower amounts of collagen I and basic fibroblast growth factor on nitrocellulose membranes than did the iodinated probe, with comparable detection times. To create more attachment sites for biotin, we covalently attached amino groups to the hydroxyl groups of heparin using 3-bromopropylamine hydrobromide. After biotinylation, the amino-rich probe detected heparin-binding proteins at the same or higher sensitivity as the biotinylated native heparin probe, using 100-fold less probe and much shorter detection times. This method of labeling is generally applicable to other polysaccharides, and would be useful when the amount of ligand is limited. We show that these three probes detect essentially the same spectrum of proteins in detergent extract of smooth muscle cell plasma membrane, and expect them to be useful probes for detection of cell-surface heparin receptors.

An enzyme- linked immunosorbent assay for heparan sulfate proteoglycans

An enzyme- linked immunosorbent assay (ELISA) for heparan sulfate proteoglycan (HSPG) was developed based on the high affinity binding profile of HSPG to lipoprotein lipase (LPL). LPL was shown to bind to precoated HSPG in dose dependent manner and was determined spectrophotometrically using specific anti- LPL antibody. This ELISA allowed to evaluate HSPG produced by PC12 cell with clear linearity at range of 10 - 500 ng/ml. Soluble chondroitin sulfate proteoglycan (CSPG) from rat brain, which was not detectable by this method, did not exhibit any inhibitory effects on affinity binding of HSPG to LPL, even if 8 times higher concentrations of CSPG to HSPG was added. The sensitivity of this ELISA was about 100 times higher than that of conventional carbazole reaction method. These findings indicated its potential usefulness of this method for measuring small amounts of HSPG capable of binding to LPL and for studying biological implications of HSPG - LPL interaction.

Sulfated malto-oligosaccharides bind to basic FGF, inhibit endothelial cell proliferation, and disrupt endothelial cell tube formation

The interaction of basic FGF (bFGF) with heparin, heparan sulfate and related sugars can potentiate or antagonize bFGF activity, depending on the size of the saccharide used. Oligosaccharides based on heparin structures, as small as six sugar residues, have been demonstrated to bind to bFGF and block its activity, while larger structures (> 10 sugar residues) tend to potentiate bFGF. In this study we have synthesized a series of compounds designed to test the requirements of size and sulfation for binding of oligosaccharides to bFGF. These oligosaccharides are not derived from heparin, but rather, are linear chains of glucose linked alpha 1-4 (malto-oligosaccharides) that have been chemically sulfated. In addition to bFGF binding, these compounds were tested for their ability to block basic functions of endothelial cells that are known to be mediated, at least in part, by bFGF. We report that the ability of sulfated malto-oligosaccharides to block binding of bFGF to heparan sulfate was dependent on the size (at least a tetrasaccharide is required), and the degree of sulfation. The activity profile in the bFGF ELISA closely correlated with the ability of these compounds to block REEC or HMVEC tube formation on Matrigel. There was a similar relationship of size and sulfation to the ability of the sulfated malto-oligosaccharides to inhibit endothelial cell growth for most human and rat EC types tested. The single exception was REEC cell growth. One isolate of these cells was stimulated by sulfated malto-oligosaccharides rather than inhibited by them, while a second isolate was neither stimulated nor inhibited. This stimulation showed no correlation with inhibition of bFGF binding in the ELISA assay, suggesting that growth of this cell type was probably not dependent on bFGF. Compounds derived from this series of sulfated, malto-oligosaccharides have the potential to function as bFGF antagonists, are relatively easy to produce, and possess relatively low anticoagulant properties.