1H and 13C NMR spectral assignments of the major sequences of twelve systematically modified heparin derivatives

Heparin Inhibits Cellular Invasion by SARS-CoV-2: Structural Dependence of the Interaction of the Spike S1 Receptor-Binding Domain with Heparin

The dependence of development and homeostasis in animals on the interaction of hundreds of extracellular regulatory proteins with the peri- and extracellular glycosaminoglycan heparan sulfate (HS) is exploited by many microbial pathogens as a means of adherence and invasion. Heparin, a widely used anticoagulant drug, is structurally similar to HS and is a common experimental proxy. Exogenous heparin prevents infection by a range of viruses, including S-associated coronavirus isolate HSR1. Here, we show that heparin inhibits severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) invasion of Vero cells by up to 80% at doses achievable through prophylaxis and, particularly relevant, within the range deliverable by nebulisation. Surface plasmon resonance and circular dichroism spectroscopy demonstrate that heparin and enoxaparin, a low-molecular-weight heparin which is a clinical anticoagulant, bind and induce a conformational change in the spike (S1) protein receptor-binding domain (S1 RBD) of SARS-CoV-2. A library of heparin derivatives and size-defined fragments were used to probe the structural basis of this interaction. Binding to the RBD is more strongly dependent on the presence of 2-O or 6-O sulfate groups than on N-sulfation and a hexasaccharide is the minimum size required for secondary structural changes to be induced in the RBD. It is likely that inhibition of viral infection arises from an overlap between the binding sites of heparin/HS on S1 RBD and that of the angiotensin-converting enzyme 2. The results suggest a route for the rapid development of a first-line therapeutic by repurposing heparin and its derivatives as antiviral agents against SARS-CoV-2 and other members of the Coronaviridae.

Characterization of heparin and severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) spike glycoprotein binding interactions

Severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) has resulted in a pandemic and continues to spread around the globe at an unprecedented rate. To date, no effective therapeutic is available to fight its associated disease, COVID-19. Our discovery of a novel insertion of glycosaminoglycan (GAG)-binding motif at S1/S2 proteolytic cleavage site (681-686 (PRRARS)) and two other GAG-binding-like motifs within SARS-CoV-2 spike glycoprotein (SGP) led us to hypothesize that host cell surface GAGs may interact SARS-CoV-2 SGPs to facilitate host cell entry. Using a surface plasmon resonance direct binding assay, we found that both monomeric and trimeric SARS-CoV-2 SGP bind more tightly to immobilized heparin (K_D = 40 pM and 73 pM, respectively) than the SARS-CoV and MERS-CoV SGPs (500 nM and 1 nM, respectively). In competitive binding studies, the IC₅₀ of heparin, tri-sulfated non-anticoagulant heparan sulfate, and non-anticoagulant low molecular weight heparin against SARS-CoV-2 SGP binding to immobilized heparin were 0.056 μM, 0.12 μM, and 26.4 μM, respectively. Finally, unbiased computational ligand docking indicates that heparan sulfate interacts with the GAG-binding motif at the S1/S2 site on each monomer interface in the trimeric SARS-CoV-2 SGP, and at another site (453-459 (YRLFRKS)) when the receptor-binding domain is in an open conformation. The current study serves a foundation to further investigate biological roles of GAGs in SARS-CoV-2 pathogenesis. Furthermore, our findings may provide additional basis for further heparin-based interventions for COVID-19 patients exhibiting thrombotic complications.

Heparin and non-anticoagulant heparin attenuate histone-induced inflammatory responses in whole blood

Cytotoxic and pro-inflammatory histones are present in neutrophil extracellular traps (NETs) and are elevated in blood in several inflammatory conditions, sepsis being a major example. Compounds which can attenuate activities of histones are therefore of interest, with heparin being one such material that has previously been shown to bind to histones. Heparin, a successful anticoagulant for nearly a century, has been shown experimentally to bind to histones and exhibit a protective effect in inflammatory conditions. In the present study carried out in whole blood, heparin and selectively desulfated heparin reduced histone induced inflammatory markers such as interleukin 6 (IL 6), interleukin 8 (IL 8) and tissue factor and C3a, a complement component. The selectively desulfated heparins, with reduced anticoagulant activities, retained a high degree of effectiveness as an anti-histone agent, whereas fully desulfated heparin was found to be ineffective. The results from this study indicate that the presence of sulfate and other specific structural features are required for heparin to attenuate the inflammatory action of histones in whole blood.

Shotgun ion mobility mass spectrometry sequencing of heparan sulfate saccharides

Despite evident regulatory roles of heparan sulfate (HS) saccharides in numerous biological processes, definitive information on the bioactive sequences of these polymers is lacking, with only a handful of natural structures sequenced to date. Here, we develop a “Shotgun” Ion Mobility Mass Spectrometry Sequencing (SIMMS²) method in which intact HS saccharides are dissociated in an ion mobility mass spectrometer and collision cross section values of fragments measured. Matching of data for intact and fragment ions against known values for 36 fully defined HS saccharide structures (from di- to decasaccharides) permits unambiguous sequence determination of validated standards and unknown natural saccharides, notably including variants with 3O-sulfate groups. SIMMS² analysis of two fibroblast growth factor-inhibiting hexasaccharides identified from a HS oligosaccharide library screen demonstrates that the approach allows elucidation of structure-activity relationships. SIMMS² thus overcomes the bottleneck for decoding the informational content of functional HS motifs which is crucial for their future biomedical exploitation.

Heparan sulfates (HS) contain functionally relevant structural motifs, but determining their monosaccharide sequence remains challenging. Here, the authors develop an ion mobility mass spectrometry-based method that allows unambiguous characterization of HS sequences and structure-activity relationships.

Multi-target approaches to CNS repair: olfactory mucosa-derived cells and heparan sulfates

Spinal cord injury (SCI) remains one of the biggest challenges in the development of neuroregenerative therapeutics. Cell transplantation is one of numerous experimental strategies that have been identified and tested for efficacy at both preclinical and clinical levels in recent years. In this Review, we briefly discuss the state of human olfactory cell transplantation as a therapy, considering both its current clinical status and its limitations. Furthermore, we introduce a mesenchymal stromal cell derived from human olfactory tissue, which has the potential to induce multifaceted reparative effects in the environment within and surrounding the lesion. We argue that no single therapy will be sufficient to treat SCI effectively and that a combination of cell-based, rehabilitation and pharmaceutical interventions is the most promising approach to aid repair. For this reason, we also introduce a novel pharmaceutical strategy based on modifying the activity of heparan sulfate, an important regulator of a wide range of biological cell functions. The multi-target approach that is exemplified by these types of strategies will probably be necessary to optimize SCI treatment.

The interaction between oxytocin and heparin

Heparin interacts with the nonapeptide oxytocin, the binding region preferentially involves the 6-O- and N-sulfates of glucosamine.

Interaction with the heparin-derived binding inhibitors destabilizes galectin-3 protein structure

The β-galactoside-binding protein, galectin-3, is extensively involved in cancer development, progression and metastasis through multiple mechanisms. Inhibition of the galectin-3-mediated actions is increasingly considered as a promising therapeutic approach for cancer treatment. Our early studies have identified several novel galectin-3 binding inhibitors from chemical modification of the anticoagulant drug heparin. These heparin-derived galectin-3 binding inhibitors, which show no anticoagulant activity and bind to the galectin-3 canonical carbohydrate-binding site, induce galectin-3 conformational changes and inhibit galectin-3-mediated cancer cell adhesion, invasion and angiogenesis in vitro and reduce metastasis in mice. In this study, we determined the binding affinities of these heparin-derived ligands to galectin-3 using an isothermal titration calorimetry (ITC) ligand displacement approach. Such ITC experiments showed that the 2-de-O-sulphated, N-acetylated (compound E) and 6-de-O-sulphated, N-acetylated (F) heparin-derived ligands and their ultra-low molecular weight sub-fractions (E3 and F3) bind to galectin-3 with K_D ranging from 0.96 to 1.32 mM.Differential scanning fluorimetry analysis revealed that, in contrast to the disaccharide ligand, N-acetyl-lactosamine, which binds to the fully folded form of galectin-3 and promotes galectin-3 thermal stability, the heparin-derived ligands preferentially bind to the unfolded state of galectin-3 and cause destabilization of the galectin-3 protein structure. These results provide molecular insights into the interaction of galectin-3 with the heparin-derived ligands and explain the previously demonstrated in vitro and in vivo effects of these binding inhibitors on galectin-3-mediated cancer cell behaviours.

Isolation and Characterization of a Heparin-Like Compound with Potent Anticoagulant and Fibrinolytic Activity from the Clam Coelomactra antiquata

Heparin from mollusks with unique sulfated glycosaminoglycan exhibits strong anti-thrombotic activities. This study reports on a purified heparinoid from Coelomactra antiquata, which shows potent anticoagulant and fibrinolytic abilities. Its structure was characterized by infrared spectroscopy, high-performance liquid chromatography, and one-dimensional and two-dimensional nuclear magnetic resonance spectroscopy. Its fibrinolytic activity was determined in vitro and in vivo. Its anticoagulant activity was determined by activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT). The results indicated that clam heparinoid was a homogeneous glycosaminoglycan with a molecular weight of 30.99 kDa, mainly composed of →4)-α-IdoA2S-(1→4)-α-GlcNS3S6S (or GlcNS6S)-(1→4)-β-GlcA-(1→4)-α-GlcNS6S (or GlcNAC)-(1→. Furthermore, this heparinoid showed a highly anticoagulant titer and fibrinolytic value of 149.63 IU/mg and 1.96 IU/mg, respectively. In summary, clam heparinoid shows great potential for application in the clinic and antithrombotic drugs industry.

Interactions between Sclerostin and Glycosaminoglycans

Sclerostin (SOST) is a glycoprotein having many important functions in the regulation of bone formation as a key negative regulator of Wnt signaling in bone. Surface plasmon resonance (SPR), which allows for a direct quantitative analysis of the label-free molecular interactions in real-time, has been widely used for the biophysical characterization of glycosaminoglycan (GAG)-protein interactions. In the present study, we report kinetics, structural analysis and the effects of physiological conditions (e.g., salt concentrations, Ca²⁺ and Zn²⁺concentrations) on the interactions between GAGs and recombinant human (rh) and recombinant mouse (rm) SOST using SPR. SPR results revealed that both SOSTs bind heparin with high affinity (rhSOST-heparin, K_D~36 nM and rmSOST-heparin, K_D~77 nM) and the shortest oligosaccharide of heparin that effectively competes with full size heparin for SOST binding is octadecasaccharide (18mer). This heparin binding protein also interacts with other highly sulfated GAGs including, disulfated-dermatan sulfate and chondroitin sulfate E. In addition, liquid chromatography-mass spectrometry was used to characterize the structure of sulfated GAGs that bound to SOST.

Tools for the Quality Control of Pharmaceutical Heparin

Heparin is a vital pharmaceutical anticoagulant drug and remains one of the few naturally sourced pharmaceutical agents used clinically. Heparin possesses a structural order with up to four levels of complexity. These levels are subject to change based on the animal or even tissue sources that they are extracted from, while higher levels are believed to be entirely dynamic and a product of their surrounding environments, including bound proteins and associated cations. In 2008, heparin sources were subject to a major contamination with a deadly compound-an over-sulphated chondroitin sulphate polysaccharide-that resulted in excess of 100 deaths within North America alone. In consideration of this, an arsenal of methods to screen for heparin contamination have been applied, based primarily on the detection of over-sulphated chondroitin sulphate. The targeted nature of these screening methods, for this specific contaminant, may leave contamination by other entities poorly protected against, but novel approaches, including library-based chemometric analysis in concert with a variety of spectroscopic methods, could be of great importance in combating future, potential threats.

Comparison of the Interactions of Different Growth Factors and Glycosaminoglycans

Most growth factors are naturally occurring proteins, which are signaling molecules implicated in cellular multiple functions such as proliferation, migration and differentiation under patho/physiological conditions by interacting with cell surface receptors and other ligands in the extracellular microenvironment. Many of the growth factors are heparin-binding proteins (HBPs) that have a high affinity for cell surface heparan sulfate proteoglycans (HSPG). In the present study, we report the binding kinetics and affinity of heparin interacting with different growth factors, including fibroblast growth factor (FGF) 2,7,10, hepatocyte growth factor (HGF) and transforming growth factor (TGF β-1), using a heparin chip. Surface plasmon resonance studies revealed that all the tested growth factors bind to heparin with high affinity (with K_D ranging from ~0.1 to 59 nM) and all the interactions are oligosaccharide size dependent except those involving TGF β-1. These heparin-binding growth factors also interact with other glycosaminoglycans (GAGs), as well as various chemically modified heparins. Other GAGs, including heparan sulfate, chondroitin sulfates A, B, C, D, E and keratan sulfate, showed different inhibition activities for the growth factor-heparin interactions. FGF2, FGF7, FGF10 and HGF bind heparin but the 2-O-sulfo and 6-O-sulfo groups on heparin have less impact on these interactions than do the N-sulfo groups. All the three sulfo groups (N-, 2-O and 6-O) on heparin are important for TGFβ-1-heparin interaction.

A Glycosaminoglycan Extract from Portunus pelagicus Inhibits BACE1, the β Secretase Implicated in Alzheimer's Disease

Therapeutic options for Alzheimer’s disease, the most common form of dementia, are currently restricted to palliative treatments. The glycosaminoglycan heparin, widely used as a clinical anticoagulant, has previously been shown to inhibit the Alzheimer’s disease-relevant β-secretase 1 (BACE1). Despite this, the deployment of pharmaceutical heparin for the treatment of Alzheimer’s disease is largely precluded by its potent anticoagulant activity. Furthermore, ongoing concerns regarding the use of mammalian-sourced heparins, primarily due to prion diseases and religious beliefs hinder the deployment of alternative heparin-based therapeutics. A marine-derived, heparan sulphate-containing glycosaminoglycan extract, isolated from the crab Portunus pelagicus, was identified to inhibit human BACE1 with comparable bioactivity to that of mammalian heparin (IC₅₀ = 1.85 μg mL⁻¹ (R² = 0.94) and 2.43 μg mL⁻¹ (R² = 0.93), respectively), while possessing highly attenuated anticoagulant activities. The results from several structural techniques suggest that the interactions between BACE1 and the extract from P. pelagicus are complex and distinct from those of heparin.

Chemically modified, non-anticoagulant heparin derivatives are potent galectin-3 binding inhibitors and inhibit circulating galectin-3-promoted metastasis

Concentrations of circulating galectin-3, a metastasis promoter, are greatly increased in cancer patients. Here we show that 2- or 6-de-O-sulfated, N-acetylated heparin derivatives are galectin-3 binding inhibitors. These chemically modified heparin derivatives inhibited galectin-3-ligand binding and abolished galectin-3-mediated cancer cell-endothelial adhesion and angiogenesis. Unlike standard heparin, these modified heparin derivatives and their ultra-low molecular weight sub-fractions had neither anticoagulant activity nor effects on E-, L- or P-selectin binding to their ligands nor detectable cytotoxicity. Intravenous injection of such heparin derivatives (with cancer cells pre-treated with galectin-3 followed by 3 subcutaneous injections of the derivatives) abolished the circulating galectin-3-mediated increase in lung metastasis of human melanoma and colon cancer cells in nude mice. Structural analysis using nuclear magnetic resonance and synchrotron radiation circular dichroism spectroscopies showed that the modified heparin derivatives bind to the galectin-3 carbohydrate-recognition domain. Thus, these chemically modified, non-anticoagulant, low-sulfated heparin derivatives are potent galectin-3 binding inhibitors with substantial potential as anti-metastasis/cancer drugs.

SPR Biosensor Probing the Interactions between TIMP-3 and Heparin/GAGs

Tissue inhibitor of metalloproteinases-3 (TIMP-3) belongs to a family of proteins that regulate the activity of matrix metalloproteinases (MMPs), which can process various bioactive molecules such as cell surface receptors, chemokines, and cytokines. Glycosaminoglycans (GAGs) interact with a number of proteins, thereby playing an essential role in the regulation of many physiological/patho-physiological processes. Both GAGs and TIMP/MMPs play a major role in many cell biological processes, including cell proliferation, migration, differentiation, angiogenesis, apoptosis, and host defense. In this report, a heparin biosensor was used to map the interaction between TIMP-3 and heparin and other GAGs by surface plasmon resonance spectroscopy. These studies show that TIMP-3 is a heparin-binding protein with an affinity of ~59 nM. Competition surface plasmon resonance analysis indicates that the interaction between TIMP-3 and heparin is chain-length dependent, and N-sulfo and 6-O-sulfo groups (rather than the 2-O-sulfo groups) in heparin are important in the interaction of heparin with TIMP-3. Other GAGs (including chondroitin sulfate (CS) type E (CS-E)and CS type B (CS-B)demonstrated strong binding to TIMP-3, while heparan sulfate (HS), CS type A (CSA), CS type C (CSC), and CS type D (CSD) displayed only weak binding affinity.

Composition, sequencing and ion mobility mass spectrometry of heparan sulfate-like octasaccharide isomers differing in glucuronic and iduronic acid content

Here we report ion mobility mass spectrometry (IMMS) separation and tandem mass spectrometry (MS(2)) sequencing methods used to analyze and differentiate six synthetically produced heparin/heparan sulfate (HS)-like octasaccharide (dp8) isomeric structures. These structures are isomeric with regard to either glucuronic acid (GlcA) or iduronic acid (IdoA) residues at various positions. IMMS analysis showed that a fully GlcA structure exhibited a more compact conformation, whereas the fully IdoA structure was more extended. Interestingly, the change from IdoA to GlcA in specific locations resulted in strong conformational distortions. MS(2) of the six isomers showed very different spectra with unique sets of diagnostic product ions. Analysis of MS(2) product ion spectra suggests that the GlcA group correlated with the formation of a glycosidic product ion under lower energy conditions. This resulted in an earlier product ion formation and more intense product ions. Importantly, this knowledge enabled a complete sequencing of the positions of GlcA and IdoA in each of the four positions located in each unique dp8 structure.

Heparin derivatives for the targeting of multiple activities in the inflammatory response

An attractive strategy for ameliorating symptoms arising from the multi-faceted processes of excessive and/or continual inflammation would be to identify compounds able to interfere with multiple effectors of inflammation. The well-tolerated pharmaceutical, heparin, is capable of acting through several proteins in the inflammatory cascade, but its use is prevented by strong anticoagulant activity. Derivatives of heparin involving the periodate cleavage of 2,3 vicinal diols in non-sulfated uronate residues (glycol-split) and replacement of N-sulphamido- with N-acetamido- groups in glucosamine residues, capable of inhibiting neutrophil elastase activity in vitro, while exhibiting attenuated anticoagulant properties, have been identified and characterised. These also interact with two other important modulators of the inflammatory response, IL-8 and TNF-alpha. It is therefore feasible in principle to modulate several activities, while minimising anticoagulant side effects, providing a platform from which improved anti-inflammatory agents might be developed.

On the catalytic mechanism of polysaccharide lyases: evidence of His and Tyr involvement in heparin lysis by heparinase I and the role of Ca2+

The structurally diverse polysaccharide lyase enzymes are distributed from plants to animals but share common catalytic mechanisms. One, heparinase I (F. heparinum), is employed in the production of the major anticoagulant drug, low molecular weight heparin, and is a mainstay of cell surface proteoglycan analysis. We demonstrate that heparinase I specificity and efficiency depend on the cationic form of the substrate. Ca(2+)-heparin, in which α-L-iduronate-2-O-sulfate residues adopt (1)C4 conformation preferentially, is a substrate, while Na(+)-heparin is an inhibitor. His and Tyr residues are identified in the catalytic step and a model based on molecular dynamics and docking is proposed, in which deprotonated His203 initiates β-elimination by abstracting the C5 proton of the α-L-iduonate-2-O-sulfate residue in the substrate, and protonated Tyr357 provides the donor to the hexosamine leaving group.

Re-visiting the structure of heparin

The sulfated polysaccharide heparin has been used as a life-saving anticoagulant in clinics well before its detailed structure was known. This mini-review is a survey of the evolution in the discovery of the primary and secondary structure of heparin. Highlights in this history include elucidation and synthesis of the specific sequence that binds to antithrombin, the development of low-molecular-weight heparins currently used as antithrombotic drugs, and the most promising start of chemo-enzymatic synthesis. Special emphasis is given to peculiar conformational properties contributing to interaction with proteins that modulate different biological properties.

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2-Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand-receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ~650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1-heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1-heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.

New targets for glycosaminoglycans and glycosaminoglycans as novel targets

Biological functions of a variety of proteins are mediated via their interaction with glycosaminoglycans (GAGs). The structural diversity within the wide GAG landscape provides individual interaction sites for a multitude of proteins involved in several pathophysiological processes. This 'GAG angle' of such proteins as well as their specific GAG ligands give rise to novel therapeutic concepts for drug development. Current glycomic technologies to elucidate the glycan structure-function relationships, methods to investigate the selectivity and specificity of glycan-protein interactions and existing therapeutic approaches to interfere with GAG-protein interactions are discussed.

A heparin-like compound isolated from a marine crab rich in glucuronic acid 2-O-sulfate presents low anticoagulant activity

A natural heparin-like compound isolated from the crab Goniopsis cruentata was structurally characterized and its anticoagulant and hemorrhagic activities were determined. Enzymatic and nuclear magnetic resonance analysis revealed that its structure is rich in disulfated disaccharides, possessing significant amounts of 2-O-sulfated-β-D-glucuronic acid units. Furthermore, low amounts of trisulfated disaccharide units containing 2-O-sulfated-α-L-iduronic acid were detected, when compared to mammalian heparin. In addition, this heparin-like structure showed negligible in vitro anticoagulant activity and low bleeding potency, facts that make it a suitable candidate for the development of structure-driven, heparin based therapeutic agents with fewer undesirable effects.

A highly efficient tree structure for the biosynthesis of heparan sulfate accounts for the commonly observed disaccharides and suggests a mechanism for domain synthesis

The form of the biosynthetic pathway of the biologically and medically important polysaccharides heparan sulfate (HS) and the closely related heparin remain obscure despite significant progress characterising the biosynthetic machinery. Considering possible biosynthetic schemes using a graph approach and applying known constraints of enzyme order and specificity, a previously unreported system with a highly efficient tree structure emerged with two features: (1) All commonly occurring HS disaccharides could be synthesised through a common route, the major branch. (2) The least common disaccharides also occurred on a separate common branch, termed here the minor branch. This suggested that the relative abundance of these two sets of structures were the result of the specificity of a single enzyme (HS epimerase) acting at an early point in the scheme, to convert GlcA-GlcNS to IdoA-GlcNS in preference to converting GlcA-GlcNAc to IdoA-GlcNAc. A third key finding was that the common substrates for 3-O-sulfation all lie on the same (major) branch. The proposed scheme is consistent with a wide body of experiments comprising both biochemical data and results from HS biosynthetic enzyme knockout experiments in the literature. The major branch also contains a bifurcation, providing a choice of two distinct backbone geometries with the same charge. Further development of this novel biosynthetic scheme, in which frame shifts in the site of action of the enzymes were permitted to occur, while maintaining their order of action, suggested a mechanism by which domains could be generated, or further modification blocked. The relationship between the proposed pathway and the geometric and charge possibilities it allows were also explored.

Heparin and heparan sulfate: analyzing structure and microheterogeneity

The structural microheterogeneity of heparin and heparan sulfate is one of the major reasons for the multifunctionality exhibited by this class of molecules. In a physiological context, these molecules primarily exert their effects extracellularly by mediating key processes of cellular cross-talk and signaling leading to the modulation of a number of different biological activities including development, cell proliferation, and inflammation. This structural diversity is biosynthetically imprinted in a nontemplate-driven manner and may also be dynamically remodeled as cellular function changes. Understanding the structural information encoded in these molecules forms the basis for attempting to understand the complex biology they mediate. This chapter provides an overview of the origin of the structural microheterogeneity observed in heparin and heparan sulfate, and the orthogonal analytical methodologies that are required to help decipher this information.

Low-molecular-weight heparins: differential characterization/physical characterization

Low-molecular-weight heparins (LMWHs), derived from unfractionated heparin (UFH) through different depolymerization processes, have advantages with respect to the parent heparin in terms of pharmacokinetics, convenience of administration, and reduced side effects. Each LMWH can be considered as an independent drug with its own activity profile, placing significance on their biophysical characterization, which will also enable a better understanding of their structure-function relationship. Several chemical and physical methods, some involving sample modification, are now available and are reviewed.

Investigating the mechanism of the assembly of FGF1-binding heparan sulfate motifs

Heparan sulfate (HS) chains play crucial biological roles by binding to various signaling molecules including fibroblast growth factors (FGFs). Distinct sulfation patterns of HS chains are required for their binding to FGFs/FGF receptors (FGFRs). These sulfation patterns are putatively regulated by biosynthetic enzyme complexes, called GAGOSOMES, in the Golgi. While the structural requirements of HS-FGF interactions have been described previously, it is still unclear how the FGF-binding motif is assembled in vivo. In this study, we generated HS structures using biosynthetic enzymes in a sequential or concurrent manner to elucidate the potential mechanism by which the FGF1-binding HS motif is assembled. Our results indicate that the HS chains form ternary complexes with FGF1/FGFR when enzymes carry out modifications in a specific manner.

Biologically Relevant Metal-Cation Binding Induces Conformational Changes in Heparin Oligosaccharides as Measured by Ion Mobility Mass Spectrometry

Heparin interacts with many proteins and is involved in biological processes such as anticoagulation, angiogenesis, and antitumorigenic activities. These heparin-protein interactions can be influenced by the binding of various metal ions to these complexes. In particular, physiologically relevant metal cations influence heparin-protein conformations through electronic interactions inherent to this polyanion. In this study, we employed ion mobility mass spectrometry (IMMS) to observe conformational changes that occur in fully-sulfated heparin octasaccharides after the successive addition of metal ions. Our results indicate that binding of positive counter ions causes a decrease in collision cross section (CCS) measurements, thus promoting a more compact octasaccharide structure.

Modulation of the proteolytic activity of the complement protease C1s by polyanions: implications for polyanion-mediated acceleration of interaction between C1s and SERPING1

The complement system plays crucial roles in the immune system, but incorrect regulation causes inflammation and targeting of self-tissue, leading to diseases such as systemic lupus erythematosus, rheumatoid arthritis and age-related macular degeneration. In vivo, the initiating complexes of the classical complement and lectin pathways are controlled by SERPING1 [(C1 inhibitor) serpin peptidase inhibitor, clade G, member 1], which inactivates the components C1s and MASP-2 (mannan-binding lectin serine peptidase 2). GAGs (glycosaminoglycan) and DXS (dextran sulfate) are able to significantly accelerate SERPING1-mediated inactivation of C1s, the key effector enzyme of the classical C1 complex, although the mechanism is poorly understood. In the present study we have shown that C1s can bind to DXS and heparin and that these polyanions enhanced C1s proteolytic activity at low concentrations and inhibited it at higher concentrations. The recent determination of the crystal structure of SERPING1 has given rise to the hypothesis that both the serpin (serine protease inhibitor)-polyanion and protease-polyanion interactions might be required to accelerate the association rate of SERPING1 and C1s. To determine what proportion of the acceleration was due to protease-polyanion interactions, a chimaeric mutant of alpha1-antitrypsin containing the P4-P1 residues from the SERPING1 RCL (reactive-centre loop) was produced. Like SERPING1, this molecule is able to effectively inhibit C1s, but is unable to bind polyanions. DXS exerted a biphasic effect on the association rate of C1s which correlated strongly with the effect of DXS on C1s proteolytic activity. Thus, whereas polyanions are able to bind C1s and modulate its activity, polyanion interactions with SERPING1 must also play a vital role in the mechanism by which these cofactors accelerate the C1s-SERPING1 reaction.

The activities of heparan sulfate and its analogue heparin are dictated by biosynthesis, sequence, and conformation

The glycosaminoglycan heparan sulfate (HS), is expressed on the surface of virtually all mammalian cells and is implicated in many crucial biological activities. The activities of HS and its close structural analogue heparin are mediated through interactions with proteins. However, the relationship between structure and activity is not simple, because the structure and conformation of HS and heparin are complex. This review surveys some of the relevant findings in HS/heparin chemistry, biochemistry, and biology.

Protein–GAG interactions: new surface-based techniques, spectroscopies and nanotechnology probes

New approaches, rooted in the physical sciences, have been developed to gain a more fundamental understanding of protein–GAG (glycosaminoglycan) interactions. DPI (dual polarization interferometry) is an optical technique, which measures real-time changes in the mass of molecules bound at a surface and the geometry of the bound molecules. QCM-D (quartz crystal microbalance-dissipation), an acoustic technique, measures the mass and the viscoelastic properties of adsorbates. The FTIR (Fourier-transform IR) amide bands I, II and III, resulting from the peptide bond, provide insight into protein secondary structure. Synchrotron radiation CD goes to much shorter wavelengths than laboratory CD, allowing access to chromophores that provide insights into the conformation of the GAG chain and of β-strand structures of proteins. To tackle the diversity of GAG structure, we are developing noble metal nanoparticle probes, which can be detected at the level of single particles and so enable single molecule biochemistry and analytical chemistry. These new approaches are enabling new insights into structure–function relationships in GAGs and together they will resolve many of the outstanding problems in this field.

Chemical Approaches to Define the Structure-Activity Relationship of Heparin-like Glycosaminoglycans

Heparin, the drug of choice for the prevention and treatment of thromboembolic disorders, has been shown to interact with many proteins. Despite its widespread medical use, little is known about the precise sequences that interact with specific proteins. The minimum heparin binding sequence for FGF1 and FGF2 necessary to promote signaling was investigated. A characteristic pentasaccharide sequence, DEFGH, is required to accelerate the inhibition of thrombin and factor Xa in the blood-coagulation cascade. The first synthetic heparin pentasaccharide drug has been approved in Europe and the US and is sold under the trade name Arixtra. Other oligosaccharides with different composition are under clinical investigation. The enormous interest in the assembly of heparin oligosaccharides will stimulate the development of new synthetic approaches. Heparin-oligosaccharide-synthesis automation similar to that of DNA or peptide synthesis will play an important role.

Heparan sulfate regulates amyloid precursor protein processing by BACE1, the Alzheimer's beta-secretase

Cleavage of amyloid precursor protein (APP) by the Alzheimer's β-secretase (BACE1) is a key step in generating amyloid β-peptide, the main component of amyloid plaques. Here we report evidence that heparan sulfate (HS) interacts with β-site APP-cleaving enzyme (BACE) 1 and regulates its cleavage of APP. We show that HS and heparin interact directly with BACE1 and inhibit in vitro processing of peptide and APP substrates. Inhibitory activity is dependent on saccharide size and specific structural characteristics, and the mechanism of action involves blocking access of substrate to the active site. In cellular assays, HS specifically inhibits BACE1 cleavage of APP but not alternative cleavage by α-secretase. Endogenous HS immunoprecipitates with BACE1 and colocalizes with BACE1 in the Golgi complex and at the cell surface, two of its putative sites of action. Furthermore, inhibition of cellular HS synthesis results in enhanced BACE1 activity. Our findings identify HS as a natural regulator of BACE1 and suggest a novel mechanism for control of APP processing.

A novel computational approach to integrate NMR spectroscopy and capillary electrophoresis for structure assignment of heparin and heparan sulfate oligosaccharides

Heparin and heparan sulfate (HS) glycosaminoglycans (GAGs) are cell surface polysaccharides that bind to a multitude of signaling molecules, enzymes, and pathogens and modulate critical biological processes ranging from cell growth and development to anticoagulation and viral invasion. Heparin has been widely used as an anticoagulant in a variety of clinical applications for several decades. The heterogeneity and complexity of HS GAGs pose significant challenges to their purification and characterization of structure-function relationships. Nuclear magnetic resonance (NMR) spectroscopy is a promising tool that provides abundant sequence and structure information for characterization of HS GAGs. However, complex NMR spectra and low sensitivity often make analysis of HS GAGs a daunting task. We report the development of a novel methodology that incorporates distinct linkage information between adjacent monosaccharides obtained from NMR and capillary electrophoresis (CE) data using a property encoded nomenclature (PEN) computational framework to facilitate a rapid and unbiased procedure for sequencing HS GAG oligosaccharides. We demonstrate that the integration of NMR and CE data sets with the help of the PEN framework dramatically reduces the number of experimental constraints required to arrive at an HS GAG oligosaccharide sequence.

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.

Identification of the heparan sulfate binding sites in the cellular prion protein

Data from cell culture and animal models of prion disease support the separate involvement of both heparan sulfate proteoglycans and copper (II) ions in prion (PrP) metabolism. Though direct interactions between prion protein and heparin have been recorded, little is known of the structural features implicit in this interaction or of the involvement of copper (II) ions. Using biosensor and enzyme-linked immunosorbent assay methodology we report direct heparin and heparan sulfate-binding activity in recombinant cellular prion protein (PrP(c)). We also demonstrate that the interaction of recombinant PrP(c) with heparin is weakened in the presence of Cu(II) ions and is particularly sensitive to competition with dextran sulfate. Competitive inhibition experiments with chemically modified heparins also indicate that 2-O-sulfate groups (but not 6-O-sulfate groups) are essential for heparin recognition. We have also identified three regions of the prion protein capable of independent binding to heparin and heparan sulfate: residues 23-52, 53-93, and 110-128. Interestingly, the interaction of an octapeptide-spanning peptide motif amino acids 53-93 with heparin is enhanced by Cu(II) ions. Significantly, a peptide of this sequence is able to inhibit the binding of full-length prion molecule to heparin, suggesting a direct role in heparin recognition within the intact protein. The collective data suggest a complex interaction between prion protein and heparin/heparan sulfate and has implications for the cellular and pathological functions of prion proteins.

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.

A novel heparan sulphate with high degree of N-sulphation and high heparin cofactor-II activity from the brine shrimp Artemia franciscana

With the aid of heparinase and heparitinases from Flavobacterium heparinum and 13C and IH NMR spectroscopy it was shown that the heparan sulphate isolated from the brine shrimp Artemia franciscana exhibits structural features intermediate between those of mammalian heparins and heparan sulphates. These include an unusually high degree of N-sulphation (with corresponding very low degree of N-acetylation), a relatively high content of iduronic acid residues (both unsulphated and 2-O-sulphated) and a relatively low degree of 6-O-sulphation of the glucosamine residues. The major sequences (glucuronic acid-->N-sulphated glucosamine and glucuronic acid-->N, 6-disulphated glucosamine) are most probably arranged in blocks. Although exhibiting negligible anticlotting activity in the APTT and anti-factor Xa assays the A. franciscana heparan sulphate has a high heparin cofactor-II activity (about 1/3 that of heparin).

"Linkage region" sequences of heparins and heparan sulfates: detection and quantification by nuclear magnetic resonance spectroscopy

The (13)C NMR spectra of most heparin and heparan sulfate preparations display minor signals not attributable to the glycosaminoglycan chains of these polysaccharides. These signals have been "concentrated" in oligosaccharides isolated from an acid hydrolyzate of heparin and shown to arise from the sequence GlcA-Gal-Gal-Xyl of the "linkage region" (LR) connecting the carbohydrate chains to the peptide chains in the original proteoglycans. Mono- and two-dimensional (1)H and (13)C NMR analysis of the major oligosaccharide (LR-OLIGO) indicated the prevalent structure GlcA-GlcNAc-GlcA-Gal-Gal-Xyl, where GlcNAc is partially 6-O-sulfated. (13)C NMR signals at 84.6 and 85.0 ppm, arising from C-3 of the two Gal residues, lend themselves to easy detection and quantification of the linkage region in heparins and heparan sulfates and can be used to assess the importance of the LR in the modulation of various biological activities of these glycosaminoglycans.

Neuropilin-1 is a placenta growth factor-2 receptor

Placenta growth factor (PlGF) belongs to the family of vascular endothelial growth factors (VEGFs). It binds to the flt-1 VEGF receptor but not to the KDR/flk-1 receptor which is thought to mediate most of the angiogenic and proliferative effects of VEGF. Three PlGF isoforms are produced by alternative splicing. PlGF-1 and PlGF-3 differ from PlGF-2 since they lack the exon 6 encoded peptide which bestows upon PlGF-2 its heparin binding properties. Cross-linking experiments revealed that 125I-PlGF-2 binds to two endothelial cell surface receptors in a heparin dependent fashion. The binding of 125I-PlGF-2 to these receptors was inhibited by an excess of PlGF-2 and by the 165-amino acid form of VEGF (VEGF165), but not at all by VEGF121 and very marginally if at all by PlGF-1. The apparent molecular weight and the binding characteristics of these receptors correspond to those of the recently identified VEGF165 specific receptor neuropilin-1, and we therefore conclude that neuropilin-1 is a receptor for PlGF-2. The binding of 125I-PlGF-2 as well as the binding of 125I-VEGF165 to these receptors was inhibited by a synthetic peptide derived from exon 6 of PlGF. Furthermore, the binding of 125I-PlGF-2, but not that of 125I-VEGF165, was also inhibited by a synthetic peptide derived from exon 7 of PlGF. These observations indicate that the peptides encoded by these exons probably participate in the formation of the domain which mediates the binding of PlGF-2 to these receptors. We have also determined, using chemically modified heparin species, that the presence of sulfate moieties on the glucosamine-O-6 and on the iduronic acid-O-2 groups of heparin was required for the potentiation of 125I-PlGF-2 binding to these receptors. To determine if PlGF-2 is able to induce biological responses that are not induced by PlGF-1, we compared the effects of PlGF-1 and PlGF-2 on the migration and proliferation of endothelial cells. Both PlGF forms induced migration of endothelial cells. However, there was no quantitative difference between the response to PlGF-2 and the response to PlGF-1. Furthermore, neither PlGF-1 nor PlGF-2 had any effect upon the proliferation of the endothelial cells.