A surface plasmon resonance-based solution affinity assay for heparan sulfate-binding proteins

Effects of Heparan Sulfate Trisaccharide Containing Oleanolic Acid in Attenuating Hyperphosphorylated Tau-Induced Cell Dysfunction Associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, marked by progressive brain degeneration and cognitive decline. A major pathological feature of AD is the accumulation of hyperphosphorylated tau (p-tau) in the form of neurofibrillary tangles (NFTs), which leads to neuronal death and neurodegeneration. P-tau also induces endoplasmic reticulum (ER) stress and activates the unfolded protein response, causing inflammation and apoptosis. Additionally, p-tau spreads in the brain through interactions with heparan sulfate (HS) proteoglycans, promoting aggregation and internalization. Targeting the tau-HS interaction offers a potential therapeutic strategy for AD. We present a novel HS mimetic with a lipophilic oleanolic acid linker and a sulfated trisaccharide, which shows strong cytoprotective effects against p-tau. Moreover, this compound alleviates p-tau-induced ER stress and inflammation. Molecular docking studies indicate that the conjugation of oleanolic acid enhances binding between the ligand and tau protofilament cores, facilitating protective interactions. These findings provide a foundation for the development of novel HS mimetics, enabling further investigation of tau-HS interactions in AD and other tauopathies.

Heparan sulfates and heparan sulfate proteoglycans in hematopoiesis

ABSTRACT

From signaling mediators in stem cells to markers of differentiation and lineage commitment to facilitators for the entry of viruses, such as HIV-1, cell surface heparan sulfate (HS) glycans with distinct modification patterns play important roles in hematopoietic biology. In this review, we provide an overview of the importance of HS and the proteoglycans (HSPGs) to which they are attached within the major cellular subtypes of the hematopoietic system. We summarize the roles of HSPGs, HS, and HS modifications within each main hematopoietic cell lineage of both myeloid and lymphoid arms. Lastly, we discuss the biological advances in the detection of HS modifications and their potential to further discriminate cell types within hematopoietic tissue.

Well-Defined Heparin Mimetics Can Inhibit Binding of the Trimeric Spike of SARS-CoV-2 in a Length-Dependent Manner

The emergence of new SARS-CoV-2 variants and the dangers of long-covid necessitate the development of broad-acting therapeutics that can reduce viral burden. SARS-CoV-2 employs heparan sulfate (HS) as an initial cellular attachment factor, and therefore, there is interest in developing heparin as a therapeutic for SARS-CoV-2. Its use is, however, complicated by structural heterogeneity and the risk of causing bleeding and thrombocytopenia. Here, we describe the preparation of well-defined heparin mimetics by a controlled head-to-tail assembly of HS oligosaccharides having an alkyne or azide moiety by copper-catalyzed azide-alkyne cycloaddition (CuAAC). Alkyne- and azide-containing sulfated oligosaccharides were prepared from a common precursor by modifying an anomeric linker with 4-pentynoic acid and by enzymatic extension with an N-acetyl-glucosamine having an azide moiety at C-6 (GlcNAc6N₃), respectively, followed by CuAAC. The process of enzymatic extension with GlcNAc6N₃ followed by CuAAC with the desired alkyne-containing oligosaccharides could be repeated to give compounds composed of 20 and 27 monosaccharides, respectively. The heparin mimetics could inhibit the binding of the SARS-CoV-2 spike or RBD to immobilized heparin or to Vero E6 cells. The inhibitory potency increased with increasing chain length, and a compound composed of four sulfated hexasaccharides linked by triazoles had a similar potency as unfractionated heparin. Sequence analysis and HS microarray binding studies with a wide range of RBDs of variants of concern indicate that they have maintained HS-binding capabilities and selectivities. The heparin mimetics exhibit no or reduced binding to antithrombin-III and platelet factor 4, respectively, which are associated with side effects.

Heparan sulfate glycomimetics via iterative assembly of "clickable" disaccharides

Heparan sulfate (HS) glycosaminoglycans are widely expressed on the mammalian cell surfaces and extracellular matrices and play important roles in a variety of cell functions. Studies on the structure-activity relationships of HS have long been hampered by the challenges in obtaining chemically defined HS structures with unique sulfation patterns. Here, we report a new approach to HS glycomimetics based on iterative assembly of clickable disaccharide building blocks that mimic the disaccharide repeating units of native HS. Variably sulfated clickable disaccharides were facilely assembled into a library of mass spec-sequenceable HS-mimetic oligomers with defined sulfation patterns by solution-phase iterative syntheses. Microarray and surface plasmon resonance (SPR) binding assays corroborated molecular dynamics (MD) simulations and confirmed that these HS-mimetic oligomers bind protein fibroblast growth factor 2 (FGF2) in a sulfation-dependent manner consistent with that of the native HS. This work established a general approach to HS glycomimetics that can potentially serve as alternatives to native HS in both fundamental research and disease models.

Heparan Sulfate Mimicking Glycopolymer Prevents Pancreatic β Cell Destruction and Suppresses Inflammatory Cytokine Expression in Islets under the Challenge of Upregulated Heparanase

Diabetes is a chronic disease in which the levels of blood glucose are too high because the body does not effectively produce insulin to meet its needs or is resistant to insulin. β Cells in human pancreatic islets produce insulin, which signals glucogen production by the liver and causes muscles and fat to uptake glucose. Progressive loss of insulin-producing β cells is the main cause of both type 1 and type 2 diabetes. Heparan sulfate (HS) is a ubiquitous polysaccharide found at the cell surface and in the extracellular matrix (ECM) of a variety of tissues. HS binds to and assembles proteins in ECM, thus playing important roles in the integrity of ECM (particularly basement membrane), barrier function, and ECM-cell interactions. Islet HS is highly expressed by the pancreatic β cells and critical for the survival of β cells. Heparanase is an endoglycosidase and cleaves islet HS in the pancreas, resulting in β-cell death and oxidative stress. Heparanase could also accelerate β-cell death by promoting cytokine release from ECM and secretion by activated inflammatory and endothelial cells. We demonstrate that HS-mimicking glycopolymer, a potent heparanase inhibitor, improves the survival of cultured mouse pancreatic β cells and protects HS contents under the challenge of heparanase in human pancreatic islets. Moreover, this HS-mimicking glycopolymer reduces the expression levels of cytokines (IL8, IL1β, and TNFα) and the gene encoding Toll-like Receptor 2 (TLR2) in human pancreatic islets.

The binding of heparin to spike glycoprotein inhibits SARS-CoV-2 infection by three mechanisms

Heparin, a naturally occurring glycosaminoglycan, has been found to have antiviral activity against SARS-CoV-2, the causative virus of COVID-19. To elucidate the mechanistic basis for the antiviral activity of heparin, we investigated the binding of heparin to the SARS-CoV-2 spike glycoprotein by means of sliding window docking, molecular dynamics simulations, and biochemical assays. Our simulations show that heparin binds at long, positively-charged patches on the spike glycoprotein, thereby masking basic residues of both the receptor binding domain (RBD) and the multifunctional S1/S2 site. Biochemical experiments corroborated the simulation results, showing that heparin inhibits the furin-mediated cleavage of spike by binding to the S1/S2 site. Our simulations also showed that heparin can act on the hinge region responsible for motion of the RBD between the inactive closed and active open conformations of the spike glycoprotein. In simulations of the closed spike homotrimer, heparin binds the RBD and the N-terminal domain of two adjacent spike subunits and hinders opening. In simulations of open spike conformations, heparin induces stabilization of the hinge region and a change in RBD motion. Taken together, our results indicate that heparin can inhibit SARS-CoV-2 infection by three mechanisms: by allosterically hindering binding to the host cell receptor, by directly competing with binding to host heparan sulfate proteoglycan co-receptors, and by preventing spike cleavage by furin. Furthermore, these simulations provide insights into how host heparan sulfate proteoglycans can facilitate viral infection. Our results will aid the rational optimization of heparin derivatives for SARS-CoV-2 antiviral therapy.

From Cancer to COVID-19: A Perspective on Targeting Heparan Sulfate-Protein Interactions

Heparan sulfate (HS) is a complex, polyanionic polysaccharide ubiquitously expressed on cell surfaces and in the extracellular matrix. HS interacts with numerous proteins to mediate a vast array of biological and pathological processes. Inhibition of HS-protein interactions is thus an attractive approach for new therapeutic development for cancer and infectious diseases, including COVID-19; however, synthesis of well-defined native HS oligosaccharides remains challenging. This has aroused significant interest in the development of HS mimetics which are more synthetically tractable and have fewer side effects, such as undesired anticoagulant activity. This account provides a perspective on the design and synthesis of different classes of HS mimetics with useful properties, and the development of various assays and molecular modelling tools to progress our understanding of their interactions with HS-binding proteins.

Probing Amyloid β Interactions with Synthetic Heparan Sulfate Oligosaccharides

Heparan sulfate (HS) can play important roles in the biology and pathology of amyloid β (Aβ), a hallmark of Alzheimer's disease. To better understand the structure-activity relationship of HS/Aβ interactions, synthetic HS oligosaccharides ranging from tetrasaccharides to decasaccharides have been utilized to study Aβ interactions. Surface plasmon resonance experiments showed that the highly sulfated HS tetrasaccharides bearing full 2-O, 6-O, and N-sulfations exhibited the strongest binding with Aβ among the tetrasaccharides investigated. Elongating the glycan length to hexa- and deca-saccharides significantly enhanced Aβ affinity compared to the corresponding HS tetrasaccharide. Solid state NMR studies of the complexes of Aβ with HS hexa- and deca-saccharides showed most significant chemical shift perturbation in the C-terminus residues of Aβ. The strong binding HS oligosaccharides could reduce the cellular toxicities induced by Aβ. This study provides new insights into HS/Aβ interactions, highlighting how synthetic structurally well-defined HS oligosaccharides can assist in biological understanding of Aβ.

The Chemokine-Based Peptide, CXCL9(74-103), Inhibits Angiogenesis by Blocking Heparan Sulfate Proteoglycan-Mediated Signaling of Multiple Endothelial Growth Factors

Simple Summary

Major angiogenic growth factors activate downstream signaling cascades by interacting with both receptor tyrosine kinases (RTKs) and cell surface proteoglycans, such as heparan sulfate proteoglycans (HSPGs). As current anti-angiogenesis regimens in cancer are often faced with resistance, alternative therapeutic strategies are highly needed. The aim of our study was to investigate the impact on angiogenic signaling when we interfered with growth factor-HSPG interactions using a CXCL9 chemokine-derived peptide with high affinity for HS.

Abstract

Growth factors such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF) and epidermal growth factor (EGF) are important angiogenesis-mediating factors. They exert their effects not only through their respective receptor tyrosine kinases (RTKs), but they also require molecular pairing with heparan sulfate proteoglycans (HSPGs). Angiogenic growth factors and their signaling pathways are commonly targeted in current anti-angiogenic cancer therapies but have unfortunately insufficient impact on patient survival. Considering their obvious role in pathological angiogenesis, HS-targeting drugs have become an appealing new strategy. Therefore, we aimed to reduce angiogenesis through interference with growth factor-HS binding and downstream signaling using a CXCL9-derived peptide with a high affinity for glycosaminoglycans (GAGs), CXCL9(74-103). We showed that CXCL9(74-103) reduced EGF-, VEGF165- and FGF-2-mediated angiogenic processes in vitro, such as endothelial cell proliferation, chemotaxis, adhesion and sprouting, without exerting cell toxicity. CXCL9(74-103) interfered with growth factor signaling in diverse ways, e.g., by diminishing VEGF165 binding to HS and by direct association with FGF-2. The dependency of CXCL9(74-103) on HS for binding to HMVECs and for exerting its anti-angiogenic activity was also demonstrated. In vivo, CXCL9(74-103) attenuated neovascularization in the Matrigel plug assay, the corneal cauterization assay and in MDA-MB-231 breast cancer xenografts. Additionally, CXCL9(74-103) reduced vascular leakage in the retina of diabetic rats. In contrast, CXCL9(86-103), a peptide with low GAG affinity, showed no overall anti-angiogenic activity. Altogether, our results indicate that CXCL9(74-103) reduces angiogenesis by interfering with multiple HS-dependent growth factor signaling pathways.

Systemically Administered Homing Peptide Targets Dystrophic Lesions and Delivers Transforming Growth Factor-β (TGFβ) Inhibitor to Attenuate Murine Muscular Dystrophy Pathology

Muscular dystrophy is a progressively worsening and lethal disease, where accumulation of functionality-impairing fibrosis plays a key pathogenic role. Transforming growth factor-β1 (TGFβ1) is a central signaling molecule in the development of fibrosis in muscular dystrophic humans and mice. Inhibition of TGFβ1 has proven beneficial in mouse models of muscular dystrophy, but the global strategies of TGFβ1 inhibition produce significant detrimental side effects. Here, we investigated whether murine muscular dystrophy lesion-specific inhibition of TGFβ1 signaling by the targeted delivery of therapeutic decorin (a natural TGFβ inhibitor) by a vascular homing peptide CAR (CARSKNKDC) would reduce skeletal muscle fibrosis and pathology and increase functional characteristics of skeletal muscle. We demonstrate that CAR peptide homes to dystrophic lesions with specificity in two muscular dystrophy models. Recombinant fusion protein consisting of CAR peptide and decorin homes selectively to sites of skeletal muscle damage in mdxDBA2/J and gamma-sarcoglycan deficient DBA2/J mice. This targeted delivery reduced TGFβ1 signaling as demonstrated by reduced nuclear pSMAD staining. Three weeks of targeted decorin treatment decreased both membrane permeability and fibrosis and improved skeletal muscle function in comparison to control treatments in the mdxD2 mice. These results show that selective delivery of decorin to the sites of skeletal muscle damage attenuates the progression of murine muscular dystrophy.

Regenerative responses of rabbit corneal endothelial cells to stimulation by fibroblast growth factor 1 (FGF1) derivatives, TTHX1001 and TTHX1114

Utilising rabbit corneal endothelial cells (CEC) in three different paradigms, two human FGF1 derivatives (TTHX1001 and TTHX1114), engineered to exhibit greater stability, were tested as proliferative agents. Primary CECs and mouse NIH 3T3 cells treated with the two FGF1 derivatives showed equivalent EC₅₀ ranges (3.3-24 vs.1.9-16. ng/mL) and, in organ culture, chemically lesioned corneas regained half of the lost endothelial layer in three days after treatment with the FGF1 derivatives as compared to controls. In vivo, following cryolesioning, the CEC monolayer, as judged by specular microscopy, regenerated 10-11 days faster when treated with TTHX1001. Over two weeks, all treated eyes showed clearing of opacity about twice that of untreated controls. In all three rabbit models, both FGF1 derivatives were effective in inducing CEC proliferation over control conditions, supporting the prediction that these stabilised FGF1 derivatives can potentially regenerate corneal endothelial deficits in humans.

PI-88 and Related Heparan Sulfate Mimetics

The heparan sulfate mimetic PI-88 (muparfostat) is a complex mixture of sulfated oligosaccharides that was identified in the late 1990s as a potent inhibitor of heparanase. In preclinical animal models it was shown to block angiogenesis, metastasis and tumor growth, and subsequently became the first heparanase inhibitor to enter clinical trials for cancer. It progressed to Phase III trials but ultimately was not approved for use. Herein we summarize the preparation, physicochemical and biological properties of PI-88, and discuss preclinical/clinical and structure-activity relationship studies. In addition, we discuss the PI-88-inspired development of related HS mimetic heparanase inhibitors with improved properties, ultimately leading to the discovery of PG545 (pixatimod) which is currently in clinical trials.

Modulating Heparanase Activity: Tuning Sulfation Pattern and Glycosidic Linkage of Oligosaccharides

Heparanase cleaves polymeric heparan sulfate (HS) molecules into smaller oligosaccharides, allowing for release of angiogenic growth factors promoting tumor development and autoreactive immune cells to reach the insulin-producing β cells. Interaction of heparanase with HS chains is regulated by specific substrate sulfation sequences. We have synthesized 11 trisaccharides that are highly tunable in structure and sulfation pattern, allowing us to determine how heparanase recognizes HS substrate and selects a favorable cleavage site. Our study shows that (1) N-SO₃^- at +1 subsite and 6-O-SO₃^- at -2 subsite of trisaccharides are critical for heparanase recognition, (2) addition of 2-O-SO₃^- at the -1 subsite and of 3-O-SO₃^- to GlcN unit is not advantageous, and (3) the anomeric configuration (α or β) at the reducing end is crucial in controlling heparanase activity. Our study also illustrates that the α-trisaccharide having N- and 6-O-SO₃^- at -2 and +1 subsites inhibited heparanase and was resistant toward hydrolysis.

Targeting Chemokine-Glycosaminoglycan Interactions to Inhibit Inflammation

Leukocyte migration into tissues depends on the activity of chemokines that form concentration gradients to guide leukocytes to a specific site. Interaction of chemokines with their specific G protein-coupled receptors (GPCRs) on leukocytes induces leukocyte adhesion to the endothelial cells, followed by extravasation of the leukocytes and subsequent directed migration along the chemotactic gradient. Interaction of chemokines with glycosaminoglycans (GAGs) is crucial for extravasation in vivo. Chemokines need to interact with GAGs on endothelial cells and in the extracellular matrix in tissues in order to be presented on the endothelium of blood vessels and to create a concentration gradient. Local chemokine retention establishes a chemokine gradient and prevents diffusion and degradation. During the last two decades, research aiming at reducing chemokine activity mainly focused on the identification of inhibitors of the interaction between chemokines and their cognate GPCRs. This approach only resulted in limited success. However, an alternative strategy, targeting chemokine-GAG interactions, may be a promising approach to inhibit chemokine activity and inflammation. On this line, proteins derived from viruses and parasites that bind chemokines or GAGs may have the potential to interfere with chemokine-GAG interactions. Alternatively, chemokine mimetics, including truncated chemokines and mutant chemokines, can compete with chemokines for binding to GAGs. Such truncated or mutated chemokines are characterized by a strong binding affinity for GAGs and abrogated binding to their chemokine receptors. Finally, Spiegelmers that mask the GAG-binding site on chemokines, thereby preventing chemokine-GAG interactions, were developed. In this review, the importance of GAGs for chemokine activity in vivo and strategies that could be employed to target chemokine-GAG interactions will be discussed in the context of inflammation.

Specific Inhibition of Heparanase by a Glycopolymer with Well-Defined Sulfation Pattern Prevents Breast Cancer Metastasis in Mice

Heparanase, the heparan sulfate polysaccharide degrading endoglycosidase enzyme, has been correlated with tumor angiogenesis and metastasis and therefore has become a potential target for anticancer drug development. In this systematic study, the sulfation pattern of the pendant disaccharide moiety on synthetic glycopolymers was synthetically manipulated to achieve optimal heparanase inhibition. Upon evaluation, a glycopolymer with 12 repeating units was determined to be the most potent inhibitor of heparanase (IC50 = 0.10 ± 0.36 nM). This glycopolymer was further examined for cross-bioactivity using a solution-based competitive biolayer interferometry assay with other HS-binding proteins (growth factors, P-selectin, and platelet factor 4), which are responsible for mediating angiogenic activity, cell metastasis, and antibody-induced thrombocytopenia. The synthetic glycopolymer has low affinity for these HS-binding proteins in comparison to natural heparin. In addition, the glycopolymer possessed no proliferative properties toward human umbilical endothelial cells (HUVECs) and a potent antimetastatic effect against 4T1 mammary carcinoma cells. Thus, our study not only establishes a specific inhibitor of heparanase with high affinity but also illustrates the high effectiveness of this multivalent heparanase inhibitor in inhibiting experimental metastasis in vivo.

Influence of the SPR Experimental Conditions on the G-Quadruplex DNA Recognition by Porphyrin Derivatives

Surface plasmon resonance (SPR) is a powerful technique to study the interactions of ligands with analytes and therefore a number of biosensor surfaces and injection methods have been developed so far. However, many experimental parameters can affect the interactions and consequently the affinity measurements. In particular, the interactions of positively charged analytes (often used for anionic nucleic acids targets) can be influenced by the sensing surfaces (e.g., negatively charged), leading to significant nonspecific interactions as well as regeneration problems. The aim of the present work is to investigate the effect of different parameters, including ionic strength, SPR biosensor (i.e., nature of the surfaces), and the injection method on the recognition of porphyrin G-quadruplex ligands. We demonstrate that the injection method does not influence the affinity whereas the ionic strength and the nature of the surface impact the recognition properties of the porphyrin for the G-quadruplex DNA. We also found that self-assembled monolayer coating surface presents many advantages in comparison with carboxymethylated dextran surface for SPR studies of G-quadruplex DNA/ligand interactions: (i) the electrostatic interaction with charged analytes is less important, (ii) its structure/composition is less sensitive to the ionic concentration and less prone to unspecific adsorption, (iii) it is easily homemade, and (iv) the cost is approximately 10 times cheaper.

Modulating the degree of fucosylation of fucosylated chondroitin sulfate enhances heparin cofactor II-dependent thrombin inhibition

Fucosylated chondroitin sulfate (FCS), an unusual glycosaminoglycan with fucose side chains, is a promising anticoagulant agent. To assess the effect of its structure on anticoagulant activity, its derivatives with various degrees of fucosylation (DF), molecular weights (Mw) and sulfation patterns were prepared and characterized. Biological tests showed that their APTT (activated partial thromboplastin time) prolonging activity and intrinsic factor Xase complex (factor IXa-VIIIa-Ca²⁺-PL complex) inhibitory activity were both reduced in FCS derivatives with lower Mw and DF. However, FCSs with DF at least 16% resulted in greater heparin cofactor II (HCII)-dependent thrombin inhibitory activity in response to decreasing DF, and these activities did not depend on Mw (Mw > 5.2 kDa). Solution competition binding assay further suggested that modulating the DF of FCS derivatives might enhance inhibition of thrombin by activating HCII. These findings imply that FCS derivatives with suitable chain length and DF value may be novel anticoagulants by activating HCII.

Comparison of Metal-Ammine Compounds Binding to DNA and Heparin. Glycans as Ligands in Bioinorganic Chemistry

We present spectroscopic and biophysical approaches to examine the affinity of metal-ammine coordination complexes for heparin as a model for heparan sulfate (HS). Similar to nucleic acids, the highly anionic nature of heparin means it is associated in vivo with physiologically relevant cations, and this work extends their bioinorganic chemistry to substitution-inert metal-ammine compounds (M). Both indirect and direct assays were developed. M compounds are competitive inhibitors of methylene blue (MB)-heparin binding, and the change in the absorbance of the dye in the presence or absence of heparin can be used as an indirect reporter of M-heparin affinity. A second indirect assay uses the change in fluorescence of TAMRA-R₉, a nonaarginine linked to a fluorescent TAMRA moiety, as a reporter for M-heparin binding. Direct assays are surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). The K_d values for TriplatinNC-heparin varied to some extent depending on the technique from 33.1 ± 2 nM (ITC) to 66.4 ± 1.3 nM (MB absorbance assay) and 340 ± 30 nM (SPR). The differences are explained by the nature of the technique and the use of heparin of differing molecular weight. Indirect probes using the displacement of ethidium bromide from DNA or, separately, fluorescently labeled oligonucleotide (DNA-Fl) can measure the relative affinities of heparin and DNA for M compounds. These assays showed essentially equivalent affinity of TriplatinNC for heparin and DNA. The generality of these methods was confirmed with a series of mononuclear cobalt, ruthenium, and platinum compounds with significantly lower affinity because of their smaller overall positive charge but in the order [Co(NH₃)₆]³⁺ > [Ru(NH₃)₆]³⁺ > [Pt(NH₃)₄]²⁺. The results on heparin can be extrapolated to glycosoaminoglycans such as HS, emphasizing the relevance of glycan interactions in understanding the biological properties of coordination compounds and the utility of the metalloglycomics concept for extending bioinorganic chemistry to this class of important biomolecules.

Interactions between depolymerized fucosylated glycosaminoglycan and coagulation proteases or inhibitors

Fucosylated glycosaminoglycan (FG) is a structurally novel glycosaminoglycan derivative, and it has potent anticoagulant activity. Depolymerized FG (dFG) is a selective factor Xase (FXase, FIXa-FVIIIa complex) inhibitor and it has antithrombotic action without major bleeding risks. In this study, we report the effects of dFG-3 (Mw ~14kDa) on the catalysis rates of factor IIa (FIIa), factor Xa (FXa) and factor IXa (FIXa) inhibition by antithrombin (AT), and the kinetic of the interactions between coagulation proteases or inhibitors and dFG-3 were also studied using biolayer interferometry (BLI) technology. We found that dFG-3 had much weaker catalysis activity of coagulation proteases inhibition by AT compared with heparin (UFH). The binding affinity of AT bound to dFG-3 was lower than UFH, and the UFH-AT interaction fitted well with biphasic-binding model while dFG-3-AT interaction was monophasic-binding, suggesting dFG-3 might not have allosteric activation effect on AT. The results are consistent with AT-independent inhibitory activities of dFG-3. dFG-3 could strongly bind to FIXa with much higher affinity than UFH, further explained the reason for its potent FXase inhibitory activity. Additionally, the binding ability of dFG-3 and FIXa decreased with decreasing molecular, and the fucose side chains and carboxyl groups of dFG-3 might be required for its high affinity binding with FIXa. Our data supports further the investigation of dFG-3 as a promising anticoagulant drug inhibiting the intrinsic FXase by binding to FIXa.

Synthesis, radiolabeling with fluorine-18 and preliminary in vivo evaluation of a heparan sulphate mimetic as potent angiogenesis and heparanase inhibitor for cancer applications

A rationally designed, fully synthetic, octasaccharide-based, HS mimetic has been synthesized, in vitro characterized, labeled with fluorine-18, and in vivo imaged with PET in rats.

Heparin/Heparan sulfate proteoglycans glycomic interactome in angiogenesis: biological implications and therapeutical use

Angiogenesis, the process of formation of new blood vessel from pre-existing ones, is involved in various intertwined pathological processes including virus infection, inflammation and oncogenesis, making it a promising target for the development of novel strategies for various interventions. To induce angiogenesis, angiogenic growth factors (AGFs) must interact with pro-angiogenic receptors to induce proliferation, protease production and migration of endothelial cells (ECs). The action of AGFs is counteracted by antiangiogenic modulators whose main mechanism of action is to bind (thus sequestering or masking) AGFs or their receptors. Many sugars, either free or associated to proteins, are involved in these interactions, thus exerting a tight regulation of the neovascularization process. Heparin and heparan sulfate proteoglycans undoubtedly play a pivotal role in this context since they bind to almost all the known AGFs, to several pro-angiogenic receptors and even to angiogenic inhibitors, originating an intricate network of interaction, the so called "angiogenesis glycomic interactome". The decoding of the angiogenesis glycomic interactome, achievable by a systematic study of the interactions occurring among angiogenic modulators and sugars, may help to design novel antiangiogenic therapies with implications in the cure of angiogenesis-dependent diseases.

Angiogenic growth factors interactome and drug discovery: The contribution of surface plasmon resonance

Angiogenesis is implicated in several pathological conditions, including cancer, and in regenerative processes, including the formation of collateral blood vessels after stroke. Physiological angiogenesis is the outcome of a fine balance between the action of angiogenic growth factors (AGFs) and anti-angiogenic molecules, while pathological angiogenesis occurs when this balance is pushed toward AGFs. AGFs interact with multiple endothelial cell (EC) surface receptors inducing cell proliferation, migration and proteases upregulation. On the contrary, free or extracellular matrix-associated molecules inhibit angiogenesis by sequestering AGFs (thus hampering EC stimulation) or by interacting with specific EC receptors inducing apoptosis or decreasing responsiveness to AGFs. Thus, angiogenesis results from an intricate network of interactions among pro- and anti-angiogenic molecules, EC receptors and various modulators. All these interactions represent targets for the development of pro- or anti-angiogenic therapies. These aims call for suitable technologies to study the countless interactions occurring during neovascularization. Surface plasmon resonance (SPR) is a label-free optical technique to study biomolecular interactions in real time. It has become the golden standard technology for interaction analysis in biomedical research, including angiogenesis. From a survey of the literature it emerges that SPR has already contributed substantially to the better understanding of the neovascularization process, laying the basis for the decoding of the angiogenesis "interactome" and the identification of "hub molecules" that may represent preferential targets for an efficacious modulation of angiogenesis. Here, the still unexploited full potential of SPR is enlightened, pointing to improvements in its use for a deeper understanding of the mechanisms of neovascularization and the identification of novel anti-angiogenic drugs.

Granulin-epithelin precursor interacts with heparan sulfate on liver cancer cells

The first study to demonstrate HS to affect GEP binding on the cell surface.

Granulin-epithelin precursor (GEP) is a pluripotent secretory growth factor which promotes cancer progression in a number of human cancers. However, how cancer cells interact with GEP remains unknown. In this study, we aimed to identify the cell surface-binding partner of GEP on liver cancer cells. Human recombinant GEP (rGEP) was expressed and purified to homogeneity. The rGEP was shown to trigger phosphorylation of AKT and ERK1/2 in liver cancer cells. We demonstrated cell surface attachment of rGEP, which was blocked by prebinding of platelet-derived growth factor-AA, platelet-derived growth factor-BB and fibroblast growth factor-2. Therefore, heparan sulfate (HS) had been reasoned as the binding partner of rGEP. Heparinase digestion validated the role of HS on supporting the attachment. The heparin-binding domain of GEP was mapped to RRH(555-557) in the C-terminal region. Suppression of the HS polymerase exostosin-1 reduced the rGEP binding and rGEP-mediated signaling transduction. Suppression of a specific HS proteoglycan, glypican-3, also showed a partial reduction of rGEP binding and an inhibition on rGEP-mediated activation of AKT. Furthermore, glypican-3 was shown to correlate with the expressions of GEP in clinical samples (Spearman’s ρ = 0.363, P = 0.001). This study identified HS, partly through glypican-3, as a novel binding partner of GEP on the surface of liver cancer cells.

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.

Determining the affinity and stoichiometry of interactions between unmodified proteins in solution using Biacore

We describe a general Biacore method for measuring equilibrium binding affinities and stoichiometries for interactions between unmodified proteins and their unmodified ligands free in solution. Mixtures of protein and ligand are preequilibrated at different ratios in solution and then analyzed by Biacore using a sensor chip surface that detects only unbound analyte. Performing the Biacore analysis under mass transport limited conditions allows the concentration of unbound analyte to be determined from the initial velocity of binding. Plots of initial velocity versus the concentration of the varied binding partner are fitted to a quadratic binding equation to give the affinity and stoichiometry of binding. We demonstrate the method using soluble Her2 extracellular domain binding to monovalent, bivalent, and trivalent forms of an anti-Her2 antibody. The affinity we measured agrees with that obtained from conventional Biacore kinetic analysis, and the stoichiometries for the resulting 1:1, 1:2, and 1:3 complexes were confirmed by gel filtration with in-line light scattering. The method is applicable over an affinity range of approximately 100 pM to 1 μM and is particularly useful when there is concern that covalently modifying one or the other binding partner might affect its binding properties or where multivalency might otherwise complicate a quantitative analysis of binding.

Perturbation of the heparin/heparin-sulfate interactome of human breast cancer cells modulates pro-tumourigenic effects associated with PI3K/Akt and MAPK/ERK signalling

Heparansulfate-proteoglycans (HSPGs) interact via their polyanionic heparansulfate (HS) side chains with a variety of proteins on the cell surface or within the extracellular matrix membrane. The large number of heparin/HS binding proteins form a highly interconnected functional network, which has been termed as the heparin/HS interactome and is functionally linked to physiological and pathological processes. The aim of this study was to investigate the global effect of these protein-HSPG interactions on the tumourigenicity of two breast cancer cell lines (MCF-7 and MDA-MB-231). Cancer cells were cultured in serum-free medium and treated with a concentration of heparin which was capable of modulating HS/ligand interaction. Microarray analysis of MCF-7 cells cultured under these conditions showed that expression of 105 of 1,357 genes potentially related to the pathogenesis of breast neoplasm was significantly altered by heparin treatment. The changes in gene expression correlated with a less tumourigenic phenotype, including reduction of cell adhesive, invasive and migratory properties. These effects were associated with an inhibition of the PI3K/Akt and Raf/MEK/ERK signalling pathways. The modulatory effect of heparin on HS-associated activity was confirmed with one example of heparin/HS interactomes, transforming growth factor β (TGFβ). The innate TGFβ activity of MCF-7 cells was reduced by heparin treatment, with specific interruption of the TGFβ-Smad signalling pathway. The pro-tumourigenic contribution of the heparin/HS interactomes was verified in cells in which HSPG synthesis was blocked using β-xyloside. In conclusion, the interaction between cell surface HPSGs and innate heparin/HS interactomes makes a significant contribution to the tumourigenicity.

Survey of the 2009 commercial optical biosensor literature

We took a different approach to reviewing the commercial biosensor literature this year by inviting 22 biosensor users to serve as a review committee. They set the criteria for what to expect in a publication and ultimately decided to use a pass/fail system for selecting which papers to include in this year's reference list. Of the 1514 publications in 2009 that reported using commercially available optical biosensor technology, only 20% passed their cutoff. The most common criticism the reviewers had with the literature was that "the biosensor experiments could have been done better." They selected 10 papers to highlight good experimental technique, data presentation, and unique applications of the technology. This communal review process was educational for everyone involved and one we will not soon forget.

Binding affinities of vascular endothelial growth factor (VEGF) for heparin-derived oligosaccharides

Heparin and HS (heparan sulfate) exert their wide range of biological activities by interacting with extracellular protein ligands. Among these important protein ligands are various angiogenic growth factors and cytokines. HS binding to VEGF (vascular endothelial growth factor) regulates multiple aspects of vascular development and function through its specific interaction with HS. Many studies have focused on HS-derived or HS-mimicking structures for the characterization of VEGF165 interaction with HS. Using a heparinase 1-prepared small library of heparin-derived oligosaccharides ranging from hexasaccharide to octadecasaccharide, we systematically investigated the heparin-specific structural features required for VEGF binding. We report the apparent affinities for the association between the heparin-derived oligosaccharides with both VEGF165 and VEGF55, a peptide construct encompassing exclusively the heparin-binding domain of VEGF165. An octasaccharide was the minimum size of oligosaccharide within the library to efficiently bind to both forms of VEGF and a tetradecasaccharide displayed an effective binding affinity to VEGF165 comparable to unfractionated heparin. The range of relative apparent binding affinities among VEGF and the panel of heparin-derived oligosaccharides demonstrate that the VEGF binding affinity likely depends on the specific structural features of these oligosaccharides, including their degree of sulfation, sugar-ring stereochemistry and conformation. Notably, the unique 3-O-sulfo group found within the specific antithrombin binding site of heparin is not required for VEGF165 binding. These findings afford new insight into the inherent kinetics and affinities for VEGF association with heparin and heparin-derived oligosaccharides with key residue-specific modifications and may potentially benefit the future design of oligosaccharide-based anti-angiogenesis drugs.

Interaction of thrombin with sucrose octasulfate

The serine protease thrombin plays multiple roles in many important physiological processes, especially coagulation, where it functions as both a pro- and anticoagulant. The polyanionic glycosaminoglycan heparin modulates thrombin's activity through binding at exosite II. Sucrose octasulfate (SOS) is often used as a surrogate for heparin, but it is not known whether it is an effective heparin mimic in its interaction with thrombin. We have characterized the interaction of SOS with thrombin in solution and determined a crystal structure of their complex. SOS binds thrombin with a K(d) of ~1.4 μM, comparable to that of the much larger polymeric heparin measured under the same conditions. Nonionic (hydrogen bonding) interactions make a larger contribution to thrombin binding of SOS than to heparin. SOS binding to exosite II inhibits thrombin's catalytic activity with high potency but with low efficacy. Analytical ultracentrifugation shows that bovine and human thrombins are monomers in solution in the presence of SOS, in contrast to their complexes with heparin, which are dimers. In the X-ray crystal structure, two molecules of SOS are bound nonequivalently to exosite II portions of a thrombin dimer, in contrast to the 1:2 stoichiometry of the heparin-thrombin complex, which has a different monomer association mode in the dimer. SOS and heparin binding to exosite II of thrombin differ on both chemical and structural levels and, perhaps most significantly, in thrombin inhibition. These differences may offer paths to the design of more potent exosite II binding, allosteric small molecules as modulators of thrombin function.

VEGF internalization is not required for VEGFR-2 phosphorylation in bioengineered surfaces with covalently linked VEGF

Vascular endothelial growth factor (VEGF) is known to activate proliferation, migration, and survival pathways in endothelial cells through phosphorylation of VEGF receptor-2 (VEGFR-2). VEGF has been incorporated into biomaterials through encapsulation, electrostatic sequestration, and covalent attachment, but the effect of these immobilization strategies on VEGF signaling has not been thoroughly investigated. Further, although growth factor internalization along with the receptor generally occurs in a physiological setting, whether this internalization is needed for receptor phosphorylation is not entirely clear. Here we show that VEGF covalently bound through a modified heparin molecule elicits an extended response of pVEGFR-2 in human umbilical vein endothelial cells (HUVECs) and that the covalent linkage reduces internalization of the growth factor during receptor endocytosis. Optical tweezer measurements show that the rupture force required to disrupt the heparin-VEGF-VEGFR-2 interaction increases from 3-8 pN to 6-12 pN when a covalent bond is introduced between VEGF and heparin. Importantly, by covalently binding VEGF to a heparin substrate, the stability (half-life) of VEGF is extended over three-fold. Here, mathematical models support the biological conclusions, further suggesting that VEGF internalization is significantly reduced when covalently bound, and indicating that VEGF is available for repeated phosphorylation events.

Evolving the use of peptides as components of biomaterials

This manuscript is part of a debate on the statement that "the use of short synthetic adhesion peptides, like RGD, is the best approach in the design of biomaterials that guide cell behavior for regenerative medicine and tissue engineering". We take the position that although there are some acknowledged disadvantages of using short peptide ligands within biomaterials, it is not necessary to discard the notion of using peptides within biomaterials entirely, but rather to reinvent and evolve their use. Peptides possess advantageous chemical definition, access to non-native chemistries, amenability to de novo design, and applicability within parallel approaches. Biomaterials development programs that require such aspects may benefit from a peptide-based strategy.

Functional divergence between 2 chemokines is conferred by single amino acid change

CXCL4 and CXCL4L1 are 2 closely related CXC chemokines that exhibit potent antiangiogenic activity. Because interactions with glycosaminoglycans play a crucial role in chemokines activity, we determined the binding parameters of CXCL4 and CXCL4L1 for heparin, heparan sulfate, and chondroitin sulfate B. We further demonstrated that the Leu67/His67 substitution is critical for the decrease in glycan binding of CXCL4L1 but also for the increase of its angiostatic activities. Using a set of mutants, we show that glycan affinity and angiostatic properties are not completely related. These data are reinforced using a monoclonal antibody that specifically recognizes structural modifications in CXCL4L1 due to the presence of His67 and that blocks its biologic activity. In vivo, half-life and diffusibility of CXCL4L1 compared with CXCL4 is strongly increased. As opposed to CXCL4L1, CXCL4 is preferentially retained at its site of expression. These findings establish that, despite small differences in the primary structure, CXCL4L1 is highly distinct from CXCL4. These observations are not only of great significance for the antiangiogenic activity of CXCL4L1 and for its potential use in clinical development but also for other biologic processes such as inflammation, thrombosis or tissue repair.

Glycosaminoglycans as polyelectrolytes

One of the barriers to understanding structure-property relations for glycosaminoglycans has been the lack of constructive interplay between the principles and methodologies of the life sciences (molecular biology, biochemistry and cell biology) and the physical sciences, particularly in the field of polyelectrolytes. To address this, we first review the similarities and differences between the physicochemical properties of GAGs and other statistical chain polyelectrolytes of both natural and abioitic origin. Since the biofunctionality and regulation of the structures of GAGs is intimately connected with interactions with their cognate proteins, we particularly compare and contrast aspects of protein binding, i.e. effects of both GAGs and other polyelectrolytes on protein stability, protein aggregation and phase behavior. The protein binding affinities and their dependences on pH and ionic strength for the two groups are discussed not only in terms of observable differences, but also with regard to contrasting descriptions of the bound state and the role of electrostatics. We conclude that early studies of the heparin-Antithromin system, proceeding to a large extent through the methods and models of protein chemistry and drug discovery, established not only many enabling precedents but also constraining paradigms. Current studies on heparan sulfate and chondroitin sulfate seem to reflect a more ecumenical view likely to be more compatible with concepts from physical and polymer chemistry.

Exploiting Surface Plasmon Resonance (SPR) Technology for the Identification of Fibroblast Growth Factor-2 (FGF2) Antagonists Endowed with Antiangiogenic Activity

Angiogenesis, the process of new blood vessel formation, is implicated in various physiological/pathological conditions, including embryonic development, inflammation and tumor growth. Fibroblast growth factor-2 (FGF2) is a heparin-binding angiogenic growth factor involved in various physiopathological processes, including tumor neovascularization. Accordingly, FGF2 is considered a target for antiangiogenic therapies. Thus, numerous natural/synthetic compounds have been tested for their capacity to bind and sequester FGF2 in the extracellular environment preventing its interaction with cellular receptors. We have exploited surface plasmon resonance (SPR) technique in search for antiangiogenic FGF2 binders/antagonists. In this review we will summarize our experience in SPR-based angiogenesis research, with the aim to validate SPR as a first line screening for the identification of antiangiogenic compounds.

The PG500 series: novel heparan sulfate mimetics as potent angiogenesis and heparanase inhibitors for cancer therapy

Heparan sulfate mimetics, which we have called the PG500 series, have been developed to target the inhibition of both angiogenesis and heparanase activity. This series extends the technology underpinning PI-88, a mixture of highly sulfated oligosaccharides which reached Phase III clinical development for hepatocellular carcinoma. Advances in the chemistry of the PG500 series provide numerous advantages over PI-88. These new compounds are fully sulfated, single entity oligosaccharides attached to a lipophilic moiety, which have been optimized for drug development. The rational design of these compounds has led to vast improvements in potency compared to PI-88, based on in vitro angiogenesis assays and in vivo tumor models. Based on these and other data, PG545 has been selected as the lead clinical candidate for oncology and is currently undergoing formal preclinical development as a novel treatment for advanced cancer.