Interaction of heparin with two synthetic peptides that neutralize the anticoagulant activity of heparin

Rational identification and characterisation of peptide ligands for targeting polysialic acid

The alpha-2,8-linked form of the polysaccharide polysialic acid (PSA) has widespread implications in physiological and pathological processes, ranging from neurological development to disease progression. Though the high electronegativity and excluded volume of PSA often promotes interference of biomolecular interactions, PSA-binding ligands have important implications for both biological processes and biotechnological applications. As such, the design, identification, and characterisation of novel ligands towards PSA is critical for expanding knowledge of PSA interactions and achieving selective glycan targeting. Here, we report on a rational approach for the identification of alpha-2,8-PSA-binding peptides, involving design from the endogenous ligand Siglec-11 and multi-platform characterisation of peptide binding. Microarray-based examination of peptides revealed charge and sequence characteristics influencing peptide affinity to PSA, and carbohydrate-peptide binding was further quantified with a novel fluorescence anisotropy assay. PSA-binding peptides exhibited specific binding to polymeric SA, as well as different degrees of selective binding in various conditions, including competition with PSA of alternating 2,8/9-linkages and screening with PSA-expressing cells. A computational study of Siglec-11 and Siglec-11-derived peptides offered synergistic insight into ligand binding. These results demonstrate the potential of PSA-binding peptides for selective targeting and highlight the importance of the approaches described herein for the study of carbohydrate interactions.

Microarrays for the screening and identification of carbohydrate-binding peptides

The development of carbohydrate-binding ligands is crucial for expanding knowledge on the glycocode and for achieving systematic carbohydrate targeting. Amongst such ligands, carbohydrate-binding peptides (CBPs) are attractive for use in bioanalytical and biomedical systems due to their biochemical and physicochemical properties; moreover, given the biological significance of lectin-carbohydrate interactions, these ligands offer an opportunity to study peptide sequence and binding characteristics to inform on natural target/ligand interactions. Here, a high-throughput microarray screening technique is described for the identification and study of CBPs, with a focus on polysialic acid (PSA), a polysaccharide found on neural stem cells. The chemical and biological uniqueness of PSA suggests that an ability to exclusively target this glycan may promote a number of diagnostic and therapeutic applications. PSA-binding peptides from phage display screening and from epitope mapping of an scFv for oligosialic acid were screened in an optimized microarray format with three ligand density conditions. Hypothesis-driven mutations were additionally applied to select peptides to modulate peptide affinity and selectivity to PSA. Peptide compositional and positional analyses revealed the significance of various residues for PSA binding and suggested the importance of basic residue positioning for PSA recognition. Furthermore, selectivity studies performed directly on microarrays with chondroitin sulfate A (CS-A) demonstrated the value of screening for both affinity and selectivity in the development of CBPs. Thus, the integrated approach described, with attention to design strategy, screening, and peptide characterization, successfully identified novel PSA-binding ligands and offers a platform for the identification and study of additional polysaccharide-binding peptides.

Preparation and characterization of functionalized heparin-loaded poly-Ɛ-caprolactone fibrous mats to prevent infection with human papillomaviruses

In this study, heparin-loaded poly-ɛ-caprolactone (PCL) fibrous mats were prepared and characterized based on their physical, cytotoxic, thermal, and biological properties. The main objective of the work described here was to test the hypothesis that incorporation of heparin into a PCL carrier could serve as bio-compatible material capable of inhibiting Human Papillomavirus (HPV) infection. The idea of firmly anchoring heparin to capture soluble virus, vs. a slow heparin release to inhibit a virus in solution was tested. Thus, one material was produced via conventional heparin matrix encapsulation and electrohydrodynamic fiber processing in one step. A second type of material was obtained via heparin crosslinking. This was achieved by running a carbodiimide/N-hydroxysuccinimide (EDC/NHS) coupling reaction on preformed PCL fibers. In vitro HPV16 L1 protein binding capacity studies were performed. Infectivity assays were done using HPV16 pseudoviruses (PsVs) carrying a GFP plasmid to directly test the ability of the fibrous mats to prevent internalization of HPV PsVs. The crosslinked heparin material presented a dissociation constant (K_d) value comparable to those found in the literature for different heparin-protein L1 peptide interactions. Both materials significantly reduced internalization of HPV PsVs, with a reduction of 94% of PsVs internalization when matrix encapsulated heparin-loaded material was present. Differences in performance between the two proposed structures are discussed.

Small Molecule Antagonist of Cell Surface Glycosaminoglycans Restricts Mouse Embryonic Stem Cells in a Pluripotent State

Recently, the field of stem cell-based regeneration has turned its attention toward chemical approaches for controlling the pluripotency and differentiation of embryonic stem cells (ESCs) using drug-like small molecule modulators. Growth factor receptors or their associated downstream kinases that regulate intracellular signaling pathways during differentiation are typically the targets for these molecules. The glycocalyx, which plays an essential role in actuating responses to growth factors at the cellular boundary, offers an underexplored opportunity for intervention using small molecules to influence differentiation. Here, we show that surfen, an antagonist of cell-surface glycosaminoglycans required for growth factor association with cognate receptors, acts as a potent and general inhibitor of differentiation and promoter of pluripotency in mouse ESCs. This finding shows that drugging the stem cell Glycome with small molecules to silence differentiation cues can provide a powerful new alternative to existing techniques for controlling stem cell fate. Stem Cells 2018;36:45-54.

Secondary structure propensity and chirality of the amyloidophilic peptide p5 and its analogues impacts ligand binding - In vitro characterization

BACKGROUND

Polybasic helical peptides, such as peptide p5, bind human amyloid extracts and synthetic amyloid fibrils. When radiolabeled, peptide p5 has been shown to specifically bind amyloid in vivo thereby allowing imaging of the disease. Structural requirements for heparin and amyloid binding have been studied using analogs of p5 that modify helicity and chirality.

METHODS

Peptide-ligand interactions were studied using CD spectroscopy and solution-phase binding assays with radiolabeled p5 analogs. The interaction of a subset of peptides was further studied by using molecular dynamics simulations.

RESULTS

Disruption of the peptide helical structure reduced peptide binding to heparin and human amyloid extracts. The all-D enantiomer and the β-sheet-structured peptide bound all substrates as well as, or better than, p5. The interaction of helical and β-sheet structured peptides with Aβ fibrils was modeled and shown to involve both ionic and non-ionic interactions.

CONCLUSIONS

The α-helical secondary structure of peptide p5 is important for heparin and amyloid binding; however, helicity is not an absolute requirement as evidenced by the superior reactivity of a β-sheet peptide. The differential binding of the peptides with heparin and amyloid fibrils suggests that these molecular interactions are different. The all-D enantiomer of p5 and the β-sheet peptide are candidates for amyloid targeting reagents in vivo.

GENERAL SIGNIFICANCE

Efficient binding of polybasic peptides with amyloid is dependent on the linearity of charge spacing in the context of an α-helical secondary structure. Peptides with an α-helix or β-sheet propensity and with similar alignment of basic residues is optimal.

A D-amino acid containing peptide as a potent, noncovalent inhibitor of α5β1 integrin in human prostate cancer invasion and lung colonization

Primary tumors often give rise to disseminated tumor cells (DTC's), which acquire full malignancy after invading distant site(s). Thus, DTC's may be a productive target for preventing prostate cancer metastasis progression. Our prior research showed that PHSCN peptide (Ac-PHSCN-NH2) targets activated α5β1 integrin to prevent invasion and metastasis in preclinical adenocarcinoma models, and disease progression in Phase I clinical trial. Here, we report that D-stereoisomer replacement of histidine and cysteine in PHSCN produces a highly potent derivative, Ac-PhScN-NH2 (PhScN). PhScN was 27,000- to 150,000-fold more potent as an inhibitor of basement membrane invasion by DU 145 and PC-3 prostate cancer cells. A large increase in invasion-inhibitory potency occurred after covalent modification of the sulfhydryl group in PHSCN to prevent disulfide bond formation; while the potency of covalently modified PhScN was not significantly increased. Thus PhScN and PHSCN invasion inhibition occurs by a noncovalent mechanism. These peptides also displayed similar cell surface binding dissociation constants (Kd), and competed for the same site. Consistent with its increased invasion-inhibitory potency, PhScN was also a highly potent inhibitor of lung extravasation and colonization in athymic nude mice: it was several hundred- or several thousand-fold more potent than PHSCN at blocking extravasation by PC-3 or DU 145 cells, and 111,000- or 379,000-fold more potent at inhibiting lung colonization, respectively. Furthermore, systemic 5 mg/kg PhScN monotherapy was sufficient to cause complete regression of established, intramuscular DU 145 tumors. PhScN thus represents a potent new family of therapeutic agents targeting metastasis by DTC's to prevent parallel progression in prostate cancer.

A spoonful of sugar makes the melanoma go: the role of heparan sulfate proteoglycans in melanoma metastasis

Heparan sulfate proteoglycans (HSPGs) have been shown to regulate signaling in many systems and are of increasing interest in cancer. While these are not the only sugars to drive melanoma metastasis, HSPGs play important roles in driving metastatic signaling cascades in melanoma. The ability of these proteins to modulate ligand-receptor interactions in melanoma has been quite understudied. Recent data from several groups indicate the importance of these ligands in modulating key signaling pathways including Wnt and fibroblast growth factor (FGF) signaling. In this review, we summarize the current knowledge regarding the structure and function of these proteoglycans and their role in melanoma. Understanding how HSPGs modulate signaling in melanoma could lead to new therapeutic approaches via the dampening or heightening of key signaling pathways.

Interaction of synthetic HPV-16 capsid peptides with heparin: thermodynamic parameters and binding mechanism

Capsid proteins binding cell surface proteoglycans is a key early event in human papillomavirus (HPV) infection. The positively charged sequences at the C-terminus of the L1 protein and the N- and C-termini of the L2 protein of HPV-16 can efficiently bind to heparin receptors, which were characterized in the present study by quantitative isothermal titration calorimetry experiments primarily, fluorescence spectroscopy, and static right-angle light scattering. The binding constant, K, was at an order of magnitude of 10(7) M(-1) for the two peptides at the N- and C-termini of HPV-16 L2 and segment b at the C-terminus of HPV-16 L1, while that for other L1 analogues were of a smaller order, illustrating that the heparin binding is a typical sequence-specific and -dependent phenomenon. These results suggest that, in addition to L1, the L2 protein may participate in cell surface attachment during HPV infection. Furthermore, the calorimetry results demonstrated that hydrophobic interactions and hydrogen bonding are involved in peptide binding to heparin in addition to the essential electrostatic interactions. Meanwhile, circular dichroism spectroscopy revealed that binding to heparin does not induce obvious secondary structural changes in the peptides.

Interaction of heparin and heparin-derived oligosaccharides with synthetic peptide analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein

BACKGROUND

Although protamine is effective as an antidote of heparin, there is a need to replace protamine due to its side effects. HIP peptide has been reported to neutralize the anticoagulant activity of heparin. The interaction of HIP analog peptides with heparin and heparin-derived oligosaccharides is investigated in this paper.

METHODS

Seven analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein (HIP) were synthesized, and their interaction with heparin was characterized by heparin affinity chromatography, isothermal titration calorimetry, and NMR.

RESULTS

NMR results indicate the imidazolium groups of the His side chains of histidine-containing Hip analog peptide interact site-specifically with heparin at pH 5.5. Heparin has identical affinities for HIP analog peptides of opposite chirality. Analysis by counterion condensation theory indicates the peptide AC-SRPKAKAKAKAKDQTK-NH2 makes on average approximately 3 ionic interactions with heparin that result in displacement of approximately 2 Na+ ions, and ionic interactions account for approximately 46% of the binding free energy at a Na+ concentration of 0.15 M.

CONCLUSIONS

The affinity of heparin for the peptides is strongly dependent on the nature of the cationic side chains and pH. The thermodynamic parameters measured for the interaction of HIP peptide analogs with heparin are strongly dependent on the peptide sequence and pH.

GENERAL SIGNIFICANCE

The information obtained in this research will be of use in the design of new agents for neutralization of the anticoagulant activity of heparin. The site-specific binding of protonated histidine side chains to heparin provides a molecular-level explanation for the pH-dependent binding of beta-amyloid peptides by heparin and heparan sulfate proteoglycan and may have implications for amyloid formation.

Surfen, a small molecule antagonist of heparan sulfate

In a search for small molecule antagonists of heparan sulfate, we examined the activity of bis-2-methyl-4-amino-quinolyl-6-carbamide, also known as surfen. Fluorescence-based titrations indicated that surfen bound to glycosaminoglycans, and the extent of binding increased according to charge density in the order heparin > dermatan sulfate > heparan sulfate > chondroitin sulfate. All charged groups in heparin (N-sulfates, O-sulfates, and carboxyl groups) contributed to binding, consistent with the idea that surfen interacted electrostatically. Surfen neutralized the anticoagulant activity of both unfractionated and low molecular weight heparins and inhibited enzymatic sulfation and degradation reactions in vitro. Addition of surfen to cultured cells blocked FGF2-binding and signaling that depended on cell surface heparan sulfate and prevented both FGF2- and VEGF(165)-mediated sprouting of endothelial cells in Matrigel. Surfen also blocked heparan sulfate-mediated cell adhesion to the Hep-II domain of fibronectin and prevented infection by HSV-1 that depended on glycoprotein D interaction with heparan sulfate. These findings demonstrate the feasibility of identifying small molecule antagonists of heparan sulfate and raise the possibility of developing pharmacological agents to treat disorders that involve glycosaminoglycan-protein interactions.

A novel assay to probe heparin-peptide interactions using pentapeptide-stabilized gold nanoparticles

In this article, we present a novel assay to probe the interactions between heparin and heparin-binding peptides based on CALNN pentapeptide-stabilized gold nanoparticles. This assay relies on rapid aggregation of gold nanoparticles and dramatic retardation in the presence of a large excess of heparin due to the binding of peptides to heparin. Using this method, the dissociation constant ( K d) and melting temperature ( T m) of three different peptides against heparin were determined. The results from capillary electrophoresis demonstrated that K d values measured by this method were comparatively accurate. It was found that the peptide with the lowest K d did not have the highest T m. Structural analysis by circular dichroism was performed to explain this phenomenon. A comparison with the results from affinity chromatography indicates that electrostatic interactions only are not the major determinant of the affinity between heparin and peptide, but other interactions such as hydrogen-bonding and hydrophobic interactions may play important roles in the overall interactions. This novel assay is inexpensive, label-free, and easy to implement in the laboratories, does not suffer precipitation of the heparin-peptide complex or their conformational changes caused by surface immobilization, and is expected to be a useful complement to other existing methods.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

Melittin interaction with sulfated cell surface sugars

Melittin is a 26-residue cationic peptide with cytolytic and antimicrobial properties. Studies on the action mechanism of melittin have focused almost exclusively on the membrane-perturbing properties of this peptide, investigating in detail the melittin-lipid interaction. Here, we report physical-chemical studies on an alternative mechanism by which melittin could interact with the cell membrane. As the outer surface of many cells is decorated with anionic (sulfated) glycosaminoglycans (GAGs), a strong Coulombic interaction between the two oppositely charged molecules can be envisaged. Indeed, the present study using isothermal titration calorimetry reveals a high affinity of melittin for several GAGs, that is, heparan sulfate (HS), dermatan sulfate, and heparin. The microscopic binding constant of melittin for HS is 2.4 x 10 (5) M (-1), the reaction enthalpy is Delta H melittin (0) = -1.50 kcal/mol, and the peptide-to-HS stoichiometry is approximately 11 at 10 mM Tris, 100 mM NaCl at pH 7.4 and 28 degrees C. Delta H melittin (0) is characterized by a molar heat capacity of Delta C P (0) = -227 cal mol (-1) K (-1). The large negative heat capacity change indicates that hydrophobic interactions must also be involved in the binding of melittin to HS. Circular dichroism spectroscopy demonstrates that the binding of the peptide to HS induces a conformational change to a predominantly alpha-helical structure. A model for the melittin-HS complex is presented. Melittin binding was compared with that of magainin 2 and nisin Z to HS. Magainin 2 is known for its antimicrobial properties, but it does not cause lysis of the eukaryotic cells. Nisin Z shows activity against various Gram-positive bacteria. Isothermal titration calorimetry demonstrates that magainin 2 and nisin Z do not bind to HS (5-50 degrees C, 10 mM Tris, and 100 mM NaCl at pH 7.4).