Inhibition of fibrinogen binding to GP IIb-IIIa by a GP IIIa peptide

Application of Funnel Metadynamics to the Platelet Integrin αIIbβ3 in Complex with an RGD Peptide

Integrin αIIbβ3 mediates platelet aggregation by binding the Arginyl-Glycyl-Aspartic acid (RGD) sequence of fibrinogen. RGD binding occurs at a site topographically proximal to the αIIb and β3 subunits, promoting the conformational activation of the receptor from bent to extended states. While several experimental approaches have characterized RGD binding to αIIbβ3 integrin, applying computational methods has been significantly more challenging due to limited sampling and the need for a priori information regarding the interactions between the RGD peptide and integrin. In this study, we employed all-atom simulations using funnel metadynamics (FM) to evaluate the interactions of an RGD peptide with the αIIb and β3 subunits of integrin. FM incorporates an external history-dependent potential on selected degrees of freedom while applying a funnel-shaped restraint potential to limit RGD exploration of the unbound state. Furthermore, it does not require a priori information about the interactions, enhancing the sampling at a low computational cost. Our FM simulations reveal significant molecular changes in the β3 subunit of integrin upon RGD binding and provide a free-energy landscape with a low-energy binding mode surrounded by higher-energy prebinding states. The strong agreement between previous experimental and computational data and our results highlights the reliability of FM as a method for studying dynamic interactions of complex systems such as integrin.

The influence of hydrogen ions on coagulation in traumatic brain injury, explored by molecular dynamics

Background: Patients in intensive care units with traumatic brain injuries (TBI) frequently present acid-base abnormalities and coagulability disorders, which complicate their condition.Objective: To identify protonation through in silico simulations of molecules involved in the process of coagulation in standard laboratory tests.Materials and methods: Ten patients with TBI were selected from the intensive care unit in addition to ten "healthy control subjects", and another nine patients as "disease control subjects"; the latter being a comparative group, corresponding to subjects with diabetes mellitus 2 (DM2). Fibrinogen, FVII, FVIII, FIX, FX, and D-dimer in the presence of acidification were evaluated in 20 healthy subjects in order to compare clinical results with molecular dynamics (MD), and to explain proton interactions and coagulation molecules.Results: The TBI group presented a slight, non-significant increase in D-dimer; but this was not present in "disease control subjects". Levels of fibrinogen, FVII, FIX, FX, and D-dimer were affected in the presence of acidification. We observed that various specific residues of coagulation factors "trap" ions.Conclusion: Protonation of tissue factor and factor VIIa may favor anticoagulant mechanisms, and protonation does not affect ligand binding sites of GPIIb/IIIa (PAC1) suggesting other causes for the low affinity to PAC1.

Analyzing Integrin-Dependent Adhesion

In this unit, methods for the analysis of integrin-dependent adhesion are described. Two major types of assays are commonly used for this analysis. The first are cell adhesion assays. A key application of this type of assay is to identify which integrin(s) mediate cell-substrate interactions; a comprehensive list of antibodies suitable for this purpose is detailed. The second are solid-phase assays in which purified integrins and integrin ligands are used. These assays can be used, e.g., to measure apparent affinities of integrins for different ligands and IC₅₀ values of pharmacological inhibitors. Curr. Protoc. Cell Biol. 53:9.4.1-9.4.17. © 2018 by John Wiley & Sons, Inc.

Effect of amino acids near the RGD sequence on binding activities between αIIbβ3 integrin and fibrinogen in the presence of RGD-containing synthetic peptides from elegantin and angustatin

Elegantin and angustatin, which were isolated from the snake venoms of Protobothrops elegans and Dendroaspis angusticeps, markedly inhibit binding between platelet integrins and fibrinogen via the Arg-Gly-Asp (RGD) sequence. Angustatin, which is a three-finger toxin containing the RGD sequence, inhibits platelet aggregation almost ten times more strongly than disintegrin isolated from the venoms of Viperidae and Crotalidae. The RGD sequences of both polypeptides are located at the top of hairpin loops, and the composition of the RGD loop is very important for binding to integrin. We investigated the effects of synthetic RGD loop peptides from angustatin and elegantin on ADP- or collagen-induced platelet aggregation and αIIbβ3-fibrinogen binding. Synthetic angustatin (PRGDMP)-type peptides inhibited platelet aggregation more strongly than elegantin (ARGDDX)-type peptides. In particular, the cyclic angustatin peptide (CPRGDMPC) inhibited ADP- and collagen-induced platelet aggregation at least 10-50 times more strongly than the other peptides. The cyclic angustatin peptide (CPRGDMPC) was also the strongest inhibitor of binding between αIIbβ3 and fibrinogen, the IC₅₀ of this peptide was approximately 2.58μM. Regarding the inhibition of binding between αIIbβ3 and fibrinogen, CPRGDMPC demonstrated a stronger inhibitory and more stable effect in the presence of Mg²⁺ than in the presence of Ca²⁺.

Molecular Mechanisms of Prothrombotic Risk Due to Genetic Variations in Platelet Genes: Enhanced Outside-In Signaling Through the Pro33 Variant of Integrin β3

In recent years inherited variations in platelet proteins have emerged as potential risk factors that could predispose individuals to arterial thrombosis. Although many studies have examined the association of platelet gene polymorphisms with particular disease states, the underlying mechanisms by which most of these polymorphisms contribute to the pathophysiology of thrombosis have remained largely unexplored. This review will focus on the cellular and molecular features by which these genetic changes affect platelet physiology. Although many genes have been investigated in this regard, only the genes encoding integrins β3 and α2, and the platelet Fc receptor, FcγRIIA, have been studied in any depth. In some cases (such as integrin α2), evidence supports a quantitative trait locus. For other genes, nonsynonymous nucleotide substitutions lead to structural and functional consequences. A large portion of this review will focus on the widely studied Leu33Pro (PIA) polymorphism of integrin β3, and will consider the potential mechanisms by which the Pro33 polymorphism could induce a prothrombotic risk. A detailed understanding of how polymorphisms modulate platelet physiology will be important for understanding individual differences in response to antiplatelet therapy.

Clinical, Biochemical, and Molecular Aspects of Glanzmann's Thrombasthenia in Humans and Dogs

Glanzmann's thrombasthenia (GT) is an inherited, intrinsic platelet function defect that involves the platelet glycoprotein complex IIb–IIIa, also known as the fibrinogen receptor and the integrin αIIbβ3. The defect was originally described by Dr. Glanzmann in humans in 1918 as a bleeding disorder that differed clinically from other known coagulopathies. Over the decades that followed, researchers determined the biochemical and molecular basis for the disease in humans. Otterhounds with thrombasthenic thrombopathia, described in the 1960s, were the only animal model that closely resembled the disease described in humans until 1996. At that time, a Great Pyrenees dog was identified with unequivocal clinical and biochemical features of Type I GT. The cDNA encoding for glycoproteins IIb and IIIa were sequenced in normal dogs in 1999, allowing for identification of specific mutations causing Type I GT in both Otterhounds and Great Pyrenees dogs. Knowing the molecular basis for Type I GT in dogs as well as the cDNA sequences in normal dogs should enhance the understanding of structure/function relationships of the αIIbβ3 integrin and provide an excellent animal model for studies aimed at correction of GT in humans. The following review focuses on the structure and function of this platelet receptor and reviews the molecular, biochemical, and clinical aspects of Glanzmann's thrombasthenia in humans and dogs.

Hemocompatibility of Inorganic Physical Vapor Deposition (PVD) Coatings on Thermoplastic Polyurethane Polymers

Biocompatibility improvements for blood contacting materials are of increasing interest for implanted devices and interventional tools. The current study focuses on inorganic (titanium, titanium nitride, titanium oxide) as well as diamond-like carbon (DLC) coating materials on polymer surfaces (thermoplastic polyurethane), deposited by magnetron sputtering und pulsed laser deposition at room temperature. DLC was used pure (a-C:H) as well as doped with silicon, titanium, and nitrogen + titanium (a-C:H:Si, a-C:H:Ti, a-C:H:N:Ti). In-vitro testing of the hemocompatibility requires mandatory dynamic test conditions to simulate in-vivo conditions, e.g., realized by a cone-and-plate analyzer. In such tests, titanium- and nitrogen-doped DLC and titanium nitride were found to be optimally anti-thrombotic and better than state-of-the-art polyurethane polymers. This is mainly due to the low tendency to platelet microparticle formation, a high content of remaining platelets in the whole blood after testing and low concentration of platelet activation and aggregation markers. Comparing this result to shear-flow induced cell motility tests with e.g., Dictostelium discoideum cell model organism reveals similar tendencies for the investigated materials.

Peptide LSARLAF induces integrin β3 dependent outside-in signaling in platelets

INTRODUCTION

Peptide LSARLAF (LSA) can bind and activate integrin αIIbβ3 in the absence of 'inside-out' signal. The active αIIbβ3 mediates 'outside-in' signaling that elicits platelet aggregation, granule secretion and TxA2 production. Here we identify the membrane glycoproteins which mediate LSA-induced platelet activation other than αIIbβ3, and determine the roles of Src, PLCγ2, FcRγ-chain, and SLP-76 in LSA-induced platelet activation.

METHOD

Ligand-receptor binding assay was performed to study the effect of peptide LSA or its control peptide FRALASL (FRA) on integrins binding to their ligands. Spreading of CHO cells expressing αIIbβ3 or αVβ3 on immobilized fibrinogen was measured in the presence of LSA or FRA. Washed β3, Src, FcRγ-chain, LAT and SLP-76 deficient platelets aggregation and secretion were tested in response to LSA.

RESULTS

Ligand-receptor binding assay indicated that LSA promoted the binding of multiple ligands to αIIbβ3 or αVβ3. LSA also enhanced CHO cells with αIIbβ3 or αVβ3 expression spreading on immobilized fibrinogen. β3 deficient platelets failed to aggregate and secrete in response to LSA. The phosphorylation of PLCγ2 and Syk was also β3 dependent. Src, FcRγ-chain, LAT and SLP-76 deficient platelets did not aggregate, secrete ATP or produce TxA2 in response to LSA.

CONCLUSION

LSA-induced platelet activation is β3 dependent, and signaling molecules Src, FcRγ-chain, SLP-76 and LAT play crucial roles in LSA-induced β3 mediated signaling.

Advantages of RGD peptides for directing cell association with biomaterials

Despite many years of in vitro research confirming the effectiveness of RGD in promoting cell attachment to a wide variety of biomaterials, animal studies evaluating tissue responses to implanted RGD-functionalized substrates have yielded more variable results. The goals of this report are to present some of the reasons why cell culture studies may not always reliably predict in vivo responses, and more importantly, to highlight potential applications that may benefit from the use of RGD peptides.

Dynamic regulation of fibrinogen: integrin αIIbβ3 binding

This study demonstrates that two orthogonal events regulate integrin αIIbβ3's interactions with fibrinogen, its primary physiological ligand: (1) conformational changes at the αIIb-β3 interface and (2) flexibility in the carboxy terminus of fibrinogen's γ-module. The first postulate was tested by capturing αIIbβ3 on a biosensor and measuring binding by surface plasmon resonance. Binding of fibrinogen to eptifibatide-primed αIIbβ3 was characterized by a k(on) of ~2 × 10(4) L mol(-1) s(-1) and a k(off) of ~8 × 10(-5) s(-1) at 37 °C. In contrast, even at 150 nM fibrinogen, no binding was detected with resting αIIbβ3. Eptifibatide competitively inhibited fibrinogen's interactions with primed αIIbβ3 (K(i) ~0.4 nM), while a synthetic γ-module peptide (HHLGGAKQAGDV) was only weakly inhibitory (K(i) > 10 μM). The second postulate was tested by measuring αIIbβ3's interactions with recombinant fibrinogen, both normal (rFgn) and a deletion mutant lacking the γ-chain AGDV sites (rFgn γΔ408-411). Normal rFgn bound rapidly, tightly, and specifically to primed αIIbβ3; no interaction was detected with rFgn γΔ408-411. Equilibrium and transition-state thermodynamic data indicated that binding of fibrinogen to primed αIIbβ3, while enthalpy-favorable, must overcome an entropy-dominated activation energy barrier. The hypothesis that fibrinogen binding is enthalpy-driven fits with structural data showing that its γ-C peptide and eptifibatide exhibit comparable electrostatic contacts with αIIbβ3's ectodomain. The concept that fibrinogen's αIIbβ3 targeting sequence is intrinsically disordered may explain the entropy penalty that limits its binding rate. In the hemostatic milieu, platelet-platelet interactions may be localized to vascular injury sites because integrins must be activated before they can bind their most abundant ligand.

Purification and characterization of a new platelet aggregation inhibitor with dissociative effect on ADP-induced platelet aggregation, from the venom of Protobothrops elegans (Sakishima-habu)

A platelet aggregation inhibitor, named snake venom platelet aggregation dissociator (SV-PAD)-1, with a dissociative reaction of ADP-induced platelet aggregation, was purified from the venom of Protobothrops elegans (Sakishima-habu) by gel-filtration employing Sephadex G-100, and ion-exchange chromatographies using DEAE-Sepharose Fast Flow, CM-Sepharose Fast Flow, and Mono S. By this procedure, about 1.5mg of purified protein was obtained from 1.0g of P. elegans venom. The purified protein showed a single protein band and the molecular weight was about 110kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. The pI of purified protein showed four-bands of 7.7, 7.8, 7.95, and 8.15. This protein strongly inhibited ADP-induced platelet aggregation in rabbit platelet-rich plasma (PRP), and its IC(50) was about 58nM. It inhibited ristocetin-induced platelet aggregation in rabbit PRP (IC(50): 100nM), but hardly blocked collagen-induced platelet aggregation. This protein promptly dissociated platelet aggregation in rabbit PRP stimulated by high-concentration ADP.

Solid phase assays for studying ECM protein-protein interactions

Solid-phase assays provide a simple, rapid and robust method for the analysis of protein-protein interactions; i.e., does protein A interacts with protein B? In this assay, protein A (here termed as 'receptor') is adsorbed to the wells of an enzyme-linked immunosorbent assay (ELISA) plate (solid phase). The plate is then blocked using bovine serum albumin (BSA), and biotin-labelled protein B (here termed as 'ligand') is added. After washing the wells to remove unbound ligand, bound ligand is detected by addition of an avidin-peroxidase conjugate followed by a colorimetric detection step. This type of assay is particularly well suited for studying the interaction of ECM proteins with integrins. The screening of antagonists of integrin-ligand interactions in the pharmaceutical industry is an important area in which this assay is finding use.

Analyzing integrin-dependent adhesion

This unit describes methods for the analysis of integrin-ligand binding in both cell-based assays and solid-phase assays. A major application of cell adhesion assays is in investigating whether a certain cell type can adhere to a specific adhesive substrate, and, if so, which receptors are involved. Particularly if the substrate is a matrix component (e.g., fibronectin), members of the integrin family are likely to play a dominant role in adhesion. Procedures are described here for assessing which integrins are involved in this process. A detailed analysis of ligand recognition by individual integrins can be performed using a solid-phase receptor-ligand binding assay. The unit also contains support protocols for integrin purification and coupling of antibodies to Sepharose for use in this purification, as well as for biotinylation of integrin ligands to be used in the solid-phase assay.

Streptococcal pyrogenic exotoxin B-induced apoptosis in A549 cells is mediated through alpha(v)beta(3) integrin and Fas

Our previous work suggested that streptococcal pyrogenic exotoxin (SPE) B-induced apoptosis is mediated through a receptor-like mechanism. In this study, we have identified alpha(v)beta(3) and Fas as the SPE B receptors for this function. The SPE B fragment without the RGD motif and G308S, a SPE B mutant with the RSD motif, induced less apoptosis than did native SPE B, suggesting that the RGD motif is critical for SPE B-induced apoptosis. Fluorescein isothiocyanate-SPE B binding assays and immunoprecipitation analysis showed that SPE B specifically interacted with alpha(v)beta(3). Anti-alpha(v)beta(3) antibody partially inhibited SPE B-induced apoptosis but had no effect on G308S-induced apoptosis. In addition, Fas binding to SPE B was verified in an affinity column and an immunoprecipitation analysis. Anti-Fas antibody inhibited SPE B- and G308S-induced apoptosis in a dose-dependent manner, suggesting that Fas-mediated SPE B-induced apoptosis also occurs RGD independently. Both anti-alpha(v)beta(3) and anti-Fas antibodies synergistically inhibited SPE B-induced apoptosis. The apoptotic cascades were activated by SPE B and G308S, with a little delay by the latter. After SPE B binding, the cell surface level of alpha(v)beta(3), but not of Fas, was decreased. The decreased alpha(v)beta(3) level was restored by treatment with the proteasome inhibitor MG132, suggesting a SPE B-mediated endocytosis of integrin alpha(v)beta(3) via the ubiquitin-proteasome system. Taken together, our results demonstrate that SPE B-induced apoptosis is mediated through alpha(v)beta(3) integrin and Fas in a synergistic manner.

Tyr178 of beta3 is critical for alphaIIb maturation and macromolecular ligand binding to alphaIIbbeta3

To explore the structural basis of ligand binding to alphaIIbbeta3, we conducted a site-directed mutagenesis of Y178, which is located in the ligand-specificity region (C177-C184) of the beta3 subunit. Two mutant beta3 constructs, Y178A and Y178I, were transfected into CHO cells and co-expressed with human alphaIIb subunit on the cell surface. Our results showed that the Y178A mutation affected processing and cell surface exposure of recombinant alphaIIbbeta3 receptor, abrogated the binding of PAC-1, a ligand-mimetic antibody, to alphaIIbbeta3 pre-treated with the known activator DTT. The Y178A mutation also resulted in reduced adhesion of alphaIIbbeta3 on immobilized fibrinogen. In contrast, the interaction of alphaIIbbeta3 with the small molecular ligand RGDS was unaffected by Y178A mutation, as evidenced by the elevated LIBS-1 epitope expression following RGDS addition. Interestingly however, Y178I mutation did not affect the receptor synthesis and function at all. As for post-receptor occupancy, neither Y178A nor Y178I prevented alphaIIbbeta3 translocation to focal adhesion contacts. These results suggest that Y178 is involved in alphaIIb maturation and alphaIIbbeta3 complex expression. This residue is also critical for alphaIIbbeta3 interaction with its macromolecular ligand or ligand-mimetic mAb, possibly due to its involvement in other ligand-binding sites distinct from the RGD-binding pocket. We also propose that a residue with appropriate side-chain size and hydrophobicity at position 178 is spatially required for formation of the correct tertiary structure of the site.

Molecular characterization and expression analysis of the gene coding for the porcine beta(3) integrin subunit (CD61)

Integrins are heterodimeric cell adhesion molecules with major roles in a variety of biological processes ranging from cell migration to tissue organization, immune and non-immune defense mechanisms and oncogenic transformation. Members of the beta(3) integrin subfamily are composed of a beta(3) subunit (CD61) non-covalently associated with two alpha subunits, alpha(IIb) (CD41) and alpha(v) (CD51), to constitute a group of transmembrane glycoproteins that participate in many physiologically important events. This investigation has focused on the molecular characterization of the cDNA encoding the porcine beta(3) integrin subunit. The deduced 762-amino acid sequence was 93, 92, 91, 89, 79 and 73% homologous to human, dog, rabbit, mouse, chicken and Xenopus laevis CD61 protein, respectively. Porcine CD61 molecule shares many structural features with human CD61, including a region containing a metal ion-dependent adhesion site (MIDAS) folding into an I domain-like structure. Through PCR-SSCP analysis and sequencing, six polymorphic positions were detected in the cDNA sequence of porcine CD61, and their frequencies were observed from a collection of 47 pigs. Expression analysis was done at two different levels: expression of the CD61 mRNA by RT-PCR and localization of the protein by immunohistochemistry. Our results show that CD61 transcripts were detected mainly in platelets and hematopoietic tissues. The immunohistochemical tissue localization of CD61 protein by a specific monoclonal antibody against CD61 recombinant protein showed that CD61 was expressed on vascular and non-vascular smooth muscle, epithelium and myeloid cells, being undetectable in cells of the lymphoid lineage. Furthermore, pulmonary intravascular macrophages (PIM), a subpopulation of macrophages which seem to play an important role in blood clearance, expressed much more CD61 when compared to pulmonary alveolar macrophages (PAM). The knowledge of the structure and distribution of the CD61 provides insight into the physiological function of the porcine beta(3) integrins and should be of importance in understanding the role of this integrin family in biological processes.

Discovery of +(2-{4-[2-(5,6,7,8-tetrahydro-1,8-naphthyridin-2-yl)ethoxy]phenyl}-cyclopropyl)acetic acid as potent and selective αvβ3 inhibitor: Design, synthesis, and optimization

The integrin alpha(v)beta(3) is expressed in a number of cell types and is thought to play a major role in several pathological conditions. Various small molecules that inhibit the integrin have been shown to suppress tumor growth and retinal angiogenesis. The tripeptide Arg-Gly-Asp (RGD), a common binding motif in several ligands that bind to alpha(v)beta(3), has been depeptidized and optimized in our efforts toward discovering a small molecule inhibitor. We recently disclosed the synthesis and biological activity of several small molecules that did not contain any peptide bond and mimic the tripeptide RGD. The phenethyl group in one of the lead compounds was successfully replaced with a cyclopropyl moiety. The new lead compound was optimized for potency, selectivity, and for its ADME properties. We describe herein the discovery, synthesis, and optimization of cyclopropyl containing analogs that are potent and selective inhibitors of alpha(v)beta(3).

Neuroglian on hemocyte surfaces is involved in homophilic and heterophilic interactions of the innate immune system of Manduca sexta

Neuroglian, a member of the L1 family of cell adhesion molecules (L1-CAMs), is expressed on surfaces of granular cells and a subset of large plasmatocytes of Manduca sexta that act as foci for hemocyte aggregation during the innate immune response. Neuroglian expressed on surfaces of transfected Sf9 cells induced their homophilic aggregation, with the aggregation being abolished in the presence of recombinant immunoglobulin (Ig) domains of neuroglian. Neuroglian and its Ig domains also can interact with hemocyte-specific integrin (HS integrin) as demonstrated with an enzyme-linked immunoassay and a surface plasmon resonance (SPR) assay. Neuroglian double-stranded (ds) RNA not only depresses expression of neuroglian in hemocytes but also depresses the cell-mediated encapsulation response of these hemocytes to foreign surfaces. After injection of a monoclonal antibody (MAb 3B11) into M. sexta larvae that recognizes the Ig domains of neuroglian, the cell-mediated encapsulation response of hemocytes was likewise inhibited. The Ig domains of neuroglian are involved in both homophilic and heterophilic interactions, and subsets of these six different Ig domains may affect different functions of neuroglian.

Natural inhibitors targeting osteoclast-mediated bone resorption

Human cathepsin K, matrix metalloproteinase 9, and alpha(V)beta(3) integrin are the key regulators in osteoclast-mediated bone resorption. In this paper, we found natural inhibitors 1-10 for them by enzyme inhibition assays. Inhibitors 1-7, 8-9, and 10 are novel inhibitors of human cathepsin K, matrix metalloproteinase 9, and alpha(V)beta(3), respectively.

Disabled-2 is a novel αIIb-integrin-binding protein that negatively regulates platelet-fibrinogen interactions and platelet aggregation

Platelet aggregation plays a pivotal role in the haemostatic process and is involved in the pathological counterpart of arterial thrombosis. We have shown that the adapter protein disabled-2 (DAB2) is expressed abundantly in platelets. In this study, DAB2 was found to distribute in the platelet α-granules and was released from the granular compartment upon platelet activation. The secreted DAB2 binds to the extracellular region of αIIbβ3 integrin on the platelet surface through the phosphotyrosine-binding domain. The DAB2-platelet interactions result in the inhibition of agonist-induced platelet aggregation with the exception of thrombin, a DAB2 protease that renders DAB2 inactive. Biochemical and mutational analysis revealed that the DAB2 cell-adhesion Arg-Gly-Asp (RGD) motif (amino acid residues 64-66) and the αIIb-integrin–fibrinogen-binding region (amino acid residues 171-464) are important for the DAB2-platelet interactions. Such interactions compete for the binding of αIIb integrin with fibrinogen and provide a mechanism for DAB2 to inhibit platelet aggregation. Accordingly, the synthetic RGD-motif-containing DAB2 peptide PDARGDKM also elicited anti-platelet aggregation activity. These findings demonstrate for the first time that DAB2 is an αIIb-integrin-binding protein that plays a novel role in the control of platelet-fibrinogen interactions and platelet aggregation.

The Specificity and Function of the Metal-binding Sites in the Integrin β3 A-domain

The A-domains within integrin beta subunits contain three metal sites termed the metal ion-dependent adhesion site (MIDAS), site adjacent to the metal ion-dependent adhesion site (ADMIDAS), and ligand-induced metal-binding site (LIMBS), and these sites are involved in ligand engagement. The selectivity of these metal sites and their role in ligand binding have been investigated by expressing a fragment corresponding to the beta3 A-domain, beta3-(109-352), and single point mutants in which each of the cation-binding sites has been disabled. Equilibrium dialysis experiments identified three Mn2+- and two Ca2+-binding sites with the LIMBS being the site that did not bind Ca2+. Although the ADMIDAS could bind Ca2+, it did not bind Mg2+. These results indicate that the Ca2+-specific site that inhibits ligand binding is the ADMIDAS. Two different assay systems, surface plasmon resonance and a microtiter plate assay, demonstrated that the beta3 A-domain fragment bound fibrinogen in the presence of 0.1 mm Ca2+ but not in 3 mm Ca2+. This behavior recapitulated the effects of Ca2+ on fibrinogen binding to alphavbeta3 but not alphaIIbbeta3. Disabling any of the three cation-binding sites abrogated fibrinogen binding. These results indicate that the specificities of the three metal-binding sites for divalent cations are distinct and that each site can regulate the ligand binding potential of the beta3 A-domain.

Mutations in GPIIIa molecule as a cause for Glanzmann thrombasthenia in Indian patients

BACKGROUND

Glanzmann thrombasthenia (GT) results from a quantitative or qualitative defect of GPIIb-IIIa complex, the fibrinogen receptor on platelets, which plays a very important role in platelet aggregation. In this report we describe the molecular studies on 22 patients with Glanzmann Thrombasthenia at our institute.

OBJECTIVES

The main objective was to identify the mutations present in our GT population in order to establish a strategy for genetic counseling and antenatal diagnosis.

METHODS

Twenty-two patients with GT were included in the present study. Complete blood count (CBC), platelet aggregation, flow cytometry, Western blot, single strand conformation polymorphism (SSCP) and denaturing gradient gel electrophoresis (DGGE) were performed in all the patients. The patients showing an abnormal migration pattern in SSCP or DGGE were sequenced further on an automated sequencer.

RESULTS

Of the 22 patients studied, mutations were detected in 12 individuals. Of these, 11 were novel mutations and one mutation Y115C was reported earlier. Flow cytometric analysis showed the absence of receptors in type I GT, highly reduced levels in type II GT and normal levels in type III GT. The DGGE analysis and SSCP analysis of the patients showed different migration patterns. Sequencing was performed in all patients showing an abnormal migration pattern. Of the 22 cases studied mutations could be detected in 12 cases of GT. We could detect six patients with point mutations, four patients with insertions and five patients with deletion mutations. Exon 4 has been found to be the most common site for mutations in our patients.

CONCLUSION

This study has shown a wide array of mutations present in our GT patients which would be extremely useful in genetic counseling and prenatal diagnosis, essential in preventing these disorders in succeeding generations.

Discovery of Novel 2,8-Diazaspiro[4.5]decanes as Orally Active Glycoprotein IIb-IIIa Antagonists

In our efforts to develop orally active GPIIb-IIIa antagonists with improved pharmaceutical properties, we have utilized a novel 2,8-diazaspiro[4.5]decane scaffold as a template. We describe here our investigation of a variety of templates including spiropiperidinyl-gamma-lactams, spiropiperidinylimide, spiropiperidinylureas, and spiropiperidinylhydantoins. With the appropriate acidic and basic pharmacophores in place, each template yielded analogues with potent GPIIb-IIIa inhibitory activity. One of the compounds, 59 (CT50787), was also used to demonstrate for the first time the use of a pharmacological agent which is alphaIIbbeta3 specific to display biological activity in a lower species such as mouse and to extend bleeding times. Evaluation of the pharmacokinetic properties of selected compounds from each series in rat, dog, and cynomolgus monkey has led to the identification of 22 (CT51464), a double prodrug, with excellent pharmacokinetic properties. It exhibited good pharmacokinetic profile across species (F% = 33 (Cyno), 73 (dog), 22 (rat); t(1/2)(beta)() = 14.2 h (Cyno), 8.97 h (dog), 1.81 h (rat)). The biologically active form, 23 (CT50728), displayed inhibition of platelet aggregation in platelet rich plasma (PRP) with an IC(50) value of 53 nM in citrate buffer, 110 nM in PPACK anticoagulated PRP, and 4 nM in solid-phase GPIIb-IIIa competition binding assay (ELISA). Both 23 and 22 were stable in human liver microsomes, did not inhibit the P450 3A4 isozyme, and had low protein binding (18.22% for 23) and a desirable log P (0.45 +/- 0.06 for 22, and -0.91 +/- 0.32 for 23). It is predicted that the high oral bioavailability for these compounds in multiple species should translate into lower intra- and intersubject variability in man. The long plasma half-life of the lead is consistent with once or twice daily administration for chronic therapy. Analogue 22 (CT51464) thus appears to be a promising oral GPIIb-IIIa inhibitor with significantly improved pharmacokinetic properties over the previously described clinical candidates and may be found useful in the treatment of arterial occlusive disorders.

Identification of a novel binding site for platelet integrins alpha IIb beta 3 (GPIIbIIIa) and alpha 5 beta 1 in the gamma C-domain of fibrinogen

The interactions of platelets with fibrinogen mediate a variety of responses including adhesion, platelet aggregation, and fibrin clot retraction. Whereas it was assumed that interactions of the platelet integrin alpha IIb beta 3 with the AGDV sequence in the gamma C-domain of fibrinogen and/or RGD sites in the A alpha chains are involved in clot retraction and adhesion, recent data demonstrated that fibrinogen lacking these sites still supported clot retraction. These findings suggested that an unknown site in fibrinogen and/or other integrins participate in clot retraction. Here we have identified a sequence within gamma C that mediates binding of fibrinogen to platelets. Synthetic peptide duplicating the 365-383 sequence in gamma C, designated P3, efficiently inhibited clot retraction in a dose-dependent manner. Furthermore, P3 supported platelet adhesion and was an effective inhibitor of platelet adhesion to fibrinogen fragments. Analysis of overlapping peptides spanning P3 and mutant recombinant gamma C-domains demonstrated that the P3 activity is contained primarily within gamma 370-383. Integrins alpha IIb beta 3 and alpha 5 beta 1 were implicated in recognition of P3, since platelet adhesion to the peptide was blocked by function-blocking monoclonal antibodies against these receptors. Direct evidence that alpha IIb beta 3 and alpha 5 beta 1 bind P3 was obtained by selective capture of these integrins from platelet lysates using a P3 affinity matrix. Thus, these data suggest that the P3 sequence in the gamma C-domain of fibrinogen defines a previously unknown recognition specificity of alpha IIb beta 3 and alpha 5 beta 1 and may function as a binding site for these integrins.

Isolation and molecular cloning of a secreted hookworm platelet inhibitor from adult Ancylostoma caninum

Hookworms, bloodfeeding intestinal nematodes, are a leading cause of iron deficiency anemia in the developing world. These parasites have evolved potent mechanisms of interfering with mammalian hemostasis, presumably for the purpose of facilitating bloodfeeding. Adult Ancylostoma caninum worm extracts contain an activity that inhibits platelet aggregation and adhesion by blocking the function of two cell surface integrin receptors, Glycoprotein IIb/IIIa and GPIa/IIa. Using rpHPLC, the hookworm platelet inhibitor activities have been purified from protein extracts of A. caninum. Because the two inhibitory activities co-purified through multiple chromatographic steps, have similar molecular masses and share identical N-terminal as well as internal amino acid sequence homology, it is likely that they represent a single gene product. A cDNA corresponding to the purified hookworm platelet inhibitor (HPI) protein has been cloned from adult A. caninum RNA, and the translated amino acid sequence shows significant homology to Neutrophil Inhibitory Factor and Ancylostoma Secreted Proteins, suggesting that these related hookworm proteins represent a novel class of integrin receptor antagonists. Polyclonal antibodies raised against the recombinant HPI protein recognize corresponding native proteins in A. caninum extracts and excretory/secretory products, and immunohistochemistry data have identified the cephalic glands as the major source of the inhibitor within the adult hookworm. These data suggest that HPI is secreted by the adult stage of the parasite at the site of intestinal attachment. As such, it may represent a viable target for a vaccine-based strategy aimed at interfering with hookworm-induced gastrointestinal hemorrhage and iron deficiency anemia.

Binding model for nonpeptide antagonists of alpha(v)beta(3) integrin

A binding model for nonpeptide antagonists of integrin alpha(v)beta(3) has been developed through docking analyses utilizing the MMFFs force field and the recently published crystal structure, 1JV2. Results of this docking study have led to the identification of a novel binding model for selective antagonists of alpha(v)beta(3) over alpha(IIb)beta(3) integrins. Four different chemical classes are shown to bind in a similar fashion providing a measure of confidence in the proposed model. All alpha(v)beta(3) and alpha(IIb)beta(3) antagonists have a basic nitrogen separated some distance from a carboxylic acid to mimic RGD. For the alpha(v)beta(3) antagonists under present consideration, these charged ends are separated by twelve bonds. The basic nitrogen of the active alpha(v)beta(3) ligands are shown to interact with D150 of alpha(v) and the ligands' carboxylic acid interact with R214 of beta(3) while adopting an extended conformation with minimal protein induced internal strain. In addition, an energetically favorable interaction is found with all of the active alpha(v)beta(3) molecules with Y178 of alpha(v) when docked to the crystallographically determined structure. This novel interaction may be characterized as pi-pi stacking for the most active of the alpha(v)beta(3) selective antagonists. The proposed model is consistent with observed activity as well as mutagenicity and photoaffinity cross-linking studies of the alpha(v)beta(3) integrin.

Coming to grips with integrin binding to ligands

Integrins are alphabeta heterodimeric cell-surface receptors that are vital to the survival and function of nucleated cells. They recognize aspartic-acid- or a glutamic-acid-based sequence motifs in structurally diverse ligands. Integrin recognition of most ligands is divalent cation dependent and conformationally sensitive. In addition to this common property, there is an underlying binding specificity between integrins and ligands for which there has been no structural basis. The recently reported crystal structures of the extracellular segment of an integrin in its unliganded state and in complex with a prototypical Arg-Gly-Asp (RGD) ligand have provided an atomic basis for cation-mediated binding of aspartic-acid-based ligands to integrins. They also serve as a basis for modelling other integrins in complex with larger physiologic ligands. These models provide new insights into the molecular basis for ligand binding specificity in integrins and its regulation by activation-driven tertiary and quaternary changes.

Eptifibatide: a potent inhibitor of the platelet receptor integrin glycoprotein IIb/IIIa

The platelet glycoprotein (GP) IIb/IIIa integrin plays a key role in mediating platelet aggregation. Blockade of the platelet GP IIb/IIIa receptor prevents arterial thrombosis in animal models much better than does aspirin. Among the most specific inhibitors in this class of drugs is eptifibatide (Integrilin(TM), Millennium Pharmaceuticals, Inc.), a cyclic heptapeptide based on a peptide recognition sequence found in snake venom. Peptide inhibitors, such as eptifibatide, bind competitively to GP IIb/IIIa and have a short half-life, allowing the effect to be rapidly reversible and providing a favourable overall safety profile. Eptifibatide has been studied in a broad range of ischaemic coronary conditions including percutaneous coronary intervention (PCI), ST-segment and non-ST-segment acute myocardial infarction (MI) and unstable angina. In PCI and non-ST-segment MI, therapy with eptifibatide has been shown to reduce acute ischaemic complications without any increased risk of life-threatening adverse events. In the recently reported Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Therapy (ESPRIT) trial, two 180 microg/kg boluses of eptifibatide, 10 min apart, followed by an 18 - 24 h infusion at 2 microg/kg/min given as adjunctive therapy in non-urgent PCI reduced the 30-day composite of death, MI and need for urgent target vessel revascularisation from 10.4 to 6.8% compared with placebo. These results were achieved under conditions of typical contemporary PCI, namely the implantation of second- and third-generation stents deployed at high balloon pressures along with modern adjunctive pharmacological treatment, particularly the universal use of thienopyridines and lower-dose heparin. Few significant pharmacological effects other than inhibition of platelet aggregation and the effect on bleeding time have been reported. Future research will focus on alternative clinical applications and combinations with other therapies to further improve cardiovascular outcomes.

A new platelet polymorphism Duv(a+), localized within the RGD binding domain of glycoprotein IIIa, is associated with neonatal thrombocytopenia

We report here the identification and characterization of a new platelet alloantigen, Duv(a+), implicated in a case of neonatal thrombocytopenia. Immunochemical studies demonstrated that the epitope was localized on glycoprotein (GP) IIIa. Sequencing of the exons 2 to 15 of GP IIIa gene polymerase chain reaction products from both parents revealed a single base substitution 517C>T (complementary DNA) present in a heterozygous state in DNA from the father leading to amino acid substitution Thr140Ile (ACC>ATC) within the Arg-Gly-Asp binding domain of GP IIIa. Flow cytometry and immunoprecipitation studies of IIb-C517 or T517 IIIa transfected Cos cells allowed us to demonstrate this mutation was responsible for expression of the Duv(a+) epitope. By polymerase chain reaction-single-strand conformational-polymorphism analysis, the mutated allele could not be detected in a population of 100 healthy unrelated donors, indicating a low frequency of occurrence. The Thr140/Ile dimorphism, localized 3 amino acids upstream from the Arg143 involved in the expression of HPA-4a, did not interfere with the binding of an anti-HPA-4a antibody in flow cytometry. Results of functional analysis of wild-type or mutated transfected CHO cells-(1) aggregation in the presence of Ca(++) and soluble fibrinogen after complex activation by dithiothreitol, (2) adhesion on coated fibrinogen, (3) binding of monoclonal antibody PAC-1 or LIBS antibody D3, and (4) outside-in signaling-all suggest that the Thr140Ile polymorphism localized in the Arg-Gly-Asp binding domain of GP IIIa does not affect significantly, if at all, the integrin function. We have shown that the anti-Duv(a+) antibody may inhibit platelet GP IIb-IIIa function.

Identification and characterization of two cation binding sites in the integrin beta 3 subunit

The midsegment of the beta(3) subunit has been implicated in the ligand and cation binding functions of the beta(3) integrins. This region may contain a metal ion-dependent adhesion site (MIDAS) and fold into an I domain-like structure. Two recombinant fragments, beta(3)-(95-373) and beta(3)-(95-301), were expressed and found to bind fibrinogen. Whereas 0.1 mm Ca(2+) supported ligand binding to both recombinant fragments, 1.0 mm Ca(2+) suppressed binding to the longer but not the shorter fragment. These properties suggest that beta(3)-(95-373) contains both the ligand-competent (LC) and inhibitory (I) cation binding sites, and beta(3)-(95-301) lacks the I site. In equilibrium dialysis experiments, beta(3)-(95-373) contained two divalent cation binding sites, one reactive with either Mg(2+) or Ca(2+) and one Ca(2+)-specific, whereas beta(3)-(95-301) lacked the Ca(2+)-specific site. Mutant forms of beta(3)-(95-373) suggested that the LC site is a MIDAS motif involving Asp(119), Ser(121), Ser(123), Asp(217), and/or Glu(220) as coordination sites, and the I site was dependent upon residues within beta(3)-(301-323). In a molecular model of beta(3)-(95-373), a second Ca(2+) could be docked onto a flexible loop in close proximity to the MIDAS. These results indicate that the ligand competent and Ca(2+)-specific inhibitory cation binding sites are distinct and reside in beta(3)-(95-373).

CD40L stabilizes arterial thrombi by a beta3 integrin--dependent mechanism

CD40L, a member of the tumor necrosis factor family of ligands, plays a major role in immune responses via its receptor, CD40. Recently, CD40L has been detected on the surfaces of activated platelets and shown to activate endothelium. Here we further addressed the function of platelet CD40L. We show that absence of CD40L affects the stability of arterial thrombi and delays arterial occlusion in vivo. Infusion of recombinant soluble (rs)CD40L restored normal thrombosis, whereas rsCD40L lacking the KGD integrin-recognition sequence did not. CD40-deficient mice exhibited normal thrombogenesis. rsCD40L specifically bound to purified integrin alphaIIbbeta3 and to activated platelets in a beta3-dependent manner and induced platelet spreading. In addition, rsCD40L promoted the aggregation of either human or mouse platelets under high shear rates. Thus, CD40L appears to be an alphaIIbbeta3 ligand, a platelet agonist, and necessary for stability of arterial thrombi.

Identification of a contact domain between echistatin and the integrin alpha(v)beta(3) by photoaffinity cross-linking

The integrin alpha(v)beta(3) is the major receptor mediating the attachment of osteoclasts to the extracellular matrix in bone and plays a critical role in bone resorption and bone remodeling. Most of the ligands interacting with the alpha(v)beta(3) receptor contain an Arg-Gly-Asp (RGD) motif. Recently, we have identified two small RGD peptides, containing a benzophenone moiety at either the carboxyl or amino terminus, that photo-cross-linked within the beta(3)[99-118] [Bitan, G., et al. (1999) Biochemistry 38, 3414-3420] or the beta(3)[167-171] [Bitan, G., et al. (2000) Biochemistry 39, 11014-11023] sequence, respectively, of the alpha(v)beta(3) receptor in a selective fashion. Here, we report the synthesis of a photoreactive analogue of echistatin (a 49-amino acid peptide), a potent RGD-containing antagonist of the alpha(v)beta(3) receptor both in vitro and in vivo. This bioactive analogue is substituted at position 45 with a p-benzoyl moiety (pBz(2)), located within the flexible C-terminal domain and removed 20 amino acid residues from the R(24)GD(26) triad. This C-terminal domain was reported to contribute to receptor binding affinity by acting as an auxiliary binding site. The radiolabeled (125)I-[Arg(35),Lys(45)(N(epsilon)-pBz(2))]-echistatin photo-cross-links effectively to a site within the beta(3)[209-220] sequence. Residues in this domain have been reported to be part of the metal ion-dependent adhesion site (MIDAS). Receptor fragments overlapping this domain were reported to bind to fibrinogen and block fibrinogen binding to alpha(IIb)beta(3), the platelet integrin receptor. Taken together, position 45 in echistatin, located within an auxiliary binding site in echistatin, cross-links to a site distinct from the two previously reported sites, beta(3)[99-118] and beta(3)[167-171], which cross-link to photophores flanking the RGD triad. These cross-linking data support the hypothesis that the ligand-bound conformation of the integrin beta(3) subunit differs from the known conformation of I domains.

Platelet integrin alphaIIbbeta3-ligand interactions: what can we learn from the structure?

Upon vascular injury, platelets initiate interaction with exposed subendothelial matrices through various receptors such as glycoprotein (GP) Ib/IX/V complex, alpha2beta1 integrin, and GPVI/FcRgamma. Although these interactions cannot sustain stable platelet thrombus formation by themselves, they ultimately lead to the activation of alphaIIbbeta3 integrin (GPIIb-IIIa complex [GPIIb-IIIa]), the most abundant receptor in platelets. The alphaIIbbeta3 integrin plays a central role in primary hemostasis by serving as a receptor for fibrinogen and von Willebrand factor (vWf). It establishes a stable interaction with vWf bound to the extracellular matrices and uses fibrinogen as a bridging molecule in platelet aggregate formation. The alphaIIbbeta3 integrin also plays an important role in the pathogenesis of thrombosis. Over the past decades, a tremendous amount of effort has been made to elucidate the ligand-binding mechanisms of alphaIIbbeta3, in part because of its clinical significance. Most of the studies have relied on biochemical analyses of purified alphaIIbbeta3 or recombinant proteins generated in vitro. With the lack of actual 3-dimensional structure, molecular modeling has provided a useful framework for interpreting such experimental data on structure-function correlation of integrin molecules. However, it has also generated disagreement between different models. The aim of this minireview is to summarize the past efforts as well as the recent accomplishments in elucidating the structure/function of alphaIIbbeta3. Finally, we will try to explain all those experimental data using the recently published crystal structure of the extracellular domains of the alphaVbeta3 heterodimeric complex.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

Pharmacological control of platelet function

Advancement in the understanding of the mechanisms of platelet activation, as well as the development of new techniques for studying platelet function, have led to the availability of new classes of platelet inhibiting drugs. Initially, characterization of arachidonic acid metabolism in platelets furthered an understanding of the utility of cyclooxygenase inhibitors, most notably aspirin. The discovery and characterization of platelet receptors such as the adenosine diphosphate (ADP) receptor and glycoprotein IIb/IIIa has been associated with the development of novel classes of anti-platelet drug, such as thienopyridine derivatives and glycoprotein IIb/IIIa receptor antagonists, respectively. Future development in receptor pathway inhibitors also includes glycoprotein Ib/IX as well as the potential use of platelet signaling pathway inhibitors.

Recognition of fibronectin by the platelet integrin alpha IIb beta 3 involves an extended interface with multiple electrostatic interactions

Normal platelet function is dependent on the ability of integrin alpha IIb beta 3 (glycoprotein IIb/IIIa) to interact with components of the subendothelial matrix, such as fibronectin (Fn), exposed at sites of vascular injury. Studies using synthetic peptides derived from human Fn sequences Asp(1373)--Thr(1383) and Arg(1493)--Asp(1495) have suggested a role for both the 9th (3fn9) and 10th (3fn10) type III repeats of this ligand in binding to alpha IIb beta 3. In this study, we have taken a charge-to-alanine mutagenesis approach to evaluate the importance of these sites, and other charged residues, within the context of recombinant 3fn9--10 modules for binding to alpha IIb beta 3. To identify residues that are involved in Fn binding to alpha IIb beta 3, recombinantly expressed 3fn9--10 module pairs with alanine substitutions introduced into each of the 38 charged residues were individually assayed for the ability to inhibit Fn binding to purified alpha IIb beta 3. Substitutions at Fn residues Arg(1493) and Asp(1495) of the RGD sequence were found to have the greatest effect on alpha IIb beta 3 binding, as expected. However, Fn residues Arg(1369), Arg(1371), Arg(1379), Arg(1445), and Arg(1448) were needed for optimal interaction of the 3fn9--10 module pair with alpha IIb beta 3. All Fn residues found to affect binding of 3fn9--10 to alpha IIb beta 3 are located on the same face and extend from the surface of the molecule. Additionally, the epitopes for two anti-Fn monoclonal antibodies that inhibit binding of this ligand to alpha IIb beta 3 were found to overlap the sites identified. These results demonstrate that alpha IIb beta 3--Fn binding involves multiple electrostatic interactions.

Platelet-fibrinogen interactions

Binding of fibrinogen to GPIIb-IIIa on agonist-stimulated platelets results in platelet aggregation, presumably by crosslinking adjacent activated platelets. Although unactivated platelets express numerous copies of GPIIb-IIIa on their surface, spontaneous, and potentially deleterious, platelet aggregation is prevented by tightly regulating the fibrinogen binding activity of GPIIb-IIIa. Preliminary evidence suggests that it is the submembranous actin or actin-associated proteins that constrains GPIIb-IIIa in a low affinity state and that relief of this constraint by initiating actin filament turnover enables GPIIb-IIIa to bind fibrinogen. Two regions of the fibrinogen alpha chain that contain an RGD motif, as well as the carboxyl-terminus of the fibrinogen gamma chain, represent potential binding sites for GPIIb-IIIa in the fibrinogen molecule. However, ultrastructural studies using purified fibrinogen and GPIIb-IIIa, and studies using recombinant fibrinogen in which the RGD and relevant gamma chain motifs were mutated indicate that sequences located at the carboxyl-terminal end of the gamma chain mediates fibrinogen binding to GPIIb-IIIa. There is evidence that fibrinogen itself binds to regions in the amino terminal portions of both GPIIb and GPIIIa and that the sites interacting with the fibrinogen gamma chain and with RGD-containing peptides are spatially distinct. Nonetheless, there appears to be allosteric linkage between these sites, accounting for the ability of RGD-containing peptides to inhibit platelet aggregation and arterial thrombosis.

Discovery of an orally active non-peptide fibrinogen receptor antagonist based on the hydantoin scaffold

Antagonists of the platelet fibrinogen receptor (GP IIb/IIIa receptor) are expected to be a promising new class of antithrombotic agents. The binding of fibrinogen to the fibrinogen receptor depends on an Arg-Gly-Asp-Ser (RGDS) tetrapeptide recognition motif. Structural modifications of the RGDS lead have led to the discovery of a non-peptide RGD mimetic GP IIb/IIIa antagonist 44 (S 1197). Compound 44 inhibited, in a dose dependent and reversible manner, human and dog platelet aggregation as well as 125I-fibrinogen binding to ADP-activated human gel filtered platelets and isolated GP IIb/IIIa with K(i) values of 9 nM and 0.17 nM, respectively. A pharmacophore mapping procedure with QXP and a 3D-QSAR analysis applying the GRID/GOLPE methodology yielded a stable, rather predictive model and revealed structural features which are important for binding. Hydrophobic substitutions both at the hydantoin nucleus and at the C-terminus increase the affinity toward the fibrinogen receptor. The crystalline ethyl ester prodrug 48 (HMR 1794) is an orally active antithrombotic agent which is a promising drug candidate for the treatment of thrombotic diseases in humans.

Intervesicle cross-linking with integrin alpha IIb beta 3 and cyclic-RGD-lipopeptide. A model of cell-adhesion processes

We report the synthesis of a new integrin alpha(IIb)beta(3)-specific cyclic hexapeptide that contains an Arg-Gly-Asp (RGD) sequence and is coupled to a dimyristoylthioglyceryl anchor. We demonstrate that this ligand is useful to study specific integrin binding to membrane surfaces. With the help of biotinylated analogues of the peptide, a spacer of optimal length between the peptide and lipid moieties was searched for by evaluating the binding strength with an enzyme-coupled immunosorbent assay (ELISA) and by surface plasmon resonance (SPR). It was found to be strongly dependent on the length of the spacer introduced between the biotin and peptide moieties of the ligands, which consisted either of epsilon-aminohexanoic acid (epsilonAhx) or of epsilonAhx with two additional glycine units. Best results were obtained with c[Arg-Gly-Asp-D-Phe-Lys(Biot-Ahx-Gly-Gly)-Gly-] with dissociation constants of K(D) = 0.158 microM from ELISA and K(D) = 1.1 microM from SPR measurements. The analogous lipopeptide, c[Arg-Gly-Asp-D-Phe-Lys([dimyristoyl-3-thioglyceryl-succinimido -propanoyl]Ahx-Gly-Gly)-Gly], was used as a membrane-anchored integrin ligand. It is shown by fluorescence microscopy and cryo electron microscopy that integrin reconstituted into phospholipid vesicles binds to vesicles decorated with the lipopeptide, forming regularly spaced bridges between the two kinds of vesicles. The novel integrin-specific ligand allows establishment of new model systems for systematic studies of the self-organization of integrin clusters and focal adhesion complexes.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

A Leu262Pro mutation in the integrin β3 subunit results in an αIIb-β3 complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, IIbβ3. In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant IIbβ3 may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable IIbβ3, findings consistent with type II GT. Genotyping of LD revealed 2 novel β3 mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among β integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Proβ3 cotransfected with wild-type IIb into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Proβ3 formed a complex with endogenous av and retracted fibrin clots similarly to wild-type β3. The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for IIbβ3 to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding β3 Leu262 may maintain β3 in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.

Multiple discontinuous ligand-mimetic antibody binding sites define a ligand binding pocket in integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3), a platelet fibrinogen receptor, is critically involved in thrombosis and hemostasis. However, how ligands interact with alpha(IIb)beta(3) has been controversial. Ligand-mimetic anti-alpha(IIb)beta(3) antibodies (PAC-1, LJ-CP3, and OP-G2) contain the RGD-like RYD sequence in their CDR3 in the heavy chain and have structural and functional similarities to native ligands. We have located binding sites for ligand-mimetic antibodies in alpha(IIb) and beta(3) using human-to-mouse chimeras, which we expect to maintain functional integrity of alpha(IIb)beta(3). Here we report that these antibodies recognize several discontinuous binding sites in both the alpha(IIb) and beta(3) subunits; these binding sites are located in residues 156-162 and 229-230 of alpha(IIb) and residues 179-183 of beta(3). In contrast, several nonligand-mimetic antibodies (e.g. 7E3) recognize single epitopes in either subunit. Thus, binding to several discontinuous sites in both subunits is unique to ligand-mimetic antibodies. Interestingly, these binding sites overlap with several (but not all) of the sequences that have been reported to be critical for fibrinogen binding (e.g. N-terminal repeats 2-3 but not repeats 4-7, of alpha(IIb)). These results suggest that ligand-mimetic antibodies and probably native ligands may make direct contact with these discontinuous binding sites in both subunits, which may constitute a ligand-binding pocket.