Regulation of ligand binding to glycoprotein IIb-IIIa (integrin alpha IIb beta 3) in isolated platelet membranes

Synthesis of Biocompatible Superparamagnetic Iron Oxide Nanoparticles (SPION) under Different Microfluidic Regimes

Superparamagnetic iron oxide nanoparticles (SPION) have a great potential in both diagnostic and therapeutic applications as they provide contrast in magnetic resonance imaging techniques and allow magnetic hyperthermia and drug delivery. Though various types of SPION are commercially available, efforts to improve the quality of SPION are highly in demand. Here, we describe a strategy for optimization of SPION synthesis under microfluidics using the coprecipitation approach. Synthesis parameters such as temperature, pH, iron salt concentration, and coating materials were investigated in continuous and segmented flows. Continuous flow allowed synthesizing particles of a smaller size and higher stability than segmented flow, while both conditions improved the quality of particles compared to batch synthesis. The most stable particles were obtained at a synthesis condition of 6.5 M NH4OH base, iron salt (Fe2+/Fe3+) concentration ratio of 4.3/8.6, carboxymethyl dextran coating of 20 mg/mL, and temperature of 70 °C. The synthesized SPION exhibited a good efficiency in labeling of human platelets and did not impair cells. Our study under flow conditions provides an optimal protocol for the synthesis of better and biocompatible SPION that contributes to the development of nanoparticles for medical applications.

Controlling Surface-Induced Platelet Activation by Agarose and Gelatin-Based Hydrogel Films

Platelet-surface interaction is of paramount importance in biomedical applications as well as in vitro studies. However, controlling platelet-surface activation is challenging and still requires more effort as they activate immediately when contacting with any nonphysiological surface. As hydrogels are highly biocompatible, in this study, we developed agarose and gelatin-based hydrogel films to inhibit platelet-surface adhesion. We found promising agarose films that exhibit higher surface wettability, better controlled-swelling properties, and greater stiffness compared to gelatin, resulting in a strong reduction of platelet adhesion. Mechanical properties and surface wettability of the hydrogel films were varied by adding magnetite (Fe3O4) nanoparticles. While all of the films prevented platelet spreading, films formed by agarose and its nanocomposite repelled platelets and inhibited platelet adhesion and activation stronger than those of gelatin. Our results showed that platelet-surface activation is modulated by controlling the properties of the films underneath platelets and that the bioinert agarose can be potentially translated to the development of platelet storage and other medical applications.

A Multiscale Model for Recruitment Aggregation of Platelets by Correlating with In Vitro Results

Introduction

We developed a multiscale model to simulate the dynamics of platelet aggregation by recruitment of unactivated platelets flowing in viscous shear flows by an activated platelet deposited onto a blood vessel wall. This model uses coarse grained molecular dynamics (CGMD) for platelets at the microscale and dissipative particle dynamics (DPD) for the shear flow at the macroscale. Under conditions of relatively low shear, aggregation is mediated by fibrinogen via αIIbβ3 receptors.

Methods

The binding of αIIbβ3 and fibrinogen is modeled by a molecular-level hybrid force field consisting of Morse potential and Hooke law for the nonbonded and bonded interactions, respectively. The force field, parametrized in two different interaction scales, is calculated by correlating with the platelet contact area measured in vitro and the detaching force between αIIbβ3 and fibrinogen.

Results

Using our model, we derived, the relationship between recruitment force and distance between the centers of mass of two platelets, by integrating the molecular-scale inter-platelet interactions during recruitment aggregation in shear flows. Our model indicates that assuming a rigid-platelet model, underestimates the contact area by 89% and the detaching force by 93% as compared to a model that takes into account the platelet deformability leading to a prediction of a significantly lower attachment during recruitment.

Conclusions

The molecular-level predictive capability of our model sheds a light on differences observed between transient and permanent platelet aggregation patterns. The model and simulation framework can be further adapted to simulate initial thrombus formation involving multiple flowing platelets as well as deposition and adhesion onto blood vessels.

Novel Role for the AnxA1-Fpr2/ALX Signaling Axis as a Key Regulator of Platelet Function to Promote Resolution of Inflammation

Supplemental Digital Content is available in the text.

Background:

Ischemia reperfusion injury (I/RI) is a common complication of cardiovascular diseases. Resolution of detrimental I/RI-generated prothrombotic and proinflammatory responses is essential to restore homeostasis. Platelets play a crucial part in the integration of thrombosis and inflammation. Their role as participants in the resolution of thromboinflammation is underappreciated; therefore we used pharmacological and genetic approaches, coupled with murine and clinical samples, to uncover key concepts underlying this role.

Methods:

Middle cerebral artery occlusion with reperfusion was performed in wild-type or annexin A1 (AnxA1) knockout (AnxA1^−/−) mice. Fluorescence intravital microscopy was used to visualize cellular trafficking and to monitor light/dye–induced thrombosis. The mice were treated with vehicle, AnxA1 (3.3 mg/kg), WRW4 (1.8 mg/kg), or all 3, and the effect of AnxA1 was determined in vivo and in vitro.

Results:

Intravital microscopy revealed heightened platelet adherence and aggregate formation post I/RI, which were further exacerbated in AnxA1^−/− mice. AnxA1 administration regulated platelet function directly (eg, via reducing thromboxane B₂ and modulating phosphatidylserine expression) to promote cerebral protection post-I/RI and act as an effective preventative strategy for stroke by reducing platelet activation, aggregate formation, and cerebral thrombosis, a prerequisite for ischemic stroke. To translate these findings into a clinical setting, we show that AnxA1 plasma levels are reduced in human and murine stroke and that AnxA1 is able to act on human platelets, suppressing classic thrombin-induced inside-out signaling events (eg, Akt activation, intracellular calcium release, and Ras-associated protein 1 [Rap1] expression) to decrease α_IIbβ₃ activation without altering its surface expression. AnxA1 also selectively modifies cell surface determinants (eg, phosphatidylserine) to promote platelet phagocytosis by neutrophils, thereby driving active resolution. (n=5–13 mice/group or 7–10 humans/group.)

Conclusions:

AnxA1 affords protection by altering the platelet phenotype in cerebral I/RI from propathogenic to regulatory and reducing the propensity for platelets to aggregate and cause thrombosis by affecting integrin (α_IIbβ₃) activation, a previously unknown phenomenon. Thus, our data reveal a novel multifaceted role for AnxA1 to act both as a therapeutic and a prophylactic drug via its ability to promote endogenous proresolving, antithromboinflammatory circuits in cerebral I/RI. Collectively, these results further advance our knowledge and understanding in the field of platelet and resolution biology.

Platelet biomechanics, platelet bioenergetics, and applications to clinical practice and translational research

The purpose of this review is to explore the relationship between platelet bioenergetics and biomechanics and how this relationship affects the clinical interpretation of platelet function devices. Recent experimental and technological advances highlight platelet bioenergetics and biomechanics as alternative avenues for collecting clinically relevant data. Platelet bioenergetics drive energy production for key biomechanical processes like adhesion, spreading, aggregation, and contraction. Platelet function devices like thromboelastography, thromboelastometry, and aggregometry measure these biomechanical processes. Platelet storage, stroke, sepsis, trauma, or the activity of antiplatelet drugs alters measures of platelet function. However, the specific mechanisms governing these alterations in platelet function and how they relate to platelet bioenergetics are still under investigation.

The role of biophysical properties of provisional matrix proteins in wound repair

Wound healing is a complex, dynamic process required for maintaining homeostasis in an organism. Along with being controlled biochemically, wound healing is also controlled through the transduction of biophysical stimuli through cell interactions with the extracellular matrix (ECM). This review provides an overview of the ECM's role in the wound healing process and subsequently expands on the variety of roles biophysical phenomenon play.

Rupture Forces among Human Blood Platelets at different Degrees of Activation

Little is known about mechanics underlying the interaction among platelets during activation and aggregation. Although the strength of a blood thrombus has likely major biological importance, no previous study has measured directly the adhesion forces of single platelet-platelet interaction at different activation states. Here, we filled this void first, by minimizing surface mediated platelet-activation and second, by generating a strong adhesion force between a single platelet and an AFM cantilever, preventing early platelet detachment. We applied our setup to measure rupture forces between two platelets using different platelet activation states, and blockade of platelet receptors. The rupture force was found to increase proportionally to the degree of platelet activation, but reduced with blockade of specific platelet receptors. Quantification of single platelet-platelet interaction provides major perspectives for testing and improving biocompatibility of new materials; quantifying the effect of drugs on platelet function; and assessing the mechanical characteristics of acquired/inherited platelet defects.

NPXY motifs control the recruitment of the alpha5beta1 integrin in focal adhesions independently of the association of talin with the beta1 chain

With the exception of the divergent beta4 and beta8 chains, the integrin beta subunit cytoplasmic domains are short and highly conserved sequences. Consensus motifs are found among the different cytoplasmic beta chains. Experiments using chimeric receptors demonstrated that the 47 amino acids of the beta1 subunit cytoplasmic domain contain sufficient information to target integrins to adhesion plaques. Three clusters of amino acids, named cyto-1, cyto-2 and cyto-3, seem to contribute to this localization. Cyto-2 and cyto-3 exhibit NPXY motifs. At present, the exact function of these motifs remains unknown but it is likely that these sequences are involved in protein-protein interactions. Although NPXY motifs often act as internalization signals at the cytoplasmic tail of membrane receptors, our previous results showed that the two NPXY motifs are not responsible for the alpha5beta1 integrin endocytosis. Herein, we address the question of the role of the two highly conserved NPXY motifs found in the beta1 cytoplasmic domain, and which correspond to the conserved domains cyto-2 and cyto-3. We demonstrate that, within the integrin beta1 cytoplasmic tail, the two NPXY motifs are required for the recruitment of the integrin in focal adhesions. In addition, our results indicate that these two motifs control but do not belong to the talin-binding sites. Finally, the analysis of the phenotypes of NPXY mutants reveals that the interaction of talin with the beta1 cytosolic domain is not sufficient to target the integrins to focal adhesions.

Changes in binding affinity of a monoclonal antibody to a platelet binding domain of fibrinogen adsorbed to biomaterials

Previously, we found that when fibrinogen-coated polyurethanes resided in a buffer for a period of time (the 'residence time') platelet adhesion to these materials decreased. Other changes in adsorbed fibrinogen such as decreases in polyclonal antibody binding and SDS elutability supported the conclusion that fibrinogen undergoes postadsorptive conformational changes. Subsequently we measured the binding of monoclonal antibodies to the three putative platelet binding sites on fibrinogen, using a single mid-range concentration of antibody. We found that binding of a monoclonal antibody to the platelet binding site at the C-terminus of the gamma chain of fibrinogen changed little with residence time, while binding of monoclonal antibodies to the other two putative binding sites on fibrinogen either increased with residence time (RGDF at A alpha 95-98), or first increased and then decreased with residence time (RGDS at A alpha 572-575). In the current study, we measured antibody binding affinity, Ka, by measuring antibody binding at a series of antibody concentrations. This is a more sensitive method for detecting changes in adsorbed fibrinogen than measuring antibody binding from a single antibody concentration. The Ka was determined for two antibodies, M1 (4A5), which binds to a platelet binding domain of fibrinogen (gamma 402-411) and R1 (155 B 1616), which binds to residues 87-100 of the A alpha chain (containing an RGDF site). A summary of the results for the M1 antibody are as follows. The Ka was higher for M1 binding to fibrinogen adsorbed to Immulon I than to Biomer, Biospan or poly(ethylene terephthalate), suggesting that fibrinogen adsorbed to Immulon I is more platelet adhesive than fibrinogen adsorbed to the other polymers. On Biospan, the Ka decreased from 2.8 x 10(9) to 1.0 x 10(9) M-1 after a 24 h 37 degrees C residence time, which correlated with the decrease in platelet adhesiveness of adsorbed fibrinogen observed previously under these conditions. The change in Ka was greater when adsorbed fibrinogen was kept under denaturing conditions. For example, the Ka decreased from 2.8 x 10(9) to 0.8 x 10(9) M-1 after a 1 h 70 degrees C residence time whereas it remained approximately the same, 2.9 x 10(9) M-1, after a 24 h 0 degree C residence time.

Heparin-binding domain of fibrin mediates its binding to endothelial cells

Spreading of human umbilical vein endothelial cells (ECs) on fibrin requires thrombin cleavage of fibrinopeptide B (FPB) and subsequent exposure of the new beta 15-42 N-terminus. To further understand the interactions between ECs and fibrin beta 15-42 sequences, binding of fibrin(ogen) to EC monolayers was measured with polyclonal anti-fibrinogen (FBG) in parallel with monoclonal anti-FBG (18C6, beta 1-21; J88B, gamma 63-78) and anti-fibrin (T2G1, beta 15-21) antibodies in an indirect enzyme-linked immunosorbent assay. To accomplish this, large, soluble fragments of fibrin were prepared by cyanogen bromide (CNBr) cleavage (fibrin-CNBr); CNBr-cleaved FBG (FBG-CNBr) served as the control ligand. N-terminal fibrin-CNBr bound to EC monolayers and cells in suspension in a dose-dependent and saturable manner. By contrast, FBG-CNBr bound only 50% as well to EC monolayers, with no significant binding of intact FBG, C-terminal FBG plasmic fragment D, or N-terminal plasmic fragment E, which lacks beta 1-53. ECs bound the peptide beta 15-42-bovine serum albumin (BSA) conjugate but neither a scrambled beta 15-42 peptide conjugate nor conjugates of beta 24-42, beta 18-27, or beta 18-31. Binding of fibrin-CNBr was inhibited 54% by the beta 15-42-BSA conjugate and 17% by the B beta 1-42-BSA conjugate but not by free beta 15-42 peptide or RGDS-cell binding peptide. Binding of fibrin-CNBr was inhibited > 95% by heparin in a concentration-dependent manner; the same concentrations of heparin inhibited binding of beta 15-42-BSA by > 75% but not the dose-dependent binding of fibronection to ECs. These data suggest that in their native conformation, FBG B beta 15-42 sequences are unavailable for binding to ECs and that thrombin-induced exposure of beta 15-42 is required for binding by a heparin-dependent, RGD-independent mechanism at the new N-terminus of fibrin.

Increased neutrophil-platelet adhesion in patients with unstable angina

BACKGROUND

Neutrophil-platelet adhesion may occur as a consequence of platelet activation. The role of this heterotypic adhesion in ischemic disorders is poorly understood thus far.

METHODS AND RESULTS

Systemic venous blood samples were taken from 25 patients with stable angina pectoris and 25 patients with unstable angina pectoris. Neutrophil activation and neutrophil-platelet adhesion were evaluated by two-color flow cytometry. Patients with unstable angina showed a significant increase in neutrophil-platelet adhesion compared with patients with stable angina (mean +/- SEM, 132.1 +/- 20.5 versus 29.8 +/- 4.7 anti-glycoprotein IIb/IIIa mean fluorescence intensity, P = .0001). Systemic neutrophil activation was found in patients with unstable angina compared with those with stable angina assessed by cell surface CD11b expression and shedding of L-selectin (115.6 +/- 10.3 versus 74.0 +/- 6.3 anti-CD11b mean fluorescence intensity, P = .002; 49.8 +/- 6.0 versus 72.1 +/- 4.0 anti-L-selectin mean fluorescence intensity, P = .006). Markers of neutrophil activation were related to the extent of neutrophil-platelet adhesion (CD11b: r = .5, P = .0005; L-selectin: r = .42, P = .012). In vitro studies revealed that binding of purified platelet membranes to control neutrophils caused a dose-dependent increase in CD11b surface expression, a decrease in surface L-selectin, and the release of superoxide anions.

CONCLUSIONS

Thus, this study demonstrates that increased neutrophil-platelet adhesion may contribute to neutrophil activation in unstable angina.

Agglutination of isolated platelet membranes

Platelet membrane glycoproteins play a central role in platelet aggregation and thus in primary hemostasis. To investigate mechanisms of platelet-platelet interaction in the absence of cellular activation events, we studied immunological and functional aspects of isolated platelet membranes. Platelet membranes contained significant amounts of the inducible fibrinogen receptor, glycoprotein (GP) IIb-IIIa, which exposes conformation-dependent LIBS1 and PMI-1 epitopes in response to fibrinogen-mimetic peptides GRGDSP and HHLG-GAKQAGDV. In the presence of soluble fibrinogen, membrane-coated latex beads showed Ca(2+)-dependent agglutination that could be partially inhibited by GRGDSP but not by the biologically inactive peptide GRGESP. Thrombospondin enhanced agglutination of membrane-coated beads, which could be inhibited by polyvalent anti-thrombospondin Fab fragments and anti-thrombospondin monoclonal antibody MA-II. Mg2+ inhibited both GPIIb-IIIa- and thrombospondin-mediated agglutination of membranes in a dose-dependent manner. The results of the present study indicate that isolated platelet membranes are a useful tool to study regulation of GPIIb-IIIa- and thrombospondin-mediated platelet-platelet interaction.

Reversible inactivation of purified leukocyte integrin CR3 (CD11b/CD18, alpha m beta 2) by removal of divalent cations from a cryptic site

Integrins exhibit reversible changes in their ability to bind ligands and these changes enable transient cell adhesion. We recently showed that leukocyte integrin CR3 (complement receptor type three, CD11b/CD18, alpha m beta 2) may be purified in a form that is either capable or incapable of binding soluble, monomeric ligand and that "inactive" CR3 may be rendered capable of binding ligand by addition of an anti-CR3 mAb known as KIM-127 (Cai and Wright, JBC. 270: 14358, 1995). Here, we demonstrate that active CR3 may be rendered inactive by treatment of immobilized receptor with EDTA. EDTA-treated CR3 failed to bind ligand even in the presence of mM Ca2+ and Mg2+, suggesting that EDTA-treatment caused a change in the receptor that is not readily reversed. EDTA-treated receptor did, however, bind ligand upon addition of KIM-127 plus Mg2+ with an affinity (17.8 +/- 4.5 nM) similar to untreated, active receptor (12.5 +/- 4.7 nM). EDTA-treated CR3 thus exhibits the properties of inactive CR3, in which the ligand binding site is cryptic but subject to exposure by KIM-127. A candidate for the cryptic ligand binding site is the I-domain, a Mg2+-binding region in the alpha chain of CR3. We found that monomeric C3bi binds directly to recombinant I-domain in a Mg(2+)-dependent fashion with an affinity of 300 +/- 113 nM. These results thus suggest that CR3 may be inactivated by removing tightly bound divalent cation from a cryptic site in CR3.

Regulation of integrin function and cellular adhesion

The integrins are a family of adhesion receptors involved in many physiological functions. These molecules are characterized by an ability to dynamically regulate their ligand binding affinity. Several integrins become "activated" or achieve the high affinity state in response to extracellular agonists or signals. High affinity ligand binding does not result from an increase in receptor number or from changes in the receptor microenvironment. Rather, evidence suggests these altered affinity states result from the varied conformations of these molecules. Understanding how these conformational changes are achieved remains an area of great interest in the field. In this review, we will discuss several means and potential mechanisms of integrin activation. First, we will focus on "activators" such as antibodies, peptides, and cations. For the most part, these agents can be viewed as nonphysiological activators that directly effect integrin conformational changes. Later we will discuss how conformational changes are achieved in a physiological sense. Many physiological activators stimulate signal transduction pathways inside the cell and are believed to transmit these signals outward to effect conformational changes. An understanding of integrin activation mechanisms is important as it might suggest ways to regulate cell adhesion in pathology and disease.

An enzyme-linked immunosorbent assay to identify inhibitors of activation of platelet integrin alpha IIb beta 3

The affinity of integrin alpha IIb beta 3 for adhesive ligands is tightly regulated by the platelet such that fibrinogen binding is observed only after platelet activation. Ligand binding is necessary for platelet aggregation, which contributes to vascular occlusion in pathological states. Therefore, we have developed an ELISA assay to screen for compounds that inhibit alpha IIb beta 3 activation. Washed platelets were incubated in microtitre wells with potential inhibitory compounds and stimulated with an agonist to activate alpha IIb beta 3. After the addition of biotin-PAC1, a fibrinogen-mimetic monoclonal antibody, the activation state of alpha IIb beta 3 was measured by sedimenting the platelets and quantitating the residual biotin-PAC1 in the cell-free supernatant in a streptavidin-based ELISA. This assay detected (1) specific PAC1 binding to activated platelets in response to a variety of agonists, and (2) dose-dependent inhibition of PAC1 binding by function-blocking anti-alpha IIb beta 3 monoclonal antibodies, by the tetrapeptide, RGDS, and by an alpha IIb beta 3-selective RGD peptidomimetic. Furthermore, the assay detected inhibition of PAC1 binding by intracellular inhibitors of platelet activation, including bisindolylmaleimide, a selective protein kinase C antagonist, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase. These studies demonstrate that this integrin activation ELISA can detect pharmacological blockade of platelet alpha IIb beta 3 by extracellular and intracellular inhibitors. Its use may facilitate the search for clinically useful anti-platelet drugs.

Tetrafibricin, a novel non-peptide fibrinogen receptor antagonist, induces conformational changes in glycoprotein IIb/IIIa

Arg-Gly-Asp (RGD) is an amino acid sequence in fibrinogen recognized by platelet glycoprotein (GP) IIb/IIIa. Recently, it was found that RGD peptide binding to GPIIb/IIIa leads to conformational changes in the complex that are associated with the acquisition of high-affinity fibrinogen-binding function. In this study, we found that tetrafibricin, a novel non-peptidic GPIIb/IIIa antagonist, induced similar conformational changes in GPIIb/IIIa as did RGD peptides. Tetrafibricin increased the binding of purified inactive GPIIb/IIIa to immobilized pl-80, a monoclonal antibody that preferentially recognizes ligand-occupied GPIIb/IIIa. Exposure of the pl-80 epitope by tetrafibricin was also observed on resting human platelets by flow cytometry. On intact platelets, the conformational changes transformed GPIIb/IIIa into a high-affinity receptor for fibrinogen and triggered subsequent platelet aggregation. Tetrafibricin is the first non-peptidic GPIIb/IIIa antagonist reported that has the capacity to induce conformational changes in GPIIb/IIIa.

Integrin cytoplasmic domains mediate inside-out signal transduction

We analyzed the binding of fibronectin to integrin alpha 5 beta 1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd approximately 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of alpha IIb beta 3 joined to the cytoplasmic domains of alpha 5 beta 1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of alpha 5 beta 1 conferred an energy-dependent high affinity state on alpha IIb beta 3 in CHO but not K562 cells. Three additional alpha cytoplasmic domains (alpha 2, alpha 6A, alpha 6B) conferred PAC1 binding in CHO cells, while three others (alpha M, alpha L, alpha v) did not. In the high affinity alpha chimeras, cotransfection with a truncated (beta 3 delta 724) or mutated (beta 3(S752-->P)) beta 3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved alpha subunit GFFKR motif, resulted in high affinity binding of ligands to alpha IIb beta 3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the beta subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the alpha subunit cytoplasmic domain maintains the default low affinity state.