Identification of the fibrinogen receptor on human platelets by photoaffinity labeling

Venous Thromboembolism: Review of Clinical Challenges, Biology, Assessment, Treatment, and Modeling

Venous thromboembolism (VTE) is a massive clinical challenge, annually affecting millions of patients globally. VTE is a particularly consequential pathology, as incidence is correlated with extremely common risk factors, and a large cohort of patients experience recurrent VTE after initial intervention. Altered hemodynamics, hypercoagulability, and damaged vascular tissue cause deep-vein thrombosis and pulmonary embolism, the two permutations of VTE. Venous valves have been identified as likely locations for initial blood clot formation, but the exact pathway by which thrombosis occurs in this environment is not entirely clear. Several risk factors are known to increase the likelihood of VTE, particularly those that increase inflammation and coagulability, increase venous resistance, and damage the endothelial lining. While these risk factors are useful as predictive tools, VTE diagnosis prior to presentation of outward symptoms is difficult, chiefly due to challenges in successfully imaging deep-vein thrombi. Clinically, VTE can be managed by anticoagulants or mechanical intervention. Recently, direct oral anticoagulants and catheter-directed thrombolysis have emerged as leading tools in resolution of venous thrombosis. While a satisfactory VTE model has yet to be developed, recent strides have been made in advancing in silico models of venous hemodynamics, hemorheology, fluid-structure interaction, and clot growth. These models are often guided by imaging-informed boundary conditions or inspired by benchtop animal models. These gaps in knowledge are critical targets to address necessary improvements in prediction and diagnosis, clinical management, and VTE experimental and computational models.

Viscoelasticity and Ultrastructure in Coagulation and Inflammation: Two Diverse Techniques, One Conclusion

The process of blood clotting has been studied for centuries. A synopsis of current knowledge pertaining to haemostasis and the blood components, including platelets and fibrin networks which are closely involved in coagulation, are discussed. Special emphasis is placed on tissue factor (TF), calcium and thrombin since these components have been implicated in both the coagulation process and inflammation. Analysis of platelets and fibrin morphology indicate that calcium, tissue factor and thrombin at concentrations used during viscoelastic analysis (with thromboelastography or TEG) bring about alterations in platelet and fibrin network ultrastructure, which is similar to that seen in inflammation. Scanning electron microscopy indicated that, when investigating platelet structure in disease, addition of TF, calcium or thrombin will mask disease-induced alterations associated with platelet activation. Therefore, washed platelets without any additives is preferred for morphological analysis. Furthermore, morphological and viscoelastic analysis confirmed that thrombin activation is the preferred method of fibrin activation when investigating fibrin network ultrastructure.

Interaction of von Willebrand factor with platelets and the vessel wall

The initiation of thrombus formation at sites of vascular injury to secure haemostasis after tissue trauma requires the interaction of surface-exposed von Willebrand factor (VWF) with its primary platelet receptor, the glycoprotein (GP) Ib-IX-V complex. As an insoluble component of the extracellular matrix (ECM) of endothelial cells, VWF can directly initiate platelet adhesion. Circulating plasma VWF en-hances matrix VWF activity by binding to structures that become exposed to flowing blood, notably collagen type I and III in deeper layers of the vessel along with microfibrillar collagen type VI in the subendothelium. Moreover, plasma VWF is required to support platelet-to-platelet adhesion - i. e. aggregation - which promotes thrombus growth and consolidation. For these reasons, understanding how plasma VWF interaction with platelet receptors is regulated, particularly any distinctive features of GPIb binding to soluble as opposed to immobilized VWF, is of paramount importance in vascular biology. This brief review will highlight knowledge acquired and key problems that remain to be solved to elucidate fully the role of VWF in normal haemostasis and pathological thrombosis.

Glycoprotein IIb/IIIa antagonists

Mortality from ischemic cardiac disease in adults has been dramatically reduced by the development of novel therapies for inhibiting platelet function. Circulating platelets are maintained in a resting state and are activated at sites of vascular injury by exquisitely controlled mechanisms, thereby maintaining vascular integrity without causing intravascular thrombosis. As it became clear that platelets play a central role in arterial thrombosis, the processes of platelet activation, adhesion, and aggregation became logical targets for the development of antithrombotic agents.

Integrin activation by talin

The development and integrity of the cardiovascular system depends on integrins, a family of adhesion receptors, vitally important for homeostasis of animal species from fruit fly to man. Integrins are critical players in cell migration, cell adhesion, cell cycle progression, differentiation, and apoptosis. Consequently, integrins have a major impact on the patterning and functions of the blood and cardiovascular system. Integrins undergo conformational changes, which alter their affinity for ligands through a process operationally defined as integrin activation. Integrin activation is important for platelet aggregation, leukocyte extravasation, and cell adhesion and migration, thus influencing such processes as hemostasis, inflammation and angiogenesis. Recently, a series of studies have begun to define the mechanism of integrin activation by demonstrating that binding of a cytoskeletal protein, talin, to integrin beta subunit cytoplasmic tail is a last common step in integrin activation. These findings indicate that talin is likely to be at the center of converging signaling pathways regulating integrin activation.

Molecular interaction studies of hemostasis: fibrinogen ligand-human platelet receptor interactions

The interactions between fibrinogen ligands and platelet receptor alpha(IIb)beta(3) were studied under physiological conditions by atomic force microscopy (AFM). Two linear peptide sequences in fibrinogen, RGD and HHLGGAKQAGDV, play central roles in the regulation of hemostasis and thrombosis by facilitating adhesion and aggregation of platelets. In order to measure the interactions (i.e., debonding force), oligopeptides, GSSSGaaa, where aaa is -RGDSPA or -HHLGGAKQAGDV, were synthesized and grafted on to the surface of AFM probe tips. The interaction forces between a peptide-modified AFM probe tip and platelet surface were determined from pN to nN levels using AFM force measurements. Our results show that the zero kinetic off-rate, K(off)(0), for RGDSPA is significantly smaller than that for HHLGGAKQAGDV, under the consideration of flexible receptor surfaces. From our analysis, the K(off)(0), the single molecular binding energy E(b), and the transition state x(b), were extracted from the data, and estimated to be 1.53s(-1), -2.64x10(-20)J and 1.03A for the RGD-alpha(IIb)beta(3) system, and 47.58s(-1), 2.67x10(-20), 1.09A for the HHLGGAKQAGDV-alpha(IIb)beta(3) system, respectively.

Carboxypeptidase U at the interface between coagulation and fibrinolysis

In 1988, Hendricks et al. first reported on the presence of carboxypeptidase U (U refers to the unstable nature of the enzyme) in human serum. One decade later, the importance of carboxypeptidase U (CPU) in the regulation of fibrin clot dissolution is well documented. CPU circulates in plasma as an inactive zymogen, proCPU, that is converted to its active form during coagulation and fibrinolysis. CPU cleaves off C-terminal lysine residues exposed on fibrin partially degraded by the action of plasmin. Because these C-terminal lysine residues are important for upregulating the fibrinolytic rate, CPU thus slows down fibrinolysis.

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.

Efficiency of platelet adhesion to fibrinogen depends on both cell activation and flow

The kinetics of adhesion of platelets to fibrinogen (Fg) immobilized on polystyrene latex beads (Fg-beads) was determined in suspensions undergoing Couette flow at well-defined homogeneous shear rates. The efficiency of platelet adhesion to Fg-beads was compared for ADP-activated versus "resting" platelets. The effects of the shear rate (100-2000 s(-1)), Fg density on the beads (24-2882 Fg/microm(2)), the concentration of ADP used to activate the platelets, and the presence of soluble fibrinogen were assessed. "Resting" platelets did not specifically adhere to Fg-beads at levels detectable with our methodology. The apparent efficiency of platelet adhesion to Fg-beads readily correlated with the proportion of platelets "quantally" activated by doses of ADP, i.e., only ADP-activated platelets appeared to adhere to Fg-beads, with a maximal adhesion efficiency of 6-10% at shear rates of 100-300 s(-1), decreasing with increasing shear rates up to 2000 s(-1). The adhesion efficiency was found to decrease by only threefold when decreasing the density of Fg at the surface of the beads by 100-fold, with only moderate decreases in the presence of physiologic concentrations of soluble Fg. These adhesive interactions were also compared using activated GPIIbIIIa-coated beads. Our studies provide novel model particles for studying platelet adhesion relevant to hemostasis and thrombosis, and show how the state of activation of the platelet and the local flow conditions regulate Fg-dependent adhesion.

Time and force dependence of the rupture of glycoprotein IIb-IIIa-fibrinogen bonds between latex spheres

We studied the shear-induced breakup of doublets of aldehyde/sulfate (A/S) latex spheres covalently linked with purified platelet GPIIb-IIIa receptor, and cross-linked by fibrinogen. Flow cytometry with fluorescein isothiocyanate-fibrinogen showed than an average of 22,500 molecules of active GPIIb-IIIa were captured per sphere, with a mean K(d) = 56 nM for fibrinogen binding. The spheres, suspended in buffered 19% Ficoll 400 containing 120 or 240 pM fibrinogen, were subjected to Couette flow in a counter-rotating cone-plate rheoscope. Doublets, formed by two-body collisions at low shear rate (G = 8 s(-1)) for < or =15 min, were subjected to shear stress from 0.6 to 2.9 Nm(-2), their rotations recorded until they broke up or were lost to view. Although breakup was time dependent, occurring mostly in the first 2 rotations after the onset of shear, the percentage of doublets broken up after 10 rotations were almost independent of normal hydrodynamic force, F(n): at 240 pN, 15.6, 16.0, and 17.0% broke up in the force range 70-150 pN, 150-230 pN, and 230-310 pN. Unexpectedly, at both [fibrinogen], the initial rate of breakup was highest in the lowest force range, and computer simulation using a stochastic model of breakup was unable to simulate the time course of breakup. When pre-sheared at low G for >15 min, no doublets broke up within 10 rotations at 70 < F(n) < 310 pN; it required >3 min shear (>1110 rotations) at F(n) = 210 pN for significant breakup to occur. Other published work has shown that binding of fibrinogen to GPIIb-IIIa immobilized on plane surfaces exhibits an initial fast reversible process with relative low affinity succeeded by transformation of GPIIb-IIIa to a stable high-affinity complex. We postulate that most doublet breakups observed within 10 rotations were from a population of young doublets having low numbers of bonds, by dissociation of the initial receptor complex relatively unresponsive to force. The remaining, older doublets with GPIIb-IIIa in the high-affinity complex were not broken up in the time or range of forces studied.

Clotting for the Clinician: An Overview of Thrombosis and Antithrombotic Therapy

The use of antithrombotic therapy has taken on central importance in the field of cardiovascular disease. Currently, anticoagulants and antiplatelet drugs are central to the treatment and the primary and secondary prevention of coronary artery disease. New insights into the "revised" coagulation cascade have highlighted new targets for intervention. In addition, the interactions between the coagulation system and platelets demonstrate ways that anticoagulants may affect platelet function and how antiplatelet agents may have anticoagulant effects. This overview will describe the present understanding of primary and secondary hemostasis, and current and future therapeutic approaches to modify these systems for therapeutic effects in cardiovascular medicine.

Endogenous mediators and thrombophilia

Platelets are one of the most important components of primary haemostasis. Since they lack a nucleus, their functional characteristics are determined at the time of production. The role of platelets in thrombosis is further modified by the interaction with vascular mediators that are endogenously produced in response to a variety of stimuli. This chapter discusses the factors that influence platelet production, the interaction with endogenous mediators, and the potential therapeutic benefits achieved by modifying this interaction in the clinical setting.

Functional role of arg-gly-asp (RGD)-binding sites on beta1 integrin in embryo implantation using mouse blastocysts and human decidua

Amino acid residues 140-164 of integrin beta1 comprise an Arg-Gly-Asp (RGD) cross-linking region. The present study was undertaken to study the role of the RGD cross-linking region of integrin beta1 subunit in embryo implantation. Decidual cells attached to fibronectin (FN)-coated dishes. A peptide corresponding to integrin beta1[140-164] (DDL; DYPIDLYYLMDLSYSMKDDLENVKS) inhibited decidual cell attachment to FN-coated dishes in a dose-dependent manner. A variant integrin peptide in which Asp 157 and Asp 158 were replaced by Ala (AAL; DYPIDLYYLMDLSYSMKAALENVKS) did not affect decidual cell attachment to FN. Inhibition by DDL peptide was reversed by prior treatment with an RGD-containing peptide but not by prior treatment with an RGE-containing peptide. Mouse blastocysts became attached to cultured human decidual cells after embryos hatched from the zona pellucida. The majority of hatched blastocysts attached to human decidual cells within 24 h of culture. Blastocysts that attached to decidual cells exhibited extensive outgrowth after 48 h. Treatment of decidual cells with synthetic peptides did not affect the rates of hatching and attachment of blastocysts. The outgrowth of embryos on decidual cells was inhibited by DDL peptide in a dose-dependent manner, but not by AAL peptide. These findings suggest that integrin beta1[140-164] on decidual cells may be important in embryonic development and differentiation following attachment.

The fibrinogen RIBS-I epitope (gamma373-385) appears proximate to the gamma408-411 adhesive domain but is not involved in interaction between receptor-bound or surface-adsorbed fibrinogen and platelet GPIIbIIIa

The carboxyl terminus of the fibrinogen (Fg) gamma chain (gamma400-411) is necessary and sufficient to support platelet aggregation and adhesion. However, a monoclonal antibody (mAb) to the Fg RIBS-I epitope (gamma373-385), the anti-Fg-RIBS-I, which binds only to platelet-bound or surface-adsorbed Fg but not soluble Fg, inhibits platelet aggregation. In this study, we showed that this same antibody also inhibits the adhesion of platelets to Fg-coated polystyrene beads. We then investigated the mechanisms by which the anti-Fg-RIBS-I antibody inhibits platelet aggregation and adhesion. The Fg RIBS-I epitope does not interact with platelet GPIIbIIIa, since recombinant Fg missing the last four amino acids, the Ala-Gly-Asp-Val, on the carboxyl terminus of its gamma chains supports neither platelet aggregation nor adhesion to surfaces, nor GPIIbIIIa binding, while it binds anti-Fg-RIBS-I normally. Purified, soluble GPIIbIIIa (265 kDa) inhibits the binding of both the anti-Fg-RIBS-I and 4A5 (a mAb specific to gamma408-411 of Fg), however, peptide G13 (1.5 kDa), corresponding to the Fg gamma chain binding domain on GPIIba (GPIIb300-312), only inhibits the binding of 4A5, and does not affect the binding of the anti-Fg-RIBS-I to Fg. The anti-Fg-RIBS-I reduces the on-rate of the 4A5 binding to Fg with no measurable changes in the dissociation of the Fg-bound 4A5. These data indicate that the inhibition of platelet aggregation and adhesion by the anti-Fg-RIBS-I antibody is due to the steric hindrance of the Fg gamma400-411 to platelet GPIIbIIIa. Thus the Fg RIBS-I epitope (gamma373-385) does not appear to be involved in direct interaction with platelet GPIIbIIIa, leaving the gamma408-411 of Fg as the sole domain mediating platelet aggregation and adhesion.

Expression of alpha IIb beta 3 integrin (GPIIb-IIIa) in myeloid cell lines and normal CD34+/CD33+ bone marrow cells

Regulation of myeloid cell proliferation and differentiation in the bone marrow is mediated, in part, by the interaction of integrins on early myeloid cells with the extracellular matrix proteins secreted by stromal cells. To further define adhesive protein receptors of early myeloid cells, we examined the expression of the integrin GPIIb-IIIa (alphaIIbbeta3) in leukemic cell lines KG-1a, KG-1, and HL-60, that represent early stages of myeloid differentiation. All three cell lines expressed surface GPIIb-IIIa as measured by flow cytometry and by binding of 125I-anti-GPIIb-IIIa monoclonal antibody. Preincubation of cells with human AB serum or platelet releasate increased GPIIb-IIIa surface expression. GPIIb transcripts were identified in all three cell lines by Northern blot analysis. Furthermore, we readily detected GPIIb transcripts in fluorescence activated cell sorted (FACS) myeloid cells from normal human bone marrow by RT-PCR. Cloning and sequencing of the PCR products established the identity of GPIIb transcripts in the leukemic cell lines and CD34+/CD33+ normal bone marrow cells. Since the normal myeloid cells also demonstrated markers corresponding to the maturational stage of KG-1a/KG-1 cells, we propose that GPIIb-IIIa may serve as a myeloid differentiation antigen and as a key integrin of myeloid precursors.

Interaction of thrombin-activated platelets with extracellular matrices (fibronectin and vitronectin): comparison of the activity of Arg-Gly-Asp-containing venom peptides and monoclonal antibodies against glycoprotein IIb/IIIa complex

Platelets adhere to fibronectin and vitronectin substrates following activation with physiological concentrations of thrombin. Adhesion of activated-platelets to either substrate is dependent upon the amount of fibronectin and vitronectin, and the duration of the adhesion assay. In this study, we showed that the Arg-Gly-Asp-containing peptides (including naturally occurring polypeptides, triflavin, trigramin and rhodostomin, synthetic peptides GRGDS, GRGDSPK, GRGDF, and GRGD and monoclonal antibodies, 7E3, 10E5 and AP2, raised against glycoprotein IIb/IIIa complex, inhibited the adhesion of activated-platelets to fibronectin and vitronectin-coated plates in a dose-dependent manner. In fibronectin-coated plates, GRGDF was shown to be much more efficient than GRGDS, GRGDSPK and GRGD at inhibiting the adhesion of activated-platelets to immobilized fibronectin. On the other hand, there were no marked differences in the abilities of these three peptides (GRGDF, GRGDS and GRGDSPK) to inhibit platelet adhesion to immobilized vitronectin. Furthermore, the RGD-containing venom peptide, triflavin was more effective than rhodostomin and trigramin at inhibiting the adhesion of activated-platelets to either substrates. The monoclonal antibodies raised against glycoprotein IIb/IIIa complex (i.e., 7E3, 10E5 and AP2) inhibited platelet adhesion to fibronectin and vitronectin in a similar dose-dependent manner. Interestingly, we found that 7E3 was more efficient than 10E5 and AP2 in this reaction. These studies suggest that the glycoprotein IIb/IIIa complex, present on activated-platelets, may interact with fibronectin and vitronectin substrates through the Arg-Gly-Asp-dependent mechanism. Since fibronectin and vitronectin are present in the subendothelial matrix, they may be involved in platelet-vessel wall interaction. The Arg-Gly-Asp containing peptide, especially triflavin, is an ideal therapeutic agent for inhibiting thrombus formation by interrupting platelet-platelet and platelet-subendothelium interactions.

Prevention of microvascular thrombosis with short-term infusion of human tissue-type plasminogen activator

A locally active thrombolytic agent, human tissue-type plasminogen activator (t-PA), given over a finite time period (24 hours) by local infusion, maintains long-term microvascular patency (7 days) in a proven thrombosis model using an arterial inversion graft in the rabbit model. Thirteen rabbits in the control group and 16 rabbits in the experimental group underwent an arterial inversion graft followed by continuous infusion (24 hours) with human tissue-type plasminogen activator (experimental) or normal saline (control). No significant clinical bleeding or alteration of coagulation parameters was noted in hematologic studies in both experimental and control groups. Scanning electron microscopy of the postoperative human tissue-type plasminogen activator-perfused arteries suggests an interaction of the human tissue-type plasminogen activator with specific platelet receptors in reversing microvascular thrombosis by decreasing or preventing further platelet aggregation and adhesion. Human tissue-type plasminogen activator infused locally for a finite period (24 hours) allows adequate time for platelet metamorphosis to occur in converting a thrombogenic to a nonthrombogenic vessel surface. The clinical ramifications in preventing or reversing microvascular thrombosis in free-tissue transfers and replantation surgery are apparent. Further study in this area will enhance our understanding of the pathogenesis and prevention of microvascular thrombosis.

Initiation of platelet adhesion by arrest onto fibrinogen or translocation on von Willebrand factor

We have identified two distinct mechanisms initiating the adhesion of flowing platelets to thrombogenic surfaces. The intergrin alpha IIb beta 3 promotes immediate arrest onto fibrinogen but is fully efficient only at wall shear rates below 600-900 s-1, perhaps because of a relatively slow rate of bond formation or low resistance to tensile stress. In contrast, glycoprotein Ib alpha binding to immobilized von Willebrand factor (vWF) appears to have fast association and dissociation rates as well as high resistance to tensile stress, supporting slow movement of platelets in continuous contact with the surface even at shear rates in excess of 6000 s-1. This eventually allows activated alpha IIb beta 3 to arrest platelets onto vWF under conditions not permissive of direct binding to fibrinogen. The coupling of these different functions may be crucial for thrombogenesis.

RGD induces conformational transition in purified platelet integrin GPIIb/IIIa-SDS system yielding multiple binding states for fibrinogen gamma-chain C-terminal peptide

Fibrinogen gamma-chain C-terminal peptide HHLG-GAKQAGDV (gamma 12) and alpha-chain peptide GRGDSP are known to inhibit fibrinogen-mediated platelet cell aggregation via competitive interactions with platelet integrin receptor GPIIb/IIIa. NMR studies of gamma 12 in the presence of purified GPIIb/IIIa in SDS/water solution have demonstrated the presence of two gamma 12 binding states, one of which is eliminated by GRGDSP (RGD) up to a RGD: gamma 12 ratio of 2:1. RGD: gamma 12 ratios greater than 2:1 produce multiple sets of gamma 12 NMR signals in TOCSY spectra. At a ratio of 4:1, two to four such resonance sets can be resolved for A405, Q407, A408, G409, D410 and V411 spin systems. The number of multiple resonances remains unchanged at ratios of 6:1 and 8:1. Addition of gamma 12 to reverse the ratio to 8:8 (1:1) has no apparent effect on the RGD-induced distribution. Results suggest that RGD irreversibly induces a conformational transition(s) in GPIIb/IIIa to produce multiple gamma 12 binding sites on the receptor.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Interaction of integrin alpha IIb beta 3 with multiple fibrinogen domains during platelet adhesion

We have investigated how modulation of integrin alpha IIb beta 3 function influences the mechanisms that initiate platelet thrombus formation onto surface-bound fibrinogen and isolated fibrinogen domains. Under stationary conditions and with full activation of platelets blocked by prostaglandin E1, the carboxyl-terminal gamma 400-411 sequence is necessary for establishing initial contact with the immobilized substrate. Molecules containing a single copy of this sequence, like the plasmin-generated fibrinogen fragment D, support platelet spreading, but the resulting attachment to the surface is loose and disrupted by minimal peeling force. In contrast, platelets adhere firmly to intact fibrinogen under the same conditions, suggesting that recognition of contact sites outside a single D domain can secure the firm interaction not supported by a single gamma 400-411 sequence. If platelets are activated, the gamma 400-411 sequence is no longer necessary to initiate the adhesion process but becomes sufficient, even as a single copy, to mediate stable surface attachment in the absence of shear stress. Under conditions of flow, however, intact fibrinogen but not fragment D can support adhesion, regardless of whether platelets have the potential to become activated or not. These results indicate the functional relevance of multiple fibrinogen domains during the initial stages of the platelet adhesion process.

Determination of kinetic constants for the interaction between the platelet glycoprotein IIb-IIIa and fibrinogen by means of surface plasmon resonance

The binding reaction between purified human platelet glycoprotein IIb-IIIa and fibrinogen was investigated by real-time measurements using the surface-plasmon-resonance sensor technology. In these experiments, either glycoprotein IIb-IIIa or fibrinogen was immobilized on a sensor surface. The time-dependent change in surface coverage that occurred immediately upon contact with a solution of the complementary protein was then detected. The ability to record this dynamic event from its initiation allowed the collection of kinetic and thermodynamic data over an extended time period. These data indicated that initially, in fast reaction, a reversible low-affinity complex with an equilibrium dissociation constant, Kd, of 155-180 nM was formed. In a subsequent slower reaction this complex was transformed into a more stable high-affinity complex with a Kd of 20-70 nM. Efficient dissociation of the high-affinity complex could only be induced in the presence of a competitive inhibitor such as RGDV. These data demonstrate that the binding between glycoprotein IIb-IIIa and fibrinogen is not a single monophasic reaction, but is composed of at least two consecutive processes both with their own kinetics.

Endogenous mediators and thrombophilia

Platelet function is a balance between factors determined at thrombopoiesis and prothrombotic and antithrombotic mediators from the vascular wall. Nitric oxide is a crucial vasodilator and inhibitor of platelet activation synthesized constitutively by the vascular endothelium. In some pathological states this synthesis may be impaired leading to a prothrombotic state. In other situations, synthesis may be increased by a second inducible enzyme. Nitric oxide synergizes with other antithrombotic vasodilators such as prostacyclin and is opposed by prothrombotic vasoconstrictor mediators such as thromboxane. Platelets are anucleate and their reactivity is partly determined at thrombopoiesis by their progenitor cell, the megakaryocyte. In thrombotic states, such as myocardial infarction, larger, more reactive platelets from larger megakaryocytes are observed.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

Identification of a nonsense mutation at amino acid 584-arginine of platelet glycoprotein IIb in patients with type I Glanzmann thrombasthenia

Using Southern blot, the restriction digests of genomic DNAs in 11 patients with Glanzmann thrombasthenia from 10 unrelated kindreds were probed with a full-length GPIIb cDNA. An additional 2.3 kb Taq I fragment and two 1.65 kb and 0.65 kb fragments with reduced band intensity were found in the genes of two affected siblings from a family originating from the city of Huang Yan in the Zhejiang province. The Taq I digest of the abnormal gene was further probed with three portions of GPIIb cDNA, revealing that the heterozygous mutation was present in the region around exons 15-17 of the GPIIb gene. Two primers for polymerase chain reaction (PCR) were then designed, and a 394 bp PCR product was generated and sequenced, indicating that a stop codon (TGA) was substituted for an Arg codon (CGA) at amino acid position 584 of GPIIb, and resulted in a premature termination of translation and production of a shortened protein. The Western blot analysis showed that GPIIIa at the platelet surface was apparently deficient, it may be ascribed to the rapid turn-over of GPIIIa uncomplexed with the truncated GPIIb. The abnormal 2.3 kb Taq I fragment was used as a specific genetic marker to detect the carrier status of the patient family. The abnormal allele was proved to be derived from the mother, the two affected siblings are double heterozygotes, and one clinically unaffected daughter has also inherited this defective allele, while the father carries another recessive abnormal allele unidentified.

Triflavin, an RGD-containing antiplatelet peptide, binds to GpIIIa of ADP-stimulated platelets

Triflavin, an Arg-Gly-Asp-containing snake venom peptide, inhibits platelet aggregation through the blockade of fibrinogen binding to the activated platelets. It binds to fibrinogen receptors associated with the glycoprotein IIb/IIIa complex with a Kd value of 7 x 10(-8) M. In this report, a chemical cross-linking approach was used to further characterize the binding components of triflavin on platelet membrane. 125I-triflavin binding was performed with the aid of a chemical cross-linking reagent, DTSSP. Analysis of the cross-linked products by SDS-PAGE (7.5% gel) and subsequent autoradiogram revealed that 125I-triflavin was cross-linked specifically to a protein with an apparent molecular weight of 1.1 x 10(5), and this reaction was inhibited by GRGDS and excess of non-labeled triflavin. This 110 KDa component was identified to be GpIIIa, recognized by AP3, a mAb against GpIIIa, by immunoblotting technique. These results indicate that the triflavin-binding sites on platelets reside at a site in close proximity to GpIIIa.

Functional role of alpha 2/beta 1 and alpha 4/beta 1 integrins in leukocyte intercellular adhesion induced through the common beta 1 subunit

Whereas all of the integrins in the VLA protein subfamily are involved in cell-extracellular matrix interactions, only VLA-4 (through the alpha 4 subunit) has been implicated in the triggering of intercellular adhesion. Here we describe that the VLA protein beta 1 subunit (CD29) is also involved in the induction of homotypic cell aggregation. We have obtained three novel anti-beta 1 monoclonal antibodies (mAb) with the ability to induce cell aggregation on different leukocyte cell types. These mAb recognize an antigenic site on the common beta 1 chain of VLA proteins which is topographically and/or functionally distinct from other epitopes previously defined by several prototype anti-beta 1 mAb. Induction of cell aggregation by anti-beta 1 mAb is epitope specific, isotype and Fc independent, and displays kinetics similar to alpha 4-mediated aggregation. This cell aggregation requires an intact cellular metabolism, the presence of divalent cations in the extracellular medium, and the integrity of the cytoskeleton. We also have found that the Na+/H+ antiporter may be essential for this process. For Ramos cells, which bear only the VLA alpha 4/beta 1 heterodimer, intercellular adhesion induced through the VLA-beta 1 chain could be selectively inhibited by other anti-beta 1 mAb as well as by anti-alpha 4 mAb. Interestingly, anti-beta 1 mAb which induced strong aggregation of VLA-alpha 2- or VLA-alpha 4-transfected K562 cells, had minimal effect on the alpha 2- alpha 4- alpha 5+ K562 cell line. Furthermore, the beta 1-mediated induction of cell aggregation on alpha 2-K562- and alpha 4-K562-transfected cells was blocked by preincubation with either anti-alpha 2 or anti-alpha 4 mAb, respectively, as well as by other anti-beta 1 mAb. Interestingly, parental K562 cells were able to interact with both alpha 2- and alpha 4-transfected K562 cells, thus suggesting that counter-receptors for both integrins (VLA-2 and VLA-4) might exist on these cells. Together these results provide strong evidence supporting the involvement of alpha 2/beta 1 and alpha 4/beta 1 heterodimers in intercellular interactions and underline the pivotal role of the common beta 1 chain of VLA proteins in the integrin-mediated induction of cell aggregation.

Endogenous platelet fibrinogen: its modulation after surface expression is related to size-selective access to and conformational changes in the bound fibrinogen

Platelet stimulation results in the release of endogenous platelet fibrinogen which binds to the platelet surface. Previous studies have demonstrated that plasma fibrinogen bound to activated platelets becomes inaccessible to a variety of probes. We have studied endogenous platelet fibrinogen binding to activated platelets by employing an immunopurified polyclonal anti-fibrinogen antibody and F26, a monoclonal anti-fibrinogen antibody, which recognizes fibrinogen only when it is bound to a surface. Employing the Ig or F(ab')2 of the poly- or monoclonal antibody we found a marked decrease of fibrinogen accessibility 30-60 min after platelet activation. In contrast, platelet-bound fibrinogen remains accessible to the Fab fragment of F26 at a constant level for 30 min and increases at 60 min. The reduction of the polyclonal Fab fragment binding at 30 and 60 min is similar to the F26 Ig. These results indicate that the decreased accessibility of bound fibrinogen is related to two mechanisms; (1) that the access route to fibrinogen in size selective for the antibody probes and only small antibody probes, e.g. Fab fragments, can gain access to fibrinogen and (2) fibrinogen undergoes a conformational change(s) after binding which exposes at least one neo-epitope in the D domain of fibrinogen and which may decrease or mask the reactivity of other fibrinogen domains. Only the F26 Fab probe has full access to and identifies fibrinogen present on the platelet surface 60 min after stimulation.

A modified Arg-Asp-Val (RDV) peptide derived during the synthesis of Arg-Glu-Asp-Val (REDV), a tetrapeptide derived from an alternatively spliced site in fibronectin, inhibits the binding of fibrinogen, fibronectin, von Willebrand factor and vitronectin to activated platelets

Characterization of a side-product obtained during the synthesis of Arg-Glu-Asp-Val (REDV) with inhibitory activity in thrombin-activated platelet aggregation was carried out. The semipreparative column fractionation of REDV peptide was rechromatographed on an analytical HPLC column and revealed two peaks which were re-tested for inhibitory activity. Using amino acid analysis with sequencing and fast atom bombardment mass spectrometry (FABMS), the first peak was determined to be REDV with molecular mass of 517 Da, and the second peak was determined to be a modified RDV with a mass of 608 Da. The modified RDV peptide inhibited thrombin-induced platelet aggregation with an IC50 of 200 microM, and complete inhibition occurred at 600 microM. However, the REDV peptide did not inhibit platelet aggregation up to 1 mM concentration. The modified RDV peptide eluted platelet glycoprotein IIb-IIIa complex that had been bound to GRGDSP-agarose. These studies show that the modified RDV peptide interacts with the platelet glycoprotein IIb-IIIa complex. Based on the collision-induced dissociation (CID) mass spectral data analysis, the modified RDV peptide has been characterized to contain an N-terminus blocking group on the Arg residue. The origin of this blocking group is presumed to have originated from decomposition products of the phenylacetamidomethyl (PAM) resin used in the solid-phase synthesis of the target peptide Arg-Glu-Asp-Val.

Activation-independent exposure of the GPIIb-IIIa fibrinogen receptor

Following platelet activation, surface receptors for fibrinogen are exposed. On the activated platelet, glycoprotein IIb-IIIa (GPIIb-IIIa) serves as the receptor for fibrinogen. However, the molecular mechanisms which regulate GPIIb-IIIa fibrinogen receptor exposure are unknown. D3GP3 is an IgG1, kappa monoclonal antibody which is specific for glycoprotein IIIa (GPIIIa). The binding of D3GP3 to GPIIIa, in intact GPIIb-IIIa complexes, induces fibrinogen binding and platelet aggregation. To determine if D3GP3 binding to GPIIIa directly caused the exposure of fibrinogen receptors or, secondarily, due to stimulus response coupling, platelet activation parameters were monitored following the addition of D3GP3 to platelets suspensions. D3GP3 binding did not induce detectable Ca++ mobilization, protein phosphorylation or activation of the pertussis toxin sensitive G-protein subunit alpha-41. Further, D3GP3-induced aggregation was not blocked by PGE1, aspirin, apyrase or the combination of all three reagents. Scanning electron microscopy of D3GP3-induced aggregates demonstrated that the aggregates were composed of discoid platelets. These data suggest that the binding of D3GP3 to GPIIIa induced a conformational change in GPIIb-IIIa such that the fibrinogen receptor was exposed in an activation-independent fashion. This provides evidence that conformational changes in the GPIIb-IIIa complex can result in the transformation of the complex to the high affinity binding competent state.

Changes in platelet function and reactivity induced by quinine in relation to quinine (drug) induced immune thrombocytopenia

Quinine, a drug known to induce immune mediated thrombocytopenia, has been postulated to mediate binding of drug dependent antibodies to a range of platelet membrane glycoproteins. Quinine may not act solely as a hapten however, as we have shown that it inhibits platelet aggregation (ex vivo and in vitro) and release and modifies the ability of activated platelets to bind the adhesive proteins fibrinogen and fibronectin in a dose dependent fashion. Studies on the effect of quinine on the binding of monoclonal antibodies HuPlml (GpIIIa) FMC25 (GpIX) and AN51 (GpIb) to platelets shows a selective reduction in AN51 binding. In addition quinine induced platelet antibodies from thrombocytopenic patients, in the presence of quinine, have been shown to inhibit binding of these monoclonal antibodies to platelets to varying degrees. These observations suggest that quinine causes widespread but specific conformational changes in platelet membrane antigens which may expose neoantigens resulting in the production of quinine induced antibodies.

Adhesive interactions of platelets and their blockade

Formation of thrombi, which constitute the main mechanism of occlusive cardiovascular diseases, is mediated by blood platelets and fibrinogen. At least three stimulatory pathways can activate platelets, yet only one is sensitive to inhibition by aspirin (cyclooxygenase). Aspirin-insensitive pathways, mediated by protein kinase C and myosin light-chain kinase, lead to a change of platelet shape, with an attendant striking increase in their surface (pseudopods) followed by exposure of receptors for fibrinogen and vWf on GPIIb-IIIa. Another receptor for vWf (GPIb), independent of known pathways of platelet activation, seems to function primarily in vessels with a high shear rate. The multistep processes of platelet activation can be circumvented by the blockade of platelet receptors for adhesive molecules, present in subendothelium and in plasma. However, platelet receptors exposed on GPIIb-IIIa share common structural features with the endothelial receptor for vitronectin. Blockade of platelet GPIIb-IIIa with synthetic peptides containing the RGD sequence, or with certain monoclonal antibodies, may inadvertently cause detachment, or prevent attachment, of endothelial cells in a zone of vascular injury. The peptide analogs of human fibrinogen gamma chain sequence 400-411 possess high selectivity for platelet GPIIb-IIIa because they do not cause detachment of endothelial cells. Thus, endothelial regrowth in the zone of vascular injury following thrombolysis and/or angioplasty will go unperturbed. The significance of adhesive proteins interacting with their receptors transcends the issue of the fundamental mechanism of platelet aggregation of platelet thrombus formation. A molecular model of the adhesive interaction between fibrinogen domains and GPIIb-IIIa will probably be the most amenable to construction. Once such a model is established and its allosteric regulation is unraveled, its utility for further development of improved antiplatelet receptor blockers as antithrombotic drugs, that are both selective and potent will become a reality.

A bispecific antibody enhances the fibrinolytic potency of single-chain urokinase

A monoclonal antibody specific for an epitope at the amino terminus of the beta chain of fibrin and a monoclonal antibody that binds both one- and two-chain high molecular weight urokinase were chemically cross-linked [using N-succinimidyl 3-(2-pyridyldithio)propionate and 2-iminothiolane]. The chemically modified material was heterogeneous, ranging in molecular size from tetramers to monomers and containing the two antibodies in various ratios. Nevertheless, fractions of a molecular size larger than a monomer were capable of binding fibrin and urokinase simultaneously in a radioimmunoassay. These fractions also enhanced fibrinolysis by high molecular weight single-chain urokinase (scuPA) by 50-fold and plasma clot lysis by 5-fold. Whereas scuPA significantly decreased the concentration of fibrinogen in plasma clot assay supernatants, scuPA in association with the bispecific antibody did not.