Thrombin interactions with platelet membrane proteins

Comparison of the effect of edoxaban, a direct factor Xa inhibitor, with a direct thrombin inhibitor, melagatran, and heparin on intracerebral hemorrhage induced by collagenase in rats

INTRODUCTION

Intracerebral hemorrhage (ICH) is a major clinical concern with anticoagulation therapy. The effect of a new oral direct FXa inhibitor, edoxaban, was determined in a rat model of ICH and compared with a direct thrombin inhibitor, melagatran, and heparin.

METHODS

To induce ICH, 0.1 U collagenase type VII was injected into the striatum of male Wistar rats under anesthesia with thiopental or halothane. Immediately after ICH induction, edoxaban, melagatran, or heparin were infused intravenously. Five hours after ICH induction, the brain was removed and ICH size was measured. To estimate the margin of safety, antithrombotic effects were evaluated in a rat venous thrombosis model.

RESULTS

Edoxaban at 6mg/kg/h significantly increased ICH volume (1.8-fold) and prolonged prothrombin time (PT) 2.8-fold compared to the vehicle group. No deaths were observed with edoxaban. Melagatran at 1mg/kg/h increased ICH volume at 1mg/kg/h (2.8-fold) with 6.1-fold PT prolongation. At 3mg/kg/h, all rats died due to severe ICH (3.9-fold). Heparin at both 100 and 500U/kg/h significantly increased ICH. At 500U/kg/h, 5 out of 8 rats died. The doses required for 50% inhibition of thrombosis of edoxaban, melagatran, and heparin were 0.045mg/kg/h, 0.14mg/kg/h, and 55U/kg/h, respectively. The safety margins between antithrombotic and ICH exacerbation effects of these anticoagulants were 133, 7.1, and 1.8, respectively.

CONCLUSION

The safety margin of edoxaban was wider than that of melagatran or heparin. These results suggest that edoxaban may be preferable from the perspective of ICH exacerbation risk.

The interaction of thrombin with blood platelets

Thrombin is a potent agonist of platelets. In the current article, the research on the interaction of thrombin with blood platelets is reviewed starting with the first studies demonstrating the direct action of thrombin on platelets and ending with an analysis of the importance of the protease-activated receptors (PARs) and the GpIb complex. The antithrombin activity of platelets is discussed in terms of the binding of thrombin to receptor(s) on the platelet surface. Evaluation of the PAR receptors and the GpIb supports a model where thrombin binds to the GpIb receptor prior to the proteolysis of the PAR receptor(s). Thus, the maximal hemostatic response requires both PAR receptors and the GpIb receptors.

Thrombin induces GPIb-IX-mediated fibrin binding to alphaIIbbeta3 in a reconstituted Chinese hamster ovary cell model

BACKGROUND

The interaction of thrombin with platelet glycoprotein (GP) Ib-IX-V has been recently suggested to induce fibrin-dependent platelet aggregation associated with signaling events. The approaches used to avoid the protease-activated receptor (PAR) thrombin receptors in platelets have provided controversial conclusions regarding the precise mechanism and molecules involved in the response.

OBJECTIVES

In the present study, we developed a cellular model to investigate the functional consequences following the binding of thrombin to GPIb-IX.

METHODS

We used Chinese hamster ovary (CHO) cells stably expressing human alpha(IIb)beta(3) and/or GPIb-IX complexes (CHO-alpha(IIb)beta(3)-IbIX cells) to analyze the effect of thrombin on the binding of polymerizing fibrin by using fluorescein isothiocyanate-fibrinogen as precursor.

RESULTS

Thrombin induces, in a dose-dependent manner, the binding of polymerizing fibrin to CHO-alpha(IIb)beta(3)-IbIX cells. This response is not observed in cells expressing only one of the receptors, and it can be blocked by monoclonal antibodies against alpha(IIb)beta(3) and GPIbalpha. We show that the reaction is not due to simple cell trapping by the fibrin clot, and provide data supporting a role of a signaling pathway in which the 14-3-3zeta adaptor and calcium-calmodulin-dependent events are involved.

CONCLUSIONS

The present data support a significant role of GPIb-IX and alpha(IIb)beta(3) receptors in an alternative fibrin-mediated pathway of platelet activation induced by thrombin.

A critical role for thrombin in platelet aggregation under high shear stress

The serine protease, thrombin, plays a crucial role in both coagulation and platelet activation. Anhydrothrombin (AhT) is a catalytically inactive derivative of thrombin in which dehydroalanine replaces the active-site serine. AhT retains affinity for natural substrates of thrombin and may be a competitive inhibitor of thrombin-mediated coagulation and platelet reactions. In the present study, thrombelastography showed that AhT not only delayed the onset and the progress of the coagulation process but impaired clot strength, indicating that AhT may have both anticoagulant and antiplatelet activity. In addition, AhT prolonged the activated partial thromboplastin time dose-dependently, but had little effect on the prothrombin time, suggesting that its principal activity was mediated in the intrinsic coagulation pathway. AhT inhibited thrombin-induced aggregation of platelet-rich plasma. Complete inhibition of aggregation was evident at a concentration of 1.85 microM AhT. Furthermore, 3.7 microM of AhT almost completely abolished shear-induced platelet aggregation in PRP. Interpretation of this in vitro study requires confirmation in vivo, but the findings suggest that thrombin plays a critical role in shear related platelet mechanisms. AhT may be a useful tool for investigating platelet-based coagulation reactions and may provide the basis for a novel class of antithrombotic agents.

Pharmacological characterization of a novel factor Xa inhibitor, FXV673

FXV673 is a novel, potent, and selective factor Xa (FXa) inhibitor. FXV673 inhibited human, dog, and rabbit FXa with a K(i) of 0.52, 1.41, and 0.27 nM, respectively. FXV673 also displayed excellent specificity toward FXa relative to other serine proteases. It showed selectivity of more than 1000-fold over thrombin, activated protein C (aPC), plasmin, and tissue-plasminogen activator (t-PA). FXV673 prolonged plasma activated partial thromboplastin time (APTT) and prothrombin time (PT) in a dose-dependent fashion. In the APTT assays, the concentrations (microM) required for doubling coagulation time were 0.41 (human), 0.65 (monkey), 1.12 (dog), 0.25 (rabbit), and 0.80 (rat). The concentrations (microM) required in the PT assays were 1.1 (human), 1.32 (monkey), 2.31 (dog), 0.92 (rabbit), and 1.69 (rat). A coupled-enzyme assay was performed to measure thrombin activity following prothrombinase conversion of prothrombin to thrombin. FXV673 showed IC(50)s of 1.38 and 2.55 nM, respectively, when artificial phosphatidylserine/phosphatidylcholine (PS/PC) liposomes or fresh platelets were used as the phospholipid source for prothrombinase complex formation. It was demonstrated that FXV673 could inhibit further thrombin generation in the prothrombinase complex using PS/PC liposomes. FXV673 dose-dependently prolonged the time to vessel occlusion and inhibited thrombus formation in well-characterized canine models of thrombosis. Interspecies extrapolation (approximately 2.5-fold higher sensitivity for FXa inhibition in human than in dog) suggested that 100 ng/ml of FXV673 would be an effective plasma concentration for clinical studies. Currently FXV673 is undergoing clinical studies to be developed as an antithrombotic agent.

Platelet glycoprotein Ib alpha binds to thrombin anion-binding exosite II inducing allosteric changes in the activity of thrombin

The glycoprotein (GP) Ib-IX complex is a platelet surface receptor that binds thrombin as one of its ligands, although the biological significance of thrombin interaction remains unclear. In this study we have used several approaches to investigate the GPIb alpha-thrombin interaction in more detail and to study its effect on the thrombin-induced elaboration of fibrin. We found that both glycocalicin and the amino-terminal fragment of GPIb alpha reduced the release of fibrinopeptide A from fibrinogen by about 50% by a noncompetitive allosteric mechanism. Similarly, GPIb alpha caused in thrombin an allosteric reduction in the rate of turnover of the small peptide substrate d-Phe-Pro-Arg-pNA. The K(d) for the glycocalicin-thrombin interaction was 1 microm at physiological ionic strength but was highly salt-dependent, decreasing to 0.19 microm at 100 mm NaCl (Gamma(salt) = -4.2). The salt dependence was characteristic of other thrombin ligands that bind to exosite II of this enzyme, and we confirmed this as the GPIb alpha-binding site on thrombin by using thrombin mutants and by competition binding studies. R68E or R70E mutations in exosite I of thrombin had little effect on its interaction with GPIb alpha. Both the allosteric inhibition of fibrinogen turnover caused by GPIb alpha binding to these mutants, and the K(d) values for their interactions with GPIb alpha were similar to those of wild-type thrombin. In contrast, R89E and K248E mutations in exosite II of thrombin markedly increased the K(d) values for the interactions of these thrombin mutants with GPIb alpha by 10- and 25-fold, respectively. Finally, we demonstrated that low molecular weight heparin (which binds to thrombin exosite II) but not hirugen (residues 54-65 of hirudin, which binds to exosite I of thrombin) inhibited thrombin binding to GPIb alpha. These data demonstrate that GPIb alpha binds to thrombin exosite II and in so doing causes a conformational change in the active site of thrombin by an allosteric mechanism that alters the accessibility of both its natural substrate, fibrinogen, and the small peptidyl substrate d-Phe-Pro-Arg-pNA.

In vitro characterization of a novel factor Xa inhibitor, RPR 130737

RPR 130737 inhibited factor Xa (FXa) with a Ki of 2.4 nM and also displayed excellent specificity toward FXa relative to other serine proteases. It showed selectivity of more than 1000-fold over thrombin, activated protein C, plasmin, tissue-plasminogen activator and trypsin. RPR 130737 prolonged plasma activated partial thromboplastin time and prothrombin time in a dose-dependent fashion. In the activated partial thromboplastin time assay, the concentrations required for doubling coagulation time were 0.32 microM (human), 0.61 microM (monkey), 0.44 microM (dog), 0.15 microM (rabbit), and 0.82 microM (rat). The concentrations required to double prothrombin time were 0.86 microM (human), and 1.26 microM (monkey), 1.15 microM (dog), 0.39 microM (rabbit) and 7.31 microM (rat). Kinetic studies revealed that RPR 130737 was a fast binding, reversible and competitive inhibitor for FXa when Spectrozyme FXa, a chromogenic substrate, was used. A coupled-enzyme assay measuring thrombin activity following prothrombinase conversion of prothrombin to thrombin indicated that RPR 130737 was a potent inhibitor for prothrombinase-bound FXa. In this assay, RPR 130737 showed IC50s of 17 nM and 35.9 nM, respectively when artificial phosphatidylserine/phosphatidylcholine (PS/PC) liposomes or gel-filtered platelets were used as the phospholipid source. An FX-deficient plasma clotting-time correction assay further demonstrated that RPR 130737 was a specific inhibitor of FXa. RPR 130737 showed no effect on platelet aggregation in vitro. These results indicate that RPR 130737 has the potential to be developed as an antithrombotic agent based on its potent and selective inhibitory effect against FXa.

The antithrombotic efficacy of the GP IIb/IIIa antagonist SR121787 is potentiated by antithrombin-dependent factor Xa inhibition without an increase in the bleeding risk in the rabbit

Current antithrombotic therapy in acute coronary symptoms is only partially effective and exhibits bleeding complications. These experiments were designed to address the antithrombotic and hemorrhagic interactions of the novel glycoprotein (GP) IIb/IIIa antagonist SR121787 in combination with the indirect inhibitor of factor Xa, SR90107/ORG31540. Thrombogenesis was initiated by electrolytic injury of the intimal surface of the carotid artery, and thrombus formation was assessed by recording carotid blood flow and by measuring thrombus weight 45 min after electrical stimulation. SR121787 was an efficacious antithrombotic agent in this model (ED50 = 16.3+/-0.3 mg/kg, p.o.), whereas heparin (4.5 mg/kg, i.v.) and SR90107/ORG31540 [1 mg/kg (850 aXa anti-units/kg), i.v.] were only marginally effective (17 and 27% inhibition of carotid blood flow reduction, respectively). Coadministrations of heparin (4.5 mg/kg, i.v.) or SR90107/ORG31540 (0.5 mg/kg, i.v.) were found to potentiate the antithrombotic efficacy of threshold doses of SR121787 (5 or 10 mg/kg, p.o.). The enhancement of the antithrombotic efficacy of SR121787 by SR90107/ORG31540 was--contrary to the association of SR121787 with heparin--not accompanied by an increased blood loss from the incised rabbit ear. Coadministrations of heparin or SR90107/ORG31540 did not influence the ex vivo antiaggregatory activity of SR121787. SR121787 coadministration did not alter the systemic anticoagulant activities in heparin or SR90107/ORG31540-treated animals. These data suggest the potential for optimized antithrombotic treatment in acute coronary syndromes when a GP IIb/IIIa antagonist (SR121787) is combined with an antithrombin-dependent factor Xa inhibitor (SR90107/ORG31540).

Flow cytometric analysis of platelet function in stored platelet concentrates

Platelet activation occurs during the collection, processing and storage of platelet concentrates. The effect of the platelet activation on the functional state of stored platelets remains however undefined. We employed flow cytometric analysis to evaluate the extent of platelet activation and the physiological response to thrombin stimulation of platelets stored for up to five days under routine blood bank conditions. Platelet surface expression of the activation markers CD62 and CD63 was examined, along with modulation of platelet membrane glycoproteins (GP) Ib and IIbIIIa. Platelet dense granule content was determined using a mepacrine uptake assay and the extent of platelet microparticle generation was quantified. Thirteen random-donor platelet concentrates prepared under routine conditions by a platelet-rich-plasma protocol were examined. Platelets were found to be activated following preparation on day 1. Although a gradual increase was seen with increasing storage time, this was not statistically significant for CD62 or CD63 expression, GPIIbIIIa or GPIb modulation or dense granule release; the generation of platelet microparticles did, however, increase with increasing storage time. The characteristic increase in surface expression of CD62, CD63 and GPIIbIIIa and decrease in GPIb and dense granule content in response to thrombin stimulation was observed with all concentrates, but these measures of platelet functional reserve showed decreasing platelet function with increasing storage time. The results indicate that platelets are activated by day 1, likely as a consequence of manipulation during collection and processing, but are not further progressively activated with increasing storage time; they do, however, become relatively hypofunctional with increasing storage.

Antithrombotic effect of YM-75466 is separated from its effect on bleeding time and coagulation time

The antithrombotic effects of YM-75466 ([N-[4-[(1-acetimidoyl-4-piperidyl)oxy]phenyl]-N-[(7-amidino-2-nap hthyl)methyl]sulfamoyl]acetic acid monomethane sulfonate), a novel orally-active factor Xa inhibitor, and its effects on bleeding time and coagulation time were studied in rats and compared with those of warfarin. Both agents were orally administered. In the venous thrombosis model, YM-75466 and warfarin inhibited thrombus formation dose-dependently, with ID50 values of 3.3 and 0.56 mg/kg, respectively. Ex vivo study showed that both YM-75466 and warfarin prolonged prothrombin time dose-dependently, with doses, causing a two-fold prolongation of prothrombin time in the control group, of 89 and 0.38 mg/kg, respectively. In bleeding time studies, YM-75466 and warfarin prolonged bleeding time dose-dependently, with doses, causing a two-fold prolongation of bleeding time in the control group, of > 100 and 0.43 mg/kg, respectively. These results show that the antithrombotic effects of YM-75466 are markedly separate from its effects on bleeding time and coagulation time compared with warfarin.

Antithrombotic effects of YM-60828 in three thrombosis models in guinea pigs

The antithrombotic effects of a novel factor Xa inhibitor, YM-60828 ([N-[4-[(1-acetimidoyl-4-piperidyl)oxy]phenyl]-N-[(7-amidino-2-nap hthyl)methyl]sulfamoyl]acetic acid dihydrochloride), in three thrombosis models in guinea pigs were studied in comparison with its effect on bleeding time. The antithrombotic effects of YM-60828 were most pronounced in the venous thrombosis and the arterio-venous shunt models but YM-60828 showed 10-fold weaker effects in the carotid thrombosis model. However, YM-60828 prolonged bleeding time at a much higher dose than that required in all thrombosis models. In conclusion, YM-60828 exerted its antithrombotic effects without prolonging bleeding time in all thrombosis models and may be of clinical value not only in venous thrombosis but also in arterial thrombosis.

Antithrombotic effects of YM-60828, a newly synthesized factor Xa inhibitor, in rat thrombosis models and its effects on bleeding time

1. The effects of YM-60828, a newly synthesized factor Xa inhibitor, were investigated to analyse the relationship between its antithrombotic effects and its prolongation of template bleeding time in rats. YM-60828 was compared with argatroban, heparin and dalteparin. All agents were intravenously administered as a bolus. 2. In ex vivo studies, YM-60828 and argatroban prolonged both prothrombin time and activated partial thromboplastin time in a dose-dependent manner, while heparin and dalteparin prolonged only activated partial thromboplastin time. 3. In a venous thrombosis model, all agents exerted antithrombotic effects in a dose-dependent manner. The ID50 values of YM-60828, argatroban, heparin and dalteparin were 0.0081 mg kg(-1), 0.011 mg kg(-1), 6.3 iu kg(-1) and 4.7 iu kg(-1), respectively. 4. In an arterio-venous shunt model, all agents exerted antithrombotic effects in a dose-dependent manner. The ID50 values of YM-60828, argatroban, heparin and dalteparin were 0.010 mg kg(-1), 0.011 mg kg(-1), 10 iu kg(-1) and 4.2 iu kg(-1), respectively. 5. In bleeding time studies, all agents prolonged template bleeding time in a dose-dependent manner. ED2 values, the doses causing a 2 fold prolongation of bleeding time in the saline group, of YM-60828, argatroban, heparin and dalteparin were 0.76 mg kg(-1), 0.081 mg kg(-1), 18 iu kg(-1) and 25 iu kg(-1), respectively. 6. The ratio (ED2/ID50) of YM-60828 was more than 30 fold greater than that of heparin and more than 10 fold greater than those of argatroban and dalteparin. 7. These data show that YM-60828 can exert its antithrombotic effects with little prolongation of bleeding time compared with the other currently used anticoagulant agents.

YM-60828, a novel factor Xa inhibitor: separation of its antithrombotic effects from its prolongation of bleeding time

The antithrombotic effects of intravenous infusions of YM-60828 ([N-[4-[(1-acetimidoyl-4-piperidyl)oxy]phenyl]-N-[(7-amidino-2-nap hthyl)methyl]sulfamoyl]acetic acid dihydrochloride), a novel factor Xa inhibitor, argatroban, heparin and dalteparin in an arterio-venous shunt model were studied in comparison with their effects on template bleeding time. In an arterio-venous shunt model, all agents exerted antithrombotic effects in a dose-dependent manner. ID50 values of YM-60828, argatroban, heparin and dalteparin were 0.0087 mg/kg/h. 0.027 mg/kg/h, 22 IU/kg/h and 11 IU/kg/h, respectively. In bleeding time studies, all agents prolonged bleeding time in a dose-dependent manner. Doses (ED2) of YM-60828, argatroban, heparin and dalteparin, which caused 2-fold prolongation of bleeding time in the saline group, were 3.0 mg/kg/h, 0.25 mg/kg/h, 18 IU/kg/h and 26 IU/kg/h. respectively. The risk-benefit ratio (ED2/ID50) of YM-60828 was much greater than that of the other agents. These data suggest that the antithrombotic effect of YM-60828 is separate from its prolongation of bleeding time and that YM-60828 is much safer than conventional anticoagulant agents.

Role of Glycoprotein V in the Formation of the Platelet High-Affinity Thrombin-Binding Site

The glycoprotein (GP) Ib-IX-V complex contains a high-affinity binding site for thrombin on the platelet surface with a poorly defined role in platelet activation by this agonist. Four polypeptides comprise the complex: GP Ibα, GP Ibβ, GP IX, and GP V. The site within the complex that binds thrombin has been localized to a 45-kD region at the amino terminus of GP Ibα, which also contains the site through which the complex interacts with von Willebrand factor. A GP Ib-IX complex that lacks GP V can be efficiently expressed on the surface of transfected cells. We examined the ability of L cells expressing the GP Ib-IX complex (L2H cells) to bind thrombin at high affinity, and found no increase over the level of thrombin binding to control L cells. Because it is one of the few substrates for thrombin on the platelet surface, GP V has also been implicated as possibly participating in thrombin's actions on the platelet. To examine the role of GP V in forming the high-affinity thrombin-binding site, we compared the binding of thrombin to L2H cells versus cells that express the entire GP Ib-IX-V complex (L2H/V cells). Surface expression of GP Ibα was equivalent in these two stable cell lines. Thrombin binding to L2H/V cells was detectable at 0.25 nmol/L thrombin and reached a plateau at 1 nmol/L. No binding to L2H cells was detectable at these concentrations. Comparable results were obtained when thrombin binding to L2H cells transiently expressing GP V was compared with its binding to sham-transfected L2H cells. Again, only cells transiently expressing GP V bound thrombin specifically. As with the platelet polypeptide, thrombin cleaved GP V from the surface of L2H/V cells. To test whether GP V cleavage was required for enhancing thrombin binding to the complex, we tested the binding of enzymatically inactive D-phenylalanyl-Lprolyl-L-arginine chloromethylketone (PPACK)-thrombin to L2H and L2H/V cells. Like native thrombin, PPACK-thrombin at 1 nmol/L bound only to L2H/V cells, indicating that GP V cleavage is not a prerequisite for the formation of the high-affinity thrombin receptor. These data provide the first indication of a physiologic function for GP V, and suggest that formation of the high-affinity thrombin receptor on the platelet surface has complex allosteric requirements.

Molecular mechanisms of platelet adhesion and activation

When a blood vessel is injured, control of bleeding starts with the rapid adhesion of circulating platelets to the site of damage. Within seconds, the adhered platelets are activated, secrete the contents of storage organelles, spread out over the damaged area and recruit more platelets to the developing thrombus. However, if this same process occurs in a diseased, sclerotic or occluded vessel, the resulting platelet thrombus may break away and block the coronary artery, causing a heart attack, or restrict blood supply to the brain, causing a stroke. The glycoprotein (GP) Ib-IX-V complex, a member of the leucine-rich protein family, is a constitutive platelet membrane receptor for von Willebrand Factor (vWF), a multimeric adhesive glycoprotein found in the matrix underlying the endothelial cell lining of the blood vessel wall and in the plasma. Binding of vWF to the GP. Ib-IX-V complex regulates adhesion of platelets to the subendothelium at high shear flow, and initiates signal transduction leading to platelet activation. The GP Ib-IX-V complex also constitutes a binding site for alpha-thrombin, an interaction that facilitates thrombin-dependent platelet activation. This review will focus on recent detailed analysis of the GP Ib-IX-V complex and vWF that has identified discrete amino acid sequences that mediate their interaction. An anionic/sulfated tyrosine sequence of the GP Ib alpha-chain that is critical for binding of the GP Ib-IX-V complex to both vWF and alpha-thrombin is analogous to sulfated anionic amino acid sequences mediating interactions of other adhesive proteins, including P-selectin binding to PSGL-1 and Factor VIII binding to vWF.

The molecular mechanism of platelet adhesion

One of the most primitive of host-defence mechanisms is haemostasis, the ability to control blood loss. In response to vascular trauma, platelets rapidly adhere to the exposed subendothelial matrix, a process that ultimately results in the sealing of the vessel by a plug of platelets stabilised by fibrin. Paradoxically, it is the same cascade of events that leads to thrombosis and vessel occlusion, resulting in heart attack and stroke. The molecular events involved in platelet adhesion have therefore been the subject of intense investigation. In all but the largest blood vessels, the initial contact adhesion of platelets is mediated by subendothelial matrix bound von Willebrand Factor (vWF) and a specific vWF receptor on platelets, the glycoprotein (GP) Ib-V-IX complex. Our understanding of this process arose from analysis of two congenital bleeding disorders, von Willebrand's disease and the Bernard-Soulier syndrome, in which vWF or the GP Ib-V-IX, respectively, are either absent or dysfunctional. This overview discusses our current molecular understanding of platelet adhesion and how engagement of vWF by the GP Ib-V-IX complex on platelets initiates the subsequent events in platelet activation leading to either haemostasis or thrombosis.

Proton nuclear magnetic resonance lipid profiling of intact platelet membranes

A combination of high-resolution one-dimensional and two-dimensional proton nuclear magnetic resonance spectroscopic methods and high pressure liquid chromatography have been used to analyse qualitatively and quantitatively the phospholipids, cholesterol and fatty acid composition of normal human blood platelets. The major phospholipids present were identified and quantified. Information on the nature and composition of the different classes of structural phospholipids and unsaturated fatty acids was obtained and was similar to that reported by others using different analytical procedures. This work showed that the proton nuclear magnetic resonance spectroscopy approach to lipid analysis could provide rapid, comprehensive and detailed information on the roles of lipids in pathological conditions in which platelets have been implicated.

The thrombin receptor

1. The thrombin receptor has now been cloned and found to be a member of the G-protein-coupled seven-transmembrane domain receptor family. 2. The receptor has been detected directly in platelets, endothelial cells and smooth muscle cells and studies using receptor-derived peptides have demonstrated that this receptor may be the one responsible for many of the actions of thrombin in platelets, endothelial cells, smooth muscle cells, fibroblasts, mesangial cells and neural cells. 3. The receptor appears to be activated by the novel mechanism of cleavage by thrombin to yield a new N-terminus which then interacts with the receptor as a tethered ligand to initiate cell activation.

Human neutrophil cathepsin G is a potent platelet activator

PURPOSE

Neutrophil activation has been implicated in the pathophysiologic condition of ischemia-reperfusion injury, the formation of arterial aneurysms, the progression of myocardial ischemia, and the initiation of deep venous thrombosis. Activated neutrophils release cathepsin G, a serine protease, from their granules, which may cause platelet activation that leads to intravascular thrombosis, tissue infarction, and systemic release of the thrombogenic products of platelet granules. This study used flow cytometry to quantify the extent of cathepsin G-induced platelet activation and degranulation through changes in the expression of platelet surface glycoproteins.

METHODS

Increasing concentrations of human neutrophil-derived cathepsin G were incubated with washed platelets or whole blood from healthy human donors. The platelet surface expression of glycoproteins, including P-selectin, a platelet membrane glycoprotein only expressed after platelet alpha granule release, were determined by quantifying the platelet binding of a panel of fluorescently labeled monoclonal antibodies. Results were compared with the effect of a maximal dose of thrombin, the most potent known platelet activator.

RESULTS

In a washed platelet system, cathepsin G increased platelet surface expression of P-selectin (an activation-dependent neutrophil binding site), the glycoprotein IIb/IIIa complex (fibrinogen receptor), and glycoprotein IV (thrombospondin receptor), and decreased surface expression of glycoprotein Ib (von Willebrand factor receptor) to an extent comparable to maximal thrombin. However, these effects were not observed in a whole blood system. Further experiments revealed that preexposure to plasma completely inhibited cathepsin G-induced washed platelet activation and degranulation. Prostacyclin treatment of washed platelets markedly inhibited cathepsin G-induced platelet activation.

CONCLUSIONS

Cathepsin G is a very potent platelet agonist and degranulator, comparable to maximal thrombin, which alters platelet surface glycoprotein expression for enhanced neutrophil binding and effective platelet aggregation. This study helps to elucidate a possible pathway through which neutrophils may directly activate platelets, leading to intravascular thrombosis, irreversible ischemia, and tissue death in cardiovascular disease states. Patients with diseased endothelium that is deficient in prostacyclin production may be particularly prone to the detrimental effects of neutrophil-derived cathepsin G platelet activation.

Reversibility of thrombin-induced decrease in platelet glycoprotein Ib function

Thrombin induces a redistribution of glycoprotein (GP) Ib/GP IX complex from the platelet surface into the surface connected canalicular system (SCCS). This redistribution results in a reduced interaction of platelet GP Ib with von Willebrand factor (vWF) bound to subendothelium leading to impaired platelet adhesion. In this study we show that the platelet aggregation and degranulation require concentrations of thrombin above 0.05 U/ml, while the decrease in GP Ib function (about 50% of control value), as determined by ristocetin induced platelet agglutination, can be induced by lower concentrations (0.01-0.04 U/ml). Moreover, we show that when adding thrombin inhibitors to the platelets preincubated with < 0.04 U/ml thrombin for 5 min, their agglutinability by ristocetin was gradually recovered within 30 min, indicating that in these conditions the decrease in platelet adhesiveness is reversible. Immuno-electromicroscopic study showed that this restoration of platelet GP Ib function was associated with a reversed translocation of GP Ib from the SCCS to the plasma membrane. The data obtained from counting gold particles showed that the ratio of GP Ib immunolabelling on the external membrane versus that on the SCCS was 3.31 +/- 0.90 for resting platelets, down-regulated to 0.84 +/- 0.13 (P < 0.05 versus resting platelets) for the platelets treated with 0.04 U/ml thrombin and returned to 2.63 +/- 2.21 (P > 0.05 versus resting platelets) after incubation for 30 min with hirudin. However, the translocation of GP Ib was poorly reversed by thrombin inhibitors when higher concentrations of thrombin were used which induced platelet aggregation and large extent of degranulation. We conclude that thrombin affects platelets in a dose dependent manner, and that at low concentrations the decrease in platelet GP Ib related function is a reversible phenomenon.

Catalytically competent human and bovine zeta-thrombin and chimeras generated from unfolded polypeptide chains

Human and bovine alpha-thrombin cleaved at the B-chain by chymotrypsin generates catalytically competent zeta-thrombins, which are comprised of two noncovalently linked fragments: a 36-(human) or 49-(bovine) residue A-chain linked by a disulfide to B-chain residues B1-148 (zeta 1-thrombin) and B-chain residues B149-259 (zeta 2-thrombin). Human and bovine D-Phe-Pro-Arg-CH2-zeta- and PhMeSO2-zeta-thrombins were prepared by reaction of the active-site histidine (H-B43) and serine (S-B205) with PPACK and PMSF, respectively. Unfolding and dissociation of the noncovalently linked polypeptide chains of either human or bovine D-Phe-Pro-Arg-CH2-zeta- and PhMeSO2-zeta-thrombins in 4.5 M guanidine-HCl and refolding upon 30-fold dilution in 50 mM sodium phosphate buffer pH 6.5, 750 mM NaCl, 0.1% PEG resulted in biphasic generation of catalytic activity. The slow phase was eliminated in the presence of the competitive inhibitor benzamidine-HCl. Unfolding and refolding mixtures of the appropriate inactive precursors generated the active chimeric thrombins bovine zeta 1-thrombin:human zeta 2-thrombin and human zeta 1-thrombin:bovine zeta 2-thrombin. Human zeta 1-thrombin and zeta 2-thrombin were isolated, and, upon recombining, the isolated fragments refolded to generate catalytically competent zeta-thrombin with an active-site content, specific activity toward Chromozym-TH, and a specificity constant (kcat/Km) for FPA release from fibrinogen that were all within 60% of those of native alpha-thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple pathways of thrombin-induced platelet activation differentiated by desensitization and a thrombin exosite inhibitor

Recently a thrombin receptor with a unique mechanism of activation was cloned from a megakaryocyte-like cell line (Vu et al., Cell 64:1057-1068, 1991). Thrombin cleaves a portion of this receptor creating a new N-terminus that acts as a "tethered-ligand" to activate the receptor. A thrombin receptor activating peptide (SFLLRNPNDKYEPF) homologous to the new N-terminus was shown to activate platelets. We synthesized this peptide and demonstrated that it desensitized platelets to activation by low concentrations of alpha-thrombin but not gamma-thrombin. We also synthesized a thrombin exosite inhibitor (BMS 180742) that inhibited platelet aggregation induced by low, but not high, concentrations of alpha-thrombin. In contrast, a thrombin active site inhibitor, N alpha-(2-naphthylsulfonyl-glycyl)-D,L-amidinophenylalanylpiperi dide, competitively inhibited thrombin-induced platelet aggregation. We conclude that thrombin-induced platelet activation is mediated by at least two pathways: one activated by low concentrations of alpha-thrombin and blocked by a thrombin exosite inhibitor that appears to be coupled to the "tethered-ligand" thrombin receptor, and another that is stimulated by higher concentrations of alpha-thrombin and by gamma-thrombin and does not require the thrombin exosite for activation. Both pathways are blocked by a thrombin active site inhibitor.

Characterization of an antiglycoprotein Ib monoclonal antibody that specifically inhibits platelet-thrombin interaction

Platelet glycoprotein Ib (GPIb) acts as a high-affinity thrombin binding site and as a receptor for von Willebrand Factor (vWF). A new anti-GPIb monoclonal antibody (mAB) VM16d was produced that specifically inhibited platelet-thrombin but not platelet-vWF interaction. The epitope for VM16d was located within the 45 kDa N-terminal region of the alpha-chain of GPIb. VM16d inhibited platelet aggregation induced by low dose thrombin (0.05 U/ml) but did not affect platelet aggregation induced by ristocetin, bovine vWF, ADP or collagen. The same inhibitory effects on thrombin-induced platelet aggregation were observed with the whole IgG molecule of VM16d and its F(ab')2 and F(ab') fragments. VM16d also inhibited 14C-serotonin secretion induced by low dose thrombin and binding of 125I-thrombin but not ristocetin-dependent binding of 125I-vWF to platelets. These data indicate that the high-affinity thrombin binding site is located on the N-terminal 45 kDa domain of GPIb and that it is topographically separated from the vWF binding site.

Molecular cloning of a functional thrombin receptor reveals a novel proteolytic mechanism of receptor activation

We isolated a cDNA encoding a functional human thrombin receptor by direct expression cloning in Xenopus oocytes. mRNA encoding this receptor was detected in human platelets and vascular endothelial cells. The deduced amino acid sequence revealed a new member of the seven transmembrane domain receptor family with a large amino-terminal extracellular extension containing a remarkable feature. A putative thrombin cleavage site (LDPR/S) resembling the activation cleavage site in the zymogen protein C (LDPR/I) was noted 41 amino acids carboxyl to the receptor's start methionine. A peptide mimicking the new amino terminus created by cleavage at R41 was a potent agonist for both thrombin receptor activation and platelet activation. "Uncleavable" mutant thrombin receptors failed to respond to thrombin but were responsive to the new amino-terminal peptide. These data reveal a novel signaling mechanism in which thrombin cleaves its receptor's amino-terminal extension to create a new receptor amino terminus that functions as a tethered ligand and activates the receptor.

Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS) is a rare autosomal bleeding disorder characterized clinically by prolonged skin bleeding time, normal clot retraction and thrombocytopenia with large and morphologically abnormal platelets, and biochemically by the absence of platelet membrane glycoproteins (GP) Ib, V and IX. GP Ib and GP IX exist in the platelet membrane as a heterodimer complex which acts as the major receptor mediating platelet adhesion to blood vessel subendothelium. Studies with BSS platelets have proved particularly rewarding in the investigation of the GP Ib-IX complex as a multifunctional receptor protein. The transmembrane complex contains binding domains for von Willebrand factor, thrombin, fibrin and quinine/quinidine drug-dependent antibodies as well as an attachment site on the cytoplasmic side of the membrane for a platelet cytoskeleton. In addition, the internal segment of the beta-chain of GP Ib contains a cyclic AMP-dependent protein kinase-associated phosphorylation site which appears to regulate platelet reactivity. Limited proteolytic cleavage of the complex, in particular the GP Ib alpha-chain, has allowed immunological and functional characterization of three distinct domains; a 45 kDa segment at the N-terminal end of the alpha-chain of GP Ib, which contains binding sites for von Willebrand factor and thrombin, a 90 kDa highly glycosylated region of GP Ib alpha and a membrane-associated region consisting of the remnant of GP Ib alpha disulphide-linked to GP Ib beta and non-covalently-complexed with GP IX. This membrane-associated region contains the antigenic epitope(s) for quinine/quinidine drug-dependent antibodies. It is highly probable that the future study of platelets from patients with the Bernard-Soulier syndrome will further clarify the role of the GP Ib-IX complex in platelet physiology.

Drug-mediated thrombocytopenia

Thrombocytopenia due to drug-dependent antibodies most frequently occurs with quinine/quinidine and with heparin. Considerable evidence has accumulated about the mechanism of action of quinine/quinidine-induced antibodies but less is known about the effect of heparin. Although there is controversy, it is likely that the action of quinine/quinidine-induced antibodies follows a loose association between drug and platelet with antibodies acting independently of the Fc receptor. There is strong evidence that the complex of glycoprotein Ib and glycoprotein IX, absent in the Bernard-Soulier syndrome, provides the binding site for quinine/quinidine-dependent antibodies. It also appears that the two glycoproteins must be present in complex form for antibody binding to occur. There is some heterogeneity of quinine/quinidine-dependent antibodies since there are reports of a proportion of patient antibodies reacting with other membrane determinants or acting independently of the drug. Heparin-induced thrombocytopenia may be the consequence of a direct effect, or a more serious condition associated with thrombosis may occur when heparin-dependent antibodies are formed. The mode of action of these antibodies and the nature of their antigenic determinants remain unclear. Recognition of heparin-associated thrombocytopenia is important so that serious bleeding or thrombotic sequelae can be forestalled.