Platelet dysfunction associated with the novel Trp29Cys thromboxane A₂ receptor variant

BACKGROUND

Genetic variations that affect the structure of the thromboxane A2 receptor (TP receptor) provide insights into the function of this key platelet and vascular receptor, but are very rare in unselected populations.

OBJECTIVES

To determine the functional consequences of the TP receptor Trp29Cys (W29C) substitution.

PATIENTS/METHODS

We performed a detailed phenotypic analysis of an index case (P1) with reduced platelet aggregation and secretion responses to TP receptor pathway activators, and a heterozygous TP receptor W29C substitution. An analysis of the variant W29C TP receptor expressed in heterologous cells was performed.

RESULTS

Total TP receptor expression in platelets from P1 was similar to that of controls, but there was reduced maximum binding and reduced affinity of binding to the TP receptor antagonist [(3) H]SQ29548. HEK293 cells transfected with W29C TP receptor cDNA showed similar total TP receptor expression to wild-type (WT) controls. However, the TP receptor agonist U46619 was less potent at inducing rises in cytosolic free Ca(2+) in HEK293 cells expressing the W29C TP receptor than in WT controls, indicating reduced receptor function. Immunofluorescence microscopy and cell surface ELISA showed intracellular retention and reduced cell surface expression of the W29C TP receptor in HEK293 cells. Consistent with the platelet phenotype, both maximum binding and the affinity of binding of [(3) H]SQ29548 to the W29C TP receptor were reduced compared to WT controls.

CONCLUSION

These findings extend the phenotypic description of the very rare disorder TP receptor deficiency, and show that the W29C substitution reduces TP receptor function by reducing surface receptor expression and by disrupting ligand binding.

Antiplatelet and antithrombotic effect of F 16618, a new thrombin proteinase-activated receptor-1 (PAR1) antagonist

BACKGROUND AND PURPOSE

New antithrombotic agents with the potential to prevent atherothrombotic complications are being developed to target receptors on platelets and other cells involved in plaque growth. The aim of this study was to investigate the antiplatelet effects of F 16618, a new non-peptidic PAR1 (thrombin receptor) antagonist.

EXPERIMENTAL APPROACH

We investigated the inhibitory effect of F 16618 on human platelet aggregation ex vivo, in whole blood and washed platelets, by using a multiple-electrode platelet aggregometer based on impedance and an optical aggregometer, respectively. Its effects on whole-blood haemostasis (clot parameters) were analysed with the ROTEM thromboelastometry device and the platelet function analyser PFA-100. A guinea-pig model of arterial thrombosis was used to investigate its effects on thrombus formation in vivo.

KEY RESULTS

F 16618 inhibited PAR1 agonist peptide (SFLLR-peptide)-induced washed platelet aggregation ex vivo. This effect was concentration-dependent and exhibited a competitive inhibition profile. Washed platelet aggregation, as well as P-selectin expression induced by thrombin, were significantly inhibited by 10 µM F 16618. In whole-blood experiments, 20 µM F 16618 inhibited SFLLR-induced platelet aggregation by 49%. In contrast, it had no effect on whole-blood haemostasis. In the guinea-pig model of carotid thrombosis, 0.32 mg·kg(-1) F 16618 doubled the occlusion time.

CONCLUSIONS AND IMPLICATIONS

F 16618 was shown to have strong antithrombotic activity in vivo and moderate antiplatelet effects ex vivo. As these effects were not associated with major effects on physiological haemostasis, this molecule is a good antiplatelet drug candidate for use either alone or in combination with current treatments.

Implication of protein S thrombin-sensitive region with membrane binding via conformational changes in the gamma-carboxyglutamic acid-rich domain

In the vitamin K-dependent protein family, only protein S (PS) contains a thrombin-sensitive region (TSR), located between the domain containing the gamma-carboxyglutamic acid and the first epidermal growth factor-like domain. To better define the role of TSR in the PS molecule, we expressed a recombinant human PS (rHPS) and its analogue lacking TSR (rTSR-less), and prepared factor Xa- and thrombin-cleaved rHPS. A peptide reproducing TSR (TSR-peptide) was also synthesized in an attempt to obtain direct evidence of the domain involvement in PS anticoagulant activity. In a coagulation assay, both rTSR-less and factor Xa-cleaved PS were devoid of activated protein C cofactor activity. The TSR-peptide did not inhibit rHPS activity, showing that TSR must be embedded in the native protein to promote interaction with activated protein C. The binding of rHPS to activated platelets and to phospholipid vesicles was not modified after factor Xa- or thrombin-mediated TSR cleavage, whereas the binding of rTSR-less was markedly reduced. This suggested a role for TSR in conferring to PS a strong affinity for phospholipid membranes. TSR-peptide did not directly bind to activated platelets or compete with rHPS for phospholipid binding. The results of the present study show that TSR may not interact directly with membranes, but probably constrains the gamma-carboxyglutamic acid-rich domain in a conformation allowing optimal interaction with phospholipids.

Thiosulfinates inhibit platelet aggregation and microparticle shedding at a calpain-dependent step

Thiosulfinates (TSs) are sulfur compounds generated through the processing of different Allium species with antiplatelet property. To further define this platelet inhibitory effect we studied diallyl-TS (Al2TS), dipropyl-TS (Pr2TS). and dimethyl-TS (Me2TS) on platelet responses. The three TSs inhibited dose-dependent platelet aggregation, with IC50 values of 15+/-2, 19+/-2, and 9+/-1 microM for Al2TS, Pr2TS and Me2TS, respectively. TSs had no effect on the expression of a platelet procoagulant surface, measured by flow cytometry as the binding of annexin V-FITC. They inhibited the microparticle shedding and clot retraction. Since the microparticle shedding is a calpain-activation dependent step, we assessed calpain activation by analysis of autoproteolysis in shorter active forms and by talin proteolysis in the presence of TSs. Calpain activation was inhibited by TSs independently of fibrinogen binding. Thus, TSs represent a new category of platelet inhibitors, acting on calpain-induced events.

A new approach in the study of the molecular and cellular events implicated in heparin-induced thrombocytopenia. Formation of leukocyte-platelet aggregates

Heparin-induced thrombocytopenia (HIT), a relatively common complication of heparin therapy, results of platelet activation, via the receptor for the Fc domain of IgG (FcgammaRIIa), by heparin-dependent-antibodies, commonly directed against the heparin-platelet factor 4 (H-PF4) antigenic complex. Our strategy was to use whole blood allowing the study of leukocyte-platelet interactions. Experiments were performed with blood from healthy donors incubated with HIT patients' plasma and different concentrations of heparin. We showed that 75% of the HIT patients' plasma induced the formation of leukocyte-platelet-aggregates in a heparin-dependent-manner. The formation of leukocyte-platelet-aggregates induced by HIT plasma in the presence of heparin was (i) independent of the healthy blood donor FcgammaRIIa polymorphism, (ii) correlated with the levels of anti H-PF4 IgG antibodies contained in the patients' plasma, and to a lesser extent to anti H-PF4 IgM antibodies, and (iii) was mediated by P-selectin. This report opens new prospects in the study of the molecular and cellular events implicated in HIT.

Platelet alpha IIb-beta 3 integrin engagement induces the tyrosine phosphorylation of Cbl and its association with phosphoinositide 3-kinase and Syk

Agonist-induced platelet activation triggers 'inside-out' signalling which activates alpha IIb-beta 3, the most abundant integrin in platelet membranes. The engagement of activated alpha IIb-beta 3 integrin by linking fibrinogen is necessary for platelet aggregation, and this induces subsequent outside-in signalling, which enhances platelet activation. Here we studied the involvement of Cbl during alpha IIb-beta 3-integrin-mediated signal transduction. During thrombin-induced platelet activation, Cbl was tyrosine phosphorylated, and phosphoinositide 3-kinase (PI 3-kinase) activity measured in Cbl immunoprecipitates was increased. Both Cbl phosphorylation and its association with PI 3-kinase were dependent on alpha IIb-beta 3 engagement by linking fibrinogen. The P256 and anti-LIBS6 (ligand-induced binding site 6) antibodies, which activate platelets directly through alpha IIb-beta 3, induced Cbl phosphorylation and increased the PI 3-kinase activity associated with Cbl. Both thrombin and antibodies to alpha IIb-beta 3 induced association of Cbl with the tyrosine kinase, Syk. Experiments performed with inhibitors of tyrosine kinases indicated that both Src-family kinases and Syk contribute to phosphorylation of Cbl and its consequent association with PI 3-kinase. The results show that, following integrin alpha IIb-beta 3 engagement, Cbl is tyrosine phosphorylated, recruits PI 3-kinase to this integrin signalling pathway and possibly enhances PI 3-kinase activity, downstream of Src-family tyrosine kinases and Syk activation.

Tyrosine phosphorylation and association of FcgammaRII and p72(Syk) are not limited to the FcgammaRII signalling pathway

The tyrosine kinase p72(Syk) plays a critical role in platelet signal transduction. It associates with the platelet receptor for the Fc domain of IgGs, FcgammaRII, following stimulation by FcgammaRII cross-linking. Here, we show that p72(Syk) and FcgammaRII tyrosine phosphorylation and association occured following platelet stimulation by: (1) two monoclonal antibodies, which form a bridge between a target antigen and FcgammaRII, and (2) the G-protein-coupled receptor agonist thrombin. The kinetics of the p72(Syk)/FcgammaRII association depended on the signalling pathway (i.e., the antigen targeted or the thrombin receptor). We established a direct relationship between the level of FcgammaRII phosphorylation and the detection of its association with p72(Syk). Inhibition of p72(Syk) by piceatannol resulted in partial or total inhibition of FcgammaRII phosphorylation, after immunological activation or addition of thrombin, respectively, suggesting that p72(Syk) participates in FcgammaRII phosphorylation. The results provide evidence that p72(Syk)/FcgammaRII association is not restricted to immunological activation.

Calpain controls the balance between protein tyrosine kinase and tyrosine phosphatase activities during platelet activation

Protein phosphorylation was studied during platelet stimulation in two ranges of ionized [Ca2+]. At ionized [Ca2+]i< or = 1 microM, proteins were phosphorylated. At ionized [Ca2+]i > or = 4 microM, phosphoproteins disappeared. Protein dephosphorylation was prevented by the combined action of calpeptin and phosphatase inhibitors. Protein tyrosine phosphatase activity was stimulated regardless of the ionized [Ca2+] level. Protein tyrosine kinase activity was stimulated at ionized [Ca2+]i < or =1 microM, whereas at ionized [Ca2+]i > or =4 microM, no protein tyrosine kinase activity was observed except in the presence of calpeptin. Thus, the massive tyrosine phosphoprotein disappearance observed at a high ionized [Ca2+]i resulted not only in protein tyrosine phosphatase activation, but also in calpain-induced protein tyrosine kinase inactivation.

Fc receptor-mediated platelet activation is dependent on phosphatidylinositol 3-kinase activation and involves p120(Cbl)

The platelet receptor for the Fc domain of IgGs (FcgammaRIIa) triggers intracellular signaling through protein tyrosine phosphorylations leading to platelet aggregation. In this study, we focused on the adaptor protein p120(cbl) (Cbl), which became tyrosine-phosphorylated after platelet activation induced by antibodies. Cbl phosphorylation was dependent on Fc receptor engagement. An association of Cbl with the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K) occurred in parallel with Cbl tyrosine phosphorylation. We showed by in vitro experiments that Cbl/p85 association was mediated by the Src homology 3 domain of p85/PI 3-K and the proline-rich region of Cbl. Inhibition of PI 3-K activity by wortmannin led to the blockade of both platelet aggregation and serotonin release mediated by FcgammaRIIa engagement, whereas it only partly inhibited those induced by thrombin. Thus, PI 3-K may play a crucial role in the initiation of platelet responses after FcgammaRIIa engagement. Our results suggest that Cbl is involved in platelet signal transduction by the recruitment of PI 3-K to the FcgammaRIIa pathway, possibly by increasing PI 3-K activity.

Calcium induces phospholipid redistribution and microvesicle release in human erythrocyte membranes by independent pathways

The increase in intracellular Ca2+ concentration in erythrocytes and platelets results in simultaneous phospholipid scrambling and microvesicle shedding. Microvesicle formation involves membrane fusion events which were proposed either to be tightly linked to phospholipid transversal redistribution or to occur by a separate mechanism. We report here that in erythrocytes incubated in high K+ medium, or in resealed ghosts, phospholipid scrambling can be fully induced by intracellular Ca2+ without microvesicle formation. Furthermore, in ghosts resealed in the presence of spermine, intracellular Ca2+, at low concentration, was able to induce microvesicles, whereas scrambling was drastically inhibited. Surprisingly, in spermine-containing ghosts prepared from erythrocytes of a patient with a bleeding disorder, due to a lack of Ca2+-induced phospholipid scrambling and vesicle shedding (characterized as a Scott syndrome), Ca2+ also promoted microvesicle release. Data show that phospholipid scrambling and microvesicle production, although closely regulated, proceed by independent pathways.

Importance of the FcgammaRIIa-Arg/His-131 polymorphism in heparin-induced thrombocytopenia diagnosis

Heparin-induced thrombocytopenia (HIT) involves heparin-dependent antibodies which induce platelet activation. In the present study, we searched for a relationship between the polymorphism of the Fc receptor (FcgammaRIIa) and the development of HIT. In this purpose, all the donors were genotyped for their FcgammaRIIA and HIT patients were selected on the basis of at least one positive answer by 14C-serotonin release assay (SRA). The frequency distribution of the FcgammaRIIa polymorphism in the HIT patient group was similar to that observed in the healthy control group. Moreover, a statistical analysis taking into account our results and those of 3 previously published studies, suggested at most only a weak association between HIT and the FcgammaRIIa-131 polymorphism. Laboratory tests used to diagnose HIT rely on the activation of normal donor platelets but fail to detect every HIT positive patient. We determined the role of FcgammaRIIa-131 polymorphism on the reactivity of control platelets to HIT plasmas. When control platelet FcgammaRIIa-131 was of Arg/Arg form, only 47% of the HIT plasmas were positive by SRA, compared to 81% and 74% for His/His or His/Arg forms, respectively. We also compared the level of anti PF4/heparin antibodies in the HIT plasmas with the response obtained by SRA. The mean anti PF4/heparin antibodies level in HIT plasma was significantly lower in negative SRA than in positive tests when using control platelets from FcgammaRIIa-Arg/Arg 131 and heterozygous donors. Thus, the variability of control platelets to respond to HIT plasmas in the SRA test is related to both the FcgammaRIIa-131 polymorphism, and to the amount of anti PF4/heparin antibodies.