Although anucleated, blood platelets are highly organized cells rich in different types of organelles. Three specific granule populations store different types of constituents, some of which are at high concentrations. Platelets thus transport some specific compounds through the whole body. During circulation, platelets are reactive to various stimuli and release the materials stored in the specific granules. This 'release reaction' is an important step of primary haemostasis. Energy and messengers required for platelet reactivity are provided by mitochondria and the dense tubular system. Each granule population has specific properties concerning both the structure and the role played by the released constituents. Dense granules contain small non-protein molecules that are secreted to recruit other platelets. alpha-Granules contain large adhesive and healing proteins. Lysosomes contain hydrolases able to eliminate the circulating platelet aggregate. The extrusion of storage granules' content to the platelet's environment occurs according to regulated secretion events: movements of granules, apposition and fusion of granule and plasma membranes. Typical platelet disorders resulting from a storage granule abnormality are referred to as a storage pool defect and are characterized by a prolonged bleeding time.

Cofilin, in its Ser3 dephosphorylated form, accelerates actin filament turnover in cells. We report here the role of cofilin in platelet actin assembly. Cofilin is primarily phosphorylated in the resting platelet as evidenced by a specific antibody directed against its Ser3 phosphorylated form. After stimulation with thrombin under nonstirring conditions, cofilin is reversibly dephosphorylated and transiently incorporates into the actin cytoskeleton. Its dephosphorylation is maximal 1-2 min after platelet stimulation, shortly after the peak of actin assembly occurs. Cofilin rephosphorylation begins 2 min after activation and exceeds resting levels by 5-10 min. Cofilin is dephosphorylated with identical kinetics but fails to become rephosphorylated when platelets are stimulated under stirring conditions. Cofilin is normally rephosphorylated when platelets are stimulated in the presence of Arg-Gly-Asp-Ser (RGDS) peptide or wortmannin to block alpha(IIb)beta3 cross-linking and signaling or in platelets isolated from a patient with Glanzmann thrombasthenia, which express only 2-3% of normal alpha(IIb)beta3 levels. Furthermore, actin assembly and Arp2/3 complex incorporation in the platelet actin cytoskeleton are decreased when alpha(IIb)beta3 is engaged. Our results suggest that cofilin is essential for actin dynamics mediated by outside-in signals in activated platelets.

Summary Aggregation, exposure of procoagulant phospholipids and shedding of microparticles are platelet responses that depend on activating conditions. To determine how these different responses are interconnected, we simultaneously measured fibrinogen (Fg) binding and aminophospholipid exposure on activated platelets by means of flow cytometry. Low calcium ionophore (A23187) concentrations induced Fg binding but not annexin V binding. In contrast, high A23187 concentrations induced annexin V binding but not Fg binding. Collagen, both alone and in the presence of thrombin, induced both Fg and annexin V binding. Dual labelling was found on 38 ± 9% of platelets stimulated by thrombin plus collagen. The regulatory role of calpain in these platelet functions was investigated. When calpain was partially inhibited by 2 μg/ml calpeptin, Fg binding still occurred but aminophospholipid exposure was limited. By contrast, complete inhibition of calpain by 100 μg/ml calpeptin or E64d decreased Fg binding but enhanced aminophospholipid exposure. In these latter conditions, cytosolic calcium‐extruding systems were inhibited. The results suggest that (i) conditions that favour aminophospholipid exposure tend to decrease the aggregation process and (ii) calpain determines the switch to either aggregation or aminophospholipid exposure by controlling intracellular calcium.

SummaryHeparin-induced thrombocytopenia (HIT), a relatively common complication of heparin therapy, results of platelet activation, via the receptor for the Fc domain of IgG (FcγRIIa), by heparin-dependentantibodies, 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 leukocyteplatelet-aggregates in a heparin-dependent-manner. The formation of leukocyteplatelet-aggregates induced by HIT plasma in the presence of heparin was (i) independent of the healthy blood donor FcγRIIa 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.