Aminophospholipid exposure, microvesiculation and abnormal protein tyrosine phosphorylation in the platelets of a patient with Scott syndrome: a study using physiologic agonists and local anaesthetics

Abstract

Summary. The Scott syndrome is a rare inherited haemorrhagic disorder characterized by the inability of blood cells to expose aminophospholipids and to shed microparticles. We have had the opportunity to study a recently reported French patient with this syndrome and have confirmed by means of a fluorescence assay for transbilayer lipid movement a reduced aminophospholipid exposure when platelets were stimulated with the calcium-ionophore ionomycin, in spite of a normal elevation of intracellular Ca 2þ . Secretion and calpain activation were also shown to be normal. Significantly, the level of phosphotyrosine-labelled proteins in platelets treated with thrombin or a thrombin þ collagen mixture and in particular the phosphorylation of a 40 kD band were severely reduced. Furthermore, inhibition of thiolcontaining enzymes, including tyrosine-phosphatases, by Nethyl maleimide did not lead to aminophospholipid exposure in the patient's platelets, in spite of increased tyrosine protein phosphorylation. In contrast, amphiphilic membrane drugs such as tetracaine and propranolol induced both surface aminophospholipid exposure in Scott platelets and the shedding of microparticles, thereby showing that membrane perturbation can lead to loss of phospholipid asymmetry in this syndrome. Our results provide the first insight that the lack of expression of procoagulant phospholipids and microparticle formation in Scott syndrome platelets is associated with a defect of intracellular signalling.

Impaired Platelet Activation in Familial High Density Lipoprotein Deficiency (Tangier Disease)

“…The distribution of lipids between both monolayers of the plasma membrane undergoes tight regulation in platelets; its perturbation may result in an impaired platelet activation. For instance, the defect of the transmembranous PS transport seen in platelets from Scott syndrome is associated with defective signaling from collagen receptors, abnormal platelet function, and prolonged bleeding time (30,31). Therefore, we considered the possibility that the defective response of Tangier platelets to collagen stimulation might be related to a distorted plasma membrane asymmetry followed by a dysregulation of intracellular signaling.…”

PAR-1-Stimulated Factor IXa Binding to a Small Platelet Subpopulation Requires a Pronounced and Sustained Increase of Cytoplasmic Calcium

“…*To whom correspondence should be addressed: Fredda London, Ph.D., Sol Sherry Thrombosis Research Center, Temple University School of Medicine, 3400 North Broad Street, Philadelphia, PA 19140, Tel: 215-707-4458; Fax: 215-707-3005; e-mail FVIIIa (4,6,11). The physiological relevance of these interactions is emphasized by the fact that occupancy of these binding sites on activated platelets is closely correlated with enhanced rates of FX activation leading to an increase in catalytic efficiency (kcat/K m ) of >2 × 10 7 -fold in the presence of the assembled complex (7), and the fact that severe, spontaneous and post-traumatic bleeding complications occur in patients with deficiencies of FIX (12), FVIII (13), FX (14) and platelet receptors for FVIIIa (15,16).…”

“…*To whom correspondence should be addressed: Fredda London, Ph.D., Sol Sherry Thrombosis Research Center, Temple University School of Medicine, 3400 North Broad Street, Philadelphia, PA 19140, Tel: 215-707-4458; Fax: 215-707-3005; e-mail FVIIIa (4,6,11). The physiological relevance of these interactions is emphasized by the fact that occupancy of these binding sites on activated platelets is closely correlated with enhanced rates of FX activation leading to an increase in catalytic efficiency (kcat/K m ) of >2 × 10 7 -fold in the presence of the assembled complex (7), and the fact that severe, spontaneous and post-traumatic bleeding complications occur in patients with deficiencies of FIX (12), FVIII (13), FX (14) and platelet receptors for FVIIIa (15,16).Although platelets respond to many agonists with functional endpoints required for primary hemostasis including adhesion to subendothelial matrix, secretion of granule contents, aggregation and platelet plug formation to stop the flow of blood through breaches of the vasculature (17,18), exposure to strong agonists, such as collagen and thrombin, results as well in membrane surface changes allowing binding and complexation of the coagulation proteins responsible for physiologically relevant intrinsic FXa and thrombin generation (19)(20)(21). Little is known of the platelet signal transduction mechanisms resulting in these membrane surface changes.…”

A hereditary bleeding disorder of dogs caused by a lack of platelet procoagulant activity

“…[15][16][17][18] Studies of Scott syndrome, a rare bleeding disorder associated with deficiency of platelet procoagulant activity, have been used to explore the basis of membrane PS movement. [19][20][21][22] Characteristics of this syndrome include a moderate to severe bleeding tendency with normal platelet aggregation and secretion response but defective membrane transbilayer movement of PS and abnormal microvesiculation. 4,23,24 Molecular analyses of the candidate gene scramblase in a single family, and in lymphocytes derived from the original patient, have not revealed the molecular basis of Scott syndrome.…”