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

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 Ca2(+). 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 + collagen mixture and in particular the phosphorylation of a 40 kD band were severely reduced. Furthermore, inhibition of thiol-containing enzymes, including tyrosine-phosphatases, by N-ethyl 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.