Abnormal phosphoinositide metabolism and protein phosphorylation in platelets from a patient with the grey platelet syndrome

Abstract

SummaryWashed platelets isolated from one patient suffering from the inherited grey platelet syndrome were studied during thrombin‐induced activation. The agonist‐induced changes in (i) morphology, (ii) typical functional cell responses, (iii) membrane phospholipid metabolism and protein phosphorylation were studied and compared with the changes obtained with normal platelets. The morphology of the platelets as visualized by electron microscopy confirmed the almost total absence of intracellular alpha‐granules and marked vacuolization. During thrombin stimulation the morphological changes were clearly delayed as compared to normal platelets, the granule centralization and aggregation occurring only 15s after thrombin addition instead of 5 s in normal platelets. After 15 s, however, even though no alpha‐granules were observed, a ring‐like structure occurred centrally, indicating that they are not a prerequisite for this reaction. The whole release reaction, i.e. liberation of [14C]ser‐otonin from dense granules and β‐n‐acetylglucosaminidase activity from lysosomes, and the thromboxane synthesis were delayed and remained lower than in normal platelets. No thrombin‐induced phosphatidyl 4,5‐bisphosphate breakdown was measurable on 32P‐prelabelled platelets although [32P]phosphatidate formation occurred normally. Phosphorylation time courses of myosin light chain (P20) and of protein P43 (mol wt 43 000) markedly differed from those of controls, being less than half of the normal during the first 15 s and remaining subnormal even after complete aggregation. These results suggest that in platelets devoid of alpha‐granules a deficient transmembrane signalling system is likely responsible for the impaired physiological responses.