PLATELET SIGNAL-TRANSDUCTION DEFECT WITH G-ALPHA SUBUNIT DYSFUNCTION AND DIMINISHED G-ALPHA(Q) IN A PATIENT WITH ABNORMAL PLATELET RESPONSES

G proteins play a major role in signal transduction upon platelet acti vation, We have previously reported a patient with impaired agonist-in duced aggregation, secretion, arachidonate release, and Ca2+ mobilizat ion, Present studies demonstrated that platelet phospholipase A(2) (cy tosolic and membrane) activity in the patient was normal, Receptor-med iated activation of glycoprotein (GP) IIb-IIa complex measured by flow cytometry using antibody PAC-I was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamm a-thio]triphosphate (GTP[gamma S]) was diminished in the patient's pla telets, suggesting a defect distal to agonist receptors, GTPase activi ty (a function of alpha-subunit) in platelet membranes was normal in r esting state but was diminished compared with normal subjects on stimu lation with thrombin, platelet-activating factor, or the thromboxane A (2) analog U46619. Binding of S-35-labeled GTP[gamma S] to platelet me mbranes was decreased under both basal and thrombin-stimulated states, Iloprost (a stable prostaglandin I-2 analog) -induced rise in cAMP (m ediated by G alpha(s)) and its inhibition (mediated by G alpha(i)) by thrombin in the patient's platelet membranes were normal, Immunoblot a nalysis of G alpha subunits in the patient's platelet membranes showed a decrease in G alpha(q) (<50%) but not G alpha(i), G alpha(z), G alp ha(12), and G alpha(13). These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evid ence for a major role of G alpha(q) in thrombin-induced responses.