Platelet signal transduction defect with Gα subunit dysfunction and diminished Gα(q) in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca(2+) mobilization. Present studies demonstrated that platelet phospholipase A(2) (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca(2+) release induced by guanosine 5′-[γ-thio]triphosphate (GTP[γS]) was diminished in the patient’s platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of α-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A(2) analog U46619. Binding of (35)S-labeled GTP[γS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I(2) analog) -induced rise in cAMP (mediated by Gα(s)) and its inhibition (mediated by Gα(i)) by thrombin in the patient’s platelet membranes were normal. Immunoblot analysis of Gα subunits in the patient’s platelet membranes showed a decrease in Gα(q) (<50%) but not Gα(i), Gα(z), Gα(12), and Gα(13). These studies provide evidence for a hitherto undescribed defect in human platelet G-protein α-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Gα(q) in thrombin-induced responses.