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

Abstract

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. Receptormediated 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-[␥-thio]triphosphate (GTP[␥S]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of ␣-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[␥S] 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 G␣ s ) and its inhibition (mediated by G␣ i ) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of G␣ subunits in the patient's platelet membranes showed a decrease in G␣ q (<50%) but not G␣ i , G␣ z , G␣ 12 , and G␣ 13 . These studies provide evidence for a hitherto undescribed defect in human platelet G-protein ␣-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of G␣ q in thrombin-induced responses.

Coactivation of two different G protein-coupled receptors is essential for ADP-induced platelet aggregation

“…Thus, signaling through G q must be essential for ADPinduced platelet aggregation. Our conclusion is supported by at least two other studies (25,26). A patient with a congenital …”

“…Sci. USA 95 (1998)defect with decreased amounts of G␣ q was reported to have abnormal ADP-induced platelet aggregation (25). Furthermore, ADP-induced platelet aggregation and shape change were abolished in G␣ q -deficient mice, confirming the role of G q in ADP-induced platelet aggregation (26).…”

Genetic analysis of mammalian G-protein signalling

“…In addition, Ga q -de®cient mice were protected from thromboembolism induced by platelet activation in vivo through intravenous injection of a collagen/epinephrine mixture. Interestingly, platelets from a patient suering from mucocutaneous bleeding diathesis have been shown to contain reduced levels of Ga q (Gabbeta et al, 1997). These data clearly showed that the G q mediated activation of PLC is essential for induction of full platelet activation by physiological platelet activators and that the lack of Ga q cannot be compensated for by a or bg subunits of the other heterotrimeric G-proteins present in platelets, e.g.…”

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

“…Activation of the G q pathway leads to PLC stimulation, which ultimately leads to activation of PKC and an increase in intracellular calcium. In the absence of G q stimulation, as in G q -deficient mice [16] or in human patients with a dysfunctional G q [17,18], agonist-induced platelet secretion and aggregation are affected. Similarly, events downstream of signalling, i.e.…”