Congenital Disorders of Platelet Signal Transduction

After injury to the blood vessel, platelets adhere to the exposed subendothelium by a process (adhesion) that involves the interaction of a plasma protein, von Willebrand factor (vWF), and a specific protein on the platelet surface, glycoprotein Ib (GPIb; the Figure⇓). Adhesion is followed by recruitment of additional platelets that form clumps, a process called aggregation (cohesion). This involves binding of fibrinogen to specific platelet surface receptors—a complex comprising glycoproteins IIb-IIIa (GPIIb-IIIa). Activated platelets release the contents of their granules (secretion or release reaction), such as ADP and serotonin from dense granules, which subsequently cause recruitment of additional platelets. In addition, platelets play a major role in coagulation mechanisms; several key enzymatic reactions occur on the platelet membrane–lipoprotein surface. A number of physiological agonists interact with specific receptors on the platelet surface to induce responses, including a change in platelet shape from discoid to spherical, aggregation, secretion, and thromboxane A2 (TxA2) production. Other agonists such as prostacyclin inhibit these responses. Ligation of the platelet receptors initiates the production or release of several intracellular messenger molecules, including Ca2+ ions, products of phosphoinositide (PI) hydrolysis by phospholipase C (PLC; diacylglycerol [DG] and inositol 1,4,5-triphosphate [InsP3]), TxA2, and cyclic nucleotides (cAMP; the Figure⇓). These subsequently induce or modulate the various platelet responses of Ca2+ mobilization, protein phosphorylation, aggregation, secretion, and liberation of arachidonic acid. The interaction between the agonist receptors and the key intracellular effector enzymes (eg, PLA2, PLC, adenylyl cyclase) is mediated by a group of GTP-binding proteins that are modulated by GTP. As in most secretory cells, platelet activation results in …