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Platelets are anucleate circulating blood particles. They circulate around the body in an inactive state until they come into contact with areas of endothelial damage or activation of the coagulation cascade. Here they adhere to the endothelial defect, change shape, release their granule contents, and stick together to form aggregates. Physiologically these processes help to limit blood loss; however, inappropriate or excessive platelet activation results in an acute obstruction of blood flow, as occurs, for example, in an acute myocardial infarction. However, activated platelets also express and release molecules that stimulate a localized inflammatory response through the activation of leukocytes and endothelial cells, and it is now clear that platelet function is not merely limited to the prevention of blood loss. Indeed, platelets have been implicated in many pathological processes including host defense, inflammatory arthritis, adult respiratory distress syndrome, and tumor growth and metastasis. In this chapter I review our current understanding of platelet physiology in order to provide a global background for the more in-depth focused chapters later in this book.

The term describes an adhesion molecule family and originates from the integrative function of these molecules between extracellular ligands and the intracellular cytoskeleton (,). Integrins mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. Integrins have two major functions: First, they mechanically couple the cytoskeleton to the extracellular matrix or to surface receptors of other cells. Second, they transmit signals from the inside of the cell to the outside of the cell and vice versa (). At least 24 different integrins are known in vertebrates (Fig. 1). Resting platelets express integrins αβ, αβ, αβ, αβ, and αβ (). In addition to these, αβ and αβ expression on activated platelets has been reported ().

Platelet adhesion and ensuing thrombus formation play a central role in normal hemostasis as well as in the pathogenesis of acute coronary syndromes and thrombotic disorders. Circulating blood platelets adhere to sites of vascular injury through specific adhesion receptors despite the hemodynamic forces in flowing blood that oppose adhesion contacts. At high shear, this process is initiated by the reversible interaction between the platelet membrane glycoprotein (GP)Ib-IX-V complex and von Willebrand factor (vWF) bound to subendothelial components, following disruption of the endothelial cell lining of a blood vessel. Unique biomechanical properties of the GPIb-IX-V/vWF interaction permit the initial capture of platelets under high shear flow conditions (,). Once tethered to the vessel wall, platelets form irreversible adhesion bonds through the interaction of platelet receptors with specific subendothelial matrix proteins and plasma proteins immobilized at the site of injury. In addition to mediating platelet adhesion, platelet receptors trigger intracellular signaling events that lead to platelet activation and the conversion of surface integrin αβ into a form that is competent to bind soluble adhesive ligands such as plasma vWF and fibrinogen, which facilitate the crosslinking (aggregation) and further activation of platelets, providing strength and stability to the growing thrombus. The formation of a platelet plug stabilized by an insoluble fibrin network serves to prevent further blood loss from a damaged vessel.

Platelet activation results in a rapid series of reproducible morphological events that transform the nonsticky, discoid circulating platelet into a sticky, spikey glue. This morphological transformation depends on actin and is a common feature of all cell-based clotting systems across evolution. An exciting recent discovery is the identification of the protein complex that mediates the actin polymerization underlying this event, Arp2/3 (). This chapter presents the background for this discovery by first reviewing the morphology of resting and activated platelets. Then examples of other proteins involved in stabilizing and reorganizing the platelet actin cytoskeleton are described, and the findings that place Arp2/3 at the center of these events are presented. Finally, a model integrating the various activities of these proteins with the morphological changes of activation is proposed. The chapter focuses on the actin cytoskeleton, although platelets also have microtubules and a vimentin-like intermediate filament protein .

Platelets play a major physiological role in control of vascular integrity at the site of vascular lesions. However, the pathophysiological role of platelets is much broader than regulation of hemostasis and thrombosis. Platelets are critical elements in linking and modulating thrombosis, inflammation, and tissue repair. Platelets are stimulated by a variety of agonists including thrombin or ADP and also by inflammatory agents such as antibodies, complement, bacteria, and others. Platelets contribute to inflammation by interacting with inflammatory cells via adhesion and secretion of prestored proinflammatory mediators. Thus, platelets are critical elements in the pathophysiology of inflammation and modulate significantly a variety of inflammatory diseases. A profound understanding of the molecular mechanisms underlying the role of platelet in inflammation may result in new therapeutic strategies in acute and chronic inflammatory diseases.

Our understanding of platelet functions has been in evolution since their discovery. Blood platelets were initially observed in the middle of the 19th century by many investigators including Zimmerman in 1860, Schultze in 1865, Osler in 1874, and Hayem in 1878 (). Studies by Bizzozero (,) were the first to recognize the adhesive qualities of platelets, their participation in thrombosis and leukocyte recruitment, and their role in blood coagulation. These monumental findings, which have withstood the test of time, have expanded at a remarkable rate and continue to be the primary focus of investigative research in the platelet arena ().

Platelets circulate freely in the blood, playing a critical role in wound healing by forming plugs and initiating repair processes and in underlying thrombotic diseases such as myocardial infarction or stroke (). Normally platelets are found in the blood in a nonadhesive, resting state. Interactions with structures in the subendothelial matrix initiate a rapid platelet activation, resulting in the formation of vascular plugs and release of intracellular substances that initiate repair processes. Genetic defects may result in bleeding disorders such as Glanzmann’s thrombasthenia and Bernard-Soulier syndrome (). The high clinical relevance of arterial thrombosis, the limited knowledge about the underlying mechanisms at the molecular level, and the small number of available antiplatelet drugs () show the necessity for gaining more profound insights into this system. One important event in regulating the platelet function is the phosphorylation/dephosphorylation of multiple proteins on various tyrosine, serine, and threonine residues within intracellular signaling cascades. To understand the exact mechanisms, it is essential to identify proteins involved in the signaling pathways and to localize their phosphorylation sites.

Platelets play an essential role in hemostasis and thrombosis (). A defect in platelet function can result in platelets that are unresponsive or hypersensitive. Characterization of a defect is an important first step in the treatment of the disorder. Although defects usually present as bleeding or bruising, the existence of a defect is not always obvious. This is particularly a problem with thrombocytopenia, owing to heparin or a glycoprotein (GP)IIb-IIIa antagonist. In these cases the bleeding may be mistaken for the pharmacological effects of the drug.

As vertebrates have evolved high-pressure, high-flow circulatory systems, an extraordinarily effective hemostatic system has developed alongside to protect these organisms from hemorrhage. More than a century’s worth of evidence indicates that platelets are the blood cells chiefly responsible for maintaining hemostasis and causing thrombosis. Platelets are geared to monitor vascular integrity and effect hemostasis in the arterial circulation, as injuries to arteries (rather than veins) are much more likely to result in circulatory collapse and death. Furthermore, deployment of the hemostatic mechanism in the setting of vascular disease—particularly that caused by atherosclerosis—is largely responsible for the tremendous disease burden associated with vascular disease, being the culminating event in myocardial infarction and stroke. In this chapter, we review the characteristics of blood flow that influence platelet function and the cellular and molecular determinants that allow the platelets to carry out their hemostatic functions under flow.

Platelet activation and aggregation are implicated in the pathophysiology of unstable angina, myocardial infarction, transient ischemic attacks, and stroke (–). Currently available oral antiplatelet agents (aspirin, ticlopidine, and clopidogrel) suppress arachidonic acid or adenosine diphosphate (ADP)-mediated platelet aggregation and have proven clinical benefit in reducing the adverse outcomes from these vascular events (–). More complete blockade of platelet function could potentially achieve even greater reductions of acute and secondary vascular events. In fact, the more potent antiplatelet agents, such as platelet glycoprotein (GP)IIb-IIIa receptor antagonists, have shown additional clinical benefit beyond that conferred by aspirin, especially in intermediate-to-high risk patients (–). Data have suggested that combination therapies targeting multiple thrombotic pathways are superior to single-agent use (, , –). The understanding of the pharmacodynamics of all agents—singly or in combination—is critically important to the treatment of patients presenting with vascular disease.