Platelets: Physiology and Biochemistry

Abstract

Platelets are specialized blood cells that play central roles in physiologic and pathologic processes of hemostasis, inflammation, tumor metastasis, wound healing, and host defense. Activation of platelets is crucial for platelet function that includes a complex interplay of adhesion and signaling molecules. This article gives an overview of the activation processes involved in primary and secondary hemostasis, for example, platelet adhesion, platelet secretion, platelet aggregation, microvesicle formation, and clot retraction/stabilization. In addition, activated platelets are predominantly involved in cross talk to other blood and vascular cells. Stimulated ''sticky'' platelets enable recruitment of leukocytes at sites of vascular injury under high shear conditions. Platelet-derived microparticles as well as soluble adhesion molecules, sP-selectin and sCD40L, shed from the surface of activated platelets, are capable of activating, in turn, leukocytes and endothelial cells. This article focuses further on the new view of receptor-mediated thrombin generation of human platelets, necessary for the formation of a stable platelet-fibrin clot during secondary hemostasis. Finally, special emphasis is placed on important stimulatory and inhibitory signaling pathways that modulate platelet function.

Activated Platelets Provide a Functional Microenvironment for the Antiangiogenic Fragment of Histidine-Rich Glycoprotein

“…At sites of blood vessel injury, platelets are activated and aggregate at the site of the damaged endothelium to prevent hemorrhage. Besides their role in hemostasis, platelets contribute to nonhemostatic processes such as immunity, tumor metastasis, and angiogenesis (9)(10)(11). Despite their content of both positive and negative regulators of blood vessel formation, platelets have in several different experimental settings been shown to stimulate angiogenesis (11)(12)(13)(14)(15).…”

Characterization of a new peptide agonist of the protease-activated receptor-1

“…Platelet activation plays a key role in the pathophysiology of thrombotic diseases [1,[32][33][34]. The major receptors on the platelet membrane include protease-activated receptors (PARs) and ADP receptors [35,36].…”

“…Platelet activation leads to shape change, aggregation, secretion from granules, and finally in the formation of a stable clot at the damaged endothelium surface [1]. Physiologically platelet activation occurs primarily by collagen in the subendothelium, but is rapidly amplified by recruitment of platelets to the site of the injury [2][3][4].…”

Microparticles released by Listeria monocytogenes-infected macrophages are required for dendritic cell-elicited protective immunity

“…It is known that following stimulation or during apoptosis, cells change their cytoskeleton structures, leading to plasma membranes encapsulating cytosolic elements that are shed into the extracellular space. [11][12][13] We hypothesized that after phagocytosis by macrophages, Lm components were encapsulated into MPs, which were subsequently released into the extracellular space. To test this, macrophages were infected with CFSE-labeled Lm and the released MPs were isolated from the supernatants by centrifugation using a widely accepted method for MP isolation.…”

“…[11][12][13] These vesicles vary from 100 to 1000 nm in size and are known as microparticles (MPs). 14 In the present study, we show that both macrophages and DCs are essential for the induction of Lm-specific Tcell responses but with different responsibilities.…”