Platelets and platelet-derived microvesicles: Phenotype, content, impact on endothelial cells

Blood platelets are circulating anuclear structures derived from megakaryocytes. Intercellular adhesion molecules, Toll-like receptors, chemokine and cytokine receptors are represented on their surface. Platelets contain biologically active molecules, including chemokines, cytokines, and growth factors. Due to their cytological features (membranes and secretory granules), the platelets are capable of fast activation and interactions with different cells. Platelets are involved in hemostasis, immune reactions, and angiogenesis, being activated by sub-endothelial components, complement proteins, secretion products from other platelets. The activation is mediated via calcium ions. Upon these events, the platelets change their morphology, release secretory granules and produce microvesicles, a relatively new target of biological research. The aim of this review is a comparative description of platelets and their microvesicles. Platelet-derived microvesicles perform platelets functions and communicate with other cells, including endothelium. Microvesicles represent a promising object of research, and the opportunities of their applications for diagnostics and therapy are being actively studied. Majority of circulating microvesicles are of platelet origin. The platelet-derived microvesicles contain cytokines and other proteins, lipids and nucleic acids (DNA, mRNA, microRNA). Microvesicles bear the surface markers of parental cells; phosphatidylserine is represented on their membrane, which additionally participates in clotting, due to deposition of blood coagulation factors. Under the influence of signals from microenvironment, the composition, phenotype of platelet microvesicles, as well as their functional abilities towards endothelium may vary, depending on the actual stimuli. Their effects upon angiogenesis and regeneration have not been sufficiently elucidated, with controversial effects reported in experimental studies. Increased levels of platelet microvesicles are observed in the disorders accompanied by endothelial dysfunction, thus suggesting their possible participation in these events. The effects of platelets and their microvesicles on endothelium, including activation of various signaling pathways in endothelial cells, still remain the subject of further research.