GP1b-IX-V

2 GP1α2 GP12 GPIX1 GPV

GPVI -Immunoglobulin superfamily-2 extracellular immunoglobulin-like domain-Binding collagen

PLATELET: 2 receptors for initial platelet adhesion and activation in flowing blood

N-terminal GP1α: major binding region for: vWF leukocyte integrin aM2 (Mac-1) -thrombin P-selectin (activated endotelium and platelet)

Le piastrine inattive si presentano come piccoli frammenti discoidali. Attivate presentano una superficie irregolare, con protrusioni piriformi.

Platelets

Platelets are blood cell fragments that originate from the cytoplasm ofmegakaryocytes in the bone marrow and circulate in blood to play amajor role in the hemostatic process and in thrombus formation afteran endothelial injury. Recent studies have provided insight into platelet functions in inflammation and atherosclerosis. A range of molecules, present on the platelet surface and/or stored in platelet granules, contributes to the cross-talk of platelets with other cells,

Following adhesion, rapid signal transduction leads to platelet activation, cytoskeletal changes associated with shape change, spreading - Secretion - Inside–out activation of integrins that support adhesion and aggregation. The major platelet integrin, aIIbh3 (GPIIb–IIIa), binds vWF or fibrinogen to mediate platelet aggregation under shear conditions

GP1b-IX-V

GPVI

-Integrin aIIb3 (GPIIb–IIIa)-binds vWF&fibrinogen

GranulesSecretion (most of the substances that are contained within the granules are synthesized in megakaryocytes, but it is possible that some of them are endocytosed from the blood plasma) - vWF- ADP (enforces activation-aggregation)-PDGF (is the major growth factor in platelets stimulating vascular smooth muscle cell migration and proliferation associated with intimalhyperplasia. Also, PDGF is chemotactic and activates monocytes. Therefore, PDGF has long been speculated to be an important participant in the development of atherosclerosis. Insertion: - P-selectin

Granules

vWF&ADPIntegrin GPIIb–IIIa P2Y1-P2Y12

P2Y1, Gq, IP3, Ca2+

P2Y12, adenyl ciclase inhibition

P-selectinPDGF

(A) Circulating platelets (a) interact with activated endothelial Cells (b) or subendothelial matrix (c) to form mural thrombi (d)providing a substrate for adhesion of leukocytes (e) whichalso adhere to activated endothelium (f) prior toextravasation through the vessel wall (g). (B) Vascularcell adhesion receptors: adhesive interactions in thevasculature [(A), a–g] are mediated by specific receptorson platelets, leukocytes, and/or endothelial cells, andtheir ligands in plasma or subendothelial matrix, such asfibrinogen, von Willebrand factor, or collagen. Theinteraction of platelet GPIba (the major ligand-bindingsubunit of GPIb-IX–V) or GPVI with von Willebrand factoror collagen, respectively, initiates activation of theintegrin, aIIbh3, that binds von Willebrand factor orfibrinogen and mediates platelet aggregation. GPIba canalso mediate platelet–endothelial cell adhesion by bindingto P-selectin, or P-selectin-bound von Willebrandfactor. GPIba can mediate platelet–leukocyte adhesionby binding to the leukocyte integrin, aMh2 (Mac-1). Theleukocyte receptors PSGL-1 and aMh2 are involved inleukocyte adhesion to endothelium by binding P-selectinor ICAM-1, respectively, on endothelial cells, or bybinding to P-selectin or GPIba, respectively, on adheredand activated platelets. This network of receptor–counterreceptor or ligand interactions provides Intricate regulation of platelet–leukocyte–endothelial cell crosstalk.

Platelet Microparticles

Platelet microparticles, released from activated platelets, contain most of the platelet adhesive molecules and proinflammatory factors, and cause a variety of inflammatory reactions, as do activated platelets. The role of activated platelets in the development of atherosclerosis may be partially attributed to platelet microparticles.

SELECTIN

- The selectins (P-, E- and L-selectin) are cell-surface glycoproteins, able to bind carbohydrates (selectin are lectins). Both selectin and selectin ligand are glycoprotein. L-selectin is expressed on granulocytes and monocytes and on most lymphocytes. P-selectin is stored in α-granules of platelets and inWeibel–Palade bodies of endothelial cells, and is translocated to the cell surface of activated endothelial cells and platelets.

- Only P-selectin glycoprotein ligand 1 (PSGL-1) has been extensively characterized. In addition to being responsible for 90% of P-selectin binding, PSGL-1 is also the most important L-selectin ligand in inflammatory settings. PSGL-1 can also bind to E-selectin, but is not the major E-selectin ligand, which remains to be discovered.

- Activated platelet and endothelial cells have both selectins and PSGL-1, whereas leucocytes have only PSGL-1.

- Monocytes express functional PSGL-1 and use selectins to leave the vascular system. The selectins participate in the capture, rolling and slow rolling of leucocytes Platelet P-selectin is required for efficient interaction with monocytes and endothelial cells.

- Inhibition of P-selectins (by antibody, inhibitors or "false" PSGL-1 may be of therapeutical use)

The role of selectins in inflammation and disease (TRENDS in Molecular Medicine 2003)

The selectins are cell-surface glycoproteins, able to bind carbohydrates (i.e., lectins)There are 3 types of selectin: E-, L- and P-selectin. L-selectin is expressed on all granulocytes and monocytes and on most lymphocytes. P-selectin is stored in α-granules of platelets and inWeibel–Palade bodies of endothelial cells, and is translocated to the cell surface of activated endothelial cells and platelets. E-selectin is not expressed under baseline conditions, except in skin microvessels, but is rapidly induced by inflammatory cytokines.

Fig. 1. Selectin structure. (a) Selectins are composed of an N-terminal lectin domain (lime green), which binds sugars, an epidermal growth factor (EGF) domain (dark green), two (L-selectin), six (E-selectin) or nine (P-selectin) consensus repeats with homology to complement regulatory (CR) proteins (yellow), a transmembrane domain (red) and a cytoplasmic domain (purple).

There are many candidate ligands for selectins, but only P-selectin glycoprotein ligand 1 (PSGL-has been extensively characterized (Fig. 2). L-selectin ligands have been identified in high endothelial venules of secondary lymphatic organs

Ligando

Fig. 2. Structure of PSGL-1 homodimer. N-terminal tyrosine sulfates (purple) are followed by a long glycoprotein backbone (lime green) with many O-linked carbohydrates (dark green) and some N-linked carbohydrates (yellow). A functionally important O-glycan is indicated. A stabilizing disulfide bond (S–S) (vertical orange line) is located near the plasma membrane.

In most organs, leukocyte recruitment proceeds in a cascade-like fashion from capture to rolling to a systematic decrease of rolling velocity to firm adhesion and transmigration. The selectins participate in the capture, rolling and slow rolling steps.

Selectin-dependent platelet functionsActivated platelets express P-selectin, which binds PSGL-1 on leukocytes and monocytes. This interaction is responsible for the recruitment of inflammatory leukocytes to thrombi, where they are thought to help organize and resolve the thrombus, An additional function of platelet P-selectin is in the recruitment of monocyte-derived microparticles, which are a rich source of the blood-clotting element ‘tissue factor’, to the forming thrombus (Fig. 3). Furthermore, platelet P-selectin is required for efficient interaction with monocytes and endothelial cells, in the context of atherosclerotic lesions. Activated platelets deposit proinflammatory chemokines on the surface of endothelial cells and monocytes and accelerate atherosclerosis; blockade or elimination of platelet P-selectin function reduces atherosclerosis in mouse models. It appears likely that some of the beneficial preventative effects of drugs such as aspirin and clopidogrel are mediated by a reduction in platelet–leukocyte and platelet–endothelial interactions.

Selectins are crucial for the innate immune response, as demonstrated in selectin-deficient and selectin-ligand deficient patients and in mouse models. Disease treatment using selectin inhibition showsappears to be promising. However, chronic selectin inhibition will probably produce unfavorable consequences by suppressing the innate immune system. The effects of transient selectin inhibition in well controlled clinical settings, such as organ transplantation, or balloon angioplasty with or without stent placement, have not yet been sufficiently explored.