Mechanisms of Platelet Activation and Integrin

Platelet aggregation is not only an essential part of hemostasis, but also initiates acute coronary syndrome or ischemic stroke. The precise understanding of the activation mechanism of platelet aggregation is fundamental for the development of more effective agents against platelet aggregation. Adenosine diphosphate, thrombin, and thromboxane $A_2$ activate platelet integrin ${\alpha}II{\beta}3$ through G protein-coupled receptors. G protein-mediated signaling pathways, which are initiated by $G_q$, $G_{12}/G_{13}$ or $G_i$, include phospholipase C with calcium signaling, Rho signaling, protein kinase C and phosphatidylinositol 3-kinase. Rap1b, $Ca^{2+}$ and diacylglycerol-regulated guanine nucleotide exchange factor I, Rap1-GTP-interacting adaptor molecule, and Akt are important proteins involved in G protein-mediated activation of integrin ${\alpha}IIb{\beta}3$. Binding of talin-1 and kindlin-3 to cytoplasmic domains of ${\beta}3$-integrin triggers a conformational change in the extracellular domains that increases its affinity for ligands, such as fibrinogen or von Willebrand factor. Fibrinogens act as bridges between adjacent platelets to generate a platelet aggregate.