GP6 Signaling Pathway

GPVI is a member of the immunoglobulin superfamily and is expressed in platelets and their precursor megakaryocytes. It serves as the major signaling receptor for collagen, which induces platelet activation and thrombus formation. GPVI can also be activated by laminin, fibrin, collagen-related peptide (CRP), convulxin, alborhagin and by low shear stress. The extracellular region of GPVI contains two Ig-like domains, which are responsible for collagen binding, and a short mucin-like Ser/Thr-rich stalk. GPVI is expressed on platelets in a mixture of monomeric and dimeric forms. The affinity of collagen for monomeric GPVI is too low to mediate activation, and dimeric GPVI is the active form with increased affinity.Platelets abundantly express sheddases of the metalloproteinase (ADAM) family, which is used to regulate the function of adhesion and signaling receptors. The sheddase responsible for GPVI proteolysis is ADAM10. The association of calmodulin with GPVI inhibits the activation of ADAM10, and is regulated by calcium signaling, which plays a key role in overall platelet activation.On the platelet plasma membrane, GPVI forms a complex with the homodimeric FcR-gamma. Each FcR-gamma chain contains one copy of an immunoreceptor tyrosine-based activation motif (ITAM) with two Tyr residues. Src family protein kinases Fyn and Lyn phosphorylate the ITAM Tyr residues which triggers GPVl-mediated signaling.Phosphorylated FcR-gamma activates tyrosine kinase Syk, which leads to a cascade of protein phosphorylation events, phosphorylating the transmembrane adapter protein LAT, cytosolic adapter protein SLP76 and GADs and other adaptor and effector proteins, which together form LAT signalosome. The recruitment and phosphorylation of these proteins leads to the activation of phospholipase PLC-gamma-2 which cleaves phosphatidylinositol 4,5-diphosphate (PIP2) into the second messengers 1,2-diacylglycerol (DG) and inositol 1,4,5-trisphosphate (IP3). IP3 binds to receptors in plasma and intracellular membranes, leading to the release of Ca2+, while DG is the activator molecule for protein kinase C (PKC). The accumulated Ca2+ and PKC contribute to integrin-mediated platelet activation, which induce thrombus formation.The phosphoinositide 3-kinase (PI3K) phosphorylates PIP2, converting it into phosphatidylinositol 3,4,5-triphosphate (PIP3), which leads to platelet activation via Akt signaling and also supports the recruitment of the tyrosine kinase BTK to the membrane, which undergoes autophosphorylation subsequent to phosphorylation by Lyn. BTK is responsible for PLC-gamma-2 phosphorylation and protein tyrosine kinase 2 (PTK2) activation. PTK2 activation leads to dense granule secretion and also has an important role in platelet activation via intracellular ROS accumulation.