Signaling mechanisms of the platelet glycoprotein Ib-IX complex

Abstract

The glycoprotein Ib-IX (GPIb-IX) complex mediates initial platelet adhesion to von Willebrand factor (VWF) immobilized on subendothelial matrix and endothelial surfaces, and transmits VWF binding-induced signals to stimulate platelet activation. GPIb-IX also functions as part of a mechanosensor to convert mechanical force received via VWF binding into intracellular signals, thereby greatly enhancing platelet activation. Thrombin binding to GPIb-IX initiates GPIb-IX signaling cooperatively with protease-activated receptors to synergistically stimulate the platelet response to low-dose thrombin. GPIb-IX signaling may also occur following the binding of other GPIb-IX ligands such as leukocyte integrin α_Mβ₂ and red cell-derived semaphorin 7A, contributing to thrombo-inflammation. GPIb-IX signaling requires the interaction between the cytoplasmic domains of GPIb-IX and 14-3-3 protein and is mediated through Src family kinases, the Rho family of small GTPases, phosphoinositide 3-kinase-Akt-cGMP-mitogen-activated protein kinase, and LIM kinase 1 signaling pathways, leading to calcium mobilization, integrin activation, and granule secretion. This review summarizes the current understanding of GPIb-IX signaling.

Keywords: Glycoprotein Ib (GPIb); integrin; intracellular signaling; platelet activation; thrombosis; von Willebrand factor.

Figure 1.

A schematic illustrating the shear-resistant and shear-responsive characteristics of GPIb-IX binding to VWF A1 domain. Left: GPIbα with folded mechanosensory domain (MSD) prior to VWF binding. Right: VWF binds to GPIbα via a shear force-resistant “catch bond”. Shear force, via GPIb-bound VWF, unfolds MSD of GPIbα, which subsequently changes the conformation of the transmembrane complex of GPIb-IX, resulting in cross-membrane signal transduction and regulating the interactions with proteins important for GPIb-IX-initiated intracellular signaling.

Figure 2.

GPIb-IX-mediated early platelet activation pathways and secondary signal amplification mechanisms. In GPIb-IX-mediated early signaling, GPIb-IX ligation by VWF induces activation of Src family kinase (SFK) Lyn via a 14-3-3-dependent mechanism and a series of downstream signaling events as depicted in the figure, leading to activation of phospholipase C (PLC). A simplified schematic of the PLC-mediated common pathway leading to integrin α_IIbβ₃ activation and granule secretion is shown in the box with thin outlines: PLC cleaves membrane phospholipids to generate inositol 1,4,5 triphosphate (IP3) and diacylglycerol (DAG), which together activate calcium release and DAG-activated guanine nucleotide exchange factor 1 (CalDAGGEF1). CalDAGEF1 activates RAS-related protein 1 (Rap1), which induces talin binding to the cytoplasmic domain of integrin α_IIbβ₃ and integrin activation. DAG and/or calcium also activate protein kinase C (PKC) isoforms to stimulate granule secretion. Top Right: Activated integrin α_IIbβ₃ mediates stable platelet adhesion and initiates outside-in signaling, which, via a Gα₁₃- and c-Src-dependent mechanism, activates NADPH oxidase 2 (NOX2) to generate reactive oxygen species (ROS) under shear stress. ROS inhibits protein tyrosine phosphatases (PTP), which are negative regulators of Spleen tyrosine kinase (Syk), facilitating Syk activation. Syk is activated by binding to immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic domain of GPVI-associated Fc receptor γ chain (FcRγ) or Fcγ receptor IIA (FcγRIIA). Activated Syk stimulates the ITAM signaling pathway (see ref for more details), to potently activate platelets via PLCγ, which, as depicted in the thick-outlined box, induces a 2^nd wave of Ca⁺⁺ release and robust platelet activation and thrombus formation. GPIbβ and GPVI also bind to TNF receptor associated factor 4 (TRAF4) in complex with Hydrogen peroxide-inducible clone 5 protein (Hic-5) and p47PHOX, a regulatory unit of NOX2, which then stimulates GPVI-mediated ITAM signaling. Other abbreviations: GPIb, Glycoprotein Ib; VWF, von Willebrand factor; Rac1, Ras-related C3 botulinum toxin substrate 1; PAK1, P21-activated kinase 1; PI3K, phosphoinositide 3-kinase; cGMP, cyclic guanosine monophosphate; PKG, protein kinase G; MAPKs, mitogen-activated protein kinases; LIMK1, LIM domain kinase 1; cPLA2, cytosolic phospholipases A2; TXA2, thromboxane A2.