Targeting Glycoprotein VI and the Immunoreceptor Tyrosine-Based Activation Motif Signaling Pathway

Abstract

Abstract-Coronary artery thrombosis and ischemic stroke are often initiated by the disruption of an atherosclerotic plaque and consequent intravascular platelet activation. Thus, antiplatelet drugs are central in the treatment and prevention of the initial, and subsequent, vascular events. However, novel pharmacological targets for platelet inhibition remain an important goal of cardiovascular research because of the negative effect of existing antiplatelet drugs on primary hemostasis. One promising target is the platelet collagen receptor glycoprotein VI. Blockade or antibody-mediated depletion of this receptor in circulating platelets is beneficial in experimental models of thrombosis and thromboinflammatory diseases, such as stroke, without impairing hemostasis. In this review, we summarize the importance of glycoprotein VI and (hem)immunoreceptor tyrosine-based activation motif signaling in hemostasis, thrombosis, and thrombo-inflammatory processes and discuss the targeting strategies currently under development for inhibiting glycoprotein VI and its signaling. (Arterioscler Thromb Vasc Biol. 2014;34:1615-1620.)

Targeted downregulation of platelet CLEC-2 occurs through Syk-independent internalization

“…2 The other pathway is triggered by the major activatory platelet collagen receptor, glycoprotein VI (GPVI), which signals via the immunoreceptor tyrosine-based activation motif (ITAM)-bearing Fc receptor (FcR) g-chain, or by the C-type lectinlike receptor-2 (CLEC-2), where signaling is initiated by tyrosine phosphorylation of a single YxxL sequence, called hemi-ITAM (hemITAM), in its cytoplasmic tail. 4 CLEC-2 is a ;32-kDa type II transmembrane protein, encoded by the Clec1b gene, 5 that was originally identified as a transcript in immune cells and later found to be highly expressed in platelets where it serves as the receptor for the powerful platelet-activating snake venom protein rhodocytin (RC). 6 Upon ligand engagement of CLEC-2, hemITAM phosphorylation of the receptor is mediated by Src-family kinases (SFKs) and spleen tyrosine kinase (Syk), which is essential for signaling and downstream phosphorylation of effector proteins, including phospholipase Cg2.…”

Tetraspanin Tspan9 regulates platelet collagen receptor GPVI lateral diffusion and activation

“…However, platelets also cause thrombosis by aggregating to block blood flow in diseased arteries following the rupture of collagen-rich atherosclerotic plaques, which can lead to heart attack and stroke. Platelet GPVI is established as the major platelet-activating collagen receptor [1–4], but also binds laminin [5] and fibrin [6, 7]. GPVI has only a minor role in haemostasis, but is important in arterial thrombosis, ischaemic stroke and maintaining vascular integrity during inflammation [8, 9].…”

“…GPVI has only a minor role in haemostasis, but is important in arterial thrombosis, ischaemic stroke and maintaining vascular integrity during inflammation [8, 9]. The GPVI signalling pathway is well characterised and is initiated by phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) in the FcRγ chain dimer that is associated with GPVI [1, 4]. This is mediated by Src family tyrosine kinases and results in the recruitment of the tyrosine kinase Syk to the ITAMs.…”

“…This is mediated by Src family tyrosine kinases and results in the recruitment of the tyrosine kinase Syk to the ITAMs. This is followed by downstream phosphorylation and assembly of a signalosome nucleated by the adapter protein LAT, phospholipase Cγ2 (PLCγ2) activation, Ca 2+ mobilisation, platelet shape change, granule secretion, integrin activation and platelet spreading and aggregation [1, 4]. GPVI recognises collagen as a dimer, and a proportion of GPVI exists as a dimer on resting platelets, with dimer levels increasing upon platelet activation [10, 11].…”

Current State and Novel Approaches of Antiplatelet Therapy

“…7 At arterial shear rates, this is enabled principally by a specialized platelet-specific adhesion-signaling system, made up of glycoprotein (GP) Ib-IX-V that binds von Willebrand factor (vWF; via the GPIbα subunit) 8 and collagen (via GPV) 9 and GPVI that binds collagen and the collagen-associated matrix protein, laminin. [10][11][12][13][14] Thus, on exposure of collagenous subendothelial matrix in the vasculature, platelet GPIb-IX-V/GPVI can rapidly induce platelet adhesion and activation. Furthermore, in human platelets, these receptor-ligand systems are highly evolved to mediate thrombus formation as the shear rate increases to high physiological or pathological, 15 as found in occluded coronary arteries with atherosclerotic plaque or where normal blood flows are disrupted, for example, in ventricular assist devices.…”