Collagen and fibrin engagement and activation of glycoprotein (GP) VI induces proteolytic cleavage of the GPVI ectodomain generating shed soluble GPVI (sGPVI). Collagen-mediated GPVI shedding requires intracellular signalling to release the sGPVI, mediated by A Disintegrin And Metalloproteinase 10 (ADAM10), however the precise mechanism by which fibrin induces GPVI shedding remains elusive. Plasma sGPVI levels are elevated in patients with coagulopathies, sepsis or inflammation and can predict onset of sepsis and sepsis-related mortality, therefore it is clinically important to understand the mechanisms of GPVI shedding under conditions of minimal collagen exposure. Our aim was to characterize mechanisms by which fibrin-GPVI interactions trigger GPVI shedding. Platelet aggregometry, sGPVI enzyme-linked immunosorbent assay (ELISA) and an ADAM10 fluorescence resonance energy transfer (FRET) assay were used to measure fibrin-mediated platelet responses. Fibrin induced αIIbβ3-independent washed platelet aggregate formation, GPVI shedding and increased ADAM10 activity, all of which were insensitive to pre-treatment with inhibitors of Src family kinases but were divalent cation- and metalloproteinase-dependent. In contrast, treatment of washed platelets with other GPVI ligands, collagen and collagen-related peptide (CRP) caused αIIbβ3-dependent platelet aggregation and GPVI release but did not increase constitutive ADAM10 activity. Thus, fibrin engages GPVI in a manner that differs from other GPVI ligands. Inclusion of polyanionic molecules disrupted fibrin-induced platelet aggregate formation and sGPVI release, suggesting that electrostatic charge may play a role in fibrin/GPVI engagement. It may be feasible to exploit this property and specifically disrupt GPVI/fibrin interactions whilst sparing GPVI/collagen engagement.
This article is protected by copyright. All rights reserved.