Platelet-specific collagen receptor glycoprotein (GP)VI is stable on the surface of circulating platelets but undergoes ectodomain cleavage on activated platelets. Activation-dependent GPVI metalloproteolysis is primarily mediated by A Disintegrin And Metalloproteinase (ADAM) 10. Regulation of platelet ADAMs activity is not well-defined however Tissue Inhibitors of Metalloproteinases (TIMPs) may play a role. As levels of TIMPs on platelets and the control of ADAMs-mediated shedding by TIMPs has not been evaluated, we quantified the levels of TIMPs on the surface of resting and activated platelets from healthy donors by flow cytometry and multiplex ELISA. Variable levels of all TIMPs could be detected on platelets. Plasma contained significant quantities of TIMP1 and TIMP2, but only trace amounts of TIMP3 and TIMP4. Recombinant TIMP3 strongly ablated resting and activated platelet ADAM10 activity, when monitored using a quenched fluorogenic peptide substrate with ADAM10 specificity. Whilst ADAM10-specific inhibitor GI254023X or ethylenediamine tetraacetic acid (EDTA) could modulate ligand-initiated shedding of GPVI, only recombinant TIMP2 achieved a modest (~20%) inhibition. We conclude that some platelet TIMPs are able to modulate platelet ADAM10 activity but none strongly regulate ligand-dependent shedding of GPVI. Our findings provide new insights into the regulation of platelet receptor sheddase activity.
Keywords: Adhesion; Timps; metalloproteinases; platelets; receptors.
What do we know? Platelet receptor GPVI initiates platelet adhesion and aggregation and is proteolytically cleaved from the activated platelet surfaceThe metalloproteinases responsible belong to the ADAMs family of enzymes which are inhibited by TIMPsWhat did we discover? Plasma contains significant amounts of TIMP1 and TIMP2Circulating platelets bear significant amounts of TIMPs 1, 2, and 3Recombinant TIMP3 strongly inhibits resting and activated platelet ADAM10 activityExogenous addition of TIMP2 mildly blocked ligand-initiated shedding of GPVIWhat is the impact? TIMPs may modulate ADAM10 activity under resting conditions and stabilize GPVI levels in response to platelet activationAnti-GPVI agents are being evaluated as anti-thrombotic agents, however, acute loss of GPVI in trauma or settings of thrombocytopenia is linked with clinical bleedingUnderstanding how GPVI levels are regulated is important as agents that modulate GPVI function are emerging as important therapeutics for clinical applications in Thrombosis and Hemostasis fields.