Controlled shedding of platelet glycoprotein (GP)VI and GPIb–IX–V by ADAM family metalloproteinases


Michael C. Berndt, Monash University, Department of Immunology, Alfred Medical Research and Education Precinct (AMREP), Commercial Road, Melbourne, Australia.
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Background: Platelet glycoprotein (GP)VI that binds collagen, and GPIb–IX–V that binds von Willebrand factor, initiate thrombus formation.Objectives: In this study, we investigated the mechanisms of metalloproteinase-mediated ectodomain shedding that regulate the surface expression of GPVI, GPIbα (the major ligand-binding subunit) and GPV (that regulates thrombin-dependent activation via GPIbα).Methods and results: Immunoblotting human platelet lysates using affinity-purified antibodies against cytoplasmic domains of GPVI, GPIbα or GPV allowed simultaneous analysis of intact and cleaved receptor, and revealed (i) that a significant fraction of GPIbα, but not GPVI, exists in a cleaved state on platelets, even when isolated in the presence of metalloproteinase inhibitor (GM6001) or EDTA; (ii) the same-sized membrane-associated fragments of GPVI or GPIbα are generated by phorbol-ester (PMA), the mitochondrial-targeting reagent CCCP, the calmodulin inhibitor W7, or the thiol-modifying reagent, N-ethylmaleimide, that directly activates ADAM10/ADAM17; and (iii) GPV is shed by both metalloproteinase- and thrombin-dependent mechanisms, depending on the concentration of thrombin. Based on the predicted cleavage area defined by these studies, ADAM10, but not ADAM17, cleaved a GPVI-based synthetic peptide within the extracellular membrane-proximal sequence (PAR^Q243YY) as analyzed by MALDI-TOF-MS. In contrast, ADAM17, but not ADAM10, cleaved within the GPIbα-based peptide (LRG^V465LQ). Both ADAM10 and ADAM17 cleaved within a GPV-based peptide (AQP^V494TT). Metalloproteinase-mediated shedding of GPIbα from GPIb-IX-transfected or GPVI-transfected cells induced by W7 or N-ethylmaleimide was inhibited by mutagenesis of sequences identified from peptide analysis.Conclusions: These findings suggest surface levels of GPVI, GPIbα and GPV may be controlled by distinct mechanisms involving ADAM10 and/or ADAM17.