title: Glycoprotein Ib (GPIb)-dependent and GPIb-independent pathways of thrombin-induced platelet activation.
abstract: In this study, the question of whether glycoprotein Ib (GPIb) mediates both high and moderate affinity pathways of alpha-thrombin-induced platelet activation was examined. Flow cytometric studies, using a panel of monoclonal antibodies (MoAbs), showed that Serratia marcescens protease treatment removed greater than 97% of the glycocalicin portion of GPIb but did not affect the changes in the expression of GPIX or GMP-140 that were induced by high concentrations of alpha-thrombin (10 nmol/L). However, Serratia treatment almost completely abolished the increase in platelet surface GMP-140 induced by low concentrations of alpha-thrombin (0.5 nmol/L) and diminished the downregulation of platelet surface GPIX by 60.9% +/- 5.6% (mean +/- SEM, n = 3). When present in 20-fold molar excess, an MoAb directed against the alpha-thrombin/von Willebrand factor (vWf) binding domains of GPIb completely blocked the ristocetin-dependent binding of vWf to platelets but inhibited only to about 50% the binding of alpha-thrombin and the activation-dependent binding of vWf. In platelets treated with Serratia marcescens protease to remove GPIb, a concentration of this MoAb 16,000-fold in excess of the maximum possible remaining copies of GPIb failed to inhibit platelet activation by alpha-thrombin. These studies demonstrate that activation of intact platelets by alpha-thrombin proceeds by both GPIb-dependent and GPIb-independent mechanisms.
authors.affiliation: Department of Cardiovascular Research, Tokyo Metropolitan Institute of Medical Science, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Yamamoto
authors.forename: N J
authors.initials: NJ
authors.lastname: Greco
authors.forename: M R
authors.initials: MR
authors.lastname: Barnard
authors.forename: K
authors.initials: K
authors.lastname: Tanoue
authors.forename: H
authors.initials: H
authors.lastname: Yamazaki
authors.forename: G A
authors.initials: GA
authors.lastname: Jamieson
authors.forename: A D
authors.initials: AD
authors.lastname: Michelson
mesh_terms: D000911:Antibodies, Monoclonal; D001665:Binding Sites; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D010450:Endopeptidases / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D015539:Platelet Activation / Q000187:drug effects*; D010980:Platelet Membrane Glycoproteins / Q000302:isolation & purification / Q000502:physiology*; D012706:Serratia marcescens / Q000201:enzymology; D013917:Thrombin / Q000494:pharmacology* / Q000502:physiology; D014841:von Willebrand Factor / Q000502:physiology
chemical_list: D000911:Antibodies, Monoclonal; D010980:Platelet Membrane Glycoproteins; D014841:von Willebrand Factor; D010450:Endopeptidases; D013917:Thrombin