title: A thrombin receptor function for platelet glycoprotein Ib-IX unmasked by cleavage of glycoprotein V.
abstract: Glycoprotein (GP) V is a major substrate cleaved by the protease thrombin during thrombin-induced platelet activation. Previous analysis of platelets from GP V-null mice suggested a role for GP V as a negative modulator of platelet activation by thrombin. We now report the mechanism by which thrombin activates GP V -/- platelets. We show that proteolytically inactive forms of thrombin induce robust stimulatory responses in GP V null mouse platelets, via the platelet GP Ib--IX--V complex. Because proteolytically inactive thrombin can activate wild-type mouse and human platelets after treatment with thrombin to cleave GP V, this mechanism is involved in thrombin-induced platelet aggregation. Platelet activation through GP Ib-IX depends on ADP secretion, and specific inhibitors demonstrate that the recently cloned P2Y(12) ADP receptor (G(i)-coupled ADP receptor) is involved in this pathway, and that the P2Y(1) receptor (G(q)-coupled ADP receptor) may play a less significant role. Thrombosis was generated in GP V null mice only in response to catalytically inactive thrombin, whereas thrombosis occurred in both genotypes (wild type and GP V null) in response to active thrombin. These data support a thrombin receptor function for the platelet membrane GP Ib--IX--V complex, and describe a novel thrombin signaling mechanism involving an initiating proteolytic event followed by stimulation of the GP Ib--IX via thrombin acting as a ligand, resulting in platelet activation.
authors.affiliation: COR Therapeutics, Inc., South San Francisco, CA 94080, USA. vramakrishnan@corr.com
authors.forename: V
authors.initials: V
authors.lastname: Ramakrishnan
authors.forename: F
authors.initials: F
authors.lastname: DeGuzman
authors.forename: M
authors.initials: M
authors.lastname: Bao
authors.forename: S W
authors.initials: SW
authors.lastname: Hall
authors.forename: L L
authors.initials: LL
authors.lastname: Leung
authors.forename: D R
authors.initials: DR
authors.lastname: Phillips
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000502:physiology*; D016466:CHO Cells; D006224:Cricetinae; D006493:Heparin / Q000378:metabolism; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D015539:Platelet Activation / Q000502:physiology; D010974:Platelet Aggregation / Q000502:physiology*; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D013917:Thrombin / Q000378:metabolism*
chemical_list: D019038:Platelet Glycoprotein GPIb-IX Complex; D006493:Heparin; D013917:Thrombin; C027962:diisopropylphosphoryl-thrombin