A thrombin receptor function for platelet glycoprotein Ib-IX unmasked by cleavage of glycoprotein V.
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.
Ramakrishnan
V
V
COR Therapeutics, Inc., South San Francisco, CA 94080, USA. vramakrishnan@corr.com
DeGuzman
F
F
Bao
M
M
Hall
S W
SW
Leung
L L
LL
Phillips
D R
DR
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*
D019038:Platelet Glycoprotein GPIb-IX Complex; D006493:Heparin; D013917:Thrombin; C027962:diisopropylphosphoryl-thrombin