GPIbα Is Essential for Platelet Adhesion during Thrombus Formation: Studies with Mutant Mice Deficient in the Extracellular Domain of GPIbα.

GPIbα binding to von Willebrand factor (vWf) exposed at a site of vascular injury is the first step in the formation of a hemostatic plug. In addition, GPIbα binding to platelet-bound vWf has previously been shown to play a key role in the incorporation of circulating platelets into a growing thrombus. However, studies in vWf-deficient mice demonstrated delayed but not absent arterial thrombus formation, suggesting that GPIbα may bind a ligand other than vWf to facilitate platelet adhesion, or that a platelet surface receptor other than GPIbα can mediate platelet adhesion in arterioles in the absence of vWf. Here we studied thrombus formation in transgenic mice expressing GPIbα in which the extracytoplasmic sequence has been replaced by an isolated domain of the human interleukin 4 receptor (IL4R-tg mice). Early platelet adhesion to ferric chloride-treated mesenteric arterioles in IL4R-tg mice was decreased by >98% when compared with controls. As a consequence, thrombus formation was completely inhibited in all of the mutant mice. To study the role of GPIbα in platelet adhesion to already adherent platelets, we studied thrombus formation in wild-type mice infused with wild-type platelets labeled with calcein-green and IL4R-tg platelets labeled with calcein-orange/red. Upon ferric chloride-induced injury, wild-type but not IL4R-tg platelets incorporated into the growing thrombus. A similar result was observed with wild-type platelets treated with O-sialoglycoprotein endopeptidase to remove the 45 kD N-terminal domain of GPIbα. In summary, our studies in GPIbα mutant mice provide additional evidence that under arterial flow conditions GPIbα is the only receptor expressed on the platelet surface that mediates initial platelet adhesion to the subendothelium as well as to already adherent platelets. Our data further suggest that a ligand other than vWf may contribute to GPIbα-dependent platelet adhesion to subendothelium and to a growing thrombus.