Lateral Clustering of Platelet GP Ib-IX Complexes Leads to Up-regulation of the Adhesive Function of Integrin αIIbβ3

Binding of von Willebrand factor (VWF) to GP Ib-IX mediates initial platelet adhesion and increases the subsequent adhesive function of αIIbβ3. Because these responses are promoted most effectively by large VWF multimers, we hypothesized that receptor clustering modulates GP Ib-IX function. To test this, GP IX was fused at its cytoplasmic tail to tandem repeats of FKBP, and GP Ib-IX(FKBP)2 and αIIbβ3 were expressed in Chinese hamster ovary cells. Under flow conditions at wall shear rates of up to 2000 s−1, GP Ib-IX(FKBP)2 mediated cell tethering to immobilized VWF, just as in platelets. Conditional oligomerization of GP Ib-IX(FKBP)2 by AP20187, a cell-permeable FKBP dimerizer, caused a decrease in cell translocation velocities on VWF (p < 0.001). Moreover, clustering of GP Ib-IX(FKBP)2 by AP20187 led to an increase in αIIbβ3 function, manifested under static conditions by increased cell adhesion to fibrinogen (p< 0.01) and under flow by increased stable cell adhesion to VWF (p < 0.04). Clustering of GP Ib-IX(FKBP)2also stimulated rapid tyrosine phosphorylation of ectopically expressed Syk, a putative downstream effector of GP Ib-IX in platelets. These studies establish that GP Ib-IX oligomerization, per se, affects the interaction of this receptor with VWF and its ability to influence the adhesive function of αIIbβ3. By extrapolation, GP Ib-IX clustering in platelets may promote thrombus formation.