GLYCOPROTEIN IA-IIA (VLA-2) AND GLYCOPROTEIN IB-IX COMPLEXES ARE PROCESSED INDEPENDENTLY DURING THROMBIN-INDUCED PLATELET ACTIVATION

We have previously shown that when human platelets are stimulated by t hrombin, glycoprotein Ib-IX (GP Ib-IX) complexes are cleared to the su rface-connected canalicular system (Blood 1990;76:1503). The question arose as to whether GP Ia-IIa complexes (VLA-2), another adhesion rece ptor thought to be linked to the membrane cytoskeleton, behaved simila rly. Monoclonal antibodies to GP Ia-IIa were used in either (1) immuno fluorescence procedures and flow cytometry or (2) immunogold staining and electron microscopy. In flow cytometry, VLA-2 was shown to have a low but variable expression in the platelets of different donors. Immu nogold staining showed that the complexes were regularly distributed o ver the platelet surface. This was best seen after staining of parafor maldehyde-fixed whole mounts, where bound antibodies were often visual ized in small clusters. The surface expression of VLA-2 receptors incr eased somewhat after thrombin stimulation, the receptors coming from a small intracellular pool revealed by flow cytometric analysis of Trit on X-100-permeabilized cells. Immunogold staining showed that after ac tivation the receptors were equally as present on pseudopods as on the peripheral zone of the platelet. Down-regulation, as seen with GP Ib- IX complexes, was not observed. Our results therefore show that two-wa y trafficking of adhesion receptors can occur during platelet activati on. This would imply that GP Ib-IX and VLA-2 are attached to different elements within the membrane cytoskeleton.