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 thrombin, glycoprotein Ib-IX (GP Ib-IX) complexes are cleared to the surface-connected canalicular system (Blood 1990;76:1503). The question arose as to whether GP Ia-IIa complexes (VLA-2), another adhesion receptor thought to be linked to the membrane cytoskeleton, behaved similarly. Monoclonal antibodies to GP Ia-IIa were used in either (1) immunofluorescence 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. Immunogold staining showed that the complexes were regularly distributed over the platelet surface. This was best seen after staining of paraformaldehyde-fixed whole mounts, where bound antibodies were often visualized in small clusters. The surface expression of VLA-2 receptors increased somewhat after thrombin stimulation, the receptors coming from a small intracellular pool revealed by flow cytometric analysis of Triton X-100-permeabilized cells. Immunogold staining showed that after activation 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-way trafficking of adhesion receptors can occur during platelet activation. This would imply that GP Ib-IX and VLA-2 are attached to different elements within the membrane cytoskeleton.