Intracellular localization of glycoprotein VI in human platelets and its surface expression upon activation

Glycoprotein (GP) VI is a major receptor for collagen and belongs to the immunoglobulin super family. Here, we examined the localization of GPVI in resting and activated human platelets by immunogold scanning and transmission electron microscopy and flow cytometry. Ultrastructural observation detected immunolabelling for GPVI that was distributed uniformly over the entire surface of resting platelets, and revealed that GPVI was also localized on both the membranes of the surface-connected open canalicular system (OCS) and alpha-granules. The OCS- and alpha-granule-associated GPVI pools were an estimated 35.4 +/- 3.2% (mean +/- standard deviation) of the total. Little GPVI labelling was observed in any part of GPVI-deficient platelets. A remarkable time-dependent increase in GPVI surface expression was found by flow cytometry when platelets were activated by collagen-related peptide (CRP) and convulxin. The GPVI-mediated activation of platelets by CRP or convulxin resulted in similar ultrastructural changes and an increased GPVI labelling density on the activated platelet surface, which was accompanied by a decreased interior expression. GPVI was also expressed on microparticles generated from activated platelets. Thus, our study demonstrates that platelets have internal pools of GPVI, and that GPVI is increasingly redistributed to the surface membrane and to microparticles during platelet activation.