Increased expression of phosphatidylinositol-specific phospholipase C resistant prion proteins on the surface of activated platelets

The surface expression of prion protein (PrPC) on human platelets, as detected by now cytometry with the monoclonal antibody 3F4, increased more than two-fold (4300 v 1800 molecules/platelet) after full activation. Maximal surface expression of PrPC occurred within 3 min of platelet activation and declined to approximately half of maximal levels by 2 h at 37 degrees C. In comparison. PrPC on the surface of platelets, activated at 22 degrees C took 10 min to reach maximum but then remained constant for 2 h, In sonicated resting platelets, PrPC and P-selectin remained in intact granules after subcellular fractionation, Both glycoproteins were found in the ruptured membranes of activated platelets, suggesting that the PrPC was translocated from internal granules to the plasma membrane during activation, as is P-selectin, Platelet PrPC was not removed from the surface of platelets by phosphatidylinositol-specific phospholipase C (PIPLC) treatment but was degraded by proteinase K. Platelets may serve as a useful model for following the cellular processing of PrPC.