Platelet aging in vivo is associated with loss of membrane phospholipid asymmetry.

The mechanism(s) involved in the clearance of senescent platelets are largely unknown. The loss of membrane phospholipid (PL) asymmetry, with phosphatidylserine (PS) exposure appears to be an important signal for the ingestion by macrophages of apoptotic nucleated cells and it has also been suggested as a signal for the removal of aged erythrocytes. Accordingly, it seems possible that the clearance of normal aged platelets from circulation might be triggered by PS exposure. To investigate this, we determined PS exposure in human aging platelets taking advantage of the relationship between platelet density and platelet age and in dog platelets in a model of platelet aging in vivo. PS exposure was determined in two experimental conditions: 1) human platelet density subpopulations obtained by centrifugation in arabinogalactan gradients; 2) circulating canine platelets during decline in platelet count after suppression of thrombopoiesis following estradiol injection. PS exposure was determined by flow cytometry after labeling the cells with FITC-conjugated annexin V. The proportion of human platelets with exposed PS was significantly higher in high density (HD) platelets compared to low density (LD) platelets (11.3 +/- 8.0% vs 5.2 +/- 3.7%; p <0.05, respectively). In dogs, the proportion of cells with exposed PS rises dramatically with age, from 3.1 +/- 0.4% before to 17.7 +/- 12.3% ten days after estradiol injection. These findings suggest that platelet aging is associated with loss of phospholipid asymmetry and PS exposure on the outer leaflet of cell membrane, which may play an important role in the recognition and subsequent removal of senescent platelets.