Phosphatidylserine exposure on the platelet plasma membrane during A23187-induced activation is independent of cytoskeleton reorganization
Gaffet, P.; Bettache, N.; Bienvenüe, A.
European Journal of Cell Biology 67(4): 336-345
1995
ISSN/ISBN: 0171-9335 PMID: 8521873 Document Number: 453234
Modifications in cytoskeleton organization (monitored by scanning electron microscopy study of platelet shape) and cytoskeleton proteolysis were investigated for their role in phosphatidylserine exposure (measured with spin-labeled analogues of phospholipids) during A23187-induced activation of human platelets. Resting platelets treated with combinations of calpeptin and cytoskeleton-disrupting agents (nocodazole or cytochalasin D) remained discoid, and there was no dense granule release, cytoskeleton proteolysis or vesicle shedding. Spin-labeled phosphatidylserine was fully and rapidly redistributed (t1/2 approximately 5 min) from the outer to the inner leaflet of the plasma membrane through ATP-dependent aminophospholipid translocase activity. In contrast, spin-labeled phosphatidylcholine was only partially and slowly redistributed (less than 20% within 60 min) to the inner leaflet. Filopod formation, vesicle shedding, and calpain-mediated proteolysis were inhibited during activation of platelets treated with calpeptin and cytoskeleton-disrupting agents. Moreover, regardless of whether platelets were treated or not, spin-labeled phosphatidylserine was rapidly (t1/2 < 1 min) and massively (50%) exposed on the outer leaflet of the plasma membrane, while the slow and slight spin-labeled phosphatidylcholine influx did not counterbalance spin-labeled phosphatidylserine outflux. These results demonstrated that phosphatidylserine exposure was not connected to the following activation-related processes: cytoskeleton modifications (actin and tubulin polymerization, submembrane skeleton proteolysis), inhibition of aminophospholipid translocase, and filopod formation. Moreover, the redistribution kinetics of spin-labeled phospholipids during activation strongly suggested the involvement of an aminophospholipid exposure mechanism that differs from a scrambling phenomenon.