Annexin V delays apoptosis while exerting an external constraint preventing the release of CD4(+) and PrPc+ membrane particles in a human T lymphocyte model

Phosphatidylserine exposure in the exoplasmic leaflet of the plasma membran e is one of the early hallmarks of cells undergoing apoptosis, The shedding of membrane particles carrying Ags testifying to their tissue origin is an other characteristic feature. Annexin V, a protein of as yet unknown specif ic physiologic function, presents a high Ca2+-dependent affinity for phosph atidylserine and forms two-dimensional arrays at the membrane surface. In t his study, we report the delaying action of annexin V on apoptosis in the C EM human T cell line expressing CD4 and the normal cellular prion protein ( PrPc), two Ags of particular relevance to cell degeneration and with differ ent attachments to the membrane. The effect of annexin V was additive to th at of z-Val-Ala-Asp-fluoromethyl ketone, a potent caspase inhibitor. Annexi n V significantly reduced the degree of proteolytic activation of caspase-3 , and totally blocked the release of CD4(+) and PrPc+ membrane particles. z -Val-Ala-Asp-fluoromethyl ketone was a more powerful antagonist of caspase- 3 processing, but prevented the shedding of CD4(+) vesicles only partially and had no effect on that of PrPc+ ones. These results suggest that an exte rnal membrane constraint, such as that exerted by annexin V, has important consequences on the course of programmed cell death and on the disseminatio n of particular Ags, In vivo, annexin V had a significant protective effect against spleen weight loss in mice treated by an alkylating agent previous ly shown to induce lymphocyte apoptosis.