Phosphatidylserine (PS) externalization occurs in sickle red blood cells (RBC) and is thought to contribute to (1) increased sickle RBC-endothelial cell adhesion, (2) increased thrombogenesis and (3) decreased sickle RBC survival. Normal phospholipid (PL) asymmetry is maintained through a normally active ATP-dependent aminophospholipid translocase (APLT) and a normally inactive phospholipid scramblase (PLSCR). When this balance is disturbed, via PLSCR activation in the presence or absence of active APLT, PS externalization occurs.
This body of work addresses relative APLT and PLSCR activities in control RBC without external phosphatidylserine (PS-) and in PS-exposing sickle RBC. We also address potential mechanisms of PS externalization via APLT inhibition, PLSCR activation, or both. Our studies show dehydration is one mechanism of APLT inhibition, and this most likely contributes to the high levels of PS externalization observed in dense sickle RBC. Our studies also provide evidence that protein kinase C (PKC) activation is a mechanism of PS externalization through activation of PLSCR. Finally, we show that APLT activity decreases as normal human RBC age in vivo.
In accordance with the generally held but previously unsubstantiated view, the organization of phospholipids in the mature RBC membrane depends on the relative activities of APLT and PLSCR. Contrary to conventional wisdom, however, we show that PS externalization can occur in the presence of normal levels of APLT activity when PLSCR is sufficiently activated. These studies indicate that that preventing or reversing PLSCR activity may be an effective way to prevent or reverse PS externalization in sickle RBC.