Membrane transformation during malaria parasite release from human red blood cells

Three opposing pathways are proposed for the release of malaria parasites [1] from infected erythrocytes [2, 3]: coordinated rupture of the two membranes surrounding mature parasites [4, 5]; fusion of erythrocyte and parasitophorus vacuolar membranes (PVM) [6-8]; and liberation of parasites enclosed within the vacuole from the erythrocyte followed by PVM disintegration [9]. Rupture by cell swelling should yield erythrocyte ghosts; membrane fusion is inhibited by inner-leaflet amphiphiles of positive intrinsic curvature [10], which contrariwise promote membrane rupture; and without protease inhibitors [9], parasites would leave erythrocytes packed within the vacuole. Therefore, we visualized erythrocytes releasing P falciparum using fluorescent microscopy of differentially labeled membranes. Release did not yield erythrocyte ghosts, positive-curvature amphiphiles did not inhibit release but promoted it, and release of packed merozoites was shown to be an artifact. Instead, two sequential morphological stages preceded a convulsive rupture of membranes and rapid radial discharge of separated merozoites, leaving segregated internal membrane fragments and plasma membrane vesicles or blebs at the sites of parasite egress. These results, together with the modulation of release by osmotic stress, suggest a pathway of parasite release that features a biochemically altered erythrocyte membrane that folds after pressure-driven rupture of membranes.