Free Ca2+ Initiates and Regulates Malaria Parasite Egress Program in Infected Erythrocytes

Abstract

Malaria parasite egress from infected erythrocytes finalizes the asexual cycle of this organism and leading to parasite dissemination and disease progression. The egress mechanism is not fully understood, but several tightly coordinated steps and pathways were described recently. Mature parasites breach two sets of membranes to escape an infected host cell. We investigated the role of free calcium in the initiation and control of individual steps in the egress program of Plasmodium falciparum. The entire egress pathway (tens of minutes before release) is independent of extracellular free calcium, using rather intracellular calcium accumulated within the parasite during the trophozoite-schizont stages of parasite development. Based on experiments with calcium chelators and ionophore, inhibitors of the parasite endoplasmic reticulum, fluorescence microscopy and morphological analysis of mature parasites we propose that the schizont endoplasmic reticulum is the calcium source for egress initiation and that several post-initiation steps in the egress program are affected by increased intracellular Ca2+. We also reveal a new calcium-dependent step in the parasite egress mechanism: swelling of the parasitophorous vacuole (PV). This major event in parasite egress leads to the rupture of critically swelled vacuoles and mature parasite extrusion from the host cell. We found that this vacuolar swelling is exaggeratedly slow in dehydrated sickle cells, leading to a severe defect in parasite egress, probably due to low levels of ions conducting osmotic driven water translocation from erythrocyte cytoplasm to PV. Parasite egress can be accelerated in mature schizonts by pharmacological intervention; the mechanism of this phenomenon involves PV swelling. However, the same treatment of immature schizonts leads to immediate parasite death within the host cell due to erythrocyte hemolysis of a critically expanded PV. Exploring this stage-dependent parasite killing may be useful for development of anti-malarials.