Calcium-Activated Potassium Channels in the Malaria Parasite Erythrocyte Cycle

Abstract

Free calcium (Ca2+) is involved in the Apicomplexan parasite Plasmodium falciparum cycle of development, both at the time of invasion and at the time of egress from host erythrocytes. We hypothesized that the two prominent and transient erythrocyte crenation events (echinocytosis), one shortly following parasite invasion and one immediately preceding parasite egress, result from the activation of a calcium-dependent potassium channel, the Gardos channel (KCNN4 / KCa3.1), native to the red blood cell (RBC) and resident in its plasma membrane.To test if a Ca2+-dependent K+ flux (followed by water) causes the observed post-invasion RBC echinocytosis, parasite invasion was observed and quantified using RBC loaded with a Ca2+ chelator (BAPTA-AM) in a new light microscopy invasion assay. We confirmed that neither invasion nor the subsequent RBC echinocytosis were affected. Furthermore, RBC were treated with several inhibitors of Gardos channels, prior to and during parasite invasion. Again, no adverse effect on invasion or echinocytosis was observed. Finally, high external KCl that eliminates the potassium gradient across the RBC membrane did not inhibit the post-invasion RBC echinocytosis. Thus, it is unlikely that activation of the Gardos channel explains the echinocytosis of RBC following parasite invasion. While exploring the effect of calcium free medium on invasion, we found that it is often aborted but still leads to echinocytosis. Contrary to invasion in the presence of calcium, echinocytosis after abortive invasion is not reversed and often leads to RBC hemolysis.We conclude that alternative calcium independent mechanisms, such as the modification of the RBC cytoskeleton or the sudden incorporation of lipoidal substances secreted during invasion by the P. falciparum merozoite, and their effect on RBC curvature stress may explain the observed echinocytosis.