Calmidazolium evokes high calcium fluctuations in Plasmodium falciparum

Calcium and Calmodulin (CaM) are important players in eukaryote cell signaling. In the present study, by using a knockin approach, we demonstrated the expression and localization of CaM in all erythrocytic stages of Plasmodium falciparum. Under extracellular CA(2+)-free conditions, calmidazolium (CZ), a potent CaM inhibitor, promoted a transient cytosolic calcium ([CA(2+)]cyt) increase in isolated trophozoites, indicating that CZ mobilizes intracellular sources of calcium. In the same extracellular CA(2+)-free conditions, the [CA(2+)]cyt rise elicited by CZ treatment was ~3.5 fold higher when the endoplasmic reticulum (ER) calcium store was previously depleted ruling out the mobilization of calcium from the ER by CZ. The effects of the CA(2+)/H(+) ionophore ionomycin (ION) and the Na(+)/H(+) ionophore monensin (MON) suggest that the [CA(2+)]cyt-increasing effect of CZ is driven by the removal of CA(2+) from at least one CA(2+)-CaM-related (CaMR) protein as well as by the mobilization of CA(2+) from intracellular acidic calcium stores. Moreover, we showed that the mitochondrion participates in the sequestration of the cytosolic CA(2+) elicited by CZ. Finally, the modulation of membrane CA(2+) channels by CZ and thapsigargin (THG) was demonstrated. The opened channels were blocked by the unspecific Calcium Channel blocker Co(2+) but not by 2-APB (capacitative calcium entry inhibitor) or nifedipine (L-type CA(2+) channel inhibitor). Taken together, the results suggested that one CaMR protein is an important modulator of calcium signaling and homeostasis during the Plasmodium intraerythrocytic cell cycle, working as a relevant intracellular CA(2+) reservoir in the Parasite.