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 Ca2 +-free conditions, calmidazolium (CZ), a potent CaM inhibitor, promoted a transient cytosolic calcium ([Ca2 +]cyt) increase in isolated trophozoites, indicating that CZ mobilizes intracellular sources of calcium. In the same extracellular Ca2 +-free conditions, the [Ca2 +]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 Ca2 +/H+ ionophore ionomycin (ION) and the Na+/H+ ionophore monensin (MON) suggest that the [Ca2 +]cyt-increasing effect of CZ is driven by the removal of Ca2 + from at least one Ca2 +-CaM-related (CaMR) protein as well as by the mobilization of Ca2 + from intracellular acidic calcium stores. Moreover, we showed that the mitochondrion participates in the sequestration of the cytosolic Ca2 + elicited by CZ. Finally, the modulation of membrane Ca2 + channels by CZ and thapsigargin (THG) was demonstrated. The opened channels were blocked by the unspecific calcium channel blocker Co2 + but not by 2-APB (capacitative calcium entry inhibitor) or nifedipine (L-type Ca2 + 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 Ca2 + reservoir in the parasite.