The genetic Ca2+ sensor GCaMP3 reveals multiple Ca2+ stores differentially coupled to Ca2+ entry in the human malaria parasite Plasmodium falciparum

Cytosolic Ca2+ regulates multiple steps in the host-cell invasion, growth, proliferation, and egress of blood-stage Plasmodium falciparum, yet our understanding of Ca2+ signaling in this endemic malaria parasite is incomplete. By using a newly generated transgenic line of P. falciparum (PfGCaMP3) that expresses constitutively the genetically encoded Ca2+ indicator GCaMP3, we have investigated the dynamics of Ca2+ release and influx elicited by inhibitors of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pumps, cyclopiazonic acid (CPA), and thapsigargin (Thg). Here we show that in isolated trophozoite phase parasites: (i) both CPA and Thg release Ca2+ from intracellular stores in P. falciparum parasites; (ii) Thg is able to induce Ca2+ release from an intracellular compartment insensitive to CPA; (iii) only Thg is able to activate Ca2+ influx from extracellular media, through a mechanism resembling store-operated Ca2+ entry, typical of mammalian cells; and (iv) the Thg-sensitive Ca2+ pool is unaffected by collapsing the mitochondria membrane potential with the uncoupler carbonyl cyanide m-chlorophenyl hydrazone or the release of acidic Ca2+ stores with nigericin. These data suggest the presence of two Ca2+ pools in P. falciparum with differential sensitivity to the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pump inhibitors, and only the release of the Thg-sensitive Ca2+ store induces Ca2+ influx. Activation of the store-operated Ca2+ entry-like Ca2+ influx may be relevant for controlling processes such as parasite invasion, egress, and development mediated by kinases, phosphatases, and proteases that rely on Ca2+ levels for their activation.