Calcium Signaling throughout the Toxoplasma gondii Lytic Cycle A STUDY USING GENETICALLY ENCODED CALCIUM INDICATORS

Background: Ca2+ signaling is important for the lytic cycle of T. gondii. Results: Genetically encoded Ca2+ indicators revealed cytosolic Ca2+ changes in real time. Conclusion: New approach highlights important features of the lytic cycle. Significance: Ca2+ influx leads to signaling that results in enhancement of important lytic cycle features. Toxoplasma gondii is an obligate intracellular parasite that invades host cells, creating a parasitophorous vacuole where it communicates with the host cell cytosol through the parasitophorous vacuole membrane. The lytic cycle of the parasite starts with its exit from the host cell followed by gliding motility, conoid extrusion, attachment, and invasion of another host cell. Here, we report that Ca2(+) oscillations occur in the cytosol of the parasite during egress, gliding, and invasion, which are critical steps of the lytic cycle. Extracellular Ca2(+) enhances each one of these processes. We used tachyzoite clonal lines expressing genetically encoded calcium indicators combined with host cells expressing transiently expressed calcium indicators of different colors, and we measured Ca2(+) changes in both parasites and host simultaneously during egress. We demonstrated a link between cytosolic Ca2(+) oscillations in the host and in the parasite. Our approach also allowed us to measure two new features of motile parasites, which were enhanced by Ca2(+) influx. This is the first study showing, inreal time, Ca2(+) signals preceding egress and their direct link with motility, an essential virulence trait.