The Role of Host Calcium Signaling in Toxoplasma gondii egress

Toxoplasma gondii , an obligate intracellular parasite, is capable of invading virtually any nucleated cell. Active invasion by the parasite is a precise process and intracellular parasites reside and replicate inside a parasitophorous vacuole (PV). The membrane of the PV functions as a molecular sieve allowing for its ionic milieu to be in equilibrium with the host cytosol. The parasite would thus be exposed to the ionic fluctuations of the host cytoplasm, such as increases in host cytosolic Ca2+ during Ca2+ signaling events. Ca2+ is a universal signaling molecule and both the host cell and T. gondii will utilize Ca2+ signaling to stimulate diverse cellular functions. Using T. gondii tachyzoites and host cells expressing genetically encoded Ca2+ indicators, we demonstrate that host Ca2+ signaling impacts intracellular Ca2+ levels of the parasite. We propose that Ca2+ influx derived from host Ca2+ signaling into the parasite is utilized to maintain the intracellular Ca2+ stores replenished as the parasite replicates within the low Ca2+ environment of the host cell. Intracellular Ca2+ store(s) release is needed to reach the initiation spike in Ca2+ that meets the threshold of Ca2+ needed to initiate the tightly coordinated process of egress. Post activation of the signal and subsequent microneme secretion, would cause breakdown of the host cell and allow for extracellular Ca2+ influx, causing a second, larger spike in Ca2+ and activation of the glideosome, the main motility machinery. To directly deliver precise concentrations of ions needed for egress, we used patch-clamped infected host cells and monitored parasite egress. In doing so, we discovered an alternative role for K+ in timing parasite egress. This is the first study using whole-cell patch clamp to study the role of ions such as K+ and Ca2+ in T. gondii egress.