Calcium tunneling through the entoplasmic reticulum and transfer to other organelles for optimal signaling in Toxoplasma gondii

Ca2+ signaling in cells begins with the opening of Ca2+ channels in either the plasma membrane (PM) or endoplasmic reticulum (ER), leading to a sharp increase of the physiologically low (<100 nM) cytosolic Ca2+ level. The temporal and spatial regulation of Ca{superscript 2} is crucial for the precise activation of key biological processes. In the apicomplexan parasite Toxoplasma gondii, which infects approximately one-third of the global population, Ca{superscript 2} signaling governs essential aspects of the parasites infection cycle. T. gondii relies on Ca{superscript 2} signals to regulate pathogenic traits, with several Ca{superscript 2}-signaling components playing critical roles. Ca2+ entry from the extracellular environment has been demonstrated in T. gondii for both, extracellular parasites, exposed to high Ca2+, and intracellular parasites, which acquire Ca{superscript 2} from host cells during host Ca{superscript 2} signaling events. Active egress, an essential step of the parasites infection cycle, is preceded by a large increase in cytosolic Ca2+, most likely initiated by release from intracellular stores. However, extracellular Ca2+ is also necessary to reach a cytosolic Ca2+ threshold required for timely egress. In this study, we investigated the mechanism of Ca{superscript 2} intracellular store replenishment and identified a central role for the SERCA-Ca2+-ATPase in maintaining Ca{superscript 2} homeostasis not only within the ER but also in other organelles. We demonstrate mitochondrial Ca2+ uptake, which occurs by transfer of Ca2+ from the ER, likely through membrane contact sites. Our findings suggest that the T. gondii ER plays a key role in sequestering and redistributing Ca{superscript 2} to intracellular organelles following Ca{superscript 2} influx at the PM. HIGHLIGHTSThe T. gondii ER takes up Ca2+ that enters the cytosol from the extracellular milieu. Filling of acidic stores in T. gondii depends on ER Ca{superscript 2} content. The mitochondrion of T. gondii has no direct access to extracellular Ca{superscript 2} but can take it up via transfer from the ER and/or acidic stores. The absence of SERCA activity results in reduced Ca{superscript 2} levels in the ER as well as in other organelles