Regulation of calcium entry by cyclic GMP signaling in Toxoplasma gondii
Miryam Andrea Hortua Triana, Karla M. Márquez‐Nogueras, Mojtaba S. Fazli, Shannon Quinn, Silvia N.J. Moreno
Abstract
Ca 2+ signaling impacts almost every aspect of cellular life. Ca 2+ signals are generated through the opening of ion channels that permit the flow of Ca 2+ down an electrochemical gradient. Cytosolic Ca 2+ fluctuations can be generated through Ca 2+ entry from the extracellular milieu or release from intracellular stores. In Toxoplasma gondii , Ca 2+ ions play critical roles in several essential functions for the parasite, like invasion of host cells, motility, and egress. Plasma membrane Ca 2+ entry in T. gondii was previously shown to be activated by cytosolic calcium and inhibited by the voltage-operated Ca 2+ channel blocker nifedipine. However, Ca 2+ entry in T. gondii did not show the classical characteristics of store regulation. In this work, we characterized the mechanism by which cytosolic Ca 2+ regulates plasma membrane Ca 2+ entry in extracellular T. gondii tachyzoites loaded with the Ca 2+ indicator Fura-2. We compared the inhibition by nifedipine with the effect of the broad spectrum TRP channel inhibitor, anthranilic acid or ACA, and we find that both inhibitors act on different Ca 2+ entry activities. We demonstrate, using pharmacological and genetic tools, that an intracellular signaling pathway engaging cyclic GMP, protein kinase G, Ca 2+ , and the phosphatidyl inositol phospholipase C affects Ca 2+ entry and we present a model for crosstalk between cyclic GMP and cytosolic Ca 2+ for the activation of T. gondii 's lytic cycle traits.