title: Ca(2+)-dependence of conoid extrusion in Toxoplasma gondii tachyzoites.
abstract: The role of Ca2+ in conoid extrusion was investigated in isolated Toxoplasma gondii tachyzoites by treatment with Ca(2+)-ionophores, Ca(2+)-chelating agents and an inhibitor of the Ca(2+)-ATPase at the endoplasmic reticulum. The results were evaluated by light phase-contrast microscopy and electron microscopy. Ionomycin (0.5-1 microM) caused an immediate and sustained extrusion of the conoid in up to 80% of the tachyzoites, depending on the concentrations of ionophore and Ca2+ in the medium. However, over 50% of the tachyzoites extruded the conoid when treated with ionomycin in Ca(2+)-free saline complemented with EGTA. The effect of ionomycin was reversible and could be induced a second time in about half of the responsive population. Similar results were obtained with A23187. Conoid extrusion induced by ionomycin in Ca(2+)-free medium was almost completely abolished when the tachyzoites were previously loaded with a permeable compound known to chelate intracellular Ca2+ (BAPTA/AM; 25 microM). On the other hand, exposure of tachyzoites to the Ca(2+)-ATPase inhibitor thapsigargin (0.5-1 microM) produced significant extrusion of the conoid. Tachyzoites loaded with BAPTA/AM as well as those treated with ionomycin, i.e. with conoids paralyzed in opposite positions, had a diminished capacity to invade cultured epithelial cells. A substantial reduction in the response to stimulation by ionomycin was found also in parasites treated with cytochalasin-D, a drug that depolymerizes actin-filaments. The results suggest that Ca(2+)-release from internal stores may act as a key signal to activate a mechanism of conoid extrusion probably mediated, at least in part, by actin-filaments.
authors.affiliation: Departamento de Biología Celular, Biofísica y Neurociencias, Centro de Investigación y de Estudios Avanzados del IPN, México.
authors.forename: R
authors.initials: R
authors.lastname: Mondragon
authors.forename: E
authors.initials: E
authors.lastname: Frixione
mesh_terms: D000199:Actins / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002460:Cell Line; D002614:Chelating Agents; D015638:Cytochalasin D / Q000494:pharmacology; D004285:Dogs; D004533:Egtazic Acid / Q000031:analogs & derivatives; D005456:Fluorescent Dyes; D006790:Host-Parasite Interactions; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D008854:Microscopy, Electron; D014122:Toxoplasma / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D000199:Actins; D002614:Chelating Agents; D005456:Fluorescent Dyes; D007476:Ionophores; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D015638:Cytochalasin D; D004533:Egtazic Acid; D015759:Ionomycin; D000252:Calcium-Transporting ATPases; D002118:Calcium