Ionophore-Resistant Mutants of Toxoplasma gondii Reveal Host Cell Permeabilization as an Early Event in Egress

Abstract

Toxoplasma gondii is an obligate intracellular pathogen within the phylum Apicomplexa. Invasion and egress by this protozoan parasite are rapid events that are dependent upon parasite motility and appear to be directed by fluctuations in intracellular [Ca 2؉ ]. Treatment of infected host cells with the calcium ionophore A23187 causes the parasites to undergo rapid egress in a process termed ionophore-induced egress (IIE). In contrast, when extracellular parasites are exposed to this ionophore, they quickly lose infectivity (termed ionophoreinduced death [IID]). From among several Iie ؊ mutants described here, two were identified that differ in several attributes, most notably in their resistance to IID. The association between the Iie ؊ and Iid ؊ phenotypes is supported by the observation that two-thirds of mutants selected as Iid ؊ are also Iie ؊ . Characterization of three distinct classes of IIE and IID mutants revealed that the Iie؊ phenotype is due to a defect in a parasite-dependent activity that normally causes infected host cells to be permeabilized just prior to egress. Iie ؊ parasites underwent rapid egress when infected cells were artificially permeabilized by a mild saponin treatment, confirming that this step is deficient in the Iie ؊ mutants. A model is proposed that includes host cell permeabilization as a critical part of the signaling pathway leading to parasite egress. The fact that Iie ؊ mutants are also defective in early stages of the lytic cycle indicates some commonality between these normal processes and IIE.

The Loss of Cytoplasmic Potassium upon Host Cell Breakdown Triggers Egress of Toxoplasma gondii

“…Similar to what we report here during normal egress, Black et al (10) also observed permeabilization of the host plasma membrane prior to ionophore-induced egress. In addition, they were able to uncouple this permeabilization event from induced egress by immobilizing the parasites with cytochalasin.…”

“…Recently, Black and co-workers (10) published the isolation of a number of Toxoplasma mutants that displayed a 4 -8-fold delayed egress response to treatment with ionophore (10). Similar to what we report here during normal egress, Black et al (10) also observed permeabilization of the host plasma membrane prior to ionophore-induced egress.…”

“…Calcium ionophores, such as A23187, have been found to be effective inducers of Toxoplasma egress (9), suggesting that alterations in the concentration of calcium in either the host cell or the parasite are involved in inducing parasite egress. Recently, Toxoplasma mutants have been isolated that display a delayed response to ionophore-induced parasite egress (10). Similarly, the treatment of Toxoplasma-infected cells with the strong reducing agent DTT 1 results in the rapid egress of the parasites (11).…”

Phosphorylation of eukaryotic initiation factor-2α promotes the extracellular survival of obligate intracellular parasite Toxoplasma gondii

“…The number of ΔKu80 or TgIF2α-S71A parasites with trails was recorded from a minimum of six independent microscope fields. Parasite egress assays were performed as previously described (27).…”

Enhanced Egress of Intracellular Eimeria tenella Sporozoites by Splenic Lymphocytes from Coccidian-Infected Chickens

“…Toxoplasma gondii egression is dependent on K ϩ ion efflux and can be mimicked experimentally using the ionophore nigericin (10). Furthermore, increasing intracellular Ca 2ϩ levels can also induce T. gondii egression (6,9,35), which can be inhibited using Ca 2ϩ chelators (4). In this report, we describe a premature egression of sporozoites from Eimeria tenella-infected host cells which occurred when the infected host cells were cocultured in vitro with spleen lymphocytes from E. tenella-infected chickens.…”