The Antibiotic Monensin Causes Cell Cycle Disruption ofToxoplasma gondiiMediated through the DNA Repair Enzyme TgMSH-1

Abstract

Monensin is a polyether ionophore antibiotic that is widely used in the control of coccidia in animals. Despite its significance in veterinary medicine, little is known about its mode of action and potential mechanisms of resistance in coccidian parasites. Here we show that monensin causes accumulation of the coccidian Toxoplasma gondii at an apparent late-S-phase cell cycle checkpoint. In addition, experiments utilizing a monensinresistant T. gondii mutant show that this effect of monensin is dependent on the function of a mitochondrial homologue of the MutS DNA damage repair enzyme (TgMSH-1). Furthermore, the same TgMSH-1-dependent cell cycle disruption is observed with the antiparasitic ionophore salinomycin and the DNA alkylating agent methyl nitrosourea. Our results suggest a novel mechanism for the mode of action of monensin and salinomycin on coccidial parasites, in which the drug activates an MSH-1-dependent cell cycle checkpoint by an unknown mechanism, ultimately leading to the death of the parasite. This model would indicate that cell cycle disruption is an important mediator of drug susceptibility and resistance to ionophoric antibiotics in coccidian parasites.

A Novel Benzodioxole-Containing Inhibitor of Toxoplasma gondii Growth Alters the Parasite Cell Cycle

“…The effect of F1792-0016 on histone-related transcript levels is very similar to that observed previously for monensin, which was found to have potent antiToxoplasma activity. Microarray analyses of monensin-treated parasites resulted in increased abundances of some of the same transcripts that are upregulated in the presence of F1792-0016, particularly members of the histone family (H2, H3, and H4) (36). Consistent with this observation, monensin also increases the number of parasites that are in the S/M phase of the cell cycle (36).…”

“…9). This suggests that a possible mechanism of action for F1792-0016 may be to arrest parasites in the S/M phase of the cell cycle, as has been seen with other inhibitors of T. gondii growth, such as monensin (36). Previously, mutants resistant to the antibiotic monensin were identified, and we also examined whether one such mutant (MSH-1) was also resistant to F1792-0016 and found that it was equally susceptible to F1792-0016 as a wild-type strain (for both strains, 48 h of F1792-0016 exposure resulted in 40% S/M parasites, compared to 29% in controls; P Ͼ 0.05).…”

“…Microarray analyses of monensin-treated parasites resulted in increased abundances of some of the same transcripts that are upregulated in the presence of F1792-0016, particularly members of the histone family (H2, H3, and H4) (36). Consistent with this observation, monensin also increases the number of parasites that are in the S/M phase of the cell cycle (36). When we tested whether F1792-0016 was acting in a similar fashion, we found that a larger percentage of compound-treated parasites than of vehicle-treated controls were in the S/M phase of the cell cycle, suggesting that cell cycle disruption may be a mechanism of action and/or a downstream effect of F1792-0016 treatment.…”

“…Parasites were used to infect monolayers for 24 h, then treated as described above with either vehicle or 2 M F1792-0016 for 24 and 48 h, and then harvested for cell cycle analyses as described previously (36). Briefly, intracellular parasites were isolated by passage of host cells through a 30-gauge needle followed by filtration through a 3.0-m-pore-size membrane (Whatman).…”

Salinomycin and Other Ionophores as a New Class of Antimalarial Drugs with Transmission-Blocking Activity

“…Recent reports highlighted their activity against Trypanosoma brucei, Toxoplasma gondii, and cytomegalovirus (33)(34)(35)(36). Considerable attention has been devoted recently to salinomycin and derivatives due to their selective activity against tumor stem cells (13,37,38).…”