Characterization of Plasmodium Atg3-Atg8 interaction inhibitors identifies novel alternative mechanisms of action in Toxoplasma gondii [PublishAheadOfPrint]

Protozoan parasites, including the apicomplexan pathogens Plasmodium falciparum (malaria) and Toxoplasma gondii (toxoplasmosis) infect millions of people world-wide and represent a major human disease burden. Despite their prevalence, therapeutic strategies to treat these infections remain limited and are threatened by the emergence of drug resistance, highlighting the need for the identification of novel drug targets. Recently, homologues of the core autophagy proteins, including Atg8 and Atg3, were identified in many protozoan parasites. Importantly, components of the Atg8 conjugation system that facilitate the lipidation of Atg8 are required for both canonical and parasite-specific functions, and are essential for parasite viability. Structural characterization of the P. falciparum Atg3-Atg8 interaction has led to the identification of compounds that block this interaction. Additionally, many of these compounds inhibit P. falciparum growth in vitro, demonstrating the viability of this pathway as a drug target. Given the essential role of the Atg8 lipidation pathway in Toxoplasma, we sought to determine whether three PfAtg3-Atg8 interaction inhibitors identified in the Medicines for Malaria Venture Malaria Box exerted a similar inhibitory effect in Toxoplasma. While all three inhibitors blocked Toxoplasma replication in vitro at sub-micromolar concentrations, they did not inhibit TgAtg8 lipidation. Rather, high concentrations of two of these compounds induced TgAtg8 lipidation and fragmentation of the parasite mitochondrion, similar to the effects seen following starvation and monensin-induced autophagy. Additionally, we report that one of the PfAtg3-Atg8 interaction inhibitors induces Toxoplasma egress, and provide evidence that this is mediated by an increase in intracellular calcium in response to drug treatment.

http://ift.tt/2hEoFcO