Validation of the ubiquinone synthesis pathway of Toxoplasma gondii as a novel drug target

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PROJECT SUMMARY/ABSTRACT Apicomplexan parasites cause persistent mortality and morbidity worldwide through diseases including malaria, toxoplasmosis, and cryptosporidiosis. The phylum member Toxoplasma gondii alone infects approximately one third of the world population. T. gondii is an opportunistic pathogen that causes serious disease in immunocompromised patients. Most human infections are asymptomatic but immunosuppression due to organ transplant, cancer chemotherapy, or infection with HIV can lead to re-activation of the infection. In addition, infection of the fetus during pregnancy causes congenital toxoplasmosis. Some exotic strains of T. gondii have been described in tropical countries that cause severe ocular disease in immunocompetent patients. Treatment for toxoplasmosis is challenged by lack of effective drugs to eradicate the chronic infection and as many as one-third of AIDS patients do not respond to the therapy. Most of the drugs currently used are poorly distributed to the central nervous system and they trigger allergic reactions in a large number of patients. There is a compelling need for safe and effective treatments for toxoplasmosis. The mitochondrion of T. gondii and of other apicomplexan parasites is critical for replication and several major antiparasitic drugs, such as atovaquone and endochin-like quinolones, act through inhibition of the mitochondrial electron transport chain at the coenzyme Q:cytochrome c reductase. The coenzyme Q (UQ) molecule consists of a water soluble quinone head (para-hydroxybenzoate or PHBA) that can accept or donate two electrons and a lipophilic isoprenoid tail that confines the UQ to membranes. Synthesis of the PHBA moiety in plants and yeast occurs through the shikimate pathway, but how it is synthesized in apicomplexan is not known. The isoprenoid unit derives from a common precursor, isopentenyl pyrophosphate (IPP), and its isomer, dimethylallyl pyrophosphate (DMAPP), which are synthesized in mammalian cells via the mevalonate pathway, but in T. gondii instead is synthesized via an apicoplast localized methylerythritol phosphate (MEP) pathway. IPP and DMAPP are condensed by the action of a unique farnesyl diphosphate synthase (TgFPPS) into farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP). Work from our laboratory demonstrated that T. gondii is able to salvage GGPP and FPP from the host and therefore intracellular parasites are vulnerable to inhibition of the host isoprenoid synthesis pathway. In this project we propose to investigate the synthesis of UQ (downstream to the salvage point) and validate the pathway as a novel target. Our preliminary data demonstrates the essentiality of three enzymatic steps and the inhibition of one of them by a novel compound. Our hypothesis is that the synthesis of UQ represents a unique and essential pathway with enzymatic steps liable to inhibition by specific compounds, some already in use in the clinics and potential new ones to be discovered. We predict that exploring this pathway will reveal unique and essential enzymes that could be targeted for improved chemotherapy against T. gondii and other apicomplexan parasites.