The Polyamine Pathway of Leishmania donovani as a Potential Therapeutic Target

Abstract

Parasites of the genus Leishmania cause a variety of diseases, including fatal visceralizing infections, and affect an estimated 12 million people worldwide. Due to the absence of effective vaccines, therapeutic treatment has offered the only avenue of defense. Unfortunately, the current arsenal of drugs used to treat this neglected disease is far from ideal and the need for new therapeutic targets and a better understanding of host‐parasite interactions is urgent. Polyamines are ubiquitous, essential cations that are especially important for rapidly growing cells and the synthesis pathway for polyamines has been validated as a potential target in the related parasite, Trypanosoma brucei. In Leishmania, the polyamine biosynthetic pathway consists of four enzymes: arginase (ARG), ornithine decarboxylase (ODC), spermidine synthase (SPD), and S‐adenosylmethionine decarboxylase (ADOMETDC). These enzymes sequentially generate ornithine, putrescine and spermidine. We have generated a complete set of gene deletion mutants in Leishmania donovani (LdΔarg, LdΔodc, LdΔspd, and LdΔadometdc), in order to evaluate the polyamine biosynthetic enzymes as therapeutic targets. In vitro studies showed that the gene deletion mutants depend on the provision of downstream metabolites to survive. An evaluation of cell growth between the different cell lines uncovered that the only vital role of ornithine is as precursor for polyamine synthesis. However, putrescine, which has previously been postulated to be merely a substrate for spermidine formation, has additional essential functions and may even be more critical for cellular proliferation than spermidine. In vivo infectivity studies revealed that Ldodc exhibits profoundly reduced infectivity compared to wild type parasites, while Ldspd and Ldadometdc show a less pronounced, albeit significant, reductions in infectivity. In contrast, the Ldarg gene deletion has no effect on virulence. The observed discrepancies in infectivity phenotypes suggest a model in which the polyamine precursor ornithine is readily salvaged by intracellular parasites, whereas the polyamines putrescine and spermidine are severely limited. Our long‐term goal is to exploit the polyamine pathway for therapeutic development against leishmaniasis.Support or Funding InformationSchool of Pharmacy Research Incentive GrantNIH R01 AI041622