Integrating ribosomal promoter vectors that offer a choice of constitutive expression profiles in Leishmania donovani

Discovery and Optimization of Tambjamines as a Novel Class of Antileishmanial Agents

Leishmaniasis is a neglected tropical disease that is estimated to afflict over 12 million people. Current drugs for leishmaniasis suffer from serious deficiencies, including toxicity, high cost, modest efficacy, primarily parenteral delivery, and emergence of widespread resistance. We have discovered and developed a natural product-inspired tambjamine chemotype, known to be effective against Plasmodium spp, as a novel class of antileishmanial agents. Herein, we report in vitro and in vivo antileishmanial activities, detailed structure-activity relationships, and metabolic/pharmacokinetic profiles of a large library of tambjamines. A number of tambjamines exhibited excellent potency against both Leishmania mexicana and Leishmania donovani parasites with good safety and metabolic profiles. Notably, tambjamine 110 offered excellent potency and provided partial protection to leishmania-infected mice at 40 and/or 60 mg/kg/10 days of oral treatment. This study presents the first account of antileishmanial activity in the tambjamine family and paves the way for the generation of new oral antileishmanial drugs.

Purine-responsive expression of the Leishmania donovani NT3 purine nucleobase transporter is mediated by a conserved RNA stem-loop

The ability to modulate gene expression in response to changes in the host environment is essential for survival of the kinetoplastid parasite Leishmania Unlike most eukaryotes, gene expression in kinetoplastids is predominately regulated posttranscriptionally. Consequently, RNA-binding proteins and mRNA-encoded sequence elements serve as primary determinants of gene regulation in these organisms; however, few have defined roles in specific stress response pathways. Leishmania species cannot synthesize purines de novo and must scavenge these essential nutrients from the host. Leishmania have evolved a robust stress response to withstand sustained periods of purine scarcity during their life cycle. The purine nucleobase transporter LdNT3 is among the most substantially up-regulated proteins in purine-starved Leishmania donovani parasites. Here we report that the posttranslational stability of the LdNT3 protein is unchanged in response to purine starvation. Instead, LdNT3 up-regulation is primarily mediated by a 33-nucleotide-long sequence in the LdNT3 mRNA 3' UTR that is predicted to adopt a stem-loop structure. Although this sequence is highly conserved within the mRNAs of orthologous transporters in multiple kinetoplastid species, putative stem-loops from L. donovani and Trypanosoma brucei nucleobase transporter mRNAs were not functionally interchangeable for purine-responsive regulation. Through mutational analysis of the element, we demonstrate that species specificity is attributable to just three variant bases within the predicted loop. Finally, we provide evidence that the abundance of the trans-acting factor that binds the LdNT3 stem-loop in vivo is substantially higher than required for regulation of LdNT3 alone, implying a potential role in regulating other purine-responsive genes.

Arginase Is Essential for Survival of Leishmania donovani Promastigotes but Not Intracellular Amastigotes

Studies of Leishmania donovani have shown that both ornithine decarboxylase and spermidine synthase, two enzymes of the polyamine biosynthetic pathway, are critical for promastigote proliferation and required for maximum infection in mice. However, the importance of arginase (ARG), the first enzyme of the polyamine pathway in Leishmania, has not been analyzed in L. donovani. To test ARG function in intact parasites, we generated Δarg null mutants in L. donovani and evaluated their ability to proliferate in vitro and trigger infections in mice. The Δarg knockout was incapable of growth in the absence of polyamine supplementation, but the auxotrophic phenotype could be bypassed by addition of either millimolar concentrations of ornithine or micromolar concentrations of putrescine or by complementation with either glycosomal or cytosolic versions of ARG. Spermidine supplementation of the medium did not circumvent the polyamine auxotrophy of the Δarg line. Although ARG was found to be essential for ornithine and polyamine synthesis, ornithine decarboxylase appeared to be the rate-limiting enzyme for polyamine production. Mouse infectivity studies revealed that the Δarg lesion reduced parasite burdens in livers by an order of magnitude but had little impact on the numbers of parasites recovered from spleens. Thus, ARG is essential for proliferation of promastigotes but not intracellular amastigotes. Coupled with previous studies, these data support a model in which L. donovani amastigotes readily salvage ornithine and have some access to host spermidine pools, while host putrescine appears to be unavailable for salvage by the parasite.