Roles for mitochondria in pentamidine susceptibility and resistance in Leishmania donovani

Particularities of mitochondrial structure in parasitic protists (Apicomplexa and Kinetoplastida)

Without mitochondria, eukaryotic cells would depend entirely on anaerobic glycolysis for ATP generation. This also holds true for protists, both free-living and parasitic. Parasitic protists include agents of human and animal diseases that have a huge impact on world populations. In the phylum Apicomplexa, several species of Plasmodium cause malaria, whereas Toxoplasma gondii is a cosmopolite parasite found on all continents. Flagellates of the order Kinetoplastida include the genera Leishmania and Trypanosoma causative agents of human leishmaniasis and (depending on the species) African trypanosomiasis and Chagas disease. Although clearly distinct in many aspects, the members of these two groups bear a single and usually well developed mitochondrion. The single mitochondrion of Apicomplexa has a dense matrix and many cristae with a circular profile. The organelle is even more peculiar in the order Kinetoplastida, exhibiting a condensed network of DNA at a specific position, always close to the flagellar basal body. This arrangement is known as Kinetoplast and the name of the order derived from it. Kinetoplastids also bear glycosomes, peroxisomes that concentrate enzymes of the glycolytic cycle. Mitochondrial volume and activity is maximum when glycosomal is low and vice versa. In both Apicomplexa and trypanosomatids, mitochondria show particularities that are absent in other eukaryotic organisms. These peculiar features make them an attractive target for therapeutic drugs for the diseases they cause.

Paromomycin: uptake and resistance in Leishmania donovani

Paromomycin is currently in phase IV clinical trials against leishmaniasis. In the present work we elucidate the effect and mechanism of uptake of paromomycin in Leishmania donovani. The in vitro sensitivities of both promastigotes and amastigotes were determined to this aminoglycoside. Association of paromomycin with L. donovani involved a rapid initial phase that was non-saturable up to 1mM of the drug. This initial phase was largely independent of temperature and not affected by metabolic inhibitors. Poly-lysine, a membrane impermeant polycation, caused profound inhibition of this association of the drug with the parasite indicating that it represented a binding of the cationic paromomycin to the negatively charged leishmanial glycocalyx. After 72h of exposure to the drug the mitochondrial membrane potential was significantly decreased, indicating that this organelle might be the ultimate target of the drug. Both cytoplasmic and mitochondrial protein synthesis were inhibited following paromomycin exposure. A line selected for resistance to the drug showed reduced paromomycin accumulation associated with a significant reduction in the initial binding to the cell surface. The drug induced reduction in membrane potential and inhibition of protein synthesis were less pronounced in the resistant strain in comparison to the wild-type.

Characterization of Leishmania (Leishmania) amazonensis promastigotes resistant to pentamidine

Pentamidine is a second-line agent used in the treatment of leishmaniasis and its mode of action and mechanism of resistance is not well understood. It was previously demonstrated that transfection of promastigotes and amastigotes with the ABC transporter PRP1 gene confers resistance to pentamidine. To further clarify this point, we generated Leishmania amazonensis mutants resistant to pentamidine. Our results indicated that this ABC transporter is not associated with pentamidine resistance in lines generated by drug pressure through amplification or overexpression mechanisms of PRP1 gene.

Antiparasitic compounds that target DNA

Designed, synthetic heterocyclic diamidines have excellent activity against eukaryotic parasites that cause diseases such as sleeping sickness and leishmania and adversely affect millions of people each year. The most active compounds bind specifically and strongly in the DNA minor groove at AT sequences. The compounds enter parasite cells rapidly and appear first in the kinetoplast that contains the mitochondrial DNA of the parasite. With time the compounds are also generally seen in the cell nucleus but are not significantly observed in the cytoplasm. The kinetoplast decays over time and disappears from the mitochondria of treated cells. At this point the compounds begin to be observed in other regions of the cell, such as the acidocalcisomes. The cells typically die in 24-48h after treatment. Active compounds appear to selectively target extended AT sequences and induce changes in kinetoplast DNA minicircles that cause a synergistic destruction of the catenated kinetoplast DNA network and cell death.

Induction of apoptosis-like cell death by pentamidine and doxorubicin through differential inhibition of topoisomerase II in arsenite-resistant L. donovani

The current study has been undertaken to investigate the sensitivity of the topoisomerase II (topo II) of wild type (Ld-Wt) and arsenite-resistant (Ld-As20) L. donovani to an anti-leishmanial agent pentamidine and an anti-cancer drug doxorubicin. We demonstrate that the cross resistance to pentamidine and doxorubicin in Ld-As20, was in part implicated through differential inhibition of topo II in Ld-Wt and Ld-As20. Further, the treatment of promastigotes at drug concentrations inhibiting 50% of topo II activity inflicted a regulated cell death sharing several apoptotic features like externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytochrome C release into the cytosol, activation of cellular proteases and DNA fragmentation. The cytotoxic potential of pentamidine and doxorubicin in L. donovani has been shown to be mediated through topoisomerase II inhibition and results in inciting programmed cell death process.

Human African trypanosomiasis: pharmacological re-engagement with a neglected disease

This review discusses the challenges of chemotherapy for human African trypanosomiasis (HAT). The few drugs registered for use against the disease are unsatisfactory for a number of reasons. HAT has two stages. In stage 1 the parasites proliferate in the haemolymphatic system. In stage 2 they invade the central nervous system and brain provoking progressive neurological dysfunction leading to symptoms that include the disrupted sleep wake patterns that give HAT its more common name of sleeping sickness. Targeting drugs to the central nervous system offers many challenges. However, it is the cost of drug development for diseases like HAT, that afflict exclusively people of the world's poorest populations, that has been the principal barrier to new drug development and has led to them becoming neglected. Here we review drugs currently registered for HAT, and also discuss the few compounds progressing through clinical trials. Finally we report on new initiatives that might allow progress to be made in developing new and satisfactory drugs for this terrible disease.

Role of the ABC transporter PRP1 (ABCC7) in pentamidine resistance in Leishmania amastigotes

Pentamidine is a second-line agent in the treatment of leishmaniasis whose mode of action and resistance mechanism are not well understood. In this work, we show that the intracellular ABC protein PRP1 (pentamidine resistance protein 1) (ABCC7) can confer resistance to pentamidine in Leishmania sp. parasites in the intracellular stage.

Flavonoid dimers as bivalent modulators for pentamidine and sodium stiboglucanate resistance in leishmania

Drug resistance by overexpression of ATP-binding cassette (ABC) transporters is an impediment in the treatment of leishmaniasis. Flavonoids are known to reverse multidrug resistance (MDR) in Leishmania and mammalian cancers by inhibiting ABC transporters. Here, we found that synthetic flavonoid dimers with three (compound 9c) or four (compound 9d) ethylene glycol units exhibited a significantly higher reversing activity than other shorter or longer ethylene glycol-ligated dimers, with approximately 3-fold sensitization of pentamidine and sodium stibogluconate (SSG) resistance in Leishmania, respectively. This modulatory effect was dosage dependent and not observed in apigenin monomers with the linker, suggesting that the modulatory effect is due to its bivalent nature. The mechanism of reversal activity was due to increased intracellular accumulation of pentamidine and total antimony in Leishmania. Compared to other MDR modulators such as verapamil, reserpine, quinine, quinacrine, and quinidine, compounds 9c and 9d were the only agents that can reverse SSG resistance. In terms of reversing pentamidine resistance, 9c and 9d have activities comparable to those of reserpine and quinacrine. Modulators 9c and 9d exhibited reversal activity on pentamidine resistance among LeMDR1(-/-), LeMDR1(+/+), and LeMDR1-overexpressed mutants, suggesting that these modulators are specific to a non-LeMDR1 pentamidine transporter. The LeMDR1 copy number is inversely related to pentamidine resistance, suggesting that it might be involved in importing pentamidine into the mitochondria. In summary, bivalency could be a useful strategy for the development of more potent ABC transporter modulators and flavonoid dimers represent a promising reversal agent for overcoming pentamidine and SSG resistance in parasite Leishmania.

A mitochondrial protein affects cell morphology, mitochondrial segregation and virulence in Leishmania

The single mitochondrion of kinetoplastids divides in synchrony with the nucleus and plays a crucial role in cell division. However, despite its importance and potential as a drug target, the mechanism of mitochondrial division and segregation and the molecules involved are only partly understood. In our quest to identify novel mitochondrial proteins in Leishmania, we constructed a hidden Markov model from the targeting motifs of known mitochondrial proteins as a tool to search the Leishmania major genome. We show here that one of the 17 proteins of unknown function that we identified, designated mitochondrial protein X (MIX), is an oligomeric protein probably located in the inner membrane and expressed throughout the Leishmania life cycle. The MIX gene appears to be essential. Moreover, even deletion of one allele from L. major led to abnormalities in cell morphology, mitochondrial segregation and, importantly, to loss of virulence. MIX is unique to kinetoplastids but its heterologous expression in Saccharomyces cerevisiae produced defects in mitochondrial morphology. Our data show that a number of mitochondrial proteins are unique to kinetoplastids and some, like MIX, play a central role in mitochondrial segregation and cell division, as well as virulence.

The role of Leishmania enriettii multidrug resistance protein 1 (LeMDR1) in mediating drug resistance is iron-dependent

In parasitic protozoan Leishmania enriettii, the role of a multidrug resistance (mdr) gene LeMDR1 (L. enriettii multidrug resistance 1) in mediating vinblastine resistance has been previously demonstrated by association, transfection and "gene knockout" studies. LeMDR1 has been shown to be located intracellularly and it was proposed to mediate drug resistance by sequestering drugs into intracellular organelles rather than by active efflux. Here we compared LeMDR1 overexpressed cell lines (Vint3 and V160), wild type (Le) and LeMDR1 "double knockout" mutant (LeMDR1-/-) and demonstrated that LeMDR1 gene copy number was associated with (1) higher level of intracellular iron, (2) increased sensitivity to an iron-dependent antibiotic, streptonigrin and (3) increased enzyme activity of an iron-sulfur-containing mitochondrial enzyme, aconitase. This result suggests that the normal function of LeMDR1 is related to mitochondrial iron homeostasis. To test such hypothesis, we have used the LeMDR1-overexpressing mutant V160 and LeMDR1-/- mutant to determine how iron level can affect its resistance level to drugs targeting either cytosol (vinblastine) or mitochondria (rhodamine 123 and pentamidine). It was found that the resistance level of V160 to vinblastine can be increased by iron whereas resistance to both rhodamine 123 and pentamidine can be increased by iron depletion and vice versa. Iron treatment can potentiate rhodamine 123 and pentamidine accumulation whereas iron deprivation can cause the reduction of rhodamine 123 accumulation. Our result highly suggests that LeMDR1's function in mediating drug resistance is iron-dependent.

Leishmania donovani polyamine biosynthetic enzyme overproducers as tools to investigate the mode of action of cytotoxic polyamine analogs

A number of anticancer and antiparasitic drugs are postulated to target the polyamine biosynthetic pathway and polyamine function, but the exact mode of action of these compounds is still being elucidated. To establish whether polyamine analogs specifically target enzymes of the polyamine pathway, a model was developed using strains of the protozoan parasite Leishmania donovani that overproduce each of the polyamine biosynthetic enzymes. Promastigotes overexpressing episomal constructs encoding ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (ADOMETDC), or spermidine synthase (SPDSYN) revealed robust overproduction of the corresponding polyamine biosynthetic enzyme. Polyamine pools, however, were either unchanged or only marginally affected, implying that regulatory mechanisms must exist. The ODC, ADOMETDC, and SPDSYN overproducer strains exhibited a high level of resistance to difluoromethylornithine, 5'-{[(Z)-4-amino-2-butenyl]methylamino}-5'-deoxyadenosine, and n-butylamine, respectively, confirming previous observations that these agents specifically target polyamine enzymes. Conversely, augmented levels of polyamine biosynthetic enzymes did not affect the sensitivity of L. donovani promastigotes to pentamidine, berenil, and mitoguazone, drugs that were postulated to target the polyamine pathway, implying alternative and/or additional targets for these agents. The sensitivities of wild-type and overproducing parasites to a variety of polyamine analogs were also tested. The polyamine enzyme-overproducing lines offer a rapid cell-based screen for assessing whether synthetic polyamine analogs exert their mechanism of action predominantly on the polyamine biosynthetic pathway in L. donovani. Furthermore, the drug resistance engendered by the amplification of target genes and the overproduction of the encoded protein offers a general strategy for evaluating and developing therapeutic agents that target specific proteins in Leishmania.

Activity of and Initial Mechanistic Studies on a Novel Antileishmanial Agent Identified through in Silico Pharmacophore Development and Database Searching

A 3D pharmacophore was generated to describe the antileishmanial activity of dinitroaniline sulfonamides by CATALYST 3D-QSAR methodology, and this pharmacophore was used to search the Maybridge database. Two compounds identified in this search, BTB 06237 and BTB 06256, were highly active with IC(50) values against L. donovani amastigotes of 0.5 +/- 0.2 and 2.3 +/- 0.8 microM, respectively. BTB 06237 also reduced parasite burdens in L. mexicana-infected J774 macrophages at low micromolar concentrations. Unlike the dinitroaniline sulfonamides, the active compounds did not display antimitotic effects against Leishmania. Transmission electron microscopy showed that the single parasite mitochondrion becomes dilated following incubation with BTB 06237, and fluorescence microscopy demonstrated that this organelle fragments into intensely staining spheres when treated with a mitochondrion-specific dye. The mitochondrial membrane potential was also dissipated in BTB 06237-treated parasites. These results indicate that BTB 06237 is an intriguing antileishmanial lead compound that likely interferes with mitochondrial function.