The topoisomerases of protozoan parasites

DNA Topoisomerases in the Unicellular Protozoan Parasites: Unwinding the Mystery

DNA topoisomerases are a group of enzymes present ubiquitously in all organisms from unicellular protozoan parasites to humans. These enzymes control the topological problems caused by DNA double helix in the cell during nucleic acid metabolism. Certain types of topoisomerases present in unicellular parasites are quite different from human topoisomerases (hTop) concerning structure, expression, and function. Many protozoan parasites causing fatal diseases have DNA topoisomerases, which play vital roles in their survival. Given the fact that the structures of the protozoan parasite topoisomerases are different from humans, DNA topoisomerase acts as an essential target for potent drug development for parasitic diseases. Moreover, various studies revealed the therapeutic potential of these drugs targeting the parasitic topoisomerases. Therefore, the characterization of parasitic topoisomerases is pivotal for the development of future potential drug targets. Considering the importance of this ubiquitous enzyme as a potential drug target, we describe in detail all the reported protozoan topoisomerases in an organized manner including Leishmania, Trypanosoma, Plasmodium, Giardia, Entamoeba, Babesia, Theileria, Crithidia, Cryptosporidium, Toxoplasma, etc. This review highlights the unique attributes associated with the structure and function of different types of DNA topoisomerases from the unicellular protozoan parasites. So, it would be beneficial for researchers to obtain awareness about the currently characterized topoisomerases and the ones that need better characterization, understand the structure-function relationship of parasitic topoisomerases, to develop the potent anti-parasitic drugs, and also provides a future platform for therapeutic development.

Inhibition of Aedes aegypti DNA topoisomerase II by etoposide: Impact on survival and morphology of larvae and pupae

DNA topoisomerase II enzymes maintain DNA stability during vital processes, such as genome replication, transcription and chromosomal segregation during mitosis and meiosis. In the present work, we analyzed functional aspects of the DNA topoisomerase II (AeTopII) enzyme of the mosquito Aedes aegypti. Here, we show that AeTopII mRNA is expressed at all stages of mosquito development. By in situ hybridization, we found that the AeTopII mRNA is concentrated along the ovarian follicular cells as well as in the region of the follicles. The observed expression profiles likely reflect increased topoisomerase II cellular requirements due to the intense ovarian growth and egg production following blood feeding in Ae. aegypti females. The drug etoposide, a classic inhibitor of topoisomerase II, was used for in vivo testing with 2nd stage larvae, in order to investigate the functional importance of this enzyme in Ae. aegypti survival and development. Inhibition of topoisomerase II activity with etoposide concentrations ranging from 10 to 200 μM did not leads to the immediate death of larvae. However, after 10 days of observation, etoposide treatments resulted in 30-40% decrease in survival, in a dose dependent manner, with persisting larvae and pupae presenting incomplete development, as well as morphological abnormalities. Also, approximately 50% of the treated larvae did not reach the pupal stage. Thus, we conclude that AeTopII is a vital enzyme in the development of Ae. aegypti and its sensitivity to inhibitors should be explored for potential chemical agents to be used in vector control.

Effects of Eugenol on Haemoproteus columbae in domestic pigeons (Columba livia domestica) from Riyadh, Saudi Arabia

Eugenol was investigated for the treatment of Haemoproteus columbae (H. columbae) infected squabs (young domestic pigeons, Columba domestica). Thirty naturally-infected squabs were divided into three groups of 10 each. One group was treated with Eugenol, while the positive and negative control groups were administered buparvaquone (Butalex®) and distilled water, respectively. The number of infected red blood cells (RBCs) was calculated in all groups before and after treatment at 4-day intervals for 16 days. The results showed a significant therapeutic effect of Eugenol, with a progressive decrease in the number of infected RBCs from 89.20 ± 2.11 before treatment to 0.90 ± 0.31 at the end of treatment (P≤0.05). Butalex® was able to suppress the number of infected RBCs from 93.70 ± 1.72 before treatment to 0.90 ± 0.35 at the end of the experiment (P≤0.05). Eugenol showed therapeutic effects against H. columbae and may be regarded as a candidate for further studies to develop new drugs against blood parasites, in both animals and humans.

Antileishmanial Mechanism of Diamidines Involves Targeting Kinetoplasts

Leishmaniasis is a disease caused by pathogenic Leishmania parasites; current treatments are toxic and expensive, and drug resistance has emerged. While pentamidine, a diamidine-type compound, is one of the treatments, its antileishmanial mechanism of action has not been investigated in depth. Here we tested several diamidines, including pentamidine and its analog DB75, against Leishmania donovani and elucidated their antileishmanial mechanisms. We identified three promising new antileishmanial diamidine compounds with 50% effective concentrations (EC₅₀s) of 3.2, 3.4, and 4.5 μM, while pentamidine and DB75 exhibited EC₅₀s of 1.46 and 20 μM, respectively. The most potent antileishmanial inhibitor, compound 1, showed strong DNA binding properties, with a shift in the melting temperature (ΔT_m) of 24.2°C, whereas pentamidine had a ΔT_m value of 2.1°C, and DB75 had a ΔT_m value of 7.7°C. Additionally, DB75 localized in L. donovani kinetoplast DNA (kDNA) and mitochondria but not in nuclear DNA (nDNA). For 2 new diamidines, strong localization signals were observed in kDNA at 1 μM, and at higher concentrations, the signals also appeared in nuclei. All tested diamidines showed selective and dose-dependent inhibition of kDNA, but not nDNA, replication, likely by inhibiting L. donovani topoisomerase IB. Overall, these results suggest that diamidine antileishmanial compounds exert activity by accumulating toward and blocking replication of parasite kDNA.

Identification of Leishmania donovani Topoisomerase 1 inhibitors via intuitive scaffold hopping and bioisosteric modification of known Top 1 inhibitors

A library of arylidenefuropyridinediones was discovered as potent inhibitors of Leishmania donovani Topoisomerase 1 (LdTop1) where the active molecules displayed considerable inhibition with single digit micromolar EC₅₀ values. This molecular library was designed via intuitive scaffold hopping and bioisosteric modification of known topoisomerase 1 inhibitors such as camptothecin, edotecarin and etc. The design was rationalized by molecular docking analysis of the compound prototype with human topoisomerase 1 (HTop1) and Leishmania donovani topoisomerase 1(LdTop1). The most active compound 4 displayed no cytotoxicity against normal mammalian COS7 cell line (~100 fold less inhibition at the EC₅₀). Similar to camptothecin, 4 interacted with free LdTop1 as observed in the preincubation DNA relaxation inhibition experiment. It also displayed anti-protozoal activity against Leishmania donovani promastigote. Crystal structure investigation of 4 and its molecular modelling with LdTop1 revealed putative binding sites in the enzyme that could be harnessed to generate molecules with better potency.

Topoisomerase II from Human Malaria Parasites: EXPRESSION, PURIFICATION, AND SELECTIVE INHIBITION

Historically, type II topoisomerases have yielded clinically useful drugs for the treatment of bacterial infections and cancer, but the corresponding enzymes from malaria parasites remain understudied. This is due to the general challenges of producing malaria proteins in functional forms in heterologous expression systems. Here, we express full-length Plasmodium falciparum topoisomerase II (PfTopoII) in a wheat germ cell-free transcription-translation system. Functional activity of soluble PfTopoII from the translation lysates was confirmed through both a plasmid relaxation and a DNA decatenation activity that was dependent on magnesium and ATP. To facilitate future drug discovery, a convenient and sensitive fluorescence assay was established to follow DNA decatenation, and a stable, truncated PfTopoII was engineered for high level enzyme production. PfTopoII was purified using a DNA affinity column. Existing TopoII inhibitors previously developed for other non-malaria indications inhibited PfTopoII, as well as malaria parasites in culture at submicromolar concentrations. Even before optimization, inhibitors of bacterial gyrase, GSK299423, ciprofloxacin, and etoposide exhibited 15-, 57-, and 3-fold selectivity for the malarial enzyme over human TopoII. Finally, it was possible to use the purified PfTopoII to dissect the different modes by which these varying classes of TopoII inhibitors could trap partially processed DNA. The present biochemical advancements will allow high throughput chemical screening of compound libraries and lead optimization to develop new lines of antimalarials.

Trypanosomatids topoisomerase re-visited. New structural findings and role in drug discovery

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There is an urgent need of new treatments against trypanosomatids-borne diseases.

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DNA topoisomerases are pointed as potential drug targets against unicellular parasites.

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Trypanosomatids have a full set of DNA topoisomerases in both nucleus and kinetoplast.

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TopII and TopIII are located in the kinetoplast and fully involved in kDNA replication.

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Tritryps TopIB differ in structure from mammalian’s pointing to an attractive target.

The Trypanosomatidae family, composed of unicellular parasites, causes severe vector-borne diseases that afflict human populations worldwide. Chagas disease, sleeping sickness, as well as different sorts of leishmaniases are amongst the most important infectious diseases produced by Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp., respectively. All these infections are closely related to weak health care services in low-income populations of less developed and least economically developed countries. Search for new therapeutic targets in order to hit these pathogens is of paramount priority, as no effective vaccine is currently in use against any of these parasites. Furthermore, present-day chemotherapy comprises old-fashioned drugs full of important side effects. Besides, they are prone to produce tolerance and resistance as a consequence of their continuous use for decades. DNA topoisomerases (Top) are ubiquitous enzymes responsible for solving the torsional tensions caused during replication and transcription processes, as well as in maintaining genomic stability during DNA recombination. As the inhibition of these enzymes produces cell arrest and triggers cell death, Top inhibitors are among the most effective and most widely used drugs in both cancer and antibacterial therapies. Top relaxation and decatenation activities, which are based on a common nicking–closing cycle involving one or both DNA strands, have been pointed as a promising drug target. Specific inhibitors that bind to the interface of DNA-Top complexes can stabilize Top-mediated transient DNA breaks. In addition, important structural differences have been found between Tops from the Trypanosomatidae family members and Tops from the host. Such dissimilarities make these proteins very interesting for drug design and molecular intervention. The present review is a critical update of the last findings regarding trypanosomatid’s Tops, their new structural features, their involvement both in the physiology and virulence of these parasites, as well as their use as promising targets for drug discovery.

Comparative multivariate analysis of codon and amino acid usage in three Leishmania genomes

Multivariate analysis of codon and amino acid usage was performed for three Leishmania species, including L. donovani, L. infantum and L. major. It was revealed that all three species are under mutational bias and translational selection. Lower GC₁₂ and higher GC_3S in all three parasites suggests that the ancestral highly expressed genes (HEGs), compared to lowly expressed genes (LEGs), might have been rich in AT-content. This also suggests that there must have been a faster rate of evolution under GC-bias in LEGs. It was observed from the estimation of synonymous/non-synonymous substitutions in HEGs that the HEG dataset of L. donovani is much closer to L. major evolutionarily. This is also supported by the higher d_N value as compared to d_S between L. donovani and L. major, suggesting the conservation of synonymous codon positions between these two species and the role of translational selection in shaping the composition of protein-coding genes.

DNA topoisomerases in apicomplexan parasites: promising targets for drug discovery

The phylum Apicomplexa includes a large group of protozoan parasites responsible for a wide range of animal and human diseases. Destructive pathogens, such as Plasmodium falciparum and Plasmodium vivax, causative agents of human malaria, Cryptosporidium parvum, responsible of childhood diarrhoea, and Toxoplasma gondii, responsible for miscarriages and abortions in humans, are frequently associated with HIV immunosuppression in AIDS patients. The lack of effective vaccines, along with years of increasing pressure to eradicate outbreaks with the use of drugs, has favoured the formation of multi-drug resistant strains in endemic areas. Almost all apicomplexan of medical interest contain two endosymbiotic organelles that contain their own mitochondrial and apicoplast DNA. Apicoplast is an attractive target for drug testing because in addition to harbouring singular metabolic pathways absent in the host, it also has its own transcription and translation machinery of bacterial origin. Accordingly, apicomplexan protozoa contain an interesting mixture of enzymes to unwind DNA from eukaryotic and prokaryotic origins. On the one hand, the main mechanism of DNA unwinding includes the scission of one-type I-or both DNA strands-type II eukaryotic topoisomerases, establishing transient covalent bonds with the scissile end. These enzymes are targeted by camptothecin and etoposide, respectively, two natural drugs whose semisynthetic derivatives are currently used in cancer chemotherapy. On the other hand, DNA gyrase is a bacterial-borne type II DNA topoisomerase that operates within the apicoplast and is effectively targeted by bacterial antibiotics like fluoroquinolones and aminocoumarins. The present review is an update on the new findings concerning topoisomerases in apicomplexan parasites and the role of these enzymes as targets for therapeutic agents.

Functional expression of a DNA-topoisomerase IB from Cryptosporidium parvum

Cryptosporidium parvum, one of the most important causative organisms of human diarrheas during childhood, contains a monomeric DNA-topoisomerase IB (CpTopIB) in chromosome 7. Heterologous expression of CpTopIB gene in a budding yeast strain lacking this activity proves that the cryptosporidial enzyme is functional in vivo. The enzymatic activity is comprised in a single polypeptide, which contains all the structural features defining a fully active TopIB. Relaxation activity of the yeast extracts was detected only when CpTopIB ORF was expressed in a yeast expression system showing time and protein dependence under steady state kinetic conditions. The susceptibility of CpTopIB-transformed yeast to the irreversible inhibitor camptothecin and its water-soluble derivatives (topotecan and SN-38) was assessed.

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.

Deletion study of DNA topoisomerase IB from Leishmania donovani: searching for a minimal functional heterodimer

The substantial differences between trypanosomal and leishmanial DNA topoisomerase IB concerning to their homologues in mammals have provided a new lead in the study of the structural determinants that can be effectively targeted. Leishmania donovani, the causative agent of visceral leishmaniasis, contains an unusual heterodimeric DNA topoisomerase IB. The catalytically active enzyme consists of a large subunit (LdTopIL), which contains the non-conserved N-terminal end and the phylogenetically conserved “core” domain, and of a small subunit (LdTopIS) which harbors the C-terminal region with the characteristic tyrosine residue in the active site. Heterologous co-expression of LdTopIL and LdTopIS genes in a topoisomerase I deficient yeast strain, reconstitutes a fully functional enzyme LdTopIL/S which can be used for structural studies. An approach by combinatorial cloning of deleted genes encoding for truncated versions of both subunits was used in order to find out structural insights involved in enzyme activity or protein-protein interaction. The role played by the non-conserved N-terminal extension of LdTopIL in both relaxation activity and CPT sensitivity has been examined co-expressing the full-length LdTopIS and a fully active LdTopIΔS deletion with several deletions of LdTopIL lacking growing sequences of the N-terminal end. The sequential deletion study shows that the first 26 amino acids placed at the N-terminal end and a variable region comprised between Ala548 to end of the C-terminal extension of LdTopIL were enzymatically dispensable. Altogether this combinatorial approach provides important structural insights of the regions involved in relaxation activity and for understanding the atypical structure of this heterodimeric enzyme.

Leishmania panamensis: Comparative inhibition of nuclear DNA topoisomerase II enzymes from promastigotes and human macrophages reveals anti-parasite selectivity of fluoroquinolones, flavonoids and pentamidine

Certain model inhibitors exerted selective action against the catalytic activity of nuclear DNA topoisomerase II (TOPII) of Leishmania panamensis promastigotes. The second-generation fluoroquinolones enoxacin and ciprofloxacin exhibited extraordinarily high anti-parasite selectivity displaying 582- and 40-fold greater potencies against L. panamensis TOPII as compared with the human macrophage enzyme. The flavonoids quercetin and ellagic acid showed inverse specificities, the former being 161-fold more potent against L. panamensis TOPII, and the latter 15.7-fold more active against macrophage TOPII. The protoberberine coralyne was a potent inhibitor of both Leishmania and macrophage TOPII. Bis-benzimidazoles and the diamidine diminazene aceturate exhibited uniformly high potencies against parasite and host TOPII, but a second diamidine pentamidine showed 17.6-fold greater specificity for Leishmania TOPII. The antimonial sodium stibogluconate was an ineffective inhibitor of parasite TOPII showing 4.3-fold greater potency against the macrophage enzyme. These findings suggest that the leishmanicidal activities of certain fluoroquinolones and pentamidine may be mediated partly through TOPII inhibition.

Synthesis, DNA Binding, andLeishmaniaTopoisomerase Inhibition Activities of a Novel Series of Anthra[1,2-d]imidazole-6,11-dione Derivatives

Nine novel anthra[1,2-d]imidazole-6,11-diones, differing in their side chain, were synthesized. UV-vis spectroscopy and viscometric titrations of these molecules with duplex DNA were used to assess their binding with DNA. Five of the nine compounds showed high inhibition activity against topoisomerase I of Leishmania donovani, with the one bearing the tetrazole side chain exhibiting an IC50 approximately 1 microM. The inhibition activities were not related with their DNA binding affinity and depended on the nature of the side chain.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Targeting atypical trypanosomatid DNA topoisomerase I

Tropical diseases produced by kinetoplastid protozoa are among humanity's costliest banes, owing to high mortality and the economic burden resulting from morbidity. Drug resistant strains of parasites, together with insecticide-resistant vectors, are contributing to their increased prevalence in the developing world. Their extension now threatens industrialized countries because of opportunistic infections in immuno-compromised individuals. Current chemotherapy is expensive, has undesirable side effects and, in many patients, is only marginally effective. Based on the clinical success of camptothecin derivatives as anticancer agents, DNA topoisomerases have been identified as targets for drug development. The substantial differences in homology between trypanosome and leishmania DNA topoisomerase IB compared with the human form provides a new lead in the study of the structural determinants that can be targeted.

DNA topoisomerase I from parasitic protozoa: A potential target for chemotherapy

The growing occurrence of drug resistant strains of unicellular prokaryotic parasites, along with insecticide-resistant vectors, are the factors contributing to the increased prevalence of tropical diseases in underdeveloped and developing countries, where they are endemic. Malaria, cryptosporidiosis, African and American trypanosomiasis and leishmaniasis threaten human beings, both for the high mortality rates involved and the economic loss resulting from morbidity. Due to the fact that effective immunoprophylaxis is not available at present; preventive sanitary measures and pharmacological approaches are the only sources to control the undesirable effects of such diseases. Current anti-parasitic chemotherapy is expensive, has undesirable side effects or, in many patients, is only marginally effective. Under this point of view molecular biology techniques and drug discovery must walk together in order to find new targets for chemotherapy intervention. The identification of DNA topoisomerases as a promising drug target is based on the clinical success of camptothecin derivatives as anticancer agents. The recent detection of substantial differences between trypanosome and leishmania DNA topoisomerase IB with respect to their homologues in mammals has provided a new lead in the study of the structural determinants that can be effectively targeted. The present report is an up to date review of the new findings on type IB DNA topoisomerase in unicellular parasites and the role of these enzymes as targets for therapeutic agents.

Leishmania donovani: Differential activities of classical topoisomerase inhibitors and antileishmanials against parasite and host cells at the level of DNA topoisomerase I and in cytotoxicity assays

Different classes of topoisomerase (TOP) inhibitors and antitrypanosomatid agents exhibited variable efficacies against Leishmania donovani parasites and human mononuclear cells both at the level of DNA topoisomerase I (TOPI) catalytic activity and in cytotoxicity assays. Bis-benzimidazoles and the diamidine diminazene aceturate exhibited uniformly high efficacies against parasite and host enzymes as well as against parasite and mononuclear cells, but pentamidine showed around 2 orders of magnitude greater specificity for Leishmania TOPI and amastigote cells (P<0.05). The protoberberine coralyne and the flavonoid quercetin were highly potent, but non-selective, inhibitors in vitro, although the latter showed slight selectivity for parasite TOPI. Camptothecin was selective for mononuclear cells at both levels (P<0.05) and sodium stibogluconate was selective only at the enzyme level displaying 30-fold greater potency against parasite TOPI (P<0.05). These data suggest that at least part of pentamidines' leishmanicidal activity may be mediated through TOPI inhibition, and support the feasibility of exploiting differences between Leishmania and human TOPs to develop modified compounds with improved selectivity.

Canine visceral leishmaniasis: Comparison of in vitro leishmanicidal activity of marbofloxacin, meglumine antimoniate and sodium stibogluconate

The control of canine leishmaniasis largely depends on the success of treatment. Drugs currently available to treat this disease are toxic and partially effective. The curative effect of marbofloxacin, a third-generation fluoroquinolone developed for veterinarian individual treatment, was evaluated in vitro in the presence of Leishmania infantum promastigotes and dog-monocyte-derived macrophages; meglumine antimoniate and sodium stibogluconate were used as comparative treatments. We observed that the killing of Leishmania promastigotes and intracellular amastigotes by marbofloxacin was dose-dependent. We demonstrated that successful treatment of canine infected macrophages for 48 h was possible with 500 microg/ml of marbofloxacin. Leishmanicidal activity acted through a TNF-alpha and nitric oxide pathway and correlated with the generation of nitric oxide (NO(2)) production by monocytes derived macrophages from infected (23+/-5 microM) or healthy (21+/-6 microM) dogs, in comparison with NO(2) concentration in infected/non-treated macrophages (< 3 microM, P<0.01). This significant induced parasiticidal effect correlated with extensive elimination of amastigotes by macrophages derived from infected (11+/-5) and healthy dogs (6+/-2), when compared to infected/non-treated macrophages (530+/-105 and 472+/-86 amastigotes, respectively, P< 0.01). Marbofloxacin was shown to be non-toxic at 500 microg/ml in vitro and no cell apoptosis was observed. The molecule was able to induce a parasitic process after significant elimination of amastigotes in leishmania-infected dog macrophages. We propose that marbofloxacin, compared to standard chemotherapeutic agents (meglumine antimoniate and sodium stibogluconate), could be an effective and pragmatic oral route alternative to treat canine leishmaniasis.

SELECTIVE ACTION OF FLUOROQUINOLONES AGAINST INTRACELLULAR AMASTIGOTES OF LEISHMANIA (VIANNIA) PANAMENSIS IN VITRO

We have demonstrated that fluoroquinolones, a class of antibacterial agents that act through inhibition of type II DNA topoisomerases, exert selective action against intracellular amastigotes of Leishmania (Viannia) panamensis at concentrations that are achievable in vivo. Drug cytotoxicity assays employing the luciferase reporter gene revealed that intracellular amastigotes were 6.6- to 25.9-fold more sensitive than human macrophages (P < 0.05) to second-generation fluoroquinolones in vitro. The most selective agents (enoxacin and ciprofloxacin) exhibited 2 orders of magnitude greater potency against parasites (50% effective dose [ED50] = 54.9-83.4 microM) than host cells (ED50 = 1,425-1,740 microM). Linear regression analysis of ED50 data confirmed a complete lack of correlation (r = 0.001) between the relative drug sensitivities of parasites and host cells. A potential relationship between the structures of fluoroquinolones and their relative leishmanicidal activities was observed. The key substituents of the basic pyridone beta-carboxylic acid nucleus accounting for enhanced antiparasite potency and selectivity appear to be a nitrogen at position 8 of the bicyclic nucleus (enoxacin), a cyclopropyl substituent at the R1 site (ciprofloxacin), and linkage of the R1 and X8 groups by a CH3CHO bridge to form a tricyclic compound (ofloxacin). These findings support the potential of fluoroquinolones and derivatives as novel antileishmanials and encourage their clinical evaluation.

Expression and characterization of the ATP-binding domain of a malarial Plasmodium vivax gene homologous to the B-subunit of the bacterial topoisomerase DNA gyrase

We have previously reported the presence of a DNA gyrase-like topoisomerase activity associated with the 35kb apicoplast DNA in the malarial parasite Plasmodium falciparum [Weissig V, Vetro-Widenhouse TS, Rowe TC. Topoisomerase II inhibitors induce cleavage of nuclear and 35kb plastid DNAs in the malarial parasite Plasmodium falciparum. DNA Cell Biol 1997;16:1483]. Sequences encoding polypeptides homologous to both the A and B subunits of bacterial DNA gyrase have been identified in the genome sequence of P. falciparum among data produced by the Malaria Genome Consortium and the University of Florida Malaria Gene Sequence Tag Project. Based on these findings, we have cloned and expressed a region of the Plasmodium vivax GyrB gene encoding a 43kDa polypeptide homologous to the ATP-binding domain of Escherichia coli DNA gyrase. The 43kDa PvGyrB polypeptide was found to have intrinsic ATPase activity with a K(m) of 0.27mM and a k(cat) of 0.051s(-1). The PvGyrB ATPase was also sensitive to the bacterial DNA gyrase inhibitor coumermycin. The implications of these findings are discussed.

Topoisomerase I amino acid substitutions, Gly185Arg and Asp325Glu, confer camptothecin resistance in Leishmania donovani

The antitumor compound camptothecin (CPT) is also recognized for its specific activity against Leishmania donovani topoisomerase I (Topo-I). In consequence, defining CPT resistance mechanisms represents an important strategic tool in the acquisition of a better understanding of its mode of action. In the present study, we selected a single highly resistant L. donovani strain termed LdRCPT.160 by stepwise exposure to CPT. Gene sequencing revealed two single nucleotide mutations in the LdRCPT.160 LdTOP1A gene, resulting in two amino acid substitutions (Gly185Arg and Asp325Glu) in the protein. Moreover, these two substitutions observed in the LdTOP1A protein were correlated with a decreased Topo-I DNA relaxation activity in these resistant parasites. Nevertheless, there was no change in the LdTOP1A gene expression level. Interestingly, transfection studies of the LdRCPT.160 LdTOP1A gene in its wild-type counterpart showed that it induced CPT resistance. Site-directed mutagenesis studies demonstrated that, despite a substantial level of resistance conferred by the Gly185Arg and Asp325Glu substitutions separately, both were essential to reach a high-resistance phenotype. Of interest, the amino acid substitutions observed in LdRCPT.160 LdTOP1A protein occurred near the amino acids previously predicted to interact with CPT, providing new insight into the mechanism of CPT molecular action.

Resistance mechanism development to the topoisomerase-I inhibitor Hoechst 33342 byLeishmania donovani

The bisbenzimidazole compound Hoechst 33342 (Ho342) has been identified as a specific Topoisomerase-I (Topo-I) inhibitor in mammalian cells. More recently, we have reported the ability of Ho342 to targetL. donovaniTopo-I, leading to parasite growth inhibitionin vitroby mechanisms involving DNA breakage and apoptosis-like phenomenon. As the Ho342 lead molecule (2,5′-Bi-1H-benzimidazole) can be used as a starting structure for derivative compounds more effective againstLeishmania, defining the Ho342 resistance mechanism(s) inLeishmaniarepresents an important strategic tool. In the present study, we selected resistant parasites to Ho342 (LdRHo.300). While we observed an increase of the Topo-I gene expression correlated by a higher Topo-I DNA relaxation activity, the Topo-I genes (LdTOP1AandLdTOP1B) sequencing did not reveal any mutation for the resistant parasites. Moreover, our results on Ho342 cellular accumulation suggested the presence of a potential energy-dependent Ho342 transporter in the wild-type parasite, and that an alteration of this transporter has occurred inLdRHo.300, leading to an altered drug accumulation. Collectively, Ho342 resistance characterization provided results supporting that the resistance developed byLdRHo.300involves complex mechanisms, most likely dominated by an altered drug accumulation, providing new insight in the Ho342 resistance mechanisms.

INHIBITION OF LEISHMANIA DONOVANI PROMASTIGOTE DNA TOPOISOMERASE I AND HUMAN MONOCYTE DNA TOPOISOMERASES I AND II BY ANTIMONIAL DRUGS AND CLASSICAL ANTITOPOISOMERASE AGENTS

We have compared the inhibitor sensitivities of DNA topoisomerase I (TOPI) from Leishmania donovani promastigotes and TOPs I and II of human monocytes using pentavalent and trivalent antimonials (SbV, SbIII) and classical TOP inhibitors. Bis-benzimidazoles (Hoechst-33258 and -33342) were potent inhibitors of both parasite and human TOPI, but Hoechst-33342 was markedly less cytotoxic to promastigotes than to monocytes in vitro. Leishmania donovani was also considerably less sensitive than monocytes to camptothecin, both at enzyme and cellular levels. Sodium stibogluconate (SSG) was the only antimonial to inhibit TOPI, exhibiting a significant (P < 0.05) 3-fold greater potency against the L. donovani enzyme but showed low cytotoxicities against intact promastigotes. The SbV meglumine antimoniate failed to inhibit TOPI and showed negligible cytotoxicities, whereas SbIII drugs were lethal to parasites and monocytes yet poor inhibitors of TOPI. Monocyte TOPII was inhibited by bis-benzimidazoles and insensitive to antimonials and camptothecin. The disparity between the high leishmanicidal activity and low anti-TOPI potency of SbIII indicates that in vivo targeting of L. donovani TOPI by the reductive pathway of antimonial activation is improbable. Nevertheless, the potent direct inhibition of TOPI by SSG and the differential interactions of camptothecin with L. donovani and human TOPI support the possibility of developing parasite-specific derivatives.

Reconstitution and functional characterization of the unusual bi-subunit type I DNA topoisomerase from Leishmania donovani

Leishmania donovani topoisomerase I is an unusual bi-subunit enzyme. The activity of the enzyme has been detected when the genes of the individual subunits were co-expressed in yeast [J. Biol. Chem. 278 (2003) 3521]. Here, we report for the first time, the in vitro reconstitution of the two recombinant proteins, LdTOP1L and LdTOP1S, corresponding to the large and small subunits and localization of the active enzyme in both the nucleus and kinetoplast. The proteins were purified from bacterial extract and the activity was measured by plasmid DNA relaxation assay. LdTOP1L and LdTOP1S form a direct 1:1 heterodimer complex through protein-protein interaction. Under standard relaxation assay condition (50 mM KCl and 10 mM Mg(2+)), reconstituted enzyme (LdTOP1LS) showed reduced processivity as well as 2-fold reduced affinity for DNA compared to eukaryotic monomeric rat liver topoisomerase I (RLTOP1). Cleavage assay at various salt concentrations reveals that Camptothecin (CPT) enhanced the formation of "cleavable complex" at low salt. Interaction between the two subunits leading to the formation of an active complex could be explored as an insight for development of new therapeutic agents with specific selectivity.

Characterization of Leishmania chagasi DNA topoisomerase II: a potential chemotherapeutic target

DNA topoisomerase II (topo II), an enzyme essential for cellular replication, is an eminent target for antimicrobial therapy against Leishmania chagasi, the major cause of visceral leishmaniasis in Latin America. The complete L. chagasi (Lch) TOP2 gene, encoding L. chagasi topo II, was isolated from genomic DNA using the polymerase chain reaction. The LchTOP2 gene revealed an open reading frame (ORF) of 3,711 base pairs predicting a protein with 1,236 amino acids and an estimated molecular weight of 140 kDA. The L. chagasi topo II sequence had high identity with the L. donovani topo II (98.8%) and L. infantum topo II (98.7%), followed by Crithidia fasciculata topo II (84.4%), Trypanosoma cruzi topo II (67.6%) and Trypanosoma brucei topo II (66.6%). Lch topo II had low identity with the human homologs htopo II alpha (26.3%) and htopo II beta (26.4%). Differences between L. chagasi TOP2 and human TOP2 genes suggest that leishmanial topo II is a potential target for the development of new antileishmanial agents.

EFFECTS OF TOPOISOMERASES INHIBITORS PROTOBERBERINE ON LEISHMANIA DONOVANI GROWTH, MACROPHAGE FUNCTION, AND INFECTION

DNA topoisomerases play a pivotal role in the regulation of cell division. Inhibition of Leishmania spp. topoisomerases represents an alternative to control parasite growth. Cancer research led to the development of several potent topoisomerase inhibitors such as topoisomerase I, topoisomerase II, or both (monobenzimidazole, terbenzimidazole, and protoberberine alkaloid-related compounds) that are effective antitumor agents. In the present study, we evaluated the efficacy of these compounds against Leishmania spp. growth in vitro. Some protoberberine compounds showed pronounced antileishmanial activity and were selected for further analysis in macrophages. These compounds did not affect macrophage viability and only slightly reduced macrophage nitric oxide generation in response to interferon-gamma. Moreover, exposure of infected macrophages to these compounds significantly reduced parasite loads. Collectively, our data suggest that protoberberine-related compounds have powerful antileishmania action and that minor structural variations among them can substantially improve their activity to restrict Leishmania spp. infection in vitro.

Cloning, functional analysis and post-transcriptional regulation of a type II DNA topoisomerase from Leishmania infantum. A new potential target for anti-parasite drugs

We identified a type II topoisomerase enzyme from Leishmania infantum, a parasite protozoon causing disease in humans. This protein, named Li topo II, which displays a variable C-terminal end, is located in the kinetoplast. The cloned gene encoding Li-TOP2 compensates for the slow growth of topo II-deficient mutants of Saccharomyces cerevisiae, resulting in a catalytically active DNA topoisomerase in yeast. Analysis of the specific mRNA levels of the Li-TOP2 gene showed variations throughout the parasite cell cycle in synchronized cells as well as between the distinct forms of the parasite. Thus, the enzyme had higher levels of mRNA expression in the highly infective intracellular form of the parasite, the amastigote, than in the extracellular promastigote form, suggesting a relation with the distinct developmental and infectious phases of the protozoon. In addition, western blot analysis showed differences in protein expression between the proliferative and non-proliferative forms of L.infantum promastigotes, which displayed similar levels of mRNA. This indicated possible post-transcriptional regulation mechanisms. The data suggest that Li topo II has a part in DNA decatenation and probably at the initial stages of proliferation in the intracellular form of L.infantum, a parasite that has to proliferate into the host macrophage to survive its hostile environment in its first moments of intracellular infection.

A novel active DNA topoisomerase I in Leishmania donovani

A common feature shared by type I DNA topoisomerases is the presence of a "serine, lysine, X, X, tyrosine" motif as conventional enzyme active site. Preliminary data have shown that Leishmania donovani DNA topoisomerase I gene (LdTOP1A) lacked this conserved motif, giving rise to different theories about the reconstitution of an active DNA topoisomerase I in this parasite. We, herein, describe the molecular cloning of a new DNA topoisomerase I gene from L. donovani (LdTOP1B) containing the highly conserved serine, lysine, X, X, tyrosine motif. DNA topoisomerase I activity was detected only when both genes (LdTOP1A and LdTOP1B) were co-expressed in a yeast expression system, suggesting the existence of a dimeric DNA topoisomerase I in Leishmania parasites.