Human African trypanosomiasis

Multitask learning-driven identification of novel antitrypanosomal compounds

Background: Chagas disease and human African trypanosomiasis cause substantial death and morbidity, particularly in low- and middle-income countries, making the need for novel drugs urgent. Methodology & results: Therefore, an explainable multitask pipeline to profile the activity of compounds against three trypanosomes (Trypanosoma brucei brucei, Trypanosoma brucei rhodesiense and Trypanosoma cruzi) were created. These models successfully discovered four new experimental hits (LC-3, LC-4, LC-6 and LC-15). Among them, LC-6 showed promising results, with IC50 values ranging 0.01-0.072 μM and selectivity indices >10,000. Conclusion: These results demonstrate that the multitask protocol offers predictivity and interpretability in the virtual screening of new antitrypanosomal compounds and has the potential to improve hit rates in Chagas and human African trypanosomiasis projects.

Antiprotozoal agents: How have they changed over a decade?

Neglected tropical diseases are a diverse group of communicable diseases that are endemic in low- or low-to-middle-income countries located in tropical and subtropical zones. The number and availability of drugs for treating these diseases are low, the administration route is inconvenient in some cases, and most of them have safety, efficacy, or adverse/toxic reaction issues. The need for developing new drugs to deal with these issues is clear, but one of the most drastic consequences of this negligence is the lack of interest in the research and development of new therapeutic options among major pharmaceutical companies. Positive changes have been achieved over the last few years, although the overall situation remains alarming. After more than one decade since the original work reviewing antiprotozoal agents came to light, now it is time to question ourselves: How has the scenario for the treatment of protozoal diseases such as malaria, leishmaniasis, human African trypanosomiasis, and American trypanosomiasis changed? This review covers the last decade in terms of the drugs currently available for the treatment of these diseases as well as the clinical candidates being currently investigated.

Does Counting Different Life Stages Impact Estimates for Extinction Probabilities for Tsetse (Glossina spp)?

As insect populations decline, due to climate change and other environmental disruptions, there has been an increased interest in understanding extinction probabilities. Generally, the life cycle of insects occurs in well-defined stages: when counting insects, questions naturally arise about which life stage to count. Using tsetse flies (vectors of trypanosomiasis) as a case study, we develop a model that works when different life stages are counted. Previous branching process models for tsetse populations only explicitly represent newly emerged adult female tsetse and use that subpopulation to keep track of population growth/decline. Here, we directly model other life stages. We analyse reproduction numbers and extinction probabilities and show that several previous models used for estimating extinction probabilities for tsetse populations are special cases of the current model. We confirm that the reproduction number is the same regardless of which life stage is counted, and show how the extinction probability depends on which life stage we start from. We demonstrate, and provide a biological explanation for, a simple relationship between extinction probabilities for the different life stages, based on the probability of recruitment between stages. These results offer insights into insect population dynamics and provide tools that will help with more detailed models of tsetse populations. Population dynamics studies of insects should be clear about life stages and counting points.

Prevalence of symbionts and trypanosome infections in tsetse flies of two villages of the "Faro and Déo" division of the Adamawa region of Cameroon

Background

Tsetse flies are vectors of human and animal African trypanosomiasis. In spite of many decades of chemotherapy and vector control, the disease has not been eradicated. Other methods like the transformation of tsetse fly symbionts to render the fly refractory to trypanosome infection are being evaluated. The aim of the present study was to evaluate the association between trypanosome infections and the presence of symbionts in these tsetse species. Tsetse flies were trapped in two villages of the “Faro and Déo” Division of the Adamawa region of Cameroon. In the field, tsetse fly species were identified and their infection by trypanosomes was checked by microscopy. In the laboratory, DNA was extracted from their midguts and the presence of symbionts (Sodalis glossinidius and Wolbachia sp.) and trypanosomes was checked by PCR. Symbionts/trypanosomes association tests were performed.

Results

Three tsetse fly species including Glossina tachinoides (90.1%), Glossina morsitans submorsitans (9.4%) and Glossina fuscipes fuscipes (0.5%) were caught. In all the population we obtained an occurrence rate of 37.2% for Sodalis glossinidius and 67.6% for Wolbachia irrespective to tsetse flies species. S. glossinidius and Wolbachia sp. occurrence rates were respectively 37 and 68% for G. tachinoides and 28.6 and 59.5% for G. m. submorsitans. Between Golde Bourle and Mayo Dagoum significant differences were observed in the prevalence of symbionts. Prevalence of trypanosomes were 34.8% for Glossina tachinoides and 40.5% for Glossina morsitans submorsitans. In G. tachinoides, the trypanosome infection rates were 11, 2.6 and 13.7%, respectively, for T. brucei s.l., T. congolense forest type and T. congolense savannah type. In G. m. submorsitans, these infection rates were 16.7, 9.5 and, 2.4% respectively, for T. brucei s.l., T. congolense forest type and T. congolense savannah type.

Conclusions

The rate of tsetse fly infection by trypanosomes was low compared to those obtained in HAT foci of south Cameroon, and this rate was not statistically linked to the rate of symbiont occurrence. This study allowed to show for the first time the presence of Wolbachia sp. in the tsetse fly sub-species Glossina morsitans submorsitans and Glossina tachinoides.

Optimization and Evaluation of Antiparasitic Benzamidobenzoic Acids as Inhibitors of Kinetoplastid Hexokinase 1

Kinetoplastid-based infections are neglected diseases that represent a significant human health issue. Chemotherapeutic options are limited due to toxicity, parasite susceptibility, and poor patient compliance. In response, we studied a molecular-target-directed approach involving intervention of hexokinase activity-a pivotal enzyme in parasite metabolism. A benzamidobenzoic acid hit with modest biochemical inhibition of Trypanosoma brucei hexokinase 1 (TbHK1, IC50 =9.1 μm), low mammalian cytotoxicity (IMR90 cells, EC50 >25 μm), and no appreciable activity on whole bloodstream-form (BSF) parasites was optimized to afford a probe with improved TbHK1 potency and, significantly, efficacy against whole BSF parasites (TbHK1, IC50 =0.28 μm; BSF, ED50 =1.9 μm). Compounds in this series also inhibited the hexokinase enzyme from Leishmania major (LmHK1), albeit with less potency than toward TbHK1, suggesting that inhibition of the glycolytic pathway may be a promising opportunity to target multiple disease-causing trypanosomatid protozoa.

Genetic diversity and population structure of the tsetse fly Glossina fuscipes fuscipes (Diptera: Glossinidae) in Northern Uganda: Implications for vector control

Uganda is the only country where the chronic and acute forms of human African Trypanosomiasis (HAT) or sleeping sickness both occur and are separated by < 100 km in areas north of Lake Kyoga. In Uganda, Glossina fuscipes fuscipes is the main vector of the Trypanosoma parasites responsible for these diseases as well for the animal African Trypanosomiasis (AAT), or Nagana. We used highly polymorphic microsatellite loci and a mitochondrial DNA (mtDNA) marker to provide fine scale spatial resolution of genetic structure of G. f. fuscipes from 42 sampling sites from the northern region of Uganda where a merger of the two disease belts is feared. Based on microsatellite analyses, we found that G. f. fuscipes in northern Uganda are structured into three distinct genetic clusters with varying degrees of interconnectivity among them. Based on genetic assignment and spatial location, we grouped the sampling sites into four genetic units corresponding to northwestern Uganda in the Albert Nile drainage, northeastern Uganda in the Lake Kyoga drainage, western Uganda in the Victoria Nile drainage, and a transition zone between the two northern genetic clusters characterized by high level of genetic admixture. An analysis using HYBRIDLAB supported a hybrid swarm model as most consistent with tsetse genotypes in these admixed samples. Results of mtDNA analyses revealed the presence of 30 haplotypes representing three main haplogroups, whose location broadly overlaps with the microsatellite defined clusters. Migration analyses based on microsatellites point to moderate migration among the northern units located in the Albert Nile, Achwa River, Okole River, and Lake Kyoga drainages, but not between the northern units and the Victoria Nile drainage in the west. Effective population size estimates were variable with low to moderate sizes in most populations and with evidence of recent population bottlenecks, especially in the northeast unit of the Lake Kyoga drainage. Our microsatellite and mtDNA based analyses indicate that G. f. fuscipes movement along the Achwa and Okole rivers may facilitate northwest expansion of the Rhodesiense disease belt in Uganda. We identified tsetse migration corridors and recommend a rolling carpet approach from south of Lake Kyoga northward to minimize disease dispersal and prevent vector re-colonization. Additionally, our findings highlight the need for continuing tsetse monitoring efforts during and after control.

Northern Uganda is an epidemiologically important region affected by human African trypanosomiasis (HAT) because it harbors both forms of the HAT disease (T. b. gambiense and T. b. rhodesiense). The geographic location of this region creates the risk that these distinct foci could merge, which would complicate diagnosis and treatment, and may result in recombination between the two parasite strains with as yet unknown consequences. Both strains require a tsetse fly vector for transmission, and in Uganda, G. f. fuscipes is the major vector of HAT. Controlling the vector remains one of the most effective strategies for controlling trypanosome parasites. However, vector control efforts may not be sustainable in terms of long term reduction in G. f. fuscipes populations due to population rebounds. Population genetics data can allow us to determine the likely source of population rebounds and to establish a more robust control strategy. In this study, we build on a previous broad spatial survey of G. f. fuscipes genetic structure in Uganda by adding more than 30 novel sampling sites that are strategically spaced across a region of northern Uganda that, for historical and political reasons, was severely understudied and faces particularly high disease risk. We identify natural population breaks, migration corridors and a hybrid zone with evidence of free interbreeding of G. f. fuscipes across the geographic region that spans the two HAT disease foci. We also find evidence of low effective population sizes and population bottlenecks in some areas that have been subjects of past control but remain regions of high tsetse density, which stresses the risk of population rebounds if monitoring is not explicitly incorporated into the control strategy. We use these results to make suggestions that will enhance the design and implementation of control activities in northern Uganda.

Crystal structures of Trypanosoma brucei oligopeptidase B broaden the paradigm of catalytic regulation in prolyl oligopeptidase family enzymes

Oligopeptidase B cleaves after basic amino acids in peptides up to 30 residues. As a virulence factor in bacteria and trypanosomatid pathogens that is absent in higher eukaryotes, this is a promising drug target. Here we present ligand-free open state and inhibitor-bound closed state crystal structures of oligopeptidase B from Trypanosoma brucei, the causative agent of African sleeping sickness. These (and related) structures show the importance of structural dynamics, governed by a fine enthalpic and entropic balance, in substrate size selectivity and catalysis. Peptides over 30 residues cannot fit the enzyme cavity, preventing the complete domain closure required for a key propeller Asp/Glu to fix the catalytic His and Arg in the catalytically competent conformation. This size exclusion mechanism protects larger peptides and proteins from degradation. Similar bacterial prolyl endopeptidase and archael acylaminoacyl peptidase structures demonstrate this mechanism is conserved among oligopeptidase family enzymes across all three domains of life.

Tsetse-Wolbachia symbiosis: comes of age and has great potential for pest and disease control

Tsetse flies (Diptera: Glossinidae) are the sole vectors of African trypanosomes, the causative agent of sleeping sickness in human and nagana in animals. Like most eukaryotic organisms, Glossina species have established symbiotic associations with bacteria. Three main symbiotic bacteria have been found in tsetse flies: Wigglesworthia glossinidia, an obligate symbiotic bacterium, the secondary endosymbiont Sodalis glossinidius and the reproductive symbiont Wolbachia pipientis. In the present review, we discuss recent studies on the detection and characterization of Wolbachia infections in Glossina species, the horizontal transfer of Wolbachia genes to tsetse chromosomes, the ability of this symbiont to induce cytoplasmic incompatibility in Glossina morsitans morsitans and also how new environment-friendly tools for disease control could be developed by harnessing Wolbachia symbiosis.

Detection and characterization of Wolbachia infections in laboratory and natural populations of different species of tsetse flies (genus Glossina)

Background

Wolbachia is a genus of endosymbiotic α-Proteobacteria infecting a wide range of arthropods and filarial nematodes. Wolbachia is able to induce reproductive abnormalities such as cytoplasmic incompatibility (CI), thelytokous parthenogenesis, feminization and male killing, thus affecting biology, ecology and evolution of its hosts. The bacterial group has prompted research regarding its potential for the control of agricultural and medical disease vectors, including Glossina spp., which transmits African trypanosomes, the causative agents of sleeping sickness in humans and nagana in animals.

Results

In the present study, we employed a Wolbachia specific 16S rRNA PCR assay to investigate the presence of Wolbachia in six different laboratory stocks as well as in natural populations of nine different Glossina species originating from 10 African countries. Wolbachia was prevalent in Glossina morsitans morsitans, G. morsitans centralis and G. austeni populations. It was also detected in G. brevipalpis, and, for the first time, in G. pallidipes and G. palpalis gambiensis. On the other hand, Wolbachia was not found in G. p. palpalis, G. fuscipes fuscipes and G. tachinoides. Wolbachia infections of different laboratory and natural populations of Glossina species were characterized using 16S rRNA, the wsp (Wolbachia Surface Protein) gene and MLST (Multi Locus Sequence Typing) gene markers. This analysis led to the detection of horizontal gene transfer events, in which Wobachia genes were inserted into the tsetse flies fly nuclear genome.

Conclusions

Wolbachia infections were detected in both laboratory and natural populations of several different Glossina species. The characterization of these Wolbachia strains promises to lead to a deeper insight in tsetse flies-Wolbachia interactions, which is essential for the development and use of Wolbachia-based biological control methods.

Polyandry is a common event in wild populations of the Tsetse fly Glossina fuscipes fuscipes and may impact population reduction measures

BACKGROUND

Glossina fuscipes fuscipes is the main vector of human and animal trypanosomiasis in Africa, particularly in Uganda. Attempts to control/eradicate this species using biological methods require knowledge of its reproductive biology. An important aspect is the number of times a female mates in the wild as this influences the effective population size and may constitute a critical factor in determining the success of control methods. To date, polyandry in G.f. fuscipes has not been investigated in the laboratory or in the wild. Interest in assessing the presence of remating in Ugandan populations is driven by the fact that eradication of this species is at the planning stage in this country.

METHODOLOGY/PRINCIPAL FINDINGS

Two well established populations, Kabukanga in the West and Buvuma Island in Lake Victoria, were sampled to assess the presence and frequency of female remating. Six informative microsatellite loci were used to estimate the number of matings per female by genotyping sperm preserved in the female spermathecae. The direct count of the minimum number of males that transferred sperm to the spermathecae was compared to Maximum Likelihood and Bayesian probability estimates. The three estimates provided evidence that remating is common in the populations but the frequency is substantially different: 57% in Kabukanga and 33% in Buvuma.

CONCLUSIONS/SIGNIFICANCE

The presence of remating, with females maintaining sperm from different mates, may constitute a critical factor in cases of re-infestation of cleared areas and/or of residual populations. Remating may enhance the reproductive potential of re-invading propagules in terms of their effective population size. We suggest that population age structure may influence remating frequency. Considering the seasonal demographic changes that this fly undergoes during the dry and wet seasons, control programmes based on SIT should release large numbers of sterile males, even in residual surviving target populations, in the dry season.

Design and screening of ASIC inhibitors based on aromatic diamidines for combating neurological disorders

Acid sensing ion channels (ASICs) are implicated in various brain functions including learning and memory and are involved in a number of neurological disorders such as pain, ischemic stroke, depression, and multiple sclerosis. We have recently defined ASICs as one of receptor targets of aromatic diamidines in neurons. Aromatic diamidines are DNA-binding agents and have long been used in the treatment of leishmaniasis, trypanosomiasis, pneumocystis pneumonia and babesiosis. Moreover, some aromatic diamidines are used as skin-care and baby products and others have potential to suppress tumor growth or to combat malaria. A large number of aromatic diamidines or analogs have been synthesized. Many efforts are being made to optimize the therapeutic spectrum of aromatic diamidines, i.e. to reduce toxicity, increase oral bioavailability and enhance their penetration of the blood-brain barrier. Aromatic diamidines therefore provide a shortcut of screening for selective ASIC inhibitors with therapeutic potential. Intriguingly nafamostat, a protease inhibitor for treating acute pancreatitis, also inhibits ASIC activities. Aromatic diamidines and nafamostat have many similarities although they belong to distinct classes of medicinal agents for curing different diseases. Here we delineate background, clinical application and drug development of aromatic diamidines that could facilitate the screening for selective ASIC inhibitors for research purposes. Further studies may lead to a drug with therapeutic value and extend the therapeutic scope of aromatic diamidines to combat neurological diseases.

Functional characterization of two novel parvulins in Trypanosoma brucei

Parvulins belong to a family of peptidyl-prolyl cis/trans isomerases (PPIases) that catalyze the cis/trans conformations of prolyl-peptidyl bonds. Herein, we characterized two novel parvulins, TbPIN1 and TbPAR42, in Trypanosoma brucei. TbPIN1, a 115 amino-acid protein, contains a single PPIase domain but lacks the N-terminal WW domain. Using NMR spectroscopy, TbPIN1 was found to exhibit PPIase activity toward a phosphorylated substrate. Overexpression of TbPIN1 can rescue the impaired temperature-sensitive phenotype in a mutant yeast strain. TbPAR42, containing 383 amino acids, comprises a novel FHA domain at its N terminus and a C-terminal PPIase domain but is a non-Pin1-type PPIase. Functionally, a knockdown of TbPAR42 in its procyclic form results in reduced proliferation rates suggesting an important role in cell growth.

High levels of genetic differentiation between Ugandan Glossina fuscipes fuscipes populations separated by Lake Kyoga

Background

Glossina fuscipes fuscipes is the major vector of human African trypanosomiasis, commonly referred to as sleeping sickness, in Uganda. In western and eastern Africa, the disease has distinct clinical manifestations and is caused by two different parasites: Trypanosoma brucei rhodesiense and T. b. gambiense. Uganda is exceptional in that it harbors both parasites, which are separated by a narrow 160-km belt. This separation is puzzling considering there are no restrictions on the movement of people and animals across this region.

Methodology and Results

We investigated whether genetic heterogeneity of G. f. fuscipes vector populations can provide an explanation for this disjunct distribution of the Trypanosoma parasites. Therefore, we examined genetic structuring of G. f. fuscipes populations across Uganda using newly developed microsatellite markers, as well as mtDNA. Our data show that G. f. fuscipes populations are highly structured, with two clearly defined clusters that are separated by Lake Kyoga, located in central Uganda. Interestingly, we did not find a correlation between genetic heterogeneity and the type of Trypanosoma parasite transmitted.

Conclusions

The lack of a correlation between genetic structuring of G. f. fuscipes populations and the distribution of T. b. gambiense and T. b. rhodesiense indicates that it is unlikely that genetic heterogeneity of G. f. fuscipes populations explains the disjunct distribution of the parasites. These results have important epidemiological implications, suggesting that a fusion of the two disease distributions is unlikely to be prevented by an incompatibility between vector populations and parasite.

The two types of sleeping sickness in West and East Africa are markedly distinct, require different treatments, and are caused by different parasites. The only country where both parasites are present is Uganda, where they are separated by a narrow 160 km disease-free belt. Because there is no restriction on the movement of humans and animals between the two disease zones, this separation is puzzling. We asked whether this disjunct distribution can be explained by variation within the tsetse fly that is largely responsible for transmitting both diseases in Uganda, Glossina fuscipes fuscipes. We therefore examined whether this tsetse subspecies is genetically uniform across Uganda. Our results indicate that G. f. fusicipes is not genetically different between the two disease zones, but there are clear genetic differences between northern and southern populations, which are separated by Lake Kyoga. Therefore, it is unlikely that variation in the tsetse fly determines the distribution of the two parasites. This implies that the two diseases may fuse in the near future, which would greatly complicate diagnosis and treatment of sleeping sickness in any potential area of overlap.

Small ncRNA transcriptome analysis from kinetoplast mitochondria of Leishmania tarentolae

Gene expression in mitochondria of kinetoplastid protozoa requires RNA editing, a post-transcriptional process which involves insertion or deletion of uridine residues at specific sites within mitochondrial pre-mRNAs. Sequence specificity of the RNA editing process is mediated by oligo-uridylated small, non-coding RNAs, designated as guide RNAs (gRNAs). In this study, we have analyzed the small ncRNA transcriptome from kinetoplast mitochondria of Leishmania tarentolae by generating specialized cDNA libraries encoding size-selected RNA species. Through this screen, a significant number of novel oligo-uridylated RNA species, which we have termed oU-RNAs, has been identified. Most novel oU-RNAs are present as stable RNA species in mitochondria as assessed by northern blot analysis. Thereby, novel oU-RNAs show similar expression levels and sizes as previously reported for canonical gRNAs. Several oU-RNAs are transcribed from both strands of the maxicircle and minicircles components of the mitochondrial genome, from regions where up till now no transcription has been reported. Two stable oU-RNAs exhibit an anchor sequence in antisense orientation to known gRNAs and thus might regulate editing of respective pre-mRNAs. A number of oU-RNAs map in antisense orientation to non-edited protein-coding genes suggesting that they might function by a different mechanism. In addition, our screen shows that all kinetoplast-derived RNAs are prone to some degree of uridylation.

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.

Overcoming resistance with designer immunotoxins

Normal human serum contains apolipoprotein L-I (apoL-I), which lyses African trypanaosomes. Resistant forms, such as Trypanosoma brucei rhodesiense express apoL-I-neutralising serum resistance-associated protein, which enables this parasite to infect humans and cause human African trypanosomiasis. This paper describes the construction of a mutant apoL-I conjugated to a nanobody that targets the variant surface glycoprotein of trypanosomes. Treatment with this engineered immunotoxin has resulted in both alleviating and curative effects on chronic and acute infections of mice with normal human serum-resistant and -sensitive trypanosomes.

RNA aptamers as potential pharmaceuticals against infections with African trypanosomes

Protozoal pathogens cause symptomatic as well as asymptomatic infections. They have a worldwide impact, which in part is reflected in the long-standing search for antiprotozoal chemotherapy. Unfortunately, effective treatments for the different diseases are by and large not available. This is especially true for African trypanosomiasis, also known as sleeping sickness. The disease is an increasing problem in many parts of sub-Saharan Africa, which is due to the lack of new therapeutics and the increasing resistance against traditional drugs such as melarsoprol, berenil and isometamidium. Considerable progress has been made over the past 10 years in the development of nucleic acid-based drug molecules using a variety of different technologies. One approach is a combinatorial technology that involves an iterative Darwinian-type in vitro evolution process, which has been termed SELEX for "systematic evolution of ligands by exponential enrichment". The procedure is a highly efficient method of identifying rare ligands from combinatorial nucleic acid libraries of very high complexity. It allows the selection of nucleic acid molecules with desired functions, and it has been instrumental in the identification of a number of synthetic DNA and RNA molecules, so-called aptamers that recognize ligands of different chemical origin. Aptamers typically bind their target with high affinity and high specificity and have successfully been converted into pharmaceutically active compounds. Here we summarize the recent examples of the SELEX technique within the context of identifying high-affinity RNA ligands against the surface of the protozoan parasite Trypanosoma brucei, which is the causative agent of sleeping sickness.

NIMA-related kinase TbNRKC is involved in basal body separation in Trypanosoma brucei

The NIMA-related kinase 2 (NEK 2) has important cell cycle functions related to centriole integrity and splitting. Trypanosoma brucei does not possess centrioles, however, cytokinesis is coupled to basal body separation events. Here we report the first functional characterisation of a T. brucei basal body-cytoskeletal NIMA-related kinase (NRK) protein, TbNRKC. The TbNRKC kinase domain has high amino acid identity with the human NEK1 kinase domain (50%) but also shares 42% identity with human NEK2. TbNRKC is expressed in bloodstream and procyclic cells and functions as a bona fide kinase in vitro. Remarkably, RNAi knockdown of TbNRKC and overexpression of kinase-dead TbNRKC in procyclic forms induces the accumulation of cells with four basal bodies, whereas overexpression of active protein produces supernumary basal bodies and blocks cytokinesis. TbNRKC is located on mature and immature basal bodies and is the first T. brucei NRK to be found associated with the basal body cytokinesis pathway.

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.

Inhibition of Trypanosoma cruzi hexokinase by bisphosphonates

Hexokinase is the first enzyme involved in glycolysis in most organisms, including the etiological agents of Chagas disease (Trypanosoma cruzi) and African sleeping sickness (Trypanosoma brucei). The T. cruzi enzyme is unusual since, unlike the human enzyme, it is inhibited by inorganic diphosphate (PPi). Here, we show that non-hydrolyzable analogues of PPi, bisphosphonates, are potent inhibitors of T. cruzi hexokinase (TcHK). We determined the activity of 42 bisphosphonates against TcHK, and the IC(50) values were used to construct pharmacophore and comparative molecular similarity indices analysis (CoMSIA) models for enzyme inhibition. Both models revealed the importance of electrostatic, hydrophobic, and steric interactions, and the IC(50) values for 17 active compounds were predicted with an average error of 2.4x by using the CoMSIA models. The compound most active against T. cruzi hexokinase was found to have a 2.2 microM IC(50) versus the clinically relevant intracellular amastigote form of T. cruzi, but only a approximately 1-2 mM IC(50) versus Dictyostelium discoideum and a human cell line, indicating selective activity versus T. cruzi.