3-Hydroxy-3-methyl-glutaryl-CoA reductase in Trypanosoma (Schizotrypanum) cruzi: subcellular localization and kinetic properties

Antiplasmodial, Trypanocidal, and Genotoxicity In Vitro Assessment of New Hybrid α,α-Difluorophenylacetamide-statin Derivatives

BACKGROUND

Statins present a plethora of pleiotropic effects including anti-inflammatory and antimicrobial responses. A,α-difluorophenylacetamides, analogs of diclofenac, are potent pre-clinical anti-inflammatory non-steroidal drugs. Molecular hybridization based on the combination of pharmacophoric moieties has emerged as a strategy for the development of new candidates aiming to obtain multitarget ligands.

METHODS

Considering the anti-inflammatory activity of phenylacetamides and the potential microbicidal action of statins against obligate intracellular parasites, the objective of this work was to synthesize eight new hybrid compounds of α,α-difluorophenylacetamides with the moiety of statins and assess their phenotypic activity against in vitro models of Plasmodium falciparum and Trypanosoma cruzi infection besides exploring their genotoxicity safety profile.

RESULTS

None of the sodium salt compounds presented antiparasitic activity and two acetated compounds displayed mild anti-P. falciparum effect. Against T. cruzi, the acetate halogenated hybrids showed moderate effect against both parasite forms relevant for human infection. Despite the considerable trypanosomicidal activity, the brominated compound revealed a genotoxic profile impairing future in vivo testing.

CONCLUSIONS

However, the chlorinated derivative was the most promising compound with chemical and biological profitable characteristics, without presenting genotoxicity in vitro, being eligible for further in vivo experiments.

The lipidome of Crithidia fasiculataand its plasticity

Crithidia fasiculata belongs to the trypanosomatidae order of protozoan parasites, bearing close relation to other kinetoplastid parasites such as Trypanosoma brucei and Leishmania spp. As an early diverging lineage of eukaryotes, the study of kinetoplastid parasites has provided unique insights into alternative mechanisms to traditional eukaryotic metabolic pathways. Crithidia are a monogenetic parasite for mosquito species and have two distinct lifecycle stages both taking place in the mosquito gut. These consist of a motile choanomastigote form and an immotile amastigote form morphologically similar to amastigotes in Leishmania. Owing to their close relation to Leishmania, Crithidia are a growing research tool, with continuing interest in its use as a model organism for kinetoplastid research with the added benefit that they are non-pathogenic to humans and can be grown with no special equipment or requirements for biological containment. Although comparatively little research has taken place on Crithidia, similarities to other kinetoplast species has been shown in terms of energy metabolism and genetics. Crithidia also show similarities to kinetoplastids in their production of the monosaccharide D-arabinopyranose similar to Leishmania, which is incorporated into a lipoarabinogalactan a major cell surface GPI-anchored molecule. Additionally, Crithidia have been used as a eukaryotic expression system to express proteins from other kinetoplastids and potentially other eukaryotes including human proteins allowing various co- and post-translational protein modifications to the recombinant proteins. Despite the obvious usefulness and potential of this organism very little is known about its lipid metabolism. Here we describe a detailed lipidomic analyses and demonstrate the possible placidity of Crithidia’s lipid metabolis. This could have important implications for biotechnology approaches and how other kinetoplastids interact with, and scavenge nutrients from their hosts.

MC, Concepción JL, Cáceres AJ. Consumption of Galactose by Trypanosoma cruzi Epimastigotes Generates Resistance against Oxidative Stress

In this study, we demonstrate that Trypanosoma cruzi epimastigotes previously grown in LIT medium supplemented with 20 mM galactose and exposed to sub-lethal concentrations of hydrogen peroxide (100 μM) showed two-fold and five-fold viability when compared to epimastigotes grown in LIT medium supplemented with two different glucose concentrations (20 mM and 1.5 mM), respectively. Similar results were obtained when exposing epimastigotes from all treatments to methylene blue 30 μM. Additionally, through differential centrifugation and the selective permeabilization of cellular membranes with digitonin, we found that phosphoglucomutase activity (a key enzyme in galactose metabolism) occurs predominantly within the cytosolic compartment. Furthermore, after partially permeabilizing epimastigotes with digitonin (0.025 mg × mg-1 of protein), intact glycosomes treated with 20 mM galactose released a higher hexose phosphate concentration to the cytosol in the form of glucose-1-phosphate, when compared to intact glycosomes treated with 20 mM glucose, which predominantly released glucose-6-phosphate. These results shine a light on T. cruzi's galactose metabolism and its interplay with mechanisms that enable resistance to oxidative stress.

Delineating transitions during the evolution of specialised peroxisomes: Glycosome formation in kinetoplastid and diplonemid protists

One peculiarity of protists belonging to classes Kinetoplastea and Diplonemea within the phylum Euglenozoa is compartmentalisation of most glycolytic enzymes within peroxisomes that are hence called glycosomes. This pathway is not sequestered in peroxisomes of the third Euglenozoan class, Euglenida. Previous analysis of well-studied kinetoplastids, the ‘TriTryps’ parasites Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp., identified within glycosomes other metabolic processes usually not present in peroxisomes. In addition, trypanosomatid peroxins, i.e. proteins involved in biogenesis of these organelles, are divergent from human and yeast orthologues. In recent years, genomes, transcriptomes and proteomes for a variety of euglenozoans have become available. Here, we track the possible evolution of glycosomes by querying these databases, as well as the genome of Naegleria gruberi, a non-euglenozoan, which belongs to the same protist supergroup Discoba. We searched for orthologues of TriTryps proteins involved in glycosomal metabolism and biogenesis. Predicted cellular location(s) of each metabolic enzyme identified was inferred from presence or absence of peroxisomal-targeting signals. Combined with a survey of relevant literature, we refine extensively our previously postulated hypothesis about glycosome evolution. The data agree glycolysis was compartmentalised in a common ancestor of the kinetoplastids and diplonemids, yet additionally indicates most other processes found in glycosomes of extant trypanosomatids, but not in peroxisomes of other eukaryotes were either sequestered in this ancestor or shortly after separation of the two lineages. In contrast, peroxin divergence is evident in all euglenozoans. Following their gain of pathway complexity, subsequent evolution of peroxisome/glycosome function is complex. We hypothesize compartmentalisation in glycosomes of glycolytic enzymes, their cofactors and subsequently other metabolic enzymes provided selective advantage to kinetoplastids and diplonemids during their evolution in changing marine environments. We contend two specific properties derived from the ancestral peroxisomes were key: existence of nonselective pores for small solutes and the possibility of high turnover by pexophagy. Critically, such pores and pexophagy are characterised in extant trypanosomatids. Increasing amenability of free-living kinetoplastids and recently isolated diplonemids to experimental study means our hypothesis and interpretation of bioinformatic data are suited to experimental interrogation.

Functional Peroxisomes Are Essential for Efficient Cholesterol Sensing and Synthesis

Cholesterol biosynthesis is a multi-step process involving several subcellular compartments, including peroxisomes. Cells adjust their sterol content by both transcriptional and post-transcriptional feedback regulation, for which sterol regulatory element-binding proteins (SREBPs) are essential; such homeostasis is dysregulated in peroxisome-deficient Pex2 knockout mice. Here, we compared the regulation of cholesterol biosynthesis in Chinese hamster ovary (CHO-K1) cells and in three isogenic peroxisome-deficient CHO cell lines harboring Pex2 gene mutations. Peroxisome deficiency activated expression of cholesterogenic genes, however, cholesterol levels were unchanged. 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) protein levels were increased in mutant cells, whereas HMGCR activity was significantly decreased, resulting in reduced cholesterol synthesis. U18666A, an inhibitor of lysosomal cholesterol export, induced cholesterol biosynthetic enzymes; yet, cholesterol synthesis was still reduced. Interestingly, peroxisome deficiency promoted ER-to-Golgi SREBP cleavage-activating protein (SCAP) trafficking even when cells were cholesterol-loaded. Restoration of functional peroxisomes normalized regulation of cholesterol synthesis and SCAP trafficking. These results highlight the importance of functional peroxisomes for maintaining cholesterol homeostasis and efficient cholesterol synthesis.

Hysteresis of pyruvate phosphate dikinase from Trypanosoma cruzi

Trypanosoma cruzi, the causative agent of Chagas' disease, belongs to the Trypanosomatidae family. The parasite undergoes multiple morphological and metabolic changes during its life cycle, in which it can use both glucose and amino acids as carbon and energy sources. The glycolytic pathway is peculiar in that its first six or seven steps are compartmentalized in glycosomes, and has a two-branched auxiliary glycosomal system functioning beyond the intermediate phosphoenolpyruvate (PEP) that is also used in the cytosol as substrate by pyruvate kinase. The pyruvate phosphate dikinase (PPDK) is the first enzyme of one branch, converting PEP, PPi, and AMP into pyruvate, Pi, and ATP. Here we present a kinetic study of PPDK from T. cruzi that reveals its hysteretic behavior. The length of the lag phase, and therefore the time for reaching higher specific activity values is affected by the concentration of the enzyme, the presence of hydrogen ions and the concentrations of the enzyme's substrates. Additionally, the formation of a more active PPDK with more complex structure is promoted by it substrates and the cation ammonium, indicating that this enzyme equilibrates between the monomeric (less active) and a more complex (more active) form depending on the medium. These results confirm the hysteretic behavior of PPDK and are suggestive for its functioning as a regulatory mechanism of this auxiliary pathway. Such a regulation could serve to distribute the glycolytic flux over the two auxiliary branches as a response to the different environments that the parasite encounters during its life cycle.

Activation of artemisinin and heme degradation in Leishmania tarentolae promastigotes: A possible link

Endoperoxides (EPs) appear to be promising drug candidates against protozoal diseases, including malaria and leishmaniasis. Previous studies have shown that these drugs need an intracellular activation to exert their pharmacological potential. The efficiency of these drugs is linked to the extensive iron demand of these intracellular protozoal parasites. An essential step of the activation mechanism of these drugs is the formation of radicals in Leishmania. Iron is a known trigger for intracellular radical formation. However, the activation of EPs by low molecular iron or by heme iron may strongly depend on the structure of the EPs themselves. In this study, we focused on the activation of artemisinin (Art) in Leishmania tarentolae promastigotes (LtP) in comparison to reference compounds. Viability assays in different media in the presence of different iron sources (hemin/fetal calf serum) showed that IC50 values of Art in LtP were modulated by assay conditions, but overall were within the low micromolar range. Low temperature electron paramagnetic resonance (EPR) spectroscopy of LtP showed that Art shifted the redox state of the labile iron pool less than the EP ascaridole questioning its role as a major activator of Art in LtP. Based on the high reactivity of Art with hemin in previous biomimetic experiments, we focused on putative heme-metabolizing enzymes in Leishmania, which were so far not well described. Inhibitors of mammalian heme oxygenase (HO; tin and chromium mesoporphyrin) acted antagonistically to Art in LtP and boosted its IC50 value for several magnitudes. By inductively coupled plasma methods (ICP-OES, ICP-MS) we showed that these inhibitors do not block iron (heme) accumulation, but are taken up and act within LtP. These inhibitors blocked the conversion of hemin to bilirubin in LtP homogenates, suggesting that an HO-like enzyme activity in LtP exists. NADPH-dependent degradation of Art and hemin was highest in the small granule and microsomal fractions of LtP. Photometric measurements in the model Art/hemin demonstrated that hemin requires reduction to heme and that subsequently an Art/heme complex (λmax 474 nm) is formed. EPR spin-trapping in the system Art/hemin revealed that NADPH, ascorbate and cysteine are suitable reductants and finally activate Art to acyl-carbon centered radicals. These findings suggest that heme is a major activator of Art in LtP either via HO-like enzyme activities and/or chemical interaction of heme with Art.

Isolation of Glycosomes from Trypanosoma cruzi

Glycosomes are peroxisome-related organelles of trypanosomatids in which the glycolytic and some other metabolic pathways are compartmentalized. We describe here two methods for the purification of glycosomes from Trypanosoma cruzi for preparative purposes, differential and isopycnic centrifugation. These are two techniques that allow the separation of different cellular compartments based on their different physicochemical characteristics. The first type of centrifugation is a rapid method that does not require large inputs and allows for fractions enriched in specific cell compartments to be obtained. The second type of centrifugation is a more elaborate method, but enables highly purified cellular compartments to be isolated. The success in obtaining these purified, intact organelles critically depends on using an appropriate method for controlled rupture of the cells.

Major Kinds of Drug Targets in Chagas Disease or American Trypanosomiasis

American Trypanosomiasis, a parasitic infection commonly named Chagas disease, affects millions of people all over Latin American countries. Presently, the World Health Organization (WHO) predicts that the number of international infected individuals extends to 7 to 8 million, assuming that more than 10,000 deaths occur annually. The transmission of the etiologic agent, Trypanosoma cruzi, through people migrating to non-endemic world nations makes it an emergent disease. The best promising targets for trypanocidal drugs may be classified into three main groups: Group I includes the main molecular targets that are considered among specific enzymes involved in the essential processes for parasite survival, principally Cruzipain, the major antigenic parasite cysteine proteinase. Group II involves biological pathways and their key specific enzymes, such as Sterol biosynthesis pathway, among others, specific antioxidant defense mechanisms, and bioenergetics ones. Group III includes the atypical organelles /structures present in the parasite relevant clinical forms, which are absent or considerably different from those present in mammals and biological processes related to them. These can be considered potential targets to develop drugs with extra effectiveness and fewer secondary effects than the currently used therapeutics. An improved distinction between the host and the parasite targets will help fight against this neglected disease.

Proteomic analysis of glycosomes from Trypanosoma cruzi epimastigotes

In Trypanosoma cruzi, the causal agent of Chagas disease, the first seven steps of glycolysis are compartmentalized in glycosomes, which are authentic but specialized peroxisomes. Besides glycolysis, activity of enzymes of other metabolic processes have been reported to be present in glycosomes, such as β-oxidation of fatty acids, purine salvage, pentose-phosphate pathway, gluconeogenesis and biosynthesis of ether-lipids, isoprenoids, sterols and pyrimidines. In this study, we have purified glycosomes from T. cruzi epimastigotes, collected the soluble and membrane fractions of these organelles, and separated peripheral and integral membrane proteins by Na₂CO₃ treatment and osmotic shock. Proteomic analysis was performed on each of these fractions, allowing us to confirm the presence of enzymes involved in various metabolic pathways as well as identify new components of this parasite's glycosomes.

Subcellular localization of glycolytic enzymes and characterization of intermediary metabolism of Trypanosoma rangeli

Trypanosoma rangeli is a hemoflagellate protist that infects wild and domestic mammals as well as humans in Central and South America. Although this parasite is not pathogenic for human, it is being studied because it shares with Trypanosoma cruzi, the etiological agent of Chagas' disease, biological characteristics, geographic distribution, vectors and vertebrate hosts. Several metabolic studies have been performed with T. cruzi epimastigotes, however little is known about the metabolism of T. rangeli. In this work we present the subcellular distribution of the T. rangeli enzymes responsible for the conversion of glucose to pyruvate, as determined by epifluorescense immunomicroscopy and subcellular fractionation involving either selective membrane permeabilization with digitonin or differential and isopycnic centrifugation. We found that in T. rangeli epimastigotes the first six enzymes of the glycolytic pathway, involved in the conversion of glucose to 1,3-bisphosphoglycerate are located within glycosomes, while the last four steps occur in the cytosol. In contrast with T. cruzi, where three isoenzymes (one cytosolic and two glycosomal) of phosphoglycerate kinase are expressed simultaneously, only one enzyme with this activity is detected in T. rangeli epimastigotes, in the cytosol. Consistent with this latter result, we found enzymes involved in auxiliary pathways to glycolysis needed to maintain adenine nucleotide and redox balances within glycosomes such as phosphoenolpyruvate carboxykinase, malate dehydrogenase, fumarate reductase, pyruvate phosphate dikinase and glycerol-3-phosphate dehydrogenase. Glucokinase, galactokinase and the first enzyme of the pentose-phosphate pathway, glucose-6-phosphate dehydrogenase, were also located inside glycosomes. Furthermore, we demonstrate that T. rangeli epimastigotes growing in LIT medium only consume glucose and do not excrete ammonium; moreover, they are unable to survive in partially-depleted glucose medium. The velocity of glucose consumption is about 40% higher than that of procyclic Trypanosoma brucei, and four times faster than by T. cruzi epimastigotes under the same culture conditions.

Kinetic and molecular characterization of the pyruvate phosphate dikinase from Trypanosoma cruzi

Trypanosoma cruzi, like other trypanosomatids analyzed so far, can use both glucose and amino acids as carbon and energy source. In these parasites, glycolysis is compartmentalized in glycosomes, authentic but specialized peroxisomes. The major part of this pathway, as well as a two-branched glycolytic auxiliary system, are present in these organelles. The first enzyme of one branch of this auxiliary system is the PPi-dependent pyruvate phosphate dikinase (PPDK) that converts phosphoenolpyruvate (PEP), inorganic pyrophosphate (PPi) and AMP into pyruvate, inorganic phosphate (Pi) and ATP, thus contributing to the ATP/ADP balance within the glycosomes. In this work we cloned, expressed and purified the T. cruzi PPDK. It kinetic parameters were determined, finding KM values for PEP, PPi and AMP of 320, 70 and 17 μM, respectively. Using molecular exclusion chromatography, two native forms of the enzyme were found with estimated molecular weights of 200 and 100 kDa, corresponding to a homodimer and monomer, respectively. It was established that T. cruzi PPDK's specific activity can be enhanced up to 2.6 times by the presence of ammonium in the assay mixture. During growth of epimastigotes in batch culture an apparent decrease in the specific activity of PPDK was observed. However, when its activity is normalized for the presence of ammonium in the medium, no significant modification of the enzyme activity per cell in time was found.

Trypanosoma evansi contains two auxiliary enzymes of glycolytic metabolism: Phosphoenolpyruvate carboxykinase and pyruvate phosphate dikinase

Trypanosoma evansi is a monomorphic protist that can infect horses and other animal species of economic importance for man. Like the bloodstream form of the closely related species Trypanosoma brucei, T. evansi depends exclusively on glycolysis for its free-energy generation. In T. evansi as in other kinetoplastid organisms, the enzymes of the major part of the glycolytic pathway are present within organelles called glycosomes, which are authentic but specialized peroxisomes. Since T. evansi does not undergo stage-dependent differentiations, it occurs only as bloodstream forms, it has been assumed that the metabolic pattern of this parasite is identical to that of the bloodstream form of T. brucei. However, we report here the presence of two additional enzymes, phosphoenolpyruvate carboxykinase and PPi-dependent pyruvate phosphate dikinase in T. evansi glycosomes. Their colocalization with glycolytic enzymes within the glycosomes of this parasite has not been reported before. Both enzymes can make use of PEP for contributing to the production of ATP within the organelles. The activity of these enzymes in T. evansi glycosomes drastically changes the model assumed for the oxidation of glucose by this parasite.

Glycosomal bromodomain factor 1 from Trypanosoma cruzi enhances trypomastigote cell infection and intracellular amastigote growth

We characterized bromodomain factor 1 from Trypanosoma cruzi (TcBDF1), a developmentally regulated protein that localizes in the glycosomes of epimastigotes. The overexpression of wild-type TcBDF1 is detrimental for epimastigotes, but favours trypomastigote infection, whereas mutant TcBDF1 diminishes the infectivity rate.

Aryloxyethyl Thiocyanates Are Potent Growth Inhibitors of Trypanosoma cruzi and Toxoplasma gondii

As a part of our project aimed at searching for new safe chemotherapeutic agents against parasitic diseases, several compounds structurally related to the antiparasitic agent WC-9 (4-phenoxyphenoxyethyl thiocyanate), which were modified at the terminal phenyl ring, were designed, synthesized, and evaluated as growth inhibitors against Trypanosoma cruzi, the etiological agent of Chagas disease, and Toxoplasma gondii, the parasite responsible of toxoplasmosis. Most of the synthetic analogues exhibited similar antiparasitic activity and were slightly more potent than our lead WC-9. For example, two trifluoromethylated derivatives exhibited ED50 values of 10.0 and 9.2 μM against intracellular T. cruzi, whereas they showed potent action against tachyzoites of T. gondii (ED50 values of 1.6 and 1.9 μM against T. gondii). In addition, analogues of WC-9 in which the terminal aryl group is in the meta position with respect to the alkyl chain bearing the thiocyanate group showed potent inhibitory action against both T. cruzi and T. gondii at the very low micromolar range, which suggests that a para-phenyl substitution pattern is not necessary for biological activity.

Hysteresis and positive cooperativity as possible regulatory mechanisms of Trypanosoma cruzi hexokinase activity

In Trypanosoma cruzi, the causal agent of Chagas disease, the first six or seven steps of glycolysis are compartmentalized in glycosomes, which are authentic but specialized peroxisomes. Hexokinase (HK), the first enzyme in the glycolytic pathway, has been an important research object, particularly as a potential drug target. Here we present the results of a specific kinetics study of the native HK from T. cruzi epimastigotes; a sigmoidal behavior was apparent when the velocity of the reaction was determined as a function of the concentration of its substrates, glucose and ATP. This behavior was only observed at low enzyme concentration, while at high concentration classical Michaelis-Menten kinetics was displayed. The progress curve of the enzyme's activity displays a lag phase of which the length is dependent on the protein concentration, suggesting that HK is a hysteretic enzyme. The hysteretic behavior may be attributed to slow changes in the conformation of T. cruzi HK as a response to variations of glucose and ATP concentrations in the glycosomal matrix. Variations in HK's substrate concentrations within the glycosomes may be due to variations in the trypanosome's environment. The hysteretic and cooperative behavior of the enzyme may be a form of regulation by which the parasite can more readily adapt to these environmental changes, occurring within each of its hosts, or during the early phase of transition to a new host.

Dronedarone, an amiodarone analog with improved anti-Leishmania mexicana efficacy

Dronedarone and amiodarone are cationic lipophilic benzofurans used to treat cardiac arrhythmias. They also have activity against the parasitic protozoan Trypanosoma cruzi, the causative agent of Chagas' disease. They function by disrupting intracellular Ca2+ homeostasis of the parasite and by inhibiting membrane sterol (ergosterol) biosynthesis. Amiodarone also has activity against Leishmania mexicana, suggesting that dronedarone might likewise be active against this organism. This might be of therapeutic interest, since dronedarone is thought to have fewer side effects in humans than does amiodarone. We show here that dronedarone effectively inhibits the growth of L. mexicana promastigotes in culture and, more importantly, has excellent activity against amastigotes inside infected macrophages (the clinically relevant form) without affecting the host cell, with the 50% inhibitory concentrations against amastigotes being 3 orders of magnitude lower than those obtained previously with T. cruzi amastigotes (0.65 nM versus 0.75 μM). As with amiodarone, dronedarone affects intracellular Ca2+ homeostasis in the parasite, inducing an elevation of intracellular Ca2+ levels. This is achieved by rapidly collapsing the mitochondrial membrane potential and inducing an alkalinization of acidocalcisomes at a rate that is faster than that observed with amiodarone. We also show that dronedarone inhibits parasite oxidosqualene cyclase, a key enzyme in ergosterol biosynthesis known to be vital for survival. Overall, our results suggest the possibility of repurposing dronedarone as a treatment for cutaneous, and perhaps other, leishmaniases.

Post-translational modification of the pyruvate phosphate dikinase from Trypanosoma cruzi

In kinetoplastids such as Trypanosoma cruzi, glycolysis is compartmentalized in peroxisome-like organelles called glycosomes. Pyruvate phosphate dikinase (PPDK), an auxiliary enzyme of glycolysis, is also located in the glycosomes. We have detected that this protein is post-translationally modified by phosphorylation and proteolytic cleavage. On western blots of T. cruzi epimastigotes, two PPDK forms were found with apparent MW of 100 kDa and 75 kDa, the latter one being phosphorylated at Thr481, a residue present in a highly conserved region. In subcellular localization assays the 75 kDa PPDK was located peripherally at the glycosomal membrane. Both PPDK forms were found in all life-cycle stages of the parasite. When probing for both PPDK forms during a growth of epimastigotes in batch culture, an increase in the level of the 75 kDa form and a decrease of the 100 kDa one were observed by western blot analysis, signifying that glucose starvation and the concomitant switch of the metabolism to amino acid catabolism may play a role in the post-translational processing of the PPDK. Either one or both of the processes, phosphorylation and proteolytic cleavage of PPDK, result in inactivation of the enzyme. It remains to be established whether the phenomenon exerts a regulatory function.

Design, synthesis and biological evaluation of WC-9 analogs as antiparasitic agents

As a part of our project pointed at the search of new safe chemotherapeutic and chemoprophylactic agents against parasitic diseases, several compounds structurally related to 4-phenoxyphenoxyethyl thiocyanate (WC-9), which were modified at the terminal aromatic ring, were designed, synthesized and evaluated as antiproliferative agents against Trypanosoma cruzi, the parasite responsible of American trypanosomiasis (Chagas disease) and Toxoplasma gondii, the etiological agent of toxoplasmosis. Most of the synthetic analogs exhibited similar antiparasitic activity being slightly more potent than the reference compound WC-9. For example, the nitro derivative 13 showed an ED₅₀ value of 5.2 μM. Interestingly, the regioisomer of WC-9, compound 36 showed similar inhibitory action than WC-9 indicating that para-phenyl substitution pattern is not necessarily required for biological activity. The biological evaluation against T. gondii was also very promising. The ED₅₀ values corresponding for 13, 36 and 37 were at the very low micromolar level against tachyzoites of T. gondii.

Trypanosoma cruzi response to sterol biosynthesis inhibitors: morphophysiological alterations leading to cell death

The protozoan parasite Trypanosoma cruzi displays similarities to fungi in terms of its sterol lipid biosynthesis, as ergosterol and other 24-alkylated sterols are its principal endogenous sterols. The sterol pathway is thus a potential drug target for the treatment of Chagas disease. We describe here a comparative study of the growth inhibition, ultrastructural and physiological changes leading to the death of T. cruzi cells following treatment with the sterol biosynthesis inhibitors (SBIs) ketoconazole and lovastatin. We first calculated the drug concentration inhibiting epimastigote growth by 50% (EC(50)/72 h) or killing all cells within 24 hours (EC(100)/24 h). Incubation with inhibitors at the EC(50)/72 h resulted in interesting morphological changes: intense proliferation of the inner mitochondrial membrane, which was corroborated by flow cytometry and confocal microscopy of the parasites stained with rhodamine 123, and strong swelling of the reservosomes, which was confirmed by acridine orange staining. These changes to the mitochondria and reservosomes may reflect the involvement of these organelles in ergosterol biosynthesis or the progressive autophagic process culminating in cell lysis after 6 to 7 days of treatment with SBIs at the EC(50)/72 h. By contrast, treatment with SBIs at the EC(100)/24 h resulted in rapid cell death with a necrotic phenotype: time-dependent cytosolic calcium overload, mitochondrial depolarization and reservosome membrane permeabilization (RMP), culminating in cell lysis after a few hours of drug exposure. We provide the first demonstration that RMP constitutes the "point of no return" in the cell death cascade, and propose a model for the necrotic cell death of T. cruzi. Thus, SBIs trigger cell death by different mechanisms, depending on the dose used, in T. cruzi. These findings shed new light on ergosterol biosynthesis and the mechanisms of programmed cell death in this ancient protozoan parasite.

When, how and why glycolysis became compartmentalised in the Kinetoplastea. A new look at an ancient organelle

A characteristic, well-studied feature of the pathogenic protists belonging to the family Trypanosomatidae is the compartmentalisation of the major part of the glycolytic pathway in peroxisome-like organelles, hence designated glycosomes. Such organelles containing glycolytic enzymes appear to be present in all members of the Kinetoplastea studied, and have recently also been detected in a representative of the Diplonemida, but they are absent from the Euglenida. Glycosomes therefore probably originated in a free-living, common ancestor of the Kinetoplastea and Diplonemida. The initial sequestering of glycolytic enzymes inside peroxisomes may have been the result of a minor mistargeting of proteins, as generally observed in eukaryotic cells, followed by preservation and its further expansion due to the selective advantage of this specific form of metabolic compartmentalisation. This selective advantage may have been a largely increased metabolic flexibility, allowing the organisms to adapt more readily and efficiently to different environmental conditions. Further evolution of glycosomes involved, in different taxonomic lineages, the acquisition of additional enzymes and pathways - often participating in core metabolic processes - as well as the loss of others. The acquisitions may have been promoted by the sharing of cofactors and crucial metabolites between different pathways, thus coupling different redox processes and catabolic and anabolic pathways within the organelle. A notable loss from the Trypanosomatidae concerned a major part of the typical peroxisomal H(2)O(2)-linked metabolism. We propose that the compartmentalisation of major parts of the enzyme repertoire involved in energy, carbohydrate and lipid metabolism has contributed to the multiple development of parasitism, and its elaboration to complicated life cycles involving consecutive different hosts, in the protists of the Kinetoplastea clade.

Low doses of simvastatin therapy ameliorate cardiac inflammatory remodeling in Trypanosoma cruzi-infected dogs

Chagas cardiomyopathy remodeling is based on the presence of Trypanosoma cruzi in heart tissue and on the complex inflammatory response leading to a myocardium fibrosis and alterations in conductive and functional heart parameters. This study aims to evaluate Simvastatin on the inflammatory response and heart functionality using dogs infected with Y strain of T. cruzi. Animals were treated daily with Simvastatin (20 mg) for 6 months and submitted to clinical and immunopathological evaluations. Simvastatin reduced heart expression and serum levels of interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) but not interleukin-10 (IL-10), possibly favoring blood parasitism but reducing inflammation and fibrosis in the left ventricle and right atrium. Simvastatin also ameliorated ejection fraction, diastolic diameter, and mass index of the left ventricle 6 months after infection. This study suggests that more investigation should be performed on the use of statins as a prophylactic therapy against cardiac remodeling because of their effects on modifying immune response and benefiting functional parameters in dogs with T. cruzi-induced ventricular dysfunctions.

Immune detection of acetylcholinesterase in subcellular compartments of Trypanosoma evansi

Trypanosoma evansi is a worldwide distributed hemoparasite with a strong economic impact in veterinary activities. Despite widespread knowledge about the etiology of the disease caused by T. evansi, there are few detailed studies about the metabolism of this parasite. The aim of this study was to determine the presence of Acetylcholinesterase (AChE) in T. evansi through a strategy of subcellular localization and confocal microscopy. The localization of the AChE by differential and isopycnic centrifugation strategy showed that this enzyme has a predominant localization in the glycosome, similar to hexokinase, and it is not present in either the cytosol or the plasma membrane. This study shows novel data that help to understand the non-neuronal role of AChE in the Trypanosomatidae family.

Rewiring and regulation of cross-compartmentalized metabolism in protists

Plastid acquisition, endosymbiotic associations, lateral gene transfer, organelle degeneracy or even organelle loss influence metabolic capabilities in many different protists. Thus, metabolic diversity is sculpted through the gain of new metabolic functions and moderation or loss of pathways that are often essential in the majority of eukaryotes. What is perhaps less apparent to the casual observer is that the sub-compartmentalization of ubiquitous pathways has been repeatedly remodelled during eukaryotic evolution, and the textbook pictures of intermediary metabolism established for animals, yeast and plants are not conserved in many protists. Moreover, metabolic remodelling can strongly influence the regulatory mechanisms that control carbon flux through the major metabolic pathways. Here, we provide an overview of how core metabolism has been reorganized in various unicellular eukaryotes, focusing in particular on one near universal catabolic pathway (glycolysis) and one ancient anabolic pathway (isoprenoid biosynthesis). For the example of isoprenoid biosynthesis, the compartmentalization of this process in protists often appears to have been influenced by plastid acquisition and loss, whereas for glycolysis several unexpected modes of compartmentalization have emerged. Significantly, the example of trypanosomatid glycolysis illustrates nicely how mathematical modelling and systems biology can be used to uncover or understand novel modes of pathway regulation.

In vitro activities of ER-119884 and E5700, two potent squalene synthase inhibitors, against Leishmania amazonensis: antiproliferative, biochemical, and ultrastructural effects

ER-119884 and E5700, novel arylquinuclidine derivatives developed as cholesterol-lowering agents, were potent in vitro growth inhibitors of both proliferative stages of Leishmania amazonensis, the main causative agent of cutaneous leishmaniasis in South America, with the 50% inhibitory concentrations (IC(50)s) being in the low-nanomolar to subnanomolar range. The compounds were very potent noncompetitive inhibitors of native L. amazonensis squalene synthase (SQS), with inhibition constants also being in the nanomolar to subnanomolar range. Growth inhibition was strictly associated with the depletion of the parasite's main endogenous sterols and the concomitant accumulation of exogenous cholesterol. Using electron microscopy, we identified the intracellular structures affected by the compounds. A large number of lipid inclusions displaying different shapes and electron densities were observed after treatment with both SQS inhibitors, and these inclusions were associated with an intense disorganization of the membrane that surrounds the cell body and flagellum, as well as the endoplasmic reticulum and the Golgi complex. Cells treated with ER-119884 but not those treated with E5700 had an altered cytoskeleton organization due to an abnormal distribution of tubulin, and many were arrested at cytokinesis. A prominent contractile vacuole and a phenotype typical of programmed cell death were frequently found in drug-treated cells. The selectivity of the drugs was demonstrated with the JC-1 mitochondrial fluorescent label and by trypan blue exclusion tests with macrophages, which showed that the IC(50)s against the host cells were 4 to 5 orders of magnitude greater that those against the intracellular parasites. Taken together, our results show that ER-119884 and E5700 are unusually potent and selective inhibitors of the growth of Leishmania amazonensis, probably because of their inhibitory effects on de novo sterol biosynthesis at the level of SQS, but some of our observations indicate that ER-119884 may also interfere with other cellular processes.

Delta24(25)-sterol methenyltransferase: intracellular localization and azasterol sensitivity in Leishmania major promastigotes overexpressing the enzyme

Trypanosomatids contain predominantly ergostane-based sterols, which differ from cholesterol, the main sterol in mammalian cells, in the presence of a methyl group in the 24 position. The methylation is initiated by S-adenosyl-L-methionine:Delta(24 (25))-sterol methenyltransferase, an enzyme present in protozoa, but absent in mammals. The importance of this enzyme is underscored by its potential as a drug target in the treatment of the leishmaniases. Here, we report studies concerning the intracellular distribution of sterol methenyltransferase in Leishmania major promastigotes and overexpressing cells using a specific antibody raised against highly purified recombinant protein. It was found by immunofluorescence and electron microscopy studies that in L. major wild-type cells sterol methenyltransferase was primarily associated to the endoplasmic reticulum. In addition to this location, the protein was incorporated into translucent vesicles presumably of the endocytic pathway. We also found in this study that cells overproducing the enzyme do not have increased resistance to the sterol methenyltransferase inhibitor 22, 26 azasterol.

Farnesyl diphosphate synthase localizes to the cytoplasm of Trypanosoma cruzi and T. brucei

The farnesyl diphosphate synthase (FPPS) has previously been characterized in trypanosomes as an essential enzyme for their survival and as the target for bisphosphonates, drugs that are effective both in vitro and in vivo against these parasites. Enzymes from the isoprenoid pathway have been assigned to different compartments in eukaryotes, including trypanosomatids. We here report that FPPS localizes to the cytoplasm of both Trypanosoma cruzi and T. brucei, and is not present in other organelles such as the mitochondria and glycosomes.

Quinuclidine derivatives as potential antiparasitics

There is an urgent need for the development of new drugs for the treatment of tropical parasitic diseases such as Chagas' disease and leishmaniasis. One potential drug target in the organisms that cause these diseases is sterol biosynthesis. This paper describes the design and synthesis of quinuclidine derivatives as potential inhibitors of a key enzyme in sterol biosynthesis, squalene synthase (SQS). A number of compounds that were inhibitors of the recombinant Leishmania major SQS at submicromolar concentrations were discovered. Some of these compounds were also selective for the parasite enzyme rather than the homologous human enzyme. The compounds inhibited the growth of and sterol biosynthesis in Leishmania parasites. In addition, we identified other quinuclidine derivatives that inhibit the growth of Trypanosoma brucei (the causative organism of human African trypanosomiasis) and Plasmodium falciparum (a causative agent of malaria), but through an unknown mode(s) of action.

Bisphosphonates as inhibitors of Trypanosoma cruzi hexokinase: kinetic and metabolic studies

Trypanosoma cruzi, the etiologic agent of Chagas disease, has an unusual ATP-dependent hexokinase (TcHK) that is not affected by D-glucose 6-phosphate, but is non-competitively inhibited by inorganic pyrophosphate (PP(i)), suggesting a heterotropic modulator effect. In a previous study we identified a novel family of bisphosphonates, metabolically stable analogs of PP(i), which are potent and selective inhibitors of TcHK as well as the proliferation of the clinically relevant intracellular amastigote form of the parasite in vitro (Hudock, M. P., Sanz-Rodriguez, C. E., Song, Y., Chan, J. M., Zhang, Y., Odeh, S., Kosztowski, T., Leon-Rossell, A., Concepcion, J. L., Yardley, V., Croft, S. L., Urbina, J. A., and Oldfield, E. (2006) J. Med. Chem. 49, 215-223). In this work, we report a detailed kinetic analysis of the effects of three of these bisphosphonates on homogeneous TcHK, as well as on the enzyme in purified intact glycosomes, peroxisome-like organelles that contain most of the glycolytic pathway enzymes in this organism. We also investigated the effects of the same compounds on glucose consumption by intact and digitonin-permeabilized T. cruzi epimastigotes, and on the growth of such cells in liver-infusion tryptose medium. The bisphosphonates investigated were several orders of magnitude more active than PP(i) as non-competitive or mixed inhibitors of TcHK and blocked the use of glucose by the epimastigotes, inducing a metabolic shift toward the use of amino acids as carbon and energy sources. Furthermore, there was a significant correlation between the IC(50) values for TcHK inhibition and those for epimastigote growth inhibition for the 12 most potent compounds of this series. Finally, these bisphosphonates did not affect the sterol composition of the treated cells, indicating that they do not act as inhibitors of farnesyl diphosphate synthase. Taken together, our results suggest that these novel bisphosphonates act primarily as specific inhibitors of TcHK and may represent a novel class of selective anti-T. cruzi agents.

A solanesyl-diphosphate synthase localizes in glycosomes of Trypanosoma cruzi

We report the cloning of a Trypanosoma cruzi gene encoding a solanesyl-diphosphate synthase, TcSPPS. The amino acid sequence (molecular mass approximately 39 kDa) is homologous to polyprenyl-diphosphate synthases from different organisms, showing the seven conserved motifs and the typical hydrophobic profile. TcSPPS preferred geranylgeranyl diphosphate as the allylic substrate. The final product, as determined by TLC, had nine isoprene units. This suggests that the parasite synthesizes mainly ubiquinone-9 (UQ-9), as described for Trypanosoma brucei and Leishmania major. In fact, that was the length of the ubiquinone extracted from epimastigotes, as determined by high-performance liquid chromatography. Expression of TcSPPS was able to complement an Escherichia coli ispB mutant. A punctuated pattern in the cytoplasm of the parasite was detected by immunofluorescence analysis with a specific polyclonal antibody against TcSPPS. An overlapping fluorescence pattern was observed using an antibody directed against the glycosomal marker pyruvate phosphate dikinase, suggesting that this step of the isoprenoid biosynthetic pathway is located in the glycosomes. Co-localization in glycosomes was confirmed by immunogold electron microscopy and subcellular fractionation. Because UQ has a central role in energy production and in reoxidation of reduction equivalents, TcSPPS is promising as a new chemotherapeutic target.

Farnesyl diphosphate synthase is a cytosolic enzyme in Leishmania major promastigotes and its overexpression confers resistance to risedronate

Farnesyl diphosphate synthase is the most likely molecular target of aminobisphosphonates (e.g., risedronate), a set of compounds that have been shown to have antiprotozoal activity both in vitro and in vivo. This protein, together with other enzymes involved in isoprenoid biosynthesis, is an attractive drug target, yet little is known about the compartmentalization of the biosynthetic pathway. Here we show the intracellular localization of the enzyme in wild-type Leishmania major promastigote cells and in transfectants overexpressing farnesyl diphosphate synthase by using purified antibodies generated towards a homogenous recombinant Leishmania major farnesyl diphosphate synthase protein. Indirect immunofluorescence, together with immunoelectron microscopy, indicated that the enzyme is mainly located in the cytoplasm of both wild-type cells and transfectants. Digitonin titration experiments also confirmed this observation. Hence, while the initial step of isoprenoid biosynthesis catalyzed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase is located in the mitochondrion, synthesis of farnesyl diphosphate by farnesyl diphosphate synthase is a cytosolic process. Leishmania major promastigote transfectants overexpressing farnesyl diphosphate synthase were highly resistant to risedronate, and the degree of resistance correlated with the increase in enzyme activity. Likewise, when resistance was induced by stepwise selection with the drug, the resulting resistant promastigotes exhibited increased levels of farnesyl diphosphate synthase. The overproduction of protein under different conditions of exposure to risedronate further supports the hypothesis that this enzyme is the main target of aminobisphosphonates in Leishmania cells.

Biphenylquinuclidines as inhibitors of squalene synthase and growth of parasitic protozoa

In this paper we describe the preparation of some biphenylquinuclidine derivatives and their evaluation as inhibitors of squalene synthase in order to explore their potential in the treatment of the parasitic diseases leishmaniasis and Chagas disease. The compounds were screened against recombinant Leishmania major squalene synthase and against Leishmania mexicana promastigotes, Leishmania donovani intracellular amastigotes and Trypanosoma cruzi intracellular amastigotes. Compounds that inhibited the enzyme, also reduced the levels of steroids and caused growth inhibition of L. mexicana promastigotes. However there was a lower correlation between inhibition of the enzyme and growth inhibition of the intracellular parasites, possibly due to delivery problems. Some compounds also showed growth inhibition of T. brucei rhodesiense trypomastigotes, although in this case alternative modes of action other than inhibition of SQS are probably involved.

Pyruvate phosphate dikinase and pyrophosphate metabolism in the glycosome of Trypanosoma cruzi epimastigotes

Pyruvate phosphate dikinase (PPDK) was recently reported in trypanosomatids, but its metabolic function is not yet known. The present work deals with the cellular localization and the function of the Trypanosoma cruzi enzyme. First, we show by digitonin titration and cell fractionation that the enzyme was essentially present in the glycosome matrix of the epimastigote form. Second, we address the issue of the direction of the reaction inside the glycosome for one part, our bibliographic survey evidenced a quite exergonic DeltaGo' (at least -5.2 kcal/mol at neutral pH and physiologic ionic strength); for another part, no pyrophosphatase (PPase) could be detected in fractions corresponding to the glycosomes; therefore, glycosomal PPDK likely works in the direction of pyruvate production. Third, we address the issue of the origin of the glycosomal pyrophosphate (PPi): several synthetic pathways known to produce PPi are already considered to be glycosomal. This work also indicates the presence of an NADP(+)-dependent beta-oxidation of palmitoyl-CoA in the glycosome. Several pyruvate-consuming activities, in particular alanine dehydrogenase (ADH) and pyruvate carboxylase (PC), were detected in the glycosomal fraction. PPDK appears therefore as a central enzyme in the metabolism of the glycosome of T. cruzi by providing a link between glycolysis, fatty acid oxidation and biosynthetic PPi-producing pathways. Indeed, PPDK seems to replace pyrophosphatase in its classical thermodynamic role of displacing the equilibrium of PPi-producing reactions, as well as in its role of eliminating the toxic PPi.

The glycosome membrane of Trypanosoma cruzi epimastigotes: protein and lipid composition

Highly purified glycosomes from Trypanosoma cruzi epimastigotes were obtained by differential centrifugation and isopycnic ultracentrifugation. Glycosomal membranes, produced by carbonate treatment of purified glycosomes, exhibited about eight main protein bands and eight minor ones. Essentially the same protein pattern was observed in the detergent-rich fraction of a Triton X-114 fractionation of whole glycosomes, indicating that most of the membrane-bound polypeptides were highly hydrophobic. The orientation of these proteins was studied by in situ labelling followed by limited pronase hydrolysis of intact glycosomes. Three glycosome membrane proteins were characterized as peripheral by comparing the protein bands patterns of membrane fractions obtained by different treatments. Noteworthy membrane polypeptides were: (1) a peripheral 75k Da membrane protein, oriented towards the cytosol, which was the most abundant glycosomal membrane protein in exponentially growing epimastigotes but was essentially absent in stationary phase cells; (2) a pair of integral membrane proteins with molecular masses in the range of 85-100 kDa, which were only present in stationary phase cells; (3) a heme-containing 36k Da protein, strongly associated to the membrane, present in both growth phases; (4) a very immunogenic 41k Da integral membrane polypeptide, oriented towards the cytosol. The lipid composition of the glycosomal membranes was also investigated. The distribution of phospholipid species in glycosomes and glycosomal membranes was very similar to that of whole cells, with phosphatidyl-ethanolamine, phosphatidyl-choline, and phosphatidyl-serine as main components and smaller proportions of sphingomyelin and with phosphatidyl-inositol. On the other hand, glycosomes were enriched in endogenous sterols (ergosterol, 24-ethyl-5,7,22-cholesta-trien-3beta-ol), and precursors, when compared with whole cells, a finding consistent with the proposal that these organelles are involved in the de novo biosynthesis of sterols in trypanosomatids.

Mitochondrial localization of the mevalonate pathway enzyme 3-Hydroxy-3-methyl-glutaryl-CoA reductase in the Trypanosomatidae

3-Hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) is a key enzyme in the sterol biosynthesis pathway, but its subcellular distribution in the Trypanosomatidae family is somewhat controversial. Trypanosoma cruzi and Leishmania HMGRs are closely related in their catalytic domains to bacterial and eukaryotic enzymes described but lack an amino-terminal domain responsible for the attachment to the endoplasmic reticulum. In the present study, digitonin-titration experiments together with immunoelectron microscopy were used to establish the intracellular localization of HMGR in these pathogens. Results obtained with wild-type cells and transfectants overexpressing the enzyme established that HMGR in both T. cruzi and Leishmania major is localized primarily in the mitochondrion and that elimination of the mitochondrial targeting sequence in Leishmania leads to protein accumulation in the cytosolic compartment. Furthermore, T. cruzi HMGR is efficiently targeted to the mitochondrion in yeast cells. Thus, when the gene encoding T. cruzi HMGR was expressed in a hmg1 hmg2 mutant of Saccharomyces cerevisiae, the mevalonate auxotrophy of mutant cells was relieved, and immunoelectron analysis showed that the parasite enzyme exhibits a mitochondrial localization, suggesting a conservation between the targeting signals of both organisms.

Mechanism of action of 4-phenoxyphenoxyethyl thiocyanate (WC-9) against Trypanosoma cruzi, the causative agent of Chagas' disease

We investigated the molecular basis of the activity of 4-phenoxyphenoxyethyl thiocyanate (WC-9) against Trypanosoma cruzi, the etiological agent of Chagas' disease. We found that growth inhibition of T. cruzi epimastigotes induced by this compound was associated with a reduction in the content of the parasite's endogenous sterols due to a specific blockade of their de novo synthesis at the level of squalene synthase.

A second gene for peroxisomal HMG-CoA reductase? A genomic reassessment

HMG-CoA reductase (HMGCR) catalyzes the conversion of HMG-CoA to mevalonate, the rate-limiting step of eukaryotic isoprenoid biosynthesis, and is the main target of cholesterol-lowering drugs. The classical form of the enzyme is a transmembrane-protein anchored to the endoplasmic reticulum. However, during the last years several lines of evidence pointed to the existence of a second isoform of HMGCR localized in peroxisomes, where mevalonate is converted further to farnesyl diphosphate. This finding is relevant for our understanding of the complex regulation and compartmentalization of the cholesterogenic pathway. Here we review experimental evidence suggesting that the peroxisomal activity might be due to a second HMGCR gene in mammals. We then present a comprehensive analysis of completely sequenced eukaryotic genomes, as well as the human and mouse genome drafts. Our results provide evidence for a large number of independent duplications of HMGCR in all eukaryotic kingdoms, but not for a second gene in mammals. We conclude that the peroxisomal HMGCR activity in mammals is due to alternative targeting of the ER enzyme to peroxisomes by an as yet uncharacterized mechanism.

Squalene synthase as a chemotherapeutic target in Trypanosoma cruzi and Leishmania mexicana

Trypanosoma cruzi and Leishmania parasites have a strict requirement for specific endogenous sterols (ergosterol and analogs) for survival and growth and cannot use the abundant supply of cholesterol present in their mammalian hosts. Squalene synthase (SQS, E.C. 2.5.1.21) catalyzes the first committed step in sterol biosynthesis and is currently under intense study as a possible target for cholesterol-lowering agents in humans, but it has not been investigated as a target for anti-parasitic chemotherapy. SQS is a membrane-bound enzyme in both T. cruzi epimastigotes and Leishmania mexicana promastigotes with a dual subcellular localization, being almost evenly distributed between glycosomes and mitochondrial/microsomal vesicles. Kinetic studies showed that the parasite enzymes display normal Michaelis-Menten kinetics and the values of the kinetic constants are comparable to those of the mammalian enzyme. We synthesized and purified 3-(biphenyl-4-yl)-3-hydroxyquinuclidine (BPQ-OH), a potent and specific inhibitor of mammalian SQS and found that it is also a powerful non-competitive inhibitor of T. cruzi and L. mexicana SQS, with K(i)'s in the range of 12-62 nM. BPQ-OH induced a dose-dependent reduction of proliferation the extracellular stages of these parasites with minimal growth inhibitory concentrations (MIC) of 10-30 microM. Growth inhibition and cell lysis induced by BPQ-OH in both parasites was associated with complete depletion of endogenous squalene and sterols, consistent with a blockade of de novo sterol synthesis at the level of SQS. BPQ-OH was able to eradicate intracellular T. cruzi amastigotes from Vero cells cultured at 37 degrees C, with a MIC of 30 microM with no deleterious effects on host cells. Taken together, these results support the notion that SQS inhibitors could be developed as selective anti-trypanosomatid agents.

The expression and intracellular distribution of phosphoglycerate kinase isoenzymes in Trypanosoma cruzi

In this paper, we report the subcellular distribution of phosphoglycerate kinase (PGK) in epimastigotes of Trypanosoma cruzi. Approximately 80% of the PGK activity was found in the cytosol, 20% in the glycosomes. Western blot analysis suggested that two isoenzymes of 56 and 48 kDa, respectively, are responsible for the glycosomal PGK activity, whereas the cytosolic activity should be attributed to a single PGK of 48 kDa. In analogy to the situation previously reported for PGK in Trypanosoma brucei, these isoenzymes were called PGKA, C and B, respectively. However, in T. cruzi, PGKA seems not to be a minor enzyme like its counterpart in T. brucei. Whereas PGKC behaved as a soluble glycosomal matrix protein, PGKA appeared to be present at the inner surface of the organelle's membrane. After alkaline carbonate treatment, the enzyme remained associated with the particulate fraction of the organelles. Upon solubilization of glycosomes with Triton X-114, PGKA was recovered from the detergent phase, indicating its (partial) hydrophobic character and therefore, a possible hydrophobic interaction with the membrane. The PGKA gene was cloned and sequenced, but the predicted amino-acid sequence did not reveal an obvious clue as to the mechanism by which the enzyme is attached to the glycosomal membrane.

Biogenesis and function of peroxisomes and glycosomes

Peroxisomes of higher eukaryotes, glycosomes of kinetoplastids, and glyoxysomes of plants are related microbody organelles that perform differing metabolic functions tailored to their cellular environments. The close evolutionary relationship of these organelles is most clearly evidenced by the conservation of proteins involved in matrix protein import and biogenesis. The glycosome can be viewed as an offshoot of the peroxisomal lineage with additional metabolic functions, specifically glycolysis and purine salvage. Within the parasitic protozoa, only kinetoplastids have been conclusively demonstrated to possess glycosomes or indeed any peroxisome-like organelle. The importance of glycosomal pathways and their compartmentation emphasizes the potential of the glycosome and glycosomal proteins as drug targets.

The non-mevalonate pathway of isoprenoids: genes, enzymes and intermediates

Although the mevalonate pathway had been considered for a long time as the unique source of biosynthetic isoprenoids, an alternative pathway has recently been discovered. The first intermediate, 1-deoxy-D-xylulose 5-phosphate, is assembled by condensation of glyceraldehyde 3-phosphate and pyruvate. A skeletal rearrangement coupled with a reduction step affords the branched-chain polyol, 2C-methyl-D-erythritol 4-phosphate, which is subsequently converted into a cyclic 2,4-diphosphate by the consecutive action of three enzymes via nucleotide diphosphate intermediates. The genes specifying these enzymes have been cloned from bacteria, plants and protozoa. Their expression in recombinant bacterial hosts has opened the way to the identification of several novel pathway intermediates.

A alpha-glycerophosphate dehydrogenase is present in Trypanosoma cruzi glycosomes

alpha-glycerophosphate dehydrogenase (alpha-GPDH-EC.1.1.1.8) has been considered absent in Trypanosoma cruzi in contradiction with all other studied trypanosomatids. After observing that the sole malate dehydrogenase can not maintain the intraglycosomal redox balance, GPDH activity was looked for and found, although in very variable levels, in epimastigotes extracts. GPDH was shown to be exclusively located in the glycosome of T. cruzi by digitonin treatment and isopycnic centrifugation. Antibody against T. brucei GPDH showed that this enzyme seemed to be present in an essentially inactive form at the beginning of the epimastigotes growth. GPDH is apparently linked to a salicylhydroxmic-sensitive glycerophosphate reoxidizing system and plays an essential role in the glycosome redox balance.

TcRho1, a farnesylated Rho family homologue from Trypanosoma cruzi: cloning, trans-splicing, and prenylation studies

Rho GTPases are members of the Ras superfamily and are involved in signal transduction pathways, including maintenance of cell morphology and motility, cell cycle progression, and transcription activation. We report the molecular identification in trypanosomatids (Trypanosoma cruzi) of the first member of the Rho family. The cloned Rho protein, TcRho1, shares approximately 40% homology with other members of the Rho family. Southern blot analysis revealed that TcRHO1 is a single copy gene per haploid genome, and Northern blot assays showed a transcript of 1200 nucleotides in length. Mapping the 5'-untranslated region of TcRHO1 transcripts revealed at least five different transcripts derived from differential trans-splicing. Three of the five transcripts contain the trans-splicing site within the coding region of the TcRHO1 gene. TcRho1 also contains the C-terminal sequence CQLF (CAAX motif), which is predicted to direct post-translation prenylation of the cysteine residue. A synthetic peptide containing this C-terminal motif, when tested against Q-Sepharose chromatography fractions from T. cruzi cytosol, was shown to be efficiently farnesylated, but not geranylgeranylated, despite the fact that the CAAX motif with X = Phe specifies geranylgeranylation by mammalian protein geranylgeranyltransferase I. Furthermore, immunoblot analyses of epimastigote protein with anti-S-farnesylcysteine methyl ester and anti-TcRho1 antisera strongly suggested that TcRho1 is farnesylated in vivo. The farnesylation of proteins such as Rho GTPases could be the basis for the selective cytotoxic action of protein farnesyltransferase inhibitors on trypanosomatids versus mammalian cells.

The biosynthetic incorporation of the intact leucine skeleton into sterol by the trypanosomatid Leishmania mexicana

The amino acid leucine is efficiently used by the trypanosomatid Leishmania mexicana for sterol biosynthesis. The incubation of [2-(13)C]leucine with L. mexicana promastigotes in the presence of ketoconazole gave 14alpha-methylergosta-8,24(24(1))-3beta-ol as the major sterol, which was shown by mass spectrometry to contain up to six atoms of (13)C per molecule. (13)C NMR analysis of the 14alpha-methylergosta-8,24(24(1))-3beta-ol revealed that it was labeled in only six positions: C-2, C-6, C-11, C-12, C-16, and C-23. This established that the leucine skeleton is incorporated intact into the isoprenoid pathway leading to sterol; it is not converted first to acetyl-CoA, as in animals and plants, with utilization of the acetyl-CoA to regenerate 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). An inhibitor of HMG-CoA synthase (L-659,699) blocked the incorporation of [1-(14)C]acetate into sterol but had no inhibitory effect on [U-(14)C]leucine incorporation. The HMG-CoA reductase inhibitor lovastatin inhibited promastigote growth and [U-(14)C]leucine incorporation into sterol. The addition of unlabeled mevalonic acid (MVA) overcame the lovastatin inhibition of growth and also diluted the incorporation of [1-(14)C]leucine into sterol. These results are compatible with two routes by which the leucine skeleton may enter intact into the isoprenoid pathway. The catabolism of leucine could generate HMG-CoA that is then directly reduced to MVA for incorporation into sterol. Alternatively, a compound produced as an intermediate in leucine breakdown to HMG-CoA (e.g. dimethylcrotonyl-CoA) could be directly reduced to produce an isoprene alcohol followed by phosphorylation to enter the isoprenoid pathway post-MVA.

Characterization and regulation of Leishmania major 3-hydroxy-3-methylglutaryl-CoA reductase

In eukaryotes the enzyme 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase catalyses the synthesis of mevalonic acid, a common precursor to all isoprenoid compounds. Here we report the isolation and overexpression of the gene coding for HMG-CoA reductase from Leishmania major. The protein from Leishmania lacks the membrane domain characteristic of eukaryotic cells but exhibits sequence similarity with eukaryotic reductases. Highly purified protein was achieved by ammonium sulphate precipitation followed by chromatography on hydroxyapatite. Kinetic parameters were determined for the protozoan reductase, obtaining K(m) values for the overall reaction of 40.3+/-5.8 microM for (R,S)-HMG-CoA and 81.4+/-5.3 microM for NADPH; V(max) was 33.55+/-1.8 units x mg(-1). Gel-filtration experiments suggested an apparent molecular mass of 184 kDa with subunits of 46 kDa. Finally, in order to achieve a better understanding of the role of this enzyme in trypanosomatids, the effect of possible regulators of isoprenoid biosynthesis in cultured promastigote cells was studied. Neither mevalonic acid nor serum sterols appear to modulate enzyme activity whereas incubation with lovastatin results in significant increases in the amount of reductase protein. Western- and Northern-blot analyses indicate that this activation is apparently performed via post-transcriptional control.