A comparative study of Leishmania mexicana amastigotes and promastigotes. Enzyme activities and subcellular locations

The Role of Inorganic Phosphate Transporters in Highly Proliferative Cells: From Protozoan Parasites to Cancer Cells

In addition to their standard inorganic phosphate (Pi) nutritional function, Pi transporters have additional roles in several cells, including Pi sensing (the so-called transceptor) and a crucial role in Pi metabolism, where they control several phenotypes, such as virulence in pathogens and tumour aggressiveness in cancer cells. Thus, intracellular Pi concentration should be tightly regulated by the fine control of intake and storage in organelles. Pi transporters are classified into two groups: the Pi transporter (PiT) family, also known as the Pi:Na⁺ symporter family; and the Pi:H⁺ symporter (PHS) family. Highly proliferative cells, such as protozoan parasites and cancer cells, rely on aerobic glycolysis to support the rapid generation of biomass, which is equated with the well-known Warburg effect in cancer cells. In protozoan parasite cells, Pi transporters are strongly associated with cell proliferation, possibly through their action as intracellular Pi suppliers for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity. Similarly, the growth rate hypothesis (GRH) proposes that the high Pi demands of tumours when achieving accelerated proliferation are mainly due to increased allocation to P-rich nucleic acids. The purpose of this review was to highlight recent advances in understanding the role of Pi transporters in unicellular eukaryotes and tumorigenic cells, correlating these roles with metabolism in these cells.

Three types of Leishmania mexicana amastigotes: Proteome comparison by quantitative proteomic analysis

Leishmania is the unicellular parasite transmitted by phlebotomine sand fly bite. It exists in two different forms; extracellular promastigotes, occurring in the gut of sand flies, and intracellular, round-shaped amastigotes residing mainly in vertebrate macrophages. As amastigotes originating from infected animals are often present in insufficient quality and quantity, two alternative types of amastigotes were introduced for laboratory experiments: axenic amastigotes and amastigotes from macrophages infected in vitro. Nevertheless, there is very little information about the degree of similarity/difference among these three types of amastigotes on proteomic level, whose comparison is crucial for assessing the suitability of using alternative types of amastigotes in experiments. In this study, L. mexicana amastigotes obtained from lesion of infected BALB/c mice were proteomically compared with alternatively cultivated amastigotes (axenic and macrophage-derived ones). Amastigotes of all three types were isolated, individually treated and analysed by LC-MS/MS proteomic analysis with quantification using TMT10-plex isobaric labeling. Significant differences were observed in the abundance of metabolic enzymes, virulence factors and proteins involved in translation and condensation of DNA. The most pronounced differences were observed between axenic amastigotes and lesion-derived amastigotes, macrophage-derived amastigotes were mostly intermediate between axenic and lesion-derived ones.

Leishmania Encodes a Bacterium-like 2,4-Dienoyl-Coenzyme A Reductase That Is Required for Fatty Acid β-Oxidation and Intracellular Parasite Survival

Leishmania spp. are protozoan parasites that cause a spectrum of important diseases in humans. These parasites develop as extracellular promastigotes in the digestive tract of their insect vectors and as obligate intracellular amastigotes that infect macrophages and other phagocytic cells in their vertebrate hosts. Promastigote-to-amastigote differentiation is associated with marked changes in metabolism, including the upregulation of enzymes involved in fatty acid β-oxidation, which may reflect adaptation to the intracellular niche. Here, we have investigated the function of one of these enzymes, a putative 2,4-dienoyl-coenzyme A (CoA) reductase (DECR), which is specifically required for the β-oxidation of polyunsaturated fatty acids. The Leishmania DECR shows close homology to bacterial DECR proteins, suggesting that it was acquired by lateral gene transfer. It is present in other trypanosomatids that have obligate intracellular stages (i.e., Trypanosoma cruzi and Angomonas) but is absent from dixenous parasites with an exclusively extracellular lifestyle (i.e., Trypanosoma brucei). A DECR-green fluorescent protein (GFP) fusion protein was localized to the mitochondrion in both promastigote and amastigote stages, and the levels of expression increased in the latter stages. A Leishmania major Δdecr null mutant was unable to catabolize unsaturated fatty acids and accumulated the intermediate 2,4-decadienoyl-CoA, confirming DECR's role in β-oxidation. Strikingly, the L. major Δdecr mutant was unable to survive in macrophages and was avirulent in BALB/c mice. These findings suggest that β-oxidation of polyunsaturated fatty acids is essential for intracellular parasite survival and that the bacterial origin of key enzymes in this pathway could be exploited in developing new therapies.IMPORTANCE The Trypanosomatidae are protozoan parasites that infect insects, plants, and animals and have evolved complex monoxenous (single host) and dixenous (two hosts) lifestyles. A number of species of Trypanosomatidae, including Leishmania spp., have evolved the capacity to survive within intracellular niches in vertebrate hosts. The adaptations, metabolic and other, that are associated with development of intracellular lifestyles remain poorly defined. We show that genomes of Leishmania and Trypanosomatidae that can survive intracellularly encode a 2,4-dienoyl-CoA reductase that is involved in catabolism of a subclass of fatty acids. The trypanosomatid enzyme shows closest similarity to the corresponding bacterial enzymes and is located in the mitochondrion and essential for intracellular growth of Leishmania The findings suggest that acquisition of this gene by lateral gene transfer from bacteria by ancestral monoxenous Trypanosomatidae likely contributed to the development of a dixenous lifestyle of these parasites.

Characterization of phosphate transporter(s) and understanding their role in Leishmania donovani parasite

Inorganic phosphate (Pi) is shown to be involved in excretion of methylglyoxal (MG) in the promastigote form of Leishmania donovani parasite. Absence of Pi leads to its accumulation inside the parasite. Accumulation of MG is toxic to the parasite and utilizes glyoxylase as well as excretory pathways for its detoxification. In addition, Pi is also reported to regulate activities of ectoenzymes and energy metabolism (glucose to pyruvate) etc. Thus, it is known to cumulatively affect the growth of Leishmania parasite. Hence the transporters, which allow the movement of Pi across the membrane, can prove to be a crucial drug target. Therefore, we characterized two phosphate transporters in Leishmania (i) H+ dependent myo-inositol transporter (LdPHO84), and (ii) Na+ dependent transporter (LdPHO89), based on similar studies done previously on other lower organisms and trypanosomatids. We tried to understand the secondary structure of these two proteins and confirm modulation in their expression with the change in Pi concentration outside. Moreover, their modes of action were also measured in the presence of specific inhibitors (LiF, CCCP). Further analysis on the physiological role of these transporters in various stages of the parasite life cycle needs to be entrenched.

Modulation of mitochondrial metabolism as a biochemical trait in blood feeding organisms: the redox vampire hypothesis redux

Hematophagous organisms undergo remarkable metabolic changes during the blood digestion process, increasing fermentative glucose metabolism, and reducing respiratory rates, both consequence of functional mitochondrial remodeling. Here, we review the pathways involved in energy metabolism and mitochondrial functionality in a comparative framework across different hematophagous species, and consider how these processes regulate redox homeostasis during blood digestion. The trend across distinct species indicate that a switch in energy metabolism might represent an important defensive mechanism to avoid the potential harmful interaction of oxidants generated from aerobic energy metabolism with products derived from blood digestion. Indeed, in insect vectors, blood feeding transiently reduces respiratory rates and oxidant production, irrespective of tissue and insect model. On the other hand, a different scenario is observed in several unrelated parasite species when exposed to blood digestion products, as respiratory rates reduce and mitochondrial oxidant production increase. The emerging picture indicates that re-wiring of energy metabolism, through reduced mitochondrial function, culminates in improved tolerance to redox insults and seems to represent a key step for hematophagous organisms to cope with the overwhelming and potentially toxic blood meal.

Evaluating the anti-leishmania activity of Lucilia sericata and Sarconesiopsis magellanica blowfly larval excretions/secretions in an in vitro model

Leishmaniasis is a vector-borne disease caused by infection by parasites from the genus Leishmania. Clinical manifestations can be visceral or cutaneous, the latter mainly being chronic ulcers. This work was aimed at evaluating Calliphoridae Lucilia sericata- and Sarconesiopsis magellanica-derived larval excretions and secretions' (ES) in vitro anti-leishmanial activity against Leishmania panamensis. Different larval-ES concentrations from both blowfly species were tested against either L. panamensis promastigotes or intracellular amastigotes using U937-macrophages as host cells. The Alamar Blue method was used for assessing parasite half maximal inhibitory concentration (IC₅₀) and macrophage cytotoxicity (LC₅₀). The effect of larval-ES on L. panamensis intracellular parasite forms was evaluated by calculating the percentage of infected macrophages, parasite load and toxicity. L. sericata-derived larval-ES L. panamensis macrophage LC₅₀ was 72.57μg/mL (65.35-80.58μg/mL) and promastigote IC₅₀ was 41.44μg/mL (38.57-44.52μg/mL), compared to 34.93μg/mL (31.65-38.55μg/mL) LC₅₀ and 23.42μg/mL (22.48-24.39μg/mL) IC₅₀ for S. magellanica. Microscope evaluation of intracellular parasite forms showed that treatment with 10μg/mL L. sericata ES and 5μg/mL S. magellanica ES led to a decrease in the percentage of infected macrophages and the amount of intracellular amastigotes. This study produced in vitro evidence of the antileishmanial activity of larval ES from both blowfly species on different parasitic stages and showed that the parasite was more susceptible to the ES than it's host cells. The antileishmanial effect on L. panamensis was more evident from S. magellanica ES.

Changes to cholesterol trafficking in macrophages by Leishmania parasites infection

Leishmania spp. are protozoan parasites that are transmitted by sandfly vectors during blood sucking to vertebrate hosts and cause a spectrum of diseases called leishmaniases. It has been demonstrated that host cholesterol plays an important role during Leishmania infection. Nevertheless, little is known about the intracellular distribution of this lipid early after internalization of the parasite. Here, pulse-chase experiments with radiolabeled cholesteryl esterified to fatty acids bound to low-density lipoproteins indicated that retention of this source of cholesterol is increased in parasite-containing subcellular fractions, while uptake is unaffected. This is correlated with a reduction or absence of detectable NPC1 (Niemann-Pick disease, type C1), a protein responsible for cholesterol efflux from endocytic compartments, in the Leishmania mexicana habitat and infected cells. Filipin staining revealed a halo around parasites within parasitophorous vacuoles (PV) likely representing free cholesterol accumulation. Labeling of host cell membranous cholesterol by fluorescent cholesterol species before infection revealed that this pool is also trafficked to the PV but becomes incorporated into the parasites' membranes and seems not to contribute to the halo detected by filipin. This cholesterol sequestration happened early after infection and was functionally significant as it correlated with the upregulation of mRNA-encoding proteins required for cholesterol biosynthesis. Thus, sequestration of cholesterol by Leishmania amastigotes early after infection provides a basis to understand perturbation of cholesterol-dependent processes in macrophages that were shown previously by others to be necessary for their proper function in innate and adaptive immune responses.

Increased Abundance of Proteins Involved in Resistance to Oxidative and Nitrosative Stress at the Last Stages of Growth and Development of Leishmania amazonensis Promastigotes Revealed by Proteome Analysis

Leishmania amazonensis is one of the major etiological agents of the neglected, stigmatizing disease termed american cutaneous leishmaniasis (ACL). ACL is a zoonosis and rodents are the main reservoirs. Most cases of ACL are reported in Brazil, Bolivia, Colombia and Peru. The biological cycle of the parasite is digenetic because sand fly vectors transmit the motile promastigote stage to the mammalian host dermis during blood meal intakes. The amastigote stage survives within phagocytes of the mammalian host. The purpose of this study is detection and identification of changes in protein abundance by 2DE/MALDI-TOF/TOF at the main growth phases of L. amazonensis promastigotes in axenic culture and the differentiation process that takes place simultaneously. The average number of proteins detected per gel is 202 and the non-redundant cumulative number is 339. Of those, 63 are differentially abundant throughout growth and simultaneous differentiation of L. amazonensis promastigotes. The main finding is that certain proteins involved in resistance to nitrosative and oxidative stress are more abundant at the last stages of growth and differentiation of cultured L. amazonensis promastigotes. These proteins are the arginase, a light variant of the tryparedoxin peroxidase, the iron superoxide dismutase, the regulatory subunit of the protein kinase A and a light HSP70 variant. These data taken together with the decrease of the stress-inducible protein 1 levels are additional evidence supporting the previously described pre-adaptative hypothesis, which consists of preparation in advance towards the amastigote stage.

Dissecting Leishmania infantum Energy Metabolism - A Systems Perspective

Leishmania infantum, causative agent of visceral leishmaniasis in humans, illustrates a complex lifecycle pertaining to two extreme environments, namely, the gut of the sandfly vector and human macrophages. Leishmania is capable of dynamically adapting and tactically switching between these critically hostile situations. The possible metabolic routes ventured by the parasite to achieve this exceptional adaptation to its varying environments are still poorly understood. In this study, we present an extensively reconstructed energy metabolism network of Leishmania infantum as an attempt to identify certain strategic metabolic routes preferred by the parasite to optimize its survival in such dynamic environments. The reconstructed network consists of 142 genes encoding for enzymes performing 237 reactions distributed across five distinct model compartments. We annotated the subcellular locations of different enzymes and their reactions on the basis of strong literature evidence and sequence-based detection of cellular localization signal within a protein sequence. To explore the diverse features of parasite metabolism the metabolic network was implemented and analyzed as a constraint-based model. Using a systems-based approach, we also put forth an extensive set of lethal reaction knockouts; some of which were validated using published data on Leishmania species. Performing a robustness analysis, the model was rigorously validated and tested for the secretion of overflow metabolites specific to Leishmania under varying extracellular oxygen uptake rate. Further, the fate of important non-essential amino acids in L. infantum metabolism was investigated. Stage-specific scenarios of L. infantum energy metabolism were incorporated in the model and key metabolic differences were outlined. Analysis of the model revealed the essentiality of glucose uptake, succinate fermentation, glutamate biosynthesis and an active TCA cycle as driving forces for parasite energy metabolism and its optimal growth. Finally, through our in silico knockout analysis, we could identify possible therapeutic targets that provide experimentally testable hypotheses.

Leptomonas seymouri: Adaptations to the Dixenous Life Cycle Analyzed by Genome Sequencing, Transcriptome Profiling and Co-infection with Leishmania donovani

The co-infection cases involving dixenous Leishmania spp. (mostly of the L. donovani complex) and presumably monoxenous trypanosomatids in immunocompromised mammalian hosts including humans are well documented. The main opportunistic parasite has been identified as Leptomonas seymouri of the sub-family Leishmaniinae. The molecular mechanisms allowing a parasite of insects to withstand elevated temperature and substantially different conditions of vertebrate tissues are not understood. Here we demonstrate that L. seymouri is well adapted for the environment of the warm-blooded host. We sequenced the genome and compared the whole transcriptome profiles of this species cultivated at low and high temperatures (mimicking the vector and the vertebrate host, respectively) and identified genes and pathways differentially expressed under these experimental conditions. Moreover, Leptomonas seymouri was found to persist for several days in two species of Phlebotomus spp. implicated in Leishmania donovani transmission. Despite of all these adaptations, L. seymouri remains a predominantly monoxenous species not capable of infecting vertebrate cells under normal conditions.

Gluconeogenesis in Leishmania mexicana: contribution of glycerol kinase, phosphoenolpyruvate carboxykinase, and pyruvate phosphate dikinase

Gluconeogenesis is an active pathway in Leishmania amastigotes and is essential for their survival within the mammalian cells. However, our knowledge about this pathway in trypanosomatids is very limited. We investigated the role of glycerol kinase (GK), phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate phosphate dikinase (PPDK) in gluconeogenesis by generating the respective Leishmania mexicana Δgk, Δpepck, and Δppdk null mutants. Our results demonstrated that indeed GK, PEPCK, and PPDK are key players in the gluconeogenesis pathway in Leishmania, although stage-specific differences in their contribution to this pathway were found. GK participates in the entry of glycerol in promastigotes and amastigotes; PEPCK participates in the entry of aspartate in promastigotes, and PPDK is involved in the entry of alanine in amastigotes. Furthermore, the majority of alanine enters into the pathway via decarboxylation of pyruvate in promastigotes, whereas pathway redundancy is suggested for the entry of aspartate in amastigotes. Interestingly, we also found that l-lactate, an abundant glucogenic precursor in mammals, was used by Leishmania amastigotes to synthesize mannogen, entering the pathway through PPDK. On the basis of these new results, we propose a revision in the current model of gluconeogenesis in Leishmania, emphasizing the differences between amastigotes and promastigotes. This work underlines the importance of studying the trypanosomatid intracellular life cycle stages to gain a better understanding of the pathologies caused in humans.

The mechanism behind the antileishmanial effect of zinc sulphate. II. Effects on the enzymes of the parasites

When used in vitro, zinc sulphate has a direct antileishmanial effect. To see if this effect involved the inhibition of the parasites' enzymes, extracts of the promastigotes and axenic amastigotes of Leishmania major (MHOM/IQ/93/MRC6) and L. tropica (MHOM/IQ/93/MRC2) were prepared. Zinc sulphate, at various concentrations, was then added to samples of these extracts before the activities, in the samples, of certain key enzymes of the Embden-Meyerhof pathway, hexose-monophosphate shunt and citric-acid cycle, and of two enzymes associated with virulence (protease and acid phosphatase), were determined. The zinc was found to inhibit every enzyme investigated, usually in a dose-dependent manner. Thus the direct antileishmanial effect of zinc may result, partially or entirely, from the inhibition of enzymes that are necessary for the parasites' carbohydrate metabolism and virulence.

Differential quantitative proteomic profiling of Leishmania infantum and Leishmania mexicana density gradient separated membranous fractions

Leishmaniasis, caused by infection with Leishmania, is a major public health concern affecting more than 20million people globally. Leishmania has a digenetic lifecycle consisting of an extracellular flagellated promastigote, adapted to live in the mid-gut of the sand fly host and an aflagellated intracellular amastigote that resides within the macrophage of the mammalian host. Leishmania mexicana and Leishmania infantum are causative agents of cutaneous and visceral leishmaniasis, respectively. Membrane proteins play a pivotal role in host-pathogen interactions and in regulatory pathways. As the genome of Leishmania is essentially constitutively expressed, regulation of protein expression during differentiation occurs post-transcriptionally and/or post-translationally. Quantitative mass spectrometry using iTRAQ labeling identified differences in the proteomes of density gradient separated membranous fractions of promastigote and amastigote life-stages. We identified 189 L. infantum and 107 L. mexicana non-redundant proteins of which 20-40% showed differential expression levels between promastigote and amastigote lifecycle stages. Differentially expressed proteins mapped to several pathways including cell motility, metabolism, and infectivity as well as virulence factors such as eEF-1α, amastin and leishmanolysin (GP63). Western blot analysis validated iTRAQ quantitation for leishmanolysin. Focusing on differentially expressed proteins essential for pathogenesis, may ultimately lead to the identification of novel potential therapeutic targets.

BIOLOGICAL SIGNIFICANCE

Leishmania, protozoan parasites of the Trypanosomatidae family, are the causative agents of leishmaniasis that represents a major public health concern affecting more than 20million people globally Membrane associated proteins play a pivotal role in host-pathogen interactions and in regulatory pathways. Quantitative proteomic analysis of the membranous fractions from L. mexicana and L. infantum (causative agents of cutaneous and visceral leishmaniasis, respectively) identified a number of proteins that may have important stage-specific functions in either the sand fly or mammalian host. The function of these proteins includes roles in virulence, as well as differences in metabolic process between life stages. Many of the proteins identified may act as virulence factors playing significant roles in parasite invasion, host-parasite interaction or parasite survival and thus may have therapeutic potential as drug target candidates.

Deletion of mitochondrial associated ubiquitin fold modifier protein Ufm1 in Leishmania donovani results in loss of β-oxidation of fatty acids and blocks cell division in the amastigote stage

Recently, we described the existence of the ubiquitin fold modifier 1 (Ufm1) and its conjugation pathway in Leishmania donovani. We demonstrated the conjugation of Ufm1 to proteins such as mitochondrial trifunctional protein (MTP) that catalyses β-oxidation of fatty acids in L. donovani. To elucidate the biological roles of the Ufm1-mediated modifications, we made an L. donovani Ufm1 null mutant (Ufm1(-/-)). Loss of Ufm1 and consequently absence of Ufm1 conjugation with MTP resulted in diminished acetyl-CoA, the end-product of the β-oxidation in the Ufm1(-/-) amastigote stage. The Ufm1(-/-) mutants showed reduced survival in the amastigote stage in vitro and ex vivo in human macrophages. This survival was restored by re-expression of wild-type Ufm1 with concomitant induction of acetyl-CoA but not by re-expressing the non-conjugatable Ufm1, indicating the essential nature of Ufm1 conjugation and β-oxidation. Both cell cycle analysis and ultrastructural studies of Ufm1(-/-) parasites confirmed the role of Ufm1 in amastigote growth. The defect in vitro growth of amastigotes in human macrophages was further substantiated by reduced survival. Therefore, these studies suggest the importance of Ufm1 in Leishmania pathogenesis with larger impact on other organisms and further provide an opportunity to test Ufm1(-/-) parasites as drug and vaccine targets.

Bioinformatic Analysis of Leishmania donovani Long-Chain Fatty Acid-CoA Ligase as a Novel Drug Target

Fatty acyl-CoA synthetase (fatty acid: CoA ligase, AMP-forming; (EC 6.2.1.3)) catalyzes the formation of fatty acyl-CoA by a two-step process that proceeds through the hydrolysis of pyrophosphate. Fatty acyl-CoA represents bioactive compounds that are involved in protein transport, enzyme activation, protein acylation, cell signaling, and transcriptional control in addition to serving as substrates for beta oxidation and phospholipid biosynthesis. Fatty acyl-CoA synthetase occupies a pivotal role in cellular homeostasis, particularly in lipid metabolism. Our interest in fatty acyl-CoA synthetase stems from the identification of this enzyme, long-chain fatty acyl-CoA ligase (LCFA) by microarray analysis. We found this enzyme to be differentially expressed by Leishmania donovani amastigotes resistant to antimonial treatment. In the present study, we confirm the presence of long-chain fatty acyl-CoA ligase gene in the genome of clinical isolates of Leishmania donovani collected from the disease endemic area in India. We predict a molecular model for this enzyme for in silico docking studies using chemical library available in our institute. On the basis of the data presented in this work, we propose that long-chain fatty acyl-CoA ligase enzyme serves as an important protein and a potential target candidate for development of selective inhibitors against leishmaniasis.

Proteomics of trypanosomatids of human medical importance

Leishmania spp., Trypanosoma cruzi, and Trypanosoma brucei are protozoan parasites that cause a spectrum of fatal human diseases around the world. Recent completion of the genomic sequencing of these parasites has enormous relevance to the study of their biology and the pathogenesis of the diseases they cause because it opens the door to high-throughput proteomic technologies. This review encompasses studies using diverse proteomic approaches with these organisms to describe and catalogue global protein profiles, reveal changes in protein expression during development, elucidate the subcellular localisation of gene products, and evaluate host-parasite interactions.

Effect of alkyl-lysophospholipids on some aspects of the metabolism of Leishmania donovani

Alkyl-lysophospholipids (ALPs), developed initially to be antitumor agents, have proved highly effective in the treatment of visceral leishmaniasis, a disease caused by the species making up the protozoan complex Leishmania donovani. Although their effectiveness is known, the mode of action against this parasite is not completely understood. In the present work, we have studied the effect of 3 derivatives, edelfosine, miltefosine, and ilmofosine. Using nuclear magnetic resonance spectroscopy ('H-NMR), we have examined the excreted catabolites from glucose metabolism in the promastigote forms treated with these compounds. The ALPs at concentrations of 19 and 38 microM inhibit the excretion of acetate, succinate, and pyruvate. The effect of edelfosine, miltefosine, and ilmofosine on the activity of the enzymes hexokinase, glycerolkinase 3-PD, phosphoglucose isomerase, superoxide dismutase, and phospholipase C were also examined. Glycerolkinase 3-PD and phosphoglucose isomerase are generally insensitive to the compounds, whereas hexokinase and superoxide dismutase are inhibited by miltefosine and ilmofosine. The ALPs exhibited an activated effect against the phospholipase C activity. Alkyl-lysophospholipids were shown to have a significant effect on several enzymes in important biochemical pathways indispensable for the survival of L. donovani promasigotes.

Retooling Leishmania metabolism: from sand fly gut to human macrophage

To survive extremely different environments, intracellular parasites require highly adaptable physiological and metabolic systems. Leishmania donovani extracellular promastigotes reside in a glucose-rich, slightly alkaline environment in the sand fly vector alimentary tract. On entry into human macrophage phagolysosomes, promastigotes differentiate into intracellular amastigotes. These cope with an acidic milieu, where glucose is scarce while amino acids are abundant. Here, we use an axenic differentiation model and a novel high-coverage, comparative proteomic methodology to analyze in detail protein expression changes throughout the differentiation process. The analysis identified and quantified 21% of the parasite proteome across 7 time points during differentiation. The data reveal a delayed increase in gluconeogenesis enzymes, coinciding with a decrease in glycolytic capacity. At the same time, beta-oxidation, amino acid catabolism, tricarboxylic acid cycle, mitochondrial respiration chain, and oxidative phosphorylation capacities are all up-regulated. The results indicate that the differentiating parasite shifts from glucose to fatty acids and amino acids as its main energy source. Furthermore, glycerol and amino acids are used as precursors for sugar synthesis, compensating for lack of exogenous sugars. These changes occur while promastigotes undergo morphological transformation. Our findings provide new insight into changes occurring in single-cell organisms during a developmental process.

Purification and characterization of hexokinase from Leishmania mexicana

Hexokinase from Leishmania mexicana was purified to homogeneity from a glycosome-enriched fraction obtained after a differential centrifugation of promastigote form. The kinetic properties of the pure enzyme were determined and the Km values for glucose (Km = 66 microM) and ATP (Km = 303 muM) were comparable to those from hexokinase of Trypanosoma cruzi. L. mexicana hexokinase was able to use fructose (Km = 142 microM), which reflects the condition found in the insect host. In contrast with hexokinases from other trypanosomatids, the enzyme exhibited a moderate sensitivity to inhibition by glucose 6-phosphate. This inhibition was competitive with respect to both ATP and glucose, indicating that an allosteric site for glucose 6-phosphate does not exist in this enzyme. The enzyme was also inhibited by inorganic pyrophosphate, the inhibition being higher than that observed for T. cruzi enzyme. As expected, the enzyme was localized, by immunofluorescence analysis, in glycosomes and is present in both promastigotes and true amastigotes obtained from hamster lesion. Hexokinase specific activity increased with the aging of promastigote culture, and this increment was related to glucose consumption. However, the level of the hexokinase protein remains constant as determined by Western blotting. Several hypotheses are discussed to explain this result.

Functional characterization and subcellular localization of the three malate dehydrogenase isozymes in Leishmania spp

As part of a study on the malate dehydrogenase isozymes (MDHs) from Trypanosomatids, three different fractions with MDH activity were obtained from crude extracts of Leishmania mexicana promastigotes combining two different chromatographic steps. Gel filtration chromatography in native conditions showed that most of the MDH activity present in the crude extracts eluted in a single peak, which corresponded to a lower apparent molecular mass ( congruent with 57kDa) than the value expected for typical MDHs. To further characterize the leishmanial isozymes, three putative MDH genes, presumably corresponding to the mitochondrial, glycosomal and cytosolic isoforms were amplified by PCR, cloned into bacterial expression vectors, and the recombinant enzymes purified. Digitonin extraction of intact L. mexicana promastigotes and immunofluorescence microscopy of L. major promastigotes confirmed the subcellular compartmentation of each of the three isozymes. Western blot analysis showed that the three MDHs are developmentally regulated. At the protein level, these isozymes are remarkably more abundant in amastigotes than in promastigotes of L. mexicana. Altogether our results demonstrate the presence of three MDH isoforms with slightly distinct biochemical properties and different subcellular localization in Leishmania spp. Presumably the functional and biochemical features of these isozymes reflect the metabolic adaptation to the different nutrient sources these parasites have to face along their life cycle. These results also emphasize the differences among Trypanosomatids in this area of metabolism, since in the case of Trypanosoma brucei the cMDH is the only isoform expressed in bloodstream trypomastigotes, whereas in Trypanosoma cruzi cMDH is absent.

Comparative proteomics of glycosomes from bloodstream form and procyclic culture form Trypanosoma brucei brucei

Peroxisomes are present in nearly every eukaryotic cell and compartmentalize a wide range of important metabolic processes. Glycosomes of Kinetoplastid parasites are peroxisome-like organelles, characterized by the presence of the glycolytic pathway. The two replicating stages of Trypanosoma brucei brucei, the mammalian bloodstream form (BSF) and the insect (procyclic) form (PCF), undergo considerable adaptations in metabolism when switching between the two different hosts. These adaptations involve also substantial changes in the proteome of the glycosome. Comparative (non-quantitative) analysis of BSF and PCF glycosomes by nano LC-ESI-Q-TOF-MS resulted in the validation of known functional aspects of glycosomes and the identification of novel glycosomal constituents.

Metabolic changes in glucose transporter-deficient Leishmania mexicana and parasite virulence

Leishmania mexicana are parasitic protozoa that express a variety of glycoconjugates that play important roles in their biology as well as the storage carbohydrate beta-mannan, which is an essential virulence factor for survival of intracellular amastigote forms in the mammalian host. Glucose transporter null mutants, which are viable as insect form promastigotes but not as amastigotes, do not take up glucose and other hexoses but are still able to synthesize these glycoconjugates and beta-mannan, although at reduced levels. Synthesis of these carbohydrate-containing macromolecules could be accounted for by incorporation of non-carbohydrate precursors into carbohydrates by gluconeogenesis. However, the significantly reduced level of the virulence factor beta-mannan in the glucose transporter null mutants compared with wild-type parasites may contribute to the non-viability of these null mutants in the disease-causing amastigote stage of the life cycle.

Niche metabolism in parasitic protozoa

Complete or partial genome sequences have recently become available for several medically and evolutionarily important parasitic protozoa. Through the application of bioinformatics complete metabolic repertoires for these parasites can be predicted. For experimentally intractable parasites insight provided by metabolic maps generated in silico has been startling. At its more extreme end, such bioinformatics reckoning facilitated the discovery in some parasites of mitochondria remodelled beyond previous recognition, and the identification of a non-photosynthetic chloroplast relic in malarial parasites. However, for experimentally tractable parasites, mapping of the general metabolic terrain is only a first step in understanding how the parasite modulates its streamlined, yet still often puzzlingly complex, metabolism in order to complete life cycles within host, vector, or environment. This review provides a comparative overview and discussion of metabolic strategies used by several different parasitic protozoa in order to subvert and survive host defences, and illustrates how genomic data contribute to the elucidation of parasite metabolism.

Molecular characterization of the hexokinase gene from Leishmania major

The coding sequence for hexokinase enzyme was cloned from Leishmania major. The sequence was found to encode an enzyme with a molecular mass of 51.74 kDa. Amino acid sequence showed maximum homology with known trypanosome and plant hexokinases. It has a calculated isoelectric point of 8.46 and contains an N-terminal peroxisome-targeting signal, the characteristics frequently associated with glycosomal proteins. The sequence indicated the presence of conserved amino acid residues and motifs that are present in plant and mammalian hexokinases; these are apparently involved in the binding of different substrates. The L. major genome was found to have 2 copies of hexokinase coding sequences in tandem with an intergenic spacer of 2.58 kb. Both the genes in the hexokinase locus were transcribed as individual transcripts in a monocistronic form, having the same size as seen by Northern blot analysis. The hexokinase gene was transcribed in large amounts in the promastigote stage, whereas there is only weak expression in the amastigote stage as determined by RT-PCR analysis. Sequencing of hexokinase loci from different Leishmania species (e.g., L. donovani, L. infantum, L. tropica, and L. mexicana) revealed that the hexokinase locus is highly conserved at the DNA and protein levels among species of Leishmania compared with trypanosomes.

Leishmania major and Leishmania tropica: II. Effect of an immunomodulator, S(2) complex on the enzymes of the parasites

S(2) complex has been reported to have a direct antileishmanial effect. The possibility that the direct antileishmanial effect may be due to inhibition of key enzymes involved in glucose metabolism and/ or enzymes associated with virulence was investigated. Cell pellets were prepared from cultures of both axenic amastigotes and promastigotes of Leishmania major (MHOM/IQ/93/MRC6) and L. tropica (MHOM/IQ/93/MRC2). S(2) complex, at various concentrations, was added to the enzyme extracts prepared from the pellets. Results show that in the Embden-Meyerhof pathway, both hexokinase and glucose-phosphate isomerase but not fructophosphokinase were dose dependently inhibited. In the hexose-monophosphate shunt both glucose-6-phosphate dehydrogenase and ribose-5-phosphate isomerase were dose dependently inhibited. Malic dehydrogenase and malic enzyme from the citric-acid cycle were both dose dependently inhibited but succinate dehydrogenase from the same pathway was not inhibited. Both enzymes associated with virulence (protease and acid phosphatase), showed activation rather than inhibition at higher doses of S(2) complex. Thus, the direct antileishmanial effect of S(2) complex may result, partially or entirely, from the inhibition of enzymes that are necessary for the parasites' carbohydrate metabolism.

Subcellular localization of an intracellular serine protease of 68 kDa in Leishmania (Leishmania) amazonensis promastigotes

Here we report the subcellular localization of an intracellular serine protease of 68 kDa in axenic promastigotes of Leishmania (Leishmania) amazonensis, using subcellular fractionation, enzymatic assays, immunoblotting, and immunocytochemistry. All fractions were evaluated by transmission electron microscopy and the serine protease activity was measured during the cell fractionation procedure using alpha-N-r-tosyl-L-arginine methyl ester (L-TAME) as substrate, phenylmethylsulphone fluoride (PMSF) and L-1-tosylamino-2-phenylethylchloromethylketone (TPCK) as specific inhibitors. The enzymatic activity was detected mainly in a membranous vesicular fraction (6.5-fold enrichment relative to the whole homogenate), but also in a crude plasma membrane fraction (2.0-fold). Analysis by SDS-PAGE gelatin under reducing conditions demonstrated that the major proteolytic activity was found in a 68 kDa protein in all fractions studied. A protein with identical molecular weight was also recognized in immunoblots by a polyclonal antibody against serine protease (anti-SP), with higher immunoreactivity in the vesicular fraction. Electron microscopic immunolocalization using the same polyclonal antibody showed the enzyme present at the cell surface, as well as in cytoplasmic membranous compartments of the parasite. Our findings indicate that the internal location of this serine protease in L. amazonensis is mainly restricted to the membranes of intracellular compartments resembling endocytic/exocytic elements.

Biogenesis of peroxisomes and glycosomes: trypanosomatid glycosome assembly is a promising new drug target

In trypanosomatids (Trypanosoma and Leishmania), protozoa responsible for serious diseases of mankind in tropical and subtropical countries, core carbohydrate metabolism including glycolysis is compartmentalized in peculiar peroxisomes called glycosomes. Proper biogenesis of these organelles and the correct sequestering of glycolytic enzymes are essential to these parasites. Biogenesis of glycosomes in trypanosomatids and that of peroxisomes in other eukaryotes, including the human host, occur via homologous processes involving proteins called peroxins, which exert their function through multiple, transient interactions with each other. Decreased expression of peroxins leads to death of trypanosomes. Peroxins show only a low level of sequence conservation. Therefore, it seems feasible to design compounds that will prevent interactions of proteins involved in biogenesis of trypanosomatid glycosomes without interfering with peroxisome formation in the human host cells. Such compounds would be suitable as lead drugs against trypanosomatid-borne diseases.

Leishmania amazonensis: early proteinase activities during promastigote-amastigote differentiation in vitro

Leishmania proteinase activity is known as parasite differentiation marker, and has been considered relevant for leishmanial survival and virulence. These properties suggest that Leishmania proteinases can be promising targets for development of anti-leishmania drugs. Here, we analyze the activities of four proteinases during the early phase of the Leishmania amazonensis promastigotes differentiation into amastigotes induced by heat shock. We have examined activities of cysteine-, metallo-, serine-, and aspartic-proteinase by hydrolysis of specific chromogenic substrates at pH 5.0 and at the optimal pH for each enzyme. Our results show that metallo-, serine-, and aspartic-proteinases activities were down-regulated during the shock-induced transformation of promastigotes into amastigotes. In contrast, cysteine-proteinase activity increased concomitantly with the promastigote differentiation. Immunocytochemical localization using two anti-cysteine-proteinase monospecific rabbit antibodies detected the enzyme in several cell compartments of both parasite stages. Our results show different proteinase activity modulation and expression during the early phases of the shock-induced parasite transformation.

Glycosomes: parasites and the divergence of peroxisomal purpose

Peroxisomes are membrane-bounded organelles that compartmentalize a variety of metabolic functions. Perhaps the most divergent peroxisomes known are the glycosomes of trypanosomes and their relatives. The glycolytic pathway of these organisms resides within the glycosome. The development of robust molecular genetic and proteomic approaches coupled with the completion of the genome sequence of the pathogens Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major provides an opportunity to determine the complement of proteins within the glycosome and the function of compartmentation. Studies now suggest that regulation of glycolysis is a strong driving force for maintenance of the glycosome.

Pentose phosphate metabolism in Leishmania mexicana

The metabolism of pentose phosphates was studied in Leishmania mexicana promastigotes. Each of the enzymes of the classical pentose phosphate pathway (PPP) has been identified and specific activities measured. Functioning of the PPP was demonstrated in non-growing cells by measuring the evolution of 14CO2 from [1-14C]D-glucose and [6-14C]D-glucose under normal conditions and also under selective stimulation of the PPP by exposure to methylene blue. The proportion of glucose which passes through the PPP increases in the latter condition, thus suggesting a protective role against oxidant stress. The incorporation into nucleic acids of ribose 5-phosphate provided via either glucose or free ribose was also determined. Results indicate that the PPP enables glucose to serve as a source of ribose 5-phosphate in nucleotide biosynthesis. Moreover, free ribose is incorporated efficiently, implying the presence of a ribose uptake system and also of ribokinase. Ribose was shown to be accumulated by a carrier mediated process in L. mexicana promastigotes and ribokinase activity was also measured in these cells.

Leishmania mexicana glycerol-3-phosphate dehydrogenase showed conformational changes upon binding a bi-substrate adduct

Certain pathogenic trypanosomatids are highly dependent on glycolysis for ATP production, and hence their glycolytic enzymes, including glycerol-3-phosphate dehydrogenase (GPDH), are considered attractive drug targets. The ternary complex structure of Leishmania mexicana GPDH (LmGPDH) with dihydroxyacetone phosphate (DHAP) and NAD(+) was determined to 1.9A resolution as a further step towards understanding this enzyme's mode of action. When compared with the apo and binary complex structures, the ternary complex structure shows an 11 degrees hinge-bending motion of the C-terminal domain with respect to the N-terminal domain. In addition, residues in the C-terminal domain involved in catalysis or substrates binding show significant movements and a previously invisible five-residue loop region becomes well ordered and participates in NAD(+) binding. Unexpectedly, DHAP and NAD(+) appear to form a covalent bond, producing an adduct in the active site of LmGPDH. Modeling a ternary complex glycerol 3-phosphate (G3P) and NAD(+) with LmGPDH identified ten active site residues that are highly conserved among all GPDHs. Two lysine residues, Lys125 and Lys210, that are presumed to be critical in catalysis, were mutated resulting in greatly reduced catalytic activity. Comparison with other structurally related enzymes found by the program DALI suggested Lys210 as a key catalytic residue, which is located on a structurally conserved alpha-helix. From the results of site-directed mutagenesis, molecular modeling and comparison with related dehydrogenases, a catalytic mechanism of LmGPDH and a possible evolutionary scenario of this group of dehydrogenases are proposed.

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Effect of amidine derivatives on nitric oxide production by Leishmania amazonensis promastigotes and axenic amastigotes

The effects of pentamidine isethionate (reference drug) and N,N'-diphenyl-4-methoxy-benzamidine (test compound) on NO. production by Leishmania amazonensis promastigotes and axenic amastigotes were investigated by measuring nitrite, a by-product of nitric oxide released into culture supernatants. The NO. production by infective promastigotes was inhibited by OCH(3)-amidine in about 23.53% and by pentamidine in only 3.78%. In axenic amastigotes, the inhibition of NO. production by OCH(3)-amidine was significantly higher (52.94%; p=0.01) than that by pentamidine, which inhibited this radical production nonsignificantly (25.29%; p=0.1). The mechanism of amidine derivatives, as an antimicrobial agent, is unknown. However, other amidines, such as a diamidine (pentamidine), contain chemical structures shared by the guanidino group of the nitric oxide synthase substrate L-arginine, suggesting the possibility of an interaction with this enzyme or electronic factors (substituent constant) that alter physical and chemical properties significant for biological activity.

Anomalous differences of light elements in determining precise binding modes of ligands to glycerol-3-phosphate dehydrogenase

Pathogenic protozoa such as Trypanosome and Leishmania species cause tremendous suffering worldwide. Because of their dependence on glycolysis for energy, the glycolytic enzymes of these organisms, including glycerol-3-phosphate dehydrogenase (GPDH), are considered attractive drug targets. Using the adenine part of NAD as a lead compound, several 2,6-disubstituted purines were synthesized as inhibitors of Leishmania mexicana GPDH (LmGPDH). The electron densities for the inhibitor 2-bromo-6-chloro-purine bound to LmGPDH using a "conventional" wavelength around 1 A displayed a quasisymmetric shape. The anomalous signals from data collected at 1.77 A clearly indicated the positions of the halogen atoms and revealed the multiple binding modes of this inhibitor. Intriguing differences in the observed binding modes of the inhibitor between very similarly prepared crystals illustrate the possibility of crystal-to-crystal variations in protein-ligand complex structures.

Leishmania donovani phosphofructokinase. Gene characterization, biochemical properties and structure-modeling studies

The characterization of the gene encoding Leishmania donovani phosphofructokinase (PFK) and the biochemical properties of the expressed enzyme are reported. L. donovani has a single PFK gene copy per haploid genome that encodes a polypeptide with a deduced molecular mass of 53 988 and a pI of 9.26. The predicted amino acid sequence contains a C-terminal tripeptide that conforms to an established signal for glycosome targeting. L. donovani PFK showed most sequence similarity to inorganic pyrophosphate (PPi)-dependent PFKs, despite being ATP-dependent. It thereby resembles PFKs from other Kinetoplastida such as Trypanosoma brucei, Trypanoplasma borreli (characterized in this study), and a PFK found in Entamoeba histolytica. It exhibited hyperbolic kinetics with respect to ATP whereas the binding of the other substrate, fructose 6-phosphate, showed slight positive cooperativity. PPi, even at high concentrations, did not have any effect. AMP acted as an activator of PFK, shifting its kinetics for fructose 6-phosphate from slightly sigmoid to hyperbolic, and increasing considerably the affinity for this substrate, whereas GDP did not have any effect. Modelling studies and site-directed mutagenesis were employed to shed light on the structural basis for the AMP effector specificity and on ATP/PPi specificity among PFKs.

Experimental studies on the evolution of antimony-resistant phenotype during the in vitro life cycle of Leishmania infantum: implications for the spread of chemoresistance in endemic areas

Pentavalent antimonial unresponsiveness is an emerging problem in endemic areas and information on factors which could modulate the transmission of drug-resistant phenotypes and parasites during life cycle are warranted. Using axenic amastigotes resistant to potassium antimonyl tartrate (Sb(III)) we investigated the modulation of antimonyl resistance during the in vitro life cycle. We assessed: (i) the stability of the drug-resistant phenotype during the in vitro life cycle; (ii) the transmission of drug-resistant clones when mixed with a wild-type clone at different susceptible/chemoresistant ratios (50/50,90/10,10/90) after one or two in vitro life cycles. We demonstrate that: (i) mutants which were 12,28,35 and 44 fold more resistant to Sb(III)-antimonial than their parental wild-type, were Glucantime Sb(V)-resistant when growing in THP-1 cells; (ii) the drug-resistant phenotype was partially retained during long-term in vitro culture (3 months) in drug free medium; (iii) the antimonyl-resistant phenotype was retained after one or more in vitro life cycles. However, when drug-resistant parasites were mixed with susceptible, mutants could not be detected in the resulting population, after one or two in vitro life cycles, whatever the initial wild-type/chemoresistant ratio. These results could be explained by the lower capacity of drug-resistant amastigotes to undergo the amastigote-promastigote differentiation process, leading probably to their sequential elimination during life cycle. Taken together, these observations demonstrate that different factors could modulate the transmission of Leishmania drug resistance during the parasite's life cycle.

Antimonial-mediated DNA fragmentation in Leishmania infantum amastigotes

The basic treatment of leishmaniasis consists in the administration of pentavalent antimonials. The mechanisms that contribute to pentavalent antimonial toxicity against the intracellular stage of the parasite (i.e., amastigote) are still unknown. In this study, the combined use of several techniques including DNA fragmentation assay and in situ and cytofluorometry terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling methods and YOPRO-1 staining allowed us to demonstrate that potassium antimonyl tartrate, an Sb(III)-containing drug, was able to induce cell death associated with DNA fragmentation in axenic amastigotes of Leishmania infantum at low concentrations (10 microg/ml). This observation was in close correlation with the toxicity of Sb(III) species against axenic amastigotes (50% inhibitory concentration of 4.75 microg/ml). Despite some similarities to apoptosis, nuclease activation was not a consequence of caspase-1, caspase-3, calpain, cysteine protease, or proteasome activation. Altogether, our results demonstrate that the antileishmanial toxicity of Sb(III) antimonials is associated with parasite oligonucleosomal DNA fragmentation, indicative of the occurrence of late events in the overall process of apoptosis. The elucidation of the biochemical pathways leading to cell death could allow the isolation of new therapeutic targets.

Identification of a novel prostaglandin f(2alpha) synthase in Trypanosoma brucei

Members of the genus Trypanosoma cause African trypanosomiasis in humans and animals in Africa. Infection of mammals by African trypanosomes is characterized by an upregulation of prostaglandin (PG) production in the plasma and cerebrospinal fluid. These metabolites of arachidonic acid (AA) may, in part, be responsible for symptoms such as fever, headache, immunosuppression, deep muscle hyperaesthesia, miscarriage, ovarian dysfunction, sleepiness, and other symptoms observed in patients with chronic African trypanosomiasis. Here, we show that the protozoan parasite T. brucei is involved in PG production and that it produces PGs enzymatically from AA and its metabolite, PGH(2). Among all PGs synthesized, PGF(2alpha) was the major prostanoid produced by trypanosome lysates. We have purified a novel T. brucei PGF(2alpha) synthase (TbPGFS) and cloned its cDNA. Phylogenetic analysis and molecular properties revealed that TbPGFS is completely distinct from mammalian PGF synthases. We also found that TbPGFS mRNA expression and TbPGFS activity were high in the early logarithmic growth phase and low during the stationary phase. The characterization of TbPGFS and its gene in T. brucei provides a basis for the molecular analysis of the role of parasite-derived PGF(2alpha) in the physiology of the parasite and the pathogenesis of African trypanosomiasis.

Comparative study of Leishmania mexicana and Trypanosoma brucei NAD-dependent glycerol-3-phosphate dehydrogenase

The NAD-dependent glycerol-3-phosphate dehydrogenases (G3PDH, EC 1.1.1.8) of Trypanosoma brucei and Leishmania mexicana are thought to have different roles in carbohydrate metabolism. Here the physicochemical and kinetic properties of natural G3PDH from T. brucei with the recombinant homologue of L. mexicana which share 63% positional identity are compared. Despite their supposed different functions in energy metabolism of the parasites the two G3PDHs have remarkably similar properties, including pH optima and K(m) value for dihydroxyacetone phosphate (DHAP) and NADH in the formation of glycerol 3-phosphate (G3P) and for NAD+ and G3P in the reverse reaction. Both enzymes are subject inhibition by dihydroxyacetone phosphate at concentrations above 0.2 mM and are inhibited by the trypanocidal drugs suramin and melarsen oxide at sub-micromolar concentrations.

Efficacy of second line drugs on antimonyl-resistant amastigotes of Leishmania infantum

In a previous paper we have demonstrated that the induction, by direct drug pressure, of a resistance to Sb(III) antimony at physiological concentration in the amastigote stage of the parasite, led to a high cross-resistance to Sb(V) species in the form of Glucantime. In this paper, further chemoresistant clones were characterized. Axenic amastigotes of Leishmania infantum were adapted to survive in culture medium containing 4, 20, 30 and 120 microg/ml of potassium antimonyl tartrate Sb(II). These mutants were 12, 28, 35 and 44-fold more resistant to Sb(III) than the parental wild-type clone. They were able to resist at concentrations of Glucantime Sb(V) as high as 160 microg/ml when growing in THP-1 cells. We have investigated the efficacy of second line drugs in clinical use (pentamidine and amphotericin B) on the antimony-resistant mutants. Amphotericin B was toxic for both wild-type and chemoresistant mutants at concentrations ranging from 0.05 to 0.15 microM. Pentamidine which is extensively used when the first course of antimonial pentavalent compounds is unsuccessful, was more toxic for all the chemoresistant organisms than for the wild-type clone. In the same way, chemoresistant amastigotes growing within THP-1 cells were more susceptible to pentamidine than the wild-type clone. Our results showed that the resistance of the mutants was restricted to the antimony containing drugs and did not led to a cross-resistance against the other clinically relevant drugs. These results confirmed that these two drugs (pentamidine and amphotericin B) are good candidates to treat pentavalent antimonial unresponsiveness.

Enzymes of glucose and glycerol catabolism in in vitro-propagated Theileria parva schizonts

Theileria parva schizonts propagated in vitro in peripheral blood lymphocytes were purified and assayed for key enzymes of glucose and glycerol catabolism and the citric acid cycle. The activities of glycolytic enzymes were in the range of 21-100 nmol/min/mg protein. Glycerol kinase and alpha -glycerophosphate dehydrogenase activities were more than 16 times lower than the activities of other enzymes catalysing the oxidation of the triose phosphates to lactate. It was suggested that the catabolism of glycerol is negligible and that glucose is catabolized to lactate via the Embden-Meyerhof pathway. The activities of the enzymes catalysing the section of the citric acid cycle that involves the formation of citrate to succinyl-CoA were consistently very low (less than 2.0 nmol/min/mg protein), indicating that this part of the cycle plays a minor role in this parasite. Enzyme activities of the cycle catalysing the formation of succinate from oxaloacetate were relatively higher than those catalysing other sections of the citric acid cycle, suggesting that this section of the cycle could be important to the parasite. Pyruvate carboxylase activity was more than 10 times that of phosphoenolpyruvate carboxykinase. It was suggested that pyruvate could be carboxylated to oxaloacetate. Taken together, these results suggest that the catabolism of glucose in Theileria parva schizonts is mainly via the Embden-Meyerhof pathway and that the citric acid cycle plays a minor role in energy production.

Axenically grown amastigotes of Leishmania infantum used as an in vitro model to investigate the pentavalent antimony mode of action

The mechanism(s) of activity of pentavalent antimony [Sb(V)] is poorly understood. In a recent study, we have shown that potassium antimonyl tartrate, a trivalent antimonial [Sb(III)], was substantially more potent than Sb(V) against both promastigotes and axenically grown amastigotes of three Leishmania species, supporting the idea of an in vivo metabolic conversion of Sb(V) into Sb(III). We report that amastigotes of Leishmania infantum cultured under axenic conditions were poorly susceptible to meglumine [Glucantime; an Sb(V)], unlike those growing inside THP-1 cells (50% inhibitory concentrations [IC50s], about 1.8 mg/ml and 22 microg/ml, respectively). In order to define more precisely the mode of action of Sb(V) agents in vivo, we first induced in vitro Sb(III) resistance by direct drug pressure on axenically grown amastigotes of L. infantum. Then we determined the susceptibilities of both extracellular and intracellular chemoresistant amastigotes to the Sb(V)-containing drugs meglumine and sodium stibogluconate plus m-chlorocresol (Pentostam). The chemoresistant amastigotes LdiR2, LdiR10, and LdiR20 were 14, 26, and 32 times more resistant to Sb(III), respectively, than the wild-type one (LdiWT). In accordance with the hypothesis described above, we found that intracellular chemoresistant amastigotes were resistant to meglumine [Sb(V)] in proportion to the initial level of Sb(III)-induced resistance. By contrast, Sb(III)-resistant cells were very susceptible to sodium stibogluconate. This lack of cross-resistance is probably due to the presence in this reagent of m-chlorocresol, which we found to be more toxic than Sb(III) to L. infantum amastigotes (IC50s, of 0.54 and 1.32 microg/ml, respectively). Collectively, these results were consistent with the hypothesis of an intramacrophagic metabolic conversion of Sb(V) into trivalent compounds, which in turn became readily toxic to the Leishmania amastigote stage.

In vitro life cycle of pentamidine-resistant amastigotes: stability of the chemoresistant phenotypes is dependent on the level of resistance induced

Using a continuous drug pressure protocol, we induced pentamidine resistance in an active and dividing population of amastigote forms of Leishmania mexicana. We selected in vitro two clones with different levels of resistance to pentamidine, with clone LmPENT5 being resistant to 5 microM pentamidine, while clone LmPENT20 was resistant to 20 microM pentamidine. Resistance indexes (50% inhibitory concentration [IC50] after drug presure/IC50 before drug pressure) of 2 (LmPENT5) and 6 (LmPENT20) were determined after drug selection. Both resistant clones expressed significant cross-resistance to diminazene aceturate and primaquine. Pentamidine resistance was not reversed by verapamil, a calcium channel blocker known to reverse multidrug resistance (A. J. Bitonti, et al., Science 242:1301-1303, 1988; A. R. C. Safa et al., J. Biol. Chem. 262:7884-7888, 1987). No difference in the in vitro infectivity for resident mouse macrophages was observed between the wild-type clone (clone LmWT) and pentamidine-resistant clones. During in vitro infectivity experiments, when the life cycle was performed starting from the intramacrophagic amastigote stage, the drug resistance of the resulting LmPENT20 amastigotes was preserved even if the intermediate promastigote stage could not be considered resistant to 20 microM pentamidine. In the same way, when a complete developmental sequence of L. mexicana was achieved axenically by manipulation of appropriate culture conditions, the resulting axenically grown LmPENT20 amastigotes remained pentamidine resistant, whereas LmPENT5 amastigotes lost their ability to resist pentamidine, with IC50s and index of resistance values close to those for the LmWT clone. These results strongly indicate that the level of pentamidine tolerated by resistant amastigotes after the life cycle was dependent on the induced level of resistance. This fact could be significant in the in vivo transmission of drug-resistant parasites by Phlebotominae. Particular attention should be given to the finding that the emergence of parasite resistance is a potential risk of the use of inadequate doses as therapy in humans.

Axenically cultured amastigote forms as an in vitro model for investigation of antileishmanial agents

Using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide microassay, previously described as a means of quantifying Leishmania amazonensis in vitro at the amastigote stage (D. Sereno and J. L. Lemesre, Parisitol. Res., in press), we have compared the activities of seven drugs, including those currently used to treat leishmaniasis, against axenically grown amastigote and promastigote forms of three Leishmania species (L. amazonensis, L. mexicana, and L. infantum, responsible for diffuse cutaneous, cutaneous, and visceral leishmaniasis, respectively). The ability of axenically cultured amastigote organisms to be used in an investigation of antileishmanial agents was first evaluated. We have confirmed the toxicities of sodium stibogluconate (Pentostam), pentamidine, and amphotericin B to active and dividing populations of axenically cultured amastigotes. The toxicity of potassium antimonyl tartrate trihydrate, which is generally higher than that of Pentostam, seemed to indicate that pentavalent antimony can be metabolized in vivo to compounds, possibly trivalent in nature, which are more active against the amastigote organisms. When the drug susceptibilities of parasites at both stages were compared, great variations were found for all the drugs studied. These major differences, which show the specific chemosusceptibility of the parasite at the mammalian stage, demonstrate the potential of using cultured amastigotes instead of promastigotes in a drug-screening procedure for early detection. This in vitro model may help in the isolation of active compounds, particularly those with low-grade activities, against the mammalian stage of the parasite.

An axenic amastigote system for drug screening

Currently available primary screens for selection of candidate antileishmanial compounds are not ideal. The choices include screens that are designed to closely reflect the situation in vivo but are labor-intensive and expensive (intracellular amastigotes and animal models) and screens that are designed to facilitate rapid testing of a large number of drugs but do not use the clinically relevant parasite stage (promastigote model). The advent of successful in vitro culture of axenic amastigotes permits the development of a primary screen which is quick and easy like the promastigote screen but still representative of the situation in vivo, since it uses the relevant parasite stage. We have established an axenic amastigote drug screening system using a Leishmania mexicana strain (strain M379). A comparison of the 50% inhibitory concentration (IC50) drug sensitivity profiles of M379 promastigotes, intracellular amastigotes, and axenic amastigotes for six clinically relevant antileishmanial drugs (sodium stibogluconate, meglumine antimoniate, pentamidine, paromomycin, amphotericin B, WR6026) showed that M379 axenic amastigotes are a good model for a primary drug screen. Promastigote and intracellular amastigote IC50s differed for four of the six drugs tested by threefold or more; axenic amastigote and intracellular amastigote IC50s differed by twofold for only one drug. This shows that the axenic amastigote susceptibility to clinically used reference drugs is comparable to the susceptibility of amastigotes in macrophages. These data also suggest that for the compounds tested, susceptibility is intrinsic to the parasite stage. This contradicts previous hypotheses that suggested that the activities of antimonial agents against intracellular amastigotes were solely a function of the macrophage.

Energy metabolism in Leishmania

Alanine plays a key role in the response of promastigotes to osmotic stress and to hypoxia. It is rapidly released in response to hypo-osmolality, is consumed from its large intracellular pool under iso-osmotic conditions even in the presence of glucose, and is synthesized under hyperosmotic conditions even in the absence of glucose. Its rate of oxidation, in the presence or absence of any of ten other amino acids tested, is strongly inhibited by hyperosmolality. Glucose oxidation is also inhibited by hyperosmolality, but to a lesser extent than that of alanine, and is inhibited by alanine, glutamate, and aspartate. Hyperosmolality also inhibits the incorporation of label from [2-14C]acetate into the putative storage carbohydrate, mannan, which occurs via the glyoxylate bypass and the as yet unexplored "mannoneogenic" pathway. The rates of glycolysis and of oxidation of several amino acids decrease with increasing culture age, but the capacity to oxidize fatty acids increases, and in cells from 3-day stationary phase cultures hyperosmolality enhances rather than inhibits alanine oxidation.