Proline transport in Leishmania donovani amastigotes: dependence on pH gradients and membrane potential

Energy metabolism as a target for cyclobenzaprine: A drug candidate against Visceral Leishmaniasis

Leishmaniases have a broad spectrum of clinical manifestations, ranging from a cutaneous to a progressive and fatal visceral disease. Chemotherapy is nowadays the almost exclusive way to fight the disease but limited by its scarce therapeutic arsenal, on its own compromised by adverse side effects and clinical resistance. Cyclobenzaprine (CBP), an FDA-approved oral muscle relaxant drug has previously demonstrated in vitro and in vivo activity against Leishmania sp., but its targets were not fully unveiled. This study aimed to define the role of energy metabolism as a target for the leishmanicidal mechanisms of CBP. Methodology to assess CBP leishmanicidal mechanism variation of intracellular ATP levels using living Leishmania transfected with a cytoplasmic luciferase. Induction of plasma membrane permeability by assessing depolarization with DiSBAC(2)3 and entrance of the vital dye SYTOX® Green. Mitochondrial depolarization by rhodamine 123 accumulation. Mapping target site within the respiratory chain by oxygen consumption rate. Reactive oxygen species (ROS) production using MitoSOX. Morphological changes by transmission electron microscopy. CBP caused on L. infantum promastigotes a decrease of intracellular ATP levels, with irreversible depolarization of plasma membrane, the collapse of the mitochondrial electrochemical potential, mild uncoupling of the respiratory chain, and ROS production, with ensuing intracellular Ca²⁺ imbalance and DNA fragmentation. Electron microscopy supported autophagic features but not a massive plasma membrane disruption. The severe and irreversible mitochondrial damage induced by CBP endorsed the bioenergetics metabolism as a relevant target within the lethal programme induced by CBP in Leishmania. This, together with the mild-side effects of this oral drug, endorses CBP as an appealing novel candidate as a leishmanicidal drug under a drug repurposing strategy.

P-type transport ATPases in Leishmania and Trypanosoma

P-type ATPases are critical to the maintenance and regulation of cellular ion homeostasis and membrane lipid asymmetry due to their ability to move ions and phospholipids against a concentration gradient by utilizing the energy of ATP hydrolysis. P-type ATPases are particularly relevant in human pathogenic trypanosomatids which are exposed to abrupt and dramatic changes in their external environment during their life cycles. This review describes the complete inventory of ion-motive, P-type ATPase genes in the human pathogenic Trypanosomatidae; eight Leishmania species (L. aethiopica, L. braziliensis, L. donovani, L. infantum, L. major, L. mexicana, L. panamensis, L. tropica), Trypanosoma cruzi and three Trypanosoma brucei subspecies (Trypanosoma brucei brucei TREU927, Trypanosoma brucei Lister strain 427, Trypanosoma brucei gambiense DAL972). The P-type ATPase complement in these trypanosomatids includes the P_1B (metal pumps), P_2A (SERCA, sarcoplasmic-endoplasmic reticulum calcium ATPases), P_2B (PMCA, plasma membrane calcium ATPases), P_2D (Na⁺ pumps), P_3A (H⁺ pumps), P₄ (aminophospholipid translocators), and P_5B (no assigned specificity) subfamilies. These subfamilies represent the P-type ATPase transport functions necessary for survival in the Trypanosomatidae as P-type ATPases for each of these seven subfamilies are found in all Leishmania and Trypanosoma species included in this analysis. These P-type ATPase subfamilies are correlated with current molecular and biochemical knowledge of their function in trypanosomatid growth, adaptation, infectivity, and survival.

The Uptake and Metabolism of Amino Acids, and Their Unique Role in the Biology of Pathogenic Trypanosomatids

Trypanosoma brucei, as well as Trypanosoma cruzi and more than 20 species of the genus Leishmania, form a group of flagellated protists that threaten human health. These organisms are transmitted by insects that, together with mammals, are their natural hosts. This implies that during their life cycles each of them faces environments with different physical, chemical, biochemical, and biological characteristics. In this work we review how amino acids are obtained from such environments, how they are metabolized, and how they and some of their intermediate metabolites are used as a survival toolbox to cope with the different conditions in which these parasites should establish the infections in the insects and mammalian hosts.

Interplay between a cytosolic and a cell surface carbonic anhydrase in pH homeostasis and acid tolerance of Leishmania

Leishmania parasites have evolved to endure the acidic phagolysosomal environment within host macrophages. How Leishmania cells maintain near-neutral intracellular pH and proliferate in such a proton-rich mileu remains poorly understood. We report here that, in order to thrive in acidic conditions, Leishmania major relies on a cytosolic and a cell surface carbonic anhydrase, LmCA1 and LmCA2, respectively. Upon exposure to acidic medium, the intracellular pH of the LmCA1^+/-, LmCA2^+/- and LmCA1^+/-:LmCA2^+/- mutant strains dropped by varying extents that led to cell cycle delay, growth retardation and morphological abnormalities. Intracellular acidosis and growth defects of the mutant strains could be reverted by genetic complementation or supplementation with bicarbonate. When J774A.1 macrophages were infected with the mutant strains, they exhibited much lower intracellular parasite burdens than their wild-type counterparts. However, these differences in intracellular parasite burden between the wild-type and mutant strains were abrogated if, before infection, the macrophages were treated with chloroquine to alkalize their phagolysosomes. Taken together, our results demonstrate that haploinsufficiency of LmCA1 and/or LmCA2 renders the parasite acid-susceptible, thereby unravelling a carbonic anhydrase-mediated pH homeostatic circuit in Leishmania cells.

Lysosomal degradation of Leishmania hexose and inositol transporters is regulated in a stage-, nutrient- and ubiquitin-dependent manner

Leishmania parasites experience variable nutrient levels as they cycle between the extracellular promastigote stage in the sandfly vector and the obligate intracellular amastigote stage in the mammalian host. Here we show that the surface expression of three Leishmania mexicana hexose and myo-inositol transporters is regulated in both a stage-specific and nutrient-dependent manner. GFP-chimeras of functionally active hexose transporters, LmGT2 and LmGT3, and the myo-inositol transporter, MIT, were primarily expressed in the cell body plasma membrane in rapidly dividing promastigote stages. However MIT-GFP was mostly rerouted to the multivesicular tubule (MVT)-lysosome when promastigotes reached stationary phase growth and all three nutrient transporters were targeted to the amastigote lysosome following transformation to in vitro differentiated or in vivo imaged amastigote stages. This stage-specific decrease in surface expression of GFP-tagged transporters correlated with decreased hexose or myo-inositol uptake in stationary phase promastigotes and amastigotes. The MVT-lysosme targeting of the MIT-GFP protein was reversed when promastigotes were deprived of myo-inositol, indicating that nutrient signals can override stage-specific changes in transporter distribution. The surface expression of the hexose and myo-inositol transporters was not regulated by interactions with the subpellicular cytoskeleton, as both classes of transporters associated with detergent-resistant membranes. LmGT3-GFP and MIT-GFP proteins C-terminally modified with mono-ubiquitin were constitutively transported to the MVT-lysosome, suggesting that ubiquitination may play a key role in regulating the subcellular distribution of these transporters and parasite adaptation to different nutrient conditions.

Origins of amino acid transporter loci in trypanosomatid parasites

Background

Large amino acid transporter gene families were identified from the genome sequences of three parasitic protists, Trypanosoma brucei, Trypanosoma cruzi and Leishmania major. These genes encode molecular sensors of the external host environment for trypanosomatid cells and are crucial to modulation of gene expression as the parasite passes through different life stages. This study provides a comprehensive phylogenetic account of the origins of these genes, redefining each locus according to a positional criterion, through the integration of phyletic identity with comparative gene order information.

Results

Each locus was individually specified by its surrounding gene order and associated with homologs showing the same position ('homoeologs') in other species, where available. Bayesian and maximum likelihood phylogenies were in general agreement on systematic relationships and confirmed several 'orthology sets' of genes retained since divergence from the common ancestor. Reconciliation analysis quantified the scale of duplication and gene loss, as well as identifying further apparent orthology sets, which lacked conservation of genomic position. These instances suggested substantial genomic restructuring or transposition. Other analyses identified clear instances of evolutionary rate changes post-duplication, the effects of concerted evolution within tandem gene arrays and gene conversion events between syntenic loci.

Conclusion

Despite their importance to cell function and parasite development, the repertoires of AAT loci in trypanosomatid parasites are relatively fluid in both complement and gene dosage. Some loci are ubiquitous and, after an ancient origin through transposition, originated through descent from the ancestral trypanosomatid. However, reconciliation analysis demonstrated that unilateral expansions of gene number through tandem gene duplication, transposition of gene duplicates to otherwise well conserved genomic positions, and differential patterns of gene loss have produced largely customised and idiosyncratic AAT repertoires in all three species. Not least in T. brucei, which seems to have retained fewer ancestral loci and has acquired novel loci through a complex mix of tandem and transpositive duplication.

A COOH-terminal domain regulates the activity of Leishmania proton pumps LDH1A and LDH1B

Leishmania donovani requires actively transporting proton efflux pumps to survive the acidic environment of macrophage phagolysosomal vacuoles and to maintain an electrogenic H(+) gradient for nutrient uptake. The L. donovani genome contains a differentially expressed pair of genes, LDH1A and LDH1B, with homology to yeast H(+)-ATPases that are 98% identical in sequence with amino acid differences concentrated at the COOH-terminus (15 of last 37 differ), a region implicated in regulation of yeast and plant proton pumps. Functional complementation of a Saccharomyces cerevisiae strain deficient in endogenous H(+)-ATPase activity, support of yeast growth at low pH, and ability to acidify media demonstrate that LDH1A and LDH1B encode proton pumps. LDH1A and LDH1B encode a COOH-terminal autoinhibitory domain as COOH-truncated peptides support increased rates of growth in yeast, enhanced media acidification, increased enzyme activity (V(max)) and decreased K(m). This regulatory domain mediates differing function properties; LDH1A, but not LDH1B, supports yeast growth at pH 3 and LDH1A shows a greater ability to acidify media. Deletion of the last eight amino acids from LDH1B permits growth at pH 3 and increases media acidification, swapping of the COOH-tails between LDH1A and LDH1B results in LDH1A (with LDH1B tail) unable to support yeast growth at pH 3 and LDH1B (with LDH1A tail) now able to support growth at pH 3. Replacement of the COOH-terminal eight amino acids of LDH1B with those from LDH1A also confers the ability to support growth at pH 3. The complementation system for the Leishmania proton pumps in yeast described here provides a means to dissect the functional properties of the two isoforms, a convenient supply of protein for structural analysis and a model amenable to screening proton pump inhibitors for potential anti-leishmanial therapeutics.

Glucose and proline transport in kinetoplastids

The parasitic protozoa belonging to the kinetoplastids can use both sugars and amino acids as carbon and energy sources. In this review, Benno ter Kuile discusses nutrient acquisition and utilization and how the metabolic strategies reflect the environment encountered in host and vector. Recent genetic and physiological evidence suggests that facilitated diffusion may be the primary uptake mechanism for glucose, and possibly for proline as well, even though there is biochemical and genetic evidence suggesting that active transport occurs, if not across the plasma membrane, then across the membranes of organelles. Trypanosoma brucei seems to have a metabolic strategy that strives for maximum energy efficiency, making no storage materials and thereby limiting the control over its internal conditions. On the other hand, Leishmania donovani does create a storage buffer, entrapping glucose in the cell. In this manner, it maintains constant internal conditions at the expense of energy, enabling it to survive more adverse conditions in the macrophage and in its vector.

Characterisation of a developmentally regulated amino acid transporter gene from Leishmania amazonensis

The metabolism of protozoan parasites of the Leishmania genus is strongly based on amino acid consumption, but little is known about amino acid uptake in these organisms. In the present work, we identified a Leishmania amazonensis gene (La-PAT1) encoding a putative amino acid transporter that belongs to the amino acid/auxin permease family, a group of H(+)/amino acid symporters. This single copy gene is upregulated in amastigotes, the life cycle stage found in the mammalian host. La-PAT1 putative orthologous sequences were identified in Leishmania infantum, Leishmania donovani, Leishmania major and Trypanosoma.

Novel motifs in amino acid permease genes from Leishmania

Eight amino acid permease genes from the protozoan parasite Leishmania donovani (AAPLDs) were cloned, sequenced, and shown to be expressed in promastigotes. Seven of these belong to the amino acid transporter-1 and one to the amino acid polyamino-choline superfamilies. Using these sequences as well as known and characterized amino acid permease genes from all kingdoms, a training set was established and used to search for motifs, using the MEME motif discovery tool. This study revealed two motifs that are specific to the genus Leishmania, four to the family trypanosomatidae, and a single motif that is common between trypanosomatidae and mammalian systems A1 and N. Interestingly, most of these motifs are clustered in two regions of 50-60 amino acids. Blast search analyses indicated a close relationship between the L. donovani and Trypanosoma brucei amino acid permeases. The results of this work describe the cloning of the first amino acid permease genes in parasitic protozoa and contribute to the understanding of amino acid permease evolution in these organisms. Furthermore, the identification of genus-specific motifs in these proteins might be useful to better understand parasite physiology within its hosts.

Choline transport in Leishmania major promastigotes and its inhibition by choline and phosphocholine analogs

Phosphatidylcholine is the most abundant phospholipid in the membranes of the human parasite Leishmania. The metabolic pathways leading to its biosynthesis are likely to play a critical role in parasite development and survival and may offer a good target for antileishmanial chemotherapy. Phosphatidylcholine synthesis via the CDP-choline pathway requires transport of the choline precursor from the host. Here, we report the first characterization of choline transport in this parasite, which is carrier-mediated and exhibits Michaelis-Menten kinetics with an apparent K(m) value of 2.5 microM for choline. This process is Na(+)-independent and requires an intact proton gradient to be fully functional. Choline transport into Leishmania is highly specific for choline and is inhibited by the choline carrier inhibitor hemicholinium-3, the channel blocker quinacrine, the antimalarial aminoquinolines quinine and quinidine, the antileishmanial phosphocholine analogs, miltefosine and edelfosine, and by choline analogs, most of which have antimalarial activities. Most importantly, choline analogs kill the promastigote form of the parasite in vitro in the low micromolar range. These results set the stage for the use of choline analogs in antileishmanial chemotherapy and shed new lights on the mechanism of action of the leishmanicidal phosphocholine analogs.

Antileishmanial activity of the antiulcer agent omeprazole

The benzimidazole compound omeprazole, used widely for the treatment of peptic ulcer disease, inhibits the growth of Leishmania donovani, the causative agent of visceral leishmaniasis. Promastigotes cultured at acidic pH and amastigotes within infected macrophages are reduced 90% or more with 150 microM omeprazole. Antiparasitic action of the drug is due to its inhibition of the P-type K(+),H(+)-ATPase on the surface membrane. This enzyme is important for pH homeostasis and the maintenance of proton motive force across the membrane in Leishmania. The drug is effective only at acidic pH, a condition that mimics the in vivo environment within the phagolysosomal vesicles where the amastigote form of the parasite resides. Omeprazole deserves consideration as an alternative to currently available chemotherapeutics, which have severe toxic side effects.

Putrescine and spermidine transport in Leishmania

The transport of putrescine and spermidine into Leishmnania donovani promastigotes and Leishmania mexicana promastigotes and amastigotes has been characterised. Polyamine transport was shown to be saturable and temperature-sensitive for both developmental stages of Leishmania. Transport was pH-dependent with pH optima of 7.4 and 5.5 for promastigotes and amastigotes, respectively. The uptake process was independent of extracellular Na+, but inhibited by protonophores and H+-ATPase inhibitors. Kinetic analyses of polyamine transport showed that Km and Vmax differed between promastigotes of the two species and between promastigotes and amastigotes of L. mexicana. Inhibition data suggest that putrescine and spermidine use different transporters. The aromatic diamidine pentamidine, the drug of choice for treatment of antimonial-resistant cases of leishmaniasis, inhibited both putrescine and spermidine transport non-competitively.

Developmental regulation of proline transport in Leishmania donovani

Leishmania donovani are the causative agents of kala azar in humans. These organisms cycle between the proline-rich environment of the sand fly vector (extracellular promastigotes) and the sugar-rich condition in the mammalian host (intracellular amastigotes). Parasites have adapted to these extreme changes in proline concentrations: promastigotes utilize proline as a carbon source, whereas amastigotes utilize sugars and fatty acids. Previous studies have suggested that promastigotes and amastigotes express distinct proline transporters. However, the information available on these transporters is limited. In this work, proline transport was investigated in axenic L. donovani cultures. Three transport systems were identified: cation-dependent and -independent proline transporters in promastigotes (systems A and B, respectively) and a single cation-independent transporter in amastigotes (system C). Systems A and C have broad specificity to almost all amino acids and obtain optimum activity at acidic pH ranges (pH 6 and 5, respectively). System B is more specific to proline, as it is inhibited by only five amino acids. Temperature response analyses indicated that the transporters of both promastigotes and amastigotes perform best at 37 degrees C. The activity of system A during parasite differentiation was assessed. The transport activity of system A disappeared 3 days after promastigotes were induced to differentiate into amastigotes. In these cells, elevated temperature and acidic pH each suppressed the activity of system A. When amastigotes were induced to differentiate back into promastigotes, system A resumed its activity 24 h after differentiation was initiated. In conclusion, L. donovani obtain proline transport systems that are stage specific, regulated by both pH and temperature. This paper constitutes the first investigation of amino acid transport in axenic L. donovani.

Leishmania sp.: growth and survival are impaired by ion channel blockers

In the present work we examined the effect of ion transport blockers on the growth and viability of Leishmania sp. and on the infection of macrophages by the parasite. 4-aminopyridine and glibenclamide block voltage-dependent and K+ ATP channels, respectively; amiloride is used to detect Na+ channels and Na+/H+ antiporters; and anthracene-9-carboxylic acid affects chloride channels. The EC50 for promastigote cultures of three strains of the Leishmania subgenus, namely, Leishmania (Leishmania) NR, Leishmania (Leishmania) amazonensis LTB0016, and Leishmania (Leishmania) major, at their stationary phase of growth, were, respectively, 39, 46, and 464 microM for 4-aminopyridine; 7, 0.8, and 10 microM for glibenclamide and 66, 170, and 10 microM for anthracene-9-carboxylic acid. The amiloride EC50 for NR was 264 microM and 10 microM for L. (L.) major, but was never reached for LTB0016. Higher concentrations of the drugs impaired the exponential growth of Leishmania promastigotes. These results suggest the susceptibility of Leishmania sp. to blockers associated with K+ and Cl- and to Na+ or Na+/H+ transport systems. Blockade of such systems might have impaired the survival of the parasites as promastigotes. In addition, it affected the persistence of parasites in host cells. Although the infection of the macrophage cell line J774 and peritoneal-exudate macrophages was not significantly decreased by concentrations of the drugs around the promastigotes' EC50, the survival of intracellular parasites decreased significantly in the presence of these drugs without affecting the viability of the macrophages. Some blockers consistently gave small EC50 and significantly decreased the infection process as well as the survival of intracellular parasites. Thus, elucidation of their mechanism of action in Leishmania is relevant, since they could represent a potential subject for the development of leishmanicidal drugs.

Leishmania spp.: mechanisms of toxicity of nitrogen oxidation products

Intracellular killing of Leishmania parasites within activated murine macrophages is thought to result from the toxic activities of nitrogen oxidation products (referred to as NO) released by the activated cells. In order to determine possible mechanisms of NO toxicity for these microorganisms, promastigotes of Leishmania major and Leishmania enriettii were exposed to NO generated chemically from acidified nitrite, S-nitrosocysteine, diethylamine NONOate, or nitroprusside. Treatment with these agents led to loss of viability (as determined from decreased motility and inhibition of [3H]TdR uptake upon reincubation in NO-free medium) with kinetics characteristic for each compound L. major was less sensitive to these effects than L. enriettii, and amastigotes displayed the same sensitivity as promastigotes of the same species. The early effects of NO toxicity could be detected within minutes of exposure to the NO donors; they included decreased respiration rate and inhibition of glucose, proline, and adenine incorporation. Inhibition of the activities of glyceraldehyde 3-phosphate dehydrogenase and of aconitase were also evidenced. In order to determine whether these phenomena reflected the mechanisms of toxicity of bona fide NO generated by macrophages, promastigotes were exposed to IFN-gamma + LPS-activated macrophages across permeable membranes. This resulted in marked inhibition of proline and adenine uptake in the parasites, which was restored, however, to control levels when macrophages were activated in the presence of the nitric oxide synthase inhibitor NGMMA. These results indicate that several cellular targets may be subject to NO toxicity in Leishmania parasites, including enzymes of glycolysis and respiratory metabolism as well as trans-membrane transport systems.

Entry and survival of Leishmania amazonensis amastigotes within phagolysosome-like vacuoles that shelter Coxiella burnetii in Chinese hamster ovary cells

Coxiella burnetii, a rickettsia, and Leishmania amazonensis, a protozoan flagellate, lodge in their host cells within large phagolysosome-like vacuoles. In the present study, C. burnetii-infected Vero or CHO cells were superinfected with L. amazonensis amastigotes to determine if these parasites can home to and survive within heterologous vacuoles. Six hours after superinfection, Leishmania amastigotes were located almost exclusively within large Coxiella-containing vacuoles. Thereafter, the numbers of parasites in the vacuoles increased at the same rate as those in cells infected with L. amazonensis alone. Furthermore, in cultures shifted to 25 degrees C, some of the amastigotes transformed into promastigote-like forms that moved their flagella within the adoptive vacuoles. Thus, L. amazonensis amastigotes not only entered Coxiella vacuoles, most likely by fusion of donor and recipient vacuoles, but temporarily survived, differentiated, and replicated therein. This appears to be the first account of the temporary cohabitation of two living pathogens within the same vacuole in a mammalian cell.

Biochemical and immunochemical characterization of a P-type ATPase from Leishmania donovani promastigote plasma membrane

An ATPase on the plasma membrane of Leishmania donovani has been characterized. An antiserum, generated against ATPase active bands from native gels, was specific for a 105 kDa protein in promastigotes. However, in plasma membrane preparations a 70 kDa protein is also recognized, suggesting proteolysis of the intact 105 kDa protein or the presence of a second similar ATPase. [gamma-32P]ATP phosphorylates two proteins (105 kDa and 70 kDa) in promastigotes and plasma membranes. Both proteins form a transient phosphorylated intermediate, characteristic of a P-type ATPase. Immunostaining of permeabilized parasites shows diffuse staining of the surface of promastigotes and amastigotes, which is consistent with a plasma membrane protein. The antiserum immunoprecipitates a 70 kDa [14C]DCCD binding protein from whole cells and plasma membranes of promastigotes. Furthermore, the antiserum immunoprecipitates a 105 kDa and 70 kDa protein which can be subsequently phosphorylated. These results indicate the presence of a 105 kDa P-type ATPase on the L. donovani plasma membrane which is similar to the mammalian and fungal cation pumps.

Surface antigens of Leishmania mexicana amastigotes: characterization of glycoinositol phospholipids and a macrophage-derived glycosphingolipid

Amastigotes of the protozoan parasite Leishmania proliferate in phagolysosomes of macrophages. They abundantly express glycoinositol phospholipids (GIPLs), which are considered necessary for parasite survival by providing a shield at the surface against lysosomal hydrolases and by serving as receptors for the interaction with host cells. The structures of four GIPLs of L. mexicana amastigotes were characterized by a combination of gas-liquid chromatography-mass spectrometry, methylation linkage analysis and enzymatic treatments. They contain the glycan structures Man alpha 1–3Man alpha 1–4GlcN (iM2), Man alpha 1–6(Man alpha 1–3)Man alpha 1–4GlcN (iM3), Man alpha 1–2Man alpha 1–6(Man alpha 1–3)-Man alpha 1–4GlcN (iM4) and (NH2-CH2CH2-PO4)Man alpha 1–6(Man alpha 1–3)Man alpha 1–4GlcN (EPiM3), which are linked to alkylacyl-phosphatidylinositol. The predominant amastigote GIPL, EPiM3 (approximately 2 × 10(7) molecules/cell), is located at the parasite cell surface, in the flagellar pocket and in lysosomal membranes, but not on host cell structures as shown by immunofluorescence and immunoelectron microscopy. In addition, amastigotes in infected Balb/c mice contain a glycolipid with similar distribution as EPiM3, which has the same characteristics as the Forssman antigen of mammalian cells. In contrast to EPiM3, there is strong evidence that this glycosphingolipid is not synthesized by amastigotes but by macrophages in the lesion. This suggests a mechanism of lipid transfer from the macrophage to the parasite.

Developmental gene expression in Leishmania donovani: differential cloning and analysis of an amastigote-stage-specific gene

Leishmania protozoans are the causative agents of leishmaniasis, a major parasitic disease in humans. During their life cycle, Leishmania protozoans exist as flagellated promastigotes in the sand fly vector and as nonmotile amastigotes in the mammalian hosts. The promastigote-to-amastigote transformation occurs in the phagolysosomal compartment of the macrophage cell and is a critical step for the establishment of the infection. To study this cytodifferentiation process, we differentially screened an amastigote cDNA library with life cycle stage-specific cDNA probes and isolated seven cDNAs representing amastigote-specific transcripts. Five of these were closely related (A2 series) and recognized, by Northern (RNA) blot analyses, a 3.5-kb transcript in amastigotes and in amastigote-infected macrophages. Expression of the amastigote-specific A2 gene was induced in promastigotes when they were transferred from culture medium at 26 degrees C and pH 7.4 to medium at 37 degrees C and pH 4.5, conditions which mimic the macrophage phagolysosomal environment. A2 genes are clustered in tandem arrays, and a 6-kb fragment corresponding to a unit of the cluster was cloned and partially sequenced. An open reading frame found within the A2-transcribed region potentially encoded a 22-kDa protein containing repetitive sequences. The recombinant A2 protein produced in Escherichia coli cells was specifically recognized by immune serum from a patient with visceral leishmaniasis. The A2 protein repetitive element has strong homology with an S antigen of Plasmodium falciparum, the protozoan parasite responsible for malaria. Both the A2 protein of Leishmania donovani and the S antigen of P. falciparum are stage specific and developmentally expressed in mammalian hosts.

Developmental gene expression in Leishmania donovani: differential cloning and analysis of an amastigote-stage-specific gene

Leishmania protozoans are the causative agents of leishmaniasis, a major parasitic disease in humans. During their life cycle, Leishmania protozoans exist as flagellated promastigotes in the sand fly vector and as nonmotile amastigotes in the mammalian hosts. The promastigote-to-amastigote transformation occurs in the phagolysosomal compartment of the macrophage cell and is a critical step for the establishment of the infection. To study this cytodifferentiation process, we differentially screened an amastigote cDNA library with life cycle stage-specific cDNA probes and isolated seven cDNAs representing amastigote-specific transcripts. Five of these were closely related (A2 series) and recognized, by Northern (RNA) blot analyses, a 3.5-kb transcript in amastigotes and in amastigote-infected macrophages. Expression of the amastigote-specific A2 gene was induced in promastigotes when they were transferred from culture medium at 26 degrees C and pH 7.4 to medium at 37 degrees C and pH 4.5, conditions which mimic the macrophage phagolysosomal environment. A2 genes are clustered in tandem arrays, and a 6-kb fragment corresponding to a unit of the cluster was cloned and partially sequenced. An open reading frame found within the A2-transcribed region potentially encoded a 22-kDa protein containing repetitive sequences. The recombinant A2 protein produced in Escherichia coli cells was specifically recognized by immune serum from a patient with visceral leishmaniasis. The A2 protein repetitive element has strong homology with an S antigen of Plasmodium falciparum, the protozoan parasite responsible for malaria. Both the A2 protein of Leishmania donovani and the S antigen of P. falciparum are stage specific and developmentally expressed in mammalian hosts.

Regulation of L-proline transport in Leishmania donovani by extracellular pH

We have previously shown that Leishmania donovani promastigotes adapted to long-term culture at acidic pH can serve as a model to study parasite development in a lysosomal-like environment. In this study we investigated the effect of growth pH on L. donovani L-proline transport systems. Reducing the pH of the growth medium causes an up to 7-fold decrease in the extent of L-proline transport. Transport resumes after switching the culture from pH 4.5 to pH 7 for 48 h by a protein synthesis-dependent process. The pH optimum for transport changes from 7.5 in promastigotes grown at pH 7 to 5.5 in cells grown at pH 4.5. In addition, kinetic analysis of L-proline transport showed that Vmax in pH 4.5-grown L. donovani promastigotes is one-tenth that of cells grown at pH 7 (4.5 and 44.7 nmol min-1 (10(8) cells)-1, respectively). The apparent Km for L-proline in pH 4.5 promastigotes is one-half of the Km in pH 7 cells (0.30 and 0.65 mM, respectively). In contrast to L-proline transport, D-glucose transport demonstrates a growth pH-independent activity: Km and Vmax as well as optimum pH of transport are similar in promastigotes grown at either pH 7 or pH 4.5. Taken together, the results indicate that in L. donovani, expression and activity of L-proline transport is regulated by culture pH. The pH-dependent expression of L-proline transporters may be of physiological significance during the promastigote-amastigote transition.

Active transport of L-proline in the protozoan parasite Trypanosoma brucei brucei

The characteristics of L-proline transport in the procyclic form of Trypanosoma brucei were studied by using L-[14C]proline and a quick separation technique by centrifugation through an oil mixture. L-Proline uptake displayed typical Michaelis-Menten kinetics, with a Km of 19 microM and a maximum transport velocity of 17 nmol/min per 10(8) cells at 27 degrees C. The maximum concentration gradient factor obtained after 1 min of incubation was 270-fold in 0.02 mM proline. Cells permeabilized with 80 microM digitonin were still able to accumulate 14C label, but to a lower extent. The temperature-dependence of proline uptake gave an apparent activation energy of 74.9 kJ.mol-1. In competition studies with a 10-fold excess of structural analogues, L-alanine, L-cysteine and L-azetidine-2-carboxylate were found to inhibit L-proline uptake. Variation of pH or addition of the protonophore carbonyl cyanide m-chlorophenylhydrazone ('CCCP') did not affect proline transport, showing that it is not driven by a protonmotive force. The absence of Na+, with or without monensin, did not affect proline transport. The absence of K+ and the addition of the Na+,K(+)-ATPase inhibitor ouabain had no significant effect on proline uptake activity. The thiol-modifying reagent iodoacetate (10 mM) decreased proline uptake by half. KCN (1 mM) inhibited proline uptake to a lesser extent, and the degree of inhibition was proportional to the intracellular ATP concentration. Preliminary experiments on proline transport in plasma-membrane vesicles of the cells, using a filtration technique, showed an uptake of proline (0.67 nmol/mg of protein) by the vesicles, but only in the presence of intravesicular ATP. The results thus obtained suggest that the proline carrier system in T. brucei is ATP-driven and independent of Na+, K+ or H+ co-transport.

Developmental life cycle of Leishmania--cultivation and characterization of cultured extracellular amastigotes

The biochemistry and immunology of Leishmania promastigotes has been extensively studied; this is due primarily to the facility with which this stage, in contrast to the amastigotes stage, can be maintained in axenic culture. Several attempts to axenically culture lines of Leishmania amastigotes have been reported in the literature. This paper summarizes methods of adaptation (low pH, elevated temperature and culture medium) and characterization of several axenic lines of Leishmania amastigotes. Based on morphological, biological, immunological and biochemical evidence, these organisms appear to resemble amastigotes from infected macrophages or tissue. The axenically cultured amastigotes appear to be distinct from shocked (heat, serum deprivation, stressed) Leishmania promastigotes in the plethora of proteins synthesized, growth (multiplication) in culture, and developmental regulation observed. These data suggest that Leishmania organisms have a significant developmental response to certain signals (pH, temperature) mimicking their in vivo macrophage milieu. The response to other environmental parameters characteristic of the host-macrophage remain to be determined. These axenically cultured amastigotes should be of interest for further immunological, biochemical and developmental investigations of the disease-maintaining stage of this parasite.

A chemostat study on proline uptake and metabolism of Leishmania donovani

Leishmania donovani grew in the chemostat on proline as its sole carbon and energy source at a maximum growth rate of 1.39 divisions per day. The efficiency of proline metabolism decreased with increasing external proline concentration. The internal concentration of proline and its intracellular metabolites was low when proline was the growth rate limiting substrate and high when proline was available in excess. In time-course experiments proline uptake leveled off after 30 min, independent of the culture conditions prior to the experiment. Proline uptake depended on the external proline concentration in a manner that is best described as the combination of an enzymatic and a diffusion component. Adaptation to different proline concentrations did not occur and no evidence was found that proline is actively transported by L. donovani.

The plasma membrane electrical gradient (membrane potential) in Leishmania donovani promastigotes and amastigotes

The equilibrium distribution of tetraphenylphosphonium bromide was used to measure the membrane potential in Leishmania donovani amastigotes and promastigotes and to investigate mechanisms underlying the maintenance of membrane potential. At pH 7.0, membrane potential ranges between -90 and -113 mV. Increasing the external concentrations of hydrogen or potassium ions decreased membrane potential as did treatments with carbonylcyanide chlorophenylhydrazone or valinomycin. These observations are consistent with a membrane potential set by hydrogen and potassium ion diffusion gradients. Anaerobiosis lowered membrane potential, suggesting the involvement of ATPase(s) in maintaining membrane potential. Membrane potential was insensitive to treatment with ouabain, demonstrating the absence of a Na+/K(+)-ATPase. Treatment with dicyclohexylcarbodiimide caused a temporary hyperpolarization of the membrane suggesting the participation of a proton ATPase in the maintenance of membrane potential. Determination of the membrane potential makes it possible to quantitate the total proton motive force which is the force for active transport across the parasite membrane.