Dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin (DC-SIGN, CD209), a C-type surface lectin in human DCs, is a receptor for Leishmania amastigotes

Dendritic cells (DCs) play a critical role in the initiation of the immunological response against Leishmania parasites. However, the receptors involved in amastigote-dendritic cell interaction are unknown, especially in absence of opsonizing antibodies. We have studied the interaction of Leishmania pifanoi axenic amastigotes with the C-type lectin DC-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin (DC-SIGN, CD209), a receptor for ICAM-2, ICAM-3, human immunodeficiency virus gp120, and Ebola virus. L. pifanoi amastigotes interact with immature human dendritic cells and CD209-transfected K562 cells in a time- and dose-dependent manner. Leishmania amastigote binding to human dendritic cells and DC-SIGN-transfected cells is inhibited by a function-blocking DC-SIGN-specific monoclonal antibody. More importantly, this monoclonal antibody dramatically reduces internalization of Leishmania amastigotes by immature human DCs. These results constitute the first description of a nonviral pathogen ligand for DC-SIGN and provide evidence for a relevant role of DC-SIGN in Leishmania amastigote uptake by dendritic cells. Our finding has important implications for Leishmania host-cell interaction and the immunoregulation of cutaneous leishmaniasis.

ICAM-L gene is conserved only in Leishmania species in the family of kinetoplastida

Attachment of Leishmania to its host macrophage is a ligand-receptor mediated event. Two abundant surface molecules, a complex carbohydrate lipophosglycan (LPG) and a glycosylphosphotidylinositol (GPI) anchored GP63 protease have been shown to independently play a critical role in this event. LPG and GP63 are widely conserved among members of the kinetoplastida family. An intercellular adhesive molecule from Leishmania (ICAM-L) that acts as a ligand of Leishmania for the macrophage has recently been described by our lab. When assessed by molecular determinations including Southern, Northern and sequencing, ICAM-L appears to be conserve only among Leishmania species but not in other closely related members of the kinetoplastida. However using a polyclonal antibody specific to ICAM-L, the molecule appears to be conserved among all the kinetoplastids. The detention by antibody of ICAM-L molecules among other kinetoplastids may due to conservation in these parasites of certain immunoreactive epitopes at the amino acid level, while the nucleotide sequence is divergent enough to preclude detection by methods employed.

A new type of high affinity folic acid transporter in the protozoan parasite Leishmania and deletion of its gene in methotrexate-resistant cells

The protozoan parasite Leishmania is a folate auxotroph and thus depends on the uptake of folate from the environment to meet its folate requirement. We show here that Leishmania contains several putative pteridine transporter genes. Some of these genes are deleted in methotrexate-resistant Leishmania cells where there is no measurable uptake of methotrexate. Transport studies suggest that Leishmania has more than one active folate transporter, and one of these, named FT5, corresponds to a very high affinity folate transporter (K(m) 84 nm). The uptake of both folate and methotrexate was impaired in an FT5 null mutant at low substrate concentrations (50 nm), although transport properties at higher concentrations (1000 nm) were not statistically different between wild-type and the FT5 null mutant. Modulation of the expression of FT5 also changes the susceptibility of Leishmania cells to methotrexate. These results have permitted the characterization of a novel class of folate transporters and suggest that the parasite Leishmania has several gene products possibly transporting folates and related molecules under varying conditions.

Multidrug resistance phenotype mediated by the P-glycoprotein-like transporter in Leishmania: a search for reversal agents

Abstract: Protozoan parasites are responsible for important diseases that threaten the lives of nearly one-quarter of the human population world-wide. Among them, leishmaniasis has become the second cause of death, mainly due to the emergence of parasite resistance to conventional drugs. P-glycoprotein (Pgp)-like transporters overexpression is a very efficient mechanism to reduce the intracellular accumulation of many drugs in cancer cells and parasitic protozoans including Plasmodium and Leishmania, thus conferring a multidrug resistance (MDR) phenotype. Therefore, there is a great clinical interest in developing inhibitors of these transporters to overcome such a resistance. Pgps are active pumps belonging to the ATP-binding cassette (ABC) superfamily of proteins, and consist of two homologous halves, each containing a transmembrane domain (TMD) involved in drug efflux, and a cytosolic nucleotide-binding domain (NBD) responsible for ATP binding and hydrolysis. Most conventional cancer MDR modulators interact with the drug-binding sites on the TMDs of Pgps, but they are also usually transported and the required concentrations for a permanent inhibition produce subsequent side-effects that hamper their clinical use. Besides, they only poorly modulate the resistance in protozoan parasites. We review here a rational strategy developed to overcome the MDR phenotype in Leishmania, consisting in: i) the selection of an MDR Leishmania tropica line that overexpresses a Pgp-like transporter; ii) the use of their cytosolic NBDs as new pharmacological targets; iii) the search of new natural compounds that revert the MDR phenotype in Leishmania by binding to the TMDs; iv) the combination of subdoses of the above selected modulators directed to both targets in the transporter, NBDs and TMDs, to accumulate their reversal effects while diminishing their toxicity. In this way, we have reverted the MDR phenotype in Leishmania, including the resistance to the most promising new antileishmania agents, the alkyl-lysophospholipids. This approach might be extrapolated to be used in other eukaryotic cells.

Promising therapeutic targets for antileishmanial drugs

Current treatments for the parasitic disease leishmaniasis are unsatisfactory due to their route of administration, toxicity and expense. Resistance is also developing to first-line antimonial drugs. Fortunately, a handful of antileishmanial agents, such as the orally available compound miltefosine, are currently in clinical trials. In addition, several promising drug targets and lead molecules are being studied with the goal of developing new antileishmanial agents. Drug candidates have been identified through the continued investigation of parasite sterol metabolism and parasite proteases. New antileishmanial molecules have also been discovered through the study of novel targets and pathways, such as the bisphosphonate inhibitors of isoprenoid biosynthesis. This review presents a synopsis of the drug targets and lead compounds that have been investigated over the last few years against leishmaniasis, gives a perspective on the chemotherapeutic potential of each and discusses some of the obstacles to antileishmanial drug development.

The effect of location and direction of an episomal gene on the restoration of a phenotype by functional complementation in Leishmania

The feasibility of genetic manipulation in trypanosomatids has allowed the introduction of molecular approaches for the investigation of gene function. The development of cosmids that carry approximately 40 kb of insert and can be easily introduced in trypanosomatids greatly increased the possibility of gene rescue by functional complementation in these parasites. Although functional complementation is widely used, some of its aspects, such as differential levels of expression along the insert clone, are not clear. We have used the DHFRTS gene as a tool to better understand the mechanisms of transcription of genes present in episomes and the results obtained via functional complementation in Leishmania. This gene was chosen not only because its inactivation in the parasite generates an easily recoverable phenotype, auxotrophy for thymidine (TdR), but also because null mutants are already available. The null mutant available contained two resistance markers (neomycin phosphotransferase-NEO and hygromycin phosphotransferase-HYG), and the loss of heterozygosity (LOH) was induced to recover clones sensitive to hygromycin B, which were necessary for the rescue of transfectants. Analyses of the Leishmania clones confirmed the loss of the HYG gene associated with unanticipated genomic rearrangements. A LOH clone was transfected with cosmids containing the DHFRTS gene in several distinct contexts in order to evaluate the levels of expression of the complementing gene. Results presented here show that the lost phenotype is rescued, irrespective to the DHFRTS location or direction of transcription indicating that functional complementation can be achieved without concern for the position of the complementing gene in a cosmid.

[Metacyclogenesis: a basic process in the biology of Leishmania]

Metacyclogenesis is a process whereby Leishmania transforms from poorly infective procyclic promastigotes into highly infective metacyclic promastigotes. In nature, metacyclogenesis occurs in the insect vector. This transformation is accompanied by an increased ability to infect and survive in the vertebrate host, where the parasite is attacked by the host's immune system. Metacyclogenesis has also been shown to occur in axenic cultures of promastigotes. Morphological changes in size and shape, and length of flagellum were first associated with differentiation in the insect gut and in different phases of growth in culture. Later, the expression of molecules such as LPG and the surface protease gp63 were associated with this process. These two molecules were observed to undergo qualitative and quantitative modifications as the promastigotes differentiated from procyclic to metacyclic forms. Using cDNA subtractive hybridization-based methods or differential amplification, previously unknown genes tightly linked to metacyclogenesis have been identified. Gene products exclusively expressed in metacyclic promastigotes included a gene B product and Mat-1--a gene associated with metacyclogenesis. Other proteins, Meta-1, SHERP and HASP, were up-regulated during the metacyclic stage. The function and stage-regulated expression of these molecules and their relationship with infectivity are now under investigation.

Comparison of the post-transcriptional regulation of the mRNAs for the surface proteins PSA (GP46) and MSP (GP63) of Leishmania chagasi

MSP (GP63) and PSA (GP46) are abundant 63- and 46-kDa glycolipid-anchored proteins on the surface of the promastigote form of most Leishmania species. MSP is a zinc metalloprotease that confers resistance to host complement-mediated lysis. PSA contains internal repeats of 24 amino acids, and its function is unknown. The steady state levels of mRNAs for both glycoproteins are regulated post-transcriptionally, resulting in about a 30-fold increase as Leishmania chagasi promastigotes grow in vitro from logarithmic phase to stationary phase. Previous studies showed the 3'-untranslated regions (3'-UTRs) of these mRNAs are essential for this post-transcriptional regulation. These two 3'-UTRs of 1.0 and 1.3 kilobases were cloned immediately downstream of a beta-galactosidase reporter gene in a plasmid, and segments were systematically deleted to examine which portions of the 3'-UTRs contribute to the post-transcriptional regulation. The 92-nucleotide segment of greatest similarity between the two 3'-UTRs was deleted without loss of regulation, but the segments flanking this similarity region have positive regulatory elements essential for the regulation. We propose that similar, but non-identical, molecular mechanisms regulate the parallel expression of these two L. chagasi mRNAs despite their lack of sequence identity. These post-transcriptional mechanisms resemble the mechanism recently suggested for the regulation of mRNAs encoding the dipeptide (EP) and pentapeptide (GPEET) repeat proteins in Trypanosoma brucei that involves interactions between positive and negative regulatory elements in the 3'-UTR.

Genetic and structural heterogeneity of proteophosphoglycans in Leishmania

Proteophosphoglycans (PPG) are a large family of extensively glycosylated proteins with some unusual and unique features. The ppg gene family is conserved in at least three Leishmania species and localises to chromosome 35. Previous studies using standard discontinuous SDS-PAGE have been incapable of resolving PPG heterogeneity with most material failing to enter the resolving gel. We have exploited a continuous electrophoretic system, which allows for the first time the separation and characterisation of a low molecular weight population of PPG polypeptides. We provide evidence of surface expressed and developmentally regulated forms. Among those, we identify for the first time the previously described membrane-bound PPG and a form of filamentous fPPG, which is altered, or absent in two of the three L. major isolates examined.

Pterin transport and metabolism in Leishmania and related trypanosomatid parasites

The folate metabolic pathway has been exploited successfully for the development of antimicrobial and antineoplasic agents. Inhibitors of this pathway, however, are not useful against Leishmania and other trypanosomatids. Work on the mechanism of methotrexate resistance in Leishmania has dramatically increased our understanding of folate and pterin metabolism in this organism. The metabolic and cellular functions of the reduced form of folates and pterins are beginning to be established and this work has led to several unexpected findings. Moreover, the currently ongoing sequencing efforts on trypanosomatid genomes are suggesting the presence of several gene products that are likely to require folates and pterins. A number of the properties of folate and pterin metabolism are unique suggesting that these pathways are valid and worthwhile targets for drug development.

Functional domains of the rDNA promoter display a differential recognition in Leishmania

A construct containing the RNA polymerase I promoter of Leishmania (L.) amazonensis, driving the expression of cloramphenicol acetyl transferase reporter gene, was better recognised by heterologous hosts species Leishmania (L.) major and Leishmania (L.) mexicana than by the homologous host L. (L.) amazonensis. The rDNA promoter domains responsible for recognition were functionally mapped. The core domain (-74 to +170) conferred a barely equal recognition on homologous or heterologous cells, slightly favouring to the later. Addition of the upstream domain (-196 to -74) repressed the expression in all cells tested. The third domain, consisting of repeated elements (upstream to -196 in L. (L.) amazonensis), enhanced by about 20 times the core activity of homologous species and by about 40 times the heterologous ones. Gel mobility shift patterns generated by the binding of core sequence of L. (L.) amazonensis to nuclear extracts of the Leishmania species suggested that the rDNA transcriptional machinery is a complex DNA-protein association particular for each species. A model is proposed to explain the mechanism and possible interactions of transcription machinery in the regulation of rDNA expression in phylogenically related organisms.

The expression system of biologically active canine interleukin-8 in Leishmania promastigotes

It has been reported that Leishmania promastigotes have ability to express foreign genes on drug selectable plasmids. To investigate further abilities of the recently described expression vector, P6.5, in the transfection of Leishmania organisms (Chen D-Q, Kolli BK, Yadava N et al. Episomal expression of specific sense and antisense mRNAs in Leishmania amazonensis: modulation of gp63 levels in promastigotes and their infection of macrophages in vitro. Infect Immun 2000;68:80--86), the constructed expression vector, which contains canine interleukin-8 (cIL-8) coding cDNA, was introduced by electroporation to promastigotes of four species of the genus Leishmania: Leishmania amazonensis, L. equatorensis, L. donovani and L. infantum. Extrachromosomal DNAs and total RNAs from the transfected promastigotes were subjected to polymerase chain reaction (PCR) and reverse transcriptase-PCR, respectively, using cIL-8 gene specific primers, and a predicted product of 330 bp was detected. Western blot analysis using a mouse monoclonal antibody raised against cIL-8 demonstrated the successful expression of cIL-8 in the transfectants and culture supernatants. Culture supernatants of the transfected L. amazonensis and L. equatorensis promastigotes showed a high chemotactic activity to both dog and mouse polymorphonuclear leukocytes. These results indicate that Leishmania promastigotes transfected with the expression vector P6.5 containing cIL-8 cDNA are capable of producing biologically active cIL-8. The Leishmania expression system using the P6.5 vector might be a useful alternative for the production of biologically active recombinant cytokines.

A microbial strategy to multiply in macrophages: the pregnant pause

Humans live in harmony with much of the microbial world, thanks to a sophisticated immune system. As the first line of defense, macrophages engulf, digest, and display foreign material, then recruit specialists to eliminate potential threats. Yet infiltrators exist: certain fungi, viruses, parasites, and bacteria thrive within sentinel macrophages. By scrutinizing the life styles of these shrewd microbes, we can deduce how macrophages routinely mount an effective immune response. The bimorphic life cycles of three pathogens have dramatic consequences for phagosome traffic. In the transmissible state, Leishmania spp., Coxiella burnetii, and Legionella pneumophila block phagosome maturation; after a pregnant pause, replicative forms emerge and thrive in lysosomes.

Unusual polypeptide synthesis in the kinetoplast-mitochondria from Leishmania tarentolae. Identification of individual de novo translation products

The de novo synthesis of cytochrome c oxidase subunits I, II (COI and COII), and apocytochrome b (Cyb) was investigated in kinetoplast-mitochondria of Leishmania. The organelles were isolated after breaking whole cells with nitrogen cavitation. Individual COI, COII, and Cyb polypeptides were identified by fractionation of the kinetoplast membranes, labeled with [(35)S]methionine and cysteine, using two-dimensional (9 versus 14% and 20 versus 11%) denaturing gel electrophoresis. The reaction did not require exogenous energy sources or amino acids. On the contrary, the presence of amino acids other than methionine somewhat inhibited the labeling reaction probably by competing with the uptake of labeled amino acids. The synthesis reaction was insensitive to 100 microg/ml chloramphenicol, gentamycin, paromomycin, lincomycin, hygromycin, and tetracycline, as well as cycloheximide. The process showed a linear increase in the amount of synthesized polypeptides during the first 2 h of incubation, followed by a slower accumulation of products for up to 4 h. The de novo synthesized polypeptides were stable for several additional hours. Their assembly into respiratory complexes, investigated using two-dimensional Blue Native/N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine-SDS gels, began early during the incubation and continued throughout the course of the synthesis. This work represents the first unequivocal identification of the polypeptide synthesis in kinetoplasts.

Ultrastructural and biochemical alterations induced by 22,26-azasterol, a delta(24(25))-sterol methyltransferase inhibitor, on promastigote and amastigote forms of Leishmania amazonensis

We report on the antiproliferative effects and the ultrastructural and biochemical alterations induced in vitro by 22,26-azasterol, a sterol Delta(24(25))-methyltransferase (24-SMT) inhibitor, on Leishmania amazonensis. When promastigotes and amastigotes were exposed to 100 nM 22,26-azasterol, complete growth arrest and cell lysis ensued after 72 (promastigotes) or 120 (amastigotes) h. Exposure of parasites to this azasterol led to the complete depletion of parasite endogenous sterols (episterol and 5-dehydroepisterol) and their replacement by 24-desalkyl sterols (zymosterol, cholesta-5,7,24-trien-3beta-ol, and cholesta-7,24-dien-3beta-ol), while 14-methyl-zymosterol and 4,14-dimethyl-zymosterol accumulated as a result of simultaneous incubation of the parasites with 22,26-azasterol and ketoconazole, a known inhibitor of the parasite's sterol C14-demethylase. These results confirmed that 24-SMT is the primary site of action of the azasterol. Profound changes were also observed in the phospholipid compositions of treated cells, in which a twofold reduction in the content of phosphatidylserine was observed; this was accompanied by a concomitant increase in the content of phosphatidylinositol. Transmission electron microscopy showed that 22,26-azasterol induced marked morphological changes, including mitochondrial swelling, invaginations of the inner mitochondrial membrane, and the appearance of large bodies containing concentric membranes. Other modifications included increases in the numbers of acidocalcisomes, megasomes, and lipid inclusions and the appearance of typical autophagic structures and cell body protrusions toward the flagellar pocket. We conclude that the dramatic alteration of the lipid composition of the parasite's membranes induced by the drug underlies the ultrastructural alterations that lead to the loss of cell viability and that 24-SMT inhibitors could be useful as selective antileishmanial agents.

Post transcriptional control of gene expression in Leishmania

Leishmania parasites are ancient eukaryotes, characterized by unusual molecular mechanisms. We have used the gene encoding for Hsp83 as a model system for studying regulatory mechanisms that control developmental gene regulation. We previously showed that protein coding genes are regulated exclusively by post-transcriptional mechanisms, while no transcriptional activation could be observed even for the conserved Hsp83 gene. We now show that processing and maturation of the Hsp83 polycistronic primary transcripts is more efficient at elevated temperatures. The mature transcripts are more stable during heat shock, with regulation conferred by 3' UTRs. Poly(A) tails of Hsp83 are approximately 30 nucleotides long, as common for other low eukaryotes. The mechanism that signals differential degradation is still unclear, since it was not possible to detect differences in deadenylation of Hsp83 transcripts at varying temperatures. Heat shock transcripts are preferentially translated at 33-37 degrees C, but unlike Drosophila, translational regulation is controlled by a region within the 3' UTR. Using this traditionally conserved system emphasizes that regulatory mechanisms in Leishmania differ from those prevailing in other eukaryotes.

Life in vacuoles--nutrient acquisition by Leishmania amastigotes

Leishmania have a digenetic life cycle, involving a motile, extracellular stage (promastigote) which parasitises the alimentary tract of a sandfly vector. Bloodfeeding activity by an infected sandfly can result in transmission of infective (metacyclic) promastigotes to mammalian hosts, including humans. Leishmania promastigotes are rapidly phagocytosed but may survive and transform into non-motile amastigote forms which can persist as intracellular parasites. Leishmania amastigotes multiply in an acidic intracellular compartment, the parasitophorous vacuole. pH plays a central role in the developmental switch between promastigote and amastigote stages, and amastigotes are metabolically most active when their environment is acidic, although the cytoplasm of the amastigote is regulated at near-neutral pH by an active process of proton extrusion. A steep proton gradient is thus maintained across the amastigote surface and all membrane processes must be adapted to function under these conditions. Amastigote uptake systems for glucose, amino acids, nucleosides and polyamines are optimally active at acidic pH. Promastigote uptake systems are kinetically distinct and function optimally at more neutral environmental pH, indicating that membrane transport activity is developmentally regulated. The nutrient environment encountered by amastigotes is not well understood but the parasitophorous vacuole can fuse with endosomes, phagosomes and autophagosomes, suggesting that a diverse range of macromolecules will be present. The parasitophorous vacuole is a hydrolytic compartment in which such material will be rapidly degraded to low molecular weight components which are typical substrates for membrane transporters. Amastigote surface transporters must compete for these substrates with equivalent host transporters in the membrane of the parasitophorous vacuole. The elaboration of accumulative transporters with high affinity will be beneficial to amastigotes in this environment. The influence of environmental pH on membrane transporter function is discussed, with emphasis on the potential role of a transmembrane proton gradient in active, high affinity transport.

ABC transporters in the protozoan parasite Leishmania

ATP-binding cassette (ABC) transporters constitute one of the biggest and most conserved protein families in the evolutionary scale. Many of them are of enormous clinical relevance, due to their relationship with genetic diseases and drug resistance during the treatment of cancer and infectious diseases. Leishmaniasis is a major and globally widespread group of parasitic diseases, whose treatment has been complicated by the expansion of resistance to conventional drugs. Here, we review the current knowledge about ABC transporters in Leishmania spp, with special attention to their relationship with the drug-resistance phenotype.

Enhanced activity of antisense phosphorothioate oligos against leishmania amastigotes: augmented uptake of oligo, ribonuclease H activation, and efficient target intervention under altered growth conditions

Leishmania, a parasitic protozoan, infects human macrophages, often causing severe morbidity and mortality. The pathogenic form of this parasite, the amastigote, lives inside the acidic phagolysosomes of infected macrophages. In our attempt to develop anti-miniexon phosphorothioate oligodeoxyribonucleotides (S-oligos) as an alternative chemotherapy against Leishmania, we found that intracellular as well as 'axenic' amastigotes were more susceptible to these S-oligos than were the cultured promastigotes. Lower pH (4.5) and elevated temperature (35 degrees) of the medium were among the direct enhancing factors for killing. Addition of the cationic polypeptide poly-l-lysine (PLL) to the growth medium further enhanced the killing effect of the S-oligo at pH 4.5. The enhancement of specific ablation of mRNA expression was directly correlated to the increased leishmanicidal activity of the S-oligo. This was shown by the increased inhibition of luciferase activity expressed in transgenic Leishmania amazonensis promastigotes by anti-miniexon S-oligo or anti-luciferase S-oligo at acidic pHs and in the presence of PLL. The leishmanicidal effects of S-oligos at acidic pH and in the presence of PLL were related to increased uptake of the S-oligos under these conditions. The rate of S-oligo uptake was enhanced up to 15-fold at pH 4.5. The addition of PLL to the assay medium at acidic pH further enhanced the uptake of S-oligo up to 80-fold. RNase H is known to accentuate the antisense action of S-oligos. We found that at an elevated temperature RNase H activity in Leishmania cell extracts increased about 5-fold. Thus, enhanced uptake of S-oligos at the acidic pH of macrophage phagolysosomes and activation of RNase H may explain the efficient killing of the parasite in macrophages, both in tissue culture and in the animal model, by antisense miniexon oligonucleotide/PLL, when targeted directly to the parasite-containing phagolysosomes.

Mechanisms involved in metalloid transport and tolerance acquisition

Toxic metalloids such as arsenic and antimony have always been an integral part of the natural environment. To survive in such a hostile habitat, it is crucial to develop strategies to exclude toxic substances from the cell and to acquire tolerance. Cells remove metalloids from the cytosol either by active efflux or by sequestration in an internal organelle. Controlling the influx appears to be another way of maintaining a low intracellular metalloid content. Inside the cell, the metalloid can be reduced to a form that is recognised by the expulsion system(s). In addition, metalloid complexation and compartmentalisation contributes to enhanced cellular tolerance. Finally, the presence of metalloids activates transcription of various cellular defence genes. Metalloid-containing drugs are currently used to treat protozoan infections and promyelocytic leukaemia. Since metalloid resistance hampers efficient treatment, interest in identifying the mechanisms involved in tolerance acquisition has arisen. The possibility of using genetic approaches has made the yeast Saccharomyces cerevisiae a compelling model system to investigate the basis of metalloid tolerance at a molecular level. This review describes the recent progress made in elucidating the mechanisms involved in metalloid transport and tolerance in yeast and other organisms.

A Mg-dependent ecto-ATPase in Leishmania amazonensis and its possible role in adenosine acquisition and virulence

The plasma membrane of cells contains enzymes whose active sites face the external medium rather than the cytoplasm. The activities of these enzymes, referred to as ectoenzymes, can be measured using living cells. In this work we describe the ability of living promastigotes of Leishmania amazonensis to hydrolyze extracellular ATP. In these intact parasites whose viability was assessed before and after the reactions by motility and by trypan blue dye exclusion, there was a low level of ATP hydrolysis in the absence of any divalent metal (5.39 +/- 0.71 nmol P(i)/h x 10(7) cells). The ATP hydrolysis was stimulated by MgCl(2) and the Mg-dependent ecto-ATPase activity was 30.75 +/- 2.64 nmol P(i)/h x 10(7) cells. The Mg-dependent ecto-ATPase activity was linear with cell density and with time for at least 60 min. The addition of MgCl(2) to extracellular medium increased the ecto-ATPase activity in a dose-dependent manner. At 5 mM ATP, half-maximal stimulation of ATP hydrolysis was obtained with 1.21 mM MgCl(2). This stimulatory activity was also observed when MgCl(2) was replaced by MnCl(2), but not by CaCl(2) or SrCl(2). The apparent K(m) for Mg-ATP(2-) was 0.98 mM and free Mg(2+) did not increase the ecto-ATPase activity. In the pH range from 6.8 to 8.4, in which the cells were viable, the acid phosphatase activity decreased, while the Mg(2+)-dependent ATPase activity increased. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities, such as oligomycin, sodium azide, bafilomycin A(1), ouabain, furosemide, vanadate, molybdate, sodium fluoride, tartrate, and levamizole. To confirm that this Mg-dependent ATPase was an ecto-ATPase, we used an impermeant inhibitor, 4,4'-diisothiocyanostylbene 2',2'-disulfonic acid as well as suramin, an antagonist of P(2) purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. A comparison between the Mg(2+)-dependent ATPase activity of virulent and avirulent promastigotes showed that avirulent promastigotes were less efficient than the virulent promastigotes in hydrolyzing ATP.

Function and assembly of the Leishmania surface coat

Like many trypanosomatids, the cell surface coat of Leishmania spp. is responsible for mediating various host-parasite interactions as well as acting as a dense physical barrier. This confers protection to the parasites in the hostile environments of the sandfly midgut and the macrophage phagolysosome. The major components of the surface coat are tethered to the cell surface via glycosylphosphatidylinositol glycolipids, and the composition of this surface coat is exquisitely regulated during the course of the parasite life-cycle. In this paper, we review what is known about the composition, biosynthesis and function of these glycosylphosphatidylinositol-containing molecules found within the parasite surface coat.

Cellular trafficking in trypanosomatids: a new target for therapies?

Pathogenic trypanosomatids cause a plethora of diseases marked by the lack of efficient vaccines and therapies. As a consequence, studies are being conducted that are geared towards the understanding of basic mechanisms and various biological aspects of these parasites that might be used as targets for new developments in these areas. One such aspect is the understanding of specific cellular trafficking mechanisms that might be attacked with the intention of disease control. In this paper, we give an overview of our current knowledge of cellular targeting mechanisms in trypanosomatids, with special emphasis on our data related to lysosomal targeting of cysteine proteinases in Leishmania.