Changes in folate and pterin metabolism after disruption of the Leishmania H locus short chain dehydrogenase gene

Delving in folate metabolism in the parasite Leishmania major through a chemogenomic screen and methotrexate selection

Most of our understanding of folate metabolism in the parasite Leishmania is derived from studies of resistance to the antifolate methotrexate (MTX). A chemical mutagenesis screen of L. major Friedlin and selection for resistance to MTX led to twenty mutants with a 2- to 400-fold decrease in MTX susceptibility in comparison to wild-type cells. The genome sequence of the twenty mutants highlighted recurrent mutations (SNPs, gene deletion) in genes known to be involved in folate metabolism but also in novel genes. The most frequent events occurred at the level of the locus coding for the folate transporter FT1 and included gene deletion and gene conversion events, as well as single nucleotide changes. The role of some of these FT1 point mutations in MTX resistance was validated by gene editing. The gene DHFR-TS coding for the dihydrofolate reductase-thymidylate synthase was the second locus with the most mutations and gene editing confirmed a role in resistance for some of these. The pteridine reductase gene PTR1 was mutated in two mutants. The episomal overexpression of the mutated versions of this gene, but also of DHFR-TS, led to parasites several fold more resistant to MTX than those overexpressing the wild-type versions. Genes with no known link with folate metabolism and coding for a L-galactolactone oxidase or for a methyltransferase were mutated in specific mutants. Overexpression of the wild-type versions of these genes in the appropriate mutants reverted their resistance. Our Mut-seq approach provided a holistic view and a long list of candidate genes potentially involved in folate and antifolate metabolism in Leishmania.

Development of Novel Anti-Leishmanials: The Case for Structure-Based Approaches

The neglected tropical disease (NTD) leishmaniasis is the collective name given to a diverse group of illnesses caused by ~20 species belonging to the genus Leishmania, a majority of which are vector borne and associated with complex life cycles that cause immense health, social, and economic burdens locally, but individually are not a major global health priority. Therapeutic approaches against leishmaniasis have various inadequacies including drug resistance and a lack of effective control and eradication of the disease spread. Therefore, the development of a rationale-driven, target based approaches towards novel therapeutics against leishmaniasis is an emergent need. The utilization of Artificial Intelligence/Machine Learning methods, which have made significant advances in drug discovery applications, would benefit the discovery process. In this review, following a summary of the disease epidemiology and available therapies, we consider three important leishmanial metabolic pathways that can be attractive targets for a structure-based drug discovery approach towards the development of novel anti-leishmanials. The folate biosynthesis pathway is critical, as Leishmania is auxotrophic for folates that are essential in many metabolic pathways. Leishmania can not synthesize purines de novo, and salvage them from the host, making the purine salvage pathway an attractive target for novel therapeutics. Leishmania also possesses an organelle glycosome, evolutionarily related to peroxisomes of higher eukaryotes, which is essential for the survival of the parasite. Research towards therapeutics is underway against enzymes from the first two pathways, while the third is as yet unexplored.

Inactivation of the cytosolic and mitochondrial serine hydroxymethyl transferase genes in Leishmania major

Leishmania has two serine hydroxylmethyl transferase (SHMT) genes, one coding for a cytosolic and the other for a mitochondrial enzyme. Trypanosoma cruzi has only the gene coding for the cytosolic enzyme and Trypanosoma brucei has no SHMT. We tested whether these genes were dispensable for growth in Leishmania major. By gene inactivation we succeeded in generating three cells lines one without the cytosolic cSHMT, one without the mitochondrial mSHMT, and finally one L. major line without any SHMT. SHMT is thus dispensable for growth of Leishmania in rich medium. The ability of the various shmt null mutants to grow in defined medium was tested and the growth of the shmt null mutant was dependent on the presence of serine. Overall this work has shown that SHMT is dispensable for Leishmania growth but it may be necessary when growing in environments poor in serine.

Folate metabolic pathways in Leishmania

Trypanosomatid parasitic protozoans of the genus Leishmania are autotrophic for both folate and unconjugated pteridines. Leishmania salvage these metabolites from their mammalian hosts and insect vectors through multiple transporters. Within the parasite, folates are reduced by a bifunctional DHFR (dihydrofolate reductase)-TS (thymidylate synthase) and by a novel PTR1 (pteridine reductase 1), which reduces both folates and unconjugated pteridines. PTR1 can act as a metabolic bypass of DHFR inhibition, reducing the effectiveness of existing antifolate drugs. Leishmania possess a reduced set of folate-dependent metabolic reactions and can salvage many of the key products of folate metabolism from their hosts. For example, they lack purine synthesis, which normally requires 10-formyltetrahydrofolate, and instead rely on a network of purine salvage enzymes. Leishmania elaborate at least three pathways for the synthesis of the key metabolite 5,10-methylene-tetrahydrofolate, required for the synthesis of thymidylate, and for 10-formyltetrahydrofolate, whose presumptive function is for methionyl-tRNAMet formylation required for mitochondrial protein synthesis. Genetic studies have shown that the synthesis of methionine using 5-methyltetrahydrofolate is dispensable, as is the activity of the glycine cleavage complex, probably due to redundancy with serine hydroxymethyltransferase. Although not always essential, the loss of several folate metabolic enzymes results in attenuation or loss of virulence in animal models, and a null DHFR-TS mutant has been used to induce protective immunity. The folate metabolic pathway provides numerous opportunities for targeted chemotherapy, with strong potential for 'repurposing' of compounds developed originally for treatment of human cancers or other infectious agents.

Glyoxalase I gene deletion mutants of Leishmania donovani exhibit reduced methylglyoxal detoxification

BACKGROUND

Glyoxalase I is a metalloenzyme of the glyoxalase pathway that plays a central role in eliminating the toxic metabolite methyglyoxal. The protozoan parasite Leishmania donovani possesses a unique trypanothione dependent glyoxalase system.

PRINCIPAL FINDINGS

Analysis of the L. donovani GLOI sequence predicted a mitochondrial targeting sequence, suggesting that the enzyme is likely to be targeted to the mitochondria. In order to determine definitively the intracellular localization of GLOI in L. donovani, a full-length GLOI gene was fused to green fluorescent protein (GFP) gene to generate a chimeric construct. Confocal microscopy of L. donovani promastigotes carrying this chimeric construct and immunofluorescence microscopy using anti-GLOI antibodies demonstrated that GLOI is localized in the kinetoplast of the parasite apart from the cytosol. To study the physiological role of GLOI in Leishmania, we first created promastigote mutants heterozygous for GLOI by targeted gene replacement using either hygromycin or neomycin phosphotransferases as selectable markers. Heterozygous mutants of L. donovani display a slower growth rate, have lower glyoxalase I activity and have reduced ability to detoxify methylglyoxal in comparison to the wild-type parasites. Complementation of the heterozygous mutant with an episomal GLOI construct showed the restoration of heterozygous mutant phenotype nearly fully to that of the wild-type. Null mutants were obtained only after GLOI was expressed from an episome in heterozygous mutants.

CONCLUSIONS

We for the first time report localization of GLOI in L. donovani in the kinetoplast. To study the physiological role of GLOI in Leishmania, we have generated GLOI attenuated strains by targeted gene replacement and report that GLOI is likely to be an important gene since GLOI mutants in L. donovani showed altered phenotype. The present data supports that the GLOI plays an essential role in the survival of this pathogenic organism and that inhibition of the enzyme potentiates the toxicity of methylglyoxal.

The role of reduced pterins in resistance to reactive oxygen and nitrogen intermediates in the protozoan parasite Leishmania

During its life cycle, the protozoan parasite Leishmania experiences oxidative stress when interacting with macrophages. Reduced pterins are known scavengers of reactive oxygen and nitrogen intermediates. Leishmania has a pteridine reductase, PTR1, whose main function is to provide reduced pterins. We investigated the role of PTR1 in resistance to oxidative and nitrosative stress in Leishmania tarentolae, Leishmania infantum, and Leishmania major PTR1(-/-) mutants. The PTR1(-/-) cells of the three species were more sensitive to H2O2- and NO-induced stress. Using a fluorescent probe allowing ROI quantification, we demonstrated an increase in intracellular oxidant molecules in the PTR1(-/-) mutants. The disruption of PTR1 increased metacyclogenesis in L. infantum and L. major. We purified metacyclic parasites from PTR1(-/-) mutants and control cells and tested their intracellular survival in the J774 mouse cell line and in human monocyte-derived macrophages. Our results showed that PTR1(-/-) null mutants survived less in both macrophage models compared to control cells and this decrease was more pronounced in macrophages activated for oxidant production. This study demonstrates that one physiological role of reduced pterins in Leishmania is to deal with oxidative and nitrosative species, and a decreased ability to provide reduced pterins leads to decreased intracellular survival.

Drug resistance in Leishmania: similarities and differences to other organisms

The main line of defense available against parasitic protozoa is chemotherapy. Drug resistance has emerged however, as a primary obstacle to the successful treatment and control of parasitic diseases. Leishmania spp., the causative agents of leishmaniasis, have served as a useful model for studying mechanisms of drug resistance in vitro. Antimonials and amphotericin B are the first line drugs to treat Leishmania followed by pentamidine and a number of other drugs. Parasites resistant against all these classes of drugs have been selected under laboratory conditions. A multiplicity of resistance mechanisms has been detected, the most prevalent being gene amplification and transport mutations. With the tools now available, it should be possible to elucidate the mechanisms that govern drug resistance in field isolates and develop more effective chemotherapeutic agents.

Stage specific gene expression and cellular localization of two isoforms of the serine hydroxymethyltransferase in the protozoan parasite Leishmania

Serine hydroxymethyltransferase (SHMT) catalyses the reversible conversion of serine and tetrahydrofolate to glycine and methylene-tetrahydrofolate. The recent completion of the genome sequence of Leishmania major revealed the presence of two genes coding for two isoforms of this protein. In silico analysis showed that one isoform had an extension at its N-terminus and was predicted to localize to the mitochondrion. The situation is different in other kinetoplastid parasites with only one SHMT encoding gene in Trypanosoma cruzi and no SHMT encoding gene in Trypanosoma brucei. The two L. major SHMT genes were cloned in frame with the green fluorescent protein and the resulting fusion proteins showed differential localization: the short form (SHMT-S) was found in the cytosol while the long one (SHMT-L) was found in an organelle that has hallmarks of the parasite mitochondrion. Indeed, SHMT-L had a similar cellular fractionation pattern as the mitochondrial HSP60 as determined by digitonin fractionation. Both SHMT-S and SHMT-L genes were expressed preferentially in the amastigote stage of the parasite and the RNA levels of SHMT-L could be modulated by glycine, serine, and folate. Overexpression of SHMT-S increased resistance to the antifolate methotrexate and to a lower level to the inhibitor thiosemicarbazide in a rich folate containing medium. These findings suggest that folate metabolism is compartmentalised in Leishmania and that SHMT RNA levels are responsive to environmental conditions.

Inactivation of the Leishmania tarentolae pterin transporter (BT1) and reductase (PTR1) genes leads to viable parasites with changes in folate metabolism and hypersensitivity to the antifolate methotrexate

The protozoan parasite Leishmania is a folate and pterin auxotroph. The main biopterin transporter (BT1) and pterin reductase (PTR1) have already been characterized in Leishmania. In this study, we have succeeded in generating a BT1 and PTR1 null mutant in the same Leishmania tarentolae strain. These cells are viable with growth properties indistinguishable from wildtype cells. However, in response to the inactivation of BT1 and PTR1, at least one of the folate transporter genes was deleted, and the level of the folylpolyglutamate synthetase activity was increased, leading to increased polyglutamylation of both folate and methotrexate (MTX). Secondary events following gene inactivation should be considered when analyzing a phenotype in Leishmania. The BT1/PTR1 null mutant is hypersensitive to MTX, but in a step-by-step fashion, we could induce resistance to MTX in these cells. Several resistance mechanisms were found to co-exist including a reduced folate and MTX accumulation, demonstrating that cells with no measurable biopterin uptake but also greatly reduced folate uptake are viable, despite their auxotrophy for each of these substrates. The resistant cells have also amplified the gene coding for the MTX target dihydrofolate reductase. Finally, we found a marked reduction in MTX polyglutamylation in resistant cells. These studies further highlight the formidable ability of Leishmania cells to bypass the blockage of key metabolic pathways.

Characterization of quinonoid-dihydropteridine reductase (QDPR) from the lower eukaryote Leishmania major

Biopterin is required for growth of the protozoan parasite Leishmania and is salvaged from the host through the activities of a novel biopterin transporter (BT1) and broad-spectrum pteridine reductase (PTR1). Here we characterize Leishmania major quinonoid-dihydropteridine reductase (LmQDPR), the key enzyme required for regeneration and maintenance of H(4)biopterin pools. LmQDPR shows good homology to metazoan quinonoid-dihydropteridine reductase and conservation of domains implicated in catalysis and regulation. Unlike other organisms, LmQDPR is encoded by a tandemly repeated array of 8-9 copies containing LmQDPR plus two other genes. QDPR mRNA and enzymatic activity were expressed at similar levels throughout the infectious cycle. The pH optima, kinetic properties, and substrate specificity of purified LmQDPR were found to be similar to that of other qDPRs, although it lacked significant activity for non-quinonoid pteridines. These and other data suggest that LmQDPR is unlikely to encode the dihydrobiopterin reductase activity (PTR2) described previously. Similarly LmQDPR is not inhibited by a series of antifolates showing anti-leishmanial activity beyond that attributable to dihydrofolate reductase or PTR1 inhibition. qDPR activity was found in crude lysates of Trypanosoma brucei and Trypanosoma cruzi, further emphasizing the importance of H(4)biopterin throughout this family of human parasites.

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.

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.

A telomere-mediated chromosome fragmentation approach to assess mitotic stability and ploidy alterations of Leishmania chromosomes

We have used a telomere-associated chromosome fragmentation strategy to induce internal chromosome-specific breakage of Leishmania chromosomes. The integration of telomeric repeats from the kinetoplastid Trypanosoma brucei into defined positions of the Leishmania genome by homologous recombination can induce chromosome breakage accompanied by the deletion of the chromosomal part that is distal to the site of the break. The cloned telomeric DNA at the end of the truncated chromosomes is functional and it can seed the formation of new telomeric repeats. We found that genome ploidy is often altered upon telomere-mediated chromosome fragmentation events resulting in large chromosomal deletions. In most cases diploidy is either preserved, or partial trisomic cells are observed, but interestingly we report here the generation of partial haploid mutants in this diploid organism. Partial haploid Leishmania mutants should facilitate studies on the function of chromosome-assigned genes. We also present several lines of evidence for the presence of sequences involved in chromosome mitotic stability and segregation during cell cycle in this parasitic protozoan. Telomere-directed chromosome fragmentation studies in Leishmania may constitute a useful tool to assay for centromere function.

Regulation of differentiation to the infective stage of the protozoan parasite Leishmania major by tetrahydrobiopterin

A critical step in the infectious cycle of Leishmania is the differentiation of parasites within the sand fly vector to the highly infective metacyclic promastigote stage. Here, we establish tetrahydrobiopterin (H4B) levels as an important factor controlling the extent of metacyclogenesis. H4B levels decline substantially during normal development, and genetic or nutritional manipulations showed that low H4B caused elevated metacyclogenesis. Mutants lacking pteridine reductase 1 (PTR1) had low levels of H4B, remained infectious to mice, and induced larger cutaneous lesions (hypervirulence). Thus, the control of pteridine metabolism has relevance to the mechanism of Leishmania differentiation and the limitation of virulence during evolution.

Pteridine salvage throughout the Leishmania infectious cycle: implications for antifolate chemotherapy

Protozoan parasites of the trypanosomatid genus Leishmania are pteridine auxotrophs, and have evolved an elaborate and versatile pteridine salvage network capable of accumulating and reducing pteridines. This includes biopterin and folate transporters (BT1 and FT1), pteridine reductase (PTR1), and dihydrofolate reductase-thymidylate synthase (DHFR-TS). Notably, PTR1 is a novel alternative pteridine reductase whose activity is resistant to inhibition by standard antifolates. In cultured promastigote parasites, PTR1 can function as a metabolic by-pass under conditions of DHFR inhibition and thus reduce the efficacy of chemotherapy. To test whether pteridine salvage occurred in the infectious stage of the parasite, we examined several pathogenic species of Leishmania and the disease-causing amastigote stage that resides within human macrophages. To accomplish this we developed a new sensitive HPLC-based assay for PTR1 activity. These studies established the existence of the pteridine salvage pathway throughout the infectious cycle of Leishmania, including amastigotes. In general, activities were not well correlated with RNA transcript levels, suggesting the occurrence of at least two different modes of post-transcriptional regulation. Thus, pteridine salvage by amastigotes may account for the clinical inefficacy of antifolates against leishmaniasis, and ultimately provide insights into how this may be overcome in the future.

Highly reliable heterologous system for evaluating resistance of clinical herpes simplex virus isolates to nucleoside analogues

Clinical resistance of herpes simplex virus (HSV) types 1 and 2 to acyclovir (ACV) is usually caused by the presence of point mutations within the coding region of the viral thymidine kinase (TK) gene. The distinction between viral TK mutations involved in ACV resistance or part of viral polymorphism can be difficult to evaluate with current methodologies based on transfection and homologous recombination. We have developed and validated a new heterologous system based on the expression of the viral TK gene by the protozoan parasite Leishmania, normally devoid of TK activity. The viral TK genes from 5 ACV-susceptible and 13 ACV-resistant clinical HSV isolates and from the reference strains MS2 (type 2) and KOS (type 1) were transfected as part of an episomal expression vector in Leishmania. The susceptibility of TK-recombinant parasites to ganciclovir (GCV), a closely related nucleoside analogue, was evaluated by a simple measurement of the absorbance of Leishmania cultures grown in the presence of the drug. Expression of the TK gene from ACV-susceptible clinical isolates resulted in Leishmania susceptibility to GCV, whereas expression of a TK gene with frameshift mutations or nucleotide substitutions from ACV-resistant isolates gave rise to parasites with high levels of GCV resistance. The expression of the HSV TK gene in Leishmania provides an easy, reliable, and sensitive assay for evaluating HSV susceptibility to nucleoside analogues and for assessing the role of specific viral TK mutations.

Chromosome structure and sequence organization between pathogenic and non-pathogenic Leishmania spp

We have used a chromosome fragmentation strategy based on systematic genomic insertions of the rare cutting yeast I-SceI endonuclease to assess structure and sequence organization of homologous chromosomes between evolutionary divergent pathogenic and non-pathogenic Leishmania species. This method was combined to physical mapping and hybridization studies using a number of specific chromosomal markers as probes. Our studies have concentrated on two different chromosomes of Leishmania major (L. major), L. donovani and L. infantum and of the non-pathogenic species L. tarentolae. Specific chromosome fragmentation events at the level of multiple I-SccI genomic integrations indicated that very similar distances separated internal genomic sequences between homologous chromosomes and that distances from chromosome ends were more variable. The order and orientation of genes along the homologous chromosomes were also conserved between species. With only few exceptions, genome organization between pathogenic and non-pathogenic Leishmania species was found to be highly conserved. Genomic comparison of pathogenic and non-pathogenic species may be useful for depicting regions involved in species-specific related pathologies.

A new developmentally regulated gene family in Leishmania amastigotes encoding a homolog of amastin surface proteins

The ability of Leishmania to survive within the phagolysosomes of mammalian macrophages is heavily dependent on the developmental regulation of a number of genes. Characterization of genes preferentially expressed during the parasite's intracellular growth would help to elucidate the mechanisms controlling stage-specific gene regulation and the intracellular life of the parasite in general. Using a genomic approach based on the differential hybridization screening of high-density filters, we have identified a new developmentally regulated gene in Leishmania, which is part of a multigene family and encodes a highly hydrophobic protein that shares homology with the Trypanosoma cruzi amastin proteins. The fusion of the Leishmania amastin gene homolog with the green fluorescent protein and analysis by confocal microscopy suggested a surface localization for this protein. The amastin gene homolog is expressed predominantly in the amastigote form of several Leishmania species and is strictly regulated by acidic pH at the post-transcriptional level. Its developmental expression involves sequences within the 3'-untranslated region.

Role of the locus and of the resistance gene on gene amplification frequency in methotrexate resistant Leishmania tarentolae

The protozoan parasite Leishmania resists the antifolate methotrexate (MTX) by amplifying the R locus dihydrofolate reductase-thymidylate synthase ( dhfr-ts ) gene, the H locus ptr1 pterin reductase gene, and finally by mutation in a common folate/MTX transporter. Amplification of dhfr-ts has never been observed in Leishmania tarentolae MTX resistant mutants while ptr1 amplification is common. We have selected a L.tarentolae ptr1 null mutant for MTX resistance and observed dhfr-ts amplification in this mutant demonstrating that once a preferred resistance mechanism is unavailable, a second one will take over. By introducing the ptr1 gene at the R locus and the dhfr-ts gene at the H locus by gene targeting, we investigated the role of the resistance gene and the locus on the rate of gene amplification. Transfection studies indicated that ptr1 gave higher levels of MTX resistance than dhfr-ts. Consistent with this, when ptr1 was present as part of either the H locus or the R locus it was invariably amplified, while dhfr-ts was only amplified when ptr1 was inactivated. When dhfr-ts was present in a ptr1 null background on both the H locus and the R locus, amplification from the H locus was more frequent suggesting that both the gene and the locus are determining the frequency of gene amplification in Leishmania.

Mutant PTR1 proteins from Leishmania tarentolae: comparative kinetic properties and active-site labeling

PTR1, the gene promoting MTX resistance following gene amplification or DNA transfection in Leishmania tarentolae and selected mutants, has been cloned and heavily overexpressed (>100 mg/liter) in Escherichia coli strain BL21 (DE3). Protein has been purified, essentially to homogeneity, in two steps, via ammonium sulfate precipitation and chromatography on DEAE-Trisacryl. The active proteins are tetramers and display optimal pteridine reductase activity at pH 6.0 using biopterin as substrate and NADPH as the reduced dinucleotide cofactor. 2,4-Diaminopteridine substrate analogues are strong competitive inhibitors (K(i) approximately 38 --> 3 nM) against the pterin substrate and both NADP(+) and folate are inhibitors although somewhat weaker. Dihydropteridines are poor substrates compared to the fully oxidized pteridine. Kinetic analysis affords the usual Michaelis constants and in addition shows that inhibition by NADP(+) allows the formation of ternary nonproductive complexes with folate. The kinetic results are consistent with a sequential ordered bi-bi kinetic mechanism in which first NADPH and then pteridine bind to the free enzyme. Sequence comparisons suggest that PTR1 belongs to the short-chain dehydrogenase/reductase (SDR) family containing an amino-terminal glycine-rich dinucleotide binding site plus a catalytic Y(Xaa)(3)K motif. In accord with this observation, the mutants K16A, Y37D, and R39A and the double mutants K17A:R39A and Y37D:R39A all show a two- to threefold lower binding affinity for NADPH and exhibit low or zero activity. Two Y(Xaa)(3)K regions are present in wild-type PTR1 at 152 and 194. Only Y194F gives protein with zero activity. This observation coupled with affinity labeling of PTR1 by oNADP(+) (2', 3'-dialdehyde derivative of NADP(+)) followed by NaBH(4) reduction, V8 protease digestion, and mass spectral analysis suggests that the motif participating in catalysis is that at 194. The mutation K198Q eliminates inactivation by oNADP(+) supporting the hypothesis that K198 is associated with nucleotide orientation, as has been demonstrated for similar lysine residues in other members of the SDR family.

Leishmania: overexpression and comparative structural analysis of the stage-regulated meta 1 gene

The meta 1 gene of Leishmania major is upregulated in metacyclic promastigotes and encodes an 11.5-kDa protein with no significant similarities to other proteins in the existing databases. In this paper, we characterize the homologous meta 1 genes in L. amazonensis and L. donovani. Proteins encoded by this gene in all three species present a high degree of identity. The meta 1 gene cannot be replaced by gene targeting in L. major, suggesting an essential role for the protein, at least in promastigotes. Overexpression of the meta 1 protein in L. amazonensis generates parasites that are more virulent than wild-type organisms in vivo.

A pteridine reductase gene ptr1 contiguous to a P-glycoprotein confers resistance to antifolates in Trypanosoma cruzi

We have isolated the pteridine reductase-1 gene (ptr1), from Trypanosoma cruzi (Y strain), located contiguous to the Trypanosoma cruzi P-glycoprotein-2 (tcpgp2). The gene encodes a member of the family of short-chain dehydrogenases, enzymes that are involved in several oxidoreduction reactions. One member of the family, pteridine reductase-1 (PTR1) has been previously described in Leishmania as being involved in antifolate resistance. The ptr1 gene from T. cruzi presents an 828 bp open reading frame, coding for a 276 amino acid protein with a predicted molecular mass of 30 kDa. The deduced amino acid sequence exhibited a remarkable homology with the ptr1 genes of Leishmania major and Leishmania tarentolae. Southern blot analysis using a specific probe indicated that T. cruzi PTR1 is encoded by a single copy gene located in two chromosomes of about 0.9 and 1.2 Mb. Western blot analysis using a polyclonal antiserum against recombinant PTR1 revealed that the protein is only expressed in the epimastigote forms of the parasite; we did not detect the protein either in the amastigote or trypomastigote forms. Purified recombinant PTR1 exhibits a NADPH-dependent pteridine reductase activity comparable with those described in Leishmania. Gene transfection experiments using the pTEX expression vector show that, under the conditions tested, T. cruzi PTR1 is involved in resistance to the methotrexate, aminopterin and trimethoprim antifolates.

Parameters controlling the rate of gene targeting frequency in the protozoan parasite Leishmania

In this study we investigated the role of several parameters governing the efficiency of gene targeting mediated by homologous recombination in the protozoan parasite Leishmania. We evaluated the relative targeting frequencies of different replacement vectors designed to target several sequences within the parasite genome. We found that a decrease in the length of homologous sequences <1 kb on one arm of the vector linearly influences the targeting frequency. No homologous recombination was detected, however, when the flanking homologous regions were <180 bp. A requirement for a very high degree of homology between donor and target sequences was found necessary for efficient gene targeting in Leishmania , as targeted recombination was strongly affected by base pair mismatches. Targeting frequency increased proportionally with copy number of the target only when the target was part of a linear amplicon, but remained unchanged when it was present on circles. Different chromosomal locations were found to be targeted with significantly variable levels of efficiency. Finally, different strains of the same species showed differences in gene targeting frequency. Overall, gene targeting mediated by homologous recombination in Leishmania shares similarities to both the yeast and the mammalian recombination systems.

Pterin and folate reduction by the Leishmania tarentolae H locus short-chain dehydrogenase/reductase PTR1

Overproduction of the short-chain dehydrogenase/reductase PTR1 confers resistance to the dihydrofolate reductase inhibitor methotrexate in the protozoan parasite Leishmania. Genetic analysis has previously implicated PTR1 in pterin and folate metabolism. PTR1 was purified from a fusion protein expressed in Escherichia coli. Purified PTR1 exhibits NADPH-dependent biopterin, dihydrobiopterin, folate, and dihydrofolate reductase activities. The highest activity was found with the most oxidized pterins. The active protein was found to be a tetramer as demonstrated by gel-filtration chromatography. Kinetic constants (K(m)), as determined by double-reciprocal plots, were calculated for NADPH and for several of PTR1's substrates. The PTR1 of Leishmania tarentolae had a K(m) of 16.9 microM for the cofactor NADPH and K(m) values ranging from 3.5 to 85 microM for the various substrates. The dissociation constant (KD), as determined by fluorescence titration, for NADPH was estimated to be 130 microM. The biochemical characterization of this important and novel enzyme involved in folate and pterin metabolism of Leishmania should be useful for structure-function analysis and for developing specific inhibitors against this putative important chemotherapeutic target.

The roles of pteridine reductase 1 and dihydrofolate reductase-thymidylate synthase in pteridine metabolism in the protozoan parasite Leishmania major

Trypanosomatid protozoans depend upon exogenous sources of pteridines (pterins or folates) for growth. A broad spectrum pteridine reductase (PTR1) was recently identified in Leishmania major, whose sequence places it in the short chain alcohol dehydrogenase protein family although its enzymatic activities resemble dihydrofolate reductases. The properties of PTR1 suggested a role in essential pteridine salvage as well as in antifolate resistance. To prove this, we have characterized further the properties and relative roles of PTR1 and dihydrofolate reductase-thymidylate synthase in Leishmania pteridine metabolism, using purified enzymes and knockout mutants. Recombinant L. major and Leishmania tarentolae, and native L. major PTR1s, were tetramers of 30-kDa subunits and showed similar catalytic properties with pterins and folates (pH dependence, substrate inhibition with H2pteridines). Unlike PTR1, dihydrofolate reductase-thymidylate synthase showed weak activity with folate and no activity with pterins. Correspondingly, studies of ptr1(-) and dhfr-ts- mutants implicated only PTR1 in the ability of L. major to grow on a wide array of pterins. PTR1 exhibited 2000-fold less sensitivity to inhibition by methotrexate than dihydrofolate reductase-thymidylate synthase, suggesting several mechanisms by which PTR1 may compromise antifolate inhibition in wild-type Leishmania and lines bearing PTR1 amplifications. We incorporate these results into a comprehensive model of pteridine metabolism and discuss its implications in chemotherapy of this important human pathogen.

Trypanosomatid shuttle vectors: new tools for the functional dissection of parasite genomes

In the past five years, gene-transfer systems have been established for each of the medically important trypanosomatids: Leishmania sp, Trypanosoma brucei and T. cruzi. Transformation can be mediated by integration, which occurs exclusively by homologous recombination, or by episomal shuttle vectors. In this article, John Kelly will focus on recent progress in the development and applications of trypanosomatid shuttle vectors, ie. vectors which are maintained extrachromosomally and which are capable of autonomous replication in both trypanosomatid and bacterial hosts.

Autonomous replication of bacterial DNA plasmid oligomers in Leishmania

Extrachromosomal amplicons are frequently observed in drug-resistant Leishmania. A dominant selectable marker, the neomycin phosphotransferase gene, was introduced by gene targeting in a circular amplicon derived from the H locus of Leishmania in a mutant cell. This recombinant amplicon was isolated and transfected in a wild-type cell. The amplicon was kept in the wild-type cells, provided the selective pressure was maintained, suggesting that it was capable of autonomous replication. Novel Leishmania expression vectors suited for stable transfections were made to isolate, by a high transformation assay, the putative origin of replication in the amplicons. However, these plasmids, which did not contain a single Leishmania nucleotide, were found as extrachromosomal circular oligomers in Leishmania transfectants. Their relative stability, in addition to changes in their methylation pattern, indicated that these plasmids were most likely replicating. No specific sequences seem to be required for replication (and expression) in Leishmania, therefore precluding the isolation of origins of replication by genetic transformation.