Study of the safety, immunogenicity and efficacy of attenuated and killed Leishmania (Leishmania) major vaccines in a rhesus monkey (Macaca mulatta) model of the human disease

We have compared the efficacy of two Leishmania (Leishmania) major vaccines, one genetically attenuated (DHFR-TS deficient organisms), the other inactivated [autoclaved promastigotes (ALM) with bacillus Calmete-Guérin (BCG)], in protecting rhesus macaques (Macaca mulatta) against infection with virulent L. (L.) major. Positive antigen-specific recall proliferative response was observed in vaccinees (79% in attenuated parasite-vaccinated monkeys, versus 75% in ALM-plus-BCG-vaccinated animals), although none of these animals exhibited either augmented in vitro gamma interferon (IFN-gamma) production or positive delayed-type hypersensitivity (DTH) response to the leishmanin skin test prior to the challenge. Following challenge, there were significant differences in blastogenic responses (p < 0.05) between attenuated-vaccinated monkeys and naïve controls. In both vaccinated groups very low levels of antibody were found before challenge, which increased after infective challenge. Protective immunity did not follow vaccination, in that monkeys exhibited skin lesion at the site of challenge in all the groups. The most striking result was the lack of pathogenicity of the attenuated parasite, which persisted in infected animals for up to three months, but were incapable of causing disease under the conditions employed. We concluded that both vaccine protocols used in this study are safe in primates, but require further improvement for vaccine application.

New Mos1 mariner transposons suitable for the recovery of gene fusions in vivo and in vitro

The Drosophila Mos1 element can be mobilized in species ranging from prokaryotes to protozoans and vertebrates, and the purified transposase can be used for in vitro transposition assays. In this report we developed a 'mini-Mos1' element and describe a number of useful derivatives suitable for transposon mutagenesis in vivo or in vitro. Several of these allow the creation and/or selection of tripartite protein fusions to a green fluorescent protein-phleomycin resistance (GFP-PHLEO) reporter/selectable marker. Such X-GFP-PHLEO-X fusions have the advantage of retaining 5' and 3' regulatory information and N- and C-terminal protein targeting domains. A Mos1 derivative suitable for use in transposon-insertion mediated linker insertion (TIMLI) mutagenesis is described, and transposons bearing selectable markers suitable for use in the protozoan parasite Leishmania were made and tested. A novel 'negative selection' approach was developed which permits in vitro assays of transposons lacking bacterial selectable markers. Application of this assay to several Mos1 elements developed for use in insects suggests that the large mariner pM[cn] element used previously in vivo is poorly active in vitro, while the Mos1-Act-EGFP transposon is highly active.

The Leishmania genome project: new insights into gene organization and function

The sequencing of Leishmania major Friedlin chromosome 1 (Chr1), Chr3, and Chr4 has been completed. and several other chromosomes are well underway. The complete genome sequence should be available by 2003. Over 1,000 full-length new genes have been identified, with the majority (approximately 75%) having unknown function. Many of these may be Leishmania (or kinetoplastid) specific. Most interestingly, the genes are organized into large (> 100-500 kb) polycistronic clusters of adjacent genes on the same DNA strand. Chr1 contains two such clusters organized in a "divergent" manner, i.e., the mRNAs for the two sets of genes are both transcribed towards the telomeres. Nuclear run-on analysis suggests that transcription is initiated in both directions within the "divergent" region. Chr3 and Chr4 contain two "convergent" clusters, with a single "divergent" gene at one telomere of Chr3. Sequence analysis of several genes from the LD1 region of Chr35 indicates a high degree of sequence conservation between L. major and L. donovani/L. infantum within protein-coding open reading frames (ORFs), with a lower degree of conservation within the non-coding regions. Immunization of mice with recombinant antigen from two of these genes, BTI (formerly ORFG) and ORFF, results in significant reduction in parasite burden following Leishmania challenge. Recombinant ORFF antigen shows promise as a serodiagnostic. We have also developed a tetracycline-regulated promoter system, which allows us to modulate gene expression in Leishmania.

A lipophosphoglycan-independent method for isolation of infective Leishmania metacyclic promastigotes by density gradient centrifugation

At the end of their growth in the sand fly, Leishmania parasites differentiate into the infective metacyclic promastigote stage, which is transmitted to the mammalian host. Thus, in experimental studies of parasite infectivity toward animals or macrophages, the use of purified metacyclics is generally preferred. While metacyclics of several Leishmania species can be efficiently purified with the aid of lectins or monoclonal antibodies, which differentially exploit stage-specific differences in the structure of the abundant surface glycolipid lipophosphoglycan (LPG), such reagents are unavailable for most species and they are unsuitable for studies involving LPG-deficient mutants. Here we describe a simple density gradient centrifugation method, which allows the rapid purification of infective metacyclic parasites from both wild-type and LPG-deficient Leishmania major. The purified metacyclic promastigotes are authentic, as judged by criteria such as their morphology, expression of the metacyclic-specific gene SHERP, and ability to invade and replicate within macrophages in vitro. Preliminary studies suggest that this method is applicable to other Leishmania species including L. donovani.

Dual role of the fringe connection gene in both heparan sulphate and fringe-dependent signalling events

The precise regulation of growth factor signalling is crucial to the molecular control of development in Drosophila. Post-translational modification of signalling molecules is one of the mechanisms that modulate developmental signalling specificity. We describe a new gene, fringe connection (frc), that encodes a nucleotide-sugar transporter that transfers UDP-glucuronic acid, UDP-N-acetylglucosamine and possibly UDP-xylose from the cytoplasm into the lumen of the endoplasmic reticulum/Golgi. Embryos with the frc mutation display defects in Wingless, Hedgehog and fibroblast growth factor signalling. Clonal analysis shows that fringe-dependent Notch signalling is disrupted in frc mutant tissue.

Pteridine reductase mechanism correlates pterin metabolism with drug resistance in trypanosomatid parasites

Pteridine reductase (PTR1) is a short-chain reductase (SDR) responsible for the salvage of pterins in parasitic trypanosomatids. PTR1 catalyzes the NADPH-dependent two-step reduction of oxidized pterins to the active tetrahydro-forms and reduces susceptibility to antifolates by alleviating dihydrofolate reductase (DHFR) inhibition. Crystal structures of PTR1 complexed with cofactor and 7,8-dihydrobiopterin (DHB) or methotrexate (MTX) delineate the enzyme mechanism, broad spectrum of activity and inhibition by substrate or an antifolate. PTR1 applies two distinct reductive mechanisms to substrates bound in one orientation. The first reduction uses the generic SDR mechanism, whereas the second shares similarities with the mechanism proposed for DHFR. Both DHB and MTX form extensive hydrogen bonding networks with NADP(H) but differ in the orientation of the pteridine.

Is lipophosphoglycan a virulence factor? A surprising diversity between Leishmania species

Lipophosphoglycan is a prominent member of the phosphoglycan-containing surface glycoconjugates of Leishmania. Genetic tests enable confirmation of its role in parasite virulence and permit discrimination between the roles of lipophosphoglycan and related glycoconjugates. When two different lipophosphoglycan biosynthetic genes from Leishmania major were knocked out, there was a clear loss of virulence in several steps of the infectious cycle but, with Leishmania mexicana, no effect on virulence was found. This points to an unexpected diversity in the reliance of Leishmania species on virulence factors, a finding underscored by recent studies showing great diversity in the host response to Leishmania species.

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.

Protective immunity against the protozoan Leishmania chagasi is induced by subclinical cutaneous infection with virulent but not avirulent organisms

Protective immunity against Leishmania major is provided by s.c. immunization with a low dose of L. major promastigotes or with dihydrofolate-thymidylate synthase gene locus (DHFR-TS) gene knockout L. major organisms. Whether these vaccine strategies will protect against infection with other Leishmania species that elicit distinct immune responses and clinical syndromes is not known. Therefore, we investigated protective immunity to Leishmania chagasi, a cause of visceral leishmaniasis. In contrast to L. major, a high dose s.c. inoculum of L. chagasi promastigotes was required to elicit protective immunity. Splenocytes from mice immunized with a high dose produced significantly greater amounts of IFN-gamma and lower TGF-beta than mice immunized with a low dose of promastigotes. The development of protective immunity did not require the presence of NK cells. Protection was not afforded by s.c. immunization with either attenuated L. chagasi or with L. major promastigotes, and s.c. L. chagasi did not protect against infection with L. major. Subcutaneous immunization with DHFR-TS gene knockouts derived from L. chagasi, L. donovani, or L. major did not protect against L. chagasi infection. We conclude that s.c. inoculation of high doses of live L. chagasi causes a subclinical infection that elicits protective immune responses in susceptible mice. However, L. chagasi that have been attenuated either by long-term passage or during the raising of recombinant gene knockout organisms do not elicit protective immunity, either because they fail to establish a subclinical infection or because they no longer express critical antigenic epitopes.

Purified mariner (Mos1) transposase catalyzes the integration of marked elements into the germ-line of the yellow fever mosquito, Aedes aegypti

Derivatives of the mariner transposable element, Mos1, from Drosophila mauritiana, can integrate into the germ-line of the yellow fever mosquito, Aedes aegypti. Previously, the transposase required to mobilize Mos1 was provided in trans by a helper plasmid expressing the enzyme under the control of the D. psuedoobscura heat-shock protein 82 promoter. Here we tested whether purified recombinant Mos1 transposase could increase the recovery of Ae. aegypti transformants. Mos1 transposase was injected into white-eyed, kh(w)/kh(w), Ae. aegypti embryos with a Mos1 donor plasmid containing a copy of the wild-type allele of the D. melanogaster cinnabar gene. Transformed mosquitoes were recognized by partial restoration of eye color in the G(1) animals and confirmed by Southern analyses of genomic DNA. At Mos1 transposase concentrations approaching 100 nM, the rate of germ-line transformants arising from independent insertions in G(0) animals was elevated 2-fold compared to that seen in experiments with helper plasmids. Furthermore, the recovery of total G(1) transformants was increased 7.5-fold over the frequency seen with co-injected helper plasmid. Southern blot analyses and gene amplification experiments confirmed the integration of the transposons into the mosquito genome, although not all integrations were of the expected cut-and-paste type transposition. The increased frequency of germ-line integrations obtained with purified transposase will facilitate the generation of Mos1 transgenic mosquitoes and the application of transgenic approaches to the biology of this important vector of multiple pathogens.

Genomewide insertional mutagenesis in Streptomyces coelicolor reveals additional genes involved in morphological differentiation

The filamentous soil bacterium Streptomyces coelicolor undergoes a complex cycle of morphological differentiation involving the formation of an aerial mycelium and the production of pigmented antibiotics. We have developed a procedure for generating insertional mutants of S. coelicolor based on in vitro transposition of a plasmid library of cloned S. coelicolor DNAs. The insertionally mutated library was introduced into S. coelicolor, and transposon insertions were recovered at widely scattered locations around the chromosome. Many of the insertions revealed previously uncharacterized genes, and several caused novel mutant phenotypes, such as altered pigment production, enhanced antibiotic sensitivity, delayed or impaired formation of aerial hyphae, and a block in spore formation. The sporulation mutant harbored an insertion in one of three adjacent genes that are apparently unique to Streptomyces but are each represented by at least 20 paralogs at dispersed locations in the chromosome. Individual members of the three families often are found grouped together in a characteristic arrangement, suggesting that they have a common function.

Lipophosphoglycan is a virulence factor distinct from related glycoconjugates in the protozoan parasite Leishmania major

Protozoan parasites of the genus Leishmania undergo a complex life cycle involving transmission by biting sand flies and replication within mammalian macrophage phagolysosomes. A major component of the Leishmania surface coat is the glycosylphosphatidylinositol (GPI)-anchored polysaccharide called lipophosphoglycan (LPG). LPG has been proposed to play many roles in the infectious cycle, including protection against complement and oxidants, serving as the major ligand for macrophage adhesion, and as a key factor mitigating host responses by deactivation of macrophage signaling pathways. However, all structural domains of LPG are shared by other major surface or secretory products, providing a biochemical redundancy that compromises the ability of in vitro tests to establish whether LPG itself is a virulence factor. To study truly lpg(-) parasites, we generated Leishmania major lacking the gene LPG1 [encoding a putative galactofuranosyl (Gal(f)) transferase] by targeted gene disruption. The lpg1(-) parasites lacked LPG but contained normal levels of related glycoconjugates and GPI-anchored proteins. Infections of susceptible mice and macrophages in vitro showed that these lpg(-) Leishmania were highly attenuated. Significantly and in contrast to previous LPG mutants, reintroduction of LPG1 into the lpg(-) parasites restored virulence. Thus, genetic approaches allow dissection of the roles of this complex family of interrelated parasite virulence factors, and definitively establish the role of LPG itself as a parasite virulence factor. Because the lpg1(-) mutant continue to synthesize bulk GPI-anchored Gal(f)-containing glycolipids other than LPG, a second pathway distinct from the Golgi-associated LPG synthetic compartment must exist.

Heterologous expression of Trypanosoma cruzi trans-sialidase in Leishmania major enhances virulence

Earlier studies showed that mice primed for a few hours with the trans-sialidase (TS) of Trypanosoma cruzi, the agent of Chagas' disease, become highly susceptible to trypanosomal infection. These studies suggest that TS affects parasite virulence independent of antigenic stimulation. Potentially, TS could enhance or reduce the virulence of heterologous microbes depending on the mechanism of TS action and on the type of immune response elicited by the particular parasite. We tested this hypothesis by expressing heterologous TS in Leishmania major, a protozoan parasite that causes cutaneous leishmaniasis and lacks TS and the TS product alpha2-3-linked sialic acid. Leishmania cells transfected with a T. cruzi TS expression construct made high levels of active enzyme, which was present in the promastigotes and shed into the extracellular milieu. TS expression did not affect L. major binding to and entry into cultured macrophages or its tropism for macrophage infection in vivo. However, TS-expressing L. major exhibited elevated virulence in BALB/c mice, as determined by lesion progression, parasite numbers, and macro- and microscopic examination of cutaneous lesions. Several genetic tests proved that the enhanced virulence was directly attributable to TS expression. The results are consistent with TS functioning to sabotage the mouse immune system to confer a growth advantage on T. cruzi and transgenic L. major. These data suggest that heterologous expression of T. cruzi virulence factors in Leishmania may provide a new approach for dissecting their function in vivo.

The Leishmania GDP-mannose transporter is an autonomous, multi-specific, hexameric complex of LPG2 subunits

LPG2 (a gene involved in lipophosphoglycan assembly) encodes the Golgi GDP-Man transporter of the protozoan parasite Leishmania and is a defining member of a new family of eukaryotic nucleotide-sugar transporters (NSTs). Although NST activities are widespread, mammalian cells lack a GDP-Man NST, thereby providing an ideal heterologous system for probing the LPG2 structure and activity. LPG2 expression constructs introduced into either mammalian cells or a Leishmania lpg2(-) mutant conferred GDP-Man, GDP-Ara, and GDP-Fuc (in Leishmania only) uptake in isolated microsomes. LPG2 is the first NST to be associated with multiple substrate specificities. Uptake activity showed latency, exhibited an antiport mechanism of transport with GMP, and was susceptible to the anion transport inhibitor DIDS. The apparent K(m) for GDP-Man uptake was similar in transfected mammalian cells (12.2 microM) or Leishmania (6.9 microM). Given the evolutionary distance between protozoans and vertebrates, these data suggest that LPG2 functions autonomously to provide transporter activity. Using epitope-tagged LPG2 proteins, we showed the existence of hexameric LPG2 complexes by immunoprecipitation experiments, glycerol gradient centrifugation, pore-limited native gel electrophoresis, and cross-linking experiments. This provides strong biochemical evidence for a multimeric complex of NSTs, a finding with important implications to the structure and specificity of NSTs in both Leishmania and other organisms. Inhibition of essential GDP-Man uptake in fungal and protozoan systems offers an attractive target for potential chemotherapy.

Virulent or avirulent (dhfr-ts-) Leishmania major elicit predominantly a type-1 cytokine response by human cells in vitro

In this study we have compared the immune response of normal human cells cultured in vitro to two virulent strains of Leishmania major (CC1 and LV39), and to an avirulent vaccine strain (dhfr-ts-) made by targeted deletion of the essential gene DHFR-TS. We utilized an in vitro system in which naive T cells from normal human donors were primed with autologous Leishmania-infected macrophages. All three parasites infected macrophages and transformed into amastigotes within the cells. However, whereas LV39 and CC1 replicated in macrophages, dhfr-ts- did not. When peripheral blood lymphocytes (PBL) were stimulated with autologous macrophages infected with any of the three parasites, the lymphocytes produced a type-1-biased cytokine response. Finally, addition of IL-12 during the first stimulation period increased the production of interferon-gamma but decreased IL-5 secretion. On the other hand, anti-IL-12 resulted in the opposite effect.

cis and trans factors affecting Mos1 mariner evolution and transposition in vitro, and its potential for functional genomics

Mos1 and other mariner / Tc1 transposons move horizon-tally during evolution, and when transplanted into heterologous species can transpose in organisms ranging from prokaryotes to protozoans and vertebrates. To further develop the Drosophila Mos1 mariner system as a genetic tool and to probe mechanisms affecting the regulation of transposition activity, we developed an in vitro system for Mos1 transposition using purified transposase and selectable Mos1 derivatives. Transposition frequencies of nearly 10(-3)/target DNA molecule were obtained, and insertions occurred at TA dinucleotides with little other sequence specificity. Mos1 elements containing only the 28 bp terminal inverted repeats were inactive in vitro, while elements containing a few additional internal bases were fully active, establishing the minimal cis -acting requirements for transposition. With increasing transposase the transposition frequency increased to a plateau value, in contrast to the predictions of the protein over-expression inhibition model and to that found recently with a reconstructed Himar1 transposase. This difference between the 'natural' Mos1 and 'reconstructed' Himar1 transposases suggests an evolutionary path for down-regulation of mariner transposition following its introduction into a naïve population. The establishment of the cis and trans requirements for optimal mariner transposition in vitro provides key data for the creation of vectors for in vitro mutagenesis, and will facilitate the development of in vivo systems for mariner transposition.

Priming of a beta-galactosidase (beta-GAL)-specific type 1 response in BALB/c mice infected with beta-GAL-transfected Leishmania major

To determine whether an ongoing response to Leishmania major would affect the response to a non-cross-reacting, non-leishmanial antigen, susceptible BALB/c mice and resistant C3H mice were infected with L. major parasites expressing Escherichia coli beta-galactosidase (beta-GAL); this parasite was designated L. major-betaGAL. BALB/c and C3H mice responded to infection with L. major-betaGAL by mounting a CD4 T-cell response to both parasite antigens and to the reporter antigen, beta-GAL. The phenotypes of these T cells were characterized after generating T-cell lines from infected mice. As expected, BALB/c mice responded to infection with L. major-betaGAL by producing interleukin 4 in response to the parasite and C3H mice produced gamma interferon (IFN-gamma) in response to the parasite and beta-GAL. Interestingly, however, BALB/c mice produced IFN-gamma in response to beta-GAL. Taken together, these results demonstrate that priming of IFN-gamma-producing cells can occur in BALB/c mice despite the fact the animals are simultaneously mounting a potent Th2 response to L. major.

The role of phosphoglycans in Leishmania-sand fly interactions

Leishmania promastigotes synthesize an abundance of phosphoglycans, either attached to the cell surface through phosphatidylinositol anchors (lipophosphoglycan, LPG) or secreted as protein-containing glycoconjugates. These phosphoglycans are thought to promote the survival of the parasite within both its vertebrate and invertebrate hosts. The relative contributions of different phosphoglycan-containing molecules in Leishmania-sand fly interactions were tested by using mutants specifically deficient in either total phosphoglycans or LPG alone. Leishmania donovani promastigotes deficient in both LPG and protein-linked phosphoglycans because of loss of LPG2 (encoding the Golgi GDP-Man transporter) failed to survive the hydrolytic environment within the early blood-fed midgut. In contrast, L. donovani and Leishmania major mutants deficient solely in LPG expression because of loss of LPG1 (involved in biosynthesis of the core oligosaccharide LPG domain) had only a slight reduction in the survival and growth of promastigotes within the early blood-fed midgut. The ability of the LPG1-deficient promastigotes to persist in the midgut after blood meal excretion was completely lost, and this defect was correlated with their inability to bind to midgut epithelial cells in vitro. For both mutants, when phosphoglycan expression was restored to wild-type levels by reintroduction of LPG1 or LPG2 (as appropriate), then the wild-type phenotype was also restored. We conclude, first, that LPG is not essential for survival in the early blood-fed midgut but, along with other secreted phosphoglycan-containing glycoconjugates, can protect promastigotes from the digestive enzymes in the gut and, second, that LPG is required to mediate midgut attachment and to maintain infection in the fly during excretion of the digested blood meal.

Isolation of genes mediating resistance to inhibitors of nucleoside and ergosterol metabolism in Leishmania by overexpression/selection

We tested a general method for the identification of drug resistance loci in the trypanosomatid protozoan parasite Leishmania major. Genomic libraries in a multicopy episomal cosmid vector were transfected into susceptible parasites, and drug selections of these transfectant libraries yielded parasites bearing cosmids mediating resistance. Tests with two antifolates led to the recovery of cosmids encoding DHFR-TS or PTR1, two known resistance genes. Overexpression/selection using the toxic nucleoside tubercidin similarly yielded the TOR (toxic nucleoside resistance) locus, as well as a new locus (TUB2) conferring collateral hypersensitivity to allopurinol. Leishmania synthesize ergosterol rather than cholesterol, making this pathway attractive as a chemotherapeutic target. Overexpression/selection using the sterol synthesis inhibitors terbinafine (TBF, targeting squalene epoxidase) and itraconazole (ITZ, targeting lanosterol C(14)-demethylase) yielded nine new resistance loci. Several conferred resistance to both drugs; several were drug-specific, and two TBF-resistant cosmids induced hypersensitivity to ITZ. One TBF-resistant cosmid encoded squalene synthase (SQS1), which is located upstream of the sites of TBF and ITZ action in the ergosterol biosynthetic pathway. This suggests that resistance to "downstream" inhibitors can be mediated by increased expression of ergosterol biosynthetic intermediates. Our studies establish the feasibility of overexpression/selection in parasites and suggest that many Leishmania drug resistance loci are amenable to identification in this manner.

The Leishmania donovani LD1 locus gene ORFG encodes a biopterin transporter (BT1)

We have previously described two genes, ORFF and ORFG, from the LD1 locus near one telomere of chromosome 35, which are frequently amplified in Leishmania isolates. In Leishmania donovani LSB-51.1, gene conversion of the rRNA gene locus on chromosome 27 with these two genes resulted in their over-expression, because of their transcription by the RNA polymerase I-mediated rRNA promoter. The predicted ORFG protein has substantial sequence homology to the ESAG10 gene product from the Trypanosoma brucei VSG expression site and both are putative membrane proteins. Using successive rounds of gene replacement of the three ORFG genes in L. donovani LSB-51.1, ORFG null mutants were obtained. These mutant cell lines show a direct relationship between ORFG mRNA, protein expression levels and active transport of biopterin into the cells. Transformation of the null mutant with a plasmid containing ORFG restores biopterin transport activity. In addition, the null mutants are unable to grow in the absence of supplemental biopterin. Thus, ORFG encodes a biopterin transporter and has been renamed BTI.

Crystallization of recombinant Leishmania major pteridine reductase 1 (PTR1)

The enzyme pteridine reductase (PTR1) has recently been discovered in the protozoan parasite Leishmania and validated as a target for therapeutic intervention. PTR1 is responsible for the salvage of pteridines and also contributes to antifolate drug resistance. Structural analysis, in combination with ongoing biochemical characterization will assist the elucidation of the structure-activity relationships of this important enzyme and support a structure-based approach to discover novel inhibitors. Recombinant L. major PTR1 has been purified from an Escherichia coli expression system and used in crystallization experiments. Orthorhombic crystals have been obtained and data to 2.8 A has been measured. The space group is P2(1)2(1)2 or P2(1)2(1)2(1) with unit-cell dimensions of a = 103.9, b = 134.7, c = 96.2 A. One homotetramer, of molecular mass approximately 120 kDa, probably constitutes the asymmetric unit and gives a Matthews coefficient, V(m), of 2.8 A(3) Da(-1) and 56% solvent volume. Self-rotation function calculations show a single well defined non-crystallographic twofold axis with features that might represent additional elements of non-crystallographic symmetry. The detail of exactly what constitutes the asymmetric unit will be resolved by structure determination.

A set of independent selectable markers for transfection of the human malaria parasite Plasmodium falciparum

Genomic information is rapidly accumulating for the human malaria pathogen, Plasmodium falciparum. Our ability to perform genetic manipulations to understand Plasmodium gene function is limited. Dihydrofolate reductase is the only selectable marker presently available for transfection of P. falciparum. Additional markers are needed for complementation and for expression of mutated forms of essential genes. We tested parasite sensitivity to different drugs for which selectable markers are available. Two of these drugs that were very effective as antiplasmodial inhibitors in culture, blasticidin and geneticin (G418), were selected for further study. The genes BSD, encoding blasticidin S deaminase of Aspergillus terreus, and NEO, encoding neomycin phosphotransferase II from transposon Tn 5, were expressed under the histidine-rich protein III (HRPIII) gene promoter and tested for their ability to confer resistance to blasticidin or G418, respectively. After transfection, blasticidin and G418-resistant parasites tested positive for plasmid replication and BSD or NEO expression. Cross-resistance assays indicate that these markers are independent. The plasmid copy number and the enzymatic activity depended directly on the concentration of the drug used for selection. These markers set the stage for new methods of functional analysis of the P. falciparum genome.

Stage-specific activity of the Leishmania major CRK3 kinase and functional rescue of a Schizosaccharomyces pombe cdc2 mutant

Cell cycle control by cdc2-related kinases (CRKs) is essential to the regulation of cell proliferation and developmental processes in many organisms. Alternating phases of growth, arrest, and differentiation are characteristics of the infectious cycle of many trypanosomatid parasites, raising the possibility that members of the trypanosomatid CRK gene family participate in the regulation of these essential processes. Here we describe properties of the CRK3 gene from Leishmania major, which encodes a 36 kDa protein kinase showing 60% amino acid sequence identity with human CDK2, including several conserved sites implicated in regulation of kinase activity. CRK3 mRNA was constitutively expressed throughout the parasite life cycle, but histone H1 kinase activity of an epitope tagged CRK3 protein was greater in log-phase than in stationary-phase promastigotes. When integrated into the genome and expressed at the optimal level, CRK3 was able to rescue the growth defect of a Schizosaccharomyces pombe cdc2 mutant (cdc2-33(ts)), indicating that CRK3 is a functional homolog of cdc2. Mutants of CRK3 at several key regulatory residues showed the expected dominant negative effects on the S. pombe mutant. This is the first example of functional expression of a trypanosomatid CRK in yeast, opening the way for further genetic studies within this amenable organism.

Cloning of Leishmania nucleoside transporter genes by rescue of a transport-deficient mutant

All parasitic protozoa studied to date are incapable of purine biosynthesis and must therefore salvage purine nucleobases or nucleosides from their hosts. This salvage process is initiated by purine transporters on the parasite cell surface. We have used a mutant line (TUBA5) of Leishmania donovani that is deficient in adenosine/pyrimidine nucleoside transport activity (LdNT1) to clone genes encoding these nucleoside transporters by functional rescue. Two such genes, LdNT1.1 and LdNT1.2, have been sequenced and shown to encode deduced polypeptides with significant sequence identity to the human facilitative nucleoside transporter hENT1. Hydrophobicity analysis of the LdNT1.1 and LdNT1.2 proteins predicted 11 transmembrane domains. Transfection of the adenosine/pyrimidine nucleoside transport-deficient TUBA5 parasites with vectors containing the LdNT1.1 and LdNT1.2 genes confers sensitivity to the cytotoxic adenosine analog tubercidin and concurrently restores the ability of this mutant line to take up [3H]adenosine and [3H]uridine. Moreover, expression of the LdNT1.2 ORF in Xenopus oocytes significantly increases their ability to take up [3H]adenosine, confirming that this single protein is sufficient to mediate nucleoside transport. These results establish genetically and biochemically that both LdNT1 genes encode functional adenosine/pyrimidine nucleoside transporters.

Leishmania donovani has distinct mannosylphosphoryltransferases for the initiation and elongation phases of lipophosphoglycan repeating unit biosynthesis

Lipophosphoglycan (LPG) is the predominant surface glycoconjugate of Leishmania promastigotes and plays several roles in the infectious cycle of this protozoan parasite. The salient feature of LPG is the presence of 15-30 copies of a disaccharide-phosphate repeating unit Gal(beta1,4)Man(alpha1-PO4), which is also found on many other secreted molecules (secretory acid phosphatase, phosphoglycan, proteophosphoglycan). This structural diversity suggests that a multiplicity of enzymes mediating repeating unit addition may exist, especially for the mannosylphosphoryltransferases (MPTs), which initiate repeating unit synthesis. This work has taken a combined biochemical-genetic approach to resolve this issue. An lpg- mutant of Leishmania donovani, JEDI, was obtained by antibody selection against cells expressing a repeating unit epitope of LPG. Metabolic and surface labeling experiments revealed that JEDI cells accumulated a truncated form of LPG bearing only a single repeating unit: [Gal(beta 1,4)Man(alpha1-PO4)][Gal(alpha1,6)Gal(alpha1,3)Gal(f)(beta1,3)[Glc(alpha 1-PO4)]Man(alpha1,3)Man(alpha1,4)GlcN(alpha1,6)]-PI. Enzymatic assays of microsomal preparations showed that JEDI lacked MPT activity when tested with a repeating unit acceptor but retained wild-type levels of the MPT activity with an LPG glycan core acceptor. These data indicate that at least two distinct MPT activities are required for LPG repeating unit synthesis: one involved in the 'initiation' of repeating unit synthesis on the LPG core (iMPT), and a second (lacking in JEDI) participating in the 'elongation' phase of repeating unit addition (eMPT), leading to the mature full-length LPG.

Leishmania major pteridine reductase 1 belongs to the short chain dehydrogenase family: stereochemical and kinetic evidence

Pteridine reductase 1 (PTR1) is a novel broad spectrum enzyme of pterin and folate metabolism in the protozoan parasite Leishmania. Overexpression of PTR1 confers methotrexate resistance to these protozoa, arising from the enzyme's ability to reduce dihydrofolate and its relative insensitivity to methotrexate. The kinetic mechanism and stereochemical course for the catalyzed reaction confirm PTR1's membership within the short chain dehydrogenase/reductase (SDR) family. With folate as a substrate, PTR1 catalyzes two rounds of reduction, yielding 5,6,7, 8-tetrahydrofolate and oxidizing 2 equiv of NADPH. Dihydrofolate accumulates transiently during folate reduction and is both a substrate and an inhibitor of PTR1. PTR1 transfers the pro-S hydride of NADPH to carbon 6 on the si face of dihydrofolate, producing the same stereoisomer of THF as does dihydrofolate reductase. Product inhibition and isotope partitioning studies support an ordered ternary complex mechanism, with NADPH binding first and NADP+ dissociating after the reduced pteridine. Identical kinetic mechanisms and NAD(P)H hydride chirality preferences are seen with other SDRs. An observed tritium effect upon V/K for reduction of dihydrofolate arising from isotopic substitution of the transferred hydride was suppressed at a high concentration of dihydrofolate, consistent with a steady-state ordered kinetic mechanism. Interestingly, half of the binary enzyme-NADPH complex appears to be incapable of rapid turnover. Fluorescence quenching results also indicate the existence of a nonproductive binary enzyme-dihydrofolate complex. The nonproductive complexes observed between PTR1 and its substrates are unique among members of the SDR family and may provide leads for developing antileishmanial therapeutics.

Lipophosphoglycan (LPG) and the identification of virulence genes in the protozoan parasite Leishmania

Leishmania exploits several strategies to survive within the phagolysosome of vertebrate macrophages and be transmitted by sand fly vectors. Recent advances in functional genetic analysis provide a new avenue for identifying genes implicated in the infectious cycle of the parasite, such as those necessary for the synthesis and expression of the key surface glycoconjugate, lipophosphoglycan (LPG).

Biochemical and genetic tests for inhibitors of Leishmania pteridine pathways

The study of antifolate-resistant mutants of the protozoan parasite Leishmania has provided useful information about genetic processes such as gene amplification and mutation and knowledge of the unique features of the pteridine metabolic pathway in this primitive eukaryote. The novel bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is an essential enzyme, yet most DHFR-TS inhibitors show little promise as potential drugs. Leishmania possess a novel alternative pteridine reductase (PTR1) which is relatively insensitive to methotrexate. We have proposed that the ability of PTR1 to serve as a metabolic bypass and thus modulate drug inhibition of DHFR-TS activity may be responsible for the poor efficacy of many antifolates. In this work, we have sought inhibitors of L. major PTR1 from a collection of 74 compounds. The most potent inhibitors were also tested against L. major DHFR-TS and human DHFR and several compounds showing good activity for PTR1 alone, or for all three reductases, were identified. The activity of these compounds was tested against wild-type promastigotes, and those which were potent inhibitors of both PTR1 and DHFR-TS (but not those active against only PTR1) showed good potencies. Growth inhibition tests of L. major mutants, lacking PTR1 or DHFR-TS (ptr1- and dhfr-ts- knockouts) or overexpressing PTR1, were used as a "genetic screen" to assess whether these two pteridine reductases were targets in vivo. Remarkably, only one compound showed a methotrexate-like pattern of inhibition. Six compounds showed good inhibition of Leishmania growth regardless of PTR1 or DHFR-TS levels. These findings suggest that Leishmania cells contain multiple targets for a diverse set of antifolates, with one or more significant targets in addition to DHFR-TS and PTR1. This emphasizes the necessity of combined biochemical and genetic screens in efforts to rationally design chemotherapeutic strategies in Leishmania.

Insertional mutagenesis by a modified in vitro Ty1 transposition system

Transposable elements are useful tools for insertional mutagenesis and have many potential applications in the characterization of complex genomes. Here we describe a system which facilitates the construction of large transposon insertion libraries useful for genome sequencing and functional genomic analysis. We developed two transposons, TyK and TyK'GFP+, which can be introduced into target DNAs by Ty1-mediated transposition in vitro, and several modifications which decrease the frequency of false transposition events and direct the recovery of transpositions into passenger rather than vector DNA. Insertions of TyK'GFP+ additionally may yield fusions to the Aequorea green fluorescent protein (GFP), useful in studies of gene expression and protein targeting. Transposition in vitro was obtained into target DNAs of up to 50 kb in size, restriction mapping showed insertion to be relatively random, and the sequence of 55 insertion sites showed neither strong site nor base compositional preference. Our data suggest that TyK-based artificial transposons will be suitable for a variety of genetic applications in many organisms.

Trans-kingdom transposition of the Drosophila element mariner within the protozoan Leishmania

Transposable elements of the mariner/Tc1 family are postulated to have spread by horizontal transfer and be relatively independent of host-specific factors. This was tested by introducing the Drosophila mauritiana element mariner into the human parasite Leishmania major, a trypanosomatid protozoan belonging to one of the most ancient eukaryotic lineages. Transposition in Leishmania was efficient, occurring in more than 20 percent of random transfectants, and proceeded by the same mechanism as in Drosophila. Insertional inactivation of a specific gene was obtained, and a modified mariner element was used to select for gene fusions, establishing mariner as a powerful genetic tool for Leishmania and other organisms. These experiments demonstrate the evolutionary range of mariner transposition in vivo and underscore the ability of this ubiquitous DNA to parasitize the eukaryotic genome.

Physical mapping across the dihydrofolate reductase-thymidylate synthase chromosome of Leishmania major

We have used a chromosome-specific approach to generate a 300 kb long 'contig' across Leishmania major 500 kb chromosome. Clones from a 13-hit genomic library served as templates to generate end-specific probes that were used in hybridization to a high density array of the library. The 'contig' generated contained 12 markers uniformly spaced. Three restriction endonucleases were mapped within the map extending its resolution. Map extension indicated a peculiar feature of sequence organization in subtelomeric regions where chromosome-specificity of mapping is lost. End-probes generated from clones mapping to the extremes of a 300 kb 'contig' rescued a high percentage of 2 types of clones from the genomic library, 1 of which showed positive hybridization to the hexameric telomere repeat. Fine mapping at these regions revealed that these 2 clones contained elements common to all chromosomes of the parasite. The physical map generated constitutes ready-to-use data for the study of many aspects of genome organization. Being cloned in a shuttle vector, the genomic sequences reordered in the map can be used to generate genetic information by transfection into the parasite.

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.

Epitope cleavage by Leishmania endopeptidase(s) limits the efficiency of the exogenous pathway of major histocompatibility complex class I-associated antigen presentation

The activation of CD8+ T cell responses is commonplace during infection with a number of nonviral pathogens. Consequently, there has been much interest in the pathways of presentation of such exogenous antigens for major histocompatibility complex class I-restricted recognition. We had previously shown that Leishmania promastigotes transfected with the ovalbumin (OVA) gene could efficiently target OVA to the parasitophorous vacuole (PV), with subsequent recognition by class II-restricted T cells. We now report the results of studies aimed at evaluating the PV as a route of entry into the exogenous class I pathway. Bone marrow-derived macrophages can present soluble OVA (albeit at high concentrations) to the OVA(257-264)-specific T cell hybridoma 13.13. In contrast, infection with OVA-transfected Leishmania promastigotes failed to result in the stimulation of this hybridoma. This appeared unrelated to variables such as antigen concentration, parasite survival, and macrophage activation status. These results prompted an analysis of the effects of promastigotes on class I peptide binding using RMA-S cells and OVA(257-264). Our data indicate that the major surface protease of Leishmania, gp63, inhibits this interaction by virtue of its endopeptidase activity against the OVA(257-264) peptide. The data suggest that this activity, if maintained within the PV, would result in loss of the OVA(257-264) epitope. Although we can therefore draw no conclusions from these studies regarding the efficiency of the PV as a site of entry of antigen into the exogenous class I pathway, we have identified a further means by which parasites may manipulate the immune repertoire of their host.

Leishmania major: promastigotes induce expression of a subset of chemokine genes in murine macrophages

Recent studies suggest that Leishmania major promastigotes infect cultured macrophages in a stealthy fashion, activating little or no host gene expression and often interfering with the host's ability to respond to further stimulation. Here we examined macrophage transcription at early times following infection, when virulent parasites must execute steps required for survival. Stationary-phase promastigotes induced rapid and transient expression of transcripts of the chemokines JE (human MCAF/MCP-1) and KC (human GRO) in bone marrow-derived macrophages from BALB/c mice. JE and KC expression rose four- to sixfold shortly after infection and returned to uninduced levels by 4-24 hr. In contrast, chemokines MIP-1alpha, C10, and RANTES were not induced, nor were TGF-beta, IL-10, IL-12, or i-NOS. Chemokine induction did not occur following ingestion of latex beads, implicating a parasite-specific stimulus. Elevated expression of a subset of chemokines is the earliest known transcriptional response of macrophages to L. major infection and potentially may provide a signal for the initiation of downstream immunological responses which occur in vivo, such as cytokine induction and chemotaxis of monocytes and macrophages. Thus, Leishmania has a remarkable ability to take an active role in either inducing or preventing the expression of distinct sets of host genes during macrophage invasion and successful intracellular parasitism.

Functional identification of a Leishmania gene related to the peroxin 2 gene reveals common ancestry of glycosomes and peroxisomes

Glycosomes are membrane-bounded microbody organelles that compartmentalize glycolysis as well as other important metabolic processes in trypanosomatids. The compartmentalization of these enzymatic reactions is hypothesized to play a crucial role in parasite physiology. Although the metabolic role of glycosomes differs substantially from that of the peroxisomes that are found in other eukaryotes, similarities in signals targeting proteins to these organelles suggest that glycosomes and peroxisomes may have evolved from a common ancestor. To examine this hypothesis, as well as gain insights into the function of the glycosome, we used a positive genetic selection procedure to isolate the first Leishmania mutant (gim1-1 [glycosome import] mutant) with a defect in the import of glycosomal proteins. The mutant retains glycosomes but mislocalizes a subset glycosomal proteins to the cytoplasm. Unexpectedly, the gim1-1 mutant lacks lipid bodies, suggesting a heretofore unknown role of the glycosome. We used genetic approaches to identify a gene, GIM1, that is able to restore import and lipid bodies. A nonsense mutation was found in one allele of this gene in the mutant line. The predicted Gim1 protein is related the peroxin 2 family of integral membrane proteins, which are required for peroxisome biogenesis. The similarities in sequence and function provide strong support for the common origin model of glycosomes and peroxisomes. The novel phenotype of gim1-1 and distinctive role of Leishmania glycosomes suggest that future studies of this system will provide a new perspective on microbody biogenesis and function.

Golgi GDP-mannose uptake requires Leishmania LPG2. A member of a eukaryotic family of putative nucleotide-sugar transporters

The synthesis of glycoconjugates within the secretory pathway of eukaryotes requires the provision of lumenal nucleotide-sugar substrates. This is particularly important for eukaryotic microbes such as Leishmania because they must synthesize considerable amounts of extracellular and cell surface glycoconjugates that play significant roles in the infectious cycle. Here we used properly oriented sealed microsomes to characterize lumenal uptake of GDP-Man in Leishmania donovani. In this system, GDP-Man uptake was saturable with an apparent Km for GDP-Man of 0.3 microM and facilitated its use as a donor substrate for lipophosphoglycan (LPG) synthesis. A lpg2(-) deletion mutant showed loss of GDP-Man but not UDP-Gal uptake, which was restored by introduction of the gene LPG2. Immunoelectron microscopy localized an active, epitope-tagged LPG2 protein to the Golgi apparatus. Thus, LPG2 is required for nucleotide-sugar transport activity and probably encodes this Golgi transporter. LPG2 belongs to a large family of eukaryotic genes that potentially encode transporters with different substrate specificities and/or cellular locations. In the future, the amenability of the Leishmania system to biochemical and genetic manipulation will assist in functional characterization of nucleotide-sugar transports from this and other eukaryotes. Furthermore, since LPG2 plays an important role in the Leishmania infectious cycle and mammalian cells lack a Golgi GDP-Man transporter, this activity may offer a new target for chemotherapy.

Leishmania donovani possess a NADPH-dependent alkylglycerol cleavage enzyme

Leishmania parasites possess an abundance of ether-linked hydrocarbons as components of phospholipids and glycosylphosphatidylinositol anchors of glycoproteins and polysaccharides, including important surface molecules such as lipophosphoglycan (LPG) and glycosylinositolphospholipids (GIPLs). Cleavage of the ether bond is an important feature in the turnover pathway of alkylglycerols. In mammals, ether lipid cleavage activity requires a pteridine cofactor (H4 biopterin), suggesting the potential for linkage between the unusual Leishmania pteridine metabolic pathways and lipid metabolism. In this study, we partially purified and characterized an activity in L. donovani capable of cleaving the ether lipid 1-O-alkyl[3H]glycol. Unlike the mammalian enzyme but like that of Tetrahymena, the Leishmania enzyme required NADPH rather than H4-biopterin. The use of divergent cofactors by the parasite and mammalian enzymes may provide a basis for the design of anti-parasitic drugs targeting ether-linked lipid metabolism.

Selection against the dihydrofolate reductase-thymidylate synthase (DHFR-TS) locus as a probe of genetic alterations in Leishmania major

The genome of the trypanosomatid protozoan genus Leishmania has been shown to undergo a number of changes relevant to drug resistance and virulence, such as gene amplification, chromosomal rearrangement, and variation in ploidy. Experimental approaches to the study of genomic changes have in some cases been limited by the fact that Leishmania cells are asexual diploids, as are some other trypanosomatids, pathogenic fungi, and cultured mammalian cells. Here we report upon a system which permits the measurement of several types of genomic change occurring at the dihydrofolate reductase-thymidylate synthase (DHFR-TS) locus. First, we show that DHFR-TS can function as a positive/negative marker. We used selection against DHFR-TS on a heterozygous line (+/HYG) to generate colonies exhibiting both loss of heterozygosity and structural mutations in DHFR-TS, permitting the first measurement of mutation frequencies in this parasite. Loss of heterozygosity occurred at a frequency ranging from 10(-4) to 10(-6) and was elevated 24-fold by treatment with gamma-irradiation, while the frequency of other events was less than 10(-6) and was increased more than 1,000-fold by nitrosoguanidine treatment. The frequency of loss of heterozygosity relative to other processes such as mutation and gene replacement has important implications for genetic variability in natural Leishmania populations and the generation of both targeted and random mutations. We also developed a protocol for null targeting of diploid cells, in which transfection of a DHFR-TS deletion construct into Leishmania cells followed by negative selection yielded parasites lacking DHFR-TS or foreign sequences. The null-targeting method can be applied to any diploid cell, at any locus for which a negative selection exists. Such marker-free auxotrophic Leishmania cells show potential as an attenuated vaccine, and the methods developed here provide a new approach for manipulating and characterizing the plasticity of the Leishmania genome.

Use of the green fluorescent protein as a marker in transfected Leishmania

We have tested the suitability of the green fluorescent protein (GFP) of Aequorea victoria as a marker for studies of gene expression and protein targeting in the trypanosomatid parasite Leishmania. Leishmania promastigotes expressing GFP from episomal pXG vectors showed a bright green fluorescence distributed throughout the cell, readily distinguishable from control parasites. Transfection of a modified GFP gene containing GC-rich synonymous codons and the S65T mutation (GFP+) yielded a much higher fluorescence. FACS analysis revealed a clear quantitative separation between GFP-transfected and control parasites, with pXG-GFP+ transfectants showing fluorescence signals more than 100-fold background. Episomal DNAs could be recovered from small numbers of fixed cells, showing that GFP could be used as a convenient screenable marker for FACS separations. GFP was fused to the C-terminus of the LPG1 protein, which retained its ability to restore LPG expression when expressed in the lpg- R2D2 mutant of L. donovani. The LPG1(GFP) fusion was localized to a region situated between the nucleus and kinetoplast; its pattern was similar to that of LPG2, which is known to be located in the Golgi apparatus. This is notable as LPG1 participates in the biosynthesis of the glycan core of the LPG GPI anchor, whereas protein GPI anchor biosynthesis occurs in the endoplasmic reticulum. These studies suggest that the GFP will be a broadly useful marker in Leishmania.

Identification of genes mediating lipophosphoglycan biosynthesis by functional complementation of Leishmania donovani mutants

A powerful approach for identifying the genes involved in the infectious cycle of pathogens is functional genetic complementation. Here, the current status of this technology in Leishmania is reviewed, focusing on the genes involved in the biosynthesis of the unique parasite surface glycolipid, lipophosphoglycan (LPG). LPG plays multiple roles in the Leishmania infectious cycle, in both the sand fly vector and in establishing successful intracellular parasitism within the vertebrate macrophage. The emerging methods for generating LPG mutations and for recovering the affected gene(s) by complementation with an episomal genomic Leishmania DNA library are reviewed. The properties and probable roles of the first two genes identified by this methodology are discussed. These methods also show great promise in the search for genes affecting other virulence factors of Leishmania as well as in the identification of new drug-resistance loci.