Thymidine kinase as a negative selectable marker in Leishmania major

Joining forces: first application of a rapamycin-induced dimerizable Cre system for conditional null mutant analysis in Leishmania

Reverse genetics in Leishmania spp has gained importance beyond basic research as efforts increase to discover and validate new drug targets. Often, the most promising targets are essential for viability of the parasites, defying a genetic analysis by current gene replacement strategies. Duncan et al. demonstrate the applicability of DiCre recombination in Leishmania for induced replacement of the kinase CRK3 gene in promastigotes. DiCre gene replacement leads to the rapid loss of the gene and allows monitoring the phenotypic effects of the loss of function, eliminating the need for prolonged cultivation and selection. Implementation of the DiCre approach will allow functional genetics of the most important of Leishmania genes and is likely to boost genetic research and drug target discovery.

The genetic toolbox for Leishmania parasites

Leishmania parasites cause a variety of devastating diseases in tropical areas around the world. Due to the lack of vaccines and limited availability of drugs, new therapeutic targets are urgently needed. A variety of genetic tools have been developed to investigate the complex biology of this parasite and its interactions with the host. One of the main techniques is the generation of knock-out parasites via targeted gene replacement, a process that takes advantage of the parasites ability to undergo homologous recombination. Studying the effect of gene deletions in vitro and in infectivity models in vivo allows understanding the function of a target gene and its potential as a therapeutic target. Other genetic manipulations available include episomal and chromosomal complementation and the generation of overproducer strains. However, there are also limitations, such as the lack of RNA interference machinery in most Leishmania species and limited options for inducible expression systems. The genomes of several Leishmania species have now been sequenced and will provide powerful resources in combination with the genetic tools that are available. The increasing knowledge of parasite biology and host parasite interactions derived from these studies will raise the number of potential therapeutic targets, which are sorely needed to combat leishmaniasis.

Molecular genetic analysis of purine nucleobase transport in Leishmania major

Leishmania major and all other parasitic protozoa are unable to synthesize purines de novo and are therefore reliant upon uptake of preformed purines from their hosts via nucleobase and nucleoside transporters. L. major expresses two nucleobase permeases, NT3 that is a high affinity transporter for purine nucleobases and NT4 that is a low affinity transporter for adenine. nt3((-/-)) null mutant promastigotes were unable to replicate in medium containing 10 microM hypoxanthine, guanine, or xanthine and replicated slowly in 10 microM adenine due to residual low affinity uptake of that purine. The NT3 transporter mediated the uptake of the anti-leishmanial drug allopurinol, and the nt3((-/-)) mutants were resistant to killing by this drug. Expression of the NT3 permease was profoundly downregulated at the protein but not the mRNA level in stationary phase compared with logarithmic phase promastigotes. The nt4((-/-)) null mutant was quantitatively impaired in survival within murine bone marrow-derived macrophages. Extensive efforts to generate an nt3((-/-))/nt4((-/-)) dual null mutant were not successful, suggesting that one of the two nucleobase permeases must be retained for robust growth of the parasite. The phenotypes of these null mutants underscore the importance of purine nucleobase transporters in the Leishmania life cycle and pharmacology.

The use of Tn5 transposable elements in a gene trapping strategy for the protozoan Leishmania

The use of transposable elements as a gene-trapping strategy is a powerful tool for gene discovery. Herein we describe the development of a transposable system, based on the bacterial Tn5 transposon, which has been used successfully in Leishmania braziliensis. The transposon carries the neomycin phosphotransferase gene, which is expressed only when inserted in-frame with a Leishmania gene present in the target DNA. Four cosmid clones from a L. braziliensis genomic library were used as targets in transposition reactions and four insertional libraries were constructed and transfected in L. braziliensis. Clones resistant to G418 were selected and analysed by immunofluorescence in order to identify the subcellular localisation of the protein coded by the trapped gene. A definitive subcellular localisation for neomycin phosphotransferase/targeted protein fusion was not obtained in any of the four Leishmania clones investigated. However, the constructed transposable element is highly efficient considering the frequency of insertion in large targets and is therefore a useful tool for functional genetic studies in Leishmania. Our data confirm the utility of the Tn5 transposon system for insertion of sequencing priming sites into target DNA. Furthermore, the high frequency of insertion and even distribution are important in studying genomic regions bearing long and polymorphic repetitive sequences.

Construction and analysis of Leishmania tarentolae transgenic strains free of selection markers

The trypanosomatid protozoan Leishmania tarentolae has been extensively used as a model system for studying causative agents of several tropical diseases and more recently as a host for recombinant protein production. Here we analyze the rates of partial or complete deletions of expression cassettes integrated into small ribosomal RNA and tubulin gene clusters as well as into ornithine decarboxylase gene of L. tarentolae. In approximately 60% of cases gene conversion was responsible for the deletion while in the rest of the cases deletion occurred within the expression cassette. We used this observation to design constitutive and inducible expression vectors that could be stably integrated into the genome and subsequently depleted of the antibiotic resistance genes using thymidine kinase or bleomycin resistance genes as negative selection markers. This enabled us to obtain L. tarentolae strains containing constitutive or inducible markerless expression cassettes. Analysis of the markerless strains demonstrated that although stability varied among clones some were stable for as many as 200 generations. We expect that this approach will be useful for the construction of strains carrying multiple expression cassettes for analysis of trypanosomatid pathogenicity mechanisms and overexpression of multi-subunit protein complexes for biochemical and structural studies.

Development of a recombinant Leishmania major strain sensitive to ganciclovir and 5-fluorocytosine for use as a live vaccine challenge in clinical trials

To provide a safer live challenge strain for use in clinical vaccine trials, a double drug sensitive strain of Leishmania major was derived using advances in gene targeting technology by stably introducing into the chromosome a modified HSV-1 thymidine kinase gene (tk), conferring increased sensitivity to ganciclovir (GCV), and a Saccharomyces cerevisiae cytosine deaminase gene (cd), conferring sensitivity to 5-fluorocytosine (5-FC). In vitro studies showed that the homozygous L. major (tk-cd+/+) promastigotes were killed by either drug alone, and together the drugs acted synergistically. In vivo infection studies showed that progressively growing lesions in BALB/c mice, caused by L. major (tk-cd+/+), were completely cured by 2 weeks of treatment with either drug alone or in combination. Treated animals showed no signs of reoccurrence of infection for at least 4 months when the experiments were terminated.

New selectable markers and single crossover integration for the highly versatile Plasmodium knowlesi transfection system

Plasmodium knowlesi provides a highly versatile transfection system for malaria, since it enables rapid genetic modification of the parasite both in vivo as well as in vitro. However, it is not possible to perform multiple genetic manipulations within one parasite line because of a lack of selectable markers. In an effort to develop additional selectable markers for this parasite, positive and negative selectable markers that have recently been successfully used in Plasmodium falciparum were tested. It was shown that the positive selectable markers human dihydrofolate reductase (hdhfr), blasticidin S deaminase (bsd) and neomycin phosphotransferase II (neo) all conferred drug resistance to P. knowlesi when introduced as episomes. The plasmid containing the hdhfr selectable marker was not only successfully introduced as circular form, but also as linear fragment, demonstrating for the first time single crossover integration in P. knowlesi. Thymidine kinase was tested for its potential as negative selectable marker and it was shown that recombinant P. knowlesi parasites expressing thymidine kinase from episomes were highly sensitive to ganciclovir compared to wild-type P. knowlesi. The availability of new positive selectable markers and a strong candidate for a negative selectable marker for P. knowlesi, in combination with the opportunity to perform targeted single crossover integration in P. knowlesi, significantly increases the flexibility of this transfection system, making it one of the most versatile systems available for Plasmodium.

Molecular determinants and regulation of Leishmania virulence

A Leishmania model to explain microbial virulence in chronic infectious diseases is proposed. All these diseases progress from infection to symptomatic phase to host death or recovery. The outcome of each phase is depicted to result from the interactions of a distinct group of parasite molecules with a specific host immune compartment. The first group consists of invasive/evasive determinants, which are largely parasite cell surface and secreted molecules. Their activities help parasites establish infection by overcoming host immunologic and non-immunologic barriers. These determinants do not cause disease per se, but are indispensable for infection necessary for the development of a disease-state. The second group of parasite molecules consists of "pathoantigenic" determinants - unique parasite epitopes present often within otherwise highly conserved cytoplasmic molecules. Immune response against these determinants is thought to result in immunopathology manifested as clinical signs or symptoms, namely the virulent phenotype. The third group of parasite molecules is hypothetically perceived as vaccine determinants. Their interactions with the host immune system lead to the elimination or reduction of parasites to effect a clinical cure. Differential expression of these determinants alone by parasites may alter their interactions with the hosts. Virulent phenotype is consequently presented as a spectrum of manifestations from asymptomatic infection to fatality. A secondary level of regulation lies in host genetic and environmental factors. The model suggests that different parasite determinants may be targeted by different strategies to achieve more effective control of leishmaniasis and other similar diseases.

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 African Green Monkey model for cutaneous and visceral leishmaniasis

Non-human primates are valuable models for biomedical research because of their similarities to human anatomy, immunology and physiology. Leishmaniasis, a disease caused by protozoan parasites, has a worldwide distribution and results in high morbidity and mortality. Availability of a non-human primate model of leishmaniasis would facilitate the study of different aspects of this disease and would accelerate the development of vaccines and new drugs. In this article, some interesting features of the vervet monkey (African Green monkey) model of human cutaneous and visceral leishmaniasis are discussed.

Genetic manipulation of kinetoplastida

During the 1980s, many kinetoplastid genes were cloned and their function inferred from homology with genes from other organisms, location of the corresponding proteins or expression in heterologous systems. Up until 1990, before the availability of DNA transfection methodology, we could not analyze the function of kinetoplastid genes within the organisms themselves. Since then, it has become possible to create and complement mutants, to overexpress foreign proteins in the parasites, to knock out genes and even to switch off essential functions. However, these methods are not equally applicable in all parasites. Here, Christine Clayton highlights the differences and similarities between the most commonly used model organisms, and assesses the relative advantages of different approaches and parasites for different types of investigation.

Inducible expression of suicide genes in Leishmania donovani amastigotes

This study tests the feasibility of using the A2 gene regulatory system to create a Leishmania cell line in which attenuation is developmentally regulated when the parasite differentiates from promastigotes to amastigotes. The Leishmania donovani- inducible A2 gene regulatory system was used to differentially express in amastigotes two potential suicide genes: a truncated version of the L. donovani 3'-nucleotidase/nuclease expressed in the cytoplasm and the herpes simplex virus thymidine kinase gene. These genes were inserted between A2 noncoding regulatory sequences for up-regulation of expression in amastigotes. The accumulation of toxic products affected L. donovani cell replication and viability both in vitro and in vivo. The inducible expression of toxic gene products represents a valuable tool for the development of safe and effective vaccines.

A cell cycle model for the tachyzoite of Toxoplasma gondii using the Herpes simplex virus thymidine kinase

Toxoplasma gondii (RH strain) tachyzoites were transfected with a plasmid containing a fusion of the chloramphenicol acetyl transferase and the Herpes simplex virus-2 thymidine kinase coding regions and transgenic parasites obtained by chloramphenicol selection. CTK11, a single high expressing clone was isolated based on immunofluorescence and contained approximately five integrated copies of the fusion sequence. Lysates prepared from this clone displayed thymidine kinase activity of 2.9 pmol min(-1) microg(-1) protein, whereas thymidine kinase activity was not detected in lysates from the parental RH strain. Growth of CTK11 tachyzoites was fully inhibited in 5 microM ganciclovir and thymidine and in 2.5 microM 5-bromo-2'-deoxyuridine. While the inhibitory effects of ganciclovir were lethal, low concentrations of thymidine (10 microM) were largely reversible. Asynchronously growing CTK11 tachyzoites were found to contain major G1 (1 N) and S phase (1 N+) distributions as determined by relative propidium iodide fluorescence and with reference to the haploid (1 N) DNA content of a T. gondii sporozoite population. CTK11 tachyzoites blocked 4 h in 10 microM thymidine exhibited mean fluorescence consistent with a 1 N complement of DNA indicating growth was arrested in G1. Following the removal of excess thymidine, parasites immediately entered S phase, thus confirming the late G1 block. Parasites with a 2 N complement of DNA (G2 + M) first appear at 2 h post-release, while 1 N (G1) parasites re-appear at 3 h suggesting the length of S phase is < or = 2 h and that of G2 + M is < or = 1 h. Within 7 h, parasites had transited G2 + M and much of G1 and re-entered S of the subsequent cell cycle--a time consistent with the doubling of these parasites in culture. Thus, the CTK11 tachyzoite cell cycle is similar to those of higher eukaryotic cells and is characterized by major G1 and S phases and a relatively short G2 + M.

Trypanosoma cruzi: use of herpes simplex virus-thymidine kinase as a negative selectable marker

Trypanosoma cruzi, the protozoan that causes Chagas' disease, was transfected with a fusion gene of hygromycin phosphotransferase and herpes simplex virus-thymidine kinase, HyTK. Transfectants selected in hygromycin had thymidine kinase activity, whereas controls did not. In vitro growth of the mammalian life-stage forms, amastigotes and trypomastigotes, was inhibited 98% by the nucleoside analogue ganciclovir (5 micrograms/ml). Growth of the insect-stage form, epimastigotes, was not inhibited by ganciclovir (up to 250 micrograms/ml) or other nucleoside analogues. Intracellular uptake of ganciclovir by epimastigotes was found to be 10-fold less than that by amastigotes. Mice infected with the HyTK-expressing parasites and treated with ganciclovir had a statistically significant reduction of parasitemia by 57%; however, complete eradication of parasites was not achieved. The parasites recovered from the treated mice continued to be susceptible to ganciclovir in vitro. Parasite clones with higher expression of thymidine kinase were more sensitive to ganciclovir, suggesting that greater expression of the thymidine kinase gene may lead to parasites that can be fully eradicated from infected experimental animals.

Selective killing of Leishmania amastigotes expressing a thymidine kinase suicide gene

The thymidine kinase gene of Herpes simplex type-1 virus was transfected into several Leishmania species to create drug-sensitive mutants. Expression of the thymidine kinase gene is not by itself harmful to Leishmania cells but it is capable of phosphorylating ganciclovir, a nucleoside analog, into a highly toxic product. In addition to the generation of Leishmania promastigotes highly sensitive to ganciclovir, the thymidine kinase gene was expressed similarly by amastigotes engulfed either by murine or by human macrophages. Leishmania major amastigotes expressing thymidine kinase were eliminated by 85% when treated with ganciclovir. Selective killing of parasites expressing suicide genes at their infective stage could suggest novel strategies for controlling parasitic infections.

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.

The viral thymidine kinase gene as a tool for the study of mutagenesis in Trypanosoma brucei

We have tested the use of thymidine kinase as a negative selection system for Trypanosoma brucei. To this end we have targeted a construct containing a Herpes simplex virus thymidine kinase (TK) gene into the ribosomal DNA array of procyclic T. brucei. This resulted in TK activity 30-50-fold above background and in susceptibility to the nucleoside analogues ganciclovir, ethyl-deoxyuridine and 1-[2-deoxy,2-fluoro-8-D-arabinofuranosyl]-5-iodouracil, all of which have no effect on wild-type trypanosomes. TK+ trypanosomes, however, reverted to a ganciclovir resistant phenotype at a rate of 10(-6) per cell-generation. A similar reversion rate was observed using the Varicella-zoster virus TK gene. Loss of TK activity was not due to detectable DNA rearrangements or a decrease in TK mRNA. Sequence analysis of the revertant genes demonstrated, however, the occurrence of point mutations and frameshifts. One revertant line had a mutation in the thymidine binding site leading to the substitution of a conserved arginine by a glycine. Other mutations included single base insertion, single base deletion and the introduction of a premature termination codon by point mutation.