An expressed sequence tag analysis of a full-length, spliced-leader cDNA library from Leishmania major promastigotes

Trypanosomatid comparative genomics: Contributions to the study of parasite biology and different parasitic diseases

In 2005, draft sequences of the genomes of Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, also known as the Tri-Tryp genomes, were published. These protozoan parasites are the causative agents of three distinct insect-borne diseases, namely sleeping sickness, Chagas disease and leishmaniasis, all with a worldwide distribution. Despite the large estimated evolutionary distance among them, a conserved core of ~6,200 trypanosomatid genes was found among the Tri-Tryp genomes. Extensive analysis of these genomic sequences has greatly increased our understanding of the biology of these parasites and their host-parasite interactions. In this article, we review the recent advances in the comparative genomics of these three species. This analysis also includes data on additional sequences derived from other trypanosmatid species, as well as recent data on gene expression and functional genomics. In addition to facilitating the identification of key parasite molecules that may provide a better understanding of these complex diseases, genome studies offer a rich source of new information that can be used to define potential new drug targets and vaccine candidates for controlling these parasitic infections.

A prime/boost DNA/Modified vaccinia virus Ankara vaccine expressing recombinant Leishmania DNA encoding TRYP is safe and immunogenic in outbred dogs, the reservoir of zoonotic visceral leishmaniasis

Previous studies demonstrated safety, immunogenicity and efficacy of DNA/modified vaccinia virus Ankara (MVA) prime/boost vaccines expressing tryparedoxin peroxidase (TRYP) and Leishmania homologue of the mammalian receptor for activated C kinase (LACK) against Leishmania major challenge in mice, which was consistent with results from TRYP protein/adjuvant combinations in non-human primates. This study aimed to conduct safety and immunogenicity trials of these DNA/MVA vaccines in dogs, the natural reservoir host of Leishmania infantum, followed-up for 4 months post-vaccination. In a cohort of 22 uninfected outbred dogs, blinded randomised administration of 1000 microg (high dose) or 100 microg (low dose) DNA prime (day 0) and 1x10(8)pfu MVA boost (day 28) was shown to be safe and showed no clinical side effects. High dose DNA/MVA vaccinated TRYP dogs produced statistically higher mean levels of the type-1 pro-inflammatory cytokine IFN-gamma than controls in whole blood assays (WBA) stimulated with the recombinant vaccine antigen TRYP, up to the final sampling at day 126, and in the absence of challenge with Leishmania. TRYP vaccinated dogs also demonstrated significantly higher TRYP-specific total IgG and IgG2 subtype titres than in controls, and positive in vivo intradermal reactions at day 156 in the absence of natural infection, observed in 6/8 TRYP vaccinated dogs. No significant increases in IFN-gamma in LACK-stimulated WBA, or in LACK-specific IgG levels, were detected in LACK vaccinated dogs compared to controls, and only 2/9 LACK vaccinated dogs demonstrated DTH responses at day 156. In all groups, IgG1 subclass responses and antigen-specific stimulation of IL-10 were similar to controls demonstrating an absence of Th2/T(reg) response, as expected in the absence of in vivo restimulation or natural/experimental challenge with Leishmania. These collective results indicate significant antigen-specific type-1 responses and in vivo memory phase cellular immune responses, consistent with superior potential for protective vaccine immunogenicity of DNA/MVA TRYP over LACK.

Generating EST libraries: trans-spliced cDNAs

Eukaryotes using trans-splicing for transcript processing incorporate a taxon-specific sequence tag (the spliced leader, SL) to a proportion (either all or a fraction) of their mRNAs. This feature may be exploited for the preparation of full-length-enriched cDNA libraries from these organisms (a diverse group including euglenozoa and dinoflagellates, as well as members from five metazoan phyla: Cnidaria, Rotifera, Nematoda, Platyhelminths and Chordata). The strategy has indeed been widely used to construct cDNA libraries for the generation of ESTs, mainly from parasitic euglenozoa and helminths.We describe a set of optimised protocols to prepare directional SL-cDNA libraries; the method involves PCR-amplification of SL-cDNA and its subsequent cloning in a plasmid vector under a specific orientation. It uses small amounts of total RNA as starting material and may be applied to a variety of samples. The approach permits the selective cloning of mRNAs tagged with a particular SL from mixtures including large amounts of non-trans-spliced mRNAs. Thus, it allows exclusion of host contamination when isolating SL-cDNAs from parasitic organisms, and has other potential applications, such as the characterisation of the trans-spliced transcriptome from organisms in mixed pools of species.

Heterologous priming-boosting with DNA and modified vaccinia virus Ankara expressing tryparedoxin peroxidase promotes long-term memory against Leishmania major in susceptible BALB/c Mice

Leishmaniasis affects 12 million people, but there are no vaccines in routine clinical use. Th1 polarizing vaccines that elicit long-term protection are required to prevent disease in susceptible populations. We recently showed that heterologous priming-boosting with tryparedoxin peroxidase (TRYP) DNA followed by TRYP-modified vaccinia virus Ankara (TRYP MVA) protected susceptible BALB/c mice from Leishmania major. Here we compared treatment with TRYP DNA with treatment with TRYP DNA/TRYP MVA. We found that equivalent levels of protection during the postvaccination effector phase correlated with equivalent levels of serum immunoglobulin G2a and gamma interferon (IFN-gamma) in draining lymph nodes. In contrast, challenge infection during the memory phase revealed that there was enhanced clinical efficacy with TRYP DNA/TRYP MVA. This correlated with higher levels of effector phase splenic IFN-gamma, sustained prechallenge levels of memory phase IFN-gamma, and a more polarized post-L. major challenge Th1 response compared to the Th2/T(reg) response. Thus, TRYP DNA/TRYP MVA, but not TRYP DNA alone, provides long-term protection against murine leishmaniasis.

Interleukin-4 (IL-4) and IL-10 collude in vaccine failure for novel exacerbatory antigens in murine Leishmania major infection

Leishmaniasis affects 12 million people but there are no vaccines in routine use. Recently, we used DNA vaccination in a susceptible BALB/c high-dose model of infection to screen 100 novel Leishmania major genes as vaccine candidates. In addition to finding novel protective antigens, we identified several antigens that reproducibly exacerbated disease. Here we examined the immune response to two of these antigens, lmd29 and 584C, that were originally identified in an expressed sequence tag cDNA sequencing project. We show that, in addition to exacerbating disease in susceptible BALB/c mice, these antigens retain a propensity to exacerbate disease in resistant C57BL/6 mice. This ability to exacerbate disease was lost when susceptible BALB/c mice were rendered resistant by disruption of the genes encoding interleukin-4 (IL-4) alone, IL-4/IL-13, or IL-4, IL-5, IL-9, and IL-13. Failure to exacerbate disease was associated with reduced IL-5 and IL-10 production in IL-4 knockout mice. Treatment of lmd29-vaccinated mice with anti-IL-10 receptor antibody prior to challenge infection converted exacerbation in wild-type BALB/c mice into highly significant antigen-specific protection. These studies demonstrate that some highly immunogenic antigens of L. major, while having an intrinsic capacity to exacerbate disease in the context of otherwise T helper 1-promoting DNA vaccine delivery, can be rendered protective by the removal of functional IL-10.

IL-10 from Regulatory T Cells Determines Vaccine Efficacy in MurineLeishmania majorInfection

Leishmaniasis affects 12 million people, but there are no vaccines. Immunological correlates of vaccine efficacy are unclear. Polarized Th1 vs Th2 responses in Leishmania major-infected mice suggested that a shift in balance from IL-4 to IFN-gamma was the key to vaccine success. Recently, a role for IL-10 and regulatory T cells in parasite persistence was demonstrated, prompting re-evaluation of vaccine-induced immunity. We compared DNA/modified vaccinia virus Ankara heterologous prime-boost with Leishmania homolog of the receptor for activated C kinase (LACK) or tryparedoxin peroxidase (TRYP). Both induced low IL-4 and high IFN-gamma prechallenge. Strikingly, high prechallenge CD4 T cell-derived IL-10 predicted vaccine failure using LACK, whereas low IL-10 predicted protection with TRYP. The ratio of IFN-gamma:IL-10 was thus a clear prechallenge indicator of vaccine success. Challenge infection caused further polarization to high IL-10/low IFN-gamma with LACK and low IL-10/high IFN-gamma with TRYP. Ex vivo quantitative RT-PCR and in vitro depletion and suppression experiments demonstrated that Ag-driven CD4+ CD25+ T regulatory 1-like cells were the primary source of IL-10 in LACK-vaccinated mice. Anti-IL-10R treatment in vivo demonstrated that IL-10 was functional in determining vaccine failure, rendering LACK protective in the presence of high IFN-gamma/low IL-5 responses.

Current understandings on the immunology of leishmaniasis and recent developments in prevention and treatment

Leishmaniasis is a major tropical disease with a wide clinical spectrum of cutaneous, mucocutaneous and visceral involvement. Presentation is often varied and diagnosis can be challenging. The outcome of infection is determined by the parasite species and the host's immunological response. The CD4+ T helper cell is critical with animal models demonstrating that cure is associated with strong IFN-gamma, interleukin (IL)-2 and IL-12 responses in the absence of classical Th2 cytokines or IL-10. Prevention has focussed on vector control, control of animal reservoirs and efforts to develop a protective vaccine. Treatment options historically have relied on antimonials though agents with better tolerability and efficacy have been developed including amphotericin and the oral agent miltefosine. Drug resistance, human immunodeficiency virus and changes in vector epidemiology threaten recent advances. Renewed impetus led by the WHO is required to co-ordinate future international effort to develop new drugs and ultimately a vaccine.

DNA-Salmonella enterica serovar Typhimurium primer-booster vaccination biases towards T helper 1 responses and enhances protection against Leishmania major infection in mice

Successful resolution of infections by intracellular pathogens requires gamma interferon (IFN-gamma). DNA vaccines promote T helper 1 (Th1) responses by triggering interleukin-12 (IL-12) release by dendritic cells (DC) through Toll-like receptor 9 (TLR9). In humans TLR9 is restricted to plasmacytoid DC. Here we show that DNA-Salmonella enterica serovar Typhimurium primer-booster vaccination, which provides alternative ligands to bind TLR4 on myeloid DC, strongly biases towards Th1 responses compared to vaccination with DNA alone. This results in higher immunoglobulin G2a (IgG2a) responses compared to IgG1 responses, higher IFN-gamma responses compared to IL-10 CD4(+)-T-cell responses, and enhanced protection against Leishmania major infection in susceptible BALB/c mice.

Generation and analysis of expressed sequence tags from Trypanosoma cruzi trypomastigote and amastigote cDNA libraries

We have generated 2771 expressed sequence tags (ESTs) from two cDNA libraries of Trypanosoma cruzi CL-Brener. The libraries were constructed from trypomastigote and amastigotes, using a spliced leader primer to synthesize the cDNA second strand, thus selecting for full-length cDNAs. Since the libraries were not normalized nor pre-screened, we compared the representation of transcripts between the two using a statistical test and identify a subset of transcripts that show apparent differential representation. A non-redundant set of 1619 reconstructed transcripts was generated by sequence clustering. This dataset was used to perform similarity searches against protein and nucleotide databases. Based on these searches, 339 sequences could be assigned a putative identity. One thousand one-hundred and sixteen sequences in the non-redundant clustered dataset (68.8%) are new expression tags, not represented in the T. cruzi epimastigote ESTs that are in the public databases. Additional information is provided online at http://genoma.unsam.edu.ar/projects/tram. To the best of our knowledge these are the first ESTs reported for the life cycle stages of T. cruzi that occur in the vertebrate host.

Trypanosoma rangeli Transcriptome Project: Generation and analysis of expressed sequence tags

Trypanosoma rangeli is an important hemoflagellate parasite of several mammalian species in Central and South America, sharing geographical areas, vectors and reservoirs with T. cruzi, the causative agent of Chagas disease. Thus, the occurrence of single and/or mixed infections, including in humans, must be expected and are of great importance for specific diagnosis and epidemiology. In comparison to several Trypanosomatidae species, the T. rangeli biology and genome are little known, reinforcing the needs of a gene discovery initiative. The T. rangeli transcriptome initiative aims to promote gene discovery through the generation of expressed sequence tags (ESTs) and Orestes (ORF ESTs) from both epimastigote and trypomastigote forms of the parasite, allowing further studies of the parasite biology, taxonomy and phylogeny.

Reverse transcription-polymerase chain reaction construction of plasmid-based, full-length cDNA libraries from Leishmania infantum for in vitro expression screening

We describe a streamlined reverse transcription-polymerase chain reaction methodology for constructing full-length cDNA libraries of trypanosomatids on the basis of conserved sequences located at the 5' and 3'ends of trans-spliced mRNAs. The amplified cDNA corresponded to full-length messengers and was amenable to in vitro expression. Fractionated libraries could be rapidly constructed in a plasmid vector by the TA cloning method (Invitrogen). We believe this is useful when there are concerns over the use of restriction enzymes and phage technology as well as in cases where expression of proteins in their native conformation is desired.

From genomes to vaccines: Leishmania as a model

The 35 Mb genome of Leishmania should be sequenced by late 2002. It contains approximately 8500 genes that will probably translate into more than 10 000 proteins. In the laboratory we have been piloting strategies to try to harness the power of the genome-proteome for rapid screening of new vaccine candidate. To this end, microarray analysis of 1094 unique genes identified using an EST analysis of 2091 cDNA clones from spliced leader libraries prepared from different developmental stages of Leishmania has been employed. The plan was to identify amastigote-expressed genes that could be used in high-throughput DNA-vaccine screens to identify potential new vaccine candidates. Despite the lack of transcriptional regulation that polycistronic transcription in Leishmania dictates, the data provide evidence for a high level of post-transcriptional regulation of RNA abundance during the developmental cycle of promastigotes in culture and in lesion-derived amastigotes of Leishmania major. This has provided 147 candidates from the 1094 unique genes that are specifically upregulated in amastigotes and are being used in vaccine studies. Using DNA vaccination, it was demonstrated that pooling strategies can work to identify protective vaccines, but it was found that some potentially protective antigens are masked by other disease-exacerbatory antigens in the pool. A total of 100 new vaccine candidates are currently being tested separately and in pools to extend this analysis, and to facilitate retrospective bioinformatic analysis to develop predictive algorithms for sequences that constitute potentially protective antigens. We are also working with other members of the Leishmania Genome Network to determine whether RNA expression determined by microarray analyses parallels expression at the protein level. We believe we are making good progress in developing strategies that will allow rapid translation of the sequence of Leishmania into potential interventions for disease control in humans.

mRNA trans-splicing in the human parasitic cestode Echinococcus multilocularis

An identical 36-nucleotide exon was identified at the 5' termini of different mRNAs from the cestode Echinococcus multilocularis. We provide evidence that this exon constitutes a new spliced leader (SL) that is obviously trans-spliced to echinococcal pre-mRNAs, donated by a non-polyadenylated, trimethylguanosine-capped SL-RNA of 104 nucleotides. Sequence comparisons indicated that cestode and trematode SLs are likely to be derived from a common ancestor gene. No conservation was, however, observed concerning the spectrum of mRNAs that is trans-spliced in cestodes and trematodes, indicating that trans-splicing of a particular flatworm mRNA is not correlated with the function of the encoded protein. We also show that the echinococcal gene elp, encoding a member of the ezrin/radixin/moesin protein family, is expressed via two alternative transcripts, spliced either cis or trans at an identical splice acceptor site. This was accompanied by the formation of different elp primary transcripts, harboring a complete or a truncated upstream intron, which supports the hypothesis that alternative cis/trans-splicing depends on the presence or absence of an upstream splice donor site. A putative SL gene was also identified on chromosomal DNA of Echinococcus granulosus, indicating widespread utilization of trans-splicing in the genus.

A European perspective on Neospora caninum

Since the identification of Neospora caninum in 1984 as a parasite separate from Toxoplasma gondii by Bjerkas et al., and its subsequent characterization and classification in 1988 by Dubey and co-workers, this parasite has attracted increasing attention, primarily as an important causative agent of abortion in cattle and neuromuscular disease in dogs, but also as a complementary model system to T. gondii for investigating the basic biology of intracellular parasitism. During November 11-14, 1999, the COST 820 Annual meeting (Vaccines against coccidioses) took place in Interlaken, Switzerland. Almost half of the papers presented at that meeting were on N. caninum and neosporosis, reflecting the increasing awareness of the importance of this parasite on part of the scientific community in Europe. On the occasion of the meeting, participants in this COST Action involved in Neospora research in Europe were asked to participate in this invited review in order to document the growing interest in N. caninum and the disease it causes. Thus, this paper is a unique collection of contributions provided by several European experts in the field. It is comprised of 10 reviews or original papers on different aspects of Neospora research including epidemiology, immunology, application and development of serological tools, and molecular characterisation of the parasite currently carried out throughout Europe. In addition, two distinguished invited speakers from overseas (Milton McAllister and John Ellis) provided valuable contributions. This invited review demonstrates that the COST 820 Action has brought together scientists from all over Europe and other parts of the world, and has laid the basis for many fruitful collaborations. The studies described here will contribute in assessing the relevance of neosporosis as a potential risk factor not only for animals, but also for human health.

GTPases in protozoan parasites: tools for cell biology and chemotherapy

Small G proteins belong to a superfamily of GTPases related to the protooncogene ras, and function as master control elements for a range of cellular functions. This ability is related to their low rate of substrate turnover; GTPases catalyse the conversion of GTP to GDP, but with a rate in the order of one substrate per second, orders of magnitude slower than 'good' enzyme catalysis, but placing the reaction into the temporal frame of many cellular processes including signal transduction, cytoskeletal reorganization and vesicle trafficking. In this article, Mark Field, Bassam Ali and Helen Field describe some recent advances in G-protein studies in the parasite field, concentrating on the protozoan parasites. Because of their numerous roles in cell biology, understanding parasite G proteins has great potential for increasing our knowledge of parasite cellular physiology, as well as providing important inroads into vital processes for potential therapeutic exploitation.

The Leishmania genome comes of Age

The Leishmania Genome Network (LGN) was born in Rio de Janeiro, Brazil in 1994. In the short period that has elapsed since then, the LGN has focused solely on the acquisition of the resources, and hence data, that have enabled a rational approach to genomic sequencing of the reference strain, Leishmania major Friedlin. This has now been achieved. In this review, Alasdair Ivens and Jennie Blackwell, secretary and chairman of the LGN, respectively, re-examine the approaches that were adopted, comment on some of the interesting data that have been obtained and introduce some genome-wide approaches that will facilitate functional studies of the parasite.

Genomics and the biology of parasites

Despite the advances of modern medicine, the threat of chronic illness, disfigurement, or death that can result from parasitic infection still affects the majority of the world population, retarding economic development. For most parasitic diseases, current therapeutics often leave much to be desired in terms of administration regime, toxicity, or effectiveness and potential vaccines are a long way from market. Our best prospects for identifying new targets for drug, vaccine, and diagnostics development and for dissecting the biological basis of drug resistance, antigenic diversity, infectivity and pathology lie in parasite genome analysis, and international mapping and gene discovery initiatives are under way for a variety of protozoan and helminth parasites. These are far from ideal experimental organisms, and the influence of biological and genomic characteristics on experimental approaches is discussed, progress is reviewed and future prospects are examined.

Leishmania major: histone H1 gene expression from the sw3 locus

Histone H1 in the parasitic protozoan Leishmania is a developmentally regulated protein encoded by the sw3 gene. Here we report that histone H1 variants exist in different Leishmania species and strains of L. major and that they are encoded by polymorphic genes. Amplification of the sw3 gene from the genome of three strains of L. major gave rise to different products in each strain, suggesting the presence of a multicopy gene family. In L. major, these genes were all restricted to a 50-kb Bg/II fragment found on a chromosomal band of 1.3 Mb (chromosome 27). The detection of RFLPs in this locus demonstrated its heterogeneity within several species and strains of Leishmania. Two different copies of sw3 (sw3.0 and sw3.1) were identified after screening a cosmid library containing L. major strain Friedlin genomic DNA. They were identical in their 5' UTRs and open reading frames, but differed in their 3' UTRs. With respect to the originally cloned copy of sw3 from L. major strain LV39, their open reading frames lacked a repeat unit of 9 amino acids. Immunoblots of L. guyanensis parasites transfected with these cosmids revealed that both copies could give rise to the histone H1 protein. The characterization of this locus will now make possible a detailed analysis of the function of histone H1 in Leishmania, as well as permit the dissection of the molecular mechanisms governing the developmental regulation of the sw3 gene.

Conserved linkage groups associated with large-scale chromosomal rearrangements between Old World and New World Leishmania genomes

The genus Leishmania can be taxonomically separated into three main groups: the Old World subgenus L. (Leishmania), the New World subgenus L. (Leishmania) and the New World subgenus L. (Viannia). The haploid genome of Old World Leishmania species has been shown to contain 36 chromosomes defined as physical linkage groups; the latter were found entirely conserved across species. In the present study, we tried to verify whether this conservation of the genome structure extends to the New World species of Leishmania. 300 loci were explored by hybridization on optimized pulsed field gel electrophoresis separations of the chromosomes of polymorphic strains of the six main pathogenic Leishmania species of the New World. When comparing these New World karyotypes with their Old World counterparts, 32 out of 36 linkage groups were found conserved among all species. Four chromosomal rearrangements were found. All species belonging to the L. (Viannia) subgenus were characterized by the presence (i) of a short sequence exchange between chromosomes 26 and 35, and (ii) more importantly, of a fused version of chromosomes 20 and 34 which are separated in all Old World species. 69 additional markers were isolated from a plasmid library specifically constructed from the rearranged chromosomes 20+34 in an attempt to detect mechanisms other than a fusion or breakage: only two markers out of 40 did not belong to the linkage groups 20 and 34. On the other hand, all strains belonging to the New World subgenus L. (Leishmania) were characterized by two different chromosomal rearrangements of the same type (fusion/breakage) as above as compared with Old World species: chromosomes 8+29 and 20+36. Consequently, these two groups of species have 35 and 34 heterologous chromosomes, respectively. Overall, these results show that large-scale chromosomal rearrangements occurred during the evolution of the genus Leishmania, and that the three main groups of pathogenic species are characterized by different chromosome numbers. Nevertheless, translocations seem particularly rare, and the conservation of the major linkage groups should be an essential feature for the compared genetics between species of this parasite.

Gene discovery through expressed sequence Tag sequencing in Trypanosoma cruzi

Analysis of expressed sequence tags (ESTs) constitutes a useful approach for gene identification that, in the case of human pathogens, might result in the identification of new targets for chemotherapy and vaccine development. As part of the Trypanosoma cruzi genome project, we have partially sequenced the 5' ends of 1, 949 clones to generate ESTs. The clones were randomly selected from a normalized CL Brener epimastigote cDNA library. A total of 14.6% of the clones were homologous to previously identified T. cruzi genes, while 18.4% had significant matches to genes from other organisms in the database. A total of 67% of the ESTs had no matches in the database, and thus, some of them might be T. cruzi-specific genes. Functional groups of those sequences with matches in the database were constructed according to their putative biological functions. The two largest categories were protein synthesis (23.3%) and cell surface molecules (10.8%). The information reported in this paper should be useful for researchers in the field to analyze genes and proteins of their own interest.

Identification and characterisation of a functional peroxidoxin from Leishmania major

Leishmania spp. encounter damaging oxygen metabolites from endogenous metabolic processes as well as from exogenous sources, such as inside the gut of the sandfly vector and within host macrophages. The recently described peroxidoxin protein family form part of a novel pathway for metabolising hydrogen peroxide that, in trypanosomatids, links peroxide reduction to NADPH oxidation via trypanothione. Here we report the cloning and characterisation of the Leishmania major peroxidoxin gene, tryparedoxin peroxidase (TryP). TryP is a multi-copy gene arranged in a complex tandem array located on the size polymorphic homologues of chromosome 15. Northern analysis showed that TryP expresses a single 1.6 kb mRNA throughout promastigote development. TryP encodes a 22-kDa protein with two conserved cysteine-containing domains that defines it as a 2-Cys peroxidoxin. Purified recombinant TryP protein catabolised hydrogen peroxide in the presence of the tryparedoxin homologue from Crithidia fasciculata (Cf-TryX), trypanothione, trypanothione reductase and NADPH. The demonstration that L. major utilises a three-protein peroxidase system confirms that this is a mechanism of protection against oxidative damage in this parasite.

Caenorhabditis elegans as a model for parasitic nematodes

Caenorhabditis elegans has become a popular model system for genetic and molecular research, since it is easy to maintain and has a very fast life-cycle. Its genome is small and a virtually complete physical map in the form of cosmids and YAC clones exists. Thus it was chosen as a model system by the Genome Project for sequencing, and it is expected that by 1998 the complete sequence (100 million bp) will be available. The accumulated wealth of information about C. elegans should be a boon for nematode parasitologists, as many aspects of gene regulation and function can be studied in this simple model system. A large array of techniques is available to study many aspects of C. elegans biology. In combination with genome projects for parasitic nematodes, conserved genes can be identified rapidly. We expect many new areas of fertile research that will lead to new insights in helminth parasitology, which are based not only on the information gained from C. elegans per se, but also from its use as a heterologous system to study parasitic genes.

A physical map of the Leishmania major Friedlin genome

An extensive physical map of the Leishmania major Friedlin genome has been assembled by the combination of fingerprint analysis of a shuttle vector cosmid library and probe hybridization. The integrated data obtained for 9004 fingerprinted clones and 974 probes have placed 91.2% of the 33.58-Mb genome into contigs representing each of the 36 chromosomes. This first-generation map has already provided a suitable framework for both high-throughput DNA sequencing and functional studies of the L. major parasite.

Using 3' untranslated sequences to identify differentially expressed genes in Leishmania

A more sensitive screen for Leishmania major genes differentially expressed as the insect stage develops into an infectious form (metacyclogenesis) has been devised. The screen expolits the observation that in kinetoplastid protozoa differentially expressed genes are often associated with unique 3' untranslated regions (UTRs). To obtain probes encoding this region, cDNA is synthesised using an oligo-dT primer containing the universal vectorette sequence in the first strand reaction and an oligonucleotide comprising the spliced leader sequence in the second strand reaction. The cDNAs are then cleaved with Sau3AI, ligated to the vectorette and the 3' UTRs polymerase chain reaction (PCR) amplified using the universal vectorette sequence as the primer. Differential screening with PCR-amplified 3' UTRs uncovered: (1) previously identified metacyclic-specific expressed genes; (2) cloned genes which had not been shown to be differentially regulated; and (3) a new gene identified only as a match to two identical L. major expressed sequence tags (ESTs) that is upregulated in the infectious stage.

A direct method for the chromosomal assignment of DNA markers in Leishmania

A simple method for the chromosomal assignment of any DNA marker would be an important tool for the ongoing project to map the genome of the protozoan parasite Leishmania. The Leishmania chromosomes enter pulsed field gel electrophoresis (PFGE) gels under current electrophoretic conditions, but their direct identification in a given strain is hampered by their stacking in a few chromosomal bands, and by the very frequent size variations of the same chromosome among parasite strains. To overcome these problems. we determined the complete karyotypes of 12 Old World Leishmania cloned strains. This enabled us to select three of these strains that display great chromosome size polymorphisms, such that every chromosome can be individualized by a specific pattern after hybridization onto these three karyotypes. The complete resolution of the genomes of these three strains can be carried out with only three electrophoretic conditions. This makes a series of three blots sufficient for the assignment of any new marker on a particular Leishmania chromosome.

Diversity in repeat-containing surface proteins of Leishmania major

The gene B protein (GBP) is one of the products of the LmcDNA16 gene family, a cluster of related but non-identical genes that are differentially-expressed during the Leishmania life cycle. This protein, which is found on the surface of infective stage parasites, contains an extensive region of proline-rich amino acid repeats, constituting 45% of the total protein. The structure and stability of these repeats have been investigated in a number of L. major strains by polymerase chain reaction (PCR) amplification and Southern blotting. Data reported in this paper demonstrate variability between strains with respect to the number of repeats encoded by GBP, although those strains isolated within adjacent geographical regions have conserved repeat structures. The data also reveal that some parasite lines have additional repeat sequences within a second, related gene in the LmcDNA16 array. Western blotting experiments have established that these sequences are expressed in vivo, indicating that L. major strains are heterogeneous in their surface complement of gene B repeat-containing proteins.

Parasite genome analysis. A global map of the Leishmania major genome: prelude to genomic sequencing

In 1994, the World Health Organization (TDR) launched a new strategic initiative in parasite genome analysis, establishing international genome networks for filariae, Schistosoma, Leishmania, Trypanosoma brucei and T. cruzi. For Leishmania, a number of different but complementary approaches have been adopted by members of the Leishmania Genome Network. Our laboratory has been using cosmid clone fingerprinting to produce a physical map of the genome. Progress towards the completion of an integrated physical and biological map of L. major, and the preparations for genomic sequencing, are described.

Parasite genome analysis. Progress in the Leishmania genome project

Genome projects have been established for 7 major groups of human parasitic infections: malaria, leishmaniasis, African trypanosomiasis, American trypanosomiasis, toxoplasmosis, schistosomiasis and filariasis. All except malaria and toxoplasmosis have come under the umbrella of the World Health Organization's Strategic Committee on Parasite Genome Analysis. The focus of this meeting of the Society was to review progress made in the Leishmania and African trypanosome genome projects. This paper introduces the genome projects and reviews briefly progress in pulsed-field gel karyotype mapping and gene identification via expressed sequence tag sequencing for the leishmaniasis genome project. The overall aim of the genome projects is to harness the latest developments in molecular genetic technology and sequence analysis for the rapid-generation of new data which may, in turn, revolutionize our approaches to the study of the biology of these organisms.

Unravelling the Leishmania genome

The past few years have been significant advances in our understanding of eukaryotic genomes. In the field of parasitology, this is best exemplified by the application of genome mapping techniques to the study of genome structure and function in the protozoan parasite, Leishmania. Although much is known about the organism and the diseases it causes, molecular genetics has only recently begun to play a major part in elucidating some of the unusual characteristics of this interesting parasite. Mapping of the small (35 Mb) genome and determination of the functional role of genes by the application of in vitro homologous gene targeting techniques are revealing novel avenues for the development of prophylactic measures.