Diversity of aspartate carbamoyltransferase genes of Trypanosoma cruzi

Fresh insights into the pyrimidine metabolism in the trypanosomatids

The trypanosomatid parasites continue their killing spree resulting in significant annual mortality due to the lack of effective treatments and the prominence of these diseases in poorer countries. These dimorphic parasites thrive unchecked in the host system, outsmarting the immune mechanisms. An understanding of biology of these parasitic forms will help in the management and elimination of these fatal diseases. Investigation of various metabolic pathways in these parasites has shed light in the understanding of the unique biology of the trypansomatids. An understanding of these pathways have helped in tracing the soft targets in the metabolic pathways, which could be used as effective drug targets which would further impact the therupeutic implications. Pyrimidine pathway is a vital metabolic pathway which yields in the formation of pyrimidines, which are then integrated in nucleic acids (DNA and RNA) in sugars (UDP sugars) and lipids (CDP lipids). A wealth of data and information has been generated in the past decades by in-depth analyses of pyrimidine pathway in the trypanosomatid parasites, which can aid in the identification of anomalies between the parasitic and host counterpart which could be further harnessed to develop therapeutic interventions for the treatment of parasitic diseases. This review presents an updated and comprehensive detailing of the pyrimidine metabolism in the trypansomatids, their uniqueness and their distinctions, and its possible outcomes that would aid in the eradication of these parasitic diseases.

Evolutionary analysis of synteny and gene fusion for pyrimidine biosynthetic enzymes in Euglenozoa: an extraordinary gap between kinetoplastids and diplonemids

A unique feature of the genome architecture in the parasitic trypanosomatid protists is large-scale synteny. We addressed the evolutionary trait of synteny in the eukaryotic group, Euglenozoa, which consists of euglenoids (earliest branching), diplonemids, and kinetoplastids (trypanosomatids and bodonids). Synteny of the pyrimidine biosynthetic (pyr) gene cluster, which constitutes part of a large syntenic cluster in trypanosomatids and includes four separate genes (pyr1-pyr4) and one fused gene (pyr6/pyr5 fusion), was conserved in the bodonid, Parabodo caudatus. In the diplonemid, Diplonema papillatum, we identified pyr4 and pyr6 genes. Phylogenetic analyses of pyr4 and pyr6 showed the separate origin of each in kinetoplastids and euglenoids/diplonemids and suggested that kinetoplastids have acquired these genes via lateral gene transfer (LGT). Because replacement of genes by non-orthologs within the syntenic cluster is highly unlikely, we concluded that, after separation of the line leading to diplonemids, the syntenic pyr gene cluster was established in the common ancestor of kinetoplastids, preceded by their acquisition via LGT. Notably, we found that diplonemid pyr6 is a stand-alone gene, inconsistent with both euglenoid pyr5/pyr6 and kinetoplastid pyr6/pyr5 fusions. Our findings provide insights into the evolutionary gaps within Euglenozoa and the evolutionary trait of rearrangement of gene fusion in this lineage.

Dihydroorotate dehydrogenase arises from novel fused gene product with aspartate carbamoyltransferase in Bodo saliens

The ACT-DHOD gene in the kinetoplastid Bodo saliens encodes aspartate carbamoyltransferase and dihydroorotate dehydrogenase, the second and fourth enzymes of pyrimidine biosynthesis. Although the single mRNA species yielded a 70-kDa ACT-DHOD protein, Western blotting with anti-DHOD-peptide antibody showed a major band of 35-kDa and minor bands. In-gel digestion and liquid chromatography-tandem mass (MS/MS) spectrometry showed that the 35-kDa band contained DHOD-specific polypeptides and an ACT-specific polypeptide, suggesting the occurrence of independent DHOD and ACT. Immunoprecipitation and MS/MS analysis identified a 70-kDa ACT-DHOD and a 35-kDa DHOD independently, and the N-terminal amino acid of 35-kDa DHOD was blocked. In vitro processing assay showed that recombinant ACT-DHOD was decreased by the B. saliens lysate, accompanying the appearance of 35-kDa DHOD and 35-kDa ACT. These results indicate that fused ACT-DHOD is the precursor to mature DHOD. Large amount of 35-kDa DHOD in B. saliens is discussed from a viewpoint of its physiological roles.

Tandem gene arrays in Trypanosoma brucei: comparative phylogenomic analysis of duplicate sequence variation

Background

The genome sequence of the protistan parasite Trypanosoma brucei contains many tandem gene arrays. Gene duplicates are created through tandem duplication and are expressed through polycistronic transcription, suggesting that the primary purpose of long, tandem arrays is to increase gene dosage in an environment where individual gene promoters are absent. This report presents the first account of the tandem gene arrays in the T. brucei genome, employing several related genome sequences to establish how variation is created and removed.

Results

A systematic survey of tandem gene arrays showed that substantial sequence variation existed across the genome; variation from different regions of an array often produced inconsistent phylogenetic affinities. Phylogenetic relationships of gene duplicates were consistent with concerted evolution being a widespread homogenising force. However, tandem duplicates were not usually identical; therefore, any homogenising effect was coincident with divergence among duplicates. Allelic gene conversion was detected using various criteria and was apparently able to both remove and introduce sequence variation. Tandem arrays containing structural heterogeneity demonstrated how sequence homogenisation and differentiation can occur within a single locus.

Conclusion

The use of multiple genome sequences in a comparative analysis of tandem gene arrays identified substantial sequence variation among gene duplicates. The distribution of sequence variation is determined by a dynamic balance of conservative and innovative evolutionary forces. Gene trees from various species showed that intraspecific duplicates evolve in concert, perhaps through frequent gene conversion, although this does not prevent sequence divergence, especially where structural heterogeneity physically separates a duplicate from its neighbours. In describing dynamics of sequence variation that have consequences beyond gene dosage, this survey provides a basis for uncovering the hidden functionality within tandem gene arrays in trypanosomatids.

Molecular cloning and characterization of ouabain-insensitive Na(+)-ATPase in the parasitic protist, Trypanosoma cruzi

Maintaining low intracellular sodium concentrations is vital for almost all organisms. Na(+) efflux is generally governed by P-type ATPases, Na(+)/K(+)-ATPase in animals and Na(+)-ATPase, called ENA, in fungi and plants. Trypanosoma cruzi, which parasitizes mammalian cells, must undergo drastic adaptations to high Na(+) concentrations outside and low Na(+) concentrations inside host cells. However, T. cruzi Na(+) efflux pumps have not been identified. We report here the cloning and characterization of the gene encoding Na(+)-ATPase in T cruzi, which resembled fungal and plant ENAs, termed TcENA. TcENA was a plasma membrane protein expressed throughout the parasite life cycle. The transcription level of TcENA was higher in insect stage epimastigotes and blood stream trypomastigotes than in intracellular amastigotes, probably reflecting the high Na(+) concentration outside the host cells. Biochemical analysis of TcENA expressed heterologously in mammalian cells demonstrated, for the fist time, that the ATPase activity of TcENA is stimulated by both Na(+) and K(+) and is insensitive to ouabain, a specific inhibitor of Na(+)/K(+)-ATPases. Furthermore, epimastigotes overproducing TcENA showed increased tolerance to high Na(+) stress. Our findings suggest that TcENA acts as a sodium pump and provide insights into the regulation of ion homeostasis in the parasitic protist.

Aspartate transport and metabolism in the protozoan parasite Trypanosoma cruzi

Aspartate is one of the compounds that induce the differentiation process of the non-infective epimastigote stage to the infective trypomastigote stage of the protozoan parasite Trypanosoma cruzi. l-aspartate is transported by both epimastigote and trypomastigote cells at the same rate, about 3.4 pmolmin(-1) per 10(7) cells. Aspartate transport is only competed by glutamate suggesting that this transport system is specific for anionic amino acids. Aspartate uptake rates increase along the parasite growth curve, by amino acids starvation or pH decrease. The metabolic fate of the transported aspartate was predicted in silico by identification of seven putative genes coding for enzymes involved in aspartate metabolism that could be related to the differentiation process.

Genetic diversity and kinetic properties of Trypanosoma cruzi dihydroorotate dehydrogenase isoforms

Dihydroorotate dehydrogenase (DHOD) is the fourth enzyme in the de novo pyrimidine biosynthetic pathway and is essential in Trypanosoma cruzi, the parasitic protist causing Chagas' disease. T. cruzi and human DHOD have different biochemical properties, including the electron acceptor capacities and cellular localization, suggesting that T. cruzi DHOD may be a potential chemotherapeutic target against Chagas' disease. Here, we report nucleotide sequence polymorphisms of T. cruzi DHOD genes and the kinetic properties of the recombinant enzymes. T. cruzi Tulahuen strain possesses three DHODgenes: DHOD1 and DHOD2, involved in the pyrimidine biosynthetic (pyr) gene cluster on an 800 and a 1000 kb chromosomal DNA, respectively, and DHOD3, located on an 800 kb DNA. The open reading frames of all three DHOD genes are comprised of 942 bp, and encode proteins of 314 amino acids. The three DHOD genes differ by 26 nucleotides, resulting in replacement of 8 amino acid residues. In contrast, all residues critical for constituting the active site are conserved among the three proteins. Recombinant T. cruzi DHOD1 and DHOD2 expressed in E. coli possess similar enzymatic properties, including optimal pH, optimal temperature, Vmax, and Km for dihydroorotate and fumarate. In contrast, DHOD3 had a higher Vmax and Km for both substrates. Orotate competitively inhibited all three DHOD enzymes to a comparable level. These results suggest that, despite their genetic variations, kinetic properties of the three T. cruziDHODs are conserved. Our findings facilitate further exploitation of T. cruzi DHOD inhibitors, as chemotherapeutic agents against Chagas' disease.

Inhibitory action of marine algae extracts on the Trypanosoma cruzi dihydroorotate dehydrogenase activity and on the protozoan growth in mammalian cells

Trypanosoma cruzi, the causative agent of Chagas' disease, replicates in mammalian cells and relies on the de novo pyrimidine biosynthetic pathway that supplies essential precursors for nucleic acid synthesis. The protozoan dihydroorotate dehydrogenase (DHOD), the fourth enzyme of the pathway catalyzing production of orotate from dihydroorotate, markedly differs from the human enzyme. This study was thus aimed to search for potent inhibitors against T. cruzi DHOD activity, and a number of methanol extracts prepared from green, brown, and red algae were assayed. The extracts from two brown algae, Fucus evanescens and Pelvetia babingtonii, yielded 59 and 58% decrease in the recombinant DHOD activity, respectively, at the concentration of 50 microg/ml. Inhibition by these extracts was noncompetitive with respect to dihydroorotate, with apparent Ki values of 35.3+/-5.9 and 10.3+/-4.4 microg/ml, respectively. Further, in an in vitro T. cruzi-HeLa cell infection system, ethanol-reconstituted F. evanescens and P. babingtonii extracts at the concentration of 1 microg/ml, respectively, decreased significantly the infection rate of host cells and the average parasite number per infected cell. These results imply that F. evanescens and P. babingtonii contain inhibitor(s) against the T. cruzi DHOD activity and against the protozoan infection and proliferation in mammalian cells. Identification of inhibitor(s) in these two brown algae and further screening of other marine algae may facilitate the discovery of new, anti-trypanosomal lead compounds.

The Origin of Dihydroorotate Dehydrogenase Genes of Kinetoplastids, with Special Reference to Their Biological Significance and Adaptation to Anaerobic, Parasitic Conditions

Trypanosoma cruzi dihydroorotate dehydrogenase (DHOD), the fourth enzyme of the de novo pyrimidine biosynthetic pathway, is localized in the cytosol and utilizes fumarate as electron acceptor (fumarate reductase activity), while the enzyme from other various eukaryotes is mitochondrial membrane-linked. Here we report that DHOD-knockout T. cruzi did not express the enzyme protein and could not survive even in the presence of pyrimidine nucleosides, substrates for the potentially active salvage pathway, suggesting a vital role of fumarate reductase activity in the regulation of cellular redox balance. Cloning and phylogenetic analysis of euglenozoan DHOD genes showed that the euglenoid Euglena gracilis had a mitochondrial DHOD and that biflagellated bodonids, a sister group of trypanosomatids within kinetoplastids, harbor the cytosolic DHOD. Further, Bodo saliens, a bodonid, had an ACT/DHOD gene fusion encoding aspartate carbamoyltransferase (ACT), the second enzyme of the de novo pyrimidine pathway, and DHOD. This is the first report of this novel gene structure. These results are consistent with suggestions that an ancient common ancestor of Euglenozoa had a mitochondrial DHOD whose descendant exists in E. gracilis and that a common ancestor of kinetoplastids (bodonids and trypanosomatids) subsequently acquired a cytosolic DHOD by horizontal gene transfer. The cytosolic DHOD gene thus acquired may have contributed to adaptation to anaerobiosis in the kinetoplastid lineage and further contributed to the subsequent establishment of parasitism in a trypanosomatid ancestor. Different molecular strategies for anaerobic adaptation in pyrimidine biosynthesis, used by kinetoplastids and by euglenoids, are discussed. Evolutionary implications of the ACT/DHOD gene fusion are also discussed.