Analyzing expression of the calmodulin and ubiquitin-fusion genes of Trypanosoma cruzi using simultaneous, independent dual gene replacements

Life-cycle and growth-phase-dependent regulation of the ubiquitin genes of Trypanosoma cruzi

BACKGROUND

Trypanosoma cruzi, the causative agent of Chagas disease, exhibits a complex life cycle that is accompanied by the stage-specific gene expression. At the molecular level, very little is known about gene regulation in trypanosomes. Complex gene organizations coupled with polycistronic transcription units make the analysis of regulated gene expression difficult in trypanosomes. The ubiquitin genes of T. cruzi are a good example of this complexity. They are organized as a single cluster containing five ubiquitin fusion (FUS) and five polyubiquitin (PUB) genes that are polycistronically transcribed but expressed differently in response to developmental and environmental changes.

METHODS

Gene replacements were used to study FUS and PUB gene expression at different stages of growth and at different points in the life cycle of T. cruzi.

RESULTS

Based on the levels of reporter gene expression, it was determined that FUS1 expression was downregulated as the parasites approached stationary phase, whereas PUB12.5 polyubiquitin gene expression increased. Conversely, FUS1 expression increases when epimastigotes and amastigotes differentiate into trypomastigotes, whereas the expression of PUB12.5 decreases when epimastigotes differentiate into amastigotes and trypomastigotes.

CONCLUSIONS

Although the level of CAT activity in logarithmic growing epimastigotes is six- to seven-fold higher when the gene was expressed from the FUS1 locus than when expressed from the PUB12.5 locus, the rate of transcription from the two loci was the same implying that post-transcriptional mechanisms play a dominant role in the regulation of gene expression.

Upregulation of sterol C14-demethylase expression in Trypanosoma cruzi treated with sterol biosynthesis inhibitors

Infection with the protozoan, Trypanosoma cruzi, is the cause of Chagas disease that occurs widely throughout Latin America. T. cruzi contains sterol biosynthesis enzymes, and produces sterol products similar to those found in fungi. Antifungal drugs that inhibit ergosterol biosynthesis have potent anti-T. cruzi activity in vitro and in animal models. In this report, we describe the effects of sterol biosynthesis inhibitors (simvistatin, zaragosic acid, terbinafine, a lanosterol synthase inhibitor, ketoconazole, and tridemorph) on the regulation of two sterol biosynthesis genes and their protein products. Culturing T. cruzi in the presence of the lanosterol synthase inhibitor, terbinafine, or ketoconazole increased mRNA levels of the sterol C14-demethylase gene approximately 7-12-fold. The sterol C14-demethylase protein levels were also elevated. The effects of the sterol biosynthesis inhibitors on hydroxymethylglutaryl-CoA reductase expression were minimal. Control of the upregulation of sterol C14-demethylase appears to be mediated through the 3'-untranslated region of the gene. The findings demonstrate that T. cruzi can specifically regulate gene expression in response to derangements in its cellular functions.

LYT1 protein is required for efficient in vitro infection by Trypanosoma cruzi

Trypanosoma cruzi invasion of host cells involves several discrete steps: attachment, parasite internalization mediated by recruitment and fusion of host cell lysosomes, and escape from the parasitophorous vacuole to liberate amastigotes to multiply freely in the cytosol. This report describes the initial characterization of the LYT1 gene and the demonstration that the gene product is involved in cell lysis and infectivity. Mutational analysis demonstrated that deletion of LYT1 resulted in attenuation of infection, which was associated with diminished hemolytic activity. Reintroduction of LYT1 restored infectivity in null mutants, confirming the critical role of LYT1 in infection. Additionally, in vitro stage transition experiments with LYT1-deficient lines showed that these parasites converted to extracellular amastigote-like cells and metacyclic trypomastigotes more rapidly than wild-type parasites, suggesting that the diminished infectivity was not a result of the LYT1 deficiency that affected the parasite's ability to complete the life cycle.

Potent anti-Trypanosoma cruzi activities of oxidosqualene cyclase inhibitors

Trypanosoma cruzi is the protozoan agent that causes Chagas' disease, a major health problem in Latin America. Better drugs are needed to treat infected individuals. The sterol biosynthesis pathway is a potentially excellent target for drug therapy against T. cruzi. In this study, we investigated the antitrypanosomal activities of a series of compounds designed to inhibit a key enzyme in sterol biosynthesis, oxidosqualene cyclase. This enzyme converts 2,3-oxidosqualene to the tetracyclic product, lanosterol. The lead compound, N-(4E,8E)-5,9, 13-trimethyl-4,8, 12-tetradecatrien-1-ylpyridinium, is an electron-poor aromatic mimic of a monocyclized transition state or high-energy intermediate formed from oxidosqualene. This compound and 27 related compounds were tested against mammalian-stage T. cruzi, and 12 inhibited growth by 50% at concentrations below 25 nM. The lead compound was shown to cause an accumulation of oxidosqualene and decreased production of lanosterol and ergosterol, consistent with specific inhibition of the oxidosqualene cyclase. The data demonstrate potent anti-T. cruzi activity associated with inhibition of oxidosqualene cyclase.

Colony polymerase chain reaction of stably transfected trypanosoma cruzi grown on solid medium

Tools for the genetic manipulation of Trypanosoma cruzi are largely unavailable, although several vectors for transfection of epimastigotes and expression of foreign or recombinant genes have been developed. We have previously constructed several plasmid vectors in which recombinant genes are expressed in T. cruzi using the rRNA promoter. In this report, we demonstrate that one of these vectors can simultaneously mediate expression of neomycin phosphotransferase and green fluorescent protein when used to stably transfect cultured epimastigotes. These stably transfected epimastigotes can be selected and cloned as unique colonies on solid medium. We describe a simple colony PCR approach to the screening of these T. cruzi colonies for relevant genes. Thus, the methodologies outlined herein provide important new tools for the genetic dissection of this important parasite.

Expression of Trypanosoma cruzi surface antigen FL-160 is controlled by elements in the 3' untranslated, the 3' intergenic, and the coding regions

The FL-160 surface antigen gene family of T. cruzi consists of hundreds of members of 160 kDa glycoproteins expressed in trypomastigotes, but not in epimastigotes. Steady-state levels of FL-160 mRNA were 80 to 100-fold higher in trypomastigotes than in epimastigotes, yet transcription rates were equivalent between the lifecycle stages. Luciferase reporter constructs demonstrated that the 3' untranslated region (UTR) and intergenic region (IR) following the coding sequence of FL-160 was sufficient to generate 8-fold higher luciferase expression in trypomastigotes compared with epimastigotes. Transfection of 3' UTR/IR deletion constructs revealed cis-acting elements which conferred a trypomastigote-specific expression pattern similar to that of FL-160. Parasites treated with translation and transcription inhibitors, cyclohexamide and Actinomycin D, respectively, displayed a stage-specific pattern of FL-160 mRNA degradation. Epimastigotes, but not trypomastigotes, treated with the inhibitors accumulated a 1.4 Kb FL-160 cleavage product. The cleavage site mapped to a 31 base poly-purine tract in the FL-160 coding region. The first 526 aa of FL-160, containing the 31 base poly-purine tract and several smaller tracts, were fused to green fluorescent protein (GFP) and expressed from the T. cruzi tubulin locus. Stable transformants expressed 4-fold more FL-160:GFP fusion mRNA and 12-fold more fusion protein in the trypomastigote stage than in the epimastigote stage suggesting post-transcriptional and translational control elements. These data reveal at least two distinct control mechanisms for trypomastigote-specific expression of FL-160 surface glycoproteins, one involving the 3' UTR/IR and one involving the coding region of FL-160.

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.

Efficient technique for screening drugs for activity against Trypanosoma cruzi using parasites expressing beta-galactosidase

A new drug screening method was devised utilizing Trypanosoma cruzi cells that express the Escherichia coli beta-galactosidase gene. Transfected parasites catalyze a colorimetric reaction with chlorophenol red beta-D-galactopyranoside as substrate. Parasite growth in the presence of drugs in microtiter plates was quantitated with an enzyme-linked immunosorbent assay reader. The assay was performed with the mammalian form of T. cruzi that requires intracellular growth on a monolayer of fibroblast cells. To determine if selective toxicity to the parasites was occurring, the viability of the host cells in the drug was assayed with AlamarBlue. The drugs benznidazole, fluconazole, and amphotericin B were shown to inhibit the parasites at concentrations similar to those previously reported. Several compounds were tested that are inhibitors of glyceraldehyde-3-phosphate dehydrogenase of the related organisms Leishmania mexicana and Trypanosoma brucei. One of these compounds, 2-guanidino-benzimidazole, had an 50% inhibitory concentration of 10 microM in our assay. Two derivatives of this compound were identified with in vitro activity at even lower concentrations. In addition, the assay was modified for testing compounds for lytic activity against the bloodstream form of the parasite under conditions used for storing blood products. Thus, an assay with beta-galactosidase-expressing T. cruzi greatly simplifies screening drugs for selective anti-T. cruzi activity, and three promising new compounds have been identified.

The calmodulin-ubiquitin (CUB) genes of Trypanosoma cruzi are essential for parasite viability

The CUB genes represent single copy genes in the diploid Trypanosoma cruzi genome. In this report data are presented which demonstrate that a single expressed CUB gene is necessary for parasite viability. Although either CUB gene could be deleted individually, repeated attempts to simultaneously delete both genes were unsuccessful. The essential nature of the CUB genes was further supported by studies which demonstrated positive selection for CUB gene expression. Positive selection was demonstrated by carrying out dual gene replacements which showed that both native CUB genes could be efficiently deleted provided the CalB1 calmodulin gene was simultaneously replaced by a CUB gene protein coding sequence. Although the function of the CUB gene product remains unknown the experiments presented here indicate the product is likely to play an important role in the parasites' life cycle.

Expression of mammalian cytokines by Trypanosoma cruzi indicates unique signal sequence requirements and processing

A vector based upon the calmodulin-ubiquitin 2.65 locus of Trypanosoma cruzi has enabled the expression and secretion of the murine cytokines interleukin-2 (IL-2) and gamma-interferon (gamma-IFN) by transfected T. cruzi. The T. cruzi-derived cytokines were bioactive and produced by both epimastigotes and mammalian forms. The native coding sequence of IL-2 was sufficient to cause secretion of the protein, but the gamma-IFN signal sequence had to be replaced by the IL-2 signal sequence (IL-2/gamma-IFN) to allow efficient secretion of gamma-IFN. The amino acid sequences at the N-termini of the secreted T. cruzi-derived cytokines were different from the expected murine secreted protein. The secreted IL-2 was cleaved six amino acids downstream from the murine signal sequence cleavage site, and the hybrid IL-2/gamma-IFN molecule was cleaved three amino acids downstream from the predicted signal cleavage site in the IL-2/gamma-IFN molecule. These apparent differences in signal peptide sequence requirements and cleavage sites most likely indicate that the signal sequence processing in trypanosomes is distinct from that of higher eukaryotes.