Organization and expression of the gene encoding an immunodominant repetitive antigen associated to the cytoskeleton of Trypanosoma cruzi

Construction, expression, and characterization of thymosin alpha 1 tandem repeats in Escherichia coli

Thymosin alpha 1 (Tα1), which is composed of 28 amino acids, has been commercialized worldwide for its immune-modulatory and antitumor effects. Tα1 can stimulate T cell proliferation and differentiation from bone marrow stem cells, augment cell-mediated immune responses, and regulate homeostasis of immune system. In this study, we developed a novel strategy to produce Tα1 concatemer (Tα1③) in Escherichia coli and compared its activity with chemically synthesized Tα1. Results showed that Tα1③ can more effectively stimulate T cell proliferation and significantly upregulate IL-2 receptor expression. We concluded that the expression system for Tα1 concatemer was constructed successfully, which could serve as an efficient tool for the production of large quantities of the active protein.

The repetitive cytoskeletal protein H49 of Trypanosoma cruzi is a calpain-like protein located at the flagellum attachment zone

Background

Trypanosoma cruzi has a single flagellum attached to the cell body by a network of specialized cytoskeletal and membranous connections called the flagellum attachment zone. Previously, we isolated a DNA fragment (clone H49) which encodes tandemly arranged repeats of 68 amino acids associated with a high molecular weight cytoskeletal protein. In the current study, the genomic complexity of H49 and its relationships to the T. cruzi calpain-like cysteine peptidase family, comprising active calpains and calpain-like proteins, is addressed. Immunofluorescence analysis and biochemical fractionation were used to demonstrate the cellular location of H49 proteins.

Methods and Findings

All of H49 repeats are associated with calpain-like sequences. Sequence analysis demonstrated that this protein, now termed H49/calpain, consists of an amino-terminal catalytic cysteine protease domain II, followed by a large region of 68-amino acid repeats tandemly arranged and a carboxy-terminal segment carrying the protease domains II and III. The H49/calpains can be classified as calpain-like proteins as the cysteine protease catalytic triad has been partially conserved in these proteins. The H49/calpains repeats share less than 60% identity with other calpain-like proteins in Leishmania and T. brucei, and there is no immunological cross reaction among them. It is suggested that the expansion of H49/calpain repeats only occurred in T. cruzi after separation of a T. cruzi ancestor from other trypanosomatid lineages. Immunofluorescence and immunoblotting experiments demonstrated that H49/calpain is located along the flagellum attachment zone adjacent to the cell body.

Conclusions

H49/calpain contains large central region composed of 68-amino acid repeats tandemly arranged. They can be classified as calpain-like proteins as the cysteine protease catalytic triad is partially conserved in these proteins. H49/calpains could have a structural role, namely that of ensuring that the cell body remains attached to the flagellum by connecting the subpellicular microtubule array to it.

Effect of repetitiveness on the immunogenicity and antigenicity of Trypanosoma cruzi FRA protein

Repetitive proteins (RP) of Trypanosoma cruzi are highly present in the parasite and are strongly recognized by sera from Chagas' disease patients. Flagelar Repetitive Antigen (FRA), which is expressed in all steps of the parasite life cycle, is the RP that displays the greatest number of aminoacids per repeat and has been indicated as one of the most suitable candidate for diagnostic test because of its high performance in immunoassays. Here we analyzed the influence of the number of repeats on the immunogenic and antigenic properties of the antigen. Recombinant proteins containing one, two, and four tandem repeats of FRA (FRA1, FRA2, and FRA4, respectively) were obtained and the immune response induced by an equal amount of repeats was evaluated in a mouse model. The reactivity of specific antibodies present in sera from patients naturally infected with T. cruzi was also assessed against FRA1, FRA2, and FRA4 proteins, and the relative avidity was analyzed. We determined that the number of repeats did not increase the humoral response against the antigen and this result was reproduced when the repeated motifs were alone or fused to a non-repetitive protein. By contrast, the binding affinity of specific human antibodies increases with the number of repeated motifs in FRA antigen. We then concluded that the high ability of FRA to be recognized by specific antibodies from infected individuals is mainly due to a favorable polyvalent interaction between the antigen and the antibodies. In accordance with experimental results, a 3D model was proposed and B epitope in FRA1, FRA2, and FRA4 were predicted.

Unique behavior of Trypanosoma dionisii interacting with mammalian cells: invasion, intracellular growth, and nuclear localization

The phylogenetic proximity between Trypanosoma cruzi and Trypanosoma (Schizotrypanum) dionisii suggests that these parasites might explore similar strategies to complete their life cycles. T. cruzi is the etiological agent of the life-threatening Chagas' disease, whereas T. dionisii is a bat trypanosome and probably not capable of infecting humans. Here we sought to compare mammalian cell invasion and intracellular traffic of both trypanosomes and determine the differences and similarities in this process. The results presented demonstrate that T. dionisii is highly infective in vitro, particularly when the infection process occurs without serum and that the invasion is similarly affected by agents known to interfere with T. cruzi invasion process. Our results indicate that the formation of lysosomal-enriched compartments is part of a cell-invasion mechanism retained by related trypanosomatids, and that residence and further escape from a lysosomal compartment may be a common requisite for successful infection. During intracellular growth, parasites share a few epitopes with T. cruzi amastigotes and trypomastigotes. Unexpectedly, in heavily infected cells, amastigotes and trypomastigotes were found inside the host cell nucleus. These findings suggest that T. dionisii, although sharing some features in host cell invasion with T. cruzi, has unique behaviors that deserve to be further explored.

Expression and cellular localization of molecules of the gp82 family in Trypanosoma cruzi metacyclic trypomastigotes

A member of the Trypanosoma cruzi gp82 family, expressed on metacyclic trypomastigote surface and identified by monoclonal antibody (MAb) 3F6, plays a key role in host cell invasion. Apart from the gp82 defined by MAb 3F6, no information is available on members of this protein family. From cDNA clones encoding gp82 proteins sharing 59.1% sequence identity, we produced the recombinant proteins J18 and C03, the former containing and the latter lacking the epitope for MAb 3F6. Polyclonal antibodies to J18 and C03 proteins were generated and used, along with MAb 3F6, to analyze the expression and cellular localization of gp82 family members in metacyclic forms of CL and G strains, which belong to highly divergent genetic groups. By two-dimensional gel electrophoresis and immunoblotting, molecules of 82 to 86 kDa, focusing at pH 4.6 to 5.4, and molecules of 72 to 88 kDa, focusing at pH 4.9 to 5.7, were visualized in CL and G strains, respectively. Flow cytometry and microscopic analysis revealed in both strains similar expression of MAb 3F6-reactive gp82 in live and permeabilized parasites, indicating its surface localization. The reaction of live parasites of both strains with anti-J18 antibodies was weaker than with MAb 3F6 and was increased by permeabilization. Anti-C03 antibodies bound predominantly to flagellar components in permeabilized G strain parasites, but in the CL strain the flagellum was not the preferential target for these antibodies. Host cell invasion of metacyclic forms was inhibited by J18 protein, as well as by MAb 3F6 and anti-J18 antibodies, but not by C03 protein or anti-C03 antibodies.

Macro, micro and nano domains in the membrane of parasitic protozoa

The structural organization of the plasma membrane of eukaryotic cells is briefly revised taking into consideration the organization of proteins and lipids and the concept of microdomains, lipid rafts and detergent resistant membranes. The biochemical data available concerning the presence of microdomains in parasitic protozoa is reviewed and emphasis is given on the identification of special domains recognized by morphological approaches, especially with the use of the freeze-fracture technique.

Effect of suramin on trypomastigote forms of Trypanosoma cruzi: changes on cell motility and on the ultrastructure of the flagellum-cell body attachment region

Suramin has been previously reported to inhibit distinct cell enzymes and to affect the synthesis and distribution of cytoskeleton proteins. Our study indicates that prolonged incubation of Trypanosoma cruzi infected-LLC-MK2 cells in the presence of 500 microM suramin during the intracellular development of the parasite caused morphological changes on trypomastigote forms characterized by a partial or complete detachment of the flagellum from the cell body, besides an accentuated decrease on parasite motility. Immunofluorescence analysis of the region of adhesion between the cell body and the flagellum on trypomastigotes obtained from suramin-treated host cells after the completion of cell cycle did not show any difference in the localization of FAZ antigens recognized by 4D9 and L3B2 monoclonal antibodies despite the presence of a detached flagellum. On the other hand, suramin caused a significant increase on the phenotypic expression of FRA antigen, which was observed throughout the surface of trypomastigotes. Cytochemical localization of cationized ferritin in trypomastigotes obtained from suramin-treated host cells showed that anionic particles gained access to the space between the cell and flagellar membranes, as well as to the flagellar pocket, indicating an alteration on extracellular components of the region of adhesion between the cell body and the flagellum.

The flagellar attachment zone of Trypanosoma cruzi epimastigote forms

The flagellar attachment zone (FAZ) is an adhesion region of Trypanosoma cruzi epimastigote forms where the flagellum emerges from the flagellar pocket and remains attached to the cell body. This region shows a junctional complex which is formed by a linear series of apposed macular structures that are separated by amorphous material and clusters of intramembranous particles. Two protein groups appear to be important in the FAZ region: a membrane glycoprotein of 72kDa and several high molecular weight proteins. To gain a better understanding of the FAZ region, we compared wild-type Y strain T. cruzi epimastigotes with a mutant cell in which the 72-kDa surface glycoprotein (Gp72), involved in cell body-flagellum adhesion, had been deleted by target gene replacement. Using immunofluorescence confocal microscopy and electron microscopy techniques to analyze the FAZ region the results suggest that, in the absence of Gp72, other proteins involved in the formation of FAZ remain concentrated in the flagellar pocket region. The analysis of a 3-D reconstruction model of wild-type epimastigotes showed that the endoplasmic reticulum and mitochondrion are in intimate association with FAZ, in contrast to the null mutant cells where the endoplasmic reticulum was not visualized.

Chagas disease: recombinant Trypanosoma cruzi antigens for serological diagnosis

Diagnosis of individuals infected by Trypanosoma cruzi is performed mainly by serological tests using crude antigens, which might crossreact with other infections. In the past ten years, many recombinant T. cruzi proteins and synthetic peptides have been described, and some are already on the market. Managers of laboratories and blood banks need to make decisions on a cost-benefit basis whether to include these new-generation tests. Here, we indicate antigens that are likely to prove most useful.

Phosphatidylinositol-specific phospholipase C (PI-PLC) cleavage of GPI-anchored surface molecules of Trypanosoma cruzi triggers in vitro morphological reorganization of trypomastigotes

Trypanosoma cruzi trypomastigotes treated with phosphatidylinositol-specific phospholipase C (PI-PLC) in vitro are rapidly induced to differentiate into round forms. Using confocal microscopy, we were able to show that trypomastigotes treated with PI-PLC initiate the process of flagellum remodeling by 30 sec after contact with the enzyme and amastigote-like forms are detected as early as 10 min after PI-PLC treatment. Scanning and transmission electron microscopy indicate that trypomastigotes undergo a previously undescribed process of flagellum circularization and internalization. Analysis of the flagellar complex with monoclonal antibody 4D9 shows heterogeneous labeling among the parasites, suggesting a remodeling of these molecules. After PI-PLC treatment, parasites rapidly lose the surface marker Ssp-3 and 24 h post-treatment they begin to exhibit a circular nucleus and a rod-shaped kinetoplast. By flow cytometry analysis and confocal microscopy, the Ssp-4 amastigote-specific epitope can be detected on the parasite surface. This indicates that the release of trypomastigote GPI-anchored molecules by exogenous PI-PLC in vitro can trigger morphological changes.

Cloning, characterization, and expression of a 200-kilodalton diagnostic antigen of Babesia bigemina

Current serological tests for Babesia bigemina use semipurified merozoite antigens derived from infected erythrocytes. One of the major drawbacks of these tests is that antigen quality can vary from batch to batch. Since the quality of the antigen contributes to the sensitivity and specificity of serological tests, the use of standardized recombinant antigens should ensure consistency in assay quality. Previously, a 200-kDa merozoite antigen (p200) was identified as a candidate diagnostic antigen for use in a serological assay for the detection of B. bigemina antibodies in infected cattle. In this study, we have cloned, characterized, and expressed p200. A 3.5-kbp cDNA clone encoding p200 was isolated and shown to be almost full length, lacking approximately 300 bp at the 5' end. The predicted amino acid sequence shows that p200 consists of a long, highly charged central repeat region of an uninterrupted alpha helix, indicative of a fibrous protein. Immunoelectron microscopy localized p200 to the merozoite cytoplasm, suggesting that the antigen may be a structural protein involved in forming filament structures within the cytoskeleton. The 3.5-kbp cDNA was expressed in bacteria as a fusion protein with glutathione S-transferase (GST), but the yield was poor. To improve the yield, cDNA fragments encoding antigenic domains of p200 were expressed as fusions with GST. One of these fusion proteins, C1A-GST, is composed of a 7-kDa fragment of the p200 repeat region and contains epitopes that react strongly with sera from cattle experimentally infected with B. bigemina. Recombinant C1A-GST should permit the development of an improved enzyme-linked immunosorbent assay for the detection of antibodies against B. bigemina.

Physical mapping of a 670-kb region of chromosomes XVI and XVII from the human protozoan parasite Trypanosoma cruzi encompassing the genes for two immunodominant antigens

As part of the Trypanosoma cruzi Genome Initiative, we have mapped a large portion of the chromosomal bands XVI (2.3 Mb) and XVII (2.6 Mb) containing the highly repetitive and immunodominant antigenic gene families h49 and jl8. Restriction mapping of the isolated chromosomal bands and hybridization with chromosome specific gene probes showed that genes h49 and jl8 are located in a pair of size-polymorphic homologous chromosomes. To construct the integrated map of the chromosomes harboring the h49 and jl8 loci, we used YAC, cosmid, and lambda phage overlapping clones, and long range restriction analysis using a variety of probes (i.e., known gene sequences, ESTs, polymorphic repetitive sequences, anonymous sequences, STSs generated from the YAC ends). The total length covered by the YAC contig was approximately 670 kb, and its map agreed and was complementary to the one obtained by long-range restriction fragment analysis. Average genetic marker spacing in a 105 kb region around h49 and jl8 genes was estimated to be 6.2 kb/marker. We have detected some polymorphism in the H49/JL8 antigens-encoding chromosomes, affecting also the coding regions. The physical map of this region, together with the isolation of specific chromosome markers, will contribute in the global effort to sequence the nuclear genome of this parasite.

The cytoskeleton of trypanosomatid parasites

Species of the trypanosomatid parasite genera Trypanosoma and Leishmania exhibit a particular range of cell shapes that are defined by their internal cytoskeletons. The cytoskeleton is characterized by a subpellicular corset of microtubules that are cross-linked to each other and to the plasma membrane. Trypanosomatid cells possess an extremely precise organization of microtubules and filaments, with some of their organelles, such as the mitochondria, kinetoplasts, basal bodies, and flagella, present as single copies in each cell. The duplication of these structures and changes in their position during life cycle differentiations provide markers and insight into events involved in determining cell form and division. We have a rapidly increasing catalog of these structures, their molecular cytology, and their ontogeny. The current sophistication of available molecular genetic techniques for use in these organisms has allowed a new functional analysis of the cytoskeleton, including functions that are intrinsic to the proliferation and pathogenicity of these parasites.

The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex

Okuda, K., Esteva, M., Segura, E. L., and Bijovsky, A. T. 1999. The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex. Experimental Parasitology 92, 223-231. Proliferative forms of Trypanosoma cruzi, amastigotes and epimastigotes, have a cytostome, a specialized structure formed by an invagination of the flagellar pocket's membrane surrounded by microtubules and frequently followed by a row of vesicles. All this assemblage penetrates deeply into the cytoplasm overpassing the nucleus. This structure, together with the flagellar pocket, appears to play an important role in the nutrition of the parasite. We demonstrated that the monoclonal antibody 2C4, made-up against isolated flagellar complex of T. cruzi epimastigotes, recognizes a protein doublet of 76 and 87 kDa in total epimastigotes homogenate. The 76-kDa polypeptide is enriched in the detergent-soluble fraction whereas the 87-kDa polypeptide is highly represented in the insoluble fractions and the purified flagella. Immuno-fluorescence assays show the antigen as a small spot at the flagellar pocket region. Immunogold labeling of ultrathin sections of epimastigote forms reveals gold particles at the opening of flagellar pocket, concentrated in the cytostome region. Immunocytochemistry of epimastigote whole-mount cytoskeletons reveals the labeling on an array of three to four microtubules that appears attached to flagellum, running in the direction of the nucleus. Ultrastructural observations have shown that the posterior region of isolated flagella, corresponding to the level of the flagellar pocket, possesses a microtubular structure compatible with that from the cytostome. The relationship between the cytostome, an endocytic organelle, and the flagellum is here described for the first time.

Tandem Amino Acid Repeats From Trypanosoma cruzi Shed Antigens Increase the Half-Life of Proteins in Blood

Proteins containing amino acid repeats are widespread among protozoan parasites. It has been suggested that these repetitive structures act as immunomodulators, but other functional aspects may be of primary importance. We have recently suggested that tandem repeats present in Trypanosoma cruzi trans-sialidase stabilize the catalytic activity in blood. Because the parasite releasestrans-sialidase, this delayed clearance of the enzyme might have implications in vivo. In the present work, the ability of repetitive units from different T. cruzi molecules in stabilizing trans-sialidase activity in blood was evaluated. It is shown that repeats present on T. cruzi shed proteins (antigens 13 and Shed-Acute-Phase-Antigen [SAPA]) increase trans-sialidase half-life in blood from 7 to almost 35 hours. Conversely, those repeats present in intracellular T. cruzi proteins only increase the enzyme half-life in blood up to 15 hours. Despite these results, comparative analysis of structural and catalytic properties of both groups of chimeric enzymes show no substantial differences. Interestingly, antigens 13 and SAPA also increase the persistence in blood of chimeric glutathione S-transferases, thus suggesting that this effect is inherent to these repeats and independent of the carrier protein. Although the molecular basis of this phenomenon is still uncertain, its biotechnological potential can be envisaged.

The Trypanosoma cruzi genome project: nuclear karyotype and gene mapping of clone CL Brener

By using improved pulsed field gel electrophoresis conditions, the molecular karyotype of the reference clone CL Brener selected for Trypanosoma cruzi genome project was established. A total of 20 uniform chromosomal bands ranging in size from 0.45 to 3.5 Megabase pairs (Mbp) were resolved in a single run. The weighted sum of the chromosomal bands was approximately 87 Mbp. Chromoblots were hybridized with 39 different homologous probes, 13 of which identified single chromosomes. Several markers showed linkage and four different linkage groups were identified, each comprising two markers. Densitometric analysis suggests that most of the chromosomal bands contain two or more chromosomes representing either homologous chromosomes and/or heterologous chromosomes with similar sizes.

Towards the physical map of the Trypanosoma cruzi nuclear genome: construction of YAC and BAC libraries of the reference clone T. cruzi CL-Brener

Strategies to construct the physical map of the Trypanosoma cruzi nuclear genome have to capitalize on three main advantages of the parasite genome, namely (a) its small size, (b) the fact that all chromosomes can be defined, and many of them can be isolated by pulse field gel electrophoresis, and (c) the fact that simple Southern blots of electrophoretic karyotypes can be used to map sequence tagged sites and expressed sequence tags to chromosomal bands. A major drawback to cope with is the complexity of T. cruzi genetics, that hinders the construction of a comprehensive genetic map. As a first step towards physical mapping, we report the construction and partial characterization of a T. cruzi CL-Brener genomic library in yeast artificial chromosomes (YACs) that consists of 2,770 individual YACs with a mean insert size of 365 kb encompassing around 10 genomic equivalents. Two libraries in bacterial artificial chromosomes (BACs) have been constructed, BACI and BACII. Both libraries represent about three genome equivalents. A third BAC library (BAC III) is being constructed. YACs and BACs are invaluable tools for physical mapping. More generally, they have to be considered as a common resource for research in Chagas disease.

Cloning and characterization of a gene encoding a novel immunodominant antigen of Trypanosoma cruzi

A Trypanosoma cruzi genomic expression library was screened with a pool of sera obtained from chronic chagasic patients. The recombinant antigen (Tc40) isolated from this library reacted with a large number of serum samples of chronic chagasic patients, suggesting that the presence of anti-Tc40 antibodies may be specifically associated to Chagas' disease. The full-length sequence of the Tc40 gene was determined after isolation of genomic and cDNA clones. The Tc40 cDNA includes a large open reading frame (2745 bp-long) that encodes a polypeptide of 100 kDa without any homology with previously described T. cruzi sequences. In contrast with other T. cruzi antigens whose immunodominant B-cell epitopes are composed by amino acid repetitive motifs, Tc40 does not show any amino acid repetition. Antibodies against the Tc40 recombinant protein reacted with three native polypeptides of 100, 41 and 38 kDa which are tightly associated with membranes or cytoskeleton and expressed in all developmental stages of the parasite life cycle. A transcript of 3.9-kb was detected in Northern blot analysis which is large enough to encode a 100 kDa polypeptide. Tc40 genes were mapped on a chromosomal band of 1.1 Mbp and in a few copies per haploid genome in the G strain.

Plasmodium chabaudi: immunogenicity of a highly antigenic glutamate-rich protein

The immunogenicity of a 93-kDa Plasmodium chabaudi protein that contains glutamate-rich tandem repeats was investigated in this study. Immunoblotting with various monoclonal antibodies indicates that this 93-kDa protein is equivalent to a potential P. chabaudi RESA analogue. However, the sequence of the P. chabaudi protein does not exhibit any significant homology to Pf155/RESA. Antibodies against the 93-kDa protein appear early during P. chabaudi infection and reach high titers. The highest antibody titers are found when the parasitemia is descending, suggesting that this protein may play some role in immunity. Immunization of mice with the recombinant protein also results in high antibody titers, indicating that the protein is quite immunogenic. However, mice immunized with recombinant protein and challenged with P. chabaudi do not exhibit a delayed appearance of parasitemia, a reduced parasitemia, or a shortened duration of parasitemia. Glutamate-rich P. falciparum proteins such as Pf155/RESA, are being considered as vaccine candidates. The studies with P. chabaudi suggest that interpretation of serological data using glutamate-rich proteins should proceed with caution. The glutamate-rich repeats, although highly immunogenic, may not be important in host immunity against malaria. However, antibodies that appear late in the P. chabaudi infection do appear to play a role in anti-malarial immunity.

Detection of polymorphism in the Trypanosoma cruzi TcP2 beta gene family by single strand conformational analysis (SSCA)

Single strand conformation analysis (SSCA) is a technique that has been used to detect point mutations. We explored its usefulness in the analysis of four different members of the Trypanosoma cruzi TcP2 beta gene family and its suitability for detection of polymorphism in different parasite strains. The availability of primers covering a 97-bp sequence at the 5' end of the genes allowed assessment of the effect of a single base substitution, while the analysis of a 321 bp long sequence permitted the evaluation of sequences differing in several bases. PCR products were analysed under four different electrophoretic conditions: with or without the addition of 10% glycerol in a 6% polyacrylamide gel run at room temperature or at 4 degrees C. Shifts in mobility were radically dependent on the migration condition. Both 97-bp and 321-bp amplicons were best resolved at 4 degrees C, without glycerol. Amplification products derived from total genomic DNA showed a pattern that resembled closely a combination of the products derived from the cloned genes. The results herein demonstrate the usefulness of SSCA to differentiate forms of a complex protozoan gene family, and to scan its polymorphic nature. Furthermore, due to the remarkable sensitivity of the technique it can generate genomic markers, such as Sequence Tagged Sites (STS), of great need in the T. cruzi genome project.