Altered expression of neuroplasticity-related genes in the brain of depressed suicides

BACKGROUND

Expression of the neuronal membrane glycoprotein M6a (GPM6A), the proteolipid protein (PLP/DM20) family member, is downregulated in the hippocampus of chronically stressed animals. Its neuroplastic function involves a role in neurite formation, filopodium outgrowth and synaptogenesis through an unknown mechanism. Disruptions in neuroplasticity mechanisms have been shown to play a significant part in the etiology of depression. Thus, the current investigation examined whether GPM6A expression is also altered in human depressed brain.

METHODS

Expression levels and coexpression patterns of GPM6A, GPM6B, and PLP1 (two other members of PLP/DM20 family) as well as of the neuroplasticity-related genes identified to associate with GPM6A were determined using quantitative polymerase chain reaction (qPCR) in postmortem samples from the hippocampus (n = 18) and the prefrontal cortex (PFC) (n = 25) of depressed suicide victims and compared with control subjects (hippocampus n = 18; PFC n = 25). Neuroplasticity-related proteins that form complexes with GPM6A were identified by coimmunoprecipitation technique followed by mass spectrometry.

RESULTS

Results indicated transcriptional downregulation of GPM6A and GPM6B in the hippocampus of depressed suicides. The expression level of calcium/calmodulin-dependent protein kinase II alpha (CAMK2A) and coronin1A (CORO1A) was also significantly decreased. Subsequent analysis of coexpression patterns demonstrated coordinated gene expression in the hippocampus and in the PFC indicating that the function of these genes might be coregulated in the human brain. However, in the brain of depressed suicides this coordinated response was disrupted.

CONCLUSIONS

Disruption of coordinated gene expression as well as abnormalities in GPM6A and GPM6B expression and expression of the components of GPM6A complexes were detected in the brain of depressed suicides.

A superfamily of Trypanosoma cruzi surface antigens

Several genes o f Trypanosoma cruzi encode surface antigens that include an amino acid motif that is conserved among bacterial neurominidases. Oscar Campetella, Daniel Sdnchez, Juan Jose Cazzulo and Alberto Carlos Frasch here suggest grouping these gene families in a superfamily.

TcUBP-1, a developmentally regulated U-rich RNA-binding protein involved in selective mRNA destabilization in trypanosomes

Developmental stages of the trypanosome life cycle differ in their morphology, biology, and biochemical properties. Consequently, several proteins have to be tightly regulated in their expression to allow trypanosomes to adapt rapidly to sudden environmental changes, a process that might be of central importance for parasite survival. However, in contrast to higher eukaryotic cells, trypanosomes do not seem to regulate gene expression through regulation of transcription initiation. These parasites make use of post-transcriptional regulatory mechanisms and modification of mRNA half-life is a relevant one. Trans-acting factors binding to cis-elements that affect mRNA stability of mature transcripts have not been identified in these cells. In this work, a novel U-rich RNA-binding protein (TcUBP-1) from Trypanosoma cruzi, the agent of Chagas disease, was identified. Its structure includes an RNA recognition motif, a nuclear export signal, and auxiliary domains with glycine- and glutamine-rich regions. TcUBP-1 recognizes the 44-nucleotide AU-rich RNA instability element located in the 3'-untranslated region of mucin SMUG mRNAs (Di Noia, J. M., D'Orso, I., Sanchez, D. O., and Frasch, A. C. (2000) J. Biol. Chem. 275, 10218-10227) as well as GU-rich sequences. Over-expression of TcUBP-1 in trypanosomes decreases the half-life of SMUG mucin mRNAs in vivo but does not affect the stability of other parasite mRNAs. Because TcUBP-1 is developmentally regulated, it might have a relevant role in regulating protein expression during trypanosome differentiation, allowing a correct expression pattern of U-rich-containing mRNAs.

Functionally different AU- and G-rich cis-elements confer developmentally regulated mRNA stability in Trypanosoma cruzi by interaction with specific RNA-binding proteins

Post-transcriptional regulatory mechanisms have been suggested to be the main point of control of gene expression in kinetoplastid parasites. We have previously shown that Trypanosoma cruzi SMUG mucin mRNA steady-state level is developmentally regulated by post-transcriptional mechanisms, being stable in the epimastigote insect vector stage, but unstable in the trypomastigote infective stage of the parasite. Its turnover is controlled by an AU-rich element (ARE) localized in the 3'-untranslated region, since a reporter gene lacking this sequence was stable in the trypomastigote stage (Di Noia, J. M., D'Orso, I., Sanchez, D. O., and Frasch, A. C. (2000) J. Biol. Chem. 275, 10218-10227). Here, we show by gel mobility shift assay that the 44-nt ARE sequence interacts with a set of stage-specific AU-rich element RNA-binding proteins (ARE-BPs). The epimastigote stage AU-rich element RNA-binding protein, named E-ARE-BP, and the trypomastigote stage ARE-BPs, named T-ARE-BPs, are efficiently competed by poly(U). UV cross-linking analysis showed that E-ARE-BP has an apparent molecular mass of 100 kDa and is different from the 45-50-kDa ARE-BPs present in other stages of the parasite. Transfection experiments allowed the identification of a novel cis-element that might be responsible for a positive effect on mRNA stability. It is a G-rich element, named GRE, composed by two contiguous CGGGG pentamers. The factors that recognize GRE were different from the ones that bind to ARE, in both molecular masses and subcellular localization. Thus, ARE and GRE are functionally different cis-elements, which might regulate mucin expression throughout the parasite life cycle.

Probing molecular function of trypanosomal sialidases: single point mutations can change substrate specificity and increase hydrolytic activity

Sialidases are present on the surface of several trypanosomatid protozoan parasites. They are highly specific for sialic acid linked in alpha-(2,3) to a terminal beta-galactose and include the strictly hydrolytic enzymes and trans-sialidases (sialyl-transferases). Based on the structural comparison of the sialidase from Trypanosoma rangeli and the trans-sialidase from T. cruzi (the agent of Chagas' disease in humans), we have explored the role of specific amino acid residues sought to be important for substrate specificity. The substitution of a conserved tryptophanyl residue in the two enzymes, Trp312/313-Ala, changed substrate specificity, rendering the point mutants capable to hydrolyze both alpha-(2,3)- and alpha-(2,6)-linked sialoconjugates. The same mutation abolished sialyl-transferase activity, indicating that transfer (but not hydrolysis) requires a precise orientation of the bound substrate. The exchange substitution of another residue that modulates oligosaccharide binding, Gln284-Pro, was found to significantly increase the hydrolytic activity of sialidase, and residue Tyr119 was confirmed to be part of a second binding site for the acceptor substrate in trans-sialidase. Together with the structural information, these results provide a consistent framework to account for the unique enzymatic properties of trypanosome trans-sialidases.

Gene discovery through genomic sequencing of Brucella abortus

Brucella abortus is the etiological agent of brucellosis, a disease that affects bovines and human. We generated DNA random sequences from the genome of B. abortus strain 2308 in order to characterize molecular targets that might be useful for developing immunological or chemotherapeutic strategies against this pathogen. The partial sequencing of 1,899 clones allowed the identification of 1,199 genomic sequence surveys (GSSs) with high homology (BLAST expect value < 10(-5)) to sequences deposited in the GenBank databases. Among them, 925 represent putative novel genes for the Brucella genus. Out of 925 nonredundant GSSs, 470 were classified in 15 categories based on cellular function. Seven hundred GSSs showed no significant database matches and remain available for further studies in order to identify their function. A high number of GSSs with homology to Agrobacterium tumefaciens and Rhizobium meliloti proteins were observed, thus confirming their close phylogenetic relationship. Among them, several GSSs showed high similarity with genes related to nodule nitrogen fixation, synthesis of nod factors, nodulation protein symbiotic plasmid, and nodule bacteroid differentiation. We have also identified several B. abortus homologs of virulence and pathogenesis genes from other pathogens, including a homolog to both the Shda gene from Salmonella enterica serovar Typhimurium and the AidA-1 gene from Escherichia coli. Other GSSs displayed significant homologies to genes encoding components of the type III and type IV secretion machineries, suggesting that Brucella might also have an active type III secretion machinery.

A random sequencing approach for the analysis of the Trypanosoma cruzi genome: general structure, large gene and repetitive DNA families, and gene discovery

A random sequence survey of the genome of Trypanosoma cruzi, the agent of Chagas disease, was performed and 11,459 genomic sequences were obtained, resulting in approximately 4.3 Mb of readable sequences or approximately 10% of the parasite haploid genome. The estimated total GC content was 50.9%, with a high representation of A and T di- and trinucleotide repeats. Out of the estimated 5000 parasite genes, 947 putative new genes were identified. Another 1723 sequences corresponded to genes detected previously in T. cruzi through expression sequence tag analysis. 7735 sequences had no matches in the database, but the presence of open reading frames that passed Fickett's test suggests that some might contain coding DNA. The survey was highly redundant, with approximately 35% of the sequences included in a few large sequence families. Some of them code for protein families present in dozens of copies, including proteins essential for parasite survival and retrotransposons. Other sequence families include repetitive DNA present in thousands of copies per haploid genome. Some families in the latter group are new, parasite-specific, repetitive DNAs. These results suggest that T. cruzi could constitute an interesting model to analyze gene and genome evolution due to its plasticity in terms of sequence amplification and divergence. Additional information can be found at http://www.iib.unsam.edu.ar/tcruzi.gss. html.

Trypanosoma cruzi surface mucins with exposed variant epitopes

The protozoan parasite Trypanosoma cruzi, the agent of Chagas disease, has a large number of mucin molecules on its surface, whose expression is regulated during the life cycle. These mucins are the main acceptors of sialic acid, a monosaccharide that is required by the parasite to infect and survive in the mammalian host. A large mucin-like gene family named TcMUC containing about 500 members has been identified previously in T. cruzi. TcMUC can be divided into two subfamilies according to the presence or absence of tandem repeats in the central region of the genes. In this work, T. cruzi parasites were transfected with one tagged member of each subfamily. Only the product from the gene with repeats was highly O-glycosylated in vivo. The O-linked oligosaccharides consisted mainly of beta-d-Galp(1-->4)GlcNAc and beta-d-Galp(1-->4)[beta-d-Galp(1-->6)]-d-GlcNAc. The same glycosyl moieties were found in endogenous mucins. The mature product was anchored by glycosylphosphatidylinositol to the plasma membrane and exposed to the medium. Sera from infected mice recognized the recombinant product of one repeats-containing gene thus showing that they are expressed during the infection. TcMUC genes encode a hypervariable region at the N terminus. We now show that the hypervariable region is indeed present in the exposed mature N termini of the mucins because sera from infected hosts recognized peptides having sequences from this region. The results are discussed in comparison with the mucins from the insect stages of the parasite (Di Noia, J. M., D'Orso, I., Sánchez, D. O., and Frasch, A. C. C. (2000) J. Biol. Chem. 275, 10218-10227) which do not have variable regions.

Functional diversity in the trans-sialidase and mucin families in Trypanosoma cruzi

Trypanosomes are unable to synthesize the monosaccharide sialic acid, but some African trypanosomes and the American Trypanosoma cruzi can incorporate sialic acid derived from the host. To do so, T. cruzi expresses a trans-sialidase, an enzyme that catalyzes the transfer of sialic acid from host glycoconjugates to mucin-like molecules located on the parasite surface membrane. The importance of the process is indicated by the fact that T. cruzi has hundreds of genes encoding trans-sialidase, trans-sialidase-like proteins and mucin core proteins. Sequence divergence of members of these families has resulted in some molecules having functions unrelated to the acquisition of sialic acid. In this article, Alberto Frasch reviews the structure and possible function of the proteins making up these families.

AU-rich elements in the 3'-untranslated region of a new mucin-type gene family of Trypanosoma cruzi confers mRNA instability and modulates translation efficiency

Trypanosoma cruzi has a complex mucin gene family of 500 members with hypervariable regions expressed preferentially in vertebrate associated stages of the parasite. In this work, a novel mucin-type gene family is reported, composed of two groups of genes organized in independent tandems and having very short open reading frames. The structures of deduced proteins share the N and C termini but differ in central regions. One group has repeats with the consensus Lys-Asn-Thr(7)-Ser-Thr(3)-Ser(Ser/Lys)-Ala-Pro and the other a Thr-rich sequence of the type Asp-Gln-Thr(17-20)-Asn-Ala-Pro-Ala-Lys-Asp-Thr(5-7)-Asn-Ala-Pro-Ala-L ys. In both cases, expected mature core proteins are around 7 kDa. Both groups, named L and S, respectively, differ in the structure of genomic loci and mRNA, with differential blocks in the 3'-untranslated region. The highest mRNA level for S and L groups are in the epimastigote stage but they show distinct developmentally regulated patterns. Transcripts are short lived and their steady-state abundance is regulated post-transcriptionally with increased mRNA stability in insect stage epimastigote. AU-rich sequences, similar to ARE motives known to cause mRNA instability in higher eukaryotes, are present in the 3'-untranslated region of the transcripts. In transfection experiments this sequence is shown to be functional for the L group destabilizing its mRNA in a stage-specific manner. Furthermore, an effect of this AU-rich region on translation efficiency is shown. To our knowledge, this is the first time that a functional ARE sequence-dependent post-transcriptional regulation mechanism is reported in a lower eukaryote.

[Organization of the network for the study of the Trypanosoma cruzi genome]

Five years ago the Special Programme for Research and Training in Tropical Diseases (TDR) from the World Health Organization (WHO) launched the Parasite Genome Project. The aims were to obtain information on genome organization and gene discovery in five parasites, namely, Schistosoma, Filaria, Leishmania and Trypanosomas brucei and cruzi. Organization of research networks for each parasite under study, promotion of international collaboration and training of researchers in developing countries, were also main objectives of the programme. After five years, a large amount of information has been obtained, which is now available to researchers in the field.

Structural basis of sialyltransferase activity in trypanosomal sialidases

The intracellular parasite Trypanosoma cruzi, the etiological agent of Chagas disease, sheds a developmentally regulated surface trans-sialidase, which is involved in key aspects of parasite-host cell interactions. Although it shares a common active site architecture with bacterial neuraminidases, the T.cruzi enzyme behaves as a highly efficient sialyltransferase. Here we report the crystal structure of the closely related Trypanosoma rangeli sialidase and its complex with inhibitor. The enzyme folds into two distinct domains: a catalytic beta-propeller fold tightly associated with a lectin-like domain. Comparison with the modeled structure of T.cruzi trans-sialidase and mutagenesis experiments allowed the identification of amino acid substitutions within the active site cleft that modulate sialyltransferase activity and suggest the presence of a distinct binding site for the acceptor carbohydrate. The structures of the Trypanosoma enzymes illustrate how a glycosidase scaffold can achieve efficient glycosyltransferase activity and provide a framework for structure-based drug design.

Enzymically inactive members of the trans-sialidase family from Trypanosoma cruzi display beta-galactose binding activity

trans-sialidase is a unique sialidase in that, instead of hydrolizing sialic acid, it preferentially transfers the monosaccharide to a terminal beta-galactose in glycoproteins and glycolipids. This enzyme, originally identified in Trypanosoma cruzi, belongs to a large family of proteins. Some members of the family lack the enzymatic activity. No function has been yet assigned to them. In this work, the gene copy number and the possible function of inactive members of the trans -sialidase family was studied. It is shown that genes encoding inactive members are not a few, but rather, are present in the same copy number (60-80 per haploid genome) as those encoding active trans -sialidases. Recombinant inactive proteins were purified and assayed for sialic acid and galactose binding activity in agglutination tests. The enzymatically inactive trans -sialidases were found to agglutinate de-sialylated erythrocytes but not untreated red blood cells. Assays made with mouse and rabbit red blood cells suggest that inactive trans -sialidases bind to beta, rather than alpha, terminal galactoses, the same specificity required by active trans -sialidases. A recombinant molecule that was made enzymatically inactive through a mutation in a single amino acid also retained the galactose binding activity. The binding was competed by lactose and was dependent on conservation of the protein native conformation. Therefore, at least some molecules in the trans -sialidase family that have lost their enzymatic function still retain their Gal-binding properties and might have a function as lectins in the parasite-host interaction.

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 releases trans-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.

Structure of the glycosylphosphatidylinositol-anchor of the trans-sialidase from Trypanosoma cruzi metacyclic trypomastigote forms

Both, culture-derived and metacyclic trypomastigotes of Trypanosoma cruzi shed a glycoprotein, the shed acute phase antigen, that is responsible for the trans-sialidase activity. In the present work the structure of the glycosylphosphatidylinositol membrane anchor of the trans-sialidase isolated from metacyclic forms was determined. Parasites were metabolically labelled with [9, 10(n)3H]-palmitic acid and the glycoprotein was purified by immunoprecipitation with a monoclonal antibody directed against the repetitive aminoacid sequence. Treatment of the glycoprotein with phosphatidylinositol phospholipase C (PI-PLC) from Bacillus thuringiensis rendered a lipid that comigrated in TLC with a standard of ceramide. No alkylglycerol was detected in contrast with the results previously obtained for the trans-sialidase isolated from culture-derived trypomastigotes where both lipids were found. Chemical and chromatographic analysis showed that the lipid moiety is palmitoyldihydrosphingosine with a minor amount of stearoyldihydrosphingosine. The glycan constituent of the glycosylphosphatidylinositol-anchor was analysed by nitrous acid deamination of the aqueous phase of the PI-PLC treatment, followed by reduction with NaBH4 and hydrolysis of the phosphodiester with aqueous hydrofluoric acid. A major oligosaccharide was obtained and enzymatic treatment with exoglycosidases and further chromatography in a high pH anion exchange system showed that the trimannosyl core backbone is substituted by an alpha-galactose. A comparison between the lipid constituent of the glycosylphosphatidylinositol anchor of several proteins and their spontaneous shedding by the action of an endogenous PI-PLC was made.

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.

Vector development for the expression of foreign proteins in the vaccine strain Brucella abortus S19

A vector for the expression of foreign antigens in the vaccine strain Brucella abortus S19 was developed by using a DNA fragment containing the regulatory sequences and the signal peptide of the Brucella bcsp31 gene. This fragment was cloned in broad-host-range plasmid pBBR4MCS, resulting in plasmid pBEV. As a reporter protein, a repetitive antigen of Trypanosoma cruzi was used. The recombinant fusion protein is stably expressed and secreted into the Brucella periplasmic space, inducing a good antibody response against the T. cruzi antigen. The expression of the repetitive antigen in Brucella neither altered its growth pattern nor generated a toxic or lethal effect during experimental infection. The application of this strategy for the generation of live recombinant vaccines and the tagging of B. abortus S19 vaccine is discussed. This is the first time that a recombinant protein has been expressed in the periplasm of brucellae.

The Trypanosoma cruzi mucin family is transcribed from hundreds of genes having hypervariable regions

In previous works we have identified genes in the protozoan parasite Trypanosoma cruzi whose structure resemble those of mammalian mucin genes. Indirect evidence suggested that these genes might encode the core protein of parasite mucins, glycoproteins that were proposed to be involved in the interaction with, and invasion of, mammalian host cells. We now show that the mucin gene family from T. cruzi is much larger and diverse than expected. A minimal number of 484 mucin genes per haploid genome is calculated for a parasite clone. Most, if not all, genes are transcribed, as deduced from cDNA analysis. Comparison of the cDNA sequences showed evidences of a high mutation rate in localized regions of the genes. Sequence conservation among members of the family is much higher in the untranslated (UTR) regions than in the sequences encoding the mature mucin core protein. Transcription units can be classified into two main subfamilies according to the sequence homologies in the 5'-UTR, whereas the 3'-UTR is highly conserved in all clones analyzed. The common origin of members of this gene family as well as their relationships can be defined by sequence comparison of different domains in the transcription units. The regions encoding the N and C termini, supposed to correspond to the leader peptide and membrane-anchoring signal, respectively, (Di Noia, J. M., Sánchez, D. O., and Frasch, A. C. C. (1995) J. Biol. Chem. 270, 24146-24149) are highly conserved. Conversely, the central regions are highly variable. These regions encode the target sites for O-glycosylation and are made of a variable number of repetitive units rich in Thr and Pro residues or are nonrepetitive but still rich in Thr/Ser and Pro residues. The region putatively coding for the N-terminal domain of the mature core protein is hypervariable, being different in most of the transcripts sequenced. Nonrepetitive central domains are unique to each gene. Gene-specific probes show that the relative abundance of different mRNAs varies greatly within the same parasite clone.

Use of trans-Sialidase inhibition assay in a population serologically negative for Trypanosoma cruzi but at a high risk of infection

trans-Sialidase inhibition assay (TIA) was employed in a population at high risk of Trypanosoma cruzi infection. From 20 serum samples that were negative by conventional serologic and parasitologic assays, 18 (90%) were reactive in TIA, providing further evidence of the higher sensitivity of TIA and suggesting that the actual prevalence of T. cruzi infection might be underestimated.

The NADP+-linked glutamate dehydrogenase from Trypanosoma cruzi: sequence, genomic organization and expression

NADP-linked glutamate dehydrogenase (NADP+-GluDH, EC 1.4.1.4) has been purified to homogeneity from epimastigotes of Trypanosoma cruzi by an improved procedure, and the amino acid sequences of 11 internal peptides obtained by digestion with trypsin, endopeptidase Lys-C, endopeptidase Arg-C or CNBr have been obtained. Using oligonucleotide primers synthesized according to the amino acid sequence of the N-terminus of the mature enzyme and to the nucleotide sequence of a clone corresponding to the C-terminus, obtained by immunological screening of an expression library, two complete open reading frames (TcGluDH1 and TcGluDH2) were isolated and sequenced. The sequences obtained are most similar to that of the NADP+-GluDH of Escherichia coli (70-72% identity), and less similar (50-56%) to those of lower eukaryotes. Using TcGluDH1 as a probe, evidence for the presence of several genes and developmental regulation of the expression of NADP+-GluDH in different parasite stages was obtained. TcGluDH1 encodes an enzymically active protein, since its expression in E. coli resulted in the production of a GluDH activity with kinetic parameters similar to those of the natural enzyme.

The repetitive domain of Trypanosoma cruzi trans-sialidase enhances the immune response against the catalytic domain

Trypanosoma cruzi trans-sialidase consists of a C-terminal domain composed essentially of immunodominant amino acid repeat units (SAPA-repeats) and an amino region responsible for the enzymatic activity (catalytic domain). To investigate the possible function(s) of SAPA-repeats, recombinant trans-sialidases either containing or lacking the C-terminal region were tested in mice. The presence of SAPA-repeats in the intravenously injected protein has two consequences. First, they enhance the persistence of the trans-sialidase activity in blood. Second, SAPA-repeats promoted the production of antibodies directed to the catalytic domain that inhibit trans-sialidase activity. These results suggest that SAPA-repeats modulate the trans-sialidase activity in blood.

Trypanosoma rangeli sialidase: cloning, expression and similarity to T. cruzi trans-sialidase

Sialidases are hydrolytic enzymes present from virus to higher eukaryotes, catalyzing the removal of sialic acid from glycoconjugates. Some protozoa Trypanosomatidae secrete high levels of sialidase into the medium. We have now purified the secreted sialidase from Trypanosoma rangeli. Its N-terminal sequence reveals 100% identity with the corresponding region of the trans-sialidase from T. cruzi. Trans-sialidase, although homologous to viral and bacterial sialidases, displays a novel sialyltransferase activity and is involved in host cell invasion. Several homologous transsialidase-like genes were cloned from genomic DNA of T. rangeli, and grouped in three subfamilies. Active sialidase-encoding genes were found in one of them. The recombinant sialidase shows similar properties to those of the native enzyme, including undetectable trans-sialidase activity. Nevertheless, it has an overall identity of 68.9% with the catalytic domain of T. cruzi trans-sialidase, increasing to 86.7% admitting conservative substitutions. Only three other eukaryotic sialidases have been previously cloned, none of them showing significant homology to trans-sialidase. The isolation of a highly similar sialidase is relevant to further identify the molecular determinants allowing trans-sialidase activity. As a first approach, chimeric constructs between sialidase and trans-sialidase were generated, one of them rendering a sialidase with three times lower Km than the natural enzyme.

The trans-sialidase of Trypanosoma cruzi is anchored by two different lipids

The trans-sialidase from the trypomastigote stage of Trypanosoma cruzi was metabolically labeled with [3H]-palmitic acid and purified by immunoprecipitation with a monoclonal antibody. The action of PI-PLC on the immunoprecipitate released a lipid that was analyzed by TLC. Lyso-1-O-hexadecylglycerol and N-palmitoyl-sphinganine were obtained in a 1:3 ratio. A comparison with the GPI anchors present in the different stages of T. cruzi was made.

Long-lasting antibodies detected by a trans-sialidase inhibition assay of sera from parasite-free, serologically cured chagasic patients

A test based on the inhibition by antibodies of the trans-sialidase was used to analyze infection by Trypanosoma cruzi, the agent of Chagas' disease. Sera collected during the longitudinal follow-up of benznidazole-treated acutely and congenitally infected patients became negative for T. cruzi as determined by tests presently used to assess cure; however, the sera remained positive for T. cruzi by the trans-sialidase inhibition assay (TIA) up to 14 years after treatment. Therefore, TIA is a highly sensitive marker for previous T. cruzi infection.

High diversity in mucin genes and mucin molecules in Trypanosoma cruzi

Mucins are highly O-glycosylated molecules which in mammalian cells accomplish essential functions, like cytoprotection and cell-cell interactions. In the protozoan parasite Trypanosoma cruzi, mucin-related glycoproteins have been shown to play a relevant role in the interaction with and invasion of host cells. We have previously reported a family of mucin-like genes in T. cruzi whose overall structure resembled that of mammalian mucin genes. We have now analyzed the relationship between these genes and mucin proteins. A monoclonal antibody specific for a mucin sugar epitope and a polyclonal serum directed to peptide epitopes in a MUC gene-encoded recombinant protein, detected identical bands in three out of seven strains of T. cruzi. Immunoprecipitation experiments confirmed these results. When expressed in eukaryotic cells, the MUC gene product is post-translationally modified, most likely, through extensive O-glycosylation. Gene sequencing showed that the central domains encoding the repeated sequences with the consensus T8KP2, varies in number from 1 to 10, and the number of Thr residues in each repeat could be 7, 8, or 10. A run of 16 to 18 Thr residues was present in some, but not all, MUC gene-derived sequences. Direct compositional analysis of mucin core proteins showed that Thr residues are much more frequent than Ser residues. The same fact occurs in MUC gene-derived protein sequences. Molecular mass determinations of the 35-kDa glycoproteins further extend the heterogeneity of the family to the natural mucin molecules. Difficulties in assigning each of the several MUC genes identified to a mucin product arise from the high diversity and partial sequence conservation of the members of this family.

A putative pyruvate dehydrogenase alpha subunit gene from Trypanosoma cruzi

A full-length DNA clone encoding a putative pyruvate dehydrogenase alpha subunit (E1 alpha) gene was isolated from a Trypanosoma cruzi (RA strain) DNA library. Sequencing of this clone revealed it to encode a 378 amino acid protein (M(r) 42774) with high sequence similarity to E1 alpha obtained from different sources. The highest score is obtained with human E1 alpha: 43,3% similarity. Southern blot analysis is consistent with the existence of a single copy of this putative T. cruzi E1 alpha gene per haploid genome in different parasite strains. Expression of this gene was demonstrated by Northern blot analysis and its trans-splicing acceptor site was identified by Polymerase Chain Reaction-mediated amplification of its cDNA.

Mapping the Trypanosoma cruzi genome: analyses of representative cosmid libraries

In order to generate contiguous cosmid coverage of the genome of the protozoan parasite Trypanosoma cruzi for large-scale sequence analysis, a cosmid library of 36864 individual, primary clones was generated. Total genomic DNA of the reference strain CL Brener was fragmented both by partial digestion with MboI and by physical shearing. For cloning, a modified cosmid vector was used that simplifies analyses such as restriction mapping. The library's representation is about 25 genome equivalents, assuming a size of 55 Mb per haploid genome. No chimerism of inserts in the clones could be detected. The colinearity between cosmid inserts and genomic DNA was verified. Also, hybridizations to the gel-separated karyotype of the organism were carried out as a quality check. Gridded onto two nylon filters, the library was analyzed with a variety of probes. Apart from being used for combined physical and transcriptional mapping of the genome, library filters and clones are also available to interested parties.

Medium scale production and purification to homogeneity of a recombinant trans-sialidase from Trypanosoma cruzi

Trypanosoma cruzi, the agent of Chagas' disease, presents an enzyme that catalyzes the transfer of sialic acid among glycoproteins and glycolipids known as trans-sialidase (TS), displaying some interesting features: 1) It differs from all other eucaryotic sialyltransferases, both kinetically and in substrate specificity and 2) it is involved in the parasite's mechanism of mammalian host cell invasion. We report here the production and purification to homogeneity of an enzymatically active recombinant TS (rTS) lacking the C-terminal amino acid repeats, using iminodiacetic metal affinity chromatography. Initial ratios of non-fusion recombinant versus total protein were very low in several expression systems tested, mainly due to high degradation rate. However, after purifying 1,330 times, we were able to obtain an essentially homogeneous preparation of rTS with a final yield of 29%. After minor changes, a modified protocol for a medium scale production was designed obtaining 0.5 mg of homogeneous rTS per liter of bacterial culture. The purified rTS behaved as a homogeneous protein in silver-stained denaturing gels, isoelectrofocusing and N-terminal sequencing having identical pH and temperature optima as the natural enzyme. Conditions to keep the rTS for long periods without a significant loss of activity were identified.

Effect of primary structure modifications in Trypanosoma cruzi neuraminidase/trans-sialidase activities

Neuraminidases have been implicated in various processes involving the interaction of pathogens and their receptor cells. Trypanosoma cruzi, the agent of Chagas disease, has an unusual neuraminidase, able to transfer bound alpha(2-3)sialic acid to a suitable acceptor rather than to water: the trans-sialidase (TcTS). This enzyme is encoded by a family of several homologous genes, some of them rendering inactive the products. We have shown that enzymatically active proteins have Tyr in position 342, whereas inactive TcTS contain a His342. We have now mutated this Tyr residue to Phe or Thr. Both mutant proteins resulted in enzymatically inactive products. Chimeras expressing parts of Salmonella typhimurium neuraminidase (NANH) and TcTS were constructed. Only the construct containing the complete NANH molecule fused to the last 272 residues of TcTS had neuraminidase activity. However this construct did not present TcTS activity. This finding suggests that other residues present in the homology region are required for TcTS activity.

The protozoan Trypanosoma cruzi has a family of genes resembling the mucin genes of mammalian cells

Mucins are heavily O-glycosylated Thr/Ser/Pro-rich molecules. Given their relevant functions, mucins and their genes have been mainly studied in higher eukaryotes. In the protozoan parasite Trypanosoma cruzi, mucin-like glycoproteins were shown to play an important role in the interaction with the surface of the mammalian cell during the invasion process. We show now that this parasite has a family of putative mucin genes, whose organization resembles the one present in mammalian cells. Different parasite isolates have different sets of genes, as defined by their central domain. Central domains, rich in codons for Thr and/or Ser and Pro residues, are made up of either a variable number of repeat units in tandem or non-repetitive sequences. Conversely, 5'- and 3'-ends from different genes in different isolates have similar sequences, suggesting their common origin. Comparison of deduced amino acid sequences revealed that all members of the family have the same putative signal peptide on the N terminus and a putative sequence for glycophosphatidylinositol anchoring on the C terminus. The deduced molecular mass of the core proteins is small (from 17 to 21 kDa), in agreement with the 1-kilobase size of the mRNA detected. Putative mucin genes in T. cruzi are located on large chromosomal bands of about 1.6-2.2 megabase pairs.

A single tyrosine differentiates active and inactive Trypanosoma cruzi trans-sialidases

Several genes encode members of the Trypanosoma cruzi (Tc) trans-sialidase (TS) family. These proteins contain an enzymatic domain on the N terminus, the only one required for TS activity, and an antigenic domain (SAPA (shed acute phase antigen) amino acid (aa) repeats) on the C terminus. Only some members of this glycoprotein family are enzymatically active. The complete sequence of two clones encoding the enzymatic domain of active and inactive protein from each of two Tc strains has now been obtained. Comparison of these sequences showed a limited divergence among them: 20 out of the 642 deduced aa in the enzymatic domain were found to differ. From these 20 aa, only one was found to be essential for enzymatic activity. A Tyr342 residue is deduced in both active proteins while a His342 is present in both inactive ones. This naturally occurring Tyr342-->His substitution completely abolished the TS activity. In addition to Tyr342, a second deduced aa, Pro231, was found to be necessary for full enzymatic TS activity; a Pro231-->Ala change rendered the TS protein partially active. Fourteen aa residues, including Tyr342, out of the 16 aa in the active site of a sialidase from Salmonella typhimurium are present at the same or very similar positions in the Tc TS.

Chromosome specific markers reveal conserved linkage groups in spite of extensive chromosomal size variation in Trypanosoma cruzi

The karyotypes of three cloned stocks, CL Brener (CL), CA I/72 (CA) and Sylvio X10/7 (X10), of Trypanosoma cruzi were studied by pulsed-field gel electrophoresis followed by ethidium bromide staining and hybridization with 35 different probes, 30 of which identified single chromosomes. The chromosome-specific probes identified between 26 and 31 chromosomal bands in the three cloned stocks, corresponding to 20 unique chromosomes in CL and 19 in CA and X10. Considering the DNA content of the parasite, it was predicted that the markers recognise at least half of all T. cruzi chromosomes. A majority of identified chromosomes showed large differences in size among different strains, in some cases by up to 50%. Interestingly, CL had in general larger chromosomes than the two other studied cloned stocks. Several of the markers showed linkage and nine different linkage groups were identified, each comprising 2-4 markers. The linkage between the markers was maintained in 8 of the 9 linkage groups when a panel comprising 26 different T. cruzi strains representing major T. cruzi populations was tested. One linkage group was found to be maintained in some strains but not in others. This result shows that chromosomal rearrangements occur in the T. cruzi genome, albeit with a low frequency. Repetitive DNA, both non-coding and in one case coding, was more abundant in the cloned stock CL Brener than in CA and X10. The information presented will make it possible to select chromosomes for the construction of physical chromosomal maps required for the T. cruzi genome project.

Trypanosoma cruzi exoantigen is a member of a 160 kDa gene family

During the chronic stage of Chagas disease a 160 kDa antigen appears in the blood of patients and remains detectable many years after the onset of the disease. This antigen is secreted by the trypomastigote form of the parasite while it is undetectable in the epimastigote form. We report here that the chronic 160 kDa exoantigen is encoded by a gene family (CEA 160 family). We describe the cloning and partial nucleotide sequence of a gene (CEA 160-1) belonging to the CEA160 family. Comparison of the gene sequence with other sequences present in the databases revealed homologies with several Trypanosoma cruzi surface antigens. Highest amino acid identity (59%) was with members of a family containing epitopes that mimic nervous tissues (Van Voorhis et al. 1993). Another related group (18-22% amino acid identity) comprises proteins of 85 or 160 kDa sharing an amino acid motif that is conserved among bacterial neuraminidases (Fouts et al. 1991; Pollevick et al. 1991; Kahn et al. 1991; Takle & Cross, 1991; Franco et al. 1993). The amino acid identities with the different antigens were not homogeneously distributed. Regions of higher identity (40-60%) were grouped in the central region of each protein.

Antibodies inhibiting Trypanosoma cruzi trans-sialidase activity in sera from human infections

Trans-sialidase, an enzyme that transfers sialic acid among macromolecules, has been implicated in invasion of host cells by Trypanosoma cruzi, the agent of Chagas' disease. Most antibodies produced in natural and experimental infections are directed to the highly antigenic C-terminal domain (shed acute-phase antigen). These antibodies do not inhibit the trans-sialidase activity, which is present in the N-terminal domain of the molecule. Antibodies able to inhibit trans-sialidase in sera from human infections have been found. TIA (trans-sialidase inhibition assay) was positive in sera from patients with acute and chronic infections. Healthy and congenitally infected infants born to mothers with Chagas' disease were also TIA-positive, but the antibody titers diminished within months after birth or after treatment. Thus, antibodies neutralizing trans-sialidase are detectable in most forms of T. cruzi human infections, and TIA may be useful in the diagnosis of Chagas' disease.

Immunoassay with recombinant Trypanosoma cruzi antigens potentially useful for screening donated blood and diagnosing Chagas disease

A new enzyme immunoassay (EIA), the Dia Kit Bio-Chagas assay (Gador S.A.), is potentially useful for detecting antibodies to Trypanosoma cruzi in the diagnosis of infected individuals and the screening of blood in blood banks. The EIA is carried out on test strips of plastic backing covered with a nitrocellulose membrane to which a mixture of recombinant T. cruzi antigens 1, 2, shed acute-phase antigen, 13, and 30 has been applied as a horizontal line. A horizontal line of human IgG is included to monitor the test procedure. The test strips exhibited homogeneity in the adsorption of the mixture of recombinant antigens (CV = 6.0%) and the human IgG (CV = 8.6%). The EIA results obtained with sera positive by xenodiagnosis showed 100% agreement between both types of tests; tested against sera with positive and negative matched results of indirect hemagglutination, immunofluorescence, and ELISA, the EIA results agreed for 99.1% (347 of 350) and 99.6% (299 of 300) of the samples, respectively.

Immunoassay with recombinant Trypanosoma cruzi antigens potentially useful for screening donated blood and diagnosing Chagas disease

A new enzyme immunoassay (EIA), the Dia Kit Bio-Chagas assay (Gador S.A.), is potentially useful for detecting antibodies to Trypanosoma cruzi in the diagnosis of infected individuals and the screening of blood in blood banks. The EIA is carried out on test strips of plastic backing covered with a nitrocellulose membrane to which a mixture of recombinant T. cruzi antigens 1, 2, shed acute-phase antigen, 13, and 30 has been applied as a horizontal line. A horizontal line of human IgG is included to monitor the test procedure. The test strips exhibited homogeneity in the adsorption of the mixture of recombinant antigens (CV = 6.0%) and the human IgG (CV = 8.6%). The EIA results obtained with sera positive by xenodiagnosis showed 100% agreement between both types of tests; tested against sera with positive and negative matched results of indirect hemagglutination, immunofluorescence, and ELISA, the EIA results agreed for 99.1% (347 of 350) and 99.6% (299 of 300) of the samples, respectively.

Mice infected with Trypanosoma cruzi produce antibodies against the enzymatic domain of trans-sialidase that inhibit its activity

trans-Sialidase (TS) is an enzymatic activity described only for trypanosomes that is involved in the invasion of host cells by Trypanosoma cruzi. The enzyme that is shed by the parasite is made of two domains, the C-terminal region containing immunodominant amino acid repeats that define the SAPA antigen and the N-terminal domain that contains the putative region for enzymatic activity. The SAPA antigen induces a strong humoral response detected shortly after infection, both in humans and in mice. This response is directed to the immunodominant domain but is irrelevant in terms of neutralization of TS activity. We now show that TS activity can be detected in sera from acutely infected mice. However, mice infected with a T. cruzi strain whose growth can be controlled by the host did not have detectable levels of TS activity in sera. In fact, sera from these mice were able to abolish TS activity. This inhibition was due to the presence of specific antibodies directed against the enzymatic domain of the protein since they also abolish the activity of a recombinant molecule lacking the immunodominant amino acid repeats. The neutralizing antibodies were present from day 30 after the infection, while antibodies to the immunodominant repeats were detected by day 8 postinoculation, suggesting that the in vivo role of these repeats is to defect the humoral response to the repeat domain until the infection is established.

Direct detection of Vibrio cholerae in stool samples

A direct method to detect Vibrio cholerae in stool samples was developed by using a PCR procedure that did not require a DNA purification step. Dilution (1/100) of stool samples prevented inhibition of the reaction by contaminants, and two consecutive PCRs, the second one with a nested primer, achieved the desired sensitivity. Comparison of the results obtained from stool swab samples processed by the two-step PCR and by an enzyme-linked immunosorbent assay using GM1 as the capture molecule showed that the former is more sensitive and gave positive results even when V. cholerae was not culturable or dead.

An unusually small gene encoding a putative mucin-like glycoprotein in Trypanosoma cruzi

The gene encoding a putative core protein of a mucin-like glycoprotein was identified in Trypanosoma cruzi. It contains five repeats of eleven amino acids each, eight of which are Thr and two of which are Pro residues. These Thr-Pro-rich repeats resemble the ones in the human MUC2 gene encoding mucin.

Trans-sialidase, SAPA amino acid repeats and the relationship between Trypanosoma cruzi and the mammalian host

During invasion of multicellular organisms, protozoan parasites expose functional molecules that become targets for the host immune response. Recent research on Trypanosoma cruzi, the agent of Chagas' disease, suggests a new model of how the parasite might deal with this problem. Several antigens of T. cruzi have tandemly repeated amino acid motifs in molecules with as yet unknown functions. In two cases, these repeats are in molecules with a defined structure or function. Both proteins are implicated in the invasion of host-cells by the parasite. One of these is the core protein of a putative mucin-like glycoprotein that has Thr/Pro-rich repeats which, by themselves, might define the structure of a highly O-glycosylated molecule. The other protein is SAPA/trans-sialidase/neuraminidase, a molecule able to transfer sialic acid, that has so far only been described in trypanosomes. The amino acid repeats present in SAPA/transsialidase/neuraminidase are unrelated to the enzymic activity and constitute an immunodominant C-terminal domain. The N-terminal domain of SAPA/trans-sialidase/neuraminidase controls the enzymic activity since a recombinant molecule lacking the repeats conserves trans-sialidase activity. That both domains are functionally independent is also indicated by experiments that show that antibodies directed against the amino acid repeats are unable to inhibit trans-sialidase activity. A large number of proteins having trans-sialidase related sequences but lacking enzymic activity are also present in the surface membrane of the parasite. The immunodominant SAPA/trans-sialidase/neuraminidase repeats, together with the complex network of cross-reacting epitopes present in related but enzymatically inactive proteins might contribute to the delay in mounting an effective antibody response.(ABSTRACT TRUNCATED AT 250 WORDS)

The action of Trypanosoma cruzi trans-sialidase on glycolipids and glycoproteins

Addition of sialic acid residues in the human pathogen Trypanosoma cruzi glycoconjugates is mediated by a trans-sialidase and not by a CMP-sialic acid:glycoconjugate sialyltransferase. Incubation of trans-sialidase with N-[galactose-14C]acetyllactosamine and O-linked oligosaccharides, N-linked glycopeptides (both obtained from fetuin) or sialyllactose showed that the last three compounds were donors of sialic acid residues to the first one. Moreover, N- and O-linked oligosaccharides in asialofetuin and asialomucin, respectively, served as acceptors of sialic acid units. Gangliosides GM3, GD1a and GT1b but not GM2, GM1a nor GD1b donated sialic acid units to N-acetyllactos amine when incubated with trans-sialidase. This showed that only sialic acid units bound to terminal galactosyl residues were transferred. GM1a was converted to GD1a, and GD1b to GT1b when incubated with the appropriate donor. The fact that asialo-GM1a was converted to a ganglioside migrating as GD1a on thin-layer chromatography suggested that sialic acid units may be transferred to internal galactosyl residues, although once linked to those residues they can not be further transferred to other glycoconjugates. Sialic acid residues linked alpha 2,3- but not alpha 2,6- or alpha 2,8- were transferred by the trans-sialidase. Methyl beta-galactoside but not methyl alpha-galactoside served as acceptor of sialic acid units, thus suggesting that terminal alpha-linked galactosyl units in T. cruzi and mammalian glycoproteins are not sialylated by the enzyme. As the trans-sialidase employed in these experiments has been shown to be located on the external surface of the parasite and to be shed to the medium, the relatively broad specificity shown by the enzyme with respect to protein- and lipid-linked oligosaccharides strongly suggests that infection by T. cruzi might alter the sialic acid distribution in glycoproteins and glycolipids of the mammalian host.

The reactivity of sera from chagasic patients against different fragments of cruzipain, the major cysteine proteinase from Trypanosoma cruzi, suggests the presence of defined antigenic and catalytic domains

Three fragments of cruzipain, expressed separately in bacterial vectors, were used to detect antibodies in sera from patients with chronic Chagas' disease. Most antibodies directed against the enzyme were found to react with the C-terminal extension, thus suggesting the presence of immunodominant B-cell epitopes within this protein domain. Immunoprecipitation with these antibodies did not impair enzyme activity. It is suggested that cruzipain consists of an enzymatic domain and a non-enzymatic, immunodominant domain, which corresponds to the C-terminal extension.

Identification of toxigenic Vibrio cholerae from the Argentine outbreak by PCR for ctx A1 and ctx A2-B

A polymerase chain reaction (PCR) to detect a region of the A1 cholera toxin gene was applied to the identification of 43 Vibrio cholerae strains isolated from the recent outbreak in Argentina. A good correlation was observed between the GM1-enzyme-linked immunosorbent assay (GM1-ELISA) to detect the B subunit of the enterotoxin and PCR. However, a V. cholerae non-01 strain that was negative by the ELISA test, was positive by the PCR assay for the A1 region. A second PCR test to detect the A2-B coding region was developed to solve this case. We propose that routine detection of toxigenic V. cholerae by PCR should include analysis of A2-B coding region or the whole cholera toxin operon.

SAPA/trans-sialidase and cruzipain: two antigens from Trypanosoma cruzi contain immunodominant but enzymatically inactive domains

Trypanosoma cruzi, the parasitic protozoan that causes the American trypanosomiasis, or Chagas disease, contains a number of antigenic molecules, some of which have tandems of amino acid repeats. One of these molecules, SAPA (shed acute phase antigen), contains a so-far unique trans-sialidase activity that is essential for penetration of the parasite into mammalian cells. The enzyme consists of two different domains, one presumably enzymatic, which contains four copies of an amino acid motif conserved in bacterial neuraminidases, and the other highly antigenic, consisting of the repeats. Another enzyme that seems to be involved in the host-parasite relationship, the cysteine proteinase cruzipain, is also made up in its mature form of a catalytic domain with high homology to cathepsin L and a COOH-terminal domain that is highly antigenic in vivo. These bifunctional molecules may have arisen by incorporation of a highly antigenic domain to an essential enzyme in order to attract the immune response, thus protecting the enzyme activity.