Neuronal membrane glycoprotein (nmgp-1) gene deficiency affects chemosensation-related behaviors, dauer exit and egg-laying in Caenorhabditis elegans

The nervous system monitors the environment to maintain homeostasis, which can be affected by stressful conditions. Using mammalian models of chronic stress, we previously observed altered brain levels of GPM6A, a protein involved in neuronal morphology. However, GPM6A's role in systemic stress responses remains unresolved. The nematode Caenorhabditis elegans expresses a GPM6A ortholog, the neuronal membrane glycoprotein 1 (NMGP-1). Because of the shared features between nematode and mammalian nervous systems and the vast genetic tools available in C. elegans, we used the worm to elucidate the role of GPM6A in the stress response. We first identified nmgp-1 expression in different amphid and phasmid neurons. To understand the nmgp-1 role, we characterized the behavior of nmgp-1(RNAi) animals and two nmgp-1 mutant alleles. Compared to control animals, mutant and RNAi-treated worms exhibited increased recovery time from the stress-resistant dauer stage, altered SDS chemosensation and reduced egg-laying rate resulting in egg retention (bag-of-worms phenotype). Silencing of nmgp-1 expression induced morphological abnormalities in the ASJ sensory neurons, partly responsible for dauer exit. These results indicate that nmgp-1 is required for neuronal morphology and for behaviors associated with chemosensation. Finally, we propose nmgp-1 mutants as a tool to screen drugs for human nervous system pathologies.

Identification of Potential Interacting Proteins With the Extracellular Loops of the Neuronal Glycoprotein M6a by TMT/MS

Nowadays, great efforts are made to gain insight into the molecular mechanisms that underlie structural neuronal plasticity. Moreover, the identification of signaling pathways involved in the development of psychiatric disorders aids the screening of possible therapeutic targets. Genetic variations or alterations in GPM6A expression are linked to neurological disorders such as schizophrenia, depression, and Alzheimer's disease. GPM6A encodes the neuronal surface glycoprotein M6a that promotes filopodia/spine, dendrite, and synapse formation by unknown mechanisms. A substantial body of evidence suggests that the extracellular loops of M6a command its function. However, the proteins that associate with them and that modulate neuronal plasticity have not been determined yet. To address this question, we generated a chimera protein that only contains the extracellular loops of M6a and performed a co-immunoprecipitation with rat hippocampus samples followed by TMT/MS. Here, we report 72 proteins, which are good candidates to interact with M6a's extracellular loops and modify its function. Gene ontology (GO) analysis showed that 63% of the potential M6a's interactor proteins belong to the category "synapse," at both sides of the synaptic cleft, "neuron projections" (51%) and "presynapse" (49%). In this sense, we showed that endogenous M6a interacts with piccolo, synaptic vesicle protein 2B, and synapsin 1 in mature cultured hippocampal neurons. Interestingly, about 28% of the proteins left were related to the "myelin sheath" annotation, suggesting that M6a could interact with proteins at the surface of oligodendrocytes. Indeed, we demonstrated the (cis and trans) interaction between M6a and proteolipid protein (PLP) in neuroblastoma N2a cells. Finally, the 72 proteins were subjected to disease-associated genes and variants screening by DisGeNET. Apart from the diseases that have already been associated with M6a, most of the proteins are also involved in "autistic disorder," "epilepsy," and "seizures" increasing the spectrum of disorders in which M6a could play a role. Data are available via ProteomeXchange with identifier PXD017347.

Alanine Scanning Mutagenesis of the C-Terminal Cytosolic End of Gpm6a Identifies Key Residues Essential for the Formation of Filopodia

Neuronal membrane glycoprotein M6a (Gpm6a) is a protein with four transmembrane regions and the N- and the C-ends facing the cytosol. It functions in processes of neuronal development, outgrowth of neurites, and formation of filopodia, spines, and synapsis. Molecular mechanisms by which Gpm6a acts in these processes are not fully comprehended. Structural similarities of Gpm6a with tetraspanins led us to hypothesize that, similarly to tetraspanins, the cytoplasmic tails function as connections with cytoskeletal and/or signaling proteins. Here, we demonstrate that the C- but not the N-terminal cytosolic end of Gpm6a is required for the formation of filopodia by Gpm6a in cultured neurons from rat hippocampus and in neuroblastoma cells N2a. Further immunofluorescence microcopy and flow cytometry analysis show that deletion of neither the N- nor the C-terminal intracellular domains interferes with the recognition of Gpm6a by the function-blocking antibody directed against the extracellular part of Gpm6a. Expression levels of both truncation mutants were not affected but we observed decrease in the amount of both truncated proteins on cell surface suggesting that the incapacity of the Gpm6a lacking C-terminus to induce filopodium formation is not due to the lower amount of Gpm6a on cell surface. Following colocalization assays shows that deletion of the C- but not the N-terminus diminishes the association of Gpm6a with clathrin implying involvement of clathrin-mediated trafficking events. Next, using comprehensive alanine scanning mutagenesis of the C-terminus we identify K250, K255, and E258 as the key residues for the formation of filopodia by Gpm6a. Substitution of these charged residues with alanine also diminishes the amount of Gpm6a on cell surface and in case of K255 and E258 leads to the lower amount of total expressed protein. Subsequent bioinformatic analysis of Gpm6a amino acid sequence reveals that highly conserved and functional residues cluster preferentially within the C- and not within the N-terminus and that K250, K255, and E258 are predicted as part of sorting signals of transmembrane proteins. Altogether, our results provide evidence that filopodium outgrowth induced by Gpm6a requires functionally critical residues within the C-terminal cytoplasmic tail.

Neural glycoprotein M6a is released in extracellular vesicles and modulated by chronic stressors in blood

Membrane neuronal glycoprotein M6a is highly expressed in the brain and contributes to neural plasticity promoting neurite growth and spine and synapse formation. We have previously showed that chronic stressors alter hippocampal M6a mRNA levels in rodents and tree shrews. We now show that M6a glycoprotein can be detected in mouse blood. M6a is a transmembrane glycoprotein and, as such, unlikely to be free in blood. Here we demonstrate that, in blood, M6a is transported in extracellular vesicles (EVs). It is also shown that M6a-containing EVs are delivered from cultured primary neurons as well as from M6a-transfected COS-7 cells. Released EVs containing M6a can be incorporated into COS-7 cells changing its phenotype through formation of membrane protrusions. Thus, M6a-containing EVs might contribute to maintain cellular plasticity. M6a presence in blood was used to monitor stress effects. Chronic restraint stress modulated M6a protein level in a sex dependent manner. Analysis of individual animals indicated that M6a level variations depend on the stressor applied. The response to stressors in blood makes M6a amenable to further studies in the stress disorder field.

Neuronal filopodium formation induced by the membrane glycoprotein M6a (Gpm6a) is facilitated by coronin-1a, Rac1, and p21-activated kinase 1 (Pak1)

Stress-responsive neuronal membrane glycoprotein M6a (Gpm6a) functions in neurite extension, filopodium and spine formation and synaptogenesis. The mechanisms of Gpm6a action in these processes are incompletely understood. Previously, we identified the actin regulator coronin-1a (Coro1a) as a putative Gpm6a interacting partner. Here, we used co-immunoprecipitation assays with the anti-Coro1a antibody to show that Coro1a associates with Gpm6a in rat hippocampal neurons. By immunofluorescence microscopy, we demonstrated that in hippocampal neurons Coro1a localizes in F-actin-enriched regions and some of Coro1a spots co-localize with Gpm6a labeling. Notably, the over-expression of a dominant-negative form of Coro1a as well as its down-regulation by siRNA interfered with Gpm6a-induced filopodium formation. Coro1a is known to regulate the plasma membrane translocation and activation of small GTPase Rac1. We show that Coro1a co-immunoprecipitates with Rac1 together with Gpm6a. Pharmacological inhibition of Rac1 resulted in a significant decrease in filopodium formation by Gpm6a. The same was observed upon the co-expression of Gpm6a with the inactive GDP-bound form of Rac1. In this case, the elevated membrane recruitment of GDP-bound Rac1 was detected as well. Moreover, the kinase activity of the p21-activated kinase 1 (Pak1), a main downstream effector of Rac1 that acts downstream of Coro1a, was required for Gpm6a-induced filopodium formation. Taken together, our results provide evidence that a signaling pathway including Coro1a, Rac1, and Pak1 facilitates Gpm6a-induced filopodium formation. Formation of filopodia by membrane glycoprotein M6a (Gpm6a) requires actin regulator coronin-1a (Coro1a), known to regulate plasma membrane localization and activation of Rac1 and its downstream effector Pak1. Coro1a associates with Gpm6a. Blockage of Coro1a, Rac1, or Pak1 interferes with Gpm6a-induced filopodium formation. Moreover, Gpm6a facilitates Rac1 membrane recruitment. Altogether, a mechanistic insight into the process of Gpm6a-induced neuronal filopodium formation is provided.

Filopodia formation driven by membrane glycoprotein M6a depends on the interaction of its transmembrane domains

Membrane glycoprotein M6a, which belongs to the tetraspan proteolipid protein family, promotes structural plasticity in neurons and cell lines by unknown mechanisms. This glycoprotein is encoded by Gpm6a, a stress-regulated gene. The hippocampus of animals chronically stressed by either psychosocial or physical stressors shows decreased M6a expression. Stressed Gpm6a-null mice develop a claustrophobia-like phenotype. In humans, de novo duplication of GPM6A results in learning/behavioral abnormalities, and two single-nucleotide polymorphisms (SNPs) in the non-coding region are linked to mood disorders. Here, we studied M6a dimerization in neuronal membranes and its functional relevance. We showed that the self-interaction of M6a transmembrane domains (TMDs) might be driving M6a dimerization, which is required to induce filopodia formation. Glycine mutants located in TMD2 and TMD4 of M6a affected its dimerization, thus preventing M6a-induced filopodia formation in neurons. In silico analysis of three non-synonymous SNPs located in the coding region of TMDs suggested that these mutations induce protein instability. Indeed, these SNPs prevented M6a from being functional in neurons, owing to decreased stability, dimerization or improper folding. Interestingly, SNP3 (W141R), which caused endoplasmic reticulum retention, is equivalent to that mutated in PLP1, W161L, which causes demyelinating Pelizaeus-Merzbacher disease. In this work we analyzed the functional contribution of transmembrane domains (TMDs) of the neuronal membrane glycoprotein M6a. We determined that certain glycines present in TMD2 and TMD4 are critical for filopodia induction in neurons. In addition, three nsSNPs located in the coding region of TMD2 and TMD3 of GPM6A impair M6a function by affecting its stability, folding and dimer formation.

Towards High-throughput Immunomics for Infectious Diseases: Use of Next-generation Peptide Microarrays for Rapid Discovery and Mapping of Antigenic Determinants

Complete characterization of antibody specificities associated to natural infections is expected to provide a rich source of serologic biomarkers with potential applications in molecular diagnosis, follow-up of chemotherapeutic treatments, and prioritization of targets for vaccine development. Here, we developed a highly-multiplexed platform based on next-generation high-density peptide microarrays to map these specificities in Chagas Disease, an exemplar of a human infectious disease caused by the protozoan Trypanosoma cruzi. We designed a high-density peptide microarray containing more than 175,000 overlapping 15mer peptides derived from T. cruzi proteins. Peptides were synthesized in situ on microarray slides, spanning the complete length of 457 parasite proteins with fully overlapped 15mers (1 residue shift). Screening of these slides with antibodies purified from infected patients and healthy donors demonstrated both a high technical reproducibility as well as epitope mapping consistency when compared with earlier low-throughput technologies. Using a conservative signal threshold to classify positive (reactive) peptides we identified 2,031 disease-specific peptides and 97 novel parasite antigens, effectively doubling the number of known antigens and providing a 10-fold increase in the number of fine mapped antigenic determinants for this disease. Finally, further analysis of the chip data showed that optimizing the amount of sequence overlap of displayed peptides can increase the protein space covered in a single chip by at least ∼threefold without sacrificing sensitivity. In conclusion, we show the power of high-density peptide chips for the discovery of pathogen-specific linear B-cell epitopes from clinical samples, thus setting the stage for high-throughput biomarker discovery screenings and proteome-wide studies of immune responses against pathogens.

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.

Tyrosine 251 at the C-terminus of neuronal glycoprotein M6a is critical for neurite outgrowth

Neuronal glycoprotein M6a is involved in neuronal plasticity, promoting neurite and filopodia outgrowth and, likely, synaptogenesis. Polymorphisms in the human M6a gene GPM6A have recently been associated with mental illnesses such as schizophrenia, bipolar disorders, and claustrophobia. Nevertheless, the molecular bases underlying these observations remain unknown. We have previously documented that, to induce filopodia formation, M6a depends on the association of membrane lipid microdomains and the activation of Src and mitogen-activated protein kinase kinases. Here, in silico analysis of the phosphorylation of tyrosine 251 (Y251) at the C-terminus of M6a showed that it could be a target of Src kinases. We examined whether phosphorylation of M6a at Y251 affects neurite and filopodia outgrowth and the targets involved in its signal propagation. This work provides evidence that the Src kinase family and the phosphatidylinositide 3-kinase (PI3K), but not Ras, participate in M6a signal cascade leading to neurite/filopodia outgrowth in hippocampal neurons and murine neuroblastoma N2a cells. Phosphorylation of M6a at Y251 is essential only for neurite outgrowth by the PI3K/AKT-mediated pathway and, moreover, rescues the inhibition caused by selective Src inhibitor and external M6a monoclonal antibody treatment. Thus, we suggest that phosphorylation of M6a at Y251 is critical for a specific stage of neuronal development and triggers redundant signaling pathways leading to neurite extension.

Prenatal stress changes the glycoprotein GPM6A gene expression and induces epigenetic changes in rat offspring brain

Prenatal stress (PS) exerts strong impact on fetal brain development and on adult offspring brain functions. Previous work demonstrated that chronic stress alters the mRNA expression of GPM6A, a neuronal glycoprotein involved in filopodium extension. In this work, we analyzed the effect of PS on gpm6a expression and the epigenetic mechanisms involved. Pregnant Wistar rats received restraint stress during the last week of gestation. Male offspring were sacrificed on postnatal days 28 and 60. Hippocampus and prefrontal cortex samples were analyzed for gene expression (qPCR for mRNAs and microRNAs), methylation status (bisulfite conversion) and protein levels. Hippocampal neurons in culture were used to analyze microRNA overexpression effects. Prenatal stress induced changes in gpm6a levels in both tissues and at both ages analyzed, indicating a persistent effect. Two CpG islands in the gpm6a gene were identified. Variations in the methylation pattern at three specific CpGs were found in hippocampus, but not in PFC samples from PS offspring. microRNAs predicted to target gpm6a were identified in silico. qPCR measurements showed that PS modified the expression of several microRNAs in both tissues, being microRNA-133b the most significantly altered. Further studies overexpressing this microRNA in neuronal cultures showed a reduction in gmp6a mRNA and protein level. Moreover filopodium density was also reduced, suggesting that GPM6A function was affected. Gestational stress affected gpm6a gene expression in offspring likely through changes in methylation status and in posttranscriptional regulation by microRNAs. Thus, our findings propose gpm6a as a novel target for epigenetic regulation during prenatal stress.

Genome-wide analysis of 3'-untranslated regions supports the existence of post-transcriptional regulons controlling gene expression in trypanosomes

In eukaryotic cells, a group of messenger ribonucleic acids (mRNAs) encoding functionally interrelated proteins together with the trans-acting factors that coordinately modulate their expression is termed a post-transcriptional regulon, due to their partial analogy to a prokaryotic polycistron. This mRNA clustering is organized by sequence-specific RNA-binding proteins (RBPs) that bind cis-regulatory elements in the noncoding regions of genes, and mediates the synchronized control of their fate. These recognition motifs are often characterized by conserved sequences and/or RNA structures, and it is likely that various classes of cis-elements remain undiscovered. Current evidence suggests that RNA regulons govern gene expression in trypanosomes, unicellular parasites which mainly use post-transcriptional mechanisms to control protein synthesis. In this study, we used motif discovery tools to test whether groups of functionally related trypanosomatid genes contain a common cis-regulatory element. We obtained conserved structured RNA motifs statistically enriched in the noncoding region of 38 out of 53 groups of metabolically related transcripts in comparison with a random control. These motifs have a hairpin loop structure, a preferred sense orientation and are located in close proximity to the open reading frames. We found that 15 out of these 38 groups represent unique motifs in which most 3'-UTR signature elements were group-specific. Two extensively studied Trypanosoma cruzi RBPs, TcUBP1 and TcRBP3 were found associated with a few candidate RNA regulons. Interestingly, 13 motifs showed a strong correlation with clusters of developmentally co-expressed genes and six RNA elements were enriched in gene clusters affected after hyperosmotic stress. Here we report a systematic genome-wide in silico screen to search for novel RNA-binding sites in transcripts, and describe an organized network of several coordinately regulated cohorts of mRNAs in T. cruzi. Moreover, we found that structured RNA elements are also conserved in other human pathogens. These results support a model of regulation of gene expression by multiple post-transcriptional regulons in trypanosomes.

A role for the membrane protein M6 in the Drosophila visual system

BACKGROUND

Members of the proteolipid protein family, including the four-transmembrane glycoprotein M6a, are involved in neuronal plasticity in mammals. Results from our group previously demonstrated that M6, the only proteolipid protein expressed in Drosophila, localizes to the cell membrane in follicle cells. M6 loss triggers female sterility, which suggests a role for M6 in follicular cell remodeling. These results were the basis of the present study, which focused on the function and requirements of M6 in the fly nervous system.

RESULTS

The present study identified two novel, tissue-regulated M6 isoforms with variable N- and C- termini, and showed that M6 is the functional fly ortholog of Gpm6a. In the adult brain, the protein was localized to several neuropils, such as the optic lobe, the central complex, and the mushroom bodies. Interestingly, although reduced M6 levels triggered a mild rough-eye phenotype, hypomorphic M6 mutants exhibited a defective response to light.

CONCLUSIONS

Based on its ability to induce filopodium formation we propose that M6 is key in cell remodeling processes underlying visual system function. These results bring further insight into the role of M6/M6a in biological processes involving neuronal plasticity and behavior in flies and mammals.

A 43-nucleotide U-rich element in 3'-untranslated region of large number of Trypanosoma cruzi transcripts is important for mRNA abundance in intracellular amastigotes

Trypanosoma cruzi, the agent of Chagas disease, does not seem to control gene expression through regulation of transcription initiation and makes use of post-transcriptional mechanisms. We report here a 43-nt U-rich RNA element located in the 3'-untranslated region (3'-UTR) of a large number of T. cruzi mRNAs that is important for mRNA abundance in the intracellular amastigote stage of the parasite. Whole genome scan analysis, differential display RT-PCR, Northern blot, and RT-PCR analyses were used to determine the transcript levels of more than 900 U-rich-containing mRNAs of large gene families as well as single and low copy number genes. Our results indicate that the 43-nt U-rich mRNA element is preferentially present in amastigotes. The cis-element of a protein kinase 3'-UTR but not its mutated version promoted the expression of the green fluorescent protein reporter gene in amastigotes. The regulatory cis-element, but not its mutated version, was also shown to interact with the trypanosome-specific RNA-binding protein (RBP) TcUBP1 but not with other related RBPs. Co-immunoprecipitation experiments of TcUBP1-containing ribonucleoprotein complexes formed in vivo validated the interaction with representative endogenous RNAs having the element. These results suggest that this 43-nt U-rich element together with other yet unidentified sequences might be involved in the modulation of abundance and/or translation of subsets of transcripts in the amastigote stage.

Neuronal glycoprotein M6a induces filopodia formation via association with cholesterol-rich lipid rafts

A neuronal integral membrane glycoprotein M6a has been suggested to be involved in a number of biological processes, including neuronal remodeling and differentiation, trafficking of mu-opioid receptors, and Ca(2+) transportation. Moreover, pathological situations such as chronic stress in animals and depression in humans have been associated with alterations in M6a sequence and expression. The mechanism of action of M6a is essentially unknown. In this work, we analyze the relevance of M6a distribution in plasma membrane, namely its lipid microdomain targeting, for its biological function in filopodia formation. We demonstrate that M6a is localized in membrane microdomains compatible with lipid rafts in cultured rat hippocampal neurons. Removal of cholesterol from neuronal membranes with methyl-β-cyclodextrin decreases M6a-induced filopodia formation, an effect that is reversed by the addition of cholesterol. Inhibition of Src kinases and MAPK prevents filopodia formation in M6a-over-expressing neurons. Src-deficient SYF cells over-expressing M6a fail to promote filopodia formation. Taken together, our findings reveal that the association of M6a with lipid rafts is important for its role in filopodia formation and Src and MAPK kinases participate in M6a signal propagation.

RNA recognition motifs involved in nuclear import of RNA-binding proteins

In eukaryotic cells, a regulated import and export of factors is required to fulfill the requirements of precise gene expression. Post-transcriptional regulation of gene expression has proven to provide ubiquitous control, as well as a quick response to environmental changes when required. RNA-binding proteins (RBP) are involved in the several steps at which mRNA biogenesis, stability, translation and decay is exerted. The most characterized RBPs contain single or multiple copies of an RNA Recognition Motif (RRM). Here, we concentrate on RRMs mediating protein nuclear import by virtue of its ability to interact with proteins, besides interacting with nucleic acids. The consensus on how RRM-protein interactions take place is non-existent, and so is the involvement of the RRM as a nuclear localization signal (NLS). Within the cases examined, the single RRM from a trypanosome RBP behaves as a structural NLS, alternating nuclear import and RNA-binding.

An RNA recognition motif mediates the nucleocytoplasmic transport of a trypanosome RNA-binding protein

RNA-binding proteins (RBPs) and RNA metabolism are considered to be important for modulating gene expression in trypanosomes, because these protozoan parasites mainly rely on post-transcriptional mechanisms to regulate protein levels. Previously, we have identified TcUBP1, a single RNA recognition motif (RRM)-type RBP from Trypanosoma cruzi. TcUBP1 is a cytoplasmic protein with roles in stabilization/degradation of mRNAs and in the protection of transcripts through their recruitment into cytoplasmic granules. We now show that TcUBP1, and the closely related protein TcUBP2, can be found in small amounts in the nucleus under normal conditions, and are able to accumulate in the nucleus under arsenite stress. The kinetics of nuclear accumulation, and export to the cytoplasm, are consistent with the shuttling of TcUBP1 between the nucleus and the cytoplasm. The sequence required for TcUBP1 nuclear accumulation was narrowed to the RRM, and point mutations affecting RNA binding abolished nuclear import. This RRM was also shown to be efficiently exported from the nucleus in unstressed parasites, a property that relied on the binding to RNA. TcUBP1 nuclear accumulation was dependent on active transcription, and colocalized with transcripts in the nucleus, suggesting nuclear binding of the mRNA. We propose that TcUBP1 could be linking the mRNA metabolism at both sides of the nuclear pore complex, using the RRM as a nuclear localization signal, and being exported as a cargo on mRNA.

Cysteine residues in the large extracellular loop (EC2) are essential for the function of the stress-regulated glycoprotein M6a

Gpm6a was identified as a stress-responsive gene in the hippocampal formation. This gene is down-regulated in the hippocampus of both socially and physically stressed animals, and this effect can be reversed by antidepressant treatment. Previously we showed that the stress-regulated protein M6a is a key modulator for neurite outgrowth and filopodium/spine formation. In the present work, mutational analysis was used to characterize the action of M6a at the molecular level. We show that four cysteines 162, 174, 192, and 202 within EC2 are functionally crucial sites. The presence of cysteines 162 and 202 is essential for the efficient cell surface expression of the M6a protein. In contrast, cysteines 174 and 192, which form a disulfide bridge as shown by biochemical analysis, are not required for the efficient surface expression of M6a. Their mutation to alanine does not interfere with the localization of M6a to filopodial protrusions in primary hippocampal neurons. The neurons expressing C174A and/or C192A mutants display decreased filopodia number. In non-permeabilized cells, these mutant proteins are not recognized by a function-blocking monoclonal antibody directed to M6a. Moreover, neurons in contact with axons expressing C174A/C192A mutant display significantly lower density of presynaptic clusters over their dendrites. Taken together, this study demonstrates that cysteines in the EC2 domain are critical for the role of M6a in filopodium outgrowth and synaptogenesis.

Functionally related transcripts have common RNA motifs for specific RNA-binding proteins in trypanosomes

Background

Trypanosomes mostly control gene expression by post-transcriptional events such as modulation of mRNA stability and translational efficiency. These mechanisms involve RNA-binding proteins (RBPs), which associate with transcripts to form messenger ribonucleoprotein (mRNP) complexes.

Results

In this study, we report the identification of mRNA targets for Trypanosoma cruzi U-rich RBP 1 (TcUBP1) and T. cruzi RBP 3 (TcRBP3), two phylogenetically conserved proteins among Kinetoplastids. Co-immunoprecipitated RBP-associated RNAs were extracted from mRNP complexes and binding of RBPs to several targets was confirmed by independent experimental assays. Analysis of target transcript sequences allowed the identification of different signature RNA motifs for each protein. Cis-elements for RBP binding have a stem-loop structure of 30–35 bases and are more frequently represented in the 3'-untranslated region (UTR) of mRNAs. Insertion of the correctly folded RNA elements to a non-specific mRNA rendered it into a target transcript, whereas substitution of the RNA elements abolished RBP interaction. In addition, RBPs competed for RNA-binding sites in accordance with the distribution of different and overlapping motifs in the 3'-UTRs of common mRNAs.

Conclusion

Functionally related transcripts were preferentially associated with a given RBP; TcUBP1 targets were enriched in genes encoding proteins involved in metabolism, whereas ribosomal protein-encoding transcripts were the largest group within TcRBP3 targets. Together, these results suggest coordinated control of different mRNA subsets at the post-transcriptional level by specific RBPs.

Relevance of the diversity among members of the Trypanosoma cruzi trans-sialidase family analyzed with camelids single-domain antibodies

The sialic acid present in the protective surface mucin coat of Trypanosoma cruzi is added by a membrane anchored trans-sialidase (TcTS), a modified sialidase that is expressed from a large gene family. In this work, we analyzed single domain camelid antibodies produced against trans-sialidase. Llamas were immunized with a recombinant trans-sialidase and inhibitory single-domain antibody fragments were obtained by phage display selection, taking advantage of a screening strategy using an inhibition test instead of the classic binding assay. Four single domain antibodies displaying strong trans-sialidase inhibition activity against the recombinant enzyme were identified. They share the same complementarity-determining region 3 length (17 residues) and have very similar sequences. This result indicates that they likely derived from a unique clone. Probably there is only one structural solution for tight binding inhibitory antibodies against the TcTS used for immunization. To our surprise, this single domain antibody that inhibits the recombinant TcTS, failed to inhibit the enzymatic activity present in parasite extracts. Analysis of individual recombinant trans-sialidases showed that enzymes expressed from different genes were inhibited to different extents (from 8 to 98%) by the llama antibodies. Amino acid changes at key positions are likely to be responsible for the differences in inhibition found among the recombinant enzymes. These results suggest that the presence of a large and diverse trans-sialidase family might be required to prevent the inhibitory response against this essential enzyme and might thus constitute a novel strategy of T. cruzi to evade the host immune system.

Kinetic and mechanistic analysis of Trypanosoma cruzi trans-sialidase reveals a classical ping-pong mechanism with acid/base catalysis

The trans-sialidase from Trypanosoma cruzi catalyzes the transfer of a sialic acid moiety from sialylated donor substrates to the terminal galactose moiety of lactose and lactoside acceptors to yield alpha-(2,3)-sialyllactose or its derivatives with net retention of anomeric configuration. Through kinetic analyses in which the concentrations of two different donor aryl alpha-sialoside substrates and the acceptor substrate lactose were independently varied, we have demonstrated that this enzyme follows a ping-pong bi-bi kinetic mechanism. This is supported for both the native enzyme and a mutant (D59A) in which the putative acid/base catalyst has been replaced by the demonstration of the half-reaction in which a sialyl-enzyme intermediate is formed. Mass spectrometric analysis of the protein directly demonstrates the formation of a covalent intermediate, while the observation of release of a full equivalent of p-nitrophenol by the mutant in a pre-steady state burst provides further support. The active site nucleophile is confirmed to be Tyr342 by trapping of the sialyl-enzyme intermediate using the D59A mutant and sequencing of the purified peptic peptide. The role of D59 as the acid/base catalyst is confirmed by chemical rescue studies in which activity is restored to the D59A mutant by azide and a sialyl azide product is formed.

Recruitment of mRNAs to cytoplasmic ribonucleoprotein granules in trypanosomes

Trypanosomes are outstanding examples of the importance of mRNA metabolism in the regulation of gene expression, as these unicellular eukaryotes mostly control protein synthesis by post-transcriptional mechanisms. Here, we show that mRNA metabolism in these organisms involves recruitment of mRNAs and proteins to microscopically visible ribonucleoprotein granules in the cytoplasm. These structures engage transcripts that are being translated and protect mRNAs from degradation. Analysis of the protein composition of trypanosomal mRNA granules indicated that they contain orthologous proteins to those present in P bodies and stress granules from metazoan organisms. Formation of mRNA granules was observed after carbon-source deprivation of parasites in axenic culture. More important, mRNA granules are formed naturally in trypanosomes present in the intestinal tract of the insect vector. We suggest that trypanosomes make use of mRNA granules for transient transcript protection as a strategy to cope with periods of starvation that they have to face during their complex life cycles.

mRNA maturation by two-step trans-splicing/polyadenylation processing in trypanosomes

Trypanosomes are unique eukaryotic cells, in that they virtually lack mechanisms to control gene expression at the transcriptional level. These microorganisms mostly control protein synthesis by posttranscriptional regulation processes, like mRNA stabilization and degradation. Transcription in these cells is polycistronic. Tens to hundreds of protein-coding genes of unrelated function are arrayed in long clusters on the same DNA strand. Polycistrons are cotranscriptionally processed by trans-splicing at the 5' end and polyadenylation at the 3' end, generating monocistronic units ready for degradation or translation. In this work, we show that some trans-splicing/polyadenylation sites may be skipped during normal polycistronic processing. As a consequence, dicistronic units or monocistronic transcripts having long 3' UTRs are produced. Interestingly, these unspliced transcripts can be processed into mature mRNAs by the conventional trans-splicing/polyadenylation events leading to translation. To our knowledge, this is a previously undescribed mRNA maturation by trans-splicing uncoupled from transcription. We identified an RNA-recognition motif-type protein, homologous to the mammalian polypyrimidine tract-binding protein, interacting with one of the partially processed RNAs analyzed here that might be involved in exon skipping. We propose that splice-site skipping might be part of a posttranscriptional mechanism to regulate gene expression in trypanosomes, through the generation of premature nontranslatable RNA molecules.

RNA-binding domain proteins in Kinetoplastids: a comparative analysis

RNA-binding proteins are important in many aspects of RNA processing, function, and destruction. One class of such proteins contains the RNA recognition motif (RRM), which consists of about 90 amino acid residues, including the canonical RNP1 octapeptide: (K/R)G(F/Y)(G/A)FVX(F/Y). We used a variety of homology searches to classify all of the RRM proteins of the three kinetoplastids Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major. All three organisms have similar sets of RRM-containing protein orthologues, suggesting common posttranscriptional processing and regulatory pathways. Of the 75 RRM proteins identified in T. brucei, only 13 had clear homologues in other eukaryotes, although 8 more could be given putative functional assignments. A comparison with the 18 RRM proteins of the obligate intracellular parasite Encephalitozoon cuniculi revealed just 3 RRM proteins which appear to be conserved at the primary sequence level throughout eukaryotic evolution: poly(A) binding protein, the rRNA-processing protein MRD1, and the nuclear cap binding protein.

Regulation of hippocampal gene expression is conserved in two species subjected to different stressors and antidepressant treatments

BACKGROUND

Chronic stress has significant effects on hippocampal structure and function. We have previously identified nerve growth factor (NGF), membrane glycoprotein 6a (M6a), the guanine nucleotide binding protein (G protein) alpha q polypeptide (GNAQ), and CDC-like kinase 1 (CLK-1) as genes regulated by psychosocial stress and clomipramine treatment in the hippocampus of tree shrews. These genes encode proteins involved in neurite outgrowth.

METHODS

To analyze whether regulation of the above-mentioned genes is conserved between different species, stressors, and antidepressant drugs, we subjected mice to repeated restraint stress and tianeptine treatment and measured hippocampal messenger RNA (mRNA) levels by real time reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Chronically stressed mice displayed a reduction in transcript levels for NGF, M6a, GNAQ, and CLK-1. In addition, other genes implicated in neuronal plasticity, such as brain-derived neurotrophic factor (BDNF), cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), protein kinase C (PKC), neural cell adhesion molecule (NCAM), and synapsin I were downregulated in stressed mice. Tianeptine treatment reversed the stress effects for the genes analyzed. Alterations in gene expression were dependent on the duration of the stress treatment and, in some cases, were only observed in male mice.

CONCLUSIONS

These results suggest that genes involved in neurite remodeling are one of the main targets for regulation by chronic stress. The finding that this regulation is conserved in different stress models and antidepressant treatments highlights the biological relevance of the genes analyzed and suggests that they might be involved in stress-related disorders.

The stress-regulated protein M6a is a key modulator for neurite outgrowth and filopodium/spine formation

Neuronal remodeling is a fundamental process by which the brain responds to environmental influences, e.g., during stress. In the hippocampus, chronic stress causes retraction of dendrites in CA3 pyramidal neurons. We have recently identified the glycoprotein M6a as a stress-responsive gene in the hippocampal formation. This gene is down-regulated in the hippocampus of both socially and physically stressed animals, and this effect can be reversed by antidepressant treatment. In the present work, we analyzed the biological function of the M6a protein. Immunohistochemistry showed that the M6a protein is abundant in all hippocampal subregions, and subcellular analysis in primary hippocampal neurons revealed its presence in membrane protrusions (filopodia/spines). Transfection experiments revealed that M6a overexpression induces neurite formation and increases filopodia density in hippocampal neurons. M6a knockdown with small interference RNA methodology showed that M6a low-expressing neurons display decreased filopodia number and a lower density of synaptophysin clusters. Taken together, our findings indicate that M6a plays an important role in neurite/filopodium outgrowth and synapse formation. Therefore, reduced M6a expression might be responsible for the morphological alterations found in the hippocampus of chronically stressed animals. Potential mechanisms that might explain the biological effects of M6a are discussed.

Chronic stress, depression and antidepressants: effects on gene transcription in the hippocampus

Depressive disorders are among the most frequent forms of mental illness. Both genetic and environmental factors, such as stress, are involved in the etiology of depression. Therefore, chronic stress paradigms in laboratory animals constitute an important tool for research in this field. The molecular bases of chronic stress/depression are largely unknown, although a large amount of information has been accumulated during recent years. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis as well as structural and physiological alterations in the hippocampus and neocortex are known to occur. Modifications in the expression level of some genes, such as brain-derived neurotrophic factor, cAMP-response-element binding protein, serotonin receptors and HPA axis components were consistently associated in a number of experimental models. However, recent results suggest that several synaptic proteins, transcription factors and proteins involved in neuronal growth/differentiation, are also modified in their expression in experimental models of chronic stress. In general, these alterations can be reversed by treatment with antidepressants. Thus, a complex pattern of gene expression leading to stress/depression is starting to emerge. We summarize here recent findings on the alterations of gene expression in the hippocampus of chronically stressed and antidepressant treated animals.

NMR structural study of TcUBP1, a single RRM domain protein from Trypanosoma cruzi: contribution of a beta hairpin to RNA binding

TcUBP1 is a trypanosome cytoplasmic RNA-binding protein containing a single, conserved RNA-recognition motif (RRM) domain involved in selective destabilization of U-rich mRNAs such as the Trypanosoma cruzi small mucin gene family mRNA, TcSMUG. TcUBP1 binds specific transcripts in vivo and co-localizes in the perinuclear part of the cell with components of the mRNA-stability determinant pathway such as poly(A)-binding protein 1 (PABP1) and TcUBP2, a closely related RRM-containing protein. In TcUBP proteins, the RRM domain is flanked by N-terminal Gln-rich and C-terminal Gly-Gln-rich extensions, which are involved in protein-protein interactions. In this work, we determined the solution structure of the TcUBP1 RRM domain by nuclear magnetic resonance (NMR) spectroscopy. The domain has a characteristic betaalphabetabetaalphabeta fold, consisting of a beta sheet composed of four antiparallel betastrands and two alpha helices packed against one face of the beta sheet. A unique aspect of TcUBP1 is the participation of a beta hairpin (beta4-beta5) in the beta sheet, resulting in an enlarged RNA-binding surface. Detailed analysis of the TcUBP1 interaction with a short single-stranded RNA derived from the 3' UTR of TcSMUG was carried out by titration experiments using both NMR spectroscopy and isothermal titration calorimetry. This analysis revealed that amino acids located within the beta hairpin (beta4-beta5) contribute to complex formation. This enlarged protein-RNA interface could compensate for the lack of additional RNA-binding domains in TcUBP1, as observed in many other RRM-containing proteins. The structure of TcUBP1 reveals new aspects of single RRM-RNA interactions and insight into how N- and C-terminal extensions can contribute to RNA binding.

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.

Trypanosoma cruzi trans-sialidase operates through a covalent sialyl-enzyme intermediate: tyrosine is the catalytic nucleophile

Modified sialic acid substrates have been used to label Trypanosoma cruzi trans-sialidase, demonstrating that the enzyme catalyses the transfer of sialic acid through a covalent glycosyl-enzyme intermediate, a mechanism common to most retaining glycosidases. Peptic digestion of labeled protein, followed by LC-MS/MS analysis of the digest, identified Tyr342 as the catalytic nucleophile. This is the first such example of a retaining glycosidase utilizing an aryl glycoside intermediate. It is suggested that this alternative choice of nucleophile is a consequence of the chemical nature of sialic acid. A Tyr/Glu couple is invoked to relay charge from a remote glutamic acid, thereby avoiding electrostatic repulsion with the sialic acid carboxylate group.

The crystal structure and mode of action of trans-sialidase, a key enzyme in Trypanosoma cruzi pathogenesis

Trans-sialidases (TS) are GPI-anchored surface enzymes expressed in specific developmental stages of trypanosome parasites like Trypanosoma cruzi, the etiologic agent of Chagas disease, and T. brucei, the causative agent of sleeping sickness. TS catalyzes the transfer of sialic acid residues from host to parasite glycoconjugates through a transglycosidase reaction that appears to be critical for T. cruzi survival and cell invasion capability. We report here the structure of the T. cruzi trans-sialidase, alone and in complex with sugar ligands. Sialic acid binding is shown to trigger a conformational switch that modulates the affinity for the acceptor substrate and concomitantly creates the conditions for efficient transglycosylation. The structure provides a framework for the structure-based design of novel inhibitors with potential therapeutic applications.

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.