Clues on the dynamics of DNA replication in Giardia lamblia.

Genomic replication is a critical, regulated process that ensures accurate genetic information duplication. In eukaryotic cells, strategies have evolved to prevent conflicts between replication and transcription. Giardia lamblia, a binucleated protozoan, alternates between tetraploid and octaploid genomes during its cell cycle. Using single-molecule techniques like DNA combing and nanopore-based sequencing, we investigated the spatio-temporal organization of DNA replication, replication fork progression and potential head-on replication-transcription collisions in Giardia trophozoites. Our findings indicate that Giardia chromosomes are replicated from only a few active origins, which are widely spaced and exhibit faster replication rates compared to those in other protozoan parasites. Immunofluorescence assays revealed that ∼20% of trophozoites show asynchronous replication between nuclei. Forksense and gene ontology analyses disclosed that genes in regions with potential head-on collisions are linked to chromatin dynamics, cell cycle regulation and DNA replication/repair pathways, possibly explaining the observed asynchronous replication in part of the population. This study offers the first comprehensive view of replication dynamics in Giardia, which is the pathogen that causes giardiasis, a diarrheal disease impacting millions worldwide.

Clues on the dynamics of DNA replication in Giardia lamblia Icon for the Forest of Biologists

Genomic replication is a critical, regulated process that ensures accurate genetic information duplication. In eukaryotic cells, strategies have evolved to prevent conflicts between replication and transcription. Giardia lamblia, a binucleated protozoan, alternates between tetraploid and octoploid genomes during its cell cycle. Using single-molecule techniques like DNA combing and nanopore-based sequencing, we investigated the spatio-temporal organization of DNA replication, replication fork progression, and potential head-on replication-transcription collisions in Giardia trophozoites. Our findings indicate that Giardia chromosomes are replicated from few active origins, which are widely spaced and exhibit faster replication rates compared to other protozoan parasites. Immunofluorescence assays revealed that around 20% of trophozoites show asynchronous replication between nuclei. Forksense and gene ontology analyses disclosed that genes in regions with potential head-on collisions are linked to chromatin dynamics, cell cycle regulation, and DNA replication/repair pathways, possibly explaining the observed asynchronous replication in part of the population. This study offers the first comprehensive view of replication dynamics in Giardia, the cause of giardiasis, a diarrheal disease impacting millions worldwide.

Efficient oral vaccination by bioengineering virus-like particles with protozoan surface proteins.

Intestinal and free-living protozoa, such as Giardia lamblia, express a dense coat of variant-specific surface proteins (VSPs) on trophozoites that protects the parasite inside the host's intestine. Here we show that VSPs not only are resistant to proteolytic digestion and extreme pH and temperatures but also stimulate host innate immune responses in a TLR-4 dependent manner. We show that these properties can be exploited to both protect and adjuvant vaccine antigens for oral administration. Chimeric Virus-like Particles (VLPs) decorated with VSPs and expressing model surface antigens, such as influenza virus hemagglutinin (HA) and neuraminidase (NA), are protected from degradation and activate antigen presenting cells in vitro. Orally administered VSP-pseudotyped VLPs, but not plain VLPs, generate robust immune responses that protect mice from influenza infection and HA-expressing tumors. This versatile vaccine platform has the attributes to meet the ultimate challenge of generating safe, stable and efficient oral vaccines.

Protein SUMOylation is Involved in Cell-cycle Progression and Cell Morphology in Giardia lamblia.

The unicellular protozoa Giardia lamblia is a food- and waterborne parasite that causes giardiasis. This illness is manifested as acute and self-limited diarrhea and can evolve to long-term complications. Successful establishment of infection by Giardia trophozoites requires adhesion to host cells and colonization of the small intestine, where parasites multiply by mitotic division. The tight binding of trophozoites to host cells occurs by means of the ventral adhesive disc, a spiral array of microtubules and associated proteins such as giardins. In this work we show that knock down of the Small Ubiquitin-like MOdifier (SUMO) results in less adhesive trophzoites, decreased cell proliferation and deep morphological alterations, including at the ventral disc. Consistent with the reduced proliferation, SUMO knocked-down trophozoites were arrested in G1 and in S phases of the cell cycle. Mass spectrometry analysis of anti-SUMO immunoprecipitates was performed to identify SUMO substrates possibly involved in these events. Among the identified SUMOylation targets, α-tubulin was further validated by Western blot and confirmed to be a SUMO target in Giardia trophozoites.

Protein SUMOylation is involved in cell-cycle progression and cell morphology in giardia lamblia

The unicellular protozoa Giardia lamblia is a food- and waterborne parasite that causes giardiasis. This illness is manifested as acute and self-limited diarrhea and can evolve to long-term complications. Successful establishment of infection by Giardia trophozoites requires adhesion to host cells and colonization of the small intestine, where parasites multiply by mitotic division. The tight binding of trophozoites to host cells occurs by means of the ventral adhesive disc, a spiral array of microtubules and associated proteins such as giardins. In this work we show that knock down of the Small Ubiquitin-like MOdifier (SUMO) results in less adhesive trophzoites, decreased cell proliferation and deep morphological alterations, including at the ventral disc. Consistent with the reduced proliferation, SUMO knocked-down trophozoites were arrested in G1 and in S phases of the cell cycle. Mass spectrometry analysis of anti-SUMO immunoprecipitates was performed to identify SUMO substrates possibly involved in these events. Among the identified SUMOylation targets, -tubulin was further validated by Western blot and confirmed to be a SUMO target in Giardia trophozoites.

Infection Strategies of Intestinal Parasite Pathogens and Host Cell Responses.

Giardia lamblia, Cryptosporidium sp., and Entamoeba histolytica are important pathogenic intestinal parasites and are amongst the leading causes worldwide of diarrheal illness in humans. Diseases caused by these organisms, giardiasis, cryptosporidiosis, and amoebiasis, respectively, are characterized by self-limited diarrhea but can evolve to long-term complications. The cellular and molecular mechanisms underlying the pathogenesis of diarrhea associated with these three pathogens are being unraveled, with knowledge of both the strategies explored by the parasites to establish infection and the methods evolved by hosts to avoid it. Special attention is being given to molecules participating in parasite-host interaction and in the mechanisms implicated in the diseases' pathophysiologic processes. This review focuses on cell mechanisms that are modulated during infection, including gene transcription, cytoskeleton rearrangements, signal transduction pathways, and cell death.

The Gp85 surface glycoproteins from Trypanosoma cruzi.

Trypanosoma cruzi strains show distinctive characteristics as genetic polymorphism and infectivity. Large repertoires of molecules, such as the Gp85 glycoproteins, members of the Gp85/Trans-sialidase superfamily, as well as multiple signaling pathways, are associated with invasion of mammalian cells by the parasite. Due to the large number of expressed members, encoded by more than 700 genes, the research focused on this superfamily conserved sequences is discussed. Binding sites to laminin have been identified at the N-terminus of the Gp85 molecules. Interestingly, the T. cruzi protein phosphorylation profile is changed upon parasite binding to laminin (or fibronectin), particularly the cytoskeletal proteins such as those from the paraflagellar rod and the tubulins, which are both markedly dephosphorylated. Detailed analysis of the signaling cascades triggered upon T. cruzi binding to extracellular matrix (ECM) proteins revealed the involvement of the MAPK/ERK pathway in this event. At the C-terminus, the conserved FLY sequence is a cytokeratin-binding domain and is involved in augmented host cell invasion in vitro and high levels of parasitemia in vivo. FLY, which is associated to tissue tropism and preferentially binds to the heart vasculature may somehow be correlated with the severe cardiac form, an important clinical manifestation of chronic Chagas' disease.

Eukaryotic initiation factor 5A dephosphorylation is required for translational arrest in stationary phase cells.

The protein known as eIF5A (eukaryotic initiation factor 5A) has an elusive role in translation. It has a unique and essential hypusine modification at a conserved lysine residue in most eukaryotes. In addition, this protein is modified by phosphorylation with unknown functions. In the present study we show that a phosphorylated state of eIF5A predominates in exponentially growing Trypanosoma cruzi cells, and extensive dephosphorylation occurs in cells in stationary phase. Phosphorylation occurs mainly at Ser(2), as shown in yeast eIF5A. In addition, a novel phosphorylation site was identified at Tyr(21). In exponential cells, T. cruzi eIF5A is partially associated with polysomes, compatible with a proposed function as an elongation factor, and becomes relatively enriched in polysomal fractions in stationary phase. Overexpression of the wild-type eIF5A, or eIF5A with Ser(2) replaced by an aspartate residue, but not by alanine, increases the rate of cell proliferation and protein synthesis. However, the presence of an aspartate residue instead of Ser(2) is toxic for cells reaching the stationary phase, which show a less-pronounced protein synthesis arrest and a decreased amount of eIF5A in dense fractions of sucrose gradients. We conclude that eIF5A phosphorylation and dephosphorylation cycles regulate translation according to the growth conditions.

Protein synthesis attenuation by phosphorylation of eIF2α is required for the differentiation of Trypanosoma cruzi into infective forms.

Chagas' disease is a potentially life-threatening illness caused by the unicellular protozoan parasite Trypanosoma cruzi. It is transmitted to humans by triatomine bugs where T. cruzi multiplies and differentiates in the digestive tract. The differentiation of proliferative and non-infective epimastigotes into infective metacyclic trypomastigotes (metacyclogenesis) can be correlated to nutrient exhaustion in the gut of the insect vector. In vitro, metacyclic-trypomastigotes can be obtained when epimastigotes are submitted to nutritional stress suggesting that metacyclogenesis is triggered by nutrient starvation. The molecular mechanism underlying such event is not understood. Here, we investigated the role of one of the key signaling responses elicited by nutritional stress in all other eukaryotes, the inhibition of translation initiation by the phosphorylation of the eukaryotic initiation factor 2α (eIF2α), during the in vitro differentiation of T. cruzi. Monospecific antibodies that recognize the phosphorylated Tc-eIF2α form were generated and used to demonstrate that parasites subjected to nutritional stress show increased levels of Tc-eIF2α phosphorylation. This was accompanied by a drastic inhibition of global translation initiation, as determined by polysomal profiles. A strain of T. cruzi overexpressing a mutant Tc-eIF2α, incapable of being phosphorylated, showed a block on translation initiation, indicating that such a nutritional stress in trypanosomatids induces the conserved translation inhibition response. In addition, Tc-eIF2α phosphorylation is critical for parasite differentiation since the overexpression of the mutant eIF2α in epimastigotes abolished metacyclogenesis. This work defines the role of eIF2α phosphorylation as a key step in T. cruzi differentiation.

Role of the gp85/trans-sialidases in Trypanosoma cruzi tissue tropism: preferential binding of a conserved peptide motif to the vasculature in vivo.

BACKGROUND: Transmitted by blood-sucking insects, the unicellular parasite Trypanosoma cruzi is the causative agent of Chagas' disease, a malady manifested in a variety of symptoms from heart disease to digestive and urinary tract dysfunctions. The reasons for such organ preference have been a matter of great interest in the field, particularly because the parasite can invade nearly every cell line and it can be found in most tissues following an infection. Among the molecular factors that contribute to virulence is a large multigene family of proteins known as gp85/trans-sialidase, which participates in cell attachment and invasion. But whether these proteins also contribute to tissue homing had not yet been investigated. Here, a combination of endothelial cell immortalization and phage display techniques has been used to investigate the role of gp85/trans-sialidase in binding to the vasculature. METHODS: Bacteriophage expressing an important peptide motif (denominated FLY) common to all gp85/trans-sialidase proteins was used as a surrogate to investigate the interaction of this motif with the endothelium compartment. For that purpose phage particles were incubated with endothelial cells obtained from different organs or injected into mice intravenously and the number of phage particles bound to cells or tissues was determined. Binding of phages to intermediate filament proteins has also been studied. FINDINGS AND CONCLUSIONS: Our data indicate that FLY interacts with the endothelium in an organ-dependent manner with significantly higher avidity for the heart vasculature. Phage display results also show that FLY interaction with intermediate filament proteins is not limited to cytokeratin 18 (CK18), which may explain the wide variety of cells infected by the parasite. This is the first time that members of the intermediate filaments in general, constituted by a large group of ubiquitously expressed proteins, have been implicated in T. cruzi cell invasion and tissue homing.

Glucose uptake in the mammalian stages of Trypanosoma cruzi.

Trypanosoma cruzi, the agent of Chagas' disease, alternates between different morphogenetic stages that face distinct physiological conditions in their invertebrate and vertebrate hosts, likely in the availability of glucose. While the glucose transport is well characterized in epimastigotes of T. cruzi, nothing is known about how the mammalian stages acquire this molecule. Herein glucose transport activity and expression were analyzed in the three developmental stages present in the vertebrate cycle of T. cruzi. The infective trypomastigotes showed the highest transport activity (V(max)=5.34+/-0.54 nmol/min per mg of protein; K(m)=0.38+/-0.01 mM) when compared to intracellular epimastigotes (V(max)=2.18+/-0.20 nmol/min per mg of protein; K(m)=0.39+/-0.01 mM). Under the conditions employed no transport activity could be detected in amastigotes. The gene of the glucose transporter is expressed at the mRNA level in trypomastigotes and in intracellular epimastigotes but not in amastigotes, as revealed by real-time PCR. In both trypomastigotes and intracellular epimastigotes protein expression could be detected by Western blot with an antibody raised against the glucose transporter correlating well with the transport activity measured experimentally. Interestingly, anti-glucose transporter antibodies showed a strong reactivity with glycosome and reservosome organelles. A comparison between proline and glucose transport among the intracellular differentiation forms is presented. The data suggest that the regulation of glucose transporter reflects different energy and carbon requirements along the intracellular life cycle of T. cruzi.

A conserved domain of the gp85/trans-sialidase family activates host cell extracellular signal-regulated kinase and facilitates Trypanosoma cruzi infection.

Chagas' disease is a chronic, debilitating and incapacitating illness, caused by the protozoan parasite Trypanosoma cruzi when infective trypomastigotes invade host cells. Although the mechanism of trypomastigotes interaction with mammalian cells has been intensively studied, a final and integrated picture of the signal transduction mechanisms involved still remains to be elucidated. Our group has previously shown that the conserved FLY domain (VTVXNVFLYNR), present in all members of the gp85/trans-sialidase glycoprotein family coating the surface of trypomastigotes, binds to cytokeratin 18 (CK18) on the surface of LLC-MK(2) epithelial cells, and significantly increases parasite entry into mammalian cells. Now it is reported that FLY, present on the surface of trypomastigotes or on latex beads binds to CK18, promotes dephosphorylation and reorganization of CK18 and activation of the ERK1/2 signaling cascade culminating in an increase of approximately 9-fold in the number of parasites/cell. Inhibition of ERK1/2 phosphorylation completely blocks the adhesion of FLY to cells and blocks by 57% the host cell infection by T. cruzi. Taken together our results indicate that the conserved FLY domain is an important tool that trypomastigotes have evolved to specific exploit the host cell machinery and guarantee a successful infection.

Post genomic analysis of permeases from the amino acid/auxin family in protozoan parasites.

The "amino acid/auxin permeases" is probably the most represented family of transporters in the Trypanosoma cruzi genome. Using a high-throughput searching routine and preliminary data from the T. cruzi genome project, more than 15,000 sequences were iteratively assembled into contigs, and 60 open reading frames corresponding to different putative amino acid transporters, clustered in 12 groups, were detected and characterized in silico. T. cruzi genomic organization of such sequences showed that these putative amino acid transporter genes are in an unusually large number and arranged in repeat clusters comprising about 0.2% of the genome. These data suggest that the family has evolved following tandem duplication events and constitutes a novel family of variable proteins in protozoan organisms. The mRNA expression of the predicted genes was demonstrated in infective and non-infective parasite forms. Orthologous sequences were also identified in other unicellular parasites such as Leishmania spp., Plasmodium spp., and Trypanosoma brucei.

L-proline is essential for the intracellular differentiation of Trypanosoma cruzi.

Using as the host cell, a proline-requiring mutant of Chinese hamster ovary cell (CHO-K1), it was possible to arrest the differentiation of amastigote forms of Trypanosoma cruzi at the intermediate intracellular epimastigote-like stage. Complete differentiation to the trypomastigote stage was obtained by addition of L-proline to the medium. This effect was more pronounced using the T. cruzi CL-14 clone that differentiates fully at 33 degrees C (permissive temperature) and poorly at 37 degrees C (restrictive temperature). A synchronous differentiation of T. cruzi inside the host-cell is then possible by temperature switching in the presence of proline. It was found that differentiation of intracellular epimastigotes and trypomastigote bursting were proline concentration dependent. The intracellular concentration of proline was measured as well as the transport capacity of proline by each stage of the parasite. Amastigotes have the highest concentration of free proline (8.09 +/- 1.46 mM) when compared to trypomastigotes (3.81 +/- 1.55) or intracellular epimastigote-like forms (0.45 +/- 0.06 mM). In spite of having the lowest content of intracellular free proline, intracellular epimastigotes maintained the highest levels of L-proline transport compared to trypomastigotes and intracellular amastigotes, providing evidence for a high turnover for the L-proline pool in that parasite stage. This is the first report to establish a relationship between proline concentration and intracellular differentiation of Trypanosoma cruzi in the mammalian host.

Active transport of L-proline in Trypanosoma cruzi.

L-proline is the main energy source in insect vector stages of most trypanosomatids, including Trypanosoma cruzi epimastigotes. This is the first biochemical description of two active proline transporter systems in T. cruzi. Uptake of this amino acid occurred by a low affinity system B and a high affinity system A. System B consistently appeared more specific than System A when excess competing amino acids were used in transport inhibition assays. Furthermore, the high affinity system is 70% inhibited by L-tryptophan, but the low affinity system is not. Both systems were found to be insensitive to the intracellular proline concentration and D-proline did not inhibit L-proline uptake showing that both systems are stereospecific. Both systems were Na+ and K+ independant but dependant on energy since ATP depletion impairs L-proline uptake. The combined action of carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) and oligomycin, and the dependence of activity on pH, further differentiated between the two systems leading to the conclusion that the high affinity system is a H+ gradient-dependant transporter whereas the low affinity system depends directly on ATP.