Dual role of signaling pathways leading to Ca(2+) and cyclic AMP elevation in host cell invasion by Trypanosoma cruzi

Nonsteroidal Anti-Inflammatory Drugs and Experimental Chagas Disease: An Unsolved Question

Chagas disease is a parasitic disease caused by the protozoan Trypanosoma cruzi with an acute, detectable blood parasites phase and a chronic phase, in which the parasitemia is not observable, but cardiac and gastrointestinal consequences are possible. Mice are the principal host used in experimental Chagas disease but reproduce the human infection depending on the animal and parasite strain, besides dose and route of administration. Lipidic mediators are tremendously involved in the pathogenesis of T. cruzi infection, meaning the prostaglandins and thromboxane, which participate in the immunosuppression characteristic of the acute phase. Thus, the eicosanoids inhibition caused by the nonsteroidal anti-inflammatory drugs (NSAIDs) alters the dynamic of the disease in the experimental models, both in vitro and in vivo, which can explain the participation of the different mediators in infection. However, marked differences are founded in the various NSAIDs existing because of the varied routes blocked by the drugs. So, knowing the results in the experimental models of Chagas disease with or without the NSAIDs helps comprehend the pathogenesis of this infection, which still needs a better understanding.

Impact of the Extracellular Vesicles Derived From Trypanosoma cruzi: A Paradox in Host Response and Lipid Metabolism Modulation

Chagas disease is a major public health problem, especially in the South and Central America region. Its incidence is related to poverty and presents a high rate of morbidity and mortality. The pathogenesis of Chagas disease is complex and involves many interactive pathways between the hosts and the Trypanosoma cruzi. Several factors have been implicated in parasite-host interactions, including molecules secreted by infected cells, lipid mediators and most recent, extracellular vesicles (EVs). The EVs of T. cruzi (EVsT) were reported for the first time in the epimastigote forms about 42 years ago. The EVsT are involved in paracrine communication during the infection and can have an important role in the inflammatory modulation and parasite escape mechanism. However, the mechanisms by which EVs employ their pathological effects are not yet understood. The EVsT seem to participate in the activation of macrophages via TLR2 triggering the production of cytokines and a range of other molecules, thus modulating the host immune response which promotes the parasite survival. Moreover, new insights have demonstrated that EVsT induce lipid body formation and PGE₂ synthesis in macrophages. This phenomenon is followed by the inhibition of the synthesis of pro-inflammatory cytokines and antigen presentation, causing decreased parasitic molecules and allowing intracellular parasite survival. Therefore, this mini review aims to discuss the role of the EVs from T. cruzi as well as its involvement in the mechanisms that regulate the host immune response in the lipid metabolism and its significance for the Chagas disease pathophysiology.

Extracellular Vesicles in Chagas Disease: A New Passenger for an Old Disease

Extracellular vesicles (EVs) are small lipid vesicles released by prokaryotic and eukaryotic cells containing nucleic acids, proteins, and small metabolites essential for cellular communication. Depending on the targeted cell, EVs can act either locally or in distant tissues in a paracrine or endocrine cell signaling manner. Released EVs from virus-infected cells, bacteria, fungi, or parasites have been demonstrated to perform a pivotal role in a myriad of biochemical changes occurring in the host and pathogen, including the modulation the immune system. In the past few years, the biology of Trypanosoma cruzi EVs, as well as their role in innate immunity evasion, has been started to be unveiled. This review article will present findings on and provide a coherent understanding of the currently known mechanisms of action of T. cruzi-EVs and hypothesize the implication of these parasite components during the acute and chronic phases of Chagas disease.

Response to Trypanosoma cruzi by Human Blood Cells Enriched with Dentritic Cells Is Controlled by Cyclooxygenase-2 Pathway

Chagas disease (Cd) or American human trypanosomiasis is caused by Trypanosoma cruzi and affects ~7 million people, mostly in Latin America. The infective trypomastigote forms of the parasite can invade several human blood cell populations, including monocytes and dendritic cells (DC). Although these cells display a wide functional diversity, their interactions with T. cruzi via cyclooxygenase (COX) and cyclic adenosine monophosphate (cAMP) dependent pathways have not been analyzed. To exploiting this mechanism, DC-enriched peripheral human blood mononuclear cell populations (DC-PBMC) were used as our model. Our results showed that the treatment of these cell populations with celecoxib (CEL), a cyclooxygenase-2 selective inhibitor or SQ 22,536, an adenilate cyclase inhibitor, significantly caused marked inhibition of T. cruzi infection. In contrast, aspirin (ASA, a non-selective COX-1 and COX-2 inhibitor) treatment did not inhibit the infection of the cells by the parasite and was independent of nitric oxide (NO) production. The expression of co-stimulatory molecules CD80 and CD86 were similar on cells treated or not with both COX-inhibitors. The infection stimulated the release of TNF-α, IL-1β, IL-6, IL-8, and IL-10 production by infected cells. Treatment with ASA or CEL did not affect TNF-α, IL-6, IL-8, IL-10, and NO production by infected cells, but increased IL-1β production by them. Our results suggest a key role of COX-2 and cAMP pathways in T. cruzi invasion process of human blood cells and these pathways may represent targets of new therapeutic options for Cd.

Trypanosoma cruzi: Inhibition of infection of human monocytes by aspirin

Cell invasion by Trypanosoma cruzi and its intracellular replication are essential for progression of the parasite life cycle and development of Chagas disease. Prostaglandin E2 (PGE₂) and other eicosanoids potently modulate host response and contribute to Chagas disease progression. In this study, we evaluated the effect of aspirin (ASA), a non-selective cyclooxygenase (COX) inhibitor on the T. cruzi invasion and its influence on nitric oxide and cytokine production in human monocytes. The pretreatment of monocytes with ASA or SQ 22536 (adenylate-cyclase inhibitor) induced a marked inhibition of T. cruzi infection. On the other hand, the treatment of monocytes with SQ 22536 after ASA restored the invasiveness of T. cruzi. This reestablishment was associated with a decrease in nitric oxide and PGE₂ production, and also an increase of interleukin-10 and interleukin-12 by cells pre-treated with ASA. Altogether, these results reinforce the idea that the cyclooxygenase pathway plays a fundamental role in the process of parasite invasion in an in vitro model of T. cruzi infection.

Synergistic cellular effects including mitochondrial destabilization, autophagy and apoptosis following low-level exposure to a mixture of lipophilic persistent organic pollutants

Humans are exposed to multiple exogenous environmental pollutants. Many of these compounds are parts of mixtures that can exacerbate harmful effects of the individual mixture components. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is primarily produced via industrial processes including incineration and the manufacture of herbicides. Both endosulfan and TCDD are persistent organic pollutants which elicit cytotoxic effects by inducing reactive oxygen species generation. Sublethal concentrations of mixtures of TCDD and endosulfan increase oxidative stress, as well as mitochondrial homeostasis disruption, which is preceded by a calcium rise and, in fine, induce cell death. TCDD+Endosulfan elicit a complex signaling sequence involving reticulum endoplasmic destalilization which leads to Ca2+ rise, superoxide anion production, ATP drop and late NADP(H) depletion associated with a mitochondrial induced apoptosis concomitant early autophagic processes. The ROS scavenger, N-acetyl-cysteine, blocks both the mixture-induced autophagy and death. Calcium chelators act similarly and mitochondrially targeted anti-oxidants also abrogate these effects. Inhibition of the autophagic fluxes with 3-methyladenine, increases mixture-induced cell death. These findings show that subchronic doses of pollutants may act synergistically. They also reveal that the onset of autophagy might serve as a protective mechanism against ROS-triggered cytotoxic effects of a cocktail of pollutants in Caco-2 cells and increase their tumorigenicity.

Distribution, classification, domain architectures and evolution of prolyl oligopeptidases in prokaryotic lineages

Background

Prolyl oligopeptidases (POPs) are proteolytic enzymes, widely distributed in all the kingdoms of life. Bacterial POPs are pharmaceutically important enzymes, yet their functional and evolutionary details are not fully explored. Therefore, current analysis is aimed at understanding the distribution, domain architecture, probable biological functions and gene family expansion of POPs in bacterial and archaeal lineages.

Results

Exhaustive sequence analysis of 1,202 bacterial and 91 archaeal genomes revealed ~3,000 POP homologs, with only 638 annotated POPs. We observed wide distribution of POPs in all the analysed bacterial lineages. Phylogenetic analysis and co-clustering of POPs of different phyla suggested their common functions in all the prokaryotic species. Further, on the basis of unique sequence motifs we could classify bacterial POPs into eight subtypes. Analysis of coexisting domains in POPs highlighted their involvement in protein-protein interactions and cellular signaling. We proposed significant extension of this gene family by characterizing 39 new POPs and 158 new α/β hydrolase members.

Conclusions

Our study reflects diversity and functional importance of POPs in bacterial species. Many genomes with multiple POPs were identified with high sequence variations and different cellular localizations. Such anomalous distribution of POP genes in different bacterial genomes shows differential expansion of POP gene family primarily by multiple horizontal gene transfer events.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-985) contains supplementary material, which is available to authorized users.

Parasite prolyl oligopeptidases and the challenge of designing chemotherapeuticals for Chagas disease, leishmaniasis and African trypanosomiasis

The trypanosomatids Trypanosoma cruzi, Leishmania spp. and Trypanosoma brucei spp. cause Chagas disease, leishmaniasis and human African trypanosomiasis, respectively. It is estimated that over 10 million people worldwide suffer from these neglected diseases, posing enormous social and economic problems in endemic areas. There are no vaccines to prevent these infections and chemotherapies are not adequate. This picture indicates that new chemotherapeutic agents must be developed to treat these illnesses. For this purpose, understanding the biology of the pathogenic trypanosomatid-host cell interface is fundamental for molecular and functional characterization of virulence factors that may be used as targets for the development of inhibitors to be used for effective chemotherapy. In this context, it is well known that proteases have crucial functions for both metabolism and infectivity of pathogens and are thus potential drug targets. In this regard, prolyl oligopeptidase and oligopeptidase B, both members of the S9 serine protease family, have been shown to play important roles in the interactions of pathogenic protozoa with their mammalian hosts and may thus be considered targets for drug design. This review aims to discuss structural and functional properties of these intriguing enzymes and their potential as targets for the development of drugs against Chagas disease, leishmaniasis and African trypanosomiasis.

Dealing with environmental challenges: mechanisms of adaptation in Trypanosoma cruzi

Protozoan parasites have a significant impact upon global health, infecting millions of people around the world. With limited therapeutic options and no vaccines available, research efforts are focused upon unraveling cellular mechanisms essential for parasite survival. During its life cycle, Trypanosoma cruzi, the causal agent of Chagas disease, is exposed to multiple external conditions and different hosts. Environmental cues are linked to the differentiation process allowing the parasite to complete its life cycle. Successful transmission depends on the ability of the cells to trigger adaptive responses and cope with stressors while regulating proliferation and transition to different life stages. This review focuses upon different aspects of the stress response in T. cruzi, proposing new hypotheses regarding cross-talk and cross-tolerance with respect to environmental changes and discussing open questions and future directions.

Mechanisms of cellular invasion by intracellular parasites

Numerous disease-causing parasites must invade host cells in order to prosper. Collectively, such pathogens are responsible for a staggering amount of human sickness and death throughout the world. Leishmaniasis, Chagas disease, toxoplasmosis, and malaria are neglected diseases and therefore are linked to socio-economical and geographical factors, affecting well-over half the world's population. Such obligate intracellular parasites have co-evolved with humans to establish a complexity of specific molecular parasite-host cell interactions, forming the basis of the parasite's cellular tropism. They make use of such interactions to invade host cells as a means to migrate through various tissues, to evade the host immune system, and to undergo intracellular replication. These cellular migration and invasion events are absolutely essential for the completion of the lifecycles of these parasites and lead to their for disease pathogenesis. This review is an overview of the molecular mechanisms of protozoan parasite invasion of host cells and discussion of therapeutic strategies, which could be developed by targeting these invasion pathways. Specifically, we focus on four species of protozoan parasites Leishmania, Trypanosoma cruzi, Plasmodium, and Toxoplasma, which are responsible for significant morbidity and mortality.

OsPOP5, a prolyl oligopeptidase family gene from rice confers abiotic stress tolerance in Escherichia coli

The prolyl oligopeptidase family, which is a group of serine peptidases, can hydrolyze peptides smaller than 30 residues. The prolyl oligopeptidase family in plants includes four members, which are prolyl oligopeptidase (POP, EC3.4.21.26), dipeptidyl peptidase IV (DPPIV, EC3.4.14.5), oligopeptidase B (OPB, EC3.4.21.83), and acylaminoacyl peptidase (ACPH, EC3.4.19.1). POP is found in human and rat, and plays important roles in multiple biological processes, such as protein secretion, maturation and degradation of peptide hormones, and neuropathies, signal transduction and memory and learning. However, the function of POP is unclear in plants. In order to study POP function in plants, we cloned the cDNA of the OsPOP5 gene from rice by nested-PCR. Sequence analysis showed that the cDNA encodes a protein of 596 amino acid residues with M_w ≈ 67.29 kD. In order to analyze the protein function under different abiotic stresses, OsPOP5 was expressed in Escherichia coli. OsPOP5 protein enhanced the tolerance of E. coli to high salinity, high temperature and simulated drought. The results indicate that OsPOP5 is a stress-related gene in rice and it may play an important role in plant tolerance to abiotic stress.

Trans-sialidase stimulates eat me response from epithelial cells

Epithelial cell invasion by the protozoan parasite Trypanosoma cruzi is enhanced by the presence of an enzyme expressed on its cell surface during the trypomastigote life cycle stage. The enzyme, trans-sialidase (TS), is a member of one of the largest gene families expressed by the parasite and the role of its activity in mediating epithelial cell entry has not hitherto been understood. Here we show that the T. cruzi TS generates an eat me signal which is capable of enabling epithelial cell entry. We have utilized purified, recombinant, active (TcTS) and inactive (TcTS2V0) TS coated onto beads to challenge an epithelial cell line. We find that TS activity acts upon G protein coupled receptors present at the epithelial cell synapse with the coated bead, thereby enhancing cell entry. By so doing, we provide evidence that TS proteins bind glycans, mediate the formation of distinct synaptic domains and promote macropinocytotic uptake of microparticles into a perinuclear compartment in a manner which may emulate entosis.

Cell signaling during Trypanosoma cruzi invasion

Cell signaling is an essential requirement for mammalian cell invasion by Trypanosoma cruzi. Depending on the parasite strain and the parasite developmental form, distinct signaling pathways may be induced. In this short review, we focus on the data coming from studies with metacyclic trypomastigotes (MT) generated in vitro and tissue culture-derived trypomastigotes (TCT), used as counterparts of insect-borne and bloodstream parasites, respectively. During invasion of host cells by MT or TCT, intracellular Ca²⁺ mobilization and host cell lysosomal exocytosis are triggered. Invasion mediated by MT surface molecule gp82 requires the activation of mammalian target of rapamycin (mTOR), phosphatidylinositol 3-kinase (PI3K), and protein kinase C (PKC) in the host cell, associated with Ca²⁺-dependent disruption of the actin cytoskeleton. In MT, protein tyrosine kinase, PI3K, phospholipase C, and PKC appear to be activated. TCT invasion, on the other hand, does not rely on mTOR activation, rather on target cell PI3K, and may involve the host cell autophagy for parasite internalization. Enzymes, such as oligopeptidase B and the major T. cruzi cysteine proteinase cruzipain, have been shown to generate molecules that induce target cell Ca²⁺ signal. In addition, TCT may trigger host cell responses mediated by transforming growth factor β receptor or integrin family member. Further investigations are needed for a more complete and detailed picture of T. cruzi invasion.

Host microtubule plus-end binding protein CLASP1 influences sequential steps in the Trypanosoma cruzi infection process

Mammalian cell invasion by the protozoan parasite Trypanosoma cruzi involves host cell microtubule dynamics. Microtubules support kinesin-dependent anterograde trafficking of host lysosomes to the cell periphery where targeted lysosome exocytosis elicits remodelling of the plasma membrane and parasite invasion. Here, a novel role for microtubule plus-end tracking proteins (+TIPs) in the co-ordination of T. cruzi trypomastigote internalization and post-entry events is reported. Acute silencing of CLASP1, a +TIP that participates in microtubule stabilization at the cell periphery, impairs trypomastigote internalization without diminishing the capacity for calcium-regulated lysosome exocytosis. Subsequent fusion of the T. cruzi vacuole with host lysosomes and its juxtanuclear positioning are also delayed in CLASP1-depleted cells. These post-entry phenotypes correlate with a generalized impairment of minus-end directed transport of lysosomes in CLASP1 knock-down cells and mimic the effects of dynactin disruption. Consistent with GSK3β acting as a negative regulator of CLASP function, inhibition of GSK3β activity enhances T. cruzi entry in a CLASP1-dependent manner and expression of constitutively active GSK3β dampens infection. This study provides novel molecular insights into the T. cruzi infection process, emphasizing functional links between parasite-elicited signalling, host microtubule plus-end tracking proteins and dynein-based retrograde transport. Highlighted in this work is a previously unrecognized role for CLASPs in dynamic lysosome positioning, an important aspect of the nutrient sensing response in mammalian cells.

Trypanosoma cruzi invades host cells through the activation of endothelin and bradykinin receptors: a converging pathway leading to chagasic vasculopathy

BACKGROUND AND PURPOSE

Independent studies in experimental models of Trypanosoma cruzi appointed different roles for endothelin-1 (ET-1) and bradykinin (BK) in the immunopathogenesis of Chagas disease. Here, we addressed the hypothesis that pathogenic outcome is influenced by functional interplay between endothelin receptors (ET(A)R and ET(B)R) and bradykinin B(2) receptors (B(2)R).

EXPERIMENTAL APPROACH

Intravital microscopy was used to determine whether ETR/B(2)R drives the accumulation of rhodamine-labelled leucocytes in the hamster cheek pouch (HCP). Inflammatory oedema was measured in the infected BALB/c paw of mice. Parasite invasion was assessed in CHO over-expressing ETRs, mouse cardiomyocytes, endothelium (human umbilical vein endothelial cells) or smooth muscle cells (HSMCs), in the presence/absence of antagonists of B(2)R (HOE-140), ET(A)R (BQ-123) and ET(B)R (BQ-788), specific IgG antibodies to each GPCRs; cholesterol or calcium-depleting drugs. RNA interference (ET(A)R or ET(B)R genes) in parasite infectivity was investigated in HSMCs.

KEY RESULTS

BQ-123, BQ-788 and HOE-140 reduced leucocyte accumulation in HCP topically exposed to trypomastigotes and blocked inflammatory oedema in infected mice. Acting synergistically, ET(A)R and ET(B)R antagonists reduced parasite invasion of HSMCs to the same extent as HOE-140. Exogenous ET-1 potentiated T. cruzi uptake by HSMCs via ETRs/B(2)R, whereas RNA interference of ET(A)R and ET(B)R genes conversely reduced parasite internalization. ETRs/B(2)R-driven infection in HSMCs was reduced in HSMC pretreated with methyl-β-cyclodextrin, a cholesterol-depleting drug, or in thapsigargin- or verapamil-treated target cells.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that plasma leakage, a neutrophil-driven inflammatory response evoked by trypomastigotes via the kinin/endothelin pathways, may offer a window of opportunity for enhanced parasite invasion of cardiovascular cells.

Membrane traffic and synaptic cross-talk during host cell entry by Trypanosoma cruzi

It is widely accepted that Trypanosoma cruzi can exploit the natural exocytic response of the host to cell damage, utilizing host cell lysosomes as important effectors. It is, though, increasingly clear that the parasite also exploits endocytic mechanisms which allow for incorporation of plasma membrane into the parasitophorous vacuole. Further, that these endocytic mechanisms are involved in cross-talk with the exocytic machinery, in the recycling of vesicles and in the manipulation of the cytoskeleton. Here we review the mechanisms by which T. cruzi exploits features of the exocytic and endocytic pathways in epithelial and endothelial cells and the evidence for cross-talk between these pathways.

Molecular and cellular mechanisms involved in the Trypanosoma cruzi/host cell interplay

The protozoan parasite Trypanosoma cruzi has a complex biological cycle that involves vertebrate and invertebrate hosts. In mammals, the infective trypomastigote form of this parasite can invade several cell types by exploiting phagocytic-like or nonphagocytic mechanisms depending on the class of cell involved. Morphological studies showed that when trypomastigotes contact macrophages, they induce the formation of plasma membrane protrusions that differ from the canonical phagocytosis that occurs in the case of noninfective epimastigotes. In contrast, when trypomastigotes infect epithelial or muscle cells, the cell surface is minimally modified, suggesting the induction of a different class of process. Lysosomal-dependent or -independent T. cruzi invasion of host cells are two different models that describe the molecular and cellular events activated during parasite entry into nonphagocytic cells. In this context, we have previously shown that induction of autophagy in host cells before infection favors T. cruzi invasion. Furthermore, we demonstrate that autophagosomes and the autophagosomal protein LC3 are recruited to the T. cruzi entry sites and that the newly formed T. cruzi parasitophorous vacuole has characteristics of an autophagolysosome. This review summarizes the current knowledge of the molecular and cellular mechanisms of T. cruzi invasion in nonphagocytic cells. Based on our findings, we propose a new model in which T. cruzi takes advantage of the upregulation of autophagy during starvation to increase its successful colonization of host cells.

Mechanisms of host cell invasion by Trypanosoma cruzi

One of the more accepted concepts in our understanding of the biology of early Trypanosoma cruzi-host cell interactions is that the mammalian-infective trypomastigote forms of the parasite must transit the host cell lysosomal compartment in order to establish a productive intracellular infection. The acidic environment of the lysosome provides the appropriate conditions for parasite-mediated disruption of the parasitophorous vacuole and release of T. cruzi into the host cell cytosol, where replication of intracellular amastigotes occurs. Recent findings indicate a level of redundancy in the lysosome-targeting process where T. cruzi trypomastigotes exploit different cellular pathways to access host cell lysosomes in non-professional phagocytic cells. In addition, the reversible nature of the host cell penetration process was recently demonstrated when conditions for fusion of the nascent parasite vacuole with the host endosomal-lysosomal system were not met. Thus, the concept of parasite retention as a critical component of the T. cruzi invasion process was introduced. Although it is clear that host cell recognition, attachment and signalling are required to initiate invasion, integration of this knowledge with our understanding of the different routes of parasite entry is largely lacking. In this chapter, we focus on current knowledge of the cellular pathways exploited by T. cruzi trypomastigotes to invade non-professional phagocytic cells and to gain access to the host cell lysosome compartment.

Dysregulation of serine biosynthesis contributes to the growth defect of a Mycobacterium tuberculosis crp mutant

Mycobacterium tuberculosis CRP(Mt), encoded by Rv3676 (crp), is a CRP-like transcription factor that binds with the serC-Rv0885 intergenic region. In the present study, we evaluated CRP(Mt) 's regulation of serC and Rv0885 in M. tuberculosis and M. bovis BCG, using site-specific mutagenesis, promoter fusions and reverse-transcriptase PCR (RT-PCR). The CRP(Mt) binding site was required for full expression of serC and Rv0885, and expression of both genes was reduced in M. tuberculosis and M. bovis BCG crp mutants. These data show that CRP(Mt) binding directly activates both serC and Rv0885 expression. M. tuberculosis serC restored the ability of an Escherichia coli serC mutant to grow in serine-dropout medium, demonstrating that M. tuberculosis serC encodes a phosphoserine aminotransferase. Serine supplementation, or overexpression of serC, accelerated the growth of M. tuberculosis and M. bovis BCG crp mutants in mycomedium, but not within macrophages. These results establish a role for CRP(Mt) in the regulation of amino acid biosynthesis, and show that reduced serine production contributes to the slow-growth phenotype of M. tuberculosis and M. bovis BCG crp mutants in vitro. Restoration of serine biosynthesis by serC expression will facilitate identification of additional CRP(Mt)-regulated factors required by M. tuberculosis during macrophage and host infection.

Differential expression and characterization of a member of the mucin-associated surface protein family secreted by Trypanosoma cruzi

We describe the characterization, purification, expression, and location of a 52-kDa protein secreted during interaction between the metacyclic form of Trypanosoma cruzi and its target host cell. The protein, which we have named MASP52, belongs to the family of mucin-associated surface proteins (MASPs). The highest levels of expression of both the protein and mRNA occur during the metacyclic and bloodstream trypomastigote stages, the forms that infect the vertebrate host cells. The protein is located in the plasma membrane and in the flagellar pockets of the epimastigote, metacyclic, and trypomastigote forms and is secreted into the medium at the point of contact between the parasite and the cell membrane, as well as into the host-cell cytosol during the amastigote stage. IgG antibodies specific against a synthetic peptide corresponding to the catalytic zone of MASP52 significantly reduce the parasite's capacity to infect the host cells. Furthermore, when the protein is adsorbed onto inert particles of bentonite and incubated with a nonphagocytic cell culture, the particles are able to induce endocytosis in the cells, which seems to demonstrate that MASP52 plays a role in a process whereby the trypomastigote forms of the parasite invade the host cell.

Signaling mechanism of poly(ADP-ribose) polymerase-1 (PARP-1) in inflammatory diseases

Poly(ADP-ribosyl)ation, attaching the ADP-ribose polymer chain to the receptor protein, is a unique posttranslational modification. Poly(ADP-ribose) polymerase-1 (PARP-1) is a well-characterized member of the PARP family. In this review, we provide a general update on molecular structure and structure-based activity of this enzyme. However, we mainly focus on the roles of PARP-1 in inflammatory diseases. Specifically, we discuss the signaling pathway context that PARP-1 is involved in to regulate the pathogenesis of inflammation. PARP-1 facilitates diverse inflammatory responses by promoting inflammation-relevant gene expression, such as cytokines, oxidation-reduction-related enzymes, and adhesion molecules. Excessive activation of PARP-1 induces mitochondria-associated cell death in injured tissues and constitutes another mechanism for exacerbating inflammation.

Visual genome-wide RNAi screening to identify human host factors required for Trypanosoma cruzi infection

The protozoan parasite Trypanosoma cruzi is the etiologic agent of Chagas disease, a neglected tropical infection that affects millions of people in the Americas. Current chemotherapy relies on only two drugs that have limited efficacy and considerable side effects. Therefore, the development of new and more effective drugs is of paramount importance. Although some host cellular factors that play a role in T. cruzi infection have been uncovered, the molecular requirements for intracellular parasite growth and persistence are still not well understood. To further study these host-parasite interactions and identify human host factors required for T. cruzi infection, we performed a genome-wide RNAi screen using cellular microarrays of a printed siRNA library that spanned the whole human genome. The screening was reproduced 6 times and a customized algorithm was used to select as hits those genes whose silencing visually impaired parasite infection. The 162 strongest hits were subjected to a secondary screening and subsequently validated in two different cell lines. Among the fourteen hits confirmed, we recognized some cellular membrane proteins that might function as cell receptors for parasite entry and others that may be related to calcium release triggered by parasites during cell invasion. In addition, two of the hits are related to the TGF-beta signaling pathway, whose inhibition is already known to diminish levels of T. cruzi infection. This study represents a significant step toward unveiling the key molecular requirements for host cell invasion and revealing new potential targets for antiparasitic therapy.

Discovery of Salmonella virulence factors translocated via outer membrane vesicles to murine macrophages

Salmonella enterica serovar Typhimurium, an intracellular pathogen and leading cause of food-borne illness, encodes a plethora of virulence effectors. Salmonella virulence factors are translocated into host cells and manipulate host cellular activities, providing a more hospitable environment for bacterial proliferation. In this study, we report a new set of virulence factors that is translocated into the host cytoplasm via bacterial outer membrane vesicles (OMV). PagK (or PagK1), PagJ, and STM2585A (or PagK2) are small proteins composed of ∼70 amino acids and have high sequence homology to each other (>85% identity). Salmonella lacking all three homologues was attenuated for virulence in a mouse infection model, suggesting at least partial functional redundancy among the homologues. While each homologue was translocated into the macrophage cytoplasm, their translocation was independent of all three Salmonella gene-encoded type III secretion systems (T3SSs)-Salmonella pathogenicity island 1 (SPI-1) T3SS, SPI-2 T3SS, and the flagellar system. Selected methods, including direct microscopy, demonstrated that the PagK-homologous proteins were secreted through OMV, which were enriched with lipopolysaccharide (LPS) and outer membrane proteins. Vesicles produced by intracellular bacteria also contained lysosome-associated membrane protein 1 (LAMP1), suggesting the possibility of OMV convergence with host cellular components during intracellular trafficking. This study identified novel Salmonella virulence factors secreted via OMV and demonstrated that OMV can function as a vehicle to transfer virulence determinants to the cytoplasm of the infected host cell.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces calcium influx through T-type calcium channel and enhances lysosomal exocytosis and insulin secretion in INS-1 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been associated with diabetes in several epidemiological studies. However, the diabetogenic action of TCDD on pancreatic cells is unclear. Here, we investigated the direct toxic effects of TCDD on a rat insulin-secreting beta cell line. We found that TCDD enhances exocytosis of MTT formazan and lysosomal proteins such as beta-hexosaminindase and Lamp-1. This TCDD-induced exocytosis was abrogated by T-type calcium channel blockers (mibefradil, flunarizine) but not by an aryl hydrocarbon receptor antagonist (alpha-naphtoflavone). Indeed, cytosolic calcium levels were increased by TCDD. Furthermore, TCDD stimulated insulin secretion, which was inhibited by flunarizine. Taken together, our results suggest that TCDD-induced calcium influx via T-type channels regulates vesicular trafficking, such as lysosomal and secretory granule exocytosis, and that TCDD might exert adverse effects on beta cells by continuous insulin release followed by beta cell exhaustion. This could contribute to the link between TCDD exposure and the risk of developing diabetes.

Cyclic AMP decreases the production of NO and CCL2 by macrophages stimulated with Trypanosoma cruzi GPI-mucins

Glycosylphosphatidylinositol-anchored mucin-like glycoproteins (tGPI-mucin) present on the surface of the cellular membrane of Trypanosoma cruzi forms activate toll-like receptors 2 (TLR2) on the surface of immune cells and induce the release of several mediators of inflammation which may be relevant in the context of Chagas disease. Here, we evaluated the ability of tGPI-mucins to activate murine peritoneal macrophages to induce nitric oxide (NO) and monocyte chemoattractant protein-1 (MCP-1/CCL2). We also investigated the ability of compounds which increase or mimic cyclic adenosine monophosphate (AMP) to modulate the production of NO and CCL2. Our data show that elevation of intracellular levels of cyclic AMP prevents the release of NO and CCL2 induced by tGPI-mucins in macrophages. Overall, the release of CCL2 was decreased to a greater extent and at lower concentrations of cyclic AMP-modifying agents than the production of NO. It is suggested that the elevation of cyclic AMP during T. cruzi infection may modify the release of pro-inflammatory mediators and alter significantly the course of T. cruzi infection.

Modulation of host cell mechanics by Trypanosoma cruzi

To investigate the effects of Trypanosoma cruzi on the mechanical properties of infected host cells, cytoskeletal stiffness and remodeling dynamics were measured in parasite-infected fibroblasts. We find that cell stiffness decreases in a time-dependent fashion in T. cruzi-infected human foreskin fibroblasts without a significant change in the dynamics of cytoskeletal remodeling. In contrast, cells exposed to T. cruzi secreted/released components become significantly stiffer within 2 h of exposure and exhibit increased remodeling dynamics. These findings represent the first direct mechanical data to suggest a physical picture in which an intact, stiff, and rapidly remodeling cytoskeleton facilitates early stages of T. cruzi invasion and parasite retention, followed by subsequent softening and disassembly of the cytoskeleton to accommodate intracellular replication of parasites. We further suggest that these changes occur through protein kinase A and inhibition of the Rho/Rho kinase signaling pathway. In the context of tissue infection, changes in host cell mechanics could adversely affect the function of the infected organs, and may play an important role on the pathophysiology of Chagas' disease.

Development of protease inhibitors for protozoan infections

PURPOSE OF REVIEW

To highlight the promise of parasite proteases as targets for development of new antiparasitic chemotherapy. Proteolytic enzymes play key roles in the life cycle of protozoan parasites or the pathogenesis of diseases they produce. These roles include processing of host or parasite surface proteins for invasion of host cells, digestion of host proteins for nutrition, and inactivation of host immune defense mediators.

RECENT FINDINGS

Drug development for other markets has shown that proteases are druggable targets, and protease inhibitors are now licensed or in clinical development to treat hypertension, diabetes, thrombosis, osteoporosis, infectious diseases, and cancer. Several protease targets have been validated by genetic or chemical knockout in protozoan parasites. Many other parasite proteases appear promising as targets, but require more work for validation, or to identify viable drug leads. Because homologous proteases function as key enzymes in several parasites, targeting these proteases may allow development of a single compound, or a set of similar compounds, that target multiple diseases including malaria, trypanosomiasis, leishmaniasis, toxoplasmosis, cryptosporidiosis, and amebiasis.

SUMMARY

Proteases have been validated as targets in a number of parasitic infections. Proteases are druggable targets as evidenced by effective antiprotease drugs for the treatment of many human diseases including hypertension and AIDS. Future drug development targeting parasite proteases will be aided by the strong foundation of biochemical, structural, and computational databases already published or available online.

Trypanosoma cruzi induces changes in cardiac connexin43 expression

Gap junction proteins (connexins) are required for myocardial function, since they allow intercellular transmission of current carrying ions and signaling molecules. Previous studies demonstrated that rat cardiac myocytes infected with Trypanosoma cruzi lost gap junctional communication and decreased automaticity. We infected mouse cardiac myocytes with trypomastigotes of the Y strain of T. cruzi and observed alterations in connexin43 (Cx43) distribution. One hour post infection Cx43 levels were significantly increased. However, at longer time points post infection there was a significant loss of Cx43 staining in membranes of infected cardiac myocytes. Interestingly, there was also a significant reduction in myocardial Cx43 protein levels during acute infection. These data indicate that T. cruzi infection alters Cx43 expression both in vitro and in vivo. Disruptions in Cx43 may contribute to the pathogenesis of cardiac electrical alterations observed in T. cruzi infection.

The Mycobacterium bovis BCG cyclic AMP receptor-like protein is a functional DNA binding protein in vitro and in vivo, but its activity differs from that of its M. tuberculosis ortholog, Rv3676

Mycobacterium tuberculosis Rv3676 encodes a cyclic AMP (cAMP) receptor-like protein (CRP(Mt)) that has been implicated in global gene regulation and may play an important role during tuberculosis infection. The CRP(Mt) ortholog in Mycobacterium bovis BCG, CRP(BCG), is dysfunctional in an Escherichia coli CRP competition assay and has been proposed as a potential source of M. bovis BCG's attenuation. We compared CRP(BCG) and CRP(Mt) in vitro and in vivo, in M. bovis BCG and M. tuberculosis, to evaluate CRP(BCG)'s potential function in a mycobacterial system. Both proteins formed dimers in mycobacterial lysates, bound to the same target DNA sequences, and were similarly affected by the presence of cAMP in DNA binding assays. However, CRP(Mt) and CRP(BCG) differed in their relative affinities for specific DNA target sequences and in their susceptibilities to protease digestion. Surprisingly, CRP(BCG) DNA binding activity was stronger than that of CRP(Mt) both in vitro and in vivo, as measured by electrophoretic mobility shift and chromatin immunoprecipitation assays. Nutrient starvation-associated regulation of several CRP(Mt) regulon members also differed between M. bovis BCG and M. tuberculosis. We conclude that CRP(BCG) is a functional cAMP-responsive DNA binding protein with an in vivo DNA binding profile in M. bovis BCG similar to that of CRP(Mt) in M. tuberculosis. However, biologically significant functional differences may exist between CRP(BCG) and CRP(Mt) with respect to gene regulation, and this issue warrants further study.

Interaction with host factors exacerbates Trypanosoma cruzi cell invasion capacity upon oral infection

Outbreaks of severe acute Chagas' disease acquired by oral infection, leading to death in some cases, have occurred in recent years. Using the mouse model, we investigated the basis of such virulence by analyzing a Trypanosoma cruzi isolate, SC, from a patient with severe acute clinical symptoms, who was infected by oral route. It has previously been shown that, upon oral inoculation into mice, T. cruzi metacyclic trypomastigotes invade the gastric mucosal epithelium by engaging the stage-specific surface glycoprotein gp82, whereas the surface molecule gp90 functions as a down-modulator of cell invasion. We found that, when orally inoculated into mice, metacyclic forms of the SC isolate, which express high levels of gp90, produced high parasitemias and high mortality, in sharp contrast with the reduced infectivity in vitro. Upon recovery from the mouse stomach 1h after oral inoculation, the gp90 molecule of the parasites was completely degraded, and their entry into HeLa cells, as well as into Caco-2 cells, was increased. The gp82 molecule was more resistant to digestive action of the gastric juice. Host cell invasion of SC isolate metacyclic trypomastigotes was augmented in the presence of gastric mucin. No alteration in infectivity was observed in T. cruzi strains CL and G which were used as references and which express gp90 molecules resistant to degradation by gastric juice. Taken together, our findings suggest that the exacerbation of T. cruzi infectivity, such as observed upon interaction of the SC isolate with the mouse stomach components, may be responsible for the severity of acute Chagas' disease that has been reported in outbreaks of oral T. cruzi infection.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

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.

Regulation of the expression of whiB1 in Mycobacterium tuberculosis: role of cAMP receptor protein

The wbl (whiB-like) genes encode putative transcription factors unique to actinomycetes. This study characterized the promoter element of one of the seven wbl genes of Mycobacterium tuberculosis, whiB1 (Rv3219c). The results reveal that whiB1 is transcribed by a class I-type cAMP receptor protein (CRP)-dependent promoter, harbouring a CRP-binding site positioned at -58.5 with respect to its transcription start point. In vivo promoter activity analysis and electrophoretic mobility shift assays suggest that the expression of whiB1 is indeed regulated by cAMP-dependent binding of CRP(M) (encoded by the M. tuberculosis gene Rv3676) to the whiB1 5' untranslated region (5'UTR). beta-Galactosidase gene fusion analysis revealed induction of the whiB1 promoter in M. tuberculosis on addition of exogenous dibutyric cAMP (a diffusible cAMP analogue) only when an intact CRP-binding site was present. These results indicate that M. tuberculosis whiB1 transcription is regulated in part by cAMP levels via direct binding of cAMP-activated CRP(M) to a consensus CRP-binding site in the whiB1 5'UTR.

Molecular cloning of a Trypanosoma cruzi cell surface casein kinase II substrate, Tc-1, involved in cellular infection

In this work, we report the cloning and characterization of the first cell surface casein kinase II (CKII) substrate (Tc-1) of Trypanosoma cruzi, the causative agent of Chagas' disease. Analysis of the gene sequence revealed a 1,653-bp open reading frame coding for 550 amino acid residues. Northern blot analysis showed a 4.5-kb transcript that is expressed in invasive trypomastigotes but not in noninvasive epimastigote forms of T. cruzi. Southern blot analysis indicates that Tc-1 is a single-copy gene. At the amino acid level, Tc-1 displayed 95% and 99% identity to two hypothetical proteins recently reported by the T. cruzi genome project. Analysis of the translated amino acid sequence indicates that the Tc-1 gene has a putative transmembrane domain with multiple cytoplasmic and extracellular CKII phosphosites. Exogenous human CKII was able to phosphorylate serine residues on both recombinant Tc-1 and Tc-1 of intact trypomastigotes. This phosphorylation was inhibited by the CKII inhibitors heparin and 4,5,6,7,-tetrabromo-2-azabenzimidazole. Immunoblots of solubilized trypomastigotes, epimastigotes, and amastigotes probed with anti-recombinant Tc-1 immunoglobulin G revealed a 62-kDa protein that is expressed only in infective trypomastigotes. Immunoprecipitation of labeled surface proteins of trypomastigotes indicated that the 62-kDa protein is a surface protein, and we found that the protein is uniformly distributed on the surface of trypomastigotes by direct immunofluorescence. Antibodies to Tc-1 effectively blocked trypomastigote invasion of host cells and consequently reduced parasite load. Preincubation of either trypomastigotes or myoblasts with CKII inhibitors blocked T. cruzi infection. Thus, for the first time, we describe a cell surface CKII substrate of a protozoan parasite that is phosphorylated by human CKII and that is involved in cellular infection.

Enhancement of tyrosine hydroxylase expression and activity by Trypanosoma cruzi parasite-derived neurotrophic factor

A parasite-derived protein, PDNF, produced by the Chagas' disease agent Trypanosoma cruzi, functionally mimics mammalian neurotrophic factors by delaying apoptotic death and promoting survival and differentiation of neurons, including dopaminergic cells, through the activation of nerve growth factor receptor TrkA. Because it is well established that neurotrophic factors regulate enzymes involved in the biosynthesis of neurotransmitters, we examined whether PDNF could also directly activate tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of dopamine and other catecholamine neurotransmitters. We found that primary cultures of rat ventral mesencephalon responded to PDNF by increasing the number of TH-positive neurons and, most importantly, preserved expression of TH in neurons treated with Parkinson disease-inducing neurotoxin 1-methyl-4-phenyl pyridinium (MPP(+)). In dopaminergic PC12 cells, PDNF induced TH transcription via CRE element in TH promoter followed by significant increase in TH protein and expansion of TH-positive cell population. Furthermore, PDNF stimulated TH enzymatic activity by enhancing phosphorylation of seryl residues 31 and 40 through the activation of MAPK/Erk1/2 and cAMP-dependent protein kinase A signaling, respectively. Therefore, our results indicate that PDNF, in addition to its functioning as survival and differentiation-promoting factor for dopaminergic neuronal cells, can directly influence activity of the rate-limiting enzyme that underlies catecholamine biosynthetic cascade. This novel feature of PDNF should help understand the mechanism of neuronal function altered by T. cruzi infection, specifically neurotransmitter secretion. In addition, the findings have potential implications in the therapy of Chagas' and other neurodegenerative disorders.

Molecular basis of mammalian cell invasion by Trypanosoma cruzi

Establishment of infection by Trypanosoma cruzi, the agent of Chagas' disease, depends on a series of events involving interactions of diverse parasite molecules with host components. Here we focus on the mechanisms of target cell invasion by metacyclic trypomastigotes (MT) and mammalian tissue culture trypomastigotes (TCT). During MT or TCT internalization, signal transduction pathways are activated both in the parasite and the target cell, leading to Ca2+ mobilization. For cell adhesion, MT engage surface glycoproteins, such as gp82 and gp35/50, which are Ca2+ signal-inducing molecules. In T. cruzi isolates that enter host cells in gp82-mediated manner, parasite protein tyrosine kinase as well as phospholipase C are activated, and Ca2+ is released from I P3-sensitive stores, whereas in T. cruzi isolates that attach to target cells mainly through gp35/50, the signaling pathway involving adenylate cyclase appears to be stimulated, with Ca2+ release from acidocalciosomes. In addition, T. cruzi isolate-dependent inhibitory signals, mediated by MT-specific gp90, may be triggered both in the host cell and the parasite. The repertoire of TCT molecules implicated in cell invasion includes surface glycoproteins of gp85 family, with members containing binding sites for laminin and cytokeratin 18, enzymes such as cruzipain, trans-sialidase, and an oligopeptidase B that generates a Ca2+-agonist from a precursor molecule.

Deletion of PREPL, a gene encoding a putative serine oligopeptidase, in patients with hypotonia-cystinuria syndrome

In 11 patients with a recessive congenital disorder, which we refer to as "the hypotonia-cystinuria syndrome," microdeletion of part of the SLC3A1 and PREPL genes on chromosome 2p21 was found. Patients present with generalized hypotonia at birth, nephrolithiasis, growth hormone deficiency, minor facial dysmorphism, and failure to thrive, followed by hyperphagia and rapid weight gain in late childhood. Since loss-of-function mutations in SLC3A1 are known to cause isolated cystinuria type I, and since the expression of the flanking genes, C2orf34 and PPM1B, was normal, the extended phenotype can be attributed to the deletion of PREPL. PREPL is localized in the cytosol and shows homology with prolyl endopeptidase and oligopeptidase B. Substitution of the predicted catalytic residues (Ser470, Asp556, and His601) by alanines resulted in loss of reactivity with a serine hydrolase-specific probe. In sharp contrast to prolyl oligopeptidase and oligopeptidase B, which require both aminoterminal and carboxyterminal sequences for activity, PREPL activity appears to depend only on the carboxyterminal domain. Taken together, these results suggest that PREPL is a novel oligopeptidase, with unique structural and functional characteristics, involved in hypotonia-cystinuria syndrome.

Characterization of Mycobacterium tuberculosis Rv3676 (CRPMt), a cyclic AMP receptor protein-like DNA binding protein

Little is known about cyclic AMP (cAMP) function in Mycobacterium tuberculosis, despite its ability to encode 15 adenylate cyclases and 10 cNMP-binding proteins. M. tuberculosis Rv3676, which we have designated CRP(Mt), is predicted to be a cAMP-dependent transcription factor. In this study, we characterized CRP(Mt)'s interactions with DNA and cAMP, using experimental and computational approaches. We used Gibbs sampling to define a CRP(Mt) DNA motif that resembles the cAMP receptor protein (CRP) binding motif model for Escherichia coli. CRP(Mt) binding sites were identified in a total of 73 promoter regions regulating 114 genes in the M. tuberculosis genome, which are being explored as a regulon. Specific CRP(Mt) binding caused DNA bending, and the substitution of highly conserved nucleotides in the binding site resulted in a complete loss of binding to CRP(Mt). cAMP enhanced CRP(Mt)'s ability to bind DNA and caused allosteric alterations in CRP(Mt) conformation. These results provide the first direct evidence for cAMP binding to a transcription factor in M. tuberculosis, suggesting a role for cAMP signal transduction in M. tuberculosis and implicating CRP(Mt) as a cAMP-responsive global regulator.

Inhibition of host connective tissue growth factor expression: a novel Trypanosoma cruzi ‐mediated response

Connective tissue growth factor (CTGF) is a secreted cytokine that plays a fundamental role in the development of tissue fibrosis by mediating many of the profibrotic effects of TGF-beta. We present the novel finding that the intracellular pathogen Trypanosoma cruzi elicits immediate and sustained repression of basal CTGF expression in dermal fibroblasts, followed by down-regulation of the extracellular matrix proteins, fibronectin, and collagen I alpha1. To address mechanisms underlying this response, the major CTGF-regulating pathways were investigated. We report that both T. cruzi trypomastigotes and secreted parasite factor(s) antagonize TGF-beta-dependent induction of CTGF in fibroblasts. Of the TGF-beta-dependent signaling pathways required for CTGF expression, we demonstrate that T. cruzi interferes with cellular Erk1/2 phosphorylation but not Smad3 signaling. While increased stimulation of Erk phosphorylation alone was insufficient to override the parasite-mediated repression of CTGF, stimulation of fibroblasts with increased concentrations of TGF-beta, which activates both Smad3 and Erk1/2, completely abrogated this inhibition. Together with the finding that T. cruzi-mediated down-regulation of CTGF expression requires de novo host cell protein synthesis, our data indicate that the unique ability of T. cruzi to interfere with the host fibrogenic response is a complex process requiring input from multiple host cell signaling pathways.

Characterization of an ABCA-like transporter involved in vesicular trafficking in the protozoan parasite Trypanosoma cruzi

Protozoan parasites are responsible of important healthy problems, among others malaria, leishmaniasis and trypanosomiasis. The present work reports the characterization of the first mammalian ATP-binding cassette transporter, subfamily A (ABCA)-like in Trypanosoma cruzi. TcABC1 is a single copy gene differentially expressed along the life cycle of the parasite, being absent in its infective form. TcABC1 localizes to the plasma membrane, flagellar pocket and intracellular vesicles. Functional studies of TcABC1 in transfected parasites suggest that the protein is implicated in intracellular trafficking, as determined by the analysis of endocytosis and exocytosis events. The accumulation of the endocytic markers FM4-64 and NBD-SM is increased in transfected parasites. Similarly, ectophosphatase and ectoATPase activities are increased in TcABC1 overproducers. Indeed, transmission electronic microscopy analysis showed a higher number of intracellular vesicles in TcABC1 transfectants. Taken together, these results suggest that the protein is involved in the endocytic and exocytic pathways of T. cruzi.

Synaptotagmin VII restricts fusion pore expansion during lysosomal exocytosis

Synaptotagmin is considered a calcium-dependent trigger for regulated exocytosis. We examined the role of synaptotagmin VII (Syt VII) in the calcium-dependent exocytosis of individual lysosomes in wild-type (WT) and Syt VII knockout (KO) mouse embryonic fibroblasts (MEFs) using total internal reflection fluorescence microscopy. In WT MEFs, most lysosomes only partially released their contents, their membrane proteins did not diffuse into the plasma membrane, and inner diameters of their fusion pores were smaller than 30 nm. In Syt VII KO MEFs, not only was lysosomal exocytosis triggered by calcium, but all of these restrictions on fusion were also removed. These observations indicate that Syt VII does not function as the calcium-dependent trigger for lysosomal exocytosis. Instead, it restricts the kinetics and extent of calcium-dependent lysosomal fusion.

Novel PI 3-kinase-dependent mechanisms of trypanosome invasion and vacuole maturation

Mammalian cell invasion by the protozoan parasite, Trypanosoma cruzi, is facilitated by the activation of host cell phosphatidylinositol 3 (PI 3)-kinases. We demonstrate that the well-characterized Ca2+-regulated lysosome-mediated parasite entry pathway is abolished by wortmannin pretreatment. In addition, we have characterized a novel route of T. cruzi invasion unexpectedly revealed in the course of this study. For over a decade, targeted exocytosis of lysosomes at the host cell plasma membrane was considered as the primary mechanism for T. cruzi entry into non-professional phagocytic cells. We now provide evidence that a significant fraction (50% or greater) of invading T. cruzi trypomastigotes exploit an alternate actin-independent entry pathway that involves formation of a tightly associated host cell plasma membrane-derived vacuole enriched in the lipid products of class I PI 3-kinases, PtdInsP3/PtdIns(3,4)P2. Initially devoid of lysosomal markers, the resultant parasite-containing vacuoles gradually acquire lysosome associated membrane protein 1 (lamp-1) and fluid phase endocytic tracer from the lysosomal compartment. In striking contrast to latex bead phagosomes, few T. cruzi vacuoles associate with the early endosomal marker, EEA1 and the 'maturation' process becomes refractory to PI 3-kinase inhibition immediately following parasite internalization. Jointly, these data provide a new paradigm for T. cruzi invasion of non-professional phagocytic cells and reveal a novel vacuole maturation process that appears to bypass the requirement for EEA1.

Many facets of mammalian lanosterol 14alpha-demethylase from the evolutionarily conserved cytochrome P450 family CYP51

Lanosterol 14alpha-demethylase is a cytochrome P450 enzyme of the cholesterol biosynthetic pathway belonging to the CYP51 gene family which is the most evolutionarily conserved member of the CYP superfamily. Mammalian (human, mouse, rat, pig) CYP51 genes are unique in sharing several common characteristics: highly conserved exon/intron borders and proximal promoter structures, ubiquitous expression at the highest level in the testis, and appearance of testis-specific transcripts that arise from differential polyadenylation site usage. CYP51 protein demethylates lanosterol to form follicular fluid meiosis-activating sterol, FF-MAS, which is, besides being an intermediate of cholesterol biosynthesis, also a signaling sterol that accumulates in ovaries. CYP51 protein resides in the endoplasmatic reticulum of most cells and also in acrosomal membranes of spermatids where transport through the Golgi apparatus is suggested. While sterol regulatory element binding protein (SREBP)-dependent transcriptional regulation of CYP51 contributes to synthesis of cholesterol, the germ-cell-specific cAMP/CREMtau-dependent upregulation might contribute to increased production of MAS.

Trypanosoma cruzi induces edematogenic responses in mice and invades cardiomyocytes and endothelial cells in vitro by activating distinct kinin receptor (B1/B2) subtypes

Trypanosoma cruzi, the protozoan that causes Chagas' heart disease, invades endothelial cells in vitro by activating the B2 kinin receptor (B2R). Here, we demonstrate that mice infected with trypomastigotes develop potent edema after treatment with the angiotensin-converting enzyme (ACE) (or kininase II) inhibitor captopril. Experiments performed with specific kinin receptor (B2R/B1R) antagonists and knockout mice revealed that the early-phase (3-h) edema is mediated by the constitutive B2R, whereas the late-phase (24-h) response depends on stimulation of the up-regulated B1R. Given previous evidence that parasite invasion of cells expressing B2R is potentiated by captopril, we investigated the prerequisites for in vitro infection of Chinese hamster ovary cells overexpressing either B1R or B2R, human umbilical vein endothelial cells activated by lipopolysaccharide, and neonatal rat cardiomyocytes. Our results indicate that captopril potentiates parasite invasion regardless of the kinin (B2/B1) activation pathways, whereas DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid (MGTA), an inhibitor of kininase I (carboxypeptidase M/N), selectively decreases parasite infectivity for B1R-expressing cells. These data suggest that formation of the B1R agonist, i.e., [des-Arg] kinins, critically depends on the processing action of kininase I, here proposed as a potential pathogenesis cofactor. Collectively, our data suggest that fluctuations in the levels of kininases may modulate parasite infectivity and pathological outcome in Chagas' disease.

Cellular signaling during the macrophage invasion by Trypanosoma cruzi

We have reported that protein tyrosine kinases play an important role in the invasion of Trypanosoma cruzi into primary resident macrophages. In the present study we carry out immunofluorescence assays, using monoclonal anti-phosphotyrosine antibodies, to reveal an accumulation of tyrosine-phosphorylated residues at the site of parasite association with the macrophage surface, colocalizing with host cell F-actin-rich domains. SDS-PAGE analysis of macrophage cell line IC-21 tyrosine phosphoproteins, labeled with [(35)S] L-methionine, revealed several peptides with increased levels of phosphorylation upon interaction with the parasite. Among them, were detected bands of 140, 120, 112, 94, 73, 67, and 56 kDa that match the molecular weights of proteins described as being tyrosine phosphorylated during events that lead to actin assembly in mononuclear phagocytes. The pretreatment of IC-21 macrophages with the tyrosine kinase inhibitor tyrphostin 23 inhibited trypomastigote uptake showing that tyrosine phosphorylation is important for the parasite penetration in this particular cell line. Immunofluorescence microscopy, using antibodies against p85, the regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase), placed this enzyme also in the same sites, in accordance to what is reported for phagocytosis. We suggest that once the components of T. cruzi trypomastigotes surface are recognized by macrophage receptors, they trigger the activation of a tyrosine phosphorylation cascade, PI 3-kinase recruitment, and assembly of actin filaments at the site of initial cell-to-cell contact, resembling the events described during phagocytosis. These achievements support the model for a phagocytic-like actin-dependent invasion mechanism for T. cruzi trypomastigotes into macrophages.

Biological roles of proteases in parasitic protozoa

Proteases from a variety of protozoan parasites have been characterized at the molecular and cellular levels, and the many roles that proteases play in these organisms are coming into focus. Central roles have been proposed for proteases in diverse processes such as host cell invasion and egress, encystation, excystation, catabolism of host proteins, differentiation, cell cycle progression, cytoadherence, and both stimulation and evasion of host immune responses. Detailed structural and functional characterization of parasite proteases has led to novel insights into the workings of these fascinating catalytic machines. The possibility of developing selective inhibitors of key proteases of pathogenic parasites into novel chemotherapeutic strategies is being vigorously explored.

Membrane proximal lysosomes are the major vesicles responsible for calcium-dependent exocytosis in nonsecretory cells

Similar to its role in secretory cells, calcium triggers exocytosis in nonsecretory cells. This calcium-dependent exocytosis is essential for repair of membrane ruptures. Using total internal reflection fluorescence microscopy, we observed that many organelles implicated in this process, including ER, post-Golgi vesicles, late endosomes, early endosomes, and lysosomes, were within 100 nm of the plasma membrane (in the evanescent field). However, an increase in cytosolic calcium led to exocytosis of only the lysosomes. The lysosomes that fused were predominantly predocked at the plasma membrane, indicating that calcium is primarily responsible for fusion and not recruitment of lysosomes to the cell surface.