High- and low-density lipoproteins enhance infection of Trypanosoma cruzi in vitro

Vaccine Design against Chagas Disease Focused on the Use of Nucleic Acids

Chagas disease is caused by the protozoan Trypanosoma cruzi and is endemic to Central and South America. However, it has spread around the world and affects several million people. Treatment with currently available drugs cause several side effects and require long treatment times to eliminate the parasite, however, this does not improve the chronic effects of the disease such as cardiomyopathy. A therapeutic vaccine for Chagas disease may be able to prevent the disease and improve the chronic effects such as cardiomyopathy. This vaccine would be beneficial for both infected people and those which are at risk in endemic and non-endemic areas. In this article, we will review the surface antigens of T. cruzi, in order to choose those that are most antigenic and least variable, to design effective vaccines against the etiological agent of Chagas disease. Also, we discuss aspects of the design of nucleic acid-based vaccines, which have been developed and proven to be effective against the SARS-CoV-2 virus. The role of co-adjuvants and delivery carriers is also discussed. We present an example of a chimeric trivalent vaccine, based on experimental work, which can be used to design a vaccine against Chagas disease.

LDL receptor and pathogen processes: Functions beyond normal lipids

Although the role of the LDL receptor concerning lipids is well known, its role in various viral and parasitic infections, and in regulating the inflammatory response is poorly understood. Several infectious agents use the LDL receptor as a port of entry, and others depend on it for their cycle of infection. In this review, we focus on the discovery, structure, and normal function of the LDL receptor, as well as its role in a selection of infections. The LDL receptor plays an important role in certain infections and is a potential target for treatment deserving further research.

The Liver and the Hepatic Immune Response in Trypanosoma cruzi Infection, a Historical and Updated View

Chagas disease was described more than a century ago and, despite great efforts to understand the underlying mechanisms that lead to cardiac and digestive manifestations in chronic patients, much remains to be clarified. The disease is found beyond Latin America, including Japan, the USA, France, Spain, and Australia, and is caused by the protozoan Trypanosoma cruzi. Dr. Carlos Chagas described Chagas disease in 1909 in Brazil, and hepatomegaly was among the clinical signs observed. Currently, hepatomegaly is cited in most papers published which either study acutely infected patients or experimental models, and we know that the parasite can infect multiple cell types in the liver, especially Kupffer cells and dendritic cells. Moreover, liver damage is more pronounced in cases of oral infection, which is mainly found in the Amazon region. However, the importance of liver involvement, including the hepatic immune response, in disease progression does not receive much attention. In this review, we present the very first paper published approaching the liver's participation in the infection, as well as subsequent papers published in the last century, up to and including our recently published results. We propose that, after infection, activated peripheral T lymphocytes reach the liver and induce a shift to a pro-inflammatory ambient environment. Thus, there is an immunological integration and cooperation between peripheral and hepatic immunity, contributing to disease control.

Mechanisms Associated with Trypanosoma cruzi Host Target Cell Adhesion, Recognition and Internalization

Chagas disease is caused by the kinetoplastid parasite Trypanosoma cruzi, which is mainly transmitted by hematophagous insect bites. The parasite's lifecycle has an obligate intracellular phase (amastigotes), while metacyclic and bloodstream-trypomastigotes are its infective forms. Mammalian host cell recognition of the parasite involves the interaction of numerous parasite and host cell plasma membrane molecules and domains (known as lipid rafts), thereby ensuring internalization by activating endocytosis mechanisms triggered by various signaling cascades in both host cells and the parasite. This increases cytoplasmatic Ca2+ and cAMP levels; cytoskeleton remodeling and endosome and lysosome intracellular system association are triggered, leading to parasitophorous vacuole formation. Its membrane becomes modified by containing the parasite's infectious form within it. Once it has become internalized, the parasite seeks parasitophorous vacuole lysis for continuing its intracellular lifecycle, fragmenting such a vacuole's membrane. This review covers the cellular and molecular mechanisms involved in T. cruzi adhesion to, recognition of and internalization in host target cells.

Lipid metabolism in Trypanosoma cruzi: A review

The cellular membranes of Trypanosoma cruzi, like all eukaryotes, contain varying amounts of phospholipids, sphingolipids, neutral lipids and sterols. A multitude of pathways exist for the de novo synthesis of these lipid families but Trypanosoma cruzi has also become adapted to scavenge some of these lipids from the host. Completion of the TriTryp genomes has led to the identification of many putative genes involved in lipid synthesis, revealing some interesting differences to higher eukaryotes. Although many enzymes involved in lipid synthesis have yet to be characterised, completed experiments have shown the indispensability of some lipid metabolic pathways. Furthermore, the bioactive lipids of Trypanosoma cruzi and their effects on the host are becoming increasingly studied. Further studies on lipid metabolism in Trypanosoma cruzi will no doubt reveal some attractive targets for therapeutic intervention as well as reveal the interplay between parasite lipids, host response and pathogenesis.

Apolipoprotein A1 and Fibronectin Fragments as Markers of Cure for the Chagas Disease

Chagas disease (CD), endemic from Latin America, affects more than 8 million people, and the disease keeps spreading around the world due to population migrations. The treatment options for CD are currently limited to two drugs, benznidazole (BZ) and nifurtimox (Nfx), which are often unsatisfactory in chronically infected patients. To date, the only accepted marker of the cure is seroconversion (the disappearance of Trypanosoma cruzi antibodies in the patient's serum), which can take decades to occur, if ever. The lack of posttreatment test-of-cure often prevents appropriate patient counseling and limits the development of new drugs. Without a doubt, reliable biomarkers for parasitological cure are urgently needed. Several pieces of evidence suggest that apolipoprotein A1 and fibronectin fragments are produced during the infection as part of the process of T. cruzi cell invasion and can thus be used as its surrogate biomarkers. In this chapter, we present a standardized method to evaluate these fragments in serum using mass spectrometry and immunoblotting in CD patients for diagnosis, prognosis, and treatment assessment purposes.

Validation of Apolipoprotein A-1 and Fibronectin Fragments as Markers of Parasitological Cure for Congenital Chagas Disease in Children Treated With Benznidazole

Background

No reliable tests or validated biomarkers exist to ensure parasitological cure following treatment of Chagas disease (CD) patients chronically infected with Trypanosoma cruzi. As seroreversion, the only marker of cure, happens more quickly in children, we investigated the correlation between previously identified biomarkers and seroreversion in children.

Methods

Thirty CD children (age 1 month to 10 years) diagnosed as T. cruzi positive (time point S0) were treated with benznidazole (BZ) 5–8 mg/kg/d for 60 days. At least 2 serological tests were used to evaluate treatment efficacy from the end of treatment (S1) until seroreversion (S2). Thirty children (age 1 month to 10 years) and 15 adults were used as healthy controls (HCs). Immunoblot and a proteomic-based assay were used to validate previously identified fragments of apolipoprotein A-1 (ApoA1) and fibronectin (FBN) as CD biomarkers.

Results

Correlation between seroreversion and absence of ApoA1 and FBN fragments by immunoblot was observed in 30/30 (100%) and 29/30 (96.6%) CD children, respectively. ApoA1 and FBN fragments were absent at the end of BZ treatment in 20/30 (66.6%) and 16/30 (53.3%) children, respectively. Absence of fragments in serum profiles was confirmed by mass spectrometry. Using intact protein analysis, a 28 109-Da protein identified as full-length ApoA1 by tandem mass spectrometry was detected in HC serum samples.

Conclusions

These data confirm that ApoA1 and FBN fragments can discriminate between healthy and T. cruzi–infected samples. Correlation with seroreversion was shown for the first time; results suggest predictive capacity potentially superior to serology, making them potentially useful as surrogate biomarkers.

Metabolic Crosstalk Between Host and Parasitic Pathogens

A complex network that embraces parasite-host intrinsic factors and the microenvironment regulated the interaction between a parasite and its host. Nutritional pressures exerted by both elements of this duet thus dictate this host-parasite niche. To survive and proliferate inside a host and a harsh nutritional environment, the parasites modulate different nutrient sensing pathways to subvert host metabolic pathways. Such mechanism is able to change the flux of distinct nutrients/metabolites diverting them to be used by the parasites. Apart from this nutritional strategy, the scavenging of nutrients, particularly host fatty acids, constitutes a critical mechanism to fulfil parasite nutritional requirements, ultimately defining the host metabolic landscape. The host metabolic alterations that result from host-parasite metabolic coupling can certainly be considered important targets to improve diagnosis and also for the development of future therapies. Metabolism is in fact considered a key element within this complex interaction, its modulation being crucial to dictate the final infection outcome.

A Brief View of the Surface Membrane Proteins from Trypanosoma cruzi

Trypanosoma cruzi is the causal agent of Chagas' disease which affects millions of people around the world mostly in Central and South America. T. cruzi expresses a wide variety of proteins on its surface membrane which has an important role in the biology of these parasites. Surface molecules of the parasites are the result of the environment to which the parasites are exposed during their life cycle. Hence, T. cruzi displays several modifications when they move from one host to another. Due to the complexity of this parasite's cell surface, this review presents some membrane proteins organized as large families, as they are the most abundant and/or relevant throughout the T. cruzi membrane.

Antagonistic effect of atorvastatin on high fat diet induced survival during acute Chagas disease

Chagasic cardiomyopathy, which is seen in Chagas disease, is the most severe and life-threatening manifestation of infection by the kinetoplastid Trypanosoma cruzi. Adipose tissue and diet play a major role in maintaining lipid homeostasis and regulating cardiac pathogenesis during the development of Chagas cardiomyopathy. We have previously reported that T. cruzi has a high affinity for lipoproteins and that the invasion rate of this parasite increases in the presence of cholesterol, suggesting that drugs that inhibit cholesterol synthesis, such as statins, could affect infection and the development of Chagasic cardiomyopathy. The dual epidemic of diabetes and obesity in Latin America, the endemic regions for Chagas disease, has led to many patients in the endemic region of infection having hyperlipidemia that is being treated with statins such as atorvastatin. The current study was performed to examine mice fed on either regular or high fat diet for effects of atorvastatin on T. cruzi infection-induced myocarditis and to evaluate the effect of this treatment during infection on adipose tissue physiology and cardiac pathology. Atorvastatin was found to regulate lipolysis and cardiac lipidopathy during acute T. cruzi infection in mice and to enhance tissue parasite load, cardiac LDL levels, inflammation, and mortality in during acute infection. Overall, these data suggest that statins, such as atorvastatin, have deleterious effects during acute Chagas disease.

Trypanosoma cruzi infection and host lipid metabolism

Trypanosoma cruzi is the causative agent of Chagas disease. Approximately 8 million people are thought to be affected worldwide. Several players in host lipid metabolism have been implicated in T. cruzi-host interactions in recent research, including macrophages, adipocytes, low density lipoprotein (LDL), low density lipoprotein receptor (LDLR), and high density lipoprotein (HDL). All of these factors are required to maintain host lipid homeostasis and are intricately connected via several metabolic pathways. We reviewed the interaction of T. cruzi with each of the relevant host components, in order to further understand the roles of host lipid metabolism in T. cruzi infection. This review sheds light on the potential impact of T. cruzi infection on the status of host lipid homeostasis.

Trypanosoma cruzi infection results in an increase in intracellular cholesterol

Chagasic cardiomyopathy caused by Trypanosoma cruzi is a major health concern in Latin America and among immigrant populations in non-endemic areas. T. cruzi has a high affinity for host lipoproteins and uses the low density lipoprotein receptor (LDLr) for invasion. Herein, we report that T. cruzi infection is associated with an accumulation of LDL and cholesterol in tissues in both acute and chronic murine Chagas disease. Similar findings were observed in tissue samples from a human case of Chagasic cardiomyopathy. T. cruzi infection of cultured cells displayed increased invasion with increasing cholesterol levels in the medium. Studies of infected host cells demonstrated alterations in their cholesterol regulation. T. cruzi invasion/infection via LDLr appears to be involved in changes in intracellular cholesterol homeostasis. The observed changes in intracellular lipids and associated oxidative stress due to these elevated lipids may contribute to the development of Chagasic cardiomyopathy.

LDL uptake by Leishmania amazonensis: involvement of membrane lipid microdomains

Leishmania amazonensis lacks a de novo mechanism for cholesterol synthesis and therefore must scavenge this lipid from the host environment. In this study we show that the L. amazonensis takes up and metabolizes human LDL(1) particles in both a time and dose-dependent manner. This mechanism implies the presence of a true LDL receptor because the uptake is blocked by both low temperature and by the excess of non-labelled LDL. This receptor is probably associated with specific microdomains in the membrane of the parasite, such as rafts, because this process is blocked by methyl-β-cyclodextrin (MCBD). Cholesteryl ester fluorescently-labeled LDL (BODIPY-cholesteryl-LDL) was used to follow the intracellular distribution of this lipid. After uptake it was localized in large compartments along the parasite body. The accumulation of LDL was analyzed by flow cytometry using FITC-labeled LDL particles. Together these data show for the first time that L. amazonensis is able to compensate for its lack of lipid synthesis through the use of a lipid importing machinery largely based on the uptake of LDL particles from the host. Understanding the details of the molecular events involved in this mechanism may lead to the identification of novel targets to block Leishmania infection in human hosts.

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.

Trypanosoma cruzi utilizes the host low density lipoprotein receptor in invasion

Background

Trypanosoma cruzi, an intracellular protozoan parasite that infects humans and other mammalian hosts, is the etiologic agent in Chagas disease. This parasite can invade a wide variety of mammalian cells. The mechanism(s) by which T. cruzi invades its host cell is not completely understood. The activation of many signaling receptors during invasion has been reported; however, the exact mechanism by which parasites cross the host cell membrane barrier and trigger fusion of the parasitophorous vacuole with lysosomes is not understood.

Methodology/Principal Findings

In order to explore the role of the Low Density Lipoprotein receptor (LDLr) in T. cruzi invasion, we evaluated LDLr parasite interactions using immunoblot and immunofluorescence (IFA) techniques. These experiments demonstrated that T. cruzi infection increases LDLr levels in infected host cells, inhibition or disruption of LDLr reduces parasite load in infected cells, T. cruzi directly binds recombinant LDLr, and LDLr-dependent T. cruzi invasion requires PIP2/3. qPCR analysis demonstrated a massive increase in LDLr mRNA (8000 fold) in the heart of T. cruzi infected mice, which is observed as early as 15 days after infection. IFA shows a co-localization of both LDL and LDLr with parasites in infected heart.

Conclusions/Significance

These data highlight, for the first time, that LDLr is involved in host cell invasion by this parasite and the subsequent fusion of the parasitophorous vacuole with the host cell lysosomal compartment. The model suggested by this study unifies previous models of host cell invasion for this pathogenic protozoon. Overall, these data indicate that T. cruzi targets LDLr and its family members during invasion. Binding to LDL likely facilitates parasite entry into host cells. The observations in this report suggest that therapeutic strategies based on the interaction of T. cruzi and the LDLr pathway should be pursued as possible targets to modify the pathogenesis of disease following infection.

Trypanosoma cruzi, an intracellular protozoan parasite that causes Chagas disease in humans and results in the development of cardiomyopathy, is a major health problem in endemic areas. This parasite can invade a wide variety of mammalian cells. The mechanisms by which these parasites invade their host cells are not completely understood. Our study highlights, for the first time, that the Low Density Lipoprotein receptor (LDLr) is important in the invasion and the subsequent fusion of the parasitophorous vacuole with host lysosomes. We demonstrate that T. cruzi directly binds to LDLr, and inhibition or disruption of LDLr significantly decreases parasite entry. Additionally, we have determined that this cross-linking triggers the accumulation of LDLr and phosphotidylinositol phosphates in coated pits, which initiates a signaling cascade that results in the recruitment of lysosomes, possibly via the sorting motif in the cytoplasmic tail of LDLr, to the site of adhesion/invasion. Studies of infected CD1 mice demonstrate that LDLs accumulate in infected heart and that LDLr co-localize with internalized parasites. Overall, this study demonstrates that LDLr and its family members, engaged mainly in lipoprotein transportation, are also involved in T. cruzi entry into host cells and this interaction likely contributes to the progression of chronic cardiomyopathy.

Do parasites express receptors for host lipoproteins?

Although cholesterol is the predominant sterol in parasite tissue, many parasites are unable to synthesize cholesterol or longchain fatty acids, de novo, and must therefore obtain these components from the host. Of particular interest are the plasma lipoproteins, a rich and abundant source of cholesterol, and other lipids that could be used by parasites inhabiting the vascular system of their host or with access to plasma proteins at extravascular sites. It is not inconceivable that parasites may have evolved a variety of receptors for lipoproteins by convergent evolution. Here, Mark Rogers discusses evidence for the presence of lipoprotein receptors in protozoan and metazoan parasites of mammals.

Trypanosoma rangeli uptakes the main lipoprotein from the hemolymph of its invertebrate host

During its life cycle Trypanosoma rangeli crosses the hemolymph of its invertebrate host. In the present study, we demonstrate for the first time the uptake of lipophorin (Lp), the main lipid-transporting particle of insect hemolymph. We observed that living T. rangeli parasites uptake lipids from both 32P- and 3H-, or 125I-labeled Lp. However, the parasites do not uptake any other hemolymphatic protein such as 32P-labeled vitellogenin. The presence of a specific receptor to Lp in the parasite surface is suggested based on experiments using 125I-Lp. We also investigated the intracellular fate of lipids using Texas Red-labeled phosphatidylethanolamine-Lp. Parasites were observed under confocal microscope and displayed fluorescent-labeled lipids close to the flagellar pocket and in vesicles at the posterior region. In conclusion, this study raises a novel set of molecular events which takes place during vector-parasite interaction.

Involvement of calyculin A-sensitive phosphatase(s) in the differentiation of Trypanosoma cruzi trypomastigotes to amastigotes

Differentiation of the non-dividing trypomastigote form of Trypanosoma cruzi, the causative agent of Chagas disease, to the dividing amastigote form normally occurs in cytoplasm of infected cells. Here we show that calyculin A. a potent inhibitor of protein phosphatases 1 and 2A, induces at pH 7.5 extracellular transformation of long slender trypomastigotes to round amastigote-like forms which acquire characteristic features observed after the normal differentiation process: repositioning and structural changes of the kinetoplast, release of surface neuraminidase, and expression of amastigote-specific epitopes. Calyculin A inhibits parasite phosphatases and changes in the phosphorylation of specific proteins occur during the transformation process. As an exposure of trypomastigotes to calyculin A concentrations as low as 1 nM and for only 1-2 h is sufficient to induce transformation, the inhibition of calyculin A-sensitive phosphatase(s) appears to play a major role in initiating the trypomastigote differentiation.

Mediation of Trypanosoma cruzi invasion by sialic acid on the host cell and trans-sialidase on the trypanosome

Trypanosoma cruzi attaches and invades a large variety of mammalian cells. The nature of the cell receptors and of the corresponding parasite counter-receptors that mediate T. cruzi-host cell interaction are not known. Three sialic acid-deficient mutants of Chinese hamster ovary (CHO) cells were used to probe the role of host sialyl residues in T. cruzi infection. All three mutants supported adhesion and infection to a much lower extent than the parental CHO cells. One of the mutants, Lec2, contains sugar chains terminating in non-reducing beta Gal residues, which are acceptors for sialylation by the T. cruzi trans-sialidase. Re-sialylation of Lec2 cells restored T. cruzi adhesion and invasion to about the same extent as wild-type cells. Digestion of wild-type cells with bacterial sialidase reduced T. cruzi interaction but after re-sialylation, the cells were almost as good as control, naturally sialylated parental cells. These results suggest that T. cruzi recognizes sialyl residues on the surface of host cells during invasion. On the other hand, affinity-purified trans-sialidase blocked T. cruzi adherence and invasion of sialylated cells, and had no effect on parasite interaction with sialic acid-deficient Lec2 mutant. Furthermore, 2,3-sialyllactose, a substrate for the trans-sialidase, competitively inhibited T. cruzi invasion of sialylated parental K1 cells, but 2,6-sialyllactose, which does not react with the trans-sialidase, was without effect, as were other sugars that do not contain alpha 2,3 sialyl residues. These results suggest that the trans-sialidase functions as a counter-receptor for trypomastigote binding to alpha 2,3-sialyl receptors on host cells as a prelude to T. cruzi invasion.

Mucin-like glycoproteins linked to the membrane by glycosylphosphatidylinositol anchor are the major acceptors of sialic acid in a reaction catalyzed by trans-sialidase in metacyclic forms of Trypanosoma cruzi

We have previously shown that 35- and 50-kDa glycoconjugates of cultured metacyclic trypomastigotes participate in the attachment of parasites to mammalian cells. Here we show that when metacyclic trypomastigotes are incubated with [3H]sialyllactose, most of the sialic acid is transferred to these 35/50-kDa molecules in a reaction catalyzed by a parasite transsialidase. The sialic acid is incorporated in oligosaccharides of about 10 glucose units in size that are released from the glycoconjugate by mild alkaline hydrolysis. Compositional analysis reveals that the 35/50-kDa molecules are highly glycosylated proteins rich in threonine, galactose, N-acetyl-glucosamine and sialic acid. These glycoproteins can be labeled in vivo with [3H]palmitate, and the labeled fatty acid is released by glycosylphosphatidylinositol specific phospholipases C. This result, associated with the fact that they contain mannose, ethanolamine, myo-inositol, and lipid, indicate that these glycoproteins are anchored to the membrane by glycosylphosphatidylinositol. During cell invasion, these molecules appear to be capped and locally released by the parasite.

Trypanosoma cruzi trans-sialidase and cell invasion

Trypanosoma cruzi does not synthesize sialic acid but does contain a trans-sialidase, an enzyme capable of transferring sialic acid between host glycoconjugates and the parasite. Sialic acids are negatively charged carbohydrates attached to the terminal non-reducing end of glycoproteins and glycolipids, and their presence can dramatically influence many cell-surface recognition processes. Since sialic acids have been implicated in several ligand-receptor interactions, including the interaction of pathogenic viruses, bacteria and protozoans with their hosts, the expression of trans-sialidase and the acquisition of sialic acid by T. cruzi may be relevant to the interaction of the parasite with the host, and consequently may influence the pathobiology of Chagas disease. In this review, Sergio Schenkman and Daniel Eichinger discuss recent data about the structure and function of T. cruzi trans-sialidase.

Developmentally-regulated virulence factors of Trypanosoma cruzi and their relationship to evasion of host defences

Developmental preadaptation of virulent stages of Trypanosoma cruzi correlates with their ability to survive and establish infection in mammalian hosts. Infective trypomastigote stages must first preadapt to survival in the extracellular milieu and then to the rigors of establishing an intracellular infection. Selected phenotypic variations in evading host defences have been correlated with expression of stage-specific proteins or functions. Resistance of trypomastigotes to complement-mediated killing correlates with the presence of a stage-specific molecule that exhibits an analogous function to mammalian decay-accelerating factor, and with the presence of a neuraminidase/trans-sialidase that transfers sialic acid moieties to the parasite surface, thereby enabling it to avoid complement activation. Trypomastigotes enter cells by a mechanism that involves sorting of cell surface receptors and avoids eliciting a respiratory burst. Once within a membrane-bound vacuole, which undergoes acidification, the neuraminidase/trans-sialidase and an acid-active, transmembrane pore-forming protein are released by the parasite and are capable of acting together to accelerate rupture of the vacuolar membrane and the parasite's escape into the cytoplasm of the host cell. Escape from the parasitophorous vacuole allows virulent stages of T. cruzi to avoid compartmental, non-oxidative killing mechanisms such as degradation by lysosomal hydrolases.

Mammalian cell sialic acid enhances invasion by Trypanosoma cruzi

We have used a Chinese hamster ovary cell mutant (Lec2) that express much less sialic acid on the surface than the parental cell line (Pro5) to investigate whether sialic acid plays a role during cell invasion by Trypanosoma cruzi. Trypomastigotes derived from a tissue culture (corresponding to bloodstream trypomastigotes) and metacyclic trypomastigotes (corresponding to infective stages of the insect vector) invaded the Lec2 mutant less efficiently than the parental cell line. Invasion of the Lec2 mutant cells could be restored to the Pro5 level by resialylation of the mutant cells with T. cruzi trans-sialidase and sialyllactose. Conversely, pretreatment of the Pro5 parental cells with bacterial neuraminidase decreased invasion. These results indicate that sialic acid associated with the host cell contributes to invasion by T. cruzi.

Chagas' disease

Chagas' disease, caused by Trypanosoma cruzi, is an important cause of morbidity in many countries in Latin America. The important modes of transmission are by the bite of the reduviid bug and blood transfusion. The organism exists in three morphological forms: trypomastigotes, amastigotes, and epimastigotes. The mechanism of transformation and differentiation is currently being explored, and signal transduction pathways of the parasites may be involved in this process. Parasite adherence to and invasion of host cells is a complex process involving complement, phospholipase, penetrin, neuraminidase, and hemolysin. Two clinical forms of the disease are recognized, acute and chronic. During the acute stage pathological damage is related to the presence of the parasite, whereas in the chronic stage few parasites are found. In recent years the roles of tumor necrosis factor, gamma interferon, and the interleukins in the pathogenesis of this infection have been reported. The common manifestations of chronic cardiomyopathy are arrhythmias and thromboembolic events. Autoimmune, neurogenic, and microvascular factors may be important in the pathogenesis of the cardiomyopathy. The gastrointestinal tract is another important target, and "mega syndromes" are common manifestations. The diagnosis and treatment of this infection are active areas of investigation. New serological and molecular biological techniques have improved the diagnosis of chronic infection. Exacerbations of T. cruzi infection have been reported for patients receiving immuno-suppressive therapy and for those with AIDS.

Desialylation of lysosomal membrane glycoproteins by Trypanosoma cruzi: a role for the surface neuraminidase in facilitating parasite entry into the host cell cytoplasm

Trypanosoma cruzi enters host cells via formation of an acidic vacuole which is subsequently disrupted, allowing the parasite access to the cytoplasm. We show that in an acid environment, release of the parasite surface neuraminidase is enhanced, and this release is likely mediated by a phosphatidylinositol-specific phospholipase C (PIPLC), since antibodies to a carbohydrate epitope (CRD) revealed in glycosylphosphatidylinositol (GPI)-anchored proteins after PIPLC cleavage remove the great majority of the soluble neuraminidase activity from culture supernatants. The neuraminidase is active at acidic pH, and is capable of desialylating known vacuolar constituents, i.e., lysosomal membrane glycoproteins. Parasite escape into the cytoplasm is significantly facilitated in terminal sialylation-defective mutant Lec 2 cells, and enzymatically desialylated membranes are more susceptible to lysis by a parasite hemolysin previously implicated in vacuole membrane rupture. These findings provide evidence that terminal sialylation on carbohydrate moieties contributes to maintaining lysosomal membrane integrity, and indicate a role for a protozoan-derived neuraminidase in facilitating parasite entry into host cells. These observations raise the possibility that other microbial neuraminidases may serve a similar function in acidic intracellular compartments.

Mapping of a B-cell epitope present in the neuraminidase of Trypanosoma cruzi

We have previously shown that a polyclonal (rabbit anti-TCNA) and a mouse monoclonal antibody (TCN-2) against the neuraminidase of Trypanosoma cruzi (TCNA) inhibit enzyme activity, immunoprecipitate active enzyme, enhance in vitro infection, and identify a subpopulation of extracellular trypomastigotes. We now report on the identification of a synthetic peptide that contains the epitope recognized by these antibodies. The synthetic peptide (TR) is a dodecamer (D-S-S-A-H-G-T-P-S-T-P-A) deduced from the DNA sequence of the long tandem repeat (LTR) domain present in the TCNA carboxyterminus. By ELISA, rabbit anti-TCNA bound to TR coupled to ovalbumin, and the binding was inhibited by soluble TR but not by BR (Y-S-V-D-D-G-E-T-W-E), a peptide derived from the N-terminal domain of the enzyme. TCN-2 recognized TR, and this reaction as well as TCN-2 binding to endogenous TCNA could be inhibited by soluble TR but not by BR. These results indicate that the rabbit anti-TCNA and TCN-2 react with the LTR region of TCNA. Antibodies to TR reacted by immunoblot with the TCNA of the Silvio X-10/4, MV-13 and Y-H6 strains, identifying the same molecular polymorphism previously observed with the rabbit anti-TCNA and TCN-2. Furthermore, anti-TR antibodies immunoprecipitated active enzyme and immunofluorescence analysis revealed that anti-TR and TCN-2 antibodies detected equally well the differential expression of their epitopes in intra- and extracellular trypomastigotes. Moreover, expression of TR and TCN-2 epitopes on the different stages of T. cruzi paralleled the stage-specificity of TCNA activity. TCN-2 prevented desialylation by TCNA of intact cells but not of soluble glycoconjugates, indicating that TCN-2 epitope is probably not associated with the enzyme catalytic site, in agreement with the predicted sequence of the TCNA gene. Finally, analysis of the humoral response of a Chagasic patient to different areas of the TCNA molecule indicated that the antibody response is predominantly against TR suggesting that the tandem repeat is the immunodominant domain of TCNA.

Trypanosoma cruzi trans-sialidase and neuraminidase activities can be mediated by the same enzymes

Trans-sialidase and neuraminidase activities have been detected on the surface membrane of trypomastigotes of Trypanosoma cruzi, and both have been implicated in the parasite's invasion of host cells. We show here that these enzymes are structurally related. They are recognized by two independently derived monoclonal antibodies, are anchored to the membrane by glycosylphosphatidylinositol, copurify by ion exchange, molecular sieving, and hydrophobic chromatography, have maximal activities between pH 6.5 and 7.5, and are inactivated by heating at 56 degrees C. Furthermore, the neuraminidase and trans-sialidase reactions are coupled. An increase of the concentration of acceptors of the transfer reaction decreases the amount of free sialic acid released through the neuraminidase reaction. We conclude that a single enzyme can catalyze the transfer or the hydrolysis of macromolecular-bound sialic acid. The predominant direction of the reaction will depend on the availability of appropriate oligosaccharide acceptors of sialic acid.

An 85-kilodalton surface antigen gene family of Trypanosoma cruzi encodes polypeptides homologous to bacterial neuraminidases

We have determined the sequence of a cDNA (Tt34c1) encoding a Trypanosoma cruzi trypomastigote stage-specific 85-kDa surface glycoprotein (gp85). Within the peptide sequence of Tt34c1 are two 8-amino acid motifs, Ser-X-Asp-X-Gly-X-Thr-Trp, that are characteristic of bacterial neuraminidases. Analysis of the Tt34c1 sequence predicts the presence of an amino-terminal signal sequence and a hydrophobic carboxy-terminus that is probably replaced by a glycosyl phosphatidylinositol membrane anchor. Gp85 is encoded by an extensive multigene family that is distributed throughout the genome and can be divided into subsets on the basis of oligonucleotide hybridisation patterns. By sequencing products of polymerase chain reaction (PCR) amplification of the 5' end of trypomastigote gp85 mRNA we show that multiple copies of the gene family are transcribed simultaneously in a trypanosome population. Comparison of the sequence of the PCR clones and another gp85 cDNA showed a highly conserved region 5' of the first methionine extending 180 nt into the coding sequence. Insertions and point mutations were observable outside these homologous sequences demonstrating the variant nature of the gp85 mRNAs.

The Trypanosoma cruzi neuraminidase contains sequences similar to bacterial neuraminidases, YWTD repeats of the low density lipoprotein receptor, and type III modules of fibronectin

Trypanosoma cruzi expresses a developmentally regulated neuraminidase (TCNA) implicated in parasite invasion of cells. We isolated full-length DNA clones encoding TCNA. Sequence analysis demonstrated an open reading frame coding for a polypeptide of 1,162 amino acids. In the N-terminus there is a cysteine-rich domain containing a stretch of 332 amino acids nearly 30% identical to the Clostridium perfringens neuraminidase, three repeat motifs highly conserved in bacterial and viral neuraminidases, and two segments with similarity to the YWTD repeats found in the low density lipoprotein (LDL) receptor and in other vertebrate and invertebrate proteins. This domain is connected by a structure characteristic of type III modules of fibronectin to a long terminal repeat (LTR) consisting of 44 full length copies of twelve amino acids rich (75%) in serine, threonine, and proline. LTR is unusual in that it contains at least 117 potential phosphorylation sites. At the extreme C-terminus is a hydrophobic segment of 35 amino acids, which could mediate anchorage of TCNA to membranes via a glycosylphosphatidylinositol linkage. This is the first time a protozoan protein has been found to contain a YWTD repeat and a fibronectin type III module. The domain structure of TCNA suggests that the enzyme may have functions additional to its catalytic activity such as in protein-protein interaction, which could play a role in T. cruzi binding to host cells.

On the interaction of Trypanosoma cruzi neuraminidase and human lipoproteins

Binding and penetration of Trypanosoma cruzi to host cells is a process that preludes infection and is mediated by specific recognition molecules. Neuraminidase is one of the parasite molecules involved in infection and, in this review, we describe some of its biochemical characteristics, its interaction with human lipoproteins and its effect on infection of mammalian cells.