Trypanosoma cruzi: ultrastructural, cytochemical and freeze-fracture studies of protein uptake

Contribution of microscopy to a better understanding of the anatomy of pathogenic protists

In this Inaugural Article the author briefly revises its scientific career and how he starts to work with parasitic protozoa. Emphasis is given to his contribution to topics such as a) the structural organization of the surface of protozoa using freeze-fracture and deep-etching; b) the cytoskeleton of protozoa, especially structures such as the subpellicular microtubules of trypanosomatids, the conoid of Toxoplasma gondii, microtubules and inner membrane complex of this protozoan, and the costa of Tritrichomonas foetus; c) the flagellulm of trypanosomatids, that in addition to the axoneme contains a complex network of filaments that constitute the paraflagellar rod; d) special organelles such as the acidocalcisome, hydrogenosome, and glycosome; and e) the highly polarized endocytic pathway found in epimastigote forms of Trypanosoma cruzi.

Protozoan phagotrophy from predators to parasites: An overview of the enigmatic cytostome-cytopharynx complex of Trypanosoma cruzi

Eating is fundamental and from this basic principle, living organisms have evolved innumerable strategies to capture energy and nutrients from their environment. As part of the world's aquatic ecosystems, the expansive family of heterotrophic protozoans uses self-generated currents to funnel prokaryotic prey into an ancient, yet highly enigmatic, oral apparatus known as the cytostome-cytopharynx complex prior to digestion. Despite its near ubiquitous presence in protozoans, little is known mechanistically about how this feeding organelle functions. Intriguingly, one class of these flagellated phagotrophic predators known as the kinetoplastids gave rise to a lineage of obligate parasitic protozoa, the trypanosomatids, that can infect a wide variety of organisms ranging from plants to humans. One parasitic species of humans, Trypanosoma cruzi, has retained this ancestral organelle much like its free-living relatives and continues to use it as its primary mode of endocytosis. In this review, we will highlight foundational observations made regarding the cytostome-cytopharynx complex and examine some of the most pressing questions regarding the mechanistic basis for its function. We propose that T. cruzi has the potential to serve as an excellent model system to dissect the enigmatic process of protozoal phagotrophy and thus enhance our overall understanding of fundamental eukaryotic biology.

Electron microscopy cytochemistry and three-dimensional reconstruction of labeled structures in Trypanosoma cruzi

Significant advances have occurred in the area of high-resolution scanning electron microscopy (SEM), especially related to methodologies that allow the observation of intracellular structures that are exposed either by successive abrasion with a gallium ion beam or by sectioning in epoxy-embedded cells. Images of series of successively exposed surfaces can then be rendered into 3D models. Here, we report our observations by combining this approach with classical cytochemical methods to facilitate the 3D reconstruction of labeled structures and organelles. We used epimastigotes of Trypanosoma cruzi whose endocytic pathway was labeled with horseradish peroxidase, followed by fixation and detection of the peroxidase activity using the classical diaminobenzidine-osmium method followed by incubation with thiocarbohydrazide, which increases the concentration of osmium at the sites where the enzyme is located as well as the contrast of lipid-containing structures. This procedure allows not only a better visualization of membranous structures and lipid inclusions but can also easily identify the endocytic tracer (HRP) inside the cell. All structures involved in the endocytic activity could be traced and reconstructed.

Identification and Localization of the First Known Proteins of the Trypanosoma cruzi Cytostome Cytopharynx Endocytic Complex

The etiological agent of Chagas disease, Trypanosoma cruzi, is an obligate intracellular parasite that infects an estimated 7 million people in the Americas, with an at-risk population of 70 million. Despite its recognition as the highest impact parasitic infection of the Americas, Chagas disease continues to receive insufficient attention and resources in order to be effectively combatted. Unlike the other parasitic trypanosomatids that infect humans (Trypanosoma brucei and Leishmania spp.), T. cruzi retains an ancestral mode of phagotrophic feeding via an endocytic organelle known as the cytostome-cytopharynx complex (SPC). How this tubular invagination of the plasma membrane functions to bring in nutrients is poorly understood at a mechanistic level, partially due to a lack of knowledge of the protein machinery specifically targeted to this structure. Using a combination of CRISPR/Cas9 mediated endogenous tagging, fluorescently labeled overexpression constructs and endocytic assays, we have identified the first known SPC targeted protein (CP1). The CP1 labeled structure co-localizes with endocytosed protein and undergoes disassembly in infectious forms and reconstitution in replicative forms. Additionally, through the use of immunoprecipitation and mass spectrometry techniques, we have identified two additional CP1-associated proteins (CP2 and CP3) that also target to this endocytic organelle. Our localization studies using fluorescently tagged proteins and surface lectin staining have also allowed us, for the first time, to specifically define the location of the intriguing pre-oral ridge (POR) surface prominence at the SPC entrance through the use of super-resolution light microscopy. This work is a first glimpse into the proteome of the SPC and provides the tools for further characterization of this enigmatic endocytic organelle. A better understanding of how this deadly pathogen acquires nutrients from its host will potentially direct us toward new therapeutic targets to combat infection.

Electron Microscopy Techniques Applied to Symbiont-Harboring Trypanosomatids: The Association of the Bacterium with Host Organelles

In this chapter we describe different electron microscopy techniques such as freeze fracture, deep etching, and three-dimensional reconstruction, obtained by electron tomography or focused ion beam scanning electron microscopy (FIB-SEM), combined with quick-freezing methods in order to reveal aspects of the cell structure in trypanosomatids. For this purpose, we chose protists that evolve in a mutualistic way with a symbiotic bacterium. Such cells represent excellent models to study the positioning and distribution of organelles, since the symbiotic bacterium interacts with different organelles of the host trypanosomatid. We demonstrate that the employment of such techniques can show the proximity and even the interaction of the symbiotic bacterium with different structures of the protist host, such as the nucleus and the glycosomes. In addition, the quick-freezing approach can reveal new aspects of the gram-negative bacterial envelope, such as the presence of a greatly reduced cell wall between the two membrane units.

Specific Endocytosis Blockade of Trypanosoma cruzi Exposed to a Poly-LAcNAc Binding Lectin Suggests that Lectin-Sugar Interactions Participate to Receptor-Mediated Endocytosis

Trypanosoma cruzi is a protozoan parasite transmitted by a triatomine insect, and causing human Chagas disease in South America. This parasite undergoes a complex life cycle alternating between non-proliferative and dividing forms. Owing to their high energy requirement, replicative epimastigotes of the insect midgut display high endocytic activity. This activity is mainly restricted to the cytostome, by which the cargo is taken up and sorted through the endosomal vesicular network to be delivered to reservosomes, the final lysosomal-like compartments. In African trypanosomes tomato lectin (TL) and ricin, respectively specific to poly-N-acetyllactosamine (poly-LacNAc) and β-D-galactose, allowed the identification of giant chains of poly-LacNAc in N-glycoproteins of the endocytic pathway. We show that in T. cruzi epimastigote forms also, glycoproteins of the endocytic pathway are characterized by the presence of N-linked glycans binding to both ricin and TL. Affinity chromatography using both TL and Griffonia simplicifolia lectin II (GSLII), specific to non-reducing terminal residue of N-acetylglucosamine (GlcNAc), led to an enrichment of glycoproteins of the trypanosomal endocytic pathway. Incubation of live parasites with TL, which selectively bound to the cytostome/cytopharynx, specifically inhibited endocytosis of transferrin (Tf) but not dextran, a marker of fluid endocytosis. Taken together, our data suggest that N-glycan modification of endocytic components plays a crucial role in receptor-mediated endocytosis of T. cruzi.

Effects of chlorate on the sulfation process of Trypanosoma cruzi glycoconjugates. Implication of parasite sulfates in cellular invasion

Sulfation, a post-translational modification which plays a key role in various biological processes, is inhibited by competition with chlorate. In Trypanosoma cruzi, the agent of Chagas' disease, sulfated structures have been described as part of glycolipids and we have reported sulfated high-mannose type oligosaccharides in the C-T domain of the cruzipain (Cz) glycoprotein. However, sulfation pathways have not been described yet in this parasite. Herein, we studied the effect of chlorate treatment on T. cruzi with the aim to gain some knowledge about sulfation metabolism and the role of sulfated molecules in this parasite. In chlorate-treated epimastigotes, immunoblotting with anti-sulfates enriched Cz IgGs (AS-enriched IgGs) showed Cz undersulfation. Accordingly, a Cz mobility shift toward higher isoelectric points was observed in 2D-PAGE probed with anti-Cz antibodies. Ultrastructural membrane abnormalities and a significant decrease of dark lipid reservosomes were shown by electron microscopy and a significant decrease in sulfatide levels was confirmed by TLC/UV-MALDI-TOF-MS analysis. Altogether, these results suggest T. cruzi sulfation occurs via PAPS. Sulfated epitopes in trypomastigote and amastigote forms were evidenced using AS-enriched IgGs by immunoblotting. Their presence on trypomastigotes surface was demonstrated by flow cytometry and IF with Cz/dCz specific antibodies. Interestingly, the percentage of infected cardiac HL-1 cells decreased 40% when using chlorate-treated trypomastigotes, suggesting sulfates are involved in the invasion process. The same effect was observed when cells were pre-incubated with dCz, dC-T or an anti-high mannose receptor (HMR) antibody, suggesting Cz sulfates and HMR are also involved in the infection process by T. cruzi.

The Role of Heme and Reactive Oxygen Species in Proliferation and Survival of Trypanosoma cruzi

Trypanosoma cruzi, the protozoan responsible for Chagas disease, has a complex life cycle comprehending two distinct hosts and a series of morphological and functional transformations. Hemoglobin degradation inside the insect vector releases high amounts of heme, and this molecule is known to exert a number of physiological functions. Moreover, the absence of its complete biosynthetic pathway in T. cruzi indicates heme as an essential molecule for this trypanosomatid survival. Within the hosts, T. cruzi has to cope with sudden environmental changes especially in the redox status and heme is able to increase the basal production of reactive oxygen species (ROS) which can be also produced as byproducts of the parasite aerobic metabolism. In this regard, ROS sensing is likely to be an important mechanism for the adaptation and interaction of these organisms with their hosts. In this paper we discuss the main features of heme and ROS susceptibility in T. cruzi biology.

Electron microscopy and cytochemistry analysis of the endocytic pathway of pathogenic protozoa

Endocytosis is essential for eukaryotic cell survival and has been well characterized in mammal and yeast cells. Among protozoa it is also important for evading from host immune defenses and to support intense proliferation characteristic of some life cycle stages. Here we focused on the contribution of morphological and cytochemical studies to the understanding of endocytosis in Trichomonas, Giardia, Entamoeba, Plasmodium, and trypanosomatids, mainly Trypanosoma cruzi, and also Trypanosoma brucei and Leishmania.

New insights into the morphology of Trypanosoma cruzi reservosome

Reservosomes are late endosomes present only in members of the Schizotrypanum subgenus of the Trypanosoma genus and are defined as the site of storage of endocytosed macromolecules and lysosomal enzymes. They have been extensively described in Trypanosoma cruzi epimastigote: are bounded by a membrane unit, present an electron-dense protein matrix with electron-lucent lipid inclusions, being devoid of inner membranes. Here we performed a detailed ultrastructural analysis of these organelles using a variety of electron microscopy techniques, including ultrathin sectioning, uranyl acetate stained preparations, and freeze fracture, either in intact epimastigotes or in isolated reservosomes. New informations were obtained. First, both isolated and in situ reservosomes presented small profiles of inner membranes that are morphologically similar to the membrane surrounding the organelle. In uranyl acetate stained preparations, internal membrane profiles turned out to be longer than they appeared in ultrathin section images and traversed the organelle diameter. Internal vesicles were also found. Second, endocytosed cargo are not associated with internal vesicles and reach reservosomes on board of vesicles that fuse with the boundary membrane, delivering cargo directly into reservosome lumen. Third, electron-lucent bodies with saturated lipid core surrounded by a membrane monolayer and with unusual rectangular shape were also observed. Fourth, it was possible to demonstrate the presence of intramembranous particles on the E face of both internal vesicles and the surrounding membrane. Collectively, these results indicate that reservosomes have a complex internal structure, which may correlate with their multiple functions.

Heme requirement and intracellular trafficking in Trypanosoma cruzi epimastigotes

Epimastigotes multiplies in the insect midgut by taking up nutrients present in the blood meal including heme bound to hemoglobin of red blood cell. During blood meal digestion by vector proteases in the posterior midgut, hemoglobin is clipped off into amino acids, peptides, and free heme. In this paper, we compared the heme and hemoglobin uptake kinetics and followed their intracellular trafficking. Addition of heme to culture medium increased epimastigote proliferation in a dose-dependent manner, while medium supplemented with hemoglobin enhanced growth after 3-day lag phase. Medium supplemented with globin-derived peptides stimulated cell proliferation in a dose-independent way. Using Palladium mesoporphyrin IX (Pd-mP) as a fluorescent heme-analog, we observed that heme internalization proceeded much faster than that observed by hemoglobin-rhodamine. Binding experiments showed that parasites accumulated the Pd-mP into the posterior region of the cell whereas hemoglobin-rhodamine stained the anterior region. Finally, using different specific inhibitors of ABC transporters we conclude that a P-glycoprotein homologue transporter is probably involved in heme transport through the plasma membrane.

Reservosomes: multipurpose organelles?

Reservosomes are endocytic organelles from Trypanosoma cruzi epimastigotes that store proteins and lipids for future use. The lack of molecular markers for the compartments of this parasite makes it difficult to clarify all reservosome functions, as they present characteristics of pre-lysosomes, lysosomes and recycling compartments.

Effects of dibucaine on the endocytic/exocytic pathways in Trypanosoma cruzi

Although local anesthetics (LA) are considered primarily Na+-channel blockers in the past decade, an alternative action of LA as inhibitors of fusion among compartments of the endocytic/exocytic pathways was described. In epimastigote forms of Trypanosoma cruzi, we observed that 50 mM dibucaine reduced the rates of uptake of bovine serum albumin (BSA) and immunoglobulin to 60% of control values in addition to the delay of exocytosis of cysteine proteases. Fusion among endocytic compartments was not inhibited in the presence of dibucaine because previously labeled reservosomes was loaded with a second label in sequential pulse-chase experiments. However, dibucaine reduced the degradation of BSA-gold complex in the reservosomes, which was not caused either by an inhibition of the whole proteolytic activity of the parasite or by a reduction on the expression levels of cruzipain. The immunocytochemical analysis suggested that the inhibition of the degradation of gold-labeled BSA in reservosomes could be due to a subversion of the regular traffic of proteases toward the reservosomes in dibucaine-treated cells.

Effects of inhibitors of Delta24(25)-sterol methyl transferase on the ultrastructure of epimastigotes of Trypanosoma cruzi

Trypanosoma cruzi is the ethiological agent of Chagas disease. New compounds are being developed based on the biosynthesis and function of sterols, because T. cruzi has a requirement for specific endogenous sterols for growth and survival. Sterol biosynthesis inhibitors (SBIs) are drugs commonly used against fungal diseases. These drugs act by depleting essential and specific membrane components and/or inducing the accumulation of toxic intermediary or lateral products of the biosynthetic pathway. In this work we present the effects of WSP488, WSP501, and WSP561, specific inhibitors of Delta24(25)-sterol methyl transferase, on the ultrastructure of T. cruzi epimastigotes. All three drugs inhibited parasite multiplication at low concentrations, with IC50 values of 0.48, 0.44, and 0.48 muM, respectively, and induced marked morphological changes including (a) blockage of cell division; (b) swelling of the mitochondrion, with several projections and depressions; (c) swelling of the perinuclear space; (d) presence of autophagosomes and myelin-like figures; (e) enlargement of the flagellar pocket and of a cytoplasmic vacuole located in close association with the flagellar pocket; (f) detachment of the membrane of the cell body; and (g) formation of a vesicle at the surface of the parasite between the flagellar pocket and the cytostome. Our results show that these drugs are potent in vitro inhibitors of growth of T. cruzi.

Characterization of RAB-like4, the first identified RAB-like protein from Trypanosoma cruzi with GTPase activity

RAB proteins, which belong to the RAS superfamily, regulate exocytic and endocytic pathways of eukaryotic cells, controlling vesicle docking and fusion. Few RAB proteins have been identified in parasites. Molecular markers for cellular compartments are important to studies concerning about the protein traffic in Trypanosoma cruzi, the causal agent of Chagas disease. In this work, we describe the characterization of TcRABL4, the first RAB-like gene identified in T. cruzi (GenBank Accession No.: ), present as a single-copy gene. TcRABL4 contains all five consensus RAB motifs but lacks cysteine residues at the C terminus, which are essential to isoprenylation, an absolute prerequisite for membrane association of these proteins. TcRABL4 is a functional GTPase that is able to bind and hydrolyze GTP, and its gene is transcribed as a single 1.2 kb mRNA in epimastigotes. TcRABL4 appears to be differentially regulated in the three cell forms of the parasite, and the protein is not associated to membranes, unlike other RAB proteins. It is possible that TcRABL4 may be a member of a novel family of small GTPases.

Characterization of a RAB5 homologue in Trypanosoma cruzi

RAB proteins are small GTPases involved in exocytic and endocytic pathways of eukaryotic cells, controlling vesicle docking and fusion. RABs show a remarkable specificity in subcellular localization, so they can be used as molecular markers for studying protein trafficking in Trypanosoma cruzi, the causal agent of Chagas' disease. RAB5 is a component of early endosomes. It has been identified in kinetoplastids such as Trypanosoma brucei and Leishmania donovani. In this work, we describe the characterization of the complete coding sequence of a RAB5 gene homologue in T. cruzi (TcRAB5, GenBank Accession No. AY730667). It is present as a single copy gene, located at chromosomal bands XIII and XIV. TcRAB5 shares the highest degrees of similarity (71%) and identity (63%) with Trypanosoma brucei rhodesiense RAB5a and contains all five characteristic RAB motifs. TcRAB5 is transcribed as a single 1.5kb mRNA in epimastigotes. Its transcript was also detected in the other two forms of the parasite, metacyclic trypomastigotes and spheromastigotes. The recombinant TcRAB5 protein was able to bind and hydrolyze GTP. The identification of proteins involved in T. cruzi endo- and exocytic pathways may generate cellular compartment markers, an invaluable tool to better understand the vesicular transport in this parasite.

Giardia lamblia: behavior of the nuclear envelope

Giardia Lamblia is a flagellar parasite possessing the unusual morphology of bearing two nuclei. New morphological observations on trophozoites and encysting Giardia nuclei using routine transmission electron microscopy, freeze fracture and cytochemistry are presented. Nuclear pores of both nuclei in the same cells were assessed on freeze-fracture replicas from different cell cycle phases, and compared. These techniques showed that (1) both nuclei in the same cell are distinct in nuclear pore number and distribution; (2) nuclear pore complexes are frequently clustered in nuclear envelope domains; (3) dividing nuclei display very few nuclear pores; (4) few ribosomes are found on the outer nuclear envelope of the trophozoite form; (5) nuclear membranes present spots of closely apposed membranes, which are different from the typical diaphragm nuclear pore complexes; (6) in addition to the nuclear pores, membrane blebs are also present in the nuclear envelope; (7) encysting cells show intranuclear inclusions, morphologically similar to the ESV (encystation-specific vesicles) and to the ER membranes, which may be the result of nuclear envelope folding. It is proposed that the two nuclei in Giardia are dissimilar in morphology and activity.

Visualization of the cytostome inTrypanosoma cruziby high resolution field emission scanning electron microscopy using secondary and backscattered electron imaging

High resolution scanning electron microscopy was used to analyze the surface of epimastigote, amastigote and trypomastigote forms of Trypanosoma cruzi. Significant differences were observed between these forms and in different areas of the same cell. The cytostome found in amastigote and epimastigote forms could be easily visualized in images, which resemble those obtained only using the freeze-fracture technique. In contrast to other areas of the cell surface, the region of the cytostome, localized close to the flagellar pocket, showed a rugous surface and an opening with a diameter of 90 nm. Gold-labeled concanavalin A binds to the whole cell surface. However, the extent of binding was much higher in the region of the cytostome. The results obtained show that high resolution scanning electron microscopy is a powerful technique for analyzing the surface of protozoa.

Biogenesis of the reservosomes of Trypanosoma cruzi

Reservosomes are endocytic compartments found in the posterior region of epimastigotes of Trypanosoma cruzi. In the differentiation from trypomastigotes to epimastigotes (reverse metacyclogenesis in vitro), one has the rare opportunity of following the biogenesis of an endocytic compartment. Metacyclic trypomastigotes incubated in LIT medium highly enriched with fetal calf serum differentiated directly to epimastigotes. In recently differentiated epimastigotes, acidic organelles were found in round compartments spread along the cell body, whereas in control epimastigotes they were found in reservosomes located in the posterior region. Ultrastructural analysis of intermediate forms showed that the cytostome and reservosomes appeared before differentiation to epimastigotes was completed. Many polymorphic reservosomes, with or without lipid inclusions, were observed from the anterior portion of the cell body, in close relationship with the Golgi complex, to the posterior region. Endocytic tracers were observed in the cytostome, flagellar pocket, vesicles, and newly formed reservosomes. Cruzipain, the main protease of T. cruzi, was localized in newly formed reservosomes and in vesicles budding from the trans-Golgi network that seem to fuse with reservosomes. Ingested gold-labeled albumin and cruzipain colocalized in recently formed reservosomes. Endocytosis and immunocytochemical analysis suggested that the endocytic and the secretory pathways may contribute to reservosome formation.

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Isolation and characterization of a reservosome fraction from Trypanosoma cruzi

Reservosomes are acidic compartments present at the posterior region of epimastigote forms of Trypanosoma cruzi that store proteins and lipids. During metacyclogenesis, they consume their contents and disappear. Reservosomes are rich in cruzipain, the main proteolytic enzyme of this parasite. By centrifugation in a sucrose gradient, we have obtained a highly purified subcellular fraction containing reservosomes from 5-day-old Y strain epimastigotes. Transmission electron microscopy showed that the fraction contained well-preserved organelles. The protein profile of the organelle analyzed by SDS-PAGE depicted a wide range of protein bands, predominating those corresponding to a triplet of 60-51 kDa and a doublet of 25-23 kDa. Protease activity in substrate-containing gels, in the presence or absence of protease inhibitors, showed that cysteine proteinase is enriched and very active in the purified fraction. Enzymatic assays demonstrated the absence of pyrophosphatase, an acidocalcisome marker, and succinate cytochrome c reductase, a mitochondrial marker, although these enzymes were active in other regions of the purification sucrose gradient. Thin layer chromatographic neutral lipid analysis of purified reservosomes demonstrated that the organelle stores large amounts of ergosterol and esterified cholesterol. Phospholipid analysis indicated phosphatidylcholine and phosphatidylethanolamine as the major constituents of reservosome membranes.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.

Trypanosoma cruzi epimastigote endocytic pathway: cargo enters the cytostome and passes through an early endosomal network before storage in reservosomes

It has been known for many years that trypanosomatids require exogenous essential growth factors in order to divide. Two surface domains are involved in starting nutrient endocytosis: the flagellar pocket and the cytostome. Although the flagellar pocket plays a fundamental role in the endocytic process occurring in several trypanosomatids, we have shown the cytostome as the main structure involved in this process in epimastigote forms of T. cruzi. After one minute of endocytosis, cargo is still found at the cytostome entry as well as along the cytopharynx. After two, five and fifteen minutes of endocytosis, cargo was seen inside vesicles and tubules, prior to fusing with reservosomes. Three-dimensional reconstruction of these tubules and vesicles showed they are interconnected, forming an intricate and branched network, distributed from the perinuclear region to the posterior end of the cell. Whole unfixed parasites that had taken up gold-protein conjugates for fifteen minutes were washed and dried on electron microscope grids. Observation with an energy-filtering transmission electron microscope revealed long gold-filled tubules at the posterior end of the cell. Parasites treated with ammonium chloride had their intracellular traffic slowed down, which allowed us to observe many events of vesicle fusion. The acidic nature of this network was evidenced using acridine orange. Based on pH and protein uptake kinetics we propose that the vesicular-tubular network is the early endosome of Trypanosoma cruzi epimastigotes.

Flagellar elongation induced by glucose limitation is preadaptive for Trypanosoma cruzi differentiation

Trypanosomes must sense and respond to environmental change in order to progress through their life cycles. The American trypanosome, Trypanosoma cruzi, differentiates from the noninfective epimastigote form to the infective metacyclic form spontaneously in axenic culture. Here, we investigate the initial stimulus for that change and demonstrate that T. cruzi epimastigotes sense limitation of glucose in the medium and respond by undergoing significant morphological and biochemical change. As part of this change, the mean flagellar length of the population triples, which is correlated with an increased ability to maintain interactions with hydrophobic substrates, a requirement for differentiation to the next life cycle stage.

The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex

Okuda, K., Esteva, M., Segura, E. L., and Bijovsky, A. T. 1999. The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex. Experimental Parasitology 92, 223-231. Proliferative forms of Trypanosoma cruzi, amastigotes and epimastigotes, have a cytostome, a specialized structure formed by an invagination of the flagellar pocket's membrane surrounded by microtubules and frequently followed by a row of vesicles. All this assemblage penetrates deeply into the cytoplasm overpassing the nucleus. This structure, together with the flagellar pocket, appears to play an important role in the nutrition of the parasite. We demonstrated that the monoclonal antibody 2C4, made-up against isolated flagellar complex of T. cruzi epimastigotes, recognizes a protein doublet of 76 and 87 kDa in total epimastigotes homogenate. The 76-kDa polypeptide is enriched in the detergent-soluble fraction whereas the 87-kDa polypeptide is highly represented in the insoluble fractions and the purified flagella. Immuno-fluorescence assays show the antigen as a small spot at the flagellar pocket region. Immunogold labeling of ultrathin sections of epimastigote forms reveals gold particles at the opening of flagellar pocket, concentrated in the cytostome region. Immunocytochemistry of epimastigote whole-mount cytoskeletons reveals the labeling on an array of three to four microtubules that appears attached to flagellum, running in the direction of the nucleus. Ultrastructural observations have shown that the posterior region of isolated flagella, corresponding to the level of the flagellar pocket, possesses a microtubular structure compatible with that from the cytostome. The relationship between the cytostome, an endocytic organelle, and the flagellum is here described for the first time.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

Structural organization of the cell surface of pathogenic protozoa

The surface of parasitic protozoa plays an important role in the process of their interaction with cells from the host. The present review analyzes the structural organization of the surface of sporozoa, trypanosomatids, Entamoeba and Trichomonas, as evaluated by conventional transmission electron microscopy, cytochemical techniques and freeze-fracture. In most protozoa, no special region of surface membrane is detected. In others, however, special membrane domains have been described. As examples, we can mention the cytostome found in epimastigote forms of Trypanosoma cruzi, the region of attachment of the flagellum to the protozoon body in Trypanosomatidae and Trichomonadidae, and the inner membrane complex of Apicomplexa.

The reservosomes of epimastigote forms of Trypanosoma cruzi: occurrence during in vitro cultivation

Reservosomes are large membrane-bound structures found mainly at the posterior end of epimastigote forms of Trypanosoma cruzi, the agent of Chagas' disease. We screened 5-day-old culture forms of the following strains at the ultrastructural level for the presence of reservosomes: T. cruzi strains Y and YuYu as well as Trypanosoma (Schizotrypanum) spp. strains M431, M504, and M519 isolated from bats (Epitesicus sp.) in Santa Catarina, Brazil. The organelles could be found in all strains analyzed, supporting a previous hypothesis that they are a marker of trypanosomes of the Schizotrypanum subgenus. Stereological analysis was carried out with the Y strain to follow the development of reservosomes and lipid droplets during in vitro cultivation of the parasites. Reservosomes were partitioned to the daughter cells during cell division such that the organelles were present in newly formed parasites. Estimation of the volume density after 3, 5, 7, 9, and 12 days of cultivation showed that it was lower in younger cultures, becoming maximal at day 9 (8.0%), but decreased in older cultures (5.9% at day 12). Morphological changes also occurred: type I reservosomes presented an electron-dense matrix with lipid droplets and were characteristic of younger cultures, whereas type II reservosomes presented a homogeneous matrix without lipid inclusions and predominated in older cultures. These organelles were absent in bloodstream trypomastigote forms isolated from infected mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a Leishmania antigen associated with cytoplasmic vesicles resembling endosomal-like structure

In the present study we have used antibodies to Leishmania major promastigote antigens which were eluted from a glutathione-agarose column (LmGbp) and could identify several parasite components among different Leishmania species by using immunoprecipitation and Western blot techniques. The results also showed that some of LmGbp are present among the molecules released into the culture medium. Moreover, immunofluorescence assays clearly demonstrated that LmGbp are expressed by intracellular amastigotes. The electron micrographs of thawed cryosections of L. major-infected cells revealed that the antigens were associated with the membrane of the phagocytic vacuole. Moreover, the Western blot technique allowed us to identify, using other Leishmania species extracts and anti-LmGbp antibodies, a major polypeptide of an apparent molecular mass of 66 kDa. Immunofluorescence studies suggested that the 66 kDa polypeptide is associated with intracytoplasmic vesicles. Cryosections of Leishmania promastigotes improved the fine structure preservation of the organelles and enabled a number of features to be seen, particularly the structures considered as vesicles, which appeared as a complex tubulo-vesicular structure resembling mammalian cell endosomes and Leishmania organelles previously named 'megasomes'. Further studies using antibodies against the native 66 kDa protein will be needed to investigate the localization of the protein at the ultrastructural level and to follow its intracellular vesicular traffic.

Endocytosis of gold-labeled proteins and LDL by Trypanosoma cruzi

Endocytosis was studied at the ultrastructural level in different developmental forms of Trypanosoma cruzi after incubation of the parasites in the presence of gold-labeled proteins (albumin-Au, peroxidase-Au and transferrin-Au) and low-density lipoprotein (LDL-Au). Epimastigote (culture) forms actively ingested LDL and proteins. Initially, gold particles were seen adhering only to the cytostome and inside the flagellar pocket. In parasites incubated at 4 degrees C with transferrin-Au or peroxidase-Au, labeling was found only at these two sites, showing that receptor-mediated endocytosis occurs in both regions. In the cytoplasm, gold particles were seen only inside two different compartments: membrane-bound vesicles and reservosomes. Incubation of epimastigotes with acridine orange followed by fluorescence microscopy revealed intense orange staining, indicating that the reservosomes have an acidic pH. This staining was abolished after incubation of the parasites in the presence of ammonium chloride. These data confirm that this compartment is the site of accumulation of ingested lipids and proteins. Little intracellular labeling with transferrin-Au was found in in vitro-derived amastigotes and trypomastigotes (both lack reservosomes). However, although in amastigotes very few gold particles were seen bound to the cells, in trypomastigotes they were observed bound to the membrane that encloses the cell body, the flagellar pocket, and the flagellum, suggesting that the receptors are more abundant in this form.

A light and electron microscopic study of Trypanosoma fallisi N. Sp. in toads (Bufo americanus) from Algonquin Park, Ontario

Trypanosoma fallisi n. sp. is described from Bufo americanus in Ontario. The parasite was observed in 65 of 94 toads examined. The trypanosomes were pleomorphic with respect to the age of infections, being longer and broader in early infections (during spring and summer) and shorter and more slender during late summer and autumn. They ranged in size from 38-76 microns in body length and 3-8 microns in width, with a free flagellum 6-30 microns long. Epizootiological and experimental evidence suggests that this trypanosome is transmitted to the toads by the leech, Batracobdella picta. Trypanosoma fallisi is morphologically similar to T. bufophlebotomi described in Bufo boreas from California, but geographic isolation, host and vector differences as well as slight morphological differences indicate that speciation has occurred. Similar trypanosomes from Bufo americanus (which were identified as T. bufophlebotomi) in Michigan, are probably T. fallisi. This species shares many ultrastructural features with trypanosomes of other lower vertebrates and also of mammals.

Freeze-fracture study of the bloodstream form of Trypanosoma brucei gambiense

The ultrastructure of Trypanosoma brucei gambiense was investigated by the freeze-fracture method. Three different regions of the continuous plasma membrane; cell body proper, flagellar pocket, and flagellum were compared in density and distribution of the intramembranous particles (IMP's). The IMP-density was highest in the flagellar pocket membrane and lowest in flagellum. Intra membranous particles of the cell body membrane were distributed uniformly on both the protoplasmic (P) and exoplasmic (E) faces. On the P face of the flagellar membrane, a single row of IMP-clusters was seen along the juncture of the flagellum to the cell body. Since the spacing of the IMP-clusters was almost equal to the spacing of the paired rivet structures observed in thin section, these clusters likely are related to the junction of flagellum and cell body. At the neck of the flagellar pocket, several linear arrays of IMP's were found on the P face of the flagellar membrane, while on the E face rows of depressions were seen. At the flagellar base, the clusters of IMP's were only seen on the P face. On the flagellar pocket membrane, particle-rich depressions and linear particle arrays were also found on the P face, while on the E face such special particle arrangements were not recognized. These particle-rich depressions may correspond to the sites of pinocytosis of the bloodstream forms which have been demonstrated in thin sections.

A stereological study of the differentiation process in Trypanosoma cruzi

When epimastigote forms of Trypanosoma cruzi grown in a rich medium (LIT) are transferred to a simple, chemically defined medium (TAU3AAG, containing Ca2+, Mg2+, K+, Na+, L-proline, L-glutamate, and L-aspartate in phosphate buffer) they transform into trypomastigote forms. Morphometric analysis of transmission electron micrographs of thin sections of parasites collected at different steps of the transformation process showed that no changes occurred in the volume density of mitochondria and cytoplasmic vacuoles. However, a significant increase in the volume density of the kinetoplast DNA network as well as the lipid inclusions and a decrease in that of the reservosome (a special type of endosome) was observed. These observations suggest that during differentiation, T. cruzi accumulates lipids and uses molecules contained in the reservosome as its main energy source.

Ultrastructural visualization of lipids in trypanosomatids

An imidazole-buffered osmium tetroxide solution was used to visualize lipids at the ultrastructural level in the following members of the family Trypanosomatidae: Trypanosoma cruzi, T. dionisii, T. vespertilionis. T. rangeli, Crithidia deanei, C. fasciculata, C. oncopelti, and Blastocrithidia culicis. Electron-dense material was seen in various lipid droplets found in all parasites and in the multivesicular structure of members of the sub-genus Schizotrypanum. High contrast of some membranes, mainly those which enclose the mitochondrion, the nucleus, and the endoplasmic reticulum, was observed even in unstained sections. X-ray microanalysis confirmed that the electron density of lipid droplets of B. culicis and membrane-bounded dense granules of C. oncopelti was due to the presence of osmium.

Freeze-fracture localization of filipin-sterol complexes in plasma- and cyto-membranes of Pneumocystis carinii

The polyene antibiotic, filipin, was used as the probe for demonstrating sterols in the freeze-fractured plasma- and cytomembranes of Pneumocystis carinii. The distribution of filipin-sterol complexes was homogeneous on the plasma membrane throughout all developmental stages from trophozoite to cyst; however, the density of the complexes gradually decreased with the progress of development. In the trophozoite, the density of the complexes was 485 +/- 42/micron2 on the P face and 341 +/- 27/micron2 on the E face. It was 249 +/- 50 on the P face and 132 +/- 48 on the E face in the precyst and 138 +/- 24 and 59 +/- 20, respectively, in the cyst. The membranes of nucleus, mitochondria, and small round bodies showed more or fewer complexes while no complexes were found in the membranes of one endoplasmic reticulum. In nuclear and mitochondrial membranes, some small scattered clusters of complexes were observed. Two types of vacuoles were distinguished: one having many complexes in its membrane and the other having none at all.

Tritrichomonas foetus: ultrastructure and cytochemistry of endocytosis

Tritrichomonas foetus ingests horseradish peroxidase, native ferritin, cationized ferritin, and 0.08 micron latex beads by a process which involves the formation of pinocytic vesicles. These vesicles fuse with each other and with lysosomes forming large vacuoles. Biochemical determinations on the ingestion of horseradish peroxidase and morphometric analysis on the ingestion of cationized ferritin covered latex beads indicated that T. foetus has high endocytic activity. The process of ingestion of the various tracers used was analyzed by transmission electron microscopy of thin sections and freeze fracture replicas.

Freeze-fracture studies on Pneumocystis carinii. I. Structural alteration of the pellicle during the development from trophozoite to cyst

Pneumocystis carinii has generally been distinguished in three developmental stages, namely, trophozoite, precyst and cyst. The fine structure of the pellicle--the plasma membrane and the outer layer existing outside this plasma membrane--of each stage was studied by freeze-fracture technique. By this technique, P. carinii was cleaved through the cytoplasm or through the hydrophobic region of the plasma membrane, and the cross-fractured face of the outer layer was revealed on the replicas. The outer layer, which is electron-dense in the thin section, consisted of numerous fine granules about 15 nm in diameter in freeze-fracture images, whereas the electron-lucent middle layer which appeared in the precyst and cyst was less granular. Measurement of the intramembranous particles (IMP) also was carried out. The number of IMP per square micrometer of the plasma membrane of the trophozoite was 1,512 +/- 125 on the P face and 417 +/- 44 on the E face. In the precyst, the IMP density decreased, and 1,037 +/- 56 on the P face and 262 +/- 22 on the E face. In the cyst, it further decreased, nd 875 +/- 59 and 150 +/- 20 respectively. It is generally assumed that the density of IMP is related to the physiological activity of the cell membrane, so that the present results obtained in P. carinii suggest that the trophozoite is the most active stage, and that metabolic activity of the pellicle gradually decreases with the progress of development to the precyst then to the cyst.

Localization of a Mg2+-activated ATPase in the plasma membrane of Trypanosoma cruzi

The Wachstein and Meisel incubation medium was used to detect ATPase activity in epimastigote, spheromastigote (amastigote), and bloodstream trypomastigote forms of Trypanosoma cruzi. Reaction product, indicative of enzyme activity, was associated with the plasma membrane covering the cell body and the flagellum of the parasite. No reaction product was found in the portion of the plasma membrane lining the flagellar pocket. The plasma membrane-associated ATPase activity was not inhibited by ouabain or oligomycin, was detected in incubation medium without K+, was inhibited by prolonged glutaraldehyde fixation, and its activity was diminished when Mg2+ was omitted from the incubation medium. The Ernst medium was used to detect Na+-K+-ATPase activity in T. cruzi. No reaction product indicative of the presence of this enzyme was detected. Reaction product indicative of 5'-nucleotidase was not detected in T. cruzi. Acid phosphatase activity was detected in lysosomes. Those results indicate that a Mg2+-activated ATPase is present in the plasma membrane of T. cruzi and that it can be used as an enzyme marker, provided that the mitochondrial and flagellar ATPases are inhibited, to assess the purity of plasma membrane fractions isolated from this parasite.

Fine structure and cytochemistry of the nucleus and the kinetoplast of epimastigotes of Trypanosoma cruzi

Ultrastructural cytochemical techniques were used to analyze the nucleus and the kinetoplast of epimastigotes of Trypanosoma cruzi. With the use of ethanolic phosphotungstic acid, which detects basic proteins, reaction product was seen in the chromatin and at the periphery of the kinetoplast. Thallium alcoholate, which interacts with DNA, stained strongly the whole kinetoplast and the chromatin. With the use of a silver impregnation method that detects acidic nucleolar proteins, silver granules were seen preferentially located in the central region of the nucleolus. With the EDTA method, which reveals the presence of ribonucleoproteins, staining was observed in the nuclear pores. Also 6-8 nm fibrils, 25 nm and 40 nm granules, which correspond to the perichromatin fibers, interchromatin granules and the perichromatin granules, respectively, were identified in the nucleus. The EDTA method also revealed the presence of 40 nm granules in the kinetoplast. These granules were seen mainly at the two extremities of the kinetoplast. Freeze-fracture images indicate that the nuclear membrane contains ca. 9 pores/microns2 of nuclear surface area. The mean diameter of the pores was 80 nm. All these results suggest that epimastigotes of T. cruzi have a very active nucleus and a high rate of nucleocytoplasmic interchange.

Tritrichomonas foetus: fine structure of freeze-fractured membranes

Freeze-fracture techniques reveal differences in fine structure between the anterior three flagella of Tritrichomonas foetus and its recurrent flagellum. The anterior flagella have rosettes of 9-12 intramembranous particles on both the P and E faces. The recurrent flagellum lacks rosettes but has ribbon-like arrays of particles along the length of the flagellum, which may be involved in the flagellum's attachment to the cell body. This flagellum is attached to the membrane of the cell body along a distinct groove that contains few discernible particles. Some large intramembranous particles are visible on the P face of the cell body membrane at the point where the flagellum emerges from the cell body. The randomly distributed particles on the P and E faces of the plasma membrane have a particle density of 919/micron2 and 468/micron2 respectively, and there are areas on both faces that are devoid of particles. Freeze-fracture techniques also reveal numerous fenestrations in the membrane of the Golgi complex and about 24 pores per micron2 in the nuclear membrane.

Possible artifacts in the radioiodination of surface proteins of trypanosomatids

Three different methods for the radioiodination of surface proteins in trypanosomatids have been comparatively analyzed. Data interpretation in the lactoperoxidase-mediated method was impaired by autoiodination of the enzyme and its tight retention by the cells. In the enzymobeads method, iodinated enzymes were released from the beads upon solubilization of the preparation with SDS, thus mixing with iodinated proteins of the trypanosomatids. Therefore, upon separation by polyacrylamide gel electrophoresis of the cell lysates, both methods yielded autoradiographic patterns in which prominent bands did not represent labeled cell surface components. In contrast, the autoradiographic patterns obtained by use of the Iodo-Gen method were apparently free of artifacts.