Trypanosoma cruzi Epimastigotes Are Able to Manage Internal Cholesterol Levels under Nutritional Lipid Stress Conditions

Trypanosoma cruzi epimastigotes store high amounts of cholesterol and cholesteryl esters in reservosomes. These unique organelles are responsible for cellular digestion by providing substrates for homeostasis and parasite differentiation. Here we demonstrate that under nutritional lipid stress, epimastigotes preferentially mobilized reservosome lipid stocks, instead of lipid bodies, leading to the consumption of parasite cholesterol reservoirs and production of ergosterol. Starved epimastigotes acquired more LDL-NBD-cholesterol by endocytosis and distributed the exogenous cholesterol to their membranes faster than control parasites. Moreover, the parasites were able to manage internal cholesterol levels, alternating between consumption and accumulation. With normal lipid availability, parasites esterified cholesterol exhibiting an ACAT-like activity that was sensitive to Avasimibe in a dose-dependent manner. This result also implies that exogenous cholesterol has a role in lipid reservoirs in epimastigotes.

On the ultrastructural organization of Trypanosoma cruzi using cryopreparation methods and electron tomography

The structural organization of Trypanosoma cruzi has been intensely investigated by different microscopy techniques. At the electron microscopy level, bi-dimensional analysis of thin sections of chemically fixed cells has been one of the most commonly used techniques, despite the known potential of generating artifacts during chemical fixation and the subsequent steps of sample preparation. In contrast, more sophisticated and elaborate techniques, such as cryofixation followed by freeze substitution that are known to preserve the samples in a more close-to-native state, have not been widely applied to T. cruzi. In addition, the 3D characterization of such cells has been carried out mostly using 3D reconstruction from serial sections, currently considered a low resolution technique when compared to electron tomography (ET). In this work, we re-visited the 3D ultrastructure of T. cruzi using a combination of two approaches: (1) analysis of both conventionally processed and cryofixed and freeze substituted cells and (2) 3D reconstruction of large volumes by serial electron tomography. The analysis of high-pressure frozen and freeze substituted parasites showed novel characteristics in a number of intracellular structures, both in their structure and content. Organelles generally showed a smooth and regular morphology in some cases presenting a characteristic electron dense content. Ribosomes and new microtubule sets showed an unexpected localization in the cell body. The improved preservation and imaging in 3D of T. cruzi cells using cryopreparation techniques has revealed some novel aspects of the ultrastructural organization of this parasite.

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.

Cryptic paraflagellar rod in endosymbiont-containing kinetoplastid protozoa

Cilia and flagella are central to many biological processes in a diverse range of organisms. The kinetoplastid protozoa are very appealing models for the study of flagellar function, particularly in the light of the availability of extensive trypanosomatid genome information. In addition to the highly conserved 9 + 2 axoneme, the kinetoplastid flagellum contains a characteristic paraflagellar rod structure (PFR). The PFR is necessary for full motility and provides support for metabolic regulators that may influence flagellar beating. However, there is an intriguing puzzle: one clade of endosymbiont-containing kinetoplastids apparently lack a PFR yet are as motile as species that possess a PFR and are able to attach to the invertebrate host epithelia. We investigated how these organisms are able to locomote despite the apparent lack of PFR. Here we have identified a PFR1 gene in the endosymbiont-bearing trypanosome Crithidia deanei. This gene is expressed in C. deanei and is able to partially complement a pfr1 null mutation in Leishmania mexicana cells, demonstrating that the encoded protein is functional. Careful reexamination of C. deanei flagellar ultrastructure revealed a greatly reduced PFR missed by many previous analyses. This affirms the PFR as a canonical organelle of kinetoplastids. Moreover, although PFR proteins have been conserved in evolution, primary sequence differences contribute to particular PFR morphotypes characteristic of different kinetoplastid species.

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.

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.