Abstract
Toxoplasma gondii is an intracellular parasite that causes chronic infection by the development of bradyzoites housed in tissue cysts, preferably in the muscles and central nervous system. The composition and the function of the cyst wall are still not fully understood. Are T. gondii cysts able to incorporate nutrients through its wall? If so, how would these nutrients be traversed to cross the cyst matrix to reach the bradyzoite forms? Herein, we tested the uptake capacity of the Toxoplasma tissue cyst wall by employing some fluid-phase endocytosis tracers as peroxidase (HRP) and bovine serum albumin (BSA). Fluorescence images revealed these molecules on the cyst wall as well as in the cyst matrix. The subcellular localization of the tracer was confirmed by ultrastructural analysis showing numerous labeled vesicles and tubules distributed within the cyst matrix in close association with intracystic bradyzoite membrane, suggesting the cyst wall as a route of nutrient uptake. Furthermore, we confirmed the presence of cytoskeleton proteins, such as tubulin, actin, and myosin, in the tissue cyst matrix that may explain the nutrient input mechanism through the cyst wall. A better understanding of the nutrient acquisition process by the cyst might potentially contribute to the development of new therapeutic targets against chronic toxoplasmosis.
Keywords
- Toxoplasma gondii
- tissue cysts
- endocytosis
- macromolecules
- cytoskeleton
1. Introduction
Toxoplasmosis is a worldwide human protozoan infection caused by an intracellular protozoan,
Despite the clinical relevance of chronic toxoplasmosis, the biology of the cyst wall as yet has not been completely elucidated [4–9]. A 116‐kDa glycoprotein termed CST1 [7] and two lectins,
Our previous ultrastructural analysis demonstrated that the cyst wall displays endocytic activity through the engulfing of negatively charged molecules in the cystic wall [8]. These molecules are incorporated by tubules and vesicles formed from the membrane that delimits the cyst wall and localized in the granular region and posteriorly in the cyst matrix. Within the cyst, the presence of vesicles containing the tracer in close contact to the bradyzoite membrane or in its neighborhood suggests that it could be one of the incorporated molecule pathways from the host cell cytoplasm to intracystic parasites [8]. The current knowledge of mechanisms involved in the process of nutrient uptake by this parasite still presents many gaps, restricted to few reports [12, 13]. As an obligate intracellular parasite,
In all eukaryotes, endocytosis and intracellular vesicle traffic are events mediated by several proteins including a complex cytoskeleton network. Cytoskeleton proteins are indispensable components of a number of vital parasite structures and functions, like cell division, membrane and cytoplasmic architectures, parasite gliding motility (glideosome), and host‐cell invasion [18–20]. Besides conventional cytoskeleton elements, many components of
Herein, the putative mechanisms employed by
2. Experimental design
2.1. Parasites
Mice infected with ME‐49 strain
2.2. Endocytosis assays
Cysts were incubated with bovine serum albumin (BSA) labeled with fluorescein isothiocyanate (BSA‐FITC), a fluid phase endocytic tracer, for 2 or 3 h at 37°C and processed for analysis in a confocal laser scanning microscopy FV300/BX51 Olympus and differential interference contrast (DIC) microscopy. Serial optical sections of each cyst incubated with BSA‐FITC were converted into a volume performing 3D reconstruction. Additionally, the ultrastructural analysis was performed with two fluid phase endocytic tracers, BSA and peroxidase (HRP), both conjugated with colloidal gold (Au) particles.
2.3. Identification of cytoskeleton proteins
Brain cryosections of
3. Results
3.1. T. gondii is able to incorporate macromolecules through the tissue cyst wall
3.2. Ultrastructural analysis confirmed distinct molecule incorporation through the cyst wall
In order to examine whether the predicted protein labeling was in fact throughout the cyst wall, an ultrastructural analysis was conducted, incubating cysts with electron dense tracers, such as BSA‐Au and HRP‐Au. The incubation for 30 min at 4°C with BSA‐Au revealed the marker at the surface as well as within invaginations of the cyst wall (Figure 2A). The internalization of BSA‐Au occurred after 2 or 4 h of incubation at 37°C. Colloidal gold particles could be found within small vesicles and tubules localized at the granular region independent of the time of BSA‐Au incubation (Figure 2B and C).
Similar results were obtained with HRP‐Au by incubation of tissue cysts for 30 min at 4°C (Figure 3A). When the temperature was elevated to 37°C, the HRP‐Au particles were observed within uncoated vesicles and tubules localized in the granular region of the cyst (Figure 3B and C).
3.3. Tissue cysts display a cytoskeleton network both in vivo and ex vivo
Aiming to reveal the cytoskeletal proteins in
DIC microscopy (Figure 5A) and the immunofluorescence with the anti‐tubulin antibody revealed a homogeneous labeling along the cyst wall in an area correspondent to the granular region (Figure 5B). This intense wall labeling was better visualized in the merged figure (Figure 5C). Different virtual confocal sections of the same cyst disclosed the presence of tubulin inside bradyzoites (Figure 5D–I). In
The most intense fluorescence labeling was for actin detection in
3.4. Ultrastructural localization of cytoskeleton proteins through the cyst matrix
The detection by ultrastructural immunocytochemical of actin and tubulin proteins in tissue cysts revealed many gold particles distributed along the granular region and dispersed in the cyst matrix of tissue cysts (Figure 8A). In bradyzoites, some particles were located in the cytoplasm, most of them inside amylopectin granules, along the cyst granular region (arrowhead) and in clusters in the parasite cytoplasm (Figure 8B). The immunogold reaction for tubulin identified some gold particles on the cyst wall membrane along the granular region and in the matrix (Figure 9A). In bradyzoites, tubulin gold particles were detected mainly inside and around the amylopectin granules as well as in the cytoplasm and membrane (Figure 9B).
4. Discussion
This study reveals the ability of the
The exposure of Toxoplasma cysts to BSA‐FITC for 2 or 3 h exhibited for the first time, the incorporation dynamics of this endocytic marker located at and attached initially to the cyst wall and afterward, the transit within vesicles toward the cyst matrix. Ultrastructural images from cysts incubated with the two different fluid‐phase endocytic tracers (BSA‐Au and HRP‐Au) demonstrated that they were capable to associate with the cystic wall and be internalized via vesicle and tubules. The incubation of tracers and cysts at 37°C for periods 1–4 h displayed a heterogeneous labeling with formation of clusters on the cyst wall. Guimarães et al. [8] obtained similar images after cyst incubation with cationized ferritin. This labeling type may be due to the motility of cyst wall components or even the presence of different surface micro domains. Some images suggested fusion between the vesicles originating from invagination of the cyst wall with the parasite membrane. However, we have been unable to visualize vesicles discharging the marker or the marker inside the parasite. The present results corroborate previous data of our group with cationized ferritin showing vesicles and tubules containing particles in close contact with the membrane bradyzoite intracystics [8]. The previous data added together with those presented in this work suggest an active process of membrane fusion involved in the bradyzoite macromolecule uptake. It could be one of the pathways for parasite nutrient acquisition through the molecules available in the host cell cytoplasm (see Scheme 1, supplementary material). Moreover, we believe that the tubules and vesicles between the filaments of the cyst wall play a key role in delivering internalized molecules from the cyst wall to the intracellular bradyzoites and vice versa.
This cyst wall property opens new perspectives to the investigation of cytoskeletal element involvement in the process of nutrient incorporation by
Until now, there has been no evidence of the fusion of these vesicles with the parasite membrane. However, we suggest that it might be an important pathway for host cell nutrient acquisition. Nutrient required for bradyzoites is extremely important, as the parasite is confined inside the tissue for long periods and can persist throughout the life of the host [1]. Di Cristina et al. [33] observed that fluorescence molecules, such as D‐luciferin, have access to bradyzoites within intact cysts both
In this way, our interest was to explain how these molecules are incorporated by the cyst wall and also how they can move within the cyst matrix considering that the cyst is not one cell but a structure that maintains various parasites in its interior. The endocytic activity of the cyst wall requires a stimulus induced by molecular signaling which involves cytoskeleton proteins as described for eukaryotic cells. Both actin and tubulin contribute to intracellular vesicle trafficking and endocytic activity in a series of dynamic events in the endomembrane system. The strong positive staining of tubulin and actin, revealed with polyclonal mammalian antibodies throughout the cyst matrix, indicated the presence of a cytoskeleton network within the cyst. Further investigation would be needed to evaluate the origin of these proteins. Eukaryotic cells exhibit tubulin dimers that can be easily altered by posttranslational modifications that differentially mark distinct microtubule subpopulations [34, 35]. Each microtubule subpopulation is organized for specific functions, such as the mitotic spindle, tracks for vesicular transport plus the basal body, and associated flagellar axoneme. In
Numerous protozoan parasites have the ability to form the cyst stage that normally allows them to survive under adverse environmental conditions. Chávez‐Munguía et al. [36] showed that during the encystation process of
Actin is a highly conserved microfilament protein that plays an important role for gliding motility and in
In conclusion,
Funding
Our work was supported by grants from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Fundação Oswaldo Cruz (Programa Estratégico de Apoio à Pesquisa em Saúde—PAPES IV e VI), Pronex—Programa de Apoio a Núcleos de Excelência—CNPq/FAPERJ, and Instituto Oswaldo Cruz/Fiocruz. MFFS is currently supported by the Re‐entry grant of the University of Cologne.
Acknowledgments
The authors thank Carlos Bizarro, Genilton Vieira, Bruno Ávila for all the image management and Dr. Maurício Paiva for the scheme designed in this work. We are grateful to Sandra Maria for technical support. English review and revision by Mitchell Raymond Lishon, native of Chicago, Illinois, USA—UCLA 1969.
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