Toxoplasma gondii: an ultrastructural study of host-cell invasion by the bradyzoite stage

An apical protein, Pcr2, is required for persistent movement by the human parasite Toxoplasma gondii

The phylum Apicomplexa includes thousands of species of unicellular parasites that cause a wide range of human and animal diseases such as malaria and toxoplasmosis. To infect, the parasite must first initiate active movement to disseminate through tissue and invade into a host cell, and then cease moving once inside. The parasite moves by gliding on a surface, propelled by an internal cortical actomyosin-based motility apparatus. One of the most effective invaders in Apicomplexa is Toxoplasma gondii, which can infect any nucleated cell and any warm-blooded animal. During invasion, the parasite first makes contact with the host cell "head-on" with the apical complex, which features an elaborate cytoskeletal apparatus and associated structures. Here we report the identification and characterization of a new component of the apical complex, Preconoidal region protein 2 (Pcr2). Pcr2 knockout parasites replicate normally, but they are severely diminished in their capacity for host tissue destruction due to significantly impaired invasion and egress, two vital steps in the lytic cycle. When stimulated for calcium-induced egress, Pcr2 knockout parasites become active, and secrete effectors to lyse the host cell. Calcium-induced secretion of the major adhesin, MIC2, also appears to be normal. However, the movement of the Pcr2 knockout parasite is spasmodic, which drastically compromises egress. In addition to faulty motility, the ability of the Pcr2 knockout parasite to assemble the moving junction is impaired. Both defects likely contribute to the poor efficiency of invasion. Interestingly, actomyosin activity, as indicated by the motion of mEmerald tagged actin chromobody, appears to be largely unperturbed by the loss of Pcr2, raising the possibility that Pcr2 may act downstream of or in parallel with the actomyosin machinery.

The protozoan pathogen Toxoplasma gondii targets the paracellular pathway to invade the intestinal epithelium

Abstract  Toxoplasma gondii is a ubiquitous parasite found within all mammals and birds worldwide that can cause fatal infections in immunocompromised persons and fetuses. The parasite causes chronic infections by residing in long-living tissues of the muscle and brain. T. gondii infects the host through contaminated meat and water consumption with the gastrointestinal tract (GI tract) being the first point of contact with the host. The mechanisms by which the parasite invades the host through the GI tract are unknown, although it has been suggested that the paracellular pathway is important for parasite dissemination. Studies indicate that epithelial tight junction-associated proteins are affected by T. gondii, although which junctional proteins are affected and the nature of host protein-parasite interactions have not been established. We have uncovered evidence that T. gondii influences the cellular distribution of occludin to transmigrate the intestinal epithelium and suggest how candidate binding partners can be identified.

The organization of the wall filaments and characterization of the matrix structures of Toxoplasma gondii cyst form

The encystation process is a key step in Toxoplasma gondii life cycle, allowing the parasite to escape from the host immune system and the transmission among the hosts. A detailed characterization of the formation and structure of the cyst stage is essential for a better knowledge of toxoplasmosis. Here we isolated cysts from mice brains and analysed the cyst wall structure and cyst matrix organization using different electron microscopy techniques. Images obtained showed that the cyst wall presented a filamentous aspect, with circular openings on its surface. The filaments were organized in two layers: a compact one, facing the exterior of the whole cyst and a more loosen one, facing the matrix. Within the cyst wall, we observed tubules and a large number of vesicles. The cyst matrix presented vesicles of different sizes and tubules, which were organized in a network connecting the bradyzoites to each other and to the cyst wall. Large vesicles, with a granular material in their lumen of glycidic nature were observed. Similar vesicles were also found associated with the posterior pole of the bradyzoites and in proximity to the cyst wall.

Interaction and cystogenesis of Toxoplasma gondii within skeletal muscle cells in vitro

Infection by the protozoan parasite Toxoplasma gondii is widely prevalent in humans and animals. To prevent human infection, all meat should be well cooked before consumption, since the parasite is present in skeletal muscle. In this context, the use of skeletal muscle cells (SkMCs) as a cellular model opens up new approaches to investigate T. gondii-host cell interactions. Immunofluorescent detection of proteins that are stage-specific for bradyzoites indicated that complete cystogenesis of T. gondii in in vitro cultures of SkMCs occurs after 96 h of infection. Ultrastructural analysis showed that, after 48 h of interaction, there were alterations on the parasitophorous vacuole membrane, including greater thickness and increased electron density at the inner face of the membrane. The present study demonstrates the potential use of primary cultures of SkMCs to evaluate different molecular aspects of T. gondii invasion and cystogenesis and presents a promising in vitro model for the screening of drug activities toward tissue cysts and bradyzoites.

Primary culture of skeletal muscle cells as a model for studies of Toxoplasma gondii cystogenesis

Toxoplasma gondii is a protozoan pathogen of birds and mammals, including humans. The infective stage, the bradyzoite, lives within cysts, which occur predominantly in cells of the central nervous system and skeletal and cardiac muscles, characterizing the chronic phase of toxoplasmosis. In the present study, we employed for the first time primary mouse culture of skeletal muscle cells (SkMC) infected with bradyzoites, as a cellular model for cystogenesis. The interconversion of bradyzoite and tachyzoite was analyzed by immunofluorescence using 2 stage-specific antibodies, i.e., anti-bradyzoite (anti-BAG1) and anti-tachyzoite (anti-SAG1). After 24 hr of interaction only bradyzoites were multiplying, as revealed by anti-BAG1 incubation; interconversion to tachyzoites was not observed. After 48 hr of infection, 2 types of vacuoles were seen, i.e., BAG1+ and SAG1+, indicating the presence of bradyzoites as well as their interconversion to tachyzoites. After 96 hr of infection, BAG1+ vacuoles presented a higher number of parasites when compared to 48 hr, indicating multiplication of bradyzoites without interconversion. Using ultrastructural analysis, bradyzoites were found to adhere to the cell membranes via both the apical and posterior regions or were associated with SkMC membrane expansions. During bradyzoite invasion of SkMC, migration of the rough endoplasmic reticulum (RER) profiles to the parasite invasion site was observed. Later, RER profiles were localized between the mitochondria and parasitophorous vacuole membrane (PVM) that contained the parasite. After 31 days of parasite-host cell infection, RER profiles and mitochondria were not observed in association with the cyst wall. Alterations of the PVM, including increased thickness and electrondensity gain on its inner membrane face, were observed 48 hr after infection. Cystogenesis was complete 96 hr after infection, resulting in the formation of the cyst wall, which displayed numerous membrane invaginations. In addition, an electron-dense granular region enriched with vesicles and tubules was present, as well as numerous intracystic bradyzoites. These results show that the in vitro T. gondii model and SkMC are potential tools for both the study of cystogenesis using molecular approaches and the drug screening action on tissue cysts and bradyzoites.

A novel rhoptry protein in Toxoplasma gondii bradyzoites and merozoites

The secretory organelles of Toxoplasma gondii orchestrate invasion of the host cell and establish the parasitophorous vacuole. Although much has been learned about the roles played by these organelles in invasion by the tachyzoite stage, little is known about the contents or functions of these organelles during bradyzoite development or pathogenesis. We identified a novel protein that localizes to the rhoptries of the bradyzoite stage, but is absent from the tachyzoite stage. This protein, BRP1, first appears in the nascent rhoptries during the first division of bradyzoite stage development. We observed secretion of BRP1 and other rhoptry proteins into the parasitophorous vacuole during bradyzoite development in vitro, but there was no evidence that this occurs in vivo. Brp1 knockout parasites did not appear to have any developmental or growth defects in vitro, and were able to establish infections in mice both as tachyzoites (via intraperitoneal injection of in vitro-derived tachyzoites) or bradyzoites (via oral gavage using cysts harvested from mouse brain). Mice infected using brain cysts from the brp1 knockout or the control strain developed similar numbers and sizes of brain cysts. Thus BRP1 does not appear to play an essential role in development of the bradyzoite stage, development of brain cysts, or oral infection of new hosts, at least in the mouse model used here. Since we also observed that BRP1 is expressed in the merozoite stages in the gut of infected cats, the coccidian phase of the life cycle may be where BRP1 plays its most important role.

Cryptosporidium excystation and invasion: getting to the guts of the matter

Cryptosporidium parvum excystation and host cell invasion have been characterized in some detail ultrastructurally. However, until recently, the biochemical and molecular basis of host-parasite interactions and parasite- and host-specific molecules involved in excystation, motility and host cell invasion have been poorly understood. This article describes our understanding of Cryptosporidium excystation and the events leading to host cell invasion, and draws from information available about these processes in other apicomplexans. Many questions remain but, once the specific mechanisms are identified, they could prove to be novel targets for drug delivery.

The role of IFN-gamma and Toll-like receptors in nephropathy induced by Toxoplasma gondii infection

The pathologic links between Toxoplasma gondii infections and renal diseases have not yet been established. Gamma interferon (IFN-gamma) and Toll-like receptors (TLRs) are involved in the host defense mechanism against T. gondii infection. The role of IFN-gamma and TLRs in renal function of T. gondii -infected mice was studied using wild type (WT), TLR2-deficient and TLR4-deficient mice perorally infected with cysts of an avirulent cyst-forming Fukaya strain of T. gondii. T. gondii was abundant in kidneys in IFN-gamma KO (GKO) mice as determined by a quantitative competitive-polymerase chain reaction (QC-PCR). But, T. gondii was not detected in kidneys in WT, TLR2-deficient and TLR4-deficient mice. Interestingly, renal function of TLR2-deficient and TLR4-deficient mice was damaged as evaluated by serum creatinine, serum blood urea nitrogen (BUN), and urine albumin/creatinine ratio (ACR), whereas renal function of GKO and WT mice was not damaged. Histopathology of TLR2-deficient mice exhibited glomerular and extracellular matrix swelling with advancing glomerular tissue proliferation, thickened Bowman's capsules and vacuolization of tubules. Renal immunofluorescence study of T. gondii -infected TLR2-deficient mice displayed positive staining of the glomerular basement membrane, mesangial areas and peritubular capillaries. The damage of kidney from TLR4-deficient mice was less severe compared to TLR2-deficient mice, and histopathological damage of kidney was not observed in WT and GKO mice. These results indicate that TLR2, but not IFN-gamma, plays a role in the protection of the renal function against T. gondii infection.

In vitro induction of Neospora caninum bradyzoites in vero cells reveals differential antigen expression, localization, and host-cell recognition of tachyzoites and bradyzoites

We report on an optimized method for the in vitro culture of tissue cyst-forming Neospora caninum bradyzoites in Vero cells and the separation of viable parasites from host cells. Treatment of tachyzoite-infected Vero cell cultures with 17 microM sodium nitroprusside for 8 days severely scaled down parasite proliferation, led to reduced expression of tachyzoite surface antigens, and induced the expression of the bradyzoite marker NcBAG1 and the cyst wall antigen recognized by the monoclonal antibody MAbCC2. Transmission electron microscopy demonstrated that intracellular parasites were located within parasitophorous vacuoles that were surrounded by a cyst wall-like structure, and the dense granule antigens NcGRA1, NcGRA2, and NcGRA7 were incorporated into the cyst wall. Adhesion-invasion assays employing purified tachyzoites and bradyzoites showed that tachyzoites adhered to, and invaded, Vero cells with higher efficiency than bradyzoites. However, removal of terminal sialic acid residues from either the host cell or the parasite surface increased the invasion of Vero cells by bradyzoites, but not tachyzoites.

Roles of IFN-gamma on stage conversion of an obligate intracellular protozoan parasite, Toxoplasma gondii

IFN-gamma downregulates the stage conversion of Toxoplasma gondii (T. gondii), from bradyzoites to tachyzoites, and the expression of heat shock protein 70 (HSP70) of T. gondii (T.g.HSP70) by tachyzoites. T.g.HSP70 has been shown to downregulate NO release from macrophages and also to induce auto-HSP70 antibody formation and IL-10 secretion by VH11-JH1 B-1 cells, resulting in the suppression of host defense responses to T. gondii infection. A novel category of virulent tachyzoite stage of T. gondii expressing T.g.HSP70 (virulent tachyzoite), which indirectly manifests its pathogenicity by downregulating host defense responses, has been proposed.

Genetic analysis of tachyzoite to bradyzoite differentiation mutants in Toxoplasma gondii reveals a hierarchy of gene induction

Developmental switching in Toxoplasma gondii, from the virulent tachyzoite to the relatively quiescent bradyzoite stage, is responsible for disease propagation and reactivation. We have generated tachyzoite to bradyzoite differentiation (Tbd-) mutants in T. gondii and used these in combination with a cDNA microarray to identify developmental pathways in bradyzoite formation. Four independently generated Tbd- mutants were analysed and had defects in bradyzoite development in response to multiple bradyzoite-inducing conditions, a stable phenotype after in vivo passages and a markedly reduced brain cyst burden in a murine model of chronic infection. Transcriptional profiles of mutant and wild-type parasites, growing under bradyzoite conditions, revealed a hierarchy of developmentally regulated genes, including many bradyzoite-induced genes whose transcripts were reduced in all mutants. A set of non-developmentally regulated genes whose transcripts were less abundant in Tbd- mutants were also identified. These may represent genes that mediate downstream effects and/or whose expression is dependent on the same transcription factors as the bradyzoite-induced set. Using these data, we have generated a model of transcription regulation during bradyzoite development in T. gondii. Our approach shows the utility of this system as a model to study developmental biology in single-celled eukaryotes including protozoa and fungi.