Organização estrutural do taquizoíto de toxoplasma gondii: [revisão] / Structural organization of the tachyzoite of toxoplasma gondii: [review]

Objetivos: fazer uma revisão dos aspectos básicos da ultraestrutura do taquizoíto de Toxoplasma gondii, agente etiológico da toxoplasmose. Fonte de dados: os dados apresentados tomam como referência resultados recentes obtidos pelos principais grupos de pesquisadores no mundo, que se dedicam ao estudo do Toxoplasma gondii, incluindo-se dados do próprio grupo de autores. Síntese dos dados: os taquizoítos de Toxoplasma gondii são responsáveis pela fase aguda da infecção, penetrando ativamente, através do complexo apical, em células dos hospedeiros onde se multiplicam. São abordadas características ultraestruturais e moleculares particulares da película, do citoesqueleto, de organelas secretórias (róptrias, micronemas e grânulos densos) e não secretórias (apicoplasto) exclusivas do filo Apicomplexa, além das peculiaridades do núcleo, mitocôndria, acidocalcisomas, retículo endoplasmático e complexo de Golgi desses parasitos intracelulares. Conclusões: estas características confirmam que o sucesso nas etapas de adesão, invasão e multiplicação do parasito possui clara correlação com suas características morfofuncionais.

Aims: To review basic aspects on the ultrastructure of the tachyzoite of Toxoplasma gondii, the causative agent of toxoplasmosis. Source of data: The data presented are based on recent publications by the most distinguished research groups in the area dedicated to the study of Toxoplasma gondii, including studies from the present authors. Summary of findings: The tachyzoites are responsible for the acute phase of the infection by actively penetrating, through the parasites? apical complex, the host cells where they multiply. Both ultrastructural and molecular particularities of the pellicle, the cytoskeleton, secretory (rhoptries, micronemas and dense granules) and non secretory (apicoplast) organelles, specific to Apicomplexa phylum, besides peculiar features of the nucleus, mitochondrion, acidocalcisomes, endoplasmic reticulum and Golgi complex of these intracellular parasites. Conclusions: These characteristics confirm that the success in the process of adhesion, invasion and multiplication of this parasite is clearly correlated to its morphology.

Anti-parasitic action and elimination of intracellular Toxoplasma gondii in the presence of novel thiosemicarbazone and its 4-thiazolidinone derivatives

Toxoplasma, which infects all eukaryotic cells, is considered to be a good system for the study of drug action and of the behavior of infected host cells. In the present study, we asked if thiosemicarbazone derivatives can be effective against tachyzoites and which morphological and ultrastructural features of host cells and parasites are associated with the destruction of Toxoplasma. The compounds were tested in infected Vero cell culture using concentration screens (0.1 to 20 mM). The final concentration of 1 mM was chosen for biological assay. The following results were obtained: 1) These new derivatives decreased T. gondii infection with an in vitro parasite IC50 percent of 0.2-0.7 mM, without a significant effect on host cells and the more efficient compounds were 2, 3 (thiosemicarbazone derivatives) and 4 (thiazolidinone derivative); 2) The main feature observed during parasite elimination was continuous morphological disorganization of the tachyzoite secretory system, progressive organelle vesiculation, and then complete disruption; 3) Ultrastructural assays also revealed that progressive vesiculation in the cytoplasm of treated parasites did not occur in the host cell; 4) Vesiculation inside the parasite resulted in death, but this feature occurred asynchronously in different intracellular tachyzoites; 5) The death and elimination of T. gondii was associated with features such as apoptosis-like stage, acidification and digestion of parasites into parasitophorous vacuoles. Our results suggest that these new chemical compounds are promising for the elimination of intracellular parasites by mainly affecting tachyzoite development at 1 mM concentration for 24 h of treatment.

Toxoplasma gondii: further studies on the subpellicular network

The association of the pellicle with cytoskeletal elements in Toxoplasma gondii allows this parasite to maintain its mechanical integrity and makes possible its gliding motility and cell invasion. The inner membrane complex (IMC) resembles the flattened membrane sacs observed in free-living protozoa and these sacs have been found to associate with cytoskeletal proteins such as articulins. We used immunofluorescence microscopy to characterise the presence and distribution of plateins, a sub-family of articulins, in T. gondii tachyzoites. A dispersed labelling of the whole protozoan body was observed. Electron microscopy of detergent-extracted cells revealed the presence of a network of 10 nm filaments distributed throughout the parasite. These filaments were labelled with anti-platein antibodies. Screening the sequenced T. gondii genome, we obtained the sequence of an IMC predicted protein with 25 percent identity and 42 percent similarity to the platein isoform alpha 1 present in Euplotes aediculatus, but with 42 percent identity and 55 percent similarity to that found in Euglena gracilis, suggesting strong resemblance to articulins.

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.

Spontaneous stage differentiation of mouse-virulent Toxoplasma gondii RH parasites in skeletal muscle cells: an ultrastructural evaluation

Although the predilection for Toxoplasma gondii to form cysts in the nervous system and skeletal and heart muscles has been described for more than fifty years, skeletal muscle cells (SkMCs) have not been explored as a host cell type to study the Toxoplasma-host cell interaction and investigate the intracellular development of the parasite. Morphological aspects of the initial events in the Toxoplasma-SkMC interaction were analysed and suggest that there are different processes of protozoan adhesion and invasion and of the subsequent fate of the parasite inside the parasitophorous vacuole (PV). Using scanning electron microscopy,Toxoplasma tachyzoites from the mouse-virulent RH strain were found to be attached to SkMCs by the anterior or posterior region of the body, with or without expansion of the SkMC membrane. This suggests that different types of parasite internalization occurred. Asynchronous multiplication and differentiation of T. gondii were observed. Importantly, intracellular parasites were seen to display high amounts of amylopectin granules in their cytoplasm, indicating that tachyzoites of the RH strain were able to differentiate spontaneously into bradyzoites in SkMCs. This stage conversion occurred in approximately 3 percent of the PVs. This is particularly intriguing as tachyzoites of virulent Toxoplasma strains are not thought to be prone to cyst formation. We discuss whether biological differences in host cells are crucial to Toxoplasma stage conversion and suggest that important questions concerning the host cell type and its relevance in Toxoplasma differentiation are still unanswered.

Morphological investigation of Toxoplasma gondii in vivo by a multiple beam interference microscope

A recently developed technique, namely multiple beam interference microscopy, has been applied to investigate the morphology of the parasite Toxoplasma gondii for the first time. The interference pattern obtained from the multiple internal reflection of a T. gondii, sandwiched between a glass plate and a cover plate, was focused on the objective of a conventional microscope. Because of the enhance contrast, several details of sub cellular structure and separating compartments are clearly visible. Details reveal the presence of a nucleus, lipid body, dense granule, rhoptry and amylopectin. The wall thickness of the membrane of the lipid body and the amylopectin is of the order of 0.02 æm and can be clearly distinguished with the help of the present technique. The same parasite has also been examined with the help of atomic force microscopy, and because of its thick membrane, the inner structural details were not observed at all. Sub cellular details of T. gondii observed with the present technique have been reported earlier only by low amplification transmission electron microscopy and not by any optical microscopic technique