Abstract
The tissue cyst of Toxoplasma gondii, found in latent infection, serves a critical role in both transmission and reactivation of this organism. Within infected cells, slowly replicating parasites (bradyzoites) are surrounded by a cyst matrix, cyst wall, and cyst membrane. The cyst wall is clearly delineated by ultrastructural analysis; however, the composition and function of this layer in host-parasite interactions are not fully understood. In order to understand the composition of the cyst wall, a proteomic analysis of purified cyst wall fragments, that were enriched with Percoll gradients and subsequently immunoprecipitated with CST1 antibody, was performed. Known cyst wall proteins, such as CST1, BPK1, MCP4, MAG1, GRA2, GRA3, and GRA5, were identified in this preparation by liquid chromatography-tandem mass spectrometry (LC-MS/MS). In addition, dense granule proteins (GRAs) not previously shown to associate with the cyst wall, as well as uncharacterized hypothetical proteins, were identified in this cyst wall preparation. Several of these hypothetical cyst wall (CST) proteins were epitope tagged, and immunofluorescence assays confirmed their localization as novel cyst matrix and cyst wall proteins. Expression of two of these newly identified cyst wall proteins was eliminated by gene knockout (CST2-KO and CST3-KO). CST2-KO parasites were highly attenuated in virulence and did not establish detectable cyst burdens. This targeted proteomic approach allowed the identification of new components of the cyst wall that probably have roles in the parasite/host interface.IMPORTANCEToxoplasma gondii is a highly prevalent parasite worldwide that presents life-threatening risks to immunocompromised and pregnant individuals. Whereas the life stage responsible for acute infection can be treated, the life stage responsible for chronic infection is refractory to currently available therapeutics. Little is known about the protein composition of the cyst wall, an amorphous structure formed by parasites that is suspected to facilitate persistence within muscle and nervous tissue during chronic (latent) infection. By implementing a refined approach to selectively purify cyst wall fragments, we identified several known and novel cyst wall proteins from our sample preparations. We confirmed the localizations of several proteins from this data set and identified one that is involved in parasite virulence. These data will propel further studies on cyst wall structure and function, leading to therapeutic strategies that can eliminate the chronic infection stage.
Highlights
The tissue cyst of Toxoplasma gondii, found in latent infection, serves a critical role in both transmission and reactivation of this organism
CST1-KO parasites have a defect in bradyzoite differentiation [7]; MAG1 expression was lower in these parasites than in the wild-type (WT) parasites (Fig. 1D, input lane)
The purified fragments from WT bradyzoite cysts contained membranous structures with small vacuoles (Fig. 1E), which bear similarities to the cyst wall structure observed in transmission electron microscopy (TEM) analysis of intact tissue cysts [4, 7]; this purified membranous material lacked the electron-dense material and tubules that have been observed in intact cysts
Summary
The tissue cyst of Toxoplasma gondii, found in latent infection, serves a critical role in both transmission and reactivation of this organism. We confirmed the localizations of several proteins from this data set and identified one that is involved in parasite virulence These data will propel further studies on cyst wall structure and function, leading to therapeutic strategies that can eliminate the chronic infection stage. The cyst wall is hypothesized to be involved in host-parasite interactions and tolerance to environmental stress In accordance with these hypotheses, genetic deletion of the BPK1 gene affected the transmission of the parasite through the GI tract [8], and genetic deletion of the CST1 gene reduced cyst sturdiness and cyst number in the brains of mice with persistent infection [7]. All of the specific functions the cyst wall serves are not yet fully understood, including how this structure is formed and which proteins are related to its structural and functional formation
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