Structure and function of the Toxoplasma gondii vacuole

Temporal gene expression during asexual development of the apicomplexan Sarcocystis neurona

Asexual replication in the apicomplexan Sarcocystis neurona involves two main developmental stages: the motile extracellular merozoite and the sessile intracellular schizont. Merozoites invade host cells and transform into schizonts that undergo replication via endopolygeny to form multiple (64) daughter merozoites that are invasive to new host cells. Given that the capabilities of the merozoite vary significantly from the schizont, the patterns of transcript levels throughout the asexual lifecycle were determined and compared in this study. RNA-Seq data were generated from extracellular merozoites and four intracellular schizont development time points. Of the 6,938 genes annotated in the S. neurona genome, 6,784 were identified in the transcriptome. Of these, 4,111 genes exhibited significant differential expression between the merozoite and at least one schizont development time point. Transcript levels were significantly higher for 2,338 genes in the merozoite and 1,773 genes in the schizont stages. Included in this list were genes encoding the secretory pathogenesis determinants (SPDs), which encompass the surface antigen and SAG-related sequence (SAG/SRS) and the secretory organelle proteins of the invasive zoite stage (micronemes, rhoptries, and dense granules). As anticipated, many of the S. neurona SPD gene transcripts were abundant in merozoites. However, several SPD transcripts were elevated in intracellular schizonts, suggesting roles unrelated to host cell invasion and the initial establishment of the intracellular niche. The hypothetical genes that are potentially unique to the genus Sarcocystis are of particular interest. Their conserved expression patterns are instructive for future investigations into the possible functions of these putative Sarcocystis-unique genes.

IMPORTANCE

The genus Sarcocystis is an expansive clade within the Apicomplexa, with the species S. neurona being an important cause of neurological disease in horses. Research to decipher the biology of S. neurona and its host-pathogen interactions can be enhanced by gene expression data. This study has identified conserved apicomplexan orthologs in S. neurona, putative Sarcocystis-unique genes, and gene transcripts abundant in the merozoite and schizont stages. Importantly, we have identified distinct clusters of genes with transcript levels peaking during different intracellular schizont development time points, reflecting active gene expression changes across endopolygeny. Each cluster also has subsets of transcripts with unknown functions, and investigation of these seemingly Sarcocystis-unique transcripts will provide insights into the interesting biology of this parasite genus.

Calcium signaling in intracellular protist parasites

Calcium ion (Ca²⁺) signaling is one of the most frequently employed mechanisms of signal transduction by eukaryotic cells, and starts with either Ca²⁺ release from intracellular stores or Ca²⁺ entry through the plasma membrane. In intracellular protist parasites Ca²⁺ signaling initiates a sequence of events that may facilitate their invasion of host cells, respond to environmental changes within the host, or regulate the function of their intracellular organelles. In this review we examine recent findings in Ca²⁺ signaling in two groups of intracellular protist parasites that have been studied in more detail, the apicomplexan and the trypanosomatid parasites.

Disruption of lipid rafts interferes with the interaction of Toxoplasma gondii with macrophages and epithelial cells

The intracellular parasite Toxoplasma gondii can penetrate any warm-blooded animal cell. Conserved molecular assemblies of host cell plasma membranes should be involved in the parasite-host cell recognition. Lipid rafts are well-conserved membrane microdomains that contain high concentrations of cholesterol, sphingolipids, glycosylphosphatidylinositol, GPI-anchored proteins, and dually acylated proteins such as members of the Src family of tyrosine kinases. Disturbing lipid rafts of mouse peritoneal macrophages and epithelial cells of the lineage LLC-MK2 with methyl-beta cyclodextrin (MβCD) and filipin, which interfere with cholesterol or lidocaine, significantly inhibited internalization of T. gondii in both cell types, although adhesion remained unaffected in macrophages and decreased only in LLC-MK2 cells. Scanning and transmission electron microscopy confirmed these observations. Results are discussed in terms of the original role of macrophages as professional phagocytes versus the LLC-MK2 cell lineage originated from kidney epithelial cells.

The toxofilin-actin-PP2C complex of Toxoplasma: identification of interacting domains

Toxofilin is a 27 kDa protein isolated from the human protozoan parasite Toxoplasma gondii, which causes toxoplasmosis. Toxofilin binds to G-actin, and in vitro studies have shown that it controls elongation of actin filaments by sequestering actin monomers. Toxofilin affinity for G-actin is controlled by the phosphorylation status of its Ser53, which depends on the activities of a casein kinase II and a type 2C serine/threonine phosphatase (PP2C). To get insights into the functional properties of toxofilin, we undertook a structure-function analysis of the protein using a combination of biochemical techniques. We identified a domain that was sufficient to sequester G-actin and that contains three peptide sequences selectively binding to G-actin. Two of these sequences are similar to sequences present in several G- and F-actin-binding proteins, while the third appears to be specific to toxofilin. Additionally, we identified two toxofilin domains that interact with PP2C, one of which contains the Ser53 substrate. In addition to characterizing the interacting domains of toxofilin with its partners, the present study also provides information on an in vivo-based approach to selectively and competitively disrupt the protein-protein interactions that are important to parasite motility.

Gene knock-outs and allelic replacements in Toxoplasma gondii: HXGPRT as a selectable marker for hit-and-run mutagenesis

The hypoxanthine-xanthine-guanine phosphoribosyl transferase (HXGPRT) gene of the protozoan parasite Toxoplasma gondii encodes a safe, practical genetic marker suitable for both positive and negative selection. Taking advantage of the ability to control homologous versus nonhomologous recombination in haploid T. gondii tachyzoites by manipulating the length of homologous DNA sequence, we have explored the possibility of 'hit-and-run' mutagenesis to introduce gene knock-outs (or allelic replacements) at loci for which no known selection or screen is available. Using the uracil phosphoribosyl transferase (UPRT) locus as a target, a genomic clone containing approximately 8 kb encompassing the UPRT gene (but lacking essential coding sequence) was fused to a cDNA-derived HXGPRT 'minigene', which lacks sufficient contiguous genomic sequence for homologous recombination. After transfection of circular plasmid DNA, positive selection for HXGPRT activity identified stable transformants, > 30% of which were found to have integrated at the UPRT locus as 'pseudodiploids' (produced by single-site homologous recombination between the circular plasmid and genomic DNA). Upon removal of mycophenolic acid, resolution of pseudodiploids by spontaneous intrachromosomal homologous recombination was selected using 6-thioxanthine, yielding a 1:1 ratio of UPRT knock-out parasites to wild-type revertants, at frequencies of approximately 10(-6) per parasite doubling. Applications of 'hit-and-run' technology relative to other gene targeting strategies are discussed.