Toxoplasma gondii Targets a Protein Phosphatase 2C to the Nuclei of Infected Host Cells

Abstract

Intracellular pathogens have evolved a wide array of mechanisms to invade and co-opt their host cells for intracellular survival. Apicomplexan parasites such as Toxoplasma gondii employ the action of unique secretory organelles named rhoptries for internalization of the parasite and formation of a specialized niche within the host cell. We demonstrate that Toxoplasma gondii also uses secretion from the rhoptries during invasion to deliver a parasite-derived protein phosphatase 2C (PP2C-hn) into the host cell and direct it to the host nucleus. Delivery to the host nucleus does not require completion of invasion, as evidenced by the fact that parasites blocked in the initial stages of invasion with cytochalasin D are able to target PP2C-hn to the host nucleus. We have disrupted the gene encoding PP2C-hn and shown that PP2C-hn-knockout parasites exhibit a mild growth defect that can be rescued by complementation with the wild-type gene. The delivery of parasite effector proteins via the rhoptries provides a novel mechanism for Toxoplasma to directly access the command center of its host cell during infection by the parasite.Toxoplasma gondii is an obligate intracellular parasite in the phylum Apicomplexa that causes severe central nervous system disorders of immunocompromised (AIDS/transplant/lymphoma) individuals and birth defects in congenitally infected neonates worldwide (16). Toxoplasma infects a wide range of mammalian hosts and is capable of infecting virtually any nucleated cell type from these organisms. The parasite actively invades its host cell, establishing a specialized parasitophorous vacuole (PV) within the host cytoplasm (22). This vacuole fails to fuse with the host endocytic or exocytic pathways, thus avoiding lysosomal destruction, and provides a residence in which parasites can replicate within the host cell (29, 37). The processes of invasion and vacuole formation therefore establish an intimate yet separate association between the parasite and its host cell.Host cell invasion and PV formation are mediated in part by the action of the rhoptries, specialized secretory organelles that release their contents at the onset of invasion (32). The club-shaped rhoptries are composed of two suborganellar domains, the bulbous rhoptry bodies and the duct-like rhoptry necks. These domains appear to carry out very different roles in host cell invasion and establishment of the intracellular niche for survival. Proteins secreted from the rhoptry necks have recently been shown to be released into the moving junction, a ring-shaped structure that forms the intersection between the invading parasite and the host plasma membrane (1, 6). Rhoptry neck proteins in the moving junction likely serve to filter host transmembrane proteins from the nascent PV during invasion, a process that contributes to the nonfusogenic nature of the vacuole within the host cell. Rhoptry proteins from the other subcompartment, the rhoptry bodies, are secreted into the nascent PV, where they are destined to remain within the vacuole or a...

Host cell manipulation by the human pathogen Toxoplasma gondii

“…Thus, nuclear-localized ROP16 most likely comes from the release of the rhoptry contents during the invasion itself, rather than from deployment after entry. A similar phenomenon was also recently seen with another ROP protein initially identified by its reaction with a monoclonal antibody and its occupation of the host cell nucleus [74]. Also consistent with delivery during invasion, nuclear localization of this protein is also observed when cells are treated with cytochalasin D (CytoD), arresting the parasite at the stage between rhoptry secretion and penetration.…”

“…Despite residing in the nucleus where it is well positioned to modulate host gene expression, no differences in host transcriptional profiles were observed comparing wild type and PPC2-deficient parasites. Nonetheless, it remains possible that host elements are postranslationally affected by PPC2-hn [74].…”

Rhoptries: an arsenal of secreted virulence factors

“…Furthermore, all of the bulb-localized proteins examined thus far are secreted into the host cell during invasion. A striking example of secretion into the host cell came with the discovery of a protein phosphatase 2C (PP2C-hn) which is secreted from the rhoptries and targeted to the host nucleus during invasion [23] (Figure 1D,E). PP2C-hn contains a nuclear localization sequence, indicating that delivery to the host cytosol is followed by uptake via the host nuclear import machinery.…”

Toxoplasmaexports dense granule proteins beyond the vacuole to the host cell nucleus and rewires the host genome expression

“…A first wave of secretion from micronemes (not represented here) is required for the initial attachment to the host cell (Carruthers and Tomley, 2008), whereas proteins from rhoptries and dense granules convert the host cell into a suitable environment for parasite growth by modulating a variety of host processes (Blader and Saeij, 2009). Rhoptry proteins are secreted directly into the forming vacuole (Hakansson et al, 2001) or the host cell cytosol such as ROP16 (Saeij et al, 2007) and protein phosphatase 2C (PP2C-hn) (Gilbert et al, 2007), which are then directed to the host cell nucleus. The dense granule resident proteins are presumably secreted outside of the parasite plasma membrane by exocytosis.…”