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A Toxoplasma dense granule protein, GRA24, modulates the early immune response to infection by promoting a direct and sustained host p38 MAPK activation

ABSTRACT: Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular protozoan parasite that resides inside a parasitophorous vacuole. During infection, Toxoplasma actively remodels the transcriptome of its hosting cells with profound and coupled impact on the host immune response. We report that Toxoplasma secretes GRA24, a novel dense granule protein which traffics from the vacuole to the host cell nucleus. Once released into the host cell, GRA24 has the unique ability to trigger prolonged autophosphorylation and nuclear translocation of the host cell p38α MAP kinase. This noncanonical kinetics of p38α activation correlates with the up-regulation of the transcription factors Egr-1 and c-Fos and the correlated synthesis of key proinflammatory cytokines, including interleukin-12 and the chemokine MCP-1, both known to control early parasite replication in vivo. Remarkably, the GRA24-p38α complex is defined by peculiar structural features and uncovers a new regulatory signaling path distinct from the MAPK signaling cascade and otherwise commonly activated by stress-related stimuli or various intracellular microbes.

ORGANISM(S): Mus musculus

PROVIDER: GSE38782 | GEO | 2013/09/01

SECONDARY ACCESSION(S): PRJNA168972

REPOSITORIES: GEO

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Project description:Toxoplasma gondii is an obligate intracellular parasite that must establish a favorable environment in the host cells in which it replicates. To do this, it controls key host cell processes by MYR-dependent translocation of dense granule proteins from the parasitophorous vacuole into the host cytosol. Using RNASeq, we previously reported that mutants lacking the MYR translocation apparatus fail to elicit a key set of host responses related to regulation of the cell cycle, including cyclin E and other targets of E2F transcription factors. As no previously reported effector was known to mediate such an effect, we report here our application of a set of criteria to proteins secreted by Toxoplasma leading to our discovery of a novel effector protein that induces the host to produce cyclin E. This inducer of Host Cyclin E (HCE1) is exported from the parasitophorous vacuole in a manner dependent on MYR1 and ASP5, another component of the export system, where it then localizes to the host’s nucleus. HCE1 expression is important for parasite growth as those lacking it produce plaques that are ~35% smaller in area than wild type parasites. RNASeq of host cells infected with parasites lacking HCE1 compared to those infected with wild type parasites shows a complete loss of the parasite’s ability to modulate key host cell cycle genes including those related to the E2F axis. Immunoprecipitation of HCE1 from infected host cells shows that HCE1 efficiently binds elements of the cyclin E regulatory complex: DP1 and its partners E2F3 and E2F4. Expression of HCE1 in Neospora caninum, which is not otherwise capable of up-regulating cyclin E, or in uninfected HFFs, shows nuclear localization of the expressed protein and results in strong cyclin E up-regulation. Thus, HCE1 is a novel, stand-alone effector protein that is necessary and sufficient to impact the E2F-axis of transcription resulting in co-opting of host functions to Toxoplasma’s advantage.