Understanding mechanisms and the role of differentiation in pathogenesis of Toxoplasma gondii: a review

Abstract

Parasite differentiation from proliferating tachyzoites into latent bradyzoites is central to pathogenesis and transmission of the intracellular protozoan pathogen Toxoplasma gondii. The presence of bradyzoite-containing cysts in human hosts and their subsequent rupture can cause life-threatening recrudescence of acute infection in the immunocompromised and cyst formation in other animals contributes to zoonotic transmission and widespread dissemination of the parasite. In this review, we discuss the evidence showing how the clinically relevant process of bradyzoite differentiation is regulated at both transcriptional and post-transcriptional levels. Specific regulatory factors implicated in modulating bradyzoite differentiation include promoter-based cis-elements, epigenetic modifications and protein translation control through eukaryotic initiation factor -2 (eIF2). In addition to a summary of the current state of knowledge in these areas we discuss the pharmacological ramifications and pose some questions for future research.

Fig. 1

Multiple levels of regulation control stress-induced differentiation in Toxoplasma. The developmental transition from proliferating tachyzoites to encysted bradyzoites, which can be induced by cellular stress, is regulated by transcriptional (A) and post-transcriptional mechanisms (B). At least one response pathway that has been linked to stress-induced bradyzoite differentiation involves the eIF2 kinases, which lead to the phosphorylation of eIF2α in response to stress. In turn, this decreases general protein synthesis in favor of a select subset of mRNAs that may encode transcription factors (TF). Other post-transcriptional modes of regulation, such as RNA transport or splicing, may also be involved (not shown). While no transcription factor has been directly implicated in Toxoplasma developmental transitions, cis-elements have been identified in bradyzoite promoters (gray box). There is also evidence that epigenetic events, such as chromatin remodeling, are associated with gene expression changes that are required for bradyzoite differentiation. An example of histone acetylation (Ac), which activates gene expression, is shown (see Fig. 2). It has yet to be determined if similar mechanisms of regulation occur in parasites that “spontaneously” differentiate, i.e. convert to bradyzoites without a stress application. TBP: TATA-binding protein.

Fig. 2

Histones upstream of bradyzoite-specific genes become acetylated during stress-induced differentiation in vitro. Chromatin immunoprecipitation using anti-acetylated histone H3 (AcH3) or H4 (AcH4) was performed on Toxoplasma tachyzoites (Tz) or a population enriched in bradyzoites (Bz) due to alkaline pH exposure for three days. Upstream regions of constitutively expressed genes like tubulin (TUB) are acetylated regardless of life cycle stage. However, those of Bz-specific genes (LDH2) become hyperacetylated after subjecting parasites to a pH stress that induces cyst development. “No Ab” refers to a no antibody control ChIP and “input” is the PCR of the DNA prior to immunoprecipitation. Methods used have been described previously (Saksouk et al. 2005).