Identification of a master regulator of differentiation in Toxoplasma

One of the main challenges that parasites face during their lifetime is the colonisation of different niches within their hosts, eventually establishing long-lasting chronic infections. For example, Toxoplasma gondii invades its vertebrate host (e.g. felines, rodents, humans) as a highly replicative form termed tachyzoite (1–3). Once the tachyzoites infect the host, a subpopulation then undergoes a developmental program leading to the differentiation into a slow-growing, cyst-forming stage called bradyzoite (Fig. 1) (3,4). The presence of the cyst allows the parasite to remain protected from its microenvironment, thus becoming refractory to chemotherapy (1,3).

Even though the process of differentiation from tachyzoites to bradyzoites is fundamental for the establishment of chronic toxoplasmosis, the mechanism controlling this process is virtually unknown. Canonical transcription factors found in other apicomplexan parasites (e.g. Plasmodiumspp.) such as ApiAP transcription factors are known to be master regulators of differentiation in Plasmodium spp., and are central for different processes such as development of sporozoites, ookinetes, and sexual stages, amongst others (5–8). However, knockdown of ApiAP2 failed to completely impair differentiation in Toxoplasmagondii (8,9), thus leading to the widely accepted view that the process of differentiation was not mediated by a single “master regulator” (4,10,11).

Taking advantage of a tachyzoite-to-bradyzoite reporter cell line, and employing a combination of a targeted CRISP/Cas9-mediated genetic screening and single-cell sequencing,  Benjamin S. Waldman and colleagues (Sebastian Lourido’s lab – Massachusetts Institute of Technology) identified a a Myb-like transcription factor as essential and sufficient for the differentiation of tachyzoites into bradyzoites. The authors found that:

1. Toxoplasma gondii lacking the expression of this transcription factor, termed by the authors as Bradyzoite Differentiation Factor (BDF1), continue to replicate and display markers typical of tachyzoites under conditions that would trigger differentiation,
2. Toxoplasma gondiioverexpressing BDF1 adquired a bradyzoite phenotype even in the absence of environmental challenges typically associated with bradyzoite differentiation (e.g. alkaline pH, chemotherapeutic agents, etc.), thus indicating that BDF1 alone is capable of coordinating differentiation in Toxoplasma.
3. BDF1 is post-transcriptionally regulated in response to environmental stressors, providing a mechanistic link between environmental sensing and cell fate in Toxoplasma gondii ( 1) (9).

Taken together, these findings provide further insights into the process of developmental regulation in apicomplexan parasites, and also provides evidence of a wider breadth of mechanisms employed by this heterogeneous phylum to control cell fate in response to environmental stimuli.

Why I like this preprint and how I believe it moves the field forward

It is clear that the authors have carefully designed and combined cutting-edge approaches to further understand how cells react and adapt to different microenvironmental challenges. Both CRISPR/Cas9-mediated genome editing and single cell sequencing techniques are revolutionizing the way we understand biological phenomena, and I particularly find it exciting to see these techniques being developed for the study of (single cell) parasites. The key findings presented in this preprint offer unprecedented insight into the complexity of molecular and transcriptional networks that protozoan parasites deploy to adjust their developmental schedule in order to establish chronic infections. Similarly, they have provided a much-needed toolkit to unravel the intrinsic heterogeneity and complexity associated with cell populations, which has been extensively overlooked by traditional parasitological methods, thus setting up a precedent for future research on these organisms. I personally believe that this article merges fast-evolving methodologies to tackle common challenges to both cell biologists and parasitologists.

Figure 1. Proposed model of cell cycle progression of wild type Toxoplasmaparasites or deficient for the Bradyzoite differentiation Factor (∆BDF1) under stress.Wild type parasites are continuously replicating as tachyzoites (depicted as red/orange/yellow parasites in the middle cycle) under normal conditions rapidly differentiate into a quiescent bradyzoite (depicted as blue parasites in the left hand cycle) when stressed. Parasites lacking BDF1 failed to acquired a resistant phenotype and failed to substain viability over time (depicted as grey parasites in the right hand cycle) (Taken from Waldman, BS, et al. (9))

References

1. Dubey JP, Lindsay DS, Speer CA. Structures of Toxoplasma gondii Tachyzoites, Bradyzoites, and Sporozoites and Biology and Development of Tissue Cysts. Clin Microbiol Rev. 1998;11(2):267–99.
2. Tentera AM, Heckerotha AR, Weissr LM. Toxoplasma gondii: from animals to humans. Int J Parasitol. 2000;30(12–13):1217–58.
3. Weiss LM, Kim K. The development and Biology of Bradyzoites of Toxoplasma gondii. Front Biosci. 2000;15:D391–D405.
4. Hong D, Radke JB, White MW. Opposing Transcriptional Mechanisms Regulate Toxoplasma Development. mSphere. 2017;2(1):1–15.
5. Jeninga MD, Quinn JE, Petter M. ApiAP2 Transcription Factors in Apicomplexan Parasites. Pathogens. 2019;8:47.
6. Usui M, Prajapati SK, Ayanful-torgby R, Acquah FK, Cudjoe E, Kakaney C, et al. Plasmodium falciparum sexual differentiation in malaria patients is associated with host factors and GDV1-dependent genes. Nat Commun. 2019;1–15.
7. Modrzynska K, Pfander C, Chappell L, Rayner JC, Choudhary J, Modrzynska K, et al. A Knockout Screen of ApiAP2 Genes Reveals Networks of Interacting Transcriptional Regulators Controlling the Plasmodium Life Cycle Article A Knockout Screen of ApiAP2 Genes Reveals Networks of Interacting Transcriptional Regulators Controlling the. Cell Host Microbe. 2017;21:11–22.
8. Radke JB, Worth D, Hong D, Huang S, Sullivan WJ, Wilson EH, et al. Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis. PLoS Pathog. 2018;14(5):e1007035.
9. Waldman BS, Schwarz D, Saeij JP, Alex K, Lourido S. Identification of a master regulator of differentiation in Toxoplasma. bioRxiv. 2019;1–23.
10. Radke JB, Lucas O, Silva EK De, Ma Y, Sullivan WJ, Weiss LM, et al. ApiAP2 transcription factor restricts development of the Toxoplasma tissue cyst. PNAS. 2013;110(17):6871–6.
11. Jeffers V, Tampaki Z, Kim K, SullivanJr. WJ. A latent ability to persist: differentiation in Toxoplasma gondii. Cell Mol Life Sci. 2018;75(13):2355–73.

Tags: #toxoplasma, #toxoplasmosis

Posted on: 10th July 2019

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