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Abscisic acid controls calcium-dependent egress and development in Toxoplasma gondii

Abstract

Calcium controls a number of critical events, including motility, secretion, cell invasion and egress by apicomplexan parasites1. Compared to animal2 and plant cells3, the molecular mechanisms that govern calcium signalling in parasites are poorly understood. Here we show that the production of the phytohormone abscisic acid (ABA) controls calcium signalling within the apicomplexan parasite Toxoplasma gondii, an opportunistic human pathogen. In plants, ABA controls a number of important events, including environmental stress responses, embryo development and seed dormancy4,5. ABA induces production of the second-messenger cyclic ADP ribose (cADPR), which controls release of intracellular calcium stores in plants6. cADPR also controls intracellular calcium release in the protozoan parasite T. gondii7,8; however, previous studies have not revealed the molecular basis of this pathway9. We found that addition of exogenous ABA induced formation of cADPR in T. gondii, stimulated calcium-dependent protein secretion, and induced parasite egress from the infected host cell in a density-dependent manner. Production of endogenous ABA within the parasite was confirmed by purification (using high-performance liquid chromatography) and analysis (by gas chromatography-mass spectrometry). Selective disruption of ABA synthesis by the inhibitor fluridone delayed egress and induced development of the slow-growing, dormant cyst stage of the parasite. Thus, ABA-mediated calcium signalling controls the decision between lytic and chronic stage growth, a developmental switch that is central in pathogenesis and transmission. The pathway for ABA production was probably acquired with an algal endosymbiont that was retained as a non-photosynthetic plastid known as the apicoplast. The plant-like nature of this pathway may be exploited therapeutically, as shown by the ability of a specific inhibitor of ABA synthesis to prevent toxoplasmosis in the mouse model.

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Figure 1: ABA induced cADPR production and calcium-dependent protein secretion by T. gondii.
Figure 2: Biochemical evidence for the production of ABA in T. gondii.
Figure 3: Fluridone blocked parasite growth by preventing egress.
Figure 4: Fluridone treatment induced development of tissue cysts and protected mice against toxoplasmosis.

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Acknowledgements

We thank J. Zeevaart for conducting the initial purification and analysis of ABA and for supplying standards; J. Boothroyd and L. Weiss for providing reagents; W. Beatty, S. Moreno, B. Striepen, A. Waters and L. Xiong for comments; and J. Nawas and D. Gill for technical assistance. This work was supported by the Uehara Medical Foundation (K.N.), the Mayo Clinic and American Heart Association (E.N.C.) and the NIH (L.D.S.).

Author Contributions K.N. performed the experiments on the effects of ABA and fluridone on the parasite, L.M.H. performed the MS studies, B.F. performed the animal studies, F.B. contributed to the analysis of ABA genes, E.N.C. performed the measurements of cADPR, L.D.S. supervised the project and wrote the manuscript with input from all the authors.

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Correspondence to L. David Sibley.

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Nagamune, K., Hicks, L., Fux, B. et al. Abscisic acid controls calcium-dependent egress and development in Toxoplasma gondii. Nature 451, 207–210 (2008). https://doi.org/10.1038/nature06478

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