“Prison Break”: A programmed event controlled by a lipid mediator

The Apicomplexa phylum includes a large group of obligate intracellular protozoan parasites responsible for important diseases in humans and animals. Toxoplasma gondii is a widespread parasite with considerable versatility and capable of infecting virtually any warm-blooded animals, including humans. The success of this parasite as an invader can at least in part be attributed to an efficient and continuous sensing of the environment with a strategy of ready-to-go during acute infection (tachyzoite stage), and an incredible quiescence during chronic infection (bradyzoite stage). The parasite appears to constantly monitor the pH, which might drop due to metabolic dysregulation in the host cell, or potassium concentration, which drops in extracellular environments or when the integrity of the host cell plasma membrane is compromised. By sensing host calcium (Ca2+) levels, T. gondii is informed about key host cell decisions such as commitment to of apoptosis, adherence of monocytes to endothelial cells or the presence of a particular hormone. All of these signals lead to a “get out of here” signalling on the parasite.

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Intriguingly, we have known for years that even in the absence of alerting signals triggering egress, the number of divisions of the parasite in a single host cell is limited. Egress occurs after 5 to 6 cycles of asexual multiplication. Two different hypothesis have been made in order to explain this phenomenon and define what was called natural egress as a passive or an active process. The passive process of natural egress would be due to a mechanical breakage of the host cell plasma membrane, leak out of potassium and induced egress, resulting from the parasite exponential replication. Alternatively, an active natural egress would position the parasite on the driving seat. An intrinsic signal produced by the parasite governs the maximum number of cycles before egress.  Such a clock-like system would be genetically encoded by the parasite and set up at the time of host cell entry.

Bisio et al, have unveiled several components of the molecular clock mastering natural egress. Synthesis of phospholipids by T. gondii not only occurs in the cytoplasm but also in the parasitophorous vacuole, an internal space where the parasite grows sheltered from host cell attacks. One particular phospholipid, phosphatidic acid, is produced from diacylglycerol by the action of the diacylglycerol kinase 2, secreted by the parasite in the vacuole. This enzyme accumulates progressively in the vacuole during infection and produces phosphatidic acid which ultimately triggers natural egress (Bisio et al, 2019).

An atypical guanylate cyclase fused to a P4-ATPase (flippase-like) domain at the surface of the parasites serves as signalling node (Brown et al, 2018). The guanylate cyclase produces cytosolic cGMP that serves as initiator of the entire signalling cascade leading to egress. It is tempting to speculate that phosphatidic acid directly activates the guanylate cyclase is via binding to the P4-ATPase domain predicted to recognize phospholipids in the outer leaflet of the plasma membrane.  

Noteworthy, passive and active natural egress are not mutually exclusive and might occur in one way or another depending on the robustness of host cell. Genetic ablation of DGK2 leads to a full shift to passive natural egress. These findings raise new questions such as the physiological contribution of natural versus induced egress during infection? How is the molecular clock set in bradyzoite cysts containing thousands of parasites? Is natural egress participating in the dissemination of chronic infection? 

Brown et al, Cell Host Microbe, 2018 Dec 12;24(6):804-816.e6.
Bisio et al, Nature Microbiology 2019 Feb 11. doi: 10.1038/s41564-018-0339-8.

Dominique Soldati-Favre

Professor, University of Geneva

cell and molecular biology of Apicomplexa