The Toxoplasma Tour de Force to Unfold its Intravacuolar Developmental Program

Abstract

Toxoplasma gondii is an obligate intracellular single-celled eukaryotic parasite with an impressive ability to invade virtually all nucleated cells from all warm-blooded animals, within a second time-scale. The invasive T. gondii tachyzoite achieves this feat by injecting a multi-unit nanodevice in the plasma membrane and underlying cortical cytoskeleton of the targeted cell that serves as an anchor point to withstand the parasite invasive force. Whether this nanodevice could also contribute at the latest step of invasion when the budding entry vesicle pinches off of the plasma membrane as a parasitophorous vacuole had not been yet addressed. Using fluorescent versions of both a parasite nanodevice component and a reporter for the target plasma membrane in conjunction with quantitative high-resolution live imaging, Pavlou et al . characterized the nanodevice toroidal shape once inserted in the membrane as well as its stretching and shrinking when accommodating the passage of the several micron-sized ellipsoid shaped tachyzoite. Tracking in real time the motion of internal eccentric markers allowed defining the tachyzoite final rotation along the long axis which imposes a twisting motion on its basal pole and directs closure of the torus hence promoting both sealing and release of the entry vesicle. Monitoring distinct host cell plasma markers allowed Pavlou et al . to propose that the twisting motion could also act as an initial mechanical trigger for the transition to the intracellular lifestyle. Their publication therefore brings evidence for a key new contribution of the nanodevice to end the high-speed multi-step invasion process. This unveils a twisting motion of the tachyzoite imposing rotation on its basal end and likely constriction/torsion of the entry vesicle neck that facilitates membrane fusion and fission upstream the torus to release a Parasitophorous Vacuole (PV) whose membrane (PVM) is rapidly remodelled. When closure is mechanically prevented by microbeads that stay tightly bound to the posterior end of the tachyzoite, the leaky ZCJ causes an osmotic response from the host cell leading to parasite lysis. This work suggests a functional similarity between the mode of action of (i) the GTPase dynamins that form helices around the neck of vesicles (endocytic buds) at the PM to apply torsion