The cortical microtubules ofToxoplasma gondiiunderlie the helicity of parasite movement

Abstract

Apicomplexans parasitize a wide range of hosts. To infect, the parasite needs to travel through different types of tissues and invade into various types of cells. In tissues and three-dimensional (3-D) matrix, the apicomplexan parasite Toxoplasma gondii moves along a helical path. While many genes, including actin and myosins, have been shown to be important for parasite motility, it remains unknown what allows the parasite to travel over a long distance along a helical path. The cortical microtubules, which are ultra-stable, sprially arranged, and form extensive lateral interaction with the parasite cortex, have been considered to be a candidate structure for guiding the long-distance movement of the parasite. In wild-type parasites, the cortical microtubules in mature parasites are impervious to destabilization by cold-treatment or depolymerizing drugs, which makes it difficult to assess their function. Previously, we generated a mutant (dubbed "TKO" for succinctness) that lacks three microtubule-associated proteins. The loss of these three proteins destabilizes the cortical microtubules in mature parasites. Here we quantify the proportion of parasites with various levels of defects in the microtubule array under different conditions. We found that ~ 80% of the non-dividing TKO parasites have severely curtailed cortical microtubules. The extent of depolymerization is further exacerbated upon the initiation of daughter construction or cold treatment. The cold-induced depolymerization is reversible, with noticeable restoration of the cortical microtubules within 20 min of temperature shift to 37 degree. While microtubule polymerization is essential for generating viable daughter cells, the destabilization of the cortical microtubules in the mature parasite does not affect parasite replication. In a 3-D Matrigel matrix, the TKO mutant parasites can travel directionally over long distances. However, their trajectories are significantly more linear than those of wild-type parasites. In tissue culture, the TKO parasite displays a defect in infection and cytolytic efficiency. Interestingly, the speed and behavior of the parasite's entry into and egress from the host cell are similar to that of the wild-type parasite. These results indicate that the cortical microtubules contribute to the helicity but not the persistence of parasite long-distance movement. Furthermore, host cell entry is less sensitive to structural changes in the parasite than overall infection efficiency, which also include extracellular migration and orient the parasite for proper contact with the host cell.