Motility-dependent processes in Toxoplasma gondii tachyzoites and bradyzoites: same same but different

During infection, Toxoplasma gondii tachyzoites must be able to move in order to migrate through tissues, cross biological barriers, and penetrate into and egress from cells they infect. Bradyzoite-stage parasites, which establish infection in naïve hosts, also require motility to escape from cysts after they are ingested and to subsequently migrate to the gut wall, where they either invade cells of the intestinal epithelium or squeeze between these cells to infect the underlying tissue. Little is known about the motility of bradyzoites, which we analyze in detail here and compare to the well-characterized motility and motility-dependent processes of tachyzoites. Unexpectedly, bradyzoites were found to be as motile as tachyzoites in a three-dimensional model extracellular matrix, and they showed increased invasion into and transmigration across monolayers of certain cell types, consistent with their need to establish infection in the gut. The motility of the two stages was inhibited to the same extent by cytochalasin D and KNX-002, compounds known to target the parasite's actomyosin-based motor. Other compounds that impact tachyzoite motility (tachyplegin and enhancer 5) have a reduced effect on bradyzoites. Furthermore, rapid bradyzoite egress from infected cells is not triggered by treatment with calcium ionophores, as it is with tachyzoites. The similarities and differences between these two life cycle stages highlight the need to characterize both tachyzoites and bradyzoites for a more complete understanding of the role of motility in the parasite life cycle and the effect that motility-targeting therapeutics will have on disease establishment and progression.

Toxoplasma gondii is a parasite that chronically infects around one-third of the world's population. Toxoplasma uses motility for multiple purposes during infection, including extracellular migration, invasion into host cells, and host cell egress. These motility-dependent processes have been extensively studied in the life cycle stage responsible for acute infection, the tachyzoite. In contrast, motility and motility-dependent processes are poorly understood in bradyzoite-stage parasites, which are responsible for both establishing infection after consumption of infected meat and initiating potentially life-threatening reactivated infections in the brains of immunocompromised individuals. We show here that the motility and motility-dependent processes of bradyzoites are similar in many respects to those of tachyzoites but markedly different in others. The results of this study highlight the need to consider both life cycle stages in attempts to develop drugs targeting parasite motility and the signaling processes that regulate motility-dependent processes during infection by these important human pathogens.