Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Microbiology and Molecular Genetics

First Advisor

Gary E. Ward


Intracellular parasites of the phylum Apicomplexa cause pervasive human diseases and are responsible for millions of deaths annually. The apicomplexan, Toxoplasma gondii, causes the disease toxoplasmosis and can infect most warm-blooded vertebrates. These parasites can cause blindness in their hosts. In immunocompromised individuals, the infection can lead to brain lesions and death. T. gondii uses a unique form of cellular motility to invade cells of its host, traverse numerous biological barriers, and disseminate throughout the host. These parasites move with an attachment-dependent mechanism called gliding. In the “linear motor model” of parasite motility, the parasite’s myosin motor, TgMyoA, is thought to be attached to the inner membrane complex and bind to short actin filaments. Transmembrane adhesins bridge the internal actin filaments to ligands on the host cell or substrate. When myosin undergoes a power stroke, the transmembrane adhesins are translocated rearward, which drives the parasite forward.

While this model currently dominates the field, recent data where key motility proteins were disrupted without completely ablating motility suggest that we still do not fully understand the mechanisms underlying T. gondii motility. One key piece of missing information is the directionality of the forces a moving parasite exerts on its surrounding environment. To address this gap in our knowledge, we developed methods for 3D traction force mapping in a fluorescent fibrin matrix. As the parasite moves, it periodically pulls on and releases the matrix. The periods of matrix deformation correlate directly with striking periodic constrictions of the parasite’s body and all matrix displacements are directed towards these constrictions. The ring-like constriction initiates at the apical end of the parasite and remains stationary relative to the matrix as the parasite moves through, similar to the moving junction during invasion. Wild-type parasites do not move without forming constrictions. Parasites lacking TgMyoA do not form constrictions or cause a detectable fibrin deformation and have a severe defect in motility. In parasites lacking the surface adhesin TgMIC2, constrictions do not form, and the parasites move in less straight trajectories compared to wildtype. Together the data suggest the constrictions are repeated invasion-like events that act as a guidance system for productive, forward-directed motility. To further characterize the shape of the trajectories in wild-type and mutant parasites, we developed a custom software package, “Bugs”, to track parasites and compare trajectories of different parasite populations. Using Bugs, we confirmed the TgMIC2 knockout parasites move in less straight trajectories than wildtype as well as determined the pattern of the non-moving periods.

Collectively, the data give new insights into the motility of T. gondii. The data suggest that the parasite exerts inward-directed forces at fixed circular locations within the extracellular matrix. The circular ring force corresponds to a circumferential attachment zone between the parasite and the matrix, through which the parasite propels itself to move forward. The combined data also suggest a closer connection between the mechanisms underlying parasite motility and host cell invasion than previously recognized, establish new methods for studying motility, and define critical questions for future study.



Number of Pages

222 p.

Available for download on Thursday, September 19, 2024