Investigating the role of peripheral nerve damage in chronic Toxoplasma infection

Project Summary Toxoplasma gondii is a highly successful intracellular parasite estimated to infect up to a third of the world’s population. T. gondii establishes a chronic infection forming intracellular cysts largely in the central nervous system (CNS) and skeletal muscle, associated with chronic inflammation and critically, a loss of motor function in mice. Notably, Toxoplasma’s tropism for the CNS and muscle is in spite of its ability to infect any nucleated cell type, which is only partially understood, but is related to the induction of differentiation from replicative lytic tachyzoites, to slow growing, lowly-immunogenic bradyzoite cysts. Prior in vitro work has shown that glycolytic cells inhibit bradyzoite differentiation, while oxidative cells promote it. While skeletal muscle features both glycolytic fast twitch muscles and oxidative slow twitch muscles, investigation into parasite distribution within skeletal muscle is limited. Additionally, while pathology in the CNS and muscle is established, neither pathology in the peripheral nervous system (PNS), nor the role of any of these tissues in motor function loss has been directly determined. In preliminary work in T. gondii-infected mice, differential cyst burden between fast and slow twitch muscle was observed, in addition to immune infiltration into peripheral nerves, accompanied by pathology to the neuromuscular junction (NMJ) which connects motor neurons and myofibers. In this work, multiple types of pathology to the NMJ were observed and intriguingly, the findings suggest that the parasite, muscle metabolic state, and immune response may drive distinct aspects of this pathology. However, how these factors interact is unclear. The main hypothesis of this proposal is that the parasite, myofiber metabolic state and immune response all contribute to the loss of motor function. In Aim 1, the impact of myofiber metabolic state on parasite burden, immune responses, and subsequent NMJ pathology of both muscle types will be investigated. In Aim 2, to tease apart the roles of the immune system versus parasite in mediating NMJ pathology, the impact of blocking CD4+ cell-mediated inflammation or blocking T. gondii replication on NMJ morphology and function will be determined. The overall goal of this research training proposal is to strengthen the candidate’s understanding of host-parasite interactions and tissue tropisms, and the contributions of the parasite and immune response(s) to the associated pathology and function of the skeletal muscle and PNS. The results of this proposal will be broadly applicable to understanding the host-parasite interactions determining tropism, and the impacts of chronic infection and inflammation on PNS health and function.