Neurophysiological Changes During Chronic Toxoplasma gondii Infection

Mammalian neuronal function is regulated by a complex matrix of neurochemical signals, cellular effectors and cell-cell communication. Many of the mechanisms governing neurobiology are poorly understood. The obligate intracellular parasite Toxoplasma gondii forms a chronic neuronal infection lasting the lifetime of the mammalian host. In this thesis, changes in catecholaminergic signalling during Toxoplasma gondii infection were investigated. Preliminary RNAseq data was used to identify catecholaminergic genes exhibiting the highest expression changes during chronic Toxoplasma gondii infection. Reduced expression of dopamine beta-hydroxylase mRNA was found during Toxoplasma gondii infection in rat catecholaminergic and human neuronal cells. This down-regulation of dopamine beta-hydroxylase was shown in newly synthesised RNA, suggesting that regulation occurs at the transcriptional level. Using methylation sensitive restriction enzyme quantitative polymerase chain reaction, hypermethylation in the 5' upstream region of the dopamine beta-hydroxylase promoter was shown in infected rat catecholaminergic cells, human neuronal cells and chronically infected mouse nuclei. Surprisingly, dopamine beta-hydroxylase mRNA down-regulation and methylation in the 5' promoter were globally altered in vivo, despite the fact that only a small number of cysts can be identified in the host brain. In order to delineate the mechanism(s) responsible for this global change, extracellular communication during chronic Toxoplasma gondii infection was examined. Dopamine beta-hydroxylase silencing was observed only in cells exposed to infected cells. Extracellular vesicles purified from infected rat catecholaminergic cells induced transcriptional silencing of dopamine beta-hydroxylase. This effect could also be generated in vivo when rats received intracerebral injections with purified extracellular vesicles from infected cells. This represents a new perspective of the host-pathogen interaction. Through this mechanism, Toxoplasma gondii may be able to induce many of neurophysiological changes associated with chronic infection. In addition, Toxoplasma gondii is a unique model to study mammalian neurological function. By examining the influence the parasite is able to exert over cell-cell communication, we may be able to further understand the mechanisms governing CNS function and dysfunction.