Maintaining the intestinal barrier : the role of the tight junction protein occludin in Toxoplasma gondii transmigration of the gastrointestinal tract

The small intestinal permeability barrier is dependent on tight junction (TJ) complexes that separate the external lumen from the underlying mucosa. Apical TJs consist of integral transmembrane proteins including occludin, claudins and ZO-1 as well as the cytoplasmic plaque of TJ-associated adaptor, scaffolding and signalling proteins. Although the function of occludin at the TJ remains unclear, the dynamic mobility of occludin, claudins and ZO-1 to and from the TJ suggests occludin may play a key role in regulation of TJ structure and function, regulated by occludin phosphorylation status. Defects in TJ barrier function have been implicated in a range of inflammatory diseases such as inflammatory bowel disease (IBD) and pathogens such as Toxoplasma gondii target this complex as a route of infection. As oral infection is the primary cause of toxoplasmosis, the first point of contact between T. gondii and the host is the small intestinal epithelium and studies by Weight, 2012 show occludin may be involved in T. gondii paracellular transmigration of the small intestinal epithelium. \nThe aim of this research was to investigate T. gondii paracellular transmigration using an in vitro model of the small intestinal epithelium and elucidate the role of occludin both in regulation of the TJ barrier and as a receptor for T. gondii infection. The results presented in this thesis demonstrate that T. gondii infects the small intestinal epithelium via the paracellular pathway and occludin was shown to play a key role both in regulation of the TJ paracellular barrier and as a receptor for T. gondii infection by parasite-mediated modulation of the occludin C-terminus phosphorylation status and direct binding to occludin ECL1; suggesting T. gondii interactions with occludin are a potential mechanism of paracellular transmigration of the small intestinal epithelium.