Cellular and subcellular co-storage of gastrointestinal hormones
AuthorFothergill, Linda Jane
AffiliationAnatomy and Neuroscience
Document TypePhD thesis
Access StatusOpen Access
© 2018 Dr. Linda Jane Fothergill
Gastrointestinal hormones regulate a diverse range of physiological processes including digestion, metabolism, and food intake, as well as maintaining mucosal integrity and mounting defensive mechanism in response to pathogens or toxins. Disruptions of these regulatory processes are associated with disorders that include diabetes, obesity, irritable bowel syndrome, and coeliac disease. Despite this clinical significance, many aspects of the cells that contain gut hormones, enteroendocrine cells (EEC), remain poorly characterised. Historically, EEC were categorised based on their hormone content, with the assumption that each cell contained a single hormone. However, hidden in some early studies were examples that the simple ‘one cell‐one hormone’ classification system was not correct. A major focus of this thesis is the characterising of coexpression patterns of hormones in mouse and human intestine. Remarkably, examples of overlap could be found for all hormones investigated, demonstrating a need for a revised classification system for EEC. Moreover, species differences in EEC expression patterns occur, highlighting a further level of complexity in the field of research. The coexpression of EEC hormones raises new questions about how these hormones are stored at a subcellular level. Super‐resolution microscopy allowed the visualisation and quantification of vesicular stores in EEC. Overlapping and non‐overlapping vesicular stores were observed. Furthermore, the relative abundance of hormone expression varied considerably between cells and, consequentially, small amounts of hormones sometimes could not be detected with low‐resolution methods. In addition, a monoclonal antioxyntomodulin antibody was characterised, expanding the examination of hormone colocalisation patterns to the investigation of proglucagon‐derived peptides. Several hormones, including 5‐HT, influence electrolyte transport, which can be measured as the shortcircuit current (Isc) in an Ussing chamber. The effects of 5‐HT and secretin, which are frequently colocalised, on electrolyte transport across the intestinal epithelium were examined. Both hormones stimulated a secretory response in all regions of the mouse intestine, including the colon where secretin was not thought to be expressed. However, secretin gene transcripts were identified and secretin immunoreactivity was localised to EEC in the mouse colon. The functional effects of TRPA1, which is an ion channel that has previously been localised to EEC containing both 5‐HT and CCK, was also examined using an Ussing chamber. While TRPA1 agonists stimulated a secretory response, this was not mediated by 5‐HT. This research has advanced knowledge of the colocalisation patterns of gastrointestinal hormones at a cell and subcellular level and explored some aspects of the functions of costored hormones in relation to water and electrolyte movement across the gut lining.
Keywordsenteroendocrine cells; gastrointestinal tract
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