Enteroendocrine cells (EECs) of the gastrointestinal (GI) tract and islets of Langerhans (islets) of the pancreas are endocrine cells capable of sensing nutrient intake or the nutrient content of the blood and responding by secreting peptide hormones to control various physiological functions. This thesis focuses on characterising the peptidome of EECs and islets by utilising liquid-chromatography coupled to mass spectrometry (LC-MS).

Due to their ability to control food intake and glucose tolerance, hormones from EECs and islets are of great therapeutic interest for the treatment of type 2 diabetes (T2D) and obesity. I hypothesised that there may be undiscovered peptide hormones derived from EECs or islets that could also be of therapeutic interest for the treatment of T2D. In order to identify potentially novel hormone candidates, peptidomic analyses of mouse EECs, mouse islets and human GI tissue were carried out utilising LC-MS. 13 interesting peptide candidates identified in the peptidomic analyses were picked out to be synthesised for in vivo and/or in vitro characterisation.

All 13 peptides were screened for activity at Gs or Gi signalling pathways in various cell lines although unfortunately no peptide seemed to exhibit activity through these signalling mechanisms in the cell lines tested. Despite this, studies were carried out to assess the ability of the 2 top peptide candidates to control food intake and glucose tolerance in vivo. 1 of these peptides, a peptide derived from progastrin, seemed to mildly improve glucose tolerance in lean mice, an effect that was seen in across several studies. I hypothesised that this progastrin derived peptide may be altering peptide secretions from islets to modulate glucose tolerance but no effect of this novel peptide was seen on insulin, somatostatin-14 or glucagon secretion from islets. The progastrin derived peptide was then administered to diet-induced obese mice to assess its therapeutic potential but unfortunately no effect was seen on glucose tolerance. More work needs to be performed to verifying my findings and investigating a possible mechanism of action before this novel progastrin derived peptide may be considered a gut hormone.

In regard to the peptidome of islets, there are some gaps in the literature that our initial peptidomic analysis did not answer. Questions such as do islets produce GLP1 (glucagon-like peptide 1) receptor agonists and how is the islet peptidome altered in response to metabolic stress. I therefore carried out an investigation to address these questions. Targeted LC-MS methodology was used to search for intra-islet GLP1 receptor agonists. No peptides derived from glucose-dependent insulinotropic polypeptide (GIP) were found in mouse islets but active GLP-1 was found in both human and mouse islets. Comparisons were made between the abundance of active GLP-1 and glucagon to assess which of these peptides might be responsible for the intra-islet “incretin” effect. My peptidomic analysis of islets from T2D humans and obese mice may not have revealed new major findings but it has provided some clarity over how metabolic stress alters the peptidome of islets in T2D an obesity.

In summary; a comprehensive analysis of the peptidome of EECs and islets was carried out, a number of novel peptide hormone candidates were synthesised and one of which exhibited a mild improvement in the glucose tolerance of lean mice, and clarity was provided on how metabolic stress alters the islet peptidome in mice and humans. I hope that the data produced in this thesis can contribute to a better understanding of the peptidome of the EECs and islets and may contribute to advances in therapeutic options for patients suffering from obesity or T2D.