Role of short chain fatty acid receptors in the gastrointestinal tract and their potential involvement in appetite control

The objective was to explore the role of GPR41 (FFAR3) and GPR43 (FFAR2) in colonic physiology and potential involvement in appetite control. The expression of SCFA receptors mRNA across the longitudinal axis of healthy mice, pig and human colon were found to be similar. Immunohistochemistry revealed FFAR2 and FFAR3 to be localised to flask shaped cells deep in colonic crypts and were identified as open type enteroendocrine cells by co-localisation with chromogranin A, a classical marker of enteroendocrine cells using both pig and human colon tissue. Further co-localisation studies with pig colonic tissue showed the SCFA receptors are co-localised with serotonin (enterochromaffin cells), and the satiety peptides, peptide YY (PYY) and glucagon like peptide 1 (GLP-1), indicative of enteroendocrine L cells. Impact of dietary fibre was assessed using colonic tissue collected from pigs fed a fermentable carbohydrate diet were compared to a control diet of less fermentable carbohydrate. Results demonstrate steady state SCFA concentrations exist in the colonic lumen, explained by enhanced uptake of SCFA due to an increased expression of SCFA transporter (MCT1). Dietary fibre was found to have minimal impact on relative abundance of the SCFA receptors mRNA across the longitudinal axis of pig colon. However, evaluation of the satiety peptides, PYY and pro-glucagon (precursor of GLP-1) mRNA have revealed a statistical significant increase in PYY but not pro-glucagon mRNA. Subsequently, attention was focused toward use of in-vitro models to explore SCFA receptors in appetite control. Initial in-vitro experiments assessed activity profile of SCFA receptors and evaluated chemical tools used to differentiate between FFAR2 and FFAR3. Murine ‘GLUTag’ and human ‘NCI-H716’ in-vitro models of enteroendocrine ‘L cells’ were characterised and SCFA induced GLP-1 secretion evaluated. 10mM butyrate and 10μM 4-CMTB induced GLP-1 release. RNA interference was used in attempt to knockdown FFAR2 in NCI-H716 cells to evaluate SCFA sensor’s involvement in GLP-1 release. In summary, luminal SCFA may stimulate SCFA receptors of enteroendocrine cells releasing peptides; GLP-1, PYY or serotonin to co-ordinate gut motility and appetite.