Understanding the mechanisms of G protein-coupled receptor mediated nutrient sensing in enteroendocrine cells

Abstract

Nutrient sensing is the process by which cells detect and respond to dynamic metabolite fluctuations. In gastrointestinal enteroendocrine cells (EECs), this process facilitates the post-prandial secretion of anorectic gut hormones. As these elicit diverse impacts on physiology, appetite and metabolism, they present potential targets for anti-obesity therapeutics. Short chain fatty acids (SCFAs) are produced as by-products of the fermentation of non-digestible carbohydrates by the gut microbiota, predominantly in the colon. SCFAs induce EECs to release anorectic hormones GLP-1 and PYY by acting as ligands at G protein-coupled receptors (GPCRs) FFAR2 and FFAR3. Thus far, it has been confirmed that FFAR2 and FFAR3 couple to Gαi/o but FFAR2 alone activates Gαq/11 calcium mobilisation. However, FFAR2 is seemingly only able to induce calcium signals in EECs when activated by synthetic FFAR2 ligands, not SCFAs. It is plausible that crosstalk between FFAR2 and FFAR3 could be preventing FFAR2 coupling to Gαq in response to SCFAs. This has been assessed by combining genome editing and functional signalling assays. Conversely, the results demonstrate that co-expression of FFAR2 and FFAR3 in heterologous systems enhances propionate-mediated calcium mobilisation. Futhermore, functional knockout of FFAR3 has no impact on propionate signalling in EECs. Interestingly, crosstalk with FFAR4 may represent a mechanism for the lack of SCFA-induced Gαq signalling in EECs. The impact of SCFAs on the expression of other GPCR signalling machinery remains poorly characterised. This project has, to date, demonstrated propionate and butyrate robustly upregulate transcripts encoding the umami taste receptor, via a Gαi/o independent mechanism. We show that overnight exposure of EECs to butyrate enhances the calcium mobilisation in response to L-Alanine when IMP is present. Therefore, I propose an alternative, indirect mechanism by which SCFAs enhance gut hormone release; namely, by inducing a hypersensitivity of EECs to L-amino acids by promoting expression of taste signalling components.