Intestinal G protein-coupled receptors (GPCRs): characterising gut GPCR-signalling with potential to treat colitis

Free fatty acid receptors (FFARs) are GPCRs known to be expressed in enteroendocrine cells and therefore likely to be involved in nutrient- and microbial metabolite signaling in the mammalian GI tract. FFA1 and FFA4 are co-activated by long-chain fatty acids (LCFAs) while FFA2 and FFA3 are preferentially co-stimulated by microbial-derived short chain fatty acids (SCFAs). The nonselectivity between these pairs of GPCRs is complicated further by other GPCRs with affinity for fatty acids. For example, SCFAs activate GPR109A (also known as HCA2; Husted et al 2017) as well as olfactory receptors Olf78 and Olf558 (Bellono et al 2017) making specific FFARs' function(s) challenging due to lack of selective ligands. Thus, the roles of this GPCR group remain obscure in terms of GI health and disease.

Using commercially available tools, Cox et al have however partially characterized FFA1-4 signaling in healthy GI mucosae and established their effects on GI transit in mice (Forbes et al 2015; Moodaley et al 2017; Tough et al 2018). This has been achieved using a combination of proven electrophysiological in vitro methods (measuring vectorial epithelial ion transport and barrier function in parallel) and measuring changes in upper and lower GI transit (Tough et al 2011). Our in-depth understanding of neuro-epithelial signaling within the GI tract, has been critical too. Notably, we see similar pharmacology and cellular mechanisms for FFA2 & FFA3 activities in normal mouse and human colon (Tough et al 2018) indicating translation of this approach. Selective, potent FFAR ligands are now critical for continued progress. With Heptares, our combined understanding of GI signalling is allowing us to characterise mucosal dysfunctions induced by acute GI inflammation. To this end we will include the mild colitis dextran sulphate sodium (DSS) model which PhD student, Evans (in Cox lab) is showing currently, recapitulates aspects of colitis pathology observed in man, allowing us to...