Signalling and Regulation of the Glucagon-like Peptide-1 receptor

Following nutrient ingestion, glucagon-like peptide 1 (GLP-1) secreted from intestinal L-cells mediates anti-diabetic effects, most notably stimulating glucose-dependent insulin release from pancreatic β-cells but also inhibiting glucagon release, promoting satiety and weight reduction and potentially enhancing or preserving β-cell mass. These effects are through the GLP-1 receptor (GLP-1R) which is a therapeutic target in type 2 diabetes. The present study focused on desensitisation and re-sensitisation of GLP-1R-mediated signalling and interactions of orthosteric and allosteric ligands. Data demonstrate GLP-1R desensitisation and subsequent re-sensitisation following removal of extracellular ligand with ligand-specific features. Following GLP-1-mediated desensitisation, re-sensitisation is dependent on receptor internalisation, endosomal acidification and receptor recycling. Re-sensitisation is also dependent on endothelin converting enzyme-1 (ECE-1) activity, possibly through proteolysis of GLP-1 in endosomes, facilitating disassociation of receptor-β-arrestin complexes leading to GLP-1R recycling and re-sensitisation. ECE-1 activity also regulates GLP-1-induced activation of extracellular signal regulated kinase (ERK) and generation of cAMP possibly through a G protein independent/β-arrestin dependent mechanism. By contrast, following GLP-1R activation by the orthosteric agonist, exendin-4, or allosteric agonist, compound 2, re-sensitisation was slow and independent of ECE-1 activity. Thus, different ligands depend on different events during GLP-1R trafficking which could be important for re-sensitisation and signalling, particularly that mediated by scaffolding around β-arrestin. As the GLP-1R is targeted therapeutically at orthosteric and allosteric sites, this study examined activation of the GLP-1R by orthosteric and allosteric agonists and in particular interactions between ligands of these sites. Challenging the GLP-1R with the allosteric ligand, compound 2, along with GLP-1 9-36 amide, a low affinity, low efficacy metabolite of GLP-1 7-36 amide, results in synergistic receptor activation. This may be important for therapeutic approaches with allosteric ligands, as metabolites of GLP-1 may be present in vivo at concentrations higher than the classic endogenous ligand. Indeed this could present a novel therapeutic approach.