Pharmacological and structure-function analysis of the human glucagon-like peptide-1 receptor

The glucagon-like peptide-1 receptor (GLP-1R) is an important regulator of insulin biosynthesis and secretion, and is one of the key therapeutic targets in the management of type II diabetes mellitus (DM). Like most GPCRs, the GLP-1R is pleiotropically coupled, with physiologically relevant signaling pathways including cAMP formation, intracellular calcium (iCa2+) mobilization and phosphorylation of extracellular signal regulated kinases 1 and 2 (pERK1/2). GLP-1R activity is further complicated by the ability to be activated by multiple endogenous peptide agonists, and the presence of single nucleotide polymorphisms (SNPs) within the receptor protein. Additionally, in the absence of complete crystal structures and limited knowledge of functionally relevant domains other than the N-terminus, this receptor system is in many respects unexplored. Consequently, the studies comprising this thesis are focused on illustrating distinct pharmacological concepts at the GLP-1R, while also addressing the role of a receptor core domain associated with biological activity. Specifically, the differential actions of allosteric modulators on endogenous peptide ligands (‘probe dependence’), the differential responses of receptor variants with respect to both orthosteric and allosteric ligands, and the capacity for both ligand- and pathway-specific effects (‘biased signaling’) are highlighted. While demonstrating the potential benefits of allosteric modulation, these studies significantly impact on the approaches and precautions that must be considered in the design, identification and development of allosteric modulators as therapeutics. In addition, these studies illustrate the importance of extracellular loop (ECL) 2 in the peptide-mediated activation transition of the GLP-1R. Collectively, this leads to enhanced understanding of the highly complex GLP-1R system, while also exemplifying the importance of extended pharmacological analysis of other GPCRs in order to gain a more comprehensive understanding of receptor systems.