The structure and function of the glucagon-like peptide-1 receptor and its ligands

Abstract

Glucagon-like peptide-1(7-36)amide (GLP-1) is a 30-residue peptide hormone released from intestinal L cells following nutrient consumption. It potentiates the glucose-induced secretion of insulin from pancreatic beta cells, increases insulin expression, inhibits beta-cell apoptosis, promotes beta-cell neogenesis, reduces glucagon secretion, delays gastric emptying, promotes satiety and increases peripheral glucose disposal. These multiple effects have generated a great deal of interest in the discovery of long-lasting agonists of the GLP-1 receptor (GLP-1R) in order to treat type 2 diabetes. This review article summarizes the literature regarding the discovery of GLP-1 and its physiological functions. The structure, function and sequence-activity relationships of the hormone and its natural analogue exendin-4 (Ex4) are reviewed in detail. The current knowledge of the structure of GLP-1R, a Family B GPCR, is summarized and discussed, before its known interactions with the principle peptide ligands are described and summarized. Finally, progress in discovering non-peptide ligands of GLP-1R is reviewed. GLP-1 is clearly an important hormone linking nutrient consumption with blood sugar control, and therefore knowledge of its structure, function and mechanism of action is of great importance.

Figure 1

A sequence alignment of GLP-1, Ex4 and several analogues mentioned at various stages in the text. All peptides are C-terminally amidated. Note that the first residue of GLP-1 is His⁷, while that of Ex4 is His¹. Residues changed from the original sequence are underlined.

Figure 2

Concentration–response curves (LANCE cAMP assay) for various peptides at hGLP-1R, expressed in HEK-293 cells, using the same cell line as described previously by Mann et al. (2010a,b). pEC₅₀ values, averaged over three independent experiments, were (A) GLP-1, 11.17 ± 0.12; A7-GLP-1,9.64 ± 0.09; A10-GLP-1, 9.45 ± 0.08; GLP-1(9-36), 6.97 ± 0.09; GLP-1(15-36), 6.72 ± 0.13. (B) Ex4, 12.14 ± 0.06; Ex4(1-30), 11.42 ± 0.03; Ex4(3-39), 8.87 ± 0.06; Ex(9-39) and Ex4(9-30) gave no response (Nasr, Wishart and Donnelly, unpubl.). Note that several peptides [A7-GLP-1, A10-GLP-1, GLP-1(15-36), Ex4(3-39)], believed to be inactive in other expression systems, display robust activity in HEK-293 cells.

Figure 3

(A) A schematic representation of the structure of the NTD of hGLP-1R based upon the crystal structure of Runge et al. (2008). The residue numbers at the boundaries of the various secondary structural elements are shown. Disulphide bonds are depicted with dashed lines. On the right is the α-helical region of Ex4(9-39) with lines connecting to contacting residues on the NTD. (B) Left: Ideal helical wheel containing the residues in the helical region of Ex4(9-39). Residues conserved with GLP-1 are filled grey. Interactions with residues in the NTD are shown with solid arrows. Intra-helical interactions, which stabilize the helix, are shown with dotted arrows. The hydrophilic and hydrophobic faces are labelled. Right: The helical region of GLP-1 on the C-terminal side of the Gly-22 kink. Note that the helix-stabilizing interactions found in Ex4(9-39) are not formed in GLP-1 since the residues are not conserved.