Class B1 GPCRs: insights into multireceptor pharmacology for the treatment of metabolic disease

Abstract

The secretin-like, class B1 subfamily of seven transmembrane-spanning G protein-coupled receptors (GPCRs) consists of 15 members that coordinate important physiological processes. These receptors bind peptide ligands and use a distinct mechanism of activation that is driven by evolutionarily conserved structural features. For the class B1 receptors, the C-terminus of the cognate ligand is initially recognized by the receptor via an N-terminal extracellular domain that forms a hydrophobic ligand-binding groove. This binding enables the N-terminus of the ligand to engage deep into a large volume, open transmembrane pocket of the receptor. Importantly, the phylogenetic basis of this ligand-receptor activation mechanism has provided opportunities to engineer analogs of several class B1 ligands for therapeutic use. Among the most accepted of these are drugs targeting the glucagon-like peptide-1 (GLP-1) receptor for the treatment of type 2 diabetes and obesity. Recently, multifunctional agonists possessing activity at the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor, such as tirzepatide, and others that also contain glucagon receptor activity, have been developed. In this article, we review members of the class B1 GPCR family with focus on receptors for GLP-1, GIP, and glucagon, including their signal transduction and receptor trafficking characteristics. The metabolic importance of these receptors is also highlighted, along with the benefit of polypharmacologic ligands. Furthermore, key structural features and comparative analyses of high-resolution cryogenic electron microscopy structures for these receptors in active-state complexes with either native ligands or multifunctional agonists are provided, supporting the pharmacological basis of such therapeutic agents.

Keywords: glucagon receptor; glucagon-like peptide-1 receptor; glucose-dependent insulinotropic polypeptide receptor; obesity; type 2 diabetes.

Figure 1.

A phylogenetic tree of human class B1 G protein-coupled receptors (GPCRs) and a common activation (two-step binding) model. A: the phylogenetic tree of human class B1 GPCRs generated by the Maximum Likelihood method and Kimura 2 parameter model using MEGA 11 is depicted (9, 10). The numbers below the branches indicate the percentage of replicate trees in which the associated taxa is clustered together in the bootstrap test 1,000 replicates (11). Bootstrap values of 50% or lower are collapsed. Sequences used for the construction of phylogenetic tree were downloaded from NCBI/GenBank. B i: activation of the class B1 receptors is initiated by binding of the C-terminal region of the peptide ligand to the N-terminal extracellular domain (ECD) of the receptor, forming an affinity trap for the ligand. ii: this initial low affinity interaction brings the N-terminal region of the ligand to be positioned within close proximity of the extracellular loops (ECLs) and the 7 transmembrane domain (TMD) of the receptor. iii: the high-affinity engagement of the ligand with the ECD, ECLs, and TMD induces conformational changes of the receptor, thereby enabling G-protein coupling at the cytoplasmic side and inducing signal transduction.

Figure 2.

Signal transduction of the glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP,) and glucagon G protein-coupled receptors (GPCRs). A: glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) signaling and downstream pathway activation in pancreatic β-cells. Incretin receptor stimulation can lead to Gαs and Gαq coupling, leading to upregulation of cyclic adenosine monophosphate (cAMP) and intracellular Ca²⁺, which results in downstream insulin production and secretion. B: activation of the glucagon receptor (GCGR) in hepatocytes enhances hepatic glucose output, lipid oxidation, and energy expenditure. Figure 2 was created with BioRender.com.

Figure 3.

Peptide sequence of native peptide agonists, dual agonists (A), and triple agonists (B) that have been studied using cryogenic electron microscopy (cryo-EM) or are currently in late stages of clinical development. Unnatural amino acids are indicated as blue text: Aib, alpha-amino isobutyric acid; Ac4c, 1-amino-cyclobutanecarboxylic acid; ameL, alpha-methyl-L-leucine. Residues with covalent modifications are indicated as red text. Glucagon-like peptide-1 (GLP-1)/ glucagon receptor (GCGR) agonists: oxyntomodulin (OXM): MW 4422.9 Da (142); peptide 15 (pep-15): MW 3356.6 Da (143); efinopegdutide (MK-6024, HM12525A): side chains of E16 and K20 forming a macrocycle and an IgG4 Fc region with a polyethylene glycol linker attached at C36. (144); mazdutide (IBI362, LY3305677): C20 diacid acyl moiety with linker attached at K20, MW 4563.1 Da (145, 146); pemvidutide (ALT-801, SP-1373): side chains of E16 and K20 forming a macrocycle with a glucuronic acid/C18 diacid acyl moiety attached at K17, MW 3873.4 Da (63, 64); survodutide (BI 456906): C18 diacid acyl moiety with linker attached at K24, MW 4231.6 Da (65, 66). glucose-dependent insulinotropic polypeptide (GIP)/ glucagon-like peptide-1 receptor (GLP-1R) agonists: peptide 19 (pep-19): C16 monoacid acyl moiety attached at K40, no linker, MW 4473.1 Da (147); tirzepatide (TZP; LY3298176): C20 diacid acyl moiety with linker attached at K20, MW 4813.5 Da (56). GLP-1/GIP/GCGR agonists: efocipegtrutide (HM15211): an IgG4 Fc region with a polyethylene glycol linker attached at C40 (67). The sequence of efocipegtrutide is reported in WHO Drug Information, Vol. 35, No. 4, 2021, Proposed INN: List 126. peptide 20 (pep-20): C16 diacid acyl moiety with linker attached at K10), MW 4543.1 Da (148); retatrutide (LY3437943): C20 diacid acyl moiety with linker attached at K17, MW 4731.4 Da (68).

Figure 4.

Structural comparison of the glucagon-like peptide-1 receptor (GLP-1R), glucose-dependent insulinotropic polypeptide receptor (GIPR), and glucagon receptor (GCGR) bound to their native ligands [glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), glucagon], dual agonists [oxyntomodulin (OXM), pep-15], and triple agonist (pep-20). A: GLP-1R bound to GLP-1 vs. OXM. B: GCGR bound to glucagon vs. pep-15. C: GLP-1R bound to GLP-1 vs. pep-20. D: GIPR bound to GIP vs. pep-20. E: GCGR bound to glucagon vs. pep-20). The 4-letter Protein Data Bank (PDB) codes are indicated in the figure. Receptors are shown as thin lines; ligands are shown as pink ribbons. Major area of difference indicated in red text. Top-down view (from the extracellular side): extracellular domain (ECD) and ligand are excluded for clarity. Pairwise comparisons of the Cα root mean square deviation (RMSD) between receptors and ligands are indicated beneath. Cα RMSDs are calculated by aligning the Cα of the receptor, excluding the ligand.

Figure 5.

Structural comparison of the glucagon-like peptide-1 receptor (GLP-1R) and the glucose-dependent insulinotropic polypeptide receptor (GIPR) bound to their native ligands [glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP)] and dual agonists [tirzepatide (TZP), pep-19]. A: GLP-1R bound to GLP-1 vs. TZP. B: GLP-1R bound to GLP-1 vs. TZP (the alternate structure). C: GLP-1R bound to GLP-1 vs. pep-19. D: GIPR bound to GIP vs. TZP. E: GIPR bound to GIP vs. TZP. The 4-letter Protein Data Bank (PDB) codes are indicated in the figure. Receptors are shown as thin lines; ligands are shown as pink ribbons. Major area of difference indicated in red text. Top-down view (from the extracellular side): extracellular domain (ECD) and ligand are excluded for clarity. Pairwise comparisons of the Cα root mean square deviation (RMSD) between receptors and ligands are indicated beneath. Cα RMSDs are calculated by aligning the Cα of the receptor, excluding the ligand.