Human glucagon-like peptide receptor (GLP-1R)

Ashley Auerbach '28 and Josie DeBruer '28

Contents:

I. Introduction

Glucagon-like peptide 1 receptor (GLP-1R) is a protein found in the membranes of multiple cell types, most notably intestinal mucosal L cells, pancreatic islet alpha cells, and neurons in the nucleus of the solitary tract. This receptor is a G protein coupled receptor (GPCR) and, with binding of a ligand, causes the coupled G protein to release its bound GDP and recruits a GTP, releasing the activated G protein from the GPCR. Abundance of activated G proteins increases production of cyclic AMP (cAMP) which activates protein kinase A (PKA), an enzyme responsible for synthesis of insulin and inhibition of glucagon in the pancreas.

Insulin and glucagon work in opposition in the body; insulin is a hormone produced when blood sugar is high that signals the liver to remove glucose from the bloodstream and store it as glycogen while the hormone glucagon stimulates the liver to release stored glycogen as glucose into the bloodstream to raise blood sugar levels. These pathways are notable for the study of both Type 1 and Type 2 diabetes mellitus (TDM) as these diseases result from either failure to produce insulin or development of insulin resistance.

In an attempt to treat diabetes, researchers identified the GLP-1R and its natural ligand GLP-1 as molecules of interest after sequencing analysis revealed the insulin-stimulating property of the peptide. Understanding of the structure and function of the receptor and its ligand yielded mass production of the pharmaceutical class of glucagon-like peptide 1 receptor agonists (GLP-1RA). This drug binds GLP-1R as an agonist and signals the insulin production pathway just as GLP-1 would. The use of GLP-1RAs increases concentration of possible ligands for GLP-1R, increasing production of insulin and inhibition of glucagon resulting in reduced gastric emptying and suppression of appetite.

II. General Structure

GLP-1R is a class B GPCR, meaning its binding to a ligand begins a signal cascade as a coupled G protein becomes activated with GTP. GLP-1R is composed of a single polypeptide chain consisting of 463 amino acids. The receptor contains two major regions, a large extracellular N-terminal domain (ECD) and a seven-transmembrane helix bundle (TMD) . The seven ⍺-helices TM1-TM7 form the V-shape central cavity that is a common motif among class B GPCRs and serves as the primary ligand binding pocket.

As a GPCR, GLP-1R binds the G protein complex G⍺ , Gβ and Gγ . The G protein complex binds the receptor through G⍺ which also binds GDP. Activation of the receptor via a ligand results in the release of GDP from G⍺ which is replaced by GTP. G⍺ then dissociates from the receptor, now activated to begin signal transduction. The Gβ-Gγ complex dissociates as well to begin a separate signalling cascade. G⍺ contacts the receptor through the ⍺5 helix at residue which interacts with L251, L356, and L359 of TM3 the receptor and with bonds to H180 and E247 of TM2 of the receptor. D312 of Gβ is stabilized through with a primary structure intracellular loop and an intracellular helix.

III. GLP-1 Binding

GLP-1R binds the endogenous molecule GLP-1 in the major binding domain. The ligand is most commonly identified as a 30 amino acid single-subunit peptide named GLP-1 (7-37), however, different enzymatic processes lead to alternative forms of the molecule including GLP-1 (1-37) and GLP-1 (7-37)NH2. The enzyme dipeptidyl peptidase IV (DPP4) degrades GLP-1 by cleaving it at the alanine residue at position 8 . This degradation occurs quickly after GLP-1 begins circulating in the bloodstream, causing a half-life of about 5 minutes.

The ligand first contacts the receptor at residues 24 through 33 of the C terminus of the ECD . The N terminus of the peptide then swings into the binding pocket in the TMD of the receptor and the ligand locks into place, signalling the insulin synthesis pathway. When GLP-1R binds a ligand, the ECD of the receptor moves upwards towards the C terminal domain of the ligand and rotates clockwise. This results in an opening of the TMD and access to the binding pocket.

The specific amino acids H7, A8, and E9 are critical for GLP-1 binding to the base of the GLP-1R binding pocket . of GLP-1 binds with bonds and interactions to TM3 and TM5 of the receptor. binds TM7 through a mediated water molecule and polar interactions with E387 and hydrophobically with L388. of GLP-1 binds to the base of the binding pocket with bonds and van der Waals forces to allow for activation and signal transduction.

IV. GLP-1RA Drug Binding

Semaglutide is one of the most commonly used GLP-1RAs in circulation today. Approved for use in 2017, the drug is now present in the prescription medications Ozempic, Wegovy, and Rybelsus. Semaglutide is a 31 amino acid peptide with a 94% structural homology to human GLP-1. The single subunit peptide forms a helical secondary structure at position 8 in semaglutide replaces the alanine at position 8 in GLP-1, which limits DPP4 degradation. In semaglutide there is an at position 34 replacing the lysine present in GLP-1. Additionally, despite the change from a lysine to arginine at position 34, the helical secondary structure terminates at this position and the remaining amino acids remain in primary structure. The same occurs in the GLP-1R-GLP-1 complex. Semaglutide possesses an at position 26 which is attached to a γGlu-2xOEG linker bound to a C18 diacid. Though the crystal structure of the derivatized lysine, γ-linker, and C18 diacid currently is not confidently modeled, comparison to structurally similar receptor-ligand complexes places the derivative complex in a downwards conformation extending out between residues 129 and 137 of the ECD. This complex at position 26 replaces the leucine in GLP-1 and allows the drug to bind to the protein albumin, freely abundant in the bloodstream, increasing the half-life of the molecule to 168 hours.

Semaglutide binds the GLP-1R receptor similarly to GLP-1; The C terminus of the peptide binds the ECD of the receptor which allows the N terminus of the ligand to swing down into the TMD and primary locus of binding of the receptor. Full binding of semaglutide causes a conformation change, an upwards movement and rotation of the ECD, and signals the release of the G proteins and transfer from GDP to GTP. The relative structure of the receptor in its activated form is highly structurally similar to the conformation of GLP-1R in complex with GLP-1 as the homology of the ligands is 94%. The main difference in the semaglutide bound and GLP-1 bound receptors occurs in the dynamism of the agonist bound receptor, resulting in movement of the TMD to accommodate binding. Similar interactions occur between H7, Aib8, and E9 with the TMD in Semaglutide as in residues H7, A8, and E9 in GLP-1.


V. Implications

Primarily GLP-1RAs are prescribed as treatment for T2DM, however, they have shown promise in treatment for T1DM and non-blood sugar related ailments such as rheumatoid arthritis, Alzheimer’s disease, addiction disorders, and some cancers due to its anti-inflammatory properties and activity in the nervous system. Today, GLP-1RAs are more commonly known by the misnomer, GLP-1s and are widely discussed as Ozempic and other GLP-1RAs have risen in cultural significance. After clinic trials displayed the side effect of significant weight loss, demand for GLP-1RAs has increased among the general population. Still a recent phenomenon, many questions on the discourse still remain: who has the right to use GLP-1RAs, are non-prescription, non-FDA approved GLP-1RAs ethical, and what does the desire for weight loss illuminate about modern society?


VI. References

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Kommu, Sharath, and Philip Whitfield. 2024. “Semaglutide.” PubMed. Treasure Island (FL): StatPearls Publishing. February 11, 2024. https://www.ncbi.nlm.nih.gov/books/NBK603723/.

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Wu, Fan, Linlin Yang, Kaini Hang, Mette Laursen, Lijie Wu, Gye Won Han, Qiansheng Ren, et al. 2020. “Full-Length Human GLP-1 Receptor Structure without Orthosteric Ligands.” Nature Communications 11 (1): 1272. https://doi.org/10.1038/s41467-020-14934-5.

Zhang, Xin, Matthew J. Belousoff, Peishen Zhao, Albert J. Kooistra, Tin T. Truong, Sheng Yu Ang, Christina Rye Underwood, et al. 2020. “Differential GLP-1R Binding and Activation by Peptide and Non-Peptide Agonists.” Molecular Cell 80 (3): 485-500.e7. https://doi.org/10.1016/j.molcel.2020.09.020.

Zheng, Zhikai, Yao Zong, Yiyang Ma, Yucheng Tian, Yidan Pang, Changqing Zhang, and Junjie Gao. 2024. “Glucagon-like Peptide-1 Receptor: Mechanisms and Advances in Therapy.” Signal Transduction and Targeted Therapy 9 (1): 1–29. https://doi.org/10.1038/s41392-024-01931-z.

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