GLP-1 receptor (revision 28)

The GLP-1 receptor (GLP1R) is a class B G-protein-coupled receptor and the molecular target of Glucagon-like peptide-1 and of every drug in the GLP-1 receptor agonist class. It belongs to the secretin-like receptor family, whose members are characterised by a large extracellular domain that captures the C-terminal portion of a peptide ligand before the ligand N-terminus engages the transmembrane bundle.^[1]

Activation couples principally to G_s, raising intracellular cAMP and activating protein kinase A and the exchange protein Epac2. In the pancreatic beta cell this cascade amplifies, but does not initiate, insulin exocytosis: the amplification requires a permissive rise in cytosolic calcium driven by glucose metabolism, which is the molecular basis of the glucose dependence that defines the class.^[2]

The receptor is expressed well beyond the pancreas, and the sites of expression explain much of the clinical profile of its agonists — gastric smooth muscle and enteric neurons for delayed emptying, the area postrema for nausea, and hypothalamic satiety circuits for reduced food intake.^[1]

GLP1R comprises an extracellular domain of about 130 residues, seven transmembrane helices and an intracellular C-terminal tail. Ligand binding follows the two-domain model characteristic of class B receptors: the C-terminal helix of the peptide is captured by the extracellular domain, which positions and concentrates the ligand, after which the peptide N-terminus inserts into a cavity formed by the transmembrane helices and drives the conformational change that couples to G protein.^[1]

Cryo-electron microscopy structures of the agonist-bound, G_s-coupled receptor have resolved the sharp kink in transmembrane helix 6 that accompanies activation. The structures also explain why the first eight residues of the peptide are indispensable while the C-terminal half tolerates extensive modification: acylation, PEGylation and fusion partners are all attached distal to the pharmacophore.^[3]

Small-molecule agonists bind a partially overlapping but distinct pocket, closer to the extracellular face of the transmembrane bundle, and do not require the extracellular-domain capture step. This is why an orally absorbable non-peptide agonist is chemically possible at all.^[1]

Beyond G_s, the activated receptor recruits β-arrestin, which terminates G-protein signalling and initiates internalisation. Agonists differ in the ratio of cAMP accumulation to β-arrestin recruitment they produce, a phenomenon described as biased agonism.^[4]

Bias has been proposed as a design lever: an agonist that produces sustained cAMP with reduced β-arrestin recruitment internalises the receptor less, maintains surface expression, and might therefore sustain insulin secretion better over prolonged exposure. Evidence for this in humans is indirect, and the in vitro bias factors reported for marketed agonists have not been shown to predict clinical differences.^[4]

Desensitisation in vivo is limited. Tachyphylaxis to the insulinotropic effect is not observed clinically at therapeutic exposures, whereas tachyphylaxis to the gastric-emptying effect is well documented and is one reason the delay in emptying attenuates over months of treatment while glycaemic effect persists.^[2]

Receptor expression has been mapped by radioligand binding, transcriptomics and, more recently, by validated monoclonal antibodies after early immunohistochemical work proved unreliable.

The area postrema lies outside the blood-brain barrier, which is why a peptide too large to cross it can nonetheless produce centrally mediated nausea. Access to the arcuate nucleus is partly via fenestrated capillaries at the median eminence and partly via vagal afferents.^[1]

The modest resting heart-rate increase of two to four beats per minute reported across the class is attributed to direct action at the sinoatrial node. It has not been associated with adverse outcomes in the completed outcome trials.^[2]

Common coding variants of GLP1R have been associated in genome-wide studies with small differences in fasting glucose and in insulin response to oral glucose. The effect sizes are small relative to the pharmacological effect of an agonist, and no variant has been shown to be useful for predicting response to treatment.^[3]

Rare loss-of-function variants have been described but are not associated with a recognised clinical syndrome, consistent with substantial redundancy in the incretin axis. Whether variation in GLP1R contributes to the wide interindividual spread in weight response to agonists is unresolved; the published attempts to associate genotype with response have been small and have not replicated consistently.^[4]

• Glucagon-like peptide-1
• GLP-1 receptor agonist
• Receptor bias
• Insulin secretion
• Arcuate nucleus

• GLP1R — Gene entry — NCBI Gene record for GLP1R.