Incretin Receptor Pharmacology: GLP-1, GIP and Glucagon Receptors

GLP-1, GIP and glucagon receptors are class B1 G-protein-coupled receptors. They share architecture, they share a primary signalling route, and they are the targets behind mono-, dual- and triple-agonist research compounds. Understanding what distinguishes them clarifies why multi-receptor peptides are designed the way they are.

Class B1 GPCRs

Class B1 receptors have a large extracellular domain that captures the C-terminal portion of a peptide ligand, while the ligand’s N-terminus inserts into the transmembrane bundle and triggers activation. This two-step mechanism explains why these receptors bind peptides rather than small molecules, and why truncating a peptide’s N-terminus usually abolishes activity while leaving binding partly intact.

All three couple primarily to Gαs. Activation raises intracellular cAMP and activates protein kinase A. Cryo-EM structures of agonist-bound receptor–Gs complexes have resolved the binding poses directly; Zhao and colleagues published five such structures across GIPR, GLP-1R and GCGR at 3.0–3.5 Å resolution (Nat Commun, 2022; PMID 35217653).

The three receptors

GLP-1 receptor

Expressed in pancreatic beta cells, the central nervous system and elsewhere. Native GLP-1 is rapidly cleaved by dipeptidyl peptidase-4 at the position-2 alanine, giving a plasma half-life of minutes. Essentially every synthetic analog addresses this — substitution at position 2, fatty-acid acylation for albumin binding, or backbone modification.

GIP receptor

The other incretin receptor, expressed in beta cells and adipose tissue. GIP is also a DPP-4 substrate. GIPR pharmacology is less straightforward than GLP-1R: both agonism and antagonism have been reported to produce overlapping receptor-signalling phenotypes in preclinical models, and the field has not fully resolved why.

Glucagon receptor

Predominantly hepatic. Glucagon opposes insulin in glucose homeostasis, and GCGR activation also increases energy expenditure in rodent models — the rationale for including glucagon agonism in a multi-receptor peptide despite its glycaemic effect running counter to the incretin arms.

Why sequences overlap

GLP-1, GIP and glucagon all derive from the same ancestral peptide family and share substantial sequence homology, particularly at the N-terminus. That homology is what makes multi-receptor agonism achievable: a single engineered sequence can retain enough of each native ligand’s recognition elements to activate more than one receptor.

Balance and bias

A dual or triple agonist is characterised not just by which receptors it activates but by relative potency at each, and by which downstream pathways it favours. Willard and colleagues characterised tirzepatide as an imbalanced and biased dual agonist, measuring competition radioligand binding, cAMP accumulation, GTPγS binding, β-arrestin-1/2 recruitment and receptor internalisation at human GIPR and GLP-1R in HEK293 and CHO-K1 cells at defined low receptor densities (JCI Insight, 2020; PMID 32730231).

Two concepts matter here:

• Imbalance — unequal potency across the targeted receptors. A compound can be near-native at one receptor and substantially weaker at another by design.
• Bias — preferential engagement of one downstream pathway over another at the same receptor, most often cAMP signalling versus β-arrestin recruitment. Reduced β-arrestin recruitment means less receptor internalisation and desensitisation, which changes the response profile under sustained exposure.

Because both properties are tunable, triple agonists are engineered against explicit potency ratios rather than maximised at every receptor. Knerr and colleagues built unimolecular triagonists with empirically tuned in vitro GLP-1R/GIPR/GCGR ratios and measured the resulting phenotypes in diet-induced obese mice (Mol Metab, 2022; PMID 35809773).

What this means for assay design

Characterising a multi-receptor compound requires each receptor tested separately in a defined expression system, at controlled receptor density — potency measurements shift substantially with overexpression — and across more than one downstream readout. A cAMP EC50 alone will not reveal bias.

Receptor architecture in more detail

The two-domain model is worth unpacking, because it governs how these receptors are assayed and why small sequence changes have outsized effects. The extracellular domain (ECD) of a class B1 receptor is a compact fold held together by three conserved disulfide bonds. It presents a hydrophobic groove that accepts the C-terminal half of the peptide ligand, usually in an alpha-helical conformation. Binding here contributes most of the affinity but none of the activation.

Activation comes from the N-terminal residues. Once the C-terminus is anchored, the local concentration of the N-terminus at the mouth of the transmembrane bundle rises sharply, and those first eight or so residues insert into a deep, largely polar pocket formed by transmembrane helices 1, 2, 3, 5, 6 and 7. Insertion breaks a conserved kink in helix 6 (the PxxG motif), the intracellular half of helix 6 swings outward by more than ten angstroms, and a cavity opens for the G-protein alpha-5 helix.

Two consequences follow directly. First, a peptide truncated at the N-terminus can retain most of its binding affinity while losing agonism entirely — which is how many class B1 antagonists were originally derived. Second, the affinity contributed by the ECD is largely independent of the activation contributed by the N-terminus, so medicinal chemistry can tune the two properties semi-independently. That separability is what makes a single sequence able to serve three receptors at deliberately unequal potencies.

Signalling beyond cAMP

Gs coupling and cAMP accumulation are the canonical readout, but they are not the whole picture, and a characterisation that stops there will miss most of what distinguishes one multi-receptor compound from another.

• Gq/11 and calcium. GLP-1R couples to Gq in addition to Gs in several cell backgrounds, producing inositol phosphate accumulation and intracellular calcium mobilisation. The extent of Gq coupling is strongly dependent on receptor density, which is one reason overexpressing systems can report pharmacology that does not reproduce at native expression levels.
• Beta-arrestin recruitment. Arrestin engagement follows GRK phosphorylation of the receptor C-terminal tail and drives desensitisation and internalisation. Compounds that recruit arrestin weakly relative to their cAMP potency keep the receptor at the surface for longer under continuous exposure.
• ERK1/2 phosphorylation. Downstream of both arms, and therefore a poor discriminator on its own, but frequently reported and worth measuring if the aim is to compare against the published record.
• Receptor internalisation and recycling. Measured by surface labelling or imaging. Internalisation rate and the balance between recycling and lysosomal sorting shape the response to sustained rather than acute exposure.

A compound characterised only by a cAMP EC50 has been described in one dimension. Bias is a ratio of ratios — the compound’s cAMP-to-arrestin balance relative to a reference agonist’s — and it cannot be inferred from any single assay.

Half-life engineering and what it does to pharmacology

Native GLP-1 and GIP are both cleaved by dipeptidyl peptidase-4 between residues 2 and 3, giving circulating half-lives measured in minutes. Every long-acting analog in the literature addresses this, and the chosen strategy changes the in vitro profile as well as the in vivo one.

• Position-2 substitution. Replacing the DPP-4-labile alanine, most often with alpha-aminoisobutyric acid (Aib), blocks cleavage. Aib also rigidifies the backbone, which can raise or lower potency depending on the receptor.
• Fatty-acid acylation. A C16 or C18 diacid attached through a linker at a lysine side chain binds serum albumin reversibly, creating a circulating depot and slowing renal clearance. Acylation also reduces apparent in vitro potency in assays containing albumin, because a fraction of the compound is sequestered — a well-known source of discrepancy between published EC50 values measured with and without serum.
• Backbone and helix stabilisation. Lactam bridges, alpha-helix-favouring residues and stapling raise proteolytic stability and can improve receptor selectivity by locking the bound conformation.
• Sequence chimerism. Multi-receptor peptides are built by grafting recognition elements from more than one native ligand onto a single scaffold, then tuning individual positions to move the potency ratio.

For anyone comparing numbers across papers, the practical point is that acylated peptides and unmodified peptides are frequently not measured under comparable conditions. Albumin content, receptor density and incubation time all move the reported EC50, often by more than the difference being discussed.

Methods used to characterise these receptors

The assays below are the ones that recur throughout the incretin literature. Each answers a different question.

• Competition radioligand binding. Displacement of a labelled reference ligand yields Ki. Still the reference method for affinity, though increasingly replaced by fluorescence-based alternatives.
• NanoBRET and TR-FRET binding. Non-radioactive, real-time, and able to report association and dissociation kinetics rather than equilibrium affinity alone. Residence time is a meaningful discriminator for peptides with slow off-rates.
• cAMP accumulation. HTRF, GloSensor or similar. Yields EC50 and Emax. Extremely sensitive to receptor reserve: in an overexpressing line, a partial agonist can look full.
• GTP-gamma-S binding. Measures the proximal G-protein activation step, less subject to amplification than cAMP, and therefore closer to intrinsic efficacy.
• Beta-arrestin recruitment. Enzyme-complementation or BRET formats. Required for any statement about bias.
• Internalisation. Surface ELISA, high-content imaging or BRET-based surface-expression sensors.

Two design choices dominate the quality of the resulting data. The first is receptor density: potency and apparent efficacy both shift with expression level, so comparisons are only meaningful within a single defined system, and cross-paper comparisons require the expression conditions to be stated. The second is the reference agonist. Bias factors are always relative, and a bias factor calculated against native GLP-1 is not comparable to one calculated against exendin-4.

Species differences

Human and rodent receptors are not interchangeable in this family. GIPR in particular shows meaningful species divergence in ligand recognition, and glucagon receptor pharmacology likewise differs enough that compound rankings established in mouse systems do not always reproduce at the human receptor. Preclinical phenotypes in diet-induced obese mice are routinely used to characterise triagonists, but the potency ratios that produced those phenotypes were tuned at human receptors in vitro, and the two sets of numbers should be read as separate lines of evidence rather than a single continuous one.

Antagonism and allosteric modulation

The GIP receptor is the interesting case. Both agonism and antagonism at GIPR have been reported to produce overlapping phenotypes in preclinical metabolic models, and the field has not settled the mechanism. Candidate explanations include profound receptor desensitisation under sustained agonism, functionally equivalent to blockade, and tissue-selective differences in receptor reserve. For anyone designing a study in this area, the implication is that GIPR direction of effect cannot be assumed from the compound class and needs to be measured in the specific system being used.

Small-molecule allosteric modulators exist for several class B1 receptors and bind outside the orthosteric peptide site, usually within the transmembrane bundle. They are useful pharmacological tools because they can change agonist efficacy without competing for the peptide binding site.

Analytical characterisation of a multi-receptor peptide

Receptor pharmacology assumes the material is what the label says. For long, often acylated sequences, that assumption needs support.

• RP-HPLC purity. Area-percent at a stated wavelength. The number is meaningful only alongside the gradient and column, since closely eluting deletion sequences can co-elute under a shallow gradient.
• Mass confirmation. ESI-MS or MALDI-TOF against the calculated monoisotopic or average mass. For acylated peptides the expected mass includes the lipid and linker, which is a common source of arithmetic error.
• Net peptide content. Lyophilised peptide carries counterions and residual water. Gross mass is not peptide mass, and a concentration calculated from vial weight without a net-content figure can be off by a fifth or more — enough to shift a reported EC50 noticeably.
• Aggregation state. Long amphipathic sequences aggregate. SEC or dynamic light scattering will reveal it; a cell assay will simply return a wrong number.

Reconstituted peptides are generally handled cold, aliquoted to avoid repeated freeze–thaw cycles, and protected from light, with stability under the chosen storage conditions treated as something to verify rather than assume. For a fuller treatment of documentation practice see keeping a peptide inventory, and for the analytical side, third-party versus in-house testing.

Reading the published numbers critically

Five questions will resolve most apparent contradictions between papers in this field:

• What was the receptor density, and was it stated?
• Was albumin present in the assay buffer?
• Which reference agonist was used, and was the same one used in the comparison being drawn?
• Was the readout proximal (GTP-gamma-S, arrestin) or amplified (cAMP)?
• Was the receptor human or rodent?

Differences of several fold in reported potency are common and usually trace to one of these rather than to a real disagreement about the compound.

Terms used here

• EC50 — concentration producing half-maximal response in a functional assay. Depends on the system, not only the compound.
• Ki — equilibrium inhibition constant from competition binding; a property of the compound–receptor pair.
• Emax — maximal response achievable, expressed relative to a reference agonist.
• Intrinsic efficacy — the compound’s ability to activate the receptor, separable from potency and obscured by receptor reserve.
• Receptor reserve — spare receptors beyond those needed for a maximal response; high reserve makes partial agonists look full.
• Signalling bias — preferential activation of one transducer over another, quantified relative to a reference agonist.

For definitions of the binding and potency measures in more depth, see Ki, IC50 and EC50 explained. For how sequence modifications change duration of action, see peptide half-life and analog modifications.

ExoLabz supplies GLP-1, GLP-1/GIP and GLP-1/GIP/Glucagon research peptides. Browse Incretin & Glucagon Peptides.

This article covers receptor pharmacology and preclinical literature only. All products referenced are supplied for laboratory research use only and are not approved for human consumption, clinical, or veterinary use.

On retatrutide specifically — the triple-receptor design, its regulatory status in Canada and why the analytical questions for a long peptide differ from those for a short one — see retatrutide in Canada.