Three Receptors, Three Signalling Routes

The GLP-1, GIP and glucagon receptors are all class B1 G-protein-coupled receptors. They share an architecture, they share a primary signaling route, and between them they account for the mono-, dual- and triple-agonist compounds that appear in the research literature. What separates them is what explains why multi-receptor peptides are engineered the way they are.

How a class B1 receptor works

These receptors operate in two stages. A large extracellular domain captures the C-terminal portion of a peptide ligand; the ligand’s N-terminus then inserts into the transmembrane bundle and triggers activation. That arrangement is why the family binds peptides rather than small molecules, and why truncating a peptide’s N-terminus usually destroys activity while leaving a good deal of binding intact.

The detail is worth unpacking, because it governs how these receptors get assayed and why small sequence changes have outsized consequences. The extracellular domain is a compact fold held by three conserved disulfide bonds, presenting a hydrophobic groove that accepts the C-terminal half of the ligand, typically as an alpha helix. Affinity comes mostly from here; activation comes from none of it.

Activation is the job of 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 roughly the first eight residues drop into a deep, largely polar pocket lined by transmembrane helices 1, 2, 3, 5, 6 and 7. That insertion breaks a conserved kink in helix 6 at the PxxG motif, the intracellular half of helix 6 swings outward by more than ten angstroms, and a cavity opens up for the G-protein alpha-5 helix.

Two things follow immediately. An N-terminally truncated peptide can hold on to most of its affinity while losing agonism completely, which is how many class B1 antagonists were originally derived. And because ECD-derived affinity is largely independent of N-terminus-derived activation, medicinal chemistry can tune the two semi-independently – which is exactly what allows one sequence to serve three receptors at deliberately unequal potencies.

All three couple primarily to Gαs, so activation raises intracellular cAMP and activates protein kinase A. Binding poses have been resolved directly by cryo-EM of agonist-bound receptor-Gs complexes; 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, and why their ligands resemble each other

GLP-1 receptor

Found on pancreatic beta cells, in the central nervous system and elsewhere. Native GLP-1 is cleaved quickly by dipeptidyl peptidase-4 at the position-2 alanine, leaving a plasma half-life of minutes. Practically every synthetic analog addresses that somehow: substitution at position 2, fatty-acid acylation for albumin binding, or backbone modification.

GIP receptor

The second incretin receptor, expressed on beta cells and in adipose tissue, and also a DPP-4 substrate. Its pharmacology is less tidy than GLP-1R: agonism and antagonism have both been reported to produce overlapping receptor-signaling phenotypes in preclinical models, and the field has yet to explain why.

Glucagon receptor

Mainly hepatic. Glucagon opposes insulin in glucose homeostasis, and GCGR activation additionally raises energy expenditure in rodent models – the stated rationale for building glucagon agonism into a multi-receptor peptide even though its glycemic effect pulls against the incretin arms.

Shared ancestry

GLP-1, GIP and glucagon all descend from one ancestral peptide family and retain substantial sequence homology, concentrated at the N-terminus. That homology is precisely what makes multi-receptor agonism feasible, since a single engineered sequence can carry enough of each native ligand’s recognition elements to activate more than one receptor.

Imbalance and bias

A dual or triple agonist is defined not only by which receptors it hits but by its relative potency at each of them, and by which downstream pathways it favors. Willard and colleagues examined an imbalanced and biased dual agonist on exactly those terms, measuring competition radioligand binding, cAMP accumulation, GTPγS binding, β-arrestin-1/2 recruitment and receptor internalization at human GIPR and GLP-1R in HEK293 and CHO-K1 cells held at defined low receptor densities (JCI Insight, 2020; PMID 32730231).

Two ideas carry the weight:

• Imbalance – potency that is unequal across the targeted receptors. A compound can sit near native potency at one receptor and well below it at another, by design rather than by accident.
• Bias – preferential engagement of one downstream pathway over another at the same receptor, most often cAMP signaling against β-arrestin recruitment. Weaker arrestin recruitment means less internalization and less desensitization, which reshapes the response under sustained exposure.

Since both properties can be dialed in, triple agonists are engineered toward explicit potency ratios rather than pushed to maximum 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 lies past cAMP

Gs coupling and cAMP accumulation are the canonical readout, but a characterization that stops at cAMP misses most of what separates one multi-receptor compound from another.

• Gq/11 and calcium. In several cell backgrounds GLP-1R couples to Gq alongside Gs, generating inositol phosphate accumulation and intracellular calcium mobilization. How much Gq coupling appears depends heavily on receptor density, which is one reason overexpressing systems can report pharmacology that fails to reproduce at native expression levels.
• Beta-arrestin recruitment, which follows GRK phosphorylation of the receptor C-terminal tail and drives desensitization and internalization. A compound recruiting arrestin weakly relative to its cAMP potency leaves the receptor at the surface longer under continuous exposure.
• ERK1/2 phosphorylation, downstream of both arms and therefore a poor discriminator in isolation – but widely reported, and worth including when the goal is comparison against the published record.
• Internalization and recycling, measured by surface labeling or imaging. Internalization rate, and the split between recycling and lysosomal sorting, shape the response to sustained exposure rather than acute exposure.

A compound described by a cAMP EC50 alone has been described in one dimension. Bias is a ratio of ratios – the compound’s cAMP-to-arrestin balance measured against a reference agonist’s – and no single assay can produce it.

Extending half-life, and what that does to the numbers

DPP-4 cleaves native GLP-1 and GIP alike between residues 2 and 3, leaving circulating half-lives of minutes. Every long-acting analog in the literature confronts this, and the route chosen alters the in vitro profile as much as the in vivo one.

• Position-2 substitution. Swapping the DPP-4-labile alanine, usually for alpha-aminoisobutyric acid (Aib), blocks the cleavage. Aib also stiffens the backbone, which can push potency up or down depending on the receptor.
• Fatty-acid acylation. A C16 or C18 diacid hung off a lysine side chain through a linker binds serum albumin reversibly, creating a circulating depot and slowing renal clearance. It also lowers apparent in vitro potency in albumin-containing assays, because part of the compound is sequestered – a well-known source of disagreement between EC50 values measured with and without serum.
• Backbone and helix stabilization. Lactam bridges, helix-favoring residues and stapling all raise proteolytic stability, and can improve selectivity by locking in the bound conformation.
• Sequence chimerism. Multi-receptor peptides are assembled by grafting recognition elements from more than one native ligand onto a single scaffold, then adjusting individual positions to shift the potency ratio.

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

The assay toolkit

Characterizing a multi-receptor compound means testing each receptor separately, in a defined expression system, at controlled receptor density, and across more than one downstream readout. The assays below recur throughout this literature, and each answers a different question.

• Competition radioligand binding. Displacement of a labeled reference ligand gives Ki. Still the reference method for affinity, though fluorescence-based alternatives are steadily displacing it.
• NanoBRET and TR-FRET binding. Non-radioactive, real-time, and capable of reporting association and dissociation kinetics rather than equilibrium affinity alone. Residence time is a meaningful discriminator among peptides with slow off-rates.
• cAMP accumulation by HTRF, GloSensor or equivalent, yielding EC50 and Emax. Acutely sensitive to receptor reserve: in an overexpressing line a partial agonist can pass for a full one.
• GTP-gamma-S binding, which reports the proximal G-protein activation step. Less amplified than cAMP and therefore closer to intrinsic efficacy.
• Beta-arrestin recruitment in enzyme-complementation or BRET formats. Mandatory before any statement about bias.
• Internalization by surface ELISA, high-content imaging or BRET-based surface-expression sensors.

Two choices dominate data quality. Receptor density comes first: both potency and apparent efficacy shift with expression level, so comparisons hold only within one defined system, and cross-paper comparison demands that expression conditions be stated. The reference agonist comes second. Bias factors are inherently relative, and one calculated against native GLP-1 does not compare with one calculated against exendin-4.

Species are not interchangeable here

Human and rodent receptors diverge meaningfully in this family. GIPR in particular differs in ligand recognition across species, and glucagon receptor pharmacology diverges enough that compound rankings established in mouse systems do not reliably reproduce at the human receptor. Preclinical phenotypes in diet-induced obese mice are a routine part of triagonist characterization, but the potency ratios that generated those phenotypes were tuned at human receptors in vitro, and the two sets of numbers are best read as separate lines of evidence rather than one continuous chain.

The GIPR direction-of-effect problem

GIPR is the genuinely puzzling case. Agonism and antagonism at this receptor have both been reported to yield overlapping phenotypes in preclinical metabolic models, and the mechanism remains unsettled. Candidate explanations on offer include profound receptor desensitization under sustained agonism, functionally equivalent to blockade, and tissue-selective differences in receptor reserve. The design implication is blunt: direction of effect at GIPR cannot be assumed from the compound class and has to be measured in the particular system in use.

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

Verifying the material itself

All of this pharmacology assumes the vial contains what the label claims. For long, frequently acylated sequences that assumption needs support.

• RP-HPLC purity as area-percent at a stated wavelength. The figure only means something next to the gradient and column, since closely eluting deletion sequences will hide under a shallow gradient.
• Mass confirmation by ESI-MS or MALDI-TOF against the calculated monoisotopic or average mass. For acylated peptides the expected mass has to include lipid and linker, a common arithmetic trap.
• Net peptide content. Lyophilized peptide carries counterions and residual water, so gross mass is not peptide mass, and a concentration derived from vial weight without a net-content figure can be off by a fifth or more – enough to move a reported EC50 visibly.
• Aggregation state. Long amphipathic sequences aggregate. SEC or dynamic light scattering will expose it; a cell assay will simply hand back a wrong number.

Reconstituted peptides are generally kept cold, aliquoted so nothing is thawed twice, and shielded from light, with stability under the chosen storage conditions treated as something to verify rather than take on trust. Documentation practice is covered at length in keeping a peptide inventory, and the analytical side in third-party versus in-house testing.

Five questions that resolve most contradictions

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

Several-fold differences in reported potency are routine in this field, and they usually trace back to one of the above rather than to any real disagreement about the compound.

Abbreviations used above

• EC50 – the concentration producing a half-maximal response in a functional assay; a property of the system as much as the compound.
• Ki – equilibrium inhibition constant from competition binding; a property of the compound-receptor pair.
• Emax – the maximal achievable response, expressed relative to a reference agonist.
• Intrinsic efficacy – the compound’s capacity 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.
• Signaling bias – preferential activation of one transducer over another, quantified against a reference agonist.

The binding and potency measures are treated in more depth in Ki, IC50 and EC50 explained, and the effect of sequence modification on duration of action in peptide half-life and analog modifications.

Research compounds in this area are listed under Incretin & Glucagon Peptides.

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