Lateral Allosterism in the Glucagon Receptor Family: Glucagon-Like Peptide 1 Induces G-Protein-Coupled Receptor Heteromer Formation

Abstract

Activation of G-protein-coupled receptors (GPCRs) results in a variety of cellular responses, such as binding to the same receptor of different ligands that activate distinct downstream cascades. Additional signaling complexity is achieved when two or more receptors are integrated into one signaling unit. Lateral receptor interactions can allosterically modulate the receptor response to a ligand, which creates a mechanism for tissue-specific fine tuning, depending on the cellular receptor coexpression pattern. GPCR homomers or heteromers have been explored widely for GPCR classes A and C but to lesser extent for class B. In the present study, we used bioluminescence resonance energy transfer (BRET) techniques, calcium flux measurements, and microscopy to study receptor interactions within the glucagon receptor family. We found basal BRET interactions for some of the receptor combinations tested that decreased upon ligand binding. A BRET increase was observed exclusively for the gastric inhibitory peptide (GIP) receptor and the glucagon-like peptide 1 (GLP-1) receptor upon binding of GLP-1 that could be reversed with GIP addition. The interactions of GLP-1 receptor and GIP receptor were characterized with BRET donor saturation studies, shift experiments, and tests of glucagon-like ligands. The heteromer displayed specific pharmacological characteristics with respect to GLP-1-induced ␤-arrestin recruitment and calcium flux, which suggests a form of allosteric regulation between the receptors. This study provides the first example of ligand-induced heteromer formation in GPCR class B. In the body, the receptors are functionally related and coexpressed in the same cells. The physiological evidence for this heteromerization remains to be determined.

Impact of G protein-coupled receptor heteromers in endocrine systems

“…Moreover, the enhanced MC3R activity observed was dependent on the decrease in basal GHSR activity, suggesting positive and negative allosteric regulation between MC3R and GHSR protomers to create functionally asymmetric complexes (26). Functionally, a change in the activity of the GLP-1R was also observed in terms of calcium response, and -arrestin recruitment (27). There has also been recent evidence for functional cross-talk between Gs and Gq-coupled receptors in the regulation of long chain fatty acid mediated incretin secretion.…”

Glucagon-like peptide-1 receptor dimerization differentially regulates agonist signaling but does not affect small molecule allostery

“…For family B receptors, there is consistent evidence for homooligomerization, together with the potential for functionally important heterooligomerization (18)(19)(20)(21)(22)(23)(25)(26)(27). Within this theme, there is an emerging paradigm of behavior in which homodimerization of receptors occurs via a TM4/TM4 interface and that this interaction is required for optimal function of the receptor, generation of high-affinity agonist binding, and signaling via formation of cAMP (17,18,31); this paradigm is true also for the GLP-1R.…”

“…With the current study, a functionally important TM4 dimeric interface has now been demonstrated in the three family B GPCRs that have been studied to date (17,18), and may imply a more general role for allosteric conformational transition across receptor protomers for this TM domain. In this vein, it is interesting to note that heterodimerization of GLP-1 and GIP receptors led to altered GLP-1-induced arrestin recruitment and intracellular calcium mobilization (27), and although the dimer interface for family B GPCR heterodimers has yet to be empirically established, it is possible that allosteric regulation of receptors across TM4 is relevant for signaling even for heteromeric complexes.…”

“…There is also emerging evidence for functionally significant heterodimerization (25)(26)(27). Furthermore, although there is an emerging theme in which dimerization contributes to high-affinity peptide binding and cAMP signaling (17,18), how dimerization contributes more globally to receptor signaling and whether it plays a role in ligand-directed stimulus bias are unknown.…”

GLP-1 Receptor Is Expressed in Human Stomach Mucosa

“…Our findings also suggest that GLP-1R is involved in the direct regulation of acid secretion in the human stomach. It is of interest that GLP-1 can induce heteromerization of its own receptor and gastric inhibitory polypeptide (GIP) receptors to promote a biological action (Schelshorn et al 2012). It is conceivable, therefore, that GLP-1R can act in concert with other established receptors to modulate the complex process of acid secretion.…”