Structural insight into antibody-mediated antagonism of the Glucagon-like peptide-1 Receptor

Binding sites and design strategies for small molecule GLP-1R agonists

Glucagon-like peptide-1 receptor (GLP-1R) is a pivotal receptor involved in blood glucose regulation and influencing feeding behavior. It has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. Peptide GLP-1 receptor agonists (GLP-1RAs) have achieved tremendous success in the market, driving the vigorous development of small molecule GLP-1RAs. Currently, several small molecules have entered the clinical research stage. Additionally, recent discoveries of GLP-1R positive allosteric modulators (PAMs) are also unveiling new regulatory patterns and treatment methods. This article reviews the structure and functional mechanisms of GLP-1R, recent reports on small molecule GLP-1RAs and PAMs, as well as the optimization process. Furthermore, it combines computer simulations to analyze structure-activity relationships (SAR) studies, providing a foundation for exploring new strategies for designing small molecule GLP-1RAs.

Antibodies expand the scope of angiotensin receptor pharmacology

G-protein-coupled receptors (GPCRs) are key regulators of human physiology and are the targets of many small-molecule research compounds and therapeutic drugs. While most of these ligands bind to their target GPCR with high affinity, selectivity is often limited at the receptor, tissue and cellular levels. Antibodies have the potential to address these limitations but their properties as GPCR ligands remain poorly characterized. Here, using protein engineering, pharmacological assays and structural studies, we develop maternally selective heavy-chain-only antibody ('nanobody') antagonists against the angiotensin II type I receptor and uncover the unusual molecular basis of their receptor antagonism. We further show that our nanobodies can simultaneously bind to angiotensin II type I receptor with specific small-molecule antagonists and demonstrate that ligand selectivity can be readily tuned. Our work illustrates that antibody fragments can exhibit rich and evolvable pharmacology, attesting to their potential as next-generation GPCR modulators.

Antibodies Expand the Scope of Angiotensin Receptor Pharmacology

G protein-coupled receptors (GPCRs) are key regulators of human physiology and are the targets of many small molecule research compounds and therapeutic drugs. While most of these ligands bind to their target GPCR with high affinity, selectivity is often limited at the receptor, tissue, and cellular level. Antibodies have the potential to address these limitations but their properties as GPCR ligands remain poorly characterized. Here, using protein engineering, pharmacological assays, and structural studies, we develop maternally selective heavy chain-only antibody ("nanobody") antagonists against the angiotensin II type I receptor (AT1R) and uncover the unusual molecular basis of their receptor antagonism. We further show that our nanobodies can simultaneously bind to AT1R with specific small-molecule antagonists and demonstrate that ligand selectivity can be readily tuned. Our work illustrates that antibody fragments can exhibit rich and evolvable pharmacology, attesting to their potential as next-generation GPCR modulators.

Rapid One-Step Capturing of Native, Cell-Free Synthesized and Membrane-Embedded GLP-1R

G protein-coupled receptors (GPCRs) are of outstanding pharmacological interest as they are abundant in cell membranes where they perform diverse functions that are closely related to the vitality of cells. The analysis of GPCRs in natural membranes is laborious, as established methods are almost exclusively cell culture-based and only a few methods for immobilization in a natural membrane outside the cell are known. Within this study, we present a one-step, fast and robust immobilization strategy of the GPCR glucagon-like peptide 1 receptor (GLP-1R). GLP-1R was synthesized in eukaryotic lysates harboring endogenous endoplasmic reticulum-derived microsomes enabling the embedment of GLP-1R in a natural membrane. Interestingly, we found that these microsomes spontaneously adsorbed to magnetic Neutravidin beads thus providing immobilized membrane protein preparations which required no additional manipulation of the target receptor or its supporting membrane. The accessibility of the extracellular domain of membrane-embedded and bead-immobilized GLP-1R was demonstrated by bead-based enzyme-linked immunosorbent assay (ELISA) using GLP-1R-specific monoclonal antibodies. In addition, ligand binding of immobilized GLP-1R was verified in a radioligand binding assay. In summary, we present an easy and straightforward synthesis and immobilization methodology of an active GPCR which can be beneficial for studying membrane proteins in general.

A Small-Molecule Oral Agonist of the Human Glucagon-like Peptide-1 Receptor

Peptide agonists of the glucagon-like peptide-1 receptor (GLP-1R) have revolutionized diabetes therapy, but their use has been limited because they require injection. Herein, we describe the discovery of the orally bioavailable, small-molecule, GLP-1R agonist PF-06882961 (danuglipron). A sensitized high-throughput screen was used to identify 5-fluoropyrimidine-based GLP-1R agonists that were optimized to promote endogenous GLP-1R signaling with nanomolar potency. Incorporation of a carboxylic acid moiety provided considerable GLP-1R potency gains with improved off-target pharmacology and reduced metabolic clearance, ultimately resulting in the identification of danuglipron. Danuglipron increased insulin levels in primates but not rodents, which was explained by receptor mutagensis studies and a cryogenic electron microscope structure that revealed a binding pocket requiring a primate-specific tryptophan 33 residue. Oral administration of danuglipron to healthy humans produced dose-proportional increases in systemic exposure (NCT03309241). This opens an opportunity for oral small-molecule therapies that target the well-validated GLP-1R for metabolic health.

MiR-27a-3p Targets GLP1R to Regulate Differentiation, Autophagy, and Release of Inflammatory Factors in Pre-Osteoblasts via the AMPK Signaling Pathway

Objective: Osteoporosis is caused by the dysregulation of bone homeostasis which is synergistically mediated by osteoclasts and osteoblasts. MiR-27a-3p is a key inhibitor of bone formation. Hence, unearthing the downstream target gene of miR-27a-3p is of great significance to understand the molecular mechanism of osteoporosis. Methods: Bioinformatics analysis was utilized to find the downstream target gene of miR-27a-3p, and dual-luciferase reporter assay was conducted to validate the interplay of miR-27a-3p and GLP1R. Besides, qRT-PCR, Western blot, and enzyme-linked immunosorbent assay (ELISA) were employed to verify the impact of miR-27a-3p on GLP1R expression and the differentiation, autophagy, and inflammatory response of MC3T3-E1 pre-osteoblasts. Results: Dual-luciferase assay validated that miR-27a-3p directly targeted GLP1R. Additionally, posttreatment of MC3T3-E1 cells with miR-27a-3p mimics resulted in a remarkable decrease in expression levels of GLP1R, cell differentiation marker gene, autophagy marker gene, and AMPK. These results indicated that miR-27a-3p targeted GLP1R to inhibit AMPK signal activation and pre-osteoblast differentiation and autophagy, while promoting the release of inflammatory factors. Conclusion: The miR-27a-3p/GLP1R regulatory axis in pre-osteoblasts contributes to understanding the molecular mechanism of osteoporosis.

Scap structures highlight key role for rotation of intertwined luminal loops in cholesterol sensing

The cholesterol-sensing protein Scap induces cholesterol synthesis by transporting membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum (ER) to the Golgi apparatus for proteolytic activation. Transport requires interaction between Scap's two ER luminal loops (L1 and L7), which flank an intramembrane sterol-sensing domain (SSD). Cholesterol inhibits Scap transport by binding to L1, which triggers Scap's binding to Insig, an ER retention protein. Here we used cryoelectron microscopy (cryo-EM) to elucidate two structures of full-length chicken Scap: (1) a wild-type free of Insigs and (2) mutant Scap bound to chicken Insig without cholesterol. Strikingly, L1 and L7 intertwine tightly to form a globular domain that acts as a luminal platform connecting the SSD to the rest of Scap. In the presence of Insig, this platform undergoes a large rotation accompanied by rearrangement of Scap's transmembrane helices. We postulate that this conformational change halts Scap transport of SREBPs and inhibits cholesterol synthesis.

Rapid identification of highly potent human anti-GPCR antagonist monoclonal antibodies

Complex cellular targets such as G protein-coupled receptors (GPCRs), ion channels, and other multi-transmembrane proteins represent a significant challenge for therapeutic antibody discovery, primarily because of poor stability of the target protein upon extraction from cell membranes. To assess whether a limited set of membrane-bound antigen formats could be exploited to identify functional antibodies directed against such targets, we selected a GPCR of therapeutic relevance (CCR1) and identified target binders using an in vitro yeast-based antibody discovery platform (Adimab^TM) to expedite hit identification. Initially, we compared two different biotinylated antigen formats overexpressing human CCR1 in a ‘scouting’ approach using a subset of the antibody library. Binders were isolated using streptavidin-coated beads, expressed as yeast supernatants, and screened using a high-throughput binding assay and flow cytometry on appropriate cell lines. The most suitable antigen was then selected to isolate target binders using the full library diversity. This approach identified a combined total of 183 mAbs with diverse heavy chain sequences. A subset of clones exhibited high potencies in primary cell chemotaxis assays, with IC₅₀ values in the low nM/high pM range. To assess the feasibility of any further affinity enhancement, full-length hCCR1 protein was purified, complementary-determining region diversified libraries were constructed from a high and lower affinity mAb, and improved binders were isolated by fluorescence-activated cell sorting selections. A significant affinity enhancement was observed for the lower affinity parental mAb, but not the high affinity mAb. These data exemplify a methodology to generate potent human mAbs for challenging targets rapidly using whole cells as antigen and define a route to the identification of affinity-matured variants if required.

The Interplay of Glucagon-Like Peptide-1 Receptor Trafficking and Signalling in Pancreatic Beta Cells

The glucagon-like peptide 1 receptor (GLP-1R) is a class B G protein-coupled receptor (GPCR) which mediates the effects of GLP-1, an incretin hormone secreted primarily from L-cells in the intestine and within the central nervous system. The GLP-1R, upon activation, exerts several metabolic effects including the release of insulin and suppression of appetite, and has, accordingly, become an important target for the treatment for type 2 diabetes (T2D). Recently, there has been heightened interest in how the activated GLP-1R is trafficked between different endomembrane compartments, controlling the spatial origin and duration of intracellular signals. The discovery of "biased" GLP-1R agonists that show altered trafficking profiles and selective engagement with different intracellular effectors has added to the tools available to study the mechanisms and physiological importance of these processes. In this review we survey early and recent work that has shed light on the interplay between GLP-1R signalling and trafficking, and how it might be therapeutically tractable for T2D and related diseases.

Molecular mechanism of an antagonistic antibody against glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone involved in regulating glucose and lipid metabolism. GIP receptor (GIPR) antagonism is believed to offer therapeutic potential for various metabolic diseases. Pharmacological intervention of GIPR, however, has limited success due to lack of effective antagonistic reagents. Previously we reported the discovery of two mouse anti-murine GIPR monoclonal antibodies (mAbs) with distinctive properties in rodent models. Here, we report the detailed structural and biochemical characterization of these two antibodies, mAb1 and mAb2. In vitro and in vivo characterizations demonstrated mAb2 is a full GIPR antagonistic antibody and mAb1 is a non-neutralizing GIPR binder. To understand the molecular basis of these two antibodies, we determined the co-crystal structures of GIPR extracellular domain in complex with mAb1 and with mAb2 at resolutions of 2.1 and 2.6 Å, respectively. While the non-neutralizing mAb1 binds to GIPR without competing with the ligand peptide, mAb2 not only partially occludes the ligand peptide binding, but also recognizes the GIPR C-terminal stalk region in a helical conformation that acts as a molecular mimic of the ligand peptide and locks GIPR in a novel auto-inhibited state. Furthermore, administration of mAb2 in diet-induced obesity mice for 7 weeks leads to both reduction in body weight gain and improvement of metabolic profiles. In contrast, mAb1 has no effect on body weight or other metabolic improvement. Together, our studies reveal the unique molecular mechanism of action underlying the superior antagonistic activity of mAb2 and signify the promising therapeutic potential of effective GIPR antagonism for the treatment of metabolic disorders.

Peptidyl and Non-Peptidyl Oral Glucagon-Like Peptide-1 Receptor Agonists

Glucagon-like peptide-1 (GLP-1) receptor agonists are efficacious glucose-lowering medications with salient benefits for body weight and cardiovascular events. This class of medications is now recommended as the top priority for patients with established cardiovascular disease or indicators of high risk. Until the advent of oral semaglutide, however, GLP-1 receptor agonists were available only in the form of subcutaneous injections. Aversion to needles, discomfort with self-injection, or skin problems at the injection site are commonly voiced problems in people with diabetes, and thus, attempts for non-invasive delivery strategies have continued. Herein, we review the evolution of GLP-1 therapy from its discovery and the development of currently approved drugs to the unprecedented endeavor to administer GLP-1 receptor agonists via the oral route. We focus on the pharmacokinetic and pharmacodynamic properties of the recently approved oral GLP-1 receptor agonist, oral semaglutide. Small molecule oral GLP-1 receptor agonists are currently in development, and we introduce how these chemicals have addressed the challenge posed by interactions with the large extracellular ligand binding domain of the GLP-1 receptor. We specifically discuss the structure and pharmacological properties of TT-OAD2, LY3502970, and PF-06882961, and envision an era where more patients could benefit from oral GLP-1 receptor agonist therapy.

Ligand-Specific Factors Influencing GLP-1 Receptor Post-Endocytic Trafficking and Degradation in Pancreatic Beta Cells

The glucagon-like peptide-1 receptor (GLP-1R) is an important regulator of blood glucose homeostasis. Ligand-specific differences in membrane trafficking of the GLP-1R influence its signalling properties and therapeutic potential in type 2 diabetes. Here, we have evaluated how different factors combine to control the post-endocytic trafficking of GLP-1R to recycling versus degradative pathways. Experiments were performed in primary islet cells, INS-1 832/3 clonal beta cells and HEK293 cells, using biorthogonal labelling of GLP-1R to determine its localisation and degradation after treatment with GLP-1, exendin-4 and several further GLP-1R agonist peptides. We also characterised the effect of a rare GLP1R coding variant, T149M, and the role of endosomal peptidase endothelin-converting enzyme-1 (ECE-1), in GLP1R trafficking. Our data reveal how treatment with GLP-1 versus exendin-4 is associated with preferential GLP-1R targeting towards a recycling pathway. GLP-1, but not exendin-4, is a substrate for ECE-1, and the resultant propensity to intra-endosomal degradation, in conjunction with differences in binding affinity, contributes to alterations in GLP-1R trafficking behaviours and degradation. The T149M GLP-1R variant shows reduced signalling and internalisation responses, which is likely to be due to disruption of the cytoplasmic region that couples to intracellular effectors. These observations provide insights into how ligand- and genotype-specific factors can influence GLP-1R trafficking.

Synthetic nanobodies as angiotensin receptor blockers

There is considerable interest in developing antibodies as functional modulators of G protein-coupled receptor (GPCR) signaling for both therapeutic and research applications. However, there are few antibody ligands targeting GPCRs outside of the chemokine receptor group. GPCRs are challenging targets for conventional antibody discovery methods, as many are highly conserved across species, are biochemically unstable upon purification, and possess deeply buried ligand-binding sites. Here, we describe a selection methodology to enrich for functionally modulatory antibodies using a yeast-displayed library of synthetic camelid antibody fragments called "nanobodies." Using this platform, we discovered multiple nanobodies that act as antagonists of the angiotensin II type 1 receptor (AT1R). Following angiotensin II infusion in mice, we found that an affinity matured nanobody antagonist has comparable antihypertensive activity to the angiotensin receptor blocker (ARB) losartan. The unique pharmacology and restricted biodistribution of nanobody antagonists may provide a path for treating hypertensive disorders when small-molecule drugs targeting the AT1R are contraindicated, for example, in pregnancy.

Exendin-4 restores airway mucus homeostasis through the GLP1R-PKA-PPARγ-FOXA2-phosphatase signaling

Goblet cell hyperplasia and metaplasia and excessive mucus are prominent pathologies of chronic airway diseases such as chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), and chronic bronchitis. Chronic infection by respiratory pathogens, including Pseudomonas aeruginosa, exacerbates cyclical proinflammatory responses and mucus hypersecretion. P. aeruginosa and its virulence factor pyocyanin contribute to these pathologies by inhibiting FOXA2, a key transcriptional regulator of mucus homeostasis, through activation of antagonistic signaling pathways EGFR-AKT/ERK1/2 and IL-4/IL-13-STAT6-SPDEF. However, FOXA2-targeted therapy has not been previously explored. Here, we examined the feasibility of repurposing the incretin mimetic Exendin-4 to restore FOXA2-mediated airway mucus homeostasis. We have found that Exendin-4 restored FOXA2 expression, attenuated mucin production in COPD and CF-diseased airway cells, and reduced mucin and P. aeruginosa burden in mouse lungs. Mechanistically, Exendin-4 activated the GLP1R-PKA-PPAR-γ-dependent phosphatases PTEN and PTP1B, which inhibited key kinases within both EGFR and STAT6 signaling cascades. Our results may lead to the repurposing of Exendin-4 and other incretin mimetics to restore FOXA2 function and ultimately regulate excessive mucus in diseased airways.

Structure-guided discovery of a single-domain antibody agonist against human apelin receptor

Developing antibody agonists targeting the human apelin receptor (APJ) is a promising therapeutic approach for the treatment of chronic heart failure. Here, we report the structure-guided discovery of a single-domain antibody (sdAb) agonist JN241-9, based on the cocrystal structure of APJ with an sdAb antagonist JN241, the first cocrystal structure of a class A G protein-coupled receptor (GPCR) with a functional antibody. As revealed by the structure, JN241 binds to the extracellular side of APJ, makes critical contacts with the second extracellular loop, and inserts the CDR3 into the ligand-binding pocket. We converted JN241 into a full agonist JN241-9 by inserting a tyrosine into the CDR3. Modeling and molecular dynamics simulation shed light on JN241-9-stimulated receptor activation, providing structural insights for finding agonistic antibodies against class A GPCRs.

Structural Basis for Allosteric Modulation of Class B G Protein-Coupled Receptors

Recent advances in our understanding of the structure and function of class B G protein-coupled receptors (GPCRs) provide multiple opportunities for targeted development of allosteric modulators. Given the pleiotropic signaling patterns emanating from these receptors in response to a variety of natural agonist ligands, modulators have the potential to sculpt the responses to meet distinct needs of different groups of patients. In this review, we provide insights into how this family of GPCRs differs from the rest of the superfamily, how orthosteric agonists bind and activate these receptors, the potential for allosteric modulators to interact with various regions of these targets, and the allosteric influence of endogenous proteins on the pharmacology of these receptors, all of which are important considerations when developing new therapies.

Therapeutic Monoclonal Antibodies to Complex Membrane Protein Targets: Antigen Generation and Antibody Discovery Strategies

Cell surface membrane proteins comprise a wide array of structurally and functionally diverse proteins involved in a variety of important physiological and homeostatic processes. Complex integral membrane proteins, which are embedded in the lipid bilayer by multiple transmembrane-spanning helices, are represented by families of proteins that are important target classes for drug discovery. Such protein families include G-protein-coupled receptors, ion channels and transporters. Although these targets have typically been the domain of small-molecule drugs, the exquisite specificity of monoclonal antibodies offers a significant opportunity to selectively modulate these target proteins. Nevertheless, the isolation of antibodies with desired pharmacological functions has proved difficult because of technical challenges in preparing membrane protein antigens for antibody drug discovery. In this review, we describe recent progress in defining strategies for the generation of membrane protein antigens. We also describe antibody-isolation strategies that identify antibodies that bind the membrane protein and modulate protein function.

α-Helix or β-Turn? An Investigation into N-Terminally Constrained Analogues of Glucagon-like Peptide 1 (GLP-1) and Exendin-4

Peptide agonists acting on the glucagon-like peptide 1 receptor (GLP-1R) promote glucose-dependent insulin release and therefore represent important therapeutic agents for type 2 diabetes (T2D). Previous data indicated that an N-terminal type II β-turn motif might be an important feature for agonists acting on the GLP-1R. In contrast, recent publications reporting the structure of the full-length GLP-1R have shown the N-terminus of receptor-bound agonists in an α-helical conformation. To reconcile these conflicting results, we prepared N-terminally constrained analogues of glucagon-like peptide 1 (GLP-1) and exendin-4 and evaluated their receptor affinity and functionality in vitro; we then examined their crystal structures in complex with the extracellular domain of the GLP-1R and used molecular modeling and molecular dynamics simulations for further investigations. We report that the peptides' N-termini in all determined crystal structures adopted a type II β-turn conformation, but in vitro potency varied several thousand-fold across the series. Potency correlated better with α-helicity in our computational model, although we have found that the energy barrier between the two mentioned conformations is low in our most potent analogues and the flexibility of the N-terminus is highlighted by the dynamics simulations.

Peptide recognition, signaling and modulation of class B G protein-coupled receptors

Class B G protein-coupled receptors (GPCRs) are important drug targets in many human diseases, including type 2 diabetes, obesity, cardiovascular disease and neurodegeneration. Peptide hormones bind to these receptors through interactions with both the extracellular domain and transmembrane domain. Despite remarkable advances in structural studies of GPCRs, structural characterization of the full-length class B receptors remains extremely challenging due to their conformational complexity. The recently solved structures of class B GPCRs reveal the structural basis of peptide ligand recognition and modulation mechanisms of small molecule allosteric modulators. Furthermore, these structures provide essential insights into molecular mechanisms of class B GPCR signal transduction and modulation.

Development and characterisation of a novel glucagon like peptide-1 receptor antibody

AIMS/HYPOTHESIS

Glucagon like peptide-1 (GLP-1) enhances glucose-dependent insulin secretion by binding to GLP-1 receptors (GLP1Rs) on pancreatic beta cells. GLP-1 mimetics are used in the clinic for the treatment of type 2 diabetes, but despite their therapeutic success, several clinical effects of GLP-1 remain unexplained at a mechanistic level, particularly in extrapancreatic tissues. The aim of this study was to generate and characterise a monoclonal antagonistic antibody for the GLP1R for use in vivo.

METHODS

A naive phage display selection strategy was used to isolate single-chain variable fragments (ScFvs) that bound to GLP1R. The ScFv with the highest affinity, Glp1R0017, was converted into a human IgG1 and characterised further. In vitro antagonistic activity was assessed in a number of assays: a cAMP-based homogenous time-resolved fluorescence assay in GLP1R-overexpressing cell lines, a live cell cAMP imaging assay and an insulin secretion assay in INS-1 832/3 cells. Glp1R0017 was further tested in immunostaining of mouse pancreas, and the ability of Glp1R0017 to block GLP1R in vivo was assessed by both IPGTT and OGTT in C57/Bl6 mice.

RESULTS

Antibodies to GLP1R were selected from naive antibody phage display libraries. The monoclonal antibody Glp1R0017 antagonised mouse, human, rat, cynomolgus monkey and dog GLP1R. This antagonistic activity was specific to GLP1R; no antagonistic activity was found in cells overexpressing the glucose-dependent insulinotropic peptide receptor (GIPR), glucagon like peptide-2 receptor or glucagon receptor. GLP-1-stimulated cAMP and insulin secretion was attenuated in INS-1 832/3 cells by Glp1R0017 incubation. Immunostaining of mouse pancreas tissue with Glp1R0017 showed specific staining in the islets of Langerhans, which was absent in Glp1r knockout tissue. In vivo, Glp1R0017 reversed the glucose-lowering effect of liraglutide during IPGTTs, and reduced glucose tolerance by blocking endogenous GLP-1 action in OGTTs.

CONCLUSIONS/INTERPRETATION

Glp1R0017 is a monoclonal antagonistic antibody to the GLP1R that binds to GLP1R on pancreatic beta cells and blocks the actions of GLP-1 in vivo. This antibody holds the potential to be used in investigating the physiological importance of GLP1R signalling in extrapancreatic tissues where cellular targets and signalling pathways activated by GLP-1 are poorly understood.

Extending the Structural View of Class B GPCRs

The secretin-like class B family of G protein-coupled receptors (GPCRs) are key players in hormonal homeostasis. Recent structures of various receptors in complex with a variety of orthosteric and allosteric ligands provide fundamental new insights into the function and mechanism of class B GPCRs, including: (i) ligand-induced changes in the relative orientation of the extracellular and transmembrane receptor domains; (ii) intramolecular interaction networks that stabilize conformational changes to accommodate intracellular G protein binding; and (iii) allosteric modulation of receptor activation. This review provides a comprehensive analysis of the structural, biochemical, and pharmacological data on class B GPCRs for understanding ligand-receptor interaction and modulation mechanisms and assessing the potential implications for drug discovery for the secretin-like GPCR family.

Opportunities for therapeutic antibodies directed at G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are activated by a diverse range of ligands, from large proteins and proteases to small peptides, metabolites, neurotransmitters and ions. They are expressed on all cells in the body and have key roles in physiology and homeostasis. As such, GPCRs are one of the most important target classes for therapeutic drug discovery. The development of drugs targeting GPCRs has therapeutic value across a wide range of diseases, including cancer, immune and inflammatory disorders as well as neurological and metabolic diseases. The progress made by targeting GPCRs with antibody-based therapeutics, as well as technical hurdles to overcome, are presented and discussed in this Review. Antibody therapeutics targeting C-C chemokine receptor type 4 (CCR4), CCR5 and calcitonin gene-related peptide (CGRP) are used as illustrative clinical case studies.

Rearrangement of a polar core provides a conserved mechanism for constitutive activation of class B G protein-coupled receptors

The glucagon receptor (GCGR) belongs to the secretin-like (class B) family of G protein-coupled receptors (GPCRs) and is activated by the peptide hormone glucagon. The structures of an activated class B GPCR have remained unsolved, preventing a mechanistic understanding of how these receptors are activated. Using a combination of structural modeling and mutagenesis studies, we present here two modes of ligand-independent activation of GCGR. First, we identified a GCGR-specific hydrophobic lock comprising Met-338 and Phe-345 within the IC3 loop and transmembrane helix 6 (TM6) and found that this lock stabilizes the TM6 helix in the inactive conformation. Disruption of this hydrophobic lock led to constitutive G protein and arrestin signaling. Second, we discovered a polar core comprising conserved residues in TM2, TM3, TM6, and TM7, and mutations that disrupt this polar core led to constitutive GCGR activity. On the basis of these results, we propose a mechanistic model of GCGR activation in which TM6 is held in an inactive conformation by the conserved polar core and the hydrophobic lock. Mutations that disrupt these inhibitory elements allow TM6 to swing outward to adopt an active TM6 conformation similar to that of the canonical β₂-adrenergic receptor complexed with G protein and to that of rhodopsin complexed with arrestin. Importantly, mutations in the corresponding polar core of several other members of class B GPCRs, including PTH1R, PAC1R, VIP1R, and CRFR1, also induce constitutive G protein signaling, suggesting that the rearrangement of the polar core is a conserved mechanism for class B GPCR activation.