An intrinsic agonist mechanism for activation of glucagon-like peptide-1 receptor by its extracellular domain

Insights into the structure and activation mechanism of some class B1 GPCR family members

In humans, 15 genes encode the class B1 family of GPCRs, which are polypeptide hormone receptors characterized by having a large N-terminal extracellular domain (ECD) and receive signals from outside the cell to activate cellular response. For example, the insulinotropic polypeptide (GIP) stimulates the glucose-dependent insulinotropic polypeptide receptor (GIPR), while the glucagon receptor (GCGR) responds to glucagon by increasing blood glucose levels and promoting the breakdown of liver glycogen to induce the production of insulin. The glucagon-like peptides 1 and 2 (GLP-1 and GLP-2) elicit a response from glucagon-like peptide receptor types 1 and 2 (GLP1R and GLP2R), respectively. Since these receptors are implicated in the pathogenesis of diabetes, studying their activation is crucial for the development of effective therapies for the condition. With more structural information being revealed by experimental methods such as X-ray crystallography, cryo-EM, and NMR, the activation mechanism of class B1 GPCRs becomes unraveled. The available crystal and cryo-EM structures reveal that class B1 GPCRs follow a two-step model for peptide binding and receptor activation. The regions close to the C-termini of hormones interact with the N-terminal ECD of the receptor while the regions close to the N-terminus of the peptide interact with the TM domain and transmit signals. This review highlights the structural details of class B1 GPCRs and their conformational changes following activation. The roles of MD simulation in characterizing those conformational changes are briefly discussed, providing insights into the potential structural exploration for future ligand designs.

New insights into the structure and function of class B1 GPCRs

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors. Class B1 GPCRs constitute a subfamily of 15 receptors that characteristically contain large extracellular domains (ECDs) and respond to long polypeptide hormones. Class B1 GPCRs are critical regulators of homeostasis, and as such, many are important drug targets. While most transmembrane proteins, including GPCRs, are recalcitrant to crystallization, recent advances in electron cryo-microscopy (cryo-EM) have facilitated a rapid expansion of the structural understanding of membrane proteins. As a testament to this success, structures for all the class B1 receptors bound to G proteins have been determined by cryo-EM in the past five years. Further advances in cryo-EM have uncovered dynamics of these receptors, ligands, and signalling partners. Here, we examine the recent structural underpinnings of the class B1 GPCRs with an emphasis on structure-function relationships.

Biased Agonism and Polymorphic Variation at the GLP-1 Receptor: Implications for the Development of Personalised Therapeutics

Glucagon-like peptide-1 receptor (GLP-1R) is a well-studied incretin hormone receptor and target of several therapeutic drugs for type 2 diabetes (T2D), obesity and, more recently, cardiovascular disease. Some signalling pathways downstream of GLP-1R may be responsible for drug adverse effects such as nausea, while others mediate therapeutic outcomes of incretin-based T2D therapeutics. Understanding the interplay between different factors that alter signalling, trafficking, and receptor activity, including biased agonism, single nucleotide polymorphisms and structural modifications is key to develop the next-generation of personalised GLP-1R agonists. However, these interactions remain poorly described, especially for novel therapeutics such as dual and tri-agonists that target more than one incretin receptor. Comparison of GLP-1R structures in complex with G proteins and different peptide and non-peptide agonists has revealed novel insights into important agonist-residue interactions and networks crucial for receptor activation, recruitment of G proteins and engagement of specific signalling pathways. Here, we review the latest knowledge on GLP-1R structure and activation, providing structural evidence for biased agonism and delineating important networks associated with this phenomenon. We survey current biased agonists and multi-agonists at different stages of development, highlighting possible challenges in their translational potential. Lastly, we discuss findings related to non-synonymous genomic variants of GLP1R and the functional importance of specific residues involved in GLP-1R function. We propose that studies of GLP-1R polymorphisms, and specifically their effect on receptor dynamics and pharmacology in response to biased agonists, could have a significant impact in delineating precision medicine approaches and development of novel therapeutics.

A Conserved Hydrophobic Moiety and Helix-Helix Interactions Drive the Self-Assembly of the Incretin Analog Exendin-4

Exendin-4 is a pharmaceutical peptide used in the control of insulin secretion. Structural information on exendin-4 and related peptides especially on the level of quaternary structure is scarce. We present the first published association equilibria of exendin-4 directly measured by static and dynamic light scattering. We show that exendin-4 oligomerization is pH dependent and that these oligomers are of low compactness. We relate our experimental results to a structural hypothesis to describe molecular details of exendin-4 oligomers. Discussion of the validity of this hypothesis is based on NMR, circular dichroism and fluorescence spectroscopy, and light scattering data on exendin-4 and a set of exendin-4 derived peptides. The essential forces driving oligomerization of exendin-4 are helix–helix interactions and interactions of a conserved hydrophobic moiety. Our structural hypothesis suggests that key interactions of exendin-4 monomers in the experimentally supported trimer take place between a defined helical segment and a hydrophobic triangle constituted by the Phe22 residues of the three monomeric subunits. Our data rationalize that Val19 might function as an anchor in the N-terminus of the interacting helix-region and that Trp25 is partially shielded in the oligomer by C-terminal amino acids of the same monomer. Our structural hypothesis suggests that the Trp25 residues do not interact with each other, but with C-terminal Pro residues of their own monomers.

Structure and dynamics of semaglutide- and taspoglutide-bound GLP-1R-Gs complexes

The glucagon-like peptide-1 receptor (GLP-1R) regulates insulin secretion, carbohydrate metabolism, and appetite and is an important target for treatment of type 2 diabetes and obesity. Multiple GLP-1R agonists have entered into clinical trials, with some, such as semaglutide, progressing to approval. Others, including taspoglutide, failed due to the high incidence of side effects or insufficient efficacy. GLP-1R agonists have a broad spectrum of signaling profiles, but molecular understanding is limited by a lack of structural information on how different agonists engage with the GLP-1R. Here, we report cryoelectron microscopy (cryo-EM) structures and cryo-EM 3D variability analysis of semaglutide- and taspoglutide-bound GLP-1R-Gs protein complexes. These reveal similar peptide interactions to GLP-1 but different motions within the receptor and bound peptides, providing insights into the molecular determinants of GLP-1R peptide engagement.

Molecular insights into ago-allosteric modulation of the human glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is a validated drug target for metabolic disorders. Ago-allosteric modulators are capable of acting both as agonists on their own and as efficacy enhancers of orthosteric ligands. However, the molecular details of ago-allosterism remain elusive. Here, we report three cryo-electron microscopy structures of GLP-1R bound to (i) compound 2 (an ago-allosteric modulator); (ii) compound 2 and GLP-1; and (iii) compound 2 and LY3502970 (a small molecule agonist), all in complex with heterotrimeric G_s. The structures reveal that compound 2 is covalently bonded to C347 at the cytoplasmic end of TM6 and triggers its outward movement in cooperation with the ECD whose N terminus penetrates into the GLP-1 binding site. This allows compound 2 to execute positive allosteric modulation through enhancement of both agonist binding and G protein coupling. Our findings offer insights into the structural basis of ago-allosterism at GLP-1R and may aid the design of better therapeutics.

The glucagon-like peptide-1 (GLP-1) receptor is a key regulator of glucose homeostasis and a drug target for type 2 diabetes but available GLP-1R agonists are suboptimal due to several side-effects. Here authors report the cryo-EM structure of GLP-1R bound to an ago-allosteric modulator in complex with heterotrimeric G_s which offers insights into the molecular details of ago-allosterism.

Differential GLP-1R Binding and Activation by Peptide and Non-peptide Agonists

Peptide drugs targeting class B1 G-protein-coupled receptors (GPCRs) can treat multiple diseases; however, there remains substantial interest in the development of orally delivered non-peptide drugs. Here, we reveal unexpected overlap between signaling and regulation of the glucagon-like peptide-1 (GLP-1) receptor by the non-peptide agonist PF 06882961 and GLP-1 that was not observed for another compound, CHU-128. Compounds from these patent series, including PF 06882961, are currently in clinical trials for treatment of type 2 diabetes. High-resolution cryoelectron microscopy (cryo-EM) structures reveal that the binding sites for PF 06882961 and GLP-1 substantially overlap, whereas CHU-128 adopts a unique binding mode with a more open receptor conformation at the extracellular face. Structural differences involving extensive water-mediated hydrogen bond networks could be correlated to functional data to understand how PF 06882961, but not CHU-128, can closely mimic the pharmacological properties of GLP-1. These findings will facilitate rational structure-based discovery of non-peptide agonists targeting class B GPCRs.

Rapid selection of a novel GLP-1/GIP dual receptor agonist with prolonged glycemic control and weight loss in rodent animals

BACKGROUND

Glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) co-agonists have emerged as treatment options for reversing diabetes and obesity. Here, we screened the high potency receptor-biased GLP-1R agonists via a newly designed high-throughput GLP-1R extracellular domain (ECD)-based system and demonstrated its in vitro and in vivo therapeutic characters.

METHODS

Twelve 9-mer peptides (named XEL1-XEL12) which were screened from a large phage-displayed peptide library were fused to the N-terminus of GIP (3-30) to generate another twelve fusion peptides, termed XEL13-24. Using the six lysine-altered XEL17 as leading sequences, eighteen fatty chain modified fusion peptides were further assessed via in vitro GLP-1R/GIPR-based cell assay. Moreover, the acute and long-acting in vivo effects of selected candidate on diabetic db/db mice and diet-induced obesity (DIO) rats were both carefully evaluated.

RESULTS

XEL17 exhibited balanced activation potency on GLP-1R/GIPR in stable cell lines, and further assessment was performed to evaluate the XEL32, a fatty chain modified XEL17 derivative. Preclinical pharmacodynamic results in diabetic db/db mice demonstrated that XEL32 held outstanding insulinotropic and glucose-lowering activities. In addition, protracted antidiabetic effects of XEL32 were also proved by the hypoglycemic test and multiple oral glucose tolerance test. Furthermore, chronic treatment of XEL32 in DIO rats exhibited outstanding beneficial effects on body weight control, fat loss, food intake control, hemoglobin A1C (HbA1C) reduction as well as the glucose tolerance.

CONCLUSIONS

XEL32, as a novel GLP-1/GIP dual receptor agonist, may supply efficient glycemic control and weight loss.

Recombinant Peptide Production Platform Coupled with Site-Specific Albumin Conjugation Enables a Convenient Production of Long-Acting Therapeutic Peptide

The number of therapeutic peptides for human treatment is growing rapidly. However, their development faces two major issues: the poor yield of large peptides from conventional solid-phase synthesis, and the intrinsically short serum half-life of peptides. To address these issues, we investigated a platform for the production of a recombinant therapeutic peptide with an extended serum half-life involving the site-specific conjugation of human serum albumin (HSA). HSA has an exceptionally long serum half-life and can be used to extend the serum half-lives of therapeutic proteins and peptides. We used glucagon-like-peptide 1 (GLP-1) as a model peptide in the present study. A “clickable” non-natural amino acid—p-azido-l-phenylalanine (AzF)—was incorporated into three specific sites (V16, Y19, and F28) of a GLP-1 variant, followed by conjugation with HSA through strain-promoted azide–alkyne cycloaddition. All three HSA-conjugated GLP-1 variants (GLP1_16HSA, GLP1_19HSA, and GLP1_28HSA) exhibited comparable serum half-lives in vivo. However, the three GLP1_HSA variants had different in vitro biological activities and in vivo glucose-lowering effects, demonstrating the importance of site-specific HSA conjugation. The platform described herein could be used to develop other therapeutic peptides with extended serum half-lives.

Discovery and pharmacology of the covalent GLP-1 receptor (GLP-1R) allosteric modulator BETP: A novel tool to probe GLP-1R pharmacology

The glucagon-like peptide-1 receptor (GLP-1R) is a significant therapeutic target for small molecule drug discovery given the therapeutic impact of peptide agonists in the diabetes sphere. We review the discovery and subsequent characterization of the small molecule GLP-1R allosteric modulator 4-(3-(Benzyloxy)phenyl)-2-(ethylsulfinyl)-6-(trifluoromethyl)pyrimidine (BETP). BETP is a covalent modulator of the GLP-1R, and we discuss the pharmacological implications and possible structural basis of this novel mode of action. We highlight the insights into class B G-protein coupled receptor pharmacology and biology provided by studies conducted with BETP. These include the descriptions of exquisite allosteric modulator probe dependence and biased signaling in vitro and in vivo. We conclude with an analysis of the utility of BETP as a chemical probe for the GLP-1R.

Activation of the GLP-1 receptor by a non-peptidic agonist

Class B G-protein-coupled receptors are major targets for the treatment of chronic diseases, including diabetes and obesity¹. Structures of active receptors reveal peptide agonists engage deep within the receptor core, leading to an outward movement of extracellular loop 3 and the tops of transmembrane helices 6 and 7, an inward movement of transmembrane helix 1, reorganization of extracellular loop 2 and outward movement of the intracellular side of transmembrane helix 6, resulting in G-protein interaction and activation^2-6. Here we solved the structure of a non-peptide agonist, TT-OAD2, bound to the glucagon-like peptide-1 (GLP-1) receptor. Our structure identified an unpredicted non-peptide agonist-binding pocket in which reorganization of extracellular loop 3 and transmembrane helices 6 and 7 manifests independently of direct ligand interaction within the deep transmembrane domain pocket. TT-OAD2 exhibits biased agonism, and kinetics of G-protein activation and signalling that are distinct from peptide agonists. Within the structure, TT-OAD2 protrudes beyond the receptor core to interact with the lipid or detergent, providing an explanation for the distinct activation kinetics that may contribute to the clinical efficacy of this compound series. This work alters our understanding of the events that drive the activation of class B receptors.

Incretin Mimetics as Rational Candidates for the Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is becoming an increasing public health issue. With an annually estimated 1.7 million TBIs in the United States (U.S) and nearly 70 million worldwide, the injury, isolated or compounded with others, is a major cause of short- and long-term disability and mortality. This, along with no specific treatment, has made exploration of TBI therapies a priority of the health system. Age and sex differences create a spectrum of vulnerability to TBI, with highest prevalence among younger and older populations. Increased public interest in the long-term effects and prevention of TBI have recently reached peaks, with media attention bringing heightened awareness to sport and war related head injuries. Along with short-term issues, TBI can increase the likelihood for development of long-term neurodegenerative disorders. A growing body of literature supports the use of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and glucagon (Gcg) receptor (R) agonists, along with unimolecular combinations of these therapies, for their potent neurotrophic/neuroprotective activities across a variety of cellular and animal models of chronic neurodegenerative diseases (Alzheimer's and Parkinson's diseases) and acute cerebrovascular disorders (stroke). Mild or moderate TBI shares many of the hallmarks of these conditions; recent work provides evidence that use of these compounds is an effective strategy for its treatment. Safety and efficacy of many incretin-based therapies (GLP-1 and GIP) have been demonstrated in humans for the treatment of type 2 diabetes mellitus (T2DM), making these compounds ideal for rapid evaluation in clinical trials of mild and moderate TBI.

Structure and dynamics of the active human parathyroid hormone receptor-1

The parathyroid hormone receptor-1 (PTH1R) is a class B G protein-coupled receptor central to calcium homeostasis and a therapeutic target for osteoporosis and hypoparathyroidism. Here we report the cryo-electron microscopy structure of human PTH1R bound to a long-acting PTH analog and the stimulatory G protein. The bound peptide adopts an extended helix with its amino terminus inserted deeply into the receptor transmembrane domain (TMD), which leads to partial unwinding of the carboxyl terminus of transmembrane helix 6 and induces a sharp kink at the middle of this helix to allow the receptor to couple with G protein. In contrast to a single TMD structure state, the extracellular domain adopts multiple conformations. These results provide insights into the structural basis and dynamics of PTH binding and receptor activation.

Structure of the glucagon receptor in complex with a glucagon analogue

Class B G-protein-coupled receptors (GPCRs), which consist of an extracellular domain (ECD) and a transmembrane domain (TMD), respond to secretin peptides to play a key part in hormonal homeostasis, and are important therapeutic targets for a variety of diseases. Previous work has suggested that peptide ligands bind to class B GPCRs according to a two-domain binding model, in which the C-terminal region of the peptide targets the ECD and the N-terminal region of the peptide binds to the TMD binding pocket. Recently, three structures of class B GPCRs in complex with peptide ligands have been solved. These structures provide essential insights into peptide ligand recognition by class B GPCRs. However, owing to resolution limitations, the specific molecular interactions for peptide binding to class B GPCRs remain ambiguous. Moreover, these previously solved structures have different ECD conformations relative to the TMD, which introduces questions regarding inter-domain conformational flexibility and the changes required for receptor activation. Here we report the 3.0 Å-resolution crystal structure of the full-length human glucagon receptor (GCGR) in complex with a glucagon analogue and partial agonist, NNC1702. This structure provides molecular details of the interactions between GCGR and the peptide ligand. It reveals a marked change in the relative orientation between the ECD and TMD of GCGR compared to the previously solved structure of the inactive GCGR-NNC0640-mAb1 complex. Notably, the stalk region and the first extracellular loop undergo major conformational changes in secondary structure during peptide binding, forming key interactions with the peptide. We further propose a dual-binding-site trigger model for GCGR activation-which requires conformational changes of the stalk, first extracellular loop and TMD-that extends our understanding of the previously established two-domain peptide-binding model of class B GPCRs.

Two distinct domains of the glucagon-like peptide-1 receptor control peptide-mediated biased agonism

G protein-coupled receptors (GPCRs) can be differentially activated by ligands to generate multiple and distinct downstream signaling profiles, a phenomenon termed biased agonism. The glucagon-like peptide-1 receptor (GLP-1R) is a class B GPCR and a key drug target for managing metabolic disorders; however, its peptide agonists display biased signaling that affects their relative efficacies. In this study, we combined mutagenesis experiments and mapping of surface mutations onto recently described GLP-1R structures, which revealed two major domains in the GLP-1/GLP-1R/G_s protein active structure that are differentially important for both receptor quiescence and ligand-specific initiation and propagation of biased agonism. Changes to the conformation of transmembrane helix (TM) 5 and TM 6 and reordering of extracellular loop 2 were essential for the propagation of signaling linked to cAMP formation and intracellular calcium mobilization, whereas ordering and packing of residues in TMs 1 and 7 were critical for extracellular signal-regulated kinase 1/2 (pERK) activity. On the basis of these findings, we propose a model of distinct peptide-receptor interactions that selectively control how these different signaling pathways are engaged. This work provides important structural insight into class B GPCR activation and biased agonism.

Conditional and Reversible Activation of Class A and B G Protein-Coupled Receptors Using Tethered Pharmacology

Understanding the activation and internalization of G protein-coupled receptors (GPCRs) using conditional approaches is paramount to developing new therapeutic strategies. Here, we describe the design, synthesis, and testing of ExONatide, a benzylguanine-linked peptide agonist of the glucagon-like peptide-1 receptor (GLP-1R), a class B GPCR required for maintenance of glucose levels in humans. ExONatide covalently binds to SNAP-tagged GLP-1R-expressing cells, leading to prolonged cAMP generation, Ca²⁺ rises, and intracellular retention of the receptor. These effects were readily switched OFF following cleavage of the introduced disulfide bridge using the cell-permeable reducing agent beta-mercaptoethanol (BME). A similar approach could be extended to a class A GPCR using GhrelON, a benzylguanine-linked peptide agonist of the growth hormone secretagogue receptor 1a (GHS-R1a), which is involved in food intake and growth. Thus, ExONatide and GhrelON allow SNAP-tag-directed activation of class A and B GPCRs involved in gut hormone signaling in a reversible manner. This tactic, termed reductively cleavable agONist (RECON), may be useful for understanding GLP-1R and GHS-R1a function both in vitro and in vivo, with applicability across GPCRs.

A calcium-sensing receptor mutation causing hypocalcemia disrupts a transmembrane salt bridge to activate β-arrestin-biased signaling

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor (GPCR) that signals through Gq/11 and Gi/o to stimulate cytosolic calcium (Ca2+i) and mitogen-activated protein kinase (MAPK) signaling to control extracellular calcium homeostasis. Studies of loss- and gain-of-function CASR mutations, which cause familial hypocalciuric hypercalcemia type 1 (FHH1) and autosomal dominant hypocalcemia type 1 (ADH1), respectively, have revealed that the CaSR signals in a biased manner. Thus, some mutations associated with FHH1 lead to signaling predominantly through the MAPK pathway, whereas mutations associated with ADH1 preferentially enhance Ca2+i responses. We report a previously unidentified ADH1-associated R680G CaSR mutation, which led to the identification of a CaSR structural motif that mediates biased signaling. Expressing CaSRR680G in HEK 293 cells showed that this mutation increased MAPK signaling without altering Ca2+i responses. Moreover, this gain of function in MAPK activity occurred independently of Gq/11 and Gi/o and was mediated instead by a noncanonical pathway involving β-arrestin proteins. Homology modeling and mutagenesis studies showed that the R680G CaSR mutation selectively enhanced β-arrestin signaling by disrupting a salt bridge formed between Arg680 and Glu767, which are located in CaSR transmembrane domain 3 and extracellular loop 2, respectively. Thus, our results demonstrate CaSR signaling through β-arrestin and the importance of the Arg680-Glu767 salt bridge in mediating signaling bias.

Control of insulin secretion by GLP-1

Stimulation of insulin secretion by glucagon-like peptide-1 (GLP-1) and other gut-derived peptides is central to the incretin response to ingesting nutriments. Analogues of GLP-1, and inhibitors of its breakdown, have found widespread clinical use for the treatment of type 2 diabetes (T2D) and obesity. The release of these peptides underlies the improvements in glycaemic control and disease remission after bariatric surgery. Given therapeutically, GLP-1 analogues can lead to side effects including nausea, which limit dosage. Greater understanding of the interactions between the GLP-1 receptor (GLP-1R) and both the endogenous and artificial ligands therefore holds promise to provide more efficacious compounds. Here, we discuss recent findings concerning the signalling and trafficking of the GLP-1R in pancreatic beta cells. Leveraging "bias" at the receptor towards cAMP generation versus the recruitment of β-arrestins and extracellular signal-regulated kinases (ERK1/2) activation may allow the development of new analogues with significantly improved clinical efficacy. We describe how, unexpectedly, relatively low-affinity agonists, which prompt less receptor internalisation than the parent compound, provoke greater insulin secretion and consequent improvements in glycaemia.

Phase-plate cryo-EM structure of a class B GPCR-G-protein complex

Class B G-protein-coupled receptors are major targets for the treatment of chronic diseases, such as osteoporosis, diabetes and obesity. Here we report the structure of a full-length class B receptor, the calcitonin receptor, in complex with peptide ligand and heterotrimeric Gαsβγ protein determined by Volta phase-plate single-particle cryo-electron microscopy. The peptide agonist engages the receptor by binding to an extended hydrophobic pocket facilitated by the large outward movement of the extracellular ends of transmembrane helices 6 and 7. This conformation is accompanied by a 60° kink in helix 6 and a large outward movement of the intracellular end of this helix, opening the bundle to accommodate interactions with the α5-helix of Gαs. Also observed is an extended intracellular helix 8 that contributes to both receptor stability and functional G-protein coupling via an interaction with the Gβ subunit. This structure provides a new framework for understanding G-protein-coupled receptor function.

Understanding the GPCR biased signaling through G protein and arrestin complex structures

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and are important drug targets for many human diseases. The determination of the 3-D structure of GPCRs and their signaling complexes has promoted our understanding of GPCR biology and provided templates for structure-based drug discovery. In this review, we focus on the recent structure work on GPCR signaling complexes, the β2-adrenoreceptor-Gs and the rhodopsin-arrestin complexes in particular, and highlight the structural features of GPCR complexes involved in G protein- and arrestin-mediated signal transduction. The crystal structures reveal distinct structural mechanisms by which GPCRs recruit a G protein and an arrestin. A comparison of the two complex structures provides insight into the molecular mechanism of functionally selective GPCR signaling, and a structural basis for the discovery of G protein- and arrestin-biased treatments of human diseases related to GPCR signal transduction.

Genetically encoded photocross-linkers determine the biological binding site of exendin-4 peptide in the N-terminal domain of the intact human glucagon-like peptide-1 receptor (GLP-1R)

The glucagon-like peptide-1 receptor (GLP-1R) is a key therapeutic target in the management of type II diabetes mellitus, with actions including regulation of insulin biosynthesis and secretion, promotion of satiety, and preservation of β-cell mass. Like most class B G protein-coupled receptors (GPCRs), there is limited knowledge linking biological activity of the GLP-1R with the molecular structure of an intact, full-length, and functional receptor·ligand complex. In this study, we have utilized genetic code expansion to site-specifically incorporate the photoactive amino acid p-azido-l-phenylalanine (azF) into N-terminal residues of a full-length functional human GLP-1R in mammalian cells. UV-mediated photolysis of azF was then carried out to induce targeted photocross-linking to determine the proximity of the azido group in the mutant receptor with the peptide exendin-4. Cross-linking data were compared directly with the crystal structure of the isolated N-terminal extracellular domain of the GLP-1R in complex with exendin(9-39), revealing both similarities as well as distinct differences in the mode of interaction. Generation of a molecular model to accommodate the photocross-linking constraints highlights the potential influence of environmental conditions on the conformation of the receptor·peptide complex, including folding dynamics of the peptide and formation of dimeric and higher order oligomeric receptor multimers. These data demonstrate that crystal structures of isolated receptor regions may not give a complete reflection of peptide/receptor interactions and should be combined with additional experimental constraints to reveal peptide/receptor interactions occurring in the dynamic, native, and full-length receptor state.

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.