Structural and functional diversity among agonist-bound states of the GLP-1 receptor

Prolonged Activation of the GLP-1 Receptor via Covalent Capture

The incretin gut hormone glucagon-like peptide-1 (GLP-1) has become a household name because of its ability to induce glucose-dependent insulin release with accompanying weight loss in patients. Indeed, derivatives of the peptide exert numerous pleiotropic actions that favorably affect other metabolic functions, and consequently, such compounds are being considered as treatments for a variety of ailments. The ability of native GLP-1 to function as a clinical drug is severely limited because of its short half-life in vivo. All of the beneficial effects of GLP-1 come from its agonism at the cognate receptor, GLP-1R. In our quest for long-lived activation of the receptor, we hypothesized that an agonist that had the ability to covalently cross-link with GLP-1R would prove useful. We here report the structure-guided design of peptide analogues containing an electrophilic warhead that could be covalently captured by a resident native nucleophile on the receptor. The compounds were evaluated using washout experiments, and resistance to such washing serves as an index of prolonged activation and covalent capture, which we use to tabulate longevity and robust long-lived GLP-1R agonism. The addition of SulF (cross-linkable warhead), an N-terminal trifluoroethyl group (for protease protection), and a C18 diacid lipid (protractor) all contributed to the increased wash resistance of GLP-1. The most effective compound based on the wash resistance metric, C2K26DAC18_K34SulF, has all three elements outlined and may serve as a blueprint and a proof-of-concept scaffold for the design of clinically useful molecules.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

Impact of secretin receptor homo-dimerization on natural ligand binding

Class B G protein-coupled receptors can form dimeric complexes important for high potency biological effects. Here, we apply pharmacological, biochemical, and biophysical techniques to cells and membranes expressing the prototypic secretin receptor (SecR) to gain insights into secretin binding to homo-dimeric and monomeric SecR. Spatial proximity between peptide and receptor residues, probed by disulfide bond formation, demonstrates that the secretin N-terminus moves from adjacent to extracellular loop 3 (ECL3) at wild type SecR toward ECL2 in non-dimerizing mutants. Analysis of fluorescent secretin analogs demonstrates stable engagement of the secretin C-terminal region within the receptor extracellular domain (ECD) for both dimeric and monomeric receptors, while the mid-region exhibits lower mobility while docked at the monomer. Moreover, decoupling of G protein interaction reduces mobility of the peptide mid-region at wild type receptor to levels similar to the mutant, whereas it has no further impact on the monomer. These data support a model of peptide engagement whereby the ability of SecR to dimerize promotes higher conformational dynamics of the peptide-bound receptor ECD and ECLs that likely facilitates more efficient G protein recruitment and activation, consistent with the higher observed functional potency of secretin at wild type SecR relative to the monomeric mutant receptor.

Palmitoylation of the Glucagon-like Peptide-1 Receptor Modulates Cholesterol Interactions at the Receptor-Lipid Microenvironment

The G protein-coupled receptor (GPCR) superfamily of cell surface receptors has been shown to be functionally modulated by post-translational modifications. The glucagon-like peptide receptor-1 (GLP-1R), which is a drug target in diabetes and obesity, undergoes agonist-dependent palmitoyl tail conjugation. The palmitoylation in the C-terminal domain of GLP-1R has been suggested to modulate the receptor-lipid microenvironment. In this work, we have performed coarse-grain molecular dynamics simulations of palmitoylated and nonpalmitoylated GLP-1R to analyze the differential receptor-lipid interactions. Interestingly, the placement and dynamics of the C-terminal domain of GLP-1R are found to be directly dependent on the palmitoyl tail. We observe that both cholesterol and phospholipids interact with the receptor but display differential interactions in the presence and absence of the palmitoyl tail. We characterize important cholesterol-binding sites and validate sites that have been previously reported in experimentally resolved structures of the receptor. We show that the receptor acts like a conduit for cholesterol flip-flop by stabilizing cholesterol in the membrane core. Taken together, our work represents an important step in understanding the molecular effects of lipid modifications in GPCRs.

Phosphorylation Sites of the Gastric Inhibitory Polypeptide Receptor (GIPR) Revealed by Trapped-Ion-Mobility Spectrometry Coupled to Time-of-Flight Mass Spectrometry (TIMS-TOF MS)

The gastric inhibitory polypeptide receptor (GIPR), a G protein-coupled receptor (GPCR) that regulates glucose metabolism and insulin secretion, is a target for the development of therapeutic agents to address type 2 diabetes and obesity. Signal transduction processes mediated by GPCR activation typically result in receptor phosphorylation, but very little is known about GIPR phosphorylation. Mass spectrometry (MS) is a powerful tool for detecting phosphorylation and other post-translational modifications of proteins and for identifying modification sites. However, applying MS methods to GPCRs is challenging because the native expression levels are low and the hydrophobicity of these proteins complicates isolation and enrichment. Here we use a widely available technique, trapped-ion-mobility spectrometry coupled to time-of-flight mass spectrometry (TIMS-TOF MS), to characterize the phosphorylation status of the GIPR. We identified eight serine residues that are phosphorylated, one in an intracellular loop and the remainder in the C-terminal domain. Stimulation with the native agonist GIP enhanced phosphorylation at four of these sites. For comparison, we evaluated tirzepatide (TZP), a dual agonist of the glucagon-like peptide-1 (GLP-1) receptor and the GIPR that has recently been approved for the treatment of type 2 diabetes. Stimulation with TZP enhanced phosphorylation at the same four sites that were enhanced with GIP; however, TZP also enhanced phosphorylation at a fifth site that is unique to this synthetic agonist. This work establishes an important and accessible tool for the characterization of signal transduction via the GIPR and reveals an unanticipated functional difference between GIP and TZP.

Effects of Replacing a Central Glycine Residue in GLP-1 on Receptor Affinity and Signaling Profile

Agonists of the glucagon-like peptide-1 receptor (GLP-1R) are used to treat diabetes and obesity. Cryo-EM structures indicate that GLP-1 is completely α-helical when bound to the GLP-1R. The mature form of this hormone, GLP-1(7-36), contains a glycine residue near the center (Gly22). Since glycine has the second-lowest α-helix propensity among the proteinogenic α-amino acid residues, and Gly22 does not appear to make direct contact with the receptor, we were motivated to explore the impact on agonist activity of altering the α-helix propensity at this position. We examined GLP-1 analogues in which Gly22 was replaced with L-Ala, D-Ala, or β-amino acid residues with varying helix propensities. The results suggest that the receptor is reasonably tolerant of variations in helix propensity, and that the functional receptor-agonist complex may comprise a conformational spectrum rather than a single fixed structure.

Structure and Molecular Mechanism of Signaling for the Glucagon-like Peptide-1 Receptor Bound to Gs Protein and Exendin-P5 Biased Agonist

The glucagon-like peptide-1 receptor (GLP-1R) is a key regulator of blood glucose and a prime target for the treatment of type II diabetes and obesity with multiple public drugs. Here we present a comprehensive computational analysis of the interactions of the activated GLP-1R-Gs signaling complex with a G protein biased agonist, Exendin P5 (ExP5), which possesses a unique N-terminal sequence responsible for the signal bias. Using a refined all-atom model of the ExP5-GLP-1R-Gs complex in molecular dynamics (MD) simulations, we propose a novel mechanism of conformation transduction in which the unique interaction network of ExP5 N-terminus propagates the binding signal across an array of conserved residues at the transmembrane domain to enhance Gs protein coupling at the cytoplasmic end of the receptor. Our simulations reveal previously unobserved interactions important for activation by ExP5 toward GDP-GTP signaling, providing new insights into the mechanism of class B G protein-coupled receptor (GPCR) signaling. These findings offer a framework for the structure-based design of more effective therapeutics.

Hormone Analogues with Unique Signaling Profiles from Replacement of α-Residue Triads with β/γ Diads

We have applied an underexplored backbone modification strategy to generate new analogues of peptides that activate two clinically important class B1 G protein-coupled receptors (GPCRs). Most peptide modification strategies involve changing side chains or, less commonly, changing the configuration at side chain-bearing carbons (i.e., l residues replaced by d residues). In contrast, backbone modifications alter the number of backbone atoms and the identities of backbone atoms relative to a poly-α-amino acid backbone. Starting from the peptide agonists PTH(1-34) (the first 34 residues of the parathyroid hormone, used clinically as the drug teriparatide) and glucagon-like peptide-1 (7-36) (GLP-1(7-36)), we replaced native α-residue triads with a diad composed of a β-amino acid residue and a γ-amino acid residue. The β/γ diad retains the number of backbone atoms in the ααα triad. Because the β and γ residue each bear a single side chain, we implemented ααα→βγ replacements at sites that contained a Gly residue (i.e., at α-residue triads that presented only two side chains). All seven of the α/β/γ-peptides derived from PTH(1-34) or GLP-1(7-36) bind to the cognate receptor (the PTHR1 or the GLP-1R), but they vary considerably in their activity profiles. Outcomes include functional mimicry of the all-α agonist, receptor-selective agonist activity, biased agonism, or strong binding with weak activation, which could lead to antagonist development. Collectively, these findings demonstrate that ααα→βγ replacements, which are easily implemented via solid-phase synthesis, can generate peptide hormone analogues that display unique and potentially useful signaling behavior.

Optimization of a Glucagon-Like Peptide 1 Receptor Antagonist Antibody for Treatment of Hyperinsulinism

Congenital hyperinsulinism (HI) is a genetic disorder in which pancreatic β-cell insulin secretion is excessive and results in hypoglycemia that, without treatment, can cause brain damage or death. Most patients with loss-of-function mutations in ABCC8 and KCNJ11, the genes encoding the β-cell ATP-sensitive potassium channel (KATP), are unresponsive to diazoxide, the only U.S. Food and Drug Administration-approved medical therapy and require pancreatectomy. The glucagon-like peptide 1 receptor (GLP-1R) antagonist exendin-(9-39) is an effective therapeutic agent that inhibits insulin secretion in both HI and acquired hyperinsulinism. Previously, we identified a highly potent antagonist antibody, TB-001-003, which was derived from our synthetic antibody libraries that were designed to target G protein-coupled receptors. Here, we designed a combinatorial variant antibody library to optimize the activity of TB-001-003 against GLP-1R and performed phage display on cells overexpressing GLP-1R. One antagonist, TB-222-023, is more potent than exendin-(9-39), also known as avexitide. TB-222-023 effectively decreased insulin secretion in primary isolated pancreatic islets from a mouse model of hyperinsulinism, Sur1-/- mice, and in islets from an infant with HI, and increased plasma glucose levels and decreased the insulin to glucose ratio in Sur1-/- mice. These findings demonstrate that targeting GLP-1R with an antibody antagonist is an effective and innovative strategy for treatment of hyperinsulinism.

ARTICLE HIGHLIGHTS

Patients with the most common and severe form of diazoxide-unresponsive congenital hyperinsulinism (HI) require a pancreatectomy. Other second-line therapies are limited in their use because of severe side effects and short half-lives. Therefore, there is a critical need for better therapies. Studies with the glucagon-like peptide 1 receptor (GLP-1R) antagonist, avexitide (exendin-(9-39)), have demonstrated that GLP-1R antagonism is effective at lowering insulin secretion and increasing plasma glucose levels. We have optimized a GLP-1R antagonist antibody with more potent blocking of GLP-1R than avexitide. This antibody therapy is a potential novel and effective treatment for HI.

Differential Responses of the GLP-1 and GLP-2 Receptors to N-Terminal Modification of a Dual Agonist

Class B1 G protein-coupled receptors (GPCRs), collectively, respond to a diverse repertoire of extracellular polypeptide agonists and transmit the encoded messages to cytosolic partners. To fulfill these tasks, these highly mobile receptors must interconvert among conformational states in response to agonists. We recently showed that conformational mobility in polypeptide agonists themselves plays a role in activation of one class B1 GPCR, the receptor for glucagon-like peptide-1 (GLP-1). Exchange between helical and nonhelical conformations near the N-termini of agonists bound to the GLP-1R was revealed to be critical for receptor activation. Here, we ask whether agonist conformational mobility plays a role in the activation of a related receptor, the GLP-2R. Using variants of the hormone GLP-2 and the designed clinical agonist glepaglutide (GLE), we find that the GLP-2R is quite tolerant of variations in α-helical propensity near the agonist N-terminus, which contrasts with signaling at the GLP-1R. A fully α-helical conformation of the bound agonist may be sufficient for GLP-2R signal transduction. GLE is a GLP-2R/GLP-1R dual agonist, and the GLE system therefore enables direct comparison of the responses of these two GPCRs to a single set of agonist variants. This comparison supports the conclusion that the GLP-1R and GLP-2R differ in their response to variations in helical propensity near the agonist N-terminus. The data offer a basis for development of new hormone analogues with distinctive and potentially useful activity profiles; for example, one of the GLE analogues is a potent agonist of the GLP-2R but also a potent antagonist of the GLP-1R, a novel form of polypharmacology.

Pharmacological hallmarks of allostery at the M4 muscarinic receptor elucidated through structure and dynamics

Allosteric modulation of G protein-coupled receptors (GPCRs) is a major paradigm in drug discovery. Despite decades of research, a molecular-level understanding of the general principles that govern the myriad pharmacological effects exerted by GPCR allosteric modulators remains limited. The M₄ muscarinic acetylcholine receptor (M₄ mAChR) is a validated and clinically relevant allosteric drug target for several major psychiatric and cognitive disorders. In this study, we rigorously quantified the affinity, efficacy, and magnitude of modulation of two different positive allosteric modulators, LY2033298 (LY298) and VU0467154 (VU154), combined with the endogenous agonist acetylcholine (ACh) or the high-affinity agonist iperoxo (Ipx), at the human M₄ mAChR. By determining the cryo-electron microscopy structures of the M₄ mAChR, bound to a cognate G_i1 protein and in complex with ACh, Ipx, LY298-Ipx, and VU154-Ipx, and applying molecular dynamics simulations, we determine key molecular mechanisms underlying allosteric pharmacology. In addition to delineating the contribution of spatially distinct binding sites on observed pharmacology, our findings also revealed a vital role for orthosteric and allosteric ligand-receptor-transducer complex stability, mediated by conformational dynamics between these sites, in the ultimate determination of affinity, efficacy, cooperativity, probe dependence, and species variability. There results provide a holistic framework for further GPCR mechanistic studies and can aid in the discovery and design of future allosteric drugs.

An HR2-Mimicking Sulfonyl-γ-AApeptide Is a Potent Pan-coronavirus Fusion Inhibitor with Strong Blood-Brain Barrier Permeability, Long Half-Life, and Promising Oral Bioavailability

Neutralizing antibodies and fusion inhibitory peptides have the potential required to combat the global pandemic caused by SARS-CoV-2 and its variants. However, the lack of oral bioavailability and enzymatic susceptibility limited their application, necessitating the development of novel pan-CoV fusion inhibitors. Herein we report a series of helical peptidomimetics, d-sulfonyl-γ-AApeptides, which effectively mimic the key residues of heptad repeat 2 and interact with heptad repeat 1 in the SARS-CoV-2 S2 subunit, resulting in inhibiting SARS-CoV-2 spike protein-mediated fusion between virus and cell membranes. The leads also displayed broad-spectrum inhibitory activity against a panel of other human CoVs and showed strong potency in vitro and in vivo. Meanwhile, they also demonstrated complete resistance to proteolytic enzymes or human sera and exhibited extremely long half-life in vivo and highly promising oral bioavailability, delineating their potential as pan-CoV fusion inhibitors with the potential to combat SARS-CoV-2 and its variants.

Molecular insights into peptide agonist engagement with the PTH receptor

The parathyroid hormone (PTH) 1 receptor (PTH1R) is a G protein-coupled receptor (GPCR) that regulates skeletal development and calcium homeostasis. Here, we describe cryo-EM structures of the PTH1R in complex with fragments of the two hormones, PTH and PTH-related protein, the drug abaloparatide, as well as the engineered tool compounds, long-acting PTH (LA-PTH) and the truncated peptide, M-PTH(1-14). We found that the critical N terminus of each agonist engages the transmembrane bundle in a topologically similar fashion, reflecting similarities in measures of Gαs activation. The full-length peptides induce subtly different extracellular domain (ECD) orientations relative to the transmembrane domain. In the structure bound to M-PTH, the ECD is unresolved, demonstrating that the ECD is highly dynamic when unconstrained by a peptide. High resolutions enabled identification of water molecules near peptide and G protein binding sites. Our results illuminate the action of orthosteric agonists of the PTH1R.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

GLP-1R Signaling and Functional Molecules in Incretin Therapy

Glucagon-like peptide-1 receptor (GLP-1R) is a critical therapeutic target for type 2 diabetes mellitus (T2DM). The GLP-1R cellular signaling mechanism relevant to insulin secretion and blood glucose regulation has been extensively studied. Numerous drugs targeting GLP-1R have entered clinical treatment. However, novel functional molecules with reduced side effects and enhanced therapeutic efficacy are still in high demand. In this review, we summarize the basis of GLP-1R cellular signaling, and how it is involved in the treatment of T2DM. We review the functional molecules of incretin therapy in various stages of clinical trials. We also outline the current strategies and emerging techniques that are furthering the development of novel therapeutic drugs for T2DM and other metabolic diseases.

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.

Kinetic and Thermodynamic Insights into Agonist Interactions with the Parathyroid Hormone Receptor-1 from a New NanoBRET Assay

Polypeptides that activate the parathyroid hormone receptor-1 (PTHR1) are important in human physiology and medicine. Most previous studies of peptide binding to this receptor have involved the displacement of a radiolabeled ligand. We report a new assay format based on bioluminescence resonance energy transfer (BRET). Fusion of a NanoLuc luciferase (nLuc) unit to the N-terminus of the PTHR1 allows the direct detection of binding by an agonist peptide bearing a tetramethylrhodamine (TMR) unit. Affinity measurements from the BRET assay align well with results previously obtained via radioligand displacement. The BRET assay offers substantial operational benefits relative to affinity measurements involving radioactive compounds. The convenience of the new assay allowed us to explore several questions raised by earlier reports. For example, we show that although the first two residues of PTH(1-34) (the drug teriparatide) are critical for PTHR1 activation, these two residues contribute little or nothing to affinity. Comparisons among the well-studied agonists PTH(1-34), PTHrP(1-34), and "long-acting PTH" (LA-PTH) reveal that the high affinity of LA-PTH arises largely from a diminished rate constant for dissociation relative to the other two. A D-peptide recently reported to be comparable to PTH(1-34) as an agonist of the PTHR1 was found not to bind detectably to the receptor and to be a very weak agonist.

Understanding VPAC receptor family peptide binding and selectivity

The vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) receptors are key regulators of neurological processes. Despite recent structural data, a comprehensive understanding of peptide binding and selectivity among different subfamily receptors is lacking. Here, we determine structures of active, Gs-coupled, VIP-VPAC1R, PACAP27-VPAC1R, and PACAP27-PAC1R complexes. Cryo-EM structural analyses and molecular dynamics simulations (MDSs) reveal fewer stable interactions between VPAC1R and VIP than for PACAP27, more extensive dynamics of VIP interaction with extracellular loop 3, and receptor-dependent differences in interactions of conserved N-terminal peptide residues with the receptor core. MD of VIP modelled into PAC1R predicts more transient VIP-PAC1R interactions in the receptor core, compared to VIP-VPAC1R, which may underlie the selectivity of VIP for VPAC1R over PAC1R. Collectively, our work improves molecular understanding of peptide engagement with the PAC1R and VPAC1R that may benefit the development of novel selective agonists.

α/Sulfono-γ-AA peptide hybrids agonist of GLP-1R with prolonged action both in vitro and in vivo

Peptides are increasingly important resources for biological and therapeutic development, however, their intrinsic susceptibility to proteolytic degradation represents a big hurdle. As a natural agonist for GLP-1R, glucagon-like peptide 1 (GLP-1) is of significant clinical interest for the treatment of type-2 diabetes mellitus, but its in vivo instability and short half-life have largely prevented its therapeutic application. Here, we describe the rational design of a series of α/sulfono-γ-AA peptide hybrid analogues of GLP-1 as the GLP-1R agonists. Certain GLP-1 hybrid analogues exhibited enhanced stability (t _1/2 > 14 days) compared to t _1/2 (<1 day) of GLP-1 in the blood plasma and in vivo. These newly developed peptide hybrids may be viable alternative of semaglutide for type-2 diabetes treatment. Additionally, our findings suggest that sulfono-γ-AA residues could be adopted to substitute canonical amino acids residues to improve the pharmacological activity of peptide-based drugs.

Targeting VIP and PACAP Receptor Signaling: New Insights into Designing Drugs for the PACAP Subfamily of Receptors

Pituitary Adenylate Cyclase-Activating Peptide (PACAP) and Vasoactive Intestinal Peptide (VIP) are neuropeptides involved in a diverse array of physiological and pathological processes through activating the PACAP subfamily of class B1 G protein-coupled receptors (GPCRs): VIP receptor 1 (VPAC1R), VIP receptor 2 (VPAC2R), and PACAP type I receptor (PAC1R). VIP and PACAP share nearly 70% amino acid sequence identity, while their receptors PAC1R, VPAC1R, and VPAC2R share 60% homology in the transmembrane regions of the receptor. PACAP binds with high affinity to all three receptors, while VIP binds with high affinity to VPAC1R and VPAC2R, and has a thousand-fold lower affinity for PAC1R compared to PACAP. Due to the wide distribution of VIP and PACAP receptors in the body, potential therapeutic applications of drugs targeting these receptors, as well as expected undesired side effects, are numerous. Designing selective therapeutics targeting these receptors remains challenging due to their structural similarities. This review discusses recent discoveries on the molecular mechanisms involved in the selectivity and signaling of the PACAP subfamily of receptors, and future considerations for therapeutic targeting.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Secretin Amino-Terminal Structure-Activity Relationships and Complementary Mutagenesis at the Site of Docking to the Secretin Receptor

Class B1 G protein-coupled receptors are activated by peptides, with amino-terminal regions critical for biologic activity. Although high resolution structures exist, understanding of key features of the peptide activation domain that drive signaling is limited. In the secretin receptor (SecR) structure, interactions are observed between peptide residues His1 and Ser2 and seventh transmembrane segment (TM7) receptor residue E373. We interrogated these interactions using systematic structure-activity analysis of peptide and receptor. His1 was critical for binding and cAMP responses, but its orientation was not critical, and substitution could independently modify affinity and efficacy. Ser2 was also critical, with all substitutions reducing peptide affinity and functional responses proportionally. Mutation of E373 to conserved acidic Asp (E373D), uncharged polar Gln (E373Q), or charge-reversed basic Arg (E373R) did not alter receptor expression, with all exhibiting secretin-dependent cAMP accumulation. All position 373 mutants displayed reduced binding affinities and cAMP potencies for many peptide analogs, although relative effects of position 1 peptides were similar whereas position 2 peptides exhibited substantial differences. The peptide including basic Lys in position 2 was active at SecR having acidic Glu in position 373 and at E373D while exhibiting minimal activity at those receptors in which an acidic residue is absent in this position (E373Q and E373R). In contrast, the peptide including acidic Glu in position 2 was equipotent with secretin at E373R while being much less potent than secretin at wild-type SecR and E373D. These data support functional importance of a charge-charge interaction between the amino-terminal region of secretin and the top of TM7. SIGNIFICANCE STATEMENT: This work refines our molecular understanding of the activation mechanisms of class B1 G protein-coupled receptors. The amino-terminal region of secretin interacts with the seventh transmembrane segment of its receptor with structural specificity and with a charge-charge interaction helping to drive functional activation.

Minimal Increments of Hydrophobic Collapse within the N-Terminus of the Neuropeptide Galanin

The neuropeptide galanin has a 35-year history as an intriguing target in drug design owing to its implication as a potential anticonvulsant and neuronal trophic factor among many other therapeutically interesting functions including analgesia and mood alteration. In this study, we report the structural characterization of three synthetic fragments of the galanin N-terminus in buffered aqueous solution: hGal(2-12)KK, hGal(1-12)KK, and hGal(1-17)KK. High-field two-dimensional ¹H-¹H nuclear magnetic resonance (NMR) data were acquired for these fragments and used to derive distance restraints. We further utilized modified hydrogen bonding and dihedral restraints to reflect chemical shift patterns in the data, which revealed the signature of a weakly folded helix. Together, these sets of restraints were used to generate NMR structures of all three fragments, which depict a core of hydrophobic residues that cluster together regardless of the presence of a helical structure, and correspond to residues in the N-terminus of galanin that have been previously shown to be critical for binding its receptors. The helical structure only appears following the inclusion of Gly(1) in the sequence, and at longer sequence lengths, unlike many other peptides, the helix does not propagate. Rather, a few turns of poorly ordered helix appear to be a secondary consequence of clusters of hydrophobic sidechains that are conserved across all of the peptides in this study; the helices themselves appear ordered as a consequence of this clustering, and these clusters compare directly to those observed recently to make contacts between galanin and two of its receptor subtypes. Collapsed hydrophobic residues therefore organize and compose the functional core of human galanin and raise interesting questions about the nature of the conformational order in ligands that bind cell surface receptors.