Molecular modeling of the three-dimensional structure of GLP-1R and its interactions with several agonists

Neuroprotective Mechanisms of Glucagon-Like Peptide-1-Based Therapies in Ischemic Stroke: An Update Based on Preclinical Research

The public and social health burdens of ischemic stroke have been increasing worldwide. Hyperglycemia leads to a greater risk of stroke. This increased risk is commonly seen among patients with diabetes and is in connection with worsened clinical conditions and higher mortality in patients with acute ischemic stroke (AIS). Therapy for stroke focuses mainly on restoring cerebral blood flow (CBF) and ameliorating neurological impairment caused by stroke. Although choices of stroke treatment remain limited, much advance have been achieved in assisting patients in recovering from ischemic stroke, along with progress of recanalization therapy through pharmacological and mechanical thrombolysis. However, it is still necessary to develop neuroprotective therapies for AIS to protect the brain against injury before and during reperfusion, prolong the time window for intervention, and consequently improve neurological prognosis. Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are broadly regarded as effective drugs in the treatment of type 2 diabetes mellitus (T2DM). Preclinical data on GLP-1 and GLP-1 RAs have displayed an impressive neuroprotective efficacy in stroke, Parkinson's disease (PD), Alzheimer's disease (AD), Amyotrophic lateral sclerosis (ALS), and other neurodegenerative diseases. Based on the preclinical studies in the past decade, we review recent progress in the biological roles of GLP-1 and GLP-1 RAs in ischemic stroke. Emphasis will be placed on their neuroprotective effects in experimental models of cerebral ischemia stroke at cellular and molecular levels.

Novel Small Molecule Glucagon-Like Peptide-1 Receptor Agonist S6 Stimulates Insulin Secretion From Rat Islets

Glucagon-like peptide-1 receptor (GLP-1R) agonist-based therapeutics for type 2 diabetes mellitus have attracted worldwide attention. However, there are challenges in the development of small molecule GLP-1R agonists owing to the complexity of ligand recognition and signal induction mechanisms. Here, we attained S6 using virtual screening and fluorescent imaging plate reader (FLIPR)-based calcium assays. The purpose of this study was to identify and characterize S6, a novel small molecule GLP-1R agonist. Data from cellular thermal shift assay (CETSA) and Bio-Layer Interferometry (BLI) indicated that S6 could bind potently with GLP-1R. Radioimmunoassay data showed that S6 potentiated insulin secretion in a glucose-dependent manner and the insulinotropic effect was mediated by GLP-1R. Calcium imaging techniques suggested that S6 elevated the intracellular calcium concentration [(Ca²⁺)_i] by activating GLP-1R. In patch-clamp experiments, we demonstrated that S6 inhibited voltage-dependent K⁺ (Kv) channels in a GLP-1R-dependent fashion. Besides, S6 significantly prolonged action potential duration but had no effect on voltage-dependent Ca²⁺ channels. In summary, these findings indicate that S6 stimulates glucose-dependent insulin secretion mainly by acting on GLP-1R, inhibiting Kv channels, increasing (Ca²⁺)_i. This study will provide direction for the screening and development of novel small-molecule agents targeting GLP-1R in the future.

Controlled Release of Pyrimidine Compound Using Polymeric Coated ZIF-8 Metal-Organic Framework as Glucagon-Like Peptide-1 Receptor Agonist Carrier

This work demonstrates synthetic strategies for the incorporation of a synthesized pyrimidine glucagon-like peptide-1 (GLP-1) agonist into alginate-coated ZIF-8. The prepared pyrimidine GLP-1 agonist used for the treatment of diabetes type II, was trapped inside polymer coated ZIF-8. The encapsulation of the GLP-1 agonist was confirmed by UV-visible and FT-IR spectroscopies. Furthermore, the release kinetics of GLP-1 agonist drug from alginate-coated ZIF-8 were investigated in phosphate-buffered saline at 37 °C at pH 8 and 1.5. The alginate-coated ZIF-8 exhibited much faster drug release at basic pH than at pH 1.5, indicating the potential of the alginate-coated ZIF-8 system to overcome the fast degradation at acidic pH of the stomach and improve the drug's activity. This study may open the way for the synthesis of new metal organic frameworks (MOFs) to enhance drug delivery systems.

Drug discovery approaches targeting the incretin pathway

Incretin pathway plays an important role in the development of diabetes medications. Interventions in DPP-4 and GLP-1 receptor have shown remarkable efficacy in experimental and clinical studies and imperatively become one of the most promising therapeutic approaches in the T2DM drug discovery pipeline. Herein, we analyzed the actionmechanismsof DPP-4 and GLP-1 receptor targeting the incretin pathway in T2DM treatment. We gave an insight into the structural requirements for the potent DPP-4 inhibitors and revealed a classification of DPP-4 inhibitors by stressing on the binding modes of these ligands to the enzyme. We then reviewed the drug discovery strategies for the development of peptide and non-peptide GLP-1 receptor agonists (GLP-1 RAs). Furthermore, the drug design strategies for DPP-4 inhibitors and GLP-1R agonists were detailed accurately. This review might provide an efficient evidence for the highly potent and selective DPP-4 inhibitors and the GLP-1 RAs, as novel medicines for patients suffering from T2DM.

Cochinchinenin C, a potential nonpolypeptide anti-diabetic drug, targets a glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is currently being explored as a therapeutic target for anti-diabetic drugs.

An Orally Active Allosteric GLP-1 Receptor Agonist Is Neuroprotective in Cellular and Rodent Models of Stroke

Diabetes is a major risk factor for the development of stroke. Glucagon-like peptide-1 receptor (GLP-1R) agonists have been in clinical use for the treatment of diabetes and also been reported to be neuroprotective in ischemic stroke. The quinoxaline 6,7-dichloro-2-methylsulfonyl-3-N-tert- butylaminoquinoxaline (DMB) is an agonist and allosteric modulator of the GLP-1R with the potential to increase the affinity of GLP-1 for its receptor. The aim of this study was to evaluate the neuroprotective effects of DMB on transient focal cerebral ischemia. In cultured cortical neurons, DMB activated the GLP-1R, leading to increased intracellular cAMP levels with an EC50 value about 100 fold that of exendin-4. Pretreatment of neurons with DMB protected against necrotic and apoptotic cell death was induced by oxygen-glucose deprivation (OGD). The neuroprotective effects of DMB were blocked by GLP-1R knockdown with shRNA but not by GLP-1R antagonism. In C57BL/6 mice, DMB was orally administered 30 min prior to middle cerebral artery occlusion (MCAO) surgery. DMB markedly reduced the cerebral infarct size and neurological deficits caused by MCAO and reperfusion. The neuroprotective effects were mediated by activation of the GLP-1R through the cAMP-PKA-CREB signaling pathway. DMB exhibited anti-apoptotic effects by modulating Bcl-2 family members. These results provide evidence that DMB, a small molecular GLP-1R agonist, attenuates transient focal cerebral ischemia injury and inhibits neuronal apoptosis induced by MCAO. Taken together, these data suggest that DMB is a potential neuroprotective agent against cerebral ischemia.

Computational insight into conformational states of glucagon-like peptide-1 receptor (GLP-1R) and its binding mode with GLP-1

It was observed that the apo-GLP-1R stabilized in the ‘closed’ state, while GLP-1R coupled with GLP-1 stabilized in the ‘open’ state.

Stereoselective Synthesis of 1,3-Diaminotruxillic Acid Derivatives: An Advantageous Combination of C-H-ortho-Palladation and On-Flow [2+2]-Photocycloaddition in Microreactors

The stereoselective synthesis of ε-isomers of dimethyl esters of 1,3-diaminotruxillic acid in three steps is reported. The first step is the ortho-palladation of (Z)-2-aryl-4-aryliden-5(4H)-oxazolones 1 to give dinuclear complexes 2 with bridging carboxylates. The reaction occurs through regioselective activation of the ortho-CH bond of the 4-arylidene ring in carboxylic acids. The second step is the [2+2]-photocycloaddition of the CC exocyclic bonds of the oxazolone skeleton in 2 to afford the corresponding dinuclear ortho-palladated cyclobutanes 3. This key step was performed very efficiently by using LED light sources with different wavelengths (465, 525 or 625 nm) in flow microreactors. The final step involved the depalladation of 3 by hydrogenation in methanol to afford the ε-1,3-diaminotruxillic acid derivatives as single isomers.

Effects of truncating van der Waals interactions in lipid bilayer simulations

In membrane simulations, it is known that truncating electrostatic interactions results in artificial ordering of lipids at the truncation distance. However, less attention has been paid to the effect of truncating van der Waals (VDW) interactions. Since the VDW potential decays as r(-6), it is frequently neglected beyond a cutoff of around 1 nm. In some cases, analytical dispersion corrections appropriate for isotropic systems are applied to the pressure and the potential energy. In this work, we systematically study the effect of truncating VDW interactions at different cutoffs in 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine bilayers with the Berger force field. We show that the area per lipid decreases systematically when the VDW cutoff (r(c)) increases. This dependence persists even when dispersion corrections are applied. Since the analytical form of the dispersion correction is only appropriate for isotropic systems, we suggest that a long VDW cutoff should be used in preference over a short VDW cutoff. To determine the appropriate cutoff, we simulate liquid pentadecane with the Berger parameters and find that r(c) ≥ 1.4 nm is sufficient to reproduce the density and the heat of vaporization of pentadecane. Bilayers simulated with r(c) ≥ 1.4 nm show an improved agreement with experiments in both the form factors and the deuterium order parameters. Finally, we report that the VDW cutoff has a significant impact on the lipid flip-flop energetics and an inappropriate short VDW cutoff results in a bilayer that is prone to form water defects across the bilayer.

Ligand binding pocket formed by evolutionarily conserved residues in the glucagon-like peptide-1 (GLP-1) receptor core domain

Glucagon-like peptide-1 (GLP-1) plays a pivotal role in glucose homeostasis through its receptor GLP1R. Due to its multiple beneficial effects, GLP-1 has gained great attention for treatment of type 2 diabetes and obesity. However, little is known about the molecular mechanism underlying the interaction of GLP-1 with the heptahelical core domain of GLP1R conferring high affinity ligand binding and ligand-induced receptor activation. Here, using chimeric and point-mutated GLP1R, we determined that the evolutionarily conserved amino acid residue Arg(380) flanked by hydrophobic Leu(379) and Phe(381) in extracellular loop 3 (ECL3) may have an interaction with Asp(9) and Gly(4) of the GLP-1 peptide. The molecular modeling study showed that Ile(196) at transmembrane helix 2, Met(233) at ECL1, and Asn(302) at ECL2 of GLP1R have contacts with His(1) and Thr(7) of GLP-1. This study may shed light on the mechanism underlying high affinity interaction between the ligand and the binding pocket that is formed by these conserved residues in the GLP1R core domain.

Molecular evolution of GPCRs: GLP1/GLP1 receptors

Glucagon-like peptide 1 (GLP1) is an intestinal incretin that regulates glucose homeostasis through stimulation of insulin secretion from pancreatic β-cells and inhibits appetite by acting on the brain. Thus, it is a promising therapeutic agent for the treatment of type 2 diabetes mellitus and obesity. Studies using synteny and reconstructed ancestral chromosomes suggest that families for GLP1 and its receptor (GLP1R) have emerged through two rounds (2R) of whole genome duplication and local gene duplications before and after 2R. Exon duplications have also contributed to the expansion of the peptide family members. Specific changes in the amino acid sequence following exon/gene/genome duplications have established distinct yet related peptide and receptor families. These specific changes also confer selective interactions between GLP1 and GLP1R. In this review, we present a possible macro (genome level)- and micro (gene/exon level)-evolution mechanisms of GLP1 and GLP1R, which allows them to acquire selective interactions between this ligand-receptor pair. This information may provide critical insight for the development of potent therapeutic agents targeting GLP1R.

Glucose-lowering drugs in patients with chronic kidney disease: a narrative review on pharmacokinetic properties

The achievement of a good glycaemic control is one of the cornerstones for preventing and delaying progression of microvascular and macrovascular complications in patients with both diabetes and chronic kidney disease (CKD). As for other drugs, the presence of an impaired renal function may significantly affect pharmacokinetics of the majority of glucose-lowering agents, thus exposing diabetic CKD patients to a higher risk of side effects, mainly hypoglycaemic episodes. As a consequence, a reduction in dosing and/or frequency of administration is necessary to keep a satisfactory efficacy/safety profile. In this review, we aim to summarize the pharmacology of the most widely used glucose-lowering agents, discuss whether and how it is altered by a reduced renal function, and the recommendations that can be made for their use in patients with different degrees of CKD.

Advances in pharmacologic therapies for type 2 diabetes

Type 2 diabetes (T2DM) is a multi-causal, heterogeneous and progressive cardiometabolic condition, with an increasing prevalence worldwide. T2DM is associated with multiple comorbidities that may impact patients' quality of life. Treatment is multifactorial, but pharmacologic treatment of hyperglycemia is still regarded as the mainstay of diabetes management. Current established therapies include metformin, sulfonylurea agents and insulin, the long-term use of which was associated with reduced micro- and macrovascular events in the United Kingdom Prospective Diabetes Study. Despite major recent advances in diabetes care, a large proportion of patients remain in poor glycemic control, necessitating the development of new therapeutic options. The recently published position statement of the American Diabetes Association and European Association for the Study of Diabetes for the management of hyperglycemia in T2DM has accommodated this wider range of therapy choices, as it is less prescriptive and advocates an individualized treatment approach, taking into account many relevant patient- and disease-related factors. This review summarizes the updates on various established agents as well as the recent developments with regard to incretin-based therapies, inhibitors of the renal tubular sodium-glucose-linked-transporter-2 and ultra-long acting basal insulin formulations.

A novel glucagon-related peptide (GCRP) and its receptor GCRPR account for coevolution of their family members in vertebrates

The glucagon (GCG) peptide family consists of GCG, glucagon-like peptide 1 (GLP1), and GLP2, which are derived from a common GCG precursor, and the glucose-dependent insulinotropic polypeptide (GIP). These peptides interact with cognate receptors, GCGR, GLP1R, GLP2R, and GIPR, which belong to the secretin-like G protein-coupled receptor (GPCR) family. We used bioinformatics to identify genes encoding a novel GCG-related peptide (GCRP) and its cognate receptor, GCRPR. The GCRP and GCRPR genes were found in representative tetrapod taxa such as anole lizard, chicken, and Xenopus, and in teleosts including medaka, fugu, tetraodon, and stickleback. However, they were not present in mammals and zebrafish. Phylogenetic and genome synteny analyses showed that GCRP emerged through two rounds of whole genome duplication (2R) during early vertebrate evolution. GCRPR appears to have arisen by local tandem gene duplications from a common ancestor of GCRPR, GCGR, and GLP2R after 2R. Biochemical ligand-receptor interaction analyses revealed that GCRP had the highest affinity for GCRPR in comparison to other GCGR family members. Stimulation of chicken, Xenopus, and medaka GCRPRs activated Gα_s-mediated signaling. In contrast to chicken and Xenopus GCRPRs, medaka GCRPR also induced Gα_q/11-mediated signaling. Chimeric peptides and receptors showed that the K¹⁶M¹⁷K¹⁸ and G¹⁶Q¹⁷A¹⁸ motifs in GCRP and GLP1, respectively, may at least in part contribute to specific recognition of their cognate receptors through interaction with the receptor core domain. In conclusion, we present novel data demonstrating that GCRP and GCRPR evolved through gene/genome duplications followed by specific modifications that conferred selective recognition to this ligand-receptor pair.

The structure and function of the glucagon-like peptide-1 receptor and its ligands

Glucagon-like peptide-1(7-36)amide (GLP-1) is a 30-residue peptide hormone released from intestinal L cells following nutrient consumption. It potentiates the glucose-induced secretion of insulin from pancreatic beta cells, increases insulin expression, inhibits beta-cell apoptosis, promotes beta-cell neogenesis, reduces glucagon secretion, delays gastric emptying, promotes satiety and increases peripheral glucose disposal. These multiple effects have generated a great deal of interest in the discovery of long-lasting agonists of the GLP-1 receptor (GLP-1R) in order to treat type 2 diabetes. This review article summarizes the literature regarding the discovery of GLP-1 and its physiological functions. The structure, function and sequence-activity relationships of the hormone and its natural analogue exendin-4 (Ex4) are reviewed in detail. The current knowledge of the structure of GLP-1R, a Family B GPCR, is summarized and discussed, before its known interactions with the principle peptide ligands are described and summarized. Finally, progress in discovering non-peptide ligands of GLP-1R is reviewed. GLP-1 is clearly an important hormone linking nutrient consumption with blood sugar control, and therefore knowledge of its structure, function and mechanism of action is of great importance.

The development of non-peptide glucagon-like peptide-1 receptor agonist for the treatment of type 2 diabetes

Glucagon-like peptide-1 (GLP-1) is the main member of the incretin family and stimulates insulin secretion by binding with its specific receptor on pancreatic β-cells. In addition, GLP-1 exerts broad beneficial effects on the glucose regulation by suppressing food intake and delaying stomach emptying. Now, long acting GLP-1 analogs including exenatide and liraglutide have been approved for the treatment of diabetes mellitus type 2, however long-term injection can limit their use for these chronic patients. In this report, the authors provide a review on the development of non-peptide GLP-1 receptor agonists and introduce a novel agonist DA-15864.

Structural and molecular conservation of glucagon-like Peptide-1 and its receptor confers selective ligand-receptor interaction

Glucagon-like peptide-1 (GLP-1) is a major player in the regulation of glucose homeostasis. It acts on pancreatic beta cells to stimulate insulin secretion and on the brain to inhibit appetite. Thus, it may be a promising therapeutic agent for the treatment of type 2 diabetes mellitus and obesity. Despite the physiological and clinical importance of GLP-1, molecular interaction with the GLP-1 receptor (GLP1R) is not well understood. Particularly, the specific amino acid residues within the transmembrane helices and extracellular loops of the receptor that may confer ligand-induced receptor activation have been poorly investigated. Amino acid sequence comparisons of GLP-1 and GLP1R with their orthologs and paralogs in vertebrates, combined with biochemical approaches, are useful to determine which amino acid residues in the peptide and the receptor confer selective ligand-receptor interaction. This article reviews how the molecular evolution of GLP-1 and GLP1R contributes to the selective interaction between this ligand-receptor pair, providing critical clues for the development of potent agonists for the treatment of diabetes mellitus and obesity.

Transmembrane α-helix 2 and 7 are important for small molecule-mediated activation of the GLP-1 receptor

Glucagon-like peptide-1 (GLP-1) activates the GLP-1 receptor (GLP-1R), which belongs to family B of the G-protein-coupled receptors. We previously identified a selective small molecule ligand, compound 2, that acted as a full agonist and allosteric modulator of GLP-1R. In this study, the structurally related small molecule, compound 3, stimulated cAMP production from GLP-1R, but not from the homologous glucagon receptor (GluR). The receptor selectivity encouraged a chimeric receptor approach to identify domains important for compound 3-mediated activation of GLP-1R. A subsegment of the GLP-1R transmembrane domain containing TM2 to TM5 was sufficient to transfer compound 3 responsiveness to GluR. Therefore, divergent residues in this subsegment of GLP-1R and GluR are responsible for the receptor selectivity of compound 3. Functional analyses of other chimeric receptors suggested that the existence of a helix-helix interface between TM1 and TM7 is important for the compound 3 response. Furthermore, site-directed mutagenesis revealed that a Phe195-Leu substitution in TM2 and a Thr391-Ala substitution in TM7 increased and decreased the efficacy of compound 3 without disturbing the potency or efficacy of GLP-1. Collectively, differential effects of receptor mutations suggest that TM2 and/or TM7 are important for compound 3-mediated activation of GLP-1R.

Evolutionarily conserved residues at glucagon-like peptide-1 (GLP-1) receptor core confer ligand-induced receptor activation

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) play important roles in insulin secretion through their receptors, GLP1R and GIPR. Although GLP-1 and GIP are attractive candidates for treatment of type 2 diabetes and obesity, little is known regarding the molecular interaction of these peptides with the heptahelical core domain of their receptors. These core domains are important not only for specific ligand binding but also for ligand-induced receptor activation. Here, using chimeric and point-mutated GLP1R/GIPR, we determined that evolutionarily conserved amino acid residues such as Ile(196) at transmembrane helix 2, Leu(232) and Met(233) at extracellular loop 1, and Asn(302) at extracellular loop 2 of GLP1R are responsible for interaction with ligand and receptor activation. Application of chimeric GLP-1/GIP peptides together with molecular modeling suggests that His(1) of GLP-1 interacts with Asn(302) of GLP1R and that Thr(7) of GLP-1 has close contact with a binding pocket formed by Ile(196), Leu(232), and Met(233) of GLP1R. This study may provide critical clues for the development of peptide and/or nonpeptide agonists acting at GLP1R.

Residues within the transmembrane domain of the glucagon-like peptide-1 receptor involved in ligand binding and receptor activation: modelling the ligand-bound receptor

The C-terminal regions of glucagon-like peptide-1 (GLP-1) bind to the N terminus of the GLP-1 receptor (GLP-1R), facilitating interaction of the ligand N terminus with the receptor transmembrane domain. In contrast, the agonist exendin-4 relies less on the transmembrane domain, and truncated antagonist analogs (e.g. exendin 9-39) may interact solely with the receptor N terminus. Here we used mutagenesis to explore the role of residues highly conserved in the predicted transmembrane helices of mammalian GLP-1Rs and conserved in family B G protein coupled receptors in ligand binding and GLP-1R activation. By iteration using information from the mutagenesis, along with the available crystal structure of the receptor N terminus and a model of the active opsin transmembrane domain, we developed a structural receptor model with GLP-1 bound and used this to better understand consequences of mutations. Mutation at Y152 [transmembrane helix (TM) 1], R190 (TM2), Y235 (TM3), H363 (TM6), and E364 (TM6) produced similar reductions in affinity for GLP-1 and exendin 9-39. In contrast, other mutations either preferentially [K197 (TM2), Q234 (TM3), and W284 (extracellular loop 2)] or solely [D198 (TM2) and R310 (TM5)] reduced GLP-1 affinity. Reduced agonist affinity was always associated with reduced potency. However, reductions in potency exceeded reductions in agonist affinity for K197A, W284A, and R310A, while H363A was uncoupled from cAMP generation, highlighting critical roles of these residues in translating binding to activation. Data show important roles in ligand binding and receptor activation of conserved residues within the transmembrane domain of the GLP-1R. The receptor structural model provides insight into the roles of these residues.

Functional association of the N-terminal residues with the central region in glucagon-related peptides

GLP-1 is an incretin peptide involved in the regulation of glucose metabolism and the glucose-dependent stimulation of insulin secretion. Ex-4 is a paralog of GLP-1 that has comparable GLP-1R potency but extended physiological action. GLP-1 and Ex-4 are helical peptides that share ∼50% sequence homology but differ at several residues, notably the second amino acid which controls susceptibility to DPP-IV cleavage. This single amino acid difference yields divergent receptor potency when studied in the context of the two hormone sequences. Ex-4 uniquely tolerates Gly2 through select amino acid differences in the middle region of the peptide that are absent in GLP-1. We report that substitution of Ex-4 amino acids Glu16, Leu21, and Glu24 to the GLP-1 sequence enabled Gly2 tolerance. The coordination of the N-terminus with these central residues shows an interaction of substantial importance not only to DPP-IV stability but also to receptor activation. Extension of this observation to glucagon-based co-agonist peptides showed different structural requirements for effective communication between the N-terminus and the mid-section of these peptides in achieving high potency agonism at the GLP-1 and GCGRs.

Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling

The glucagon-like peptide 1 (GLP1) receptor is an important drug target within the B family of G protein-coupled receptors. Its natural agonist ligand, GLP1, has incretin-like actions and the receptor is a recognized target for management of type 2 diabetes mellitus. Despite recent solution of the structure of the amino terminus of the GLP1 receptor and several close family members, the molecular basis for GLP1 binding to and activation of the intact receptor remains unclear. We previously demonstrated molecular approximations between amino- and carboxyl-terminal residues of GLP1 and its receptor. In this work, we study spatial approximations with the mid-region of this peptide to gain insights into the orientation of the intact receptor and the ligand-receptor complex. We have prepared two new photolabile probes incorporating a p-benzoyl-l-phenylalanine into positions 16 and 20 of GLP1(7-36). Both probes bound to the GLP1 receptor specifically and with high affinity. These were each fully efficacious agonists, stimulating cAMP accumulation in receptor-bearing CHO cells in a concentration-dependent manner. Each probe specifically labeled a single receptor site. Protease cleavage and radiochemical sequencing identified receptor residue Leu(141) above transmembrane segment one as its site of labeling for the position 16 probe, whereas the position 20 probe labeled receptor residue Trp(297) within the second extracellular loop. Establishing ligand residue approximation with this loop region is unique among family members and may help to orient the receptor amino-terminal domain relative to its helical bundle region.

A novel human-based receptor antagonist of sustained action reveals body weight control by endogenous GLP-1

Ex-4 (9-39)a is a well characterized GLP-1 receptor antagonist that suffers from two notable limitations, its nonhuman amino acid sequence and its relatively short in vivo duration of action. Comparable N-terminal shortening of human GLP-1 lessens agonism but does not provide a high potency antagonist. Through a series of GLP-1/Ex-4 hybrid peptides, the minimal structural changes required to generate a pure GLP-1-based antagonist were identified as Glu16, Val19, and Arg20, yielding an antagonist of approximately 3-fold greater in vitro potency compared with Ex-4 (9-39)a. The structural basis of antagonism appears to result from stabilization of the α helix combined with enhanced electrostatic and hydrophobic interactions with the extracellular domain of the receptor. Site-specific acylation of the human-based antagonist yielded a peptide of increased potency as a GLP-1 receptor antagonist and 10-fold greater selectivity relative to the GIP receptor. The acylated antagonist demonstrated sufficient duration of action to maintain inhibitory activity when administered as a daily subcutaneous injection. The sustained pharmacokinetics and enhanced human sequence combine to form an antagonist optimized for clinical study. Daily administration of this antagonist by subcutaneous injection to diet-induced obese mice for 1 week caused a significant increase in food intake, body weight, and glucose intolerance, demonstrating endogenous GLP-1 as a relevant hormone in mammalian energy balance in the obese state.

Novel small molecule glucagon-like peptide-1 receptor agonist stimulates insulin secretion in rodents and from human islets

OBJECTIVE

The clinical effectiveness of parenterally-administered glucagon-like peptide-1 (GLP-1) mimetics to improve glucose control in patients suffering from type 2 diabetes strongly supports discovery pursuits aimed at identifying and developing orally active, small molecule GLP-1 receptor agonists. The purpose of these studies was to identify and characterize novel nonpeptide agonists of the GLP-1 receptor.

RESEARCH DESIGN AND METHODS

Screening using cells expressing the GLP-1 receptor and insulin secretion assays with rodent and human islets were used to identify novel molecules. The intravenous glucose tolerance test (IVGTT) and hyperglycemic clamp characterized the insulinotropic effects of compounds in vivo.

RESULTS

Novel low molecular weight pyrimidine-based compounds that activate the GLP-1 receptor and stimulate glucose-dependent insulin secretion are described. These molecules induce GLP-1 receptor-mediated cAMP signaling in HEK293 cells expressing the GLP-1 receptor and increase insulin secretion from rodent islets in a dose-dependent manner. The compounds activate GLP-1 receptor signaling, both alone or in an additive fashion when combined with the endogenous GLP-1 peptide; however, these agonists do not compete with radiolabeled GLP-1 in receptor-binding assays. In vivo studies using the IVGTT and the hyperglycemic clamp in Sprague Dawley rats demonstrate increased insulin secretion in compound-treated animals. Further, perifusion assays with human islets isolated from a donor with type 2 diabetes show near-normalization of insulin secretion upon compound treatment.

CONCLUSIONS

These studies characterize the insulinotropic effects of an early-stage, small molecule GLP-1 receptor agonist and provide compelling evidence to support pharmaceutical optimization.

Near-infrared fluorescent probe for imaging of pancreatic beta cells

The ability to image and ultimately quantitate beta-cell mass in vivo will likely have far reaching implications in the study of diabetes biology, in the monitoring of disease progression or response to treatment, and for drug development. Here, using animal models, we report on the synthesis, characterization, and intravital microscopic imaging properties of a near-infrared fluorescent exendin-4 analogue with specificity for the GLP-1 receptor on beta cells (E4(K12)-Fl). The agent demonstrated subnanomolar EC(50) binding concentrations, with high specificity and binding that could be inhibited by GLP-1R agonists. Following intravenous administration to mice, pancreatic islets were readily distinguishable from exocrine pancreas, achieving target-to-background ratios within the pancreas of 6:1, as measured by intravital microscopy. Serial imaging revealed rapid accumulation kinetics (with initial signal within the islets detectable within 3 min and peak fluorescence within 20 min of injection), making this an ideal agent for in vivo imaging.

Non-peptidic glucose-like peptide-1 receptor agonists: aftermath of a serendipitous discovery

Glucagon-like peptide-1 (GLP-1) receptor is an ideal target in the development of incretin-based therapies for diabetes and obesity. Two approaches have been adopted: GLP-1 receptor agonists that mimic the effects of native GLP-1 and dipeptidyl peptidase-4 inhibitors that increase endogenous GLP-1 levels. During the past two decades, search for orally active, non-peptidic GLP-1 receptor agonists has been the focal point of research and development activities in many multinational pharmaceutical companies. Such efforts have not resulted in any success thus far. Serendipitous discovery of substituted cyclobutanes represented by Boc5 as a new class of GLP-1 receptor agonists led us to believe that a small molecule approach to class B G-protein coupled receptor agonism is no longer a fantasy but a reality. However, major obstacles still pose great challenges, and the reasons of which are discussed in this perspectives.

Spatial approximations between residues 6 and 12 in the amino-terminal region of glucagon-like peptide 1 and its receptor: a region critical for biological activity

Understanding the molecular basis of natural ligand binding and activation of the glucagon-like peptide 1 (GLP1) receptor may facilitate the development of agonist drugs useful for the management of type 2 diabetes mellitus. We previously reported molecular approximations between carboxyl-terminal residues 24 and 35 within GLP1 and its receptor. In this work, we have focused on the amino-terminal region of GLP1, known to be critical for receptor activation. We developed two high-affinity, full agonist photolabile GLP1 probes having sites of covalent attachment in positions 6 and 12 of the 30-residue peptide (GLP1(7-36)). Both probes bound to the receptor specifically and covalently labeled single distinct sites. Chemical and protease cleavage of the labeled receptor identified the juxtamembrane region of its amino-terminal domain as the region of covalent attachment of the position 12 probe, whereas the region of labeling by the position 6 probe was localized to the first extracellular loop. Radiochemical sequencing identified receptor residue Tyr(145), adjacent to the first transmembrane segment, as the site of labeling by the position 12 probe, and receptor residue Tyr(205), within the first extracellular loop, as the site of labeling by the position 6 probe. These data provide support for a common mechanism for natural ligand binding and activation of family B G protein-coupled receptors. This region of interaction of peptide amino-terminal domains with the receptor may provide a pocket that can be targeted by small molecule agonists.

Comparative effects of the endogenous agonist glucagon-like peptide-1 (GLP-1)-(7-36) amide and the small-molecule ago-allosteric agent "compound 2" at the GLP-1 receptor

Glucagon-like peptide-1 (GLP-1) mediates antidiabetogenic effects through the GLP-1 receptor (GLP-1R), which is targeted for the treatment of type 2 diabetes. Small-molecule GLP-1R agonists have been sought due to difficulties with peptide therapeutics. Recently, 6,7-dichloro-2-methylsulfonyl-3-N-tert-butylaminoquinoxaline (compound 2) has been described as a GLP-1R allosteric modulator and agonist. Using human embryonic kidney-293 cells expressing human GLP-1Rs, we extended this work to consider the impact of compound 2 on G protein activation, Ca(2+) signaling and receptor internalization and particularly to compare compound 2 and GLP-1 across a range of functional assays in intact cells. GLP-1 and compound 2 activated Galpha(s) in cell membranes and increased cellular cAMP in intact cells, with compound 2 being a partial and almost full agonist, respectively. GLP-1 increased intracellular [Ca(2+)] by release from intracellular stores, which was mimicked by compound 2, with slower kinetics. In either intact cells or membranes, the orthosteric antagonist exendin-(9-39), inhibited GLP-1 cAMP generation but increased the efficacy of compound 2. GLP-1 internalized enhanced green fluorescent protein-tagged GLP-1Rs, but the speed and magnitude evoked by compound 2 were less. Exendin-(9-39) inhibited internalization by GLP-1 and also surprisingly that by compound 2. Compound 2 displays GLP-1R agonism consistent with action at an allosteric site, although an orthosteric antagonist increased its efficacy on cAMP and blocked compound 2-mediated receptor internalization. Full assessment of the properties of compound 2 was potentially hampered by damaging effects that were particularly manifest in either longer term assays with intact cells or in acute assays with membranes.

In silico point mutation and evolutionary trace analysis applied to nicotinic acetylcholine receptors in deciphering ligand-binding surfaces

The nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily and contain ligand gated ion channels (LGIC). These receptors are located mostly in the central nervous system (CNS) and peripheral nervous system (PNS). nAChRs reside at pre-synaptic regions to mediate acetylcholine neurotransmission and in the post synaptic membrane to propagate nerve impulses through neurons via acetylcholine. Malfunction of this neurotransmitter receptor is believed to cause various neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and schizophrenia, and nAChRs are thus important drug targets. In the present work, starting from an earlier model of pentameric alpha7nAChR, a considerable effort has been taken to investigate interaction with ligands by performing docking studies with a diverse array of agonists and antagonists. Analysis of these docking complexes reveals identification of possible ligand-interacting residues. Some of these residues, e.g. Ser34, Gln55, Ser146, and Tyr166, which are evolutionarily conserved, were specifically subjected to virtual mutations based on their amino acid properties and found to be highly sensitive in the presence of antagonists by docking. Further, the study was extended using evolutionary trace analysis, revealing conserved and class-specific residues close to the putative ligand-binding site, further supporting the results of docking experiments.

Emerging cardiovascular actions of the incretin hormone glucagon-like peptide-1: potential therapeutic benefits beyond glycaemic control?

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted by the small intestine in response to nutrient ingestion. It has wide-ranging effects on glucose metabolism, including stimulation of insulin release, inhibition of glucagon secretion, reduction of gastric emptying and augmentation of satiety. Importantly, the insulinotropic actions of GLP-1 are uniquely dependent on ambient glucose concentrations, and it is this particular characteristic which has led to its recent emergence as a treatment for type 2 diabetes. Although the major physiological function of GLP-1 appears to be in relation to glycaemic control, there is growing evidence to suggest that it may also play an important role in the cardiovascular system. GLP-1 receptors (GLP-1Rs) are expressed in the heart and vasculature of both rodents and humans, and recent studies have demonstrated that GLP-1R agonists have wide-ranging cardiovascular actions, such as modulation of heart rate, blood pressure, vascular tone and myocardial contractility. Importantly, it appears that these agents may also have beneficial effects in the setting of cardiovascular disease (CVD). For example, GLP-1 has been found to exert cardioprotective actions in experimental models of dilated cardiomyopathy, hypertensive heart failure and myocardial infarction (MI). Preliminary clinical studies also indicate that GLP-1 infusion may improve cardiac contractile function in chronic heart failure patients with and without diabetes, and in MI patients after successful angioplasty. This review will discuss the current understanding of GLP-1 biology, examine its emerging cardiovascular actions in both health and disease and explore the potential use of GLP-1 as a novel treatment for CVD.

Structural basis for ligand recognition of incretin receptors

The glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor are homologous G-protein-coupled receptors (GPCRs). Incretin receptor agonists stimulate the synthesis and secretion of insulin from pancreatic β-cells and are therefore promising agents for the treatment of type 2 diabetes. It is well established that the N-terminal extracellular domain (ECD) of incretin receptors is important for ligand binding and ligand specificity, whereas the transmembrane domain is involved in receptor activation. Structures of the ligand-bound ECD of incretin receptors have been solved recently by X-ray crystallography. The crystal structures reveal a similar fold of the ECD and a similar mechanism of ligand binding, where the ligand adopts an α-helical conformation. Residues in the C-terminal part of the ligand interact directly with the ECD and hydrophobic interactions appear to be the main driving force for ligand binding to the ECD of incretin receptors. Obviously, the-still missing-structures of full-length incretin receptors are required to construct a complete picture of receptor function at the molecular level. However, the progress made recently in structural analysis of the ECDs of incretin receptors and related GPCRs has shed new light on the process of ligand recognition and binding and provided a basis to disclose some of the mechanisms underlying receptor activation at high resolution.

Design, synthesis and in vitro characterization of Glucagon-Like Peptide-1 derivatives for pancreatic beta cell imaging by SPECT

Novel Glucagon-Like Peptide-1 (GLP-1) derivatives containing the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and naturally occurring Indium ((113/115)In) were prepared using solid-phase Fmoc methods. All synthesized peptides contained d-Ala-8, a modification known to improve resistance towards degradation by dipeptidyl peptidase-IV. The effect of increased distance between DOTA and the peptide chain was investigated using an (aminoethyl) ethoxy acetyl linker, in order to reduce steric effects imposed by DOTA. Placement of linker and DOTA moieties were also varied within the GLP-1 sequence to test for optimal metal-complex location. The binding affinity of the peptide derivatives was determined in vitro with Chinese hamster ovary cells stably transfected with a human GLP-1 receptor (CHO/GLP-1R) cell line and was shown to be in the nM range. Gamma camera imaging of an insulinoma cell line was carried out using (111)In-labeled peptides. Our results suggest that the prepared GLP-1 derivatives are suitable imaging probes for studying pancreatic islet function in vivo.

Role of the signal peptide in the synthesis and processing of the glucagon-like peptide-1 receptor

BACKGROUND AND PURPOSE

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B of the G protein-coupled receptor superfamily and is a target for treatment of type 2 diabetes. Family B G protein-coupled receptors contain a putative N-terminal signal peptide, but its role in receptor synthesis and trafficking are unclear. Further, the signal peptide is not cleaved in at least one family member.

EXPERIMENTAL APPROACH

We examined receptor glycosylation and the role of the signal peptide in GLP-1R synthesis and trafficking using constructs containing epitope tags at the N- and/or C-terminus and in which the signal peptide sequence was either present or absent.

KEY RESULTS

The signal peptide was absolutely required for GLP-1R synthesis but could be substituted to some extent by increasing positive charge in the N-terminal region of the receptor flanking the signal peptide. The signal peptide is cleaved during synthesis and processing of the receptor. An enhanced GFP-epitope tag at the N-terminus of the receptor permitted synthesis of the receptor but blocked signal peptide cleavage and prevented trafficking to the plasma membrane. Cleavage site mutation allowed synthesis of a full-length receptor, blocked signal peptide cleavage and caused retention within the endoplasmic reticulum.

CONCLUSIONS AND IMPLICATIONS

Signal peptide cleavage was not essential for receptor synthesis but was obligatory for processing and trafficking of receptors to the plasma membrane. Further, the GLP-1R is subject to N-linked glycosylation and only the mature, fully glycosylated form of the receptor is present in the plasma membrane. Inhibition of glycosylation prevents processing and cell surface expression of the GLP-1R.

Crystal structure of glucagon-like peptide-1 in complex with the extracellular domain of the glucagon-like peptide-1 receptor

GLP-1 (glucagon-like peptide-1) is an incretin released from intestinal L-cells in response to food intake. Activation of the GLP-1 receptor potentiates the synthesis and release of insulin from pancreatic β-cells in a glucose-dependent manner. The GLP-1 receptor belongs to class B of the G-protein-coupled receptors, a subfamily characterized by a large N-terminal extracellular ligand binding domain. Exendin-4 and GLP-1 are 50% identical, and exendin-4 is a full agonist with similar affinity and potency for the GLP-1 receptor. We recently solved the crystal structure of the GLP-1 receptor extracellular domain in complex with the competitive antagonist exendin-4(9–39). Interestingly, the isolated extracellular domain binds exendin-4 with much higher affinity than the endogenous agonist GLP-1. Here, we have solved the crystal structure of the extracellular domain in complex with GLP-1 to 2.1 Åresolution. The structure shows that important hydrophobic ligand-receptor interactions are conserved in agonist- and antagonist-bound forms of the extracellular domain, but certain residues in the ligand-binding site adopt a GLP-1-specific conformation. GLP-1 is a kinked but continuous α-helix from Thr¹³ to Val³³ when bound to the extracellular domain. We supplemented the crystal structure with site-directed mutagenesis to link the structural information of the isolated extracellular domain with the binding properties of the full-length receptor. The data support the existence of differences in the binding modes of GLP-1 and exendin-4 on the full-length GLP-1 receptor.

Molecular basis of glucagon-like peptide 1 docking to its intact receptor studied with carboxyl-terminal photolabile probes

The glucagon-like peptide 1 (GLP1) receptor is a member of Family B G protein-coupled receptors and represents an important drug target for type 2 diabetes. Despite recent solution of the structure of the amino-terminal domain of this receptor and that of several close family members, understanding of the molecular basis of natural ligand GLP1 binding to its intact receptor remains limited. The goal of this study was to explore spatial approximations between specific receptor residues within the carboxyl terminus of GLP1 and its receptor as normally docked. Therefore, we developed and characterized two high affinity, full-agonist photolabile GLP1 probes having sites for covalent attachment in positions 24 and 35. Both probes labeled the receptor specifically and saturably. Subsequent peptide mapping using chemical and proteinase cleavages of purified wild-type and mutant GLP1 receptor identified that the Arg(131)-Lys(136) segment at the juxtamembrane region of the receptor amino terminus contained the site of labeling for the position 24 probe, and the specific receptor residue labeled by this probe was identified as Glu(133) by radiochemical sequencing. Similarly, nearby residue Glu(125) within the same region of the receptor amino-terminal domain was identified as the site of labeling by the position 35 probe. These data represent the first direct demonstration of spatial approximation between GLP1 and its intact receptor as docked, providing two important constraints for the modeling of this interaction. This should expand our understanding of the molecular basis of natural agonist ligand binding to the GLP1 receptor and may be relevant to other family members.