Structural basis of natural ligand binding and activation of the Class II G-protein-coupled secretin receptor

Secretin Receptor as a Target in Gastrointestinal Cancer: Expression Analysis and Ligand Development

Secretin was originally discovered as a gastrointestinal peptide that stimulates fluid secretion from the pancreas and liver and delays gastric emptying. In disease, a secretin receptor (SCTR) was found to occur as a splice variant in gastrinoma and pancreatic adenocarcinoma. Overexpression of SCTR has been described for gastrinomas, carcinoid tumors of the lung and cholangiocarcinoma. SCTR therefore is considered a candidate target for molecular tumor imaging as well as for peptide receptor radioligand therapy (PRRT) in a number of oncological indications. The aim of this study was to characterize SCTR expression in esophageal and pancreatic cancer, demonstrating for the first time high SCTR overexpression in these tumor types. In total, 65 of 70 pancreatic ductal adenocarcinoma tissues stained strongly positive for SCTR in immunohistochemistry, as did most of the 151 esophageal cancer samples, with minor influence of grading in both entities. In addition, the aim of this study was to further delineate residues in human secretin that are critical for binding to and activation of human SCTR. For a potential development of short and metabolically stable analogs for clinical use, it was intended to probe the peptide for its capacity to incorporate deletions and substitutions without losing its affinity to SCTR. In a systematic approach, a library of 146 secretin variants containing single amino acid substitutions as well as truncations on either end was tested in β-arrestin2-GFP translocation and fluorescent ligand internalization assays employing high-content analysis, in cAMP assays which run in agonist and antagonist mode, and in radioligand binding. The main structural determinants of SCTR binding and activation were localized to the N-terminus, with His1, Asp3 being among the most sensitive positions, followed by Phe6, Thr7 and Leu10. Aminoterminal truncation caused a rapid decline in receptor activity and most of these variants proved to be partial agonists showing antagonistic properties. In this study, the most potent novel antagonist showed an IC50 of 309 ± 74 nM in the β-arrestin2-GFP translocation assay on human SCTR while remaining a weak partial agonist. Future studies will have to demonstrate the utility of further enhanced secretin analogues as tracers for in vivo imaging and therapy.

Generation of a highly diverse panel of antagonistic chicken monoclonal antibodies against the GIP receptor

Raising functional antibodies against G protein-coupled receptors (GPCRs) is challenging due to their low density expression, instability in the absence of the cell membrane's lipid bilayer and frequently short extracellular domains that can serve as antigens. In addition, a particular therapeutic concept may require an antibody to not just bind the receptor, but also act as a functional receptor agonist or antagonist. Antagonizing the glucose-dependent insulinotropic polypeptide (GIP) receptor may open up new therapeutic modalities in the treatment of diabetes and obesity. As such, a panel of monoclonal antagonistic antibodies would be a useful tool for in vitro and in vivo proof of concept studies. The receptor is highly conserved between rodents and humans, which has contributed to previous mouse and rat immunization campaigns generating very few usable antibodies. Switching the immunization host to chicken, which is phylogenetically distant from mammals, enabled the generation of a large and diverse panel of monoclonal antibodies containing 172 unique sequences. Three-quarters of all chicken-derived antibodies were functional antagonists, exhibited high-affinities to the receptor extracellular domain and sampled a broad epitope repertoire. For difficult targets, including GPCRs such as GIPR, chickens are emerging as valuable immunization hosts for therapeutic antibody discovery.

Use of Cysteine Trapping to Map Spatial Approximations between Residues Contributing to the Helix N-capping Motif of Secretin and Distinct Residues within Each of the Extracellular Loops of Its Receptor

Amino-terminal regions of secretin-family peptides contain key determinants for biological activity and binding specificity, although the nature of interactions with receptors is unclear. A helix N-capping motif within this region has been postulated to directly contribute to agonist activity while also stabilizing formation of a helix extending toward the peptide carboxyl terminus and docking within the receptor amino terminus. We used cysteine trapping to systematically explore spatial approximations between cysteines replacing each residue in this motif of secretin (sec), Phe(6), Thr(7), and Leu(10), and cysteines incorporated into the extracellular face of the receptor. Each peptide was a full agonist for cAMP, but had a lower binding affinity than natural hormone. These bound to COS cells expressing 61 receptor constructs incorporating cysteines in every position along each extracellular loop (ECL) and adjacent parts of transmembrane (TM) segments. Patterns of covalent labeling were distinct for each probe, with Cys(6)-sec labeling multiple residues in the carboxyl-terminal half of ECL2 and throughout ECL3, Cys(7)-sec predominantly labeling only single residues in the carboxyl-terminal end of ECL2 and the amino-terminal end of ECL3, and Cys(10)-sec not efficiently labeling any of these residues. These spatial constraints were used to refine our model of secretin bound to its receptor, now bringing ECL3 above the amino terminus of the ligand and revealing possible charge-charge interactions between this part of secretin and receptor residues in TM5, TM6, ECL2, and ECL3, which can orient and stabilize the peptide-receptor complex. This was validated by testing predicted approximations by mutagenesis and residue-residue complementation studies.

Computational identification of novel natural inhibitors of glucagon receptor for checking type II diabetes mellitus

BACKGROUND

Interaction of the small peptide hormone glucagon with glucagon receptor (GCGR) stimulates the release of glucose from the hepatic cells during fasting; hence GCGR performs a significant function in glucose homeostasis. Inhibiting the interaction between glucagon and its receptor has been reported to control hepatic glucose overproduction and thus GCGR has evolved as an attractive therapeutic target for the treatment of type II diabetes mellitus.

RESULTS

In the present study, a large library of natural compounds was screened against 7 transmembrane domain of GCGR to identify novel therapeutic molecules that can inhibit the binding of glucagon with GCGR. Molecular dynamics simulations were performed to study the dynamic behaviour of the docked complexes and the molecular interactions between the screened compounds and the ligand binding residues of GCGR were analysed in detail. The top scoring compounds were also compared with already documented GCGR inhibitors- MK-0893 and LY2409021 for their binding affinity and other ADME properties. Finally, we have reported two natural drug like compounds PIB and CAA which showed good binding affinity for GCGR and are potent inhibitor of its functional activity.

CONCLUSION

This study contributes evidence for application of these compounds as prospective small ligand molecules against type II diabetes. Novel natural drug like inhibitors against the 7 transmembrane domain of GCGR have been identified which showed high binding affinity and potent inhibition of GCGR.

Fish genomes provide novel insights into the evolution of vertebrate secretin receptors and their ligand

The secretin receptor (SCTR) is a member of Class 2 subfamily B1 GPCRs and part of the PAC1/VPAC receptor subfamily. This receptor has long been known in mammals but has only recently been identified in other vertebrates including teleosts, from which it was previously considered to be absent. The ligand for SCTR in mammals is secretin (SCT), an important gastrointestinal peptide, which in teleosts has not yet been isolated, or the gene identified. This study revises the evolutionary model previously proposed for the secretin-GPCRs in metazoan by analysing in detail the fishes, the most successful of the extant vertebrates. All the Actinopterygii genomes analysed and the Chondrichthyes and Sarcopterygii fish possess a SCTR gene that shares conserved sequence, structure and synteny with the tetrapod homologue. Phylogenetic clustering and gene environment comparisons revealed that fish and tetrapod SCTR shared a common origin and diverged early from the PAC1/VPAC subfamily group. In teleosts SCTR duplicated as a result of the fish specific whole genome duplication but in all the teleost genomes analysed, with the exception of tilapia (Oreochromis niloticus), one of the duplicates was lost. The function of SCTR in teleosts is unknown but quantitative PCR revealed that in both sea bass (Dicentrarchus labrax) and tilapia (Oreochromis mossambicus) transcript abundance is high in the gastrointestinal tract suggesting it may intervene in similar processes to those in mammals. In contrast, no gene encoding the ligand SCT was identified in the ray-finned fishes (Actinopterygii) although it was present in the coelacanth (lobe finned fish, Sarcopterygii) and in the elephant shark (holocephalian). The genes in linkage with SCT in tetrapods and coelacanth were also identified in ray-finned fishes supporting the idea that it was lost from their genome. At present SCTR remains an orphan receptor in ray-finned fishes and it will be of interest in the future to establish why SCT was lost and which ligand substitutes for it so that full characterization of the receptor can occur.

Regulation of glucose homeostasis by GLP-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is a secreted peptide that acts as a key determinant of blood glucose homeostasis by virtue of its abilities to slow gastric emptying, to enhance pancreatic insulin secretion, and to suppress pancreatic glucagon secretion. GLP-1 is secreted from L cells of the gastrointestinal mucosa in response to a meal, and the blood glucose-lowering action of GLP-1 is terminated due to its enzymatic degradation by dipeptidyl-peptidase-IV (DPP-IV). Released GLP-1 activates enteric and autonomic reflexes while also circulating as an incretin hormone to control endocrine pancreas function. The GLP-1 receptor (GLP-1R) is a G protein-coupled receptor that is activated directly or indirectly by blood glucose-lowering agents currently in use for the treatment of type 2 diabetes mellitus (T2DM). These therapeutic agents include GLP-1R agonists (exenatide, liraglutide, lixisenatide, albiglutide, dulaglutide, and langlenatide) and DPP-IV inhibitors (sitagliptin, vildagliptin, saxagliptin, linagliptin, and alogliptin). Investigational agents for use in the treatment of T2DM include GPR119 and GPR40 receptor agonists that stimulate the release of GLP-1 from L cells. Summarized here is the role of GLP-1 to control blood glucose homeostasis, with special emphasis on the advantages and limitations of GLP-1-based therapeutics.

Regulation of glucose homeostasis by GLP-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is a secreted peptide that acts as a key determinant of blood glucose homeostasis by virtue of its abilities to slow gastric emptying, to enhance pancreatic insulin secretion, and to suppress pancreatic glucagon secretion. GLP-1 is secreted from L cells of the gastrointestinal mucosa in response to a meal, and the blood glucose-lowering action of GLP-1 is terminated due to its enzymatic degradation by dipeptidyl-peptidase-IV (DPP-IV). Released GLP-1 activates enteric and autonomic reflexes while also circulating as an incretin hormone to control endocrine pancreas function. The GLP-1 receptor (GLP-1R) is a G protein-coupled receptor that is activated directly or indirectly by blood glucose-lowering agents currently in use for the treatment of type 2 diabetes mellitus (T2DM). These therapeutic agents include GLP-1R agonists (exenatide, liraglutide, lixisenatide, albiglutide, dulaglutide, and langlenatide) and DPP-IV inhibitors (sitagliptin, vildagliptin, saxagliptin, linagliptin, and alogliptin). Investigational agents for use in the treatment of T2DM include GPR119 and GPR40 receptor agonists that stimulate the release of GLP-1 from L cells. Summarized here is the role of GLP-1 to control blood glucose homeostasis, with special emphasis on the advantages and limitations of GLP-1-based therapeutics.

Key role of CRF in the skin stress response system

The discovery of corticotropin-releasing factor (CRF) or CRH defining the upper regulatory arm of the hypothalamic-pituitary-adrenal (HPA) axis, along with the identification of the corresponding receptors (CRFRs 1 and 2), represents a milestone in our understanding of central mechanisms regulating body and local homeostasis. We focused on the CRF-led signaling systems in the skin and offer a model for regulation of peripheral homeostasis based on the interaction of CRF and the structurally related urocortins with corresponding receptors and the resulting direct or indirect phenotypic effects that include regulation of epidermal barrier function, skin immune, pigmentary, adnexal, and dermal functions necessary to maintain local and systemic homeostasis. The regulatory modes of action include the classical CRF-led cutaneous equivalent of the central HPA axis, the expression and function of CRF and related peptides, and the stimulation of pro-opiomelanocortin peptides or cytokines. The key regulatory role is assigned to the CRFR-1α receptor, with other isoforms having modulatory effects. CRF can be released from sensory nerves and immune cells in response to emotional and environmental stressors. The expression sequence of peptides includes urocortin/CRF→pro-opiomelanocortin→ACTH, MSH, and β-endorphin. Expression of these peptides and of CRFR-1α is environmentally regulated, and their dysfunction can lead to skin and systemic diseases. Environmentally stressed skin can activate both the central and local HPA axis through either sensory nerves or humoral factors to turn on homeostatic responses counteracting cutaneous and systemic environmental damage. CRF and CRFR-1 may constitute novel targets through the use of specific agonists or antagonists, especially for therapy of skin diseases that worsen with stress, such as atopic dermatitis and psoriasis.

Structure of the human glucagon class B G-protein-coupled receptor

Binding of the glucagon peptide to the glucagon receptor (GCGR) triggers the release of glucose from the liver during fasting; thus GCGR plays an important role in glucose homeostasis. Here we report the crystal structure of the seven transmembrane helical domain of human GCGR at 3.4 Å resolution, complemented by extensive site-specific mutagenesis, and a hybrid model of glucagon bound to GCGR to understand the molecular recognition of the receptor for its native ligand. Beyond the shared seven transmembrane fold, the GCGR transmembrane domain deviates from class A G-protein-coupled receptors with a large ligand-binding pocket and the first transmembrane helix having a 'stalk' region that extends three alpha-helical turns above the plane of the membrane. The stalk positions the extracellular domain (~12 kilodaltons) relative to the membrane to form the glucagon-binding site that captures the peptide and facilitates the insertion of glucagon's amino terminus into the seven transmembrane domain.

Strategies for studying the ligand binding site of GPCRs: photoaffinity labeling of the VPAC1 receptor, a prototype of class B GPCRs

G protein-coupled receptors (GPCRs) are crucial receptors acting as molecular sensors for many physiological and pathological processes. Class B GPCRs represent a small GPCR subfamily encompassing 15 members, and are very promising targets for the development of drugs to improve many diseases such as chronic inflammation, neurodegeneration, diabetes, stress, and osteoporosis. Over the past decade, structure-function relationship studies have demonstrated that the N-terminal ectodomain (N-ted) of class B GPCRs plays a pivotal role in natural ligand recognition. The N-ted structure of some class B GPCRs folds into a Sushi domain consisting of two antiparallel β sheets stabilized by three disulfide bonds and a salt bridge. The VPAC1 receptor is an archetype of class B GPCRs that binds vasoactive intestinal peptide (VIP), a neuropeptide modulating many physiological processes. The structure-function relationship of VPAC1 has been extensively studied. The use of a photoaffinity labeling strategy has been a powerful approach to determine the physical contacts between the functional receptor and its ligand. Those studies, coupled with 3D molecular modeling techniques, have clearly demonstrated the crucial role of the VPAC1 receptor N-ted in VIP recognition.

Ligand binding and activation of the secretin receptor, a prototypic family B G protein-coupled receptor

The secretin receptor is a prototypic member of family B G protein-coupled receptors that binds and responds to a linear 27-residue peptide natural ligand. The carboxyl-terminal region of this peptide assumes a helical conformation that occupies the peptide-binding cleft within the structurally complex disulphide-bonded amino-terminal domain of this receptor. The amino terminus of secretin is directed toward the core helical bundle domain of this receptor that seems to be structurally distinct from the analogous region of family A G protein-coupled receptors. This amino-terminal region of secretin is critical for its biological activity, to stimulate Gs coupling and the agonist-induced cAMP response. While the natural peptide ligand is known to span the two key receptor domains, with multiple residue-residue approximation constraints well established, the orientation of the receptor amino terminus relative to the receptor core helical bundle domain is still unclear. Fluorescence studies have established that the mid-region and carboxyl-terminal end of secretin are protected by the receptor peptide-binding cleft and the amino terminus of secretin is most exposed to the aqueous milieu as it is directed toward the receptor core, with the mid-region of the peptide becoming more exposed upon receptor activation. Like other family B peptide hormone receptors, the secretin receptor is constitutively present in a structurally specific homo-dimeric complex built around the lipid-exposed face of transmembrane segment four. This complex is important for facilitating G protein association and achieving the high affinity state of this receptor.

Characterization of chicken secretin (SCT) and secretin receptor (SCTR) genes: a novel secretin-like peptide (SCT-LP) and secretin encoded in a single gene

Secretin and the secretin receptor have been reported to play an important role in regulating pancreatic water and bicarbonate secretion in mammals; however, little is known about their expression, structure, and biological functions in non-mammalian vertebrates including birds. In this study, the full-length cDNAs encoding secretin and secretin receptor have first been cloned from duodenum of adult chickens. The putative chicken secretin receptor (cSCTR) is 449 amino acids in length and shares high sequence identity (58-63%) with its mammalian counterparts. Interestingly, chicken secretin cDNA encodes not only the secretin peptide (cSCT), but also a novel secretin-like peptide (cSCT-LP), which shares high amino acid identity with chicken (56%) and mammalian (48-52%) secretin. Using a pGL3-CRE-luciferase reporter system, we further demonstrated that both cSCT (EC(50): 0.31nM) and cSCT-LP (EC(50): 1.10nM), but not other structurally-related peptides, could potently activate cSCTR expressed in CHO cells, suggesting that both peptides may function as potential ligands for cSCTR. Using RT-PCR, the expression of secretin and secretin receptor in adult chicken tissues was also examined. Secretin was detected to be predominantly expressed in small intestine, while the mRNA expression of cSCTR was restricted to several tissues including gastrointestinal tract, liver, testis, pancreas and several brain regions. Collectively, results from present study not only established a molecular basis to elucidate the physiological roles of SCT, SCT-LP and SCTR in chickens, but also provide critical insights into structural and functional changes of secretin and its receptor during vertebrate evolution.

Receptor oligomerization: from early evidence to current understanding in class B GPCRs

Dimerization or oligomerization of G protein-coupled receptors (GPCRs) are known to modulate receptor functions in terms of ontogeny, ligand-oriented regulation, pharmacological diversity, signal transduction, and internalization. Class B GPCRs are receptors to a family of hormones including secretin, growth hormone-releasing hormone, vasoactive intestinal polypeptide and parathyroid hormone, among others. The functional implications of receptor dimerization have extensively been studied in class A GPCRs, while less is known regarding its function in class B GPCRs. This article reviews receptor oligomerization in terms of the early evidence and current understanding particularly of class B GPCRs.

Secretin is not necessary for exocrine pancreatic development and growth in mice

Adaptive exocrine pancreatic growth is mediated primarily by dietary protein and the gastrointestinal hormone cholecystokinin (CCK). Feeding trypsin inhibitors such as camostat (FOY-305) is known to induce CCK release and stimulate pancreatic growth. However, camostat has also been reported to stimulate secretin release and, because secretin often potentiates the action of CCK, it could participate in the growth response. Our aim was to test the role of secretin in pancreatic development and adaptive growth through the use of C57BL/6 mice with genetic deletion of secretin or secretin receptor. The lack of secretin in the intestine or the secretin receptor in the pancreas was confirmed by RT-PCR. Other related components, such as vasoactive intestinal polypeptide (VIP) receptors (VPAC(1) and VPAC(2)), were not affected. Secretin increased cAMP levels in acini from wild-type (WT) mice but had no effect on acini from secretin receptor-deleted mice, whereas VIP and forskolin still induced a normal response. Secretin in vivo failed to induce fluid secretion in receptor-deficient mice. The pancreas of secretin or secretin receptor-deficient mice was of normal size and histology, indicating that secretin is not necessary for normal pancreatic differentiation or maintenance. When WT mice were fed 0.1% camostat in powdered chow, the pancreas doubled in size in 1 wk, accompanied by parallel increases in protein and DNA. Camostat-fed littermate secretin and secretin receptor-deficient mice had similar pancreatic mass to WT mice. These results indicate that secretin is not required for normal pancreatic development or adaptive growth mediated by CCK.

Photo-cross-linkers incorporated into G-protein-coupled receptors in mammalian cells: a ligand comparison

Capturing the right ligand at the right spot: a well-balanced system for non-natural amino acid mutagenesis allows the ligand binding sites of a class II G-protein coupled receptor to be mapped and distinct binding domains to be identified for different ligands in the native environment of mammalian cells.

Origin of secretin receptor precedes the advent of tetrapoda: evidence on the separated origins of secretin and orexin

At present, secretin and its receptor have only been identified in mammals, and the origin of this ligand-receptor pair in early vertebrates is unclear. In addition, the elusive similarities of secretin and orexin in terms of both structures and functions suggest a common ancestral origin early in the vertebrate lineage. In this article, with the cloning and functional characterization of secretin receptors from lungfish and X. laevis as well as frog (X. laevis and Rana rugulosa) secretins, we provide evidence that the secretin ligand-receptor pair has already diverged and become highly specific by the emergence of tetrapods. The secretin receptor-like sequence cloned from lungfish indicates that the secretin receptor was descended from a VPAC-like receptor prior the advent of sarcopterygians. To clarify the controversial relationship of secretin and orexin, orexin type-2 receptor was cloned from X. laevis. We demonstrated that, in frog, secretin and orexin could activate their mutual receptors, indicating their coordinated complementary role in mediating physiological processes in non-mammalian vertebrates. However, among the peptides in the secretin/glucagon superfamily, secretin was found to be the only peptide that could activate the orexin receptor. We therefore hypothesize that secretin and orexin are of different ancestral origins early in the vertebrate lineage.

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.

Passing the baton in class B GPCRs: peptide hormone activation via helix induction?

G-protein-coupled receptors (GPCRs) represent the largest constellation of validated drug targets. Crystal structures of class A GPCRs have facilitated major advances in understanding the principles underlying GPCR activation. By contrast, relatively little is known about class B GPCRs, a family of receptors for a variety of therapeutically relevant peptide hormones. Encouraging progress has recently been made through the structural elucidation of several extracellular hormone-binding domains of class B GPCRs in complex with their natural ligands or synthetic analogues. The structures reveal similar modes of ligand binding, with concomitant alpha-helical structuring of the ligand. The latter suggests an attractive mechanical model for class B GPCR activation.

Family Resemblances? Ligand Binding and Activation of Family A and B G-Protein-Coupled Receptors

In April 2007, the Biochemical Society held a meeting to compare and contrast ligand binding and activation of Family A and B GPCRs (G-protein-coupled receptors). Being the largest class, Family A GPCRs usually receive the most attention, although a previous Biochemical Society meeting has focused on Family B GPCRs. The aim of the present meeting was to bring researchers of both families together in order to identify commonalities between the two. The present article introduces the proceedings of the meeting, briefly commenting on the focus of each of the following articles.