Two basic residues of the h-VPAC1 receptor second transmembrane helix are essential for ligand binding and signal transduction

Molecular cloning, sequence characteristics, and tissue expression analysis of glucagon receptor gene in Bama minipig

Recent studies have shown that the glucagon receptor (GCGR) plays an important role in the development of type 2 diabetes mellitus. Both pigs and humans exhibit significantly similar behaviors in their glucose and lipid metabolism. In this study, the obtained Bama minipig GCGR coding sequence was 1437 bp encoding 479 amino acids (AA), which demonstrated higher sequence homology with humans than other species. It showed the highest expression profile in the liver, followed by the lung and kidney. In addition, the three-dimensional structure analysis showed that the porcine GCGR protein also had a classic sevenfold transmembrane region and a stalk region at the N-terminus for ligand binding. The stalk region of GCGR possessed five AA variations. The ligand binding pocket of GCGR has one AA variation in the key region, none of which affected the glucagon binding verified by the crystal structure mutagenesis in humans. There was no variation found in the region of membrane anchoring, hydrophobic bond, salt bridge, and hydrogen bond. However, the Gly40Ser mutation in mice resulted in major diseases, meaning that pigs are more suitable for the evaluation of GCGR-related drugs than mice.

Cryo-EM structure of an activated VIP1 receptor-G protein complex revealed by a NanoBiT tethering strategy

Vasoactive intestinal polypeptide receptor (VIP1R) is a widely expressed class B G protein-coupled receptor and a drug target for the treatment of neuronal, metabolic, and inflammatory diseases. However, our understanding of its mechanism of action and the potential of drug discovery targeting this receptor is limited by the lack of structural information of VIP1R. Here we report a cryo-electron microscopy structure of human VIP1R bound to PACAP27 and Gs heterotrimer, whose complex assembly is stabilized by a NanoBiT tethering strategy. Comparison with other class B GPCR structures reveals that PACAP27 engages VIP1R with its N-terminus inserting into the ligand binding pocket at the transmembrane bundle of the receptor, which subsequently couples to the G protein in a receptor-specific manner. This structure has provided insights into the molecular basis of PACAP27 binding and VIP receptor activation. The methodology of the NanoBiT tethering may help to provide structural information of unstable complexes.

Vasoactive intestinal polypeptide receptor (VIP1R) is a widely expressed class B G protein-coupled receptor and a drug target for the treatment of inflammatory diseases. Here authors report a cryoelectron microscopy structure of human VIP1R bound to PACAP27 and Gs heterotrimer, which provides insights into PACAP27 binding and VIP receptor activation.

A Molecular Dynamics Study of Vasoactive Intestinal Peptide Receptor 1 and the Basis of Its Therapeutic Antagonism

Vasoactive intestinal peptide receptor 1 (VPAC1) is a member of a secretin-like subfamily of G protein-coupled receptors. Its endogenous neuropeptide (VIP), secreted by neurons and immune cells, modulates various physiological functions such as exocrine and endocrine secretions, immune response, smooth muscles relaxation, vasodilation, and fetal development. As a drug target, VPAC1 has been selected for therapy of inflammatory diseases but drug discovery is still hampered by lack of its crystal structure. In this study we presented the homology model of this receptor constructed with the well-known web service GPCRM. The VPAC1 model is composed of extracellular and transmembrane domains that form a complex with an endogenous hormone VIP. Using the homology model of VPAC1 the mechanism of action of potential drug candidates for VPAC1 was described. Only two series of small-molecule antagonists of confirmed biological activity for VPAC1 have been described thus far. Molecular docking and a series of molecular dynamics simulations were performed to elucidate their binding to VPAC1 and resulting antagonist effect. The presented work provides the basis for the possible binding mode of VPAC1 antagonists and determinants of their molecular recognition in the context of other class B GPCRs. Until the crystal structure of VPAC1 will be released, the presented homology model of VPAC1 can serve as a scaffold for drug discovery studies and is available from the author upon request.

An Overview of VPAC Receptors in Rheumatoid Arthritis: Biological Role and Clinical Significance

The axis comprised by the Vasoactive Intestinal Peptide (VIP) and its G protein-coupled receptors (GPCRs), VPAC1, and VPAC2, belong to the B1 family and signal through Gs or Gq proteins. VPAC receptors seem to preferentially interact with Gs in inflammatory cells, rather than Gq, thereby stimulating adenylate cyclase activity. cAMP is able to trigger various downstream pathways, mainly the canonical PKA pathway and the non-canonical cAMP-activated guanine nucleotide exchange factor (EPAC) pathway. Classically, the presence of VPACs has been confined to the plasma membrane; however, VPAC1 location has been described in the nuclear membrane in several cell types such as activated Th cells, where they are also functional. VPAC receptor signaling modulates a number of biological processes by tipping the balance of inflammatory mediators in macrophages and other innate immune cells, modifying the expression of TLRs, and inhibiting MMPs and the expression of adhesion molecules. Receptor signaling also downregulates coagulation factors and acute-phase proteins, promotes Th2 over Th1, stimulates Treg abundance, and finally inhibits a pathogenic Th17 profile. Thus, the VIP axis signaling regulates both the innate and adaptive immune responses in several inflammatory/autoimmune diseases. Rheumatoid arthritis (RA) is a complex autoimmune disease that develops on a substrate of genetically susceptible individuals and under the influence of environmental factors, as well as epigenetic mechanisms. It is a heterogeneous disease with different pathogenic mechanisms and variable clinical forms between patients with the same diagnosis. The knowledge of VIP signaling generated in both animal models and human ex vivo studies can potentially be translated to clinical reality. Most recently, the beneficial effects of nanoparticles of VIP self-associated with sterically stabilized micelles have been reported in a murine model of RA. Another novel research area is beginning to define the receptors as biomarkers in RA, with their expression levels shown to be associated with the activity of the disease and patients-reported impairment. Therefore, VPAC expression together VIP genetic variants could allow patients to be stratified at the beginning of the disease with the purpose of guiding personalized treatment decisions.

Extending the Structural View of Class B GPCRs

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

Expression and localization of VPAC1, the major receptor of vasoactive intestinal peptide along the length of the intestine

Vasoactive intestinal peptide (VIP) is an endogenous neuropeptide with a broad array of physiological functions in many organs including the intestine. Its actions are mediated via G protein-coupled receptors, and vasoactive intestinal peptide receptor 1 (VPAC1) is the key receptor responsible for majority of VIP's biological activity. The distribution of VPAC1 along the length of the gastrointestinal tract and its subcellular localization in intestinal epithelial cells have not been fully characterized. The current studies were undertaken to determine VPAC1 distribution and localization so that VIP-based therapies can be targeted to specific regions of the intestine. The results indicated that the mRNA levels of VPAC1 showed an abundance pattern of colon > ileum > jejunum in the mouse intestine. In parallel, the VPAC1 protein levels were higher in the mouse colon, followed by the ileum and jejunum. Immunofluorescence studies in mouse colon demonstrated that the receptor was specifically localized to the luminal surface, as was evident by colocalization with the apical marker villin but not with the basolateral marker Na+/K+-ATPase. In the human intestine, VPAC1 mRNA expression exhibited a distribution similar to that in mouse intestine and was highest in the sigmoid colon. Furthermore, in the human colon, VPAC1 also showed predominantly apical localization. The physiological relevance of the expression and apical localization of VPAC1 remains elusive. We speculate that apical VPAC1 in intestinal epithelial cells may have relevance in recognizing secreted peptides in the intestinal lumen and therefore supports the feasibility of potential therapeutic and targeting use of VIP formulations via oral route to treat gastrointestinal diseases.NEW & NOTEWORTHY These studies for the first time present comprehensive data on the relative characterization of vasoactive intestinal peptide (VIP) receptors in the intestinal mucosa. Vasoactive intestinal peptide receptor 1 (VPAC1) was identified as the predominant receptor with higher levels in the colon compared with the small intestine and was mainly localized to the apical membrane. In addition, the findings in the human tissues were consistent with VPAC1 expression in the mouse intestine and open possibilities to target colonic tissues with VIP for treating diseases such as inflammatory bowel disease.

High affinity binding of the peptide agonist TIP-39 to the parathyroid hormone 2 (PTH2) receptor requires the hydroxyl group of Tyr-318 on transmembrane helix 5

TIP39 ("tuberoinfundibular peptide of 39 residues") acts via the parathyroid hormone 2 receptor, PTH2, a Family B G protein-coupled receptor (GPCR). Despite the importance of GPCRs in human physiology and pharmacotherapy, little is known about the molecular details of the TIP39-PTH2 interaction. To address this, we utilised the different pharmacological profiles of TIP39 and PTH(1-34) at PTH2 and its related receptor PTH1: TIP39 being an agonist at the former but an antagonist at the latter, while PTH(1-34) activates both. A total of 23 site-directed mutations of PTH2, in which residues were substituted to the equivalent in PTH1, were made and pharmacologically screened for agonist activity. Follow-up mutations were analysed by radioligand binding and cAMP assays. A model of the TIP39-PTH2 complex was built and analysed using molecular dynamics. Only Tyr318-Ile displayed reduced TIP39 potency, despite having increased PTH(1-34) potency, and further mutagenesis and analysis at this site demonstrated that this was due to reduced TIP39 affinity at Tyr318-Ile (pIC50=6.01±0.03) compared with wild type (pIC50=7.81±0.03). The hydroxyl group of the Tyr-318's side chain was shown to be important for TIP39 binding, with the Tyr318-Phe mutant displaying 13-fold lower affinity and 35-fold lower potency compared with wild type. TIP39 truncated by up to 5 residues at the N-terminus was still sensitive to the mutations at Tyr-318, suggesting that it interacts with a region within TIP39(6-39). Molecular modelling and molecular dynamics simulations suggest that the selectivity is based on an interaction between the Tyr-318 hydroxyl group with the carboxylate side chain of Asp-7 of the peptide.

Structural Determinants of Binding the Seven-transmembrane Domain of the Glucagon-like Peptide-1 Receptor (GLP-1R)

The glucagon-like peptide-1 receptor (GLP-1R) belongs to the secretin-like (class B) family of G protein-coupled receptors. Members of the class B family are distinguished by their large extracellular domain, which works cooperatively with the canonical seven-transmembrane (7TM) helical domain to signal in response to binding of various peptide hormones. We have combined structure-based site-specific mutational studies with molecular dynamics simulations of a full-length model of GLP-1R bound to multiple peptide ligand variants. Despite the high sequence similarity between GLP-1R and its closest structural homologue, the glucagon receptor (GCGR), nearly half of the 62 stably expressed mutants affected GLP-1R in a different manner than the corresponding mutants in GCGR. The molecular dynamics simulations of wild-type and mutant GLP-1R·ligand complexes provided molecular insights into GLP-1R-specific recognition mechanisms for the N terminus of GLP-1 by residues in the 7TM pocket and explained how glucagon-mimicking GLP-1 mutants restored binding affinity for (GCGR-mimicking) GLP-1R mutants. Structural analysis of the simulations suggested that peptide ligand binding mode variations in the 7TM binding pocket are facilitated by movement of the extracellular domain relative to the 7TM bundle. These differences in binding modes may account for the pharmacological differences between GLP-1 peptide variants.

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

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.

A Hydrogen-Bonded Polar Network in the Core of the Glucagon-Like Peptide-1 Receptor Is a Fulcrum for Biased Agonism: Lessons from Class B Crystal Structures

The glucagon-like peptide 1 (GLP-1) receptor is a class B G protein-coupled receptor (GPCR) that is a key target for treatments for type II diabetes and obesity. This receptor, like other class B GPCRs, displays biased agonism, though the physiologic significance of this is yet to be elucidated. Previous work has implicated R2.60(190), N3.43(240), Q7.49(394), and H6.52(363) as key residues involved in peptide-mediated biased agonism, with R2.60(190), N3.43(240), and Q7.49(394) predicted to form a polar interaction network. In this study, we used novel insight gained from recent crystal structures of the transmembrane domains of the glucagon and corticotropin releasing factor 1 (CRF1) receptors to develop improved models of the GLP-1 receptor that predict additional key molecular interactions with these amino acids. We have introduced E6.53(364)A, N3.43(240)Q, Q7.49(394)N, and N3.43(240)Q/Q7.49(394)N mutations to probe the role of predicted H-bonding and charge-charge interactions in driving cAMP, calcium, or extracellular signal-regulated kinase (ERK) signaling. A polar interaction between E6.53(364) and R2.60(190) was predicted to be important for GLP-1- and exendin-4-, but not oxyntomodulin-mediated cAMP formation and also ERK1/2 phosphorylation. In contrast, Q7.49(394), but not R2.60(190)/E6.53(364) was critical for calcium mobilization for all three peptides. Mutation of N3.43(240) and Q7.49(394) had differential effects on individual peptides, providing evidence for molecular differences in activation transition. Collectively, this work expands our understanding of peptide-mediated signaling from the GLP-1 receptor and the key role that the central polar network plays in these events.

The peptide agonist-binding site of the glucagon-like peptide-1 (GLP-1) receptor based on site-directed mutagenesis and knowledge-based modelling

Mutagenesis and molecular pharmacological analysis of the glucagon-like peptide-1 (GLP-1) receptor highlighted several residues involved in peptide agonist recognition. Coupled with a new molecular model of the full-length agonist-docked receptor, the binding site and a pharmacophore for agonist peptides are described.

Glucagon-like peptide-1 (7–36)amide (GLP-1) plays a central role in regulating blood sugar levels and its receptor, GLP-1R, is a target for anti-diabetic agents such as the peptide agonist drugs exenatide and liraglutide. In order to understand the molecular nature of the peptide–receptor interaction, we used site-directed mutagenesis and pharmacological profiling to highlight nine sites as being important for peptide agonist binding and/or activation. Using a knowledge-based approach, we constructed a 3D model of agonist-bound GLP-1R, basing the conformation of the N-terminal region on that of the receptor-bound NMR structure of the related peptide pituitary adenylate cyclase-activating protein (PACAP21). The relative position of the extracellular to the transmembrane (TM) domain, as well as the molecular details of the agonist-binding site itself, were found to be different from the model that was published alongside the crystal structure of the TM domain of the glucagon receptor, but were nevertheless more compatible with published mutagenesis data. Furthermore, the NMR-determined structure of a high-potency cyclic conformationally-constrained 11-residue analogue of GLP-1 was also docked into the receptor-binding site. Despite having a different main chain conformation to that seen in the PACAP21 structure, four conserved residues (equivalent to His-7, Glu-9, Ser-14 and Asp-15 in GLP-1) could be structurally aligned and made similar interactions with the receptor as their equivalents in the GLP-1-docked model, suggesting the basis of a pharmacophore for GLP-1R peptide agonists. In this way, the model not only explains current mutagenesis and molecular pharmacological data but also provides a basis for further experimental design.

Transmembrane signal transduction by peptide hormones via family B G protein-coupled receptors

Although family B G protein-coupled receptors (GPCRs) contain only 15 members, they play key roles in transmembrane signal transduction of hormones. Family B GPCRs are drug targets for developing therapeutics for diseases ranging from metabolic to neurological disorders. Despite their importance, the molecular mechanism of activation of family B GPCRs remains largely unexplored due to the challenges in expression and purification of functional receptors to the quantity for biophysical characterization. Currently, there is no crystal structure available of a full-length family B GPCR. However, structures of key domains, including the extracellular ligand binding regions and seven-helical transmembrane regions, have been solved by X-ray crystallography and NMR, providing insights into the mechanisms of ligand recognition and selectivity, and helical arrangements within the cell membrane. Moreover, biophysical and biochemical methods have been used to explore functions, key residues for signaling, and the kinetics and dynamics of signaling processes. This review summarizes the current knowledge of the signal transduction mechanism of family B GPCRs at the molecular level and comments on the challenges and outlook for mechanistic studies of family B GPCRs.

Structural and functional insights into the juxtamembranous amino-terminal tail and extracellular loop regions of class B GPCRs

Class B guanine nucleotide-binding protein GPCRs share heptahelical topology and signalling via coupling with heterotrimeric G proteins typical of the entire superfamily of GPCRs. However, they also exhibit substantial structural differences from the more extensively studied class A GPCRs. Even their helical bundle region, most conserved across the superfamily, is predicted to differ from that of class A GPCRs. Much is now known about the conserved structure of the amino-terminal domain of class B GPCRs, coming from isolated NMR and crystal structures, but the orientation of that domain relative to the helical bundle is unknown, and even less is understood about the conformations of the juxtamembranous amino-terminal tail or of the extracellular loops linking the transmembrane segments. We now review what is known about the structure and function of these regions of class B GPCRs. This comes from indirect analysis of structure-function relationships elucidated by mutagenesis and/or ligand modification and from the more direct analysis of spatial approximation coming from photoaffinity labelling and cysteine trapping studies. Also reviewed are the limited studies of structure of some of these regions. No dominant theme was recognized for the structures or functional roles of distinct regions of these juxtamembranous portions of the class B GPCRs. Therefore, it is likely that a variety of molecular strategies can be engaged for docking of agonist ligands and for initiation of conformational changes in these receptors that would be expected to converge to a common molecular mechanism for activation of intracellular signalling cascades.

Insights into the structure of class B GPCRs

The secretin-like (class B) family of G protein-coupled receptors (GPCRs) are key players in hormonal homeostasis and are interesting drug targets for the treatment of several metabolic disorders (such as type 2 diabetes, osteoporosis, and obesity) and nervous system diseases (such as migraine, anxiety, and depression). The recently solved crystal structures of the transmembrane domains of the human glucagon receptor and human corticotropin-releasing factor receptor 1 have opened up new opportunities to study the structure and function of class B GPCRs. The current review shows how these structures offer more detailed explanations to previous biochemical and pharmacological studies of class B GPCRs, and provides new insights into their interactions with ligands.

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.

Mapping spatial approximations between the amino terminus of secretin and each of the extracellular loops of its receptor using cysteine trapping

While it is evident that the carboxyl-terminal region of natural peptide ligands bind to the amino-terminal domain of class B GPCRs, how their biologically critical amino-terminal regions dock to the receptor is unclear. We utilize cysteine trapping to systematically explore spatial approximations among residues in the first five positions of secretin and in every position within the receptor extracellular loops (ECLs). Only Cys(2) and Cys(5) secretin analogues exhibited full activity and retained moderate binding affinity (IC(50): 92±4 and 83±1 nM, respectively). When these peptides probed 61 human secretin receptor cysteine-replacement mutants, a broad network of receptor residues could form disulfide bonds consistent with a dynamic ligand-receptor interface. Two distinct patterns of disulfide bond formation were observed: Cys(2) predominantly labeled residues in the amino terminus of ECL2 and ECL3 (relative labeling intensity: Ser(340), 94±7%; Pro(341), 84±9%; Phe(258), 73±5%; Trp(274) 62±8%), and Cys(5) labeled those in the carboxyl terminus of ECL2 and ECL3 (Gln(348), 100%; Ile(347), 73±12%; Glu(342), 59±10%; Phe(351), 58±11%). These constraints were utilized in molecular modeling, providing improved understanding of the structure of the transmembrane bundle and interconnecting loops, the orientation between receptor domains, and the molecular basis of ligand docking. Key spatial approximations between peptide and receptor predicted by this model (H(1)-W(274), D(3)-N(268), G(4)-F(258)) were supported by mutagenesis and residue-residue complementation studies.

Calcitonin and calcitonin receptor-like receptors: common themes with family B GPCRs?

The calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR) are two of the 15 human family B (or Secretin-like) GPCRs. CTR and CLR are of considerable biological interest as their pharmacology is moulded by interactions with receptor activity-modifying proteins. They also have therapeutic relevance for many conditions, such as osteoporosis, diabetes, obesity, lymphatic insufficiency, migraine and cardiovascular disease. In light of recent advances in understanding ligand docking and receptor activation in both the family as a whole and in CLR and CTR specifically, this review reflects how applicable general family B GPCR themes are to these two idiosyncratic receptors. We review the main functional domains of the receptors; the N-terminal extracellular domain, the juxtamembrane domain and ligand interface, the transmembrane domain and the intracellular C-terminal domain. Structural and functional findings from the CLR and CTR along with other family B GPCRs are critically appraised to gain insight into how these domains may function. The ability for CTR and CLR to interact with receptor activity-modifying proteins adds another level of sophistication to these receptor systems but means careful consideration is needed when trying to apply generic GPCR principles. This review encapsulates current thinking in the realm of family B GPCR research by highlighting both conflicting and recurring themes and how such findings relate to two unusual but important receptors, CTR and CLR.

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.

Conformational switches in the VPAC(1) receptor

The vasoactive intestinal peptide receptor 1 (VPAC(1) ) belongs to family B of GPCRs and is activated upon binding of vasoactive intestinal peptide (VIP) and pituitary AC-activating polypeptide neuropeptides. Widely distributed throughout body, VPAC(1) plays important regulatory roles in human physiology and physiopathology. Like most members of the GPCR-B family, VPAC(1) receptor is predicted to follow the actual paradigm of a common 'two-domain' model of natural ligand action. However the precise structural basis for ligand binding, receptor activation and signal transduction are still incompletely understood due in part to the absence of X-ray crystal structure of the whole receptor and to significant structural differences with the most extensively studied family of receptor, the GPCR-A/rhodopsin family. Here, we try to summarize the current knowledge of the molecular mechanisms involved in VPAC(1) receptor activation and signal transduction. This includes search for amino acids involved in the two-step process of VIP binding, in the stabilization of VPAC(1) inactive and active conformations, and in binding and activation of G proteins.

The VPAC1 receptor: structure and function of a class B GPCR prototype

The class B G protein-coupled receptors (GPCRs) represents a small sub-family encompassing 15 members, and are very promising targets for the development of drugs to treat many diseases such as chronic inflammation, neurodegeneration, diabetes, stress, and osteoporosis. The VPAC1 receptor which is an archetype of the class B GPCRs binds Vasoactive Intestinal Peptide (VIP), a neuropeptide widely distributed in central and peripheral nervous system modulating many physiological processes including regulation of exocrine secretions, hormone release, foetal development, immune response … VIP appears to exert beneficial effect in neurodegenerative and inflammatory diseases. This article reviews the current knowledge regarding the structure and molecular pharmacology of VPAC1 receptors. Over the past decade, structure-function relationship studies have demonstrated that the N-terminal ectodomain (N-ted) of VPAC1 plays a pivotal role in VIP recognition. The use of different approaches such as directed mutagenesis, photoaffinity labeling, Nuclear Magnetic Resonance (NMR), molecular modeling, and molecular dynamic simulation has led to demonstrate that: (1) the central and C-terminal part of the VIP molecule interacts with the N-ted of VPAC1 receptor which is itself structured as a « Sushi » domain; (2) the N-terminal end of the VIP molecule interacts with the first transmembrane domain of the receptor where three residues (K(143), T(144), and T(147)) play an important role in VPAC1 interaction with the first histidine residue of VIP.

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.

Evidence that interaction between conserved residues in transmembrane helices 2, 3, and 7 are crucial for human VPAC1 receptor activation

The VPAC(1) receptor belongs to family B of G protein-coupled receptors (GPCR-B) and is activated upon binding of the vasoactive intestinal peptide (VIP). Despite the recent determination of the structure of the N terminus of several members of this receptor family, little is known about the structure of the transmembrane (TM) region and about the molecular mechanisms leading to activation. In the present study, we designed a new structural model of the TM domain and combined it with experimental mutagenesis experiments to investigate the interaction network that governs ligand binding and receptor activation. Our results suggest that this network involves the cluster of residues Arg(188) in TM2, Gln(380) in TM7, and Asn(229) in TM3. This cluster is expected to be altered upon VIP binding, because Arg(188) has been shown previously to interact with Asp(3) of VIP. Several point mutations at positions 188, 229, and 380 were experimentally characterized and were shown to severely affect VIP binding and/or VIP-mediated cAMP production. Double mutants built from reciprocal residue exchanges exhibit strong cooperative or anticooperative effects, thereby indicating the spatial proximity of residues Arg(188), Gln(380), and Asn(229). Because these residues are highly conserved in the GPCR-B family, they can moreover be expected to have a general role in mediating function.

The serendipitous origin of chordate secretin peptide family members

Background

The secretin family is a pleotropic group of brain-gut peptides with affinity for class 2 G-protein coupled receptors (secretin family GPCRs) proposed to have emerged early in the metazoan radiation via gene or genome duplications. In human, 10 members exist and sequence and functional homologues and ligand-receptor pairs have been characterised in representatives of most vertebrate classes. Secretin-like family GPCR homologues have also been isolated in non-vertebrate genomes however their corresponding ligands have not been convincingly identified and their evolution remains enigmatic.

Results

In silico sequence comparisons failed to retrieve a non-vertebrate (porifera, cnidaria, protostome and early deuterostome) secretin family homologue. In contrast, secretin family members were identified in lamprey, several teleosts and tetrapods and comparative studies revealed that sequence and structure is in general maintained. Sequence comparisons and phylogenetic analysis revealed that PACAP, VIP and GCG are the most highly conserved members and two major peptide subfamilies exist; i) PACAP-like which includes PACAP, PRP, VIP, PH, GHRH, SCT and ii) GCG-like which includes GCG, GLP1, GLP2 and GIP. Conserved regions flanking secretin family members were established by comparative analysis of the Takifugu, Xenopus, chicken and human genomes and gene homologues were identified in nematode, Drosophila and Ciona genomes but no gene linkage occurred. However, in Drosophila and nematode genes which flank vertebrate secretin family members were identified in the same chromosome.

Conclusions

Receptors of the secretin-like family GPCRs are present in protostomes but no sequence homologues of the vertebrate cognate ligands have been identified. It has not been possible to determine when the ligands evolved but it seems likely that it was after the protostome-deuterostome divergence from an exon that was part of an existing gene or gene fragment by rounds of gene/genome duplication. The duplicate exon under different evolutionary pressures originated the chordate PACAP-like and GCG-like subfamily groups. This event occurred after the emergence of the metazoan secretin GPCRs and led to the establishment of novel peptide-receptor interactions that contributed to the generation of novel physiological functions in the chordate lineage.

Identification of determinants of glucose-dependent insulinotropic polypeptide receptor that interact with N-terminal biologically active region of the natural ligand

Glucose-dependent insulinotropic polypeptide receptor (GIPR), a member of family B of the G-protein coupled receptors, is a potential therapeutic target for which discovery of nonpeptide ligands is highly desirable. Structure-activity relationship studies indicated that the N-terminal part of glucose-dependent insulinotropic polypeptide (GIP) is crucial for biological activity. Here, we aimed at identification of residues in the GIPR involved in functional interaction with N-terminal moiety of GIP. A homology model of the transmembrane core of GIPR was constructed, whereas a three-dimensional model of the complex formed between GIP and the N-terminal extracellular domain of GIPR was taken from the crystal structure. The latter complex was docked to the transmembrane domains of GIPR, allowing in silico identification of putative residues of the agonist binding/activation site. All mutants were expressed at the surface of human embryonic kidney 293 cells as indicated by flow cytometry and confocal microscopy analysis of fluorescent GIP binding. Mutation of residues Arg183, Arg190, Arg300, and Phe357 caused shifts of 76-, 71-, 42-, and 16-fold in the potency to induce cAMP formation, respectively. Further characterization of these mutants, including tests with alanine-substituted GIP analogs, were in agreement with interaction of Glu3 in GIP with Arg183 in GIPR. Furthermore, they strongly supported a binding mode of GIP to GIPR in which the N-terminal moiety of GIP was sited within transmembrane helices (TMH) 2, 3, 5, and 6 with biologically crucial Tyr1 interacting with Gln224 (TMH3), Arg300 (TMH5), and Phe357 (TMH6). These data represent an important step toward understanding activation of GIPR by GIP, which should facilitate the rational design of therapeutic agents.

Structural diversity of G protein-coupled receptors and significance for drug discovery

G protein-coupled receptors (GPCRs) are the largest family of membrane-bound receptors and also the targets of many drugs. Understanding of the functional significance of the wide structural diversity of GPCRs has been aided considerably in recent years by the sequencing of the human genome and by structural studies, and has important implications for the future therapeutic potential of targeting this receptor family. This article aims to provide a comprehensive overview of the five main human GPCR families--Rhodopsin, Secretin, Adhesion, Glutamate and Frizzled/Taste2--with a focus on gene repertoire, general ligand preference, common and unique structural features, and the potential for future drug discovery.

Crystal structure of the ligand-bound glucagon-like peptide-1 receptor extracellular domain

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B1 of the seven-transmembrane G protein-coupled receptors, and its natural agonist ligand is the peptide hormone glucagon-like peptide-1 (GLP-1). GLP-1 is involved in glucose homeostasis, and activation of GLP-1R in the plasma membrane of pancreatic beta-cells potentiates glucose-dependent insulin secretion. The N-terminal extracellular domain (nGLP-1R) is an important ligand binding domain that binds GLP-1 and the homologous peptide Exendin-4 with differential affinity. Exendin-4 has a C-terminal extension of nine amino acid residues known as the "Trp cage", which is absent in GLP-1. The Trp cage was believed to interact with nGLP-1R and thereby explain the superior affinity of Exendin-4. However, the molecular details that govern ligand binding and specificity of nGLP-1R remain undefined. Here we report the crystal structure of human nGLP-1R in complex with the antagonist Exendin-4(9-39) solved by the multiwavelength anomalous dispersion method to 2.2A resolution. The structure reveals that Exendin-4(9-39) is an amphipathic alpha-helix forming both hydrophobic and hydrophilic interactions with nGLP-1R. The Trp cage of Exendin-4 is not involved in binding to nGLP-1R. The hydrophobic binding site of nGLP-1R is defined by discontinuous segments including primarily a well defined alpha-helix in the N terminus of nGLP-1R and a loop between two antiparallel beta-strands. The structure provides for the first time detailed molecular insight into ligand binding of the human GLP-1 receptor, an established target for treatment of type 2 diabetes.

The vasoactive intestinal peptide (VIP) alpha-Helix up to C terminus interacts with the N-terminal ectodomain of the human VIP/Pituitary adenylate cyclase-activating peptide 1 receptor: photoaffinity, molecular modeling, and dynamics

The neuropeptide vasoactive intestinal peptide (VIP) strongly impacts on human pathophysiology and does so through interaction with class II G protein-coupled receptors. We characterized the C terminus-binding site of VIP in the N-terminal ectodomain (N-ted) of the human VPAC1 receptor: 1) The probe [(125)I-Bpa(28)]VIP in which the C-terminal residue (Asn(28)) is substituted by a photoreactive p-benzoyl-l-Phe (Bpa) was used to photolabel the receptor. After receptor cleavage and Edman sequencing, it was shown that Asn(28) of VIP is in contact with Lys(127) in the receptor N-ted. Taking into account previous data, it follows that the C-terminal and central parts of VIP from Asn(28) to Phe(6) lie in the N-ted. 2) A three-dimensional model of the N-ted was constructed, the fold being identified as a Sushi domain with two antiparallel beta-sheets and three disulfide bonds. The nuclear magnetic resonance structure of VIP was then docked into this model by taking into account the constraint provided by photoaffinity experiments with [(125)I-Bpa(28)]VIP. It appeared that VIP runs parallel to the beta3-beta4 antiparallel sheets. 3) We performed molecular dynamic simulations over 14 nsec of the complex between VIP and receptor N-ted and the free N-ted. The structural model of the free N-ted is stable, and VIP tends to further stabilize the N-ted structure more especially in the loops connecting the beta-sheets. These structural studies provide a detailed molecular understanding of the VIP-receptor interaction.

Molecular mechanisms involved in vasoactive intestinal peptide receptor activation and regulation: current knowledge, similarities to and differences from the A family of G-protein-coupled receptors

An actual paradigm for activation and regulation of the GPCR (G-protein-coupled receptors)/seven-transmembrane helix family of receptors essentially emerges from extensive studies of the largest family of receptors, the GPCR-A/rhodopsin family. The mechanisms regulating the GPCR-B family signal transduction are less precisely understood due in part to the lack of the conserved signatures of the GPCR-A family (E/DRY, NPXXY) and in part to the absence of a reliable receptor modelling, although some studies suggest that both families share similar features. Here, we try to highlight the current knowledge of the activation and the regulation of the VIP (vasoactive intestinal peptide) receptors, namely VPAC (VIP/pituitary adenylate cyclase-activating peptide receptor) 1 and 2. This includes search for amino acids involved in the stabilization of the receptor active conformation and in coupling to G-proteins, signalling pathways activated in response to VIP, agonist-dependent receptor down-regulation, phosphorylation and internalization as well as pharmacological consequences of receptor hetero-dimerization.

Solution structure and mutational analysis of pituitary adenylate cyclase-activating polypeptide binding to the extracellular domain of PAC1-RS

The pituitary adenylate cyclase-activating polypeptide (PACAP) receptor is a class II G protein-coupled receptor that contributes to many different cellular functions including neurotransmission, neuronal survival, and synaptic plasticity. The solution structure of the potent antagonist PACAP (residues 6'-38') complexed to the N-terminal extracellular (EC) domain of the human splice variant hPAC1-R-short (hPAC1-R(S)) was determined by NMR. The PACAP peptide adopts a helical conformation when bound to hPAC1-R(S) with a bend at residue A18' and makes extensive hydrophobic and electrostatic interactions along the exposed beta-sheet and interconnecting loops of the N-terminal EC domain. Mutagenesis data on both the peptide and the receptor delineate the critical interactions between the C terminus of the peptide and the C terminus of the EC domain that define the high affinity and specificity of hormone binding to hPAC1-R(S). These results present a structural basis for hPAC1-R(S) selectivity for PACAP versus the vasoactive intestinal peptide and also differentiate PACAP residues involved in binding to the N-terminal extracellular domain versus other parts of the full-length hPAC1-R(S) receptor. The structural, mutational, and binding data are consistent with a model for peptide binding in which the C terminus of the peptide hormone interacts almost exclusively with the N-terminal EC domain, whereas the central region makes contacts to both the N-terminal and other extracellular parts of the receptor, ultimately positioning the N terminus of the peptide to contact the transmembrane region and result in receptor activation.

Epithelial Na+ channel stimulation by n-3 fatty acids requires proximity to a membrane-bound A-kinase-anchoring protein complexed with protein kinase A and phosphodiesterase

Essential polyunsatured fatty acids have been shown to modulate enzymes, channels and transporters, to interact with lipid bilayers and to affect metabolic pathways. We have previously shown that eicosapentanoic acid (EPA, C20:5, n-3) activates epithelial sodium channels (ENaCs) in a cAMP-dependent manner involving stimulation of cAMP-dependent protein kinase (PKA). In the present study, we explored further the mechanism of EPA stimulation of ENaC in A6 cells. Fluorescence resonance energy transfer experiments confirmed activation of PKA by EPA. Consistent with our previous studies, EPA had no further stimulatory effect on amiloride-sensitive transepithelial current (INa) in the presence of CPT-cAMP. Thus, we investigated the effect of EPA on cellular pathways which produce cAMP. EPA did not stimulate adenylate cyclase activity or total cellular cAMP accumulation. However, membrane-bound phosphodiesterase activity was inhibited by EPA from 2.46 pmol/mg of protein/min to 1.3 pmol/mg of protein/min. To investigate the potential role of an A-kinase-anchoring protein (AKAP), we used HT31, an inhibitor of the binding between PKA and AKAPs as well as cerulenin, an inhibitor of myristoylation and palmitoylation. Both agents prevented the stimulatory effect of EPA and CPT-cAMP on INa and drastically decreased the amount of PKA in the apical membrane. Colocalization experiments in A6 cells cotransfected with fluorescently labeled ENaC beta subunit and PKA regulatory subunit confirmed the close proximity of the two proteins and the membrane anchorage of PKA. Last, in A6 cells transfected with a dead mutant of Sgk, an enzyme which up-regulates ENaCs, EPA did not stimulate Na+ current. Our results suggest that stimulation of ENaCs by EPA occurs via SGK in membrane-bound compartments containing an AKAP, activated PKA, and a phosphodiesterase.

Peptide agonist docking in the N-terminal ectodomain of a class II G protein-coupled receptor, the VPAC1 receptor. Photoaffinity, NMR, and molecular modeling

The neuropeptide vasoactive intestinal peptide (VIP) strongly impacts on human pathophysiology and does so through interaction with class II G protein-coupled receptors named VIP pituitary adenylate cyclase-activating peptide (PACAP) receptors (VPACs). The molecular nature of VIP binding to receptors remains elusive. In this work, we have docked VIP in the human VPAC1 receptor by the following approach. (i) VIP probes containing photolabile residues in positions 6, 22, and 24 of VIP were used to photolabel the receptor. After receptor cleavage and Edman sequencing of labeled receptor fragments, it was shown that Phe6, Tyr22, and Asn24 of VIP are in contact with Asp107, Gly116, and Cys122 in the N-terminal ectodomain (N-ted) of the receptor, respectively. (ii) The structure of VIP was determined by NMR showing a central alpha helix, a disordered N-terminal His1-Phe6 segment and a 3(10) Ser25-Asn28 helix termination. (iii) A three-dimensional model of the N-ted of hVPAC1 was constructed by using the NMR structure of the N-ted of corticotropin-releasing factor receptor 2beta as a template. As expected, the fold is identified as a short consensus repeat with two antiparallel beta sheets and is stabilized by three disulfide bonds. (iv) Taking into account the constraints provided by photoaffinity, VIP was docked into the hVPAC1 receptor N-ted. The 6-28 fragment of VIP nicely lies in the N-ted C-terminal part, but the N terminus region of VIP is free for interacting with the receptor transmembrane region. The data provide a structural rationale to the proposed two-step activation mechanism of VPAC receptor and more generally of class II G protein-coupled receptors.

Asn229 in the third helix of VPAC1 receptor is essential for receptor activation but not for receptor phosphorylation and internalization: comparison with Asn216 in VPAC2 receptor

After stimulation with agonist, G protein coupled receptors (GPCR) undergo conformational changes that allow activation of G proteins to transduce the signal, followed by phosphorylation by kinases and arrestin binding to promote receptor internalization. Actual paradigm, based on a study of GPCR-A/rhodopsin family, suggests that a network of interactions between conserved residues located in transmembrane (TM) domains (mainly TM3, TM6 and TM7) is involved in the molecular switch leading to GPCR activation. We evaluated in CHO cells expressing the VPAC(1) receptor the role of the third transmembrane helix in agonist signalling by point mutation into Ala of the residues highly conserved in the secretin-family of receptors: Y(224), N(229), F(230), W(232), E(236), G(237), Y(239), L(240). N(229)A VPAC(1) mutant was characterized by a decrease in both potency and efficacy of VIP stimulated adenylate cyclase activity, by the absence of agonist stimulated [Ca(2+)](i) increase, by a preserved receptor recognition of agonists and antagonist and by a preserved sensitivity to GTP suggesting the importance of that residue for efficient G protein activation. N(229)D mutant was not expressed at the membrane, and the N(229)Q with a conserved mutation was less affected than the A mutant. Agonist stimulated phosphorylation and internalization of N(229)A and N(229)Q VPAC(1) were unaffected. However, the re-expression of internalized mutant receptors, but not that of the wild type receptor, was rapidly reversed after VIP washing. Receptor phosphorylation, internalization and re-expression may be thus dissociated from G protein activation and linked to another active conformation that may influence its trafficking. Mutation of that conserved amino acid in VPAC(2) could be investigated only by a conservative mutation (N(216)Q) and led to a receptor with a low VIP stimulation of adenylate cyclase, receptor phosphorylation and internalization. This indicated the importance of the conserved N residue in the TM3 of that family of receptors.

Diffuse pharmacophoric domains of vasoactive intestinal peptide (VIP) and further insights into the interaction of VIP with the N-terminal ectodomain of human VPAC1 receptor by photoaffinity labeling with [Bpa6]-VIP

The widespread 28-amino acid neuropeptide vasoactive intestinal peptide (VIP) exerts its many biological effects through interaction with serpentine class II G protein-coupled receptors named VPAC receptors. We previously provided evidence for a physical contact between the side chain at position 22 of VIP and the N-terminal ectodomain of the hVPAC1 receptor (Tan, Y. V., Couvineau, A., Van Rampelbergh, J., and Laburthe, M. (2003) J. Biol. Chem. 278, 36531-36536). We explored here the contact site between hVPAC1 receptor and the side chain at position 6 of VIP by photoaffinity labeling. The photoreactive para-benzoyl-l-Phe (Bpa) was substituted for Phe(6) in VIP resulting in [Bpa(6)]-VIP, which was shown to be a hVPAC1 receptor agonist in Chinese hamster ovary cells stably expressing the recombinant receptor. After obtaining the covalent (125)I-[Bpa(6)-VIP].hVPAC1 receptor complex, it was sequentially cleaved by cyanogen bromide, peptide N-glycosidase F, endopeptidase Glu-C, and trypsin, and the cleavage products were analyzed by electrophoresis. The data demonstrated that (125)I-[Bpa(6)-VIP] were covalently attached to the short 104-108 fragment within the N-terminal ectodomain of the receptor. The data were confirmed by creation of a receptor mutant with new CNBr cleavage site. In a three-dimensional model of the receptor N-terminal ectodomain, this fragment was located on one edge of the putative VIP-binding groove and was adjacent to the fragment covalently attached to the side chain at position 22 of VIP. Altogether these data showed that the central part of VIP, at least between Phe(6) and Tyr(22), interacts with the N-terminal ectodomain of the hVPAC1 receptor.

Identification of a contact site for residue 19 of parathyroid hormone (PTH) and PTH-related protein analogs in transmembrane domain two of the type 1 PTH receptor

Recent functional studies have suggested that position 19 in PTH interacts with the portion of the PTH-1 receptor (P1R) that contains the extracellular loops and seven transmembrance helices (TMs) (the J domain). We tested this hypothesis using the photoaffinity cross-linking approach. A PTHrP(1-36) analog and a conformationally constrained PTH(1-21) analog, each containing para-benzoyl-l-phenylalanine (Bpa) at position 19, each cross-linked efficiently to the P1R expressed in COS-7 cells, and digestive mapping analysis localized the cross-linked site to the interval (Leu232-Lys240) at the extracellular end of TM2. Point mutation analysis identified Ala234, Val235, and Lys240 as determinants of cross-linking efficiency, and the Lys240-->Ala mutation selectively impaired the binding of PTH(1-21) and PTH(1-19) analogs, relative to that of PTH(1-15) analogs. The findings support the hypothesis that residue 19 of the receptor-bound ligand contacts, or is close to, the P1R J domain-specifically, Lys240 at the extracellular end of TM2. The findings also support a molecular model in which the 1-21 region of PTH binds to the extracellular face of the P1R J domain as an alpha-helix.

Photoaffinity labeling demonstrates physical contact between vasoactive intestinal peptide and the N-terminal ectodomain of the human VPAC1 receptor

Vasoactive intestinal peptide (VIP) is a prominent neuropeptide whose actions are mediated by VPAC receptors belonging to class II G protein-coupled receptors. To identify contact sites between VIP and its VPAC1 receptor, an analog of VIP substituted with a photoreactive para-benzoyl-l-Phe (Bpa) at position 22 has been synthesized and evaluated in Chinese hamster ovary cells stably expressing the recombinant human receptor. Bpa22-VIP and native VIP are equipotent in stimulating adenylyl cyclase activity in cell membranes. Cyanogen bromide cleavage of the covalent 125I-[Bpa22-VIP]-hVPAC1R complex yielded a single labeled fragment of 30 kDa that shifted to 11 after deglycosylation, most consistent with the 67-137 fragment of the receptor N-terminal ectodomain. Further cleavage of this fragment with V8 endoproteinase and creation of receptor mutants with new CNBr cleavage sites (XàMet), demonstrated that 125I-[Bpa22-VIP] was covalently attached to the short receptor 109-120 fragment (GWTHLEPGPYPI). In a three-dimensional model of the receptor N-terminal ectodomain, this fragment is located on one edge of the putative VIP binding groove and encompasses several amino acids previously shown to be crucial for VIP binding (reviewed in Laburthe, M., Couvineau, A., and Marie, J. C. (2002) Receptors Channels 8, 137-153). Our data provide the first direct evidence for a physical contact between VIP and the N-terminal ectodomain of the hVPAC1 receptor.

N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine inhibits ligand binding to certain G protein-coupled receptors

N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) is used widely in biological systems to chelate certain heavy metals, particularly Zn2+. Here we show that TPEN inhibits ligand binding to certain G protein-coupled receptors and is an antagonist at muscarinic receptors. In intact human neuroblastoma SH-SY5Y cells, the binding of the muscarinic receptor ligand [N-methyl-3H]scopolamine methyl chloride was inhibited by TPEN (Ki approximately 26 microM), as was muscarinic receptor agonist-induced inositol 1,4,5-trisphosphate formation (Ki approximately 26 microM). This antagonism was not due to metal ion chelation, indicating that it resulted from a direct interaction of TPEN with muscarinic receptors. Examination of the effects of TPEN on other receptors in SH-SY5Y cell membrane preparations showed that the binding of the nonpeptide opioid receptor ligand [15,16-3H]diprenorphine was strongly inhibited, whereas binding of [125I]vasoactive intestinal polypeptide was not. This pattern of selectivity was also seen in AR4-2J rat pancreatoma cell membranes, in which TPEN inhibited ligand binding to muscarinic receptors, but not that to cholecystokinin receptors. In conclusion, these data show that TPEN inhibits ligand binding to certain G protein-coupled receptors and exhibits selectivity towards those receptors whose transmembrane helices form the predominant site for ligand interaction. TPEN may have widespread antagonistic activity towards G protein-coupled receptors of this kind.

Three distinct epitopes on the extracellular face of the glucagon receptor determine specificity for the glucagon amino terminus

The glucagon and glucagon-like peptide-1 (GLP-1) receptors are homologous family B seven-transmembrane (7TM) G protein-coupled receptors, and they selectively recognize the homologous peptide hormones glucagon (29 amino acids) and GLP-1 (30-31 amino acids), respectively. The amino-terminal extracellular domain of the glucagon and GLP-1 receptors (140-150 amino acids) determines specificity for the carboxyl terminus of glucagon and GLP-1, respectively. In addition, the glucagon receptor core domain (7TM helices and connecting loops) strongly determines specificity for the glucagon amino terminus. Only 4 of 15 residues are divergent in the glucagon and GLP-1 amino termini; Ser2, Gln3, Tyr10, and Lys12 in glucagon and the corresponding Ala8, Glu9, Val16, and Ser18 in GLP-1. In this study, individual substitution of these four residues of glucagon with the corresponding residues of GLP-1 decreased the affinity and potency at the glucagon receptor relative to glucagon. Substitution of distinct segments of the glucagon receptor core domain with the corresponding segments of the GLP-1 receptor rescued the affinity and potency of specific glucagon analogs. Site-directed mutagenesis identified the Asp385 --> Glu glucagon receptor mutant that specifically rescued Ala2-glucagon. The results show that three distinct epitopes of the glucagon receptor core domain determine specificity for the N terminus of glucagon. We suggest a glucagon receptor binding model in which the extracellular ends of TM2 and TM7 are close to and determine specificity for Gln3 and Ser2 of glucagon, respectively. Furthermore, the second extracellular loop and/or proximal segments of TM4 and/or TM5 are close to and determine specificity for Lys12 of glucagon.

Molecular determinants of glucagon receptor signaling

A 29-amino acid polypeptide hormone, glucagon has been one of the most prolific models in the study of hormone action. The key biologic function of glucagon is to counterbalance the actions of insulin and maintain a normal level of serum glucose. Diabetes mellitus can thus be considered a bihormonal disorder with an excess of glucagon contributing to the hyperglycemic state. The effects of glucagon are mediated by the glucagon receptor, which is itself a prototypical member of a distinct category called family B receptors within the G protein-coupled superfamily of seven-helical transmembrane receptors (GPCRs). At the structural level, the peptide ligands of family B receptors are highly homologous, in particular in the N-terminal region of the molecules. The mechanism by which highly homologous peptide ligands selectively recognize their receptors involves distinct molecular interactions that are gradually being elucidated. This review focuses on structural determinants of the glucagon receptor that are important for its activity with respect to interaction with its ligand and G proteins. Information about the glucagon receptor is presented within the context of what is known about other members of the family B GPCRs.

Evidence for a direct interaction between the Thr11 residue of vasoactive intestinal polypeptide and Tyr184 located in the first extracellular loop of the VPAC2 receptor

We developed previously VPAC(1) [vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) receptor]>VPAC(2) receptor selective ligands. Replacement of the VIP-Thr(11) by an Arg(11) in these ligands contributed to their selectivity: Arg(11)-VIP had a 200-fold lower affinity when compared with VIP at VPAC(2) receptors as opposed to 3- to 5-fold higher affinity at VPAC(1) receptors. Comparison of the binding and functional properties of related VIP analogues suggested that the VPAC(1) selectivity of Arg(11)-VIP was due to the loss of a hydrogen bond between the hydroxy group of Thr residue and the VPAC(2) receptor, steric hindrance between the Arg side chain and the VPAC(2) receptor and charge attraction by the VPAC(1) receptor. Comparison of the ability of VIP analogues to activate adenylate cyclase through chimaeric VPAC(1)/VPAC(2) and VPAC(2)/VPAC(1) receptors indicated that the first extracellular receptor loop carried most of the VPAC(2) receptors' ability to discriminate VIP from Arg(11)-VIP. Based on results obtained for a truncated VPAC(2) receptor and the closely related PACAP-preferring receptor (PAC(1)) and secretin receptors, we hypothesized that Thr(11) interacted with the VPAC(2) receptor Tyr(184) (similar to the VPAC(1) receptor Phe(200) residue). The Y184F (Tyr(184)-->Phe) VPAC(2) mutant lost the ability to discriminate VIP from Val(11)-VIP, and the F200Y VPAC(1) mutant acquired the ability to discriminate the natural peptide from Val(11)-VIP. These results support the hypothesis that the hydroxy group of the native VIP-Thr(11) side chain can indeed form a hydrogen bond with the Tyr side chain in the VPAC(2) receptor.

Different domains of the glucagon and glucagon-like peptide-1 receptors provide the critical determinants of ligand selectivity

(1) Glucagon and glucagon-like peptide-1 (GLP-1) are homologous peptide hormones with important functions in glucose metabolism. The receptors for glucagon and GLP-1 are homologous family B G-protein coupled receptors. The GLP-1 receptor amino-terminal extracellular domain is a major determinant of glucagon/GLP-1 selectivity of the GLP-1 receptor. However, the divergent residues in glucagon and GLP-1 that determine specificity for the GLP-1 receptor amino-terminal extracellular domain are not known. Less is known about how the glucagon receptor distinguishes between glucagon and GLP-1. (2) We analysed chimeric glucagon/GLP-1 peptides for their ability to bind and activate the glucagon receptor, the GLP-1 receptor and chimeric glucagon/GLP-1 receptors. The chimeric peptide GLP-1(7-20)/glucagon(15-29) was unable to bind and activate the glucagon receptor. Substituting the glucagon receptor core domain with the GLP-1 receptor core domain (chimera A) completely rescued the affinity and potency of GLP-1(7-20)/glucagon(15-29) without compromising the affinity and potency of glucagon. Substituting transmembrane segment 1 (TM1), TM6, TM7, the third extracellular loop and the intracellular carboxy-terminus of chimera A with the corresponding glucagon receptor segments re-established the ability to distinguish GLP-1(7-20)/glucagon(15-29) from glucagon. Corroborant results were obtained with the opposite chimeric peptide glucagon(1-14)/GLP-1(21-37). (3) The results suggest that the glucagon and GLP-1 receptor amino-terminal extracellular domains determine specificity for the divergent residues in the glucagon and GLP-1 carboxy-terminals respectively. The GLP-1 receptor core domain is not a critical determinant of glucagon/GLP-1 selectivity. Conversely, the glucagon receptor core domain contains two or more sub-segments which strongly determine specificity for divergent residues in the glucagon amino-terminus.

Lysine 195 and aspartate 196 in the first extracellular loop of the VPAC1 receptor are essential for high affinity binding of agonists but not of antagonists

The role in ligand recognition and receptor activation of two adjacent charged residues (lysine 195 and aspartate 196) in the first extracellular loop of the human VPAC(1) receptor was investigated in stably transfected CHO cells expressing the wild type or point mutated receptors.Replacement of lysine 195 by glutamine or of aspartate 196 by asparagine reduced the agonists' ability to stimulate adenylate cyclase activity; VIP behaved like a partial agonist and a partial agonist behaved as an antagonist. The receptor's capacity to recognize agonists was reduced but antagonists' affinity was unaffected. Both results suggesting that the two charged residues are essential for VPAC(1) receptor activation. On the other hand, the double mutant was less severely affected than single mutants suggesting that hydrogen bonds may partially compensate the loss of charged residues. But the inversion of the residues affected receptor recognition and activation more markedly suggesting that the two charged residues do not interact directly.

Roles of specific extracellular domains of the glucagon receptor in ligand binding and signaling

To identify structural determinants of ligand binding in the glucagon receptor, eight receptor chimeras and additional receptor point mutants were prepared and studied. Amino acid residues 103-117 and 126-137 in the extracellular N-terminal tail and residues 206-219 and 220-231 in the first extracellular loop of the glucagon receptor were replaced with the corresponding segments of the glucagon-like peptide-1 receptor or the secretin receptor. Specific segments of both the N-terminal tail and the first extracellular loop of the glucagon receptor are required for hormone binding. The 206-219 segment of the first loop appears to be important for both glucagon binding and receptor activation. Functional studies with a synthetic chimeric peptide consisting of the N-terminal 14 residues of glucagon and the C-terminal 17 residues of glucagon-like peptide 1 suggest that hormone binding specificity may involve this segment of the first loop. The binding selectivity may arise in part from aspartic acid residues in this segment. Mutation of R-202 located at the junction between the second transmembrane helix and the first loop resulted in a mutant receptor that failed to bind glucagon or signal. We conclude that high-affinity glucagon binding requires multiple contacts with residues in the N-terminal tail and first extracellular loop domain of the glucagon receptor, with hormone specificity arising primarily from the amino acid 206-219 segment. The data suggest a model whereby glucagon first interacts with the N-terminal domain of the receptor followed by more specific interactions between the N-terminal half of the peptide and the first extracellular loop of the receptor, leading to activation.

Two tyrosine residues in the first transmembrane helix of the human vasoactive intestinal peptide receptors play a role in supporting the active conformation

1: We investigated the human vasoactive intestinal polypeptide (VIP) receptors VPAC(1) and VPAC(2) mutated at conserved tyrosine residues in the first transmembrane helix (VPAC(1) receptor Y146A and Y150A and VPAC(2) receptor Y130A and Y134A). 2: [(125)I]-Acetyl-His(1) [D-Phe(2), K(15), R(16), L(27)]-VIP (1-7)/GRF (8-27) (referred to as [(125)I]-VPAC(1) antagonist) labelled VPAC(1) binding sites, that displayed high and low affinities for VIP (IC(50) values and per cent of high affinity binding sites: wild-type, 1 nM (57+/-9%) and 160 nM; Y146A, 30 nM (40+/-8%) and 800 nM; Y150A, 4 nM (27+/-8%) and 300 nM). [R(16)]-VIP behaved as a "super agonist" at both mutated VPAC(1) receptors and the efficacies of VIP analogues modified in positions 1, 3 and 6 were significantly decreased. 3: VIP was less potent at the Y130A and Y134A mutated VPAC(2) receptors (EC(50) 200 and 400 nM, respectively) than at the wild-type VPAC(2) receptor (EC(50) 7 nM). Furthermore, [hexanoyl-His(1)]-VIP behaved as a "super agonist" at the two mutated VPAC(2) receptors, and VIP analogues modified in positions 1, 3 and 6 were less potent and efficient at the mutated than at wild-type VPAC(2) receptors. However, the Y130A and Y134A mutants could not be studied in binding assays. 4: Our results suggest that the conserved tyrosine residues do not interact directly with the VIP His(1), Asp(3) or Phe(6) residues (that are necessary for receptor activation), but stabilize the correct active receptor conformation.

VPAC(1) receptors have different agonist efficacy profiles on membrane and intact cells

The vasoactive intestinal peptide receptor VPAC(1) is preferentially coupled to G(alpha s) protein but also increases [Ca(2+)](i) through interaction with G(alpha i)/G(alpha q) protein. We evaluated a panel of full, partial and null agonists for their capability to stimulate adenylate cyclase activity in both intact cells and membrane and [Ca(2+)](i) in intact cells transfected with the reporter gene aequorin. In intact cells, the agonists efficacy for cAMP and calcium increase were well, but not linearly correlated: VPAC(1) receptors activated G(alpha s) protein more efficiently but with the same pharmacological profile as the other G proteins. In contrast, there was a difference between cAMP increase in intact and broken cell membranes: EC(50) values were generally lower in intact cells whereas the efficacy was higher. There was, however, no correlation between the shift in the EC(50) value and the intrinsic activity. Of interest, the (4-28) fragment, a reported antagonist on cell membrane, was a full agonist in intact cells. We concluded that the active states of the VPAC(1) receptor resulting from the coupling to different effector are undistinguishable by the VIP analogs tested but that receptor properties are different when evaluated in intact cells or cell membranes.

A small sequence in the third intracellular loop of the VPAC(1) receptor is responsible for its efficient coupling to the calcium effector

The stimulatory effect of VIP on intracellular calcium concentration ([Ca(2+)](i)) has been investigated in Chinese hamster ovary cells stably transfected with the reporter gene aequorin, and expressing human VPAC(1), VPAC(2), chimeric VPAC(1)/VPAC(2), or mutated receptors. The VIP-induced [Ca(2+)](i) increase was linearly correlated with receptor density and was higher in cells expressing VPAC(1) receptors than in cells expressing a similar VPAC(2) receptor density. The study was performed to establish the receptor sequence responsible for that difference. VPAC(1)/VPAC(2) chimeric receptors were first used for a broad positioning: those having the third intracellular loop (IC(3)) of the VPAC(1) or of the VPAC(2) receptor behaved, in that respect, phenotypically like VPAC(1) and VPAC(2) receptor, respectively. Replacement in the VPAC(2) receptor of the sequence 315-318 (VGGN) within the IC(3) by its VPAC(1) receptor counterpart 328-331 (IRKS) and the introduction of VGGN in state of IRKS in VPAC(1) was sufficient to mimic the VPAC(1) and VPAC(2) receptor characteristics, respectively. Thus, a small sequence in the IC(3) of the VPAC(1) receptor, probably through interaction with G(alphai) and G(alphaq) proteins, is responsible for the efficient agonist-stimulated [Ca(2+)](i) increase.

Mutational analysis of the glucagon receptor: similarities with the vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP)/secretin receptors for recognition of the ligand's third residue

Receptor recognition by the Asp(3) residues of vasoactive intestinal peptide and secretin requires the presence of a lysine residue close to the second transmembrane helix (TM2)/first extracellular loop junction and an ionic bond with an arginine residue in TM2. We tested whether the glucagon Gln(3) residue recognizes the equivalent positions in its receptor. Our data revealed that the binding and functional properties of the wild-type glucagon receptor and the K188R mutant were not significantly different, whereas all agonists had markedly lower potencies and affinities at the I195K mutated receptor. In contrast, glucagon was less potent and the Asp(3)-, Asn(3)- and Glu(3)-glucagon mutants were more potent and efficient at the double-mutated K188R/I195K receptor. Furthermore, these alterations were selective for position 3 of glucagon, as shown by the functional properties of the mutant Glu(9)- and Lys(15)-glucagon. Our results suggest that although the Gln(3) residue of glucagon did not interact with the equivalent binding pocket as the Asp(3) residue of vasoactive intestinal peptide or secretin, the Asp(3)-glucagon analogue was able to interact with position 188 of the K188R/I195K glucagon receptor. Nevertheless, the Gln(3) side chain of glucagon probably binds very close to this region in the wild-type receptor.

(N-stearyl, norleucine17)VIPhybrid is a broad spectrum vasoactive intestinal peptide receptor antagonist

The effects of a (N-stearyl, Norleucine17) vasoactive intestinal peptide hybrid ((SN)VIPhybrid) on cells stably transfected with VPAC,, VPAC2, or PAC1 receptors were investigated. (SN)VIPhybrid inhibited specific 125I-VIP binding to membranes derived from CHO cells transfected with VPAC, or VPAC2 receptors with high affinity (IC50 = 30 and 50 nM). (SN)VIPhyb inhibited specific 125I-PACAP-27 binding to membranes derived from NIH/3T3 cells transfected with PAC1 receptors with high affinity (IC50 = 65 nM). PACAP-27 caused cAMP elevation in NIH/3T3 cells transfected with PAC1 receptors and the increase cAMP caused by pituitary adenylated cyclase (PACAP) was inhibited by (SN)VIPhyb. Also, the increase in cAMP caused by VIP using CHO cells transfected with VPAC1 or VPAC2 receptors was antagonized by (SN)VIPhyb. These results indicate that (SN)VIPhyb is an antagonist for VPAC1, VPAC2, and PAC1 receptors.