Molecular approximation between a residue in the amino-terminal region of calcitonin and the third extracellular loop of the class B G protein-coupled calcitonin receptor

Molecular mechanism of an antagonistic antibody against glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone involved in regulating glucose and lipid metabolism. GIP receptor (GIPR) antagonism is believed to offer therapeutic potential for various metabolic diseases. Pharmacological intervention of GIPR, however, has limited success due to lack of effective antagonistic reagents. Previously we reported the discovery of two mouse anti-murine GIPR monoclonal antibodies (mAbs) with distinctive properties in rodent models. Here, we report the detailed structural and biochemical characterization of these two antibodies, mAb1 and mAb2. In vitro and in vivo characterizations demonstrated mAb2 is a full GIPR antagonistic antibody and mAb1 is a non-neutralizing GIPR binder. To understand the molecular basis of these two antibodies, we determined the co-crystal structures of GIPR extracellular domain in complex with mAb1 and with mAb2 at resolutions of 2.1 and 2.6 Å, respectively. While the non-neutralizing mAb1 binds to GIPR without competing with the ligand peptide, mAb2 not only partially occludes the ligand peptide binding, but also recognizes the GIPR C-terminal stalk region in a helical conformation that acts as a molecular mimic of the ligand peptide and locks GIPR in a novel auto-inhibited state. Furthermore, administration of mAb2 in diet-induced obesity mice for 7 weeks leads to both reduction in body weight gain and improvement of metabolic profiles. In contrast, mAb1 has no effect on body weight or other metabolic improvement. Together, our studies reveal the unique molecular mechanism of action underlying the superior antagonistic activity of mAb2 and signify the promising therapeutic potential of effective GIPR antagonism for the treatment of metabolic disorders.

Calcitonin Receptor N-Glycosylation Enhances Peptide Hormone Affinity by Controlling Receptor Dynamics

The class B G protein-coupled receptor (GPCR) calcitonin receptor (CTR) is a drug target for osteoporosis and diabetes. N-glycosylation of asparagine 130 in its extracellular domain (ECD) enhances calcitonin hormone affinity with the proximal GlcNAc residue mediating this effect through an unknown mechanism. Here, we present two crystal structures of salmon calcitonin-bound, GlcNAc-bearing CTR ECD at 1.78 and 2.85 Å resolutions and analyze the mechanism of the glycan effect. The N130 GlcNAc does not contact the hormone. Surprisingly, the structures are nearly identical to a structure of hormone-bound, N-glycan-free ECD, which suggested that the GlcNAc might affect CTR dynamics not observed in the static crystallographic snapshots. Hydrogen-deuterium exchange mass spectrometry and molecular dynamics simulations revealed that glycosylation stabilized a β-sheet adjacent to the N130 GlcNAc and the N-terminal α-helix near the peptide-binding site while increasing flexibility of the peptide-binding site turret loop. These changes due to N-glycosylation increased the ligand on-rate and decreased its off-rate. The glycan effect extended to RAMP-CTR amylin receptor complexes and was also conserved in the related CGRP receptor. These results reveal that N-glycosylation can modulate GPCR function by altering receptor dynamics.

Current understanding of the structure and function of family B GPCRs to design novel drugs

Family B of G-protein-coupled receptors (GPCRs) and their ligands play a central role in a number of homeostatic mechanisms in the endocrine, gastrointestinal, skeletal, immune, cardiovascular and central nervous systems. Alterations in family B GPCR-regulated homeostatic mechanisms may cause a variety of potentially life-threatening conditions, signifying the necessity to develop novel ligands targeting these receptors. Obtaining structural and functional information on family B GPCRs will accelerate the development of novel drugs to target these receptors. Family B GPCRs are proteins that span the plasma membrane seven times, thus forming seven transmembrane domains (TM1-TM7) which are connected to each other by three extracellular (EL) and three intracellular (IL) loops. In addition, these receptors have a long extracellular N-domain and an intracellular C-tail. The upper parts of the TMs and ELs form the J-domain of receptors. The C-terminal region of peptides first binds to the N-domain of receptors. This 'first-step' interaction orients the N-terminal region of peptides towards the J-domain of receptors, thus resulting in a 'second-step' of ligand-receptor interaction that activates the receptor. Activation-associated structural changes of receptors are transmitted through TMs to their intracellular regions and are responsible for their interaction with the G proteins and activation of the latter, thus resulting in a biological effect. This review summarizes the current information regarding the structure and function of family B GPCRs and their physiological and pathophysiological roles.

Type II Turn of Receptor-bound Salmon Calcitonin Revealed by X-ray Crystallography

Calcitonin is a peptide hormone consisting of 32 amino acid residues and the calcitonin receptor is a Class B G protein-coupled receptor (GPCR). The crystal structure of the human calcitonin receptor ectodomain (CTR ECD) in complex with a truncated analogue of salmon calcitonin ([BrPhe(22)]sCT(8-32)) has been determined to 2.1-Å resolution. Parallel analysis of a series of peptide ligands showed that the rank order of binding of the CTR ECD is identical to the rank order of binding of the full-length CTR, confirming the structural integrity and relevance of the isolated CTR ECD. The structure of the CTR ECD is similar to other Class B GPCRs and the ligand binding site is similar to the binding site of the homologous receptors for the calcitonin gene-related peptide (CGRP) and adrenomedulin (AM) recently published (Booe, J. M., Walker, C. S., Barwell, J., Kuteyi, G., Simms, J., Jamaluddin, M. A., Warner, M. L., Bill, R. M., Harris, P. W., Brimble, M. A., Poyner, D. R., Hay, D. L., and Pioszak, A. A. (2015) Mol. Cell 58, 1040-1052). Interestingly the receptor-bound structure of the ligand [BrPhe(22)]sCT(8-32) differs from the receptor-bound structure of the homologous ligands CGRP and AM. They all adopt an extended conformation followed by a C-terminal β turn, however, [BrPhe(22)]sCT(8-32) adopts a type II turn (Gly(28)-Thr(31)), whereas CGRP and AM adopt type I turns. Our results suggest that a type II turn is the preferred conformation of calcitonin, whereas a type I turn is the preferred conformation of peptides that require RAMPs; CGRP, AM, and amylin. In addition the structure provides a detailed molecular explanation and hypothesis regarding ligand binding properties of CTR and the amylin receptors.

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.

Structure-activity relationships of the N-terminus of calcitonin gene-related peptide: key roles of alanine-5 and threonine-6 in receptor activation

BACKGROUND AND PURPOSE

The N-terminus of calcitonin gene-related peptide (CGRP) is important for receptor activation, especially the disulphide-bonded ring (residues 1-7). However, the roles of individual amino acids within this region have not been examined and so the molecular determinants of agonism are unknown. This study has examined the role of residues 1, 3-6 and 8-9, excluding Cys-2 and Cys-7.

EXPERIMENTAL APPROACH

CGRP derivatives were substituted with either cysteine or alanine; further residues were introduced at position 6. Their affinity was measured by radioligand binding and their efficacy by measuring cAMP production in SK-N-MC cells and β-arrestin 2 translocation in CHO-K1 cells at the CGRP receptor.

KEY RESULTS

Substitution of Ala-5 by cysteine reduced affinity 270-fold and reduced efficacy for production of cAMP in SK-N-MCs. Potency at β-arrestin translocation was reduced by ninefold. Substitution of Thr-6 by cysteine destroyed all measurable efficacy of both cAMP and β-arrestin responses; substitution with either alanine or serine impaired potency. Substitutions at positions 1, 4, 8 and 9 resulted in approximately 10-fold reductions in potency at both responses. Similar observations were made at a second CGRP-activated receptor, the AMY(1(a)) receptor.

CONCLUSIONS AND IMPLICATIONS

Ala-5 and Thr-6 are key determinants of agonist activity for CGRP. Ala-5 is also very important for receptor binding. Residues outside of the 1-7 ring also contribute to agonist activity.

Prolonged calcitonin receptor signaling by salmon, but not human calcitonin, reveals ligand bias

Salmon calcitonin (sCT) and human calcitonin (hCT) are pharmacologically distinct. However, the reason for the differences is unclear. Here we analyze the differences between sCT and hCT on the human calcitonin receptor (CT_(a)R) with respect to activation of cAMP signaling, β-arrestin recruitment, ligand binding kinetics and internalization. The study was conducted using mammalian cell lines heterologously expressing the human CT_(a) receptor. CT_(a)R downstream signaling was investigated with dose response profiles for cAMP production and β-arrestin recruitment for sCT and hCT during short term (<2 hours) and prolonged (up to 72 hours) stimulation. CT_(a)R kinetics and internalization was investigated with radio-labeled sCT and hCT ligands on cultured cells and isolated membrane preparations from the same cell line. We found that sCT and hCT are equipotent during short-term stimulations with differences manifesting themselves only during long-term stimulation with sCT inducing a prolonged activation up to 72 hours, while hCT loses activity markedly earlier. The prolonged sCT stimulation of both cAMP accumulation and β-arrestin recruitment was attenuated, but not abrogated by acid wash, suggesting a role for sCT activated internalized receptors. We have demonstrated a novel phenomenon, namely that two distinct CT_(a)R downstream signaling activation patterns are activated by two related ligands, thereby highlighting qualitatively different signaling responses in vitro that could have implications for sCT use in vivo.

Genetically encoded chemical probes in cells reveal the binding path of urocortin-I to CRF class B GPCR

Molecular determinants regulating the activation of class B G-protein-coupled receptors (GPCRs) by native peptide agonists are largely unknown. We have investigated here the interaction between the corticotropin releasing factor receptor type 1 (CRF1R) and its native 40-mer peptide ligand Urocortin-I directly in mammalian cells. By incorporating unnatural amino acid photochemical and new click-chemical probes into the intact receptor expressed in the native membrane of live cells, 44 intermolecular spatial constraints have been derived for the ligand-receptor interaction. The data were analyzed in the context of the recently resolved crystal structure of CRF1R transmembrane domain and existing extracellular domain structures, yielding a complete conformational model for the peptide-receptor complex. Structural features of the receptor-ligand complex yield molecular insights on the mechanism of receptor activation and the basis for discrimination between agonist and antagonist function.

Receptor activity-modifying protein-dependent impairment of calcitonin receptor splice variant Δ(1-47)hCT((a)) function

BACKGROUND AND PURPOSE

Alternative splicing expands proteome diversity to GPCRs. Distinct receptor variants have been identified for a secretin family GPCR, the calcitonin receptor (CTR). The possible functional contributions of these receptor variants are further altered by their potential interactions with receptor activity-modifying proteins (RAMPs). One variant of the human CTR lacks the first 47 residues at its N terminus [Δ(1-47)hCT((a)) ]. However, very little is known about the pharmacology of this variant or its ability to interact with RAMPs to form amylin receptors.

EXPERIMENTAL APPROACH

Δ(1-47)hCT((a)) was characterized both with and without RAMPs in Cos7 and/or HEK293S cells. The receptor expression (ELISA assays) and function (cAMP and pERK1/2 assays) for up to six agonists and two antagonists were determined.

KEY RESULTS

Despite lacking 47 residues at the N terminus, Δ(1-47)hCT((a)) was still able to express at the cell surface, but displayed a generalized reduction in peptide potency. Δ(1-47)hCT((a)) retained its ability to interact with RAMP1 and formed a functional amylin receptor; this also appeared to be the case with RAMP3. On the other hand, its interaction with RAMP2 and resultant amylin receptor was reduced to a greater extent.

CONCLUSIONS AND IMPLICATIONS

Δ(1-47)hCT((a)) acts as a functional receptor at the cell surface. It exhibits altered receptor function, depending on whether it associates with a RAMP and which RAMP it interacts with. Therefore, the presence of this variant in tissues will potentially contribute to altered peptide binding and signalling, depending on the RAMP distribution in tissues.

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.

The role of the extracellular loops of the CGRP receptor, a family B GPCR

The CGRP (calcitonin gene-related peptide) receptor is a family B GPCR (G-protein-coupled receptor). It consists of a GPCR, CLR (calcitonin receptor-like receptor) and an accessory protein, RAMP1 (receptor activity-modifying protein 1). RAMP1 is needed for CGRP binding and also cell-surface expression of CLR. There have been few systematic studies of the ECLs (extracellular loops) of family B GPCRs. However, they are likely to be especially important for the interaction of the N-termini of the peptide agonists that are the natural agonists for these receptors. We have carried out alanine scans on all three ECLs of CLR, as well as their associated juxtamembrane regions. Residues within all three loops influence CGRP binding and receptor activation. Mutation of Ala203 and Ala206 on ECL1 to leucine increased the affinity of CGRP. Residues at the top of TM (transmembrane) helices 2 and 3 influenced CGRP binding and receptor activation. L351A and E357A in TM6/ECL3 reduced receptor expression and may be needed for CLR association with RAMP1. ECL2 seems especially important for CLR function; of the 16 residues so far examined in this loop, eight residues reduce the potency of CGRP at stimulating cAMP production when mutated to alanine.

Lifting the lid on GPCRs: the role of extracellular loops

GPCRs exhibit a common architecture of seven transmembrane helices (TMs) linked by intracellular loops and extracellular loops (ECLs). Given their peripheral location to the site of G-protein interaction, it might be assumed that ECL segments merely link the important TMs within the helical bundle of the receptor. However, compelling evidence has emerged in recent years revealing a critical role for ECLs in many fundamental aspects of GPCR function, which supported by recent GPCR crystal structures has provided mechanistic insights. This review will present current understanding of the key roles of ECLs in ligand binding, activation and regulation of both family A and family B GPCRs.

Physiology and emerging biochemistry of the glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is one of the best validated therapeutic targets for the treatment of type 2 diabetes mellitus (T2DM). Over several years, the accumulation of basic, translational, and clinical research helped define the physiologic roles of GLP-1 and its receptor in regulating glucose homeostasis and energy metabolism. These efforts provided much of the foundation for pharmaceutical development of the GLP-1 receptor peptide agonists, exenatide and liraglutide, as novel medicines for patients suffering from T2DM. Now, much attention is focused on better understanding the molecular mechanisms involved in ligand induced signaling of the GLP-1 receptor. For example, advancements in biophysical and structural biology techniques are being applied in attempts to more precisely determine ligand binding and receptor occupancy characteristics at the atomic level. These efforts should better inform three-dimensional modeling of the GLP-1 receptor that will help inspire more rational approaches to identify and optimize small molecule agonists or allosteric modulators targeting the GLP-1 receptor. This article reviews GLP-1 receptor physiology with an emphasis on GLP-1 induced signaling mechanisms in order to highlight new molecular strategies that help determine desired pharmacologic characteristics for guiding development of future nonpeptide GLP-1 receptor activators.

Second extracellular loop of human glucagon-like peptide-1 receptor (GLP-1R) has a critical role in GLP-1 peptide binding and receptor activation

The glucagon-like peptide-1 receptor (GLP-1R) is a therapeutically important family B G protein-coupled receptor (GPCR) that is pleiotropically coupled to multiple signaling effectors and, with actions including regulation of insulin biosynthesis and secretion, is one of the key targets in the management of type II diabetes mellitus. However, there is limited understanding of the role of the receptor core in orthosteric ligand binding and biological activity. To assess involvement of the extracellular loop (ECL) 2 in ligand-receptor interactions and receptor activation, we performed alanine scanning mutagenesis of loop residues and assessed the impact on receptor expression and GLP-1(1-36)-NH(2) or GLP-1(7-36)-NH(2) binding and activation of three physiologically relevant signaling pathways as follows: cAMP formation, intracellular Ca(2+) (Ca(2+)(i)) mobilization, and phosphorylation of extracellular signal-regulated kinases 1 and 2 (pERK1/2). Although antagonist peptide binding was unaltered, almost all mutations affected GLP-1 peptide agonist binding and/or coupling efficacy, indicating an important role in receptor activation. However, mutation of several residues displayed distinct pathway responses with respect to wild type receptor, including Arg-299 and Tyr-305, where mutation significantly enhanced both GLP-1(1-36)-NH(2)- and GLP-1(7-36)-NH(2)-mediated signaling bias for pERK1/2. In addition, mutation of Cys-296, Trp-297, Asn-300, Asn-302, and Leu-307 significantly increased GLP-1(7-36)-NH(2)-mediated signaling bias toward pERK1/2. Of all mutants studied, only mutation of Trp-306 to alanine abolished all biological activity. These data suggest a critical role of ECL2 of the GLP-1R in the activation transition(s) of the receptor and the importance of this region in the determination of both GLP-1 peptide- and pathway-specific effects.

Site of action of a pentapeptide agonist at the glucagon-like peptide-1 receptor. Insight into a small molecule agonist-binding pocket

The development of small molecule agonists for class B G protein-coupled receptors (GPCRs) has been quite challenging. With proof-of-concept that exenatide, the parenterally administered peptide agonist of the glucagon-like peptide-1 (GLP1) receptor, is an effective treatment for patients with diabetes mellitus, the development of small molecule agonists could have substantial advantages. We previously reported a lead for small molecule GLP1 receptor agonist development representing the pentapeptide NRTFD. In this work, we have prepared an NRTFD derivative incorporating a photolabile benzoylphenylalanine and used it to define its site of action. This peptide probe was a full agonist with potency similar to NRTFD, which bound specifically and saturably to a single, distinct site within the GLP1 receptor. Peptide mapping using cyanogen bromide and endoproteinase Lys-C cleavage of labeled wild type and M397L mutant receptor constructs identified the site of covalent attachment of NRTFD within the third extracellular loop above the sixth transmembrane segment (TM6). This region is the same as that identified using an analogous photolabile probe based on secretin receptor sequences, and has been shown in mutagenesis studies to be important for natural agonist action of several members of this family. While these observations suggest that small molecule ligands can act at a site bordering the third extracellular loop to activate this class B GPCR, the relationship of this site to the site of action of the amino-terminal end of the natural agonist peptide is unclear.

Lactam constraints provide insights into the receptor-bound conformation of secretin and stabilize a receptor antagonist

The natural ligands for family B G protein-coupled receptors are moderate-length linear peptides having diffuse pharmacophores. The amino-terminal regions of these ligands are critical for biological activity, with their amino-terminal truncation leading to production of orthosteric antagonists. The carboxyl-terminal regions of these peptides are thought to occupy a ligand-binding cleft within the disulfide-bonded amino-terminal domains of these receptors, with the peptides in amphipathic helical conformations. In this work, we have characterized the binding and activity of a series of 11 truncated and lactam-constrained secretin(5-27) analogues at the prototypic member of this family, the secretin receptor. One peptide in this series with lactam connecting residues 16 and 20 [c[E(16),K(20)][Y(10)]sec(5-27)] improved the binding affinity of its unconstrained parental peptide 22-fold while retaining the absence of endogenous biological activity and competitive antagonist characteristics. Homology modeling with molecular mechanics and molecular dynamics simulations established that this constrained peptide occupies the ligand-binding cleft in an orientation similar to that of natural full-length secretin and provided insights into why this peptide was more effective than other truncated conformationally constrained peptides in the series. This lactam bridge is believed to stabilize an extended α-helical conformation of this peptide while in solution and not to interfere with critical residue-residue approximations while docked to the receptor.

Molecular basis of secretin docking to its intact receptor using multiple photolabile probes distributed throughout the pharmacophore

The molecular basis of ligand binding and activation of family B G protein-coupled receptors is not yet clear due to the lack of insight into the structure of intact receptors. Although NMR and crystal structures of amino-terminal domains of several family members support consistency in general structural motifs that include a peptide-binding cleft, there are variations in the details of docking of the carboxyl terminus of peptide ligands within this cleft, and there is no information about siting of the amino terminus of these peptides. There are also no empirical data to orient the receptor amino terminus relative to the core helical bundle domain. Here, we prepared a series of five new probes, incorporating photolabile moieties into positions 2, 15, 20, 24, and 25 of full agonist secretin analogues. Each bound specifically to the receptor and covalently labeled single distinct receptor residues. Peptide mapping of labeled wild-type and mutant receptors identified that the position 15, 20, and 25 probes labeled residues within the distal amino terminus of the receptor, whereas the position 24 probe labeled the amino terminus adjacent to TM1. Of note, the position 2 probe labeled a residue within the first extracellular loop of the receptor, a region not previously labeled, providing an important new constraint for docking the amino-terminal region of secretin to its receptor core. These additional experimentally derived constraints help to refine our understanding of the structure of the secretin-intact receptor complex and provide new insights into understanding the molecular mechanism for activation of family B G protein-coupled receptors.

Importance of each residue within secretin for receptor binding and biological activity

Secretin is a linear 27-residue peptide hormone that stimulates pancreatic and biliary ductular bicarbonate and water secretion by acting at its family B G protein-coupled receptor. While, like other family members, the carboxyl-terminal region of secretin is most important for high affinity binding and its amino-terminal region is most important for receptor selectivity and receptor activation, determinants for these activities are distributed throughout the entire length of this peptide. In this work, we have systematically investigated changing each residue within secretin to alanine and evaluating the impact on receptor binding and biological activity. The residues most critical for receptor binding were His1, Asp3, Gly4, Phe6, Thr7, Ser8, Leu10, Asp15, Leu19, and Leu23. The residues most critical for biological activity included His1, Gly4, Thr7, Ser8, Glu9, Leu10, Leu19, Leu22, and Leu23, with Asp3, Phe6, Ser11, Leu13, Asp15, Leu26, and Val27 also contributing. While the importance of residues in positions analogous to His1, Asp3, Phe6, Thr7, and Leu23 is conserved for several closely related members of this family, Leu19 is uniquely important for secretin. We, therefore, have further studied this residue by molecular modeling and molecular dynamics simulations. Indeed, the molecular dynamics simulations showed that mutation of Leu19 to alanine was destabilizing, with this effect greater than that observed for the analogous position in the other close family members. This could reflect reduced contact with the receptor or an increase in the solvent-accessible surface area of the hydrophobic residues in the carboxyl terminus of secretin as bound to its receptor.

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

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

Elucidation of the active conformation of the amino terminus of receptor-bound secretin using intramolecular disulfide bond constraints

Family B G protein-coupled receptors include several potentially important drug targets, yet our understanding of the molecular basis of ligand binding to and activation of these receptors is incomplete. While NMR and crystal structures exist for peptide ligand-associated amino-terminal domains of several family members, these only provide insights into the conformation of the carboxyl-terminal region of the peptides. The amino-terminal region of these peptides, critical for biological activity, is believed to interact with the helical bundle domain, and is, therefore, unconstrained in these structures. The aim of the current study was to provide insights into the conformation of the amino terminus of secretin as bound to its receptor. We prepared a series of conformationally constrained secretin peptides containing intramolecular disulfide bonds that were predicted by molecular modeling to approximate the conformation of the analogous region of PACAP bound to its receptor that had been determined using transfer-NOE NMR techniques. Secretin peptides with pairs of cysteine residues in positions 2-7, 3-5, 3-6, 4-7, 7-9, and 4-10 were studied as linear and disulfide-bonded forms. The analog with a disulfide bond connecting positions 7-9 had binding affinity and biological activity similar to natural secretin, supporting the relevance of this constraint to its active conformation. While this feature is shared between secretin and PACAP, absence of activity in other constrained peptides in this series also suggest that there are differences between these receptor-bound conformations. It will be critical to extend similar studies to other family members to learn what structural elements might be most conserved in this family.

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

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

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

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

Effects of pH and temperature on photoaffinity labeling of Family B G protein-coupled receptors

The efficiency of covalent labeling of a receptor by a photolabile analogue of its natural ligand is dependent on the spatial approximation of the probe and its target. Systematic application of intrinsic photoaffinity labeling to the secretin receptor, a prototypic Family B G protein-coupled receptor, demonstrated reduced efficiency of labeling for amino-terminal and mid-region sites of labeling relative to carboxyl-terminal sites. Reduction of pH from 7.4 to 5.5 and reduction of temperature from 25 degrees C to 4 degrees C improved the efficiency of covalent labeling of the receptor with these probes. This correlated with sites of labeling at the interface between the receptor amino terminus and the receptor core, a region containing histidine residues that have their ionization affected in this pH range. Application to the calcitonin receptor, another Family B G protein-coupled receptor, yielded analogous results. These results support the consistent mode of docking peptide ligands to this group of receptors.

Spatial approximation between secretin residue five and the third extracellular loop of its receptor provides new insight into the molecular basis of natural agonist binding

The amino terminus of class II G protein-coupled receptors plays an important role in ligand binding and receptor activation. Understanding of the conformation of the amino-terminal domain of these receptors has been substantially advanced with the solution of nuclear magnetic resonance and crystal structures of this region of receptors for corticotrophin-releasing factor, pituitary adenylate cyclase-activating polypeptide, and gastric inhibitory polypeptide. However, the orientation of the amino terminus relative to the receptor core and how the receptor gets activated upon ligand binding remain unclear. In this work, we have used photoaffinity labeling to identify a critical spatial approximation between residue five of secretin and a residue within the proposed third extracellular loop of the secretin receptor. This was achieved by purification, deglycosylation, cyanogen bromide cleavage, and sequencing of labeled wild-type and mutant secretin receptors. This constraint has been used to refine our evolving molecular model of secretin docked at the intact receptor, which for the first time includes refined helical bundle and loop regions and reflects a peptide-binding groove within the receptor amino terminus that directs the amino terminus of the peptide toward the receptor body. This model is fully consistent with the endogenous agonist mechanism for class II G protein-coupled receptor activation, where ligand binding promotes the interaction of a portion of the receptor amino terminus with the receptor body to activate it.

Exploration of the endogenous agonist mechanism for activation of secretin and VPAC1 receptors using synthetic glycosylated peptides

Current understanding of the molecular basis of activation of class II G protein-coupled receptors remains limited, despite recent solution of NMR and crystal structures of amino-terminal domains of several family members. One mechanism proposed for the activation of these receptors involves an agonist-stimulated change in conformation of the receptor amino terminus. This results in the exposure of a "hidden endogenous agonist" (WDN sequence in secretin and VPAC1 receptors) within the receptor amino terminus that interacts with the receptor core, thereby changing its conformation and exposing its G protein-binding region. The Asn in this WDN sequence is known to be glycosylated in both secretin and VPAC1 receptors, raising concern about whether this posttranslational modification might interfere with the proposed mechanism. Therefore, we prepared glycosylated forms of cyclic WDN and the longer cyclic peptide, LWDNM, and tested them for agonist activity at secretin and VPAC1 receptor-bearing cell lines. Both glycosylated peptides stimulated full cAMP responses in the cell lines. Clearly, glycosylation did not interfere with this mechanism and may actually facilitate the correct orientation of the pharmacophore of the endogenous agonist ligand. These data provide further evidence for this proposed mechanism for the activation of this family of receptors.

Insights into the structural basis of endogenous agonist activation of family B G protein-coupled receptors

Agonist drugs targeting the glucagon-like peptide-1 (GLP1) receptor represent important additions to the clinical management of patients with diabetes mellitus. In the current report, we have explored whether the recently described concept of a receptor-active endogenous agonist sequence within the amino terminus of the secretin receptor may also be applicable to the GLP1 receptor. If so, this could provide a lead for the development of additional small molecule agonists targeting this and other important family members. Indeed, the region of the GLP1 receptor analogous to that containing the active WDN within the secretin receptor was found to possess full agonist activity at the GLP1 receptor. The minimal fragment within this region that had full agonist activity was NRTFD. Despite having no primary sequence identity with the WDN, it was also active at the secretin receptor, where it had similar potency and efficacy to WDN, suggesting common structural features. Molecular modeling demonstrated that an intradomain salt bridge between the side chains of arginine and aspartate could yield similarities in structure with cyclic WDN. This directly supports the relevance of the endogenous agonist concept to the GLP1 receptor and provides new insights into the rational development and refinement of new types of drugs activating this important receptor.

Mapping ligand-receptor interfaces: approaching the resolution limit of benzophenone-based photoaffinity scanning

Photoaffinity crosslinking has yielded important insights in the study of G protein-coupled receptors and the mode of ligand binding. The most widely used photolabile moiety is p-benzoylphenylalanine largely because of its reportedly high site specificity, reduced reactivity to water and light, photokinetics, and ease of incorporation into peptide ligands during synthesis. However, in the course of our studies directed at characterizing the binding of parathyroid hormone to its cognate G protein-coupled receptor, we find that inherent properties of p-benzoylphenylalanine, such as its size and conformational flexibility, limit the resulting resolution of the ligand-receptor structure. Here, we examine and define these limits.

Fluorescence Resonance Energy Transfer Analysis of Secretin Docking to Its Receptor

Full structural characterization of G protein-coupled receptors has been limited to rhodopsin, with its uniquely stable structure and ability to be crystallized. For other members of this important superfamily, direct structural insights have been limited to NMR structures of soluble domains. Two members of the Class II family have recently had the structures of their isolated amino-terminal regions solved by NMR, yet it remains unclear how that domain is aligned with the heptahelical transmembrane bundle domain of those receptors. Indeed, three distinct orientations have been suggested for different members of this family. In the current work, we have utilized fluorescence resonance energy transfer to establish the distances between four residues distributed throughout fully biologically active, high affinity analogues of secretin and distinct residues in each of four extracellular regions of the intact secretin receptor. These 16 distance constraints were utilized along with nine photoaffinity labeling spatial approximation constraints to study the three proposed orientations of the peptide-binding amino terminus and helical bundle domains of this receptor. In the best model, the carboxyl terminus of secretin was found to bind in a groove above the beta-hairpin region of the receptor amino terminus, with its amino-terminal end adjacent to the third extracellular loop and top of transmembrane segment VI. This refined model of the intact receptor was also fully consistent with the spatial approximation of the Trp(48)-Asp(49)-Asn(50) endogenous agonist segment with the third extracellular loop region that it has been shown to photolabel. This provides strong evidence for the orientation of peptide-binding and signaling domains of a prototypic Class II G protein-coupled receptor.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Agonist-dependent consequences of proline to alanine substitution in the transmembrane helices of the calcitonin receptor

BACKGROUND AND PURPOSE

Transmembrane proline (P) residues in family A G protein-coupled receptors (GPCRs) form functionally important kinks in their helices. These residues are little studied in family B GPCRs but experiments with the VPAC1 receptor and calcitonin receptor-like receptor (CL) show parallels with family A receptors. We sought to determine the function of these residues in the insert negative form of the human calcitonin receptor, a close relative of CL.

EXPERIMENTAL APPROACH

Proline residues within the transmembrane domains of the calcitonin receptor (P246, P249, P280, P326, P336) were individually mutated to alanine (A) using site-directed mutagenesis. Receptors were transiently transfected into Cos-7 cells using polyethylenimine and salmon and human calcitonin-induced cAMP responses measured. Salmon and human calcitonin competition binding experiments were also performed and receptor cell-surface expression assessed by whole cell ELISA.

KEY RESULTS

P246A, P249A and P280A were wild-type in terms of human calcitonin-induced cAMP activation. P326A and P336A had reduced function (165 and 12-fold, respectively). In membranes, human calcitonin binding was not detectable for any mutant receptor but in whole cells, binding was detected for all mutants apart from P326A. Salmon calcitonin activated mutant and wild-type receptors equally, although B(max) values were reduced for all mutants apart from P326A.

CONCLUSIONS AND IMPLICATIONS

P326 and P336 are important for the function of human calcitonin receptors and are likely to be involved in generating receptor conformations appropriate for agonist binding and receptor activation. However, agonist-specific effects were observed , implying distinct conformations of the human calcitonin receptor.

Structural Determinants of Salmon Calcitonin Bioactivity

Salmon calcitonin (sCT) forms an amphipathic helix in the region 9-19, with the C-terminal decapeptide interacting with the helix (Amodeo, P., Motta, A., Strazzullo, G., Castiglione Morelli, M. A. (1999) J. Biomol. NMR 13, 161-174). To uncover the structural requirements for the hormone bioactivity, we investigated several sCT analogs. They were designed so as to alter the length of the central helix by removal and/or replacement of flanking residues and by selectively mutating or deleting residues inside the helix. The helix content was assessed by circular dichroism and NMR spectroscopies; the receptor binding affinity in human breast cancer cell line T 47D and the in vivo hypocalcemic activity were also evaluated. In particular, by NMR spectroscopy and molecular dynamics calculations we studied Leu(23),Ala(24)-sCT in which Pro(23) and Arg(24) were replaced by helix inducing residues. Compared with sCT, it assumes a longer amphipathic alpha-helix, with decreased binding affinity and one-fifth of the hypocalcemic activity, therefore supporting the idea of a relationship between a definite helix length and bioactivity. From the analysis of other sCT mutants, we inferred that the correct helix length is located in the 9-19 region and requires long range interactions and the presence of specific regions of residues within the sequence for high binding affinity and hypocalcemic activity. Taken together, the structural and biological data identify well defined structural parameters of the helix for sCT bioactivity.

Differential Spatial Approximation between Secretin and Its Receptor Residues in Active and Inactive Conformations Demonstrated by Photoaffinity Labeling

Understanding of the conformational changes in G protein-coupled receptors associated with activation and inactivation is of great interest. We previously used photoaffinity labeling to elucidate spatial approximations between photolabile residues situated throughout the pharmacophore of secretin agonist probes and this receptor. The aim of the current work was to develop analogous photolabile secretin antagonist probes and to explore their spatial approximations. The most potent secretin antagonist reported is a pseudopeptide ([ψ4, 5]secretin) in which the peptide bond between residues 4 and 5 was replaced by a ψ(CH2-NH) peptide bond isostere. We have developed a series of [ψ4, 5]secretin analogs incorporating photolabile benzoyl phenylalanine residues in positions 6, 22, and 26. Each bound to the secretin receptor saturably and specifically, with affinity similar to their parental peptide. At concentrations with no measurable agonist activity, each probe covalently labeled the secretin receptor. Peptide mapping using proteolytic cleavage, immunoprecipitation, and radiochemical sequencing identified that each of these three probes labeled the amino terminus of the secretin receptor. Whereas the position 22 probe labeled the same residue as its analogous agonist probe and the position 6 probe labeled a residue within two residues of that labeled by its analogous agonist probe, the position 26 probe labeled a site 16 residues away from that labeled by its analogous agonist probe. Thus, whereas structurally related agonist and antagonist probes dock in the same general region of this receptor, conformational differences in active and inactive states result in substantial differences in spatial approximation at the carboxyl-terminal end of secretin analogs.

Possible endogenous agonist mechanism for the activation of secretin family G protein-coupled receptors

The class B family of G protein-coupled receptors contains several potentially important drug targets, yet our understanding of the molecular basis of ligand binding and receptor activation remains incomplete. Although a key role is recognized for the cysteine-rich, disulfide-bonded amino-terminal domain of these receptors, detailed insights into ligand docking and resultant conformational changes are not clear. We postulate that binding natural ligands to this domain results in a conformational change that exposes an endogenous ligand which interacts with the body of the receptor to activate it. In this work, we examined whether a synthetic peptide corresponding to a candidate region between the first and third conserved cysteines could act as an agonist. Indeed, this peptide was a weakly potent but fully efficacious agonist, stimulating a concentration-dependent cAMP response in secretin receptor-bearing cells. This effect was maintained as the peptide length was reduced from 30 to 5, and ultimately, three residues focused on the conserved residue Asp49. The agonist potency was enhanced by cyclization through a diaminopropionic acid linker and by amino-terminal fatty acid acylation. Both ends of the cyclic peptide were shown to interact with the top of transmembrane segment 6 of the receptor, using probes with a photolabile benzoyl-phenylalanine on each end. Analogous observations were also made for two other members of this family, the vasoactive intestinal polypeptide type 1 and calcitonin receptors. These data may provide a unique molecular mechanism and novel leads for the development of small-molecule agonists acting at potential drug targets within this physiologically important receptor family.

Insights into interactions between the alpha-helical region of the salmon calcitonin antagonists and the human calcitonin receptor using photoaffinity labeling

Fish-like calcitonins (CTs), such as salmon CT (sCT), are widely used clinically in the treatment of bone-related disorders; however, the molecular basis for CT binding to its receptor, a class II G protein-coupled receptor, is not well defined. In this study we have used photoaffinity labeling to identify proximity sites between CT and its receptor. Two analogues of the antagonist sCT(8-32) containing a single photolabile p-benzoyl-l-phenylalanine (Bpa) residue in position 8 or 19 were used. Both analogues retained high affinity for the CT receptor and potently inhibited agonist-induced cAMP production. The [Bpa(19)]sCT(8-32) analogue cross-linked to the receptor at or near the equivalent cross-linking site of the full-length peptide, within the fragment Cys(134)-Lys(141) (within the amino terminus of the receptor, adjacent to transmembrane 1) (Pham, V., Wade, J. D., Purdue, B. W., and Sexton, P. M. (2004) J. Biol. Chem. 279, 6720-6729). In contrast, proteolytic mapping and mutational analysis identified Met(49) as the cross-linking site for [Bpa(8)]sCT(8-32). This site differed from the previously identified cross-linking site of the agonist [Bpa(8)]human CT (Dong, M., Pinon, D. I., Cox, R. F., and Miller, L. J. (2004) J. Biol. Chem. 279, 31177-31182) and may provide evidence for conformational differences between interaction with active and inactive state receptors. Molecular modeling suggests that the difference in cross-linking between the two Bpa(8) analogues can be accounted for by a relatively small change in peptide orientation. The model was also consistent with cooperative interaction between the receptor amino terminus and the receptor core.

Insights into the structure and molecular basis of ligand docking to the G protein-coupled secretin receptor using charge-modified amino-terminal agonist probes

The amino terminus and third loop regions of class B G protein-coupled receptors play critical roles in ligand docking and action. For the prototypic secretin receptor, the hormone amino terminus is spatially approximated with receptor region high in transmembrane segment 6 (TM6), whereas residues ranging from position 6 through 26 label the amino terminus. Here, we focus on the role of charge of the secretin amino terminus, using a series of full-agonist, acetylated probes. Sites of covalent labeling were examined using sequential purification, chemical and enzymatic cleavage, and Edman degradation. High-affinity amino-terminally-blocked probes labeled the distal amino-terminal tail, rather than TM6, while adding a basic residue, again labeled TM6. These data suggest that the secretin amino terminus docks between the amino terminus and TM6 of the receptor, with this region of secretin likely interacting with an acidic residue within the receptor TM6 and the third extracellular loop. To explore this, candidate acidic residues were mutated to Ala (E341A, D342A, E345A, E351A). The E351A mutant markedly interfered with binding, biological activity, and internalization, whereas all others bound secretin and signaled and internalized normally. This supports the possibility that there is a charge-charge interaction between this residue and the amino terminus of secretin that is critical to its normal docking.