Use of N,O-bis-Fmoc-D-Tyr-ONSu for introduction of an oxidative iodination site into cholecystokinin family peptides

Cholesterol-dependent dynamic changes in the conformation of the type 1 cholecystokinin receptor affect ligand binding and G protein coupling

Development of optimal therapeutics for disease states that can be associated with increased membrane cholesterol requires better molecular understanding of lipid modulation of the drug target. Type 1 cholecystokinin receptor (CCK1R) agonist actions are affected by increased membrane cholesterol, enhancing ligand binding and reducing calcium signaling, while agonist actions of the closely related CCK2R are not. In this work, we identified a set of chimeric human CCK1R/CCK2R mutations that exchange the cholesterol sensitivity of these 2 receptors, providing powerful tools when expressed in CHO and HEK-293 model cell lines to explore mechanisms. Static, low energy, high-resolution structures of the mutant CCK1R constructs, stabilized in complex with G protein, were not substantially different, suggesting that alterations to receptor dynamics were key to altered function. We reveal that cholesterol-dependent dynamic changes in the conformation of the helical bundle of CCK receptors affects both ligand binding at the extracellular surface and G protein coupling at the cytosolic surface, as well as their interrelationships involved in stimulus-response coupling. This provides an ideal setting for potential allosteric modulators to correct the negative impact of membrane cholesterol on CCK1R.

Impact of secretin receptor homo-dimerization on natural ligand binding

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

Mechanism of Action and Structure-Activity Relationships of Tetracyclic Small Molecules Acting as Universal Positive Allosteric Modulators of the Cholecystokinin Receptor

As part of an ongoing effort to develop a drug targeting the type 1 cholecystokinin receptor (CCK1R) to help prevent and/or treat obesity, we recently performed a high throughput screening effort of small molecules seeking candidates that enhanced the action of the natural agonist, CCK, thus acting as positive allosteric modulators without exhibiting intrinsic agonist action. Such probes would be expected to act in a temporally finite way to enhance CCK action to induce satiety during and after a meal and potentially even modulate activity at the CCK1R in a high cholesterol environment present in some obese patients. The current work focuses on the best scaffold, representing tetracyclic molecules identified through high throughput screening we previously reported. Extensive characterization of the two top "hits" from the previous effort demonstrated them to fulfill the desired pharmacologic profile. We undertook analog-by-catalog expansion of this scaffold using 65 commercially available analogs. In this effort, we were able to eliminate an off-target effect observed for this scaffold while retaining its activity as a positive allosteric modulator of CCK1R in both normal and high cholesterol membrane environments. These insights should be useful in the rational medicinal chemical enhancement of this scaffold and in the future development of candidates to advance to pre-clinical proof-of-concept and to clinical trials.

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

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

Structures of the human cholecystokinin 1 (CCK1) receptor bound to Gs and Gq mimetic proteins provide insight into mechanisms of G protein selectivity

G protein-coupled receptors (GPCRs) are critical regulators of cellular function acting via heterotrimeric G proteins as their primary transducers with individual GPCRs capable of pleiotropic coupling to multiple G proteins. Structural features governing G protein selectivity and promiscuity are currently unclear. Here, we used cryo-electron microscopy (cryo-EM) to determine structures of the cholecystokinin (CCK) type 1 receptor (CCK1R) bound to the CCK peptide agonist, CCK-8 and 2 distinct transducer proteins, its primary transducer Gq, and the more weakly coupled Gs. As seen with other Gq/11-GPCR complexes, the Gq-α5 helix (αH5) bound to a relatively narrow pocket in the CCK1R core. Surprisingly, the backbone of the CCK1R and volume of the G protein binding pocket were essentially equivalent when Gs was bound, with the Gs αH5 displaying a conformation that arises from "unwinding" of the far carboxyl-terminal residues, compared to canonically Gs coupled receptors. Thus, integrated changes in the conformations of both the receptor and G protein are likely to play critical roles in the promiscuous coupling of individual GPCRs.

Discovery of a Positive Allosteric Modulator of Cholecystokinin Action at CCK1R in Normal and Elevated Cholesterol

Drugs useful in prevention/treatment of obesity could improve health. Cholecystokinin (CCK) is a key regulator of appetite, working through the type 1 CCK receptor (CCK1R); however, full agonists have not stimulated more weight loss than dieting. We proposed an alternate strategy to target this receptor, while reducing likelihood of side effects and/or toxicity. Positive allosteric modulators (PAMs) with minimal intrinsic agonist activity would enhance CCK action, while maintaining spatial and temporal characteristics of physiologic signaling. This could correct abnormal stimulus-activity coupling observed in a high-cholesterol environment observed in obesity. We utilized high-throughput screening to identify a molecule with this pharmacological profile and studied its basis of action. Compound 1 was a weak partial agonist, with PAM activity to enhance CCK action at CCK1R, but not CCK2R, maintained in both normal and high cholesterol. Compound 1 (10 µM) did not exhibit agonist activity or stimulate internalization of CCK1R. It enhanced CCK activity by slowing the off-rate of bound hormone, increasing its binding affinity. Computational docking of Compound 1 to CCK1R yielded plausible poses. A radioiodinatable photolabile analogue retained Compound 1 pharmacology and covalently labeled CCK1R Thr²¹¹, consistent with one proposed pose. Our study identifies a novel, selective, CCK1R PAM that binds to the receptor to enhance action of CCK-8 and CCK-58 in both normal and disease-mimicking high-cholesterol environments. This facilitates the development of compounds that target the physiologic spatial and temporal engagement of CCK1R by CCK that underpins its critical role in metabolic regulation.

Structure and dynamics of the active Gs-coupled human secretin receptor

The class B secretin GPCR (SecR) has broad physiological effects, with target potential for treatment of metabolic and cardiovascular disease. Molecular understanding of SecR binding and activation is important for its therapeutic exploitation. We combined cryo-electron microscopy, molecular dynamics, and biochemical cross-linking to determine a 2.3 Å structure, and interrogate dynamics, of secretin bound to the SecR:Gs complex. SecR exhibited a unique organization of its extracellular domain (ECD) relative to its 7-transmembrane (TM) core, forming more extended interactions than other family members. Numerous polar interactions formed between secretin and the receptor extracellular loops (ECLs) and TM helices. Cysteine-cross-linking, cryo-electron microscopy multivariate analysis and molecular dynamics simulations revealed that interactions between peptide and receptor were dynamic, and suggested a model for initial peptide engagement where early interactions between the far N-terminus of the peptide and SecR ECL2 likely occur following initial binding of the peptide C-terminus to the ECD.

Rational development of a high-affinity secretin receptor antagonist

The secretin receptor is a prototypic class B GPCR with substantial and broad pharmacologic importance. The aim of this project was to develop a high affinity selective antagonist as a new and important pharmacologic tool and to aid stabilization of this receptor in an inactive conformation for ultimate structural characterization. Amino-terminal truncation of the natural 27-residue ligand reduced biological activity, but also markedly reduced binding affinity. This was rationally and experimentally overcome with lactam stabilization of helical structure and with replacement of residues with natural and unnatural amino acids. A key new step in this effort was the replacement of peptide residue Leu²² with L-cyclohexylalanine (Cha) to enhance potential hydrophobic interactions with receptor residues Leu³¹, Val³⁴, and Phe⁹² that were predicted from molecular modeling. Alanine-replacement mutagenesis of these residues markedly affected ligand binding and biological activity. The optimal antagonist ligand, (Y¹⁰,c[E¹⁶,K²⁰],I¹⁷,Cha²²,R²⁵)sec(6-27), exhibited high binding affinity (4 nM), similar to natural secretin, and exhibited no demonstrable biological activity to stimulate cAMP accumulation, intracellular calcium mobilization, or β-arrestin-2 translocation. It acts as an orthosteric competitive antagonist, predicted to bind within the peptide-binding groove in the receptor extracellular domain. The analogous peptide that was one residue longer, retaining Thr⁵, exhibited partial agonist activity, while further truncation of even a single residue (Phe⁶) reduced binding affinity. This sec(6-27)-based peptide will be an important new tool for pharmacological and structural studies.

Use of Fluorescence Indicators in Receptor Ligands

Fluorescence techniques can provide insights into the environment of fluorescence indicators incorporated within a ligand as it is bound to its receptor. Fluorescence indicators of different sizes and chemical characteristics can provide insights into the nature of the binding environment, the surrounding structures, and even into conformational changes associated with receptor activation. Methods for determining fluorescence spectral analysis, fluorescence quenching, fluorescence anisotropy, fluorescence lifetimes, and red edge excitation shifts of the ligand probes are described. The applications of these techniques to the CCK1 receptor occupied by alexa(488)-CCK and aladan-CCK, as examples of probes developed (1) by derivatization of an existing peptide and (2) by incorporation during peptide synthesis, are utilized as examples. These methods represent powerful tools to expand our understanding of the structure and molecular basis of ligand activation of G protein-coupled receptors.

Beneficial effects of β-sitosterol on type 1 cholecystokinin receptor dysfunction induced by elevated membrane cholesterol

BACKGROUND & AIMS

The type 1 cholecystokinin receptor (CCK1R) mediates the actions of CCK to support nutritional homeostasis, including post-cibal satiety. However, elevated levels of membrane cholesterol, such as have been observed in metabolic syndrome, interfere with CCK stimulus-activity coupling at the CCK1R, thereby disrupting this important servomechanism. We hypothesize that reversal of the negative impact of cholesterol on this receptor could be useful in the management of obesity.

METHODS

We have studied the effects of β-sitosterol, a phytosterol structurally related to cholesterol, on CCK receptor function. This included CCK binding and biological activity at wild type CCK1R and CCK2R, as well as at CCK1R in a high cholesterol environment, and at a CCK1R mutant, Y140A, which mimics the behavior of wild type receptor in high cholesterol.

RESULTS

β-sitosterol (100 μM and 10 μM) significantly improved the defective signaling of the CCK1R present in high cholesterol (p < 0.05), without affecting CCK binding affinity. This effect was absent at the CCK1R present in a normal cholesterol environment, as well as at the structurally-related CCK2R. Furthermore, the cholesterol-insensitive Y140A mutant of CCK1R was resistant to the effects of β-sitosterol.

CONCLUSION

These data suggest that β-sitosterol affects CCK1R function in high cholesterol by competing with cholesterol at a receptor cholesterol-binding site and may shift its conformation toward normal. This phytosterol extends our understanding of the structure-activity relationships for developing a drug that can target the external surface of CCK1R. Since the concentrations of β-sitosterol shown to be effective in this study are similar to serum levels of this compound achievable during oral administration, it may be worthwhile to study possible beneficial effects of β-sitosterol in metabolic syndrome.

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.

Impact of ursodeoxycholic acid on a CCK1R cholesterol-binding site may contribute to its positive effects in digestive function

Dysfunction of the type 1 cholecystokinin (CCK) receptor (CCK1R) as a result of increased gallbladder muscularis membrane cholesterol has been implicated in the pathogenesis of cholesterol gallstones. Administration of ursodeoxycholic acid, which is structurally related to cholesterol, has been shown to have beneficial effects on gallstone formation. Our aims were to explore the possible direct effects and mechanism of action of bile acids on CCK receptor function. We studied the effects of structurally related hydrophobic chenodeoxycholic acid and hydrophilic ursodeoxycholic acid in vitro on CCK receptor function in the setting of normal and elevated membrane cholesterol. We also examined their effects on a cholesterol-insensitive CCK1R mutant (Y140A) disrupting a key site of cholesterol action. The results show that, similar to the impact of cholesterol on CCK receptors, bile acid effects were limited to CCK1R, with no effects on CCK2R. Chenodeoxycholic acid had a negative impact on CCK1R function, while ursodeoxycholic acid had no effect on CCK1R function in normal membranes but was protective against the negative impact of elevated cholesterol on this receptor. The cholesterol-insensitive CCK1R mutant Y140A was resistant to effects of both bile acids. These data suggest that bile acids compete with the action of cholesterol on CCK1R, probably by interacting at the same site, although the conformational impact of each bile acid appears to be different, with ursodeoxycholic acid capable of correcting the abnormal conformation of CCK1R in a high-cholesterol environment. This mechanism may contribute to the beneficial effect of ursodeoxycholic acid in reducing cholesterol gallstone formation.

Elimination of a cholecystokinin receptor agonist 'trigger' in an effort to develop positive allosteric modulators without intrinsic agonist activity

Cholecystokinin (CCK) acts at the type 1 cholecystokinin receptor (CCK1R) to elicit satiety and is a well-established drug target for obesity. To date, small molecule agonists have been developed, but have failed to demonstrate adequate efficacy in clinical trials, and concerns about side effects and potential toxicity have limited further development of full agonists. The use of positive allosteric modulators (PAMs) without intrinsic agonist activity that are active only for a brief period of time after a meal might represent a safer alternative. Here, we propose a possible novel strategy to develop such compounds by modifying the agonist 'trigger' of an existing small molecule agonist. We have studied analogues of the 1,5-benzodiazepine agonist, GI181771X, in which the N1-isopropyl agonist 'trigger' was modified. While agonist activity was greatly reduced in these compounds, they acted as negative, rather than positive modulators. The parent drug was also found to exhibit no positive modulation of CCK action. Receptor structure-activity relationship studies demonstrated that the mode of docking these derivatives was distinct from that of the parent compound, perhaps explaining their action as negative allosteric modulators. We conclude that this outcome is likely characteristic of the parental agonist, and that this strategy may be more successfully utilized with a parental ago-PAM, possessing intrinsic positive modulatory activity.

Development of a highly selective allosteric antagonist radioligand for the type 1 cholecystokinin receptor and elucidation of its molecular basis of binding

Understanding the molecular basis of ligand binding to receptors provides insights useful for rational drug design. This work describes development of a new antagonist radioligand of the type 1 cholecystokinin receptor (CCK1R), (2-fluorophenyl)-2,3-dihydro-3-[(3-isoquinolinylcarbonyl)amino]-6-methoxy-2-oxo-l-H-indole-3-propanoate (T-0632), and exploration of the molecular basis of its binding. This radioligand bound specifically with high affinity within an allosteric pocket of CCK1R. T-0632 fully inhibited binding and action of CCK at this receptor, while exhibiting no saturable binding to the closely related type 2 cholecystokinin receptor (CCK2R). Chimeric CCK1R/CCK2R constructs were used to explore the molecular basis of T-0632 binding. Exchanging exonic regions revealed the functional importance of CCK1R exon 3, extending from the bottom of transmembrane segment (TM) 3 to the top of TM5, including portions of the intramembranous pocket as well as the second extracellular loop region (ECL2). However, CCK1R mutants in which each residue facing the pocket was changed to that present in CCK2R had no negative impact on T-0632 binding. Extending the chimeric approach to ECL2 established the importance of its C-terminal region, and site-directed mutagenesis of each nonconserved residue in this region revealed the importance of Ser(208) at the top of TM5. A molecular model of T-0632-occupied CCK1R was consistent with these experimental determinants, also identifying Met(121) in TM3 and Arg(336) in TM6 as important. Although these residues are conserved in CCK2R, mutating them had a distinct impact on the two closely related receptors, suggesting differential orientation. This establishes the molecular basis of binding of a highly selective nonpeptidyl allosteric antagonist of CCK1R, illustrating differences in docking that extend beyond determinants attributable to distinct residues lining the intramembranous pocket in the two receptor subtypes.

A type 1 cholecystokinin receptor mutant that mimics the dysfunction observed for wild type receptor in a high cholesterol environment

Cholecystokinin (CCK) stimulates the type 1 CCK receptor (CCK1R) to elicit satiety after a meal. Agonists with this activity, although potentially useful for treatment of obesity, can also have side effects and toxicities of concern, making the development of an intrinsically inactive positive allosteric modulator quite attractive. Positive allosteric modulators also have the potential to correct the defective receptor-G protein coupling observed in the high membrane cholesterol environment described in metabolic syndrome. Current model systems to study CCK1R in such an environment are unstable and expensive to maintain. We now report that the Y140A mutation within a cholesterol-binding motif and the conserved, class A G protein-coupled receptor-specific (E/D)RY signature sequence results in ligand binding and activity characteristics similar to wild type CCK1R in a high cholesterol environment. This is true for natural CCK, as well as ligands with distinct chemistries and activity profiles. Additionally, the Y140A construct also behaved like CCK1R in high cholesterol in regard to its internalization, sensitivity to a nonhydrolyzable GTP analog, and anisotropy of a bound fluorescent CCK analog. Chimeric CCK1R/CCK2R constructs that systematically changed the residues in the allosteric ligand-binding pocket were studied in the presence of Y140A. This established increased importance of unique residues within TM3 and reduced the importance of TM2 for binding in the presence of this mutation, with the agonist trigger likely pulled away from its Leu(356) target on TM7. The distinct conformation of this intramembranous pocket within Y140A CCK1R provides an opportunity to normalize this by using a small molecule allosteric ligand, thereby providing safe and effective correction of the coupling defect in metabolic syndrome.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Direct demonstration of unique mode of natural peptide binding to the type 2 cholecystokinin receptor using photoaffinity labeling

Direct analysis of mode of peptide docking using intrinsic photoaffinity labeling has provided detailed insights for the molecular basis of cholecystokinin (CCK) interaction with the type 1 CCK receptor. In the current work, this technique has been applied to the closely related type 2 CCK receptor that also binds the natural full agonist peptide, CCK, with high affinity. A series of photolabile CCK analog probes with sites of covalent attachment extending from position 26 through 32 were characterized, with the highest affinity analogs that possessed full biological activity utilized in photoaffinity labeling. The position 29 probe, incorporating a photolabile benzoyl-phenylalanine in that position, was shown to bind with high affinity and to be a full agonist, with potency not different from that of natural CCK, and to covalently label the type 2 CCK receptor in a saturable, specific and efficient manner. Using proteolytic peptide mapping, mutagenesis, and radiochemical Edman degradation sequencing, this probe was shown to establish a covalent bond with type 2 CCK receptor residue Phe¹²⁰ in the first extracellular loop. This was in contrast to its covalent attachment to Glu³⁴⁵ in the third extracellular loop of the type 1 CCK receptor, directly documenting differences in mode of docking this peptide to these receptors.

Insights into the impact of phenolic residue incorporation at each position along secretin for receptor binding and biological activity

Understanding of the structural importance of each position along a peptide ligand can provide important insights into the molecular basis for its receptor binding and biological activity. This has typically been evaluated using serial replacement of each natural residue with an alanine. In the current report, we have further complemented alanine scanning data with the serial replacement of each residue within secretin-27, the natural ligand for the prototypic class B G protein-coupled secretin receptor, using a photolabile phenolic residue. This not only provided the opportunity to probe spatial approximations between positions along a docked ligand with its receptor, but also provided structure-activity insights when compared with tolerance for alanine replacement of the same residues. The pattern of sensitivity to phenolic residue replacement was periodic within the carboxyl-terminal region of this peptide ligand, corresponding with alanine replacements in that region. This was supportive of the alpha-helical conformation of the peptide in that region and its docking within a groove in the receptor amino-terminal domain. In contrast, the pattern of sensitivity to phenolic residue replacement was almost continuous in the amino-terminal region of this peptide ligand, again similar to alanine replacements, however, there were key positions in which either the phenolic residue or alanine was differentially preferred. This provided insights into the receptor environment of the portion of this ligand most critical for its biological activity. As the structure of the intact receptor is elucidated, these data will provide a guide for ligand docking to the core domain and to help clarify the molecular basis of receptor activation.

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.

Molecular basis for binding and subtype selectivity of 1,4-benzodiazepine antagonist ligands of the cholecystokinin receptor

Allosteric binding pockets in peptide-binding G protein-coupled receptors create opportunities for the development of small molecule drugs with substantial benefits over orthosteric ligands. To gain insights into molecular determinants for this pocket within type 1 and 2 cholecystokinin receptors (CCK1R and CCK2R), we prepared a series of receptor constructs in which six distinct residues in TM2, -3, -6, and -7 were reversed. Two novel iodinated CCK1R- and CCK2R-selective 1,4-benzodiazepine antagonists, differing only in stereochemistry at C3, were used. When all six residues within CCK1R were mutated to corresponding CCK2R residues, benzodiazepine selectivity was reversed, yet peptide binding selectivity was unaffected. Detailed analysis, including observations of gain of function, demonstrated that residues 6.51, 6.52, and 7.39 were most important for binding the CCK1R-selective ligand, whereas residues 2.61 and 7.39 were most important for binding CCK2R-selective ligand, although the effect of substitution of residue 2.61 was likely indirect. Ligand-guided homology modeling was applied to wild type receptors and those reversing benzodiazepine binding selectivity. The models had high predictive power in enriching known receptor-selective ligands from related decoys, indicating a high degree of precision in pocket definition. The benzodiazepines docked in similar poses in both receptors, with C3 urea substituents pointing upward, whereas different stereochemistry at C3 directed the C5 phenyl rings and N1 methyl groups into opposite orientations. The geometry of the binding pockets and specific interactions predicted for ligand docking in these models provide a molecular framework for understanding ligand selectivity at these receptor subtypes. Furthermore, the strong predictive power of these models suggests their usefulness in the discovery of lead compounds and in drug development programs.

Differential determinants for coupling of distinct G proteins with the class B secretin receptor

The secretin receptor is a prototypic class B G protein-coupled receptor that is activated by binding of its natural peptide ligand. The signaling effects of this receptor are mediated by coupling with Gs, which activates cAMP production, and Gq, which activates intracellular calcium mobilization. We have explored the molecular basis for the coupling of each of these G proteins to this receptor using systematic site-directed mutagenesis of key residues within each of the intracellular loop regions, and studying ligand binding and secretin-stimulated cAMP and calcium responses. Mutation of a conserved histidine in the first intracellular loop (H157A and H157R) markedly reduced cell surface expression, resulting in marked reduction in cAMP and elimination of measurable calcium responses. Mutation of an arginine (R153A) in the first intracellular loop reduced calcium, but not cAMP responses. Mutation of a dibasic motif in the second intracellular loop (R231A/K232A) had no significant effects on any measured responses. Mutations in the third intracellular loop involving adjacent lysine and leucine residues (K302A/L303A) or two arginine residues separated by a leucine and an alanine (R318A/R321A) significantly reduced cAMP responses, while the latter also reduced calcium responses. Additive effects were elicited by combining the effective mutations, while combining all the effective mutations resulted in a construct that continued to bind secretin normally, but that elicited no significant cAMP or calcium responses. These data suggest that, while some receptor determinants are clearly shared, there are also distinct determinants for coupling with each of these G proteins.

Ligand-induced internalization of the type 1 cholecystokinin receptor independent of recognized signaling activity

Receptor ligands, identified as antagonists, based on the absence of stimulation of signaling, can rarely stimulate receptor internalization. d-Tyr-Gly-[(Nle(28,31),d-Trp(30))CCK-26-32]-2-phenylethyl ester (d-Trp-OPE) is such a ligand that binds to the cholecystokinin (CCK) receptor and stimulates internalization. Here, the molecular basis of this trafficking event is explored, with the assumption that ligand binding initiates conformational change, exposing an epitope to direct endocytosis. Ligand-stimulated internalization was studied morphologically using fluorescent CCK and d-Trp-OPE. d-Trp-OPE occupation of Chinese hamster ovary cell receptors stimulated internalization into the same region as CCK. Arrestin-biased action was ruled out using morphological translocation of fluorescent arrestin 2 and arrestin 3, moving to the membrane in response to CCK, but not d-Trp-OPE. Possible roles of the carboxyl terminus were studied using truncated receptor constructs, eliminating the proline-rich distal tail, the serine/threonine-rich midregion, and the remainder to the vicinal cysteines. None of these constructs disrupted d-Trp-OPE-stimulated internalization. Possible contributions of transmembrane segments were studied using competitive inhibition with peptides that also had no effect. Intracellular regions were studied with a similar strategy using coexpressing cell lines. Peptides corresponding to ends of each loop region were studied, with only the peptide at the carboxyl end of the third loop inhibiting d-Trp-OPE-stimulated internalization but having no effect on CCK-stimulated internalization. The region contributing to this effect was refined to peptide 309-323, located below the recognized G protein-association motif. While a receptor in which this segment was deleted did internalize in response to d-Trp-OPE, it exhibited abnormal ligand binding and did not signal in response to CCK, suggesting an abnormal conformation and possible mechanism of internalization distinct from that being studied. This interpretation was further supported by the inability of peptide 309-323 to inhibit its d-Trp-OPE-stimulated internalization. Thus the 309-323 region of the type 1 CCK receptor affects antagonist-stimulated internalization of this receptor, although its mechanism and interacting partner are not yet clear.

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.

Fluorescence polarization screening for allosteric small molecule ligands of the cholecystokinin receptor

The success in screening for drug candidates is highly dependent on the power of the strategy implemented. In this work, we report and characterize a novel fluorescent benzodiazepine antagonist of the type 1 cholecystokinin receptor (3-(3-(7-fluoro-1-(2-isopropyl(4-methoxyphenyl)amino)-2-oxoethyl)-2,4-dioxo-5-phenyl-2,3,4,5-tetrahydro-1H-benzo[b][1,4]-diazepin-3-yl)ureido)benzoic acid) that can be used as a receptor ligand in a fluorescence polarization assay, which is ideally suited for the identification of small molecule allosteric modulators of this physiologically important receptor. By binding directly to the small molecule-docking region within the helical bundle of this receptor, this indicator can be displaced by many small molecule candidate drugs, even those that might not affect the binding of an orthosteric cholecystokinin-like peptide ligand. The biological, pharmacological, and fluorescence properties of this reagent are described, and proof-of-concept is provided in a fluorescence polarization assay utilizing this fluorescent benzodiazepine ligand.

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.

Refinement of the pharmacophore of an agonist ligand of the secretin receptor using conformationally constrained cyclic hexapeptides

There is a compelling need for the development of small molecule agonists acting at family B G protein-coupled receptors. A possible lead for the development of such drugs was reported when it was recognized that sequences endogenous to the amino terminus of the secretin receptor and certain other receptors in this family possess weak full agonist activity (Dong et al. Mol Pharmacol 2006;70:206-213). In the current report, we extended those observations by building the active dipeptide motif found in the secretin receptor (WD) into each position around a conformationally constrained d-amino acid-containing cyclic hexapeptide, and determining the biological activity of each peptide at the secretin receptor. Indeed, only two positions for WD around this constrained ring resulted in biological activity at the receptor, providing further insights into the structural specificity of this phenomenon. Molecular modeling supported the presence of a unique WD backbone conformation shared only by these active peptides, and provided a more constrained template for future receptor-active agonist drug development.

Modulation of cell surface expression of nonactivated cholecystokinin receptors using bivalent ligand-induced internalization

CCK(2) receptor antagonists potentiate pain relief by MOP receptor agonists. In an attempt to enhance this effect, we prepared bivalent ligands incorporating CCK(2) receptor antagonist and MOP receptor agonist pharmacophores. (9) Ligands with 16- to 22-atom spacers could simultaneously bind both receptors but provided no advantage in activity over individual ligands. We now examine the effect of these ligands on receptor internalization as a mechanism of receptor regulation. We prepared CHO cell lines expressing nonfluorescent halves (YN and YC) of yellow fluorescent protein attached to each receptor. Spatial approximation of constructs was needed to yield fluorescence. Monovalent MOP agonist 1 signaled normally and internalized the MOP receptor. Monovalent CCK(2) antagonist 2 did not stimulate receptor internalization. In the dual receptor-bearing cells, bivalent ligands 3a-c capable of simultaneously binding both receptors resulted in cell surface fluorescence and internalization of the fluorescent complex in a time- and temperature-dependent manner. Bivalent ligand 4 with spacer too short to occupy both receptors simultaneously yielded no signal. Receptor tethering with appropriate bivalent ligands can down-regulate signaling by moving a nonactivated receptor into the endocytic pathway.

Secretin occupies a single protomer of the homodimeric secretin receptor complex: insights from photoaffinity labeling studies using dual sites of covalent attachment

The secretin receptor, a prototypic family B G protein-coupled receptor, forms a constitutive homodimeric complex that is stable even in the presence of hormone. Recently, a model of this agonist-bound receptor was built based on high resolution structures reported for amino-terminal domains of other family members. Although this model provided the best solution for all extant data, including 10 photoaffinity labeling constraints, a new such constraint now obtained with a position 16 photolabile probe was inconsistent with this model. As the secretin receptor forms constitutive homodimers, we explored whether secretin might dock across both protomers of the complex, an observation that could also contribute to the negative cooperativity observed. To directly explore this, we prepared six secretin analogue probes that simultaneously incorporated two photolabile benzoylphenylalanines as sites of covalent attachment, in positions known to label distinct receptor subdomains. Each bifunctional probe was a full agonist that labeled the receptor specifically and saturably, with electrophoretic migration consistent with labeling a single protomer of the homodimeric secretin receptor. No band representing radiolabeled receptor dimer was observed with any bifunctional probe. The labeled monomeric receptor bands were cleaved with cyanogen bromide to demonstrate that both of the photolabile benzoylphenylalanines within a single probe had established covalent adducts with a single receptor in the complex. These data are consistent with a model of secretin occupying a single secretin receptor protomer within the homodimeric receptor complex. A new molecular model accommodating all constraints is now proposed.

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.

Elucidation of the molecular basis of cholecystokinin Peptide docking to its receptor using site-specific intrinsic photoaffinity labeling and molecular modeling

G protein-coupled receptors represent the largest family of receptors and the major target of current drug development efforts. Understanding of the mechanisms of ligand binding and activation of these receptors remains limited, despite recent advances in structural determination of family members. This work focuses on the use of photoaffinity labeling and molecular modeling to elucidate the structural basis of binding a natural peptide ligand to a family A G protein-coupled receptor, the type 1 cholecystokinin receptor. Two photolabile cholecystokinin analogues were developed and characterized as representing high-affinity, fully biologically active probes with sites of covalent attachment at positions 28 and 31. The sites of receptor labeling were identified by purification, proteolytic peptide mapping, and radiochemical sequencing of labeled wild-type and mutant cholecystokinin receptors. The position 28 probe labeled second extracellular loop residue Leu(199), while the position 31 probe labeled first extracellular loop residue Phe(107). Along with five additional spatial approximation constraints coming from previous photoaffinity labeling studies and 12 distance restraints from fluorescence resonance energy transfer studies, these were built into two homology models of the cholecystokinin receptor, based on the recent crystal structures of the beta2-adrenergic receptor and A2a-adenosine receptor. The resultant agonist ligand-occupied receptor models fully accommodate all existing experimental data and represent the best refined models of a peptide hormone receptor in this important family.

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.

Juxtamembranous region of the amino terminus of the family B G protein-coupled calcitonin receptor plays a critical role in small-molecule agonist action

The Family B G protein-coupled calcitonin receptor is an important drug target. The aim of this work was to elucidate the molecular mechanism of action of small-molecule agonist ligands acting at this receptor, comparing it with the action mechanism of the receptor's natural peptide ligand. cAMP responses to four non-peptidyl ligands and calcitonin were studied in COS-1 cells expressing wild-type and chimeric calcitonin-secretin receptors. All compounds were full agonists at the calcitonin receptor with no activity at the secretin receptor. Only chimeric constructs including the calcitonin receptor amino terminus exhibited responses to any of these ligands. We progressively truncated this domain and tested constructs for cAMP responses. Although calcitonin was able to activate the calcitonin receptor fully with the first 58 residues absent, its potency was 3 orders of magnitude lower than that at the wild-type receptor. After truncation of 114 residues, there was no response to calcitonin. In contrast, small-molecule ligands were fully active at receptors having up to 149 amino-terminal residues absent. Those compounds finally became inactive after truncation of 153 residues. Deletion and/or alanine replacement of the region of the calcitonin receptor between residues 150 and 153 resulted in marked reduction in cAMP responses to these compounds, with some compound-specific differences observed, supporting a critical role for this region. Binding studies further supported distinct sites of action of small molecules relative to that of calcitonin. These findings focus attention on the potential importance of the juxtamembranous region of the amino terminus of the Family B calcitonin receptor for agonist drug action.

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.

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.

Mapping the architecture of secretin receptors with intramolecular fluorescence resonance energy transfer using acousto-optic tunable filter-based spectral imaging

The molecular structure and agonist-induced conformational changes of class II G protein-coupled receptors are poorly understood. In this work, we developed and characterized a series of dual cyan fluorescent protein (CFP)-tagged and yellow fluorescent protein (YFP)-tagged secretin receptor constructs for use in various functional and fluorescence analyses of receptor structural variants. CFP insertions within the first or second intracellular loop domains of this receptor were tolerated poorly or partially, respectively, in receptors tagged with a carboxyl-terminal yellow fluorescent protein that itself had no effect on secretin binding or cAMP production. A similar CFP insertion into the third intracellular loop resulted in a plasma membrane-localized receptor that bound secretin and signaled normally. This fully active third-loop variant exhibited a significant decrease in fluorescence resonance energy transfer signals that were recorded with an acousto-optic tunable filter microscope after exposure to secretin agonist but not to a receptor antagonist. These data demonstrate changes in the relative positions of intracellular structures that support a model for secretin receptor activation.

Role of lysine187 within the second extracellular loop of the type A cholecystokinin receptor in agonist-induced activation. Use of complementary charge-reversal mutagenesis to define a functionally important interdomain interaction

Activation of guanine nucleotide-binding protein (G protein)-coupled receptors is believed to involve conformational change that exposes a domain for G protein coupling at the cytosolic surface of the helical confluence, although the mechanisms for achieving this are not well understood. This conformational change can be achieved by docking a diverse variety of agonist ligands, known to occur by interacting with different regions of these receptors. In this study, we focus on the importance of a specific basic residue (Lys187) within the second extracellular loop of the receptor for the peptide hormone, cholecystokinin. Alanine-replacement and charge-reversal mutagenesis of this residue showed that it had no effect on the binding of natural peptide and nonpeptidyl ligands of this receptor but markedly interfered with agonist-stimulated signaling. It was demonstrated that this negative effect on biological activity could be eliminated with the truncation of the first 30 residues of the amino-terminal tail of this receptor. Complementary charge-reversal mutagenesis of each of the five conserved acidic residues within this region of the receptor in the presence of the charge-reversed Lys187 revealed that only the Asp5 mutant fully reversed the negative functional impact of the Lys187 charge reversal. Thus, we have demonstrated that a basic residue within the second extracellular loop of the cholecystokinin receptor interacts with a specific acidic residue within the amino terminus of this receptor. This residue-residue interaction is nicely accommodated within a new molecular model of the agonist-occupied cholecystokinin receptor.

Transmembrane segment peptides can disrupt cholecystokinin receptor oligomerization without affecting receptor function

Oligomerization of the G protein-coupled cholecystokinin (CCK) receptor has been demonstrated, but its molecular basis and functional importance are not clear. We now examine contributions of transmembrane (TM) segments to oligomerization of this receptor using a peptide competitive inhibition strategy. Oligomerization of CCK receptors tagged at the carboxyl terminus with Renilla luciferase or yellow fluorescent protein was quantified using bioluminescence resonance energy transfer (BRET). Synthetic peptides representing TM I, II, V, VI, and VII of the CCK receptor were utilized as competitors. Of these, only TM VI and VII peptides disrupted receptor BRET. Control studies established that the beta2-adrenergic receptor TM VI peptide that disrupts oligomerization of that receptor had no effect on CCK receptor BRET. Notably, disruption of CCK receptor oligomerization had no effect on agonist binding, biological activity, or receptor internalization. To gain insight into the face of TM VI contributing to oligomerization, we utilized analogous peptides with alanines in positions 315, 319, and 323 (interhelical face) or 317, 321, and 325 (external lipid-exposed face). The Ala317,321,325 peptide eliminated the disruptive effect on CCK receptor BRET, whereas the other mutant peptide behaved like wild-type TM VI. This suggests that the lipid-exposed face of the CCK receptor TM VI most contributes to oligomerization and supports external contact dimerization of helical bundles, rather than domain-swapped dimerization. Fluorescent CCK receptor mutants with residues 317, 321, and 325 replaced with alanines were also prepared and failed to yield significant resonance transfer signals using either BRET or a morphological FRET assay, further supporting this interpretation.

Fluorescent Indicators Distributed throughout the Pharmacophore of Cholecystokinin Provide Insights into Distinct Modes of Binding and Activation of Type A and B Cholecystokinin Receptors

Ligand probes with fluorescent indicators positioned throughout the pharmacophoric domain can provide important insights into the molecular basis of receptor binding and activation as reflected in the microenvironment of each indicator while docked at a receptor. We developed three cholecystokinin-like probes with Aladan situated at the N terminus, in the mid-region, and at the C terminus (positions 24, 29, and 33, respectively). These were studied in solution and docked at type A and B cholecystokinin receptors. This study demonstrated clear differences in mechanisms of cholecystokinin binding and activation of these structurally related receptors with distinct agonist structure-activity relationships. The fluorescence characteristics of Aladan are highly sensitive to the polarity of its microenvironment. The mid-region probe was least accessible to the aqueous milieu as determined by fluorescence emission spectra and iodide quenching, which was not altered by changes in conformation from active to inactive. Accessibility of the N- and C-terminal probes was affected by receptor conformation. The position 24 probe was more easily quenched in the active than in the G protein-uncoupled conformation for both receptors. However, the position 33 probe docked at the type A cholecystokinin receptor was more easily quenched in the active conformation, whereas the same probe docked at the type B cholecystokinin receptor was more easily quenched in the inactive conformation. Fluorescence anisotropy and red edge excitation shift determinations confirmed these observations and supported the proposed movements. Although both type A and B cholecystokinin receptors bind cholecystokinin with high affinity, resulting in fully efficacious biological responses, these receptors utilize distinct molecular modes of binding.

Secretin receptor oligomers form intracellularly during maturation through receptor core domains

Oligomerization of numerous G protein-coupled receptors has been documented, including the prototypic family B secretin receptor. The clinical significance of oligomerization of this receptor became clear with the recent observation that a misspliced form present in pancreatic cancer could associate with the wild-type receptor and act as a dominant negative inhibitor of its normal growth inhibitory function. Our goal was to explore the molecular mechanism of this interaction using bioluminescence (BRET) and fluorescence (FRET) resonance energy transfer and fluorescence microscopy with a variety of receptor constructs tagged with luciferase or cyan or yellow fluorescent proteins. BRET signals comparable to those obtained from cells coexpressing differentially tagged wild-type receptors were observed for similarly tagged secretin receptors in which all or part of the amino-terminal domain was deleted. As expected, neither of these constructs bound secretin, and only the partially truncated construct sorted to the plasma membrane. Receptors lacking the majority of the carboxyl-terminal domain, including that important for phosphorylation-mediated desensitization, also produced BRET signals above background. These findings suggested that the receptor's membrane-spanning core is responsible for secretin receptor oligomerization. Interestingly, alanine substitutions for a -GxxxG- helix interaction motif in transmembrane segment 7 created nonfunctional receptors that were capable of forming oligomers. Furthermore, treatment of receptor-expressing cells with brefeldin A did not eliminate the BRET signals, and morphologic FRET experiments confirmed the expected subcellular localizations of receptor oligomers. We conclude that secretin receptor oligomerization occurs through -GxxxG- motif-independent interactions of transmembrane segments during the maturation of nascent molecules.

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.

Novel benzodiazepine photoaffinity probe stereoselectively labels a site deep within the membrane-spanning domain of the cholecystokinin receptor

An understanding of the molecular basis of drug action provides opportunities for refinement of drug properties and for development of more potent and selective molecules that act at the same biological target. In this work, we have identified the active enantiomers in racemic mixtures of structurally related benzophenone derivatives of 1,5-benzodiazepines, representing both antagonist and agonist ligands of the type A cholecystokinin receptor. The parent compounds of the 1,5-benzodiazepine CCK receptor photoaffinity ligands were originally prepared in an effort to develop orally active drugs. The enantiomeric compounds reported in this study selectively photoaffinity-labeled the CCK receptor, resulting in the identification of a site of attachment for the photolabile moiety of the antagonist probe deep within the receptor's membrane-spanning region at Leu(88), a residue within transmembrane segment two. In contrast, the agonist probe labeled a region including extracellular loop one and a portion of transmembrane segment three. The antagonist covalent attachment site to the receptor served as a guide in the construction of theoretical three-dimensional molecular models for the antagonist-receptor complex. These models provided a means for visualization of physically plausible ligand-receptor interactions in the context of all currently available biological data that address small molecule interactions with the CCK receptor. Our approach, featuring the use of novel photolabile compounds targeting the membrane-spanning receptor domain to probe the binding site region, introduces powerful tools and a strategy for direct and selective investigation of nonpeptidyl ligand binding to peptide receptors.

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.