His381 of the rat CCKB receptor is essential for CCKB versus CCKA receptor antagonist selectivity

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.

Identification of novel amino acid derived CCK-2R antagonists as potential antiulcer agent: homology modeling, design, synthesis, and pharmacology

The present study revisited the three-dimensional (3D) homology model of CCK-2R using human A(2a) adenosine receptor and the resolved NMR based structure of the third extracellular loop of the CCK-2R as templates. Further in order to identify novel antiulcer agents, rational designing have been performed utilizing the substructure of a well-known CCK-2R antagonist benzotript as a lead molecule and submitted to the combined docking and simulation studies. This led to the understanding of the essential structure requirement as well as variation of binding mode among conformational isomers of small molecule CCK-2R antagonists. In the next step, preparation of each configurational isomer of these molecules was carried out and submitted for their in vitro activity followed by in vivo screening into antiulcer rat model. The biological screening of these compounds has not only validated the developed homology model of CCK-2R but also led to the identification of highly potent CCK-2R antagonist 6a as an orally active and safe candidate molecule having better antiulcer properties than the well-known drug benzotript.

Toward the identification of a reliable 3D QSAR pharmacophore model for the CCK2 receptor antagonism

The present study describes application of computational approaches to identify a validated and reliable 3D QSAR pharmacophore model for the CCK-2R antagonism through integrated ligand and structure based studies using anthranilic sulfonamide and 1,3,4-benzotriazepine based CCK-2R antagonists. The best hypothesis consisted five features viz. two aliphatic hydrophobic, one aromatic hydrophobic, one H-bond acceptor, and one ring aromatic feature with an excellent correlation for 34 training set (r²(training) = 0.83) and 58 test set compounds (r²(test) = 0.74). This model was validated through F-test and docking studies at the active site of the plausible CCK-2R where the 99% significance and well corroboration with the pharmacophore model respectively describes the model's reliability. The model also predicts well to other known clinically effective CCK-2R antagonists. Therefore, the developed model may useful in finding new scaffolds that may aid in design and develop new chemical entities (NCEs) as potent CCK-2R antagonists before their synthesis.

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.

Structural basis of cholecystokinin receptor binding and regulation

Two structurally-related guanine nucleotide-binding protein-coupled receptors for two related peptides, cholecystokinin (CCK) and gastrin, have evolved to exhibit substantial diversity in specificity of ligand recognition, in their molecular basis of binding these ligands, and in their mechanisms of biochemical and cellular regulation. Consistent with this, the CCK1 and CCK2 receptors also play unique and distinct roles in physiology and pathophysiology. The paradigms for ligand recognition and receptor regulation and function are reviewed in this article, and should be broadly applicable to many members of this remarkable receptor superfamily. This degree of specialization is instructive and provides an encouraging basis for the diversity of potential drugs targeting these receptors and their actions that can be developed.

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.

Evidence That Interspecies Polymorphism in the Human and Rat Cholecystokinin Receptor-2 Affects Structure of the Binding Site for the Endogenous Agonist Cholecystokinin

The cholecystokinin (CCK) receptor-2 exerts very important central and peripheral functions by binding the neuropeptides cholecystokinin or gastrin. Because this receptor is a potential therapeutic target, great interest has been devoted to the identification of efficient antagonists. However, interspecies genetic polymorphism that does not alter cholecystokinin-induced signaling was shown to markedly affect activity of synthetic ligands. In this context, precise structural study of the agonist binding site on the human cholecystokinin receptor-2 is a prerequisite to elucidating the molecular basis for its activation and to optimizing properties of synthetic ligands. In this study, using site-directed mutagenesis and molecular modeling, we delineated the binding site for CCK on the human cholecystokinin receptor-2 by mutating amino acids corresponding to that of the rat homolog. By doing so, we demonstrated that, although resembling that of rat homolog, the human cholecystokinin receptor-2 binding site also displays important distinct structural features that were demonstrated by susceptibility to several point mutations (F120A, Y189A, H207A). Furthermore, docking of CCK in the human and rat cholecystokinin receptor-2, followed by dynamic simulations, allowed us to propose a plausible structural explanation of the experimentally observed difference between rat and human cholecystokinin-2 receptors.

Modeled structure of a G-protein-coupled receptor: the cholecystokinin-1 receptor

The Cholecystokinin-1 receptor (CCK1R) mediates actions of CCK in areas of the central nervous system and of the gut. It is a potential target to treat a number of diseases. As for all G-protein-coupled receptors, docking of ligands into modeled CCK1R binding site should greatly help to understand intrinsic mechanisms of activation. Here, we describe the procedure we used to progressively build a structural model for the CCK1R, to integrated, and on the basis of site-directed mutagenesis data on its binding site. Reliability of the CCK1R model was confirmed by interaction networks that involved conserved and functionally crucial motifs in G-protein-coupled receptors, such as Glu/Asp-Arg-Tyr and Asn-Pro-Xaa-Xaa-Tyr motifs. In addition, the 3-D structure of CCK1R-bound CCK resembled that determined by NMR in a lipid environment. The derived computational model was also used for revealing binding modes of several nonpeptide ligands and for rationalizing ligand structure-activity relationships known from experiments. Our findings indeed support that our "validated CCK1R model" could be used to study the intrinsic mechanism of CCK1R activation and design new ligands.

Structure-activity function for binding and signaling in CHO-K1 and COS-7 cells expressing the cholecystokinin A receptor

Key amino acids of the cholecystokinin (CCK) peptide for receptor binding are sulfated Y27, W30, D32, and F33-NH(2). Three-dimensional modeling showed that the CCK-A receptor (CCK-AR) antagonist devazepide penetrated into the transmembrane (TM) domains, whereas CCK was placed on the surface of the CCK-AR. Four types of rat CCK-AR cDNAs were transfected into CHO-K1 and COS-7 cells: normal CCK-AR cDNA transfected cells (wild type, WT); K120 substituted with V; K130V; and R352V. Binding of [3H]CCK-8 was observed in WT and K130V, but not in K120V and R352V. CCK caused Ca(2+) spiking in WT and K130V, whereas K120V and R352V had no effect. Three chimeras including the CCK-AR/3ibeta2 adrenergic receptor (beta2AR), 3Nibeta2AR, and 3Cibeta2AR were constructed. Two groups of point mutations in the CCK-AR3i were also made: Y252V, S274V, S281V, and S289V (non-phospho-acceptor Y or S); S260V, S264V, S271V, and S275V (phospho-acceptor S). WT and CCK-AR/3Cibeta2AR increased [Ca(2+)](i) in response to CCK; 3Nibeta2AR was vice versa. CCK failed to increase [IP(3)] in phospho-acceptor S to V without affecting binding. Non-phospho-acceptor S or Y to V showed normal response. Thus, Lys120 outside the TM2 and Arg352 outside the TM6 of the CCK-AR are amino acids interacting with Tyr[SO(3)H]27 and Asp32 of the CCK peptide for binding. Phospho-acceptor Ser groups in the CCK-AR 3Ni are amino acids for initiating cell signaling.

Intermolecular interactions between peptidic and nonpeptidic agonists and the third extracellular loop of the cholecystokinin 1 receptor

Intermolecular interactions were determined between a synthetic peptide corresponding to the third extracellular loop and several residues from the adjoining sixth and seventh transmembrane domains of the human cholecystokinin-1 receptor, CCK(1)-R(329-357), and the synthetic agonists Ace-Trp-Lys[NH(epsilon)CONH-o-(MePh)]-Asp-MePhe-NH(2) (GI5269) and the C1 N-isopropyl-N-(4-methoxyphenyl)acetamide derivative of 3-(1H-Indazol-3ylmethyl)-3-methyl-5-pyridin-3-yl-1,5-benzodiazepine (GI0122), using high-resolution nuclear magnetic resonance spectroscopy and computer simulations. Addition of the ligands to CCK(1)-R(329-357) in an aqueous solution of DPC micelles produced a number of intermolecular nuclear Overhauser enhancements (NOEs) to residues in TMs 6 and 7 of the receptor fragment. NOE-restrained molecular models of the GI5269 and GI0122/CCK(1)-R complexes provide evidence for overlapping ligand-binding sites for peptidic and nonpeptidic agonists. The proposed binding modes of GI5269 and GI0122 are supported by the structure-activity relationship of analogues and mutagenesis data for the CCK(1)-R selective antagonist L-364,718.

Identification of tyrosine 189 and asparagine 358 of the cholecystokinin 2 receptor in direct interaction with the crucial C-terminal amide of cholecystokinin by molecular modeling, site-directed mutagenesis, and structure/affinity studies

The cholecystokinin (CCK) receptors CCK1R and CCK2R exert important central and peripheral functions by binding the neuropeptide cholecystokinin. Because these receptors are potential therapeutic targets, great interest has been devoted to the identification of efficient ligands that selectively activate or inhibit these receptors. A complete mapping of the CCK binding site in these receptors would help to design new CCK ligands and to optimize their properties. In this view, a molecular model of the CCK2R occupied by CCK was built to identify CCK2R residues that interact with CCK functional groups. No such study has yet been reported for the CCK2R. Docking of CCK in the receptor was performed by taking into account our previous mutagenesis data and by using, as constraint, the direct interaction that we demonstrated between His207 in the CCK2R and Asp8 of CCK (Mol Pharmacol 54:364-371, 1998; J Biol Chem 274:23191-23197, 1999). Two residues that had not been revealed in our previous mutagenesis studies, Tyr189 (Y4.60) and Asn358 (N6.55), were identified in interaction via hydrogen bonds with the C-terminal amide of CCK, a crucial functional group of the peptide. Mutagenesis of Tyr189 (Y4.60) and Asn358 (N6.55) as well as structure-affinity studies with modified CCK analogs validated these interactions and the involvement of both residues in the CCK binding site. These results indicate that the present molecular model is an important tool to identify direct contact points between CCK and the CCK2R and to rapidly progress in mapping of the CCK2R binding site. Moreover, comparison of the present CCK2R.CCK molecular model with that of CCK1R.CCK, which we have previously published and validated, clearly argues that the positioning of CCK in these receptors is different.

Conformational and molecular modeling studies of sulfated cholecystokinin-15

Conformational features of the C-terminal carboxyamidated pentadecapeptide of CCK (S(19)HRISDRD[SO(4)]-YMGWMDF(33)-NH(2)) were determined by NMR spectroscopy in a zwitterionic membrane-mimetic solvent system, composed of DPC micelles. The C-terminal octapeptide consisted of a well-defined pseudohelix that was nearly identical to the structure previously reported for nonsulfated CCK-8 in the same solvent system. N-terminal amino acids of CCK-15 were highly disordered, with no clear conformational preference. Extensive NOE-restrained molecular dynamics simulations of the CCK-15/CCK(1)-R complex suggested that almost all the experimentally determined intermolecular contact points provided by NMR, site-directed mutagenesis, and photoaffinity labeling could be simultaneously satisfied, when the N-terminus of the ligand is placed in close spatial proximity to the N-terminus of the receptor.

Intermolecular interactions between cholecystokinin-8 and the third extracellular loop of the cholecystokinin-2 receptor

The structure of the third extracellular loop of the human cholecystokinin-2 receptor, CCK2-R(352-379), and its interactions with the C-terminal octapeptide of cholecystokinin (CCK-8) have been determined by high-resolution NMR and computer simulations. In the presence of dodecylphosphocholine micelles, the structure of the receptor fragment consisted of three helices, with the first and third corresponding to residues of the extracellular ends of transmembrane helices (TM) 6 and 7, respectively. The central, extracellular helix, consisting of residues 363-368, was found to be closely associated with the membrane mimetic used during the spectroscopic studies and molecular dynamics (MD) simulations. Upon titration of CCK-8 to the receptor domain, chemical shift perturbation and intermolecular NOEs (Trp30, Met31 of CCK-8 and P371, F374 of CCK2-R) indicated the formation of a stable complex and specific ligand/receptor interactions. Using the NOE-generated intermolecular contact points, extensive MD simulations of CCK-8 bound to the CCK2 receptor were carried out. The results, with CCK-8 in close proximity to TM7, differ from previous structural studies of CCK-8 association with CCK1-R, in which the ligand formed a number of interactions with TM6. These differences may play a role in the ligand specificity displayed by the CCK1 and CCK2 receptor subtypes.

Peptide/benzodiazepine hybrids as ligands of CCK(A) and CCK(B) receptors

The (neuro)hormones gastrin and cholecystokinin (CCK) share a common C-terminal tetrapeptide amide sequence that has been recognized as the message portion while the N-terminal extensions are responsible for the CCK(A) and CCK(B) receptor subtype selectivity and avidity. 1,4-Benzodiazepine derivatives are potent and selective antagonists of these receptors, and according to comparative molecular field analysis, the structures of these nonpeptidic compounds could well mimic the message sequence of the peptide agonists at least in terms of spatial array of the aromatic residues. Docking of a larger series of low molecular weight nonpeptide antagonists to a homology modeling derived CCK(B) receptor structure revealed a consensus binding mode that is further validated by data from site-directed mutagenesis studies of the receptors. Whether this putative binding pocket of the nonpeptide antagonists is identical to that of the message portion of the peptide agonists, or whether it is distinct and spatially separated, or overlapping, but with distinct interaction sites, is still object of debate. Using a 1,4-benzodiazepine core amino-functionalized at the C3 position, related tryptophanyl derivatives were synthesized as mimics of the tetrapeptide and subsequently extended N-terminally with gastrin and CCK address sequences. All hybrid constructs were recognized as antagonists by the CCK(A) and CCK(B) receptors, but their address portions were incapable of enhancing in significant manner selectivity and avidity. Consequently, the binding of the peptide/benzodiazepine hybrids has to be dictated mainly by the benzodiazepine moiety, which apparently prevents optimal interactions of the address peptides with extracellular receptor subdomains. These findings would strongly support the view of distinct binding sites for the message portion of the peptide agonists and the benzodiazepine-based nonpeptide antagonists.

Activation of phospholipase C by cholecystokinin receptor subtypes with different G-protein-coupling specificities in hormone-secreting pancreatic cell lines

Phospholipase C (PLC) activity was investigated by stimulation of membrane preparations obtained from insulin (beta-TC3)-, somatostatin (Rin 1027-B2)-, and glucagon (INR1-G9)-producing pancreatic cell lines using the non-hydrolyzable GTP analogue GTPgammaS alone, the C-terminal octapeptide cholecystokinin (CCK-8), or gastrin. All compounds caused a significant 2- to 4.4-fold stimulation of PLC activity in the different cell lines, which was diminished by the non-hydrolyzable GDP analogue GDPbetaS. CCK receptor subtypes were characterized by radioligand binding experiments. High-affinity binding sites for tritiated CCK(A) receptor antagonist L-364,718 (K(d) = 0.24 nM) and tritiated CCK(B) receptor antagonist L-365,260 (K(d) = 0.13 nM) were only present in Rin 1027-B2 cells. High-affinity binding sites for both ligands were not found in beta-TC3 or INR1-G9 cells. Competition binding experiments with non-labeled CCK receptor antagonists CR 1505 (CCK(A) receptor-selective) and CR 2945 (CCK(B) receptor-selective), as well as microphysiometry experiments, resulted in the same receptor distribution. Reverse transcriptase-polymerase chain reaction confirmed the CCK receptor distribution pattern for Rin 1027-B2 cells, but in addition showed the existence of CCK(B) receptors in beta-TC3 cells. Immunoblocking experiments with C-terminal antibodies against different G-protein alpha-subunits demonstrated inhibition of CCK-stimulated PLC activity in beta-TC3 cells by G(q/11)alpha antiserum (70%), in Rin 1027-B2 cells by G(q/11)alpha antiserum (70%) and G(i)-3alpha antiserum (23%), and in INR1-G9 cells by G(q/11)alpha antiserum (60%) and G(o)alpha antiserum (45%). We conclude that CCK receptor subtypes with different G-protein-coupling specificities to PLC are present in the different hormone-secreting cells of the endocrine pancreas.

Contrasting roles of leu(356) in the human CCK(1) receptor for antagonist SR 27897 and agonist SR 146131 binding

A new highly specific, potent non-peptide agonist for the cholecystokinin subtype 1 receptor (CCK(1)), SR 146131 (2-[4-(4-chloro-2, 5-dimethoxyphenyl)-5-(2-cyclohexyl-ethyl)-thiazol-2-ylcarbamoyl ]-5, 7-dimethyl-indol-1-yl-1-acetic acid) was recently described [Bignon, E., Bachy, A., Boigegrain, R., Brodin, R., Cottineau, M., Gully, D., Herbert, J.-M., Keane, P., Labie, C., Molimard, J.-C., Olliero, D., Oury-Donat, F., Petereau, C., Prabonneaud, V., Rockstroh, M.-P., Schaeffer, P., Servant, O.Thurneyssen, O., Soubrié, P., Pascal, M., Maffrand, J.-P., Le Fur, G., 1999. SR 146131: a new, potent, orally active and selective non-peptide cholecystokinin subtype I receptor agonist: I. In vitro studies. J. Pharmacol. Exp. Ther. 289, 742-751]. From binding and activity assays with chimeric constructs of human CCK(1) and the cholecystokinin subtype 2 receptor (CCK(2)) and receptors carrying point mutations, we show that Leu(356), situated in transmembrane domain seven in the CCK(1) receptor, is a putative contact point for SR 146131. In contrast, Leu(356) is probably not in contact with the CCK(1) receptor specific antagonist SR 27897 (1-[2-(4-(2-chlorophenyl)thiazol-2-yl)aminocarbonyl indoyl]acetic acid), a compound structurally related to SR 146131, since its replacement by alanine, histidine or asparagine gave receptors having wild-type CCK(1) receptor SR 27897 binding affinity. Previous mutational analysis of His(381), the cognate position in the rat CCK(2) receptor, had implicated it as being involved in subtype specificity for SR 27897, results which we confirm with corresponding mutations in the human CCK(2) receptor. Moreover, binding and activity assays with the natural CCK receptor agonist, CCK-8S, show that CCK-8S is more susceptible to the mutations in that position in the CCK(1) receptor than in the CCK(2) receptor. The results suggest different binding modes for SR 27897, SR 146131 and CCK-8S in each CCK receptor subtype.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

Direct identification of the agonist binding site in the human brain cholecystokininB receptor

In investigating the agonist binding site of the human brain cholecystokininB receptor (CCKBR), we employed the direct protein chemical approach using a photoreactive tritiated analogue of sulfated cholecystokinin octapeptide, which contains the p-benzoylbenzoyl moiety at the N-terminus, followed by purification of the affinity-labeled receptor to homogeneity. This probe bound specifically, saturably, and with high affinity (KD = 1.2 nM) to the CCKBR and has full agonistic activity. As the starting material for receptor purification, we used stably transfected HEK 293 cells overexpressing functional CCKBR. Covalent labeling of the WGA-lectin-enriched receptor revealed a 70-80 kDa glycoprotein with a protein core of about 50 kDa. Identification of the agonist binding site was achieved by the application of subsequent chemical and enzymatical cleavage to the purified receptor. A radiolabeled peptide was identified by Edman degradation amino acid sequence analysis combined with MALDI-TOF mass spectrometry. The position of the radioactive probe within the identified peptide was determined using combined tandem electrospray mass spectrometry and peptide mapping. The probe was covalently attached within the sequence L52ELAIRITLY61 that represents the transition between the N-terminal domain and predicted transmembrane domain 1. Using this interaction as a constraint to orientate the ligand within the putative receptor binding site, a model of the CCK-8s-occupied CCKBR was constructed. The hormone was found to be placed in a binding pocket built from both extracellular and transmembrane domains of CCKBR with its N-terminus mainly interacting with residues Arg57 and Tyr61.

Role of the extracellular domains of the cholecystokinin receptor in agonist binding

The cholecystokinin (CCK) receptor types A and B (CCKAR and CCKBR) are G protein-coupled receptors with approximately 50% amino acid identity; both have high affinity for the sulfated CCK octapeptide (CCK-8), whereas only the CCKBR has high affinity for gastrin. Previously, we identified five amino acids in the second extracellular loop (ECL) of the CCKBR that were essential for gastrin selectivity. Subsequent mutagenesis of one of these five amino acids (H207F) resulted in the loss of radiolabeled CCK-8 binding. CCK-8 stimulated total inositol phosphate accumulation in COS-1 cells transiently expressing the CCKBR-H207F with full efficacy and a 3044-fold reduced potency, which suggests that the loss of radioligand binding was caused by a loss in affinity. Alanine scanning mutagenesis was performed on the amino terminus near the top of transmembrane domain I (TMI) and on ECL1, two extracellular domains implicated in ligand binding by previous mutagenesis studies. 125I-Bolton-Hunter-CCK-8 binding to mutant receptors transiently expressed in COS-1 identified one nonconserved amino acid, R57A, at the top of TMI that caused a 21-fold reduction in CCK-8 affinity and four conserved amino acids, N115A, L116A, F120A and F122A, in the ECL1 that caused a 15.6-, 6-, 440-, and 8-fold reduction in affinity or efficacy. Alanine substitution of the equivalent amino acids in the CCKAR corresponding to each of the five amino acids in ECL1 and ECL2 affecting CCK-8 affinity for the CCKBR revealed only two mutations, L103A and F107A, that decreased CCK-8 affinity (68- and 2885-fold, respectively). These data suggest that CCK-8 interacts at multiple contact points in the extracellular domains of CCK receptors and that the CCKAR and CCKBR have distinct binding sites despite their shared high affinity for CCK-8.

Cholecystokinin (CCK) regulates somatostatin secretion through both the CCK-A and CCK-B/gastrin receptors in sheep

1. Cholecystokinin (CCK) and gastrin both stimulate gastric somatostatin (SOM) secretion in vitro and thus have the potential to modulate their direct effects on the parietal cell. However, the relative potencies and the mechanisms of action of CCK and gastrin on SOM secretion in vivo have not been determined. 2. The objectives of the present study were to compare the in vivo potencies of the sulphated(s) and non-sulphated (ns) forms of gastrin heptadecapeptide (G-17) and CCK octapeptide (CCK-8) on SOM secretion, and to determine the nature of the receptors involved by repeating the studies in the presence of the CCK-A and CCK-B/gastrin receptor antagonists L-364,718 and L-365,260, respectively. All experiments were performed in the chronically cannulated sheep. 3. Dose-response experiments revealed the following potencies for SOM secretion: G-17s = CCK-8s > G-17 ns >> CCK-8ns. However, based on the plasma levels achieved and a higher metabolic clearance rate (MCR) for CCK, CCK-8s was the most potent. 4. Both the CCK-A and CCK-B/gastrin receptor antagonists suppressed CCK-8s-stimulated SOM output. In contrast, G-17s-stimulated SOM output was inhibited by only the CCK-B/gastrin receptor antagonist. 5. Both receptor antagonists increased basal plasma gastrin and CCK levels. 6. The predominant circulating SOM molecular form after both gastrin and CCK stimulation was SOM-14. 7. In conclusion, the sulphated forms of CCK and gastrin are more potent than the non-sulphated forms. Despite sharing a common biologically active carboxy terminus, CCK stimulates SOM secretion by both the CCK-A and CCK-B/gastrin receptors, while gastrin acts via the CCK-B/gastrin receptor alone. These findings explain in part why CCK is a net inhibitor of gastric acid secretion in vivo.

5-(Piperidin-2-yl)- and 5-(homopiperidin-2-yl)-1,4-benzodiazepines: high-affinity, basic ligands for the cholecystokinin-B receptor

The design, synthesis, and biological activity of a series of high-affinity, basic ligands for the cholecystokinin-B receptor are described. The compounds, which incorporate a piperidin-2-yl or a homopiperidin-2-yl group attached to C5 of a benzodiazepine core structure, are substantially more basic (e.g., 9d, pKa = 9.48) than previously reported antagonists based on 5-amino-1,4-benzodiazepines (e.g., 5, pKa = 7.1) and have improved aqueous solubility. In view of their basicity, it would be tempting to speculate that the present series of compounds might be binding to the CCK-B receptor in their protonated form. Compounds such as 9d, e and 10d showed high affinity for this receptor (IC50 < 2.5 nM) and very good selectivity over CCK-A (CCK-A/CCK-B > 2000), even as the racemates. Additionally, a significantly improved in vivo half-life was observed for a selection of compounds compared to the clinical candidate L-365, -260 (1).

Mapping of ligand binding sites of the cholecystokinin-B/gastrin receptor with lipo-gastrin peptides and molecular modeling

Double-tailed lipo-tetragastrin derivatives of increasing fatty acid chain length were used to identify the minimum size of the fatty acid moieties (> or = C10) that restricts the access to the CCK-B/gastrin (CCK: cholecystokinin) receptor via a membrane-bound pathway. Then dimyristoyl-mercaptoglycerol/maleoyl-gastrin adducts of increasing peptide chain length were synthesized to define the minimal peptide size required for receptor binding affinities comparable, to those of underivatized gastrin peptides despite anchorage of the lipid tails in the membrane bilayer. The experimental results indicated that most of the little-gastrin sequence, i.e., 2-17, is needed for optimal interaction of the molecule with the binding cleft of the receptor. From these data experimentally based restraints could be derived for docking of lipo-gastrin onto a CCK-B/gastrin receptor model applying molecular dynamics simulations and energy minimizations. In the receptor-bound state some of the secondary structure elements of gastrin as determined by nmr analysis of gastrin-peptides in low dielectric constant media are retained. The N-terminal gastrin portion interacts in a more or less extended conformation with the receptor surface, and upon a sharp kink at the Ala-Tyr dipeptide portion the C-terminal pentapeptide amide part inserts deeply into the helix bundle. Besides Arg-57 on top of helix 1 of the receptor, for which no potential interaction with the ligand could be detected, the other amino acid residues identified by mutagenesis studies as involved in gastrin recognition were found to interact with the C-terminal portion of gastrin. Even taking into account the strong limitations of such a model system, it represents an interesting tool for rationalizing the experimental results of the extensive structure-function studies performed previously on gastrin and to delineate more precisely the putative ligand binding site on the extracellular face of the receptor.

Mutational analysis of the putative devazepide binding site of the CCK(A) receptor

Recently a molecular model was proposed for the binding site of the antagonist 3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl) -1H-indole-2-carboxamide (devazepide) on the cholecystokinin-A (CCK(A)) receptor (Van der Bent et al., 1994. Drug Design Discov. 12, 129-148). Fifteen amino acids were identified, including hydrophilic ones such as Ser139, Asn349 and Ser379, that might interact with the carboxamide moiety in devazepide. To provide mutational evidence for this model, wild-type and mutant receptors (S139A, N349A and S379A) were transiently expressed and compared with respect to the ability of devazepide to inhibit binding of radiolabelled cholecystokinin-(26-33)-peptide amide (CCK-8) and CCK-8-evoked Ca2+ mobilization. The data presented suggest the involvement of the three residues in antagonist binding, although to a different extent. However, it does not seem likely that hydrogen bonds are the driving force in view of the relatively minor changes in receptor affinity and activity.

Structure-based design of new constrained cyclic agonists of the cholecystokinin CCK-B receptor

New constrained cyclic pseudopeptide cholecystokinin-B (CCK-B) agonists have been designed on the basis of conformational characteristics of the potent and selective CCK-B agonist Boc-Trp-(NMe)Nle-Asp-Phe-NH2 (Ki = 0.8 nM, selectivity ratio CCK-A/CCK-B > 6000) (Goudreau et al. Biopolymers, 1994, 34, 155-169). These compounds are among the first successful examples of macrocyclic constrained CCK4 analogs endowed with agonist properties and as such may be of value for the development of nonpeptide CCK-B agonists. The affinities and selectivities of these compounds for CCK-B and CCK-A receptors have been determined in vitro by measuring the displacement of [3H]pCCK8 binding to guinea pig cortex and pancreas membranes, respectively. The most potent compound, 8b, N-(cycloamido)-alpha-Me(R)Trp-[(2S)-2-amino-9- ((cycloamido)carbonyl)nonanoyl]-Asp-Phe-NH2, has a Ki value of 15 +/- 1 nM for guinea pig cortex membranes with a good CCK-B selectivity ratio (CCK-A/CCK-B = 147). Furthermore, 8b behaved as a potent and full agonist in a functional assay which measures the stimulation of inositol phosphate accumulation in CHO cells transfected with the rat CCK-B receptor (EC50 = 7 nM). The in vivo affinity of 8b for mouse brain CCK-B receptors was determined following intracerebroventricular injection (ID50 approximately 29 nmol/kg). 8b was also shown to cross the blood-brain barrier (0.16%), after intravenous administration in mice. 8b also increased gastric acid secretion measured in anesthetized rats after intravenous injection. Therefore, 8b appears to be an interesting pharmacological tool and is currently under investigation as a lead for further development of nonpeptide CCK-B agonists.

Identification of two amino acids of the human cholecystokinin-A receptor that interact with the N-terminal moiety of cholecystokinin

A region between residues 38 and 42 of the human cholecystokinin-A (CCK-A) receptor was shown to be involved in the binding of CCK but not in that of JMV 179 and JMV 180, two peptides closely related to CCK (Kennedy, K., Escrieut, C., Dufresne, M., Clerc, P., Vaysse, N., and Fourmy, D. (1995) Biochem. Biophys. Res. Commun. 213, 845-852). In the present study, we have identified the residues of both the receptor and the ligand responsible for this differential binding. Residues Trp-39 and Gln-40 of the receptor were crucial for binding of the C-terminal nonapeptide of CCK as W39F and Q40N mutants demonstrated parallel decreases in both affinity and potency to induce accumulation of inositol phosphates (12.9- and 20.9-fold). The W39F and Q40N mutant receptors bound CCK analogues modified at their C-terminal end, including JMV 179 and JMV 180, as well as the C-terminal amidated heptapeptide of CCK, with identical affinities to the wild-type receptor. In contrast, W39F and Q40N mutants bound CCK octapeptide with the same decreased affinity as the CCK nonapeptide. The modeling of the CCK-A receptor and the docking of the peptide agonists [Thr,Nle]CCK9 and CCK-8 indicated that their N terminus was connected to the receptor through a strong bond network involving Trp-39 and Gln-40 thus confirming experimental data. These first molecular data identifying the agonist binding site of the human CCK-A receptor represent an important step toward the complete delineation of the agonist binding site and the understanding of the molecular mechanisms that govern differential activation of this receptor by CCK-related peptides.

Role of N- and C-terminal substituents on the CCK-B agonist-antagonist pharmacological profile of Boc-Trp-Phg-Asp-Nal-NH2 derivatives

Among the CCK derivatives, the tetrapeptide Boc-Trp-Phg-Asp-Nal-NH2 (1) behaves as a short potent CCK-B agonist which led to the development of an efficient peptidase-resistant CCK-B antagonist by bismethylation of its terminal CONH2 group. Further modifications of the N- and C-terminal moieties of 1 have been performed and are described in this paper, together with the pharmacological profile of the novel synthetized compounds. Introduction of more bulky substituents than NalNH2 on the C-terminal part decreased the CCK-B receptor binding affinity. In the series of N-protected tetrapeptides X30-Phg31-Asp32-Nal33-N(CH3)2, the Boc-substituent was shown to be optimal among the N-protecting groups Boc, 2Adoc, propionyl or acetyl when X = Trp. On the other hand, when X = alpha MeTrp, its optimal N-protecting group was 2Adoc and its configuration was preferentially D. In the newly synthesized compounds, 13: 2Adoc-D-alpha MeTrp-Phg-Asp-NalN(CH3)2 and 16: 2Adoc-D-alpha MeTrp-Phg-Asp-NalNH2 had the best CCK-B receptor affinities (KI = 3.5 and 3.4 nM, respectively) and were selected for further biological evaluation. Interestingly, when tested for their capacity to influence inositol phosphate formation, induced by CCK8 in CHO cells transfected with the rat CCK-B receptor, compound 13 behaved as a full CCK-B antagonist with an IC50 value of 18 +/- 1 nM, being as potent as the antagonist L-365,260 and PD-134,308 (IC50 values respectively, 39 +/- 17 and 30 +/- 2 nM), whereas compound 16 was found to behave as a partial CCK-B agonist. Indeed 16 behaved as an antagonist on the firing rate of rat CA1 hippocampal neurons and acted as an agonist in the pentagastrin stimulated gastric acid secretion (EC50 = 12 nmol/kg) in anesthetized rats. Compound 13 in contrast, was found to inhibit the pentagastrin action at a dose (ID50 = 0.56 mumol/kg) similar to the potent antagonist PD-134,308 (ID50 = 0.4 mumol/kg). The antagonist/agonist properties of compounds 13 and 16 show that both N- and C-terminal substituents modulate the pharmacological properties in the Boc-CCK4 derivatives presented here.

Comparative conformational analysis of CCK-B agonist Boc-Trp-Phg-Asp-(1-Nal)-NH2 and CCK1-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 using 1HNMR spectroscopy and restrained molecular dynamics

The tetrapeptide Boc-Trp-Phg-Asp-(1-Nal)-NH2 is a potent CCK-B agonist. Interestingly, bis-methylation of the C-terminal carboxamide group of this compound leads to Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 which behaves as a CCK-B antagonist in electrophysiological studies on hippocampal neurones (Corringer et al., 1993). In order to ascertain whether bismethylation of the terminal carboxamide group has an influence on the conformational preferences of the peptide, we have undertaken a comparative conformational analysis of the two tetrapeptides by the combined use of 2D NMR spectroscopy and restrained molecular dynamics. The solution conformation of the two peptides were examined by 1H NMR in a d6-DMSO/H2O (80:20) mixture. 1H-1H distance constraints, derived from 2D NOESY and ROESY experiments, were used as inputs for subsequent restrained molecular dynamics simulations. Comparison of the NMR and molecular modeling data indicates different conformational preferences for these two peptides. Interestingly, the aromatic side chains of the CCK-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 in its preferential conformation, overlap their corresponding moieties in the two non peptide CCK-B antagonists L-362,260 and LY-288,513. The differences in conformational behaviour of the studied tetrapeptides could, at least in part, account for their opposite agonist/antagonist profile, a findings which could serve for the design of new conformationally restricted CCK-B analogs.