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

A Biomimetic C-Terminal Extension Strategy for Photocaging Amidated Neuropeptides

Photoactivatable neuropeptides offer a robust stimulus-response relationship that can drive mechanistic studies into the physiological mechanisms of neuropeptidergic transmission. The majority of neuropeptides contain a C-terminal amide, which offers a potentially general site for installation of a C-terminal caging group. Here, we report a biomimetic caging strategy in which the neuropeptide C-terminus is extended via a photocleavable amino acid to mimic the proneuropeptides found in large dense-core vesicles. We explored this approach with four prominent neuropeptides: gastrin-releasing peptide (GRP), oxytocin (OT), substance P (SP), and cholecystokinin (CCK). C-terminus extension greatly reduced the activity of all four peptides at heterologously expressed receptors. In cell type-specific electrophysiological recordings from acute brain slices, subsecond flashes of ultraviolet light produced rapidly activating membrane currents via activation of endogenous G protein-coupled receptors. Subsequent mechanistic studies with caged CCK revealed a role for extracellular proteases in shaping the temporal dynamics of CCK signaling, and a striking switch-like, cell-autonomous anti-opioid effect of transient CCK signaling in hippocampal parvalbumin interneurons. These results suggest that C-terminus extension with a photocleavable linker may be a general strategy for photocaging amidated neuropeptides and demonstrate how photocaged neuropeptides can provide mechanistic insights into neuropeptide signaling that are inaccessible using conventional approaches.

Elucidating the molecular determinants in the process of gastrin C-terminal pentapeptide amide end activating cholecystokinin 2 receptor by Gaussian accelerated molecular dynamics simulations

Gastrin plays important role in stimulating the initiation and development of many gastrointestinal diseases through interacting with the cholecystokinin 2 receptor (CCK2R). The smallest bioactive unit of gastrin activating CCK2R is the C-terminal tetrapeptide capped with an indispensable amide end. Understanding the mechanism of this smallest bioactive unit interacting with CCK2R on a molecular basis could provide significant insights for designing CCK2R antagonists, which can be used to treat gastrin-related diseases. To this end, we performed extensive Gaussian accelerated molecular dynamics simulations to investigate the interaction between gastrin C-terminal pentapeptide capped with/without amide end and CCK2R. The amide cap influences the binding modes of the pentapeptide with CCK2R by weakening the electrostatic attractions between the C-terminus of the pentapeptide and basic residues near the extracellular domain in CCK2R. The C-terminus with the amide cap penetrates into the transmembrane domain of CCK2R while floating at the extracellular domain without the amide cap. Different binding modes induced different conformational dynamics of CCK2R. Residue pairs in CCK2R had stronger correlated motions when binding with the amidated pentapeptide. Key residues and interactions important for CCK2R binding with the amidated pentagastrin were also identified. Our results provide molecular insights into the determinants of the bioactive unit of gastrin activating CCK2R, which would be of great help for the design of CCK2R antagonists.

Understanding Peptide Binding in Class A G Protein-Coupled Receptors

Many physiologic processes are controlled through the activation of G protein-coupled receptors (GPCRs) by regulatory peptides, making peptide GPCRs particularly useful targets for major human diseases such as diabetes and cancer. Peptide GPCRs are also being evaluated as next-generation targets for the development of novel antiparasite agents and insecticides in veterinary medicine and agriculture. Resolution of crystal structures for several peptide GPCRs has advanced our understanding of peptide-receptor interactions and fueled interest in correlating peptide heterogeneity with receptor-binding properties. In this review, the knowledge of recently crystalized peptide-GPCR complexes, previously accumulated peptide structure-activity relationship studies, receptor mutagenesis, and sequence alignment are integrated to better understand peptide binding to the transmembrane cavity of class A GPCRs. Using SAR data, we show that peptide class A GPCRs can be divided into groups with distinct hydrophilic residues. These characteristic residues help explain the preference of a receptor to bind the C-terminal free carboxyl group, the C-terminal amidated group, or the N-terminal ammonium group of peptides.

Cholecystokinin receptors in Atlantic salmon: molecular cloning, gene expression, and structural basis

The peptide hormone cholecystokinin (CCK) exerts a wide range of digestive and CNS-related physiological signaling via CCK receptors in brain and gut. There is very limited information available on these receptors in Atlantic salmon. The aim of this study was to characterize CCK receptors in gut and brain of salmon. We have identified and cloned one CCK-1 receptor and duplicates of CCK-2 receptor in salmon. The phylogenetic analysis indicates the existence of one common ancestor gene for all CCK receptors. CCK-1R mRNA is highly expressed in pancreas followed by midgut, hindgut, gallbladder, and stomach indicating an involvement in pancreatic regulation and gallbladder contractions. CCK-2R1/gastrin mRNA is expressed at high levels in midgut and at relatively low levels in stomach, gallbladder, and pancreas. We postulate CCK-2R1/gastrin receptor to have gastrin-related functions because of its distribution and abundance in gastro-intestinal (GI) tissues. CCK-2R2 is relatively abundant in brain but has low expression levels in gut tissues supporting the hypothesis for involvement in the gut-brain signaling. Major functional motifs and ligand interaction sites in salmon are conserved with that of mammals. This information will be instrumental for comparative studies and further targeting receptor activation and selectivity of biological responses of CCK in salmon.

Distinct CCK-2 receptor conformations associated with β-arrestin-2 recruitment or phospholipase-C activation revealed by a biased antagonist

Seven-transmembrane receptors (7TMRs), also termed G protein-coupled receptors (GPCRs), form the largest class of cell surface membrane receptors, involving several hundred members in the human genome. Nearly 30% of marketed pharmacological agents target 7TMRs. 7TMRs adopt multiple conformations upon agonist binding. Biased agonists, in contrast to non-biased agonists, are believed to stabilize conformations preferentially activating either G-protein- or β-arrestin-dependent signaling pathways. However, proof that cognate conformations of receptors display structural differences within their binding site where biased agonism initiates, are still lacking. Here, we show that a non-biased agonist, cholecystokinin (CCK) induces conformational states of the CCK2R activating Gq-protein-dependent pathway (CCK2R(G)) or recruiting β-arrestin2 (CCK2R(β)) that are pharmacologically and structurally distinct. Two structurally unrelated antagonists competitively inhibited both pathways. A third ligand (GV150013X) acted as a high affinity competitive antagonist on CCK2R(G) but was nearly inefficient as inhibitor of CCK2R(β). Several structural elements on both GV150013X and in CCK2R binding cavity, which hinder binding of GV150013X only to the CCK2R(β) were identified. At last, proximity between two conserved amino acids from transmembrane helices 3 and 7 interacting through sulfur-aromatic interaction was shown to be crucial for selective stabilization of the CCK2R(β) state. These data establish structural evidence for distinct conformations of a 7TMR associated with β-arrestin-2 recruitment or G-protein coupling and validate relevance of the design of biased ligands able to selectively target each functional conformation of 7TMRs.

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.

Development of a new radioligand for cholecystokinin receptor subtype 2 scintigraphy: from molecular modeling to in vivo evaluation

To improve the targeting to tumors expressing the cholecystokinin receptor subtype 2 (CCK2R) with limited kidney uptake, we synthesized a novel cholecystokinin C-terminal tetrapeptide (CCK4)-based derivative conjugated to an original bipyridine-chelator (BPCA), 111In-BPCA-(Ahx)2-CCK4. To our knowledge this is the first CCK4-based radioligand that presents a high affinity for the CCK2R, a high and specific tumor uptake, a low renal accumulation and a very good visualization of tumors in vivo compared with an internal control, 111Indium-trans-cyclohexyldiethylenetriaminepenta-acetic acid-cholecystokinin octapeptide (111In-CHX-A''-DTPA-CCK8). These properties make 111In-BPCA-(Ahx)2-CCK4, a promising candidate for imaging and peptide receptor radionuclide therapy of CCK2R positive tumors.

The family of G protein-coupled receptors: an example of membrane proteins

The G protein coupled receptors belong to the largest group of membrane proteins that regulates many essential physiological properties and represents an important class of drug targets. In this chapter, we show how the synergy between a laboratory experiment and computational modeling leads to structural delineation of the ligand binding pocket and how the knowledge of ligand-protein interactions is used for rational local and global drug design in which the structural knowledge of a particular receptor and its ligands is used for drug design of this particular GPCR and others.

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.

Mechanism of Activation of a G Protein-coupled Receptor, the Human Cholecystokinin-2 Receptor

G protein-coupled receptors (GPCRs) represent a major focus in functional genomics programs and drug development research, but their important potential as drug targets contrasts with the still limited data available concerning their activation mechanism. Here, we investigated the activation mechanism of the cholecystokinin-2 receptor (CCK2R). The three-dimensional structure of inactive CCK2R was homology-modeled on the basis of crystal coordinates of inactive rhodopsin. Starting from the inactive CCK2R modeled structure, active CCK2R (namely cholecystokinin-occupied CCK2R) was modeled by means of steered molecular dynamics in a lipid bilayer and by using available data from other GPCRs, including rhodopsin. By comparing the modeled structures of the inactive and active CCK2R, we identified changes in the relative position of helices and networks of interacting residues, which were expected to stabilize either the active or inactive states of CCK2R. Using targeted molecular dynamics simulations capable of converting CCK2R from the inactive to the active state, we delineated structural changes at the atomic level. The activation mechanism involved significant movements of helices VI and V, a slight movement of helices IV and VII, and changes in the position of critical residues within or near the binding site. The mutation of key amino acids yielded inactive or constitutively active CCK2R mutants, supporting this proposed mechanism. Such progress in the refinement of the CCK2R binding site structure and in knowledge of CCK2R activation mechanisms will enable target-based optimization of nonpeptide ligands.

Insights into the Cholecystokinin 2 Receptor Binding Site and Processes of Activation

The cholecystokinin (CCK) 2 receptor (CCK2R) appears as a pharmacological target for the treatment of many major diseases. To complete the mapping of the CCK2R binding site and its activation processes, we have looked for the receptor residues that interact with Trp6, an essential residue for CCK binding and activity. In our molecular model of the CCK-occupied CCK2R, the indole group of Trp6 stacked with the phenyl ring of Phe120 (ECL1) and interacted with the imidazole group of His381(H7.39) and the phenyl ring of Tyr385(H7.43). Mutagenesis and pharmacological studies validated these interactions. It is noteworthy that the mutation of Phe120 to Trp conferred constitutive activity to the CCK2R. Molecular modeling and experimental works identified the residues involved in the activation cascade initiated by Trp6 and revealed that the constitutively active F120W mutation mimics the conformational changes induced by Trp6 resulting in: 1) the exposure of Glu151(E3.49) of the conserved E/DRY motif 2) the formation of an amphiphatic pocket involving protonated Glu151(E3.49) and Leu330 (ICL3), and 3) the opening of the intracellular loops 2 and 3 and the release of Arg158 (ICL2). The R158A mutation was shown to affect inositol phosphate production, whereas the E151A and L330E mutations induced constitutive inositol phosphate production. Given that a constitutively active variant of the CCK2R has been identified in different cancers and the fact that the E151A mutant has been reported to induce tumors, these studies should help in the development of potent inverse agonists to inhibit the constitutive activation of the CCK2R.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Partial Agonism, Neutral Antagonism, and Inverse Agonism at the Human Wild-Type and Constitutively Active Cholecystokinin-2 Receptors

Cholecystokinin receptor-2 (CCK2R) is a G protein receptor that regulates a number of physiological functions. Activation of CCK2R and/or expression of a constitutively active CCK2R variant may contribute to human diseases, including digestive cancers. Search for antagonists of the CCK2R has been an important challenge during the last few years, leading to discovery of a set of chemically distinct compounds. However, several early-discovered antagonists turned out to be partial agonists. In this context, we carried out pharmacological characterization of six CCK2R antagonists using COS-7 cells expressing the human CCK2R or a CCK2R mutant having a robust constitutive activity on inositol phosphates production, and we investigated the molecular mechanisms which, at a CCK2R binding site, account for these features. Results indicated that three compounds, 3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl)-N'-(3-methylphenyl)urea (L365,260), 4-{[2-[[3-(lH-indol-3-yl)-2-methyl-1-oxo-2-[[[1.7.7-trimethyl-bicyclo[2.2.1]hept-2-yl)-oxy]carbonyl]amino]propyl]amino]-1-phenylethyl]amino-4-oxo-[lS-la.2[S*(S*)]4a]}-butanoate N-methyl-D-glucamine (PD135,158), and (R)-1-naphthalenepropanoic acid, b-[2-[[2-(8-azaspiro-[4.5]dec-8-ylcarbonyl)-4,6-dimethylphenyl]amino]-2-oxoethyl] (CR2945), were partial agonists; one molecule, 1-[(R)-2,3-dihydro-1-(2,3-dihydro-1-(2-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl]-3-(3-methylphenyl)urea (YM022), was a neutral antagonist; and two compounds, N-(+)-[1-(adamant-1-ylmethyl)-2,4-dioxo-5-phenyl2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-3-yl]-N'-phenylurea (GV150,013X) and ([(N-[methoxy-3 phenyl] N-[N-methyl N-phenyl carbamoylmethyl], carbomoyl-methyl)-3 ureido]-3-phenyl)2-propionic acid (RPR101,048), were inverse agonists. Furthermore, target- and pharmacophore-based docking of ligands followed by molecular dynamic simulation experiments resulted in consistent motion of aromatic residues belonging to a network presumably important for activation, thus providing the first structural explanations for the different pharmacological profiles of tested compounds. This study confirms that several referenced so-called antagonists are in fact partial agonists, and because of this undesired activity, we suggest that newly generated molecules should be preferred to efficiently block CCK2R-related physiological effects. Furthermore, data on the structural basis for the different pharmacological features of CCK2R ligands will serve to further clarify CCK2R mechanism of activation.

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.

Hexanoylation of a VPAC2 receptor-preferring ligand markedly increased its selectivity and potency

We synthesized a VIP analog that combines mutations that decrease the affinity for the VPAC1 receptor but maintain a high affinity for the VPAC2 receptor with an amino-terminal hexanoylation that increases the affinity for the VPAC2 receptor with a limited decrease in the affinity of the VPAC1 receptor. The resulting Hexanoyl[A19,K(27,28)]VIP had the expected properties of a high affinity for the VPAC2 receptor and a low affinity for the VPAC1 receptor and also a low affinity for the PAC1 and secretin receptors. With a 1000-fold preference for the VPAC2 receptor and a IC50 value of binding of 1 nM, this compound is the most potent and the most selective agonist presently described.