Separate agonist and peptide antagonist binding sites of the oxytocin receptor defined by their transfer into the V2 vasopressin receptor

The N-Terminal Juxtamembrane Segment of the V1a Vasopressin Receptor Provides Two Independent Epitopes Required for High-Affinity Agonist Binding and Signaling

It is fundamentally important to define how agonist-receptor interaction differs from antagonist-receptor interaction. The V1a vasopressin receptor (V1aR) is a member of the neurohypophysial hormone subfamily of G protein-coupled receptors. Using alanine-scanning mutagenesis of the N-terminal juxtamembrane segment of the V1aR, we now establish that Glu54 (1.35) is critical for arginine vasopressin binding. The mutant [E54A]V1aR exhibited decreased arginine vasopressin affinity (1700-fold) and disrupted signaling, but antagonist binding was unaffected. Mutation of Glu54 had an almost identical pharmacological effect as mutation of Arg46, raising the possibility that agonist binding required a mutual interaction between Glu54 and Arg46. The role of these two charged residues was investigated by 1) substituting Glu54; 2) inserting additional Glu/Arg in transmembrane helix (TM) 1; 3) repositioning the Glu/Arg in TM1; and 4) characterizing the reciprocal mutant [R46E/E54R]V1aR. We conclude that 1) the positive/negative charges need to be precisely positioned in this N terminus/TM1 segment; and 2) Glu54 and Arg46 function independently, providing two discrete epitopes required for high-affinity agonist binding and signaling. This study explains why Glu and Arg, part of an -R(X3)L/V(X3)E(X3)L- motif, are conserved at these loci throughout this G protein-coupled receptor subfamily and provides molecular insight into key differences between agonist and antagonist binding requirements.

A Gly/Ala switch contributes to high affinity binding of benzoxazinone-based non-peptide oxytocin receptor antagonists

Non-peptide antagonists of the oxytocin receptor (OTR) have been developed to prevent pre-term labour. The benzoxazinone-based antagonists L-371,257 and L-372,662 display pronounced species-dependent pharmacology with respect to selectivity for the OTR over the V(1a) vasopressin receptor. Examination of receptor sequences from different species identified Ala(318) in helix 7 of the human OTR as a candidate discriminator required for high affinity binding. The mutant receptor [A318G]OTR was engineered and characterised using ligands representing many different chemical classes. Of all the ligands investigated, only the benzoxazinone-based antagonists had decreased affinity for [A318G]OTR. Molecular modelling revealed that Ala(318) provides a direct hydrophobic contact with a methoxy group of L-371,257 and L-372,662.

Binding domains of the oxytocin receptor for the selective oxytocin receptor antagonist barusiban in comparison to the agonists oxytocin and carbetocin

We have analyzed binding domains of the oxytocin receptor for barusiban, a highly selective oxytocin receptor antagonist, in comparison to the combined vasopressin V1A/oxytocin receptor antagonist atosiban and the agonists oxytocin and carbetocin. For this purpose, chimeric 'gain-in function' oxytocin/vasopressin V2 receptors were expressed in COS-7 cells. These recombinant receptors have been produced by transfer of domains from the oxytocin receptor into the related vasopressin V2 receptor and have already been successfully employed for the identification of ligand binding domains at the oxytocin receptor (Postina, R., Kojro, E., Fahrenholz, F., 1996. Separate agonist and peptide antagonist binding sites of the oxytocin receptor defined by their transfer into the V2 vasopressin receptor. J. Biol. Chem. 271, 31593-31601). In displacement studies with 10 chimeric receptor constructs, the binding profile of barusiban was compared with the binding profiles of the ligands oxytocin, [Arg8]vasopressin, carbetocin, and atosiban. The binding profiles for the agonists oxytocin and carbetocin were found to be similar. For both agonists, important binding domains were the extracellular N-terminus (=E1) and the extracellular loops E2 and E3 from the oxytocin receptor. For the vasopressin V1A/oxytocin receptor antagonist atosiban, none of the receptor constructs were able to provide a binding with higher affinity than the starting vasopressin V2 receptor. In contrast, the binding of barusiban was significantly improved when the transmembrane domains 1 and 2 were transferred from the oxytocin receptor to the vasopressin V2 receptor. The binding domain of barusiban differs from the binding domain of the agonists and the nonselective oxytocin receptor antagonist d(CH2)5[Tyr-(Me)2,Thr4,Orn8,Tyr9]vasotocin that has been used in previous studies. Overall, the data supported the concept of a central pocket site within the oxytocin receptor.

Oxytocin-Receptor Binding: Why Divalent Metals Are Essential

Biologists have observed that the presence of divalent metal is essential for the binding of the hormone oxytocin (OT) to its cellular receptor. However, this interaction is not understood on the molecular level. Because conformation is a key factor controlling ligand binding in biomolecule systems, we have used ion mobility experiments and molecular modeling to probe the conformation of the oxytocin-zinc complex. Results show that Zn2+ occupies an octahedral site in the interior of the OT peptide that frees the N-terminus and creates a structured hydrophobic binding site on the peptide exterior; both factors are conducive to binding oxytocin to its receptor.

Non-peptide oxytocin agonists

A library of compounds targeted to the vasopressin/oxytocin family of receptors was screened for activity at a cloned human oxytocin receptor using a reporter gene assay. Potency and selectivity were optimised to afford compound 39, EC50 = 33 nM. This series of compounds represents the first disclosed, non-peptide, low molecular weight agonists of the hormone oxytocin (OT).

An arginyl in the N-terminus of the V1a vasopressin receptor is part of the conformational switch controlling activation by agonist

Defining how the agonist-receptor interaction differs from that of the antagonist-receptor and understanding the mechanisms of receptor activation are fundamental issues in cell signalling. The V1a vasopressin receptor (V1aR) is a member of a family of related G-protein coupled receptors that are activated by neurohypophysial peptide hormones, including vasopressin (AVP). It has recently been reported that an arginyl in the distal N-terminus of the V1aR is critical for binding agonists but not antagonists. To determine specific features required at this locus to support high affinity agonist binding and second messenger generation, Arg46 was substituted by all other 19 encoded amino acids. Our data establish that there is an absolute requirement for arginyl, as none of the [R46X]V1aR mutant constructs supported high affinity agonist binding and all 19 had defective signalling. In contrast, all of the mutant receptors possessed wildtype binding for both peptide and nonpeptide antagonists. The ratio of Ki to EC50, an indicator of efficacy, was increased for all substitutions. Consequently, although [R46X]V1aR constructs have a lower affinity for agonist, once AVP has bound all 19 are more likely than the wildtype V1aR to become activated. Therefore, in the wildtype V1aR, Arg46 constrains the inactive conformation of the receptor. On binding AVP this constraint is alleviated, promoting the transition to active V1aR. Our findings explain why arginyl is conserved at this locus throughout the evolutionary lineage of the neurohypophysial peptide hormone receptor family of G-protein coupled receptors.

Vasopressin-induced contraction of uterus is mediated solely by the oxytocin receptor in mice, but not in humans

In the non-pregnant mouse myometrium, both arginine vasopressin and oxytocin induced contractions (pD(2)=8.55+/-0.13 and 9.23+/-0.09, respectively). The effect of oxytocin was the most potent, while the maximum contractions induced by these two peptides were almost of the same magnitude. Both vasopressin- and oxytocin-induced contractions were strongly inhibited by an oxytocin receptor antagonist, CL-12-42 (d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2)(9)]OVT), and weakly inhibited by a vasopressin V(1a) receptor antagonist, SR49059 ((2S)1-[(2R,3S)-5-chloro-3-(2-chlorophenyl)-1-(3,4-dimethoxybenzene-sulfonyl)-3-hydroxy-2,3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2-carboxamide). Similar results were obtained in the pregnant mouse myometrium. These results suggest that not only oxytocin- but also vasopressin-induced contraction is mediated by the activation of oxytocin receptors in the mouse myometrium. A reverse transcription polymerase chain reaction study failed to reveal mRNA of the vasopressin V(1a) receptor in the mouse myometrium. In contrast, in the non-pregnant human myometrium, vasopressin-induced contraction was inhibited by SR49059. Oxytocin showed no effect on the myometrium. These results suggest that there are significant differences in the functional receptors and contractile responses to vasopressin and oxytocin in the human and mouse uteri.

Expression of human vasopressin and oxytocin receptors in Escherichia coli

In order to produce large amounts of human vasopressin and oxytocin receptors compatible with direct structural biology approaches such as X-ray crystallography, NMR or mass spectrometry, we have expressed these neurohypophysial hormone receptors in Escherichia coli. To facilitate the level of expression, the coding sequence for the V1a vasopressin receptor and the oxytocin receptor were first optimized for bacterial expression. The resulting 'bacterial receptor cDNAs' were then subcloned into pET/T7-driven prokaryotic expression vectors. Different constructs have been prepared: each cDNA was incorporated alone or in fusion with a T7 tag sequence or a glutathione-S-transferase tag sequence at the N-terminus end. Moreover, a 6 x His tag sequence has been added at the C-terminus end for one-step purification of the receptors. Screening of BL21(DE3) and BL21(DE3)pLysS bacterial strains transformed with the different constructions was achieved by Coomassie blue-stained SDS-polyacrylamide gels and by 6 x His antibody Western blotting. Several clones were selected for purification of the receptors. Expression levels of the receptors are now encouraging and will be optimized for further structural and functional studies. Moreover, at the same time, the construction of the bacterial-optimized sequence of the V2 vasopressin receptor and its expression will be performed.

Cholesterol and steroid hormones: modulators of oxytocin receptor function

The function and physiological regulation of the oxytocin-receptor system is strongly steroid-dependent. This is, unexpectedly, only partially reflected by the promoter sequences in the oxytocin receptor and favors the idea that posttranscriptional mechanisms may also play a significant role for the physiological regulation of the oxytocin-receptor system. Our data indicate that cholesterol acts as an allosteric modulator of the oxytocin receptor and stabilizes both membrane-associated and solubilized OT receptors in a high-affinity state for agonists and antagonists. Moreover, high-affinity OT receptors are 2-fold enriched in cholesterol-rich plasma membrane domains in HEK293 fibroblasts stably expressing the human OT receptor. Biochemical data suggest a direct and cooperative molecular interaction of cholesterol molecules with OT receptors. To localize the cholesterol interacting domain of the oxytocin receptor the C-terminal part including the last two transmembrane domains have been exchanged by the corresponding sequences of the cholecystokinin type B receptor, which is functionally not dependent on cholesterol. Concerning its ligand-binding behavior this chimeric receptor protein showed the same dependence on cholesterol and its analogues as the wild type oxytocin receptor. From mutagenesis experiments and studies with receptor chimera between the OTR and cholecystokinin type B receptor, we conclude that a major part of the cholesterol interacting domain may be localized in the first part of the oxytocin receptor, possibly in a domain nearby the agonist binding site. Progesterone is considered to be essential to maintain the uterine quiescence. High concentrations of progesterone (> 10 microM) attenuate or block the signaling of several GPCRs, including the OT receptor via a fast, reversible and non-genomic pathway. Progesterone is known to inhibit both cholesterol biosynthesis and the intracellular trafficking of cholesterol. We therefore test the hypothesis that progesterone affects the signal transduction and subdomain localization of receptors via its influence on cholesterol trafficking. Since cholesterol-rich subdomains (rafts) are considered to be organization centers for cellular signal transduction, changes of the level or distribution of cholesterol may have profound effects on receptor-mediated signaling in general. Using fluorescence recovery after photobleaching (FRAP) measurements with GFP-tagged oxytocin receptors the influence of steroids on the mobility and distribution of the oxytocin receptor in the plasma membrane was analyzed. Progesterone had no effect on the lateral mobility of the oxytocin receptor, but it led to marked inhibition of cellular motility such as vesicle trafficking and movements of filopodia. Non-genomic effects of progesterone and estradiol with respect to receptor signaling as well as the influence of cholesterol on signal transduction will be discussed in more detail.

Molecular pharmacology and modeling of vasopressin receptors

AVP receptors represent a logical target for drug development. As a new class of therapeutic agents, orally active AVP analogs could be used to treat several human pathophysiological conditions including neurogenic diabetes insipidus, the syndrome of inappropriate secretion of AVP (SIADH), congestive heart failure, arterial hypertension, liver cirrhosis, nephrotic syndrome, dysmenorrhea, and ocular hypertension. By immunoprecipitation and immunoblotting, we elucidated the phosphorylation pattern of green fluorescent protein-tagged AVP receptors and showed interactions with the specific kinases PKC and GRK5 that are agonist-, time- and receptor subtype-dependent. The tyrosine residue of the NPWIY motif present in the 7th helix of AVP receptors is rapidly and transiently phosphorylated after agonist stimulation. This phosphorylation is instrumental in the genesis of the mitogenic cascade linked to the activation of this receptor, presumably by establishing key intramolecular contacts and by participating in the creation of a scaffold of proteins that produce the activation of downstream kinases. The random screening of chemical entities and optimization of lead compounds recently resulted in the development of orally active non-peptide AVP receptor agonists and antagonists. Furthermore, the identification of the molecular determinants of receptor-ligand interactions should facilitate the development of more potent and very selective orally active compounds via the approach of structure-based drug design. We developed three-dimensional molecular docking models of peptide and non-peptide ligands to the human V1 vascular, V2 renal and V3 pituitary AVP receptors. Docking of the peptide hormone AVP to the receptor ligand binding pockets reflects its dual polar and non-polar structure, but is receptor subtype-specific. The characteristics of non-peptide AVP analogs docking to the receptors are clearly distinct from those of peptide analogs docking. Molecular modeling of the results of site-directed mutagenesis experiments performed in CHO cells stably transfected with the human AVP receptor subtypes revealed that non-peptide antagonists establish key contacts with a few amino acid residues of the receptor subtypes that are different from those involved in agonist binding. Moreover, these interactions are species-specific. These findings provide further understanding of the signal transduction pathways of AVP receptors and new leads for elucidation of drug-receptor interactions and optimization of drug design. NOTE TO THE READER: The recent cloning and molecular characterization of AVP/OT receptor subtypes call for the revision of their nomenclature. For the sake of clarity and reference to their main site of expression, we call the V1a receptor the V1 vascular receptor, the V2 receptor the V2 renal receptor and the V1b or V3 receptor the V3 pituitary receptor in the present review.

Agonist-specific, high-affinity binding epitopes are contributed by an arginine in the N-terminus of the human oxytocin receptor

The effects of the peptide hormone oxytocin (OT) are mediated by the oxytocin receptor, which is a member of the G-protein-coupled receptor family. Defining differences between the binding of agonists and antagonists to the OTR, at the molecular level, is of fundamental importance to understanding OTR activation and to rational drug design. Previous reports have indicated that the N-terminus of the OTR is required for OT binding. The aim of this study was to identify which individual residues within the N-terminal domain of the human OTR provided these OT binding epitopes. A series of truncated OTRs and mutant receptor constructs with systematic alanine substitution were characterized with respect to their pharmacological profile and intracellular signaling capability. Although a number of residues within the OTR will be required for optimal OT-OTR interaction, our data establish that Arg(34) within the N-terminal domain contributes to high-affinity OT binding. Removal of Arg(34) by truncation or substitution resulted in a 2000-fold decrease in OT affinity. In addition, we show that the arginyl at this locus is required for high-affinity binding of agonists in general. However, the importance of Arg(34) is restricted to agonist interaction with the OTR, as it was not required for binding peptide antagonist or non-peptide antagonist. It is noteworthy that the corresponding Arg in the related rat V(1a) vasopressin receptor is also required for high-affinity agonist binding. This study defines, at the molecular level, the role of the N-terminus of the OTR in high-affinity agonist binding and identifies a key residue for this function.

The social deficits of the oxytocin knockout mouse

Numerous studies have implicated oxytocin (OT) and oxytocin receptors in the central mediation of social cognition and social behavior. Much of our understanding of OT's central effects depends on pharmacological studies with OT agonists and antagonists. Recently, our knowledge of OT's effects has been extended by the development of oxytocin knockout (OTKO) mice. Mice with a null mutation of the OT gene manifest several interesting cognitive and behavioral changes, only some of which were predicted by pharmacological studies. Contrary to studies in rats, mice do not appear to require OT for normal sexual or maternal behavior, though OT is necessary for the milk ejection reflex during lactation. OTKO pups thrive if raised by a lactating female, but OTKO pups emit fewer ultrasonic vocalizations with maternal separation and OTKO adults are more aggressive than WT mice. Remarkably, OTKO mice fail to recognize familiar conspecifics after repeated social encounters, though olfactory and non-social memory functions appear to be intact. Central OT administration into the amygdala restores social recognition. The development of transgenic mice with specific deficits in social memory represents a promising approach to examine the cellular and neural systems of social cognition. These studies may provide valuable new perspectives on diseases characterized by social deficits, such as autism or reactive attachment disorder.

Docking ligands to vasopressin and oxytocin receptors via genetic algorithm

The aim of the study was to computer-dock selected ligands to neurophyseal receptors in order to identify amino acid residues responsible for ligand-receptor interactions. To this aim, reliable oxytocin receptor (OTR) and arginine-vasopressin receptor (V1aR/V2R) models were built. The OTR-selective agonist [Thr4,Gly7]OT, the OTR-selective cyclohexapeptide antagonist L-366,948 and OT itself were docked via genetic algorithm to OTR, V1aR, and V2R and relaxed using a constrained simulated annealing protocol. For the analysis of receptor/ligand interactions a subset of initial conformations was chosen using energetic and steric criteria. All three ligands seem to prefer similar modes of binding to the receptors, manifested by repetitive residues of the receptors which directly interact with the ligands. Taking into account that many aspects of mechanisms of G protein-coupled receptor (GPCR) action are still unsolved, the results obtained with the docking simulations may propose future experimental research, especially in site-directed mutagenesis analysis and searching for key amino acid residues responsible for drug activities.

Design, synthesis and pharmacological characterization of a potent radioiodinated and photoactivatable peptidic oxytocin antagonist

Using a segment strategy, we have synthesized four iodinated photoactivatable cyclic peptidic ligands of oxytocin, bearing a beta-mercapto-betabeta-cyclopentamethylene propionic group (Pmp) on their N-terminus. All the syntheses were RP-HPLC monitored, and the compounds were HPLC purified. They were characterized by 1H NMR, MALDI-TOF, or FAB mass spectrometries. The affinities of Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (20), Pmp-Tyr-Ile-Thr-Asn-Cys-Gly-Orn-Phe(3I,4N3)-NH2 (21), Pmp-Tyr(Me)-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (22), and Pmp-Tyr-Ile-Thr-Asn-Cys-Pro-Orn-Phe(3I,4N3)-NH2 (23) were evaluated as inhibition constants (K(i), in nM) for the human oxytocin receptor expressed in Chinese hamster ovary cells by displacement of a radioiodinated disulfide-cyclized antagonist (Elands et al. Eur. J. Pharmacol. 1987, 147, 197-207). The most potent of them, compound 22, was synthesized by another method in order to allow its radiolabeling by 125I. Its dissociation constant (K(d)) for the human oxytocin receptor, directly measured in saturation studies, was 0.25 +/- 0.04 nM, and its antagonist properties were determined by inactivation of phospholipase C, thus obtaining an inactivation constant (K(inact)) of 0.18 +/- 0.02 nM, evaluated by inositol phosphate accumulation. This compound is a very good tool for the mapping of peptidic antagonist binding sites in the human oxytocin receptor.

Direct identification of human oxytocin receptor-binding domains using a photoactivatable cyclic peptide antagonist: comparison with the human V1a vasopressin receptor

Understanding of the molecular determinants responsible for antagonist binding to the oxytocin receptor should provide important insights that facilitate rational design of potential therapeutic agents for the treatment of preterm labor. To study ligand/receptor interactions, we used a novel photosensitive radioiodinated antagonist of the human oxytocin receptor, d(CH(2))(5) [Tyr(Me)(2),Thr(4),Orn(8),Phe(3(125)I,4N(3))-NH(2)9]vasotocin. This ligand had an equivalent high affinity for human oxytocin and V(1a) vasopressin receptors expressed in Chinese hamster ovary cells. Taking advantage of this dual specificity, we conducted photoaffinity labeling experiments on both receptors. Photolabeled oxytocin and V(1a) receptors appeared as a unique protein band at 70-75 kDa and two labeled protein bands at 85-90 and 46 kDa, respectively. To identify contact sites between the antagonist and the receptors, the labeled 70-75- and the 46-kDa proteins were cleaved with CNBr and digested with Lys-C and Arg-C endoproteinases. The fragmentation patterns allowed the identification of a covalently labeled region in the oxytocin receptor transmembrane domain III consisting of the residues Leu(114)-Val(115)-Lys(116). Analysis of contact sites in the V(1a) receptor led to the identification of the homologous region consisting of the residues Val(126)-Val(127)-Lys(128). Binding domains were confirmed by mutation of several CNBr cleavage sites in the oxytocin receptor and of one Lys-C cleavage site in the V(1a) receptor. The results are in agreement with previous experimental data and three-dimensional models of agonist and antagonist binding to members of the oxytocin/vasopressin receptor family.

Role of the human V1 vasopressin receptor COOH terminus in internalization and mitogenic signal transduction

We studied the role played by the intracellular COOH-terminal region of the human arginine vasopressin (AVP) V1-vascular receptor (V1R) in ligand binding, trafficking, and mitogenic signal transduction in Chinese hamster ovary cells stably transfected with the human AVP receptor cDNA clones that we had isolated previously. Truncations, mutations, or chimeric alterations of the V1R COOH terminus did not alter ligand binding, but agonist-induced V1R internalization and recycling were reduced in the absence of the proximal region of the V(1)R COOH terminus. Coupling to phospholipase C was altered as a function of the COOH-terminal length. Deletion of the proximal portion of the V1R COOH terminus or its replacement by the V2-renal receptor COOH terminus prevented AVP stimulation of DNA synthesis and progression through the cell cycle. Mutation of a kinase consensus motif in the proximal region of the V1R COOH terminus also abolished the mitogenic response. Thus the V1R cytoplasmic COOH terminus is not involved in ligand specificity but is instrumental in receptor trafficking and facilitates the interaction between the intracellular loops of the receptor, G protein, and phospholipase C. It is absolutely required for transmission of the mitogenic action of AVP, probably via a specific kinase phosphorylation site.

The basic and clinical pharmacology of nonpeptide vasopressin receptor antagonists

The neurohypophysial hormone arginine vasopressin (AVP) is a cyclic nonpeptide whose actions are mediated by the stimulation of specific G protein--coupled membrane receptors pharmacologically classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptor subtypes. The random screening of chemical compounds and optimization of lead compounds recently resulted in the development of orally active nonpeptide AVP receptor antagonists. Potential therapeutic uses of AVP receptor antagonists include (a) the blockade of V1-vascular AVP receptors in arterial hypertension, congestive heart failure, and peripheral vascular disease; (b) the blockade of V2-renal AVP receptors in the syndrome of inappropriate vasopressin secretion, congestive heart failure, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent dilutional hyponatremia; (c) the blockade of V3-pituitary AVP receptors in adrenocorticotropin-secreting tumors. The pharmacological and clinical profile of orally active nonpeptide vasopressin receptor antagonists is reviewed here.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Identification of an extracellular segment of the oxytocin receptor providing agonist-specific binding epitopes

The effects of the peptide hormone oxytocin are mediated by oxytocin receptors (OTRs) expressed by the target tissue. The OTR is a member of the large family of G-protein-coupled receptors. Defining differences between the interaction of agonists and antagonists with the OTR at the molecular level is of fundamental importance, and is addressed in this study. Using truncated and chimaeric receptor constructs, we establish that a small 12-residue segment in the distal portion of the N-terminus of the human OTR provides important epitopes which are required for agonist binding. In contrast, this segment does not contribute to the binding site for antagonists, whether peptide or non-peptide. It does, however, have a role in agonist-induced OTR signalling. Oxytocin is also an agonist at the vasopressin V(1a) receptor (V(1a)R). A chimaeric receptor (V(1a)R(N)-OTR) was engineered in which the N-terminus of the OTR was substituted by the corresponding, but unrelated, sequence from the N-terminus of the V(1a)R. We show that the V(1a)R N-terminus present in V(1a)R(N)-OTR fully restored both agonist binding and intracellular signalling to a dysfunctional truncated OTR construct. The N-terminal segment does not, however, contribute to receptor-selective agonism between the OTR and the V(1a)R. Our data establish a key role for the distal N-terminus of the OTR in providing agonist-specific binding epitopes.

Critical role of a subdomain of the N-terminus of the V1a vasopressin receptor for binding agonists but not antagonists; functional rescue by the oxytocin receptor N-terminus

A fundamental issue in molecular pharmacology is to define how agonist:receptor interaction differs from that of antagonist:receptor. The V(1a) receptor (V(1a)R) is a member of a family of related G-protein-coupled receptors that are activated by the neurohypophysial peptide hormone arginine-vasopressin (AVP). Here we define a short subdomain of the N-terminus of the V(1a)R from Glu(37) to Asn(47) that is an absolute requirement for binding AVP and other agonists. In marked contrast to the situation for agonists, deleting this segment has little or no effect on the binding of either peptide or non-peptide antagonists. In addition, we established that this subdomain was crucial for receptor activation and second messenger generation. The oxytocin receptor (OTR) also binds AVP with high affinity but exhibits a different pharmacological profile to the V(1a)R. Substitution of the N-terminus of the V(1a)R with the corresponding sequence from the OTR generated a chimeric receptor (OTR(N)-V(1a)R). The presence of the OTR N-terminus recovered high affinity agonist binding such that the OTR(N)-V(1a)R possessed almost wild-type V(1a)R pharmacology and signaling. Consequently, a domain within the N-terminus is required for agonist binding but it does not provide the molecular discriminator for subtype-selective agonist recognition. Cotransfection and peptide mimetic studies demonstrated that this N-terminal subdomain had to be contiguous with the receptor polypeptide to be functional. This study establishes that a segment of the V(1a)R N-terminus has a pivotal role in the mechanism of agonist binding and provides molecular insight into key differences between the interaction of agonists and antagonists with a peptide receptor family.

Conserved aromatic residues in the transmembrane region VI of the V1a vasopressin receptor differentiate agonist vs. antagonist ligand binding

Despite their opposite effects on signal transduction, the nonapeptide hormone arginine-vasopressin (AVP) and its V1a receptor-selective cyclic peptide antagonist d(CH2)5[Tyr(Me)2]AVP display homologous primary structures, differing only at residues 1 and 2. These structural similarities led us to hypothesize that both ligands could interact with the same binding pocket in the V1a receptor. To determine receptor residues responsible for discriminating binding of agonist and antagonist ligands, we performed site-directed mutagenesis of conserved aromatic and hydrophilic residues as well as nonconserved residues, all located in the transmembrane binding pocket of the V1a receptor. Mutation of aromatic residues of transmembrane region VI (W304, F307, F308) reduced affinity for the d(CH2)5[Tyr(Me)2]AVP and markedly decreased affinity for the unrelated strongly hydrophobic V1a-selective nonpeptide antagonist SR 49059. Replacement of these aromatic residues had no effect on AVP binding, but increased AVP-induced coupling efficacy of the receptor for its G protein. Mutating hydrophilic residues Q108, K128 and Q185 in transmembrane regions II, III and IV, respectively, led to a decrease in affinity for both agonists and antagonists. Finally, the nonconserved residues T333 and A334 in transmembrane region VII, controlled the V1a/V2 binding selectivity for both nonpeptide and cyclic peptide antagonists. Thus, because conserved aromatic residues of the V1a receptor binding pocket seem essential for antagonists and do not contribute at all to the binding of agonists, we propose that these residues differentiate agonist vs. antagonist ligand binding.

Misfolded vasopressin V2 receptors caused by extracellular point mutations entail congential nephrogenic diabetes insipidus

Vasopressin V2 receptor mutants from three different patients with congenital nephrogenic diabetes insipidus phenotypes were investigated after expression in COS cells. The amino acid exchanges within the human V2 receptor are located in the second extracellular loop (T204N, Y205C and V206D). Confocal microscopy showed that all receptor mutants were strongly expressed but mainly located within the cell. Residual binding capacity for the antidiuretic hormone arginine vasopressin (AVP) could only be detected for the T204N mutant and was 10-fold lower than for the wild-type receptor. Stimulation of transfected cells with 1 microM AVP showed that the T204N mutant was able to activate the adenylyl cyclase pathway. In contrast, the Y205C mutant was almost inactive and stimulation of the V206D mutant increased the cAMP accumulation only slightly. Dose dependent stimulation of cells expressing the T204N mutant with AVP and with the therapeutic AVP analogue 1-deamino[D-Arg8]vasopressin (dDAVP) revealed that AVP was 50-fold more potent than dDAVP. This indicates that the ligand binding selectivity of the T204N mutant has changed as compared with the wild-type receptor where AVP is only 2.3-fold more potent than dDAVP. Despite its defects in membrane localization, ligand binding affinity and selectivity, the T204N receptor could be activated with high concentrations of dDAVP. Our results indicate that in cases of congenital nephrogenic diabetes insipidus with residual V2 receptor activities the use of antidiuretic drugs, such as dDAVP, might be beneficial for patients.

Human oxytocin receptors in cholesterol-rich vs. cholesterol-poor microdomains of the plasma membrane

We analyzed the properties of a G protein-coupled receptor localized in cholesterol-poor vs. cholesterol-rich microdomains of the plasma membrane. For this purpose, the human oxytocin receptor, which is very sensitive against alterations of the membrane cholesterol level, was stably expressed in HEK293 cells. To calculate the total number of receptors independent of ligand binding studies, the oxytocin receptor was tagged with an enhanced green fluorescent protein (EGFP) which did not change the functional properties of the receptor. Only 1% of the oxytocin receptors were present in cholesterol-rich detergent-insoluble domains. In contrast, employing a detergent-free fractionation scheme that preserves the functional activity of the receptor, we detected 10-15% of the receptors in cholesterol-rich low-density membranes and therein the high-affinity state receptors were twofold enriched. In cholesterol-poor vs. cholesterol-rich domains, high-affinity oxytocin receptors behaved similar with respect to their agonist binding kinetics and GTP sensitivity. However, high-affinity oxytocin receptors localized in cholesterol-rich low-density membranes showed a markedly enhanced (t (1/2) approximately threefold) stability at 37 degrees C as compared with the oxytocin receptors localized in the cholesterol-poor high-density membranes. Addition of cholesterol to the high-density membranes fully protected the oxytocin receptors against loss of function. The importance of cholesterol to stabilize the oxytocin receptor was supported in experiments with solubilized receptors. Cholesterol markedly delayed the inactivation of oxytocin receptors solubilized with Chapso. In conclusion, the data of this report suggest that functional properties of heptahelical receptor proteins could differ in dependence of their localization in different membrane microdomains.

A mutation in the second intracellular loop of the pituitary adenylate cyclase activating polypeptide type I receptor confers constitutive receptor activation

The pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor belongs to the glucagon/secretin/vasoactive intestinal polypeptide (VIP) receptor family. We mutated and deleted an amino acid residue (E261) which is located within the second intracellular loop of the rat PACAP type I receptor and which is highly conserved among the receptor family. The wild-type receptor and the mutant receptors were efficiently expressed at the surface of COS-7 cells at nearly the same level and revealed the same high affinity for the agonist PACAP-27. The cAMP contents of COS cells transfected with the E261A, E261Q, and the deletion mutant receptor were 4.6-, 5.7-, and 6.7-fold higher as compared with COS cells transfected with the wild-type receptor. Thus, all the mutant PACAP receptors were constitutively active. The data suggest that the glutamic acid in the second intracellular loop of the PACAP receptor may be a key residue to constrain the receptor in the inactive conformation with respect to its coupling to G(s) proteins.

Molecular basis of ligand binding and receptor activation in the oxytocin and vasopressin receptor family

Although it is now widely accepted that G-protein-coupled receptors exist in at least two allosteric states, inactive and active, and that the spontaneous equilibrium between the two is regulated by various events including the binding of specific agonists and antagonists, the molecular counterparts of these functionally different states are still poorly understood. In this paper, we review our current knowledge concerning the structure-function relationships of the oxytocin and vasopressin receptors, focusing in particular on the process of receptor activation. Using a combined approach of site-directed mutagenesis and molecular modelling, we investigated the molecular events leading to agonist-dependent and -independent receptor activation in the human oxytocin receptor. Our analysis allows us to propose that the active conformations of this receptor are characterised by similar rearrangements of its cytosolic regions that ultimately lead to the opening of a putative docking site for the G-protein. Furthermore, the dynamics of these motions are similar to that observed in the alpha1B-adrenergic receptor, thus suggesting that, although activated by different ligands, the process of receptor isomerization in these two receptors is regulated by the same cluster of highly conserved residues and that common molecular events are responsible for receptor activation in different G-protein-coupled receptors.

Identification of residues involved in neurotensin binding and modeling of the agonist binding site in neurotensin receptor 1

The neurotensin receptor 1 (NTR1) subtype belongs to the family of G protein-coupled receptors and mediates most of the known effects of the neuropeptide including modulation of central dopaminergic transmission. This suggested that nonpeptide agonist mimetics acting at the NTR1 might be helpful in the treatment of Parkinson's disease and schizophrenia. Here, we attempted to define the molecular interactions between neurotensin-(8-13), the pharmacophore of neurotensin, and the rat NTR1. Mutagenesis of the NTR1 identified residues that interact with neurotensin. Structure-activity studies with neurotensin-(8-13) analogs identified the peptide residues that interact with the mutated amino acids in the receptor. By taking these data into account, computer-assisted modeling techniques were used to build a tridimensional model of the neurotensin-(8-13)-binding site in which the N-terminal tetrapeptide of neurotensin-(8-13) fits in the third extracellular loop and the C-terminal dipeptide binds to residues at the junction between the extracellular and transmembrane domains of the receptor. Interestingly, the agonist binding site lies on top of the previously described NTR1-binding site for the nonpeptide neurotensin antagonist SR 48692. Our data provide a basis for understanding at the molecular level the agonist and antagonist binding modes and may help design nonpeptide agonist mimetics of the NTR1.

Structural requirements for V2 vasopressin receptor proteolytic cleavage

The ligand-induced proteolytic cleavage of the V2 vasopressin receptor transiently expressed in COS cells was investigated. After incubation of the cell membranes with a photoreactive ligand possessing full agonistic properties for V2 receptors, approximately 90% of the porcine and bovine V2 vasopressin receptors were cleaved in the upper part of transmembrane helix 2 at a heptapeptide sequence conserved in both vasopressin and oxytocin receptors. The oxytocin receptor was completely resistant to proteolysis after binding the same photoreactive ligand, which is only a partial agonist for this receptor. Chimeric V2/oxytocin receptors obtained by transfer of extracellular domains of the oxytocin receptor into the V2 receptor showed an increase in binding affinity for oxytocin versus vasopressin and a diminished cleavage. The proteolysis-resistant chimeric V2/oxytocin receptor, which contains the first three extracellular domains of the oxytocin receptor, stimulated cAMP accumulation to a larger extent in response to vasopressin than the wild-type receptor and showed impaired desensitization of the adenylate cyclase system. Our data indicate that the proteolytic cleavage of the V2 receptor requires a defined conformation, especially of the first two extracellular domains that is induced by agonist binding. Furthermore, the results suggest that the proteolytic V2 receptor cleavage might play a role in signal termination at elevated hormone concentrations.

Previous maternal experience potentiates the effect of parturition on oxytocin receptor mRNA expression in the paraventricular nucleus

In sheep, central oxytocin release at parturition induces maternal behaviour which is thought to be mediated by changes in the expression of central oxytocin receptors. The distribution, effects of parturition, previous maternal experience and hormonal status on the distribution of an oxytocin receptor was investigated using immunocytochemistry and in situ hybridization. In ewes with no previous maternal experience, parturition induced significant increases in oxytocin receptor mRNA expression in the anterior olfactory nucleus, medial preoptic area, ventromedial hypothalamus, lateral septum, medial amygdala, bed nucleus of the stria terminalis and diagonal band of Broca. In maternally experienced ewes, parturition induced additional increases in two areas, the paraventricular nucleus and the Islands of Calleja. The changes in progesterone and oestrogen that occur during late pregnancy and parturition appear to contribute to increases in expression in the anterior olfactory nucleus, Islands of Calleja, medial preoptic area, ventromedial hypothalamus, bed nucleus of the stria terminalis and diagonal band of Broca, but not in the paraventricular nucleus, lateral septum and medial amygdala. These results demonstrate that progesterone and oestrogen priming enhance oxytocin receptor mRNA expression in a number of regions in the olfactory system, hypothalamus and limbic brain. These effects appear to be independent of maternal experience. Parturition increases oxytocin receptor mRNA expression in all the areas influenced by hormonal priming and the lateral septum, medial amygdala and paraventricular nucleus. Maternal experience also enhances expression of oxytocin receptor mRNA in the paraventricular nucleus and the Islands of Calleja. Because the paraventricular nucleus is the main source of oxytocin release in the brain, this upgrading of autoreceptors as a result of maternal experience may serve to enhance release of this peptide in projection sites regulating maternal behaviour.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Docking of linear peptide antagonists into the human V(1a) vasopressin receptor. Identification of binding domains by photoaffinity labeling

A novel photoactivatable linear peptide antagonist selective for the V(1a) vasopressin receptor, [(125)I][Lys(3N(3) Phpa)(8)]HO-LVA, was synthesized, characterized, and used to photolabel the human receptor expressed in Chinese hamster ovary cells. Two specific glycosylated protein species at 85-90 and 46 kDa were covalently labeled, a result identical to that obtained with a previous photosensitive ligand, [(125)I]3N(3)Phpa-LVA (Phalipou, S., Cotte, N. , Carnazzi, E., Seyer, R., Mahe, E., Jard, S., Barberis, C., and Mouillac, B. (1997) J. Biol. Chem. 272, 26536-26544). To identify contact sites between the new photoreactive analogue and the V(1a) receptor, the labeled receptors were digested with Lys-C or Asp-N endoproteinases and chemically cleaved with CNBr. Fragmentation with CNBr, Lyc-C, and Asp-N used alone or in combination, led to the identification of a restricted receptor region spanning the first extracellular loop. The results established that sequence Asp(112)-Pro(120) could be considered as the smallest covalently labeled fragment with [(125)I][Lys(3N(3)Phpa)(8)]HO-LVA. Based on the present experimental result and on previous photoaffinity labeling data obtained with [(125)I]3N(3)Phpa-LVA (covalent attachment to transmembrane domain VII), three-dimensional models of the antagonist-bound receptors were constructed and then verified by site-directed mutagenesis studies. Strikingly, these two linear peptide antagonists, when bound to the V(1a) receptor, could adopt a pseudocyclic conformation similar to that of the cyclic agonists. Despite divergent functional properties, these peptide antagonists could interact with a transmembrane-binding site significantly overlapping that of the natural hormone vasopressin.

Activation mechanism of human oxytocin receptor: a combined study of experimental and computer-simulated mutagenesis

The aim of this study was to investigate the molecular changes associated with the transition of the human oxytocin receptor from its inactive to its active states. Mutation of the conserved arginine of the glutamate/aspartate-arginine-tyrosine motif located in the second intracellular domain gave rise to the first known constitutively active oxytocin receptor (R137A), whereas mutation of the aspartic acid located in the second transmembrane domain led to an inactive receptor (D85A). The structural features of the constitutively active and inactive receptor mutants were compared with those of the wild type in its free and agonist-bound states. The results suggest that, although differently triggered, the activation process induced by the agonist and the activating mutation are characterized by the opening of a solvent exposed site formed by the 2nd intracellular loop, the cytosolic extension of helix 5, and the 3rd intracellular loop; on the contrary, the D85A mutation prevents oxytocin from triggering the opening of a cytosolic site. On the basis of these findings, we hypothesize that this cytosolic crevice plays an important role in G protein recognition. Finally, comparative analysis of the free- and agonist-bound forms of the wild-type oxytocin receptor and alpha1B adrenergic receptor suggests that the highly conserved polar amino acids and the seven helices play similar mechanistic roles in the different G protein-coupled receptors.

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.

Identification of neurohypophysial hormone receptor domains involved in ligand binding and G protein coupling

Chimeric vasopressin V2/OT receptors were constructed and investigated to identify receptor regions involved in ligand binding or G protein coupling. The fusion sites for one series of hybrid receptors were either located at the C-terminal end of the third extracellular domain or in the centre of the third transmembrane helix, respectively. In each pair of the resulting symmetrical hybrids only one receptor was able to bind arginine vasopressin (AVP) and/or oxytocin (OT). In both cases a major part of the vasopressin V2 receptor (V2R) was needed for ligand binding. A chimeric OT/V2 receptor including OT receptor (OTR) sequences from its N-terminus to the middle of transmembrane region three showed both high-affinity OT binding (Ki = 3 nM) and activation of the adenylyl cyclase. In contrast, a hybrid containing OTR sequences reaching from transmembrane helix five to its C-terminus showed the V2 receptor's ligand binding profile and was unable to couple to G alpha s. These results indicate (i) that the third and/or the fourth intracellular domain of the V2R are involved in G protein coupling and (ii) for high-affinity OT binding the N-terminal third of the OTR plays an important role. By detailed binding studies on a second series of chimeric V2/OT receptors with AVP, OT and the two hybrid hormone derivatives arginine vasotocin and oxypressin it was further demonstrated that the first two extracellular domains of the OTR are involved in binding to the C-terminal tripeptide of OT. Moreover, the third extracellular domain of the OTR is able to contact the cyclic part of OT and the fourth outer domain does not interact with the two variable amino acid residues of AVP and OT. Thus, the first three extracellular domains of the OTR provide an essential part of the OT binding site. The other part is most probably contributed by the OTR's transmembrane helices 3 and 4. Photoaffinity labeling and ligand binding studies demonstrated that the binding site for the OT antagonist d(CH2)5[Tyr(Me)2, Thr4, Orn8, Tyr9]vasotocin is located in the helices 1, 2 and 7. Our results provide evidence for the existence of separate domains of a peptide hormone receptor involved in binding and selectivity for agonists and peptide antagonists.

Molecular dynamics of a vasopressin V2 receptor in a phospholipid bilayer membrane

Molecular dynamics simulations were carried out for a V2 receptor (V2R) model embedded in a dimyristoylphosphatidylcholine (DMPC) bilayer. Both free and ligand-bound states of V2R were modeled. Our initial V2R model was obtained using a rule-based automated method for GPCR modeling and refined using constrained simulated annealing in vacuo. The docking site of the native vasopressin ligand was selected and justified upon consideration of ligand-receptor interactions and structure-activity data. The primary purpose of this work was to investigate the usefulness of MD simulation of an integral membrane protein like a GPCR receptor, upon inclusion of a carefully parameterized surrounding lipid membrane and water. Physical properties of the system were evaluated and compared with the fully hydrated pure DMPC bilayer membrane. The solvation interactions, individual lipid-protein interaction and fluctuations of the protein, the lipid, and water were analyzed in detail. As expected, the membrane-spanning helices of the protein fluctuate less than the peripheral loops do. The protein appears to disturb the local lipid structure. Simulations were carried out using AMBER 4.1 package upon constant number-pressure-temperature (NPT) conditions on massively parallel computers Cray T3E and IBM SP2.

Identification of residues responsible for the selective binding of peptide antagonists and agonists in the V2 vasopressin receptor

To improve our understanding of the functional architecture of G protein-coupled receptors, we have taken advantage of differences among mammalian species in ligand binding to search for the rat versus human selectivity determinants of the V2 vasopressin receptor and of its peptide ligands. Our data indicate that residue 2 of species-selective peptide antagonists such as d(CH2)5-[D-Ile2,Ile4, Tyr-NH29]arginine vasopressin controls their rat versus human selectivity. For species-selective agonists such as desmopressin, residues 1 and 8 modulate the binding selectivity. Among residues different between rat and human V2 receptors, those localized in the upper part of the human V2 receptor have been substituted with their rat V2 homologs. Pharmacological analysis of mutant receptors revealed that residues 202 and 304 fully control the species selectivity of the discriminating antagonists in an independent and additive manner. A third residue (position 100) is necessary to observe an equivalent phenomenon for the discriminating agonists. The substitution of these three residues does not modify the affinity of the nonselective agonists and antagonists. In conclusion, extracellular loops and the top of the transmembrane domains of V2 vasopressin receptors may provide the molecular basis for peptide ligand-binding species selectivity. Very few residues in these regions may control the binding mode of both agonists and antagonists.

Agonist and antagonist-dependent internalization of the human vasopressin V2 receptor

In this report we demonstrate that in HEK293 cells stably expressing the human V2 vasopressin receptor, ligand-induced internalization of the hormone receptor occurs via the clathrin-dependent pathway. Studies of receptor trafficking either by direct visualization of the V2 receptor by confocal microscopy or binding experiments show a rapid internalization (half-time 6-7 min). Blocking of the clathrin-dependent pathway by hypertonic sucrose increased vasopressin-induced cellular cAMP production and decreased the desensitization of the V2 receptor-adenylyl cyclase system. Thus, internalization appears to be a major regulatory mechanism terminating vasopressin action in HEK293 cells. Two antagonists of the vasopressin V2 receptor exerted different effects on receptor internalization, as determined by confocal fluorescence microscopy. The nonpeptidic antagonist OPC31260 did not induce any visible receptor internalization, whereas the peptidic antagonist d(CH2)5[D-Tyr(Et)2,Val4,Lys8,Tyr-NH29]VP induced a slow but substantial receptor internalization. These results suggest that long-term treatment with peptidic V2 receptor antagonists might lead to desensitization.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Molecular modeling of the human vasopressin V2 receptor/agonist complex

The V2 vasopressin renal receptor (V2R), which controls antidiuresis in mammals, is a member of the large family of heptahelical transmembrane (7TM) G protein-coupled receptors (GPCRs). Using the automated GPCR modeling facility available via Internet (http:/(/)expasy.hcuge.ch/swissmod/SWISS-MODEL.+ ++html) for construction of the 7TM domain in accord with the bovine rhodopsin (RD) footprint, and the SYBYL software for addition of the intra- and extracellular domains, the human V2R was modeled. The structure was further refined and its conformational variability tested by the use of a version of the Constrained Simulated Annealing (CSA) protocol developed in this laboratory. An inspection of the resulting structure reveals that the V2R (likewise any GPCR modeled this way) is much thicker and accordingly forms a more spacious TM cavity than most of the hitherto modeled GPCR constructs do, typically based on the structure of bacteriorhodopsin (BRD). Moreover, in this model the 7TM helices are arranged differently than they are in any BRD-based model. Thus, the topology and geometry of the TM cavity, potentially capable of receiving ligands, is in this model quite different than it is in the earlier models. In the subsequent step, two ligands, the native [arginine8]vasopressin (AVP) and the selective agonist [D-arginine8]vasopressin (DAVP) were inserted, each in two topologically non-equivalent ways, into the TM cavity and the resulting structures were equilibrated and their conformational variabilities tested using CSA as above. The best docking was selected and justified upon consideration of ligand-receptor interactions and structure-activity data. Finally, the amino acid residues were indicated, mainly in TM helices 3-7, as potentially important in both AVP and DAVP docking. Among those Cys112, Val115-Lys116, Gln119, Met123 in helix 3; Glu174 in helix 4; Val206, Ala210, Val213-Phe214 in helix 5; Trp284, Phe287-Phe288, Gln291 in helix 6; and Phe307, Leu310, Ala314 and Asn317 in helix 7 appeared to be the most important ones. Many of these residues are invariant for either the GPCR superfamily or the neurophyseal (vasopressin V2R, V1aR and V1bR and oxytocin OR) subfamily of receptors. Moreover, some of the equivalent residues in V1aR have already been found critical for the ligand affinity.