Identification of Two Serine Residues Involved in Agonist Activation of the β-Adrenergic Receptor

SER-7b, a constitutively active Galphas coupled 5-HT7-like receptor expressed in the Caenorhabditis elegans M4 pharyngeal motorneuron

Serotonin plays a key role in the regulation of pharyngeal pumping in nematodes. We have isolated a Caenorhabditis elegans cDNA (C09B7.1b, ser-7b) with greatest identity to the 5-HT7 receptor family. Membranes from COS-7 cells expressing SER-7b exhibit saturable [3H]-LSD binding (Kd = 45 nm) that is inhibited by serotonin (5-HT) and tryptamine, but not by other physiological biogenic amines. Expression of SER-7b in COS-7 cells results in dramatic increase in basal cAMP levels over untransfected cells that is dependent on expression level. 5-HT further elevates cAMP levels in a dose-dependent manner (pEC50 = 7.5 +/- 0.5). Mammalian 5-HT7 receptor inverse agonists reduce constitutive activity, with methiothepin the most potent (pIC50 = 7.8 +/- 0.1). Ser-7::GFP transcriptional fusions reveal that SER-7b appears to be expressed solely in the M4 pharyngeal motorneuron after hatching. This is the first report of a Galphas coupled biogenic amine receptor in nematodes and the localization of SER-7b in the M4 pharyngeal motorneuron suggests that SER-7b may play a role in the regulation of pharyngeal pumping.

Molecular mechanisms of ligand-receptor interactions in transmembrane domain V of the alpha2A-adrenoceptor

1. The structural determinants of catechol hydroxyl interactions with adrenergic receptors were examined using 12 alpha2-adrenergic agonists and a panel of mutated human alpha2A-adrenoceptors. The alpha2ASer201 mutant had a Cys --> Ser201 (position 5.43) amino-acid substitution, and alpha2ASer201Cys200 and alpha2ASer201Cys204 had Ser --> Cys200 (5.42) and Ser --> Cys204 (5.46) substitutions, respectively, in addition to the Cys --> Ser201 substitution. 2. Automated docking methods were used to predict the receptor interactions of the ligands. Radioligand-binding assays and functional [35S]GTPgammaS-binding assays were performed using transfected Chinese hamster ovary cells to experimentally corroborate the predicted binding modes. 3. The hydroxyl groups of phenethylamines were found to have different effects on ligand affinity towards the activated and resting forms of the wild-type alpha2A-adrenoceptor. Substitution of Ser200 or Ser204 with cysteine caused a deterioration in the capability of catecholamines to activate the alpha2A-adrenoceptor. The findings indicate that (i) Cys201 plays a significant role in the binding of catecholamine ligands and UK14,304 (for the latter, by a hydrophobic interaction), but Cys201 is not essential for receptor activation; (ii) Ser200 interacts with the meta-hydroxyl group of phenethylamine ligands, affecting both catecholamine binding and receptor activation; while (iii) substituting Ser204 with a cysteine interferes both with the binding of catecholamine ligands and with receptor activation, due to an interaction between Ser204 and the para-hydroxyl group of the catecholic ring.

Application of thin-layer chromatographic data in quantitative structure-activity relationship assay of thiazole and benzothiazole derivatives with H1-antihistamine activity. I

A quantitative structure-activity relationship analysis of H1-antihistamine activity and chromatographic data of 2-[2-(phenylamino)thiazol-4-yl]ethanamine; 2-(2-benzyl-4-thiazolyl)ethanamine; 2-(2-benzhydrylthiazol-4-yl)ethylamine derivative; 2-(1-piperazinyl- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazole derivatives was made. The RP2 thin-layer chromatography (TLC) plates (silica gel RP2 60F254 silanised precoated), impregnated with solutions of selected amino acid mixtures (L-Asp, L-Asn, L-Thr and L-Lys), were used in two developing solvents as hH1R antagonistic interaction models. Using regression analysis, the relationships between chromatographic and biological activity data were found. The correlations obtained in regression analysis for the examined thiazole and benzothiazole derivatives with H1-antihistamine activity [pA2(H1)] represent their interaction with all the proposed biochromatographic models (S1-S7). Some of the calculated equations can be applied to predict the pharmacological activity of new drug candidates. The best multivariate relationships useful in predicting the pharmacological activity of thiazole and benzothiazole derivatives were obtained under the condition of experiment with RP2 TLC plates using the developing solvent acetonitrile-methanol-buffer (40:40:20, v/v). The log P values of particular compounds are extremely important for this kind of activity.

B96Bom encodes a Bombyx mori tyramine receptor negatively coupled to adenylate cyclase

A cDNA encoding a biogenic amine receptor (B96Bom) was isolated from silkworm (Bombyx mori) larvae, and the ligand response of the receptor stably expressed in HEK-293 cells was examined. Tyramine (TA) at 0.1-100 micro m reduced forskolin (10 micro m)-stimulated intracellular cAMP levels by approximately 40%. The inhibitory effect of TA at 1 micro m was abolished by yohimbine and chlorpromazine (each 10 micro m). Although octopamine (OA) also reduced the cAMP levels, the potency was at least two orders of magnitude lower than that of TA. Furthermore, unlabelled TA (IC50 = 5.2 nm) inhibited specific [3H]TA binding to the membranes of B96Bom-transfected HEK-293 cells more potently than did OA (IC50 = 1.4 micro m) and dopamine (IC50 = 1.7 micro m). Taken together with the result of phylogenetic analysis, these findings indicate that the B96Bom receptor is a B. mori TA receptor, which is negatively coupled to adenylate cyclase. The use of this expression system should facilitate physiological studies of TA receptors as well as structure-activity studies of TA receptor ligands.

The unique ligand-binding pocket for the human prostacyclin receptor. Site-directed mutagenesis and molecular modeling

The human prostacyclin receptor is a seven-transmembrane alpha-helical G-protein coupled receptor, which plays important roles in both vascular smooth muscle relaxation as well as prevention of blood coagulation. The position of the native ligand-binding pocket for prostacyclin as well as other derivatives of the 20-carbon eicosanoid, arachidonic acid, has yet to be determined. Through the use of prostanoid receptor sequence alignments, site-directed mutagenesis, and the 2.8-A x-ray crystallographic structure of bovine rhodopsin, we have developed a three-dimensional model of the agonist-binding pocket within the seven-transmembrane (TM) domains of the human prostacyclin receptor. Upon mutation to alanine, 11 of 29 candidate residues within TM domains II, III, IV, V, and VII exhibited a marked decrease in agonist binding. Of this group, four amino acids, Arg-279 (TMVII), Phe-278 (TMVII), Tyr-75 (TMII), and Phe-95 (TMIII), were identified (via receptor amino acid sequence alignment, ligand structural comparison, and computer-assisted homology modeling) as having direct molecular interactions with ligand side-chain constituents. This binding pocket is distinct from that of the biogenic amine receptors and rhodopsin where the native ligands (also composed of a carbon ring and a carbon chain) are accommodated in an opposing direction. These findings should assist in the development of novel and highly specific ligands including selective antagonists for further molecular pharmacogenetic studies of the human prostacyclin receptor.

Site-directed mutagenesis studies on the Drosophila octopamine/tyramine receptor

The cloned Drosophila octopamine/tyramine receptor can be coupled to second messenger pathways in an agonist-specific fashion by the endogenously occurring biogenic amines, octopamine and tyramine, when expressed in Chinese hamster ovary cells. We have mutated to alanine a range of receptor amino acids that could potentially form hydrogen bonds with the beta-hydroxyl group of octopamine based on homologies with alpha- and beta-adrenergic receptor subtypes. After stable expression of the mutant receptors in CHO cells we have compared the ability of octopamine and tyramine to displace [(3)H]yohimbine binding to membrane fractions from the mutant cell lines with their ability to modulate adenylyl cyclase activity in intact cells. The results suggest that none of the mutated amino acids residues, at least in isolation, are likely to be involved in interactions with the beta-hydroxyl group of the octopamine side chain. It is possible that amino acids not mutated in the present study are somehow involved in this interaction. Alternatively, it is also possible that the beta-hydroxyl group of the octopamine side chain is capable of interacting with more than one of the amino acids mutated in the present study.

Protein-based virtual screening of chemical databases. II. Are homology models of G-Protein Coupled Receptors suitable targets?

The aim of the current study is to investigate whether homology models of G-Protein-Coupled Receptors (GPCRs) that are based on bovine rhodopsin are reliable enough to be used for virtual screening of chemical databases. Starting from the recently described 2.8 A-resolution X-ray structure of bovine rhodopsin, homology models of an "antagonist-bound" form of three human GPCRs (dopamine D3 receptor, muscarinic M1 receptor, vasopressin V1a receptor) were constructed. The homology models were used to screen three-dimensional databases using three different docking programs (Dock, FlexX, Gold) in combination with seven scoring functions (ChemScore, Dock, FlexX, Fresno, Gold, Pmf, Score). Rhodopsin-based homology models turned out to be suitable, indeed, for virtual screening since known antagonists seeded in the test databases could be distinguished from randomly chosen molecules. However, such models are not accurate enough for retrieving known agonists. To generate receptor models better suited for agonist screening, we developed a new knowledge- and pharmacophore-based modeling procedure that might partly simulate the conformational changes occurring in the active site during receptor activation. Receptor coordinates generated by this new procedure are now suitable for agonist screening. We thus propose two alternative strategies for the virtual screening of GPCR ligands, relying on a different set of receptor coordinates (antagonist-bound and agonist-bound states).

Mutation of Asn293 to Asp in transmembrane helix VI abolishes agonist-induced but not constitutive activity of the beta(2)-adrenergic receptor

The beta(2)-adrenergic receptor has been shown to display significant constitutive activity (i.e., in the absence of agonist) in addition to agonist-induced activation. Various studies have suggested that a movement in transmembrane helix VI plays a role in activation of various G-protein-coupled receptors. Here we show that a mutation in this domain of the beta(2)-adrenergic receptor abolishes agonist activation but not constitutive activity. An Asn293Asp mutant of the human beta(2)-adrenergic receptor was expressed either transiently in COS-7 cells or stably in Chinese hamster ovary cells. The mutant receptors were unable to couple to G(s), as seen by the lack of high-affinity agonist binding as well as a reduction of the affinities of several agonists correlating with their intrinsic activities. The mutant receptors caused only minimal activation of adenylyl cyclase (2.5% of wild-type activity) and also failed to show agonist-induced phosphorylation by G-protein-coupled receptor kinase 2. In contrast, the mutant receptors were much less affected in their constitutive activity: transient transfection of wild-type and mutant receptors into COS-7 cells caused an increase in intracellular cAMP-levels that was dependent on the level of receptor expression and was maximally 5.4-fold for the mutant and 6.8-fold for the wild-type receptors (67% of wild-type activity). Introduction of the Asn293Asp mutation into a constitutively active mutant receptor did not affect the constitutive activity of this mutant. These results underscore the importance of transmembrane helix VI in controlling agonist-induced activation of the receptor and suggest that constitutive activity is different from agonist-induced activity. Furthermore, they indicate that Asn293 is a key residue in transferring conformational information from the agonist-binding site to the intracellular surface.

Update on current concepts of the molecular basis of beta2-adrenergic receptor signaling

The proposed manner by which beta(2)-adrenergic receptors signal has dramatically changed from earlier concepts that centered on a lock-and-key mechanism in which the receptor acts as a simple switch. We now know that beta(2)-adrenergic receptors spontaneously toggle to an activated state (R*) and that the equilibrium between R (the inactive state) and R* can be altered by ligands. In addition, the R* conformation is likely to consist of multiple subspecies that may favor certain signaling pathways or regulatory events. Changes in agonist structure alter the abundance of certain subspecies of R*. Indeed, multifunctional coupling is common with many G-protein-coupled receptors and can be modulated pharmacologically to attain specific outcomes. In addition to providing the basis for development of new beta-agonists for unique signaling, these properties can be extended such that beta(2)-adrenergic receptors, or highly modified "designer" receptors, can be used for gene therapy with highly specific effects.

Mechanisms of ligand binding and efficacy at the human D2(short) dopamine receptor

Mechanisms of ligand binding and receptor activation for the human D2(short) dopamine receptor have been probed using two homologous series of monohydroxylated and dihydroxylated agonists (phenylethylamines and 2-dipropylaminotetralins). In ligand binding studies, the majority of compounds exhibited competition curves versus [3H]spiperone that were best fitted using a two site binding model. The compounds had different abilities (potencies and maximal effects) to stimulate [35S]GTPgammaS binding and to inhibit forskolin-stimulated cAMP accumulation. From the data it can be concluded that: (i) the ability of an agonist to stabilize receptor/G protein coupling can be used to predict agonist efficacy for some groups of compounds (2-dipropylaminotetralins) but not for others (phenylethylamines); (ii) the receptor may be activated by unhydroxylated compounds; (iii) single hydroxyl groups or pairs of hydroxyl groups on the agonist may contribute to binding affinity, potency and efficacy; and (iv) for the 2-dipropylaminotetralin series two modes of agonist/receptor interaction have been identified associated with different relative efficacy.

Characterization of a tyramine receptor from Caenorhabditis elegans

Octopamine (OA) plays an important role in the regulation of a number of key processes in nematodes, including pharyngeal pumping, locomotion and egg-laying. However, while putative OA receptors can be tentatively identified in the Caenorhabditis elegans database, no OA receptors have been functionally characterized from any nematode. We have isolated two cDNAs, ser-2 and ser-2a, encoding putative C.elegans serotonin/OA receptors (C02D4.2, ser-2). The sequences of these cDNAs differ from that predicted by GeneFinder and lack 42 bp of exon 2. In addition, ser-2a appears to be alternatively spliced and lacks a predicted 23 amino acids in the third intracellular loop. COS-7 cells expressing SER-2 bind [3H]LSD in the low nM range and exhibit Kis for tyramine, octopamine and serotonin of 0.07, 2, and 13.7 micro m, respectively. Significantly, tyramine reduces forskolin-stimulated cAMP levels in HEK293 cells stably expressing SER-2 with an IC50 of about 360 nm, suggesting that SER-2 is a tyramine receptor.

Molecular modeling and site-specific mutagenesis of the histamine-binding site of the histamine H4 receptor

The histamine H4 receptor is a novel G-protein-coupled receptor with a unique pharmacological profile. The distribution of H4 mRNA suggests that it may play a role in the regulation of immune function, particularly with respect to allergy and asthma. To define the histamine-binding site of this receptor, molecular modeling and site-directed mutagenesis were used to predict and alter amino acids residing in the histamine-binding pocket. The effects of these alterations on histamine binding and receptor activation were then assessed. Our results indicate that Asp94 (3.32) in transmembrane region (TM) 3 and Glu182 (5.46) in TM5 are critically involved in histamine binding. Asp94 probably serves as a counter-anion to the cationic amino group of histamine, whereas Glu182 (5.46) interacts with the N(tau) nitrogen atom of the histamine imidazole ring via an ion pair. In contrast, Thr178 (5.42) and Ser179 (5.43) in TM5 are not significantly involved in either histamine binding or receptor activation. These results resemble those for the analogous residues in the H1 histamine receptor but contrast with findings regarding the H2 histamine receptor. Our results also demonstrate that Asn147 (4.57) in TM4 and Ser320 (6.52) in TM6 play a role in receptor activation but are not involved in histamine binding. Taken together, these data indicate that although histamine seems to bind to the H4 receptor in a fashion similar to that predicted for the other histamine receptor subtypes, there are also important differences that can probably be exploited for the discovery of novel H4-selective compounds.

Discovery of an ectopic activation site on the M(1) muscarinic receptor

Receptors have well-conserved regions that are recognized and activated by hormones and neurotransmitters. Most drugs bind to these sites and mimic or block the action of the native ligands. Using a high-throughput functional screen, we identified a potent and selective M(1) muscarinic receptor agonist from a novel structural class. Using a series of chimeric receptors, we demonstrated that this ligand activates the receptor through a region that is not conserved among receptor subtypes, explaining its unprecedented selectivity. This region of the receptor is distinct from the conserved region that is recognized by traditional ligands. The finding that receptors for small-molecule transmitters can have multiple, structurally distinct activation sites has broad implications for the study of receptor structure/function and the potential for the discovery of novel ligands with high selectivity.

The role of transmembrane helix 5 in agonist binding to the human H3 receptor

We have used alanine scanning mutagenesis to identify residues in transmembrane domain 5 of the histamine H3 receptor that are important for agonist binding. All of the mutants generated were functionally expressed as demonstrated by their ability to bind [(125)I]iodoproxyfan with comparable affinity to the wild-type receptor and their ability to inhibit forskolin-stimulated cAMP formation when activated by histamine. Many mutations produced small changes in the potency of histamine, but the most pronounced reduction in potency and affinity of the agonists, histamine, R-alpha-methylhistamine, imetit, and impentamine, was seen with mutation of glutamate 206. Our modeling suggests that this residue plays a key role in ligand binding by interacting with the imidazole ring of histamine. Interestingly, L199A greatly reduced agonist potency in functional assays but had only minor effects on agonist affinity, implicating a role for this residue in the mechanism of receptor activation. We also studied the functional effects of the mutations by linking the receptor to calcium signaling using a chimeric G protein. A comparison of the two functional assays demonstrated contrasting effects on agonist activity. Histamine, imetit, and impentamine were full agonists in the cAMP assay, but imetit exhibited only partial agonist activity through the chimeric G protein. Furthermore, impentamine, another potent agonist in the cAMP assay, was only able to activate the E206A mutant in the calcium assay despite being inactive at the wild-type receptor. These observations suggest that the agonist receptor complexes formed by these three different H3 agonists are not conformationally equivalent.

The binding site of aminergic G protein-coupled receptors: the transmembrane segments and second extracellular loop

In the current chapter, we review approaches to the identification of the residues forming the binding sites for agonists, antagonists, and allosteric modulators in the family of aminergic G protein-coupled receptors (GPCRs). We then review the structural bases for ligand binding and pharmacological specificity based on the application of these methods to muscarinic cholinergic, adrenergic, dopaminergic, serotonergic, and histaminergic receptors, using the high resolution rhodopsin structure as a template. Furthermore, we propose a critical role of the second extracellular loop in forming the binding site for small molecular weight aminergic ligands, much as this loop dives down into the binding-site crevice and contacts retinal in rhodopsin.

Mutations inducing divergent shifts of constitutive activity reveal different modes of binding among catecholamine analogues to the beta(2)-adrenergic receptor

1. We compared the changes in binding energy generated by two mutations that shift in divergent directions the constitutive activity of the human beta(2) adrenergic receptor (beta(2)AR). 2. A constitutively activating mutant (CAM) and the double alanine replacement (AA mutant) of catechol-binding serines (S204A, S207A) in helix 5 were stably expressed in CHO cell lines, and used to measure the binding affinities of more than 40 adrenergic ligands. Moreover, the efficacy of the same group of compounds was determined as intrinsic activity for maximal adenylyl cyclase stimulation in wild-type beta(2)AR. 3. Although the two mutations had opposite effects on ligand affinity, the extents of change were in both cases largely correlated with the degree of ligand efficacy. This was particularly evident if the extra loss of binding energy due to hydrogen bond deletion in the AA mutant was taken into account. Thus the data demonstrate that there is an overall linkage between the configuration of the binding pocket and the intrinsic equilibrium between active and inactive receptor forms. 4. We also found that AA mutation-induced affinity changes for catecholamine congeners gradually lacking ethanolamine substituents were linearly correlated to the loss of affinity that such modifications of the ligand cause for wild-type receptor. This indicates that the strength of bonds between catechol ring and helix 5 is critically dependent on the rest of interactions of the beta-ethanolamine tail with other residues of the beta(2)-AR binding pocket.

Beta(1)-selective agonist (-)-1-(3,4-dimethoxyphenetylamino)-3-(3,4-dihydroxy)-2-propanol [(-)-RO363] differentially interacts with key amino acids responsible for beta(1)-selective binding in resting and active states

(-)-1-(3,4-Dimethoxyphenetylamino)-3-(3,4-dihydroxy)-2-propanol [(-)-RO363] is a highly selective beta(1)-adrenergic receptor (beta(1)AR) agonist. To study the binding site of beta(1)-selective agonist, chimeric beta(1)/beta(2)ARs and Ala-substituted beta(1)ARs were constructed. Several key residues of beta(1)AR [Leu(110) and Thr(117) in transmembrane domain (TMD) 2], and Phe(359) in TMD 7] were found to be responsible for beta(1)-selective binding of (-)-RO363, as determined by competitive binding. Based on these results, we built a three-dimensional model of the binding domain for (-)-RO363. The model indicated that TMD 2 and TMD 7 of beta(1)AR form a binding pocket; the methoxyphenyl group of N-substituent of (-)-RO363 seems to locate within the cavity surrounded by Leu(110), Thr(117), and Phe(359). The amino acids Leu(110) and Phe(359) interact with the phenyl ring of (-)-RO363, whereas Thr(117) forms hydrogen bond with the methoxy group of (-)-RO363. To examine the interaction of these residues with beta(1)AR in an active state, each of the amino acids was changed to Ala in a constitutively active (CA)-beta(1)AR mutant. The degree of decrease in the affinity of CA-beta(1)AR for (-)-RO363 was essentially the same as that of wild-type beta(1)AR when mutated at Leu(110) and Thr(117). However, the affinity was decreased in Ala-substituted mutant of Phe(359) compared with that of wild-type beta(1)AR. These results indicated that Leu(110) and Thr(117) are necessary for the initial binding of (-)-RO363 with beta(1)-selectivity, and interaction of Phe(359) with the N-substituent of (-)-RO363 in an active state is stronger than in the resting state.

Dopamine induces intracellular Ca2+ signals mediated by alpha1B-adrenoceptors in rat pineal cells

We have studied the functional interaction of dopamine with alpha1-adrenoceptor subtypes by measuring intracellular Ca2+ levels in pineal cells, a cell type where adrenoceptors are well characterized. We show that dopamine induces transient intracellular Ca2+ signals in only 70% of cells responding to phenylephrine. Dopamine-induced Ca2+ signals desensitise faster than Ca2+ transients elicited with phenylephrine and are selectively blocked by desipramine, imipramine, and alpha1B-adrenoceptor antagonists. These results suggest that dopamine induced Ca2+ signals are mainly due to the activation of one subtype of alpha-adrenoceptor, the alpha1B.

Assessment of the intraocular pressure-lowering activity of bicyclic derivatives of 1-substituted benzyloctahydroisoquinoline

In our study of IOP-lowering agents, we have synthesized several bicyclic analogs of 1-benzyloctahydroisoquinoline. The target molecules were synthesized in an eleven-step process. Structures were proved through spectrometry, elemental analysis and, in selected cases, high resolution mass spectrometry. The final products were secondary or tertiary amines containing a 1-benzyl moiety substituted at the p-position with a methoxy, methyl or chloro group. All target molecules were analyzed in 1% solution in distilled water in normotensive rabbits. After topical administration, IOP was monitored in both eyes for up to seven hours. The 1-p-methoxybenzyl molecule 2 was the most active, and caused a maximal IOP drop of 8.8 +/- 1.9 (n = 7) mm Hg in the ipsilateral eye at 4 hours post-administration, with only partial recovery at seven hours. All other compounds tested either showed very weak activity (3-6) or were inactive (1). All compounds produced a contralateral effect, and 5 induced rebound ocular hypertension. We conclude that selected tertiary bicyclic 1-p-methoxybenzyl-octahydroisoquinolines, particularly N-methylated structures, exhibit a significant IOP-lowering effect in normotensive rabbits.

Beta-adrenoceptors: three-dimensional structures and binding sites for ligands

Recent progress in analyzing the structures and functions of G-protein coupled receptors (GPCRs) including beta-adrenoceptors (beta-ARs) has been made by pharmacological, physiological and molecular biological techniques. The three-dimensional (3D) structures, interaction sites with ligands and conformational changes of these receptor subtypes due to ligand binding are now better understood by the simulation of these receptors using computer-aided molecular modeling. Based on these techniques, numbers and conformations of amino acid sequences of each subtype (beta1-, beta2- and beta3-ARs) were defined and also interaction sites or modes of interaction between ligands and beta-ARs could be analyzed three-dimensionally. In addition, simulation of 3D structures of beta-ARs by molecular modeling could clearly determine the limited size, space or pocket for fitting with ligands. These studies will give some clues for the clarification of other GPCRs. Thus, this review summarizes current findings on chemical structures of ligands, amino acid sequences, 3D structures and important amino acids of beta-AR subtypes for interacting with ligands obtained from mutagenesis, chimeric studies and molecular modeling techniques.

Modeling the 3D structure of GPCRs from sequence

G-protein-coupled receptors (GPCRs) are a large and functionally diverse protein superfamily, which form a seven transmembrane (TM) helices bundle with alternating extra-cellular and intracellular loops. GPCRs are considered to be one of the most important groups of drug targets because they are involved in a broad range of body functions and processes and are related to major diseases. In this paper we present a new technology, named PREDICT, for modeling the 3D structure of any GPCR from its amino acid sequence. This approach takes into account both internal protein properties (i.e., the amino acid sequence) and the properties of the membrane environment. Unlike competing approaches, the new technology does not rely on the single known structure of rhodopsin, and is thus capable of predicting novel GPCR conformations. We demonstrate the capabilities of PREDICT in reproducing the known experimental structure of rhodopsin. In principle, PREDICT-generated models offer new opportunities for structure-based drug discovery towards GPCR targets.

Modification of the beta 2-adrenergic receptor to engineer a receptor-effector complex for gene therapy

Depressed G-protein-coupled receptor (GPCR) signaling has been implicated as a component of the pathophysiology of a number of complex diseases including heart failure and asthma, and augmentation or restoration of signaling by various means has been shown to improve organ function. Because some properties of native GPCRs are disadvantageous for ectopic therapeutic expression, we utilized the beta(2)-adrenergic receptor (beta(2)AR) as a scaffold to construct a highly modified therapeutic receptor-effector complex (TREC) suitable for gene therapy. Altogether, 19 modifications were made to the receptor. The ligand-binding site was re-engineered in TM-3 so that a beta-hydroxylmethyl side chain acts as a proton donor for the binding of a novel ligand. In addition, sites critical for agonist-promoted down-regulation in the amino terminus and for phosphorylation by GPCR kinases, and protein kinases A and C, in the third intracellular loop and the carboxyl terminus of the receptor were altered. These modifications of the receptor resulted in depressed agonist-stimulated adenylyl cyclase activity (26.8 +/- 2.1 versus 41.4 +/- 8 pmol/min/mg for wild-type beta(2)AR). This was fully restored by fusing the carboxyl terminus of the modified receptor to G alpha(s) (43.3 +/- 2.7 pmol/min/mg). The fully modified fused receptor was not activated by beta-agonists but rather by a nonbiogenic amine agonist that itself failed to activate the wild-type beta(2)AR. This two-way selectivity thus provides targeted activation based on physiologic status. Furthermore, the TREC did not display tachyphylaxis to prolonged agonist exposure (desensitization was 1 +/- 5% versus 55 +/- 4% for wild-type beta(2)AR). Thus, despite extensive alterations in regions of conformational lability, the beta(2)AR can be tailored to have optimal signaling characteristics for gene therapy. As a general paradigm, TRECs for enhancement of other G-protein signaling appear to be feasible for modification of other pathologic states.

Modeling and mutational analysis of a putative sodium-binding pocket on the dopamine D2 receptor

A homology model of the dopamine D2 receptor was constructed based on the crystal structure of rhodopsin. A putative sodium-binding pocket identified in an earlier model (PDB ) was revised. It is now defined by Asn-419 backbone oxygen at the apex of a pyramid and Asp-80, Ser-121, Asn-419, and Ser-420 at each vertex of the planar base. Asn-423 stabilizes this pocket through hydrogen bonds to two of these residues. Highly conserved Asn-52 is positioned near the sodium pocket, where it hydrogen-bonds with Asp-80 and the backbone carbonyl of Ser-420. Mutation of three of these residues, Asn-52 in helix 1, Ser-121 in helix 3, and Ser-420 in helix 7, profoundly altered the properties of the receptor. Mutants in which Asn-52 was replaced with Ala or Leu or Ser-121 was replaced with Leu exhibited no detectable binding of radioligands, although receptor immunoreactivity in the membrane was similar to that in cells expressing the wild-type D2L receptor. A mutant in which Asn-52 was replaced with Gln, preserving hydrogen-bonding capability, was similar to D2L in affinity for ligands and ability to inhibit cAMP accumulation. Mutants in which either Ser-121 or Ser-420 was replaced with Ala or Asn had decreased affinity for agonists (Ser-121), but increased affinity for the antagonists haloperidol and clozapine. Interestingly, the affinity of these Ser-121 and Ser-420 mutants for substituted benzamide antagonists showed little or no dependence on sodium, consistent with our hypothesis that Ser-121 and Ser-420 contribute to the formation of a sodium-binding pocket.

Phe-308 and Phe-312 in transmembrane domain 7 are major sites of alpha 1-adrenergic receptor antagonist binding. Imidazoline agonists bind like antagonists

Although agonist binding in adrenergic receptors is fairly well understood and involves residues located in transmembrane domains 3 through 6, there are few residues reported that are involved in antagonist binding. In fact, a major docking site for antagonists has never been reported in any G-protein coupled receptor. It has been speculated that antagonist binding is quite diverse depending upon the chemical structure of the antagonist, which can be quite different from agonists. We now report the identification of two phenylalanine residues in transmembrane domain 7 of the alpha(1a)-adrenergic receptor (Phe-312 and Phe-308) that are a major site of antagonist affinity. Mutation of either Phe-308 or Phe-312 resulted in significant losses of affinity (4-1200-fold) for the antagonists prazosin, WB4101, BMY7378, (+) niguldipine, and 5-methylurapidil, with no changes in affinity for phenethylamine-type agonists such as epinephrine, methoxamine, or phenylephrine. Interestingly, both residues are involved in the binding of all imidazoline-type agonists such as oxymetazoline, cirazoline, and clonidine, confirming previous evidence that this class of ligand binds differently than phenethylamine-type agonists and may be more antagonist-like, which may explain their partial agonist properties. In modeling these interactions with previous mutagenesis studies and using the current backbone structure of rhodopsin, we conclude that antagonist binding is docked higher in the pocket closer to the extracellular surface than agonist binding and appears skewed toward transmembrane domain 7.

Dopamine signaling in Caenorhabditis elegans-potential for parkinsonism research

The nematode Caenorhabditis elegans is an attractive model system for the study of many biological processes. It possesses a simple nervous system with known anatomy and connectivity, is conveniently and cheaply cultured in the laboratory, and is amenable to many genetic manipulations that are impossible in mammalian systems. The recent completion of the C. elegans genome sequence provides a rich resource of genomic and bioinformatic data to researchers in diverse fields. This organism, however, has been underexploited in the studies of many basic processes related to nervous system function, neuropsychiatric disorders and neuromuscular function. Anatomical, biochemical, behavioral, pharmacological and genetic evidence accumulated to date strongly suggests that dopamine is used as a neurotransmitter by C. elegans, and that its effects are mediated through pathway(s) that share many features with those of mammals. DNA sequence analysis reveals genes highly homologous to those encoding mammalian dopamine receptors. Probably, C. elegans has dopamine receptors that transduce environmental cues into behaviors, and these receptors pharmacologically most closely resemble the D2 family. Here we present a review of the current state of research into the dopamine system of the worm, focussing on its potential for use in the study of biological processes related to parkinsonism.

The Ile164 beta(2)-adrenoceptor polymorphism alters salmeterol exosite binding and conventional agonist coupling to G(s)

beta(2)-adrenoceptors (beta(2)AR) are polymorphic at amino acid 164 (Thr or Ile) of the fourth transmembrane domain. In transfected fibroblasts, six agonists commonly used in the treatment of bronchospasm were studied. Isoproterenol, albuterol, metaproterenol, terbutaline, formoterol, and salmeterol displayed decreased binding affinities (K(i)s were 1.2-3.0-fold higher) and a significant degree of impaired maximal stimulation of adenylyl cyclase ( approximately 40%), was observed with all agonists for the Ile164 receptor. The ratios of signal transduction efficiencies (Tau function, Ile164/Thr164) varied from a low of 0.17 for terbutaline to 0.49 for salmeterol. In addition, Ile164 bound salmeterol at the exosite, as delineated in perfusion washout studies, at a decreased level (31+/-4.8% vs. 49+/-4.4% retained salmeterol, respectively, P=0.02). In cAMP production studies under perfusion conditions, this decreased exosite binding caused a approximately 50% decrease in the duration of action of salmeterol at Ile164 (t(1/2)=21.0+/-3.6 vs. 46.8+/-4.1 min for Thr164, P=0.001). The durations of action for isoproterenol and formoterol under similar perfusion conditions were not different between the two receptors. These in vitro results indicate the Ile164 polymorphic receptor represents a pharmacogenetic locus for the most commonly utilized agonists in the treatment of asthma with a unique phenotype for salmeterol.

Molecular mechanism for agonist-promoted alpha(2A)-adrenoceptor activation by norepinephrine and epinephrine

We present a mechanism for agonist-promoted alpha(2A)-adrenergic receptor (alpha(2A)-AR) activation based on structural, pharmacological, and theoretical evidence of the interactions between phenethylamine ligands and alpha(2A)-AR. In this study, we have: 1) isolated enantiomerically pure phenethylamines that differ both in their chirality about the beta-carbon, and in the presence/absence of one or more hydroxyl groups: the beta-OH and the catecholic meta- and para-OH groups; 2) used [(3)H]UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; agonist] and [(3)H]RX821002 [2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline; antagonist] competition binding assays to determine binding affinities of these ligands to the high- and low-affinity forms of alpha(2A)-AR; 3) tested the ability of the ligands to promote receptor activation by measuring agonist-induced stimulation of [(35)S]GTPgammaS binding in isolated cell membranes; and 4) used automated docking methods and our alpha(2A)-AR model to predict the binding modes of the ligands inside the alpha(2A)-AR binding site. The ligand molecules are sequentially missing different functional groups, and we have correlated the structural features of the ligands and ligand-receptor interactions with experimental ligand binding and receptor activation data. Based on the analysis, we show that structural rearrangements in transmembrane helix (TM) 5 could take place upon binding and subsequent activation of alpha(2A)-AR by phenethylamine agonists. We suggest that the following residues are important in phenethylamine interactions with alpha(2A)-AR: Asp113 (D(3.32)), Val114 (V(3.33)), and Thr118 (T(3.37)) in TM3; Ser200 (S(5.42)), Cys201 (C(5.43)), and Ser204 (S(5.46)) in TM5; Phe391 (F(6.52)) and Tyr394 (Y(6.55)) in TM6; and Phe411 (F(7.38)) and Phe412 (F(7.39)) in TM7.

Serine residues 110 and 114 are required for agonist binding but not antagonist binding to the melatonin MT(1) receptor

Site-directed mutation of serine 110 (Ser(3.35)) and serine 114 (Ser(3.39)) in the human melatonin MT(1) receptor to alanine residues reduced ligand binding affinities of seven known melatonin receptor agonists and partial agonists by 3- to 15-fold. These mutants also displayed a relative reduction in their affinities for melatonin-mediated functional responses of 30- and 14-fold, respectively. In contrast to the observed effects of the agonists and partial agonists, the melatonin receptor antagonist luzindole was found to bind to mutants Ser(3.35)Ala and Ser(3.39)Ala with affinities equivalent to that determined for the wild-type melatonin MT(1) receptor. Luzindole was subsequently confirmed as an antagonist of melatonin-mediated functional responses for both mutant receptors. These studies have identified that in the human melatonin MT(1) receptor, Ser(3.35) and Ser(3.39), in transmembrane domain 3, are critical for the formation of the high-affinity ligand binding site for agonists and partial agonists but not for the antagonist luzindole.

Catecholamine-induced vasoconstriction is sensitive to carbonic anhydrase I activation

We studied the relationship between alpha- and beta-adrenergic agonists and the activity of carbonic anhydrase I and II in erythrocyte, clinical and vessel studies. Kinetic studies were performed. Adrenergic agonists increased erythrocyte carbonic anhydrase as follows: adrenaline by 75%, noradrenaline by 68%, isoprenaline by 55%, and orciprenaline by 62%. The kinetic data indicated a non-competitive mechanism of action. In clinical studies carbonic anhydrase I from erythrocytes increased by 87% after noradrenaline administration, by 71% after orciprenaline and by 82% after isoprenaline. The increase in carbonic anhydrase I paralleled the increase in blood pressure. Similar results were obtained in vessel studies on piglet vascular smooth muscle. We believe that adrenergic agonists may have a dual mechanism of action: the first one consists of a catecholamine action on its receptor with the formation of a stimulus-receptor complex. The second mechanism proposed completes the first one. By this second component of the mechanism, the same stimulus directly acts on the carbonic anhydrase I isozyme (that might be functionally coupled with adrenergic receptors), so that its activation ensures an adequate pH for stimulus-receptor coupling for signal transduction into the cell, resulting in vasoconstriction.

The forgotten serine. A critical role for Ser-2035.42 in ligand binding to and activation of the beta 2-adrenergic receptor

Previous work in the beta(2)-adrenergic receptor demonstrated critical interactions between Ser-204 and Ser-207 in the fifth membrane-spanning segment and the meta-OH and para-OH, respectively, of catecholamine agonists (Strader, C. D., Candelore, M. R., Hill, W. S., Sigal, I. S., and Dixon, R. A. (1989) J. Biol. Chem. 264, 13572-13578). Using the substituted cysteine accessibility method in the beta(2)-adrenergic receptor, we have found that in addition to Ser-204 and Ser-207, Ser-203 is also accessible on the surface of the binding-site crevice and is occluded by bound agonist. Mutation of Ser-203 to Ala, Val, or Cys reduced the binding affinity and adenylyl cyclase-activating potency of agonists containing a meta-OH, whereas their affinities and potencies were largely preserved by mutation of Ser-203 to Thr, which maintained an OH at this position. Thus both Ser-203 and Ser-204 appear to interact with the meta-OH of catecholamines, perhaps through a bifurcated H bond. Furthermore, the removal of the OH at position 203 led to a significant loss of affinity of antagonists with nitrogen in their heterocyclic ring structure. The greatest effect was seen with pindolol, a partial agonist, suggesting that a H bond between the heterocyclic ring and Ser-203 may play a role in partial agonism. In contrast, the affinities of antagonists such as propranolol or alprenolol, which have cyclic structures without H-bonding capability, were unaltered after mutation of Ser-203.

A comparison of agonist-specific coupling of cloned human alpha(2)-adrenoceptor subtypes

The agonist-specific coupling properties of the three cloned human alpha(2)-adrenoceptor subtypes have been compared, when expressed at similar levels in Chinese hamster ovary (CHO) cell lines, using noradrenaline and (+/-)-meta-octopamine as agonists. Noradrenaline can couple the receptor to both the inhibition and stimulation of forskolin-stimulated cyclic AMP production in all three receptor subtypes, with the relative strength of the coupling to the pathways varying for each of the receptor subtypes. meta-Octopamine selectively couples the alpha(2A)-adrenoceptor only to the inhibition of forskolin-stimulated cyclic AMP production. However, meta-octopamine couples the alpha(2B)- and alpha(2C)-adrenoceptors to both the inhibition and stimulation of forskolin-stimulated cyclic AMP production. The relative potency of meta-octopamine to noradrenaline varies between the different alpha(2)-adrenoceptor subtypes. The effects of meta-octopamine are around two orders of magnitude less potent than those of noradrenaline on both the alpha(2A)- and alpha(2B)-adrenoceptor subtypes. In contrast, in the case of the alpha(2C)-adrenoceptor, meta-octopamine is only one order of magnitude less potent than noradrenaline in the stimulation of forskolin-stimulated cyclic AMP production and, in addition, is equipotent with noradrenaline in the inhibition of forskolin-stimulated cyclic AMP production and has an increased maximal response. This raises the possibility that meta-octopamine may have physiologically important actions via alpha(2C)-adrenoceptors in vivo. The results show that the modulation of cyclic AMP production occurs in both a subtype- and agonist-specific manner for alpha(2A)-adrenoceptors and in a subtype specific manner for alpha(2B)- and alpha(2C)-adrenoceptors.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

5-ht6 receptors as emerging targets for drug discovery

5-ht6 receptors are the latest serotonin receptors to be identified by molecular cloning. Their high affinity for a wide range of drugs used in psychiatry, coupled with their intriguing distribution in the brain, has stimulated significant interest. Antisense oligonucleotides, antipeptide antibodies, selective radioligands, knockout mice, and selective antagonists of the 5-ht6 receptor have recently become available. Surprisingly, 5-ht6 receptors appear to regulate cholinergic neurotransmission in the brain, rather than the expected interaction as modulators of dopaminergic transmission. This interaction predicts a possible role for 5-ht6 receptor antagonists in the treatment of learning and memory disorders. Furthermore, polymorphisms in the sequence of the 5-ht6 receptor gene may provide a genetic tool to further our understanding of the differential responses of patients to antipsychotic medications.

Recent advances in structure, binding sites with ligands and pharmacological function of beta-adrenoceptors obtained by molecular biology and molecular modeling

The structure, binding sites interacting with ligands and the physiological functions of G-protein coupled beta-adrenoceptors (beta-ARs) are being elucidated by molecular biology and molecular modeling studies. The definition given amino acid sequences of beta-ARs in molecular biology and the analysis of three-dimensional and functional binding sites interacting with ligands by molecular modeling may be important for identifying other functional beta-ARs in various tissues and discovering new drugs. Thus, this review focuses on the interaction sites for receptor-ligand and roles of functional beta-ARs as studied by molecular biology and molecular modeling.

Chimeric melatonin mt1 and melatonin-related receptors. Identification of domains and residues participating in ligand binding and receptor activation of the melatonin mt1 receptor

Melatonin receptors bind and become activated by melatonin. The melatonin-related receptor, despite sharing considerable amino acid sequence identity with melatonin receptors, does not bind melatonin and is currently an orphan G protein-coupled receptor. To investigate the structure and function of both receptors, we engineered a series of 14 chimeric receptor constructs, allowing us to determine the relative contribution of each transmembrane domain to ligand binding and receptor function. Results identified that when sequences encoding transmembrane domains 1, 2, 3, 5, or 7 of the melatonin mt(1) receptor were replaced by the corresponding domains of the melatonin-related receptor, the resultant chimeric receptors all displayed specific 2-[(125)I]iodomelatonin binding. Replacement of sequences incorporating transmembrane domains 4 or 6, however, resulted in chimeric receptors that displayed no detectable 2-[(125)I]iodomelatonin binding. The subsequent testing of a "reverse" chimeric receptor in which sequences encoding transmembrane domains 4 and 6 of the melatonin-related receptor were replaced by the corresponding melatonin mt(1) receptor sequences identified specific 2-[(125)I]iodomelatonin binding and melatonin-mediated modulation of cyclic AMP levels. To further investigate these findings, site-directed mutagenesis was performed on residues within transmembrane domain 6 of the melatonin mt(1) receptor. This identified Gly(258) (Gly(6.55)) as a critical residue required for high affinity ligand binding and receptor function.

Novel aromatic residues in transmembrane domains IV and V involved in agonist binding at alpha(1a)-adrenergic receptors

We examined the role that aromatic residues located in the transmembrane helices of the alpha(1a)-adrenergic receptor play in promoting antagonist binding. Since alpha(1)-antagonists display low affinity binding at beta(2)-adrenergic receptors, two phenylalanine residues, Phe-163 and Phe-187, of the alpha(1a)-AR were mutated to the corresponding beta(2)-residue. Neither F163Q nor F187A mutations of the alpha(1a) had any effect on the affinity of the alpha(1)-antagonists. However, the affinity of the endogenous agonist epinephrine was reduced 12.5- and 8-fold by the F163Q and F187A mutations, respectively. An additive loss in affinity (150-fold) for epinephrine was observed at an alpha(1a) containing both mutations. The loss of agonist affinity scenario could be reversed by a gain of affinity with mutation of the corresponding residues in the beta(2) to the phenylalanine residues in the alpha(1a). We propose that both Phe-163 and Phe-187 are involved in independent aromatic interactions with the catechol ring of agonists. The potency but not the efficacy of epinephrine in stimulating phosphatidylinositol hydrolysis was reduced 35-fold at the F163Q/F187A alpha(1a) relative to the wild type receptor. Therefore, Phe-163 and Phe-187 represent novel binding contacts in the agonist binding pocket of the alpha(1a)-AR, but are not involved directly in receptor activation.

Ligand recognition of serine-cysteine amino acid exchanges in transmembrane domain 5 of alpha2-adrenergic receptors by UK 14,304

Ligand binding of UK 14,304 reveals notable species (i.e., human-rodent) and receptor-subtype differences of alpha2-adrenergic receptors (alpha2-ARs). To study the molecular basis of the selectivity of UK 14,304, we compared a series of conservative serine-cysteine exchange mutants at ligand-accessible positions in transmembrane domain 5 of the human and mouse alpha2A-ARs. UK 14,304 bound with approximately 200-fold higher affinity to the human alpha2A-AR wild-type receptor compared with the human alpha2A-ARSer201 mutant, but only an approximately fivefold difference was seen with the corresponding mouse alpha2A-AR variant. These effects of cysteine-serine exchanges only involved the agonist low-affinity forms of the receptors, as the affinity of [3H]UK 14,304 for the agonist high-affinity receptor populations was not influenced. The apparent affinities of a set of eight structurally diverse alpha2-AR ligands (six agonists and two antagonists) were not influenced significantly by the cysteine-serine exchanges (except for oxymetazoline and yohimbine, with up to nine- and eightfold differences in affinity, respectively). We conclude that position 201 (a) plays a primary role in determining observed subtype/species selectivity of UK 14,304 in competitive antagonist radioligand binding assays and (b) does not determine the subtype selectivity of chlorpromazine.

Biochemical and functional characterization of 1-benzyl substituted trimetoquinol affinity analogs on rat and human beta-adrenoceptors

The site of interaction for the 1-(3',4',5'-trimethoxybenzyl) group of trimetoquinol (TMQ) with beta-adrenoceptors (beta-ARs) is important for the rational design of highly potent and beta3-AR-selective analogs. 1-Benzyl ring-substituted TMQ analogs were evaluated for binding affinities and biochemical activities (cyclic AMP accumulations) in Chinese hamster ovary (CHO) cells expressing the rat and human beta3-AR, and for functional activities on isolated rat tissues. Binding affinities (K1 approximately 0.055 to 1.5 microM) for the rat beta3-AR and potencies for adenylyl cyclase activation (K(act) approximately 0.43 to 2;5 nM) of the 3'-monoiodo or 3',5'-diiodo derivatives with 4'-isothiocyanato-, 4'-amino, 4'-acetamido, or 4'-alpha-haloacetamido substitutions were higher than those of (-)-isoproterenol, and comparable to those of BRL 37344 [(+/-)-(R*,R*-[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]ph enoxy]-acetic acid sodium]. A similar rank order of binding affinities (K(i) approximately 0.11 to 2.5 microM) and potencies (K(act) approximately 0.45 to 9.5 nM) was obtained for TMQ analogs on the human beta3-AR. The 4'-acetamido and 4'-alpha-chloroacetamido analogs of 3',5'-diiodoTMQ were more potent than (-)-isoproterenol in rat atria (beta1-AR) and rat trachea (beta2-AR) and exhibited partial agonist activities, whereas full agonist activities were observed in rat esophageal smooth muscle (EC50 approximately 2-8 nM, beta3-AR). 4'-alpha-Chloroacetamido-3',5'-diiodoTMQ-mediated chronotropic responses in atria were sustained and resistant to washout. Further, the 4'-alpha-chloroacetamido and 4'-alpha-bromoacetamido analogs of 3',5'-diiodoTMQ demonstrated significant concentration-dependent irreversible binding to the rat beta3-AR. Reversible beta-AR agonists such as (-)-isoproterenol, BRL 37344, and 4'-acetamido-3',5'-diiodoTMQ or nucleophilic 1-amino acids (lysine, glutathione, cysteine) did not protect against this irreversible binding. Thus, the lipophilic 1-benzyl ring of TMQ analogs interacts with a hydrophobic region of the beta-AR that may represent an exo-site or an allosteric binding site.

Amtyr1: characterization of a gene from honeybee (Apis mellifera) brain encoding a functional tyramine receptor

Biogenic amine receptors are involved in the regulation and modulation of various physiological and behavioral processes in both vertebrates and invertebrates. We have cloned a member of this gene family from the CNS of the honeybee, Apis mellifera. The deduced amino acid sequence is homologous to tyramine receptors cloned from Locusta migratoria and Drosophila melanogaster as well as to an octopamine receptor cloned from Heliothis virescens. Functional properties of the honeybee receptor were studied in stably transfected human embryonic kidney 293 cells. Tyramine reduced forskolin-induced cyclic AMP production in a dose-dependent manner with an EC50 of approximately 130 nM. A similar effect of tyramine was observed in membrane homogenates of honeybee brains. Octopamine also reduced cyclic AMP production in the transfected cell line but was both less potent (EC50 of approximately 3 microM) and less efficacious than tyramine. Receptor-encoding mRNA has a wide-spread distribution in the brain and subesophageal ganglion of the honeybee, suggesting that this tyramine receptor is involved in sensory signal processing as well as in higher-order brain functions.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Effect of multiple serine/alanine mutations in the transmembrane spanning region V of the D2 dopamine receptor on ligand binding

Three conserved serine residues (Ser193, Ser194, and Ser197) in transmembrane spanning region (TM) V of the D2 dopamine receptor have been mutated to alanine, individually and in combination, to explore their role in ligand binding and G protein coupling. The multiple Ser -->Ala mutations had no effect on the binding of most antagonists tested, including [3H]spiperone, suggesting that the multiple mutations did not affect the overall conformation of the receptor protein. Double or triple mutants containing an Ala197 mutation showed a decrease in affinity for domperidone, whereas Ala193 mutants showed an increased affinity for a substituted benzamide, remoxipride. However, dopamine showed large decreases in affinity (>20-fold) for each multiple mutant receptor containing the Ser193Ala mutation, and the high-affinity (coupled) state of the receptor (in the absence of GTP) could not be detected for any of the multiple mutants. A series of monohydroxylated phenylethylamines and aminotetralins was tested for their binding to the native and multiple mutant D2 dopamine receptors. The results obtained suggest that Ser193 interacts with the hydroxyl of S-5-hydroxy-2-dipropylaminotetralin (OH-DPAT) and Ser197 with the hydroxyl of R-5-OH-DPAT. We predict that Ser193 interacts with the hydroxyl of R-7-OH-DPAT and the 3-hydroxyl (m-hydroxyl) of dopamine. Therefore, the conserved serine residues in TMV of the D2 dopamine receptor are involved in hydrogen bonding interactions with selected antagonists and most agonists tested and also enable agonists to stabilise receptor-G protein coupling.

Modelling G-protein-coupled receptors for drug design

The G-protein coupled receptors form a large and diverse multi-gene superfamily with many important physiological functions. As such, they have become important targets in pharmaceutical research. Molecular modelling and site-directed mutagenesis have played an important role in our increasing understanding of the structural basis of drug action at these receptors. Aspects of this understanding, how these techniques can be used within a drug-design programme, and remaining challenges for the future are reviewed.

beta(1)- and beta(2)-Adrenoceptor antagonist activity of a series of para-substituted N-isopropylphenoxypropanolamines

To further explore the structure-activity relationships of beta-adrenoceptor (beta-AR) antagonists, a series of 25 para-substituted N-isopropylphenoxy-propanolamines were synthesised, nine of which are new compounds. All have been examined for their ability to antagonise beta(1)-ARs in rat atria and beta(2)-ARs in rat trachea. Substitution in the para-position of the phenyl ring is thought to confer beta(3)-specificity and the selectivity of these compounds for the beta(1)-AR ranges from 1.5-234. None of the compounds tested were selective for the beta(2)-AR. Of the 25 compounds studied, 22 had reasonable (pA(2) > 7) potencies for the rat beta(1)-AR. Only compound 1 displayed reasonable (pA(2) > 7) potency for the rat beta(2)-AR. Twenty two compounds were used as the training set for comparative molecular field analysis (CoMFA) of antagonist potency (pA(2)) at the rat beta(1)- and beta(2)-ARs. The inclusion of a number of additional physical characteristics improved the QSAR analysis over models derived solely using the CoMFA electrostatic and steric fields. The final models predicted the beta(1)- and beta(2)-AR potency of the compounds in the training set with high accuracy (r(2) = 0.93 and 0.86 respectively). The final beta(1)-AR model predicted the beta(1)-potencies of two out of the three test compounds, not included in the training set, with residual pA(2) values < -0.14, whereas the test compounds were not as well predicted by our final beta(2)-AR model (residual pA(2) values < -0.38).

Study of interaction between agonists and asn293 in helix VI of human beta(2)-adrenergic receptor

Previously, we demonstrated the involvement of Asn293 in helix VI of the human beta(2)-adrenergic receptor in stereoselective agonist recognition and activation. In the present study, we have further explored the role of this residue by synthesizing derivatives of isoproterenol and clenbuterol, two beta-adrenergic receptor agonists. We analyzed their efficacy and affinity on the wild-type and a mutant receptor (Asn293Leu). Each compound had similar efficacy (tau values) on both the wild-type and mutant receptor, although tau values varied considerably among the eight compounds studied. It appeared that one derivative of isoproterenol, but not of clenbuterol, showed a gain in affinity from the wild type to the mutant receptor. This derivative had a methyl substituent instead of the usual beta-OH group in the aliphatic side chain of isoproterenol, compatible with the more lipophilic nature of the leucine side chain. Such a "gain of function" approach through a combination of synthetic chemistry with molecular biology, may be useful to enhance our insight into the precise atomic events that govern ligand-receptor interactions.