Disparate ligand-mediated Ca(2+) responses by wild-type, mutant Ser(200)Ala and Ser(204)Ala alpha(2A)-adrenoceptor: G(alpha15) fusion proteins: evidence for multiple ligand-activation binding sites

Neuropharmacological Effects of the Dichloromethane Extract from the Stems of Argemone ochroleuca Sweet (Papaveraceae) and Its Active Compound Dihydrosanguinarine

Argemone ochroleuca Sweet (Papaveraceae) is used in folk medicine as a sedative and hypnotic agent. This study aimed to evaluate the anxiolytic-like, sedative, antidepressant-like, and anticonvulsant activities of a dichloromethane extract of A. ochroleuca stems (AOE), chemically standardized using gas chromatography-mass spectrometry (GC-MS), and its active compound dihydrosanguinarine (DHS). The anxiolytic-like, sedative, antidepressant-like, and anticonvulsant activities of the AOE (0.1-50 mg/kg p.o.) and DHS (0.1-10 mg/kg p.o.) were evaluated using murine models. A possible mechanism for the neurological actions induced by the AOE or DHS was assessed using inhibitors of neurotransmission pathways and molecular docking. Effective dose 50 (ED50) values were calculated by a linear regression analysis. The AOE showed anxiolytic-like activity in the cylinder exploratory test (ED50 = 33 mg/kg), and antidepressant-like effects in the forced swimming test (ED50 = 3 mg/kg) and the tail suspension test (ED50 = 23 mg/kg), whereas DHS showed anxiolytic-like activity (ED50 = 2 mg/kg) in the hole board test. The AOE (1-50 mg/kg) showed no locomotive affectations or sedation in mice. A docking study revealed the affinity of DHS for α2-adrenoreceptors and GABAA receptors. The anxiolytic-like and anticonvulsant effects of the AOE are due to GABAergic participation, whereas the antidepressant-like effects of the AOE are due to the noradrenergic system. The noradrenergic and GABAergic systems are involved in the anxiolytic-like actions of DHS.

Ligands of Adrenergic Receptors: A Structural Point of View

Adrenergic receptors are G protein-coupled receptors for epinephrine and norepinephrine. They are targets of many drugs for various conditions, including treatment of hypertension, hypotension, and asthma. Adrenergic receptors are intensively studied in structural biology, displayed for binding poses of different types of ligands. Here, we summarized molecular mechanisms of ligand recognition and receptor activation exhibited by structure. We also reviewed recent advances in structure-based ligand discovery against adrenergic receptors.

Activation of the α2B adrenoceptor by the sedative sympatholytic dexmedetomidine

The α₂ adrenergic receptors (α₂ARs) are G protein-coupled receptors (GPCRs) that respond to adrenaline and noradrenaline and couple to the Gi/o family of G proteins. α₂ARs play important roles in regulating the sympathetic nervous system. Dexmedetomidine is a highly selective α₂AR agonist used in post-operative patients as an anxiety-reducing, sedative medicine that decreases the requirement for opioids. As is typical for selective αAR agonists, dexmedetomidine consists of an imidazole ring and a substituted benzene moiety lacking polar groups, which is in contrast to βAR-selective agonists, which share an ethanolamine group and an aromatic system with polar, hydrogen-bonding substituents. To better understand the structural basis for the selectivity and efficacy of adrenergic agonists, we determined the structure of the α_2BAR in complex with dexmedetomidine and Go at a resolution of 2.9 Å by single-particle cryo-EM. The structure reveals the mechanism of α₂AR-selective activation and provides insights into Gi/o coupling specificity.

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

Analysis of functional selectivity through G protein-dependent and -independent signaling pathways at the adrenergic α(2C) receptor

Although G protein-coupled receptors (GPCRs) are traditionally categorized as Gs-, Gq-, or Gi/o-coupled, their signaling is regulated by multiple mechanisms. GPCRs can couple to several effector pathways, having the capacity to interact not only with more than one G protein subtype but also with alternative signaling or effector proteins such as arrestins. Moreover, GPCR ligands can have different efficacies for activating these signaling pathways, a characteristic referred to as biased agonism or functional selectivity. In this work our aim was to detect differences in the ability of various agonists acting at the α2C type of adrenergic receptors (α2C-ARs) to modulate cAMP accumulation, cytoplasmic Ca(2+) release, β-arrestin recruitment and receptor internalization. A detailed comparative pharmacological characterization of G protein-dependent and -independent signaling pathways was carried out using adrenergic agonists (norepinephrine, phenylephrine, brimonidine, BHT-920, oxymetazoline, clonidine, moxonidine, guanabenz) and antagonists (MK912, yohimbine). As initial analysis of agonist Emax and EC50 values suggested possible functional selectivity, ligand bias was quantified by applying the relative activity scale and was compared to that of the endogenous agonist norepinephrine. Values significantly different from 0 between pathways indicated an agonist that promoted different level of activation of diverse effector pathways most likely due to the stabilization of a subtly different receptor conformation from that induced by norepinephrine. Our results showed that a series of agonists acting at the α2C-AR displayed different degree of functional selectivity (bias factors ranging from 1.6 to 36.7) through four signaling pathways. As signaling via these pathways seems to have distinct functional and physiological outcomes, studying all these stages of receptor activation could have further implications for the development of more selective therapeutics with improved efficacy and/or fewer side effects.

Remote control of neuronal signaling

A significant challenge for neuroscientists is to determine how both electrical and chemical signals affect the activity of cells and circuits and how the nervous system subsequently translates that activity into behavior. Remote, bidirectional manipulation of those signals with high spatiotemporal precision is an ideal approach to addressing that challenge. Neuroscientists have recently developed a diverse set of tools that permit such experimental manipulation with varying degrees of spatial, temporal, and directional control. These tools use light, peptides, and small molecules to primarily activate ion channels and G protein-coupled receptors (GPCRs) that in turn activate or inhibit neuronal firing. By monitoring the electrophysiological, biochemical, and behavioral effects of such activation/inhibition, researchers can better understand the links between brain activity and behavior. Here, we review the tools that are available for this type of experimentation. We describe the development of the tools and highlight exciting in vivo data. We focus primarily on designer GPCRs (receptors activated solely by synthetic ligands, designer receptors exclusively activated by designer drugs) and microbial opsins (e.g., channelrhodopsin-2, halorhodopsin, Volvox carteri channelrhodopsin) but also describe other novel techniques that use orthogonal receptors, caged ligands, allosteric modulators, and other approaches. These tools differ in the direction of their effect (activation/inhibition, hyperpolarization/depolarization), their onset and offset kinetics (milliseconds/minutes/hours), the degree of spatial resolution they afford, and their invasiveness. Although none of these tools is perfect, each has advantages and disadvantages, which we describe, and they are all still works in progress. We conclude with suggestions for improving upon the existing tools.

Engineering a GPCR-ligand pair that simulates the activation of D(2L) by Dopamine

In the past decade, engineered G-protein-coupled receptors activated solely by synthetic ligands (RASSLs) have been implemented as a new means to study neurotransmission, which is controlled by G-protein-coupled receptors in vitro and in vivo. In this study, we report an engineered dopamine receptor D(2L) F390(6.52)W, which is the first identified RASSL for the dopamine receptor family. The mutant receptor is characterized by a disrupted ligand binding and complete loss of efficacy for the endogenous ligand, dopamine, which is putatively due to a sterically induced perturbation of H-bonding with conserved serine residues in TM5. Based on this model, we rationally developed an aminoindane-derived set of agonists. Because these agonists forgo analogous H-bonding functionalities, their binding energy does not depend on the respective interactions. Binding affinity and potency were optimized by ligand modifications bearing molecular appendages that obviously interact with a secondary recognition site provided by four hydrophobic residues in TM2 and TM3. Thus, the ferrocenyl carboxamide 5b (FAUC 185) was identified as a synthetic agonist that is able to stimulate the mutant receptor in a manner similar to that by which endogenous dopamine activates the D(2L) wild-type receptor. The engineered dopamine receptor D(2L) F390(6.52)W in combination with FAUC 185 (5b) provides a new tool to probe GPCR functions selectively in specific cell populations in vitro and in vivo.

Engineered G-protein Coupled Receptors are Powerful Tools to Investigate Biological Processes and Behaviors

Understanding how discreet tissues and neuronal circuits function in relation to the whole organism to regulate physiological processes and behaviors is a fundamental goal of modern biological science. Powerful and important new tools in this discovery process are modified G-protein coupled receptors (GPCRs) known as ‘Receptors Activated Solely by Synthetic Ligands (RASSLs),’ and ‘Designer Receptors Exclusively Activated by a Designer Drug (DREADDs).’ Collectively, these are GPCRs modified either through rational design or directed molecular evolution, that do not respond to native ligand, but functionally respond only to synthetic ligands. Importantly, the utility of these receptors is not limited to examination of the role of GPCR-coupled effector signal transduction pathways. Due to the near ubiquitous expression of GPCRs throughout an organism, this technology, combined with whole animal transgenics to selectively target expression, has the ability to regulate activity of discreet tissues and neuronal circuits through effector pathway modulation to study function and behavior throughout the organism. Advantages over other systems currently used to modify in vivo function include the ability to rapidly, selectively and reversibly manipulate defined signal transduction pathways both in short term and long term studies, and no need for specialized equipment due to convenient systemic treatment with activating ligand.

Comparative pharmacology of adrenergic alpha(2C) receptors coupled to Ca(2+) signaling through different Galpha proteins

Adrenergic alpha(1), alpha(2) and beta receptors are members of the G-protein-coupled receptor families (GPCRs) mediating physiological responses to adrenaline (epinephrine) and noradrenaline (norepinephrine). Since GPCRs are major targets for potential therapeutic agents, development of robust, reliable and cost effective functional screening methods for these receptors is in the focus of pharmacological research. For this reason, the aim of the present study was to develop an intracellular calcium assay for investigating the pharmacology of the alpha(2C) type of adrenergic receptors (alpha(2C)-AR). Although activation of alpha(2C)-AR is not linked to calcium mobilization, co-expression of these receptors with the chimeric Galpha(qi5) protein, containing the five carboxyl-terminal amino acids from G(i), or promiscuosus Galpha(16) protein can divert receptor signaling to the G(q) pathway generating Ca(2+) release from intracellular stores. In order to assess the functional potency of alpha(2)-AR agonists and antagonists, we established a fluorometric Ca(2+) assay using cell lines stably and constitutively co-expressing alpha(2C)-AR and Galpha(qi5) or Galpha(16) proteins (Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C)). As part of the pharmacological characterization, we measured the changes in cytoplasmic Ca(2+) levels due to activation of the chimeric Galpha(qi5) or Galpha(16) coupled recombinant alpha(2C) receptors as a function of increasing concentration of several agonists (noradrenaline, brimonidine, oxymetazoline, clonidine, moxonidine) and antagonists (MK912, yohimbine). The binding affinities of alpha(2)-AR agonist and antagonists and the inhibition of the forskolin-stimulated cAMP accumulation in alpha(2C)-AR expressing cells were also measured. These results confirmed that the Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C) recombinant systems can be useful for modelling the native G(i)-coupled system. Our results indicate that a plate-reader based fluorometric Ca(2+) assay may be suitable in high-throughput screening for alpha(2C)-AR ligands as well.

Evolving the lock to fit the key to create a family of G protein-coupled receptors potently activated by an inert ligand

We evolved muscarinic receptors in yeast to generate a family of G protein-coupled receptors (GPCRs) that are activated solely by a pharmacologically inert drug-like and bioavailable compound (clozapine-N-oxide). Subsequent screening in human cell lines facilitated the creation of a family of muscarinic acetylcholine GPCRs suitable for in vitro and in situ studies. We subsequently created lines of telomerase-immortalized human pulmonary artery smooth muscle cells stably expressing all five family members and found that each one faithfully recapitulated the signaling phenotype of the parent receptor. We also expressed a G(i)-coupled designer receptor in hippocampal neurons (hM(4)D) and demonstrated its ability to induce membrane hyperpolarization and neuronal silencing. We have thus devised a facile approach for designing families of GPCRs with engineered ligand specificities. Such reverse-engineered GPCRs will prove to be powerful tools for selectively modulating signal-transduction pathways in vitro and in vivo.

Bring your own G protein

G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.

Dissimilar pharmacological responses by a new series of imidazoline derivatives at precoupled and ligand-activated alpha 2A-adrenoceptor states: evidence for effector pathway-dependent differential antagonism

Whereas agonist-directed differential signaling at a single receptor subtype has become an accepted pharmacological concept, distinct behaviors by ligands that are assumed to be antagonists is less documented. The intrinsic activity and capacity of antagonism for a new series of imidazoline-derived adrenergic ligands analogous to dexefaroxan were investigated by measuring two distinct signaling pathways at the recombinant human alpha 2A-adrenoceptor (alpha 2A AR): 1) pertussis toxin-resistant guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding responses mediated by either a recombinant G alpha oCys351Ile or G alpha i2Cys352Ile protein in CHO-K1 cells, and 2) inhibition of cAMP formation in a stably transfected C6-glial cell line. Ligands could be differentiated as inverse agonists [i.e., 2-(4-methoxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851062], neutral antagonists [i.e., 2-(4-hydroxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851057], partial [i.e., 2-(4-chloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821008], and high-efficacy [i.e., 2-(6,7-dichloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821010] agonists at a precoupled alpha 2A AR state in the copresence of a G alpha oCys351Ile protein but not G alpha i2Cys352Ile protein by monitoring [35S]GTP gamma S binding responses. Neither positive nor negative efficacy was observed for these compounds by monitoring the adenylate cyclase pathway at a presumably low-affinity alpha 2A AR state. The capacity of the dexefaroxan analogs to antagonize the (-)-epinephrine-mediated [35S]GTP gamma S binding response at a G alpha oCys351Ile protein was inversely correlated with their magnitude of intrinsic activity and unrelated to their ligand binding affinity for the alpha 2A AR. On the other hand, their capacity to antagonize either (-)-epinephrine or 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline tartrate (UK 14304)-mediated inhibition of forskolin-stimulated cAMP formation was not related with the rank order of antagonist capacity for the (-)-epinephrine-mediated [35S]GTP gamma S binding response. In conclusion, these data demonstrate that certain alpha2 AR ligands that are assumed to be antagonists, may yield dissimilar pharmacological responses, dependent on the investigated agonist-stimulated effector pathway.

Evidence for protean agonism of RX 831003 at alpha 2A-adrenoceptors by co-expression with different G alpha protein subunits

Intrinsic properties of alpha(2) AR ligands were investigated by measuring two distinct signalling pathways via the alpha(2A) AR protein in CHO-K1 cells: (i) a Ca(2+) response mediated by a promiscuous G(alpha 15) protein; and (ii) a pertussis toxin-resistant [(35)S]GTP gamma S binding response mediated by a G(alpha o)Cys(351)Ile protein. The dexefaroxan analogue RX 831003 was virtually without intrinsic activity at the wt alpha(2A) AR via a G(alpha 15) protein, but induced a partial positive Ca(2+) response [pEC(50): 7.79 (0.17), E(max): 38+/-1% vs (-)-adrenaline] at the mutant Thr(373L)ys alpha(2A) AR. RX 831003 displayed a similar potency (pIC(50): 7.68 (0.21) for both the wt (E(max): -18+/-4%) and Thr(373)Lys alpha(2A) AR (E(max): -19+/-4%) inhibition of basal [(35)S]GTP gamma S binding via a G(alpha o)Cys(351)Ile protein. These data indicate that the alpha(2) AR ligand RX 831003 behaves as a protean agonist at the alpha(2A) AR and that its activity is highly dependent on the co-expressed G(alpha) protein subunit.

Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits

The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.

Partial to complete antagonism by putative antagonists at the wild-type alpha(2C)-adrenoceptor based on kinetic analyses of agonist:antagonist interactions

1. Activation of the recombinant human alpha(2C)-adrenoceptor (alpha(2C) AR) by (-)-adrenaline in CHO-K1 cells transiently co-expressing a chimeric G(alpha q/i1) protein induced a rapid, transient Ca(2+) response with a high-magnitude followed by a low-magnitude phase which continued throughout the recorded time period (15 min). 2. Activation of the alpha(2C) AR by various alpha(2) AR agonists revealed the following rank order of high-magnitude Ca(2+) response [E(max) (%) versus 10 microM (-)-adrenaline]: UK 14304 (102+/-4)=talipexole (101+/-3)=(-)-adrenaline (100)=d-medetomidine (98+/-1)>oxymetazoline (81+/-4) reverse similarclonidine (75+/-5). 3. The methoxy- (RX 821002) and ethoxy-derivatives (RX 811059) of idazoxan and the dexefaroxan analogue atipamezole were fully effective as antagonists of both the high- and the low-magnitude Ca(2+) response. However, though acting as full antagonists of the high-magnitude response, the further putative alpha(2) AR antagonists idazoxan (27%), SKF 86466 (29%) and dexefaroxan (59%) reversed the low-magnitude response only partially. 4. In conclusion, kinetic analyses of agonist : antagonist interactions at the alpha(2C) AR demonstrate a wide spectrum of partial to complete antagonism of the low-magnitude Ca(2+) response for structurally related alpha(2) AR ligands.