Functional expression of adrenergic and opioid receptors in Xenopus oocytes: interaction between alpha 2- and beta 2-adrenergic receptors

A nonolfactory shark adenosine receptor activates CFTR with unique pharmacology and structural features

Adenosine receptors (ADORs) are G protein-coupled purinoceptors that have several functions including regulation of chloride secretion via cystic fibrosis transmembrane conductance regulator (CFTR) in human airway and kidney. We cloned an ADOR from Squalus acanthias (shark) that likely regulates CFTR in the rectal gland. Phylogenic and expression analyses indicate that elasmobranch ADORs are nonolfactory and appear to represent extant predecessors of mammalian ADORs. We therefore designate the shark ADOR as the A₀ receptor. We coexpressed A₀ with CFTR in Xenopus laevis oocytes and characterized the coupling of A₀ to the chloride channel. Two-electrode voltage clamping was performed, and current-voltage (I-V) responses were recorded to monitor CFTR status. Only in A₀- and CFTR-coinjected oocytes did adenosine analogs produce a significant concentration-dependent activation of CFTR consistent with its electrophysiological signature. A pharmacological profile for A₀ was obtained for ADOR agonists and antagonists that differed markedly from all mammalian ADOR subtypes [agonists: R-phenyl-isopropyl adenosine (R-PIA) > S-phenyl-isopropyl adenosine (S-PIA) > CGS21680 > N⁶-cyclopentyladenosine (CPA) > 2-chloroadenosine (2ClAdo) > CV1808 = N⁶-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA) > N-ethyl-carboxyl adenosine (NECA); and antagonists: 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > PD115199 > 1,3-dimethyl-8-phenylxanthine (8PT) > CGS15943]. Structures of human ADORs permitted a high-confidence homology model of the shark A₀ core that revealed unique structural features of ancestral receptors. We conclude that 1) A₀ is a novel and unique adenosine receptor ancestor by functional and structural criteria; 2) A₀ likely activates CFTR in vivo, and this receptor activates CFTR in oocytes, indicating an evolutionary coupling between ADORs and chloride secretion; and 3) A₀ appears to be a nonolfactory evolutionary ancestor of all four mammalian ADOR subtypes.

High throughput electrophysiology with Xenopus oocytes

Voltage-clamp techniques are typically used to study the plasma membrane proteins, such as ion channels and transporters that control bioelectrical signals. Many of these proteins have been cloned and can now be studied as potential targets for drug development. The two approaches most commonly used for heterologous expression of cloned ion channels and transporters involve either transfection of the genes into small cells grown in tissue culture or the injection of the genetic material into larger cells. The standard large cells used for the expression of cloned cDNA or synthetic RNA are the egg progenitor cells (oocytes) of the African frog, Xenopus laevis. Until recently, cellular electrophysiology was performed manually by a single operator, one cell at a time. However, methods of high throughput electrophysiology have been developed which are automated and permit data acquisition and analysis from multiple cells in parallel. These methods are breaking a bottleneck in drug discovery, useful in some cases for primary screening as well as for thorough characterization of new drugs. Increasing throughput of high-quality functional data greatly augments the efficiency of academic research and pharmaceutical drug development. Some examples of studies that benefit most from high throughput electrophysiology include pharmaceutical screening of targeted compound libraries, secondary screening of identified compounds for subtype selectivity, screening mutants of ligand-gated channels for changes in receptor function, scanning mutagenesis of protein segments, and mutant-cycle analysis. We describe here the main features and potential applications of OpusXpress, an efficient commercially available system for automated recording from Xenopus oocytes. We show some types of data that have been gathered by this system and review realized and potential applications.

Alpha2C-adrenergic receptors exhibit enhanced surface expression and signaling upon association with beta2-adrenergic receptors

The alpha(2C)-adrenergic receptor (alpha(2C)AR) is known to be poorly trafficked to the cell surface when expressed in a variety of cell types. We tested the hypothesis that the surface expression and signaling of alpha(2C)AR might be enhanced by heterodimerization with other G protein-coupled receptors (GPCRs). Cotransfection of alpha(2C)AR with more than 25 related GPCRs revealed that only coexpression with the beta(2)-adrenergic receptor (beta(2)AR) increased the surface localization of alpha(2C)AR in human embryonic kidney-293 cells. Coimmunoprecipitation of alpha(2C)AR with beta(2)AR confirmed a physical interaction between the two receptors. Confocal microscopy studies demonstrated that alpha(2C)AR expressed alone was mainly intracellular, whereas alpha(2C)AR coexpressed with beta(2)AR was predominantly localized to the plasma membrane. Ligand binding studies revealed a significant increase in alpha(2C)AR binding sites upon coexpression with beta(2)AR, with no apparent change in affinity for alpha(2)AR ligands. Functional assays with the alpha(2)AR-specific agonist brimonidine (UK 14,304) revealed that coexpression of beta(2)AR with alpha(2C)AR enhanced alpha(2C)AR-mediated activation of extracellular signal-regulated kinase 1/2. Furthermore, analyses of agonist-promoted receptor endocytosis demonstrated enhanced alpha(2C)AR internalization in response to alpha(2)AR agonists when alpha(2C)AR and beta(2)AR were coexpressed. In addition, substantial cointernalization of alpha(2C)AR in response to betaAR agonists was observed when alpha(2C)AR was coexpressed with beta(2)AR. These data reveal that alpha(2C)AR can interact with beta(2)AR in cells in a manner that regulates alpha(2C)AR surface expression, internalization, and functionality.

Co-operation of Gsalpha and Gbetagamma in maintaining G2 arrest in Xenopus oocytes

Progesterone-induced oocyte maturation is thought to involve the inhibition of an oocyte adenylyl cyclase and reduction of intracellular cAMP. Our previous studies demonstrated that injection of inhibitors of G protein betagamma complex induces hormone-independent oocyte maturation. In contrast, over-expression of Xenopus Gbeta1 (xGbeta1), alone or together with bovine Ggamma2, elevates oocyte cAMP and inhibits progesterone-induced oocyte maturation. To further investigate the mechanism of Gbetagamma-induced oocyte maturation, we generated a mutant xGbeta1, substituting Asp-228 for Gly (D228G). An equivalent mutation in the mammalian Gbeta1 results in the loss of its ability to activate adenylyl cyclases. Indeed, co-injection of xGbeta1D228G with Ggamma2 failed to increase oocyte cAMP or inhibit progesterone-induced oocyte maturation. To directly demonstrate that oocytes contained a Gbetagamma-regulated adenylyl cyclase, we analyzed cAMP formation in vitro by using oocyte membrane preparations. Purified brain Gbetagamma complexes significantly activated membrane-bound adenylyl cyclase activities. Multiple adenylyl cyclase isoforms were identified in frog oocytes by PCR using degenerate primers corresponding to highly conserved catalytic amino acid sequences. Among these we identified a partial Xenopus adenylyl cyclase 7 (xAC7) that was 65% identical in amino acid sequence to human AC7. A dominant-negative mutant of xAC7 induced hormone-independent oocyte maturation and accelerated progesterone-induced oocyte maturation. Theses findings suggest that xAC7 is a major component of the G2 arrest mechanism in Xenopus oocytes.

Heterodimerization of alpha 2A- and beta 1-adrenergic receptors

beta- and alpha(2)-adrenergic receptors are known to exhibit substantial cross-talk and mutual regulation in tissues where they are expressed together. We have found that the beta(1)-adrenergic receptor (beta(1)AR) and alpha(2A)-adrenergic receptor (alpha(2A)AR) heterodimerize when coexpressed in cells. Immunoprecipitation studies with differentially tagged beta(1)AR and alpha(2A)AR expressed in HEK-293 cells revealed robust co-immunoprecipitation of the two receptors. Moreover, agonist stimulation of alpha(2A)AR was found to induce substantial internalization of coexpressed beta(1)AR, providing further evidence for a physical association between the two receptors in a cellular environment. Ligand binding assays examining displacement of [(3)H]dihydroalprenolol binding to the beta(1)AR by various ligands revealed that beta(1)AR pharmacological properties were significantly altered when the receptor was coexpressed with alpha(2A)AR. Finally, beta(1)AR/alpha(2A)AR heterodimerization was found to be markedly enhanced by a beta(1)AR point mutation (N15A) that blocks N-linked glycosylation of the beta(1)AR as well as by point mutations (N10A/N14A) that block N-linked glycosylation of the alpha(2A)AR. These data reveal an interaction between beta(1)AR and alpha(2A)AR that is regulated by glycosylation and that may play a key role in cross-talk and mutual regulation between these receptors.

The cAMP-dependent kinase pathway does not sensitize the cloned vanilloid receptor type 1 expressed in xenopus oocytes or Aplysia neurons

Capsaicin-activated channels present in sensory neurons are ligand-gated cation channels that largely account for mediating some types of pain. The cAMP-dependent protein kinase (PKA) signal pathway was suggested to mediate the prostaglandin-induced enhancement of capsaicin-evoked inward current (I(CAP)) in rat sensory neurons. It is not clear, however, whether PKA acts directly on the capsaicin-sensitive channel that is responsible for I(CAP). To address this issue, we overexpressed the cloned capsaicin receptor, VR1, in heterologous expression systems such as Xenopus oocytes or Aplysia R2 neuron and stimulated PKA pathways. As a result, activation of PKA by applying either 8-bromo-cAMP or forskolin with 3-isobutyl-1-methylxanthine or through activation of beta(2) adrenergic receptors failed to enhance I(CAP) in oocytes or R2 neurons expressing VR1. Our results raise two possibilities. (1) Direct phosphorylation of VR1 by PKA may not be responsible for the sensitization; instead, phosphorylation of regulatory proteins associated with VR1 would account for the sensitization of I(CAP) evoked by prostaglandin E(2) in dorsal root ganglion (DRG) neurons. (2) DRG neurons may have a different PKA signaling mechanism that is not replicable in Xenopus oocytes or Aplysia R2 neurons.

The alpha2a adrenergic receptor subtype mediates spinal analgesia evoked by alpha2 agonists and is necessary for spinal adrenergic-opioid synergy

Agonists acting at alpha2 adrenergic and opioid receptors have analgesic properties and act synergistically when co-administered in the spinal cord; this synergy may also contribute to the potency and efficacy of spinally administered morphine. The lack of subtype-selective pharmacological agents has previously impeded the definition of the adrenergic receptor subtype(s) mediating these effects. We therefore exploited a genetically modified mouse line expressing a point mutation (D79N) in the alpha2a adrenergic receptor (alpha2aAR) to investigate the role of the alpha2aAR in alpha2 agonist-evoked analgesia and adrenergic-opioid synergy. In the tail-flick test, intrathecal administration of UK 14,304, a nonsubtype-selective alpha2AR agonist, had no analgesic effect in D79N mice, whereas the analgesic potency of morphine (intrathecal) in this assay was not affected by the mutation. The mutation also decreased alpha2-agonist-mediated spinal analgesia and blocked the synergy seen in wild-type mice with both the delta-opioid agonist deltorphin II and the micro-opioid agonist [D-ALA2,N-Me-Phe4, Gly-ol5]-Enkephalin (DAMGO) in the substance P behavioral test. In addition, the potency of spinally administered morphine was decreased in this test, suggesting that activation of descending noradrenergic systems impinging on the alpha2aAR contributes to morphine-induced spinal inhibition in this model. These results demonstrate that the alpha2aAR subtype is the primary mediator of alpha2 adrenergic spinal analgesia and is necessary for analgesic synergy with opioids. Thus, combination therapies targeting the alpha2aAR and opioid receptors may prove useful in maximizing the analgesic efficacy of opioids while decreasing total dose requirements.

Ultra-low doses of naltrexone or etorphine increase morphine's antinociceptive potency and attenuate tolerance/dependence in mice

In previous studies we showed that low (pM) concentrations of naloxone (NLX), naltrexone (NTX) or etorphine selectively antagonize excitatory, but not inhibitory, opioid receptor-mediated functions in nociceptive types of sensory neurons in culture. Cotreatment of these neurons with pM NTX or etorphine not only results in marked enhancement of the inhibitory potency of acutely applied nM morphine [or other bimodally-acting (inhibitory/excitatory) opioid agonists], but also prevents development of cellular manifestations of tolerance and dependence during chronic exposure to microM morphine. These in vitro studies were confirmed in vivo by demonstrating that acute cotreatment of mice with morphine plus a remarkably low dose of NTX (ca. 10 ng/kg) does, in fact, enhance the antinociceptive potency of morphine, as measured by hot-water tail-flick assays. Furthermore, chronic cotreatment of mice with morphine plus low doses of NTX markedly attenuates development of naloxone-precipitated withdrawal-jumping in physical dependence assays. The present study provides systematic dose-response analyses indicating that NTX elicited optimal enhancement of morphine's antinociceptive potency in mice when co-administered (i.p.) at about 100 ng/kg together with morphine (3 mg/kg). Doses of NTX as low as 1 ng/kg or as high as 1 microg/kg were still effective, but to a lesser degree. Oral administration of NTX in the drinking water of mice was equally effective as i.p. injections in enhancing the antinociceptive potency of acute morphine injections and even more effective in attenuating development of tolerance and NLX-precipitated withdrawal-jumping during chronic cotreatment. Cotreatment with a subanalgesic dose of etorphine (10 ng/kg) was equally effective as NTX in enhancing morphine's antinociceptive potency and attenuating withdrawal-jumping after chronic exposure. These studies provide a rationale for the clinical use of ultra-low-dose NTX or etorphine so as to increase the antinociceptive potency while attenuating the tolerance/dependence liability of morphine or other conventional bimodally-acting opioid analgesics.

Interaction of the delta-opioid receptor with GM1 ganglioside: conversion from inhibitory to excitatory mode

Previous studies have shown GM1 ganglioside to play a crucial role in regulating excitatory opioid receptor function, which may underlie some aspects of opioid dependence, tolerance, and supersensitivity. To study the mechanism of this receptor modulation we have employed CHO cells containing a single, transfected opioid receptor of the delta-type. When forskolin was employed to elevate cAMP the reduction affected by 10 microM DADLE was counteracted by preincubation of the cells with GM1. No effect was observed with GD1a, GD1b, GT1b GM3, or the GM1 derivative, GM1-OH. In pertussis toxin-treated cells 10 nM DADLE increased basal levels of cAMP after preincubation with as little as 10 nM GM1. The results suggest conformational alteration of the opioid receptor from a form coupled primarily to G(i)/G(o) to one also capable of interacting with G(s).

mu-opioid receptor regulates CFTR coexpressed in Xenopus oocytes in a cAMP independent manner

The objective of this study was to characterize the signaling mechanisms of the mu-opioid receptor in its coupling to the cystic fibrosis transmembrane conductance regulator (CFTR) when coexpressed in Xenopus oocytes. Because oocytes do not contain endogenous cAMP-regulated ion channels, the cAMP-modulated CFTR was coexpressed with receptors as a 'reporter' channel. Agonist treatment of oocytes coexpressing mu-opioid receptors, beta2-adrenergic receptors and CFTR produced Cl- currents in a dose-related manner and immunocytochemical analysis confirmed receptor expression. These data suggest that opioid agonists could activate adenylyl cyclase in this system to elevate cAMP levels. Heterotrimeric G protein betagamma-subunits acting on adenylyl cyclase type II would increase cAMP levels. The probable presence of adenylyl cyclase type II and other components of opioid signal transduction such as G(i alpha2), were demonstrated by RT-PCR. However, measurement of cAMP levels in individual oocytes by radioimmunoassay showed that opioid agonist application to oocytes expressing mu-opioid receptors, beta2-adrenergic receptors and CFTR did not increase cAMP levels, whereas application of the beta2-adrenergic agonist, isoproterenol, or IBMX alone did increase cAMP levels. Opioid-induced CFTR activation was not affected by either application of the broad spectrum kinase inhibitor, H7, nor by application of the specific PKA inhibitor, KT5720. Injection of free betagamma-subunits, which could activate the endogenous type II cyclase, was unable to produce measurable currents in oocytes expressing the CFTR. These studies indicate that opioid activation of the CFTR is not mediated through a cAMP/PKA pathway, by either betagamma-subunit activation of an adenylyl cyclase type II or promiscuous coupling to G(s alpha).

Etorphine elicits anomalous excitatory opioid effects on sensory neurons treated with GM1 ganglioside or pertussis toxin in contrast to its potent inhibitory effects on naive or chronic morphine-treated cells

The ultra-potent opioid analgesic, etorphine, elicits naloxone-reversible, dose-dependent inhibitory effects, i.e., shortening of the action potential duration (APD) of naive and chronic morphine-treated sensory dorsal root ganglion (DRG) neurons, even at low (pM-nM) concentrations. In contrast, morphine and most other opioid agonists elicit excitatory effects, i.e., APD prolongation, at these low opioid concentrations, require much higher (ca. 0.1-1 microM) concentrations to shorten the APD of naive neurons, and evoke only excitatory effects on chronic morphine-treated cells even at high > 1-10 microM concentrations. In addition to the potent agonist action of etorphine at mu-, delta- and kappa-inhibitory opioid receptors in vivo and on DRG neurons in culture, this opioid has also been shown to be a potent antagonist of excitatory mu-, delta- and kappa-receptor functions in naive and chronic morphine-treated DRG neurons. The present study demonstrates that the potent inhibitory APD-shortening effects of etorphine still occur in DRG neurons tested in the presence of a mixture of selective antagonists that blocks all mu-, delta- and kappa-opioid receptor-mediated functions, whereas addition of the epsilon (epsilon)-opioid-receptor antagonist, beta-endorphin(1-27) prevents these effects of etorphine. Furthermore, after markedly enhancing excitatory opioid receptor functions in DRG neurons by treatment with GM1 ganglioside or pertussis toxin, etorphine shows excitatory agonist action on non-mu-/delta-/kappa-opioid receptor functions in these sensory neurons, in contrast to its usual potent antagonist action on mu-, delta- and kappa-excitatory receptor functions in naive and even in chronic morphine-treated cells which become supersensitive to the excitatory effects of mu-, delta- and kappa-opioid agonists. This weak excitatory agonist action of etorphine on non-mu-/delta-/kappa-opioid receptor functions may account for the tolerance and dependence observed after chronic treatment with extremely high doses of etorphine in vivo.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

Modulatory effects of Gs-coupled excitatory opioid receptor functions on opioid analgesia, tolerance, and dependence

Electrophysiologic studies of opioid effects on nociceptive types of dorsal root ganglion (DRG) neurons in organotypic cultures have shown that morphine and most mu, delta, and kappa opioid agonists can elicit bimodal excitatory as well as inhibitory modulation of the action potential duration (APD) of these cells. Excitatory opioid effects have been shown to be mediated by opioid receptors that are coupled via Gs to cyclic AMP-dependent ionic conductances that prolong the APD, whereas inhibitory opioid effects are mediated by opioid receptors coupled via Gi/Go to ionic conductances that shorten the APD. Selective blockade of excitatory opioid receptor functions by low (ca. pM) concentrations of naloxone, naltrexone, etorphine and other specific agents markedly increases the inhibitory potency of morphine or other bimodally acting agonists and attenuates development of tolerance/dependence. These in vitro studies have been confirmed by tail-flick assays showing that acute co-treatment of mice with morphine plus ultra-low-dose naltrexone or etorphine remarkably enhances the antinociceptive potency of morphine whereas chronic co-treatment attenuates development of tolerance and naloxone-precipitated withdrawal-jumping symptoms.

Mutation of a conserved serine in TM4 of opioid receptors confers full agonistic properties to classical antagonists

The involvement of a conserved serine (Ser196 at the mu-, Ser177 at the delta-, and Ser187 at the kappa-opioid receptor) in receptor activation is demonstrated by site-directed mutagenesis. It was initially observed during our functional screening of a mu/delta-opioid chimeric receptor, mu delta2, that classical opioid antagonists such as naloxone, naltrexone, naltriben, and H-Tyr-Tic[psi,CH2NH]Phe-Phe-OH (TIPPpsi; Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) could inhibit forskolin-stimulated adenylyl cyclase activity in CHO cells stably expressing the chimeric receptor. Antagonists also activated the G protein-coupled inward rectifying potassium channel (GIRK1) in Xenopus oocytes coexpressing the mu delta2 opioid receptor and the GIRK1 channel. By sequence analysis and back mutation, it was determined that the observed antagonist activity was due to the mutation of a conserved serine to leucine in the fourth transmembrane domain (S196L). The importance of this serine was further demonstrated by analogous mutations created in the mu-opioid receptor (MORS196L) and delta-opioid receptor (DORS177L), in which classical opioid antagonists could inhibit forskolin-stimulated adenylyl cyclase activity in CHO cells stably expressing either MORS196L or DORS177L. Again, antagonists could activate the GIRK1 channel coexpressed with either MORS196L or DORS177L in Xenopus oocytes. These data taken together suggest a crucial role for this serine residue in opioid receptor activation.

Identification of a protein that confers calcitonin gene-related peptide responsiveness to oocytes by using a cystic fibrosis transmembrane conductance regulator assay

An expression-cloning strategy was used to isolate a cDNA that encodes a protein that confers calcitonin gene-related peptide (CGRP) responsiveness to Xenopus laevis oocytes. A guinea pig organ of Corti (the mammalian hearing organ) cDNA library was screened by using an assay based on the cystic fibrosis transmembrane conductance regulator (CFTR). The CFTR is a chloride channel that is activated upon phosphorylation; this channel activity was used as a sensor for CGRP-induced activation of intracellular kinases. A cDNA library from guinea pig organ of Corti was screened by using this oocyte-CFTR assay. A cDNA was identified that contained an open reading frame coding for a small hydrophilic protein that is presumed to be either a CGRP receptor or a component of a CGRP receptor complex. This CGRP receptor component protein confers CGRP-specific activation to the CFTR assay, as no activation was detected upon application of calcitonin, amylin, neuropeptide Y, vasoactive intestinal peptide, or beta-endorphin. In situ hybridization demonstrated that the CGRP receptor component protein is expressed in outer hair cells of the organ of Corti and is colocalized with CGRP-containing efferent nerve terminals.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.