Expression cloning of cDNA encoding a seven-helix receptor from human placenta with affinity for opioid ligands

The impact of opioids on the transcriptional landscape of human villous trophoblasts

INTRODUCTION

Opioid use disorder (OUD) is implicated in major obstetrical diseases such as fetal growth restriction. Whether or not opioids directly impact placental trophoblast development and function remains unclear. We sought to examine the expression of opioid receptors (OPRs) in villous trophoblasts and the effect of opioids on placental transcriptomics.

METHODS

Trophoblast stem (TS) cells and primary human trophoblast (PHT) cells from healthy term placentas were used to assess OPR expression in conditions that enhance trophoblast stemness vs differentiation. Placental RNAseq was conducted using our retrospective cohorts of pregnant people with OUD vs controls, both without major obstetrical complications. RT-qPCR was used to determine the effect of fentanyl on the expression of putative opioid targets and stemness or differentiation-associated genes in TS and PHT cells.

RESULTS

Three main OPRs, including OPRM1, OPRD1, and OPRK1 were expressed in term PHT cells cultured in the stemness medium, whereas only OPRD1 and OPRK1 were expressed in TS cells. Interestingly, upon induction of differentiation, the expressed OPR mRNAs in TS or in PHT cells were downregulated. We found 286 differentially expressed long RNAs in placentas from the OUD participants vs controls. While three putative opioid targets differed their expression in stemness vs differentiation states of trophoblasts, fentanyl had no effect on their expression or the expression of major stemness or differentiation-relevant genes in TS and PHT cells.

DISCUSSION

Trophoblastic expression of OPRs and opioid RNA targets is impacted by cell differentiation, suggesting differential susceptibility of villous trophoblasts to the effect of opioids.

Contributions of the International Narcotics Research Conference to Opioid Research Over the Past 50 years

The International Narcotics Research Conference (INRC), founded in 1969, has been a successful forum for research into the actions of opiates, with an annual conference since 1971. Every year, scientists from around the world have congregated to present the latest data on novel opiates, opiate receptors and endogenous ligands, mechanisms of analgesic activity and unwanted side effects, etc. All the important discoveries in the opiate field were discussed, often first, at the annual INRC meeting. With an apology to important events and participants not discussed, this review presents a short history of INRC with a discussion of groundbreaking discoveries in the opiate field and the researchers who presented from the first meeting up to the present.

The placenta as a target of opioid drugs†

Opioid drugs are analgesics increasingly being prescribed to control pain associated with a wide range of causes. Usage of pregnant women has dramatically increased in the past decades. Neonates born to these women are at risk for neonatal abstinence syndrome (also referred to as neonatal opioid withdrawal syndrome). Negative birth outcomes linked with maternal opioid use disorder include compromised fetal growth, premature birth, reduced birthweight, and congenital defects. Such infants require lengthier hospital stays necessitating rising health care costs, and they are at greater risk for neurobehavioral and other diseases. Thus, it is essential to understand the genesis of such disorders. As the primary communication organ between mother and conceptus, the placenta itself is susceptible to opioid effects but may be key to understanding how these drugs affect long-term offspring health and potential avenue to prevent later diseases. In this review, we will consider the evidence that placental responses are regulated through an endogenous opioid system. However, maternal consumption of opioid drugs can also bind and act through opioid receptors express by trophoblast cells of the placenta. Thus, we will also discuss the current human and rodent studies that have examined the effects of opioids on the placenta. These drugs might affect placental hormones associated with maternal recognition of pregnancy, including placental lactogens and human chorionic gonadotropin in rodents and humans, respectively. A further understanding of how such drugs affect the placenta may open up new avenues for early diagnostic and remediation approaches.

New challenges in the study of the mammalian tachykinins

There is an expanding repertoire of mammalian tachykinins produced by a variety of tachykinin genes, gene splicing events and peptide processing. Novel tachykinin-binding molecules/receptors are proposed, but only, three tachykinin receptors are identified with certainty. The question remains - do more tachykinin receptors exist or is there just the need to reappraise our understanding of the known receptors? The tachykinin NK1 receptor, the preferred receptor for both substance P and the peripheral SP-like endokinins, exists in several tissue-specific conformations and isoforms and may provide some clues. This review addresses recent advances in this exciting field and raises challenging new concepts.

Discovery of new G protein-coupled receptors for lipid mediators

Successful sequencing of the human genome has opened a new era in the life sciences and has greatly accelerated biomedical research. Among various research endeavors benefiting from established genomic information, one of the most fruitful areas is the research on orphan G protein-coupled receptors (GPCRs). Many intercellular mediators, including peptides, lipids, and other small molecules, have found their GPCRs in the plasma membrane, e.g., relaxin and tyramine. In the past 14 months, more than one dozen papers have been published reporting the finding of intercellular lipid mediators acting on rhodopsin family GPCRs. This review focuses primarily on intercellular lipid mediators and their recently discovered GPCRs.

The human tachykinin NK1 (short form) and tachykinin NK4 receptor: a reappraisal

Excessive secretion of placental neurokinin B into the circulation during the third trimester of pregnancy is seen in women with preeclampsia. To determine a role for neurokinin B, we have used a number of different animal models to ascertain the expression of the three tachykinin receptors (NK1--both short and long forms, NK2 and NK3) and the putative human tachykinin NK4 receptor in the placenta. Human and rat placenta express all three classical tachykinin receptors. However, we failed to reveal the expression of the short tachykinin NK1 receptor or the tachykinin NK4 receptor in any of 24 human tissues examined including the placenta. We conclude that the proposed short form of the tachykinin NK1 receptor is a truncated genomic clone and that the human tachykinin NK4 receptor is in fact, the guinea pig tachykinin NK3 receptor.

Structure and regulation of opioid receptors

Significant advances have been made in understanding the structure, function, and regulation of opioid receptors and endogenous opioid peptides since their discovery approximately 25 years ago. This review summarizes recent studies aimed at identifying key amino acids that confer ligand selectivity to the opioid receptors and that are critical constituents of the ligand binding sites. A molecular model of the delta receptor based on the crystal structure of rhodopsin is presented. Agonist-induced down regulation of opioid receptors is discussed, highlighting recent evidence for the involvement of the ubiquitin/proteasome system in this process.

A regulatory role for neurokinin B in placental physiology and pre-eclampsia

Tachykinin dogma has assumed, so far, that neurokinin B (NKB) is a neuropeptide that is not produced in any peripheral tissue even though its endogenous receptor, NK3, has been found in a number of locations throughout the human body. We have found an abundant source of peripheral NKB in the human and rat placenta. In this review we describe the discovery of NKB in the placenta and examine its possible role in placental physiology and pre-eclampsia (PE). Excessive secretion of placental NKB into the maternal circulation during the third trimester of pregnancy has been found in women suffering from PE. This may provide the key to the cause of the multiple and complex symptoms associated with this potentially life-threatening illness. We also reveal the structural organisation of the human NKB gene for the first time as well as discussing putative mechanisms for its control.

Evidence that the proposed novel human "neurokinin-4" receptor is pharmacologically similar to the human neurokinin-3 receptor but is not of human origin

There have been proposals that the tachykinin receptor classification should be extended to include a novel receptor, the "neurokinin-4" receptor (NK-4R), which has a close homology with the human NK-3 receptor (hNK-3R). We compared the pharmacological and molecular biological characteristics of the hNK-3R and NK-4R. Binding experiments, with (125)I-[MePhe(7)]-NKB binding to HEK 293 cell membranes transiently expressing the hNK-3R (HEK 293-hNK-3R) or NK-4R (HEK 293-NK-4R), and functional studies (Ca(2+) mobilization in the same cells) revealed a similar profile of sensitivity to tachykinin agonists and antagonists for both receptors; i.e., in binding studies with the hNK-3R, MePhe(7)-NKB > NKB > senktide >> NKA = Substance P; with the NK-4R, MePhe(7)-NKB > NKB = senktide >> Substance P = NKA; and with antagonists, SB 223412 = SR 142801 > SB 222200 >> SR 48968 >> CP 99994 for both hNK-3R and NK-4R. Thus, the pharmacology of the two receptors was nearly identical. However, attempts to isolate or identify the NK-4R gene by using various molecular biological techniques were unsuccessful. Procedures, including nested polymerase chain reaction studies, that used products with restriction endonuclease sites specific for either hNK-3R or NK-4R, failed to demonstrate the presence of NK-4R in genomic DNA from human, monkey, mouse, rat, hamster, or guinea pig, and in cDNA libraries from human lung, brain, or heart, whereas the hNK-3R was detectable in the latter libraries. In view of the failure to demonstrate the presence of the putative NK-4R it is thought to be premature to extend the current tachykinin receptor classification.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Neurokinin-3 receptor distribution in rat and human brain: an immunohistochemical study

Autoradiographic and immunohistochemical studies have shown that the neurokinin-3 receptor is widely distributed in the rodent CNS. Expression of the neurokinin-3 receptor in human brain, however, has been debated. These conflicting findings, as well as the poor resolution of autoradiographic images, prompted us to develop a polyclonal antibody against an oligopeptide derived from the carboxy-terminus consensus sequence of both the rat and human neurokinin-3 receptor ([C]ASTTSSFISSPYTSVDEYS, amino acids 434-452 of the rat neurokinin-3 receptor). Western blot analysis of both human and rat brain tissue revealed a major band in the molecular weight range 65,000-67,000, the proposed molecular weight of the neurokinin-3 receptor based on its amino acid sequence and presumed glycosylation state. The distribution of selective high affinity neurokinin-3 receptor agonist [3H]senktide binding and neurokinin-3 receptor immunoreactivity were virtually identical in the brains of male Fischer 344 rats. The highest concentrations of neurokinin-3 receptors were observed in cortical layers IV-V; the basolateral amygdaloid nucleus; the hypothalamic paraventricular, perifornical and supraoptic nuclei; the zona incerta; and the entopeduncular and interpeduncular nuclei. [3H]senktide binding and neurokinin-3 receptor immunoreactivity were compared in homologous cortical areas of the human and rat brain. In contrast to the rat, autoradiographic analysis of normal control human brains (35-75 years) revealed a distinct and predominant superficial cortical labeling in the glia limitans and the cortical layer I. However, neurokinin-3 receptor immunoreactivity could be found not only in the superficial cortical layers, but also on pyramidal neurons and astrocytes in the neuropil and white matter. These findings suggest species differences in both the cellular and anatomical distribution of the neurokinin-3 receptor.

Natural agonist enhancing bis-His zinc-site in transmembrane segment V of the tachykinin NK3 receptor

In the wild-type tachykinin NK3A receptor histidyl residues are present at two positions in TM-V, V:01 and V:05, at which Zn2+ functions as an antagonist in NK1 and kappa-opioid receptors with engineered metal-ion sites. Surprisingly, in the NK3A receptor Zn2+ instead increased the binding of the agonist 125I-[MePhe7]neurokinin B to 150%. [MePhe7]neurokinin B bound to the NK3A receptor in a two-component mode of which Zn2+ eliminated the subnanomolar binding mode but induced a higher binding capacity of the nanomolar binding mode. Signal transduction was not induced by ZnCl2 but 10 microM ZnCl2 enhanced the effect of neurokinin B. Ala-substitution of HisV:01 eliminated the enhancing effect of Zn2+ on peptide binding. It is concluded that physiological concentrations of Zn2+ have a positive modulatory effect on the binding and function of neurokinin B on the NK3A receptor through a bis-His site in TM-V.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Functional expression of a novel human neurokinin-3 receptor homolog that binds [3H]senktide and [125I-MePhe7]neurokinin B, and is responsive to tachykinin peptide agonists

In 1992, Xie et al. identified a cDNA sequence in the expression cloning search for the kappa opioid receptor. When the cDNA was expressed in Cos-7 cells, binding of opioid compounds was observed to be of low affinity and without kappa, mu, or delta selectivity [Xie, G.-X., Miyajima, A. and Goldstein, A. (1992) Proc. Natl. Acad. Sci. USA 89, 4124-4128]. This cDNA was highly homologous to the human neurokinin-3 (NK-3) receptor sequence, and displayed lower homology to NK-1 and NK-2 sequences. This sequence was stably expressed in Chinese hamster ovary cells, which do not express neurokinin receptors naturally, and ligand binding and second messenger characteristics were compared with a human NK-3 receptor. The NK-3 receptor homolog bound [3H] senktide with a Kd of 39 nM, similar to that of the NK-3 receptor. The rank order of tachykinin peptides competing for [3H]senktide binding at the NK-3 receptor homolog was [MePhe7]neurokinin B > senktide > substance P = neurokinin A > neurokinin B. This cell line also bound [125I-MePhe7]neurokinin B; however, neurokinin B was an effective competitor. Tachykinin peptides stimulated both inositol phospholipid hydrolysis and arachidonic acid release at NK-3 and NK-3 receptor homolog cell lines, with similar rank orders of potency of [MePhe7] neurokinin B = neurokinin B = senktide > NKA = substance P. These results indicate that expression of the NK-3 receptor homolog cDNA in the Chinese hamster ovary cell system induces the expression of a receptor site with many similarities but certain key differences from that of the human NK-3 receptor. The results are discussed with reference to the existence of a novel human tachykinin receptor.

Functional characterization by heterologous expression of a novel cloned tachykinin peptide receptor

An orphan receptor resembling the neurokinin 3 tachykinin receptor (NK3), initially claimed to be an atypical opioid receptor, is shown herein to respond potently to the physiological NK3 receptor ligand, neurokinin B. This 'NK4' receptor did not give functional responses in Xenopus oocytes to opioid agonists. However, NK4 receptor activation was inhibited by nanomolar concentrations of dynorphin. The NK4 receptor is therefore a tachykinin receptor which is functionally antagonized by an endogenous opioid peptide.

Cloning and functional expression of a cDNA encoding a human type 2 neuropeptide Y receptor

Neuropeptide Y (NPY) is a 36-amino acid polypeptide that is widely distributed in the central nervous system and periphery. Pharmacological studies have suggested that there are at least three receptor subtypes, Y1, Y2, and Y3. Cloning of the Y1 subtype has been reported previously. Here we report the isolation by expression cloning of a cDNA encoding a human NPY receptor displaying a pharmacology typical of a Y2 receptor. COS-7 cells transfected with the cDNA express high affinity binding sites for NPY, peptide YY, and NPY13-36, whereas [Leu31,Pro34]NPY binds with lower affinity. The receptor is 381 amino acids in length and has seven putative transmembrane regions typical of G-protein-coupled receptors. Comparison of the amino acid sequence of this Y2 receptor to that of the human Y1 receptor indicates that the two receptors are 31% identical at the amino acid level. Northern blot analyses reveal a single 4-kilobase mRNA species and indicate that the messenger RNA is present in many areas of the central nervous system. NPY induced calcium mobilization and inhibited forskolin-stimulated cAMP accumulation in Chinese hamster ovary cells that stably express the Y2 receptor cDNA, indicating that the recombinant Y2 receptor is functionally coupled to second messenger systems.

Endogenous opioid peptides suppress cytokine-mediated upregulation of HIV-1 expression in the chronically infected promonocyte clone U1

Opioid peptides appear to have an immunomodulatory activity. Using the chronically HIV-1-infected promonocyte clone U1, we investigated the effect of endogenous and synthetic opioid agonists on cytokine-induced HIV-1 expression. None of the endogenous or synthetic opioid agonists had an effect on constitutive HIV-1 expression. Opioid agonists such as methionine-enkephalin, dynorphin, and the kappa receptor agonist, U50,488, dose-dependently suppressed (> 40%) interleukin (IL)-6-induced upregulation of HIV-1 expression. Interestingly, opioid receptor antagonists (mu, delta, and kappa types) also inhibited (> 60%) IL-6-induced upregulation of HIV-1 expression. All opioid agonists and antagonists tested only modestly suppressed (< 20%) tumor necrosis factor-alpha-induced upregulation of HIV-1 expression in U1 cell cultures. These data suggest that certain opioid peptides alter an IL-6-induced signal transduction pathway which triggers HIV-1 expression in the chronically infected promonocyte U1.

Primary structure and functional expression of a guinea pig kappa opioid (dynorphin) receptor

A full-length cDNA encoding the guinea pig kappa opioid (dynorphin) receptor has been isolated. The deduced protein contains 380 aa and seven hydrophobic alpha-helices characteristic of the G protein-coupled receptors. This receptor is 90% identical to the mouse and rat kappa receptors, with the greatest level of divergence in the N-terminal region. When expressed in COS-7 cells, the receptor displays high affinity and stereospecificity toward dynorphin peptides and other kappa-selective opioid ligands such as U50, 488. It does not bind the mu- and delta-selective opioid ligands. The expressed receptor is functionally coupled to G protein(s) to inhibit adenylyl cyclase and Ca2+ channels. The guinea pig kappa receptor mRNA is expressed in many brain areas, including the cerebellum, a pattern that agrees well with autoradiographic maps of classical guinea pig kappa binding sites. Species differences in the pharmacology and mRNA distribution between the cloned guinea pig and rat kappa receptors may be worthy of further examination.

Cloning and pharmacological characterization of a rat kappa opioid receptor

A full-length cDNA was isolated from a rat striatal library by using low-stringency screening with two PCR fragments, one spanning transmembrane domains 3-6 of the mouse delta opioid receptor and the other unidentified but homologous to the mouse delta receptor from rat brain. The novel cDNA had a long open reading frame encoding a protein of 380 residues with 59% identity to the mouse delta receptor and topography consistent with a seven-helix guanine nucleotide-binding protein-coupled receptor. COS-1 cells transfected with the coding region of this clone showed high-affinity binding to kappa opioid receptor-selective ligands such as dynorphin A and U-50,488 and also nonselective opioid ligands such as bremazocine, ethylketocyclazocine, and naloxone. Not bound at all (or bound with low affinity) were dynorphin A-(2-13), enantiomers of naloxone and levophanol [i.e., (+)-naloxone and dextrorphan], and selective mu and delta opioid receptor ligands. Activation of the expressed receptor by kappa receptor agonists led to inhibition of cAMP. Finally, in situ hybridization revealed a mRNA distribution in rat brain that corresponded well to the distribution of binding sites labeled with kappa-selective ligands. These observations indicate that we have cloned a cDNA encoding a rat kappa receptor of the kappa 1 subtype.

Molecular cloning and expression of a rat kappa opioid receptor

At least three types of opioid receptors have been identified in the nervous system. In this paper we report molecular cloning and expression of a rat kappa opioid receptor. PCR was performed on double-stranded cDNA derived from poly(A)+ RNA of the rat striatum with primers similar to those of Libert and co-workers [Libert, Parmentier, Lefort, Dinsart, Van Sande, Maenhaut, Simons, Dumont and Vassart (1989) Science 244, 569-572]. One of the PCR products, which had 65% sequence similarity to the mouse delta opioid receptor, was used to screen a rat striatum cDNA library. Two positive clones were isolated and found to be identical. The clone had a 2.1-kb insert, which was termed RKOR-1. RKOR-1 has an open reading frame of 1140 bp and encodes a 380-amino-acid protein. Hydropathy analysis indicates that RKOR-1 has seven putative transmembrane domains with short intra- and extra-cellular loops. Membranes of Cos-7 cells transfected with RKOR-1 exhibited high specific binding for [3H]diprenorphine ([3H]DIP), a non-selective opioid ligand. Naloxone inhibited [3H]DIP binding with stereospecificity. [3H]DIP binding was potently inhibited by selective kappa opioid ligands, with Ki values in the nanomolar or subnanomolar range, but much less potently inhibited by drugs selective for mu or delta receptors. Thus, RKOR-1 represents an opioid receptor with kappa characteristics.

mu opiate receptor: cDNA cloning and expression

mu opiate receptors recognize morphine with high affinity. A 2.1-kb rat brain cDNA whose predicted translation product displays 63% identity with recently described delta and kappa opiate receptor sequences was identified through polymerase chain reaction and cDNA homology approaches. This cDNA recognizes a 10.5-kb mRNA that is expressed in thalamic neurons. COS-cell expression confers naloxonazine-, Na(+)-, and GTP-sensitive binding of mu but not delta or kappa opioid ligands. Expressing cells bind morphine, [D-Ala2,N-methyl-Phe4,glyol5]enkephalin (DAMGO), and [D-Ala2,D-Leu5]enkephalin (DADLE) with nanomolar or subnanomolar affinities, defining a mu opiate receptor that avidly recognizes analgesic and euphoric opiate drugs and opioid peptides.

Regional expression and chromosomal localization of the delta opiate receptor gene

The delta opiate receptor gene has been cloned from the mouse neuroblastoma-rat glioma hybrid cell NG108-15. The clone that we isolated is apparently identical to that reported by Evans et al. [Evans, C. J., Keith, D. E., Jr., Morrison, H., Magendzo, K. & Edwards, R. H. (1992) Science 258, 1952-1955] and essentially identical with that of Kieffer et al. [Kieffer, B. L., Befort, K., Gaveriaux-Ruff, C. & Hirth, C. G. (1992) Proc. Natl. Acad. Sci. USA 89, 12048-12052]. We have found full-length transcripts of the gene in mouse brain but in no other tissues examined. Within the brain the gene is expressed at low levels in many regions but transcripts are found in particularly large amounts in the anterior pituitary and pineal glands. Since these tissues are located outside the blood-brain barrier, opioid peptides easily can reach receptors in these areas from the blood. The gene, which is present as a single copy, has been mapped to the distal region of mouse Chromosome 4.

Solubilization of high-affinity, guanine nucleotide-sensitive mu-opioid receptors from 7315c cell membranes

High-affinity mu-opioid receptors have been solubilized from 7315c cell membranes. Occupancy of the membrane-associated receptors with morphine before their solubilization in the detergent 3-[(3-cholamidopropyl) dimethyl]-1-propane sulfonate was critical for stabilization of the receptor. The solubilized opioid receptor bound [3H]-etorphine with high affinity (KD = 0.304 +/- 0.06 nM; Bmax = 154 +/- 33 fmol/mg of protein). Of the membrane-associated [3H]etorphine binding sites, 40 +/- 5% were recovered in the solubilized fraction. Both mu-selective and non-selective enkephalins competed with [3H]etorphine for the solubilized binding sites; in contrast, delta- and kappa-opioid enkephalins failed to compete with [3H]etorphine for the solubilized binding sites at concentrations of < 1 microM. The mu-selective ligand [3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin also bound with high affinity (KD = 0.79 nM; Bmax = 108 +/- 17 fmol/mg of protein) to the solubilized material. Of the membrane-associated [3H][D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin binding sites, 43 +/- 3% were recovered in the solubilized material. Guanosine 5'-O-(3-thiotriphosphate), GTP, and guanosine 5'-O-(2-thiodiphosphate), but not adenylylimidodiphosphate, diminished [3H][D-Ala2,N-Me-Phe4,Gly5-ol] enkephalin binding in a concentration-dependent manner. Finally, mu-opioid receptors from rat brain membranes were also solubilized in a high-affinity, guanine nucleotide-sensitive state if membrane-associated receptors were occupied with morphine before and during their solubilization with the detergent 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate.

Cloning and functional comparison of kappa and delta opioid receptors from mouse brain

While trying to identify new members of the somatostatin receptor family of G protein-coupled receptors, we isolated cDNAs from a mouse brain library encoding two related receptor-like proteins, designated msl-1 and msl-2, of 380 and 372 amino acids, respectively. There was 61% identity and 71% similarity between the sequences of msl-1 and msl-2. Among members of the G protein-coupled receptor superfamily, the sequences of both msl-1 and msl-1 were most closely related to those of the somatostatin receptors (SSTRs), having approximately 35% identity with the sequence of SSTR1. Transient expression in COS-1 cells showed that msl-1 and msl-2 did not bind somatostatin. Rather they bound opioids selectively and with high affinity and had the pharmacological properties of kappa and delta opioid receptors, respectively. Indeed, the sequence of msl-2 was identical to that of a delta opioid receptor recently cloned by other workers. Functional characterization of kappa/msl-1 and delta/msl-2 opioid receptors showed that they were coupled to G proteins and mediated opioid receptor class-specific agonist inhibition of forskolin-stimulated cAMP formation. RNA blotting studies and in situ hybridization histochemistry showed that kappa opioid receptor mRNA was expressed at high levels in brain in the neocortex, hippocampus, amygdala, medial habenula, hypothalamus (arcuate and paraventricular nuclei), locus ceruleus, and parabrachial nucleus, suggesting that this receptor may play a role in arousal and regulation of autonomic and neuroendocrine functions.

The E. coli envY gene encodes a high affinity opioid binding site

In an attempt to isolate a cDNA encoding an opioid receptor, a cDNA library was constructed in the lambda ZAP vector using NG108-15 mRNA as template. Using an in vitro transcription-translation assay and a sib selection strategy, a single phage was isolated. An RNA transcribed from this cDNA was able to direct in vitro translation of opioid binding sites. The insert was sequenced and comparison with data banks showed a 100% homology with the E. coli envY gene. We assume that the presence of the envY sequence in the NG108-15 cDNA library was due to a contamination of the lambda ZAP vector with E. coli DNA. A search for opioid binding sites on E. coli strains showed that envY+ strains, but not envY- mutants were able to bind opiates. On envY+ cells, the sites are stereospecific, saturable and of high affinity for the opiate ligands. These sites bind opiate agonists and antagonists but neither mu nor delta opioid peptides. In contrast, rabbit reticulocyte lysate primed with RNA transcribed in vitro from the envY sequence elicited the synthesis of an opioid binding site with mixed mu and delta properties. In addition, transfection of the envY sequence into mammalian cells resulted in the expression of opioid binding sites. Depending on the type of cells transfected, these sites were selective for either the mu or delta ligands.

Cloning of a delta opioid receptor by functional expression

Opiate drugs have potent analgesic and addictive properties. These drugs interact with receptors that also mediate the response to endogenous opioid peptide ligands. However, the receptors for opioids have eluded definitive molecular characterization. By transient expression in COS cells and screening with an iodinated analog of the opioid peptide enkephalin, a complementary DNA clone encoding a functional delta opioid receptor has been identified. The sequence shows homology to G protein-coupled receptors, in particular the receptors for somatostatin, angiotensin, and interleukin-8.

The delta-opioid receptor: isolation of a cDNA by expression cloning and pharmacological characterization

A random primed expression cDNA library was constructed from the RNA of NG 108-15 cells. Pools of plasmid DNA were transfected into COS cells, which were screened for their ability to bind 3H-labeled Tyr-D-Thr-Gly-Phe-Leu-Thr, a tritiated agonist for the delta-opioid receptor. A cDNA was isolated that encodes a 371-amino acid-residue protein presenting all the structural characteristics of receptors that interact with guanine nucleotide-binding proteins. Noticeable features are (i) the high hydrophobicity of the encoded protein, (ii) its low sequence similarity to both catecholamine receptors and peptide-binding receptors, although it presents the typical aspartate residue involved in catecholamine binding of the first group and the characteristic short third cytoplasmic loop of the second group. When expressed in COS cells, the receptor exhibits pharmacological properties similar to those of the native receptor: high-affinity binding sites for 3H-labeled Tyr-D-Thr-Gly-Phe-Leu-Thr (Kd = 1.4 nM), stereospecific binding sites for the - enantiomers of levorphanol and naloxone, and the selectivity profile of a delta receptor, as determined by competition experiments with a set of mu-, delta-, and kappa-opioid ligands.

Regulation of an opioid-binding protein in NG108-15 cells parallels regulation of delta-opioid receptors

An opioid-binding protein has recently been purified from bovine brain and cloned, and its cDNA sequence has been obtained. Indirect evidence suggests that this protein has a role in opioid-receptor function. However, because direct testing of its function by expression of its cDNA has not yet been possible and because its structure bears no resemblance to G protein-coupled receptors, the role of this protein in opioid-receptor activity is still in question. An antibody raised to a portion of the predicted amino acid sequence of opioid-binding cell-adhesion molecule (OBCAM) specifically labeled the surface of NG108-15 cells, as visualized by immunofluorescence with confocal microscopy. Furthermore, chronic treatment of these cells with opioid agonist, which down-regulates opioid receptors, reduced OBCAM immunoreactivity (ir). Down-regulation of both opioid receptors and OBCAM-ir was greatest after chronic treatment of NG108-15 cells with delta-opioid agonists, as well as with nonselective agonists such as etorphine, whereas other agonists including [D-Ala2-N-MePhe4-Gly-ol]enkephalin, morphine, levorphanol, dynorphin A-(1-13), and U-50,488H were less effective or ineffective. Chronic treatment of NG108-15 cells with muscarinic agonists had no effect on OBCAM-ir. Furthermore, NG108-15 cells transfected with an antisense construct to OBCAM have a reduced density of opioid-binding sites as well as reduced OBCAM-ir. Taken together, these results strongly suggest that OBCAM has a role in opioid-receptor function in NG108-15 cells.