Mutational analysis of the structure and function of opioid receptors

Food-Derived Opioid Peptides in Human Health: A Review

World Health Organization data suggest that stress, depression, and anxiety have a noticeable prevalence and are becoming some of the most common causes of disability in the Western world. Stress-related disorders are considered to be a challenge for the healthcare system with their great economic and social impact. The knowledge on these conditions is not very clear among many people, as a high proportion of patients do not respond to the currently available medications for targeting the monoaminergic system. In addition, the use of clinical drugs is also associated with various side effects such as vomiting, dizziness, sedation, nausea, constipation, and many more, which prevents their effective use. Therefore, opioid peptides derived from food sources are becoming one of the safe and natural alternatives because of their production from natural sources such as animals and plant proteins. The requirement for screening and considering dietary proteins as a source of bioactive peptides is highlighted to understand their potential roles in stress-related disorders as a part of a diet or as a drug complementing therapeutic prescription. In this review, we discussed current knowledge on opioid endogenous and exogenous peptides concentrating on their production, purification, and related studies. To fully understand their potential in stress-related conditions, either as a drug or as a therapeutic part of a diet prescription, the need to screen more dietary proteins as a source of novel opioid peptides is emphasized.

Bioinformatics and evolution of vertebrate nociceptin and opioid receptors

G protein-coupled receptors (GPCRs) are ancestrally related membrane proteins on cells that mediate the pharmacological effect of most drugs and neurotransmitters. GPCRs are the largest group of membrane receptor proteins encoded in the human genome. One of the most famous types of GPCRs is the opioid receptors. Opioid family receptors consist of four closely related proteins expressed in all vertebrate brains and spinal cords examined to date. The three classical types of opioid receptors shown unequivocally to mediate analgesia in animal models and in humans are the mu- (MOR), delta- (DOR), and kappa-(KOR) opioid receptor proteins. The fourth and most recent member of the opioid receptor family discovered is the nociceptin or orphanin FQ receptor (ORL). The role of ORL and its ligands in producing analgesia is not as clear, with both analgesic and hyperalgesic effects reported. All four opioid family receptor genes were cloned from expressed mRNA in a number of vertebrate species, and there are enough sequences presently available to carry out bioinformatic analysis. This chapter presents the results of a comparative analysis of vertebrate opioid receptors using pharmacological studies, bioinformatics, and the latest data from human whole-genome studies. Results confirm our initial hypotheses that the four opioid receptor genes most likely arose by whole-genome duplication, that there is an evolutionary vector of opioid receptor type divergence in sequence and function, and that the hMOR gene shows evidence of positive selection or adaptive evolution in Homo sapiens.

Morphine-induced internalization of the L83I mutant of the rat μ-opioid receptor

BACKGROUND AND PURPOSE

Naturally occurring single-nucleotide polymorphisms (SNPs) within GPCRs can result in alterations in various pharmacological parameters. Understanding the regulation and function of endocytic trafficking of the μ-opioid receptor (MOP receptor) is of great importance given its implication in the development of opioid tolerance. This study has compared the agonist-dependent trafficking and signalling of L83I, the rat orthologue of a naturally occurring variant of the MOP receptor.

EXPERIMENTAL APPROACH

Cell surface elisa, confocal microscopy and immunoprecipitation assays were used to characterize the trafficking properties of the MOP-L83I variant in comparison with the wild-type receptor in HEK 293 cells. Functional assays were used to compare the ability of the L83I variant to signal to several downstream pathways.

KEY RESULTS

Morphine-induced internalization of the L83I MOP receptor was markedly increased in comparison with the wild-type receptor. The altered trafficking of this variant was found to be specific to morphine and was both G-protein receptor kinase- and dynamin-dependent. The enhanced internalization of L83I variant in response to morphine was not due to increased phosphorylation of serine 375, arrestin association or an increased ability to signal.

CONCLUSIONS AND IMPLICATIONS

These results suggest that morphine promotes a specific conformation of the L83I variant that makes it more liable to internalize in response to morphine, unlike the wild-type receptor that undergoes significantly less morphine-stimulated internalization, providing an example of a ligand-selective biased receptor. The presence of this SNP within an individual may consequently affect the development of tolerance and analgesic responses.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Identification of amino acids that account for long-range interactions in two triosephosphate isomerases from pathogenic trypanosomes

For a better comprehension of the structure-function relationship in proteins it is necessary to identify the amino acids that are relevant for measurable protein functions. Because of the numerous contacts that amino acids establish within proteins and the cooperative nature of their interactions, it is difficult to achieve this goal. Thus, the study of protein-ligand interactions is usually focused on local environmental structural differences. Here, using a pair of triosephosphate isomerase enzymes with extremely high homology from two different organisms, we demonstrate that the control of a seventy-fold difference in reactivity of the interface cysteine is located in several amino acids from two structurally unrelated regions that do not contact the cysteine sensitive to the sulfhydryl reagent methylmethane sulfonate, nor the residues in its immediate vicinity. The change in reactivity is due to an increase in the apparent pKa of the interface cysteine produced by the mutated residues. Our work, which involved grafting systematically portions of one protein into the other protein, revealed unsuspected and multisite long-range interactions that modulate the properties of the interface cysteines and has general implications for future studies on protein structure-function relationships.

Discovery of dermorphin-based affinity labels with subnanomolar affinity for mu opioid receptors

A series of potent electrophilic affinity labels (IC(50) = 0.1-5 nM) containing either a bromoacetamide or isothiocyanate based on the mu opioid receptor (MOR) selective peptide dermorphin were prepared. All four analogues exhibited wash resistant inhibition of [(3)H]DAMGO binding at subnanomolar to nanomolar concentrations, suggesting that these analogues bind covalently to MOR. To our knowledge, these peptides are the highest affinity peptide-based affinity labels for MOR reported to date.

Design, synthesis, and biological evaluation of 6alpha- and 6beta-N-heterocyclic substituted naltrexamine derivatives as mu opioid receptor selective antagonists

Opioid receptor selective antagonists are important pharmacological probes in opioid receptor structural characterization and opioid agonist functional study. Thus far, a nonpeptidyl, highly selective and reversible mu opioid receptor (MOR) antagonist is unavailable. On the basis of our modeling studies, a series of novel naltrexamine derivatives have been designed and synthesized. Among them, two compounds were identified as leads based on the results of in vitro and in vivo assays. Both of them displayed high binding affinity for the MOR (K(i) = 0.37 and 0.55 nM). Compound 6 (NAP) showed over 700-fold selectivity for the MOR over the delta receptor (DOR) and more than 150-fold selectivity over the kappa receptor (KOR). Compound 9 (NAQ) showed over 200-fold selectivity for the MOR over the DOR and approximately 50-fold selectivity over the KOR. Thus these two novel ligands will serve as leads to further develop more potent and selective antagonists for the MOR.

Docking studies suggest ligand-specific delta-opioid receptor conformations

An automated docking procedure was used to study binding of a series of delta-selective ligands to three models of the delta-opioid receptor. These models are thought to represent the three ligand-specific receptor conformations. Docking results are in agreement with point mutation studies and suggest that different ligands--agonists and antagonists--may bind to the same binding site under different receptor conformations. Docking to different receptor models (conformations) also suggests that by changing to a receptor-specific conformation, the receptor may open or close different binding sites to other ligands.

Src phosphorylation of micro-receptor is responsible for the receptor switching from an inhibitory to a stimulatory signal

Recent studies have revealed that in G protein-coupled receptor signalings switching between G protein- and beta-arrestin (betaArr)-dependent pathways occurs. In the case of opioid receptors, the signal is switched from the initial inhibition of adenylyl cyclase (AC) to an increase in AC activity (AC activation) during prolonged agonist treatment. The mechanism of such AC activation has been suggested to involve the switching of G proteins activated by the receptor, phosphorylation of signaling molecules, or receptor-dependent recruitment of cellular proteins. Using protein kinase inhibitors, dominant negative mutant studies and mouse embryonic fibroblast cells isolated from Src kinase knock-out mice, we demonstrated that mu-opioid receptor (OPRM1)-mediated AC activation requires direct association and activation of Src kinase by lipid raft-located OPRM1. Such Src activation was independent of betaArr as indicated by the ability of OPRM1 to activate Src and AC after prolonged agonist treatment in mouse embryonic fibroblast cells lacking both betaArr-1 and -2. Instead the switching of OPRM1 signals was dependent on the heterotrimeric G protein, specifically Gi2 alpha-subunit. Among the Src kinase substrates, OPRM1 was phosphorylated at Tyr336 within NPXXY motif by Src during AC activation. Mutation of this Tyr residue, together with mutation of Tyr166 within the DRY motif to Phe, resulted in the complete blunting of AC activation. Thus, the recruitment and activation of Src kinase by OPRM1 during chronic agonist treatment, which eventually results in the receptor tyrosine phosphorylation, is the key for switching the opioid receptor signals from its initial AC inhibition to subsequent AC activation.

Chronic morphine treatment up-regulates mu opioid receptor binding in cells lacking filamin A

We investigated the effects of morphine and other agonists on the human mu opioid receptor (MOP) expressed in M2 melanoma cells, lacking the actin cytoskeleton protein filamin A and in A7, a subclone of the M2 melanoma cells, stably transfected with filamin A cDNA. The results of binding experiments showed that after chronic morphine treatment (24 h) of A7 cells, MOP-binding sites were down-regulated to 63% of control, whereas, unexpectedly, in M2 cells, MOP binding was up-regulated to 188% of control naive cells. Similar up-regulation was observed with the agonists methadone and levorphanol. The presence of antagonists (naloxone or CTAP) during chronic morphine treatment inhibited MOP down-regulation in A7 cells. In contrast, morphine-induced up-regulation of MOP in M2 cells was further increased by these antagonists. Chronic morphine desensitized MOP in A7 cells, i.e., it decreased DAMGO-induced stimulation of GTPgammaS binding. In M2 cells DAMGO stimulation of GTPgammaS binding was significantly greater than in A7 cells and was not desensitized by chronic morphine. Pertussis toxin treatment abolished morphine-induced receptor up-regulation in M2 cells, whereas it had no effect on morphine-induced down-regulation in A7 cells. These results indicate that, in the absence of filamin A, chronic treatment with morphine, methadone or levorphanol leads to up-regulation of MOP, to our knowledge, the first instance of opioid receptor up-regulation by agonists in cell culture.

Changes in opioid receptor proteins during mitochondrial impairment in differentiated SK-N-SH cells

Aging and neurodegenerative diseases are associated with oxidative damage that may contribute to changes in neurosensory processing, including pain. The effects of neuronal oxidation on the opioid receptor system are poorly understood. Earlier, we have reported that 3-nitroproprionic acid (3-NPA)-induced oxidative stress and impairment of mitochondrial energy metabolism significantly reduced the function of mu but not delta opioid receptors [A. Raut, M. Iglewski, A. Ratka, Differential effects of impaired mitochondrial energy production on the function of mu and delta opioid receptors in neuronal SK-N-SH cells, Neurosci. Lett. 404 (2006) 242-246]. In the present study, we studied the effects of 3-NPA-induced oxidative stress on protein levels of the mu, delta, and kappa opioid receptors (MOR, DOR, and KOR, respectively). The opioid-responsive differentiated SK-N-SH neuronal cells were used as an in vitro model. Cells were exposed to 0, 5, 10, and 20mM of 3-NPA for 0, 1, 2, 12, and 24h. After the 3-NPA treatments, plasma membrane preparations were made and used for the Western blot assay. There was a significant reduction in the level of the MOR protein while levels of DOR and KOR proteins remained unaffected after exposure to 3-NPA. These findings demonstrate for the first time that there is a selective impairment of the MOR protein under conditions of mitochondrial oxidative damage at the neuronal level. The reduction in the level of the MOR protein may contribute to the impairment of MOR function under oxidative damage conditions shown in our previous study.

Conformation of a double-membrane-spanning fragment of a G protein-coupled receptor: Effects of hydrophobic environment and pH

Overcoming the problems associated with the expression, purification and in vitro handling of membrane proteins requires an understanding of the factors governing the folding and stability of such proteins in detergent solutions. As a sequel to our earlier report (Biochim. Biophys. Acta 1747(2005), 133-140), we describe an improved purification procedure and a detailed structural analysis of a fragment of the mu-opioid receptor ('TM2-3') that comprises the second and third transmembrane segments and the extracellular loop that connects them. Circular dichroism (CD) spectroscopy of TM2-3 in 2,2,2-trifluoroethanol gave a helical content similar to that predicted by published homology models, while spectra acquired in several detergents showed significantly lower helical contents. This indicates that this part of the mu-opioid receptor has an intrinsic propensity to be highly helical in membrane-like environments, but that in detergent solutions, this helical structure is not fully formed. Proteolysis of TM2-3 with trypsin showed that the helical portions of TM2 and TM3 are both shorter than their predicted lengths, indicating that helix-helix interactions in the full-length receptor are apparently important for stabilizing their conformation. Lengthening the alkyl chain of the detergent led to a small but significant increase in the helicity of TM2-3, suggesting that hydrophobic mismatch could play an important role in the stabilization of transmembrane helices by detergents. Protonation of aspartic acid residues in detergent-solubilized TM2-3 also caused a significant increase in helicity. Our results thus suggest that detergent alkyl chain-length and pH may influence membrane protein stability by modulating the stability of individual transmembrane segments.

The adaptor Grb7 links netrin-1 signaling to regulation of mRNA translation

We previously reported a novel biological activity of Netrin-1 in translational stimulation of kappa opioid receptor (KOR). We now identify Grb7 as a new RNA-binding protein that serves as the molecular adaptor for transmitting Netrin-1 signals, through focal adhesion kinase (FAK), to the translation machinery. Grb7 binds specifically to the first stem loop of kor mRNA 5'-UTR through a new RNA-binding domain located in its amino terminus. Upon binding to its capped, target mRNA, Grb7 blocks the recruitment of eIF4E, rendering mRNA untranslatable. The RNA-binding and translation-repressive activity is reduced by FAK-mediated hyperphosphorylation on two tyrosine residues of its carboxyl terminus. This study reports an adaptor protein Grb7 that transmits the stimulating signals of Netrin-1 to the translational machinery to rapidly regulate mRNA translation.

Netrin-1 signaling regulates de novo protein synthesis of kappa opioid receptor by facilitating polysomal partition of its mRNA

The expression of kappa opioid receptor (KOR) is subjected to both transcriptional and posttranscriptional controls. We report that KOR translation is regulated by netrin-1 in primary neurons of dorsal root ganglion (DRG) and in P19 embryonal carcinoma cells. Without stimulation, a significant portion of KOR mRNA is maintained in a dormant state and partitions in the translationally inactive, post-polysomal fraction. During netrin-1 stimulation, which activates its downstream target focal adhesion kinase (FAK), KOR mRNA rapidly partitions to the translationally active polysomal fraction. Functionally, the newly synthesized KOR proteins in DRG neurons are able to bind to specific ligands. This report describes the first example of netrin-1 signaling in the translational control of a drug receptor KOR, which involves the mediator of netrin-1, FAK, and a novel mechanism that enhances the association of target mRNA with polysomes for translational activation.

Differential effects of impaired mitochondrial energy production on the function of mu and delta opioid receptors in neuronal SK-N-SH cells

Oxidative stress contributes to changes in neurosensory processing, including pain, that occur during aging and neurodegeneration. The effects of neuronal oxidation on the opioid system are poorly understood. In this in vitro study, oxidative stress was induced by 3-nitroproprionic acid (3-NPA) in opioid-responsive differentiated SK-N-SH cells. Changes in the inhibitory effects of opioid receptor agonists on intracellular cAMP were used as a marker of the function of mu and delta opioid receptors (MOR and DOR, respectively). Cells were treated with morphine and selective MOR and DOR agonists and antagonists to characterize the function of each receptor subtype. Cyclic AMP (cAMP) was measured by enzyme immunoassay. Levels of reactive oxygen species (ROS) were assessed using the 2',7'-dichlorofluorescin diacetate assay. Exposure of cells to 3-NPA resulted in an increase in ROS. After 3-NPA exposure, there was a significant attenuation of the inhibitory effect of morphine and DAMGO but not of DPDPE on cAMP. In cells pretreated with CTOP, 3-NPA did not change the inhibitory effect on cAMP. These findings demonstrate for the first time that under conditions of mitochondrial damage, the function of MOR is significantly decreased, while the function of DOR does not change, suggesting that the effect of 3-NPA on opioid receptors is subtype-specific.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

Partial and full agonism in endomorphin derivatives: comparison by null and operational model

The partial mu-opioid receptor pool inactivation strategy in isolated mouse vas deferens was used to determine partial agonism of endomorphins and their analogs (endomorphin-1-ol, 2',6'-dimethyltyrosine (Dmt)-endomorphin-1, endomorphin-2-ol and (D-Met2)-endomorphin-2) using morphine, normorphine, morphiceptin, (D-Ala2,MePhe4,Gly5-ol)-enkephalin (DAMGO) and its amide (DAMGA) as reference opioid agonists. Agonist affinities (KA) and efficacies were assessed both by the "null" and the "operational" method. The KA values determined by the two methods correlated significantly with each other and also with the displacing potencies against 3H-naloxone in the receptor binding assay in the presence of Na+. DAMGO and DAMGA were full agonist prototypes, morphine, endomorphin-1, endomorphin-1-ol, Dmt-endomorphin-1, endomorphin-2-ol and (D-Met2)-endomorphin-2 were found by both methods to be partial agonists whereas the parameters for normorphine, morphiceptin and endomorphin-2 were intermediate.

Expression of the third intracellular loop of the delta-opioid receptor inhibits signaling by opioid receptors and other G protein-coupled receptors

To explore the feasibility of developing inhibitors of signaling by opioid receptors and other G protein-coupled receptors (GPCRs) that use the same G protein pool, we investigated the capacity of a minigene encoding the third intracellular loop of the delta-opioid receptor (delta-i3L) to act as competitive antagonist of the receptor-G protein interface interaction. In delta-i3L-expressing cells, the peptide blocked high-affinity agonist binding to both the delta- and the mu-opioid (delta-OR and mu-OR) and attenuated opioid and alpha2-adrenergic receptor (alpha2AR)-dependent [35S]guanosine-5'-O-(3-thio)triphosphate binding. Furthermore, delta-i3L expression resulted in inhibition of delta-, mu-OR-, and alpha2AR-receptor-mediated cAMP accumulation, whereas the cAMP response produced by activation of the beta2-adrenergic receptor was unaffected, suggesting that the inhibitory effects of delta-i3L expression were selective for Gi/Go proteins. Moreover, although delta-i3L expression also attenuated drastically phospholipase C accumulation and Ca2+ release following mu- and delta-OR stimulation, it failed to inhibit carbachol-mediated stimulation of inositol phosphate accumulation in M1-muscarinic receptor-expressing human embryonic kidney 293 cells. Finally, we also examined the effects of delta-i3L expression on the regulation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase pathway. Our results demonstrate that, although ERK activation by mu- and delta-ORs is attenuated by the presence of delta-i3L, ERK activation mediated by alpha2AR remained unaffected. Collectively, our data demonstrate that the delta-i3L can be used as potent inhibitor of G protein signaling for various GPCRs that use a common pool of G proteins.

Engineering and functional immobilization of opioid receptors

Opioid receptors, like many G protein-coupled receptors (GPCRs), are notoriously unstable in detergents. We have now developed a more stable variant of the mu-opioid receptor (MOR) and also a method for the immobilization of solubilized, functional opioid receptors on a solid phase (magnetic beads). Starting with the intrinsically more stable kappa-opioid receptor (KOR), we optimized the conditions (i.e. detergents and stabilizing ligands) for receptor extraction from lipid bilayers of HEK293T cells to obtain maximal amounts of functional, immobilized receptor. After immobilization, the ligand binding profile remains the same as observed for the membrane-embedded receptor. For the immobilized wild-type mu-opioid receptor, however, no conditions were found under which ligand binding capacity was retained. To solve this problem, we engineered the receptor chimera KKM where the N-terminus and the first transmembrane helix (TM1) of wild-type MOR is exchanged for the homologous receptor parts of the wild-type KOR. This hybrid receptor behaves exactly as the wild-type MOR in functional assays. Interestingly, the modified MOR is expressed at six times higher levels than wild-type MOR and is similarly stable as wild-type KOR after immobilization. Hence the immobilized MOR, represented by the chimera KKM, is now also amenable for biophysical characterization. These results are encouraging for future stability engineering of GPCRs.

Homology Modeling and Molecular Dynamics Simulations of the Mu Opioid Receptor in a Membrane-Aqueous System

Three types of opioid receptors-mu, delta, and kappa-belong to the rhodopsin subfamily in the G protein-coupled receptor superfamily. With the recent characterization of the high-resolution X-ray crystal structure of bovine rhodopsin, considerable attention has been focused on molecular modeling of these transmembrane proteins. In this study, a homology model of the mu opioid receptor was constructed based on the X-ray crystal structure of bovine rhodopsin. A phospholipid bilayer was built around the receptor, and two water layers were placed on both surfaces of the lipid bilayer. Molecular-dynamics simulations were carried out by using CHARMM for the entire system, which consisted of 316 amino acid residues, 92 phospholipid molecules, 8327 water molecules, and 11 chloride counter ions-40 931 atoms altogether. The whole system was equilibrated for 250 ps followed by another 2 ns dynamic simulation. The opioid ligand naltrexone was docked into the optimized model, and the critical amino acid residues for binding were identified. The mu opioid receptor homology model optimized in a complete membrane-aqueous system should provide a good starting point for further characterization of the binding modes for opioid ligands. Furthermore, the method developed herein will be applicable to molecular model building to other opioid receptors as well as other GPCRs.

Agonist-specific regulation of the delta-opioid receptor

Delta opioid receptor (DOR) agonists are attractive potential analgesics, since these compounds exhibit strong antinociceptive activity with relatively few side effects. In the past decade, several novel classes of delta-opioid agonists have been synthesized. Recent experimental data indicate that structurally distinct opioid agonists interact differently with the delta-opioid receptor. Consequently, individual agonist-bound DOR conformations may interact differently with intracellular proteins. In the present paper, after a brief review of the cellular processes that contribute to homologous desensitization of the DOR signaling, we shall focus on experimental data demonstrating that chemically different agonists differ in their ability to phosphorylate, internalize, and/or down-regulate the DOR. Homologous regulation of the opioid receptor signaling is thought to play an important role in the development of opioid tolerance. Therefore, agonist-specific differences in DOR regulation suggest that by further chemical modification, delta-selective opioid analgesics can be designed that exhibit a reduced propensity for analgesic tolerance.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

The molecular mechanisms of cellular tolerance to delta-opioid agonists. A minireview

Chronic treatment with deltaopioid agonists, similar to other agonist drugs, causes tolerance. Tolerance is a complex adaptation process that consists of multiple, cellular and neural-system adaptations. Cellular tolerance to delta-opioid agonists involves feedback-regulation of the function, concentration, and localization of the delta-opioid receptors (receptor desensitization) as well as of intracellular effectors (functional desensitization). We are using a recombinant Chinese hamster ovary cell line expressing the human delta-opioid receptors (hDOR/CHO) to investigate the molecular mechanisms of cellular tolerance. We found that the structurally distinct delta-opioid agonists mediate receptor down-regulation by different mechanisms. Thus, truncation of the last 35 C-terminal amino acids of the hDOR completely abolished DPDPE, but not SNC 80-mediated receptor down-regulation. In addition, down-regulation of the wild type-, and the truncated hDORs exhibited different inhibitor sensitivity-profile. Chronic delta-opioid agonist treatment also causes functional desensitization of forskolin-stimulated cAMP formation and cAMP overshoot in the hDOR/CHO cells. We have demonstrated that chronic SNC 80 treatment also causes concurrent phosphorylation of the adenylyl cyclase (AC) VI isoenzyme hDOR/CHO cells. Both AC superactivation and AC VI phosphorylation were SNC 80 dose-dependent, naltrindole-sensitive, and exhibited similar time course-, and protein kinase inhibitor-sensitivity profile. We hypothesize that phosphorylation of AC VI plays an important role in delta-opioid agonist-mediated AC superactivation in hDOR/CHO cells.

Decreased immunodensities of micro-opioid receptors, receptor kinases GRK 2/6 and beta-arrestin-2 in postmortem brains of opiate addicts

The homologous regulation of opioid receptors, through G protein-coupled receptor kinases (GRKs) and beta-arrestins, is an initial step in the complex molecular mechanisms leading to opiate tolerance and dependence. This study was designed to evaluate in parallel the contents of immunolabeled micro-opioid receptors (glycosylated proteins), two representative GRKs (GRK 2 and GRK 6) and beta-arrestin-2 in brains of opiate addicts who had died of an opiate overdose (heroin or methadone). The immunodensities of micro-opioid receptors were decreased (66 kDa protein: 24%, n=24, P<0.0001; 85 kDa protein: 16%, n=24, P<0.05) in the prefrontal cortex of opiate addicts compared with sex-, age-, and PMD-matched controls. This down-regulation of brain micro-opioid receptors was more pronounced in opiate addicts dying of a heroin overdose (27-30%, n=13) than in those who died of a methadone overdose (5-16%, n=11). In the same brains, significant decreases in the immunodensities of GRK 2 (19%, n=24, P<0.05), GRK 6 (25%, n=24, P<0.002) and beta-arrestin-2 (22%, n=24, P< 0.0005) were also quantitated. In contrast, the content of alpha-internexin (a neuronal marker used as a negative control) was not changed in brains of opiate addicts. In these subjects, there was a significant correlation between the densities of GRK 6 and beta-arrestin-2 (r=0.63, n=24, P=0.001), suggesting that both proteins are regulated in a coordinated manner by opiate drugs in the brain. The results indicate that opiate addiction in humans (tolerant state) is associated with down-regulation of brain micro-opioid receptors and regulatory GRK 2/6 and beta-arrestin-2 proteins. These molecular adaptations may be relevant mechanisms for the induction of opiate tolerance in brains of opiate addicts.

An affinity label for δ-opioid receptors derived from [d-Ala2]deltorphin I*

A series of potential affinity label derivatives of the amphibian opioid peptide [D-Ala2]deltorphin I were prepared by incorporation at the para position of Phe3 (in the 'message' sequence) or Phe5 (in the 'address' sequence) of an electrophilic group (i.e. isothiocyanate or bromoacetamide). The introduction of the electrophile was accomplished by incorporating Fmoc-Phe(p-NHAlloc) into the peptide, followed later in the synthesis by selective deprotection of the Alloc group and modification of the resulting amine. While para substitution decreased the delta-opioid receptor affinity, selected analogs retained nanomolar affinity for delta receptors. [D-Ala2,Phe(p-NCS)3]deltorphin I exhibited moderate affinity (IC50=83 nM) and high selectivity for delta receptors, while the corresponding amine and bromoacetamide derivatives showed pronounced decreases in delta-receptor affinity (80- and >1200-fold, respectively, compared with [D-Ala2]deltorphin I). In the 'address' sequence, the Phe(p-NH2)5 derivative showed the highest delta-receptor affinity (IC50=32 nM), while the Phe(p-NHCOCH2Br)5 and Phe(p-NCS)5 peptides displayed four- and tenfold lower delta-receptor affinities, respectively. [D-Ala2,Phe(p-NCS)3]deltorphin I exhibited wash-resistant inhibition of [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) binding to delta receptors at a concentration of 80 nM. [D-Ala2, Phe(p-NCS)3]deltorphin I represents the first affinity label derivative of one of the potent and selective amphibian opioid peptides, and the first electrophilic affinity label derivative of an agonist containing the reactive functionality in the 'message' sequence of the peptide.

Synthesis and evaluation of derivatives of leucine enkephalin as potential affinity labels for δ opioid receptors

As part of an effort to develop peptide-based affinity labels for opioid receptors, [Leu(5)]enkephalin (LeuEnk) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), potent agonists for delta receptors, were selected as the parent peptides for further modification. The affinity label derivatives were prepared using standard Fmoc solid-phase peptide synthesis in conjunction with Fmoc-Phe(p-NHAlloc) (Fmoc: 9-flourenylmethoxycarbonyl;) and selective modification of the p-amino group on this residue. The electrophilic isothiocyanate and bromoacetamide groups were introduced into the para position of Phe(4); the corresponding free amine-containing peptides were also prepared for comparison. The pure peptides were evaluated in radioligand binding assays using Chinese hamster ovary (CHO) cells expressing delta and micro opioid receptors. Modification of Phe(4) in LeuEnk and DTLET significantly decreased delta-receptor binding affinity (40 to >2,000-fold). Among the synthesized analogues, [Phe(p-NH(2))(4)]DTLET showed the highest delta-receptor binding affinity (IC(50) = 39 nM) and enhanced selectivity for delta receptors compared to DTLET while other derivatives exhibited much lower delta receptor affinity. The differences in affinities between the two series of analogues and between the derivatives of LeuEnk and N,N-dibenzyl[Leu(5)]Enk reported previously suggest subtle differences in interactions of Phe(4) with delta receptors depending on other modifications in the sequences.

Effect of opioid receptor ligands on the μ-S196A knock-in and μ knockout mouse vas deferens

We have determined the effect of naltrexone, naloxone, [D-Ala2,D-Leu5]enkephalin (DADLE), and morphine on the mu-S196A opioid receptor knock-in and mu-opioid receptor knockout mouse vas deferens preparations. The antagonists, naltrexone and naloxone, exhibited agonist activity and possessed IC50 values that were 14- and 37-fold greater than morphine on the S196A preparation. Morphine was found to be threefold more potent at S196A relative to wild-type mu-opioid receptor. The mouse vas deferens data suggest that S196 in transmembrane helix 4 of the mu-opioid receptor modulates efficacy. It is proposed that this may be due to decreased dimerization of the receptor. Identical IC50 values of DADLE obtained on the wild-type, S196A knock-in, and mu-opioid receptor knockout preparations support the absence of mu-delta heterodimers in the mouse vas deferens.

N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine inhibits ligand binding to certain G protein-coupled receptors

N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) is used widely in biological systems to chelate certain heavy metals, particularly Zn2+. Here we show that TPEN inhibits ligand binding to certain G protein-coupled receptors and is an antagonist at muscarinic receptors. In intact human neuroblastoma SH-SY5Y cells, the binding of the muscarinic receptor ligand [N-methyl-3H]scopolamine methyl chloride was inhibited by TPEN (Ki approximately 26 microM), as was muscarinic receptor agonist-induced inositol 1,4,5-trisphosphate formation (Ki approximately 26 microM). This antagonism was not due to metal ion chelation, indicating that it resulted from a direct interaction of TPEN with muscarinic receptors. Examination of the effects of TPEN on other receptors in SH-SY5Y cell membrane preparations showed that the binding of the nonpeptide opioid receptor ligand [15,16-3H]diprenorphine was strongly inhibited, whereas binding of [125I]vasoactive intestinal polypeptide was not. This pattern of selectivity was also seen in AR4-2J rat pancreatoma cell membranes, in which TPEN inhibited ligand binding to muscarinic receptors, but not that to cholecystokinin receptors. In conclusion, these data show that TPEN inhibits ligand binding to certain G protein-coupled receptors and exhibits selectivity towards those receptors whose transmembrane helices form the predominant site for ligand interaction. TPEN may have widespread antagonistic activity towards G protein-coupled receptors of this kind.

Rescuing the traffic-deficient mutants of rat mu-opioid receptors with hydrophobic ligands

Deletion of a sequence near the fifth transmembrane domain (258RLSKV262, i3-1 mutant) and a motif residing at the proximal carboxyl tail (344KFCTR348, C-2 mutant) resulted in mu-opioid receptor mutants that were poorly expressed on the surface of transfected human embryonic kidney 293 cells. Treatment with the opioid antagonist naloxone, the agonist etorphine, and other hydrophobic ligands enhanced cell surface expression of i3-1 and C-2 mutants. The observed enhancement was time- and concentration-dependent, required the ligands to be membrane permeable, and was not the result of the reversal of the constitutive activities of the mutant receptors. The binding of the ligands resulted in the trafficking of the mutant receptors retained in the endoplasmic reticulum to the cell surface. The cell surface-expressed mutant C-2, but not i3-1, fully retained ability to mediate inhibition of adenylyl cyclase activity. Furthermore, the Golgi-disturbing agents brefeldin A and monensin completely blocked naloxone-enhanced expression of i3-1 and C-2 mutants. Results of these studies suggest that intracellular interactions of agonist and antagonist with mutant receptors can serve as chaperones in the trafficking of the mutants to the cell surface.

Enhanced binding of nor-binaltorphimine to kappa-opioid receptors in rats dependent on butorphanol

Autoradiographic characterization of binding for brain kappa(1) ([(3)H]CI-977) and kappa(2) ([(3)H]bremazocine) in the presence of DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin), DPDPE ([D-Pen(2), D-Pen(5)]-enkephalin), and U-69,593 opioid receptors, in the presence of different concentrations of a selective unlabeled kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI), was performed in rats in which dependence on or withdrawal from butorphanol had been established. Dependence was induced by a 72 hr intracerebroventricular (i.c.v.) infusion with butorphanol (26 nmol/microl/hr; butorphanol dependent). Butorphanol withdrawal was produced by terminating the infusion of butorphanol in dependent animals. Responses were studied 7 hr following termination (butorphanol withdrawal). IC(50) values from competition studies were estimated by fitting inhibition curves for both kappa(1)- and kappa(2)-opioid receptor assays. In both dependent and withdrawal groups, the IC(50) values obtained against [(3)H]CI-977 or [(3)H]bremazocine with nor-BNI were decreased (ratios of approximately 0.03-0.21 and approximately 0.05-0.42 vs. control, respectively) in brain regions, including frontal cortex, nucleus accumbens, claustrum, dorsal endopiriform nucleus, caudate putamen, parietal cortex, posterior basolateral amygdaloid nucleus, dorsomedial hypothalamus, hippocampus, posterior paraventricular thalamic nucleus, periaqueductal gray, substantia nigra, superficial gray layer of the superior colliculus, ventral tegmental area, and locus coeruleus, compared with control. These results indicate that, in butorphanol-dependent and butorphanol-withdrawal rats, the brain kappa(1)- and kappa(2)-opioid receptors developed a supersensitivity to antagonist binding.

Ligand-selective activation of mu-oid receptor: demonstrated with deletion and single amino acid mutations of third intracellular loop domain

The mechanism for the differential regulation of the mu-opioid receptor by agonists is investigated by identifying the receptor domains used to define the relative efficacies of three mu-opioid receptor-selective agonists: [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), morphine, and [N-MePhe3,D-Pro4]-morphiceptin (PL017) to inhibit forskolin-stimulated intracellular cAMP production in human embryonic kidney 293 cells. This was accomplished by systematically deleting four to five amino acids clusters within the third intracellular loop of rat mu-opioid receptor, Arg258 to Arg280, followed by Ala substitution and scanning studies of the 276RRITR280 sequence, the putative G protein-coupling motif. Deletion of the four to five amino acid clusters resulted in differential effects on the affinities of the agonists and antagonists, and also on the potencies and coupling efficiencies of the three opioid agonists. Ala scanning studies of the 276RRITR280 sequence revealed also the differences between [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), morphine, and PL017. Substitution of Arg276 or Ile278 with Ala reduced the potency of DAMGO but not that of morphine PL017. Meanwhile, mutation of Thr279 to Ala increased the potencies of morphine and PL017 but not that of DAMGO. The I278A mutation decreased the DAMGO coupling efficiency but increased the PL017 coupling efficiency. The R280A mutation resulted in the increase in PL017 potency and coupling efficiency without altering those of DAMGO and morphine. Thus, these mutation studies suggested that the activation of mu-opioid receptor and interaction between the critical domains such as RRITR within third intracellular loop and the G proteins are agonist-selective.

Unpredictable stereochemical preferences for mu opioid receptor activity in an exhaustively stereodiversified library of 1,4-enediols

Using olefin cross-metathesis, we synthesized a novel stereodiversified library of compounds 3 containing a trans-1,4-enediol. Screening this library for mu opioid receptor (MOR) affinity identified multiple high-affinity ligands and revealed that the stereochemical configuration varied widely among those ligands having the highest affinity. It was not possible to predict the configurations of the most active compounds 3 on the basis of the configuration of endomorphin-2, a known MOR peptide ligand, validating the diversity-based approach to ligand discovery.

Serine 329 of the mu-opioid receptor interacts differently with agonists

To investigate the effect of the hydrophilic Ser amino acid in position 329 of the human mu-opioid receptor (hMORwt) on the potency of various agonists, we mutated this residue to Ala (hMORS329A). Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying potassium channel (GIRK1/GIRK2), either the wild-type hMOR or the mutated receptor (hMORS329A) was functionally coexpressed with GIRK1 and GIRK2 channels together with a regulator of G-protein signaling (RGS4) in Xenopus laevis oocytes. The two-microelectrode voltage-clamp technique was used to measure the opioid receptor activated GIRK1/GIRK2 channel responses. The potency of the peptide agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO) decreased as measured via hMORS329A, whereas the potency of nonpeptide agonists like morphine, fentanyl, and beta-hydroxyfentanyl (R004333) increased via the mutated receptor. Our results are indicative for the existence of hydrophilic interactions between Ser(329) and DAMGO, thereby decreasing the potency of DAMGO via the mutated receptor, whereas hydrophobic interactions between the mutated receptor and the N-phenylethyl of morphine and fentanyl can explain the increased potency. We conclude that the hydroxyl group of Ser(329) is not involved in the formation of a hydrogen bond with the beta-hydroxy group of fentanyl and that mutation of this residue to alanine caused dual effects depending on the nature of the ligand.

Synthesis and evaluation of potential affinity labels derived from endomorphin-2

In an attempt to identify potential peptide-based affinity labels for opioid receptors, endomorphin-2 (Tyr-Pro-Phe-PheNH2), a potent and selective endogenous ligand for mu-opioid receptors, was chosen as the parent peptide for modification. The tetrapeptide analogs were prepared using standard Fmoc-solid phase peptide synthesis in conjunction with incorporation of Fmoc-Phe(p-NHAlloc) and modification of the p-amino group. The electrophilic groups isothiocyanate and bromoacetamide were introduced into the para position on either Phe3 or Phe4; the corresponding free amine-containing peptides were also prepared for comparison. The peptides bearing an affinity label group and their free amine analogs were evaluated in a radioligand-binding assay using Chinese hamster ovary (CHO) cells expressing mu- and delta-opioid receptors. Modification on Phe4 was better tolerated than on Phe3 for mu-receptor binding. Among the analogs tested, [Phe(p-NH2)4]endomorphin-2 showed the highest affinity (IC50 = 37 nm) for mu-receptors. The Phe(p-NHCOCH2Br)4 analog displayed the highest mu-receptor affinity (IC50 = 158 nm) among the peptides containing an affinity label group. Most of the compounds exhibited negligible binding affinity for delta-receptors, similar to the parent peptide.

Dermorphin-based potential affinity labels for mu-opioid receptors

Dermorphin and [Lys7]dermorphin, selective micro -opioid receptor ligands originating from amphibian skin, have been modified with various electrophiles in either the 'message' or 'address' sequences as potential peptide-based affinity labels for micro -receptors. Introduction of the electrophilic isothiocyanate and bromoacetamide groups on the para position of Phe3 and Phe5 was accomplished by incorporating Fmoc-Phe(p-NHAlloc) into the peptide followed by selective deprotection and modification. The corresponding amine-containing peptides were also prepared. The pure peptides were evaluated in radioligand binding experiments using Chinese hamster ovary (CHO) cells expressing micro - and delta-opioid receptors. In dermorphin, introduction of the electrophilic groups in the 'message' domain lowered the binding affinity by > 1000-fold; only [Phe(p-NH2)3]dermorphin retained nanomolar affinity for micro -receptors. Modifications in the 'address' region of both dermorphin and [Lys7]dermorphin were relatively well tolerated. In particular, [Phe(p-NH2)5,Lys7]dermorphin showed similar affinity to dermorphin, with almost 2-fold higher selectivity for micro -receptors. [Phe(p-NHCOCH2Br)5]- and [Phe(p-NHCOCH2Br)5,Lys7]dermorphin exhibited relatively high affinity (IC50 = 27.7 and 15.1 nm, respectively) for micro -receptors. However, neither of these peptides inhibited [3H]DAMGO binding in a wash-resistant manner.

The role of the hydrophilic Asn230 residue of the mu-opioid receptor in the potency of various opioid agonists

1. To investigate the effect of the hydrophilic Asn amino acid at position 230 of the human mu-opioid receptor (hMOR230) on the potency of various agonists, we mutated this residue to Thr and Leu (hMORN230T and hMORN230L respectively). 2. Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying K(+) (GIRK1/GIRK2) channel, either the wild type hMOR or one of the mutated receptors (hMORN230L or hMORN230T) were functionally coexpressed with GIRK1/GIRK2 channels and a regulator of G-protein signalling (RGS4) in Xenopus laevis oocytes. 3. The two-microelectrode voltage clamp technique was used to measure the opioid receptor-activated GIRK1/GIRK2 channel responses. The potency of [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO), remained unaffected as measured via hMORN230T and hMORN230L, while the potency of fentanyl and morphine significantly increased via these mutated receptors. 4. Our results are indicative for the existence of hydrophobic interactions between a methyl-group of the side chain of Thr or Leu on the one hand and the piperidine-ring of fentanyl and the hexene-ring of morphine on the other. The mutations also had no influence on the potency of morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G). 5. We conclude that the hydrophilic side chain of Asn in position 230 is not involved in the formation of a H-bond with the aliphatic alcohol of morphine and that an enhancement of the potency of morphine and fentanyl can be explained by mutating this residue towards more hydrophobic amino acids.

Genetic variations in human G protein-coupled receptors: implications for drug therapy

Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.