DAMGO recognizes four residues in the third extracellular loop to discriminate between mu- and kappa-opioid receptors

Deciphering the molecular basis of the kappa opioid receptor selectivity: A Molecular Dynamics study

Selective antagonists for the kappa opioid receptor (KOP) have the potential to treat opiate and alcohol addiction, as well as depression and other CNS disorders. Over the years, the development of KOP-selective antagonists yielded very few successful compounds. Recently, N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines have emerged as a novel class of pure opioid receptor antagonists, including the marketed Mu opioid receptor (MOP) peripheral antagonist Alvimopan and the potent KOP antagonist JDTic. However, the selectivity determinants of this class of compounds towards the opioid receptor subtypes are still vague and understudied. In this work, we have performed Molecular Dynamics (MD) simulation to gain insights into the differential binding of this class of compounds into KOP, as exemplified by Alvimopan and JDTic. Our study indicated that the selectivity largely depends on ligands interaction with the selectivity pocket formed by Val108, Thr111, and Val118, supported by two additional polar and hydrophobic contacts with Asp138 and Trp287, respectively. Our results also demonstrate, for the first time, that non-morphinan ligands can still adopt the "message-address model" for KOP efficacy and selectivity by binding to Glu297.

Bias-inducing allosteric binding site in mu-opioid receptor signaling

Abstract

G-protein-biased agonism of the mu-opioid receptor (μ-OR) is emerging as a promising strategy in analgesia. A deep understanding of how biased agonists modulate and differentiate G-protein-coupled receptors (GPCR) signaling pathways and how this is transferred into the cell are open questions. Here, using extensive all-atom molecular dynamics simulations, we analyzed the binding recognition process and signaling effects of three prototype μ-OR agonists. Our suggested structural mechanism of biased signaling in μ-OR involves an allosteric sodium ion site, water networks, conformational rearrangements in conserved motifs and collective motions of loops and transmembrane helices. These analyses led us to highlight the relevance of a bias-inducing allosteric binding site in the understanding of μ-OR’s G-protein-biased signaling. These results also suggest a competitive equilibrium between the agonists and the allosteric sodium ion, where the bias-inducing allosteric binding site can be modulated by this ion or an agonist such as herkinorin. Notably, herkinorin arises as the archetype modulator of μ-OR and its interactive pattern could be used for screening efforts via protein–ligand interaction fingerprint (PLIF) studies.

Article highlights

Functional expression and pharmacological modulation of TRPM3 in human sensory neurons

BACKGROUND AND PURPOSE

The transient receptor potential (TRP) ion channel TRPM3 functions as a noxious heat sensor, plays a key role in acute pain sensation and inflammatory hyperalgesia in rodents. Despite its potential as a novel analgesic drug target, little is known about the expression, function and modulation in the humans.

EXPERIMENTAL APPROACH

We studied TRPM3 in freshly isolated human dorsal root ganglion (hDRG) neurons and human stem cell-derived sensory (hSCDS) neurons. Expression was analysed at the mRNA level using RT-qPCR. Channel function was assessed using Fura-2-based calcium imaging and whole-cell patch-clamp recordings.

KEY RESULTS

TRPM3 was detected at the mRNA level in both hDRG and hSCDS neurons. The TRPM3 agonists pregnenolone sulphate (PS) and CIM0216 evoked robust intracellular Ca²⁺ responses in 52% of hDRG and 58% of hSCDS neurons. Whole-cell patch-clamp recordings in hSCDS neurons revealed pregnenolone sulphate (PS)- and CIM0216-evoked currents exhibiting the characteristic current-voltage relation of TRPM3. PS-induced calcium responses in hSCDS neurons were reversed in a dose-dependent manner by the flavonoid isosakuranetin and by antiseizure drug primidone. Finally, the μ-opioid receptor agonist DAMGO and the GABA_B receptor agonist baclofen inhibited PS-evoked TRPM3 responses in a subset of hSCDS neurons.

CONCLUSION AND IMPLICATIONS

These results provide the first direct evidence of functional expression of the pain receptor TRPM3 in human sensory neurons, largely mirroring the channel's properties observed in mouse sensory neurons. hSCDS neurons represent a valuable and readily accessible in vitro model to study TRPM3 regulation and pharmacology in a relevant human cellular context.

Peptide ligand recognition by G protein-coupled receptors

The past few years have seen spectacular progress in the structure determination of G protein-coupled receptors (GPCRs). We now have structural representatives from classes A, B, C, and F. Within the rhodopsin-like class A, most structures belong to the α group, whereas fewer GPCR structures are available from the β, γ, and δ groups, which include peptide GPCRs such as the receptors for neurotensin (β group), opioids, chemokines (γ group), and protease-activated receptors (δ group). Structural information on peptide GPCRs is restricted to complexes with non-peptidic drug-like antagonists with the exception of the chemokine receptor CXCR4 that has been crystallized in the presence of a cyclic peptide antagonist. Notably, the neurotensin receptor 1 is to date the only peptide GPCR whose structure has been solved in the presence of a peptide agonist. Although limited in number, the current peptide GPCR structures reveal great diversity in shape and electrostatic properties of the ligand binding pockets, features that play key roles in the discrimination of ligands. Here, we review these aspects of peptide GPCRs in view of possible models for peptide agonist binding.

K303⁶·⁵⁸ in the μ opioid (MOP) receptor is important in conferring selectivity for covalent binding of β-funaltrexamine (β-FNA)

β-funaltrexamine (β-FNA) is an irreversible μ opioid (MOP) receptor antagonist and a reversible agonist of κ opioid (KOP) receptor. β-FNA binds covalently to the MOP receptor at Lys233(5.39), which is conserved among opioid receptors. Molecular docking of β-FNA showed that K303(6.58) in the MOP receptor and E297(6.58) in the KOP receptor played distinct roles in positioning β-FNA. K303(6.58)E MOP receptor and E297(6.58)K KOP receptor mutants were generated. The mutations did not affect β-FNA affinity or efficacy. K303(6.58)E mutation in the MOP receptor greatly reduced covalent binding of [(3)H]β-FNA; however, E297(6.58)K did not enable the KOP receptor to bind irreversibly to β-FNA. Molecular modeling demonstrated that the ε-amino group of K303(6.58) in the MOP receptor interacted with CO of the acetate group of β-FNA to facilitate covalent bond formation with Lys233(5.39). Replacement of K303(6.58) with Glu in the MOP receptor resulted in repulsion between the COOH of Glu and the CO of β-FNA and increased the distance between K233(5.39) and the fumarate group, making it impossible for covalent bond formation. These findings will be helpful for design of selective non-peptide MOP receptor antagonists.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Binding mode characterization of 6α- and 6β-N-heterocyclic substituted naltrexamine derivatives via docking in opioid receptor crystal structures and site-directed mutagenesis studies: application of the 'message-address' concept in development of mu opioid receptor selective antagonists

Highly selective opioid receptor antagonists are essential pharmacological probes in opioid receptor structural characterization and opioid agonist functional studies. Currently, there is no highly selective, nonpeptidyl and reversible mu opioid receptor antagonist available. Among a series of naltrexamine derivatives that have been designed and synthesized, two compounds, NAP and NAQ, were previously identified as novel leads for this purpose based on their in vitro and in vivo pharmacological profiles. Both compounds displayed high binding affinity and selectivity to the mu opioid receptor. To further study the interaction of these two ligands with the three opioid receptors, the recently released opioid receptor crystal structures were employed in docking studies to further test our original hypothesis that the ligands recognize a unique 'address' domain in the mu opioid receptor involving Trp318 that facilitates their selectivity. These modeling results were supported by site-directed mutagenesis studies on the mu opioid receptor, where the mutants Y210A and W318A confirmed the role of the latter in binding. Such work not only enriched the 'message-address' concept, also facilitated our next generation ligand design and development.

Crystal structure of the µ-opioid receptor bound to a morphinan antagonist

Opium is one of the world's oldest drugs, and its derivatives morphine and codeine are among the most used clinical drugs to relieve severe pain. These prototypical opioids produce analgesia as well as many undesirable side effects (sedation, apnoea and dependence) by binding to and activating the G-protein-coupled µ-opioid receptor (µ-OR) in the central nervous system. Here we describe the 2.8 Å crystal structure of the mouse µ-OR in complex with an irreversible morphinan antagonist. Compared to the buried binding pocket observed in most G-protein-coupled receptors published so far, the morphinan ligand binds deeply within a large solvent-exposed pocket. Of particular interest, the µ-OR crystallizes as a two-fold symmetrical dimer through a four-helix bundle motif formed by transmembrane segments 5 and 6. These high-resolution insights into opioid receptor structure will enable the application of structure-based approaches to develop better drugs for the management of pain and addiction.

Development of a cyclic adenosine monophosphate assay for Gi-coupled G protein-coupled receptors by utilizing the endogenous calcitonin activity in Chinese hamster ovary cells

Activation of G(i)-coupled G protein-coupled receptor (GPCRs) by their ligands leads to inhibition of adenylyl cyclase (AC) and reduction of cyclic adenosine monophosphate (cAMP) levels in cells. The traditional cAMP assay for G(i)-coupled GPCRs commonly uses forskolin, a nonspecific AC activator, to increase the basal cAMP level in cells to create an assay window for ligand detection. However, there is still a need to develop a nonforskolin-based cAMP assay because of the challenges inherent in titrating the concentration of forskolin to achieve a reliable assay window, along with issues related to the cAMP-independent effects of forskolin. Herein, we describe such an assay by utilizing the endogenous activity of the calcitonin receptor in Chinese hamster ovary (CHO) cells. The calcitonin receptor is a G(s)-coupled GPCR that, when activated by calcitonin, leads to the stimulation of AC and increases cAMP in cells. Thus, we use calcitonin, instead of forskolin, to increase the basal cAMP level in CHO cells to achieve an assay window. We demonstrated that calcitonin peptides robustly increased cAMP accumulation in several CHO cell lines stably expressing well-known G(i)-coupled GPCRs, such as the Dopamine D2 receptor, the Opioid μ receptor, or the Cannabinoid receptor-1. Agonists of these G(i)-coupled GPCRs attenuated calcitonin-induced cAMP production in their receptor stable cell lines. On the other hand, antagonists and/or inverse agonists blocked the effects of their agonists on calcitonin-induced cAMP production. This calcitonin-based cAMP assay has been demonstrated to be sensitive and robust and exhibited acceptable assay windows (signal/noise ratio) and, thus, can be applied to screen for agonists and antagonists/inverse agonists of G(i)-coupled GPCRs in high-throughput screening formats.

A pharmacological comparison of the cloned frog and human mu opioid receptors reveals differences in opioid affinity and function

This study presents a direct comparison of the ligand binding and signaling profiles of a mammalian and non-mammalian mu opioid receptor. Opioid ligand binding and agonist potencies were determined for an amphibian (Rana pipiens) mu opioid receptor (rpMOR) and the human mu opioid receptor (hMOR) in transfected, intact Chinese hamster ovary (CHO) cells. Identical conditions were employed such that statistically meaningful differences between the two receptors could be determined. Identifying these differences is an important first step in understanding how evolutionary changes affect ligand binding and signaling in vertebrate opioid receptors. As expected, the rank of opioid ligand affinity for rpMOR and hMOR was consistent with the ligands' previously characterized type-selectivity. However, most of the opioid ligands tested had significant differences in affinity for rpMOR and hMOR. For example, the mu-selective agonist, DAMGO ([d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin), had a 10.9-fold greater affinity (K(i)) for hMOR (K(i)=268 nM) than rpMOR (K(i)=2914 nM). In addition, differences in signaling between these receptors were found by measuring inhibition of cAMP accumulation by morphine or DAMGO. DAMGO was significantly more potent (13.6-fold) in CHO cells expressing hMOR versus those expressing rpMOR. In addition, a significantly greater maximal inhibition was elicited by both opioid agonists in cells expressing hMOR. In summary, this study supports an ongoing effort to better understand how vertebrate evolution has shaped opioid receptor properties and function.

Steric interactions and the activity of fentanyl analogs at the μ-opioid receptor

Fentanyl is a highly potent and clinically widely used narcotic analgesic. The synthesis of its analogs remains a challenge in the attempt to develop highly selective mu-opioid receptor agonists with specific pharmacological properties. In this paper, the use of flexible molecular docking in a study of the formation of complexes between a series of active fentanyl analogs and the mu-opioid receptor is described. The optimal position and orientation of fourteen fentanyl analogs in the binding pocket of the mu-receptor were determined. The major receptor amino acids and the ligand functional groups participating in the complex formation were identified. Stereochemical effects on the potency and binding are explained. The proposed model of ligand-receptor binding is in agreement with point mutation experiments explaining the role of the amino acids: Asp147, Tyr148, Asn230, His297, Trp318, His319, Cys321, and Tyr326 in the complex formation. In addition, the following amino acids were identified as being important for ligand binding or receptor activation: Ile322, Gly325, Val300, Met203, Leu200, Val143, and Ile144.

Construction and characterization of a kappa opioid receptor devoid of all free cysteines

We have constructed an optimized mutant of the kappa opioid receptor (KOR), which is devoid of its 10 free cysteines. It was necessary to test different amino acid replacements at various positions and we used a structural model and homology with other receptor family members as a guide. This mutant binds ligands and couples to the cognate G-proteins in a very similar fashion to wild-type KOR. The addition of the antagonist naloxone during cell growth greatly enhances heterogeneous expression of the mutant in mammalian cells, such that amounts similar to wild-type could be produced. We showed by fluorescence microscopy that naloxone stabilizes the mutant in the plasma membrane. This mutant, which now permits the insertion of single cysteines, was designed for use in spectroscopic studies of ligand-induced receptor conformational changes as well as to simplify folding studies.

Anti-mu opioid antiserum against the third external loop of the cloned mu-opioid receptor acts as a mu receptor neutral antagonist

The region from the third external loop to the C terminus of MOR-1 appeared to be critical to the selective binding of MOR-1 ligands as DAMGO and morphine to MOR-1. To study the pharmacological properties of the third extracellular loop an antibody was raised in rabbits against the sequence 304-316 which is unique to MOR-1 and includes the third external loop; the anti-MOR-1 antibody was affinity purified against the immunogen sequence and characterized by [3H]DAMGO and Western blotting; [3H]DPDPE binding assay remained unchanged in the presence of the antibody. Anti-MOR-1 IgG was characterized as a neutral antagonist in Chinese hamster ovary (CHO) cells hyperexpressing constitutively active MOR-1s; in fact, anti-MOR-1 IgG completely reversed the inhibition induced by the MOR-1 agonist endomorphin1, endomorphin2, DAMGO and morphine on forskolin stimulated cyclic AMP (cAMP) accumulation and attenuated both the action of the selective MOR-1 agonist DAMGO to increase [35S]GTPgammaS binding and the action of the MOR-1 inverse agonist beta-chlornaltrexamine (CNA) to decrease [35S]GTPgammaS binding. Radioligand binding assay using membrane suspensions from CHO cells hyperexpressing MOR-1 revealed a significant decreased binding affinity and capacity of all the tested MOR-1 selective ligands after preincubation with anti-MOR-1 IgG. Therefore, the third extracellular loop of MOR-1 appeared to be a key element for the binding of MOR-1 ligands.

[Molecular pharmacology of opioid receptors]

We cloned kappa and mu opioid receptor cDNAs. Using these cDNAs, first, we examined the molecular mechanism for the subtype selectivity of opioid ligands, especially a mu-selective ligand DAMGO. Binding experiments using various chimera and mutated receptors revealed that DAMGO discriminates between mu and delta receptors by recognizing the difference in only one amino acid residue, that is, N(127) in mu and K(108) in delta, at the first extracellular loop, and that it distinguishes between mu and kappa receptors by the difference in four amino acid residues at the third extracellular loop. Second, we established the cell lines expressing the cloned mu, delta, or kappa receptor and elucidated the pharmacological properties, that is, binding affinity and agonistic activity of several opioid agonists. Third, distribution of the mRNAs for mu, delta, and kappa receptors in the brain, spinal cord, and DRG was examined by in situ hybridization histochemistry (ISHH). Double ISHH demonstrated that most of the substance P-producing DRG neurons express the micro receptor. Recently, we are interested in the emotional aspect of pain and its regulation by opioids. Behavioral and microdialysis studies showed that sustained pain evoked by the intraplanter injection of formalin induced conditioned place aversion through the increment of glutamate release followed by the activation of NMDA receptors in the basolateral nucleus of amygdala (BLA). Intra-BLA injection of morphine suppressed the place aversion by inhibiting the glutamate release.

Integrity of extracellular loop 1 of the human cannabinoid receptor 1 is critical for high-affinity binding of the ligand CP 55,940 but not SR 141716A

Like other G-protein coupled receptors with hydrophobic ligands, the human cannabinoid receptor 1 (CB1) is thought to bind its ligands within the transmembrane region of the receptor. However, for some of these receptors the extracellular loops (ECs) have also been shown to play a role in ligand recognition and selectivity. We have taken a mutagenesis approach to examine the role of the amino terminus, EC1, and EC3 of CB1 in ligand binding. Eight mutant receptors, each with a dipeptide insertion, were constructed, expressed, and evaluated for binding to the cannabinoid ligands (-)-cis-3[2-hydroxy-4-(1',1'-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP 55,940) and N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR 141716A). Mutants with insertions in the membrane distal region of the amino terminus or EC3 maintained affinity for both ligands. Those with insertions in the membrane proximal region of the amino terminus or EC1 exhibited a loss of affinity for CP 55,940 while retaining wild-type affinity for SR 141716A. Representative mutants were analyzed for agonist-induced inhibition of cyclic AMP accumulation, and the results indicated that G-protein coupling remained intact. Alanine substitution mutants were made to address whether it was the perturbation of the overall structure of the region or the displacement of particular side chains that was responsible for the loss of CP 55,940 binding. We conclude that a structurally intact EC1, but not the comparably short EC3, is essential for high-affinity CP 55,940 binding.

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.

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.

Comparison of the amino acid residues in the sixth transmembrane domains accessible in the binding-site crevices of mu, delta, and kappa opioid receptors

We have mapped the residues in the sixth transmembrane domains (TMs 6) of the mu, delta, and kappa opioid receptors that are accessible in the binding-site crevices by the substituted cysteine accessibility method (SCAM). We previously showed that ligand binding to the C7.38S mutants of the mu and kappa receptors and the wild-type delta receptor was relatively insensitive to methanethiosulfonate ethylammonium (MTSEA), a positively charged sulfhydryl-specific reagent. These MTSEA-insensitive constructs were used as the templates, and 22 consecutive residues in TM6 (excluding C6.47) of each receptor were mutated to cysteine, 1 at a time. Most mutants retained binding affinities for [3H]diprenorphine, a nonselective opioid antagonist, similar to that of the template receptors. Treatment with MTSEA significantly inhibited [3H]diprenorphine binding to 11 of 22 mutants of the rat mu receptor and 9 of 22 mutants of the human delta receptor and 10 of 22 mutants of the human kappa receptor. Naloxone or diprenorphine protected all sensitive mutants, except the A6.42(287)C mu mutant. Thus, V6.40, F6.44, W6.48, I6.51, Y6.54, V6.55, I6.56, I6.57, K6.58, and A6.59 of the mu receptor; F6.44, I6.51, F6.54, V6.55, I6.56, V6.57, W6.58, T6.59, and L6.60 of the delta receptor; and F6.44, W6.48, I6.51, F6.54, I6.55, L6.56, V6.57, E6.58, A6.59, and L6.60 of the kappa receptor are on the water-accessible surface of the binding-site crevices. The accessibility patterns of residues in the TMs 6 of the mu, delta, and kappa opioid receptors are consistent with the notion that the TMs 6 are in alpha-helical conformations with a narrow strip of accessibility on the intracellular side of 6.54 and a wider area of accessibility on the extracellular side of 6.54, likely due to a proline kink at 6.50 that bends the helix in toward the binding pocket and enables considerable motion in this region. The wider exposure of residues 6.55-6.60 to the binding-site crevice, combined with the divergent amino acid sequences, is consistent with the inferred role of residues in this region in determining ligand binding selectivity. The conservation of the accessibility pattern on the cytoplasmic side of 6.54 suggests that this region may be important for receptor activation. This accessibility pattern is similar to that of the D2 dopamine receptor, the only other GPCR in which TM6 has been mapped by SCAM. That opioid receptors and the remotely related D2 dopamine receptor have similar accessibility patterns in TM6 suggest that these segments of GPCRs in the rhodopsin-like subfamily not only share secondary structure but also are packed similarly into the transmembrane bundle and thus have similar tertiary structure.

Identification of mu-opioid receptor epitopes recognized by agonistic IgG

We have previously reported the presence of IgG antibodies with a morphine-like activity in the serum of healthy individuals. The agonistic activity of IgG was dependent on their binding to the first and the third extracellular loops of the human mu opioid receptor. In this study we show that IgG antibodies obtained by immunizing rats with peptides corresponding to these two loops exhibited a similar morphine-like activity. Residues corresponding to Y(130), M(132), G(133), T(134) within the first and F(315) within the third extracellular segment were required for antibody binding and conferred to IgG a high mu-opioid selectivity.

Selectivity of mu-opioid receptor determined by interfacial residues near third extracellular loop

We hypothesized that the selectivity profile of the rat mu-opioid receptor for opioid receptor-selective ligands is determined by the nature of the amino acid residues at highly divergent sites in the ligand-binding pocket. To determine which characteristics of these residues contribute to opioid receptor ligand selectivity, we made various mutant receptors that replaced the Lys(303) and Trp(318) residues near the extracellular interface of transmembrane domains VI and VII, respectively. Ligand binding determinations using transiently transfected monkey kidney epithelial (COS-1) cells show that Lys(303) mutations cause little change in the receptor binding profile, whereas the Trp(318) mutant receptors have considerably lower affinity for micro-opioid receptor-selective ligands and greatly increased affinity for delta-opioid receptor-selective ligands. The nature of these mutations show that this effect is not due to sterics or charge alone. [35S]guanosine-5'-O-(3-thio)-triphosphate ([35S]GTPgammaS) activity assays show that these residues may influence functional, as well as binding selection. We conclude that a primary role for Trp(318) is to form a basis for ligand selectivity.

Endomorphin-1 discriminates the mu-opioid receptor from the delta- and kappa-opioid receptors by recognizing the difference in multiple regions

Endomorphin-1 is a novel endogenous peptide that is highly selective for the mu-opioid receptor over the delta- and kappa-opioid receptors. The structural basis of high selectivity of endomorphin-1 to the mu-opioid receptor was examined using chimeric receptors between mu- and delta-opioid receptors and those between mu- and kappa-opioid receptors. The chimeric receptors were constructed by using restriction enzyme sites intrinsically possessed by or introduced to the mu-, delta- and kappa-opioid receptor cDNAs. The junctions for the construction were located at the first intracellular loop (Bbs I site), third transmembrane domain (Afl III site) and fifth transmembrane domain (Bgl II site). The competitive binding assay using chimeric receptors revealed that the region from the Bbs I site to the Afl III site, including the first extracellular loop, contributes to the discrimination between mu- and delta-opioid receptors by endomorphin-1 more than any other regions. However, the region from the Afl III site to the Bgl II site and that from the Bgl II site to the carboxy terminal also somewhat contribute to the discrimination between mu- and delta-opioid receptors. For the discrimination between mu- and kappa-opioid receptors, two regions, that is, the region from the Bbs I site to the Afl III site and that from the Bgl II site to the carboxy terminal, were shown to be important. The present results show that endomorphin-1 discriminates the mu-opioid receptor from the other two types of opioid receptors by recognizing the differences in several amino acid residues widely distributed through the receptor structure. We previously reported that DAMGO, a synthetic highly mu-selective peptide, discriminates between mu- and delta-opioid receptors by recognizing the difference in only one amino acid residue and discriminates between mu- and kappa-opioid receptors by recognizing the difference in four residues localized in the restricted region. Although both endomorphin-1 and DAMGO are mu-opioid receptor selective peptides, molecular mechanisms for mu-selectivity are different between these peptides.

Mutational analysis of the structure and function of opioid receptors

The cloning of the opioid receptors allows the investigation of receptor domains involved in the peptidic and nonpeptidic ligand interaction and activation of the opioid receptors. Receptor chimera studies and mutational analysis of the primary sequences of the opioid receptors have provided insights into the structural domains required for the ligand recognition and receptor activation. In the current review, we examine the current reports on the possible involvement of extracellular domains and transmembrane domains in the high-affinity binding of peptidic and nonpeptidic ligands to the opioid receptor. The structural requirement for the receptors' selectivity toward different ligands is discussed. The receptor domains involved in the activation and subsequent cellular regulation of the receptors' activities as determined by mutational analysis will also be discussed. Finally, the validity of the conclusions based on single amino acid mutations is examined.

Pharmacological properties of TRK-820 on cloned mu-, delta- and kappa-opioid receptors and nociceptin receptor

We analyzed the pharmacological properties of 17-cyclopropylmethyl-3,14beta-dihydroxy-4,5alpha-epoxy-6b eta-[N-methyl-trans-3-(3-furyl)acrylamido]morphinan hydrochloride (TRK-820) using Chinese hamster ovary (CHO) cells expressing cloned rat mu-, delta- and kappa-opioid receptors and human nociceptin receptor. TRK-820 showed high affinity for the kappa-opioid receptor, with a Ki value of 3.5 +/- 0.9 nM. In CHO cells expressing kappa-opioid receptors, TRK-820 inhibited forskolin-stimulated cAMP accumulation, and the maximal inhibitory effect was equivalent to that of (+)-(5alpha,7alpha,8beta)-N-methyl-N-[7-(1-pyrrolidiny l)-1-oxaspiro-(4,5)dec-8-yl]benzeneacetamide (U69,593), a full agonist of kappa-opioid receptor. In CHO cells expressing mu-opioid receptors, TRK-820 inhibited cAMP accumulation, but the maximal inhibitory effect was significantly smaller than that of [D-Ala2, N-MePhe4, Gly-ol5]enkephalin (DAMGO), a full agonist of mu-opioid receptor. In CHO cells expressing delta-opioid receptor, the inhibitory effect of TRK-820 on cAMP accumulation was very weak. Using site-directed mutagenesis, the high affinity of TRK-820 for the kappa-opioid receptor was revealed to require Glu297. TRK-820 bound to the nociceptin receptor with a Ki value of 380 +/- 50 nM. TRK-820 by itself had no effect on cAMP accumulation in CHO cells expressing nociceptin receptors, but significantly antagonized the nociceptin (10 nM)-mediated inhibition of cAMP accumulation at high concentrations. These results indicate that TRK-820 acts as a full agonist for the kappa-opioid receptor, a partial agonist for the mu-opioid receptor and a low-affinity antagonist for the nociceptin receptor.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

Bremazocine recognizes the difference in four amino acid residues to discriminate between a nociceptin/orphanin FQ receptor and opioid receptors

We investigated the molecular basis of the discrimination between nociceptin/orphanin FQ receptor (NociR) and opioid receptors (OPRs) by bremazocine, a non-type-selective opioid ligand. Construction of several chimeric receptors between NociR and kappa-opioid receptor (KOPR) and mutant NociRs followed by binding experiments with [3H]bremazocine showed that the mutation of only four amino acid residues of NociR, Ala216, Val279, Gln280 and Val281, to the amino acid residues located at the corresponding position of KOPR, Lys227, Ile290, His291 and Ile292, made it possible for the resultant mutant NociR to bind bremazocine with high affinity. Considering that these four amino acid residues are conserved among mu-, delta- and kappa-OPRs, the present result suggests that bremazocine recognizes the difference in these four amino acid residues to discriminate between NociR and OPRs.