Amide alkaloids from Scopolia tangutica

Four new hydroxycinnamic acid amides, scotanamines A-D (1-4), and seven known alkaloids, including N (1),N (10)-di-dihydrocaffeoylspermidine (5), scopolamine (6), anisodamine (7), hyoscyamine (8), anisodine (9), caffeoylputrescine (10), and N (1)-caffeoyl-N (3)-dihydrocaffeoylspermidine (11), were obtained from the roots of Scopolia tangutica. The present study represents the first recognition of hydroxycinnamic acid amides containing putrescine or spermidine in S. tangutica. Compound 1, in particular, contains a moiety resulting from the condensation of nortropinone and putrescine. Compound 2 exhibited moderate agonist activity at the µ-opioid receptor (EC50=7.3 µM). Compound 2 was tested in vivo and induced analgesia in mice. The analgesic effect was recorded using the tail-flick assay and was reversed by naloxone.

Scalable production of highly sensitive nanosensors based on graphene functionalized with a designed G protein-coupled receptor

We have developed a novel, all-electronic biosensor for opioids that consists of an engineered μ-opioid receptor protein, with high binding affinity for opioids, chemically bonded to a graphene field-effect transistor to read out ligand binding. A variant of the receptor protein that provided chemical recognition was computationally redesigned to enhance its solubility and stability in an aqueous environment. A shadow mask process was developed to fabricate arrays of hundreds of graphene transistors with average mobility of ∼1500 cm(2) V(-1) s(-1) and yield exceeding 98%. The biosensor exhibits high sensitivity and selectivity for the target naltrexone, an opioid receptor antagonist, with a detection limit of 10 pg/mL.

Solubilization and reconstitution of the mu-opioid receptor expressed in human neuronal SH-SY5Y and CHO cells

The zwitterionic detergent CHAPS was used to solubilize the human mu-opioid receptor (hMOR) from SH-SY5Y neuroblastoma cells and recombinant hMOR-CHO (CHO-T7-hMOR) and hMOR-SH-SY5Y (SH-SY5Y-T7-hMOR) cell membranes. Agonist stimulation and G-protein activation by the mu-selective opioid agonist DAMGO ([D-Ala2, N-MePhe4, Gly-ol]-enkephalin) were recovered after removing of CHAPS after polyethylene glycol (PEG) precipitation. Binding assays show that hMOR solubilized and reconstituted this way was functional and able to interact with both agonist peptides and with G-protein. The effective solubilization and reconstitution of hMOR from mammalian cells, without truncation and extensive modification, represent an essential step toward the purification of a receptor bearing important post-translational modifications.

Differential stability of the crystallographic interfaces of mu- and kappa-opioid receptors

The recent mu-opioid receptor (MOPr) and kappa-opioid receptor (KOPr) crystal structures have inspired hypotheses of physiologically relevant dimerization contacts, specifically: a closely packed interface involving transmembrane (TM) helices TM5 and TM6, and a less compact interface, involving TM1, TM2, and helix 8 (H8). While the former was only found in MOPr crystals, similar arrangements of the latter were identified for both KOPr and MOPr. The relevance of these interfaces outside of a crystal lattice is called into question by the possibility that they might be influenced by the specific crystallization conditions. In this study, we have employed umbrella sampling molecular dynamics simulations of coarse-grained representations of the interacting MOPr or KOPr crystallographic structures, in the absence of the T4 lysozyme, and in an explicit lipid-water environment, to determine the strength of receptor dimerization at the different crystallographic interfaces. We note that the shape of the interface plays a dominant role in the strength of the interaction, and the pattern of contacting residues defines the shape of the potential of mean force. This information can be used to guide experiments aimed at exploring the role of dimerization in opioid receptor function.

Co-expression of GRK2 reveals a novel conformational state of the µ-opioid receptor

Agonists at the µ-opioid receptor are known to produce potent analgesic responses in the clinical setting, therefore, an increased understanding of the molecular interactions of ligands at this receptor could lead to improved analgesics. As historically morphine has been shown to be a poor recruiter of β-arrestin in recombinant cell systems and this can be overcome by the co-expression of GRK2, we investigated the effects of GRK2 co-expression, in a recombinant µ-opioid receptor cell line, on ligand affinity and intrinsic activity in both β-arrestin recruitment and [(35)S]GTPγS binding assays. We also investigated the effect of receptor depletion in the β-arrestin assay. GRK2 co-expression increased both agonist Emax and potency in the β-arrestin assay. The increase in agonist potency could not be reversed using receptor depletion, supporting that the effects were due to a novel receptor conformation not system amplification. We also observed a small but significant effect on agonist KL values. Potency values in the [(35)S]GTPγS assay were unchanged; however, inverse agonist activity became evident with GRK2 co-expression. We conclude that this is direct evidence that the µ-opioid receptor is an allosteric protein and the co-expression of signalling molecules elicits changes in its conformation and thus ligand affinity. This has implications when describing how ligands interact with the receptor and how efficacy is determined.

Design, synthesis, and biological evaluation of 14-heteroaromatic-substituted naltrexone derivatives: pharmacological profile switch from mu opioid receptor selectivity to mu/kappa opioid receptor dual selectivity

On the basis of a mu opioid receptor (MOR) homology model and the isosterism concept, three generations of 14-heteroaromatically substituted naltrexone derivatives were designed, synthesized, and evaluated as potential MOR-selective ligands. The first-generation ligands appeared to be MOR-selective, whereas the second and the third generation ones showed MOR/kappa opioid receptor (KOR) dual selectivity. Docking of ligands 2 (MOR selective) and 10 (MOR/KOR dual selective) to the three opioid receptor crystal structures revealed a nonconserved-residue-facilitated hydrogen-bonding network that could be responsible for their distinctive selectivity profiles. The MOR/KOR dual-selective ligand 10 showed no agonism and acted as a potent antagonist in the tail-flick assay. It also produced less severe opioid withdrawal symptoms than naloxone in morphine-dependent mice. In conclusion, ligand 10 may serve as a novel lead compound to develop MOR/KOR dual-selective ligands, which might possess unique therapeutic value for opioid addiction treatment.

A computationally designed water-soluble variant of a G-protein-coupled receptor: the human mu opioid receptor

G-protein-coupled receptors (GPCRs) play essential roles in various physiological processes, and are widely targeted by pharmaceutical drugs. Despite their importance, studying GPCRs has been problematic due to difficulties in isolating large quantities of these membrane proteins in forms that retain their ligand binding capabilities. Creating water-soluble variants of GPCRs by mutating the exterior, transmembrane residues provides a potential method to overcome these difficulties. Here we present the first study involving the computational design, expression and characterization of water-soluble variant of a human GPCR, the human mu opioid receptor (MUR), which is involved in pain and addiction. An atomistic structure of the transmembrane domain was built using comparative (homology) modeling and known GPCR structures. This structure was highly similar to the subsequently determined structure of the murine receptor and was used to computationally design 53 mutations of exterior residues in the transmembrane region, yielding a variant intended to be soluble in aqueous media. The designed variant expressed in high yield in Escherichia coli and was water soluble. The variant shared structural and functionally related features with the native human MUR, including helical secondary structure and comparable affinity for the antagonist naltrexone (K_d  = 65 nM). The roles of cholesterol and disulfide bonds on the stability of the receptor variant were also investigated. This study exemplifies the potential of the computational approach to produce water-soluble variants of GPCRs amenable for structural and functionally related characterization in aqueous solution.

Effect of anchoring 4-anilidopiperidines to opioid peptides

We report here the design, synthesis, and in vitro characterization of new opioid peptides featuring a 4-anilidopiperidine moiety. Despite the fact that the chemical structures of fentanyl surrogates have been found suboptimal per se for the opioid activity, the corresponding conjugates with opioid peptides displayed potent opioid activity. These studies shed an instructive light on the strategies and potential therapeutic values of anchoring the 4-anilidopiperidine scaffold to different classes of opioid peptides.

A facile synthesis for novel loperamide analogs as potential μ opioid receptor agonists

A facile synthesis for novel loperamide analogs as potential μ opioid receptors is described. The synthetic procedure for compound 5, which contains two 4-phenyl piperidine scaffolds, was optimized, and this compound was synthesized in excellent yield. We also describe a mild and highly efficient protocol for the synthesis of compounds 6 and 7.

Dual-opioid therapy: changing the paradigm. Introduction

INTRODUCTION

Morphine sulfate and oxycodone hydrochloride are commonly used for pain management because of their pharmacologic profile, pharmacokinetics, and analgesic potency. However, opioids are associated with a significant adverse event (AE) burden that limits their use in both the acute and the chronic pain settings.

SUMMARY

Co-administration of opioids demonstrated synergistic analgesia and reduced side effects. Thus, dual-opioid therapy has the potential to enhance the positive analgesic benefits of opioids, while limiting the burden of opioid-related AEs. This symposium proceedings was based on presentations at the 13th World Congress on Pain in August 2010.

CONCLUSIONS

This program will review the rationale for dual-opioid therapy based on preclinical findings and data from clinical studies showing the efficacy and tolerability profile of a dual-opioid formulation when used to treat acute postoperative pain.

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.

Structural basis for μ-opioid receptor binding and activation

Opioids that stimulate the μ-opioid receptor (MOR1) are the most frequently prescribed and effective analgesics. Here we present a structural model of MOR1. Molecular dynamics simulations show a ligand-dependent increase in the conformational flexibility of the third intracellular loop that couples with the G protein complex. These simulations likewise identified residues that form frequent contacts with ligands. We validated the binding residues using site-directed mutagenesis coupled with radioligand binding and functional assays. The model was used to blindly screen a library of ∼1.2 million compounds. From the 34 compounds predicted to be strong binders, the top three candidates were examined using biochemical assays. One compound showed high efficacy and potency. Post hoc testing revealed this compound to be nalmefene, a potent clinically used antagonist, thus further validating the model. In summary, the MOR1 model provides a tool for elucidating the structural mechanism of ligand-initiated cell signaling and for screening novel analgesics.

Consensus 3D model of μ-opioid receptor ligand efficacy based on a quantitative Conformationally Sampled Pharmacophore

Despite being studied for over 30 years, a consensus structure-activity relationship (SAR) that encompasses the full range peptidic and nonpeptidic μ-opioid receptor ligands is still not available. To achieve a consensus SAR the Conformationally Sampled Pharmacophore (CSP) method was applied to develop a predictive model of the efficacy of μ-opioid receptor ligands. Emphasis was placed on predicting the efficacy of a wide range of agonists, partial agonists, and antagonists as well as understanding their mode of interaction with the receptor. Inclusion of all accessible conformations of each ligand, a central feature of the CSP method, enabled structural features between diverse μ-opioid receptor ligands that dictate efficacy to be identified. The models were validated against a diverse collection of peptidic and nonpeptidic ligands, including benzomorphans, fentanyl (4-anilinopiperidine), methadone (3,3-diphenylpropylamines), etonitazene (benzimidazole derivatives), funaltrexamine (C6-substituted 4,5-epoxymorphinan), and herkinorin. The model predicts (1) that interactions of ligands with the B site, as with the 19-alkyl substituents of oripavines, modulate the extent of agonism; (2) that agonists with long N-substituents, as with fentanyl and N-phenethylnormorphine, can bind in an orientation such that the N substitutent interacts with the B site that also allows the basic N-receptor Asp interaction essential for agonism; and (3) that the μ agonist herkinorin, that lacks a basic nitrogen, binds to the receptor in a manner similar to the traditional opioids via interactions mediated by water or a ion. Importantly, the proposed CSP model can be reconciled with previously published SAR models for the μ receptor.

Identification of a µ opiate receptor signaling mechanism in human placenta

BACKGROUND

Previous studies report that genes in the morphine biosynthetic pathway have been found in placental tissue. Prior researchers have shown that kappa opioid receptors are present in human placenta. We determined if a µ opiate receptor was present and which subtype was expressed in human placenta. We also sought to demonstrate a functional µ opiate receptor in human placenta.

MATERIAL/METHODS

Polymerase chain reactions as well as DNA sequencing were performed to identify the µ opiate receptor subtypes present in human placenta. The functionality of the receptor was demonstrated by real time amperometric measurements of morphine induced NO release.

RESULTS

The µ4 opiate receptor sequence was present as well as the µ1 opioid receptor transcript. The addition of morphine to placental tissue resulted in immediate nitric oxide release and this effect was blocked by naloxone.

CONCLUSIONS

In the present study, an intact morphine signaling system has been demonstrated in human placenta. Morphine signaling in human placenta probably functions to regulate the immune, vascular, and endocrine functions of this organ via NO.

The genetic influence on the cortical processing of experimental pain and the moderating effect of pain status

BACKGROUND

Research suggests that the COMT Val(158)Met, BDNF Val(66)Met and OPRM1 A(118)G polymorphisms moderate the experience of pain. In order to obtain experimental confirmation and extension of findings, cortical processing of experimentally-induced pain was used.

METHOD

A sample of 78 individuals with chronic low back pain complaints and 37 healthy controls underwent EEG registration. Event-Related Potentials were measured in response to electrical nociceptive stimuli and moderation by COMT Val(158)Met, BDNF Val(66)Met and OPRM1 A(118)G polymorphisms was assessed.

RESULTS

Genetic variation did not have a direct effect on cortical processing of experimental pain. However, genetic effects (COMT Val(158)Met and BDNF Val(66)Met) on experimental pain were moderated by the presence of chronic pain. In the presence of chronic pain, the COMT Met allele and the BDNF Met allele augmented cortical pain processing, whilst reducing pain processing in pain-free controls. No significant effects were found concerning the OPRM1 A(118)G polymorphism.

CONCLUSIONS

The current study suggests that chronic experience of pain enhances genetic sensitivity to experimentally induced mildly painful stimuli, possibly through a process of epigenetic modification.

Endogenous morphine and nitric oxide coupled regulation of mitochondrial processes

The widespread expression of morphine by plants, invertebrate and vertebrate cells/organ systems strongly indicates a high level of evolutionary conservation of morphine and related morphinan alkaloids as essential chemical factors required for normal growth and development. The prototype catecholamine dopamine (DA) serves as an essential chemical intermediate in morphine biosynthesis both in plants and animals. We surmise primordial, multi-potential cell types, before the emergence of specialized plant and animal cells/organ systems, required selective mechanisms to limit their responsiveness to environmental noise. Accordingly, cellular systems that emerged with the potential for recruitment of the free radical gas nitric oxide (NO) as a multi-faceted autocrine/paracrine signaling molecule were provided with extremely positive evolutionary advantages. Endogenous "morphinergic" in concert with NO-coupled signaling systems have evolved as autocrine/paracrine regulators of metabolic homeostasis, energy metabolism, mitochondrial respiration and energy production. Basic physiological processes involving "morphinergic"/NO-coupled regulation of mitochondrial function, with special emphasis on the cardiovascular system, are critical to all organismic survival. Critical to this concept may be the phenomenon of mitochondrial enslavement in eukaryotic evolution via morphine.

Synthesis and evaluation of new endomorphin analogues modified at the Pro(2) residue

Six new endomorphin analogues, incorporating constrained amino acids in place of native proline have been synthesized. Residues of (S)-azetidine-2-carboxylic acid (Aze), 3,4-dehydro-(S)-proline (Delta(3)Pro), azetidine-3-carboxylic acid (3Aze) and dehydro-alanine (DeltaAla) have been used to prepare [Delta(3)Pro(2)]EM-2 (1), [Aze(2)]EM-1 (2), [Aze(2)]EM-2 (3), [3Aze(2)]EM-1 (4), [3Aze(2)]EM-2 (5) and [DeltaAla(2)]EM-2 (6). Binding assays and functional bioactivities for mu- and delta-receptors are reported. The highest affinity, bioactivity and selectivity are shown by peptides 2 and 3 containing the Aze residue.

Membrane functional organisation and dynamic of mu-opioid receptors

The activation and signalling activity of the membrane mu-opioid receptor (MOP-R) involve interactions among the receptor, G-proteins, effectors and many other membrane or cytosolic proteins. Decades of investigation have led to identification of the main biochemical processes, but the mechanisms governing the successive protein-protein interactions have yet to be established. We will need to unravel the dynamic membrane organisation of this complex and multifaceted molecular machinery if we are to understand these mechanisms. Here, we review and discuss advances in our understanding of the signalling mechanism of MOP-R resulting from biochemical or biophysical studies of the organisation of this receptor in the plasma membrane.

14-O-Heterocyclic-substituted naltrexone derivatives as non-peptide mu opioid receptor selective antagonists: design, synthesis, and biological studies

Mu opioid receptor antagonists have clinical utility and are important research tools. To develop non-peptide and highly selective mu opioid receptor antagonist, a series of 14-O-heterocyclic-substituted naltrexone derivatives were designed, synthesized, and evaluated. These compounds showed subnanomolar-to-nanomolar binding affinity for the mu opioid receptor. Among them, compound 1 exhibited the highest selectivity for the mu opioid receptor over the delta and kappa receptors. These results implicated an alternative 'address' domain in the extracellular loops of the mu opioid receptor.

Computational study of the heterodimerization between mu and delta receptors

A growing body of evidence indicated that the G protein coupled receptors exist as homo- or hetero-dimers in the living cell. The heterodimerization between mu and delta opioid receptors has attracted researchers' particular interests, it is reported to display novel pharmacological and signalling regulation properties. In this study, we construct the full-length 3D-model of mu and delta opioid receptors using the homology modelling method. Threading program was used to predict the possible templates for the N- and C-terminus domains. Then, a 30 ns molecular dynamics simulations was performed with each receptor embedded in an explicit membrane-water environment to refine and explore the conformational space. Based on the structures extracted from the molecular dynamics, the likely interface of mu-delta heterodimer was investigated through the analysis of protein-protein docking, cluster, shape complementary and interaction energy. The computational modelling works revealed that the most likely interface of heterodimer was formed between the transmembrane1,7 (TM1,7) domains of mu receptor and the TM(4,5) domains of delta receptor, with emphasis on mu-TM1 and delta-TM4, the next likely interface was mu(TM6,7)-delta(TM4,5), with emphasis on mu-TM6 and delta-TM4. Our results were consistent with previous reports.

Isolation and characterization of new exon 11-associated N-terminal splice variants of the human mu opioid receptor gene

Alternative splicing of the mu opioid receptor genes to create multiple mu receptor subtypes has been demonstrated in animals and humans. Previously, we identified a number of C-terminal variants in mice, rats and human, followed by several N-terminal variants associated with a new upstream exon in mice (exon 11). Behavioral studies in exon 11 knockout mice suggest an important role for the exon 11 variants in the analgesic actions of heroin and morphine-6beta-glucuronide, but not morphine or methadone. We now have identified a homologous human exon 11 and three similar human exon 11-associated variants, suggesting conservation of exon 11 and its associated variants across species. hMOR-1i has an additional 93 amino acids at the tip of the N-terminus but is otherwise identical to hMOR-1. When expressed in Chinese hamster ovary cells, the additional 93 amino acids in hMOR-1i had little effect on opioid binding, but significantly altered agonist-induced G-protein activation. hMOR-1G1 and hMOR-1G2 predicted six transmembrane domain variants, similar to those seen in mice. The regional expression of these exon 11-associated variants, as determined by RT-PCR, varied markedly, implying region-specific alternative splicing. The presence of exon 11-associated variants in humans raises questions regarding their potential role in heroin and morphine-6beta-glucuronide actions in people as they do in mice.

Plasmon-waveguide resonance studies of ligand binding to integral proteins in membrane fragments derived from bacterial and mammalian cells

A procedure has been developed for directly depositing membrane fragments derived from bacterial cells (chromatophores from Rhodopseudomonas sphaeroides) and mammalian cells (mu-opioid receptor- and MC4 receptor-transfected human embryonic kidney (HEK) cells and rat trigeminal ganglion cells) on the silica surface of a plasmon-waveguide resonance (PWR) spectrometer. Binding of ligands (cytochrome c(2) for the chromatophores, the peptide agonists DAMGO and melanotan-II that are specific for the mu-opioid and MC4 receptors, and two nonpeptide agonists that are specific for the CB1 receptor) to these membrane fragments has been observed and characterized with high sensitivity using PWR spectral shifts. The K(D) values obtained are in excellent agreement with conventional pharmacological assays and with prior PWR studies using purified receptors inserted into deposited lipid bilayer membranes. These studies provide a new tool for obtaining useful biological information about receptor-mediated processes in real biological membranes.

Heterodimerization of hypothalamic G-protein-coupled receptors involved in weight regulation

BACKGROUND

Melanocortin 3 and 4 receptors (MC3R and MC4R) are known to play an essential role in hypothalamic weight regulation. In addition to these two G-protein-coupled receptors (GPCRs), a huge number of other GPCRs are expressed in hypothalamic regions, and some of them are involved in weight regulation. So far, homodimerization was shown for a few of these receptors. Heterodimerization of unrelated receptors may have profound functional consequence but heterodimerization of GPCRs involved in weight regulation was not reported yet.

METHODS

A selective number of hypothalamically expressed GPCRs were cloned into a eukaryotic expression vector. Cell surface expression was demonstrated by an ELISA approach. Subcellular distribution was investigated by confocal laser microscopy. A sandwich ELISA and fluorescence resonance energy transfer (FRET) were used to determine protein-protein interaction.

RESULTS

Via sandwich ELISA and FRET approach we could demonstrate a robust interaction of the MC4R with GPR7, both of which are expressed in the hypothalamic nucleus paraventricularis. Moreover, we determined a strong interaction of MC3R with the growth hormone secretagogue receptor expressed in the nucleus arcuatus.

CONCLUSION

Identification GPCR heterodimerization adds to the understanding of the complexity of weight regulation and may provide important information to develop therapeutic strategies to treat obesity.

Conformation-opioid activity relationships of bicyclic guanidines from 3D similarity analysis

Conformation of bicyclic guanidines with kappa-opioid receptor activity derived in our laboratory from a positional scanning synthetic combinatorial library is presented in this work. We propose a common bioactive conformation and putative pharmacophoric features by means of 3D similarity methods. Our 'Y' shape molecular binding model explains structure-activity relationships and suggests that the guanidine functionality and a 4-methoxybenzyl group may be involved in key interactions with the receptor. Comparison of our model with known opiates suggest a similar binding mode showing that the bicyclic guanidines presented in this work are suitable scaffolds for further development of new opioid receptors ligands.

Persistence of evolutionary memory: primordial six-transmembrane helical domain mu opiate receptors selectively linked to endogenous morphine signaling

Biochemical, molecular and pharmacological evidence for two unique six-transmembrane helical (TMH) domain opiate receptors expressed from the micro opioid receptor (MOR) gene have been shown. Designated micro3 and micro4 receptors, both protein species are Class A rhodopsin-like members of the superfamily of G-protein coupled receptors but are selectively tailored to mediate the cellular regulatory effects of endogenous morphine and related morphinan alkaloids via stimulation of nitric oxide (NO) production and release. Both micro3 and micro4 receptors lack an amino acid sequence of approximately 90 amino acids that constitute the extracellular N-terminal and TMH1 domains and part of the first intracellular loop of the micro1 receptor, but retain the empirically defined ligand binding pocket distributed across conserved TMH2, TMH3, and TMH7 domains of the micro1 sequence. Additionally, the receptor proteins are terminated by unique intracellular C-terminal amino acid sequences that serve as putative coupling or docking domains required for constitutive NO synthase activation. Because the recognition profile of micro3 and micro4 receptors is restricted to rigid benzylisoquinoline alkaloids typified by morphine and its extended family of chemical congeners, it is hypothesized that conformational stabilization provided by interaction of extended extracellular N-terminal protein domains and the extracellular loops is required for binding of endogenous opioid peptides as well as synthetic flexible opiate alkaloids.