A mu-opioid receptor single nucleotide polymorphism in rhesus monkey: association with stress response and aggression

Variations in the human mu-opioid receptor gene have driven exploration of their biochemical, physiological and pathological relevance. We investigated the existence of variations in the nonhuman primate mu-opioid receptor gene to determine whether nonhuman primates can model genotype/phenotype associations of relevance to humans. Similar to the A118G single nucleotide polymorphism (SNP) in the human mu-opioid receptor gene, a SNP discovered in the rhesus monkey mu-opioid receptor gene (C77G) alters an amino acid in the N-terminal arm of the receptor (arginine for proline at position 26). Two mu-opioid receptor coding regions isolated from a single heterozygous (C77/G77) rhesus monkey brain were expressed in HEK-293 cells and characterized in radioreceptor assays. Paralleling the findings of increased affinity of beta-endorphin by the A118G allele in the human, the rhesus monkey mu-opioid receptor protein derived from the G77-containing clone demonstrated a 3.5-fold greater affinity for beta-endorphin than the receptor derived from the C77-containing clone. An assay developed to assess the incidence of the C77G SNP in a behaviorally and physiologically characterized cohort of rhesus monkeys (n=32) indicated that 44% were homozygous for C77-containing alleles, 50% were heterozygous and 6% were homozygous for G77-containing alleles. The presence of G77-containing alleles was associated with significantly lower basal and ACTH-stimulated plasma cortisol levels (P<0.03-0.05 and P<0.02, respectively) and a significantly higher aggressive threat score (P<0.05) in vivo. In a cohort of 20 monkeys, a trend towards an inverse correlation between aggressive threat and plasma cortisol levels was observed. The findings suggest that mu-opioid receptor haplotypes in monkeys can contribute to individual variability in stress response and related aggression. The data support the use of nonhuman primates to investigate mu-opioid receptor genotype/phenotype relations of relevance to humans.

Use of NMR and Fluorescence Spectroscopy as well as Theoretical Conformational Analysis in Conformation-activity Studies of Cyclic Enkephalin Analogues

In this review the conformational studies of natural enkephalins (H-Tyr-Gly-Gly-Phe-Met-OH; the [Met(5)]enkephalin and H-Tyr-Gly-Gly-Phe-Leu-OH; the [Leu(5)]enkephalin), their acyclic and cyclic analogues, including those carried out in our laboratory, performed by experimental and theoretical methods and their combination, are described. Emphasis is given on the role of conformational constraints introduced by cyclization on activity at the micro and delta opioid receptors. Comparison of the conformations of cyclic enkephalin analogues with high delta-receptor activity with those of potent rigid non-peptide delta-receptor agonists indicates that the proximity of the aromatic side chains in positions 1 and 4 as well as the N-terminal amino group is desirable for the activity at the delta opioid receptors; early conformational studies also suggested that spatial separation of the aromatic side chains and rigidity of the cyclic backbone is desirable for micro-receptor activity. The results of our recent conformational studies performed with the use of fluorescence and NMR spectroscopy as well as theoretical calculations indicate, however, that these structural features are not necessary for activity at the micro opioid receptors. Methods applied to the determination of the conformation of flexible peptides, such as Nuclear Magnetic Resonance (NMR), fluorescence spectroscopy, and theoretical conformational analysis are also discussed briefly.

Syntheses of Immobilized G Protein-Coupled Receptor Chromatographic Stationary Phases: Characterization of Immobilized μ and κ Opioid Receptors

Opioid receptors are members of the superfamily of G protein-coupled receptors (GPCRs). Liquid chromatographic stationary phases containing either the human mu or kappa opioid receptor subtypes have been developed to study the binding between the opioid receptors and their ligands. The receptors were obtained from Chinese hamster ovary cells stably transfected with human mu or kappa receptor subtypes. The receptors were isolated through partial solubilization with sodium cholate detergent, and the partially purified receptor complex was immobilized in the phospholipid analogue monolayer of an immobilized artificial membrane liquid chromatographic stationary phase. The resulting phase was packed into a glass column (1.8 x 0.5 (i.d.) cm) and used in the on-line chromatographic determination of drug/ligand binding affinities to the immobilized opioid receptors. Preliminary on-line binding studies employing frontal chromatographic techniques were conducted with the known mu antagonist (naloxone) and a kappa agonist (U69593). The calculated dissociation constants (Kd) were 110 nM for naloxone and 84 nM for U69593. The results indicate that the immobilized receptors retained their ability to specifically bind ligands and were active for 1200 column volumes applied over two weeks. Zonal chromatographic experiments were also conducted, and competitive displacements of the marker ligands were observed. The results suggest that the immobilized opioid receptor stationary phases can be used to qualitatively assess the binding affinities of compounds to the immobilized receptors and represent the first example of the use of immobilized GPCRs in a chromatographic system.

Selective interactions between helix VIII of the human mu-opioid receptors and the C terminus of periplakin disrupt G protein activation

Analysis of interactions between the C-terminal tail of the MOP-1 and MOP-1A variants of the human mu-opioid receptor with proteins derived from a human brain cDNA library resulted in identification of the actin and intermediate filament-binding protein periplakin. Mapping of this interaction indicated that the predicted fourth intracellular loop/helix VIII of the receptor interacts with the C-terminal rod and linker region of periplakin. Periplakin is widely expressed in the central nervous system of both man and rat and demonstrated an overlapping but not identical distribution with mu-opioid (MOP) receptors. Co-expression of periplakin with MOP-1 or a MOP-1-eYFP fusion construct in HEK293 cells did not interfere with agonist-mediated internalization of the receptor. When co-expressed with a MOP-1-Gi1 alpha fusion protein periplakin significantly reduced the capacity of the agonist to stimulate binding of 35S-labeled guanosine 5'-3-O-(thio)triphosphate ([35S]GTP gamma S) to the receptor-associated G protein. By contrast, periplakin did not interfere with agonist-stimulation of [35S]GTP gamma S binding to either an alpha 2A-adrenoreceptor-Gi1 alpha fusion protein or a beta2-adrenoreceptor-Gs alpha fusion protein, indicating its selectivity of function. This represents the first example of an opioid receptor-interacting protein that functions to disrupt agonist-mediated G protein activation.

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.

Synthesis and Biological activity of kappa opioid receptor agonists. Part 2: preparation of 3-aryl-2-pyridone analogues generated by solution- and solid-phase parallel synthesis methods

New analogues of the previously described 3-aryl pyridone KOR agonists have been synthesised by parallel synthetic methods, both in solution- and with solid-phase chemistry, making use of the well known and versatile Mitsunobu, Suzuki and Buchwald reactions. Opioid receptor binding data for the compounds produced is reported.

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.

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.

Confined diffusion without fences of a g-protein-coupled receptor as revealed by single particle tracking

Single particle tracking is a powerful tool for probing the organization and dynamics of the plasma membrane constituents. We used this technique to study the micro -opioid receptor belonging to the large family of the G-protein-coupled receptors involved with other partners in a signal transduction pathway. The specific labeling of the receptor coupled to a T7-tag at its N-terminus, stably expressed in fibroblastic cells, was achieved by colloidal gold coupled to a monoclonal anti T7-tag antibody. The lateral movements of the particles were followed by nanovideomicroscopy at 40 ms time resolution during 2 min with a spatial precision of 15 nm. The receptors were found to have either a slow or directed diffusion mode (10%) or a walking confined diffusion mode (90%) composed of a long-term random diffusion and a short-term confined diffusion, and corresponding to a diffusion confined within a domain that itself diffuses. The results indicate that the confinement is due to an effective harmonic potential generated by long-range attraction between the membrane proteins. A simple model for interacting membrane proteins diffusion is proposed that explains the variations with the domain size of the short-term and long-term diffusion coefficients.

Untangling ligand induced activation and desensitization of G-protein-coupled receptors

Long-term treatment with a drug to a G-protein-coupled receptor (GPCR) often leads to receptor-mediated desensitization, limiting the therapeutic lifetime of the drug. To better understand how this therapeutic window might be controlled, we created a mechanistic Monte Carlo model of the early steps in GPCR signaling and desensitization. Using this model we found that the rates of G-protein activation and receptor phosphorylation can be partially decoupled by varying the drug-receptor dissociation rate constant, k(off), and the drug's efficacy, alpha. The maximum ratio of G-protein activation to receptor phosphorylation (GARP) was found for drugs with an intermediate k(off) value and small alpha-value. Changes to the cellular environment, such as changes in the diffusivity of membrane molecules and the G-protein inactivation rate constant, affected the GARP value of a drug but did not change the characteristic shape of the GARP curve. These model results are examined in light of experimental data for a number of GPCRs and are found to be in good agreement, lending support to the idea that the desensitization properties of a drug might be tailored to suit a specific application.

High-affinity mu opioid receptor ligands discovered by the screening of an exhaustively stereodiversified library of 1,5-enediols

In this communication, we report the synthesis of an exhaustively stereodiversified library of 16 1,5-enediols (2) and the screening of these compounds for mu opioid receptor (MOR) binding. The stereochemical configuration of 2 strongly impacted the binding affinity, and (S,S,S,R)-2 exhibited a Ki of 8.8 nM for MOR, comparable to that of endomorphin-2 (Ki = 1.2 nM). Moreover, compounds 2 exhibited 5-86-fold selectivity for MOR over delta opioid receptor (DOR) and 16-150-fold selectivity for MOR over kappa opioid receptor (KOR). Additionally, analogues of 2 were synthesized which showed the importance of the trans olefin for receptor binding but that modifications of the C-terminal amino acid were well tolerated. Ligand 11 is noteworthy because it retains only one of the amide bonds present in 1, but binds MOR with an affinity of 10 nM and 110- and 600-fold selectivity for MOR over DOR and KOR. These results demonstrate the utility of stereochemical diversity in the discovery of bioactive small molecules.

The local environment at the cytoplasmic end of TM6 of the mu opioid receptor differs from those of rhodopsin and monoamine receptors: introduction of an ionic lock between the cytoplasmic ends of helices 3 and 6 by a L6.30(275)E mutation inactivates the mu opioid receptor and reduces the constitutive activity of its T6.34(279)K mutant

Activation of rhodopsin and monoamine G protein-coupled receptors (GPCRs) has been proposed to involve in part the disruption of a conserved E6.30-R3.50 ionic interaction between transmembrane segments (TMs) 3 and 6. However, this interaction does not occur in the opioid receptors, which have L275 at 6.30. On the basis of our findings that mutations of T6.34(279) to K and D produced, respectively, a constitutively active and an inactive form of the mu opioid receptor, we previously suggested that the functional role of the 6.30(275) residue could be assumed by T6.34(279), but the interplay between residues at positions 6.30 and 6.34 remained unresolved. In this study, we examined the effects of introducing an E in position 6.30(275) of the wild type (WT) and of the T6.34(279) mutants of the mu opioid receptor to compare the participation of the 6.30 locus in molecular events during activation in this receptor with its role in other GPCRs. The L6.30(275)E and the L6.30(275)E/T6.34(279)D mutants displayed no constitutive activity and could not be activated by the agonist DAMGO or morphine. The L6.30(275)E/T6.34(279)K mutant had some constitutive activity, but much less than the T6.34(279)K mutant, and could be activated by both agonists. The rank order of affinity for the agonist DAMGO is as follows: T6.34(279)K > WT congruent with L6.30(275)E/T6.34(279)K > L6.30(275)E congruent with T6.34(279)D > L6.30(275)E/T6.34(279)D; however, all constructs have a similar affinity for the antagonist [(3)H]diprenorphine. These data are interpreted in the context of interactions with the conserved R3.50(165) in TM3. When L6.30(275) is mutated to E, the favorable E6.30(275)-R3.50(165) interaction stabilizes an inactive state, as in rhodopsin, and hence reduces the activities of T6.34(279) mutants. Thus, the mu opioid receptor is shown to be different from rhodopsin and monoamine GPCRs, of which the WTs with native E6.30 can be activated, and the 6.34D or 6.34K mutants display enhanced constitutive activities. Our molecular modeling results suggest that some specific differences in local geometry at the cytoplasmic ends of TM5 and TM6 may account in part for the observed differences in the molecular mechanisms of receptor activation.

Homodimerization of human mu-opioid receptor overexpressed in Sf9 insect cells

In this study, we demonstrate that human mu-opioid receptors do form SDS-resistant homodimers and examine the ability of human mu-opioid receptors to dimerize and the role of agonists in the dimerization. Increasing concentrations and longer exposure of agonists reduce the levels of dimmer with a corresponding increase in the levels of monomer. This effect is achieved with both peptide and alkaloid opioid agonists and it is antagonist reversible. These results suggest that human mu-opioid receptors are present as receptor oligomers and interconversion between dimeric and monomeric forms may be important for biological activity.

Crystal structure of biphalin sulfate: a multireceptor opioid peptide

Biphalin is a dimeric opioid peptide, composed of two tetrapeptides connected 'tail-to-tail', that exhibits a high affinity for all three opioid receptor types (i.e. mu, delta and kappa). This study presents the X-ray crystal structure of biphalin sulfate and compares it to other opioids that interact with the same biological targets. Both halves of the molecule have a folded backbone conformation but differ significantly from one another. Residues 1-4 in biphalin, which compare well with the delta selective opioid peptide DADLE, fold into a random coil. Residues 5-8, which can be fit to the mu selective peptide D-TIPP-NH2, exhibit a fairly normal type III' beta bend. Biphalin also exhibits structural similarities with two naltrexone analogs, naltrexonazine and norbinaltorphamine, that are specific to mu and kappa receptor sites.

Solubilization of mu-opioid receptors enriched from bovine brain membranes

Solubilization is the most critical step in the purification of opioid receptors as these proteins are highly sensitive to detergents and get inactivated even with very mild detergents. Membranes enriched with micro-opioid receptors from bovine corpus striatum were solubilized by various methods to obtain the active soluble receptor suitable for affinity purification. Solubilization by digitonin resulted in marginal yields. CHAPS in presence of NaCl could extract active receptor into the solution. The detergent and NaCl were removed by either polyethylene glycol precipitation or by desalting on Sephadex G50. The polyethylene glycol precipitation resulted in the formation of liposomes into which the receptor protein was incorporated. Liposome formation was not observed in desalting method and the recovery of the receptor was partial.

Side chain modifications change the binding and agonist properties of endomorphin 2

Side chain modifications were introduced to endomorphin 2 (E2) to improve its binding properties and biological activity. A number of C-terminal modifications decreased the binding affinity to the mu-opioid receptor and the intrinsic activity in rat brain membranes. The exception was E2-ol, which showed increased binding affinity to MOR and higher potency in stimulating [(35)S]GTPgammaS binding. N-methylation of Phe(3) (MePhe(3)) attenuated the binding affinity and produced a rightward shift of [(35)S]GTPgammaS binding curves. All derivatives had lower intrinsic activity than E2. Some of the modified peptides partially inhibited, while YPF-benzyl-allyl-amide fully inhibited, the E2 or [d-Ala(2),MePhe(4),Gly(5)ol]enkephalin stimulated [(35)S]GTPgammaS binding. Marked differences were found between the results obtained using tritiated E2, tritiated naloxone, and [(35)S]GTPgammaS binding, indicating the possible involvement of multiple binding sites. The data presented demonstrate that the C-terminal amide group has an essential role in the regulation of the binding and the agonist/antagonist properties of E2.

Enantiomers of 2-[(Acylamino)ethyl]-1,4-benzodiazepines, potent ligands of kappa-opioid receptor: chiral chromatographic resolution, configurational assignment and biological activity

Compounds 2a and 3a-e are racemic 2-[(acylamino)ethyl]-1,4-benzodiazepines, tifluadom analogs, with high affinity and selectivity towards the kappa-opioid receptor. We describe the enantiomeric separation of all compounds through liquid chromatography with chiral stationary phases, as well as the resolution of the enantiomers of the most interesting compounds, 2a and 3a, by the semipreparative column Chiralpak AD. The configuration of the resolved enantiomers was investigated: the comparative study of CD and (1)H NMR spectra shows that compounds (-)-2a and (-)-3a have the same absolute configuration of (+)-(S)-tifluadom. A study on the stereoselective interaction with opiate receptors is reported.

Functional role of a conserved motif in TM6 of the rat mu opioid receptor: constitutively active and inactive receptors result from substitutions of Thr6.34(279) with Lys and Asp

Mutations within the "X1BBX2X3B" motif or its variants in the junction of the third intracellular (i3) loop and the sixth transmembrane domain (TM6) have been shown to lead to constitutive activation of several G protein-coupled receptors (GPCRs). In this study, T6.34(279) at the X3 locus of the rat mu opioid receptor was mutated to Lys and Asp, and the mutants were examined for binding and signaling properties. The T6.34(279)K mutant was poorly expressed, and pretreatment with naloxone greatly enhanced its expression. This construct exhibited properties identified previously with constitutive activation: (1) compared with the wild type, it produced much higher agonist-independent [35S]GTPgammaS binding, which was abolished by pertussis toxin treatment; (2) it displayed an enhanced affinity for the agonist DAMGO similar to that of the high-affinity state of the wild type, which was not altered by GTPgammaS, while having unchanged affinity for the antagonist diprenorphine. The T6.34(279)K mutant displayed a higher intracellular receptor pool than the wild type. Naloxone inhibited the basal [35S]GTPgammaS binding of the T6.34(279)K mutant, demonstrating inverse agonist activity at this mutant receptor. In contrast, the T6.34(279)D substitution did not increase basal [35S]GTPgammaS binding, greatly reduced agonist-promoted [35S]GTPgammaS binding, and markedly decreased affinity for DAMGO. Thus, the T6.34(279)D mutant adopts conformations corresponding to inactive states of the receptor. The results were interpreted in the structural context of a model for the mu opioid receptor that incorporates the information from the crystal structure of rhodopsin. The interaction of T6.34(279) with R3.50(165) in the mu opioid receptor is considered to stabilize the inactive conformations. The T6.34(279)K substitution would then disrupt this interaction and support agonist-free activation, while T6.34(279)D mutation should strengthen this interaction which keeps the receptor in inactive states. T6.34(279) may, in addition, interact with the neighboring R6.35(280) to help constrain the receptor in inactive states, and T6.34(279)K and T6.34(279)D mutations would affect this interaction by disrupting or strengthening it, respectively. To the best of our knowledge, the results presented here represent the first structurally rationalized demonstration that mutations of this locus can lead to dramatically different properties of a GPCR.

From hit to lead. Combining two complementary methods for focused library design. Application to mu opiate ligands

Compound 1 obtained by random screening and displaying a micromolar activity on the mu opiate receptor was chosen as a starting point for optimization. Two complementary concepts of similarity were used for the design of analogues and compared. These are based, respectively, on a computer-aided comparison of pharmacophoric patterns and on topological similarity. The structure-activity relationships are discussed in light of both similarity concepts. Compound 40, an N-methyl-3-(4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]decyl)acetamide derivative, designed by combining the structure-activity relationships enlightened by each method, has a subnanomolar affinity for mu (h) receptor (IC(50) = 0.9 nM). It is a promising lead, allowing the design of a new series of analogues substituted at the N-3 of the spirocycle moiety.

Constitutive activation of the mu opioid receptor by mutation of D3.49(164), but not D3.32(147): D3.49(164) is critical for stabilization of the inactive form of the receptor and for its expression

The roles of conserved aspartates in the third transmembrane domain of the rat mu opioid receptor (RMOR) were explored with mutations of D3.32(147) and D3.49(164). D3.49(164) in the highly conserved DRY motif was mutated to 13 amino acids. Except for the D3.49(164)E mutant, each mutant displayed little or no detectable [(3)H]diprenorphine binding, and pretreatment with naloxone greatly enhanced binding. D3.49(164)H, -Q, -Y, -M, and -E mutants were further studied. D3.32(147) was substituted with A or N. All seven mutants exhibited similar binding affinities for the antagonist [(3)H]diprenorphine as the wild-type. The D3.49(164)H, -Q, -Y, and -M mutants, but not the D3.49(164)E and D3.32(147) mutants, exhibited enhanced basal [(35)S]GTPgammaS binding which was comparable to the maximally activated level of the wild-type and was related to expression levels. Naloxone, naltrexone, and naloxone methiodide significantly inhibited the basal [(35)S]GTPgammaS binding of the D3.49(164) mutants, indicating inverse agonist activities. Treatment of the D3.49(164)Y mutant with pertussis toxin greatly reduced the basal [(35)S]GTPgammaS binding, demonstrating constitutive activation of Galpha(i)/Galpha(o). The D3.49(164)H, -Y, -M, and -Q mutants had higher affinities for DAMGO than the wild-type, which were not significantly lowered by GTPgammaS. Thus, mutation of D3.49(164) to H, Y, M, or Q in RMOR resulted in receptor assuming activated conformations. In contrast, the D3.49(164)E mutant displayed significantly lower basal [(35)S]GTPgammaS binding and reduced affinity for DAMGO. Upon incubation of membranes at 37 degrees C, the constitutively active D3.49(164)Y mutant was structurally less stable, whereas the inactivated D3.49(164)E mutant was more stable, than the wild-type. Computational simulations showed that the E3.49 side chain interacted strongly with the conserved R3.50 in the DRY motif and stabilized the inactive form of the receptor. Taken together, these results indicate that D3.49 plays an important role in constraining the receptor in inactive conformations.

C-terminal Splice Variants of the Mouse µ-Opioid Receptor Differ in Morphine-induced Internalization and Receptor Resensitization

The main analgesic effects of the opioid alkaloid morphine are mediated by the mu-opioid receptor. In contrast to endogenous opioid peptides, morphine activates the mu-opioid receptor without causing its rapid endocytosis. Recently, three novel C-terminal splice variants (MOR1C, MOR1D, and MOR1E) of the mouse mu-opioid receptor (MOR1) have been identified. In the present study, we show that these receptors differ substantially in their agonist-selective membrane trafficking. MOR1 and MOR1C stably expressed in human embryonic kidney 293 cells exhibited phosphorylation, internalization, and down-regulation in the presence of the opioid peptide [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO) but not in response to morphine. In contrast, MOR1D and MOR1E exhibited robust phosphorylation, internalization, and down-regulation in response to both DAMGO and morphine. DAMGO elicited a similar desensitization (during an 8-h exposure) and resensitization (during a 50-min drug-free interval) of all four mu-receptor splice variants. After morphine treatment, however, MOR1 and MOR1C showed a faster desensitization and no resensitization as compared with MOR1D and MOR1E. These results strongly reinforce the hypothesis that receptor phosphorylation and internalization are required for opioid receptor reactivation thus counteracting agonist-induced desensitization. Our findings also suggest a mechanism by which cell- and tissue-specific C-terminal splicing of the mu-opioid receptor may significantly modulate the development of tolerance to the various effects of morphine.

Stepwise Modulation of Neurokinin-3 and Neurokinin-2 Receptor Affinity and Selectivity in Quinoline Tachykinin Receptor Antagonists

A stepwise chemical modification from human neurokinin-3 receptor (hNK-3R)-selective antagonists to potent and combined hNK-3R and hNK-2R antagonists using the same 2-phenylquinoline template is described. Docking studies with 3-D models of the hNK-3 and hNK-2 receptors were used to drive the chemical design and speed up the identification of potent and combined antagonsits at both receptors. (S)-(+)-N-(1-Cyclohexylethyl)-3-[(4-morpholin-4-yl)piperidin-1-yl]methyl-2-phenylquinoline-4-carboxamide (compound 25, SB-400238: hNK-3R binding affinity, K(i) = 0.8 nM; hNK-2R binding affinity, K(i) = 0.8 nM) emerged as the best example in this approach. Further studies led to the identification of (S)-(+)-N-(1,2,2-trimethylpropyl)-3-[(4-piperidin-1-yl)piperidin-1-yl]methyl-2-phenylquinoline-4-carboxamide (compound 28, SB-414240: hNK-3R binding affinity, K(i) = 193 nM; hNK-2R binding affinity, K(i) = 1.0 nM) as the first hNK-2R-selective antagonist belonging to the 2-phenylquinoline chemical class. Since some members of this chemical series showed a significant binding affinity for the human mu-opioid receptor (hMOR), docking studies were also conducted on a 3-D model of the hMOR, resulting in the identification of a viable chemical strategy to avoid any significant micro-opioid component. Compounds 25 and 28 are therefore suitable pharmacological tools in the tachykinin area to elucidate further the pathophysiological role of NK-3 and NK-2 receptors and the therapeutic potential of selective NK-2 (28) or combined NK-3 and NK-2 (25) receptor antagonists.

Phosphorylation of Ser363, Thr370, and Ser375 residues within the carboxyl tail differentially regulates mu-opioid receptor internalization

Prolonged activation of opioid receptors leads to their phosphorylation, desensitization, internalization, and down-regulation. To elucidate the relationship between mu-opioid receptor (MOR) phosphorylation and the regulation of receptor activity, a series of receptor mutants was constructed in which the 12 Ser/Thr residues of the COOH-terminal portion of the receptor were substituted to Ala, either individually or in combination. All these mutant constructs were stably expressed in human embryonic kidney 293 cells and exhibited similar expression levels and ligand binding properties. Among those 12 Ser/Thr residues, Ser(363), Thr(370), and Ser(375) have been identified as phosphorylation sites. In the absence of the agonist, a basal phosphorylation of Ser(363) and Thr(370) was observed, whereas [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO)-induced receptor phosphorylation occurs at Thr(370) and Ser(375) residues. Furthermore, the role of these phosphorylation sites in regulating the internalization of MOR was investigated. The mutation of Ser(375) to Ala reduced the rate and extent of receptor internalization, whereas mutation of Ser(363) and Thr(370) to Ala accelerated MOR internalization kinetics. The present data show that the basal phosphorylation of MOR could play a role in modulating agonist-induced receptor internalization kinetics. Furthermore, even though mu-receptors and delta-opioid receptors have the same motif encompassing agonist-induced phosphorylation sites, the different agonist-induced internalization properties controlled by these sites suggest differential cellular regulation of these two receptor subtypes.

Synthesis and evaluation of 4-(N,N-diarylamino)piperidines with high selectivity to the delta-opioid receptor: a combined 3D-QSAR and ligand docking study

A series of 4-(N,N-diarylamino)piperidines are synthesized and evaluated for high affinity binding and selectivity to the delta-opioid receptor using a combination of 3D-QSAR and molecular docking techniques. Based on experimental ligand binding data to both mu- and delta- opioid receptors, CoMFA fields are generated and applied to identify potential ligand modifications to further optimize lead compounds. Molecular docking experiments to the delta-receptor are also reported that explain the CoMFA trends predicted as well as the differential binding and selectivity displayed by various compounds in the series. An analysis of the binding site model proposed indicates the piperidines take advantage of 3 key sites or binding domains within the delta-receptor. These include an aromatic pocket (approximately 1/3 into the receptor cavity), an aspartic acid residue (which serves as a docking point for the piperidinyl cationic amine) and a hydrophobic pocket at the extracellular boundary of the receptor cavity. Links are established between ligand modification and amino acid composition at these sites in mu and delta, providing new insight to the structural basis to binding and selectivity across the series and for related piperazines (i.e. SNC80 and BW373U86). Results are also presented that indicate delta- and mu-selectivity may be determined at alternate sites, suggesting opioid receptors may display multiple binding domains. The model is further supported by comparisons with opiate binding modes and site directed mutagenesis studies and is finally applied to suggest new strategies in ligand design.

Threonine 180 is required for G-protein-coupled receptor kinase 3- and beta-arrestin 2-mediated desensitization of the mu-opioid receptor in Xenopus oocytes

To determine the sites in the mu-opioid receptor (MOR) critical for agonist-dependent desensitization, we constructed and coexpressed MORs lacking potential phosphorylation sites along with G-protein activated inwardly rectifying potassium channels composed of K(ir)3.1 and K(ir)3.4 subunits in Xenopus oocytes. Activation of MOR by the stable enkephalin analogue, [d-Ala(2),MePhe(4),Glyol(5)]enkephalin, led to homologous MOR desensitization in oocytes coexpressing both G-protein-coupled receptor kinase 3 (GRK3) and beta-arrestin 2 (arr3). Coexpression with either GRK3 or arr3 individually did not significantly enhance desensitization of responses evoked by wild type MOR activation. Mutation of serine or threonine residues to alanines in the putative third cytoplasmic loop and truncation of the C-terminal tail did not block GRK/arr3-mediated desensitization of MOR. Instead, alanine substitution of a single threonine in the second cytoplasmic loop to produce MOR(T180A) was sufficient to block homologous desensitization. The insensitivity of MOR(T180A) might have resulted either from a block of arrestin activation or arrestin binding to MOR. To distinguish between these alternatives, we expressed a dominant positive arrestin, arr2(R169E), that desensitizes G protein-coupled receptors in an agonist-dependent but phosphorylation-independent manner. arr2(R169E) produced robust desensitization of MOR and MOR(T180A) in the absence of GRK3 coexpression. These results demonstrate that the T180A mutation probably blocks GRK3- and arr3-mediated desensitization of MOR by preventing a critical agonist-dependent receptor phosphorylation and suggest a novel GRK3 site of regulation not yet described for other G-protein-coupled receptors.