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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Med Chem. Author manuscript; available in PMC 2010 December 10.
Published in final edited form as:
PMCID: PMC2788680
NIHMSID: NIHMS150015

Pentapeptides displaying mu opioid receptor agonist and delta opioid receptor partial agonist/antagonist properties

Abstract

Chronic use of mu-opioid agonists has been shown to cause neurochemical adaptations resulting in tolerance and dependence. While the analgesic effects of these drugs are mediated by mu-opioid receptors (MOR), several studies have shown that antagonism or knockdown of delta-opioid receptors (DOR) can lessen or prevent development of tolerance and dependence. Based on computational modeling of putative active and inactive conformations of MOR and DOR, we have synthesized a series of pentapeptides with the goal of developing a MOR agonist/DOR antagonist peptide with similar affinity at both receptors as a tool to probe functional opioid receptor interaction(s). The eight resulting naphthylalanine-substituted cyclic pentapeptides displayed variable mixed-efficacy profiles. The most promising peptide (9; Tyr-c(S-CH2-S)[D-Cys-Phe-2-Nal-Cys]NH2) displayed a MOR agonist and DOR partial agonist/antagonist profile and bound with equipotent affinity (Ki ~ 0.5 nM) to both receptors, but also showed kappa opioid receptor (KOR) agonist activity.

Introduction

Mu-opioid receptor (MOR) agonists such as morphine are commonly used in the treatment of moderate to severe pain. However, use of such drugs is associated with side effects including the development of tolerance, limiting the usefulness of these compounds. It has been hypothesized that opioid compounds displaying MOR agonism paired with a selective delta- or kappa-opioid receptor effect could lessen the severity of limiting side effects surrounding current MOR agonist use1, including respiratory depression and constipation as well as tolerance. In particular, studies pointing to a role of the delta opioid receptor (DOR) in modulating the development of MOR tolerance have led to the hypothesis that both MOR and DOR play major roles in the development of tolerance after chronic morphine exposure. For example, work in DOR knockout rodent models24 or using DOR antagonists58 was shown to prevent or lessen the severity of tolerance development to chronic morphine exposure. More recent in vivo work also points to a role of DOR in modulating morphine-induced behavioral sensitization and conditioned place preference in rodents.911 It has been hypothesized that the formation of homo- or heterodimers of MOR and DOR leads to changes in their pharmacological behaviors including alteration in tolerance or dependence development.6, 1214

The growing body of evidence implicating a role of DOR in modulating MOR-induced tolerance suggests that opioid ligands with similar affinities at MOR and DOR, but displaying agonism at MOR and antagonism at DOR might be of great clinical potential, especially for the treatment of chronic pain conditions. Consequently, many groups have developed compounds with MOR and DOR affinity, including peptidic1519 and non-peptidic2024 ligands displaying MOR agonism and DOR antagonism. However, many of these compounds, while displaying the desired efficacy profile, do not have equivalent binding affinities to both MOR and DOR, thus limiting their usefulness in probing MOR-DOR interactions.

Our previous work led to the synthesis of peptide 1 (Tyr-c(S-CH2-S)[D-Cys-Phe-Phe-Cys]NH2).25 Peptide 1 displayed a promising mixed-efficacy profile at MOR and DOR, binding with high affinity for both MOR and DOR while exhibiting full agonism at MOR and the kappa opioid receptor (KOR) but only partial agonism at DOR. We wished to improve peptide 1 by decreasing efficacy at DOR while increasing affinity for this receptor, retaining both efficacy and affinity at MOR, and reducing affinity at KOR. To pursue this aim, we examined the docking of 1 into computational models of MOR and DOR. Based on modeling of putative active and inactive conformations of MOR and DOR2629 and docking of 1 to these models, we focused on steric constraints surrounding the third and fourth Phe residues of 1. We hypothesized that replacement of these Phe residues with bulkier side chains would decrease ligand affinity to the DOR active state, but not the DOR inactive state and not affect binding to MOR, thus favoring the desired MOR agonist/DOR antagonist profile. Consequently, we designed eight analogues of peptide 1 containing naphthylalanine in place of Phe3 or Phe4 to more fully explore the steric limits of the receptor binding pocket at either of these positions. We have previously used naphthylalanine substitution to add steric bulk in cyclic peptides30 and this has been more recently applied to linear peptides.31 In vitro, our cyclic peptides displayed variable MOR efficacies and had decreased DOR efficacy. One compound (peptide 9; Tyr-c(S-CH2-S)[D-Cys-Phe-2-Nal-Cys]NH2) displayed full agonism at MOR (99% stimulation compared with the full MOR agonist DAMGO) while acting as an antagonist at DOR in the [35S]GTPγS assay but with partial agonist activity in the adenylyl cyclase inhibition assay, and bound with similar subnanomolar affinity to MOR and DOR stably and independently expressed in C6 rat glioma cells. Compound 9 also bound with high affinity to KOR and was an agonist at that receptor. Thus, incorporation of a substitution based on rational design and intended to highlight putative steric constraints resulted in a compound that had similar affinity for MOR and DOR but decreased DOR efficacy without compromising MOR agonism. This is an important step forward in the development of novel ligands presenting MOR agonist and DOR antagonist effects.

Results

Rationale for the design of pentapeptides displaying MOR agonism and DOR antagonism

Previous work by our group led to the synthesis of the high-affinity, MOR-selective cyclic pentapeptide 1 (Tyr-c(S-CH2-S)[D-Cys-Phe-Phe-Cys]NH2),25 which has picomolar affinity for MOR (Ki = 0.016 nM) and nanomolar affinities for DOR (Ki = 1.8 nM) and KOR (Ki = 2.5 nM). When evaluated for efficacy at the three opioid receptors using the [35S]GTPγS assay32, 1 displayed full agonism at MOR and KOR, but only partial agonism at DOR (Table 1). In order to understand the molecular mechanism underlying the mixed-efficacy profile of 1, computational models of MOR and DOR were utilized. Peptide 1 was virtually docked in models of active and inactive conformations of MOR and DOR2629 (Figure 1) in a similar manner as MOR and DOR-selective tetrapeptides, JOM6 and JOM13 respectively, which were previously positioned in MOR and DOR models based on their structure-activity relationships and receptor mutagenesis data.26, 27, 33, 34 These active and inactive receptor models were designed based on the published crystal structure of the β2-adrenergic receptor35 (as described in the Experimental section). Though the new adrenoreceptor-based models of opioid receptors differ from our previously published rhodopsin-based models26, 28, 29, 33 by some helix shifts and an outward movement of extracellular loop (EXL) 2 (r.m.s.d. in the range 2.3–2.6 Å for all Cα-atoms, excluding EXL2), the ligand binding mode and receptor-ligand interactions with residues from transmembrane helixes (TMs) 3, 5, 6 and 7 of either DOR or MOR remained essentially the same. Thus, we found that the docking mode of peptide ligands is more influenced by its interactions with helix residues than residues from EXL2, the modeling of which is expected to be less accurate.

Figure 1
Modeling of peptide 1 in the binding pocket of putative active and inactive conformations of mouse MOR and human DOR. Peptide 1 docked in the putative active (A) and inactive (B) conformation of MOR shows no noticeable unfavorable interactions between ...
Table 1
Binding affinities and efficacies of cyclic opioid pentapeptides 1–9 at MOR, DOR, and KOR.

Examination of the position of pentapeptide 1 inside the binding pocket of active and inactive MOR (Figure 1A, B) allowed us to obtain a low-energy conformation of 1 that did not have steric hindrances or other adverse interactions with residues in the receptor binding pocket in either receptor state. Peptide 1 docked in MOR showed favorable aromatic interactions between its Phe3 side chain and the Trp318 side chain in TM7, which were more pronounced in the active conformation of MOR. These aromatic interactions may explain the preferential binding of 1 to MOR, as compared to DOR or KOR, which have Leu300 or Tyr312 at the corresponding position. The same conformation of 1, when fitted into the active DOR model (Figure 1C), demonstrated some steric overlap between Phe4 of the ligand and Trp284 from the TM6 of DOR which was not observed in the inactive DOR model (Figure 1D). The docking of peptide 1 to both active and inactive conformations of MOR and its better compatibility with the inactive conformation of DOR is consistent with the MOR agonist/DOR partial agonist profile of 1.

Based on the above, we hypothesized that incorporation of a bulkier naphthylalanine side chain in either the third or fourth position of pentapeptide 1 would affect its binding and efficacy properties differentially at MOR and DOR and could result in potent MOR agonist/DOR antagonist ligands. To test this hypothesis, we replaced Phe3 and Phe4 of 1 with the bulkier 3-(1-naphthyl)alanine or 3-(2-naphthyl)alanine (Figure 2) to provide eight new cyclic pentapeptides (2–9; Table 1, Table 2). Peptides were cyclized via a disulfide bond (S-S) or methylene dithioether (S-CH2-S) linkage to allow for altered size and flexibility of the cycle.

Figure 2
Naphthylalanine-containing analogs of peptide 1.
Table 2
Analytical data of peptides 2–9.

Analysis of synthesized pentapeptides 2–9 for binding affinity and efficacy at opioid receptors

Most naphthylalanine peptides demonstrated relatively high binding affinities to MOR, DOR, and KOR as measured by competitive displacement of the radiolabeled non-selective opioid antagonist [3H]diprenorphine. However, some loss in affinity to all three opioid receptors was generally noted as compared to the highly potent pentapeptide 1. At MOR, the most significant loss of affinity occurred with peptide 6 (p < 0.001) while at DOR four peptides had ≥ 10-fold decreased affinity (2, 3, 4 p < 0.001, and 5 p < 0.05). At KOR, peptides 4 and 5 had > 10-fold decreased binding affinity when compared to 1 (p < 0.001) (Table 1). Of the eight peptides synthesized, only peptide 9 (Tyr-c(S-CH2-S)[D-Cys-Phe-2-Nal-Cys]NH2) showed similar low nanomolar binding affinity to MOR, DOR, and KOR (Ki = 0.47 nM, 0.48 nM, and 1.3 nM, respectively).

Peptides 2–9 were also analyzed for efficacy at MOR, DOR, and KOR as determined by maximal stimulation of [35S]GTPγS binding32 as a percentage of 10 µM standard compounds DAMGO, SNC80, and U69,593 (Table 1). Peptides 3, 6, and 8 showed equivalent agonism at MOR as peptide 1 (~ 90% maximal stimulation), while peptides 2, 4, and 5 had decreased efficacy at MOR (31–34% stimulation; p < 0.001). In contrast, peptides 7 and 9 showed greater stimulation of [35S]GTPγS binding at MOR than 1 (p < 0.05), displaying the same maximal stimulation. It is noteworthy that all peptides displaying decreased efficacy at MOR (2, 4, and 5) were substituted at position 3 with naphthylalanine. In contrast, analogs with naphthylalanine substitution at position 4 (6–9) showed equal or greater efficacy at MOR, indicating substitution at position 3 by residues with bulky side chains is deleterious for MOR agonism. One exception was peptide 3, which has naphthylalanine substitution at position 3 and displayed high efficacy at MOR. Peptide 3 was cyclized via a methylene dithioether, possibly allowing greater flexibility of the peptide ring structure and leading to increased stimulation over the disulfide-cyclized counterpart 2. At DOR, peptides 2, 4, 8, and 9 behaved essentially as antagonists, providing very little or no [35S]GTPγS stimulation. Compounds 3, 5, 6, and 7 displayed partial agonism with maximal stimulation varying from 15–23%. The eight peptides displayed varying efficacy profiles at KOR, with most compounds behaving as partial to full agonists (maximal stimulation 35–100%).

Docking of peptide 9 to modeled MOR and DOR active and inactive conformations

The binding and efficacy studies described above identify peptide 9 as a candidate ligand displaying the desired MOR agonism and DOR antagonism profile. In order to better understand the mechanism of the decreased efficacy of 9 at DOR, we docked this peptide in the modeled active and inactive conformations of MOR and DOR similar to the docking of peptide 1 (Figure 3).

Figure 3
Pentapeptide 9 docked in the binding pocket of putative active and inactive conformations of mouse MOR and human DOR. The 2-naphthylalanine4 side chain of peptide 9 shows minimal hindrance with receptor residue Lys303 in the MOR active conformation (A), ...

The lowest energy conformation of peptide 9 exhibited no hindrance in the binding pocket of either active or inactive conformations of MOR (Figure 3A, B) nor in the inactive DOR conformation (Figure 3D). However, when compared to peptide 1 (Figure 2C), 9 shows greater overlap between its 2-naphthylalanine4 and Trp284 in the active conformation of DOR (Figure 3C). This reduced compatibility of 9 relative to 1 for the active state of DOR supports the decreased agonism at DOR. Modeling results also explain the high agonist efficacy of 9 at MOR, because 9 fits both receptor states well and may promote a conformational shift toward the active conformation of MOR where favorable aromatic interactions between Phe3 and Trp318 of MOR are more prominent.

Characterization of the functional properties of peptide 9 at opioid receptors

In the [35S]GTPγS binding assay, peptide 9 behaved as a full agonist at MOR with EC50 of 1.2 ± 0.05 nM (Figure 4A). On the other hand, at 10 µM concentration 9 produced only 7% of SNC80-induced stimulation of DOR-mediated [35S]GTPγS binding (Table 1).

Figure 4
Pharmacological analysis of peptide 9. (A) Activity of peptide 9 in the [35S]GTPγS binding assay at MOR, DOR and KOR. Results are plotted as percent stimulation compared to a 10 µM concentration of standard compounds (MOR standard agonist ...

The properties of 9 were further evaluated by measuring its ability to inhibit SNC80-stimulated binding of [35S]GTPγS to G-proteins. Peptide 9 produced a 3.1-fold rightward shift in the dose-response curve of SNC80 in C6-DOR cells (Figure 4B); the EC50 for SNC80 was shifted from 75 ± 3.8 nM to 188 ± 31 nM in the presence of 100 nM 9 (p = 0.02). However, this shift and the calculated Ke value (48 ± 9.5 nM) for 9 was not consistent with its high binding affinity to DOR (Ki = 2.1 nM), indicating 9 may have some partial agonist efficacy at DOR which cannot be fully observed using the high efficacy-requiring [35S]GTPγS binding assay. To more fully assess the extent of this partial agonism, we measured the ability of 9 to inhibit cAMP accumulation as a measure of adenylyl cyclase activity.36 Due to downstream signaling amplification, it is easier to visualize partial agonism using this system. Peptide 9 was shown to be more potent (EC50: 36 ± 4.8 nM) than SNC80 (EC50: 166 ± 43 nM; p = 0.01) and behaved as a partial agonist, able to produce 55% inhibition of that seen with SNC80 (Figure 4C). The DOR-selective antagonist naltrindole (NTI) was without effect in this assay.

Discussion

The current studies were aimed toward the development of a potent compound with mixed MOR agonist/DOR antagonist properties, a profile that would be valuable to probe interactions of MOR and DOR and that has considerable clinical promise. The eight newly synthesized cyclic pentapeptides represent modifications of our previously reported pentapeptide 1 (Tyr-c(S-CH2-S)[D-Cys-Phe-Phe-Cys]NH2)25, which was characterized by high affinity binding to all opioid receptors and a mixed efficacy profile. Based on our computational modeling of active and inactive conformations of MOR and DOR and docking of 1 to these models (Figure 1), we focused on receptor-ligand interactions surrounding Phe3 and Phe4 residues of 1. We designed several derivative peptides containing naphthylalanine substitution to more fully explore the steric limits of the receptor binding pocket at either of these positions. All peptides were evaluated for their potential to interact at both MOR and DOR via receptor binding and in vitro functional studies. The newly synthesized peptides demonstrated MOR agonism with variable efficacies and had greatly decreased DOR efficacy in the [35S]GTPγS binding assay. One compound, peptide 9 (Tyr-c(S-CH2-S)[D-Cys-Phe-2-Nal-Cys]NH2), bound with similar subnanomolar affinity to MOR and DOR stably expressed in rat glioma cells and was characterized as an agonist at MOR and an antagonist or partial agonist at DOR depending on the assay used. This latter difference highlights the importance of the choice of assay in efficacy determination.37

The development of pentapeptide 9 represents a significant step forward in the development of a mixed-efficacy MOR agonist/DOR antagonist ligand. Previously reported mixed-efficacy ligands did not show the same equipotent affinity for both MOR and DOR1517, 1924 or the same full MOR agonist properties.18, 21, 22 The results also represent a validation of our receptor models and a novel demonstration of the use of differences in modeled active and inactive states to design ligands with prescribed properties. In this example, steric differences in the binding site of the active and inactive DOR models were exploited by incorporating bulkier naphthylalanine in place of phenylalanine in residues 3 and 4 of lead peptide 1 to generate ligands with the desired MOR agonist/DOR antagonist profile.

Although peptide 9 displays the desired MOR/DOR mixed-efficacy profile, it also acts as a full agonist at KOR in the [35S]GTPγS binding assay with EC50 of 12 ± 0.1 nM although it is 10-fold selective in potency for MOR over KOR (Figure 4A). This residual KOR activity is not surprising, as compound 1 was initially developed in a series aiming to improve KOR binding and efficacy.25 Using computational models of KOR developed as described above for putative active and inactive conformation models of MOR and DOR, we are designing analogs that are intended to exhibit reduced KOR affinity while retaining the desired MOR agonist/DOR antagonist profile. Such compounds will allow us to characterize MOR and DOR interactions without the potential complication of concomitant KOR activation or antagonism.

Experimental Section

Materials

All Fmoc-protected amino acids were obtained from Advanced ChemTech (Louisville, KY) or Sigma-Aldrich (St. Louis, MO). All other reagents for peptide synthesis and characterization were from Sigma-Aldrich (St. Louis, MO) unless otherwise indicated. Fetal bovine serum and all cell culture media and additives were purchased from Gibco Life Sciences (Grand Island, NY). [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) and all other biochemicals were obtained from Sigma-Aldrich and were of analytical grade. 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80) was obtained from the Narcotic Drug and Opioid Peptide Basic Research Center at the University of Michigan (Ann Arbor, MI). [35S] - guanosine - 5' - O - (3 - thio) triphosphate ([35S] - GTPγS; 1250Ci (46.2TBq)/mmol) and [3H]-diprenorphine were purchased from Perkin Elmer (Boston, MA). Cyclic AMP accumulation was determined by radioimmunoassay (GE Healthcare; Piscataway, NJ).

Structural modeling

The homology modeling of the inactive conformation of human DOR (residues 45–338, UniProt accession code P41143) and mouse MOR (residues 64–354, UniProt accession code P42866) was performed as previously described26, 27, 38 using the more recent structure of the β2-adrenergic receptor fused with T4 lysozyme (2rh1 PDB code).35 Distance geometry calculations with DIANA39 were used to provide helix shift and loop modeling. To model the active receptor conformation, we included several structural constraints between TM 3, 5, and 6 that have been shown to be compatible with active states of different GPCRs.27 Introduction of these constraints allowed us to reproduce the significant movement of TM6 that has been suggested based on numerous experimental studies of different GPCRs, as well as on the comparison of the rhodopsin (1f88)40 and opsin (3dqb)41 crystal structures.

3D structures of cyclic pentapeptides were generated by QUANTA (Accelrys Inc.) using residue substitution of previously modeled pentapeptides25 followed by molecular mechanics computations using the CHARMm force field. Several conformations of disulfide or methylene dithioether bridges and different rotamers of residues in the third and fourth positions of pentapeptides were tested during ligand docking. Several low energy conformations (within 2 kcal/mol) were manually positioned inside the receptor binding cavity similarly to tetrapeptides JOM6 and JOM13 whose docking in DOR and MOR has been previously justified using conformational and mutagenesis analysis.26, 27, 33, 34 During ligand docking, we selected those low energy conformations that better satisfied the following criteria: (1) provided interactions between Tyr1 and Phe3 and functionally important receptor residues (Asp147 from TM3, His297 from TM6 and Trp318 from TM7 of MOR27 or corresponding Asp128, His278, and Leu300 in DOR); (2) had minimal steric overlap; and (3) formed more hydrogen-bonds between receptor and ligand polar groups. The docking pose of each ligand was subsequently refined using the Solid Docking module of QUANTA. The opioid receptor models are available upon request.

Solid Phase Peptide Synthesis

All peptides were synthesized by solid phase methods42 on an ABI Model 431A solid phase peptide synthesizer (Applied Biosystems, Foster City, CA) as previously published.25 Rink resin (Advanced ChemTech) was used as the solid support for C-terminal carboxamide peptides. Peptide elongation on the peptide-resin involved treating resin with piperidine (Sigma-Aldrich) to cleave the Fmoc-protecting group, diisopropylethylamine (DIEA) activation, followed by coupling of the next amino acid with o-benzotriazol-1-yl-N,N,N',N'-tetramethyl uronium hexafluorophosphate (HBTU) and 1-hydroxybenzotriazole (HOBt; Applied Biosystems). These steps were repeated until the entire peptide was assembled. A solution of trifluoroacetic acid/H2O/triisopropylsilane (9:0.5:0.5, v/v/v) was used to cleave the linear peptide from the resin and simultaneously remove the side chain-protecting groups. The peptide solution was filtered from the resin and then subjected to semi-preparative reverse phase high-performance liquid chromatography (RP-HPLC) to afford the linear disulfhydryl-containing peptide (Table 2). Final product confirmation was obtained by high resolution mass spectroscopy (HRMS) (Protein Structure Facility, University of Michigan, Ann Arbor MI).

General method for disulfide cyclization of peptides

To obtain disulfide-cyclized peptides, linear disulfhydryl-containing peptide was dissolved in a 1% (v/v) acetic acid (AcOH) in H2O solution (saturated with N2) at 5°C (1mg linear peptide/mL of aqueous AcOH solution). The pH of the peptide solution was raised to 8.5 using NH4OH, followed by the addition of 4 equiv of K3Fe(CN)6. The reaction mixture was stirred for 2 min and quenched by adjusting the pH to 3.5 with AcOH. The mixture was then subjected to semi-preparative RP-HPLC to afford the disulfide-cyclized peptide.

General method for methylene dithioether cyclization of peptides

To form methylene dithioether-containing cyclic peptides, linear disulfhydryl peptide was added to dimethylformamide (DMF) and maintained at 5°C under a N2 atmosphere (0.1mg linear peptide/mL DMF). Approximately 10 equiv of potassium t-butoxide was added to the peptide solution, followed by the addition of 10 equiv of dibromomethane. The reaction was quenched with 5 mL AcOH after 2 h and the solvent removed in vacuo. The residue was dissolved in water, filtered, and subjected to semi-preparative RP-HPLC to obtain the methylene dithioether cyclized peptide.

Characterization of Peptides

All final product peptides were >97% pure as assessed by analytical RP-HPLC on a Vydac 218TP C-18 column (The Nest Group, Southboro, MA) using two solvent systems; (A): 0.1% trifluoroacetic acid (TFA; w/v) in water/0.1% TFA (w/v) in acetonitrile with a gradient of 0–70% organic component in 70 min and (B): 0.1% TFA (w/v) in water/0.1% TFA (w/v) in methanol with a gradient of 20–70% organic component in 50 min, monitored at 230 nm. All peptides displayed the appropriate molecular weights as determined by HRMS (Table 2).

Cell Lines and Membrane Preparations

C6-rat glioma cells stably transfected with a rat mu (C6-MOR) or rat delta (C6-DOR) opioid receptor43 and Chinese hamster ovary (CHO) cells stably expressing a human kappa (CHO-KOR) opioid receptor44 were used. Cells were grown to confluence at 37°C in 5% CO2 in either Dulbecco's Modified Eagle's Medium (DMEM; C6 cells), or DMEM-F12 Medium (CHO-κ) containing 10% fetal bovine serum. To prepare membranes for biochemical assays as described previously45, confluent cells were washed twice with ice-cold phosphate-buffered saline (0.9% NaCl; 0.61mM Na2HPO4; and 0.38mM KH2PO4, pH 7.4), detached from the plates by incubation in harvesting buffer (20mM HEPES, pH 7.4; 150mM NaCl; and 0.68mM EDTA) at room temperature, and pelleted by centrifugation at 200×g for 3 min. The cell pellet was suspended in ice-cold 50mM Tris-HCl buffer, pH 7.4 and homogenized with a Tissue Tearor (Biospec Products, Inc., Bartlesville, OK) for 20 s at setting 4. The homogenate was centrifuged at 20,000×g for 20 min at 4°C and the pellet re-homogenized in 50mM Tris-HCl with a Tissue Tearor for 10 s at setting 2, followed by re-centrifugation. The final pellet was re-suspended in 50mM Tris-HCl, to 0.5–1.0 mg/ml protein and frozen in aliquots at −80°C. Protein concentration was determined using the BCA protein assay46 (Thermo Fisher Scientific, Rockford, IL) using bovine serum albumin as the standard.

Radioligand Binding Assays

Opioid ligand-binding assays were based on the competitive displacement of [3H]diprenorphine by the test compound from membrane preparations containing opioid receptors as described above.25 The assay mixture, containing membrane suspension (10–20 µg protein/tube) in 50mM Tris-HCl buffer (pH 7.4), [3H]diprenorphine (0.2 nM), and increasing concentrations of test peptide, was incubated at 25°C for 1 h to allow binding to reach equilibrium. Subsequently, the samples were filtered rapidly through GF/C filters (Whatman, Middlesex, UK) using a Brandel harvester and washed three times with ice-cold 50mM Tris-HCl buffer. The radioactivity retained on dried filters was determined by liquid scintillation counting after saturation with EcoLume liquid scintillation cocktail (MP Biomedicals, Solon, OH) in a Wallac 1450 MicroBeta (PerkinElmer, Waltham, MA). Non-specific binding was determined using 10 µM naloxone. Ki values were determined from nonlinear regression analysis to fit a logistic equation to the competition data using GraphPad Prism 5.01 software (GraphPad Software, La Jolla, CA). The results presented are the mean from at least three separate assays, each performed in duplicate.

[35S]GTPγS Binding Assay

Agonist stimulation of [35S]GTPγS binding was measured as described previously.32 Briefly, membranes (20–30 µg of protein/tube) were incubated in GTPγS binding buffer (50mM Tris-HCl, pH 7.4; 100mM NaCl; and 5mM MgCl2) containing 0.1 nM [35S]GTPγS, 100 µM GDP, and varying concentrations (0.001–10,000 nM) or a maximum dose (10 µM) of opioid peptides, compared with standards DAMGO, SNC80, or U69,593 (10 µM) in a total volume of 500 µl for 1 h at 25°C. The reaction was terminated by rapidly filtering through GF/C filters and washing four times with 2 ml ice-cold GTPγS binding buffer. Retained radioactivity was measured as described in radioligand binding assay methods. Experiments were performed at least three times in duplicate. EC50 values were determined by nonlinear regression analysis using GraphPad Prism 5.01 software as described above. To determine antagonism of 9, [35S]GTPγS binding was determined for SNC80 in the presence or absence of 100 nM 9. The IC50 value in the presence of 100 nM 9 was divided by the IC50 value for SNC80 alone, and this ratio (DR) was employed to calculate the Ke value using the equation Ke = [antagonist]/(DR-1).

Whole cell acute inhibition of adenylyl cyclase

Inhibition of adenylyl cyclase by opioid standards or test peptides was measured in C6-DOR cells grown to confluence in 24-well plates.36 Cells were washed in serum-free DMEM at least 30 min prior to the start of the assay and incubated with vehicle or various concentrations (0.1 – 1000 nM) of SNC80, naltrindole, or peptide 9 in serum-free media containing 5 µM forskolin (FSK) and 1 mM 3-isobutyl-1-methylxanthine for 10 min at 37°C. The assay was quenched by replacing media with 1 ml ice-cold 3% perchloric acid and 30 min incubation at 4°C. A 400 µl aliquot of sample was neutralized with 2.5M KHCO3 and centrifuged 1 min at 11,000×g. Cyclic AMP (cAMP) was measured from the supernatant using a radioimmunoassay kit from GE Healthcare (Piscataway, NJ), according to the manufacturer’s instructions. Inhibition of cAMP accumulation by 9 or standard opioid ligands was calculated as a percent of FSK-stimulated cAMP accumulation in vehicle-treated cells. EC50 values were calculated for each compound using GraphPad Prism 5.01 software. Experiments were performed in duplicate and repeated a minimum of three times.

Statistical Analysis

Data were analyzed using Student’s two-tailed t test or a one-way analysis of variance followed by Bonferroni’s post-hoc test using GraphPad Prism version 5.01 for Windows (GraphPad Software, San Diego, CA www.graphpad.com). p values less than 0.05 were considered to be significant.

Acknowledgements

This work was funded by NIH grants DA04087 (JRT) and DA03910 (HIM). LCP was supported by DA007281 and DA007267. The authors would like to thank Hui-Fang Song for technical assistance with the [3H]diprenorphine binding assays and Jessica Anand for assistance with the HRMS analyses.

Abbreviations

Symbols and abbreviations are in accordance with recommendations of the IUPAC-IUB Joint Commission on Biochemical Nomenclature: Nomenclature and Symbolism for Amino Acids and Peptides. Biochem. J. 1984, 219, 345–373. All other abbreviations are as follows:

1-Nal
3-(1-naphthyl)alanine
2-Nal
3-(2-naphthyl)alanine
AcOH
acetic acid
C6-MOR
C6 rat glioma cells stably expressing the mu-opioid receptor
C6-DOR
C6 rat glioma cells stably expressing the delta-opioid receptor
cAMP
cyclic adenosine monophosphate
CHO-KOR
Chinese hamster ovary cells stably expressing the kappa-opioid receptor
DAMGO
[D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin
DIEA
diisopropylethylamine
DMEM
Dulbecco’s Modified Eagle Medium
DMF
dimethylformamide
DOR
delta opioid receptor
EXL
extracellular loop
Fmoc
9-fluorenylmethyloxycarbonyl
FSK
forskolin
GPCR
G protein-coupled receptor
[35S]GTPγS
[35S]-guanosine-5'-O-(3-thio)triphosphate
HBTU
o-benzotriazole-N,N,N',N'-tetramethyl-uronium-hexafluoro-phosphate
HOBT
1-hydroxybenzotriazole
KOR
kappa opioid receptor
MOR
mu opioid receptor
NTI
naltrindole
r.m.s.d.
root mean square deviation
RP-HPLC
reverse phase-high performance liquid chromatography
SNC80
4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)- methyl]-N,N-diethylbenzamide
TFA
trifluoroacetic acid
TM
transmembrane helixes
U69,593
(5α, 7α, 8β)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]benzeneacetamide

Footnotes

This work is submitted in celebration of the 100th anniversary of the Division of Medicinal Chemistry.

References

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