Real-time imaging of mu opioid receptors by total internal reflection fluorescence microscopy.

Receptor trafficking and signaling are intimately linked, especially in the Mu opioid receptor (MOR) where ligand-dependent endocytosis and recycling have been associated with opioid tolerance and dependence. Ligands of MOR can induce receptor endocytosis and recycling within minutes of exposure in heterologous systems and cultured neurons. Endocytosis removes desensitized receptors after their activation from the plasma membrane, while recycling promotes resensitization by delivering functional receptors to the cell surface. These rapid mechanisms can escape traditional analytical methods where only snapshots are obtained from highly dynamic events.Total internal reflection fluorescence (TIRF) microscopy is a powerful tool that can be used to investigate, in real time, surface trafficking events at the single molecule level. The restricted excitation of fluorophores located at or near the plasma membrane in combination with high sensitivity quantitative cameras makes it possible to record and analyze individual endocytic and recycling event in real time. In this chapter, we describe a TIRF microscopy protocol to investigate in real time, the ligand-dependent MOR trafficking in Human Embryonic Kidney 293 cells and dissociated striatal neuronal cultures. This approach can provide unique spatio-temporal resolution to understand the fundamental events controlling MOR trafficking at the plasma membrane.

Detection of mu opioid receptor (MOPR) and its glycosylation in rat and mouse brains by western blot with anti-µC, an affinity-purified polyclonal anti-MOPR antibody.

Our experience demonstrates that it is difficult to identify MOPR in rat and mouse brains by western blot, in part due to low abundance of the receptor and a wide relative molecular mass (Mr) range of the receptor associated with its heterogeneous glycosylation states. Here, we describe generation and purification of anti-µC (a rabbit polyclonal anti-MOPR antibody), characterization of its specificity in immunoblotting of HA-tagged MOPR expressed in a cell line, and ultimately, unequivocal detection of the MOPR in brain tissues by western blot with multiple rigorous controls. In particular, using brain tissues from MOPR knockout (K/O) mice as the negative controls allowed unambiguous identification of the MOPR band, since the anti-MOPR antibody, even after affinity purification, recognizes nonspecific protein bands. The MOPR was resolved as a faint, broad, and diffuse band with a wide Mr range of 58-84 kDa depending on brain regions and species. Upon deglycosylation to remove N-linked glycans by PNGase F (but not Endo H), the MOPR became a dense and sharp band with Mr of ~43 kDa, close to the theoretical Mr of its deduced amino acid sequences. Thus, MOPRs in rodent brains are differentially glycosylated by complex type of N-linked glycans in brain region- and species-specific manners. Furthermore, we characterized the MOPR in an A112G/N38D-MOPR knockin mouse model that possesses the equivalent substitution of the A118G/N40D SNP in the human MOPR gene. The substitution removes one of the four and five N-linked consensus glycosylation sites of the mouse and human MOPR, respectively. We demonstrated that the Mr of the MOPR in A112G mouse brains was lower than that in wild-type mouse brains, and that the difference was due to lower degrees of N-linked glycosylation.

A real-time, fluorescence-based assay for measuring µ-opioid receptor modulation of adenylyl cyclase activity in Chinese hamster ovary cells.

Inhibition of adenylyl cyclase (AC) activity is frequently used to measure µ-opioid receptor (MOR) activation. We sought to develop a simple, rapid assay of AC activity in whole cells that could be used to study MOR signaling. Chinese hamster ovary cells expressing human MOR (CHO-MOR cells) were grown in 96-well plates and loaded with membrane potential-sensitive fluorescent dye. CHO-MOR cells were treated with the AC activator forskolin (FSK), with or without simultaneous application of MOR agonists, and the resulting change in fluorescence was measured. CHO-MOR cells hyperpolarized in response to application of FSK (pEC50, 7.3) or calcitonin (pEC50, 9.4). A submaximally effective concentration of FSK (300 nM) caused a 52% ± 2% decrease in fluorescence. Simultaneous application of the opioids DAMGO (pEC50, 7.4; E(max), 56%), morphine (pEC50, 7.0; E(max), 61%); and buprenorphine (pEC50, 8.6; E(max), 24%) inhibited the FSK response in a dose-dependent manner while having no effect by themselves. The effects of DAMGO were blocked by pertussis toxin. This assay represents a simple, robust method for real-time observation of AC inhibition by MOR in CHO cells. It represents an appealing alternative to end-point assays that rely on cAMP accumulation and can avoid potential confounds associated with rapid desensitization of MOR signaling.

Expression and purification of functional human mu opioid receptor from E.coli.

N-terminally his-tagged human mu opioid receptor, a G protein-coupled receptor was produced in E.coli employing synthetic codon-usage optimized constructs. The receptor was expressed in inclusion bodies and membrane-inserted in different E.coli strains. By optimizing the expression conditions the expression level for the membrane-integrated receptor was raised to 0.3-0.5 mg per liter of culture. Milligram quantities of receptor could be enriched by affinity chromatography from IPTG induced cultures grown at 18°C. By size exclusion chromatography the protein fraction with the fraction of alpha-helical secondary structure expected for a 7-TM receptor was isolated, by CD-spectroscopy an alpha-helical content of ca. 45% was found for protein solubilised in the detergent Fos-12. Receptor in Fos-12 micelles was shown to bind endomorphin-1 with a K(D) of 61 nM. A final yield of 0.17 mg functional protein per liter of culture was obtained.

The N-terminal end truncated mu-opioid receptor: from expression to circular dichroism analysis.

In order to evaluate the biochemical, biophysical, and pharmacological implication of the N-terminal domain of the human mu-opioid receptor (HuMOR), deletion mutants lacking 64 amino acids from the amino terminus of HuMOR were constructed and expressed in the yeast Pichia pastoris. The recombinant proteins differed with respect to the presence of the Saccharomyces cerevisiae alpha-factor prepropeptide and the enhanced green fluorescent protein fused to the N terminus of the receptor. Pharmacological studies indicated that deletion of the N-terminal domain produced little effect on ligand affinities. The N-terminal end truncated and c-myc/6his-tagged receptor was subsequently purified to homogeneity and a yield of 5 mg/l was obtained after purification. The N-terminal end truncated receptor was further characterized by circular dichroism in trifluoroethanol and showed a characteristic pattern of alpha-helical structure. A pH effect on the structure of the receptor was observed when it was solubilized in sodium dodecyl sulfate micelles, with an increase of helicity at low pH.

Detection of the endogenous mu opioid receptor (mopr) in brain

In general, it has been difficult to obtain antibodies which are useful for immunoblotting of endogenous seven-transmembrane receptors (7TMRs) despite the claims made by many companies on commercially available antibodies. In this review, we will use the mu opioid receptor (MOPR) in brain as an example to underscore the importance of using knock-out (K/O) mice and multiple independent approaches (ligand affinity-labeling, receptor phosphorylation and immunoblotting) in identifying 7TMRs following sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE). The rigor and convergence of pharmacological and biochemical data provide confidence on the unequivocal identification of MOPR. The distinct relative molecular masses (Mr's) and band patterns are largely due to variations in the extent of N-glycosylation in different cell lines, brain regions and species.

Purification and functional reconstitution of monomeric mu-opioid receptors: allosteric modulation of agonist binding by Gi2.

Despite extensive characterization of the mu-opioid receptor (MOR), the biochemical properties of the isolated receptor remain unclear. In light of recent reports, we proposed that the monomeric form of MOR can activate G proteins and be subject to allosteric regulation. A mu-opioid receptor fused to yellow fluorescent protein (YMOR) was constructed and expressed in insect cells. YMOR binds ligands with high affinity, displays agonist-stimulated [(35)S]guanosine 5'-(gamma-thio)triphosphate binding to Galpha(i), and is allosterically regulated by coupled G(i) protein heterotrimer both in insect cell membranes and as purified protein reconstituted into a phospholipid bilayer in the form of high density lipoprotein particles. Single-particle imaging of fluorescently labeled receptor indicates that the reconstituted YMOR is monomeric. Moreover, single-molecule imaging of a Cy3-labeled agonist, [Lys(7), Cys(8)]dermorphin, illustrates a novel method for studying G protein-coupled receptor-ligand binding and suggests that one molecule of agonist binds per monomeric YMOR. Together these data support the notion that oligomerization of the mu-opioid receptor is not required for agonist and antagonist binding and that the monomeric receptor is the minimal functional unit in regard to G protein activation and strong allosteric regulation of agonist binding by G proteins.

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.

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

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

Solubilization, purification, and mass spectrometry analysis of the human mu-opioid receptor expressed in Pichia pastoris.

The human mu-opioid receptor was expressed in Pichia pastoris with or without EGFP at the N-terminal end. Expression yields of the recombinant proteins reached several tens of milligram of receptor per liter of culture medium in shacked flasks. Pharmacological studies using specific ligands demonstrated a typical opioid profile for the HuMOR-c-myc-his-tag construct, whereas the GFP-HuMOR-c-myc-his-tag receptor was unable to bind opioid drugs. The hexahistidine epitope-tagged receptors were purified by immobilized-nickel affinity chromatography. The identity of the purified mu-opioid receptor proteins was confirmed by Western blot and mass spectrometry analysis. In conclusion, the expression, solubilization, and purification strategies described herein allow to isolate very high quantities of purified receptor, up to 12 mg/L.

Expression of MOR1C-like mu-opioid receptor mRNA in rats.

The reverse transcriptase-polymerase chain reaction (RT-PCR) was used to clone a cDNA fragment of a putative G-protein-coupled receptor from rat brain total RNA. Nucleotide sequencing of this cDNA fragment showed it to be homologous to that of the mu-opioid receptor splice variant MOR(1C) from mice. We used the cDNA to make an RNA probe for a ribonuclease protection assay (RPA). The results from the RPA showed a protected fragment of the size expected for MOR(1C) mRNA, as well as other RNase-protected fragments that may indicate the existence of other MOR1 transcripts. We then used the RNA probe for in situ hybridization (ISH) experiments. We detected strong autoradiographic labeling over much of the rat telencephalon, diencephalon, mesencephalon, cerebellum, spinal cord, and dorsal root ganglia. These findings suggest that MOR(1C), and possibly other MOR1 splice variants, are important components of the system by which the actions of opioids are transduced.

Purification and mass spectrometric analysis of the mu opioid receptor.

A mouse mu opioid receptor was engineered to contain a FLAG epitope at the amino-terminus and a hexahistidine tag at the carboxyl-terminus to facilitate purification. Selection of transfected human embryonic kidney (HEK) 293 cells yielded a cell line that expressed the receptor with a B(max) of 10 pmol/mg protein. 3[H]Bremazocine exhibited high affinity binding to the epitope-tagged mu opioid receptor with a KD of 1.0 nM. The agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), morphine and [D-Ala(2),D-Leu(5)]enkephalin (DADL) competitively inhibited bremazocine binding to the tagged mu receptor with KI's of 3.5, 17 and 70 nM, respectively. Chronic treatment of cells expressing the epitope-tagged mu receptor with DAMGO resulted in down-regulation of the receptor, indicating that the tagged receptor retained the capacity to mediate signal transduction. The mu receptor was solubilized from HEK 293 cell membranes with n-dodecyl-beta-D-maltoside in an active form that maintained high affinity bremazocine binding. Sequential use of wheat germ agglutinin (WGA)-agarose chromatography, Sephacryl S300 gel filtration chromatography, immobilized metal affinity chromatography, immunoaffinity chromatography, and sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE) permitted purification of the receptor. The purified mu opioid receptor was a glycoprotein that migrated on SDS/PAGE with an apparent molecular mass of 80 kDa. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry was used to identify and characterize peptides derived from the mu opioid receptor following in-gel digestion with trypsin or chymotrypsin, and precursor-derived tandem mass spectrometry (ms/ms) confirmed the identity of several peptides derived from enzymatic digestion of the mu opioid receptor.

Dimerization of morphine and orphanin FQ/nociceptin receptors: generation of a novel opioid receptor subtype.

Although orphanin FQ/nociceptin (OFQ/N) receptors are a member of the opioid receptor family of receptors, they bind traditional opioids with very poor affinity. We now demonstrate that mu opioid receptors can physically associate with OFQ/N receptors, resulting in a complex with a unique binding selectivity profile. Immunoprecipitation of epitope-tagged OFQ/N receptors co-precipitates mu receptors. When the two receptors were co-expressed in CHO cells, [3H]OFQ/N retained its high binding affinity for its receptor. However, co-expression of the two receptors increased by up to 250-fold the affinity of a series of opioids in [3H]OFQ/N binding assays. This enhanced affinity was limited to agonists with high affinity for mu receptors. Selective kappa(1) and delta opioids did not lower binding. Despite the dramatic increase in affinity for the opioid agonists in co-expressing cells, the opioid antagonists naloxone and diprenorphine failed to compete [3H]OFQ/N binding.

Heterologous expression of a deuterated membrane-integrated receptor and partial deuteration in methylotrophic yeasts.

Methylotrophic yeast has previously been shown to be an excellent system for the cost-effective production of perdeuterated biomass and for the heterologous expression of membrane receptors. A protocol for the expression of 85% deuterated, functional human mu-opiate receptor was established. For partially deuterated biomass, deuteration level and distribution were determined for fatty acids, amino acids and carbohydrates. It was shown that prior to biosynthesis of lipids and amino acids (and of carbohydrates, to a lower extent), exchange occurs between water and methanol hydrogen atoms, so that 80%-90% randomly deuterated biomass and over-expressed proteins may be obtained using only deuterated water.

Calmodulin binding to G protein-coupling domain of opioid receptors.

The ubiquitous intracellular Ca(2+) sensor calmodulin (CaM) regulates numerous proteins involved in cellular signaling of G protein-coupled receptors, but most known interactions between GPCRs and CaM occur downstream of the receptor. Using a sequence-based motif search, we have identified the third intracellular loop of the opioid receptor family as a possible direct contact point for interaction with CaM, in addition to its established role in G protein activation. Peptides derived from the third intracellular loop of the mu-opioid (OP(3)) receptor strongly bound CaM and were able to reduce binding interactions observed between CaM and immunopurified OP(3) receptor. Functionally, CaM reduced basal and agonist-stimulated (35)S-labeled guanosine 5'-3-O-(thio)triphosphate incorporation, a measure of G protein activation, in membranes containing recombinant OP(3) receptor. Changes in CaM membrane levels as a result of overexpression or antisense CaM suppression inversely affected basal and agonist-induced G protein activation. The ability of CaM to abolish high affinity binding sites of an agonist at OP(3) further supports the hypothesis of a direct interaction between CaM and opioid receptors. An OP(3) receptor mutant with a Lys(273) --> Ala substitution (K273A-OP(3)), an amino acid predicted to play a critical role in CaM binding based on motif structure, was found to be unaffected by changes in CaM levels but coupled more efficiently to G proteins than the wild-type receptor. Stimulation of both the OP(1) (delta-opioid) and OP(3) wild-type receptors, but not the K273A-OP(3) mutant, induced release of CaM from the plasma membrane. These results suggest that CaM directly competes with G proteins for binding to opioid receptors and that CaM may itself serve as an independent second messenger molecule that is released upon receptor stimulation.

Identification and characterization of three new alternatively spliced mu-opioid receptor isoforms.

We have identified four new mu-opiod receptor (MOR)-1 exons, indicating that the gene now contains at least nine exons spanning more than 200 kilobases. Replacement of exon 4 by combinations of the new exons yields three new receptors. When expressed in Chinese hamster ovary cells, all three variants displayed high affinity for mu-opioid ligands, but kappa and delta drugs were inactive. However, there were subtle, but significant, differences in the binding profiles of the three variants among themselves and from MOR-1. Immunohistochemically, the major variant, MOR-1C, displayed a regional distribution quite distinct from that of MOR-1. Region-specific processing also was seen at the mRNA level. Antisense mapping revealed that the four new exons were all involved in morphine analgesia. Together with two other variants generated from alternative splicing of exon 4, there are now six distinct MOR-1 receptors.

Use of zonal centrifugation method for the preparation of mu-opioid receptor enriched membranes from bovine corpus striatum.

Earlier attempts to purify the opioid receptors met with limited success because of the use of brain crude membranes. Subcellular fractionation procedures adopted now to get enriched membranes resulted in 2-3 fold enrichment. However, a procedure has been developed in our laboratory in which a crude membrane fraction obtained at 17,500 x g was lysed with 1 mM sodium bicarbonate and later subjected to zonal fractionation, employing a density gradient of 0.6-1.2 M sucrose. This could yield a membrane fraction highly enriched with mu-opioid receptors which is 9.3 fold higher than the crude membranes.