Heterologous expression of G-protein-coupled receptors: human opioid receptors under scrutiny

Advances in cell membrane chromatography

Cell membrane chromatography (CMC) is a biomimetic chromatographic method based on the ability of membrane receptors to selectively interact with their ligands in vivo. Using membrane receptors as a stationary phase, the CMC method helps in determining the binding characteristics between ligands and membrane receptors and in efficiently identifying specific target components in a complex sample that produce the cellular biological effects of ligands (drugs, antibodies, enzymes, cytokines, etc.). CMC is an analytical tool for revealing characteristics of ligand-receptor interactions, screening and discovering target substances, and accurately controlling the quality of drugs. Since establishment of CMC in the early 1990s, with the rapid development of cell biology, significant progress has been made in the development of high-expression receptors, engineered cell cultures, and standardized preparations, which allowed in vitro immobilization of cell membrane receptors and miniaturization of binding assays. A variety of CMC models have been established using different membrane receptors as a stationary phase, and many new methods have been developed by combining CMC with high-performance liquid chromatography (HPLC)/mass spectrometry or HPLC-IT-TOF technologies. CMC methods have been widely used to study drug-receptor interactions and to screen complex samples for effective or harmful components.

Molecular details of dimerization kinetics reveal negligible populations of transient µ-opioid receptor homodimers at physiological concentrations

Various experimental and computational techniques have been employed over the past decade to provide structural and thermodynamic insights into G Protein-Coupled Receptor (GPCR) dimerization. Here, we use multiple microsecond-long, coarse-grained, biased and unbiased molecular dynamics simulations (a total of ~4 milliseconds) combined with multi-ensemble Markov state models to elucidate the kinetics of homodimerization of a prototypic GPCR, the µ-opioid receptor (MOR), embedded in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol lipid bilayer. Analysis of these computations identifies kinetically distinct macrostates comprising several different short-lived dimeric configurations of either inactive or activated MOR. Calculated kinetic rates and fractions of dimers at different MOR concentrations suggest a negligible population of MOR homodimers at physiological concentrations, which is supported by acceptor photobleaching fluorescence resonance energy transfer (FRET) experiments. This study provides a rigorous, quantitative explanation for some conflicting experimental data on GPCR oligomerization.

Multiresidue Method for Analysis of β Agonists in Swine Urine by Enzyme Linked Receptor Assay Based on β2 Adrenergic Receptor Expressed in HEK293 Cells

A novel enzyme-linked receptor assay (ELRA) based on β₂-adrenergic receptor (β₂-AR) has been developed for rapid and high-throughput detection of β-adrenergic agonists (β-agonists) in urine. Human embryonic kidney cells (HEK293) were introduced as the expression system to enhance the functionality of the recombinant β₂-AR, and the attempt to detect β-agonists in swine urine using such approaches was accomplished unprecedentedly. In this article, a recombinant porcine β₂-AR was produced in the inner membrane of HEK293 cells and purified from crude membrane protein by nickel-nitrilotriacetic acid affinity chromatography. After activity identification, the recombinant receptor was used in the development of direct competitive ELRA. Several parameters such as blocking buffer and blocking process were optimized and the performance of the system was determined. The IC50 concentrations of clenbuterol, salbutamol, and ractopamine were 34, 53 and 63 μg/L, and the average recovery rates were 68.2%, 60.3% and 65.5%, respectively. ELRA based on β₂-AR shows a series of advantages such as safety, easy operation, and high efficiency, making it promising for the rapid screening of β-agonists in animal urine.

Rapid establishment of G-protein-coupled receptor-expressing cell lines by site-specific integration

The establishment of mammalian cell lines reliably expressing G-protein-coupled receptors (GPCRs) can be a tedious and often time-consuming process. A strategy has been developed to allow the rapid production of such cell lines. The first step of this approach was the generation of a specialized master cell line, characterized by optimized stable expression of a membrane-bound reporter protein. In the second step, this reporter gene was exchanged for that of the GPCR of interest by a DNA recombinase "cut-and-paste" engineering step. It has been demonstrated that the resulting GPCR cell lines inherit the advantages of the master cell line, expressing the GPCR in a homogeneous and stable manner. The case studies presented demonstrate the functionality of the established GPCR cell lines, and most important, because of the highly efficient integration event, these recombinant GPCR-expressing cell lines were generated within a timeframe of 2 to 4 weeks. The advantages of this cut-and-paste approach versus other strategies such as Flp-In or Jump-In are compared.

The p1D4-hrGFP II expression vector: a tool for expressing and purifying visual pigments and other G protein-coupled receptors

The heterologous expression of membrane proteins such as G protein-coupled receptors can be a notoriously difficult task. We have engineered an expression vector, p1D4-hrGFP II, in order to efficiently express visual pigments in mammalian cell culture. This expression vector is based on pIRES-hrGFP II (Stratagene), with the addition of a C-terminal 1D4 epitope tag for immunoblotting and immunoaffinity purification. This vector employs the CMV promoter and hrGFP II, a co-translated reporter gene. We measured the effectiveness of pIRES-hrGFP II in expressing bovine rhodopsin, and showed a 3.9- to 5.7-fold increase in expression as measured by absorbance spectroscopy as compared with the pMT vector, a common choice for visual pigment expression. We then expressed zebrafish RH2-1 using p1D4-hrGFP II in order to assess its utility in expressing cone opsins, known to be less stable and more difficult to express than bovine rhodopsin. We show a λ(280)/λ(MAX) value of 3.3, one third of that reported in previous studies, suggesting increased expression levels and decreased levels of misfolded, non-functional visual pigment. Finally, we monitored HEK293T cell growth following transfection with pIRES-hrGFP II using fluorescence microscopy to illustrate the benefits of having a co-translated reporter during heterologous expression studies.

High-level production and characterization of a G-protein coupled receptor signaling complex

Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.

Analysis of cell surface proteome changes via label-free, quantitative mass spectrometry

We present a mass spectrometry-based strategy for the specific detection and quantification of cell surface proteome changes. The method is based on the label-free quantification of peptide patterns acquired by high mass accuracy mass spectrometry using new software tools and the cell surface capturing technology that selectively enriches glycopeptides exposed to the cell exterior. The method was applied to monitor dynamic protein changes in the cell surface glycoproteome of Drosophila melanogaster cells. The results led to the construction of a cell surface glycoprotein atlas consisting of 202 cell surface glycoproteins of D. melanogaster Kc167 cells and indicated relative quantitative changes of cell surface glycoproteins in four different cellular states. Furthermore we specifically investigated cell surface proteome changes upon prolonged insulin stimulation. The data revealed insulin-dependent cell surface glycoprotein dynamics, including insulin receptor internalization, and linked these changes to intracellular signaling networks.

Heterologous GPCR expression: a bottleneck to obtaining crystal structures

G protein-coupled receptors (GPCRs) are an important, medically relevant class of integral membrane proteins. Laboratories throughout all disciplines of science devote time and energy into developing practical methods for the discovery, isolation, and characterization of these proteins. Since the crystal structure of rhodopsin was solved 6 years ago, the race to determine high-resolution structures of more GPCRs has gained momentum. Since certain GPCRs are currently produced at sufficient levels for X-ray crystallography trials, it is speculated that heterologous expression of GPCRs may no longer be a bottleneck in obtaining crystal structures. This Review focuses on the current approaches in heterologous expression of GPCRs and explores the problems associated with obtaining crystal structures from GPCRs expressed in different systems. Although milligram amounts of certain GPCRs are attainable, the majority of GPCRs are still either produced at very low levels or not at all. Developing reliable expression techniques for GPCRs is still a major priority for the structural characterization of GPCRs.

The synthesis and high-level expression of a beta2-adrenergic receptor gene in a tetracycline-inducible stable mammalian cell line

High-level expression of G-protein-coupled receptors (GPCRs) in functional form is required for structure-function studies. The main goal of the present work was to improve expression levels of beta2-adrenergic receptor (beta2-AR) so that biophysical studies involving EPR, NMR, and crystallography can be pursued. Toward this objective, the total synthesis of a codon-optimized hamster beta2-AR gene suitable for high-level expression in mammalian systems has been accomplished. Transient expression of the gene in COS-1 cells resulted in 18 +/- 3 pmol beta2-AR/mg of membrane protein, as measured by saturation binding assay using the beta2-AR antagonist [3H] dihydroalprenolol. Previously, we reported the development of an HEK293S tetracycline-inducible system for high-level expression of rhodopsin. Here, we describe construction of beta2-AR stable cell lines using the HEK293S-TetR-inducible system, which, after induction, express wild-type beta2-AR at levels of 220 +/- 40 pmol/mg of membrane protein corresponding to 50 +/- 8 microg/15-cm plate. This level of expression is the highest reported so far for any wild-type GPCR, other than rhodopsin. The yield of functional receptor using the single-step affinity purification is 12 +/- 3 microg/15-cm plate. This level of expression now makes it feasible to pursue structure-function studies using EPR. Furthermore, scale-up of beta2-AR expression using suspension cultures in a bioreactor should now allow production of enough beta2-AR for the application of biophysical techniques such as NMR spectroscopy and crystallography.

Characterization of 16 human G protein-coupled receptors expressed in baculovirus-infected insect cells

Understanding the three-dimensional structure of G protein-coupled receptors (GPCRs) has been limited by the technical challenges associated with expression, purification, and crystallization of membrane proteins, and their low abundance in native tissue. In the first large-scale comparative study of GPCR protein production using recombinant baculovirus, we report the characterization of 16 human receptors. The GPCRs were produced in three insect cell lines and functional protein levels monitored over 72 h using radioligand binding assays. Different GPCRs exhibited widely different expression levels, ranging from less than 1 pmol receptor/mg protein to more than 250 pmol/mg. No single set of conditions was suitable for all GPCRs, and large differences were seen for the expression of individual GPCRs in different cell lines. Closely related GPCRs did not share similar expression profiles; however, high expression (greater than 20 pmol/mg) was achieved for over half the GPCRs in our study. Overall, the levels of protein production compared favourably to other published systems.

Molecular Characterization of a Purified 5-HT4 Receptor

Serotonin 5-HT(4(a)) receptor, a G-protein-coupled receptor (GPCR), was produced as a functional isolated protein using Escherichia coli as an expression system. The isolated receptor was characterized at the molecular level by circular dichroism (CD) and steady-state fluorescence. A specific change in the near-UV CD band associated with the GPCR disulfide bond connecting the third transmembrane domain to the second extracellular loop (e2) was observed upon agonist binding to the purified receptor. This is a direct experimental evidence for a change in the conformation of the e2 loop upon receptor activation. Different variations were obtained depending whether the ligand was an agonist (partial or full) or an inverse agonist. In contrast, antagonist binding did not induce any variation. These observations provide a first direct evidence for the fact that free (or antagonist-occupied), active (partial- or full agonist-occupied) and silent (inverse agonist-occupied) states of the receptor involve different arrangements of the e2 loop. Finally, ligand-induced changes in the fluorescence emission profile of the purified receptor confirmed that the partial agonist stabilized a single, well-defined, conformational state and not a mixture of different states. This result is of particular interest in a pharmacological perspective since it directly demonstrates that the efficacy of a drug is likely due to the stabilization of a ligand-specific state rather than selection of a mixture of different conformational states of the receptor.

Transient overexpression of κ and μ opioid receptors using recombinant adenovirus vectors

In order to study the trafficking and signal transduction mechanisms of the multiple opioid receptors, these receptors are expressed either transiently or stably in cell lines. Often, it is difficult to express receptors at a sufficiently high density to obtain reproducible results. To achieve a high density of receptors, replication-defective adenovirus (rAd5) vectors encoding the mu (MOR) and kappa (KOR) opioid receptors, both in their native form and as fusion proteins bearing the green fluorescent protein (GFP) at their C-terminus, were constructed. These vectors efficiently and reproducibly infected Chinese hamster ovary (CHO) cells that stably express the human coxsackie-adenovirus receptor (hCAR), with up to 90% of cells becoming infected at a low multiplicity of infection (MOI). Saturation receptor binding studies using mu- and kappa-selective agonists, [3H][D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) and [3H](5alpha7alpha,8beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl)benzeneacetamide (U69,593), respectively, and a nonselective antagonist, [3H]diprenorphine, revealed that rAd5-transduced cells expressed MOR and KOR for at least 3 days, at levels which exceeded those present on widely-used CHO sublines that stably express MOR or KOR. Expression levels were highest for the vectors encoding native MOR or KOR, and slightly reduced for the GFP fusion proteins. These findings demonstrate the feasibility of using rAd5 vectors to express opioid receptors at high densities, which may facilitate opioid receptor studies.

A novel yeast expression system for the overproduction of quality-controlled membrane proteins

Saturation of the cell's protein folding capacity and accumulation of inactive incompletely folded protein often accompanying the overexpression of membrane proteins (MPs) presents an obstacle to their efficient purification in a functional form for structural studies. We present a novel strategy for optimization of functional MP expression in Saccharomyces cerevisiae. This approach exploits the unfolded protein response (UPR) pathway, a stress signaling mechanism that senses the accumulation of unfolded proteins in the endoplasmic reticulum. We demonstrate that a high level of UPR induction upon expression of a MP reflects impaired functional expression of that protein. Tuning the expression level of the protein so as to avoid or minimize UPR induction results in its increased functional expression. UPR status can therefore serve as a proxy variable for the extent of impaired expression of a MP that may even be applicable in the absence of knowledge of the protein's biological function.

Structure-based analysis of GPCR function: conformational adaptation of both agonist and receptor upon leukotriene B4 binding to recombinant BLT1

We produced the human leukotriene B(4) (LTB(4)) receptor BLT1, a G-protein-coupled receptor, in Escherichia coli with yields that are sufficient for the first structural characterization of this receptor in solution. Overexpression was achieved through codon optimization and the search for optimal refolding conditions of BLT1 recovered from inclusion bodies. The detergent-solubilized receptor displays a 3D-fold compatible with a seven transmembrane (TM) domain with ca 50% alpha-helix and an essential disulfide bridge (circular dichroism evidence); it binds LTB(4) with K(a)=7.8(+/-0.2)x10(8)M(-1) and a stoichiometric ratio of 0.98(+/-0.02). Antagonistic effects were investigated using a synthetic molecule that shares common structural features with LTB(4). We report evidence that both partners, LTB(4) and BLT1, undergo a rearrangement of their respective conformations upon complex formation: (i) a departure from planarity of the LTB(4) conjugated triene moiety; (ii) a change in the environment of Trp234 (TM-VI helix) and in the exposure of the cytoplasmic region of this transmembrane helix.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Green fluorescent protein as a reporter of human mu-opioid receptor overexpression and localization in the methylotrophic yeast Pichia pastoris

The human mu-opioid receptor (HuMOR) was fused in its N-terminus end to the green fluorescent protein (GFP) or/and to the c-myc and six histidines tags in its C-terminus end, and expressed in the methylotrophic yeast Pichia pastoris. Neither the C- nor the N-terminal tagging of the receptor does modify its pharmacological properties as compared to the untagged receptor. Expression levels of fusion receptors determined by GFP fluorescence measurements strongly correlates with the number of sites expressed per cell detected through saturation studies (Bmax value), thus showing that GFP is an efficient and reliable reporter of the HuMOR functional expression. The N- and C-terminus tags have allowed to show that the entire molecule is overexpressed. They have permitted in-situ localization experiments using fluorescence and electron microscopy techniques and have shown a dense intracellular labelling. Above all, the quantification of expression levels made possible through fluorescence intensity analysis, have revealed that huge amounts of receptor are produced that could not be detected through classical binding experiments: for a Bmax value of 1 pmol mg(-1) of receptor determined through binding studies, 16 pmol were found in membrane preparations using fluorescence and 100 pmol in whole cells. These results should be very useful for large-scale production and structural biology of HuMOR, and other G-protein coupled receptors (GPCRs).

Expression of the human mu opioid receptor in a stable Sf9 cell line

The cDNA that encodes the human mu opioid receptor (hMOR) has been cloned and expressed in Spodoptera frugiperda (Sf9) cells using a nonlytic vector system. The coding sequence fused to the cleavable glycoprotein signal peptide gp 64, and a C-terminal histidine tag was placed under the transcriptional control of the Orgyia pseudotsugata multicapsid nucleopolyhedrosis virus immediate-early 2 (OpIE2) promoter. Transfected cells were selected using Zeocin resistance and the receptor was constitutively expressed at approximately 12000 receptors per cell. Immunofluorescence images illustrated that more than 75% of the Sf9 cells expressed hMOR at the plasma membrane. This is the first report of the constitutive and heterologous expression of a G protein-coupled receptor in a stably transfected Sf9 cell line, under the control of the OpIE2 promoter.

Optimizing functional versus total expression of the human mu-opioid receptor in Pichia pastoris

The expression of the EGFP-human mu-opioid receptor fusion protein in the methylotrophic yeast Pichia pastoris was optimized and monitored using both fluorescence and ligand-binding experiments. A set of parameters, including gene copy number, strain type, temperature, pH, and methanol inducer levels, was studied for its effect on the production of the recombinant protein. We show here that the expression level is optimal after 10 h of promoter induction and that the maximum is reached at a lower temperature and a higher pH than normally used. The optimized conditions have allowed a fourfold increase of the ligand-binding active form of the receptor, whereas the total expression level determined by EGFP fluorescence measurements was not modified.

Purification and characterization of the human adenosine A(2a) receptor functionally expressed in Escherichia coli

The adenosine A(2a) receptor belongs to the seven transmembrane helix G-protein-coupled receptor family, is abundant in striatum, vasculature and platelets and is involved in several physiological processes such as blood pressure regulation and protection of cells during anoxia. For structural and biophysical studies we have expressed the human adenosine A(2a) receptor (hA2aR) at high levels inserted into the Escherichia coli inner membrane, and established a purification scheme. Expression was in fusion with the periplasmic maltose-binding protein to levels of 10-20 nmol of receptor per L of culture, as detected with the specific antagonist ligand [(3)H]ZM241385. As the receptor C-terminus was proteolyzed upon solubilization, a protease-resistant but still functional receptor was created by truncation to Ala316. Addition of the sterol, cholesteryl hemisuccinate, allowed a stable preparation of functional hA2aR solubilized in dodecylmaltoside to be obtained, and, increased the stability of the receptor solubilized in other alkylmaltosides. Purification to homogeneity was achieved in three steps, including ligand affinity chromatography based on the antagonist xanthine amine congener. The purified hA2aR fusion protein bound [(3)H]ZM241385 with a K(d) of 0.19 nm and an average B(max) of 13.7 nmol x mg(-1) that suggests 100% functionality. Agonist affinities for the purified solubilized receptor were higher than those for the membrane-bound form. Sufficient pure, functional hA2aR can now be prepared regularly for structural studies.

Role of sterols in modulating the human mu-opioid receptor function in Saccharomyces cerevisiae

This study provides evidence that the differences in membrane composition found from one cell type to another can represent a limiting factor to recovering the functionality of transmembrane proteins when expressed in heterologous systems. Restoring the properties of the human mu-opioid receptor in yeast (Saccharomyces cerevisiae), similar to those observed in native cells, was achieved by replacing ergosterol from yeast by cholesterol, which is normally found in mammalian plasma membranes. The results suggest that these two sterols have opposite effects with respect to the ligand binding function of the receptor. Ergosterol was found to constrain the mu-opioid receptor in an inactive state in yeast plasma membranes and cannot replace cholesterol in activating it. These data differ from previous works dealing with the function of related G-protein-coupled receptors (GPCR) in ergosterol-enriched membranes. This suggests that structural requirements of GPCR with respect to their modulation by lipid components differ from one protein to another. As a consequence, we assume that the presence of appropriate lipids around transmembrane proteins determines their function. This highlights the functional significance of lateral heterogeneities of membrane components within biological membranes.

Enhanced expression, native purification, and characterization of CCR5, a principal HIV-1 coreceptor

Seven-transmembrane segment, G protein-coupled receptors (GPCRs) play important roles in many biological processes in which pharmaceutical intervention may be useful. High level expression and native purification of GPCRs are important steps in the biochemical and structural characterization of these molecules. Here, we describe enhanced mammalian cell expression and purification of a codon-optimized variant of the chemokine receptor CCR5, a GPCR that plays a central role in the entry of the human immunodeficiency virus-1 (HIV-1) into immune cells. CCR5 could be solubilized in its native state as determined by its ability to be precipitated by 2D7, a conformation-dependent anti-CCR5 antibody, and by the HIV-1 gp120 envelope glycoprotein. The 2D7 antibody recognized immature and mature forms of CCR5 equally, whereas gp120 preferentially recognized the mature form, a result that underscores a role for posttranslational modification of CCR5 in its HIV-1 coreceptor function. The methods described herein contribute to the analysis of CCR5 and are likely to be applicable to many other GPCRs.