Ligand regulation of the quaternary organization of cell surface M3 muscarinic acetylcholine receptors analyzed by fluorescence resonance energy transfer (FRET) imaging and homogeneous time-resolved FRET

Oligomerization of yeast α-factor receptor detected by fluorescent energy transfer between ligands

G-protein-coupled receptors (GPCRs) comprise a large superfamily of transmembrane receptors responsible for transducing responses to the binding of a wide variety of hormones, neurotransmitters, ions, and other small molecules. There is extensive evidence that GPCRs exist as homo-and hetero-oligomeric complexes; however, in many cases, the role of oligomerization and the extent to which it occurs at low physiological levels of receptor expression in cells remain unclear. We report here the use of flow cytometry to detect receptor-receptor interactions based on fluorescence resonance energy transfer between fluorescently labeled cell-impermeant ligands bound to yeast α-mating pheromone receptors that are members of the GPCR superfamily. A novel, to our knowledge, procedure was used to analyze energy transfer as a function of receptor occupancy by donor and acceptor ligands. Measurements of loss of donor fluorescence due to energy transfer in cells expressing high levels of receptors were used to calibrate measurements of enhanced acceptor emission due to energy transfer in cells expressing low levels of receptors. The procedure allows determination of energy transfer efficiencies over a 50-fold range of expression of full-length receptors at the surface of living cells without the need to create fluorescent or bioluminescent fusion proteins. Energy transfer efficiencies for fluorescently labeled derivatives of the receptor agonist α-factor do not depend on receptor expression level and are unaffected by C-terminal truncation of receptors. Fluorescently labeled derivatives of α-factor that act as receptor antagonists exhibit higher transfer efficiencies than those for labeled agonists. Although the approach cannot determine the number of receptors per oligomer, these results demonstrate that ligand-bound, native α-factor receptors exist as stable oligomers in the cell membranes of intact yeast cells at normal physiological expression levels and that the extent of oligomer formation is not dependent on the concentration of receptors in the membrane.

Single Proteoliposome High-Content Analysis Reveals Differences in the Homo-Oligomerization of GPCRs

G-protein-coupled receptors (GPCRs) control vital cellular signaling pathways. GPCR oligomerization is proposed to increase signaling diversity. However, many reports have arrived at disparate conclusions regarding the existence, stability, and stoichiometry of GPCR oligomers, partly because of cellular complexity and ensemble averaging of intrareconstitution heterogeneities that complicate the interpretation of oligomerization data. To overcome these limitations, we exploited fluorescence-microscopy-based high-content analysis of single proteoliposomes. This allowed multidimensional quantification of intrinsic monomer-monomer interactions of three class A GPCRs (β₂-adrenergic receptor, cannabinoid receptor type 1, and opsin). Using a billion-fold less protein than conventional assays, we quantified oligomer stoichiometries, association constants, and the influence of two ligands and membrane curvature on oligomerization, revealing key similarities and differences for three GPCRs with decidedly different physiological functions. The assays introduced here will assist with the quantitative experimental observation of oligomerization for transmembrane proteins in general.

Practical Considerations for the Use of DREADD and Other Chemogenetic Receptors to Regulate Neuronal Activity in the Mammalian Brain

Chemogenetics is the process of genetically expressing a macromolecule receptor capable of modulating the activity of the cell in response to selective chemical ligand. This chapter will cover the chemogenetic technologies that are available to date, focusing on the commonly available engineered or otherwise modified ligand-gated ion channels and G-protein-coupled receptors in the context of neuromodulation. First, we will give a brief overview of each chemogenetic approach as well as in vitro/in vivo applications, then we will list their strengths and weaknesses. Finally, we will provide tips for ligand application in each case.Each technology has specific limitations that make them more or less suitable for different applications in neuroscience although we will focus mainly on the most commonly used and versatile family named designer receptors exclusively activated by designer drugs or DREADDs. We here describe the most common cases where these can be implemented and provide tips on how and where these technologies can be applied in the field of neuroscience.

Wild-type p53 oligomerizes more efficiently than p53 hot-spot mutants and overcomes mutant p53 gain-of-function via a "dominant-positive" mechanism

Human p53 protein acts as a transcription factor predominantly in a tetrameric form. Single residue changes, caused by hot-spot mutations of the TP53 gene in human cancer, transform wild-type (wt) p53 tumor suppressor proteins into potent oncoproteins - with gain-of-function, tumor-promoting activity. Oligomerization of p53 allows for a direct interplay between wt and mutant p53 proteins if both are present in the same cells - where a mutant p53's dominant-negative effect known to inactivate wt p53, co-exists with an opposite mechanism - a "dominant-positive" suppression of the mutant p53's gain-of-function activity by wt p53. In this study we determine the oligomerization efficiency of wt and mutant p53 in living cells using FRET-based assays and describe wt p53 to be more efficient than mutant p53 in entering p53 oligomers. The biased p53 oligomerization helps to interpret earlier reports of a low efficiency of the wt p53 inactivation via the dominant-negative effect, while it also implies that the "dominant-positive" effect may be more pronounced. Indeed, we show that at similar wt:mutant p53 concentrations in cells - the mutant p53 gain-of-function stimulation of gene transcription and cell migration is more efficiently inhibited than the wt p53's tumor-suppressive transactivation and suppression of cell migration. These results suggest that the frequent mutant p53 accumulation in human tumor cells does not only directly strengthen its gain-of-function activity, but also protects the oncogenic p53 mutants from the functional dominance of wt p53.

Muscarinic receptor oligomerization

G protein-coupled receptors (GPCRs) have been classically described as monomeric entities that function by binding in a 1:1 stoichiometric ratio to both ligand and downstream signalling proteins. However, in recent years, a growing number of studies has supported the hypothesis that these receptors can interact to form dimers and higher order oligomers although the molecular basis for these interactions, the overall quaternary arrangements and the functional importance of GPCR oligomerization remain topics of intense speculation. Muscarinic acetylcholine receptors belong to class A of the GPCR family. Each muscarinic receptor subtype has its own particular distribution throughout the central and peripheral nervous systems. In the central nervous system, muscarinic receptors regulate several sensory, cognitive, and motor functions while, in the peripheral nervous system, they are involved in the regulation of heart rate, stimulation of glandular secretion and smooth muscle contraction. Muscarinic acetylcholine receptors have long been used as a model for the study of GPCR structure and function and to address aspects of GPCR dimerization using a broad range of approaches. In this review, the prevailing knowledge regarding the quaternary arrangement for the various muscarinic acetylcholine receptors has been summarized by discussing work ranging from initial results obtained using more traditional biochemical approaches to those generated with more modern biophysical techniques. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Methods used to study the oligomeric structure of G-protein-coupled receptors

G-protein-coupled receptors (GPCRs), which constitute the largest family of cell surface receptors, were originally thought to function as monomers, but are now recognized as being able to act in a wide range of oligomeric states and indeed, it is known that the oligomerization state of a GPCR can modulate its pharmacology and function. A number of experimental techniques have been devised to study GPCR oligomerization including those based upon traditional biochemistry such as blue-native PAGE (BN-PAGE), co-immunoprecipitation (Co-IP) and protein-fragment complementation assays (PCAs), those based upon resonance energy transfer, FRET, time-resolved FRET (TR-FRET), FRET spectrometry and bioluminescence resonance energy transfer (BRET). Those based upon microscopy such as FRAP, total internal reflection fluorescence microscopy (TIRFM), spatial intensity distribution analysis (SpIDA) and various single molecule imaging techniques. Finally with the solution of a growing number of crystal structures, X-ray crystallography must be acknowledged as an important source of discovery in this field. A different, but in many ways complementary approach to the use of more traditional experimental techniques, are those involving computational methods that possess obvious merit in the study of the dynamics of oligomer formation and function. Here, we summarize the latest developments that have been made in the methods used to study GPCR oligomerization and give an overview of their application.

M3 muscarinic acetylcholine receptor facilitates the endocytosis of mu opioid receptor mediated by morphine independently of the formation of heteromeric complexes

Morphine inefficiency to induce the internalization of mu opioid (MOP) receptors observed in numerous experimental models constitutes a paradigm of G-protein coupled receptor (GPCR) functional selectivity. We recently described that activation of G_αq/11 proteins through 5-HT_2A serotonin receptors co-expressed in the same cells facilitates MOP receptor endocytosis promoted by morphine. In order to explore whether a different G_αq/11 coupled GPCR would emulate this effect, a double stable Flp-In T-REx HEK293 cell line permanently expressing MOP-YFP receptors along with FLAG-M3-Cerulean receptors expressed in an inducible manner was generated. Fluorescence microscopy examination of these cells revealed a co-distribution of both receptors mainly compartmentalized in plasma membrane. Concurrent stimulation with carbachol and morphine promoted MOP receptor internalization, desensitization and down-regulation and this facilitation was not dependent on PKC activation. Co-immunoprecipitation experiments demonstrated that FLAG-M3-Cerulean/MOP-YFP receptors interact forming heteromeric complexes in a time depending manner, i.e. the strongest interaction was detected after 96h of FLAG-M3-Cerulean induced expression. Under these experimental conditions, treatment of cells with carbachol plus morphine resulted in the internalization of both receptors within separated endocytic vesicles as visualized by confocal microscopy. This trafficking segregation observed for FLAG-M3-Cerulean and MOP-YFP receptors upon agonist stimulation suggests that this protein-protein interaction presents temporal and dynamic properties. Moreover, MOP-YFP receptor internalization facilitated by FLAG-M3-Cerulean receptors is independent of the constitution of heteromeric complexes.

Novel signaling of dynorphin at κ-opioid receptor/bradykinin B2 receptor heterodimers

The κ-opioid receptor (KOR) and bradykinin B2 receptor (B2R) are involved in a variety of important physiological processes and share many similar characteristics in terms of their distribution and functions in the nervous system. We first demonstrated the endogenous expression of KOR and B2R in human SH-SY5Y cells and their co-localization on the membrane of human embryonic kidney 293 (HEK293) cells. Bioluminescence and fluorescence resonance energy transfer and the proximity ligation assay were exploited to demonstrate the formation of functional KOR and B2R heteromers in transfected cells. KOR/B2R heteromers triggered dynorphin A (1-13)-induced Gαs/protein kinase A signaling pathway activity, including upregulation of intracellular cAMP levels and cAMP-response element luciferase reporter activity, resulting in increased cAMP-response element-binding protein (CREB) phosphorylation, which could be dampened by the protein kinase A (PKA) inhibitor H89. This indicated that the co-existence of KOR and B2R is critical for CREB phosphorylation. In addition, dynorphin A (1-13) induced a significantly higher rate of proliferation in HEK293-KOR/B2R and human SH-SY5Y cells than in the control group. These results indicate that KOR can form a heterodimer with B2R and this leads to increased protein kinase A activity by the CREB signaling pathway, leading to a significant increase in cell proliferation. The nature of this signaling pathway has significant implications for the role of dynorphin in the regulation of neuroprotective effects.

Dynamic Regulation of Quaternary Organization of the M1 Muscarinic Receptor by Subtype-selective Antagonist Drugs

Although rhodopsin-like G protein-coupled receptors can exist as both monomers and non-covalently associated dimers/oligomers, the steady-state proportion of each form and whether this is regulated by receptor ligands are unknown. Herein we address these topics for the M₁ muscarinic acetylcholine receptor, a key molecular target for novel cognition enhancers, by using spatial intensity distribution analysis. This method can measure fluorescent particle concentration and assess oligomerization states of proteins within defined regions of living cells. Imaging and analysis of the basolateral surface of cells expressing some 50 molecules·μm⁻² human muscarinic M₁ receptor identified a ∼75:25 mixture of receptor monomers and dimers/oligomers. Both sustained and shorter term treatment with the selective M₁ antagonist pirenzepine resulted in a large shift in the distribution of receptor species to favor the dimeric/oligomeric state. Although sustained treatment with pirenzepine also resulted in marked up-regulation of the receptor, simple mass action effects were not the basis for ligand-induced stabilization of receptor dimers/oligomers. The related antagonist telenzepine also produced stabilization and enrichment of the M₁ receptor dimer population, but the receptor subtype non-selective antagonists atropine and N-methylscopolamine did not. In contrast, neither pirenzepine nor telenzepine altered the quaternary organization of the related M₃ muscarinic receptor. These data provide unique insights into the selective capacity of receptor ligands to promote and/or stabilize receptor dimers/oligomers and demonstrate that the dynamics of ligand regulation of the quaternary organization of G protein-coupled receptors is markedly more complex than previously appreciated. This may have major implications for receptor function and behavior.

Experimental verification of the kinetic theory of FRET using optical microspectroscopy and obligate oligomers

Förster resonance energy transfer (FRET) is a nonradiative process for the transfer of energy from an optically excited donor molecule (D) to an acceptor molecule (A) in the ground state. The underlying theory predicting the dependence of the FRET efficiency on the sixth power of the distance between D and A has stood the test of time. In contrast, a comprehensive kinetic-based theory developed recently for FRET efficiencies among multiple donors and acceptors in multimeric arrays has waited for further testing. That theory has been tested in the work described in this article using linked fluorescent proteins located in the cytoplasm and at the plasma membrane of living cells. The cytoplasmic constructs were fused combinations of Cerulean as donor (D), Venus as acceptor (A), and a photo-insensitive molecule (Amber) as a nonfluorescent (N) place holder: namely, NDAN, NDNA, and ADNN duplexes, and the fully fluorescent quadruplex ADAA. The membrane-bound constructs were fused combinations of GFP2 as donor (D) and eYFP as acceptor (A): namely, two fluorescent duplexes (i.e., DA and AD) and a fluorescent triplex (ADA). According to the theory, the FRET efficiency of a multiplex such as ADAA or ADA can be predicted from that of analogs containing a single acceptor (e.g., NDAN, NDNA, and ADNN, or DA and AD, respectively). Relatively small but statistically significant differences were observed between the measured and predicted FRET efficiencies of the two multiplexes. While elucidation of the cause of this mismatch could be a worthy endeavor, the discrepancy does not appear to question the theoretical underpinnings of a large family of FRET-based methods for determining the stoichiometry and quaternary structure of complexes of macromolecules in living cells.

Non-equivalence of Key Positively Charged Residues of the Free Fatty Acid 2 Receptor in the Recognition and Function of Agonist Versus Antagonist Ligands

Short chain fatty acids (SCFAs) are produced in the gut by bacterial fermentation of poorly digested carbohydrates. A key mediator of their actions is the G protein-coupled free fatty acid 2 (FFA2) receptor, and this has been suggested as a therapeutic target for the treatment of both metabolic and inflammatory diseases. However, a lack of understanding of the molecular determinants dictating how ligands bind to this receptor has hindered development. We have developed a novel radiolabeled FFA2 antagonist to probe ligand binding to FFA2, and in combination with mutagenesis and molecular modeling studies, we define how agonist and antagonist ligands interact with the receptor. Although both agonist and antagonist ligands contain negatively charged carboxylates that interact with two key positively charged arginine residues in transmembrane domains V and VII of FFA2, there are clear differences in how these interactions occur. Specifically, although agonists require interaction with both arginine residues to bind the receptor, antagonists require an interaction with only one of the two. Moreover, different chemical series of antagonist interact preferentially with different arginine residues. A homology model capable of rationalizing these observations was developed and provides a tool that will be invaluable for identifying improved FFA2 agonists and antagonists to further define function and therapeutic opportunities of this receptor.

Muscarinic drugs regulate the PKG-II-dependent phosphorylation of M3 muscarinic acetylcholine receptors at plasma membranes from airway smooth muscle

Muscarinic agonists induce the activation of the airway smooth muscle (ASM) leading to smooth muscle contraction, important in asthma. This activation is mediated through M2/M3 muscarinic acetylcholine receptors (mAChRs). Muscarinic receptor activity, expressed as [(3)H]QNB binding at plasma membranes from bovine tracheal smooth muscle (BTSM), increased with cGMP and was augmented significantly cGMP plus ATP but diminished with the PKG-II inhibitor, Sp-8-pCPT-cGMPS. The [(3)H]-QNB binding was accelerated by okadaic acid, (OKA), a protein phosphatase (PPase) inhibitor. These two results indicated the involvement of a membrane-bound PPase. Moreover, a cGMP-dependent-[(32)P]γATP phosphorylation of plasma membranes from BTSM was stimulated at low concentrations of muscarinic agonist carbamylcholine (CC). However, higher amounts of CC produced a significant decrement of [(32)P]-labeling. A selective M3mAChR antagonist, 4-DAMP produced a dramatic inhibition of the basal and CC-dependent [(32)P]-labeling. The [(32)P] labeled membrane sediments were detergent solubilized and immunoprecipitated with specific M2/M3mAChR antibodies. The M3mAChR immuno-precipitates exhibited the highest cGMP-dependent [(32)P]-labeling, indicating it is a PKG-II substrate. Experiments using synthetic peptides from the C-terminal of the third intracellular loop (i3) of both M2mAChR (356-369) and M3mAChR (480-493) as external PKG-II substrates resulted in the i3M3-peptide being heavily phosphorylated. These results indicated that PKG-II phosphorylated the M3mAChR at the i3M3 domain ((480)MSLIKEKK(485)), suggesting that Ser(481) may be the target. Finally, this phosphorylation site seems to be regulated by a membrane-bound PPase linked to muscarinic receptor. These findings are important to understand the role of M3mAChR in the patho-physiology of ASM involved in asthma and COPD.

Distinct Agonist Regulation of Muscarinic Acetylcholine M2-M3 Heteromers and Their Corresponding Homomers

Each subtype of the muscarinic receptor family of G protein-coupled receptors is activated by similar concentrations of the neurotransmitter acetylcholine or closely related synthetic analogs such as carbachol. However, pharmacological selectivity can be generated by the introduction of a pair of mutations to produce Receptor Activated Solely by Synthetic Ligand (RASSL) forms of muscarinic receptors. These display loss of potency for acetylcholine/carbachol alongside a concurrent gain in potency for the ligand clozapine N-oxide. Co-expression of a form of wild type human M₂ and a RASSL variant of the human M₃ receptor resulted in concurrent detection of each of M₂-M₂ and M₃-M₃ homomers alongside M₂-M₃ heteromers at the surface of stably transfected Flp-In^TM T-REx^TM 293 cells. In this setting occupancy of the receptors with a muscarinic antagonist was without detectable effect on any of the muscarinic oligomers. However, selective agonist occupancy of the M₂ receptor resulted in enhanced M₂-M₂ homomer interactions but decreased M₂-M₃ heteromer interactions. By contrast, selective activation of the M₃ RASSL receptor did not significantly alter either M₃-M₃ homomer or M₂-M₃ heteromer interactions. Selectively targeting closely related receptor oligomers may provide novel therapeutic opportunities.

Regulation of oligomeric organization of the serotonin 5-hydroxytryptamine 2C (5-HT2C) receptor observed by spatial intensity distribution analysis

The questions of whether G protein-coupled receptors exist as monomers, dimers, and/or oligomers and if these species interconvert in a ligand-dependent manner are among the most contentious current issues in biology. When employing spatial intensity distribution analysis to laser scanning confocal microscope images of cells stably expressing either a plasma membrane-associated form of monomeric enhanced green fluorescent protein (eGFP) or a tandem version of this fluorophore, the eGFP tandem was identified as a dimer. Similar studies on cells stably expressing an eGFP-tagged form of the epidermal growth factor receptor demonstrated that, although largely a monomer in the basal state, this receptor rapidly became predominantly dimeric upon the addition of its ligand epidermal growth factor. In cells induced to express an eGFP-tagged form of the serotonin 5-hydroxytryptamine 2C (5-HT2C) receptor, global analysis of construct quantal brightness was consistent with the predominant form of the receptor being dimeric. However, detailed spatial intensity distribution analysis demonstrated the presence of multiple forms ranging from monomers to higher-order oligomers. Furthermore, treatment with chemically distinct 5-HT2C receptor antagonists resulted in a time-dependent change in the quaternary organization to one in which there was a preponderance of receptor monomers. This antagonist-mediated effect was reversible, because washout of the ligand resulted in the regeneration of many of the oligomeric forms of the receptor.

The molecular basis of oligomeric organization of the human M3 muscarinic acetylcholine receptor

G protein-coupled receptors, including the M3 muscarinic acetylcholine receptor, can form homo-oligomers. However, the basis of these interactions and the overall organizational structure of such oligomers are poorly understood. Combinations of site-directed mutagenesis and homogenous time-resolved fluorescence resonance energy transfer studies that assessed interactions between receptor protomers at the surface of transfected cells indicated important contributions of regions of transmembrane domains I, IV, V, VI, and VII as well as intracellular helix VIII to the overall organization. Molecular modeling studies based on both these results and an X-ray structure of the inactive state of the M3 receptor bound by the antagonist/inverse agonist tiotropium were then employed. The results could be accommodated fully by models in which a proportion of the cell surface M3 receptor population is a tetramer with rhombic, but not linear, orientation. This is consistent with previous studies based on spectrally resolved, multiphoton fluorescence resonance energy transfer. Modeling studies furthermore suggest an important role for molecules of cholesterol at the dimer + dimer interface of the tetramer, which is consistent with the presence of cholesterol at key locations in many G protein-coupled receptor crystal structures. Mutants that displayed disrupted quaternary organization were often poorly expressed and showed immature N-glycosylation. Sustained treatment of cells expressing such mutants with the muscarinic receptor inverse agonist atropine increased cellular levels and restored both cell surface delivery and quaternary organization to many of the mutants. These observations suggest that organization as a tetramer may occur before plasma membrane delivery and may be a key step in cellular quality control assessment.

Approaches to Characterize and Quantify Oligomerization of GPCRs

Fluorescence resonance energy transfer (FRET) is an approach widely used to detect protein-protein interactions in live cells. This approach is based on the sensitization of an "acceptor" molecule by the energy transfer from a "donor" when there is an overlap between the emission spectrum of the "donor" and the excitation spectrum of the "acceptor" and close proximity between the two fluorophore species (in the region of 8 nm). Various methods exist to quantify FRET signals: here, we describe the application of homogeneous time-resolved FRET (htrFRET) combined with Tag-lite™ technology and its application to determine not only protein-protein interactions but also the capability of GPCR mutant variants to form homomers compared to the wild type GPCR within the plasma membrane of transfected cells.

Considerations and protocols for the synthesis of custom protein labeling probes

Chemists and biologists have long recognized small molecule probes as powerful tools for functional genomics and proteomics studies. The possibility of specifically attaching chemical probes to individual proteins with spatial and temporal resolution has greatly improved our ability to visualize and characterize proteins in their native environment. The continued development of novel molecular probes for protein labeling is, therefore, of fundamental importance to gain new insights into biological processes in living cells and organisms. Several excellent approaches for the site-specific labeling of fusion proteins with chemical probes exist. Herein I discuss the design and generation of chemical probes for the SNAP-tag and CLIP-tag systems. The first part of this chapter is dedicated to reviewing the principles of the SNAP-tag technology, followed by a section dedicated to the development of chemical probes for unique applications, such as super-resolution imaging, protein trafficking and recycling, protein-protein interactions, and biomolecular sensing. The last part of the chapter contains experimental protocols and technical notes for the synthesis of selected SNAP-tag substrates and labeling of SNAP-tag fusion proteins in vitro and in living cells.

DREADDs (designer receptors exclusively activated by designer drugs): chemogenetic tools with therapeutic utility

In the past decade, emerging synthetic biology technologies such as chemogenetics have dramatically transformed how pharmacologists and systems biologists deconstruct the involvement of G protein-coupled receptors (GPCRs) in a myriad of physiological and translational settings. Here we highlight a specific chemogenetic application that extends the utility of the concept of RASSLs (receptors activated solely by synthetic ligands): We have dubbed it DREADDs (designer receptors exclusively activated by designer drugs). As we show in this review, DREADDs are now used ubiquitously to modulate GPCR activity noninvasively in vivo. Results from these studies have directly implicated GPCR signaling in a large number of therapeutically relevant contexts. We also highlight recent applications of DREADD technology that have illuminated GPCR signaling processes that control pathways relevant to the treatment of eating disorders, obesity, and obesity-associated metabolic abnormalities. Additionally, we provide an overview of the potential utility of chemogenetic technologies for transformative therapeutics.

Chemogenetic tools to interrogate brain functions

Elucidating the roles of neuronal cell types for physiology and behavior is essential for understanding brain functions. Perturbation of neuron electrical activity can be used to probe the causal relationship between neuronal cell types and behavior. New genetically encoded neuron perturbation tools have been developed for remotely controlling neuron function using small molecules that activate engineered receptors that can be targeted to cell types using genetic methods. Here we describe recent progress for approaches using genetically engineered receptors that selectively interact with small molecules. Called "chemogenetics," receptors with diverse cellular functions have been developed that facilitate the selective pharmacological control over a diverse range of cell-signaling processes, including electrical activity, for molecularly defined cell types. These tools have revealed remarkably specific behavioral physiological influences for molecularly defined cell types that are often intermingled with populations having different or even opposite functions.

Roundabout 1 exists predominantly as a basal dimeric complex and this is unaffected by binding of the ligand Slit2

Robo (Roundabout) receptors and their Slit polypeptide ligands are known to play key roles in neuronal development and have been implicated in both angiogenesis and cancer. Like the other family members, Robo1 is a large single transmembrane domain polypeptide containing a series of well-defined extracellular elements. However, the intracellular domain lacks structural definition and little is known about the quaternary structure of Robo receptors or how binding of a Slit might affect this. To address these questions combinations of both autofluorescent protein-based FRET imaging and time-resolved FRET were employed. Both approaches identified oligomeric organization of Robo1 that did not require the presence of the intracellular domain. SpIDA (spatial intensity distribution analysis) of eGFP-tagged forms of Robo1 indicated that for a C-terminally deleted version approximately two-thirds of the receptor was present as a dimer and one-third as a monomer. By contrast, full-length Robo1 was present almost exclusively as a dimer. In each case this was unaffected by the addition of Slit2, although parallel studies demonstrated the biological activity of Slit2 and its interaction with Robo1. Deletion of both the immunoglobulin and fibronectin type III extracellular repeats prevented dimer formation, with the immunoglobulin repeats providing the bulk of the protein-protein interaction affinity.

FRET spectrometry: a new tool for the determination of protein quaternary structure in living cells

Förster resonance energy transfer (FRET) is an exquisitely sensitive method for detection of molecular interactions and conformational changes in living cells. The recent advent of fluorescence imaging technology with single-molecule (or molecular-complex) sensitivity, together with refinements in the kinetic theory of FRET, provide the necessary tool kits for determining the stoichiometry and relative disposition of the protomers within protein complexes (i.e., quaternary structure) of membrane receptors and transporters in living cells. In contrast to standard average-based methods, this method relies on the analysis of distributions of apparent FRET efficiencies, E(app), across the image pixels of individual cells expressing proteins of interest. The most probable quaternary structure of the complex is identified from the number of peaks in the E(app) distribution and their dependence on a single parameter, termed pairwise FRET efficiency. Such peaks collectively create a unique FRET spectrum corresponding to each oligomeric configuration of the protein. Therefore, FRET could quite literally become a spectrometric method--akin to that of mass spectrometry--for sorting protein complexes according to their size and shape.

Role of receptor activity modifying protein 1 in function of the calcium sensing receptor in the human TT thyroid carcinoma cell line

The Calcium Sensing Receptor (CaSR) plays a role in calcium homeostasis by sensing minute changes in serum Ca(2+) and modulating secretion of calciotropic hormones. It has been shown in transfected cells that accessory proteins known as Receptor Activity Modifying Proteins (RAMPs), specifically RAMPs 1 and 3, are required for cell-surface trafficking of the CaSR. These effects have only been demonstrated in transfected cells, so their physiological relevance is unclear. Here we explored CaSR/RAMP interactions in detail, and showed that in thyroid human carcinoma cells, RAMP1 is required for trafficking of the CaSR. Furthermore, we show that normal RAMP1 function is required for intracellular responses to ligands. Specifically, to confirm earlier studies with tagged constructs, and to provide the additional benefit of quantitative stoichiometric analysis, we used fluorescence resonance energy transfer to show equal abilities of RAMP1 and 3 to chaperone CaSR to the cell surface, though RAMP3 interacted more efficiently with the receptor. Furthermore, a higher fraction of RAMP3 than RAMP1 was observed in CaSR-complexes on the cell-surface, suggesting different ratios of RAMPs to CaSR. In order to determine relevance of these findings in an endogenous expression system we assessed the effect of RAMP1 siRNA knock-down in medullary thyroid carcinoma TT cells, (which express RAMP1, but not RAMP3 constitutively) and measured a significant 50% attenuation of signalling in response to CaSR ligands Cinacalcet and neomycin. Blockade of RAMP1 using specific antibodies induced a concentration-dependent reduction in CaSR-mediated signalling in response to Cinacalcet in TT cells, suggesting a novel functional role for RAMP1 in regulation of CaSR signalling in addition to its known role in receptor trafficking. These data provide evidence that RAMPs traffic the CaSR as higher-level oligomers and play a role in CaSR signalling even after cell surface localisation has occurred.

Luminescent lanthanide cryptates: from the bench to the bedside

The design and application of luminescent lanthanide cryptates for sensing biological interactions is highlighted through the review of the work performed in our laboratory and with academic collaborations. The path from the initial applications probing biochemical interaction in vitro to "state-of-the-art" cellular assays toward clinical applications using homogeneous time-resolved fluorescence technology is described. An overview of the luminescent lanthanide macrocyclic compounds developed at Cisbio in the recent past is given with an emphasis on specific constraints required by specific applications. Recent assays for drug-discovery and diagnostic purposes using both antibody-based and suicide-enzyme-based technology are illustrated. New perspectives in the field of molecular medicine and time-resolved microscopy are discussed.

Cyclic GMP regulates M₃AChR activity at plasma membranes from airway smooth muscle

Muscarinic acetylcholine receptors MAChRs from Bovine Tracheal Smooth Muscle (BTSM) plasma membranes are responsible for the cGMP rise and signal-amplitude peaks associated with smooth muscle contraction present in bronchial asthma. These MAChRs bind [(3)H]QNB and exhibit the classic G Protein Coupled-Receptor (GPCR) behavior towards muscarinic agonist and antagonists that is sensitive to sensitive to GTP analogs. Interestingly, the [(3)H]QNB binding activity was stimulated by cGMP and ATP, and was enhanced by IBMX and Zaprinast, inhibitors of cGMP-PDE. Cyclic GMP plus ATP affected the agonist-antagonist muscarinic binding activities. Thus, the high affinity agonist (Carbamylcholine) binding sites disappeared, whereas, 4-DAMP, a M3 selective antagonist displayed an additional high affinity-binding site. In contrast, non-selective (atropine) and M2-selective (methoctramine and gallamine) antagonists revealed one low binding site. Moreover, the 4-DAMP-mustard alkylation of the MAChRs blocked the cGMP effect indicating that the M3AChR is the main receptor target of cGMP. Interestingly, these cGMP effects were potentiated by an activator (Sp-8-pCPT-cGMPS), and diminished by an inhibitor (Rp-8-pCPT-CGMPS), of cGMP-dependent protein kinase (PKG-II), which was detected by Western blotting using specific PKG II antibodies. Finally, plasma membrane M3AChRs were phosphorylated in a cGMP-dependent manner and this novel post-translational reversible modification at M3AChRs may act as a feedback mechanism to terminate the cGMP dependent muscarinic signal transduction cascades at the sarcolema of BTSM.

Fluorescence correlation spectroscopy analysis of serotonin, adrenergic, muscarinic, and dopamine receptor dimerization: the oligomer number puzzle

The issue of G protein-coupled receptor (GPCR) oligomer status has not been resolved. Although many studies have provided evidence in favor of receptor-receptor interactions, there is no consensus as to the exact oligomer size of class A GPCRs. Previous studies have reported monomers, dimers, tetramers, and higher-order oligomers. In the present study, this issue was examined using fluorescence correlation spectroscopy (FCS) with photon counting histogram (PCH) analysis, a sensitive method for monitoring diffusion and oligomer size of plasma membrane proteins. Six different class A GPCRs were selected from the serotonin (5-HT2A), adrenergic (α1b-AR and β2-AR), muscarinic (M1 and M2), and dopamine (D1) receptor families. Each GPCR was C-terminally labeled with green fluorescent protein (GFP) or yellow fluorescent protein (YFP) and expressed in human embryonic kidney 293 cells. FCS provided plasma membrane diffusion coefficients on the order of 7.5 × 10(-9) cm(2)/s. PCH molecular brightness analysis was used to determine the GPCR oligomer size. Known monomeric (CD-86) and dimeric (CD-28) receptors with GFP and YFP tags were used as controls to determine the molecular brightness of monomers and dimers. PCH analysis of fluorescence-tagged GPCRs revealed molecular brightness values that were twice the monomeric controls and similar to the dimeric controls. Reduced χ(2) analyses of the PCH data best fit a model for a homogeneous population of homodimers, without tetramers or higher-order oligomers. The homodimer configuration was unaltered by agonist treatment and was stable over a 10-fold range of receptor expression level. The results of this study demonstrate that biogenic amine receptors freely diffusing within the plasma membrane are predominantly homodimers.

Novel designer receptors to probe GPCR signaling and physiology

Muscarinic receptor-based designer receptors have emerged as powerful novel tools to study G-protein-coupled receptor (GPCR) signaling and physiology. These new designer GPCRs, which are most frequently referred to as DREADDs (designer receptors exclusively activated by designer drug), are unable to bind acetylcholine, the endogenous muscarinic receptor agonist, but can be activated by clozapine-N-oxide (CNO), an otherwise pharmacologically inert compound, with high potency and efficacy. The various DREADDs differ primarily in their G protein coupling preference. More recently, an arrestin-biased DREADD has also been developed. The expression of DREADDs in distinct tissues or cell types has enabled researchers to study the outcome of selective stimulation of distinct GPCR (or arrestin) signaling pathways in a temporally and spatially controlled fashion in vivo. In this review, we provide an up-to-date snapshot of where this field currently stands and which important novel insights have been gained using this new technology.

Functional significance of serotonin receptor dimerization

The original model of G-protein activation by a single G-protein-coupled receptor (GPCR) is giving way to a new model, wherein two protomers of a GPCR dimer interact with a single G-protein. This article will review the evidence suggesting that 5-HT receptors form dimers/oligomers and will compare the findings with the results obtained from the studies with other biogenic amine receptors. Topics to be covered include the origin or biogenesis of dimer formation, potential dimer interface(s), and oligomer size (dimer vs. tetramer or higher order). The functional significance will be discussed in terms of G-protein activation following ligand binding to one or two protomers in a dimeric structure, the formation of heterodimers, and the development of bivalent ligands.

The muscarinic M3 acetylcholine receptor exists as two differently sized complexes at the plasma membrane

The literature on GPCR (G-protein-coupled receptor) homo-oligomerization encompasses conflicting views that range from interpretations that GPCRs must be monomeric, through comparatively newer proposals that they exist as dimers or higher-order oligomers, to suggestions that such quaternary structures are rather ephemeral or merely accidental and may serve no functional purpose. In the present study we use a novel method of FRET (Förster resonance energy transfer) spectrometry and controlled expression of energy donor-tagged species to show that M3Rs (muscarinic M3 acetylcholine receptors) at the plasma membrane exist as stable dimeric complexes, a large fraction of which interact dynamically to form tetramers without the presence of trimers, pentamers, hexamers etc. That M3R dimeric units interact dynamically was also supported by co-immunoprecipitation of receptors synthesized at distinct times. On the basis of all these findings, we propose a conceptual framework that may reconcile the conflicting views on the quaternary structure of GPCRs.

The prevalence, maintenance, and relevance of G protein-coupled receptor oligomerization

Over the past decade, ideas and experimental support for the hypothesis that G protein-coupled receptors may exist as dimeric or oligomeric complexes moved initially from heresy to orthodoxy, to the current situation in which the capacity of such receptors to interact is generally accepted but the prevalence, maintenance, and relevance of such interactions to both pharmacology and function remain unclear. A vast body of data obtained following transfection of cultured cells is still to be translated to native systems and, even where this has been attempted, results often remain controversial and contradictory. This review will consider approaches that are currently being applied and why these might be challenging to interpret, and will suggest means to overcome these limitations.

Structural and biophysical characterisation of G protein-coupled receptor ligand binding using resonance energy transfer and fluorescent labelling techniques

The interaction between ligands and the G protein-coupled receptors (GPCRs) to which they bind has long been the focus of intensive investigation. The signalling cascades triggered by receptor activation, due in most cases to ligand binding, are of great physiological and medical importance; indeed, GPCRs are targeted by in excess of 30% of small molecule therapeutic medicines. Attempts to identify further pharmacologically useful GPCR ligands, for receptors with known and unknown endogenous ligands, continue apace. In earlier days direct assessment of such interactions was restricted largely to the use of ligands incorporating radioactive isotope labels as this allowed detection of the ligand and monitoring its interaction with the GPCR. This use of such markers has continued with the development of ligands labelled with fluorophores and their application to the study of receptor-ligand interactions using both light microscopy and resonance energy transfer techniques, including homogenous time-resolved fluorescence resonance energy transfer. Details of ligand-receptor interactions via X-ray crystallography are advancing rapidly as methods suitable for routine production of substantial amounts and stabilised forms of GPCRs have been developed and there is hope that this may become as routine as the co-crystallisation of serine/threonine kinases with ligands, an approach that has facilitated widespread use of rapid structure-based ligand design. Conformational changes involved in the activation of GPCRs, widely predicted by biochemical and biophysical means, have inspired the development of intramolecular FRET-based sensor forms of GPCRs designed to investigate the events following ligand binding and resulting in a signal propagation across the cell membrane. Finally, a number of techniques are emerging in which ligand-GPCR binding can be studied in ways that, whilst indirect, are able to monitor its results in an unbiased and integrated manner. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

GPCR oligomerization and receptor trafficking

The effects of oligomerization of G protein-coupled receptors (GPCRs) upon their trafficking around the cell are considerable, and this raises the potential of significant impact upon the use of existing pharmacological agents and the development of new ones. Herein, we describe a number of different techniques that can be used to study receptor dimerization/oligomerization and trafficking, beginning with a cellular system which allows the expression of two GPCRs simultaneously, one under inducible control. Subsequently, we describe means to visualize and monitor the movement of GPCRs within the cell, detect oligomerization by both resonance energy transfer and more traditional biochemical approaches, and to measure the internalization of GPCRs as part of the process of receptor regulation.

Pharmacosynthetics: Reimagining the pharmacogenetic approach

Pharmacology, in its broadest interpretation, is defined as the study of the interaction between physiological entities and drugs. In modern neuropsychopharmacology, this interaction is viewed as the drug itself on one side and signal transducer (receptor), the signal transduction cascade (effector proteins, second messengers), the cellular response (transcriptional regulation, activity modulation), the organ response (brain circuitry modulation), and, finally, the whole organism response (behavior) on the other. In other words, pharmacology has structured itself around the idea that the exogenous molecule (the drug) encodes a "signal" leading to everything on the other side including, in extreme renditions, a physiological response. The inference is that engaging a particular signal transduction pathway in a defined cell type leads inexorably to a prototypic physiological response. Thus, for instance, serotonergic activation of 5-HT(2A) receptors in rat aortic smooth muscle cells leads to an increase in intracellular Ca(++) (via IP₃ release) and smooth muscle contraction (Roth et al., 1986). Here, we suggest that the invention of synthetic ligand--GPCR pairs (aka DREADDs, RASSLS, 'pharmacogenetics') permits the study of pharmacology using a shifted equation: more of the signal transduction elements moved to the left and, subsequently, under experimental control. For the purposes of disambiguation and to clarify this new interpretation as a creation of pharmacological manipulation, we present the term pharmacosynthetics to describe what has heretofore been called pharmacogenetics or chemicogenetics. This review discusses this new interpretation and reviews recent applications of the technology and considerations of the approach. This article is part of a Special Issue entitled Optogenetics (7th BRES).

The orexin OX(1) receptor exists predominantly as a homodimer in the basal state: potential regulation of receptor organization by both agonist and antagonist ligands

It is unclear what proportion of a G-protein-coupled receptor is present in cells as dimers or oligomers. Saturation bioluminescence resonance energy transfer studies demonstrated the orexin OX(1) receptor to be present in such complexes. Forms of this receptor containing a minimal epitope tag, with the C-terminus linked to yellow fluorescent protein or modified at the N-terminus to incorporate a SNAP tag, migrated in SDS/PAGE gels as monomers, indicating a lack of covalent interactions. Solubilization with dodecylmaltoside, followed by Blue native-PAGE, indicated that the receptor constructs migrated predominantly as anticipated for dimeric species with evidence for further, higher-order, complexes, and this was true over a wide range of expression levels. Addition of SDS prior to separation by Blue native-PAGE resulted in much of the previously dimeric, and all of the higher-order, complexes being dissociated and now migrating at the size predicted for monomeric species. Expression of forms of the OX(1) receptor capable of generating enzyme complementation confirmed that solubilization itself did not result in interaction artefacts. Addition of the endogenous agonist orexin A enhanced the proportion of higher-order OX(1) receptor complexes, whereas selective OX(1) antagonists increased the proportion the OX(1) receptor migrating in Blue native-PAGE as a monomer. The antagonist effects were produced in a concentration-dependent manner, consistent with the affinity of the ligands for the receptor. Homogeneous time-resolved fluorescence resonance energy transfer studies using Tag-Lite™ reagents on cells expressing the SNAP-tagged OX(1) receptor identified cell-surface OX(1) homomers. Predominantly at low receptor expression levels, orexin A increased such fluorescence resonance energy transfer signals, also consistent with ligand-induced reorganization of the homomeric complex.

G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand?

Most cells express a panel of different G protein-coupled receptors (GPCRs) allowing them to respond to at least a corresponding variety of extracellular ligands. In order to come to an integrative well-balanced functional response these ligand-receptor pairs can often cross-regulate each other. Although most GPCRs are fully capable to induce intracellular signalling upon agonist binding on their own, many GPCRs, if not all, appear to exist and function in homomeric and/or heteromeric assemblies for at least some time. Such heteromeric organization offers unique allosteric control of receptor pharmacology and function between the protomers and might even unmask 'new' features. However, it is important to realize that some functional consequences that are proposed to originate from heteromeric receptor interactions may also be observed due to intracellular crosstalk between signalling pathways of non-associated GPCRs.

Heteromultimerization of cannabinoid CB(1) receptor and orexin OX(1) receptor generates a unique complex in which both protomers are regulated by orexin A

Agonist-induced internalization was observed for both inducible and constitutively expressed forms of the cannabinoid CB(1) receptor. These were also internalized by the peptide orexin A, which has no direct affinity for the cannabinoid CB(1) receptor, but only when the orexin OX(1) receptor was co-expressed along with the cannabinoid CB(1) receptor. This effect of orexin A was concentration-dependent and blocked by OX(1) receptor antagonists. Moreover, the ability of orexin A to internalize the CB(1) receptor was also blocked by CB(1) receptor antagonists. Remarkably, orexin A was substantially more potent in producing internalization of the CB(1) receptor than in causing internalization of the bulk OX(1) receptor population, and this was true in cells in which the CB(1) receptor was maintained at a constant level, whereas levels of OX(1) could be varied and vice versa. Both co-immunoprecipitation and cell surface, homogenous time-resolved fluorescence resonance energy transfer based on covalent labeling of N-terminal "SNAP" and "CLIP" tags present in the extracellular N-terminal domain of the receptors confirmed the capacity of these two receptors to heteromultimerize. These studies confirm the capacity of the CB(1) and OX(1) receptors to interact directly and demonstrate that this complex has unique regulatory characteristics. The higher potency of the agonist orexin A to regulate the CB(1)-OX(1) heteromer compared with the OX(1)-OX(1) homomer present in the same cells and the effects of CB(1) receptor antagonists on the function of orexin A suggest an interplay between these two systems that may modulate appetite, feeding, and wakefulness.

Developing chemical genetic approaches to explore G protein-coupled receptor function: validation of the use of a receptor activated solely by synthetic ligand (RASSL)

Molecular evolution and chemical genetics have been applied to generate functional pairings of mutated G protein-coupled receptors (GPCRs) and nonendogenous ligands. These mutant receptors, referred to as receptors activated solely by synthetic ligands (RASSLs) or designer receptors exclusively activated by designer drugs (DREADDs), have huge potential to define physiological roles of GPCRs and to validate receptors in animal models as therapeutic targets to treat human disease. However, appreciation of ligand bias and functional selectivity of different ligands at the same receptor suggests that RASSLs may signal differently than wild-type receptors activated by endogenous agonists. We assessed this by generating forms of wild-type human M(3) muscarinic receptor and a RASSL variant that responds selectively to clozapine N-oxide. Although the RASSL receptor had reduced affinity for muscarinic antagonists, including atropine, stimulation with clozapine N-oxide produced effects very similar to those generated by acetylcholine at the wild-type M(3)-receptor. Such effects included the relative movement of the third intracellular loop and C-terminal tail of intramolecular fluorescence resonance energy transfer sensors and the ability of the wild type and evolved mutant to regulate extracellular signal-regulated kinase 1/2 phosphorylation. Each form interacted similarly with β-arrestin 2 and was internalized from the cell surface in response to the appropriate ligand. Furthermore, the pattern of phosphorylation of specific serine residues within the evolved receptor in response to clozapine N-oxide was very similar to that produced by acetylcholine at the wild type. Such results provide confidence that, at least for the M(3) muscarinic receptor, results obtained after transgenic expression of this RASSL are likely to mirror the actions of acetylcholine at the wild type receptor.

Allostery at G protein-coupled receptor homo- and heteromers: uncharted pharmacological landscapes

For many years seven transmembrane domain G protein-coupled receptors (GPCRs) were thought to exist and function exclusively as monomeric units. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can both traffic and signal within higher-order complexes. As for other protein-protein interactions, conformational changes in one polypeptide, including those resulting from binding of pharmacological ligands, have the capacity to alter the conformation and therefore the response of the interacting protein(s), a process known as allosterism. For GPCRs, allosterism across homo- or heteromers, whether dimers or higher-order oligomers, represents an additional topographical landscape that must now be considered pharmacologically. Such effects may offer the opportunity for novel therapeutic approaches. Allosterism at GPCR heteromers is particularly exciting in that it offers additional scope to provide receptor subtype selectivity and tissue specificity as well as fine-tuning of receptor signal strength. Herein, we introduce the concept of allosterism at both GPCR homomers and heteromers and discuss the various questions that must be addressed before significant advances can be made in drug discovery at these GPCR complexes.

Remote control of neuronal signaling

A significant challenge for neuroscientists is to determine how both electrical and chemical signals affect the activity of cells and circuits and how the nervous system subsequently translates that activity into behavior. Remote, bidirectional manipulation of those signals with high spatiotemporal precision is an ideal approach to addressing that challenge. Neuroscientists have recently developed a diverse set of tools that permit such experimental manipulation with varying degrees of spatial, temporal, and directional control. These tools use light, peptides, and small molecules to primarily activate ion channels and G protein-coupled receptors (GPCRs) that in turn activate or inhibit neuronal firing. By monitoring the electrophysiological, biochemical, and behavioral effects of such activation/inhibition, researchers can better understand the links between brain activity and behavior. Here, we review the tools that are available for this type of experimentation. We describe the development of the tools and highlight exciting in vivo data. We focus primarily on designer GPCRs (receptors activated solely by synthetic ligands, designer receptors exclusively activated by designer drugs) and microbial opsins (e.g., channelrhodopsin-2, halorhodopsin, Volvox carteri channelrhodopsin) but also describe other novel techniques that use orthogonal receptors, caged ligands, allosteric modulators, and other approaches. These tools differ in the direction of their effect (activation/inhibition, hyperpolarization/depolarization), their onset and offset kinetics (milliseconds/minutes/hours), the degree of spatial resolution they afford, and their invasiveness. Although none of these tools is perfect, each has advantages and disadvantages, which we describe, and they are all still works in progress. We conclude with suggestions for improving upon the existing tools.

Ligand-induced internalization of the orexin OX(1) and cannabinoid CB(1) receptors assessed via N-terminal SNAP and CLIP-tagging

BACKGROUND AND PURPOSE

Many G protein-coupled receptors internalize following agonist binding. The studies were designed to identify novel means to effectively quantify this process using the orexin OX(1) receptor and the cannabinoid CB(1) receptor as exemplars.

EXPERIMENTAL APPROACH

The human OX(1) and CB(1) receptors were modified to incorporate both epitope tags and variants (SNAP and CLIP) of the enzyme O(6)-alkylguanine-DNA-alkyltransferase within their extracellular, N-terminal domain. Cells able to regulate expression of differing amounts of these constructs upon addition of an antibiotic were developed and analysed.

KEY RESULTS

Cell surface forms of each receptor construct were detected by both antibody recognition of the epitope tags and covalent binding of fluorophores to the O(6)-alkylguanine-DNA-alkyltransferase variants. Receptor internalization in response to agonists but not antagonists could be monitored by each approach but sensitivity was up to six- to 10-fold greater than other approaches when employing a novel, time-resolved fluorescence probe for the SNAP tag. Sensitivity was not enhanced, however, for the CLIP tag, possibly due to higher levels of nonspecific binding.

CONCLUSIONS AND IMPLICATIONS

These studies demonstrate that highly sensitive and quantitative assays that monitor cell surface CB(1) and OX(1) receptors and their internalization by agonists can be developed based on introduction of variants of O(6)-alkylguanine-DNA-alkyltransferase into the N-terminal domain of the receptor. This should be equally suitable for other G protein-coupled receptors.