Scholarly article on topic 'G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches'

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Academic research paper on topic "G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches"


Molecular Pharmacology

Copyright © 2015 by The American Society for Pharmacology and Experimental Therapeutics Mol Pharmacol 88:561-571, September 2015


G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches

Henry F. Vischer, Marian Castro, and Jean-Philippe Pin

Amsterdam Institute for Molecules, Medicines and Systems, Division of Medicinal Chemistry, Faculty of Sciences, VU University Amsterdam, Amsterdam, The Netherlands (H.F.V.); Molecular Pharmacology Laboratory, Biofarma Research Group (GI-1685), University of Santiago de Compostela, Center for Research in Molecular Medicine and Chronic Diseases, Santiago de Compostela, Spain (M.C.); and Centre National de la Recherche Scientifique, Institut de Génomique Fonctionnelle, Université de Montpellier, Montpellier, France (J.-P.P.) Received April 11, 2015; accepted July 2, 2015

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Heteromerization of G protein-coupled receptors (GPCRs) can significantly change the functional properties of involved receptors. Various biochemical and biophysical methodologies have been developed in the last two decades to identify and functionally evaluate GPCR heteromers in heterologous cells, with recent approaches focusing on GPCR complex stoichiometry and stability. Yet validation of these observations in native tissues is still lagging behind forthe majority of GPCR heteromers. Remarkably,

recent studies, particularly some involving advanced fluorescence microscopy techniques, are contributing to our current knowledge of aspects that were not well known until now, such as GPCR complex stoichiometry and stability. In parallel, a growing effort is being applied to move the field forward into native systems. This short review will highlight recent developments to study the stoichiometry and stability of GPCR complexes and methodologies to detect native GPCR dimers.

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G protein-coupled receptors (GPCRs) play a key role in the regulation of cells in our body by activating intracellular signaling in response to a wide variety of specific agonists (Lagerström and Schiöth, 2008; Alexander et al., 2011). Most cells express multiple GPCR subtypes and are consequently able to respond to at least a corresponding number of different agonists (Vassilatis et al., 2003; Regard et al., 2008; Insel et al., 2012; Fève et al., 2014). Distinct GPCRs can affect each other's functions to come to an integrative cellular response through direct physical interactions in heteromeric complexes. Over the last 20 years, dimerization and/or oligomerization was reported

This work is supported by the Spanish Ministry of Economy and Competitiveness (MINECO) [SAF2014-57138-C2-1-R] to Marian Castro. 115.099440.

for nearly all tested GPCR subtypes using mostly engineered GPCR constructs expressed in heterologous systems (Fig. 1) (Khelashvili et al., 2010; Cottet et al., 2012).

Class C GPCRs require dimerization to transduce transmembrane signaling in response to agonists (Kniazeff et al., 2011; El Moustaine et al., 2012). The most studied example is the GABAB receptor, for which GABAB1 and GABAB2 functionally complement each other by forming the receptor heterodimer. Binding of GABA to the N-terminal extracellular domain (NTED) of GABAB1 results in allosteric transactivation of the GABAB2 protomer and subsequent G protein coupling to the activated 7 transmembrane domain (7TMD) of GABAB2 (Galvez et al., 2001; Duthey et al., 2002; Kniazeff et al., 2002). Moreover, GABAB1 requires heteromerization with GABAB2 to traffic to the cell surface (Margeta-Mitrovic et al., 2000; Pagano et al., 2001; Brock et al., 2005). Heteromerization of the taste 1

ABBREVIATIONS: AMD3100, 1,1'-[1,4-phenylenebis-(methylene)]-bis-(1,4,8,11-tetraazacyclotetradecane) octahydrochloride; BRET, bioluminescence resonance energy transfer; CODA-RET, complemented donor-acceptor resonance energy transfer; DOR, 5-opioid receptor; eGFP, enhanced green fluorescent protein; FCS, fluorescence correlation spectroscopy; FRET, fluorescence resonance energy transfer; FSHR, follicle-stimulating hormone receptor; GPCR, G protein-coupled receptor; 5-HT, 5-hydroxytryptamine; IL3, intracellular loop 3; LHR, luteinizing hormone receptor; mGlu, metabotropic glutamate; MOR, m-opioid receptor; NTED, N-terminal extracellular domain; PLA, proximity ligation assay; RET, resonance energy transfer; SCTR, secretin receptor; TAK779, dimethyl-[[4-[[3-(4-methylphenyl)-8,9-dihydro-7H-benzo[7]annulene-6-carbonyl]amino]phenyl] methyl]-(oxan-4-yl)azanium; TC14012, L-arginyl-L-arginyl-3-(2-naphthalenyl)-L-alanyl-L-cysteinyl-L-tyrosyl-N5-(aminocarbonyl)-L-ornithyl-L-lysyl-N5-(aminocarbonyl)-D-ornithyl-L-prolyl-L-tyrosyl-L-arginyl-N5-(aminocarbonyl)-L-ornithyl-L-cysteinyl-cyclic (4^13)-disulfide-L-argininamide; TIRFM, total internal reflection fluorescence microscopy; TM, transmembrane; 7TMD, 7 transmembrane domain; TSHR, thyroid-stimulating hormone receptor; VUF10661, (3S)-N-[(1S)-5-amino-1-[[(2,2-diphenylethyl)amino]carbonyl]pentyl]-2-(1,4-dioxo-4-phenylbutyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide.

Fig. 1. Detection of GPCR dimers. Antibodies targeting epitope-tagged (depicted) or native GPCRs specifically label receptor populations to subsequently allow detection of physical interaction by coimmunoprecipitation (co-IP) (A) or close proximity using in situ proximity ligation of DNA-conjugated secondary antibodies (B) and time-resolved (TR) FRET between fluorophore-conjugated antibodies (C). Time-resolved FRET between fluorescent ligands allows detection of receptor complexes in native tissue (D). Genetic fusion of luminescent or fluorescent donor and acceptor proteins or nonfunctional fragments of these proteins to the C-terminal tail of GPCRs allows detection of close proximity by RET (E) and bimolecular fluorescence (BiFC) or luminescence (BiLC) complementation (F). Heteromer identification technology (HIT): RET between b-arrestin and one GPCR subtype that is fused to donor or acceptor proteins upon agonist activation of another untagged GPCR subtype indicates that both GPCRs are in close proximity (G). CODA-RET detects the interaction of GPCR dimers with intracellular signaling partners using the combination of bimolecular fluorescence complementation with sequential FRET (H).

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receptor (T1R) 3 with T1Ri or T1R2 results in the sensation of umami taste or sweeteners, respectively (Zhao et al., 2003). Indeed, knockout of the individual T1R1-, T1R2-, or T1R3-encoding genes in mice largely attenuates umami, sweet, or both flavors, respectively (Damak et al., 2003; Zhao et al., 2003). Dimerization of purified metabotropic glutamate (mGlu) 2 receptors in nanodiscs is required for G protein activation in response to the endogenous agonist glutamate (El Moustaine et al., 2012). In contrast, refolding of purified class A GPCRs in detergent micelles or nanodiscs revealed that monomers are fully able to bind their cognate ligand, activate G proteins, and/or recruit G protein-coupled receptor kinases and arrestins (Bayburt et al., 2007; Hanson et al., 2007; White et al., 2007; Whorton et al., 2007, 2008; Kuszak et al., 2009; Arcemisbehere et al., 2010; Tsukamoto et al., 2010; Bayburt

et al., 2011). In addition, the class B parathyroid hormone receptor activates G proteins when expressed as monomers, as observed using dimer-disrupting mutations (Pioszak et al., 2010).

Hence, dimerization is not required for tested class A and B GPCRs to transduce agonist-induced intracellular signaling. The question of why do these GPCRs dimerize and oligo-merize obviously rises. Physically interacting GPCRs may modulate each other's activities. However, unambiguous discrimination between GPCR crosstalk as a consequence of receptor heteromerization and that resulting from their intracellular signaling events (Schmidlin et al., 2002; Vázquez-Prado et al., 2003; Natarajan et al., 2006; Kelly et al., 2008; Rives et al., 2009; Nijmeijer et al., 2010) is experimentally challenging and requires experimental perturbation of these

GPCRs to form heteromers. Moreover, in situ validation of GPCR heteromerization and their specific functional properties in native tissues is difficult and consequently often lacking (Pin et al., 2007). Hence, the physiologic relevance of many identified GPCR heteromers remains a topic of debate, not least because recent methodologies shed distinct light on the size and stability of GPCR heteromers (Lambert and Javitch, 2014). In this review, we will first focus on recent developments to determine the size, stability, and proximal signaling of GPCR complexes and secondly on evidence for GPCR heteromerization in native tissue.

Proportion, Size, and Stability of GPCR Dimers and Oligomers

GPCR oligomerization has been the subject of significant research over the last two decades by using a number of biochemical and biophysical approaches mostly involving engineered GPCR constructs, among other approaches. Hence, receptors harboring N-terminal epitope (e.g., hemagglutinin, FLAG, or cMyc) and/or SNAP/CLIP/Halo tags can be detected using specific high-affinity antibodies and covalent labeling, respectively. Subsequent coimmunoprecipitation and time-resolved FRET reveals the physical association and close proximity of GPCRs in heterologous cells (Fig. 1, AC) (Milligan and Bouvier, 2005; Maurel et al., 2008; Faklaris et al., 2015). Moreover, fusion of bioluminescent, fluorescent proteins, or nonfunctional fragments of these proteins to the C-terminal tail of GPCRs allows close-proximity detection of GPCR dimers and/or oligomers in living cells using bioluminescence resonance energy transfer (BRET), fluorescence resonance energy transfer (FRET), or bimolecular complementation, respectively (Fig. 1, E and F), and approaches such as the heteromer identification technology or complemented donor-acceptor resonance energy transfer (CODA-RET) have also been implemented for the investigation of complexes involving three interacting partners (Fig. 1, G and H) (Ciruela et al., 2010; Cottet et al., 2012; Kaczor et al., 2014).

Although most of these techniques have retrieved qualitative information on the formation of GPCR dimers, the proportion of GPCRs that are engaged in dimers and/or oligomers as well as the stoichiometry and stability of such complexes are generally not well characterized and require carefully controlled quantitative measurements. Saturation FRET between purified ^-adrenergic receptors (b2-ARs) site-specifically labeled with fluorophores and reconstituted in lipid bilayers suggested the predominant formation of tetramers (Fung et al., 2009). Three-color sequential BRET-FRET and bimolecular luminescence/ fluorescence complementation in combination with resonance energy transfer (RET) revealed the formation of GPCR hetero-multimers, consisting of at least three or four individual GPCRs, when expressed at physiologic levels (Lopez-Gimenez et al., 2007; Carriba et al., 2008; Guo et al., 2008; Nijmeijer et al., 2010; Armando et al., 2014). However, the interpretation of quantitative RET approaches between membrane-associated proteins has been challenged (James et al., 2006; Lambert and Javitch, 2014; Lan et al., 2015), and revised experimental designs (Szalai et al., 2014) as well as third-party RET approaches (Kuravi et al., 2010) have been proposed to improve the interpretation of results from RET experiments.

In this context, intensity-based FRET approaches (i.e., those based on measurements of emission intensity of

the fluorophores rather than of their fluorescence lifetimes), where spectral datasets are acquired, can provide quantitative information not only of the apparent FRET efficiency of a sample, but also on donor-acceptor stoichiometry of their interactions. These methods can be combined with spectral imaging microscopy for spatial resolution and have allowed investigation of the quaternary structure of GPCRs in a more quantitative manner than other previously employed nonspectral intensity-based FRET approaches based on average measurements of apparent FRET efficiency (Zeug et al., 2012; Raicu and Singh, 2013). This is possible because spectrally resolved FRET approaches allow the accurate measurement of concentrations of donors and acceptors with overlapping emission spectra. Contaminations of the FRET signal as a consequence of the donor's bleedthrough or direct acceptor excitation are corrected in spectrally resolved FRET, and possible contributions of an unpaired donor and acceptors in the sample are taken into account by applying specific algorithms. Some drawbacks of these techniques can be the instrumentation requirements, such as spectral imaging detectors for certain applications, or the requirement of donor and acceptor reference samples of a known concentration (i.e., purified fluorescent proteins) for spectral unmixing. Different spectrally resolved FRET approaches revealed the proportion of serotonin 5-hydroxytryptamine (5-HT) 1A receptors that were engaged in oligomers in transfected cells (Gorinski et al., 2012) as well as the formation of transient tetramers by stable M3 muscarinic acetylcholine receptor dimers at the cell surface, with minimal interference from a bystander RET signal coming from nearby noninteracting partners (Patowary et al., 2013).

Fluorescence correlation spectroscopy (FCS) allows the detection of fluctuations in fluorescent intensity that result from the diffusion of fluorescent molecules in and out of an open, diffraction-limited, observation volume. Suitable for extracting two-dimensional information on membrane protein dynamics with submicrosecond temporal resolution, it constitutes a powerful approach to monitor the diffusion of GPCRs in the plasma membrane of single living cells in real time. To this end, receptors fused to a particular fluorescent protein are heterologously expressed in cells (Herrick-Davis et al., 2012, 2013; Teichmann et al., 2014). As a variant, the expression of receptors fused to complementary fragments of a fluorescent protein will allow the exclusive detection of diffusing receptor complexes that become fluorescent upon bimolecular fluorescence complementation (Briddon et al., 2008). FCS measurements achieve single molecule sensitivity and are more accurately performed in samples with very low expression levels, such as those found for some GPCRs in native cells. Hence, endogenous GPCRs can be labeled with fluorescent ligands (Cordeaux et al., 2008; Corriden et al.,

2014) or specific antireceptor fragment antigen binding proteins fused to fluorescent moieties (Herrick-Davis et al.,

2015) for FCS measurements (Fig. 2). In the latter case and due to the monovalent nature ofthe fragment antigen binding fragments opposite to antibodies, potential artifacts due to antibody-induced clustering ofreceptors are avoided. FCS can give information on a protein that is likely to be in the same complex based on the changes in the diffusion coefficients by the formation of larger codiffusing entities and on the brightness of the diffusing particles. However, it should be kept in mind that FCS measures codiffusing proteins that are

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Fig. 2. Fluorescence correlation spectroscopy combined with a photon counting histogram analysis to investigate the oligomerization status of native serotonin 5-HT2c receptors in living cells. Native serotonin 5-HT2C receptors in choroid plexus epithelial cells were labeled with monoclonal anti-5-HT2C fragment antigen binding proteins fused to green fluorescent protein (A). FCS measurements were made on the apical surface of living cells using a one-photon excitation microscope equipped with a sensitive photon counting detector, creating an observation volume of less than 1 fl (<1 mm3), and green-fluorescent protein fluorescence emission was registered for 100 seconds as 10 consecutive 10-second intervals (B). Autocorrelation analysis (ACA) of the recorded fluorescence intensity traces (FIT) from 10-second observation periods informs on the number of molecules in the observation volume and diffusion time (C), whereas a photon-counting histogram (PCH) of the FCS recordings provides a quantification of the photons emitted from the individual fluorescent molecules. This allows determination of the average molecular brightness of the sample and identification of the number of green fluorescent protein-labeled protomers that take part of a codiffusing complex [FCS traces and autocorrelation/PCH analysis taken from Herrick-Davis et al. (2015), with permission].

situated within the same microdomain but do not provide proof for direct physical protein-protein interaction. In addition, due to the detection limit, FCS is not able to discriminate between diffusion times of molecules with similar masses. At least a difference by a factor of four in the molecular mass of the particles is required to resolve their diffusion times by FCS (Meseth et al., 1999). This poses a limitation of FCS to distinguish among GPCR monomers, dimers, or tetramers on the basis of diffusion times alone. Moreover, factors such as heterogeneity of membrane viscosity in microdomains or the interaction of receptors with other signaling or scaffolding proteins might affect diffusion times. Autocorrelation analysis of FCS measurements reveals the average number of molecules within the observation volume and informs on the number of diffusing entities but not on the number of fluorescent protomers within diffusing complexes. Combining FCS with a photon counting histogram analysis enables quantification of the intensity of fluorescence fluctuations, which provides information with regards to the stoichiometry of protein complexes (Fig. 2C) (Chen et al., 1999). By doing this, the molecular brightness of the diffusing particles and consequently the number of fluorophores codiffusing in each particle can be determined. Subsequent comparison with appropriate reference proteins with a known monomeric/oligomeric status allows quantification of the oligomerization state of proteins of interest (Herrick-Davis et al., 2012, 2013). These combined techniques suggest the existence of native serotonin 5-HT2C receptor homomeric complexes in the apical membrane of living choroid plexus epithelial cells (Herrick-Davis et al., 2015). For some class A and B GPCRs, an equilibrium between monomeric and possible dimeric species was found, with a low proportion of dimers, by using fluorescence crosscorrelation spectroscopy (Teichmann et al., 2014), a development of FCS for dual-color applications that allows discrimination of single-labeled entities carrying two different fluorophores from the duallabeled diffusing particles resulting from their interaction. In this approach, the emission of the two fluorophores are separately registered by two different detectors, and when the two differently labeled partners comigrate, the fluctuations in the intensity of the two fluorophores will correlate as they

diffuse into and out of the observation volume together. The crosscorrelation function provides more sensitive information than the diffusion coefficient alone for the detection of interactions. The information gathered in the work of Teichmann et al. (2014) confirmed single molecule total internal reflection fluorescence microscopy (TIRFM) studies at b1-AR or M1 muscarinic acetylcholine receptors (M1Rs) (Hern et al., 2010; Calebiro et al., 2013). However, a homogeneous population of dimers without evidence of the coexistence of the monomeric species in equilibrium was found in the case of serotonin 5-HT2C and also b1-AR or M1R in other FCS/photon-counting histogram-based studies (HerrickDavis et al., 2012, 2013, 2015). These discrepancies might be related to different receptor expression levels in the cellular models employed, where very low expression levels of the receptors compatible with single molecule resolution might favor the occurrence of monomeric species in dynamic equilibrium with the dimer population.

Recently, super-resolution dual-color photoactivation localization microscopy using photoactivatable dye-photoactivated localization microscopy allowed imaging of the spatial arrangement of individual GPCR molecules in dimers and oligomers at the plasma membrane with a resolution of ~8 nm. To this end, hemagglutinin- and FLAG-tagged luteinizing hormone receptor (LHR) mutants that were either ligand binding (LHRB_) or signaling (LHRS_) deficient (Rivero-Muller et al., 2010) were expressed in human embryonic kidney 293 cells and specifically labeled with antibodies conjugated with CAGE 552 and CAGE 500 photoswitchable dyes for dual-color visualization (Fig. 3A) (Jonas et al., 2015). The number and identity (either LHRB_ or LHRS_) of GPCRs within a 50-nm radius of each single GPCR are determined, and the irreversible activation and bleaching of the dyes allowed quantification of dimer and oligomer complexes (Fig. 3, B and C). Expression of wild-type LHR at levels in the physiologic range observed in the ovary and testis revealed a diverse organization in monomers, dimers (14.6%), and oligomers (26.8%, varying from trimers to oligomers consisting of more than nine protomers) (Fig. 3D). In this study, agonist stimulation did not elicit any change in either the percentage of associated molecules or relative

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Fig. 3. Dual-color photoactivatable dyes and localization microscopy (PD-PALM) to investigate spatial and structural organization of GPCR complexes. Epitope-tagged binding-deficient (LHRB~) and signaling-deficient (LHRS~) receptors were labeled with specific antibodies conjugated with CAGE 552 or 500 dyes, and cells were subsequently fixed (A). PD-PALM images were acquired using a total internal reflection fluorescence-equipped microscope by iterative cycles of photo-conversion of the CAGE dyes upon UV illumination and simultaneous dual-channel single-molecule imaging and irreversible photo-bleaching (B). Localization coordinates (x-y) were assigned to individual fluorescent particles detected in each imaged channel. Analysis of the number of associated protomers was conducted based on a recursive search for neighborhood particles within a radius of50 nm from each individual protomer (C). In combination with molecular modeling studies, this approach allowed us to resolve the composition and spatial arrangement of each associated group of molecules (dimers/ oligomers) visualized by PD-PALM (D). The structural models, depicted from the extracellular side, are taken from Jonas et al. (2015) under Creative Commons Attribution Unported License to Author Choice articles.

proportions of dimers and oligomers (Jonas et al., 2015). Yet, in spite of the detailed information on the size and spatial arrangement of GPCRs in oligomeric complexes by photo-activatable dye-photoactivated localization microscopy, this technique involves the acquisition of a time series of images that needs cell fixation, and therefore it is not well suited for the investigation of complex stability and real-time dynamics.

The stability and dynamics of GPCR dimers and oligomers has been monitored by real-time single-molecule imaging and tracking using fluorescent ligands (Hern et al., 2010; Kasai et al., 2011) or SNAP-tagged GPCR constructs (Calebiro et al., 2013). In the former approach, one has to consider the number of ligands bound per dimer. If agonist binding of an agonist to one protomer prevents ligand binding to the associated protomer, then the dynamics of the dimer rather than monomers are observed (Albizu et al., 2010). Single-molecule TIRFM studies reported the existence of an equilibrium between monomeric and oligomeric species for different GPCRs, with fast dynamics of less than 1 second (Hern et al., 2010; Kasai et al., 2011; Calebiro et al., 2013). In these approaches and differently to PALM, the x-y spatial resolution is limited by the optical diffraction limit (~220 nm) so that the possibility of interpreting two monomers within the resolution limit as an apparent dimer must be considered. In particular, two-color TIRFM revealed the transient nature of M1R and N-formylpeptide receptor homodimers at the cell surface (Hern et al., 2010; Kasai et al., 2011). Yet, the stability of the interactions might differ between GPCR subtypes. For example, b1-AR displays more transient interactions than

b2-AR, as revealed by both TIRFM and dual-color fluorescence recovery after photobleaching studies (Dorsch et al., 2009; Calebiro et al., 2013). Affinity-based corecruitment of differentially tagged mGlu receptors into forced microdomains on the cell surface confirmed the stable nature of class C GPCR dimerization, which is mediated by disulfide bridges between the NTED of the protomers (Gavalas et al., 2013). On the other hand, differentially tagged b2-AR or m-opioid receptor (MOR) fully segregated into distinct artificial microdomains, suggesting that homodimerization between these class A GPCRs is not sufficiently stable (Gavalas et al., 2013). Similar segregation was observed as a reduced BRET signal between wild-type and binding-deficient b2-AR upon agonist-induced internalization of only active (wild-type) b2-AR, whereas the inactive mutant remained at the cell surface (Lan et al., 2011). In contrast, coexpression of the wild-type b2-AR with receptors activated solely by a synthetic ligand (i.e. RASSL) b2-AR mutant revealed that agonist stimulation of either protomer induced internalization of the dimer (Sartania et al., 2007). Altogether, a discrepancy in the transient nature of at least some GPCR dimers is observed between single molecule labeling strategies and studies demonstrating cointernaliza-tion and cotrafficking of receptors (Milligan, 2010).

Several GPCRs have arranged as dimers or oligomers in recent high-resolution crystal structures, whereas others have crystallized as monomers. These crystal structures suggest that GPCRs can assemble in multiple ways, which might explain the possible formation of complexes larger than dimers. Antagonist-bound MOR crystallizes as oligomers, with a large

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contact interface involving transmembrane (TM) 5/TM6 and a smaller interface formed by TM1/TM2/helix 8 (Manglik et al., 2012), whereas a ligand-free brAR is arranged as oligomers via TM4/TM5 and TM1/TM2/helix8 interfaces (Huang et al., 2013). The antagonist-bound k-opioid receptor is arranged as a dimer via a TM1, TM2, and helix 8 interface (Wu et al., 2012). CXCR4 crystallizes as dimers via a TM5/TM6 interface when occupied by antagonistic chemokine vMIPII or small-molecule (IT1t) and cyclic peptide (CVX15) antagonists (Wu et al., 2010; Qin et al., 2015), whereas antagonist-bound histamine H1 receptor has a TM4 dimerization interface (Shimamura et al., 2011). However, it should be kept in mind that receptor modifications, bound ligand, and crystal packing conditions might affect (forced) dimerization interfaces and require systematic experimental validation using site-directed mutagenesis and/or interfering peptides. Indeed, TM1 forms the dimer interface in the mGlu1 7TMD crystal structure, whereas a TM4/TM5 interface is actually observed in full-length mGlu receptors in cells (Xue et al., 2015).

Hence, observed discrepancies in both the quaternary structure and stability of GPCR complexes between different studies emphasize the need for further refinement and systematic comparison of methods to monitor GPCR interactions in time.

Proximal Evidence for GPCR Heteromer Specific Signaling

GPCR heteromerization can significantly affect signaling and/or trafficking characteristics of individual GPCR subtypes (Jordan and Devi, 1999). However, unambiguous separation of heteromer-specific signaling from downstream crosstalk is difficult and requires experimental disruption of dimer formation (Prezeau et al., 2010; Vischer et al., 2011). Supportive evidence for heteromer-specific signaling comes from RET-based detection of signaling events very proximal to GPCRs in transfected cells. For example, agonist stimulation of a given (unmodified) GPCR subtype changes BRET between another coexpressed GPCR subtype and specific G proteins and/or b-arrestins (Fig. 1G) (See et al., 2011; Mustafa et al., 2012; Watts et al., 2013; Jonas et al., 2015). In addition, fusion of GPCRs to bimolecular luminescence or fluorescence complementation protein fragments and subsequent coex-pression with compatible G protein or b-arrestin RET fusion constructs (i.e., CODA-RET) allowed simultaneous detection of GPCR heteromerization and engagement of intracellular signaling partners upon agonist stimulation (Fig. 1H) (Urizar et al., 2011; Armando et al., 2014; Guitart et al., 2014; Bellot et al., 2015; Frederick et al., 2015). Heteromerization of Gs-coupled D1R and the Gi/o-coupled dopamine D2 receptor (D2R) induces intracellular Ca21 mobilization upon agonist activation, which could be impaired by the Gq/11 inhibitor YM254890 (Lee et al., 2004; Rashid et al., 2007). Moreover, agonist-induced recruitment of Gq-green fluorescent protein to DjR-Renilla luciferase requires coexpression of D2R and could be inhibited by a membrane-permeable peptide that disrupts D1R-D2R heteromerization (Hasbi et al., 2014). In contrast, however, a recent study reported the lack of both Gq recruitment to D1R-D2R heteromers in a CODA-RET assay and Gq activation, as measured by BRET, between Gaq-Renilla luciferase 8 and Gg2-Venus (Frederick et al., 2015). Although these engineered biosensor-expressing cells can

reveal the potential of GPCRs to modify each other's signaling upon heteromerization, it should be kept in mind that protein expression levels should be kept to a minimum to avoid nonspecific interactions as a consequence of membrane (microdomain) overcrowding.

Pharmacological Evidence for GPCR Dimers and Oligomers in Native Tissues

Ligand binding to one GPCR within a dimer or oligomer can rapidly change the conformation of an associated GPCR, as shown by the inhibition of norepinephrine-induced intramolecular FRET in an engineered a2A-AR upon stimulation of the MOR with morphine (Vilardaga et al., 2008). This transconformational change is slightly faster than the rate for G protein activation, indicating direct allosterism between both receptor subtypes within the heteromer (Xue et al., 2015). Negative-binding cooperativity was observed in the 1970s for native b-AR and the thyroid-stimulating hormone receptor (TSHR) in membrane preparations of frog erythrocytes and human thyroid samples, respectively, already suggesting the existence of GPCR dimers (Limbird et al., 1975; Limbird and Lefkowitz, 1976; Powell-Jones et al., 1979).

TSHR forms homomers in transfected cells (Urizar et al., 2005). Equilibrium and dissociation binding on these cells using both wild-type and engineered TSHR chimeras confirmed negative cooperativity between their orthosteric-binding sites, which is negatively correlated to the level of constitutive activity of the protomer (Zoenen et al., 2012). The chemokine receptors CCR2, CCR5, and CXCR4 form heteromers and display negative-binding cooperativity for their cognate chemokines in transfected cells (El-Asmar et al., 2005; Springael et al., 2006; Sohy et al., 2007, 2009). Importantly, similar negative-binding cooperativity between chemokines was observed on intact human CD41 T lymphocytes and purified monocytes endogenously expressing CCR2, CCR5, and CXCR4, consistent with the existence of chemokine receptor heteromers on native cells. Moreover, the CCR2/CCR5 antagonist TAK779 [dimethyl-[[4-[[3-(4-methyl-phenyl)-8,9-dihydro-7H-benzo[7]annulene-6-carbonyl] amino] phenyl]methyl]-(oxan-4-yl)azanium] reduced CXCR4-mediated immune cell recruitment toward CXCL12 in both ex vivo and in vivo models (Sohy et al., 2009). Similarly, the CXCR4 antagonist AMD3100 [1,1'-[1,4-phenylenebis-(methylene)]-bis-(1,4,8,11-tetraazacyclotetradecane) octahydrochloride] reduced CCR2- and CCR5-mediated ex vivo chemotaxis of CD41 T lymphocytes toward CCL2 and CCL4, respectively, by cross-inhibiting chemokine binding (Sohy et al., 2007, 2009). In contrast, the CXCR4 inverse agonist TC14012 [L-arginyl-L-arginyl-3-(2-naphthalenyl)-L-alanyl-L-cysteinyl-L-tyrosyl-N5-(aminocarbonyl)-L-ornithyl-L-lysyl-Ns-(aminocarbonyl)-D-ornithyl-L-prolyl-L-tyrosyl-L-arginyl-Ns-(aminocarbonyl)-L-ornithyl-L-cysteinyl-cyclic (4^13)-disulfide-L-argininamide] was unable to crossantagonize CCL2-induced b-arrestin2 recruitment to CCR2-CXCR4 heteromers in a CODA-RET assay (Armando et al., 2014). Cannabinoid CB1 and CB2 receptor antagonists inhibited signaling in response to agonists that activate the opposite receptor within cannabinoid CB1/CB2 heteromers in both heterologous cells and globus pallidus slices from a rat brain (Callén et al., 2012).

This antagonist crossinhibition offers great therapeutical potential but is not observed for other dimers that only display binding cooperativity between agonists (Albizu et al.,

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2006, 2010). For example, the CXCR4 antagonist AMD3100 did not crossinhibit binding of chemerin and CXCL10 to ChemR23 and CXCR3, respectively, which form heteromers with CXCR4 (de Poorter et al., 2013; Watts et al., 2013). However, the small-molecule CXCR3 agonist VUF10661 [(3S)-N-[(1S)-5-amino-1-[[(2,2-diphenylethyl)amino]carbonyl] pentyl]-2-(1,4-dioxo-4-phenylbutyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide], but not the CXCR3 antagonist TAK779, attenuated CXCL12 binding to membranes expressing CXCR3 and CXCR4. In addition, positive-binding cooper-ativity has been observed between the antagonist and agonist on the D2R and oxytocin receptor heteromers in nucleus accumbens membranes (Romero-Fernandez et al., 2013).

Hence, binding cooperativity between ligands acting at different GPCRs suggests the existence of GPCR heteromers in native tissues. However, concerns have been raised on the interpretation of binding cooperativity since agonist binding to GPCRs can be G protein dependent (Chabre et al., 2009; Birdsall, 2010). Consequently, competition for a shared pool of G proteins between GPCRs could result in the observation of apparent negative agonist binding cooperativity. This G protein depletion is, in particular, possible in equilibrium competition binding on membrane preparations as G protein coupling to activated GPCRs might be almost irreversible in the absence of free GTP (Chabre et al., 2009) but can be prevented by overexpression of G proteins (Nijmeijer et al., 2010).

Importantly, differential spatiotemporal expression of GPCR subtypes in tissues or even disease states may result in distinct heteromer-specific pharmacology. This may be particularly challenging for drug discovery programs that generally measure drug activity at a single target, and such initial screens may consequently not be adequate predictors for the in vivo effectiveness of drugs.

Detection of GPCR Dimers and Oligomers in Native Tissues

Although recombinant technologies using engineered GPCRs provide supportive evidence that many GPCRs might exist as dimers and/or oligomers or at least exist in close proximity, these approaches are not easily applicable to identify GPCR complexes in native tissues. However, in a recent study, double knock-in mice expressing the MOR and d-opioid receptor (DOR) fused in frame to mCherry and enhanced green fluorescent protein (eGFP), respectively, showed colocalization of these receptors in the midbrain and hindbrain (Erbs et al., 2015). Coimmunoprecipitation using anti-mCherry and anti-eGFP antibodies revealed that MOR-mCherry and DOR-eGFP belong to the same complex in the hippocampus from these mice, which confirms earlier detection of these MOR-DOR complexes in this brain region using a heteromer-specific antibody (Gupta et al., 2010). Interestingly, this antibody revealed increasing MOR-DOR complex levels in the cortex of animals chronically treated with morphine, whereas no coexpression of MOR-mCherry and DOR-eGFP was observed in the cortex of double knock-in mice in the absence or presence of the DOR agonist (Erbs et al., 2015). This discrepancy might be related to the difference in the receptor expression level and/or detection sensitivity.

Hitherto, only a limited number of the GPCR dimers and oligomers that were identified in heterologous cells have been validated in native tissues due to the absence of highly specific

antibodies for most GPCR subtypes or heteromers (Michel et al., 2009). Indeed, coimmunoprecipitation of GPCR complexes from solubilized native tissues has been used for nearly two decades (Fig. 1A) (Kaupmann et al., 1998; González-Maeso et al., 2008; Pei et al., 2010) but requires critical analysis to ensure that physically interacting GPCRs are detected rather than aggregation artifacts due to the hydrophobic nature of GPCRs (Milligan and Bouvier, 2005). More recently, in situ GPCR complexes have also been detected using immunohistochemical antibodies. Labeling of native GPCR subtypes with specific primary antibodies followed by matching secondary antibodies that are conjugated to unique oligonucleotide sequences allows enzymatic ligation of these DNA strands if secondary antibodies are in close proximity (<16 nm), which corresponds to a theoretical distance of <40 nm between the GPCR subtypes (i.e., epitopes). The formed circle DNA strand is subsequently o amplified and hybridized with fluorescent complementary n oligonucleotide probes for high-sensitivity fluorescence mi- lo croscopy analysis (Fig. 1B) (Weibrecht et al., 2010). This so- e called proximity ligation assay (PLA) confirmed the close f proximity of various GPCR pairs in the central nervous om system. In situ PLA between D2R and adenosine A2A receptor m (A2AR) in the striatum of mice, rats, and monkeys (Trifilieff olp et al., 2011; Bonaventura et al., 2014; Fernández-Dueñas a et al., 2015) confirmed D2R-A2AR heteromer coimmunopreci- . pitation from the rat striatum (Cabello et al., 2009) as well p as detection by coimmunoprecipitation, pull-down, FRET, etj BRET, and sequential BRET-FRET in heterologous cells our (Canals et al., 2003; Kamiya et al., 2003; Cabello et al., 2009). nal Interestingly, in the striatum of an experimental Parkinson- .o ism rat model, this PLA signal was significantly decreased g as a consequence of reduced codistribution and proximity A between D2R and A2AR (Fernández-Dueñas et al., 2015). In p addition, PLA was detected between native cannabinoid CB1 T and CB2 receptors in the rat brain pineal gland and nucleus Jo accumbens (Callén et al., 2012) and between D2R and oxytocin ^ receptors in the rat dorsal striatum and the neuropil als of nucleus accumbens (Romero-Fernandez et al., 2013). n Interestingly, PLA between the dopamine D4 receptor and S a1B- or b1-AR was observed in a rat pineal gland dissected e 1 hour after sunrise, but not when pineal glands were isolated m at sunset, which reflects the circadian variation in dopamine er D4 receptor expression levels (González et al., 2012). Dopa- 3 mine inhibits adrenergic receptor signaling within these 1 heteromers and consequently limits serotonin and melatonin 5 synthesis and release in the pineal gland. Dopamine D1 receptor (D1R) and D2R form heteromers in heterologous cells (So et al., 2005; Frederick et al., 2015). D1R/D2R heteromers have been proposed to play an important role in various neuropsychiatric disorders. However, PLA was absent in the shell of the nucleus accumbens, despite coexpression of native D1R and D2R in these cells, but readily observed upon D1R/D2R overexpression using viral gene transfer (Frederick et al., 2015). On the other hand, native D1R/D2R heteromers were detected in situ in rat striatal neurons using antibody-based confocal FRET analysis (Fig. 1C) (Hasbi et al., 2009, 2014; Perreault et al., 2010; Verma et al., 2010). The discrepancy between these antibody-based techniques requires further systematic comparison on similar regions. Importantly, specificity of D1R- and D2R-primary antibodies in these studies was confirmed in cells heterologously expressing dopamine receptor subtypes and in

D1R or D2R knockout mice in situ (Lee et al., 2004; Perreault et al., 2010).

In addition to antibodies, fluorescent ligands have been used to detect native GPCR heteromers (Fig. 1D). For example, ghrelin receptor heteromerization with D2R was shown in a mice hypothalamus by confocal FRET and time-resolved FRET between fluorescently labeled agonist ghrelin and D2R antibody-secondary antibody complexes, with the latter conjugated to Cy3 fluophore or cryptate, respectively (Kern et al., 2012). TrFRET between the D2R and A2AR antagonists that were conjugated to Lumi4-Terbium and a red acceptor (dy647), respectively, confirmed the PLA signal in the rat striatum (Fernández-Dueñas et al., 2015). Likewise, native oxytocin receptor dimers were detected in the mammary gland using fluorescent antagonists, but to a much lesser extent with fluorescent agonists due to negative-binding cooperativity between agonists (Albizu et al., 2010).

Functional Evidence for GPCR Dimers in Native Tissue

Coexpression of two nonfunctional GPCRs to form a functional receptor provides convincing evidence for GPCR heteromerization, as exemplified by the functional complementation of native class C GPCRs GABAB1 and GABAB2, which is strictly required for cell-surface expression of a functional GABAB receptor in vitro and in vivo (Prosser et al., 2001; Gassmann et al., 2004). The a1D-AR is retained in the endoplasmic reticulum when individually expressed in heterologous cells. Systematic coexpression with 28 other class A GPCRs revealed that heteromerization with a1B- or b2-AR is required for the cell surface of a1D-AR (Hague et al., 2004; Uberti et al., 2005). However, a1D-AR stimulates the contraction of carotid arteries in a1B-AR knockout mice (Deighan et al., 2005), suggesting that effective trafficking of a1D-AR to the cell surface is mediated by, for example, native b2-AR (Pernomian et al., 2013).

In contrast to the majority of class A GPCRs, the follicle-stimulating hormone receptor (FSHR), TSHR, and LHR are characterized by a large leucine-rich repeat-containing NTED, which is exclusively involved in the selective and high-affinity binding of their corresponding glycoprotein hormones (Osuga et al., 1997; Vischer et al., 2003a,b; Fan and Hendrickson, 2005). All three glycoprotein hormone receptors form homo-mers and heteromers in transfected cells (Urizar et al., 2005; Feng et al., 2013). However, only heteromerization between FSHR-LHR might be physiologically relevant as both receptors are shortly coexpressed in granulosa cells during follicle maturation (Thiruppathi et al., 2001). Taking advantage of the modular architecture of these glycoprotein hormone receptors, nonfunctional LHR mutants were created to impair either hormone binding to the NTED (i.e., LHRB_) or G protein activation by the 7TMD (i.e., LHRS") (Osuga et al., 1997; Ji et al., 2002; Lee et al., 2002). Coexpression of LHRB" with LHRS" in transfected cells rescued hormone-induced cAMP production, suggesting that both nonfunctional constructs are at least organized as dimers. Similar functional complementation was observed in transfected cells coexpressing FSHRB_ and FSHRS" (Ji et al., 2004) and TSHRB" and TSHRS" (Urizar et al., 2005). Moreover, coexpression of LHRB_ with LHRS_ in LHR knockout mice using a bacterial artificial chromosome to preserve correct spatiotemporal expression rescued both

gonadal development and lull spermatogenesis (Rivero-Muller et al., 2010). In contrast, expression of the individual loss-of-function mutants was ineffective. However, the same LHRB" mutant (i.e., C22A) was more recently reported to induce some cAMP signaling in transiently transfected human embryonic kidney 293 cells (Zhang et al., 2012), which contrasts with earlier in vitro and in vivo observations (Ji et al., 2002; Lee et al., 2002; Rivero-Muller et al., 2010).

Perturbation of GPCR Heteromers in Native Tissue

D1R/D2R heteromerization involves the C-tail of D1R and intracellular loop 3 (IL3) of the long D2R isoform. Membrane-permeable fusion constructs consisting of a TAT sequence and D1R G396-L413 (TAT-D1c) or D2R M257-E271 (TAT-D2LIL3) peptides disrupted D1R/D2R complexes and heteromer-induced signaling in striatal neurons (Pei et al., 2010; Hasbi et al., 2014). D1R/D2R coimmunoprecipitation was enhanced from brain tissue derived from patients suffering major depression as compared with healthy persons (Pei et al., 2010). Importantly, TAT-D2LIL3 has antidepressant-like effects when injected in the brain of rats, as revealed by increased mobility in forced swim tests and reduced escape failures in learned helplessness tests, leading the authors to suggest a prominent role for D1R/D2R heteromers in this neurologic disorder (Pei et al., 2010). The class A angiotensin receptor type 1a (AT1aR) and class B secretin receptor (SCTR) are coexpressed in osmoregulatory brain centers and form heteromeric complexes in heterologous cells. AT1aR/SCTR heteromerization was specifically inhibited by peptides derived from AT1aR-TM1 and SCTR-TM2, whereas both homomerization and heteromerization were inhibited by AT1aR-TM4 and SCTR-TM4 (Lee et al., 2014). Injection of AT1aR-TM1 in mice brains reduced hyperosmolality-induced drinking, confirming the physiologic role of this class A/class B GPCR heteromer in the regulation of water homeostasis.


Dimerization is essential for class C GPCR functioning, whereas class A and B GPCRs can activate G proteins and recruit b-arrestins as monomers in response to agonists. Nevertheless, most tested GPCRs form dimers and oligomers in heterologous cells, resulting in an apparent plethora of functional consequences. However, the stability as well as the stoichiometry of GPCR complexes appear to vary considerably, with only class C GPCRs forming stable complexes. Hitherto, only a small percentage of GPCR dimers and oligomers have been validated in native tissues, despite the guidelines proposed by the International Union of Basic and Clinical Pharmacology in 2007 (Pin et al., 2007). Ex vivo and in vivo detection of native GPCR dimers largely rely on the availability of specific antibodies and/or fluorescent ligands. Recent progress in the generation of llama-derived nano-bodies targeting GPCRs might facilitate future detection of native GPCR dimers and oligomers in situ. The higher affinities of engineered bivalent and biparatopic nanobodies in comparison with their monovalent counterparts might suggest binding to receptor dimers, although experimental proof for these observations is still required (Jahnichen et al., 2010; Maussang et al., 2013). Binding cooperativity might be

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a feasible pharmacological approach to detect GPCR homo-mers and heteromers in ex vivo samples; however, influence from signaling molecules should be considered if using agonists, whereas both the absence and presence of binding cooperativity has been observed for antagonists. In vivo functional complementation has so far been performed for one class A GPCR subtype and requires well characterized mutants that are present at physiologic levels with correct spatio-temporal expression patterns. Finally, confirmed disruption of heteromerization using interfering peptides followed by changes in the phenotypical response provided evidence for the presence as well as (patho)-physiologic function of some GPCR heteromers. Translation of in vitro observations for more GPCR heteromers to native tissues is required in the near future to confirm that GPCR dimers and oligomers exist beyond engineered model systems.


The authors thank the organizers and participants of the 2014 Lorentz Center Workshop on "Exploring the biology of GPCRs - from in vitro to in vivo" for inspiring discussions on GPCRs, which formed the basis of this minireview.

Authorship Contributions

Wrote or contributed to the writing of the manuscript: Vischer, Castro, Pin.


Albizu L, Balestre M-N, Breton C, Pin J-P, Manning M, Mouillac B, Barberis C, and Durroux T (2006) Probing the existence of G protein-coupled receptor dimers by positive and negative ligand-dependent cooperative binding. Mol Pharmacol 70: 1783-1791.

Albizu L, Cottet M, Kralikova M, Stoev S, Seyer R, Brabet I, Roux T, Bazin H, Bourrier E, and Lamarque L et al. (2010) Time-resolved FRET between GPCR ligands reveals oligomers in native tissues. Nat Chem Biol 6:587-594. Alexander SPH, Mathie A, and Peters JA (2011) Guide to receptors and channels

(GRAC), 5th edition. Br J Pharmacol 164 (Suppl 1):S1-S2. Arcemisbéhere L, Sen T, Boudier L, Balestre M-N, Gaibelet G, Detouillon E, Orcel H, Mendre C, Rahmeh R, and Granier S et al. (2010) Leukotriene BLT2 receptor monomers activate the G(i2) GTP-binding protein more efficiently than dimers. J Biol Chem 285:6337-6347. Armando S, Quoyer J, Lukashova V, Maiga A, Percherancier Y, Heveker N, Pin J-P, Prézeau L, and Bouvier M (2014) The chemokine CXC4 and CC2 receptors form homo- and heterooligomers that can engage their signaling G-protein effectors and ßarrestin. FASEB J 28:4509-4523. Bayburt TH, Leitz AJ, Xie G, Oprian DD, and Sligar SG (2007) Transducin activation by nanoscale lipid bilayers containing one and two rhodopsins. J Biol Chem 282: 14875-14881.

Bayburt TH, Vishnivetskiy SA, McLean MA, Morizumi T, Huang C-C, Tesmer JJG, Ernst OP, Sligar SG, and Gurevich VV(2011) Monomeric rhodopsin is sufficient for normal rhodopsin kinase (GRK1) phosphorylation and arrestin-1 binding. J Biol Chem 286:1420-1428. Bellot M, Galandrin S, Boularan C, Matthies HJ, Despas F, Denis C, Javitch J, Mazeres S, Sanni SJ, and Pons V et al. (2015) Dual agonist occupancy of AT1-R-a2C-AR heterodimers results in atypical Gs-PKA signaling. Nat Chem Biol 11: 271-279.

Birdsall NJM (2010) Class A GPCR heterodimers: evidence from binding studies.

Trends Pharmacol Sci 31:499-508. Bonaventura J, Rico AJ, Moreno E, Sierra S, Sánchez M, Luquin N, Farré D, Müller CE, Martínez-Pinilla E, and Cortés A et al. (2014) L-DOPA-treatment in primates disrupts the expression of A(2A) adenosine-CB(1) cannabinoid-D(2) dopamine receptor heteromers in the caudate nucleus. Neuropharmacology 79:90-100. Briddon SJ, Gandía J, Amaral OB, Ferré S, Lluís C, Franco R, Hill SJ, and Ciruela F (2008) Plasma membrane diffusion of G protein-coupled receptor oligomers. Bio-chim Biophys Acta 1783:2262-2268. Brock C, Boudier L, Maurel D, Blahos J, and Pin J-P (2005) Assembly-dependent surface targeting of the heterodimeric GABAB receptor is controlled by COPI but not 14-3-3. Mol Biol Cell 16:5572-5578. Cabello N, Gandía J, Bertarelli DCG, Watanabe M, Lluís C, Franco R, Ferré S, Luján R, and Ciruela F (2009) Metabotropic glutamate type 5, dopamine D2 and aden-osine A2a receptors form higher-order oligomers in living cells. J Neurochem 109: 1497-1507.

Calebiro D, Rieken F, Wagner J, Sungkaworn T, Zabel U, Borzi A, Cocucci E, Zürn A, and Lohse MJ (2013) Single-molecule analysis of fluorescently labeled G-protein-coupled receptors reveals complexes with distinct dynamics and organization. Proc Natl Acad Sci USA 110:743-748. Callén L, Moreno E, Barroso-Chinea P, Moreno-Delgado D, Cortés A, Mallol J, Casadó V, Lanciego JL, Franco R, and Lluis C et al. (2012) Cannabinoid receptors CB1 and CB2 form functional heteromers in brain. J Biol Chem 287:20851-20865.

Canals M, Marcellino D, Fanelli F, Ciruela F, de Benedetti P, Goldberg SR, Neve K, Fuxe K, Agnati LF, and Woods AS et al. (2003) Adenosine A2A-dopamine D2 receptor-receptor heteromerization: qualitative and quantitative assessment by fluorescence and bioluminescence energy transfer. J Biol Chem 278:46741-46749.

Carriba P, Navarro G, Ciruela F, Ferré S, Casadó V, Agnati L, Cortés A, Mallol J, Fuxe K, and Canela EI et al. (2008) Detection of heteromerization of more than two proteins by sequential BRET-FRET. Nat Methods 5:727-733.

Chabre M, Deterre P, and Antonny B (2009) The apparent cooperativity of some GPCRs does not necessarily imply dimerization. Trends Pharmacol Sci 30: 182-187.

Chen Y, Müller JD, So PT, and Gratton E (1999) The photon counting histogram in fluorescence fluctuation spectroscopy. Biophys J 77:553-567.

Ciruela F, Vilardaga J-P, and Fernández-Dueñas V (2010) Lighting up multiprotein complexes: lessons from GPCR oligomerization. Trends Biotechnol 28:407-415.

Cordeaux Y, Briddon SJ, Alexander SPH, Kellam B, and Hill SJ (2008) Agonist-occupied A3 adenosine receptors exist within heterogeneous complexes in membrane microdomains of individual living cells. FASEB J 22:850-860.

Corriden R, Kilpatrick LE, Kellam B, Briddon SJ, and Hill SJ (2014) Kinetic analysis of antagonist-occupied adenosine-A3 receptors within membrane microdomains of individual cells provides evidence ofreceptor dimerization and allosterism. FASEB J 28:4211-4222.

Cottet M, Faklaris O, Maurel D, Scholler P, Doumazane E, Trinquet E, Pin J-P, and Durroux T (2012) BRET and time-resolved FRET strategy to study GPCR olig-omerization: from cell lines toward native tissues. Front Endocrinol (Lausanne) 3:92.

Damak S, Rong M, Yasumatsu K, Kokrashvili Z, Varadarajan V, Zou S, Jiang P, Ninomiya Y, and Margolskee RF (2003) Detection of sweet and umami taste in the absence of taste receptor T1r3. Science 301:850-853.

de Poorter C, Baertsoen K, Lannoy V, Parmentier M, and Springael J-Y (2013) Consequences of ChemR23 heteromerization with the chemokine receptors CXCR4 and CCR7. PLoS One 8:e58075.

Deighan C, Methven L, Naghadeh MM, Wokoma A, Macmillan J, Daly CJ, Tanoue A, Tsujimoto G, and McGrath JC (2005) Insights into the functional roles of alpha(1)-adrenoceptor subtypes in mouse carotid arteries using knockout mice. Br J Pharmacol 144:558-565.

Dorsch S, Klotz K-N, Engelhardt S, Lohse MJ, and Bünemann M (2009) Analysis of receptor oligomerization by FRAP microscopy. Nat Methods 6:225-230.

Duthey B, Caudron S, Perroy J, Bettler B, Fagni L, Pin J-P, and Prézeau L (2002) A single subunit (GB2) is required for G-protein activation by the heterodimeric GABA(B) receptor. J Biol Chem 277:3236-3241.

El-Asmar L, Springael J-Y, Ballet S, Andrieu EU, Vassart G, and Parmentier M (2005) Evidence for negative binding cooperativity within CCR5-CCR2b hetero-dimers. Mol Pharmacol 67:460-469.

El Moustaine D, Granier S, Doumazane E, Scholler P, Rahmeh R, Bron P, Mouillac B, Banères J-L, Rondard P, and Pin J-P (2012) Distinct roles of metabotropic glutamate receptor dimerization in agonist activation and G-protein coupling. Proc Natl Acad Sci USA 109:16342-16347.

Erbs E, Faget L, Scherrer G, Matifas A, Filliol D, Vonesch J-L, Koch M, Kessler P, Hentsch D, and Birling M-C et al. (2015) A mu-delta opioid receptor brain atlas reveals neuronal co-occurrence in subcortical networks. Brain Struct Fund 220: 677-702.

Faklaris O, Cottet M, Falco A, Villier B, Laget M, Zwier JM, Trinquet E, Mouillac B, Pin J-P, and Durroux T (2015) Multicolor time-resolved Förster resonance energy transfer microscopy reveals the impact of GPCR oligomerization on internalization processes. FASEB J 29:2235-2246.

Fan QR and Hendrickson WA (2005) Structure of human follicle-stimulating hormone in complex with its receptor. Nature 433:269-277.

Feng X, Zhang M, Guan R, and Segaloff DL (2013) Heterodimerization between the lutropin and follitropin receptors is associated with an attenuation of hormone-dependent signaling. Endocrinology 154:3925-3930.

Fernández-Dueñas V, Taura JJ, Cottet M, Gómez-Soler M, López-Cano M, Ledent C, Watanabe M, Trinquet E, Pin J-P, and Luján R et al. (2015) Untangling dopamine-adenosine receptor-receptor assembly in experimental parkinsonism in rats. Dis Model Mech 8:57-63.

Fève M, Saliou J-M, Zeniou M, Lennon S, Carapito C, Dong J, Van Dorsselaer A, Junier M-P, Chneiweiss H, and Cianférani S et al. (2014) Comparative expression study of the endo-G protein coupled receptor (GPCR) repertoire in human glio-blastoma cancer stem-like cells, U87-MG cells and non malignant cells of neural origin unveils new potential therapeutic targets. PLoS One 9:e91519.

Frederick AL, Yano H, Trifilieff P, Vishwasrao HD, Biezonski D, Mészáros J, Urizar E, Sibley DR, Kellendonk C, and Sonntag KC et al. (2015) Evidence against dopamine D1/D2 receptor heteromers. Mol Psychiatry DOI: 10.1038/mp.2014.166.

Fung JJ, Deupi X, Pardo L, Yao XJ, Velez-Ruiz GA, Devree BT, Sunahara RK, and Kobilka BK (2009) Ligand-regulated oligomerization of beta(2)-adrenoceptors in a model lipid bilayer. EMBO J 28:3315-3328.

Galvez T, Duthey B, Kniazeff J, Blahos J, Rovelli G, Bettler B, Prézeau L, and Pin JP (2001) Allosteric interactions between GB1 and GB2 subunits are required for optimal GABA(B) receptor function. EMBO J 20:2152-2159.

Gassmann M, Shaban H, Vigot R, Sansig G, Haller C, Barbieri S, Humeau Y, Schuler V, Müller M, and Kinzel B et al. (2004) Redistribution of GABAB(1) protein and atypical GABAB responses in GABAB(2)-deficient mice. JNeurosci 24:6086-6097.

Gavalas A, Lan T-H, Liu Q, Corrêa IR, Jr, Javitch JA, and Lambert NA (2013) Segregation of family A G protein-coupled receptor protomers in the plasma membrane. Mol Pharmacol 84:346-352.

González S, Moreno-Delgado D, Moreno E, Pérez-Capote K, Franco R, Mallol J, Cortés A, Casadó V, Lluís C, and Ortiz J et al. (2012) Circadian-related heteromerization of adrenergic and dopamine D4 receptors modulates melatonin synthesis and release in the pineal gland. PLoS Biol 10:e1001347.

González-Maeso J, Ang RL, Yuen T, Chan P, Weisstaub NV, López-Giménez JF, Zhou M, Okawa Y, Callado LF, and Milligan G et al. (2008) Identification of a se-rotonin/glutamate receptor complex implicated in psychosis. Nature 452:93-97.

o. ft o.

CT" ft

Gorinski N, Kowalsman N, Renner U, Wirth A, Reinartz MT, Seifert R, Zeug A, Ponimaskin E, and Niv MY (2012) Computational and experimental analysis of the transmembrane domain 4/5 dimerization interface of the serotonin 5-HT(1A) receptor. Mol Pharmacol 82:448-463.

Guitart X, Navarro G, Moreno E, Yano H, Cai N-S, Sánchez-Soto M, Kumar-Barodia S, Naidu YT, Mallol J, and Cortés A et al. (2014) Functional selectivity of allosteric interactions within G protein-coupled receptor oligomers: the dopamine D1-D3 receptor heterotetramer. Mol Pharmacol 86:417-429.

Guo W, Urizar E, Kralikova M, Mobarec JC, Shi L, Filizola M, and Javitch JA (2008) Dopamine D2 receptors form higher order oligomers at physiological expression levels. EMBO J 27:2293-2304.

Gupta A, Mulder J, Gomes I, Rozenfeld R, Bushlin I, Ong E, Lim M, Maillet E, Junek M, and Cahill CM et al. (2010) Increased abundance of opioid receptor heteromers after chronic morphine administration. Sci Signal 3:ra54.

Hague C, Uberti MA, Chen Z, Hall RA, and Minneman KP (2004) Cell surface expression of alpha1D-adrenergic receptors is controlled by heterodimerization with alpha1B-adrenergic receptors. J Biol Chem 279:15541-15549.

Hanson SM, Gurevich EV, Vishnivetskiy SA, Ahmed MR, Song X, and Gurevich VV (2007) Each rhodopsin molecule binds its own arrestin. Proc Natl Acad Sci USA 104:3125-3128.

Hasbi A, Fan T, Alijaniaram M, Nguyen T, Perreault ML, O'Dowd BF, and George SR (2009) Calcium signaling cascade links dopamine D1-D2 receptor heteromer to striatal BDNF production and neuronal growth. Proc Natl Acad Sci USA 106: 21377-21382.

Hasbi A, Perreault ML, Shen MYF, Zhang L, To R, Fan T, Nguyen T, Ji X, O'Dowd BF, and George SR (2014) A peptide targeting an interaction interface disrupts the dopamine D1-D2 receptor heteromer to block signaling and function in vitro and in vivo: effective selective antagonism. FASEB J 28:4806-4820.

Hern JA, Baig AH, Mashanov GI, Birdsall B, Corrie JET, Lazareno S, Molloy JE, and Birdsall NJM (2010) Formation and dissociation of M1 muscarinic receptor dimers seen by total internal reflection fluorescence imaging of single molecules. Proc Natl Acad Sci USA 107:2693-2698.

Herrick-Davis K, Grinde E, Cowan A, and Mazurkiewicz JE (2013) Fluorescence correlation spectroscopy analysis of serotonin, adrenergic, muscarinic, and dopa-mine receptor dimerization: the oligomer number puzzle. Mol Pharmacol 84: 630-642.

Herrick-Davis K, Grinde E, Lindsley T, Cowan A, and Mazurkiewicz JE (2012) Oligomer size of the serotonin 5-hydroxytryptamine 2C (5-HT2C) receptor revealed by fluorescence correlation spectroscopy with photon counting histogram analysis: evidence for homodimers without monomers or tetramers. J Biol Chem 287: 23604-23614.

Herrick-Davis K, Grinde E, Lindsley T, Teitler M, Mancia F, Cowan A, and Mazurkiewicz JE (2015) Native serotonin 5-HT2C receptors are expressed as homodimers on the apical surface of choroid plexus epithelial cells. Mol Pharmacol 87:660-673.

Huang J, Chen S, Zhang JJ, and Huang X-Y (2013) Crystal structure of oligomeric ß1-adrenergic G protein-coupled receptors in ligand-free basal state. Nat Struct Mol Biol 20:419-425.

Insel PA, Snead A, Murray F, Zhang L, Yokouchi H, Katakia T, Kwon O, Dimucci D, and Wilderman A (2012) GPCR expression in tissues and cells: are the optimal receptors being used as drug targets? Br J Pharmacol 165:1613-1616.

Jähnichen S, Blanchetot C, Maussang D, Gonzalez-Pajuelo M, Chow KY, Bosch L, De Vrieze S, Serruys B, Ulrichts H, and Vandevelde W et al. (2010) CXCR4 nanobodies (VHH-based single variable domains) potently inhibit chemotaxis and HIV-1 replication and mobilize stem cells. Proc Natl Acad Sci USA 107:20565-20570.

James JR, Oliveira MI, Carmo AM, Iaboni A, and Davis SJ (2006) A rigorous experimental framework for detecting protein oligomerization using bioluminescence resonance energy transfer. Nat Methods 3:1001-1006.

Ji I, Lee C, Jeoung M, Koo Y, Sievert GA, and Ji TH (2004) Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one oftwo hormone signals. Mol Endocrinol 18:968-978.

Ji I, Lee C, Song Y, Conn PM, and Ji TH (2002) Cis- and trans-activation ofhormone receptors: the LH receptor. Mol Endocrinol 16:1299-1308.

Jonas KC, Fanelli F, Huhtaniemi IT, and Hanyaloglu AC (2015) Single molecule analysis of functionally asymmetric G protein-coupled receptor (GPCR) oligomers reveals diverse spatial and structural assemblies. J Biol Chem 290:3875-3892.

Jordan BA and Devi LA (1999) G-protein-coupled receptor heterodimerization modulates receptor function. Nature 399:697-700.

Kaczor AA, Makarska-Bialokoz M, Selent J, de la Fuente RA, Martí-Solano M, and Castro M (2014) Application of BRET for studying G protein-coupled receptors. Mini Rev Med Chem 14:411-425.

Kamiya T, Saitoh O, Yoshioka K, and Nakata H (2003) Oligomerization ofadenosine A2A and dopamine D2 receptors in living cells. Biochem Biophys Res Commun 306:544-549.

Kasai RS, Suzuki KGN, Prossnitz ER, Koyama-Honda I, Nakada C, Fujiwara TK, and Kusumi A (2011) Full characterization of GPCR monomer-dimer dynamic equilibrium by single molecule imaging. J Cell Biol 192:463-480.

Kaupmann K, Malitschek B, Schuler V, Heid J, Froestl W, Beck P, Mosbacher J, Bischoff S, Kulik A, and Shigemoto R et al. (1998) GABA(B)-receptor subtypes assemble into functional heteromeric complexes. Nature 396:683-687.

Kelly E, Bailey CP, and Henderson G (2008) Agonist-selective mechanisms ofGPCR desensitization. Br J Pharmacol 153 (Suppl 1):S379-S388.

Kern A, Albarran-Zeckler R, Walsh HE, and Smith RG (2012) Apo-ghrelin receptor forms heteromers with DRD2 in hypothalamic neurons and is essential for an-orexigenic effects of DRD2 agonism. Neuron 73:317-332.

Khelashvili G, Dorff K, Shan J, Camacho-Artacho M, Skrabanek L, Vroling B, Bouvier M, Devi LA, George SR, and Javitch JA et al. (2010) GPCR-OKB: the G protein coupled receptor oligomer knowledge base. Bioinformatics 26:1804-1805.

Kniazeff J, Galvez T, Labesse G, and Pin J-P (2002) No ligand binding in the GB2 subunit of the GABA(B) receptor is required for activation and allosteric interaction between the subunits. J Neurosci 22:7352-7361.

Kniazeff J, Prézeau L, Rondard P, Pin J-P, and Goudet C (2011) Dimers and beyond: the functional puzzles of class C GPCRs. Pharmacol Ther 130:9-25.

Kuravi S, Lan T-H, Barik A, and Lambert NA (2010) Third-party bioluminescence resonance energy transfer indicates constitutive association of membrane proteins: application to class a G-protein-coupled receptors and G-proteins. Biophys J 98: 2391-2399.

Kuszak AJ, Pitchiaya S, Anand JP, Mosberg HI, Walter NG, and Sunahara RK

(2009) Purification and functional reconstitution of monomeric mu-opioid receptors: allosteric modulation of agonist binding by Gi2. J Biol Chem 284: 26732-26741.

Lagerstrom MC and Schioth HB (2008) Structural diversity of G protein-coupled receptors and significance for drug discovery. Nat Rev Drug Discov 7:339-357.

Lambert NA and Javitch JA (2014) CrossTalk opposing view: weighing the evidence for class A GPCR dimers, the jury is still out. J Physiol 592:2443-2445.

Lan T-H, Kuravi S, and Lambert NA (2011) Internalization dissociates (b2-adrenergic receptors. PLoS One 6:e17361.

Lan T-H, Liu Q, Li C, Wu G, Steyaert J, and Lambert NA (2015) BRET evidence that b2 adrenergic receptors do not oligomerize in cells. Sci Rep 5:10166.

Lee C, Ji I, Ryu K, Song Y, Conn PM, and Ji TH (2002) Two defective heterozygous luteinizing hormone receptors can rescue hormone action. J Biol Chem 277: 15795-15800.

Lee LTO, Ng SYL, Chu JYS, Sekar R, Harikumar KG, Miller LJ, and Chow BKC (2014) Transmembrane peptides as unique tools to demonstrate the in vivo action of a cross-class GPCR heterocomplex. FASEB J 28:2632-2644.

Lee SP, So CH, Rashid AJ, Varghese G, Cheng R, Lança AJ, O'Dowd BF, and George SR (2004) Dopamine D1 and D2 receptor co-activation generates a novel phos-pholipase C-mediated calcium signal. J Biol Chem 279:35671-35678.

Limbird LE and Lefkowitz RJ (1976) Negative cooperativity among beta-adrenergic receptors in frog erythrocyte membranes. J Biol Chem 251:5007-5014.

Limbird LE, Meyts PD, and Lefkowitz RJ (1975) Beta-adrenergic receptors: evidence for negative cooperativity. Biochem Biophys Res Commun 64:1160-1168.

Lopez-Gimenez JF, Canals M, Pediani JD, and Milligan G (2007) The alpha1b-adrenoceptor exists as a higher-order oligomer: effective oligomerization is required for receptor maturation, surface delivery, and function. Mol Pharmacol 71: 1015-1029.

Manglik A, Kruse AC, Kobilka TS, Thian FS, Mathiesen JM, Sunahara RK, Pardo L, Weis WI, Kobilka BK, and Granier S (2012) Crystal structure of the m-opioid receptor bound to a morphinan antagonist. Nature 485:321-326.

Margeta-Mitrovic M, Jan YN, and Jan LY (2000) A trafficking checkpoint controls GABA(B) receptor heterodimerization. Neuron 27:97-106.

Maurel D, Comps-Agrar L, Brock C, Rives M-L, Bourrier E, Ayoub MA, Bazin H, Tinel N, Durroux T, and Prézeau L et al. (2008) Cell-surface protein-protein interaction analysis with time-resolved FRET and snap-tag technologies: application to GPCR oligomerization. Nat Methods 5:561-567.

Maussang D, Mujic-Delic A, Descamps FJ, Stortelers C, Vanlandschoot P, Stigter-van Walsum M, Vischer HF, van Roy M, Vosjan M, and Gonzalez-Pajuelo M et al. (2013) Llama-derived single variable domains (nanobodies) directed against chemokine receptor CXCR7 reduce head and neck cancer cell growth in vivo. J Biol Chem 288: 29562-29572.

Meseth U, Wohland T, Rigler R, and Vogel H (1999) Resolution of fluorescence correlation measurements. Biophys J 76:1619-1631.

Michel MC, Wieland T, and Tsujimoto G (2009) How reliable are G-protein-coupled receptor antibodies? Naunyn Schmiedebergs Arch Pharmacol 379:385-388.

Milligan G (2010) The role of dimerisation in the cellular trafficking of G-protein-coupled receptors. Curr Opin Pharmacol 10:23-29.

Milligan G and Bouvier M (2005) Methods to monitor the quaternary structure of G protein-coupled receptors. FEBS J 272:2914-2925.

Mustafa S, See HB, Seeber RM, Armstrong SP, White CW, Ventura S, Ayoub MA, and Pfleger KDG (2012) Identification and profiling of novel a1A-adrenoceptor-CXC chemokine receptor 2 heteromer. J Biol Chem 287:12952-12965.

Natarajan M, Lin K-M, Hsueh RC, Sternweis PC, and Ranganathan R (2006) A global analysis of cross-talk in a mammalian cellular signalling network. Nat Cell Biol 8:571-580.

Nijmeijer S, Leurs R, Smit MJ, and Vischer HF (2010) The Epstein-Barr virus-encoded G protein-coupled receptor BILF1 hetero-oligomerizes with human CXCR4, scavenges Gai proteins, and constitutively impairs CXCR4 functioning. J Biol Chem 285:29632-29641.

Osuga Y, Hayashi M, Kudo M, Conti M, Kobilka B, and Hsueh AJ (1997) Co-expression of defective luteinizing hormone receptor fragments partially reconstitutes ligand-induced signal generation. J Biol Chem 272:25006-25012.

Pagano A, Rovelli G, Mosbacher J, Lohmann T, Duthey B, Stauffer D, Ristig D, Schuler V, Meigel I, and Lampert C et al. (2001) C-terminal interaction is essential for surface trafficking but not for heteromeric assembly of GABA(b) receptors. J Neurosci 21:1189-1202.

Patowary S, Alvarez-Curto E, Xu T-R, Holz JD, Oliver JA, Milligan G, and Raicu V (2013) The muscarinic M3 acetylcholine receptor exists as two differently sized complexes at the plasma membrane. Biochem J 452:303-312.

Pei L, Li S, Wang M, Diwan M, Anisman H, Fletcher PJ, Nobrega JN, and Liu F

(2010) Uncoupling the dopamine D1-D2 receptor complex exerts antidepressant-like effects. Nat Med 16:1393-1395.

Pernomian L, Gomes MS, Restini CBA, Pupo AS, and de Oliveira AM (2013) Crosstalk with p2 -adrenoceptors enhances ligand affinity properties from endothelial alpha1 D -adrenoceptors that mediates carotid relaxation. J Pharm Pharmacol 65: 1337-1346.

Perreault ML, Hasbi A, Alijaniaram M, Fan T, Varghese G, Fletcher PJ, Seeman P, O'Dowd BF, and George SR (2010) The dopamine D1-D2 receptor heteromer localizes in dynorphin/enkephalin neurons: increased high affinity state following amphetamine and in schizophrenia. J Biol Chem 285:36625-36634.

Pin J-P, Neubig R, Bouvier M, Devi L, Filizola M, Javitch JA, Lohse MJ, Milligan G, Palczewski K, and Parmentier M et al. (2007) International Union of Basic and

o. ft o.

CT" ft

Clinical Pharmacology. LXVII. Recommendations for the recognition and nomenclature of G protein-coupled receptor heteromultimers. Pharmacol Rev 59:5-13.

Pioszak AA, Harikumar KG, Parker NR, Miller LJ, and Xu HE (2010) Dimeric arrangement of the parathyroid hormone receptor and a structural mechanism for ligand-induced dissociation. J Biol Chem 285:12435-12444.

Powell-Jones CH, Thomas CG, Jr, and Nayfeh SN (1979) Contribution of negative cooperativity to the thyrotropin-receptor interaction in normal human thyroid: kinetic evaluation. Proc Natl Acad Sci USA 76:705-709.

Prezeau L, Rives M-L, Comps-Agrar L, Maurel D, Kniazeff J, and Pin J-P (2010) Functional crosstalk between GPCRs: with or without oligomerization. Curr Opin Pharmacol 10:6-13.

Prosser HM, Gill CH, Hirst WD, Grau E, Robbins M, Calver A, Soffin EM, Farmer CE, Lanneau C, and Gray J et al. (2001) Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice. Mol Cell Neurosci 17:1059-1070.

Qin L, Kufareva I, Holden LG, Wang C, Zheng Y, Zhao C, Fenalti G, Wu H, Han GW, and Cherezov V et al. (2015) Structural biology. Crystal structure of the chemokine receptor CXCR4 in complex with a viral chemokine. Science 347:1117-1122.

Raicu V and Singh DR (2013) FRET spectrometry: a new tool for the determination of protein quaternary structure in living cells. Biophys J 105:1937-1945.

Rashid AJ, So CH, Kong MMC, Furtak T, El-Ghundi M, Cheng R, O'Dowd BF, and George SR (2007) D1-D2 dopamine receptor heterooligomers with unique pharmacology are coupled to rapid activation of Gq/11 in the striatum. Proc Natl Acad Sci USA 104:654-659.

Regard JB, Sato IT, and Coughlin SR (2008) Anatomical profiling of G proteincoupled receptor expression. Cell 135:561-571.

Rivero-Müller A, Chou Y-Y, Ji I, Lajic S, Hanyaloglu AC, Jonas K, Rahman N, Ji TH, and Huhtaniemi I (2010) Rescue ofdefective G protein-coupled receptor function in vivo by intermolecular cooperation. Proc Natl Acad Sci USA 107:2319-2324.

Rives M-L, Vol C, Fukazawa Y, Tinel N, Trinquet E, Ayoub MA, Shigemoto R, Pin J-P, and Prezeau L (2009) Crosstalk between GABAB and mGlu1a receptors reveals new insight into GPCR signal integration. EMBO J 28:2195-2208.

Romero-Fernandez W, Borroto-Escuela DO, Agnati LF, and Fuxe K (2013) Evidence for the existence of dopamine D2-oxytocin receptor heteromers in the ventral and dorsal striatum with facilitatory receptor-receptor interactions. Mol Psychiatry 18:849-850.

Sartania N, Appelbe S, Pediani JD, and Milligan G (2007) Agonist occupancy of a single monomeric element is sufficient to cause internalization of the dimeric beta2-adrenoceptor. Cell Signal 19:1928-1938.

Schmidlin F, Dery O, Bunnett NW, and Grady EF (2002) Heterologous regulation of trafficking and signaling of G protein-coupled receptors: beta-arrestin-dependent interactions between neurokinin receptors. Proc Natl Acad Sci USA 99:3324-3329.

See HB, Seeber RM, Kocan M, Eidne KA, and Pfleger KDG (2011) Application of G protein-coupled receptor-heteromer identification technology to monitor ß-arrestin recruitment to G protein-coupled receptor heteromers. Assay Drug Dev Technol 9:21-30.

Shimamura T, Shiroishi M, Weyand S, Tsujimoto H, Winter G, Katritch V, Abagyan R, Cherezov V, Liu W, and Han GW et al. (2011) Structure of the human histamine H1 receptor complex with doxepin. Nature 475:65-70.

So CH, Varghese G, Curley KJ, Kong MMC, Alijaniaram M, Ji X, Nguyen T, O'dowd BF, and George SR (2005) D1 and D2 dopamine receptors form heterooligomers and cointernalize after selective activation of either receptor. Mol Pharmacol 68: 568-578.

Sohy D, Parmentier M, and Springael J-Y (2007) Allosteric transinhibition by specific antagonists in CCR2/CXCR4 heterodimers. J Biol Chem 282:30062-30069.

Sohy D, Yano H, de Nadai P, Urizar E, Guillabert A, Javitch JA, Parmentier M, and Springael J-Y (2009) Hetero-oligomerization of CCR2, CCR5, and CXCR4 and the protean effects of "selective" antagonists. J Biol Chem 284:31270-31279.

Springael J-Y, Le Minh PN, Urizar E, Costagliola S, Vassart G, and Parmentier M (2006) Allosteric modulation of binding properties between units of chemokine receptor homo- and hetero-oligomers. Mol Pharmacol 69:1652-1661.

Szalai B, Hoffmann P, Prokop S, Erdelyi L, Varnai P, and Hunyady L (2014) Improved methodical approach for quantitative BRET analysis of G protein coupled receptor dimerization. PLoS One 9:e109503.

Teichmann A, Gibert A, Lampe A, Grzesik P, Rutz C, Furkert J, Schmoranzer J, Krause G, Wiesner B, and Schülein R (2014) The specific monomer/dimer equilibrium of the corticotropin-releasing factor receptor type 1 is established in the endoplasmic reticulum. J Biol Chem 289:24250-24262.

Thiruppathi P, Shatavi S, Dias JA, Radwanska E, and Luborsky JL (2001) Gonad-otrophin receptor expression on human granulosa cells of low and normal re-sponders to FSH. Mol Hum Reprod 7:697-704.

Trifilieff P, Rives M-L, Urizar E, Piskorowski RA, Vishwasrao HD, Castrillon J, Schmauss C, Slättman M, Gullberg M, and Javitch JA (2011) Detection of antigen interactions ex vivo by proximity ligation assay: endogenous dopamine D2-adenosine A2A receptor complexes in the striatum. Biotechniques 51:111-118.

Tsukamoto H, Sinha A, DeWitt M, and Farrens DL (2010) Monomeric rhodopsin is the minimal functional unit required for arrestin binding. J Mol Biol 399:501-511.

Uberti MA, Hague C, Oller H, Minneman KP, and Hall RA (2005) Heterodimeriza-tion with beta2-adrenergic receptors promotes surface expression and functional activity of alpha1D-adrenergic receptors. J Pharmacol Exp Ther 313:16-23.

Urizar E, Montanelli L, Loy T, Bonomi M, Swillens S, Gales C, Bouvier M, Smits G, Vassart G, and Costagliola S (2005) Glycoprotein hormone receptors: link between receptor homodimerization and negative cooperativity. EMBO J 24:1954-1964.

Urizar E, Yano H, Kolster R, Galés C, Lambert N, and Javitch JA (2011) CODA-RET reveals functional selectivity as a result of GPCR heteromerization. Nat Chem Biol 7:624-630.

Vassilatis DK, Hohmann JG, Zeng H, Li F, Ranchalis JE, Mortrud MT, Brown A, Rodriguez SS, Weller JR, and Wright AC et al. (2003) The G protein-coupled receptor repertoires of human and mouse. Proc Natl Acad Sci USA 100:4903-4908.

Vázquez-Prado J, Casas-González P, and García-Sáinz JA (2003) G protein-coupled receptor cross-talk: pivotal roles of protein phosphorylation and protein-protein interactions. Cell Signal 15:549-557.

Verma V, Hasbi A, O'Dowd BF, and George SR (2010) Dopamine D1-D2 receptor heteromer-mediated calcium release is desensitized by D1 receptor occupancy with or without signal activation: dual functional regulation by G protein-coupled receptor kinase 2. J Biol Chem 285:35092-35103.

Vilardaga J-P, Nikolaev VO, Lorenz K, Ferrandon S, Zhuang Z, and Lohse MJ (2008) Conformational cross-talk between alpha2A-adrenergic and mu-opioid receptors controls cell signaling. Nat Chem Biol 4:126-131.

Vischer HF, Granneman JCM, and Bogerd J (2003a) Opposite contribution of two ligand-selective determinants in the N-terminal hormone-binding exodomain of human gonadotropin receptors. Mol Endocrinol 17:1972-1981.

Vischer HF, Granneman JCM, Noordam MJ, Mosselman S, and Bogerd J (2003b) Ligand selectivity of gonadotropin receptors. Role of the beta-strands of extracellular leucine-rich repeats 3 and 6 of the human luteinizing hormone receptor. J Biol Chem 278:15505-15513.

Vischer HF, Watts AO, Nijmeijer S, and Leurs R (2011) G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand? Br J Pharmacol 163:246-260.

Watts AO, van Lipzig MM, Jaeger WC, Seeber RM, van Zwam M, Vinet J, van der Lee MM, Siderius M, Zaman GJ, and Boddeke HW et al. (2013) Identification and profiling of CXCR3-CXCR4 chemokine receptor heteromer complexes. Br J Pharmacol 168:1662-1674.

Weibrecht I, Leuchowius K-J, Clausson C-M, Conze T, Jarvius M, Howell WM, Kamali-Moghaddam M, and Söderberg O (2010) Proximity ligation assays: a recent addition to the proteomics toolbox. Expert Rev Proteomics 7:401-409.

White JF, Grodnitzky J, Louis JM, Trinh LB, Shiloach J, Gutierrez J, Northup JK, and Grisshammer R (2007) Dimerization of the class A G protein-coupled neuro-tensin receptor NTS1 alters G protein interaction. Proc Natl Acad Sci USA 104: 12199-12204.

Whorton MR, Bokoch MP, Rasmussen SGF, Huang B, Zare RN, Kobilka B, and Sunahara RK (2007) A monomeric G protein-coupled receptor isolated in a high-density lipoprotein particle efficiently activates its G protein. Proc Natl Acad Sci USA 104:7682-7687.

Whorton MR, Jastrzebska B, Park PS-H, Fotiadis D, Engel A, Palczewski K, and Sunahara RK (2008) Efficient coupling of transducin to monomeric rhodopsin in a phospholipid bilayer. J Biol Chem 283:4387-4394.

Wu B, Chien EYT, Mol CD, Fenalti G, Liu W, Katritch V, Abagyan R, Brooun A, Wells P, and Bi FC et al. (2010) Structures of the CXCR4 chemokine GPCR with small-molecule and cyclic peptide antagonists. Science 330:1066-1071.

Wu H, Wacker D, Mileni M, Katritch V, Han GW, Vardy E, Liu W, Thompson AA, Huang X-P, and Carroll FI et al. (2012) Structure of the human K-opioid receptor in complex with JDTic. Nature 485:327-332.

Xue L, Rovira X, Scholler P, Zhao H, Liu J, Pin J-P, and Rondard P (2015) Major ligand-induced rearrangement of the heptahelical domain interface in a GPCR dimer. Nat Chem Biol 11:134-140.

Zeug A, Woehler A, Neher E, and Ponimaskin EG (2012) Quantitative intensity-based FRET approaches-a comparative snapshot. Biophys J 103:1821-1827.

Zhang M, Guan R, and Segaloff DL (2012) Revisiting and questioning functional rescue between dimerized LH receptor mutants. Mol Endocrinol 26:655-668.

Zhao GQ, Zhang Y, Hoon MA, Chandrashekar J, Erlenbach I, Ryba NJP, and Zuker CS (2003) The receptors for mammalian sweet and umami taste. Cell 115:255-266.

Zoenen M, Urizar E, Swillens S, Vassart G, and Costagliola S (2012) Evidence for activity-regulated hormone-binding cooperativity across glycoprotein hormone receptor homomers. Nat Commun 3:1007.

Address correspondence to: Henry F. Vischer, Amsterdam Institute for

Molecules, Medicines and Systems, Division of Medicinal Chemistry, Faculty

of Sciences, VU University Amsterdam, De Boelelaan 1083, 1081 HV

Amsterdam, The Netherlands. E-mail:

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