Crosstalk in G protein-coupled receptors: changes at the transmembrane homodimer interface determine activation

The melanocortin 4 receptor: oligomer formation, interaction sites and functional significance

This study involves the structural and functional properties of the recombinant melanocortin 4 receptor (MC(4)R) expressed in the HEK-293 cell line. Using co-immuno-purification approaches, the receptor appears to be an oligomer, which can be crosslinked through disulphide bonds involving a native cysteine residue (84) to give a dimeric species. This position is located near the cytosolic region of transmembrane segment 2 and it is suggested that this is an interacting interface between MC(4)R monomers. Using co-expression of the native protein and a C84A mutant, it appears that the receptor also forms higher order oligomers via alternative interfaces. Interestingly, disulphide crosslink formation does not occur if the receptor is uncoupled from its G-protein, even though the oligomeric state is preserved. This suggests that the conformational changes, which occur on activation, affect the TM2 interface. The pharmacology of the agonist, NDP-MSH, indicates that the MC(4)R retains high affinity for the ligand in the absence of the G-protein but occupancy for the ligand is increased. The data can be interpreted to suggest that the G-protein exerts a negative allosteric effect on the receptor. Co-expression of one receptor lacking the ability to signal with another, which cannot bind the agonist, restored ligand-dependent activation of the G-protein to situations in which neither receptor on its own could activate the G-protein. Such transactivation suggests meaningful cross talk between the receptor subunits in the oligomeric complex. These studies demonstrate further unique features of the MC(4)R.

Glucagon-like peptide-1 receptor dimerization differentially regulates agonist signaling but does not affect small molecule allostery

The glucagon-like peptide-1 receptor (GLP-1R) is a family B G protein-coupled receptor and an important drug target for the treatment of type II diabetes, with activation of pancreatic GLP-1Rs eliciting glucose-dependent insulin secretion. Currently, approved therapeutics acting at this receptor are peptide based, and there is substantial interest in small molecule modulators for the GLP-1R. Using a variety of resonance energy transfer techniques, we demonstrate that the GLP-1R forms homodimers and that transmembrane helix 4 (TM4) provides the primary dimerization interface. We show that disruption of dimerization using a TM4 peptide, a minigene construct encoding TM4, or by mutation of TM4, eliminates G protein-dependent high-affinity binding to GLP-1(7-36)NH(2) but has selective effects on receptor signaling. There was <10-fold decrease in potency in cAMP accumulation or ERK1/2 phosphorylation assays but marked loss of intracellular calcium mobilization by peptide agonists. In contrast, there was near-complete abrogation of the cAMP response to an allosteric agonist, compound 2, but preservation of ERK phosphorylation. Collectively, this indicates that GLP-1R dimerization is important for control of signal bias. Furthermore, we reveal that two small molecule ligands are unaltered in their ability to allosterically modulate signaling from peptide ligands, demonstrating that these modulators act in cis within a single receptor protomer, and this has important implications for small molecule drug design.

Complex patterns of divergence among green-sensitive (RH2a) African cichlid opsins revealed by Clade model analyses

BACKGROUND

Gene duplications play an important role in the evolution of functional protein diversity. Some models of duplicate gene evolution predict complex forms of paralog divergence; orthologous proteins may diverge as well, further complicating patterns of divergence among and within gene families. Consequently, studying the link between protein sequence evolution and duplication requires the use of flexible substitution models that can accommodate multiple shifts in selection across a phylogeny. Here, we employed a variety of codon substitution models, primarily Clade models, to explore how selective constraint evolved following the duplication of a green-sensitive (RH2a) visual pigment protein (opsin) in African cichlids. Past studies have linked opsin divergence to ecological and sexual divergence within the African cichlid adaptive radiation. Furthermore, biochemical and regulatory differences between the RH2aα and RH2aβ paralogs have been documented. It thus seems likely that selection varies in complex ways throughout this gene family.

RESULTS

Clade model analysis of African cichlid RH2a opsins revealed a large increase in the nonsynonymous-to-synonymous substitution rate ratio (ω) following the duplication, as well as an even larger increase, one consistent with positive selection, for Lake Tanganyikan cichlid RH2aβ opsins. Analysis using the popular Branch-site models, by contrast, revealed no such alteration of constraint. Several amino acid sites known to influence spectral and non-spectral aspects of opsin biochemistry were found to be evolving divergently, suggesting that orthologous RH2a opsins may vary in terms of spectral sensitivity and response kinetics. Divergence appears to be occurring despite intronic gene conversion among the tandemly-arranged duplicates.

CONCLUSIONS

Our findings indicate that variation in selective constraint is associated with both gene duplication and divergence among orthologs in African cichlid RH2a opsins. At least some of this variation may reflect an adaptive response to differences in light environment. Interestingly, these patterns only became apparent through the use of Clade models, not through the use of the more widely employed Branch-site models; we suggest that this difference stems from the increased flexibility associated with Clade models. Our results thus bear both on studies of cichlid visual system evolution and on studies of gene family evolution in general.

Apo-ghrelin receptor forms heteromers with DRD2 in hypothalamic neurons and is essential for anorexigenic effects of DRD2 agonism

We identified subsets of neurons in the brain that coexpress the dopamine receptor subtype-2 (DRD2) and the ghrelin receptor (GHSR1a). Combination of FRET confocal microscopy and Tr-FRET established the presence of GHSR1a:DRD2 heteromers in hypothalamic neurons. To interrogate function, mice were treated with the selective DRD2 agonist cabergoline, which produced anorexia in wild-type and ghrelin⁻/⁻ mice; intriguingly, ghsr⁻/⁻ mice were refractory illustrating dependence on GHSR1a, but not ghrelin. Elucidation of mechanism showed that formation of GHSR1a:DRD2 heteromers allosterically modifies canonical DRD2 dopamine signaling resulting in Gβγ subunit-dependent mobilization of [Ca²⁺](i) independent of GHSR1a basal activity. By targeting the interaction between GHSR1a and DRD2 in wild-type mice with a highly selective GHSR1a antagonist (JMV2959) cabergoline-induced anorexia was blocked. Inhibiting dopamine signaling in subsets of neurons with a GHSR1a antagonist has profound therapeutic implications by providing enhanced selectivity because neurons expressing DRD2 alone would be unaffected.

G-protein-coupled receptor dynamics: dimerization and activation models compared with experiment

Our previously derived models of the active state of the β2-adrenergic receptor are compared with recently published X-ray crystallographic structures of activated GPCRs (G-protein-coupled receptors). These molecular dynamics-based models using experimental data derived from biophysical experiments on activation were used to restrain the receptor to an active state that gave high enrichment for agonists in virtual screening. The β2-adrenergic receptor active model and X-ray structures are in good agreement over both the transmembrane region and the orthosteric binding site, although in some regions the active model is more similar to the active rhodopsin X-ray structures. The general features of the microswitches were well reproduced, but with minor differences, partly because of the unexpected X-ray results for the rotamer toggle switch. In addition, most of the interacting residues between the receptor and the G-protein were identified. This analysis of the modelling has also given important additional insight into GPCR dimerization: re-analysis of results on photoaffinity analogues of rhodopsin provided additional evidence that TM4 (transmembrane helix 4) resides at the dimer interface and that ligands such as bivalent ligands may pass between the mobile helices. A comparison, and discussion, is also carried out between the use of implicit and explicit solvent for active-state modelling.

Structural determinants of the supramolecular organization of G protein-coupled receptors in bilayers

The G protein-coupled receptor (GPCR) rhodopsin self-assembles into supramolecular structures in native bilayers, but the structural determinants of receptor oligomerization are not known. We carried out multiple self-assembly coarse-grained molecular dynamics (CGMD) simulations of model membranes containing up to 64 molecules of the visual receptor rhodopsin over time scales reaching 100 μs. The simulations show strong preferential interaction modes between receptors. Two primary modes of receptor-receptor interactions are consistent with umbrella sampling/potential of mean force (PMF) calculations as a function of the distance between a pair of receptors. The preferential interfaces, involving helices (H) 1/8, 4/5 and 5, present no energy barrier to forming a very stable receptor dimer. Most notably, the PMFs show that the preferred rhodopsin dimer exists in a tail-to-tail conformation, with the interface comprising transmembrane H1/H2 and amphipathic H8 at the extracellular and cytoplasmic surfaces, respectively. This dimer orientation is in line with earlier electron microscopy, X-ray, and cross-linking experiments of rhodopsin and other GPCRs. Less stable interfaces, involving H4 and H6, have a free energy barrier for desolvation (delipidation) of the interfaces and appear to be designed to stabilize "lubricated" (i.e., lipid-coated) dimers. The overall CGMD strategy used here is general and can be applied to study the homo- and heterodimerization of GPCRs and other transmembrane proteins. Systematic extension of the work will deepen our understanding of the forces involved in the membrane organization of integral membrane proteins.

Computational and experimental analysis of the transmembrane domain 4/5 dimerization interface of the serotonin 5-HT(1A) receptor

Experimental evidence suggests that most members of class A G-protein coupled receptors (GPCRs) can form homomers and heteromers in addition to functioning as single monomers. In particular, serotonin (5-HT) receptors were shown to homodimerize and heterodimerize with other GPCRs, although the details and the physiological role of the oligomerization has not yet been fully elucidated. Here we used computational modeling of the 5-HT(1A) receptor monomer and dimer to predict residues important for dimerization. Based on these results, we carried out rationally designed site-directed mutagenesis. The ability of the mutants to dimerize was evaluated using different FRET-based approaches. The reduced levels of acceptor photobleaching-Förster resonance energy transfer (FRET) and the lower number of monomers participating in oligomers, as assessed by lux-FRET, confirmed the decreased ability of the mutants to dimerize and the involvement of the predicted contacts (Trp175(4.64), Tyr198(5.41), Arg151(4.40), and Arg152(4.41)) at the interface. This information was reintroduced as constraints for computational protein-protein docking to obtain a high-quality dimer model. Analysis of the refined model as well as molecular dynamics simulations of wild-type (WT) and mutant dimers revealed compensating interactions in dimers composed of WT and W175A mutant. This provides an explanation for the requirement of mutations of Trp175(4.64) in both homomers for disrupting dimerization. Our iterative computational-experimental study demonstrates that transmembrane domains TM4/TM5 can form an interaction interface in 5-HT(1A) receptor dimers and indicates that specific amino acid interactions maintain this interface. The mutants and the optimized model of the dimer structure may be used in functional studies of serotonin dimers.

Ligand-dependent conformations and dynamics of the serotonin 5-HT(2A) receptor determine its activation and membrane-driven oligomerization properties

From computational simulations of a serotonin 2A receptor (5-HT_2AR) model complexed with pharmacologically and structurally diverse ligands we identify different conformational states and dynamics adopted by the receptor bound to the full agonist 5-HT, the partial agonist LSD, and the inverse agonist Ketanserin. The results from the unbiased all-atom molecular dynamics (MD) simulations show that the three ligands affect differently the known GPCR activation elements including the toggle switch at W6.48, the changes in the ionic lock between E6.30 and R3.50 of the DRY motif in TM3, and the dynamics of the NPxxY motif in TM7. The computational results uncover a sequence of steps connecting these experimentally-identified elements of GPCR activation. The differences among the properties of the receptor molecule interacting with the ligands correlate with their distinct pharmacological properties. Combining these results with quantitative analysis of membrane deformation obtained with our new method (Mondal et al, Biophysical Journal 2011), we show that distinct conformational rearrangements produced by the three ligands also elicit different responses in the surrounding membrane. The differential reorganization of the receptor environment is reflected in (i)-the involvement of cholesterol in the activation of the 5-HT_2AR, and (ii)-different extents and patterns of membrane deformations. These findings are discussed in the context of their likely functional consequences and a predicted mechanism of ligand-specific GPCR oligomerization.

The 5-HT_2A receptor for the neurotransmitter serotonin (5-HT) belongs to family A (rhodopsin-like) G-protein coupled receptors (GPCRs), one of the most important classes of membrane proteins that are targeted by an extensive and diverse collection of external stimuli. Recently we learned that different ligands targeting the same GPCR can elicit different biological responses, but the mechanisms remain unknown. We address this fundamental question for the serotonin 5-HT_2A receptor, because it is known to respond to the binding of structurally diverse ligands by producing similar stimuli in the cell, and to the binding of quite similar ligands with dramatically different responses. Molecular dynamics simulations of molecular models of the serotonin 5-HT_2A receptor in complex with pharmacologically distinct ligands show the dynamic rearrangements of the receptor molecule to be different for these ligands, and the nature and extents of the rearrangements reflect the known pharmacological properties of the ligands as full, partial or inverse activators of the receptor. The different rearrangements of the receptor molecule are shown to produce different rearrangements of the surrounding membrane, a remodeling of the environment that can have differential ligand-determined effects on receptor function and association in the cell's membrane.

Palmitoylation and membrane cholesterol stabilize μ-opioid receptor homodimerization and G protein coupling

BACKGROUND

A cholesterol-palmitoyl interaction has been reported to occur in the dimeric interface of the β₂-adrenergic receptor crystal structure. We sought to investigate whether a similar phenomenon could be observed with μ-opioid receptor (OPRM1), and if so, to assess the role of cholesterol in this class of G protein-coupled receptor (GPCR) signaling.

RESULTS

C3.55(170) was determined to be the palmitoylation site of OPRM1. Mutation of this Cys to Ala did not affect the binding of agonists, but attenuated receptor signaling and decreased cholesterol associated with the receptor signaling complex. In addition, both attenuation of receptor palmitoylation (by mutation of C3.55[170] to Ala) and inhibition of cholesterol synthesis (by treating the cells with simvastatin, a HMG-CoA reductase inhibitor) impaired receptor signaling, possibly by decreasing receptor homodimerization and Gαi2 coupling; this was demonstrated by co-immunoprecipitation, immunofluorescence colocalization and fluorescence resonance energy transfer (FRET) analyses. A computational model of the OPRM1 homodimer structure indicated that a specific cholesterol-palmitoyl interaction can facilitate OPRM1 homodimerization at the TMH4-TMH4 interface.

CONCLUSIONS

We demonstrate that C3.55(170) is the palmitoylation site of OPRM1 and identify a cholesterol-palmitoyl interaction in the OPRM1 complex. Our findings suggest that this interaction contributes to OPRM1 signaling by facilitating receptor homodimerization and G protein coupling. This conclusion is supported by computational modeling of the OPRM1 homodimer.

Mapping human protease-activated receptor 4 (PAR4) homodimer interface to transmembrane helix 4

Thrombin activates platelets by binding and cleaving protease-activated receptors 1 and 4 (PAR1 and PAR4). Because of the importance of PAR4 activation on platelets in humans and mice and emerging roles for PAR4 in other tissues, experiments were done to characterize the interaction between PAR4 homodimers. Bimolecular fluorescence complementation and bioluminescence resonance energy transfer (BRET) were used to examine the PAR4 homodimer interface. In bimolecular fluorescence complementation experiments, PAR4 formed homodimers that were disrupted by unlabeled PAR4 in a concentration-dependent manner, but not by rhodopsin. In BRET experiments, the PAR4 homodimers showed a specific interaction as indicated by a hyperbolic BRET signal in response to increasing PAR4-GFP expression. PAR4 did not interact with rhodopsin in BRET assays. The threshold maximum BRET signal was disrupted in a concentration-dependent manner by unlabeled PAR4. In contrast, rhodopsin was unable to disrupt the BRET signal, indicating that the disruption of the PAR4 homodimer is not due to nonspecific interactions. A panel of rho-PAR4 chimeras and PAR4 point mutants has mapped the dimer interface to hydrophobic residues in transmembrane helix 4. Finally, mutations that disrupted dimer formation had reduced calcium mobilization in response to the PAR4 agonist peptide. These results link the loss of dimer formation to a loss of PAR4 signaling.

Identification of residues involved in homodimer formation located within a β-strand region of the N-terminus of a Yeast G protein-coupled receptor

G protein-coupled receptors (GPCRs) are members of a superfamily of cell surface signaling proteins that play critical roles in many physiological functions; malfunction of these proteins is associated with multiple diseases. Understanding the structure-function relationships of these proteins is important, therefore, for GPCR-based drug discovery. The yeast Saccharomyces cerevisiae tridecapeptide pheromone α-factor receptor Ste2p has been studied as a model to explore the structure-function relationships of this important class of cell surface receptors. Although transmembrane domains of GPCRs have been examined extensively, the extracellular N-terminus and loop regions have received less attention. We have used substituted cysteine accessibility method to probe the solvent accessibility of single cysteine residues engineered to replace residues Gly20 through Gly33 of the N-terminus of Ste2p. Unexpectedly, our analyses revealed that the residues Ser22, Ile24, Tyr26, and Ser28 in the N-terminus were solvent inaccessible, whereas all other residues of the targeted region were solvent accessible. The periodicity of accessibility from residues Ser22-Ser28 is indicative of an underlying structure consistent with a β-strand that was predicted computationally in this region. Moreover, a number of these Cys-substituted Ste2p receptors (G20C, S22C, I24C, Y26C, S28C and Y30C) were found to form increased dimers compared to the Cys-less Ste2p. Based on these data, we propose that part of the N-terminus of Ste2p is structured and that this structure forms a dimer interface for Ste2p molecules. Dimerization mediated by the N-terminus was affected by ligand binding, indicating an unanticipated conformational change in the N-terminus upon receptor activation.

Mutually opposite signal modulation by hypothalamic heterodimerization of ghrelin and melanocortin-3 receptors

Interaction and cross-talk of G-protein-coupled receptors (GPCRs) are of considerable interest because an increasing number of examples implicate a profound functional and physiological relevance of homo- or hetero-oligomeric GPCRs. The ghrelin (growth hormone secretagogue receptor (GHSR)) and melanocortin-3 (MC3R) receptors are both known to have orexigenic effects on the hypothalamic control of body weight. Because in vitro studies indicate heterodimerization of GHSR and MC3R, we investigated their functional interplay. Combined in situ hybridization and immunohistochemistry indicated that the vast majority of GHSR-expressing neurons in the arcuate nucleus also express MC3R. In vitro coexpression of MC3R and GHSR promoted enhanced melanocortin-induced intracellular cAMP accumulation compared with activation of MC3R in the absence of GHSR. In contrast, agonist-independent basal signaling activity and ghrelin-induced signaling of GHSR were impaired, most likely due to interaction with MC3R. By taking advantage of naturally occurring GHSR mutations and an inverse agonist for GHSR, we demonstrate that the observed enhanced MC3R signaling capability depends directly on the basal activity of GHSR. In conclusion, we demonstrate a paradigm-shifting example of GPCR heterodimerization allowing for mutually opposite functional influence of two hypothalamic receptors controlling body weight. We found that the agonist-independent active conformation of one GPCR can determine the signaling modalities of another receptor in a heterodimer. Our discovery also implies that mutations within one of two interacting receptors might affect both receptors and different pathways simultaneously. These findings uncover mechanisms of important relevance for pharmacological targeting of GPCR in general and hypothalamic body weight regulation in particular.

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.

Quantitative modeling of membrane deformations by multihelical membrane proteins: application to G-protein coupled receptors

The interpretation of experimental observations of the dependence of membrane protein function on the properties of the lipid membrane environment calls for a consideration of the energy cost of protein-bilayer interactions, including the protein-bilayer hydrophobic mismatch. We present a novel (to our knowledge) multiscale computational approach for quantifying the hydrophobic mismatch-driven remodeling of membrane bilayers by multihelical membrane proteins. The method accounts for both the membrane remodeling energy and the energy contribution from any partial (incomplete) alleviation of the hydrophobic mismatch by membrane remodeling. Overcoming previous limitations, it allows for radially asymmetric bilayer deformations produced by multihelical proteins, and takes into account the irregular membrane-protein boundaries. The approach is illustrated by application to two G-protein coupled receptors: rhodopsin in bilayers of different thickness, and the serotonin 5-HT(2A) receptor bound to pharmacologically different ligands. Analysis of the results identifies the residual exposure that is not alleviated by bilayer adaptation, and its quantification at specific transmembrane segments is shown to predict favorable contact interfaces in oligomeric arrays. In addition, our results suggest how distinct ligand-induced conformations of G-protein coupled receptors may elicit different functional responses through differential effects on the membrane environment.

Development of a bivalent dopamine D₂ receptor agonist

Bivalent D₂ agonists may function as useful molecular probes for the discovery of novel neurological therapeutics. On the basis of our recently developed bivalent dopamine D₂ receptor antagonists of type 1, the bivalent agonist 2 was synthesized when a spacer built from 22 atoms was employed. Compared to the monovalent control compound 6 containing a capped spacer, the bis-aminoindane derivative 2 revealed substantial steepening of the competition curve, indicating a bivalent binding mode. Dimer-specific Hill slopes were not a result of varying functional properties because both the dopaminergic 2 and the monovalent control agent 6 proved to be D₂ agonists substantially inhibiting cAMP accumulation and inducing D₂ receptor internalization. Investigation of the heterobivalent ligands 8 and 9, containing an agonist and a phenylpiperazine-based antagonist pharmacophore, revealed moderate steepening of the displacement curves and antagonist to very weak partial agonist properties.

Structural aspects of M₃ muscarinic acetylcholine receptor dimer formation and activation

To explore the structural mechanisms underlying the assembly and activation of family A GPCR dimers, we used the rat M(3) muscarinic acetylcholine receptor (M3R) as a model system. Studies with Cys-substituted mutant M3Rs expressed in COS-7 cells led to the identification of several mutant M3Rs that exclusively existed as cross-linked dimers under oxidizing conditions. The cross-linked residues were located at the bottom of transmembrane domain 5 (TM5) and within the N-terminal portion of the third intracellular loop (i3 loop). Studies with urea-stripped membranes demonstrated that M3R disulfide cross-linking did not require the presence of heterotrimeric G proteins. Molecular modeling studies indicated that the cross-linking data were in excellent agreement with the existence of a low-energy M3R dimer characterized by a TM5-TM5 interface. [(35)S]GTPγS binding/Gα(q/11) immunoprecipitation assays revealed that an M3R dimer that was cross-linked within the N-terminal portion of the i3 loop (264C) was functionally severely impaired (∼50% reduction in receptor-G-protein coupling, as compared to control M3R). These data support the novel concept that agonist-induced activation of M3R dimers requires a conformational change of the N-terminal segment of the i3 loop. Given the high degree of structural homology among family A GPCRs, these findings should be of broad significance.

Bivalent molecular probes for dopamine D2-like receptors

Merging two arylamidoalkyl substituted phenylpiperazines as prototypical recognition elements for dopamine D(2)-like receptors by oligoethylene glycol linkers led to a series of bivalent ligands. These dimers were investigated in comparison to their monomeric analogues for their dopamine D(2long), D(2short), D(3) and D(4) receptor binding. Radioligand binding experiments revealed strong bivalent effects for some para-substituted benzamide derivatives. For the D(3) subtype, the target compounds 32, 34 and 36 showed an up to 70-fold increase of affinity and a substantial enhancement of subtype selectivity when compared to the monovalent analogue 24. Analysis of the binding curves displayed Hill slopes very close to one indicating that the bivalent ligands displace 1equiv of radioligand. Obviously, the two pharmacophores occupy an orthosteric and an allosteric binding site rather than adopting a receptor-bridging binding mode.

Light on the structure of thromboxane A₂receptor heterodimers

The structure-based design of a mutant form of the thromboxane A(2) prostanoid receptor (TP) was instrumental in characterizing the structural determinants of the hetero-dimerization process of this G protein coupled receptor (GPCR). The results suggest that the hetero-dimeric complexes between the TPα and β isoforms are characterized by contacts between hydrophobic residues in helix 1 from both monomers. Functional characterization confirms that TPα-TPβ hetero-dimerization serves to regulate TPα function through agonist-induced internalization, with important implications in cardiovascular homeostasis. The integrated approach employed in this study can be adopted to gain structural and functional insights into the dimerization/oligomerization process of all GPCRs for which the structural model of the monomer can be achieved at reasonable atomic resolution.

Structural basis of M3 muscarinic receptor dimer/oligomer formation

Class A G protein-coupled receptors (GPCRs) are known to form dimers and/or oligomeric arrays in vitro and in vivo. These complexes are thought to play important roles in modulating class A GPCR function. Many studies suggest that residues located on the "outer" (lipid-facing) surface of the transmembrane (TM) receptor core are critically involved in the formation of class A receptor dimers (oligomers). However, no clear consensus has emerged regarding the identity of the TM helices or TM subsegments involved in this process. To shed light on this issue, we have used the M(3) muscarinic acetylcholine receptor (M3R), a prototypic class A GPCR, as a model system. Using a comprehensive and unbiased approach, we subjected all outward-facing residues (70 amino acids total) of the TM helical bundle (TM1-7) of the M3R to systematic alanine substitution mutagenesis. We then characterized the resulting mutant receptors in radioligand binding and functional studies and determined their ability to form dimers (oligomers) in bioluminescence resonance energy transfer saturation assays. We found that M3R/M3R interactions are not dependent on the presence of one specific structural motif but involve the outer surfaces of multiple TM subsegments (TM1-5 and -7) located within the central and endofacial portions of the TM receptor core. Moreover, we demonstrated that the outward-facing surfaces of most TM helices play critical roles in proper receptor folding and/or function. Guided by the bioluminescence resonance energy transfer data, molecular modeling studies suggested the existence of multiple dimeric/oligomeric M3R arrangements, which may exist in a dynamic equilibrium. Given the high structural homology found among all class A GPCRs, our results should be of considerable general relevance.

Heteromerization of human cytomegalovirus encoded chemokine receptors

Human cytomegalovirus (HCMV) is a widespread pathogen that infects up to 80% of the human population and causes severe complications in immunocompromised patients. HCMV expresses four seven transmembrane (7TM) spanning/G protein-coupled receptors (GPCRs) – US28, US27, UL33 and UL78 – that show close homology to human chemokine receptors. While US28 was shown to bind several chemokines and to constitutively activate multiple signaling cascades, the function(s) of US27, UL33 and UL78 in the viral life cycle have not yet been identified. Here we investigated the possible interaction/heteromerization of US27, UL33 and UL78 with US28 and the functional consequences thereof. We provide evidence that these receptors not only co-localize, but also heteromerize with US28 in vitro. While the constitutive activation of the US28-mediated Gαq/phospholipase C pathway was not affected by receptor heteromerization, UL33 and UL78 were able to silence US28-mediated activation of the transcription factor NF-κB. Summarized, we provide evidence that these orphan viral receptors have an important regulatory capacity on the function of US28 and as a consequence, may ultimately impact on the viral life cycle of HCMV.

Bivalent dopamine D2 receptor ligands: synthesis and binding properties

Dopamine D(2) receptor homodimers might be of particular importance in the pathophysiology of schizophrenia and, thus, serve as promising target proteins for the discovery of atypical antipsychotics. A highly attractive approach to investigate and control GPCR dimerization may be provided by the exploration and characterization of bivalent ligands, which can act as molecular probes simultaneously binding two adjacent binding sites of a dimer. The synthesis of bivalent dopamine D(2) receptor ligands of type 1 is presented, incorporating the privileged structure of 1,4-disubstituted aromatic piperidines/piperazines (1,4-DAPs) and triazolyl-linked spacer elements. Radioligand binding studies provided diagnostic insights when Hill slopes close to two for bivalent ligands with particular spacer lengths and a comparative analysis with respective monovalent control ligands and unsymmetrically substituted analogues indicated a bivalent binding mode with a simultaneous occupancy of two neighboring binding sites.

GPCR oligomers in pharmacology and signaling

G protein-coupled receptors (GPCRs) represent one of the largest families of cell surface receptors, and are the target of more than half of the current therapeutic drugs on the market. When activated by an agonist, the GPCR undergoes conformational changes that facilitate its interaction with heterotrimeric G proteins, which then relay signals to downstream intracellular effectors. Although GPCRs were thought to function as monomers, many studies support the hypothesis that G protein coupling involves the formation of GPCR homo- and/or hetero-complexes. These complex systems have been suggested to exhibit specific signaling cascades, pharmacological, internalization, and recycling properties. In this review, we summarize recent advances in our understanding of the structure, function and dynamics of GPCR complexes, as well as the findings obtained in animal models.

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.

Making structural sense of dimerization interfaces of delta opioid receptor homodimers

Opioid receptors, like other members of the G protein-coupled receptor (GPCR) family, have been shown to associate to form dimers and/or oligomers at the plasma membrane. Whether this association is stable or transient is not known. Recent compelling evidence suggests that at least some GPCRs rapidly associate and dissociate. We have recently calculated binding affinities from free energy estimates to predict transient association between mouse delta opioid receptor (DOR) protomers at a symmetric interface involving the fourth transmembrane (TM4) helix (herein termed “4” dimer). Here we present disulfide cross-linking experiments with DOR constructs with cysteines substituted at the extracellular ends of TM4 or TM5 that confirm the formation of DOR complexes involving these helices. Our results are consistent with the involvement of TM4 and/or TM5 at the DOR homodimer interface, but possibly with differing association propensities. Coarse-grained (CG) well-tempered metadynamics simulations of two different dimeric arrangements of DOR involving TM4 alone or with TM5 (herein termed “4/5” dimer) in an explicit lipid−water environment confirmed the presence of two structurally and energetically similar configurations of the 4 dimer, as previously assessed by umbrella sampling calculations, and revealed a single energetic minimum of the 4/5 dimer. Additional CG umbrella sampling simulations of the 4/5 dimer indicated that the strength of association between DOR protomers varies depending on the protein region at the interface, with the 4 dimer being more stable than the 4/5 dimer.

In silico analysis of the binding of agonists and blockers to the β2-adrenergic receptor

Activation of G protein-coupled receptors (GPCRs) is a complex phenomenon. Here, we applied Induced Fit Docking (IFD) in tandem with linear discriminant analysis (LDA) to generate hypotheses on the conformational changes induced to the β(2)-adrenergic receptor by agonist binding, preliminary to the sequence of events that characterize activation of the receptor. This analysis, corroborated by a follow-up molecular dynamics study, suggested that agonists induce subtle movements to the fifth transmembrane domain (TM5) of the receptor. Furthermore, molecular dynamics also highlighted a correlation between movements of TM5 and the second extracellular loop (EL2), suggesting that freedom of motion of EL2 is required for the agonist-induced TM5 displacement. Importantly, we also showed that the IFD/LDA procedure can be used as a computational means to distinguish agonists from blockers on the basis of the differential conformational changes induced to the receptor. In particular, the two most predictive models obtained are based on the RMSD induced to Ser207 and on the counterclockwise rotation induced to TM5.

Fluorescent protein complementation assays: new tools to study G protein-coupled receptor oligomerization and GPCR-mediated signaling

G protein-coupled receptor (GPCR) signaling is mediated by protein-protein interactions at multiple levels. The characterization of the corresponding protein complexes is therefore paramount to the basic understanding of GPCR-mediated signal transduction. The number of documented interactions involving GPCRs is rapidly growing, and appreciating the functional significance of these complexes is clearly the next challenge. New experimental approaches including protein complementation assays (PCAs) have recently been used to examine the composition, plasma membrane targeting, and desensitization of protein complexes involved in GPCR signaling. These methods also hold promise for better understanding of drug-induced effects on GPCR interactions. This review focuses on the application of fluorescent PCAs for the study of GPCR signaling. Potential applications of PCAs in high-content screens are also presented. Non-fluorescent PCA techniques as well as combined assays for the detection of ternary and quaternary protein complexes are briefly discussed.

G protein-coupled receptor oligomerization for what?

Although the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last decade, under some premises the existence of a supramolecular organization of these receptors begins now to be widely accepted by the scientific community. Indeed, GPCR oligomers may enhance the diversity and performance by which extracellular signals are transferred to the G proteins in the process of receptor transduction, although the mechanism that underlie this phenomenon remains still unexplained. Recently, a trans-conformational switching model has been proposed as a mechanism allowing direct inhibition of receptor activation. Thus, heterotropic receptor-receptor allosteric regulations are behind the GPCR oligomeric function. Accordingly, we revise here how GPCR oligomerization impinge in several important receptor functions like biosynthesis, plasma membrane diffusion or velocity, pharmacology and signaling. Overall, the rationale of receptor oligomerization might lie in the cellular need of sensing complex extracellular signals and to translate into a simple computational mode.

Emerging roles for the FSH receptor adapter protein APPL1 and overlap of a putative 14-3-3τ interaction domain with a canonical G-protein interaction site

The interaction of cytoplasmic proteins with intracellular domains of membrane receptors can occur at several opportunities, including: during biosynthesis, while in membrane residency and during internalization and recycling following ligand binding. Since the initial discovery that it interacts with the FSH receptor (FSHR) together with additional members of a potential signaling complex, APPL1 has been shown to interact with a variety of membrane receptors. Recent subcellular localizations of APPL1 place it in dynamic and varied venues in the cell, including at the cell membrane, the nucleus and the early endosomes. Another adapter protein family the 14-3-3 proteins, are largely recognized as binding to phosphorylation sites but recent work demonstrated that in the case of FSHR, the 14-3-3 site overlaps with the canonical G-protein binding site. G-proteins appear to sample the environment and exchange between the membrane and intracellular locales and this binding could be mediated by or modulated by receptor interactions at the 14-3-3 binding site. Observations that multiple proteins can interact with cytoplasmic domains of GPCRs leads to the inescapable conclusion that either the interactions occur via orderly replacement or exchange, or that receptors are simultaneously occupied by a variety of adapters and effectors or even that oligomers of dimeric GPCRs provide for platforms that can simultaneously interact with effectors and adaptors.

Docking-based virtual screening for ligands of G protein-coupled receptors: not only crystal structures but also in silico models

G protein-coupled receptors (GPCRs) regulate a wide range of physiological functions and hold great pharmaceutical interest. Using the β(2)-adrenergic receptor as a case study, this article explores the applicability of docking-based virtual screening to the discovery of GPCR ligands and defines methods intended to improve the screening performance. Our controlled computational experiments were performed on a compound dataset containing known agonists and blockers of the receptor as well as a large number of decoys. The screening based on the structure of the receptor crystallized in complex with its inverse agonist carazolol yielded excellent results, with a clearly delineated prioritization of ligands over decoys. Blockers generally were preferred over agonists; however, agonists were also well distinguished from decoys. A method was devised to increase the screening yields by generating an ensemble of alternative conformations of the receptor that accounts for its flexibility. Moreover, a method was devised to improve the retrieval of agonists, based on the optimization of the receptor around a known agonist. Finally, the applicability of docking-based virtual screening also to homology models endowed with different levels of accuracy was proved. This last point is of uttermost importance, since crystal structures are available only for a limited number of GPCRs, and extends our conclusions to the entire superfamily. The outcome of this analysis definitely supports the application of computer-aided techniques to the discovery of novel GPCR ligands, especially in light of the fact that, in the near future, experimental structures are expected to be solved and become available for an ever increasing number of GPCRs.

Lessons from free energy simulations of delta-opioid receptor homodimers involving the fourth transmembrane helix

Several G protein-coupled receptors (GPCRs), including opioid receptors deltaOR, muOR, and kappaOR, have been reported to form stable dimers or oligomers in lipid bilayers and cell membranes. This notion has been recently challenged by imaging data supporting a transient nature of GPCR association. Here we use umbrella sampling reconstructed free energies of deltaOR homodimers involving the fourth transmembrane helix to predict their association constant. The results of these simulations, combined with estimates of diffusion-limited association rates, suggest a short lifetime for deltaOR homodimers in the membrane, in agreement with recent trends.

Role of aggregation in rhodopsin signal transduction

Many G protein-coupled receptors (GPCRs) are known to form dimers or even oligomers, and these aggregated states have been proposed as functional units responsible for signal transduction and G protein activation. However, the nature of their involvement has remained elusive. Here, we have investigated the role of aggregation in the signal transduction for dimeric forms of the prototypical GPCR rhodopsin using molecular dynamics simulations. The early steps after photoexcitation are characterized by a tandem mechanism in which one monomer is responsible for light detection while the other serves as the G protein activation site. Dimerization ensures efficient cross-talk between the two units within a few tens of nanoseconds following photoexcitation. This interface-mediated pathway suggests oligomerization-aided signal transduction as a crucial biological strategy to enhance activation efficiency across the entire family of GPCRs.

Interactions between intracellular domains as key determinants of the quaternary structure and function of receptor heteromers

G protein-coupled receptor (GPCR) heteromers are macromolecular complexes with unique functional properties different from those of its individual protomers. Little is known about what determines the quaternary structure of GPCR heteromers resulting in their unique functional properties. In this study, using resonance energy transfer techniques in experiments with mutated receptors, we provide for the first time clear evidence for a key role of intracellular domains in the determination of the quaternary structure of GPCR heteromers between adenosine A(2A), cannabinoid CB(1), and dopamine D(2) receptors. In these interactions, arginine-rich epitopes form salt bridges with phosphorylated serine or threonine residues from CK1/2 consensus sites. Each receptor (A(2A), CB(1), and D(2)) was found to include two evolutionarily conserved intracellular domains to establish selective electrostatic interactions with intracellular domains of the other two receptors, indicating that these particular electrostatic interactions constitute a general mechanism for receptor heteromerization. Mutation experiments indicated that the interactions of the intracellular domains of the CB(1) receptor with A(2A) and D(2) receptors are fundamental for the correct formation of the quaternary structure needed for the function (MAPK signaling) of the A(2A)-CB(1)-D(2) receptor heteromers. Analysis of MAPK signaling in striatal slices of CB(1) receptor KO mice and wild-type littermates supported the existence of A(1)-CB(1)-D(2) receptor heteromer in the brain. These findings allowed us to propose the first molecular model of the quaternary structure of a receptor heteromultimer.

Importance of lipid-exposed residues in transmembrane segment four for family B calcitonin receptor homo-dimerization

Dimerization of the prototypic family B G protein-coupled secretin receptor is determined by the lipid-exposed face of transmembrane segment four (TM4), and has substantial functional importance, facilitating G protein coupling. Recently, we demonstrated that the human secretin receptor elicits an inter-receptor bioluminescence resonance energy transfer (BRET) signal with most other human family B peptide receptors, except for the calcitonin receptor. In this study we have explored the occurrence and importance of calcitonin receptor oligomerization. Static and saturation receptor BRET were utilized to demonstrate that, unlike the human calcitonin receptor that does not yield a significant homomeric BRET signal, the rabbit calcitonin receptor exhibits strong resonance energy transfer. Within the lipid-exposed face of TM4, rabbit and human calcitonin receptors differ by a single amino acid (Arg236 in human; His in rabbit), while Thr253 that occurs in human and rabbit calcitonin receptors is unique across family B receptors. Mutating Arg236 or Thr253 of the human calcitonin receptor to residues found in the rabbit calcitonin receptor or the human secretin receptor (R236H, R236Y and T253A) resulted in generation of significant BRET signals. Similarly, mutation of Val250 of the human calcitonin receptor to another key lipid-facing residue found in the secretin receptor (V250I) also increased the receptor BRET signal. These data support the consistent theme of lipid-exposed residues of TM4 being important for the dimerization of the calcitonin receptor. However, rabbit and human calcitonin receptor constructs bound calcitonin and stimulated cAMP similarly, suggesting that differences in BRET could reflect differences in orientation or in the stability of homo-dimeric receptor complexes, which were nevertheless similarly effective in eliciting the functions attributed to that complex. The likelihood of human calcitonin receptor dimerization, even in the absence of a significant BRET signal, was further supported by data demonstrating that the peptide representing TM4 of this receptor that disrupts the rabbit receptor BRET signal, produced a right shift in the cAMP concentration-response curves for both rabbit and human receptors.

Heterodimerisation of G protein-coupled receptors: implications for drug design and ligand screening

IMPORTANCE OF THE FIELD

In recent times many G protein-coupled receptors (GPCRs) have been shown to dimerise/oligomerise and, in some cases, such structural organization has been found to be essential for receptor function or to play a modulatory role in living cells. The fact that these complexes may display differential pharmacology through, for example, the formation of a new binding pocket or signalling properties, as well as different functions or regulation in physiological tissues, offers novel opportunities for drug discovery. As a consequence, it seems necessary to develop new approaches suitable for GPCR heterodimer identification and selective ligand screening.

AREAS COVERED IN THIS REVIEW

This review gives an overview of new strategies that have been developed in an effort to incorporate the possibilities added by GPCR hetero-oligomerisation on the screening of compounds as drug candidates.

WHAT THE READER WILL GAIN

The reader will gain a wider knowledge about how the current understanding of GPCR oligomeric structure and function has mandated that hetero-oligomeric receptors must be considered as novel targets in the identification of future lead compounds.

TAKE HOME MESSAGE

For the improvement of novel drug discovery, more structural and functional information on the process of receptor oligomerisation is needed, and the realisation that the function of GPCRs can be greatly influenced by other interacting receptors or proteins also demands consideration in the lead-compound developing process in order to achieve better therapeutic agents.

Cocaine produces D2R-mediated conformational changes in the adenosine A(2A)R-dopamine D2R heteromer

Adenosine A(2A) receptors (A(2A)Rs) and dopamine D(2) receptors (D(2)Rs) form constitutive heteromers in living cells and exhibit a strong functional antagonistic interaction. Recent findings give neurochemical evidence that extended cocaine self-administration in the rat give rise to an up-regulation of functional A(2A)Rs in the nucleus accumbens that return to baseline expression levels during cocaine withdrawal. In the present work, the acute in vitro effects of a concentration of cocaine known to fully block the dopamine (DA) transporter without exerting any toxic actions were investigated on A(2A)R and D(2L)R formed heteromers in transiently co-transfected HEK-293T cells. In vitro treatment of cocaine was found to produce changes in D(2)R homodimers and in A(2A)R-D(2)R heterodimers detected through bioluminescent energy transfer (BRET). Cocaine was found to produce a time- and concentration-dependent reduction in the BRET(max) between A(2A)R-D(2L)R heterodimers and D(2L)R homodimers, but not A(2A)R homodimers, indicating its effect on D(2)R. Cocaine was evaluated with regard to D(2)R binding using a human D(2L)R stable expressing CHO cell line and was found to produce an increase in the affinity of hD(2L)R for DA. At the level of G protein-coupling, cocaine produced a small, but significant increase in DA-stimulated binding of GTPgammaS. However, cocaine failed to modulate D(2)R agonist-induced inhibition of cAMP in stable hD(2L)R CHO cells or the gating of GIRK channels in oocytes. Taken together, these results indicate a direct and specific effect of a moderate concentration of cocaine on the DA D(2L)R, that results in enhanced agonist recognition, G protein-coupling and an altered conformational state of D(2)R homodimers and A(2A)R-D(2)R heterodimers.

Progress in elucidating the structural and dynamic character of G Protein-Coupled Receptor oligomers for use in drug discovery

G Protein-Coupled Receptors (GPCRs) are the most targeted group of proteins for the development of therapeutic drugs. Until the last decade, structural information about this family of membrane proteins was relatively scarce, and their mechanisms of ligand binding and signal transduction were modeled on the assumption that GPCRs existed and functioned as monomeric entities. New crystal structures of native and engineered GPCRs, together with important biochemical and biophysical data that reveal structural details of the activation mechanism(s) of this receptor family, provide a valuable framework to improve dynamic molecular models of GPCRs with the ultimate goal of elucidating their allostery and functional selectivity. Since the dynamic movements of single GPCR protomers are likely to be affected by the presence of neighboring interacting subunits, oligomeric arrangements should be taken into account to improve the predictive ability of computer-assisted structural models of GPCRs for effective use in drug design.

Identification of specific transmembrane residues and ligand-induced interface changes involved in homo-dimer formation of a yeast G protein-coupled receptor

The Saccharomyces cerevisiae alpha-factor pheromone receptor, Ste2p, has been studied as a model for G protein-coupled receptor (GPCR) structure and function. Dimerization has been demonstrated for many GPCRs, although the role(s) of dimerization in receptor function is disputed. Transmembrane domains one (TM1) and four (TM4) of Ste2p were shown previously to play a role in dimerization. In this study, single cysteine substitutions were introduced into a Cys-less Ste2p, and disulfide-mediated dimerization was assessed. Six residues in TM1 (L64 to M69) that had not been previously investigated and 19 residues in TM7 (T278 to A296) of which 15 were not previously investigated were mutated to create 25 single Cys-containing Ste2p molecules. Ste2p mutants V68C in TM1 and nine mutants in TM7 (cysteine substituted into residues 278, 285, 289, and 291 to 296) showed increased dimerization upon addition of an oxidizing agent in comparison to the background dimers formed by the Cys-less receptor. The formation of dimers was decreased for TM7 mutant receptors in the presence of alpha-factor indicating that ligand binding resulted in a conformational change that influenced dimerization. The effect of ligand on dimer formation suggests that dimers are formed in the resting state and the activated state of the receptor by different TM interactions.

The impact of GPCR structures on pharmacology and structure-based drug design

After many years of effort, recent technical breakthroughs have enabled the X-ray crystal structures of three G-protein-coupled receptors (GPCRs) (beta1 and beta2 adrenergic and adenosine A(2a)) to be solved in addition to rhodopsin. GPCRs, like other membrane proteins, have lagged behind soluble drug targets such as kinases and proteases in the number of structures available and the level of understanding of these targets and their interaction with drugs. The availability of increasing numbers of structures of GPCRs is set to greatly increase our understanding of some of the key issues in GPCR biology. In particular, what constitutes the different receptor conformations that are involved in signalling and the molecular changes which occur upon receptor activation. How future GPCR structures might alter our views on areas such as agonist-directed signalling and allosteric regulation as well as dimerization is discussed. Knowledge of crystal structures in complex with small molecules will enable techniques in drug discovery and design, which have previously only been applied to soluble targets, to now be used for GPCR targets. These methods include structure-based drug design, virtual screening and fragment screening. This review considers how these methods have been used to address problems in drug discovery for kinase and protease targets and therefore how such methods are likely to impact GPCR drug discovery in the future.

Structural determinants underlying constitutive dimerization of unoccupied human follitropin receptors

The human follitropin receptor (hFSHR) is a G protein-coupled receptor (GPCR) central to reproductive physiology that is composed of an extracellular domain (ECD) fused to a serpentine region. Using bioluminescence resonance energy transfer (BRET) in living cells, we show that hFSHR dimers form constitutively during their biosynthesis. Mutations in TM1 and TM4 had no effect on hFSHR dimerization, alone or when combined with mutation of Tyr(110) in the ECD, a residue predicted to mediate dimerization of the soluble hormone-binding portion of the ECD complexed with FSH (Q. Fan and W. Hendrickson, Nature 433:269-277, 2005). Expressed individually, the serpentine region and a membrane-anchored form of the hFSHR ECD each exhibited homodimerization, suggesting that both domains contribute to dimerization of the full-length receptor. However, even in the context of only the membrane-anchored ECD, mutation of Tyr(110) to alanine did not inhibit dimerization. The full-length hFSHR and the membrane-anchored ECD were then each engineered to introduce a consensus site for N-linked glycosylation at residue 110. Despite experimental validation of the presence of carbohydrate on residue 110, we failed to observe disruption of dimerization of either the full-length hFSHR or membrane-anchored ECD containing the inserted glycan wedge. Taken altogether, our data suggest that both the serpentine region and the ECD contribute to hFSHR dimerization and that the dimerization interface of the unoccupied hFSHR does not involve Tyr(110) of the ECD.

Ligand-regulated oligomerization of beta(2)-adrenoceptors in a model lipid bilayer

The β₂-adrenoceptor (β₂AR) was one of the first Family A G protein-coupled receptors (GPCRs) shown to form oligomers in cellular membranes, yet we still know little about the number and arrangement of protomers in oligomers, the influence of ligands on the organization or stability of oligomers, or the requirement for other proteins to promote oligomerization. We used fluorescence resonance energy transfer (FRET) to characterize the oligomerization of purified β₂AR site-specifically labelled at three different positions with fluorophores and reconstituted into a model lipid bilayer. Our results suggest that the β₂AR is predominantly tetrameric following reconstitution into phospholipid vesicles. Agonists and antagonists have little effect on the relative orientation of protomers in oligomeric complexes. In contrast, binding of inverse agonists leads to significant increases in FRET efficiencies for most labelling pairs, suggesting that this class of ligand promotes tighter packing of protomers and/or the formation of more complex oligomers by reducing conformational fluctuations in individual protomers. The results provide new structural insights into β₂AR oligomerization and suggest a possible mechanism for the functional effects of inverse agonists.

Hetero-oligomerization of CCR2, CCR5, and CXCR4 and the protean effects of "selective" antagonists

Chemokine receptors constitute an attractive family of drug targets in the frame of inflammatory diseases. However, targeting specific chemokine receptors may be complicated by their ability to form dimers or higher order oligomers. Using a combination of luminescence complementation and bioluminescence resonance energy transfer assays, we demonstrate for the first time the existence of hetero-oligomeric complexes composed of at least three chemokine receptors (CCR2, CCR5, and CXCR4). We show in T cells and monocytes that negative binding cooperativity takes place between the binding pockets of these receptors, demonstrating their functional interaction in leukocytes. We also show that specific antagonists of one receptor (TAK-779 or AMD3100) lead to functional cross-inhibition of the others. Finally, using the air pouch model in mice, we show that the CCR2 and CCR5 antagonist TAK-779 inhibits cell recruitment promoted by the CXCR4 agonist SDF-1 alpha, demonstrating that cross-inhibition by antagonists also occurs in vivo. Thus, antagonists of the therapeutically important chemokine receptors regulate the functional properties of other receptors to which they do not bind directly with important implications for the use of these agents in vivo.

Molecular dynamics simulation of the heterodimeric mGluR2/5HT(2A) complex. An atomistic resolution study of a potential new target in psychiatric conditions

Homo- and heterodimerization is becoming an assessed concept in G-protein coupled receptor (GPCR) pharmacology, and the notion that GPCRs may dimerize or oligomerize is allowing for a reinterpretation of some inconsistencies or anomalies and is providing medicinal chemists with potentially relevant novel molecular targets for a variety of therapeutic conditions. Recently, it has been reported that two unrelated GPCRs, namely class C metabotropic glutamate receptor type-2 (mGluR2) and class A 5HT(2A) serotoninergic receptor, can heterodimerize at the transmembrane domain level. We performed a 40 ns molecular dynamics simulation of the mGluR2/5HT(2A) heterocomplex constructed around a TM4/TM5 interface and embedded in an explicit phospholipidic bilayer surrounded by water molecules. In a separate experiment, the monomeric 5HT(2A) receptor was simulated for additional 40 ns under the same conditions. The analysis and the comparison of the two simulations allowed us to clearly identify a cross-talk between the two protomers and to put forward an effect of the heterodimerization on the shape of the binding pocket of 5HT(2A). This result provides the first molecular explanation for the reported allosteric effect of mGluR2 on 5HT(2A)-mediated response and suggests that the heterocomplex can be a more suitable target for in silico screening than the monomeric protomers.

Allosteric communication between protomers of dopamine class A GPCR dimers modulates activation

A major obstacle to understanding the functional importance of dimerization between Class A G protein-coupled receptors (GPCRs) has been the methodological limitation in achieving control of the identity of the components comprising the signaling unit. We have developed a functional complementation assay that enables such control and illustrate it for the human dopamine D2 receptor. The minimal signaling unit, two receptors and a single G protein, is maximally activated by agonist binding to a single protomer, which suggests an asymmetrical activated dimer. Inverse agonist binding to the second protomer enhances signaling, whereas agonist binding to the second protomer blunts signaling. Ligand-independent constitutive activation of the second protomer also inhibits signaling. Thus, GPCR dimer function can be modulated by the activity state of the second protomer, which for a heterodimer may be altered in pathological states. Our novel methodology also makes possible the characterization of signaling from a defined heterodimer unit.