A day in the life of a G protein-coupled receptor: the contribution to function of G protein-coupled receptor dimerization

BRET Analysis of GPCR Dimers in Neurons and Non-Neuronal Cells: Evidence for Inactive, Agonist, and Constitutive Conformations

G-protein-coupled receptors (GPCRs) are dimeric proteins, but the functional consequences of the process are still debated. Active GPCR conformations are promoted either by agonists or constitutive activity. Inverse agonists decrease constitutive activity by promoting inactive conformations. The histamine H₃ receptor (H₃R) is the target of choice for the study of GPCRs because it displays high constitutive activity. Here, we study the dimerization of recombinant and brain H₃R and explore the effects of H₃R ligands of different intrinsic efficacy on dimerization. Co-immunoprecipitations and Western blots showed that H₃R dimers co-exist with monomers in transfected HEK 293 cells and in rodent brains. Bioluminescence energy transfer (BRET) analysis confirmed the existence of spontaneous H₃R dimers, not only in living HEK 293 cells but also in transfected cortical neurons. In both cells, agonists and constitutive activity of the H₃R decreased BRET signals, whereas inverse agonists and GTPγS, which promote inactive conformations, increased BRET signals. These findings show the existence of spontaneous H₃R dimers not only in heterologous systems but also in native tissues, which are able to adopt a number of allosteric conformations, from more inactive to more active states.

Combinatorial expression of GPCR isoforms affects signalling and drug responses

G-protein-coupled receptors (GPCRs) are membrane proteins that modulate physiology across human tissues in response to extracellular signals. GPCR-mediated signalling can differ because of changes in the sequence1,2 or expression3 of the receptors, leading to signalling bias when comparing diverse physiological systems4. An underexplored source of such bias is the generation of functionally diverse GPCR isoforms with different patterns of expression across different tissues. Here we integrate data from human tissue-level transcriptomes, GPCR sequences and structures, proteomics, single-cell transcriptomics, population-wide genetic association studies and pharmacological experiments. We show how a single GPCR gene can diversify into several isoforms with distinct signalling properties, and how unique isoform combinations expressed in different tissues can generate distinct signalling states. Depending on their structural changes and expression patterns, some of the detected isoforms may influence cellular responses to drugs and represent new targets for developing drugs with improved tissue selectivity. Our findings highlight the need to move from a canonical to a context-specific view of GPCR signalling that considers how combinatorial expression of isoforms in a particular cell type, tissue or organism collectively influences receptor signalling and drug responses.

β-Arrestin 1 in Thyrotropin Receptor Signaling in Bone: Studies in Osteoblast-Like Cells

A direct action of thyrotropin (TSH, thyroid-stimulating hormone) on bone precursors in humans is controversial. Studies in rodent models have provided conflicting findings. We used cells derived from a moderately differentiated osteosarcoma stably overexpressing human TSH receptors (TSHRs) as a model of osteoblast precursors (U2OS-TSHR cells) to investigate TSHR-mediated effects in bone differentiation in human cells. We review our findings that (1) TSHR couples to several different G proteins to induce upregulation of genes associated with osteoblast activity-interleukin 11 (IL-11), osteopontin (OPN), and alkaline phosphatase (ALPL) and that the kinetics of the induction and the G protein-mediated signaling pathways involved were different for these genes; (2) TSH can stimulate β-arrestin-mediated signal transduction and that β-arrestin 1 in part mediates TSH-induced pre-osteoblast differentiation; and (3) TSHR/insulin-like growth factor 1 (IGF1) receptor (IGF1R) synergistically increased OPN secretion by TSH and IGF1 and that this crosstalk was mediated by physical association of these receptors in a signaling complex that uses β-arrestin 1 as a scaffold. These findings were complemented using a novel β-arrestin 1-biased agonist of TSHR. We conclude that TSHR can signal via several transduction pathways leading to differentiation of this model system of human pre-osteoblast cells and, therefore, that TSH can directly regulate these bone cells.

Attenuation of Oxytocin and Serotonin 2A Receptor Signaling through Novel Heteroreceptor Formation

The oxytocin receptor (OTR) and the 5-hydroxytryptamine 2A receptor (5-HTR_2A) are expressed in similar brain regions modulating central pathways critical for social and cognition-related behaviors. Signaling crosstalk between their endogenous ligands, oxytocin (OT) and serotonin (5-hydroxytryptamine, 5-HT), highlights the complex interplay between these two neurotransmitter systems and may be indicative of the formation of heteroreceptor complexes with subsequent downstream signaling changes. In this study, we assess the possible formation of OTR-5HTR_2A heteromers in living cells and the functional downstream consequences of this receptor-receptor interaction. First, we demonstrated the existence of a physical interaction between the OTR and 5-HTR_2A in vitro, using a flow cytometry-based FRET approach and confocal microscopy. Furthermore, we investigated the formation of this specific heteroreceptor complex ex vivo in the brain sections using the Proximity Ligation Assay (PLA). The OTR-5HTR_2A heteroreceptor complexes were identified in limbic regions (including hippocampus, cingulate cortex, and nucleus accumbens), key regions associated with cognition and social-related behaviors. Next, functional cellular-based assays to assess the OTR-5HTR_2A downstream signaling crosstalk showed a reduction in potency and efficacy of OT and OTR synthetic agonists, carbetocin and WAY267464, on OTR-mediated Gαq signaling. Similarly, the activation of 5-HTR_2A by the endogenous agonist, 5-HT, also revealed attenuation in Gαq-mediated signaling. Finally, altered receptor trafficking within the cell was demonstrated, indicative of cotrafficking of the OTR/5-HTR_2A pair. Overall, these results constitute a novel mechanism of specific interaction between the OT and 5-HT neurotransmitters via OTR-5HTR_2A heteroreceptor formation and provide potential new therapeutic strategies in the treatment of social and cognition-related diseases.

Bioluminescence Resonance Energy Transfer as a Method to Study Protein-Protein Interactions: Application to G Protein Coupled Receptor Biology

The bioluminescence resonance energy transfer (BRET) approach involves resonance energy transfer between a light-emitting enzyme and fluorescent acceptors. The major advantage of this technique over biochemical methods is that protein-protein interactions (PPI) can be monitored without disrupting the natural environment, frequently altered by detergents and membrane preparations. Thus, it is considered as one of the most versatile technique for studying molecular interactions in living cells at "physiological" expression levels. BRET analysis has been applied to study many transmembrane receptor classes including G-protein coupled receptors (GPCR). It is well established that these receptors may function as dimeric/oligomeric forms and interact with multiple effectors to transduce the signal. Therefore, they are considered as attractive targets to identify PPI modulators. In this review, we present an overview of the different BRET systems developed up to now and their relevance to identify inhibitors/modulators of protein⁻protein interaction. Then, we introduce the different classes of agents that have been recently developed to target PPI, and provide some examples illustrating the use of BRET-based assays to identify and characterize innovative PPI modulators in the field of GPCRs biology. Finally, we discuss the main advantages and the limits of BRET approach to characterize PPI modulators.

Reinterpreting anomalous competitive binding experiments within G protein-coupled receptor homodimers using a dimer receptor model

An increasing number of G protein-coupled receptors (GPCRs) have been reported to be expressed in the plasma membrane as dimers. Since most ligand binding data are currently fitted by classical equations developed only for monomeric receptors, the interpretation of data could be misleading in the presence of GPCR dimers. On the other hand, the equations developed from dimer receptor models assuming the existence of two orthosteric binding sites within the dimeric molecule offer the possibility to directly calculate macroscopic equilibrium dissociation constants for the two sites, an index of cooperativity (D_C) that reflects the molecular communication within the dimer and, importantly, a constant of radioligand-competitor allosteric interaction (K_DAB) in competitive assays. Here, we provide a practical way to fit competitive binding data that allows the interpretation of apparently anomalous results, such as competition curves that could be either bell-shaped, monophasic or biphasic depending on the assay conditions. The consideration of a radioligand-competitor allosteric interaction allows fitting these curve patterns both under simulation conditions and in real radioligand binding experiments, obtaining competitor affinity parameters closer to the actual values. Our approach is the first that, assuming the formation of receptor homodimers, is able to explain several experimental results previously considered erroneous due to their impossibility to be fitted. We also deduce the radioligand concentration responsible for the conversion of biphasic to monophasic or to bell-shaped curves in competitive radioligand binding assays. In conclusion, bell-shaped curves in competitive binding experiments constitute evidence for GPCR homodimerization.

Pharmacoperones as Novel Therapeutics for Diverse Protein Conformational Diseases

After synthesis, proteins are folded into their native conformations aided by molecular chaperones. Dysfunction in folding caused by genetic mutations in numerous genes causes protein conformational diseases. Membrane proteins are more prone to misfolding due to their more intricate folding than soluble proteins. Misfolded proteins are detected by the cellular quality control systems, especially in the endoplasmic reticulum, and proteins may be retained there for eventual degradation by the ubiquitin-proteasome system or through autophagy. Some misfolded proteins aggregate, leading to pathologies in numerous neurological diseases. In vitro, modulating mutant protein folding by altering molecular chaperone expression can ameliorate some misfolding. Some small molecules known as chemical chaperones also correct mutant protein misfolding in vitro and in vivo. However, due to their lack of specificity, their potential as therapeutics is limited. Another class of compounds, known as pharmacological chaperones (pharmacoperones), binds with high specificity to misfolded proteins, either as enzyme substrates or receptor ligands, leading to decreased folding energy barriers and correction of the misfolding. Because many of the misfolded proteins are misrouted but do not have defects in function per se, pharmacoperones have promising potential in advancing to the clinic as therapeutics, since correcting routing may ameliorate the underlying mechanism of disease. This review will comprehensively summarize this exciting area of research, surveying the literature from in vitro studies in cell lines to transgenic animal models and clinical trials in several protein misfolding diseases.

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

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

Dynamic roles for the N-terminus of the yeast G protein-coupled receptor Ste2p

The Saccharomyces cerevisiae α-factor receptor Ste2p has been used extensively as a model to understand the molecular mechanism of signal transduction by G protein-coupled receptors (GPCRs). Single and double cysteine mutants of Ste2p were created and served as surrogates to detect intramolecular interactions and dimerization of Ste2p using disulfide cross-linking methodology. When a mutation was introduced into the phylogenetically conserved tyrosine residue at position 26 (Y26C) in the N-terminus of Ste2p, dimerization was increased greatly. The amount of dimer formed by this Y26C mutant was greatly reduced by ligand binding even though the ligand binding site is far removed from the N-terminus; the lowering of the dimer formation was consistent with a conformational change in the N-terminus of the receptor upon activation. Dimerization was decreased by double mutations Y26C/V109C or Y26C/T114C indicating that Y26 is in close proximity to V109 and T114 of extracellular loop 1 in native Ste2p. Combined with earlier studies, these results indicate previously unrecognized roles for the N-terminus of Ste2p, and perhaps of GPCRs in general, and reveal a specific N-terminus residue or region, that is involved in GPCR signaling, intrareceptor interactions, and receptor dimerization.

Unsaturated fatty acyl recognition by Frizzled receptors mediates dimerization upon Wnt ligand binding

Frizzled (FZD) receptors mediate Wnt signaling in diverse processes ranging from bone growth to stem cell activity. Moreover, high FZD receptor expression at the cell surface contributes to overactive Wnt signaling in subsets of pancreatic, ovarian, gastric, and colorectal tumors. Despite the progress in biochemical understanding of Wnt-FZD receptor interactions, the molecular basis for recognition of Wnt cis-unsaturated fatty acyl groups by the cysteine-rich domain (CRD) of FZD receptors remains elusive. Here, we determined a crystal structure of human FZD7 CRD unexpectedly bound to a 24-carbon fatty acid. We also report a crystal structure of human FZD5 CRD bound to C16:1 cis-Δ9 unsaturated fatty acid. Both structures reveal a dimeric arrangement of the CRD. The lipid-binding groove exhibits flexibility and spans both monomers, adopting a U-shaped geometry that accommodates the fatty acid. Re-evaluation of the published mouse FZD8 CRD structure reveals that it also shares the same architecture as FZD5 and FZD7 CRDs. Our results define a common molecular mechanism for recognition of the cis-unsaturated fatty acyl group, a necessary posttranslational modification of Wnts, by multiple FZD receptors. The fatty acid bridges two CRD monomers, implying that Wnt binding mediates FZD receptor dimerization. Our data uncover possibilities for the arrangement of Wnt-FZD CRD complexes and shed structural insights that could aide in the identification of pharmacological strategies to modulate FZD receptor function.

Theoretical Aspects of GPCR-Ligand Complex Pharmacology

Over the past 50 years in pharmacology, an understanding of seven transmembrane (7TMR) function has been gained from the comparison of experimental data to receptor models. These models have been constructed from building blocks composed of systems consisting of series and parallel mass action binding reactions. Basic functions such as the the isomerization of receptors upon ligand binding, the sequential binding of receptors to membrane coupling proteins, and the selection of multiple receptor conformations have been combined in various ways to build receptor systems such as the ternary complex, extended ternary complex, and cubic ternary complex models for 7TMR function. Separately, the Black/Leff operational model has furnished an extremely valuable method of quantifying drug agonism. In the past few years, incorporation of the basic allosteric nature of 7TMRs has led to additional useful models of functional receptor allosteric mechanisms; these models yield valuable methods for quantifying allosteric effects. Finally, molecular dynamics has provided yet another new set of models describing the probability of formation of multiple receptor states; these radically new models are extremely useful in the prediction of functionally selective drug effects.

Human adenosine A2A receptor binds calmodulin with high affinity in a calcium-dependent manner

Understanding how ligands bind to G-protein-coupled receptors and how binding changes receptor structure to affect signaling is critical for developing a complete picture of the signal transduction process. The adenosine A2A receptor (A2AR) is a particularly interesting example, as it has an exceptionally long intracellular carboxyl terminus, which is predicted to be mainly disordered. Experimental data on the structure of the A2AR C-terminus is lacking, because published structures of A2AR do not include the C-terminus. Calmodulin has been reported to bind to the A2AR C-terminus, with a possible binding site on helix 8, next to the membrane. The biological meaning of the interaction as well as its calcium dependence, thermodynamic parameters, and organization of the proteins in the complex are unclear. Here, we characterized the structure of the A2AR C-terminus and the A2AR C-terminus-calmodulin complex using different biophysical methods, including native gel and analytical gel filtration, isothermal titration calorimetry, NMR spectroscopy, and small-angle X-ray scattering. We found that the C-terminus is disordered and flexible, and it binds with high affinity (Kd = 98 nM) to calmodulin without major conformational changes in the domain. Calmodulin binds to helix 8 of the A2AR in a calcium-dependent manner that can displace binding of A2AR to lipid vesicles. We also predicted and classified putative calmodulin-binding sites in a larger group of G-protein-coupled receptors.

Disruption of Rhodopsin Dimerization with Synthetic Peptides Targeting an Interaction Interface

Although homo- and heterodimerizations of G protein-coupled receptors (GPCRs) are well documented, GPCR monomers are able to assemble in different ways, thus causing variations in the interactive interface between receptor monomers among different GPCRs. Moreover, the functional consequences of this phenomenon, which remain to be clarified, could be specific for different GPCRs. Synthetic peptides derived from transmembrane (TM) domains can interact with a full-length GPCR, blocking dimer formation and affecting its function. Here we used peptides corresponding to TM helices of bovine rhodopsin (Rho) to investigate the Rho dimer interface and functional consequences of its disruption. Incubation of Rho with TM1, TM2, TM4, and TM5 peptides in rod outer segment (ROS) membranes shifted the resulting detergent-solubilized protein migration through a gel filtration column toward smaller molecular masses with a reduced propensity for dimer formation in a cross-linking reaction. Binding of these TM peptides to Rho was characterized by both mass spectrometry and a label-free assay from which dissociation constants were calculated. A BRET (bioluminescence resonance energy transfer) assay revealed that the physical interaction between Rho molecules expressed in membranes of living cells was blocked by the same four TM peptides identified in our in vitro experiments. Although disruption of the Rho dimer/oligomer had no effect on the rates of G protein activation, binding of Gt to the activated receptor stabilized the dimer. However, TM peptide-induced disruption of dimer/oligomer decreased receptor stability, suggesting that Rho supramolecular organization could be essential for ROS stabilization and receptor trafficking.

Submicroscopic deletions at 13q32.1 cause congenital microcoria

Congenital microcoria (MCOR) is a rare autosomal-dominant disorder characterized by inability of the iris to dilate owing to absence of dilator pupillae muscle. So far, a dozen MCOR-affected families have been reported worldwide. By using whole-genome oligonucleotide array CGH, we have identified deletions at 13q32.1 segregating with MCOR in six families originating from France, Japan, and Mexico. Breakpoint sequence analyses showed nonrecurrent deletions in 5/6 families. The deletions varied from 35 kbp to 80 kbp in size, but invariably encompassed or interrupted only two genes: TGDS encoding the TDP-glucose 4,6-dehydratase and GPR180 encoding the G protein-coupled receptor 180, also known as intimal thickness-related receptor (ITR). Unlike TGDS which has no known function in muscle cells, GPR180 is involved in the regulation of smooth muscle cell growth. The identification of a null GPR180 mutation segregating over two generations with iridocorneal angle dysgenesis, which can be regarded as a MCOR endophenotype, is consistent with the view that deletions of this gene, with or without the loss of elements regulating the expression of neighboring genes, are the cause of MCOR.

Oligomerisation of C. elegans olfactory receptors, ODR-10 and STR-112, in yeast

It is widely accepted that vertebrate G-Protein Coupled Receptors (GPCRs) associate with each other as homo- or hetero-dimers or higher-order oligomers. The C. elegans genome encodes hundreds of olfactory GPCRs, which may be expressed in fewer than a dozen chemosensory neurons, suggesting an opportunity for oligomerisation. Here we show, using three independent lines of evidence: co-immunoprecipitation, bioluminescence resonance energy transfer and a yeast two-hybrid assay that nematode olfactory receptors (ORs) oligomerise when heterologously expressed in yeast. Specifically, the nematode receptor ODR-10 is able to homo-oligomerise and can also form heteromers with the related nematode receptor STR-112. ODR-10 also oligomerised with the rat I7 OR but did not oligomerise with the human somatostatin receptor 5, a neuropeptide receptor. In this study, the question of functional relevance was not addressed and remains to be investigated.

Functionally biased signalling properties of 7TM receptors - opportunities for drug development for the ghrelin receptor

The ghrelin receptor is a 7 transmembrane (7TM) receptor involved in a variety of physiological functions including growth hormone secretion, increased food intake and fat accumulation as well as modulation of reward and cognitive functions. Because of its important role in metabolism and energy expenditure, the ghrelin receptor has become an important therapeutic target for drug design and the development of anti-obesity compounds. However, none of the compounds developed so far have been approved for commercial use. Interestingly, the ghrelin receptor is able to signal through several different signalling pathways including Gαq , Gαi/o , Gα12/13 and arrestin recruitment. These multiple signalling pathways allow for functionally biased signalling, where one signalling pathway may be favoured over another either by selective ligands or through mutations in the receptor. In the present review, we have described how ligands and mutations in the 7TM receptor may bias the receptors to favour either one G-protein over another or to promote G-protein independent signalling pathways rather than G-protein-dependent pathways. For the ghrelin receptor, both agonist and inverse agonists have been demonstrated to signal more strongly through the Gαq -coupled pathway than the Gα12/13 -coupled pathway. Similarly a ligand that promotes Gαq coupling over Gαi coupling has been described and it has been suggested that several different active conformations of the receptor may exist dependent on the properties of the agonist. Importantly, ligands with such biased signalling properties may allow the development of drugs that selectively modulate only the therapeutically relevant physiological functions, thereby decreasing the risk of side effects.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

Putative kappa opioid heteromers as targets for developing analgesics free of adverse effects

It is now generally recognized that upon activation by an agonist, β-arrestin associates with G protein-coupled receptors and acts as a scaffold in creating a diverse signaling network that could lead to adverse effects. As an approach to reducing side effects associated with κ opioid agonists, a series of β-naltrexamides 3-10 was synthesized in an effort to selectively target putative κ opioid heteromers without recruiting β-arrestin upon activation. The most potent derivative 3 (INTA) strongly activated KOR-DOR and KOR-MOR heteromers in HEK293 cells. In vivo studies revealed 3 to produce potent antinociception, which, when taken together with antagonism data, was consistent with the activation of both heteromers. 3 was devoid of tolerance, dependence, and showed no aversive effect in the conditioned place preference assay. As immunofluorescence studies indicated no recruitment of β-arrestin2 to membranes in coexpressed KOR-DOR cells, this study suggests that targeting of specific putative heteromers has the potential to identify leads for analgesics devoid of adverse effects.

The physical association of the P2Y12 receptor with PAR4 regulates arrestin-mediated Akt activation

It is now well accepted that protease activated receptor (PAR) 1 and PAR4 have differential roles in platelet activation. PAR4, a low-affinity thrombin receptor in human platelets, participates in sustained platelet activation in a P2Y12-dependent manner; however, the mechanisms are not defined. Our previous studies demonstrated that thrombin induces the association of PAR4 with P2Y12, together with arrestin recruitment to the complex. Here we show that PAR4 and P2Y12 directly interact to coregulate Akt signaling after PAR4 activation. We observed direct and specific interaction of P2Y12 with PAR4 but not PAR1 by bioluminescent resonance energy transfer when the receptors were coexpressed in human embryonic kidney 293T cells. PAR4-P2Y12 dimerization was promoted by PAR4-AP and inhibited by P2Y12 antagonist. By using sequence comparison of the transmembrane domains of PAR1 and PAR4, we designed a mutant form of PAR4, "PAR4SFT," by replacing LGL194-196 at the base of transmembrane domain 4 with the corresponding aligned PAR1 residues SFT 220-222. PAR4SFT supported only 8.74% of PAR4-P2Y12 interaction, abolishing P2Y12-dependent arrestin recruitment to PAR4 and Akt activation. Nonetheless, PAR4SFT still supported homodimerization with PAR4. PAR4SFT failed to induce a calcium flux when expressed independently; however, coexpression of increasing concentrations of PAR4SFT, together with PAR4 potentiated PAR4-mediated calcium flux, suggested that PAR4 act as homodimers to signal to Gq-coupled calcium responses. In conclusion, PAR4 LGL (194-196) governs agonist-dependent association of PAR4 with P2Y12 and contributes to Gq-coupled calcium responses. PAR4-P2Y12 association supports arrestin-mediated sustained signaling to Akt. Hence, PAR4-P2Y12 dimerization is likely to be important for the PAR4-P2Y12 dependent stabilization of platelet thrombi.

β-Arrestin-1 mediates thyrotropin-enhanced osteoblast differentiation

Thyrotropin (TSH) activation of the TSH receptor (TSHR), a 7-transmembrane-spanning receptor (7TMR), may have osteoprotective properties by direct effects on bone. TSHR activation by TSH phosphorylates protein kinases AKT1, p38α, and ERK1/2 in some cells. We found TSH-induced phosphorylation of these kinases in 2 cell lines engineered to express TSHRs, human embryonic kidney HEK-TSHR cells and human osteoblastic U2OS-TSHR cells. In U2OS-TSHR cells, TSH up-regulated pAKT1 (7.1±0.5-fold), p38α (2.9±0.4-fold), and pERK1/2 (3.1±0.2-fold), whereas small molecule TSHR agonist C2 had no or little effect on pAKT1 (1.8±0.08-fold), p38α (1.2±0.09-fold), and pERK1/2 (1.6±0.19-fold). Furthermore, TSH increased expression of osteoblast marker genes ALPL (8.2±4.6-fold), RANKL (21±5.9-fold), and osteopontin (OPN; 17±5.3-fold), whereas C2 had little effect (ALPL, 1.7±0.5-fold; RANKL, 1.3±0.6-fold; and OPN, 2.2±0.7-fold). β-Arrestin-1 and -2 can mediate activatory signals by 7TMRs. TSH stimulated translocation of β-arrestin-1 and -2 to TSHR, whereas C2 failed to translocate either β-arrestin. Down-regulation of β-arrestin-1 by siRNA inhibited TSH-stimulated phosphorylation of ERK1/2, p38α, and AKT1, whereas down-regulation of β-arrestin-2 increased phosphorylation of AKT1 in both cell types and of ERK1/2 in HEK-TSHR cells. Knockdown of β-arrestin-1 inhibited TSH-stimulated up-regulation of mRNAs for OPN by 87 ± 1.7% and RANKL by 73 ± 2.4%, and OPN secretion by 74 ± 10%. We conclude that TSH enhances osteoblast differentiation in U2OS cells that is, in part, caused by activatory signals mediated by β-arrestin-1.

Association of immune and metabolic receptors C5aR and C5L2 with adiposity in women

Adipose tissue receptors C5aR and C5L2 and their heterodimerization/functionality and interaction with ligands C5a and acylation stimulating protein (ASP) have been evaluated in cell and rodent studies. Their contribution to obesity factors in humans remains unclear. We hypothesized that C5a receptors, classically required for host defense, are also associated with adiposity. Anthropometry and fasting blood parameters were measured in 136 women divided by body mass index (BMI): normal/overweight (≤30 kg/m(2); n = 34), obese I (≤45 kg/m(2); n = 33), obese II (≤51 kg/m(2); n = 33), and obese III (≤80 kg/m(2); n = 36). Subcutaneous and omental adipose tissue C5aR and C5L2 expression were analysed. C5L2 expression was comparable between subcutaneous and omental across all BMI groups. Plasma ASP and ASP/omental C5L2 expression increased with BMI (P < 0.001 and P < 0.01, resp.). While plasma C5a was unchanged, C5aR expression decreased with increasing BMI in subcutaneous and omental tissues (P < 0.01 and P < 0.05, resp.), with subcutaneous omental depots. Omental C5L2/C5aR ratio increased with BMI (P < 0.01) with correlations between C5L2/C5aR and waist circumference, HDL-C, and adiponectin. Tissue and BMI differences in receptors and ligands, particularly in omental, suggest relationship to metabolic disturbances and highlight adipose-immune interactions.

Luminescent lanthanide cryptates: from the bench to the bedside

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

Segregation of family A G protein-coupled receptor protomers in the plasma membrane

G protein-coupled receptors (GPCRs) transduce many important physiological signals and are targets for a large fraction of therapeutic drugs. Members of the largest family of GPCRs (family A) are thought to self-associate as dimers and higher-order oligomers, although the significance of such quaternary structures for signaling or receptor trafficking is known for only a few examples. One outstanding question is the physical stability of family A oligomers in cell membranes. Stable oligomers would be expected to move through cellular compartments and membrane domains as intact groups of protomers. Here, we test this prediction by recruiting subsets of affinity-tagged family A protomers into artificial microdomains on the surface of living cells and asking if untagged protomers move into these domains (are corecruited) at the same time. We find that tagged β₂ adrenergic and μ-opioid protomers are unable to corecruit untagged protomers into microdomains. In contrast, tagged metabotropic glutamate receptor protomers do corecruit untagged protomers into such microdomains, which is consistent with the known covalent mechanism whereby these family C receptors dimerize. These observations suggest that interactions between these family A protomers are too weak to directly influence subcellular location, and that mechanisms that move these receptors between subcellular compartments and domains must operate on individual protomers.

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

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

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

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

Crystallization of G protein-coupled receptors

Oligomerization is one of several mechanisms that can regulate the activity of G protein-coupled receptors (GPCRs), but little is known about the structure of GPCR oligomers. Crystallography and NMR are the only methods able to reveal the details of receptor-receptor interactions at an atomic level, and several GPCR homodimers already have been described from crystal structures. Two clusters of symmetric interfaces have been identified from these structures that concur with biochemical data, one involving helices I, II, and VIII and the other formed mainly by helices V and VI. In this chapter, we describe the protocols used in our laboratory for the crystallization of rhodopsin and the β2-adrenergic receptor (β2-AR). For bovine rhodopsin, we developed a new purification strategy including a (NH4)2SO4-induced phase separation that proved essential to obtain crystals of photoactivated rhodopsin containing parallel dimers. Crystallization of native bovine rhodopsin was achieved by the classic vapor-diffusion technique. For β2-AR, we developed a purification strategy based on previously published protocols employing a lipidic cubic phase to obtain diffracting crystals of a β2-AR/T4-lysozyme chimera bound to the antagonist carazolol.

GPCR oligomerization and receptor trafficking

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

Detecting single photons: a supramolecular matter?

Rod photoreceptors detect single photons through a tradeoff of light collecting ability, amplification and speed. Key roles are played by rhodopsin (Rh) and transducin (G(t)), whose complex supramolecular organization in outer segment disks begs for a functional interpretation. Here we review past and recent evidence of a temperature-dependence of photon detection by mammalian rods, and link this phenomenon with the putative oligomeric organization of Rh and new ideas on the dynamics of Rh-G(t) interaction. Identifying an electrophysiological correlate of the supramolecular organization of Rh and G(t) may shed light on the evolutionary advantage it confers to night vision.

Drug-like actions of autoantibodies against receptors of the autonomous nervous system and their impact on human heart function

Antibodies against cholinergic and adrenergic receptors (adrenoceptors) are frequent in serum of patients with chronic heart failure. Their prevalence is associated with Chagas' disease, idiopathic dilated cardiomyopathy (DCM), and ischaemic heart disease. Among the epitopes targeted are first and second extracellular loops of the β-adrenergic (β-adrenoceptor) and M2 muscarinic receptor. β₁-adrenoceptor autoantibodies affect radioligand binding and cardiomyocyte function similar to agonists. Corresponding rodent immunizations induce symptoms compatible with chronic heart failure that are reversible upon removal of the antibodies, transferable via the serum and abrogated by adrenergic antagonists. In DCM patients, prevalence and stimulatory efficacy of β₁-adrenoceptor autoantibodies are correlated to the decline in cardiac function, ventricular arrhythmia and higher incidence of cardiac death. In conclusion, such autoantibodies seem to cause or promote chronic human left ventricular dysfunction by acting on their receptor targets in a drug-like fashion. However, the pharmacology of this interaction is poorly understood. It is unclear how the autoantibodies trigger changes in receptor activity and second messenger coupling and how that is related to the pathogenesis and severity of the associated diseases. Here, we summarize the available evidence regarding these issues and discuss these findings in the light of recent knowledge about the conformational activation of the human β₂-adrenoceptor and the properties of bona fide cardiopathogenic autoantibodies derived from immune-adsorption therapy of DCM patients. These considerations might contribute to the conception of therapy regimen aimed at counteracting or neutralizing cardiopathogenic receptor autoantibodies.

Assessing the relative stability of dimer interfaces in g protein-coupled receptors

Considerable evidence has accumulated in recent years suggesting that G protein-coupled receptors (GPCRs) associate in the plasma membrane to form homo- and/or heteromers. Nevertheless, the stoichiometry, fraction and lifetime of such receptor complexes in living cells remain topics of intense debate. Motivated by experimental data suggesting differing stabilities for homomers of the cognate human β1- and β2-adrenergic receptors, we have carried out approximately 160 microseconds of biased molecular dynamics simulations to calculate the dimerization free energy of crystal structure-based models of these receptors, interacting at two interfaces that have often been implicated in GPCR association under physiological conditions. Specifically, results are presented for simulations of coarse-grained (MARTINI-based) and atomistic representations of each receptor, in homodimeric configurations with either transmembrane helices TM1/H8 or TM4/3 at the interface, in an explicit lipid bilayer. Our results support a definite contribution to the relative stability of GPCR dimers from both interface sequence and configuration. We conclude that β1- and β2-adrenergic receptor homodimers with TM1/H8 at the interface are more stable than those involving TM4/3, and that this might be reconciled with experimental studies by considering a model of oligomerization in which more stable TM1 homodimers diffuse through the membrane, transiently interacting with other protomers at interfaces involving other TM helices.

Biased signalling and allosteric machines: new vistas and challenges for drug discovery

Seven transmembrane receptors (7TMRs) are nature's prototype allosteric proteins made to bind molecules at one location to subsequently change their shape to affect the binding of another molecule at another location. This paper attempts to describe the divergent 7TMR behaviours (i.e. third party allostery, receptor oligomerization, biased agonism) observed in pharmacology in terms of a homogeneous group of allosteric behaviours. By considering the bodies involved as a vector defined by a modulator, conduit and guest, these activities can all be described by a simple model of functional allostery made up of the Ehlert allosteric model and the Black/Leff operational model. It will be shown how this model yields parameters that can be used to characterize the activity of any ligand or protein producing effect through allosteric interaction with a 7TMR.

Allosteric interactions within the AT₁ angiotensin receptor homodimer: role of the conserved DRY motif

G protein coupled receptor (GPCR) dimerization has a remarkable impact on the diversity of receptor signaling. Allosteric communication between the protomers of the dimer can alter ligand binding, receptor conformation and interactions with different effector proteins. In this study we investigated the allosteric interactions between wild type and mutant protomers of type 1 angiotensin receptor (AT₁R) dimers transiently expressed in CHO cells. In our experimental setup, one protomer of the dimer was selectively stimulated and the β-arrestin2 binding and conformation alteration of the other protomer was followed. The interaction between β-arrestin2 and the non-stimulated protomer was monitored through a bioluminescence resonance energy transfer (BRET) based method. To measure the conformational alterations in the non-stimulated protomer directly, we also used a BRET based intramolecular receptor biosensor, which was created by inserting yellow fluorescent protein (YFP) into the 3rd intracellular loop of AT₁R and fusing Renilla luciferase (RLuc) to its C terminal region. We have detected β-arrestin2 binding, and altered conformation of the non-stimulated protomer. The cooperative ligand binding of the receptor homodimer was also observed by radioligand dissociation experiments. Mutation of the conserved DRY sequence in the activated protomer, which is also required for G protein activation, abolished all the observed allosteric effects. These data suggest that allosteric interactions in the homodimers of AT₁R significantly affect the function of the non-stimulated protomer, and the conserved DRY motif has a crucial role in these interactions.

Identification and profiling of novel α1A-adrenoceptor-CXC chemokine receptor 2 heteromer

We have provided the first evidence for specific heteromerization between the α(1A)-adrenoceptor (α(1A)AR) and CXC chemokine receptor 2 (CXCR2) in live cells. α(1A)AR and CXCR2 are both expressed in areas such as the stromal smooth muscle layer of the prostate. By utilizing the G protein-coupled receptor (GPCR) heteromer identification technology on the live cell-based bioluminescence resonance energy transfer (BRET) assay platform, our studies in human embryonic kidney 293 cells have identified norepinephrine-dependent β-arrestin recruitment that was in turn dependent upon co-expression of α(1A)AR with CXCR2. These findings have been supported by co-localization observed using confocal microscopy. This norepinephrine-dependent β-arrestin recruitment was inhibited not only by the α(1)AR antagonist Terazosin but also by the CXCR2-specific allosteric inverse agonist SB265610. Furthermore, Labetalol, which is marketed for hypertension as a nonselective β-adrenoceptor antagonist with α(1)AR antagonist properties, was identified as a heteromer-specific-biased agonist exhibiting partial agonism for inositol phosphate production but essentially full agonism for β-arrestin recruitment at the α(1A)AR-CXCR2 heteromer. Finally, bioluminescence resonance energy transfer studies with both receptors tagged suggest that α(1A)AR-CXCR2 heteromerization occurs constitutively and is not modulated by ligand. These findings support the concept of GPCR heteromer complexes exhibiting distinct pharmacology, thereby providing additional mechanisms through which GPCRs can potentially achieve their diverse biological functions. This has important implications for the use and future development of pharmaceuticals targeting these receptors.

Revisiting and questioning functional rescue between dimerized LH receptor mutants

The glycoprotein hormone receptors are G protein-coupled receptors containing a large extracellular domain fused to a prototypical serpentine domain. cis-activation occurs when binding of hormone to the extracellular domain stabilizes the serpentine domain in an active conformation. Studies by others suggested that these receptors can also signal by trans-activation, where hormone binding to one receptor protomer activates the serpentine domain of an associated protomer, as documented by the partial rescue of hormone-dependent signaling when a binding defective mutant is coexpressed with a signaling defective mutant. However, our characterizations of several LH receptor (LHR) mutants used in previous studies differ markedly from those originally reported. Also, when examining a pair of LHR mutants previously shown to functionally rescue in vitro as well as in vivo, in addition to finding that the properties of the individual mutants differ significantly from those originally described, we determined that when this pair of mutants was coexpressed in vitro, quantitative analyses did not indicate functional rescue. Additional data are presented that provide a plausible alternate explanation for the apparent in vivo trans-activation that was reported. Finally, using LHR mutants that we have documented to be expressed at the cell surface but to lack human chorionic gonadotropin binding activity or to be severely impaired in their ability to activate Gs, we did not observe functional rescue of human chorionic gonadotropin-stimulated cAMP when the mutants were coexpressed, even though bioluminescence resonance energy transfer analyses confirmed that the coexpressed mutants formed dimers. Taken altogether, our data substantively question the concept of functional rescue between LHR mutants.

Homobivalent ligands of the atypical antipsychotic clozapine: design, synthesis, and pharmacological evaluation

To date all typical and atypical antipsychotics target the dopamine D(2) receptor. Clozapine represents the best-characterized atypical antipsychotic, although it displays only moderate (submicromolar) affinity for the dopamine D(2) receptor. Herein, we present the design, synthesis, and pharmacological evaluation of three series of homobivalent ligands of clozapine, differing in the length and nature of the spacer and the point of attachment to the pharmacophore. Attachment of the spacer at the N4' position of clozapine yielded a series of homobivalent ligands that displayed spacer-length-dependent gains in affinity and activity for the dopamine D(2) receptor. The 16 and 18 atom spacer bivalent ligands were the highlight compounds, displaying marked low nanomolar receptor binding affinity (1.41 and 1.35 nM, respectively) and functional activity (23 and 44 nM), which correspond to significant gains in affinity (75- and 79-fold) and activity (9- and 5-fold) relative to the original pharmacophore, clozapine. As such these ligands represent useful tools with which to investigate dopamine receptor dimerization and the atypical nature of clozapine.

GPCR oligomerization: contribution to receptor biogenesis

G protein-coupled receptor (GPCR) export to the plasma membrane is considered to follow the default secretory pathway. Several observations indicate that trafficking from the endoplasmic reticulum to the plasma membrane is strictly regulated and involves interactions with specific proteins, such as resident ER chaperones. These interactions help with GPCR folding, but more importantly, they ensure that only properly folded proteins proceed from the ER to the trans-golgi network. The assembly of several GPCRs into a quaternary structure is started in the ER, before cell surface delivery, and helps in the correct expression of the GPCRs. This review will mainly focus on the role of GPCR oligomerization in receptor biogenesis.

Heterodimerization of human apelin and kappa opioid receptors: roles in signal transduction

Apelin receptor (APJ) and kappa opioid receptor (KOR) are members of the family A of G protein-coupled receptors (GPCRs). These two receptors are involved in the central nervous system regulation of the cardiovascular system. Here, we explore the possibility of heterodimerization between APJ and KOR and investigate their novel signal transduction characteristics. Co-immunoprecipitation (Co-IP), co-localization and bioluminescence resonance energy transfer (BRET) assays confirmed the heterodimerization of APJ and KOR. In APJ and KOR stably transfected HEK293 cells, treatment with APJ ligand apelin-13 or KOR ligand dynorphinA (1-13) resulted in higher phosphorylation levels of extracellular-regulated kinases 1/2 (ERK1/2) compared to HEK293 cells transfected with either APJ or KOR alone. The siRNA knockdown of either APJ or KOR receptor in human umbilical vein endothelial cells (HUVECs) resulted in significant reduction of the apelin-13 induced ERK activation. Additionally both forskolin (FSK)-induced cAMP levels and cAMP response element reporter activities were significantly reduced, whereas the serum response element luciferase (SRE-luc) reporter activity was significantly upregulated. Moreover, the ERK phosphorylation and SRE-luc activity were abrogated by the protein kinase C (PKC) inhibitor. These results demonstrate for the first time that human APJ forms a heterodimer with KOR and leads to increased PKC and decreased protein kinase A activity leading to a significant increase in cell proliferation, which may translate to the regulation of diverse biological actions and offers the potential for the development of more selective and tissue specific drug therapies.

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

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

Differential pharmacological actions of methadone and buprenorphine in human embryonic kidney 293 cells coexpressing human μ-opioid and opioid receptor-like 1 receptors

Methadone and buprenorphine are used in maintenance therapy for heroin addicts. In this study, we compared their effects on adenylate cyclase (AC) activity in human embryonic kidney (HEK) 293 cells stably overexpressing human μ-opioid receptor (MOR) and nociceptin/opioid receptor-like 1 receptor (ORL1) simultaneously. After acute exposure, methadone inhibited AC activity; however, buprenorphine induced compromised AC inhibition. When naloxone was introduced after 30 min incubation with methadone, the AC activity was enhanced. This was not observed in the case of buprenorphine. Enhancement of the AC activity was more significant when the incubation lasted for 4 h, and prolonged exposure to buprenorphine elevated the AC activity as well. The removal of methadone and buprenorphine by washing also obtained similar AC superactivation as that revealed by naloxone challenge. The study demonstrated that methadone and buprenorphine exert initially different yet eventually convergent adaptive changes of AC activity in cells coexpressing human MOR and ORL1 receptors.

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.

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

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

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.

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.