Hetero-oligomerization between β2- and β3-Adrenergic Receptors Generates a β-Adrenergic Signaling Unit with Distinct Functional Properties

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.

Dominant negative effects of human follicle-stimulating hormone receptor expression-deficient mutants on wild-type receptor cell surface expression. Rescue of oligomerization-dependent defective receptor expression by using cognate decoys

Current evidence indicates that G protein-coupled receptors form dimers that may affect biogenesis and membrane targeting of the complexed receptors. We here analyzed whether expression-deficient follicle-stimulating hormone receptor (FSHR) mutants exert dominant negative actions on wild-type FSHR cell surface membrane expression. Co-transfection of constant amounts of wild-type receptor cDNA and increasing quantities of mutant (R556A or R618A) FSHR cDNAs progressively decreased agonist-stimulated cAMP accumulation, [(125)I]-FSH binding, and plasma membrane expression of the mature wild-type FSHR species. Co-transfection of wild-type FSHR fragments involving transmembrane domains 5-6, or transmembrane domain 7 and/or the carboxyl-terminus specifically rescued wild-type FSHR expression from the transdominant inhibition by the mutants. Mutant FSHRs also inhibited function of the luteinizing hormone receptor but not that of the thyrotropin receptor or non-related receptors. Defective intracellular transport and/or interference with proper maturation due to formation of misfolded mutant:wild-type receptor complexes may explain the negative effects provoked by the altered FSHRs.

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.

Ligand-directed signalling at beta-adrenoceptors

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

ADP-ribosylation factor 6 modulates adrenergic stimulated lipolysis in adipocytes

ADP-ribosylation factor 6 (Arf6) is a small GTPase that influences membrane receptor trafficking and the actin cytoskeleton. In adipocytes, Arf6 regulates the trafficking of the glucose transporter type 4 (GLUT4) and consequently insulin-stimulated glucose transport. Previous studies also indicated a role of Arf6 in adrenergic receptor trafficking, but whether this contributes to the control of lipolysis in adipocytes remains unknown. This was examined in the present study by using RNA interference (RNAi) and pharmaceutical inhibition in murine cultured 3T3-L1 adipocytes. Downregulation of Arf6 by RNAi impairs isoproterenol-stimulated lipolysis specifically but does not alter triacylglycerol (TAG) synthesis or the insulin signaling pathway. Neither total TAG amounts nor TAG fatty acid compositions are altered. The inhibitory effect on lipolysis is mimicked by dynasore, a specific inhibitor for dynamin, which is required for endocytosis. In contrast, lipolysis triggered by reagents that bypass events at the plasma membrane (e.g., forskolin, isobutylmethylxanthine or 8-bromo-cAMP) is not affected. Moreover, Arf6 protein levels in white adipose tissues are markedly increased in ob/ob mice, whereas they are decreased in obesity-resistant CD36 null mice. These changes reflect at least in part alterations in Arf6 mRNA levels. Collectively, these results suggest a role of the endocytic pathway and its regulation by Arf6 in adrenergic stimulation of lipolysis in adipocytes and potentially in the development of obesity.

The delta(1) opioid receptor is a heterodimer that opposes the actions of the delta(2) receptor on alcohol intake

BACKGROUND

Opioid receptors are clinically important targets for both pain and alcohol abuse. Three opioid receptors have been cloned: mu, delta, and kappa, all of which effect alcohol consumption in animal models. Naltrexone is a nonselective opioid antagonist used for alcoholism, the clinical utility of which is limited by poor efficacy and adverse side effects. Here, we demonstrate that the therapeutic limitations of naltrexone may reflect its poor selectivity. Despite decades of research, several mysteries surround the pharmacology of these receptors. For example, two pharmacologically defined subtypes of delta receptors exist in vivo.

METHODS

Effects of delta subtype-selective ligands (naltrindole, naltriben, tan-67, 7-benzylidene naltrexone) were measured on ethanol consumption in C57BL/6 wildtype and opioid receptor knockout mice using a limited access two-bottle choice paradigm. Affinity and efficacy of naltriben, 7-benzylidenenaltrexone and tan-67 was measured in vitro using radioligand binding and Ca(2+)-mobilizationa assays.

RESULTS

We show that the subtypes of the delta receptor, delta(1) and delta(2), have opposing effects on ethanol consumption. We find that these effects are synergistic; thereby suggesting that delta(1) and delta(2) receptors are distinct molecular targets. Indeed, we provide both in vitro as well as in vivo evidence that the delta(1) subtype is a micro-delta heterodimer and that the delta(2) subtype is most likely a delta homomer.

CONCLUSIONS

Together these data provide insight into the limited actions of the clinically important drug naltrexone and identify a novel target with improved specificity and efficacy for the development of new therapeutics for the treatment of alcoholism.

Nanoscale aggregation of cellular beta2-adrenergic receptors measured by plasmonic interactions of functionalized nanoparticles

Adrenergic signaling that controls the contraction of cardiac myocyte cells and the beating of the mammalian heart is initiated by ligand binding to adrenergic receptors contained in nanoscale multiprotein complexes at the cellular membrane. Here we demonstrate that the surface-enhanced Raman scattering (SERS) of functionalized silver nanoparticles can be used to report on the receptor aggregation state of specifically label beta(2)-adrenergic receptors on mouse cardiac myocyte cells. Furthermore, multimodal imaging including Raman, Rayleigh scattering, scanning electron microscopy, and luminescence imaging was combined to fully characterize the beta(2)-adrenergic receptor-mediated aggregation of silver nanoparticles on the membrane of cardiac myocytes. Scanning electron microscopy analysis reveals distinct SERS active clusters of between 10 and 70 nanoparticles per signaling domain from ultra-high-resolution images of beta(2)-adrenergic receptor clusters on the cellular membrane. These techniques can be generally applied to study the aggregation of other cell surface receptors and explore their distribution on cell surfaces.

Enhanced targeting with heterobivalent ligands

A novel approach to specifically target tumor cells for detection and treatment is the proposed use of heteromultivalent ligands, which are designed to interact with, and noncovalently crosslink, multiple different cell surface receptors. Although enhanced binding has been shown for synthetic homomultivalent ligands, proof of cross-linking requires the use of ligands with two or more different binding moieties. As proof-of-concept, we have examined the binding of synthetic heterobivalent ligands to cell lines that were engineered to coexpress two different G-protein-coupled human receptors, i.e., the human melanocortin 4 receptor (MC4R) expressed in combination with either the human delta-opioid receptor (deltaOR) or the human cholecystokinin-2 receptor (CCK2R). Expression levels of these receptors were characterized by time-resolved fluorescence saturation binding assays using Europium-labeled ligands; Eu-DPLCE, Eu-NDP-alpha-MSH, and Eu-CCK8 for the deltaOR, MC4R, and CCK2R, respectively. Heterobivalent ligands were synthesized to contain a MC4R agonist connected via chemical linkers to either a deltaOR or a CCK2R agonist. In both cell systems, the heterobivalent constructs bound with much higher affinity to cells expressing both receptors, compared with cells with single receptors or to cells where one of the receptors was competitively blocked. These results indicate that synthetic heterobivalent ligands can noncovalently crosslink two unrelated cell surface receptors, making feasible the targeting of receptor combinations. The in vitro cell models described herein will lead to the development of multivalent ligands for target combinations identified in human cancers.

The functional significance behind expressing two IL-8 receptor types on PMN

PMN are critical to innate immunity and are fundamental to antibacterial defense. To localize to sites of infection, PMN possess receptors that detect chemoattractant stimuli elicited at the site, such as chemokines, complement split products, or bioactive lipids. Signaling through these receptors stimulates chemotaxis toward the site of infection but also activates a number of biochemical processes, with the result that PMN kill invading bacteria. PMN possess two receptors, CXCR1 and CXCR2, for the N-terminal ELR motif-containing CXC chemokines, although only two chemokine members bind both receptors and the remainder binding only CXCR2. This peculiar pattern in receptor specificity has drawn considerable interest and investigation into whether signaling through each receptor might impart unique properties on the PMN. Indeed, at first glance, CXCR1 and CXCR2 appear to be functionally redundant; however, there are differences. Considering these proinflammatory activities of activating PMN through chemokine receptors, there has been great interest in the possibility that blocking CXCR1 and CXCR2 on PMN will provide a therapeutic benefit. The literature examining CXCR1 and CXCR2 in PMN function during human and modeled diseases will be reviewed, asking whether the functional differences can be perceived based on alterations in the role PMN play in these processes.

Effects of beta3-adrenergic receptor activation on rat urinary bladder hyperactivity induced by ovariectomy

Voiding dysfunctions, including increased voiding frequency, urgency, or incontinence, are prevalent in the postmenopausal population. Beta(3)-adrenergic receptor (beta(3)AR) agonists, which relax bladder smooth muscle, are being developed to treat these conditions. We utilized the rat ovariectomy (OVX) model to investigate the effect of ovarian hormone depletion on bladder function and the potential for beta(3)AR agonists to treat bladder hyperactivity in this setting. OVX increased voiding frequency and decreased bladder capacity by approximately 25% in awake rats and induced irregular cystometrograms in urethane-anesthetized rats. Reverse transcription-polymerase chain reaction revealed three betaARs subtypes (beta(1,2,3)) in bladder tissue, and immunostaining indicated beta(3)AR localization in urothelium and detrusor. Receptor expression was not different in OVX and SHAM rats. The beta(3)AR agonist selectivity of BRL37344 [(+/-)-(R(*),R(*))-[4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]phenoxy]acetic acid sodium hydrate], TAK-677 [(3-((2R)-(((2R)-(3-chlorophenyl)-2-hydroxyethyl)amino)propyl)-1H-indol-7-yloxy)acetic acid], and FK175 [acetic acid, 2-[[(8S)-8-[[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl]amino]-6,7,8,9-tetrahydro-5H-benzocyclohepten-2-yl]oxy], ethyl ester, hydrochloride] was confirmed by examining the relative potency for elevation of cAMP in CHOK1 cells overexpressing the various rat betaARs. Intravenous injection of each of the beta(3)AR agonists (0.1-500 microg/kg) in anesthetized rats decreased voiding frequency, bladder pressure, and amplitude of bladder contractions. In bladder strips, beta(3)AR agonists (10(-12)-10(-4) M) decreased baseline tone and reduced spontaneous contractions. BRL37344 (5 mg/kg) and TAK-677 (5 mg/kg) injected intraperitoneally in awake rats decreased voiding frequency by 40 to 70%. These effects were not altered by OVX. The results indicate that OVX-induced bladder dysfunction, including decreased bladder capacity and increased voiding frequency, is not associated with changes in beta(3)AR expression or the bladder inhibitory effects of beta(3)AR agonists. This suggests that beta(3)AR agonists should prove effective for the treatment of overactive bladder symptoms in the postmenopausal population.

Brain receptor mosaics and their intramembrane receptor-receptor interactions: molecular integration in transmission and novel targets for drug development

The concept of intramembrane receptor-receptor interactions and evidence for their existence was introduced by Agnati and Fuxe in 1980/81 suggesting the existence of heteromerization of receptors. In 1982, they proposed the existence of aggregates of multiple receptors in the plasma membrane and coined the term receptor mosaics (RM). In this way, cell signaling becomes a branched process beginning at the level of receptor recognition at the plasma membrane where receptors can directly modify the ligand recognition and signaling capacity of the receptors within a RM. Receptor-receptor interactions in RM are classified as operating either with classical cooperativity, when consisting of homomers or heteromers of similar receptor subtypes having the same transmitter, or non-classical cooperativity, when consisting of heteromers. It has been shown that information processing within a RM depends not only on its receptor composition, but also on the topology and the order of receptor activation determined by the concentrations of the ligands and the receptor properties. The general function of RM has also been demonstrated to depend on allosteric regulators (e.g., homocysteine) of the receptor subtypes present. RM as integrative nodes for receptor-receptor interactions in conjunction with membrane associated proteins may form horizontal molecular networks in the plasma membrane coordinating the activity of multiple effector systems modulating the excitability and gene expression of the cells. The key role of electrostatic epitope-epitope interactions will be discussed for the formation of the RM. These interactions probably represent a general molecular mechanism for receptor-receptor interactions and, without a doubt, indicate a role for phosphorylation-dephosphorylation events in these interactions. The novel therapeutic aspects given by the RMs will be discussed in the frame of molecular neurology and psychiatry and combined drug therapy appears as the future way to go.

P2Y6 receptors require an intact cysteinyl leukotriene synthetic and signaling system to induce survival and activation of mast cells

Cysteinyl leukotrienes (cys-LTs) induce inflammatory responses through type 1 (CysLT1R) and type 2 (CysLT2R) cys-LT receptors and activate mast cells in vitro. We previously demonstrated that cys-LTs cross-desensitized IL-4-primed primary human mast cells (hMCs) to stimulation with the nucleotide uridine diphosphate (UDP). We now report that hMCs, mouse bone marrow-derived mast cells (mBMMCs), and the human MC line LAD2 all express UDP-selective P2Y6 receptors that cooperate with CysLT1R to promote cell survival and chemokine generation by a pathway involving reciprocal ligand-mediated cross-talk. Leukotriene (LT) D4, the most potent CysLT1R ligand, and UDP both induced phosphorylation of ERK and prolonged the survival of cytokine-starved hMCs and mBMMCs. ERK activation and cytoprotection in response to either ligand were attenuated by treatment of the cells with a selective P2Y6 receptor antagonist (MRS2578), which did not interfere with signaling through recombinant CysLT1R. Surprisingly, both UDP and LTD4-mediated ERK activation and cytoprotection were absent in mBMMCs lacking CysLT1R and the biosynthetic enzyme LTC4 synthase, implying a requirement for a cys-LT-mediated autocrine loop. In IL-4-primed LAD2 cells, LTD4 induced the generation of MIP-1beta, a response blocked by short hairpin RNA-mediated knockdown of CysLT1R or P2Y6 receptors, but not of CysLT2R. Thus, CysLT1R and P2Y6 receptors, which are coexpressed on many cell types of innate immunity, reciprocally amplify one another's function in mast cells through endogenous ligands.

Studies on the role of the receptor protein motifs possibly involved in electrostatic interactions on the dopamine D1 and D2 receptor oligomerization

We investigated the influence of an epitope from the third intracellular loop (ic3) of the dopamine D(2) receptor, which contains adjacent arginine residues (217RRRRKR222), and an acidic epitope from the C-terminus of the dopamine D(1) receptor (404EE405) on the receptors' localization and their interaction. We studied receptor dimer formation using fluorescence resonance energy transfer. Receptor proteins were tagged with fluorescence proteins and expressed in HEK293 cells. The degree of D(1)-D(2) receptor heterodimerization strongly depended on the number of Arg residues replaced by Ala in the ic3 of D(2)R, which may suggest that the indicated region of ic3 in D(2)R might be involved in interactions between two dopamine receptors. In addition, the subcellular localization of these receptors in cells expressing both receptors D(1)-cyan fluorescent protein, D(2)-yellow fluorescent protein, and various mutants was examined by confocal microscopy. Genetic manipulations of the Arg-rich epitope induced alterations in the localization of the resulting receptor proteins, leading to the conclusion that this epitope is responsible for the cellular localization of the receptor. The lack of energy transfer between the genetic variants of yellow fluorescent protein-tagged D(2)R and cyan fluorescent protein-tagged D(1)R may result from differing localization of these proteins in the cell rather than from the possible role of the D(2)R basic domain in the mechanism of D(1)-D(2) receptor heterodimerization. However, we find that the acidic epitope from the C-terminus of the dopamine D(1) receptor is engaged in the heterodimerization process.

Postischemic brain injury is attenuated in mice lacking the beta2-adrenergic receptor

BACKGROUND

Several beta-adrenergic receptor (betaAR) antagonists have been shown to have neuroprotective effects against cerebral ischemia. However, clenbuterol, a beta(2)AR agonist, was shown to have neuroprotective activity by increasing nerve growth factor expression. We used beta(2)AR knockout mice and a beta(2) selective antagonist to test the effect of loss of beta(2)ARs on outcome from transient focal cerebral ischemia.

METHODS

Ischemia was induced by the intraluminal suture method, for 60 min of middle cerebral artery occlusion (MCAO) followed by 24 h reperfusion. Neurological score was determined at 24 h reperfusion and infarct size was determined by cresyl violet or 2,3,5-triphenyltetrazolium chloride staining. beta(2)AR knockout mice and wild-type congenic FVB/N controls were studied, as well as 2 groups of wild type mice given either ICI 118,551 (0.2 mg/kg) or 0.9% saline intraperitoneally 30 min before MCAO (n = 10 per group). Changes in expression of heat shock protein (Hsp)72 after ischemia were examined by immunohistochemistry and western blots.

RESULTS

Compared with wild type littermates, infarct volume was decreased by 22.3% in beta(2)AR knockout mice (39.7 +/- 10.7 mm(3) vs 51.0 +/- 11.4 mm(3), n = 10/group, P = 0.034) after 60 min of MCAO followed by 24 h reperfusion. Pretreatment with a beta(2)AR selective antagonist, ICI 118,551, also decreased infarct size significantly, by 25.1%, compared with the saline control (32.8 +/- 11.9 mm(3) vs 43.8 +/- 10.3 mm(3), n = 10/group, P = 0.041). Neurological scores were also significantly improved in mice lacking the beta(2)AR or pretreated with ICI 118,551. After cerebral ischemia, total levels of Hsp72 and the number of Hsp72 immunopositive cells were greater in mice lacking beta(2) AR.

CONCLUSION

Brain injury is reduced and neurological outcome improved after MCAO in mice lacking the beta(2)AR, or in wild type mice pretreated with a selective beta(2)AR antagonist. This is consistent with a shift away from prosurvival signaling to prodeath signaling in the presence of beta(2)AR activation in cerebral ischemia. Protection is associated with higher levels of Hsp72, a known antideath protein. The effect of beta(2)AR signaling in the setting of cerebral ischemia is complex and warrants further study.

Analysis of protein-protein interactions using bioluminescence resonance energy transfer

Knowledge of how and when proteins interact in living cells is fundamental to our understanding of cellular biology, and bioluminescence resonance energy transfer (BRET) provides an increasingly popular mechanism for studying these interactions in real time. The technique utilises heterologously expressed fusion proteins linking a bioluminescent donor or complementary acceptor fluorophore to proteins of interest. Resonance energy transfer between these fusion proteins is then detected when they are in close proximity, indicative of association either directly or as part of a complex. BRET is particularly useful for real-time monitoring of ligand-modulated interactions as dynamic changes in protein complex assembly can be observed in a live cell environment.

Analysis of the goldfish Carassius auratus olfactory epithelium transcriptome reveals the presence of numerous non-olfactory GPCR and putative receptors for progestin pheromones

Background

The goldfish (Carassius auratus) uses steroids and prostaglandins as pheromone cues at different stages of the reproductive cycle to facilitate spawning synchronization. Steroid progestin pheromone binding has been detected in goldfish olfactory membranes but the receptors responsible for this specific binding remain unknown. In order to shed some light on the olfactory epithelium transcriptome and search for possible receptor candidates a large set of EST from this tissue were analysed and compared to and combined with a similar zebrafish (Danio rerio) resource.

Results

We generated 4,797 high quality sequences from a normalized cDNA library of the goldfish olfactory epithelium, which were clustered in 3,879 unique sequences, grouped in 668 contigs and 3,211 singletons. BLASTX searches produced 3,243 significant (E-value < e^-10) hits and Gene Ontology (GO) analysis annotated a further 1,223 of these genes (37.7%). Comparative analysis with zebrafish olfactory epithelium ESTs revealed 1,088 identical unigenes. The transcriptome size of both species was estimated at about 16,400 unigenes, based on the proportion of genes identified involved in Glucose Metabolic Process. Of 124 G-protein coupled receptors identified in the olfactory epithelium of both species, 56 were olfactory receptors. Beta and gamma membrane progestin receptors were also isolated by subcloning of RT-PCR products from both species and an olfactory epithelium specific splice form identified.

Conclusion

The high similarity between the goldfish and zebrafish olfactory systems allowed the creation of a 'cyprinid' olfactory epithelium library estimated to represent circa 70% of the transcriptome. These results are an important resource for the identification of components of signalling pathways involved in olfaction as well as putative targets for pharmacological and histochemical studies. The possible function of the receptors identified in the olfactory system is described. Moreover, the role of olfactory epithelium specific isoforms of classical membrane progestin receptor genes as candidates for preovulatory pheromone sensing is discussed.

Dimerization in the absence of higher-order oligomerization of the G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been proposed to affect receptor function and regulation; however, little is known about the molecular nature of such complexes. We previously utilized bioluminescence resonance energy transfer (BRET) to demonstrate that the prototypic Family B secretin receptor can form oligomers. We now explore the order of oligomerization present utilizing unique bimolecular fluorescence complementation and energy transfer techniques. The non-fluorescent carboxyl-terminal and amino-terminal halves of yellow fluorescent protein (YFP) were fused to the carboxyl terminus of the secretin receptor. These constructs bound secretin normally and signaled in response to secretin like wild type receptor. When co-expressed on COS cells, these constructs physically interacted to yield typical YFP fluorescence in biosynthetic compartments and at the plasma membrane, reflecting receptor homo-dimerization. However, the addition of another potential partner in form of Rlu- or CFP-tagged secretin receptor yielded no significant BRET or FRET signal, respectively, under conditions in which intact YFP-tagged secretin receptor yielded such a signal. Absence of higher-order receptor oligomers was further confirmed using saturation BRET techniques. Absence of significant resonance transfer to the secretin receptor homo-dimer was true for carboxyl-terminally-tagged secretin receptor, as well as for receptor incorporating the transfer partner into each of the three distinct intracellular loop domains. These results suggest that the secretin receptor can exist only as a structurally-specific homo-dimer, without being present as higher-order oligomers.

CXCR4 dimerization and beta-arrestin-mediated signaling account for the enhanced chemotaxis to CXCL12 in WHIM syndrome

WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome is an immune deficiency linked in many cases to heterozygous mutations causing truncations in the cytoplasmic tail of CXC chemokine receptor 4 (CXCR4). Leukocytes expressing truncated CXCR4 display enhanced responses to the receptor ligand CXCL12, including chemotaxis, which likely impair their trafficking and contribute to the immunohematologic clinical manifestations of the syndrome. CXCR4 desensitization and endocytosis are dependent on beta-arrestin (betaarr) recruitment to the cytoplasmic tail, so that the truncated CXCR4 are refractory to these processes and so have enhanced G protein-dependent signaling. Here, we show that the augmented responsiveness of WHIM leukocytes is also accounted for by enhanced betaarr2-dependent signaling downstream of the truncated CXCR4 receptor. Indeed, the WHIM-associated receptor CXCR4(1013) maintains association with betaarr2 and triggers augmented and prolonged betaarr2-dependent signaling, as revealed by ERK1/2 phosphorylation kinetics. Evidence is also provided that CXCR4(1013)-mediated chemotaxis critically requires betaarr2, and disrupting the SHSK motif in the third intracellular loop of CXCR4(1013) abrogates betaarr2-mediated signaling, but not coupling to G proteins, and normalizes chemotaxis. We also demonstrate that CXCR4(1013) spontaneously forms heterodimers with wild-type CXCR4. Accordingly, we propose a model where enhanced functional interactions between betaarr2 and receptor dimers account for the altered responsiveness of WHIM leukocytes to CXCL12.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Cloning, expression, and functional characterization of relaxin receptor (leucine-rich repeat-containing g protein-coupled receptor 7) splice variants from human fetal membranes

The relaxin receptor [leucine-rich repeat-containing G protein-coupled receptor 7 (LGR7)] belongs to the leucine-rich repeat containing G protein-coupled receptors subgroup C. Three new LGR7 splice variants have been cloned from the human fetal membranes and shown to be truncated versions of the full-length receptor, encoded by different lengths of the extracellular domain. The expression of their mRNAs has been confirmed by both qualitative and quantitative PCR and shown to be higher in the chorion and decidua before, compared with after, spontaneous labor. When HEK293 cells were transfected with each LGR7 splice variant, their proteins were retained within the endoplasmic reticulum. However, the protein for the shortest variant was also secreted into the medium. We have characterized the intracellular functions and effects of these LGR7 variants on the function of the wild-type (WT)-LGR7. In coexpression studies, each splice variant interacted directly with the WT-LGR7 and exerted a dominant-negative effect on cAMP accumulation by the WT-LGR7 after relaxin treatment. This interaction resulted in the sequestration of the WT-LGR7 inside the cells by down-regulation of its maturation and cell surface delivery. The constitutive homodimerization of WT-LGR7 has been shown here to take place in the endoplasmic reticulum, and the presence of any one of the splice variants decreased this by the formation of heterodimers with the WT-LGR7, supporting the view that homodimerization is a prerequisite for receptor trafficking to the cell surface. These data suggest that the dominant-negative effects of the LGR7 splice variants expressed in the chorion and decidua could be functionally significant in the peripartal period by inhibiting the function of WT-LGR7 and dampening the responsiveness of these tissues to endogenous relaxin.

Hetero-oligomerization of the P2Y11 receptor with the P2Y1 receptor controls the internalization and ligand selectivity of the P2Y11 receptor

Nucleotides signal through purinergic receptors such as the P2 receptors, which are subdivided into the ionotropic P2X receptors and the metabotropic P2Y receptors. The diversity of functions within the purinergic receptor family is required for the tissue-specificity of nucleotide signalling. In the present study, hetero-oligomerization between two metabotropic P2Y receptor subtypes is established. These receptors, P2Y1 and P2Y11, were found to associate together when co-expressed in HEK293 cells. This association was detected by co-pull-down, immunoprecipitation and FRET (fluorescence resonance energy transfer) experiments. We found a striking functional consequence of the interaction between the P2Y11 receptor and the P2Y1 receptor where this interaction promotes agonist-induced internalization of the P2Y11 receptor. This is remarkable because the P2Y11 receptor by itself is not able to undergo endocytosis. Co-internalization of these receptors was also seen in 1321N1 astrocytoma cells co-expressing both P2Y11 and P2Y1 receptors, upon stimulation with ATP or the P2Y1 receptor-specific agonist 2-MeS-ADP. 1321N1 astrocytoma cells do not express endogenous P2Y receptors. Moreover, in HEK293 cells, the P2Y11 receptor was found to functionally associate with endogenous P2Y1 receptors. Treatment of HEK293 cells with siRNA (small interfering RNA) directed against the P2Y1 receptor diminished the agonist-induced endocytosis of the heterologously expressed GFP-P2Y11 receptor. Pharmacological characteristics of the P2Y11 receptor expressed in HEK293 cells were determined by recording Ca2+ responses after nucleotide stimulation. This analysis revealed a ligand specificity which was different from the agonist profile established in cells expressing the P2Y11 receptor as the only metabotropic nucleotide receptor. Thus the hetero-oligomerization of the P2Y1 and P2Y11 receptors allows novel functions of the P2Y11 receptor in response to extracellular nucleotides.

Quantification of dynamic protein complexes using Renilla luciferase fragment complementation applied to protein kinase A activities in vivo

The G protein-coupled receptor (GPCR) superfamily represents the most important class of pharmaceutical targets. Therefore, the characterization of receptor cascades and their ligands is a prerequisite to discovering novel drugs. Quantification of agonist-induced second messengers and downstream-coupled kinase activities is central to characterization of GPCRs or other pathways that converge on GPCR-mediated signaling. Furthermore, there is a need for simple, cell-based assays that would report on direct or indirect actions on GPCR-mediated effectors of signaling. More generally, there is a demand for sensitive assays to quantify alterations of protein complexes in vivo. We describe the development of a Renilla luciferase (Rluc)-based protein fragment complementation assay (PCA) that was designed specifically to investigate dynamic protein complexes. We demonstrate these features for GPCR-induced disassembly of protein kinase A (PKA) regulatory and catalytic subunits, a key effector of GPCR signaling. Taken together, our observations show that the PCA allows for direct and accurate measurements of live changes of absolute values of protein complex assembly and disassembly as well as cellular imaging and dynamic localization of protein complexes. Moreover, the Rluc-PCA has a sufficiently high signal-to-background ratio to identify endogenously expressed Galpha(s) protein-coupled receptors. We provide pharmacological evidence that the phosphodiesterase-4 family selectively down-regulates constitutive beta-2 adrenergic- but not vasopressin-2 receptor-mediated PKA activities. Our results show that the sensitivity of the Rluc-PCA simplifies the recording of pharmacological profiles of GPCR-based candidate drugs and could be extended to high-throughput screens to identify novel direct modulators of PKA or upstream components of GPCR signaling cascades.

Emerging concepts of guanine nucleotide-binding protein-coupled receptor (GPCR) function and implications for high throughput screening

Guanine nucleotide binding protein (G protein) coupled receptors (GPCRs) comprise one of the largest families of proteins in the human genome and are a target for 40% of all approved drugs. GPCRs have unique structural motifs that allow them to interact with a wide and diverse series of extracellular ligands, as well as intracellular proteins, G proteins, receptor activity-modifying proteins, arrestins, and indeed other receptors. This distinctive structure has led to numerous efforts to discover drugs against GPCRs with targeted therapeutic uses. Such "designer" drugs currently include allosteric regulators, inverse agonists, and drugs targeting hetero-oligomeric complexes. Moreover, the large family of orphan GPCRs provides a rich and novel field of targets to discover drugs with unique therapeutic properties. The numerous technologies to discover GPCR drugs have also greatly advanced over the years, facilitating compound screening against known and orphan GPCRs, as well as in the identification of unique designer GPCR drugs. Indeed, high throughput screening (HTS) technologies employing functional cell-based approaches are now widely used. These include measurement of second messenger accumulation such as cyclic AMP, calcium ions, and inositol phosphates, as well as mitogen-activated protein kinase activation, protein-protein interactions, and GPCR oligomerization. This review focuses on how the improved understanding of the molecular pharmacology of GPCRs, coupled with a plethora of novel HTS technologies, is leading to the discovery and development of an entirely new generation of GPCR-based therapeutics.

CysLT2 receptors interact with CysLT1 receptors and down-modulate cysteinyl leukotriene dependent mitogenic responses of mast cells

Cysteinyl leukotrienes (cys-LTs) induce inflammation through 2 G protein-coupled receptors (GPCRs), CysLT(1) and CysLT(2), which are coexpressed by most myeloid cells. Cys-LTs induce proliferation of mast cells (MCs), transactivate c-Kit, and phosphorylate extracellular signal-regulated kinase (ERK). Although MCs express CysLT(2), their responses to cys-LTs are blocked by antagonists of CysLT(1). We demonstrate that CysLT(2) interacts with CysLT(1), and that knockdown of CysLT(2) increases CysLT(1) surface expression and CysLT(1)-dependent proliferation of cord blood-derived human MCs (hMCs). Cys-LT-mediated responses were absent in MCs from mice lacking CysLT(1) receptors, but enhanced by the absence of CysLT(2) receptors. CysLT(1) and CysLT(2) receptors colocalized to the plasma membranes and nuclei of a human MC line, LAD2. Antibody-based fluorescent lifetime imaging microscopy confirmed complexes containing both receptors based on fluorescence energy transfer. Negative regulation of CysLT(1)-induced mitogenic signaling responses of MCs by CysLT(2) demonstrates physiologically relevant functions for GPCR heterodimers on primary cells central to inflammation.

Unraveling G protein-coupled receptor endocytosis pathways using real-time monitoring of agonist-promoted interaction between beta-arrestins and AP-2

The most widely studied pathway underlying agonist-promoted internalization of G protein-coupled receptors (GPCRs) involves beta-arrestin and clathrin-coated pits. However, both beta-arrestin- and clathrin-independent processes have also been reported. Classically, the endocytic routes are characterized using pharmacological inhibitors and various dominant negative mutants, resulting sometimes in conflicting results and interpretational difficulties. Here, taking advantage of the fact that beta-arrestin binding to the beta2 subunit of the clathrin adaptor AP-2 (beta2-adaptin) is needed for the beta-arrestin-mediated targeting of GPCRs to clathrin-coated pits, we developed a bioluminescence resonance energy transfer-based approach directly assessing the molecular steps involved in the endocytosis of GPCRs in living cells. For 10 of the 12 receptors tested, including some that were previously suggested to internalize via clathrin-independent pathways, agonist stimulation promoted beta-arrestin 1 and 2 interaction with beta2-adaptin, indicating a beta-arrestin- and clathrin-dependent endocytic process. Detailed analyses of beta-arrestin interactions with both the receptor and beta2-adaptin also allowed us to demonstrate that recruitment of beta-arrestins to the receptor and the ensuing conformational changes are the leading events preceding AP-2 engagement and subsequent clathrin-mediated endocytosis. Among the receptors tested, only the endothelin A and B receptors failed to promote interaction between beta-arrestins and beta2-adaptin. However, both receptors recruited beta-arrestins upon agonist stimulation, suggesting a beta-arrestin-dependent but clathrin-independent route of internalization for these two receptors. In addition to providing a new tool to dissect the molecular events involved in GPCR endocytosis, the bioluminescence resonance energy transfer-based beta-arrestin/beta2-adaptin interaction assay represents a novel biosensor to assess receptor activation.

Modulation of TSHR signaling by posttranslational modifications

Posttranslational modifications of seven transmembrane receptors (7TMRs) affect their function to a large extent. Many studies of glycosylation or phosphorylation of 7TMRs have shown that these modifications influence the cell-surface expression or signaling of the receptor. Recently, other types of posttranslational modifications of the thyrotropin-stimulating hormone receptor (TSHR) have been characterized, including sialylation and dimerization. Increased TSHR sialylation results in increased TSHR cell-surface expression. Furthermore, TSHR oligomerization and the probable modification of TSHR signaling in lipid rafts require further clarification with regard to their functional consequences. In addition to its known coupling to Galphas and Galphaq, and possibly other G proteins, the TSHR also couples to further signaling pathways, such as the mitogen-activated protein kinase (MAPK) pathway, which involves G-protein-coupled receptor kinases (GRKs) and arrestins. We discuss these emerging new findings and their implications for signaling of the TSHR.

G protein-coupled receptor dimerisation: Molecular basis and relevance to function

The belief that G protein-coupled receptors exist and function as monomeric, non-interacting species has been largely supplanted in recent years by evidence, derived from a range of approaches, that indicate they can form dimers and/or higher-order oligomeric complexes. Key roles for receptor homo-dimerisation include effective quality control of protein folding prior to plasma membrane delivery and interactions with hetero-trimeric G proteins. Growing evidence has also indicated the potential for many co-expressed G protein-coupled receptors to form hetero-dimers/oligomers. The relevance of this to physiology and function is only beginning to be unravelled but may offer great potential for more selective therapeutic intervention.

Opsin oligomerization in a heterologous cell system

Using bioluminescence resonance energy transfer (BRET) we studied opsin oligomerization in heterologous expression systems and quantitatively assessed its oligomerization state. BRET2 saturation and competition experiments were performed with live COS-7 cells expressing Rluc-and GFP2-tagged receptor constructs. BRET2 saturation curves obtained were hyperbolic, and the calculated oligomerization state (N = 1 for dimers) suggested that opsin (N = 1.34 +/- 0.25) forms higher oligomers. Very high BRET2 values obtained for the opsin homo-dimer pair indicated a large energy transfer efficiency (E) and for cases where E >> 0.1 a modified saturation curve was proposed. The existence of homo-dimer complexes was additionally supported by competition assay results and was also observed in HEK-293 cells. Furthermore, evidence was provided for homo-and hetero-dimerization of family A (beta2-adrenergic) and B (gastric inhibitory polypeptide, GIP) receptors. In summary, these experiments demonstrate homo-and hetero-dimerization for opsin, beta 2-adrenergic, and GIP receptors.

Each rhodopsin molecule binds its own arrestin

Arrestins (Arrs) are ubiquitous regulators of the most numerous family of signaling proteins, G protein-coupled receptors. Two models of the Arr-receptor interaction have been proposed: the binding of one Arr to an individual receptor or to two receptors in a dimer. To determine the binding stoichiometry in vivo, we used rod photoreceptors where rhodopsin (Rh) and Arr are expressed at comparably high levels and where Arr localization in the light is determined by its binding to activated Rh. Genetic manipulation of the expression of both proteins shows that the maximum amount of Arr that moves to the Rh-containing compartment exceeds 80%, but not 100%, of the molar amount of Rh present. In vitro experiments with purified proteins confirm that Arr "saturates" Rh at a 1:1 ratio. Thus, a single Rh molecule is necessary and sufficient to bind Arr. Remarkable structural conservation among receptors and Arrs strongly suggests that all Arr subtypes bind individual molecules of their cognate receptors.

Dimerization between vasopressin V1b and corticotropin releasing hormone type 1 receptors

1. Increasing evidence indicates that guanyl protein coupled receptors (GPCRs), including members of the vasopressin (VP) receptor family can act as homo- and heterodimers. Regulated expression and interaction of pituitary VP V1b receptor (V1bR) and corticotropin releasing hormone receptor type 1 (CRHR1) are critical for hypothalamic pituitary adrenal (HPA) axis adaptation, but it is unknown whether this involves physical interaction between these receptors.2. Bioluminescence resonance energy transfer (BRET) experiments using V1bR and CRHR1 fused to either Renilla luciferase (Rluc) or yellow fluorescent protein (YFP) at the N-terminus, but not the carboxyl-terminus, revealed specific interaction (BRET(50) = 0.39 +/- 0.08, V1bR) that was inhibited by untagged V1b or CRHR1 receptors, suggesting homo- and heterodimerization. The BRET data were confirmed by coimmunoprecipitation experiments using fully bioactive receptors tagged at the aminoterminus with c-myc and Flag epitopes, demonstrating specific homodimerization of the V1b receptor and heterodimerization of the V1b receptor with CRHR1 receptors.3. Heterodimerization between V1bR and CRHR1 is not ligand dependent since stimulation with CRH and AVP had no effect on coimmunoprecipitation. In membranes obtained from cells cotransfected with CRHR1 and V1bR, incubation with the heterologous nonpeptide antagonist did not alter the binding affinity or capacity of the receptor.4. The data demonstrate that V1bR and CRHR1 can form constitutive homo- and heterodimers and suggests that the heterodimerization does not influence the binding properties of these receptors.

Current issues with beta2-adrenoceptor agonists: pharmacology and molecular and cellular mechanisms

Beta2-adrenoceptors are widely, almost ubiquitously, expressed. Activation of these receptors on bronchial smooth muscle by short- and long-acting beta2-adrenoceptor agonists causes bronchodilation. Here, the beta2-adrenoceptor is linked by the G protein, Gs, to adenylyl cyclase, which increases cyclic adenosine monophosphate (cAMP), thus activating protein kinase A, which affects calcium levels and reduces the efficiency of myosin light-chain kinase, causing relaxation. Activation also entrains numerous acute and longer term downregulation responses affecting the number, location, and net efficiency of signaling of the receptor. Synthetic beta2-agonists are all "partial agonists," incompletely able to optimally stimulate cAMP signal transduction. However, compared with some cells (such as mast cells) involved in exercise- induced asthma induction, airway smooth muscle is privileged in that transduction efficiency is intrinsically high and the tissue is very resistant to complete downregulation. Glucocorticosteroids have broadly beneficial interactions with beta2-adrenoceptors. Researchers have recently discovered that the beta2-adrenoceptor may function as a homodimer and that it can form heterodimers with both the beta1- and beta3-adrenoceptors, and possibly other receptors. This further complicates interpretation of the effect of beta2-adrenoceptor polymorphisms, but it is unknown whether this occurs in humans in vivo. Researchers have known for some time that strong contraction involving receptors coupled to the Gq G protein (e.g., cholinergic and leukotriene receptors via negative biochemical crosstalk), virus infection (via uncoupling), and inflammation (via kinases) can impair relaxation. Most recently, researchers have discovered that the beta2-adrenoceptor can also send potentially adverse signals after "atypical coupling" to Gq rather than Gs. The clinical implications of these uncouplings, crosstalk, and atypical coupling possibilities are not well-understood.

Physical association between neuropeptide FF and micro-opioid receptors as a possible molecular basis for anti-opioid activity

Neuropeptide FF (NPFF) modulates the opioid system by exerting functional anti-opioid activity on neurons, the mechanism of which is unknown. By using a model of SH-SY5Y cells, we recently postulated that anti-opioid activity likely takes place upstream from the signaling cascade, suggesting that NPFF receptors could block opioid receptors by physical interaction. In the present study, fluorescence techniques were used to monitor the physical association and the dynamic of NPFF2 and micro-opioid (MOP) receptors tagged with variants of the green fluorescent protein. Importantly, cyan fluorescent protein-tagged NPFF2 receptors retained their capacity to antagonize opioid receptors. Fluorescence resonance energy transfer (FRET) and coimmunoprecipitation studies indicate that NPFF and MOP receptors are close enough to generate a basal FRET signal. The opioid agonist Tyr-D-Ala-Gly-NMe-Phe-Gly-ol disrupts by 20-30% this FRET signal, mainly because it concomitantly induces 40% internalization of receptors. In contrast, the NPFF analog 1DMe significantly increases by 10-15% the basal FRET signal, suggesting an association between both receptors. In addition, 1DMe reduces, by half, MOP receptor internalization, indicating that, besides a functional blockade of opioid receptors, the NPFF analog also inhibits their internalization. Finally, as a first report showing the modulation of the mobility of a G-protein-coupled receptor by another one, fluorescence recovery after photobleaching analysis reveals that 1DMe modifies the lateral diffusion of MOP receptors in the cell membrane, changing them from a confined to a freely diffusing state. By promoting NPFF-MOP receptor heteromerization, 1DMe could disrupt the domain organization of MOP receptors in the membrane, resulting in a reduction of opioid response.

Orphan seven transmembrane receptor screening

Drug discovery has successfully exploited the superfamily of seven transmembrane receptors (7TMR), with over 35% of clinically marketed drugs targeting them. However, it is clear that there remains an undefined potential within this protein family for successful drugs of the future. The human genome sequencing project identified approximately 720 genes that belong to the 7TMR superfamily. Around half of these genes encode sensory receptors, while the other half are potential drug targets. Natural ligands have been identified for approximately 215 of these, leaving 155 receptors classified as orphan 7TMRs having no known ligand. Deorphanisation of these receptors by identification of natural ligands has been the traditional method enabling target validation by use of these ligands as tools to define biological relevance and disease association. Such ligands have been paired with their cognate receptor experimentally by screening of small molecule and peptide ligands, reverse pharmacology and the use of bioinformatics to predict candidate ligands. In this manuscript, we review the methodologies developed for the identification of ligands at orphan 7TMRs and exemplify these with case studies.

A rigorous experimental framework for detecting protein oligomerization using bioluminescence resonance energy transfer

Bioluminescence resonance energy transfer (BRET), which relies on nonradiative energy transfer between luciferase-coupled donors and GFP-coupled acceptors, is emerging as a useful tool for analyzing the quaternary structures of cell-surface molecules. Conventional BRET analyses are generally done at maximal expression levels and single acceptor/donor ratios. We show that under these conditions substantial energy transfer arises from random interactions within the membrane. The dependence of BRET efficiency on acceptor/donor ratio at fixed surface density, or expression level at a defined acceptor/donor ratio, can nevertheless be used to correctly distinguish between well-characterized monomeric and oligomeric proteins, including a very weak dimer. The pitfalls associated with the nonrigorous treatment of BRET data are illustrated for the case of G protein-coupled receptors (GPCRs) proposed to form homophilic and/or mixed oligomers on the basis of previous, conventional BRET experiments.

Superoxide production in human neutrophils is enhanced by treatment with transmembrane peptides derived from human formyl peptide receptor

Formyl peptide receptor (FPR) mediates a number of important host defense functions. Although studies have been performed on the ligand binding site of FPR, FPR dynamic behavior such as receptor dimerization on the cell surface remains unknown. Recently, peptides derived from the transmembrane (TM) domains of GPCRs were shown to disrupt dimer formation by receptors and to result in specific regulation of receptor function. To reveal the function of FPR TM domains, hFPRTM peptides derived from FPR were synthesized, and their biological activities were evaluated with human neutrophils. Synthetic peptides did not exhibit agonistic or antagonistic activity toward superoxide anion production. However, Neutrophils treated with hFPRTM4 produced 4-fold superoxide anion compared with untreated cells when stimulated with FPR agonist fMLP. Short peptide fragments derived from the fourth TM region of FPR did not enhance superoxide anion production, which suggests that hFPRTM4 did not behave as a ligand. CD and fluorescence spectra suggested that hFPRTM peptides were inserted into the membrane. The addition of hFPRTM4 increased the intracellular calcium concentration, which meant the peptide activated some membrane protein on the cell surface. The present study suggests that the fourth TM domain of FPR has a function related to a priming effect.

The natural inverse agonist agouti-related protein induces arrestin-mediated endocytosis of melanocortin-3 and -4 receptors

Agouti-related protein (Agrp), one of the two naturally occurring inverse agonists known to inhibit G protein-coupled receptor activity, regulates energy expenditure by decreasing basal and blocking agonist-promoted melanocortin receptor (MCR) signaling. Here we report that, in addition to its inverse agonistic activities, Agrp exhibits agonistic properties on the endocytosis pathway of melanocortin receptors. Sustained exposure of human embryonic kidney 293 cells to Agrp induced endocytosis of the MC3R or the MC4R. The extent and kinetics of Agrp-promoted MCR endocytosis were similar to the endocytosis induced by melanocortins. Using the bioluminescence resonance energy transfer technique, we further showed that after binding of Agrp both MCRs interacted with beta-arrestins. In line with this observation, in COS-7 cells co-expression of beta-arrestins enhanced Agrp-induced MCR endocytosis, whereas in human embryonic kidney 293 cells co-transfection of beta-arrestin-specific small interference RNAs diminished Agrp-promoted endocytosis. This new regulatory mechanism was likewise detectable in a cell line derived from murine hypothalamic neurons endogenously expressing MC4R, pointing to the physiological relevance of Agrp-promoted receptor endocytosis. In conclusion, we demonstrated that Agrp does not solely act by directly blocking MCR signaling but also by reducing the amount of MCR molecules accessible to melanocortins at the cell surface. This beta-arrestin-dependent mechanism reveals a new aspect of MCR signaling in particular and refines the concept of G protein-coupled receptor antagonism in general.

Heterodimerization and surface localization of G protein coupled receptors

G protein coupled receptors (GPCRs) are one of the largest human gene families, and are targets for many important therapeutic drugs. Over the last few years, there has been a major paradigm shift in our understanding of how these receptors function. Formerly, GPCRs were thought to exist as monomers that, upon agonist occupation, activated a heterotrimeric G protein to alter the concentrations of specific second messengers. Until recently, this relatively linear cascade has been the standard paradigm for signaling by these molecules. However, it is now clear that this model is not adequate to explain many aspects of GPCR function. We now know that many, if not most, GPCRs form homo- and/or hetero-oligomeric complexes and interact directly with intracellular proteins in addition to G proteins. It now appears that many GPCRs may not function independently, but might more accurately be described as subunits of large multi-protein signaling complexes. These observations raise many important new questions; some of which include: (1) how many functionally and pharmacologically distinct receptor subtypes exist in vivo? (2) Which GPCRs physically associate, and in what stochiometries? (3) What are the roles of individual subunits in binding ligand and activating responses? (4) Are the pharmacological or signaling properties of GPCR heterodimers different from monomers? Since these receptors are the targets for a large number of clinically useful compounds, such information is likely to be of direct therapeutic importance, both in understanding how existing drugs work, but also in discovering novel compounds to treat disease.

Probing the activation-promoted structural rearrangements in preassembled receptor-G protein complexes

Activation of heterotrimeric G proteins by their cognate seven transmembrane domain receptors is believed to involve conformational changes propagated from the receptor to the G proteins. However, the nature of these changes remains unknown. We monitored the conformational rearrangements at the interfaces between receptors and G proteins and between G protein subunits by measuring bioluminescence resonance energy transfer between probes inserted at multiple sites in receptor-G protein complexes. Using the data obtained for the alpha(2A)AR-G alpha(i1) beta1gamma2 complex and the available crystal structures of G alpha(i1) beta1gamma2, we propose a model wherein agonist binding induces conformational reorganization of a preexisting receptor-G protein complex, leading the G alpha-G betagamma interface to open but not dissociate. This conformational change may represent the movement required to allow nucleotide exit from the G alpha subunit, thus reflecting the initial activation event.

Adrenoceptors and signal transduction in neurons

The adrenergic system is an essential regulator of neuronal, endocrine, cardiovascular, vegetative, and metabolic functions. The endogenous catecholamines epinephrine and norepinephrine activate G-protein-coupled receptors to transmit their signal across the plasma membrane. These adrenoceptors can be divided into three different groups: the alpha(1)-receptors (alpha(1A), alpha(1B), alpha(1D)), alpha(2)-receptors (alpha(2A), alpha(2B), alpha(2C)), and beta-receptors (beta(1), beta(2), beta(3)). This review summarizes recent findings in the field of adrenoceptor signaling in neurons and includes a discussion of receptor-associated proteins, receptor dimerization, subcellular trafficking, and fluorescence optical methods for studying the kinetics of adrenergic signaling. Spatio-temporal imaging may become an important future tool for identifying the physiological significance of these complex signaling mechanisms in vivo. Gene-targeted mouse models carrying deletions in alpha(2)-adrenoceptor have provided detailed insights into specific neuronal functions of the three alpha(2)-receptor subtypes.

Fluorescence Studies Reveal Heterodimerization of Dopamine D1 and D2 Receptors in the Plasma Membrane

Evidence for hetero-oligomerization has recently been provided for various G protein-coupled receptors. In this paper, we have studied the possibility that dopamine D(1) and D(2) receptors physically interact with each other. Human dopamine D(1) and D(2) receptors were fluorescently tagged with derivatives of green fluorescence protein and transiently coexpressed in the membrane of human embryonic kidney 293 cells. Using qualitative fluorescence spectroscopy, as well as quantitative Förster resonance energy transfer (FRET) analysis, performed in a single cell by confocal microscopy and fluorescence lifetime microscopy, we show that dopamine D(1) and D(2) receptors can form hetero-oligomers in the plasma membrane. The degree of receptor protein-protein interaction is significantly enhanced by concomitant addition of D(1) and D(2) receptor subtype-specific agonists. Our investigations extend biochemical and electrophysiological studies and give insights into the regulation and synergistic mode of operation of dopamine receptors.

Airway smooth muscle prostaglandin-EP1 receptors directly modulate beta2-adrenergic receptors within a unique heterodimeric complex

Multiple and paradoxical effects of airway smooth muscle (ASM) 7-transmembrane-spanning receptors activated during asthma, or by treatment with bronchodilators such as beta(2)-adrenergic receptor (beta(2)AR) agonists, indicate extensive receptor crosstalk. We examined the signaling of the prostanoid-EP(1) receptor, since its endogenous agonist prostaglandin E(2) is abundant in the airway, but its functional implications are poorly defined. Activation of EP(1) failed to elicit ASM contraction in mouse trachea via this G(alphaq)-coupled receptor. However, EP(1) activation markedly reduced the bronchodilatory function of beta(2)AR agonist, but not forskolin, indicating an early pathway interaction. Activation of EP(1) reduced beta(2)AR-stimulated cAMP in ASM but did not promote or augment beta(2)AR phosphorylation or alter beta(2)AR trafficking. Bioluminescence resonant energy transfer showed EP(1) and beta(2)AR formed heterodimers, which were further modified by EP(1) agonist. In cell membrane [(35)S]GTPgammaS binding studies, the presence of the EP(1) component of the dimer uncoupled beta(2)AR from G(alphas), an effect accentuated by EP(1) agonist activation. Thus alone, EP(1) does not appear to have a significant direct effect on airway tone but acts as a modulator of the beta(2)AR, altering G(alphas) coupling via steric interactions imposed by the EP(1):beta(2)AR heterodimeric signaling complex and ultimately affecting beta(2)AR-mediated bronchial relaxation. This mechanism may contribute to beta-agonist resistance found in asthma.

Receptor heterodimerization: a new level of cross-talk

Most G protein-coupled receptors (GPCRs) probably exist as homodimers, but it is increasingly recognized that GPCRs may also dimerize with other types of GPCRs and that this physical interaction may affect the function of either receptor. A study in this issue of the JCI demonstrates how heterodimerization between prostaglandin E receptors and beta(2)-adrenergic receptors (beta(2)ARs) in airway smooth muscle cells results in uncoupling of beta(2)ARs and a diminished bronchodilator response to beta(2)AR agonists (see the related article beginning on page 1400). This illustrates what we believe to be a novel mechanism of receptor cross-talk and highlights the potential importance of GPCR heterodimerization in diseases such as asthma and how this could lead to the development of more specific therapies in the future.

Alpha2C-adrenergic receptors exhibit enhanced surface expression and signaling upon association with beta2-adrenergic receptors

The alpha(2C)-adrenergic receptor (alpha(2C)AR) is known to be poorly trafficked to the cell surface when expressed in a variety of cell types. We tested the hypothesis that the surface expression and signaling of alpha(2C)AR might be enhanced by heterodimerization with other G protein-coupled receptors (GPCRs). Cotransfection of alpha(2C)AR with more than 25 related GPCRs revealed that only coexpression with the beta(2)-adrenergic receptor (beta(2)AR) increased the surface localization of alpha(2C)AR in human embryonic kidney-293 cells. Coimmunoprecipitation of alpha(2C)AR with beta(2)AR confirmed a physical interaction between the two receptors. Confocal microscopy studies demonstrated that alpha(2C)AR expressed alone was mainly intracellular, whereas alpha(2C)AR coexpressed with beta(2)AR was predominantly localized to the plasma membrane. Ligand binding studies revealed a significant increase in alpha(2C)AR binding sites upon coexpression with beta(2)AR, with no apparent change in affinity for alpha(2)AR ligands. Functional assays with the alpha(2)AR-specific agonist brimonidine (UK 14,304) revealed that coexpression of beta(2)AR with alpha(2C)AR enhanced alpha(2C)AR-mediated activation of extracellular signal-regulated kinase 1/2. Furthermore, analyses of agonist-promoted receptor endocytosis demonstrated enhanced alpha(2C)AR internalization in response to alpha(2)AR agonists when alpha(2C)AR and beta(2)AR were coexpressed. In addition, substantial cointernalization of alpha(2C)AR in response to betaAR agonists was observed when alpha(2C)AR was coexpressed with beta(2)AR. These data reveal that alpha(2C)AR can interact with beta(2)AR in cells in a manner that regulates alpha(2C)AR surface expression, internalization, and functionality.

G-protein-coupled receptor heterodimers: pharmacology, function and relevance to drug discovery

The growing recognition that members of the rhodopsin-like family A G-protein-coupled receptors (GPCRs) exist and function as dimers or higher-order oligomers, and that GPCR hetero-dimers and -oligomers are present in physiological tissues, offers novel opportunities for drug discovery. Differential pharmacology, function and regulation of GPCR hetero-dimers and -oligomers suggest means to selectively target GPCRs in different tissues and hint that the mechanism of function of several pharmacological agents might be different in vivo than anticipated from simple ligand-screening programmes that rely on heterologous expression of a single GPCR.

Illuminating insights into protein-protein interactions using bioluminescence resonance energy transfer (BRET)

Bioluminescence resonance energy transfer (BRET) is a straightforward biophysical technique for studying protein-protein interactions. It requires: (1) that proteins of interest and suitable controls be labeled with either a donor or acceptor molecule, (2) placement of these labeled proteins in the desired environment for assessing their potential interaction, and (3) use of suitable detection instrumentation to monitor resultant energy transfer. There are now several possible applications, combinations of donor and acceptor molecules, potential assay environments and detection system perturbations. Therefore, this review aims to demystify and clarify the important aspects of the BRET methodology that should be considered when using this technique.

Simultaneous activation of the delta opioid receptor (deltaOR)/sensory neuron-specific receptor-4 (SNSR-4) hetero-oligomer by the mixed bivalent agonist bovine adrenal medulla peptide 22 activates SNSR-4 but inhibits deltaOR signaling

Hetero-oligomerization among G protein-coupled receptors has been proposed to contribute to signal integration. Because sensory neuron-specific receptors (SNSRs) and the opioid receptors (OR) share a common ligand, the bovine adrenal medulla peptide (BAM) 22, and have opposite effects on pain modulation, we investigated the possible consequences of deltaOR/SNSR-4 hetero-oligomerization on the signaling properties of both receptor subtypes. Bioluminescence resonance energy transfer revealed that the human deltaOR has similar propensity to homo-oligomerize and to form hetero-oligomers with human SNSR-4 when coexpressed in human embryonic kidney 293 cells. The hetero-oligomerization leads to a receptor form displaying unique functional properties. Individual activation of either deltaOR or SNSR-4 in cells coexpressing the two receptors led to the modulation of their respective signaling pathways; inhibition of adenylyl cyclase and activation of phospholipase C, respectively. In contrast, the deltaOR/SNSR-4 bivalent agonist BAM22, which could activate each receptor expressed individually, fully activated the SNSR-4-dependent phospholipase C but did not promote deltaOR-mediated inhibition of adenylyl cyclase in deltaOR/SNSR-4-coexpressing cells. Likewise, concomitant activation of the deltaOR/SNSR-4 hetero-oligomer by selective deltaOR and SNSR-4 agonists promoted SNSR-4 but not deltaOR signaling, revealing an agonist-dependent dominant-negative effect of SNSR-4 on deltaOR signaling. Furthermore, the deltaOR selective antagonist naltrexone trans-inhibited the SNSR-4-promoted phospholipase C activation mediated by BAM22 but not by the SNSR-4-selective agonists, suggesting a bivalent binding mode of BAM22 to the deltaOR/SNSR-4 hetero-oligomer. The observation that BAM22 inhibited the Leu-enkephalin-promoted cAMP inhibition in rat dorsal root ganglia neurons supports the potential physiological implication of such regulatory mechanism.