Promiscuous coupling at receptor-Galpha fusion proteins. The receptor of one covalent complex interacts with the alpha-subunit of another

Benzodiazepine diazepam regulates cell surface β1-adrenergic receptor density in human monocytes

Downregulation of cell surface β-adrenergic receptors (β-AR) is an important adaptive response that prevents deleterious effects of receptor overstimulation. Various factors including reactive oxygen species cause β-AR downregulation. In this study, we evaluated the effects of ligands of the peripheral benzodiazepine receptor (PBR), a key protein in regulating oxidative stress, on surface density of endogenous β₁-and β₂-ARs in highly differentiated cells such as human monocytes, which express both β-AR subtypes. β-AR expression in human monocytes was evaluated by flow cytometry, qPCR and western blotting. Monocyte treatment with β-AR agonist isoproterenol did not change surface β₁-AR density while downregulating surface β₂-AR density. This effect was antagonized by the β-blocker propranolol. An opposite response was observed with benzodiazepine diazepam that led to a time-dependent reduction in β₁-AR density. In particular, while no significant downregulation was observed after 3 h of treatment, only 63% of β₁-ARs were still present on the cell surface after 48 h of treatment with diazepam at 1 μM. Treatment with the PBR antagonist PK11195, but not with propranolol, antagonized the effects of diazepam. No change in β₁-AR-mRNA or protein levels was observed at any time after diazepam treatment. We also found that diazepam did not affect Gs-protein or β-arrestin-2 recruitment for both β-ARs in engineered fibroblasts, further suggesting that diazepam activity on β₁-AR density is mediated by PBR. Finally, no sex-related differences were found. Collectively, these results indicate that monocyte β₁-ARs are resistant to catecholamine-mediated downregulation and suggest that PBR plays an important role in regulating β₁-AR density.

Fusion of the β2-adrenergic receptor with either Gαs or βarrestin-2 produces constitutive signaling by each pathway and induces gain-of-function in BEAS-2B cells

The β2AR is a prototypical G protein-coupled receptor (GPCR) known to orchestrate different cellular responses by the stimulation of specific signaling pathways. The best-established signaling pathways for the β2AR are the canonical Gs pathway and the alternative β arrestin 2 (βarr2) pathway. Exploring each pathway separately remains a challenging task due to the dynamic nature of the receptor. Here, we fused the β2AR with its cognate transducers, Gαs and βarr2, using short linkers as a novel approach for restricting the conformation of the receptor and preferentially activating one of its two signaling pathways. We characterized the behavior of our fusion proteins β2AR-Gαs and β2AR-βarr2 in HEK293 cells by measuring their constitutive activity, transducer recruitment, and pharmacological modulation. Our fusion proteins show (a) steric hindrance from the reciprocal endogenous transducers, (b) constitutive activity of the β2AR for the signaling pathway activated by the tethered transducer, and (c) pharmacologic modulation by β2AR ligands. Based on these characteristics, we further explored the possibility of a gain-of-function mechanism in the human lung non-tumorigenic epithelial cell line, BEAS-2B cells. This immortalized human bronchial epithelial cell line has immunomodulatory properties through cytokine release mediated by β2AR stimulation. Our findings suggest that each signaling pathway of the β2AR is biased toward either the Th1 or Th2 inflammatory response suggesting a role in regulating the immune phenotype of respiratory diseases. Our data imply that our fusion proteins can be used as tools to isolate the function elicited by a single signaling pathway in physiologically relevant cell types.

Tritium-labelled isovaleryl-RYYRIK-NH2 as potential antagonist probe for ORL1 nociceptin receptor

IsoVa-RYYRIK-NH2 is a highly specific antagonist ligand of the opioid receptor-like 1 (ORL1) receptor, an endogenous ligand of which is 17-mer peptide nociceptin. ORL1 antagonists have potential for clinical use as analgesic and antineuropathic drugs, and thus information on the receptor-binding characteristics of antagonists is very important for rational drug design. In the present study, we prepared tritium-labelled isova-RYYRIK-NH2 from its precursor with the 3-methylcrotonyl (CH3)2CCHCO group by a catalytic reduction using tritium gas. The resulting [(3)H]isoVa-RYYRIK-NH2 was evaluated in a saturation binding assay using the COS-7 cell membrane preparations of transiently expressed ORL1. It exhibited more than 90% specific binding with a dissociation constant of 1.21±0.03nM. From the mutual heterologous binding assays using [(3)H]isoVa-RYYRIK-NH2 and [(3)H]nociceptin, isoVa-RYYRIK-NH2 and nociceptin were found to share the receptor-binding site, but each also had a separate specific binding site of its own. They differentiated the two different binding states or conformations of ORL1, which might represent the agonist-active and antagonist-inactive conformations of ORL1. [(3)H]isoVa-RYYRIK-NH2 is thus a key tracer to uncover the amino acid residues important for receptor inactivation.

In vivo opioid receptor heteromerization: where do we stand?

Opioid receptors are highly homologous GPCRs that modulate brain function at all levels of neural integration, including autonomous, sensory, emotional and cognitive processing. Opioid receptors functionally interact in vivo, but the underlying mechanisms involving direct receptor-receptor interactions, affecting signalling pathways or engaging different neuronal circuits, remain unsolved. Heteromer formation through direct physical interaction between two opioid receptors or between an opioid receptor and a non-opioid one has been postulated and can be characterized by specific ligand binding, receptor signalling and trafficking properties. However, despite numerous studies in heterologous systems, evidence for physical proximity in vivo is only available for a limited number of opioid heteromers, and their physiopathological implication remains largely unknown mostly due to the lack of appropriate tools. Nonetheless, data collected so far using endogenous receptors point to a crucial role for opioid heteromers as a molecular entity that could underlie human pathologies such as alcoholism, acute or chronic pain as well as psychiatric disorders. Opioid heteromers therefore stand as new therapeutic targets for the drug discovery field.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Interaction of the muscarinic acetylcholine receptor M₂ subtype with G protein Gα(i/o) isotypes and Gβγ subunits as studied with the maltose-binding protein-M₂-Gα(i/o) fusion proteins expressed in Escherichia coli

We expressed the fusion proteins of the muscarinic acetylcholine receptor M2 subtype (M2 receptor) with a maltose-binding protein (MBP) and various G protein α subunits (Gα(i1-i3/o)) at its N- and C-terminals, respectively (MBP-M2-Gα(i/o)), in Escherichia coli, and examined the effect of G protein βγ subunits (Gβγ) on the receptor-Gα interaction as assessed by agonist- and GDP-dependent [(35)S]GTPγS binding of the fusion proteins. We found that (i) Gβγ promoted both the agonist-dependent and -independent [(35)S]GTPγS binding with little effect on the guanine nucleotide-sensitive high-affinity agonist binding, (ii) the specific [(35)S]GTPγS binding activity was much greater for MBP-M2-Gα(oA) than for MBP-M2-Gα(i1-i3) in the absence of Gβγ, whereas Gβγ preferentially promoted the agonist-dependent decrease in the affinity for GDP of MBP-M2-Gα(i1-i3) rather than of MBP-M2-Gα(oA), and (iii) the proportion of agonist-dependent [(35)S]GTPγS binding was roughly 50% irrespective of species of Gα and the presence or absence of Gβγ. These results demonstrate that receptor-Gα fusion proteins expressed in E. coli could be useful for studies of receptor-G interaction.

Divergent transducer-specific molecular efficacies generate biased agonism at a G protein-coupled receptor (GPCR)

The concept of "biased agonism" arises from the recognition that the ability of an agonist to induce a receptor-mediated response (i.e. "efficacy") can differ across the multiple signal transduction pathways (e.g. G protein and β-arrestin (βarr)) emanating from a single GPCR. Despite the therapeutic promise of biased agonism, the molecular mechanism(s) whereby biased agonists selectively engage signaling pathways remain elusive. This is due in large part to the challenges associated with quantifying ligand efficacy in cells. To address this, we developed a cell-free approach to directly quantify the transducer-specific molecular efficacies of balanced and biased ligands for the angiotensin II type 1 receptor (AT1R), a prototypic GPCR. Specifically, we defined efficacy in allosteric terms, equating shifts in ligand affinity (i.e. KLo/KHi) at AT1R-Gq and AT1R-βarr2 fusion proteins with their respective molecular efficacies for activating Gq and βarr2. Consistent with ternary complex model predictions, transducer-specific molecular efficacies were strongly correlated with cellular efficacies for activating Gq and βarr2. Subsequent comparisons across transducers revealed that biased AT1R agonists possess biased molecular efficacies that were in strong agreement with the signaling bias observed in cellular assays. These findings not only represent the first measurements of the thermodynamic driving forces underlying differences in ligand efficacy between transducers but also support a molecular mechanism whereby divergent transducer-specific molecular efficacies generate biased agonism at a GPCR.

Smoothened is a fully competent activator of the heterotrimeric G protein G(i)

Smoothened (Smo) is a 7-transmembrane protein essential to the activation of Gli transcription factors (Gli) by hedgehog morphogens. The structure of Smo implies interactions with heterotrimeric G proteins, but the degree to which G proteins participate in the actions of hedgehogs remains controversial. We posit that the G(i) family of G proteins provides to hedgehogs the ability to expand well beyond the bounds of Gli. In this regard, we evaluate here the efficacy of Smo as it relates to the activation of G(i), by comparing Smo with the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor, a quintessential G(i)-coupled receptor. We find that with use of [(35)S]guanosine 5'-(3-O-thio)triphosphate, first, with forms of G(i) endogenous to human embryonic kidney (HEK)-293 cells made to express epitope-tagged receptors and, second, with individual forms of Gα(i) fused to the C terminus of each receptor, Smo is equivalent to the 5-HT(1A) receptor in the assay as it relates to capacity to activate G(i). This finding is true regardless of subtype of G(i) (e.g., G(i2), G(o), and G(z)) tested. We also find that Smo endogenous to HEK-293 cells, ostensibly through inhibition of adenylyl cyclase, decreases intracellular levels of cAMP. The results indicate that Smo is a receptor that can engage not only Gli but also other more immediate effectors.

Divergent agonist selectivity in activating β1- and β2-adrenoceptors for G-protein and arrestin coupling

The functional selectivity of adrenergic ligands for activation of β1- and β2-AR (adrenoceptor) subtypes has been extensively studied in cAMP signalling. Much less is known about ligand selectivity for arrestin-mediated signalling pathways. In the present study we used resonance energy transfer methods to compare the ability of β1- and β2-ARs to form a complex with the G-protein β-subunit or β-arrestin-2 in response to a variety of agonists with various degrees of efficacy. The profiles of β1-/β2-AR selectivity of the ligands for the two receptor-transducer interactions were sharply different. For G-protein coupling, the majority of ligands were more effective in activating the β2-AR, whereas for arrestin coupling the relationship was reversed. These data indicate that the β1-AR interacts more efficiently than β2-AR with arrestin, but less efficiently than β2-AR with G-protein. A group of ligands exhibited β1-AR-selective efficacy in driving the coupling to arrestin. Dobutamine, a member of this group, had 70% of the adrenaline (epinephrine) effect on arrestin via β1-AR, but acted as a competitive antagonist of adrenaline via β2-AR. Thus the structure of such ligands appears to induce an arrestin-interacting form of the receptor only when bound to the β1-AR subtype.

Cleavage-resistant fusion proteins of the M(2) muscarinic receptor and Gα(i1). Homotropic and heterotropic effects in the binding of ligands

BACKGROUND

G protein-coupled receptors fused to a Gα-subunit are functionally similar to their unfused counterparts. They offer an intriguing view into the nature of the receptor-G protein complex, but their usefulness depends upon the stability of the fusion.

METHODS

Fusion proteins of the M(2) muscarinic receptor and the α-subunit of G(i1) were expressed in CHO and Sf9 cells, extracted in digitonin-cholate, and examined for their binding properties and their electrophoretic mobility on western blots.

RESULTS

Receptor fused to native α(i1) underwent proteolysis near the point of fusion to release a fragment with the mobility of α(i1). The cleavage was prevented by truncation of the α-subunit at position 18. Binding of the agonist oxotremorine-M to the stable fusion protein from Sf9 cells was biphasic, and guanylylimidodiphosphate promoted an apparent interconversion of sites from higher to lower affinity. With receptor from CHO cells, the apparent capacity for N-[(3)H]methylscopolamine was 60% of that for [(3)H]quinuclidinylbenzilate; binding at saturating concentrations of the latter was inhibited in a noncompetitive manner at low concentrations of unlabeled N-methylscopolamine.

CONCLUSIONS

A stable fusion protein of the M(2) receptor and truncated α(i1) resembles the native receptor-G protein complex with respect to the guanyl nucleotide-sensitive binding of agonists and the noncompetitive binding of antagonists.

GENERAL SIGNIFICANCE

Release of the α-subunit is likely to occur with other such fusion proteins, rendering the data ambiguous or misleading. The properties of a chemically stable fusion protein support the notion that signaling proceeds via a stable multimeric complex of receptor and G protein.

Morphine-like opiates selectively antagonize receptor-arrestin interactions

The addictive potential of opioids may be related to their differential ability to induce G protein signaling and endocytosis. We compared the ability of 20 ligands (sampled from the main chemical classes of opioids) to promote the association of mu and delta receptors with G protein or beta-arrestin 2. Receptor-arrestin binding was monitored by bioluminescence resonance energy transfer (BRET) in intact cells, where pertussis toxin experiments indicated that the interaction was minimally affected by receptor signaling. To assess receptor-G protein coupling without competition from arrestins, we employed a cell-free BRET assay using membranes isolated from cells expressing luminescent receptors and fluorescent Gbeta(1). In this system, the agonist-induced enhancement of BRET (indicating shortening of distance between the two proteins) was G alpha-mediated (as shown by sensitivity to pertussis toxin and guanine nucleotides) and yielded data consistent with the known pharmacology of the ligands. We found marked differences of efficacy for G protein and arrestin, with a pattern suggesting more restrictive structural requirements for arrestin efficacy. The analysis of such differences identified a subset of structures showing a marked discrepancy between efficacies for G protein and arrestin. Addictive opiates like morphine and oxymorphone exhibited large differences both at delta and mu receptors. Thus, they were effective agonists for G protein coupling but acted as competitive enkephalins antagonists (delta) or partial agonists (mu) for arrestin. This arrestin-selective antagonism resulted in inhibition of short and long term events mediated by arrestin, such as rapid receptor internalization and down-regulation.

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

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

Efficient silkworm expression of human GPCR (nociceptin receptor) by a Bombyx mori bacmid DNA system

Guanine nucleotide-binding protein (G protein) coupled receptors (GPCRs) are frequently expressed by a baculovirus expression vector system (BEVS). We recently established a novel BEVS using the bacmid system of Bombyx mori nucleopolyhedrovirus (BmNPV), which is directly applicable for protein expression in silkworms. Here, we report the first example of GPCR expression in silkworms by the simple injection of BmNPV bacmid DNA. Human nociceptin receptor, an inhibitory GPCR, and its fusion protein with inhibitory G protein alpha subunit (G(i)alpha) were both successfully expressed in the fat bodies of silkworm larvae as well as in the BmNPV viral fraction. Its yield was much higher than that from Sf9 cells. The microsomal fractions including the nociceptin receptor fusion, which are easily prepared by only centrifugation steps, exhibited [35S]GTPgammaS-binding activity upon specific stimulation by nociceptin. Therefore, this rapid method is easy-to-use and has a high expression level, and thus will be an important tool for human GPCR production.

Coupling of beta2-adrenoceptors to XLalphas and Galphas: a new insight into ligand-induced G protein activation

Galpha(s) and extra-large Galpha(s) (XLalpha(s)) can both transduce receptor activation into intracellular cAMP generation. It is unknown, however, whether these two GNAS-locus products display distinct properties with respect to receptor coupling. Here, we show that XLalpha(s) couples to the beta2-adrenoceptor more efficiently than Galpha(s). In transfected human embryonic kidney 293 cells and mouse embryonic fibroblasts null for both Galpha(s) and XLalpha(s) (2B2 cells), basal cAMP accumulation mediated by XLalpha(s) was higher than that mediated by Galpha(s). Inverse agonist treatment reduced Galpha(s)-mediated basal activity, whereas its effect was markedly blunted on XLalpha(s)-mediated basal activity. Rank order of ligand efficacies regarding cAMP accumulation was the same when the receptor was coupled to XLalpha(s) or Galpha(s). However, ligand-induced and XLalpha(s)-mediated cAMP generation was higher than that mediated by Galpha(s). The relatively high efficiency of XLalpha(s)-mediated cAMP generation was conditional, disappearing with increased level of receptor expression or increased efficacy of ligand. Full-agonist responses in XLalpha(s)- and Galpha(s)-expressing cells were comparable even at low receptor levels, whereas partial agonist responses became comparable only when the receptor expression was increased (>3 pmol/mg). Radioligand binding studies showed that the high-affinity component in agonist binding to beta2-adrenoceptor was more pronounced in cells expressing XLalpha(s) than those expressing Galpha(s). We discuss these findings in the framework of current receptor-G protein activation models and offer an extended ternary complex model that can fully explain our observations.

Delayed internalization and lack of recycling in a beta2-adrenergic receptor fused to the G protein alpha-subunit

BACKGROUND

Chimeric proteins obtained by the fusion of a G protein-coupled receptor (GPCR) sequence to the N-terminus of the G protein alpha-subunit have been extensively used to investigate several aspects of GPCR signalling. Although both the receptor and the G protein generally maintain a fully functional state in such polypeptides, original observations made using a chimera between the beta2-adrenergic receptor (beta2AR) and Galphas indicated that the fusion to the alpha-subunit resulted in a marked reduction of receptor desensitization and down-regulation. To further investigate this phenomenon, we have compared the rates of internalization and recycling between wild-type and Galphas-fused beta2AR.

RESULTS

The rate of agonist-induced internalization, measured as the disappearance of cell surface immunofluorescence in HEK293 cells permanently expressing N-terminus tagged receptors, was reduced three-fold by receptor-G protein fusion. However, both fused and non-fused receptors translocated to the same endocytic compartment, as determined by dual-label confocal analysis of cells co-expressing both proteins and transferrin co-localization. Receptor recycling, determined as the reversion of surface immunofluorescence following the addition of antagonist to cells that were previously exposed to agonist, markedly differed between wild-type and fused receptors. While most of the internalized beta2AR returned rapidly to the plasma membrane, beta2AR-Galphas did not recycle, and the observed slow recovery for the fusion protein immunofluorescence was entirely accounted for by protein synthesis.

CONCLUSION

The covalent linkage between beta2AR and Galphas does not appear to alter the initial endocytic translocation of the two proteins, although there is reduced efficiency. It does, however, completely disrupt the process of receptor and G protein recycling. We conclude that the physical separation between receptor and Galpha is not necessary for the transit to early endosomes, but is an essential requirement for the correct post-endocytic sorting and recycling of the two proteins.

Non-gonadotropin-releasing hormone-mediated transcription and secretion of large human glycoprotein hormone alpha-subunit in human embryonic kidney-293 cells

To identify genes that are most responsive to a sustained activation of a G(s) protein-coupled receptor, HEK293 cells were stably transfected with the beta(2)-adrenergic receptor and stimulated with agonist isoproterenol (1 mum). A microarray study indicated that the gene with the highest stimulation index (500-fold) encoded the common alpha-subunit of human glycoprotein hormones (GPHalpha). Induction of GPHalpha transcription in response to cAMP elevations resulted in a dramatic increase (600-fold) of protein secretion as shown by RT-PCR and a highly specific time-resolved immunofluorometric assay. Cloning and sequencing of the GPHalpha cDNA and mass spectrometric analysis of HPLC-purified GPHalpha derived from serum-free HEK293-beta(2)-adrenergic receptor-stimulated cells verified the nature of the molecule. Enzymatic deglycosylation with subsequent Western blots revealed that this was a large hyperglycosylated form of GPHalpha that had not been associated with a beta-subunit previously. This uncombined variant is known to be either cosecreted with GPHs from the pituitary, the placenta, and a variety of tumors or secreted without GPHs from APUD cells and rare tumors. Moreover, it is similar to GPHalpha found at high concentrations in seminal plasma. As shown by a panel of endogenous or transfected G protein-coupled receptors in HEK293 cells, the expression of large GPHalpha was controlled by G(s)- and G(q)- but not G(i)-dependent receptors and mediated via cAMP and Ca(++) release. This suggests that Gq- or G(s)-coupled receptors other than the classical GnRH receptor may play a role in the regulation of nonpituitary, nonplacental GPHalpha secretion under physiological and pathological conditions.

Synthesis and pharmacological evaluation of 1,2-dihydrospiro[isoquinoline-4(3H),4'-piperidin]-3-ones as nociceptin receptor agonists

Some synthesized 1,2-dihydrospiro[isoquinoline-4(3 H),4'-piperidin]-3-ones were evaluated as ligands for nociceptin receptor (NOP receptor). Their affinity was established by binding studies, and efficacy was investigated by GTP binding experiments. Selectivity toward DOP, KOP, and MOP receptors was assessed, and structural requirements affecting affinity and selectivity were remarked. Most notably, compound 6d displayed nanomolar NOP receptor affinity and showed more than 800-fold selectivity. The new structures exerted full or partial agonistic activity.

Co-expression of mu and delta opioid receptors as receptor-G protein fusions enhances both mu and delta signalling via distinct mechanisms

Mu and delta opioid receptors (MORs and DORs) were co-expressed as fusion proteins between a receptor and a pertussis insensitive mutant Galpha(i/o) protein in human embryonic kidney 293 cells. Signalling efficiency was then monitored following inactivation of endogenous Galpha(i/o) proteins by pertussis toxin. Co-expression resulted in increased delta opioid signalling which was insensitive to the mu specific antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2. Under these conditions, mu opioid signalling was also increased and insensitive to the delta specific antagonist Tic-deltorphin. In this latter case, however, no G protein activation was observed in the presence of the delta specific inverse agonist N,N(CH3)2-Dmt-Tic-NH2. When a MOR fused to a non-functional Galpha subunit was co-expressed with the DOR-Galpha protein fusion, delta opioid signalling was not affected whereas mu opioid signalling was restored. Altogether our results suggest that increased delta opioid signalling is due to enhanced DOR coupling to its tethered Galpha subunit. On the other hand, our data indicate that increased mu opioid signalling requires an active conformation of the DOR and also results in activation of the Galpha subunit fused the DOR.

Functional complementation of high-efficiency resonance energy transfer: a new tool for the study of protein binding interactions in living cells

Green bioluminescence in Renilla species is generated by a ∼100% efficient RET (resonance energy transfer) process that is caused by the direct association of a blue-emitting luciferase [Rluc (Renilla luciferase)] and an RGFP (Renilla green fluorescent protein). Despite the high efficiency, such a system has never been evaluated as a potential reporter of protein–protein interactions. To address the question, we compared and analysed in mammalian cells the bioluminescence of Rluc and RGFP co-expressed as free native proteins, or as fused single-chain polypeptides and tethered partners of self-assembling coiled coils. Here, we show that: (i) no spontaneous interactions generating detectable BRET (bioluminescence RET) signals occur between the free native proteins; (ii) high-efficiency BRET similar to that observed in Renilla occurs in both fusion proteins and self-interacting chimaeras, but only if the N-terminal of RGFP is free; (iii) the high-efficiency BRET interaction is associated with a dramatic increase in light output when the luminescent reaction is triggered by low-quantum yield coelenterazine analogues. Here, we propose a new functional complementation assay based on the detection of the high-efficiency BRET signal that is generated when the reporters Rluc and RGFP are brought into close proximity by a pair of interacting proteins to which they are linked. To demonstrate its performance, we implemented the assay to measure the interaction between GPCRs (G-protein-coupled receptors) and β-arrestins. We show that complementation-induced BRET allows detection of the GPCR–β-arrestin interaction in a simple luminometric assay with high signal-to-noise ratio, good dynamic range and rapid response.

Designed modification of partial agonist of ORL1 nociceptin receptor for conversion into highly potent antagonist

Nociceptin is an endogenous agonist ligand of the ORL1 (opioid receptor-like 1) receptor, and its antagonist is a potential target of therapeutics for analgesic and antineuropathy drugs. Ac-RYYRIK-NH(2) is a hexapeptide isolated from the peptide library as an antagonist that inhibits the nociceptin activities mediated through ORL1. However, the structural elements required for this antagonist activity are still indeterminate. In the present study, we evaluated the importance of the acetyl-methyl group in receptor binding and activation, examining the peptides acyl-RYYRIK-NH(2), where acyl (R-CO) possesses a series of alkyl groups, R=C(n)H(2n+1) (n=0-5). The isovaleryl derivative with the C(4)H(9) (=(CH(3))(2)CHCH(2)-) group was found to reveal a high receptor-binding affinity and a strong antagonist nature. This peptide achieved a primary goal of eliminating the agonist activity of Ac-RYYRIK-NH(2) and producing pure antagonist activity.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Rapid activation of inwardly rectifying potassium channels by immobile G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) mediate slow synaptic transmission and many other effects of small molecule and peptide neurotransmitters. In the standard model of GPCR signaling, receptors and G-proteins diffuse laterally within the plane of the plasma membrane and encounter each other by random collision. This model predicts that signaling will be most efficient if both GPCRs and G-proteins are free to diffuse, thus maximizing collision frequency. However, neuronal GPCRs are often recruited to and enriched at specific synaptic locations, suggesting receptor mobility is restricted in these cells. Here, we test the hypothesis that restricting GPCR mobility impairs signaling in neurons by limiting the frequency of collisions between receptors and G-proteins. Mu-opioid receptors (MORs) were immobilized on the surface of cerebellar granule neurons by avidin-mediated cross-linking, and inwardly rectifying potassium (GIRK) channels were used as rapid indicators of G-protein activation. Mobile and immobile MORs activated GIRK channels with the same onset kinetics and agonist sensitivity in these neurons. In a heterologous expression system, GFP (green fluorescent protein)-tagged G alpha(oA) subunits remained mobile after cross-linking, but their mobility was reduced in the presence of immobile MORs, suggesting that these receptors and subunits were transiently precoupled. In addition, channel activation could be reconstituted with immobile GPCRs, G-protein heterotrimers, and GIRK channels. These results show that collision frequency is not rate-limiting for G-protein activation in CNS neurons, and are consistent with the idea that signaling components are compartmentalized or preassembled.

Mu-delta opioid receptor functional interaction: Insight using receptor-G protein fusions

Fusion proteins between a receptor and a pertussis toxin-insensitive G(i)alpha subunit were used to gain insight into the molecular interactions that take place upon mu and delta opioid receptor heterodimerization. When mu opioid receptor-G(i1)alpha fusions were coexpressed with nonfused delta opioid receptors in human embryonic kidney 293 cells, or vice versa, receptor heterodimers were detected by coimmunoprecipitation. In pertussis toxin-treated cells, receptor coexpression decreased the amount of guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) incorporated in the fused G alpha protein after the addition of agonists specific for the receptor-G(i1)alpha fusion. In addition, activation of the G alpha protein occurred in heterodimers upon addition of an agonist specific for the nonfused receptor. It remained unaffected by an inverse agonist specific for the receptor-G(i1)alpha fusion. These data suggest that signaling through the receptor-G(i1)alpha fusion protein is impaired in heterodimers and support a mechanism in which activation of the G alpha subunit is promoted by a direct interaction with the nonfused receptor. Alternatively, receptor coexpression did not modify the ligand binding properties for the high-affinity state of the receptor-G(i1)alpha fusion nor the EC50 values for agonist-induced [35S]GTPgammaS incorporation in the G(i1)alpha subunit. In addition, no binding competition was observed between delta and mu ligands. Together, the data point to mu-delta opioid receptor heterodimers formed by contact interactions between monomers that retain their structural integrity.

Synthesis and evaluation as NOP ligands of some spiro[piperidine-4,2'(1'H)-quinazolin]-4'(3'H)-ones and spiro[piperidine-4,5'(6'H)-[1,2,4]triazolo[1,5-c]quinazolines]

Some spiro[piperidine-4,2'(1'H)-quinazolin]-4'(3'H)-ones 3 and spiro[piperidine-4,5'(6'H)-[1,2,4]triazolo[1,5-c]quinazolines] 4 were synthesized and evaluated as ligands of the nociceptin receptor. The examined compounds showed partial agonistic activity, except compounds 3, 4n that proved to be pure antagonists.

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.

Bring your own G protein

G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.

Guanine nucleotide exchange-independent activation of Gs protein by beta2-adrenoceptor

beta2-adrenoceptor-mediated activation of Gs and adenylyl cyclase or other receptor-mediated G protein activations is believed to occur by receptor-catalyzed replacement of GDP with GTP on the alpha-subunit of the G protein. Here we showed that a beta2-adrenoceptor-Gs system, heterologously expressed in cyc- or human embryonic kidney (HEK)-293 cells, can be activated in the presence of GDP or its phosphorylation-resistant analog, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS). The potency and maximal ability of GDP to activate Gs and adenylyl cyclase were identical to those of GTP. GDP-mediated activation of adenylyl cyclase, similar to that mediated by GTP, was concentration-dependent, required high magnesium concentrations, was inhibited by inverse agonists, and was correlated with the efficacy of receptor ligands used to stimulate the receptor. UDP did not block the GDP-mediated activation, although it completely blocked the formation of a small amount of GTP ( approximately 5% GDP) from GDP. Moreover, the activation of Gs in the presence of GDP was insensitive to cholera toxin treatment of the cells, whereas that observed in the presence of GTP was amplified by the treatment, which showed that the activation observed in the presence of GDP was not mediated by GTP. Therefore, we concluded that GDP itself could mediate beta-adrenoceptor-induced activation of Gs-adenylyl cyclase system as much as GTP. We discuss the results in the context of the current paradigm of receptor-mediated G protein activation and propose an additional mode of activation for beta2-adrenoceptor-G(s) adenylyl cyclase system where nucleotide exchange is not necessary and GDP and GTP play identical roles in receptor-induced Gs protein activation.

Functional complementation and the analysis of opioid receptor homodimerization

Complementation of function after coexpression of pairs of nonfunctional G protein-coupled receptors that contain distinct inactivating mutations supports the hypothesis that such receptors exist as dimers. Chimeras between members of the metabotropic glutamate receptor-like family have been particularly useful because the N-terminal ligand binding and heptahelical transmembrane elements can be considered distinct domains. To examine the utility of a related approach for opioid receptors, fusion proteins were generated in which a pertussis toxin-resistant (Cys351Ile) variant of the G protein Gi1alpha was linked to the C-terminal tails of the delta opioid peptide (DOP), kappa opioid peptide, and mu opioid peptide receptors. Each was functional as measured by agonist stimulation of guanosine 5'-([gamma-35S]thio)triphosphate ([35S]GTPgammaS) binding in Gialpha immunoprecipitates from membranes of pertussis toxin-treated HEK293 cells. Agonist function was eliminated either by fusion of the receptors to Gi1alphaGly202Ala,Cys351Ile or mutation of a pair of conserved Val residues in intracellular loop 2 of each receptor. Coexpression, but not simple mixing, of the two inactive fusion proteins reconstituted agonist-loading of [35S]GTPgammaS for each receptor. At equimolar amounts, reconstitution of DOP receptor function was more extensive than kappa or mu opioid receptor. Reconstitution of DOP function required two intact receptors and was not achieved by provision of extra Gi1alphaCys351Ile membrane anchored by linkage to DOP transmembrane domain 1. Inactive forms of all G protein alpha subunits can be produced by mutations equivalent to Gi1alphaGly202Ala. Because the amino acids modified in the opioid receptors are highly conserved in most rhodopsin-like receptors, this approach should be widely applicable to study the existence and molecular basis of receptor dimerization.

Methods to monitor the quaternary structure of G protein-coupled receptors

A wide range of approaches has been applied to examine the quaternary structure of G protein-coupled receptors, the basis of such protein-protein interactions and how such interactions might modulate the pharmacology and function of these receptors. These include co-immunoprecipitation, various adaptations of resonance energy transfer techniques, functional complementation studies and the analysis of ligand-binding data. Each of the available techniques has limitations that restrict interpretation of the data. However, taken together, they provide a coherent body of evidence indicating that many, if not all, G protein-coupled receptors exist and function as dimer/oligomers. Herein we assess the widely applied techniques and discuss the relative benefits and limitations of these approaches.

Different structural requirements for the constitutive and the agonist-induced activities of the beta2-adrenergic receptor

We converted Ser-207, located in helix 5 of the beta2-adrenergic receptor, into all other natural amino acids. To quantify receptor activation as a receptor number-independent parameter and directly related to G(s) activation, we expressed the mutants in a G alpha(s)-tethered form. GTP exchange in such constructs is restricted to the fused alpha-subunit and is a linear function of the receptor concentration. Except S207R, all other mutants were expressed to a suitable level for investigation. All mutations reduced the binding affinities of the catechol agonists, epinephrine and isoproterenol, and the extent of reduction was unrelated to the residue ability to form hydrogen bonds. Instead, both enhancements and reductions of affinity were observed for the partial agonist halostachin and the antagonist pindolol. The mutations also enhanced and diminished ligand-induced receptor activation, but the effects were strictly ligand-specific. Polar residues such as Asp and His exalted the activation by full agonists but suppressed that induced by the partial agonists halostachin and dichloroisoproterenol. In contrast, hydrophobic residues such as Ile and Val augmented partial agonist activation. Only Ile and Lys produced a significant increase of constitutive activity. The effects on binding and activity were not correlated, nor did such parameters show any clear correlation with up to 78 descriptors of amino acid physicochemical properties. Our data question the idea that Ser-207 is exposed to the polar crevice in the unbound receptor. They also suggest that the active receptor form induced by a full agonist might be substantially different from that caused by constitutive activation.

Hetero-oligomers of alpha2A-adrenergic and mu-opioid receptors do not lead to transactivation of G-proteins or altered endocytosis profiles

Complexes of alpha(2A)-ARs (alpha(2A)-adrenergic receptors) and MORs (mu-opioid receptors), probably hetero-oligomers, were detected by co-immunoisolation after extraction from HEK-293 cells (human embryonic kidney 293 cells). Functional communication between these receptors is revealed by alpha(2A)-AR activation of a pertussis toxin-insensitive G(i)alpha subunit (termed as G(i)1) when fused with the MOR and evaluated in membranes from pertussis toxin-treated cells. However, the alpha(2A)-AR does not require transactivation through MOR, since quantitatively indistinguishable results were observed in cells co-expressing alpha(2A)-AR and a fusion protein of G(i)1 with the first transmembrane span of MOR (myc-MOR-TM1). Functional cross-talk among these alpha(2A)-AR-MOR complexes does not occur for internalization profiles; incubation with adrenaline (epinephrine) leads to endocytosis of alpha(2A)-AR but not MOR, while incubation with DAMGO ([D-Ala,NMe-Phe,Gly-ol]enkephalin) leads to endocytosis of MOR but not alpha(2A)-AR in cells co-expressing both the receptors. Hence, alpha(2A)-AR and MOR hetero-oligomers, although they occur, do not have an obligatory functional influence on one another in the paradigms studied.

Allosteric modulation and constitutive activity of fusion proteins between the adenosine A1 receptor and different 351Cys-mutated Gi alpha-subunits

We studied fusion proteins between the human adenosine A1 receptor and different 351Cys-mutated G(i1) alpha-subunits (A1-Gialpha) with respect to two important concepts in receptor pharmacology, i.e. allosteric modulation and constitutive activity/inverse agonism. The aim of our study was twofold. We first analysed whether such fusion products are still subject to allosteric modulation, and, secondly, we investigated the potential utility of the fusion proteins to study constitutive receptor activity. We determined the pharmacological profile of nine different A1-Gialpha fusion proteins in radioligand binding studies. In addition, we performed [35S]GTPgammaS binding experiments to study receptor and G protein activation of selected A1-Gialpha fusion proteins. Compared to unfused adenosine A1 receptors, the affinity of N6-cyclopentyladenosine (CPA) at wild-type A1-Gialpha fusion proteins (351Cys) increased more than eightfold, while the affinity of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) did not change significantly. Furthermore, we showed that the allosteric enhancer of agonist binding, PD81,723 (2-amino-4,5-dimethyl-3-thienyl-[3-(trifluoromethyl)-phenyl]methanone), elicited similar effects on ligand binding; i.e. CPA binding to the A1-Gialpha fusion proteins was enhanced, whereas the affinity of DPCPX was hardly affected. Moreover, sodium ions were unable to decrease agonist binding to the majority of the A1-Gialpha fusion proteins, presumably because they exhibit their effect through uncoupling of the R-G complex. From [35S]GTPgammaS binding experiments, we learned that all the A1-Gialpha fusion proteins tested had a higher basal receptor activity than the unfused adenosine A1 receptor, thereby providing improved conditions to observe inverse agonism. Moreover, the maximal CPA-induced stimulation of basal [35S]GTPgammaS binding was increased for the five A1-Gialpha fusion proteins tested, whereas the inhibition induced by 8-cyclopentyltheophylline (CPT) was more pronounced at 351Cys, 351Ile, and 351Val A1-Gialpha fusion proteins. Thus, the maximal receptor (de)activation depended on the amino acid at position 351 of the Gi alpha-subunit. In conclusion, A1-Gialpha fusion proteins, especially with 351Cys and 351Ile, can be used as research tools to investigate inverse agonism, due to their increased readout window in [35S]GTPgammaS binding experiments.

A new synthetic approach of N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl)benzamide (JTC-801) and its analogues and their pharmacological evaluation as nociceptin receptor (NOP) antagonists

A series of 4-amino-2-methylquinoline and 4-aminoquinazoline derivatives, including the reference NOP antagonist JTC-801, were synthesized by an alternative pathway and their in vitro pharmacological properties were investigated. 3-Substitution of the quinoline ring resulted very critical for affinity. So 3-methyl derivative 4j showed a similar potency compared with the reference 4h while bulky lipophilic or electron withdrawing groups in the same position strongly decreased affinity. Structural and conformational requirements for affinity were outlined by NOE NMR and computational methods and suggestions for a pharmacophore model design were provided.

Characterization of glucagon-like peptide-1 receptor beta-arrestin 2 interaction: a high-affinity receptor phenotype

To dissect the interaction between beta-arrestin ((beta)arr) and family B G protein-coupled receptors, we constructed fusion proteins between the glucagon-like peptide 1 receptor and (beta)arr2. The fusion constructs had an increase in apparent affinity selectively for glucagon, suggesting that (beta)arr2 interaction locks the receptor in a high-affinity conformation, which can be explored by some, but not all, ligands. The fusion constructs adopted a signaling phenotype governed by the tethered (beta)arr2 with an attenuated G protein-mediated cAMP signal and a higher maximal internalization compared with wild-type receptors. This distinct phenotype of the fusion proteins can not be mimicked by coexpressing wild-type receptor with (beta)arr2. However, when the wild-type receptor was coexpressed with both (beta)arr2 and G protein-coupled receptor kinase 5, a phenotype similar to that observed for the fusion constructs was observed. We conclude that the glucagon-like peptide 1 fusion construct mimics the natural interaction of the receptor with (beta)arr2 with respect to binding peptide ligands, G protein-mediated signaling and internalization, and that this distinct molecular phenotype is reminiscent of that which has previously been characterized for family A G protein-coupled receptors, suggesting similarities in the effect of (beta)arr interaction between family A and B receptors also at the molecular level.

High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.

Comparative analysis of high-affinity ligand binding and G protein coupling of the human CXCR1 chemokine receptor and of a CXCR1-Galpha fusion protein after heterologous production in baculovirus-infected insect cells

In order to perform biochemical and pharmacological characterization of CXCR1, we designed several CXCR1 constructs. All constructs, including a CXCR1-G(i2)alpha fusion protein, were produced in insect cells after infection with recombinant baculovirus. The recombinant receptors exhibited specific high-affinity binding of (125)I-labelled interleukin-8, and Scatchard transformation of the binding data indicated the presence of a population of single homogenous binding sites. Furthermore, the pharmacological profiles for the different CXCR1 constructs produced in the baculovirus-infected insect cells were almost identical to those reported for CXCR1 on human neutrophils. Interestingly, when the CXCR1 constructs were coproduced with G(i2) protein as a result of coinfection with baculoviruses encoding the G(i2)alpha-, the beta- and the gamma- subunits, the B(max) values were significantly increased. Hence, the level of FlagCXCR1Bio, after coproduction with G(i2) protein, was found to be almost 10 times higher than that of the FlagCXCR1Bio alone. However, no differences in the K(i) values were observed of the receptor constructs produced either after single infection or coinfection of insect cells. The addition of guanyl-5'-yl imidodiphosphate resulted in a dramatic reduction of the number of binding sites; however, the K(i) values remained unchanged, indicating coupling of the receptor to the guanine nucleotide-binding protein.

Dimers of class A G protein-coupled receptors function via agonist-mediated trans-activation of associated G proteins

The histamine H1 receptor and the alpha1b-adrenoreceptor are G protein-coupled receptors that elevate intracellular [Ca2+] via activation of Gq/G11. Assessed by co-immunoprecipitation and time-resolved fluorescence resonance energy transfer they both exist as homo-dimers. The addition of the G protein G11alpha to the C terminus of these receptors did not prevent dimerization. Agonists produced a large stimulation of guanosine 5'-3-O-([35S]thio)triphosphate ([35S]GTPgammaS) binding to receptor-G protein fusions containing wild type forms of both polypeptides. For both receptors this was abolished by incorporation of G208AG11alpha into the fusions. Mutation of a highly conserved leucine in intracellular loop 2 of each receptor also eliminated agonist function but not binding. Co-expression of the two non-functional but complementary fusion constructs reconstituted agonist-mediated binding of [35S]GTPgammaS in membranes of HEK293 cells and elevation of [Ca2+]i in mouse embryo fibroblasts lacking both Gq and G11. Co-expression of the histamine H1 receptor- and the alpha1b-adrenoreceptor-G11alpha fusions allowed detection of functional hetero-dimeric complexes, whereas co-expression of histamine H1 receptor-G11alpha with increasing amounts of L151Dalpha1b-adrenoreceptor resulted in decreasing levels of histamine-stimulated [35S]GTPgammaS binding. Co-expression of the alpha1b-adrenoreceptor with a fusion protein incorporating the N-terminal domain and transmembrane helix 1 of the alpha1b-adrenoreceptor and G11alpha did not result in agonist activation of the G protein but did indicate a role for transmembrane helix 1 in dimerization. These data demonstrate that dimers of these class A receptors function via trans-activation of associated G proteins.

Co-expression of the beta2-adrenoceptor and dopamine D1-receptor with Gsalpha proteins in Sf9 insect cells: limitations in comparison with fusion proteins

The G-protein G(salpha) exists in three isoforms, the G(salpha) splice variants G(salphashort) (G(salphaS)) and G(salphalong) (G(salphaL)), and the G-protein G(alphaolf) that is not only involved in olfactory signaling but also in extrapyramidal motor regulation. Studies with beta(2)-adrenoceptor (beta(2)AR)-G(salpha) fusion proteins showed that G(salpha) proteins activate adenylyl cyclase (AC) in the order of efficacy G(salphaS)>G(salphaL) approximately G(alphaolf) and that G(salpha) proteins confer the hallmarks of constitutive activity to the beta(2)AR in the order of efficacy G(salphaL)>G(alphaolf)>G(salphaS). However, it is unclear whether such differences between G(salpha) proteins also exist in the nonfused state. In the present study, we co-expressed the beta(2)AR and dopamine D(1)-receptor (D(1)R) with G(salpha) proteins at different ratios in Sf9 insect cells. In agreement with the fusion protein studies, nonfused G(alphaolf) was less efficient than nonfused G(salphaS) and G(salphaL) at activating AC, but otherwise, we did not observe differences between the three G(salpha) isoforms. Thus, it is much easier to dissect differences between G(salpha) isoforms using beta(2)AR-G(salpha) fusion proteins than nonfused G(salpha) isoforms.