G Protein-Coupled Receptor Fusion Proteins in Drug Discovery

Detecting Secretory Proteins by Acoustic Droplet Ejection in Multiplexed High-Throughput Applications

Nearly one-third of the encoded proteome is comprised of secretory proteins that enable communication between cells and organ systems, playing a ubiquitous role in human health and disease. High-throughput detection of secreted proteins would enhance efforts to identify therapies for secretion-related diseases. Using the Z mutant of alpha-1 antitrypsin as a human secretory model, we have developed 1536-well high-throughput screening assays that utilize acoustic droplet ejection to transfer nanoliter volumes of sample for protein quantification. Among them, the acoustic reverse phase protein array (acoustic RPPA) is a multiplexable, low-cost immunodetection technology for native, endogenously secreted proteins from physiologically relevant model systems like stem cells that is compatible with plate-based instrumentation. Parallel assay profiling with the LOPAC¹²⁸⁰ chemical library validated performance and orthogonality between a secreted bioluminescent reporter and acoustic RPPA method by consistently identifying secretory modulators with comparable concentration response relationships. Here, we introduce a robust, multiplexed drug discovery platform coupling extracellular protein quantification by acoustic RPPA with intracellular and cytotoxicity analyses from single wells, demonstrating proof-of-principle applications for human induced pluripotent stem cell-derived hepatocytes.

Three classes of ligands each bind to distinct sites on the orphan G protein-coupled receptor GPR84

Medium chain fatty acids can activate the pro-inflammatory receptor GPR84 but so also can molecules related to 3,3′-diindolylmethane. 3,3′-Diindolylmethane and decanoic acid acted as strong positive allosteric modulators of the function of each other and analysis showed the affinity of 3,3′-diindolylmethane to be at least 100 fold higher. Methyl decanoate was not an agonist at GPR84. This implies a key role in binding for the carboxylic acid of the fatty acid. Via homology modelling we predicted and confirmed an integral role of arginine¹⁷², located in the 2nd extracellular loop, in the action of decanoic acid but not of 3,3′-diindolylmethane. Exemplars from a patented series of GPR84 antagonists were able to block agonist actions of both decanoic acid and 3,3′-diindolylmethane at GPR84. However, although a radiolabelled form of a related antagonist, [³H]G9543, was able to bind with high affinity to GPR84, this was not competed for by increasing concentrations of either decanoic acid or 3,3′-diindolylmethane and was not affected adversely by mutation of arginine¹⁷². These studies identify three separable ligand binding sites within GPR84 and suggest that if medium chain fatty acids are true endogenous regulators then co-binding with a positive allosteric modulator would greatly enhance their function in physiological settings.

Low affinity GPCRs for metabolic intermediates: challenges for pharmacologists

The discovery that a number of metabolites and metabolic intermediates can act through G protein-coupled receptors has attracted great interest in the field and has led to new therapeutic targets for diseases such as hypertension, type 2 diabetes, inflammation, and metabolic syndrome. However, the low apparent affinity of these ligands for their cognate receptors poses a number of challenges for pharmacologists interested in investigating receptor structure, function or physiology. Furthermore, the endogenous ligands matched to their receptors have other, well established metabolic roles and thus selectivity is difficult to achieve. This review discusses some of the issues researchers face when working with these receptors and highlights the ways in which a number of these obstacles have been overcome.

Allostery at G protein-coupled receptor homo- and heteromers: uncharted pharmacological landscapes

For many years seven transmembrane domain G protein-coupled receptors (GPCRs) were thought to exist and function exclusively as monomeric units. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can both traffic and signal within higher-order complexes. As for other protein-protein interactions, conformational changes in one polypeptide, including those resulting from binding of pharmacological ligands, have the capacity to alter the conformation and therefore the response of the interacting protein(s), a process known as allosterism. For GPCRs, allosterism across homo- or heteromers, whether dimers or higher-order oligomers, represents an additional topographical landscape that must now be considered pharmacologically. Such effects may offer the opportunity for novel therapeutic approaches. Allosterism at GPCR heteromers is particularly exciting in that it offers additional scope to provide receptor subtype selectivity and tissue specificity as well as fine-tuning of receptor signal strength. Herein, we introduce the concept of allosterism at both GPCR homomers and heteromers and discuss the various questions that must be addressed before significant advances can be made in drug discovery at these GPCR complexes.

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.

Selective and brain penetrant neuropeptide y y2 receptor antagonists discovered by whole-cell high-throughput screening

The role of neuropeptide Y Y2 receptor (Y2R) in human diseases such as obesity, mood disorders, and alcoholism could be better resolved by the use of small-molecule chemical probes that are substantially different from the currently available Y2R antagonist, N-[(1S)-4-[(aminoiminomethyl)amino]-1-[[[2-(3,5-dioxo-1,2-diphenyl-1,2,4-triazolidin-4-yl)ethyl]amino]carbonyl]butyl]-1-[2-[4-(6,11-dihydro-6-oxo-5H-dibenz[b,e]azepin-11-yl)-1-piperazinyl]-2-oxoethyl]-cyclopentaneacetamide) (BIIE0246). Presented here are five potent, selective, and publicly available Y2R antagonists identified by a high-throughput screening approach. These compounds belong to four chemical scaffolds that are structurally distinct from the peptidomimetic BIIE0246. In functional assays, IC(50) values between 199 and 4400 nM against the Y2R were measured, with no appreciable activity against the related NPY-Y1 receptor (Y1R). Compounds also displaced radiolabeled peptide YY from the Y2R with high affinity (K(i) values between 1.55 and 60 nM) while not displacing the same ligand from the Y1R. In contrast to BIIE0246, Schild analysis with NPY suggests that two of the five compounds behave as competitive antagonists. Profiling against a panel of 40 receptors, ion channels, and transporters found in the central nervous system showed that the five Y2R antagonists demonstrate greater selectivity than BIIE0246. Furthermore, the ability of these antagonists to penetrate the blood-brain barrier makes them better suited for pharmacological studies of Y2R function in both the brain and periphery.

High-level production and characterization of a G-protein coupled receptor signaling complex

Elucidation of the molecular details of signal transduction through G-protein coupled receptors (GPCRs) awaits the solution of high-resolution structures of the receptor species involved in passing the extracellular information across the plasma membrane. The critical challenge in this effort is the production of sufficient quantities of active and homogeneous receptor species amenable to crystallization screening. We describe here the high-level expression in mammalian cells and characterization of a fusion complex between the kappa opioid receptor and its cognate G-protein alpha subunit, G alpha(i1). Optimization of growth conditions resulted in the highest level of active binding sites reported to date for either opioid receptors or GPCR-G alpha fusions. In cells, the kappa opioid receptor was stabilized against proteolysis in the context of the fusion protein and was competent to bind both agonists and antagonists. Coupling of the kappa opioid receptor with the G alpha subunit was demonstrated by changes in agonist affinity in the presence of guanine nucleotides and by agonist-induced increases in the rate of guanine nucleotide hydrolysis. In addition to representing a physiologically relevant signaling complex, the additional hydrophilic surface area provided by the G-protein may enhance the chances of producing well-diffracting crystals from the purified complex.

Controlled coupling of peptides at their C-termini

Fusion of two proteins has become an important tool in biotechnology. Whereas biotechnological methods easily can produce C-terminal to N-terminal fused compounds, methods to couple two proteins to each of their C-termini are not easily accessible. Herein, peptides are used as models for larger proteins. A method is described exploiting the possibility to attach different reactive handles to their C-termini using a reaction catalyzed by the enzyme carboxypeptidase Y (CPY). It is possible to attach pairs of reaction handles which can react with each other to each of the peptides to be coupled. In a second step, the two modified peptides can be linked together by a chemical reaction, such as an oxime-forming reaction or a copper(I) catalyzed [2+3]-cycloaddition reaction of an azide with an alkyne.

New insights into GPCR function: implications for HTS

G protein-coupled receptors (GPCRs) are a large family of proteins that represent targets for approximately 40% of all approved drugs. They possess unique structural motifs that allow them to interact with a diverse series of extracellular ligands, as well as intracellular signaling proteins, such as G proteins, RAMPs, arrestins, and indeed other receptors. Extensive efforts are under way to discover new generations of drugs against GPCRs with unique targeted therapeutic uses, including "designer" drugs such as allosteric regulators, inverse agonists, and drugs targeting hetero-oligomeric complexes. This has been facilitated by the development of new screening technologies to identify novel drugs against both known and orphan GPCRs.

A homogeneous G protein-coupled receptor ligand binding assay based on time-resolved fluorescence resonance energy transfer

Fluorescence resonance energy transfer (FRET) has emerged as a powerful tool to the study of protein-protein interactions, such as receptor-ligand binding. However, the application of FRET to the study of G protein-coupled receptors (GPCRs) has been limited by the method of labeling receptor with fluorescence probes. Here we described a novel time-resolved (TR)-FRET method to study GPCR-ligand binding by using human complement 5a (C5a) receptor (C5aR) as a model system. Human C5aR was expressed in human embryonic kidney 293 cells with a hemagglutinin (HA) epitope at the N-terminus. Purified human C5a was labeled with terbium chelate and used as the fluorescence donor. Monoclonal anti-HA antibody conjugated with Alexa Fluor 488 was used as the fluorescence acceptor. Robust FRET signal was observed when the labeled ligand and C5aR membrane were mixed in the presence of the conjugated anti-HA antibody. This FRET signal was specific and saturable. C5a binding affinity to C5aR measured by the FRET assay was consistent with the data as determined by competition binding analysis using radiolabeled C5a. The FRET assay was also used to determine affinity of C5aR antagonists by competition binding analysis, and the data are similar to those from radioligand binding studies. Compared to the commonly used radioligand binding assay, this TR-FRET-based assay provides a nonradioactive, faster, and sensitive homogeneous assay format that could be easily adapted to high-throughput screening. The principle of this assay should also be applicable to other GPCRs, especially to those receptors with peptide or protein ligands.

mu-Opioid receptor forms a functional heterodimer with cannabinoid CB1 receptor: electrophysiological and FRET assay analysis

Interactions between mu-opioid receptor (muOR) and cannabinoid CB1 receptor (CB1R) were examined by morphological and electrophysiological methods. In baby hamster kidney (BHK) cells coexpressing muOR fused to the yellow fluorescent protein Venus and CB1R fused to the cyan fluorescent protein Cerulean, both colors were detected on the cell surface; and fluorescence resonance energy transfer (FRET) analysis revealed that muOR and CB1R formed a heterodimer. Coimmunoprecipitation and Western blotting analyses also confirmed the heterodimers of muOR and CB1R. [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) or CP55,940 elicited K+ currents in Xenopus oocytes expressing muOR or CB1R together with G protein activated-inwardly rectifying K+ channels (GIRKs), respectively. In oocytes coexpressing both receptors, either of which was fused to the chimeric Galpha protein Gqi5 that activates the phospholipase C pathway, both DAMGO and CP55,940 elicited Ca2+-activated Cl(-) currents, indicating that each agonist can induce responses through Gqi5 fused to either its own receptor or the other. Experiments with endogenous Gi/o protein inactivation by pertussis toxin (PTX) supported the functional heterodimerization of muOR/CB1R through PTX-insensitive Gqi5(m) fused to each receptor. Thus, muOR and CB1R form a heterodimer and transmit a signal through a common G protein. Our electrophysiological method could be useful for determination of signals mediated through heterodimerized G protein-coupled receptors.

CXCR2 chemokine receptor antagonism enhances DOP opioid receptor function via allosteric regulation of the CXCR2-DOP receptor heterodimer

Opioid agonists have a broad range of effects on cells of the immune system, including modulation of the inflammatory response, and opioid and chemokine receptors are co-expressed by many white cells. Hetero-oligomerization of the human DOP opioid and chemokine CXCR2 receptors could be detected following their co-expression by each of co-immunoprecipitation, three different resonance energy transfer techniques and the construction of pairs of individually inactive but potentially complementary receptor G-protein α subunit fusion proteins. Although DOP receptor agonists and a CXCR2 antagonist had no inherent affinity for the alternative receptor when either receptor was expressed individually, use of cells that expressed a DOP opioid receptor construct constitutively, and in which expression of a CXCR2 receptor construct could be regulated, demonstrated that the CXCR2 antagonist enhanced the function of DOP receptor agonists only in the presence of CXCR2. This effect was observed for both enkephalin- and alkaloid-based opioid agonists, and the effective concentrations of the CXCR2 antagonist reflected CXCR2 receptor occupancy. Entirely equivalent results were obtained in cells in which the native DOP opioid receptor was expressed constitutively and in which expression of the isolated CXCR2 receptor could be induced. These results indicate that a CXCR2 receptor antagonist can enhance the function of agonists at a receptor for which it has no inherent direct affinity by acting as an allosteric regulator of a receptor that is a heterodimer partner for the CXCR2 receptor. These results have novel and important implications for the development and use of small-molecule therapeutics.

Ligand sensitivity in dimeric associations of the serotonin 5HT2c receptor

G-protein-coupled receptors (GPCRs) respond to external stimuli by activating heterotrimeric G proteins inside the cell. There is increasing evidence that many GPCRs exist as dimers or higher oligomers, but the biochemical nature of such dimers and what roles they have, if any, in signal transduction remains unclear. We conducted a comprehensive study of dimerization of the 5HT2c serotonin receptor using disulphide-trapping experiments. We found a dimer interface between transmembrane (TM) helices IV and V that is markedly sensitive to the state of receptor activation. This dimer seems to be quasisymmetrical in interfacial geometry and asymmetrical in its association with its cognate Gα protein. We also found a second interface at TM I helices, which is insensitive to the state of activation.

FPMOD: a modeling tool for sampling the conformational space of fusion proteins

Fusion proteins are an important class of proteins with diverse applications in biotechnology. They consist of 2 or more rigid domains joined by a flexible linker. Understanding the conformational space of fusion proteins conferred by the flexible linkers is important to predicting its behavior. In this paper, we introduce a modeling tool called FPMOD (Fusion Protein MODeller) which samples the conformational space of fusion proteins by treating all domains as rigid bodies and rotating each of them around their flexible linkers. As a demonstration, FPMOD was used to predict the fluorescence resonance energy transfer (FRET) efficiency of three different fusion protein biosensors. The simulation results of the FRET efficiency prediction were consistent with the in vitro experimental data, which verified that FPMOD is a valid tool to predicting the behavior of fusion proteins.

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.

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.

Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1,Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D(2) receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [(35)S]GTPgammaS onto each of Galpha(i1),Galpha(i2),Galpha(i3), and Galpha(o1). By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when Galpha(o1) was the target G protein but an antagonist/inverse agonist at Galpha(i1),Galpha(i2), and Galpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D(2) receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D(2) receptor was fused to Galpha(i1),Galpha(i2), or Galpha(i3). However, it bound to distinct high- and low-affinity states of the D(2) receptor-Galpha(o1) fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D(2) receptor when expression of pertussis toxin-resistant forms of each of Galpha(i1), Galpha(i2), Galpha(i3), and Galpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of Galpha(o1). p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D(2) receptor and may explain in vivo actions of this ligand.

Uncovering the pharmacology of the G protein-coupled receptor GPR40: high apparent constitutive activity in guanosine 5'-O-(3-[35S]thio)triphosphate binding studies reflects binding of an endogenous agonist

In cells lacking expression of Ca(2+)-mobilizing G proteins, coexpression of human GPR40 and Galpha(q) allowed medium- and long-chain fatty acids to elevate intracellular [Ca(2+)]. This was also observed when human embryonic kidney (HEK) 293 cells were transfected with a GPR40-Galpha(q) fusion protein. The kinetic of elevation of intracellular [Ca(2+)] slowed with increasing fatty acid chain length, suggesting different ligand on-rates, whereas the addition of fatty acid-free bovine serum albumin reduced signals, presumably by binding the fatty acids. To allow effective ligand equilibration, GPR40-Galpha(q) was used in guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assays. After expression of GPR40-Galpha(q) in HEK293 cells and membrane preparation basal binding of [(35)S]GTPgammaSinGalpha(q) immunoprecipitates was high and not elevated substantially by fatty acids. However, treatment of membranes with fatty acid-free bovine serum albumin reduced the basal [(35)S]GTPgammaS binding in a concentration-dependent manner and allowed the responsiveness and pharmacology at GPR40 of each of the fatty acids thiazolidinediones and a novel small-molecule agonist to be uncovered. Membranes of rat INS-1E cells that express GPR40 endogenously provided similar observations. The high apparent constitutive activity of GPR40-Galpha(q) was also reversed by a small-molecule GPR40 antagonist, and basal [(35)S]GTPgammaS binding was prevented by the selective Galpha(q)/Galpha(11) inhibitor YM-254890. The current studies provide novel insights into the pharmacology of GPR40 and indicate that G protein-coupled receptors which respond to fatty acids, and potentially to other lipid ligands, can be occupied by endogenous agonists before assay and that this may mask the pharmacology of the receptor and may be mistaken for high levels of constitutive activity.

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.

Functional interactions between the alpha1b-adrenoceptor and Galpha11 are compromised by de-palmitoylation of the G protein but not of the receptor

Both the alpha1b-adrenoceptor and Galpha11 are targets for post-translational thio-acylation that is regulated by agonist occupancy of the receptor [P.A. Stevens, J. Pediani, J.J. Carrillo, G. Milligan, J. Biol. Chem. 276 (2001) 35883]. In co-expression studies mutation of the sites of thio-acylation in the G protein or treatment of cell membranes with hydroxylamine greatly reduced agonist stimulation of guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTPgammaS) binding. In alpha1b-adrenoceptor-Galpha11 fusion proteins mutation of thio-acylation sites in receptor or G protein did not alter the binding affinity of the antagonist [3H]prazosin or the agonist phenylephrine. Although the potency of phenylephrine to stimulate binding of [35S]GTPgammaS to alpha1b-adrenoceptor-Galpha11 fusion proteins was unaffected by the thio-acylation potential of either element, the maximal effect was reduced by some 50% when the G protein but not the receptor was mutated to prevent thio-acylation. This reflected lack of thio-acylation of the G protein rather than mutation of Cys9 and Cys10 to Ser because treatment with hydroxylamine mimicked this in fusions containing the wild type G protein but was without effect in those mutated to prevent thio-acylation. Mutation to reduce binding of beta/gamma to Galpha11 markedly reduced phenylephrine stimulation of [35S]GTPgammaS binding. Combination of mutations to prevent thio-acylation and beta/gamma binding did not, however, have an additive effect on [35S]GTPgammaS binding. These results indicate that the thio-acylation status of the alpha1b-adrenoceptor does not regulate G protein activation whereas thio-acylation of Galpha11 plays a key role in activation by the receptor beyond providing membrane association and proximity.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Activation of an alpha2A-adrenoceptor-Galphao1 fusion protein dynamically regulates the palmitoylation status of the G protein but not of the receptor

Post-translational thio-acylation of a fusion protein between the alpha2A-adrenoceptor and the alpha subunit of the G protein G(o1) is both dynamic and regulated by agonist binding. Incorporation of [3H]palmitate into the fusion protein was reduced substantially in the presence of the agonist adrenaline. This was dependent on the concentration of adrenaline and correlated with occupancy of the ligand binding site. Both the receptor and G-protein elements of the fusion construct incorporated [3H]palmitate but this occurred more rapidly for the G-protein element and regulation of acylation by the agonist occurred only for the G protein. The kinetics of de-palmitoylation of the alpha2A-adrenoceptor-Galpha(o1) fusion were accelerated markedly by agonist. Again, this reflected modulation of the G protein but not of the receptor. Agonist-induced regulation of the kinetics of thio-acylation of the G protein was abolished, however, in a mutant unable to bind guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTP[S]) in response to adrenaline. Despite the dynamic nature of the post-translational acylation and its regulation by agonist, the ability of adrenaline to activate the G protein, monitored by stimulation of the binding of [35S]GTP[S] to such fusion constructs, was unaffected by the palmitoylation potential of either the receptor or G-protein element.

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.

Techniques: GPCR assembly, pharmacology and screening by flow cytometry

Flow cytometers are well known for their ability to analyze and sort cells at high rates based on physiological responses and expression of protein markers. The potential for flow cytometry in G-protein-coupled receptor (GPCR) research, however, is less well appreciated. Potential applications include: (i) the homogenous discrimination of free and bound ligands or proteins in both cellular and microsphere-based assays; and (ii) multiplexed ('suspension array') analysis of cell responses and protein-protein interactions. Innovative sample-handling systems also provide sub-second resolution of interaction kinetics and 1 second per well throughput of microliter-sized samples from multiwell plates. Flow cytometric methods using microspheres for analysis of GPCRs that interact with intracellular and extracellular binding partners such as ligands, G proteins and kinases have been established. These analyses can produce quantitative pharmacological data analogous to radioligand assays, and, in some cases, the probes can be integrated into the assembly as fluorescent fusion proteins.

Interactions between G-protein-coupled receptors and periplakin: a selective means to regulate G-protein activation

A substantial number of G-protein-coupled receptor-interacting proteins have been identified initially by the use of yeast two-hybrid screens. Using the C-terminal tail of both opioid receptors and the melanin concentrating hormone receptor-1 as bait, the actin and intermediate filament-binding protein periplakin was isolated. In each case, the site of interaction is within helix VIII of the receptor and periplakin limits agonist-mediated G-protein activation potentially by competing with G-protein for this region of the receptor.