Galpha(16/z) chimeras efficiently link a wide range of G protein-coupled receptors to calcium mobilization

Inhibition of adenylyl cyclase by GTPase-deficient Gαi is mechanistically different from that mediated by receptor-activated Gαi

Signal transduction through G protein-coupled receptors (GPCRs) has been a major focus in cell biology for decades. Numerous disorders are associated with GPCRs that utilize Gi proteins to inhibit adenylyl cyclase (AC) as well as regulate other effectors. Several early studies have successfully defined the AC-interacting domains of several members of Gαi by measuring the loss of activity upon homologous replacements of putative regions of constitutive active Gαi mutants. However, whether such findings can indeed be translated into the context of a receptor-activated Gαi have not been rigorously verified. To address this issue, an array of known and new chimeric mutations was introduced into GTPase-deficient Q204L (QL) and R178C (RC) mutants of Gαi1, followed by examinations on their ability to inhibit AC. Surprisingly, most chimeras failed to abolish the constitutive activity brought on by the QL mutation, while some were able to eliminate the inhibitory activity of RC mutants. Receptor-mediated inhibition of AC was similarly observed in the same chimeric constructs harbouring the pertussis toxin (PTX)-resistant C351I mutation. Moreover, RC-bearing loss-of-function chimeras appeared to be hyper-deactivated by endogenous RGS protein. Molecular docking revealed a potential interaction between AC and the α3/β5 loop of Gαi1. Subsequent cAMP assays support a cooperative action of the α3/β5 loop, the α4 helix, and the α4/β6 loop in mediating AC inhibition by Gαi1-i3. Our results unveiled a notable functional divergence between constitutively active mutants and receptor-activated Gαi1 to inhibit AC, and identified a previously unknown AC-interacting domain of Gαi subunits. These results collectively provide valuable insights on the mechanism of AC inhibition in the cellular environment.

Cardiac human bitter taste receptors contain naturally occurring variants that alter function

Bitter taste receptors (T2R) are a subfamily of G protein-coupled receptors that enable humans to detect aversive and toxic substances. The ability to discern bitter compounds varies between individuals and is attributed mainly to naturally occurring T2R polymorphisms. T2Rs are also expressed in numerous non-gustatory tissues, including the heart, indicating potential contributions to cardiovascular physiology. In this study. T2Rs that have previously been identified in human cardiac tissues (T2Rs - 10, 14, 30, 31, 46 and 50) and their naturally occurring polymorphisms were functionally characterised. The ligand-dependent signaling responses of some T2R variants were completely abolished (T2R30 Leu252 and T2R46 Met228), whereas other receptor variants had moderate changes in their maximal response, but not potency, relative to wild type. Using a cAMP fluorescent biosensor, we reveal the productive coupling of T2R14, but not the T2R14 Phe201 variant, to endogenous Gαi. Modeling revealed that these variants resulted in altered interactions that generally affected ligand binding (T2R30 Leu252) or Gα protein interactions (T2R46 Met228 and T2R14 Phe201), rather than receptor structural stability. Interestingly, this study is the first to show a difference in signaling for T2R50 Tyr203 (rs1376251) which has been associated with cardiovascular disease. The observation of naturally occurring functional variation in the T2Rs with the greatest expression in the heart is important, as their discovery should prove useful in deciphering the role of T2Rs within the cardiovascular system.

Probing the orphan receptors: Tools and directions

The endogenous ligands activating a large fraction of the G Protein Coupled Receptor (GPCR) family members have yet to be identified. These receptors are commonly labeled as orphans (oGPCRs), and because of the absence of available pharmacological tools they are currently understudied. Nonetheless, genome wide association studies, together with research using animal models identified many physiological functions regulated by oGPCRs. Similarly, mutations in some oGPCRs have been associated with rare genetic disorders or with an increased risk of developing pathologies. The once underestimated pharmacological potential of targeting oGPCRs is increasingly being exploited by the development of novel tools to understand their biology and by drug discovery endeavors aimed at identifying new modulators of their activity. Here, we summarize recent advancements in the field of oGPCRs and future directions.

Lipoxin Mimetics and the Resolution of Inflammation

Inflammation and its timely resolution are critical to ensure effective host defense and appropriate tissue repair after injury and or infection. Chronic, unresolved inflammation typifies many prevalent pathologies. The key mediators that initiate and drive the inflammatory response are well defined and targeted by conventional anti-inflammatory therapeutics. More recently, there is a growing appreciation that specific mediators, including arachidonate-derived lipoxins, are generated in self-limiting inflammatory responses to promote the resolution of inflammation and endogenous repair mechanisms without compromising host defense. We discuss the proresolving biological actions of lipoxins and recent efforts to harness their therapeutic potential through the development of novel, potent lipoxin mimetics generated via efficient, modular stereoselective synthetic pathways. We consider the evidence that lipoxin mimetics may have applications in limiting inflammation and reversing fibrosis and the underlying mechanisms.

Negative allosteric modulators of group II metabotropic glutamate receptors: A patent review (2015 - present)

INTRODUCTION

Group II metabotropic glutamate (mGlu) receptors have emerged as an attractive potential target for the development of novel CNS therapeutics in areas such as Alzheimer's disease (AD), anxiety, cognitive disorders, depression, and others. Several small molecules that act as negative allosteric modulators (NAMs) on these receptors have demonstrated efficacy and/or target engagement in animal models, and one molecule (decoglurant) has been advanced into clinical trials.

AREAS COVERED

This review summarizes patent applications published between January 2015 and November 2020. It is divided into three sections: (1) small molecule nonselective mGlu_2/3 NAMs, (2) small molecule selective mGlu₂ NAMs, and (3) small molecule selective mGlu₃ NAMs.

EXPERT OPINION

Much progress has been made in the discovery of novel small molecule mGlu₂ NAMs. Still, chemical diversity remains somewhat limited and room for expansion remains. Progress with mGlu₃ NAMs has been more limited; however, some promising molecules have been disclosed. The process of elucidating the precise role of each receptor in the diseases associated with group II receptors has begun. Continued studies in animals with selective NAMs for both receptors will be critical in the coming years to inform researchers on the right compound profile and patient population for clinical development.

Activation of the μ-opioid receptor by alicyclic fentanyls: Changes from high potency full agonists to low potency partial agonists with increasing alicyclic substructure

Fentanyl analogs represent an important group of new psychoactive substances and knowing their efficacy and potency might assist in interpreting observed concentrations. The potency of fentanyl analogs can be estimated from in vitro studies and can be used to establish structure–activity relationships. In this study, recombinant CHO‐K1 cells (AequoScreen) expressing the human μ‐opioid receptor were used to establish dose–response curves via luminescent analysis for cyclopropyl‐, cyclobutyl‐, cyclopentyl‐, cyclohexyl‐, and 2,2,3,3‐tetramethylcyclopropylfentanyl (TMCPF), on three separate occasions, using eight different concentrations in an eight‐fold serial dilution in triplicates starting at ~60 μM. Fentanyl was used as a full agonist reference while morphine and buprenorphine were included for comparison. Cyclopropylfentanyl (EC₅₀ = 4.3 nM), cyclobutylfentanyl (EC50 = 6.2 nM), and cyclopentylfentanyl (EC₅₀ = 13 nM) were full agonists slightly less potent than fentanyl (EC₅₀ = 1.7 nM). Cyclohexylfentanyl (EC₅₀ = 3.1 μM, efficacy 48%) and TMCPF (EC₅₀ = 1.5 μM, efficacy 65%) were partial agonists less potent than morphine (EC₅₀ = 430 nM). Based on the results, cyclopropyl‐, cyclobutyl‐, and cyclopentylfentanyl would be expected to induce intoxication or cause fatal poisonings at similar concentrations to fentanyl, while the toxic or fatal concentrations of cyclohexylfentanyl and TMCPF would be expected to be much higher.

Analyzing kinetic signaling data for G-protein-coupled receptors

In classical pharmacology, bioassay data are fit to general equations (e.g. the dose response equation) to determine empirical drug parameters (e.g. EC50 and Emax), which are then used to calculate chemical parameters such as affinity and efficacy. Here we used a similar approach for kinetic, time course signaling data, to allow empirical and chemical definition of signaling by G-protein-coupled receptors in kinetic terms. Experimental data are analyzed using general time course equations (model-free approach) and mechanistic model equations (mechanistic approach) in the commonly-used curve-fitting program, GraphPad Prism. A literature survey indicated signaling time course data usually conform to one of four curve shapes: the straight line, association exponential curve, rise-and-fall to zero curve, and rise-and-fall to steady-state curve. In the model-free approach, the initial rate of signaling is quantified and this is done by curve-fitting to the whole time course, avoiding the need to select the linear part of the curve. It is shown that the four shapes are consistent with a mechanistic model of signaling, based on enzyme kinetics, with the shape defined by the regulation of signaling mechanisms (e.g. receptor desensitization, signal degradation). Signaling efficacy is the initial rate of signaling by agonist-occupied receptor (kτ), simply the rate of signal generation before it becomes affected by regulation mechanisms, measurable using the model-free analysis. Regulation of signaling parameters such as the receptor desensitization rate constant can be estimated if the mechanism is known. This study extends the empirical and mechanistic approach used in classical pharmacology to kinetic signaling data, facilitating optimization of new therapeutics in kinetic terms.

A Bitter Taste in Your Heart

The human genome contains ∼29 bitter taste receptors (T2Rs), which are responsible for detecting thousands of bitter ligands, including toxic and aversive compounds. This sentinel function varies between individuals and is underpinned by naturally occurring T2R polymorphisms, which have also been associated with disease. Recent studies have reported the expression of T2Rs and their downstream signaling components within non-gustatory tissues, including the heart. Though the precise role of T2Rs in the heart remains unclear, evidence points toward a role in cardiac contractility and overall vascular tone. In this review, we summarize the extra-oral expression of T2Rs, focusing on evidence for expression in heart; we speculate on the range of potential ligands that may activate them; we define the possible signaling pathways they activate; and we argue that their discovery in heart predicts an, as yet, unappreciated cardiac physiology.

Molecular basis defining the selectivity of substituted isoquinolinones for the melatonin MT2 receptor

Melatonin MT₁ and MT₂ receptors represent attractive drug targets for the treatment of various disorders. However, the high conservation of the melatonin binding pocket has hindered the development of subtype-selective compounds. By leveraging on the recently resolved crystal structures of MT₁ and MT₂ receptors, this study aims to elucidate the structural basis of MT₂-selectivity of a panel of isoquinolinone derivatives. Molecular modelling and ligand docking approaches were employed to predict residues involved in forming interactions with the MT₂-selective isoquinolinones. Seven conserved residues (Asn¹⁷⁵, His²⁰⁸, Trp²⁶⁴, Asn²⁶⁸, Gly²⁷¹, Tyr²⁹⁴ and Tyr²⁹⁸) were selected as targets for site-directed mutagenesis. Ca²⁺ mobilization, cAMP inhibition, phosphorylation of extracellular signal-regulated kinase, and ligand binding assays were performed to functionally characterize the receptor mutants in transfected CHO cells. Unlike melatonin, isoquinolinones bearing a 3-methoxybenzyloxyl substituent were unaffected by alanine substitution at His²⁰⁸ of MT₂. Although alanine substitutions at Tyr²⁹⁴ or Tyr²⁹⁸ reduced the potency of melatonin and some isoquinolinones on MT₂, similar mutations on MT₁ allowed five hitherto ineffective isoquinolinones to act as agonists. An isoquinolinone antagonist bearing a 4-methoxybenzyloxyl moiety turned into an agonist at MT₂ mutants with alanine substitutions at His²⁰⁸, Tyr²⁹⁴ or Tyr²⁹⁸. A subset of residues is apparently involved in forming a hydrophobic binding cavity to confer selectivity upon the aromatic substituent of isoquinolinone compounds. Two conserved tyrosine residues on transmembrane helix 7 may confer ligand selectivity at MT₁ and MT₂ receptors, while a conserved histidine on transmembrane helix 5 is apparently involved in receptor activation.

Celastrol is a novel selective agonist of cannabinoid receptor 2 with anti-inflammatory and anti-fibrotic activity in a mouse model of systemic sclerosis

BACKGROUND

Increasing evidence indicated that the cannabinoid receptors were involved in the pathogenesis of organ fibrogenesis.

PURPOSE

The purpose of this study was to discover novel cannabinoid receptor 2 (CB2) agonist and assess the potential of CB2 activation in treating systemic sclerosis.

METHODS

A gaussia princeps luciferase-based split luciferase complementation assay (SLCA) was developed for detection of the interaction between CB2 and β-arrestin2. A library of 366 natural products was then screened as potential CB2 agonist using SLCA approach. Several GPCR functional assays, including HTRF-based cAMP assay and calcium mobilization were also utilized to evaluated CB2 activation. Bleomycin-induced experimental systemic sclerosis was used to assess the in vivo anti-fibrotic effects. Dermal thickness and collagen content were evaluated via H&E and sirius red staining.

RESULTS

Celastrol was identified as a new agonist of CB2 by using SLCA. Furthermore, celastrol triggers several CB2-mediated downstream signaling pathways, including calcium mobilization, inhibition of cAMP accumulation, and receptor desensitization in a dose-dependent manner, and it has a moderate selectivity on CB1. In addition, celastrol exhibited the anti-inflammatory properties on lipopolysaccharide (LPS) treated murine Raw 264.7 macrophages and primary macrophages. Finally, we found that celastrol exerts anti-fibrotic effects in the bleomycin-induced systemic sclerosis mouse model accompanied by reduced inflammatory conditions.

CONCLUSION

Taken together, celastrol is identified a novel selective CB2 agonist using a new developed arrestin-based SLCA, and CB2 activation by celastrol reduces the inflammatory response, and prevents the development of dermal fibrosis in bleomycin-induced systemic sclerosis mouse model.

Mel1c Mediated Monochromatic Light-Stimulated IGF-I Synthesis through the Intracellular Gαq/PKC/ERK Signaling Pathway

Previous studies have demonstrated that monochromatic light affects plasma melatonin (MEL) levels, which in turn regulates hepatic insulin-like growth factor I (IGF-I) secretion via the Mel1c receptor. However, the intracellular signaling pathway initiated by Mel1c remains unclear. In this study, newly hatched broilers, including intact, sham operation, and pinealectomy groups, were exposed to either white (WL), red (RL), green (GL), or blue (BL) light for 14 days. Experiments in vivo showed that GL significantly promoted plasma MEL formation, which was accompanied by an increase in the MEL receptor, Mel1c, as well as phosphorylated extracellular regulated protein kinases (p-ERK1/2), and IGF-I expression in the liver, compared to the other light-treated groups. In contrast, this GL stimulation was attenuated by pinealectomy. Exogenous MEL elevated the hepatocellular IGF-I level, which is consistent with increases in cyclic adenosine monophosphate (cAMP), G_αq, phosphorylated protein kinase C (p-PKC), and p-ERK1/2 expression. However, the Mel1c selective antagonist prazosin suppressed the MEL-induced expression of IGF-I, G_αq, p-PKC, and p-ERK1/2, while the cAMP concentration was barely affected. In addition, pretreatment with Ym254890 (a G_αq inhibitor), Go9863 (a PKC inhibitor), and PD98059 (an ERK1/2 inhibitor) markedly attenuated MEL-stimulated IGF-I expression and p-ERK1/2 activity. These results indicate that Mel1c mediates monochromatic GL-stimulated IGF-I synthesis through intracellular G_αq/PKC/ERK signaling.

Activation of airway epithelial bitter taste receptors by Pseudomonas aeruginosa quinolones modulates calcium, cyclic-AMP, and nitric oxide signaling

Bitter taste receptors (taste family 2 bitter receptor proteins; T2Rs), discovered in many tissues outside the tongue, have recently become potential therapeutic targets. We have shown previously that airway epithelial cells express several T2Rs that activate innate immune responses that may be important for treatment of airway diseases such as chronic rhinosinusitis. It is imperative to more clearly understand what compounds activate airway T2Rs as well as their full range of functions. T2R isoforms in airway motile cilia (T2R4, -14, -16, and -38) produce bactericidal levels of nitric oxide (NO) that also increase ciliary beating, promoting clearance of mucus and trapped pathogens. Bacterial quorum-sensing acyl-homoserine lactones activate T2Rs and stimulate these responses in primary airway cells. Quinolones are another type of quorum-sensing molecule used by Pseudomonas aeruginosa To elucidate whether bacterial quinolones activate airway T2Rs, we analyzed calcium, cAMP, and NO dynamics using a combination of fluorescent indicator dyes and FRET-based protein biosensors. T2R-transfected HEK293T cells, several lung epithelial cell lines, and primary sinonasal cells grown and differentiated at the air-liquid interface were tested with 2-heptyl-3-hydroxy-4-quinolone (known as Pseudomonas quinolone signal; PQS), 2,4-dihydroxyquinolone, and 4-hydroxy-2-heptylquinolone (HHQ). In HEK293T cells, PQS activated T2R4, -16, and -38, whereas HHQ activated T2R14. 2,4-Dihydroxyquinolone had no effect. PQS and HHQ increased calcium and decreased both baseline and stimulated cAMP levels in cultured and primary airway cells. In primary cells, PQS and HHQ activated levels of NO synthesis previously shown to be bactericidal. This study suggests that airway T2R-mediated immune responses are activated by bacterial quinolones as well as acyl-homoserine lactones.

Kinetic operational models of agonism for G-protein-coupled receptors

The application of kinetics to research and therapeutic development of G-protein-coupled receptors has become increasingly valuable. Pharmacological models provide the foundation of pharmacology, providing concepts and measurable parameters such as efficacy and potency that have underlain decades of successful drug discovery. Currently there are few pharmacological models that incorporate kinetic activity in such a way as to yield experimentally-accessible drug parameters. In this study, a kinetic model of pharmacological response was developed that provides a kinetic descriptor of efficacy (the transduction rate constant, k_τ) and allows measurement of receptor-ligand binding kinetics from functional data. The model assumes: (1) receptor interacts with a precursor of the response ("Transduction potential") and converts it to the response. (2) The response can decay. Familiar response vs time plots emerge, depending on whether transduction potential is depleted and/or response decays. These are the straight line, the "association" exponential curve, and the rise-and-fall curve. Convenient, familiar methods are described for measuring the model parameters and files are provided for the curve-fitting program Prism (GraphPad Software) that can be used as a guide. The efficacy parameter k_τ is straightforward to measure and accounts for receptor reserve; all that is required is measurement of response over time at a maximally-stimulating concentration of agonist. The modular nature of the model framework allows it to be extended. Here this is done to incorporate antagonist-receptor binding kinetics and slow agonist-receptor equilibration. In principle, the modular framework can incorporate other cellular processes, such as receptor desensitization. The kinetic response model described here can be applied to measure kinetic pharmacological parameters than can be used to advance the understanding of GPCR pharmacology and optimize new and improved therapeutics.

Genetic variation in glia-neuron signalling modulates ageing rate

The rate of behavioural decline in the ageing population is remarkably variable among individuals. Despite the considerable interest in studying natural variation in ageing rate to identify factors that control healthy ageing, no such factor has yet been found. Here we report a genetic basis for variation in ageing rates in Caenorhabditis elegans. We find that C. elegans isolates show diverse lifespan and age-related declines in virility, pharyngeal pumping, and locomotion. DNA polymorphisms in a novel peptide-coding gene, named regulatory-gene-for-behavioural-ageing-1 (rgba-1), and the neuropeptide receptor gene npr-28 influence the rate of age-related decline of worm mating behaviour; these two genes might have been subjected to recent selective sweeps. Glia-derived RGBA-1 activates NPR-28 signalling, which acts in serotonergic and dopaminergic neurons to accelerate behavioural deterioration. This signalling involves the SIR-2.1-dependent activation of the mitochondrial unfolded protein response, a pathway that modulates ageing. Thus, natural variation in neuropeptide-mediated glia-neuron signalling modulates the rate of ageing in C. elegans.

Use of Caenorhabditis elegans Gαq Chimeras to Detect G-Protein-Coupled Receptor Signals

G-protein-coupled receptors (GPCRs) activate heterotrimeric G-proteins (Gi-, Gs-, Gq-, or G12-like) to generate specific intracellular responses, depending on the receptor/G-protein coupling. The aim was to enable a majority of GPCRs to generate a predetermined output by signaling through a single G-protein-supported pathway. The authors focused on calcium responses as the output, then engineered Gαq to promote promiscuous receptor interactions. Starting with a human Gαq containing 5 Gαz residues in the C-terminal receptor recognition domain (hGαq/z5), they evaluated agonist-stimulated calcium responses for 33 diverse GPCRs (Gi-, Gs-, and Gq-coupled) and found 20 of 33 responders. In parallel, they tested Caenorhabditis elegans Gαq containing 5 or 9 C-terminal Gαz residues (cGαq/z5, cGαq/z9). Signal detection was enhanced with cGαq/z5 and cGαq/z9 (yielding 25/33 and 26/33 responders, respectively). In a separate study of Gαs-coupled receptors, the authors compared hGαq/s5 versus hGαq/s9, cGαq/s9, andcGαq/s21 and observed optimal function with cGαq/s9. Cotransfection of an engineered Gαq “cocktail” (cGαq/z5 plus cGαq/s9) provided a powerful and efficient screening platform. When the chimeras included N-terminal myristoylation sites (to promote membrane localization), calcium responses were sustained or improved, depending on the receptor. This approach toward a “universal functional assay” is particularly useful for orphan GPCRs whose signaling pathways are unknown.

Multiplex GPCR Assay in Reverse Transfection Cell Microarrays

G protein-coupled receptors (GPCRs) are a superfamily of proteins that include some of the most important drug targets in the pharmaceutical industry. Despite the success of this group of drugs, there remains a need to identify GPCR-targeted drugs with greater selectivity, to develop screening assays for validated targets, and to identify ligands for orphan receptors. To address these challenges, the authors have created a multiplexed GPCR assay that measures greater than 3000 receptor: ligand interactions in a single microplate. The multiplexed assay is generated by combining reverse transfection in a 96-well plate format with a calcium flux readout. This assay quantitatively measures receptor activation and inhibition and permits the determination of compound potency and selectivity for entire families of GPCRs in parallel. To expand the number of GPCR targets that may be screened in this system, receptors are cotransfected with plasmids encoding a promiscuous G protein, permitting the analysis of receptors that do not normally mobilize intracellular calcium upon activation. The authors demonstrate the utility of reverse transfection cell microarrays to GPCR-targeted drug discovery with examples of ligand selectivity screening against a panel of GPCRs as well as dose-dependent titrations of selected agonists and antagonists.

Synthesis and functional characterization of substituted isoquinolinones as MT2-selective melatoninergic ligands

A series of substituted isoquinolinones were synthesized and their binding affinities and functional activities towards human melatonin MT₁ and MT₂ receptors were evaluated. Structure-activity relationship analysis revealed that substituted isoquinolinones bearing a 3-methoxybenzyloxyl group at C5, C6 or C7 position respectively (C5>C6>C7 in terms of their potency) conferred effective binding and selectivity toward the MT₂ receptor, with 15b as the most potent compound. Most of the tested compounds were MT₂-selective agonists as revealed in receptor-mediated cAMP inhibition, intracellular Ca²⁺ mobilization and phosphorylation of extracellular signal-regulated protein kinases. Intriguingly, compounds 7e and 7f bearing a 4-methoxybenzyloxyl group or 4-methylbenzyloxyl at C6 behaved as weak MT₂-selective antagonists. These results suggest that substituted isoquinolinones represent a novel family of MT₂-selective melatonin ligands. The position of the substituted benzyloxyl group, and the substituents on the benzyl ring appeared to dictate the functional characteristics of these compounds.

Activation state-dependent interaction between Gαq subunits and the Fhit tumor suppressor

Background

The FHIT tumor suppressor gene is arguably the most commonly altered gene in cancer since it is inactivated in about 60% of human tumors. The Fhit protein is a member of the ubiquitous histidine triad proteins which hydrolyze dinucleoside polyphosphates such as Ap₃A. Despite the fact that Fhit functions as a tumor suppressor, the pathway through which Fhit inhibits growth of cancer cells remains largely unknown. Phosphorylation by Src tyrosine kinases provides a linkage between Fhit and growth factor signaling. Since many G proteins can regulate cell proliferation through multiple signaling components including Src, we explored the relationship between Gα subunits and Fhit.

Results

Several members of the Gα_q subfamily (Gα₁₆, Gα₁₄, and Gα_q) were found to co-immunoprecipitate with Fhit in their GTP-bound active state in HEK293 cells. The binding of activated Gα_q members to Fhit appeared to be direct and was detectable in native DLD-1 colon carcinoma cells. The use of Gα_16/z chimeras further enabled the mapping of the Fhit-interacting domain to the α2-β4 region of Gα₁₆. However, Gα_q/Fhit did not affect either Ap₃A binding and hydrolysis by Fhit, or the ability of Gα_q/16 to regulate downstream effectors including phospholipase Cβ, Ras, ERK, STAT3, and IKK. Functional mutants of Fhit including the H96D, Y114F, L25W and L25W/I10W showed comparable abilities to associate with Gα_q. Despite the lack of functional regulation of G_q signaling by Fhit, stimulation of G_q-coupled receptors in HEK293 and H1299 cells stably overexpressing Fhit led to reduced cell proliferation, as opposed to an enhanced cell proliferation typically seen with parental cells.

Conclusions

Activated Gα_q members interact with Fhit through their α2-β4 region which may result in enhancement of the growth inhibitory effect of Fhit, thus providing a possible avenue for G protein-coupled receptors to modulate tumor suppression.

Angiotensin-[1-12] interacts with angiotensin type I receptors

Angiotensin-(1-12) [Ang-(1-12)], a newer member of angiotensin peptides, is proposed to be converted enzymatically to angiotensin I (Ang I) and to angiotensin II (Ang II); the latter being the bioactive peptide. We studied the Ang-(1-12) and Ang II responses in COS-7 cells or CHO cells transfected with 5 μg AT1R by monitoring [Ca(2+)]i using the Fluo-4. Ang II (1 pM-1 μM) and Ang-(1-12) (5 pM-5 μM) increased [Ca(2+)]i with an EC50 of 0.19 nM and 24 nM in COS-7 cells; and 0.65 nM and 28.7 nM in CHO cells. The AT1R antagonist losartan (1 nM-10 μM) suppressed [Ca(2+)]i induced by Ang-(1-12) and Ang II. In CHO cells transfected with 5 μg AT2R, Ang II (1 pM-1 μM) increased [Ca(2+)]i, with an EC50 of 9.68 nM; whereas, Ang-(1-12) (5 pM-5 μM) failed to elicit a significant change in [Ca(2+)]i. In CHO cells transfected with AT1R, Ang-(1-12) stimulated ERK phosphorylation with a potency 300-fold less than that of Ang II. To evaluate the activity of Ang-(1-12) on native AT1R, whole cell patch recordings were made from neurons in the rat hypothalamic slices. Ang II or Ang-(1-12) ejected by pressure from a micropipette elicited a membrane depolarization; the latter was blocked by losartan (10 μM), and not affected by the AT2R antagonist PD123319 (10 μM), nor by the angiotensin converting enzyme inhibitor captopril (10 μM). Our result shows that Ang-(1-12) may produce its biological activity by acting directly on AT1R, albeit at a concentration higher than that of Ang II.

Hierarchical organization of multi-site phosphorylation at the CXCR4 C terminus

The chemokine receptor CXCR4 regulates cell migration during ontogenesis and disease states including cancer and inflammation. Upon stimulation by the endogenous ligand CXCL12, CXCR4 becomes phosphorylated at multiple sites in its C-terminal domain. Mutations in the CXCR4 gene affecting C-terminal phosphorylation sites are a hallmark of WHIM syndrome, a genetic disorder characterized by a gain-of-CXCR4-function. To better understand how multi-site phosphorylation of CXCR4 is organized and how perturbed phosphorylation might affect CXCR4 function, we developed novel phosphosite-specific CXCR4 antibodies and studied the differential regulation and interaction of three C-terminal phosphorylation sites in human embryonic kidney cells (HEK293). CXCL12 promoted a robust phosphorylation at S346/347 which preceded phosphorylation at S324/325 and S338/339. After CXCL12 washout, the phosphosites S338/339 and S324/325 were rapidly dephosphorylated whereas phosphorylation at S346/347 was long-lasting. CXCL12-induced phosphorylation at S346/347 was staurosporine-insensitive and mediated by GRK2/3. WHIM syndrome-associated CXCR4 truncation mutants lacking the S346/347 phosphosite and the recently identified E343K WHIM mutant displayed strongly impaired phosphorylation at S324/325 and S338/339 as well as reduced CXCL12-induced receptor internalization. Relevance of the S346-S348 site was confirmed by a S346-348A mutant showing strongly impaired CXCL12-promoted phosphorylation at S324/325 and S338/339, defective internalization, gain of calcium mobilization, and reduced desensitization. Thus, the triple serine motif S346-S348 contains a major initial CXCR4 phosphorylation site and is required for efficient subsequent multi-site phosphorylation and receptor regulation. Hierarchical organization of CXCR4 phosphorylation explains why small deletions at the extreme CXCR4 C terminus typically associated with WHIM syndrome severely alter CXCR4 function.

Rapid uptake and degradation of CXCL12 depend on CXCR7 carboxyl-terminal serine/threonine residues

CXCL12 signaling through G protein-coupled CXCR4 regulates cell migration during ontogenesis and disease states including cancer and inflammation. The second CXCL12-receptor CXCR7 modulates the CXCL12/CXCR4 pathway by acting as a CXCL12 scavenger and exerts G protein-independent functions. Given the distinct properties of CXCR4 and CXCR7, we hypothesized that the distinct C-terminal domains differently regulate receptor trafficking and stability. Here, we examined epitope-tagged wild type and C-terminal mutant receptors in human embryonic kidney cells (HEK293) with respect to trafficking, stability, (125)I-CXCL12 degradation, and G protein-coupling. The 24 CXCR7 C-terminal residues were sufficient to promote rapid spontaneous internalization. Replacement of the CXCR7 C terminus with that of CXCR4 (CXCR7-4tail mutant) abolished spontaneous internalization but permitted ligand-induced internalization and phosphorylation at the heterologous domain. The reverse tail-swap caused ligand-independent internalization of the resulting CXCR4-7tail mutant. Receptor-mediated (125)I-CXCL12 uptake and release of (125)I-CXCL12 degradation products were accelerated with receptors bearing the CXCR7 C terminus and impaired after conversion of CXCR7 C-terminal serine/threonine residues into alanines. C-terminal lysine residues were dispensable for plasma membrane targeting and the CXCL12 scavenger function but involved in constitutive degradation of CXCR7. Although the CXCR7 C terminus abolished G protein coupling in the CXCR4-7tail mutant, replacement of the CXCR7 C terminus, CXCR7 second intracellular loop, or both domains with the corresponding CXCR4 domain did not result in a G protein-coupled CXCR7 chimera. Taken together, we provide evidence that the CXCR7 C terminus influences the ligand-uptake/degradation rate, G protein coupling, and receptor stability. Regulatory pathways targeting CXCR7 C-terminal serine/threonine sites may control the CXCL12 scavenger activity of CXCR7.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

The Aqueous Extract of Radix Glycyrrhizae Stimulates Mitogen-Activated Protein Kinases and Nuclear Factor-κB in Jurkat T-Cells and THP-1 Monocytic Cells

Radix Glycyrrhizae (RG) is a medicinal herb extensively utilized in numerous Chinese medical formulae for coordinating the actions of various components in the recipes and strengthening the body functions. In this report, we demonstrate that the aqueous extract of Radix Glycyrrhizae is capable of stimulating the c-Jun N-terminal kinase and p38 subgroups of mitogen-activated protein kinases (MAPKs), and the nuclear factor-κB ( NF κ B ) in Jurkat T-lymphocytes. The activation magnitudes of MAPKs and NF κ B were dose-dependent ( EC 50 ≈ 1 mg/ml ) and time-dependent (maximal around 15–30 minutes). Stimulations of MAPKs and NF κ B were not associated with changes in intracellular Ca 2+ mobilization. Similar activation profiles of MAPK and NF κ B were obtained from THP-1 monocytes treated with the extract. In terms of chemotactic activity, the SDF-induced chemotaxis of Jurkat cells and THP-1 cells were inhibited by RG extract at 1–10 mg/ml, while a lower RG concentration (0.1–0.3 mg/ml) potentiated the SDF-induced chemotaxis for the former, but not the latter cell type. Given the fact that MAPKs and NF κ B are important signaling intermediates for lymphocyte activities, our results suggest that Radix Glycyrrhizae may contain active constituents capable of modulating immuno-responses through various intracellular signaling pathways.

Development and optimization of FLIPR high throughput calcium assays for ion channels and GPCRs

Ca(2+) permeable ion channels and GPCRs linked to Ca(2+) release are important drug targets, with modulation of Ca(2+) signaling increasingly recognized as a valid therapeutic strategy in a range of diseases. The FLIPR is a high throughput imaging plate reader that has contributed substantially to drug discovery efforts and pharmacological characterization of receptors and ion channels coupled to Ca(2+). Now in its fourth generation, the FLIPR(TETRA) is an industry standard for high throughput Ca(2+) assays. With an increasing number of excitation LED banks and emission filter sets available; FLIPR Ca(2+) assays are becoming more versatile. This chapter describes general methods for establishing robust FLIPR Ca(2+) assays, incorporating practical aspects as well as suggestions for assay optimization, to guide the reader in the development and optimization of high throughput FLIPR assays for ion channels and GPCRs.

Performance of mouse neural stem cells as a screening reagent: characterization of PAC1 activity in medium-throughput functional assays

The self-renewal and phenotypic properties of neural stem cells make them an abundant and more physiologically relevant alternative to recombinant cell lines for drug screens to identify ligands acting at neural targets. Here, the authors use high-throughput phenotypic and signaling assays to test the ability of neural stem cells isolated from postnatal mouse hippocampus (mNSCs) to deliver high-content and physiologically relevant data on native peptide receptor activity. The authors find that mNSCs express PAC1 but not the related VPAC1 and VPAC2 receptors. PAC1 promotes both the proliferation of mNSCs and their differentiation into neuronal-like cells. In addition, the authors show that PAC1 stimulates markedly different extracellular signal-regulated kinase signals in mNSCs than in recombinant CHO-PAC1 cells and is able to couple to Ca(2+) elevation only in CHO-PAC1 cells. These data suggest that G-protein coupling in CHO-PAC1 cells is nonphysiological, which may affect the ligand binding properties of the receptor and thus distort the results of a screen by increasing numbers of false positives/negatives. This work reinforces the emerging pharmacological theory that recombinant cell lines are often inappropriate models of natively expressing primary cells, and the authors conclude that mNSCs are a viable and relevant physiological alternative for use in high-throughput drug screens.

Comparative pharmacology of adrenergic alpha(2C) receptors coupled to Ca(2+) signaling through different Galpha proteins

Adrenergic alpha(1), alpha(2) and beta receptors are members of the G-protein-coupled receptor families (GPCRs) mediating physiological responses to adrenaline (epinephrine) and noradrenaline (norepinephrine). Since GPCRs are major targets for potential therapeutic agents, development of robust, reliable and cost effective functional screening methods for these receptors is in the focus of pharmacological research. For this reason, the aim of the present study was to develop an intracellular calcium assay for investigating the pharmacology of the alpha(2C) type of adrenergic receptors (alpha(2C)-AR). Although activation of alpha(2C)-AR is not linked to calcium mobilization, co-expression of these receptors with the chimeric Galpha(qi5) protein, containing the five carboxyl-terminal amino acids from G(i), or promiscuosus Galpha(16) protein can divert receptor signaling to the G(q) pathway generating Ca(2+) release from intracellular stores. In order to assess the functional potency of alpha(2)-AR agonists and antagonists, we established a fluorometric Ca(2+) assay using cell lines stably and constitutively co-expressing alpha(2C)-AR and Galpha(qi5) or Galpha(16) proteins (Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C)). As part of the pharmacological characterization, we measured the changes in cytoplasmic Ca(2+) levels due to activation of the chimeric Galpha(qi5) or Galpha(16) coupled recombinant alpha(2C) receptors as a function of increasing concentration of several agonists (noradrenaline, brimonidine, oxymetazoline, clonidine, moxonidine) and antagonists (MK912, yohimbine). The binding affinities of alpha(2)-AR agonist and antagonists and the inhibition of the forskolin-stimulated cAMP accumulation in alpha(2C)-AR expressing cells were also measured. These results confirmed that the Galpha(qi5)/alpha(2C) and Galpha(16)/alpha(2C) recombinant systems can be useful for modelling the native G(i)-coupled system. Our results indicate that a plate-reader based fluorometric Ca(2+) assay may be suitable in high-throughput screening for alpha(2C)-AR ligands as well.

Mutations on the Switch III region and the alpha3 helix of Galpha16 differentially affect receptor coupling and regulation of downstream effectors

Background

Gα₁₆ can activate phospholipase Cβ (PLCβ) directly like Gα_q. It also couples to tetratricopeptide repeat 1 (TPR1) which is linked to Ras activation. It is unknown whether PLCβ and TPR1 interact with the same regions on Gα₁₆. Previous studies on Gα_q have defined two minimal clusters of amino acids that are essential for the coupling to PLCβ. Cognate residues in Gα₁₆ might also be essential for interacting with PLCβ, and possibly contribute to TPR1 interaction and other signaling events.

Results

Alanine mutations were introduced to the two amino acid clusters (246–248 and 259–260) in the switch III region and α3 helix of Gα₁₆. Regulations of PLCβ and STAT3 were partially weakened by each cluster mutant. A mutant harboring mutations at both clusters generally produced stronger suppressions. Activation of Jun N-terminal kinase (JNK) by Gα₁₆ was completely abolished by mutating either clusters. Contrastingly, phosphorylations of extracellular signal-regulated kinase (ERK) and nuclear factor κB (NF-κB) were not significantly affected by these mutations. The interactions between the mutants and PLCβ2 and TPR1 were also reduced in co-immunoprecipitation assays. Coupling between G₁₆ and different categories of receptors was impaired by the mutations, with the effect of switch III mutations being more pronounced than those in the α3 helix. Mutations of both clusters almost completely abolished the receptor coupling and prevent receptor-induced Gβγ release.

Conclusion

The integrity of the switch III region and α3 helix of Gα₁₆ is critical for the activation of PLCβ, STAT3, and JNK but not ERK or NF-κB. Binding of Gα₁₆ to PLCβ2 or TPR1 was reduced by the mutations of either cluster. The same region could also differentially affect the effectiveness of receptor coupling to G₁₆. The studied region was shown to bear multiple functionally important roles of G₁₆.

Novel GPCR Screening Approach: Indirect Identification of S1P Receptor Agonists in Antagonist Screening Using a Calcium Assay

To elucidate the physiological function of sphingosine 1-phosphate receptors 1-3 (S1P1-3) we aimed to identify selective ligands for these GPCRs. S1P2 and S1P3 are coupled to Gq, and are, therefore, linked to the phospholipase C/IP3/calcium pathway. S1P1 is solely coupled to Gi and was artificially linked to calcium signaling by coexpression of Galpha 16. The three receptors desensitized on challenge of cells with an agonist (i.e., agonists appeared as antagonists in a second calcium measurement). We screened a compound library for inhibitors of S1P-stimulated calcium signals, and we could identify agonists and antagonists with a single measurement. Agonism and antagonism were confirmed by recording compound-and S1P-induced calcium signals from the same assay well. For the three receptors, we found a reciprocal correlation of agonism and "apparent" antagonism of agonists. In addition, agonists indirectly discovered by this approach do not promote calcium mobilization through endogenous GPCRs.

Discovery and validation of novel peptide agonists for G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent an important group of targets for pharmaceutical therapeutics. The completion of the human genome revealed a large number of putative GPCRs. However, the identification of their natural ligands, and especially peptides, suffers from low discovery rates, thus impeding development of therapeutics based on these potential drug targets. We describe the discovery of novel GPCR ligands encrypted in the human proteome. Hundreds of potential peptide ligands were predicted by machine learning algorithms. In vitro screening of selected 33 peptides on a set of 152 GPCRs, including a group of designated orphan receptors, was conducted by intracellular calcium measurements and cAMP assays. The screening revealed eight novel peptides as potential agonists that specifically activated six different receptors in a dose-dependent manner. Most of the peptides showed distinct stimulatory patterns targeted at designated and orphan GPCRs. Further analysis demonstrated a significant in vivo effect for one of the peptides in a mouse inflammation model.

Use of Cryopreserved Cells for Enabling Greater Flexibility in Compound Profiling

Measurement of intracellular calcium release following agonist challenge within cells expressing the relevant membrane protein is a commonly used format to derive structure-activity relationship (SAR) data within a compound profiling assay. The Fluorometric Imaging Plate Reader (FLIPR) has become the gold standard for this purpose. FLIPR traditionally uses cells that are maintained in continuous culture for compound profiling of iterative chemistry campaigns. This supply dictates that assays can only be run on 4 of 5 weekdays, or alternative cell culture machinery is required such that plating can occur remotely at the weekend. The data reported here demonstrate that high-quality compound profiling data can be generated from the use of cryopreserved cells and that these cells can also be plated at various densities to generate equivalent data between 24 and 72 h post-plating. Hence, the authors report a method that allows data generation throughout the week and without the requirement of highly automated cell culture or continuous culture. ( Journal of Biomolecular Screening 2008:354-362)

Cell-based assays in GPCR drug discovery

G protein-coupled receptors (GPCRs) transmit extracellular signals into the intracellular space, and play key roles in the physiological regulation of virtually every cell and tissue. Characteristic for the GPCR superfamily of cell surface receptors are their seven transmembrane-spanning alpha-helices, an extracellular N terminus and intracellular C-terminal tail. Besides transmission of extracellular signals, their activity is modulated by cellular signals in an auto- or transregulatory fashion. The molecular complexity of GPCRs and their regulated signaling networks triggered the interest in academic research groups to explore them further, and their drugability and role in pathophysiology triggers pharmaceutical research towards small molecular weight ligands and therapeutic antibodies. About 30% of marketed drugs target GPCRs, which underlines the importance of this target class. This review describes current and emerging cellular assays for the ligand discovery of GPCRs.

Development of a universal high-throughput calcium assay for G-protein- coupled receptors with promiscuous G-protein Galpha15/16

AIM

To develop a universal high-throughput screening assay based on Galpha15/16- mediated calcium mobilization for the identification of novel modulators of Gprotein- coupled receptors (GPCR).

METHODS

In the present study, CHO-K1 or HEK293 cells were co-transfected with plasmids encoding promiscuous G-protein Galpha15/16 and various receptors originally coupled to Galphas, Galphai, or Galphaq pathways. Intracellular calcium change was monitored with fluorescent dye Fluo-4.

RESULTS

We found out for all the receptors tested, Galpha15/16 could shift the receptorso coupling to the calcium mobilization pathway, and the EC50 values of the ligands generated with this method were comparable with reported values that were obtained using traditional methods. This assay was validated and optimized with the zeta-opioid receptor, which originally coupled to Galphai and was recently found to play important roles in neurodegenerative and autoimmune diseases. A largescale screening of 48 000 compounds was performed based on this system. Several new modulators were identified and confirmed with the traditional GTPgammaS binding assay.

CONCLUSION

This cell-based calcium assay was proved to be robust and easy to automate, and could be used as a universal method in searching for GPCR modulators.

High-throughput screening assays for the identification of chemical probes

High-throughput screening (HTS) assays enable the testing of large numbers of chemical substances for activity in diverse areas of biology. The biological responses measured in HTS assays span isolated biochemical systems containing purified receptors or enzymes to signal transduction pathways and complex networks functioning in cellular environments. This Review addresses factors that need to be considered when implementing assays for HTS and is aimed particularly at investigators new to this field. We discuss assay design strategies, the major detection technologies and examples of HTS assays for common target classes, cellular pathways and simple cellular phenotypes. We conclude with special considerations for configuring sensitive, robust, informative and economically feasible HTS assays.

A robust, target-driven, cell-based assay for checkpoint kinase 1 inhibitors

Checkpoint kinase 1 (Chk1), a serine/threonine kinase, plays an important role in DNA damage checkpoint control and is an attractive target for cancer treatment. To develop a Chk1-specific cell-based assay, stable clones were established in which Chk1 kinase domain fused at its N-terminus with p53 through 4 tandem repeats of Gly-Gly-Gly-Gly-Ser was expressed in an inducible manner. Chk1 kinase specificity of the phosphorylation of fused p53 was confirmed by the experiments with a kinase-inactive Chk1. Only in the presence of an inducer molecule was phosphorylation of p53 at Ser-15 in the stable clones induced. Furthermore, its assay performance proved acceptable for high-throughput screening applications, judging from the Z' factor values (> 0.77). Finally, the cell-based assay thus established yielded structure-activity relationship data for a small set of test inhibitors of Chk1 within cells. Collectively, these results demonstrate that the established cell-based assay provides a novel and highly sensitive cellular platform for Chk1 inhibitor discovery.

Identification of human dopamine receptors agonists from Chinese herbs

AIM

To find human dopamine receptors, especially D1-like receptor specific agonists from Chinese herbs as potential antihypertension drug leads.

METHODS

Two D1-like receptor cell lines carrying a beta-lactamase reporter gene, and a D2 receptor cell line coexpressing a promiscuous G protein G15 were constructed using HEK293 cells. A natural compound library made from fractionated samples of herbal extracts was used for high-throughput screening (HTS) against one of the cell lines, HEK/D5R/CRE-blax. The interested hits were evaluated for their activities against various dopamine receptors.

RESULTS

Fourteen hits were identified from primary screening, of which 2 of the better hit samples, HD0522 and HD0059, were selected for further material and activity analysis, and to obtain 2 compounds that appeared as 2 single peaks in HPLC, HD0522H01 and HD0059H01. HD0059H01 could activate D1, D2, and D5 receptors, with EC(50 ) values of 2.28 microg/mL, 0.85 microg/mL, and 1.41 microg/mL, respectively. HD0522H01 could only activate D1R and D5R with EC(50 ) values of 2.95 microg/mL and 8.38 microg/mL.

CONCLUSION

We established cellbased assays for 3 different human dopamine receptors and identified specific agonists HD0522H01 and HD0059H01 through HTS. The specific agonist to D1-like receptors, HD0522H01, may become a new natural product-based drug lead for antihypertension treatment.

Molecular Engineering of G Protein-Coupled Receptors and G Proteins for Cell-Free Biosensing

The ability to express and purify modified recombinant proteins, so they retain their biological function in a cell-free format, has provided a basis for development of molecular biosensors. Here we utilize recombinant G Protein-coupled receptors (GPCRs) and their G proteins for cell-free detection of various binding partners. Fusion peptides were used to improve surface-attachment and fluorescent-labelling capabilities. A novel homogeneous fluorescence resonance energy transfer (FRET)-based assay was developed to detect rearrangements in the G protein heterotrimer. By using this heterotrimeric ‘molecular switch’, we are developing a generic technology such that multiple GPCRs could be assayed for ligand-mediated activation while tethered to surfaces or in solution, with increased throughput compared to current assay platforms.

Functional Characterization of Opioid Receptor Ligands by Aequorin Luminescence-Based Calcium Assay

A functional assay, based on aequorin-derived luminescence triggered by receptor-mediated changes in intracellular calcium levels, was used to examine relative potency and efficacy of the mu-opioid agonists endomorphin-1, endomorphin-2, morphiceptin, and their position 3-substituted analogs, as well as the delta-agonist deltorphin-II. The results of the aequorin assay, performed on recombinant cell lines, were compared with those obtained in the functional assay on isolated tissue preparations (guinea pig ileum and mouse vas deferens). A range of nine opioid peptide ligands produced a similar rank order of potency for the mu- and delta-opioid receptor agonists in both functional assays. The highest potency at the mu-receptor was observed for endomorphin-1, endomorphin-2, and [D-1-Nal3]morphiceptin, whereas deltorphin-II was the most potent delta-receptor agonist. In the aequorin assay, the mu- and delta-agonist-triggered luminescence was inhibited by the opioid antagonists naloxone and naltrindole, respectively. We can conclude that the use of the aequorin assay for new mu- and delta-receptor-selective opioid analogs gives pharmacologically relevant data and allows high-throughput compound screening, which does not involve radioactivity or animal tissues. This is the first study that validates the application of this assay in the screening of opioid analogs.

GABAB heterodimeric receptors promote Ca2+ influx via store-operated channels in rat cortical neurons and transfected Chinese hamster ovary cells

The GABAB receptors are generally considered to be classical Gi-coupled receptors that lack the ability to mobilize intracellular Ca2+ without the aid of promiscuous G proteins. Here, we report the ability of GABAB receptors to promote calcium influx into primary cultures of rat cortical neurons and transfected Chinese hamster ovary cells. Chinese hamster ovary cells were transfected with GABAB1(a) or GABAB1(b) subunits along with GABAB2 subunits. In experiments using the fluorometric imaging plate reader platform, GABA and selective agonists promoted increases in intracellular Ca2+ levels in transfected Chinese hamster ovary cells and cortical neurons with the expected order of potency. These effects were fully antagonized by selective GABAB receptor antagonists. To investigate the intracellular pathways responsible for mediating these effects we employed several pharmacological inhibitors. Pertussis toxin abolished GABAB mediated Ca2+ increases, as did the phospholipase Cbeta inhibitor U73122. Inhibitor 2-aminethoxydiphenyl borane acts as an antagonist at inositol 1,4,5-trisphosphate receptors and at store-operated channels. In all cell types, 2-aminethoxydiphenyl borane prevented Ca2+ mobilization. The selective store-operated channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy]ethyl-1H-imidazole hydrochloride prevented increases in intracellular Ca2+ levels as did performing the assays in Ca2+ free buffers. In conclusion, GABAB receptors expressed in Chinese hamster ovary cells and endogenously expressed in rat cortical neurons promote Ca2+ entry into the cell via the activation of store-operated channels, using a mechanism that is dependent on Gi/o heterotrimeric proteins and phospholipase Cbeta. These findings suggest that the neuronal effects mediated by GABAB receptors may, in part, rely on the receptor's ability to promote Ca2+ influx.

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.

Techniques: promiscuous Galpha proteins in basic research and drug discovery

Assay technologies that measure the activation of heterotrimeric (alphabetagamma) G proteins by G-protein-coupled receptors (GPCRs) are well established within the pharmaceutical industry, either for pharmacological characterization or for the identification of natural or surrogate receptor ligands. Despite recent evidence indicating that GPCR-linked signalling events might not be mediated exclusively by G proteins, G-protein activation remains a common benchmark for assessing GPCR family members. Thus, assay systems that translate ligand-mediated modulation of GPCRs into G-protein-dependent intracellular responses still represent key components of both basic research and the drug discovery process. In this article, the current knowledge and recent progress of integrating Galpha subunits into assay systems for GPCR drug discovery will be reviewed. Emphasis is given to novel promiscuous and chimeric Galpha proteins. Because of their ability to interact with a wide range of GPCRs, such novel G proteins are likely to be incorporated rapidly into drug discovery programmes.

G-protein-coupled receptor screening technologies

The choice of assay for high throughput screening (HTS) is of strategic importance to the successful identification of chemical entities that can be developed into drugs. During the past decade several technologies have emerged permitting large compound collections to be screened against biologically relevant models in a high throughput fashion. In this review, we summarise the technologies that are available for screening G-protein-coupled receptors (GPCRs) and discuss the issues that impact upon the choice of screening methodology.:

Identification of a stretch of six divergent amino acids on the alpha5 helix of Galpha16 as a major determinant of the promiscuity and efficiency of receptor coupling

A broad repertory of G-protein-coupled receptors shows effective coupling with the haematopoietic G16 protein. In the present study, individual residues along the C-terminal alpha5 helix of Galpha16 were examined for their contributions in defining receptor-coupling specificity. Residues that are relatively conserved within, but diverse between, the subfamilies of cloned Galpha subunits were mutated into the corresponding Galpha(z) residues. Six G(i)-linked receptors with different coupling efficiencies to Galpha16 were examined for their ability to utilize the various Galpha16 mutants to mediate agonist-induced inositol phosphate accumulation and Ca2+ mobilization. Co-operative enhancements of receptor coupling were observed with chimaeras harbouring multiple mutations at Glu350, Lys357 and Leu364 of Galpha16. Mutation of Leu364 into isoleucine appeared to be more efficient in enhancing receptor recognition compared with mutations at the other two sites. Mutation of a stretch of six consecutive residues (362-367) lying towards the end of the alpha5 helix was found to broaden significantly the receptor-coupling profile of Galpha16, and the effect was mediated partly through interactions with the beta2-beta3 loop. These results suggested that a stretch of six distinctive residues at the alpha5 helix of Galpha16 is particularly important, whereas other discrete residues spreading along the alpha5 helix function co-operatively for determining the specificity of receptor recognition.

G16-mediated activation of nuclear factor kappaB by the adenosine A1 receptor involves c-Src, protein kinase C, and ERK signaling

The G(i)-linked adenosine A1 receptor has been shown to mediate anti-inflammatory actions, possibly via modulation of the transcription factor nuclear factor-kappaB (NFkappaB). Here we demonstrate that an adenosine A1 agonist, N(6)-cyclohexyladenosine (CHA), activated IKKalpha/beta phosphorylation through PTX-insensitive G proteins in human lymphoblastoma Reh cells. To delineate the mechanism of action, different PTX-insensitive G proteins were expressed in human embryonic kidney 293 cells. Only Galpha(16) supported the CHA-induced IKK phosphorylation and NFkappaB-driven luciferase activity in time-dependent, dose-dependent, and PTX-insensitive manners. Gbetagamma subunits also modulated IKK/NFkappaB, as indicated by the stimulatory actions of Gbeta(1)gamma(2) and the abrogation of CHA-induced response by transducin. The participation of phospholipase Cbeta, protein kinase C, and calmodulin-dependent kinase II in CHA-induced IKK/NFkappaB activation were demonstrated by employing specific inhibitors and dominant-negative mutants. Inhibition of c-Src and numerous intermediates along the extracellular signal-regulated (ERK) kinase cascade including Ras, Raf-1 kinase, and MEK1/2 abolished the CHA-induced IKK/NFkappaB activation. Although c-Jun N-terminal kinase and p38 MAPK were also activated by CHA, they were not required for the IKK/NFkappaB regulation. Similar results were obtained using Reh cells. These data suggest that the G(16)-mediated activation of IKK/NFkappaB by CHA required a complex signaling network composed of multiple intermediates.