Cellular Dielectric Spectroscopy: A Label-Free Comprehensive Platform for Functional Evaluation of Endogenous Receptors

GzESTY as an optimized cell-based assay for initial steps in GPCR deorphanization

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to Gi/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce GzESTY, a sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of Gi/o/z-coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing cloning methods to ensure the correct expression ratio of GzESTY components. GzESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, and responses from endogenous GPCRs. Notably, with GzESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.

Gz Enhanced Signal Transduction assaY (GZESTY) for GPCR deorphanization

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to Gi/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce GZESTY, a highly sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of Gi/o/z-coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing novel cloning methods to ensure the correct expression ratio of GZESTY components. GZESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, as well as responses from endogenous GPCRs. Notably, with GZESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.

Label-free biomolecular and cellular methods in small molecule epigallocatechin-gallate research

Small molecule natural compounds are gaining popularity in biomedicine due to their easy access to wide structural diversity and their proven health benefits in several case studies. Affinity measurements of small molecules below 100 Da molecular weight in a label-free and automatized manner using small amounts of samples have now become a possibility and reviewed in the present work. We also highlight novel label-free setups with excellent time resolution, which is important for kinetic measurements of biomolecules and living cells. We summarize how molecular-scale affinity data can be obtained from the in-depth analysis of cellular kinetic signals. Unlike traditional measurements, label-free biosensors have made such measurements possible, even without the isolation of specific cellular receptors of interest. Throughout this review, we consider epigallocatechin gallate (EGCG) as an exemplary compound. EGCG, a catechin found in green tea, is a well-established anti-inflammatory and anti-cancer agent. It has undergone extensive examination in numerous studies, which typically rely on fluorescent-based methods to explore its effects on both healthy and tumor cells. The summarized research topics range from molecular interactions with proteins and biological films to the kinetics of cellular adhesion and movement on novel biomimetic interfaces in the presence of EGCG. While the direct impact of small molecules on living cells and biomolecules is relatively well investigated in the literature using traditional biological measurements, this review also highlights the indirect influence of these molecules on the cells by modifying their nano-environment. Moreover, we underscore the significance of novel high-throughput label-free techniques in small molecular measurements, facilitating the investigation of both molecular-scale interactions and cellular processes in one single experiment. This advancement opens the door to exploring more complex multicomponent models that were previously beyond the reach of traditional assays.

A review of electrochemical impedance as a tool for examining cell biology and subcellular mechanisms: merits, limits, and future prospects

Herein the development of cellular impedance biosensors, electrochemical impedance spectroscopy, and the general principles and terms associated with the cell-electrode interface is reviewed. This family of techniques provides quantitative and sensitive information into cell responses to stimuli in real-time with high temporal resolution. The applications of cell-based impedance biosensors as a readout in cell biology is illustrated with a diverse range of examples. The current state of the field, its limitations, the possible available solutions, and the potential benefits of developing biosensors are discussed.

Identification of monoclonal antibody drug substances using non-destructive Raman spectroscopy

Monoclonal antibodies provide highly specific and effective therapies for the treatment of chronic diseases. These protein-based therapeutics, or drug substances, are transported in single used plastic packaging to fill finish sites. According to good manufacturing practice guidelines, each drug substance needs to be identified before manufacturing of the drug product. However, considering their complex structure, it is challenging to correctly identify therapeutic proteins in an efficient manner. Common analytical techniques for therapeutic protein identification are SDS-gel electrophoresis, enzyme linked immunosorbent assays, high performance liquid chromatography and mass spectrometry-based assays. Although effective in correctly identifying the protein therapeutic, most of these techniques need extensive sample preparation and removal of samples from their containers. This step not only risks contamination but the sample taken for the identification is destroyed and cannot be re-used. Moreover, these techniques are often time consuming, sometimes taking several days to process. Here, we address these challenges by developing a rapid and non-destructive identification technique for monoclonal antibody-based drug substances. Raman spectroscopy in combination with chemometrics were used to identify three monoclonal antibody drug substances. This study explored the impact of laser exposure, time out of refrigerator and multiple freeze thaw cycles on the stability of monoclonal antibodies. and demonstrated the potential of using Raman spectroscopy for the identification of protein-based drug substances in the biopharmaceutical industry.

Label-Free Analysis with Multiple Parameters Separates G Protein-Coupled Receptor Signaling Pathways

Real-time label-free techniques are used to profile G protein-coupled receptor (GPCR) signaling pathways in living cells. However, interpreting the label-free signal responses is challenging, and previously reported methods do not reliably separate pathways from each other. In this study, a continuous angular-scanning surface plasmon resonance (SPR) technique is utilized for measuring label-free GPCR signal profiles. We show how the continuous angular-scanning ability, measuring up to nine real-time label-free parameters simultaneously, results in more information-rich label-free signal profiles for different GPCR pathways, providing a more accurate pathway separation. For this, we measured real-time full-angular SPR response curves for G_s, G_q, and G_i signaling pathways in living cells. By selecting two of the most prominent label-free parameters: the full SPR curve angular and intensity shifts, we present how this analysis approach can separate each of the three signaling pathways in a straightforward single-step analysis setup, without concurrent use of signal inhibitors or other response modulating compounds.

A Differential Hypofunctionality of Gαi Proteins Occurs in Adolescent Idiopathic Scoliosis and Correlates with the Risk of Disease Progression

Adolescent idiopathic scoliosis is the most prevalent spine deformity and the molecular mechanisms underlying its pathophysiology remain poorly understood. We have previously found a differential impairment of melatonin receptor signaling in AIS osteoblasts allowing the classification of patients into three biological endophenotypes or functional groups (FG1, FG2 and FG3). Here, we provide evidence that the defect characterizing each endophenotype lies at the level of Gαi proteins leading to a systemic and generalized differential impairment of Gi-coupled receptor signaling. The three Gαi isoforms exhibited a selective serine phosphorylation patterns for each AIS endophenotype resulting in a differential reduction in Gαi protein activity as determined by cellular dielectric spectroscopy and small interfering RNA methods. We found that one endophenotype (FG2) with phosphorylated Gαi₁ and Gαi₂ was consistently associated with a significantly high risk of spinal deformity progression when compared to the other two endophenotypes (FG1 and FG3). We further demonstrated that each endophenotype is conserved among affected family members. This study expands our understanding of the mechanism underlying the Gi-coupled receptor signaling dysfunction occurring in AIS and provides the first evidence for its hereditary nature. Collectively, our findings offers a new perspective on Gαi hypofunctionality in a human disease by revealing specific serine phosphorylation signatures of Gαi isoforms that may facilitate the identification of AIS patients at risk of spinal deformity progression.

Label-Free Whole Cell Biosensing for High-Throughput Discovery of Activators and Inhibitors Targeting G Protein-Activated Inwardly Rectifying Potassium Channels

Dynamic mass redistribution (DMR) and cellular dielectric spectroscopy (CDS) are label-free biosensor technologies that capture real-time integrated cellular responses upon exposure to extra- and intracellular stimuli. They register signaling routes that are accompanied by cell shape changes and/or molecular movement of cells proximal to the biosensor to which they are attached. Here, we report the unexpected observation that robust DMR and CDS signatures are also elicited upon direct stimulation of G protein-activated inwardly rectifying potassium (GIRK) channels, which are involved in the regulation of excitability in the heart and brain. Using ML297, a small-molecule GIRK activator, along with channel blockers and cytoskeletal network inhibitors, we found that GIRK activation exerts its effects on cell shape by a mechanism which depends on actin but not the microtubule network. Because label-free real-time biosensing (i) quantitatively determines concentration dependency of GIRK activators, (ii) accurately assesses the impact of GIRK channel blockers, (iii) is high throughput-compatible, and (iv) visualizes previously unknown cellular consequences downstream of direct GIRK activation, we do not only provide a novel experimental strategy for identification of GIRK ligands but also an entirely new angle to probe GIRK (ligand) biology. We envision that DMR and CDS may add to the repertoire of technologies for systematic exploitation of ion channel function and, in turn, to the identification of novel GIRK ligands in order to treat cardiovascular and neurological disorders.

Assessments of cellular melatonin receptor signaling pathways: β-arrestin recruitment, receptor internalization, and impedance variations

Melatonin receptors belong to the family of G-protein coupled receptors. Agonist-induced receptor activation is terminated with the recruitment of β-arrestin, which leads to receptor internalization. Furthermore, agonist binding induces a shift in cellular shape that translates into a change in the electric impedance of the cell. In the present study, we employed engineered cells to study these internalization-related processes in the context of the two melatonin receptors, MT₁ and MT₂. To assess these three receptor internalization-related functions and validate the results, we employed four classical ligands of melatonin receptors: the natural agonist melatonin; the super-agonist 2-iodo-melatonin and the two antagonists luzindole and 4-phenyl-2-propionamidotetralin. The assessments confirmed the nature of the agonistic ligands but showed that 4-phenyl-2-propionamidotetralin, a described antagonist, is a biased partial agonist at MT₂ with poorer affinity for MT₁. The methods are now available to be applied to any receptor system for which multiple signaling pathways must be evaluated for new molecules.

Conformational Change in the Ligand-Binding Pocket via a KISS1R Mutation (P147L) Leads to Isolated Gonadotropin-Releasing Hormone Deficiency

Context:

Kisspeptin receptor (KISS1R) is expressed in hypothalamic gonadotropin-releasing hormone neurons and responsible for pubertal onset and reproductive functions. KISS1R mutations remain a rare cause of congenital hypogonadotropic hypogonadism (CHH).

Objective:

The aim of this study was to identify the genetic cause of CHH in a patient and to functionally characterize a KISS1R mutation.

Design:

The patient was a 47-year-old Japanese man whose parents were first cousins. He lacked secondary sexual characteristics owing to normosmic CHH. Exon segments for the KISS1R gene in this patient were screened for mutations. Functional analyses were performed using HEK293 cells expressing KISS1R mutants. Molecular dynamics simulations were performed to compare the ligand-KISS1R mutant complex with those of wild-type KISS1R variants.

Results:

A homozygous mutation (c.440C>T, p.P147L) in KISS1R was identified. The P147L mutation did not affect either receptor expression level or subcellular localization in the recombinant expression system. Intracellular calcium measurements and cellular dielectric spectroscopy demonstrated that the P147L mutation impaired receptor function to an extent more severe than that of a previously reported L148S mutation. A receptor-ligand binding assay showed the P147L mutation causes a substantial loss of ligand-binding affinity. Molecular dynamics simulations revealed the P147L mutation decreases the contact surface area of the ligand-receptor complex in an expanded ligand-binding pocket.

Conclusion:

We identified a loss-of-function mutation in KISS1R associated with CHH. Our results demonstrated that the P147L mutation causes a severe phenotype and functional impairment resulting from the loss of ligand-binding affinity due to an expanded ligand-binding pocket.

Label-free technology and patient cells: from early drug development to precision medicine

Drug development requires physiologically more appropriate model systems and assays to increase understanding of drug action and pathological processes in individual humans. Specifically, patient-derived cells offer great opportunities as representative cellular model systems. Moreover, with novel label-free cellular assays, it is often possible to investigate complex biological processes in their native environment. Combining these two offers distinct opportunities for increasing physiological relevance. Here, we review impedance-based label-free technologies in the context of patient samples, focusing on commonly used cell types, including fibroblasts, blood components, and stem cells. Applications extend as far as tissue-on-a-chip models. Thus, applying label-free technologies to patient samples can produce highly biorelevant data and, with them, unique opportunities for drug development and precision medicine.

Label-Free Dynamic Mass Redistribution and Bio-Impedance Methods for Drug Discovery

Label-free biosensors are increasingly employed in drug discovery. Cell-based biosensors provide valuable insights into the biological consequences of exposing cells and tissues to chemical agents and the underlying molecular mechanisms associated with these effects. Optical biosensors based on the detection of dynamic mass redistribution (DMR) and impedance biosensors using cellular dielectric spectroscopy (CDS) capture changes of the cytoskeleton of living cells in real time. Because signal transduction correlates with changes in cell morphology, DMR and CDS biosensors are exquisitely suited for recording integrated cell responses in an unbiased, yet pathway-specific manner without the use of labels that may interfere with cell function. Described in this unit are several experimental approaches utilizing optical label-free system capturing dynamic mass redistribution (DMR) in living cells (Epic System) and an impedance-based CDS technology (CellKey). In addition, potential pitfalls associated with these assays and alternative approaches for overcoming such technical challenges are discussed. © 2017 by John Wiley & Sons, Inc.

Dielectrophoresis assisted loading and unloading of microwells for impedance spectroscopy

Dielectric spectroscopy (DS) is a noninvasive, label-free, fast, and promising technique for measuring dielectric properties of biological cells in real time. We demonstrate a microchip that consists of electro-activated microwell arrays for positive dielectrophoresis assisted cell capture, DS measurements, and negative dielectrophoresis driven cell unloading; thus, providing a high-throughput cell analysis platform. To the best of our knowledge, this is the first microfluidic chip that combines electro-activated microwells and DS to analyze biological cells. Device performance is tested using Saccharomyces cerevisiae (yeast) cells. DEP response of yeast cells is determined by measuring their Clausius-Mossotti factor using biophysical models in parallel plate microelectrode geometry. This information is used to determine the excitation frequency to load and unload wells. Effect of yeast cells on the measured impedance spectrum was examined both experimentally and numerically. Good match between the numerical and experimental results establishes the potential use of the microchip device for extracting subcellular properties of biological cells in a rapid and nonexpensive manner.

The Orphan Receptor GPR17 Is Unresponsive to Uracil Nucleotides and Cysteinyl Leukotrienes

Pairing orphan G protein–coupled receptors (GPCRs) with their cognate endogenous ligands is expected to have a major impact on our understanding of GPCR biology. It follows that the reproducibility of orphan receptor ligand pairs should be of fundamental importance to guide meaningful investigations into the pharmacology and function of individual receptors. GPR17 is an orphan receptor characterized by some as a dualistic uracil nucleotide/cysteinyl leukotriene receptor and by others as inactive toward these stimuli altogether. Whereas regulation of central nervous system myelination by GPR17 is well established, verification of activity of its putative endogenous ligands has proven elusive so far. Herein we report that uracil nucleotides and cysteinyl leukotrienes do not activate human, mouse, or rat GPR17 in various cellular backgrounds, including primary cells, using eight distinct functional assay platforms based on labelfree pathway-unbiased biosensor technologies, as well as canonical second-messenger or biochemical assays. Appraisal of GPR17 activity can neither be accomplished with co-application of both ligand classes, nor with exogenous transfection of partner receptors (nucleotide P2Y12, cysteinyl-leukotriene CysLT1) to reconstitute the elusive pharmacology. Moreover, our study does not support the inhibition of GPR17 by the marketed antiplatelet drugs cangrelor and ticagrelor, previously suggested to antagonize GPR17. Whereas our data do not disagree with a role of GPR17 per se as an orchestrator of central nervous system functions, they challenge the utility of the proposed (ant)agonists as tools to imply direct contribution of GPR17 in complex biologic settings.

Green tea polyphenol tailors cell adhesivity of RGD displaying surfaces: multicomponent models monitored optically

The interaction of the anti-adhesive coating, poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) and its Arg-Gly-Asp (RGD) functionalized form, PLL-g-PEG-RGD, with the green tea polyphenol, epigallocatechin-gallate (EGCg) was in situ monitored. After, the kinetics of cellular adhesion on the EGCg exposed coatings were recorded in real-time. The employed plate-based waveguide biosensor is applicable to monitor small molecule binding and sensitive to sub-nanometer scale changes in cell membrane position and cell mass distribution; while detecting the signals of thousands of adhering cells. The combination of this remarkable sensitivity and throughput opens up new avenues in testing complicated models of cell-surface interactions. The systematic studies revealed that, despite the reported excellent antifouling properties of the coatings, EGCg strongly interacted with them, and affected their cell adhesivity in a concentration dependent manner. Moreover, the differences between the effects of the fresh and oxidized EGCg solutions were first demonstrated. Using a semiempirical quantumchemical method we showed that EGCg binds to the PEG chains of PLL-g-PEG-RGD and effectively blocks the RGD sites by hydrogen bonds. The calculations supported the experimental finding that the binding is stronger for the oxidative products. Our work lead to a new model of polyphenol action on cell adhesion ligand accessibility and matrix rigidity.

Label-free versus conventional cellular assays: Functional investigations on the human histamine H1 receptor

A set of histamine H₁ receptor (H₁R) agonists and antagonists was characterized in functional assays, using dynamic mass redistribution (DMR), electric cell-substrate impedance sensing (ECIS) and various signaling pathway specific readouts (Fura-2 and aequorin calcium assays, arrestin recruitment (luciferase fragment complementation) assay, luciferase gene reporter assay). Data were gained from genetically engineered HEK293T cells and compared with reference data from GTPase assays and radioligand binding. Histamine and the other H₁R agonists gave different assay-related pEC₅₀ values, however, the order of potency was maintained. In the luciferase fragment complementation assay, the H₁R preferred β-arrestin2 over β-arrestin1. The calcium and the impedimetric assay depended on G_q coupling of the H₁R, as demonstrated by complete inhibition of the histamine-induced signals in the presence of the G_q inhibitor FR900359 (UBO-QIC). Whereas partial inhibition by FR900359 was observed in DMR and the gene reporter assay, pertussis toxin substantially decreased the response in DMR, but increased the luciferase signal, reflecting the contribution of both, G_q and G_i, to signaling in these assays. For antagonists, the results from DMR were essentially compatible with those from conventional readouts, whereas the impedance-based data revealed a trend towards higher pK_b values. ECIS and calcium assays apparently only reflect G_q signaling, whereas DMR and gene reporter assays appear to integrate both, G_q and G_i mediated signaling. The results confirm the value of the label-free methods, DMR and ECIS, for the characterization of H₁R ligands. Both noninvasive techniques are complementary to each other, but cannot fully replace reductionist signaling pathway focused assays.

A generic screening platform for inhibitors of virus induced cell fusion using cellular electrical impedance

Fusion of the viral envelope with host cell membranes is an essential step in the life cycle of all enveloped viruses. Despite such a clear target for antiviral drug development, few anti-fusion drugs have progressed to market. One significant hurdle is the absence of a generic, high-throughput, reproducible fusion assay. Here we report that real time, label-free measurement of cellular electrical impedance can quantify cell-cell fusion mediated by either individually expressed recombinant viral fusion proteins, or native virus infection. We validated this approach for all three classes of viral fusion and demonstrated utility in quantifying fusion inhibition using antibodies and small molecule inhibitors specific for dengue virus and respiratory syncytial virus.

Detection and Quantification of Allosteric Modulation of Endogenous M4 Muscarinic Acetylcholine Receptor Using Impedance-Based Label-Free Technology in a Neuronal Cell Line

Allosteric modulators of G protein–coupled receptors have the potential to achieve greater receptor subtype selectivity compared with ligands targeting the orthosteric site of this receptor family. However, the high attrition rate in GPCR drug discovery programs has highlighted the need to better characterize lead compounds in terms of their allosteric action, as well as the signals they elicit. Recently, the use of label-free technologies has been proposed as an approach to overcome some limitations of endpoint-based assays and detect global changes in the ligand-stimulated cell. In this study, we assessed the ability of an impedance-based label-free technology, xCELLigence, to detect allosteric modulation in a neuronal cell line natively expressing rodent M4 muscarinic acetylcholine receptors. We were able to demonstrate that positive allosteric modulation of the endogenous M4 muscarinic acetylcholine receptor can be detected using this technology. Importantly, the allosteric parameters estimated from the label-free approach are comparable to those estimated from endpoint-based assays.

Label-free drug discovery

Current drug discovery is dominated by label-dependent molecular approaches, which screen drugs in the context of a predefined and target-based hypothesis in vitro. Given that target-based discovery has not transformed the industry, phenotypic screen that identifies drugs based on a specific phenotype of cells, tissues, or animals has gained renewed interest. However, owing to the intrinsic complexity in drug-target interactions, there is often a significant gap between the phenotype screened and the ultimate molecular mechanism of action sought. This paper presents a label-free strategy for early drug discovery. This strategy combines label-free cell phenotypic profiling with computational approaches, and holds promise to bridge the gap by offering a kinetic and holistic representation of the functional consequences of drugs in disease relevant cells that is amenable to mechanistic deconvolution.

Decoding signaling and function of the orphan G protein-coupled receptor GPR17 with a small-molecule agonist

Replacement of the lost myelin sheath is a therapeutic goal for treating demyelinating diseases of the central nervous system (CNS), such as multiple sclerosis (MS). The G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) GPR17, which is phylogenetically closely related to receptors of the "purinergic cluster," has emerged as a modulator of CNS myelination. However, whether GPR17-mediated signaling positively or negatively regulates this critical process is unresolved. We identified a small-molecule agonist, MDL29,951, that selectively activated GPR17 even in a complex environment of endogenous purinergic receptors in primary oligodendrocytes. MDL29,951-stimulated GPR17 engaged the entire set of intracellular adaptor proteins for GPCRs: G proteins of the Gα(i), Gα(s), and Gα(q) subfamily, as well as β-arrestins. This was visualized as alterations in the concentrations of cyclic adenosine monophosphate and inositol phosphate, increased Ca²⁺ flux, phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), as well as multifeatured cell activation recorded with label-free dynamic mass redistribution and impedance biosensors. MDL29,951 inhibited the maturation of primary oligodendrocytes from heterozygous but not GPR17 knockout mice in culture, as well as in cerebellar slices from 4-day-old wild-type mice. Because GPCRs are attractive targets for therapeutic intervention, inhibiting GPR17 emerges as therapeutic strategy to relieve the oligodendrocyte maturation block and promote myelin repair in MS.

Cell-based assay protocol for the prognostic prediction of idiopathic scoliosis using cellular dielectric spectroscopy

This protocol details the experimental and analytical procedure for a cell-based assay developed in our laboratory as a functional test to predict the prognosis of idiopathic scoliosis in asymptomatic and affected children. The assay consists of the evaluation of the functional status of Gi and Gs proteins in peripheral blood mononuclear cells (PBMCs) by cellular dielectric spectroscopy (CDS), using an automated CDS-based instrument, and the classification of children into three functional groups (FG1, FG2, FG3) with respect to the profile of imbalance between the degree of response to Gi and Gs proteins stimulation. The classification is further confirmed by the differential effect of osteopontin (OPN) on response to Gi stimulation among groups and the severe progression of disease is referenced by FG2. Approximately, a volume of 10 ml of blood is required to extract PBMCs by Ficoll-gradient and cells are then stored in liquid nitrogen. The adequate number of PBMCs to perform the assay is obtained after two days of cell culture. Essentially, cells are first incubated with phytohemmaglutinin (PHA). After 24 hr incubation, medium is replaced by a PHA-free culture medium for an additional 24 hr prior to cell seeding and OPN treatment. Cells are then spectroscopically screened for their responses to somatostatin and isoproterenol, which respectively activate Gi and Gs proteins through their cognate receptors. Both somatostatin and isoproterenol are simultaneously injected with an integrated fluidics system and the cells' responses are monitored for 15 min. The assay can be performed with fresh or frozen PBMCs and the procedure is completed within 4 days.

The A, B, Cs of G-protein-coupled receptor pharmacology in assay development for HTS

G-protein-coupled receptors represent one of the most important areas of research in the pharmaceutical industry, being one of the largest druggable gene families. Recognising this fact, manufacturers have developed a huge variety of homogeneous assay technologies that facilitate the quantification of receptor ligand binding events and their downstream signalling cascades. However, while early emphasis was placed on the most sensitive, high-throughput and cost-effective screening technologies to enable identification of the most lead matter for further development, in recent years emphasis has shifted to a focus on maximising the identification of compounds that are new and developing assays that are more biologically/pharmacologically relevant. Therefore, this review provides an overview of the binding and functional techniques available for high-throughput screening, with particular attention on how assay application and configuration can be maximised to ensure their successful identification of relevant chemical matter and thereby optimising project success.

Troubleshooting and deconvoluting label-free cell phenotypic assays in drug discovery

INTRODUCTION

Central to drug discovery and development is to comprehend the target(s), potency, efficacy and safety of drug molecules using pharmacological assays. Owing to their ability to provide a holistic view of drug actions in native cells, label-free biosensor-enabled cell phenotypic assays have been emerging as new generation phenotypic assays for drug discovery. Despite the benefits associated with wide pathway coverage, high sensitivity, high information content, non-invasiveness and real-time kinetics, label-free cell phenotypic assays are often viewed to be a blackbox in the era of target-centric drug discovery.

METHODS

This article first reviews the biochemical and biological complexity of drug-target interactions, and then discusses the key characteristics of label-free cell phenotypic assays and presents a five-step strategy to troubleshooting and deconvoluting the label-free cell phenotypic profiles of drugs.

RESULTS

Drug-target interactions are intrinsically complicated. Label-free cell phenotypic signatures of drugs mirror the innate complexity of drug-target interactions, and can be effectively deconvoluted using the five-step strategy.

DISCUSSION

The past decades have witnessed dramatic expansion of pharmacological assays ranging from molecular to phenotypic assays, which is coincident with the realization of the innate complexity of drug-target interactions. The clinical features of a drug are defined by how it operates at the system level and by its distinct polypharmacology, ontarget, phenotypic and network pharmacology. Approaches to examine the biochemical, cellular and molecular mechanisms of action of drugs are essential to increase the efficiency of drug discovery and development. Label-free cell phenotypic assays and the troubleshooting and deconvoluting approach presented here may hold great promise in drug discovery and development.

Activation of prostaglandin E receptor 4 triggers secretion of gut hormone peptides GLP-1, GLP-2, and PYY

Prostaglandins E1 and E2 are synthesized in the intestine and mediate a range of gastrointestinal functions via activation of the prostanoid E type (EP) family of receptors. We examined the potential role of EP receptors in the regulation of gut hormone secretion from L cells. Analysis of mRNA expression in mouse enteroendocrine GLUTag cells demonstrated the abundant expression of EP4 receptor, whereas expression of other EP receptors was much lower. Prostaglandin E1 and E2, nonselective agonists for all EP receptor subtypes, triggered glucagon like peptide 1 (GLP-1) secretion from GLUTag cells, as did the EP4-selective agonists CAY10580 and TCS2510. The effect of EP4 agonists on GLP-1 secretion was blocked by incubation of cells with the EP4-selective antagonist L161,982 or by down-regulating EP4 expression with specific small interfering RNA. Regulation of gut hormone secretion with EP4 agonists was further studied in mice. Administration of EP4 agonists to mice produced a significant elevation of plasma levels of GLP-1, glucagon like peptide 2 (GLP-2) and peptide YY (PYY), whereas gastric inhibitory peptide (GIP) levels were not increased. Thus, our data demonstrate that activation of the EP4 receptor in enteroendocrine L cells triggers secretion of gut hormones.

Approaches for probing allosteric interactions at 7 transmembrane spanning receptors

In recent years, allosteric modulation of 7 transmembrane spanning receptors (7TMRs) has become a highly productive and exciting field of receptor pharmacology and drug discovery efforts. Positive and negative allosteric modulators (PAMs and NAMs, respectively) present a number of pharmacological and therapeutic advantages over conventional orthosteric ligands, including improved receptor-subtype selectivity, a lower propensity to induce receptor desensitization, the preservation of endogenous temporal and spatial activation of receptors, greater chemical flexibility for optimization of drug metabolism and pharmacokinetic parameters, and saturability of effect at target receptors, thus improving safety concerns and risk of overdose. Additionally, the relatively new concept of allosteric modulator-mediated receptor signal bias opens up a number of intriguing possibilities for PAMs, NAMs, and allosteric agonists, including the potential to selectively activate therapeutically beneficial signaling cascades, which could yield a superior tissue selectivity and side effect profile of allosteric modulators. However, there are a number of considerations and caveats that must be addressed when screening for and characterizing the properties of 7TMR allosteric modulators. Mode of pharmacology, methodology used to monitor receptor activity, detection of appropriate downstream analytes, selection of orthosteric probe, and assay time-course must all be considered when implementing any high-throughput screening campaign or when characterizing the properties of active compounds. Yet compared to conventional agonist/antagonist drug discovery programs, these elements of assay design are often a great deal more complicated when working with 7TMRs allosteric modulators. Moreover, for classical pharmacological methodologies and analyses, like radioligand binding and the assessment of compound affinity, the properties of allosteric modulators yield data that are more nuanced than orthosteric ligand-receptor interactions. In this review, we discuss the current methodologies being used to identify and characterize allosteric modulators, lending insight into the approaches that have been most successful in accurately and robustly identifying hit compounds. New label-free technologies capable of detecting phenotypic cellular changes in response to receptor activation are powerful tools well suited for assessing subtle or potentially masked cellular responses to allosteric modulation of 7TMRs. Allosteric modulator-induced receptor signal bias and the assay systems available to probe the various downstream signaling outcomes of receptor activation are also discussed.

Description of the constitutive activity of cloned human melatonin receptors hMT(1) and hMT(2) and discovery of inverse agonists

Melatonin receptors have been described to activate different G protein-dependent signaling pathways, both in laboratory, heterologous, cellular models and in physiological conditions. Furthermore, the constitutive activity of G protein-coupled receptors has been shown to be key in physiological and pathological conditions. In the case of melatonin receptors, information is rather scare and concerns only MT1 receptors. In the present report, we show that the G protein-coupled melatonin receptors do have a constitutive, nonmelatonin-induced signaling activity using two cellular models of different origins, the Chinese hamster ovary cell line and Neuro2A, a neuroblastoma cell line. Furthermore, we show that this constitutive activity involves mainly Gi proteins, which is consistent with the common knowledge on the melatonin receptors. Importantly, we also describe, for the first time, inverse agonist properties for melatonin ligands. Although it is clear than more in-depth, biochemistry-based studies will be required to better understand by which pathway(s) the constitutively active melatonin receptors transfer melatonin information into intracellular biochemical events; our data open interesting perspectives for understanding the importance of the constitutive activity of melatonin receptors in physiological conditions.

Tools for GPCR drug discovery

G-protein-coupled receptors (GPCRs) mediate many important physiological functions and are considered as one of the most successful therapeutic targets for a broad spectrum of diseases. The design and implementation of high-throughput GPCR assays that allow the cost-effective screening of large compound libraries to identify novel drug candidates are critical in early drug discovery. Early functional GPCR assays depend primarily on the measurement of G-protein-mediated 2nd messenger generation. Taking advantage of the continuously deepening understanding of GPCR signal transduction, many G-protein-independent pathways are utilized to detect the activity of GPCRs, and may provide additional information on functional selectivity of candidate compounds. With the combination of automated imaging systems and label-free detection systems, such assays are now suitable for high-throughput screening (HTS). In this review, we summarize the most widely used GPCR assays and recent advances in HTS technologies for GPCR drug discovery.

The significance of G protein-coupled receptor crystallography for drug discovery

Crucial as molecular sensors for many vital physiological processes, seven-transmembrane domain G protein-coupled receptors (GPCRs) comprise the largest family of proteins targeted by drug discovery. Together with structures of the prototypical GPCR rhodopsin, solved structures of other liganded GPCRs promise to provide insights into the structural basis of the superfamily's biochemical functions and assist in the development of new therapeutic modalities and drugs. One of the greatest technical and theoretical challenges to elucidating and exploiting structure-function relationships in these systems is the emerging concept of GPCR conformational flexibility and its cause-effect relationship for receptor-receptor and receptor-effector interactions. Such conformational changes can be subtle and triggered by relatively small binding energy effects, leading to full or partial efficacy in the activation or inactivation of the receptor system at large. Pharmacological dogma generally dictates that these changes manifest themselves through kinetic modulation of the receptor's G protein partners. Atomic resolution information derived from increasingly available receptor structures provides an entrée to the understanding of these events and practically applying it to drug design. Supported by structure-activity relationship information arising from empirical screening, a unified structural model of GPCR activation/inactivation promises to both accelerate drug discovery in this field and improve our fundamental understanding of structure-based drug design in general. This review discusses fundamental problems that persist in drug design and GPCR structural determination.

In vitro safety pharmacology profiling: what else beyond hERG?

One of the main reasons for drug failures in clinical development, or postmarket launch, is lacking or compromised safety margins at therapeutic doses. Organ toxicity with poorly defined mechanisms and adverse drug reactions associated with on- and off-target effects are the major contributors to safety-related shortfalls of many clinical drug candidates. Therefore, to avoid high attrition rates in clinical trials, it is imperative to test compounds for potential adverse reactions during early drug discovery. Beyond a small number of targets associated with clinically acknowledged adverse drug reactions, there is little consensus on other targets that are important to consider at an early stage for in vitro safety pharmacology assessment. We consider here a limited number of safety-related targets, from different target families, which were selected as part of in vitro safety pharmacology profiling panels integrated in the drug-development process at Novartis. The best way to assess these targets, using a biochemical or a functional readout, is discussed. In particular, the importance of using cell-based profiling assays for the characterization of an agonist action at some GPCRs is highlighted. A careful design of in vitro safety pharmacology profiling panels allows better prediction of potential adverse effects of new chemical entities early in the drug-discovery process. This contributes to the selection of the best candidate for clinical development and, ultimately, should contribute to a decreased attrition rate.

Agonist-biased signaling at the histamine H4 receptor: JNJ7777120 recruits β-arrestin without activating G proteins

The G(i/o)-coupled histamine H(4) receptor is highly expressed in hemopoietic cells and is a promising new target for the treatment of chronic inflammatory diseases. 1-[(5-Chloro-1H-indol-2-yl)carbonyl]-4-methyl-piperazine (JNJ7777120) has been described as a selective antagonist at the H(4) receptor and is widely used to characterize the physiological role of the H(4) receptor. We have investigated the pharmacological properties of JNJ7777120 using two distinct downstream signaling measurements, G protein activation and β-arrestin recruitment. The H(4) receptor agonists histamine and clobenpropit, but not JNJ7777120, were able to induce [(35)S]GTPγS binding in membranes prepared from U2OS-H(4) cells. Thioperamide, a dual H(3)/H(4) receptor antagonist, and JNJ7777120 were both able to inhibit the [(35)S]GTPγS binding induced by clobenpropit. Agonists and antagonists specific for other members of the histamine receptor family had no effect in this assay format. Histamine and clobenpropit increased β-arrestin recruitment to the H(4) receptor in a concentration-dependent manner. This β-arrestin recruitment could be inhibited by preincubation with thioperamide. We were surprised to find that preincubation with the H(4)-selective antagonist JNJ7777120 potentiated rather than antagonized the response to a submaximal concentration of clobenpropit. JNJ7777120 treatment alone resulted in an increase in β-arrestin recruitment, which again could be inhibited by preincubation with thioperamide. Schild analysis demonstrated competitive antagonism between thioperamide and both clobenpropit and JNJ7777120. Histamine and clobenpropit had comparable potencies for both [(35)S]GTPγS binding and β-arrestin recruitment, suggesting little difference in the levels of receptor reserve between the two assays. In conclusion, we have demonstrated that JNJ7777120 recruits β-arrestin to the H(4) receptor, independent of G protein activation.

Label-Free Biosensors for Cell Biology

Label-free biosensors for studying cell biology have finally come of age. Recent developments have advanced the biosensors from low throughput and high maintenance research tools to high throughput and low maintenance screening platforms. In parallel, the biosensors have evolved from an analytical tool solely for molecular interaction analysis to powerful platforms for studying cell biology at the whole cell level. This paper presents historical development, detection principles, and applications in cell biology of label-free biosensors. Future perspectives are also discussed.

Label-free screening assays: a strategy for finding better drug candidates

The last 10 years have seen advances in automation and high-throughput biochemistry in the drug-discovery arena. However, these advances have not led to improvements in drug-discovery success. Drug programs must find new ways to identify superior compounds. Advances in label-free assay technologies may provide advantages needed for improved drug discovery. In this article, we will discuss high-throughput MS, a technology that allows screening with native substrates and with targets inaccessible to standard assay formats. We will then discuss cell-based label-free biosensors, focusing on the increased information content available when using these platforms. We will conclude with speculation on the future and ways to obtain relevant biological information early in development to ensure the best compounds are promoted to medicinal chemistry campaigns.

Meganuclease-driven targeted integration in CHO-K1 cells for the fast generation of HTS-compatible cell-based assays

The development of cell-based assays for high-throughput screening (HTS) approaches often requires the generation of stable transformant cell lines. However, these cell lines are essentially created by random integration of a gene of interest (GOI) with no control over the level and stability of gene expression. The authors developed a targeted integration system in Chinese hamster ovary (CHO) cells, called the cellular genome positioning system (cGPS), based on the stimulation of homologous gene targeting by meganucleases. Five different GOIs were knocked in at the same locus in cGPS CHO-K1 cells. Further characterization revealed that the cGPS CHO-K1 system is more rapid (2-week protocol), efficient (all selected clones expressed the GOI), reproducible (GOI expression level variation of 12%), and stable over time (no change in GOI expression after 23 weeks of culture) than classical random integration. Moreover, in all cGPS CHO-K1 targeted clones, the recombinant protein was biologically active and its properties similar to the endogenous protein. This fast and robust method opens the door for creating large collections of cell lines of better quality and expressing therapeutically relevant GOIs at physiological levels, thereby enhancing the potential scope of HTS.

Cell-based assays: fuelling drug discovery

It has been estimated that over a billion dollars in resources can be consumed to obtain clinical approval, and only a few new chemical entities are approved by the US Food and Drug Administration (FDA) each year. Therefore it is of utmost importance to obtain the maximum amount of information about biological activity, toxicological profile, biochemical mechanisms, and off-target interactions of drug-candidate leads in the earliest stages of drug discovery. Cell-based assays, because of their peculiar advantages of predictability, possibility of automation, multiplexing, and miniaturization, seem the most appealing tool for the high demands of the early stages of the drug-discovery process. Nevertheless, cellular screening, relying on different strategies ranging from reporter gene technology to protein fragment complementation assays, still presents a variety of challenges. This review focuses on main advantages and limitations of different cell-based approaches, and future directions and trends in this fascinating field.

Cell-based screening test for idiopathic scoliosis using cellular dielectric spectroscopy

STUDY DESIGN

A cell-based assay was developed to identify asymptomatic children at risk of developing idiopathic scoliosis (IS) and to stratify IS patients at an earlier stage in order to better predict their clinical outcome. Clinical validation of this assay was performed by testing IS patients at different stages, healthy control subjects, and asymptomatic offspring, born from at least one scoliotic parent, who are considered at risk of developing this disorder.

OBJECTIVE

Our goal was to develop and validate a clinical test for IS using cellular dielectric spectroscopy (CDS) and peripheral blood mononuclear cells (PBMCs).

SUMMARY OF BACKGROUND DATA

We have previously demonstrated the occurrence of a melatonin signaling dysfunction in osteoblasts obtained from severely affected IS patients using a cAMP assay. This led us to stratify IS patients into 3 functional subgroups.

METHODS

A group of 44 patients with IS was compared with 42 healthy control subjects and 31 asymptomatic at-risk children. PBMCs were obtained after centrifugation on a Ficoll-gradient. Melatonin signal transduction was measured by CDS in the presence of varying concentrations of melatonin or iodomelatonin.

RESULTS

Osteoblasts from distinct functional subgroups were retested using CDS, allowing their classification into the same functional subgroups with both ligands as initially demonstrated using a cAMP assay. Clinical data obtained with CDS and PBMCs showed 100% specificity and 100% sensitivity because melatonin signaling impairment was observed only in IS patients and not in healthy controls. Assessment of the risk of developing a scoliosis in asymptomatic children was determined by CDS in 33% of asymptomatic children at risk, which was confirmed clinically within 24 months.

CONCLUSION

This cell-based assay can serve as a presymptomatic screening test to identify asymptomatic children at risk of developing IS and may be used to improve stratification of patients, which in turn allow clinicians to predict their clinical outcome. Moreover, this functional blood test is advantageous because it can be performed without prior knowledge of specifically mutated genes causing IS.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.

Detection and quantification of beta2AR internalization in living cells using FAP-based biosensor technology

Ligand-dependent receptor internalization is a feature of numerous signaling systems. In this article, the authors describe a new kind of live-cell biosensor of receptor internalization that takes advantage of fluorogen-activating protein (FAP) technology. Recombinant genes that express the human beta2 adrenergic receptor (beta2AR) with FAP domains at their extracellular N-termini were transduced into mammalian cells. Exposure of the cells to membrane-impermeant fluorogens led to a strong fluorescent signal from the cell surface. Agonist-dependent translocation of the receptor from the surface to the cell interior was readily observed and quantified by fluorescence microscopy or flow cytometry in a homogeneous format without wash or separation steps. The approach described here is generalizable to other receptors and cell surface proteins and is adaptable to a variety of fluorescence-based high-throughput screening platforms.

Cellular basis for bimatoprost effects on human conventional outflow

PURPOSE

Bimatoprost is a widely used ocular hypotensive agent to treat glaucoma. It lowers intraocular pressure in humans by increasing both pressure-independent (uveoscleral) and pressure-dependent (conventional) aqueous humor outflow. The present study specifically examines bimatoprost effects on the cells that populate human outflow tissues.

METHODS

The authors tested for prostamide receptor activation in primary cultures of human trabecular meshwork (TM), Schlemm's canal (SC), and ciliary smooth muscle (CSM) cells using cellular dielectric spectroscopy (CDS).

RESULTS

The authors observed that bimatoprost produced an immediate and concentration-dependent increase in cell monolayer impedance for TM, SC, and CSM cells with EC(50) values of 4.3, 1.2, and 1.7 nM, respectively; corresponding to decreased cell contractility. Notably, in TM, SC, and CSM cells, bimatoprost was approximately equipotent to the selective FP receptor agonists fluprostenol and 17-phenyl PGF(2α). Bimatoprost effects were insensitive to cholera toxin and pertussis toxin but were abolished by phorbol 12-myristate 13-acetate pretreatment, suggesting Gq-involvement in cell signaling. The effects of bimatoprost on TM and SC cells were inhibited by the prostamide receptor antagonist AGN211334, with IC(50) values of 1.2 and 3.3 μM, respectively. Interestingly, AGN211334 behaved as an apparent inverse agonist in CDS assays involving TM cells but as a neutral prostamide antagonist with SC cells.

CONCLUSIONS

Taken together, results suggest that bimatoprost specifically activates receptors in both cell types of the human conventional outflow pathway to modify intraocular pressure. However, only TM cell monolayers appear to have autocrine, or agonist-independent, receptor signaling that is sensitive to a prostamide receptor antagonist.

Molecular and cellular pharmacological properties of 5-methoxycarbonylamino-N-acetyltryptamine (MCA-NAT): a nonspecific MT3 ligand

5-Methoxycarbonylamino-N-acetyltryptamine (MCA-NAT) has been initially described as a ligand at non MT(1), non MT(2) melatonin binding site (MT3) selective versus MT(1) and MT(2), two membrane melatonin receptors. MCA-NAT activity has been reported by others in different models, in vivo, particularly in the intra-ocular pressure (IOP) models in rabbits and monkeys. Its activity was systematically linked to either MT3 or to a new, yet unknown, melatonin receptor. In this article, the melatonin receptor pharmacology of MCA-NAT is described. MCA-NAT has micromolar range affinities at the melatonin receptors MT(1) and MT(2), while in functional studies, MCA-NAT proved to be a powerful MT(1)/MT(2) partial agonist in the sub-micromolar range. These data strongly suggest that MCA-NAT actions might be mediated by these receptors in vivo. Finally, as described by others, we show that MCA-NAT is unable to elicit any type of receptor-like functional responses from Chinese hamster ovary cells over-expressing quinone reductase 2, the MT3.