Cellular Dielectric Spectroscopy: A Powerful New Approach to Label-Free Cellular Analysis

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.

Rutaecarpin reduces lipids by DGKθ-dependent activation of PPARα

OBJECTIVE

Lipid metabolic disorders pose a serious threat to human health, and currently no good treatments exist. In earlier studies by the authors, HepG2 cells with diacylglycerol kinase theta (DGKθ) knockout were found to cause significant lipid accumulation, suggesting that DGKθ may be a potential target for treating lipid metabolic disorders.

METHODS

A high-throughput screening of natural products targeting the potential signaling pathway of lipid metabolism was carried out in the DGKθ-T2A-luciferase knock-in HepG2 cell. RNA-sequencing and bioinformatic approaches were used to analyze the potential pathway by which rutaecarpin decreases lipids. Western blot and quantitative polymerase chain reaction were performed to investigate the mechanisms of rutaecarpin's reduction in lipid levels.

RESULTS

Rutaecarpin was found to significantly enhance DGKθ expression, and the potential mechanisms by which rutaecarpin accelerates lipid metabolism by targeting DGKθ was explored in vitro and in vivo. The results indicated that rutaecarpin could markedly reduce lipid accumulation in oleic acid-induced HepG2 cells and in high-fat diet-induced obese C57BL/6J mice by targeting the hepatocyte nuclear factor 1-beta (HNF1B)-DGKθ-peroxisome proliferator-activated receptor alpha (PPARα)-apolipoprotein C3 (APOC3) pathway.

CONCLUSION

Rutaecarpin is effective in reducing lipid accumulation, and the development of a high-throughput screening platform based on a reporter knock-in cell line may facilitate the discovery of effective drugs for lipid metabolic disorders based on the DGKθ target.

MT1 Receptor Signaling Pathways by Impedance Measurement

Melatonin exerts its classical effects of relay of the circadian rhythm through two G protein-coupled receptors, MT1 and MT2. The functions attributed to melatonin are so numerous that the action of this neurohormone should be through several protein targets or through new coupled biochemistry routes at its receptors. In order to better explore and understand these melatonin-dependent activities, we enlarged the functional pathways linked to the activation of the receptors in living system. Impedance has been shown to rely on the shape-shifting capacity of receptor-associated mechanisms. Those changes elicited by an agonist lead to changes in the actual shape of the cells, and thus to their electric conductivity. The impact of those changes onto the physiology of the cells is not completely understood from a mechanistic point of view, but the measure of these changes associated with various ligands at the melatonin receptor(s) might bring new information on melatonin-dependent cell reactivity. The following chapter is a detailed account of the way impedance can be measured in MT1-experssing cells.

A 4 × 4 Array of Complementary Split-Ring Resonators for Label-Free Dielectric Spectroscopy

In this study, complementary split-ring resonator (CSRR) metamaterial structures are proposed for label-free dielectric spectroscopy of liquids in microplates. This novel combination of an array of sensors and microplates is readily scalable and thus offers a great potential for non-invasive, rapid, and label-free dielectric spectroscopy of liquids in large microplate arrays. The proposed array of sensors on a printed circuit board consists of a microstrip line coupled to four CSRRs in cascade with resonant frequencies ranging from 7 to 10 GHz, spaced around 1 GHz. The microwells were manufactured and bonded to the CSRR using polydimethylsiloxane, whose resonant frequency is dependent on a complex relative permittivity of the liquid loaded in the microwell. The individual microstrip lines with CSRRs were interconnected to the measurement equipment using two electronically controllable microwave switches, which enables microwave measurements of the 4 × 4 CSRR array using only a two-port measurement system. The 4 × 4 microwell sensor arrays were calibrated and evaluated using water-ethanol mixtures with different ethanol concentrations. The proposed measurement setup offers comparable results to ones obtained using a dielectric probe, confirming the potential of the planar sensor array for large-scale microplate experiments.

An Approach for the Real-Time Quantification of Cytosolic Protein-Protein Interactions in Living Cells

In recent years, cell-based assays have been frequently used in molecular interaction analysis. Cell-based assays complement traditional biochemical and biophysical methods, as they allow for molecular interaction analysis, mode of action studies, and even drug screening processes to be performed under physiologically relevant conditions. In most cellular assays, biomolecules are usually labeled to achieve specificity. In order to overcome some of the drawbacks associated with label-based assays, we have recently introduced "cell-based molography" as a biosensor for the analysis of specific molecular interactions involving native membrane receptors in living cells. Here, we expand this assay to cytosolic protein-protein interactions. First, we created a biomimetic membrane receptor by tethering one cytosolic interaction partner to the plasma membrane. The artificial construct is then coherently arranged into a two-dimensional pattern within the cytosol of living cells. Thanks to the molographic sensor, the specific interactions between the coherently arranged protein and its endogenous interaction partners become visible in real time without the use of a fluorescent label. This method turns out to be an important extension of cell-based molography because it expands the range of interactions that can be analyzed by molography to those in the cytosol of living cells.

Advantages and shortcomings of cell-based electrical impedance measurements as a GPCR drug discovery tool

G Protein-Coupled Receptors (GPCRs) transduce extracellular signals and activate intracellular pathways, usually through activating associated G proteins. Due to their involvement in many human diseases, they are recognized worldwide as valuable drug targets. Many experimental approaches help identify small molecules that target GPCRs, including in vitro cell-based reporter assays and binding studies. Most cell-based assays use one signaling pathway or reporter as an assay readout. Moreover, they often require cell labeling or the integration of reporter systems. Over the last decades, cell-based electrical impedance biosensors have been explored for drug discovery. This label-free method holds many advantages over other cellular assays in GPCR research. The technology requires no cell manipulation and offers real-time kinetic measurements of receptor-mediated cellular changes. Instead of measuring the activity of a single reporter, the impedance readout includes information on multiple signaling events. This is beneficial when screening for ligands targeting orphan GPCRs since the signaling cascade(s) of the majority of these receptors are unknown. Due to its sensitivity, the method also applies to cellular models more relevant to disease, including patient-derived cell cultures. Despite its advantages, remaining issues regarding data comparability and interpretability has limited implementation of cell-based electrical impedance (CEI) in drug discovery. Future optimization must include both full exploitation of CEI response data using various ways of analysis as well as further exploration of its potential to detect biased activities early on in drug discovery. Here, we review the contribution of CEI technology to GPCR research, discuss its comparative benefits, and provide recommendations.

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.

Synthesis and Evaluation of a New 18F-Labeled Radiotracer for Studying the GABAB Receptor in the Mouse Brain

New GABA_B agonists, fluoropyridyl ether analogues of baclofen, have been synthesized as potential PET radiotracers. The compound with highest inhibition binding affinity as well as greatest agonist response, ( R)-4-amino-3-(4-chloro-3-((2-fluoropyridin-4-yl)methoxy)phenyl)butanoic acid (1b), was radiolabeled with ¹⁸F with good radiochemical yield, high radiochemical purity, and high molar radioactivity. The regional brain distribution of the radiolabeled ( R)-4-amino-3-(4-chloro-3-((2-[¹⁸F]fluoropyridin-4-yl)methoxy)phenyl)butanoic acid, [¹⁸F]1b, was studied in CD-1 male mice. The study demonstrated that [¹⁸F]1b enters the mouse brain (1% ID/g tissue). The accumulation of [¹⁸F]1b in the mouse brain was inhibited (35%) by preinjection of GABA_B agonist 1a, suggesting that the radiotracer brain uptake is partially mediated by GABA_B receptors. The presented data demonstrate a feasibility of imaging of GABA_B receptors in rodents and justify further development of GABA_B PET tracers with improved specific binding and greater blood-brain barrier permeability.

Functional, RF-Trilayer Sensors for Tooth-Mounted, Wireless Monitoring of the Oral Cavity and Food Consumption

Wearable devices have emerged as powerful tools for personalized healthcare in spite of some challenges that limit their widespread applicability as continuous monitors of physiological information. Here, a materials-based strategy to add utility to traditional dielectric sensors by developing a conformal radiofrequency (RF) construct composed of an active layer encapsulated between two reverse-facing split ring resonators is applied. These small (down to 2 mm × 2 mm) passive dielectric sensors possess enhanced sensitivity and can be further augmented by functionalization of this interlayer material. Demonstrator devices are shown where the interlayer is: (i) a porous silk film, and (ii) a modified PNIPAM hydrogel that swells with pH or temperature. In vivo use is demonstrated by adhesion of the device on tooth enamel to detect foods during human ingestion. Such sensors can be easily multiplexed and yield data-rich temporal information during the diffusion of analytes within the trilayer structure. This format could be extended to a suite of interlayer materials for sensing devices of added use and specificity.

KK-92A, a novel GABAB receptor positive allosteric modulator, attenuates nicotine self-administration and cue-induced nicotine seeking in rats

RATIONALE

GABA_B receptors (GABA_BR) play a critical role in GABAergic neurotransmission in the brain and are thought to be one of the most promising targets for the treatment of drug addiction. GABA_BR positive allosteric modulators (PAMs) have shown promise as potential anti-addictive therapies, as they lack the sedative and muscle relaxant properties of full GABA_B receptor agonists such as baclofen.

OBJECTIVES

The present study was aimed at developing novel, selective, and potent GABA_BR PAMs with efficacy on abuse-related effects of nicotine.

RESULTS

We synthetized ~100 analogs of BHF177, a GABA_BR PAM that has been shown to inhibit nicotine taking and seeking, and tested their activity in multiple cell-based functional assays. Among these compounds, KK-92A displayed superior PAM properties at the GABA_BR. Interestingly, our results revealed the existence of pathway-selective differential modulation of GABA_BR signaling by the structurally related GABA_BR allosteric modulators BHF177 and KK-92A. In vivo, similarly to BHF177, KK-92A inhibited intravenous nicotine self-administration under both fixed- and progressive-ratio schedules of reinforcement in rats. In contrast to BHF177, KK-92A had no effect on food self-administration. Furthermore, KK-92A decreased cue-induced nicotine-seeking behavior without affecting food seeking.

CONCLUSIONS

These results indicate that KK-92A is a selective GABA_BR PAM with efficacy in inhibition of the primary reinforcing and incentive motivational effects of nicotine, and attenuation of nicotine seeking, further confirming that GABA_BR PAMs may be useful antismoking medications.

GABAB receptor allosteric modulators exhibit pathway-dependent and species-selective activity

Positive modulation of the GABA_B receptor (GABA_BR) represents a potentially useful therapeutic approach for the treatment of nicotine addiction. The positive allosteric modulators (PAMs) of GABA_BR GS39783 and BHF177 enhance GABA‐stimulated [³⁵S]GTPγS‐binding, and have shown efficacy in a rodent nicotine self‐administration procedure reflecting aspects of nicotine dependence. Interestingly, the structural related analog, NVP998, had no effect on nicotine self‐administration in rats despite demonstrating similar pharmacokinetic properties. Extensive in vitro characterization of GS39783, BHF177, and NVP998 activity on GABA_BR‐regulated signaling events, including modulation of cAMP, intracellular calcium levels, and ERK activation, revealed that these structurally related molecules display distinct pathway‐specific signaling activities that correlate with the dissimilarities observed in rodent models and may be predictive of in vivo efficacy. Furthermore, these GABA_BR allosteric modulators exhibit species‐dependent activity. Collectively, these data will be useful in guiding the development of GABA_BR allosteric modulators that display optimal in vivo efficacy in a preclinical model of nicotine dependence, and will identify those that have the potential to lead to novel antismoking therapies.

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.

Cellular Dielectric Spectroscopy: A Label-Free Technology for Drug Discovery

Cellular dielectric spectroscopy (CDS) provides realtime, label-free, universal measurements, enabling comprehensive pharmacological evaluation of cell surface receptors in living cells. The sensitivity of the measurement allows monitoring of ligand-mediated activation of endogenous receptors, therefore generating physiologically relevant data. Activation of receptors results in CDS response profiles that are characteristic of main subsets of G-protein coupled receptors (GPCRs) within a cell line. This allows cluster analysis of response profiles that may be used in several important applications, which include identification of the G-protein coupling of orphan GPCRs and the cataloging of active endogenous receptors in cells. In this study, CDS technology is used in the pharmacological evaluation of multiple receptors in many cell types, including primary cells. Specifically, data is presented demonstrating hit confirmation, receptor selectivity analysis, ligand potency, and Schild analysis of receptor-selective antagonists. CDS results compare favorably to other cell-based assays, and the robustness and reproducibility of CDS assays are reflected by low assay coefficient of variation (CVs) and reliable Z'-scores of the data. Because CDS requires no stable or transiently transfected cells or special reagents, assay development and data acquisition is simple and fast. The ease of use, universality, and label-free nature of the CDS-based platform make it well suited to secondary screening applications in drug discovery.

Label-free monitoring of μ-opioid receptor-mediated signaling

In this study, we used a combination of traditional signaling investigation approaches, bioluminescence resonance energy transfer (BRET) biosensors, and the label-free approach surface plasmon resonance (SPR) spectroscopy to monitor the signaling cascades of the μ-opioid receptor (MOP). In human embryonic kidney cells stably expressing a Flag-tagged version of human MOP, we compared the signals triggered by the noninternalizing and internalizing MOP agonists morphine and DAMGO (Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol), respectively. We studied three major and well described components of MOP signaling: receptor internalization, G protein coupling, and activation of extracellular signal-regulated kinase ERK1/ERK2. Our results show that morphine and DAMGO display different profiles of receptor internalization and a similar ability to trigger the phosphorylation of ERK1/ERK2. Our SPR analyses revealed that morphine and DAMGO evoke similar SPR signatures and that Gαi, cAMP-dependent pathways, and ERK1/ERK2 have key roles in morphine- and DAMGO-mediated signaling. Most interestingly, we found that the so-called MOP neutral antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)), naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) induces similar conformational changes as naltrexone at the Gαi-βγ interface, whereas it appears as an inverse agonist based on its SPR response thus indicating distinct signaling mechanisms for the two ligands. Taken together, our results support the usefulness of label-free methods such as SPR to study whole-cell responses and signaling cascades triggered by G protein-coupled receptors and complement the conventional approaches by revealing cellular responses that would have been otherwise undetectable.

Development of a high-throughput screening-compatible cell-based functional assay to identify small molecule probes of the galanin 3 receptor (GalR3)

The galanin 3 receptor (GalR3) belongs to the large G protein-coupled receptor (GPCR) family of proteins. GalR3 and two other closely related receptors, GalR1 and GalR2, together with their endogenous ligand galanin, are involved in a variety of physiological and pathophysiological processes. GalR3 in particular has been strongly implicated in addiction and mood-related disorders such as anxiety and depression. It has been the target of many drug discovery programs within the pharmaceutical industry, but despite the significant resources and effort devoted to discovery of galanin receptor subtype selective small molecule modulators, there have been very few reports for the discovery of such molecules. GalR3 has proven difficult to enable in cell-based functional assays due to its apparent poor cell surface expression in recombinant systems. Here, we describe the generation of a modified GalR3 that facilitates its cell surface expression while maintaining wild-type receptor pharmacology. The modified GalR3 has been used to develop a high-throughput screening-compatible, cell-based, cAMP biosensor assay to detect selective small molecule modulators of GalR3. The performance of the assay has been validated by challenging it against a test library of small molecules with known pharmacological activities (LOPAC; Sigma Aldrich). This approach will enable identification of GalR3 selective modulators (chemical probes) that will facilitate dissection of the biological role(s) that GalR3 plays in normal physiological processes as well as in disease states.

Present and future approaches to screening of G-protein-coupled receptors

As G-protein-coupled receptors (GPCRs) mediate a multitude of cellular signal transduction events, affecting more or less all human disease areas, it is, therefore, no surprise that they comprise the largest family of current drug targets. Screening of compounds interacting with GPCRs has developed during the past decade from receptor binding assays, to various functional determination of coupling to G-proteins, and, more recently, G-protein-independent signal transduction events. Additional opportunities have been presented in drug discovery through novel pharmacological properties obtained for receptor dimers and by identification of ligands for orphan GPCRs. Furthermore, high-throughput formats and automation has substantially facilitated and accelerated the screening process providing powerful tools in improving modern drug discovery.

Update on in vitro cytotoxicity assays for drug development

BACKGROUND

in vitro cytotoxicity testing provides a crucial means of ranking compounds for consideration in drug discovery. The choice of using a particular viability or cytotoxicity assay technology may be influenced by specific research goals.

OBJECTIVE

Although the high-throughput screening (HTS) utility is typically dependent upon sensitivity and scalability, it is also impacted by signal robustness and resiliency to assay interferences. Further consideration should be given to data quality, ease-of-use, reagent stability, and matters of cost-effectiveness.

METHODS

Here we focus on three main classes of assays that are at present the most popular, useful, and practical for HTS drug discovery efforts. These methods measure: i) viability by metabolism reductase activities; ii) viability by bioluminescent ATP assays; or iii) cytotoxicity by enzymes 'released' into culture medium. Multi-parametric technologies are also briefly discussed.

RESULTS/CONCLUSION

Each of these methods has its relative merits and detractions; however multi-parametric methods using both viability and cytotoxicity markers may mitigate the inherent shortcomings of single parameter measures.

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 development of label-free cellular assays for drug discovery

INTRODUCTION

The need to improve drug research and development productivity continues to drive innovation in pharmacological assays. Technologies that can leverage the advantages of both molecular and phenotypic assays would hold great promise for discovery of new medicines.

AREAS COVERED

This article briefly reviews current label-free platforms for cell-based assays and is primarily focused on fundamental aspects of these assays using dynamic mass redistribution technology as an example. The article also presents strategies for relating label-free profiles to molecular modes of actions of drugs.

EXPERT OPINION

Emerging evidence suggests that label-free cellular assays are phenotypic in nature, yet permit molecular mechanistic deconvolution. Together with unique competency in throughput, sensitivity and pathway coverages, label-free cellular assays allow users to screen drugs against endogenous receptors in native cells (including disease relevant primary cells) and determine the molecular modes of action of drug molecules. However, there are challenges for label-free in both basic research and drug discovery: the deconvolution of the cellular and molecular mechanisms for the biosensor signatures of receptor-drug interactions, new methodologies for data analysis and the development of new biosensor technologies. These challenges will need to be met for the wide adoption of these assays in drug discovery.

Applying label-free dynamic mass redistribution technology to frame signaling of G protein-coupled receptors noninvasively in living cells

Label-free dynamic mass redistribution (DMR) is a cutting-edge assay technology that enables real-time detection of integrated cellular responses in living cells. It relies on detection of refractive index alterations on biosensor-coated microplates that originate from stimulus-induced changes in the total biomass proximal to the sensor surface. Here we describe a detailed protocol to apply DMR technology to frame functional behavior of G protein-coupled receptors that are traditionally examined with end point assays on the basis of detection of individual second messengers, such as cAMP, Ca(2+) or inositol phosphates. The method can be readily adapted across diverse cellular backgrounds (adherent or suspension), including primary human cells. Real-time recordings can be performed in 384-well microtiter plates and be completed in 2 h, or they can be extended to several hours depending on the biological question to be addressed. The entire procedure, including cell harvesting and DMR detection, takes 1-2 d.

NIR-mbc94, a fluorescent ligand that binds to endogenous CB(2) receptors and is amenable to high-throughput screening

High-throughput screening (HTS) of chemical libraries is often used for the unbiased identification of compounds interacting with G protein-coupled receptors (GPCRs), the largest family of therapeutic targets. However, current HTS methods require removing GPCRs from their native environment, which modifies their pharmacodynamic properties and biases the screen toward false positive hits. Here, we developed and validated a molecular imaging (MI) agent, NIR-mbc94, which emits near infrared (NIR) light and selectively binds to endogenously expressed cannabinoid CB(2) receptors, a recognized target for treating autoimmune diseases, chronic pain and cancer. The precision and ease of this assay allows for the HTS of compounds interacting with CB(2) receptors expressed in their native environment.

Monitoring cellular stress responses to nanoparticles using a lab-on-a-chip

As nanotechnology moves towards widespread commercialization, new technologies are needed to adequately address the potential health impact of nanoparticles (NPs). Assessing the safety of over 30,000 NPs through animal testing would not only be expensive, but it would also raise a number of ethical considerations. Furthermore, existing in vitro cell-based assays are not sufficient in scope to adequately address the complexity of cell-nanoparticle interactions including NP translocation, accumulation and co-transport of e.g. allergens. In particular, classical optical/fluorescent endpoint detection methods are known to provide irreproducible, inaccurate and unreliable results since these labels can directly react with the highly catalytic surfaces of NP. To bridge this technological gap we have developed a lab-on-a-chip capable of continuously and non-invasively monitoring the collagen production of primary human fibroblast cells (NHDF) using contactless dielectric microsensors. Human dermal fibroblast cells are responsible for the maintenance of soft tissue integrity, are found throughout the human body and their primary function is collagen expression. We show that cellular collagen production can be readily detected and used to assess cellular stress responses to a variety of external stimuli, including exposure to nanoparticles. Results of the study showed a 20% and 95% reduction of collagen production following 4 hour exposure to 10 μg mL(-1) gold and silver nanoparticles (dia.10 nm), respectively. Furthermore a prolonged perfusion of sub-toxic concentrations (0.1 μg mL(-1)) of silver NP reduced NHDF collagen production by 40% after 10 h indicating increased NP take up and accumulation. We demonstrate that the application of microfluidics for the tailored administration of different NP treatments constitutes a powerful new tool to study cell-nanoparticle interactions and nanoparticle accumulation effects in small cell populations.

Nonlinear impedance of whole cells near an electrode as a probe of mitochondrial activity

By simultaneously measuring the bulk media and electrode interface voltages of a yeast (Saccharomyces cerevisiae) suspension subjected to an AC voltage, a yeast-dependent nonlinear response was found only near the current injection electrodes. Computer simulation of yeast near a current injection electrode found an enhanced voltage drop across the yeast near the electrode due to slowed charging of the electrode interfacial capacitance. This voltage drop is sufficient to induce conformation change in membrane proteins. Disruption of the mitochondrial electron transport chain is found to significantly change the measured nonlinear current response, suggesting nonlinear impedance can be used as a non-invasive probe of cellular metabolic activity.

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.

Continuous differential impedance spectroscopy of single cells

A device for continuous differential impedance analysis of single cells held by a hydrodynamic cell trapping is presented. Measurements are accomplished by recording the current from two closely-situated electrode pairs, one empty (reference) and one containing a cell. We demonstrate time-dependent measurement of single cell impedance produced in response to dynamic chemical perturbations. First, the system is used to assay the response of HeLa cells to the effects of the surfactant Tween, which reduces the impedance of the trapped cells in a concentration dependent way and is interpreted as gradual lysis of the cell membrane. Second, the effects of the bacterial pore-forming toxin, Streptolysin-O are measured: a transient exponential decay in the impedance is recorded as the cell membrane becomes increasingly permeable. The decay time constant is inversely proportional to toxin concentration (482, 150, and 30 s for 0.1, 1, and 10 kU/ml, respectively). ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10404-009-0534-2) contains supplementary material, which is available to authorized users.

Comparison of label-free biosensing in microplate, microfluidic, and spot-based affinity capture assays

Using both experimental assays and fluid-dynamic finite element simulation models, we directly compared the achievable performance limits of four distinct assay configurations for label-free detection of an analyte from a test sample on a biosensor surface. The assay configurations studied in this work included a biosensor incorporated into the bottom surface of a microplate well and a microfluidic channel. For each configuration, we compared assay performance for the scenario in which the entire bottom surface of the fluid-handling vessel is coated with capture ligands with assay performance for the scenario in which the capture ligands are applied in the form of localized spots. As a model system, we used detection of the protein biomarker tumor necrosis factor-alpha (TNF-alpha) using immobilized TNF-alpha capture antibody. Results show that the microfluidic assay format dramatically reduces the time required to establish a stable equilibrium. Spot-based assays are advantageous for microplate-based detection for reducing the time required for equilibrium sensor response. The results derived are generally applicable to any label-free biosensor technology and any ligand-analyte system with adjustable variables that include sensor mass density sensitivity, analyte-ligand adsorption/desorption rate constants, immobilized ligand density, flow channel geometry, flow rate, and spot size.

Development of a GPR23 cell-based β-lactamase reporter assay

GPR23 is a G protein-coupled receptor (GPCR) proposed to play a vital role in neurodevelopment processes such as neurogenesis and neuronal migration. To date, no small molecule GPR23 agonists or antagonists have been reported, except for the natural ligand, lysophosphatic acid, and its analogs. Identification of ligands selective for GPR23 would provide valuable tools for studying the pharmacology, physiological function, and pathophysiological implications of this receptor. This report describes how a tetracycline-inducible system was utilized in conjunction with a sensitive β-lactamase reporter gene to develop an assay in which constitutive activity of the receptor could be monitored. This assay was then utilized to screen a 1.1 million compound library to identify the first small molecule inverse agonists for the receptor. We believe that these compounds will be invaluable tools in the further study of GPR23. In addition, we believe that the assay development techniques utilized in this report are broadly applicable to other receptors exhibiting constitutive activity.

Development of a microfluidic biochip for online monitoring of fungal biofilm dynamics

Microfabricated biochips are developed to continuously monitor cell population dynamics in a non-invasive manner. In the presented work we describe the novel combination of contact-less dielectric microsensors and microfluidics to promote biofilm formation for quantitative cell analysis. The cell chip consists of a polymeric fluidic (PDMS) system bonded to a glass wafer containing the electrodes while temperature and fluid flow are controlled by external heating and pumping stations. The high-density interdigitated capacitors (microIDES) are isolated by a 550 nm multi-passivation layer of defined dielectric property and provide stable, robust and non-drifting measurement conditions. The performance of this detector is evaluated using various bacterial and yeast strains. The high sensitivity of the developed dielectric microsensors allows direct identification of microbial strains based on morphological differences and biological composition. The novel biofilm analysis platform is used to continuously monitor the dynamic responses of C. albicans and P. pastoris biofilms to increased shear stress and antimicrobial agent concentration. While the presence of shear stress triggers significant changes in yeast growth profiles, the addition of 0.5 microg mL(-1) amphotericin B revealed two distinct dynamic behaviors of the C. albicans biofilm. Initially, impedance spectra increased linearly at 30 Omega h(-1) for two hours followed by 10 Omega h(-1) (at 50 kHz) over 10 hours while cell viability remained above 95% during fungicide administration. These results demonstrate the ability to directly monitor dielectric changes of sub-cellular components within a living cell population.

Evolution of cell-based reagent provision

Cell-based screening is now part of all stages of drug discovery, and therefore, cell supply is a rate-limiting step. Reagent provision groups have responded by exploiting automation and new concepts such as frozen cells to ease the constraint and increase quality and flexibility of cell supply. With increasing numbers of projects to support, reagent development is now perceived as a new bottleneck. In this review, we describe a new operating model that has emerged at Pfizer UK addressing reagent development and cell supply issues without growing headcount, by complementing the application of internal expertise with use of contract research organisations.

Assessment of cytotoxicity by impedance spectroscopy

This paper describes a simple and convenient method to monitor on-line cell adhesion by electrical impedance measurements. Immortalized mouse fibroblasts, BALB/3T3, were cultured onto interdigitated electrode structures integrated into the bottom of an in-house fabricated device. Impedance modulus, phase, real and imaginary parts were considered separately and plotted as function of frequency and time to better understand and select the component giving more information on cell adhesion changes. For cytotoxicity assessment, the cells were treated with different concentrations of sodium arsenite used as model toxicant and their responses were monitored on-line. The half inhibition concentration, the required concentration to achieve 50% inhibition, derived from the measurements fall between the results obtained using standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test and colony forming efficiency assay confirming the good sensitivity of the system. In term of impedance signal, the modulus results was found to be the most sensitive of the considered components for cytotoxicity testing of chemicals.

Evaluation of cellular dielectric spectroscopy, a whole-cell, label-free technology for drug discovery on Gi-coupled GPCRs

Cellular dielectric spectroscopy (CDS) is an emerging technology capable of detecting a range of whole-cell responses in a label-free manner. A new CDS-based instrument, CellKey, has been developed that is optimized for G-protein coupled receptor (GPCR) detection and has automated liquid handling in microplate format, thereby making CDS accessible to lead generation/optimization drug discovery. In addition to having sufficient throughput, new assay technologies must pass rigorous standards for assay development, signal window, dynamic range, and reproducibility to effectively support drug discovery SAR studies. Here, the authors evaluated CellKey with 3 different G(i)-coupled GPCRs for suitability in supporting SAR studies. Optimized assay conditions compatible with the precision, reproducibility, and throughput required for routine screening were quickly achieved for each target. Across a 1000-fold range in compound potencies, CellKey results correlated with agonist and antagonist data obtained using classical methods ([(35)S]GTPgammaS binding and cAMP production). For partial agonists, relative efficacy measurements also correlated with GTPgammaS data. CellKey detection of positive allosteric modulators appeared superior to GTPgammaS methodology. Agonist and antagonist activity could be accurately quantified under conditions of low receptor expression. CellKey is a new technology platform that uses label-free detection in a homogeneous assay that is unaffected by color quenching and is easily integrated into existing microtiter-based compound testing and data analysis procedures for drug discovery.

Protein influence on the plasma membrane dielectric properties: in vivo study utilizing dielectric spectroscopy and fluorescence microscopy

We have investigated the origin of the dielectric response of the plasma membrane of living yeast cells (Saccharomyces cerevisiae) by using radiofrequency dielectric spectroscopy. The cells were genetically engineered to overexpress in the membrane of yeast cells a G protein-coupled receptor--the Sterile2-alpha factor receptor protein (Ste2p)--fused to the green fluorescent protein (GFP). Presence of the Ste2-GFP proteins in the plasma membrane was confirmed by exciting the cells at 476 nm and observing with a confocal microscope the emission characteristic of the GFP from individual cells. The dielectric behavior of cells suspended in KCl solution was analyzed over the frequency range 40 Hz-110 MHz and compared to the behavior of control cells that lacked the ability to express Ste2p. A two-shell electrical cell model was used to fit the data starting from known structural parameters and adjustable electrical phase parameters. The best-fit value for the relative permittivity of the plasma membrane showed no significant difference between cells expressing Ste2p (1.63+/-0.11) and the control cells (1.75+/-0.16). This result confirmed earlier predictions that the dielectric properties of the plasma membrane in the radiofrequency range mostly reflect the properties of the hydrophobic layer of the membrane, which is populated by the hydrocarbon tails of the phospholipids and hydrophobic segments of integral membrane proteins. We discuss ways by which dielectric spectroscopy can be improved to be used for tag-free detection of proteins on the membrane.

Dynamic and label-free cell-based assays using the real-time cell electronic sensing system

Cell-based assays have become an integral part of the preclinical drug development process. Recently, noninvasive label-free cell-based assay technologies have taken center stage, offering important and distinct advantages over and in addition to traditional label-based endpoint assays. Dynamic monitoring of live cells, the preclusion of label, and kinetics are some of the fundamental features of cell-based label-free technologies. In this article we will discuss the real-time cell electronic sensing (RT-CES, ACEA Biosciences Inc., San Diego, CA) system and some of its key applications for cell-based assays such as cell proliferation and cytotoxicity, functional assays for receptor-ligand analysis, cell adhesion and spreading assays, dynamic monitoring of endothelial barrier function, and dynamic monitoring of cell migration and invasion. Also, where appropriate we will briefly discuss other label-free technologies in an application-specific manner.

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

The CellKey (MDS Sciex, South San Francisco, CA) system enables comprehensive pharmacological evaluation of cell surface receptors, including G-protein coupled receptors (GPCRs) and tyrosine kinase receptors, using adherent and suspension cell lines and primary cells. A unique application enabled by the ability of the CellKey system to reliably quantify activation of endogenous receptors is receptor panning. This application allows investigators to easily screen disease-relevant cell types for functionally active target receptors by treating cells with a panel of receptor-specific ligands. Receptor panning of multiple cell types including Chinese hamster ovary, human embryonic kidney 293, HeLa, U-937, U-2 OS, and TE671 cells resulted in the identification of many functionally active, differently coupled endogenous GPCRs, some of which have not been previously documented in the literature. Upon detecting GPCR activation in live cells, unique cellular dielectric spectroscopy (CDS) response profiles are generated within minutes that reflect the signaling pathways utilized and have been shown to be characteristic of Gs, Gq, and Gi GPCRs. The fact that the CDS response profiles are predictive of the G-protein coupling mechanism of the receptor was demonstrated by using examples of subtype-selective agonists/antagonists to identify the subtypes of the endogenous histamine and beta-adrenergic receptors expressed in U-2 OS cells. A direct correlation is shown between receptor subtype G-protein coupling and CDS response profile. In addition, complex pharmacology, including detection of partial agonism and Schild analysis for endogenous receptors, is presented. The CellKey system allows investigators to conduct studies using endogenously expressed receptors to generate data that are physiologically relevant and in disease context.

Real-time measurement of PMA-induced cellular alterations by microelectrode array-based impedance spectroscopy

For a feasible and cost-effective impedance measurement of cellular alterations in real-time, we combined commercially available microelectrode arrays (MEAs), consisting of 60 micro-electrodes, with a conventional impedance analyzer. For proof of principle, a breast carcinoma cell line (MCF-7) was cultured on MEAs, and cellular alterations were measured by impedance spectroscopy at a frequency ranging from 10 Hz to 1 MHz. Cells were stimulated with phorbol 12-myristate 13-acetate (PMA) at different concentrations to activate protein kinase C (PKC)-mediated extra- and intracellular changes. By addition of 0.03 µM PMA, an increase of the relative impedance (Zrel) was observed after 10 min with a maximum at 1 kHz. Moreover, a gradual elevation of the impedance was measured 60 min after stimulation with PMA. If 0.3 µM PMA was applied, the maximal amplitude of the relative impedance after 60 min shifted from 1 kHz (0.03 µM PMA) to 150 Hz. Subsequently, the impedance was further increased up to 90 min after PMA application, after which the impedance reduced after 240 min. Since we could use MEAs for at least 10 times without affecting the sensitivity, our study revealed that commercially available MEAs comprising nanocolumnar titanium nitrite electrodes are suitable microstructures for a highly reproducible and cost-effective multisite measurement of intracellular processes by impedance spectroscopy.

HTS technologies in biopharmaceutical discovery

The concepts and philosophies of HTS can be productively applied to the discovery of new biopharmaceuticals. It is now possible, comprehensively and systematically, to enumerate, clone, produce and screen all secreted proteins, by building upon knowledge accumulated over the past two decades in HTS, genomics and parallel protein expression technologies. Each of the crucial operational components (comprehensive and high-quality cDNA library construction, proper protein-sequence classification, high-throughput protein production, medically relevant assays, state-of-the-art screening and data management) must be optimized to increase the chances of success. In this review, we draw comparisons between small-molecule and protein screening to illuminate common underlying principles as well as differences between the two operations.

Label-free and real-time cell-based kinase assay for screening selective and potent receptor tyrosine kinase inhibitors using microelectronic sensor array

Kinases are the 2nd largest group of therapeutic targets in the human genome. In this article, a label-free and real-time cell-based receptor tyrosine kinase (RTK) assay that addresses limitation of existing kinase assays and can be used for high-throughput screening and lead optimization studies was validated and characterized. Using impedance, growth factor-induced morphological changes were quantitatively assessed in real time and used as a measure of RTK activity. COS7 cells treated with epidermal growth factor (EGF) and insulin results in a rapid increase in cell impedance. Assessment of these growth factor-induced morphological changes and levels of receptor autophosphorylation using fluorescent microscopy and enzyme-linked immunosorbent assay, respectively, demonstrates that these changes correlate with changes in impedance. This assay was used to screen, identify, and characterize a potent EGF receptor inhibitor from a compound library. This report describes an assay that is simple in that it does not require intensive optimization or special reagents such as peptides, antibodies, or probes. More important, because the assay is cell based, the studies are done in a physiologically relevant environment, allowing for concurrent assessment of a compound's solubility, stability, membrane permeability, cytotoxicity, and off-target interaction effects.