Development of a Ca(2+)-activated photoprotein, Photina, and its application to high-throughput screening

Imaging mitochondrial calcium dynamics in the central nervous system

Mitochondrial calcium handling is a particularly active research area in the neuroscience field, as it plays key roles in the regulation of several functions of the central nervous system, such as synaptic transmission and plasticity, astrocyte calcium signaling, neuronal activity… In the last few decades, a panel of techniques have been developed to measure mitochondrial calcium dynamics, relying mostly on photonic microscopy, and including synthetic sensors, hybrid sensors and genetically encoded calcium sensors. The goal of this review is to endow the reader with a deep knowledge of the historical and latest tools to monitor mitochondrial calcium events in the brain, as well as a comprehensive overview of the current state of the art in brain mitochondrial calcium signaling. We will discuss the main calcium probes used in the field, their mitochondrial targeting strategies, their key properties and major drawbacks. In addition, we will detail the main roles of mitochondrial calcium handling in neuronal tissues through an extended report of the recent studies using mitochondrial targeted calcium sensors in neuronal and astroglial cells, in vitro and in vivo.

Volatile allosteric antagonists of mosquito odorant receptors inhibit human-host attraction

Odorant-dependent behaviors in insects are triggered by the binding of odorant ligands to the variable subunits of heteromeric olfactory receptors. Previous studies have shown, however, that specific odor binding to ORco, the common subunit of odorant receptor heteromers, may allosterically alter olfactory receptor function and profoundly affect subsequent behavioral responses. Using an insect cell-based screening platform, we identified and characterized several antagonists of the odorant receptor coreceptor of the African malaria vector Anopheles gambiae (AgamORco) in a small collection of natural volatile organic compounds. Because some of the identified antagonists were previously shown to strongly repel Anopheles and Culex mosquitoes, we examined the bioactivities of the identified antagonists against Aedes, the third major genus of the Culicidae family. The tested antagonists inhibited the function of Ae. aegypti ORco ex vivo and repelled adult Asian tiger mosquitoes (Ae. albopictus). Binary mixtures of specific antagonists elicited higher repellency than single antagonists, and binding competition assays suggested that this enhanced repellence is due to antagonist interaction with distinct ORco sites. Our results also suggest that the enhanced mosquito repellency by antagonist mixtures is due to additive rather than synergistic effects of the specific antagonist combinations on ORco function. Taken together, these findings provide novel insights concerning the molecular aspects of odorant receptor function. Moreover, our results demonstrate that a simple screening assay may be used for the identification of allosteric modifiers of olfactory-driven behaviors capable of providing enhanced personal protection against multiple mosquito-borne infectious diseases.

Mitochondrial Ca2+ concentrations in live cells: quantification methods and discrepancies

Intracellular Ca²⁺ signaling controls numerous cellular functions. Mitochondria respond to cytosolic Ca²⁺ changes by adapting mitochondrial functions and, in some cell types, shaping the spatiotemporal properties of the cytosolic Ca²⁺ signal. Numerous methods have been developed to specifically and quantitatively measure the mitochondrial-free Ca²⁺ concentrations ([Ca²⁺ ]_m ), but there are still significant discrepancies in the calculated absolute values of [Ca²⁺ ]_m in stimulated live cells. These discrepancies may be due to the distinct properties of the methods used to measure [Ca²⁺ ]_m , the calcium-free/bound ratio, and the cell-type and stimulus-dependent Ca²⁺ dynamics. Critical processes happening in the mitochondria, such as ATP generation, ROS homeostasis, and mitochondrial permeability transition opening, depend directly on the [Ca²⁺ ]_m values. Thus, precise determination of absolute [Ca²⁺ ]_m values is imperative for understanding Ca²⁺ signaling. This review summarizes the reported calibrated [Ca²⁺ ]_m values in many cell types and discusses the discrepancies among these values. Areas for future research are also proposed.

An overview of Ca2+ mobilization assays in GPCR drug discovery

INTRODUCTION

Calcium ions (Ca²⁺) serve as a second messenger or universal signal transducer implicated in the regulation of a wide range of physiological processes. A change in the concentration of intracellular Ca²⁺ is an important step in intracellular signal transduction. G protein-coupled receptors (GPCRs), the largest and most versatile group of cell surface receptors, transduce extracellular signals into intracellular responses via their coupling to heterotrimeric G proteins. Since Ca²⁺ plays a crucial role in GPCR-induced signaling, measurement of intracellular Ca²⁺ has attracted more and more attention in GPCR-targeted drug discovery. Areas covered: This review focuses on the most popular functional assays measuring GPCRs-induced intracellular Ca²⁺ signaling. These include photoprotein-based, synthetic fluorescent indicator-based and genetically encoded calcium indicator (GECI)-based Ca²⁺ mobilization assays. A brief discussion of the design strategy of fluorescent probes in GPCR studies is also presented. Expert opinion: GPCR-mediated intracellular signaling is multidimensional. There is an urgent need for the development of multiple-readout screening assays capable of simultaneous detection of biased signaling and screening of both agonists and antagonists in the same assay. It is also necessary to develop GECIs offering low cost and consistent assays suitable for investigating GPCR activation in vivo.

Fluorescent Protein-photoprotein Fusions and Their Applications in Calcium Imaging

Calcium-activated photoproteins, such as aequorin, have been used as luminescent Ca²⁺ indicators since 1967. After the cloning of aequorin in 1985, microinjection was substituted by its heterologous expression, which opened the way for a widespread use. Molecular fusion of green fluorescent protein (GFP) to aequorin recapitulated the nonradiative energy transfer process that occurs in the jellyfish Aequorea victoria, from which these two proteins were obtained, resulting in an increase of light emission and a shift to longer wavelength. The abundance and location of the chimera are seen by fluorescence, whereas its luminescence reports Ca²⁺ levels. GFP-aequorin is broadly used in an increasing number of studies, from organelles and cells to intact organisms. By fusing other fluorescent proteins to aequorin, the available luminescence color palette has been expanded for multiplexing assays and for in vivo measurements. In this report, we will attempt to review the various photoproteins available, their reported fusions with fluorescent proteins and their biological applications to image Ca²⁺ dynamics in organelles, cells, tissue explants and in live organisms.

A luminescent hybridoma-based biosensor for rapid detection of V. cholerae upon induction of calcium signaling pathway

In this study, a hybridoma based biosensor was developed for rapid, sensitive and selective detection of Vibrio cholerae O1 which converts the antibody-antigen binding to bioluminescence light. After investigation on hybridoma performance, the biosensor was constructed by transfecting specific hybridoma cells with aequorin reporter gene and the bioluminescence activities of stable biosensor were measured. The sensitivity of biosensor was as few as 50 CFU/ml and it showed no responses to other entric bacteria. Moreover, the response time of biosensor was estimated in 7th second which means this method is considerably faster than many available detection assays. In addition, this biosensor was successfully applied to V. cholerae detection in environmental samples with no significant loss in sensitivity, demonstrating our proposed biosensor provides a sensitive and reliable method for detection of V. cholerae in natural samples. The application of whole hybridoma cell directly as a sensing element in biosensor construction which mentioned for the first time in present study suggests that hybridoma cells could provide a valuable tool for future studies in both basic and diagnostic sciences and could be considered as a fast and specific sensing element for detection of other pathogens in different applications.

Inhibition of Anopheles gambiae odorant receptor function by mosquito repellents

The identification of molecular targets of insect repellents has been a challenging task, with their effects on odorant receptors (ORs) remaining a debatable issue. Here, we describe a study on the effects of selected mosquito repellents, including the widely used repellent N,N-diethyl-meta-toluamide (DEET), on the function of specific ORs of the African malaria vector Anopheles gambiae. This study, which has been based on quantitative measurements of a Ca(2+)-activated photoprotein biosensor of recombinant OR function in an insect cell-based expression platform and a sequential compound addition protocol, revealed that heteromeric OR (ORx/Orco) function was susceptible to strong inhibition by all tested mosquito repellents except DEET. Moreover, our results demonstrated that the observed inhibition was due to efficient blocking of Orco (olfactory receptor coreceptor) function. This mechanism of repellent action, which is reported for the first time, is distinct from the mode of action of other characterized insect repellents including DEET.

Propensity scores for prediction and characterization of bioluminescent proteins from sequences

Bioluminescent proteins (BLPs) are a class of proteins with various mechanisms of light emission such as bioluminescence and fluorescence from luminous organisms. While valuable for commercial and medical applications, identification of BLPs, including luciferases and fluorescent proteins (FPs), is rather challenging, owing to their high variety of protein sequences. Moreover, characterization of BLPs facilitates mutagenesis analysis to enhance bioluminescence and fluorescence. Therefore, this study proposes a novel methodological approach to estimating the propensity scores of 400 dipeptides and 20 amino acids in order to design two prediction methods and characterize BLPs based on a scoring card method (SCM). The SCMBLP method for predicting BLPs achieves an accuracy of 90.83% for 10-fold cross-validation higher than existing support vector machine based methods and a test accuracy of 82.85%. A dataset consisting of 269 luciferases and 216 FPs is also established to design the SCMLFP prediction method, which achieves training and test accuracies of 97.10% and 96.28%, respectively. Additionally, four informative physicochemical properties of 20 amino acids are identified using the estimated propensity scores to characterize BLPs as follows: 1) high transfer free energy from inside to the protein surface, 2) high occurrence frequency of residues in the transmembrane regions of the protein, 3) large hydrophobicity scale from the native protein structure, and 4) high correlation coefficient (R = 0.921) between the amino acid compositions of BLPs and integral membrane proteins. Further analyzing BLPs reveals that luciferases have a larger value of R (0.937) than FPs (0.635), suggesting that luciferases tend to locate near the cell membrane location rather than FPs for convenient receipt of extracellular ions. Importantly, the propensity scores of dipeptides and amino acids and the identified properties facilitate efforts to predict, characterize, and apply BLPs, including luciferases, photoproteins, and FPs. The web server is available at http://iclab.life.nctu.edu.tw/SCMBLP/index.html.

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.

Aequorin-based genetic approaches to visualize Ca2+ signaling in developing animal systems

BACKGROUND

In recent years, as our understanding of the various roles played by Ca2+ signaling in development and differentiation has expanded, the challenge of imaging Ca2+ dynamics within living cells, tissues, and whole animal systems has been extended to include specific signaling activity in organelles and non-membrane bound sub-cellular domains.

SCOPE OF REVIEW

In this review we outline how recent advances in genetics and molecular biology have contributed to improving and developing current bioluminescence-based Ca2+ imaging techniques. Reporters can now be targeted to specific cell types, or indeed organelles or domains within a particular cell.

MAJOR CONCLUSIONS

These advances have contributed to our current understanding of the specificity and heterogeneity of developmental Ca2+ signaling. The improvement in the spatial resolution that results from specifically targeting a Ca2+ reporter has helped to reveal how a ubiquitous signaling messenger like Ca2+ can regulate coincidental but different signaling events within an individual cell; a Ca2+ signaling paradox that until now has been hard to explain.

GENERAL SIGNIFICANCE

Techniques used to target specific reporters via genetic means will have applications beyond those of the Ca2+ signaling field, and these will, therefore, make a significant contribution in extending our understanding of the signaling networks that regulate animal development. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Establishment of a new cell-based assay to measure the activity of sweeteners in fluorescent food extracts

Taste receptors have been defined at the molecular level in the past decade, and cell-based assays have been developed using cultured cells heterologously expressing these receptors. The most popular approach to detecting the cellular response to a tastant is to measure changes in intracellular Ca(2+) concentration using Ca(2+)-sensitive fluorescent dyes. However, this method cannot be applied to food-derived samples that contain fluorescent substances. To establish an assay system that would be applicable to fluorescent samples, we tested the use of Ca(2+)-sensitive photoproteins, such as aequorin and mitochondrial clytin-II, as Ca(2+) indicators in a human sweet taste receptor assay. Using these systems, we successfully detected receptor activation in response to sweetener, even when fluorescent compounds coexisted. This luminescence-based assay will be a powerful tool to objectively evaluate the sweetness of food-derived samples even at an industry level.

Primary cells and stem cells in drug discovery: emerging tools for high-throughput screening

Many drug discovery screening programs employ immortalized cells, recombinantly engineered to express a defined molecular target. Several technologies are now emerging that render it feasible to employ more physiologically, and clinically relevant, cell phenotypes. Consequently, numerous approaches use primary cells, which retain many functions seen in vivo, as well as endogenously expressing the target of interest. Furthermore, stem cells, of either embryonic or adult origin, as well as those derived from differentiated cells, are now finding a place in drug discovery. Collectively, these cells are expanding the utility of authentic human cells, either as screening tools or as therapeutics, as well as providing cells derived directly from patients. Nonetheless, the growing use of phenotypically relevant cells (including primary cells or stem cells) is not without technical difficulties, particularly when their envisioned use lies in high-throughput screening (HTS) protocols. In particular, the limited availability of homogeneous primary or stem cell populations for HTS mandates that novel technologies be developed to accelerate their adoption. These technologies include detection of responses with very few cells as well as protocols to generate cell lines in abundant, homogeneous populations. In parallel, the growing use of changes in cell phenotype as the assay readout is driving greater use of high-throughput imaging techniques in screening. Taken together, the greater availability of novel primary and stem cell phenotypes as well as new detection technologies is heralding a new era of cellular screening. This convergence offers unique opportunities to identify drug candidates for disorders at which few therapeutics are presently available.

Elusive equilibrium: the challenge of interpreting receptor pharmacology using calcium assays

Calcium is a key intracellular signal that controls manifold cellular processes over a wide temporal range. The development of calcium-sensitive fluorescent dyes and proteins revolutionized our ability to visualize this important second messenger and its complex signalling characteristics. The subsequent advent of high throughput plate-based fluorescence readers has resulted in the calcium assay becoming the most widely utilized assay system for the characterization of novel receptor ligands. In this review we discuss common approaches to calcium assays, paying particular attention to the potential issues associated with interpretation of receptor pharmacology using this system. Topics covered include dye saturation and forced-coupling of receptors to the calcium pathway, but special consideration is given to the influence of non-equilibrium conditions in this rapid signalling system. Modelling the calcium transient in a kinetic mode allows the influence of ligand kinetics, receptor reserve and read time to be explored. This demonstrates that observed ligand pharmacology at very early time points can be quite different to that determined after longer incubations, even resulting in reversal of agonist potency orders that may be misinterpreted as agonist biased signalling. It also shows that estimates of antagonist affinity, whether by Schild analysis or inhibition curves, are similarly affected by hemi-equilibrium conditions. Finally we end with a discussion on practical approaches to accurately estimate the affinity of insurmountable antagonists using calcium assays.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

From c-Photina® Mouse Embryonic Stem Cells to High-Throughput Screening of Differentiated Neural Cells via an Intermediate Step Enriched in Neural Precursor Cells

The use of engineered mouse embryonic stem (mES) cells in high-throughput screening (HTS) can offer new opportunities for studying complex targets in their native environment, increasing the probability of discovering more meaningful hits. The authors have generated and developed a mouse embryonic stem cell line called c-Photina® mES stably expressing a Ca2+-activated photoprotein as a reporter gene. This reporter cell line retains the ability to differentiate into any cell lineage and can be used for miniaturized screening processes in 384-well microplates. The c-Photina® mES cell line is particularly well suited for the study of the pharmacological modulation of target genes that induce Ca2+ mobilization. The authors differentiated this mES reporter cell line into neuronal cells and screened the LOPAC1280™ library monitoring the agonistic or antagonistic activities of compounds. They also demonstrate the possibility to generate and freeze bulk preparations of cells at an intermediate stage of differentiation and enriched in neural precursor cells, which retain the ability to form fully functional neural networks once thawed. The proposed cell model is of high value for HTS purposes because it offers a more physiological environment to the targets of interest and the possibility of using frozen batches of neural precursor cells.

A photoprotein in mouse embryonic stem cells measures Ca2+ mobilization in cells and in animals

Exogenous expression of pharmacological targets in transformed cell lines has been the traditional platform for high throughput screening of small molecules. However, exogenous expression in these cells is limited by aberrant dosage, or its toxicity, the potential lack of interaction partners, and alterations to physiology due to transformation itself. Instead, primary cells or cells differentiated from precursors are more physiological, but less amenable to exogenous expression of reporter systems. To overcome this challenge, we stably expressed c-Photina, a Ca²⁺-sensitive photoprotein, driven by a ubiquitous promoter in a mouse embryonic stem (mES) cell line. The same embryonic stem cell line was also used to generate a transgenic mouse that expresses c-Photina in most tissues. We show here that these cells and mice provide an efficient source of primary cells, cells differentiated from mES cells, including cardiomyocytes, neurons, astrocytes, macrophages, endothelial cells, pancreatic islet cells, stably and robustly expressing c-Photina, and may be exploited for miniaturized high throughput screening. Moreover, we provide evidence that the transgenic mice may be suitable for ex-vivo bioimaging studies in both cells and tissues.

The use of aequorins to record and visualize Ca(2+) dynamics: from subcellular microdomains to whole organisms

In this chapter, we describe the practical aspects of measuring [Ca(2+)] transients that are generated in a particular cytoplasmic domain, or within a specific organelle or its periorganellar environment, using bioluminescent, genetically encoded and targeted Ca(2+) reporters, especially those based on apoaequorin. We also list examples of the organisms, tissues, and cells that have been transfected with apoaequorin or an apoaequorin-BRET complex, as well as of the organelles and subcellular domains that have been specifically targeted with these bioluminescent Ca(2+) reporters. In addition, we summarize the various techniques used to load the apoaequorin cofactor, coelenterazine, and its analogs into cells, tissues, and intact organisms, and we describe recent advances in the detection and imaging technologies that are currently being used to measure and visualize the luminescence generated by the aequorin-Ca(2+) reaction within these various cytoplasmic domains and subcellular compartments.

Natural products as tools for neuroscience: discovery and development of novel agents to treat drug abuse

Much of what we know about the neurosciences is the direct result of studying psychoactive natural products. Unfortunately, there are many gaps in our understanding of the basic biological processes that contribute to the etiology of many CNS disorders. The investigation of psychoactive natural products offers an excellent approach to identify novel agents to treat CNS disorders and to find new chemical tools to better elucidate their biological mechanisms. This review will detail recent progress in a program directed toward investigating psychoactive natural products with the goal of treating drug abuse by targeting kappa opioid receptors.

Photoproteins: important new tools in drug discovery

The G protein-coupled receptor (GPCR) family is a major target for drug discovery, and most, if not all, GPCRs can couple to Ca2+ signaling. Consequently, there are a number of cellbased, primary, high-throughput screening (HTS) assays used for drug discovery that assess changes in intracellular Ca2+ as a functional readout of GPCR activation. Historically, changes in intracellular Ca2+ levels have been readily detected using fluorescent dyes that emit light in proportion to changes in intracellular Ca2+ concentration. An alternative approach to indirectly measure changes in Ca2+ concentrations involves the use of recombinantly expressed biosensor photoproteins, of which aequorin is a prototypic example. These biosensors have the advantage that they provide an intense luminescent signal in response to elevations in intracellular Ca2+. This exquisite sensitivity, the high signal-to-noise ratios, and the ability to target expression to discrete subcellular sites (in order to detect Ca2+ microdomains) have made photoproteins a principal choice in a wide range of GPCR drug discovery programs. Photoproteins are also finding increasing use in detecting activation of other molecular target classes such as ligand-gated ion channels and transporters. This review focuses upon the use of calcium photoproteins principally for use in GPCR drug discovery and HTS.

Screening G protein-coupled receptors: measurement of intracellular calcium using the fluorometric imaging plate reader

G protein-coupled receptors (GPCRs) are the target of approximately 40% of all approved drugs and continue to represent a significant portion of drug discovery portfolios across the pharmaceutical industry. As a result, GPCRs are the focus of many high-throughput screening (HTS) campaigns. Historically, ligand-binding assays were used to identify compounds that targeted GPCRs. Current GPCR drug discovery efforts have moved toward the utilization of functional cell-based assays for HTS. Many of these assays monitor the accumulation of a second messenger such as cAMP or calcium in response to GPCR activation. Calcium stores are released from the endoplasmic reticulum when Galphaq-coupled GPCRs are activated. Although Galphai- and Galphas-coupled receptors do not normally result in this mobilization of intracellular calcium, they can often be engineered to do so by expressing a promiscuous or a chimeric Galphaprotein, which couples to the calcium pathway. Thus calcium mobilization is a readout that can theoretically be used to assess activation of all GPCRs. The fluorometric imaging plate reader (FLIPR) has facilitated the ability to monitor calcium mobilization in the HTS setting. This assay format allows one to monitor activation and inhibition of a GPCR in a single assay and has been one of the most heavily utilized formats for screening GPCRs.

New insights into GPCR function: implications for HTS

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

Cell-based assays in GPCR drug discovery

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