Cellular functions of cholesterol probed with optical biosensors

Label-Free Profiling of Ligands for Endogenous Gpcrs Using a Cell-Based High-Throughput Screening Technology

This article reports the use of Corning Epic system— a label-free and noninvasive optical system that is centered on resonant waveguide grating biosensors—to profile endogenous G protein-coupled receptors (GPCRs) in living cells under physiologically relevant conditions. The endogenous GPCRs examined were bradykinin B2 receptor in A431 cells and protease-activated receptor subtype 1 (PAR1) in Chinese hamster ovary (CHO) cells. The activation of either receptor led to Gq-mediated signaling in the respective cells, as confirmed by Fluo-3 assays. Stimulation of CHO cells with thrombin, a PAR1 natural agonist, resulted in an optical response relating to dynamic mass redistribution that is similar to that induced by bradykinin in A431 cells. Based on the kinetics of agonist-mediated optical signatures, two time points, one before and another 5 min after the stimulation, were chosen to develop high-throughput (HT) screening assays. Results showed that such endpoint measurements enable not only HT screening of compounds using endogenous GPCRs, but also determining the efficacies of agonists. Those results suggested that the Corning Epic label-free system is an easily scaleable biosensor, amenable as an HTS platform for GPCR drug discovery and deorphanization. (JALA 2006;11:181–7)

Recent Advances in Biosensing With Photonic Crystal Surfaces: A Review

Photonic crystal surfaces that are designed to function as wavelength-selective optical resonators have become a widely adopted platform for label-free biosensing, and for enhancement of the output of photon-emitting tags used throughout life science research and in vitro diagnostics. While some applications, such as analysis of drug-protein interactions, require extremely high resolution and the ability to accurately correct for measurement artifacts, others require sensitivity that is high enough for detection of disease biomarkers in serum with concentrations less than 1 pg/ml. As the analysis of cells becomes increasingly important for studying the behavior of stem cells, cancer cells, and biofilms under a variety of conditions, approaches that enable high resolution imaging of live cells without cytotoxic stains or photobleachable fluorescent dyes are providing new tools to biologists who seek to observe individual cells over extended time periods. This paper will review several recent advances in photonic crystal biosensor detection instrumentation and device structures that are being applied towards direct detection of small molecules in the context of high throughput drug screening, photonic crystal fluorescence enhancement as utilized for high sensitivity multiplexed cancer biomarker detection, and label-free high resolution imaging of cells and individual nanoparticles as a new tool for life science research and single-molecule diagnostics.

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.

Evaluation of dynamic mass redistribution technology for pharmacological studies of recombinant and endogenously expressed g protein-coupled receptors

The Epic cell assay technology (Corning Inc., Corning, NY) uses a resonant waveguide grating optical biosensor to measure cellular response to ligands manifested through dynamic mass redistribution (DMR) of cellular contents. The DMR measurement is a noninvasive, label-free assay that can be used to assess the pharmacological properties of compounds. In this study, a panel of 12 compounds was evaluated against two G protein-coupled receptor (GPCR) targets in recombinant expressed cell lines using the Corning Epic system in 384-well microplates. The evaluation was performed in a double-blinded fashion such that the identity and properties of both the GPCR targets and compounds were unknown to the researchers at the time of the study. Analysis of the DMR response from cell stimulation was used to identify compounds that functioned as agonists or antagonists and to evaluate the associated efficacy and potency. DMR results were shown to have good agreement with data obtained from cyclic AMP and calcium flux assays for compounds evaluated. A further analysis was performed and successfully identified the signaling pathways that the two GPCRs activated. In addition, the DMR measurement was able to detect responses from an endogenous receptor in these cells. The Epic DMR technology provides a generic platform amenable to pharmacological evaluation of cellular responses to GPCR activation in a label-free live cell assay format.

Non-invasive Optical Biosensor for Probing Cell Signaling

Cell signaling mediated through a cellular target is encoded by spatial andtemporal dynamics of downstream signaling networks. The coupling of temporal dynamicswith spatial gradients of signaling activities guides cellular responses upon stimulation.Monitoring the integration of cell signaling in real time, if realized, would provide a newdimension for understanding cell biology and physiology. Optical biosensors includingresonant waveguide grating (RWG) biosensor manifest a physiologically relevant andintegrated cellular response related to dynamic redistribution of cellular matters, thusproviding a non-invasive means for cell signaling study. This paper reviews recentprogresses in biosensor instrumentation, and theoretical considerations and potentialapplications of optical biosensors for whole cell sensing.

Optical biosensor differentiates signaling of endogenous PAR1 and PAR2 in A431 cells

Background

Protease activated receptors (PARs) consist of a family of four G protein-coupled receptors. Many types of cells express several PARs, whose physiological significance is mostly unknown.

Results

Here, we show that non-invasive resonant waveguide grating (RWG) biosensor differentiates signaling of endogenous protease activated receptor subtype 1 (PAR₁) and 2 (PAR₂) in human epidermoid carcinoma A431 cells. The biosensor directly measures dynamic mass redistribution (DMR) resulted from ligand-induced receptor activation in adherent cells. In A431, both PAR₁ and PAR₂ agonists, but neither PAR₃ nor PAR₄ agonists, trigger dose-dependent Ca²⁺ mobilization as well as G_q-type DMR signals. Both Ca²⁺ flux and DMR signals display comparable desensitization patterns upon repeated stimulation with different combinations of agonists. However, PAR₁ and PAR₂ exhibit distinct kinetics of receptor re-sensitization. Furthermore, both trypsin- and thrombin-induced Ca²⁺ flux signals show almost identical dependence on cell surface cholesterol level, but their corresponding DMR signals present different sensitivities.

Conclusion

Optical biosensor provides an alternative readout for examining receptor activation under physiologically relevant conditions, and differentiates the signaling of endogenous PAR₁ and PAR₂ in A431.

Label-free cell-based assays with optical biosensors in drug discovery

Once viewed solely as a tool for low throughput and kinetic analysis of biomolecular interactions, optical biosensors are gaining widespread uses in drug discovery because of recent advances in instrumentation and experimental design. These advances have expanded the capabilities of optical biosensors to meet the needs at many points in the drug discovery process. Concurrent shifts in drug discovery paradigms have seen the growing use of whole cell systems for drug screens, thus creating both a need in drug discovery and a solution in optical biosensors. This article reviews important advances in optical biosensor instrumentation, and highlights the potential of optical biosensors for drug discovery with an emphasis on whole cell sensing in both high throughput and high content fashions.

Resonant waveguide grating biosensor for living cell sensing

This article presents theoretical analysis and experimental data for the use of resonant waveguide grating (RWG) biosensors to characterize stimulation-mediated cell responses including signaling. The biosensor is capable of detecting redistribution of cellular contents in both directions that are perpendicular and parallel to the sensor surface. This capability relies on online monitoring cell responses with multiple optical output parameters, including the changes in incident angle and the shape of the resonant peaks. Although the changes in peak shape are mainly contributed to stimulation-modulated inhomogeneous redistribution of cellular contents parallel to the sensor surface, the shift in incident angle primarily reflects the stimulation-triggered dynamic mass redistribution (DMR) perpendicular to the sensor surface. The optical signatures are obtained and used to characterize several cellular processes including cell adhesion and spreading, detachment and signaling by trypsinization, and signaling through either epidermal growth factor receptor or bradykinin B2 receptor. A mathematical model is developed to link the bradykinin-mediated DMR signals to the dynamic relocation of intracellular proteins and the receptor internalization during B2 receptor signaling cycle. This model takes the form of a set of nonlinear, ordinary differential equations that describe the changes in four different states of B2 receptors, diffusion of proteins and receptor-protein complexes, and the DMR responses. Classical analysis shows that the system converges to a unique optical signature, whose dynamics (amplitudes, transition time, and kinetics) is dependent on the bradykinin signal input, and consistent with those observed using the RWG biosensors. This study provides fundamentals for probing living cells with the RWG biosensors, in general, optical biosensors.