Ligand-Receptor Interactions in the Membrane of Cultured Cells Monitored by Fluorescence Correlation Spectroscopy

Optimization of Peptide Linker-Based Fluorescent Ligands for the Histamine H1 Receptor

The histamine H₁ receptor (H₁R) has recently been implicated in mediating cell proliferation and cancer progression; therefore, high-affinity H₁R-selective fluorescent ligands are desirable tools for further investigation of this behavior in vitro and in vivo. We previously reported a H₁R fluorescent ligand, bearing a peptide-linker, based on antagonist VUF13816 and sought to further explore structure-activity relationships (SARs) around the linker, orthostere, and fluorescent moieties. Here, we report a series of high-affinity H₁R fluorescent ligands varying in peptide linker composition, orthosteric targeting moiety, and fluorophore. Incorporation of a boron-dipyrromethene (BODIPY) 630/650-based fluorophore conferred high binding affinity to our H₁R fluorescent ligands, remarkably overriding the linker SAR observed in corresponding unlabeled congeners. Compound 31a, both potent and subtype-selective, enabled H₁R visualization using confocal microscopy at a concentration of 10 nM. Molecular docking of 31a with the human H₁R predicts that the optimized peptide linker makes interactions with key residues in the receptor.

Fluorescence Correlation and Cross-Correlation Spectroscopy in Zebrafish

There has been increasing interest in biophysical studies on live organisms to gain better insights into physiologically relevant biological events at the molecular level. Zebrafish (Danio rerio) is a viable vertebrate model to study such events due to its genetic and evolutionary similarities to humans, amenability to less invasive fluorescence techniques owing to its transparency and well-characterized genetic manipulation techniques. Fluorescence techniques used to probe biomolecular dynamics and interactions of molecules in live zebrafish embryos are therefore highly sought-after to bridge molecular and developmental events. Fluorescence correlation and cross-correlation spectroscopy (FCS and FCCS) are two robust techniques that provide molecular level information on dynamics and interactions respectively. Here, we detail the steps for applying confocal FCS and FCCS, in particular single-wavelength FCCS (SW-FCCS), in live zebrafish embryos, beginning with sample preparation, instrumentation, calibration, and measurements on the FCS/FCCS instrument and ending with data analysis.

Studying GPCR Pharmacology in Membrane Microdomains: Fluorescence Correlation Spectroscopy Comes of Age

G protein-coupled receptors (GPCRs) are organised within the cell membrane into highly ordered macromolecular complexes along with other receptors and signalling proteins. Understanding how heterogeneity in these complexes affects the pharmacology and functional response of these receptors is crucial for developing new and more selective ligands. Fluorescence correlation spectroscopy (FCS) and related techniques such as photon counting histogram (PCH) analysis and image-based FCS can be used to interrogate the properties of GPCRs in these membrane microdomains, as well as their interaction with fluorescent ligands. FCS analyses fluorescence fluctuations within a small-defined excitation volume to yield information about their movement, concentration and molecular brightness (aggregation). These techniques can be used on live cells with single-molecule sensitivity and high spatial resolution. Once the preserve of specialist equipment, FCS techniques can now be applied using standard confocal microscopes. This review describes how FCS and related techniques have revealed novel insights into GPCR biology.

Fluorescence correlation spectroscopy: molecular complexing in solution and in living cells

This chapter describes how the microscope can be used to measure a fluorescence signal from a small, confined volume of the sample-the confocal volume-and how these measurements are used to quantitate the dynamics and complexing of molecules, the technique of fluorescence correlation spectroscopy (FCS). FCS represents a significant example of how the microscope can be used to extract information beyond the resolution limit of classical optics. FCS enables studying events at the level of single molecules. With FCS, one can measure the diffusion times and the interaction of macromolecules, the absolute concentration of fluorescently labeled particles, and the kinetics of chemical reactions. Practical applications of FCS include studies on ligand-receptor binding, protein-protein and protein-DNA interactions, and the aggregation of fluorescently labeled particles. The chapter focuses on the principles of FCS, demonstrates how FCS is used to study macromolecular interactions in solution and in living cells, and examines critical experimental parameters that must be considered. The chapter also discusses the minimum requirements for building a microscope-based FCS instrument and illustrates the key criteria for both instrument sensitivity and analysis of FCS data. It can be used to study single molecules both in solution and in living cells and can be used to monitor a variety of macromolecular interactions. When used as an in vitro technique, FCS measurements are easy to conduct and can be made on simplified instrumentation. When used in vivo on living cells, many additional factors must be considered when evaluating experimental data. Despite these concerns, FCS represents a new approach that has broad applicability for the determination of molecular stoichiometry both in vivo and in vitro for a variety of membrane and soluble receptor systems.

Physical-chemical principles underlying RTK activation, and their implications for human disease

RTKs, the second largest family of membrane receptors, exert control over cell proliferation, differentiation and migration. In recent years, our understanding of RTK structure and activation in health and disease has skyrocketed. Here we describe experimental approaches used to interrogate RTKs, and we review the quantitative biophysical frameworks and structural considerations that shape our understanding of RTK function. We discuss current knowledge about RTK interactions, focusing on the role of different domains in RTK homodimerization, and on the importance and challenges in RTK heterodimerization studies. We also review our understanding of pathogenic RTK mutations, and the underlying physical-chemical causes for the pathologies. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Quantification of receptor targeting aptamer binding characteristics using single-molecule spectroscopy

This experimental design presents a single molecule approach based on fluorescence correlation spectroscopy (FCS) for the quantification of outer membrane proteins which are receptors to an aptamer specifically designed to target the surface receptors of live Salmonella typhimurium. By using correlation analysis, we also show that it is possible to determine the associated binding kinetics of these aptamers on live single cells. Aptamers are specific oligonucleotides designed to recognize conserved sequences that bind to receptors with high affinity, and therefore can be integrated into selective biosensor platforms. In our experiments, aptamers were constructed to bind to outer membrane proteins of S. typhimurium and were assessed for specificity against Escherichia coli. By fluorescently labeling aptamer probes and applying FCS, we were able to study the diffusion dynamics of bound and unbound aptamers and compare them to determine the dissociation constants and receptor densities of the bacteria for each aptamer at single molecule sensitivity. The dissociation constants for these aptamer probes calculated from autocorrelation data were 0.1285 and 0.3772 nM and the respective receptor densities were 42.27 receptors per µm(2) and 49.82 receptors per µm(2). This study provides ample evidence that the number of surface receptors is sufficient for binding and that both aptamers have a high-binding affinity and can therefore be used in detection processes. The methods developed here are unique and can be generalized to examine surface binding kinetics and receptor quantification in live bacteria at single molecule sensitivity levels. The impact of this study is broad because our approach can provide a methodology for biosensor construction and calculation of live single cell receptor-ligand kinetics in a variety of environmental and biological applications.

Ligand-macromolecule interactions in live cells by fluorescence correlation spectroscopy

The receptor concept is the primary theoretical basis for modern pharmacology. Drugs, hormones, neurotransmitters, toxin, and other biologically active substances are referred to as ligands. Ligands exert their actions by way of interaction with receptors/macromolecules. The resulting receptor/macromolecule-ligand complexes produce alterations in physiological processes. Receptor/macromolecule-binding studies most often require the use of radioactively labeled ligands. When the numbers of receptors/macromolecules are few per cell, it is impossible to detect the specific binding because of a high background. Specific interactions between certain ligands and their receptors/macromolecules are, therefore, often overlooked by the conventional binding technique. Fluorescence correlation spectroscopy (FCS) allows detection a ligand-macromolecule interaction in live cells in a tiny confocal volume element (0.2 femtoliter (fL)) at single-molecule detection sensitivity. FCS permits the identification of macromolecules that were not possible to detect before by isotope labeling. The beauty of the FCS technique is that there is no need for separating an unbound ligand from a bound one to calculate the macromolecule bound and free ligand fractions. This study will demonstrate FCS as a sensitive and a rapid technique to study ligand-macromolecule interaction in live cells using fluorescently labeled ligands (Fl-L). This study is of pharmaceutical significance since FCS assay of ligand-macromolecule interactions in live cells is one step forward toward a high throughput drug screening in cell cultures.

Measurement of molecular mobility with fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a fluctuation method established three decades ago, whose application to cellular systems became popular in the last decade. Fluctuations of fluorescence emission are observed from a small, femtoliter to sub-femtoliter, usually confocal volume at high time resolution. A time-dependent autocorrelation function is generated and evaluated to obtain time constants of photophysical and photochemical reactions, as well as of molecular diffusion and in the observation volume. Molecules in various subcellular compartments-including the nucleus, the cytoplasm, and the membrane-can be observed after labeling them with antibodies, ligands, or fluorescent proteins. The anomaly of diffusion, the local concentration, and the average fluorescence per diffusing particle can also be determined, all of which can be characteristic of molecular interactions. A two-color version of FCS, fluorescence cross-correlation spectroscopy, can also be applied to observe co-diffusion, i.e., stable association of two distinct molecular species in their cellular environment.

Mapping receptor density on live cells by using fluorescence correlation spectroscopy

Study of the density, spatial distribution, and molecular interactions of receptors on the cell membrane provides the knowledge required to understand cellular behavior and biological functions, as well as to discover, design, and screen novel therapeutic agents. However, the mapping of receptor distribution and the monitoring of ligand-receptor interactions on live cells in a spatially and temporally ordered manner are challenging tasks. In this paper, we apply fluorescence correlation spectroscopy (FCS) to map receptor densities on live cell membranes by introducing fluorescently marked aptamer molecules, which specifically bind to certain cell-surface receptors. The femtoliter-sized (0.4 fL) observation volume created by FCS allows fluorescent-aptamer detection down to 2 molecules and appears to be an ideal and highly sensitive biophysical tool for studying molecular interactions on live cells. Fluorophore-labeled aptamers were chosen for receptor recognition because of their high binding affinity and specificity. Aptamer sgc8, generated for specific cell recognition by a process called cell systematic evolution of ligands by exponential enrichment, was determined by FCS to have a binding affinity in the picomolar range (dissociation constant K(d)=790+/-150 pM) with its target membrane receptor, human protein tyrosine kinase-7 (PTK7), a potential cancer biomarker. We then constructed a cellular model and applied this aptamer-receptor interaction to estimate receptor densities and distributions on the cell surface. Specifically, different expression levels of PTK7 were studied by using human leukemia CCRF-CEM cells (1300+/-190 receptors microm(-2)) and HeLa cervical cancer cells (550+/-90 receptors microm(-2)). Competition studies with excess nonlabeled aptamers and proteinase treatment studies proved the validity of the density-estimation approach. With its intrinsic advantages of direct measurement, high sensitivity, fast analysis, and single-cell measurement, this FCS density-estimation approach holds potential for future applications in molecular-interaction studies and density estimations for subcellular structures and membrane receptors.

Accurate determination of membrane dynamics with line-scan FCS

Here we present an efficient implementation of line-scan fluorescence correlation spectroscopy (i.e., one-dimensional spatio-temporal image correlation spectroscopy) using a commercial laser scanning microscope, which allows the accurate measurement of diffusion coefficients and concentrations in biological lipid membranes within seconds. Line-scan fluorescence correlation spectroscopy is a calibration-free technique. Therefore, it is insensitive to optical artifacts, saturation, or incorrect positioning of the laser focus. In addition, it is virtually unaffected by photobleaching. Correction schemes for residual inhomogeneities and depletion of fluorophores due to photobleaching extend the applicability of line-scan fluorescence correlation spectroscopy to more demanding systems. This technique enabled us to measure accurate diffusion coefficients and partition coefficients of fluorescent lipids in phase-separating supported bilayers of three commonly used raft-mimicking compositions. Furthermore, we probed the temperature dependence of the diffusion coefficient in several model membranes, and in human embryonic kidney cell membranes not affected by temperature-induced optical aberrations.

New concepts for fluorescence correlation spectroscopy on membranes

Fluorescence correlation spectroscopy (FCS) is a powerful tool to measure useful physical quantities such as concentrations, diffusion coefficients, diffusion modes or binding parameters, both in model and cell membranes. However, it can suffer from severe artifacts, especially in non-ideal systems. Here we assess the potential and limitations of standard confocal FCS on lipid membranes and present recent developments which facilitate accurate and quantitative measurements on such systems. In particular, we discuss calibration-free diffusion and concentration measurements using z-scan FCS and two focus FCS and present several approaches using scanning FCS to accurately measure slow dynamics. We also show how surface confined FCS enables the study of membrane dynamics even in presence of a strong cytosolic background and how FCS with a variable detection area can reveal submicroscopic heterogeneities in cell membranes.

Fluorescence correlation spectroscopy and its application to the characterization of molecular properties and interactions

Fluorescence correlation spectroscopy (FCS) utilizes temporal fluctuations in fluorescence emission to extract quantitative measures of inter- or intramolecular dynamics or molecular motions of probe molecules, which occur on submicrosecond to second timescales. In typical experiments, one can readily obtain the probe's diffusion coefficient and concentration from small volumes of sample. Recent FCS applications have yielded information on interactions of the probe with changing or structured solvent, binding with other molecules, photophysical or conformational changes in the probe, polymerization, and other changes in the dynamics of the probe. In cross-correlation mode FCS promises to attract more applications as the technique can monitor interactions in a system with two or more probes with different fluorophores.

Pharmacology under the microscope: the use of fluorescence correlation spectroscopy to determine the properties of ligand-receptor complexes

Recent years have revealed a high degree of structural organisation in the way in which cell-surface receptors and their associated signalling complexes interact at a molecular level. Fluorescence-based techniques have been at the forefront of methodologies used to investigate this organisation and dissect the pharmacology of drug–receptor interactions at the single-cell level. One such technique, fluorescence correlation spectroscopy (FCS), in conjunction with a fluorescent ligand or receptor, is capable of providing quantitative information about the number of receptors and their mobilities within small areas of the cell membrane that approach the size of some signalling domains. This article describes the use of FCS to perform subcellular quantitative pharmacology, with particular reference to G-protein-coupled receptors (GPCRs). In conjunction with other forms of fluctuation analysis, such as two-colour cross-correlation FCS and molecular brightness analysis, FCS provides the first opportunity to investigate the domain-specific nature of GPCR pharmacology.

Investigation of the dimerization of proteins from the epidermal growth factor receptor family by single wavelength fluorescence cross-correlation spectroscopy

Single wavelength fluorescence cross-correlation spectroscopy (SW-FCCS), introduced to study biomolecular interactions, has recently been reported to monitor enzyme activity by using a newly developed fluorescent protein variant together with cyan fluorescent protein. Here, for the first time to our knowledge, SW-FCCS is applied to detect interactions between membrane receptors in vivo by using the widely used enhanced green fluorescent protein and monomeric red fluorescent protein. The biological system studied here is the epidermal growth factor/ErbB receptor family, which plays pivotal roles in the development of organisms ranging from worms to humans. It is widely thought that a ligand binds to the monomeric form of the receptor and induces its dimeric form for activation. By using SW-FCCS and Förster resonance energy transfer, we show that the epidermal growth factor receptor and ErbB2 have preformed homo- and heterodimeric structures on the cell surface and quantitation of dimer fractions is performed by SW-FCCS. These receptors are major targets of anti-cancer drug development, and the receptors' homo- and heterodimeric structures are relevant for such developments.

Binding of the Alzheimer amyloid β-peptide to neuronal cell membranes by fluorescence correlation spectroscopy

The deposition of the Alzheimer amyloid beta-peptide (Abeta) fibrils in brain is a key step in Alzheimer's disease. The aggregated Abeta is found to be toxic to neurons since cells die when the aggregated Abeta is added to the cell culture medium. However, target of action of Abeta to cells is unknown. We have applied the fluorescence correlation spectroscopy (FCS) technique to study the existence of a receptor or target molecule for the Alzheimer amyloid beta-peptide (Abeta) in cultured human cerebral cortical neurons. FCS measurement of the fluorophore rhodamine-labeled Abeta (Rh-Abeta) shows diffusion times: 0.1 ms, 1.1 ms and 5.9 ms. Thus, 0.1 ms corresponds to the unbound Rh-Abeta, and 1.1 ms and 5.9 ms correspond to slowly diffusing complexes of Rh-Abeta bound to a kind of receptor or target molecule for Abeta. Addition of excess non-labeled Abeta is accompanied by a competitive displacement, showing that the Abeta binding is specific. Full saturation of the Abeta binding is obtained at nanomolar concentrations, indicating that the Abeta binding is of high affinity. The notion that using FCS we have found a kind of receptor or target molecule for Abeta makes an important point that Abeta kills cells possibly by affecting cell membranes via a receptor or target molecule. This study is of highly significance since it suggests that Abeta possibly affects neuronal cell membranes of Alzheimer patients via a receptor or target molecule.

Imaging single events at the cell membrane

The ability to sense and respond to the environment is a hallmark of living systems. These processes occur at the levels of the organism, cells and individual molecules. Sensing of extracellular changes could result in a structural or chemical alteration in a molecule, which could in turn trigger a cascade of intracellular signals or regulated trafficking of molecules at the cell surface. These and other such processes allow cells to sense and respond to environmental changes. Often, these changes and the responses to them are spatially and/or temporally localized, and visualization of such events necessitates the use of high-resolution imaging approaches. Here we discuss optical imaging approaches and tools for imaging individual events at the cell surface with improved speed and resolution.

New fluorescent adenosine A1-receptor agonists that allow quantification of ligand-receptor interactions in microdomains of single living cells

Fluorescence spectroscopy is becoming a valuable addition to the array of techniques available for scrutinizing ligand-receptor interactions in biological systems. In particular, scanning confocal microscopy and fluorescence correlation spectroscopy (FCS) allow the noninvasive imaging and quantification of these interactions in single living cells. To address the emerging need for fluorescently labeled ligands to support these technologies, we have developed a series of red-emitting agonists for the human adenosine A1-receptor that, collectively, are N6-aminoalkyl derivatives of adenosine or adenosine 5'-N-ethyl carboxamide. The agonists, which incorporate the commercially available fluorophore BODIPY [630/650], retain potent and efficacious agonist activity, as demonstrated by their ability to inhibit cAMP accumulation in chinese hamster ovary cells expressing the human adenosine A1-receptor. Visualization and confirmation of ligand-receptor interactions at the cell membrane were accomplished using confocal microscopy, and their suitability for use in FCS was demonstrated by quantification of agonist binding in small areas of cell membrane.

Integrated multimodal microscopy, time-resolved fluorescence, and optical-trap rheometry: toward single molecule mechanobiology

Cells respond to forces through coordinated biochemical signaling cascades that originate from changes in single-molecule structure and dynamics and proceed to large-scale changes in cellular morphology and protein expression. To enable experiments that determine the molecular basis of mechanotransduction over these large time and length scales, we construct a confocal molecular dynamics microscope (CMDM). This system integrates total-internal-reflection fluorescence (TIRF), epifluorescence, differential interference contrast (DIC), and 3-D deconvolution imaging modalities with time-correlated single-photon counting (TCSPC) instrumentation and an optical trap. Some of the structures hypothesized to be involved in mechanotransduction are the glycocalyx, plasma membrane, actin cytoskeleton, focal adhesions, and cell-cell junctions. Through analysis of fluorescence fluctuations, single-molecule spectroscopic measurements [e.g., fluorescence correlation spectroscopy (FCS) and time-resolved fluorescence] can be correlated with these subcellular structures in adherent endothelial cells subjected to well-defined forces. We describe the construction of our multimodal microscope in detail and the calibrations necessary to define molecular dynamics in cell and model membranes. Finally, we discuss the potential applications of the system and its implications for the field of mechanotransduction.

A Simple and Powerful Flow Cytometric Method for the Simultaneous Determination of Multiple Parameters at G Protein-Coupled Receptor Subtypes

The quantification of pharmacological parameters at G protein-coupled receptors (GPCRs) is indispensable in drug research but costly and time-consuming when conventional methods are sequentially applied. With neuropeptide Y (NPY) Y(1), Y(2), and Y(5) receptors as model systems, a homogenous flow cytometric method for the simultaneous determination of the affinity, selectivity, and activity of GPCR ligands was developed. Mixtures of cells expressing the receptors of interest and cyanine-labeled NPY as a universal fluorescent Y(1), Y(2), and Y(5) receptor agonist were used. Calcium mobilization was measured in different channels with the aid of fluo-4 and fura red. A combination of dye-loaded HEL-Y(1) and CHO-Y(2)-Galpha(qi5) cells with unloaded HEC-1B-Y(5) cells allowed the simultaneous determination of Y(1), Y(2), and Y(5) receptor selectivity preceded by the Y(1) and Y(2) receptor-mediated response with one and the same sample. The data are in good agreement with those determined by radioligand binding and spectrofluorimetry. The convenient, robust, and inexpensive multiparametric procedure offers a broad range of applications in the pharmacological characterization of GPCR ligands.

Electron Multiplying Charge-Coupled Device Camera Based Fluorescence Correlation Spectroscopy

A fluorescence correlation spectroscopy (FCS) setup is built with an electron multiplying charge-coupled device camera. Although the instrument has a limited time resolution of 4 ms, compared to 0.1-0.2 mus for common instruments using avalanche photodiodes, it allows multiplexing of FCS measurements, has a software-adjustable pinhole after data collection, performs flow speed as well as flow direction measurements in microchannels and could be used to do spectral FCS. Measurements are performed on fluorescent dyes and polystyrene beads in high-viscosity media and on epidermal growth factor receptors in Chinese hamster ovary cells. Using real measurements on single spots, multiplexing of focal spots and detection elements are simulated and the results are discussed.

FCS cell surface measurements—Photophysical limitations and consequences on molecular ensembles with heterogenic mobilities

BACKGROUND

Fluorescence Correlation Spectroscopy is a powerful method to analyze densities and diffusive behavior of molecules in membranes, but effects of photodegradation can easily be overlooked.

METHOD

Based on experimental photophysical parameters, calculations were performed to analyze the consequences of photobleaching in fluorescence correlation spectroscopy (FCS) cell surface experiments, covering a range of standard measurement conditions.

RESULTS

Cumulative effects of photobleaching can be prominent, although an absolute majority of the fluorescent molecules would pass the laser excitation beam without being photo-bleached. Given a distribution of molecules on a cell surface with different diffusive properties, the fraction of molecules that is actually analyzed depends strongly on the excitation intensities and measurement times, as well as on the size of the reservoir of freely diffusing molecules. Both the slower and the faster diffusing molecules can be disfavored.

CONCLUSIONS

Apart from quantifying photobleaching effects, the calculations suggest that the effects can be used to extract additional information, for instance about the size of the reservoirs of free diffusion. By certain choices of measurement conditions, it may be possible to more specifically analyze certain species within a population, based on their different diffusive properties, different areas of free diffusion, or different kinetics of possible transient binding.

Preparation and characterization of Alexa Fluor 594-labeled epidermal growth factor for fluorescence resonance energy transfer studies: application to the epidermal growth factor receptor

We have prepared and characterized a new fluorescent derivative of murine epidermal growth factor (EGF), Alexa Fluor 594-labeled EGF (A-EGF), for fluorescence studies of EGF-EGF receptor interactions. We describe the synthesis of this derivative and its physical and biological characterization. The significant overlap between the excitation and the emission spectra of A-EGF makes this probe well suited to fluorescence resonance energy homo-transfer. Using time-resolved fluorescence to examine the oligomeric state of the EGF receptor, we have observed resonance energy homo-transfer of A-EGF bound to EGF receptors in cells, but not of A-EGF bound to EGF receptors in membrane vesicles. Our results, interpreted in the context of recent crystallographic studies of the ligand-binding domains of EGF receptors, suggest that observed fluorescence resonance energy transfer does not result from transfer within receptor dimers, but rather results from transfer within higher-order oligomers. Furthermore, our results support a structural model for oligomerization of EGF receptors in which dimers are positioned head-to-head with respect to the ligand-binding site, consistent with the head-to-head interactions observed between adjacent receptor dimers by X-ray crystallography.

Dye-Labeled Benzodiazepines: Development of Small Ligands for Receptor Binding Studies Using Fluorescence Correlation Spectroscopy

To investigate benzodiazepine receptor binding studies by fluorescence correlation spectroscopy (FCS), the four fluorophores fluorescein, tetramethylrhodamine, Oregon Green 488, and Alexa 532 were coupled to the benzodiazepine Ro 07-1986/602 (Ro). Binding assays to polyclonal antibodies to benzodiazepines and at the native benzodiazepine receptor on the membrane of rat hippocampal neurons were established to examine the dye-labeled ligands for their benzodiazepine character and their binding behavior. Both the fluorescein and the Oregon Green488 moiety led to a loss of the benzodiazepine receptor binding of the corresponding Ro derivatives. Antibody recognition and interactions to the receptor were observed for the tetramethylrhodamine derivative (K(D) = 96.0 +/- 9.5 nM) but with a high amount of nonspecific binding at the cell membrane of about 50%. In saturation experiments a K(D) value of 97.2 +/- 8.5 nM was found for the Alexa Fluor 532 derivative-antibody interaction. Investigation of the binding of this ligand to the benzodiazepine receptor in FCS cell measurements led to confirmation of high specific binding behavior with a K(D) value of 9.9 +/- 1.9 nM. A nonspecific binding of <10% was observed after coincubation with 1 microM of midazolam. The different properties of the labeled benzodiazepine derivatives and the requirements of the fluorophore in small dye-labeled ligands in FCS binding studies, at the membrane of living cells, are discussed.

Quantitative analysis of the formation and diffusion of A1-adenosine receptor-antagonist complexes in single living cells

The A1-adenosine receptor (A1-AR) is a G protein-coupled receptor that mediates many of the physiological effects of adenosine in the brain, heart, kidney, and adipocytes. Currently, ligand interactions with the A1-AR can be quantified on large cell populations only by using radioligand binding. To increase the resolution of these measurements, we have designed and characterized a previously undescribed fluorescent antagonist for the A1-AR, XAC-BY630, based on xanthine amine congener (XAC). This compound has been used to quantify ligand-receptor binding at a single cell level using fluorescence correlation spectroscopy (FCS). XAC-BY630 was a competitive antagonist of A1-AR-mediated inhibition of cAMP accumulation [log10 of the affinity constant (pKb) = 6.7)] and stimulation of inositol phosphate accumulation (pKb = 6.5). Specific binding of XAC-BY630 to cell surface A1-AR could also be visualized in living Chinese hamster ovary (CHO)-A1 cells by using confocal microscopy. FCS analysis of XAC-BY630 binding to the membrane of CHO-A1 cells revealed three components with diffusion times (tauD) of 62 micros (tauD1, free ligand), 17 ms (tauD2, A1-AR-ligand), and 320 ms (tauD3). Confirmation that tauD2 resulted from diffusion of ligand-receptor complexes came from the similar diffusion time observed for the fluorescent A1-AR-Topaz fusion protein (15 ms). Quantification of tauD2 showed that the number of receptor-ligand complexes increased with increasing free ligand concentration and was decreased by the selective A1-AR antagonist, 8-cyclopentyl-1,3-dipropylxanthine. The combination of FCS with XAC-BY630 will be a powerful tool for the characterization of ligand-A1-AR interactions in single living cells in health and disease.

Fluorescence correlation spectroscopy: molecular complexing in solution and in living cells

FCS is an important technique for biophysicists, biochemists, and cell biologists. FCS represents an example of how one can make use of the microscope and electronics to extract information beyond the resolution limit of classical optics. It can be used to study single-molecules both in solution and in living cells and can be used to monitor a wide variety of macromolecular interactions. When used as an in vitro technique, FCS measurements are easy to conduct and can be made on simplified instrumentation. When used in vivo on living cells, many additional factors must be considered when evaluating experimental data. Despite these concerns, FCS represents a new approach that has broad applicability for the determination of molecular stoichiometry both in vivo and in vitro for a variety of membrane and soluble receptor systems.

Ultrasensitive investigations of biological systems by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) extracts information about molecular dynamics from the tiny fluctuations that can be observed in the emission of small ensembles of fluorescent molecules in thermodynamic equilibrium. Employing a confocal setup in conjunction with highly dilute samples, the average number of fluorescent particles simultaneously within the measurement volume (approximately 1 fl) is minimized. Among the multitude of chemical and physical parameters accessible by FCS are local concentrations, mobility coefficients, rate constants for association and dissociation processes, and even enzyme kinetics. As any reaction causing an alteration of the primary measurement parameters such as fluorescence brightness or mobility can be monitored, the application of this noninvasive method to unravel processes in living cells is straightforward. Due to the high spatial resolution of less than 0.5 microm, selective measurements in cellular compartments, e.g., to probe receptor-ligand interactions on cell membranes, are feasible. Moreover, the observation of local molecular dynamics provides access to environmental parameters such as local oxygen concentrations, pH, or viscosity. Thus, this versatile technique is of particular attractiveness for researchers striving for quantitative assessment of interactions and dynamics of small molecular quantities in biologically relevant systems.

Measuring diffusion in cell membranes by fluorescence correlation spectroscopy

Fluorescence Correlation Spectroscopy (FCS) can measure diffusion on the cell surface with unparalleled sensitivity. In appropriate situations, this can be the most sensitive and accurate method for measuring receptor interaction and oligomerization. Here we attempt to describe FCS in sufficient detail so that the reader is able to judge when there is a compelling reason to choose this technique, understand the basic theory behind it, construct a FCS spectrometer in the laboratory, and analyze the data to obtain a meaningful estimate of the physical parameters.