Plasmon-waveguide resonance studies of ligand binding to the human beta 2-adrenergic receptor

Predictors of appropriate therapies delivered by the implantable cardioverter-defibrillator in patients with coronary artery disease during long-term period

This prospective study aimed to investigate the ability of cardiac autonomic nervous system (CANS) activity assessment to predict appropriate implantable cardioverter-defibrillator (ICD) therapy in patients with coronary artery disease (CAD) during long-term follow-up period. We enrolled patients with CAD and ICD implantation indications that included both secondary and primary prevention of sudden cardiac death. Before ICD implantation CANS was assessed by using heart rate variability (HRV), myocardium scintigraphy with 123I-meta-iodobenzylguanidine (123I-MIBG) and erythrocyte membranes β-adrenoreactivity (EMA). The study's primary endpoint was the documentation of appropriate ICD therapy. Of 45 (100.0%) patients, 15 (33.3%) had appropriate ICD therapy during 36 months follow-up period. Patients with appropriate ICD therapy were likely to have a higher summed 123I-MIBG score delayed (p < 0.001) and lower 123I-MIBG washout rate (p = 0.008) indicators. These parameters were independently associated with endpoint in univariable and multivariable logistic regression. We created a logistic equation and calculated a cut-off value. The resulting ROC curve revealed a discriminative ability with AUC of 0.933 (95% confidence interval 0.817-0.986; sensitivity 100.00%; specificity 93.33%). Combined CANS activity assessment is useful in prediction of appropriate ICD therapy in patients with CAD during long-term follow-up period after device implantation.

Plasmon Waveguide Resonance: Principles, Applications and Historical Perspectives on Instrument Development

Plasmon waveguide resonance (PWR) is a variant of surface plasmon resonance (SPR) that was invented about two decades ago at the University of Arizona. In addition to the characterization of the kinetics and affinity of molecular interactions, PWR possesses several advantages relative to SPR, namely, the ability to monitor both mass and structural changes. PWR allows anisotropy information to be obtained and is ideal for the investigation of molecular interactions occurring in anisotropic-oriented thin films. In this review, we will revisit main PWR applications, aiming at characterizing molecular interactions occurring (1) at lipid membranes deposited in the sensor and (2) in chemically modified sensors. Among the most widely used applications is the investigation of G-protein coupled receptor (GPCR) ligand activation and the study of the lipid environment’s impact on this process. Pioneering PWR studies on GPCRs were carried out thanks to the strong and effective collaboration between two laboratories in the University of Arizona leaded by Dr. Gordon Tollin and Dr. Victor J. Hruby. This review provides an overview of the main applications of PWR and provides a historical perspective on the development of instruments since the first prototype and continuous technological improvements to ongoing and future developments, aiming at broadening the information obtained and expanding the application portfolio.

Illuminating the Path to Target GPCR Structures and Functions

G-Protein-coupled receptors (GPCRs) are ubiquitous within eukaryotes, responsible for a wide array of physiological and pathological processes. Indeed, the fact that they are the most drugged target in the human genome is indicative of their importance. Despite the clear interest in GPCRs, most information regarding their activity has been so far obtained by analyzing the response from a "bulk medium". As such, this Perspective summarizes some of the common methods for this indirect observation. Nonetheless, by inspecting approaches applying super-resolution imaging, we argue that imaging is perfectly situated to obtain more detailed structural and spatial information, assisting in the development of new GPCR-targeted drugs and clinical strategies. The benefits of direct optical visualization of GPCRs are analyzed in the context of potential future directions in the field.

Improved Detection of Plasmon Waveguide Resonance Using Diverging Beam, Liquid Crystal Retarder, and Application to Lipid Orientation Determination

Plasmon waveguide resonance (PWR) sensors exhibit narrow resonances at the two orthogonal polarizations, transverse electric (TE) and transverse magnetic (TM), which are narrower by almost an order of a magnitude than the standard surface plasmon resonance (SPR), and thus the figure of merit is enhanced. This fact is useful for measuring optical anisotropy of materials on the surface and determining the orientation of molecules with high resolution. Using the diverging beam approach and a liquid crystal retarder, we present experimental results by simultaneous detection of TE and TM polarized resonances as well as using fast higher contrast serial detection with a variable liquid crystal retarder. While simultaneous detection makes the system simpler, a serial one has the advantage of obtaining a larger contrast of the resonances and thus an improved signal-to-noise ratio. Although the sensitivity of the PWR resonances is smaller than the standard SPR, the angular width is much smaller, and thus the figure of merit is improved. When the measurement methodology has a high enough angular resolution, as is the one presented here, the PWR becomes advantageous over other SPR modes. The possibility of carrying out exact numerical simulations for anisotropic molecules using the 4 × 4 matrix approach brings another advantage of the PWR over SPR on the possibility of extracting the orientation of molecules adsorbed to the surface. High sensitivity of the TE and TM signals to the anisotropic molecules orientation is found here, and comparison to the experimental data allowed detection of the orientation of lipids on the sensor surface. The molecular orientations cannot be fully determined from the TM polarization alone as in standard SPR, which underlines the additional advantage of the PWR technique.

Investigation on temperature-induce conformational change of immobilized β2 adrenergic receptor

The β₂ adrenergic receptor (β₂-AR) is a prototypical family A G protein-coupled receptor (GPCR) and an excellent model system for studying the mechanism of GPCR activation. Purified β₂-AR was immobilized on macroporous silica gel to obtain liquid chromatographic stationary phase. The resulting phase was packed into a stainless steel column (4.6 × 50 mm, 7 μm) and used for on-line chromatographic system. When column oven temperature increased from 20.0 °C to 40.0 °C, uncomplete separate chromatographic peaks of ephedrine and pseudoephedrine as receptor conformational probe were gradually merged into one peak, meanwhile retention time and resolution of the probes were reduced correspondingly, which suggested that temperature could regulate protein conformation. Temperature-induced conformational change of immobilized β₂-AR, especially changes at higher temperatures, indicated that constructed receptor chromatography could simulate fever disease state of human body and clarify receptor conformation change at pathological condition. At the same time this study could also provide new ideas for screening active components in pathological conditions.

Ligand activation induces different conformational changes in CXCR3 receptor isoforms as evidenced by plasmon waveguide resonance (PWR)

The chemokine receptor CXCR3 plays important roles in angiogenesis, inflammation and cancer. Activation studies and biological functions of CXCR3 are complex due to the presence of spliced isoforms. CXCR3-A is known as a pro-tumor receptor whereas CXCR3-B exhibits anti-tumor properties. Here, we focused on the conformational change of CXCR3-A and CXCR3-B after agonist or antagonist binding using Plasmon Waveguide Resonance (PWR). Agonist stimulation induced an anisotropic response with very distinct conformational changes for the two isoforms. The CXCR3 agonist bound CXCR3-A with higher affinity than CXCR3-B. Using various concentrations of SCH546738, a CXCR3 specific inhibitor, we demonstrated that low SCH546738 concentrations (≤1 nM) efficiently inhibited CXCR3-A but not CXCR3-B’s conformational change and activation. This was confirmed by both, biophysical and biological methods. Taken together, our study demonstrates differences in the behavior of CXCR3-A and CXCR3-B upon ligand activation and antagonist inhibition which may be of relevance for further studies aimed at specifically inhibiting the CXCR3A isoform.

Combining ε-Near-Zero Behavior and Stopped Light Energy Bands for Ultra-Low Reflection and Reduced Dispersion of Slow Light

We investigate media which exhibits epsilon-near-zero (ENZ) behavior while simultaneously sustaining stopped light energy bands which contain multiple points of zero group velocity (ZGV). This allows the merging of state-of-the-art phenomena that was hitherto attainable in media that demonstrated these traits separately. Specifically, we demonstrate the existence of Ferrell-Berreman (FB) modes within frequency bands bounded by points of ZGV with the goal to improve the coupling efficiency and localization of light in the media. The FB mode is formed within a double layer, thin-film stack where at subwavelength thicknesses the structure exhibits a very low reflection due to ENZ behavior. In addition, the structure is engineered to promote a flattened frequency dispersion with a negative permittivity able to induce multiple points of ZGV. For proof-of-concept, we propose an oxide-semiconductor-oxide-insulator stack and discuss the useful optical properties that arise from combining both phenomena. A transfer matrix (TM) treatment is used to derive the reflectivity profile and dispersion curves. Results show the ability to reduce reflection below 0.05% in accordance with recent experimental data while simultaneously exciting a polariton mode exhibiting both reduced group velocity and group velocity dispersion (GVD).

Real time monitoring of membrane GPCR reconstitution by plasmon waveguide resonance: on the role of lipids

G-protein coupled receptors (GPCRs) are important therapeutic targets since more than 40% of the drugs on the market exert their action through these proteins. To decipher the molecular mechanisms of activation and signaling, GPCRs often need to be isolated and reconstituted from a detergent-solubilized state into a well-defined and controllable lipid model system. Several methods exist to reconstitute membrane proteins in lipid systems but usually the reconstitution success is tested at the end of the experiment and often by an additional and indirect method. Irrespective of the method used, the reconstitution process is often an intractable and time-consuming trial-and-error procedure. Herein, we present a method that allows directly monitoring the reconstitution of GPCRs in model planar lipid membranes. Plasmon waveguide resonance (PWR) allows following GPCR lipid reconstitution process without any labeling and with high sensitivity. Additionally, the method is ideal to probe the lipid effect on receptor ligand binding as demonstrated by antagonist binding to the chemokine CCR5 receptor.

New insights into the molecular mechanisms of biomembrane structural changes and interactions by optical biosensor technology

Biomolecular-membrane interactions play a critical role in the regulation of many important biological processes such as protein trafficking, cellular signalling and ion channel formation. Peptide/protein-membrane interactions can also destabilise and damage the membrane which can lead to cell death. Characterisation of the molecular details of these binding-mediated membrane destabilisation processes is therefore central to understanding cellular events such as antimicrobial action, membrane-mediated amyloid aggregation, and apoptotic protein induced mitochondrial membrane permeabilisation. Optical biosensors have provided a unique approach to characterising membrane interactions allowing quantitation of binding events and new insight into the kinetic mechanism of these interactions. One of the most commonly used optical biosensor technologies is surface plasmon resonance (SPR) and there have been an increasing number of studies reporting the use of this technique for investigating biophysical analysis of membrane-mediated events. More recently, a number of new optical biosensors based on waveguide techniques have been developed, allowing membrane structure changes to be measured simultaneously with mass binding measurements. These techniques include dual polarisation interferometry (DPI), plasmon waveguide resonance spectroscopy (PWR) and optical waveguide light mode spectroscopy (OWLS). These techniques have expanded the application of optical biosensors to allow the analysis of membrane structure changes during peptide and protein binding. This review provides a theoretical and practical overview of the application of biosensor technology with a specific focus on DPI, PWR and OWLS to study biomembrane-mediated events and the mechanism of biomembrane disruption. This article is part of a Special Issue entitled: Lipid-protein interactions.

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

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

Slow receptor dissociation is not a key factor in the duration of action of inhaled long-acting β2-adrenoceptor agonists

BACKGROUND AND PURPOSE

β(2) -Adrenoceptor agonists are important bronchodilators used for the treatment of chronic obstructive pulmonary disease and asthma. Clinical data on β(2) -adrenoceptor agonists show a range of onset and duration of action. We have investigated whether the receptor binding kinetics of β(2) -adrenoceptor agonists can explain their observed onset of action and duration of effect in the clinic.

EXPERIMENTAL APPROACH

[(3) H]-DHA was used to label β(2) -adrenoceptors expressed in CHO-cell membranes (K(d) of 0.084 nM). Competition kinetic experiments were performed in the presence of unlabelled β(2) agonists at 37°C in HBSS containing GTP. To determine the kinetic parameters, three concentrations (10, 3 and 1 ×K(i) ) of the unlabelled compound were employed against a fixed concentration of [(3) H]-DHA (0.6 nM).

KEY RESULTS

The clinically used β(2) -adrenoceptor agonists exhibited a range of association and dissociation rates. The kinetic K(d) and the competition K(i) values of the eight β(2) -adrenoceptor agonists examined were strongly correlated, suggesting that the method had produced accurate k(off) and k(on) rates. The kinetic on-rate was highly correlated with equilibrium binding affinity.

CONCLUSIONS AND IMPLICATIONS

Although the β(2) -adrenoceptor agonists displayed a range of kinetic rate parameters, simulations at relevant drug concentrations suggest that receptor kinetics do not play an important role in determining onset of action in the clinic. In addition, it is unlikely that receptor kinetics exert an important influence on the duration of action of these agonists, as indacaterol (once daily dosing) had a shorter residency time at the receptor than salmeterol (twice daily dosing).

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

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

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

Introduction to membrane proteins

Membrane proteins (MPs) mediate important physiological processes for the cell via extracellular and intracellular interactions. To better understand the biochemical and structural bases of these interactions, well-characterized preparations of purified MPs are required. This introduction reviews common problems encountered in MP preparation.

Conformational dynamics of single G protein-coupled receptors in solution

G protein-coupled receptors (GPCRs) comprise a large family of seven-helix transmembrane proteins which regulate cellular signaling by sensing light, ligands, and binding proteins. The GPCR activation process, however, is not a simple on-off switch; current models suggest a complex conformational landscape in which the active, signaling state includes multiple conformations with similar downstream activity. The present study probes the conformational dynamics of single β(2)-adrenergic receptors (β(2)ARs) in the solution phase by Anti-Brownian ELectrokinetic (ABEL) trapping. The ABEL trap uses fast electrokinetic feedback in a microfluidic configuration to allow direct observation of a single fluorescently labeled β(2)AR for hundreds of milliseconds to seconds. By choosing a reporter dye and labeling site sensitive to ligand binding, we observe a diversity of discrete fluorescence intensity and lifetime levels in single β(2)ARs, indicating a varying radiative lifetime and a range of discrete conformational states with dwell times of hundreds of milliseconds. We find that the binding of agonist increases the dwell times of these states, and furthermore, we observe millisecond fluctuations within states. The intensity autocorrelations of these faster fluctuations are well-described by stretched exponential functions with a stretching exponent β ~ 0.5, suggesting protein dynamics over a range of time scales.

Reconstitution of G protein-coupled receptors into a model bilayer system: reconstituted high-density lipoprotein particles

Reconstituted high-density lipoprotein particles (rHDL) are powerful platforms used as a model phospholipid bilayer system to study membrane proteins. They consist of a discoidal-shaped planar bilayer of phospholipids that is surrounded by a dimer of apolipoprotein A-I (apoA-I). The amphipathic nature of apoA-1 shields the hydrophobic acyl chains of the lipids from solvent and keeps the particles soluble in aqueous environments. These monodispersed, nanoscale discoidal HDL particles are approximately 10-11 nm in diameter with a thickness that is dependent on the length of the phospholipid acyl chain. Reconstituted HDL particles can be assembled in vitro using purified apoA-1 and purified lipids. Investigators have utilized this model bilayer system to co-reconstitute membrane proteins, and take advantage of the small size and its monodispersion. Our laboratory and others have utilized the rHDL approach to study the behavior of G protein-coupled receptors. In this chapter, we describe strategies for the preparation of rHDL particles containing GPCRs in their monomeric form and discuss various methodologies used to analyze the reconstituted receptor function.

Conserved binding mode of human beta2 adrenergic receptor inverse agonists and antagonist revealed by X-ray crystallography

G protein-coupled receptors (GPCRs) represent a large fraction of current pharmaceutical targets, and of the GPCRs, the beta(2) adrenergic receptor (beta(2)AR) is one of the most extensively studied. Previously, the X-ray crystal structure of beta(2)AR has been determined in complex with two partial inverse agonists, but the global impact of additional ligands on the structure or local impacts on the binding site are not well-understood. To assess the extent of such ligand-induced conformational differences, we determined the crystal structures of a previously described engineered beta(2)AR construct in complex with two inverse agonists: ICI 118,551 (2.8 A), a recently described compound (2.8 A) (Kolb et al, 2009), and the antagonist alprenolol (3.1 A). The structures show the same overall fold observed for the previous beta(2)AR structures and demonstrate that the ligand binding site can accommodate compounds of different chemical and pharmacological properties with only minor local structural rearrangements. All three compounds contain a hydroxy-amine motif that establishes a conserved hydrogen bond network with the receptor and chemically diverse aromatic moieties that form distinct interactions with beta(2)AR. Furthermore, receptor ligand cross-docking experiments revealed that a single beta(2)AR complex can be suitable for docking of a range of antagonists and inverse agonists but also indicate that additional ligand-receptor structures may be useful to further improve performance for in-silico docking or lead-optimization in drug design.

Quantitative modeling of GRK-mediated beta2AR regulation

We developed a unified model of the GRK-mediated β2 adrenergic receptor (β2AR) regulation that simultaneously accounts for six different biochemical measurements of the system obtained over a wide range of agonist concentrations. Using a single deterministic model we accounted for (1) GRK phosphorylation in response to various full and partial agonists; (2) dephosphorylation of the GRK site on the β2AR; (3) β2AR internalization; (4) recycling of the β2AR post isoproterenol treatment; (5) β2AR desensitization; and (6) β2AR resensitization. Simulations of our model show that plasma membrane dephosphorylation and recycling of the phosphorylated receptor are necessary to adequately account for the measured dephosphorylation kinetics. We further used the model to predict the consequences of (1) modifying rates such as GRK phosphorylation of the receptor, arrestin binding and dissociation from the receptor, and receptor dephosphorylation that should reflect effects of knockdowns and overexpressions of these components; and (2) varying concentration and frequency of agonist stimulation “seen” by the β2AR to better mimic hormonal, neurophysiological and pharmacological stimulations of the β2AR. Exploring the consequences of rapid pulsatile agonist stimulation, we found that although resensitization was rapid, the β2AR system retained the memory of the previous stimuli and desensitized faster and much more strongly in response to subsequent stimuli. The latent memory that we predict is due to slower membrane dephosphorylation, which allows for progressive accumulation of phosphorylated receptor on the surface. This primes the receptor for faster arrestin binding on subsequent agonist activation leading to a greater extent of desensitization. In summary, the model is unique in accounting for the behavior of the β2AR system across multiple types of biochemical measurements using a single set of experimentally constrained parameters. It also provides insight into how the signaling machinery can retain memory of prior stimulation long after near complete resensitization has been achieved.

The β2 adrenergic receptor (β2AR) is involved in regulating many cellular processes such as smooth muscle relaxation in the airways and the vasculature. Drugs that activate the β2AR are used in treating asthma and chronic obstructive pulmonary disorder (COPD), and prolonged use of these drugs leads to the loss of their effects. Thus, a dynamic model of how the β2AR responds to different drugs is fundamental to their rational use. In this study a consensus model of G protein coupled receptor kinase (GRK)-mediated receptor regulation was formulated based on quantitative measures of six processes involved in β2AR regulation. This model was then used to simulate the consequences of manipulating key rates associated with the GRK-mediated β2AR regulation, leading to predictions which will provide a useful framework for further tests and elaborations of the model in basic and clinical research.

Chapter 6. Plasmon resonance methods in membrane protein biology applications to GPCR signaling

Plasmon waveguide resonance (PWR) spectroscopy, a variant of surface plasmon resonance (SPR) spectrometry, allows one to examine changes in conformation of anisotropic structures such as membranes and membrane-associated proteins such as G-protein-coupled receptors (GPCRs). The binding and resulting structural changes that accompany interactions of membrane protein with ligands (agonists, antagonists, inverse agonist, etc.), G-proteins, and other effectors and modulators of signaling can be directly examined with this technique. In this chapter we outline the instrumentation used for these studies, the experimental methods that allow determination of the structural changes, and thermodynamic and kinetic parameters that can be obtained from these studies.

Cellular assays as portals to seven-transmembrane receptor-based drug discovery

As technology advances to the point at which various behaviours of seven-transmembrane (7TM) receptors (also known as G protein-coupled receptors (GPCRs)) can be observed individually, it is clear that, rather than being 'on-off' switches, 7TM receptors are more akin to 'microprocessors' of information. This has introduced the phenomenon of functional selectivity, whereby certain ligands initiate only portions of the signalling mechanisms mediated by a given receptor, which has opened new horizons for drug discovery. The need to discover new 7TM receptor-ligand behaviours and quantify the effect of the drug on these complex systems, to guide medicinal chemistry, puts the pharmacological assay into the spotlight. This Perspective outlines the return to whole-system assays from reductionist recombinant systems, and discusses how the efficacy of a drug is linked to the particular assay used to observe its effects. It also highlights how these new assays are adding value to the drug discovery process.

Structural basis for ligand binding and specificity in adrenergic receptors: implications for GPCR-targeted drug discovery

Crystal structures of engineered human beta 2-adrenergic receptors (ARs) in complex with an inverse agonist ligand, carazolol, provide three-dimensional snapshots of the disposition of seven transmembrane helices and the ligand-binding site of an important G protein-coupled receptor (GPCR). As expected, beta 2-AR shares substantial structural similarities with rhodopsin, the dim-light photoreceptor of the rod cell. However, although carazolol and the 11- cis-retinylidene moiety of rhodopsin are situated in the same general binding pocket, the second extracellular (E2) loop structures are quite distinct. E2 in rhodopsin shows beta-sheet structure and forms part of the chromophore-binding site. In the beta 2-AR, E2 is alpha-helical and seems to be distinct from the receptor's active site, allowing a potential entry pathway for diffusible ligands. The structures, together with extensive structure-activity relationship (SAR) data from earlier studies, provide insight about possible structural determinants of ligand specificity and how the binding of agonist ligands might alter receptor conformation. We review key features of the new beta 2-AR structures in the context of recent complementary work on the conformational dynamics of GPCRs. We also report 600 ns molecular dynamics simulations that quantified beta 2-AR receptor mobility in a membrane bilayer environment and show how the binding of an agonist ligand, adrenaline (epinephrine), causes conformational changes to the ligand-binding pocket and neighboring helices.

Label-free optical biosensor for ligand-directed functional selectivity acting on beta(2) adrenoceptor in living cells

Recent realization of ligand-directed functional selectivity demands high-resolution tools for studying receptor biology and ligand pharmacology. Here we use label-free optical biosensor to examine the dynamic mass redistribution of human epidermoid A431 cells in response to diverse beta(2)-adrenoceptor ligands. Multi-parameter analysis reveals distinct patterns in activation and signaling of the receptor induced by different agonists. Sequential and co-stimulation assays categorize various ligands for their ability to modulate signaling induced by catechol, a structural component of catecholamines. This study documents multiple ligand-specific states of the beta(2)-adrenoceptor and highlights the power of the biosensor assays for screening pathway-biased ligands.

Unique agonist-bound cannabinoid CB1 receptor conformations indicate agonist specificity in signaling

Cannabinoid drugs differ in their rank order of potency to produce analgesia versus other central nervous system effects. We propose that these differences are due to unique agonist-bound cannabinoid CB1 receptor conformations that exhibit different affinities for individual subsets of intracellular signal transduction pathways. In order to test this hypothesis, we have used plasmon-waveguide resonance (PWR) spectroscopy, a sensitive method that can provide direct information about ligand-protein and protein-protein interactions, and can detect conformational changes in lipid-embedded proteins. A recombinant epitope-tagged human cannabinoid CB1 receptor was expressed in insect Sf9 cells, solubilized and purified using two-step affinity chromatography. The purified receptor was incorporated into a lipid bilayer on the surface of the PWR resonator. PWR spectroscopy demonstrated that cannabinoid agonists exhibit high affinity (KD=0.2+/-0.03 nM and 2+/-0.4 nM for CP 55,940 and WIN 55,212-2, respectively) for the purified epitope tagged hCB(1) receptor. Interestingly however, these structurally different cannabinoid agonists shifted the PWR spectra in opposite directions, indicating that CP 55,940 and WIN 55,212-2 binding leads to different hCB1 receptor conformations. Furthermore, PWR experiments also indicated that these CP 55,940-and WIN 55,212-bound hCB1 receptor conformations exhibit slightly different affinities to an inhibitory G protein heterotrimer, Gi1 (KD=27+/-8 nM and KD=10.7+/-4.7 nM, respectively), whereas they strikingly differ in their ability to activate this G protein type.

Plasmon-waveguide resonance (PWR) spectroscopy for directly viewing rates of GPCR/G-protein interactions and quantifying affinities

Plasmon-waveguide resonance (PWR) spectroscopy is an optical technique that has been developed in our laboratories and applied to the study of membrane-associated proteins, especially GPCRs. It has high sensitivity and requires no labeling of materials, and it can monitor changes in proteolipid mass density and conformation in real time using plasmon excitation by light polarized both perpendicular and parallel to the resonator surface. Direct measurements will be described of the association of ligands and G-proteins to GPCRs incorporated into a self-assembled lipid bilayer deposited on the silica surface of a PWR resonator. These studies have provided new insights into the functioning of this important class of signaling proteins.

A monomeric G protein-coupled receptor isolated in a high-density lipoprotein particle efficiently activates its G protein

G protein-coupled receptors (GPCRs) respond to a diverse array of ligands, mediating cellular responses to hormones and neurotransmitters, as well as the senses of smell and taste. The structures of the GPCR rhodopsin and several G proteins have been determined by x-ray crystallography, yet the organization of the signaling complex between GPCRs and G proteins is poorly understood. The observations that some GPCRs are obligate heterodimers, and that many GPCRs form both homo- and heterodimers, has led to speculation that GPCR dimers may be required for efficient activation of G proteins. However, technical limitations have precluded a definitive analysis of G protein coupling to monomeric GPCRs in a biochemically defined and membrane-bound system. Here we demonstrate that a prototypical GPCR, the beta2-adrenergic receptor (beta2AR), can be incorporated into a reconstituted high-density lipoprotein (rHDL) phospholipid bilayer particle together with the stimulatory heterotrimeric G protein, Gs. Single-molecule fluorescence imaging and FRET analysis demonstrate that a single beta2AR is incorporated per rHDL particle. The monomeric beta2AR efficiently activates Gs and displays GTP-sensitive allosteric ligand-binding properties. These data suggest that a monomeric receptor in a lipid bilayer is the minimal functional unit necessary for signaling, and that the cooperativity of agonist binding is due to G protein association with a receptor monomer and not receptor oligomerization.

Molecular mechanism of the constitutive activation of the L250Q human melanocortin-4 receptor polymorphism

The Melanocortin-4 Receptor is a G-protein coupled receptor that has been physiologically linked to participate in the regulation of energy homeostasis. The Melanocortin-4 Receptor is stimulated by endogenous melanocortin agonists derived from the pro-opiomelanocortin gene transcript and antagonized by the endogenous antagonist agouti-related protein. Central administration of melanocortin agonists has been demonstrated to decrease food intake and conversely, treatment with antagonists resulted in increased food intake. Deletion of the Melanocortin-4 Receptor gene from the mouse genome results in an obese and hyperphagic phenotype. Polymorphisms of the human Melanocortin-4-Receptor have been found in severely obese individuals, suggesting that Melanocortin-4 Receptor malfunction might be involved in human obesity and obesity-associated diabetes. Herein, we have performed experiments to understand the molecular mechanisms associated with the L250Q human Melanocortin-4-Receptor polymorphism discovered in an extremely obese woman. This L250Q human Melanocortin-4-Receptor has been pharmacologically characterized to result in a constitutively active receptor. The fact that a constitutively active human Melanocortin-4-Receptor mutation was found in an obese person is a physiologic contradiction, as chronic activation of the human Melanocortin-4-Receptor and subsequently high cyclic adenosine monophosphate levels should theoretically result in a normal or lean phenotype. In this study, we demonstrated that agouti-related protein acts as an inverse agonist at this constitutively active receptor, and we propose a mechanism by which agouti-related protein might contribute to the obese phenotype in the L250Q patient. In addition, using receptor mutagenesis, pharmacology, and computer modeling approaches, we investigated the molecular mechanism by which modification of the L250 residue results in constitutive activation of the human Melanocortin-4-Receptor.

Waveguide-modulated surface plasmon-coupled emission of Nile blue in poly(vinyl alcohol) thin films

Surface plasmon-coupled emission (SPCE) phenomenon is the coupling of excited fluorophores near a silver film with surface plasmons, resulting in directional emission into the underlying glass substrates. We report a complex coupling of Nile Blue fluorophore with 50 nm silver mirror, resulting in emission at several angles in the glass substrate, with either s or p polarization. This complex pattern of directional and polarized emission appears to be due to optical waveguide effects occurring when the sample thickness becomes comparable to the emission wavelength. We expect waveguide-modulated SPCE to have applications to biophysics and sensing.

Dynamic behavior of fully solvated beta2-adrenergic receptor, embedded in the membrane with bound agonist or antagonist

Recently we predicted the 3D structure of the human beta2-adrenergic receptor (beta2AR) and of the binding site of several agonists and antagonists to beta2AR. These predictions (MembStruk and HierDock) included no explicit water and only a few lipid molecules. Here we include explicit H(2)O and an infinite lipid bilayer membrane in molecular dynamics (MD) simulations of three systems: apo-beta2AR, epinephrine-bound beta2AR, and butoxamine-bound beta2AR (epinephrine is an endogenous agonist, and butoxamine is a beta2AR selective antagonist). The predicted structures for apo-beta2AR and butoxamine-beta2AR are stable in MD, but in epinephrine-beta2AR, extracellular water trickles into the binding pocket to mediate hydrogen bonding between the catechol of epinephrine and Ser-204 on helix 5. The epinephrine-beta2AR structure shows dynamic flexibility with small, piston-like movements of helices 3 and 6 and transient interhelical hydrogen bonding between Ser-165 on transmembrane 4 and Ser-207 on transmembrane 5. These couplings and motions may play a role in protein activation. The apo-beta2AR shows less dynamic flexibility, whereas the antagonist-beta2AR structure is quite rigid. This MD validation of the structure predictions for G protein-coupled receptors in explicit lipid and water suggests that these methods can be trusted for studying the mechanism of activation and the design of subtype-specific agonists and antagonists.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Homology modeling of opioid receptor-ligand complexes using experimental constraints

Opioid receptors interact with a variety of ligands, including endogenous peptides, opiates, and thousands of synthetic compounds with different structural scaffolds. In the absence of experimental structures of opioid receptors, theoretical modeling remains an important tool for structure-function analysis. The combination of experimental studies and modeling approaches allows development of realistic models of ligand-receptor complexes helpful for elucidation of the molecular determinants of ligand affinity and selectivity and for understanding mechanisms of functional agonism or antagonism. In this review we provide a brief critical assessment of the status of such theoretical modeling and describe some common problems and their possible solutions. Currently, there are no reliable theoretical methods to generate the models in a completely automatic fashion. Models of higher accuracy can be produced if homology modeling, based on the rhodopsin X-ray template, is supplemented by experimental structural constraints appropriate for the active or inactive receptor conformations, together with receptor-specific and ligand-specific interactions. The experimental constraints can be derived from mutagenesis and cross-linking studies, correlative replacements of ligand and receptor groups, and incorporation of metal binding sites between residues of receptors or receptors and ligands. This review focuses on the analysis of similarity and differences of the refined homology models of mu, delta, and kappa-opioid receptors in active and inactive states, emphasizing the molecular details of interaction of the receptors with some representative peptide and nonpeptide ligands, underlying the multiple modes of binding of small opiates, and the differences in binding modes of agonists and antagonists, and of peptides and alkaloids.

Plasmon resonance methods in GPCR signaling and other membrane events

The existence of surface guided electromagnetic waves has been theoretically predicted from Maxwell's equations and investigated during the first decades of the 20th century. However, it is only since the late 1960's that they have attracted the interest of surface physicists and earned the moniker of "surface plasmon". With the advent of commercially available instruments and well established theories, the technique has been used to study a wide variety of biochemical and biotechnological phenomena. Spectral response of the resonance condition serves as a sensitive indicator of the optical properties of thin films immobilized within a wavelength of the surface. This enhanced surface sensitivity has provided a boon to the surface sciences, and fosters collaboration between surface chemistry, physics and the ongoing biological and biotechnological revolution. Since then, techniques based on surface plasmons such as Surface Plasmon Resonance (SPR), SPR Imaging, Plasmon Waveguide Resonance (PWR) and others, have been increasingly used to determine the affinity and kinetics of a wide variety of real time molecular interactions such as protein-protein, lipid-protein and ligand-protein, without the need for a molecular tag or label. The physical-chemical methodologies used to immobilize membranes at the surface of these optical devices are reviewed, pointing out advantages and limitations of each method. The paper serves to summarize both historical and more recent developments of these technologies for investigating structure-function aspects of these molecular interactions, and regulation of specific events in signal transduction by G-protein coupled receptors (GPCRs).

Immobilization of native membrane-bound rhodopsin on biosensor surfaces

In this paper, we evaluated the grafting of G-protein-coupled receptors (GPCRs) onto functionalized surfaces, which is a primary requirement to elaborate receptor-based biosensors, or to develop novel GPCR assays. Bovine rhodopsin, a prototypical GPCR, was used in the form of receptor-enriched membrane fraction. Quantitative immobilization of the membrane-bound rhodopsin either non-specifically on a carboxylated dextran surface grafted with long alkyl groups, or specifically on a surface coated with anti-rhodopsin antibody was demonstrated by surface plasmon resonance. In addition, a new substrate based on mixed self-assembled multilayer that anchors specific anti-receptor antibodies was developed. Electrochemical impedance spectroscopy performed upon deposition of membrane-bound rhodopsin of increasing concentration exhibited a significant change, until a saturation level was reached, indicating optimum receptor immobilization on the substrate. The structures obtained with this new immobilization procedure of the rhodopsin in its native membrane environment are stable, with a controlled density of specific anchoring sites. Therefore, such receptor immobilization method is attractive for a range of applications, especially in the field of GPCR biosensors.

Beta-arrestin binding to the beta2-adrenergic receptor requires both receptor phosphorylation and receptor activation

Homologous desensitization of beta2-adrenergic receptors has been shown to be mediated by phosphorylation of the agonist-stimulated receptor by G-protein-coupled receptor kinase 2 (GRK2) followed by binding of beta-arrestins to the phosphorylated receptor. Binding of beta-arrestin to the receptor is a prerequisite for subsequent receptor desensitization, internalization via clathrin-coated pits, and the initiation of alternative signaling pathways. In this study we have investigated the interactions between receptors and beta-arrestin2 in living cells using fluorescence resonance energy transfer. We show that (a) the initial kinetics of beta-arrestin2 binding to the receptor is limited by the kinetics of GRK2-mediated receptor phosphorylation; (b) repeated stimulation leads to the accumulation of GRK2-phosphorylated receptor, which can bind beta-arrestin2 very rapidly; and (c) the interaction of beta-arrestin2 with the receptor depends on the activation of the receptor by agonist because agonist withdrawal leads to swift dissociation of the receptor-beta-arrestin2 complex. This fast agonist-controlled association and dissociation of beta-arrestins from prephosphorylated receptors should permit rapid control of receptor sensitivity in repeatedly stimulated cells such as neurons.

Plasmon-waveguide resonance spectroscopy applied to three potential drug targets: cyclooxygenase-2, hepatitis C virus RNA polymerase and integrin αVβ3

Plasmon-waveguide resonance (PWR) spectroscopy has been used to study the interactions between ligands that correspond to inhibitors, activators or substrates and three integral membrane proteins representing potential drug targets; cyclooxygenases 1 and 2 (COX-1 and -2), integrin alphaVbeta3, and hepatitis C virus RNA polymerase. The proteins were incorporated into an egg phosphatidylcholine bilayer deposited onto the surface of the PWR resonator, and changes in the amplitude and position of the PWR spectra due to mass density increases and conformational transitions have been used to characterize the kinetics and binding affinities corresponding to these interactions. Although the partition of COX-2 into the bilayer was not as efficient as was the case with the other two proteins, sufficient protein could be incorporated to allow ligand binding to be observed. It was also possible to incorporate COX-1 into a lipid bilayer by adding a suspension of microsomal membrane fragments containing this enzyme to a preformed bilayer, and to observe binding of an inhibitory ligand. The interactions between integrin alphaVbeta3 and two ligands with different in vivo efficacies could be distinguished by both spectral measurements and binding kinetics. In the case of the RNA polymerase, the kinetics of PWR spectral changes upon adding a substrate solution to an enzyme-template complex were consistent with those obtained from direct measurements of enzymatic turnover. These experiments demonstrate the utility of PWR spectroscopy to provide novel information regarding drug interactions with membrane proteins in a lipid environment and to distinguish conformational changes induced by binding of various drug molecules.

Selectivity, cooperativity, and reciprocity in the interactions between the delta-opioid receptor, its ligands, and G-proteins

A better understanding of signal transduction mechanisms is of critical importance. Methodologies that allow studies to be done while receptors are incorporated into lipid bilayers are advantageous. One such technique is plasmon-waveguide resonance (PWR) spectroscopy, which can follow changes in conformation accompanying protein-ligand, protein-protein, and protein-lipid interactions occurring in G-protein-coupled receptors in real time with high sensitivity and without the need for molecular labeling. Here we investigated several aspects of human delta-opioid receptor (hDOR)-G-protein interactions: 1) the effect of different types of agonists on the interaction with individual G-protein subtypes; 2) the affinities of the separate G-protein alpha and betagamma subunits to different ligand-occupied states of the receptor; and 3) the effect of the presence of the G-protein on the interactions of the ligand with the receptor. To accomplish this we have incorporated the receptor into a solid supported lipid bilayer in the presence of ligand or G-protein and monitored the PWR spectral changes induced by the reciprocal G-protein or ligand interactions. We found a high degree of selectivity in the interactions of different agonist-bound states of the receptor with the different G-protein subtypes. This has important implications for agonist-directed trafficking and selective drug design. Studies with the separated alpha and betagamma subunits show that cooperativity exists in these interactions. The high affinities of the separated subunits to the receptor point to the possibility of independent promotion of specific signaling events. The presence of G-proteins increased the affinity of agonists to the hDOR, and caused faster binding kinetics and different ligand-induced conformational changes. Because ligand also influences G-protein binding, reciprocity exists between these two binding processes.

Different structural states of the proteolipid membrane are produced by ligand binding to the human delta-opioid receptor as shown by plasmon-waveguide resonance spectroscopy

Understanding structure-function relationships and mechanisms of signal transduction in G-protein-coupled receptors (GPCRs) is becoming increasingly important, both as a fundamental problem in membrane biology and as a consequence of their central role as pharmacological targets. Their integral membrane nature and rather low natural abundance present many challenging problems. Using a recently developed technique, plasmon-waveguide resonance (PWR) spectroscopy, we investigated the structural changes accompanying the binding of ligands to the human delta-opioid receptor (hDOR) immobilized in a solid-supported lipid bilayer. This highly sensitive technique can directly monitor changes in mass density, conformation, and orientation occurring in such thin proteolipid films. Without requiring labeling protocols, PWR allows the direct determination of binding constants in a system very close to the receptor's natural environment. In the present study, conformational changes of a proteolipid membrane containing the hDOR were investigated upon binding of a variety of peptide and nonpeptide agonists, partial agonists, antagonists, and inverse agonists. Distinctly different structural states of the membrane were observed upon binding of each of these classes of ligands, reflecting different receptor conformational states, and the formation of each state was characterized by different kinetic properties. Binding constants, obtained by quantifying the extent of conformational change as a function of the amount of ligand bound, were in good agreement with published values determined by radiolabeling methods. The results provide new insights into ligand-induced GPCR functioning and illustrate a powerful new protocol for drug development.