Single-molecule detection technologies in miniaturized high throughput screening: binding assays for g protein-coupled receptors using fluorescence intensity distribution analysis and fluorescence anisotropy.

G Protein-coupled receptors (GPCRs) represent one of the most important target classes for drug discovery. Various assay formats are currently applied to screen large compound libraries for agonists or antagonists. However, the development of nonradioactive, miniaturizable assays that are compatible with the requirements of ultra-high throughput screening (uHTS) has so far been slow. In this report we describe homogeneous fluorescence-based binding assays that are highly amenable to miniaturization. Fluorescence intensity distribution analysis (FIDA) is a single-molecule detection method that is sensitive to brightness changes of individual particles, such as those induced by binding of fluorescent ligands to membrane particles with multiple receptor sites. As a confocal detection technology, FIDA inherently allows reduction of the assay volume to the microliter range and below without any loss of signal. Binding and displacement experiments are demonstrated for various types of GPCRs, such as chemokine, peptide hormone, or small-molecule ligand receptors, demonstrating the broad applicability of this method. The results correlate quantitatively with radioligand binding data. We compare FIDA with fluorescence anisotropy (FA), which is based on changes of molecular rotation rates upon binding of fluorescent ligands to membranes. While FA requires a higher degree of binding, FIDA is sensitive down to lower levels of receptor expression. Both methods are, within these boundary conditions, applicable to uHTS.