An overview of Ca2+ mobilization assays in GPCR drug discovery.

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