FRET Reporter Assays for cAMP and Calcium in a 96-well Format Using Genetically Encoded Biosensors Expressed in Living Cells

Stimulation of G protein-coupled receptors (GPCR) by hormones and neurotransmitters elicits cellular responses, many of which result from alterations in the concentrations of cytosolic cAMP and Ca²⁺. Here, we describe a microplate reader fluorescence resonance energy transfer (FRET) assay that uses the genetically encoded biosensors H188 and YC3.60 so that it is possible to monitor the kinetics with which alterations of [cAMP] or [Ca²⁺] occur in monolayers or suspensions of living cells exposed to GPCR agonists. This protocol uses HEK293 cell lines doubly transfected with a FRET biosensor and a recombinant GPCR of interest (e.g., glucagon receptors, CCK2 receptors, or NPY2R receptors). The protocol allows for rapid screening of small molecule GPCR agonists and antagonists, and it is also useful for discovery of synthetic mono-, dual-, and tri- agonist peptides with GPCR activating properties.