Monitoring Gq-coupled receptor response through inositol phosphate quantification with the IP-One assay

Introduction: G-protein-coupled receptors (GPCRs) are transmembrane proteins that play a key role in the signal transduction of extracellular stimuli. GPCRs associate to a complex assembly of intracellular proteins regulating a large variety of signaling pathways. In particular, the production of inositol 1,4,5 triphosphate (IP3) signs the activation of Gq-coupled receptors. However, its very short half-life makes its assessment too challenging for drug screening operations and the monitoring of calcium release, triggered by IP3, has been extensively used as a downstream readout of this signaling pathway. Recently, a new homogeneous time-resolved fluorescence (HTRF (R)) assay, detecting a downstream metabolite of IP3, inositol monophosphate (IP1), has overcome the drawbacks of the IP3 quantification, allowing its use in primary or secondary screening. Areas covered: This review provides an overview of the use of the IP-One assay in screening processes, providing comparisons with results obtained with other existing techniques traditionally used to investigate Gq-coupled receptors. Moreover, the review highlights two key features of the IP-One assay, the discrimination of slow acting compounds and the characterization of inverse agonists, which are impossible to achieve using calcium release. Expert opinion: The IP-One assay is well established to perform screening in the pharmaceutical industry. A number of criteria can be taken into account, including the impact of the sensitivity improvement of the assay, to position the IP-One assay in the different stages of the drug screening process. Moreover, the IP-One assay can be used as a valuable solution to investigate new research concepts such as ligand-biased signaling or receptor heteromerization.