10.2 Biosensors
An extensive toolbox of biosensors for GPCR signalling is available to researchers (Kim et al., 2022; Olsen & English, 2023). Here, we showcase only a few important examples (Figure 4h-l). Nanobodies initially developed as a structural tool have become widely used in the imaging of GPCR signalling (Figure 4h). These small single-chain antibodies, originally derived from camelids, selectively bind to proteins in particular conformations or protein complexes in particular compositions (Rasmussen et al., 2011). A closely related group of sensors are miniG proteins (Nehme et al., 2017; Wan et al., 2018), nanobody-like proteins based on the GTPase domain of Gα subunits, which bind to activated GPCRs and mimic G protein binding (Figure 4i). Nanobodies and miniG proteins have been used for the detection and regulation of many GPCR signalling aspects (Gil et al., 2020; Gormal et al., 2020) and the determination of subcellular localisation of signalling molecules (Irannejad et al., 2013). Another notable group of GPCR-based biosensors include dopamine receptors with inserted circularly permuted FPs which allow the quantitation of neurotransmitter release at a wide range of concentrations in live cells and animals (Figure 4l) (Patriarchi et al., 2018; Sun et al., 2018). Intracellularly recently developed FRET-based GPCR nanoruler sensors measure the concentration of second messengers, cAMP in particular (Anton et al., 2022), for the accurate detection of distances at which signalling effects propagate inside cells (Figure 4j). Multiple existing optogenetic GPCR-based sensors activate (Airan et al., 2009) and regulate (Lockyer et al., 2023) GPCR signalling using light pulses instead of chemical compounds (Figure 4k). Photomanipulation can also be used to control GPCR localisation and induce receptor confinement (Sanchez et al., 2021). BRET sensor systems such as BERKY and TRUPATH allow sensitive measurements of specific GPCR and G protein activity (Maziarz et al., 2020; Olsen et al., 2020). These and many other sensors provide an extensive set of tools for the detection and targeted regulation of GPCR signalling.