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.