Branched photoswitchable tethered ligands enable ultra-efficient optical control and detection of class C G protein-coupled receptors

The limitations of classical, soluble drugs in terms of subtype-specificity, spatiotemporal precision, and genetic targeting have spurred the development of advanced pharmacological techniques, including the use of covalently-tethered photoswitchable ligands. However, a major shortcoming of tethered photopharmacology is the inability to obtain optical control with a comparable efficacy to the native ligand. To overcome the limitations of photoisomerization efficiency and tethered ligand affinity, we have developed a family of branched photoswitchable compounds to target G protein-coupled metabotropic glutamate receptors (mGluRs). These compounds permit photo-agonism of Gi/o-coupled group II mGluRs with near-complete efficiency relative to saturating glutamate when attached to receptors via a range of orthogonal, multiplexable modalities including SNAP-, CLIP-, and Halo-tags, as well as via receptor-targeting nanobodies. Through a chimeric approach, branched ligands also allow efficient optical control of Gq-coupled mGluR5 with precise, dynamic subcellular targeting. Finally, branched ligands enabled the development of dual photoswitch-fluorophore compounds that allow simultaneous imaging and manipulation of receptors via the same attachment point. Together this work provides a new design framework for photoswitchable ligands and demonstrates a toolset suitable for quantitative, mechanistic study of neuromodulatory receptors at the molecular, cellular and circuit levels.