A far-red hybrid voltage indicator enabled by bioorthogonal engineering of rhodopsin on live neurons

Membrane potential is a key aspect of cellular signalling and is dynamically regulated by an array of ion-selective pumps and channels. Fluorescent voltage indicators enable non-invasive optical recording of the cellular membrane potential with high spatial resolution. Here, we report a palette of bright and sensitive hybrid voltage indicators (HVIs) with fluorescence intensities sensitive to changes in membrane potential via electrochromic Forster resonance energy transfer. Enzyme-mediated site-specific incorporation of a probe, followed by an inverse-electron-demand Diels–Alder cycloaddition, was used to create enhanced voltage-sensing rhodopsins with hybrid dye–protein architectures. The most sensitive indicator, HVI-Cy3, displays high voltage sensitivity (−39% ΔF/F0 per 100 mV) and millisecond response kinetics, enabling optical recording of action potentials at a sampling rate of 400 Hz over 10 min across a large neuronal population. The far-red indicator HVI-Cy5 could be paired with optogenetic actuators and green/red-emitting fluorescent indicators, allowing an all-optical investigation of neuronal electrophysiology. Voltage imaging is a powerful technique for studying electrical signalling in neurons. A palette of bright and sensitive voltage indicators has now been developed via enzyme-mediated ligation and Diels–Alder cycloaddition. Among these, a far-red indicator faithfully reports neuronal action potential dynamics with an excitation spectrum orthogonal to optogenetic actuators and green/red-emitting biosensors.