Photophysics-informed two-photon voltage imaging using FRET-opsin voltage indicators | Science Advances

Microbial rhodopsin–derived genetically encoded voltage indicators (GEVIs) are powerful tools for mapping bioelectrical dynamics in cell culture and in live animals. Förster resonance energy transfer (FRET)–opsin GEVIs use voltage-dependent quenching of an attached fluorophore, achieving high brightness, speed, and voltage sensitivity. However, the voltage sensitivity of most FRET-opsin GEVIs has been reported to decrease or vanish under two-photon (2P) excitation. Here, we investigated the photophysics of the FRET-opsin GEVIs Voltron1 and Voltron2. We found that the previously reported negative-going voltage sensitivities of both GEVIs came from photocycle intermediates, not from the opsin ground states. The voltage sensitivities of both GEVIs were nonlinear functions of illumination intensity; for Voltron1, the sensitivity reversed the sign under low-intensity illumination. Using photocycle-optimized 2P illumination protocols, we demonstrate 2P voltage imaging with Voltron2 in the barrel cortex of a live mouse. These results open the door to high-speed 2P voltage imaging of FRET-opsin GEVIs in vivo.