Rhodopsin-based voltage imaging tools for use in excitable cells of Caenorhabditis elegans

Abstract

Genetically encoded voltage indicators (GEVIs) based on microbial rhodopsins utilize the voltage-sensitive fluorescence of the all-trans retinal (ATR) cofactor, while in electrochromic (eFRET) sensors, donor fluorescence drops when the rhodopsin acts as depolarization-sensitive acceptor. We systematically assessed Arch(D95N), Archon, and QuasAr, as well as the eFRET sensors MacQ-mCitrine and QuasAr-mOrange, in C. elegans. ATR-bearing rhodopsins reported on voltage changes in body wall muscles (BWMs) and the pharynx, the feeding organ, where Arch(D95N) showed ca. 125 % {Delta}F/F increase per 100 mV. The ATR fluorescence is very dim, however, using the retinal analog dimethylaminoretinal (DMAR), it was boosted 250-fold. eFRET sensors provided sensitivities of 45 % to 78 % {Delta}F/F per 100 mV, induced by BWM action potentials (APs). All sensors reported differences in muscle depolarization induced by a voltage-gated Ca2+-channel mutant. Optogenetically evoked de-or hyperpolarization of motor neurons increased or eliminated AP activity and caused a rise or drop in BWM sensor fluorescence. Last, we could analyze voltage dynamics across the entire pharynx, showing uniform depolarization but compartmentalized repolarization of anterior and posterior parts. Our work establishes all-optical, non-invasive electrophysiology in intact C. elegans.