Multimodal characterization and optogenetic potential of the bistable G _i/o -coupled vertebrate ancient opsin from the flashlight fish Anomalops katoptron

Vertebrate ancient long opsin, or VAL opsin, is a light-sensitive protein that is found within and outside the visual system in vertebrates. In accordance with its wide distribution in the retina, brain, testis and skin, VAL is suggested to play a role in light-dependent physiological processes that are beyond vision. However, many aspects of the physiological properties and specific functions of VAL remain unclear. Here we identified and characterized the VAL opsin from the flashlight fish Anomalops katoptron ( _Ak VAL) and show that this opsin is bistable and reversibly converts between active and inactive states by responding to cycles of green and blue/UV lights. We further show that _Ak VAL couples to the G _i/o pathway and controls the activity of GIRK channels in a bistable manner. In line with this, we demonstrated that _Ak VAL modulates neuronal activity in cerebellar Purkinje cells, where neuronal activity is reduced by UV/blue light and increased by green/red light illumination. In addition, upon the in vivo expression of _Ak VAL in neurons innervating body muscles of Caenorhabditis elegans the worm’s body movement can be bidirectionally controlled altering blue/UV and green illuminations. These data highlight the potential of _Ak VAL as an optogenetic tool to control cells in vitro and in vivo , in a bistable manner.