Mager, T. et al. Nat. Commun. 9, 1750 (2018).

Optogenetic tools come in a variety of flavors: they can be activated by blue light or red light, and they can be slow or fast. Chrimson is activated by red light, which makes it particularly suitable for in vivo applications, as red light is less phototoxic than blue light. However, Chrimson’s slow channel-closing kinetics are not ideal for applications that require high stimulation rates. Mager et al. report two Chrimson variants, f-Chrimson and vf-Chrimson, that have closing kinetics five to ten times faster than those of the original protein, potentially allowing neuronal stimulation at up to 600 Hz. This increase in speed is achieved through the mutation of residues in Chrimson’s helix F, which moves during the transition from the open to the closed state of the channel. The researchers demonstrate the utility of the fast Chrimson variants in the auditory system of mice. Upon expression of f-Chrimson in spiral ganglion neurons (SGNs), the researchers were able to record responses in the auditory brainstem while illuminating the SGNs.