Ultrafast optogenetic stimulation of the auditory pathway by targeting‐optimized Chronos

Abstract

Optogenetic tools, providing non‐invasive control over selected cells, have the potential to revolutionize sensory prostheses for humans. Optogenetic stimulation of spiral ganglion neurons (SGNs) in the ear provides a future alternative to electrical stimulation used in cochlear implants. However, most channelrhodopsins do not support the high temporal fidelity pertinent to auditory coding because they require milliseconds to close after light‐off. Here, we biophysically characterized the fast channelrhodopsin Chronos and revealed a deactivation time constant of less than a millisecond at body temperature. In order to enhance neural expression, we improved its trafficking to the plasma membrane (Chronos‐ES/TS). Following efficient transduction of SGNs using early postnatal injection of the adeno‐associated virus AAV‐PHP.B into the mouse cochlea, fiber‐based optical stimulation elicited optical auditory brainstem responses (oABR) with minimal latencies of 1 ms, thresholds of 5 μJ and 100 μs per pulse, and sizable amplitudes even at 1,000 Hz of stimulation. Recordings from single SGNs demonstrated good temporal precision of light‐evoked spiking. In conclusion, efficient virus‐mediated expression of targeting‐optimized Chronos‐ES/TS achieves ultrafast optogenetic control of neurons.

Optimized Chronos sets the clock for optogenetic hearing restoration

“…The development of light‐based actuators, sensors, and genetic targeting‐strategies has dramatically broadened the experimental toolbox at disposal of neurobiological and medical research. The work by Keppeller et al () demonstrates that this revolutionary transition is sufficiently mature to facilitate the translation of fundamental discoveries into clinical hearing applications.…”

“…Keppeler et al () postulated that this was mainly due to low plasma membrane expression of the original Chronos in SGNs. While adopting vector and transduction strategies described for the use of the ChR2 variant CatCh before, neural population responses were rarely detected with Chronos.…”

“…While adopting vector and transduction strategies described for the use of the ChR2 variant CatCh before, neural population responses were rarely detected with Chronos. Based on previous work (Gradinaru et al , ; Deverman et al , ), Keppeler et al () coupled Chronos to ER‐exiting (ES) and trafficking signals (TS) taken from potassium channels to improve the opsin targeting to the cell membrane, in addition to employing a highly potent viral vector, AAV‐PHP.B.…”

“…It will be interesting to explore ways to further speed up Chronos and to continue optimizing its surface expression to achieve the full temporal fidelity of acoustic hearing. Still, Keppeler et al () present an important breakthrough toward a potential clinical translation of optogenetics in hearing restoration.…”

“…Particularly, while it is undeniable that the design of new opsins through genomic screening and mutagenesis approaches is essential to optimize opsin properties (e.g., timing fidelity, absorption bandwidth, bistable control), it is also clear that true breakthroughs in fundamental and medical research necessitate the joint optimization of membrane targeting and viral expression strategies (Gradinaru et al , ) as well as light‐delivery approaches (Emiliani et al , ). This paradigm was successfully demonstrated in Keppeler et al () paving the way to a large number of applications requiring high temporal fidelity of neural stimulation.…”