Regulation of deafness by optogenetic stimulation of nonexcitable nonglial cells in the inner ear

[Speaker] Mitsuo P Sato:1,2
[Co-author] Fumiaki Nin:1,3, Masatsugu Masuda:4, Takahisa Watabe:5, Kaoru Ogawa:5, Katsumi Doi:2, Kenji F. Tanaka:6, Hiroshi Hibino:1,3,7
1:Department of Molecular Physiology, Niigata University School of Medicine, Japan, 2:Department of Otolaryngology, Kindai University Faculty of Medicine, Japan, 3:Center for Transdisciplinary Research, Niigata University, Japan, 4:Department of Otolaryngology, Kyorin University School of Medicine, Japan, 5:Department of Otolaryngology, Head and Neck Surgery, Keio University School of Medicine, Japan, 6:Department of Neuropsychiatry, Keio University School of Medicine, Japan, 7:AMED-CREST, AMED, Niigata, Japan

Optogenetics has been utilized primarily to excitable cell types to analyze their physiological and pathological significance. Nevertheless, nonexcitable cell types have not yet been sufficiently studied in vivo with this technology. To expand the application of optogenetics, we analyzed the cochlea of the inner ear in a mouse line expressing channelrhodopsin-2 (ChR2) under control of an oligodendrocyte-specific proteolipid protein (PLP) promoter with the KENGE-tet system (Tanaka et al., Cell Rep 2012).
Distribution of ChR2 in the cochlea was examined by immunolabeling technique. Change of hearing level was monitored with auditory brainstem response (ABR) with or without blue light illumination to the cochlea. Functional significance of ChR2 was tested by patch-clamp assay with cells isolated from the cochlea as well as in vivo electrophysiological experiment with glass microelectrodes. Furthermore, electrical ABR (eABR) measurement was used to evaluate the effect of activated ChR2 in the cochlea nervous system.
Immunolabeling detected ChR2 in in nonglial cells-melanocytes-of an epithelial-like tissue in the cochlea. The membrane potential of these cells underlies a positive potential in an extracellular solution, endolymph, which is essential for hearing. Blue light stimulation of the cochlea caused moderate deafness in 3 min with elevation of ABR threshold by ~20 dB. After cessation of the illumination, the hearing recovered to the initial level within 5 min. Whole-cell patch-clamp experiments showed that the illumination rapidly depolarized the melanocytes by approximately 20 mV and this change lasted for 3 min. In in vivo electrophysiological recordings, the endolymphatic potential was reduced by ~20 mV within 3 min in response to the optical stimulation. These three photoreactions were reversible and repetitive. Finally, results of eABR measurements ruled out the involvement of the activation of ChR2 in the deafness, and therefore this impairment depended upon the reduction of the endolymphatic potential.
The acute-onset, reversible, and repeatable phenotype seems to at least partially resemble the symptom in a population of patients with sensorineural hearing loss. This animal model, of which optogenetic target is nonexcitable nonglial cell, may increase the repertoire of optogenetic approach and contribute to translational medicine of inner ear diseases.

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