Developing Fast, Red-Light Optogenetic Stimulation of Spiral Ganglion Neurons for Future Optical Cochlear Implants.

Antoine Tarquin Huet,Vladan Rankovic,Tobias Dombrowski,Tobias Moser,Anupriya Thirumalai

doi:10.3389/fnmol.2021.635897

Abstract

Optogenetic stimulation of type I spiral ganglion neurons (SGNs) promises an alternative to the electrical stimulation by current cochlear implants (CIs) for improved hearing restoration by future optical CIs (oCIs). Most of the efforts in using optogenetic stimulation in the cochlea so far used early postnatal injection of viral vectors carrying blue-light activated channelrhodopsins (ChRs) into the cochlea of mice. However, preparing clinical translation of the oCI requires (i) reliable and safe transduction of mature SGNs of further species and (ii) use of long-wavelength light to avoid phototoxicity. Here, we employed a fast variant of the red-light activated channelrhodopsin Chrimson (f-Chrimson) and different AAV variants to implement optogenetic SGN stimulation in Mongolian gerbils. We compared early postnatal (p8) and adult (>8 weeks) AAV administration, employing different protocols for injection of AAV-PHP.B and AAV2/6 into the adult cochlea. Success of the optogenetic manipulation was analyzed by optically evoked auditory brainstem response (oABR) and immunohistochemistry of mid-modiolar cryosections of the cochlea. In order to most efficiently evaluate the immunohistochemical results a semi-automatic procedure to identify transduced cells in confocal images was developed. Our results indicate that the rate of SGN transduction is significantly lower for AAV administration into the adult cochlea compared to early postnatal injection. SGN transduction upon AAV administration into the adult cochlea was largely independent of the chosen viral vector and injection approach. The higher the rate of SGN transduction, the lower were oABR thresholds and the larger were oABR amplitudes. Our results highlight the need to optimize viral vectors and virus administration for efficient optogenetic manipulation of SGNs in the adult cochlea for successful clinical translation of SGN-targeting gene therapy and of the oCI.

Highlights

Sensorineural hearing impairment, the most common form of hearing impairment, is typically characterized by a dysfunction or loss of hair cells transducing sound signals and generating a neural code at their synapses with spiral ganglion neurons (SGNs)
10 weeks after injection, the SGN density and their expression of f-Chrimson-enhanced yellow fluorescent protein (EYFP) were analyzed by confocal microscopy of mid-modiolar cryosections immunolabeled for GFP and calretinin (Figure 1A and Supplementary Figure S1A) in both groups, regardless of the presence or absence of optically evoked auditory brainstem response (oABR)
We used the Mongolian gerbil as preclinical model to further develop f-Chrimson-mediated optogenetic stimulation of SGNs

Summary

Introduction

Sensorineural hearing impairment, the most common form of hearing impairment, is typically characterized by a dysfunction or loss of hair cells transducing sound signals and generating a neural code at their synapses with spiral ganglion neurons (SGNs). The electrical cochlear implant (eCI) directly stimulates SGNs electrically (Zeng et al, 2008; Lenarz, 2018). Hearing in more complex acoustic scenes such as speech in noise or music is impeded by the wide spread of electrical current from each electrode contact, recruiting a large portion of SGNs and limiting the maximum number of independent channels to ≤10 (Kral et al, 1998; Zeng and Galvin, 1999; Friesen et al, 2001; Middlebrooks, 2005)

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