Engineering olivocochlear inhibition to reduce acoustic trauma

**Background:** Age-related hearing loss (presbycusis) affects one in three people aged 65–74 and nearly half of those over 75. Efferent brain-stem neurons release acetylcholine to inhibit cochlear hair cells and can protect the inner ear from acoustic trauma. This protection is absent in α9-knockout mice and stronger in mice with a gain-of-function point mutation (α9L9′T) of the hair cell nicotinic acetylcholine receptor. The present study explores whether viral transduction of α9L9′T can restore acoustic prophylaxis in α9-null mice.

**Methods:** α9-null mice (C57BL/6J background) were injected at postnatal days 0–2 (P0–P2) via the posterior semicircular canal with AAV2.7m8 virus carrying either GFP alone or α9L9′T plus GFP (1:1 ratio, ~10^13 GC/mL). Control groups included uninjected mice and mice injected with GFP-only virus. At ~4 weeks of age, baseline auditory brain-stem responses (ABRs) were recorded. Two days later, awake mice were exposed to 90 dB SPL broadband noise (2–20 kHz) for 2 hours to induce a temporary threshold shift. ABRs were repeated 1 day and 14 days post-exposure. Synaptic labeling was performed 14–15 days post-trauma (P45–P46) using antibodies against CtBP2 (ribbons), GluA2/3 (postsynaptic AMPAR clusters), and synaptophysin (efferent terminals). Live tissue from a subset of mice was labeled with Cy3-RgIA-5727, a fluorophore-conjugated conopeptide that binds specifically to α9-containing nAChRs, to visualize transgene expression. Statistical analyses included multiple unpaired t-tests with Welch correction, two-way ANOVA, and mixed-effects linear models (REML).

**Key Results:** Cy3-RgIA-5727 labeling revealed bright puncta at the base of outer hair cells (OHCs) in α9L9′T-injected mice, confirming postsynaptic expression of the transgene. Baseline ABR thresholds in α9L9′T-injected mice were on average 10 dB higher than in uninjected or GFP-injected controls (p < 0.0001 for both comparisons, mixed-effects model). One day post-trauma, threshold shifts were significantly smaller in α9L9′T-injected mice compared to controls (p = 0.0013 vs. uninjected; p < 0.0001 vs. GFP-injected). For example, at 16 kHz the shift was 12.1 dB in α9L9′T-injected mice vs. 21.9 dB in uninjected and 20.2 dB in GFP-injected mice. At 14 days, thresholds in α9L9′T-injected mice remained significantly higher than controls (p < 0.0001 vs. uninjected; p = 0.0004 vs. GFP-injected), indicating sustained protection. Wave 1 amplitude of the ABR (evoked by loud clicks/tones) fell by ~50% in all groups 1 day post-trauma, but recovered to near baseline only in α9L9′T-injected mice by day 14 (ratio of day 14 to pre-trauma amplitude ~1.0 vs. ~0.5 in controls; p < 0.0001 vs. GFP-injected, p = 0.0116 vs. uninjected). Wave 1 latency increased less in α9L9′T-injected mice after trauma, though differences among groups were not significant. IHC ribbon synapse counts (colocalized CtBP2/GluA2/3 puncta) ranged from 6 to 18 per IHC. No significant synapse loss was observed in any group except at 32 kHz in GFP-injected cochleas. OHC ribbons showed a tonotopic gradient in controls (more apical, fewer basal) that was absent in α9L9′T-injected mice; after trauma, α9L9′T-injected cochleas had significantly larger efferent synaptic volumes on OHCs (p = 0.0004, two-way ANOVA).

**Clinical Implications:** This study provides proof of principle that gene therapy to enhance efferent cochlear feedback can reduce temporary noise-induced hearing loss and promote recovery of afferent signaling. The approach leverages the ear's own sound-evoked protective mechanism, offering frequency- and intensity-dependent protection that could complement existing strategies such as ear coverings, hearing aids, or small-molecule therapies. Future work must assess efficacy in wild-type mice, evaluate long-term expression, and test behavioral hearing in complex acoustic environments. If successful, this strategy could benefit individuals with genetic predisposition to early-onset hearing loss, those with unavoidable noise exposure (e.g., military, industrial), and potentially those with hyperacusis or tinnitus.