Behavioral and electrophysiological assessment of hearing function in mice with deficient sound encoding at inner hair cell ribbon synapse

Abstract

Auditory synaptopathy in humans is characterized by poor speech perception especially in noisy environments despite of preserved active cochlear amplification. The ABR of affected patients is absent or shows a significant reduction of wave I amplitude caused by abnormal function of inner hair cell ribbon synapses. Here, I characterized auditory function in mouse models of auditory synaptopathy by comparing electrophysiological (ABR, ASSR, and DPOAE) and behavioral experiments (acoustic startle reflexes, operant conditioning in the shuttle box and Intellicage). The best behavioral results were obtained in the Intellicage, which produced consistent repeatable results in thresholds, gap detection, and frequency discrimination experiments. The subsequent testing of the same mice in the threshold and threshold in noise experiment was possible and demonstrated the expected masking effect in WT animals. In contrast, in Ribeye mutants, the introduction of background noise resulted in a decrease in discrimination ability. Ribeye KO mice: Analysis of ABR traces recorded from Ribeye KO revealed a significant reduction in wave I and III amplitudes, suggesting impaired activation of the auditory nerve and superior olivary complex. The behavioral threshold estimation in quiet for the Ribeye KO mice was comparable to the mild threshold shift seen in their ABRs. Introduction of noise had the expected masking effect in WT and heterozygous mice, while Ribeye KO mice showed a severe deterioration in discrimination performance when noise was introduced. Auditory phenotype of OtofQX mice and Otof2M mutant mice: OtofQX -/- mice showed absence of ABR waves with preservation of the summating potential. The hearing deficits in OtofQX -/- mice resembled the auditory phenotype described in human patients with the same mutation, making this mouse model a good candidate for gene therapy trials. Otof2M mutants exhibited a moderate hearing deficit with a significant amplitude reduction in ABR waves I and III. Otof2M mutants showed an impaired gap detection ability in comparison to their WT littermates. Such pathology could explain the deficient speech perception in patients with OTOF mutations. Otof -/- rescue: Viral gene therapy partially restored hearing function in Otof -/- mice with clearly distinguishable ABR waves II, IV and V and partial restoration of wave I and III, achieving click thresholds of ~ 60 dB SPL in 66% of Otof -/- mice injected with AAV-PHP.B or AAV-PHP.eB carrying DNA coding full-length otoferlin. Rescued Otof -/- mice were capable of detecting sound stimuli in the Intellicage. Results suggested that viral gene therapy of DFNB9 using a single overloaded AAV vector is feasible and might reduce the complexity of gene transfer when compared to dual-AAV approaches. CAPS exon 2 deletion mice: Deletion of CAPS1 exon2 was detected in a complex human phenotype in patients suffering from bipolar disorder. In mice, this mutation caused no obvious hearing impairment as evidenced by DPOAE and ABR findings; preserved ABR thresholds and comparable amplitude to their WT littermates. Interestingly, there was a reduction in ASSR amplitudes at lower modulation frequencies (110-400). Those findings suggest a role for CAPS1 protein in auditory brainstem neurons. ABR results for mutants with an auditory synaptopathy phenotype showed a reduction in the amplitude of ABR waves I and III in several mutant mouse lines (Ribeye, Otof2M, Otit CBA and Otof rescue). The reduction was less obvious for waves II and IV which might be explained by central compensatory mechanisms augmenting their response.