Otoferlin Is Required for Proper Synapse Maturation and for Maintenance of Inner and Outer Hair Cells in Mouse Models for DFNB9.

Ursula Stalmann,Nicola Strenzke,Hanan Al-Moyed,Ellen Reisinger,Albert Justin Franke

doi:10.3389/fncel.2021.677543

Abstract

Deficiency of otoferlin causes profound prelingual deafness in humans and animal models. Here, we closely analyzed developmental deficits and degenerative mechanisms in Otof knock-out (Otof–/–) mice over the course of 48 weeks. We found otoferlin to be required for proper synapse development in the immature rodent cochlea: In absence of otoferlin, synaptic pruning was delayed, and postsynaptic boutons appeared enlarged at 2 weeks of age. At postnatal day 14 (P14), we found on average ∼15 synapses per inner hair cell (IHC) in Otof–/– cochleae as well as in wild-type controls. Further on, the number of synapses in Otof–/– IHCs was reduced to ∼7 at 8 weeks of age and to ∼6 at 48 weeks of age. In the same period, the number of spiral ganglion neurons (SGNs) declined in Otof–/– animals. Importantly, we found an age-progressive loss of IHCs to an overall number of 75% of wildtype IHCs. The IHC loss more prominently but not exclusively affected the basal aspects of the cochlea. For outer hair cells (OHCs), we observed slightly accelerated age-dependent degeneration from base to apex. This was associated with a progressive decay in DPOAE amplitudes for high frequency stimuli, which could first be observed at the age of 24 weeks in Otof–/– mice. Our data will help to plan and predict the outcome of a gene therapy applied at various ages of DFNB9 patients.

Highlights

Mutations in the gene OTOF encoding for the protein otoferlin cause hearing impairment with autosomal recessive inheritance, DFNB9 (Yasunaga et al, 1999)
At the age of 8 weeks, Otof−/− mice display a near-normal complement of a single row of inner hair cell (IHC) and three rows of outer hair cells (OHCs) over the entire cochlea (Figures 1C,D)
While OHC numbers remain normal in the apical aspects of the cochlea in both genotypes, there is age-dependent loss of OHCs progressing from the base toward the middle of the cochlea, which is accelerated in Otof knock-out (Otof−/−)mice compared to Otof +/+ controls

Summary

Introduction

Mutations in the gene OTOF encoding for the protein otoferlin cause hearing impairment with autosomal recessive inheritance, DFNB9 (Yasunaga et al, 1999). The deficiency in otoferlin causes a defect in synaptic transmission from auditory sensory cells, the inner hair cells (IHCs), to the SGNs (Roux et al, 2006; Pangrsic et al, 2010; Strenzke et al, 2016; Michalski et al, 2017). OHCs, the active elements of the cochlear amplifier, are intact. This can be readily assessed by recordings of otoacoustic emissions (OAE). Due to the presence of OAEs but the absence of signal transmission in the auditory pathway, this form of hearing impairment was classified as “auditory neuropathy,” and since identification of the primary defect at the level of IHC synapses more precisely as “auditory synaptopathy.”

Methods

Results

Conclusion