SUMMARYMost forms of hearing loss are associated with loss of cochlear outer hair cells (OHCs). OHCs require the tectorial membrane (TM) for stereociliary bundle stimulation (forward transduction) and active feedback (reverse transduction). Alpha tectorin is a protein constituent of the TM and the C1509G mutation in alpha tectorin in humans results in autosomal dominant hearing loss. We engineered and validated this mutation in mice and found that the TM was shortened in heterozygous Tecta C1509G/+ mice, reaching only the first row of OHCs. Thus, deficient forward transduction renders OHCs within the second and third rows non-functional, producing partial hearing loss. Surprisingly, both Tecta C1509G/+ and Tecta C1509G/C1509G mice were found to have increased reverse transduction as assessed by sound-and electrically-evoked otoacoustic emissions. We show that an increase in prestin, a protein necessary for electromotility, in all three rows of OHCs underlies this phenomenon. This mouse model demonstrates a human hearing loss mutation in which OHC function is altered through a non-cell-autonomous variation in prestin.

Mutations within OTOF encoding otoferlin lead to a recessive disorder called DFNB9. Several studies have indicated otoferlin's association with ribbon synapses of cochlear sensory hair cells, as well as data showing the protein's presence in neurons, nerve fibers and hair cells, suggesting a more ubiquitous function. Otoferlin's co-localization not only with ribbon synaptic proteins, but also with additional endosomal (EEA1) or Golgi proteins (GM130) were motivation for a search for further binding partners of otoferlin by a yeast two-hybrid screen in a rodent cochlear cDNA library (P3-P15). This screen identified Rab8b GTPase as a novel interacting partner, substantiated by transient co-expression and co-localization in HEK 293 cells and co-immunoprecipitation of the complex using tagged proteins in vitro and native proteins from cochlea. This finding implies that otoferlin could be a part of components contributing to trans-Golgi trafficking.

Background: Genes evolved in mammals for specialization of hearing. Results: CEA cell adhesion molecule 16 (CEACAM16) is a structural component of the tectorial membrane and necessary for hearing at low and high frequencies. Conclusion: CEACAM16 has evolved in mammals to broaden the auditory frequency range. Significance: Mutation of CEACAM16 is responsible for human autosomal dominant hearing loss (DFNA4).

Thyroid hormone receptor ␤ (TR␤) dysfunction leads to deafness in humans and mice. Deafness in TR␤Ϫ/Ϫ mutant mice has been attributed to TR␤-mediated control of voltage-and Ca 2ϩ -activated K ϩ (BK) channel expression in inner hair cells (IHCs). However, normal hearing in young constitutive BK␣ Ϫ/Ϫ mutants contradicts this hypothesis. Here, we show that mice with hair cell-specific deletion of TR␤ after postnatal day 11 (P11) have a delay in BK␣ expression but normal hearing, indicating that the origin of hearing loss in TR␤ Ϫ/Ϫ mutant mice manifested before P11. Analyzing the phenotype of IHCs in constitutive TR␤ Ϫ/Ϫ mice, we found normal Ca 2ϩ current amplitudes, exocytosis, and shape of compound action potential waveforms. In contrast, reduced distortion product otoacoustic emissions and cochlear microphonics associated with an abnormal structure of the tectorial membrane and enhanced tectorin levels suggest that disturbed mechanical performance is the primary cause of deafness resulting from TR␤ deficiency.

Mutations in GJB2, a gene encoding the gap junction protein connexin 26 (Cx26), are a major cause for inherited and sporadic non-syndromic hearing loss, albeit with highly variable clinical effects. To determine new mutations and their frequencies in a Southern Egyptian population restriction fragment length polymorphism, gene sequencing, and single strand conformational polymorphism revealed only 2 mutations for GJB2: c.35delG and p.I71N. The allelic frequency of the c.35delG mutation was 8.7% (found in 27 out of 310 investigated alleles) resulting in a relatively low carrier frequency (1.6%) in Upper Egypt. The new mutation, a substitution of isoleucin (I) (a non-polar amino acid) by the polar amino acid asparagin (N), was localized within the conserved Cx26 structure. The functional significance of p.I71N was tested by injection of cRNA into Xenopus laevis oocytes. Cx26 hemi-channel activity was measured by depolarization activated conductance in non-coupled oocytes. As a result, the p.I71N mutated channel was non-functional. The study discloses a novel, functionally relevant GJB2 mutation and defines the contribution of Cx26 alterations to the hearing loss in the Southern Egyptian population.