Mutations in the potassium channel gene KCNQ4 underlie DFNA2, an autosomal dominant form of progressive hearing loss in humans. In the mouse cochlea, the transcript has been found exclusively in the outer hair cells. By using specific antibodies, we now show that KCNQ4 is situated at the basal membrane of these sensory cells. In the vestibular organs, KCNQ4 is restricted to the type I hair cells and the afferent calyx-like nerve endings ensheathing these sensory cells. Several lines of evidence suggest that KCNQ4 underlies the I _K,n and g _K,L currents that have been described in the outer and type I hair cells, respectively, and that are already open at resting potentials. KCNQ4 is also expressed in neurons of many, but not all, nuclei of the central auditory pathway, and is absent from most other brain regions. It is present, e.g., in the cochlear nuclei, the nuclei of the lateral lemniscus, and the inferior colliculus. This is the first ion channel shown to be specifically expressed in a sensory pathway. Moreover, the expression pattern of KCNQ4 in the mouse auditory system raises the possibility of a central component in the DFNA2 hearing loss.

Autosomal recessive genetic forms (DFNB) account for most cases of profound congenital deafness. Adeno-associated virus (AAV)-based gene therapy is a promising therapeutic option, but is limited by a potentially short therapeutic window and the constrained packaging capacity of the vector. We focus here on the otoferlin gene underlying DFNB9, one of the most frequent genetic forms of congenital deafness. We adopted a dual AAV approach using two different recombinant vectors, one containing the 5′ and the other the 3′ portions of otoferlin cDNA, which exceed the packaging capacity of the AAV when combined. A single delivery of the vector pair into the mature cochlea of Otof^−/− mutant mice reconstituted the otoferlin cDNA coding sequence through recombination of the 5′ and 3′ cDNAs, leading to the durable restoration of otoferlin expression in transduced cells and a reversal of the deafness phenotype, raising hopes for future gene therapy trials in DFNB9 patients. Significance In humans, inner ear development is completed in utero, with hearing onset at ∼20 weeks of gestation. However, genetic forms of congenital deafness are typically diagnosed during the neonatal period. Gene therapy approaches in animal models should therefore be tested after the period of hearing onset, to determine whether they can reverse an existing deafness phenotype. Here, we used a mouse model of DFNB9, a human deafness form accounting for 2–8% of all cases of congenital genetic deafness. We show that local gene therapy in the mutant mice not only prevents deafness when administered to immature hearing organs, but also durably restores hearing when administered at a mature stage, raising hopes for future gene therapy trials in DFNB9 patients.

Cochlear inner hair cells (IHCs), the mammalian auditory sensory cells, encode acoustic signals with high fidelity by Graded variations of their membrane potential trigger rapid and sustained vesicle exocytosis at their ribbon synapses. The kinetics of glutamate release allows proper transfer of sound information to the primary afferent auditory neurons. Understanding the physiological properties and underlying molecular mechanisms of the IHC synaptic machinery, and especially its high temporal acuity, which is pivotal to speech perception, is a central issue of auditory science. During the past decade, substantial progress in high-resolution imaging and electrophysiological recordings, as well as the development of genetic approaches both in humans and in mice, has produced major insights regarding the morphological, physiological, and molecular characteristics of this synapse. Here we review this recent knowledge and discuss how it enlightens the way the IHC ribbon synapse develops and functions.

Defects in myosin VIIA are responsible for deafness in the human and mouse. The role of this unconventional myosin in the sensory hair cells of the inner ear is not yet understood. Here we show that the C‐terminal FERM domain of myosin VIIA binds to a novel transmembrane protein, vezatin, which we identified by a yeast two‐hybrid screen. Vezatin is a ubiquitous protein of adherens cell–cell junctions, where it interacts with both myosin VIIA and the cadherin–catenins complex. Its recruitment to adherens junctions implicates the C‐terminal region of α‐catenin. Taken together, these data suggest that myosin VIIA, anchored by vezatin to the cadherin–catenins complex, creates a tension force between adherens junctions and the actin cytoskeleton that is expected to strengthen cell–cell adhesion. In the inner ear sensory hair cells vezatin is, in addition, concentrated at another membrane–membrane interaction site, namely at the fibrillar links interconnecting the bases of adjacent stereocilia. In myosin VIIA‐defective mutants, inactivity of the vezatin–myosin VIIA complex at both sites could account for splaying out of the hair cell stereocilia.