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KCNQ4, a voltage-gated potassium channel, plays an important role in maintaining cochlear ion homoeostasis and regulating hair cell membrane potential, both essential for normal auditory function. Mutations in the KCNQ4 gene lead to DFNA2, a subtype of autosomal dominant non-syndromic deafness that is characterized by progressive sensorineural hearing loss across all frequencies. Despite recent advances in the identification of pathogenic KCNQ4 mutations, the molecular aetiology of DFNA2 remains unknown. We report here that decreased cell surface expression and impaired conductance of the KCNQ4 channel are two mechanisms underlying hearing loss in DFNA2. In HEK293T cells, a dramatic decrease in cell surface expression was detected by immunofluorescent microscopy and confirmed by Western blot for the pathogenic KCNQ4 mutants L274H, W276S, L281S, G285C, G285S, G296S and G321S, while their overall cellular levels remained normal. In addition, none of these mutations affected tetrameric assembly of KCNQ4 channels. Consistent with these results, all mutants showed strong dominant-negative effects on the wild-type ( WT) channel function. Most importantly, overexpression of HSP90β, a key component of the molecular chaperone network that controls the KCNQ4 biogenesis, significantly increased cell surface expression of the KCNQ4 mutants L281S, G296S and G321S. KCNQ4 surface expression was restored or considerably improved in HEK293T cells mimicking the heterozygous condition of these mutations in DFNA2 patients. Finally, our electrophysiological studies demonstrated that these mutations directly compromise the conductance of the KCNQ4 channel, since no significant change in KCNQ4 current was observed after KCNQ4 surface expression was restored or improved. [ABSTRACT FROM AUTHOR]

An outcome of the intricate K+ regulation in the cochlear duct is the endocochlear potential (EP), ∼80 mV, the "battery" that runs hair-cell transduction; however, the detailed molecular mechanisms for the generation of the EP remain unclear. We provide strong evidence indicating that the intermediate cells (ICs) of the stria vascularis (StV) express outward K+ current that rectifies inwardly at positive potentials. The channel belongs to the ether-a-go-go-related gene (erg) family of K+ channels. We cloned an ERG1a channel in the mouse inner ear (MERG1a). The cellular distribution of MERG1a in the cochlea displayed the highest levels of immunoreactivity in the ICs and modest reactivity in the marginal cells as well as in several extrastrial cells (e.g., hair cells). Functional expression of the StV-specific MERG1a channel reveals a current that activates at relatively negative potentials (approximately -50 mV) and shows rapid inactivation reflected as inward rectification at depolarized potentials. The current was sensitive to the methanesulfonanilide drug E-4031 (IC50, ∼165 nM) and the recombinant peptide rBeKm-1 (IC50, ∼16 nM), and the single-channel conductance in symmetrical K+ was ∼14 pS. The site of expression of MERG1a and its functional phenotype (e.g., modulation of the current by external K+) make it one of the most likely candidates for establishing the high throughput of K+ ions across ICs to generate EP. In addition, the property of the channel that produces marked K+ extrusion in increased external K+ may be important in shaping the dynamics of K+ cycling in the inner ear. [ABSTRACT FROM AUTHOR]

Multiple Ca2+ channels confer diverse functions to hair cells of the auditory and vestibular organs in the mammalian inner ear. We used gene-targeting technology to generate alpha1D Ca2+ channel-deficient mice to determine the physiological role of these Ca2+ channels in hearing and balance. Analyses of auditory-evoked brainstem recordings confirmed that alpha1D-/- mice were deaf and revealed that heterozygous (alpha1D+/-) mice have increased hearing thresholds. However, hearing deficits in alpha1D+/- mice were manifested mainly by the increase in threshold of low-frequency sounds. In contrast to impaired hearing, alpha1D-/- mice have balance performances equivalent to their wild-type littermates. Light and electron microscope analyses of the inner ear revealed outer hair cell loss at the apical cochlea, but no apparent abnormality at the basal cochlea and the vestibule. We determined the mechanisms underlying the auditory function defects and the normal vestibular functions by examining the Ba2+ currents in cochlear inner and outer hair cells versus utricular hair cells in alpha1D+/- mice. Whereas the whole-cell Ba2+ currents in inner hair cells consist mainly of the nimodipine-sensitive current (approximately 85%), the utricular hair cells express only approximately 50% of this channel subtype. Thus, differential expression of alpha1D channels in the cochlear and utricular hair cells confers the phenotype of the alpha1D null mutant mice. Because vestibular and cochlear hair cells share common features and null deletion of several genes have yielded both deafness and imbalance in mice, alpha1D null mutant mice may serve as a model to disentangle vestibular from auditory-specific functions. [ABSTRACT FROM AUTHOR]