Impairment of SLC17A8 encoding vesicular glutamate transporter-3, VGLUT3, underlies nonsyndromic deafness DFNA25 and inner hair cell dysfunction in null mice.

Autosomal-dominant sensorineural hearing loss is genetically heterogeneous, with a phenotype closely resembling presbycusis, the most common sensory defect associated with aging in humans. We have identified SLC17A8, which encodes the vesicular glutamate transporter-3 (VGLUT3), as the gene responsible for DFNA25, an autosomal-dominant form of progressive, high-frequency nonsyndromic deafness. In two unrelated families, a heterozygous missense mutation, c.632C-->T (p.A211V), was found to segregate with DFNA25 deafness and was not present in 267 controls. Linkage-disequilibrium analysis suggested that the families have a distant common ancestor. The A211 residue is conserved in VGLUT3 across species and in all human VGLUT subtypes (VGLUT1-3), suggesting an important functional role. In the cochlea, VGLUT3 accumulates glutamate in the synaptic vesicles of the sensory inner hair cells (IHCs) before releasing it onto receptors of auditory-nerve terminals. Null mice with a targeted deletion of Slc17a8 exon 2 lacked auditory-nerve responses to acoustic stimuli, although auditory brainstem responses could be elicited by electrical stimuli, and robust otoacoustic emissions were recorded. Ca(2+)-triggered synaptic-vesicle turnover was normal in IHCs of Slc17a8 null mice when probed by membrane capacitance measurements at 2 weeks of age. Later, the number of afferent synapses, spiral ganglion neurons, and lateral efferent endings below sensory IHCs declined. Ribbon synapses remaining by 3 months of age had a normal ultrastructural appearance. We conclude that deafness in Slc17a8-deficient mice is due to a specific defect of vesicular glutamate uptake and release and that VGLUT3 is essential for auditory coding at the IHC synapse.

EYA1 expression in the developing inner ear.

OBJECTIVES: We sought to determine the developmental anatomy and EYA1 protein distribution in the inner ear of Xenopus laevis. METHODS: Xenopus laevis embryos were stained with monoclonal antibodies and imaged with confocal microscopy. RESULTS: At stage 27, the otocyst fully forms, with strong tubulin staining of early sensory cells at its ventromedial aspect. Neuronal ingrowth follows at stage 33/34. At stage 50, the semicircular canals are complete. EYA1 localizes to the anterior aspect of the otocyst from stages 37 to 44. By stage 50, EYA1 distribution is localized primarily to the sensory maculae and the endolymphatic duct of the developing inner ear. CONCLUSIONS: Whole mount confocal imaging of the developing Xenopus inner ear delineates the exact timing of otic development, sensory cell differentiation, and innervation. EYA1 protein expression has a distinct distribution pattern at the anterior aspect of the developing otocyst in stages 41 and 44. Later stages have a more localized pattern, in which EYA1 is detected only in the sensory epithelium and endolymphatic duct. This specific pattern of expression indicates a possible role in the determination of the anterior-posterior orientation of the inner ear, as well as a later role in sensory cell differentiation.