VGLUT3-p.A211V variant fuses stereocilia bundles and elongates synaptic ribbons.

Key points The vesicular glutamate transporter type 3 (VGLUT3) loads glutamate into synaptic vesicles of auditory sensory cells, the inner hair cells (IHCs). The VGLUT3-p.A211V variant is associated with the human deafness DFNA25 Mutant mice carrying the VGLUT3-p.A211V variant show a progressive hearing loss. IHCs from mutant mice harbor distorted stereocilary bundles, which detect incoming sound-stimulation followed by oversized synaptic ribbons, which release glutamate onto the afferent nerve fibers. These results suggest that DFNA25 stems from the failure of auditory sensory cells to faithfully transduce acoustic cues into neural message. Abstract DFNA25 is an autosomal-dominant and progressive form of human deafness caused by mutations in the SLC17A8 gene, which encodes the vesicular glutamate transporter type 3 (VGLUT3). To resolve the mechanisms underlying DFNA25, we studied phenotypes of mice harboring the p.A221V mutation in human (corresponding to p.A224V in mouse). Using auditory brainstem response and distortion products otoacoustic emissions, we showed progressive hearing loss with intact cochlear amplification in the VGLUT3A224V/A224V mouse. The summating potential was reduced indicating the alteration of inner hair cells (IHCs) receptor potential. Scanning electron microscopy examinations demonstrated the collapse of stereocilia bundle in IHCs, leaving those from outer hair cells unaffected. In addition, IHC ribbon synapses underwent structural and functional modifications at later stages. Using super-resolution microscopy, we observed oversized synaptic ribbons and patch-clamp membrane capacitance measurements showed an increase in the rate of the sustained releasable pool of exocytosis. These results suggest that DFNA25 stems from a failure in the mechano-transduction followed by a change in synaptic transfer. The VGLUT3A224V/A224V mouse model opens the way to a deeper understanding and to a potential treatment of DFNA25. This article is protected by copyright. All rights reserved.