Synaptic and Functional Changes in Cochlear and Vestibular Hair Cells of Tmc Mutant Mice

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Project Summary/Abstract Transmembrane channel-like proteins 1 and 2 (TMC1 and TMC2) are required for sensory transduction in auditory and vestibular hair cells. Mice with targeted deletion of Tmc1 (Tmc1Δ/Δ) exhibit deafness and those lacking both (Tmc1Δ/ΔTmc2Δ/Δ) display deafness, vestibular dysfunction, and absent sensory transduction. Hair cells exclusively expressing Tmc1 have smaller single-channel conductance and lower calcium permeability than those expressing only Tmc2. While hair bundle morphology, mechanotransduction, and hair cell survival have been assessed in Tmc mutant mice, consequences of absent or impaired sensory transduction on the maintenance of ribbon synapses have not been examined. Cochlear and vestibular hair cells utilize these synapses to mediate synchronous release of glutamate-filled vesicles for temporally precise transmission of auditory cues. Absent or altered numbers of synaptic ribbons can contribute to abnormal synaptic transmission, impaired speech-in-noise discrimination, and perceptual anomalies like tinnitus. This research proposal seeks to characterize the synaptic and functional consequences of impaired/absent sensory transduction in the cochlea and vestibular organs. The focus of Aim 1 is to characterize inner hair cell synapses at various developmental timepoints in Tmc mutant mice and wildtype mice. Preliminary results demonstrate decreased synapse numbers in Tmc1Δ/Δ and Tmc1Δ/ΔTmc2Δ/Δ mice at later time points but not in Tmc2Δ/Δ mice, suggesting the absence of Tmc1 accounts for the observed changes in inner hair cell ribbon synapses. Tmc1 and Tmc2 are not only required for hearing but also necessary for normal vestibular function. Tmc1Δ/ΔTmc2Δ/Δ mice demonstrate abnormal behaviors including head-bobbing, inability to right from supine positions, and circling. While behavior has been characterized in Tmc mutant mice, the status of the vestibular periphery has not been examined in detail. The focus of Aim 2 is to analyze vestibular hair cell synapses in utricles and saccules of Tmc mutant mice and to directly assess vestibular function using vestibular sensory evoked potentials (VsEPs). Studies in recent years have verified the safety and efficacy of viral vectors for delivery into mouse cochlea and utilized these viral gene therapies to restore auditory function in mice lacking Tmc1. However, recovery of vestibular function and synapses via gene therapy has not yet been examined. The focus of Aim 3 is to assess synaptic and functional recovery in Tmc mutant mice following gene therapy by comparing data acquired after injections to those collected from Aims 1 and 2. Data generated from this proposal will provide novel insight into the mechanisms underlying ribbon synapse maintenance, enhance our understanding of the vestibular periphery, and inform continued development of gene therapies for restoration of hearing and balance.