Roles of the Synapse in Hair-Cell Pathology

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Project Summary: Loud or extended noise exposure damages cochlear hair cells often resulting in either loss of synaptic connections with auditory nerve fibers or hair-cell death. One mechanism for this damage occurs through overstimulation of hair cells, which leads to a surge of Ca2+ at hair-cell synapses, metabolic stress on the hair cell, and excessive release of the neurotransmitter glutamate. The overall goal of this proposal is to understand how pathological changes at the hair-cell synapse stemming from excessive noise exposure ultimately contribute to noise-induced damage. Our objectives are to 1) test the hypothesis that hair-cell overstimulation triggers synaptic-ribbon loss, 2) determine the role of glutamate signaling in hair-cell pathology and repair of acoustic overexposure, and 3) identify the downstream effectors of glutamate-receptor mediated hair-cell death. Current gaps in our understanding of how hair-cell synapse overstimulation contributes to hair- cell damage are in large part due our inability to follow the time course of dynamic intracellular processes in noise-exposed hair cells and to separate glutamate induced damage to hair-cells versus innervating nerve terminals in mammalian cochlea. This project will circumvent these issues by investigating noise-induced hair-cell damage using live-cell imaging, tissue-specific genetic ablation, and pharmacological tools in the zebrafish lateral line— a mechanosensory organ which is made up of clusters of innervated hair cells. Zebrafish lateral-line hair cells are similar to mammalian hair cells at the molecular and cellular level, but are pharmacologically and optically accessible within the whole larvae. This feature allows for direct environmental manipulation and phenotypic observations in a live, intact preparation, which is currently not feasible in the mammalian cochlea. The results of each of our objectives will reveal how specific pathological changes at hair-cell synapses contribute to multiple pathologies resulting from excess noise exposure, thereby furthering our understanding of the underlying causes of sensorineural hearing loss.