Project Summary / Abstract The mouse cochlea harbors only 800 inner hair cells and about three times as many outer hair cells. This low number of cells has hampered progress specifically in the study of the molecular biology of inner ear cells. The use of single hair cell gene expression profiling has the potential to turn this disadvantage into a major advantage because it allows for oversampling the organ by generating high resolution quantitative gene expression maps for thousands of inner and outer hair cells of the organ of Corti, which is the first Aim of this grant application. It is anticipated that these maps, based on single cell RNA-Seq data will miss little information when compared for example with other sensory systems such as the retina, where analysis of a few thousand photoreceptor cells would represent <0.05% of the sensory cell population. It is anticipated that the maps will reveal how gradients of gene expression contribute to functional tonotopy in inner and outer hair cells. Moreover, the generated maps will serve as base line for identifying distinct changes in cochlear hair cells after noise-induced temporary threshold shift. Here, it is anticipated that the analysis will reveal functional modules of co-regulated hair bundle genes as well as candidate gene regulatory networks for susceptibility to noise-induced hearing loss. A second series of experiments aims to identify the molecular mechanisms by which non-traumatic sound exposure temporarily reduces susceptibility for permanent noise induced threshold shift. This research has the potential of elucidating genes and mechanisms involved in susceptibility to noise. Finally, it is proposed to investigate at the molecular level compensatory changes in inner and outer hair cells in response to sustained changes in cochlear integration such as lack of functional afferent or efferent innervation or lack of connection between outer hair cells and the tectorial membrane. Beside identification of co-regulated gene groups for example involved in cochlear amplification, it is expected that this research will reveal how hair cells are affected by pathological situations that do not trigger immediate hair cell loss.