Molecular and functional characterization of type I and II vestibular hair cells in adult mice

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Project Abstract The sensory receptors for mammalian vestibular organs, called hair cells, encode head movements and relay information along the vestibular nerve to the brainstem and cerebellum. Mammals have two types of hair cell, type I and type II, which different morphological and physiological properties. Type I and II hair cells also have distinct forms of afferent innervation that are differentially distributed across the epithelial zones. Vestibular hair cells die during normal aging and after exposure to ototoxins, and this loss can lead to profound vestibular deficits. After vestibular hair cell damage in adult mice, supporting cells regenerate ~50% of type ll hair cells, but no type I hair cells are replaced. This natural hair cell regeneration does not restore the vestibular system to normal function, as measured by the vestibulo-ocular reflex and other behavioral tests. One interpretation of this finding is that type I hair cells must be replaced in mammals for vestibular function to return. While this is tempting to speculate, there is little evidence that type I hair cells are required for specific vestibular functions. Indeed, we understand little about the respective contributions of type I and II hair cells to our vestibular sense. One step toward solving this puzzle is to use molecular biology to better understand the unique features of each hair cell type and to determine if subtypes of type I and II hair cells exist. Working with Drs. Neil Segil and Litao Tao, I found from single cell RNA sequencing that there seem to be four molecularly distinct populations of hair cells in adult mouse utricles: two type l-like populations, one type ll-like population, and one unknown group of hair cells. In Aim 1, I will continue these studies, examining expression of candidate cell-specific genes in all vestibular organs to determine if four distinct hair cell groups are identifiable in each organ and how these groups are distributed across epithelial zones. I will generate novel insights into hair cell features across vestibular organs, classify new markers for each cell type, and identify genes that may be used to drive cell-selective gene misexpression in future studies. Another step toward testing the different functions of type I and II hair cells is to remove each hair cell population and assess impacts on vestibular function. In Aim 2, I will use CreLoxP technology to ablate all type I hair cells or peripheral type I hair cells in all adult vestibular organs. I will determine the impact of these ablations upon animal behaviors and brainstem electrical responses to vestibular stimuli, which to my knowledge has not been done before in adult mice. I expect to gain new insights into the general function of type I hair cells and the specific functions of type I hair cells in each epithelial zone. Overall, this research provides new information about properties and functions of type I and II hair cells and inform on new therapies to treat balance disorders. Further, this project enables me to receive top-notch research training in a collaborative setting using cutting-edge scientific approaches.