From Cell Fate to Terminal Differentiation – Insights into Inner Ear Hair Cell Development from Cell Type-Specific Transcriptomic Analyses

Abstract

Hair cells (HCs), which are specialized neuroepithelial sensory cells of the inner ear, are responsible for conveying information about the environment, such as sound and movement, to the brain. In adult mammals, HCs do not spontaneously regenerate. Therefore, damage that occurs to HCs after noise or ototoxic drug exposure often leads to a permanent loss of inner ear function. It is estimated that 1/3 of individuals over the age of 65 experience hearing loss that could benefit from use of a hearing aid, representing a significant healthcare burden in the United States. It is for these reasons that the NIH National Institute on Deafness and Other Communication Disorders has set understanding the normal development of the auditory and vestibular systems as the Hearing and Balance Research Priority Area 1 of their 2017-2021 Strategic Plan, so that regenerative therapies can be better formulated. Here I have utilized cell type-specific ribosomal immunoprecipitation analyses to study HC gene expression, with the goal of furthering our understanding of their normal development. First, research into control of inner ear development has indicated that early HC differentiation appears to be dependent on the expression and normal functioning of the transcription factors ATOH1, POU4F3, and GFI1. While Pou4f3 and Gfi1 expression seems to be downstream of ATOH1, and the expression of Gfi1 is dependent on POU4F3, the signaling cascades downstream of GFI1 and its role in HC development are completely unknown. Here I have identified a critical role for GFI1 in HC maturation, namely through repression of a neuronal transcriptional program that is normally present in cochlear HCs during early embryonic development. Later in this developmental program, cochlear HC precursors must further differentiate into either inner or outer HCs, however, apart from the recently described INSM1 in embryonic development, no transcription factors have been implicated in the differentiation of outer HCs. It was therefore my second goal to identify and validate candidate downstream targets of helios, a newly identified postnatal outer HC transcription factor. Finally, I have developed and validated a model for ribosome immunoprecipitation and translatome analysis to study zebrafish HC gene expression – the Tg(myo6b:RiboTag) zebrafish.