Divide and convert: a decoupled mechanism for hair cell regeneration in the zebrafish inner ear

Abstract

Loss of hearing and balance in humans presents a massive and increasing global health burden. In humans and other mammals, dysfunction of the auditory and vestibular systems is often caused by the death of mechanosensory hair cells in the inner ear, which adult mammals have a limited ability to regenerate. In contrast, non-mammalian vertebrates including zebrafish can robustly regenerate functional hair cells throughout life. Zebrafish hair cell regeneration has been studied extensively in the lateral line, an external sensory system with hair cells and associated supporting cells that are analogous to cells of the inner ear. In comparison, the zebrafish inner ear has been understudied, despite being a highly tractable and regenerative model system that is highly conserved with the inner ear of other vertebrates. The work presented in this dissertation aims to characterize cell types in the zebrafish inner ear and determine when and how zebrafish inner ear hair cells are regenerated. First, single-cell RNA sequencing was used to describe multiple hair and supporting cell subtypes in the zebrafish inner ear, and fluorescent in situ hybridization was used to validate the identities of these cell types in vivo. Addition during growth and regeneration of both hair cell subtypes was then quantified during the larval phase of growth. Finally, by using a marker of cell division, the broad mechanism of regeneration was determined to be a two-step, decoupled process in which supporting cells divide to produce hair cell precursors and, independently, transdifferentiate into new hair cells. This work has substantially advanced the zebrafish inner ear as a model system for studying hair cell regeneration that will be of immense value in the effort to facilitate hearing and vestibular restoration in mammals.