Project Summary/Abstract: Sensory hair cell regeneration does not occur spontaneously in the mature mammalian organ of Corti, making hearing loss permanent. The goal of this proposal is to identify the causes and mechanisms behind the failure of hair cell regeneration, as well as ways to stimulate regeneration in surviving populations of inner ear supporting cells in deafened individuals. Our primary hypothesis is that regeneration requires the re- engagement of developmental gene networks to guide supporting cells to a hair cell fate, and that during postnatal inner ear maturation, epigenetic barriers arise that block the re-activation of these gene networks. The goal is to develop methods to overcome these epigenetic barriers, and to establish new cell fates with regenerative potential within the organ of Corti. Experimentally, perinatal mice retain a latent capacity for direct supporting cell transdifferentiation to a hair cell-like fate, but this capacity is rapidly lost in the first weeks after birth. This age-dependent change in regenerative potential provides a window through which to investigate the transition from a permissive to a non-permissive state for this form of regeneration in the normally maturing organ of Corti. The work of this proposal is to elucidate the mechanism(s) of epigenetic control of transdifferentiation in perinatal supporting cells (Aim1) and to identify the machinery of maturation-related changes in epigenetic/chromatin structure in supporting cells of the inner ear. (Aim 2). We hypothesize that these changes are responsible for the failure of regeneration. Finally, to investigate the epigenetic structure of adult supporting cell chromatin in normal and deafened mice (Aim 3). We hypothesize that manipulation of the epigenetic state is an important approach for future regenerative medicine approaches to restoring lost hair cells.