SIX1 is required for adult cochlear sensory epithelium homeostasis and auditory function

The mammalian auditory sensory epithelium consists of hair cells (HCs), whose stereociliary bundles convert sound-evoked vibrations into electrical signals, and supporting cells (SCs), which provide mechanical stability. The transcription factor SIX1 is essential during otic neurosensory cell specification and HC commitment, but its contribution after birth has remained elusive. Here, we show that SIX1 is required for maintaining the structure and function of the adult cochlear sensory epithelium. Conditional deletion of Six1 in SCs or HCs of young adult mice causes profound deafness. SC-specific deletion disrupts epithelial organization and promotes SC loss with secondary HC degeneration, whereas HC-specific loss results in stereociliary-bundle defects and progressive HC death. To define early molecular defects, we performed single-nucleus RNA-seq before widespread degeneration and found early collapse of mature HC identity and functional gene programs, including mechanotransduction, stereociliary-bundle, and synaptic networks. SIX1 ChIP-seq analysis in the mature cochleamapped SIX1 occupancy at regulatory elements near genes involved in epithelial junctions, actin/cytoskeletal regulation, ion transport, mechanotransduction, and cell survival, and overlapped with a subset of Six1-dependent transcripts and deafness loci. Together, these results identify SIX1 as a key transcriptional regulator of adult cochlear homeostasis and reveal downstream gene networks that preserve sensory epithelial integrity and hearing throughout life.