Thyroid hormone augmentation accelerates hair cell maturation in human cochlear organoids

Outer and inner hair cells in the cochlea are essential for hearing, but are vulnerable to damage by genetic mutations and environmental insults, resulting in irreversible sensorineural hearing loss. Investigating these processes requires access to large numbers of human cochlear hair cells and the ability to precisely control genetic, biochemical, and environmental conditions. We recently established a new organoid system that recapitulates human cochlear differentiation. However, maturation of cochlear organoid cultures is inefficient, requiring several months to express the mature outer hair cell marker Prestin. To efficiently obtain fully functional outer hair cells, we tested the effects of the thyroid hormone thyroxine on hair cell maturation in human cochlear organoids. Thyroxine supplementation significantly increased the percentages of Prestin-positive hair cells in an age-dependent manner. Additionally, transcriptomic analyses revealed upregulation of mature outer hair cell genes and downregulation of immature hair cell genes in thyroxine-treated hair cells. Furthermore, some of the treated hair cells exhibited electromotility, a functional hallmark of mature outer hair cells. These results suggest that thyroid hormones accelerate the maturation of hair cells in human cochlear organoids, thereby useful for establishing an improved in vitro model for pre-clinical investigation on sensorineural hearing loss.