The Notch ligand Jagged1 plays a dual role in cochlear hair cell regeneration.

Hair cells within the inner ear cochlea are specialized mechanoreceptors required for hearing. Hair cells are not regenerated in mammals, and their loss is a leading cause of deafness in humans. Cochlear supporting cells in newborn mice have the capacity to regenerate hair cells, but persistent Notch signaling, presumably activated by the Notch ligand Jagged1, prevents supporting cells from converting into hair cells. Employing a cochlear organoid platform, we show that while Jagged1 participates in hair cell-fate repression, Jagged1's primary function is to preserve the progenitor-like characteristics of supporting cells. Transcriptomic and mechanistic studies reveal that Jagged1/Notch signaling maintains progenitor and metabolic gene expression in supporting cells and sustains pro-growth pathways, including phosphoinositide-3-kinase/Akt /mammalian target of rapamycin signaling, a function that is Notch1 and Notch2-receptor mediated. Finally, we show that Jagged1/Notch signaling stimulation with Jagged1-Fc peptide enhances the hair cell-forming capacity of supporting cells in cochlear explants and in vivo.