Mammalian hair cells do not regenerate, and their loss is a major cause of deafness. We recently identified leucine-rich repeat containing, G-protein-coupled receptor 5 (Lgr5)-expressing cochlear supporting cells with the capacity for self-renewal and hair cell differentiation in vitro. We found that these cells, a subset of cochlear supporting cells, were responsive to Wnt signaling. Here we asked whether these Lgr5-positive cells, despite their lack of contribution to hair cell replacement after degenerative loss, could be driven by forced expression of β-catenin to act as hair cell progenitors in vivo. We showed that forced stabilization of β-catenin in supporting cells in neonatal animals resulted in proliferation of supporting cells and generation of hair cells. Although β-catenin expression was increased by genetic means in all supporting cells, entry to the cell cycle and differentiation to hair cells of the normally postmitotic cells was restricted to the Lgr5-positive population. Our finding suggests that Wnt/β-catenin can drive Lgr5-positive cells to act as hair cell progenitors, even after their exit from the cell cycle and apparent establishment of cell fate.

The development of hair cells in the auditory system can be separated into steps; first, the establishment of progenitors for the sensory epithelium, and second, the differentiation of hair cells. Although the differentiation of hair cells is known to require the expression of basic helix-loop-helix transcription factor, Atoh1, the control of cell proliferation in the region of the developing cochlea that will ultimately become the sensory epithelium and the cues that initiate Atoh1 expression remain obscure. We assessed the role of Wnt/␤-catenin in both steps in gain-and loss-of-function models in mice. The canonical Wnt pathway mediator, ␤-catenin, controls the expression of Atoh1. Knock-out of ␤-catenin inhibited hair-cell, as well as pillar-cell, differentiation from sensory progenitors but was not required to maintain a hair-cell fate once specified. Constitutive activation of ␤-catenin expanded sensory progenitors by inducing additional cell division and resulted in the differentiation of extra hair cells. Our data demonstrate that ␤-catenin plays a role in cell division and differentiation in the cochlear sensory epithelium.