SUFU and GLI regulate the differentiation of mammalian cochlear hair cells

Abstract

The temporal uncoupling of cell cycle exit and differentiation is a unique phenomenon in the developing auditory sensory epithelium of the mammalian cochlea. Precise spatiotemporal coordination of proliferation, cell cycle exit and differentiation is essential for the exquisite patterning of the auditory epithelium and hearing functions. Sonic Hedgehog signaling was proposed to be required for separating the cell cycle exit and differentiation of the auditory progenitors. However, its molecular mechanisms remain elusive. Gli transcription factors are mediators of Shh signaling. Sufu, a negative regulator of Shh signaling, sequesters Gli full-length activator(GliA) in the cytoplasm, allowing them to undergo proteolytic cleavage to form Gli repressor(GliR). Spop, an E3 ubiquitin ligase adaptor, promotes GliA degradation. Here, we use Sufu, Gli and Spop mouse mutants to investigate how Shh signaling regulates differentiation of auditory prosensory cells. We report that loss of Sufu in the cochlear epithelium leads to a significant delay in hair cell differentiation due to the lack of Atoh1 protein expression. Normally, Gli2 is not expressed in the differentiating organ of Corti. However, removal of Sufu causes ectopic expression of Gli2 in the organ of Corti. Deletion of Gli2 in Sufu mutant context could restore the delay of hair cell differentiation. The delay, but not inhibition of hair cell differentiation, is caused by the down-regulation of Gli2 levels. Loss of Spop in Sufu mutant background results in elevated Gli2 levels and severely inhibits hair cell differentiation. Moreover, elevated Gli2 levels lead to a significant expansion and maintenance of Sox2 expression in the cochlear epithelium. Overall, these results indicate the key inhibitory role of Gli2 and the availability of Gli2, subjected to an intricate regulation, is critical for mediating the hair cell differentiation. This work is supported by the Research Grant Council in Hong Kong (RGC GRF 17159316).