Neurog1 can partially replace Atoh1 to differentiate and maintain hair cells in a disorganized organ of Corti

Atoh1, a basic helix-loop-helix transcription factor (TF), is essential for the differentiation of hair cells (HCs), mechanotransducers that convert sound into auditory signals in the mammalian organ of Corti (OC). Previous work demonstrated that replacing mouse Atoh1 with the fly ortholog atonal rescues HC differentiation, indicating functional replacement by other bHLH genes. However, replacing Atoh1 with Neurog1 resulted in reduced HC differentiation compared to transient Atoh1 expression in a ‘self-terminating’ Atoh1 conditional null mouse (Atoh1-Cre; Atoh1^f/f). We now show that combining Neurog1 in one allele with removal of floxed Atoh1 in a ‘self-terminating’ conditional mutant (Atoh1-Cre; Atoh1^f/kiNeurog1) mouse results in significantly more differentiated inner HCs and outer HCs that have a prolonged longevity of nine months compared to Atoh1 ‘self-terminating’ littermates. Stereocilia bundles are partially disorganized, disoriented and not HC type specific. Replacement of Atoh1 with Neurog1 maintains limited expression of Pou4f3 and Barhl1 and rescues HCs quantitatively, but not qualitatively. OC patterning as well as supporting cell differentiation is also partially disrupted. Diffusible factors involved in patterning are reduced (Fgf8) and factors involved in cell interactions are changed (Jag1, Hes5). Despite presence of many HCs with stereocilia these mice are deaf, possibly related to HC and OC patterning defects. This study provides a novel approach to disrupt OC development through modulating the HC specific intracellular TF network. The resulting disorganized OC indicates that normally differentiated HCs act as ‘self-organizers’ for OC development and that Atoh1 plays a critical role to initiate HC stereocilia differentiation independent of HC viability.