Casz1 is required for both inner hair cell fate stabilization and outer hair cell survival
Cochlear inner hair cells (IHCs) and outer hair cells (OHCs) require different transcription factors for their cell fate stabilization and survival, suggesting separate mechanisms are involved. Here, we found that the transcription factor Casz1 was crucial for early IHC fate consolidation and for OHC survival during mouse development. Loss of Casz1 resulted in transdifferentiation of IHCs into OHCs, without affecting OHC production. However, long-term OHC survival was compromised in Casz1 mutant mice. In addition,  the transcription factor Gata3 was downregulated in Casz1-deleted IHCs and overexpressing Gata3 partially rescued IHC properties, OHC numbers, and hearing in Casz1-deleted mice. Thus, Casz1 plays critical roles in early IHC fate stabilization and OHC survival and could potentially provide a lead for therapies aimed at regenerating both IHCs and OHCs. Overall design: Casz1 -/- IHCs were manually picked using the genetic mouse model: Atoh1Cre/+; Casz1flox/flox; Rosa26-CAG-Loxp-stop-Loxp-3xGFP(Ai47)/Rosa26-CAG-Frt-stop-Frt-tdTomato; Fgf8-Flpo/+. In this model, Casz1 -/- IHCs can be simultaneously labelled with GFP and Tdtomato. In total, 73 GFP+/Tdtomato+ Casz1-/- IHCs at P14 (P14_Casz1-/- IHCs in brief) were manually picked under the fluorescent microscope. The de-novo picked 73 Casz1-/- IHCs at P14 along with the 55 wild type IHCs at P14 in our previous study (GSE199369), were pooled together for further smart-seq based single-cell transcriptomic assay.