Revisiting the potency of Tbx2 expression in transforming outer hair cells into inner hair cells at multiple ages in vivo
The mouse auditory organ cochlea contains two types of sound receptors: inner hair cells (IHCs) and outer hair cells (OHCs). Tbx2 is expressed in IHCs but repressed in OHCs, and neonatal OHCs that misexpress Tbx2 transdifferentiate into IHC-like cells. However, the extent of this switch from OHCs to IHC-like cells and the underlying molecular mechanism remain poorly understood. Furthermore, whether Tbx2 can transform fully mature adult OHCs into IHC-like cells is unknown. Here, our single-cell transcriptomic analysis revealed that in neonatal OHCs misexpressing Tbx2, 85.6% of IHC genes, including Slc17a8, are upregulated, but only 38.6% of OHC genes, including Ikzf2 and Slc26a5, are downregulated. This suggests that Tbx2 cannot fully reprogram neonatal OHCs into IHCs. Moreover, Tbx2 also failed to completely reprogram cochlear progenitors into IHCs. Lastly, restoring Ikzf2 expression alleviated the abnormalities detected in Tbx2+ OHCs, which supports the notion that Ikzf2 repression by Tbx2 contributes to the transdifferentiation of OHCs into IHC-like cells. Our study evaluates the effects of ectopic Tbx2 expression on OHC lineage development at distinct stages of either male or female mice and provides molecular insights into how Tbx2 disrupts the gene-expression profile of OHCs. This research also lays the groundwork for future studies on OHC regeneration.Significance Statement Elucidation of the molecular and genetic mechanisms governing the determination and stability of cochlear inner hair cells (IHCs) and outer hair cells (OHCs) should provide valuable insights into the regeneration of damaged IHCs and OHCs. Here, we conditionally overexpress Tbx2 in vivo in cochlear sensory progenitors, neonatal OHCs, or adult OHCs. Our results show that Tbx2 overexpression alone can partially destabilize the OHC fate but cannot fully convert OHCs into IHCs. Specifically, we demonstrate that Ikzf2 repression due to Tbx2 overexpression is one of the key pathways disrupting the OHC fate. Overall design: The smart-seq based single-cell RNA-seq approach was applied to wild type cochlear outer hair cells (OHCs) at P30 as well as the converted IHCs (cIHCs) at P42. The cIHCs are the cochlear OHCs expressing ectopic Tbx2 and transdifferentiating into IHCs. The wild type OHCs are obtained from the Slc26a5-CreER/+; Ai9/+ mice at P30, whereas the cIHCs are from theSlc26a5-CreER/+; Rosa26Tbx2/+ mice at P42. Briefly, both wild type OHCs and cIHCs express Tdtomato and are manually picked under the fluorescent microscope. In this study, 32 wild type OHCs at P30 were de-novo picked. Together with the 17 OHCs at P30 reported in our previous study, we totally have 49 OHCs. All the 67 cIHCs were  de-novo picked in this study.