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.