Revisiting the potency of Tbx2 expression in transforming outer hair cells into inner hair cells at multiple ages in vivo

The cochlea, the auditory organ, contains two types of sound receptors: inner hair cells (IHCs) and outer hair cells (OHCs). Tbx2 is expressed in IHCs but repressed in OHCs. The neonatal OHCs with Tbx2 misexpression transdifferentiate into IHC-like cells. However, the extent of the switch from OHCs to IHC-like cells and the underlying molecular mechanism remain poorly understood. Furthermore, it is unknown whether Tbx2 can transform fully mature adult OHCs into IHC-like cells. In this study, we employ single-cell transcriptomic analysis, revealing that 85.6% of IHC genes, including Slc17a8, are upregulated, whereas only 38.6% of OHC genes, including Ikzf2 and Slc26a5, are downregulated in neonatal OHCs with Tbx2 misexpression. Thus, our findings suggest that Tbx2 cannot fully reprogram neonatal OHCs into IHCs, contrary to previous assumptions. Consistently, Tbx2 also fails to fully reprogram cochlear progenitors into IHCs. Finally, Ikzf2 restoration alleviates the abnormalities present in the Tbx2+ OHCs, supporting the notion that Ikzf2 repression by Tbx2 contributes to the transdifferentiation of OHCs into IHC-like cells. Overall, our study reevaluates the effects of ectopic Tbx2 expression on OHC lineage development at different stages and provides molecular insights into how Tbx2 disrupts the gene expression profiles of OHCs. This research also lays the groundwork for future studies on OHC regeneration. Significance StatementUnderstanding the molecular and genetic mechanisms that govern the determination and stability of cochlear inner hair cells (IHCs) and outer hair cells (OHCs) would provide valuable insights into regenerating damaged IHCs and OHCs. In this manuscript, we conduct in vivo conditional overexpression of Tbx2 in cochlear sensory progenitors, neonatal OHCs and adult OHCs, respectively. Our findings challenge previous assumptions by revealing that Tbx2 expression alone can partially destabilize OHC fates but cannot fully convert OHCs into IHCs. Specifically, we demonstrate that the repression of Ikzf2 expression due to Tbx2 overexpression is one of the key pathways disrupting OHC fate.