Generation of p27icreER transgenic mice: A tool for inducible gene expression in supporting cells in the cochlea.

The loss of cochlear hair cells (HCs) is an important cause of sensorineural hearing loss, and finding ways to regenerate HCs would be the ideal way forward for restoring hearing. In this research field, tamoxifen-inducible Cre recombinase (iCreER) transgenic mice and the Cre-loxp system are widely used to manipulate gene expression in supporting cells (SCs), which lie beneath the sensory HCs and are a natural source for HC regeneration. However, many iCreER transgenic lines are of limited utility because they cannot target all subtypes of SCs or they cannot be used in the adult stage. In this study, a new line of iCreER transgenic mice, the p27-P2A-iCreERT2 knock-in mouse strain, was generated by inserting the P2A-iCreERT2 cassette immediately in front of the stop codon of p27, which kept the endogenous expression and function of p27 intact. Using a reporter mouse line with tdTomato fluorescence, we showed that the p27iCreER transgenic line can target all subtypes of cochlear SCs, including Claudius cells. p27-CreER activity in SCs was observed in both the postnatal and the adult stage, suggesting that this mouse strain can be useful for research work in adult cochlear HC regeneration. We then overexpressed Gfi1, Pou4f3, and Atoh1 in p27+ SCs of P6/7 mice using this strain and successfully induced many new Myo7a/tdTomato double-positive cells, further confirming that the p27-P2A-iCreERT2 mouse strain is a new and reliable tool for cochlear HC regeneration and hearing restoration.