Transdifferentiation in the Cochlea

Project Summary The developing mammalian cochlea generates two types of mechanosensors: Inner hair cells (IHCs), endowed with a prominent presynaptic apparatus, transmit sound information to neurons and the brain; outer hair cells (OHCs), endowed for electromotility, are used for amplification and sharp frequency discrimination of sounds. We found that the T-box transcription factor TBX2, expressed in IHCs, is a master regulator of their differentiation, preventing IHCs from transdifferentiating into OHCs. Elimination of TBX2 from IHCs at various embryonic and postnatal stages results in their conversion into OHCs. We hypothesize that TBX2 not only induces IHC differentiation, but that it also functions subsequently for them to maintain their fate. We also hypothesize that TBX2, in addition to being necessary, is also sufficient to elicit the IHC vs OHC differentiation pattern. We will identify the genes regulated by TBX2 and driving IHC vs OHC differentiation, including the chain of gene-expression events leading from one cell type to the other. We also inquire whether TBX2 acts as a transcriptional activator or repressor, and whether it acts by modifying the epigenome of IHCs, either opening regulatory elements required for IHC differentiation or closing those required for OHC differentiation. To test these and alternative hypotheses, we will perform ATAC-seq on developing and mature IHCs, OHCs, and IHCs lacking TBX2 (all of which transdifferentiate into OHCs). Finally, we will identify the TBX2-binding sites in the promoters and enhancers of the genes it regulates in order to trigger and/or maintain IHC differentiation. These studies use the transdifferentiation of IHCs into OHCs (and vice versa) triggered by TBX2 missregulation as a means for elucidating the molecular mechanisms (transcriptomic and epigenomic) by which the complementary IHCs and OHCs are generated in the developing cochlea.