The RNA-binding protein LIN28B regulates developmental timing in the mammalian cochlea

Proper tissue development requires strict coordination of proliferation, growth, and differentiation. Strict coordination is particularly important for the auditory sensory epithelium, where deviations from the normal spatial and temporal pattern of auditory progenitor cell (prosensory cell) proliferation and differentiation result in abnormal cellular organization and, thus, auditory dysfunction. The molecular mechanisms involved in the timing and coordination of auditory prosensory proliferation and differentiation are poorly understood. Here we identify the RNA-binding protein LIN28B as a critical regulator of developmental timing in the murine cochlea. We show that Lin28b and its opposing let-7 miRNAs are differentially expressed in the auditory sensory lineage, with Lin28b being highly expressed in undifferentiated prosensory cells and let-7 miRNAs being highly expressed in their progeny-hair cells (HCs) and supporting cells (SCs). Using recently developed transgenic mouse models for LIN28B and let-7g, we demonstrate that prolonged LIN28B expression delays prosensory cell cycle withdrawal and differentiation, resulting in HC and SC patterning and maturation defects. Surprisingly, let-7g overexpression, although capable of inducing premature prosensory cell cycle exit, failed to induce premature HC differentiation, suggesting that LIN28B's functional role in the timing of differentiation uses let-7 independent mechanisms. Finally, we demonstrate that overexpression of LIN28B or let-7g can significantly alter the postnatal production of HCs in response to Notch inhibition; LIN28B has a positive effect on HC production, whereas let-7 antagonizes this process. Together, these results implicate a key role for the LIN28B/let-7 axis in regulating postnatal SC plasticity.