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

Abstract

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 RNAbinding 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.Lin28b | Let-7 | hair cell | cochlea | regeneration

Lin28a uses distinct mechanisms of binding to RNA and affects miRNA levels positively and negatively

“…Other mechanisms for the control of let-7a that operate at the level of DGCR8/Drosha processing have also been suggested (Piskounova et al 2008). Additionally, close homolog Lin28b also controls let-7 levels in vivo (Shyh-Chang and Daley 2013;Golden et al 2015).…”

Cochlear hair cell regeneration after noise-induced hearing loss: Does regeneration follow development?

“…2B). In addition to Pou4f3, Lin28b is an Atoh1 target that, when overexpressed, can itself induce HC formation in explant culture (Golden et al, 2015). The overexpression of Sox4/11 can similarly convert SCs to HC-like cells in utricle explants, whereas their activity in the cochlea is unknown (Gnedeva et al, 2015).…”

“…Further studies identified the specific miRNAs and their downstream targets. In these studies, the miRNA 183 family, miRNA 124, miRNA 210, and let7 were shown to play essential roles in HC formation during inner ear development (Golden et al, 2015; Huyghe et al, 2015; Li et al, 2010; Weston et al, 2011). The miRNA 183 family also facilitates Atoh1-promoted HC regeneration in vitro (Ebeid et al, 2016).…”

Insights into inner ear-specific gene regulation: Epigenetics and non-coding RNAs in inner ear development and regeneration

“…In turn, LIN28 RBPs bind let-7 primary and precursor transcript and interfere with their processing by Drosha and Dicer1 (reviewed in [123]). In the developing inner ear, Lin28b is selectively expressed in pro-sensory cells, whereas let-7 miRNAs are expressed in terminally differentiated cells including hair cells and supporting cells [124]. Using inducible LIN28B and let-7g transgenic mouse lines the authors found that higher than normal levels of let-7 miRNAs resulted in premature pro-sensory cell cycle exit, whereas lower than normal let-7 levels, due to LIN28B overexpression delayed pro-sensory cell cycle exit [124].…”

“…In the developing inner ear, Lin28b is selectively expressed in pro-sensory cells, whereas let-7 miRNAs are expressed in terminally differentiated cells including hair cells and supporting cells [124]. Using inducible LIN28B and let-7g transgenic mouse lines the authors found that higher than normal levels of let-7 miRNAs resulted in premature pro-sensory cell cycle exit, whereas lower than normal let-7 levels, due to LIN28B overexpression delayed pro-sensory cell cycle exit [124]. LIN28B overexpression led to an increase in cochlear N-Myc (Mycn) and cyclin D1 (Ccnd1) expression.…”