LIN28B/ let-7 control the ability of neonatal murine auditory supporting cells to generate hair cells through mTOR signaling

Abstract

Mechano-sensory hair cells within the inner ear cochlea are essential for the detection of sound. In mammals, cochlear hair cells are only produced during development and their loss, due to disease or trauma, is a leading cause of deafness. In the immature cochlea, prior to the onset of hearing, hair cell loss stimulates neighboring supporting cells to act as hair cell progenitors and produce new hair cells. However, for reasons unknown, such regenerative capacity (plasticity) is lost once supporting cells undergo maturation. Here, we demonstrate that the RNA binding protein LIN28B plays an important role in the production of hair cells by supporting cells and provide evidence that the developmental drop in supporting cell plasticity in the mammalian cochlea is, at least in part, a product of declining LIN28B-mammalian target of rapamycin (mTOR) activity. Employing murine cochlear organoid and explant cultures to model mitotic and nonmitotic mechanisms of hair cell generation, we show that loss of LIN28B function, due to its conditional deletion, or due to overexpression of the antagonistic miRNA let-7g, suppressed Akt-mTOR complex 1 (mTORC1) activity and renders young, immature supporting cells incapable of generating hair cells. Conversely, we found that LIN28B overexpression increased Akt-mTORC1 activity and allowed supporting cells that were undergoing maturation to de-differentiate into progenitor-like cells and to produce hair cells via mitotic and nonmitotic mechanisms. Finally, using the mTORC1 inhibitor rapamycin, we demonstrate that LIN28B promotes supporting cell plasticity in an mTORC1-dependent manner.

mTOR Signaling in the Inner Ear as Potential Target to Treat Hearing Loss

“…Accordingly, the deletion of Lin28a/b or let-7 miRNA overexpression decreased the phosphorylation levels of AKT, S6, and 4E-BP in neonatal mice cochleae and organoids. In contrast, higher phosphorylation levels of AKT and S6 have been detected in Lin28b overexpressing organoids than in control organoids [125]. Notably, rapamycin attenuated supporting cell proliferation and hair cell generation promoted by Lin28b overexpression in organoids and cochlear explants.…”

“…In contrast, the deletion of Lin28a/b or the overexpression of their inhibitor let-7 miRNA decreased the hair cell generation capabilities of supporting cells [125]. Most interestingly, both the phosphorylation of the S6 protein at Ser240/244 and AKT phosphorylation at Ser473 decreased during postnatal development in cochlear explants (in parallel with the decline in hair cell generation capabilities) [125]. Accordingly, the deletion of Lin28a/b or let-7 miRNA overexpression decreased the phosphorylation levels of AKT, S6, and 4E-BP in neonatal mice cochleae and organoids.…”

“…Overexpressing Lin28b in cochlear explants moreover increased the production of new hair cells by nonmitotic mechanisms. In contrast, the deletion of Lin28a/b or the overexpression of their inhibitor let-7 miRNA decreased the hair cell generation capabilities of supporting cells [125]. Most interestingly, both the phosphorylation of the S6 protein at Ser240/244 and AKT phosphorylation at Ser473 decreased during postnatal development in cochlear explants (in parallel with the decline in hair cell generation capabilities) [125].…”

“…Recent work by Li and Doetzlhofer has shown that the RNA binding protein LIN28B promotes the proliferation capacity and reprogramming of supporting cells to generate hair cells via the AKT-mTORC1 pathway [125]. The intrinsic ability of cochlear supporting cells to (re)generate hair cells is quickly lost during the first postnatal days.…”

“…The intrinsic ability of cochlear supporting cells to (re)generate hair cells is quickly lost during the first postnatal days. Already epithelial cells of postnatal day (P)5 mice fail to expand and produce hair cells [125]. Using both an organoid culture system and cochlear explant cultures from P5 transgenic mice overexpressing Lin28b, Li and Doetzlhofer showed that organoids formed an increased number of hair cell clusters than the control organoids.…”