Retinoic Acid Signaling Mediates Hair Cell Regeneration by Repressingp27kip andsox2in Supporting Cells

Abstract

During development, otic sensory progenitors give rise to hair cells and supporting cells. In mammalian adults, differentiated and quiescent sensory cells are unable to generate new hair cells when these are lost due to various insults, leading to irreversible hearing loss. Retinoic acid (RA) has strong regenerative capacity in several organs, but its role in hair cell regeneration is unknown. Here, we use genetic and pharmacological inhibition to show that the RA pathway is required for hair cell regeneration in zebrafish. When regeneration is induced by laser ablation in the inner ear or by neomycin treatment in the lateral line, we observe rapid activation of several components of the RA pathway, with dynamics that position RA signaling upstream of other signaling pathways. We demonstrate that blockade of the RA pathway impairs cell proliferation of supporting cells in the inner ear and lateral line. Moreover, in neuromast, RA pathway regulates the transcription of p27 kip and sox2 in supporting cells but not fgf3. Finally, genetic cell-lineage tracing using Kaede photoconversion demonstrates that de novo hair cells derive from FGF-active supporting cells. Our findings reveal that RA has a pivotal role in zebrafish hair cell regeneration by inducing supporting cell proliferation, and shed light on the underlying transcriptional mechanisms involved. This signaling pathway might be a promising approach for hearing recovery.

Insights into sensory hair cell regeneration from the zebrafish lateral line

“…Thus, RA has a pro-proliferative function. RA is also required for support cell proliferation and HC regeneration in the zebrafish ear and lateral line [58 ]. Ectopic expression of a dominant negative RA receptor (dnRAR) blocks regeneration by preventing support cell proliferation, possibly by maintaining expression of the cell cycle inhibitor cdkn1bb ( p27 kip ) [58 ], which is normally downregulated immediately after neuromast HC ablation [27 ].…”

“…RA is also required for support cell proliferation and HC regeneration in the zebrafish ear and lateral line [58 ]. Ectopic expression of a dominant negative RA receptor (dnRAR) blocks regeneration by preventing support cell proliferation, possibly by maintaining expression of the cell cycle inhibitor cdkn1bb ( p27 kip ) [58 ], which is normally downregulated immediately after neuromast HC ablation [27 ]. Intriguingly, Notch downregulation also reduces cdkn1bb [16 ], suggesting that the downregulation of Notch and the activation of RA signaling may act together or in parallel to regulate cell proliferation during regeneration (Figure 2e).…”

“…Notch inhibits proliferation via cdkn1bb and by inhibiting Wnt signaling through the activation of dkk2. Retinoic acid (RA) promotes proliferation possibly via inhibition of cdkn1bb[58 ]. Wnt signaling activates proliferation of hair cell progenitors in the center and support cell progenitors in the poles.…”

In Vitro Differentiation of Human Bone Marrow Mesenchymal Stem Cells to Hair Cells Using Growth Factors

“…The RA pathway is activated immediately after the loss of hair cells, accelerating the proliferation of cell precursors, and regulating the expression of two key genes, P27kip and SOX2. The results have indicated that RA can be used as an essential signal for the regeneration of hair cells 34 . In the cochlea of rats fed with all-trans retinoic acid (ATRA), the protection of the organ of corti is more likely to occur than in the cochlea of those feds with oil and saline.…”

Combined Growth Factor and Gene Therapy: An Approach for Hair Cell Regeneration and Hearing Recovery

“…RA has the additional capacity to be reproduced in several organs, but its role in the reproduction of HCs has not been fully recognized. This signaling pathway is likely to be a promising method for hearing restoration [60].…”