Changes in the regulation of the Notch signaling pathway are temporally correlated with regenerative failure in the mouse cochlea

Abstract

Sensorineural hearing loss is most commonly caused by the death of hair cells in the organ of Corti, and once lost, mammalian hair cells do not regenerate. In contrast, other vertebrates such as birds can regenerate hair cells by stimulating division and differentiation of neighboring supporting cells. We currently know little of the genetic networks which become active in supporting cells when hair cells die and that are activated in experimental models of hair cell regeneration. Several studies have shown that neonatal mammalian cochlear supporting cells are able to trans-differentiate into hair cells when cultured in conditions in which the Notch signaling pathway is blocked. We now show that the ability of cochlear supporting cells to trans-differentiate declines precipitously after birth, such that supporting cells from six-day-old mouse cochlea are entirely unresponsive to a blockade of the Notch pathway. We show that this trend is seen regardless of whether the Notch pathway is blocked with gamma secretase inhibitors, or by antibodies against the Notch1 receptor, suggesting that the action of gamma secretase inhibitors on neonatal supporting cells is likely to be by inhibiting Notch receptor cleavage. The loss of responsiveness to inhibition of the Notch pathway in the first postnatal week is due in part to a down-regulation of Notch receptors and ligands, and we show that this down-regulation persists in the adult animal, even under conditions of noise damage. Our data suggest that the Notch pathway is used to establish the repeating pattern of hair cells and supporting cells in the organ of Corti, but is not required to maintain this cellular mosaic once the production of hair cells and supporting cells is completed. Our results have implications for the proposed used of Notch pathway inhibitors in hearing restoration therapies.

Notch-Wnt-Bmp crosstalk regulates radial patterning in the mouse cochlea in a spatiotemporal manner

“…Studies have shown that upon Wnt stimulation, regeneration is possible in the neonatal cochlea, but this potential is lost after postnatal day 3 (Chai et al, 2012;Cox et al, 2014). Such regenerative failure is linked to changes in the Notch pathway (Maass et al, 2015). Therefore, understanding the gene networks operating from early to late ages might prove informative in assessing feasible approaches to regeneration.…”

Epigenetic regulation of Atoh1 guides hair cell development in the mammalian cochlea

“…However, the ability to transdifferentiate in response to Notch inhibition rapidly diminishes following P1 (Maass et al, 2015). In spite of this, we show that bivalency is maintained through P6, consistent with the maintenance of limited transdifferentiation potential in response to Atoh1 ectopic expression (Kelly et al, 2012;Liu et al, 2012a).…”

“…Over the next several days, Atoh1 mRNA levels rise to peak expression around E17.5, before again falling to low levels by P6 (Fig. 1A) (Driver et al, 2013;Maass et al, 2015). To correlate changes in epigenetic status with changes in Atoh1 expression in specific cell types and at different ages, we used fluorescence-activated cell sorting (FACS) to separate prosensory cells prior to the onset of hair cell differentiation, from nascent hair cells that have begun to upregulate Atoh1.…”

“…In birds, hair cell regeneration correlates with increased levels of the basic helix-loop-helix (bHLH) transcription factor Atoh1 in surviving supporting cells (Cafaro et al, 2007), followed by their subsequent proliferation and/or direct transdifferentiation. Interestingly, although hair cell loss does not lead to widespread supporting cell regeneration in mature mammals, a latent potential for direct transdifferentiation of supporting cells to hair cells persists in the newborn mouse organ of Corti (Bramhall et al, 2014;Doetzlhofer et al, 2009;Takebayashi et al, 2007;White et al, 2006), though recent reports indicate that this potential is lost during the first week after birth (Liu et al, 2012b;Maass et al, 2015).…”

“…Through Notch-mediated lateral inhibition, Atoh1 expression in nascent hair cells represses Atoh1 expression in surrounding progenitors, and stimulates supporting cell differentiation (Kelley, 2006;Woods et al, 2004). Although Atoh1 is required for the differentiation of hair cells, it is subsequently downregulated, starting at about E17.5 and reduced to barely detectable levels by postnatal day (P) 6 (Driver et al, 2013;Maass et al, 2015) (Fig. 1A).…”