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Regeneration of Auditory Hair Cells
cochlea, inhibition of Notch signaling results in a significant increase in the number of hair cells (e.g., Hayashi et al., 2008; Doetzlhofer et al., 2009). Similar effects of Notch inhibition have been documented during hair cell regeneration in fish- es (Ma et al., 2008), birds (Daudet et al., 2009), and mouse vestibular organs (Lin et al., 2011). One study suggests that infusion of Notch inhibitors into live mice can promote sup- porting cells to convert into hair cells in the organ of Corti of adult mice after hair cell damage (Mizutari et al., 2013). However, another study clearly describes a precipitous loss of efficacy of Notch inhibitors to stimulate hair cell regen- eration (Maass et al., 2015). Hopefully, these apparently con- flicting interpretations of Notch inhibition will be resolved in future studies.
Lifting the Blockade on Supporting Cell Division
in Native Progenitors
As discussed above, supporting cells in the mature organ of Corti are strongly inhibited from dividing even after hair cells have been killed. Although Atoh1 misexpression and/ or Notch inhibition appears to encourage supporting cells to form hair cell-like cells in mature animals, neither treatment has a significant effect on supporting cell division. Therefore, as a therapy alone, either manipulation would likely deplete supporting cells, which would almost certainly reduce the function of the organ of Corti. Investigators are attempt- ing to determine how to promote supporting cells to divide mitotically and either replace themselves or form new hair cells. At this point, there are no known manipulations that have these effects in the mature organ of Corti. However, we know some ways in which supporting cell division can be promoted in the young cochlea.
For cochlear supporting cells to divide, they must exit their normal state of mitotic inactivity and enter the cell cycle. p27Kip1 is a molecule that blocks progenitor cells (or sup- porting cells) in the organ of Corti of mice from dividing during embryonic and postnatal development. Embryonic deletion of the gene encoding p27Kip1 causes an excess of cells to be formed in the organ of Corti, including hair cells (Chen and Segil, 1999; Löwenheim et al., 1999). In mature mice, blocking the synthesis of p27Kip1 causes a small but sig- nificant increase in cell division in some types of supporting cells in the organ of Corti (Oesterle et al., 2011). Inhibition of p27Kip1 and similar molecules is under investigation as a way to promote mammalian hair cell regeneration. It is par- ticularly important at this stage that investigators determine
if p27Kip1 deletion in adult rodents leads to the production of functional, stable hair cells.
Activity of p27Kip1 and other regulators of cell division is controlled by extracellular signaling molecules. One set of molecules that drives cell division in many tissues is Wnts, which binds receptors on the surface of cells and activates a transcriptional coactivator called ß-catenin (reviewed in Jansson et al., 2015). Wnt/ß-catenin signaling is required for progenitor cell division during cochlear development; when inhibited, significantly fewer hair cells form (Shi et al., 2014). Forced overexpression of Wnt promotes supporting cells in the organ of Corti to divide in very young mice but not in mature mice (Chai et al., 2012; Shi et al., 2013). Therefore, activation of Wnt alone cannot overcome other inhibitory signals present in the mature mammalian organ of Corti. In contrast, pharmacological activation of Wnt promotes hair cell regeneration in lateral line functional neuromasts of lar- val zebrafish (Head et al., 2013; Jacques et al., 2014).
Epidermal growth factor (EGF) is another molecule that drives supporting cell division in the supporting cells in the organ of Corti of neonatal mice as well as in supporting cells in the regenerating auditory epithelium of mature chickens (White et al., 2012). Treatment of cultured organs of Corti with EGF in newborn rats increases the formation of super- numerary hair cells (Lefebvre et al., 2000). Once again, this effect rapidly declines with age (Hume et al., 2003).
Could Transient or Combinatorial Treatments
Improve Hair Cell Regeneration?
As discussed above, we now know several powerful genes or signaling pathways that, when manipulated in very young rodents, cause supporting cells to divide and form new hair cells. But these same manipulations have very little effect or even deleterious effects in mature rodents. These findings tell us that promotion of hair cell regeneration in mature hu- mans will be more challenging than originally thought. One strategy that scientists are testing is whether transient acti- vation or suppression of gene activity has a better outcome than sustained alterations. During development, signals turn on and off in cells, whereas many of the manipulations discussed above are permanent and therefore unnatural. Modern techniques for transient gene silencing, such as siRNA, might enhance the effects of treatment by better recapitulating nature. Another hypothesis being tested is whether combinatorial manipulations of genes and path- ways can more effectively promote regeneration than single
44 | Acoustics Today | Summer 2016