In VivoProliferative Regeneration of Balance Hair Cells in Newborn Mice

Abstract

The regeneration of mechanoreceptive hair cells occurs throughout life in non-mammalian vertebrates and allows them to recover from hearing and balance deficits that affect humans and other mammals permanently. The irreversibility of comparable deficits in mammals remains unexplained, but often has been attributed to steep embryonic declines in cellular production. However, recent results suggest that gravity-sensing hair cells in murine utricles may increase in number during neonatal development, raising the possibility that young mice might retain sufficient cellular plasticity for mitotic hair cell regeneration. To test for this we used neomycin to kill hair cells in utricles cultured from mice of different ages and found that proliferation increased ten-fold in damaged utricles from the youngest neonates. To kill hair cells in vivo, we generated a novel mouse model that uses an inducible, hair-cell-specific CreER allele to drive expression of diptheria toxin fragment A (DTA). In newborns, induction of DTA expression killed hair cells and resulted in significant, mitotic hair cell replacement in vivo, which occurred days after the normal cessation of developmental mitoses that produce hair cells. DTA expression induced in five-day-old mice also caused hair cell loss, but no longer evoked mitotic hair cell replacement. These findings show that regeneration limits arise in vivo during the postnatal period when the mammalian balance epithelium’s supporting cells differentiate unique cytological characteristics and lose plasticity, and they support the notion that the differentiation of those cells may directly inhibit regeneration or eliminate an essential, but as yet unidentified pool of stem cells.

Spontaneous regeneration of cochlear supporting cells after neonatal ablation ensures hearing in the adult mouse

“…This same cell specificity was described previously using the Rosa26 LacZ reporter (27). Activation of the DTA transgene leads to cell-autonomous death via apoptosis (29)(30)(31). Triple-transgenic mice [PlpCreER T ;Ai14:Rosa26 tdTom ;Rosa26 DTA (PlpTomDTA)] and their PlpTom littermates were injected with tamoxifen at P0 and P1, and their cochleae were analyzed at P7.…”

Sensory hair cell development and regeneration: similarities and differences

“…In the absence of damage, pharmacological inhibition of Notch signaling in the adult utricle failed to promote a hair cell fate among supporting cells (Collado et al, 2011). However, after hair cell loss, the inhibition of Notch signaling via either inhibition of the target gene Hes5 or pharmacological inhibition of γ-secretase was able to significantly increase Atoh1 expression, with a subset of supporting cells maturing to myosin VIIa-expressing hair cells (Burns et al, 2012;Jung et al, 2013;Lin et al, 2011). Similarly, more hair cells were detected following Notch inhibition in the damaged crista ampullaris (Slowik and Bermingham-McDonogh, 2013).…”

Over Half the Hair Cells in the Mouse Utricle First Appear After Birth, with Significant Numbers Originating from Early Postnatal Mitotic Production in Peripheral and Striolar Growth Zones

“…In a separate article, we report that the utricles of mice that are beyond the age when proliferation occurs during normal postnatal growth of the macula retain the capacity for regenerating hair cells in vivo. However, that capacity is lost just a few days later (Burns et al 2012).…”