Department of Neuroscience, University of Virginia School of Medicine Charlottesville, VA 22908, USA.
J Neurosci. 2012 May 9;32(19):6570-7. doi: 10.1523/JNEUROSCI.6274-11.2012.
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 tenfold 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 diphtheria 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 5-d-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.
机械感受毛细胞在非哺乳动物的整个生命过程中都会再生，使它们能够从听力和平衡缺陷中恢复过来，而这些缺陷会对人类和其他哺乳动物造成永久性的影响。哺乳动物中类似缺陷的不可逆转性仍未得到解释，但通常归因于细胞产生的急剧胚胎下降。然而，最近的结果表明，鼠类椭圆囊中的重力感受毛细胞在新生儿发育过程中数量可能会增加，这增加了年轻老鼠可能保留足够的细胞可塑性以进行有丝分裂毛细胞再生的可能性。为了验证这一点，我们使用新霉素杀死从小鼠不同年龄培养的椭圆囊中毛细胞，发现最年幼的新生儿受损的椭圆囊中增殖增加了十倍。为了在体内杀死毛细胞，我们生成了一种新型小鼠模型，该模型使用可诱导的、毛细胞特异性 CreER 等位基因来驱动白喉毒素片段 A (DTA) 的表达。在新生儿中，DTA 表达的诱导杀死了毛细胞，并导致体内有丝分裂毛细胞的显著替代，这发生在产生毛细胞的发育有丝分裂正常停止几天后。在 5 天大的小鼠中诱导的 DTA 表达也导致了毛细胞的丧失，但不再引起有丝分裂毛细胞的替代。这些发现表明，在出生后时期，当哺乳动物平衡上皮的支持细胞分化出独特的细胞学特征并丧失可塑性时，体内的再生限制就会出现，并且它们支持这些细胞的分化可能直接抑制再生或消除一个必要的、但尚未确定的干细胞池的观点。
