Aging, disease, and trauma can lead to loss of vestibular hair cells and permanent vestibular dysfunction. Previous work showed that, following acute destruction of ~95% of vestibular hair cells in adult mice, ~20% regenerate naturally (without exogenous factors) through supporting cell transdifferentiation. There is, however, no evidence for the recovery of vestibular function. To gain insight into the lack of functional recovery, we assessed functional differentiation in regenerated hair cells for up to 15 months, focusing on key stages in stimulus transduction and transmission: hair bundles, voltage-gated conductances, and synaptic contacts. Regenerated hair cells had many features of mature type II vestibular hair cells, including polarized mechanosensitive hair bundles with zone-appropriate stereocilia heights, large voltage-gated potassium currents, basolateral processes, and afferent and efferent synapses. Regeneration failed, however, to recapture the full range of properties of normal populations, and many regenerated hair cells had some properties of immature hair cells, including small transduction currents, voltage-gated sodium currents, and small or absent HCN (hyperpolarization-activated cyclic nucleotide-gated) currents. Furthermore, although mouse vestibular epithelia normally have slightly more type I hair cells than type II hair cells, regenerated hair cells acquired neither the low-voltage-activated potassium channels nor the afferent synaptic calyces that distinguish mature type I hair cells from type II hair cells and confer distinctive physiology. Thus, natural regeneration of vestibular hair cells in adult mice is limited in total cell number, cell type diversity, and extent of cellular differentiation, suggesting that manipulations are needed to promote full regeneration with the potential for recovery of vestibular function.

SIGNIFICANCE STATEMENT Death of inner ear hair cells in adult mammals causes permanent loss of hearing and balance. In adult mice, the sudden death of most vestibular hair cells stimulates the production of new hair cells but does not restore balance. We investigated whether the lack of systems-level function reflects functional deficiencies in the regenerated hair cells. The regenerated population acquired mechanosensitivity, voltage-gated channels, and afferent synapses, but did not reproduce the full range of hair cell types. Notably, no regenerated cells acquired the distinctive properties of type I hair cells, a major functional class in amniote vestibular organs. To recover vestibular system function in adults, we may need to solve how to regenerate the normal variety of mature hair cells.

衰老、疾病和创伤可导致前庭毛细胞丢失和永久性前庭功能障碍。先前的研究表明，成年小鼠约 95% 的前庭毛细胞被急性破坏后，约 20% 会通过支持细胞转分化自然再生（无外源因素）。然而，没有证据表明前庭功能恢复。为了深入了解功能恢复的缺乏，我们评估了长达 15 个月的再生毛细胞的功能分化，重点关注刺激转导和传输的关键阶段：毛束、电压门控电导和突触接触。再生的毛细胞具有成熟的 II 型前庭毛细胞的许多特征，包括具有适合区域的静纤毛高度的极化机械敏感毛束，大电压门控钾电流、基底外侧突以及传入和传出突触。然而，再生未能重新获得正常种群的全部特性，许多再生毛细胞具有未成熟毛细胞的一些特性，包括小转导电流、电压门控钠电流和小或不存在 HCN（超极化激活循环核苷酸门控）电流。此外，尽管小鼠前庭上皮通常具有比 II 型毛细胞略多的 I 型毛细胞，但再生的毛细胞既没有获得低压激活的钾通道，也没有获得区分成熟 I 型毛细胞和 II 型毛细胞的传入突触花萼。并赋予独特的生理学。因此，

意义声明成年哺乳动物内耳毛细胞的死亡会导致听力和平衡的永久性丧失。在成年小鼠中，大多数前庭毛细胞的突然死亡会刺激新毛细胞的产生，但不会恢复平衡。我们调查了系统级功能的缺乏是否反映了再生毛细胞的功能缺陷。再生后的种群获得了机械敏感性、电压门控通道和传入突触，但没有重现所有毛细胞类型。值得注意的是，没有再生细胞获得 I 型毛细胞的独特特性，I 型毛细胞是羊膜前庭器官的主要功能类别。为了恢复成人的前庭系统功能，我们可能需要解决如何再生正常种类的成熟毛细胞。